Myosin binding protein-H like (MyBP-HL) has recently been identified as a novel component of myofilaments in the cardiac atria. Loss of MyBP-HL is associated with dilated cardiomyopathy and arrhythmia in humans and mice. MyBP-HL is highly homologous to cardiac myosin binding protein-C (cMyBP-C), which has been studied in the context of genetic cardiomyopathy. Prior work using mouse atrial tissue lysates established MyBP-HL and cMyBP-C bind competitively and maintain a stoichiometric ratio compared to myosin heavy chain. We now confirm this competitive binding using neonatal rat ventricular cardiomyocytes (NRVMs) transfected with mouse Mybphl . Using confocal microscopy with internal controls, we found a linear, inverse relationshib between myosin binding protein levels with NRVMs with high levels of MyBP-HL expression showing a reduction in cMyBP-C levels compared to untransfected NRVMs that only express cMyBP-C. While we show that these two myosin binding proteins bind comparatively with each other, the functional consequences are unclear. We performed force-calcium measurements of permeabilized single atrial cardiomyocytes and found that loss of MyBP-HL does not significantly alter the calcium sensitivity of force development, or the specific force generated by isometric contraction. Because cMyBP-C is a known regulator of crossbridge sliding velocity, we evaluated contraction and relaxation kinetics of Mybphl single-myofibrils from WT and Mybphl homozygous null mice. Atrial myofibrils lacking Mybphl showed a significantly faster rate and shorter duration of their linear phase of relaxation, which is governed by the off-rate of myosin from actin. This suggests that extra cMyBP-C in the atria can hasten myofibril relaxation. In healthy atria with MyBP-HL replacing approximately half the cMyBP-C, the slow phase of relaxation is prolonged. While the physiological importance of this is not explored, it provides a mechanism by which loss of function mutations in MYBPHL cause contractile dysfunction.
Pythons are infrequent feeders that can ingest meals equal to their own body mass. The extreme metabolic response required to digest such large meals is associated with a dramatic increase in the mass of most organs, including the heart. Recently, we have been able to assess functional effects of feeding using isolated python cardiomyocytes and myofibrils, advancing our understanding of extreme cardiac adaptation in python ( Python regius ). Twenty-four hours after feeding, python cardiomyocytes showed prolonged Ca 2+ transients, increased maximal tension and Ca 2+ sensitivity of myofibrils as compared to fasted pythons. Post-prandial positive inotropy was accompanied by enhanced metabolic output via increased mitochondrial ATP production rate and by AMP-dependent kinase (AMPK) activation and phosphofructokinase-2 reduction, suggesting a key role for fatty acid, but not glucose, metabolism after feeding. In addition, 24h post-fed hearts had significantly reduced tissue stiffness and myofibril passive tension. Finally, chromatin condensation was reduced about 30% after feeding in python cardiomyocytes and confirmed by increased histone acetylation, indicating a predominant role for epigenetics in post-prandial adaptation. These results suggest that feeding promotes positive cardiac inotropy in python via a number of coordinated mechanisms to enhance energy production, increase myofibril and tissue compliance, and increase chromatin accessibility. As heart failure is commonly characterized by depressed contractility, compromised energetics, and increased tissue stiffness, assessing post-prandial adaptation in python hearts provides us with powerful insights that could inform the development of therapeutics for human heart diseases.
Background: Pediatric dilated cardiomyopathy (DCM) is a disease with a poor prognosis that affects 1 in 100,000 children. Girls with DCM have worse outcomes than boys, and the mechanisms that lead to these differences are not clear. We have identified a cytokine, midkine (MDK) that is significantly upregulated in the serum from pediatric DCM patients. Circulating MDK is significantly higher in girls with DCM requiring heart transplantation compared to girls with DCM who are stable. Objective: The objective of this study is to determine if MDK impacts cardiomyocyte function in a sex-specific manner in juvenile male and female cells. Methods: Cardiomyocytes isolated from 3 week old juvenile male and female rats (JRVMs) were treated with 1μg MDK for 48 hours. Cardiomyocyte function and calcium dynamics were assessed using an IonOptix system. Myofibril mechanics and calcium sensitivity were measured. In addition, RNA sequencing was completed to identify differentially regulated pathways in juvenile male and female cardiomyocytes in response to MDK. Results: Female JRVMs treated with MDK had higher peak calcium and slower calcium reuptake compared to vehicle-treated female JRVMs. In contrast, male JRVMs treated with MDK did not demonstrate a change in peak calcium and had faster calcium reuptake compared to vehicle-treated male JRVMs. Myofibril calcium sensitivity was decreased in female JRVMs in response to MDK whereas calcium sensitivity was unchanged in male MDK-treated JRVMs. Analysis of the genes that were differentially expressed in cardiomyocytes in response to MDK demonstrated a sex-dependent regulation. Specifically, pathways which regulate calcium handling were only altered in female JRVMs treated with MDK but not in MDK-treated male JRVMs. Conclusions: This study demonstrates sex-specific differences in cardiomyocyte function in response to MDK. Particularly, female cardiomyocytes respond to MDK by regulating genes involved in calcium handling pathways. This suggests that elevated circulating MDK in pediatric DCM patients may lead to different cardiac responses in male and female patients. Therefore, elucidating these sex-specific disease mechanisms is critical to define therapies focused on male and female pediatric DCM patients.
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