Mitochondrial uncoupling has no effect on microvascular complications in type 2 diabetes

"Resilience to diabetic retinopathy"

Chronic elevation of blood glucose at first causes relatively minor changes to the neural and vascular components of the retina. As the duration of hyperglycemia persists, the nature and extent of damage increases and becomes readily detectable. While this second, overt manifestation of diabetic retinopathy (DR) has been studied extensively, what prevents maximal damage from the very start of hyperglycemia remains largely unexplored. Recent studies indicate that diabetes (DM) engages mitochondria-based defense during the retinopathy-resistant phase, and thereby enables the retina to remain healthy in the face of hyperglycemia. Such resilience is transient, and its deterioration results in progressive accumulation of retinal damage. The concepts that co-emerge with these discoveries set the stage for novel intellectual and therapeutic opportunities within the DR field. Identification of biomarkers and mediators of protection from DM-mediated damage will enable development of resilience-based therapies that will indefinitely delay the onset of DR.

Manifestation of Pathology in Animal Models of Diabetic Retinopathy Is Delayed from the Onset of Diabetes

Diabetic retinopathy (DR) is the most common complication that develops in patients with diabetes mellitus (DM) and is the leading cause of blindness worldwide. Fortunately, sight-threatening forms of DR develop only after several decades of DM. This well-documented resilience to DR suggests that the retina is capable of protecting itself from DM-related damage and also that accumulation of such damage occurs only after deterioration of this resilience. Despite the enormous translational significance of this phenomenon, very little is known regarding the nature of resilience to DR. Rodent models of DR have been used extensively to study the nature of the DM-induced damage, i.e., cardinal features of DR. Many of these same animal models can be used to investigate resilience because DR is delayed from the onset of DM by several weeks or months. The purpose of this review is to provide a comprehensive overview of the literature describing the use of rodent models of DR in type-1 and type-2 diabetic animals, which most clearly document the delay between the onset of DM and the appearance of DR. These readily available experimental settings can be used to advance our current understanding of resilience to DR and thereby identify biomarkers and targets for novel, prevention-based approaches to manage patients at risk for developing DR.

Complex II ambiguities-FADH2 in the electron transfer system

The prevailing notion that reduced cofactors NADH and FADH2 transfer electrons from the tricarboxylic acid cycle to the mitochondrial electron transfer system creates ambiguities regarding respiratory Complex II (CII). CII is the only membrane-bound enzyme in the tricarboxylic acid cycle and is part of the electron transfer system of the mitochondrial inner membrane feeding electrons into the coenzyme Q-junction. The succinate dehydrogenase subunit SDHA of CII oxidizes succinate and reduces the covalently bound prosthetic group FAD to FADH2 in the canonical forward tricarboxylic acid cycle. However, several graphical representations of the electron transfer system depict FADH2 in the mitochondrial matrix as a substrate to be oxidized by CII. This leads to the false conclusion that FADH2 from the β-oxidation cycle in fatty acid oxidation feeds electrons into CII. In reality, dehydrogenases of fatty acid oxidation channel electrons to the Q-junction but not through CII. The ambiguities surrounding Complex II in the literature and educational resources call for quality control, to secure scientific standards in current communications of bioenergetics, and ultimately support adequate clinical applications. This review aims to raise awareness of the inherent ambiguity crisis, complementing efforts to address the well-acknowledged issues of credibility and reproducibility.

Head-to-head comparison of BAM15, semaglutide, rosiglitazone, NEN, and calorie restriction on metabolic physiology in female db/db mice

Metabolic disorders such as type 2 diabetes, fatty liver disease, hyperlipidemia, and obesity commonly co-occur but clinical treatment options do not effectively target all disorders. Calorie restriction, semaglutide, rosiglitazone, and mitochondrial uncouplers have all demonstrated efficacy against one or more obesity-related metabolic disorders, but it currently remains unclear which therapeutic strategy best targets the combination of hyperglycaemia, liver fat, hypertriglyceridemia, and adiposity. Herein we performed a head-to-head comparison of 5 treatment interventions in the female db/db mouse model of severe metabolic disease. Treatments included ∼60 % calorie restriction (CR), semaglutide, rosiglitazone, BAM15, and niclosamide ethanolamine (NEN). Results showed that BAM15 and CR improved body weight and liver steatosis to levels superior to semaglutide, NEN, and rosiglitazone, while BAM15, semaglutide, and rosiglitazone improved glucose tolerance better than CR and NEN. BAM15, CR, semaglutide, and rosiglitazone all had efficacy against hypertriglyceridaemia. These data provide a comprehensive head-to-head comparison of several key treatment strategies for metabolic disease and highlight the efficacy of mitochondrial uncoupling to correct multiple facets of the metabolic disease milieu in female db/db mice.

New perspectives in diabetic neuropathy

Diabetes prevalence continues to climb with the aging population. Type 2 diabetes (T2D), which constitutes most cases, is metabolically acquired. Diabetic peripheral neuropathy (DPN), the most common microvascular complication, is length-dependent damage to peripheral nerves. DPN pathogenesis is complex, but, at its core, it can be viewed as a state of impaired metabolism and bioenergetics failure operating against the backdrop of long peripheral nerve axons supported by glia. This unique peripheral nerve anatomy and the injury consequent to T2D underpins the distal-to-proximal symptomatology of DPN. Earlier work focused on the impact of hyperglycemia on nerve damage and bioenergetics failure, but recent evidence additionally implicates contributions from obesity and dyslipidemia. This review will cover peripheral nerve anatomy, bioenergetics, and glia-axon interactions, building the framework for understanding how hyperglycemia and dyslipidemia induce bioenergetics failure in DPN. DPN and painful DPN still lack disease-modifying therapies, and research on novel mechanism-based approaches is also covered.

Targeting negative energy balance with calorie restriction and mitochondrial uncoupling in db/db mice

OBJECTIVE

Calorie restriction is a first-line treatment for overweight individuals with metabolic impairments. However, few patients can adhere to long-term calorie restriction. An alternative approach to calorie restriction that also causes negative energy balance is mitochondrial uncoupling, which decreases the amount of energy that can be extracted from food. Herein we compare the metabolic effects of calorie restriction with the mitochondrial uncoupler BAM15 in the db/db mouse model of severe hyperglycemia, obesity, hypertriglyceridemia, and fatty liver.

METHODS

Male db/db mice were treated with ∼50% calorie restriction, BAM15 at two doses of 0.1% and 0.2% (w/w) admixed in diet, or 0.2% BAM15 with time-restricted feeding from 5 weeks of age. Mice were metabolically phenotyped over 4 weeks with assessment of key readouts including body weight, glucose tolerance, and liver steatosis. At termination, liver tissues were analysed by metabolomics and qPCR.

RESULTS

Calorie restriction and high-dose 0.2% BAM15 decreased body weight to a similar extent, but mice treated with BAM15 had far better improvement in glucose control. High-dose BAM15 treatment completely normalized fasting glucose and glucose tolerance to levels similar to lean db/+ control mice. Low-dose 0.1% BAM15 did not affect body mass but partially improved glucose tolerance to a similar degree as 50% calorie restriction. Both calorie restriction and high-dose BAM15 significantly improved hyperglucagonemia and liver and serum triglyceride levels. Combining high-dose BAM15 with time-restricted feeding to match the time that calorie restricted mice were fed resulted in the best metabolic phenotype most similar to lean db/+ controls. BAM15-mediated improvements in glucose control were associated with decreased glucagon levels and decreased expression of enzymes involved in hepatic gluconeogenesis.

CONCLUSIONS

BAM15 and calorie restriction treatments improved most metabolic disease phenotypes in db/db mice. However, mice fed BAM15 had superior effects on glucose control compared to the calorie restricted group that consumed half as much food. Submaximal dosing with BAM15 demonstrated that its beneficial effects on glucose control are independent of weight loss. These data highlight the potential for mitochondrial uncoupler pharmacotherapies in the treatment of metabolic disease.

Identification of Immune Infiltration and the Potential Biomarkers in Diabetic Peripheral Neuropathy through Bioinformatics and Machine Learning Methods

Diabetic peripheral neuropathy (DPN) is one of the most common chronic complications in diabetes. Previous studies have shown that chronic neuroinflammation was associated with DPN. However, further research is needed to investigate the exact immune molecular mechanism underlying the pathogenesis of DPN. Expression profiles were downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were screened by R software. After functional enrichment analysis of DEGs, a protein-protein interaction (PPI) network analysis was performed. The CIBERSORT algorithm was used to evaluate the infiltration of immune cells in DPN. Next, the least absolute shrinkage and selection operator (LASSO) logistic regression and support vector machine-recursive feature elimination (SVM-RFE) algorithms were applied to identify potential DPN diagnostic markers. Finally, the results were further validated by qRT-PCR. A total of 1308 DEGs were screened in this study. Enrichment analysis identified that DEGs were significantly enriched in immune-related biological functions and pathways. Immune cell infiltration analysis found that M1 and M2 macrophages, monocytes, resting mast cells, resting CD4 memory T cells and follicular helper T cells were involved in the development of DPN. LTBP2 and GPNMB were identified as diagnostic markers of DPN. qRT-PCR results showed that 15 mRNAs, including LTBP2 and GPNMB, were differentially expressed, consistent with the microarray results. In conclusion, LTBP2 and GPNMB can be used as novel candidate molecular diagnostic markers for DPN. Furthermore, the infiltration of immune cells plays an important role in the progression of DPN.

Hyperglycemia Promotes Mitophagy and Thereby Mitigates Hyperglycemia-Induced Damage

The observation that diabetic retinopathy (DR) typically takes decades to develop suggests the existence of an endogenous system that protects from diabetes-induced damage. To investigate the existance of such a system, primary human retinal endothelial cells were cultured in either normal glucose (5 mmol/L) or high glucose (30 mmol/L; HG). Prolonged exposure to HG was beneficial instead of detrimental. Although tumor necrosis factor-α-induced expression of vascular cell adhesion molecule 1 and intercellular adhesion molecule 1 was unaffected after 1 day of HG, it waned as the exposure to HG was extended. Similarly, oxidative stress-induced death decreased with prolonged exposure to HG. Furthermore, mitochondrial functionality, which was compromised by 1 day of HG, was improved by 10 days of HG, and this change required increased clearance of damaged mitochondria (mitophagy). Finally, antagonizing mitochondrial dynamics compromised the cells' ability to endure HG: susceptibility to cell death increased, and basal barrier function and responsiveness to vascular endothelial growth factor deteriorated. These observations indicate the existence of an endogenous system that protects human retinal endothelial cells from the deleterious effects of HG. Hyperglycemia-induced mitochondrial adaptation is a plausible contributor to the mechanism responsible for the delayed onset of DR; loss of hyperglycemia-induced mitochondrial adaptation may set the stage for the development of DR.

Long-term exposure to residential surrounding greenness and incidence of diabetes: A prospective cohort study

Exposure to residential greenness might affect population health through increasing physical activity and social engagement, improving mental health, and reducing harmful environmental exposure. However, evidence on the association of greenness with risk of diabetes is still controversial. In this study, we recruited a total of 22,535 participants aged ≥18 years from Yinzhou District, Ningbo, Zhejiang Province, China to investigate the associations between residential greenness and risk of diabetes incidence. Residential greenness was estimated using Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), and Vegetation Continuous Field (VCF). We also calculated cumulative average NDVI, EVI and VCF values, and changes in NDVI, EVI and VCF during the follow-up period. We used Cox proportional hazards models controlling for demographic characteristics, lifestyles, individual socioeconomic status, history of diseases and particulate matter with an aerodynamic diameter ≤2.5 μm (PM_2.5) to examine hazard ratios (HRs) and 95% confidence intervals (95% CIs) and assessed physical activity, body mass index (BMI) or PM_2.5 as potential mediators. During 84,992.64 person-years of follow-up, a total of 1,154 incident cases of diabetes occurred. In multivariable models, living in the highest quartile of cumulative average NDVI, EVI and VCF within 250-m buffer was associated with 57% (HR = 0.43, 95% CI: 0.36, 0.52), 62% (HR = 0.38, 95% CI: 0.32, 0.45), and 55% (HR = 0.45, 95% CI: 0.38, 0.54) reduction in diabetes risk compared with the lowest quartile, respectively. Results remained similar for NDVI, EVI, and VCF within 500-m and 1000-m buffers. Stratified analyses showed stronger association for residential greenness and diabetes among older people. The association between greenness and diabetes did not appear to be mediated by physical activity, PM_2.5 or BMI. Our findings suggested that higher residential greenness was significantly associated with lower risk of diabetes.

Plasma lipid metabolites associate with diabetic polyneuropathy in a cohort with type 2 diabetes

Objective

The global rise in type 2 diabetes is associated with a concomitant increase in diabetic complications. Diabetic polyneuropathy is the most frequent type 2 diabetes complication and is associated with poor outcomes. The metabolic syndrome has emerged as a major risk factor for diabetic polyneuropathy; however, the metabolites associated with the metabolic syndrome that correlate with diabetic polyneuropathy are unknown.

Methods

We conducted a global metabolomics analysis on plasma samples from a subcohort of participants from the Danish arm of Anglo‐Danish‐Dutch study of Intensive Treatment of Diabetes in Primary Care (ADDITION‐Denmark) with and without diabetic polyneuropathy versus lean control participants.

Results

Compared to lean controls, type 2 diabetes participants had significantly higher HbA1c (p = 0.0028), BMI (p = 0.0004), and waist circumference (p = 0.0001), but lower total cholesterol (p = 0.0001). Out of 991 total metabolites, we identified 15 plasma metabolites that differed in type 2 diabetes participants by diabetic polyneuropathy status, including metabolites belonging to energy, lipid, and xenobiotic pathways, among others. Additionally, these metabolites correlated with alterations in plasma lipid metabolites in type 2 diabetes participants based on neuropathy status. Further evaluating all plasma lipid metabolites identified a shift in abundance, chain length, and saturation of free fatty acids in type 2 diabetes participants. Importantly, the presence of diabetic polyneuropathy impacted the abundance of plasma complex lipids, including acylcarnitines and sphingolipids.

Interpretation

Our explorative study suggests that diabetic polyneuropathy in type 2 diabetes is associated with novel alterations in plasma metabolites related to lipid metabolism.

High Dietary Fat Consumption Impairs Axonal Mitochondrial Function In Vivo

Peripheral neuropathy (PN) is the most common complication of prediabetes and diabetes. PN causes severe morbidity for Type 2 diabetes (T2D) and prediabetes patients, including limb pain followed by numbness resulting from peripheral nerve damage. PN in T2D and prediabetes is associated with dyslipidemia and elevated circulating lipids; however, the molecular mechanisms underlying PN development in prediabetes and T2D are unknown. Peripheral nerve sensory neurons rely on axonal mitochondria to provide energy for nerve impulse conduction under homeostatic conditions. Models of dyslipidemia in vitro demonstrate mitochondrial dysfunction in sensory neurons exposed to elevated levels of exogenous fatty acids. Herein, we evaluated the effect of dyslipidemia on mitochondrial function and dynamics in sensory axons of the saphenous nerve of a male high-fat diet (HFD)-fed murine model of prediabetes to identify mitochondrial alterations that correlate with PN pathogenesis in vivo We found that the HFD decreased mitochondrial membrane potential (MMP) in axonal mitochondria and reduced the ability of sensory neurons to conduct at physiological frequencies. Unlike mitochondria in control axons, which dissipated their MMP in response to increased impulse frequency (from 1 to 50 Hz), HFD mitochondria dissipated less MMP in response to axonal energy demand, suggesting a lack of reserve capacity. The HFD also decreased sensory axonal Ca²⁺ levels and increased mitochondrial lengthening and expression of PGC1α, a master regulator of mitochondrial biogenesis. Together, these results suggest that mitochondrial dysfunction underlies an imbalance of axonal energy and Ca²⁺ levels and impairs impulse conduction within the saphenous nerve in prediabetic PN.SIGNIFICANCE STATEMENT Diabetes and prediabetes are leading causes of peripheral neuropathy (PN) worldwide. PN has no cure, but development in diabetes and prediabetes is associated with dyslipidemia, including elevated levels of saturated fatty acids. Saturated fatty acids impair mitochondrial dynamics and function in cultured neurons, indicating a role for mitochondrial dysfunction in PN progression; however, the effect of elevated circulating fatty acids on the peripheral nervous system in vivo is unknown. In this study, we identify early pathogenic events in sensory nerve axons of mice with high-fat diet-induced PN, including alterations in mitochondrial function, axonal conduction, and intra-axonal calcium, that provide important insight into potential PN mechanisms associated with prediabetes and dyslipidemia in vivo.

Therapeutic potential of mitochondrial uncouplers for the treatment of metabolic associated fatty liver disease and NASH

Background

Mitochondrial uncouplers shuttle protons across the inner mitochondrial membrane via a pathway that is independent of adenosine triphosphate (ATP) synthase, thereby uncoupling nutrient oxidation from ATP production and dissipating the proton gradient as heat. While initial toxicity concerns hindered their therapeutic development in the early 1930s, there has been increased interest in exploring the therapeutic potential of mitochondrial uncouplers for the treatment of metabolic diseases.

Scope of review

In this review, we cover recent advances in the mechanisms by which mitochondrial uncouplers regulate biological processes and disease, with a particular focus on metabolic associated fatty liver disease (MAFLD), nonalcoholic hepatosteatosis (NASH), insulin resistance, and type 2 diabetes (T2D). We also discuss the challenges that remain to be addressed before synthetic and natural mitochondrial uncouplers can successfully enter the clinic.

Major conclusions

Rodent and non-human primate studies suggest that a myriad of small molecule mitochondrial uncouplers can safely reverse MAFLD/NASH with a wide therapeutic index. Despite this, further characterization of the tissue- and cell-specific effects of mitochondrial uncouplers is needed. We propose targeting the dosing of mitochondrial uncouplers to specific tissues such as the liver and/or developing molecules with self-limiting properties to induce a subtle and sustained increase in mitochondrial inefficiency, thereby avoiding systemic toxicity concerns.

Differential Effects of Empagliflozin on Microvascular Complications in Murine Models of Type 1 and Type 2 Diabetes

Microvascular complications account for the significant morbidity associated with diabetes. Despite tight glycemic control, disease risk remains especially in type 2 diabetes (T2D) patients and no therapy fully prevents nerve, retinal, or renal damage in type 1 diabetes (T1D) or T2D. Therefore, new antidiabetic drug classes are being evaluated for the treatment of microvascular complications. We investigated the effect of empagliflozin (EMPA), an inhibitor of the sodium/glucose cotransporter 2 (SGLT2), on diabetic neuropathy (DPN), retinopathy (DR), and kidney disease (DKD) in streptozotocin-induced T1D and db/db T2D mouse models. EMPA lowered blood glycemia in T1D and T2D models. However, it did not ameliorate any microvascular complications in the T2D model, which was unexpected, given the protective effect of SGLT2 inhibitors on DKD progression in T2D subjects. Although EMPA did not improve DKD in the T1D model, it had a potential modest effect on DR measures and favorably impacted DPN as well as systemic oxidative stress. These results support the concept that glucose-centric treatments are more effective for DPN in T1D versus T2D. This is the first study that provides an evaluation of EMPA treatment on all microvascular complications in a side-by-side comparison in T1D and T2D models.

Diabetic Retinopathy: The Role of Mitochondria in the Neural Retina and Microvascular Disease

Diabetic retinopathy (DR), a common chronic complication of diabetes mellitus and the leading cause of vision loss in the working-age population, is clinically defined as a microvascular disease that involves damage of the retinal capillaries with secondary visual impairment. While its clinical diagnosis is based on vascular pathology, DR is associated with early abnormalities in the electroretinogram, indicating alterations of the neural retina and impaired visual signaling. The pathogenesis of DR is complex and likely involves the simultaneous dysregulation of multiple metabolic and signaling pathways through the retinal neurovascular unit. There is evidence that microvascular disease in DR is caused in part by altered energetic metabolism in the neural retina and specifically from signals originating in the photoreceptors. In this review, we discuss the main pathogenic mechanisms that link alterations in neural retina bioenergetics with vascular regression in DR. We focus specifically on the recent developments related to alterations in mitochondrial metabolism including energetic substrate selection, mitochondrial function, oxidation-reduction (redox) imbalance, and oxidative stress, and critically discuss the mechanisms of these changes and their consequences on retinal function. We also acknowledge implications for emerging therapeutic approaches and future research directions to find novel mitochondria-targeted therapeutic strategies to correct bioenergetics in diabetes. We conclude that retinal bioenergetics is affected in the early stages of diabetes with consequences beyond changes in ATP content, and that maintaining mitochondrial integrity may alleviate retinal disease.

Differential effects of minocycline on microvascular complications in murine models of type 1 and type 2 diabetes

Diabetes is a global healthcare problem associated with enormous healthcare and personal costs. Despite glucose lowering agents that control glycaemia, both type 1 (T1D) and type (T2D) diabetes patients often develop microvascular complications that increase morbidity and mortality. Current interventions rely on careful glycemic control and healthy lifestyle choices, but these are ineffective at reversing or completely preventing the major microvascular complications, diabetic peripheral neuropathy (DPN), diabetic retinopathy (DR), and diabetic kidney disease (DKD). Minocycline, a tetracycline antibiotic with anti-inflammatory and anti-apoptotic properties, has been proposed as a protective agent in diabetes. However, there are no reported studies evaluating the therapeutic efficacy of minocycline in T1D and T2D models for all microvascular complications (DPN, DR, and DKD). Therefore, we performed metabolic profiling in streptozotocin-induced T1D and db/db T2D models and compared the efficacy of minocycline in preventing complications to that of insulin and pioglitazone in both models. Minocycline partially ameliorated DR and DKD in T1D and T2D animals, but was less effective than insulin or pioglitazone, and failed to improve DPN in either model. These results suggest that minocycline is unlikely to improve outcomes beyond that achieved with current available therapies in patients with T1D or T2D associated microvascular complications.

The emerging role of dyslipidemia in diabetic microvascular complications

PURPOSE OF REVIEW

To summarize recent advancements in our understanding of the impact of dyslipidemia on microvascular complications in type 2 diabetes (T2D), with an emphasis on peripheral neuropathy and nephropathy.

RECENT FINDINGS

Mounting evidence suggests that rigorous glycemic control only mitigates certain microvascular complications in T2D patients. Particularly, well regulated blood glucose levels only marginally improve peripheral neuropathy in the T2D setting. Dyslipidemia, an abnormal lipid profile, is emerging as a key factor in peripheral neuropathy. Furthermore, although glycemic control may prevent or slow nephropathy, recent developments demonstrate that dyslipidemia can also affect kidney outcomes in normoglycemic patients. Transcriptomic, epigenomic, and lipidomic investigations, as well as integrative approaches, are shedding light on potential pathomechanisms. These molecular studies are identifying possible targets for therapeutic intervention. Complementing molecular research, lifestyle interventions are on-going to assess whether dietary choices and/or exercise, weight-loss, or surgical interventions, such as bariatric surgery, can ameliorate peripheral neuropathy and nephropathy in T2D patients.

SUMMARY

Dyslipidemia is an emerging mechanism in microvascular complications in T2D. Elucidating the molecular pathomechanisms may pinpoint potential lipid-centric treatments. Interventional studies of dietary changes, exercise, or weight-loss surgery may also positively impact these highly prevalent and morbid complications.

Evaluation of the andrographolides role and its indoleamine 2,3-dioxygenase inhibitory potential and attendant molecular mechanism against STZ-induced diabetic rats

The study is to scrutinize andrographolides with Indoleamine 2,3-dioxygenase (IDO) inhibitory potential, its molecular mechanism against streptozotocin (STZ) diabetic retinopathy (DR) in Wistar rats. Oxidative stress markers such as Kynurenine metabolites, retinal histopathological changes have been studied. Further, IDO gene expression and docking studies have been performed. Andrographolide treated rats have been reducing the level of thiobarbituric acid reactive substances and protein carbonyls Kynurenine metabolites with an improvement in the level of GSH and expression of IDO as revealed by morphological changes in inner and outer nuclear layer of the retina. The current results of this study have been generated information about an activity of the andrographolide in the essential pocket of IDO. Our results explain, involving IDO and andrographolide would constitute an attempt to identify natural products with therapeutic value and further studies in this direction would be of immense significance in the administration of diabetes and its related problems.

Ranirestat Improved Nerve Conduction Velocities, Sensory Perception, and Intraepidermal Nerve Fiber Density in Rats with Overt Diabetic Polyneuropathy

Distal sensory-motor polyneuropathy is one of the most frequent diabetic complications. However, few therapies address the etiology of neurodegeneration in the peripheral nervous systems of diabetic patients. Several metabolic mechanisms have been proposed as etiologies of this polyneuropathy. In this study, we revisited one of those mechanisms, the polyol pathway, and investigated the curative effects of a novel strong aldose reductase inhibitor, ranirestat, in streptozotocin-induced diabetic rats with preexisting polyneuropathy. Twelve weeks after the onset of diabetes, rats which had an established polyneuropathy were treated once daily with a placebo, ranirestat, or epalrestat, over 6 weeks. Before and after the treatment, nerve conduction velocities and thermal perception threshold of hindlimbs were examined. After the treatment, intraepidermal fiber density was evaluated. As an ex vivo assay, murine dorsal root ganglion cells were dispersed and cultured with or without 1 μmol/l ranirestat for 48 hours. After the culture, neurite outgrowth was quantified using immunological staining. Sensory nerve conduction velocity increased in diabetic rats treated with ranirestat (43.3 ± 3.6 m/s) compared with rats treated with placebo (39.8 ± 2.3). Motor nerve conduction velocity also increased in the ranirestat group (45.6 ± 3.9) compared with the placebo group (38.9 ± 3.5). The foot withdrawal latency to noxious heating was improved in the ranirestat group (17.7 ± 0.6 seconds) compared with the placebo group (20.6 ± 0.6). The decrease in the intraepidermal fiber density was significant in the diabetic placebo group (21.6 ± 1.7/mm) but not significant in the diabetic ranirestat group (26.2 ± 1.2) compared with the nondiabetic placebo group (30.3 ± 1.5). Neurite outgrowth was promoted in the neurons supplemented with ranirestat (control 1446 ± 147 μm/neuron, ranirestat 2175 ± 149). Ranirestat improved the peripheral nervous dysfunctions in rats with advanced diabetic polyneuropathy. Ranirestat could have potential for regeneration in the peripheral nervous system of diabetic rats.