nubbin/POU domain protein 1

DEVELOPMENTAL BIOLOGY

Embryonic

See the embryonic expression pattern of nub at the Berkeley Drosophila Genome Project Patterns of Gene Expression Site.

pdm-1 is first expressed in the cellular blastoderm stage as two bands, each 8-10 cells wide, in the primordia of the abdominal segments, and in the head region in the anlage of the clypeolabium [Images]. The pattern of expression is the same as that of pdm-2. pdm-1 later exhibits a striped pattern at germ band extention where its pattern overlaps that of ftz. This expression is in the region of the neuroectoderm. During germ band retraction, pdm-1 transcripts are detected in the endoderm in the developing anterior and posterior midgut primordia (Affolter, 1993). pdm-1 is expressed later in a subset of CNS and PNS cells (Dick, 1991 and Lloyd, 1991). Thus, very specific neuroblasts express pdm-1. In the early part of the NB4-2 lineage, from which the RP2 motor neuron is derived, pdm-1 and pdm-2 are expressed. These genes are not required for the birth of the first ganglion mother cell (GMC4-2a) but both are involved in specifying its identity (Yeo, 1995).

In addition to its early regulatory functions during segmentation, Hunchback is also expressed in the developing nervous system. One possible CNS regulatory target for Hb is the POU gene pdm-1. Hb regulates pdm-1 expression at the cellular blastoderm stage, and may play a similar role in the CNS. Since Hb and Castor bind similar promoter target sequences, an exploration was carried out of the embryonic distribution of the three proteins using polyclonal antibodies. It is suggested that Hb and Cas act in a cooperative, non-overlapping manner to control POU gene expression during Drosophila CNS development. By silencing pdm expression in early and late neuroblast (NB) sublineages, Hb and Cas establish three pan-CNS compartments whose cellular constituents are marked by the expression of either Hb, Pdm, or Cas (see Lateral views of Drosophila CNS). During the initial S1 and S2 waves of NB delaminations, Pdm-1 is expressed in most, if not all, neuroectoderm cells. However, no Pdm-1 is detected in fully delaminated NBs and during stage 9 only a small subset of ventral cord GMCs express detectable levels. At this time, Hb expression is detected in all fully delaminated NBs and in many of their GMCs but not in neuroectoderm cells. Starting at late stage 9, Hb immunoreactivity is progressively lost from NBs; by late stage 10 only a small subset of ventral cord NBs express Hb. However, Hb is detected in many GMC and in their progeny generated during the first rounds of GMC production. These early sublineages reside predominantly along the inner/dorsal surfaces of the developing ganglia. The reduction in Hb NB expression coincides with the activation of Pdm-1 NB expression; by late stage 10, Pdm-1 is detected in many cephalic lobe (see Views of cephalic lobe neuroblasts) and ventral cord NBs and in GMCs. Similar to the dynamics of Hb expression, Pdm-1 NB expression is transient. However, many GMCs and their progeny arising from the Pdm-expressing NBs maintain high levels of Pdm-1 (Kambadur, 1998).

Onset of Cas expression in both ventral cord and cephalic lobe NBs parallels the loss of Pdm-1 NB expression, suggesting a transient overlap in their expression. NBs containing detectable levels of both Pdm-1 and Cas are observed during this period. However, no Pdm-1/Cas co-expression is detected in GMCs or in their progeny. Hb/Pdm-1 co-expression is also detected at a similar frequency in early S1 and S2 NBs but not in their progeny. By stage 11, ventral cord Pdm-1-expressing cells are juxtaposed to the more dorsal or internal Hb-positive sublineages and flanked on their ventral/ventral-lateral side by the superficially positioned Cas-positive NBs and GMCs. The same relative positioning of Hb, Pdm-1 and Cas subpopulations is also observed in the cephalic lobes, since Cas expressing NBs and their offspring predominantly cover the outer flanks of Pdm-1 sublineages while Hb positive cells occupy deeper internal positions. Although Hb and Cas immunopositive cells together make up >50% of the cells present in stage 12 ganglia, no Hb/Cas co-expressing cells are detected in NBs or in their progeny. In fact, no cell at any stage of embryonic development is observed co-expressing these Zn-finger proteins. Simultaneous labeling of Hb, Pdm-1 and Cas reveals that most, if not all, NB lineages express at least one of these transcription factors. The absence of prolonged overlap between Hb/Pdm-1 co-expression or Pdm-1/Cas co-expression in early and late sublineages respectively, suggests that Hb and Cas may control, via repression, the temporal boundaries of pdm expression during CNS development (Kambadur, 1998).

During Drosophila embryonic CNS development, the sequential neuroblast (NB) expression of four proteins, Hunchback (Hb), Pou-homeodomain proteins 1 and 2 (referred to collectively as Pdm), and Castor (Cas), identifies a transcription factor network regulating the temporal development of all ganglia. The Zn-finger proteins Hb and Cas, acting as repressors, confine Pdm expression to a narrow intermediate temporal window; this results in the generation of three panneural domains whose cellular constituents are marked by expression of Hb, Pdm, or Cas. Seeking to identify the cellular mechanisms that generate these expression compartments, the lineage development of isolated NBs in culture was studied. The Hb, Pdm, and Cas expression domains are generated by transitions in NB gene expression that are followed by gene product perdurance within sequentially produced sublineages. These results also indicate that following Cas expression, many CNS NBs continue their asymmetric divisions and generate additional progeny, which can be identified by the expression of the bHLH transcription factor Grainyhead (Gh). Gh appears to be a terminal embryonic CNS lineage marker. Taken together, these studies indicate that once NBs initiate lineage development, no additional signaling between NBs and the neuroectoderm and/or mesoderm is required to trigger the temporal progression of Hb followed by Pdm and then Cas, and subsequently Gh expression during NB outgrowth (Brody, 2000).

Underpinning the formation of NB lineages are spatially and temporally regulated transcription factor networks that play pivotal roles in establishing the unique cellular identities of NBs and their progeny. Prior to NB delamination, during the initial specification of NBs, two spatially regulated transcription factor networks subdivide the ventral neuroectoderm along its anterior/posterior (A/P) x axis and dorsal/ventral (D/V) y axis. Later, during NB lineage development, at least one additional network, acting over several hours, gives rise to sequentially formed multilayered basal (inner or dorsal) to apical (outer or ventral) neuronal subpopulations. Along the basal/apical z axis, neuronal subpopulations in all ganglia can be identified by their expression of the transcription factors Hb, Pdm and Cas. Hb marks a deeper, basally distributed population of neurons that are born early, Cas marks a superficial, apically distributed population of neurons that are born late, and Pdm marks an intermediate population arrayed between the Hb- and the Cas-expressing cells. Both genetic and molecular analysis indicates that two Zn-finger proteins, Hb and Cas, act as repressors to silence pdm expression. By restricting pdm expression primarily to intermediate-born neuronal precursors these structurally different Zn-finger proteins help establish three pan-CNS neural subpopulations whose cellular constituents are marked by the expression of Hb, Pdm, or Cas (Brody, 2000).

To what extent are the z axis expression domains generated successively by invariant gene expression programs, maintained in different NBs, versus sequential gene expression programs within sublineages of single NBs? To better understand the nature of the temporal components regulating the CNS z axis network, attempts were made to determine if the sequential expression of Hb, Pdm-1, and Cas occurs during NB lineage development in vitro. In order to analyze the capacity of individual NBs to generate a full repertoire of Hb-, Pdm-, or Cas-expressing sublineages, overnight cultures were simultaneously immuno-stained for all three factors and the percentage of cells within a clone that were positive for each factor was subsequently determined. Not all clones contained cells expressing each of the transcription factors. The majority of clones containing Cas-expressing cells also contain additional NB descendants marked by the expression of Hb or Pdm-1. Triple-immunolabeling studies have revealed that clones expressing only Cas are the exception. Taken together the results indicate that many isolated S1 and S2 NBs, when maintained in culture, will generate neuronal descendants that are marked by Hb, Pdm, or Cas expression. Given that Hb and Cas are repressors of pdm gene NB expression, these observations also suggest that the overlapping Hb/Pdm and Pdm/Cas expressions, both in vivo and in culture represent transition states in NB gene expression. In other words, NBs undergo sequential transitions in gene expression, thus generating the multiple cell layers seen in vivo (Brody, 2000).

Triple-immunolabeling studies have revealed that many of the overnight NB clones contain a subset of cells that do not contain detectable levels of Hb, Pdm-1, or Cas. In many of these in vitro lineages the putative NB is also unstained. The bHLH transcription factor Gh is known to be expressed in CNS NBs but only after stage 14. In view of the late onset of Gh expression in NBs and the triple-staining results identifying cells in o/n clones that do not express Hb, Pdm-1, or Cas, it was hypothesized that these negative cells may represent an additional late NB expression window marked by Gh expression. To test this hypothesis, the spatial/temporal expression dynamics of Gh were compared to other members of the z axis network. Similar to its late activation during in vivo development, Gh expression was observed only in overnight cultures; when more than one Gh-positive cell was detected in a clone they were consistently found clustered together. Two-thirds of the Cas+ clones had at least one Gh+ cell and the average number of Gh+ cells in all clones was 2.3. Approximately 2/3 of the Gh+ clones also contained Hb-immunopositive cells. While no Hb-Gh coexpressing cells were observed, approximately 20% of the Gh+ cells also expressed Cas. Given the late onset of Gh expression in both the embryo and the cultured NB clones and the overlapping Cas and Gh expression, it is likely that Gh marks a fourth temporal window for NB transcription factor expression. In addition, because there was an average of more than one cell in an o/n clone that was immunopositive for Gh, it is likely that Gh is also expressed/maintained in a sublineage(s) born after the one marked by Cas expression (Brody, 2000).

The principle finding of this study is that built on top of the x and y axis neural identity systems is an additional temporal network that defines successive stages of lineage maturation in an apical/basal z axis. This global CNS network, identified by the temporal cascade of Hb followed by Pdm and subsequently Cas NB expression, most likely ensures in part that each NB generates a column of uniquely specified neuronal subtypes. The shared transcription factor expression within a given temporal layer also suggests that the cellular constituents of these expression domains may also have similar patterns of downstream target gene expression (Brody. 2000).

The following model for the origin of the layer sublineages marked by these transcription factors has been suggested. As each NB divides, generating a succession of GMCs, it undergoes multiple transitions in transcription factor expression. In succession, the NBs express Hb, Pdm, Cas, and Gh. The first progeny generated by the early S1 and S2 NBs express Hb, and the presence of Hb protein persists in their neural progeny. These early S1 and S2 NBs go on to activate the expression of the Pdms that, like Hb, persist in neural sublineages generated during this temporal window. Subsequently Cas is activated in NBs, represses Pdm transcription, and likewise persists in neural sublineages. After Cas expression, a fourth neural subpopulation, generated by dividing NBs, expresses Gh. This Gh subpopulation most likely represents the terminal sublineage of the embryonic NB. The data also reveal that not all NBs generate cells that occupy all four layers, a result that reflects the unique set of lineages, generated by each NB. Most likely, each NB has a preprogrammed time of delamination, but the timing of transitions is synchronized in a global fashion. The model further suggests that late delaminating NBs can be distinguished from early NBs by their inability to activate Hb. Although Hb is activated shortly after the S1s and S2s have delaminated, Hb is never seen in the proliferative zone during late delaminations (Brody, 2000).

What mechanism drives transitions in transcription factor expression in NBs and in their GMC progeny? It has been shown that Hb, Cas, and Pdm are involved in a regulatory circuit in which Hb and Cas repress Pdm in a cooperative, nonoverlapping fashion both early and late within NB lineages. In addition, Pdm is also required for the proper expression of Cas. It is likely, therefore, that this Hb to Pdm followed by Cas network is responsible for temporal transitions in transcription factors, related to the generation of multiple cellular layers. This conclusion must be tempered by the observation that less than 50% of the cells in clones and, by implication, in the CNS, are positive for even one of these transcription factors. There must be other factors involved in sublineage determination related to CNS layering. If the transitions observed are not caused by the partitioning of mRNA and proteins between NBs and their GMC, but by regulatory interactions within the cells themselves, then there must be additional mechanisms that are involved in the rapid disappearance of these molecules. Expression of transcription factors restricted to one or two generations of NB development could be accomplished if these transcription factors were autoregulatory, repressing their own expression in NBs and in their progeny (Brody. 2000).

For more information on Drosophila neuroblast lineages, see Linking neuroblasts to their corresponding lineage, a site carried by Flybrain, an online atlas and database of the Drosophila nervous system.

The peripheral nervous system of Drosophila offers a powerful system to precisely identify individual cells and dissect their genetic pathways of development. The mode of specification of a subset of larval PNS cells, the multiple dendritic (md) neurons (or type II neurons), is complex and still poorly understood. A morphological categorization of md neurons reveals three subpopulations: md-da neurons are the most abundant subclass, which have extensive dendritic arborizations; md-bd neurons have bipolar dendrites; and md-td neurons extend their dendrites along tracheal branches. Within the dorsal thoracic and abdominal segments, two md neurons, dbd and dda1, apparently require the proneural gene amos but not atonal or Achaete-Scute-Complex (ASC) genes. ASC normally acts via the neural selector gene cut to specify appropriate sensory organ identities. Dbd- and dda1-type differentiation is suppressed by cut in dorsal ASC-dependent md neurons. Thus, cut is not only required to promote an ASC-dependent mode of differentiation, but also represses an ASC- and ato-independent fate that leads to dbd and dda1 differentiation (Brewster, 2001).

pdm1 and pdm2, two closely linked genes belonging to the POU family of transcription factors, are co-expressed in two PNS neurons that are potentially coincident with ASC/ato-independent neurons: dbd and a dorsal md-da neuron. In addition, the ligament cells of lch5 also weakly express these genes. To determine if the pdm-expressing cells are ASC-and ato-independent, the pdm1 expression pattern was examined in ASC and ato single and double mutant embryos. In all three mutant configurations pdm1 expression is present in dbd and a dorsal md-da neuron. The latter will be referred to henceforth as dda1 (dorsal da neuron #1). Thus, pdm specifically marks the ASC/ato-independent subclass of PNS neurons (Brewster, 2001).

In order to further characterize ASC/ato-independent md neurons, other markers expressed in these cells were sought. en and the lacZ reporter gene from the E7-3-49 enhancer trap line were identifed. In the PNS, en is expressed in one dorsal md neuron as early as stage 11. The E7-3-49 line confers lacZ expression to several PNS cells, including dbd and 2-4 dorsal da-md neurons. Co-incidence of expression of pdm1 with that of en and E7-3-49 was examined in the PNS. Double-labeling for expression of pdm1 and E7-3-49-lacZ reveals that they overlap in dda1 and dbd. Similarly, pdm and en are co-expressed in dda1 but not in dbd (Brewster, 2001).

The expression pattern of the homeobox neural selector gene cut encompasses all es organs and a large number of md neurons (the majority of which are related to es organs by lineage). cut is clearly not expressed in the readily identifiable dbd neuron. Since dbd and dda1 co-express the markers described above and perhaps are specified by the same proneural gene(s), whether dda1 is indeed negative for cut expression was examined. Embryos double-labeled for cut and E7-3-49-lacZ or pdm show that the pattern of cut expression in the dorsal PNS cluster is complementary to these markers. These results indicate that, unlike the majority of md neurons, dda1 and dbd (along with its sibling glial cell) are specified in a cut-independent fashion (Brewster, 2001).

The identity of the transformed md-da neurons in cut mutants was examined with markers for ASC/ato-independent neurons. Similar to E7-3-49, the expression of pdm1 is expanded to additional neurons in the dorsal and lateral PNS clusters. The cells expressing pdm1 ectopically are also positive for a marker, the E7-2-36 enhancer trap line, which is specific for all md neurons, suggesting that the extra Pdm1-positive neurons are indeed md neurons. The mechanism for restricting ectopic pdm1 but not E7-3-49- lacZ expression to the dorsal and lateral clusters is not known. Overall, these findings are consistent with the interpretation that in cut mutants many ASC-dependent md neurons are transformed towards an ASC/ato-independent rather than an ato-dependent fate (Brewster, 2001).

In contrast to these findings with E7-3-49-lacZ and pdm1 expression, when en expression was examined in cut mutants, the pattern of en-expressing PNS cells is unaltered, i.e. there is only one En-positive cell per dorsal cluster. Since en is not expressed in dbd but pdm1 is, the possibility that the invariance of en expression reflects the acquisition of a dbd rather than a dda1 cell fate in cut mutants cannot be ruled out. This possibility seems unlikely, however, since the morphology of supernumerary Pdm1-positive cells is unlike that of dbd, and a marker for the dbd-associated glial cell (repo) is not ectopically expressed in cut mutants (Brewster, 2001).

Taken together, it appears that in cut mutants the md neurons that normally depend on ASC in dorsal and lateral clusters are transformed towards an ASC/ato-independent fate, as determined by pdm and E7-3-49-lacZ expression. However, the postulated cell fate change may be incomplete due to the lack of ectopic en expression. This partial phenotype is not surprising, since cut (null) mutants also exhibit variability and incomplete phenotypic penetrance with respect to es organ transformation towards a ch fate. It is thus likely that gene functions other than cut also contribute to the restriction of en and pdm1, similar perhaps to the situation of ato-dependent md neurons, which do not express cut or pdm/en (Brewster, 2001).

During Drosophila neurogenesis, glial differentiation depends on the expression of glial cells missing. Understanding how glial fate is achieved thus requires knowledge of the temporal and spatial control mechanisms directing gcm expression. In the adult bristle lineage, gcm expression is negatively regulated by Notch signaling. The effect of Notch activation on gliogenesis is context-dependent. In the dorsal bipolar dendritic (dbd) sensory lineage in the embryonic peripheral nervous system (PNS), asymmetric cell division of the dbd precursor produces a neuron and a glial cell, where gcm expression is activated in the glial daughter. Within the dbd lineage, Notch is specifically activated in one of the daughter cells and is required for gcm expression and a glial fate. Thus Notch activity has opposite consequences on gcm expression in two PNS lineages. Ectopic Notch activation can direct gliogenesis in a subset of embryonic PNS lineages, suggesting that Notch-dependent gliogenesis is supported in certain developmental contexts. Evidence is presented that POU-domain protein Nubbin/PDM-1 is one of the factors that provides such context (Umesono, 2002).

Coexpression of constitutively active Notch with Nubbin generates ectopic glia outside dbd and dda lineages. This raises the possibility that Nubbin may be a part of the developmental context that allows Notch to promote gliogenesis. Within the embryonic PNS, dbd and dda neurons are the only two neurons that express Nubbin. In both lineages, Nubbin is present in both SOP daughter cells, at the time of glia versus neuron cell fate choice. Furthermore, temporal activation of Nubbin has been detected in presumptive glial cells derived from the NB1-1A lineage. Nubbin thus might create a permissive environment for the activation of gcm expression by the Notch signal. Since coexpression of Nubbin and constitutively active Notch does not cause glial transformation of all neurons, additional factors must exist that create a Notch-dependent gliogenic context (Umesono, 2002).

Nubbin is a POU-domain transcription factor with sequence-specific DNA-binding activity. The contextual role of Nubbin in Notch-dependent expression of gcm could employ a similar mechanism to the modulation of Notch activity in wing development, where Nubbin and Su(H) bind on the same enhancer element of Notch target genes. It will be interesting to further analyze the role of Nubbin in gliogenic lineages (Umesono, 2002).

The Drosophila brain develops from the procephalic neurogenic region of the ectoderm. About 100 neural precursor cells (neuroblasts) delaminate from this region on either side in a reproducible spatiotemporal pattern. Neuroblast maps have been prepared from different stages of the early embryo (stages 9, 10 and 11, when the entire population of neuroblasts has formed), in which about 40 molecular markers representing the expression patterns of 34 different genes are linked to individual neuroblasts. In particular, a detailed description is presented of the spatiotemporal patterns of expression in the procephalic neuroectoderm and in the neuroblast layer of the gap genes empty spiracles, hunchback, huckebein, sloppy paired 1 and tailless; the homeotic gene labial; the early eye genes dachshund, eyeless and twin of eyeless; and several other marker genes (including castor, pdm1, fasciclin 2, klumpfuss, ladybird, runt and unplugged). Based on the combination of genes expressed, each brain neuroblast acquires a unique identity, and it is possible to follow the fate of individual neuroblasts through early neurogenesis. Furthermore, despite the highly derived patterns of expression in the procephalic segments, the co-expression of specific molecular markers discloses the existence of serially homologous neuroblasts in neuromeres of the ventral nerve cord and the brain. Taking into consideration that all brain neuroblasts are now assigned to particular neuromeres and individually identified by their unique gene expression, and that the genes found to be expressed are likely candidates for controlling the development of the respective neuroblasts, these data provide a basic framework for studying the mechanisms leading to pattern and cell diversity in the Drosophila brain, and for addressing those mechanisms that make the brain different from the truncal CNS (Urbach, 2003).

The two closely related Drosophila POU-domain genes, pdm1 (nubbin) and pdm2, are co-expressed in the developing CNS (before stage 13) and have been shown (at least with respect to the specification of the first ganglion mother cell of the truncal NB4-2) to be functionally redundant. pdm1 is expressed in the trunk neuroectoderm during the first and second wave of NB segregation (stage 8/9), and transiently in most NBs at stage 10 and 11. In the procephalon the expression of the Pdm1 protein is highly dynamic. Until stage 10, Pdm1 is roughly restricted to the neuroectoderm of the antennal and ocular segments. Later, it is also found in the intercalary and labral ectoderm. At stage 9, NBs derived from Pdm1-positive neuroectoderm appear to be Pdm1 negative and initiate pdm1 expression at stage 10 or stage 11. At late stage 11, approximately one half of the brain NBs (about 52 NBs) express pdm1, including most deuto- and trito-cerebral NBs, as well as central ocular NBs and part of the labral NBs (Urbach, 2003).

Proximodistal subdivision of Drosophila legs and wings: The elbow-no ocelli gene complex functions upstream of Nub in the formation of the wing primordium

Appendages are thought to have arisen during evolution as outgrowths from the body wall of primitive bilateria. In Drosophila, subsets of body wall cells are set aside as appendage precursors through the action of secreted signaling proteins that direct localized expression of transcription factors. The Drosophila homeodomain protein Distal-less is expressed in the leg primordia and required for formation of legs, but not wings. The homeodomain protein Nubbin is expressed in the wing primordia and required for formation of wings, but not legs. Given that insect legs and wings have a common developmental and evolutionary origin, attempts were made to identify genes that underlie the specification of all appendage primordia. Evidence is presented that the zinc-finger proteins encoded by the elbow and no ocelli genes act in leg and wing primordia to repress body wall-specifying genes and thereby direct appendage formation (Weihe, 2004).

Evidence suggests that the el and noc genes serve as mediators of the function of the Wg and Dpp signaling systems in specification of the appendage field within the imaginal discs. El and Noc are induced by Wg and Dpp and are required to repress the proximally expressed proteins Hth and Tsh. Previous work had identified Dll as a gene required for appendage formation in leg and antenna, and nub as a gene required for wing. This report identifies El and Noc as a pair of zinc-finger proteins that function in both ventral and dorsal appendages. However, there are interesting differences in the way that they do so, when examined in detail (Weihe, 2004).

Dll expression is required for the formation of all leg and antenna elements in the ventral (leg) discs, and until this work Dll was the earliest known marker for the distal region leg disc. Previous work has shown that repression of Hth and Tsh by Dpp and Wg was not required for expression of Dll in the leg, nor could Dll repress Hth and Tsh. Thus an essential mediator of the effects of Wg and Dpp was missing. The current results present evidence that El and Noc serve this function, since their removal leads to ectopic expression of Hth and Tsh. Removal of El and Noc does not cause loss of Dll expression, so it is concluded that Wg and Dpp act independently to induce El and Noc expression and Dll to define the distal region of the leg disc (Weihe, 2004).

The situation differs slightly in the wing. Repression of Tsh is the earliest marker for specification of the distal wing region, preceding the onset of Hth repression or of Nub induction. Loss of Tsh and Hth are required to allow Nub expression. Ectopic expression of Hth and Tsh and loss of Nub is observed in clones lacking El and Noc activity. Thus in the wing, expression of the distal marker Nub cannot be demonstrated to be independent of El and Noc (because ectopic Hth can repress Nub, but not Dll). The vestigial gene is also important for wing development and has been proposed to be a wing specifying gene. However, Vestigial is expressed all along the DV boundary of the wing, both in the wing primordium and in the body wall. This led to the suggestion that while Vestigial is essential for wing development, its expression cannot be taken as a molecular marker for wing identity per se, particularly at early stages. For this reason analysis of the relationship between El, Noc and Vestigial was not performed in this study (Weihe, 2004).

Larval

The pdm-1 gene is expressed in wing and leg discs. In the wing it appears to be required for the hinge, suggesting an involvement in proximal-distal growth control (Ng, 1995).

Flexible joints separate the rigid sections of the insect leg, allowing them to move. In Drosophila, the initial patterning of these joints is apparent in the larval imaginal discs from which the adult legs will develop. The later patterning and morphogenesis of the joints, which occurs after pupariation (AP), is described. In the tibial/tarsal joint, the apodeme insertion site provides a fixed marker for the boundary between proximal and distal joint territories (the P/D boundary). Cells on either side of this boundary behave differently during morphogenesis. Morphogenesis begins with the apical constriction of distal joint cells, about 24 h AP. Distal cells then become columnar, causing distal tissue nearest the P/D boundary to fold into the leg. In the last stage of joint morphogenesis, the proximal joint cells closest to the P/D boundary align and elongate to form a 'palisade' (a row of columnar cells) over the distal joint cells. The proximal and distal joint territories are characterized by the differential organization of cytoskeletal and extracellular matrix proteins, and by the differential expression of enhancer trap lines and other gene markers. These markers also define a number of more localised territories within the pupal joint (Mirth, 2002).

To identify distinct cell populations in the joints, the expression patterns of 10 joint markers were examined with respect to a posterior marker (engrailed lacZ) and a ventral marker (wingless lacZ). The leg discs of wandering larvae, and pupal legs at 24-28 and 34-38 h AP, were examined. Four of the joint markers were previously reported to be expressed in L3 and prepupal joints (Notch, disconnected lacZ, Nubbin, and odd-skipped lacZ). The rest were isolated for this study by screening Gal4 enhancer trap lines for those that drive expression of GFP in pupal leg joints (ckm78, ckm90, ckm239, ckm175, ok388, and ok483). Most of the joint markers do not change their expression domains between 24-28 and 34-38 h AP. Therefore, data is presented from wandering L3 discs only, and from legs at 34-38 h AP (Mirth, 2002).

In the L3 leg disc, joint markers fall into one of two categories, marking either the proximal joint territories (e.g., Nubbin) or the distal territories (e.g., Notch and odd-skipped lacZ). Of all the markers examined, only Nubbin (Nub), disconnected lacZ (disco lacZ), and odd-skipped lacZ (odd lacZ) are expressed in more than two joints in the L3 stage. Others mark one or two joints at this stage but are expressed in all joints during the pupal stage. Studies examining the expression of Notch and other elements of the Notch patterning cascade have also found that the joint seems to be divided into proximal and distal territories at this stage. Thus, proximal and distal joint domains have already been established by the late L3 (Mirth, 2002).

By 34-38 h AP, patterns of marker expression define three additional territories. First, a proximal-dorsal patch is high-lighted by two joint markers, ckm90 and ckm175, that drive GFP expression only in a patch above and includes the most proximal cells of the dorsal apodeme. The expression of GFP driven by ckm175 includes a greater number of cells than that driven by ckm90. The second domain identified was a mid-distal domain. Odd lacZ expression becomes largely restricted to a mid-distal group of cells in all but the tarsal joints. This corresponds to the region that does not accumulate collagen IV and marks the cells that push underneath the proximal joint cells. Odd lacZ is also expressed in the apodemes. Lastly, ok388 expresses GFP in the lateral anterior and posterior parts of the distal tibial/tarsal (but not tarsal) joint, but is excluded from the dorsal and ventral domains. This expression domain corresponds with the region of elongating cells seen in longitudinal sections of the leg (Mirth, 2002).

Two of the joint markers are expressed in both the proximal and distal portions in the developing adult joint: ckm239 and disco lacZ. Disco lacZ is expressed throughout the entire joint, and ckm239 is excluded from the ventralmost region (wingless lacZ-expressing region) (Mirth, 2002).

It seems likely that the domains of gene expression observed in the L3 leg disc correspond with those of the same genes in the developing adult joint, though this has not been verified directly. If so, proximal and distal joint domains are established before pupariation. These two joint territories separate cells that will invaginate [the cells in the odd lacZ domain, expressing the Notch target E(SPL)Mß] from those that will form the proximal palisade (the cells expressing Delta, Serrate, and Nubbin). During pupal development, the proximal and distal domains of the joint become further subdivided. Most of the enhancer trap markers identified are expressed in specific groups of cells within either the proximal or distal domain in the tibial/tarsal joint at 34-38 h AP. At the same time, the expression of some earlier markers becomes restricted to more specific territories. odd lacZ, which is expressed in some joints in the L3, is expressed most strongly in the mid-distal joint cells at 34-38 h AP. Ok388 expresses in the distalmost but not mid-distal joint cells, and is restricted to the lateral anterior and posterior sides. In the proximal joint, markers such as ckm90 and ckm175 express in only a small group of cells on the dorsal side. Thus, it seems that the tibial/tarsal joint may divided into three proximodistal domains based both on cell behavior and gene expression: proximal, mid-distal, and distalmost regions. Later during pupal development, the distalmost region subdivides into lateral anterior/posterior and dorsal/ventral domains and the proximal joint also subdivides into smaller territories. That further patterning and subdivision of the joint occurs after the prepupal stages is hardly surprising: the adult joint is too complex a structure to be derived simply from the proximodistal interactions that occur before pupal development (Mirth, 2002).

Effects of Mutation, Deletion or Over-expression

The nubbin gene of Drosophila was originally identified as a viable spontaneous mutation which results in a dramatic reduction in the size of wings and halteres, but which does not otherwise affect adult morphology. Cloning of the DNA responsible for this phenotype, shows that the gene responsible is pdm-1 (Ng, 1995).

The role of pdm1 has been investigated during the elaboration of the GMC-1-->RP2/sib lineage. Also studied in this lineage was the functional relationship between pdm1 and pdm2. Deletion of pdm1 causes a partially penetrant GMC-1 defect, while deletion of both pdm2 and pdm1 results in a fully penetrant defect. This GMC-1 defect in pdm2 and pdm1 mutant embryos can be rescued by the pdm1 or pdm2 transgene. Rescue is observed only when these genes are expressed at the time of GMC-1 formation. Overexpression of pdm1 or pdm2 well after GMC-1 is formed results in the duplication of RP2 and/or sib cells. These results indicate that both genes are required for the normal development of this lineage and that the two collaborate during the specification of GMC-1 identity (Bhat, 1995).

The phenotype for mutations of the nubbin gene consists of a severe wing size reduction and pattern alterations, such as transformations of distal elements into proximal ones. nub expression is restricted to the wing pouch cells in wing discs from the early stages of larval development. These effects are also observed in genetic mosaics where cell proliferation is reduced in all wing blade regions autonomously, and transformation into proximal elements is observed in distal clones. Mutant clones are approximately 50% smaller than control clones or else they fail to grow in 50% of the cases. Clones located in the proximal region of the wing blade cause an additional nonautonomous reduction of the whole wing. Cell lineage experiments in a nub mutant background show that clones respect neither the anterior-posterior nor the dorsal-ventral boundary but that the selector genes decapentaplegic and engrailed are correctly expressed from early larval development. The phenotypes of nub elbow and nub dpp genetic combinations are synergistic and the overexpression of dpp in clones in nub wings does not result in overproliferation of the surrounding wild-type cells (Cifuentes, 1997).

Upregulation of Mitimere and Nubbin acts through Cyclin E to confer self-renewing asymmetric division potential to neural precursor cells

In the Drosophila CNS, neuroblasts undergo self-renewing asymmetric divisions, whereas their progeny, ganglion mother cells (GMCs), divide asymmetrically to generate terminal postmitotic neurons. It is not known whether GMCs have the potential to undergo self-renewing asymmetric divisions. It is also not known how precursor cells undergo self-renewing asymmetric divisions. Maintaining high levels of Mitimere or Nubbin, two POU proteins, in a GMC causes it to undergo self-renewing asymmetric divisions. These asymmetric divisions are due to upregulation of Cyclin E in late GMC and its unequal distribution between two daughter cells. GMCs in an embryo overexpressing Cyclin E, or in an embryo mutant for archipelago, also undergo self-renewing asymmetric divisions. Although the GMC self-renewal is independent of inscuteable and numb, the fate of the differentiating daughter is inscuteable and numb-dependent. These results reveal that regulation of Cyclin E levels, and asymmetric distribution of Cyclin E and other determinants, confer self-renewing asymmetric division potential to precursor cells, and thus define a pathway that regulates such divisions. These results add to our understanding of maintenance and loss of pluripotential stem cell identity (Bhat, 2004).

Maintenance of a self-renewing fate can be viewed as a state where activities of certain genes maintain that state. Once the activity of such genes is switched off, the cells become committed to a differentiation pathway. The results reported in this study indeed support this type of mechanism. That POU genes might be a class of genes that maintain a self-renewing capacity is indicated by the fact that the Oct4 POU gene (Pou5f1 -- Mouse Genome Informatics), which is expressed in pluripotent stem cells of the mouse early embryo, is turned off when these cells begin to differentiate (Rosner, 1990). Similarly, SCIP is expressed in the progenitors of oligodendrocytes, but it is downregulated when these cells are induced to differentiate (Collarini, 1992). The current results provide direct evidence that these genes can induce a cell that is committed to a differentiation pathway to acquire a self-renewing capability in a lineage specific manner. Moreover, studies undertaken in the past several years using the Drosophila nervous system as a paradigm have revealed how asymmetry can be generated during cell division to produce two distinct postmitotic cells. However, there is very little information on how an asymmetric self-renewing division pattern is determined. In this paper, results are presented that provide insight into this particular process. (Bhat, 2004).

The strongest evidence that a GMC-1 undergoes a self-renewing type of asymmetric division in embryos overexpressing miti/nub or CycE, and in embryos mutant for ago, comes from the presence of hemisegments with two sibs and one RP2. There are two ways the second sib cell can be generated: (1) a self-renewed GMC-1 generates another sib when it divides, and (2) some other cell is transformed into a sib. The following set of evidence indicates the former scenario: (1) the second sib cell always appears later in development, i.e. at ~8.5 hours of age (as opposed to in wild type where the GMC-1 terminally divides by ~7.5 hours of age into an RP2 and a sib); (2) the dynamics of Eve expression itself in the sib -- expression of eve is switched off in a sib during the asymmetric division of GMC-1 and there is no de novo synthesis of Eve thereafter. If a postmitotic cell from an Eve-negative lineage transforms into a sib, it would be negative for Eve and would not be detected. The development of the other Eve-positive neuronal lineages is normal in these embryos, thus it is unlikely that a cell from those Eve-positive lineages is transformed into a sib. (3) The Eve and Spectrin staining of UAS-nub; ftz-GAL4 embryos provides more direct evidence for the self-renewal of GMC-1. In ~8. 5-hour-old UAS-nub; ftz GAL4 embryos, the larger GMC-1 (this Eve-positive cell is Zfh1 negative, indicating that it is indeed a GMC-1) can be observed undergoing asymmetric cytokinesis for the second time. From the heat-shock induction experiments of nub or miti mutant embryos, it can be argued that higher levels of these proteins in the parental NB4-2 cause later born GMCs to adopt a GMC-1 fate. However, the GMC-1 self-renewing phenotype observed following targeted expression of nub using the ftz-GAL driver makes this scenario unlikely. (4) The results obtained with the miti^P; insc and miti^P; nb double mutant embryos (P referring to prolonged expression), and the mis-localization of Insc in GMC-1 of these embryos, are also consistent with this conclusion. (Bhat, 2004).

These results indicate that the level, timing and duration of presence of Miti or Nub proteins determine the dynamics of the GMC-1 division pattern. For example, the asymmetric divisions (which generate the 3-cell phenotypes) and the symmetric divisions (which generate the 4-cell phenotype) were observed when the transgenes were induced for 20-25 minutes. However, the multiple cell-phenotype was observed only when the transgenes were induced for 90 minutes. Once the induction was stopped and the levels returned to normal, the two GMC-1s appeared to exit from the cell cycle to generate postmitotic cells. Similarly, when the transgene was induced with ftz-GAL4, only the 3-cell phenotypes, and not the 4-cell or multi-cell phenotypes were observed. Thus, the following picture emerges from these results. Although high levels of Miti and Nub proteins are required for the specification of GMC-1 identity, their level must be downregulated in order for the GMC-1 to divide asymmetrically into postmitotic RP2 and sib. Maintaining a high level of these proteins in GMC-1 commits that cell to adopt a self-renewing stem cell type of division pattern. The results described here also show that Miti and Nub prevent GMC-1 from exiting the cell cycle by upregulation of CycE (Bhat, 2004).

The results clearly show that upregulation of CycE in late GMC-1 is the cause for the adoption of a self-renewing asymmetric division pattern. In other words, presence of high levels of CycE in late GMC-1 and its unequal distribution to one of the two daughter cells prevents this cell from exiting the cell cycle. Since this daughter cell still maintains the GMC-1 identity and has sufficient CycE to divide again, a further asymmetric division(s) is ensured. The cell that has lower amounts of CycE becomes committed to a differentiation pathway (RP2 or sib) (Bhat, 2004).

What lines of evidence support this conclusion? (1) In contrast with wild type, there is a significant amount of CycE present in a late GMC-1 in embryos overexpressing miti or nub. This CycE preferentially segregates to one of the two daughters of that GMC-1, usually the larger cell. When miti or nub genes are overexpressed only briefly, the level of CycE is downregulated after just one additional round of division, whereas with prolonged induction, the level is maintained at high levels in one or two cells of the multi-cell cluster for a prolonged duration of time (Bhat, 2004).

(2) Upregulation of CycE in a late GMC-1 is also observed in embryos mutant for ago, which is known to regulate CycE levels. In ago mutants, the two daughter cells of such a GMC-1 have unequal CycE levels accompanied by a self-renewing asymmetric division phenotype. The CycE is always downregulated after one additional GMC-1 division, which is consistent with the finding that the self-renewal occurs only once in these embryos. Since penetrance in ago mutants is partial, and CycE is downregulated in this lineage after just one additional division, there must be additional factors that mediate the downregulation of CycE in this lineage (Bhat, 2004).

(3) Embryos expressing high levels of CycE from a CycE transgene exhibit the same GMC-1 phenotypes as embryos expressing high levels of Miti or Nub. Thus, these results indicate that upregulation of CycE alone is sufficient for the GMC-1 to adopt a self-renewing type of division pattern. Finally, miti^P phenotypes are found to be dependent on CycE. That is, no multi-cell clusters were observed in miti^P; CycE double mutant embryos (Bhat, 2004).

In wild type, the downregulation of CycE in GMCs appears to occur through switching off CycE transcription and degradation of the protein by factors such as Ago. At what level does Miti or Nub regulate CycE? Since POU genes are thought to be transcriptional activators, they can regulate transcription of CycE either directly or indirectly. However, this does not seem to be the case since expressing high levels of miti does not have a discernible effect on the levels of CycE mRNA in GMC-1, as assessed by whole-mount RNA in situ hybridization. In addition, the putative promoter/enhancer region of CycE gene does not contain any consensus POU protein-binding sites. Therefore, it seems likely that Miti and Nub regulate factors that are involved in the degradation of CycE in late GMC-1 (Bhat, 2004).

The question arises as to how only one cell has a high level of CycE. There are several ways this can happen. There might be an asymmetric degradation of CycE. This scenario seems unlikely since there is only one of two daughter cells with high levels of CycE in ago mutants. Given that Ago downregulates CycE via a protein degradation mechanism, if there was an asymmetric degradation, in those hemisegments where the levels of CycE was elevated in GMC-1, it would initially be expected that both the daughter cells would have high CycE levels. However, this was not the case. An asymmetric transcription of the CycE gene also seems unlikely since the transcription of CycE ceases prior to GMC-1 division, as judged by whole-mount RNA in situ hybridization. The most likely possibility is that CycE is unequally distributed between the two daughter cells of GMC-1. The unequal distribution of CycE could be a passive process due to the size difference between daughter cells, especially in the GMC-1-->RP2/sib lineage. Moreover, no cytoplasmic crescent of CycE was observed during mitosis. By contrast, it could also be an active process. For instance, the size difference between an aCC and a pCC (or between a GMC1-1a and an aCC) is very small, and the fact that GMC1-1a undergoes a self-renewing asymmetric division suggests that the segregation of CycE may not be entirely a passive process (Bhat, 2004).

Finally, the results indicate that while a GMC that does not normally express Miti or Nub is insensitive to its ectopic expression (e.g., GMC1-1a of NB1-1; this GMC produces an aCC/pCC pair of neurons), a brief induction of CycE in the same GMC causes it to undergo self-renewing asymmetric division. Therefore, CycE can confer a stem cell type of division potential to more than one GMC. Another important conclusion one can draw from this result is that the segregation of CycE may be an active process. In the case of GMC1-->RP2/sib lineage, the cytokinesis of GMC-1 is asymmetric, and the size difference between an RP2 and a sib is significant. Thus, CycE can be asymmetrically segregated because of this size difference. However, the size difference between an aCC and a pCC (or between a GMC1-1a and an aCC) is very small, and the fact that GMC1-1a undergoes a self-renewing asymmetric division suggests that the segregation of CycE may not be entirely a passive process. It is possible that the difference between the levels of CycE needed to retain a cell within the cell cycle and the levels that do not support maintaining the cell within the cell cycle are quite small. Thus, even a minor change in the amount that a cell receives during division might be sufficient to make a difference. Thus, the segregation of CycE can still be a passive process. Nonetheless, these results reveal how a cell can adopt a self-renewing asymmetric division potential through CycE. (Bhat, 2004).

Pros has been implicated in inhibiting the ability of GMCs to divide more than once by preventing continued expression of cell-cycle genes. The caveat of this study, however, is that none of the GMC lineage was examined using cell-specific markers to determine whether GMCs continue to divide in embryos mutant for pros. The conclusion that Pros inhibits GMC division was mainly based on the presence of additional BrdU-positive cells in late stage (post 15-hours-old) pros mutant embryos. Pros is expressed in GMC-1 of the RP2/sib lineage and, in null alleles, this GMC-1 identity is not specified. In pros¹⁷, a loss-of-function allele, ~5% of the hemisegments had an RP2/sib lineage specified. In these hemisegments, the GMC-1 divides only once to generate an RP2 and a sib cell as in wild type. Moreover, specification of U and CQ lineages was observed in ~20% and ~13% of the hemisegments, respectively, and no additional cell division appeared to occur in these lineages. A previous study found that the aCC/pCC neurons (from GMC1-1a) have an abnormal axon morphology, but it did not find any additional neurons in this lineage. Similarly, NB6-4 of the thoracic segment produced the normal number of progeny in pros mutant embryos. These results suggest that Pros does not regulate cell division in RP2/sib, U and CQ lineages, and possibly not in many other neuronal lineages, and therefore it is unlikely to function in the miti/nub pathway (Bhat, 2004).

How is the specification of identity of one of the two progeny, either as an RP2 or as a sib, from a self-renewing asymmetric division of GMC-1 regulated? (Specification of the other progeny as GMC-1 is by high levels of CycE.) The results indicate that specification of an RP2 versus a sib identity to this differentiating cell is through a combination of low levels of CycE and localization of Insc. This is indicated by the finding that overexpression of Miti and Nub causes localization of Insc to be non-asymmetric. Non-asymmetric Insc also causes non-asymmetric localization of Numb. The cell that has lower levels of CycE and also has Numb becomes an RP2. Whenever the cell with lower levels of CycE fails to inherit Numb (the effect of overexpression of Miti or Nub on the localization of Insc is partially penetrant) that cell will become a sib. That the generation of an RP2 during the asymmetric division of GMC-1 is tied to Numb is also indicated by the analysis of miti^P; numb embryos. Although the self-renewal of GMC-1 in miti^P embryos is numb-independent, the commitment of a progeny to become a sib is numb-dependent. Thus, in ~13-hour-old miti^P; numb embryos, multiple cells are observed adopting a sib fate. An often overlooked fact is that in insc mutants the GMC-1 division is normal in ~30% of the hemisegments despite having no insc. Similarly, the penetrance of the symmetrical division of GMC-1 in pins (where Insc localization is affected as in miti^P embryos) is also partial, indicating the presence of additional (partially redundant) pathways for Insc that mediate asymmetric fate specification. These very same additional pathways must also influence the choice between a sib and an RP2 when the GMC-1 in miti^P embryos undergoes a self-renewing type of asymmetric division (Bhat, 2004).

CycE and Ago are part of a mechanism that converts a normal cell into a cancer cell. In ago mutants, CycE protein is not degraded and a number of cancer cell lines carry a mutation in ago. The current results showing that these genes are also involved in a stem cell type of division suggest a commonality between stem cells and cancer cells. These results also provide a molecular mechanism of how self-renewing asymmetric divisions are possible (Bhat, 2004).

Affolter, M., Walldorf, U., Kloter, U., Schier, A.F. and Gehring, W.J. (1993). Regional repression of a Drosophila POU box gene in the endoderm involves inductive interactions between germ layers. Development 117: 1199-1210

Andersen, B., et al. (1997). Functions of the POU domain genes Skn-1a/i and Tst-1/Oct-6/SCIP in epidermal differentiation. Genes Dev. 11(14): 1873-1884. Medline abstract: 97384999

Averof, M. and Cohen, S. M. (1997). Evolutionary origin of insect wings from ancestral gills. Nature 385: 627-630

Babb, R., Cleary, M. A. and Herr, W. (1997). OCA-B is a functional analog of VP16 but targets a separate surface of the Oct-1 POU domain. Mol. Cell. Biol. 17(12): 7295-305. Medline abstract: 98038798

Bachmann, A. and Knust, E. (1998b). Positive and negative control of Serrate expression during early development of the Drosophila wing. Mech. Dev. 76(1-2): 67-78. Medline abstract: 98440272

Bhat, K. M. and Apsel, N. (2004). Upregulation of Mitimere and Nubbin acts through Cyclin E to confer self-renewing asymmetric division potential to neural precursor cells. Development 131: 1123-1134. Medline abstract: 14973280

Belting, H.-G., et al. (2001). spiel ohne grenzen/pou2 is required during establishment of the zebrafish midbrain-hindbrain boundary organizer. Development 128: 4165-4176. Medline abstract: 11684654

Bertolino, E. and Singh, H. (2002). POU/TBP cooperativity: a mechanism for enhancer action from a distance. Molec. Cell 10: 397-407. Medline abstract: 12191484

Bhat, K. M., Poole, S. J. and Schedl, P. (1995). The miti-mere and pdm1 genes collaborate during specification of the RP2/sib lineage in Drosophila neurogenesis. Mol Cell Biol 15: 4052-4063 Medline abstract

Boehm, J., et al. (2001). Regulation of BOB.1/OBF.1 stability by SIAH. EMBO J. 20: 4153-4162. Medline abstract: 11483518

Botfield, M. C., Jancso, A. and Weiss, M. A. (1994). An invariant asparagine in the POU-specific homeodomain regulates the specificity of the Oct-2 POU motif. Biochemistry 33: 8113-8121. Medline abstract: 94297028

Brewster, R., et al. (2001), The selector gene cut represses a neural cell fate that is specified independently of the Achaete-Scute-Complex and atonal. Mech. Dev. 105: 57-68. Medline abstract: 11429282

Brody, T. and Odenwald, W. F. (2000). Programmed transformations in neuroblast gene expression during Drosophila CNS lineage development. Dev. Biol. 226: 34-44. Medline abstract: 20450848

Brugnera, E., et al. (1992). POU-specific domain of Oct-2 factor confers 'octamer' motif DNA binding specificity on heterologous Antennapedia homeodomain. FEBS Lett. 314: 361-5. Medline abstract: 93106185

Chang, J. F., et al. (1999). Oct-1 POU and octamer DNA co-operate to recognise the Bob-1 transcription co-activator via induced folding. J. Mol. Biol. 288(5): 941-52. Medline abstract: 99264415

Chasman, D., et al. (1999). Crystal structure of an OCA-B peptide bound to an Oct-1 POU domain/octamer DNA complex: specific recognition of a protein-DNA interface. Genes Dev. 13: 2650-2657. Medline abstract: 20009500

Cifuentes, F. J. and Garcia-Bellido, A. (1997). Proximo-distal specification in the wing disc of Drosophila by the nubbin gene. Proc. Natl. Acad. Sci. 94(21): 11405-11410. Medline abstract: 97470979

Cimbora, D. M. and Sakonju, S. (1995). Drosophila midgut morphogenesis requires the function of the segmentation gene odd-paired. Dev. Biol. 169: 580-595. Medline abstract: 95301100

Cockerill, K.A., Billin, A.N. and Poole, S.J. (1993). Regulation of expression domains and effects of ectopic expression reveal gap gene-like properties of the linked pdm genes of Drosophila. Mech Dev. 41: 139-153

Collarini, E. J., Kuhn, R., Marshall, C. J., Monuki, E. S., Lemke, G. and Richardson, W. D. (1992). Down-regulation of the POU transcription factor SCIP is an early event in oligodendrocyte differentiation in vivo. Development 116: 193-200. Medline abstract: 1483387

Collins, R. T. and Treisman, J. E. (2000). Osa-containing Brahma chromatin remodeling complexes are required for the repression of Wingless target genes. Genes Dev. 14: 3140-3152. Medline abstract: 11124806

Dick, T., et al. (1991). Two closely linked Drosophila POU domain genes are expressed in neuroblasts and sensory elements. Proc. Natl. Acad. Sci. 88: 7645-7649

Ford, E., Strubin, M. and Hernandez, N. (1998). The oct-1 POU domain activates snRNA gene transcription by contacting a region in the SNAPc largest subunit that bears sequence similarities to the oct-1 coactivator OBF-1. Genes Dev. 12(22): 3528-40. Medline abstract: 99051316

Halder, G., et al. (1998). The Vestigial and Scalloped proteins act together to directly regulate wing-specific gene expression in Drosophila. Genes Dev. 12(24): 3900-9. Medline abstract: 99088039

Hauptmann, G., et al. (2002). spiel ohne grenzen/pou2 is required for zebrafish hindbrain segmentation. Development 129: 1645-1655. Medline abstract: 11923201

Herr, W. and Cleary, M. A. (1995). The POU domain: versatility in transcriptional regulation by a flexible two-in-one DNA-binding domain. Genes Dev. 9:1679-93

Hovde, S., et al. (2002). Activator recruitment by the general transcription machinery: X-ray structural analysis of the Oct-1 POU domain/human U1 octamer/SNAP190 peptide ternary complex. Genes Dev. 16: 2772-2777. Medline abstract: 12414730

Hu, N. and Castelli-Gair, J. (1999). Study of the posterior spiracles of Drosophila as a model to understand the genetic and cellular mechanisms controlling morphogenesis. Dev. Biol. 214(1): 197-210. Medline abstract: 99423659

Inamoto, S., et al. (1997). The cyclin-dependent kinase-activating kinase (CAK) assembly factor, MAT1, targets and enhances CAK activity on the POU domains of octamer transcription factors. J. Biol. Chem. 272(47): 29852-29858. Medline abstract: 98037817

Kambadur, R., et al., (1998). Regulation of POU genes by castor and hunchback establishes layered compartments in the Drosophila CNS. Genes Dev. 12(2): 246-260. Medline abstract: 98099749

Kim, U., et al. (1996). The B-cell-specific transcription coactivator OCA-B/OBF-1/Bob-1 is essential for normal production of immunoglobulin isotypes. Nature 383(6600): 542-7. Medline abstract: 97002328

Kitamoto, T. and Salvaterra, P.M. (1995). A POU homeo domain protein related to dPOU-19/pdm-1 binds to the regulatory DNA necessary for vital expression of the Drosophila choline acetyltransferase gene. J. Neurosci. 15 (5 Pt 1): 3509-3518 Medline abstract

Knight, J. C., et al. (1999). A polymorphism that affects OCT-1 binding to the TNF promoter region is associated with severe malaria. Nat. Genet. 22(2): 145-50. Medline abstract: 99295929

Levavasseur, F., et al. (1998). Comparison of sequence and function of the Oct-6 genes in zebrafish, chicken and mouse. Mech. Dev. 74(1-2): 89-98

Liberg, D., Sigvardsson, M. and Leanderson, T. (1997). Oct proteins are qualitative rather than quantitative regulators of kappa transcription. Mol. Immunol. 34(14): 979-86. Medline abstract: 98147372

Lloyd, A. and Sakonju, S. (1991). Characterization of two Drosophila POU domain genes related to oct1 and oct2, and the regulation of their expression pattern. Mech Dev 36: 87-102

Luo, Y., et al. (1998). Coactivation by OCA-B: definition of critical regions and synergism with general cofactors. Mol. Cell. Biol. 18(7): 3803-10. Medline abstract: 98298224

McDonald, J. A., et al. (2003). Specification of motoneuron fate in Drosophila: Integration of positive and negative transcription factor inputs by a minimal eve enhancer. J. Neurobiol. 57(2): 193-203. Medline abstract: 14556285

Mehta, B. and Bhat, K. M. (2001). Slit signaling promotes the terminal asymmetric division of neural precursor cells in the Drosophila CNS. Development 128: 3161-3168. Medline abstract: 11688564

Mihailescu, D., Küry, P. and Monard, D. (1999). An octamer-binding site is crucial for the activity of an enhancer active at the embryonic met-/mesencephalic junction. Mec. Dev. 84 (1-2): 55-67. Medline abstract: 99400099

Mirth, C. and Akam, M. (2002). Joint development in the Drosophila leg: cell movements and cell populations. Dev. Biol. 246: 391-406. Medline abstract: 12051824

Mittal, V., Ma, B. and Hernandez, N. (1999). SNAP_c: a core promoter factor with a built-in DNA-binding damper that is deactivated by the Oct-1 POU domain. Genes Dev. 13: 1807-1821. Medline abstract: 99350323

Neumann, C. J. and Cohen, S. M. (1998). Boundary formation in Drosophila wing: Notch activity attenuated by the POU protein Nubbin. Science 281(5375): 409-413. Medline abstract: 98332777

Ng, M., Diaz-Benjumea, F.J., and Cohen, S. M. (1995). Nubbin encodes a POU-domain protein required for proximal-distal patterning in the Drosophila wing. Development 121: 589-599 Medline abstract

Ng, M., et al. (1996). Specification of the wing by localized expression of wingless protein. Nature 381: 316-318

Nielsen, P. J., et al. (1998). B lymphocytes are impaired in mice lacking the transcriptional co-activator Bob1/OCA-B/OBF1. Eur. J. Immunol. 26(12): 3214-8. Medline abstract: 97131850

Prefontaine, G. G., et al. (1998). Recruitment of octamer transcription factors to DNA by glucocorticoid receptor. Mol. Cell. Biol. 18(6): 3416-30. Medline abstract: 98252929

Prefontaine, G. G., et al. (1999). Selective binding of steroid hormone receptors to octamer transcription factors determines transcriptional synergism at the mouse mammary tumor virus promoter. J. Biol. Chem. 274(38): 26713-9. Medline abstract: 99410400

Qin, X. F., et al. (1998). OCA-B integrates B cell antigen receptor-, CD40L- and IL 4-mediated signals for the germinal center pathway of B cell development. EMBO J. 17(17): 5066-75. Medline abstract: 98393547

Rauskolb, C. and Irvine, K. D. (1999). Notch-mediated segmentation and growth control of the Drosophila leg. Dev. Biol. 210(2): 339-50. Medline abstract: 99287780

Remenyi, A., et al. (2001. Differential dimer activities of the transcription factor Oct-1 by DNA-induced interface swapping. Molec. Cell 8: 569-580. Medline abstract: 11583619

Rodriguez, D. d. A., et al. (2002). Different mechanisms initiate and maintain wingless expression in the Drosophila wing hinge. Development 129: 3995-4004. Medline abstract: 12163403

Roberts, S. B., Segil, N. and Heintz, N. (1991). Differential phosphorylation of the transcription factor Oct1 during the cell cycle. Science 253: 1022-6. Medline abstract: 91361057

Rosner, M. H., Vigano, M. A., Ozato, K., Timmons, P. M., Poirier, F., Rigby, P. W. J. and Staudt, L. M. (1990). A POU-domain transcription factor in early stem cells and germ cells of mammalian embryo. Nature 345: 686-692. Medline abstract: 1972777

Sauter, P. and Matthias, P. (1998). Coactivator OBF-1 makes selective contacts with both the POU-Specific domain and the POU homeodomain and acts as a molecular clamp on DNA. Mol. Cell. Biol. 18(12): 7397-7409. Medline abstract: 99038244

Schubart, D. B., et al. (1996). B-cell-specific coactivator OBF-1/OCA-B/Bob1 required for immune response and germinal centre formation. Nature 383(6600): 538-42. Medline abstract: 97002327

Shah, P. C., Bertolino, E. and Singh, H. (1997). Using altered specificity Oct-1 and Oct-2 mutants to analyze the regulation of immunoglobulin gene transcription. EMBO J. 16(23): 7105-17. Medline abstract: 98046039

Si, I., et al. (1997). Dissociation of Oct-1 from the nuclear peripheral structure induces the cellular aging-associated collagenase gene expression. Mol. Biol. Cell 8(12): 2407-2419

Tiedt, R., et al. (2001). The RING finger protein Siah-1 regulates the level of the transcriptional coactivator OBF-1. EMBO J. 20: 4143-4152. Medline abstract: 11483517

Tomilin, A., et al. (2000). Synergism with the coactivator OBF-1 (OCA-B, BOB-1) is mediated by a specific POU dimer configuration. Cell 103: 853-864

Umesono, Yl, Hiromi, Y. and Hotta, Y. (2002). Context-dependent utilization of Notch activity in Drosophila glial determination. Development 129: 2391-2399. Medline abstract: 11973271

Urbach, R. and Technau, G. M. (2003). Molecular markers for identified neuroblasts in the developing brain of Drosophila. Development 130: 3621-3637. Medline abstract: 12835380

Verrijzer, C. P. and Van der Vliet, P. C. (1993). POU domain transcription factors. Biochim. Biophys. Acta 1173: 1-21

Wang, J. M., et al. (1999). Developmental effects of ectopic expression of the glucocorticoid receptor DNA binding domain are alleviated by an amino acid substitution that interferes with homeodomain binding. Mol. Cell. Biol. 19(10): 7106-7122. Medline abstract: 99421992

Weihe, U., et al. (2004). Proximodistal subdivision of Drosophila legs and wings: The elbow-no ocelli gene complex. Development 131: 767-774

Whitworth, A. J. and Russell, S. (2003). Temporally dynamic response to Wingless directs the sequential elaboration of the proximodistal axis of the Drosophila wing, Dev. Bio. 254: 277-288. Medline abstract: 12591247

Wolf, I., et al. (1998). Downstream activation of a TATA-less promoter by Oct-2, Bob1, and NF-kappaB directs expression of the homing receptor BLR1 to mature B cells. J. Biol. Chem. 273(44): 28831-6. Medline abstract: 99003230

Wong, M. W., et al. (1998). The large subunit of basal transcription factor SNAPc is a Myb domain protein that interacts with Oct-1. Mol. Cell. Biol. 18(1): 368-77. Medline abstract: 98078693

Wood, J. N., et al. (1992). Regulation of expression of the neuronal POU protein Oct-2 by nerve growth factor. J. Biol. Chem. 267: 17787-91 Medline abstract

Yang, X. et al. (1993). The role of a Drosophila POU homeodomain gene in the specification of neural precursor cell identity in the developing embryonic central nervous system. Genes and Dev. 7: 504-516

Yeo, S.L., Lloyd, A., Kozak, K., Dinh, A., Dick, T., Yang, X., Sakonju, S. and Chia, W. (1995). On the functional overlap between two Drosophila POU homeo domain genes and the cell fate specification of a CNS neural precursor. Genes Dev. 9: 1223-1236 Medline abstract

Zhao, X., Pendergrast, P. S. and Hernandez, N. (2001). A positioned nucleosome on the human U6 promoter allows recruitment of SNAPc by the Oct-1 POU domain. Molec. Cell 7: 539-549

Zwilling, S., Annweiler, A. and Wirth, T. (1994). The POU domains of the Oct1 and Oct2 transcription factors mediate specific interaction with TBP. Nucleic Acids Res. 22: 1655-62 Medline abstract

Zwilling, S., Konig, H. and Wirth, T. (1995). High mobility group protein 2 functionally interacts with the POU domains of octamer transcription factors. EMBO J. 14: 1198-1208 Medline abstract

Zwilling, S., et al. (1997). Inducible expression and phosphorylation of coactivator BOB.1/OBF.1 in T cells. Science 277(5323): 221-5. Medline abstract: 97362321

date revised: 10 August 2004

 

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