Periodontal ligament (PDL) cells, located between the tooth root and alveolar bone, comprise heterogeneous cells that support periodontal homeostasis. Periodontal ligament stem cells (PDLSCs) play an important role in maintaining and regenerating the periodontium (Tomokiyo et al., 2019, Trubiani et al., 2019). Seo et al. (2004) showed that human PDL-derived cells isolated using a stro-1 antibody express mesenchymal stem cell-specific markers, such as CD146/MUC18, and are capable of differentiating into cementoblast-like cells, adipocytes, and PDL-fibroblasts under defined conditions. Since then, numerous studies have explored PDLSCs and clarified their characteristics. As an indication of their progress, PDLSCs express specific cell surface molecules associated with mesenchymal stem cells (MSCs) such as CD29, CD73, CD90, CD146, and stro-1 (Seo et al., 2004, Tomokiyo et al., 2019, Trubiani et al., 2019). Furthermore, advances in the genetic engineering technology, such as the cre/loxP system, have made it possible to trace specific cell populations and better understand their behavior (Hsu, 2015). Notably, using the cre/loxP system, novel cell populations such as α-smooth muscle actin (αSMA)+, Axin2+, Gli1+, and leptin receptor (Lepr)+ cells in PDL were identified to play roles in the maintenance and regeneration of the periodontium as PDLSCs. Populations of αSMA+ and Gli1+ PDL cells, which reside in the perivascular region, have the capacity to differentiate into osteoblasts, cementoblasts, and fibroblasts in a healthy state or after injury (Roguljic et al., 2013, Shalehin et al., 2022). Wnt-responsive Axin2+ PDL cells contribute to bone regeneration in the tooth extraction socket and bone formation during orthodontic tooth movement (Wang et al., 2022, Yuan et al., 2018). Furthermore, LepR+ PDL cells form the alveolar bone after osteoblast differentiation, followed by differentiation into osteocytes within the bone matrix (Oka et al., 2023). As LepR+ cells also become cementoblasts and cementocytes, they are believed to be the origin of hard tissue-forming cells in the PDL.

Osteogenesis in MSCs is induced by the expression of transcription factors such as runt-related transcription factor 2 (Runx2) and osterix (Osx), which are activated by the bone morphogenetic protein (BMP) signaling pathway (Hojo et al., 2017, Nishimura et al., 2012). In contrast, a receptor activator of the Nf-κB ligand (RANKL) signal is provided by osteoblasts and MSCs, and induces osteoclast differentiation from osteoclast precursors by binding to RANKL (Elson et al., 2022). These cytokines and transcription factors are essential for osteoblast and osteoclast differentiation. However, the regulatory factors involved in these processes remain unclear.

Notably, genetic studies have identified an association between low-density lipoprotein receptor-related protein 1 (LRP1) and bone characteristics, such as bone mineral density and bone mass (Cao et al., 2012, Sims et al., 2008). LRP1, a multifunctional member of the low-density lipoprotein receptor family, binds to several ligands with high affinity and is involved in various biological processes such as cell differentiation, cell migration, and endocytosis (Lillis et al., 2008). In studies on osteoblast- or osteoclast-specific LRP1 knockout mice, LRP1 was shown to control osteoclast differentiation and inhibit bone resorption (Lu et al., 2018, Bartelt et al., 2018). Although these findings suggest that LRP1 is critical in osteoclast differentiation for maintaining bone mass, its effects on bone formation have not yet been elucidated.

Here, we investigated whether LRP1 expression in PDLSCs induces alveolar bone formation using conditional knockout (cKO) mice with LRP1-deficient LepR+ cells. Through analyses in cKO mice and cKO-derived PDL cells, we aimed to demonstrate that LRP1 expression in PDLSCs plays an important role in regulating bone formation.