To date most studies of localized protein translation have been performed in cultured rodent or human stem cell–derived myocytes, but Haddad et al. (9) made a major scientific leap by experimentally demonstrating that RPSA-dependent localization of sarcomere protein translation decreased cardiac function and caused ventricular dilation in mice that had a loss of RPSA function, either globally or in a cardiac-specific mosaic condition. To achieve mosaic deletion of RPSA, Cas9 nuclease–expressing mice were transduced with global or cardiac-specific AAV-expressing RPSA-targeting sgRNAs, generating a mutation of the RPSA gene. In the cardiac-specific mosaic knockout, this dysfunction was not seen until the mice reached six months of age. Fascinatingly, the myocytes that received the Rpsa-targeting sgRNAs were smaller than control myocytes, and the unedited myocytes in RPSA-mosaic mice hypertrophied in length and width relative to control myocytes, presumably as compensation for the loss of myocyte volume in the edited cells. Importantly, the authors showed no edit-induced cell death occurred as the mice aged. The lack of growth of edited cells was accompanied by increased expression of markers of the fetal gene program, decreased sarcomeric protein translation, and decreased global protein translation. Although global protein translation was decreased, only sarcomeric proteins had a lower protein expression level compared with that of control mice, revealing the key finding that RPSA-associated ribosome translation at the Z-line was most responsible for the translation of sarcomeric-specific proteins. Finally, Haddad et al. (9) showed a trend of decreased RPSA expression in mice with a MYPBC3 mutation causing hypertrophic cardiomyopathy (HCM), yet those mice showed no difference in subcellular RPSA localization. These data indicate that RPSA function is upstream of pathological sarcomere effects and that the disruption of RPSA-mediated localized translation negatively affects cardiomyocyte structure and function.

Though many have postulated that Z-line–localized protein translation is essential for sarcomere structure, Haddad et al. (9) provide direct evidence that loss of Z-line translation specifically negatively affects sarcomere content in cardiomyocytes. As previous work by this group and others has shown that the cytoskeleton is critical for localized translation (5, 7), Haddad et al. (9) demonstrate that nocodazole- or colchicine-mediated dissolution of the microtubules prevents RPSA peripheralization, which suggests that RPSA is the key driver of the dependence of localized translation on the microtubule network. Furthermore, the C-terminus of RPSA, previously known as a laminin receptor, is sufficient for Z-line localization. Collectively, these data support a model whereby RPSA maneuvers along microtubules to position its C-terminus on the Z-line and the N-terminus on ribosomes to translate sarcomeric proteins locally (Figure 1).