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The function of tissues and organs declines with aging. This is accompanied by a progressive decrease in the regenerative capacity of their adult stem cells, which are essential for maintenance of tissue homeostasis and for sustaining regeneration in response to damage or stress. Here we review the current literature on the potential causes leading to stem cell aging, and we discuss the emerging view according to which age-associated changes in extrinsic factors impinge on cell-intrinsic mechanisms and lead to stem cell dysfunction with aging. We will focus on skeletal muscle, mesenchymal, and hematopoietic stem cells, highlighting common mechanisms that mediate age-dependent dysfunctions and their current and future applications in the fight against aging.

Age-related decline in skeletal muscle regenerative capacity is multifactorial, yet, the contribution of immune dysfunction to regenerative failure is unknown. Here, we uncover a new mechanism of immune modulation operating during skeletal muscle regeneration that is disrupted in aged animals and relies on the regulation of macrophage function. The immune modulator MANF is induced following muscle injury in young mice, but not in aged animals, and its expression is essential for regenerative success. Regenerative impairments in the aged muscle are associated with defects in the repair-associated myeloid response similar to those found in MANF-deficient models and could be improved through MANF delivery. We propose that restoring the myeloid response through immune modulation is a promising therapeutic strategy to improve the regenerative capacity of aged muscles.

In adult epithelial stem cell lineages, the precise differentiation of daughter cells is critical to maintain tissue homeostasis. Notch signaling controls the choice between absorptive and entero-endocrine cell differentiation in both the mammalian small intestine and the Drosophila midgut, yet how Notch promotes lineage restriction remains unclear. Here, we describe a role for the transcription factor Klumpfuss (Klu) in restricting the fate of enteroblasts (EBs) in the Drosophila intestine. Klu is induced in Notch-positive EBs and its activity restricts cell fate towards the enterocyte (EC) lineage. Transcriptomics and DamID profiling show that Klu suppresses enteroendocrine (EE) fate by repressing the action of the proneural gene Scute, which is essential for EE differentiation. Loss of Klu results in differentiation of EBs into EE cells. Our findings provide mechanistic insight into how lineage commitment in progenitor cell differentiation can be ensured downstream of initial specification cues., Notch signaling mediates intestinal enteroblast specification in Drosophila but the molecular mechanism as to how this is regulated is unclear. Here, the authors show that the transcription factor Klumpfuss ensures enteroblast commitment through repression of enteroendocrine cell fate downstream of Notch.

© Springer Nature Limited 2021, Skeletal muscle contains a designated population of adult stem cells, called satellite cells, which are generally quiescent. In homeostasis, satellite cells proliferate only sporadically and usually by asymmetric cell division to replace myofibres damaged by daily activity and maintain the stem cell pool. However, satellite cells can also be robustly activated upon tissue injury, after which they undergo symmetric divisions to generate new stem cells and numerous proliferating myoblasts that later differentiate to muscle cells (myocytes) to rebuild the muscle fibre, thereby supporting skeletal muscle regeneration. Recent discoveries show that satellite cells have a great degree of population heterogeneity, and that their cell fate choices during the regeneration process are dictated by both intrinsic and extrinsic mechanisms. Extrinsic cues come largely from communication with the numerous distinct stromal cell types in their niche, creating a dynamically interactive microenvironment. This Review discusses the role and regulation of satellite cells in skeletal muscle homeostasis and regeneration. In particular, we highlight the cell-intrinsic control of quiescence versus activation, the importance of satellite cell–niche communication, and deregulation of these mechanisms associated with ageing. The increasing understanding of how satellite cells are regulated will help to advance muscle regeneration and rejuvenation therapies., ork in the authors’ laboratory was supported in part by the Spanish MINECO (RTI2018-096068), ERC-AdG-741966, LaCaixa-HEALTH-HR17-00040, MDA, UPGRADE-H2020-825825, MWRF, FundacióLaMaratóTV3, AFM and DPP-Spain (P.M.-C). P.M.-C’s laboratories at Pompeu Fabra University and the Spanish National Center for Cardiovascular Research are recipients of grants from the María de Maeztu Units of Excellence programme to Pompeu Fabra University (MDM-2014-0370) and the Severo Ochoa Centers of Excellence programme to the Spanish National Center for Cardiovascular Research (SEV-2015-0505) for P.M.C. Work in the P.S.-V. laboratory was supported by the Fundação para a Ciência e Tecnologia Project (PTDC/MED-OUT/8010/2020), an EMBO installation grant (IG4448) and the ‘la Caixa’ Foundation for a junior leader fellowship (LCF/BQ/PI19/11690006) to P.S.-V. L.G.-P. was supported by an EMBO long-term fellowship (ALTF 420-2017), a Benjamin Pearl fellowship, and a CIHR fellowship (201910MFE-430959-284655).

Aging is characterized by the functional and regenerative decline of tissues and organs. This regenerative decline is a consequence of the numerical and functional loss of adult stem cells, which are the corner stone of tissue homeostasis and repair. A palpable example of this decline is provided by skeletal muscle, a specialized tissue composed of postmitotic myofibers that contract to generate force. Skeletal muscle stem cells (satellite cells) are long-lived and support muscle regeneration throughout life, but at advanced age they fail for largely undefined reasons. Here, we discuss recent advances in the understanding of how satellite cells integrate diverse intrinsic and extrinsic processes to ensure optimal homeostatic function and how this integration is perturbed during aging, causing regenerative failure. With this increased understanding, it is now feasible to design and test interventions that delay satellite cell aging. We discuss the exciting new therapeutic potential of integrating and combining distinct anti-aging strategies for regenerative medicine.

Summary Sophisticated mechanisms that preserve genome integrity are critical to ensure the maintenance of regenerative capacity while preventing transformation of somatic stem cells (SCs), yet little is known about mechanisms regulating genome maintenance in these cells. Here, we show that intestinal stem cells (ISCs) induce the Argonaute family protein Piwi in response to JAK/STAT signaling during acute proliferative episodes. Piwi function is critical to ensure heterochromatin maintenance, suppress retrotransposon activation, and prevent DNA damage in homeostasis and under regenerative pressure. Accordingly, loss of Piwi results in the loss of actively dividing ISCs and their progenies by apoptosis. We further show that Piwi expression is sufficient to allay age-related retrotransposon expression, DNA damage, apoptosis, and mis-differentiation phenotypes in the ISC lineage, improving epithelial homeostasis. Our data identify a role for Piwi in the regulation of somatic SC function, and they highlight the importance of retrotransposon control in somatic SC maintenance.

Recent advances in our understanding of tissue regeneration and the development of efficient approaches to induce and differentiate pluripotent stem cells for cell replacement therapies promise exciting avenues for treating degenerative age-related diseases. However, clinical studies and insights from model organisms have identified major roadblocks that normal aging processes impose on tissue regeneration. These new insights suggest that specific targeting of environmental niche components, including growth factors, ECM, and immune cells, and intrinsic stem cell properties that are affected by aging will be critical for the development of new strategies to improve stem cell function and optimize tissue repair processes.

© The Author(s) 2020. Open Access: This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/., A unique property of skeletal muscle is its ability to adapt its mass to changes in activity. Inactivity, as in disuse or aging, causes atrophy, the loss of muscle mass and strength, leading to physical incapacity and poor quality of life. Here, through a combination of transcriptomics and transgenesis, we identify sestrins, a family of stress-inducible metabolic regulators, as protective factors against muscle wasting. Sestrin expression decreases during inactivity and its genetic deficiency exacerbates muscle wasting; conversely, sestrin overexpression suffices to prevent atrophy. This protection occurs through mTORC1 inhibition, which upregulates autophagy, and AKT activation, which in turn inhibits FoxO-regulated ubiquitin-proteasome-mediated proteolysis. This study reveals sestrin as a central integrator of anabolic and degradative pathways preventing muscle wasting. Since sestrin also protected muscles against aging-induced atrophy, our findings have implications for sarcopenia., The authors acknowledge funding from the Spanish Ministry of Science, Innovation and Universities, Spain (grants SAF2015-67369-R, RTI2018-096068-B-I00, and SAF 2015-70270-REDT, a María de Maeztu Unit of Excellence award to UPF [MDM-2014-0370], and the UPF-CNIC collaboration agreement, ERC-2016-AdG-741966, La Caixa-HEALTH (HR17-00040), MDA, UPGRADE-H2020-825825, AFM and DPP-E. The CNIC is supported by the Instituto de Salud Carlos III (ISCIII), the Ministerio de Ciencia, Innovación y Universidades (MCNU) and the Pro CNIC Foundation, and is a Severo Ochoa Center of Excellence (SEV-2015-0505). Work was also supported by the Ellison Medical Foundation (AG-SS-2440-10 to M.K. and AG-NS-0932-12 to J.H.L.), NIH (R01DK114131, R01DK111465, and R01DK102850 to J.H.L.), CARIPARO and H2020-MSCA-RISE-2014 (to M.S.), and the Russian Science Foundation (Grant 17-14-01420 to A.V.B.). J.S. acknowledges funding from a Juan de La Cierva Postdoctoral Fellowship.

Aging is characterized by the progressive dysfunction of most tissues and organs, which has been linked to the regenerative decline of their resident stem cells over time. Skeletal muscle provides a stark example of this decline. Its stem cells, also called satellite cells, sustain muscle regeneration throughout life, but at advanced age they fail for largely undefined reasons. Here, we discuss current understanding of the molecular processes regulating satellite cell maintenance throughout life and how age-related failure of these processes contributes to muscle aging. We also highlight the emerging field of rejuvenating biology to restore features of youthfulness in satellite cells, with the ultimate goal of slowing down or reversing the age-related decline in muscle regeneration. Work in the authors’ laboratories has been supported by the following funding sources: The Spanish Ministry of Science, Innovation and Universities, Spain (grants RTI2018-096068-B-I00 and SAF 2015-70270-REDT, a María de Maeztu Unit of Excellence award to UPF [MDM-2014-0370], and a Severo Ochoa Center of Excellence award to the CNIC [SEV-2015-0505]), ERC-2016-AdG-741966, La Caixa-HEALTH (HR17-00040), MDA, UPGRADE-H2020-825825, AFM and DPP-E to PMC; JN and PSV acknowledge support from iMM start-up funding and “la Caixa” Foundation for the Junior Leader Fellowship for PSV (LCF/BQ/PI19/11690006) Sí

Aging is accompanied by altered intercellular communication, deregulated metabolic function, and inflammation. Interventions that restore a youthful state delay or reverse these processes, prompting the search for systemic regulators of metabolic and immune homeostasis. Here we identify MANF, a secreted stress-response protein with immune modulatory properties, as an evolutionarily conserved regulator of systemic and in particular liver metabolic homeostasis. We show that MANF levels decline with age in flies, mice and humans, and MANF overexpression extends lifespan in flies. MANF deficient flies exhibit enhanced inflammation and shorter lifespans, and MANF heterozygous mice exhibit inflammatory phenotypes in various tissues, as well as progressive liver damage, fibrosis, and steatosis. We show that immune cell-derived MANF protects against liver inflammation and fibrosis, while hepatocyte-derived MANF prevents hepatosteatosis. Liver rejuvenation by heterochronic parabiosis in mice further depends on MANF, while MANF supplementation ameliorates several hallmarks of liver aging, prevents hepatosteatosis induced by diet, and improves age-related metabolic dysfunction. Our findings identify MANF as a systemic regulator of homeostasis in young animals, suggesting a therapeutic application for MANF in age-related metabolic diseases.

Stem cell (SC) lineages in barrier epithelia exhibit a high degree of plasticity. Mechanisms that govern the precise specification of SC daughter cells during regenerative episodes are therefore critical to maintain homeostasis. One such common mechanism is the transient activation of the Notch (N) signaling pathway. N controls the choice between absorptive and entero-endocrine cell fates in both the mammalian small intestine and theDrosophilamidgut, yet how precisely N signaling promotes lineage restriction in progenitor cells remains unclear. Here, we describe a role for the WT1-like transcription factor Klumpfuss (Klu) in restricting the fate ofDrosophilaenteroblasts (EBs) downstream of N activation. Klu is transiently induced in Notch-positive EBs and its transient activity restricts cell fate towards the enterocyte (EC) lineage. Transcriptomics and DamID profiling show that Klu suppresses enteroendocrine (EE) cell fates by repressing E(Spl)m8-HLH and Phyllopod, both negative regulators of the proneural gene Scute, which is essential for EE differentiation. At the same time, Klu suppresses cell cycle genes, committing EBs to differentiation. Klu-mediated repression of its own transcription further sets up a negative feedback loop that ensures temporal restriction of Klu-mediated gene regulation, and is essential for subsequent differentiation of ECs. Our findings define a transient cell state in which EC lineage restriction is cemented, and establish a hierarchy of transcriptional programs critical in executing a differentiation program downstream of initial induction events governed by N signaling.

Skeletal muscle mass and function decline with aging, a process known as sarcopenia, which restrains posture maintenance, mobility and quality of life in the elderly. Sarcopenia is also linked to a progressive reduction in the regenerative capacity of the skeletal muscle stem cells (satellite cells), which are critical for myofiber formation in early life stages and for sustaining repair in response to muscle damage or trauma. Here we will review the most recent findings on the causes underlying satellite cell functional decline with aging, and will discuss the prevalent view whereby age-associated extrinsic factor alterations impact negatively on satellite cell-intrinsic mechanisms, resulting in deficient muscle regeneration with aging. Further understanding of the interplay between satellite cell extrinsic and intrinsic factors in sarcopenia will facilitate therapies aimed at improving muscle repair in the increasing aging population.

© 2019 Federation of European Biochemical Societies., Aging is accompanied by a decline in physiological integrity and a loss of regenerative capacity in many tissues. The development of interventions that prevent or reverse age-related disease requires a better understanding of the interplay of cell intrinsic, inter-cellular communication and systemic deregulations that underlie the aging process. Immune dysfunction and changes in inflammatory pathways are transversal contributors to the aging process and are essential propagators of tissue deterioration. Here, we propose and discuss the rejuvenation potential of interventions that target chronic inflammation and how modulation of tissue repair capacity could be an important mediator of such anti-aging strategies. We highlight how current knowledge on the systemic nature of inflammatory dysregulation in old organisms, together with the development of new animal models that allow for the isolation of the inflammatory component of aging, could provide new targets for interventions in aging based on the modulation of inflammatory pathways., JN and PS-V acknowledge support from iMM start-up funding and “la caixa” Foundation for the Junior Leader Fellowship for PS-V (LCF/BQ/PI19/11690006)

Stem cells must preserve their function in order to sustain organ and tissue formation, homeostasis and repair. Adult stem cells, particularly those resident in tissues with little turnover, remain quiescent for most of their life, activating only in response to regenerative demands. Among the best studied long-lived quiescent stem cells are skeletal muscle stem cells, which are fully equipped to sustain repair in response to tissue trauma. Recent evidence indicates that the preservation of muscle stem-cell quiescence and regenerative capacity depends on intracellular networks linking metabolism and protein homeostasis. Here, we review recent research into how these networks control stem cell function and how their dysregulation contributes to aging, with a particular focus on senescence entry in extreme old age. We also discuss the implications of these new findings for anti-aging research in muscle stem-cell-based regenerative medicine. Work in the P.M.-C. lab is supported by MEIC (grant SAF2015-67369-R, a Maria de Maeztu Unit of Excellence award to UPF [MDM-2014-0370], and a Severo Ochoa Center of Excellence award to the CNIC [SEV-2015-0505]), the UPF-CNIC collaboration agreement, ERC-2016-ADG-741966, AFM, E-Rare/Eranet, MDA, Fundacio MaratoTV3, and DPP-E.P.S.-V. is supported by the Glenn Foundation for Medical Research. Laura Garcia-Prat is supported by an EMBO Postdoctoral Fellowship (ALTF 420-2017). The authors apologize for omitting citations to some studies owing to space restrictions. Sí

Regeneration of skeletal muscle relies on a population of quiescent stem cells (satellite cells) and is impaired in very old (geriatric) individuals undergoing sarcopenia. Stem cell function is essential for organismal homeostasis, providing a renewable source of cells to repair damaged tissues. In adult organisms, age-dependent loss-of-function of tissue-specific stem cells is causally related with a decline in regenerative potential. Although environmental manipulations have shown good promise in the reversal of these conditions, recently we demonstrated that muscle stem cell aging is, in fact, a progressive process that results in persistent and irreversible changes in stem cell intrinsic properties. Global gene expression analyses uncovered an induction of p16(INK4a) in satellite cells of physiologically aged geriatric and progeric mice that inhibits satellite cell-dependent muscle regeneration. Aged satellite cells lose the repression of the INK4a locus, which switches stem cell reversible quiescence into a pre-senescent state; upon regenerative or proliferative pressure, these cells undergo accelerated senescence (geroconversion), through Rb-mediated repression of E2F target genes. p16(INK4a) silencing rejuvenated satellite cells, restoring regeneration in geriatric and progeric muscles. Thus, p16(INK4a)/Rb-driven stem cell senescence is causally implicated in the intrinsic defective regeneration of sarcopenic muscle. Here we discuss on how cellular senescence may be a common mechanism of stem cell aging at the organism level and show that induction of p16(INK4a) in young muscle stem cells through deletion of the Polycomb complex protein Bmi1 recapitulates the geriatric phenotype.

Regeneration of skeletal muscle depends on a population of adult stem cells (satellite cells) that remain quiescent throughout life. Satellite cell regenerative functions decline with ageing. Here we report that geriatric satellite cells are incapable of maintaining their normal quiescent state in muscle homeostatic conditions, and that this irreversibly affects their intrinsic regenerative and self-renewal capacities. In geriatric mice, resting satellite cells lose reversible quiescence by switching to an irreversible pre-senescence state, caused by derepression of p16INK4a (also called Cdkn2a). On injury, these cells fail to activate and expand, undergoing accelerated entry into a full senescence state (geroconversion), even in a youthful environment. p16INK4a silencing in geriatric satellite cells restores quiescence and muscle regenerative functions. Our results demonstrate that maintenance of quiescence in adult life depends on the active repression of senescence pathways. As p16INK4a is dysregulated in human geriatric satellite cells, these findings provide the basis for stem-cell rejuvenation in sarcopenic muscles. This study shows that ageing satellite cells undergo an irreversible transition from a quiescent to a pre-senescent state that results in the loss of muscle regeneration in sarcopenia; furthermore, increased expression of p16INK4a is identified as a common feature of senescent satellite cells. One of the properties crucial to the function of adult mammalian stem cells is the capacity to remain in a quiescent state for prolonged periods — and to respond when the need to regenerate arises. Loss of skeletal muscle mass and function are common features of advanced ageing in humans, associated with a loss of regenerative capacity of the skeletal muscle stem cells, known as satellite cells. Pura Munoz-Canoves and colleagues show that ageing satellite cells undergo an irreversible transition from quiescence to a pre-senescence state associated with increased expression of p16INK4a, a tumour-suppressor protein that has been identified as a marker for senescence. Repression of p16INK4a during adult life is shown to maintain satellite cells in a reversible quiescence state that allows muscle regeneration; p16INK4a is dysregulated in human geriatric satellite cells and the potential for muscle regeneration is lost.

Aging of an organism is associated with the functional decline of tissues and organs, as well as a sharp decline in the regenerative capacity of stem cells. A prevailing view holds that the aging rate of an individual depends on the ratio of tissue attrition to tissue regeneration. Therefore, manipulations that favor the balance towards regeneration may prevent or delay aging. Skeletal muscle is a specialized tissue composed of postmitotic myofibers that contract to generate force. Satellite cells are the adult stem cells responsible for skeletal muscle regeneration. Recent studies on the biology of skeletal muscle and satellite cells in aging have uncovered the critical impact of systemic and niche factors on stem cell functionality and demonstrated the capacity of aged satellite cells to rejuvenate and increase their regenerative potential when exposed to a youthful environment. Here we review the current literature on the coordinated relationship between cell extrinsic and intrinsic factors that regulate the function of satellite cells, and ultimately determine tissue homeostasis and repair during aging, and which encourage the search for new anti-aging strategies.

MKP-1 limits p38 MAPK signaling in macrophages to promote the resolution of the inflammatory response and tissue repair in muscle through an AKT-dependent mechanism., Repair of damaged tissue requires the coordinated action of inflammatory and tissue-specific cells to restore homeostasis, but the underlying regulatory mechanisms are poorly understood. In this paper, we report new roles for MKP-1 (mitogen-activated protein kinase [MAPK] phosphatase-1) in controlling macrophage phenotypic transitions necessary for appropriate muscle stem cell–dependent tissue repair. By restricting p38 MAPK activation, MKP-1 allows the early pro- to antiinflammatory macrophage transition and the later progression into a macrophage exhaustion-like state characterized by cytokine silencing, thereby permitting resolution of inflammation as tissue fully recovers. p38 hyperactivation in macrophages lacking MKP-1 induced the expression of microRNA-21 (miR-21), which in turn reduced PTEN (phosphatase and tensin homologue) levels, thereby extending AKT activation. In the absence of MKP-1, p38-induced AKT activity anticipated the acquisition of the antiinflammatory gene program and final cytokine silencing in macrophages, resulting in impaired tissue healing. Such defects were reversed by temporally controlled p38 inhibition. Conversely, miR-21–AKT interference altered homeostasis during tissue repair. This novel regulatory mechanism involving the appropriate balance of p38, MKP-1, miR-21, and AKT activities may have implications in chronic inflammatory degenerative diseases.

Quiescent adult skeletal muscle stem cells (satellite cells) are the main players of myogenesis assuring the possibility of growth and regeneration of the muscle tissue throughout adult life. The environmental stimuli that activate satellite cells induce their proliferation, leading on one hand to self-renewal and maintenance of the muscle stem cell reservoir, and on the other hand to the production of progenitor cells that further proliferate, differentiate, and fuse to form new muscle fibers. Hence, satellite cells constitute a perfect system to study the transitions involved in stem cell differentiation. The multistep process of myogenesis is orchestrated by specific regulatory proteins, such as Pax3/Pax7 or members of the MyoD family of transcription factors. However, findings published over the past few years indicate that epigenetic mechanisms, such as covalent modification of histones, DNA methylation, or regulation of gene expression by microRNAs, also critically repress, maintain, or induce the muscle-specific transcriptional program during myogenesis. These studies have increased our understanding of how the information encoding the muscle lineage is molecularly controlled.

Adult skeletal muscle provides a unique paradigm for studying stem to differentiated cell transitions. In response to environmental stress, quiescent muscle stem cells (satellite cells) are activated and proliferative, at which stage they can either differentiate and fuse to form new muscle fibers or alternatively self-renew and maintain the muscle stem cell reservoir. This multi-step myogenic process is orchestrated by muscle regulatory proteins such as Pax3/Pax7 and members of the MyoD family of transcription factors. Findings published over the past few years have uncovered that epigenetic mechanisms critically repress, maintain or induce muscle-specific transcriptional programs during myogenesis. These studies are increasing our understanding of how muscle lineage-specific information encoded in chromatin merges with muscle regulatory factors to drive muscle stem cells through transitions during myogenesis.

INTRODUCTION Regenerative therapies based on cell replacement hold promise for the treatment of a range of age-related degenerative diseases but are limited by unfavorable microenvironments in degenerating tissues. A promising strategy to improve success is to harness endogenous repair mechanisms that promote tissue integrity and function. Innate immune cells are central to such repair mechanisms because they coordinate local and systemic responses to tissue injury by secreting inflammatory and anti-inflammatory signals in a context-dependent manner. A proper balance between these opposing phenotypes of innate immune cells is essential for efficient tissue repair, and immune modulation may be an effective way to promote repair and enhance regenerative therapies. Here, we identified a new evolutionarily conserved immune modulatory function for mesencephalic astrocyte-derived neurotrophic factor (MANF) that biases immune cells toward an anti-inflammatory phenotype, thereby promoting tissue repair in both vertebrates and invertebrates and enhancing retinal regenerative therapy. RATIONALE In Drosophila , interactions between damaged tissues and hemocytes are essential for tissue repair. We used this model to identify immune cell–derived factors with immune modulatory activity that promote tissue repair after retinal injury. The identification of MANF as such a factor prompted us to test its role in mammalian retinal repair and ask whether its immune modulatory activity helped cell replacement therapies in degenerating retinas. RESULTS Using a combination of transcriptome analysis and genetic studies, we identified MANF as a hemocyte-derived factor that is induced by platelet-derived growth factor (PDGF)– and vascular endothelial growth factor (VEGF)–related factor 1 (Pvf-1)/PDGF- and VEGF-receptor related (PvR) signaling. MANF was necessary and sufficient to promote retinal repair after ultraviolet-light–induced retinal injury in Drosophila. MANF also had an autocrine immune-modulatory function in fly hemocytes, which was necessary for its tissue repair–promoting activity. This regulation and function of MANF was evolutionarily conserved: Mouse photoreceptors expressed PDGF-A (a Pvf-1 homolog) in response to damage signals, which promoted MANF expression in innate immune cells. This PDGF-A/MANF signaling cascade was required to limit photoreceptor apoptosis in the retina. Exogenously supplied recombinant MANF protected photoreceptors in several paradigms of retinal injury and degeneration. As in flies, this prorepair function was associated with alternative activation of macrophages and microglia in the retina. Ablation of CD11b + immune cells and deletion of Cx3Cr1, a chemokine receptor required for MANF-induced alternative activation, prevented MANF-induced repair. Thus, the protective effects of MANF in retinal injury rely on its immune modulatory activity. Finally, MANF supplementation to photoreceptors transplanted into congenitally blind mice increased integration efficiency and accelerated and improved visual function recovery. CONCLUSION Combining genetic studies in invertebrates and vertebrates has rapidly identified factors with promising therapeutic potential. Immune modulation is a promising strategy to optimize regenerative therapies. With its conserved immune modulatory function, MANF is a particularly promising molecule that is likely to be useful for the treatment of inflammatory conditions in many different disease contexts. MANF in retinal repair. In Drosophila (left) or mouse (right), the damaged retina secretes Pvf-1/PDGF-A, which acts on innate immune cells. MANF derived from innate immune cells (and other sources) promotes phenotypic changes in immune cells as part of a mechanism required for tissue repair. Therapeutically, MANF supplementation can delay retinal degeneration and improve the success of cell-replacement regenerative therapies in the retina.

The p38 mitogen-activated protein kinase (MAPK) pathway plays a critical role in skeletal muscle differentiation. However, the relative contribution of the four p38 MAPKs (p38alpha, p38beta, p38gamma and p38delta) to this process is unknown. Here we show that myoblasts lacking p38alpha, but not those lacking p38beta or p38delta, are unable to differentiate and form multinucleated myotubes, whereas p38gamma-deficient myoblasts exhibit an attenuated fusion capacity. The defective myogenesis in the absence of p38alpha is caused by delayed cell-cycle exit and continuous proliferation in differentiation-promoting conditions. Indeed, activation of JNK/cJun was enhanced in p38alpha-deficient myoblasts leading to increased cyclin D1 transcription, whereas inhibition of JNK activity rescued the proliferation phenotype. Thus, p38alpha controls myogenesis by antagonizing the activation of the JNK proliferation-promoting pathway, before its direct effect on muscle differentiation-specific gene transcription. More importantly, in agreement with the defective myogenesis of cultured p38alpha(Delta/Delta) myoblasts, neonatal muscle deficient in p38alpha shows cellular hyperproliferation and delayed maturation. This study provides novel evidence of a fundamental role of p38alpha in muscle formation in vitro and in vivo.

Background: Extracellular stimuli induce gene expression responses through intracellular signaling mediators. The p38 signaling pathway is a paradigm of the mitogen-activated protein kinase (MAPK) family that, although originally identified as stress-response mediator, contributes to establishing stem cell differentiation fates. p38α is central for induction of the differentiation fate of the skeletal muscle stem cells (satellite cells) through not fully characterized mechanisms. Methods: To investigate the global gene transcription program regulated by p38α during satellite cell differentiation (myogenesis), and to specifically address whether this regulation occurs through direct action of p38α on gene promoters, we performed a combination of microarray gene expression and genome-wide binding analyses. For experimental robustness, two myogenic cellular systems with genetic and chemical loss of p38α function were used: (1) satellite cells derived from mice with muscle-specific deletion of p38α, and (2) the C2C12 murine myoblast cell line cultured in the absence or presence of the p38α/β inhibitor SB203580. Analyses were performed at cell proliferation and early differentiation stages. Results: We show that p38α binds to a large set of active promoters during the transition of myoblasts from proliferation to differentiation stages. p38α-bound promoters are enriched with binding motifs for several transcription factors, with Sp1, Tcf3/E47, Lef1, FoxO4, MyoD, and NFATc standing out in all experimental conditions. p38α association with chromatin correlates very well with high levels of transcription, in agreement with its classical function as an activator of myogenic differentiation. Interestingly, p38α also associates with genes repressed at the onset of differentiation, thus highlighting the relevance of p38-dependent chromatin regulation for transcriptional activation and repression during myogenesis. Conclusions: These results uncover p38α association and function on chromatin at novel classes of target genes during skeletal muscle cell differentiation. This is consistent with this MAPK isoform being a transcriptional regulator. The authors acknowledge funding from MINECO, Spain (SAF2012-38547, PLE2009-0124, SAF2015-67369-R, "María de Maeztu" Programme for Units of Excellence in R&D MDM-2014-0370), AFM, MDA, DDP-Netherlands, E-RARE, Fundació Marató TV3 and EU-FP7 (Myoage, Optistem and Endostem). JS is recipient of a Juan de la Cierva postdoctoral fellowship.

Aging is characterized by a progressive decline of physiological integrity leading to the loss of tissue function and vulnerability to disease, but its causes remain poorly understood. Skeletal muscle has an outstanding regenerative capacity that relies on its resident stem cells (satellite cells). This capacity declines with aging, and recent discoveries have redefined our view of why this occurs. Here, we discuss how an interconnection of extrinsic changes in the systemic and local environment and cell-intrinsic mechanisms might provoke failure of normal muscle stem cell functions with aging. We focus particularly on the emergent biology of rejuvenation of old satellite cells, including cells of geriatric age, by restoring traits of youthfulness, with the final goal of improving human health during aging.

Src family kinases (SFKs) have long been implicated in tumorigenesis, but the exact requirement for individual kinase members, and their specific spatially and temporally defined roles in the maintenance of epithelial homeostasis remained unclear. A study by Cordero et al (2014) now combines the strengths of Drosophila and mouse models for intestinal epithelial regeneration to characterize the role of individual SFKs in epithelial stem cells, tissue regeneration, and tumorigenesis.

Adult skeletal muscle is a very stable tissue containing a small population of myofiber-associated quiescent satellite cells compared with late embryonic/neonatal skeletal muscle, which contains highly proliferating myoblasts and small actively growing myofibers, suggesting that specific regulatory pathways may control myogenesis at distinct developmental stages. The p38 MAPK signaling pathway is central for myogenesis, based on studies using immortalized and neonatal primary myoblasts in vitro. However, the contribution of this pathway to adult myogenesis has never been investigated. Four p38 isoforms (p38alpha, p38beta, p38gamma and p38delta) exist in mammalian cells, being p38alpha and p38gamma the most abundantly expressed isoforms in adult skeletal muscle. Given the embryonic/neonatal lethality of p38alpha-deficient mice, here we investigate the relative contribution of p38beta, p38gamma and p38delta to adult myogenesis. Regeneration and myofiber growth of adult muscle proceeds with similar efficiency in mice lacking p38beta, p38gamma and p38delta as in wild-type control mice. In agreement with this, there is no difference in adult primary myoblasts behavior in vitro among the different genotypes. Importantly, the pattern of p38 activation (ascribed to p38alpha) remains unperturbed during satellite cell-mediated myogenesis in vitro and adult muscle regeneration in wild type and p38beta-, p38gamma- and p38delta-deficient mice, rendering p38alpha as the essential p38 isoform sustaining adult myogenesis. This study constitutes the first analysis addressing the functionality of p38beta, p38gamma and p38delta in satellite cell-dependent adult muscle regeneration and growth.

Over the past decade, our understanding of the molecular and cellular mechanisms presiding over cellular and tissue decline with aging has greatly advanced. Classical hallmarks of aging cell include increasing levels of reactive oxygen species, DNA damage and senescence entry, which disrupt tissue architecture and function. Tissue dysfunction with aging has been shown to correlate with a cellular switch from a G0 reversible quiescence state into a G0 irreversible senescence state (geroconversion), causing a permanent proliferative block. The TOR (target of rapamycin) kinase has been shown to promote geroconversion. Rapamycin and other rapalogs specifically suppress activity of the mammalian TOR (mTOR) complex 1 (mTORC1) -but not mTOR complex 2 (mTORC2)- and decrease senescence entry, thus preserving proliferative potential. In this perspective, we briefly comment recent progress of Leontieva and colleagues showing a new class of non-rapalog drugs that target simultaneously mTORC1 and mTORC2 and prevent geroconversion in a more efficient way than rapamycin. Its potential future use as rejuvenating, anti-aging therapeutics is therefore proposed.