Fig 2: Sitagliptin inhibits the pathological features of CD26+ wound fibroblasts in vitro. (A) The expression of CD26 and Col1 in WFs and CD26+ WFs was measured by western blot following a concentration gradient of sitagliptin treatment. Representative images are shown. (B) Immunofluorescent staining of CD26 in WFs and CD26+ WFs after sitagliptin treatment. Representative images are shown. (C) The migration ability of CD26+ WFs was significantly reduced following sitagliptin treatment (20 nM) in wound healing assays. (D) The migration ability of CD26+ WFs was significantly reduced after sitagliptin treatment (20 nM) in transwell assays. (E, F) An evident increase in apoptosis within the WFs and CD26+ WFs following sitagliptin treatment (20 nM) as assayed by Annexin V-PI staining and its quantification (n=5). Scale bar: 100 μm. t test, * P<0.05 vs WF group, ** P<0.01 vs WF group.

Fig 3: Pharmacological inhibition of CD26 inhibits collagen biosynthesis during skin wound healing and reduces scar formation. (A) Immunohistochemistry staining of CD26 in samples of normal skin, burn wounds, and wounds treated with sitagliptin. Scale bar: 100 μm. (B) Masson staining in samples of normal skin, burn wounds, and wounds treated with sitagliptin and its quantification (n=5). Scale bar: 200 μm. (C) The protein expressions levels of CD26 and Col1 in tissue samples following sitagliptin treatment were measured by western blot. Representative images are shown. (D) The mRNA expression levels of CD26 and Col1 were significantly decreased in samples of skin burn wounds following sitagliptin treatment (n=5). t test, * P<0.05 vs Normal group, ** P<0.01 vs Normal group.

Fig 4: CD26 promotes cell proliferation, migration, and collagen biosynthesis in vitro. (A) Cell proliferation was remarkably increased in CD26+ WFs (CCK-8 viability assay). (B) The expressions levels of CD26 and Col1 were significantly increased in CD26+ WFs. Representative images of WB are shown. (C) The concentrations of CD26 and Col1 in the supernatant were determined by ELISA (n=5). * P<0.05 vs CD26+ NF group, ** P<0.01 vs CD26+ NF group. (D) Significant enhancement of cell migration in CD26+ WFs as gauged by wound healing assay and quantification (n=5). * P<0.05 vs CD26− NF group, ** P<0.01 vs CD26− WF group. (E) Significant enhancement of cell migration in CD26+ WFs as gauged by transwell assay and its quantification (n=5). ** P<0.01 vs CD26− NF/WF group. Scale bar: 100 μm, t test.

Fig 5: Systemic Dpp4 depletion results in significant behavioral abnormalities in mice and DPP4 is predominantly expressed in nonaxonal cells of the mouse sciatic nerve. (a) Tail suspension assay demonstrating that Dpp4 knockdown mice show hindlimb clasping, indicating impaired neurological function. (b) Open field test results with tracking of mouse movement (red lines). (c-e) Immunofluorescence staining of cross-sections from adult mouse sciatic nerve, showing DPP4 localization in comparison to markers for myelinating Schwann cells (MBP, myelin basic protein, c), nonmyelinating Schwann cells (NCAM1, neural cell adhesion molecule, d), and axons (TUJ1, β3-tubulin, e). DPP4 shows ring-like pattern (arrows) and patch-like pattern (arrowheads). Scale bar, 10 µm (left) or 5 μm (right, magnified). (f) Quantification of ratio of non-TUJ1-overlapping “ring”-like DPP4 staining and TUJ1-overlapping “patch”-like DPP4 staining (n = 4 of unit areas from the individual mouse sections, ****P < .0001). (g) Embryonic DRG tissues from E12.5 mice were dissected, dissociated, and cultured. The +FDU group represents a pure neuronal culture where fluorodeoxyuridine was added to eliminate dividing non-neuronal cells, while the −FDU group allowed non-neuronal cells to survive alongside neurons (scale bar, 100 µm). (h) Western blot analysis of total lysates collected on day 6 from the cell cultures shown in (g). Cases 1 and 2 represent independent biological replicates.