Primary cilia on muscle stem cells are critical to maintain regenerative capacity and are lost during aging

Expression and Functional Analysis of the Smo Protein in Apis mellifera

Smoothened (Smo) is a critical component regulating the Hedgehog signaling pathway. However, whether Smo is associated with the modulation of olfactory recognition capabilities of bees remains unclear. In this study, we amplified Smo from Apis mellifera. The coding sequence of Smo was 2952 bp long, encoded 983 amino acids. Smo was most highly expressed in the antennae. Cyclopamine (200 μg/mL) significantly reduced but purmorphamine (800 μg/mL) significantly increased Smo expression (p < 0.05). OR152 and OR2 expression in the cyclopamine group significantly decreased, whereas OR152 expression in the purmorphamine group significantly increased (p < 0.05). A significant decrease in the relative values of electroantennography was observed in the cyclopamine group exposed to neral. Behavioral tests indicated a significant decrease in the attractive rates of neral, VUAA1, linalool, and methyl heptenone in the cyclopamine group. Conversely, the selection rates of linalool and methyl heptenone in the purmorphamine group significantly increased. Our findings indicate that Smo may play a role in modulating olfactory receptors in bees.

Interleukin 25 promotes muscle regeneration in sarcopenia by regulating macrophage-mediated Sonic Hedgehog signaling

OBJECTIVE

Sarcopenia manifests as a chronic, low-level inflammation along with multiple inflammatory cells infiltration. Interleukin (IL)-25 can regulate the function of macrophages. However, the specific role and mechanisms through which IL-25 functions in sarcopenia are still not fully understood and require further investigation.

METHODS

Aged mice were utilized as sarcopenia models and examined the expression of inflammatory factors. To investigate the effects of IL-25 on sarcopenia, the model mice received IL-25 treatment and underwent in vivo adoptive transfer of IL-25-induced macrophages. Meanwhile, RAW264.7 cells, bone marrow-derived macrophages, satellite cells and C2C12 cells were used in vitro. Shh insufficiency was induced through intramuscular administration of SHH-shRNA adenoviruses. Then, various assays including scratch wound, cell counting kit-8 and Transwell assays, as well as histological staining and molecular biological methods, were conducted.

RESULTS

Aged mice exhibited an accelerated decline in muscle strength and mass, along with an increased muscle lipid droplets and macrophage infiltration, and decreased IL-25 levels compared to the young group. IL-25 therapy and the transfer of IL-25-preconditioned macrophages could improve these conditions by promoting M2 macrophage polarization in vivo as well as in vitro. M2 macrophage conditioned medium enhanced satellite cell proliferation and migration, as well as the vitality, migration, and differentiation of C2C12 cells in vitro. Furthermore, IL-25 enhanced Shh expression in macrophages in vitro, and activated the Shh signaling pathway in muscle tissue of aged mice, which could be suppressed by either the inhibitor cyclopamine or Shh knockdown. Mechanistic studies showed that Shh insufficiency suppressed the activation of Akt/mTOR signaling pathway in muscle tissue of aged mice.

CONCLUSION

IL-25 promotes the secretion of Shh by M2 macrophages and activates the Shh/Akt/mTOR signaling pathway, which improves symptoms and function in sarcopenia mice. This suggests that IL-25 has potential as a therapeutic agent for treating sarcopenia.

Lin28a forms an RNA-binding complex with Igf2bp3 to regulate m6A-modified stress response genes in stress granules of muscle stem cells

In the early embryonic stages, Lin-28 homologue A (Lin28a) is highly expressed and declines as the embryo matures. As an RNA-binding protein, Lin28a maintains some adult muscle stem cells (MuSCs) in an embryonic-like state, but its RNA metabolism regulation mechanism remains unclear. BioGPS analysis revealed that Lin28a expression is significantly higher in muscle tissues than in other tissues. Lin28a-positive muscle stem cells (Lin28a+ MuSCs) were sorted from Lin28a-CreERT2; LSL-tdTomato mouse skeletal muscle tissue, which exhibited a higher proliferation rate than the control group. Lin28a-bound transcripts are enriched in various biological processes such as DNA repair, cell cycle, mitochondrial tricarboxylic acid cycle and oxidative stress response. The expression of insulin-like growth factor 2 mRNA-binding protein 3 (Igf2bp3) was markedly elevated in the presence of Lin28a. Co-immunoprecipitation analysis further demonstrated that Lin28a associates with Igf2bp3. Immunofluorescence analyses confirmed that Lin28a, Igf2bp3 and G3bp1 colocalize to form stress granules (SG), and N6-methyladenosine (m6A) modification promotes the formation of Lin28a-SG. Sequencing of the transcriptome and RNAs immunoprecipitated by Lin28a, Igf2bp3 and m6A antibodies in Lin28a+ MuSCs further revealed that Lin28a and Igf2bp3 collaboratively regulate the expression of DNA repair-related genes, including Fancm and Usp1. Lin28a stabilises Igf2bp3, Usp1, and Fancm mRNAs, enhancing DNA repair against oxidative or proteotoxic stress, thus promoting MuSCs self-renewal. Understanding the intricate mechanisms through which Lin28a and Igf2bp3 regulate MuSCs provides a deeper understanding of stem cell self-renewal, with potential implications for regenerative medicine.

The involvement of signaling pathways in the pathogenesis of osteoarthritis: An update

Osteoarthritis (OA) is one of the most common disabling pathologies, characterized by joint pain and reduced function, significantly worsening the quality of life. Even if important progresses have been made in OA research, little is yet known about the precise cellular and molecular mechanisms underlying OA. Understanding dysregulated signaling networks and their crosstalk in OA may offer a strong opportunity for the development of combined targeted therapies. Hence, this review highlights the recent findings on the main pathways involved in OA development, including Wnt, Notch, Hedgehog, MAPK, AMPK, and JAK/STAT, providing insights on current targeted therapies in OA patients' management.

The translational potential of this article

The identification of key signaling pathways involved in OA development and the investigation of their signaling crosstalk could pave the way for more effective treatments and improved management of OA patients in the future.

YAP/Aurora A-mediated ciliogenesis regulates ionizing radiation-induced senescence via Hedgehog pathway in tumor cells

Primary cilia are antenna-like organelles that play critical roles in sensing and responding to various signals. Nevertheless, the function of primary cilia in cellular response to ionizing radiation (IR) in tumor cells remains unclear. Here, we show that primary cilia are frequently expressed in tumor cells and tissues. Notably, IR promotes cilia formation and elongation in time- and dose-dependent manners. Mechanistic study shows that the suppression of YAP/Aurora A pathway contributes to IR-induced ciliogenesis, which is diminished by Aurora A overexpression. The ciliated tumor cells undergo senescence but not apoptosis in response to IR and the abrogation of cilia formation is sufficient to elevate the lethal effect of IR. Furthermore, we show that IR-induced ciliogenesis leads to the activation of Hedgehog signaling pathway to drive senescence and resist apoptosis, and its blockage enhances cellular radiosensitivity by switching senescence to apoptosis. In summary, this work shows evidence of primary cilia in coordinating cellular response to IR in tumor cells, which may help to supply a novel sensitizing target to improve the outcome of radiotherapy.

Cilia loss on distinct neuron populations differentially alters cocaine-induced locomotion and reward

BACKGROUND

Neuronal primary cilia are being recognized for their role in mediating signaling associated with a variety of neurobehaviors, including responses to drugs of abuse. They function as signaling hubs, enriched with a diverse array of G-protein coupled receptors (GPCRs), including several associated with motivation and drug-related behaviors. However, our understanding of how cilia regulate neuronal function and behavior is still limited.

AIMS

The objective of the current study was to investigate the contributions of primary cilia on specific neuronal populations to behavioral responses to cocaine.

METHODS

To test the consequences of cilia loss on cocaine-induced locomotion and reward-related behavior, we selectively ablated cilia from dopaminergic or GAD2-GABAergic neurons in mice.

RESULTS

Cilia ablation on either population of neurons failed to significantly alter acute locomotor responses to cocaine at a range of doses. With repeated administration, mice lacking cilia on GAD2-GABAergic neurons showed no difference in locomotor sensitization to cocaine compared to wild-type (WT) littermates, whereas mice lacking cilia on dopaminergic neurons exhibited reduced locomotor sensitization to cocaine at 10 and 30 mg/kg. Mice lacking cilia on GAD2-GABAergic neurons showed no difference in cocaine conditioned place preference (CPP), whereas mice lacking cilia on dopaminergic neurons exhibited reduced CPP compared to WT littermates.

CONCLUSIONS

Combined with previous findings using amphetamine, our results show that behavioral effects of cilia ablation are cell- and drug type-specific, and that neuronal cilia contribute to modulation of both the locomotor-inducing and rewarding properties of cocaine.

Regulation of Satellite Cells Functions during Skeletal Muscle Regeneration: A Critical Step in Physiological and Pathological Conditions

Skeletal muscle regeneration is a complex process involving the generation of new myofibers after trauma, competitive physical activity, or disease. In this context, adult skeletal muscle stem cells, also known as satellite cells (SCs), play a crucial role in regulating muscle tissue homeostasis and activating regeneration. Alterations in their number or function have been associated with various pathological conditions. The main factors involved in the dysregulation of SCs' activity are inflammation, oxidative stress, and fibrosis. This review critically summarizes the current knowledge on the role of SCs in skeletal muscle regeneration. It examines the changes in the activity of SCs in three of the most common and severe muscle disorders: sarcopenia, muscular dystrophy, and cancer cachexia. Understanding the molecular mechanisms involved in their dysregulations is essential for improving current treatments, such as exercise, and developing personalized approaches to reactivate SCs.

The muscle stem cell niche at a glance

Skeletal muscle stem cells (MuSCs, also called satellite cells) are the source of the robust regenerative capability of this tissue. The hallmark property of MuSCs at homeostasis is quiescence, a reversible state of cell cycle arrest required for long-term preservation of the stem cell population. MuSCs reside between an individual myofiber and an enwrapping basal lamina, defining the immediate MuSC niche. Additional cell types outside the basal lamina, in the interstitial space, also contribute to niche function. Quiescence is actively maintained by multiple niche-derived signals, including adhesion molecules presented from the myofiber surface and basal lamina, as well as soluble signaling factors produced by myofibers and interstitial cell types. In this Cell Science at a Glance article and accompanying poster, we present the most recent information on how niche signals promote MuSC quiescence and provide perspectives for further research.

Advanced glycation end products inhibit proliferation and primary cilia formation of myoblasts through receptor for advanced glycation end products pathway

The loss of skeletal muscle mass leads to various adverse conditions and shortened lifespan. The inhibition of myoblast proliferation is one of the causes that trigger muscle atrophy. Advanced glycation end products (AGEs) contribute to muscle atrophy. Since primary cilia are crucial organelles for proliferation, AGEs may inhibit primary cilia formation of myoblasts, thereby leading to impaired proliferation. Therefore, we aimed to clarify whether AGEs impeded the proliferation and formation of primary cilia of C2C12 skeletal muscle cells. AGE treatment inhibited the proliferation and formation of primary cilia. However, the inhibitor of the receptor for advanced glycosylation end products (RAGEs) abolished the inhibition of the proliferation and the primary cilia formation of C2C12 cells by AGEs, suggesting that AGEs cause these inhibitions through the RAGE pathway. In summary, our findings suggested that AGEs suppress the proliferation and formation of primary cilia of myoblasts through the RAGE pathway.

Aging Associates with Cilium Elongation and Dysfunction in Kidney and Pancreas

Cilia are best known and most studied for their manifold functions enabling proper embryonic development. Loss of cilia or dysfunction thereof results in a great variety of congenital malformations and syndromes. However, there are also cilia-driven conditions, which manifest only later in life, such as polycystic kidney disease. Even degenerative diseases in the central nervous system have recently been linked to alterations in cilia biology. Surprisingly though, there is very little knowledge regarding cilia in normally aged organisms absent any disease. Here, it is provided evidence that cilia in naturally aged mice are considerably elongated in the kidney and pancreas, respectively. Moreover, such altered cilia appear to have become dysfunctional as indicated by changes in cellular signaling.

Absence of the primary cilia formation gene Talpid3 impairs muscle stem cell function

Skeletal muscle stem cells (MuSC) are crucial for tissue homoeostasis and repair after injury. Following activation, they proliferate to generate differentiating myoblasts. A proportion of cells self-renew, re-enter the MuSC niche under the basal lamina outside the myofiber and become quiescent. Quiescent MuSC have a primary cilium, which is disassembled upon cell cycle entry. Ex vivo experiments suggest cilia are important for MuSC self-renewal, however, their requirement for muscle regeneration in vivo remains poorly understood. Talpid3 (TA3) is essential for primary cilia formation and Hedgehog (Hh) signalling. Here we use tamoxifen-inducible conditional deletion of TA3 in MuSC (iSC-KO) and show that regeneration is impaired in response to cytotoxic injury. Depletion of MuSC after regeneration suggests impaired self-renewal, also consistent with an exacerbated phenotype in TA3iSC-KO mice after repeat injury. Single cell transcriptomics of MuSC progeny isolated from myofibers identifies components of several signalling pathways, which are deregulated in absence of TA3, including Hh and Wnt. Pharmacological activation of Wnt restores muscle regeneration, while purmorphamine, an activator of the Smoothened (Smo) co-receptor in the Hh pathway, has no effect. Together, our data show that TA3 and primary cilia are important for MuSC self-renewal and pharmacological treatment can efficiently restore muscle regeneration.

Gli1 marks a sentinel muscle stem cell population for muscle regeneration

Adult skeletal muscle regeneration is mainly driven by muscle stem cells (MuSCs), which are highly heterogeneous. Although recent studies have started to characterize the heterogeneity of MuSCs, whether a subset of cells with distinct exists within MuSCs remains unanswered. Here, we find that a population of MuSCs, marked by Gli1 expression, is required for muscle regeneration. The Gli1+ MuSC population displays advantages in proliferation and differentiation both in vitro and in vivo. Depletion of this population leads to delayed muscle regeneration, while transplanted Gli1+ MuSCs support muscle regeneration more effectively than Gli1- MuSCs. Further analysis reveals that even in the uninjured muscle, Gli1+ MuSCs have elevated mTOR signaling activity, increased cell size and mitochondrial numbers compared to Gli1- MuSCs, indicating Gli1+ MuSCs are displaying the features of primed MuSCs. Moreover, Gli1+ MuSCs greatly contribute to the formation of GAlert cells after muscle injury. Collectively, our findings demonstrate that Gli1+ MuSCs represents a distinct MuSC population which is more active in the homeostatic muscle and enters the cell cycle shortly after injury. This population functions as the tissue-resident sentinel that rapidly responds to injury and initiates muscle regeneration.

Development of two mouse strains conditionally expressing bright luciferases with distinct emission spectra as new tools for in vivo imaging

In vivo bioluminescence imaging (BLI) has been an invaluable noninvasive method to visualize molecular and cellular behaviors in laboratory animals. Bioluminescent reporter mice harboring luciferases for general use have been limited to a classical luciferase, Luc2, from Photinus pyralis, and have been extremely powerful for various in vivo studies. However, applicability of reporter mice for in vivo BLI could be further accelerated by increasing light intensity through the use of other luciferases and/or by improving the biodistribution of their substrates in the animal body. Here we created two Cre-dependent reporter mice incorporating luciferases oFluc derived from Pyrocoeli matsumurai and Akaluc, both of which had been reported previously to be brighter than Luc2 when using appropriate substrates; we then tested their bioluminescence in neural tissues and other organs in living mice. When expressed throughout the body, both luciferases emitted an intense yellow (oFluc) or far-red (Akaluc) light easily visible to the naked eye. oFluc and Akaluc were similarly bright in the pancreas for in vivo BLI; however, Akaluc was superior to oFluc for brain imaging, because its substrate, AkaLumine-HCl, was distributed to the brain more efficiently than the oFluc substrate, D-luciferin. We also demonstrated that the lights produced by oFluc and Akaluc were sufficiently spectrally distinct from each other for dual-color imaging in a single living mouse. Taken together, these novel bioluminescent reporter mice are an ideal source of cells with bright bioluminescence and may facilitate in vivo BLI of various tissues/organs for preclinical and biomedical research in combination with a wide variety of Cre-driver mice.

Adult tissue-specific stem cell interaction: novel technologies and research advances

Adult tissue-specific stem cells play a dominant role in tissue homeostasis and regeneration. Various in vivo markers of adult tissue-specific stem cells have been increasingly reported by lineage tracing in genetic mouse models, indicating that marked cells differentiation is crucial during homeostasis and regeneration. How adult tissue-specific stem cells with indicated markers contact the adjacent lineage with indicated markers is of significance to be studied. Novel methods bring future findings. Recent advances in lineage tracing, synthetic receptor systems, proximity labeling, and transcriptomics have enabled easier and more accurate cell behavior visualization and qualitative and quantitative analysis of cell-cell interactions than ever before. These technological innovations have prompted researchers to re-evaluate previous experimental results, providing increasingly compelling experimental results for understanding the mechanisms of cell-cell interactions. This review aimed to describe the recent methodological advances of dual enzyme lineage tracing system, the synthetic receptor system, proximity labeling, single-cell RNA sequencing and spatial transcriptomics in the study of adult tissue-specific stem cells interactions. An enhanced understanding of the mechanisms of adult tissue-specific stem cells interaction is important for tissue regeneration and maintenance of homeostasis in organisms.

Sox11 is enriched in myogenic progenitors but dispensable for development and regeneration of the skeletal muscle

Transcription factors (TFs) play key roles in regulating differentiation and function of stem cells, including muscle satellite cells (MuSCs), a resident stem cell population responsible for postnatal regeneration of the skeletal muscle. Sox11 belongs to the Sry-related HMG-box (SOX) family of TFs that play diverse roles in stem cell behavior and tissue specification. Analysis of single-cell RNA-sequencing (scRNA-seq) datasets identify a specific enrichment of Sox11 mRNA in differentiating but not quiescent MuSCs. Consistent with the scRNA-seq data, Sox11 levels increase during differentiation of murine primary myoblasts in vitro. scRNA-seq data comparing muscle regeneration in young and old mice further demonstrate that Sox11 expression is reduced in aged MuSCs. Age-related decline of Sox11 expression is associated with reduced chromatin contacts within the topologically associating domains. Unexpectedly, Myod1Cre-driven deletion of Sox11 in embryonic myoblasts has no effects on muscle development and growth, resulting in apparently healthy muscles that regenerate normally. Pax7CreER- or Rosa26CreER- driven (MuSC-specific or global) deletion of Sox11 in adult mice similarly has no effects on MuSC differentiation or muscle regeneration. These results identify Sox11 as a novel myogenic differentiation marker with reduced expression in quiescent and aged MuSCs, but the specific function of Sox11 in myogenesis remains to be elucidated.

In vitro-generated human muscle reserve cells are heterogeneous for Pax7 with distinct molecular states and metabolic profiles

BACKGROUND

The capacity of skeletal muscles to regenerate relies on Pax7+ muscle stem cells (MuSC). While in vitro-amplified MuSC are activated and lose part of their regenerative capacity, in vitro-generated human muscle reserve cells (MuRC) are very similar to quiescent MuSC with properties required for their use in cell-based therapies.

METHODS

In the present study, we investigated the heterogeneity of human MuRC and characterized their molecular signature and metabolic profile.

RESULTS

We observed that Notch signaling is active and essential for the generation of quiescent human Pax7+ MuRC in vitro. We also revealed, by immunofluorescence and flow cytometry, two distinct subpopulations of MuRC distinguished by their relative Pax7 expression. After 48 h in differentiation medium (DM), the Pax7High subpopulation represented 35% of the total MuRC pool and this percentage increased to 61% after 96 h in DM. Transcriptomic analysis revealed that Pax7High MuRC were less primed for myogenic differentiation as compared to Pax7Low MuRC and displayed a metabolic shift from glycolysis toward fatty acid oxidation. The bioenergetic profile of human MuRC displayed a 1.5-fold decrease in glycolysis, basal respiration and ATP-linked respiration as compared to myoblasts. We also observed that AMPKα1 expression was significantly upregulated in human MuRC that correlated with an increased phosphorylation of acetyl-CoA carboxylase (ACC). Finally, we showed that fatty acid uptake was increased in MuRC as compared to myoblasts, whereas no changes were observed for glucose uptake.

CONCLUSIONS

Overall, these data reveal that the quiescent MuRC pool is heterogeneous for Pax7 with a Pax7High subpopulation being in a deeper quiescent state, less committed to differentiation and displaying a reduced metabolic activity. Altogether, our data suggest that human Pax7High MuRC may constitute an appropriate stem cell source for potential therapeutic applications in skeletal muscle diseases.

New insights into the centrosome-associated spliceosome components as regulators of ciliogenesis and tissue identity

Biomolecular condensates are membrane-less assemblies of proteins and nucleic acids. Centrosomes are biomolecular condensates that play a crucial role in nuclear division, cytoskeletal remodeling, and cilia formation in animal cells. Spatial omics technology is providing new insights into the dynamic exchange of spliceosome components between the nucleus and the centrosome/cilium. Intriguingly, centrosomes are emerging as cytoplasmic sites for information storage, enriched with RNA molecules and RNA-processing proteins. Furthermore, growing evidence supports the view that nuclear spliceosome components assembled at the centrosome function as potential coordinators of splicing subprograms, pluripotency, and cell differentiation. In this article, we first discuss the current understanding of the centrosome/cilium complex, which controls both stem cell differentiation and pluripotency. We next explore the molecular mechanisms that govern splicing factor assembly and disassembly around the centrosome and examine how RNA processing pathways contribute to ciliogenesis. Finally, we discuss numerous unresolved compelling questions regarding the centrosome-associated spliceosome components and transcript variants within the cytoplasm as sources of RNA-based secondary messages in the regulation of cell identity and cell fate determination. This article is categorized under: RNA-Based Catalysis > RNA Catalysis in Splicing and Translation RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Processing > Splicing Regulation/Alternative Splicing RNA Processing > RNA Processing.

Neural cell state shifts and fate loss in ageing and age-related diseases

Most age-related neurodegenerative diseases remain incurable owing to an incomplete understanding of the disease mechanisms. Several environmental and genetic factors contribute to disease onset, with human biological ageing being the primary risk factor. In response to acute cellular damage and external stimuli, somatic cells undergo state shifts characterized by temporal changes in their structure and function that increase their resilience, repair cellular damage, and lead to their mobilization to counteract the pathology. This basic cell biological principle also applies to human brain cells, including mature neurons that upregulate developmental features such as cell cycle markers or glycolytic reprogramming in response to stress. Although such temporary state shifts are required to sustain the function and resilience of the young human brain, excessive state shifts in the aged brain might result in terminal fate loss of neurons and glia, characterized by a permanent change in cell identity. Here, we offer a new perspective on the roles of cell states in sustaining health and counteracting disease, and we examine how cellular ageing might set the stage for pathological fate loss and neurodegeneration. A better understanding of neuronal state and fate shifts might provide the means for a controlled manipulation of cell fate to promote brain resilience and repair.

Hedgehog signaling via its ligand DHH acts as cell fate determinant during skeletal muscle regeneration

Successful muscle regeneration relies on the interplay of multiple cell populations. However, the signals required for this coordinated intercellular crosstalk remain largely unknown. Here, we describe how the Hedgehog (Hh) signaling pathway controls the fate of fibro/adipogenic progenitors (FAPs), the cellular origin of intramuscular fat (IMAT) and fibrotic scar tissue. Using conditional mutagenesis and pharmacological Hh modulators in vivo and in vitro, we identify DHH as the key ligand that acts as a potent adipogenic brake by preventing the adipogenic differentiation of FAPs. Hh signaling also impacts muscle regeneration, albeit indirectly through induction of myogenic factors in FAPs. Our results also indicate that ectopic and sustained Hh activation forces FAPs to adopt a fibrogenic fate resulting in widespread fibrosis. In this work, we reveal crucial post-developmental functions of Hh signaling in balancing tissue regeneration and fatty fibrosis. Moreover, they provide the exciting possibility that mis-regulation of the Hh pathway with age and disease could be a major driver of pathological IMAT formation.

Polycomb Ezh1 maintains murine muscle stem cell quiescence through non-canonical regulation of Notch signaling

Organismal homeostasis and regeneration are predicated on committed stem cells that can reside for long periods in a mitotically dormant but reversible cell-cycle arrest state defined as quiescence. Premature escape from quiescence is detrimental, as it results in stem cell depletion, with consequent defective tissue homeostasis and regeneration. Here, we report that Polycomb Ezh1 confers quiescence to murine muscle stem cells (MuSCs) through a non-canonical function. In the absence of Ezh1, MuSCs spontaneously exit quiescence. Following repeated injuries, the MuSC pool is progressively depleted, resulting in failure to sustain proper muscle regeneration. Rather than regulating repressive histone H3K27 methylation, Ezh1 maintains gene expression of the Notch signaling pathway in MuSCs. Selective genetic reconstitution of the Notch signaling corrects stem cell number and re-establishes quiescence of Ezh1-/- MuSCs.

Primary cilia support cartilage regeneration after injury

In growing children, growth plate cartilage has limited self-repair ability upon fracture injury always leading to limb growth arrest. Interestingly, one type of fracture injuries within the growth plate achieve amazing self-healing, however, the mechanism is unclear. Using this type of fracture mouse model, we discovered the activation of Hedgehog (Hh) signaling in the injured growth plate, which could activate chondrocytes in growth plate and promote cartilage repair. Primary cilia are the central transduction mediator of Hh signaling. Notably, ciliary Hh-Smo-Gli signaling pathways were enriched in the growth plate during development. Moreover, chondrocytes in resting and proliferating zone were dynamically ciliated during growth plate repair. Furthermore, conditional deletion of the ciliary core gene Ift140 in cartilage disrupted cilia-mediated Hh signaling in growth plate. More importantly, activating ciliary Hh signaling by Smoothened agonist (SAG) significantly accelerated growth plate repair after injury. In sum, primary cilia mediate Hh signaling induced the activation of stem/progenitor chondrocytes and growth plate repair after fracture injury.

Biomarkers of aging

Aging biomarkers are a combination of biological parameters to (i) assess age-related changes, (ii) track the physiological aging process, and (iii) predict the transition into a pathological status. Although a broad spectrum of aging biomarkers has been developed, their potential uses and limitations remain poorly characterized. An immediate goal of biomarkers is to help us answer the following three fundamental questions in aging research: How old are we? Why do we get old? And how can we age slower? This review aims to address this need. Here, we summarize our current knowledge of biomarkers developed for cellular, organ, and organismal levels of aging, comprising six pillars: physiological characteristics, medical imaging, histological features, cellular alterations, molecular changes, and secretory factors. To fulfill all these requisites, we propose that aging biomarkers should qualify for being specific, systemic, and clinically relevant.

The cilium-centrosome axis in coupling cell cycle exit and cell fate

The centrosome is an evolutionarily conserved, ancient organelle whose role in cell division was first described over a century ago. The structure and function of the centrosome as a microtubule-organizing center, and of its extracellular extension - the primary cilium - as a sensory antenna, have since been extensively studied, but the role of the cilium-centrosome axis in cell fate is still emerging. In this Opinion piece, we view cellular quiescence and tissue homeostasis from the vantage point of the cilium-centrosome axis. We focus on a less explored role in the choice between distinct forms of mitotic arrest - reversible quiescence and terminal differentiation, which play distinct roles in tissue homeostasis. We outline evidence implicating the centrosome-basal body switch in stem cell function, including how the cilium-centrosome complex regulates reversible versus irreversible arrest in adult skeletal muscle progenitors. We then highlight exciting new findings in other quiescent cell types that suggest signal-dependent coupling of nuclear and cytoplasmic events to the centrosome-basal body switch. Finally, we propose a framework for involvement of this axis in mitotically inactive cells and identify future avenues for understanding how the cilium-centrosome axis impacts central decisions in tissue homeostasis.

Primary cilia shape hallmarks of health and aging

Primary cilia are specialized organelles that sense changes in extracellular milieu, and their malfunction is responsible for several disorders (ciliopathies). Increasing evidence shows that primary cilia regulate tissue and cellular aging related features, which led us to review the evidence on their role in potentiating and/or accelerating the aging process. Primary cilia malfunction is associated with some age-related disorders, from cancer to neurodegenerative and metabolic disorders. However, there is limited understanding of molecular pathways underlying primary cilia dysfunction, resulting in scarce ciliary-targeted therapies available. Here, we discuss the findings on primary cilia dysfunction as modulators of the health and aging hallmarks, and the pertinence of ciliary pharmacological targeting to promote healthy aging or treat age-related diseases.

Sox11 is enriched in myogenic progenitors but dispensable for development and regeneration of skeletal muscle

Transcription factors (TFs) play key roles in regulating the differentiation and function of stem cells, including muscle satellite cells (MuSCs), a resident stem cell population responsible for postnatal regeneration of the skeletal muscle. Sox11 belongs to the Sry-related HMG-box (SOX) family of TFs that play diverse roles in stem cell behavior and tissue specification. Analysis of single-cell RNA-sequencing (scRNA-seq) datasets identify a specific enrichment of Sox11 mRNA in differentiating but not quiescent MuSCs. Consistent with the scRNA-seq data, Sox11 levels increase during differentiation of murine primary myoblasts in vitro. scRNA-seq data comparing muscle regeneration in young and old mice further demonstrate that Sox11 expression is reduced in aged MuSCs. Age-related decline of Sox11 expression is associated with reduced chromatin contacts within the topologically associated domains. Unexpectedly, Myod1 Cre -driven deletion of Sox11 in embryonic myoblasts has no effects on muscle development and growth, resulting in apparently healthy muscles that regenerate normally. Pax7 CreER or Rosa26 CreER driven (MuSC-specific or global) deletion of Sox11 in adult mice similarly has no effects on MuSC differentiation or muscle regeneration. These results identify Sox11 as a novel myogenic differentiation marker with reduced expression in quiescent and aged MuSCs, but the specific function of Sox11 in myogenesis remain to be elucidated.

Three-dimensional chromatin re-organization during muscle stem cell aging

Age-related skeletal muscle atrophy or sarcopenia is a significant societal problem that is becoming amplified as the world's population continues to increase. The regeneration of damaged skeletal muscle is mediated by muscle stem cells, but in old age muscle stem cells become functionally attenuated. The molecular mechanisms that govern muscle stem cell aging encompass changes across multiple regulatory layers and are integrated by the three-dimensional organization of the genome. To quantitatively understand how hierarchical chromatin architecture changes during muscle stem cell aging, we generated 3D chromatin conformation maps (Hi-C) and integrated these datasets with multi-omic (chromatin accessibility and transcriptome) profiles from bulk populations and single cells. We observed that muscle stem cells display static behavior at global scales of chromatin organization during aging and extensive rewiring of local contacts at finer scales that were associated with variations in transcription factor binding and aberrant gene expression. These data provide insights into genome topology as a regulator of molecular function in stem cell aging.

The role of Hedgehog and Notch signaling pathway in cancer

Notch and Hedgehog signaling are involved in cancer biology and pathology, including the maintenance of tumor cell proliferation, cancer stem-like cells, and the tumor microenvironment. Given the complexity of Notch signaling in tumors, its role as both a tumor promoter and suppressor, and the crosstalk between pathways, the goal of developing clinically safe, effective, tumor-specific Notch-targeted drugs has remained intractable. Drugs developed against the Hedgehog signaling pathway have affirmed definitive therapeutic effects in basal cell carcinoma; however, in some contexts, the challenges of tumor resistance and recurrence leap to the forefront. The efficacy is very limited for other tumor types. In recent years, we have witnessed an exponential increase in the investigation and recognition of the critical roles of the Notch and Hedgehog signaling pathways in cancers, and the crosstalk between these pathways has vast space and value to explore. A series of clinical trials targeting signaling have been launched continually. In this review, we introduce current advances in the understanding of Notch and Hedgehog signaling and the crosstalk between pathways in specific tumor cell populations and microenvironments. Moreover, we also discuss the potential of targeting Notch and Hedgehog for cancer therapy, intending to promote the leap from bench to bedside.

Ciliary control of adipocyte progenitor cell fate regulates energy storage

The primary cilium is a cellular sensory organelle found in most cells in our body. This includes adipocyte progenitor cells in our adipose tissue, a complex organ involved in energy storage, endocrine signaling, and thermogenesis. Numerous studies have shown that the primary cilium plays a critical role in directing the cell fate of adipocyte progenitor cells in multiple adipose tissue types. Accordingly, diseases with dysfunctional cilia called ciliopathies have a broad range of clinical manifestations, including obesity and diabetes. This review summarizes our current understanding of how the primary cilium regulates adipocyte progenitor cell fate in multiple contexts and illustrates the importance of the primary cilium in regulating energy storage and adipose tissue function.

Plasticity of muscle stem cells in homeostasis and aging

We are living longer, but our healthspan has not increased. The goal of regenerative medicine is to increase quality of life through an understanding of the cellular and molecular processes that underlie effective tissue repair in order to restore damaged tissues. The drivers of muscle regeneration are the muscle stem cells that cycle between quiescent and activated states to meet tissue regenerative demands. Here we review recent findings on the role of the niche, or tissue microenvironment, in the modulation of muscle stem cell plasticity and the mechanisms responsible for the drastic loss of stem cell function with aging. These new studies unveil fundamental mechanisms of stem cell plasticity with broad relevance to other tissues and lay the foundation for the development of therapeutic strategies to boost the regenerative potential of aged muscle stem cells.

TLR4 Modulates Senescence and Paracrine Action in Placental Mesenchymal Stem Cells via Inhibiting Hedgehog Signaling Pathway in Preeclampsia

Preeclampsia (PE) is a heterogeneous disease closely associated with the accelerated senescence of the placentas. Placental mesenchymal stem cells (PMSCs) modulate placental development, which is abnormally senescent in PE together with abnormal paracrine. Both pivotal in the placenta development, Toll-like receptor 4 (TLR4) and Hedgehog (HH) pathway are also tightly involved in regulating cellular senescence. This study was aimed at demonstrating that TLR4/HH pathway modulated senescence of placentas and PMSCs in vitro and in vivo. Preeclamptic and normal PMSCs were isolated. Smoothed agonist (SAG) and cyclopamine were used to activate and inhibit HH pathway, respectively. Lipopolysaccharide (LPS) was used to activate TLR4 in vitro and establish the classic PE-like rat model. qRT-PCR, Western blotting, and immunofluorescence were used to detect the expression of TLR4 and HH components (SHH, SMO, and Gli1). Cellular biological function such as proliferation, apoptosis, and migration was compared. Cell cycle analysis, β-galactosidase staining, and the protein expressions of p16 and p53 were detected to analyze the cellular senescence. The secretion levels of human matrix metalloproteinase 9 (MMP-9) and soluble fms-like tyrosine kinase-1 (sFlt-1) were measured in the conditioned medium. Cell migration, invasion, and tube formation were analyzed in HTR8/SVneo cells or human umbilical vein endothelial cells (HUVECs). Our study demonstrated that activation of TLR4 accelerated senescence of PMSCs via suppressing HH pathway both in vitro and in vivo, accompanied by the detrimental paracrine to impair the uterine spiral artery remodeling and placental angiogenesis. Meanwhile, induction of HH pathway could alleviate PE-like manifestations, improve pregnancy outcomes, and ameliorate multiorgan injuries, suggesting that strengthening the HH pathway may serve as a potential therapy in PE.