Human dental pulp stem cells and its applications in regenerative medicine – A literature review

Regeneration of dentin/pulp complex

The regeneration of dentin pulp complex is based on vascularization. Vascular endothelial growth factor administration promotes vascularization but has a short half-life. This can be increased by binding to heparin.^[19] Treating stem cells under hypoxic conditions induce the cells to secrete vascularizing agents.^[20] Stem cells differentiate into various types of cells; hence, they have to be controlled using growth factors like soluble protein of the dentin matrix. DPSCs were mixed with a carrier and filled in a root canal treated extracted tooth, and the DPSC filled tooth was transplanted into dorsal surface of immune-compromised mice. Regenerated dentin deposited along to existing dentin and connective tissues beneath the de novo dentin contains blood vessels.^[21]

Autologous transplantation of DPSCs is clinically tried to regenerate the dentin-pulp complex. Tubular dentin formation was observed when human pulp stem cells with scaffold (HA/tricalcium phosphate) were implanted in immunocompromised mice.^[1] Reparative dentin formation on amputated pulp was found when stem cells were combined with recombinant human bone morphogenetic protein 2 in experimental studies on animal models.^[22]

Periodontal regeneration

Kawaguchi et al. used BMSCs for their ability to produce alveolar bone, PDL, and in vivo cementum after implantation into the periodontal defects thereby proving to be an alternative source in the treatment of periodontal diseases.^[23,24] Marei et al. in their experiment on goat was able to regenerate periodontal tissues around titanium implant using autologous bone marrow stem cells with the scaffold.^[25] Transplantation of PDL derived cells into animal models was shown to regenerate periodontal tissue.^[26]

Iwata et al. harvested and expanded primary canine PDL cells in vitro and also made into transplantable constructs containing PGA scaffold and PDL cell sheets. The transplantable constructs in combination with porous b-tricalcium phosphate induced regeneration of periodontal structures, including alveolar bone, cementum, and periodontal fibers.^[27]

Liu et al. regenerated periodontal tissue in miniature swine using scaffolds seeded with PDL derived stem cells (PDLSCs).^[28] PDLSCs can differentiate into cells that can colonize on the biocompatible scaffold, suggesting an easy and efficient autologous source of stem cells for regeneration of dental tissues.^[29]

Root regeneration

SCAP has remarkable cell migration activity; which is considered to involve root growth in tooth development. When SCAP-immersed root-formed HA/TCP carriers were subcutaneously grafted into the dorsal surface of immune- compromised mice, dentin/pulp-complex-like structure is formed in the root-formed carrier. In addition, when a root formed carrier containing SCAPs covered with PDLSC- immersed absorbable gelatin sponge is implanted into a socket of the mandibular bone of a swine, the root-form carrier is reconstructed with newly formed dentin/pulp-complex and is surrounded by regenerated PDL on de novo cementum. The regenerated tooth root-like structure works functionally as a masticatory organ likely natural porcine teeth.^[10]

Bone regeneration

TH [4-(4-methoxyphenyl)pyrido[40,30:4,5]thieno[2,3-b] pyridine-2-carboxamide], a helioxanthin derivative, induces osteogenic differentiation of preosteoblastic and mesenchymal cells^[30] in vitro and in vivo,^[31-33] and the optimal concentration for producing the osteogenic effects of TH on MC3T3-E1 and C3H10T1/2 cell lines is 10^–6 M.^[33]

d’Aquino et al. evaluated bone regeneration by DPSCs both clinically and radiographically, using a collagen scaffold. Their results showed that within 3 months of colonization on the scaffold, complete radiographic bone regeneration could be observed.^[34] de Mendonça Costa et al. evaluated the capacity of human DPSCs to reconstruct large cranial defects in non- immunosuppressed rats and found that a more mature bone was formed in the cranial defects, supplied with collagen membrane and HDPSCs.^[35] Chadipiralla et al. studied the osteogenic differentiation of stem cells derived from human PDLs and the pulp of human exfoliated deciduous teeth and suggested that PDLSC is a better osteogenic stem cell source.^[36]

Lymper et al. suggested that positioning of a biocomplex of collagen sponge filled with DPSCs in the extracted site of mandibular third molar resulted in a higher rate of mineralization and cortical levels leading to complete regeneration. The samples also showed a well-organized and vascularized bone with a lamellar architecture surrounding the Haversian canal was observed.^[37] They also prove to be a useful tool for the treatment of degenerative diseases involving the maxilla and mandible.^[37]

Tooth regeneration

Duailibi et al., in their experimental studies, were able to form tooth structures from single-cell suspensions of cultured rat tooth bud cells. They demonstrated bioengineered rat teeth developed in 12 weeks with PGA and PLGA scaffold.^[38] Honda et al. developed tissue-engineered teeth when implanted into omentum of the rat using porcine tooth bud cells and PGA fiber mesh scaffold resembling odontogenesis. Histological analysis showed that the pattern of tissue-engineered odontogenesis was similar to that of natural tooth development with significant regeneration of enamel, dentin, and cementum.^[39]

Central nervous system

The CNS typically has a poor ability to repair and regenerate new neurons because of its limited pool of precursor cells.^[40] Exogenous stem cells (DPSCs) will lead to both regeneration of new neural precursor cells and their enhanced neuronal and glial differentiation. They will also lead to survival and maintenance of existing neural cells through secretion of trophic factors.^[41,42]

SCI

SCI involves an initial primary tissue disruption (e.g., mechanical damage to nerve cells and blood vessels) and then a secondary injury caused by neuroinflammatory responses (e.g., excitotoxicity, blood-brain barrier disruption, oxidative stress, and apoptosis).^[43] DPSCs differentiating into neuron-like and oligodendrocyte-like cells that may promote axonal regeneration and tissue repair after SCI.^[44-46] DPSCs also reduce secondary inflammatory injury, which facilitates axonal regeneration and reduces progressive hemorrhagic necrosis associated with IL-1β, ras homolog gene family member A, and sulfonylurea receptor 1 expression.^[47] DPSCs when transplanted together with artificial scaffolds like chitosan promotes motor functional recovery and inhibits cell apoptosis after SCI by secreting BDNF, GDNF, and NT-3 and reducing the expression of active-caspase 3.^[48,49]

Stroke

Stroke is an ischemic cerebrovascular condition that leads to brain damage, long-term disability, and even death. DPSCs promote functional recovery after ischemic stroke by immunomodulation. Some in vivo studies have shown that transplantation of DPSCs into the ischemic areas of middle cerebral artery occlusion in Sprague-Dawley rats promoted locomotor functional recovery and decreased infarct areas by their differentiation into dopaminergic (DA) neurons and secretion of neurotrophic factors.^[50,51] DPSC transplantation into ischemic areas of focal cerebral ischemia in rats led to the expression of proangiogenic factors that supported dense capillary formation and renormalization of blood flow.^[52] Intracerebral transplantation of DPSCs into regions of focal cerebral ischemia in rodent models promoted forelimb sensory and motor functional recovery at 4 weeks post-treatment.^[53] DPSCs also provided cytoprotection for astrocytes by reducing reactive gliosis and preventing free radical and IL-1β secretion within in vitro ischemic models.^[54]

Parkinson’s

PDs is a progressive neurodegenerative condition associated with loss of nigrostriatal DA neurons leading to muscle rigidity, bradykinesia, postural instability, and resting tremor.^[55] Intrathecal transplantation of DPSCs into the 1-methyl-4- phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced old-aged mouse model of PD, promoted recovery of behavioral deficits, restored DA functions, and attenuated MPTP-induced damage by reducing the secretion of proinflammatory factors such as IL-1α, IL-1β, IL6, IL8, and tumor necrosis factor (TNF)-α and by upregulating the expression levels of anti- inflammatory factors such as IL2, IL4, and TNF-β.^[56] DPSCs also showed neuroimmunomodulatory activity in an in vitro model of PD by reducing MPTP-induced deficits associated with reactive oxygen species, DNA damage, and nitric oxide release.^[57]

Alzheimer’s

Alzheimer’s disease (AD) is a progressive neurodegenerative condition caused by the loss of neurons, intracellular neurofibrillary tangles, and deposition of insoluble β-amyloid peptides in the brain.^[58] Clinical symptoms of AD include memory loss, cognitive deficits, and linguistic disorders.^[59] DPSCs promoted neuronal repair and regeneration by restoring cytoskeletal structure, protecting microtubule stability, and reducing tau phosphorylation in the okadaic acid-induced cellular model of AD.^[60] DPSCs can also reduce amyloid-beta (Aβ) peptide-induced cytotoxicity and apoptosis in the AD cellular model by secreting higher levels of VEGF, fractalkine, RANTES, fms-related tyrosine kinase 3, and monocyte chemotactic protein 1.^[61,62]

Retinal injury

The retina is a part of the CNS and is composed of photoreceptors, bipolar cells, and retinal ganglion cells (RGCs).^[63] Head injuries can cause traumatic optic neuropathy while ocular chronic degenerative diseases such as glaucoma lead to the slow loss of RGCs.^[64] DPSC transplantation into the vitreous of optic nerve injury rat model could promote axonal regeneration and RGC survival by a neurotrophin-mediated mechanism.^[65] Intravitreal transplantation of DPSCs in an animal model of glaucoma maintained visual function up to 35 days after treatment by preventing RGC death.^[66]

Peripheral nerve injury

Peripheral nerve injury caused by traumatic accidents and iatrogenic damage often accompanies physical disability and neuropathic pain. Autologous nerve grafting is the preferred treatment choice for a long gap of peripheral nerve deficits. Studies suggest that DPSC-embedded biomaterial nerve conduits such as polylactic glycolic acid tubes have the ability to promote regeneration of injured facial nerve and to improve functional recovery comparable to that of autografts.^[67] Collagen conduits loaded with Schwann-like cells induced from DPSCs in vitro have facilitated repair and regeneration of 15 mm sciatic nerve defect.^[68]

Autoimmune diseases

The major mechanisms may involve the secretion of soluble factors, such as prostaglandin E2, IDO, TGF-β, and human leukocyte antigen G5, and interactions between MSCs and immune cells such as T cells, B cells, macrophages, and dendritic cells.^[69] SHED has significant effects on inhibiting T helper 17 cells compared to BMMSCs. SHED transplantation was capable of effectively reversing systemic lupus erythematosus (SLE)-associated disorders in SLE-like mice.^[14] DPSCs could inhibit acute allogeneic immune responses by the release of TGF-β as a result of allogeneic stimulation of T lymphocytes.^[70]

Systemic transplantation of SHED and DPSC in autoimmune disease mice model including SLE and inflammatory bowel disease ameliorated the tissue damages induced by hypersensitive immune response.^[14,17,18]

Bone diseases

Systemic transplantation of mesenchymal stem cells could ameliorate bone loss and autoimmune disorders in a MRL/lpr mouse SLE mode by suppression of Interleukin-17 and maintaining a regular positive bone metabolism.^[11] Systemic transplantation of SHED through the tail vein ameliorates ovariectomy (OVX)-induced osteopenia by reducing T-helper 1 and T-helper 17 cell numbers in the recipient OVX mice.^[71]

Liver diseases

Terai et al.^[72] administered bone marrow cells derived from GFP-labeled mice to carbon tetrachloride (CCl4)-induced liver injury model mice and found that these bone marrow cells engrafted in the injured liver, resulting in the absorption of fibrosis and the improvement of prognosis. The tooth germ progenitor cells prevented the progression of liver fibrosis in the liver of CCl4-treated rats and contributed to the restoration of liver function, as assessed by the measurement of hepatic serum markers aspartate aminotransferase and alanine aminotransferase.^[12] Engraftment of DPSCs and SHED morphologically and functionally ameliorate acute and chronic injury of livers in CCl4-treated rats.^[73]

Muscular

DPSCs can differentiate into dystrophin-producing multinucleated muscle cells and can be utilized in disorders such as muscular dystrophy, wherein, the body is unable to produce dystrophin. Utilization of myogenic progenitor cells derived from dental pulp produced more dystrophin as compared to the heterogeneously present stem cells.^[74] Thereby proving to be a potential alternative for stem cell therapy in muscular dystrophy patients. Kerkis et al. used human DPSCs for the treatment of muscular dystrophy in golden retriever dogs, transplanted by arterial or muscular injections.^[75]

Diabetes

Diabetes is a chronic degenerative disease. One of the treatments for diabetes includes transplantation of pancreatic islet cells. Chen et al. demonstrated that insulin-producing cells can be derived from monoclonal and polyclonal DPSCs.^[76] Govindasamy et al. demonstrated that DPSCs have the capacity to differentiate into islet-like aggregates.^[77]

Infertility

The potential of DPSCs can also be used in the treatment of infertility. Leake and Templeton isolated HDPSCs and injected them into the testes of live male mice. The mice were killed at various intervals after the injection, and their testes were examined to see whether the stem cells survived. It was found that stem cells settled in the testes and also differentiated into cells that were producing viable sperm.^[78]