Prolonged Relaxation Kinetics in Distal Arthrogryposis Skeletal Muscle Myofibrils with a MYH3 R672C Mutation

Abstract

The mechanisms underlying most forms of Distal Arthrogryposis (DA), a group of congenital contracture syndromes, are unknown. Our previous functional studies from adult individuals with DA caused by the heterozygous myosin heavy chain mutation, MYH3 R672C, showed that the time required by DA skinned skeletal myofibers to relax completely after calcium-induced contraction was several-fold longer than controls (Racca et al. (2010) Biophys J 98:542-3a). Here we measured force and kinetics of activation & relaxation, and compared chemomechanical analysis using myofibrils and myofibers sampled from gastrocnemius muscle from two affected individuals vs. three control (non-DA) individuals. The prolonged relaxation was reflected in isolated myofibrils from DA patients, as 50% relaxation time from maximal activation was 36% longer, and 90% relaxation time was prolonged by 58%. The kinetics of the slow phase of relaxation were significantly slower, in both the rate and duration (DA: kREL,SLOW=0.36±0.07s−1;tREL,SLOW=285±21ms vs. Control: 0.79±0.18s−1;199±23ms), implying slower cross-bridge release. Use of ADP prolonged relaxation of control samples to a greater extent than DA preparations, suggesting that slower ADP release from myosin in DA myofibrils may be the mechanism of slower relaxation and cross-bridge release. We also found that both mRNA and protein for this embryonic myosin (gene MYH3) were present in adult skeletal muscle, such that a small amount of this slower myosin may prolong relaxation. Although the mechanism that leads to the congenital contractures must begin prenatally, these results suggest that MYH3 R672H also affects ongoing function of adult skeletal muscle. Understanding the mechanism by which myosin mutations affect muscle cell contractility could provide a model for exploring the pathogenesis of more common contractures such as idiopathic clubfoot and facilitate the development of novel therapeutic approaches. Supported by F31AR06300(A.R.), 5K23HD057331(A.B.), HD048895(M.B.,M.R.).