Interactions of UNC-34 Enabled With Rac GTPases and the NIK Kinase MIG-15 in Caenorhabditis elegans Axon Pathfinding and Neuronal Migration

Abstract

Many genes that affect axon pathfinding and cell migration have been identified. Mechanisms by which these genes and the molecules they encode interact with one another in pathways and networks to control developmental events are unclear. Rac GTPases, the cytoskeletal signaling molecule Enabled, and NIK kinase have all been implicated in regulating axon pathfinding and cell migration. Here we present evidence that, in Caenorhabditis elegans, three Rac GTPases, CED-10, RAC-2, and MIG-2, define three redundant pathways that each control axon pathfinding, and that the NIK kinase MIG-15 acts in each Rac pathway. Furthermore, we show that the Enabled molecule UNC-34 defines a fourth partially redundant pathway that acts in parallel to Rac/MIG-15 signaling in axon pathfinding. Enabled and the three Racs also act redundantly to mediate AQR and PQR neuronal cell migration. The Racs and UNC-34 Ena might all control the formation of actin-based protrusive structures (lamellipodia and filopodia) that mediate growth cone outgrowth and cell migration. MIG-15 does not act with the three Racs in execution of cell migration. Rather, MIG-15 affects direction of PQR neuronal migration, similar to UNC-40 and DPY-19, which control initial Q cell polarity, and Wnt signaling, which acts later to control Q cell-directed migration. MIG-2 Rac, which acts with CED-10 Rac, RAC-2 Rac, and UNC-34 Ena in axon pathfinding and cell migration, also acts with MIG-15 in PQR directional migration.

Control of developmental networks by Rac/Rho small GTPases: How cytoskeletal changes during embryogenesis are orchestrated

“…In addition to this pathway, gastrulation is regulated by the unc‐34 gene, the worm homolog of the human Enabled gene (Ena), which belongs to the Ena/VASP family, formed by evolutionarily conserved actin‐modulating proteins 36. In fact, CED‐10 /Rac and UNC‐34 / Ena might have partly overlapping roles in this event.…”

“…In fact, CED‐10 /Rac and UNC‐34 / Ena might have partly overlapping roles in this event. Null unc‐34 mutants are viable and fertile, but embryos of double mutants unc‐34(lq17); ced‐10(n1993) (both hypomorphic alleles) arrest with the Gex phenotype, suggesting that UNC‐34/Ena and CED‐10/Rac have overlapping roles in gastrulation 36. Additionally, interactions between unc‐34/Ena and other rac genes were checked: ced‐10(n1993) and rac‐2(RNAi) synergized with unc‐34 in gastrulation and embryonic elongation.…”

“…Additionally, interactions between unc‐34/Ena and other rac genes were checked: ced‐10(n1993) and rac‐2(RNAi) synergized with unc‐34 in gastrulation and embryonic elongation. However, unc‐34/Ena; mig‐2/RhoG double mutants were viable and fertile, suggesting that unc‐34/Ena and mig‐2/RhoG might act in a common pathway in parallel to ced‐10/Rac and rac‐2/Rac in gastrulation 36.…”

“…Later on, at the 350 cell‐stage, epidermal cells undergo ventral enclosure, a process by which epidermal cells located on the dorsal side of the embryo go through an epiboly movement to the ventral midline to cover the embryo 36 (Supplementary Movie S1). Simultaneously, on the dorsal side of the embryo, epidermal cells undergo cell intercalation and the embryo elongates 33, 37, 38.…”

“…Both need actin cytoskeleton rearrangements in the growth cone or at the leading edges of the plasma membrane of migrating cells 36, 44, 45. In C. elegans , in response to UNC‐6/Netrin signaling, Rho GTPase CDC‐42 activates the C. elegans GEF TIAM (T‐cell lymphoma Invasion and Metastasis Factor 1), which is a GTP exchange factor specific for Rac and which activates Rac signaling, in lamellipodia and filopodia formation and axon guidance 46, 47.…”