{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\nScleraxis (SCX) is a pivotal basic helix‐loop‐helix transcription factor that directs mesenchymal cell fate towards the tendon/ligament lineage during embryogenesis and postnatal repair. SCX expression in human mesenchymal stem cells drives their commitment into tendon progenitors, as evidenced by the up‐regulation of collagen type I, decorin, fibromodulin, and tenomodulin, and by profound changes in cell morphology and growth kinetics."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " In addition, in pathological settings affecting musculoskeletal tissues, aberrant proinflammatory signals and HIF-2α activation suppress SCX, thereby diverting tendon stem/progenitor cells from their normal tenogenic fate towards osteochondrogenic differentiation, which may contribute to soft tissue calcification."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": " SCX is also expressed in the periodontal ligament, where its presence in ligament cells and progenitors appears to be intimately involved in their differentiation and tissue homeostasis."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nIn the context of musculoskeletal and cardiac development, SCX collaborates with other key transcription factors to fine-tune cell lineage decisions. SCX works cooperatively with the ubiquitous bHLH partner E47 and the chondrogenic driver Sox9 to synergistically activate the transcription of chondrocyte-specific genes, including collagen type II, by binding to conserved E-box motifs."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "5"}]}, {"type": "t", "text": " In cardiac fibroblasts, SCX is upregulated by TGF-β/Smad3 signaling and is both sufficient and necessary to transactivate the COL1A2 gene via interactions at specific E-boxes, thereby contributing to the fibrotic remodeling process."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": " A similar TGF-β–dependent SCX induction is observed in the valvular interstitial cells during heart valve remodeling, where SCX regulates proteoglycan synthesis and may influence myxomatous pathology."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": " Furthermore, SCX directly transactivates genes that drive epithelial-to-mesenchymal transition, such as Twist1 and Snai1, and is essential for the TGF-β–mediated conversion of fibroblasts into myofibroblasts, while also modulating matrix metalloproteinase expression."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "8"}]}, {"type": "t", "text": "\n"}]}, {"type": "t", "text": "\n\n"}, {"type": "p", "children": [{"type": "t", "text": "\nSCX’s regulation of extracellular matrix dynamics extends to various fibrotic conditions and regenerative processes. Elevated levels of SCX in serum correlate with fibrotic phenotypes in idiopathic pulmonary fibrosis and systemic sclerosis, suggesting its potential as a biomarker for fibrotic disease severity."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "10"}]}, {"type": "t", "text": " In the musculoskeletal system, SCX-expressing cells proximal to the myotendinous junction enhance myoblast fusion and metabolic activity, indicating a role in the communication between tendon and muscle that may facilitate muscle regeneration."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "11"}]}, {"type": "t", "text": " Moreover, in cardiac fibroblasts, SCX drives the metabolic reprogramming necessary for fibrosis by inducing glutaminolysis through upregulation of the rate-limiting enzyme GLS1, and its upregulation is reflected in hypertrophic cardiomyopathy patients, further supporting its role in pathological matrix remodeling."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "12"}]}, {"type": "t", "text": " Finally, transcriptomic comparisons in adult and fetal tenocytes following SCX knockdown confirm that SCX modulates extracellular matrix synthesis and turnover, with distinct effects across developmental stages."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "14"}]}, {"type": "t", "text": "\n"}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Paolo Alberton, Cvetan Popov, Markus Prägert, et al. 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