Muscle stem cells get a new look: Dynamic cellular projections as sensors of the stem cell niche

Abstract

Cellular mechanisms whereby quiescent stem cells sense tissue injury and transition to an activated state are largely unknown. Quiescent skeletal muscle stem cells (MuSCs, also called satellite cells) have elaborate, heterogeneous projections that rapidly retract in response to muscle injury. They may therefore act as direct sensors of their niche environment. Retraction is driven by a Rac-to-Rho GTPase activity switch that promotes downstream MuSC activation events. These and other observations lead to several hypotheses: (1) projections are morphologically dynamic at quiescence, providing a surveillance function for muscle damage; (2) quiescent projection dynamics are regulated by the relative balance of Rac and Rho activities promoted by niche-derived cues; (3) projections, particularly their associated filopodia, sense tissue damage via changes to the biomechanical properties of the niche and/or detection of signaling cues released by damaged myofibers; and (4) the dynamic nature of projections results in a population of MuSCs with heterogeneous functional properties. These concepts may extend to other types of quiescent stem cells, as well as prove useful in translational research settings.

Keywords: Rac1; Rho; filopodia; mechanosignaling; muscle stem cell; quiescence; regeneration.

FIGURE 1

Quiescent muscle stem cells (MuSCs) in vivo have long projections with filopodia at their ends. A mouse EDL muscle was subjected to tissue clearing techniques and visualized by expression of a MuSC-specific TdTomato reporter (the Lookup Table indicates signal intensity). The boxed region in the left panel is enlarged, inverted, and decolorized in the right panel. Red arrowheads indicate filopodia at the ends of projection branches. Filopodia are observed on approximately 30% of MuSCs via tissue clearing of fixed whole muscle, but because filopodia are often lost upon fixation,^[81] we suspect that nearly all quiescent MuSC projections possess them in vivo. See Kann et al. for details.^[20]

FIGURE 2

Model for muscle stem cell (MuSC) quiescence as a dynamic state of surveillance. During quiescence, Rac activity is relatively high and Rho activity is relatively low. It is hypothesized that MuSC projections are in a dynamically motile state, growing outward in a Rac1-dependent manner and partially retracting under the influence of localized Rho signaling (A); dynamic motility of projections could enable surveillance of the niche environment for muscle damage-derived signals. Sensation of such signals induces the quiescence-to-activation (Q-A) transition, including a GTPase activity switch from Rac1^high/Rho^low to Rac1^low/Rho^high (indicated by the opposing triangles on the left). Among the earliest steps of MuSC activation are projection retraction, formation of a centrosomal microtubule organizing center (cMTOC) and nuclear translocation of the transcriptional regulator MRTFA (MRTFA^Nuc), which initiates expression of FOS (B). Continued high Rho activity yields a new, “activated” morphology; such cells have initiated expression of MYOD protein (C). During quiescence, some MuSCs cells represented on the right side of the equilibrium in (A) possess a cMTOC. MRTFA^Nuc and FOS⁺ MuSCs are not generally observed in quiescent muscle but it is hypothesized that cells with a phenotype like that shown in (B) may return to a fully quiescent, projection-bearing state represented on the left side of the equilibrium in (A). In contrast, cells in (C) are hypothesized to have committed to fully enter the cell cycle and divide.