An α-cardiac myosin heavy chain gene mutation impairs contraction and relaxation function of cardiac myocytes

Abstract

Left Ventricular (LV) myocytes were isolated from 15-wk-old male mice bearing the Arg403 → Gln α-cardiac myosin heavy chain missense mutation (α-MHC403/+), a model of familial hypertrophic cardiomyopathy. LV myocytes were classified morphologically: type I, rod shaped with parallel myofibrils; type II, irregularly shaped, shorter and wider than wild-type (WT) control cells, with parallel myofibrils; and type III, irregularly shaped with disoriented myofibrils. Compared with WT myocytes, α-MHC403/+ myocytes had fewer type I cells (WT = 74 ± 3%, α-MHC403/+ = 41 ± 4%, P < 0.01) and more type III cells (WT= 12 ± 3%, α-MHC403/+ = 49 ± 7%, P < 0.01). In situ histology also demonstrated marked myofibrillar disarray in the α-MHC403/+ hearts. With the use of video edge detection, myocytes were paced at 1 Hz (37°C) to determine the effects of the mutation on myocyte function. End-diastolic length was reduced in mutant myocytes, but fractional shortening (% contraction) and sarcomere length were not. Velocity of contraction (−d L/d t max) was depressed in mutant cells, but more in type II and III cells (−31%) than in type I cells (−18%). Velocity of relaxation (+d L/d t) was also depressed more in type II and III cells (−38%) than in type I cells (−16%). Using fura 2 dye with intracellular Ca2+ transients, we demonstrated that in α-MHC403/+ myocytes, the amplitude of the Ca2+ signal during contraction was unchanged but that the time required for decay of the signal to decrease 70% from its maximum was delayed significantly (WT = 159 ± 8 ms; α-MHC403/+ = 217 ± 14 ms, P < 0.01). Sarco(endo)plasmic reticulum Ca2+-ATPase mRNA levels in α-MHC403/+ and WT mice were similar. These data indicate that the altered cardiac dysfunction of α-MHC403/+ myocytes is directly due to defective myocyte function rather than to secondary changes in global cardiac function and/or loading conditions.

Single-stranded DNA-binding Proteins PURα and PURβ Bind to a Purine-rich Negative Regulatory Element of the α-Myosin Heavy Chain Gene and Control Transcriptional and Translational Regulation of the Gene Expression

“…These hearts had abnormal Frank-Starling pressure/force (length/ tension) relations, suggesting propensity toward cardiac failure (9). Similar conclusions were also drawn from studies in which a mis-sense mutation was created in the cardiac ␣-MHC gene (10). Data showing correlation between declines of MHCs content and reduced intrinsic contractile characteristics of the myocardium also support the idea that a critical level of ␣-MHC content is essential for normal cardiac pump function (3,4).…”

The R403Q Myosin Mutation Implicated in Familial Hypertrophic Cardiomyopathy Causes Disorder at the Actomyosin Interface

“…The FHC mutations studied (including the R403Q), all of which are known to affect myosin motor activity, disrupted the myofibril organization in a manner that is characteristic of this disease. Similarly, cardiomyocytes isolated from heterozygous a-myosin R403Q/+ trangenic mice showed far fewer well-aligned parallel myofibrils than the wild-type control [37]. These data are consistent with a direct effect of the R403Q myosin mutation on the organization of the contractile cytoskeleton.…”

Differential regulation of inotropy and lusitropy in overexpressed Gsα myocytes through cAMP and Ca2+ channel pathways

“…Cardiac myocytes were prepared from WT and Gsα-overexpressed (Gsα) mice 15 ± 2 weeks old, as described previously (18). In brief, the heart was rapidly excised and submerged in Ca 2+ -free Tyrode's solution containing (in mmol/l): 140 NaCl, 5.4 KCl, 1 MgCl 2 , 0.33 NaH 2 PO 4 , 10 glucose, and 5 HEPES (pH 7.4).…”