PROJECT SUMMARY / ABSTRACT Cardiomyocytes (CMs) generate contractile forces to pump blood throughout the body. Contraction begins at the sarcomere and is coordinated by actin-myosin filament interactions. Coordinated CM contraction is maintained by adaptive responses to external stimuli. During postnatal heart development, changes in environmental oxygen levels and metabolites require CMs to undergo drastic remodeling to sustain adult functionality. Numerous of these changes are driven by transcriptional networks and RNA-processing mechanisms. Alternative splicing is an RNA-processing mechanism that allows single genes to produce more than one transcript, and potentially different protein isoforms with distinct roles in specific cells and tissues. Previously, our group demonstrated that the RNA-binding protein (RBP) called Fragile X messenger ribonucleoprotein 1 autosomal homolog 1 (FXR1) is regulated by alternative splicing in a tissue- and developmental stage specific manner: Fxr1 exon 15 is skipped in all fetal tissues but is highly included only in adult striated muscles. Recessive mutations in FXR1 exon 15 are linked to congenital multi-minicore myopathy, an inherited neuromuscular condition apparent at birth where individuals often experience cardiac failure. Others have demonstrated that sarcomere RNAs undergo local translation in CMs. During local translation, instead of protein production occurring in the perinuclear region, large cells like CMs shuttle the translational machinery, mRNAs, and RBPs to the intracellular compartment wherein the coded protein functions. FXR1 regulates the translation of numerous sarcomere mRNAs and localizes to domains far from the nucleus and endoplasmic reticulum. I hypothesize that FXR1 controls the organization and contractile capacity of adult CMs via regulation of local translation and inclusion of exon 15. First, I will establish FXR1's role in local translation at the sarcomere in cultured cardiomyocytes (AIM 1). Second, I will determine the role of FXR1 in the activation mechanosensitive transcriptional changes and signaling cascades in cultured cardiomyocytes (AIM 2). Third, I will identify the consequences of the developmentally regulated and striated muscle specific Fxr1 exon 15 on cardiac morphology and function by using our unpublished mouse model where exon 15 was deleted via CRISPR/Cas9 editing (AIM 3). My research will provide novel insights on the role of FXR1 in cardiac biology, which will have potential in the development of therapies for cardiovascular diseases. After completion of my F31 fellowship, I will have received multidisciplinary training from my sponsor, co-sponsor, and collaborators at the University of North Carolina at Chapel Hill, which will aid in my development as an independent cardiovascular researcher.