Yaohui Nie

Impaired exercise tolerance, mitochondrial biogenesis, and muscle fiber maintenance in miR-133a-deficient mice.

Exercise promotes multiple beneficial effects on muscle function, including induction of mitochondrial biogenesis. miR‐133a is a muscle‐enriched microRNA that regulates muscle development and function. The role of miR‐133a inexercise tolerance hasnotbeenfully elucidated. Inthecurrent study, mice that were deficient in miR‐133a demonstrated low maximal exercise capacity and low resting metabolic rate. Transcription of the mitochondrial biogenesis regulators peroxisome proliferator‐activated receptor‐γ coactivator 1‐α, peroxisome proliferator‐activated receptor‐γ coactivator 1‐β, nuclear respiratory factor‐1, and transcription factor A, mitochondrial were lower in miR‐133a–deficient muscle, which was consistent with lower mitochondrial mass and impaired exercise capacity. Six weeks of endurance exercise training increased the transcriptional level of miR‐133a and stimulated mitochondrial biogenesis in wild‐type mice, but failed to improve mitochondrial function in miR‐133a–deficient mice. Further mechanistic analysis showed an increase in the miR‐133a potential target, IGF‐1 receptor, alongwith hyperactivation of Akt signaling, in miR‐133a–deficient mice, which was consistent with lower transcription of the mitochondrial biogenesis regulators. These findings indicate an essential role of miR‐133a in skeletal muscle mitochondrial biogenesis, exercise tolerance, and response to exercise training.—Nie, Y., Sato, Y., Wang, C., Yue, F., Kuang, S., Gavin, T. P. Impaired exercise tolerance, mitochondrial biogenesis, and muscle fiber maintenance in miR‐133a–deficient mice. FASEB J. 30, 3745–3758 (2016) www.fasebj.org

Pten is necessary for the quiescence and maintenance of adult muscle stem cells

Satellite cells (SCs) are myogenic stem cells required for regeneration of adult skeletal muscles. A proper balance among quiescence, activation and differentiation is essential for long-term maintenance of SCs and their regenerative function. Here we show a function of Pten (phosphatase and tensin homologue) in quiescent SCs. Deletion of Pten in quiescent SCs leads to their spontaneous activation and premature differentiation without proliferation, resulting in depletion of SC pool and regenerative failure. However, prior to depletion, Pten-null activated SCs can transiently proliferate upon injury and regenerate injured muscles, but continually decline during regeneration, suggesting an inability to return to quiescence. Mechanistically, Pten deletion increases Akt phosphorylation, which induces cytoplasmic translocation of FoxO1 and suppression of Notch signalling. Accordingly, constitutive activation of Notch1 prevents SC depletion despite Pten deletion. Our findings delineate a critical function of Pten in maintaining SC quiescence and reveal an interaction between Pten and Notch signalling.

Heat therapy promotes the expression of angiogenic regulators in human skeletal muscle.

Heat therapy has been shown to promote capillary growth in skeletal muscle and in the heart in several animal models, but the effects of this therapy on angiogenic signaling in humans are unknown. We evaluated the acute effect of lower body heating (LBH) and unilateral thigh heating (TH) on the expression of angiogenic regulators and heat shock proteins (HSPs) in healthy young individuals. Exposure to LBH (n = 18) increased core temperature (Tc) from 36.9 ± 0.1 to 37.4 ± 0.1°C (P < 0.01) and average leg skin temperature (Tleg) from 33.1 ± 0.1 to 39.6 ± 0.1°C (P < 0.01), but did not alter the levels of circulating angiogenic cytokines and bone marrow-derived proangiogenic cells (CD34(+)CD133(+)). In skeletal muscle, the change in mRNA expression from baseline of vascular endothelial growth factor (VEGF), angiopoietin 2 (ANGPT2), chemokines CCL2 and CX3CL1, platelet factor-4 (PF4), and several members of the HSP family was higher 30 min after the intervention in the individuals exposed to LBH (n = 11) compared with the control group (n = 12). LBH also reduced the expression of transcription factor FOXO1 (P = 0.03). Exposure to TH (n = 14) increased Tleg from 32.8 ± 0.2 to 40.3 ± 0.1°C (P < 0.05) but Tc remained unaltered (36.8 ± 0.1°C at baseline and 36.9 ± 0.1°C at 90 min). This intervention upregulated the expression of VEGF, ANGPT1, ANGPT2, CCL2, and HSPs in skeletal muscle but did not affect the levels of CX3CL1, FOXO-1, and PF4. These findings suggest that both LBH and TH increase the expression of factors associated with capillary growth in human skeletal muscle.

Loss of MyoD Promotes Fate Transdifferentiation of Myoblasts Into Brown Adipocytes.

Brown adipose tissue (BAT) represents a promising agent to ameliorate obesity and other metabolic disorders. However, the abundance of BAT decreases with age and BAT paucity is a common feature of obese subjects. As brown adipocytes and myoblasts share a common Myf5 lineage origin, elucidating the molecular mechanisms underlying the fate choices of brown adipocytes versus myoblasts may lead to novel approaches to expand BAT mass. Here we identify MyoD as a key negative regulator of brown adipocyte development. CRISPR/CAS9-mediated deletion of MyoD in C2C12 myoblasts facilitates their adipogenic transdifferentiation. MyoD knockout downregulates miR-133 and upregulates the miR-133 target Igf1r, leading to amplification of PI3K–Akt signaling. Accordingly, inhibition of PI3K or Akt abolishes the adipogenic gene expression of MyoD null myoblasts. Strikingly, loss of MyoD converts satellite cell-derived primary myoblasts to brown adipocytes through upregulation of Prdm16, a target of miR-133 and key determinant of brown adipocyte fate. Conversely, forced expression of MyoD in brown preadipocytes blocks brown adipogenesis and upregulates the expression of myogenic genes. Importantly, miR-133a knockout significantly blunts the inhibitory effect of MyoD on brown adipogenesis. Our results establish MyoD as a negative regulator of brown adipocyte development by upregulating miR-133 to suppress Akt signaling and Prdm16.

Ascl2 inhibits myogenesis by antagonizing the transcriptional activity of myogenic regulatory factors

ABSTRACT Myogenic regulatory factors (MRFs), including Myf5, MyoD (Myod1) and Myog, are muscle-specific transcription factors that orchestrate myogenesis. Although MRFs are essential for myogenic commitment and differentiation, timely repression of their activity is necessary for the self-renewal and maintenance of muscle stem cells (satellite cells). Here, we define Ascl2 as a novel inhibitor of MRFs. During mouse development, Ascl2 is transiently detected in a subpopulation of Pax7+ MyoD+ progenitors (myoblasts) that become Pax7+ MyoD− satellite cells prior to birth, but is not detectable in postnatal satellite cells. Ascl2 knockout in embryonic myoblasts decreases both the number of Pax7+ cells and the proportion of Pax7+ MyoD− cells. Conversely, overexpression of Ascl2 inhibits the proliferation and differentiation of cultured myoblasts and impairs the regeneration of injured muscles. Ascl2 competes with MRFs for binding to E-boxes in the promoters of muscle genes, without activating gene transcription. Ascl2 also forms heterodimers with classical E-proteins to sequester their transcriptional activity on MRF genes. Accordingly, MyoD or Myog expression rescues myogenic differentiation despite Ascl2 overexpression. Ascl2 expression is regulated by Notch signaling, a key governor of satellite cell self-renewal. These data demonstrate that Ascl2 inhibits myogenic differentiation by targeting MRFs and facilitates the generation of postnatal satellite cells. Summary: The transcription factor Ascl2 is identified as a novel inhibitor of myogenic regulatory factors in mice, acting to inhibit myogenic differentiation and facilitate the generation of postnatal satellite cells.

Skeletal Muscle-Specific Deletion of MKP-1 Reveals a p38 MAPK/JNK/Akt Signaling Node that Regulates Obesity-Induced Insulin Resistance

Stress responses promote obesity and insulin resistance, in part, by activating the stress-responsive mitogen-activated protein kinases (MAPKs), p38 MAPK, and c-Jun NH2-terminal kinase (JNK). Stress also induces expression of MAPK phosphatase-1 (MKP-1), which inactivates both JNK and p38 MAPK. However, the equilibrium between JNK/p38 MAPK and MKP-1 signaling in the development of obesity and insulin resistance is unclear. Skeletal muscle is a major tissue involved in energy expenditure and glucose metabolism. In skeletal muscle, MKP-1 is upregulated in high-fat diet–fed mice and in skeletal muscle of obese humans. Mice lacking skeletal muscle expression of MKP-1 (MKP1-MKO) showed increased skeletal muscle p38 MAPK and JNK activities and were resistant to the development of diet-induced obesity. MKP1-MKO mice exhibited increased whole-body energy expenditure that was associated with elevated levels of myofiber-associated mitochondrial oxygen consumption. miR-21, a negative regulator of PTEN expression, was upregulated in skeletal muscle of MKP1-MKO mice, resulting in increased Akt activity consistent with enhanced insulin sensitivity. Our results demonstrate that skeletal muscle MKP-1 represents a critical signaling node through which inactivation of the p38 MAPK/JNK module promotes obesity and insulin resistance.

High Incomplete Skeletal Muscle Fatty Acid Oxidation Explains Low Muscle Insulin Sensitivity in Poorly Controlled T2D.

Context Almost 50% of type 2 diabetic (T2D) patients are poorly controlled [glycated hemoglobin (HbA1c) ≥ 7%]; however, the mechanisms responsible for progressively worsening glycemic control are poorly understood. Lower skeletal muscle mitochondrial respiratory capacity is associated with low insulin sensitivity and the development of T2D. Objective We investigated if skeletal muscle insulin sensitivity (SI) was different between well-controlled T2D (WCD) and poorly controlled T2D (PCD) and if the difference was associated with differences resulting from mitochondrial respiratory function. Design Vastus lateralis muscle mitochondrial respiration, mitochondrial content, mitochondrial enzyme activity, and fatty acid oxidation (FAO) were measured. SI and the acute response to glucose (AIRg) were calculated by MINMOD analysis from glucose and insulin obtained during a modified, frequently sampled, intravenous glucose tolerance test. Results SI and AIRg were lower in PCD than WCD. Muscle incomplete FAO was greater in PCD than WCD and greater incomplete FAO was associated with lower SI and higher HbA1c. Hydroxyacyl-coenzyme A dehydrogenase expression and activity were greater in PCD than WCD. There was no difference in maximal mitochondrial respiration or content between WCD and PCD. Conclusion The current results suggest that greater skeletal muscle incomplete FAO in poorly controlled T2D is due to elevated β oxidation and is associated with worsening muscle SI.