Muscle-specific isoforms of FXR1 are necessary for miR-1-mediated repression of connexin 43 and are down-regulated in pediatric dilated cardiomyopathy

The Fragile-X (FraX) protein family regulates RNA metabolism, muscle development, and neuronal plasticity. These proteins are crucial for translation regulation, interacting with ribosomal subunits and RNA through specific domains. FXR1 has seven isoforms, including isoforms mostly expressed in skeletal and cardiac tissue, and play a significant role in heart development and function. Additionally, FXR1 modulates microRNA function, impacting gene expression. Given FXR1's crucial role in cardiac differentiation, we evaluated if expression of the muscle-specific isoforms of FXR1 was dysregulated in pediatric dilated cardiomyopathy (DCM) and sought to determine the impact of these isoforms on the function of miR-1, an important cardiac miRNA, and its regulation of the 3'UTR of the gap junction protein, connexin 43 (Cx43). Our results show that FXR1 protein levels are decreased in pediatric DCM left ventricular tissue when compared to age-matched non-failing controls. We investigated the function of muscle-specific isoforms FXR1-G and FXR1-E in an in vitro model of myocyte differentiation. H9c2 cells, differentiated to cardiomyocyte-like cells, show a significant increase in FXR1-G/E protein expression compared to H9c2 myoblasts. Furthermore, we show that FXR1G/E are essential for miR-1-mediated repression of Cx43 3'UTR, emphasizing the importance of miR binding proteins in myocyte homeostasis. Lastly, we show that FXR1-G promotes interaction between miR-1 and the Cx43 3'UTR. Overall, we demonstrate miR-1 regulation of the Cx43 3'UTR relies on muscle-specific isoforms of FXR1. Significantly, we are the first to report a reduction in the muscle-specific isoforms of FXR1 in pediatric DCM patients, underscoring an age-specific regulation of FXR1 expression.