Summary: Osteoclasts are large multinucleated cells from hematopoietic origin and are responsible for bone resorption. A balance between osteoclastic bone resorption and osteoblastic bone formation is critical to maintain bone homeostasis. The alveolar bone, also called the alveolar process, is the part of the jawbone that holds the teeth and supports oral functions. It differs from other skeletal bones in several aspects: its embryonic cellular origin, the form of ossification, and the presence of teeth and periodontal tissues; hence, understanding the unique characteristic of the alveolar bone remodeling is important to maintain oral homeostasis. Excessive osteoclastic bone resorption is one of the prominent features of bone diseases in the jaw such as periodontitis. Therefore, inhibiting osteoclast formation and bone resorptive process has been the target of therapeutic intervention. Understanding the mechanisms of osteoclastic bone resorption is critical for the effective treatment of bone diseases in the jaw. In this review, we discuss basic principles of alveolar bone remodeling with a specific focus on the osteoclastic bone resorptive process and its unique functions in the alveolar bone. Lastly, we provide perspectives on osteoclast‐targeted therapies and regenerative approaches associated with bone diseases in the jaw.

Aims/Introduction: Dental pulp stem cells (DPSCs) are thought to be an attractive candidate for cell therapy. We recently reported that the transplantation of DPSCs increased nerve conduction velocity and nerve blood flow in diabetic rats. In the present study, we investigated the immunomodulatory effects of DPSC transplantation on diabetic peripheral nerves.Materials and Methods: DPSCs were isolated from the dental pulp of Sprague–Dawley rats and expanded in culture. Eight weeks after the streptozotocin injection, DPSCs were transplanted into the unilateral hindlimb skeletal muscles. Four weeks after DPSC transplantation, neurophysiological measurements, inflammatory gene expressions and the number of CD68‐positive cells in sciatic nerves were assessed. To confirm the immunomodulatory effects of DPSCs, the effects of DPSC‐conditioned media on lipopolysaccharide‐stimulated murine macrophage RAW264.7 cells were investigated.Results: Diabetic rats showed significant delays in sciatic nerve conduction velocities and decreased sciatic nerve blood flow, all of which were ameliorated by DPSC transplantation. The number of CD68‐positive monocytes/macrophages and the gene expressions of M1 macrophage‐expressed cytokines, tumor necrosis factor‐α and interleukin‐1β, were increased in the sciatic nerves of the diabetic rats. DPSC transplantation significantly decreased monocytes/macrophages and tumor necrosis factor‐α messenger ribonucleic acid expression, and increased the gene expression of the M2 macrophage marker, CD206, in the sciatic nerves of the diabetic rats. The in vitro study showed that DPSC‐conditioned media significantly increased the gene expressions of interleukin‐10 and CD206 in lipopolysaccharide‐stimulated RAW264.7 cells.Conclusions: These results suggest that DPSC transplantation promoted macrophages polarization towards anti‐inflammatory M2 phenotypes, which might be one of the therapeutic mechanisms for diabetic polyneuropathy.

Although previous reports have revealed the therapeutic potential of stem cell transplantation in diabetic polyneuropathy, the effects of cell transplantation on long-term diabetic polyneuropathy have not been investigated. In this study, we investigated whether the transplantation of dental pulp stem cells (DPSCs) ameliorated long-term diabetic polyneuropathy in streptozotocin (STZ)-induced diabetic rats. Forty-eight weeks after STZ injection, we transplanted DPSCs into the unilateral hindlimb skeletal muscles. Four weeks after DPSC transplantation (i.e., 52 weeks after STZ injection) the effects of DPSC transplantation on diabetic polyneuropathy were assessed. STZ-induced diabetic rats showed significant reductions in the sciatic motor/sensory nerve conduction velocity, increases in the current perception threshold, and decreases in capillary density in skeletal muscles and intra-epidermal nerve fiber density compared with normal rats, all of which were ameliorated by DPSC transplantation. Furthermore, sural nerve morphometrical analysis revealed that the transplantation of DPSCs significantly increased the myelin thickness and area. DPSC-conditioned media promoted the neurite outgrowth of dorsal root ganglion neurons and increased the viability and myelin-related protein expression of Schwann cells. These results indicated that the transplantation of DPSCs contributed to the neurophysiological and neuropathological recovery from a long duration of diabetic polyneuropathy.

The aim of this study was to assess the efficacy of a self-assembling peptide hydrogel as a scaffold for bone regeneration. We used a neutral and injectable self-assembling peptide hydrogel, SPG-178-Gel. Bone defects (5 mm in diameter) in rat calvarial bones were filled with a mixture of alpha-modified Eagle's medium and peptide hydrogel. Three weeks after surgery, soft X-ray and microcomputed tomography (micro-CT) images of the gel-treated bones showed new bone formations in the periphery and in central areas of the defects. Next, we evaluated the three-dimensional osteogenic induction of dental pulp stem cells (DPSCs), a type of mesenchymal stem cell, in SPG-178-Gel. We first confirmed that the osteogenic differentiation of DPSCs was significantly promoted by osteogenic induction medium containing recombinant human bone morphogenetic protein-4 (rhBMP-4) in a two-dimensional cell culture. Then, we verified DPSC proliferation and osteogenic differentiation in a three-dimensional cell culture using SPG-178-Gel. The gene expression levels of osteopontin, osteocalcin, and collagen type I were significantly increased when DPSCs were cultured in SPG-178-Gel with the osteogenic induction medium. Micro-CT observations showed the formation of widespread calcium deposition. In conclusion, SPG-178-Gel was adequately effective as a scaffold and can be a suitable tool for bone formation in vivo and in vitro. These findings suggest that the self-assembling peptide hydrogel, SPG-178-Gel, could be a promising tool for bone tissue engineering.

Tissue engineering aims to repair, restore, and/or replace tissues in the human body as an alternative to grafts and prostheses. Biomaterial scaffolds can be utilized to provide a three-dimensional microenvironment to facilitate tissue regeneration. Previously, we reported that scaffold pore size influences vascularization and extracellular matrix composition both in vivo and in vitro, to ultimately influence tissue phenotype for regenerating cranial suture and bone tissues, which have markedly different tissue properties despite similar multipotent stem cell populations. To rationally design biomaterials for specific cell and tissue fate specification, it is critical to understand the molecular processes governed by cell-biomaterial interactions, which guide cell fate specification. Building on our previous work, in this report we investigated the hypothesis that scaffold pore curvature, the direct consequence of pore size, modulates the differentiation trajectory of mesenchymal stem cells (MSCs) through alterations in the cytoskeleton. First, we demonstrated that sufficiently small pores facilitate cell clustering in subcutaneous explants cultured in vivo, which we previously reported to demonstrate stem tissue phenotype both in vivo and in vitro. Based on this observation, we cultured cell-scaffold constructs in vitro to assess early time point interactions between cells and the matrix as a function of pore size. We demonstrate that principle curvature directly influences nuclear aspect and cell aggregation in vitro. Scaffold pores with a sufficiently low degree of principle curvature enables cell differentiation; pharmacologic inhibition of actin cytoskeleton polymerization in these scaffolds decreased differentiation, indicating a critical role of the cytoskeleton in transducing cues from the scaffold pore microenvironment to the cell nucleus. We fabricated a macropore model, which allows for three-dimensional confocal imaging and demonstrates that a higher principle curvature facilitates cell aggregation and the formation of a potentially protective niche within scaffold macropores which prevents MSC differentiation and retains their stemness. Sufficiently high principle curvature upregulates yes-associated protein (YAP) phosphorylation while decreased principle curvature downregulates YAP phosphorylation and increases YAP nuclear translocation with subsequent transcriptional activation towards an osteogenic differentiation fate. Finally, we demonstrate that the inhibition of the YAP/TAZ pathway causes a defect in differentiation, while YAP/TAZ activation causes premature differentiation in a curvature-dependent way when modulated by verteporfin (VP) and 1-oleyl-lysophosphatidic acid (LPA), respectively, confirming the critical role of biomaterials-mediated YAP/TAZ signaling in cell differentiation and fate specification. Our data support that the principle curvature of scaffold macropores is a critical design criterion which guides the differentiation trajectory of mesenchymal stem cells’ scaffolds. Biomaterial-mediated regulation of YAP/TAZ may significantly contribute to influencing the regenerative outcomes of biomaterials-based tissue engineering strategies through their specific pore design.