BIOLOGICAL OVERVIEW

Classifying brainiac according to its role in development presents a problem. It has been classified as a neurogenic gene because of the neural hyperplasia (too many cells) induced in brainiac mutants. It is becoming clear, however, that brainiac besides being implicated in the Notch pathway, also interacts with Epidermal growth factor receptor (Egfr) signaling. The key to understanding the biological role of Brainiac comes from research into the role of Brn in oogenesis. Here Brainiac shows genetic interaction with Egf-r and gurken. However, the hallmark of the oocytic Egr-r-gurken interaction is zygotic dorsal-ventral polarity; with regard to this critical developmental cusp, Brn appears to be uninvolved, apparently having no effect. What then is the role of Brainiac in oogenesis, and how does a component of Egfr signaling come to be grouped with genes involved in Notch signaling?

brainiac mutants show a defect in the dorsal-ventral patterning of the ovarian follicle that results in defective dorsal appendages (anatomical features of Drosophila eggs). In mutants, dorsal appendages are shifted to the dorsal midline and become fused. A similar phenotype is apparent in mutants of gurken and Egf-r, the receptor ligand pair involved in determination of follicle cell identity. Gurken is made by the oocyte, and is involved in signaling to follicle cells, through the Egfr. Gurken signals are involved early in determining posterior follicle cell fate and later in determining dorsal follicle cell fate. In brainiac, Egf-r and gurken mutants, gaps appear in the follicular epithelium. It is believed that Gurken, acting through the Egfr, brings about the successful migration of prefollicular cells to surround each nurse cell-oocyte complex, forming a continuous follicle cell epithelium. Brainiac, a novel secreted protein produced by oocytes, interacts with this pathway either directly, through interference with the Gurken/Egfr interaction, or indirectly through an unknown receptor, thus modifying the results of Gurken signaling (Goode, 1996a).

Now ask the question above from a different perspective: how then does a protein classified as belonging in the Notch pathway become involved in Egfr signaling? One must look to the role of Notch in oogenesis to understand brainiac function. Like brainiac mutants, the animals mutant for Notch frequently develop egg chambers containing multiple oocyte-nurse cell complexes. In these mutants Notch is localized on the apical and lateral surface of all follicular epithelial cells. Stage 5 - 10 Notch mutant egg chambers have epithelial defects similar to those described for brainiac. The most striking phenotype observed for all three genes is a loss of apical-basal polarity and accumulation of follicular epithelial cells in multiple layers around the oocyte. Thus Notch is required in follicle cells for maintenance of the follicular epithelium. The observation that follicle cells show a propensity to detach from the oocyte rather than nurse cells an brainiac and Notch mutant egg chambers, starting at stage 4 and continuing through stage 9 of oogenesis, suggests that Brainiac and Notch are components of at least one oocyte-follicle cell adhesive system during stages 4 to 9 of oogenesis (Goode, 1996b)

Functional tests are consistent with the interdependence of the neurogenic and Egfr signaling process. brainiac and Notch mutations show dominant and synergistic interactions with mutations in components of the Egfr signaling process, for dorsal-ventral patterning and induction of cell proliferation and/or migration (Goode, 1992). These observations are consistent with the idea that cell adhesion, maintained by the Notch system, with the involvement of Brainiac, is required for the execution of inductive signals, carried out by the Egfr system. If Brainiac function in embryogenesis is the same as its function in oogenesis, then it is reasonable to assume that Brainiac cooperates with the Egfr in the development and maintenance of epithelial structure. Brainiac's involvement in lateral inhibition during early neurogenesis may be related to its involvement in maintaining epithelial structure within the neurogenic ectoderm during neuroblast segregation. Disruption of the continuity of the ectodermal epithelium may cause a neurogenic phenotype due to a compromised intercellular signaling capacity of neuroectodermal cells (Goode, 1996a).

GENE STRUCTURE

Exons - 1

Bases in 3' UTR - 331

PROTEIN STRUCTURE

Amino Acids - 325

Structural Domains

Brainiac protein is novel. The hydrophilic cluster at the N-terminus appears to be a signal sequence, acting to facilitate secretion (Goode, 1996a).

EVOLUTIONARY HOMOLOGS

Fringe, a secreted protein involved in boundary formation, and Braniac, required for proper contact or adhesion between germline and follicle cells, may both be part of a large family of glycosyltransferases. Brn and FNG share several features: 1) they are developmentally regulated, secreted signaling molecules without known receptors, 2) they are required during epithelial patterning, 3) they interact genetically with the Notch and/or EGF receptor pathways, suggesting that they might modify the signaling mediated by these receptors, and 4) FNG and Brn both have at least two C. elegans homologs and several vertebrate homologs as well, suggesting the presence of multigene families. FNG and Brn show conserved sequence regions homologous with Haemophilus influenzae Lex1, essential for the biosynthesis of parasitic bacterium's extracellular lipooligosaccharides (LOS). Lex1 is a galactosyltransferase, adding galactose to glucose or N-acetylglucosamine residues of LOS. Secreted glycosyltransferases may use their ability to recognize specific carbohydrate moieties on cell surface molecules to trigger particular receptors; they might also play a crucial role in epithelial pattern formation by modifying these carbohydrate moieties at particular locations recognizable by various carbohydrate-binding domains of extracellular proteins. The carbohydrate status of the cell during development might even be a function of neighboring cells and not only of its own expression set of glycosyltransferases (Yuan, 1997).

A murine homolog of the Drosophila brainiac gene has been isolated and its highly specific expression pattern during development and adult life has been delineated. Particularly strong expression is found in the developing central nervous system, in the developing retina, and in the adult hippocampus. Targeted deletion of mouse Brainiac 1 expression leads to embryonic lethality prior to implantation. Null embryos can be recovered as blastocysts but do not appear to implant, indicating that mouse Brainiac 1, likely a glycosyltransferase, is crucial for very early development of the mouse embryo (Vollrath, 2001).

A murine cDNA coding for a beta1,3-N-acetylglucosaminyltransferase enzyme (beta3GnT) has been isolated. This enzyme is similar in sequence to Drosophila melanogaster Brainiac and to the murine and human beta1,3-galactosyltransferase family of proteins. The mouse beta 3GnT protein is 397 amino acids in length and contains 7 cysteine residues that are conserved in the human ortholog. beta 3GnT is a type II membrane protein localized to the Golgi apparatus. Enzyme assays with recombinant mouse beta 3GnT reveal that it has a preference for acceptors with Gal(beta1-4)Glc(NAc) at the non-reducing termini. Proton NMR analysis of product shows incorporation of GlcNAc in beta1,3 linkage to the terminal Gal of Gal(beta1-4)Glc(beta1-O-benzyl). Northern blot analysis reveals the presence of a single 3.0 kb transcript in all adult mouse and human organs tested, with highest levels in the kidney, liver, heart and placenta. The beta 3GnT gene is also expressed in a number of tumor cell lines. The human orthologue of beta 3GnT is located on chromosome 2pl5 (Egan, 2001).

DEVELOPMENTAL BIOLOGY

Embryonic

Highest expression of brainiac occurs during the 0 to 4 hour interval and 4 to 8 hour interval of embryogenesis (Goode, 1996a).

Adult

Early egg chambers in the vitellarium show uniform expression of brn in germ cells. brn does not appear to be expressed in follicle cells. brn transcription begins at the time that the cyst becomes surrounded by a monolayer of follicle cells. At stage 6, egg chambers shows uniform levels of expression in nurse cells as well as the oocyte. At stage 10 of oogenesis, Brn transcript becomes even more abundant in nurse cells, probably reflecting the maternal contribution of brn transcripts to early neurogenesis (Goode, 1996a).

Effects of mutation or deletion

brainiac mutant mothers produce embryos with epidermal hypoplasia and neural hyperplasia. This phenotype is consistent with the classification of brainiac as a neurogenic gene. Embryos from mutant mothers are zygotically rescuable. As for the well-characterized zygotic neurogenic loci, brainiac gene activity is thus required zygotically to prevent all cells of the neurogenic region from differentiating into neuroblasts (Goode, 1992).

The dorsal appendages are located at the dorso-lateral surface of the eggshell, and are derived from dorsal-lateral follicle cells. Homozygous brn females lay eggs with fused dorsal appendages, a phenotype associated with mutants of the Egf receptor (Egfr) and with gurken mutants. brn is required for determining the dorsal-ventral polarity of the ovarian follicle. In brn and grk mutants, dorsal appendages are fused along the dorsal midline, indicating that dorsal-lateral follicle cells are shifted to a more dorsal position. Surprisingly, although brn influences follicle fates, it has no effect on embryonic DV polarity. This is the first instance of a Drosophila gene required for determination of dorsal-ventral follicle cell fates that is not required for determination of embryonic dorsal-ventral cell fates. The temperature-sensitive period for brn effect on follicle cell fate begins at the inception of vitellogenesis (Goode, 1992).

brainiac and Egfr interact in the initial formation of the follicle cells in the germarium. brainiac/Egfr double mutants lay only a few or no eggs, in contrast to single mutants which lay hundreds of eggs. In double mutants most follicles have more than one oocyte and more than 15 nurse cells. These follicles are formed within the germarium and proceed to late stages of oogenesis. Occasionally such chambers will proceed with chorion synthesis and form an egg in the shape of a ball, lacking overt polarity. Prefollicular cells fail to migrate between each oocyte/nurse cell complex, resulting in follicles with multiple sets of oocytes and nurse cells. brn and Egfr function is also required for establishing and/or maintaining a continuous follicular epithelium around each oocyte/nurse cell complex (Goode, 1992).

Embryos from mature mutant brn eggs also develop a neurogenic phenotype that can be zygotically rescued if a wild-type sperm fertilizes the egg. The zygotic brn functions, as well as the brn requirement for determination of follicle cell fate, appear to be genetically separable functions of the brn locus. Genetic mosaic experiments show that brn is required in the germline for establishment of follicle cell fate whereas the Egfr is required in the follicle cells. Thus brn may be part of a germline signaling pathway differentially regulating successive Egfr-dependent follicle cell activities of migration, division and/or adhesion and determination during oogenesis. It appears that the functions of brn in oogenesis are distinct from those of Notch and Delta, two other neurogenic loci that are known to be required for follicular development (Goode, 1992).

gurken and brainiac exhibit dosage-sensitive interactions. Double heterozygotes of weak mutants lay eggs with partially fused dorsal appendages, while single mutants are completely wildtype. Animals heterozygous for strong alleles show slight defects in patterning of follicular epithelium, while double mutants lay completely ventralized eggs completely lacking dorsal appendages. Ovarioles from females homozygous for weak alleles of grk or brn resemble wild type in terms of follicular cell development. In contrast, egg chambers from animals homozygous for strong alleles consistently display both fused egg chambers and gaps in the follicular epithelium, frequently uncovering over half of the egg chamber. It is concluded that grk acts in concert with brn to achieve the migration of prefollicular cells to surround each nurse cell-oocyte complex and to form a continuous epithelium (Goode, 1996a).

The neurogenic gene brainiac is specifically required for epithelial development. egghead (egh), a gene with phenotypes identical to brn, encodes for a novel, putative secreted or transmembrane protein. By comparing the function of germline egh and brn to Notch during oogenesis, direct evidence has been found for the involvement of Notch in maintenance of the follicle cell epithelium, and the specificity of brn and egh in epithelial development during oogenesis. The most striking phenotype observed for all three genes is a loss of apical-basal polarity and accumulation of follicular epithelial cells in multiple layers around the oocyte. The spatiotemporal onset of this adenoma-like phenotype correlates with the differential accumulation of egh transcripts in the oocyte at stage 4 of oogenesis. In contrast to N, it is found that brn and egh are essential for the organization, but not specification, of stalk and polar cells (Goode, 1996b).

Both egh and brn are required for concerted border cell-main body epithelium migration. Polar cells at the anterior of the egg chamber are part of a larger group of cells, termed border cells, that break from the main body epithelium (referred to as MBE, that portion of the epithelium that maintains a cuboidal/columnar phenotype and ends up covering the oocyte). These cells become mesenchymal-like, and migrate through the center of the egg chamber at stage 9 of oogenesis. slow border cells (slbo) is specifically expressed in border cells starting at stage 8 of oogenesis. Although follicle cells at the posterior of brn and egh mutant egg chambers become mesenchymal-like, they do not express slbo, indicating that they have not adopted a border cell fate (Goode, 1996b).

Instead, analysis of slbo expression from stage 8 to stage 10 in these mutants reveals that the migration of the MBE and border cells is not always concerted, and is not normal. The MBE frequently moves ahead of border cells, and completes its migration well before their migration. Once the MBE has completed its migration, slbo expression initiates at the anterior and posterior of the oocyte, but expression appears weaker as compared to that in wild type. All of these defects are independent of whether multiple layers of follicle cells accumulate around the posterior of the oocyte, and are consistent with a requirement for brn, egh and Notch in regulating the relative epithelial state of the follicular epithelium during a dramatic morphogenetic reorganization (Goode, 1996b).

The expression patterns and functional requirements of brn, egh, and N lead to a proposal that these genes mediate follicular morphogenesis by regulating germline-follicle cell adhesion. This proposal offers explanations for (1) the involvement of egh and brn in N-mediated epithelial development, but not lateral specification; (2) why brn and egh embryonic neurogenic phenotypes are not as severe as N phenotypes, and (3) how egh and brn influence Egfr-mediated processes. The correlation between the differential expression of egh in the oocyte and the differential requirement for brn, egh, and N in maintaining the follicular epithelium around the oocyte, suggests that Egghead is a critical component of a differential oocyte-follicle cell adhesive system (Goode, 1996b).

A screen was performed for female sterile mutations on the X chromosome of Drosophila and new loci were identified that are required for developmental events in oogenesis: new alleles of previously described genes were identified as well. The screen has identified genes that are involved in cell cycle control, intracellular transport, cell migration, maintenance of cell membranes, epithelial monolayer integrity and cell survival or apoptosis. New roles are described for the genes dunce, brainiac and fs(1)Yb, and new alleles of Sex lethal, ovarian tumor, sans filles, fs(1)K10, singed, and defective chorion-1 have been identified (Swan, 2001).

REFERENCES

Egan, S., et al. (2001). Molecular cloning and expression analysis of a mouse UDP-GlcNAc:Gal(beta1-4)Glc(NAc)-R beta1,3-N-acetylglucosaminyltransferase homologous to Drosophila melanogaster Brainiac and the beta1,3-galactosyltransferase family. Glycoconj. J. 17: 867-75. Medline abstract: 11511811

Goode, S., Wright, D. and Mahowald, A. P. (1992). The neurogenic locus brainiac cooperates with the Drosophila EGF receptor to establish the ovarian follicle and to determine its dorsal-ventral polarity. Development 116: 177-92 Medline abstract

Goode, S., et al. (1996a). brainiac encodes a novel, putative secreted protein that cooperates with Grk TGFalpha in the genesis of the follicular epithelium. Dev. Biol. 178: 35-50. Medline abstract: 8812107

Goode, S., et al. (1996b). The neurogenic genes egghead and brainiac define a novel signaling pathway essential for epithelial morphogenesis during Drosophila oogenesis. Development 122: 3863-79. Medline abstract: 9012507

Swan, A., et al. (2001). Identification of new X-chromosomal genes required for Drosophila oogenesis and novel roles for fs(1)Yb, brainiac and dunce. Genome Res. 11(1): 67-77. Medline abstract: 11156616

Vollrath, B., Fitzgerald, K. J. and Leder, P. (2001). A murine homologue of the Drosophila brainiac gene shows homology to glycosyltransferases and is required for preimplantation development of the mouse. Mol. Cell. Bio. 21: 5688-5697. Medline abstract: 11463849

Yuan, Y. P., et al. (1997). Secreted Fringe-like signaling moecules may be glycosyltransferases. Cell 88: 9-11. Medline abstract: 9019410

date revised:  30 January 2002 

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