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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 ?-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.

Background Myopalladin (MYPN) is a striated muscle‐specific, immunoglobulin‐containing protein located in the Z‐line and I‐band of the sarcomere as well as the nucleus. Heterozygous MYPN gene mutations are associated with hypertrophic, dilated, and restrictive cardiomyopathy, and homozygous loss‐of‐function truncating mutations have recently been identified in patients with cap myopathy, nemaline myopathy, and congenital myopathy with hanging big toe. Methods Constitutive MYPN knockout (MKO) mice were generated, and the role of MYPN in skeletal muscle was studied through molecular, cellular, biochemical, structural, biomechanical, and physiological studies in vivo and in vitro. Results MKO mice were 13% smaller compared with wild‐type controls and exhibited a 48% reduction in myofibre cross‐sectional area (CSA) and significantly increased fibre number. Similarly, reduced myotube width was observed in MKO primary myoblast cultures. Biomechanical studies showed reduced isometric force and power output in MKO mice as a result of the reduced CSA, whereas the force developed by each myosin molecular motor was unaffected. While the performance by treadmill running was similar in MKO and wild‐type mice, MKO mice showed progressively decreased exercise capability, Z‐line damage, and signs of muscle regeneration following consecutive days of downhill running. Additionally, MKO muscle exhibited progressive Z‐line widening starting from 8 months of age. RNA‐sequencing analysis revealed down‐regulation of serum response factor (SRF)‐target genes in muscles from postnatal MKO mice, important for muscle growth and differentiation. The SRF pathway is regulated by actin dynamics as binding of globular actin to the SRF‐cofactor myocardin‐related transcription factor A (MRTF‐A) prevents its translocation to the nucleus where it binds and activates SRF. MYPN was found to bind and bundle filamentous actin as well as interact with MRTF‐A. In particular, while MYPN reduced actin polymerization, it strongly inhibited actin depolymerization and consequently increased MRTF‐A‐mediated activation of SRF signalling in myogenic cells. Reduced myotube width in MKO primary myoblast cultures was rescued by transduction with constitutive active SRF, demonstrating that MYPN promotes skeletal muscle growth through activation of the SRF pathway. Conclusions Myopalladin plays a critical role in the control of skeletal muscle growth through its effect on actin dynamics and consequently the SRF pathway. In addition, MYPN is important for the maintenance of Z‐line integrity during exercise and aging. These results suggest that muscle weakness in patients with biallelic MYPN mutations may be associated with reduced myofibre CSA and SRF signalling and that the disease phenotype may be aggravated by exercise.