A PTHrP Gradient Drives Mandibular Condylar Chondrogenesis via Runx2

The mandibular condylar cartilage (MCC) is an essential component of the temporomandibular joint, which orchestrates the vertical growth of the mandibular ramus through endochondral ossification with distinctive modes of cell differentiation. Parathyroid hormone-related protein (PTHrP) is a master regulator of chondrogenesis; in the long bone epiphyseal growth plate, PTHrP expressed by resting zone chondrocytes promotes chondrocyte proliferation in the adjacent layer. However, how PTHrP regulates chondrogenesis in the MCC remains largely unclear. In this study, we used a Pthrp-mCherry knock-in reporter strain to map the localization of PTHrP+ cells in the MCC and define the function of PTHrP in the growing mandibular condyle. In the postnatal MCC of PthrpmCherry/+ mice, PTHrP-mCherry was specifically expressed by cells in the superficial layer immediately adjacent to RUNX2-expressing cells in the polymorphic layer. PTHrP ligands diffused across the polymorphic and chondrocyte layers where its cognate receptor PTH1R was abundantly expressed. We further analyzed the mandibular condyle of PthrpmCherry/mCherry mice lacking functional PTHrP protein (PTHrP-KO). At embryonic day (E) 18.5, the condylar process and MCC were significantly truncated in the PTHrP-KO mandible, which was associated with a significant reduction in cell proliferation across the polymorphic layer and a loss of SOX9+ cells in the chondrocyte layers. The PTHrP-KO MCC showed a transient increase in the number of Col10a1+ hypertrophic chondrocytes at E15.5, followed by a significant loss of these cells at E18.5, indicating that superficial layer-derived PTHrP prevents premature chondrocyte exhaustion in the MCC. The expression of Runx2, but not Sp7, was significantly reduced in the polymorphic layer of the PTHrP-KO MCC. Therefore, PTHrP released from cells in the superficial layer directly acts on cells in the polymorphic layer to promote proliferation of chondrocyte precursor cells and prevent their premature differentiation by maintaining Runx2 expression, revealing a unique PTHrP gradient-directed mechanism that regulates MCC chondrogenesis.

下颌髁突软骨（mandibular condylar cartilage, MCC）是颞下颌关节的重要组成部分，其通过软骨内骨化过程调控下颌支的垂直生长，且具备独特的细胞分化模式。甲状旁腺激素相关蛋白（parathyroid hormone-related protein, PTHrP）是软骨发生的关键调控因子；在长骨骨骺生长板中，静止区软骨细胞表达的PTHrP可促进邻近层软骨细胞的增殖。然而，PTHrP如何调控下颌髁突软骨的软骨发生过程，目前尚未完全明确。本研究利用Pthrp-mCherry敲入报告基因品系，绘制了PTHrP阳性细胞在下颌髁突软骨中的定位图谱，并明确了PTHrP在生长发育期下颌髁突中的功能。在Pthrp-mCherry/+小鼠的出生后下颌髁突软骨中，PTHrP-mCherry特异性表达于紧邻多形层中RUNX2阳性细胞的表层细胞。PTHrP配体可扩散至多形层与软骨细胞层，而其同源受体PTH1R在这两层中均呈高表达。我们进一步分析了缺失功能性PTHrP蛋白的Pthrp-mCherry/mCherry小鼠（PTHrP敲除，PTHrP-KO）的下颌髁突。在胚胎第18.5天（E18.5），PTHrP-KO小鼠的髁突与下颌髁突软骨显著缩短，这与多形层细胞增殖显著降低、软骨细胞层中SOX9阳性细胞丢失密切相关。PTHrP-KO的下颌髁突软骨在胚胎第15.5天（E15.5）时，Col10a1阳性肥大软骨细胞数量短暂升高，随后在E18.5时这些细胞显著丢失，表明表层来源的PTHrP可阻止下颌髁突软骨中软骨细胞过早耗竭。PTHrP-KO下颌髁突软骨的多形层中，Runx2的表达显著降低，但Sp7的表达无明显变化。综上，表层细胞释放的PTHrP可直接作用于多形层细胞，促进软骨前体细胞增殖，并通过维持Runx2的表达阻止其过早分化，揭示了一种独特的、由PTHrP梯度介导的调控下颌髁突软骨发生的机制。