# The uncoordinated‐5 homologue A is a key receptor in netrin‐ligand‐mediated fast‐twitch myotube formation in male mice

## TL;DR

This study identifies UNC5A as a key receptor that helps form fast-twitch muscle fibers in male mice through interactions with netrin ligands.

## Contribution

The study reveals UNC5A's novel role in promoting fast-twitch myotube formation via netrin ligand signaling in myoblasts.

## Key findings

- UNC5A knockdown significantly inhibits fast-twitch myotube formation and reduces fast-type myosin heavy chain synthesis.
- Recombinant netrin-1 and netrin-4 promote fast-type MyHC mRNA expression, but this effect is suppressed by UNC5A knockdown.
- UNC5A is identified as a key receptor in netrin-mediated fast-twitch myotube formation in EDL myoblasts.

## Abstract

Myoblasts autonomously govern myofiber‐type specification of newly formed myotubes through autocrine–paracrine‐dependent manners mediated by multipotent modulators. Netrin‐1, which is particularly produced in myoblasts isolated from the extensor digitorum longus (EDL; fast‐twitch myofiber‐abundant) rather than the soleus (slow‐twitch myofiber‐abundant), and netrin‐4, which is abundantly expressed during myogenic differentiation initiation, stimulate the synthesis of fast‐type myosin heavy chain (MyHC) isoforms. However, the mechanisms by which netrin‐1 and netrin‐4 promote fast‐twitch myotube formation remain unclear. Here, we investigated the roles of netrin receptors, uncoordinated‐5 homologues (UNC5A, ‐B, ‐C, and ‐D), deleted in colorectal cancer (DCC), and the DCC paralog (neogenin) during myogenic differentiation, focusing on fast‐twitch myotube formation. We confirmed that UNC5A, UNC5B, UNC5C, and neogenin synthesis patterns in EDL myoblasts showed no marked differences compared with those in soleus myoblasts. Notably, UNC5A knockdown severely inhibited fast‐twitch myotube formation compared with other receptor knockdown treatments and significantly reduced the synthesis of fast‐type MyHC isoforms. Additional treatment with recombinant netrin‐1 or netrin‐4 induced fast‐type MyHC mRNA expression; however, this effect was suppressed by UNC5A knockdown. These findings revealed that UNC5A is involved in fast‐twitch myotube formation via netrin ligands, highlighting an autonomous fast‐type myofiber commitment system within myoblasts.

Unc5A, a netrin ligand‐receptor expressed in satellite cell‐derived myoblasts, may regulate the formation of fast‐twitch myotubes by stimulating fast‐type myosin heavy chain synthesis.