Mechanisms of diabetic neuropathy: Schwann cells.

As ensheathing and secretory cells, Schwann cells are a ubiquitous and vital component of the endoneurial microenvironment of peripheral nerves. The interdependence of axons and their ensheathing Schwann cells predisposes each to the impact of injury in the other. Further, the dependence of the blood-nerve interface on trophic support from Schwann cells during development, adulthood, and after injury suggests these glial cells promote the structural and functional integrity of nerve trunks. Here, the developmental origin, injury-induced changes, and mature myelinating and nonmyelinating phenotypes of Schwann cells are reviewed prior to a description of nerve fiber pathology and consideration of pathogenic mechanisms in human and experimental diabetic neuropathy. A fundamental role for aldose-reductase-containing Schwann cells in the pathogenesis of diabetic neuropathy, as well as the interrelationship of pathogenic mechanisms, is indicated by the sensitivity of hyperglycemia-induced biochemical alterations, such as polyol pathway flux, formation of reactive oxygen species, generation of advanced glycosylation end products (AGEs) and deficient neurotrophic support, to blocking polyol pathway flux.

Repeated monitoring of corneal nerves by confocal microscopy as an index of peripheral neuropathy in type-1 diabetic rodents and the effects of topical insulin.

We developed a reliable imaging and quantitative analysis method for in vivo corneal confocal microscopy (CCM) in rodents and used it to determine whether models of type 1 diabetes replicate the depletion of corneal nerves reported in diabetic patients. Quantification was reproducible between observers and stable across repeated time points in two rat strains. Longitudinal studies were performed in normal and streptozotocin (STZ)-diabetic rats, with innervation of plantar paw skin quantified using standard histological methods after 40 weeks of diabetes. Diabetic rats showed an initial increase, then a gradual reduction in occupancy of nerves in the sub-basal plexus so that values were significantly lower at week 40 (68 ± 6%) than age-matched controls (80 ± 2%). No significant loss of stromal or intra-epidermal nerves was detected. In a separate study, insulin was applied daily to the eye of control and STZ-diabetic mice and this treatment prevented depletion of nerves of the sub-basal plexus. Longitudinal studies are viable in rodents using CCM and depletion of distal corneal nerves precedes detectable loss of epidermal nerves in the foot, suggesting that diabetic neuropathy is not length dependent. Loss of insulin-derived neurotrophic support may contribute to the pathogenesis of corneal nerve depletion in type 1 diabetes.

Electrodiagnostic testing and histopathologic changes confirm peripheral nervous system myelin abnormalities in the feline model of niemann-pick disease type C.

Niemann-Pick disease type C (NPC disease) is an incurable, neurodegenerative, autosomal recessive disease caused by mutations in either the NPC1 or the NPC2 gene. These mutations affect the intracellular trafficking of lipids and cholesterol, resulting in the intralysosomal accumulation of unesterified cholesterol and glycosphingolipids. These abnormalities are associated with clinical ataxia and impaired motor and intellectual development, and death frequently occurs in adolescence. The incidence of peripheral neuropathy in NPC patients is not known. We investigated peripheral nerves in the naturally occurring feline model of NPC disease, which has proven to be critical for understanding both disease pathogenesis and for evaluating experimental therapies. Electrodiagnostic studies revealed significantly slowed motor and sensory nerve conduction velocities in affected cats in the absence of altered M-wave amplitude. Histologic and ultrastructural analyses showed thin myelin sheaths, membranous debris, myelin figures, lipid vacuolization of Schwann cell cytoplasm, and expanded paranodal areas. Axonal degeneration was not identified. There was a shift to small myelinated fibers in affected cats, and there were significant decreases in fiber diameter, axon diameter, and myelin thickness. These changes were similar to those described in the murine NPC disease model and in rare patients in whom nerve biopsy has been performed. Characterization of the demyelinating neuropathy is necessary for evaluating clinical trials that target only the CNS aspects of NPC.

Hypertension-induced peripheral neuropathy and the combined effects of hypertension and diabetes on nerve structure and function in rats.

Diabetic neuropathy includes damage to neurons, Schwann cells and blood vessels. Rodent models of diabetes do not adequately replicate all pathological features of diabetic neuropathy, particularly Schwann cell damage. We, therefore, tested the hypothesis that combining hypertension, a risk factor for neuropathy in diabetic patients, with insulin-deficient diabetes produces a more pertinent model of peripheral neuropathy. Behavioral, physiological and structural indices of neuropathy were measured for up to 6 months in spontaneously hypertensive and age-matched normotensive rats with or without concurrent streptozotocin-induced diabetes. Hypertensive rats developed nerve ischemia, thermal hyperalgesia, nerve conduction slowing and axonal atrophy. Thinly myelinated fibers with supernumerary Schwann cells indicative of cycles of demyelination and remyelination were also identified along with reduced nerve levels of myelin basic protein. Similar disorders were noted in streptozotocin-diabetic rats, except that thinly myelinated fibers were not observed and expression of myelin basic protein was normal. Superimposing diabetes on hypertension compounded disorders of nerve blood flow, conduction slowing and axonal atrophy and increased the incidence of thinly myelinated fibers. Rats with combined insulinopenia, hyperglycemia and hypertension provide a model for diabetic neuropathy that offers an opportunity to study mechanisms of Schwann cell pathology and suggests that hypertension may contribute to the etiology of diabetic neuropathy.

Degenerative myelopathy associated with a missense mutation in the superoxide dismutase 1 (SOD1) gene progresses to peripheral neuropathy in Pembroke Welsh corgis and boxers.

Canine degenerative myelopathy (DM) is an adult-onset, fatal neurodegenerative disease with many similarities to an upper-motor-neuron-onset form of human amyotrophic lateral sclerosis (ALS), that results from mutations in the superoxide dismutase (SOD1) gene. DM occurs in many dog breeds, including the Pembroke Welsh Corgi and Boxer. The initial upper motor neuron degeneration produces spastic paraparesis and affected dogs develop general proprioceptive ataxia in the pelvic limbs. Dog owners usually elect euthanasia when their dog becomes paraplegic. When euthanasia is delayed, lower motor neuron signs including ascending tetraparesis, flaccid paralysis and widespread muscle atrophy emerge. For this study, muscle and peripheral nerve specimens were evaluated at varying disease stages from DM-affected Pembroke Welsh Corgis and Boxers that were homozygous for the SOD1 mutation and had spinal cord histopathology consistent with DM. Comparisons were made with age- and breed-matched control dogs. Here we provide evidence that Pembroke Welsh Corgis and Boxers with chronic DM develop muscle atrophy consistent with denervation, peripheral nerve pathology consistent with an axonopathy, and to a lesser degree demyelination. Canine DM has been proposed as a potential spontaneous animal disease model of human ALS. The results of this study provide further support that canine DM recapitulates one form of the corresponding human disorder and should serve as a valuable animal model to develop therapeutic strategies.

The MMP-9/TIMP-1 axis controls the status of differentiation and function of myelin-forming Schwann cells in nerve regeneration.

BACKGROUND: Myelinating Schwann cells (mSCs) form myelin in the peripheral nervous system. Because of the works by us and others, matrix metalloproteinase-9 (MMP-9) has recently emerged as an essential component of the Schwann cell signaling network during sciatic nerve regeneration. METHODOLOGY/PRINCIPAL FINDINGS: In the present study, using the genome-wide transcriptional profiling of normal and injured sciatic nerves in mice followed by extensive bioinformatics analyses of the data, we determined that an endogenous, specific MMP-9 inhibitor [tissue inhibitor of metalloproteinases (TIMP)-1] was a top up-regulated gene in the injured nerve. MMP-9 capture followed by gelatin zymography and Western blotting of the isolated samples revealed the presence of the MMP-9/TIMP-1 heterodimers and the activated MMP-9 enzyme in the injured nerve within the first 24 h post-injury. MMP-9 and TIMP-1 co-localized in mSCs. Knockout of the MMP-9 gene in mice resulted in elevated numbers of de-differentiated/immature mSCs in the damaged nerve. Our comparative studies using MMP-9 knockout and wild-type mice documented an aberrantly enhanced proliferative activity and, accordingly, an increased number of post-mitotic Schwann cells, short internodes and additional nodal abnormalities in remyelinated nerves of MMP-9 knockout mice. These data imply that during the first days post-injury MMP-9 exhibits a functionally important anti-mitogenic activity in the wild-type mice. Pharmacological inhibition of MMP activity suppressed the expression of Na(v)1.7/1.8 channels in the crushed nerves. CONCLUSION/SIGNIFICANCE: Collectively, our data established an essential role of the MMP-9/TIMP-1 axis in guiding the mSC differentiation and the molecular assembly of myelin domains in the course of the nerve repair process. Our findings of the MMP-dependent regulation of Na(v) channels, which we document here for the first time, provide a basis for therapeutic intervention in sensorimotor pathologies and pain.

The blood-nerve barrier: structure and functional significance.

The blood-nerve barrier (BNB) defines the physiological space within which the axons, Schwann cells, and other associated cells of a peripheral nerve function. The BNB consists of the endoneurial microvessels within the nerve fascicle and the investing perineurium. The restricted permeability of these two barriers protects the endoneurial microenvironment from drastic concentration changes in the vascular and other extracellular spaces. It is postulated that endoneurial homeostatic mechanisms regulate the milieu intérieur of peripheral axons and associated Schwann cells. These mechanisms are discussed in relation to nerve development, Wallerian degeneration and nerve regeneration, and lead neuropathy. Finally, the putative factors responsible for the cellular and molecular control of BNB permeability are discussed. Given the dynamic nature of the regulation of the permeability of the perineurium and endoneurial capillaries, it is suggested that the term blood-nerve interface (BNI) better reflects the functional significance of these structures in the maintenance of homeostasis within the endoneurial microenvironment.

Homeostatic regulation of the endoneurial microenvironment during development, aging and in response to trauma, disease and toxic insult.

The endoneurial microenvironment, delimited by the endothelium of endoneurial vessels and a multi-layered ensheathing perineurium, is a specialized milieu intérieur within which axons, associated Schwann cells and other resident cells of peripheral nerves function. The endothelium and perineurium restricts as well as regulates exchange of material between the endoneurial microenvironment and the surrounding extracellular space and thus is more appropriately described as a blood-nerve interface (BNI) rather than a blood-nerve barrier (BNB). Input to and output from the endoneurial microenvironment occurs via blood-nerve exchange and convective endoneurial fluid flow driven by a proximo-distal hydrostatic pressure gradient. The independent regulation of the endothelial and perineurial components of the BNI during development, aging and in response to trauma is consistent with homeostatic regulation of the endoneurial microenvironment. Pathophysiological alterations of the endoneurium in experimental allergic neuritis (EAN), and diabetic and lead neuropathy are considered to be perturbations of endoneurial homeostasis. The interactions of Schwann cells, axons, macrophages, and mast cells via cell-cell and cell-matrix signaling regulate the permeability of this interface. A greater knowledge of the dynamic nature of tight junctions and the factors that induce and/or modulate these key elements of the BNI will increase our understanding of peripheral nerve disorders as well as stimulate the development of therapeutic strategies to treat these disorders.

MTM1 mutation associated with X-linked myotubular myopathy in Labrador Retrievers.

Mutations in the MTM1 gene encoding myotubularin cause X-linked myotubular myopathy (XLMTM), a well-defined subtype of human centronuclear myopathy. Seven male Labrador Retrievers, age 14-26 wk, were clinically evaluated for generalized weakness and muscle atrophy. Muscle biopsies showed variability in fiber size, centrally placed nuclei resembling fetal myotubes, and subsarcolemmal ringed and central dense areas highlighted with mitochondrial specific reactions. Ultrastructural studies confirmed the centrally located nuclei, abnormal perinuclear structure, and mitochondrial accumulations. Wild-type triads were infrequent, with most exhibiting an abnormal orientation of T tubules. MTM1 gene sequencing revealed a unique exon 7 variant in all seven affected males, causing a nonconservative missense change, p.N155K, which haplotype data suggest derives from a recent founder in the local population. Analysis of a worldwide panel of 237 unaffected Labrador Retrievers and 59 additional control dogs from 25 other breeds failed to identify this variant, supporting it as the pathogenic mutation. Myotubularin protein levels and localization were abnormal in muscles from affected dogs, and expression of GFP-MTM1 p.N155K in COS-1 cells showed that the mutant protein was sequestered in proteasomes, where it was presumably misfolded and prematurely degraded. These data demonstrate that XLMTM in Labrador Retrievers is a faithful genetic model of the human condition.

Nesprin 1 is critical for nuclear positioning and anchorage.

Nesprin 1 is an outer nuclear membrane protein that is thought to link the nucleus to the actin cytoskeleton. Recent data suggest that mutations in Nesprin 1 may also be involved in the pathogenesis of Emery-Dreifuss muscular dystrophy. To investigate the function of Nesprin 1 in vivo, we generated a mouse model in which all isoforms of Nesprin 1 containing the C-terminal spectrin-repeat region with or without KASH domain were ablated. Nesprin 1 knockout mice are marked by decreased survival rates, growth retardation and increased variability in body weight. Additionally, nuclear positioning and anchorage are dysfunctional in skeletal muscle from knockout mice. Physiological testing demonstrated no significant reduction in stress production in Nesprin 1-deficient skeletal muscle in either neonatal or adult mice, but a significantly lower exercise capacity in knockout mice. Nuclear deformation testing revealed ineffective strain transmission to nuclei in muscle fibers lacking Nesprin 1. Overall, our data show that Nesprin 1 is essential for normal positioning and anchorage of nuclei in skeletal muscle.

Vacuolar myopathy in a dog resembling human sporadic inclusion body myositis.

Sporadic inclusion body myositis (sIBM) is the most common myopathy in people over the age of 50 years. While immune-mediated inflammatory myopathies are well documented in dogs, sIBM has not been described. An 11-year-old dog with chronic and progressive neuromuscular dysfunction was evaluated for evidence of sIBM using current pathologic, immunohistochemical and electron microscopic diagnostic criteria. Vacuoles and congophilic intracellular inclusions were identified in cryostat sections of multiple muscle biopsies and immunostained with antibodies against amyloid-beta peptide, amyloid-beta precursor protein, and proteosome 20S of the ubiquitin-proteosome system. Cellular infiltration and increased expression of MHC Class I antigen were observed. Cytoplasmic filamentous inclusions, membranous structures, and myeloid bodies were identified ultrastructurally. These observations constitute the first evidence that both the inflammatory and degenerative features of human sIBM can occur in a non-human species.

Peripheral neuropathy in rats exposed to dichloroacetate.

The use of dichloroacetate (DCA) for treating patients with mitochondrial diseases is limited by the induction of peripheral neuropathy. The mechanisms of DCA-induced neuropathy are not known. Oral DCA treatment (50-500 mg/kg per day for up to 16 weeks) induced tactile allodynia in both juvenile and adult rats; concurrent thermal hypoalgesia developed at higher doses. Both juvenile and adult rats treated with DCA developed nerve conduction slowing that was more pronounced in adult rats. No overt axonal or glial cell abnormalities were identified in peripheral nerves or spinal cord of any DCA-treated rat, but morphometric analysis identified a reduction of mean axonal caliber of peripheral nerve myelinated fibers. Dichloroacetate treatment also caused accumulation of oxidative stress markers in the nerves. These data indicate that behavioral, functional, and structural indices of peripheral neuropathy may be induced in both juvenile and adult rats treated with DCA at doses similar to those in clinical use. Dichloroacetate-induced peripheral neuropathy primarily afflicts axons and involves both metabolic and structural disorders. The DCA-treated rat may provide insight into the pathogenesis of this peripheral neuropathy and facilitate development of adjuvant therapeutics to prevent this disorder that currently restricts the clinical use of DCA.

Secretion of amyloidogenic gelsolin progressively compromises protein homeostasis leading to the intracellular aggregation of proteins.

Familial amyloidosis of Finnish type (FAF) is a systemic amyloid disease associated with the deposition of proteolytic fragments of mutant (D187N/Y) plasma gelsolin. We report a mouse model of FAF featuring a muscle-specific promoter to drive D187N gelsolin synthesis. This model recapitulates the aberrant endoproteolytic cascade and the aging-associated extracellular amyloid deposition of FAF. Amyloidogenesis is observed only in tissues synthesizing human D187N gelsolin, despite the presence of full-length D187N gelsolin and its 68-kDa cleavage product in blood-demonstrating the importance of local synthesis in FAF. Loss of muscle strength was progressive in homozygous D187N gelsolin mice. The presence of misfolding-prone D187N gelsolin appears to exacerbate the age-associated decline in cellular protein homeostasis (proteostasis), reflected by the intracellular deposition of numerous proteins, a characteristic of the most common degenerative muscle disease of aging humans, sporadic inclusion body myositis.

Demyelinating polyneuropathy with focally folded myelin sheaths in a family of Miniature Schnauzer dogs.

A spontaneous demyelinating polyneuropathy in two young Miniature Schnauzer dogs was characterized clinically, electrophysiologically and histopathologically. Both dogs were related and a third dog, belonging to the same family, had similar clinical signs. On presentation, clinical signs were restricted to respiratory dysfunction. Electrophysiological tests showed a dramatic decrease in both motor and sensory nerve conduction velocities. Microscopic examination of peripheral nerve biopsies (light and electron microscopy, teased nerve fibers), showed that this neuropathy was characterized by segmental demyelination and focally folded myelin sheaths. Various clinical syndromes associated with tomacula or focal thickening of the myelin sheath of the peripheral nerves have been described in humans and shown to be caused by gene mutations affecting the myelin proteins, such as the hereditary neuropathy with liability to pressure palsies or the demyelinating forms of Charcot-Marie-Tooth disease. In animals, a tomaculous neuropathy has been reported in cattle and chickens but not in carnivores. Here we report a demyelinating peripheral neuropathy with tomacula in two Miniature Schnauzer dogs.

Dissociation of thermal hypoalgesia and epidermal denervation in streptozotocin-diabetic mice.

The quantification of epidermal innervation, which consists primarily of heat-sensitive C-fibers, is emerging as a tool for diagnosing and staging diabetic neuropathy. However, the relationship between changes in heat sensitivity and changes in epidermal innervation has not yet been adequately explored. Therefore, we assessed epidermal nerve fiber density and thermal withdrawal latency in the hind paw of Swiss Webster mice after 2 and 4 weeks of streptozotocin-induced diabetes. Thermal hypoalgesia developed after only 2 weeks of diabetes, but a measurable reduction in PGP9.5-immunoreactive epidermal nerve fiber density did not appear until 4 weeks. These data suggest that impaired epidermal nociceptor function contributes to early diabetes-induced thermal hypoalgesia prior to the loss of peripheral terminals.

Impaired prosaposin secretion during nerve regeneration in diabetic rats and protection of nerve regeneration by a prosaposin-derived peptide.

Prosaposin is both a precursor of sphingolipid activator proteins and a secreted neurotrophic and myelinotrophic factor. Because peripheral nerve regeneration is impaired in diabetes mellitus, we measured prosaposin protein levels from control and streptozotocin-diabetic rats by collecting endoneurial fluid secreted into a bridging tube connecting the ends of transected sciatic nerve. Prosaposin protein levels were significantly reduced in endoneurial fluid from diabetic rats and increased in the proximal nerve stump compared to controls. To investigate whether a prosaposin-derived peptide could improve nerve regeneration, rats were treated with prosaptide TX14(A) after sciatic nerve crush. In control rats, TX14(A) was without effect in the uninjured nerve but shortened toe spread recovery time after nerve crush. In diabetic rats, efficacy of prosaptide TX14(A) was confirmed by correction of thermal hypoalgesia, formalin-evoked hyperalgesia, and conduction slowing in the uninjured nerve. The peptide also prevented diabetes-induced abnormalities in nerve regeneration distance and mean axonal diameter of regenerated axons, whereas delayed recovery of toe spread was not improved. Muscle denervation atrophy was attenuated by TX14(A) in both control and diabetic rats. These results suggest that reduced prosaposin secretion after nerve injury may contribute to impaired regeneration rates in diabetic rats, and that prosaptide TX14(A) can improve aspects of nerve regeneration.

Elevated lipid peroxidation and DNA oxidation in nerve from diabetic rats: effects of aldose reductase inhibition, insulin, and neurotrophic factors.

We investigated the effect of treatment with an aldose reductase inhibitor, insulin, or select neurotrophic factors on the generation of oxidative damage in peripheral nerve. Rats were either treated with streptozotocin to induce insulin-deficient diabetes or fed with a diet containing 40% d-galactose to promote hexose metabolism by aldose reductase. Initial time course studies showed that lipid peroxidation and DNA oxidation were significantly elevated in sciatic nerve after 1 week or 2 weeks of streptozotocin-induced diabetes, respectively, and that both remained elevated after 12 weeks of diabetes. The increase in nerve lipid peroxidation was completely prevented or reversed by treatment with the aldose reductase inhibitor, ICI 222155, or by insulin, but not by the neurotrophic factors, prosaptide TX14(A) or neurotrophin-3. The increase in nerve DNA oxidation was significantly prevented by insulin treatment. In contrast, up to 16 weeks of galactose feeding did not alter nerve lipid peroxidation or protein oxidation, despite evidence of ongoing nerve conduction deficits. These observations demonstrate that nerve oxidative damage develops early after the onset of insulin-deficient diabetes and that it is not induced by increased hexose metabolism by aldose reductase per se, but rather is a downstream consequence of flux through this enzyme. Furthermore, the beneficial effect of prosaptide TX14(A) and neurotrophin-3 on nerve function and structure in diabetic rats is not due to amelioration of increased lipid peroxidation.

Epidermal nerve fiber quantification in the assessment of diabetic neuropathy.

Assessment of cutaneous innervation in skin biopsies is emerging as a valuable means of both diagnosing and staging diabetic neuropathy. Immunolabeling, using antibodies to neuronal proteins such as protein gene product 9.5, allows for the visualization and quantification of intraepidermal nerve fibers. Multiple studies have shown reductions in intraepidermal nerve fiber density in skin biopsies from patients with both type 1 and type 2 diabetes. More recent studies have focused on correlating these changes with other measures of diabetic neuropathy. A loss of epidermal innervation similar to that observed in diabetic patients has been observed in rodent models of both type 1 and type 2 diabetes and several therapeutics have been reported to prevent reductions in intraepidermal nerve fiber density in these models. This review discusses the current literature describing diabetes-induced changes in cutaneous innervation in both human and animal models of diabetic neuropathy.

The ER-bound RING finger protein 5 (RNF5/RMA1) causes degenerative myopathy in transgenic mice and is deregulated in inclusion body myositis.

Growing evidence supports the importance of ubiquitin ligases in the pathogenesis of muscular disorders, although underlying mechanisms remain largely elusive. Here we show that the expression of RNF5 (aka RMA1), an ER-anchored RING finger E3 ligase implicated in muscle organization and in recognition and processing of malfolded proteins, is elevated and mislocalized to cytoplasmic aggregates in biopsies from patients suffering from sporadic-Inclusion Body Myositis (sIBM). Consistent with these findings, an animal model for hereditary IBM (hIBM), but not their control littermates, revealed deregulated expression of RNF5. Further studies for the role of RNF5 in the pathogenesis of s-IBM and more generally in muscle physiology were performed using RNF5 transgenic and KO animals. Transgenic mice carrying inducible expression of RNF5, under control of beta-actin or muscle specific promoter, exhibit an early onset of muscle wasting, muscle degeneration and extensive fiber regeneration. Prolonged expression of RNF5 in the muscle also results in the formation of fibers containing congophilic material, blue-rimmed vacuoles and inclusion bodies. These phenotypes were associated with altered expression and activity of ER chaperones, characteristic of myodegenerative diseases such as s-IBM. Conversely, muscle regeneration and induction of ER stress markers were delayed in RNF5 KO mice subjected to cardiotoxin treatment. While supporting a role for RNF5 Tg mice as model for s-IBM, our study also establishes the importance of RNF5 in muscle physiology and its deregulation in ER stress associated muscular disorders.

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.