Diabetic neuropathy is the most common and debilitating complication of diabetes mellitus with over half of all patients developing altered sensation as a result of damage to peripheral sensory neurons. Hyperglycemia results in altered nerve conduction velocities, loss of epidermal innervation, and the development of painful or painless signs and symptoms in the feet and hands. Current research has been unable to determine if a patient will develop insensate or painful neuropathy or be protected from peripheral nerve damage all together. One of the mechanisms that has been recognized to have a role in the pathogenesis of sensory neuron damage is the process of reactive dicarbonyls forming advanced glycation endproducts (AGEs) as a direct result of hyperglycemia. The glyoxalase system, composed of the enzymes glyoxalase I (GLO1) and glyoxalase II, is the main detoxification pathway involved in breaking down toxic reactive dicarbonyls before producing carbonyl stress and forming AGEs on proteins, lipids, or nucleic acids. This review discusses AGEs, GLO1, their role in diabetic neuropathy, and potential therapeutic targets of the AGE pathway.

Current experiments investigated whether a ketogenic diet impacts neuropathy associated with obesity and prediabetes. Mice challenged with a ketogenic diet were compared to mice fed a high-fat diet or a high-fat diet plus exercise. Additionally, an intervention switching to a ketogenic diet following 8 weeks of high-fat diet was performed to compare how a control diet, exercise, or a ketogenic diet affects metabolic syndrome-induced neural complications. When challenged with a ketogenic diet, mice had reduced bodyweight and fat mass compared to high-fat-fed mice, and were similar to exercised, high-fat-fed mice. High-fat-fed, exercised and ketogenic-fed mice had mildly elevated blood glucose; conversely, ketogenic diet-fed mice were unique in having reduced serum insulin levels. Ketogenic diet-fed mice never developed mechanical allodynia contrary to mice fed a high-fat diet. Ketogenic diet fed mice also had increased epidermal axon density compared all other groups. When a ketogenic diet was used as an intervention, a ketogenic diet was unable to reverse high-fat fed-induced metabolic changes but was able to significantly reverse a high-fat diet-induced mechanical allodynia. As an intervention, a ketogenic diet also increased epidermal axon density. In vitro studies revealed increased neurite outgrowth in sensory neurons from mice fed a ketogenic diet and in neurons from normal diet-fed mice given ketone bodies in the culture medium. These results suggest a ketogenic diet can prevent certain complications of prediabetes and provides significant benefits to peripheral axons and sensory dysfunction.

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Diabetic neuropathy is a common complication of diabetes mellitus with over half of all patients developing neuropathy symptoms due to sensory nerve damage. Diabetes-induced hyperglycemia leads to the accelerated production of advanced glycation end products (AGEs) that alter proteins, thereby leading to neuronal dysfunction. The glyoxalase enzyme system, specifically glyoxalase I (GLO1), is responsible for detoxifying precursors of AGEs, such as methylglyoxal and other reactive dicarbonyls. The purpose of our studies was to determine if expression differences of GLO1 may play a role in the development of diabetic sensory neuropathy. BALB/cJ mice naturally express low levels of GLO1, while BALB/cByJ express approximately 10-fold higher levels on a similar genetic background due to increased copy numbers of GLO1. Five weeks following STZ injection, diabetic BALB/cJ mice developed a 68% increase in mechanical thresholds, characteristic of insensate neuropathy or loss of mechanical sensitivity. This behavior change correlated with a 38% reduction in intraepidermal nerve fiber density (IENFD). Diabetic BALB/cJ mice also had reduced expression of mitochondrial oxidative phosphorylation proteins in Complex I and V by 83% and 47%, respectively. Conversely, diabetic BALB/cByJ mice did not develop signs of neuropathy, changes in IENFD, or alterations in mitochondrial protein expression. Reduced expression of GLO1 paired with diabetes-induced hyperglycemia may lead to neuronal mitochondrial damage and symptoms of diabetic neuropathy. Therefore, AGEs, the glyoxalase system, and mitochondrial dysfunction may play a role in the development and modulation of diabetic peripheral neuropathy.

Aims/Hypothesis Diabetic peripheral neuropathy (DN) is a common complication of diabetes; however, the mechanisms leading to positive or negative symptoms are not well understood. GLO1 is an enzyme that detoxifies reactive carbonyls that form advanced glycation endproducts (AGEs), and GLO1 may affect how sensory neurons respond to heightened AGE levels in DN. We hypothesize differential GLO1 expression levels in sensory neurons may lead to differences in AGE formation and modulate the phenotype of DN. Methods Inbred strains of mice were used to assess the variability of GLO1 expression by qRT-PCR. Nondiabetic C57BL/6 mice were used to characterize the expression of GLO1 in neural tissues by immunofluorescence. Behavioral assessments were conducted on nondiabetic and diabetic A/J and C57BL/6 mice to determine mechanical sensitivity and GLO1 expression was determined by Western blot. Results GLO1immunoreactivity is found through the central and peripheral nervous system, but selectively in small, unmyelinated peptidergic DRG neurons that are involved in pain transmission. GLO1 protein levels are present in varied levels in the DRG from different inbred mice strains. Diabetic A/J and C57BL/6 mice, two mouse strains that express divergent levels of GLO1, display dramatically different behavioral responses to mechanical stimuli. Diabetic C57BL/6 also show reduced expression of GLO1 following diabetes induction. Conclusions/Interpretations These findings suggest certain DRG neurons are better able to protect against hyperglycemia-induced damage, while others are more susceptible to toxic effects of reactive dicarbonyls. This may have important implications on modulation of mechanical sensitivity and lead to the development of different symptoms of neuropathy.