BIOLOGICAL OVERVIEW

Among neurogenic genes in Drosophila, big brain is an enigma. The term neurogenic creates its own confusion, as it refers to the mutant phenotype of a family of genes. The function of neurogenic genes is to reduce, rather than engender, the number of neural precursors; therefore, the so-called neurogenic genes may be thought of as functionally anti-neurogenic. big brain does not interact genetically with other neurogenic genes (such as Notch and Delta) suggesting that Bib does not act in the same pathway as the classic neurogenic genes. In addition, Bib structurally resembles aquaporins (water pores), the channel proteins that facilitate uptake of small molecules into cells and cell-cell communication. Compounding the confusion, Bib has no hydrophobic transmembrane sequences, suggesting that the protein, unlike other aquaporins, may be totally intracellular (Rao, 1990 and references).

What is the evidence that Bib is neurogenic? Ventral clusters of sensory neurons and their support cells are missing in bib mutants probably due to the overgrowth of the central nervous system. Lateral and dorsal clusters show an increase of sensory neurons and support cells. All three types of embryonic sensory neurons are increased by up to two fold in bib mutants: (1) external sensory neurons (sn), which innervate mechansosensory or chemosensory structures; (2) the chordotonal neurons (ch) that innervate an elongated internal sensory structure, and (3) multiple dendritic (md) organs. The duplication of sensory neurons, their support cells and the external sensory structures formed by the support cells suggests that the primary effect of bib mutation is an increase in the number of precursor cells that give rise to both neurons and the support cells. Examination of the division rate of precursors confirms that sensory neurons are more numerous in mutant embryos but that the division patterns of precursors are similar to those in wild-type embryos (Rao, 1992).

Drosophila adult sensory organs develop independently during the pupal stage. To determine whether bib also gives an adult neurogenic phenotype, clones of bib mutant cells were generated. bib mutant clones show an increased number of macrochaetae in all of the body parts examined including the notum (derived from the wing imaginal disc) the scutellum, the sternopleura, the head and wings. Supernumerary bristles are found in locations where bristles normally form (Rao, 1992).

Previous studies of big brain genetic interactions and expression suggest that bib acts as a channel protein in proneural cluster cells that adopt the epidermal cell fate and serves a necessary function in the response of these cells to the lateral inhibition signal. These genetic studies had not revealed any interaction between big brain and the other neurogenic genes. bib cannot rescue the phenotype in embryos mutant for other neurogenic genes. Ectopic bib expression does not rescue the cuticle-defect of embryos mutant for Dl, N, E(spl), mam or neu. In reciprocal experiments, ectopic Dl and neu do not rescue the neurogenic phenotype in bib mutant embryos. In contrast, ectopically activated N still has an antineurogenic effect in bib mutant embryos. These results indicate that bib, Dl and neu cannot functionally replace one another and that bib functions upstream of or parallel to activated N. Using mosaic analysis in the adult, it has been demonstrated that big brain activity is required autonomously in epidermal precursors to prevent neural development. Ectopically expressed big brain acts synergistically with ectopically expressed Delta and Notch, providing the first evidence that big brain may function by augmenting the activity of the Delta-Notch pathway (Doherty, 1997).

big brain mutants are different from other neurogenic phenotypes in two ways:

• bib is not required for the refinement of the entire neurogenic cluster. bib causes only a two fold increase in the number of sensory organ precursors as compared to the 8 to 12-fold increase in N or Dl null mutants. Not all cells within a proneural cluster become sensory organ precursors simultaneously. It is likely that bib is required for only a subset of proneural cells and not for all the cells of a proneural cluster.
  
  
• Notch functions both at the level of proneural precursors and at the level of sensory neurons and their support cells. bib functions only at the level of proneural precursors (Rao, 1992).

The enigma remains. Does Bib brain function have anything to do with its aquaporin structure? If Bib has an aquaporin function, is this reflected in the corresponding effect of Bib on cell size or cytoskeletal dynamics, and how is this translated into altered cell fate?

GENE STRUCTURE

Bases in 5' UTR - 302

Exons -

Bases in 3' UTR - 915

PROTEIN STRUCTURE

Amino Acids - 700

Structural Domains

The N-terminal half of Bib protein is highly hydrophobic, whereas the C-terminal portion is generally hydrophilic and glutamine-rich. There are six hydrophobic stretches of more than 21 amino acid residues that could potentially span the membrane. Since there is not signal peptide, it is thought that Bib is intracellular (Rao, 1990).

The Bib protein has significant sequence homology to the major intrinsic protein (MIP) of bovine lens fiber cell membrane; 40% of the 224 amino acids in the N-terminal half are identical to those in MIP. This region has sequence similarity to a soybean membrane protein, nodulin (with 27% amino-acid identity), and an E. coli protein, GlpF (with 23% identity). The Bib protein is about twice as large as the other proteins; the C-terminal half of Bib shows no significant homology to any known protein (Rao, 1990).

The Drosophila Big Brain (Bib) protein plays a critical role in the determination of neuroblasts in the embryonic ectoderm and many other cell types. Bib is a member of the MIP family of transmembrane channel proteins. The conserved channel domain of Bib is flanked by amino- and carboxy-terminal cytoplasmic domains of unique sequence, which comprise over two-thirds of the protein. To determine whether the cytoplasmic domains of Bib are important for Bib function, the bib gene of D. virilis has been cloned and sequenced and compared with that of D. melanogaster. The channel domain and both cytoplasmic domains are highly conserved between the two species. The conservation of the cytoplasmic domains indicates that they are critical to Bib function. bib transcripts are found in similar patterns in both species, indicating that the developmental function(s) of Bib has also been conserved (Burris, 1999).

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