Ablation of Pyrophosphate Regulators Promotes Periodontal Regeneration

Amorphous 1-D nanowires of calcium phosphate/pyrophosphate: A demonstration of oriented self-growth of amorphous minerals

Amorphous inorganic solids are traditionally isotropic, thus, it is believed that they only grow in a non-preferential way without the assistance of regulators, leading to the morphologies of nanospheres or irregular aggregates of nanoparticles. However, in the presence of (ortho)phosphate (Pi) and pyrophosphate ions (PPi) which have synergistic roles in biomineralization, the highly elongated amorphous nanowires (denoted ACPPNs) form in a regulator-free aqueous solution (without templates, additives, organics, etc). Based on thorough characterization and tracking of the formation process (e.g., Cryo-TEM, spherical aberration correction high resolution TEM, solid state NMR, high energy resolution monochromated STEM-EELS), the microstructure and its preferential growth behavior are elucidated. In ACPPNs, amorphous calcium orthophosphate and amorphous calcium pyrophosphate are distributed at separated but close sites. The ACPPNs grow via either the preferential attachment of ∼2 nm nanoclusters in a 1-dimension way, or the transformation of bigger nanoparticles, indicating an inherent driving force-governed process. We propose that the anisotropy of ACPPNs microstructure, which is corroborated experimentally, causes their oriented growth. This study proves that, unlike the conventional view, amorphous minerals can form via oriented growth without external regulation, demonstrating a novel insight into the structures and growth behaviors of amorphous minerals.

The inhibition of mineralisation by fibroblast growth factor 2 is associated with the altered expression of genes regulating phosphate balance

The study aimed to determine whether inhibitory effects of fibroblast growth factor 2 (FGF2) on mineralisation in dental pulp (DP) cultures were associated with changes in the expression of genes regulating phosphate balance (Enpp1, Ank, Slc20a2, Alpl, Phospho1, and Xpr1). DP cultures growing under mineralisation-inducing conditions were exposed to FGF2 and inhibitors of the FGFR and MEK/ERK1/2 signaling pathways. Mineralisation, culture cellularity, and gene expression were examined at various time points. Statistical analysis was performed using analysis of variance followed by the Holm-Šídák test. Control cultures exhibited transient increases in Enpp1 and Ank, continuous increases in Alpl, Phospho1, and Xpr1, and continuous decreases in Slc20a2. FGF2 increased Enpp1, Ank, and Slc20a2 and decreased Alpl, Phospho1, and Xpr1, whereas the FGF2 withdrawal and inhibition of FGFR and MEK/ERK1/2 exerted opposite effects. These changes suggest that FGF2-mediated decreases in mineralisation could be functionally coupled to the altered regulation of phosphate formation and transport.

Loss of function mutation in Ank causes aberrant mineralization and acquisition of osteoblast-like-phenotype by the cells of the intervertebral disc

Pathological mineralization of intervertebral disc is debilitating and painful and linked to disc degeneration in a subset of human patients. An adenosine triphosphate efflux transporter, progressive ankylosis (ANK) is a regulator of extracellular inorganic pyrophosphate levels and plays an important role in tissue mineralization. However, the function of ANK in intervertebral disc has not been fully explored. Herein we analyzed the spinal phenotype of Ank mutant mice (ank/ank) with attenuated ANK function. Micro-computed tomography and histological analysis showed that loss of ANK function results in the aberrant annulus fibrosus mineralization and peripheral disc fusions with cranial to caudal progression in the spine. Vertebrae in ank mice exhibit elevated cortical bone mass and increased tissue non-specific alkaline phosphatase-positive endplate chondrocytes with decreased subchondral endplate porosity. The acellular dystrophic mineral inclusions in the annulus fibrosus were localized adjacent to apoptotic cells and cells that acquired osteoblast-like phenotype. Fourier transform infrared spectral imaging showed that the apatite mineral in the outer annulus fibrosus had similar chemical composition to that of vertebral bone. Transcriptomic analysis of annulus fibrosus and nucleus pulposus tissues showed changes in several biological themes with a prominent dysregulation of BMAL1/CLOCK circadian regulation. The present study provides new insights into the role of ANK in the disc tissue compartments and highlights the importance of local inorganic pyrophosphate metabolism in inhibiting the mineralization of this important connective tissue.

Deficiency of G protein-coupled receptor Gpr111/Adgrf2 causes enamel hypomineralization in mice by alteration of the expression of kallikrein-related peptidase 4 (Klk4) during pH cycling process

Enamel is formed by the repetitive secretion of a tooth-specific extracellular matrix and its decomposition. Calcification of the enamel matrix via hydroxyapatite (HAP) maturation requires pH cycling to be tightly regulated through the neutralization of protons released during HAP synthesis. We found that Gpr115, which responds to changes in extracellular pH, plays an important role in enamel formation. Gpr115-deficient mice show partial enamel hypomineralization, suggesting that other pH-responsive molecules may be involved. In this study, we focused on the role of Gpr111/Adgrf2, a duplicate gene of Gpr115, in tooth development. Gpr111 was highly expressed in mature ameloblasts. Gpr111-KO mice showed enamel hypomineralization. Dysplasia of enamel rods and high carbon content seen in Gpr111-deficient mice suggested the presence of residual enamel matrices in enamel. Depletion of Gpr111 in dental epithelial cells induced the expression of ameloblast-specific protease, kallikrein-related peptidase 4 (Klk4), suggesting that Gpr111 may act as a suppressor of Klk4 expression. Moreover, reduction of extracellular pH to 6.8 suppressed the expression of Gpr111, while the converse increased Klk4 expression. Such induction of Klk4 was synergistically enhanced by Gpr111 knockdown, suggesting that proper enamel mineralization may be linked to the modulation of Klk4 expression by Gpr111. Furthermore, our in vitro suppression of Gpr111 and Gpr115 expression indicated that their suppressive effect on calcification was additive. These results suggest that both Gpr111 and Gpr115 respond to extracellular pH, contribute to the expression of proteolytic enzymes, and regulate the pH cycle, thereby playing important roles in enamel formation.

Bone Sialoprotein Is Critical for Alveolar Bone Healing in Mice

Bone sialoprotein (BSP) is an extracellular matrix (ECM) protein associated with mineralized tissues, particularly bone and cementum. BSP includes functional domains implicated in collagen binding, hydroxyapatite nucleation, and cell signaling, although its function(s) in osteoblast and osteoclast differentiation and function remain incompletely understood. Genetic ablation of BSP in Ibsp knockout (Ibsp-/-) mice results in developmental bone mineralization and remodeling defects, with alveolar bone more severely affected than the femurs and tibias of the postcranial skeleton. The role of BSP in alveolar bone healing has not been studied. We hypothesized that BSP ablation would cause defective alveolar bone healing. We employed a maxillary first molar extraction socket healing model in 42-d postnatalIbsp-/- and wild-type (WT) control mice. Tissues were collected at 0, 7, 14, 21, and 56 d postprocedure (dpp) for analysis by micro-computed tomography (microCT), histology, in situ hybridization (ISH), immunohistochemistry (IHC), and quantitative polymerase chain reaction (qPCR) array. As expected, alveolar bone healing progressed in WT mice with increasing bone volume fraction (BV/TV), bone mineral density (BMD), and tissue mineral density (TMD), transitioning from woven to mature bone from 7 to 56 dpp. Ibsp messenger RNA (mRNA) and BSP protein were strongly expressed during alveolar bone healing in parallel with other osteogenic markers. Compared to WT, Ibsp-/- mice exhibited 50% to 70% reduced BV/TV and BMD at all time points, 7% reduced TMD at 21 dpp, abnormally increased Col1a1 and Alpl mRNA expression, and persistent presence of woven bone and increased bone marrow in healing sockets. qPCR revealed substantially dysregulated gene expression in alveolar bone of Ibsp-/- versus WT mice, with significantly disrupted expression of 45% of tested genes in functional groups, including markers for osteoblasts, osteoclasts, mineralization, ECM, cell signaling, and inflammation. We conclude that BSP is a critical and nonredundant factor for alveolar bone healing, and its absence disrupts multiple major pathways involved in appropriate healing.

Dual peptide-functionalized hydrogels differentially control periodontal cell function and promote tissue regeneration

Restoring the tooth-supporting tissues lost during periodontitis is a significant clinical challenge, despite advances in both biomaterial and cell-based approaches. This study investigated poly(ethylene glycol) (PEG) hydrogels functionalized with integrin-binding peptides RGD and GFOGER for controlling periodontal ligament cell (PDLC) activity and promoting periodontal tissue regeneration. Dual presentation of RGD and GFOGER within PEG hydrogels potentiated two key PDLC functions, alkaline phosphatase (ALP) activity and matrix mineralization, over either peptide alone and could be tuned to differentially promote each function. Hydrogel matrix mineralization, fostered by high concentrations of GFOGER together with RGD, identified a PDLC phenotype with accelerated matrix adhesion formation and expression of cementoblast and osteoblast genes. In contrast, maximizing ALP activity through high RGD and low GFOGER levels resulted in minimal hydrogel mineralization, in part, through altered PDLC pyrophosphate regulation. Transplantation of PDLCs in hydrogels optimized for either outcome promoted cementum formation in rat periodontal defects; however, only hydrogels optimized for in vitro mineralization improved new bone formation. Overall, these results highlight the utility of engineered hydrogel systems for controlling PDLC functions and their promise for promoting periodontal tissue regeneration.

Between a rock and a hard place: Regulation of mineralization in the periodontium

The periodontium supports and attaches teeth via mineralized and nonmineralized tissues. It consists of two, unique mineralized tissues, cementum and alveolar bone. In between these tissues, lies an unmineralized, fibrous periodontal ligament (PDL), which distributes occlusal forces, nourishes and invests teeth, and harbors progenitor cells for dentoalveolar repair. Many unanswered questions remain regarding periodontal biology. This review will focus on recent research providing insights into one enduring mystery: the precise regulation of the hard-soft tissue borders in the periodontium which define the interfaces of the cementum-PDL-alveolar bone structure. We will focus on advances in understanding the molecular mechanisms that maintain the unmineralized PDL "between a rock and a hard place" by regulating the mineralization of cementum and alveolar bone.

Temporal induction of Lhx8 by optogenetic control system for efficient bone regeneration

BACKGROUND

The spatiotemporal regulation of essential genes is crucial for controlling the growth and differentiation of cells in a precise manner during regeneration. Recently, optogenetics was considered as a potent technology for sophisticated regulation of target genes, which might be a promising tool for regenerative medicine. In this study, we used an optogenetic control system to precisely regulate the expression of Lhx8 to promote efficient bone regeneration.

METHODS

Quantitative real-time PCR and western blotting were used to detect the expression of Lhx8 and osteogenic marker genes. Alkaline phosphatase staining and alizarin red staining were used to detect alkaline phosphatase activity and calcium nodules. A customized optogenetic expression system was constructed to regulate Lhx8, of which the expression was activated in blue light but not in dark. We also used a critical calvarial defect model for the analysis of bone regeneration in vivo. Moreover, micro-computed tomography (micro-CT), three-dimensional reconstruction, quantitative bone measurement, and histological and immunohistochemistry analysis were performed to investigate the formation of new bone in vivo.

RESULTS

During the osteogenic differentiation of BMSCs, the expression levels of Lhx8 increased initially but then decreased thereafter. Lhx8 promoted the early proliferation of BMSCs but inhibited subsequent osteogenic differentiation. The optogenetic activation of Lhx8 in BMSCs in the early stages of differentiation by blue light stimulation led to a significant increase in cell proliferation, thus allowing a sufficient number of differentiating BMSCs to enter the later osteogenic differentiation stage. Analysis of the critical calvarial defect model revealed that the pulsed optogenetic activation of Lhx8 in transplanted BMSCs over a 5-day period led to a significant increase in the generation of bone in vivo.

CONCLUSIONS

Lhx8 plays a critical role in balancing proliferation and osteogenic differentiation in BMSCs. The optogenetic activation of Lhx8 expression at early stage of BMSCs differentiation led to better osteogenesis, which would be a promising strategy for precise bone regeneration.

Delivery of Alkaline Phosphatase Promotes Periodontal Regeneration in Mice

Factors regulating the ratio of pyrophosphate (PP_i) to phosphate (P_i) modulate biomineralization. Tissue-nonspecific alkaline phosphatase (TNAP) is a key promineralization enzyme that hydrolyzes the potent mineralization inhibitor PP_i. The goal of this study was to determine whether TNAP could promote periodontal regeneration in bone sialoprotein knockout mice (Ibsp^-/- mice), which are known to have a periodontal disease phenotype. Delivery of TNAP was accomplished either systemically (through a lentiviral construct expressing a mineral-targeted TNAP-D₁₀ protein) or locally (through addition of recombinant human TNAP to a fenestration defect model). Systemic TNAP-D₁₀ delivered by intramuscular injection at 5 d postnatal (dpn) increased circulating alkaline phosphatase (ALP) levels in Ibsp^-/- mice by 5-fold at 30 dpn, with levels returning to normal by 60 dpn when tissues were evaluated by micro-computed tomography and histology. Local delivery of recombinant human TNAP to fenestration defects in 5-wk-old wild type (WT) and Ibsp^-/- mice did not alter long-term circulating ALP levels, and tissues were evaluated by micro-computed tomography and histology at postoperative day 45. Systemic and local delivery of TNAP significantly increased alveolar bone volume (20% and 37%, respectively) and cementum thickness (3- and 42-fold) in Ibsp^-/- mice, with evidence for periodontal ligament attachment and bone/cementum marker localization. Local delivery significantly increased regenerated cementum and bone in WT mice. Addition of 100-μg/mL bovine intestinal ALP to culture media to increase ALP in vitro increased media P_i concentration, mineralization, and Spp1 and Dmp1 marker gene expression in WT and Ibsp^-/- OCCM.30 cementoblasts. Use of phosphonoformic acid, a nonspecific inhibitor of sodium P_i cotransport, indicated that effects of bovine intestinal ALP on mineralization and marker gene expression were in part through P_i transport. These findings show for the first time through multiple in vivo and in vitro approaches that pharmacologic modulation of P_i/PP_i metabolism can overcome periodontal breakdown and accomplish regeneration.