Myosin Heavy Chain Isoform Switching in Skeletal Muscles in an A8V-Troponin C Hypertrophic Cardiomyopathy Knock-In Mouse Model

Abstract

Missense mutations in the TNNC1 gene encoding cardiac and slow skeletal troponin C (c/ssTnC) are associated with phenotypic outcomes of hypertrophic cardiomyopathy (HCM). Since the impact of HCM c/ssTnC mutations during skeletal muscle development and function is unknown, we addressed whether changes in the myosin heavy chain (MHC) isoforms can occur outside of the heart. We hypothesized that the ratio of MHC I to MHC II in various skeletal muscles change due to an HCM-associated c/ssTnC A8V mutation in a knock-in homozygous mouse (KI-TnC-A8V+/+). The distribution of MHC I/II isoforms in diaphragm (D), soleus (S), tibialis anterior (TA), extensor digitorum longus (EDL) and quadriceps (Q) isolated from 3- and 9-month (mo) old male WT and KI-TnC-A8V+/+ mice were analyzed by glycerol SDS-PAGE. The muscle/body weight ratio (M/BW) and running capacity were also measured. At 3 mo, MHC isoform switching was observed in D (increased MHCIIb), S (decreased MHCIIa, increased MHCI) and EDL (increased MHCIIa/x, decreased MHCIIb) of KI-TnC-A8V+/+ compared to WT. TA M/BW is increased in 3- and 9-mo KI-TnC-A8V+/+ mice compared to WT at the same age. Pairwise comparison of M/BW from 3- vs 9-mo KI-TnC-A8V+/+ mice showed a significant decrease for TA and Q and an increase for EDL, and no change for any WT muscles. In running tests, KI-TnC-A8V+/+ mice had a lower capacity to run distances compared to WT (281 vs 413 meters). Thus, changes in MHC isoforms in the skeletal muscles of mice expressing c/ssTnC A8V are associated with reduced exercise endurance, suggesting that skeletal muscle remodeling may contribute to the development of HCM phenotype. Future studies will examine MHC isoforms at 9 mo and Ca2+-activated myofibrilar Mg2+-ATPase rates from skeletal muscles in KI-TnC-A8V+/+ mice.