Dual effect of methylglyoxal on the intracellular Ca2+ signaling and neurite outgrowth in mouse sensory neurons

Use of physiologically based kinetic modeling to predict neurotoxicity and genotoxicity of methylglyoxal in humans

This study aimed to evaluate human neurotoxicity and genotoxicity risks from dietary and endogenous methylglyoxal (MGO), utilizing physiologically based kinetic (PBK) modeling-facilitated reverse dosimetry as a new approach methodology (NAM) to extrapolate in vitro toxicity data to in vivo dose-response predictions. A human PBK model was defined based on a newly developed and evaluated mouse model enabling the translation of in vitro toxicity data for MGO from human stem cell-derived neurons and WM-266-4 melanoma cells into quantitative human in vivo toxicity data and subsequent risk assessment by the margin of exposure (MOE) approach. The results show that the MOEs resulting from daily dietary intake did not raise a concern for endpoints for neurotoxicity including mitochondrial function, cytotoxicity, and apoptosis, while those for DNA adduct formation could not exclude a concern over genotoxicity. Endogenous MGO formation, especially under diabetic conditions, resulted in MOEs that raised concern not only for genotoxicity but also for some of the neurotoxicity endpoints evaluated. Thus, the results also point to the importance of taking the endogenous levels into account in the risk assessment of MGO.

3D human stem-cell-derived neuronal spheroids for in vitro neurotoxicity testing of methylglyoxal, highly reactive glycolysis byproduct and potent glycating agent

Human-derived three-dimensional (3D) in vitro models are advanced human cell-based model for their complexity, relevance and application in toxicity testing. Intracellular accumulation of methylglyoxal (MGO), the most potent glycating agent in humans, mainly generated as a by-product of glycolysis, is associated with age-related diseases including neurodegenerative disorders. In our study, 3D human stem-cell-derived neuronal spheroids were set up and applied to evaluate cytotoxic effects after short-term (5 to 48 h) treatments with different MGO concentrations, including low levels, taking into consideration several biochemical endpoints. In MGO-treated neurospheroids, reduced cell growth proliferation and decreased cell viability occurred early from 5-10 μM, and their compactness diminished starting from 100 μM, apparently without affecting spheroid size. MGO markedly caused loss of the neuronal markers MAP-2 and NSE from 10-50 μM, decreased the detoxifying Glo1 enzyme from 50 μM, and activated NF-kB by nuclear translocation. The cytochemical evaluation of the 3D sections showed the presence of necrotic cells with loss of nuclei. Apoptotic cells were observed from 50 μM MGO after 48 h, and from 100 μM after 24 h. MGO (50-10 µM) also induced modifications of the cell-cell and cell-ECM interactions. These effects worsened at the higher concentrations (300-500 µM). In 3D neuronal spheroids, MGO tested concentrations comparable to human samples levels measured in MGO-associated diseases, altered neuronal key signalling endpoints relevant for the pathogenesis of neurodegenerative diseases and aging. The findings also demonstrated that the use of 3D neuronal spheroids of human origin can be useful in a strategy in vitro for testing MGO and other dicarbonyls evaluation.

Methylglyoxal-induced neurotoxic effects in primary neuronal-like cells transdifferentiated from human mesenchymal stem cells: Impact of low concentrations

In the last decades, advanced glycation end-products (AGEs) have aroused the interest of the scientific community due to the increasing evidence of their involvement in many pathophysiological processes including various neurological disorders and cognitive decline age related. Methylglyoxal (MG) is one of the reactive dicarbonyl precursors of AGEs, mainly generated as a by-product of glycolysis, whose accumulation induces neurotoxicity. In our study, MG cytotoxicity was evaluated employing a human stem cell-derived model, namely, neuron-like cells (hNLCs) transdifferentiated from mesenchymal stem/stromal cells, which served as a source of human based species-specific "healthy" cells. MG increased ROS production and induced the first characteristic apoptotic hallmarks already at low concentrations (≥10 μM), decreased the cell growth (≥5-10 μM) and viability (≥25 μM), altered Glo-1 and Glo-2 enzymes (≥25 μM), and markedly affected the neuronal markers MAP-2 and NSE causing their loss at low MG concentrations (≥10 μM). Morphological alterations started at 100 μM, followed by even more marked effects and cell death after few hours (5 h) from 200 μM MG addition. Substantially, most effects occurred as low as 10 μM, concentration much lower than that reported from previous observations using different in vitro cell-based models (e.g., human neuroblastoma cell lines, primary animal cells, and human iPSCs). Remarkably, this low effective concentration approaches the level range measured in biological samples of pathological subjects. The use of a suitable cellular model, that is, human primary neurons, can provide an additional valuable tool, mimicking better the physiological and biochemical properties of brain cells, in order to evaluate the mechanistic basis of molecular and cellular alterations in CNS.

Overexpressing glyoxalase 1 attenuates acute hyperglycemia-exacerbated neurological deficits of ischemic stroke in mice

Stress-induced hyperglycemia (SIH) is associated with poor functional recovery and high mortality in patients with acute ischemic stroke (AIS). However, intensive controlling of blood glucose by using insulin was not beneficial in patients with AIS and acute hyperglycemia. This study investigated the therapeutic effects of the overexpression of glyoxalase I (GLO1), a detoxifying enzyme of glycotoxins, on acute hyperglycemia-aggravated ischemic brain injury.  In the present study, adeno-associated viral (AAV)-mediated GLO1 overexpression reduced infarct volume and edema level but did not improve neurofunctional recovery in the mice with middle cerebral artery occlusion (MCAO). AAV-GLO1 infection significantly enhanced neurofunctional recovery in the MCAO mice with acute hyperglycemia but not in the mice with normoglycemia. Methylglyoxal (MG)-modified proteins expression significantly increased in the ipsilateral cortex of the MCAO mice with acute hyperglycemia. AAV-GLO1 infection attenuated the induction of MG-modified proteins, ER stress formation, and caspase 3/7 activation in MG-treated Neuro-2A cells, and reductions in synaptic plasticity and microglial activation were mitigated in the injured cortex of the MCAO mice with acute hyperglycemia. Treatment with ketotifen, a potent GLO1 stimulator, after surgery, alleviated neurofunctional deficits and ischemic brain damage in the MCAO mice with acute hyperglycemia.  Altogether, our data substantiate that, in ischemic brain injury, GLO1 overexpression can alleviate pathologic alterations caused by acute hyperglycemia. Upregulation of GLO1 may be a therapeutic strategy for alleviating SIH-aggravated poor functional outcomes in patients with AIS.

Glyoxal-induced disruption of tumor cell progression in breast cancer

Breast cancer is the most common malignant tumor in women and remains a major global challenge, with ~1.4 million cases per year, worldwide. Numerous studies have shown that changes in cell metabolism are associated with the regulation of tumor progression. In the present study, the anti-cancer properties of glyoxal (GO), which is the smallest dialdehyde formed in the oxidation-reduction reaction and involved in electron transfer and energy metabolism, in breast cancer was investigated. The biological functions and molecular mechanisms of GO were investigated in breast cancer cell lines using MTT and crystal violet assays, flow cytometry, western blot analysis, 3D laser scanning confocal microscopy and transmission electron microscopy. The results showed that GO strongly inhibited cell proliferation, promoted cell apoptosis and cell cycle G₂/M arrest, induced the disappearance of cellular microvilli, and enlarged mitochondria. In addition, the protein expression level of AKT, mTOR and p70-S6K decreased in the AKT-mTOR pathway, accompanied by an increase in p-ERK and p-MEK in the MAPK pathway. The results from the present study indicate that GO suppressed breast cancer progression via the MAPK and AKT-mTOR pathways. Taken together, these results provide the basis for a potential therapeutic strategy for breast cancer.

The Type 2 Diabetes Factor Methylglyoxal Mediates Axon Initial Segment Shortening and Alters Neuronal Function at the Cellular and Network Levels

Recent evidence suggests that alteration of axon initial segment (AIS) geometry (i.e., length or location along the axon) contributes to CNS dysfunction in neurological diseases. For example, AIS length is shorter in the prefrontal cortex of type 2 diabetic mice with cognitive impairment. To determine the key type 2 diabetes-related factor that produces AIS shortening we modified levels of insulin, glucose, or the reactive glucose metabolite methylglyoxal in cultures of dissociated cortices from male and female mice and quantified AIS geometry using immunofluorescent imaging of the AIS proteins AnkyrinG and βIV spectrin. Neither insulin nor glucose modification altered AIS length. Exposure to 100 but not 1 or 10 μm methylglyoxal for 24 h resulted in accumulation of the methylglyoxal-derived advanced glycation end-product hydroimidazolone and produced reversible AIS shortening without cell death. Methylglyoxal-evoked AIS shortening occurred in both excitatory and putative inhibitory neuron populations and in the presence of tetrodotoxin (TTX). In single-cell recordings resting membrane potential was depolarized at 0.5-3 h and returned to normal at 24 h. In multielectrode array (MEA) recordings methylglyoxal produced an immediate ∼300% increase in spiking and bursting rates that returned to normal within 2 min, followed by a ∼20% reduction of network activity at 0.5-3 h and restoration of activity to baseline levels at 24 h. AIS length was unchanged at 0.5-3 h despite the presence of depolarization and network activity reduction. Nevertheless, these results suggest that methylglyoxal could be a key mediator of AIS shortening and disruptor of neuronal function during type 2 diabetes.

Glucagon Prevents Cytotoxicity Induced by Methylglyoxal in a Rat Neuronal Cell Line Model

Although diabetic polyneuropathy (DPN) is a frequent diabetic complication, no effective therapeutic approach has been established. Glucagon is a crucial hormone for glucose homeostasis but has pleiotropic effects, including neuroprotective effects in the central nervous system. However, the importance of glucagon in the peripheral nervous system (PNS) has not been clarified. Here, we hypothesized that glucagon might have a neuroprotective function in the PNS. The immortalized rat dorsal root ganglion (DRG) neuronal cell line 50B11 was treated with methylglyoxal (MG) to mimic an in vitro DPN model. The cells were cultured with or without glucagon or MG. Neurotoxicity, survival, apoptosis, neurite projection, cyclic adenosine monophosphate (cAMP), and protein kinase A (PKA) were examined. Glucagon had no cytotoxicity and rescued the cells from neurotoxicity. Cell survival was increased by glucagon. The ratio of apoptotic cells, which was increased by MG, was reduced by glucagon. Neurite outgrowth was accelerated in glucagon-treated cells. Cyclic AMP and PKA accumulated in the cells after glucagon stimulation. In conclusion, glucagon protected the DRG neuronal cells from MG-induced cellular stress. The cAMP/PKA pathway may have significant roles in those protective effects of glucagon. Glucagon may be a potential target for the treatment of DPN.

Methylglyoxal interaction with superoxide dismutase 1

Methylglyoxal (MG) is a highly reactive aldehyde spontaneously formed in human cells mainly as a by-product of glycolysis. Such endogenous metabolite reacts with proteins, nucleotides and lipids forming advanced glycation end-products (AGEs). MG binds to arginine, lysine and cysteine residues of proteins causing the formation of stable adducts that can interfere with protein function. Among the proteins affected by glycation, MG has been found to react with superoxide dismutase 1 (SOD1), a fundamental anti-oxidant enzyme that is abundantly expressed in neurons. Considering the high neuronal susceptibility to MG-induced oxidative stress, we sought to investigate by mass spectrometry and NMR spectroscopy which are the structural modifications induced on SOD1 by the reaction with MG. We show that MG reacts preferentially with the disulfide-reduced, demetallated form of SOD1, gradually causing its unfolding, and to a lesser extent, with the intermediate state of maturation – the reduced, zinc-bound homodimer – causing its gradual monomerization. These results suggest that MG could impair the correct maturation of SOD1 in vivo, thus both increasing cellular oxidative stress and promoting the cytotoxic misfolding and aggregation process of SOD1.

•

MG forms stable adducts with the immature forms of SOD1.

•

MG causes the unfolding of the apo-SOD1^SH form.

•

MG causes the monomerization of the E,Zn-SOD1^SH form.

•

In both forms, arginine 143 is more prone to interact with MG.

•

The structural modifications caused by MG impair the correct maturation of SOD1.

Electrophilic Signaling: The Role of Reactive Carbonyl Compounds

Reactive carbonyl compounds (RCC) are a group of compounds with clearly pronounced electrophilic properties that facilitate their spontaneous reactions with numerous nucleophilic reaction sites in proteins, lipids, and nucleic acids. The biological functions of RCC are determined by their concentration and governed by the hormesis (biphasic reaction) principle. At low concentrations, RCC act as signaling molecules activating defense systems against xenobiotics and oxidizers, and at high concentrations, they exhibit the cytotoxic effect. RCC participate in the formation of cell adaptive response via intracellular signaling pathways involving regulation of gene expression and cytoplasmic mechanisms related to the structure-functional rearrangements of proteins. Special attention in this review is given to the functioning of electrophiles as mediators of cell general adaption syndrome manifested as the biphasic response. The hypothesis is proposed that electrophilic signaling can be a proto-signaling system.

Activation of the integrated stress response in nociceptors drives methylglyoxal-induced pain

Methylglyoxal (MGO) is a reactive glycolytic metabolite associated with painful diabetic neuropathy at plasma concentrations between 500 nM and 5 μM. The mechanisms through which MGO causes neuropathic pain at these pathological concentrations are not known. Because MGO has been linked to diabetic neuropathic pain, which is prevalent and poorly treated, insight into this unsolved biomedical problem could lead to much needed therapeutics. Our experiments provide compelling evidence that ∼1-μM concentrations of MGO activate the integrated stress response (ISR) in IB4-positive nociceptors in the dorsal root ganglion (DRG) of mice in vivo and in vitro. Blocking the integrated stress response with a specific inhibitor (ISRIB) strongly attenuates and reverses MGO-evoked pain. Moreover, ISRIB reduces neuropathic pain induced by diabetes in both mice and rats. Our work elucidates the mechanism of action of MGO in the production of pain at pathophysiologically relevant concentrations and suggests a new pharmacological avenue for the treatment of diabetic and other types of MGO-driven neuropathic pain.

Hormetic potential of methylglyoxal, a side-product of glycolysis, in switching tumours from growth to death

Metabolic reprogramming toward aerobic glycolysis unavoidably favours methylglyoxal (MG) and advanced glycation end products (AGEs) formation in cancer cells. MG was initially considered a highly cytotoxic molecule with potential anti-cancer value. However, we have recently demonstrated that MG enhanced tumour growth and metastasis. In an attempt to understand this dual role, we explored MG-mediated dicarbonyl stress status in four breast and glioblastoma cancer cell lines in relation with their glycolytic phenotype and MG detoxifying capacity. In glycolytic cancer cells cultured in high glucose, we observed a significant increase of the conversion of MG to D-lactate through the glyoxalase system. Moreover, upon exogenous MG challenge, glycolytic cells showed elevated amounts of intracellular MG and induced de novo GLO1 detoxifying enzyme and Nrf2 expression. Thus, supporting the adaptive nature of glycolytic cancer cells to MG dicarbonyl stress when compared to non-glycolytic ones. Finally and consistent with the pro-tumoural role of MG, we showed that low doses of MG induced AGEs formation and tumour growth in vivo, both of which can be reversed using a MG scavenger. Our study represents the first demonstration of a hormetic effect of MG defined by a low-dose stimulation and a high-dose inhibition of tumour growth.

New isatin derivative inhibits neurodegeneration by restoring insulin signaling in brain

Diabetes is associated with neurodegeneration. Glycation ensues in diabetes and glycated proteins cause insulin resistance in brain resulting in amyloid plaques and NFTs. Also glycation enhances gliosis by promoting neuroinflammation. Currently there is no therapy available to target neurodegenration in brain therefore, development of new therapy that offers neuroprotection is critical. The objective of this study was to evaluate mechanistic effect of isatin derivative URM-II-81, an anti-glycation agent for improvement of insulin action in brain and inhibition of neurodegenration. Methylglyoxal induced stress was inhibited by treatment with URM-II-81. Also, Ser473 and Ser9 phosphorylation of Akt and GSK-3β respectively were restored by URM-II-81. Effect of URM-II-81 on axonal integrity was studied by differentiating Neuro2A using retinoic acid. URM-II-81 restored axonal length in MGO treated cells. Its effects were also studied in high fat and low dose streptozotocin induced diabetic mice where it reduced RBG levels and inhibited glycative stress by reducing HbA1c. URM-II-81 treatment also showed inhibition of gliosis in hippocampus. Histological analysis showed reduced NFTs in CA3 hippocampal region and restoration of insulin signaling in hippocampii of diabetic mice. Our findings suggest that URM-II-81 can be developed as a new therapeutic agent for treatment of neurodegenration.

Cross-talk between lipid and protein carbonylation in a dynamic cardiomyocyte model of mild nitroxidative stress

Reactive oxygen and nitrogen species (ROS/RNS) play an important role in the regulation of cardiac function. Increase in ROS/RNS concentration results in lipid and protein oxidation and is often associated with onset and/or progression of many cardiovascular disorders. However, interplay between lipid and protein modifications has not been simultaneously studied in detail so far. Biomolecule carbonylation is one of the most common biomarkers of oxidative stress. Using a dynamic model of nitroxidative stress we demonstrated rapid changes in biomolecule carbonylation in rat cardiomyocytes. Levels of carbonylated species increased as early as 15min upon treatment with the peroxynitrite donor, 3-morpholinosydnonimine (SIN-1), and decreased to values close to control after 16h. Total (lipids+proteins) vs. protein-specific carbonylation showed different dynamics, with a significant increase in protein-bound carbonyls at later time points. Treatment with SIN-1 in combination with inhibitors of proteasomal and autophagy/lysosomal degradation pathways allowed confirmation of a significant role of the proteasome in the degradation of carbonylated proteins, whereas lipid carbonylation increased in the presence of autophagy/lysosomal inhibitors. Electrophilic aldehydes and ketones formed by lipid peroxidation were identified and relatively quantified using LC-MS/MS. Molecular identity of reactive species was used for data-driven analysis of their protein targets. Combination of different enrichment strategies with LC-MS/MS analysis allowed identification of more than 167 unique proteins with 332 sites modified by electrophilic lipid peroxidation products. Gene ontology analysis of modified proteins demonstrated enrichment of several functional categories including proteins involved in cytoskeleton, extracellular matrix, ion channels and their regulation. Using calcium mobilization assays, the effect of nitroxidative stress on the activity of several ion channels was further confirmed.

Methylglyoxal, a glycolysis side-product, induces Hsp90 glycation and YAP-mediated tumor growth and metastasis

Metabolic reprogramming toward aerobic glycolysis unavoidably induces methylglyoxal (MG) formation in cancer cells. MG mediates the glycation of proteins to form advanced glycation end products (AGEs). We have recently demonstrated that MG-induced AGEs are a common feature of breast cancer. Little is known regarding the impact of MG-mediated carbonyl stress on tumor progression. Breast tumors with MG stress presented with high nuclear YAP, a key transcriptional co-activator regulating tumor growth and invasion. Elevated MG levels resulted in sustained YAP nuclear localization/activity that could be reverted using Carnosine, a scavenger for MG. MG treatment affected Hsp90 chaperone activity and decreased its binding to LATS1, a key kinase of the Hippo pathway. Cancer cells with high MG stress showed enhanced growth and metastatic potential in vivo. These findings reinforce the cumulative evidence pointing to hyperglycemia as a risk factor for cancer incidence and bring renewed interest in MG scavengers for cancer treatment.

Can Diabetic Neuropathy Be Modeled In Vitro?

Diabetic neuropathy is a common secondary complication of diabetes that impacts on patient's health and well-being. Distal axon degeneration is a key feature of diabetic neuropathy, but the pathological changes which underlie axonal die-back are incompletely understood; despite decades of research a treatment has not yet been identified. Basic research must focus on understanding the complex mechanisms underlying changes that occur in the nervous system during diabetes. To this end, tissue culture techniques are invaluable as they enable researchers to examine the intricate mechanistic responses of cells to high glucose or other factors in order to better understand the pathogenesis of nerve dysfunction. This chapter describes the use of in vitro models to study a wide range of specific cellular effects pertaining to diabetic neuropathy including apoptosis, neurite outgrowth, neurodegeneration, activity, and bioenergetics. We consider problems associated with in vitro modeling and future refinement such as use of induced pluripotent stem cells and microfluidic technology.

Are they in or out? The elusive interaction between Qtracker®800 vascular labels and brain endothelial cells

Aim: Qtracker®800 Vascular labels (Qtracker®800) are promising biomedical tools for high-resolution vasculature imaging; their effects on mouse and human endothelia, however, are still unknown. Materials & methods: Qtracker®800 were injected in Balb/c mice, and brain endothelium uptake was investigated by transmission electron microscopy 3-h post injection. We then investigated, in vitro, the effects of Qtracker®800 exposure on mouse and human endothelial cells by calcium imaging. Results: Transmission electron microscopy images showed nanoparticle accumulation in mouse brain endothelia. A subset of mouse and human endothelial cells generated intracellular calcium transients in response to Qtracker®800. Conclusion: Qtracker®800 nanoparticles elicit endothelial functional responses, which prompts biomedical safety evaluations and may bias the interpretation of experimental studies involving vascular imaging.

Methylglyoxal, the dark side of glycolysis

Glucose is the main energy substrate for the brain. There is now extensive evidence indicating that the metabolic profile of neural cells with regard to glucose utilization and glycolysis rate is not homogenous, with a marked propensity for glycolytic glucose processing in astrocytes compared to neurons. Methylglyoxal, a highly reactive dicarbonyl compound, is inevitably formed as a by-product of glycolysis. Methylglyoxal is a major cell-permeant precursor of advanced glycation end-products (AGEs), which are associated with several pathologies including diabetes, aging and neurodegenerative diseases. In normal situations, cells are protected against methylglyoxal toxicity by different mechanisms and in particular the glyoxalase system, which represents the most important pathway for the detoxification of methylglyoxal. While the neurotoxic effects of methylglyoxal and AGEs are well characterized, our understanding the glyoxalase system in the brain is more scattered. Considering the high energy requirements (i.e., glucose) of the brain, one should expect that the cerebral glyoxalase system is adequately fitted to handle methylglyoxal toxicity. This review focuses on our actual knowledge on the cellular aspects of the glyoxalase system in brain cells, in particular with regard to its activity in astrocytes and neurons. A main emerging concept is that these two neural cell types have different and energetically adapted glyoxalase defense mechanisms which may serve as protective mechanism against methylglyoxal-induced cellular damage.

Advanced type 1 diabetes is associated with ASIC alterations in mouse lower thoracic dorsal root ganglia neurons

Acid-sensing ion channels (ASICs) from dorsal root ganglia (DRG) neurons are proton sensors during ischemia and inflammation. Little is known about their role in type 1 diabetes (T1D). Our study was focused on ASICs alterations determined by advanced T1D status. Primary neuronal cultures were obtained from lower (T9-T12) thoracic DRG neurons from Balb/c and TCR-HA(+/-)/Ins-HA(+/-) diabetic male mice (16 weeks of age). Patch-clamp recordings indicate a change in the number of small DRG neurons presenting different ASIC-type currents. Multiple molecular sites of ASICs are distinctly affected in T1D, probably due to particular steric constraints for glycans accessibility to the active site: (i) ASIC1 current inactivates faster, while ASIC2 is slower; (ii) PcTx1 partly reverts diabetes effects against ASIC1- and ASIC2-inactivations; (iii) APETx2 maintains unaltered potency against ASIC3 current amplitude, but slows ASIC3 inactivation. Immunofluorescence indicates opposite regulation of different ASIC transcripts while qRT-PCR shows that ASIC mRNA ranking (ASIC2 > ASIC1 > ASIC3) remains unaltered. In conclusion, our study has identified biochemical and biophysical ASIC changes in lower thoracic DRG neurons due to advanced T1D. As hypoalgesia is present in advanced T1D, ASICs alterations might be the cause or the consequence of diabetic insensate neuropathy.

Methylglyoxal reduces mitochondrial potential and activates Bax and caspase-3 in neurons: Implications for Alzheimer's disease

Alzheimer's disease (AD) is characterized by the oxidative stress generated from amyloid β-peptide (Aβ) aggregates. It produces protein nitrotyrosination, after the reaction with nitric oxide to form peroxynitrite, being triosephosphate isomerase (TPI) one of the most affected proteins. TPI is a glycolytic enzyme that catalyzes the interconversion between glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). Methylglyoxal (MG) is a by-product of TPI activity whose production is triggered when TPI is nitrotyrosinated. MG is harmful to cells because it glycates proteins. Here we found protein glycation when human neuroblastoma cells were treated with Aβ. Moreover glycation was also observed when neuroblastoma cells overexpressed mutated TPI where Tyr165 or Tyr209, the two tyrosines close to the catalytic center, were changed by Phe in order to mimic the effect of nitrotyrosination. The pathological relevance of these findings was studied by challenging cells with Aβ oligomers and MG. A significant decrease in mitochondrial transmembrane potential, one of the first apoptotic events, was obtained. Therefore, increasing concentrations of MG were assayed searching for MG effect in neuronal apoptosis. We found a decrease of the protective Bcl2 and an increase of the proapoptotic caspase-3 and Bax levels. Our results suggest that MG is triggering apoptosis in neurons and it would play a key role in AD neurodegeneration.

Transient receptor potential ankyrin 1 (TRPA1) ion channel in the pathophysiology of peripheral diabetic neuropathy

Background

Transient receptor potential ankyrin 1 (TRPA1) is a non-selective cation channel permeable to calcium that is expressed on pain-mediating primary afferent nerve fibers. Here we review recent experimental evidence supporting the hypothesis that activation of the TRPA1 channel by reactive compounds generated in diabetes mellitus, such as 4-hydroxynonenal and methylglyoxal, exerts an important role in the pathophysiology of peripheral diabetic neuropathy (PDN). The hypothesis includes development of the early diabetic pain hypersensitivity and the later loss of cutaneous nerve endings of pain fibers and their dysfunction, which are hallmarks of peripheral diabetic neuropathy (PDN).

Methods

The evidence for a role of the TRPA1 channel in PDN consists of in vitro patch clamp and calcium imaging data and assessments of pain behavior, axon reflex measurements, and immunohistochemical analyses of cutaneous innervation in an experimental animal model of diabetes. The experiments were combined with blocking the TRPA1 channel with selective antagonists Chembridge-5861528 or A-967079.

Results

In vitro studies indicate that under physiological concentration of Ca2+, methylglyoxal and 4-hydroxynonenal produce sustained activation of the TRPA1 channel and sustained inflow of calcium. In vivo studies indicate that diabetic pain hypersensitivity is maintained by the TRPA1 channel as indicated by the antihypersensitivity effect induced by acute blocking of the TRPA1 channel. Moreover, TRPA1 channel is involved in the development of diabetic hypersensitivity as indicated by prevention of the development of pain hypersensitivity in diabetic animals treated daily with Chembridge-5861528. The diabetes-induced loss of substance P-like cutaneous innervation and that of the TRPA1 channel-mediated cutaneous axon reflex function during the later phase of diabetes were also prevented or delayed by prolonged blocking of the TRPA1 channel. No motor impairment or other obvious side-effects were observed following block of the TRPA1 channel.

Conclusions

Together the in vitro and in vivo results indicate that reactive compounds generated in diabetes exert, through action on the TRPA1 channel, an important role in the pathophysiology of PDN. Sustained activation of the TRPA1 channel is a plausible mechanism that contributes to the early diabetic pain hypersensitivity and the later loss of cutaneous pain fiber endings and their dysfunction with prolonged diabetes.

Implications

Blocking the TRPA1 channel with a selective antagonist provides a promising disease-modifying treatment for PDN, with only minor, if any, side-effects.

TRPV1 properties in thoracic dorsal root ganglia neurons are modulated by intraperitoneal capsaicin administration in the late phase of type-1 autoimmune diabetes

Pharmacological therapies in type 1 diabetes for efficient control of glycemia and changes in pain alterations due to diabetic neuropathy are a continuous challenge. Transient receptor potential vanilloid type 1 (TRPV1) from dorsal root ganglia (DRG) neurons is one of the main pharmacological targets in diabetes, and its ligand capsaicin can be a promising compound for blood-glucose control. Our goal is to elucidate the effect of intraperitoneal (i.p.) capsaicin administration in type 1 diabetic mice against TRPV1 receptors from pancreatic DRG primary afferent neurons. A TCR(+/-)/Ins-HA(+/-) diabetic mice (dTg) was used, and patch-clamp and immunofluorescence microscopy measurements have been performed on thoracic T(9)-T(12) DRG neurons. Capsaicin (800 μg/kg, i.p. three successive days) administration in the late-phase diabetes reduces blood-glucose levels, partly reverses the TRPV1 current density and recovery time constant, without any effect on TRPV1 expression general pattern, in dTg mice. A TRPV1 hypoalgesia profile was observed in late-phase diabetes, which was partly reversed to normoalgesic profile upon capsaicin i.p. administration. According to the soma dimensions of the thoracic DRG neurons, a detailed analysis of the TRPV1 expression upon capsaicin i.p. treatment was done, and the proportion of large A-fiber neurons expressing TRPV1 increased in dTg capsaicin-treated mice. In conclusion, the benefits of low-dose capsaicin intraperitoneal treatment in late-phase type-1 diabetes should be further exploited.