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 tradeoff for the acquisition of endothermy.

Fatty acid synthesis in the central nervous system is implicated in the control of food intake and energy expenditure. An intermediate in this pathway, malonyl-CoA, mediates these effects. Malonyl-CoA is an established inhibitor of carnitine palmitoyltransferase-1 (CPT1), an outer mitochondrial membrane enzyme that controls entry of fatty acids into mitochondria and, thereby, fatty acid oxidation. CPT1c, a brain-specific enzyme with high sequence similarity to CPT1a (liver) and CPT1b (muscle) was recently discovered. All three CPTs bind malonyl-CoA, and CPT1a and CPT1b catalyze acyl transfer from various fatty acyl-CoAs to carnitine, whereas CPT1c does not. These findings suggest that CPT1c has a unique function or activation mechanism. We produced a targeted mouse knockout (KO) of CPT1c to investigate its role in energy homeostasis. CPT1c KO mice have lower body weight and food intake, which is consistent with a role as an energy-sensing malonyl-CoA target. Paradoxically, CPT1c KO mice fed a high-fat diet are more susceptible to obesity, suggesting that CPT1c is protective against the effects of fat feeding. CPT1c KO mice also exhibit decreased rates of fatty acid oxidation, which may contribute to their increased susceptibility to diet-induced obesity. These findings indicate that CPT1c is necessary for the regulation of energy homeostasis.acetyl-CoA carboxylase ͉ fatty acid synthase ͉ food intake ͉ malonyl-CoA ͉ obesity B ody weight is maintained by regulating food intake and energy expenditure. This balance is monitored by the central nervous system (CNS) in response to cytokine and endocrine signals, including leptin, ghrelin, obestatin, insulin, cholecystokinin, and peptide YY secreted by peripheral tissues. Concomitantly, parallel pathways in the CNS regulate energy balance by monitoring the availability of neuronal energy-rich metabolic substrates. Integration of these signals occurs in the hypothalamus and, ultimately, in higher brain centers where feeding behavior and energy expenditure are adjusted. Two primary indicators of energy surplus, glucose and fatty acids, are also monitored by subsets of hypothalamic neurons that modulate feeding behavior and energy expenditure (1). Fatty acids (2) and de novo fatty acid synthesis from glucose (3) are known to mediate these effects. Indeed, food intake and body weight have been shown to be altered by manipulating the activities of the enzymes involved in fatty acid synthesis, e.g., fatty acid synthase (FAS) (3), malonyl-CoA decarboxylase (4, 5), acetyl-CoA carboxylase (ACC) (6, 7), stearoyl-CoA desaturase (8, 9), and 5Ј-AMP kinase (10, 11).Inhibition of FAS in the CNS, for example, reduces body weight by rapidly provoking a reduction in food intake and an increase in peripheral energy expenditure (3,12). This inhibition can reverse the weight gain caused by diet-induced obesity (13,14) or mutations in leptin (ob͞ob) or its receptor (db͞db) (3, 15), suggesting that it acts independently of STAT3, which is known to be essential for leptin 's action (16, 17). I...

Summary Durable antibody production after vaccination or infection is mediated by long-lived plasma cells (LLPCs). Pathways that specifically allow LLPCs to persist remain unknown. Through bioenergetic profiling, we found that human and mouse LLPCs could robustly engage pyruvate-dependent respiration whereas their short-lived counterparts could not. LLPCs took up more glucose than did short-lived plasma cells (SLPCs) in vivo, and this glucose was essential for the generation of pyruvate. Glucose was primarily used to glycosylate antibodies, but glycolysis could be promoted by stimuli such as low ATP levels and the resultant pyruvate used for respiration by LLPCs. Deletion of Mpc2, which encodes an essential component of the mitochondrial pyruvate carrier, led to a progressive loss of LLPCs and of vaccine-specific antibodies in vivo. Thus, glucose uptake and mitochondrial pyruvate import prevent bioenergetic crises and allow LLPCs to persist. Immunizations which maximize these plasma cell metabolic properties may thus provide enduring antibody-mediated immunity.

Long-chain fatty acid (LCFA) oxidation has been shown to play an important role in interleukin-4 (IL-4)-mediated macrophage polarization (M(IL-4)). However, many of these conclusions are based on the inhibition of carnitine palmitoyltransferase-1 with high concentrations of etomoxir that far exceed what is required to inhibit enzyme activity (EC < 3 μM). We employ genetic and pharmacologic models to demonstrate that LCFA oxidation is largely dispensable for IL-4-driven polarization. Unexpectedly, high concentrations of etomoxir retained the ability to disrupt M(IL-4) polarization in the absence of Cpt1a or Cpt2 expression. Although excess etomoxir inhibits the adenine nucleotide translocase, oxidative phosphorylation is surprisingly dispensable for M(IL-4). Instead, the block in polarization was traced to depletion of intracellular free coenzyme A (CoA), likely resulting from conversion of the pro-drug etomoxir into active etomoxiryl CoA. These studies help explain the effect(s) of excess etomoxir on immune cells and reveal an unappreciated role for CoA metabolism in macrophage polarization.

Signal transducer and activator of transcription (STAT)3 is widely expressed in the CNS during development and adulthood. STAT3 has been implicated in the control of neuron͞glial differentiation and leptin-mediated energy homeostasis, but the physiological role and degree of involvement of STAT3 in these processes is not defined and controversial because of the lack of a direct genetic model. To address this, we created mice with a neural-specific disruption of STAT3 (STAT3 N؊/؊ ). Surprisingly, homozygous mutants were born at the expected Mendelian ratio without apparent developmental abnormalities but susceptible to neonatal lethality. Mutants that survived the neonatal period were hyperphagic, obese, diabetic, and infertile. Administering a melanocortin-3͞4 receptor agonist abrogated the hyperphagia and hypothalamic immunohistochemistry showed a marked reduction in proopiomelanocortin with an increase in neuropeptide Y and agouti-related protein. Mutants had reduced energy expenditure and became hypothermic after fasting or cold stress. STAT3 N؊/؊ mice are hyperleptinemic, suggesting a leptin-resistant condition. Concomitant with neuroendocrine defects such as decreased linear growth and infertility with accompanying increased corticosterone levels, this CNS knockout recapitulates the unique phenotype of db͞db and ob͞ob obese models and distinguishes them from other genetic models of obesity. Thus, STAT3 in the CNS plays essential roles in the regulation of energy homeostasis and reproduction. Signal transducer and activator of transcription (STAT) proteins are a group of cytokine-activated signaling molecules that can directly bind to DNA and activate or repress transcription of target genes. A myriad of cytokines activate STAT proteins through receptor-associated kinases. Activation of STATs occurs through tyrosine phosphorylation that is required for SH2 domain-mediated dimerization and DNA binding. Most STAT proteins have specific effects, but STAT3, the most ancient STAT, is broadly expressed and activated by a diverse array of cytokines and stresses (1-3). It was initially described as an acute phase protein involved in various biological and pathological processes. STAT3 deletion causes embryonic lethality before gastrulation via an unknown mechanism (4). STAT3 has unusually pleiotropic effects regulating murine embryonic stem cell maintenance (5), macrophage function (6, 7), immune regulation (8), and peripheral neuron survival after axotomy (9, 10) among others.In the CNS, STAT3 is expressed during embryonic development, mostly at ventricular areas, where neuronal proliferation and differentiation take place. Consistently, STAT3 is strongly suggested by in vitro studies to play an instructive role in glial and neuron differentiation (11)(12)(13)(14). In adults, STAT3 has long been implicated in the regulation of energy homeostasis through the fat-derived cytokine leptin. Although leptin can activate several STAT proteins, including STAT3, 5, and 6, in vitro (15), only STAT3 is activated in the hypoth...

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Using a newly synthesized gibberellin analog (GA3-AM) and its binding proteins, we developed a novel and efficient chemically induced dimerization (CID) system, that is completely orthogonal to the existing rapamycin-mediated protein dimerization. Combining the two systems should allow applications that were difficult or impossible with only one CID system. By using both chemical inputs (rapamycin and GA3-AM), we designed and synthesized Boolean logic gates in living mammalian cells. These gates produced output signals such as fluorescence and membrane ruffling on a timescale of seconds, a significant improvement over previous intracellular logic gates. The use of two orthogonal dimerization systems in the same cell also allows for finer modulation of protein perturbations than is possible with a single dimerizer.

Central nervous system control of energy balance affects susceptibility to obesity and diabetes, but how fatty acids, malonyl-CoA, and other metabolites act at this site to alter metabolism is poorly understood. Pharmacological inhibition of fatty acid synthase (FAS), rate limiting for de novo lipogenesis, decreases appetite independently of leptin but also promotes weight loss through activities unrelated to FAS inhibition. Here we report that the conditional genetic inactivation of FAS in pancreatic β cells and hypothalamus produced lean, hypophagic mice with increased physical activity and impaired hypothalamic PPARα signaling. Administration of a PPARα agonist into the hypothalamus increased PPARα target genes and normalized food intake. Inactivation of β cell FAS enzyme activity had no effect on islet function in culture or in vivo. These results suggest a critical role for brain FAS in the regulation of not only feeding, but also physical activity, effects that appear to be mediated through the provision of ligands generated by FAS to PPARα. Thus, 2 diametrically opposed proteins, FAS (induced by feeding) and PPARα (induced by starvation), unexpectedly form an integrative sensory module in the central nervous system to orchestrate energy balance. IntroductionHigher organisms adapt to changes in energy needs by assimilating peripheral hormonal and nutritional cues and integrating them in the central nervous system (1, 2). Even subtle defects in this system have deleterious consequences since modest excess weight in humans is associated with increased mortality (3, 4). The most thermodynamically efficient strategy for weight loss is appetite suppression, a difficult goal given the diversity of factors regulating food intake, ranging from amines and peptides to metabolites and fatty acids (reviewed in ref. 5).Fatty acid metabolism affects feeding behavior. Malonyl-CoA, an intermediary substrate controlling fatty acid flux, and carnitine palmitoyltransferase-1 (CPT-1), which allows fatty acids access to mitochondria for β-oxidation, have been independently implicated in regulating appetite (6, 7). Pharmacological inhibition of fatty acid synthase (FAS), the multifunctional enzyme that utilizes malonyl-CoA for the first committed step in fatty acid biosynthesis (8), with the compound C75 produces anorexia and weight loss in mice in the setting of increased malonyl-CoA (9). However, recent studies indicate that these effects on malonyl-CoA alone may not be sufficient to induce anorexia, as C75 also has an impact on the sympathetic nervous system and metabolic mediators, including PPARα and PPARγ coactivator-1 α (PGC1α) (10, 11). In addition,