Periodontal tissue engineering and regeneration: current approaches and expanding opportunities

Stem cell-delivery therapeutics for periodontal tissue regeneration

Periodontitis, an inflammatory disease, is the most common cause of tooth loss in adults. Attempts to regenerate the complex system of tooth-supporting apparatus (i.e., the periodontal ligament, alveolar bone and root cementum) after loss/damage due to periodontitis have made some progress recently and provide a useful experimental model for the evaluation of future regenerative therapies. Concentrated efforts have now moved from the use of guided tissue/bone regeneration technology, a variety of growth factors and various bone grafts/substitutes toward the design and practice of endogenous regenerative technology by recruitment of host cells (cell homing) or stem cell-based therapeutics by transplantation of outside cells to enhance periodontal tissue regeneration and its biomechanical integration. This shift is driven by the general inability of conventional therapies to deliver satisfactory outcomes, particularly in cases where the disease has caused large tissue defects in the periodontium. Cell homing and cell transplantation are both scientifically meritorious approaches that show promise to completely and reliably reconstitute all tissue and connections damaged through periodontal disease, and hence research into both directions should continue. In view of periodontal regeneration by paradigms that unlock the body's innate regenerative potential has been reviewed elsewhere, this paper specifically explores and analyses the stem cell types and cell delivery strategies that have been or have the potential to be used as therapeutics in periodontal regenerative medicine, with particular emphasis placed on the efficacy and safety concerns of current stem cell-based periodontal therapies that may eventually enter into the clinic.

Human periodontal ligament cells reaction on a novel hydroxyapatite-collagen scaffold

BACKGROUND

Periodontal tissue regeneration presents a highly promising method for restoring periodontal structures. The development of a suitable bioactive scaffold that promotes cell proliferation and differentiation is critical in periodontal tissue engineering. The aim of this study was to evaluate the biocompatibility of a novel 3-dimensional hydroxyapatite-collagen scaffold with human periodontal ligament (hPDL) cell culture.

METHODS

The scaffold was produced from a natural collagen matrix - purified porcine acellular dermal matrix (PADM), which was then treated with hydroxyapatite (HA) through a biomimetic chemical process to obtain hydroxyapatite-porcine acellular dermal matrix (HA-PADM) scaffold. The hPDL cells were cultured with HA-PADM scaffolds for 1, 3, 6, 14, and 28 days. The cell viability assay, scanning electron microscopy (SEM), hematoxylin and eosin (H&E) staining, immunohistochemistry, and confocal microscopy were employed in different time points to evaluate the biocompatibility of the scaffolds with hPDL cells.

RESULTS

The cell viability assay (WST-1 test) verified cell proliferation on the HA-PADM scaffolds. The SEM study showed unique morphology of hPDL cells, which attach and spread on the surface of the scaffolds. The H&E staining, immunohistochemistry, and confocal microscopy demonstrated that hPDL cells were able to grow into the HA-PADM scaffolds and maintain viability after prolonged culture.

CONCLUSIONS

This study proved that HA-PADM scaffold is -biocompatible for hPDL cells. The cells were able to proliferate and migrate into the scaffold. These observations suggest that HA-PADM is a potential cell carrier for periodontal tissue regeneration.

Tissue engineering: from research to dental clinics

Tissue engineering is an interdisciplinary field that combines the principles of engineering, material and biological sciences toward the development of therapeutic strategies and biological substitutes that restore, maintain, replace or improve biological functions. The association of biomaterials, stem cells, growth and differentiation factors has yielded the development of new treatment opportunities in most of the biomedical areas, including Dentistry. The objective of this paper is to present the principles underlying tissue engineering and the current scenario, the challenges and the perspectives of this area in Dentistry.

SIGNIFICANCE

The growth of tissue engineering as a research field has provided a novel set of therapeutic strategies for biomedical applications. Indeed, tissue engineering may lead to new strategies for the clinical management of patients with dental and craniofacial needs in the future.

The use of calcium phosphate-based biomaterials in implant dentistry

Since calcium phosphates (CaPs) were first proposed, a wide variety of formulations have been developed and continuously optimized, some of which (e.g. calcium phosphate cements, CPCs) have been successfully commercialized for clinical applications. These CaP-based biomaterials have been shown to be very attractive bone substitutes and efficient drug delivery vehicles across diverse biomedical applications. In this article, CaP biomaterials, principally CPCs, are addressed as alternatives/complements to autogenous bone for grafting in implant dentistry and as coating materials for enhancing the osteoinductivity of titanium implants, highlighting their performance benefits simultaneously as carriers for growth factors and as scaffolds for cell proliferation, differentiation and penetration. Different strategies for employing CaP biomaterials in dental implantology aim to ultimately reach the same goal, namely to enhance the osseointegration process for dental implants in the context of immediate loading and to augment the formation of surrounding bone to guarantee long-term success.

Insulin-like growth factor 1 enhances the proliferation and osteogenic differentiation of human periodontal ligament stem cells via ERK and JNK MAPK pathways

Insulin-like growth factor 1 (IGF-1) is a potent mitogenic protein which can enhance the osteogenic differentiation of periodontal ligament (PDL) fibroblasts. However, it remains unclear whether IGF-1 can stimulate the osteogenic differentiation and osteogenesis of human periodontal ligament stem cells (PDLSCs). In this study, STRO-1(+) PDLSCs were isolated from human PDL tissues, treated with IGF-1, and their osteogenic capacity was investigated in vitro and in vivo. Dimethyl-thiazol-diphenyl tetrazolium bromide assay and flow cytometry results demonstrated that 10-200 ng/mL IGF-1 can stimulate the proliferation ability of PDLSCs and 100 ng/mL is the optimal concentration. Exogenous IGF-1 can modify the ultrastructure, enhance the alkaline phosphatase activity, the mineralization ability of PDLSCs, and increase the expression of osteogenic markers (runt-related transcription factor 2, osterix, and osteocalcin) at mRNA and protein levels. In vivo transplantation illustrated that IGF-1 treated implants generated more mineralized tissues, and presented stronger expression of RUNX2, OSX, and OCN than control group. Moreover, the expression of phosphor-ERK and phosphor-JNK in these stem cells was upregulated by IGF-1, indicating that MAPK signaling pathway was activated during the osteogenic differentiation of PDLSCs mediated by IGF-1. Together, the results showed that IGF-1 can promote the osteogenic differentiation and osteogenesis of STRO-1(+) PDLSCs via ERK and JNK MAPK pathway, suggesting that IGF-1 is a potent agent for stem cell-based periodontal tissue regeneration.

Prospects for translational regenerative medicine

Translational medicine is an evolutional concept that encompasses the rapid translation of basic research for use in clinical disease diagnosis, prevention and treatment. It follows the idea "from bench to bedside and back", and hence relies on cooperation between laboratory research and clinical care. In the past decade, translational medicine has received unprecedented attention from scientists and clinicians and its fundamental principles have penetrated throughout biomedicine, offering a sign post that guides modern medical research toward a patient-centered focus. Translational regenerative medicine is still in its infancy, and significant basic research investment has not yet achieved satisfactory clinical outcomes for patients. In particular, there are many challenges associated with the use of cell- and tissue-based products for clinical therapies. This review summarizes the transformation and global progress in translational medicine over the past decade. The current obstacles and opportunities in translational regenerative medicine are outlined in the context of stem cell therapy and tissue engineering for the safe and effective regeneration of functional tissue. This review highlights the requirement for multi-disciplinary and inter-disciplinary cooperation to ensure the development of the best possible regenerative therapies within the shortest timeframe possible for the greatest patient benefit.

Designing biomaterials for in situ periodontal tissue regeneration

The regeneration of periodontal tissue poses a significant challenge to biomaterial scientists, tissue engineers and periodontal clinicians. Recent advances in this field have shifted the focus from the attempt to recreate tissue replacements/constructs ex vivo to the development of biofunctionalized biomaterials that incorporate and release regulatory signals in a precise and near-physiological fashion to achieve in situ regeneration. The molecular and physical information coded within the biomaterials define a local biochemical and mechanical niche with complex and dynamic regulation that establishes key interactions with host endogenous cells and, hence, may help to unlock latent regenerative pathways in the body by instructing cell homing and regulating cell proliferation/differentiation. In the future, these innovative principles and biomaterial devices promise to have a profound impact on periodontal reconstructive therapy and are also likely to reconcile the clinical and commercial pressures on other tissue engineering endeavors.

In vitro comparative analysis of cryopreservation of undifferentiated mesenchymal cells derived from human periodontal ligament

Cryopreservation aims to cease all biological functions of living tissues in a reversible and controlled manner, i.e., to permit the recovery of cells by maintaining a high degree of their viability and functional integrity. The objective of this study was to evaluate in vitro the influence of cryopreservation on undifferentiated mesenchymal cells derived from the periodontal ligament of human third molars. Mesenchymal cells were isolated from six healthy teeth and cultured in α-MEM medium supplemented with antibiotics and 15% FBS in a humid atmosphere with 5% CO(2) at 37°C. The cells isolated from each tooth were divided into two groups: group I (fresh, non-cryopreserved cells) was immediately cultured, and group II was submitted to cryopreservation for 30 days. The rates of cell adhesion and proliferation were analyzed in the two groups by counting the cells adhered to the wells at 24, 48 and 72 h after plating. The number of cells per well was obtained by counting viable cells in a hemocytometer using the Trypan blue exclusion method. Differences between groups at each time point were evaluated by the Wilcoxon test. The Friedman test was used to determine differences between time points and, if detected, the Wilcoxon test with Bonferroni correction was applied. The results showed no significant difference in the in vitro growth capacity of undifferentiated mesenchymal cells between the two groups. In conclusion, cryopreservation for 30 days had no influence on periodontal ligament mesenchymal cells.

Stem cell-biomaterial interactions for regenerative medicine

The synergism of stem cell biology and biomaterial technology promises to have a profound impact on stem-cell-based clinical applications for tissue regeneration. Biomaterials development is rapidly advancing to display properties that, in a precise and physiological fashion, could drive stem-cell fate both in vitro and in vivo. Thus, the design of novel materials is trying to recapitulate the molecular events involved in the production, clearance and interaction of molecules within tissue in pathologic conditions and regeneration of tissue/organs. In this review we will report on the challenges behind translating stem cell biology and biomaterial innovations into novel clinical therapeutic applications for tissue and organ replacements (graphical abstract).

Comparison of different tissue-derived stem cell sheets for periodontal regeneration in a canine 1-wall defect model

Cytotherapeutic approaches have been investigated to overcome the limitations of existing procedures for periodontal regeneration. In this study, cell sheet transplantation was performed using three kinds of mesenchymal tissue (periodontal ligament, alveolar periosteum, and bone marrow)-derived cells to compare the differences between cell sources in a canine severe defect model (one-wall intrabony defect). Periodontal ligament cells (PDLCs), iliac bone marrow mesenchymal stromal cells (BMMSCs), and alveolar periosteal cells (APCs) were obtained from each dog; a total of four dogs were used. Three-layered cell sheets of each cell source supported with woven polyglycolic acid were autologously transplanted to the denuded root surface. One-wall intrabony defects were filled with a mixture of β-tricalcium phosphate (β-TCP) and collagen. Eight weeks after the transplantation, periodontal regeneration was significantly observed with both newly formed cementum and well-oriented PDL fibers more in the PDLC group than in the other groups. In addition, nerve filament was observed in the regenerated PDL tissue only in the PDLC group. The amount of alveolar bone regeneration was highest in the PDLC group, although it did not reach statistical significance among the groups. These results indicate that PDLC sheets combined with β-TCP/collagen scaffold serve as a promising tool for periodontal regeneration.

Dental pulp and dentin tissue engineering and regeneration: advancement and challenge

Hard tissue is difficult to repair especially dental structures. Tooth enamel is incapable of self-repairing whereas dentin and cementum can regenerate with limited capacity. Enamel and dentin are commonly under the attack by caries. Extensive forms of caries destroy enamel and dentin and can lead to dental pulp infection. Entire pulp amputation followed by the pulp space disinfection and filling with an artificial rubber-like material is employed to treat the infection -- commonly known as root canal or endodontic therapy. Regeneration of dentin relies on having vital pulps; however, regeneration of pulp tissue has been difficult as the tissue is encased in dentin without collateral blood supply except from the root apical end. With the advent of modern tissue engineering concept and the discovery of dental stem cells, regeneration of pulp and dentin has been tested. This article will review the recent endeavor on pulp and dentin tissue engineering and regeneration. The prospective outcomes of current advancements and challenges in this line of research are discussed.

Biological approaches toward dental pulp regeneration by tissue engineering

Root canal therapy has been the predominant approach in endodontic treatment, wherein the entire pulp is cleaned out and replaced with a gutta-percha filling. However, living pulp is critical for the maintenance of tooth homeostasis and essential for tooth longevity. An ideal form of therapy, therefore, might consist of regenerative approaches in which diseased/necrotic pulp tissues are removed and replaced with regenerated pulp tissues to revitalize the teeth. Dental pulp regeneration presents one of the most challenging issues in regenerative dentistry due to the poor intrinsic ability of pulp tissues for self-healing and regrowth. With the advent of modern tissue engineering and the discovery of dental stem cells, biological therapies have paved the way to utilize stem cells, delivered or internally recruited, to generate dental pulp tissues, where growth factors and a series of dentine extracellular matrix molecules are key mediators that regulate the complex cascade of regeneration events to be faithfully fulfilled.

A review on endogenous regenerative technology in periodontal regenerative medicine

Periodontitis is a globally prevalent inflammatory disease that causes the destruction of the tooth-supporting apparatus and potentially leads to tooth loss. Currently, the methods to reconstitute lost periodontal structures (i.e. alveolar bone, periodontal ligament, and root cementum) have relied on conventional mechanical, anti-infective modalities followed by a range of regenerative procedures such as guided tissue regeneration, the use of bone replacement grafts and exogenous growth factors (GFs), and recently developed tissue engineering technologies. However, all current or emerging paradigms have either been shown to have limited and variable outcomes or have yet to be developed for clinical use. To accelerate clinical translation, there is an ongoing need to develop therapeutics based on endogenous regenerative technology (ERT), which can stimulate latent self-repair mechanisms in patients and harness the host's innate capacity for regeneration. ERT in periodontics applies the patient's own regenerative 'tools', i.e. patient-derived GFs and fibrin scaffolds, sometimes in association with commercialized products (e.g. Emdogain and Bio-Oss), to create a material niche in an injured site where the progenitor/stem cells from neighboring tissues can be recruited for in situ periodontal regeneration. The choice of materials and the design of implantable devices influence therapeutic potential and the number and invasiveness of the associated clinical procedures. The interplay and optimization of each niche component involved in ERT are particularly important to comprehend how to make the desired cell response safe and effective for therapeutics. In this review, the emerging opportunities and challenges of ERT that avoid the ex vivo culture of autologous cells are addressed in the context of new approaches for engineering or regeneration of functional periodontal tissues by exploiting the use of platelet-rich products and its associated formulations as key endogenous resources for future clinical management of periodontal tissue defects.

Toward delivery of multiple growth factors in tissue engineering

Inspired by physiological events that accompany the "wound healing cascade", the concept of developing a tissue either in vitro or in vivo has led to the integration of a wide variety of growth factors (GFs) in tissue engineering strategies in an effort to mimic the natural microenvironments of tissue formation and repair. Localised delivery of exogenous GFs is believed to be therapeutically effective for replication of cellular components involved in tissue development and the healing process, thus making them important factors for tissue regeneration. However, any treatment aiming to mimic the critical aspects of the natural biological process should not be limited to the provision of a single GF, but rather should release multiple therapeutic agents at an optimised ratio, each at a physiological dose, in a specific spatiotemporal pattern. Despite several obstacles, delivery of more than one GF at rates mimicking an in vivo situation has promising potential for the clinical management of severely diseased tissues. This article summarises the concept of and early approaches toward the delivery of dual or multiple GFs, as well as current efforts to develop sophisticated delivery platforms for this ambitious purpose, with an emphasis on the application of biomaterials-based deployment technologies that allow for controlled spatial presentation and release kinetics of key biological cues. Additionally, the use of platelet-rich plasma or gene therapy is addressed as alternative, easy, cost-effective and controllable strategies for the release of high concentrations of multiple endogenous GFs, followed by an update of the current progress and future directions of research utilising release technologies in tissue engineering and regenerative medicine.