Exercise remains the most effective way to promote physical and metabolic wellbeing, but molecular mechanisms underlying exercise tolerance and its plasticity are only partially understood. In this study we identify musclin-a peptide with high homology to natriuretic peptides (NP)-as an exercise-responsive myokine that acts to enhance exercise capacity in mice. We use human primary myoblast culture and in vivo murine models to establish that the activity-related production of musclin is driven by Ca 2+ -dependent activation of Akt1 and the release of musclin-encoding gene (Ostn) transcription from forkhead box O1 transcription factor inhibition. Disruption of Ostn and elimination of musclin secretion in mice results in reduced exercise tolerance that can be rescued by treatment with recombinant musclin. Reduced exercise capacity in mice with disrupted musclin signaling is associated with a trend toward lower levels of plasma atrial NP (ANP) and significantly smaller levels of cyclic guanosine monophosphate (cGMP) and peroxisome proliferator-activated receptor gamma coactivator 1-α in skeletal muscles after exposure to exercise. Furthermore, in agreement with the established musclin ability to interact with NP clearance receptors, but not with NP guanyl cyclase-coupled signaling receptors, we demonstrate that musclin enhances cGMP production in cultured myoblasts only when applied together with ANP. Elimination of the activity-related musclin-dependent boost of ANP/ cGMP signaling results in significantly lower maximum aerobic capacity, mitochondrial protein content, respiratory complex protein expression, and succinate dehydrogenase activity in skeletal muscles. Together, these data indicate that musclin enhances physical endurance by promoting mitochondrial biogenesis.osteocrin | mitochondria | skeletal muscle | exercise | natriuretic peptide

g Lytic activation of Epstein-Barr virus (EBV) is central to its life cycle and to most EBV-related diseases. However, not every EBVinfected B cell is susceptible to lytic activation. This lack of uniform susceptibility to lytic activation also directly impacts the success of viral oncolytic therapy for EBV cancers, yet determinants of susceptibility to lytic induction signals are not well understood. To determine if host factors influence susceptibility to EBV lytic activation, we developed a technique to separate lytic from refractory cells and reported that EBV lytic activation occurs preferentially in cells with lower levels of signal transducer and activator of transcription 3 (STAT3). Using this tool to detect single cells, we now extend the correlation between STAT3 and lytic versus refractory states to EBV-infected circulating B cells in patients with primary EBV infection, leading us to investigate whether STAT3 controls susceptibility to EBV lytic activation. In loss-of-function and gain-of-function studies in EBV-positive B lymphoma and lymphoblastoid cells, we found that the levels of functional STAT3 regulate susceptibility to EBV lytic activation. This prompted us to identify a pool of candidate cellular genes that might be regulated by STAT3 to limit EBV lytic activation. From this pool, we confirmed increases in transcript levels in refractory cells of a set of genes known to participate in transcription repression. Taken together, our findings place STAT3 at a critical crossroads between EBV latency and lytic activation, processes fundamental to EBV lymphomagenesis. E pstein-Barr virus (EBV) infects most humans and persists silently in B lymphocytes. Primary infection with EBV can cause infectious mononucleosis (IM). Under certain circumstances, EBV can cause B-cell lymphomas and epithelial cell cancers (1). Several studies suggest that EBV lytic activation is an important step in the pathogenesis of such EBV-related diseases (2-5). From a therapeutic standpoint, efforts to eliminate EBV-positive tumors using nucleoside analogues after induction of viral lytic activation have shown promise (6-8). However, EBV-infected tumor cells are not fully permissive to lytic induction, as only a fraction of EBV-infected B cells exposed to lytic cycle-inducing agents enters the lytic cycle; the remainder of the population is refractory to lytic induction (9, 10). These refractory cells are not susceptible to oncolytic therapy, necessitating further investigations into the physiology underlying both lytic and refractory states.A general problem with investigating EBV lytic activation is that a mixed population of refractory and lytic cells results from exposure to lytic cycle-inducing stimuli. We therefore developed a technique to separate lytic cells from refractory cells in a mixed population of EBV-infected B cells (9). Our previous studies using this technique showed that host cell determinants regulate susceptibility of EBV-infected B cells to lytic cycle-inducing stimuli (9, 11) and that higher levels o...

DNA damage response (DDR) is a signaling network that senses DNA damage and activates response pathways to coordinate cellcycle progression and DNA repair. Thus, DDR is critical for maintenance of genome stability, and presents a powerful defense against tumorigenesis. Therefore, to drive cell-proliferation and transformation, viral and cellular oncogenes need to circumvent DDRinduced cell-cycle checkpoints. Unlike in hereditary cancers, mechanisms that attenuate DDR and disrupt cell-cycle checkpoints in sporadic cancers are not well understood. Using Epstein-Barr virus (EBV) as a source of oncogenes, we have previously shown that EBV-driven cell proliferation requires the cellular transcription factor STAT3. EBV infection is rapidly followed by activation and increased expression of STAT3, which mediates relaxation of the intra-S phase cell-cycle checkpoint; this facilitates viral oncogene-driven cell proliferation. We now show that replication stress-associated DNA damage, which results from EBV infection, is detected by DDR. However, signaling downstream of ATR is impaired by STAT3, leading to relaxation of the intra-S phase checkpoint. We find that STAT3 interrupts ATR-to-Chk1 signaling by promoting loss of Claspin, a protein that assists ATR to phosphorylate Chk1. This loss of Claspin which ultimately facilitates cell proliferation is mediated by caspase 7, a protein that typically promotes cell death. Our findings demonstrate how STAT3, which is constitutively active in many human cancers, suppresses DDR, fundamental to tumorigenesis. This newly recognized role for STAT3 in attenuation of DDR, discovered in the context of EBV infection, is of broad interest as the biology of cell proliferation is central to both health and disease.autosomal dominant hyper-IgE syndrome | infectious mononucleosis | latent membrane protein 1 | Epstein-Barr nuclear antigen