Restoration of CPEB4 prevents muscle stem cell senescence during aging

Cellular senescence in age-related musculoskeletal diseases

Aging is typically associated with decreased musculoskeletal function, leading to reduced mobility and increased frailty. As a hallmark of aging, cellular senescence plays a crucial role in various age-related musculoskeletal diseases, including osteoporosis, osteoarthritis, intervertebral disc degeneration, and sarcopenia. The detrimental effects of senescence are primarily due to impaired regenerative capacity of stem cells and the pro-inflammatory environment created by accumulated senescent cells. The secreted senescence-associated secretory phenotype (SASP) can induce senescence in neighboring cells, further amplifying senescent signals. Although the removal of senescent cells and the suppression of SASP factors have shown promise in alleviating disease progression and restoring musculoskeletal health in mouse models, clinical trials have yet to demonstrate significant efficacy. This review summarizes the mechanisms of cellular senescence in age-related musculoskeletal diseases and discusses potential therapeutic strategies targeting cellular senescence.

Autophagy in adult stem cell homeostasis, aging, and disease therapy

Autophagy is a crucial cellular process that facilitates the degradation of damaged organelles and protein aggregates, and the recycling of cellular components for the energy production and macromolecule synthesis. It plays an indispensable role in maintaining cellular homeostasis. Over recent decades, research has increasingly focused on the role of autophagy in regulating adult stem cells (SCs). Studies suggest that autophagy modulates various cellular processes and states of adult SCs, including quiescence, proliferation, self-renewal, and differentiation. The primary role of autophagy in these contexts is to sustain homeostasis, withstand stressors, and supply energy. Notably, the dysfunction of adult SCs during aging is correlated with a decline in autophagic activity, suggesting that autophagy is also involved in SC- and aging-associated disorders. Given the diverse cellular processes mediated by autophagy and the intricate mechanisms governing adult SCs, further research is essential to elucidate both universal and cell type-specific regulatory pathways of autophagy. This review discusses the role of autophagy in regulating adult SCs during quiescence, proliferation, self-renewal, and differentiation. Additionally, it summarizes the relationship between SC aging and autophagy, providing therapeutical insights into treating and ameliorating aging-associated diseases and cancers, and ultimately promoting longevity.

Metabolic regulation in adult and aging skeletal muscle stem cells

Adult stem cells maintain homeostasis and enable regeneration of most tissues. Quiescence, proliferation, and differentiation of stem cells and their progenitors are tightly regulated processes governed by dynamic transcriptional, epigenetic, and metabolic programs. Previously thought to merely reflect a cell's energy state, metabolism is now recognized for its critical regulatory functions, controlling not only energy and biomass production but also the cell's transcriptome and epigenome. In this review, we explore how metabolic pathways, metabolites, and transcriptional and epigenetic regulators are functionally interlinked in adult and aging skeletal muscle stem cells.

Combinational regenerative inductive effect of bio-adhesive hybrid hydrogels conjugated with hiPSC-derived myofibers and its derived EVs for volumetric muscle regeneration

In regenerative medicine, extracellular vesicles (EVs) possess the potential to repair injured cells by delivering modulatory factors. However, the therapeutic effect of EVs in large-scale tissue defects, which are subject to prolonged timelines for tissue architecture and functional restoration, remains poorly understood. In this study, we introduce EVs and cell-tethering hybrid hydrogels composed of tyramine-conjugated gelatin (GelTA) that can be in-situ crosslinked with EVs derived from human induced pluripotent stem cell-derived myofibers (hiPSC-myofibers) and hiPSC-muscle precursor cells. This hybrid hydrogel sustains the release of EVs and provides a beneficial nano-topography and mechanical properties for creating a favorable extracellular matrix. Secreted EVs from the hiPSC-myofibers contain specific microRNAs, potentially improving myogenesis and angiogenesis. Herein, we demonstrate increased myogenic markers and fusion/differentiation indexes through the combinatory effects of EVs and integrin-mediated adhesions in the 3D matrix. Furthermore, we observe a unique impact of EVs, which aid in maintaining the viability and phenotype of myofibers under harsh environments. The hybrid hydrogel in-situ crosslinked with hiPSCs and EVs is facilely used to fabricate large-scale muscle constructs by the stacking of micro-patterned hydrogel domains. Later, we confirmed a combinational effect, whereby muscle tissue regeneration and functional restoration were improved, via an in vivo murine volumetric muscle loss model.

Mitochondrial Dynamics Drive Muscle Stem Cell Progression from Quiescence to Myogenic Differentiation

From quiescence to activation and myogenic differentiation, muscle stem cells (MuSCs) experience drastic alterations in their signaling activity and metabolism. Through balanced cycles of fission and fusion, mitochondria alter their morphology and metabolism, allowing them to affect their decisive role in modulating MuSC activity and fate decisions. This tightly regulated process contributes to MuSC regulation by mediating changes in redox signaling pathways, cell cycle progression, and cell fate decisions. In this review, we discuss the role of mitochondrial dynamics as an integral modulator of MuSC activity, fate, and maintenance. Understanding the influence of mitochondrial dynamics in MuSCs in health and disease will further the development of therapeutics that support MuSC integrity and thus may aid in restoring the regenerative capacity of skeletal muscle.

Loss of Tob1 promotes muscle regeneration through muscle stem cell expansion

Muscle stem cells (MuSCs) play an indispensable role in postnatal muscle growth and hypertrophy in adults. MuSCs also retain a highly regenerative capacity and are therefore considered a promising stem cell source for regenerative therapy for muscle diseases. In this study, we identify tumor-suppressor protein Tob1 as a Pax7 target protein that negatively controls the population expansion of MuSCs. Tob1 protein is undetectable in the quiescent state but is upregulated during activation in MuSCs. Tob1 ablation in mice accelerates MuSC population expansion and boosts muscle regeneration. Moreover, inactivation of Tob1 in MuSCs ameliorates the efficiency of MuSC transplantation in a murine muscular dystrophy model. Collectively, selective targeting of Tob1 might be a therapeutic option for the treatment of muscular diseases, including muscular dystrophy and age-related sarcopenia.

Navigating translational control of gene expression in satellite cells

Satellite cells, named for their satellite position around the sarcolemma of the myofibre, are responsible for skeletal muscle regeneration. Satellite cells normally reside in a quiescent state, but rapidly activate the myogenic program and the cell cycle in response to injury. Translational control of gene expression has emerged as an important regulator of satellite cell activity. Quiescent satellite cells maintain low levels of protein synthesis and selectively translate specific mRNAs to conserve limited energy. Activated satellite cells rapidly restore global protein synthesis to meet the demands of proliferating myogenic progenitors that participate in muscle repair. We propose a model by which translational control enables rapid protein level changes in response to injury-induced environmental shifts, serving as both a brake mechanism during quiescence and an accelerator for injury response. In this Chapter, we navigate the processing, translation and metabolism of newly transcribed mRNAs. We review the modifications of mRNA that occur during mRNA processing in the nucleus of satellite cells, and illustrate how these modifications impact the translation and stability of mRNAs. In the cytoplasm, we review how pathways work in concert to regulate protein synthesis globally, while trans acting microRNAs and RNA binding proteins modify specific mRNA translation within a context of tightly regulated protein synthesis. While navigating translational control of gene expression in satellite cells, this chapter reveals that despite significant progress, the field remains nascent in the broader scope of translational control in cell biology. We propose that future investigations will benefit from incorporating emerging global analyses to study translational control of gene expression in rare satellite cells, and we pose unanswered questions that warrant future exploration.

Protocol for low-input proteomic analysis of in situ fixed adult murine muscle stem cells

Studying skeletal muscle stem cells (MuSCs) quiescence is challenging as they quickly activate within hours of isolation from muscle. Here, we present a protocol to disassociate and characterize fixed peptides from quiescent MuSCs using trapped ion-mobility time-of-flight mass spectrometry (MS). We describe steps for mouse perfusion, fluorescence-activated cell sorting preparation and sorting, protein extraction, digestion, and liquid chromatography MS analysis. This protocol can be applied to other less-abundant somatic stem cell types using mouse lines with a reporter. For complete details on the use and execution of this protocol, please refer to Zeng et al. (2022, 2023).1,2.