Enhanced NCLX-dependent mitochondrial Ca2+ efflux attenuates pathological remodeling in heart failure.

Mitochondrial calcium (mCa2+) uptake couples changes in cardiomyocyte energetic demand to mitochondrial ATP production. However, excessive mCa2+ uptake triggers permeability transition and necrosis. Despite these established roles during acute stress, the involvement of mCa2+ signaling in cardiac adaptations to chronic stress remains poorly defined. Changes in NCLX expression are reported in heart failure (HF) patients and models of cardiac hypertrophy. Therefore, we hypothesized that altered mCa2+ homeostasis contributes to the hypertrophic remodeling of the myocardium that occurs upon a sustained increase in cardiac workload. The impact of mCa2+ flux on cardiac function and remodeling was examined by subjecting mice with cardiomyocyte-specific overexpression (OE) of the mitochondrial Na+/Ca2+ exchanger (NCLX), the primary mediator of mCa2+ efflux, to several well-established models of hypertrophic and non-ischemic HF. Cardiomyocyte NCLX-OE preserved contractile function, prevented hypertrophy and fibrosis, and attenuated maladaptive gene programs in mice subjected to chronic pressure overload. Hypertrophy was attenuated in NCLX-OE mice, prior to any decline in cardiac contractility. NCLX-OE similarly attenuated deleterious cardiac remodeling in mice subjected to chronic neurohormonal stimulation. However, cardiomyocyte NCLX-OE unexpectedly reduced overall survival in mice subjected to severe neurohormonal stress with angiotensin II + phenylephrine. Adenoviral NCLX expression limited mCa2+ accumulation, oxidative metabolism, and de novo protein synthesis during hypertrophic stimulation of cardiomyocytes in vitro. Our findings provide genetic evidence for the contribution of mCa2+ to early pathological remodeling in non-ischemic heart disease, but also highlight a deleterious consequence of increasing mCa2+ efflux when the heart is subjected to extreme, sustained neurohormonal stress.

mTORC1 restrains adipocyte lipolysis to prevent systemic hyperlipidemia.

OBJECTIVE: Pharmacological agents targeting the mTOR complexes are used clinically as immunosuppressants and anticancer agents and can extend the lifespan of model organisms. An undesirable side effect of these drugs is hyperlipidemia. Although multiple roles have been described for mTOR complex 1 (mTORC1) in lipid metabolism, the etiology of hyperlipidemia remains incompletely understood. The objective of this study was to determine the influence of adipocyte mTORC1 signaling in systemic lipid homeostasis in vivo. METHODS: We characterized systemic lipid metabolism in mice lacking the mTORC1 subunit Raptor (RaptoraKO), the key lipolytic enzyme ATGL (ATGLaKO), or both (ATGL-RaptoraKO) in their adipocytes. RESULTS: Mice lacking mTORC1 activity in their adipocytes failed to completely suppress lipolysis in the fed state and displayed prominent hypertriglyceridemia and hypercholesterolemia. Blocking lipolysis in their adipose tissue restored normal levels of triglycerides and cholesterol in the fed state as well as the ability to clear triglycerides in an oral fat tolerance test. CONCLUSIONS: Unsuppressed adipose lipolysis in the fed state interferes with triglyceride clearance and contributes to hyperlipidemia. Adipose tissue mTORC1 activity is necessary for appropriate suppression of lipolysis and for the maintenance of systemic lipid homeostasis.

Impaired mitochondrial calcium efflux contributes to disease progression in models of Alzheimer's disease.

Impairments in neuronal intracellular calcium (iCa2+) handling may contribute to Alzheimer's disease (AD) development. Metabolic dysfunction and progressive neuronal loss are associated with AD progression, and mitochondrial calcium (mCa2+) signaling is a key regulator of both of these processes. Here, we report remodeling of the mCa2+ exchange machinery in the prefrontal cortex of individuals with AD. In the 3xTg-AD mouse model impaired mCa2+ efflux capacity precedes neuropathology. Neuronal deletion of the mitochondrial Na+/Ca2+ exchanger (NCLX, Slc8b1 gene) accelerated memory decline and increased amyloidosis and tau pathology. Further, genetic rescue of neuronal NCLX in 3xTg-AD mice is sufficient to impede AD-associated pathology and memory loss. We show that mCa2+ overload contributes to AD progression by promoting superoxide generation, metabolic dysfunction and neuronal cell death. These results provide a link between the calcium dysregulation and metabolic dysfunction hypotheses of AD and suggest mCa2+ exchange as potential therapeutic target in AD.

Skeletal Muscle-specific G Protein-coupled Receptor Kinase 2 Ablation Alters Isolated Skeletal Muscle Mechanics and Enhances Clenbuterol-stimulated Hypertrophy.

GRK2, a G protein-coupled receptor kinase, plays a critical role in cardiac physiology. Adrenergic receptors are the primary target for GRK2 activity in the heart; phosphorylation by GRK2 leads to desensitization of these receptors. As such, levels of GRK2 activity in the heart directly correlate with cardiac contractile function. Furthermore, increased expression of GRK2 after cardiac insult exacerbates injury and speeds progression to heart failure. Despite the importance of this kinase in both the physiology and pathophysiology of the heart, relatively little is known about the role of GRK2 in skeletal muscle function and disease. In this study we generated a novel skeletal muscle-specific GRK2 knock-out (KO) mouse (MLC-Cre:GRK2fl/fl) to gain a better understanding of the role of GRK2 in skeletal muscle physiology. In isolated muscle mechanics testing, GRK2 ablation caused a significant decrease in the specific force of contraction of the fast-twitch extensor digitorum longus muscle yet had no effect on the slow-twitch soleus muscle. Despite these effects in isolated muscle, exercise capacity was not altered in MLC-Cre:GRK2fl/fl mice compared with wild-type controls. Skeletal muscle hypertrophy stimulated by clenbuterol, a ß2-adrenergic receptor (ß2AR) agonist, was significantly enhanced in MLC-Cre:GRK2fl/fl mice; mechanistically, this seems to be due to increased clenbuterol-stimulated pro-hypertrophic Akt signaling in the GRK2 KO skeletal muscle. In summary, our study provides the first insights into the role of GRK2 in skeletal muscle physiology and points to a role for GRK2 as a modulator of contractile properties in skeletal muscle as well as ß2AR-induced hypertrophy.

MCUR1 Is a Scaffold Factor for the MCU Complex Function and Promotes Mitochondrial Bioenergetics.

Mitochondrial Ca(2+) Uniporter (MCU)-dependent mitochondrial Ca(2+) uptake is the primary mechanism for increasing matrix Ca(2+) in most cell types. However, a limited understanding of the MCU complex assembly impedes the comprehension of the precise mechanisms underlying MCU activity. Here, we report that mouse cardiomyocytes and endothelial cells lacking MCU regulator 1 (MCUR1) have severely impaired [Ca(2+)]m uptake and IMCU current. MCUR1 binds to MCU and EMRE and function as a scaffold factor. Our protein binding analyses identified the minimal, highly conserved regions of coiled-coil domain of both MCU and MCUR1 that are necessary for heterooligomeric complex formation. Loss of MCUR1 perturbed MCU heterooligomeric complex and functions as a scaffold factor for the assembly of MCU complex. Vascular endothelial deletion of MCU and MCUR1 impaired mitochondrial bioenergetics, cell proliferation, and migration but elicited autophagy. These studies establish the existence of a MCU complex that assembles at the mitochondrial integral membrane and regulates Ca(2+)-dependent mitochondrial metabolism.