Effect of transitory glucose deprivation on mitochondrial structure and functions in cultured cerebellar granule neurons

Mitochondria dysfunction and bipolar disorder: From pathology to therapy

Bipolar disorder (BD) is one of the major psychiatric diseases in which the impairment of mitochondrial functions has been closely connected or associated with the disease pathologies. Different lines of evidence of the close connection between mitochondria dysfunction and BD were discussed with a particular focus on (1) dysregulation of energy metabolism, (2) effect of genetic variants, (3) oxidative stress, cell death and apoptosis, (4) dysregulated calcium homeostasis and electrophysiology, and (5) current as well as potential treatments targeting at restoring mitochondrial functions. Currently, pharmacological interventions generally provide limited efficacy in preventing relapses or recovery from mania or depression episodes. Thus, understanding mitochondrial pathology in BD will lead to novel agents targeting mitochondrial dysfunction and formulating new effective therapy for BD.

Reduction of Deuterium Level Supports Resistance of Neurons to Glucose Deprivation and Hypoxia: Study in Cultures of Neurons and on Animals

The effect of a reduced deuterium (D) content in the incubation medium on the survival of cultured neurons in vitro and under glucose deprivation was studied. In addition, we studied the effect of a decrease in the deuterium content in the rat brain on oxidative processes in the nervous tissue, its antioxidant protection, and training of rats in the T-shaped maze test under hypoxic conditions. For experiments with cultures of neurons, 7-8-day cultures of cerebellar neurons were used. Determination of the rate of neuronal death in cultures was carried out using propidium iodide. Acute hypoxia with hypercapnia was simulated in rats by placing them in sealed vessels with a capacity of 1 L. The effect on oxidative processes in brain tissues was assessed by changes in the level of free radical oxidation and malondialdehyde. The effect on the antioxidant system of the brain was assessed by the activity of catalase. The study in the T-maze was carried out in accordance with the generally accepted methodology, the skill of alternating right-sided and left-sided loops on positive reinforcement was developed. This work has shown that a decrease in the deuterium content in the incubation medium to a level of -357‱ has a neuroprotective effect, increasing the survival rate of cultured neurons under glucose deprivation. When exposed to hypoxia, a preliminary decrease in the deuterium content in the rat brain to -261‱ prevents the development of oxidative stress in their nervous tissue and preserves the learning ability of animals in the T-shaped maze test at the level of the control group. A similar protective effect during the modification of the 2H/1H internal environment of the body by the consumption of DDW can potentially be used for the prevention of pathological conditions associated with the development of oxidative stress with damage to the central nervous system.

Oxysterol-binding protein-related protein (ORP) 6 localizes to the ER and ER-plasma membrane contact sites and is involved in the turnover of PI4P in cerebellar granule neurons

Oxysterol-binding protein (OSBP)-related proteins (ORPs) are conserved lipid binding proteins found in organisms ranging from yeast to mammals. Recent findings have indicated that these proteins mainly localize to contact sites of 2 different membranous organelles. ORP6, a member of the ORP subfamily III, is one of the least studied ORPs. Using approaches in molecular cell biology, we attempted to study the characteristics of ORP6 and found that ORP6 is abundantly expressed in mouse cultured neurons. Deconvolution microscopy of cultured cerebellar granular cells revealed that ORP6 is localized to the endoplasmic reticulum (ER) and ER-plasma membrane (PM) contact sites, where it co-localized with extended synaptotagmin2 (E-Syt2), a well-known ER-PM contact site marker. E-Syt2 also co-localized with ORP3, another subfamily III member, and ORP5, a subfamily IV member. However, ORP5 does not distribute to the same ER-PM contact sites as subfamily III members. Also, the co-expression of ORP3 but not ORP5 altered the distribution of ORP6 into the processes of cerebellar neurons. Immunoprecipitation demonstrated binding between the intermediate region of ORP6 and ORP3 or ORP6 itself. Additionally, the localization of ORP6 in the PM decreased when co-expressed with the intermediate region of ORP6, in which the pleckstrin homology (PH) domain and OSBP-related ligand binding domain (ORD) are deleted. Over-expression of this intermediate region shifted the location of a phophtidylinositol-4-phosphate (PI4P) marker from the Golgi to the PM. Knockdown of ORP6 resulted in the same shift of the PI4P marker. Collectively, our data suggests that the recruitment of ORP6 to ER-PM contact sites is involved in the turnover of PI4P in cerebellar granular neurons.

Streptozotocin toxicity in vitro depends on maturity of neurons

Streptozotocin (STZ) is a glucosamine-nitrosourea compound that is particularly toxic to the insulin-producing beta-cells of the pancreas in mammals; it is used for experimental simulation of sporadic Alzheimer's disease by means of intracerebroventricular administration in vivo. Here we show that the application of 3-4 mM STZ to primary culture for 48 h induces neuronal death in immature (2-3 days in vitro) cultures of rat cerebellar granule cells. Mature cultures (7-8 days in vitro) were poorly sensitive to this toxic treatment. Immature cultures demonstrated a high expression of the protein PSA-NCAM, the marker of immature neurons, and they were insensitive to the toxic effect of glutamate. In mature cultures, this protein was poorly expressed, whereas neurons showed a very high sensitivity to the toxic effect of glutamate. Measurements of the concentration of intracellular free calcium ions ([Ca²⁺]_i) showed that the STZ-induced [Ca²⁺]_i increase in young neurons was six times higher than that in mature neurons. Our results show that STZ is very toxic for immature neurons and probably it can significantly impair neurogenesis.

N-Acetyl-L-cysteine in the Presence of Cu2+ Induces Oxidative Stress and Death of Granule Neurons in Dissociated Cultures of Rat Cerebellum

Addition into the culture medium of the antioxidant N-acetylcysteine (NAC, 1 mM) in the presence of Cu2+ (0.0005-0.001 mM) induced intensive death of cultured rat cerebellar granule neurons, which was significantly decreased by the zinc ion chelator TPEN (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine). However, the combined action of NAC and Zn2+ did not induce destruction of the neurons. Measurement of the relative intracellular concentration of Zn2+ with the fluorescent probe FluoZin-3 AM or of free radical production using a CellROX Green showed that incubation of the culture for 4 h with Cu2+ and NAC induced an intensive increase in the fluorescence of CellROX Green but not of FluoZin-3. Probably, the protective effect of TPEN in this case could be mediated by its ability to chelate Cu2+. Incubation of cultures in a balanced salt solution in the presence of 0.01 mM Cu2+ caused neuronal death already after 1 h if the NAC concentration in the solution was within 0.005-0.05 mM. NAC at higher concentrations (0.1-1 mM) together with 0.01 mM Cu2+ did not cause the death of neurons. These data imply that the antioxidant NAC can be potentially harmful to neurons even in the presence of nanomolar concentrations of variable valence metals.

Streptozotocin causes neurotoxic effect in cultured cerebellar granule neurons

Streptozotocin (STZ) is a glucosamine-nitrosourea compound used for experimental simulation of sporadic Alzheimer's disease at intracerebroventricular administration in vivo. The studies of STZ influence on neurons of central nervous system performed on the primary cultures are practically absent. We have shown the application of STZ (1-5mM) in primary culture for 48h induced strong dose-dependent death in cultured cerebellar granule neurons. This toxic effect was decreased by pyruvate, insulin partially. Using the indicator Fluo-4 AM for measurements of intracellular calcium ions and tetramethylrhodamine ethyl ester (TMRE) for detection of changes of mitochondrial membrane potential in live cells we have shown that 5 h-exposure to STZ induced intensive increase of Fluo-4 and decrease TMRE fluorescence in neurons. STZ exposure caused considerable ultrastructural alterations in granule neurons: chromatin clumping, swelling of the endoplasmic reticulum and mitochondria, and disruption of the mitochondrial cristae. Probably, STZ significantly impaired glucose metabolism and mitochondrial function that, in turn, resulted in mitochondrial membrane potential damage, excessive calcium overload and neuronal death.

Glucose starvation stimulates Zn2+ toxicity in cultures of cerebellar granule neurons

Zinc chloride (0.02 mM, 3h) did not have any influence on the survival of cerebellar granule neurons (CGNs) incubated in balanced salt solution (BSS). However, in the absence of glucose ZnCl(2) caused severe neuronal damage, decreasing cell survival to 12±2%. Either the blockade of ionotropic glutamate NMDA-receptors with MK-801 or APV or supplementation the medium with ruthenium red (mitochondrial Ca(2+) uniporter blocker) almost entirely protected CGNs from the toxic effect of ZnCl(2) during glucose deprivation (GD). However, NBQX (AMPA/kainate glutamate receptor blocker) did not show protective effect. Measurements of intracellular calcium ions concentration using fluorescent probe (Fluo-4 AM) and zinc ions (FluoZin-3AM) demonstrated that 1.5h-exposure to GD induced intensive increase of Fluo-4 fluorescence and small increase of FluoZin-3 fluorescence in neurons. The supplementation of medium with ZnCl(2) caused equal increase of FluoZin-3 fluorescence at both GD and normoglycemia, whereas the potentiation of Fluo-4 fluorescence by zinc was observed only under GD and could be prevented by MK-801. However, neither MK-801 nor NBQX could influence [Zn(2+)](i) increase caused by zinc addition under GD, while ruthenium red did cause significant increase of [Zn(2+)](i). This data implies that zinc ions during GD induce an additional overload of CGNs with calcium ions that get transported through activated NMDA-channel. Zinc and calcium ions accumulate in mitochondria and amplify individual destructive action on these organelles leading to neuronal death.

Stimulation of kainate toxicity by zinc in cultured cerebellar granule neurons and the role of mitochondria in this process

Zinc chloride (0.01 mM kept for 3h) is not toxic to cultured cerebellar granule neurons (CGNs) while kainate (0.1mM kept for 3h) demonstrates some but very low toxicity towards these cells. Measurements of the relative intraneuronal zinc ion concentration showed that increase in [Zn(2+)](i) under the simultaneous action of ZnCl(2) and kainate was significantly stronger compared to their separate action. Simultaneous treatment of CGNs with kainate and zinc chloride caused the swelling of neuronal mitochondria and consequent intensive neuronal death, which was totally prevented by NBQX (an AMPA/kainate-receptors blocker) or ruthenium red (a mitochondrial Ca(2+) uniporter blocker). These data imply that Zn(2+) synergistically to kainate increase their separate toxic effects on mitochondria leading to rapid neuronal death.

Glutamine effect on cultured granule neuron death induced by glucose deprivation and chemical hypoxia

Using a specific fluorescent probe of mitochondrial membrane potential (tetramethylrhodamine ethyl ester), we have shown that glucose deprivation (GD) of cultured cerebellar granule neurons (CGN) for 3 h lowers mitochondrial membrane potential in these cells. Longer glucose starvation (24 h) causes CGN death that is not prevented by blockers of ionotropic glutamate receptors (MK-801 (10 µM) and NBQX (10 µM)). Glutamine or pyruvate (2 mM) maintain membrane potential of mitochondria and decrease CGN death under GD conditions. In the presence of glucose the mitochondrial respiratory chain blocker rotenone induces neuron death potentiated by glutamine. The potentiation effect is completely prevented by blockers of ionotropic glutamate receptors. These results show that glutamine under conditions of GD can be utilized by mitochondria as substrate, but at the same time, in the case of mitochondrial function deterioration, metabolism of this amino acid results in glutamate accumulation to toxic level.

Mitochondrial dysfunction and pathology in bipolar disorder and schizophrenia

Bipolar disorder (BPD) and schizophrenia (SZ) are severe psychiatric illnesses with a combined prevalence of 4%. A disturbance of energy metabolism is frequently observed in these disorders. Several pieces of evidence point to an underlying dysfunction of mitochondria: (i) decreased mitochondrial respiration; (ii) changes in mitochondrial morphology; (iii) increases in mitochondrial DNA (mtDNA) polymorphisms and in levels of mtDNA mutations; (iv) downregulation of nuclear mRNA molecules and proteins involved in mitochondrial respiration; (v) decreased high-energy phosphates and decreased pH in the brain; and (vi) psychotic and affective symptoms, and cognitive decline in mitochondrial disorders. Furthermore, transgenic mice with mutated mitochondrial DNA polymerase show mood disorder-like phenotypes. In this review, we will discuss the genetic and physiological components of mitochondria and the evidence for mitochondrial abnormalities in BPD and SZ. We will furthermore describe the role of mitochondria during brain development and the effect of current drugs for mental illness on mitochondrial function. Understanding the role of mitochondria, both developmentally as well as in the ailing brain, is of critical importance to elucidate pathophysiological mechanisms in psychiatric disorders.