Spermatogenesis is a temperature-dependent process, and increases in scrotal temperature can disrupt its progression. We previously showed that heat stress causes DNA damage in germ cells, an increase in germ cell death (as seen on TUNEL staining), and subfertility. The present study evaluated the stress response in mouse testes following a single mild transient scrotal heat exposure (40 degrees C or 42 degrees C for 30 min). We investigated markers of three types of stress response, namely, hypoxia, oxidative stress, and apoptosis. Heat stress caused an increase in expression of hypoxia-inducible factor 1 alpha (Hif1a) mRNA expression and translocation of HIF1A protein to the germ cell nucleus, consistent with hypoxic stress. Increased expression of heme oxygenase 1 (Hmox1) and the antioxidant enzymes glutathione peroxidase 1 (GPX1) and glutathione S-transferase alpha (GSTA) was consistent with a robust oxidative stress response. Germ cell death was associated with an increase in expression of the effector caspase cleaved caspase 3 and a decrease in expression of the protein inhibitor of caspase-activated DNase (ICAD). Reduced expression of ICAD contributes to increased activity of caspase-activated DNase and is consistent with the increased rates of DNA fragmentation that have been detected previously using TUNEL staining. These studies confirmed that transient mild testicular hyperthermia results in temperature-dependent germ cell death and demonstrated that elevated temperature results in a complex stress response, including induction of genes associated with oxidative stress and hypoxia.

Recent research reveals that dysfunction and subsequent loss of mitochondria (mitophagy) is a potent inducer of skeletal muscle wasting. However, the molecular mechanisms that govern the deregulation of mitochondrial function during muscle wasting are unclear. In this report, we show that different muscle-wasting stimuli upregulated mitochondrial E3 ubiquitin protein ligase 1 (Mul1), through a mechanism involving FoxO1/3 transcription factors. Overexpression of Mul1 in skeletal muscles and myoblast cultures was sufficient for the induction of mitophagy. Consistently, Mul1 suppression not only protected against mitophagy but also partially rescued the muscle wasting observed in response to muscle-wasting stimuli. In addition, upregulation of Mul1, while increasing mitochondrial fission, resulted in ubiquitination and degradation of the mitochondrial fusion protein Mfn2. Collectively, these data explain the molecular basis for the loss of mitochondrial number during muscle wasting.

Background: PPAR␤/␦ has been implicated in muscle regeneration; however the signaling mechanism(s) is unclear. Results: Activation of PPAR␤/␦-promoted Gasp-1 expression blocked myostatin activity and enhanced myogenesis. Conclusion: Activation of PPAR␤/␦ led to inhibition of myostatin activity and thus increased myogenesis. Significance: PPAR␤/␦ agonists are novel myostatin antagonists that have potential benefits toward improving postnatal muscle growth and repair.Classically, peroxisome proliferator-activated receptor ␤/␦ (PPAR␤/␦) function was thought to be restricted to enhancing adipocyte differentiation and development of adipose-like cells from other lineages. However, recent studies have revealed a critical role for PPAR␤/␦ during skeletal muscle growth and regeneration. Although PPAR␤/␦ has been implicated in regulating myogenesis, little is presently known about the role and, for that matter, the mechanism(s) of action of PPAR␤/␦ in regulating postnatal myogenesis. Here we report for the first time, using a PPAR␤/␦-specific ligand (L165041) and the PPAR␤/␦-null mouse model, that PPAR␤/␦ enhances postnatal myogenesis through increasing both myoblast proliferation and differentiation. In addition, we have identified Gasp-1 (growth and differentiation factor-associated serum protein-1) as a novel downstream target of PPAR␤/␦ in skeletal muscle. In agreement, reduced Gasp-1 expression was detected in PPAR␤/␦-null mice muscle tissue. We further report that a functional PPAR-responsive element within the 1.5-kb proximal Gasp-1 promoter region is critical for PPAR␤/␦ regulation of Gasp-1. Gasp-1 has been reported to bind to and inhibit the activity of myostatin; consistent with this, we found that enhanced secretion of Gasp-1, increased Gasp-1 myostatin interaction and significantly reduced myostatin activity upon L165041-mediated activation of PPAR␤/␦. Moreover, we analyzed the ability of hGASP-1 to regulate myogenesis independently of PPAR␤/␦ activation. The results revealed that hGASP-1 protein treatment enhances myoblast proliferation and differentiation, whereas silencing of hGASP-1 results in defective myogenesis. Taken together these data revealed that PPAR␤/␦ is a positive regulator of skeletal muscle myogenesis, which functions through negatively modulating myostatin activity via a mechanism involving Gasp-1.In the late 1960s, work performed by De Duve et al.(1) led to the identification of a series of compounds that promote peroxisome proliferation. These compounds were subsequently grouped into a family known as peroxisome proliferators. Peroxisome proliferators were shown to elicit biological function through binding to ligand-inducible nuclear hormone receptors, of which the first receptor, cloned from mouse liver, was termed peroxisome proliferator-activated receptor (PPAR) 2 (2). Upon ligand binding, PPARs become activated and bind to their target genes by forming heterodimeric complexes with retinoid-X receptors (RXR) (3, 4). The activated PPAR-RXR complex then binds to consensus peroxisome pr...

This article has been withdrawn by the authors. In this article, we reported that PPAR␤/␦ positively regulates myogenesis. After a thorough investigation by the Nanyang Technological University in Singapore, data falsifications have been found in some of the in vitro laboratory studies, which invalidate the results reported. Hence, the co-authors wish to withdraw this publication and offer our sincere apologies to all those investigators who may have been affected and misled by this.

(Cell Metabolism 16, 613–624; November 7, 2012) In the article, we reported that the expression of a mitochondrial specific E3 ligase, Mul1, was induced in response to wasting conditions, and that increased Mul1 expression caused mitophagy and muscle wasting. Following an investigation by Nanyang Technological University, it was determined that the first author, Sudarsanareddy Lokireddy, falsified data in Figures 2D, 4B, 6D, 6E, and S5B. Although other work has linked Mul1 with mitophagy, in order to protect the integrity of science as well as of our laboratory and institutes, we are retracting the paper. We sincerely apologize to our colleagues and readers for any adverse consequences that this may have caused. The co-authors agree with this statement, with the exception of the first author, Sudarsanareddy Lokireddy.