Systemic treatment of diabetic rats with a novel human growth factor, vascular endothelial growth factor (VEGF)-A165b, reversed neuropathic pain and peripheral nerve damage.

Key points Diabetes is thought to induce neuropathic pain through activation of dorsal horn sensory neurons in the spinal cord.Here we explore the impact of hyperglycaemia on the blood supply supporting the spinal cord and chronic pain development.In streptozotocin‐induced diabetic rats, neuropathic pain is accompanied by a decline in microvascular integrity in the dorsal horn. Hyperglycaemia‐induced degeneration of the endothelium in the dorsal horn was associated with a loss in vascular endothelial growth factor (VEGF)‐A165b expression. VEGF‐A165b treatment prevented diabetic neuropathic pain and degeneration of the endothelium in the spinal cord.Using an endothelial‐specific VEGFR2 knockout transgenic mouse model, the loss of endothelial VEGFR2 signalling led to a decline in vascular integrity in the dorsal horn and the development of hyperalgesia in VEGFR2 knockout mice.This highlights that vascular degeneration in the spinal cord could be a previously unidentified factor in the development of diabetic neuropathic pain. AbstractAbnormalities of neurovascular interactions within the CNS of diabetic patients is associated with the onset of many neurological disease states. However, to date, the link between the neurovascular network within the spinal cord and regulation of nociception has not been investigated despite neuropathic pain being common in diabetes. We hypothesised that hyperglycaemia‐induced endothelial degeneration in the spinal cord, due to suppression of vascular endothelial growth factor (VEGF)‐A/VEGFR2 signalling, induces diabetic neuropathic pain. Nociceptive pain behaviour was investigated in a chemically induced model of type 1 diabetes (streptozotocin induced, insulin supplemented; either vehicle or VEGF‐A165b treated) and an inducible endothelial knockdown of VEGFR2 (tamoxifen induced). Diabetic animals developed mechanical allodynia and heat hyperalgesia. This was associated with a reduction in the number of blood vessels and reduction in Evans blue extravasation in the lumbar spinal cord of diabetic animals versus age‐matched controls. Endothelial markers occludin, CD31 and VE‐cadherin were downregulated in the spinal cord of the diabetic group versus controls, and there was a concurrent reduction of VEGF‐A165b expression. In diabetic animals, VEGF‐A165b treatment (biweekly i.p., 20 ng g−1) restored normal Evans blue extravasation and prevented vascular degeneration, diabetes‐induced central neuron activation and neuropathic pain. Inducible knockdown of VEGFR2 (tamoxifen treated Tie2CreERT2‐vegfr2flfl mice) led to a reduction in blood vessel network volume in the lumbar spinal cord and development of heat hyperalgesia. These findings indicate that hyperglycaemia leads to a reduction in the VEGF‐A/VEGFR2 signalling cascade, resulting in endothelial dysfunction in the spinal cord, which could be an undiscovered contributing factor to diabetic neuropathic pain.

Chronic pain can develop in response to conditions such as inflammatory arthritis. The central mechanisms underlying the development and maintenance of chronic pain in humans are not well elucidated although there is evidence for a role of microglia and astrocytes. However in pre-clinical models of pain, including models of inflammatory arthritis, there is a wealth of evidence indicating roles for pathological glial reactivity within the CNS. In the spinal dorsal horn of rats with painful inflammatory arthritis we found both a significant increase in CD11b microglia-like cells and GFAP astrocytes associated with blood vessels, and the number of activated blood vessels expressing the adhesion molecule ICAM-1, indicating potential glio-vascular activation. Using pharmacological interventions targeting VEGFR2 in arthritic rats, to inhibit endothelial cell activation, the number of dorsal horn ICAM-1 blood vessels, CD11b microglia and the development of secondary mechanical allodynia, an indicator of central sensitization, were all prevented. Targeting endothelial VEGFR2 by inducible Tie2-specific VEGFR2 knock-out also prevented secondary allodynia in mice and glio-vascular activation in the dorsal horn in response to inflammatory arthritis. Inhibition of VEGFR2 in vitro significantly blocked ICAM-1-dependent monocyte adhesion to brain microvascular endothelial cells, when stimulated with inflammatory mediators TNF-α and VEGF-Aa. Taken together our findings suggest that a novel VEGFR2-mediated spinal cord glio-vascular mechanism may promote peripheral CD11b circulating cell transmigration into the CNS parenchyma and contribute to the development of chronic pain in inflammatory arthritis. We hypothesise that preventing this glio-vascular activation and circulating cell translocation into the spinal cord could be a new therapeutic strategy for pain caused by rheumatoid arthritis.

Diabetic retinopathy (DR) is one of the leading causes of blindness in the developed world. Characteristic features of DR are retinal neurodegeneration, pathological angiogenesis and breakdown of both the inner and outer retinal barriers of the retinal vasculature and retinal pigmented epithelial (RPE)–choroid respectively. Vascular endothelial growth factor (VEGF-A), a key regulator of angiogenesis and permeability, is the target of most pharmacological interventions of DR. VEGF-A can be alternatively spliced at exon 8 to form two families of isoforms, pro- and anti-angiogenic. VEGF-A165a is the most abundant pro-angiogenic isoform, is pro-inflammatory and a potent inducer of permeability. VEGF-A165b is anti-angiogenic, anti-inflammatory, cytoprotective and neuroprotective. In the diabetic eye, pro-angiogenic VEGF-A isoforms are up-regulated such that they overpower VEGF-A165b. We hypothesized that this imbalance may contribute to increased breakdown of the retinal barriers and by redressing this imbalance, the pathological angiogenesis, fluid extravasation and retinal neurodegeneration could be ameliorated. VEGF-A165b prevented VEGF-A165a and hyperglycaemia-induced tight junction (TJ) breakdown and subsequent increase in solute flux in RPE cells. In streptozotocin (STZ)-induced diabetes, there was an increase in Evans Blue extravasation after both 1 and 8 weeks of diabetes, which was reduced upon intravitreal and systemic delivery of recombinant human (rh)VEGF-A165b. Eight-week diabetic rats also showed an increase in retinal vessel density, which was prevented by VEGF-A165b. These results show rhVEGF-A165b reduces DR-associated blood–retina barrier (BRB) dysfunction, angiogenesis and neurodegeneration and may be a suitable therapeutic in treating DR.