A pathway to bone: signaling molecules and transcription factors involved in chondrocyte development and maturation

PTPN11 in cartilage development, adult homeostasis, and diseases

The SH2 domain-containing protein tyrosine phosphatase 2 (SHP2, also known as PTP2C), encoded by PTPN11, is ubiquitously expressed and has context-specific effects. It promotes RAS/MAPK signaling downstream of receptor tyrosine kinases, cytokine receptors, and extracellular matrix proteins, and was shown in various lineages to modulate cell survival, proliferation, differentiation, and migration. Over the past decade, PTPN11 inactivation in chondrocytes was found to cause metachondromatosis, a rare disorder characterized by multiple enchondromas and osteochondroma-like lesions. Moreover, SHP2 inhibition was found to mitigate osteoarthritis pathogenesis in mice, and abundant but incomplete evidence suggests that SHP2 is crucial for cartilage development and adult homeostasis, during which its expression and activity are tightly regulated transcriptionally and posttranslationally, and by varying sets of functional partners. Fully uncovering SHP2 actions and regulation in chondrocytes is thus fundamental to understanding the mechanisms underlying both rare and common cartilage diseases and to designing effective disease treatments. We here review current knowledge, highlight recent discoveries and controversies, and propose new research directions to answer remaining questions.

Dynamics of postnatal bone development and epiphyseal synostosis in the caprine autopod

BACKGROUND

Bones develop to structurally balance strength and mobility. Bone developmental dynamics are influenced by whether an animal is ambulatory at birth. Precocial species, which are ambulatory at birth, develop advanced skeletal maturity in utero and experience postnatal development under mechanical loading. Here, we characterized postnatal bone development in the lower forelimbs of precocial goats using microcomputed tomography and histology. Our analysis focused on the two phalanges 1 (P1) bones and the partially fused metacarpal bone of the goat autopod from birth through adulthood.

RESULTS

P1 cortical bone densified rapidly after birth, but cortical thickness increased continually through adulthood. Upon normalization by body mass, the P1 normalized polar moment of inertia was constant over time, suggestive of changes correlating with ambulatory loading. P1 trabecular bone increased in trabecular number and thickness until sexual maturity (12 months), while metacarpal trabeculae grew primarily through trabecular thickening. Unlike prenatal synostosis (i.e., bone fusion) of the metacarpal diaphysis, synostosis of the epiphyses occurred postnatally, prior to growth plate closure, through a unique fibrocartilaginous endochondral ossification.

CONCLUSIONS

These findings implicate ambulatory loading in postnatal bone development of precocial goats and identify a novel postnatal synostosis event in the caprine metacarpal epiphysis.

METTL14 regulates chondrogenesis through the GDF5-RUNX-extracellular matrix gene axis during limb development

m6A RNA methylation is essential for many aspects of mammalian development but its roles in chondrogenesis remain largely unknown. Here, we show that m6A is necessary for chondrogenesis and limb morphogenesis using limb progenitor-specific knockout mice of Mettl14, an essential subunit in the m6A methyltransferase complex. The knockout disrupts cartilage anlagen formation in limb buds with 11 downregulated proteins known to dysregulate chondrogenesis and shorten limb skeletons upon mutation in mice and humans. Further studies show a gene regulatory hierarchy among the 11 proteins. m6A stabilizes the transcript and increases the protein level of GDF5, a BMP family member. This activates the chondrogenic transcription factor genes Runx2 and Runx3, whose mRNAs are also stabilized by m6A. They promote the transcription of six collagen genes and two other chondrogenic genes, Ddrgk1 and Pbxip1. Thus, this study uncovers an m6A-based cascade essential for chondrogenesis during limb skeletal development.

FAM20B-Catalyzed Glycosylation Regulates the Chondrogenic and Osteogenic Differentiation of the Embryonic Condyle by Controlling IHH Diffusion and Release

Although the roles of proteoglycans (PGs) have been well documented in the development and homeostasis of the temporomandibular joint (TMJ), how the glycosaminoglycan (GAG) chains of PGs contribute to TMJ chondrogenesis and osteogenesis still requires explication. In this study, we found that FAM20B, a hexokinase essential for attaching GAG chains to the core proteins of PGs, was robustly activated in the condylar mesenchyme during TMJ development. The inactivation of Fam20b in craniofacial neural crest cells (CNCCs) dramatically reduced the synthesis and accumulation of GAG chains rather than core proteins in the condylar cartilage, which resulted in a hypoplastic condylar cartilage by severely promoting chondrocyte hypertrophy and perichondral ossification. In the condyles of Wnt1-Cre;Fam20bf/f mouse embryos, enlarged Ihh- and COL10-expressing domains indicated premature hypertrophy resulting from an attenuated IHH-PTHRP negative feedback in condylar chondrocytes, while increased osteogenic markers, canonical Wnt activity, and type-H angiogenesis verified the enhanced osteogenesis in the perichondrium. Further ex vivo investigations revealed that the loss of Fam20b decreased the domain area but increased the activity of HH signaling in the embryonic condylar mesenchyme. Moreover, the abrogation of GAG chains in heparan sulfate and chondroitin sulfate proteoglycans led to a rapid up- and then downregulation of HH signaling in condylar chondrocytes, implicating a "slow-release" manner of growth factors controlled by GAG chains. Overall, this study revealed a comprehensive role of the FAM20B-catalyzed GAG chain synthesis in the chondrogenic and osteogenic differentiation of the embryonic TMJ condyle.

miRNA-based regulation in growth plate cartilage: mechanisms, targets, and therapeutic potential

MicroRNAs (miRNAs) are critical regulators of the skeleton. In the growth plate, these small non-coding RNAs modulate gene networks that drive key stages of chondrogenesis, including proliferation, differentiation, extracellular matrix synthesis and hypertrophy. These processes are orchestrated through the interaction of pivotal pathways including parathyroid hormone-related protein (PTHrP), Indian hedgehog (IHH), and bone morphogenetic protein (BMP) signaling. This review highlights the miRNA-mRNA target networks essential for chondrocyte differentiation. Many miRNAs are differentially expressed in resting, proliferating and hypertrophic cartilage zones. Moreover, differential enrichment of specific miRNAs in matrix vesicles is also observed, providing means for chondrocytes to influence the function and differentiation of their neighbors by via matrix vesicle protein and RNA cargo. Notably, miR-1 and miR-140 emerge as critical modulators of chondrocyte proliferation and hypertrophy by regulating multiple signaling pathways, many of them downstream from their mutual target Hdac4. Demonstration that a human gain-of-function mutation in miR-140 causes skeletal dysplasia underscores the clinical relevance of understanding miRNA-mediated regulation. Further, miRNAs such as miR-26b have emerged as markers for skeletal disorders such as idiopathic short stature, showcasing the translational relevance of miRNAs in skeletal health. This review also highlights some miRNA-based therapeutic strategies, including innovative delivery systems that could target chondrocytes via cartilage affinity peptides, and potential applications related to treatment of physeal bony bridge formation in growing children. By synthesizing current research, this review offers a nuanced understanding of miRNA functions in growth plate biology and their broader implications for skeletal health. It underscores the translational potential of miRNA-based therapies in addressing skeletal disorders and aims to inspire further investigations in this rapidly evolving field.

Callus organoids reveal distinct cartilage to bone transition mechanisms across donors and a role for biological sex

Clinical translation of tissue-engineered advanced therapeutic medicinal products is hindered by a lack of patient-dependent and independent in-process biological quality controls that are reflective of in vivo outcomes. Recent insights into the mechanism of native bone repair highlight a robust path dependence. Organoid-based bottom-up developmental engineering mimics this path-dependence to design personalized living implants scaffold-free, with in-build outcome predictability. Yet, adequate (noninvasive) quality metrics of engineered tissues are lacking. Moreover, insufficient insight into the role of donor variability and biological sex as influencing factors for the mechanism toward bone repair hinders the implementation of such protocols for personalized bone implants. Here, male and female bone-forming organoids were compared to non-bone-forming organoids regarding their extracellular matrix composition, transcriptome, and secreted proteome signatures to directly link in vivo outcomes to quality metrics. As a result, donor variability in bone-forming callus organoids pointed towards two distinct pathways to bone, through either a hypertrophic cartilage or a fibrocartilaginous template. The followed pathway was determined early, as a biological sex-dependent activation of distinct progenitor populations. Independent of donor or biological sex, a cartilage-to-bone transition was driven by a common panel of secreted factors that played a role in extracellular matrix remodeling, mineralization, and attraction of vasculature. Hence, the secreted proteome is a source of noninvasive biomarkers that report on biological potency and could be the missing link toward data-driven decision-making in organoid-based bone tissue engineering.

Prg4 and Osteoarthritis: Functions, Regulatory Factors, and Treatment Strategies

Proteoglycan 4 (PRG4), also known as lubricin, plays a critical role in maintaining joint homeostasis by reducing friction between articular cartilage surfaces and preventing cartilage degradation. Its deficiency leads to early-onset osteoarthritis (OA), while overexpression can protect against cartilage degeneration. Beyond its lubricating properties, PRG4 exerts anti-inflammatory effects by interacting with Toll-like receptors, modulating inflammatory responses within the joint. The expression of Prg4 is regulated by various factors, including mechanical stimuli, inflammatory cytokines, transcription factors such as Creb5 and FoxO, and signaling pathways like TGF-β, EGFR, and Wnt/β-catenin. Therapeutic strategies targeting PRG4 in OA have shown promising results, including recombinant PRG4 protein injections, gene therapies, and small molecules that enhance endogenous Prg4 expression or mimic its function. Further research into the molecular mechanisms regulating Prg4 expression will be essential in developing more effective OA treatments. Understanding the interplay between Prg4 and other signaling pathways could reveal novel therapeutic targets. Additionally, advancements in gene therapy and biomaterials designed to deliver PRG4 in a controlled manner may hold potential for the long-term management of OA, improving patient outcomes and delaying disease progression.

Pax1a-EphrinB2a pathway in the first pharyngeal pouch controls hyomandibular plate formation by promoting chondrocyte formation in zebrafish

The hyomandibular (HM) cartilage securing the lower jaw to the neurocranium in fish is a craniofacial skeletal element whose shape and function have changed dramatically in vertebrate evolution, yet the genetic mechanisms shaping this cartilage are less understood. Using mutants and rescue experiments in zebrafish, we reveal a previously unappreciated role of Pax1a in the anterior HM plate formation through EphrinB2a. During craniofacial development, pax1a is expressed in the pharyngeal endoderm from the pharyngeal segmentation stage to chondrocyte formation. Loss of pax1a leads to defects in the first pouch and to the absence of chondrocytes in the anterior region of the HM plate caused by increased cell death in differentiating osteochondral progenitors. In pax1 mutants, a forced expression of pax1a by the heat shock before pouch formation rescues the defects in the first pouch and HM plate together, whereas a forced expression of pax1a after pouch formation rescues only the defects in the HM plate without rescuing the first pouch defects. In pax1a mutants, ephrinb2a expressed in the first pouch is downregulated when adjacent osteochondral progenitors differentiate into the chondrocytes, with mutations in ephrinb2a causing hyomandibular plate defects. Lastly, pax1 mutants rescue the anterior hyomandibular plate defects by pouch-specific restoration of EphrinB2a or a heat-shock-treated expression of ephrinb2a after pouch formation. We propose that the Pax1a-EphrinB2a pathway in the first pouch is directly required to shape the HM plate in addition to the early role of Pax1a in the first pouch formation.

Distinct gene regulatory dynamics drive skeletogenic cell fate convergence during vertebrate embryogenesis

Cell type repertoires have expanded extensively in metazoan animals, with some clade-specific cells being crucial to evolutionary success. A prime example are the skeletogenic cells of vertebrates. Depending on anatomical location, these cells originate from three different precursor lineages, yet they converge developmentally towards similar cellular phenotypes. Furthermore, their 'skeletogenic competency' arose at distinct evolutionary timepoints, thus questioning to what extent different skeletal body parts rely on truly homologous cell types. Here, we investigate how lineage-specific molecular properties are integrated at the gene regulatory level, to allow for skeletogenic cell fate convergence. Using single-cell functional genomics, we find that distinct transcription factor profiles are inherited from the three precursor states and incorporated at lineage-specific enhancer elements. This lineage-specific regulatory logic suggests that these regionalized skeletogenic cells are distinct cell types, rendering them amenable to individualized selection, to define adaptive morphologies and biomaterial properties in different parts of the vertebrate skeleton.

Strategies for promoting neurovascularization in bone regeneration

Bone tissue relies on the intricate interplay between blood vessels and nerve fibers, both are essential for many physiological and pathological processes of the skeletal system. Blood vessels provide the necessary oxygen and nutrients to nerve and bone tissues, and remove metabolic waste. Concomitantly, nerve fibers precede blood vessels during growth, promote vascularization, and influence bone cells by secreting neurotransmitters to stimulate osteogenesis. Despite the critical roles of both components, current biomaterials generally focus on enhancing intraosseous blood vessel repair, while often neglecting the contribution of nerves. Understanding the distribution and main functions of blood vessels and nerve fibers in bone is crucial for developing effective biomaterials for bone tissue engineering. This review first explores the anatomy of intraosseous blood vessels and nerve fibers, highlighting their vital roles in bone embryonic development, metabolism, and repair. It covers innovative bone regeneration strategies directed at accelerating the intrabony neurovascular system over the past 10 years. The issues covered included material properties (stiffness, surface topography, pore structures, conductivity, and piezoelectricity) and acellular biological factors [neurotrophins, peptides, ribonucleic acids (RNAs), inorganic ions, and exosomes]. Major challenges encountered by neurovascularized materials during their clinical translation have also been highlighted. Furthermore, the review discusses future research directions and potential developments aimed at producing bone repair materials that more accurately mimic the natural healing processes of bone tissue. This review will serve as a valuable reference for researchers and clinicians in developing novel neurovascularized biomaterials and accelerating their translation into clinical practice. By bridging the gap between experimental research and practical application, these advancements have the potential to transform the treatment of bone defects and significantly improve the quality of life for patients with bone-related conditions.

Cartilage Forming Tumors of the Skeleton

Cartilage-forming tumors are a broad and diverse group of neoplasms frequently affecting the skeleton. Distinguishing between the members of this group is important because of significant differences in treatment and prognosis. Accurate diagnosis can be challenging because of similarities in their clinical, radiographic, and pathologic features. Immunohistochemistry and molecular tools are helpful in select instances. Therefore, careful evaluation and correlation of these features are essential in arriving at the correct diagnosis and appropriate patient management. This review provides an overview of the current literature, emphasizing helpful features in diagnosis.

Parallel Chondrogenesis and Osteogenesis Tissue Morphogenesis in Muscle Tissue via Combinations of TGF-β Supergene Family Members

OBJECTIVE

This study aimed to decipher the temporal and spatial signaling code for clinical cartilage and bone regeneration. We investigated the effects of continuous equal dosages of a single, dual, or triplicate growth factor combination of bone morphogenetic protein (BMP)-2, transforming growth factor (TGF)-β3, and/or BMP-7 on muscle tissue over a culturing period. The hypothesis was that specific growth factor combinations at specific time points direct tissue transformation toward endochondral bone or cartilage formation.

DESIGN

The harvested muscle tissues from F-344 adult male rats were cultured in 96-well plates maintained in a specific medium and cultured at specific conditions. And the multidimensional and multi-time point analyses were performed at both the genetic and protein levels.

RESULTS

The results insinuate that the application of growth factor stimulates a chaotic tissue response that does not follow a chronological signaling cascade. Both osteogenic and chondrogenic genes showed upregulation after induction, a similar result was also observed in the semiquantitative analysis after immunohistochemical staining against different antigens.

CONCLUSIONS

The study showed that multiple TGF-β superfamily proteins applied to tissue stimulate developmental tissue processes that do not follow current tissue formation rules. The findings contribute to the understanding of the chronological order of signals and expression patterns needed to achieve chondrogenesis, articular chondrogenesis, or osteogenesis, which is crucial for the development of treatments that can regrow bone and articular cartilage clinically.

Active cell proliferation contributes to the enlargement of the nascent nucleus pulposus

BACKGROUND

The notochord is an embryonic organ involved in forming and patterning the spinal column. The mechanism by which the notochord transforms from a continuous rod to a segmented structure excluded from the vertebrae and residing solely as the nucleus pulposus within the intervertebral disc is understudied. The current model of notochordal segmentation suggests that swelling through formation and maturation of the vertebrate cartilage squeezes the notochord cells from the vertebra.

RESULTS

Analysis of Collagen 10, a marker for hypertrophic differentiation, as well as evaluation of changes in cell density, reveal that the expansion of the vertebral precursor cells occurs after notochord segmentation has already taken place. We find that the bulk of the nucleus pulposus is derived from accelerated proliferation within the nucleus pulposus itself. In a model of cell proliferation, the increased proliferation at the nucleus pulposus importantly contributes to expand the nucleus pulposus area.

CONCLUSIONS

Our data is consistent with the hypothesis that notochord cell proliferation contributes to the enlargement of the nucleus pulposus before the vertebra undergo hypertrophy.

Learning Latent Trajectories in Developmental Time Series with Hidden-Markov Optimal Transport

Deriving the sequence of transitions between cell types, or differentiation events, that occur during organismal development is one of the fundamental challenges in developmental biology. Single-cell and spatial sequencing of samples from different developmental timepoints provide data to investigate differentiation but inferring a sequence of differentiation events requires: (1) finding trajectories, or ancestor:descendant relationships, between cells from consecutive timepoints; (2) coarse-graining these trajectories into a differentiation map , or collection of transitions between cell types , rather than individual cells. We introduce Hidden-Markov Optimal Transport ( HM - OT ), an algorithm that simultaneously groups cells into cell types and learns transitions between these cell types from developmental transcriptomics time series. HM - OT uses low-rank optimal transport to simultaneously align samples in a time series and learn a sequence of clusterings and a differentiation map with minimal total transport cost. We assume that the law governing cell-type trajectories is characterized by the joint law on consecutive time points, tantamount to a Markov assumption on these latent trajectories. HM - OT can learn these clusterings in a fully unsupervised manner or can generate the least-cost cell type differentiation map consistent with a given set of cell type labels. We validate the unsupervised clusters and cell type differentiation map output by HM - OT on a Stereo-seq dataset of zebrafish development, and we demonstrate the scalability of HM - OT to a massive Stereo-seq dataset of mouse embryonic development.

Code availability

Software is available at https://github.com/raphael-group/HM-OT.

Wnt2bb signaling promotes pharyngeal chondrogenic precursor proliferation and chondrocyte maturation by activating Yap expression in zebrafish

Pharyngeal cartilage morphogenesis is crucial for the formation of craniofacial structures. Cranial neural crest cells are specified at the neural plate border, migrate to pharyngeal arches, and differentiate into pharyngeal chondrocytes, which subsequently flatten, elongate, and stack like coins during maturation. Although the developmental processes prior to chondrocyte maturation have been extensively studied, their subsequent changes in morphology and organization remain largely elusive. Here, we show that wnt2bb is expressed in the pharyngeal ectoderm adjacent to the chondrogenic precursor cells in zebrafish. Inactivation of Wnt2bb leads to a reduction in nuclear β-catenin, which impairs chondrogenic precursor proliferation and disrupts chondrocyte morphogenesis and organization, eventually causing a severe shrinkage of pharyngeal cartilages. Moreover, the decrease of β-catenin in wnt2bb-/- mutants is accompanied by the reduction of Yap expression. Reactivation of Yap can restore the proliferation of chondrocyte progenitors as well as the proper size, shape, and stacking of pharyngeal chondrocytes. Our findings suggest that Wnt/β-catenin signaling promotes Yap expression to regulate pharyngeal cartilage formation in zebrafish.

Exosomal non-coding RNAs in the regulation of bone metabolism homeostasis: Molecular mechanism and therapeutic potential

Bone metabolism is a dynamic balance between bone formation and absorption regulated by osteoblasts/osteoclasts. Bone metabolic disorders can lead to metabolic bone disease. Osteoporosis (OP), osteoarthritis (OA) and femoral head necrosis (ONFH) are common metabolic bone diseases. At present, the treatment of metabolic bone disease is still mainly to relieve pain and improve joint function. However, surgical treatment does not apply to the vast majority of high-risk groups, including postmenopausal women, patients with diabetes, cirrhosis, etc. Exosomes (Exos) are nanoscale membrane vesicles that are released by almost all cells. Exos are rich in a variety of bioactive substances, such as non-coding RNAs, nucleic acids, proteins and lipids. In view of the structure of Exos, it can protect the biologically active molecules can be smoothly delivered to the target cells and involved in the regulation of cell function. In this review, we focus on the regulation mechanism and function of bone homeostasis mediated by exosomal ncRNAs (Exos-ncRNAs), including macrophage polarization, autophagy, angiogenesis, signal transduction and competing endogenous RNA (ceRNA). We summarized the therapeutic strategies and potential drugs of Exos-ncRNAs in metabolic bone disease. Moreover, we discussed the shortcomings and potential research directions of Exos as carrier to deliver ncRNAs to play a role.

Emerging biological functions of Twist1 in cell differentiation

Twist1 is required for embryonic development and expresses after birth in mesenchymal stem cells derived from mesoderm, where it governs mesenchymal cell development. As a well-known regulator of epithelial-mesenchymal transition or embryonic organogenesis, Twist1 is important in a variety of developmental systems, including mesoderm formation, neurogenesis, myogenesis, cranial neural crest cell migration, and differentiation. In this review, we first highlight the physiological significance of Twist1 in cell differentiation, including osteogenic, chondrogenic, and myogenic differentiation, and then detail its probable molecular processes and signaling pathways. On this premise, we summarize the significance of Twist1 in distinct developmental disorders and diseases to provide a reference for studies on cell differentiation/development-related diseases.

Modeling of skeletal development and diseases using human pluripotent stem cells

Human skeletal elements are formed from distinct origins at distinct positions of the embryo. For example, the neural crest produces the facial bones, the paraxial mesoderm produces the axial skeleton, and the lateral plate mesoderm produces the appendicular skeleton. During skeletal development, different combinations of signaling pathways are coordinated from distinct origins during the sequential developmental stages. Models for human skeletal development have been established using human pluripotent stem cells (hPSCs) and by exploiting our understanding of skeletal development. Stepwise protocols for generating skeletal cells from different origins have been designed to mimic developmental trails. Recently, organoid methods have allowed the multicellular organization of skeletal cell types to recapitulate complicated skeletal development and metabolism. Similarly, several genetic diseases of the skeleton have been modeled using patient-derived induced pluripotent stem cells and genome-editing technologies. Model-based drug screening is a powerful tool for identifying drug candidates. This review briefly summarizes our current understanding of the embryonic development of skeletal tissues and introduces the current state-of-the-art hPSC methods for recapitulating skeletal development, metabolism, and diseases. We also discuss the current limitations and future perspectives for applications of the hPSC-based modeling system in precision medicine in this research field.

Dynamics of postnatal bone development and epiphyseal synostosis in the caprine autopod

Bones develop to structurally balance strength and mobility. Bone developmental dynamics are influenced by whether an animal is ambulatory at birth (i.e., precocial). Precocial species, such as goats, develop advanced skeletal maturity in utero, making them useful models for studying the dynamics of bone formation under mechanical load. Here, we used microcomputed tomography and histology to characterize postnatal bone development in the autopod of the caprine lower forelimb. The caprine autopod features two toes, fused by metacarpal synostosis (i.e., bone fusion) prior to birth. Our analysis focused on the phalanges 1 (P1) and metacarpals of the goat autopod from birth through adulthood (3.5 years). P1 cortical bone densified rapidly after birth (half-life using one-phase exponential decay model (τ1/2 = 1.6 ± 0.4 months), but the P1 cortical thickness increased continually through adulthood (τ1/2 = 7.2 ± 2.7 mo). Upon normalization by body mass, the normalized polar moment of inertia of P1 cortical bone was constant over time, suggestive of structural load adaptation. P1 trabecular bone increased in trabecular number (τ1/2 = 6.7 ± 2.8 mo) and thickness (τ1/2 = 6.6 ± 2.0 mo) until skeletal maturity, while metacarpal trabeculae grew primarily through trabecular thickening (τ1/2 = 7.9 ± 2.2 mo). Unlike prenatal fusion of the metacarpal diaphysis, synostosis of the epiphyses occurred postnatally, prior to growth plate closure, through a unique fibrocartilaginous endochondral ossification. These findings implicate ambulatory loading in postnatal bone development of precocial goats and identify a novel postnatal synostosis event in the caprine metacarpal epiphysis.

Functional analysis of a novel pathogenic variant in CREBBP associated with bone development

BACKGROUND

CREBBP has been extensively studied in syndromic diseases associated with skeletal dysplasia. However, there is limited research on the molecular mechanisms through which CREBBP may impact bone development. We identified a novel pathogenic CREBBP variant (c.C3862T/p.R1288W, which is orthologous to mouse c.3789 C > T/p.R1289W) in a patient with non-syndromic polydactyly.

METHODS

We created a homozygous Crebbp p.R1289W mouse model and compared their skeletal phenotypes to wild-type (WT) animals. Bone marrow stem cells (BMSCs) were isolated and assessed for their proliferative capacity, proportion of apoptotic cells in culture, and differentiation to chondrocytes and osteocytes.

RESULTS

We observed a significant decrease in body length in 8-week-old homozygous Crebbp p.R1289W mice. The relative length of cartilage of the digits of Crebbp p.R1289W mice was significantly increased compared to WT mice. BMSCs derived from Crebbp p.R1289W mice had significantly decreased cell proliferation and an elevated rate of apoptosis. Consistently, cell proliferative capacity was decreased and the proportion of apoptotic cells was increased in the distal femoral growth plate of Crebbp p.R1289W compared to WT mice. Chemical induction of BMSCs indicated that Crebbp p.R1289W may promote chondrocyte differentiation.

CONCLUSION

The Crebbp p.R1289W variant plays a pathogenic role in skeletal development in mice.

IMPACT

CREBBP has been extensively studied in syndromic diseases characterized by skeletal dysplasia. There is limited research regarding the molecular mechanism through which CREBBP may affect bone development. To our knowledge, we generated the first animal model of a novel Crebbp variant, which is predicted to be pathogenic for skeletal diseases. Certain pathogenic variants, such as Crebbp p.R1289W, can independently lead to variant-specific non-syndromic skeletal dysplasia.

Deficiencies in corin and atrial natriuretic peptide-mediated signaling impair endochondral ossification in bone development

Corin is a protease that activates atrial natriuretic peptide (ANP), a hormone in cardiovascular homeostasis. Structurally, ANP is similar to C-type natriuretic peptide (CNP) crucial in bone development. Here, we examine the role of corin and ANP in chondrocyte differentiation and bone formation. We show that in Corin and Nppa (encoding ANP) knockout (KO) mice, chondrocyte differentiation is impaired, resulting in shortened limb long bones. In adult mice, Corin and Nppa deficiency impairs bone density and microarchitecture. Molecular studies in cartilages from newborn Corin and Nppa KO mice and in cultured chondrocytes indicate that corin and ANP act in chondrocytes via cGMP-dependent protein kinase G signaling to inhibit mitogen-activated protein kinase phosphorylation and stimulate glycogen synthase kinase-3β phosphorylation and β-catenin upregulation. These results indicate that corin and ANP signaling regulates chondrocyte differentiation in bone development and homeostasis, suggesting that enhancing ANP signaling may improve bone quality in patients with osteoporosis.

A mathematical model of signalling molecule-mediated processes during regeneration of osteochondral defects after chondrocyte implantation

Treating bone-cartilage defects is a fundamental clinical problem. The ability of damaged cartilage to self-repair is limited due to its avascularity. Left untreated, these defects can lead to osteoarthritis. Details of osteochondral defect repair are elusive, but animal models indicate healing occurs via an endochondral ossification-like process, similar to that in the growth plate. In the growth plate, the signalling molecules parathyroid hormone-related protein (PTHrP) and Indian Hedgehog (Ihh) form a feedback loop regulating chondrocyte hypertrophy, with Ihh inducing and PTHrP suppressing hypertrophy. To better understand this repair process and to explore the regulatory role of signalling molecules on the regeneration process, we formulate a reaction-diffusion mathematical model of osteochondral defect regeneration after chondrocyte implantation. The drivers of healing are assumed to be chondrocytes and osteoblasts, and their interaction via signalling molecules. We model cell proliferation, migration and chondrocyte hypertrophy, and matrix production and conversion, spatially and temporally. We further model nutrient and signalling molecule diffusion and their interaction with the cells. We consider the PTHrP-Ihh feedback loop as the backbone mechanisms but the model is flexible to incorporate extra signalling mechanisms if needed. Our mathematical model is able to represent repair of osteochondral defects, starting with cartilage formation throughout the defect. This is followed by chondrocyte hypertrophy, matrix calcification and bone formation deep inside the defect, while cartilage at the surface is maintained and eventually separated from the deeper bone by a thin layer of calcified cartilage. The complete process requires around 48 months. A key highlight of the model demonstrates that the PTHrP-Ihh loop alone is insufficient and an extra mechanism is required to initiate chondrocyte hypertrophy, represented by a critical cartilage density. A parameter sensitivity study reveals that the timing of the repair process crucially depends on parameters, such as the critical cartilage density, and those describing the actions of PTHrP to suppress hypertrophy, such as its diffusion coefficient, threshold concentration and degradation rate.

Ex vivo organotypic bone slice culture reveals preferential chondrogenesis after sustained growth plate injury

Postnatal bone growth primarily relies on chondrocyte proliferation and osteogenic differentiation within the growth plate (GP) via endochondral ossification. Despite its importance, the GP is vulnerable to injuries, affecting 15-30 % of bone fractures. These injuries may lead to growth discrepancies, influence bone length and shape, and negatively affecting the patient's quality of life. This study aimed to investigate the molecular and cellular physiological and pathophysiological regeneration following sustained growth plate injury (GPI) in an ex vivo rat femur organotypic culture (OTC) model. Specifically, focusing on postnatal endochondral ossification process. 300 μm thick ex vivo bone cultures with a 2 mm long horizontal GPI was utilized. After 15 days of cultivation, gene expression analysis, histological and immunohistochemistry staining's were conducted to analyze key markers of endochondral ossification. In our OTCs we observed a significant increase in Sox9 expression due to GPI at day 15. The Ihh-PTHrP feedback loop was affected, favoring chondrocyte proliferation and maturation. Ihh levels increased significantly on day 7 and day 15, while PTHrP was downregulated on day 7. GPI had no impact on osteoclast number and activity, but gene expression analysis indicated OTCs' efforts to inhibit osteoclast differentiation and activation, thereby reducing bone resorption. In conclusion, our study provides novel insights into the molecular and cellular mechanisms underlying postnatal bone growth and regeneration following growth plate injury (GPI). We demonstrate that chondrocyte proliferation and differentiation play pivotal roles in the regeneration process, with the Ihh-PTHrP feedback loop modulating these processes. Importantly, our ex vivo rat femur organotypic culture model allows for the detailed investigation of these processes, providing a valuable tool for future research in the field of skeletal biology and regenerative medicine.

Involvement of the Wnt/β-catenin signalling pathway in heterotopic ossification and ossification-related diseases

Heterotopic ossification (HO) is a pathological condition characterized by the formation of bone within soft tissues. The development of HO is a result of abnormal activation of the bone formation programs, where multiple signalling pathways, including Wnt/β-catenin, BMP and hedgehog signalling, are involved. The Wnt/β-catenin signalling pathway, a conserved pathway essential for various fundamental activities, has been found to play a significant role in pathological bone formation processes. It regulates angiogenesis, chondrocyte hypertrophy and osteoblast differentiation during the development of HO. More importantly, the crosstalk between Wnt signalling and other factors including BMP, Hedgehog signalling, YAP may contribute in a HO-favourable manner. Moreover, several miRNAs may also be involved in HO formation via the regulation of Wnt signalling. This review aims to summarize the role of Wnt/β-catenin signalling in the pathogenesis of HO, its interactions with related molecules, and potential preventive and therapeutic measures targeting Wnt/β-catenin signalling.

A protocol to differentiate the chondrogenic ATDC5 cell-line for the collection of chondrocyte-derived extracellular vesicles

Skeletal growth and fracture healing rely on the mineralization of cartilage in a process called endochondral ossification. Chondrocytes firstly synthesize and then modify cartilage by the release of a wide range of particles into their extracellular space. Extracellular vesicles (EVs) are one type of such particles, but their roles in endochondral ossification are yet to be fully understood. It remains a challenge to obtain representative populations of chondrocyte-derived EVs, owing to difficulties both in preserving the function of primary chondrocytes in culture and in applying the serum-free conditions required for EV production. Here, we used the ATDC5 cell-line to recover chondrocyte-derived EVs from early- and late-differentiation stages, representing chondrocytes before and during cartilage mineralization. After screening different culture conditions, our data indicate that a serum-free Opti-MEM-based culture medium preserves chondrocyte identity and function, matrix mineralization and cell viability. We subsequently scaled-up production and isolated EVs from conditioned medium by size-exclusion chromatography. The obtained chondrocyte-derived EVs had typical ultrastructure and expression of classical EV markers, at quantities suitable for downstream experiments. Importantly, chondrocyte-derived EVs from late-differentiation stages had elevated levels of alkaline phosphatase activity. Hence, we established a method to obtain functional chondrocyte-derived EVs before and during cartilage mineralization that may aid the further understanding of their roles in endochondral bone growth and fracture healing.

The Effects of the Addition of Strontium on the Biological Response to Calcium Phosphate Biomaterials: A Systematic Review

Strontium is known for enhancing bone metabolism, osteoblast proliferation, and tissue regeneration. This systematic review aimed to investigate the biological effects of strontium-doped calcium phosphate biomaterials for bone therapy. A literature search up to May 2024 across Web of Science, PubMed, and Scopus retrieved 759 entries, with 42 articles meeting the selection criteria. The studies provided data on material types, strontium incorporation and release, and in vivo and in vitro evidence. Strontium-doped calcium phosphate biomaterials were produced via chemical synthesis and deposited on various substrates, with characterization techniques confirming successful strontium incorporation. Appropriate concentrations of strontium were non-cytotoxic, stimulating cell proliferation, adhesion, and osteogenic factor production through key signaling pathways like Wnt/β-catenin, BMP-2, Runx2, and ERK. In vivo studies identified novel bone formation, angiogenesis, and inhibition of bone resorption. These findings support the safety and efficacy of strontium-doped calcium phosphates, although the optimal strontium concentration for desired effects is still undetermined. Future research should focus on optimizing strontium release kinetics and elucidating molecular mechanisms to enhance clinical applications of these biomaterials in bone tissue engineering.

FOXC1 and FOXC2 regulate growth plate chondrocyte maturation towards hypertrophy in the embryonic mouse limb skeleton

The Forkhead box transcription factors FOXC1 and FOXC2 are expressed in condensing mesenchyme cells at the onset of endochondral ossification. We used the Prx1-cre mouse to ablate Foxc1 and Foxc2 in limb skeletal progenitor cells. Prx1-cre;Foxc1Δ/Δ;Foxc2Δ/Δ limbs were shorter than controls, with worsening phenotypes in distal structures. Cartilage formation and mineralization was severely disrupted in the paws. The radius and tibia were malformed, whereas the fibula and ulna remained unmineralized. Chondrocyte maturation was delayed, with fewer Indian hedgehog-expressing, prehypertrophic chondrocytes forming and a smaller hypertrophic chondrocyte zone. Later, progression out of chondrocyte hypertrophy was slowed, leading to an accumulation of COLX-expressing hypertrophic chondrocytes and formation of a smaller primary ossification center with fewer osteoblast progenitor cells populating this region. Targeting Foxc1 and Foxc2 in hypertrophic chondrocytes with Col10a1-cre also resulted in an expanded hypertrophic chondrocyte zone and smaller primary ossification center. Our findings suggest that FOXC1 and FOXC2 direct chondrocyte maturation towards hypertrophic chondrocyte formation. At later stages, FOXC1 and FOXC2 regulate function in hypertrophic chondrocyte remodeling to allow primary ossification center formation and osteoblast recruitment.

The role of Clec11a in bone construction and remodeling

Bone is a dynamically active tissue whose health status is closely related to its construction and remodeling, and imbalances in bone homeostasis lead to a wide range of bone diseases. The sulfated glycoprotein C-type lectin structural domain family 11 member A (Clec11a) is a key factor in bone mass regulation that significantly promotes the osteogenic differentiation of bone marrow mesenchymal stem cells and osteoblasts and stimulates chondrocyte proliferation, thereby promoting longitudinal bone growth. More importantly, Clec11a has high therapeutic potential for treating various bone diseases and can enhance the therapeutic effects of the parathyroid hormone against osteoporosis. Clec11a is also involved in the stress/adaptive response of bone to exercise via mechanical stimulation of the cation channel Pieoz1. Clec11a plays an important role in promoting bone health and preventing bone disease and may represent a new target and novel drug for bone disease treatment. Therefore, this review aims to explore the role and possible mechanisms of Clec11a in the skeletal system, evaluate its value as a potential therapeutic target against bone diseases, and provide new ideas and strategies for basic research on Clec11a and preventing and treating bone disease.

Regenerative capacity of human pluripotent stem cell-derived articular chondrocytes in vitro

The ideal cell source for articular cartilage repair remains elusive. Using developmentally inspired differentiation protocols, we induced human pluripotent stem cells (hPSCs) toward articular chondrocytes capable of joint cartilage repair in rodent models, which were distinct from growth plate chondrocytes, fated to be replaced by bone in vivo. Working toward clinical translation, we demonstrated controlled differentiation into chondrocytes by comprehensive gene expression analysis at each step of the differentiation. Articular chondrocytes derived from hPSCs could be expanded several passages in vitro without losing chondrogenic potential. Furthermore, chondrocytes isolated from these articular cartilage tissues had the potential to serially regenerate new articular and growth plate cartilage tissues. Finally, the ability to cryopreserve articular chondrocytes with the desired phenotype is critical for clinical translation and here we report no loss in cell viability or regenerative potential following cryopreservation. These results support the immense potential of hPSC-derived articular chondrocytes as a cell-based therapy for cartilage repair.

Increased Wnt5a/ROR2 signaling is associated with chondrogenesis in meniscal degeneration

The aim of the present study was to investigate the association between chondrogenic differentiation and Wnt signal expression in the degenerative process of the human meniscus. Menisci were obtained from patients with and without knee osteoarthritis (OA), and degeneration was histologically assessed using a grading system. Immunohistochemistry, real-time polymerase chain reaction (PCR), and Western blot analysis were performed to examine the expressions of chondrogenic markers and of the components of Wnt signaling. Histological analyses showed that meniscal degeneration involved a transition from a fibroblastic to a chondrogenic phenotype with the upregulation of SOX9, collagen type II, collagen type XI, and aggrecan, which were associated with increased Wnt5a and ROR2 and decreased TCF7 expressions. OA menisci showed significantly higher expressions of Wnt5a and ROR2 and significantly lower expressions of AXIN2 and TCF7 than non-OA menisci on real-time PCR and Western blot analysis. These results potentially demonstrated that increased expression of Wnt5a/ROR2 signaling promoted chondrogenesis with decreased expression in downstream Wnt/β-catenin signaling. This study provides insights into the role of Wnt signaling in the process of meniscal degeneration, shifting to a chondrogenic phenotype. The findings suggested that the increased expression of Wnt5a/ROR2 and decreased expression of the downstream target of Wnt/β-catenin signaling are associated with chondrogenesis in meniscal degeneration.

RanGAP1 maintains chromosome stability in limb bud mesenchymal cells during bone development

BACKGROUND

Bone development involves the rapid proliferation and differentiation of osteogenic lineage cells, which makes accurate chromosomal segregation crucial for ensuring cell proliferation and maintaining chromosomal stability. However, the mechanism underlying the maintenance of chromosome stability during the rapid proliferation and differentiation of Prx1-expressing limb bud mesenchymal cells into osteoblastic precursor cells remains unexplored.

METHODS

A transgenic mouse model of RanGAP1 knockout of limb and head mesenchymal progenitor cells was constructed to explore the impact of RanGAP1 deletion on bone development by histomorphology and immunostaining. Subsequently, G-banding karyotyping analysis and immunofluorescence staining were used to examine the effects of RanGAP1 deficiency on chromosome instability. Finally, the effects of RanGAP1 deficiency on chromothripsis and bone development signaling pathways were elucidated by whole-genome sequencing, RNA-sequencing, and qPCR.

RESULTS

The ablation of RanGAP1 in limb and head mesenchymal progenitor cells expressing Prx1 in mice resulted in embryonic lethality, severe cartilage and bone dysplasia, and complete loss of cranial vault formation. Moreover, RanGAP1 loss inhibited chondrogenic or osteogenic differentiation of mesenchymal stem cells (MSCs). Most importantly, we found that RanGAP1 loss in limb bud mesenchymal cells triggered missegregation of chromosomes, resulting in chromothripsis of chromosomes 1q and 14q, further inhibiting the expression of key genes involved in multiple bone development signaling pathways such as WNT, Hedgehog, TGF-β/BMP, and PI3K/AKT in the chromothripsis regions, ultimately disrupting skeletal development.

CONCLUSIONS

Our results establish RanGAP1 as a critical regulator of bone development, as it supports this process by preserving chromosome stability in Prx1-expressing limb bud mesenchymal cells.

Ptip safeguards the epigenetic control of skeletal stem cell quiescence and potency in skeletogenesis

Stem cells remain in a quiescent state for long-term maintenance and preservation of potency; this process requires fine-tuning regulatory mechanisms. In this study, we identified the epigenetic landscape along the developmental trajectory of skeletal stem cells (SSCs) in skeletogenesis governed by a key regulator, Ptip (also known as Paxip1, Pax interaction with transcription-activation domain protein-1). Our results showed that Ptip is required for maintaining the quiescence and potency of SSCs, and loss of Ptip in type II collagen (Col2)+ progenitors causes abnormal activation and differentiation of SSCs, impaired growth plate morphogenesis, and long bone dysplasia. We also found that Ptip suppressed the glycolysis of SSCs through downregulation of phosphoglycerate kinase 1 (Pgk1) by repressing histone H3 lysine 27 acetylation (H3K27ac) at the promoter region. Notably, inhibition of glycolysis improved the function of SSCs despite Ptip deficiency. To the best of our knowledge, this is the first study to establish an epigenetic framework based on Ptip, which safeguards skeletal stem cell quiescence and potency through metabolic control. This framework is expected to improve SSC-based treatments of bone developmental disorders.

NOX4 and its association with myeloperoxidase and osteopontin in regulating endochondral ossification

IMPORTANCE

Endochondral ossification plays an important role in skeletal development. Recent studies have suggested a link between increased intracellular reactive oxygen species (ROS) and skeletal disorders. Moreover, previous studies have revealed that increasing the levels of myeloperoxidase (MPO) and osteopontin (OPN) while inhibiting NADPH oxidase 4 (NOX4) can enhance bone growth. This investigation provides further evidence by showing a direct link between NOX4 and MPO, OPN in bone function.

OBJECTIVE

This study investigates NOX4, an enzyme producing hydrogen peroxide, in endochondral ossification and bone remodeling. NOX4's role in osteoblast formation and osteogenic signaling pathways is explored.

METHODS

Using NOX4-deficient (NOX4-/-) and ovariectomized (OVX) mice, we identify NOX4's potential mediators in bone maturation.

RESULTS

NOX4-/- mice displayed significant differences in bone mass and structure. Compared to the normal Control and OVX groups. Hematoxylin and eosin staining showed NOX4-/- mice had the highest trabecular bone volume, while OVX had the lowest. Proteomic analysis revealed significantly elevated MPO and OPN levels in bone marrow-derived cells in NOX4-/- mice. Immunohistochemistry confirmed increased MPO, OPN, and collagen II (COLII) near the epiphyseal plate. Collagen and chondrogenesis analysis supported enhanced bone development in NOX4-/- mice.

CONCLUSIONS AND RELEVANCE

Our results emphasize NOX4's significance in bone morphology, mesenchymal stem cell proteomics, immunohistochemistry, collagen levels, and chondrogenesis. NOX4 deficiency enhances bone development and endochondral ossification, potentially through increased MPO, OPN, and COLII expression. These findings suggest therapeutic implications for skeletal disorders.

Pre-hypertrophic chondrogenic enhancer landscape of limb and axial skeleton development

Chondrocyte differentiation controls skeleton development and stature. Here we provide a comprehensive map of chondrocyte-specific enhancers and show that they provide a mechanistic framework through which non-coding genetic variants can influence skeletal development and human stature. Working with fetal chondrocytes isolated from mice bearing a Col2a1 fluorescent regulatory sensor, we identify 780 genes and 2'704 putative enhancers specifically active in chondrocytes using a combination of RNA-seq, ATAC-seq and H3K27ac ChIP-seq. Most of these enhancers (74%) show pan-chondrogenic activity, with smaller populations being restricted to limb (18%) or trunk (8%) chondrocytes only. Notably, genetic variations overlapping these enhancers better explain height differences than those overlapping non-chondrogenic enhancers. Finally, targeted deletions of identified enhancers at the Fgfr3, Col2a1, Hhip and, Nkx3-2 loci confirm their role in regulating cognate genes. This enhancer map provides a framework for understanding how genes and non-coding variations influence bone development and diseases.

Sclerostin modulates mineralization degree and stiffness profile in the fibrocartilaginous enthesis for mechanical tissue integrity

Fibrocartilaginous entheses consist of tendons, unmineralized and mineralized fibrocartilage, and subchondral bone, each exhibiting varying stiffness. Here we examined the functional role of sclerostin, expressed in mature mineralized fibrochondrocytes. Following rapid mineralization of unmineralized fibrocartilage and concurrent replacement of epiphyseal hyaline cartilage by bone, unmineralized fibrocartilage reexpanded after a decline in alkaline phosphatase activity at the mineralization front. Sclerostin was co-expressed with osteocalcin at the base of mineralized fibrocartilage adjacent to subchondral bone. In Scx-deficient mice with less mechanical loading due to defects of the Achilles tendon, sclerostin+ fibrochondrocyte count significantly decreased in the defective enthesis where chondrocyte maturation was markedly impaired in both fibrocartilage and hyaline cartilage. Loss of the Sost gene, encoding sclerostin, elevated mineral density in mineralized zones of fibrocartilaginous entheses. Atomic force microscopy analysis revealed increased fibrocartilage stiffness. These lines of evidence suggest that sclerostin in mature mineralized fibrochondrocytes acts as a modulator for mechanical tissue integrity of fibrocartilaginous entheses.

Growth deficiency in a mouse model of Kabuki syndrome 2 bears mechanistic similarities to Kabuki syndrome 1

Growth deficiency is a characteristic feature of both Kabuki syndrome 1 (KS1) and Kabuki syndrome 2 (KS2), Mendelian disorders of the epigenetic machinery with similar phenotypes but distinct genetic etiologies. We previously described skeletal growth deficiency in a mouse model of KS1 and further established that a Kmt2d-/- chondrocyte model of KS1 exhibits precocious differentiation. Here we characterized growth deficiency in a mouse model of KS2, Kdm6atm1d/+. We show that Kdm6atm1d/+ mice have decreased femur and tibia length compared to controls and exhibit abnormalities in cortical and trabecular bone structure. Kdm6atm1d/+ growth plates are also shorter, due to decreases in hypertrophic chondrocyte size and hypertrophic zone height. Given these disturbances in the growth plate, we generated Kdm6a-/- chondrogenic cell lines. Similar to our prior in vitro model of KS1, we found that Kdm6a-/- cells undergo premature, enhanced differentiation towards chondrocytes compared to Kdm6a+/+ controls. RNA-seq showed that Kdm6a-/- cells have a distinct transcriptomic profile that indicates dysregulation of cartilage development. Finally, we performed RNA-seq simultaneously on Kmt2d-/-, Kdm6a-/-, and control lines at Days 7 and 14 of differentiation. This revealed surprising resemblance in gene expression between Kmt2d-/- and Kdm6a-/- at both time points and indicates that the similarity in phenotype between KS1 and KS2 also exists at the transcriptional level.

IL21 inhibits miR-361-5p to promote MAP3K9 and further aggravate the progression of shoulder arthritis

OBJECTIVE

This research aimed to explore IL-21/miR-361-5p/MAP3K9 expression in shoulder arthritis and identify its regulatory pathways.

METHODS

We established a rat shoulder arthritis model, then quantified IL21 and miR-361-5p in synovial fluid using ELISA and monitored the arthritis development. Additionally, IL21's effect on miR-361-5p levels in cultured human chondrocytes (HC-a) was assessed. Chondrocyte cell cycle status and apoptosis were measured via flow cytometry. Interactions between miR-361-5p and MAP3K9 were confirmed through dual-luciferase reporting and bioinformatic scrutiny. Protein levels of MAP3K9, p-ERK1/2, p-NF-κB, MMP1, and MMP9 were analyzed by Western blots.

RESULTS

IL21 levels were elevated, while miR-361-5p was reduced in the synovial fluid from arthritic rats compared to healthy rats. IL21 was shown to suppress miR-361-5p in chondrocytes leading to hindered cell proliferation and increased apoptosis. Western blots indicated that miR-361-5p curbed MAP3K9 expression, reducing MMP activity by attenuating the ERK1/2/NF-κB pathway in chondrocytes.

CONCLUSION

IL21 upregulation and miR-361-5p downregulation characterize shoulder arthritis, resulting in MAP3K9 overexpression. This chain of molecular events boosts MMP expression in chondrocytes and exacerbates the condition's progression.

The effects of microgravity on stem cells and the new insights it brings to tissue engineering and regenerative medicine

Conventional two-dimensional (2D) cell culture techniques may undergo modifications in the future, as life scientists have widely acknowledged the ability of three-dimensional (3D) in vitro culture systems to accurately simulate in vivo biology. In recent years, researchers have discovered that microgravity devices can address many challenges associated with 3D cell culture. Stem cells, being pluripotent cells, are regarded as a promising resource for regenerative medicine. Recent studies have demonstrated that 3D culture in microgravity devices can effectively guide stem cells towards differentiation and facilitate the formation of functional tissue, thereby exhibiting advantages within the field of tissue engineering and regenerative medicine. Furthermore, We delineate the impact of microgravity on the biological behavior of various types of stem cells, while elucidating the underlying mechanisms governing these alterations. These findings offer exciting prospects for diverse applications.

Chondrocyte hypertrophy in the growth plate promotes stress anisotropy affecting long bone development through chondrocyte column formation

The length of long bones is determined by column formation of proliferative chondrocytes and subsequent chondrocyte hypertrophy in the growth plate during bone development. Despite the importance of mechanical loading in long bone development, the mechanical conditions of the cells within the growth plate, such as the stress field, remain unclear owing to the difficulty in investigating spatiotemporal changes within dynamically growing tissues. In this study, the mechanisms of longitudinal bone growth were investigated from a mechanical perspective through column formation of proliferative chondrocytes within the growth plate before secondary ossification center formation using continuum-based particle models (CbPMs). A one-factor model, which simply describes essential aspects of a biological signaling cascade regulating cell activities within the growth plate, was developed and incorporated into CbPM. Subsequently, the developmental process and maintenance of the growth plate structure and resulting bone morphogenesis were simulated. Thus, stress anisotropy in the proliferative zone that affects bone elongation through chondrocyte column formation was identified and found to be promoted by chondrocyte hypertrophy. These results provide further insights into the mechanical regulation of multicellular dynamics during bone development.

BMP signaling maintains auricular chondrocyte identity and prevents microtia development by inhibiting protein kinase A

Elastic cartilage constitutes a major component of the external ear, which functions to guide sound to the middle and inner ears. Defects in auricle development cause congenital microtia, which affects hearing and appearance in patients. Mutations in several genes have been implicated in microtia development, yet, the pathogenesis of this disorder remains incompletely understood. Here, we show that Prrx1 genetically marks auricular chondrocytes in adult mice. Interestingly, BMP-Smad1/5/9 signaling in chondrocytes is increasingly activated from the proximal to distal segments of the ear, which is associated with a decrease in chondrocyte regenerative activity. Ablation of Bmpr1a in auricular chondrocytes led to chondrocyte atrophy and microtia development at the distal part. Transcriptome analysis revealed that Bmpr1a deficiency caused a switch from the chondrogenic program to the osteogenic program, accompanied by enhanced protein kinase A activation, likely through increased expression of Adcy5/8. Inhibition of PKA blocked chondrocyte-to-osteoblast transformation and microtia development. Moreover, analysis of single-cell RNA-seq of human microtia samples uncovered enriched gene expression in the PKA pathway and chondrocyte-to-osteoblast transformation process. These findings suggest that auricle cartilage is actively maintained by BMP signaling, which maintains chondrocyte identity by suppressing osteogenic differentiation.

Stem-Cell-Driven Chondrogenesis: Perspectives on Amnion-Derived Cells

Regenerative medicine harnesses stem cells' capacity to restore damaged tissues and organs. In vitro methods employing specific bioactive molecules, such as growth factors, bio-inductive scaffolds, 3D cultures, co-cultures, and mechanical stimuli, steer stem cells toward the desired differentiation pathways, mimicking their natural development. Chondrogenesis presents a challenge for regenerative medicine. This intricate process involves precise modulation of chondro-related transcription factors and pathways, critical for generating cartilage. Cartilage damage disrupts this process, impeding proper tissue healing due to its unique mechanical and anatomical characteristics. Consequently, the resultant tissue often forms fibrocartilage, which lacks adequate mechanical properties, posing a significant hurdle for effective regeneration. This review comprehensively explores studies showcasing the potential of amniotic mesenchymal stem cells (AMSCs) and amniotic epithelial cells (AECs) in chondrogenic differentiation. These cells exhibit innate characteristics that position them as promising candidates for regenerative medicine. Their capacity to differentiate toward chondrocytes offers a pathway for developing effective regenerative protocols. Understanding and leveraging the innate properties of AMSCs and AECs hold promise in addressing the challenges associated with cartilage repair, potentially offering superior outcomes in tissue regeneration.

Compensatory growth and recovery of cartilage cytoarchitecture after transient cell death in fetal mouse limbs

A major question in developmental and regenerative biology is how organ size and architecture are controlled by progenitor cells. While limb bones exhibit catch-up growth (recovery of a normal growth trajectory after transient developmental perturbation), it is unclear how this emerges from the behaviour of chondroprogenitors, the cells sustaining the cartilage anlagen that are progressively replaced by bone. Here we show that transient sparse cell death in the mouse fetal cartilage is repaired postnatally, via a two-step process. During injury, progression of chondroprogenitors towards more differentiated states is delayed, leading to altered cartilage cytoarchitecture and impaired bone growth. Then, once cell death is over, chondroprogenitor differentiation is accelerated and cartilage structure recovered, including partial rescue of bone growth. At the molecular level, ectopic activation of mTORC1 correlates with, and is necessary for, part of the recovery, revealing a specific candidate to be explored during normal growth and in future therapies.

MAGP2 promotes osteogenic differentiation during fracture healing through its crosstalk with the β-catenin pathway

Osteogenic differentiation is important for fracture healing. Microfibrial-associated glycoprotein 2 (MAGP2) is found to function as a proangiogenic regulator in bone formation; however, its role in osteogenic differentiation during bone repair is not clear. Here, a mouse model of critical-sized femur fracture was constructed, and the adenovirus expressing MAGP2 was delivered into the fracture site. Mice with MAGP2 overexpression exhibited increased bone mineral density and bone volume fraction (BV/TV) at Day 14 postfracture. Within 7 days postfracture, overexpression of MAGP2 increased collagen I and II expression at the fracture callus, with increasing chondrogenesis. MAGP2 inhibited collagen II level but elevated collagen I by 14 days following fracture, accompanied by increased endochondral bone formation. In mouse osteoblast precursor MC3T3-E1 cells, MAGP2 treatment elevated the expression of osteoblastic factors (osterix, BGLAP and collagen I) and enhanced ALP activity and mineralization through activating β-catenin signaling after osteogenic induction. Besides, MAGP2 could interact with lipoprotein receptor-related protein 5 (LRP5) and upregulated its expression. Promotion of osteogenic differentiation and β-catenin activation mediated by MAGP2 was partially reversed by LRP5 knockdown. Interestingly, β-catenin/transcription factor 4 (TCF4) increased MAGP2 expression probably by binding to MAGP2 promoter. These findings suggest that MAGP2 may interact with β-catenin/TCF4 to enhance β-catenin/TCF4's function and activate LRP5-activated β-catenin signaling pathway, thus promoting osteogenic differentiation for fracture repair. mRNA sequencing identified the potential targets of MAGP2, providing novel insights into MAGP2 function and the directions for future research.

Glaucocalyxin A delays the progression of OA by inhibiting NF-κB and MAPK signaling pathways

BACKGROUND

Osteoarthritis (OA) is a common degenerative joint condition marked by inflammation and cartilage breakdown. Currently, there is a dearth of treatment medications that can clearly slow the course of OA. Glaucocalyxin A (GLA) is a diterpene chemical identified and extracted from Rabdosia japonica with antithrombotic, anticoagulant, anti-tumor, anti-inflammatory, anti-oxidant, and other pharmacological properties. Previous research has linked inflammation to abnormalities in the homeostasis of the extracellular matrix (ECM). Although GLA has been shown to have anti-inflammatory qualities, its effects on the progression of OA are unknown. As a result, the goal of this study was to see if GLA could slow the course of OA.

METHODS

ATDC5 cells were stimulated by IL-1β to create an inflammatory chondrocyte damage model. Quantitative polymerase chain reaction, Western Blot, high-density culture, and immunofluorescence were used to detect the expression levels of associated gene phenotypes. We also created a mouse model of OA induced by destabilization of the medial meniscus (DMM) instability, and GLA was administered intraperitoneally once every two days for eight weeks. Mice knee specimens were stained with hematoxylin-eosin, Safranin O/fast green, and immunohistochemical, and the Osteoarthritis Research Society International grade system and Mankin's score were used to assess the protective effect of GLA on cartilage.

RESULTS

In vitro and in vivo, we explored the effects and molecular processes of GLA as a therapy for OA. The findings demonstrated that GLA might reduce the expression of associated inflammatory mediators and protect the ECM by inhibiting the NF-κB and MAPK signaling pathways. Animal research revealed that GLA could protect against the DMM-induced OA model mice by stabilizing ECM.

CONCLUSION

Taken together, our findings show that GLA has a protective impact on cartilage throughout OA progression, implying that GLA could be employed as a possible therapeutic agent for OA, thus giving a new therapeutic method for the treatment of OA.

Role of the NF-kB signalling pathway in heterotopic ossification: biological and therapeutic significance

Heterotopic ossification (HO) is a pathological process in which ectopic bone develops in soft tissues within the skeletal system. Endochondral ossification can be divided into the following types of acquired and inherited ossification: traumatic HO (tHO) and fibrodysplasia ossificans progressiva (FOP). Nuclear transcription factor kappa B (NF-κB) signalling is essential during HO. NF-κB signalling can drive initial inflammation through interactions with the NOD-like receptor protein 3 (NLRP3) inflammasome, Sirtuin 1 (SIRT1) and AMP-activated protein kinase (AMPK). In the chondrogenesis stage, NF-κB signalling can promote chondrogenesis through interactions with mechanistic target of rapamycin (mTOR), phosphatidylinositol-3-kinase (PI3K)/AKT (protein kinase B, PKB) and other molecules, including R-spondin 2 (Rspo2) and SRY-box 9 (Sox9). NF-κB expression can modulate osteoblast differentiation by upregulating secreted protein acidic and rich in cysteine (SPARC) and interacting with mTOR signalling, bone morphogenetic protein (BMP) signalling or integrin-mediated signalling under stretch stimulation in the final osteogenic stage. In FOP, mutated ACVR1-induced NF-κB signalling exacerbates inflammation in macrophages and can promote chondrogenesis and osteogenesis in mesenchymal stem cells (MSCs) through interactions with smad signalling and mTOR signalling. This review summarizes the molecular mechanism of NF-κB signalling during HO and highlights potential therapeutics for treating HO.

Supersulfides support bone growth by promoting chondrocyte proliferation in the growth plates

OBJECTIVES

While chondrocytes have mitochondria, they receive little O2 from the bloodstream. Sulfur respiration, an essential energy production system in mitochondria, uses supersulfides instead of O2. Supersulfides are inorganic and organic sulfides with catenated sulfur atoms and are primarily produced by cysteinyl tRNA synthetase-2 (CARS2). Here, we investigated the role of supersulfides in chondrocyte proliferation and bone growth driven by growth plate chondrocyte proliferation.

METHODS

We examined the effects of NaHS, an HS-/H2S donor, and cystine, the cellular source of cysteine, on the proliferation of mouse primary chondrocytes and growth of embryonic mouse tibia in vitro. We also examined the effect of RNA interference acting on the Cars2 gene on chondrocyte proliferation in the presence of cystine.

RESULTS

NaHS (30 μmol/L) enhanced tibia longitudinal growth in vitro with expansion of the proliferating zone of their growth plates. While NaHS (30 μmol/L) also promoted chondrocyte proliferation only under normoxic conditions (20 % O2), cystine (0.5 mmol/L) promoted it under both normoxic and hypoxic (2 % O2) conditions. Cars2 gene knockdown abrogated the ability of cystine (0.5 mmol/L) to promote chondrocyte proliferation under normoxic conditions, indicating that supersulfides produced by CARS2 were responsible for the cystine-dependent promotion of bone growth.

CONCLUSIONS

The presented results indicate that supersulfides play a vital role in bone growth achieved by chondrocyte proliferation in the growth plates driven by sulfur respiration.

Prenatal amoxicillin exposure induces developmental toxicity in fetal mice and its characteristics

Amoxicillin, a widely used antibiotic in human and veterinary pharmaceuticals, is now considered as an "emerging contaminant" because it exists widespreadly in the environment and brings a series of adverse outcomes. Currently, systematic studies about the developmental toxicity of amoxicillin are still lacking. We explored the potential effects of amoxicillin exposure on pregnancy outcomes, maternal/fetal serum phenotypes, and fetal multiple organ development in mice, at different doses (75, 150, 300 mg/(kg·day)) during late-pregnancy, or at a dose of 300 mg/(kg·day) during different stages (mid-/late-pregnancy) and courses (single-/multi-course). Results showed that prenatal amoxicillin exposure (PAmE) had no significant influence on the body weights of dams, but it could inhibit the physical development and reduce the survival rate of fetuses, especially during the mid-pregnancy. Meanwhile, PAmE altered multiple maternal/fetal serum phenotypes, especially in fetuses. Fetal multi-organ function results showed that PAmE inhibited testicular/adrenal steroid synthesis, long bone/cartilage and hippocampal development, and enhanced ovarian steroid synthesis and hepatic glycogenesis/lipogenesis, and the order of severity might be gonad (testis, ovary) > liver > others. Further analysis found that PAmE-induced multi-organ developmental and functional alterations had differences in stages, courses and fetal gender, and the most obvious changes might be in high-dose, late-pregnancy and multi-course, but there was no typical rule of a dose-response relationship. In conclusion, this study confirmed that PAmE could cause abnormal development and multi-organ function alterations, which deepens our understanding of the risk of PAmE and provides an experimental basis for further exploration of the long-term harm.

Skeletal growth is enhanced by a shared role for SOX8 and SOX9 in promoting reserve chondrocyte commitment to columnar proliferation

SOX8 was linked in a genome-wide association study to human height heritability, but roles in chondrocytes for this close relative of the master chondrogenic transcription factor SOX9 remain unknown. We undertook here to fill this knowledge gap. High-throughput assays demonstrate expression of human SOX8 and mouse Sox8 in growth plate cartilage. In situ assays show that Sox8 is expressed at a similar level as Sox9 in reserve and early columnar chondrocytes and turned off when Sox9 expression peaks in late columnar and prehypertrophic chondrocytes. Sox8-/- mice and Sox8fl/flPrx1Cre and Sox9fl/+Prx1Cre mice (inactivation in limb skeletal cells) have a normal or near normal skeletal size. In contrast, juvenile and adult Sox8fl/flSox9fl/+Prx1Cre compound mutants exhibit a 15 to 20% shortening of long bones. Their growth plate reserve chondrocytes progress slowly toward the columnar stage, as witnessed by a delay in down-regulating Pthlh expression, in packing in columns and in elevating their proliferation rate. SOX8 or SOX9 overexpression in chondrocytes reveals not only that SOX8 can promote growth plate cell proliferation and differentiation, even upon inactivation of endogenous Sox9, but also that it is more efficient than SOX9, possibly due to greater protein stability. Altogether, these findings uncover a major role for SOX8 and SOX9 in promoting skeletal growth by stimulating commitment of growth plate reserve chondrocytes to actively proliferating columnar cells. Further, by showing that SOX8 is more chondrogenic than SOX9, they suggest that SOX8 could be preferred over SOX9 in therapies to promote cartilage formation or regeneration in developmental and degenerative cartilage diseases.

The salamander limb: a perfect model to understand imperfect integration during skeletal regeneration

Limb regeneration in salamanders is achieved by a complex coordination of various biological processes and requires the proper integration of new tissue with old. Among the tissues found inside the limb, the skeleton is the most prominent component, which serves as a scaffold and provides support for locomotion in the animal. Throughout the years, researchers have studied the regeneration of the appendicular skeleton in salamanders both after limb amputation and as a result of fracture healing. The final outcome has been widely seen as a faithful re-establishment of the skeletal elements, characterised by a seamless integration into the mature tissue. The process of skeletal integration, however, is not well understood, and several works have recently provided evidence of commonly occurring flawed regenerates. In this Review, we take the reader on a journey through the course of bone formation and regeneration in salamanders, laying down a foundation for critically examining the mechanisms behind skeletal integration. Integration is a phenomenon that could be influenced at various steps of regeneration, and hence, we assess the current knowledge in the field and discuss how early events, such as tissue histolysis and patterning, influence the faithful regeneration of the appendicular skeleton.

Circadian protein expression patterns in healthy young adults

OBJECTIVES

To explore how the blood plasma proteome fluctuates across the 24-hour day and identify a subset of proteins that show endogenous circadian rhythmicity.

METHODS

Plasma samples from 17 healthy adults were collected hourly under controlled conditions designed to unmask endogenous circadian rhythmicity; in a subset of 8 participants, we also collected samples across a day on a typical sleep-wake schedule. A total of 6916 proteins were analyzed from each sample using the SomaScan aptamer-based multiplexed platform. We used differential rhythmicity analysis based on a cosinor model with mixed effects to identify a subset of proteins that showed circadian rhythmicity in their abundance.

RESULTS

One thousand and sixty-three (15%) proteins exhibited significant daily rhythmicity. Of those, 431 (6.2%) proteins displayed consistent endogenous circadian rhythms on both a sleep-wake schedule and under controlled conditions: it included both known and novel proteins. When models were fitted with two harmonics, an additional 259 (3.7%) proteins exhibited significant endogenous circadian rhythmicity, indicating that some rhythmic proteins cannot be solely captured by a simple sinusoidal model. Overall, we found that the largest number of proteins had their peak levels in the late afternoon/evening, with another smaller group peaking in the early morning.

CONCLUSIONS

This study reveals that hundreds of plasma proteins exhibit endogenous circadian rhythmicity in humans. Future analyses will likely reveal novel physiological pathways regulated by circadian clocks and pave the way for improved diagnosis and treatment for patients with circadian disorders and other pathologies. It will also advance efforts to include knowledge about time-of-day, thereby incorporating circadian medicine into personalized medicine.