Skeletal stem cells: origins, definitions, and functions in bone development and disease

NELL2, a novel osteoinductive factor, regulates osteoblast differentiation and bone homeostasis through fibronectin 1/integrin-mediated FAK/AKT signaling

Neural EGFL-like 2 (NELL2) is a secreted protein known for its regulatory functions in the nervous and reproductive systems, yet its role in bone biology remains unexplored. In this study, we observed that NELL2 was diminished in the bone of aged and ovariectomized (OVX) mice, as well as in the serum of osteopenia and osteoporosis patients. In vitro loss-of-function and gain-of-function studies revealed that NELL2 facilitated osteoblast differentiation and impeded adipocyte differentiation from stromal progenitor cells. In vivo studies further demonstrated that the deletion of NELL2 in preosteoblasts resulted in decreased cancellous bone mass in mice. Mechanistically, NELL2 interacted with the FNI-type domain located at the C-terminus of Fibronectin 1 (Fn1). Moreover, we found that NELL2 activated the focal adhesion kinase (FAK)/AKT signaling pathway through Fn1/integrin β1 (ITGB1), leading to the promotion of osteogenesis and the inhibition of adipogenesis. Notably, administration of NELL2-AAV was found to ameliorate bone loss in OVX mice. These findings underscore the significant role of NELL2 in osteoblast differentiation and bone homeostasis, suggesting its potential as a therapeutic target for managing osteoporosis.

Harnessing the diversity and potential of endogenous skeletal stem cells for musculoskeletal tissue regeneration

In our aging society, the degeneration of the musculoskeletal system and adjacent tissues is a growing orthopedic concern. As bones age, they become more fragile, increasing the risk of fractures and injuries. Furthermore, tissues like cartilage accumulate damage, leading to widespread joint issues. Compounding this, the regenerative capacity of these tissues declines with age, exacerbating the consequences of fractures and cartilage deterioration. With rising demand for fracture and cartilage repair, bone-derived stem cells have attracted significant research interest. However, the therapeutic use of stem cells has produced inconsistent results, largely due to ongoing debates and uncertainties regarding the precise identity of the stem cells responsible for musculoskeletal growth, maintenance and repair. This review focuses on the potential to leverage endogenous skeletal stem cells (SSCs)-a well-defined population of stem cells with specific markers, reliable isolation techniques, and functional properties-in bone repair and cartilage regeneration. Understanding SSC behavior in response to injury, including their activation to a functional state, could provide insights into improving treatment outcomes. Techniques like microfracture surgery, which aim to stimulate SSC activity for cartilage repair, are of particular interest. Here, we explore the latest advances in how such interventions may modulate SSC function to enhance bone healing and cartilage regeneration.

The characterization and comparison of femoral bone-derived skeletal stem cells

Skeletal stem cells (SSCs) reside in various niche locations within long bones to maintain bone homeostasis and facilitate fracture repair. Bone fragility, associated with ageing, increases the susceptibility of the femoral head to fractures due to an increase in bone adipocytes and concomitant loss of structural integrity. However, the specific contribution of epiphyseal SSCs to fragility is unknown. To explore this, a comparative analysis was performed on the transcriptional profiles and lineage commitment of Wistar rat femoral SSCs derived from the bone marrow (BM-), diaphyseal cortical bone (CB-) and proximal epiphyseal trabecular bone (PF-SSCs) isolated from the same long bones. SSCs were characterized based on morphology, immunophenotype (CD90/CD45), growth rate (population doubling time), gene expression profiles and differentiation capacity (Oil Red O, Alizarin Red S). qRT-PCR micro-arrays were performed on SSCs to evaluate the expression of stemness, SSC and lineage-specific markers in both undifferentiated and differentiated states. Our findings support the hypothesis that SSCs from different bone regions exhibit distinct transcriptional profiles, reflecting their specific niche environments. CB-SSCs displayed superior osteogenic potential as evidenced by the expression of key osteogenic genes and higher levels of mineralization. In contrast, PF-SSCs had a reduced osteogenic capacity with a higher adipogenic potential. Overall, the study revealed the importance of niche-specific stem cell properties for use in regenerative medicine applications and provides insight into the potential role of PF-SSCs in bone fragility and fracture risk.

Relationship of Chronic Stress and Hypertension with Bone Resorption

Background/Objectives: Chronic exposure to stress has been considered a risk factor for hypertension, which is also associated with increased bone resorption. This review aimed to investigate the effect of acute and chronic stress, associated with hypertension, on the skeletal system. Methods: A comprehensive search was conducted across multiple databases, focusing on peer-reviewed articles published in English. We include experimental, clinical, and peer-reviewed studies focused on the relationship between stress, hypertension, and bone resorption. Searches were conducted in MEDLINE via PubMed, Embase and Scopus, with the last search completed on 10 September 2024. Results: The main topics include situations that favor bone loss, such as psychological stress, which can lead to osteoporotic fractures through immunological and endocrine mechanisms. The relationship between psychological stress and loss of bone density, as in osteoporosis, occurs due to the reduction in the number of osteoblasts and loss in the balance between physiological formation/resorption. Conclusions: Chronic stress significantly affects cardiovascular health and bone resorption. This narrative review study highlights the vulnerability of the skeletal system, along with the cardiovascular system, to prolonged stress, emphasizing the need for multidisciplinary strategies in preventing stress-related conditions. Effective stress management can help reduce the risks of cardiovascular disease and bone resorption, emphasizing their role in comprehensive health care.

Microenvironment-optimized gastrodin-functionalized scaffolds orchestrate asymmetric division of recruited stem cells in endogenous bone regeneration

The regeneration of osteoporotic bone defects remains challenging as the critical stem cell function is impaired by inflammatory microenvironment. Synthetic materials that intrinsically direct osteo-differentiation versus self-renewal of recruited stem cell represent a promising alternative strategy for endogenous bone formation. Therefore, a microenvironmentally optimized polyurethane (PU) /n-HA scaffold to enable sustained delivery of gastrodin is engineered to study its effect on the osteogenic fate of stem cells. It exhibited interconnected porous networks and an elevated sequential gastrodin release pattern to match immune-osteo cascade concurrent with progressive degradation of materials. In a critical-sized femur defect model of osteoporotic rat, 5% gastrodin-PU/n-HA potently promoted neo-bone regeneration by facilitating M2 macrophage polarization and CD146+ host stem cell recruitment to defective site. The implantation time-dependently increased the bone marrow mesenchymal stem cell (BMSC) population, and further culture of BMSCs showed a robust ability of proliferation, migration, and mitochondrial resurgence. Of note, some of cell pairs produced one stemness daughter cell while the other committed to osteogenic lineage in an asymmetric cell division (ACD) manner, and a much more compelling ACD response was triggered when 5% gastrodin-PU/n-HA implanted. Further investigation revealed that one-sided concentrated presentation of aPKC and β-catenin in dividing cells effectively induced asymmetric distribution, which polarized aPKC biased the response of the daughter cells to Wnt signal. The asymmetric cell division in skeletal stem cells (SSCs) was mechanically comparable to BMSCs and also governed by distinct aPKC and β-catenin biases. Concomitantly, delayed bone loss adjacent to the implant partly alleviated development of osteoporosis. In conclusion, our findings provide insight into the regulation of macrophage polarization combined with osteogenic commitment of recruited stem cells in an ACD manner, advancing scaffold design strategy for endogenous bone regeneration.

The Hippo signalling pathway in bone homeostasis: Under the regulation of mechanics and aging

The Hippo signalling pathway is a conserved kinase cascade that orchestrates diverse cellular processes, such as proliferation, apoptosis, lineage commitment and stemness. With the onset of society ages, research on skeletal aging-mechanics-bone homeostasis has exploded. In recent years, aging and mechanical force in the skeletal system have gained groundbreaking research progress. Under the regulation of mechanics and aging, the Hippo signalling pathway has a crucial role in the development and homeostasis of bone. We synthesize the current knowledge on the role of the Hippo signalling pathway, particularly its downstream effectors yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), in bone homeostasis. We discuss the regulation of the lineage specification and function of different skeletal cell types by the Hippo signalling pathway. The interactions of the Hippo signalling pathway with other pathways, such as Wnt, transforming growth factor beta and nuclear factor kappa-B, are also mentioned because of their importance for modulating bone homeostasis. Furthermore, YAP/TAZ have been extensively studied as mechanotransducers. Due to space limitations, we focus on reviewing how mechanical forces and aging influence cell fate, communications and homeostasis through a dysregulated Hippo signalling pathway.

Nicotinamide mononucleotide enhances fracture healing by promoting skeletal stem cell proliferation

The process of skeletal regeneration initiated by stem cells following injury, especially in fractures, is significantly impaired by aging and adverse factors. Nicotinamide mononucleotide (NMN), a critical endogenous precursor of nicotinamide adenine dinucleotide (NAD), has garnered extensive attention for its multifaceted regulatory functions in living organisms and its wide-ranging therapeutic potential. However, whether NMN contributes to trauma-induced skeletal regeneration remains unclear. Methods: The transverse femoral shaft fracture model was employed to evaluate the potential advantages of NMN administration for overall repair during the initial fracture stages in male mice through micro-CT analysis, histochemistry, and biomechanical testing. The pro-proliferative function of NMN on skeletal stem cells (SSCs) was investigated through flow cytometry, qRT-PCR, NAD content measurement, and cell proliferation assay. Results: In this study, we observed that the administration of NMN during the initial phase of fracture in mice led to a larger callus and corresponding improvement in micro-CT parameters. NMN enhances the cartilaginous component of the callus by elevating the NAD content, consequently accelerating subsequent endochondral ossification and the fracture healing process. Subsequent analyses elucidated that NMN was beneficial in promoting the expansion of diverse stem cells in vivo and in vitro potentially via modulation of the Notch signaling pathway. Moreover, the depletion of macrophages profoundly obstructs the proliferation of SSCs. Conclusion: Our discoveries provide a potential strategy for enhancing fracture healing through stimulation of callus SSC proliferation at an early stage, shedding light on the translational value of NMN as an enhancer for skeletal regeneration and highlighting the pivotal role of macrophage-stem cell interactions in governing the regenerative influence of NMN on stem cells.

Metabolic regulation of skeletal cell fate and function

Bone development and bone remodelling during adult life are highly anabolic processes requiring an adequate supply of oxygen and nutrients. Bone-forming osteoblasts and bone-resorbing osteoclasts interact closely to preserve bone mass and architecture and are often located close to blood vessels. Chondrocytes within the developing growth plate ensure that bone lengthening occurs before puberty, but these cells function in an avascular environment. With ageing, numerous bone marrow adipocytes appear, often with negative effects on bone properties. Many studies have now indicated that skeletal cells have specific metabolic profiles that correspond to the nutritional microenvironment and their stage-specific functions. These metabolic networks provide not only skeletal cells with sufficient energy, but also biosynthetic intermediates that are necessary for proliferation and extracellular matrix synthesis. Moreover, these metabolic pathways control redox homeostasis to avoid oxidative stress and safeguard cell survival. Finally, several intracellular metabolites regulate the activity of epigenetic enzymes and thus control the fate and function of skeletal cells. The metabolic profile of skeletal cells therefore not only reflects their cellular state, but can also drive cellular activity. Insight into skeletal cell metabolism will thus not only advance our understanding of skeletal development and homeostasis, but also of skeletal disorders, such as osteoarthritis, diabetic bone disease and bone malignancies.

2-N, 6-O sulfated chitosan evokes periosteal stem cells for bone regeneration

Musculoskeletal injuries and bone defects represent a significant clinical challenge, necessitating innovative approaches for effective bone tissue regeneration. In this study, we investigated the potential of harnessing periosteal stem cells (PSCs) and glycosaminoglycan (GAG)-mimicking materials for in situ bone regeneration. Our findings demonstrated that the introduction of 2-N, 6-O sulfated chitosan (26SCS), a GAG-like polysaccharide, enriched PSCs and promoted robust osteogenesis at the defect area. Mechanistically, 26SCS amplifies the biological effect of endogenous platelet-derived growth factor-BB (PDGF-BB) through enhancing the interaction between PDGF-BB and its receptor PDGFRβ abundantly expressed on PSCs, resulting in strengthened PSC proliferation and osteogenic differentiation. As a result, 26SCS effectively improved bone defect repair, even in an osteoporotic mouse model with lowered PDGF-BB level and diminished regenerative potential. Our findings suggested the significant potential of GAG-like biomaterials in regulating PSC behavior, which holds great promise for addressing osteoporotic bone defect repair in future applications.

Human ESC-derived vascular cells promote vascular regeneration in a HIF-1α dependent manner

Hypoxia-inducible factor (HIF-1α), a core transcription factor responding to changes in cellular oxygen levels, is closely associated with a wide range of physiological and pathological conditions. However, its differential impacts on vascular cell types and molecular programs modulating human vascular homeostasis and regeneration remain largely elusive. Here, we applied CRISPR/Cas9-mediated gene editing of human embryonic stem cells and directed differentiation to generate HIF-1α-deficient human vascular cells including vascular endothelial cells, vascular smooth muscle cells, and mesenchymal stem cells (MSCs), as a platform for discovering cell type-specific hypoxia-induced response mechanisms. Through comparative molecular profiling across cell types under normoxic and hypoxic conditions, we provide insight into the indispensable role of HIF-1α in the promotion of ischemic vascular regeneration. We found human MSCs to be the vascular cell type most susceptible to HIF-1α deficiency, and that transcriptional inactivation of ANKZF1, an effector of HIF-1α, impaired pro-angiogenic processes. Altogether, our findings deepen the understanding of HIF-1α in human angiogenesis and support further explorations of novel therapeutic strategies of vascular regeneration against ischemic damage.

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.