Alterations in cardiac energy metabolism contribute to the severity of heart failure. However, the energy metabolic changes that occur in heart failure are complex and are dependent not only on the severity and type of heart failure present but also on the co-existence of common comorbidities such as obesity and type 2 diabetes. The failing heart faces an energy deficit, primarily because of a decrease in mitochondrial oxidative capacity. This is partly compensated for by an increase in ATP production from glycolysis. The relative contribution of the different fuels for mitochondrial ATP production also changes, including a decrease in glucose and amino acid oxidation, and an increase in ketone oxidation. The oxidation of fatty acids by the heart increases or decreases, depending on the type of heart failure. For instance, in heart failure associated with diabetes and obesity, myocardial fatty acid oxidation increases, while in heart failure associated with hypertension or ischemia, myocardial fatty acid oxidation decreases. Combined, these energy metabolic changes result in the failing heart becoming less efficient (ie, a decrease in cardiac work/O 2 consumed). The alterations in both glycolysis and mitochondrial oxidative metabolism in the failing heart are due to both transcriptional changes in key enzymes involved in these metabolic pathways, as well as alterations in NAD redox state (NAD + and nicotinamide adenine dinucleotide levels) and metabolite signaling that contribute to posttranslational epigenetic changes in the control of expression of genes encoding energy metabolic enzymes. Alterations in the fate of glucose, beyond flux through glycolysis or glucose oxidation, also contribute to the pathology of heart failure. Of importance, pharmacological targeting of the energy metabolic pathways has emerged as a novel therapeutic approach to improving cardiac efficiency, decreasing the energy deficit and improving cardiac function in the failing heart.

Big mitogen-activated protein kinase 1 (BMK1), also known as ERK5, is a member of the MAPK family. Genetic ablation of BMK1 in mice leads to embryonic lethality, precluding the exploration of pathophysiological roles of BMK1 in adult mice. We generated a BMK1 conditional mutation in mice in which disruption of the BMK1 gene is under the control of the inducible Mx1-Cre transgene. Ablation of BMK1 in adult mice led to lethality within 2-4 weeks after the induction of Cre recombinase. Physiological analysis showed that the blood vessels became abnormally leaky after deletion of the BMK1 gene. Histological analysis revealed that, after BMK1 ablation, hemorrhages occurred in multiple organs in which endothelial cells lining the blood vessels became round, irregularly aligned, and, eventually, apoptotic. In vitro removal of BMK1 protein also led to the death of endothelial cells partially due to the deregulation of transcriptional factor MEF2C, which is a direct substrate of BMK1. Additionally, endothelial-specific BMK1-KO leads to cardiovascular defects identical to that of global BMK1-KO mutants, whereas, surprisingly, mice lacking BMK1 in cardiomyocytes developed to term without any apparent defects. Taken together, the data provide direct genetic evidence that the BMK1 pathway is critical for endothelial function and for maintaining blood vessel integrity. 1138The Nonstandard abbreviations used: big mitogen-activated protein kinase-1 (BMK1); BMK1 flox/flox Mx1-Cre mice treated with pIpC (BMK1-CKO); cardiomyocyte-specific BMK1-KO (BMK1-cmKO); EC-specific BMK1-KO (BMK1-ecKO); embryonic day (E); endothelial cell (EC); endothelial cell growth supplement (ECGS); extracellular signalregulated kinases-1 and -2 (ERK1/2); lacZ/alkaline phosphatase (Z/AP); MAPK kinase (MEK); MAPK kinase kinase (MEKK); mouse lung capillary endothelial cell (MLCEC); myocyte enhancer factor-2 (MEF2); polyinosinic-polycytidylic acid (pIpC).

To investigate the role of insulin signaling on postnatal cardiac development, physiology, and cardiac metabolism, we generated mice with a cardiomyocyte-selective insulin receptor knockout (CIRKO) using cre/loxP recombination. Hearts of CIRKO mice were reduced in size by 20-30% due to reduced cardiomyocyte size and had persistent expression of the fetal beta-myosin heavy chain isoform. In CIRKO hearts, glucose transporter 1 (GLUT1) expression was reduced by about 50%, but there was a twofold increase in GLUT4 expression as well as increased rates of cardiac glucose uptake in vivo and increased glycolysis in isolated working hearts. Fatty acid oxidation rates were diminished as a result of reduced expression of enzymes that catalyze mitochondrial beta-oxidation. Although basal rates of glucose oxidation were reduced, insulin unexpectedly stimulated glucose oxidation and glycogenolysis in CIRKO hearts. Cardiac performance in vivo and in isolated hearts was mildly impaired. Thus, insulin signaling plays an important developmental role in regulating postnatal cardiac size, myosin isoform expression, and the switching of cardiac substrate utilization from glucose to fatty acids. Insulin may also modulate cardiac myocyte metabolism through paracrine mechanisms by activating insulin receptors in other cell types within the heart.

The retina is one of the most metabolically active tissues in the body and utilizes glucose to produce energy and intermediates required for daily renewal of photoreceptor cell outer segments. Glucose transporter 1 (GLUT1) facilitates glucose transport across outer blood retinal barrier (BRB) formed by the retinal pigment epithelium (RPE) and the inner BRB formed by the endothelium. We used conditional knockout mice to study the impact of reducing glucose transport across the RPE on photoreceptor and Müller glial cells. Transgenic mice expressing Cre recombinase under control of the Bestrophin1 ( Best1) promoter were bred with Glut1flox/flox mice to generate Tg-Best1-Cre:Glut1flox/flox mice ( RPEΔGlut1). The RPEΔGlut1 mice displayed a mosaic pattern of Cre expression within the RPE that allowed us to analyze mice with ~50% ( RPEΔGlut1m) recombination and mice with >70% ( RPEΔGlut1h) recombination separately. Deletion of GLUT1 from the RPE did not affect its carrier or barrier functions, indicating that the RPE utilizes other substrates to support its metabolic needs thereby sparing glucose for the outer retina. RPEΔGlut1m mice had normal retinal morphology, function, and no cell death; however, where GLUT1 was absent from a span of RPE greater than 100 µm, there was shortening of the photoreceptor cell outer segments. RPEΔGlut1h mice showed outer segment shortening, cell death of photoreceptors, and activation of Müller glial cells. The severe phenotype seen in RPEΔGlut1h mice indicates that glucose transport via the GLUT1 transporter in the RPE is required to meet the anabolic and catabolic requirements of photoreceptors and maintain Müller glial cells in a quiescent state.