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

Table 1 Summary of pathophysiological mechanisms in painful diabetic neuropathy

Category	Mechanism/component	Pathways and molecular mediators	Pathological consequences	Functional/clinical correlates	Synthetic insight: Cross-mechanism interaction and therapeutic implication
Peripheral mechanisms of nerve injury	Polyol pathway activation	Aldose reductase (glucose → sorbitol); sorbitol dehydrogenase (sorbitol → fructose); NADPH depletion; NADH generation	Osmotic stress; myo-inositol depletion; impaired Na+/K+-ATPase; reduced glutathione; mitochondrial superoxide production	Increased oxidative stress; axonal dysfunction; reduced antioxidant capacity	Represents the earliest measurable metabolic insult; NADPH depletion directly links polyol pathway activation to downstream mitochondrial dysfunction (row below) by impairing glutathione regeneration. Exosomal cargo from metabolically stressed neurons under polyol pathway activation is the proposed upstream trigger for Schwann cell exosomal reprogramming (Table 2)
	AGEs	Non-enzymatic glycation by methylglyoxal; RAGE activation; NF-κB signaling	Direct structural modification of myelin and cytoskeletal proteins; accelerated protein degradation; chronic sterile neuroinflammation	Progressive protein dysfunction; sustained inflammatory state; Schwann cell, endothelial cell, and macrophage activation	AGE/RAGE activation provides the mechanistic bridge between chronic hyperglycemia and sustained neuroinflammation; RAGE activation on Schwann cells directly reprograms their exosomal secretome (Table 2) toward a pro-inflammatory, neurotrophic-depleted profile, linking metabolic and inflammatory pathological arms
	Mitochondrial dysfunction	Impaired oxidative phosphorylation; reduced ATP; increased ROS; PGC-1α suppression; excessive mitochondrial fission	Reduced neuronal viability; impaired axonal transport; intrinsic apoptosis pathway activation; nitration/inactivation of mitochondrial enzymes	Energy failure in DRG neurons; progressive distal axonopathy; impaired nerve regeneration	Mitochondrial dysfunction in DRG neurons is both a downstream consequence of polyol/AGE pathology and an independent amplifier; impaired axonal transport reduces retrograde neurotrophic signaling to Schwann cells, accelerating their phenotypic reprogramming and exosomal cargo shift. MSC-derived exosomes carrying HSP70/90 and SIRT1 specifically target this node (Table 5)
	Axonal degeneration and demyelination	Progressive IENFD loss; Schwann cell phenotypic transition	Release of MMPs; pro-inflammatory cytokine secretion; altered Schwann cell exosomal cargo (pro-inflammatory miRNAs, depleted neurotrophic factors)	IENFD loss correlates with pain severity and diabetes duration; predominant vulnerability of small unmyelinated C-fibers and thinly myelinated A-delta fibers	IENFD reduction is the structural readout integrating all upstream metabolic and inflammatory insults; its correlation with exosomal miR-21-5p and NfL levels (Table 3) positions it as the primary composite endpoint bridging mechanistic and diagnostic domains, and its reversibility in response to MSC-derived exosome therapy (35%-80% recovery; Table 5) makes it the most relevant disease-modification endpoint for future clinical trials
	Peripheral nociceptor sensitization	Voltage-gated sodium channels (Nav1.7, Nav1.8, Nav1.9); TRP channels (TRPV1, TRPA1); HCN channels; potassium channel subtypes	Lowered action potential threshold; increased spontaneous ectopic discharge from injured axons and DRG; channel upregulation and sensitization in neuroinflammation/oxidative stress	Thermal hyperalgesia; mechanical allodynia; spontaneous burning pain	Nav1.7 upregulation and TRPV1 sensitization are directly regulated by Schwann cell-derived exosomal let-7 downregulation and ceramide/sphingosine-1-phosphate delivery (Table 2), mechanistically connecting the glial exosomal secretome to nociceptor hyperexcitability; exosomal let-7a mimic and anti-TRPV1 siRNA therapies (Table 5) specifically target this node without systemic cardiac conduction side effects
Neuroinflammation	Macrophages	Transition from M2-like to M1-like phenotype; RAGE activation; TLR4 signaling	The iNOS upregulation; enhanced TNF-α, IL-1β, IL-6, IL-18; increased matrix metalloproteinase secretion	Degradation of extracellular matrix; Schwann cell dysfunction; axonal degeneration; nociceptor sensitization	M1 macrophage polarization and Schwann cell pathology form a bidirectional feed-forward loop mediated by exosomal miR-155/SOCS1/JAK-STAT signaling (Table 2); this loop, once established, is self-sustaining independently of glycemic level, which may explain why PDN symptoms persist after glycemic control is achieved
	Schwann cells	Hyperglycemia-induced phenotypic reprogramming; p38 MAPK activation	Reduced NGF, NT-3, BDNF; upregulation of MCP-1 and TNF-α; altered exosomal repertoire (pro-inflammatory miRNAs, depleted neurotrophic cargo)	Impaired neurotrophic support for small-diameter sensory fibers; direct nociceptor sensitization; exosome-mediated propagation of inflammatory signals	Schwann cell exosomal cargo remodeling is the central pathogenic hub connecting vascular injury (endothelial exosomal dysfunction), metabolic stress (AGE/mitochondrial pathway), macrophage activation, and peripheral nociceptor sensitization; it is therefore also the primary target of MSC-derived exosome therapy (Table 5), which restores the physiological Schwann cell exosomal phenotype
	SGCs	Reactive phenotype in response to peripheral nerve injury and inflammatory signals	Increased GFAP, connexin 43; IL-1β, TNF-α production	Paracrine inflammatory microenvironment within DRG; lowered excitability threshold of sensory neuron somata; promoted ectopic discharge; sustained peripheral sensitization	SGC activation amplifies peripheral sensitization by lowering DRG neuron excitability thresholds via connexin 43-mediated gap junctions; this mechanism is currently underrepresented in exosomal biomarker panels (Tables 3, 4 and 7) and represents a significant gap in the PDN exosome literature
	Endoneurial vasculature	Dysregulation of vasa nervorum; blood-nerve barrier disruption	Altered exosomal cargo (pro-coagulant factors); impaired tight junction integrity	Facilitated macrophage infiltration; entry of systemic inflammatory mediators into endoneurial compartment	Vascular injury is the earliest initiating node in the pathological cascade (Table 2), upstream of Schwann cell and macrophage pathology; adipose-derived MSC exosomes with pro-angiogenic VEGF/miR-132-3p cargo (Table 5) specifically target this node, but are expected to only partially attenuate downstream pathology occurring through vasculature-independent routes
Central sensitization	Spinal dorsal horn synaptic plasticity	Wind-up and long-term potentiation at C-fiber synapses; NMDA receptor activation; substance P; CGRP; neurokinin-1 receptor	Calcium influx; post-synaptic kinase activation; immediate early gene expression; synaptic structural remodeling	Sustained enhancement of synaptic efficacy; transition to sensitized central state	Spinal LTP at C-fiber synapses has been directly demonstrated in STZ-diabetic rodent models; however, the contribution of peripherally derived pathological exosomes to this process (Table 2) remains inferential and has not been directly demonstrated in PDN models – distinguishing it from the better-evidenced peripheral mechanisms above
	Spinal microglia	Activation via ATP (P2X4, P2X7), fractalkine (CX3CR1), CSF-1; BDNF release	BDNF → TrkB → KCC2 (reduce) → converts GABAergic inhibition into net excitation	Loss of inhibitory tone; enhanced nociceptive transmission; central disinhibition	Microglial BDNF/KCC2 suppression is the most directly evidenced central sensitization mechanism in PDN models and the most tractable central exosomal therapeutic target; IL-4-stimulated macrophage exosomes enriched with IL-10, TGF-β, and miR-124 restore KCC2 expression in diabetic rodent spinal cord (Table 5)
	Spinal astrocytes	Activation; downregulation of GLT-1, GLAST	Reduced GLT-1/GLAST expression; glutamate spillover; D-serine and TNF-α release; extrasynaptic NMDA receptor activation	Potentiated synaptic transmission; sustained central sensitization	Astrocytic GLT-1/GLAST downregulation by exosomal miRNAs has been demonstrated in general neuroinflammatory models but not specifically in PDN spinal cord tissue; this mechanistic inference requires direct PDN-model confirmation before astrocyte-targeting exosomal therapies can be rationally designed
	Descending pain modulatory system	Impaired descending inhibition (reduced noradrenaline, reduced alpha-2 adrenergic receptors); enhanced descending facilitation (increased 5-HT on 5-HT3 receptors)	Imbalance between descending inhibition and facilitation; permissive central environment for pain amplification	Allodynia and hyperalgesia; pain signals amplified rather than gated; largely refractory to peripherally targeted analgesics	Descending modulatory pathway dysfunction explains the frequent inadequacy of peripherally targeted therapies in PDN and creates the strongest mechanistic rationale for intranasal exosomal delivery strategies capable of accessing supraspinal pain-modulatory circuits; however, direct exosomal evidence for this pathway in PDN is currently absent

PDN: Painful diabetic neuropathy; NADPH: Nicotinamide adenine dinucleotide phosphate; Na⁺/K⁺-ATPase: Sodium-potassium adenosine triphosphatase; NADH: Nicotinamide adenine dinucleotide (reduced form); AGEs: Advanced glycation end-products; RAGE: Receptor for advanced glycation end products; NF-κB: Nuclear factor kappa B; ATP: Adenosine triphosphate; ROS: Reactive oxygen species; PGC-1α: Peroxisome proliferator-activated receptor gamma coactivator 1-alpha; IENFD: Intraepidermal nerve fiber density; DRG: Dorsal root ganglion; Nav1.7, Nav1.8, Nav1.9: Voltage-gated sodium channel subtypes 1.7, 1.8, and 1.9; TRPV1: Transient receptor potential vanilloid 1; TRPA1: Transient receptor potential ankyrin 1; HCN: Hyperpolarization-activated cyclic nucleotide-gated channels; iNOS: Inducible nitric oxide synthase; TNF-α: Tumor necrosis factor-alpha; IL-1β: Interleukin-1 beta; IL-6: Interleukin-6; IL-18: Interleukin-18; TLR4: Toll-like receptor 4; NGF: Nerve growth factor; NT-3: Neurotrophin-3; BDNF: Brain-derived neurotrophic factor; MCP-1: Monocyte chemoattractant protein-1; p38 MAPK: P38 mitogen-activated protein kinase; GFAP: Glial fibrillary acidic protein; NMDA: N-methyl-D-aspartate; CGRP: Calcitonin gene-related peptide; P2X4, P2X7: Purinergic receptor subtypes P2X4 and P2X7; CX3CR1: CX3C chemokine receptor 1; CSF-1: Colony-stimulating factor 1; TrkB: Tropomyosin receptor kinase B; KCC2: Potassium-chloride cotransporter 2; GABA: Gamma-aminobutyric acid; GLT-1: Glutamate transporters-1; GLAST: Glutamate aspartate transporter; 5-HT: Serotonin (5-hydroxytryptamine); 5-HT₃: Serotonin receptor subtype 3; miRNAs: MicroRNAs; MSC: Mesenchymal stem cell; NfL: Neurofilament light chain; HSP70: Heat shock protein 70; MMPs: Matrix metalloproteinases; SGCs: Satellite glial cells; SOCS1: Suppressor of cytokine signaling 1; siRNA: Small interfering RNA; VEGF: Vascular endothelial growth factor; JAK-STAT: Janus kinase-signal transducer and activator of transcription; STZ: Streptozotocin; LTP: Long-term potentiation; M1: Classically activated macrophage.