The L27 Domain of MPP7 enhances TAZ-YY1 Cooperation to Renew Muscle Stem Cells.

Stem cells regenerate differentiated cells to maintain and repair tissues and organs. They also replenish themselves, i.e. self-renewal, for the regenerative process to last a lifetime. How stem cells renew is of critical biological and medical significance. Here we use the skeletal muscle stem cell (MuSC) to study this process. Using a combination of genetic, molecular, and biochemical approaches, we show that MPP7, AMOT, and TAZ/YAP form a complex that activates a common set of target genes. Among these targets, Carm1 can direct MuSC renewal. In the absence of MPP7, TAZ can support regenerative progenitors and activate Carm1 expression, but not to a level needed for self-renewal. Facilitated by the actin polymerization-responsive AMOT, TAZ recruits the L27 domain of MPP7 to up-regulate Carm1 to the level necessary to drive MuSC renewal. The promoter of Carm1, and those of other common downstream genes, also contain binding site(s) for YY1. We further demonstrate that the L27 domain of MPP7 enhances the interaction between TAZ and YY1 to activate Carm1. Our results define a renewal transcriptional program embedded within the progenitor program, by selectively up-regulating key gene(s) within the latter, through the combination of protein interactions and in a manner dependent on the promoter context.

TrkA-mediated sensory innervation of injured mouse tendon supports tendon sheath progenitor cell expansion and tendon repair.

Peripheral neurons terminate at the surface of tendons partly to relay nociceptive pain signals; however, the role of peripheral nerves in tendon injury and repair remains unclear. Here, we show that after Achilles tendon injury in mice, there is new nerve growth near tendon cells that express nerve growth factor (NGF). Conditional deletion of the Ngf gene in either myeloid or mesenchymal mouse cells limited both innervation and tendon repair. Similarly, inhibition of the NGF receptor tropomyosin receptor kinase A (TrkA) abrogated tendon healing in mouse tendon injury. Sural nerve transection blocked the postinjury increase in tendon sensory innervation and the expansion of tendon sheath progenitor cells (TSPCs) expressing tubulin polymerization promoting protein family member 3. Single cell and spatial transcriptomics revealed that disruption of sensory innervation resulted in dysregulated inflammatory signaling and transforming growth factor-ß (TGFß) signaling in injured mouse tendon. Culture of mouse TSPCs with conditioned medium from dorsal root ganglia neuron further supported a role for neuronal mediators and TGFß signaling in TSPC proliferation. Transcriptomic and histologic analyses of injured human tendon biopsy samples supported a role for innervation and TGFß signaling in human tendon regeneration. Last, treating mice after tendon injury systemically with a small-molecule partial agonist of TrkA increased neurovascular response, TGFß signaling, TSPC expansion, and tendon tissue repair. Although further studies should investigate the potential effects of denervation on mechanical loading of tendon, our results suggest that peripheral innervation is critical for the regenerative response after acute tendon injury.

The L27 Domain of MPP7 enhances TAZ-YY1 Cooperation to Renew Muscle Stem Cells.

Stem cells regenerate differentiated cells to maintain and repair tissues and organs. They also replenish themselves, i.e. self-renewal, for the regenerative process to last a lifetime. How stem cells renew is of critical biological and medical significance. Here we use the skeletal muscle stem cell (MuSC) to study this process. Using a combination of genetic, molecular, and biochemical approaches, we show that MPP7, AMOT, and TAZ/YAP form a complex that activates a common set of target genes. Among these targets, Carm1 can direct MuSC renewal. In the absence of MPP7, TAZ can support regenerative progenitors and activate Carm1 expression, but not to a level needed for self-renewal. Facilitated by the actin polymerization-responsive AMOT, TAZ recruits the L27 domain of MPP7 to up-regulate Carm1 to the level necessary to drive MuSC renewal. The promoter of Carm1, and those of other common downstream genes, also contain binding site(s) for YY1. We further demonstrate that the L27 domain of MPP7 enhances the interaction between TAZ and YY1 to activate Carm1. Our results define a renewal transcriptional program embedded within the progenitor program, by selectively up-regulating key gene(s) within the latter, through the combination of protein interactions and in a manner dependent on the promoter context.

Tppp3+ synovial/tendon sheath progenitor cells contribute to heterotopic bone after trauma.

Heterotopic ossification (HO) is a pathological process resulting in aberrant bone formation and often involves synovial lined tissues. During this process, mesenchymal progenitor cells undergo endochondral ossification. Nonetheless, the specific cell phenotypes and mechanisms driving this process are not well understood, in part due to the high degree of heterogeneity of the progenitor cells involved. Here, using a combination of lineage tracing and single-cell RNA sequencing (scRNA-seq), we investigated the extent to which synovial/tendon sheath progenitor cells contribute to heterotopic bone formation. For this purpose, Tppp3 (tubulin polymerization-promoting protein family member 3)-inducible reporter mice were used in combination with either Scx (Scleraxis) or Pdgfra (platelet derived growth factor receptor alpha) reporter mice. Both tendon injury- and arthroplasty-induced mouse experimental HO models were utilized. ScRNA-seq of tendon-associated traumatic HO suggested that Tppp3 is an early progenitor cell marker for either tendon or osteochondral cells. Upon HO induction, Tppp3 reporter+ cells expanded in number and partially contributed to cartilage and bone formation in either tendon- or joint-associated HO. In double reporter animals, both Pdgfra+Tppp3+ and Pdgfra+Tppp3- progenitor cells gave rise to HO-associated cartilage. Finally, analysis of human samples showed a substantial population of TPPP3-expressing cells overlapping with osteogenic markers in areas of heterotopic bone. Overall, these data demonstrate that synovial/tendon sheath progenitor cells undergo aberrant osteochondral differentiation and contribute to HO after trauma.

Mechanical compression creates a quiescent muscle stem cell niche.

Tissue stem cell niches are regulated by their mechanical environment, notably the extracellular matrix (ECM). Skeletal muscles consist of bundled myofibers for force transmission. Within this macroscopic architecture, quiescent Pax7-expressing (Pax7+) muscle stem cells (MuSCs) are compressed between ECM basally and myofiber apically. Muscle injury causes MuSCs to lose apical compression from the myofiber and re-enter the cell cycle for regeneration. While ECM elasticities have been shown to affect MuSC's renewal, the significance of apical compression remains unknown. To investigate the role of apical compression, we simulate the MuSCs' in vivo mechanical environment by applying physical compression to MuSCs' apical surface. We demonstrate that compression drives activated MuSCs back to a quiescent stem cell state, regardless of basal elasticities and chemistries. By mathematical modeling and cell tension manipulation, we conclude that low overall tension combined with high axial tension generated by compression leads to MuSCs' stemness and quiescence. Unexpectedly, we discovered that apical compression results in up-regulation of Notch downstream genes, accompanied by the increased levels of nuclear Notch1&3 in a Delta ligand (Dll) and ADAM10/17 independent manner. Our results fill a knowledge gap on the role of apical compression for MuSC fate and have implications to stem cells in other tissues.

Skeletal Muscle Satellite Cells Co-Opt the Tenogenic Gene Scleraxis to Instruct Regeneration.

Skeletal muscles connect bones and tendons for locomotion and posture. Understanding the regenerative processes of muscle, bone and tendon is of importance to basic research and clinical applications. Despite their interconnections, distinct transcription factors have been reported to orchestrate each tissue's developmental and regenerative processes. Here we show that Scx expression is not detectable in adult muscle stem cells (also known as satellite cells, SCs) during quiescence. Scx expression begins in activated SCs and continues throughout regenerative myogenesis after injury. By SC-specific Scx gene inactivation (ScxcKO), we show that Scx function is required for SC expansion/renewal and robust new myofiber formation after injury. We combined single-cell RNA-sequencing and CUT&RUN to identify direct Scx target genes during muscle regeneration. These target genes help explain the muscle regeneration defects of ScxcKO, and are not overlapping with Scx -target genes identified in tendon development. Together with a recent finding of a subpopulation of Scx -expressing connective tissue fibroblasts with myogenic potential during early embryogenesis, we propose that regenerative and developmental myogenesis co-opt the Scx gene via different mechanisms.

Extracellular fluid viscosity enhances cell migration and cancer dissemination.

Cells respond to physical stimuli, such as stiffness1, fluid shear stress2 and hydraulic pressure3,4. Extracellular fluid viscosity is a key physical cue that varies under physiological and pathological conditions, such as cancer5. However, its influence on cancer biology and the mechanism by which cells sense and respond to changes in viscosity are unknown. Here we demonstrate that elevated viscosity counterintuitively increases the motility of various cell types on two-dimensional surfaces and in confinement, and increases cell dissemination from three-dimensional tumour spheroids. Increased mechanical loading imposed by elevated viscosity induces an actin-related protein 2/3 (ARP2/3)-complex-dependent dense actin network, which enhances Na+/H+ exchanger 1 (NHE1) polarization through its actin-binding partner ezrin. NHE1 promotes cell swelling and increased membrane tension, which, in turn, activates transient receptor potential cation vanilloid 4 (TRPV4) and mediates calcium influx, leading to increased RHOA-dependent cell contractility. The coordinated action of actin remodelling/dynamics, NHE1-mediated swelling and RHOA-based contractility facilitates enhanced motility at elevated viscosities. Breast cancer cells pre-exposed to elevated viscosity acquire TRPV4-dependent mechanical memory through transcriptional control of the Hippo pathway, leading to increased migration in zebrafish, extravasation in chick embryos and lung colonization in mice. Cumulatively, extracellular viscosity is a physical cue that regulates both short- and long-term cellular processes with pathophysiological relevance to cancer biology.

Examining the lineage autonomous role of ß3-integrin in muscle regeneration.

Skeletal muscles can regenerate over the lifetime from resident muscle stem cells (MuSCs). Interactions between MuSCs and extracellular matrix (ECM) proteins are essential for muscle regeneration. The best-known receptors for ECM proteins are integrins, a family composed of twenty-some heterodimeric combinations of an α- and a ß-subunit. ß1-integrin (encoded by Itgb1) is required for quiescence, proliferation, migration, and fusion of Pax7+ MuSCs in the mouse model. ß3-integrin (encoded by Itgb3) has been reported to be critical for the myogenic differentiation of C2C12 myoblasts, and Itgb3 germline mutant mice were shown to regenerate few if any myofibers after injury. To investigate the autonomous role of Itgb3 in the myogenic lineage in vivo, we conditionally inactivated a floxed Itgb3 allele (Itgb3F ) by constitutive Pax7-Cre and tamoxifen-inducible Pax7-CreERT2 drivers. Unexpectedly, we found no defects in muscle regeneration in both conditional knockout models. In vitro studies using Itgb3 mutant myoblasts or RNAi knockdown of Itgb3 in myoblasts also did not reveal a role for myogenic differentiation. As ß1- and ß3-integrins share ECM ligands and downstream signaling effectors, we further examined Itgb3's role in a Itgb1 haploid background. Still, we found no evidence for an autonomous role of Itgb3 in muscle regeneration in vivo. Thus, while Itgb3 is critical for the differentiation of C2C12 cells, the regenerative defects reported for the Itgb3 germline mutant are not due to its role in the MuSC. We conclude that if ß3-integrin does have a role in Pax7+ MuSCs, it is compensated by ß1- and/or another ß-integrin(s).

Regulation of Myogenesis by a Na/K-ATPase α1 Caveolin-Binding Motif.

The N-terminal caveolin-binding motif (CBM) in Na/K-ATPase (NKA) α1 subunit is essential for cell signaling and somitogenesis in animals. To further investigate the molecular mechanism, we have generated CBM mutant human-induced pluripotent stem cells (iPSCs) through CRISPR/Cas9 genome editing and examined their ability to differentiate into skeletal muscle (Skm) cells. Compared with the parental wild-type human iPSCs, the CBM mutant cells lost their ability of Skm differentiation, which was evidenced by the absence of spontaneous cell contraction, marker gene expression, and subcellular myofiber banding structures in the final differentiated induced Skm cells. Another NKA functional mutant, A420P, which lacks NKA/Src signaling function, did not produce a similar defect. Indeed, A420P mutant iPSCs retained intact pluripotency and ability of Skm differentiation. Mechanistically, the myogenic transcription factor MYOD was greatly suppressed by the CBM mutation. Overexpression of a mouse Myod cDNA through lentiviral delivery restored the CBM mutant cells' ability to differentiate into Skm. Upstream of MYOD, Wnt signaling was demonstrated from the TOPFlash assay to have a similar inhibition. This effect on Wnt activity was further confirmed functionally by defective induction of the presomitic mesoderm marker genes BRACHYURY (T) and MESOGENIN1 (MSGN1) by Wnt3a ligand or the GSK3 inhibitor/Wnt pathway activator CHIR. Further investigation through immunofluorescence imaging and cell fractionation revealed a shifted membrane localization of ß-catenin in CBM mutant iPSCs, revealing a novel molecular component of NKA-Wnt regulation. This study sheds light on a genetic regulation of myogenesis through the CBM of NKA and control of Wnt/ß-catenin signaling.

Evolutionarily conserved inhibitory uORFs sensitize Hox mRNA translation to start codon selection stringency.

Translation start site selection in eukaryotes is influenced by context nucleotides flanking the AUG codon and by levels of the eukaryotic translation initiation factors eIF1 and eIF5. In a search of mammalian genes, we identified five homeobox (Hox) gene paralogs initiated by AUG codons in conserved suboptimal context as well as 13 Hox genes that contain evolutionarily conserved upstream open reading frames (uORFs) that initiate at AUG codons in poor sequence context. An analysis of published cap analysis of gene expression sequencing (CAGE-seq) data and generated CAGE-seq data for messenger RNAs (mRNAs) from mouse somites revealed that the 5' leaders of Hox mRNAs of interest contain conserved uORFs, are generally much shorter than reported, and lack previously proposed internal ribosome entry site elements. We show that the conserved uORFs inhibit Hox reporter expression and that altering the stringency of start codon selection by overexpressing eIF1 or eIF5 modulates the expression of Hox reporters. We also show that modifying ribosome homeostasis by depleting a large ribosomal subunit protein or treating cells with sublethal concentrations of puromycin leads to lower stringency of start codon selection. Thus, altering global translation can confer gene-specific effects through altered start codon selection stringency.

Sim1-expressing cells illuminate the origin and course of migration of the nucleus of the lateral olfactory tract in the mouse amygdala.

We focus this report on the nucleus of the lateral olfactory tract (NLOT), a superficial amygdalar nucleus receiving olfactory input. Mixed with its Tbr1-expressing layer 2 pyramidal cell population (NLOT2), there are Sim1-expressing cells whose embryonic origin and mode of arrival remain unclear. We examined this population with Sim1-ISH and a Sim1-tauLacZ mouse line. An alar hypothalamic origin is apparent at the paraventricular area, which expresses Sim1 precociously. This progenitor area shows at E10.5 a Sim1-expressing dorsal prolongation that crosses the telencephalic stalk and follows the terminal sulcus, reaching the caudomedial end of the pallial amygdala. We conceive this Sim1-expressing hypothalamo-amygdalar corridor (HyA) as an evaginated part of the hypothalamic paraventricular area, which participates in the production of Sim1-expressing cells. From E13.5 onwards, Sim1-expressing cells migrated via the HyA penetrate the posterior pallial amygdalar radial unit and associate therein to the incipient Tbr1-expressing migration stream which swings medially past the amygdalar anterior basolateral nucleus (E15.5), crosses the pallio-subpallial boundary (E16.5), and forms the NLOT2 within the anterior amygdala by E17.5. We conclude that the Tbr1-expressing NLOT2 cells arise strictly within the posterior pallial amygdalar unit, involving a variety of required gene functions we discuss. Our results are consistent with the experimental data on NLOT2 origin reported by Remedios et al. (Nat Neurosci 10:1141-1150, 2007), but we disagree on their implication in this process of the dorsal pallium, observed to be distant from the amygdala.

A microfluidic cell-migration assay for the prediction of progression-free survival and recurrence time of patients with glioblastoma.

Clinical scores, molecular markers and cellular phenotypes have been used to predict the clinical outcomes of patients with glioblastoma. However, their clinical use has been hampered by confounders such as patient co-morbidities, by the tumoral heterogeneity of molecular and cellular markers, and by the complexity and cost of high-throughput single-cell analysis. Here, we show that a microfluidic assay for the quantification of cell migration and proliferation can categorize patients with glioblastoma according to progression-free survival. We quantified with a composite score the ability of primary glioblastoma cells to proliferate (via the protein biomarker Ki-67) and to squeeze through microfluidic channels, mimicking aspects of the tight perivascular conduits and white-matter tracts in brain parenchyma. The assay retrospectively categorized 28 patients according to progression-free survival (short-term or long-term) with an accuracy of 86%, predicted time to recurrence and correctly categorized five additional patients on the basis of survival prospectively. RNA sequencing of the highly motile cells revealed differentially expressed genes that correlated with poor prognosis. Our findings suggest that cell-migration and proliferation levels can predict patient-specific clinical outcomes.

An unusual microbiome characterises a spatially-aggressive crustose alga rapidly overgrowing shallow Caribbean reefs.

Several species of crustose coralline algae (CCA) and their associated microbial biofilms play important roles in determining the settlement location of scleractinian corals on tropical reefs. In recent decades, peyssonnelid algal crusts (PAC) have become spatial dominants across large areas of shallow Caribbean reefs, where they appear to deter the recruitment of scleractinians. Our genetic investigations of PAC in St. John, US Virgin Islands, amplifying the large-subunit ribosomal RNA and psbA protein D1 marker genes, revealed them to be identical to Ramicrusta textilis previously reported overgrowing corals in Jamaica. Specimens of PAC sampled from the Honduras were likewise identical, confirming that this crustose alga inhabits the easternmost and westernmost regions of the Caribbean. We also analysed 16S rDNA tag amplicon libraries of the biofilms associated with PAC and sympatric CCA, which is favoured for coral settlement. Our results show that the microbial communities on PAC (vs. CCA) are characterized by significantly lower numbers of the epibiotic bacterial genus Pseudoalteromonas, which facilitates the recruitment and settlement of marine invertebrates. From these data, we infer that PAC are therefore unlikely to be attractive as settlement sites for coral larvae. Given the significant ecological change anticipated on these reefs due to increasing cover of PAC, there is an urgent need to further investigate competitive interactions between PAC and scleractinian corals, and elucidate the role of PAC and their associated microbiomes in accentuating phase shifts from coral to algae on tropical reefs.

Lineage dynamics of the endosymbiotic cell type in the soft coral Xenia.

Many corals harbour symbiotic dinoflagellate algae. The algae live inside coral cells in a specialized membrane compartment known as the symbiosome, which shares the photosynthetically fixed carbon with coral host cells while host cells provide inorganic carbon to the algae for photosynthesis1. This endosymbiosis-which is critical for the maintenance of coral reef ecosystems-is increasingly threatened by environmental stressors that lead to coral bleaching (that is, the disruption of endosymbiosis), which in turn leads to coral death and the degradation of marine ecosystems2. The molecular pathways that orchestrate the recognition, uptake and maintenance of algae in coral cells remain poorly understood. Here we report the chromosome-level genome assembly of a Xenia species of fast-growing soft coral3, and use this species as a model to investigate coral-alga endosymbiosis. Single-cell RNA sequencing identified 16 cell clusters, including gastrodermal cells and cnidocytes, in Xenia sp. We identified the endosymbiotic cell type, which expresses a distinct set of genes that are implicated in the recognition, phagocytosis and/or endocytosis, and maintenance of algae, as well as in the immune modulation of host coral cells. By coupling Xenia sp. regeneration and single-cell RNA sequencing, we observed a dynamic lineage progression of the endosymbiotic cells. The conserved genes associated with endosymbiosis that are reported here may help to reveal common principles by which different corals take up or lose their endosymbionts.

A Tppp3+Pdgfra+ tendon stem cell population contributes to regeneration and reveals a shared role for PDGF signalling in regeneration and fibrosis.

Tendon injuries cause prolonged disability and never recover completely. Current mechanistic understanding of tendon regeneration is limited. Here, we use single-cell transcriptomics to identify a tubulin polymerization-promoting protein family member 3-expressing (Tppp3+) cell population as potential tendon stem cells. Through inducible lineage tracing, we demonstrate that these cells can generate new tenocytes and self-renew upon injury. A fraction of Tppp3+ cells expresses platelet-derived growth factor receptor alpha (Pdfgra). Ectopic platelet-derived growth factor-AA (PDGF-AA) protein induces new tenocyte production while inactivation of Pdgfra in Tppp3+ cells blocks tendon regeneration. These results support Tppp3+Pdgfra+ cells as tendon stem cells. Unexpectedly, Tppp3-Pdgfra+ fibro-adipogenic progenitors coexist in the tendon stem cell niche and give rise to fibrotic cells, revealing a clandestine origin of fibrotic scars in healing tendons. Our results explain why fibrosis occurs in injured tendons and present clinical challenges to enhance tendon regeneration without a concurrent increase in fibrosis by PDGF application.

Muscle stem cell renewal suppressed by Gas1 can be reversed by GDNF in mice.

Muscle undergoes progressive weakening and regenerative dysfunction with age due in part to the functional decline of skeletal muscle stem cells (MuSCs). MuSCs are heterogeneous but whether their gene expression changes with age and the implication of such changes are unclear. Here we show that in mice, Growth arrest-specific gene 1 (Gas1) is expressed in a small subset of young MuSCs with its expression progressively increasing in larger fractions of MuSCs later in life. Over-expression of Gas1 in young MuSCs and inactivation of Gas1 in aged MuSCs support that Gas1 reduces the quiescence and self-renewal capacity of MuSCs. Gas1 reduces Ret signaling, which is required for MuSC quiescence and self-renewal. Indeed, we show that the Ret ligand, Glial Cell-Derived Neurotrophic Factor (GDNF), can counteract Gas1 by stimulating Ret signaling and enhancing MuSC self-renewal and regeneration, thus improving muscle function. We propose that strategies aimed to target this pathway can be exploited to improve the regenerative decline of muscle stem cells.

A CREB-MPP7-AMOT Regulatory Axis Controls Muscle Stem Cell Expansion and Self-Renewal Competence.

Skeletal muscle regeneration requires resident muscle stem cells, termed satellite cells (SCs). SCs are largely quiescent during homeostasis yet become activated upon injury to supply myonuclei and self-renewed SCs. Molecular mechanisms underlying the competence of SCs to proliferate and self-renew in response to injury remain unclear. Here, we show that CREB activity establishes proliferative potential during SC quiescence. SCs with inhibited CREB activity remain quiescent and positioned in their niche, but upon injury, they cannot enter or maintain a proliferative state for expansion and self-renewal. We demonstrate mechanistically that Mpp7 is a CREB target and its functional mediator. MPP7 loss affects the level and sub-cellular localization of AMOT and YAP1 in quiescent SCs. Furthermore, MPP7 and AMOT are required for YAP1 nuclear accumulation, and the three are individually required for a proliferative state in myoblasts. We propose that the CREB-MPP7-AMOT-YAP1 axis establishes the competence of quiescent SCs to expand and self-renew, thereby preserving stem cell function.

Genetic interactions between the hedgehog co-receptors Gas1 and Boc regulate cell proliferation during murine palatogenesis.

Abnormal regulation of Sonic hedgehog (Shh) signaling has been described in a variety of human cancers and developmental anomalies, which highlights the essential role of this signaling molecule in cell cycle regulation and embryonic development. Gas1 and Boc are membrane co-receptors for Shh, which demonstrate overlapping domains of expression in the early face. This study aims to investigate potential interactions between these co-receptors during formation of the secondary palate. Mice with targeted mutation in Gas1 and Boc were used to generate Gas1; Boc compound mutants. The expression of key Hedgehog signaling family members was examined in detail during palatogenesis via radioactive in situ hybridization. Morphometric analysis involved computational quantification of BrdU-labeling and cell packing; whilst TUNEL staining was used to assay cell death. Ablation of Boc in a Gas1 mutant background leads to reduced Shh activity in the palatal shelves and an increase in the penetrance and severity of cleft palate, associated with failed elevation, increased proliferation and reduced cell death. Our findings suggest a dual requirement for Boc and Gas1 during early development of the palate, mediating cell cycle regulation during growth and subsequent fusion of the palatal shelves.

Targeting ß1-integrin signaling enhances regeneration in aged and dystrophic muscle in mice.

Interactions between stem cells and their microenvironment, or niche, are essential for stem cell maintenance and function. Our knowledge of the niche for the skeletal muscle stem cell, i.e., the satellite cell (SC), is incomplete. Here we show that ß1-integrin is an essential niche molecule that maintains SC homeostasis, and sustains the expansion and self-renewal of this stem cell pool during regeneration. We further show that ß1-integrin cooperates with fibroblast growth factor 2 (Fgf2), a potent growth factor for SCs, to synergistically activate their common downstream effectors, the mitogen-activated protein (MAP) kinase Erk and protein kinase B (Akt). Notably, SCs in aged mice show altered ß1-integrin activity and insensitivity to Fgf2. Augmenting ß1-integrin activity with a monoclonal antibody restores Fgf2 sensitivity and improves regeneration after experimentally induced muscle injury. The same treatment also enhances regeneration and function of dystrophic muscles in mdx mice, a model for Duchenne muscular dystrophy. Therefore, ß1-integrin senses the SC niche to maintain responsiveness to Fgf2, and this integrin represents a potential therapeutic target for pathological conditions of the muscle in which the stem cell niche is compromised.

Loss of fibronectin from the aged stem cell niche affects the regenerative capacity of skeletal muscle in mice.

Age-related changes in the niche have long been postulated to impair the function of somatic stem cells. Here we demonstrate that the aged stem cell niche in skeletal muscle contains substantially reduced levels of fibronectin (FN), leading to detrimental consequences for the function and maintenance of muscle stem cells (MuSCs). Deletion of the gene encoding FN from young regenerating muscles replicates the aging phenotype and leads to a loss of MuSC numbers. By using an extracellular matrix (ECM) library screen and pathway profiling, we characterize FN as a preferred adhesion substrate for MuSCs and demonstrate that integrin-mediated signaling through focal adhesion kinase and the p38 mitogen-activated protein kinase pathway is strongly de-regulated in MuSCs from aged mice because of insufficient attachment to the niche. Reconstitution of FN levels in the aged niche remobilizes stem cells and restores youth-like muscle regeneration. Taken together, we identify the loss of stem cell adhesion to FN in the niche ECM as a previously unknown aging mechanism.