Regeneration of dental-pulp-like tissue by chemotaxis-induced cell homing

Prolyl hydroxylase inhibitors increase the production of vascular endothelial growth factor in dental pulp-derived cells

INTRODUCTION

Prolyl hydroxylase (PHD) inhibitors can induce a proangiogenic response that stimulates regeneration in soft and hard tissues. However, the effect of PHD inhibitors on the dental pulp is unclear. The purpose of this study was to evaluate the effects of PHD inhibitors on the proangiogenic capacity of human dental pulp-derived cells.

METHODS

To test the response of dental pulp-derived cells to PHD inhibitors, the cells were exposed to dimethyloxalylglycine, desferrioxamine, L-mimosine, and cobalt chloride. To assess the response of dental pulp cells to a capping material supplemented with PHD inhibitors, the cells were treated with supernatants from calcium hydroxide. Viability, proliferation, and protein synthesis were assessed by formazan formation, (3)[H]thymidine, and (3)[H]leucine incorporation assays. The effect on the proangiogenic capacity was measured by immunoassays for vascular endothelial growth factor (VEGF).

RESULTS

We found that all 4 PHD inhibitors can reduce viability, proliferation, and protein synthesis at high concentrations. At nontoxic concentrations and in the presence of supernatants from calcium hydroxide, PHD inhibitors stimulated the production of VEGF in dental pulp-derived cells. When calcium hydroxide was supplemented with the PHD inhibitors, the supernatants from these preparations did not significantly elevate VEGF levels.

CONCLUSIONS

These results show that PHD inhibitors can stimulate VEGF production of dental pulp-derived cells, suggesting a corresponding increase in their proangiogenic capacity. Further studies will be required to understand the impact that this might have on pulp regeneration.

Constructs and scaffolds employed to regenerate dental tissue

Dental tissue injury and regeneration affects the daily lives of almost everyone. Tissue engineering is emerging as a promising therapy to regenerate missing teeth and dental tissues. The aim of regenerative dental therapies is to restore patients to full oral health. This means restoring normal function to missing or damaged tissue. Regeneration approaches use a combination of scaffolds, stem cells, growth factors, tissue engineering, organ tissue culture, transplantation, and tissue grafting. There are 8 key elements to create and use tissue constructs for tissue regeneration. These will be described in detail in this article.

Partial characterization of the Sox2+ cell population in an adult murine model of digit amputation

Tissue regeneration in response to injury in adult mammals is generally limited to select tissues. Nonmammalian species such as newts and axolotls undergo regeneration of complex tissues such as limbs and digits via recruitment and accumulation of local and circulating multipotent progenitors preprogrammed to recapitulate the missing tissue. Directed recruitment and activation of progenitor cells at a site of injury in adult mammals may alter the default wound-healing response from scar tissue toward regeneration. Bioactive molecules derived from proteolytic degradation of extracellular matrix (ECM) proteins have been shown to recruit a variety of progenitor cells in vitro and in vivo to the site of injury. The present study further characterized the population of cells accumulating at the site of injury after treatment with ECM degradation products in a well-established model of murine digit amputation. After a mid-second phalanx digit amputation in 6-8-week-old adult mice, treatment with ECM degradation products resulted in the accumulation of a heterogeneous population of cells, a subset of which expressed the transcription factor Sox2, a marker of pluripotent and adult progenitor cells. Sox2+ cells were localized lateral to the amputated P2 bone and coexpressed progenitor cell markers CD90 and Sca1. Transgenic Sox2 eGFP/+ and bone marrow chimeric mice showed that the bone marrow and blood circulation did not contribute to the Sox2+ cell population. The present study showed that, in addition to circulating progenitor cells, resident tissue-derived cells also populate at the site of injury after treatment with ECM degradation products. Although future work is necessary to determine the contribution of Sox2+ cells to functional tissue at the site of injury, recruitment and/or activation of local tissue-derived cells may be a viable approach to tissue engineering of more complex tissues in adult mammals.

Biomaterial selection for tooth regeneration

Biomaterials are native or synthetic polymers that act as carriers for drug delivery or scaffolds for tissue regeneration. When implanted in vivo, biomaterials should be nontoxic and exert intended functions. For tooth regeneration, biomaterials have primarily served as a scaffold for (1) transplanted stem cells and/or (2) recruitment of endogenous stem cells. This article critically synthesizes our knowledge of biomaterial use in tooth regeneration, including the selection of native and/or synthetic polymers, three-dimensional scaffold fabrication, stem cell transplantation, and stem cell homing. A tooth is a complex biological organ. Tooth loss represents the most common organ failure. Tooth regeneration encompasses not only regrowth of an entire tooth as an organ, but also biological restoration of individual components of the tooth including enamel, dentin, cementum, or dental pulp. Regeneration of tooth root represents perhaps more near-term opportunities than the regeneration of the whole tooth. In the adult, a tooth owes its biological vitality, arguably more, to the root than the crown. Biomaterials are indispensible for the regeneration of tooth root, tooth crown, dental pulp, or an entire tooth.

Regeneration of dental pulp following pulpectomy by fractionated stem/progenitor cells from bone marrow and adipose tissue

Pulp stem/progenitor cells can induce complete pulp regeneration. However, due to the limited availability of pulp tissue with age, there is a need to examine other sources for fractions of side population (SP) cells. In the present investigation bone marrow and adipose tissues of the same individual were evaluated as alternate sources. Pulp CD31(-) SP cells have higher migration activity and higher expression of angiogenic/neurotrophic factors than bone marrow and adipose CD31(-) SP cells. Adipose tissue CD31(-) SP cell transplantation yielded the same amount of regenerated tissue as pulp derived cells. However, bone marrow CD31(-) SP cell transplantation yielded significantly less regenerated tissue in pulpectomized root canals in dogs. The rate of matrix formation was much higher in adipose CD31(-) SP cell transplantation compared to pulp CD31(-) SP cell transplantation on day 28. Microarray analysis demonstrated similar qualitative and quantitative patterns of mRNA expression characteristic of pulp in the regenerated tissues from all three cell sources. Expression of many angiogenic/neurotrophic factors in the transplanted cells demonstrated trophic effects. Our results demonstrate that bone marrow and adipose CD31(-) SP cells might be suitable alternative cell sources for pulp 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.

Local regeneration of dentin-pulp complex using controlled release of fgf-2 and naturally derived sponge-like scaffolds

Restorative and endodontic procedures have been recently developed in an attempt to preserve the vitality of dental pulp after exposure to external stimuli, such as caries infection or traumatic injury. When damage to dental pulp is reversible, pulp wound healing can proceed, whereas irreversible damage induces pathological changes in dental pulp, eventually requiring its removal. Nonvital teeth lose their defensive abilities and become severely damaged, resulting in extraction. Development of regeneration therapy for the dentin-pulp complex is important to overcome limitations with presently available therapies. Three strategies to regenerate the dentin-pulp complex have been proposed; regeneration of the entire tooth, local regeneration of the dentin-pulp complex from amputated dental pulp, and regeneration of dental pulp from apical dental pulp or periapical tissues. In this paper, we focus on the local regeneration of the dentin-pulp complex by application of exogenous growth factors and scaffolds to amputated dental pulp.

Combined systemic and local delivery of stem cell inducing/recruiting factors for in situ tissue regeneration

Whereas the conventional tissue engineering strategy involves the use of scaffolds combined with appropriate cell types to restore normal functions, the concept of in situ tissue regeneration uses host responses to a target-specific scaffold to mobilize host cells to a site of injury without the need for cell seeding. For this purpose, local delivery of bioactive molecules from scaffolds has been generally used. However, this approach has limited stem cell recruitment into the implants. Thus, we developed a combination of systemic delivery of substance P (SP) and local release of stromal-derived factor-1α (SDF-1α) from an implant. In this study, we examined whether this combined system would significantly enhance recruitment of host stem cells into the implants. Flow cytometry and immunohistochemistry for CD29/CD45, CD146/α-smooth muscle actin, and c-kit demonstrated that this system significantly increased the number of stem cell-like cells within the implants when compared with other systems. In vitro culture of the cells that had infiltrated into the scaffolds from the combined system confirmed that host stem cells were recruited into these implants and indicated that they were capable of differentiation into multiple lineages. These results indicate that this combined system may lead to more efficient tissue regeneration.

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.

Recommendations for using regenerative endodontic procedures in permanent immature traumatized teeth

The regeneration of immature permanent teeth following trauma could be beneficial to reduce the risk of fracture and loss of millions of teeth each year. Regenerative endodontic procedures include revascularization, partial pulpotomy, and apexogenesis. Several case reports give these procedures a good prognosis as an alternative to apexification. Care is needed to deliver regenerative endodontic procedures that maintain or restore the vitality of teeth, but which also disinfect and remove necrotic tissues. Regeneration can be accomplished through the activity of the cells from the pulp, periodontium, vascular, and immune system. Most therapies use the host's own pulp or vascular cells for regeneration, but other types of dental stem cell therapies are under development. There are no standardized treatment protocols for endodontic regeneration. The purpose of this article is to review the recent literature and suggest guidelines for using regenerative endodontic procedures for the treatment of permanent immature traumatized teeth. Recommendations for the selection of regenerative and conventional procedures based on the type of tooth injury, fracture type, presence of necrosis or infection, periodontal status, presence of periapical lesions, stage of tooth development, vitality status, patient age, and patient health status will be reviewed. Because of the lack of long-term evidence to support the use of regenerative endodontic procedures in traumatized teeth with open apices, revascularization regeneration procedures should only be attempted if the tooth is not suitable for root canal obturation, and after apexogenesis, apexification, or partial pulpotomy treatments have already been attempted and have a poor prognosis.

Chondrogenesis by chemotactic homing of synovium, bone marrow, and adipose stem cells in vitro

Cell transplantation has been well explored for cartilage regeneration. We recently showed that the entire articular surface of a synovial joint can regenerate by endogenous cell homing and without cell transplantation. However, the sources of endogenous cells that regenerate articular cartilage remain elusive. Here, we studied whether cytokines not only chemotactically recruit adipose stem cells (ASCs), mesenchymal stem cells (MSCs), and synovium stem cells (SSCs) but also induce chondrogenesis of the recruited cells. Recombinant human transforming growth factor-β3 (TGF-β3; 100 ng) and/or recombinant human stromal derived factor-1β (SDF-1β; 100 ng) was control released into an acellular collagen sponge cube with underlying ASCs, MSCs, or SSCs in monolayer culture. Although all cell types randomly migrated into the acellular collagen sponge cube, TGF-β3 and/or SDF-1β recruited significantly more cells than the cytokine-free control group. In 6 wk, TGF-β3 alone recruited substantial numbers of ASCs (558±65) and MSCs (302±52), whereas codelivery of TGF-β3 and SDF-1β was particularly chemotactic to SSCs (400±120). Proliferation of the recruited cells accounted for some, but far from all, of the observed cellularity. TGF-β3 and SDF-1β codelivery induced significantly higher aggrecan gene expression than the cytokine-free group for ASCs, MSCs, and SSCs. Type II collagen gene expression was also significantly higher for ASCs and SSCs by SDF-1 and TGF-β3 codelivery. Remarkably, the expression of aggrecan and type II collagen was detected among all cell types. Thus, homing of multiple stem/progenitor cell populations may potentially serve as an alternative or adjunctive approach to cell transplantation for cartilage regeneration.

Recruitment of progenitor cells by an extracellular matrix cryptic peptide in a mouse model of digit amputation

Biologic scaffolds composed of extracellular matrix (ECM) have been used successfully in preclinical models and humans for constructive remodeling of functional, site-appropriate tissue after injury. The mechanisms underlying ECM-mediated constructive remodeling are not completely understood, but scaffold degradation and site-directed recruitment of both differentiated and progenitor cells are thought to play critical roles. Previous studies have shown that degradation products of ECM scaffolds can recruit a population of progenitor cells both in vitro and in vivo. The present study identified a single cryptic peptide derived from the α subunit of the collagen III molecule that is chemotactic for a well-characterized perivascular stem cell in vitro and causes the site-directed accumulation of progenitor cells in vivo. The oligopeptide was additionally chemotactic for human cortical neural stem cells, rat adipocyte stem cells, C2C12 myoblast cells, and rat Schwann cells in vitro. In an adult murine model of digit amputation, treatment with this peptide after mid-second phalanx amputation resulted in a greater number of Sox2+ and Sca1+,Lin- cells at the site of injury compared to controls. Since progenitor cell activation and recruitment are key prerequisites for epimorphic regeneration in adult mammalian tissues, endogenous site-directed recruitment of such cells has the potential to alter the default wound healing response from scar tissue toward regeneration.

Dentin-Pulp Complex Regeneration

Dentistry is entering an exciting era in which many of the advances in biotechnology offer opportunities for exploitation in novel and more effective therapies. Pulp healing is complex and dependent on the extent of injury, among many other factors. Many of the molecular and cellular processes involved in these healing events recapitulate developmental processes. The regulation of odontoblast activity is clearly central to pulp healing, and an understanding of the mechanisms involved in these processes is necessary to enable laboratory studies to be translated to clinic application. Transcriptome analysis has identified changes in many odontoblast genes during the life-cycle of this cell and its responses to injurious challenge. The p38 MAPKinase pathway appears to be central to the transcriptional control of odontoblasts and may provide a key target for therapeutic intervention. The many recent advances in knowledge of pulpal stem cells and molecular signaling molecules within the tooth, now provide exciting opportunities for clinical translation to novel therapies. Such translation will require the partnership of researchers and skilled clinicians who can effectively apply advances in knowledge to appropriate clinical cases and develop novel therapies which can be realistically introduced into the clinic.

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.