Increased peripheral nerve microvessels in early experimental diabetic neuropathy: quantitative studies of nerve and dorsal root ganglia

Reactive dicarbonyl compounds cause Calcitonin Gene-Related Peptide release and synergize with inflammatory conditions in mouse skin and peritoneum

The plasmas of diabetic or uremic patients and of those receiving peritoneal dialysis treatment have increased levels of the glucose-derived dicarbonyl metabolites like methylglyoxal (MGO), glyoxal (GO), and 3-deoxyglucosone (3-DG). The elevated dicarbonyl levels can contribute to the development of painful neuropathies. Here, we used stimulated immunoreactive Calcitonin Gene-Related Peptide (iCGRP) release as a measure of nociceptor activation, and we found that each dicarbonyl metabolite induces a concentration-, TRPA1-, and Ca²⁺-dependent iCGRP release. MGO, GO, and 3-DG were about equally potent in the millimolar range. We hypothesized that another dicarbonyl, 3,4-dideoxyglucosone-3-ene (3,4-DGE), which is present in peritoneal dialysis (PD) solutions after heat sterilization, activates nociceptors. We also showed that at body temperatures 3,4-DGE is formed from 3-DG and that concentrations of 3,4-DGE in the micromolar range effectively induced iCGRP release from isolated murine skin. In a novel preparation of the isolated parietal peritoneum PD fluid or 3,4-DGE alone, at concentrations found in PD solutions, stimulated iCGRP release. We also tested whether inflammatory tissue conditions synergize with dicarbonyls to induce iCGRP release from isolated skin. Application of MGO together with bradykinin or prostaglandin E₂ resulted in an overadditive effect on iCGRP release, whereas MGO applied at a pH of 5.2 resulted in reduced release, probably due to an MGO-mediated inhibition of transient receptor potential (TRP) V1 receptors. These results indicate that several reactive dicarbonyls activate nociceptors and potentiate inflammatory mediators. Our findings underline the roles of dicarbonyls and TRPA1 receptors in causing pain during diabetes or renal disease.

Diabetic neuropathy and the sensory neuron: New aspects of pathogenesis and their treatment implications

Diabetic polyneuropathy (DPN) continues to be generally considered as a "microvascular" complication of diabetes mellitus alongside nephropathy and retinopathy. The microvascular hypothesis, however, might be tempered by the concept that diabetes directly targets dorsal root ganglion sensory neurons. This neuron-specific concept, supported by accumulating evidence, might account for important features of DPN, such as its early sensory neuron degeneration. Diabetic sensory neurons develop neuronal atrophy alongside a series of messenger ribonucleic acid (RNA) changes related to declines in structural proteins, increases in heat shock protein, increases in the receptor for advanced glycation end-products, declines in growth factor signaling and other changes. Insulin is recognized as a potent neurotrophic factor, and insulin ligation enhances neurite outgrowth through activation of the phosphoinositide 3-kinase-protein kinase B pathway within sensory neurons and attenuates phenotypic features of experimental DPN. Several interventions, including glucagon-like peptide-1 agonism, and phosphatase and tensin homolog inhibition to activate growth signals in sensory neurons, or heat shock protein overexpression, prevent or reverse neuropathic abnormalities in experimental DPN. Diabetic sensory neurons show a unique pattern of microRNA alterations, a key element of messenger RNA silencing. For example, let-7i is widely expressed in sensory neurons, supports their growth and is depleted in experimental DPN; its replenishment improves features of DPN models. Finally, impairment of pre-messenger RNA splicing in diabetic sensory neurons including abnormal nuclear RNA metabolism and structure with loss of survival motor neuron protein, a neuron survival molecule, and overexpression of CWC22, a splicing factor, offer further novel insights. The present review addresses these new aspects of DPN sensory neurodegeneration.

Mechanisms of diabetic neuron damage: Molecular pathways

Diabetic polyneuropathy (DPN) is a common but intractable degenerative disorder of peripheral neurons. DPN first results in retraction and loss of sensory terminals in target organs such as the skin, whereas the perikarya (cell bodies) of neurons are relatively preserved. This is important because it implies that regrowth of distal terminals, rather than neuron replacement or rescue, may be useful clinically. Although a number of neuronal molecular abnormalities have been examined in experimental DPN, several are prominent: loss of structural proteins, neuropeptides, and neurotrophic receptors; upregulation of "stress" and "repair" proteins; elevated nitric oxide synthesis; increased AGE-RAGE signaling, NF-κB and PKC; altered neuron survival pathways; changes of pain-related ion channel investment. There is also a role for abnormalities of direct signaling of neurons by insulin, an important trophic factor for neurons that express its receptors. While evidence implicating each of these pathways has emerged, how they link together and result in neuronal degeneration remains unclear. However, several offer interesting new avenues for more definitive therapy of this condition.

Sensory Neurodegeneration in Diabetes: Beyond Glucotoxicity

Diabetic polyneuropathy in humans is of gradual, sometimes insidious onset, and is more likely to occur if glucose control is poor. Arguments that the disorder arises chiefly from glucose toxicity however ignore the greater complexity of a unique neurodegenerative disorder. For example, sensory neurons regularly thrive in media with levels of glucose at or exceeding those of poorly controlled diabetic persons. Also, all of the linkages between hyperglycemia and neuropathy develop in the setting of altered insulin availability or sensitivity. Insulin itself is recognized as a potent growth, or trophic factor for adult sensory neurons. Low doses of insulin, insufficient to alter blood glucose levels, reverse features of diabetic neurodegeneration in animal models. Insulin resistance, as occurs in diabetic adipose tissue, liver, and muscle, also develops in sensory neurons, offering a mechanism for neurodegeneration in the setting of normal or elevated insulin levels. Other interventions that "shore up" sensory neurons prevent features of diabetic polyneuropathy from developing despite persistent hyperglycemia. More recently evidence has emerged that a series of subtle molecular changes in sensory neurons can be linked to neurodegeneration including epigenetic changes in the control of gene expression. Understanding the new complexity of sensory neuron degeneration may give rise to therapeutic strategies that have a higher chance of success in the clinical trial arena.

Impaired Hedgehog signalling-induced endothelial dysfunction is sufficient to induce neuropathy: implication in diabetes

AIMS

Microangiopathy, i.e. endothelial dysfunction, has long been suggested to contribute to the development of diabetic neuropathy, although this has never been fully verified. In the present paper, we have identified the role of Hedgehog (Hh) signalling in endoneurial microvessel integrity and evaluated the impact of impaired Hh signalling in endothelial cells (ECs) on nerve function.

METHODS AND RESULTS

By using Desert Hedgehog (Dhh)-deficient mice, we have revealed, that in the absence of Dhh, endoneurial capillaries are abnormally dense and permeable. Furthermore, Smoothened (Smo) conditional KO mice clarified that this increased vessel permeability is specifically due to impaired Hh signalling in ECs and is associated with a down-regulation of Claudin5 (Cldn5). Moreover, impairment of Hh signalling in ECs was sufficient to induce hypoalgesia and neuropathic pain. Finally in Lepr(db/db) type 2 diabetic mice, the loss of Dhh expression observed in the nerve was shown to be associated with increased endoneurial capillary permeability and decreased Cldn5 expression. Conversely, systemic administration of the Smo agonist SAG increased Cldn5 expression, decreased endoneurial capillary permeability, and restored thermal algesia to diabetic mice, demonstrating that loss of Dhh expression is crucial in the development of diabetic neuropathy.

CONCLUSION

The present work demonstrates the critical role of Dhh in maintaining blood nerve barrier integrity and demonstrates for the first time that endothelial dysfunction is sufficient to induce neuropathy.

Emerging therapy for diabetic neuropathy: cell therapy targeting vessels and nerves

Diabetic neuropathy (DN), the most common complication of diabetes, frequently leads to foot ulcers and may progress to limb amputations. Despite continuous increase in incidence, there is no clinical therapy to effectively treat DN. Pathogenetically, DN is characterized by reduced vascularity in peripheral nerves and deficiency in angiogenic and neurotrophic factors. We will briefly review the pathogenetic mechanism of DN and address the effects and the mechanisms of cell therapies for DN. To reverse the changes of DN, studies have attempted to deliver neurotrophic or angiogenic factors for treatment in the form of protein or gene therapy; however, the effects turned out to be very modest if not ineffective. Recent studies have demonstrated that bone marrow (BM)-derived cells such as mononuclear cells or endothelial progenitor cells (EPCs) can effectively treat various cardiovascular diseases through their paracrine effects. As BM-derived cells include multiple angiogenic and neurotrophic cytokines, these cells were used for treating experimental DN and found to reverse manifestations of DN. Particularly, EPCs were shown to exert favorable therapeutic effects through enhanced neural neovascularization and neuro-protective effects. These findings clearly indicate that DN is a complex disorder with pathogenetic involvement of both vascular and neural components. Studies have shown that cell therapies targeting both vascular and neural elements are shown to be advantageous in treating DN.

Sensory nerve conduction in the caudal nerves of rats with diabetes

PURPOSE: To investigate sensory nerve conduction of the caudal nerve in normal and diabetic rats. METHODS: Diabetes was induced in twenty 8-weeks old Wistar male rats. Twenty normal rats served as controls. Caudal nerve conduction studies were made before diabetes induction and the end of each week for six consecutive weeks. The caudal nerve was stimulated distally and nerve potentials were recorded proximally on the animal's tail using common "alligator" clips as surface electrodes. RESULTS: After induction, nerve conduction velocities (NCV) increased slower in the diabetic than in the control group. Sensory nerve action potentials (SNAP) conduction velocities increased slower in the diabetic than in the control group (slope of regression line: 0.5 vs 1.3m/s per week; NCV in the 15th week = 39±3m/s vs 44±4m/s). Tukey's tests showed differences between groups at the 11th, 13th and 15th weeks old. From the 10th week on, SNAP amplitudes increased faster in the diabetic than in the control group (slopes of the regression line: 10 vs 8µV per week; SNAP amplitudes in the 15th week: 107±23µV vs 85±13µV). Differences at the 12th, 13th and 15th weeks were significant. CONCLUSION: In diabetic rats nerve conduction velocities were slower whereas amplitudes were larger than in normal rats.

Endoneurial microvascular pathology in feline diabetic neuropathy

Endoneurial capillaries in nerve biopsies from 12 adult diabetic cats with varying degrees of neurological dysfunction were examined for evidence of microvascular pathology and compared to nerves obtained at necropsy from 7 adult non-diabetic cats without clinical evidence of neurological dysfunction. As reported previously [Mizisin, A.P., Nelson, R.W., Sturges, B.K., Vernau, K.M., LeCouteur, R.A., Williams, D.C., Burgers, M.L., Shelton, G.D., 2007. Comparable myelinated nerve pathology in feline and human diabetes mellitus. Acta Neuropathol. 113, 431-442.], the diabetic cats had elevated glycosylated hemoglobin and serum fructosamine levels, decreased motor nerve conduction velocity and compound muscle action potential (CMAP) amplitude, and markedly decreased myelinated nerve fiber densities. Compared to non-diabetic cats, there was a non-significant 26% increase in capillary density and a significant (P<0.009) 45% increase in capillary size in diabetic cats. Capillary luminal size was also significantly (P<0.001) increased, while an index of vasoconstriction was significantly decreased (P<0.001) in diabetic cats compared to non-diabetic controls. No differences in endothelial cell size, endothelial cell number or pericyte size were detected between non-diabetic and diabetic cats. In diabetic cats, basement membrane thickening, seen as a reduplication of the basal lamina, was significantly (P<0.0002) increased by 73% compared to non-diabetic controls. Regression analysis of either myelinated nerve fiber density or CMAP amplitude against basement membrane size demonstrated a negative correlation with significant slopes (P<0.03 and P<0.04, respectively). These data demonstrate that myelinated nerve fiber injury in feline diabetic neuropathy is associated with microvascular pathology and that some of these changes parallel those documented in experimental rodent and human diabetic neuropathy.

Diabetes mellitus and the peripheral nervous system: manifestations and mechanisms

Diabetes targets the peripheral nervous system with several different patterns of damage and several mechanisms of disease. Diabetic polyneuropathy (DPN) is a common disorder involving a large proportion of diabetic patients, yet its pathophysiology is controversial. Mechanisms considered have included polyol flux, microangiopathy, oxidative stress, abnormal signaling from advanced glycation endproducts and growth factor deficiency. Although some clinical trials have demonstrated modest benefits in disease stabilization or pain therapy in DPN, robust therapy capable of reversing the disease is unavailable. In this review, general aspects of DPN and other diabetic neuropathies are examined, including a summary of recent therapeutic trials. A particular emphasis is placed on the evidence that the neurobiology of DPN reflects a unique yet common and disabling neurodegenerative disorder.

Lesão por esmagamento do nervo isquiático de ratos: estudo da vascularização

Este trabalho teve como objetivo estudar as alterações microvasculares intraneurais aguda em nervo isquiático de rato submetido a esmagamento por diferentes cargas. Foram utilizados 60 ratos machos da linhagem Wistar, distribuídos em grupos experimentais de acordo com a injeção de vasos e com a carga de esmagamento. Os nervos isquiáticos direitos foram isolados e submetidos ao esmagamento com cargas (0,5 Kg, 1 Kg, 5 Kg, 10 kg e 15 kg) por 10 minutos e os nervos isquiáticos esquerdos foram utilizados como controle. Após esmagamento, os animais foram submetidos à cateterização da aorta abdominal e injeção dos vasos, em seguida 30 nervos direitos e esquerdos foram fixados em formol 10%, desidratados e diafanizados para análise longitudinal dos vasos intraneurais e os restantes retirados em toda a sua extensão, cortados em 3 fragmentos, congelados em isopentano em gelo seco e armazenados em freezer -70°C, seccionados transversalmente para análise e contagem dos vasos intraneurais. As análises macroscópica e microscópica mostraram regiões de hematoma endoneural e epineural nas diferentes cargas de esmagamento. A análise morfométrica sugere que a lesão aos vasos intraneurais foi proporcional à carga de esmagamento, causando hematoma endoneural e epineural, que cria microambiente desfavorável para a regeneração das fibras nervosas.

Nerve and ganglion blood flow in diabetes: an appraisal

Vasa nervorum, the vascular supply to peripheral nerve trunks, and their associated cell bodies in ganglia have unique anatomical and physiological characteristics. Several different experimental approaches toward understanding the changes in vase nervorum following injury and disease have been used. Quantative techniques most widely employed have been microelectrode hydrogen clearance palarography and [14C]iodoantipyrine autoradiographic distribution, whereas estimates of red blood cell flux using a fiber-optic laser Doppler probe offer real time data at different sites along the nerve trunk. There are important caveats about the use of these techniques, their advantages, and their limitations. Reports of nerve blood flow require careful documentation of physiological variables, including mean arterial pressure and nerve temperature during the recordings. Several ischemic models of the peripheral nerve trunk have addressed the ischemic threshold below which axonal degeneration ensues (< 5ml/100 g/min). Following injury, rises in local blood flow reflect acitons of vasoactive peptides, nitric oxide, and the development of angiogenesis. In experimental diabetes, a large number of studies have documented reductions in nerve blood flow and tandem corrections of nerve blood flow and conduction slowing. A significant proportions, however, of the work can be criticized on the basis of methodology and interpretation. Similarly, not all work has confirmed that reductions of nerve blood flow are an invariable feature of experimental or human diabetic polyneuropathy. Therefore, while there is disagreement as to whether early declines in nerve blood flow "account" for diabetic polyneuropathy, there is unquestioned eveidence of early microangiopathy. Abnormalities of vase nervorum and micorvessels supplying ganglia at the very least develop parallel to and together with changes in neurons, Schwann cells, and axons.

Influence of experimental diabetes on the microcirculation of injured peripheral nerve: functional and morphological aspects

Regeneration of diabetic axons has delays in onset, rate, and maturation. It is possible that microangiopathy of vasa nervorum, the vascular supply of the peripheral nerve, may render an unfavorable local environment for nerve regeneration. We examined local nerve blood flow proximal and distal to sciatic nerve transection in rats with long-term (8 month) experimental streptozotocin diabetes using laser Doppler flowmetry and microelectrode hydrogen clearance polarography. We then correlated these findings, using in vivo perfusion of an India ink preparation, by outlining the lumens of microvessels from unfixed nerve sections. There were no differences in baseline nerve blood flow between diabetic and nondiabetic uninjured nerves, and vessel number, density, and area were unaltered. After transection, there were greater rises in blood flow in proximal stumps of nondiabetic nerves than in diabetic animals associated with a higher number, density, and caliber of epineurial vessels. Hyperemia also developed in distal stumps of nondiabetic nerves but did not develop in diabetic nerves. In these stumps, diabetic rats had reduced vessel numbers and smaller mean endoneurial vessel areas. Failed or delayed upregulation of nerve blood flow after peripheral nerve injury in diabetes may create a relatively ischemic regenerative microenvironment.

Do denervated peripheral nerve trunks become ischemic? The impact of chronic denervation on vasa nervorum

The long-term relationship between the peripheral nerve trunk and its vascular supply, the vasa nervorum, has not been considered in the context of denervation and regeneration. While the microvessels of peripheral nerve are not thought to influence Wallerian degeneration itself, in this work we explored how vasa nervorum respond to denervation of the nerve trunk. Our hypotheses were that the presence of axons had a significant impact on the vasa nervorum and that the absence of reinnervation might eventually lead to an unfavorable ischemic regenerative microenvironment. We studied rat sciatic nerve trunks for up to 6 months following transection and either prevented regeneration or allowed it to proceed. Vasa nervorum were studied in several ways: (i) measurements of local endoneurial blood flow using microelectrode hydrogen clearance polarography; (ii) measurements of erythrocyte flux (flow) in the extrinsic nerve plexus using laser Doppler flowmetry; (iii) India ink perfusion of microvessels in unfixed nerve; (iv) mRNA expression of vascular endothelial growth factor (VEGF) using reverse transcription polymerase chain reaction. Early after injury, there were rises in endoneurial and extrinsic flow, microvessel numbers, and VEGF mRNA expression. Angiogenesis was apparently confined to the epineurial and perineurial compartments. Later, however, there were substantial declines in flow observed in long-term (6-month) denervated sciatic nerve trunks associated with declines in the caliber of new microvessels. Reinnervated sciatic nerves had restored endoneurial blood flow. The findings confirm important relationships between axon presence and local blood flow. Angiogenesis is a feature of the injured peripheral nerve, but long term denervated nerve trunks have declines of flow despite retaining new microvessels.

The role of protein kinase C in the development of the complications of diabetes

Diabetes mellitus produces a state of chronic hyperglycemia which in turn leads to the development of severe complications including retinopathy, nephropathy, neuropathy, and atherosclerosis. Many different mechanisms have been put forward to attempt to explain how glucose elevations can damage these various organ systems. Protein kinase C activation is one of the sequelae of hyperglycemia and is thought to play a role in the development of diabetic complications. There are multiple mechanisms for its activation in the diabetic state and multiple downstream effects attributable to that activation. The role of protein kinase C activation in the development of the above-mentioned complications of diabetes is discussed in this chapter. In addition, the potential use of isoform-specific inhibitors of protein kinase C for the treatment of diabetic complications is proposed.