Aminoguanidine effects on nerve blood flow, vascular permeability, electrophysiology, and oxygen free radicals

Ketogenic diet prevents methylglyoxal-evoked nociception by scavenging methylglyoxal

ABSTRACT

Methylglyoxal (MGO) is a reactive dicarbonyl byproduct of glycolysis implicated in a growing number of neuropathic pain conditions, including chemotherapy-induced peripheral neuropathy, diabetic peripheral neuropathy, and radiculopathy with lumbar disk herniation. Recent studies show success in preclinical models treating these disorders with an interventional ketogenic diet. Here, we tested the hypothesis that a ketogenic diet modifies pathological MGO signaling as a mechanism underlying neuropathy improvement. We found that mice injected with MGO displayed nocifensive behaviors, whereas mice prefed a ketogenic diet were resistant to mechanical allodynia elicited by MGO. In addition, levels of circulating MGO were reduced in ketogenic diet-fed mice and negatively correlated with levels of the ketone body β-hydroxybutyrate (β-HB). Methylglyoxal is normally scavenged by the glyoxalase system, and ketogenic diet-fed mice displayed increased glyoxalase 1 activity compared with chow-fed control mice. Recent studies also suggest that ketone bodies contribute to MGO detoxification, consistent with a negative correlation between β-HB and MGO. To assess whether ketone bodies modified MGO-evoked nociception through direct MGO detoxification, we coincubated either acetoacetate or β-HB with MGO before injection. Mice receiving intraplantar MGO injection exhibit increased nociceptive behavior (lifting, licking, biting, and scratching), which was significantly reduced by coincubation with either acetoacetate or β-HB. Methylglyoxal increased phospho-extracellular signal-regulated kinase-positive cells in the spinal dorsal horn, and this evoked spinal activation was ameliorated by preincubation with acetoacetate or β-HB. These results suggest that a ketogenic diet and ketone bodies ameliorate MGO-evoked nociception, partially through detoxification of MGO, and provide rationale for therapeutic intervention with a ketogenic diet in MGO-driven pathologies.

Glucose and Blood Pressure-Dependent Pathways-The Progression of Diabetic Kidney Disease

The major clinical associations with the progression of diabetic kidney disease (DKD) are glycemic control and systemic hypertension. Recent studies have continued to emphasize vasoactive hormone pathways including aldosterone and endothelin which suggest a key role for vasoconstrictor pathways in promoting renal damage in diabetes. The role of glucose per se remains difficult to define in DKD but appears to involve key intermediates including reactive oxygen species (ROS) and dicarbonyls such as methylglyoxal which activate intracellular pathways to promote fibrosis and inflammation in the kidney. Recent studies have identified a novel molecular interaction between hemodynamic and metabolic pathways which could lead to new treatments for DKD. This should lead to a further improvement in the outlook of DKD building on positive results from RAAS blockade and more recently newer classes of glucose-lowering agents such as SGLT2 inhibitors and GLP1 receptor agonists.

Methylglyoxal, a Highly Reactive Dicarbonyl Compound, in Diabetes, Its Vascular Complications, and Other Age-Related Diseases

The formation and accumulation of methylglyoxal (MGO), a highly reactive dicarbonyl compound, has been implicated in the pathogenesis of type 2 diabetes, vascular complications of diabetes, and several other age-related chronic inflammatory diseases such as cardiovascular disease, cancer, and disorders of the central nervous system. MGO is mainly formed as a byproduct of glycolysis and, under physiological circumstances, detoxified by the glyoxalase system. MGO is the major precursor of nonenzymatic glycation of proteins and DNA, subsequently leading to the formation of advanced glycation end products (AGEs). MGO and MGO-derived AGEs can impact on organs and tissues affecting their functions and structure. In this review we summarize the formation of MGO, the detoxification of MGO by the glyoxalase system, and the biochemical pathways through which MGO is linked to the development of diabetes, vascular complications of diabetes, and other age-related diseases. Although interventions to treat MGO-associated complications are not yet available in the clinical setting, several strategies to lower MGO have been developed over the years. We will summarize several new directions to target MGO stress including glyoxalase inducers and MGO scavengers. Targeting MGO burden may provide new therapeutic applications to mitigate diseases in which MGO plays a crucial role.

Glucotoxic Mechanisms and Related Therapeutic Approaches

Neuropathy is the earliest and commonest complication of diabetes. With increasing duration of diabetes, frequency and severity of neuropathy are worsened. Long-term hyperglycemia is therefore implicated in the development of this disorder. Nerve tissues require glucose energy to function and survive. Upon excessive glucose entry into the peripheral nerve, the glycolytic pathway and collateral glucose-utilizing pathways are overactivated and initiate adverse effects on nerve tissues. During hyperglycemia, flux through the polyol pathway, formation of advanced glycation end-products, production of free radicals, flux into the glucosamine pathway, and protein kinase C activity are all enhanced to negatively influence nerve function and structure. Suppression of these aberrant metabolic pathways has succeeded in prevention and inhibition of the development of neuropathy in animal models with diabetes. Satisfactory results were not attained, however, in patients with diabetes and further clinical trials are required. In this review, the author summarizes the hitherto proposed theories on the pathogenesis of diabetic neuropathy related to glucose metabolism and future prospects for the effective treatment of neuropathy.

The role of methylglyoxal and the glyoxalase system in diabetes and other age-related diseases. Clin Sci (Lond). 2015;128:839-861. [PMID: 25818485 DOI: 10.1042/cs20140683]

The formation and accumulation of advanced glycation endproducts (AGEs) are related to diabetes and other age-related diseases. Methylglyoxal (MGO), a highly reactive dicarbonyl compound, is the major precursor in the formation of AGEs. MGO is mainly formed as a byproduct of glycolysis. Under physiological circumstances, MGO is detoxified by the glyoxalase system into D-lactate, with glyoxalase I (GLO1) as the key enzyme in the anti-glycation defence. New insights indicate that increased levels of MGO and the major MGO-derived AGE, methylglyoxal-derived hydroimidazolone 1 (MG-H1), and dysfunctioning of the glyoxalase system are linked to several age-related health problems, such as diabetes, cardiovascular disease, cancer and disorders of the central nervous system. The present review summarizes the mechanisms through which MGO is formed, its detoxification by the glyoxalase system and its effect on biochemical pathways in relation to the development of age-related diseases. Although several scavengers of MGO have been developed over the years, therapies to treat MGO-associated complications are not yet available for application in clinical practice. Small bioactive inducers of GLO1 can potentially form the basis for new treatment strategies for age-related disorders in which MGO plays a pivotal role.

Mechanism of diabetic neuropathy: Where are we now and where to go?

Neuropathy is the most common complication of diabetes. As a consequence of longstanding hyperglycemia, a downstream metabolic cascade leads to peripheral nerve injury through an increased flux of the polyol pathway, enhanced advanced glycation end‐products formation, excessive release of cytokines, activation of protein kinase C and exaggerated oxidative stress, as well as other confounding factors. Although these metabolic aberrations are deemed as the main stream for the pathogenesis of diabetic microvascular complications, organ‐specific histological and biochemical characteristics constitute distinct mechanistic processes of neuropathy different from retinopathy or nephropathy. Extremely long axons originating in the small neuronal body are vulnerable on the most distal side as a result of malnutritional axonal support or environmental insults. Sparse vascular supply with impaired autoregulation is likely to cause hypoxic damage in the nerve. Such dual influences exerted by long‐term hyperglycemia are critical for peripheral nerve damage, resulting in distal‐predominant nerve fiber degeneration. More recently, cellular factors derived from the bone marrow also appear to have a strong impact on the development of peripheral nerve pathology. As evident from such complicated processes, inhibition of single metabolic factors might not be sufficient for the treatment of neuropathy, but a combination of several inhibitors might be a promising approach to overcome this serious disorder. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00070.x, 2010)

Exercise training enhances insulin-stimulated nerve arterial vasodilation in rats with insulin-treated experimental diabetes

Insulin stimulates nerve arterial vasodilation through a nitric oxide (NO) synthase (NOS) mechanism. Experimental diabetes reduces vasa nervorum NO reactivity. Studies investigating hyperglycemia and nerve arterial vasodilation typically omit insulin treatment and use sedentary rats resulting in severe hyperglycemia. We tested the hypotheses that 1) insulin-treated experimental diabetes and inactivity (DS rats) will attenuate insulin-mediated nerve arterial vasodilation, and 2) deficits in vasodilation in DS rats will be overcome by concurrent exercise training (DX rats; 75–85% V̇o2 max, 1 h/day, 5 days/wk, for 10 wk). The baseline index of vascular conductance values (VCi = nerve blood flow velocity/mean arterial blood pressure) were similar ( P ≥ 0.68), but peak VCi and the area under the curve (AUCi) for the VCi during a euglycemic hyperinsulinemic clamp (EHC; 10 mU·kg−1·min−1) were lower in DS rats versus control sedentary (CS) rats and DX rats ( P ≤ 0.01). Motor nerve conduction velocity (MNCV) was lower in DS rats versus CS rats and DX rats ( P ≤ 0.01). When compared with DS rats, DX rats expressed greater nerve endothelial NOS (eNOS) protein content ( P = 0.04). In a separate analysis, we examined the impact of diabetes in exercise-trained rats alone. When compared with exercise-trained control rats (CX), DX rats had a lower AUCi during the EHC, lower MNCV values, and lower sciatic nerve eNOS protein content ( P ≤ 0.03). Therefore, vasa nervorum and motor nerve function are impaired in DS rats. Such deficits in rats with diabetes can be overcome by concurrent exercise training. However, in exercise-trained rats (CX and DX groups), moderate hyperglycemia lowers vasa nervorum and nerve function.

Neuropathy induced by exogenously administered advanced glycation end-products in rats

Aims/Introduction:  Advanced glycation end‐products (AGE) have been implicated in the development of diabetic neuropathy. It still remains unknown, however, how AGE cause functional and structural changes of the peripheral nerve in diabetes. To explore the role of AGE in diabetic neuropathy, we examined the peripheral nerve by injecting AGE into normal Wistar rats.

Materials and Methods:  Young, normal male Wistar rats were injected intraperitoneally (i.p.) daily for 12 weeks with purified AGE prepared by incubating D‐glucose with bovine serum albumin (BSA). A control group received BSA alone. A group of rats given AGE were co‐treated with aminoguanidine (50 mg/kg/day, i.p.). Peripheral nerve function and structure, as well as nerve Na⁺,K⁺‐ATPase activity, were examined in these rats. Immunohistochemical expressions of 8‐hydroxy‐2′‐deoxyguanosine (8OHdG) and nuclear factor‐κB (NF‐κB)p65 were also examined.

Results:  Serum AGE levels were increased two to threefold in the AGE‐treated group compared with those in the BSA‐treated control group. AGE‐treated rats showed a marked slowing of motor nerve conduction velocity (MNCV) and decreased nerve Na⁺,K⁺‐ATPase activity compared with those in the BSA‐treated group. These changes were accompanied by intensified expressions of 8OHdG and NF‐κBp65 in endothelial cells and Schwann cells. Aminoguanidine treatment corrected MNCV delay, Na⁺,K⁺‐ATPase activity, and suppressed the expression of 8OHdG and NF‐κB, despite there being no influence on serum AGE levels.

Conclusions:  The results suggest that an elevated concentration of blood AGE might be one of the contributing factors to the development of neuropathic changes in diabetes.

Biology of diabetic neuropathy. Handb Clin Neurol. 2013;115:591-606. [PMID: 23931804 DOI: 10.1016/b978-0-444-52902-2.00034-5]

More than half of all patients with diabetes develop neuropathic disorders affecting the distal sensory and/or motor nerves, or autonomic or cranial nerve functions. Glycemic control can decrease the incidence of neuropathy but is not adequate alone to prevent or treat the disease. This chapter introduces diabetic neuropathy with a morphological description of the disease then describes our current understanding of metabolic and molecular mechanisms that contribute to neurovascular dysfunctions. Key mechanisms include glucose and lipid imbalances and insulin resistance that are interconnected via oxidative stress, inflammation, and altered gene expression. These complex interactions should be considered for the development of new treatment strategies against the onset or progression of neuropathy. Advances in understanding the combined metabolic stressors and the novel study of epigenetics suggest new therapeutic targets to combat this morbid and intractable disease affecting millions of patients with type 1 or type 2 diabetes.

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.

Advanced glycation end products inhibit Na+ K+ ATPase in proximal tubule epithelial cells: role of cytosolic phospholipase A2alpha and phosphatidylinositol 4-phosphate 5-kinase gamma

Chronic hyperglycaemia during diabetes leads to non-enzymatic glycation of proteins to form advanced glycation end products (AGEs) that contribute to nephropathy. In diabetes, renal Na+ K+ ATPase (NKA) activity is downregulated and phosphoinositide metabolism is upregulated. We examined the effects of AGEs on NKA activity in porcine LLC-PK1 and human HK2 proximal tubule epithelial cells. AGE-BSA increased cellular phosphoinositol 4,5 bisphosphate (PIP2) production as determined by immunofluorescence microscopy and thin layer chromatography. AGE-BSA (40 microM) induced 3H-arachidonic acid release and reactive oxygen species (ROS) production via cytosolic phospholipase A2 (cPLA2) activation. Within minutes, AGE-BSA significantly inhibited NKA surface expression and activity in a dose- and time-dependent manner as determined by immunofluorescence staining and [86Rb+] uptake, respectively, suggesting AGEs inhibit NKA by stimulating its endocytosis. The AGE-BSA-induced decrease in cell surface NKA was reversed by a cPLA2alpha inhibitor, neomycin, a PIP2 inhibitor, and PP2, a Src inhibitor. AGE-BSA increased binding of NKA to the alpha-adaptin but not beta2- or mu2-adaptin subunits of the AP-2 clathrin pit adaptor complex. Transfection of HK2 cells with PIP5Kgamma siRNA prevented AGE-BSA inhibition of NKA activity. AGEs may stimulate PIP5Kgamma to increase PIP2 production, which may enhance AP-2 localisation to clathrin pits, increase clathrin pit formation, enhance NKA cargo recognition by AP-2 and/or stimulate cPLA2alpha activity. These results suggest AGEs modulate arachidonic acid and phosphoinositide metabolism to inhibit NKA via clathrin-mediated endocytosis. Elucidation of new intracellular AGE signaling pathways may lead to improved therapies for diabetic nephropathy.

PPARgamma-mediated advanced glycation end products regulation of neural stem cells

Hyperglycemia is accompanied by an accelerated rate of advanced glycation end products (AGEs) formation, which is found to be associated with the pathogenesis of diabetic cognitive deficit, including Alzheimer's disease (AD). Peroxisome proliferator-activated receptor gamma (PPARgamma) plays an important role in controlling the proliferation of neural stem cells (NSCs) and their neuronal differentiation. We investigate the hypothesis that PPARgamma could mediate AGEs-related regulation of NSCs, by which AGEs possibly fulfill important roles in diabetic-related cognitive impairment. We found that AGEs down-regulated the proliferation and neurogenic differentiation of NSCs, and protein level of PPARgamma. PPARgamma agonist reversed the proliferation through the aid of AGE-BSA, with the exclusion of the neuronal differentiation of the NSCs which were also downregulated by AGE-BSA. These findings extend our understanding of the central role of PPARgamma in AGEs-related neurogenesis impairment, which probably increased risks of cognitive deficits or AD in diabetic patients.

Inhibition of inducible nitric oxide synthase reduces an acute peripheral motor neuropathy produced by dermal burn injury in mice

The systemic inflammatory response produced by a full-thickness dermal burn injury is associated with a peripheral motor neuropathy. We previously reported that a 20% body surface area (BSA) full-thickness dermal burn in C57BL6 mice produced structural and functional deficits in motor axons at a distance from the burn site. The etiology of the neuropathy, however, is not well characterized. Burn injury leads to an increase in production of a number of proinflammatory mediators, including nitric oxide (NO). We tested the hypothesis that dermal burn-induced motor neuropathy is mediated by increased production of NO. NO synthase (NOS) activity was inhibited following a 20% BSA full-thickness burn by injection of non-specific NOS inhibitor, nitro-L-arginine methyl ester or inducible NOS (iNOS) inhibitors, L-N6-(1-iminoethyl) lysine, and aminoguanidine. NOS inhibitors also prevented the reduction in ventral roots mean axon caliber and the decrease in a motor nerve conduction velocity (MCV) following burn. iNOS knockout mice prevented MCV decrease in the first 3 days post-burn, but iNOS knockout MCV was significantly reduced at 7-14 days post-burn. These results suggest that an increase in NO production generated by systemic inflammatory response pathways after burn injury contributes to the development of structural and functional deficits in peripheral motor axons.

The pathologic continuum of diabetic vascular disease

Hyperglycemia can promote vascular complications by multiple mechanisms, with formation of advanced glycation end products and increased oxidative stress proposed to contribute to both macrovascular and microvascular complications. Many of the earliest pathologic responses to hyperglycemia are manifest in the vascular cells that directly encounter elevated blood glucose levels. In the macrovasculature, these include endothelial cells and vascular smooth muscle cells. In the microvasculature, these include endothelial cells, pericytes (in retinopathy), and podocytes (in renal disease). Additionally, neovascularization arising from the vasa vasorum may promote atherosclerotic plaque progression and contribute to plaque rupture, thereby interconnecting macroangiopathy and microangiopathy.

The role of inflammatory cytokines in endothelial dysfunction

Clinical and experimental data support a link between endothelial dysfunction and inflammation. Inflammatory cytokines are important protagonists in formation of atherosclerotic plaque, eliciting effects throughout the atherosclerotic vessel. Importantly, the development of atherosclerotic lesions, regardless of the risk factor, e.g., diabetes, hypertension, obesity, is characterized by disruption in normal function of the endothelial cells. Endothelial cells, which line the internal lumen of the vasculature, are part of a complex system that regulates vasodilation and vasoconstriction, growth of vascular smooth muscle cells, inflammation, and hemostasis, maintaining a proper blood supply to tissues and regulating inflammation and coagulation. Current concepts suggest that the earliest event in atherogenesis is endothelial dysfunction, manifested by deficiencies in the production of nitric oxide (NO) and prostacyclin. The focus of this review is to summarize recent evidence showing the effects of inflammation on vascular dysfunction in ischemic-heart disease, which may prompt new directions for targeting inflammation in future therapies.

AGE/RAGE produces endothelial dysfunction in coronary arterioles in type 2 diabetic mice

We hypothesized that impaired nitric oxide (NO)-dependent dilation (endothelial dysfunction) in type 2 diabetes results, in part, from elevated production of superoxide (O(2)(*-)) induced by the interaction of advanced glycation end products (AGE)/receptor for AGE (RAGE) and TNF-alpha signaling. We assessed the role of AGE/RAGE and TNF-alpha signaling in endothelial dysfunction in type 2 diabetic (Lepr(db)) mice by evaluation of endothelial function in isolated coronary resistance vessels of normal control (nondiabetic, m Lepr(db)) and diabetic mice. Although dilation of vessels to the endothelium-independent vasodilator sodium nitroprusside (SNP) was not different between diabetic and control mice, dilation to the endothelium-dependent agonist acetylcholine (ACh) was reduced in diabetic vs. control mice. The activation of RAGE with RAGE agonist S100b eliminated SNP-potentiated dilation to ACh in Lepr(db) mice. Administration of a soluble form of RAGE (sRAGE) partially restored dilation in diabetic mice but did not affect dilation in control mice. The expression of RAGE in coronary arterioles was markedly increased in diabetic vs. control mice. We also observed in diabetic mice that augmented RAGE signaling augmented expression of TNF-alpha, because this increase was attenuated by sRAGE or NF-kappaB inhibitor MG132. Protein and mRNA expression of NAD(P)H oxidase subunits including NOX-2, p22(phox), and p40(phox) increased in diabetic compared with control mice. sRAGE significantly inhibited the expression of NAD(P)H oxidase in diabetic mice. These results indicate that AGE/RAGE signaling plays a pivotal role in regulating the production/expression of TNF-alpha, oxidative stress, and endothelial dysfunction in type 2 diabetes.

Advanced glycation endproducts (AGEs): pharmacological inhibition in diabetes

AGE inhibitors may act by various mechanisms at different steps of advanced glycation endproduct (AGE) formation (depending on oxidative stress and/or carbonyl stress) and AGE-mediated damage: trapping of reactive dicarbonyl species; antioxidant activity by transition metal chelation; other antioxidant activity including free radical scavenging; AGE cross-link breaking; AGE receptor (RAGE) blocking; RAGE signaling blocking; glycemia reduction by anti-diabetic therapy; aldose reductase inhibition; shunting of trioses-P towards the pentose-P pathway by transketolase activation. Most of the inhibitors have several sites of action. Practically one can distinguish drugs specifically developed as AGE inhibitors or AGE breakers; RAGE and receptor signaling blockers; other therapeutic compounds which were found subsequently to possess also AGE inhibitor activity, including dietary antioxidants. Encouraging results obtained in studies of various AGE inhibitors, conducted in vitro and in diabetic animals, are summarized in this review. However most of the clinical trials have been more or less disappointing, in part because of side effects; the long-term therapeutic interest of the most recently developed AGE inhibitors or breakers remains to be demonstrated in diabetes.

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.

Effects of Advanced Glycation End Products on Ezrin-Dependent Functions in LLC-PK1 Proximal Tubule Cells

We have recently shown that advanced glycation products (AGEs) bind to the ERM (ezrin, radixin, moesin) family of proteins. ERM proteins act as cross-linkers between cell membrane proteins and the actin cytoskeleton. They are also involved in signal transduction pathways. They therefore have a critical role in normal cell processes, including modulation of cell shape, adhesion, and motility. We postulate that AGEs may contribute to diabetic complications by disrupting ERM function. In support of this hypothesis, AGEs inhibit ezrin-dependent tubulogenesis of proximal tubule cells. Phosphorylation is an important activating mechanism for ERM proteins, and AGEs inhibit ezrin phosphorylation mediated by the epidermal growth factor receptor.

Inhibitors of Advanced Glycation End Product Formation and Neurovascular Dysfunction in Experimental Diabetes

Advanced glycation and lipoxidation end products (AGEs/ALEs) have been implicated in the pathogenesis of the major microvascular complications of diabetes mellitus: nephropathy, neuropathy, and retinopathy. This article reviews the evidence regarding the peripheral nerve and its vascular supply. Most investigations done to assess the role of AGEs/ALEs in animal models of diabetic neuropathy have used aminoguanidine as a prototypic inhibitor. Preventive or intervention experiments have shown treatment benefits for motor and sensory nerve conduction velocity, autonomic nitrergic neurotransmission, nerve morphometry, and nerve blood flow. The latter depends on improvements in nitric oxide-mediated endothelium-dependent vasodilation and is responsible for conduction velocity improvements. A mechanistic interpretation of aminoguanidine's action in terms of AGE/ALE inhibition is made problematic by the relative lack of specificity. However, other unrelated compounds, such as pyridoxamine and pyridoxamine analogues, have recently been shown to have beneficial effects similar to aminoguanidine, as well as to improve pain-related measures of thermal hyperalgesia and tactile allodynia. These data also stress the importance of redox metal ion-catalyzed AGE/ALE formation. A further approach is to decrease substrate availability by reducing the elevated levels of hexose and triose phosphates found in diabetes. Benfotiamine is a transketolase activator that directs these substrates to the pentose phosphate pathway, thus reducing tissue AGEs. A similar spectrum of improvements in nerve and vascular function were noted when using benfotiamine in diabetic rats. Taken together, the data provide strong support for an important role for AGEs/ALEs in the etiology of diabetic neuropathy.

Localization of the Ezrin Binding Epitope for Glycated Proteins

ERM proteins (ezrin, radixin, and moesin) have recently been identified as a new class of AGE-binding proteins. ERM proteins link the plasma membrane with the actin cytoskeleton and regulate cell shape, motility, adhesion, and signal transduction. ERM proteins have three structural domains: the N-terminal domain, a coiled midregion, and the C-terminal domain. The N-terminal domain binds to a number of plasma membrane ligands and is involved in signal transduction, while the C-domain binds to actin filaments. Binding studies with isolated structural domains showed that glycated proteins bind to an epitope within the N-terminal domain of ezrin (aa 1-324). It is postulated that some of the cellular effects of AGEs leading to diabetic complications may be mediated by binding to this region of ezrin, thereby interrupting the cross-linking between the plasma membrane and actin cytoskeleton and downstream signaling pathways. Indeed, changes in actin arrangement, cell shape, and adhesion have been described in diabetes, and AGE-BSA inhibits ezrin-dependent tubulogenesis of LLC-PK1 proximal tubular cells. For future development of antagonists, further identification of the ezrin-binding epitope for glycated proteins is required.

Transmission electron microscopy and autofluorescence findings in the cornea of diabetic rats treated with aminoguanidine

BACKGROUND

The accumulation of advanced glycation end products (AGEs) has been implicated in the pathogenesis of diabetic keratopathy. The present study was aimed to understand if aminoguanidine (AG), an AGE inhibitor, was protective against the development of corneal complications in a diabetic rat model.

METHODS

Wistar rats were divided into three experimental groups: control, diabetic, and AG-treated diabetic. Diabetes was induced in rats via a single intraperitoneal injection (60 mg/kg) of streptozocin (STZ) and AG was administered in drinking water at a dose of 1 g/L. All animals were sacrificed at the end of 10 weeks and corneas from diabetic and nondiabetic rats were analyzed via transmission electron microscopy (TEM). Corneal autofluorescence measurements were also performed in all experimental groups.

RESULTS

Electron microscopic evaluation revealed that aminoguanidine treatment in diabetic rats prevented the formation of intracellular spaces between neighbouring cells in the superficial corneal epithelium. Hyperglycemia-induced degeneration of intracellular organelles and formation of cytoplasmic vacuoles in the corneal stroma was also prevented with the treatment of AG. Corneal autofluorescence detected in the diabetic group (5.98 +/- 2.17 Fi/mg protein) was found to be significantly greater than the control (3.92 +/- 0.56 Fi/mg protein) and the AG-treated diabetic group (4.18 +/- 0.59 Fi/mg protein) (p < 0.05).

INTERPRETATION

The presented data provide evidence that AG is preventive against corneal alterations in experimental diabetes.

Advanced Glycation End Products Inhibit Tubulogenesis and Migration of Kidney Epithelial Cells in an Ezrin-Dependent Manner

Nonenzymatic glycation of proteins to form advanced glycation end products (AGE) is implicated in diabetic complications, including nephropathy. It was shown recently that AGE bind to the ERM (ezrin, radixin, and moesin) family of membrane-cytoskeletal linker proteins in renal homogenates. Herein is reported the effects of AGE-BSA on ezrin-dependent LLC-PK1 kidney epithelial cellular functions: migration and hepatocyte growth factor (HGF)-induced tubulogenesis. LLC-PK1 cells were stably transfected with cDNA for ezrin sense, ezrin antisense, and N-ezrin. Transfection of LLC-PK1 cells with ezrin antisense and dominant negative N-ezrin decreased basal tubulogenesis and migration relative to vector-only transfection, establishing the ezrin dependency of these processes. AGE-BSA (20 or 40 microM) significantly decreased HGF-induced tubulogenesis and basal migration in two vector control lines relative to BSA-treated cells. However, AGE-BSA inhibition of both HGF-induced tubulogenesis and migration was overcome by overexpressing ezrin. These results demonstrate that the AGE-ezrin interaction significantly alters cellular function. These changes may be relevant to detrimental renal consequences as a result of diabetes.

Peripheral nerve dysfunction in experimental diabetes is mediated by cyclooxygenase-2 and oxidative stress

Glucose-mediated oxidative stress and alterations in cyclooxygenase (COX) pathway activity with secondary deficits of endoneurial perfusion have been implicated in the pathogenesis of experimental diabetic neuropathy (EDN). We have previously reported that activation of the COX-2 pathway is an important mediator of neurochemical and neurovascular defects in EDN in a rat model. Considering that chemical COX inhibition may exert other pharmacological effects in addition to inhibition of COX activity, the aim of this study was to explore the role of COX-2 in experimental diabetic neuropathy, using a COX-2 knockout mouse model. Here we provide evidence that COX-2 inactivation had a protective effect against diabetes-induced motor and sensory nerve conduction slowing and impaired nerve antioxidative defense that were clearly manifest in the wild-type (COX-2(+/+)) diabetic mice. These preliminary data support the role of the activation of the COX-2 pathway in mediating sensory and motor nerve conduction velocity deficits in EDN. These findings also suggest that the COX-2 pathway seems to be an important modulator of oxidative stress in EDN.

Increased sorbitol pathway activity generates oxidative stress in tissue sites for diabetic complications

Chronic diabetic complications, in particular, nephropathy, peripheral and autonomic neuropathy, "diabetic foot," retinopathy, and cardiovascular disease, remain the major cause of morbidity and mortality in patients with diabetes mellitus. Growing evidence indicates that both increased activity of the sorbitol pathway of glucose metabolism and enhanced oxidative stress are the leading factors in the pathogenesis of diabetic complications. The relation between the two mechanisms remains the area of controversy. One group has reported that increased sorbitol pathway activity has a protective rather than detrimental role in complication-prone tissues because the pathway detoxifies toxic lipid peroxidation products. Others put forward a so-called "unifying hypothesis" suggesting that activation of several major pathways implicated in diabetic complications (e.g., sorbitol pathway) occurs due to increased production of superoxide anion radicals in mitochondria and resulting poly(ADP-ribose) polymerase activation. This review (a) presents findings supporting a key role for the sorbitol pathway in oxidative stress and oxidative stress-initiated downstream mechanisms of diabetic complications, and (b) summarizes experimental evidence against a detoxifying role of the sorbitol pathway, as well as the "unifying concept."

Role of advanced glycation end products and their receptors in development of diabetic neuropathy

Diabetic neuropathy is a life-threatening complication involving both peripheral and autonomic nerves. The hyperglycemia-induced polyol pathway as well as enhanced oxidative stress are among the factors implicated in the pathogenesis of diabetic neuropathy. Their effects are possibly exerted by direct nerve tissue damage or mediated by endothelial injury or vascular dysfunction. Formation of advanced glycation end product (AGE) is another important candidate for the cause of peripheral neuropathy. Indeed, the levels of AGEs were increased in the serum and also in the peripheral nerves obtained from diabetic patients. Structural and functional proteins of those nerves are also glycated, resulting in impaired nerve function and characteristic pathologic alterations. In addition, interaction between AGEs and their receptors induce biological effects on the target tissues for diabetic complications. In the peripheral nerve, the receptor for AGE (RAGE) is expressed in endothelial and Schwann cells. It is thus anticipated that interactions between AGEs and RAGE facilitate endoneural vascular dysfunction, leading to microangiopathy in the peripheral nerve. The roles of these mechanisms, in particular on the molecular mechanisms of AGE-RAGE interactions in the development of diabetic neuropathy are largely still speculative and yet to be explored.

Genetic difference in susceptibility to the blood-retina barrier breakdown in diabetes and oxygen-induced retinopathy

The breakdown of the blood-retina barrier (BRB) is a common feature of diabetic retinopathy. The purpose of the present study is to determine whether there are genetic differences in susceptibility to the breakdown of the BRB in diabetic retinopathy using two rat models. In streptozotocin (STZ)-induced diabetes, Brown Norway (BN) rats developed sustained vascular hyperpermeability in the retina during the entire experimental period (16 weeks of diabetes), while diabetic Sprague Dawley (SD) rats only showed retinal hyperpermeability from 3 to 10 days after the onset of diabetes. The strain difference in permeability was not correlated with the blood glucose levels in these two strains. In oxygen-induced retinopathy (OIR), BN rats developed retinal vascular hyperpermeability from postnatal day 12 (P12) to P22 with a peak at P16, which was 8.7-fold higher than that in the age-matched normal controls. In OIR-SD rats, however, hyperpermeability was observed from P14 to P18, with a peak only 2.2-fold higher than that in the controls. The strain difference in vascular hyperpermeability was correlated with the different overexpression of vascular endothelial growth factor (VEGF) in the retina of these two models. This finding suggests that genetic backgrounds contribute to the susceptibility to diabetic retinopathy.

Clinical experience with thioctacid (thioctic acid) in the treatment of distal symmetric polyneuropathy in Korean diabetic patients

OBJECTIVE

This open-label study was performed to evaluate the efficacy and safety of oral treatment with the antioxidant alpha-lipoic acid (ALA, thioctic acid) in Korean diabetic patients with distal symmetric polyneuropathy (DSP).

SUBJECTS AND METHODS

Thioctic acid was administered orally using 600 mg once daily for 8 weeks in 61 diabetic patients with symptomatic polyneuropathy. Neuropathic symptoms (pain, burning sensation, paresthesia, and numbness) were scored at baseline as well as at 4 and 8 weeks following treatment. In addition, neurological assessment was carried out before and after 8 weeks of treatment, and an overall evaluation was performed at the end of treatment. The primary endpoint was the response rate after 8 weeks of treatment, defined as an improvement in the Total Symptom Score (TSS) of > or =30%.

RESULTS

Efficacy was evaluated in 38 patients who had completed the study according to the protocol. Safety was evaluated in all 61 patients who had taken the study medication. Fasting blood glucose and HbA(1)c did not change during the study. The response rate after 8 weeks was 71.4%. At 4 weeks, the response rate was 47.4%. The TSS significantly decreased at 4 weeks, which decreased further at 8 weeks (P<.05). All the individual scores for neuropathic symptoms (pain, burning sensation, paresthesia, and numbness) were also significantly reduced at 4 weeks and further decreased at 8 weeks (P<.05). The duration of diabetes, severity and duration of diabetic polyneuropathy, and all the other demographic and metabolic parameters did not demonstrate an effect on the response rate. The parameters of neurological assessment (ankle reflexes, pin-prick test, 10-g monofilament test) and quantitative sensory tests (vibration, warm and cold sensation) were not influenced by 8 weeks of treatment with 600 mg of oral thioctic acid per day. Overall efficacy rated as "good/fair" was 86.8% by the physician and 76.3% by the patients at the end of an 8-week treatment period. Eleven episodes (18.0%) of adverse events (possibly, probably, definitely related) were reported in seven patients (11.5%).

CONCLUSION

These findings indicate that oral treatment with thioctic acid at a dose of 600 mg/day for 8 weeks improved symptoms of polyneuropathy in Korean diabetic patients without causing serious adverse events.

Use of aminoguanidine (Pimagedine) to prevent the formation of advanced glycation endproducts

Aminoguanidine (AG) is a prototype therapeutic agent for the prevention of formation of advanced glycation endproducts. It reacts rapidly with alpha,beta-dicarbonyl compounds such as methylglyoxal, glyoxal, and 3-deoxyglucosone to prevent the formation of advanced glycation endproducts (AGEs). The adducts formed are substituted 3-amino-1,2,4-triazine derivatives. Inhibition of disease mechanisms, particularly vascular complications in experimental diabetes, by AG has provided evidence that accumulation of AGEs is a risk factor for disease progression. AG has other pharmacological activities, inhibition of nitric oxide synthase and semicarbazide-sensitive amine oxidase (SSAO), at pharmacological concentrations achieved in vivo for which controls are required in anti-glycation studies. AG is a highly reactive nucleophilic reagent that reacts with many biological molecules (pyridoxal phosphate, pyruvate, glucose, malondialdehyde, and others). Use of high concentrations of AG in vitro brings these reactions and related effects into play. It is unadvisable to use concentrations of AG in excess of 500 microM if selective prevention of AGE formation is desired. The peak plasma concentration of AG in clinical therapy was ca. 50 microM. Clinical trial of AG to prevent progression of diabetic nephropathy was terminated early due to safety concerns and apparent lack of efficacy. Pharmacological scavenging of alpha-oxoaldehydes or stimulation of host alpha-oxoaldehyde detoxification remains a worthy therapeutic strategy to prevent diabetic complications and other AGE-related disorders.

Neuroprotective effect of docosahexaenoic acid-enriched phospholipids in experimental diabetic neuropathy

A deficiency in essential fatty acid metabolism has been widely reported in both human and animal diabetes. Fish oil supplementations (n-3 fatty acids), containing docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), were less effective on diabetic neuropathy than (n-6) fatty acids. This partial effect of (n-3) fatty acids might be attributed to the presence of EPA, a competitor of arachidonic acid, which enhanced the diabetes-induced decrease of this fatty acid in serum and tissues. For determining whether a supplementation with DHA alone could prevent neuropathy in streptozotocin-induced diabetes, diabetic rats were given daily, by gavage, liposomes containing DHA phospholipids, at a dose of 60 mg/kg. Eight weeks of diabetes induced significant decreases in nerve conduction velocity (NCV), nerve blood flow (NBF), and sciatic nerve and erythrocyte (red blood cells [RBCs]) Na,K-ATPase activities. DHA phospholipids totally prevented the decrease in NCV and NBF observed during diabetes when compared with the nonsupplemented diabetic group. DHA phospholipids also prevented the Na,K-ATPase activity decrease in RBC but not in sciatic nerve. Moreover, DHA level in sciatic nerve membranes was correlated with NCV. These results demonstrate a protective effect of daily doses of DHA on experimental diabetic neuropathy. Thus, treatment with DHA phospholipids could be suitable for evaluation in clinical trials.

The amino-terminal domains of the ezrin, radixin, and moesin (ERM) proteins bind advanced glycation end products, an interaction that may play a role in the development of diabetic complications

The presence of advanced glycation end products (AGEs) formed because of hyperglycemia in diabetic patients has been strongly linked to the development of diabetic complications and disturbances in cellular function. In this report, we describe the isolation and identification of novel AGE-binding proteins from diabetic rat kidneys. The proteins were purified by cation exchange and AGE-modified bovine serum albumin (AGE-BSA) affinity chromatography. NH2-terminal and internal sequencing identified the proteins as the NH2-terminal domains of ezrin, radixin, and moesin (ERM proteins). Using BIAcore biosensor analysis, human N-ezrin-(1-324) bound to immobilized AGE-BSA with a KD of 5.3 +/- 2.1 x 10 -7 m, whereas full-length ezrin-(1-586) and C-ezrin-(323-586) did not bind. Other glycated proteins such as AGE-RNase, N in -carboxymethyllysine (CML)-BSA, and glycated human serum albumin isolated from hyperglycemic diabetic sera competed with the immobilized AGE-BSA for binding to N-ezrin, but non-glycated BSA and RNase did not. Thus N-ezrin binds to AGEs in a glycation- and concentration-dependent manner. Phosphorylated ezrin plays a crucial role in cell shape changes, cell attachment, and cell adhesion. The effect of AGE-BSA on ezrin function was studied in a tubulogenesis model in which LLC-PK1 cell tubule formation is dependent on phosphorylated ezrin. Addition of AGE-BSA completely inhibited the ability of the cells to produce tubules. Furthermore, in vitro tyrosine phosphorylation of N-ezrin and ezrin was also inhibited by AGE-BSA. These proteins represent a novel family of intracellular binding molecules for glycated proteins and provide a potential new target for therapeutic intervention in the prevention or treatment of diabetic complications.

Oxidative stress and diabetic neuropathy: pathophysiological mechanisms and treatment perspectives

Increased oxidative stress is a mechanism that probably plays a major role in the development of diabetic complications, including peripheral neuropathy. This review summarises recent data from in vitro and in vivo studies that have been performed both to understand this aspect of the pathophysiology of diabetic neuropathy and to develop therapeutic modalities for its prevention or treatment. Extensive animal studies have demonstrated that oxidative stress may be a final common pathway in the development of diabetic neuropathy, and that antioxidants can prevent or reverse hyperglycaemia-induced nerve dysfunction. Most probably, the effects of antioxidants are mediated by correction of nutritive blood flow, although direct effects on endoneurial oxidative state are not excluded. In a limited number of clinical studies, antioxidant drugs including alpha-lipoic acid and vitamin E were found to reduce neuropathic symptoms or to correct nerve conduction velocity. These data are promising, and additional larger studies with alpha-lipoic acid are currently being performed.

Generation of superoxide anions during the reaction of guanidino compounds with methylglyoxal

Uremic toxins are accumulated in the blood of patients with chronic renal failure (CRF), although alteration of the toxicity by the interaction of various uremic retention products has not been precisely clarified. In this study, we found that cytochrome c added to incubation mixtures containing guanidino compounds and methylglyoxal in phosphate buffer solution (pH 7.4) resulted in reduction of cytochrome c. Superoxide anions were generated from incubation mixtures of each guanidino compound with methylglyoxal, because the reduction was inhibited by the addition of superoxide dismutase. The incubation mixture containing each guanidino compound and methylglyoxal had different rates of generation of the superoxide anion from other mixtures. A relatively higher superoxide anion formation rate was observed in the incubation mixture containing Arg and methylglyoxal (7.9 +/- 0.5nmol x m(-1) x min(-1)), or in the incubation mixture containing methylguanidine and methylglyoxal (6.3 +/- 0.6 nmol x ml(-1) min(-1)). These findings suggest that interactions of various uremic retention products which accumulate in the blood of uremic patients may generate reactive oxygen species and may be involved in the oxidative stress observed in CRF patients. The addition of aminoguanidine, which is known to inhibit the formation of advanced glycation end products, to a mixture of guanidino compounds and methylglyoxal inhibited reactions between guanidino compounds and methylglyoxal.

An aldose reductase inhibitor reverses early diabetes-induced changes in peripheral nerve function, metabolism, and antioxidative defense

Aldose reductase inhibitors (ARIs) prevent peripheral nerve dysfunction and morphological abnormalities in diabetic animal models. However, some experimental intervention studies and clinical trials of ARIs on diabetic neuropathy appeared disappointing because of either 1) their inadequate design and, in particular, insufficient correction of the sorbitol pathway activity or 2) the inability to reverse established functional and metabolic deficits of diabetic neuropathy by AR inhibition in general. We evaluated whether diabetes-induced changes in nerve function, metabolism, and antioxidative defense are corrected by the dose of ARI (sorbinil, 65 mg/kg/d in the diet), resulting in complete inhibition of increased sorbitol pathway activity. The groups included control rats and streptozotocin-diabetic rats treated with/without ARI for 2 weeks after 4 weeks of untreated diabetes. ARI treatment corrected diabetes-induced nerve functional changes; that is, decrease in endoneurial nutritive blood flow, motor and sensory nerve conduction velocities, and metabolic abnormalities (i.e., mitochondrial and cytosolic NAD+/NADH redox imbalances and energy deficiency). ARI restored nerve concentrations of two major non-enzymatic antioxidants, reduced glutathione (GSH) and ascorbate, and completely arrested diabetes-induced lipid peroxidation. In conclusion, treatment with adequate doses of ARIs (that is, doses that completely inhibit increased sorbitol pathway activity) is an effective approach for reversal of, at least, early diabetic neuropathy.

Therapeutic potential of AGE inhibitors and breakers of AGE protein cross-links

Glucose and other reducing sugars react non-enzymatically with proteins leading to the formation of advanced glycosylation end products (AGEs) and AGE-derived protein cross-linking. Formation of AGEs is a normal physiological process, which is accelerated under the hyperglycaemic condition in diabetes. Under normal conditions, AGEs build up slowly and accumulate as one ages. Numerous studies have indicated that AGEs contribute to the pathological events leading to diabetic complications, such as age-related diseases, including nephropathy, retinopathy, vasculopathy and neuropathy. Potential therapeutic approaches to prevent these complications include pharmacological inhibition of AGE formation and disruption of pre-formed AGE-protein cross-links. Studies using animal models and preliminary clinical trials have shown the ability of the AGE-inhibitor, pimagedine and the cross-link breaker, ALT-711, to reduce the severity of pathologies of advanced glycosylation. These agents offer potential treatments for glucose-derived complications of diabetes and ageing.

Taurine counteracts oxidative stress and nerve growth factor deficit in early experimental diabetic neuropathy

Oxidative stress has a key role in the pathogenesis of diabetic complications. We have previously reported that taurine (T), which is known to counteract oxidative stress in tissues (lens, kidney, retina) of diabetic rats, attenuates nerve blood flow and conduction deficits in early experimental diabetic neuropathy (EDN). The purpose of this study was to evaluate whether dietary T supplementation counteracts oxidative stress and the nerve growth factor (NGF) deficit in the diabetic peripheral nerve. The experiments were performed in control rats and streptozotocin-diabetic rats fed standard or 1% T-supplemented diets for 6 weeks. All measurements were performed in the sciatic nerve. Malondialdehyde (MDA) plus 4-hydroxyalkenals (4-HA) were quantified with N-methyl-2-phenylindole. GSH, GSSG, dehydroascorbate (DHAA), and ascorbate (AA) were assayed spectrofluorometrically, T by reverse-phase HPLC, and NGF by ELISA. MDA plus 4-HA concentration (mean +/- SEM) was increased in diabetic rats (0.127 +/- 0.006 vs 0.053 +/- 0.003 micromol/g in controls, P < 0.01), and this increase was partially prevented by T (0.096 +/- 0.004, P < 0.01 vs untreated diabetic group). GSH levels were similarly decreased in diabetic rats treated with or without taurine vs controls. GSSG levels were similar in control and diabetic rats but were lower in diabetic rats treated with T (P < 0.05 vs controls). AA levels were decreased in diabetic rats (0.133 +/- 0.015 vs 0.219 +/- 0.023 micromol/g in controls, P < 0.05), and this deficit was prevented by T. DHAA/AA ratio was increased in diabetic rats vs controls (P < 0.05), and this increase was prevented by T. T levels were decreased in diabetic rats (2.7 +/- 0.16 vs 3.8 +/- 0.1 micromol/g in controls, P < 0.05) and were repleted by T supplementation (4.2 +/- 0.3). NGF levels were decreased in diabetic rats (2.35 +/- 0.20 vs 3.57 +/- 0.20 ng/g in controls, P < 0.01), and this decrease was attenuated by T treatment (3.16 +/- 0.28, P < 0.05 vs diabetic group). In conclusion, T counteracts oxidative stress and the NGF deficit in early EDN. Antioxidant effects of T in peripheral nerve are, at least in part, mediated through the ascorbate system of antioxidative defense. The findings are consistent with the important role for oxidative stress in impaired neurotrophic support in EDN.

Pharmacological inhibition of diabetic retinopathy: aminoguanidine and aspirin

Effects of aminoguanidine and aspirin on the development of retinopathy have been examined in 5-year studies of diabetic dogs. Either agent was administered daily in doses of 20-25 mg. kg(-1). day(-1). Because severity of hyperglycemia greatly influences development of the retinopathy, special effort was devoted to maintaining comparable glycemia in experimental and control groups. The retinal vasculature was isolated by the trypsin digest method, and retinopathy was assessed by light microscopy. Diabetes for 5 years resulted, as expected, in saccular capillary aneurysms, pericyte ghosts, acellular capillaries, retinal hemorrhages, and other lesions. Administration of aminoguanidine essentially prevented the retinopathy, significantly inhibiting the development of retinal microaneurysms, acellular capillaries, and pericyte ghosts compared with diabetic controls. Aspirin significantly inhibited the development of retinal hemorrhages and acellular capillaries over the 5 years of study, but had less effect on other lesions. Although diabetes resulted in significantly increased levels of advanced glycation end products (AGEs) (namely, pentosidine in tail collagen and aorta, and Hb-AGE), aminoguanidine had no significant influence on these parameters of glycation. Nitration of a retinal protein was significantly increased in diabetes and inhibited by aminoguanidine. The biochemical mechanism by which aminoguanidine has inhibited retinopathy thus is not clear. Aminoguanidine (but not aspirin) inhibited a diabetes-induced defect in ulnar nerve conduction velocity, but neither agent was found to influence kidney structure or albumen excretion.

Effects of OPB-9195, anti-glycation agent, on experimental diabetic neuropathy

BACKGROUND

Nonenzymatic glycation of neural proteins and their end-products (advanced glycation end-products, AGE) have been implicated in the pathogenesis of diabetic neuropathy. We need a development of effective ant-glycation agents for future clinical use.

MATERIALS AND METHODS

We examined the effects of OPB-9195 (OPB), a new inhibitor of glycation, on the peripheral nerve structure and function in diabetic rats. Eight-week-old Wistar rats were made diabetic by streptozotocin (40 mg kg(-1), i.v.) and OPB (60 mg kg(-1) day(-1)) was given by gavage for 24 weeks. Age- and sex-matched normal Wistar rats were used for comparison.

RESULTS

During the experimental period, OPB treatment did not affect the reduced body weight, elevated levels of blood glucose and glycated haemoglobin in diabetic rats. At the end of the experiment, delayed tibial motor nerve conduction velocity was significantly improved (by 60%) in treated diabetic rats, with reduction of serum AGE levels. Expression of immunoreactive AGE in the sciatic nerve was reduced in treated diabetic rats compared with those in untreated rats. Sciatic nerve (Na+, K+)-ATPase activity was also restored in treated diabetic rats. On the cross-sectioned sciatic nerves, positive cells with oxidative stress-related DNA damage, as expressed by 8-hydroxy-2'-deoxyguanosine, were less in the peripheral nerve of treated diabetic rats compared with those of untreated rats.

CONCLUSION

The current study suggested that OPB is beneficial for the reduction of serum AGE and the prevention of diabetic neuropathy.

Depletion of taurine in experimental diabetic neuropathy: implications for nerve metabolic, vascular, and functional deficits

In diabetes, increased oxidative stress, disruption of signal transduction pathways, and endothelial dysfunction have been critically implicated in the pathogenesis of experimental diabetic neuropathy (EDN). The development of nerve conduction slowing in diabetes is accompanied by depletion of the beta-amino acid taurine. Since taurine functions as an antioxidant, calcium modulator, and vasodilator, taurine depletion may provide a pathogenetic link between nerve metabolic, vascular, and functional deficits complicating diabetes. The mechanism(s) of nerve taurine depletion, the localization of critical taurine deficits, and its pathophysiological significance in EDN are however unknown. This study explored the pathophysiological effects of selective nerve taurine replacement in streptozotocin-diabetic (STZ-D) rats. A polyclonal human taurine transporter (TT) antibody was also generated in order to determine potential loci of critical taurine depletion. Two weeks of STZ-D reduced sciatic motor nerve conduction velocity (NCV) by 23% (P < 0.01), decreased composite nerve blood flow by 38% (P < 0.01), and reduced nerve taurine content by 29% (P < 0.05). In STZ-D rats, a 1% taurine diet corrected nerve taurine depletion, prevented motor NCV slowing, and partially attenuated composite nerve blood flow deficits. After 6 weeks of STZ-D, a 1% taurine diet ameliorated motor NCV slowing and endoneurial nutritive blood flow deficits, prevented digital sensory NCV slowing, and reduced ouabain-sensitive nerve (Na,K)-ATPase activity. Immunohistochemical studies localized taurine and the TT to the vascular endothelium and Schwann cells of the sciatic nerve. In conclusion, taurine depletion in the vascular endothelium and Schwann cells of the sciatic nerve may contribute to the neurovascular and metabolic deficits in EDN.

Effects of aminoguanidine and desferrioxamine on some vascular and biochemical changes associated with streptozotocin-induced hyperglycaemia in rats

The effects of aminoguanidine (AG; 100 mg x kg(-1)) and desferrioxamine (DFO; 50 mg x kg(-1)) on some vascular and biochemical changes associated with streptozotocin (STZ; 65 mg x kg(-1); i.p.)-induced hyperglycaemia were investigated in rats. Both AG and DFO were administered i.p., once daily, for 14 consecutive days to normal and hyperglycaemic animals. The responsiveness of the isolated aortic rings to phenylephrine (PE) was tested. In addition, biochemical markers for oxidative stress such as plasma levels of lipid peroxides and total thiols, as well as the activities of erythrocytic superoxide dismutase (SOD) and whole blood glutathione peroxidase (GSH-Px) were assessed. Results of the present study indicated that induction of hyperglycaemia was associated with increased aortic ring responsiveness to PE, loss in body weight, increase in urine volume, elevation of plasma total thiols and lipid peroxide levels and elevated SOD and GSH-Px enzymatic activities. Treatment of normal rats with AG reduced the response of their aortae to PE. Furthermore, a profound increase in body weight without any significant change in the measured biochemical parameters was observed. In hyperglycaemic animals, AG tended to normalize the enhanced aortic response to PE and modulated STZ-induced biochemical changes without affecting the elevated plasma glucose level. Treatment of normal rats with DFO reduced the response of their aortae to PE and decreased their body weight without altering any of the chosen biochemical parameters. In hyperglycaemic animals, DFO attenuated the responsiveness of their aortae to PE and at the same time, did not affect the loss in body weight and the elevation of plasma glucose level observed in the hyperglycaemic group. Additionally, DFO normalized the elevated plasma level of total thiols and exerted a modulatory influence on the enhanced activities of SOD and GSH-Px as well as on the increased levels of lipid peroxides. Our data lend further credence for the contribution of oxidative stress in the vascular and biochemical changes associated with STZ-induced hyperglycaemia. It is also apparent that advanced glycosylation end products and nitric oxide might be involved. Until clinical studies prove the efficacy and safety of these drugs, specific agents which could scavenge free radicals and block protein glycosylation seem beneficial as a helpful adjunct to the therapy of diabetes.

Effect of alpha-tocopherol supplementation on the ultrastructural abnormalities of peripheral nerves in experimental diabetes

Ultrastructural observations were made on myelinated fibers in the tibial nerves in order to investigate the beneficial effects of alpha-tocopherol administration in streptozotocin-diabetic rats. Male Wistar rats, aged 12 weeks and weighing between 250 g to 300 g were studied. Six onset control rats were used to obtain the baseline parameters for this strain and age. Further 3 groups--untreated diabetic animals, diabetic animals treated with alpha-tocopherol, and age-matched controls--were studied over a 3-month period. In the diabetic animal, administration of alpha-tocopherol resulted in a significant increase (p < 0.05) in total plasma vitamin E levels when compared with other groups. Myelinated fiber cross-sectional area (p < 0.05), axonal area (p < 0.01) and myelin sheath area (p < 0.05) were significantly less in the tibial nerve of diabetic animals than in age-matched controls, but not different from those of onset controls. In the alpha-tocopherol treated diabetic animals, the values for these parameters were intermediate without showing significant difference when compared with age-matched controls and untreated diabetics. The "g" ratio (axon to fiber area) did not differ between any experimental groups. The number of large myelinated fibers were less in the untreated diabetic animals, but in the alpha-tocopherol-treated diabetics, the values were significantly higher (p < 0.05) than with untreated diabetics and were similar to those of age-matched controls. In conclusion, this ultrastructural study reiterated the fact that structural abnormalities of myelinated fibers occur in experimental diabetes and that alpha-tocopherol administration may be useful in preventing the development of these abnormalities.

Diabetic neuropathy--a continuing enigma

In this article we will review the clinical signs and symptoms of diabetic somatic polyneuropathy (DPN), its prevalence and clinical management. Staging and classification of DPN will be exemplified by various staging paradigms of varied sophistication. The results of therapeutic clinical trials will be summarized. The pathogenesis of diabetic neuropathy reviews an extremely complex issue that is still not fully understood. Various recent advances in the understanding of the disease will be discussed, particularly with respect to the differences between neuropathy in the two major types of diabetes. The neuropathology and natural history of diabetic neuropathy will be discussed pointing out the heterogeneities of the disease. Finally, the various prospective therapeutic avenues will be dealt with and discussed.