Mechanisms of Ephrin Receptor Protein Kinase-Independent Signaling in Amphid Axon Guidance in Caenorhabditis elegans

Abstract

Eph receptors and their ephrin ligands are key conserved regulators of axon guidance and can function in a variety of signaling modes. Here we analyze the genetic and cellular requirements for Eph signaling in a Caenorhabditis elegans axon guidance choice point, the ventral guidance of axons in the amphid commissure. The C. elegans Eph receptor EFN-1 has both kinase-dependent and kinase-independent roles in amphid ventral guidance. Of the four C. elegans ephrins, we find that only EFN-1 has a major role in amphid axon ventral guidance, and signals in both a receptor kinase-dependent and kinase-independent manner. Analysis of EFN-1 and EFN-1 expression and tissue-specific requirements is consistent with a model in which VAB-1 acts in amphid neurons, interacting with EFN-1 expressed on surrounding cells. Unexpectedly, left-hand neurons are more strongly affected than right-hand neurons by loss of Eph signaling, indicating a previously undetected left-right asymmetry in the requirement for Eph signaling. By screening candidate genes involved in Eph signaling, we find that the Eph kinase-independent pathway involves the ABL-1 nonreceptor tyrosine kinase and possibly the phosphatidylinositol 3-kinase pathway. Overexpression of ABL-1 is sufficient to rescue EFN-1 ventral guidance defects cell autonomously. Our results reveal new aspects of Eph signaling in a single axon guidance decision in vivo.E PHRINS and their cell surface receptors, the Eph receptor tyrosine kinases (EphR), play critical roles in many axon guidance processes, including midline guidance and growth cone collapse (Drescher et al. 1995;Cowan et al. 2000). In contrast to long-range guidance cues, Eph signaling involves short-range interactions between a transmembrane receptor and transmembrane (ephrin-B) or GPI-linked (ephrin-A) ligands. Eph signaling is complex and multifunctional, capable of mediating both repulsion and attraction depending on ephrin concentration even in the same neurons (Hansen et al. 2004). Many of the signaling pathways downstream of Eph receptors and ephrins regulate cell movement or cell adhesion (Kullander and Klein 2002;Pasquale 2005).Because Eph receptors and ephrins are cell surface molecules, they can operate in a variety of signaling modes (Kullander and Klein 2002;Egea and Klein 2007;Pasquale 2008). Eph receptors can generate kinase-dependent "forward" signals, in which ligand binding triggers receptor dimerization, activating the intrinsic kinase activity of the receptor, and initiating responses in the receptor-expressing cell. Kinase-dependent forward Eph signaling contributes to many processes including retinotopic mapping (Hindges et al. 2002), axonal midline avoidance after crossing (Yokoyama et al. 2001), neural crest cell migration (Smith et al. 1997), and migration of neural progenitors (Catchpole and Henkemeyer 2011). This regulation of diverse developmental processes occurs in part via kinase-dependent interactions with downstream effectors including Src-family kinases (Zisch et al. 1998;Knoll and Dr...

Regulation of Axonal Midline Guidance by Prolyl 4-Hydroxylation inCaenorhabditis elegans

“…Interestingly, we observed that the VNC defects in dpy-18 mutant animals have a strong left-right bias where left axons cross over to the right fascicle. This leftright bias in neuronal patterning has also been reported recently where loss of Eph signaling causes asymmetric defects in amphid neuron guidance (Grossman et al, 2013). Such findings suggest the asymmetric expression of particular guidance cues and postulate a role for DPY-18 in their localization.…”

“…Therefore, we tested whether HSN axon guidance is affected in emb-9 and let-2 mutants. As emb-9 and let-2 mutants are lethal, we used the temperature-sensitive alleles emb-9(hc70) and let-2(b246) for this analysis (Guo et al, 1991;Sibley et al, 1994). Both alleles exhibit embryonic lethality at 25°C, whereas at 20°C they are able to grow.…”

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

“…efn-2, efn-3, and efn-4 also act redundantly in PLM extension (Mohamed and Chin-Sang 2006). EFN-1 functions to guide amphid axons ventrally through the amphid commissure Grossman et al 2013); amphid axon ventral guidance also involves partly redundant SAX-3 and netrin signals. EFN-4 also signals via noncanonical mechanisms that do not involve VAB-1 (Chin-Sang et al 2002), and interacts with the L1CAM LAD-2 to provide a cue for the dorsal migration of the SDQ axons (Dong et al 2016).…”

The Caenorhabditis elegans Ephrin EFN-4 Functions Non-cell Autonomously with Heparan Sulfate Proteoglycans to Promote Axon Outgrowth and Branching

“…Previous work has identified multiple roles for the C. elegans Eph receptor VAB-1 during development, including embryonic morphogenesis, oocyte maturation, and amphid neuron axon guidance (George et al 1998;Miller et al 2003;Mohamed and Chin-Sang 2006;Ikegami et al 2012;Grossman et al 2013). VAB-1 can bind EFN-4 in vitro, making it an obvious candidate receptor (Wang et al 1999).…”