Exosomes as emerging mediators and therapeutic vehicles in painful diabetic neuropathy: From pathophysiology to clinical translation

Table 3 Exosomal diagnostic biomarkers in painful diabetic neuropathy

Biomarker category	Specific biomarker	Expression change in PDN	Mechanistic/clinical correlate	Diagnostic utility	Validation status and key specificity limitation
MiRNAs	MiR-21-5p	Upregulated	Targets PTEN (reduced) and PDCD4 (reduced) → enhanced PI3K-Akt survival signaling	Correlates positively with neuropathic pain scores, negatively with IENFD	Limited human PDN cohort data only; not PDN-specific – also upregulated in diabetic nephropathy, retinopathy, cardiomyopathy, and chemotherapy-induced neuropathy; no comparative cohort study has distinguished PDN from these conditions using this marker alone
	MiR-146a-5p	Upregulated	Negative feedback regulator of TLR/IL-1R signaling (targets IRAK1, TRAF6); parallels increased IL-6 and TNF-α	Dynamic indicator of neuroinflammatory activity; correlates with disease duration and nerve conduction deficits	Supporting evidence derived from a DPN cohort using a ceRNA framework, not a pain-stratified PDN population; also elevated in diabetic nephropathy (a frequent PDN comorbidity), representing a critical confound for clinical diagnostic use without tissue-source enrichment
	Let-7a-5p, let-7b-5p	Downregulated	Derepresses HMGA2 → increased Nav1.7 transcription; derepresses IL-6Rα and STAT3 → increased IL-6 signaling	Molecular readout of peripheral nociceptor sensitization and neuroinflammation	Evidence is primarily preclinical (rodent DRG/spinal cord); no published human PDN cohort has confirmed circulating exosomal let-7 downregulation with clinical pain correlates; it is also dysregulated in diabetic retinopathy and hereditary neuropathies, limiting current specificity
	MiR-9-5p	Reduced	Impairs post-transcriptional suppression of NF-κB signaling in Schwann cells	Reflects Schwann cell pro-inflammatory phenotype	Mechanistic evidence only; no published PDN-specific human biomarker cohort data; lowest validation tier among listed miRNAs
	MiR-29 family	Downregulated	Promotes extracellular matrix accumulation and endoneurial fibrosis	Correlates with structural nerve remodeling	Mechanistic evidence only; endoneurial fibrosis correlation is inferred from general fibrosis biology rather than PDN-specific cohort data
	MiR-132-3p	Reduced	Derepresses RhoGAP32 → compromises axonal branching and synaptic plasticity	Indicates impaired axonal regeneration capacity	Dual role: Pathogenic exosomal cargo in PDN (Table 2) and therapeutic cargo in MSC-derived exosomes (Table 5); this context-dependency must be considered when interpreting circulating levels as a biomarker
	MiR-223-3p	Elevated	Derived from M1-polarized macrophages	Marker of endoneurial macrophage pro-inflammatory activation	Not PDN-specific; elevated in systemic inflammatory states; utility as a PDN biomarker depends on demonstrating neural-compartment specificity via L1CAM or MOG immunoaffinity enrichment (Table 4)
Proteins	NfL	Elevated (exosomal fraction)	Axonal cytoskeletal damage; exosomal NfL correlates more strongly with neuropathy severity than total plasma NfL	Indicator of axonal degeneration; correlates with nerve conduction studies and IENFD	Best-validated protein biomarker in this table; however, NfL elevation is not PDN-specific (also elevated in AD, MS, ALS); the exosomal fraction advantage over total plasma NfL has not yet been validated in large independent PDN cohorts
	HSP70	Reduced	Neuroprotective chaperone normally enriched in healthy Schwann cell exosomes	Inversely correlates with neuropathic pain severity; reflects depletion of neuroprotective Schwann cell cargo	Reduction is consistent with hyperglycemia-induced Schwann cell dysfunction; relatively more PDN-contextual than miRNA markers since HSP70 enrichment is a Schwann cell-specific exosomal feature, though formal specificity studies across PDN comorbidities are lacking
	GAP-43	Variable (higher in regenerative phenotype)	Expressed in regenerating axonal growth cones	Prognostic indicator of residual regenerative capacity; may guide timing of neuroprotective interventions	Prognostic rather than diagnostic marker; variable expression pattern limits use as a single threshold biomarker; most useful as part of a composite panel with NfL (degeneration) and HSP70 (Schwann cell support) to characterize individual patient disease trajectory
	MBP fragments, PMP22	Elevated	Released from degenerating myelin sheaths; packaged into exosomes by activated macrophages	Correlates with slowing of motor and sensory nerve conduction velocities; marker of demyelinating neuropathy	Relevant specifically to large-fiber and demyelinating PDN subtypes; complementary to NfL (small-fiber/axonal marker); combined NfL + MBP/PMP22 panel could enable fiber-type-specific PDN subtyping, which has direct implications for therapeutic target selection

PDN: Painful diabetic neuropathy; miRNAs: MicroRNAs; MSC: Mesenchymal stem cell; NfL: Neurofilament light chain; HSP70: Heat shock protein 70; PMP22: Peripheral myelin protein 22; L1CAM: L1 cell adhesion molecule; MOG: Myelin oligodendrocyte glycoprotein; PTEN: Phosphatase and tensin homolog; PDCD4: Programmed cell death protein 4; PI3K-Akt: Phosphatidylinositol 3-kinase-protein kinase B; IENFD: Intraepidermal nerve fiber density; TLR4: Toll-like receptor 4; IL-1R: Interleukin-1 receptor; IRAK1: Interleukin-1 receptor-associated kinase 1; TRAF6: Tumor necrosis factor receptor-associated factor 6; TNF-α: Tumor necrosis factor-alpha; IL: Interleukin; DPN: Diabetic peripheral neuropathy; HMGA2: High mobility group AT-hook 2; STAT3: Signal transducer and activator of transcription 3; DRG: Dorsal root ganglion; NF-κB: Nuclear factor kappa B; RhoGAP32: Rho GTPase activating protein 32; M1: Classically activated macrophage; MS: Multiple sclerosis; ALS: Amyotrophic lateral sclerosis; GAP-43: Growth-associated protein 43; AD: Alzheimer’s disease.