Experimental diabetic neuropathy: impairment of slow transport with changes in axon cross-sectional area

Alzheimer's disease: an axonal injury disease?

Alzheimer's disease (AD) is the primary cause of dementia and is anticipated to impose a substantial economic burden in the future. Over a significant period, the widely accepted amyloid cascade hypothesis has guided research efforts, and the recent FDA approval of an anti- amyloid-beta (Aβ) protofibrils antibody, believed to decelerate AD progression, has further solidified its significance. However, the excessive emphasis placed on the amyloid cascade hypothesis has overshadowed the physiological nature of Aβ and tau proteins within axons. Axons, specialized neuronal structures, sustain damage during the early stages of AD, exerting a pivotal influence on disease progression. In this review, we present a comprehensive summary of the relationship between axonal damage and AD pathology, amalgamating the physiological roles of Aβ and tau proteins, along with the impact of AD risk genes such as APOE and TREM2. Furthermore, we underscore the exceptional significance of axonal damage in the context of AD.

Axonal transport deficits in the pathogenesis of diabetic peripheral neuropathy

Diabetic peripheral neuropathy (DPN) is a chronic and prevalent metabolic disease that gravely endangers human health and seriously affects the quality of life of hyperglycemic patients. More seriously, it can lead to amputation and neuropathic pain, imposing a severe financial burden on patients and the healthcare system. Even with strict glycemic control or pancreas transplantation, peripheral nerve damage is difficult to reverse. Most current treatment options for DPN can only treat the symptoms but not the underlying mechanism. Patients with long-term diabetes mellitus (DM) develop axonal transport dysfunction, which could be an important factor in causing or exacerbating DPN. This review explores the underlying mechanisms that may be related to axonal transport impairment and cytoskeletal changes caused by DM, and the relevance of the latter with the occurrence and progression of DPN, including nerve fiber loss, diminished nerve conduction velocity, and impaired nerve regeneration, and also predicts possible therapeutic strategies. Understanding the mechanisms of diabetic neuronal injury is essential to prevent the deterioration of DPN and to develop new therapeutic strategies. Timely and effective improvement of axonal transport impairment is particularly critical for the treatment of peripheral neuropathies.

AAV1.NT-3 gene therapy in a CMT2D model: phenotypic improvements in GarsP278KY/+ mice

Glycyl-tRNA synthetase mutations are associated to the Charcot-Marie-Tooth disease type-2D. The Gars^P278KY/+ model for Charcot-Marie-Tooth disease type-2D is known best for its early onset severe neuropathic phenotype with findings including reduced axon size, slow conduction velocities and abnormal neuromuscular junction. Muscle involvement remains largely unexamined. We tested the efficacy of neurotrophin 3 gene transfer therapy in two Gars mutants with severe (Gars^P278KY/+ ) and milder (Gars^ΔETAQ/+ ) phenotypes via intramuscular injection of adeno-associated virus setoype-1, triple tandem muscle creatine kinase promoter, neurotrophin 3 (AAV1.tMCK.NT-3) at 1 × 10¹¹ vg dose. In the Gars^P278KY/+ mice, the treatment efficacy was assessed at 12 weeks post-injection using rotarod test, electrophysiology and detailed quantitative histopathological studies of the peripheral nervous system including neuromuscular junction and muscle. Neurotrophin 3 gene transfer therapy in Gars^P278KY/+ mice resulted in significant functional and electrophysiological improvements, supported with increases in myelin thickness and improvements in the denervated status of neuromuscular junctions as well as increases in muscle fibre size along with attenuation of myopathic changes. Improvements in the milder phenotype Gars^ΔETAQ/+ was less pronounced. Furthermore, oxidative enzyme histochemistry in muscles from Gars mutants revealed alterations in the content and distribution of oxidative enzymes with increased expression levels of Pgc1a. Cox1, Cox3 and Atp5d transcripts were significantly decreased suggesting that the muscle phenotype might be related to mitochondrial dysfunction. Neurotrophin 3 gene therapy attenuated these abnormalities in the muscle. This study shows that neurotrophin 3 gene transfer therapy has disease modifying effect in a mouse model for Charcot-Marie-Tooth disease type-2D, leading to meaningful improvements in peripheral nerve myelination and neuromuscular junction integrity as well as in a unique myopathic process, associated with mitochondria dysfunction, all in combination contributing to functional outcome. Based on the multiple biological effects of this versatile molecule, we predict neurotrophin 3 has the potential to be beneficial in other aminoacyl-tRNA synthetase-linked Charcot-Marie-Tooth disease subtypes.

Association of aerobic glycolysis with the structural connectome reveals a benefit-risk balancing mechanism in the human brain

Aerobic glycolysis (AG), that is, the nonoxidative metabolism of glucose, contributes significantly to anabolic pathways, rapid energy generation, task-induced activity, and neuroprotection; yet high AG is also associated with pathological hallmarks such as amyloid-β deposition. An important yet unresolved question is whether and how the metabolic benefits and risks of brain AG is structurally shaped by connectome wiring. Using positron emission tomography and magnetic resonance imaging techniques as well as computational models, we investigate the relationship between brain AG and the macroscopic connectome. Specifically, we propose a weighted regional distance-dependent model to estimate the total axonal projection length of a brain node. This model has been validated in a macaque connectome derived from tract-tracing data and shows a high correspondence between experimental and estimated axonal lengths. When applying this model to the human connectome, we find significant associations between the estimated total axonal projection length and AG across brain nodes, with higher levels primarily located in the default-mode and prefrontal regions. Moreover, brain AG significantly mediates the relationship between the structural and functional connectomes. Using a wiring optimization model, we find that the estimated total axonal projection length in these high-AG regions exhibits a high extent of wiring optimization. If these high-AG regions are randomly rewired, their total axonal length and vulnerability risk would substantially increase. Together, our results suggest that high-AG regions have expensive but still optimized wiring cost to fulfill metabolic requirements and simultaneously reduce vulnerability risk, thus revealing a benefit-risk balancing mechanism in the human brain.

Diabetes Mellitus-Related Dysfunction of the Motor System

Although motor deficits in humans with diabetic neuropathy have been extensively researched, its effect on the motor system is thought to be lesser than that on the sensory system. Therefore, motor deficits are considered to be only due to sensory and muscle impairment. However, recent clinical and experimental studies have revealed that the brain and spinal cord, which are involved in the motor control of voluntary movement, are also affected by diabetes. This review focuses on the most important systems for voluntary motor control, mainly the cortico-muscular pathways, such as corticospinal tract and spinal motor neuron abnormalities. Specifically, axonal damage characterized by the proximodistal phenotype occurs in the corticospinal tract and motor neurons with long axons, and the transmission of motor commands from the brain to the muscles is impaired. These findings provide a new perspective to explain motor deficits in humans with diabetes. Finally, pharmacological and non-pharmacological treatment strategies for these disorders are presented.

Impairment of Axonal Transport in Diabetes: Focus on the Putative Mechanisms Underlying Peripheral and Central Neuropathies

Diabetes mellitus is a chronic disease with numerous complications that severely impact on the quality of life of patients. Different neuropathies may arise as complications associated with the nervous system, both peripherally and at the central level. The mechanisms behind these neuronal complications are far from being clarified, but axonal transport impairment, a vital process for neuronal physiology, has been described in the context of experimental diabetes. Alterations in neuronal cytoskeleton and motor proteins, deficits in ATP supply or neuroinflammation, as processes that disturb the effective transport of cargoes along the axon, were reported as putative causes of axonal impairment, ultimately leading to axonal degeneration. The main goal of the present review is to reunite the main studies in the literature exploring diabetes-induced alterations likely involved in axonal transport deficits, and call the attention for the uttermost importance of further exploring the field. Understanding the mechanisms underlying neuronal deficits in diabetes is crucial for the development of new therapeutic strategies to prevent neuronal degeneration in diabetes and related neuropathies.

Effect of streptozotocin-induced diabetes on motor representations in the motor cortex and corticospinal tract in rats

Motor disorders in patients with diabetes are associated with diabetic peripheral neuropathy, which can lead to symptoms such as lower extremity weakness. However, it is unclear whether central motor system disorders can disrupt motor function in patients with diabetes. In a streptozotocin-induced rat model of type 1 diabetes, we used intracortical microstimulation to evaluate motor representations in the motor cortex, recorded antidromic motor cortex responses to spinal cord stimulation to evaluate the function of corticospinal tract (CST) axons, and used retrograde labeling to evaluate morphological alterations of CST neurons. The diabetic rats exhibited size reductions in the hindlimb area at 4 weeks and in trunk and forelimb areas after 13 weeks, with the hindlimb and trunk area reductions being the most severe. Other areas were unaffected. Additionally, we observed reduced antidromic responses in CST neurons with axons projecting to lumbar spinal segments (CST-L) but not in those with axons projecting to cervical segments (CST-C). This was consistent with the observation that retrograde-labeled CST-L neurons were decreased in number following tracer injection into the spinal cord in diabetic animals but that CST-C neurons were preserved. These results show that diabetes disrupts the CST system components controlling hindlimb and trunk movement. This disruption may contribute to lower extremity weakness in patients.

In Vivo Evaluation of the Visual Pathway in Streptozotocin-Induced Diabetes by Diffusion Tensor MRI and Contrast Enhanced MRI

Visual function has been shown to deteriorate prior to the onset of retinopathy in some diabetic patients and experimental animal models. This suggests the involvement of the brain's visual system in the early stages of diabetes. In this study, we tested this hypothesis by examining the integrity of the visual pathway in a diabetic rat model using in vivo multi-modal magnetic resonance imaging (MRI). Ten-week-old Sprague-Dawley rats were divided into an experimental diabetic group by intraperitoneal injection of 65 mg/kg streptozotocin in 0.01 M citric acid, and a sham control group by intraperitoneal injection of citric acid only. One month later, diffusion tensor MRI (DTI) was performed to examine the white matter integrity in the brain, followed by chromium-enhanced MRI of retinal integrity and manganese-enhanced MRI of anterograde manganese transport along the visual pathway. Prior to MRI experiments, the streptozotocin-induced diabetic rats showed significantly smaller weight gain and higher blood glucose level than the control rats. DTI revealed significantly lower fractional anisotropy and higher radial diffusivity in the prechiasmatic optic nerve of the diabetic rats compared to the control rats. No apparent difference was observed in the axial diffusivity of the optic nerve, the chromium enhancement in the retina, or the manganese enhancement in the lateral geniculate nucleus and superior colliculus between groups. Our results suggest that streptozotocin-induced diabetes leads to early injury in the optic nerve when no substantial change in retinal integrity or anterograde transport along the visual pathways was observed in MRI using contrast agent enhancement. DTI may be a useful tool for detecting and monitoring early pathophysiological changes in the visual system of experimental diabetes non-invasively.

Axon Transport and Neuropathy: Relevant Perspectives on the Etiopathogenesis of Familial Dysautonomia

Peripheral neuropathies are highly prevalent and are most often associated with chronic disease, side effects from chemotherapy, or toxic-metabolic abnormalities. Neuropathies are less commonly caused by genetic mutations, but studies of the normal function of mutated proteins have identified particular vulnerabilities that often implicate mitochondrial dynamics and axon transport mechanisms. Hereditary sensory and autonomic neuropathies are a group of phenotypically related diseases caused by monogenic mutations that primarily affect sympathetic and sensory neurons. Here, I review evidence to indicate that many genetic neuropathies are caused by abnormalities in axon transport. Moreover, in hereditary sensory and autonomic neuropathies. There may be specific convergence on gene mutations that disrupt nerve growth factor signaling, upon which sympathetic and sensory neurons critically depend.

Mechanisms of distal axonal degeneration in peripheral neuropathies

Peripheral neuropathy is a common complication of a variety of diseases and treatments, including diabetes, cancer chemotherapy, and infectious causes (HIV, hepatitis C, and Campylobacter jejuni). Despite the fundamental difference between these insults, peripheral neuropathy develops as a combination of just six primary mechanisms: altered metabolism, covalent modification, altered organelle function and reactive oxygen species formation, altered intracellular and inflammatory signaling, slowed axonal transport, and altered ion channel dynamics and expression. All of these pathways converge to lead to axon dysfunction and symptoms of neuropathy. The detailed mechanisms of axon degeneration itself have begun to be elucidated with studies of animal models with altered degeneration kinetics, including the slowed Wallerian degeneration (Wld(S)) and Sarm knockout animal models. These studies have shown axonal degeneration to occur through a programmed pathway of injury signaling and cytoskeletal degradation. Insights into the common disease insults that converge on the axonal degeneration pathway promise to facilitate the development of therapeutics that may be effective against other mechanisms of neurodegeneration.

Axon diameter and axonal transport: In vivo and in vitro effects of androgens

Testosterone is a sex hormone involved in brain maturation via multiple molecular mechanisms. Previous human studies described age-related changes in the overall volume and structural properties of white matter during male puberty. Based on this work, we have proposed that testosterone may induce a radial growth of the axon and, possibly, modulate axonal transport. In order to determine whether this is the case we have used two different experimental approaches. With electron microscopy, we have evaluated sex differences in the structural properties of axons in the corpus callosum (splenium) of young rats, and tested consequences of castration carried out after weaning. Then we examined in vitro the effect of the non-aromatizable androgen Mibolerone on the structure and bidirectional transport of wheat-germ agglutinin vesicles in the axons of cultured sympathetic neurons. With electron microscopy, we found robust sex differences in axonal diameter (males>females) and g ratio (males>females). Removal of endogenous testosterone by castration was associated with lower axon diameter and lower g ratio in castrated (vs. intact) males. In vitro, Mibolerone influenced the axonal transport in a time- and dose-dependent manner, and increased the axon caliber as compared with vehicle-treated neurons. These findings are consistent with the role of testosterone in shaping the axon by regulating its radial growth, as predicted by the initial human studies.

Surgical decompression of painful diabetic peripheral neuropathy: the role of pain distribution

Objective

To investigate the effect of surgical decompression on painful diabetic peripheral neuropathy (DPN) patients and discuss the role which pain distribution and characterization play in the management of painful DPN as well as the underlying mechanism involved.

Methods

A total of 306 patients with painful diabetic lower-extremity neuropathy were treated with Dellon surgical nerve decompression in our department. Clinical evaluation including Visual analogue scale (VAS), Brief Pain Inventory Short Form for diabetic peripheral neuropathy (BPI-DPN) questionnaire, two-point discrimination (2-PD), nerve conduction velocity (NCV) and high-resolution ultrasonography (cross-sectional area, CSA) were performed in all cases preoperatively, and at 6 month intervals for 2 years post-decompression. The patients who underwent surgery were retrospectively assigned into two subgroups (focal and diffuse pain) according to the distribution of the diabetic neuropathic pain. The control group included 92 painful DPN patients without surgery.

Results

The levels of VAS, scores in BPI-DPN, 2-PD, NCV results and CSA were all improved in surgical group when compared to the control group (P<0.05). More improvement of VAS, scores in BPI-DPN and CSA was observed in focal pain group than that in diffuse group (P<0.05).

Conclusions

Efficacy of decompression of multiple lower-extremity peripheral nerves in patients with painful diabetic neuropathy was confirmed in this study. While both focal and diffuse group could benefit from surgical decompression, pain relief and morphological restoration could be better achieved in focal group.

Mitochondrial dynamics in peripheral neuropathies

SIGNIFICANCE

Mitochondrial dynamics describes the continuous change in the position, size, and shape of mitochondria within cells. The morphological and functional complexity of neurons, the remarkable length of their processes, and the rapid changes in metabolic requirements arising from their intrinsic excitability render these cells particularly dependent on effective mitochondrial function and positioning. The rules that govern these changes and their functional significance are not fully understood, yet the dysfunction of mitochondrial dynamics has been implicated as a pathogenetic factor in a number of diseases, including disorders of the central and peripheral nervous systems.

RECENT ADVANCES

In recent years, a number of mutations of genes encoding proteins that play important roles in mitochondrial dynamics and function have been discovered in patients with Charcot-Marie-Tooth (CMT) disease, a hereditary peripheral neuropathy. These findings have directly linked mitochondrial pathology to the pathology of peripheral nerve and have identified certain aspects of mitochondrial dynamics as potential early events in the pathogenesis of CMT. In addition, mitochondrial dysfunction has now been implicated in the pathogenesis of noninherited neuropathies, including diabetic and inflammatory neuropathies.

CRITICAL ISSUES

The role of mitochondria in peripheral nerve diseases has been mostly examined in vitro, and less so in animal models.

FUTURE DIRECTIONS

This review examines available evidence for the role of mitochondrial dynamics in the pathogenesis of peripheral neuropathies, their relevance in human diseases, and future challenges for research in this field.

Signaling mechanisms and disrupted cytoskeleton in the diphenyl ditelluride neurotoxicity

Evidence from our group supports that diphenyl ditelluride (PhTe)₂ neurotoxicity depends on modulation of signaling pathways initiated at the plasma membrane. The (PhTe)₂-evoked signal is transduced downstream of voltage-dependent Ca²⁺ channels (VDCC), N-methyl-D-aspartate receptors (NMDA), or metabotropic glutamate receptors activation via different kinase pathways (protein kinase A, phospholipase C/protein kinase C, mitogen-activated protein kinases (MAPKs), and Akt signaling pathway). Among the most relevant cues of misregulated signaling mechanisms evoked by (PhTe)₂ is the cytoskeleton of neural cells. The in vivo and in vitro exposure to (PhTe)₂ induce hyperphosphorylation/hypophosphorylation of neuronal and glial intermediate filament (IF) proteins (neurofilaments and glial fibrillary acidic protein, resp.) in different brain structures of young rats. Phosphorylation of IFs at specific sites modulates their association/disassociation and interferes with important physiological roles, such as axonal transport. Disrupted cytoskeleton is a crucial marker of neurodegeneration and is associated with reactive astrogliosis and apoptotic cell death. This review focuses the current knowledge and important results on the mechanisms of (PhTe)₂ neurotoxicity with special emphasis on the cytoskeletal proteins and their differential regulation by kinases/phosphatases and Ca²⁺-mediated mechanisms in developmental rat brain. We propose that the disrupted cytoskeletal homeostasis could support brain damage provoked by this neurotoxicant.

Myosin Va but Not nNOSα is Significantly Reduced in Jejunal Musculomotor Nerve Terminals in Diabetes Mellitus

Nitric oxide (NO) mediated slow inhibitory junction potential and mechanical relaxation after electrical field stimulation (EFS) is impaired in diabetes mellitus. Externally added NO donor restore nitrergic function, indicating that this reduction result from diminution of NO synthesis within the pre-junctional nerve terminals. The present study aimed to investigate two specific aims that may potentially provide pathophysiological insights into diabetic nitrergic neuropathy. Specifically, alteration in nNOSα contents within jejunal nerve terminals and a local subcortical transporter myosin Va was tested 16 weeks after induction of diabetes by low dose streptozotocin (STZ) in male Wistar rats. The results show that diabetic rats, in contrast to vehicle treated animals, have: (a) nearly absent myosin Va expression in nerve terminals of axons innervating smooth muscles and (b) significant decrease of myosin Va in neuronal soma of myenteric plexus. In contrast, nNOSα staining in diabetic jejunum neuromuscular strips showed near intact expression in neuronal cell bodies. The space occupancy of nitrergic nerve fibers was comparable between groups. Normal concentration of nNOSα was visualized within a majority of nitrergic terminals in diabetes, suggesting intact axonal transport of nNOSα to distant nerve terminals. These results reveal the dissociation between presences of nNOSα in the nerve terminals but deficiency of its transporter myosin Va in the jejunum of diabetic rats. This significant observation of reduced motor protein myosin Va within jejunal nerve terminals may potentially explain impairment of pre-junctional NO synthesis during EFS of diabetic gut neuromuscular strips despite presence of the nitrergic synthetic enzyme nNOSα.

Optic nerve crush induces spatial and temporal gene expression patterns in retina and optic nerve of BALB/cJ mice

Background

Central nervous system (CNS) trauma and neurodegenerative disorders trigger a cascade of cellular and molecular events resulting in neuronal apoptosis and regenerative failure. The pathogenic mechanisms and gene expression changes associated with these detrimental events can be effectively studied using a rodent optic nerve crush (ONC) model. The purpose of this study was to use a mouse ONC model to: (a) evaluate changes in retina and optic nerve (ON) gene expression, (b) identify neurodegenerative pathogenic pathways and (c) discover potential new therapeutic targets.

Results

Only 54% of total neurons survived in the ganglion cell layer (GCL) 28 days post crush. Using Bayesian Estimation of Temporal Regulation (BETR) gene expression analysis, we identified significantly altered expression of 1,723 and 2,110 genes in the retina and ON, respectively. Meta-analysis of altered gene expression (≥1.5, ≤-1.5, p < 0.05) using Partek and DAVID demonstrated 28 up and 20 down-regulated retinal gene clusters and 57 up and 41 down-regulated optic nerve clusters. Regulated gene clusters included regenerative change, synaptic plasticity, axonogenesis, neuron projection, and neuron differentiation. Expression of selected genes (Vsnl1, Syt1, Synpr and Nrn1) from retinal and ON neuronal clusters were quantitatively and qualitatively examined for their relation to axonal neurodegeneration by immunohistochemistry and qRT-PCR.

Conclusion

A number of detrimental gene expression changes occur that contribute to trauma-induced neurodegeneration after injury to ON axons. Nrn1 (synaptic plasticity gene), Synpr and Syt1 (synaptic vesicle fusion genes), and Vsnl1 (neuron differentiation associated gene) were a few of the potentially unique genes identified that were down-regulated spatially and temporally in our rodent ONC model. Bioinformatic meta-analysis identified significant tissue-specific and time-dependent gene clusters associated with regenerative changes, synaptic plasticity, axonogenesis, neuron projection, and neuron differentiation. These ONC induced neuronal loss and regenerative failure associated clusters can be extrapolated to changes occurring in other forms of CNS trauma or in clinical neurodegenerative pathological settings. In conclusion, this study identified potential therapeutic targets to address two key mechanisms of CNS trauma and neurodegeneration: neuronal loss and regenerative failure.

Surgical treatment of superimposed, lower extremity, peripheral nerve entrapments with diabetic and idiopathic neuropathy

BACKGROUND

Although it is recognized that people with peripheral neuropathy have an increased prevalence of chronic nerve entrapment, controversy still exists over their management. The present report details the evaluation, surgical approach, and outcome of a large cohort of people with diabetic and with idiopathic neuropathy.

METHODS

A retrospective review of 158 consecutive patients, 96 with diabetic and 62 with idiopathic neuropathy, was done to analyze the results of neurolysis of multiple sites of chronic nerve compression in the lower extremity. Of these patients, 50 had a contralateral limb decompressed for a total of 208 limbs included in the study. Outcomes included visual analog scale (VAS) for pain in the 109 patients who had pain level greater than 8.0, measurement of the cutaneous pressure threshold for sensibility, self-reported change in pain medication usage, and self-reported change in balance.

RESULTS

With a minimum follow-up of 1 year, 88% of patients with preoperative numbness reported improvement in sensation (P < 0.001). Of the 84 patients with impaired balance, 81% reported improvement in balance. Of those whose VAS was greater than 8, 83% reported an improvement in VAS (P < 0.001). There was a concomitant reduction in pain medication usage. There was no difference in outcomes between patients with diabetic versus idiopathic neuropathy in response to nerve decompression.

CONCLUSIONS

Neurolysis of lower extremity chronic nerve compressions in patients with neuropathy and superimposed nerve compressions is an effective method for relieving pain, restoring sensation, and improving balance.

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.

Impaired slow axonal transport in diabetic peripheral nerve is independent of RAGE

Diabetic peripheral nerve dysfunction is a common complication occurring in 30-50% of long-term diabetic patients. The pathogenesis of this dysfunction remains unclear but growing evidence suggests that it might be attributed, in part, to alteration in axonal transport. Our previous studies demonstrated that RAGE (Receptor for Advanced Glycation Endproducts) contributes to the pathogenesis of diabetic peripheral neuropathy and impairs nerve regeneration consequent to sciatic nerve crush, particularly in diabetes. We hypothesize that RAGE plays a role in axonal transport impairment via the interaction of its cytoplasmic domain with mammalian Diaphanous 1 (mDia1) - actin interacting molecule. Studies showed that mDia1-RAGE interaction is necessary for RAGE-ligand-dependent cellular migration, AKT phosphorylation, macrophage inflammatory response and smooth muscle migration. Here, we studied RAGE, mDia1 and markers of axonal transport rates in the peripheral nerves of wild-type C57BL/6 and RAGE null control and streptozotocin-injected diabetic mice at 1, 3 and 6 h after sciatic nerve crush. The results show that in both control and diabetic nerves, the amount of RAGE accumulated at the proximal and distal side of the crush area is similar, indicating that the recycling rate for RAGE is very high and that it is evenly transported from and towards the neuronal cell body. Furthermore, we show that slow axonal transport of proteins such as Neurofilament is affected by diabetes in a RAGE-independent manner. Finally, our study demonstrates that mDia1 axonal transport is impaired in diabetes, suggesting that diabetes-related changes affecting actin binding proteins occur early in the course of the disease.

Impaired nerve fiber regeneration in axotomized peripheral nerves in streptozotocin-diabetic rats

Aim/Introduction

Impaired nerve fiber regeneration is a salient feature of diabetic neuropathy. Its pathogenesis is still unclear. We attempted to characterize the structure of regenerated myelinated fibers after transection in streptozotocin‐diabetic rats.

Materials and Methods

Streptozotocin‐diabetic rats underwent transection of the sciatic nerve. Two and 4 weeks post‐axotomy, regenerated myelinated fibers of the cut end and fibers at its proximal site were morphometrically examined. Non‐diabetic control rats with axotomy were also examined for comparison.

Results

At 4 weeks post‐axotomy, diabetic rats showed an increased myelinated fiber density and total fiber number with a trend toward reduced fiber size at the cut end compared with those in control rats. The average number of myelin lamellae relative to axonal size in regenerated fibers at the cut end was significantly reduced in diabetic rats compared with that in control rats. The proximal site showed a reduced size of fibers and axons in both diabetic and control rats to a similar extent compared with those in a non‐axotomized state. At 2 weeks post‐axotomy, these findings were less apparent.

Conclusions

The nerves of diabetic rats when axotomized undergo impaired regeneration characterized by increased fiber density with hypomyelination.

Diabetes alters KIF1A and KIF5B motor proteins in the hippocampus

Diabetes mellitus is the most common metabolic disorder in humans. Diabetic encephalopathy is characterized by cognitive and memory impairments, which have been associated with changes in the hippocampus, but the mechanisms underlying those impairments triggered by diabetes, are far from being elucidated. The disruption of axonal transport is associated with several neurodegenerative diseases and might also play a role in diabetes-associated disorders affecting nervous system. We investigated the effect of diabetes (2 and 8 weeks duration) on KIF1A, KIF5B and dynein motor proteins, which are important for axonal transport, in the hippocampus. The mRNA expression of motor proteins was assessed by qRT-PCR, and also their protein levels by immunohistochemistry in hippocampal slices and immunoblotting in total extracts of hippocampus from streptozotocin-induced diabetic and age-matched control animals. Diabetes increased the expression and immunoreactivity of KIF1A and KIF5B in the hippocampus, but no alterations in dynein were detected. Since hyperglycemia is considered a major player in diabetic complications, the effect of a prolonged exposure to high glucose on motor proteins, mitochondria and synaptic proteins in hippocampal neurons was also studied, giving particular attention to changes in axons. Hippocampal cell cultures were exposed to high glucose (50 mM) or mannitol (osmotic control; 25 mM plus 25 mM glucose) for 7 days. In hippocampal cultures incubated with high glucose no changes were detected in the fluorescence intensity or number of accumulations related with mitochondria in the axons of hippocampal neurons. Nevertheless, high glucose increased the number of fluorescent accumulations of KIF1A and synaptotagmin-1 and decreased KIF5B, SNAP-25 and synaptophysin immunoreactivity specifically in axons of hippocampal neurons. These changes suggest that anterograde axonal transport mediated by these kinesins may be impaired in hippocampal neurons, which may lead to changes in synaptic proteins, thus contributing to changes in hippocampal neurotransmission and to cognitive and memory impairments.

The size of motoneurons of the gastrocnemius muscle in rats with diabetes

Alterations in the number and size of motoneurons were studied in the medial gastrocnemius (MG) motor nucleus of diabetic rats (12 or 22 weeks after injection of storeptozotocin) and age-matched controls. Each group contained 6 animals. MG motoneurons were retrogradely labeled by dextran-fluorescein and the number and size of cell bodies were examined. Significantly fewer labeled MG motoneurons were found in the 22-week diabetic rats as compared with age-matched control animals. The mean soma diameter of MG motoneurons was significantly smaller in the 12- and 22-week diabetic animals. Furthermore the soma size for 22-week diabetic animals was smaller than for 12-week diabetic animals. The distribution of average soma diameters in the MG nucleus of control animals was bimodal; cells with larger average diameter were presumed to be alpha-motoneurons and those with smaller diameters were presumed to be gamma. Compared to control animals, the number of smaller MG motoneurons was reduced in 12 week diabetic animals. By 22 weeks, diabetic animals had no small MG motoneurons and the size distribution became unimodal. We conclude that there is a significant decrease in the absolute number and size of MG motoneurons in diabetic rats, with the possibility that the decrease occurred predominantly among the smaller gamma-motoneurons.

Down regulation of the proliferation and apoptotic pathways in the embryonic brain of diabetic rats

Compelling evidence shows that the offspring subjected to uncontrolled hyperlycemia during gestation display behavioral, neurochemical, and cellular abnormalities during adulthood. However, the molecular mechanisms underlying these defects remain elusive. Previous studies have shown an increased rate of apoptosis and a decreased index of neuronal proliferation associated with diabetic embryopathy. The aim of the present study was to determine whether impairments in apoptotic related proteins also occur in the developing central nervous system from non-malformed embryos exposed to uncontrolled gestational hyperglycemia. Pregnant rats injected with either streptozotocin or vehicle were killed on gestational day 19. Offspring brains were quickly removed to evaluate protein expression by Western blotting. Embryonic brains from diabetic rats exhibited a decrease in the cell survival p-Akt expression (52.83 ± 24.35%) and in the pro-apoptotic protein Bax (56.16 ± 6.47%). Moreover, the anti-apoptotic protein Bcl-2 showed a non-significant increase while there were no changes in Procaspase 3 or cleaved Caspase 3 proteins. The cytoskeleton proteins NF-200 and GFAP were also examined. Neither NF-200 nor GFAP showed differences in embryonic brains from diabetic rats compared to controls. Altogether, these results indicate that both proliferation and apoptotic pathways are decreased in the brain from the developing offspring of diabetic rats. Since selective neuronal apoptosis, as well as selective cell proliferation, are specifically involved in brain organogenesis, it is possible that simultaneous impairments during the perinatal period contribute to the long lasting alterations observed in the adult brain.

Neuroprotective effect of the glucagon-like peptide-1 receptor agonist, synthetic exendin-4, in streptozotocin-induced diabetic rats

BACKGROUND AND PURPOSE

Glucagon-like peptide-1 (GLP-1) receptors are widely expressed in neural tissues and diminish neuronal degeneration or induce neuronal differentiation. The aim of this study was to investigate the effect of the GLP-1 pathway on peripheral nerves in streptozotocin-induced diabetic rats.

EXPERIMENTAL APPROACH

Diabetic and nondiabetic rats were treated with the GLP-1 receptor agonist, synthetic exendin-4 (i.p., 1 nmol·kg(-1)·day(-1)) or placebo for 24 weeks, and current perception threshold values, cAMP levels and nerve fibre size in the sciatic nerve were measured. We also investigated GLP-1 receptor expression, quantitative changes in PGP9.5-positive intraepidermal nerve fibres and cleaved caspase 3-stained Schwann cells by immunohistochemistry.

KEY RESULTS

GLP-1 receptor expression was detected in the sciatic nerve and skin. After exendin-4 treatment, the increase seen in current perception threshold values at 2000 and 250 Hz in diabetic rats was reduced. Also, the decrease in myelinated fibre size or axon/fibre area ratio in the sciatic nerve and the loss of intraepidermal nerve fibre in the skin of diabetic rats were ameliorated. These responses were closely associated with the attenuation of Schwann cell apoptosis and improvement in the cAMP level in exendin-4-treated diabetic rats, compared with placebo-treated animals.

CONCLUSION AND IMPLICATIONS

Synthetic exendin-4 may prevent peripheral nerve degeneration induced by diabetes in an animal model, supporting the hypothesis that GLP-1 may be useful in peripheral neuropathy. The neuroprotection is probably attributable to GLP-1 receptor activation, antiapoptotic effects and restoration of cAMP content.

A brief review of in vitro models of diabetic neuropathy

The neuropathies of the peripheral, central and autonomic nervous systems are known to be caused by hyperglycemia, a consequence of the deregulation of glucose in diabetes. Several in vivo models such as streptozotocin-induced diabetic rats, mice and Chinese hamsters have been used to study the pathogenesis of diabetic neuropathy because of their resemblance to human pathology. However, these in vivo models have met with strong ethical oppositions. Further, the system complexity has inherent limitations of inconvenience of analyzing ephemeral molecular events and crosstalk of signal transduction pathways. Alternative in vitro models have been selected and put to effective use in diabetic studies. We critically review the use of these in vitro models such as primary cultures of dorsal root ganglia, Schwann cells and neural tissue as well as neural cell lines which have proved to be excellent systems for detailed study. We also assess the use of embryo cultures for the study of hyperglycemic effects on development, especially of the nervous system. These systems function as useful models to scrutinize the molecular events underlying hyperglycemia-induced stress in neuronal systems and have been very effectively used for the same. This comprehensive overview of advantages and disadvantages of in vitro systems that are currently in use will be of interest especially for comparative assessment of results and for appropriate choice of models for experiments in diabetic neuropathy.

The involvement of aldose reductase in alterations to neurotrophin receptors and neuronal cytoskeletal protein mRNA levels in the dorsal root ganglion of streptozotocin-induced diabetic rats

Dorsal root ganglia (DRG) are recognized as one of the organs which are damaged in peripheral sensory diabetic neuropathy. In an experimental animal model, the alteration of the mRNA expression level of neurotrophins, their receptors and neuronal cytoskeletal protein have been reported. In this study, we examined whether these changes are improved by treatment with the aldose reductase inhibitor, zenarestat, in early-stage diabetic neuropathy of streptozotocin (STZ)-induced diabetic rats. Two weeks after the induction of diabetes mellitus by STZ treatment, zenarestat or a vehicle were given orally for two weeks. After the zenarestat treatment, the mRNA expression levels of neurotrophin receptors and neuronal cytoskeletal proteins in dorsal root ganglia were determined with a real-time polymerase chain reaction (PCR) method. Compared with the expression level of normal rats, a significant increase in Trk-C and Talpha1 alpha-tubulin and a decrease in neurofilament H mRNA expression level were observed in the DRG of STZ rats, while there were no significant changes in Trk-A, Trk-B, p75, neurofilament L, neurofilament M and betaIII tubulin mRNA expression. Zenarestat treatment significantly ameliorated the abnormal increase in Trk-C mRNA expression level. These data suggest that hyperactivation of the polyol pathway induces a deficit in neurotropism on peripheral sensory diabetic neuropathy.

Myelination and the trophic support of long axons

In addition to their role in providing myelin for rapid impulse propagation, the glia that ensheath long axons are required for the maintenance of normal axon transport and long-term survival. This presumably ancestral function seems to be independent of myelin membrane wrapping. Here, I propose that ensheathing glia provide trophic support to axons that are metabolically isolated, and that myelin itself might cause such isolation. This glial support of axonal integrity may be relevant for a number of neurological and psychiatric diseases.

Neuronal intermediate filaments and neurodegenerative disorders

Intermediate filaments represent the most abundant cytoskeletal element in mature neurons. Mutations and/or accumulations of neuronal intermediate filament proteins are frequently observed in several human neurodegenerative disorders. Although it is now admitted that disorganization of the neurofilament network may be directly involved in neurodegeneration, certain type of perikaryal intermediate filament aggregates confer protection in motor neuron disease. The use of various mouse models provided a better knowledge of the role played by the disorganization of intermediate filaments in the pathogenesis of neurodegenerative disorders, but the mechanisms leading to the formation of these aggregates remain elusive. Here, we will review some neurodegenerative diseases involving intermediate filaments abnormalities and possible mechanisms susceptible to provoke them.

Expression of hexokinase isoforms in the dorsal root ganglion of the adult rat and effect of experimental diabetes

The effect of streptozotocin (STZ)-induced diabetes on expression and activity of hexokinase, the first enzyme and rate-limiting step in glycolysis, was studied in sensory neurons of lumbar dorsal root ganglia (DRG). The DRG and sciatic nerve of adult rats expressed the hexokinase I isoform only. Immunofluorescent staining of lumbar DRG demonstrated that small-medium neurons and satellite cells exhibited high levels of expression of hexokinase I. Large, mainly proprioceptive neurons, had very low or negative staining for hexokinase I. Intracellular localization and biochemical studies on intact DRG from adult rats and cultured adult rat sensory neurons revealed that hexokinase I was almost exclusively found in the mitochondrial compartment. Duration of STZ-diabetes of 6 or 12 weeks diminished hexokinase activity by 28% and 30%, respectively, in lumbar DRG compared with age matched controls (P<0.05). Quantitative Western blotting showed no effect of diabetes on hexokinase I protein expression in homogenates or mitochondrial preparations from DRG. Immunofluorescent staining for hexokinase I showed no diabetes-dependent change in small-medium neuron expression in DRG, however, large neurons became positive for hexokinase I (P<0.05). Such complex effects of diabetes on hexokinase I expression in the DRG may be due to glucose-driven up-regulation of expression or the result of impaired axonal transport and perikaryal accumulation in the large neuron sub-population. Because hexokinase is the rate-limiting enzyme of glycolysis these results imply that metabolic flux through the glycolytic pathway is reduced in diabetes. This finding, therefore, questions the role of high glucose-induced metabolic flux as a key driving force in reactive oxygen species generation by mitochondria.

Identification of changes in gene expression in dorsal root ganglia in diabetic neuropathy: correlation with functional deficits

This study aimed to correlate the onset of functional deficits in diabetic neuropathy with changes in gene expression in rat dorsal root ganglia (DRG). After 1, 4, or 8 weeks of streptozotocin-induced diabetes, sensory and motor nerve conduction velocities (NCV) were measured as an indicator of neuropathy and changes in gene expression were measured using Affymetrix oligonucleotide microarrays. No significant changes in NCV were found after 1 week of diabetes, but after 4 and 8 weeks, there was a significant reduction in both sensory and motor NCV. Global gene expression changes in diabetic rat DRG were evident from principal component analysis of microarray data after 1, 4, and 8 weeks. Expression changes in individual genes were relatively small in line with a gradual degenerative neuropathy indirectly resulting from diabetes. Sets of differentially expressed genes have been identified and quantitative reverse transcriptase-polymerase chain reaction has been used to confirm the microarray data for several genes. Gene ontology overrepresentation analysis was performed on the microarray data to identify biologic processes altered in diabetic DRG. The genes identified in this study may be responsible for causing the functional deficits and suggest pathways/processes that require further investigation as possible targets for therapeutic intervention.

Diabetic neuropathies: features and mechanisms

Diabetic neuropathies include both focal neuropathies and diffuse polyneuropathy. Polyneuropathy, the most common of the diabetic neuropathies excluding focal entrapment, has not yet been explained by a single disease mechanism despite intensive investigation. A number of abnormalities appear to cascade into a 'vicious cycle' of progressive microvascular disease associated with motor, sensory and autonomic fiber loss. These abnormalities include excessive polyol (sugar alcohol) flux through the aldose reductase pathway, functional and structural alterations of nerve microvessels, nerve and ganglia hypoxia, oxidative stress, nonspecific glycosylation of axon and microvessel proteins, and impairment in the elaboration of trophic factors critical for peripheral nerves and their ganglia. While an initiating role for nerve ischemia in the development of polyneuropathy has been proposed, the evidence for it can be questioned. The role of sensory and autonomic ganglia in the development of polyneuropathy has had relatively less attention despite the possibility that they may be vulnerable to a variety of insults, particularly neurotrophin deficiency. Superimposed on the deficits of polyneuropathy is the failure of diabetic nerves to regenerate as effectively as nondiabetics. Polyneuropathy has not yet yielded to specific forms of treatment but a variety of new trials addressing plausible hypotheses have been initiated. This review will summarize some of the clinical, pathological and experimental work applied toward understanding human diabetic neuropathy and will emphasize ideas on pathogenesis.

Alterations in neurofilament protein(s) in human leprous nerves: morphology, immunohistochemistry and Western immunoblot correlative study

Using a specific antibody (SMI 31), the state of phosphorylation of high and medium molecular weight neurofilaments (NF-H and NF-M) was studied in 22 leprous and four nonleprous human peripheral nerves by means of immunohistochemistry, sodium dodecyl sulfate-poly acrylamide gel electrophoresis (SDS-PAGE) and Western immunoblot (WB). The results thus obtained were compared with morphological changes in the respective nerves studied through light and electron microscopy. Many of the leprous nerves showing minimal pathology revealed lack of or weak staining with SMI 31, denoting dephosphorylation. Remyelinated fibres stained intensely with SMI 31 antibody. The WB analysis of Triton X-100 insoluble cytoskeletal preparation showed absence of regular SMI 31 reactive bands corresponding to 200 and 150 kDa molecular weight (NF-H and NF-M, respectively) in 10 nerves. Three of the 10 nerves revealed presence of NF protein bands in SDS-PAGE but not in WB. Presence of additional protein band (following NF-M) was seen in four nerves. Two nerves revealed NF-H band but not NF-M band and one nerve showed trace positivity. In the remaining five nerves presence of all the three NF bands was seen. Thus, 77.3% (17/22) of human leprous nerves studied showed abnormal phosphorylation of NF protein(s). The ultrastructural study showed abnormal compaction and arraying of NF at the periphery of the axons in the fibres with altered axon to myelin thickness ratio (atrophied fibres) as well as at the Schmidt-Lantermann (S-L) cleft region. Such NF changes were more pronounced in the severely atrophied axons suggesting a direct correlation. The observed well-spaced NF in the remyelinated fibres under ultrastructural study was in keeping with both intense SMI 31 staining and presence of NF triplet bands seen in WBs in four of leprous nerves that showed a large number of regenerating fibres suggesting reversal of changes with regeneration. Findings in the present study suggest that atrophy, that is, the reduction in axonal calibre and paranodal demyelination, seen in leprous nerves may result from dephosphorylation of NF-H and NF-M proteins.

Diabetic neuropathy and nerve regeneration

Diabetic neuropathy is the most common peripheral neuropathy in western countries. Although every effort has been made to clarify the pathogenic mechanism of diabetic neuropathy, thereby devising its ideal therapeutic drugs, neither convinced hypotheses nor unequivocally effective drugs have been established. In view of the pathologic basis for the treatment of diabetic neuropathy, it is important to enhance nerve regeneration as well as prevent nerve degeneration. Nerve regeneration or sprouting in diabetes may occur not only in the nerve trunk but also in the dermis and around dorsal root ganglion neurons, thereby being implicated in the generation of pain sensation. Thus, inadequate nerve regeneration unequivocally contributes to the pathophysiologic mechanism of diabetic neuropathy. In this context, the research on nerve regeneration in diabetes should be more accelerated. Indeed, nerve regenerative capacity has been shown to be decreased in diabetic patients as well as in diabetic animals. Disturbed nerve regeneration in diabetes has been ascribed at least in part to all or some of decreased levels of neurotrophic factors, decreased expression of their receptors, altered cellular signal pathways and/or abnormal expression of cell adhesion molecules, although the mechanisms of their changes remain almost unclear. In addition to their steady-state changes in diabetes, nerve injury induces injury-specific changes in individual neurotrophic factors, their receptors and their intracellular signal pathways, which are closely linked with altered neuronal function, varying from neuronal survival and neurite extension/nerve regeneration to apoptosis. Although it is essential to clarify those changes for understanding the mechanism of disturbed nerve regeneration in diabetes, very few data are now available. Rationally accepted replacement therapy with neurotrophic factors has not provided any success in treating diabetic neuropathy. Aside from adverse effects of those factors, more rigorous consideration for their delivery system may be needed for any possible success. Although conventional therapeutic drugs like aldose reductase (AR) inhibitors and vasodilators have been shown to enhance nerve regeneration, their efficacy should be strictly evaluated with respect to nerve regenerative capacity. For this purpose, especially clinically, skin biopsy, by which cutaneous nerve pathology including nerve regeneration can be morphometrically evaluated, might be a safe and useful examination.

Therapeutic efficacy of sonic hedgehog protein in experimental diabetic neuropathy

Hedgehog proteins modulate development and patterning of the embryonic nervous system. As expression of desert hedgehog and the hedgehog receptor, patched-1, persist in the postnatal and adult peripheral nerves, the hedgehog pathway may have a role in maturation and maintenance of the peripheral nervous system in normal and disease states. We measured desert hedgehog expression in the peripheral nerve of maturing diabetic rats and found that diabetes caused a significant reduction in desert hedgehog mRNA. Treating diabetic rats with a sonic hedgehog-IgG fusion protein fully restored motor- and sensory-nerve conduction velocities and maintained the axonal caliber of large myelinated fibers. Diabetes-induced deficits in retrograde transport of nerve growth factor and sciatic-nerve levels of calcitonin gene-related product and neuropeptide Y were also ameliorated by treatment with the sonic hedgehog-IgG fusion protein, as was thermal hypoalgesia in the paw. These studies implicate disruption of normal hedgehog function in the etiology of diabetes-induced peripheral-nerve dysfunction and indicate that delivery of exogenous hedgehog proteins may have therapeutic potential for the treatment of diabetic neuropathy.

Accumulation of imidazolone, pentosidine and N(epsilon)-(carboxymethyl)lysine in hippocampal CA4 pyramidal neurons of aged human brain

Previous studies from our laboratory demonstrated that N(epsilon)-(carboxymethyl)lysine (CML), one of the major advanced glycation end products (AGE), was accumulated in human pyramidal neurons in the hippocampus in an age-dependent manner. This suggests a potential link between AGE-accumulation and the aging process in neurons. The purpose of the present study was to examine whether this notion could be extended to other AGE structures, such as imidazolone and pentosidine. This was done using 19 human brains that were not affected by dementia. The immunohistochemical survey on distribution in brain tissues of imidazolone and pentosidine was carried out with monoclonal antibodies specific for imidazolone and pentosidine. A parallel control experiment was carried out with anti-CML antibody. The results showed that pentosidine and imidazolone were localized in neurons in different areas of human brain tissue, especially in neurons of CA4 in the hippocampus. The characteristic distribution of pentosidine and imidazolone is very similar to that of CML. Furthermore, when the accumulation of these AGE structures was compared with the age of individual brains it was found that accumulation of imidazolone, pentosidine and CML in the CA4 region increased with age. These findings taken together support the notion that the accumulation of AGE structures in the CA4 region might be closely related to the aging process in neurons.

Streptozotocin-induced diabetes reduces retrograde axonal transport in the afferent and efferent vagus nerve

Diabetes-induced alterations in nerve function include reductions in the retrograde axonal transport of neurotrophins. A decreased axonal accumulation of endogenous nerve growth factor (NGF) and neurotrophin-3 (NT-3) in the vagus nerve of streptozotocin (STZ)-induced diabetic rats was previously shown. In the current study, no changes in the NGF and NT-3 protein or mRNA levels in the stomach or atrium, two vagally innervated organs, were noted after 16 or 24 weeks of diabetes. Moreover, the amounts of neurotrophin receptor (p75, TrkA, TrkC) mRNAs in the vagus nerve and vagal afferent nodose ganglion were not reduced in diabetic rats. These data suggest that neither diminished access to target-derived neurotrophins nor the loss of relevant neurotrophin receptors accounts for the diabetes-induced alteration in the retrograde axonal transport of neurotrophins. To assess whether diabetes causes a defect in axonal transport that may not be specific to neurotrophin transport, we studied the ability of a neuronal tracer (FluoroGold, FG) to be retrogradely transported by vagal neurons of control and diabetic rats. After vagal target tissue (stomach) injections of FG, the numbers of FG-labeled afferent and efferent vagal neurons were counted in the nodose ganglion and in the dorsal motor nucleus of the vagus, respectively. After 24 weeks of diabetes, FG was retrogradely transported to more than 50% fewer afferent and efferent vagal neurons in the STZ-diabetic compared to control rats. The diabetes-induced deficit in retrograde axonal transport of FG is likely to reflect alterations in basic axonal transport mechanisms in both the afferent and efferent vagus nerve that contribute to the previously observed reductions in neurotrophin transport.

Morphometry of dorsal root ganglion in chronic experimental diabetic neuropathy

Chronic hyperglycemia results in a predominantly sensory neuropathy. Recent studies suggest that dorsal root ganglion (DRG) neurons comprise a specific target and may be responsible for the important complication of diabetic sensory neuropathy, since hyperglycemia for longer than 6 months results in a vacuolar ganglionopathy with associated radiculopathy and distal sensory neuropathy. We undertook morphometric analysis of L5 DRG neurons in seven diabetic rats and six age- and sex-matched littermates. Nerve conduction studies were also performed, and neuropathy was confirmed. Diabetes was induced with streptozotocin; duration of diabetes was 12 months. The DRG count for control rats was 15,304 +/- 991 neurons. Two of seven diabetic DRG counts were reduced, but the group mean count at 14,847 plus minus 1,524 was not significantly reduced. The number of small neurons (type B) considerably exceeded that of large neurons (type A), at a ratio of 71:29. The percentage of large cells was significantly reduced in diabetic compared with control rats (P = 0.01). The large-diameter population can be subdivided into two groups; with this subdivision, the number of neurons < 50 microm was not reduced in samples from diabetic rats, but the neurons of largest size (> or = 50 microm) were significantly reduced (by 41%).

Abnormal centrifugal axons in streptozotocin-diabetic rat retinas

PURPOSE

To characterize the effects of diabetes on the expression of histidine decarboxylase mRNA and on the morphology of the histaminergic centrifugal axons in the rat retina.

METHODS

Rats were made diabetic by streptozotocin. After 3 months, retinal histidine decarboxylase expression was analyzed by in situ hybridization in radial sections. Flatmount retinas from a second group of rats were labeled with an antiserum to histamine or an antibody to phosphorylated neurofilament protein.

RESULTS

Histidine decarboxylase mRNA was expressed in cells in the inner and outer nuclear layers of diabetic retinas, but not in normal retinas. However, immunoreactive (IR) histamine was not localized to perikarya in either the normal or the diabetic retinas. Instead, a population of centrifugal axons was labeled. These axons emerged from the optic disc and had varicose terminal branches in the inner plexiform layer (IPL) of the peripheral retina. Some branches ended on large retinal blood vessels and others in dense clusters in the IPL. In rats with streptozotocin-induced diabetes, the centrifugal axon terminals developed many large swellings that contained neurofilament immunoreactivity; these swellings were rare in normal retinas.

CONCLUSIONS

Diabetes perturbs the retinal histaminergic system, causing increases in histidine decarboxylase mRNA expression in neurons or glia and abnormal focal swellings on the centrifugal axons.

Cilostazol prevents impairment of slow axonal transport in streptozotocin-diabetic rats

We studied the effects of cilostazol, an antiplatelet and vasodilating agent, on axonal transport patterns of cytoskeletal proteins in the motor fibers of sciatic nerve of streptozotocin-induced diabetic rats. Proteins labeled with L-[35S]methionine in 6-mm consecutive segments of the nerve were analyzed electrophoretically following fractionation into Triton-soluble and-insoluble subpopulations. Transport rates of proteins (particularly neurofilaments) in slow component a were reduced by 50% 2 weeks after labeling (4 weeks after streptozotocin). An apparent reduction of tubulin and actin was observed at later intervals after induction of diabetes. Actin transported in slow component b was also impaired, though to a lesser extent than in component a. Cilostazol prevented transport impairment of both slow components a and b without affecting hyperglycemia or reduction in body weight gain. These results suggest that in sciatic motor fibers early defects in slowly transported proteins are more marked in slow component a, and that impairment may be caused primarily by hemodynamic abnormalities.

Caracterização de um modelo experimental de neuropatia em ratos diabéticos induzidos pela aloxana

Cem ratos norvégicus, machos, com aproximadamente 3 meses de idade foram distribuídos por sorteio em 2 grupos experimentais: Grupo Controle (GC): com 50 ratos sadios, não diabéticos e Grupo Diabético (GD): com 50 ratos diabéticos, induzidos pela aloxana, sem qualquer tratamento. Cada grupo foi dividido em 5 subgrupos com 10 ratos cada e sacrificados com 1, 3, 6, 9 e 12 meses de seguimento, respectivamente. Parâmetros clínicos (peso, ingestão hídrica e alimentar, e diurese) e laboratoriais (glicemia, glicose urinária e insulina) foram documentados em todos os momentos de avaliação. Um segmento do nervo ciático foi obtido de cada animal, em ambos os grupos, para estudo à MO. e ME. Alterações clínicas e laboratoriais significativas (P<0,01), compatíveis com diabetes grave, foram observadas em todos os animais do GD a partir do 4o dia após a indução. Ratos de ambos os grupos apresentaram alterações no número de fibras mielínicas e nos depósitos intraaxonais de glicogênio que não diferiram, estatisticamente, aos 1, 3 e 6 meses de seguimento. Entretanto, aos 9 e 12 meses, ratos do GD apresentaram diminuição significativa no número de fibras mielínicas, com aumento do número de fibras mielínicas de menor calibre, quando comparados com ratos do GC (P<0,05). Grânulos de glicogênio intraaxonais também foram mais acentuados em ratos do GD no 9o e 12o mês de seguimento. Não foram observadas diferenças na densidade de fibras amielínicas ou alterações ultraestruturais significativas entre os dois grupos, em relação aos espaços intraaxonais e endoneurais, bainhas de mielina e células de Schwann durante todo o estudo.

Tolerance to beta,beta'-iminodipropionitrile (IDPN)-induced neurobehavioural and vestibular toxicity in diabetic rats

The present investigation was undertaken to study the neurotoxic effects of beta,beta'-iminodipropionitrile (IDPN) in normal, diabetic and insulin-treated diabetic rats. Sprague-Dawley male rats were divided into five groups: control, IDPN, diabetes, diabetes plus IDPN and diabetes plus insulin plus IDPN. The diabetes was induced with a single i.p. injection of streptozotocin (50 mg kg(-1)). One month after the induction of diabetes, the rats were treated with IDPN (100 mg kg(-1), i.p.) daily for 11 days. One of the diabetic groups treated with IDPN also received daily injection of insulin (25 U kg(-1), s.c.), 1 h before IDPN. The rats were observed daily for abnormal head movements and circling. The grip strength of the forelimbs was also measured. In the IDPN group the dyskinetic symptoms appeared on the 8th day, whereas the onset of dyskinesia was on the 12th day in IDPN-treated diabetic rats. The incidence and severity of dyskinesia were significantly higher in IDPN-treated normal (non-diabetic) rats as compared to IDPN-treated diabetic rats. The treatment of diabetic rats with insulin normalized striatal dopamine (DA) turnover but partially reversed diabetes-induced protection against IDPN dyskinesia. There was severe degeneration of sensory hair cells in crista ampullaris of IDPN-treated normal rats, whereas the diabetic rats showed significant protection against IDPN-induced vestibular hair cell degeneration. In conclusion, our study clearly demonstrates that diabetic rats are resistant to IDPN-induced neurobehavioural and vestibular toxicity. The results also show that diabetes-induced protection against IDPN-induced dyskinesia can be partially reversed by insulin. The mechanism behind the decreased vulnerability of diabetic animals to IDPN remains to be resolved. Further studies are warranted to investigate this paradoxical phenomenon.