Dental pulp stem cells: Novel cell-based and cell-free therapy for peripheral nerve repair

Table 1 Studies retrieved from PubMed database evaluating the effect of DPSCs on peripheral nerve repair or regeneration both in-vivo and in-vitro

Author (publication year)		Source of stem cells	Target tissue	Study model	Objective	Outcome
Carnevale et al[14], 2018	In-vivo	Human STRO-1+ /c-Kit+ /CD34+DPSCs expressing P75NTR, nestin and SOX-10	Sciatic nerve defect	Animal rat model	To demonstrate the ability of human STRO-1+ /c-Kit+ /CD34+ DPSCs expressing P75NTR, nestin and SOX-10 to promote axonal regeneration.	The cells promoted regeneration and functional recovery of sciatic nerve defects after injury.
	In-vitro	Human STRO-1+ /c-Kit+ /CD34+DPSCs expressing P75NTR, nestin and SOX-10	To differentiate into SC-like cells	In-vitro culturing of DPSCs and their differentiation to SCs	To demonstrate the ability of Human STRO-1+ /c-Kit+ /CD34+ DPSCs expressing P75NTR, nestin and SOX-10 to differentiate into SC-like cells.	Under appropriate conditions, the cells differentiated into SC-like cells
Kolar et al[75], 2017	In-vivo	Adult rat SCs; Human SCAP, DPSCs and PDLSC	10 mm nerve gap defect in a rat sciatic nerve	Sciatic nerve injury model	To demonstrate the ability of human SCAP, DPSCs and PDLSC to promote axonal regeneration using nerve guidance conduit of 14 mm length.	All the stem cell types significantly enhanced axon regeneration after two weeks. SCAP are the optimal dental stem cell type for peripheral nerve repair.
	In-vitro	CM from unstimulated or stimulated human SCAP, DPSCs and PDLSC	Differentiated human neuroblastoma SH-SY5Y cell line	In-vitro neurite outgrowth assay	To examine the biological activity of the conditioned medium for unstimulated and stimulated human SCAP, DPSC and PDLSC.	Quantification of the neurite outgrowth showed that unstimulated and stimulated human SCAP, DPSCs and PDLSC increased both the percentage of cells producing neurites and the total neurite outgrowth length.
Omi et al[76], 2017	In-vivo	DPSCs isolated from the incisor teeth of 6-wk-old male rats	Sciatic nerve; Sensory nerve fibers; Sural nerves	Streptozotocin-induced diabetic rats.	Investigated whether the transplantation of DPSCs ameliorated long-term diabetic polyneuropathy in streptozotocin-induced diabetic rats.	Significant reductions in the sciatic motor/sensory nerve conduction velocity, increases in the current perception threshold, and decreases in capillary density in skeletal muscles and intra-epidermal nerve fiber density. Sural nerve morphometrical analysis revealed that the transplantation of DPSCs significantly increased the myelin thickness.
	In-vitro	DPSCs isolated from the incisor teeth of 6-wk-old male rats	Dorsal root ganglion neuron were cultured for use in neurite outgrowth with DPSC-CM; Immortalized adult Fischer rat SCs were cultured with DPSC-CM	In-vitro neurite outgrowth assay; Cell viability assay	Evaluation of neurite outgrowth. Analysis of myelin-related protein formation in immortalized adult Fischer rat SCs.	DPSCs-CM promoted the neurite outgrowth of dorsal root ganglion neurons. Increased the viability and myelin-related protein expression of SCs.
Sanen et al[77], 2017	In-vivo	SCs derived from differentiated human DPSCs	15-mm rat sciatic nerve defects	Sciatic nerve injury model	Evaluated the performance of SCs derived from differentiated human DPSCs in a rat model of PNI.	Immunohistochemistry and ultrastructural analysis revealed in-growing neurites, myelinated nerve fibres and blood vessels along the construct.
	In-vitro	SCs derived from differentiated human DPSCs	Human microvascular endothelial cell line (HMEC-1)	Alamar Blue cell proliferation assay; Transwell migration assay; Tube formation assay	Investigated the neuroregenerative and the proangiogenic capacities of SCs derived from differentiated human DPSCs.	The endothelial cell line HMEC-1 had proliferated significantly more in the presence of conditioned medium from human DPSCs and differentiated human DPSCs compared with those in control medium.
Hei et al[78], 2016	In-vivo	Schwann-like cells were derived from human DPSCs; Human DPSCs	3 mm - wide crush injury was inflicted at a distance of approximately 10 mm from the mental foramen	Male Sprague-Dawley rats crush-injury site	To investigate the effect of Schwann-like cells combined with pulsed electromagnetic field on peripheral nerve regeneration.	Schwann-like cells, human DPSCs with or without pulsed electromagnetic field, and pulsed electromagnetic field only improved peripheral nerve regeneration.
	In-vitro	Schwann-like cells were derived from human DPSCs; Human DPSCs	Schwann Cells	Cell culture dishes	To demonstrate the ability of hDPSCs to differentiate into Schwann - like cells and demonstrate glial character with expression of CD104, S100, GFAP, laminin and p75NTR.	Successful morphological differentiation of hDPSCs toward Schwann - like cells.
Yamamoto et al[79], 2016	In-vivo	Human mobilized DPSCs	5-mm gap of the left sciatic nerve	Rat sciatic nerve defect model	To investigate the effects of human mobilized DPSC transplantation on peripheral nerve regeneration using 9-mm collagen conduit.	Human mobilized DPSCs promote axon regeneration through trophic functions, acting on SCs and promote angiogenesis.
	In-vitro	CM of human mobilized DPSCs	Rat SCs (RT4-D6P2T)	Migration, proliferation, and anti-apoptotic assays	To investigate the trophic effects of mobilized human DPSCs on proliferation, migration and anti-apoptosis in SCs	The human mobilized DPSCs-CM significantly enhanced proliferation and migratory activity and decreased apoptosis of RT4-D6P2T cells.
Askari et al[80], 2014	In-vivo	Human DPSCs transfected with a tetracycline-inducible system expressing oligodendrocyte lineage transcription factor 2 gene	Sciatic nerve demyelination experiment	Mouse model of local sciatic demyelination damage by lysolecithin	To investigate if the tetracycline-regulated expression of oligodendrocyte lineage transcription factor 2 gene transfected in human DPSCs can lead to mouse sciatic nerve regeneration upon transplantation.	Human DPSCs-derived oligodendrocyte progenitor cells have relevant therapeutic potential in the animal model of sciatic nerve injury.
	In-vitro	Human DPSCs	Oligodendrocyte	In-vitro plasmid construct and transfection	DPSCs were transfected with oligodendrocyte transcription factor 2 which play important role in differentiation of DPSCs to oligodendrocyte progenitor cells.	Exogenous expression of the oligodendrocyte lineage transcription factor 2 gene by a tetracycline-regulated system could be used as an efficient way to induce the differentiation of DPSCs into functional oligodendrocytes.
Dai et al[81], 2013	In-vivo	SCs, AMSCs, DPSCs, and the combination of SCs with AMSCs or DPSCs	15-mm-long critical gap defect of rat sciatic nerve	Sciatic nerve injury model	To test their efficacy in repairing PNI 17-mm nerve conduit.	Co-culture of SCs with AMSCs or DPSCs in a conduit promoted peripheral nerve regeneration over a critical gap defect.
	In-vitro	SCs, AMSCs, DPSCs, and the combination of SCs with AMSCs or DPSCs	Neuronal cells	RT-PCR analysis of the coculture in-vitro	To verify if the combination of cells led to synergistic neurotrophic effects NGF, BDNF, and GDNF.	Results confirmed the synergistic NGF production from the co-culture of SCs and ASCs.

SCAP: Stem cells of apical papillae; DPSC: Dental pulp stem cells; PDLSC: Periodontal ligament stem cells; CM: Conditioned medium; AMSCs: Adipose mesenchymal stem cells; SCs: Stem cells; NGF: Nerve growth factor; BDNF: Brain derived neurotrophic factor; GDNF: Glial derived neurotrophic factor.

Table 2 Studies retrieved from PubMed database evaluating the effect of DPSCs on nerve repair or regeneration in-vivo

Author	Publication year	Source of stem cells	Target nerves	Study model	Objective	Outcome
Ullah et al[82], 2017	2017	Human DPSCs; Differentiated neuronal cells from DPSCs	5-mm gap sciatic nerve transection	Animal rat model	To evaluate the in-vivo peripheral nerve regeneration potential of human DPSCs and differentiated neuronal cells from DPSCs.	In-vivo transplantation of the undifferentiated hDPSCs could exhibit sufficient and excellent peripheral nerve regeneration potential.
Spyridopoulos et al[83], 2015	2015	DPSCs isolated from second lateral incisor pigs	Transected fifth and sixth intercostal nerves	Animal pig model	Examined the potential of DPSCs for peripheral nerve regeneration, using biodegradable collagen conduits.	The nerves where DPSCs were injected exhibited morphological and functional recovery.

DPSCs: Dental pulp stem cells; hDPSCs: Human dental pulp stem cells.

Table 3 Studies retrieved from PubMed database evaluating the effect of DPSCs on nerve repair or regeneration in-vitro

Author	Publication year	Source of stem cells	Target tissues	Objective	Outcome
Geng et al[84], 2017	2017	Human DPSCs	Differentiation of hDPSCs.	To demonstrate the differentiating ability of resveratrol on DPSCs.	Resveratrol induced DPSCs differentiation into neuroprogenitor cells. DPSCs might be an important cell population for neurological disease treatment.
Hafner et al[85], 2017	2017	Human DPSCs	Spider dragline silk fibers	To evaluating adhesion and alignment of dental pulp stem cells to a spider silk substrate for tissue engineering applications.	Natural drawn spider silk acted as an effective substrate for cellular adhesion and alignment of DPSCs and could be used in neural differentiation applications.
Chang et al[86], 2014	2014	Human DPSCs	Medium preparation for the induction of spinal motor neuronal differentiation; Medium preparation for the induction of dopaminergic neuronal differentiation	To evaluate the efficacy of dopaminergic and motor neuronal inductive media on transdifferentiation of human DPSCs (hDPSCs) into neuron-like cells.	Human DPSCs-derived dopaminergic and spinal motor neuron cells after induction expressed a higher density of neuron cell markers than those before induction.
Mead et al[60], 2014	2014	Human DPSC, human BMSCs human AMSCs	Axotomised adult rat retinal ganglion cells	To evaluate the therapeutic potential for neurodegenerative conditions of retinal ganglion cells.	Human DPSCs promoted significant multi-factorial paracrine-mediated retinal ganglion cell survival and neurite outgrowth compared with Human BMSCs/Human AMSCs.
Martens et al[23], 2014	2014	Human DPSCs	Dorsal root ganglia	Evaluated the differentiation potential of human DPSCs toward SCs, together with their functional capacity with regard to myelination and support of neurite outgrowth.	Human DPSCs are able to undergo SCs differentiation and support neural outgrowth.

DPSCs: Dental pulp stem cells; BMSCs: Bone marrow stem cells; AMSCs: Adipose mesenchymal stem cells.