Changes in mitochondrial Ca2+ homeostasis in primary sensory neurons of diabetic mice

Oxidative Stress and Mitochondrial Dysfunction Associated with Peripheral Neuropathy in Type 1 Diabetes

Significance: This review highlights the many intracellular processes generating reactive oxygen species (ROS) in the peripheral nervous system in the context of type 1 diabetes. The major sources of superoxide and hydrogen peroxide (H₂O₂) are described, and scavenging systems are explained. Important roles of ROS in regulating normal redox signaling and in a disease setting, such as diabetes, contributing to oxidative stress and cellular damage are outlined. The primary focus is the role of hyperglycemia in driving elevated ROS production and oxidative stress contributing to neurodegeneration in diabetic neuropathy (within the dorsal root ganglia [DRG] and peripheral nerve). Recent Advances: Contributors to ROS production under high intracellular glucose concentration such as mitochondria and the polyol pathway are discussed. The primarily damaging impact of ROS on multiple pathways including mitochondrial function, endoplasmic reticulum (ER) stress, autophagy, and epigenetic signaling is covered. Critical Issues: There is a strong focus on mechanisms of diabetes-induced mitochondrial dysfunction and how this may drive ROS production (in particular superoxide). The mitochondrial sites of superoxide/H₂O₂ production via mitochondrial metabolism and aerobic respiration are reviewed. Future Directions: Areas for future development are highlighted, including the need to clarify diabetes-induced changes in autophagy and ER function in neurons and Schwann cells. In addition, more clarity is needed regarding the sources of ROS production at mitochondrial sites under high glucose concentration (and lack of insulin signaling). New areas of study should be introduced to investigate the role of ROS, nuclear lamina function, and epigenetic signaling under diabetic conditions in peripheral nerve.

Reductions in Calcium Signaling Limit Inhibition to Diabetic Retinal Rod Bipolar Cells

Purpose

The balance of neuronal excitation and inhibition is important for proper retinal signaling. A previous report showed that diabetes selectively reduces light-evoked inhibition to the retinal dim light rod pathway, changing this balance. Here, changes in mechanisms of retinal inhibitory synaptic transmission after 6 weeks of diabetes are investigated.

Methods

Diabetes was induced in C57BL/6J mice by three intraperitoneal injections of streptozotocin (STZ, 75 mg/kg), and confirmed by blood glucose levels more than 200 mg/dL. After 6 weeks, whole-cell voltage-clamp recordings of electrically evoked inhibitory postsynaptic currents from rod bipolar cells and light-evoked excitatory postsynaptic currents from A17-amacrine cells were made in dark-adapted retinal slices.

Results

Diabetes shortened the timecourse of directly activated lateral GABAergic inhibitory amacrine cell inputs to rod bipolar cells. The timing of GABA release onto rod bipolar cells depends on a prolonged amacrine cell calcium signal that is reduced by slow calcium buffering. Therefore, the effects of calcium buffering with EGTA-acetoxymethyl ester (AM) on diabetic GABAergic signaling were tested. EGTA-AM reduced GABAergic signaling in diabetic retinas more strongly, suggesting that diabetic amacrine cells have reduced calcium signals. Additionally, the timing of release from reciprocal inhibitory inputs to diabetic rod bipolar cells was reduced, but the activation of the A17 amacrine cells responsible for this inhibition was not changed.

Conclusions

These results suggest that reduced light-evoked inhibitory input to rod bipolar cells is due to reduced and shortened calcium signals in presynaptic GABAergic amacrine cells. A reduction in calcium signaling may be a common mechanism limiting inhibition in the retina.

Vildagliptin, a DPP4 inhibitor, alleviates diabetes-associated cognitive deficits by decreasing the levels of apoptosis-related proteins in the rat hippocampus

Cognitive impairment is a prevalent but underestimated complication of diabetes, which can cause spatial memory and learning deficits. In the present study, a streptozotocin-induced type 2 diabetic rat model was employed to investigate the effects of vildagliptin, a new oral hypoglycemic agent that acts by inhibiting dipeptidyl peptidase-4, on diabetes-associated cognitive impairments, as well as the molecular mechanisms involved. The present findings demonstrated that vildagliptin treatment prevented memory impairment and decreased the apoptosis of hippocampal neurons. It also attenuated the abnormal expression of caspase-3, B cell lymphoma-2 (Bcl-2) and Bcl-2 associated X protein in the diabetic model. Vildagliptin treatment also reversed diabetes-induced decreases in phosphorylated (p)-protein kinase B (Akt) and p-glycogen synthase kinase 3β (GSK3β), brain-derived neurotrophic factor and nerve growth factor expression levels. The results indicated that the administration of vildagliptin exerts a protective effect against cognitive deficits by decreasing the expression of apoptosis-related proteins in the hippocampus and that this protective effect was mediated via the Akt/GSK3β signaling pathway.

Vildagliptin, a DPP4 inhibitor, alleviates diabetes-associated cognitive deficits by decreasing the levels of apoptosis-related proteins in the rat hippocampus

Cognitive impairment is a prevalent but underestimated complication of diabetes, which can cause spatial memory and learning deficits. In the present study, a streptozotocin-induced type 2 diabetic rat model was employed to investigate the effects of vildagliptin, a new oral hypoglycemic agent that acts by inhibiting dipeptidyl peptidase-4, on diabetes-associated cognitive impairments, as well as the molecular mechanisms involved. The present findings demonstrated that vildagliptin treatment prevented memory impairment and decreased the apoptosis of hippocampal neurons. It also attenuated the abnormal expression of caspase-3, B cell lymphoma-2 (Bcl-2) and Bcl-2 associated X protein in the diabetic model. Vildagliptin treatment also reversed diabetes-induced decreases in phosphorylated (p)-protein kinase B (Akt) and p-glycogen synthase kinase 3β (GSK3β), brain-derived neurotrophic factor and nerve growth factor expression levels. The results indicated that the administration of vildagliptin exerts a protective effect against cognitive deficits by decreasing the expression of apoptosis-related proteins in the hippocampus and that this protective effect was mediated via the Akt/GSK3β signaling pathway.

Role of calcium in regulating primary sensory neuronal excitability

The fundamental role of calcium ions (Ca(2+)) in an excitable tissue, the frog heart, was first demonstrated in a series of classical reports by Sydney Ringer in the latter part of the nineteenth century (1882a, b; 1893a, b). Even so, nearly a century elapsed before it was proven that Ca(2+) regulated the excitability of primary sensory neurons. In this chapter we review the sites and mechanisms whereby internal and external Ca(2+) can directly or indirectly alter the excitability of primary sensory neurons: excitability changes being manifested typically by variations in shape of the action potential or the pattern of its discharge.

Taurine replacement attenuates hyperalgesia and abnormal calcium signaling in sensory neurons of STZ-D rats

The etiology of painful diabetic neuropathy is poorly understood, but may result from neuronal hyperexcitability secondary to alterations of Ca2+ signaling in sensory neurons. The naturally occurring amino acid taurine functions as an osmolyte, antioxidant, Ca2+ modulator, inhibitory neurotransmitter, and analgesic such that its depletion in diabetes may predispose one to neuronal hyperexcitability and pain. This study reports the effects of taurine replacement on hyperalgesia and sensory neuron Ca2+ homeostasis in streptozotocin-diabetic (STZ-D) rats. Nondiabetic and STZ-D rats were treated with a 2% taurine-supplemented diet for 6-12 wk. Thermal hyperalgesia and mechanical allodynia were determined by measuring hindpaw withdrawal latency to radiant heat and the withdrawal threshold to the von Frey anesthesiometer. Intracellular Ca2+ signaling was explored in neurons from L4-L6 dorsal root ganglia (DRG), using fura 2 fluorescence. Taurine replacement of diabetic rats attenuated deficits of nerve conduction and prevented reductions of mechanical and thermal withdrawal threshold and latency, respectively. In small DRG sensory neurons from diabetic rats, recovery of intracellular Ca2+ concentration ([Ca2+]i) in response to KCl was slowed and 73% corrected by taurine. The amplitudes of caffeine and ATP-induced [Ca2+]i transients were decreased by 47 and 27% (P < 0.05), respectively, in diabetic rat DRG sensory neurons and corrected by 74 and 93% (P < 0.05), respectively, by taurine replacement. These data indicate that taurine is important in the regulation of neuronal Ca2+ signaling and that taurine deficiency may predispose one to nerve hyperexcitability and pain, complicating diabetes.

Diabetes-induced abnormalities in ER calcium mobilization in primary and secondary nociceptive neurons

Development of diabetic sensory polyneuropathy is associated with alterations in intracellular calcium homeostasis in primary and secondary nociceptive neurons. We have shown previously that in a model of streptozotocin (STZ)-induced diabetes, the calcium signal is prolonged and calcium release from ryanodine-sensitive calcium stores down-regulated in neurons of the nociceptive system. The aim of the present study was a more detailed characterization of calcium homeostasis in primary (dorsal root ganglia, DRG) and secondary (dorsal horn, DH) nociceptive neurons in STZ-induced diabetes. Fluorescence video-imaging was used to measure free cytosolic [Ca2+] ([Ca2+]i) in lumbar nociceptive neurons of control and streptozotocin-diabetic rats. Resting [Ca2+]i rose progressively in these neurons with the duration of diabetes and calcium mobilization from the endoplasmic reticulum (ER) decreased during diabetes. The amplitude of calcium release from both ryanodine- and IP3-sensitive calcium stores induced by caffeine, ionomycin, ATP or glutamate was significantly (P<0.01) lower in DRG and DH neurons from 6-week STZ-diabetic rats. Diabetes-induced changes in the calcium homeostasis were similar in DRG and DH neurons indicating that they might be general for many types of neurons from the central and peripheral nervous systems.

Diabetes is not a potent inducer of neuronal cell death in mouse sensory ganglia, but it enhances neurite regeneration in vitro

We examined the effects of diabetes on the morphological features and regenerative capabilities of adult mouse nodose ganglia (NG) and dorsal root ganglia (DRG). By light and electron microscopy, no apoptotic cell death was detected in the ganglia obtained from either streptozotocin (STZ)-induced diabetic or normal C57BL/6J mice in vivo. Neurite regeneration from transected nerve terminals of NG and DRG explants in culture at normal (10 mM) and high (30 mM) glucose concentrations was significantly enhanced in the diabetic mice. Chromatolytic changes (i.e. swelling and migration of the nucleus to an eccentric position in the neurons, and a loss of Nissl substance in the neuronal perikarya) and apoptotic cell death (less than one-fifth of the neurons) in the cultured ganglia were present, but neither hyperglycemia in vivo nor high glucose conditions in vitro altered the morphological features of the ganglia or the ratios of apoptotic cells at 3 days in culture. By semiquantitative RT-PCR analysis, the mRNA expressions of ciliary neurotrophic factor (CNTF) in DRG from both mice were down-regulated at 1 day in culture. The expression in diabetic DRG, but not in control DRG, was significantly up-regulated at later stages (3 and 7 days) in culture. In summary, hyperglycemia is unlikely to induce cell death in the sensory ganglia, but enhances the regenerative capability of vagal and spinal sensory nerves in vitro. The up-regulation of CNTF mRNA expression during the culture of diabetic DRG may play a role in the enhanced neurite regeneration.

Role of mitochondria in intracellular calcium signaling in primary and secondary sensory neurones of rats

The participation of different calcium-regulated mechanisms in the generation of cytosolic Ca(2+) transients during neuronal excitation has been compared in isolated large and small primary (dorsal root ganglia (DRG)) and secondary (spinal dorsal horn (DH)) rat sensory neurones. As it was shown before in murine primary sensory neurones the application of mitochondrial protonophore CCCP by itself induced only small elevation of [Ca(2+)](i). However, its preceding application substantially increased the peak amplitude of depolarization-induced transients. Application of CCCP immediately after termination of the depolarizing pulse induced in both types of primary neurones a massive release of Ca(2+) from mitochondria into the cytosol. In secondary neurones application of CCCP by itself induced a substantial release of Ca(2+) from the mitochondria, but its preceding application resulted in only an insignificant increase in the peak amplitude of depolarization-triggered calcium transients. Application of CCCP immediately after termination of depolarization elicited a small release of Ca(2+), which became more pronounced when the application was delayed. Preceding application of CCCP increased the amplitude of the transients induced by caffeine-triggered Ca(2+) release from the endoplasmic reticulum in secondary neurones and did not affect those in large primary neurones. These findings may be explained by substantial differences in the density and distribution of mitochondria in the cytosol of primary and secondary sensory neurones. This suggestion was confirmed electronmicroscopically, showing a much lower density of mitochondria near plasmalemma in secondary sensory neurones and predominant clustered location of mitochondria beneath the plasmalemma in the primary cells. The possible functional importance of these differences is discussed.

The endoplasmic reticulum and mitochondria as elements of the mechanism of intracellular signaling in the nerve cell

Experimental data obtained in our laboratory from studies of intracellular signals arising within nerve cells during excitation are summarized. Measurements of transmembrane ion currents in conditions of fixed membrane potential and intracellular free Ca ion concentrations, using fluorescent probes, yielded the time and spatial characteristics of transient elevations in the Ca concentration (the "calcium signal") in various types of mouse and rat neurons. These studies showed that intracellular structures-the endoplasmic reticulum and mitochondria-had significant roles in forming these signals; these structures can take up Ca from the cytosol and liberate Ca into the cytosol; the contribution of these processes was extremely variable, depending on the internal organization of different functional types of neurons.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Changes in calcium signalling in dorsal horn neurons in rats with streptozotocin-induced diabetes

Intracellular calcium signalling was studied in the dorsal horn from neurons of rats with streptozotocin-induced diabetes versus control animals. The cytoplasmic Ca2+ concentration ([Ca2+]i) was measured in Fura-2 acetoxymethyl ester-loaded dorsal horn neurons from acutely isolated spinal cord slices using a fluorescence technique. The recovery of depolarization-induced [Ca2+]i increase was delayed in diabetic neurons compared with normal animals. In normal neurons, [Ca2+]i after the end of KCl depolarization recovered to the basal level monoexponentially with a time constant of 8.0+/-0.5 s (n = 23), while diabetic neurons showed two exponentials in the [Ca2+]i recovery. The time constants of these exponentials were 7.2+/-0.5 and 23.0+/-0.6 s (n = 19), respectively. The amplitude of calcium release from caffeine-sensitive endoplasmic reticulum calcium stores became significantly smaller in diabetic neurons. The amplitudes of [Ca2+]i transients evoked by 30 mM caffeine were 268+/-29 nM (n = 13) and 31+/-9 nM (n = 17) in control and diabetic neurons, respectively. We conclude that streptozotocin-induced diabetes is associated with prominent changes in the mechanisms responsible for [Ca2+]i regulation, which presumably include a slowdown of Ca2+ elimination from the cytoplasm by the endoplasmic reticulum.