Fragile X mental retardation protein regulates skeletal muscle stem cell activity by regulating the stability of Myf5 mRNA

Abstract

BackgroundRegeneration of adult tissues relies on adult stem cells that are primed to enter a differentiation program, while typically remaining quiescent. In mouse skeletal muscle, these features are reconciled by multiple translational control mechanisms that ensure primed muscle stem cells (MuSCs) are not activated. In quiescent MuSCs, this concept is illustrated by reversible microRNA silencing of Myf5 translation, mediated by microRNA-31 and fragile X mental retardation protein (FMRP).MethodsIn this work, we take advantage of FMRP knockout (Fmr1 −/−) mice to support the role for FMRP in maintaining stem cell properties of the MuSC. We compare the activity of MuSCs in vivo after acute injury and engraftment, as well as ex vivo during culture. We use RNA immunoprecipitation and 3’UTR poly-adenine (poly(A)) length assays to assess the impact of FMRP on the stability of transcripts for myogenic regulatory factors.ResultsWe show that RNA-binding FMRP is required to maintain the MuSC pool. More specifically, FMRP is required for stem cell properties of muscle stem cells, which include MuSC capacity to prime the myogenic program, their self-renewal, and their capacity to efficiently regenerate muscle. We provide evidence that FMRP regulation of MuSC activity occurs in part by the capacity of FMRP to directly bind Myf5 transcripts and impact rates of Myf5 deadenylation.ConclusionsOur results provide further evidence supporting a role for post-transcriptional silencing platforms by RNA-binding proteins in maintaining stemness properties of adult stem cells. In addition, deregulated MuSC activity in the absence of Fmr1 may have implications for fragile X syndrome, which is associated with muscle hypotonia during infancy.

mRNP Granule Proteins Fmrp and Dcp1a Differentially Regulate Muscle Stem Cell Quiescence via Reciprocally Regulating mRNP Complexes

“…Notably, during G0, translationally repressive complexes (Fmrp+) dominate, consistent with the enrichment of Fmrp+ storage granules in quiescent muscle stem cells in vivo [17] (Fig 1). Our ndings con rm the recent report [22] that Fmrp is required for MuSC function in vivo.…”

“…As Fmrp knockdown leads to reduced proliferative capacity, reduced differentiation may re ect the reduced number of cells available for myogenic commitment. It has been reported [22] that Fmr1 -/-MuSCs show lower accumulation of MyoD and Myf5 proteins through translational silencing, delaying entry into the differentiation program. Our study suggests that this effect could be at multiple regulatory nodes where Fmrp either directly or indirectly participates in decisions regarding cell fate.…”

“…Despite the ubiquitous expression of Fmrp, little is known of its speci c role in non-neuronal tissues, including skeletal muscle [17]. Fmrp has been detected in quiescent MuSC [13], and regulates MuSC function via control of the myogenic determinant Myf5 mRNA [17,22].…”

“…By contrast, quiescence is reversible and is characterized by a broad suppression of the differentiation program and increased expression of the MuSC-speci c transcription factor Pax7, which are reversed during cell cycle activation, along with re-induction of determination factors MyoD and Myf5 [25]. Indeed, quiescent MuSC exhibit translational control of lineage determinants [27], with Myf5 transcripts held in non-translating mRNPs [17,22].…”

Dynamic changes in copper homeostasis and post-transcriptional regulation ofAtp7aduring myogenic differentiation

“…A similar model of hypotonia in Fragile X Syndrome was recently hypothesized when the fragile X mental retardation protein was shown to be critical for translational regulation in satellite cells. 92 This study demonstrates an important role for Cu homeostasis in muscle progenitor cell function and provides the basis for future studies linking muscle progenitors and Cu homeostasis diseases.…”