EGF Receptor Signaling: A Prickly Proposition

The L1-type transmembrane protein Echinoid [1] is essential for the proper spatial patterning of the compound eye in the fruitfly Drosophila. Two recent papers [2,3] report that, in echinoid mutant flies, the primary defect is in patterning a specific cell type, the R8 photoreceptor cell. During normal eye development in Drosophila, a single R8 cell founds each ommatidium, but in echinoid mutants ommatidia have clusters of two or three R8 cells instead [2,3]. The effects of losing Echinoid are dependent entirely on EGF receptor activity [2], which is elevated in echinoid mutants [2,3]. In comparison to the many other developmental roles of the EGF receptor, the defect in echinoid mutants is quite specific, and much less extensive than the effect of wholesale EGF receptor activation. Spencer and Cagan [3] propose that Echinoid participates in negative feedback regulation of EGF receptor activity, because removing Echinoid prolongs EGF-receptor-dependent MAP kinase phosphorylation [3], and because the intracellular domain of Echinoid is tyrosine phosphorylated in response to EGF receptor activity [3]. The phosphorylation may well be direct, as it does not occur after activation of downstream components such as Ras1 [3]. Although the importance of Echinoid phosphorylation has not been shown directly, large experimental increases in echinoid transcription do not affect EGF receptor signaling, suggesting that Echinoid activity may depend on such post-translational mechanisms [3]. The proposal is that Echinoid provides negative feedback control upstream of Ras1, when phosphorylated in response to EGF receptor activity [3]. Echinoid protein coimmunoprecipitates with the EGF receptor, suggesting that Echinoid regulates EGF receptor through forming a complex with it [3]. Echinoid lacks the intracellular motifs that mediate endocytosis in other L1-adhesion proteins, however, making the mechanism by which the complex reduces EGF receptor activity uncertain. Mutations in echinoid suppress the effects of loss-of-function mutations in the gene corkscrew [4], suggesting that Echinoid may act through Corkscrew, an SH2-domain-containing protein tyrosine phosphatase that acts positively in signal transduction from multiple receptor tyrosine kinases [5]. Echinoid is reported to bind Corkscrew [3]; if Echinoid indeed acts on Corkscrew directly, then it resembles a class of molecules called signal regulatory proteins (SIRPs) [6], which are known from vertebrates but apparently absent from Drosophila [3]. Like Echinoid, SIRPs contain multiple extracellular immunoglobulin repeats and an intracellular domain with potential tyrosine phosphorylation sites [6]. Receptor tyrosine kinases phosphorylate SIRPs, creating sites that sequester Shp-2, the vertebrate homolog of Corkscrew, and reduce signal transduction [6,7]. It is possible that Echinoid functions like a Drosophila SIRP, even though it is not a SIRP by the current sequence definition [3]. Echinoid differs from the negative regulators of EGF receptor that are already known — Argos, Sprouty and Kekkon 1 [8–10]. Although mutations in Argos, Sprouty and Kekkon 1 hyperactivate the EGF receptor and lead to exaggerations of EGF receptor’s normal functions, including recruitment of various retinal cell types, they do not affect R8-cell number [8–10]. The effect of losing Echinoid shows unequivocally that excess EGF receptor activity can also recruit additional R8 cells, even though R8 is one of the few retinal cell types that can be specified in the complete absence of the EGF receptor [2,3]. This had already been shown from overexpression experiments [11,12], but the fact that Drosophila uses Echinoid to prevent this happening in normal development demonstrates its physiological relevance [2]. As Rawlins et al. [2] further point out, there is at least one tissue in which EGF receptor normally does something very similar by recruiting a cluster of neural cells. During the development of chordotonal organs (part of the peripheral nervous system), a primary neural precursor is specified independently of EGF receptor, but subsequently it uses local EGF receptor activation to recruit several neighboring cells into a small cluster of neurons [13,14]. It is possible that Echinoid functions to make eye development different from chordotonal organ development by short-circuiting the second step, preventing recruitment of further R8-like neurons [2]. Why should echinoid mutants be different from the other negative regulators? One difference is in their expression: whereas transcription of Argos, Sprouty and Kekkon-1 are all induced by EGF receptor activity [8–10], Echinoid seems to be widely and uniformly expressed regardless of the EGF receptor activity level [1,3]. Unlike Argos, Sprouty and Kekkon-1, preexisting Echinoid might be able to modify EGF receptor activity very rapidly, even before any transcriptional response has occurred. Furthermore, Echinoid appears to have effects on neighboring cells [2], whereas Sprouty and Kekkon-1 act autonomously and Argos shows long-range nonautonomy. This suggests EGF receptor signaling could be influenced Dispatch Current Biology, Vol. 13, 773–774, September 30, 2003, ©2003 Elsevier Science Ltd. All rights reserved.