Mechanical and energetic properties of papillary muscle fromACTCE99K transgenic mouse models of hypertrophic cardiomyopathy

Abstract

We compared the contractile performance of papillary muscle from a mouse model of hypertrophic cardiomyopathy [α-cardiac actin (ACTC) E99K mutation] with nontransgenic (non-TG) littermates. In isometric twitches, ACTC E99K papillary muscle produced three to four times greater force than non-TG muscle under the same conditions independent of stimulation frequency and temperature, whereas maximum isometric force in myofibrils from these muscles was not significantly different. ACTC E99K muscle relaxed slower than non-TG muscle in both papillary muscle (1.4×) and myofibrils (1.7×), whereas the rate of force development after stimulation was the same as non-TG muscle for both electrical stimulation in intact muscle and after a Ca²⁺ jump in myofibrils. The EC₅₀ for Ca²⁺ activation of force in myofibrils was 0.39 ± 0.33 μmol/l in ACTC E99K myofibrils and 0.80 ± 0.11 μmol/l in non-TG myofibrils. There were no significant differences in the amplitude and time course of the Ca²⁺ transient in myocytes from ACTC E99K and non-TG mice. We conclude that hypercontractility is caused by higher myofibrillar Ca²⁺ sensitivity in ACTC E99K muscles. Measurement of the energy (work + heat) released in actively cycling heart muscle showed that for both genotypes, the amount of energy turnover increased with work done but with decreasing efficiency as energy turnover increased. Thus, ACTC E99K mouse heart muscle produced on average 3.3-fold more work than non-TG muscle, and the cost in terms of energy turnover was disproportionately higher than in non-TG muscles. Efficiency for ACTC E99K muscle was in the range of 11-16% and for non-TG muscle was 15-18%.

Modeling Hypertrophic Cardiomyopathy: Mechanistic Insights and Pharmacological Intervention

“…Genomic correction of the diseased line restored the abnormalities back to baseline levels, which renders this mutation as 'necessary but not sufficient' to cause HCM. These results are in line with studies on a previous transgenic mouse model [79,80] which also reported enlarged atria and increased interstitial fibrosis. However, it is likely that the noncardiomyocyte cell types and full embryonic development that are absent from the hPSC-CM model play an important role in developing HCM phenotypes.…”

Why Is there a Limit to the Changes in Myofilament Ca2+-Sensitivity Associated with Myopathy Causing Mutations?

“…Figure 2 shows a real life example: in a mouse model of HCM (ACTC E99K) we measured both the Ca 2+ -activation curve for myofibrils and the contractility of intact papillary muscle as well as the Ca 2+ -transient (Song et al, 2013 ). Under the conditions of this experiment the Ca 2+ transient was the same in Wild-type and ACTC E99K muscle, Ca 2+ sensitivity was 0.8 μM for wild-type and 0.34 μM for ACTC E99K with a Hill coefficient of about 4.…”

Molecular defects in cardiac myofilament Ca2+- regulation due to cardiomyopathy-linked mutations can be reversed by small molecules binding to troponin

“…Skeletal muscle F-actin and myosin were isolated from rabbit fast-twitch skeletal muscle as previously described (Messer et al, 2007). TnC G159D mutant troponin was extracted from a mutant human heart sample (Dyer et al, 2009), E99K mutant cardiac actin was extracted from the E99K transgenic mouse heart (Song et al, 2013).…”