SHP2 regulates chondrocyte terminal differentiation, growth plate architecture and skeletal cell fates

Protein phosphatase SCP4 regulates cartilage development and endochondral osteogenesis via FoxO3a dephosphorylation

The regulatory mechanisms involved in embryonic development are complex and yet remain unclear. SCP4 represents a novel nucleus-resident phosphatase identified in our previous study. The primary aim of this study was to elucidate the function of SCP4 in the progress of cartilage development and endochondral osteogenesis. SCP4-/- and SCP4Col2ER mice were constructed to assess differences in bone formation using whole skeleton staining. ABH/OG staining was used to compare chondrocyte differentiation and cartilage development. Relevant biological functions were analysed using RNA-sequencing and GO enrichment, further validated by immunohistochemical staining, Co-IP and Western Blot. Global SCP4 knockout led to abnormal embryonic development in SCP4-/- mice, along with delayed endochondral osteogenesis. In parallel, chondrocyte-specific removal of SCP4 yielded more severe embryonic deformities in SCP4Col2ER mice, including limb shortening, reduced chondrocyte number in the growth plate, disorganisation and cell enlargement. Moreover, RNA-sequencing analysis showed an association between SCP4 and chondrocyte apoptosis. Notably, Tunnel-positive cells were indeed increased in the growth plates of SCP4Col2ER mice. The deficiency of SCP4 up-regulated the expression levels of pro-apoptotic proteins both in vivo and in vitro. Additionally, phosphorylation of FoxO3a (pFoxO3a), a substrate of SCP4, was heightened in chondrocytes of SCP4Col2ER mice growth plate, and the direct interaction between SCP4 and pFoxO3a was further validated in chondrocytes. Our findings underscore the critical role of SCP4 in regulating cartilage development and endochondral osteogenesis during embryonic development partially via inhibition of chondrocytes apoptosis regulated by FoxO3a dephosphorylation.

Targeting KRAS and SHP2 signaling pathways for immunomodulation and improving treatment outcomes in solid tumors

Historically, KRAS has been considered 'undruggable' inspite of being one of the most frequently altered oncogenic proteins in solid tumors, primarily due to the paucity of pharmacologically 'druggable' pockets within the mutant isoforms. However, pioneering developments in drug design capable of targeting the mutant KRAS isoforms especially KRASG12C-mutant cancers, have opened the doors for emergence of combination therapies comprising of a plethora of inhibitors targeting different signaling pathways. SHP2 signaling pathway, primarily known for activation of intracellular signaling pathways such as KRAS has come up as a potential target for such combination therapies as it emerged to be the signaling protein connecting KRAS and the immune signaling pathways and providing the link for understanding the overlapping regions of RAS/ERK/MAPK signaling cascade. Thus, SHP2 inhibitors having potent tumoricidal activity as well as role in immunomodulation have generated keen interest in researchers to explore its potential as combination therapy in KRAS mutant solid tumors. However, the excitement with these combination therapies need to overcome challenges thrown up by drug resistance and enhanced toxicity. In this review, we will discuss KRAS and SHP2 signaling pathways and their roles in immunomodulation and regulation of tumor microenvironment and also analyze the positive effects and drawbacks of the different combination therapies targeted at these signaling pathways along with their present and future potential to treat solid tumors.

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.

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

Skeletal stem cells (SSCs) are tissue-specific stem cells that can self-renew and sit at the apex of their differentiation hierarchy, giving rise to mature skeletal cell types required for bone growth, maintenance, and repair. Dysfunction in SSCs is caused by stress conditions like ageing and inflammation and is emerging as a contributor to skeletal pathology, such as the pathogenesis of fracture nonunion. Recent lineage tracing experiments have shown that SSCs exist in the bone marrow, periosteum, and resting zone of the growth plate. Unraveling their regulatory networks is crucial for understanding skeletal diseases and developing therapeutic strategies. In this review, we systematically introduce the definition, location, stem cell niches, regulatory signaling pathways, and clinical applications of SSCs.

The fate of early perichondrial cells in developing bones

In endochondral bone development, bone-forming osteoblasts and bone marrow stromal cells have dual origins in the fetal cartilage and its surrounding perichondrium. However, how early perichondrial cells distinctively contribute to developing bones remain unidentified. Here we show using in vivo cell-lineage analyses that Dlx5+ fetal perichondrial cells marked by Dlx5-creER do not generate cartilage but sustainably contribute to cortical bone and marrow stromal compartments in a manner complementary to fetal chondrocyte derivatives under the regulation of Hedgehog signaling. Postnatally, Dlx5+ fetal perichondrial cell derivatives preferentially populate the diaphyseal marrow stroma with a dormant adipocyte-biased state and are refractory to parathyroid hormone-induced bone anabolism. Therefore, early perichondrial cells of the fetal cartilage are destined to become an adipogenic subset of stromal cells in postnatal diaphyseal bone marrow, supporting the theory that the adult bone marrow stromal compartments are developmentally prescribed within the two distinct cells-of-origins of the fetal bone anlage.

A Cd9+Cd271+ stem/progenitor population and the SHP2 pathway contribute to neonatal-to-adult switching that regulates tendon maturation

Tendon maturation lays the foundation for postnatal tendon development, its proper mechanical function, and regeneration, but the critical cell populations and the entangled mechanisms remain poorly understood. Here, by integrating the structural, mechanical, and molecular properties, we show that post-natal days 7-14 are the crucial transitional stage for mouse tendon maturation. We decode the cellular and molecular regulatory networks at the single-cell level. We find that a nerve growth factor (NGF)-secreting Cd9⁺Cd271⁺ tendon stem/progenitor cell population mainly prompts conversion from neonate to adult tendon. Through single-cell gene regulatory network analysis, in vitro inhibitor identification, and in vivo tendon-specific Shp2 deletion, we find that SHP2 signaling is a regulator for tendon maturation. Our research comprehensively reveals the dynamic cell population transition during tendon maturation, implementing insights into the critical roles of the maturation-related stem cell population and SHP2 signaling pathway during tendon differentiation and regeneration.

The Extracellular Signal-Regulated Kinase Mitogen-Activated Protein Kinase Pathway in Osteoblasts

Extracellular signal-regulated kinases (ERKs) are evolutionarily ancient signal transducers of the mitogen-activated protein kinase (MAPK) family that have long been linked to the regulation of osteoblast differentiation and bone formation. Here, we review the physiological functions, biochemistry, upstream activators, and downstream substrates of the ERK pathway. ERK is activated in skeletal progenitors and regulates osteoblast differentiation and skeletal mineralization, with ERK serving as a key regulator of Runt-related transcription factor 2, a critical transcription factor for osteoblast differentiation. However, new evidence highlights context-dependent changes in ERK MAPK pathway wiring and function, indicating a broader set of physiological roles associated with changes in ERK pathway components or substrates. Consistent with this importance, several human skeletal dysplasias are associated with dysregulation of the ERK MAPK pathway, including neurofibromatosis type 1 and Noonan syndrome. The continually broadening array of drugs targeting the ERK pathway for the treatment of cancer and other disorders makes it increasingly important to understand how interference with this pathway impacts bone metabolism, highlighting the importance of mouse studies to model the role of the ERK MAPK pathway in bone formation.

Protein tyrosine phosphatases in skeletal development and diseases

Skeletal development and homeostasis in mammals are modulated by finely coordinated processes of migration, proliferation, differentiation, and death of skeletogenic cells originating from the mesoderm and neural crest. Numerous molecular mechanisms are involved in these regulatory processes, one of which is protein posttranslational modifications, particularly protein tyrosine phosphorylation (PYP). PYP occurs mainly through the action of protein tyrosine kinases (PTKs), modifying protein enzymatic activity, changing its cellular localization, and aiding in the assembly or disassembly of protein signaling complexes. Under physiological conditions, PYP is balanced by the coordinated action of PTKs and protein tyrosine phosphatases (PTPs). Dysregulation of PYP can cause genetic, metabolic, developmental, and oncogenic skeletal diseases. Although PYP is a reversible biochemical process, in contrast to PTKs, little is known about how this equilibrium is modulated by PTPs in the skeletal system. Whole-genome sequencing has revealed a large and diverse superfamily of PTP genes (over 100 members) in humans, which can be further divided into cysteine (Cys)-, aspartic acid (Asp)-, and histidine (His)-based PTPs. Here, we review current knowledge about the functions and regulatory mechanisms of 28 PTPs involved in skeletal development and diseases; 27 of them belong to class I and II Cys-based PTPs, and the other is an Asp-based PTP. Recent progress in analyzing animal models that harbor various mutations in these PTPs and future research directions are also discussed. Our literature review indicates that PTPs are as crucial as PTKs in supporting skeletal development and homeostasis.

4PBA reduces growth deficiency in osteogenesis imperfecta by enhancing transition of hypertrophic chondrocytes to osteoblasts

Short stature is a major skeletal phenotype in osteogenesis imperfecta (OI), a genetic disorder mainly caused by mutations in genes encoding type I collagen. However, the underlying mechanism is poorly understood, and no effective treatment is available. In OI mice that carry a G610C mutation in COL1A2, we previously found that mature hypertrophic chondrocytes (HCs) are exposed to cell stress due to accumulation of misfolded mutant type I procollagen in the endoplasmic reticulum (ER). By fate mapping analysis of HCs in G610C OI mice, we found that HCs stagnate in the growth plate, inhibiting translocation of HC descendants to the trabecular area and their differentiation to osteoblasts. Treatment with 4-phenylbutyric acid (4PBA), a chemical chaperone, restored HC ER structure and rescued this inhibition, resulting in enhanced longitudinal bone growth in G610C OI mice. Interestingly, the effects of 4PBA on ER dilation were limited in osteoblasts, and the bone fragility was not ameliorated. These results highlight the importance of targeting HCs to treat growth deficiency in OI. Our findings demonstrate that HC dysfunction induced by ER disruption plays a critical role in the pathogenesis of OI growth deficiency, which lays the foundation for developing new therapies for OI.

Molecular diagnosis for growth hormone deficiency in Chinese children and adolescents and evaluation of impact of rare genetic variants on treatment efficacy of growth hormone

BACKGROUND

Growth hormone is an effective therapy for growth hormone deficiency (GHD) but with a rather variable individual sensitivity. It is unclear whether rare genetic variants may contribute to the differential GH responsiveness.

METHODS

The present study aims to investigate the molecular etiology of GHD in Chinese children and adolescents and evaluate the impact of rare variants on therapeutic efficacies of GH.

RESULTS

Twenty-one rare heterozygous variant were classified as promising uncertain significance (n = 14), pathogenic (n = 5) or likely pathogenic (n = 2) for 21 of the 93 GHD patients. After GHD patients harboring these rare variants were excluded, inter-individual variability in the response to GH therapy obviously reduced and the negative correlation between initiation age of treatment and height SDS change became stronger in the group without rare variants. Among rare variants, 7 (likely) pathogenic variants (7.5%, 7/93) involved a total of 6 genes not only associated with GH secretion (PROKR2, LZTR1), but also growth plate chondrocyte signaling (ACAN, FBN1, COL9A1) or genetic syndromes (PTPN11).

CONCLUSIONS

Rare genetic variants are an important factor contributing to differential GH responsiveness and genetic testing should be factored into accurate diagnosis and treatment decision making in the future.

CLINICAL TRIAL REGISTRATION NUMBER

ChiCTR1900026510.

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.

Type-I collagen produced by distinct fibroblast lineages reveals specific function during embryogenesis and Osteogenesis Imperfecta

Type I collagen (Col1) is the most abundant protein in mammals. Col1 contributes to 90% of the total organic component of bone matrix. However, the precise cellular origin and functional contribution of Col1 in embryogenesis and bone formation remain unknown. Single-cell RNA-sequencing analysis identifies Fap⁺ cells and Fsp1⁺ cells as the major contributors of Col1 in the bone. We generate transgenic mouse models to genetically delete Col1 in various cell lineages. Complete, whole-body Col1 deletion leads to failed gastrulation and early embryonic lethality. Specific Col1 deletion in Fap⁺ cells causes severe skeletal defects, with hemorrhage, edema, and prenatal lethality. Specific Col1 deletion in Fsp1⁺ cells results in Osteogenesis Imperfecta-like phenotypes in adult mice, with spontaneous fractures and compromised bone healing. This study demonstrates specific contributions of mesenchymal cell lineages to Col1 production in organogenesis, skeletal development, and bone formation/repair, with potential insights into cell-based therapy for patients with Osteogenesis Imperfecta.

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.

RASopathies: The musculoskeletal consequences and their etiology and pathogenesis

The RASopathies comprise an ever-growing number of clinical syndromes resulting from germline mutations in components of the RAS/MAPK signaling pathway. While multiple organs and tissues may be affected by these mutations, this review will focus on how these mutations specifically impact the musculoskeletal system. Herein, we review the genetics and musculoskeletal phenotypes of these syndromes in humans. We discuss how mutations in the RASopathy syndromes have been studied in translational mouse models. Finally, we discuss how signaling molecules within the RAS/MAPK pathway are involved in normal and abnormal bone biology in the context of osteoblasts, osteoclasts and chondrocytes.

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.

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.

Etiology and Treatment of Growth Delay in Noonan Syndrome

Noonan syndrome is characterized by multiple phenotypic features, including growth retardation, which represents the main cause of consultation to the clinician. Longitudinal growth during childhood and adolescence depends on several factors, among them an intact somatotrophic axis, which is characterized by an adequate growth hormone (GH) secretion by the pituitary, subsequent binding to its receptor, proper function of the post-receptor signaling pathway for this hormone (JAK-STAT5b and RAS/MAPK), and ultimately by the production of its main effector, insulin like growth factor 1 (IGF-1). Several studies regarding the function of the somatotrophic axis in patients with Noonan syndrome and data from murine models, suggest that partial GH insensitivity at a post-receptor level, as well as possible derangements in the RAS/MAPK pathway, are the most likely causes for the growth failure in these patients. Treatment with recombinant human growth hormone (rhGH) has been used extensively to promote linear growth in these patients. Numerous treatment protocols have been employed so far, but the published studies are quite heterogeneous regarding patient selection, length of treatment, and dose of rhGH utilized, so the true benefit of GH therapy is somewhat difficult to establish. This review will discuss the possible etiologies for the growth delay, as well as the outcomes following rhGH treatment in patients with Noonan syndrome.

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.

BRAF and MEK inhibition in melanoma patients enables reprogramming of tumor infiltrating lymphocytes

BACKGROUND

Combined inhibition of BRAF/MEK is an established therapy for melanoma. In addition to its canonical mode of action, effects of BRAF/MEK inhibitors on antitumor immune responses are emerging. Thus, we investigated the effect of these on adaptive immune responses.

PATIENTS, METHODS AND RESULTS

Sequential tumor biopsies obtained before and during BRAF/MEK inhibitor treatment of four (n = 4) melanoma patients were analyzed. Multiplexed immunofluorescence staining of tumor tissue revealed an increased infiltration of CD4⁺ and CD8⁺ T cells upon therapy. Determination of the T-cell receptor repertoire usage demonstrated a therapy induced increase in T-cell clonotype richness and diversity. Application of the Grouping of Lymphocyte Interactions by Paratope Hotspots algorithm revealed a pre-existing immune response against melanoma differentiation and cancer testis antigens that expanded preferentially upon therapy. Indeed, most of the T-cell clonotypes found under BRAF/MEK inhibition were already present in lower numbers before therapy. This expansion appears to be facilitated by induction of T-bet and TCF7 in T cells, two transcription factors required for self-renewal and persistence of CD8⁺ memory T cells.

CONCLUSIONS

Our results suggest that BRAF/MEK inhibition in melanoma patients allows an increased expansion of pre-existing melanoma-specific T cells by induction of T-bet and TCF7 in these.

A method for colocalizing lineage tracing reporter and RNAscope signals on skeletal tissue section

Fluorescent reporters have been widely used in modern biology as a powerful tool in cell lineage tracing during development and in studying the pathogenesis of diseases. RNAscope is a recently developed RNA in situ hybridization method with high specificity and sensitivity. Combined application of these two techniques on skeletal tissue is difficult and has not been done before; the reporter fluorophores in the tissue specimen bleach quickly and mRNAs degrade rapidly due to the decalcification process typically used in processing skeletal samples. Therefore, we developed a method that can simultaneously detect and colocalize both the fluorescent lineage tracing reporter signal and the RNAscope signal in the same skeletal section without compromising the fidelity, sensitivity, and specificity of lineage tracing and RNAscope. This was achieved by cryosectioning bone and cartilage tissue without decalcification, thus allowing the fluorescent reporter signal and RNA in the sections to be well-preserved so that RNAscope can be carried out in situ, and these two signals can be colocalized. Our method of colocalization has versatile applications, e.g., determination of gene knockout efficacy at the mRNA level in a specific cell lineage in situ, detection of alterations in target gene transcripts in reporter-positive cells caused by a specific gene mutation, studies of the disease pathology by examining the transcript-level expression of genes of interest in the cell lineage in vivo.

RNA-seq in Skeletal Biology

PURPOSE OF REVIEW

The goal of this paper is to review state-of-the-art transcriptome profiling methods and their recent applications in the field of skeletal biology.

RECENT FINDINGS

Next-generation sequencing of mRNA (RNA-seq) methods have been established and routinely used in skeletal biology research. RNA-seq has led to the identification of novel genes and transcription factors involved in skeletal development and disease, through its application in small and large animal models, as well as human tissue and cells. With the availability of advanced techniques such as single-cell RNA-seq, novel cell types in skeletal tissues are being identified. As the sequencing technologies are rapidly evolving, the exciting discoveries supported by transcriptomics will continue to emerge and improve our understanding of the biology of the skeleton.

Inactivation of Lkb1 in postnatal chondrocytes leads to epiphyseal growth-plate abnormalities and promotes enchondroma-like formation

Liver serine-threonine kinase B1 (LKB1) is a tumor suppressor that has been linked to many types of tumors. However, the role of LKB1 in cartilaginous tumorigenesis is still poorly understood. In this study, we find that cartilage-specific, tamoxifen-inducible Lkb1 knockout results in multiple enchondroma-like lesions adjacent to the disorganized growth plates. We showed that chondrocytes retain an immature status caused by loss of Lkb1, which may lead to the dramatic expansion of growth-plate cartilage and the formation of enchondroma-like lesions. Additionally, increased mammalian target of rapamycin complex 1 (mTORC1) activity is observed in the Lkb1 conditional knockout (cKO) chondrocytes, and rapamycin (mTORC1 inhibitor) treatment significantly alleviates the expansion of growth-plate cartilage and eliminates the enchondroma-like lesions in Lkb1 cKO mice. Thus, our findings indicate that loss of Lkb1 leads to the expansion of chondrocytes and the formation of enchondroma-like lesions during postnatal cartilage development, and that the up-regulated mTORC1-signaling pathway is implicated in this process. Our findings suggest that modulation of LKB1 and related signaling is a potential therapy in cartilaginous tumorigenesis.-Zhou, S., Li, Y., Qiao, L., Ge, Y., Huang, X., Gao, X., Ju, H., Wang, W., Zhang, J., Yan, J., Teng, H., Jiang, Q. Inactivation of Lkb1 in postnatal chondrocytes leads to epiphyseal growth-plate abnormalities and promotes enchondroma-like formation.

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.

Endoplasmic reticulum stress is induced in growth plate hypertrophic chondrocytes in G610C mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a hereditary bone disorder most commonly caused by autosomal dominant mutations in genes encoding type I collagen. In addition to bone fragility, patients suffer from impaired longitudinal bone growth. It has been demonstrated that in OI, an accumulation of mutated type I collagen in the endoplasmic reticulum (ER) induces ER stress in osteoblasts, causing osteoblast dysfunction leading to bone fragility. We hypothesize that ER stress is also induced in the growth plate where bone growth is initiated, and examined a mouse model of dominant OI that carries a G610C mutation in the procollagen α2 chain. The results demonstrated that G610C OI mice had significantly shorter long bones with growth plate abnormalities including elongated total height and hypertrophic zone. Moreover, we found that mature hypertrophic chondrocytes expressed type I collagen and ER dilation was more pronounced compared to wild type littermates. The results from in vitro chondrocyte cultures demonstrated that the maturation of G610C OI hypertrophic chondrocytes was significantly suppressed and ER stress related genes were upregulated. Given that the alteration of hypertrophic chondrocyte activity often causes dwarfism, our findings suggest that hypertrophic chondrocyte dysfunction induced by ER stress may be an underlying cause of growth deficiency in G610C OI mice.

SHP2 regulates skeletal cell fate by modifying SOX9 expression and transcriptional activity

Chondrocytes and osteoblasts differentiate from a common mesenchymal precursor, the osteochondroprogenitor (OCP), and help build the vertebrate skeleton. The signaling pathways that control lineage commitment for OCPs are incompletely understood. We asked whether the ubiquitously expressed protein-tyrosine phosphatase SHP2 (encoded by Ptpn11) affects skeletal lineage commitment by conditionally deleting Ptpn11 in mouse limb and head mesenchyme using "Cre-loxP"-mediated gene excision. SHP2-deficient mice have increased cartilage mass and deficient ossification, suggesting that SHP2-deficient OCPs become chondrocytes and not osteoblasts. Consistent with these observations, the expression of the master chondrogenic transcription factor SOX9 and its target genes Acan, Col2a1, and Col10a1 were increased in SHP2-deficient chondrocytes, as revealed by gene expression arrays, qRT-PCR, in situ hybridization, and immunostaining. Mechanistic studies demonstrate that SHP2 regulates OCP fate determination via the phosphorylation and SUMOylation of SOX9, mediated at least in part via the PKA signaling pathway. Our data indicate that SHP2 is critical for skeletal cell lineage differentiation and could thus be a pharmacologic target for bone and cartilage regeneration.

Noonan syndrome: an update on growth and development

PURPOSE OF REVIEW

To provide an update on recent developments on Noonan syndrome with a special focus on endocrinology, bone, and metabolism aspects. The key issues still to be resolved and the future therapeutic perspectives will be discussed.

RECENT FINDINGS

The discovery of the molecular genetic causes of Noonan syndrome and Noonan-syndrome-related disorders has permitted us to better understand the mechanisms underlying the different symptoms of these diseases and to establish genotype-phenotype correlations (in growth patterns for example). In addition to the classical clinical hallmarks of Noonan syndrome, new important aspects include decreased fertility in men, lean phenotype with increased energy expenditure and possible impact on carbohydrate metabolism/insulin sensitivity, and impaired bone health. Further clinical studies are needed to investigate the long-term impact of these findings and their possible interconnections. Finally, the understanding of the crucial role of RAS/mitogen-activated protein kinases dysregulation in the pathophysiology of Noonan syndrome allows us to devise new therapeutic approaches. Some agents are currently undergoing clinical trials in Noonan syndrome patients.

SUMMARY

On the last 10 years, our knowledge of the molecular basis and the pathophysiology of Noonan syndrome has greatly advanced allowing us to gain insight in all the aspects of this disease and to devise new specific therapeutic strategies.

Identification of human age-associated gene co-expressions in functional modules using liquid association

Aging is a major risk factor for age-related diseases such as certain cancers. In this study, we developed Age Associated Gene Co-expression Identifier (AAGCI), a liquid association based method to infer age-associated gene co-expressions at thousands of biological processes and pathways across 9 human tissues. Several hundred to thousands of gene pairs were inferred to be age co-expressed across different tissues, the genes involved in which are significantly enriched in functions like immunity, ATP binding, DNA damage, and many cancer pathways. The age co-expressed genes are significantly overlapped with aging genes curated in the GenAge database across all 9 tissues, suggesting a tissue-wide correlation between age-associated genes and co-expressions. Interestingly, age-associated gene co-expressions are significantly different from gene co-expressions identified through correlation analysis, indicating that aging might only contribute to a small portion of gene co-expressions. Moreover, the key driver analysis identified biologically meaningful genes in important function modules. For example, IGF1, ERBB2, TP53 and STAT5A were inferred to be key genes driving age co-expressed genes in the network module associated with function “T cell proliferation”. Finally, we prioritized a few anti-aging drugs such as metformin based on an enrichment analysis between age co-expressed genes and drug signatures from a recent study. The predicted drugs were partially validated by literature mining and can be readily used to generate hypothesis for further experimental validations.

Inducible Activation of FGFR2 in Adult Mice Promotes Bone Formation After Bone Marrow Ablation

Apert syndrome is one of the most severe craniosynostoses, resulting from gain-of-function mutations in fibroblast growth factor receptor 2 (FGFR2). Previous studies have shown that gain-of-function mutations of FGFR2 (S252W or P253R) cause skull malformation of human Apert syndrome by affecting both chondrogenesis and osteogenesis, underscoring the key role of FGFR2 in bone development. However, the effects of FGFR2 on bone formation at the adult stage have not been fully investigated. To investigate the role of FGFR2 in bone formation, we generated mice with tamoxifen-inducible expression of mutant FGFR2 (P253R) at the adult stage. Mechanical bone marrow ablation (BMX) was performed in both wild-type and Fgfr2 mutant (MT) mice. Changes in newly formed trabecular bone were assessed by micro-computed tomography and bone histomorphometry. We found that MT mice exhibited increased trabecular bone formation and decreased bone resorption after BMX accompanied with a remarkable increase in bone marrow stromal cell recruitment and proliferation, osteoblast proliferation and differentiation, and enhanced Wnt/β-catenin activity. Furthermore, pharmacologically inhibiting Wnt/β-catenin signaling can partially reverse the increased trabecular bone formation and decreased bone resorption in MT mice after BMX. Our data demonstrate that gain-of-function mutation in FGFR2 exerts a Wnt/β-catenin-dependent anabolic effect on trabecular bone by promoting bone formation and inhibiting bone resorption at the adult stage. © 2017 American Society for Bone and Mineral Research.

Ptpn11 Deletion in CD4+ Cells Does Not Affect T Cell Development and Functions but Causes Cartilage Tumors in a T Cell-Independent Manner

The ubiquitously expressed tyrosine phosphatase Src homology region 2 domain-containing phosphatase-2 (SHP-2, encoded by Ptpn11) is required for constitutive cellular processes including proliferation, differentiation, and the regulation of immune responses. During development and maturation, subsets of T cells express a variety of inhibitory receptors known to associate with phosphatases, which in turn, dephosphorylate key players of activating receptor signaling pathways. We hypothesized that SHP-2 deletion would have major effects on T cell development by altering the thresholds for activation, as well as positive and negative selection. Surprisingly, using mice conditionally deficient for SHP-2 in the T cell lineage, we show that the development of these lymphocytes is globally intact. In addition, our data demonstrate that SHP-2 absence does not compromise T cell effector functions, suggesting that SHP-2 is dispensable in these cells. Unexpectedly, in aging mice, Ptpn11 gene deletion driven by CD4 Cre recombinase leads to cartilage tumors in wrist bones in a T cell-independent manner. These tumors resemble miniature cartilaginous growth plates and contain CD4-lineage positive chondrocyte-like cells. Altogether these results indicate that SHP-2 is a cartilage tumor suppressor during aging.

SHP2 Regulates the Osteogenic Fate of Growth Plate Hypertrophic Chondrocytes

Transdifferentiation of hypertrophic chondrocytes into bone-forming osteoblasts has been reported, yet the underlying molecular mechanism remains incompletely understood. SHP2 is an ubiquitously expressed cytoplasmic protein tyrosine phosphatase. SHP2 loss-of-function mutations in chondroid cells are linked to metachondromatosis in humans and mice, suggesting a crucial role for SHP2 in the skeleton. However, the specific role of SHP2 in skeletal cells has not been elucidated. To approach this question, we ablated SHP2 in collagen 2α1(Col2α1)-Cre- and collagen 10α1(Col10α1)-Cre-expressing cells, predominantly proliferating and hypertrophic chondrocytes, using "Cre-loxP"-mediated gene excision. Mice lacking SHP2 in Col2α1-Cre-expressing cells die at mid-gestation. Postnatal SHP2 ablation in the same cell population caused dwarfism, chondrodysplasia and exostoses. In contrast, mice in which SHP2 was ablated in the Col10α1-Cre-expressing cells appeared normal but were osteopenic. Further mechanistic studies revealed that SHP2 exerted its influence partly by regulating the abundance of SOX9 in chondrocytes. Elevated and sustained SOX9 in SHP2-deficient hypertrophic chondrocytes impaired their differentiation to osteoblasts and impaired endochondral ossification. Our study uncovered an important role of SHP2 in bone development and cartilage homeostasis by influencing the osteogenic differentiation of hypertrophic chondrocytes and provided insight into the pathogenesis and potential treatment of skeletal diseases, such as osteopenia and osteoporosis.

Tumor Syndromes That Include Bone Tumors: An Update

Tumor syndromes, including bone neoplasias, are genetic predisposing conditions characterized by the development of a pattern of malignancies within a family at an early age of onset. Occurrence of bilateral, multifocal, or metachronous neoplasias and specific histopathologic findings suggest a genetic predisposition syndrome. Additional clinical features not related to the neoplasia can be a hallmark of specific genetic syndromes. Mostly, those diseases have an autosomal dominant pattern of inheritance with variable percentage of penetrance. Some syndromic disorders with an increased tumor risk may show an autosomal recessive transmission or are related to somatic mosaicism. Many genetic tumor syndromes are known. This update is specifically focused on syndromes predisposing to osteosarcoma and chondrosarcoma.

Aberrant perichondrial BMP signaling mediates multiple osteochondromagenesis in mice

Multiple hereditary exostoses (MHE) is characterized by the development of numerous benign bony tumors (osteochondromas). Although it has been well established that MHE is caused by mutations in EXT1 and EXT2, which encode glycosyltransferase essential for heparan sulfate (HS) biosynthesis, the cellular origin and molecular mechanisms of MHE remain elusive. Here, we show that in Ext1 mutant mice, osteochondromas develop from mesenchymal stem cell-like progenitor cells residing in the perichondrium, and we show that enhanced BMP signaling in these cells is the primary signaling defect that leads to osteochondromagenesis. We demonstrate that progenitor cells in the perichondrium, including those in the groove of Ranvier, highly express HS and that Ext1 ablation targeted to the perichondrium results in the development of osteochondromas. Ext1-deficient perichondrial progenitor cells show enhanced BMP signaling and increased chondrogenic differentiation both in vitro and in vivo. Consistent with the functional role for enhanced BMP signaling in osteochondromagenesis, administration of the small molecule BMP inhibitor LDN-193189 suppresses osteochondroma formation in two MHE mouse models. Together, our results demonstrate a role for enhanced perichondrial BMP signaling in osteochondromagenesis in mice, and they suggest the possibility of pharmacological treatment of MHE with BMP inhibitors.

Extracellular Signal-Regulated Kinase Signaling in CD4-Expressing Cells Inhibits Osteochondromas

Defects in cartilage homeostasis can give rise to various skeletal disorders including osteochondromas. Osteochondromas are benign bone tumors caused by excess accumulation of chondrocytes, the main cell type of cartilage. The extracellular signal-regulated kinase (ERK) pathway is a major signaling node that functions within chondrocytes to regulate their growth and differentiation. However, it is not known whether the ERK pathway in other cell types regulates cartilage homeostasis. We show here that mice with a germline deficiency of Erk1 and a conditional deletion of Erk2 in cells that express CD4, or expressed CD4 at one point in development, unexpectedly developed bone deformities. The bone lesions were due to neoplastic outgrowths of chondrocytes and disordered growth plates, similar to tumors observed in the human disease, osteochondromatosis. Chondrocyte accumulation was not due to deletion of Erk2 in the T cells. Rather, CD4cre was expressed in cell types other than T cells, including a small fraction of chondrocytes. Surprisingly, the removal of T cells accelerated osteochondroma formation and enhanced disease severity. These data show for the first time that T cells impact the growth of osteochondromas and describe a novel model to study cartilage homeostasis and osteochondroma formation.

Modeling RASopathies with Genetically Modified Mouse Models

The RAS/MAPK signaling pathway plays key roles in development, cell survival and proliferation, as well as in cancer pathogenesis. Molecular genetic studies have identified a group of developmental syndromes, the RASopathies, caused by germ line mutations in this pathway. The syndromes included within this classification are neurofibromatosis type 1 (NF1), Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NS-ML, formerly known as LEOPARD syndrome), Costello syndrome (CS), cardio-facio-cutaneous syndrome (CFC), Legius syndrome (LS, NF1-like syndrome), capillary malformation-arteriovenous malformation syndrome (CM-AVM), and hereditary gingival fibromatosis (HGF) type 1. Although these syndromes present specific molecular alterations, they are characterized by a large spectrum of functional and morphological abnormalities, which include heart defects, short stature, neurocognitive impairment, craniofacial malformations, and, in some cases, cancer predisposition. The development of genetically modified animals, such as mice (Mus musculus), flies (Drosophila melanogaster), and zebrafish (Danio rerio), has been instrumental in elucidating the molecular and cellular bases of these syndromes. Moreover, these models can also be used to determine tumor predisposition, the impact of different genetic backgrounds on the variable phenotypes found among the patients and to evaluate preventative and therapeutic strategies. Here, we review a wide range of genetically modified mouse models used in the study of RASopathies and the potential application of novel technologies, which hopefully will help us resolve open questions in the field.

NFAT restricts osteochondroma formation from entheseal progenitors

Osteochondromas are common benign osteocartilaginous tumors in children and adolescents characterized by cartilage-capped bony projections on the surface of bones. These tumors often cause pain, deformity, fracture, and musculoskeletal dysfunction, and they occasionally undergo malignant transformation. The pathogenesis of osteochondromas remains poorly understood. Here, we demonstrate that nuclear factor of activated T cells c1 and c2 (NFATc1 and NFATc2) suppress osteochondromagenesis through individual and combinatorial mechanisms. In mice, conditional deletion of NFATc1 in mesenchymal limb progenitors, Scleraxis-expressing (Scx-expressing) tendoligamentous cells, or postnatally in Aggrecan-expressing cells resulted in osteochondroma formation at entheses, the insertion sites of ligaments and tendons onto bone. Combinatorial deletion of NFATc1 and NFATc2 gave rise to larger and more numerous osteochondromas in inverse proportion to gene dosage. A population of entheseal NFATc1- and Aggrecan-expressing cells was identified as the osteochondroma precursor, previously believed to be growth plate derived or perichondrium derived. Mechanistically, we show that NFATc1 restricts the proliferation and chondrogenesis of osteochondroma precursors. In contrast, NFATc2 preferentially inhibits chondrocyte hypertrophy and osteogenesis. Together, our findings identify and characterize a mechanism of osteochondroma formation and suggest that regulating NFAT activity is a new therapeutic approach for skeletal diseases characterized by defective or exaggerated osteochondral growth.

Conditional Deletion of Fgfr3 in Chondrocytes leads to Osteoarthritis-like Defects in Temporomandibular Joint of Adult Mice

Osteoarthritis (OA) in the temporomandibular joint (TMJ) is a common degenerative disease in adult, which is characterized by progressive destruction of the articular cartilage. To investigate the role of FGFR3 in the homeostasis of TMJ cartilage during adult stage, we generated Fgfr3^f/f; Col2a1-CreER^T2 (Fgfr3 cKO) mice, in which Fgfr3 was deleted in chondrocytes at 2 months of age. OA-like defects were observed in Fgfr3 cKO TMJ cartilage. Immunohistochemical staining and quantitative real-time PCR analyses revealed a significant increase in expressions of COL10, MMP13 and AMAMTS5. In addition, there was a sharp increase in chondrocyte apoptosis at the Fgfr3 cKO articular surface, which was accompanied by a down-regulation of lubricin expression. Importantly, the expressions of RUNX2 and Indian hedgehog (IHH) were up-regulated in Fgfr3 cKO TMJ. Primary Fgfr3 cKO chondrocytes were treated with IHH signaling inhibitor, which significantly reduced expressions of Runx2, Col10, Mmp13 and Adamts5. Furthermore, the IHH signaling inhibitor partially alleviated OA-like defects in the TMJ of Fgfr3 cKO mice, including restoration of lubricin expression and improvement of the integrity of the articular surface. In conclusion, our study proposes that FGFR3/IHH signaling pathway plays a critical role in maintaining the homeostasis of TMJ articular cartilage during adult stage.

SHP2-Deficiency in Chondrocytes Deforms Orofacial Cartilage and Ciliogenesis in Mice

Congenital orofacial abnormalities are clinically seen in human syndromes with SHP2 germline mutations such as LEOPARD and Noonan syndrome. Recent studies demonstrate that SHP2-deficiency leads to skeletal abnormalities including scoliosis and cartilaginous benign tumor metachondromatosis, suggesting that growth plate cartilage is a key tissue regulated by SHP2. The role and cellular mechanism of SHP2 in the orofacial cartilage, however, remains unknown. Here, we investigated the postnatal craniofacial development by inducible disruption of Shp2 in chondrocytes. Shp2 conditional knockout (cKO) mice displayed severe deformity of the mandibular condyle accompanied by disorganized, expanded cartilage in the trabecular bone region, enhanced type X collagen, and reduced Erk production. Interestingly, the length of primary cilia, an antenna like organelle sensing environmental signaling, was significantly shortened, and the number of primary cilia was reduced in the cKO mice. The expression levels of intraflagellar transports (IFTs), essential molecules in the assembly and function of primary cilia, were significantly decreased. Taken together, lack of Shp2 in orofacial cartilage led to severe defects of ciliogenesis through IFT reduction, resulting in mandibular condyle malformation and cartilaginous expansion. Our study provides new insights into the molecular pathogenesis of SHP2-deficiency in cartilage and helps to understand orofacial and skeletal manifestations seen in patients with SHP2 mutations.

SHP2 sails from physiology to pathology

Over the two past decades, mutations of the PTPN11 gene, encoding the ubiquitous protein tyrosine phosphatase SHP2 (SH2 domain-containing tyrosine phosphatase 2), have been identified as the causal factor of several developmental diseases (Noonan syndrome (NS), Noonan syndrome with multiple lentigines (NS-ML), and metachondromatosis), and malignancies (juvenile myelomonocytic leukemia). SHP2 plays essential physiological functions in organism development and homeostasis maintenance by regulating fundamental intracellular signaling pathways in response to a wide range of growth factors and hormones, notably the pleiotropic Ras/Mitogen-Activated Protein Kinase (MAPK) and the Phosphoinositide-3 Kinase (PI3K)/AKT cascades. Analysis of the biochemical impacts of PTPN11 mutations first identified both loss-of-function and gain-of-function mutations, as well as more subtle defects, highlighting the major pathophysiological consequences of SHP2 dysregulation. Then, functional genetic studies provided insights into the molecular dysregulations that link SHP2 mutants to the development of specific traits of the diseases, paving the way for the design of specific therapies for affected patients. In this review, we first provide an overview of SHP2's structure and regulation, then describe its molecular roles, notably its functions in modulating the Ras/MAPK and PI3K/AKT signaling pathways, and its physiological roles in organism development and homeostasis. In the second part, we describe the different PTPN11 mutation-associated pathologies and their clinical manifestations, with particular focus on the biochemical and signaling outcomes of NS and NS-ML-associated mutations, and on the recent advances regarding the pathophysiology of these diseases.

Expression and clinical significance of tyrosine phosphatase SHP2 in thyroid carcinoma

Protein-tyrosine phosphatase SHP2 is encoded by the gene PTPN11. SHP2 is hypothesized to have a critical role in cancer, via the activation of mutations that have been detected in several types of leukaemia and in certain solid tumours, including liver, breast, gastric and cervical cancer. However, to the best of our knowledge, there have been no previous reports evaluating the significance of SHP2 expression in thyroid cancer. The present study evaluated SHP2 expression in 65 thyroid cancer specimens, 40 specimens of self-matched adjacent peritumour tissues and 40 specimens of normal thyroid tissue, using immunohistochemical and western blot analyses with an anti-SHP2 antibody. Western blotting was also used to assess SHP2 expression in thyroid cancer cell lines (SW579, IHH-4, FTC-133, TPC-1, DRO, TA-K, and ML-1) and Nthy-ori3-1 normal thyroid cells. In addition, SHP2 antisense oligonucleotides were used to block SHP2 expression in SW579 cells, and growth inhibition assays were conducted. Increased SHP2 expression was detected in the tumour tissues compared with that of the normal thyroid tissues (P<0.05). SHP2 expression was significantly correlated with poor tumour differentiation (P<0.05), late TNM stage (P<0.05) and lymph node metastasis (P<0.05), suggesting that SHP2 may represent a potential target for thyroid cancer therapy.

FGFR3 Deficiency Causes Multiple Chondroma-like Lesions by Upregulating Hedgehog Signaling

Most cartilaginous tumors are formed during skeletal development in locations adjacent to growth plates, suggesting that they arise from disordered endochondral bone growth. Fibroblast growth factor receptor (FGFR)3 signaling plays essential roles in this process; however, the role of FGFR3 in cartilaginous tumorigenesis is not known. In this study, we found that postnatal chondrocyte-specific Fgfr3 deletion induced multiple chondroma-like lesions, including enchondromas and osteochondromas, adjacent to disordered growth plates. The lesions showed decreased extracellular signal-regulated kinase (ERK) activity and increased Indian hedgehog (IHH) expression. The same was observed in Fgfr3-deficient primary chondrocytes, in which treatment with a mitogen-activated protein kinase (MEK) inhibitor increased Ihh expression. Importantly, treatment with an inhibitor of IHH signaling reduced the occurrence of chondroma-like lesions in Fgfr3-deficient mice. This is the first study reporting that the loss of Fgfr3 function leads to the formation of chondroma-like lesions via downregulation of MEK/ERK signaling and upregulation of IHH, suggesting that FGFR3 has a tumor suppressor-like function in chondrogenesis.

Gpr126/Adgrg6 deletion in cartilage models idiopathic scoliosis and pectus excavatum in mice

Adolescent idiopathic scoliosis (AIS) and pectus excavatum (PE) are common pediatric musculoskeletal disorders. Little is known about the tissue of origin for either condition, or about their genetic bases. Common variants near GPR126/ADGRG6 (encoding the adhesion G protein-coupled receptor 126/adhesion G protein-coupled receptor G6, hereafter referred to as GPR126) were recently shown to be associated with AIS in humans. Here, we provide genetic evidence that loss of Gpr126 in osteochondroprogenitor cells alters cartilage biology and spinal column development. Microtomographic and x-ray studies revealed several hallmarks of AIS, including postnatal onset of scoliosis without malformations of vertebral units. The mutants also displayed a dorsal-ward deflection of the sternum akin to human PE. At the cellular level, these defects were accompanied by failure of midline fusion within the developing annulus fibrosis of the intervertebral discs and increased apoptosis of chondrocytes in the ribs and vertebrae. Molecularly, we found that loss of Gpr126 upregulated the expression of Gal3st4, a gene implicated in human PE, encoding Galactose-3-O-sulfotransferase 4. Together, these data uncover Gpr126 as a genetic cause for the pathogenesis of AIS and PE in a mouse model.

ERK1 and ERK2 regulate chondrocyte terminal differentiation during endochondral bone formation

Chondrocytes in the epiphyseal cartilage undergo terminal differentiation prior to their removal through apoptosis. To examine the role of ERK1 and ERK2 in chondrocyte terminal differentiation, we generated Osterix (Osx)-Cre; ERK1(-/-) ; ERK2(flox/flox) mice (conditional knockout Osx [cKOosx]), in which ERK1 and ERK2 were deleted in hypertrophic chondrocytes. These cKOosx mice were grossly normal in size at birth, but by 3 weeks of age exhibited shorter long bones. Histological analysis in these mice revealed that the zone of hypertrophic chondrocytes in the growth plate was markedly expanded. In situ hybridization and quantitative real-time PCR analyses demonstrated that Matrix metalloproteinase-13 (Mmp13) and Osteopontin expression was significantly decreased, indicating impaired chondrocyte terminal differentiation. Moreover, Egr1 and Egr2, transcription factors whose expression is restricted to the last layers of hypertrophic chondrocytes in wild-type mice, were also strongly downregulated in these cKOosx mice. In transient transfection experiments in the RCS rat chondrosarcoma cell line, the expression of Egr1, Egr2, or a constitutively active mutant of MEK1 increased the activity of an Osteopontin promoter, whereas the MEK1-induced activation of the Osteopontin promoter was inhibited by the coexpression of Nab2, an Egr1 and Egr2 co-repressor. These results suggest that MEK1-ERK signaling activates the Osteopontin promoter in part through Egr1 and Egr2. Finally, our histological analysis of cKOosx mice demonstrated enchondroma-like lesions in the bone marrow that are reminiscent of human metachondromatosis, a skeletal disorder caused by mutations in PTPN11. Our observations suggest that the development of enchondromas in metachondromatosis may be caused by reduced extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK MAPK) signaling.