Abstract
The periodontal ligament (PDL) comprises a fibrous tissue that connects teeth to alveolar bone and is essential for periodontal function. The transcription factor mohawk homeobox (Mkx) is expressed in the PDL where it plays an important role in the development and maintenance of the PDL. However, the precise and critical functions of Mkx in the cell populations comprising PDL have not yet been elucidated. The present study aimed to clarify the effects of a Mkx deficiency on PDL cellular heterogeneity and differences between gene expression in PDL tissues from wild-type (WT) (Mkx +/+ ) and Mkx knockout (Mkx −/− ) rats using single-cell RNA sequencing. We identified 12 cell clusters comprising mesenchymal cells and macrophages. The expression of Mkx and scleraxis (Scx; another key transcription factor of PDL), was mutually exclusive, and partitioned mesenchymal cell clusters into Mkx and Scx types that dominantly expressed proteoglycans and elastic fibers, and type 1 and 3 collagen, respectively. Ossification-related genes were upregulated in mesenchymal cell and osteoblast clusters with more Mkx −/− than Mkx +/+ PDLs. Increased number of cells and inflammatory mediators were observed in macrophage clusters of Mkx −/− PDL. These results suggested that Mkx plays an important role in maintaining PDL homeostasis by regulating specific cell populations and gene expression.
Highlights
The periodontal ligament is a fibrous tissue that connects alveolar bone to the cementum that covers teeth
We identified CD31 (Pecam1)+ and Tie-2 (Tek)+ vascular endothelial cells (C8_Endo), Myelin basic protein (Mbp)+, Sry-related HMg-Box gene 10 (Sox10)+ and Peripheral myelin protein 22 (Pmp22)+ neural cells, including oligodendrocytes and Schwann cells (C12_NCs), CD146 (Mcam)+, Platelet-derived growth factor receptor beta (Pdgfrb)+ and Thy-1 cell surface antigen (Thy1)+ pericytes (C10_Peri)
mohawk homeobox (Mkx) and Scx were described as transcription factors involved in tendon development (Cserjesi et al, 1995; Ito et al, 2010; Liu et al, 2010; Schweitzer et al, 2001), and they play important roles in the development and homeostasis of tendon cells
Summary
The periodontal ligament is a fibrous tissue that connects alveolar bone to the cementum that covers teeth. The periodontal ligament comprises fibroblasts, osteoblasts, blood vessels, nerves, epithelial cells, and a rich extracellular matrix (ECM) (Beertsen et al, 1997). Fibroblasts derived from mesenchymal cells are important for maintaining space between the teeth and alveolar bone by producing an ECM in response to mechanical loading (Chukkapalli and Lele, 2018). Mechanical loading of the PDL by orthodontic and occlusal forces induces the production of ECM and the activation of osteoclasts and osteoblasts (Harrel, 2003). Inflammation induced by oral bacteria and mechanical loading in PDL elicits ECM and bone remodeling, which is important for PDL homeostasis (Lerner, 2006; Chukkapalli and Lele, 2018)
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