FGF receptor signaling plays a role in many varied biological processes including cell viability, cell migration, and cell differentiation. The C. elegans FGF receptor is encoded by a single gene, EGL-15. The functions of EGL-15, like mammalian FGF receptors, are biologically diverse. The 5A isoform of EGL-15 functions in the M lineage of the developing hermaphrodite to regulate migration of the sex muscle precursors, and can also play a role in the subsequent differentiation of the sex muscles. Hyperactivation of the C. elegans FGFR inhibits muscle differentiation. Our lab has generated a hyperactivated EGL-15(5A) isoform bearing the transmembrane region of oncogenic NEU/Her-2. This construct is known as
egl-15(5A*) . The
egl-15(5A*) construct causes constitutive ligand-independent dimerization of the receptor and stimulation of downstream signaling. The sex muscle precursors in
egl-15(5A*) animals fail to differentiate to functional sex muscles, and the animals are severely egg-laying defective. This failure of the sex muscles to differentiate can be visualized by the lack of muscle actin filaments in these cells and by disorganized expression of the vulval muscle-specific marker 16Nde::RFP (modified from Harfe and Fire, 1998 1 ). The downstream effectors of EGL-15 in the M-lineage and the mechanism by which
egl-15(5A*) inhibits sex muscle differentiation remain unknown. To identify additional factors which influence sex muscle differentiation, we are looking for genes which, when compromised, can restore sex muscle differentiation in the presence of
egl-15(5A*). To do this, we are utilizing the recently available genome-wide RNA interference library 2 to perform a suppressor screen. To simplify the screening process, we have generated a strain bearing an integrated array containing the vulval muscle-specific marker 16Nde::RFP , the
egl-15(5A*) construct, and the marker of differentiated body wall muscles P
myo-3 ::GFP-NLS . This strain allows rapid visual scoring of vulval muscle and body wall muscle differentiation in live animals using a dissecting microscope and appropriate fluorescent light filters. Sex muscle differentiation in C. elegans acts as a model for the differentiation of mammalian muscle cells in vitro . In mammalian muscle precursors, activation of the FGF receptor can inhibit muscle differentiation, whereas the PI3-kinase pathway is thought to promote muscle differentiation. A candidate gene approach has already shown that activation of the C. elegans PI3-kinase signaling pathway can partially suppress muscle differentiation in the presence of
egl-15(5A*). Thus, sex muscle differentiation in C. elegans is regulated by pathways similar to those that affect mammalian sex muscle differentiation. The suppressors of
egl-15(5A*) are expected to include both the components of the
egl-15(5A*) pathway responsible for inhibiting muscle differentiation, and signaling components in other myogenic pathways. Candidate pathways are those involved in muscle differentiation in mammalian systems, including the
p38 MAP kinase and
p70S6-kinase pathways. Suppressors will be tested for potential roles in EGL-15 signaling with respect to sex myoblast migration, sex muscle differentiation, and other FGF signaling-dependent processes. Suppressors will also be tested for roles in regulating the differentiation of other muscle types in C. elegans . Thus, this project provides an opportunity to study both FGF signaling in the M lineage and the process of muscle differentiation in general. 1. Harfe, BD., and Fire, A. 1998 Development 125 : 421-429. 2. Kamath RS., et al. 2003 Nature. 421 : 231-237.