# FBXL3 serves as a suppressor of regenerative myogenesis

## TL;DR

FBXL3 limits muscle regeneration by degrading TCF12, and its deletion boosts muscle repair by increasing TCF12 and MEF2C activity.

## Contribution

FBXL3 is identified as a novel negative regulator of myogenic regeneration through its regulation of TCF12 and MEF2C.

## Key findings

- FBXL3 promotes TCF12 ubiquitination and degradation, limiting myogenic differentiation.
- FBXL3 deletion increases TCF12 and MEF2C activity, enhancing muscle regeneration.
- TCF12 binds to and activates the MEF2C promoter, linking it to myogenic gene expression.

## Abstract

Muscle regeneration hinges on the proliferation and differentiation of satellite cells. FBXL3, a member of the F-box protein family known for its role as a negative regulator of the circadian clock, is implicated in myogenesis. In this study, we demonstrate the expression of FBXL3 in satellite cells of adult mice, where it acts as a negative regulator of myogenic regeneration. This regulation occurs through the promotion of ubiquitination and degradation of TCF12, a transcription factor crucial for differentiation. Loss of FBXL3 activates MyoD and myogenin, thereby augmenting myogenic differentiation and regeneration. The role of FBXL3 in muscle regeneration was also confirmed using the tamoxifen-inducible Pax7-CreER recombination system. To unravel the regulatory mechanism of MyoD and myogenin by FBXL3, we conducted RNA sequencing on Fbxl3+/+
and Fbxl3-/-
primary myoblasts. Gene set enrichment analysis (GSEA) revealed that FBXL3 deficiency enriches the gene set associated with striated muscle cell development, including MEF2C, a regulator of myogenin expression. Through a search in the ChEA3 database, TCF12 emerged as the downstream candidate gene regulated by FBXL3 to modulate MEF2C. ChIP-PCR assays confirmed the enrichment of TCF12 on MEF2C promoter at three consensus sites. Dual-luciferase reporter assay validated that TCF12 activates the MEF2C promoter. This comprehensive study underscores the crucial role of FBXL3 in satellite cell-mediated myogenic regeneration and provides insights into the intricate regulatory network involving TCF12 and MEF2C.

FBXL3 plays a pivotal role in regulating the proliferation and differentiation of satellite cells, thereby facilitating regenerative myogenesis. The SCF (SKP1-CUL1-F-box) complex, consisting of Skp1, Cullin1, Rbx1, and the F-box protein, is integral to this process. The F-box protein, with its COOH terminal, specifically recognizes various substrate proteins, including the transcription factor protein TCF12. FBXL3, a member of the F-box protein family, encodes the F-box domain and serves as one of the four subunits in the SCF complex. Deletion of FBXL3 in the satellite cells of mice leads to a reduction in the degradation of TCF12. Consequently, there is an accumulation of TCF12, which activates the MEF2C promoter, ultimately promoting myogenic proliferation and differentiation.

Diagram comparing normal and FBXL3-deleted satellite cells in mice after muscle injury. The left side shows the control: FBXL3 targets TCF12 for degradation, limiting satellite cell proliferation. The right side shows FBXL3 deletion: TCF12 is not degraded, enhancing satellite cell proliferation and muscle regeneration. Both include Cullin1 complexes and promote transcription in the nucleus.