Mitochondrial dynamics and cell death in heart failure.

The highly regulated processes of mitochondrial fusion (joining), fission (division) and trafficking, collectively called mitochondrial dynamics, determine cell-type specific morphology, intracellular distribution and activity of these critical organelles. Mitochondria are critical for cardiac function, while their structural and functional abnormalities contribute to several common cardiovascular diseases, including heart failure (HF). The tightly balanced mitochondrial fusion and fission determine number, morphology and activity of these multifunctional organelles. Although the intracellular architecture of mature cardiomyocytes greatly restricts mitochondrial dynamics, this process occurs in the adult human heart. Fusion and fission modulate multiple mitochondrial functions, ranging from energy and reactive oxygen species production to Ca(2+) homeostasis and cell death, allowing the heart to respond properly to body demands. Tightly controlled balance between fusion and fission is of utmost importance in the high energy-demanding cardiomyocytes. A shift toward fission leads to mitochondrial fragmentation, while a shift toward fusion results in the formation of enlarged mitochondria and in the fusion of damaged mitochondria with healthy organelles. Mfn1, Mfn2 and OPA1 constitute the core machinery promoting mitochondrial fusion, whereas Drp1, Fis1, Mff and MiD49/51 are the core components of fission machinery. Growing evidence suggests that fusion/fission factors in adult cardiomyocytes play essential noncanonical roles in cardiac development, Ca(2+) signaling, mitochondrial quality control and cell death. Impairment of this complex circuit causes cardiomyocyte dysfunction and death contributing to heart injury culminating in HF. Pharmacological targeting of components of this intricate network may be a novel therapeutic modality for HF treatment.

Mitochondrial DNA maintenance: an appraisal.

Mitochondria play a crucial role in a variety of cellular processes ranging from energy metabolism, generation of reactive oxygen species (ROS), and Ca(2+) handling to stress responses, cell survival, and death. Malfunction of the organelle may contribute to the pathogenesis of neuromuscular disorders, cancer, premature aging, and cardiovascular diseases, including myocardial ischemia, cardiomyopathy, and heart failure. Mitochondria are unique as they contain their own genome organized into DNA-protein complexes, so-called mitochondrial nucleoids, along with multiprotein machineries, which promote mitochondrial DNA (mtDNA) replication, transcription, and repair. Although the organelle possesses almost all known nuclear DNA repair pathways, including base excision repair, mismatch repair, and recombinational repair, the proximity of mtDNA to the main sites of ROS production and the lack of protective histones may result in increased susceptibility to oxidative stress and other types of mtDNA damage. Defects in the components of these highly organized machineries, which mediate mtDNA maintenance (replication and repair), may result in accumulation of point mutations and/or deletions in mtDNA and decreased mtDNA copy number impairing mitochondrial function. This review will focus on the mechanisms of mtDNA maintenance with emphasis on the proteins implicated in these processes and their functional role in various disease conditions and aging.

Epigenetics of the failing heart.

With the impressive advancement in high-throughput 'omics' technologies over the past two decades, epigenetic mechanisms have emerged as the regulatory interface between the genome and environmental factors. These mechanisms include DNA methylation, histone modifications, ATP-dependent chromatin remodeling and RNA-based mechanisms. Their highly interdependent and coordinated action modulates the chromatin structure controlling access of the transcription machinery and thereby regulating expression of target genes. Given the rather limited proliferative capability of human cardiomyocytes, epigenetic regulation appears to play a particularly important role in the myocardium. The highly dynamic nature of the epigenome allows the heart to adapt to environmental challenges and to respond quickly and properly to cardiac stress. It is now becoming evident that histone-modifying and chromatin-remodeling enzymes as well as numerous non-coding RNAs play critical roles in cardiac development and function, while their dysregulation contributes to the onset and development of pathological cardiac remodeling culminating in HF. This review focuses on up-to-date knowledge about the epigenetic mechanisms and highlights their emerging role in the healthy and failing heart. Uncovering the determinants of epigenetic regulation holds great promise to accelerate the development of successful new diagnostic and therapeutic strategies in human cardiac disease.

Mitochondrial oxidative metabolism and uncoupling proteins in the failing heart.

Despite significant progress in cardiovascular medicine, myocardial ischemia and infarction, progressing eventually to the final end point heart failure (HF), remain the leading cause of morbidity and mortality in the USA. HF is a complex syndrome that results from any structural or functional impairment in ventricular filling or blood ejection. Ultimately, the heart's inability to supply the body's tissues with enough blood may lead to death. Mechanistically, the hallmarks of the failing heart include abnormal energy metabolism, increased production of reactive oxygen species (ROS) and defects in excitation-contraction coupling. HF is a highly dynamic pathological process, and observed alterations in cardiac metabolism and function depend on the disease progression. In the early stages, cardiac remodeling characterized by normal or slightly increased fatty acid (FA) oxidation plays a compensatory, cardioprotective role. However, upon progression of HF, FA oxidation and mitochondrial oxidative activity are decreased, resulting in a significant drop in cardiac ATP levels. In HF, as a compensatory response to decreased oxidative metabolism, glucose uptake and glycolysis are upregulated, but this upregulation is not sufficient to compensate for a drop in ATP production. Elevated mitochondrial ROS generation and ROS-mediated damage, when they overwhelm the cellular antioxidant defense system, induce heart injury and contribute to the progression of HF. Mitochondrial uncoupling proteins (UCPs), which promote proton leak across the inner mitochondrial membrane, have emerged as essential regulators of mitochondrial membrane potential, respiratory activity and ROS generation. Although the physiological role of UCP2 and UCP3, expressed in the heart, has not been clearly established, increasing evidence suggests that these proteins by promoting mild uncoupling could reduce mitochondrial ROS generation and cardiomyocyte apoptosis and ameliorate thereby myocardial function. Further investigation on the alterations in cardiac UCP activity and regulation will advance our understanding of their physiological roles in the healthy and diseased heart and also may facilitate the development of novel and more efficient therapies.

Mitochondria in heart failure: the emerging role of mitochondrial dynamics.

Over the past decade, mitochondria have emerged as critical integrators of energy production, generation of reactive oxygen species (ROS), multiple cell death, and signaling pathways in the constantly beating heart. Clarification of the molecular mechanisms, underlying mitochondrial ROS generation and ROS-induced cell death pathways, associated with cardiovascular diseases, by itself remains an important aim; more recently, mitochondrial dynamics has emerged as an important active mechanism to maintain normal mitochondria number and morphology, both are necessary to preserve cardiomyocytes integrity. The two opposing processes, division (fission) and fusion, determine the cell type-specific mitochondrial morphology, the intracellular distribution and activity. The tightly controlled balance between fusion and fission is of particular importance in the high energy demanding cells, such as cardiomyocytes, skeletal muscles, and neuronal cells. A shift toward fission will lead to mitochondrial fragmentation, observed in quiescent cells, while a shift toward fusion will result in the formation of large mitochondrial networks, found in metabolically active cardiomyocytes. Defects in mitochondrial dynamics have been associated with various human disorders, including heart failure, ischemia reperfusion injury, diabetes, and aging. Despite significant progress in our understanding of the molecular mechanisms of mitochondrial function in the heart, further focused research is needed to translate this knowledge into the development of new therapies for various ailments.

Myocardial regeneration of the failing heart.

Human heart failure (HF) is one of the leading causes of morbidity and mortality worldwide. Currently, heart transplantation and implantation of mechanical devices represent the only available treatments for advanced HF. Two alternative strategies have emerged to treat patients with HF. One approach relies on transplantation of exogenous stem cells (SCs) of non-cardiac or cardiac origin to induce cardiac regeneration and improve ventricular function. Another complementary strategy relies on stimulation of the endogenous regenerative capacity of uninjured cardiac progenitor cells to rebuild cardiac muscle and restore ventricular function. Various SC types and delivery strategies have been examined in the experimental and clinical settings; however, neither the ideal cell type nor the cell delivery method for cardiac cell therapy has yet emerged. Although the use of bone marrow (BM)-derived cells, most frequently exploited in clinical trials, appears to be safe, the results are controversial. Two recent randomized trials have failed to document any beneficial effects of intracardiac delivery of autologous BM mononuclear cells on cardiac function of patients with HF. The remarkable discovery that various populations of cardiac progenitor cells (CPCs) are present in the adult human heart and that it possesses limited regeneration capacity has opened a new era in cardiac repair. Importantly, unlike BM-derived SCs, autologous CPCs from myocardial biopsies cultured and subsequently delivered by coronary injection to patients have given positive results. Although these data are promising, a better understanding of how to control proliferation and differentiation of CPCs, to enhance their recruitment and survival, is required before CPCs become clinically applicable therapeutics.

Polycyclic aromatic hydrocarbons impair function of ß2-adrenergic receptors in airway epithelial and smooth muscle cells.

Incomplete combustion produces a pollutant mixture that includes polycyclic aromatic hydrocarbons (PAHs). Previous work by the Columbia Center for Children's Environmental Health (CCCEH) and others linked exposure to PAH with symptoms of asthma and other adverse health effects in young children. Inhaled ß(2)-adrenergic agonists are mainstays in the treatment of reactive airway diseases. These exogenous catecholamines engage membrane-bound ß(2)-adrenergic receptors (ß(2)AR) on airway epithelial and smooth muscle cells to cause airway dilation. We hypothesized that exposure to PAH might similarly interfere with the function of ß(2)AR in airway epithelial or smooth muscle cells, reducing the efficacy of a medication important for the treatment of asthma symptoms. A PAH mixture was devised, based on ambient levels measured prenatally among a cohort of pregnant women participating at the CCCEH. Primary airway epithelial and smooth muscle cells were exposed to varying concentrations of the PAH mixture, and expression, function, and signaling of ß(2)AR were assessed. Murine tracheal epithelial cells and human airway smooth muscle cells, after exposure to a PAH mixture, exhibited reduced expression and function of ß(2)AR. These findings support our hypothesis that environmentally relevant PAHs can impede ß(2)AR-mediated airway relaxation, and suggest a new paradigm where air pollutants not only contribute to the pathogenesis of childhood asthma, but also diminish responsiveness to standard therapy.

Adenosine regulation of alveolar fluid clearance.

Adenosine is a purine nucleoside that regulates cell function through G protein-coupled receptors that activate or inhibit adenylyl cyclase. Based on the understanding that cAMP regulates alveolar epithelial active Na(+) transport, we hypothesized that adenosine and its receptors have the potential to regulate alveolar ion transport and airspace fluid content. Herein, we report that type 1 (A(1)R), 2a (A(2a)R), 2b (A(2b)R), and 3 (A(3)R) adenosine receptors are present in rat and mouse lungs and alveolar type 1 and 2 epithelial cells (AT1 and AT2). Rat AT2 cells generated and produced cAMP in response to adenosine, and micromolar concentrations of adenosine were measured in bronchoalveolar lavage fluid from mice. Ussing chamber studies of rat AT2 cells indicated that adenosine affects ion transport through engagement of A(1)R, A(2a)R, and/or A(3)R through a mechanism that increases CFTR and amiloride-sensitive channel function. Intratracheal instillation of low concentrations of adenosine (< or =10(-8)M) or either A(2a)R- or A(3)R-specific agonists increased alveolar fluid clearance (AFC), whereas physiologic concentrations of adenosine (> or =10(-6)M) reduced AFC in mice and rats via an A(1)R-dependent pathway. Instillation of a CFTR inhibitor (CFTR(inh-172)) attenuated adenosine-mediated down-regulation of AFC, suggesting that adenosine causes Cl(-) efflux by means of CFTR. These studies report a role for adenosine in regulation of alveolar ion transport and fluid clearance. These findings suggest that physiologic concentrations of adenosine allow the alveolar epithelium to counterbalance active Na(+) absorption with Cl(-) efflux through engagement of the A(1)R and raise the possibility that adenosine receptor ligands can be used to treat pulmonary edema.

Interdependency of beta-adrenergic receptors and CFTR in regulation of alveolar active Na+ transport.

Beta-adrenergic receptors (betaAR) regulate active Na+ transport in the alveolar epithelium and accelerate clearance of excess airspace fluid. Accumulating data indicates that the cystic fibrosis transmembrane conductance regulator (CFTR) is important for upregulation of the active ion transport that is needed to maintain alveolar fluid homeostasis during pulmonary edema. We hypothesized that betaAR regulation of alveolar active transport may be mediated via a CFTR dependent pathway. To test this hypothesis we used a recombinant adenovirus that expresses a human CFTR cDNA (adCFTR) to increase CFTR function in the alveolar epithelium of normal rats and mice. Alveolar fluid clearance (AFC), an index of alveolar active Na+ transport, was 92% greater in CFTR overexpressing lungs than controls. Addition of the Cl- channel blockers NPPB, glibenclamide, or bumetanide and experiments using Cl- free alveolar instillate solutions indicate that the accelerated AFC in this model is due to increased Cl- channel function. Conversely, CFTR overexpression in mice with no beta1- or beta2-adrenergic receptors had no effect on AFC. Overexpression of a human beta2AR in the alveolar epithelium significantly increased AFC in normal mice but had no effect in mice with a non-functional human CFTR gene (Deltaphi508 mutation). These studies indicate that upregulation of alveolar CFTR function speeds clearance of excess fluid from the airspace and that CFTRs effect on active Na+ transport requires the betaAR. These studies reveal a previously undetected interdependency between CFTR and betaAR that is essential for upregulation of active Na+ transport and fluid clearance in the alveolus.