Contractile Properties of Human Fetal Skeletal Myofibrils

Congenital contractures such as clubfoot are present in ?1/250 live births. Several congenital contractures syndromes are caused by mutations in genes that code for skeletal myofilament proteins, including MYH3 and MYH8. Because embryonic (MYH3) and perinatal (MYH8) myosin heavy chains are unique to the prenatal development of muscle, it is important to understand the contractile properties of human embryonic (HE) myosin and human fetal (HF) muscle to determine how mutations affect performance and development. However, information on HF skeletal muscle function is lacking. We previously reported HE myosin crossbridge cycling (measured by in vitro motility) was much slower that rabbit psoas (RP) myosin (Biophys. J. (2010), 98:542a). Here we characterized the contraction and relaxation properties of HF muscle using myofibril mechanics techniques. HF skeletal muscle and myofibrils were isolated from a 15.4 week gestation fetus. During maximal calcium activation (15°C) HF myofibrils produced much lower force (FMAX=5.9±1.2mN/mm2) as compared to myofibrils from human adult (HA) skeletal muscle (84±34mN/mm2) or RP muscle (220±40mN/mm2). Unlike control HA and RP fibrils, no striation pattern was apparent for HF myofibrils, suggesting that immature sarcomeres could explain the lower force production. HF myofibrils had slower kinetics of force development (kACT=0.66±0.1s?1) vs. HA (7.5±6.3s?1) and RP (5.7±0.5s?1) myofibrils. The initial (slow) phase of relaxation upon return to low calcium solution was slower and prolonged (kREL,SLOW=0.59±0.22s?1; tREL,SLOW=174±13ms) vs. HA (2.9±1.7s?1; 71±18ms) or RP (2.1±0.4s?1; 73±14ms) myofibrils. The larger, faster phase of relaxation was also slower (HF=1.5±0.2s?1 vs. HA=12±5s?1 and RP=21±4s?1). Our previous in vitro motility experiments indicated a similar inhibition of filament speed by increasing [ADP] for HE and RP myosin, but this was under low load, thus ongoing experiments will determine if ADP release is responsible for the slower kinetics of HF muscle. Funded by F31AR06300(A.R.), 5K23HD057331(A.B.), HD048895(M.B., M.R.)