Mechanisms and Functions of RPL3L Ribosomes

Dilated cardiomyopathy (DCM) is a major cause of heart failure. Despite intense study, ~50% of all DCM cases and 66% of pediatric cases are of unknown cause, underscoring the importance of discovering and understanding new pathogenetic mechanisms of DCM to advance future diagnosis and targeted treatment. We and others recently reported ribosomal protein L3-like (RPL3L) as a novel causal gene for neonatal DCM. Specifically, bi-allelic heterozygous missense mutations in RPL3L co-segregate with severe neonatal DCM in three unrelated families. RPL3L is a RPL3 paralog upregulated around birth exclusively in striated muscle (cardiac and skeletal muscle), where the ubiquitous paralog RPL3, the closest protein to ribosome catalytic center, is downregulated. To our knowledge, this is the first case implicating cytoplasmic ribosomal protein in cardiomyopathy and the first human disease caused by mutations in tissue-specific ribosomes. While the developmental and tissue-specific RPL3-to-RPL3L ribosome switch has been well documented, little is known about the role of RPL3L-containing ribosomes in the heart, hampering our understanding of how RPL3L mutations cause DCM. Our overall aim is to elucidate the functions of the specialized RPL3L-ribosomes in cardiomyocytes and how mutations in DCM patients disrupt RPL3L functions. Our preliminary data suggested that the specialized RPL3L ribosome may play a key role in the protein homeostasis (proteostasis) of the sarcomere, one of the largest and most complicated protein complexes only found in striated muscle cells, which also harbors most of DCM mutations. To test this hypothesis, we propose a multidisciplinary approach utilizing a combination of imaging, biochemical, genomic, and proteomic assays in cardiomyocyte cell lines, patient- specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs), explanted heart tissue from patient, and both knockout and knockin mouse models. Completion of this project will not only answer the long-standing question of why the heart needs a specialized ribosome, but also shed lights on the pathogenesis of dilated cardiomyopathy and lay the foundation for the development of new therapies.