Xianghong Luan

Evolution and Development of Hertwig’s Epithelial Root Sheath

Periodontal regeneration and tissue engineering has re‐awakened interest in the role of Hertwigs Epithelial Root Sheath (HERS), an epithelial tissue layer first discovered in amphibians more than a century ago. Using developmental, evolutionary, and cell biological approaches, we have, therefore, performed a careful analysis of the role of HERS in root formation and compared our data with clinical findings. Our developmental studies revealed HERS as a transient structure assembled in the early period of root formation and elongation and, subsequently, fenestrated and reduced to epithelial rests of Malassez (ERM). Our comparative evolutionary studies indicated that HERS fenestration was closely associated with the presence of a periodontal ligament and a gomphosis‐type attachment apparatus in crocodilians and mammals. Based on these studies, we are proposing that HERS plays an important role in the regulation and maintenance of periodontal ligament space and function. Additional support for this hypothesis was rendered by our meta‐analysis of recent clinical reports related to HERS function. Developmental Dynamics 235:1167–1180, 2006.

Extracellular matrix-mediated differentiation of periodontal progenitor cells

The periodontal ligament (PDL) is a specialized connective tissue that connects the surface of the tooth root with the bony tooth socket. The healthy PDL harbors stem cell niches and extracellular matrix (ECM) microenvironments that facilitate periodontal regeneration. During periodontal disease, the PDL is often compromised or destroyed, reducing the life-span of the tooth. In order to explore new approaches toward the regeneration of diseased periodontal tissues, we have tested the effect of periodontal ECM signals, fibroblast growth factor 2 (FGF2), connective tissue growth factor (CTGF), and the cell adhesion peptide Arg-Gly-Asp (RGD) on the differentiation of two types of periodontal progenitor cells, PDL progenitor cells (PDLPs) and dental follicle progenitor cells (DFCs). Our studies documented that CTGF and FGF2 significantly enhanced the expression of collagens I & III, biglycan and periostin in tissue engineered regenerates after 4 weeks compared to untreated controls. Specifically, CTGF promoted mature PDL-like tissue regeneration as demonstrated by dense periostin localization in collagen fiber bundles. CTGF and FGF2 displayed synergistic effects on collagen III and biglycan gene expression, while effects on mineralization were antagonistic to each other: CTGF promoted while FGF2 inhibited mineralization in PDL cell cultures. Incorporation of RGD peptides in hydrogel matrices significantly enhanced attachment, spreading, survival and mineralization of the encapsulated DFCs, suggesting that RGD additives might promote the use of hydrogels for periodontal mineralized tissue engineering. Together, our studies have documented the effect of three key components of the periodontal ECM on the differentiation of periodontal progenitor populations.

Successful periodontal ligament regeneration by periodontal progenitor preseeding on natural tooth root surfaces.

The regeneration of lost periodontal ligament (PDL) and alveolar bone is the purpose of periodontal tissue engineering. The goal of the present study was to assess the suitability of 3 odontogenic progenitor populations from dental pulp, PDL, and dental follicle for periodontal regeneration when exposed to natural and synthetic apatite surface topographies. We demonstrated that PDL progenitors featured higher levels of periostin and scleraxis expression, increased adipogenic and osteogenic differentiation potential, and pronounced elongated cell shapes on barren root chips when compared with dental pulp and dental follicle cells. When evaluating the effect of surface characteristics on PDL progenitors, natural root surfaces resulted in elongated PDL cell shapes, whereas PDL progenitors on synthetic apatite surfaces were rounded or polygonal. In addition, surface coatings affected PDL progenitor gene expression profiles: collagen I coatings enhanced alkaline phosphatase and osteocalcin expression levels and laminin-1 coatings increased epidermal growth factor (EGF), nestin, cadherin 1, and keratin 8 expression. PDL progenitors seeded on natural tooth root surfaces in organ culture formed new periodontal fibers after 3 weeks of culture. Finally, replantation of PDL progenitor-seeded tooth roots into rat alveolar bone sockets resulted in the complete formation of a new PDL and stable reattachment of teeth over a 6-month period. Together, these findings indicate that periodontal progenitor cell type as well as mineral surface topography and molecular environment play crucial roles in the regeneration of true periodontal anchorage.

Platelet-Rich Fibrin Promotes Periodontal Regeneration and Enhances Alveolar Bone Augmentation

In the present study we have determined the suitability of platelet-rich fibrin (PRF) as a complex scaffold for periodontal tissue regeneration. Replacing PRF with its major component fibrin increased mineralization in alveolar bone progenitors when compared to periodontal progenitors, suggesting that fibrin played a substantial role in PRF-induced osteogenic lineage differentiation. Moreover, there was a 3.6-fold increase in the early osteoblast transcription factor RUNX2 and a 3.1-fold reduction of the mineralization inhibitor MGP as a result of PRF application in alveolar bone progenitors, a trend not observed in periodontal progenitors. Subcutaneous implantation studies revealed that PRF readily integrated with surrounding tissues and was partially replaced with collagen fibers 2 weeks after implantation. Finally, clinical pilot studies in human patients documented an approximately 5 mm elevation of alveolar bone height in tandem with oral mucosal wound healing. Together, these studies suggest that PRF enhances osteogenic lineage differentiation of alveolar bone progenitors more than of periodontal progenitors by augmenting osteoblast differentiation, RUNX2 expression, and mineralized nodule formation via its principal component fibrin. They also document that PRF functions as a complex regenerative scaffold promoting both tissue-specific alveolar bone augmentation and surrounding periodontal soft tissue regeneration via progenitor-specific mechanisms.

RANKL, osteopontin, and osteoclast homeostasis in a hyperocclusion mouse model.

The biological mechanisms that maintain the position of teeth in their sockets establish a dynamic equilibrium between bone resorption and apposition. In order to reveal some of the dynamics involved in the tissue responses towards occlusal forces on periodontal ligament (PDL) and alveolar bone homeostasis, we developed the first mouse model of hyperocclusion. Swiss-Webster mice were kept in hyperocclusion for 0, 3, 6, and 9 d. Morphological and histological changes in the periodontium were assessed using micro-computed tomography (micro-CT) and ground sections with fluorescent detection of vital dye labels. Sections were stained for tartrate-resistant acid phosphatase, and the expression of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteopontin (OPN) was analyzed by immunohistochemistry and real-time polymerase chain reaction (PCR). Traumatic occlusion resulted in enamel surface abrasion, inhibition of alveolar bone apposition, significant formation of osteoclasts at 3, 6 and 9 d, and upregulation of OPN and RANKL. Data from this study suggest that both OPN and RANKL contribute to the stimulation of bone resorption in the hyperocclusive state. In addition, we propose that the inhibition of alveolar bone apposition by occlusal forces is an important mechanism for the control of occlusal height that might work in synergy with RANKL-induced bone resorption to maintain normal occlusion.

Extracellular Matrix-mediated Tissue Remodeling Following Axial Movement of Teeth

Tooth eruption is a multifactorial process involving movement of existing tissues and formation of new tissues coordinated by a complex set of genetic events. We have used the model of the unopposed rodent molar to study morphological and genetic mechanisms involved in axial movement of teeth. Following extraction of opposing upper molars, lower molars supererupted by 0.13 mm. Labeled tissue sections revealed significant amounts of new bone and cementum apposition at the root apex of the unopposed side following supereruption for 12 days. Newly apposited cementum and alveolar bone layers were approximately 3-fold thicker in the experimental vs the control group, whereas periodontal ligament width was maintained. Tartrate-resistant acid phosphatase staining indicated bone resorption at the mesial alveolar walls of unopposed molars and provided in tandem with new bone formation at the distal alveolar walls an explanation for the distal drift of molars in this model. Microarray analysis and semiquantitative RT-PCR demonstrated a significant increase in collagen I, integrin β5, and SPARC gene expression as revealed by comparison between the unopposed molar group and the control group. Immunohistochemical verification revealed increased levels of integrin β5 and SPARC labeling in the periodontal ligament of the unopposed molar. Together our findings suggest that posteruptive axial movement of teeth was accomplished by significant formation of new root cementum and alveolar bone at the root apex in tandem with upregulation of collagen I, integrin β5, and SPARC gene expression.

Differentiation of Neural-Crest-Derived Intermediate Pluripotent Progenitors into Committed Periodontal Populations Involves Unique Molecular Signature Changes, Cohort Shifts, and Epigenetic Modifications

Intermediate progenitor populations play a crucial role in the regional specification and differentiation of the cranial neural crest. On the basis of global gene expression profiles, gene cohort expression levels, and epigenetic modifications, we have defined key factors involved in the differentiation of dental follicle (DF) intermediate progenitors into periodontal lineages, including alveolar bone (AB) osteoblasts, cementoblasts, and periodontal ligament (PDL) cells. When comparing differentially expressed genes, PDL cells most closely resembled DF progenitors, followed by AB osteoblasts and cementoblasts as the most distant population. According to gene ontology analyses, extracellular matrix-adhesion proteins were substantially increased in PDL cells, osteogenesis factors were elevated in AB osteoblasts, and gene expression levels were lower in cementoblasts, especially in the cytokine group. Unique signature proteins included interleukin 6, paired-like homeodomain transcription factor 2, thrombospondin 2, and glial cell line-derived neurotrophic factor for DF progenitors; asporin and prostaglandin-H2 D-isomerase for AB osteoblasts; and keratin 18, Netrin 4, Jagged 1, and Dickkopf1 for cementoblasts, as verified by western blot analysis. Secreted frizzled-related protein 1 was preferentially expressed in PDL cells, whereas matrix Gla-protein, bone sialoprotein, and insulin-like growth factor binding protein 5 were higher in AB osteoblasts than in cementoblasts. On an epigenetic level, DF progenitors featured high levels of the euchromatin marker H3K4me3, whereas PDL cells, AB osteoblasts, and cementoblasts contained high levels of the transcriptional repressor H3K9me3. Together, our data indicate that in addition to changes in signature gene expression, unique shifts in gene cohort expression levels, epigenetic modifications, and changes in cell morphology contribute to the individuation of tissue populations from a common neural-crest-derived ancestor.

Elongated Polyproline Motifs Facilitate Enamel Evolution through Matrix Subunit Compaction

How does proline-repeat motif length in the proteins of teeth and bones relate to the evolution of vertebrates? Counterintuitively, longer repeat stretches are associated with smaller aggregated subunits within a supramolecular matrix, resulting in enhanced crystal length in mammalian versus amphibian tooth enamel.

Epigenetic Marks Define the Lineage and Differentiation Potential of Two Distinct Neural Crest-Derived Intermediate Odontogenic Progenitor Populations

Epigenetic mechanisms, such as histone modifications, play an active role in the differentiation and lineage commitment of mesenchymal stem cells. In the present study, epigenetic states and differentiation profiles of two odontogenic neural crest-derived intermediate progenitor populations were compared: dental pulp (DP) and dental follicle (DF). ChIP on chip assays revealed substantial H3K27me3-mediated repression of odontoblast lineage genes DSPP and dentin matrix protein 1 (DMP1) in DF cells, but not in DP cells. Mineralization inductive conditions caused steep increases of mineralization and patterning gene expression levels in DP cells when compared to DF cells. In contrast, mineralization induction resulted in a highly dynamic histone modification response in DF cells, while there was only a subdued effect in DP cells. Both DF and DP progenitors featured H3K4me3-active marks on the promoters of early mineralization genes RUNX2, MSX2, and DLX5, while OSX, IBSP, and BGLAP promoters were enriched for H3K9me3 or H3K27me3. Compared to DF cells, DP cells expressed higher levels of three pluripotency-associated genes, OCT4, NANOG, and SOX2. Finally, gene ontology comparison of bivalent marks unique for DP and DF cells highlighted cell-cell attachment genes in DP cells and neurogenesis genes in DF cells. In conclusion, the present study indicates that the DF intermediate odontogenic neural crest lineage is distinguished from its DP counterpart by epigenetic repression of DSPP and DMP1 genes and through dynamic histone enrichment responses to mineralization induction. Findings presented here highlight the crucial role of epigenetic regulatory mechanisms in the terminal differentiation of odontogenic neural crest lineages.

MicroRNA-138 Inhibits Periodontal Progenitor Differentiation under Inflammatory Conditions.

Inflammatory conditions as they occur during periodontal disease often result in decreased alveolar bone levels and a loss of connective tissue homeostasis. Here we have focused on the effect of microRNA-138 (miR-138) as a potential regulator of periodontal stem cells as they affect homeostasis during inflammatory conditions. Our data indicate that miR-138 was significantly upregulated in our periodontal disease animal model. Interaction of miR-138 with a predicted targeting site on the osteocalcin (OC) promoter resulted in a 3.7-fold reduction of luciferase activity in promoter assays compared with controls; and miR-138 overexpression in periodontal progenitors significantly inhibited OC (3.4-fold), Runx2 (2.8-fold), and collagen I (2.6-fold). Moreover, treatment with inflammatory modulators such as interleukin (IL)–6 and lipopolysaccharide (LPS) resulted in a significant 2.2-fold (IL-6) or 1.9-fold (LPS) increase in miR-138 expression, while OC and Runx2 expression was significantly decreased as a result of treatment with each inflammatory mediator. Further defining the role of miR-138 in the OC-mediated control of mineralization, we demonstrated that the LPS-induced downregulation of OC expression was partially reversed after miR-138 knockdown. LPS, miR-138 mimic, and OC small interfering RNA inhibited osteoblast differentiation marker alkaline phosphatase activity, while miR-138 inhibitor and OC protein addition enhanced alkaline phosphatase activity. Supporting the role of OC as an essential modulator of osteoblast differentiation, knockdown of miR-138 or addition of OC protein partially rescued alkaline phosphatase activity in periodontal ligament (PDL) cells subjected to LPS treatment. Our data establish miR-138 inhibitor as a potential therapeutic agent for the prevention of the bone loss associated with advanced periodontal disease.