MICU1 is a component of the mitochondrial calcium uniporter, a multiprotein complex that also includes MICU2, MCU, and EMRE. Here, we describe a mouse model of MICU1 deficiency. MICU1−/− mitochondria demonstrate altered calcium uptake and deletion of MICU1 results in significant, but not complete, perinatal mortality. Similar to afflicted patients, viable MICU1−/− mice manifest marked ataxia and muscle weakness. Early in life, these animals display a range of biochemical abnormalities including increased resting mitochondrial calcium levels, altered mitochondrial morphology, and reduced ATP. Older MICU1−/− mice show marked, spontaneous improvement, coincident with improved mitochondrial calcium handling and an age-dependent reduction in EMRE expression. Remarkably, deleting one allele of EMRE helps normalize calcium uptake while simultaneously rescuing the high perinatal mortality observed in young MICU1−/− mice. Together, these results demonstrate that MICU1 serves as a molecular gatekeeper preventing calcium overload and suggests that modulating the calcium uniporter could have widespread therapeutic benefits.

This study developed an approach to quantify frailty with a frailty index (FI) and investigated whether age-related changes in contractions, calcium transients, and ventricular myocyte length were more prominent in mice with a high FI. The FI combined 31 variables that reflect different aspects of health in middle-aged (∼12 months) and aged (∼30 months) mice of both sexes. Aged animals had a higher FI than younger animals (FI = 0.43 ± 0.03 vs 0.08 ± 0.02, p < .001, n = 12). Myocyte hypertrophy increased by 30%-50% as the FI increased in aged animals. Peak contractions decreased more than threefold from lowest to highest FI values in aged mice (p < .037), but calcium transients were unaffected. Similar results were seen with an FI based on eight noninvasive variables identified as underlying factors. These results show that an FI can be developed for murine models and suggest that age-associated changes in myocytes are more prominent in animals with a high FI.

Calcium is thought to play an important role in regulating mitochondrial function. Evidence suggests that an increase in mitochondrial calcium can augment ATP production by altering the activity of calciumsensitive mitochondrial matrix enzymes. In contrast, the entry of large amounts of mitochondrial calcium in the setting of ischemia-reperfusion injury is thought to be a critical event in triggering cellular necrosis. For many decades, the details of how calcium entered the mitochondria remained a biological mystery. In the past few years, significant progress has been made in identifying the molecular components of the mitochondrial calcium uniporter complex. Here, we review how calcium enters and leaves the mitochondria, the growing insight into the topology, stoichiometry and function of the uniporter complex, and the early lessons learned from some initial mouse models that genetically perturb mitochondrial calcium homeostasis. (Circ Res.

The incidence and expression of cardiovascular diseases differs between the sexes. This is not surprising, as cardiac physiology differs between men and women. Clinical and basic science investigations have shown important sex differences in cardiac structure and function. The pervasiveness of sex differences suggests that such differences must be fundamental, likely operating at a cellular level. Indeed, studies have shown that isolated ventricular myocytes from female animals have smaller and slower contractions and underlying calcium transients compared to males. Recent evidence suggests that this arises from sex differences in components of the cardiac excitation–contraction coupling pathway, the sequence of events linking myocyte depolarization to calcium release from the sarcoplasmic reticulum and subsequent contraction. The concept that sex hormones may regulate intracellular calcium at the level of the cardiomyocyte is important, as levels of these hormones decline in both men and women as the incidence of cardiovascular disease rises. This review focuses on the impact of sex on cardiac contraction, in particular at the cellular level, and highlights specific components of the excitation–contraction coupling pathway that differ between the sexes. Understanding sex hormone regulation of calcium homeostasis in the heart may reveal new avenues for therapeutic strategies to treat cardiac dysfunction and cardiovascular diseases.

Mitochondrial calcium is thought to play an important role in the regulation of cardiac bioenergetics and function. The entry of calcium into the mitochondrial matrix requires that the divalent cation pass through the inner mitochondrial membrane via a specialized pore known as the mitochondrial calcium uniporter (MCU). Here, we use mice deficient for MCU expression to rigorously assess the role of mitochondrial calcium in cardiac function. Mitochondria isolated from MCU-/- mice have reduced matrix calcium levels, impaired calcium uptake and a defect in calcium-stimulated respiration. Nonetheless, we find that the absence of MCU expression does not affect basal cardiac function at either 12 or 20 months of age. Moreover, the physiological response of MCU-/- mice to isoproterenol challenge or transverse aortic constriction appears similar to control mice. Thus, while mitochondria derived from MCU-/- mice have markedly impaired mitochondrial calcium handling, the hearts of these animals surprisingly appear to function relatively normally under basal conditions and during stress.

Previous studies have shown that ventricular myocytes from female rats have smaller contractions and Ca(2+) transients than males. As cardiac contraction is regulated by the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway, we hypothesized that sex differences in cAMP contribute to differences in Ca(2+) handling. Ca(2+) transients (fura-2) and ionic currents were measured simultaneously (37°C, 2Hz) in ventricular myocytes from adult male and female C57BL/6 mice. Under basal conditions, diastolic Ca(2+), sarcoplasmic reticulum (SR) Ca(2+) stores, and L-type Ca(2+) current did not differ between the sexes. However, female myocytes had smaller Ca(2+) transients (26% smaller), Ca(2+) sparks (6% smaller), and excitation-contraction coupling gain in comparison to males (23% smaller). Interestingly, basal levels of intracellular cAMP were lower in female myocytes (0.7±0.1 vs. 1.7±0.2fmol/μg protein; p<0.001). Importantly, PKA inhibition (2μM H-89) eliminated male-female differences in Ca(2+) transients and gain, as well as Ca(2+) spark amplitude. Western blots showed that PKA inhibition also reduced the ratio of phospho:total RyR2 in male hearts, but not in female hearts. Stimulation of cAMP production with 10μM forskolin abolished sex differences in cAMP levels, as well as differences in Ca(2+) transients, sparks, and gain. To determine if the breakdown of cAMP differed between the sexes, phosphodiesterase (PDE) mRNA levels were measured. PDE3 expression was similar in males and females, but PDE4B expression was higher in female ventricles. The inhibition of cAMP breakdown by PDE4 (10μM rolipram) abolished differences in Ca(2+) transients and gain. These findings suggest that female myocytes have lower levels of basal cAMP due, in part, to higher expression of PDE4B. Lower cAMP levels in females may attenuate PKA phosphorylation of Ca(2+) handling proteins in females, and may limit positive inotropic responses to stimulation of the cAMP/PKA pathway in female hearts.

In contrast to ischemic cardiomyopathies which are more common in men, women are over-represented in diabetic cardiomyopathies. Diabetes is a risk factor for cardiovascular disease; however, there is a sexual dimorphism in this risk factor: heart disease is five times more common in diabetic women but only two-times more common in diabetic men. Heart failure with preserved ejection fraction, which is associated with metabolic syndrome, is also more prevalent in women. This review will examine potential mechanisms for the sex differences in metabolic cardiomyopathies. Sex differences in metabolism, calcium handling, nitric oxide, and structural proteins will be evaluated. Nitric oxide synthase and PPARα exhibit sex differences and have also been proposed to mediate the development of hypertrophy and heart failure. We focused on a role for these signalling pathways in regulating sex differences in metabolic cardiomyopathies.

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