Abstract
Cell migration plays essential roles in animal development, homeostasis, and disease states. Migrating cells often rely on extracellular signals to accurately navigate, and these signaling dynamics are tightly regulated in space and time to control cell migration. While many key genes involved in cell migration have been identified through genetic approaches, much less is understood about how these components act in vivo to steer cells to precise destinations. To determine how an extracellular Fibroblast Growth Factor (FGF) cue disperses and directs migrating cells, I combined CRISPR-based endogenous tagging, live confocal imaging, and spatially controlled protein depletion to elucidate mechanisms underlying postembryonic mesodermal progenitor migration in C. elegans. During larval development, two muscle progenitors called sex myoblasts (SMs) migrate from the posterior to the center of the worm, where they divide to generate the egg-laying muscles. Previous genetic screens identified the FGF homolog egl-17 and its receptor egl-15 as essential for SM migration. However, the mode of FGF dispersal and the signal transduction mechanisms by which cells migrate towards an FGF signal were not known. To investigate this, I first visualized FGF source cells using an endogenous transcriptional reporter and endogenously tagged the FGF ligand to visualize native protein localization. Surprisingly, I identified multiple, previously unknown sources of FGF and found that FGF protein does not form a visible long-range gradient during SM migration. To test whether FGF diffusion is functionally required to direct the SMs, I used receptor-based trapping, mislocalization, and membrane-tethering strategies. These experiments showed that FGF acts as a bona fide instructive cue for migrating SMs, and FGF diffusion is required for SM migration. Next, to determine the downstream signaling mechanisms that translate the FGF signal into directed cell migration, I generated a genetic toolkit of endogenous tagged proteins in the Ras-ERK and PI3K-Akt-mTOR pathways. Strikingly, while homologs of GRB2, SOS, and Ras were required cell-autonomously for SM migration, canonical effectors of Ras, including ERK, PI3K, Akt, and mTOR were not required. Spatial manipulations of SOS and FGF signaling demonstrated that polarized SOS activity orients migrating cells towards an FGF source, while Ras gain-of-function alleles act permissively. Along with collaborators, I then characterized an intragenic revertant mutation in Ras that uncouples Ras-dependent migration from other Ras-ERK-dependent developmental processes. Together, this work provides a mechanistic framework that integrates FGF expression and dispersal, receptor-proximal decoding, and directed cell motility during cell migration in vivo.
