THE ROLE OF THE SARCOMERIC PROTEIN MYOPALLADIN IN SKELETAL AND CARDIAC MUSCLE STRUCTURE, FUNCTION AND DISEASE

Myopalladin (MYPN) is a striated muscle-specific sarcomeric protein belonging to the small palladin/myopalladin/myotilin family of actin-associated immunoglobulin-containing proteins [1]. Within the sarcomeric Z-line, MYPN binds to α-actinin, nebulin, and PDZ-LIM proteins [1, 2]. Furthermore, it is present in the nucleus and the I-band where it binds to the stress-inducible transcriptional cofactor CARP/Ankrd1, which, in turn, binds to the I-band region of titin, suggesting a role of MYPN in mechanosensing [1, 3]. The important role of MYPN in striated muscle is illustrated by the identification of MYPN mutations in human patients with dilated (DCM), hypertrophic (HCM), and restrictive (RCM) cardiomyopathy [4-6]. In our biochemical studies we demonstrated that MYPN, like PALLD, can bind and bundle filamentous actin (F-actin), thereby promoting actin polymerization. Moreover, we found that, similar to PALLD, MYPN interacts with MRTF-A and strongly increases MRTF-A-mediated activation of serum response factor (SRF) signaling, required for skeletal and cardiac muscle growth, maturation, and differentiation [7-11]. To determine the role of MYPN in vivo, we generated MYPN knockout (MKO) mice. MKO mice were significantly smaller compared to wildtype (WT) mice and had an about 30% reduction in skeletal muscle cross-sectional area (CSA) at all ages. Consistently, reduced differentiation rate and myotube width was observed in primary skeletal muscle cultures derived from MKO mice. Furthermore, studies of muscle performance in 2-month-old MKO mice showed reduced isometric force and power during isotonic shortening at any load as a result of the reduced cross sectional area, whereas the force developed by each myosin molecular motor was unaffected. By up- and downhill treadmill running, MKO and WT mice performed similarly. However, while the performance of WT mice was unaffected following four consecutive days of downhill running, the performance of MKO mice decreased progressively and Z-line damage was observed. Consistent with a higher susceptibility to muscle damage, progressive Z-line widening was observed in MKO skeletal muscle from about 8 months of age. RNAseq revealed downregulation of actin isoforms and other SRF-target genes in MKO muscle both at 2 and 4 weeks of age, suggesting that the smaller skeletal muscle fiber size in MKO mice is due to reduced SRF activity. Cardiac analyses of MKO mice showed no cardiac phenotype at young age but the development of progressive cardiac dilation and decreased fractional shortening. On the other hand, in response to mechanical pressure overload induced by transaortic constriction (TAC), MKO mice quickly developed cardiac dilation and reduced cardiac function accompanied by activation of the MAPK and AKT signaling pathways. Ongoing investigations are focused on studying the precise mechanism by which MYPN modulates actin dynamics through the Rho-MRTF-SRF signaling pathway as well as understanding the mechanisms leading from MYPN mutations to cardiomyopathy.