Growth plate chondrocytes play central roles in the proper development and growth of endochondral bones. Particularly, a population of chondrocytes in the resting zone expressing parathyroid hormone-related protein (PTHrP) is now recognized as skeletal stem cells, defined by their ability to undergo self-renewal and clonally give rise to columnar chondrocytes in the postnatal growth plate. These chondrocytes also possess the ability to differentiate into a multitude of cell types including osteoblasts and bone marrow stromal cells during skeletal development. Using single-cell transcriptomic approaches and in vivo lineage tracing technology, it is now possible to further elucidate their molecular properties and cellular fate changes. By discovering the fundamental molecular characteristics of these cells, it may be possible to harness their functional characteristics for skeletal growth and regeneration. Here, we discuss our current understanding of the molecular signatures defining growth plate chondrocytes.

Formation of functional skeletal tissues requires highly organized steps of mesenchymal progenitor cell differentiation. The dental follicle (DF) surrounding the developing tooth harbors mesenchymal progenitor cells for various differentiated cells constituting the tooth root–bone interface and coordinates tooth eruption in a manner dependent on signaling by parathyroid hormone-related peptide (PTHrP) and the PTH/PTHrP receptor (PPR). However, the identity of mesenchymal progenitor cells in the DF and how they are regulated by PTHrP-PPR signaling remain unknown. Here, we show that the PTHrP-PPR autocrine signal maintains physiological cell fates of DF mesenchymal progenitor cells to establish the functional periodontal attachment apparatus and orchestrates tooth eruption. A single-cell RNA-seq analysis revealed cellular heterogeneity of PTHrP ⁺ cells, wherein PTHrP ⁺ DF subpopulations abundantly express PPR. Cell lineage analysis using tamoxifen-inducible PTHrP-creER mice revealed that PTHrP ⁺ DF cells differentiate into cementoblasts on the acellular cementum, periodontal ligament cells, and alveolar cryptal bone osteoblasts during tooth root formation. PPR deficiency induced a cell fate shift of PTHrP ⁺ DF mesenchymal progenitor cells to nonphysiological cementoblast-like cells precociously forming the cellular cementum on the root surface associated with up-regulation of Mef2c and matrix proteins, resulting in loss of the proper periodontal attachment apparatus and primary failure of tooth eruption, closely resembling human genetic conditions caused by PPR mutations. These findings reveal a unique mechanism whereby proper cell fates of mesenchymal progenitor cells are tightly maintained by an autocrine system mediated by PTHrP-PPR signaling to achieve functional formation of skeletal tissues.

Orthodontic tooth movement (OTM) is achieved by the simultaneous activation of bone resorption by osteoclasts and bone formation by osteoblasts. When orthodontic forces are applied, osteoclast-mediated bone resorption occurs in the alveolar bone on the compression side, creating space for tooth movement. Therefore, controlling osteoclastogenesis is the fundamental tenet of orthodontic treatment. Orthodontic forces are sensed by osteoblast lineage cells such as periodontal ligament (PDL) cells and osteocytes. Of several cytokines produced by these cells, the most important cytokine promoting osteoclastogenesis is the receptor activator of nuclear factor-κB ligand (RANKL), which is mainly supplied by osteoblasts. Additionally, osteocytes embedded within the bone matrix, T lymphocytes in inflammatory conditions, and PDL cells produce RANKL. Besides RANKL, inflammatory cytokines, such as interleukin-1, tumor necrosis factor-α, and prostaglandin E2 promote osteoclastogenesis under OTM. On the downside, excessive osteoclastogenesis activation triggers orthodontically-induced external root resorption (ERR) through pro-osteoclastic inflammatory cytokines. Therefore, understanding the mechanisms of osteoclastogenesis during OTM is essential in reducing the adverse effects of orthodontic treatment. Here, we review the current concepts of the mechanisms underlying osteoclastogenesis in OTM and orthodontically induced ERR.

Tooth eruption is a unique biological process by which highly mineralized tissues emerge into the outer world, and it occurs concomitantly with tooth root formation. These 2 processes have been considered independent phenomena; however, recent studies support the theory that they are indeed intertwined. Dental mesenchymal progenitor cells in the dental follicle lie at the heart of the coupling of these 2 processes, providing a source for diverse mesenchymal cells that support formation of the highly functional tooth root and the periodontal attachment apparatus, while facilitating formation of osteoclasts. These cells are regulated by autocrine signaling by parathyroid hormone–related protein (PTHrP) and its parathyroid hormone/PTHrP receptor PPR. This PTHrP-PPR signaling appears to crosstalk with other signaling pathways and regulates proper cell fates of mesenchymal progenitor cell populations. Disruption of this autocrine PTHrP-PPR signaling in these cells leads to defective formation of the periodontal attachment apparatus, tooth root malformation, and failure of tooth eruption in molars, which essentially recapitulate primary failure of eruption in humans, a rare genetic disorder exclusively affecting tooth eruption. Diversity and distinct functionality of these mesenchymal progenitor cell populations that regulate tooth eruption and tooth root formation are beginning to be unraveled.

The parathyroid hormone receptor (PTH1R) mediates the biologic actions of parathyroid hormone (PTH) and parathyroid hormone related protein (PTHrP). Here, we showed that salt inducible kinases (SIKs) are key kinases that control the skeletal actions downstream of PTH1R and that this GPCR, when activated, inhibited cellular SIK activity. Sik gene deletion led to phenotypic changes that were remarkably similar to models of increased PTH1R signaling. In growth plate chondrocytes, PTHrP inhibited SIK3 and ablation of this kinase in proliferating chondrocytes rescued perinatal lethality of PTHrP-null mice. Combined deletion of Sik2/Sik3 in osteoblasts and osteocytes led to a dramatic increase in bone mass that closely resembled the skeletal and molecular phenotypes observed when these bone cells express a constitutively active PTH1R that causes Jansen’s metaphyseal chondrodysplasia. Finally, genetic evidence demonstrated that class IIa HDACs were key PTH1R-regulated SIK substrates in both chondrocytes and osteocytes. Taken together, our findings established that SIK inhibition is central to PTH1R action in bone development and remodeling. Furthermore, this work highlighted the key role of cAMP-regulated salt inducible kinases downstream of GPCR action.