CDK4-E2F3 signals enhance oxidative skeletal muscle fiber numbers and function to affect myogenesis and metabolism.

Understanding how skeletal muscle fiber proportions are regulated is vital to understanding muscle function. Oxidative and glycolytic skeletal muscle fibers differ in their contractile ability, mitochondrial activity, and metabolic properties. Fiber-type proportions vary in normal physiology and disease states, although the underlying mechanisms are unclear. In human skeletal muscle, we observed that markers of oxidative fibers and mitochondria correlated positively with expression levels of PPARGC1A and CDK4 and negatively with expression levels of CDKN2A, a locus significantly associated with type 2 diabetes. Mice expressing a constitutively active Cdk4 that cannot bind its inhibitor p16INK4a, a product of the CDKN2A locus, were protected from obesity and diabetes. Their muscles exhibited increased oxidative fibers, improved mitochondrial properties, and enhanced glucose uptake. In contrast, loss of Cdk4 or skeletal muscle-specific deletion of Cdk4's target, E2F3, depleted oxidative myofibers, deteriorated mitochondrial function, and reduced exercise capacity, while increasing diabetes susceptibility. E2F3 activated the mitochondrial sensor PPARGC1A in a Cdk4-dependent manner. CDK4, E2F3, and PPARGC1A levels correlated positively with exercise and fitness and negatively with adiposity, insulin resistance, and lipid accumulation in human and rodent muscle. All together, these findings provide mechanistic insight into regulation of skeletal muscle fiber-specification that is of relevance to metabolic and muscular diseases.

A distinct hypothalamus-to-ß cell circuit modulates insulin secretion.

The central nervous system has long been thought to regulate insulin secretion, an essential process in the maintenance of blood glucose levels. However, the anatomical and functional connections between the brain and insulin-producing pancreatic ß cells remain undefined. Here, we describe a functional transneuronal circuit connecting the hypothalamus to ß cells in mice. This circuit originates from a subpopulation of oxytocin neurons in the paraventricular hypothalamic nucleus (PVNOXT), and it reaches the islets of the endocrine pancreas via the sympathetic autonomic branch to innervate ß cells. Stimulation of PVNOXT neurons rapidly suppresses insulin secretion and causes hyperglycemia. Conversely, silencing of these neurons elevates insulin levels by dysregulating neuronal signaling and secretory pathways in ß cells and induces hypoglycemia. PVNOXT neuronal activity is triggered by glucoprivation. Our findings reveal that a subset of PVNOXT neurons form functional multisynaptic circuits with ß cells in mice to regulate insulin secretion, and their function is necessary for the ß cell response to hypoglycemia.

Protection from ß-cell apoptosis by inhibition of TGF-ß/Smad3 signaling.

Prevailing insulin resistance and the resultant hyperglycemia elicits a compensatory response from pancreatic islet beta cells (ß-cells) that involves increases in ß-cell function and ß-cell mass. However, the sustained metabolic stress eventually leads to ß-cell failure characterized by severe ß-cell dysfunction and progressive loss of ß-cell mass. Whereas, ß-cell dysfunction is relatively well understood at the mechanistic level, the avenues leading to loss of ß-cell mass are less clear with reduced proliferation, dedifferentiation, and apoptosis all potential mechanisms. Butler and colleagues documented increased ß-cell apoptosis in pancreas from lean and obese human Type 2 diabetes (T2D) subjects, with no changes in rates of ß-cell replication or neogenesis, strongly suggesting a role for apoptosis in ß-cell failure. Here, we describe a permissive role for TGF-ß/Smad3 in ß-cell apoptosis. Human islets undergoing ß-cell apoptosis release increased levels of TGF-ß1 ligand and phosphorylation levels of TGF-ß's chief transcription factor, Smad3, are increased in human T2D islets suggestive of an autocrine role for TGF-ß/Smad3 signaling in ß-cell apoptosis. Smad3 phosphorylation is similarly increased in diabetic mouse islets undergoing ß-cell apoptosis. In mice, ß-cell-specific activation of Smad3 promotes apoptosis and loss of ß-cell mass in association with ß-cell dysfunction, glucose intolerance, and diabetes. In contrast, inactive Smad3 protects from apoptosis and preserves ß-cell mass while improving ß-cell function and glucose tolerance. At the molecular level, Smad3 associates with Foxo1 to propagate TGF-ß-dependent ß-cell apoptosis. Indeed, genetic or pharmacologic inhibition of TGF-ß/Smad3 signals or knocking down Foxo1 protects from ß-cell apoptosis. These findings reveal the importance of TGF-ß/Smad3 in promoting ß-cell apoptosis and demonstrate the therapeutic potential of TGF-ß/Smad3 antagonism to restore ß-cell mass lost in diabetes.

Age-associated repression of type 1 inositol 1, 4, 5-triphosphate receptor impairs muscle regeneration.

Skeletal muscle mass and power decrease with age, leading to impairment of mobility and metabolism in the elderly. Ca2+ signaling is crucial for myoblast differentiation as well as muscle contraction through activation of transcription factors and Ca2+-dependent kinases and phosphatases. Ca2+ channels, such as dihydropyridine receptor (DHPR), two-pore channel (TPC) and inositol 1,4,5-triphosphate receptor (ITPR), function to maintain Ca2+ homeostasis in myoblasts. Here, we observed a significant decrease in expression of type 1 IP3 receptor (ITPR1), but not types 2 and 3, in aged mice skeletal muscle and isolated myoblasts, compared with those of young mice. ITPR1 knockdown using shRNA-expressing viruses in C2C12 myoblasts and tibialis anterior muscle of mice inhibited myotube formation and muscle regeneration after injury, respectively, a typical phenotype of aged muscle. This aging phenotype was associated with repression of muscle-specific genes and activation of the epidermal growth factor receptor (EGFR)-Ras-extracellular signal-regulated kinase (ERK) pathway. ERK inhibition by U0126 not only induced recovery of myotube formation in old myoblasts but also facilitated muscle regeneration after injury in aged muscle. The conserved decline in ITPR1 expression in aged human skeletal muscle suggests utility as a potential therapeutic target for sarcopenia, which can be treated using ERK inhibition strategies.

miR-431 promotes differentiation and regeneration of old skeletal muscle by targeting Smad4.

The myogenic capacity of myoblasts decreases in skeletal muscle with age. In addition to environmental factors, intrinsic factors are important for maintaining the regenerative potential of muscle progenitor cells, but their identities are largely unknown. Here, comparative analysis of microRNA (miRNA) expression profiles in young and old myoblasts uncovered miR-431 as a novel miRNA showing markedly reduced abundance in aged myoblasts. Importantly, elevating miR-431 improved the myogenic capacity of old myoblasts, while inhibiting endogenous miR-431 lowered myogenesis. Bioinformatic and biochemical analyses revealed that miR-431 directly interacted with the 3' untranslated region (UTR) of Smad4 mRNA, which encodes one of the downstream effectors of TGF-ß signaling. In keeping with the low levels of miR-431 in old myoblasts, SMAD4 levels increased in this myoblast population. Interestingly, in an in vivo model of muscle regeneration following cardiotoxin injury, ectopic miR-431 injection greatly improved muscle regeneration and reduced SMAD4 levels. Consistent with the finding that the mouse miR-431 seed sequence in the Smad4 3' UTR is conserved in the human SMAD4 3' UTR, inhibition of miR-431 also repressed the myogenic capacity of human skeletal myoblasts. Taken together, our results suggest that the age-associated miR-431 plays a key role in maintaining the myogenic ability of skeletal muscle with age.

Nucleoredoxin promotes adipogenic differentiation through regulation of Wnt/ß-catenin signaling.

Nucleoredoxin (NRX) is a member of the thioredoxin family of proteins that controls redox homeostasis in cell. Redox homeostasis is a well-known regulator of cell differentiation into various tissue types. We found that NRX expression levels were higher in white adipose tissue of obese ob/ob mice and increased in the early adipogenic stage of 3T3-L1 preadipocyte differentiation. Knockdown of NRX decreased differentiation of 3T3-L1 cells, whereas overexpression increased differentiation. Adipose tissue-specific NRX transgenic mice showed increases in adipocyte size as well as number compared with WT mice. We further confirmed that the Wingless/int-1 class (Wnt)/ß-catenin pathway was also involved in NRX-promoted adipogenesis, consistent with a previous report showing NRX regulation of this pathway. Genes involved in lipid metabolism were downregulated, whereas inflammatory genes, including those encoding macrophage markers, were significantly upregulated, likely contributing to the obesity in Adipo-NRX mice. Our results therefore suggest that NRX acts as a novel proadipogenic factor and controls obesity in vivo.

Peroxiredoxin 3 has a crucial role in the contractile function of skeletal muscle by regulating mitochondrial homeostasis.

Antioxidant systems against reactive oxygen species (ROS) are important factors in regulating homeostasis in various cells, tissues, and organs. Although ROS are known to cause to muscular disorders, the effects of mitochondrial ROS in muscle physiology have not been fully understood. Here, we investigated the effects of ROS on muscle mass and function using mice deficient in peroxiredoxin 3 (Prx3), which is a mitochondrial antioxidant protein. Ablation of Prx3 deregulated the mitochondrial network and membrane potential of myotubes, in which ROS levels were increased. We showed that the DNA content of mitochondria and ATP production were also reduced in Prx3-KO muscle. Of note, the mitofusin 1 and 2 protein levels decreased in Prx3-KO muscle, a biochemical evidence of impaired mitochondrial fusion. Contractile dysfunction was examined by measuring isometric forces of isolated extensor digitorum longus (EDL) and soleus muscles. Maximum absolute forces in both the EDL and the soleus muscles were not significantly affected in Prx3-KO mice. However, fatigue trials revealed that the decrease in relative force was greater and more rapid in soleus from Prx3-KO compared to wild-type mice. Taken together, these results suggest that Prx3 plays a crucial role in mitochondrial homeostasis and thereby controls the contractile functions of skeletal muscle.

Quantitative proteome analysis of age-related changes in mouse gastrocnemius muscle using mTRAQ.

Aging is associated with a progressive loss of skeletal muscular function that often leads to progressive disability and loss of independence. Although muscle aging is well documented, the molecular mechanisms of this condition still remain unclear. To gain greater insight into the changes associated with aging of skeletal muscle, we performed quantitative proteomic analyses on young (6 months) and aged (27 months) mouse gastrocnemius muscles using mTRAQ stable isotope mass tags. We identified and quantified a total of 4585 peptides corresponding to 236 proteins (protein probability >0.9). Among them, 33 proteins were more than 1.5-fold upregulated and 20 proteins were more than 1.5-fold downregulated in aged muscle compared with young muscle. An ontological analysis revealed that differentially expressed proteins belonged to distinct functional groups, including ion homeostasis, energy metabolism, protein turnover, and Ca(2+) signaling. Identified proteins included aralar1, ß-enolase, fatty acid-binding protein 3, 3-hydroxyacyl-CoA dehydrogenase (Hadh), F-box protein 22, F-box, and leucine-rich repeat protein 18, voltage-dependent L-type calcium channel subunit beta-1, ryanodine receptor (RyR), and calsequestrin. Ectopic expression of calsequestrin in C2C12 myoblast resulted in decreased activity of nuclear factor of activated T-cells and increased levels of atrogin-1 and MuRF1 E3 ligase, suggesting that these differentially expressed proteins are involved in muscle aging.

Proteomic analysis of oxidative stress-induced neuronal cell death by using two-dimensional fluorescence difference gel electrophoresis.

Oxidative stress has been implicated in a number of neurological disorders, including cerebral ischemia and neuro-degenerative diseases. Comprehensive proteomic studies were carried out using an immortalized mouse hippocampal cell line, HT22, exhibiting oxidative stress-mediated cell death upon glutamate treatment. Two-dimensional fluorescence difference gel electrophoresis (2D DIGE) of subcellular organelle fractions revealed that significant numbers of proteins showed quantitative changes during HT22 cell death, among which a total of 51 proteins were identified by mass spectrometry. The identified proteins indicate that HT22 cell death occurs through perturbations in mitochondrial function, changes in translational elongation machinery, and translocation of proteins across subcellular organelles. This list of proteins may shed light on oxidative stress-mediated neuronal cell death.

Structure-based de novo design and biochemical evaluation of novel Cdc25 phosphatase inhibitors.

Cdc25 phosphatases have been considered as attractive drug targets for anticancer therapy due to the correlation of their overexpression with a wide variety of cancers. We have been able to identify 32 novel Cdc25 phosphatase inhibitors with micromolar activity by means of a structure-based de novo design method with the two known inhibitor scaffolds. Because the newly discovered inhibitors are structurally diverse and have desirable physicochemical properties as a drug candidate, they deserve further investigation as anticancer drugs. The differences in binding modes of the identified inhibitors in the active sites of Cdc25A and B are addressed in detail.

Identification of novel inhibitors of extracellular signal-regulated kinase 2 based on the structure-based virtual screening.

Extracellular signal-regulated kinase 2 (ERK2) has become an attractive target for the development of therapeutics for the treatment of cancer. We have been able to identify eight new inhibitors of ERK2 by means of a drug design protocol involving the virtual screening with docking simulations and in vitro enzyme assay. The newly discovered inhibitors can be categorized into three structural classes and reveal a significant potency with IC(50) values ranging from 1 to 30 microM. Therefore, all of the three inhibitor scaffolds deserve further development by structure-activity relationship or de novo design methods. Structural features relevant to the stabilizations of the newly identified inhibitors in the ATP-binding site of ERK2 are discussed in detail.

Discovery of novel Cdc25 phosphatase inhibitors with micromolar activity based on the structure-based virtual screening.

Cdc25 phosphatases have been considered as attractive drug targets for anticancer therapy because of the correlation of their overexpression with a wide variety of cancers. We have been able to identify five novel Cdc25 phosphatase inhibitors with micromolar activity by means of a computer-aided drug design protocol involving the homology modeling of Cdc25A and the virtual screening with the automated AutoDock program implementing the effects of ligand solvation in the scoring function. Because the newly discovered inhibitors are structurally diverse and reveal a significant potency with IC 50 values lower than 10 microM, they can be considered for further development by structure-activity relationship studies or de novo design methods. The differences in binding modes of the identified inhibitors in the active sites of Cdc25A and B are discussed in detail.

A proteomic analysis of the effect of MAPK pathway activation on L-glutamate-induced neuronal cell death.

Oxidative stress has been implicated in the pathogenesis of neuronal degenerative diseases. It is also widely known that oxidative stress induces mitogen-activated protein kinase (MAPK) signaling cascades. In this study, we used proteomic analysis to investigate the role of the MAPK pathway in oxidative stress-induced neuronal cell death. The results demonstrated that several proteins, including eukaryotic translation elongation factor 2 (eEF2) and enolase I, showed a differential expression pattern during the neuronal cell death process, and this was MAPK pathway dependent. Several chaperone and cytoskeletal proteins including heat shock protein 70, calreticulin, vimentin, prolyl 4-hydroxylase beta polypeptide, and transgelin 2 were up-or down-regulated, despite their expressions not depending on the MAPK pathway. These findings strongly suggest that the expressions of proteins which play protective roles are independent of the MAPK pathway. On the other hand, eEF2 and enolase I may be the downstream targets of the MAPK pathway.

Transition to an irreversible state of senescence in HeLa cells arrested by repression of HPV E6 and E7 genes.

Inhibition of human papillomavirus (HPV) E6 and E7 transcription by means of the E2 protein of bovine papillomavirus 1 (BPV1) has been shown to induce acute growth arrest in HPV-positive cervical carcinoma cells. This state of arrest is marked by the expression of senescence phenotypes including SA beta-Gal activity and lipofuscin accumulation. In this study, we examined the reversibility of these phenotypes by exogenously expressing the E6 and E7 genes into HeLa cells growth-arrested by the depletion of E6/E7. Re-expression of E7 (but not E6) in 2 days following E2 transduction induced the cells to resume growth. The proliferating cells manifested the phenotype of untreated HeLa cells, suggesting that E7 is the major factor responsible for the continued proliferation and the suppression of the senescence phenotype in cervical carcinoma cells. However, E7 in 5 days following E2 transduction did not prevent HeLa cells from entering the senescent state, indicating that the arrested state becomes irreversible. Our results suggest that, upon depletion of the viral oncoproteins, a senescent state is irreversibly induced in HeLa cells after a period of commitment. The status and cellular location of certain factors involved in signal transduction and cell cycle control was altered as well along with this irreversibility transition.