Pedro Sousa-Victor

Geriatric muscle stem cells switch reversible quiescence into senescence

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.

Genetic analysis of p38 MAP kinases in myogenesis: fundamental role of p38α in abrogating myoblast proliferation

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 (p38α, p38β, p38γ and p38δ) to this process is unknown. Here we show that myoblasts lacking p38α, but not those lacking p38β or p38δ, are unable to differentiate and form multinucleated myotubes, whereas p38γ‐deficient myoblasts exhibit an attenuated fusion capacity. The defective myogenesis in the absence of p38α is caused by delayed cell‐cycle exit and continuous proliferation in differentiation‐promoting conditions. Indeed, activation of JNK/cJun was enhanced in p38α‐deficient myoblasts leading to increased cyclin D1 transcription, whereas inhibition of JNK activity rescued the proliferation phenotype. Thus, p38α 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 p38αΔ/Δ myoblasts, neonatal muscle deficient in p38α shows cellular hyperproliferation and delayed maturation. This study provides novel evidence of a fundamental role of p38α in muscle formation in vitro and in vivo.

p38/MKP-1–regulated AKT coordinates macrophage transitions and resolution of inflammation during tissue repair

The authors acknowledge funding from The Ministry of Science and Innovation (PLE2009-0124, SAF2009-09782, FIS-PS09/01267, and SAF2010-21682), Association Francaise contre les Myopathies, Fundacion Marato-TV3/R-Pascual, Muscular Dystrophy Association, and European Union Seventh Framework Programme (Myoage, Optistem, and Endostem). P. Sousa-Victor was supported by a predoctoral fellowship from Fundacao para a Ciencia e a Tecnologia

Epigenetic regulation of myogenesis

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.

Functional dysregulation of stem cells during aging: a focus on skeletal muscle stem cells

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.

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

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. 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.

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.

Efficient adult skeletal muscle regeneration in mice deficient in p38β, p38γ and p38δ MAP kinases

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 (p38α, p38β, p38γ and p38δ) exist in mammalian cells, being p38α and p38γ the most abundantly expressed isoforms in adult skeletal muscle. Given the embryonic/neonatal lethality of p38α-deficient mice, here we investigate the relative contribution of p38β, p38γ and p38δ to adult myogenesis. Regeneration and myofiber growth of adult muscle proceeds with similar efficiency in mice lacking p38β, p38γ and p38δ as in wild-type control mice. In agreement with this, there is no difference in adult satellite cell behavior in vitro among the different genotypes. Importantly, the pattern of p38 activation (ascribed to p38α) remains unperturbed during satellite myogenesis in vitro and adult muscle regeneration in wild type and p38β-, p38γ- and p38δ-deficient mice, rendering p38α as the essential p38 isoform sustaining adult myogenesis. This study constitutes the first analysis addressing the functionality of p38β, p38γ and p38δ in satellite cell-dependent adult muscle regeneration and growth.

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.

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.