Antagonism of EGFR and notch signalling in the reiterative recruitment of Drosophila adult chordotonal sense organ precursors

The EGF receptor and N signalling pathways act antagonistically in Drosophila mesothorax bristle patterning

An early step in the development of the large mesothoracic bristles (macrochaetae) of Drosophila is the expression of the proneural genes of the achaete-scute complex (AS-C) in small groups of cells (proneural clusters) of the wing imaginal disc. This is followed by a much increased accumulation of AS-C proneural proteins in the cell that will give rise to the sensory organ, the SMC (sensory organ mother cell). This accumulation is driven by cis-regulatory sequences, SMC-specific enhancers, that permit self-stimulation of the achaete, scute and asense proneural genes. Negative interactions among the cells of the cluster, triggered by the proneural proteins and mediated by the Notch receptor (lateral inhibition), block this accumulation in most cluster cells, thereby limiting the number of SMCs. Here we show that the proneural proteins trigger, in addition, positive interactions among cells of the cluster that are mediated by the Epidermal growth factor receptor (EGFR) and the Ras/Raf pathway. These interactions, which we denominate ‘lateral co-operation’, are essential for macrochaetae SMC emergence. Activation of the EGFR/Ras pathway appears to promote proneural gene self-stimulation mediated by the SMC-specific enhancers. Excess EGFR signalling can overrule lateral inhibition and allow adjacent cells to become SMCs and sensory organs. Thus, the EGFR and Notch pathways act antagonistically in notum macrochaetae determination.

Sense organ identity in the Drosophila antenna is specified by the expression of the proneural gene atonal

We have shown that the basic helix-loop-helix transcription factor Atonal is sufficient for specification of one of the three subsets of olfactory sense organs on the Drosophila antenna. Misexpression of Atonal in all sensory precursors in the antennal disc results in their conversion to coeloconic sensilla. The mechanism by which specific sense organ fate is triggered remains unclear. We have shown that the homeodomain transcription factor Cut which acts in the chordotonal-external sense organ choice does not play a role in olfactory sense organ development. The expression of atonal in specific domains of the antennal disc is regulated by an interplay of the patterning genes, Hedgehog and Wingless, and Drosophila epidermal growth factor receptor pathway.

Notch signaling in the development of the inner ear: lessons from Drosophila

The sensory patches in the ear of a vertebrate can be compared with the mechanosensory bristles of a fly. This comparison has led to the discovery that lateral inhibition mediated by the Notch cell-cell signaling pathway, first characterized in Drosophila and crucial for bristle development, also has a key role in controlling the pattern of sensory hair cells and supporting cells in the ear. We review the arguments for considering the sensory patches of the vertebrate ear and bristles of the insect to be homologous structures, evolved from a common ancestral mechanosensory organ, and we examine more closely the role of Notch signaling in each system. Using viral vectors to misexpress components of the Notch pathway in the chick ear, we show that a simple lateral-inhibition model based on feedback regulation of the Notch ligand Delta is inadequate for the ear just as it is for the fly bristle. The Notch ligand Serrate1, expressed in supporting cells in the ear, is regulated by lateral induction, not lateral inhibition; commitment to become a hair cell is not simply controlled by levels of expression of the Notch ligands Delta1, Serrate1, and Serrate2 in the neighbors of the nascent hair cell; and at least one factor, Numb, capable of blocking reception of lateral inhibition is concentrated in hair cells. These findings reinforce the parallels between the vertebrate ear and the fly bristle and show how study of the insect system can help us understand the vertebrate.

cato encodes a basic helix-loop-helix transcription factor implicated in the correct differentiation of Drosophila sense organs

In Drosophila neurogenesis, proneural genes encode bHLH proteins that are required for neural precursor selection. But many vertebrate homologues are expressed later and are postulated to have multiple roles during neurogenesis. We have isolated a new Drosophila gene, cato, which encodes a protein with a bHLH domain that is closely related to that of the proneural protein Atonal. cato expression is restricted to the developing PNS, where it is expressed in between the stages of precursor selection and terminal differentiation (and therefore later than the proneural genes). We present evidence from loss-of-function and misexpression experiments that cato is involved in sensory neurone morphology. Moreover, in prospero mutants, in which axon and dendrite outgrowth is defective, cato is strongly derepressed in the developing CNS.

Drosophila atonal controls photoreceptor R8-specific properties and modulates both receptor tyrosine kinase and Hedgehog signalling

During Drosophila eye development, the proneural gene atonal specifies founding R8 photoreceptors of individual ommatidia, evenly spaced relative to one another in a pattern that prefigures ommatidial organisation in the mature compound eye. Beyond providing neural competence, however, it has remained unclear to what extent atonal controls specific R8 properties. We show here that reduced Atonal function gives rise to R8 photoreceptors that are functionally compromised: both recruitment and axon pathfinding defects are evident. Conversely, prolonged Atonal expression in R8 photoreceptors induces defects in inductive recruitment as a consequence of hyperactive EGFR signalling. Surprisingly, such prolonged expression also results in R8 pattern formation defects in a process associated with both Hedgehog and Receptor Tyrosine Kinase signalling. Our results strongly suggest that Atonal regulates signalling and other properties of R8 precursors.

split ends encodes large nuclear proteins that regulate neuronal cell fate and axon extension in the Drosophila embryo

split ends (spen) encodes nuclear 600 kDa proteins that contain RNA recognition motifs and a conserved C-terminal sequence. These features define a new protein family, Spen, which includes the vertebrate MINT transcriptional regulator. Zygotic spen mutants affect the growth and guidance of a subset of axons in the Drosophila embryo. Removing maternal and zygotic protein elicits cell-fate and more general axon-guidance defects that are not seen in zygotic mutants. The wrong number of chordotonal neurons and midline cells are generated, and we identify defects in precursor formation and EGF receptor-dependent inductive processes required for cell-fate specification. The number of neuronal precursors is variable in embryos that lack Spen. The levels of Suppressor of Hairless, a key transcriptional effector of Notch required for precursor formation, are reduced, as are the nuclear levels of Yan, a transcriptional repressor that regulates cell fate and proliferation downstream of the EGF receptor. We propose that Spen proteins regulate the expression of key effectors of signaling pathways required to specify neuronal cell fate and morphology.

atonal regulates neurite arborization but does not act as a proneural gene in the Drosophila brain

Drosophila atonal (ato) is the proneural gene of the chordotonal organs (CHOs) in the peripheral nervous system (PNS) and the larval and adult photoreceptor organs. Here, we show that ato is expressed at multiple stages during the development of a lineage of central brain neurons that innervate the optic lobes and are required for eclosion. A novel fate mapping approach shows that ato is expressed in the embryonic precursors of these neurons and that its expression is reactivated in third instar larvae (L3). In contrast to its function in the PNS, ato does not act as a proneural gene in the embryonic brain. Instead, ato performs a novel function, regulating arborization during larval and pupal development by interacting with Notch.

amos, a proneural gene for Drosophila olfactory sense organs that is regulated by lozenge

In a variety of organisms, early neurogenesis requires the function of basic-helix-loop-helix (bHLH) transcription factors. For the Drosophila PNS, such transcription factors are encoded by the proneural genes (atonal and the achaete-scute complex, AS-C). We have identified a proneural gene, amos, that has strong similarity with atonal in its bHLH domain. We present evidence that amos is required for olfactory sensilla and is regulated by the prepattern gene lozenge. Between them, amos, atonal, and the AS-C can potentially account for the origin of the entire PNS.