# Differentially expressed fusogens specify myocyte states to drive myogenesis

## TL;DR

This study identifies two distinct muscle cell states during embryonic development that differ in their fusion capabilities, shedding light on how muscle fibers form.

## Contribution

The discovery of two myocyte populations with distinct fusogen expression patterns challenges the traditional view of muscle precursor homogeneity.

## Key findings

- Two myocyte populations, Mc1 and Mc2, were identified based on their expression of myomaker and myomixer.
- Mc1 cells persist during secondary myogenesis, while Mc2 cells arise from Mc1 and do not persist.
- Mef2 and Rxr factors regulate Mymx expression, and RXRG modulates MYOD1 and MYOG transcriptional activity.

## Abstract

During myogenesis, myocyte fusion leads to the formation of multinucleated muscle fibers, but how exactly this process is initiated remains poorly understood. Here, we performed single-cell RNA-sequencing on mouse somites from E9.5-E11.5 embryos, revealing multiple differentiation states during primary myogenesis. Among these, we identified two unexpected myocyte populations: one expressing both myomaker (Mymk) and myomixer (Mymx) (termed Mc1) and another expressing only Mymk (termed Mc2). Fluorescence in situ hybridization demonstrated that both populations are mononucleated and co-exist within the same somites, with only Mc1 persisting during secondary myogenesis. Lineage tracing using Mymx:Cre; RosaTdT mice demonstrated that the Mc2 cells arise from the Mc1. Mechanistically, we show that Mef2 and Rxr factors positively and negatively regulate Mymx expression, respectively. Additionally, RXRG interacts with MYOD1 and MYOG, modulating their transcriptional activity in luciferase assays. Collectively, our findings uncover two populations among the myocytes that drive primary and secondary fiber formation, challenging the traditional view of vertebrate muscle precursor homogeneity.

Summary: Single-cell RNA-sequencing reveals two fusion-competent muscle cell states that differentially express the fusogens myomaker and myomixer, offering new insight into how muscle cells coordinate fusion during embryonic development.