BIOLOGICAL OVERVIEW

In spite of the structural and functional similarities between gooseberry proximal and gooseberry distal, they show considerable divergence. Their exon structures have diverged, as has their regulation. gsb-n has five exons while gsb-d has two. They are each regulated by different proximal promoter regions that interact with different enhancers. gsb-d is induced by pair rule genes and is involved with wingless in an autoregulatory loop. gsb-n's role in differentiation can be carried out by gsb. gsb-n is regulated by gooseberry distal and has a more minor role in neuroblast committment. It would seem that gsb-n is redundant.

There is precedence for such overlap in function. An example is found in the working relationship between engrailed and invected. engrailed has a broader regulatory role, and invected is specialized for carrying out the tasks dictated by engrailed. A similar division of labor is found for gsb and gsb-n. Apparently gsb behaves as a director-general, leaving the more specialized tasks to gsb. What additional function might gsb-n serve that cannot be carried out by gsb? To date, the answer is unknown.

GENE STRUCTURE

The gooseberry (gsb) locus contains two closely linked genes, gsb and gsb-n transcribed in opposite directions. They are separated by about 10 kb of a common upstream region. gooseberry-neural has five exons (Baumgartner, 1987).

PROTEIN STRUCTURE

Amino Acids - 452

Structural Domains

The two gooseberrys are structurally related to each other and to the paired (prd) gene. The structural homology between these two putative proteins and the PRD protein consists essentially of two domains forming most of the amino-terminal halves of the proteins: the PRD domain of 128 amino acids and a PRD-type homeodomain of 60 amino acids, plus an additional 18 amino acids at the amino-terminal end of the homeodomain. There is no C terminal PRD domain, as in paired (Baumgartner, 1987).

Despite the functional difference and the considerably diverged coding sequence of the two gooseberry genes, their proteins have conserved the same function. The finding that the essential difference between genes may reside in their cis-regulatory regions exemplifies an important evolutionary mechanism of how function diversifies after gene duplication (Li,1994b).

There are seven Pax genes in Drosophila and nine Pax genes known in mouse and human. Different Pax proteins use multiple combinations of the HTH motifs to recognize several types of target sites. Drosophila Paired protein can bind, in vitro exclusively through its PAI domain (the N-terminal portion of the bipartite paired domain), or through a dimer of its Homeodomain, or through cooperative interaction between PAI domain and HD. However, paired function in vivo requires the synergistic action of both the PAI domain and the HD. Pax proteins with only a PD (such as Pax-5) appear to require both PAI and RED domains, while a Pax-6 isoform and a new Pax protein Lune, may rely on the RED domain and HD. Thus Pax protein appear to recognize different target genes in vivo through various combinations of their DNA binding domains, thus expanding their recognition repertoire (Jun, 1996).

Promoter Structure

Although gsb and gsb-n are closely linked, their regulation is independent. Different non-overlapping enhancer or upstream control elements drive the specific expression of gsb and gsb-n. Specificity of these enhancers for their respective genes is indicated by their inability to activate transcription in genetically engineered combination with the heterologous promoter of the other gene (Li, 1994a).

Transcriptional Regulation

P-element-mediated transformation with the gooseberry gene has been used to demonstrate that GSB-D transactivates gsb-n and is sufficient to rescue the gooseberry cuticular phenotype in the absence of gsb-n (Gutjhar, 1993).

Patched targets gooseberry distal and gooseberry-proximal in neuroblast determination. The RP2 neuron is a motoneuron and innervates muscle number 2 of the dorsal musculature. This neuron originates along with its sibling cell from the first ganglion mother cell derived from NB4-2, and occupies the anterior commissure along with several other RP2 neurons. NB4-2 itself is formed during the second wave of neuroblast delamination in stage 9. Gooseberry and Patched participate in the Wingless-mediated specification of NB4-2 by controlling the response to the wingless signal. In gsb mutants, WG-positive NB5-3 is transformed to NB4-2 in a Wg-dependent manner, suggesting that GSB normally represses the capacity to respond to the wingless signal. In ptc mutants, gsb is ectopically expressed in normally Wg-reponsive cells, thus preventing the response to Wingless and consequently the correct specification of NB4-2 does not take place. The timing of the response to GSB suggests that the specification of neuroblast identities takes place within the neuroectoderm, prior to neuroblast delamination (Bhat, 1996).

Krüppel is coexpressed with engrailed in a subset of neurons and glia that include the medial-lateral cluster of en-expressing neurons and the dorsal channel glia cells. In Kr mutants, the medial-lateral cluster is either absent or fails to express en, but the dorsal channel glia cells are not affected. These medial-lateral cluster cells gives rise to serotoninergic neurons, and almost no neurons synthesizing serotonin remain in these mutant embryos. In Kr mutants, the number of gooseberry neural-expressing cells increases from 10% to 50%. Ectopic Kr expression leads to a strong reduction in gsb-n expressing neurons (Romani, 1996).

DEVELOPMENTAL BIOLOGY

Embryonic

The Gsb-n protein first appears during germ band extension in cells of the central nervous system. Much later it appears in epidermal stripes and in a small number of muscle cells. Like gooseberry, gsb-n is expressed in a complex pattern in the head [Image] and tail (Gutjhar, 1993).

Ectopic expression of either Gooseberry protein causes cell fate transformations that are reciprocal to those of a deletion mutant of both gooseberry genes. The Gsb protein is required for the specification of naked cuticle in the epidermis and specific neuroblasts in the central nervous system. These roles may reflect independent functions in neuroblasts and epidermal cells or a single function in the common ectodermal precursor cells. The Gsb-n protein is also found in the same neuroblasts as GSB-D and in the descendants of these cells (Zhang, 1994).

REFERENCES

Baumgartner, S., Bopp, D., Burri, M. and Noll, M. (1987). Structure of two genes at the gooseberry locus related to the paired gene and their spatial expression during Drosophila embryogenesis. Genes & Dev 1: 1247-67

Bhat, K. M. (1996). The patched signaling pathway mediates repression of gooseberry allowing neuroblast specification by wingless during Drosophila neurogenesis. Development 122: 2921-32.

Gutjahr T., Patel, N. H., Li, X., Goodman, C. S. and Noll, N. (1993). Analysis of the gooseberry locus in Drosophila embryos: gooseberry determines the cuticular pattern and activates gooseberry neuro. Development 118: 21-31

Jun, S., and Desplan, C. (1996). Cooperative interactions between paired domain and homeodomain. Development 122: 2639-50

Li, X. and Noll, M., (1994a). Compatibility between enhancers and promoters determines the transcriptional specificity of gooseberry and gooseberry neuro in the Drosophila embryo. EMBO J. 13: 400-6 Medline abstract

Li, X. and Noll, M. (1994b). Evolution of distinct developmental functions of three Drosophila genes by acquisition of different cis-regulatory regions. Nature 367: 83-87

Romani, S., et al. (1996). Krüppel, a Drosophila segmentation gene, participates in the specification of neurons and glial cells. Mech. Dev. 60: 95-107

Zhang, Y., Ungar, A., Fresquez, C. and Holmgren, R. (1994). Ectopic expression of either the Drosophila gooseberry-distal or proximal gene causes alterations of cell fate in the epidermis and central nervous system. Development 120: 1151-1161 Medline abstract

date revised:  17 feb 96

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