Skeletal muscle aging results in a gradual loss of skeletal muscle mass, skeletal muscle function and decreased regenerative capacity, which can lead to sarcopenia and increased mortality. While the mechanisms underlying sarcopenia remain unclear, the skeletal muscle stem cell, or satellite cell, is required for muscle regeneration. Therefore, identification of signaling pathways affecting satellite cell function during aging may provide insights into therapeutic targets for combating sarcopenia. Here, we show that a cell-autonomous loss in self-renewal occurs via alterations in FGF Receptor 1 and p38αβ MAPK signaling in aged satellite cells. We further demonstrate that pharmacological manipulation of these pathways can ameliorate age-associated self-renewal defects. Thus, our data highlight an age-associated deregulation of a satellite cell homeostatic network and reveal potential therapeutic opportunities for the treatment of progressive muscle wasting.

Aging is characterized by the development of metabolic dysfunction and frailty. Recent studies show that a reduction in nicotinamide adenine dinucleotide (NAD) is a key factor for the development of age-associated metabolic decline. We recently demonstrated that the NADase CD38 has a central role in age-related NAD decline. Here we show that a highly potent and specific thiazoloquin(az)olin(on)e CD38 inhibitor, 78c, reverses age-related NAD decline and improves several physiological and metabolic parameters of aging, including glucose tolerance, muscle function, exercise capacity, and cardiac function in mouse models of natural and accelerated aging. The physiological effects of 78c depend on tissue NAD levels and were reversed by inhibition of NAD synthesis. 78c increased NAD levels, resulting in activation of pro-longevity and health span-related factors, including sirtuins, AMPK, and PARPs. Furthermore, in animals treated with 78c we observed inhibition of pathways that negatively affect health span, such as mTOR-S6K and ERK, and attenuation of telomere-associated DNA damage, a marker of cellular aging. Together, our results detail a novel pharmacological strategy for prevention and/or reversal of age-related NAD decline and subsequent metabolic dysfunction.

Topoisomerase poisons are chemotherapeutic agents that are used extensively for treating human malignancies. These drugs can be highly effective, yet tumors are frequently refractory to treatment or become resistant upon tumor relapse. Using a pool-based RNAi screening approach and a well characterized mouse model of lymphoma, we explored the genetic basis for heterogeneous responses to topoisomerase poisons in vitro and in vivo. These experiments identified Top2A expression levels as major determinants of response to the topoisomerase 2 poison doxorubicin and showed that suppression of Top2A produces resistance to doxorubicin in vitro and in vivo. Analogously, using a targeted RNAi approach, we demonstrated that suppression of Top1 produces resistance to the topoisomerase 1 poison camptothecin yet hypersensitizes cancer cells to doxorubicin. Importantly, lymphomas relapsing after treatment display spontaneous changes in topoisomerase levels as predicted by in vitro gene knockdown studies. These results highlight the utility of pooled shRNA screens for identifying genetic determinants of chemotherapy response and suggest strategies for improving the effectiveness of topoisomerase poisons in the clinic.A myriad of genetic factors influence the efficacy of cancer chemotherapy, including both somatic changes in the tumor itself as well as genetic polymorphisms present in the patient. These factors include increased expression of detoxification pumps that prevent access of the drug to its target (1), point mutations that disrupt the drug-target interaction (2, 3), and mutations in stress response pathways [e.g., p53 loss (4)]. To tailor treatment successfully to the individual patient, a more complete understanding of the genetic determinants of therapy response is necessary.RNA interference (RNAi) exploits a mechanism of gene regulation whereby double-stranded RNAs are processed by a conserved cellular machinery to suppress the expression of genes containing homologous sequences (5). Importantly, libraries of DNA-based vectors encoding short hairpin RNAs (shRNAs) capable of targeting most genes in the human and mouse genomes have been produced and enable forward genetic screens to be performed in mammalian cells. Indeed, by using human tumor-derived cell lines treated in vitro, RNAi has been used to evaluate potential drug targets (6) or to investigate mechanisms of drug action and drug resistance by screening for new molecules that modulate the response of tumor-derived cell lines to a given chemotherapeutic agent (7-10).Here, we evaluate the suitability of combining mouse models and RNAi to identify genetic modifiers of drug action in tumors in their natural site. Initially, we chose to investigate resistance to doxorubicin in the E -Myc mouse lymphoma system. Doxorubicin (Adriamycin) is an anthracycline DNA-damaging agent that exerts its effects primarily by targeting of the topoisomerase 2 activity and DNA intercalation (11). Along with etoposide and the camptothecin derivatives, doxorubicin is one of several t...

The development of cancer drug resistance is a persistent clinical problem limiting the successful treatment of disseminated malignancies. However, the molecular mechanisms by which initially chemoresponsive tumors develop therapeutic resistance remain poorly understood. Error-prone translesional DNA synthesis (TLS) is known to underlie the mutagenic effects of numerous anticancer agents, but little is known as to whether mutation induced by this process is ultimately relevant to tumor drug resistance. Here, we use a tractable mouse model of B-cell lymphoma to interrogate the role of error-prone translesional DNA synthesis in chemotherapyinduced mutation and resistance to front-line chemotherapy. We find that suppression of Rev1, an essential TLS scaffold protein and dCMP transferase, inhibits both cisplatin-and cyclophosphamideinduced mutagenesis. Additionally, by performing repeated cycles of tumor engraftment and treatment, we show that Rev1 plays a critical role in the development of acquired cyclophosphamide resistance. Thus, chemotherapy not only selects for drug-resistant tumor population but also directly promotes the TLS-mediated acquisition of resistance-causing mutations. These data provide an example of an alteration that prevents the acquisition of drug resistance in tumors in vivo. Because TLS also represents a critical mechanism of DNA synthesis in tumor cells following chemotherapy, these data suggest that TLS inhibition may have dual anticancer effects, sensitizing tumors to therapy as well as preventing the emergence of tumor chemoresistance.DNA polymerase | cancer | chemotherapy | relapse T he development of acquired chemoresistance is a persistent clinical problem limiting the successful treatment of disseminated malignancies. Tumors that relapse following initial treatment frequently are refractory to subsequent administration of the initial drug regimen as well as to distinct sets of chemotherapeutics. Although a number of key pathways have been implicated in resistance to conventional chemotherapeutics, including enhanced drug efflux, increased drug metabolism, drug inactivation, enhanced DNA repair, and defects in apoptosis programs (1, 2), the mechanisms by which tumors develop drug resistance-causing mutations remains unclear.At its core, acquired chemoresistance represents the emergence of subpopulations of drug-resistant tumor cells, a phenomenon rooted in the inherent genetic heterogeneity of the tumor itself. This heterogeneity may occur as a consequence of tumor genetic instability, a process known to underlie tumor development in numerous malignancies. Alternatively, cancer therapy itself may promote mutation and subsequent chemoresistance in relapsed tumors. Support for the latter hypothesis comes from several observations. First, conventional chemotherapeutics can be highly mutagenic (3). In fact, considerable work has gone into highlighting the mutagenic properties of platinum-based and other DNA adduct-forming chemotherapeutics as well as the genes that act in the cellular res...

Platinum-based chemotherapeutic drugs are front-line therapies for the treatment of non-small cell lung cancer. However, intrinsic drug resistance limits the clinical efficacy of these agents. Recent evidence suggests that loss of the translesion polymerase, Polζ, can sensitize tumor cell lines to cisplatin, although the relevance of these findings to the treatment of chemoresistant tumors in vivo has remained unclear. Here, we describe a tumor transplantation approach that enables the rapid introduction of defined genetic lesions into a preclinical model of lung adenocarcinoma. Using this approach, we examined the effect of impaired translesion DNA synthesis on cisplatin response in aggressive late-stage lung cancers. In the presence of reduced levels of Rev3, an essential component of Polζ, tumors exhibited pronounced sensitivity to cisplatin, leading to a significant extension in overall survival of treated recipient mice. Additionally, treated Rev3-deficient cells exhibited reduced cisplatin-induced mutation, a process that has been implicated in the induction of secondary malignancies following chemotherapy. Taken together, our data illustrate the potential of Rev3 inhibition as an adjuvant therapy for the treatment of chemoresistant malignancies, and highlight the utility of rapid transplantation methodologies for evaluating mechanisms of chemotherapeutic resistance in preclinical settings.

Altered stem cell homeostasis is linked to organismal aging. However, the mechanisms involved remain poorly understood. Here we report novel alterations in hair follicle stem cells during skin aging, including increased numbers, decreased function, and an inability to tolerate stress. Performing high-throughput RNA sequencing on aging stem cells, cytokine arrays, and functional assays, we identify an age-associated imbalance in epidermal Jak-Stat signaling that inhibits stem cell function. Collectively, this study reveals a role for the aging epidermis in the disruption of cytokine and stem cell homeostasis, suggesting that stem cell decline during aging may be part of broader tumor-suppressive mechanisms.[Keywords: stem cells; aging; epidermis; Jak-Stat] Supplemental material is available for this article. Adult tissue homeostasis requires continual replacement of cells lost due to normal turnover, injury, and disease. However, aging is accompanied by decreased tissue regeneration and homeostasis, both of which are frequently associated with impaired stem cell function. As some stem cell compartments undergo dramatic ageassociated changes (including alterations in cell number, decreased regenerative capacity, and fate change), a decline in stem cell function is suggested to contribute to the aging process (Conboy et al. 2003;Rossi et al. 2005;Molofsky et al. 2006). Therefore, characterizing the regulatory mechanisms that mediate normal stem cell aging is critical to understanding age-associated pathologies and disease.The skin is one of the most obvious tissues to undergo aging-associated phenotypical and functional changes, including decreased hair cycling, epidermal thinning, diminished sebaceous gland function, and an impaired wound response (Balin and Pratt 1989). Although some studies in mice suggest that epidermal stem cells are retained during aging (Stern and Bickenbach 2007;Giangreco et al. 2008), aged human keratinocyte stem cells exhibit decreased colony-forming ability (Barrandon and Green 1987), suggesting that undiscovered stem cell changes may be involved in epidermal aging. Indeed, the skin contains many individual, well-characterized stem cell populations, which not only highlights the complexity of the skin hierarchy, but also suggests that discrete populations may undergo age-associated changes that might ultimately impact tissue function. We set out to investigate this hypothesis using well-established murine hair follicle stem cell models. ResultsKeratin-15 (Krt-15)-positive hair follicle cells are one of the best-characterized stem cell populations in the skin. Specifically, studies using Krt-15 promoter reporter mouse models have demonstrated that these cells possess stem cell properties, as they are label-retaining cells with multipotent capacity, contributing to hair follicle cycling, sebaceous gland function, and wound repair (Morris et al. 2004;Ito et al. 2005;Petersson et al. 2011). In agreement with published work, whole-mount immunostaining for GFP in the tails of young Krt-15-GFP ...

Skeletal muscle satellite cells in their niche are quiescent and upon muscle injury, exit quiescence, proliferate to repair muscle tissue, and self-renew to replenish the satellite cell population. To understand the mechanisms involved in maintaining satellite cell quiescence, we identified gene transcripts that were differentially expressed during satellite cell activation following muscle injury. Transcripts encoding RNA binding proteins were among the most significantly changed and included the mRNA decay factor Tristetraprolin. Tristetraprolin promotes the decay of MyoD mRNA, which encodes a transcriptional regulator of myogenic commitment, via binding to the MyoD mRNA 3′ untranslated region. Upon satellite cell activation, p38α/β MAPK phosphorylates MAPKAP2 and inactivates Tristetraprolin, stabilizing MyoD mRNA. Satellite cell specific knockdown of Tristetraprolin precociously activates satellite cells in vivo, enabling MyoD accumulation, differentiation and cell fusion into myofibers. Regulation of mRNAs by Tristetraprolin appears to function as one of several critical post-transcriptional regulatory mechanisms controlling satellite cell homeostasis.DOI: http://dx.doi.org/10.7554/eLife.03390.001

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