Characterization of isolated human SHEDs and DPSCs for use in transplantation studies. Flow cytometry analysis showed that the SHEDs and DPSCs expressed a set of mesenchymal stem cell (MSC) markers (i.e., CD90, CD73, and CD105), but not endothelial/hematopoietic markers (i.e., CD34, CD45, CD11b/c, and HLA-DR) (Table 1). Like human BMSCs, both the SHEDs and DPSCs exhibited adipogenic, chondrogenic, and osteogenic differentiation as described previously (refs. 16, 17, and data not shown). The majority of SHEDs and DPSCs coexpressed several neural lineage markers: nestin (neural stem cells), doublecortin (DCX; neuronal progenitor cells), III-tubulin (early neuronal cells), NeuN (mature neurons), GFAP (neural stem cells and astrocytes), S-100 (Schwann cells), and A2B5 and CNPase (oligodendrocyte progenitor cells), but not adenomatous polyposis coli (APC) or myelin basic protein (MBP) (mature oligodendrocytes) (Figure 1A and Table 1). This expression profile was confirmed by immunohistochemical analyses (Figure 1B).

Characterization of the SHEDs and DPSCs used for transplantation. (A) Flow cytometry analysis of the neural cell lineage markers expressed in SHEDs. Note that most of the SHEDs and DPSCs coexpressed neural stem and multiple progenitor markers, but not mature oligodendrocytes (APC and MBP). (B) Confocal images showing SHEDs coexpressed nestin, GFAP, and DCX. SHEDs also expressed markers for oligodendrocyte progenitor cells (A2B5 and CNPase), but not for mature oligodendrocytes (APC and MBP). Scale bar: 10 m. (C) Real-time RT-PCR analysis of the expression of neurotrophic factors. Results are expressed as fold increase compared with the level expressed in skin fibroblasts. Data represent the average measurements for each cell type from 3 independent donors. This set of experiments was repeated twice and yielded similar results. Data represent the mean SEM. *P < 0.01 compared with BMSCs and fibroblasts (Fbs).

Flow cytometry of stem cells from humans

Next, we examined the expression of representative neurotrophic factors by real-time PCR. Both the SHEDs and DPSCs expressed glial cellderived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and ciliary neurotrophic factor (CNTF) at more than 3 to 5 times the levels expressed by skin-derived fibroblasts or BMSCs (Figure 1C).

We further characterized the transcriptomes of SHEDs and BMSCs by cDNA microarray analysis. This gene expression analysis revealed a 2.0-fold difference in the expression of 3,318 of 41,078 genes between SHEDs and BMSCs. Of these, 1,718 genes were expressed at higher levels in the SHEDs and 1,593 genes were expressed at lower levels (data not shown). The top 30 genes showing higher expression in the SHEDs were in the following ontology categories: extracellular and cell surface region, cell proliferation, and tissue/embryonic development (Table 2).

Functional gene classification in SHEDs versus BMSCs

SHEDs and DPSCs promoted locomotor recovery after SCI. To compare the neuroregenerative activities of human SHEDs and DPSCs with those of human BMSCs and human skin fibroblasts, we transplanted the cells into the completely transected SCs, as described in Methods, and evaluated locomotion recovery using the Basso, Beattie, Bresnahan locomotor rating scale (BBB scale) (24). Remarkably, the animals that received SHEDs or DPSCs exhibited a significantly higher BBB score during the entire observation period, compared with BMSC-transplanted, fibroblast-transplanted, or PBS-injected control rats (Figure 2A). Importantly, their superior recoveries were evident soon after the operation, during the acute phase of SCI. After the recovery period (5 weeks after the operation), the rats that had received SHEDs were able to move 3 joints of hind limb coordinately and walk without weight support (P < 0.01; Supplemental Videos 1 and 2), while the BMSC- or fibroblast-transplanted rats exhibited only subtle movements of 12 joints. These results demonstrate that the transplantation of SHEDs or DPSCs during the acute phase of SCI significantly improved the recovery of hind limb locomotor function. Since the level of recovery was similar in the SHED- and DPSC-transplanted rats, we focused on the phenotypical examination of SHED-transplanted rats to elucidate how tooth-derived stem cells promoted the regeneration of the completely transected rat SC.

Engrafted SHEDs promote functional recovery of the completely transected SC. (A) Time course of functional recovery of hind limbs after complete transection of the SC. A total of 1 106 SHEDs, DPSCs, BMSCs, or fibroblasts were transplanted into the SCI immediately after transection. Data represent the mean SEM. **P < 0.001, *P < 0.01 compared with SCI models injected with PBS. (BD) Representative images (B and C) and quantification (D) of NF-Mpositive nerve fibers in sagittal sections of a completely transected SC, at 8 weeks after SCI. Dashed lines outline the SC. Insets are magnified images of boxed areas in B and C. (D) Nerve fiber quantification, representing the average of 3 experiments performed under the same conditions. The x axis indicates specific locations along the rostrocaudal axis of the SC (3 mm rostral and caudal to the epicenter), and y axis indicates the percentage of NF-Mpositive fibers compared with that of the sham-operated SCs at the ninth thoracic spinal vertebrate (Th9) level. Data represent the mean SEM. *P < 0.05 compared with SCI models injected with PBS. Scale bars: 100 m and inset 20 m (B) and 50 m (C). Asterisks in B and C indicate the epicenter of the lesion.

SHEDs regenerated the transected corticospinal tract and raphespinal serotonergic axons. To examine whether engrafted SHEDs affect the preservation of neurofilaments, we performed immunohistochemical analyses with an antineurofilament M (NF-M) mAb, 8 weeks after transection. Compared with the PBS-treated control SCs, the SHED-transplanted SCs exhibited greater preservation of NF-positive axons from 3 mm rostral to 3 mm caudal to the transected lesion site (Figure 2, B and C; asterisk indicates epicenter). The percentages of NF-positive axons in the epicenter of the SHED-transplanted and control SCs were 35.8% 13.0% and 8.7% 3.4%, respectively, relative to sham-treated SCs (Figure 2D).

Regeneration of both the corticospinal tract (CST) and the descending serotonergic raphespinal axons is important for the recovery of hind limb locomotor function in rat SCI. We therefore examined whether these axons had extended beyond the epicenter in the SHED-transplanted SCs. The CST axons were traced with the anterograde tracer biotinylated dextran amine (BDA), which was injected into the sensorimotor cortex. The serotonergic raphespinal axons were immunohistochemically detected by a mAb that specifically reacts with serotonin (5-hydroxytryptamine [5-HT]), which is synthesized within the brain stem. We found that both BDA- and 5-HTpositive fibers extended as far as 3 mm caudal to the epicenter in the SHED-transplanted but not the control group (Figures 3 and 4). Furthermore, some BDA- and 5-HTpositive boutons could be seen apposed to neurons in the caudal stump (Figure 3D and Figure 4C), suggesting that the regenerated axons had established new neural connections. Notably, although the number of descending axons extending beyond the epicenter was small, we observed many of them penetrating the scar tissue of the rostral stump (Figure 3A and Figure 4A). The percentages of 5-HTpositive axons of the SHED-transplanted SCs at 1 and 3 mm rostral to the epicenter were 58.9% 3.9% and 78.3% 7.4% relative to sham-treated SC, respectively (Figure 4D). These results demonstrate that the engrafted SHEDs promoted the recovery of hind limb locomotion via the preservation and regeneration of transected axons, even in the microenvironment of the damaged CNS.

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Human Dental Pulp-Derived Stem Cells Promote Locomotor ...