Neuronal development in the Drosophila retina: the sextra gene defines an inhibitory component in the developmental pathway of R7 photoreceptor cells.

Abstract

Mutations in a gene called sextra (sxt) have been isolated. Loss of one copy of sxt promotes R7 photoreceptor cell development in a genetically sensitized background, while loss of both copies results in precursors of non-neuronal cone cells transforming into R7 cells. The requirement for sxt function is cell-autonomous. The transformation of cone-cell precursors into R7 cells occurs independently of the sevenless signal. However, the R7 precursor becomes neuronal in an sxt/sxt mutant only in a wild-type sevenless background. The genetic analysis of sxt suggests that it plays an inhibitory role, preventing cone cells from becoming neuronal. Additionally, sxt functions in R7 precursors, but the sevenless sial is essential for specification of this fate, since oss of sextra alone is unable to impart a neural fate to this cell.Characterization of a set ofgenes controlling R7 development has provided much insight into the process of induction and signal transduction (1)(2)(3)(4). Genetic and molecular analysis of sevenless (sev) (5-8) and bride of sevenless (boss) (9, 10) has demonstrated that R8 induces a neighboring cell to take on the R7 fate (9). In boss or sev mutants, R7 cells are missing within the eye. While the boss protein, a membrane-bound ligand, is required in the inducing R8 cell for normal R7 development (11), the sev gene encodes a tyrosine kinase receptor required in the R7 precursor for it to assume an R7 fate (12, 13). The boss protein has been shown to bind directly to the sevenless protein (11), initiating a molecular cascade that causes development of the R7 neuron.Recently, two more genes, Son of sevenless (Sos) 78C/D were mated to either sxtlu6I/sxtBJ6' or sxtRI6I,bossl/ sxtMu6l,bossl males. Progeny from these crosses were irradiated with -y-rays (1200 rads; 1 rad = 0.01 Gy) between 24 and 48 hr of development. Mosaic eyes were generated at a frequency of -0.01. Histology. Preparation of samples for transmission electron microscopy (TEM) was essentially as described (19), except that uranyl acetate staining en bloc was omitted. The sections were stained for 30 min with uranyl acetate and 10 min with lead citrate and were analyzed on a Phillips 300 electron microscope operating at 60 kV. For light-level microscopy, the fixation conditions were modified as described (9). To facilitate scoring of pigment granules, flies were exposed to bright light for 10 min before dissection. Cobalt sulfide staining was done as described (4).Immunohistochemistry. Adult heads were dissected into halves and fixed for 1 hr in 0.8% glutaraldehyde in phosphatebuffered saline. The retinas were transferred to fresh fixative, dissected out of the surrounding cuticle, allowed to fix for another 30 min, stained with antibodies (8), and embedded for sectioning (9). Eye imaginal discs were stained with monoclonal antibody (mAb) 22C10 and processed for TEM (11). The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accord...

Interactions of Drosophila Cbl with Epidermal Growth Factor Receptors and Role of Cbl in R7 Photoreceptor Cell Development

“…The above-described genetic background is extremely sensitive to the dosage of genes participating in receptor tyrosine kinase signaling, and the fraction of ommatidia developing R7 cells is a readout for the strength of the transduced signal. We have previously shown that removing a single copy of a positive regulator such as Ras or Sos abolishes the development of R7 cells in this genetic background, while removing one copy of a negative regulator, such as GAP1, will increase this number significantly (29). Removing a copy of the EGF receptor in this background also completely abrogates R7 differentiation (Fig.…”

A Drosophila SH2-SH3 adaptor protein implicated in coupling the sevenless tyrosine kinase to an activator of Ras guanine nucleotide exchange, Sos

“…The response of ommatidial precursor cells to either a partially functional, or a constitutively active sev kinase is dosage sensitive. This observation has been exploited to identify genetically downstream components of the sev signaling pathway, establishing that Gapl, Ras1, and Son of sevenless (Sos), a putative guanine nucleotide-releasing protein (GNRP), play essential roles in signaling from the sev receptor (Rogge et al, 1992;Gaul et al 1992;Sinion et al, 1991: Rogge et al, 1991. Similarly, in C. elegans it has been shown that a receptor tyrosine kinase encoded by the let-23 gene and a Ras protein encoded by let-60 are required for inducrion of the vulval cell fate (Aroian et al, 1990;Beitel et al, 1990;Han and Sternberg, 1990).…”

“…In principle, Ras GTP binding could be induced either by stimulation of guanine nucleotide exchange by a GNRP or by inhibition of a Ras GTPaseactivating protein (GAP). In the sev pathway, it has been proposed that Ras1 activation is achieved both by stimulation of the Sos GNRP arid by inhibition of Gap1 (Rogge et al, 1992;Gaul et al, 1992). Biochemical data from mammalian cells have shown that growth factors increase the fraction of Ras in the GTP-bound state (Gibbs et al, 1990;Satoh et al, 1990aSatoh et al, , 1990b, and have suggested that regulation of GNRP activity may be the predominant mechanism by which growth factor receptors control Ras activation (Zhang et al, 1992;Medema et al, 1993).…”

tramtrack is a transcriptional repressor required for cell fate determination in the Drosophila eye.

“…1991). The recent isolation of mutations in Rasl, and the positive and negative regulators of Rasl, Sos, and Gapl, have demonstrated that R7 cell determination involves activation of Rasl (Rogge et al 1991(Rogge et al , 1992Simon et al 1991;Bonfini et al 1992;Buckles et al 1992;Gaul et al 1992). Although the direct effector for Rasl has not been identified, the Drosophila homolog of the mammalian serine/threonine kinase Rafl has been shown to act downstream of Rasl (Dickson et al 1992).…”