# Regulation of Muscle Stem Cells by Netrin Signaling

> **NIH NIH R21** · ICAHN SCHOOL OF MEDICINE AT MOUNT SINAI · 2024 · $223,080

## Abstract

Skeletal muscle regenerates following injury, with muscle stem cells (MuSCs) the source of new myofibers.
MuSCs are quiescent during homeostasis. Quiescence is an actively maintained state, supported by signals
from the MuSC niche. Upon muscle injury, MuSCs activate and enter the cell cycle, proliferate as myoblasts,
and differentiate and fuse to form new myofibers. How MuSCs maintain quiescence is not well understood, and
there is little known about the earliest events in the transition from quiescence to activation (the Q-A transition).
Quiescent MuSCs in vivo have long, heterogeneous cellular projections that rapidly retract in response to
muscle injury. Projections may therefore act as direct sensors of the niche environment. Projection retraction is
driven by a Rac-to-Rho GTPase activity switch that promotes downstream MuSC activation events. These
observations lead to several hypotheses: 1) MuSC projections are morphologically dynamic at quiescence,
providing a surveillance function for muscle damage; 2) MuSC dynamics during quiescence are regulated by
the relative balance of Rac and Rho activities promoted by niche-derived cues; and 3) there exist factors in
muscle tissue that signal to stimulate MuSC projection outgrowth and, consequently, promote MuSC
quiescence. Such factors are anticipated to be critical regulators of MuSC quiescence and the Q-A transition,
but their identities are unknown. Moreover, new approaches are required to screen for candidate factors. We
have developed an ex vivo live imaging assay for MuSCs within muscle bundles and used it to identify the
axonal chemoattractant, netrin-1, as a candidate niche-derived regulator of MuSC projection dynamics.
Multiple cell types in adult muscle express netrin-1, and MuSCs express the netrin-1 receptors, Neogenin
(Neo1) and Dcc. Furthermore, MuSCs extend projections in response to recombinant netrin-1, as visualized in
the ex vivo live imaging assay. It is proposed here to determine the role of netrin-1 signaling in MuSC
quiescence and the Q-A transition. We will use conditional genetic removal of netrin-1 receptors from MuSCs
to test directly the role of netrin-1 signaling in MuSC morphology, quiescence, and the Q-A transition. A
combination of in vivo and ex vivo techniques will be used, including tissue clearing, in vivo analyses of
uninjured and injured muscles, analysis of MuSCs on single myofibers; and live imaging of MuSCs within
muscle bundles. Successful completion of the proposed work will provide strong evidence that: 1) regulation of
MuSC projection dynamics is a key process in maintenance of quiescence; and 2) netrin-1 is the first member
of a new class of MuSC quiescence regulator. Such findings will validate the use of our new ex vivo live
imaging assay as a means to identify secreted signaling cues involved in this process.

## Key facts

- **NIH application ID:** 10951165
- **Project number:** 1R21AR084632-01
- **Recipient organization:** ICAHN SCHOOL OF MEDICINE AT MOUNT SINAI
- **Principal Investigator:** Robert S. Krauss
- **Activity code:** R21 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $223,080
- **Award type:** 1
- **Project period:** 2024-08-01 → 2026-05-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10951165

## Citation

> US National Institutes of Health, RePORTER application 10951165, Regulation of Muscle Stem Cells by Netrin Signaling (1R21AR084632-01). Retrieved via AI Analytics 2026-05-24 from https://api.ai-analytics.org/grant/nih/10951165. Licensed CC0.

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