Aging disrupts MANF-mediated immune modulation during skeletal muscle regeneration.

Age-related decline in skeletal muscle regenerative capacity is multifactorial, yet the contribution of immune dysfunction to regenerative failure is unknown. Macrophages are essential for effective debris clearance and muscle stem cell activity during muscle regeneration, but the regulatory mechanisms governing macrophage function during muscle repair are largely unexplored. 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 mesencephalic astrocyte-derived neurotrophic factor (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 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 MANF levels is a viable strategy to improve myeloid response and regenerative capacity in aged muscle.

Control of satellite cell function in muscle regeneration and its disruption in ageing.

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.

Muscle stem cell aging: identifying ways to induce tissue rejuvenation.

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.

Sestrin prevents atrophy of disused and aging muscles by integrating anabolic and catabolic signals.

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.

Regulation of inflammation as an anti-aging intervention.

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.

Understanding muscle regenerative decline with aging: new approaches to bring back youthfulness to aged stem cells.

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.

MANF regulates metabolic and immune homeostasis in ageing and protects against liver damage.

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.

The WT1-like transcription factor Klumpfuss maintains lineage commitment of enterocyte progenitors in the Drosophila intestine.

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.

Trophic Factors in Inflammation and Regeneration: The Role of MANF and CDNF.

Regeneration is an important process in multicellular organisms, responsible for homeostatic renewal and repair of different organs after injury. Immune cell activation is observed at early stages of the regenerative response and its regulation is essential for regenerative success. Thus, immune regulators play central roles in optimizing regenerative responses. Neurotrophic factors (NTFs) are secreted molecules, defined by their ability to support neuronal cell types. However, emerging evidence suggests that they can also play important functions in the regulation of immune cell activation and tissue repair. Here we discuss the literature supporting a role of NTFs in the regulation of inflammation and regeneration. We will focus, in particular, in the emerging roles of mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) in the regulation of immune cell function and in the central role that immune modulation plays in their biological activity in vivo. Finally, we will discuss the potential use of these factors to optimize regenerative success in vivo, both within and beyond the nervous system.

New mechanisms driving muscle stem cell regenerative decline with aging.

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.

Piwi Is Required to Limit Exhaustion of Aging Somatic Stem Cells.

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.

Proteostatic and Metabolic Control of Stemness.

Adult stem cells, particularly those resident in tissues with little turnover, are largely quiescent and only activate in response to regenerative demands, while embryonic stem cells continuously replicate, suggesting profoundly different regulatory mechanisms within distinct stem cell types. In recent years, evidence linking metabolism, mitochondrial dynamics, and protein homeostasis (proteostasis) as fundamental regulators of stem cell function has emerged. Here, we discuss new insights into how these networks control potency, self-renewal, differentiation, and aging of highly proliferative embryonic stem cells and quiescent adult stem cells, with a focus on hematopoietic and muscle stem cells and implications for anti-aging research.

Rejuvenating Strategies for Stem Cell-Based Therapies in Aging.

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.

Immune modulation by MANF promotes tissue repair and regenerative success in the retina.

Regenerative therapies are limited by unfavorable environments in aging and diseased tissues. A promising strategy to improve success is to balance inflammatory and anti-inflammatory signals and enhance endogenous tissue repair mechanisms. Here, we identified a conserved immune modulatory mechanism that governs the interaction between damaged retinal cells and immune cells to promote tissue repair. In damaged retina of flies and mice, platelet-derived growth factor (PDGF)-like signaling induced mesencephalic astrocyte-derived neurotrophic factor (MANF) in innate immune cells. MANF promoted alternative activation of innate immune cells, enhanced neuroprotection and tissue repair, and improved the success of photoreceptor replacement therapies. Thus, immune modulation is required during tissue repair and regeneration. This approach may improve the efficacy of stem-cell-based regenerative therapies.

Chromatin-wide and transcriptome profiling integration uncovers p38α MAPK as a global regulator of skeletal muscle differentiation.

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/ß 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.

Regenerative decline of stem cells in sarcopenia.

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.

Dual mTORC1/C2 inhibitors: gerosuppressors with potential anti-aging effect.

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.

Muscle stem cell aging: regulation and rejuvenation.

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.

Geroconversion of aged muscle stem cells under regenerative pressure.

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.