CXCL12 and diabetes mellitus: TLR4: toll-like receptor 4; TNF: tumor necrosis factor; DPN: diabetic peripheral neuropathy; ICAM-1: intercellular adhesion molecule 1; T2DM: type 2 diabetes mellitus; IL-6: interleukin 6; PAD: peripheral artery disease; AGE: advanced glycation end-product; ALA: alpha-lipoic acid; PGRN: progranulin; CPT: current perception threshold; GRO: growth-regulated oncogene; DSPN: diabetic sensorimotor polyneuropathy; MNSI: Michigan Neuropathy Screening Instrument; EPC: endothelial progenitor cells; SDF-1: stromal cell-derived factor 1