SHP2 Regulates the Osteogenic Fate of Growth Plate Hypertrophic Chondrocytes

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.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Skeletal defects and bone metabolism in Noonan, Costello and cardio-facio-cutaneous syndromes

Noonan, Costello and Cardio-facio-cutaneous syndromes belong to a group of disorders named RASopathies due to their common pathogenetic origin that lies on the Ras/MAPK signaling pathway. Genetics has eased, at least in part, the distinction of these entities as they are presented with overlapping clinical features which, sometimes, become more pronounced with age. Distinctive face, cardiac and skeletal defects are among the primary abnormalities seen in these patients. Skeletal dysmorphisms range from mild to severe and may include anterior chest wall anomalies, scoliosis, kyphosis, short stature, hand anomalies, muscle weakness, osteopenia or/and osteoporosis. Patients usually have increased serum concentrations of bone resorption markers, while markers of bone formation are within normal range. The causative molecular defects encompass the members of the Ras/MAPK/ERK pathway and the adjacent cascades, important for the maintenance of normal bone homeostasis. It has been suggested that modulation of the expression of specific molecules involved in the processes of bone remodeling may affect the osteogenic fate decision, potentially, bringing out new pharmaceutical targets. Currently, the laboratory imprint of bone metabolism on the clinical picture of the affected individuals is not clear, maybe due to the rarity of these syndromes, the small number of the recruited patients and the methods used for the description of their clinical and biochemical profiles.

Genome-wide CRISPR screening of chondrocyte maturation newly implicates genes in skeletal growth and height-associated GWAS loci

Alterations in the growth and maturation of chondrocytes can lead to variation in human height, including monogenic disorders of skeletal growth. We aimed to identify genes and pathways relevant to human growth by pairing human height genome-wide association studies (GWASs) with genome-wide knockout (KO) screens of growth-plate chondrocyte proliferation and maturation in vitro. We identified 145 genes that alter chondrocyte proliferation and maturation at early and/or late time points in culture, with 90% of genes validating in secondary screening. These genes are enriched in monogenic growth disorder genes and in KEGG pathways critical for skeletal growth and endochondral ossification. Further, common variants near these genes capture height heritability independent of genes computationally prioritized from GWASs. Our study emphasizes the value of functional studies in biologically relevant tissues as orthogonal datasets to refine likely causal genes from GWASs and implicates new genetic regulators of chondrocyte proliferation and maturation.

Setting sail: Maneuvering SHP2 activity and its effects in cancer

Since the discovery of tyrosine phosphorylation being a critical modulator of cancer signaling, proteins regulating phosphotyrosine levels in cells have fast become targets of therapeutic intervention. The nonreceptor protein tyrosine phosphatase (PTP) coded by the PTPN11 gene "SHP2" integrates phosphotyrosine signaling from growth factor receptors into the RAS/RAF/ERK pathway and is centrally positioned in processes regulating cell development and oncogenic transformation. Dysregulation of SHP2 expression or activity is linked to tumorigenesis and developmental defects. Even as a compelling anti-cancer target, SHP2 was considered "undruggable" for a long time owing to its conserved catalytic PTP domain that evaded drug development. Recently, SHP2 has risen from the "undruggable curse" with the discovery of small molecules that manipulate its intrinsic allostery for effective inhibition. SHP2's unique domain arrangement and conformation(s) allow for a truly novel paradigm of inhibitor development relying on skillful targeting of noncatalytic sites on proteins. In this review we summarize the biological functions, signaling properties, structural attributes, allostery and inhibitors of SHP2.

From Stem to Sternum: The Role of Shp2 in the Skeleton

Src homology-2 domain-containing phosphatase 2 (SHP2) is a ubiquitously expressed phosphatase that is vital for skeletal development and maintenance of chondrocytes, osteoblasts, and osteoclasts. Study of SHP2 function in small animal models has led to insights in phenotypes observed in SHP2-mutant human disease, such as Noonan syndrome. In recent years, allosteric SHP2 inhibitors have been developed to specifically target the protein in neoplastic processes. These inhibitors are highly specific and have great potential for disease modulation in cancer and other pathologies, including bone disorders. In this review, we discuss the importance of SHP2 and related signaling pathways (e.g., Ras/MEK/ERK, JAK/STAT, PI3K/Akt) in skeletal development. We review rodent models of pathologic processes caused by germline mutations that activate SHP2 enzymatic activity, with a focus on the skeletal phenotype seen in these patients. Finally, we discuss SHP2 inhibitors in development and their potential for disease modulation in these genetic diseases, particularly as it relates to the skeleton.

Rotating Magnetic Field Mitigates Ankylosing Spondylitis Targeting Osteocytes and Chondrocytes via Ameliorating Immune Dysfunctions

Ankylosing spondylitis (AS) is clinically characterized by bone fusion that is induced by the pathological formation of extra bone. Unfortunately, the fundamental mechanism and related therapies remain unclear. The loss of SHP-2 (encoded by Ptpn11) in CD4-Cre;Ptpn11^f/f mice resulted in the induction of AS-like pathological characteristics, including spontaneous cartilage and bone lesions, kyphosis, and arthritis. Hence, this mouse was utilized as an AS model in this study. As one of the basic physical fields, the magnetic field (MF) has been proven to be an effective treatment method for articular cartilage degeneration. In this study, the effects of a rotating magnetic field (RMF; 0.2 T, 4 Hz) on an AS-like mouse model were investigated. The RMF treatment (2 h/d, 0.2 T, 4 Hz) was performed on AS mice from two months after birth until the day before sampling. The murine specimens were subjected to transcriptomics, immunomics, and metabolomics analyses, combined with molecular and pathological experiments. The results demonstrated that the mitigation of inflammatory deterioration resulted in an increase in functional osteogenesis and a decrease in dysfunctional osteolysis due to the maintenance of bone homeostasis via the RANKL/RANK/OPG signaling pathway. Additionally, by regulating the ratio of CD4+ and CD8+ T-cells, RMF treatment rebalanced the immune microenvironment in skeletal tissue. It has been observed that RMF interventions have the potential to alleviate AS, including by decreasing pathogenicity and preventing disease initiation. Consequently, RMF, as a moderately physical therapeutic strategy, could be considered to alleviate the degradation of cartilage and bone tissue in AS and as a potential option to halt the progression of AS.

Purine metabolites promote ectopic new bone formation in ankylosing spondylitis

Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease that mainly affects the axial skeleton, whose typical features are inflammatory back pain, bone structural damage and pathological new bone formation. The pathology of ectopic new bone formation is still little known. In this study, we found increased purine metabolites in plasma of patients with AS. Similarly, metabolome analysis indicated increased purine metabolites in both serum of CD4-Cre; Ptpn11^fl/fl and SHP2-deficient chondrocytes. SHP2-deficient chondrocytes promoted the growth of wild type chondrocytes and differentiation of osteoblasts in CD4-Cre; Ptpn11^fl/fl mice, which spontaneously developed AS-like bone disease. Purine metabolites, along with PTHrP derived from SHP2-deficient chondrocytes, accelerated the growth of chondrocytes and ectopic new bone formation through PKA/CREB signaling. Moreover, Suramin, a purinergic receptor antagonist, suppressed pathological new bone formation in AS-like bone disease. Overall, these results highlight the potential role of targeting purinergic signaling in retarding ectopic new bone formation in AS.

The Cell-Specific Role of SHP2 in Regulating Bone Homeostasis and Regeneration Niches

Src homology-2 containing protein tyrosine phosphatase (SHP2), encoded by PTPN11, has been proven to participate in bone-related diseases, such as Noonan syndrome (NS), metachondromatosis and osteoarthritis. However, the mechanisms of SHP2 in bone remodeling and homeostasis maintenance are complex and undemonstrated. The abnormal expression of SHP2 can influence the differentiation and maturation of osteoblasts, osteoclasts and chondrocytes. Meanwhile, SHP2 mutations can act on the immune system, vasculature and nervous system, which in turn affect bone development and remodeling. Signaling pathways regulated by SHP2, such as mitogen-activated protein kinase (MAPK), Indian hedgehog (IHH) and phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)/protein kinase B (AKT), are also involved in the proliferation, differentiation and migration of bone functioning cells. This review summarizes the recent advances of SHP2 on osteogenesis-related cells and niche cells in the bone marrow microenvironment. The phenotypic features of SHP2 conditional knockout mice and underlying mechanisms are discussed. The prospective applications of the current agonists or inhibitors that target SHP2 in bone-related diseases are also described. Full clarification of the role of SHP2 in bone remodeling will shed new light on potential treatment for bone related diseases.

A Shifting Paradigm: Transformation of Cartilage to Bone during Bone Repair

While formation and regeneration of the skeleton have been studied for a long period of time, significant scientific advances in this field continue to emerge based on an unmet clinical need to improve options to promote bone repair. In this review, we discuss the relationship between mechanisms of bone formation and bone regeneration. Data clearly show that regeneration is not simply a reinduction of the molecular and cellular programs that were used for development. Instead, the mechanical environment exerts a strong influence on the mode of repair, while during development, cell-intrinsic processes drive the mode of skeletal formation. A major advance in the field has shown that cell fate is flexible, rather than terminal, and that chondrocytes are able to differentiate into osteoblasts and other cell types during development and regeneration. This is discussed in a larger context of regeneration in vertebrates as well as the clinical implication that this shift in understanding presents.

A comprehensive review of SHP2 and its role in cancer

Src homology 2-containing protein tyrosine phosphatase 2 (SHP2) is a non-receptor protein tyrosine phosphatase ubiquitously expressed mainly in the cytoplasm of several tissues. SHP2 modulates diverse cell signaling events that control metabolism, cell growth, differentiation, cell migration, transcription and oncogenic transformation. It interacts with diverse molecules in the cell, and regulates key signaling events including RAS/ERK, PI3K/AKT, JAK/STAT and PD-1 pathways downstream of several receptor tyrosine kinases (RTKs) upon stimulation by growth factors and cytokines. SHP2 acts as both a phosphatase and a scaffold, and plays prominently oncogenic functions but can be tumor suppressor in a context-dependent manner. It typically acts as a positive regulator of RTKs signaling with some inhibitory functions reported as well. SHP2 expression and activity is regulated by such factors as allosteric autoinhibition, microRNAs, ubiquitination and SUMOylation. Dysregulation of SHP2 expression or activity causes many developmental diseases, and hematological and solid tumors. Moreover, upregulated SHP2 expression or activity also decreases sensitivity of cancer cells to anticancer drugs. SHP2 is now considered as a compelling anticancer drug target and several classes of SHP2 inhibitors with different mode of action are developed with some already in clinical trial phases. Moreover, novel SHP2 substrates and functions are rapidly growing both in cell and cancer. In view of this, we comprehensively and thoroughly reviewed literatures about SHP2 regulatory mechanisms, substrates and binding partners, biological functions, roles in human cancers, and different classes of small molecule inhibitors target this oncoprotein in cancer.

The role of the protein tyrosine phosphatase SHP2 in ossification

SHP2, encoded by the PTPN11 gene, participates in multiple cell functions including cell proliferation, movement, and differentiation. PTPN11 loss-of-function and gain-of-function mutations are both associated with diseases, such as Noonan syndrome, whose manifestations include bone defects, suggesting a crucial role for SHP2 in the skeleton. However, the exact mechanisms by which SHP2 regulates bone development remain unclear. This review focuses on the current understanding of the regulation of SHP2 and highlights the vital roles of SHP2 in skeletal development, especially its roles in ossification. Overall, a better understanding of the functions of SHP2 in ossification will provide a new avenue to treat-related skeletal diseases.

Targeting chondrocytes for arresting bony fusion in ankylosing spondylitis

Bony fusion caused by pathological new bone formation manifests the clinical feature of ankylosing spondylitis (AS). However, the underlying mechanism remains elusive. Here we discovered spontaneous kyphosis, arthritis and bony fusion in mature CD4-Cre;Ptpn11^f/f mice, which present the pathophysiological features of AS. A population of CD4-Cre-expressing proliferating chondrocytes was SHP2 deficient, which could differentiate into pre-hypertrophic and hypertrophic chondrocytes. Functionally, SHP2 deficiency in chondrocytes impeded the fusion of epiphyseal plate and promoted chondrogenesis in joint cavity and enthesis. Mechanistically, aberrant chondrocytes promoted ectopic new bone formation through BMP6/pSmad1/5 signaling. It is worth emphasizing that such pathological thickness of growth plates was evident in adolescent humans with enthesitis-related arthritis, which could progress to AS in adulthood. Targeting dysfunctional chondrogenesis with Smo inhibitor sonidegib significantly alleviated the AS-like bone disease in mice. These findings suggest that blockade of chondrogenesis by sonidegib would be a drug repurposing strategy for AS treatment.

Current treatments cannot significantly alleviate the radiographic progression in ankylosing spondylitis (AS), which results in joints stiffness and bony fusion of AS. Smo inhibitor sonidegib retards the pathological new bone formation in AS through targeting dysfunctional chondrogenesis.

SHP-2 deletion in CD4Cre expressing chondrocyte precursors leads to tumor development with wrist tropism

Due to redundancy with other tyrosine phosphatases, the ubiquitously expressed tyrosine phosphatase SHP-2 (encoded by Ptpn11) is not required for T cell development. However, Ptpn11 gene deletion driven by CD4 Cre recombinase leads to cartilage tumors in the wrist. Using a fate mapping system, we demonstrate that wrist tumor development correlates with increased frequency and numbers of non-hematopoietic lineage negative CD45 negative cells with a bone chondrocyte stromal cell precursor cell (BCSP) phenotype. Importantly, the BCSP subset has a history of CD4 expression and a marked wrist location tropism, explaining why the wrist is the main site of tumor development. Mechanistically, we found that in SHP-2 absence, SOX-9 is no longer regulated, leading to an uncontrolled proliferation of the BCSP subset. Altogether, these results identify a unique subset of chondrocyte precursors tightly regulated by SHP-2. These findings underscore the need for the development of methods to therapeutically target this subset of cells, which could potentially have an impact on treatment of SHP-2 dysfunction linked debilitating diseases.

SHP2 inhibition attenuates osteoarthritis by maintaining homeostasis of cartilage metabolism via the DOK1/UPP1/uridine cascade

OBJECTIVE

Protein tyrosine kinases (PTKs) regulate osteoarthritis (OA) progression by activating a series of signal transduction pathways. However, the roles of protein tyrosine phosphatases (PTPs) in OA remain obscure.

METHODS

The expression of 107 PTP genes in human OA cartilage was analyzed based on a single-cell sequencing dataset. The enzyme activity of the PTP SHP2 was detected in primary chondrocytes after interleukin (IL)-1β treatment and in human OA cartilage. Destabilized medial meniscus (DMM) model and IL-1β-stimulated primary mouse chondrocytes were treated with an SHP2 inhibitor and celecoxib (a clinical drug for the treatment of OA). The function of SHP2 in OA pathogenesis was further verified in Aggrecan-Cre^ERT ; SHP2 ^flox/flox mice. The downstream protein expression profile and dephosphorylated substrate of SHP2 were examined by tandem mass tag (TMT) labeling-based global proteomic and stable isotope labeling using amino acids in cell culture (SILAC)-labeled tyrosine phosphoproteomic analysis, respectively.

RESULTS

SHP2 enzyme activity significantly increased in human OA samples with serious articular cartilage injury and in IL-1β-stimulated chondrocytes. Pharmacological inhibition or genetic deletion of SHP2 ameliorated OA progression. SHP2 inhibitors dramatically reduced the expression of cartilage degradation-related genes and simultaneously promoted the expression of cartilage synthesis-related genes. Mechanistically, SHP2 inhibition suppressed the dephosphorylation of DOK1 and subsequently reduced the expression of uridine phosphorylase 1 and increased uridine level, thereby contributing to the homeostasis of cartilage metabolism.

CONCLUSIONS

SHP2 is a novel accelerator of the imbalance in the cartilage homeostasis. Specific inhibition of SHP2 may ameliorate OA by maintaining the anabolic and catabolic balance.

Comparative Study of DHA with Different Molecular Forms for Ameliorating Osteoporosis by Promoting Chondrocyte-to-Osteoblast Transdifferentiation in the Growth Plate of Ovariectomized Mice

Osteoblasts play a key role in bone remodeling. Recent studies have reported that some hypertrophic chondrocytes co-expressing collagen I(Col I) and collagen X (ColX) could directly transdifferentiate into osteoblasts during endochondral ossification. However, whether nutrition intervention is beneficial to this transformation to improve osteoporosis (OP) remains unknown. In this study, ovariectomy (OVX)-induced OP mice were orally administered with docosahexaenoic acid (DHA) in different molecular forms for 13 weeks. The results showed that both DHA-triglyceride (DHA-TG) and DHA-phosphatidylcholine (DHA-PC) increased the bone mineral density and bone mineral apposition rate in ovariectomized mice, while DHA-ethyl esters (DHA-EE) had little effect. Interestingly, we found that both DHA-PC and DHA-TG increased the height of the growth plate, mainly increasing the number of hypertrophic chondrocytes. Further investigation by simultaneously labeling ColX and ColI indicated that DHA-PC and DHA-TG promoted the number of chondrocyte-transdifferentiated osteoblasts in the growth plate close to the diaphysis, in which DHA-PC performed better than DHA-TG. Apoptosis was not the only fate of hypertrophic chondrocytes. Western blot results showed that both DHA-TG and DHA-PC downregulated the Bax and cleaved-caspase3 expression and upregulated Bcl-2 expression in the growth plate, suggesting that chondrocyte apoptosis is inhibited. Runx2, the key regulator of chondrocyte-to-osteoblast transdifferentiation, was significantly increased by DHA-TG and DHA-PC, while DHA-EE had no effect on the above indicators. To our best knowledge, this is the first report that both DHA-PC and DHA-TG enhanced bone formation via promoting the chondrocyte-to-osteoblast transdifferentiation in the growth plate, contributing to the amelioration of OP. These activities depend on the molecular forms of DHA and their bioavailabilities. Our results provide guidance for the application of fish oil for bone health.

Enchondromatosis and Growth Plate Development

PURPOSE OF REVIEW

Enchondroma is a common cartilage benign tumor that develops from dysregulation of chondrocyte terminal differentiation during growth plate development. Here we provide an overview of recent progress in understanding causative mutations for enchondroma, dysregulated signaling and metabolic pathways in enchondroma, and the progression from enchondroma to malignant chondrosarcoma.

RECENT FINDINGS

Several signaling pathways that regulate chondrocyte differentiation are dysregulated in enchondromas. Somatic mutations in the metabolic enzymes isocitrate dehydrogenase 1 and 2 (IDH1/2) are the most common findings in enchondromas. Mechanisms including metabolic regulation, epigenetic regulation, and altered signaling pathways play a role in enchondroma formation and progression. Multiple pathways regulate growth plate development in a coordinated manner. Deregulation of the process can result in chondrocytes failing to undergo differentiation and the development of enchondroma.

Targeted Ptpn11 deletion in mice reveals the essential role of SHP2 in osteoblast differentiation and skeletal homeostasis

The maturation and function of osteoblasts (OBs) rely heavily on the reversible phosphorylation of signaling proteins. To date, most of the work in OBs has focused on phosphorylation by tyrosyl kinases, but little has been revealed about dephosphorylation by protein tyrosine phosphatases (PTPases). SHP2 (encoded by PTPN11) is a ubiquitously expressed PTPase. PTPN11 mutations are associated with both bone and cartilage manifestations in patients with Noonan syndrome (NS) and metachondromatosis (MC), although the underlying mechanisms remain elusive. Here, we report that SHP2 deletion in bone gamma-carboxyglutamate protein-expressing (Bglap⁺) bone cells leads to massive osteopenia in both trabecular and cortical bones due to the failure of bone cell maturation and enhanced osteoclast activity, and its deletion in Bglap⁺ chondrocytes results in the onset of enchondroma and osteochondroma in aged mice with increased tubular bone length. Mechanistically, SHP2 was found to be required for osteoblastic differentiation by promoting RUNX2/OSTERIX signaling and for the suppression of osteoclastogenesis by inhibiting STAT3-mediated RANKL production by osteoblasts and osteocytes. These findings are likely to explain the compromised skeletal system in NS and MC patients and to inform the development of novel therapeutics to combat skeletal disorders.

Apoptosis in the Extraosseous Calcification Process

Extraosseous calcification is a pathologic mineralization process occurring in soft connective tissues (e.g., skin, vessels, tendons, and cartilage). It can take place on a genetic basis or as a consequence of acquired chronic diseases. In this last case, the etiology is multifactorial, including both extra- and intracellular mechanisms, such as the formation of membrane vesicles (e.g., matrix vesicles and apoptotic bodies), mitochondrial alterations, and oxidative stress. This review is an overview of extraosseous calcification mechanisms focusing on the relationships between apoptosis and mineralization in cartilage and vascular tissues, as these are the two tissues mostly affected by a number of age-related diseases having a progressively increased impact in Western Countries.

Pathomechanisms of Posttraumatic Osteoarthritis: Chondrocyte Behavior and Fate in a Precarious Environment

Traumatic injuries of the knee joint result in a wide variety of pathomechanisms, which contribute to the development of so-called posttraumatic osteoarthritis (PTOA). These pathogenetic processes include oxidative stress, excessive expression of catabolic enzymes, release of damage-associated molecular patterns (DAMPs), and synovial inflammation. The present review focuses on the underlying pathomechanisms of PTOA and in particular the behavior and fate of the surviving chondrocytes, comprising chondrocyte metabolism, regulated cell death, and phenotypical changes comprising hypertrophy and senescence. Moreover, possible therapeutic strategies, such as chondroanabolic stimulation, anti-oxidative and anti-inflammatory treatment, as well as novel therapeutic targets are discussed.

A Second Career for Chondrocytes-Transformation into Osteoblasts

PURPOSE OF REVIEW

The goal of the review is to summarize the current knowledge on the process of chondrocyte-to-osteoblast transdifferentiation during endochondral bone formation and its potential implications in fracture healing and disease.

RECENT FINDINGS

Lineage tracing experiments confirmed the transdifferentiation of chondrocytes into osteoblasts. More recent studies lead to the discovery of molecules involved in this process, as well as to the hypothesis that these cells may re-enter a stem cell-like phase prior to their osteoblastic differentiation. This review recapitulates the current knowledge regarding chondrocyte transdifferentiating into osteoblasts, the developmental and postnatal events where transdifferentiation appears to be relevant, and the molecules implicated in this process.

SHP2 regulates intramembranous ossification by modifying the TGFβ and BMP2 signaling pathway

SHP2 is a ubiquitously expressed protein tyrosine phosphatase, which is involved in many signaling pathways to regulate the skeletal development. In endochondral ossification, SHP2 is known to modify the osteogenic fate of osteochondroprogenitors and to impair the osteoblastic transdifferentiation of hypertrophic chondrocytes. However, how SHP2 regulates osteoblast differentiation in intramembranous ossification remains incompletely understood. To address this question, we generated a mouse model to ablate SHP2 in the Prrx1-expressing mesenchymal progenitors by using "Cre-loxP"-mediated gene excision and examined the development of calvarial bone, in which the main process of bone formation is intramembranous ossification. Phenotypic characterization showed that SHP2 mutants have severe defects in calvarial bone formation. Cell lineage tracing and in situ hybridization data showed less osteoblast differentiation of mesenchymal cells and reduced osteogenic genes expression, respectively. Further mechanistic studies revealed enhanced TGFβ and suppressed BMP2 signaling in SHP2 ablated mesenchymal progenitors and their derivatives. Our study uncovered the critical role of SHP2 in osteoblast differentiation through intramembranous ossification and might provide a potential target to treat craniofacial skeleton disorders.

Noonan syndrome-causing SHP2 mutants impair ERK-dependent chondrocyte differentiation during endochondral bone growth

Growth retardation is a constant feature of Noonan syndrome (NS) but its physiopathology remains poorly understood. We previously reported that hyperactive NS-causing SHP2 mutants impair the systemic production of insulin-like growth factor 1 (IGF1) through hyperactivation of the RAS/extracellular signal-regulated kinases (ERK) signalling pathway. Besides endocrine defects, a direct effect of these mutants on growth plate has not been explored, although recent studies have revealed an important physiological role for SHP2 in endochondral bone growth. We demonstrated that growth plate length was reduced in NS mice, mostly due to a shortening of the hypertrophic zone and to a lesser extent of the proliferating zone. These histological features were correlated with decreased expression of early chondrocyte differentiation markers, and with reduced alkaline phosphatase staining and activity, in NS murine primary chondrocytes. Although IGF1 treatment improved growth of NS mice, it did not fully reverse growth plate abnormalities, notably the decreased hypertrophic zone. In contrast, we documented a role of RAS/ERK hyperactivation at the growth plate level since 1) NS-causing SHP2 mutants enhance RAS/ERK activation in chondrocytes in vivo (NS mice) and in vitro (ATDC5 cells) and 2) inhibition of RAS/ERK hyperactivation by U0126 treatment alleviated growth plate abnormalities and enhanced chondrocyte differentiation. Similar effects were obtained by chronic treatment of NS mice with statins. In conclusion, we demonstrated that hyperactive NS-causing SHP2 mutants impair chondrocyte differentiation during endochondral bone growth through a local hyperactivation of the RAS/ERK signalling pathway, and that statin treatment may be a possible therapeutic approach in NS.

The Chondro-Osseous Continuum: Is It Possible to Unlock the Potential Assigned Within?

Endochondral ossification (EO), by which long bones of the axial skeleton form, is a tightly regulated process involving chondrocyte maturation with successive stages of proliferation, maturation, and hypertrophy, accompanied by cartilage matrix synthesis, calcification, and angiogenesis, followed by osteoblast-mediated ossification. This developmental sequence reappears during fracture repair and in osteoarthritic etiopathology. These similarities suggest that EO, and the cells involved, are of great clinical importance for bone regeneration as it could provide novel targeted approaches to increase specific signaling to promote fracture healing, and if regulated appropriately in the treatment of osteoarthritis. The long-held accepted dogma states that hypertrophic chondrocytes are terminally differentiated and will eventually undergo apoptosis. In this mini review, we will explore recent evidence from experiments that revisit the idea that hypertrophic chondrocytes have pluripotent capacity and may instead transdifferentiate into a specific sub-population of osteoblast cells. There are multiple lines of evidence, including our own, showing that local, selective alterations in cartilage extracellular matrix (ECM) remodeling also indelibly alter bone quality. This would be consistent with the hypothesis that osteoblast behavior in long bones is regulated by a combination of their lineage origins and the epigenetic effects of chondrocyte-derived ECM which they encounter during their recruitment. Further exploration of these processes could help to unlock potential novel targets for bone repair and regeneration and in the treatment of osteoarthritis.