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Tissue regenerative potential displays striking divergence across phylogeny and ontogeny, but the underlying mechanisms remain enigmatic. Loss of mammalian cardiac regenerative potential correlates with cardiomyocyte cell-cycle arrest and polyploidization as well as the development of postnatal endothermy. We reveal that diploid cardiomyocyte abundance across 41 species conforms to Kleiber's law-the ¾-power law scaling of metabolism with bodyweight-and inversely correlates with standard metabolic rate, body temperature, and serum thyroxine level. Inactivation of thyroid hormone signaling reduces mouse cardiomyocyte polyploidization, delays cell-cycle exit, and retains cardiac regenerative potential in adults. Conversely, exogenous thyroid hormones inhibit zebrafish heart regeneration. Thus, our findings suggest that loss of heart regenerative capacity in adult mammals is triggered by increasing thyroid hormones and may be a trade-off for the acquisition of endothermy.

Syntrophus aciditrophicus is a model syntrophic bacterium that degrades key intermediates in anaerobic decomposition, such as benzoate, cyclohexane-1-carboxylate, and certain fatty acids, to acetate when grown with hydrogen-/formate-consuming microorganisms. ATP formation coupled to acetate production is the main source for energy conservation by S. aciditrophicus However, the absence of homologs for phosphate acetyltransferase and acetate kinase in the genome of S. aciditrophicus leaves it unclear as to how ATP is formed, as most fermentative bacteria rely on these two enzymes to synthesize ATP from acetyl coenzyme A (CoA) and phosphate. Here, we combine transcriptomic, proteomic, metabolite, and enzymatic approaches to show that S. aciditrophicus uses AMP-forming, acetyl-CoA synthetase (Acs1) for ATP synthesis from acetyl-CoA. acs1 mRNA and Acs1 were abundant in transcriptomes and proteomes, respectively, of S. aciditrophicus grown in pure culture and coculture. Cell extracts of S. aciditrophicus had low or undetectable acetate kinase and phosphate acetyltransferase activities but had high acetyl-CoA synthetase activity under all growth conditions tested. Both Acs1 purified from S. aciditrophicus and recombinantly produced Acs1 catalyzed ATP and acetate formation from acetyl-CoA, AMP, and pyrophosphate. High pyrophosphate levels and a high AMP-to-ATP ratio (5.9 ± 1.4) in S. aciditrophicus cells support the operation of Acs1 in the acetate-forming direction. Thus, S. aciditrophicus has a unique approach to conserve energy involving pyrophosphate, AMP, acetyl-CoA, and an AMP-forming, acetyl-CoA synthetase. Bacteria use two enzymes, phosphate acetyltransferase and acetate kinase, to make ATP from acetyl-CoA, while acetate-forming archaea use a single enzyme, an ADP-forming, acetyl-CoA synthetase, to synthesize ATP and acetate from acetyl-CoA. Syntrophus aciditrophicus apparently relies on a different approach to conserve energy during acetyl-CoA metabolism, as its genome does not have homologs to the genes for phosphate acetyltransferase and acetate kinase. Here, we show that S. aciditrophicus uses an alternative approach, an AMP-forming, acetyl-CoA synthetase, to make ATP from acetyl-CoA. AMP-forming, acetyl-CoA synthetases were previously thought to function only in the activation of acetate to acetyl-CoA.

Author(s): Yuji Nakada [1]; Diana C. Canseco [1]; SuWannee Thet [1]; Salim Abdisalaam [2]; Aroumougame Asaithamby [2]; Celio X. Santos [3]; Ajay M. Shah [3]; Hua Zhang [4]; James E. [...]

Chronic hyperglycemia causes structural alterations of proteins through the Maillard reaction. In diabetes, methylglyoxal (MGO)-induced hydroimidazolones are the predominant modification. In contrast to acute hyperglycemia, mitochondrial respiration is depressed in chronic diabetes. To determine whether MGO-derived protein modifications result in abnormalities in mitochondrial bioenergetics and superoxide formation, proteomics and functional studies were performed in renal cortical mitochondria isolated from rats with 2, 6, and 12 mo of streptozotocin-induced diabetes. MGO-modified proteins belonged to the following two pathways: 1) oxidative phosphorylation and 2) fatty acid [beta]-oxidation. Two of these proteins were identified as components of respiratory complex III, the major site of superoxide production in health and disease. Mitochondria from rats with diabetes exhibited a diminution of oxidative phosphorylation. A decrease in the respiratory complex III activity was significantly correlated with the quantity of MGO-derived hydroimidazolone present on mitochondrial proteins in both diabetic and control animals. In diabetes, isolated renal mitochondria produced significantly increased quantities of superoxide and showed evidence of oxidative damage. Administration of aminoguanidine improved mitochondrial respiration and complex III activity and decreased oxidative damage to mitochondrial proteins. Therefore, posttranslational modifications of mitochondrial proteins by MGO may represent pathogenic events leading to mitochondria-induced oxidative stress in the kidney in chronic diabetes. methylglyoxal; nephropathy; complex III; proteomics

Familial hypertrophic cardiomyopathy (HCM) is an autosomal dominant disease characterized by varying degrees of ventricular hypertrophy and myofibrillar disarray. Mutations in cardiac contractile proteins cause HCM. However, there is an unexplained wide variability in the clinical phenotype, and it is likely that there are multiple contributing factors. Because mitochondrial dysfunction has been described in heart disease, we tested the hypothesis that mitochondrial dysfunction contributes to the varying HCM phenotypes. Mitochondrial function was assessed in two transgenic models of HCM: mice with a mutant myosin heavy chain gene (MyHC) or with a mutant cardiac troponin T(R92Q) gene. Despite mitochondrial ultrastructural abnormalities in both models, the rate of state 3 respiration was significantly decreased only in the mutant MyHC mice by ~23%. Notably, this decrease in state 3 respiration preceded hemodynamic dysfunction. The maximum activity of [alpha]-ketogutarate dehydrogenase as assayed in isolated disrupted mitochondria was decreased by 28% compared with isolated control mitochondria. In addition, complexes I and IV were decreased in mutant MyHC transgenic mice. Inhibition of [beta]-adrenergic receptor kinase, which is elevated in mutant MyHC mouse hearts, can prevent mitochondrial respiratory impairment in mutant MyHC mice. Thus our results suggest that mitochondria may contribute to the hemodynamic dysfunction seen in some forms of HCM and offer a plausible mechanism responsible for some of the heterogeneity of the disease phenotypes. mitochondria; transgenic

Alterations in renal mitochondrial respiration in response to the reactive oxoaldehyde methylglyoxal. Am J Physiol Renal Physiol 283: F52-F59, 2002. First published January 29, 2002; 10.1152/ ajprenal.00302.2001.--Chronic hyperglycemia has been linked to alterations in mitochondrial function, suggesting an important role in the pathophysiology of the complications of diabetes mellitus. In the diabetic kidney, ultrastructural changes in mitochondria are associated with impaired tubular function. The goal of this study was to determine if methylglyoxal (MGO), a dicarbonyl compound reaching high levels in hyperglycemic conditions, has direct toxicity for renal mitochondria. Intact mitochondria isolated from the renal cortex of rats were incubated with MGO to determine 1) its effect on mitochondrial respiration, 2) the conditions under which MGO exerts these effects, and 3) the potential mitochondrial targets of MGO influence. This study demonstrates that MGO has an inhibitory effect on both the tricarboxylic acid cycle and the electron respiratory chain. The modifications appear to be specific to certain mitochondrial proteins. Alterations of these proteins lead to disturbances in mitochondria that may play an important role in renal cellular toxicity and in the development of diabetic nephropathy. dicarbonyl; diabetic nephropathy; mitochondria

We previously reported that cardiac reperfusion results in declines in mitochondrial NADH-linked respiration. The degree of inactivation increased with age and was paralleled by modification of protein by the lipid peroxidation product 4-hydroxy-2-nonenal. To gain insight into potential sites of oxidative damage, the present study was undertaken to identify specific mitochondrial protein(s) inactivated during ischemia and reperfusion and to determine which of these losses in activity are responsible for observed declines in mitochondrial respiration. Using a Langendorff rat heart perfusion protocol, we observed age-dependent inactivation of complex I during ischemia and complex IV and [Alpha]-ketoglutarate dehydrogenase during reperfusion. Although losses in complex I and IV activities were found not to be of sufficient magnitude to cause declines in mitochondrial respiration, an age-related decrease in complex I activity during ischemia may predispose old animals to more severe oxidative damage during reperfusion. It was determined that inactivation of [Alpha]-ketoglutarate dehydrogenase is responsible, in large part, for observed reperfusion-induced declines in NADH-linked respiration, [Alpha]-Ketoglutarate dehydrogenase is highly susceptible to 4-hydroxy-2-nonenal inactivation in vitro. Thus, our results suggest a plausible mechanism for age-dependent, reperfusion-induced declines in mitochondrial function and identify [Alpha]-ketoglutarate dehydrogenase as a likely site of free radical-mediated damage.