Cardiac myosin-binding protein-C is a critical mediator of diastolic function
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- Tong, C.W., Nair, N., Doersch, K.M. et al. Pflugers Arch - Eur J Physiol (2014) 466: 451. doi:10.1007/s00424-014-1442-1
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Diastolic dysfunction prominently contributes to heart failure with preserved ejection fraction (HFpEF). Owing partly to inadequate understanding, HFpEF does not have any effective treatments. Cardiac myosin-binding protein-C (cMyBP-C), a component of the thick filament of heart muscle that can modulate cross-bridge attachment/detachment cycling process by its phosphorylation status, appears to be involved in the diastolic dysfunction associated with HFpEF. In patients, cMyBP-C mutations are associated with diastolic dysfunction even in the absence of hypertrophy. cMyBP-C deletion mouse models recapitulate diastolic dysfunction despite in vitro evidence of uninhibited cross-bridge cycling. Reduced phosphorylation of cMyBP-C is also associated with diastolic dysfunction in patients. Mouse models of reduced cMyBP-C phosphorylation exhibit diastolic dysfunction while cMyBP-C phosphorylation mimetic mouse models show enhanced diastolic function. Thus, cMyBP-C phosphorylation mediates diastolic function. Experimental results of both cMyBP-C deletion and reduced cMyBP-C phosphorylation causing diastolic dysfunction suggest that cMyBP-C phosphorylation level modulates cross-bridge detachment rate in relation to ongoing attachment rate to mediate relaxation. Consequently, alteration in cMyBP-C regulation of cross-bridge detachment is a key mechanism that causes diastolic dysfunction. Regardless of the exact molecular mechanism, ample clinical and experimental data show that cMyBP-C is a critical mediator of diastolic function. Furthermore, targeting cMyBP-C phosphorylation holds potential as a future treatment for diastolic dysfunction.
KeywordsCardiac myosin-binding protein-CMyBPC3Diastolic dysfunctionHeart failure with preserved ejection fractionHFpEF
Heart failure occurs when cardiac output cannot meet the body’s demand. It has an estimated global prevalence of 23 M . Lifetime risks for developing heart failure of a 55-year-old European and a 40-year-old American are 30.2 and 20 %, respectively [2, 15]. Despite treatment advances, 5-year mortality of heart failure patients remains high at 42–80 % . Heart failure can occur with left ventricular ejection fraction (EF) of ≥50 %, which is defined as heart failure with persevered ejection fraction (HFpEF) [29, 49]. Prevalence of HFpEF has increased to 47 % of all heart failure cases . Diastolic dysfunction is the generally accepted cause of HFpEF . Diastolic dysfunction also occurs with heart failure with reduced ejection fraction (HFrEF) , defined as EF < 40 % . Hypertrophic cardiomyopathy (HCM) patients progress to heart failure with type distribution of 48 % HFpEF, 30 % HFrEF, and 22 % outflow obstruction . HCM patients with primarily diastolic dysfunction and without outflow obstruction experience the shortest progression from HCM diagnosis to heart failure . Mere diagnosis of mild diastolic dysfunction carries >eightfold increase in mortality over 5 years . Unfortunately, pathogenic mechanisms that cause diastolic dysfunction remain enigmatic. With this perspective, this review summarizes evidence that cardiac myosin-binding protein-C mediates diastolic function.
Need for cMyBP-C
Cardiac myosin-binding protein-C (cMyBP-C) is a part of the thick filament of the heart muscle . Although cMyBP-C is believed to repress myosin–actin interaction by different mechanisms [12, 18], an important mechanism is that cMyBP-C binding to the rod region of myosin can slow cross-bridge detachment to impair relaxation [1, 12, 26]. Thus, cMyBP-C mutations may lead to diastolic dysfunction. Mutations in cMyBP-C are a leading cause of hypertrophic cardiomyopathy (HCM) . HCM patients, a significant portion of whom carry cMyBP-C mutations, can present with diastolic dysfunction (demonstrated by slowed heart muscle relaxation velocity Ea) before the onset of hypertrophy [19, 33, 34]. A cohort of pediatric HCM patients, 19/27 of whom have cMyBP-C mutations, demonstrates diastolic dysfunction without hypertrophy . Another cohort of patients with three common cMyBP-C mutations found in the Netherlands exhibits hypertrophy with diastolic dysfunction or prehypertrophy with TD evidence of impaired relaxation . The presentation of diastolic dysfunction before the onset of hypertrophy suggests that cMyBP-C mutations cause diastolic dysfunction independent of hypertrophy. Furthermore, a single nucleotide polymorphism in cMyBP-C has been found in diastolic heart failure patients . Thus, clinical evidence suggests that nonmutated/normal cMyBP-C is needed for normal diastolic function.
Mediation of diastolic function by posttranslational modification of cMyBP-C
cMyBP-C phosphorylation levels have been found to be decreased by >50 % in explanted hearts from patients with end-stage heart failure during heart transplant [8, 11, 21, 25]. End-stage failing hearts have severe diastolic and systolic dysfunction along with calcium and metabolic derangements; therefore, it is difficult to assess the impact of cMyBP-C phosphorylation. Samples obtained during myomectomy surgery to relieve outflow obstruction showed that HCM hearts have decreased cMyBP-C phosphorylation levels [8, 10, 21]. HCM hearts exhibit predominantly diastolic dysfunction, implying that reduced cMyBP-C phosphorylation is an underlying cause.
Animal models suggest that cMyBP-C phosphorylation mediates diastolic function. Protein kinase A (PKA) can phosphorylate human cMyBP-C at S275, S284, and S304  and their mouse equivalents (S273, S282, S302) as confirmed by mass spectrometry . Expressing cMyBP-C with S273A, S282A, and S302A and S273D, S282D, and S302D mutations onto cMyBP-C(-/-, Ex3-10) background created cMyBP-C(t3SA) (phosphorylation deficient)  and cMyBP-C(t3SD) (phosphorylation mimetic) [7, 26] mouse models, respectively. These mouse models allow one to elucidate the impact of cMyBP-C phosphorylation at its known PKA sites. Myosin-binding protein C (cMyBP-C)(t3SA) hearts exhibited similar EF [7, 26, 44], reduced Ea (slowed heart muscle relaxation TD velocity, Fig. 2), and increased E/Ea ratio (diastolic dysfunction) [26, 44] in comparison to its wild-type equivalent cMyBP-C(tWT) control, suggesting that reduced cMyBP-C phosphorylation causes predominantly diastolic dysfunction. Furthermore, cMyBP-C(t3SA) mice resemble human HFpEF with shorter voluntary running distances, pulmonary edema, and elevated brain natriuretic peptide levels . Another cMyBP-C phosphorylation-deficient mouse model cMyBP-C(t/t,AllP-) was made by expressing cMyBP-C with five mutations (T272A, S273A, T281A, S282A, S302A) onto the cMyBP-C truncation background of cMyBP-C(t/t) . Unlike cMyBP-C(t3SA), cMyBP-C(t/t, AllP-) hearts showed ~50 % reduction in fractional shortening and severely dilated ventricles in comparison to its cMyBP-C(t/t, WT) control , suggesting that cMyBP-C phosphorylation also mediates systolic function. Differences in mutations and mouse backgrounds probably caused the different phenotypes in these two cMyBP-C phosphorylation-deficient mouse models. Subsequently, expressing combinatorial phosphorylation site mutations (S282A-SAS, S273A/S282D/S302A-ADA, and S273D/S282A/S302D-DAD) onto the cMyBP-C(t/t) background made mutant hearts that exhibit similar EF as their control cMyBP-C(t/t, WT), providing evidence that cMyBP-C phosphorylation has greater impact on diastolic function . More recently, expressing phosphorylation-deficient cMyBP-C mutants of AAD(T272A,S273A,T281A,S282A,S302D) and DAA(T272D,S273D,T281A,S282A,S302A) onto cMyBP-C(t/t) background led to reduced EF and impaired relaxation as evidenced by slowed heart muscle relaxation TD velocity Ea . Conversely, the phosphorylation-mimetic cMyBP-C(t3SD) demonstrated enhanced diastolic function by faster heart muscle relaxation TD velocity Ea (Fig. 2) and reduced E/Ea ratio (enhanced diastolic function) . Together, these findings indicate that cMyBP-C phosphorylation mediates diastolic function.
Posttranslational modifications of cMyBP-C other than phosphorylation may also affect diastolic function. Unilateral nephrectomy and chronic deoxycorticosterone acetate (DOCA) salt treatment will cause diastolic dysfunction . Diastolic dysfunction in this mouse model was attributed to altered myofilament calcium sensitivity due to increased glutathionylation of cMyBP-C . Tetrahydrobiopterin treatment decreased glutathionylation and increased cross-bridge cycling rate to reverse diastolic dysfunction independent of cMyBP-C phosphorylation . Thus, glutathionylation of cMyBP-C may also mediate diastolic dysfunction.
Clinical evidence and animal models demonstrate that cMyBP-C mediates diastolic function. The correlation of intact papillary muscle experiments and in vivo TD measurements suggests that cMyBP-C phosphorylation modulates relative cross-bridge detachment rate with respect to attachment rate to mediate diastolic function. Thus, targeting cMyBP-C phosphorylation holds great potential for the treatment of diastolic dysfunction.
This effort is supported in part by AHA-BGIA7750035, NIH/NHLBI K08HL114877, and Texas A&M University/Baylor Scott & White startup funds to CT.
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