Fragile X Related Protein-1 (FXR1) Regulates RNA Metabolism in Striated Muscle

Cardiac muscle function necessitates the meticulous assembly and interactions of several cytoskeletal and regulatory proteins into specialized structures that orchestrate contraction and transmission forces. Despite extensive studies identifying the protein components responsible for these important aspects of heart development, putative RNA based mechanisms remain poorly understood, even with their demonstrated importance in other tissues. Evidence suggests that post-transcriptional regulation is critical for muscle function, but the molecular players involved (RNA binding proteins and mRNA targets) have remained elusive. We investigated the molecular mechanisms and targets of the muscle-specific Fragile X Related protein-1 (FXR1), an RNA binding protein whose absence leads to perinatal lethality in mice. Loss of FXR1 results in global protein level alterations. Morphological and biochemical analyses of Fxr1^(-/-) mice revealed severe disruption of intercalated disc and costamere architecture and composition. We identified several candidate mRNAs specifically enriched in the FXR1 protein complex. Two targets that likely contribute to the architectural defects are desmoplakin (dsp) and talin2 (tln2). In vitro assays indicate that FXR1 binds to these mRNA targets directly and represses their translation. Additionally, we provide preliminary evidence that the Fxr1^(-/-) mice mimic a hypothyroid state of cardiac gene expression, with alterations in myosin heavy chain and troponin I isoforms. Our findings reveal the first mRNA targets of FXR1 in muscle and support translational repression as a novel mechanism for cardiac muscle development and function