Correcting diastolic dysfunction by Ca2+ desensitizing troponin in a transgenic mouse model of restrictive cardiomyopathy

Abstract

Several cardiac troponin I (cTnI) mutations are associated with restrictive cardiomyopathy (RCM) in humans. We have created transgenic mice (cTnI193His mice) that express the corresponding human RCM R192H mutation. Phenotype of this RCM animal model includes restrictive ventricles, biatrial enlargement and sudden cardiac death, which are similar to those observed in RCM patients carrying the same cTnI mutation. In the present study, we modified the overall cTnI in cardiac muscle by crossing cTnI193His mice with transgenic mice expressing an N-terminal truncated cTnI (cTnI-ND) that enhances relaxation. Protein analyses determined that wild type cTnI was replaced by cTnI-ND in the heart of double transgenic mice (Double TG), which express only cTnI-ND and cTnI R193H in cardiac myocytes. The presence of cTnI-ND effectively rescued the lethal phenotype of RCM mice by reducing the mortality rate. Cardiac function was significantly improved in Double TG mice when measured by echocardiography. The hypersensitivity to Ca2+ and the prolonged relaxation of RCM cTnI193His cardiac myocytes were completely reversed by the presence of cTnI-ND in RCM hearts. The results demonstrate that myofibril hypersensitivity to Ca2+ is a key mechanism that causes impaired relaxation in RCM cTnI mutant hearts and Ca2+ desensitization by cTnI-ND can correct diastolic dysfunction and rescue the RCM phenotypes, suggesting that Ca2+ desensitization in myofibrils is a therapeutic option for treatment of diastolic dysfunction without interventions directed at the systemic β-adrenergic-PKA pathways.

Engineered Troponin C Constructs Correct Disease-related Cardiac Myofilament Calcium Sensitivity

“…Although the etiology of the cardiomyopathies remains unclear, experimental evidence shows promise that improving cardiac muscle Ca 2ϩ sensitivity through either pharmacological or genetic approaches can relieve disease-related symptoms (27,28,30,49). Ca 2ϩ sensitizers have attracted growing clinical interest for their potential therapeutic value in treating heart failure and cardiomyopathies that desensitize cardiac muscle to Ca 2ϩ (31).…”

“…As an alternative to pharmaceuticals, genetic approaches that directly modulate contractile proteins have recently received increasing attention. Excitingly, chimeric tropomyosin, N-terminal truncated TnI, and fetal TnT have all been shown to improve disease-related abnormal cardiac muscle Ca 2ϩ sensitivity and in vivo function (28,30,49). However, it is not clear how applicable these proteins will be to correcting the wide assortment of Ca 2ϩ -sensitizing and -desensitizing cardiac diseases because it is unknown how to specifically tune their performance.…”

“…Strikingly, for any particular class of inherited cardiomyopathies, the apparent Ca 2ϩ sensitivity of TnC and force development are typically altered in a qualitatively similar manner (20,22,25). Furthermore, pharmacological and genetic interventions that rectify the apparent Ca 2ϩ sensitivity of cardiac muscle in transgenic animal models harboring cardiomyopathic genes show promise of alleviating the disease symptoms (27)(28)(29). For instance, modulating TnI, TnT, or tropomyosin (each of which can indirectly tune the Ca 2ϩ sensitivity of TnC) counteracted the abnormal cardiac muscle Ca 2ϩ sensitivities and ameliorated the disease symptoms (28 -30).…”

Rescue of Familial Hypertrophic Cardiomyopathy by Altering Sarcomeric Exposure and Response to Calcium

“…To determine whether it is possible to rescue FHC mice from their lethality and cardiac hypertrophic phenotype, we and others have taken several approaches (29)(30)(31)(32). The first approach was to target the myofilaments themselves through the incorporation of proteins that counteract the properties exhibited by the FHC mutant Tm 180 protein since myofilaments obtained from the hearts of FHC patients and animal models usually exhibit an increased sensitivity to calcium with respect to tension development.…”

“…Under hemodynamic stress and heart failure, the cTnI-ND isoform is increased in expression (40). The double transgenic mice (cardiac TnI 193His x cTnI-ND) rescued the lethal phenotype of the cardiac TnI 193His RCM mutant mice; cardiac function was significantly improved and the myofilaments' increased sensitivity to calcium was reversed (32). These results, which are similar to those found with the FHC Tm180 rescued mice, suggest that calcium desensitization in myofibrils is a therapeutic option for the treatment of diastolic dysfunction.…”