Role of the mitochondrial sodium/calcium exchanger in neuronal physiology and in the pathogenesis of neurological diseases

Pivotal role of mitochondrial Na⁺₋Ca²⁺ exchange in antigen receptor mediated Ca²⁺ signalling in DT40 and A20 B lymphocytes

Cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) increases upon activation of antigen-receptor in lymphocytes. Mitochondria have been suggested to regulate the [Ca(2+)](i) response, but the molecular mechanisms and the roles are poorly understood. To clarify them, we carried out a combination study of mathematical simulations and knockout or knockdown of NCLX, a gene candidate for the mitochondrial Na(+)-Ca(2+) exchanger (NCX(mit)), in B lymphocytes. A mathematical model of Ca(2+) dynamics in B lymphocytes demonstrated that NCX(mit) inhibition reduces basal Ca(2+) content of endoplasmic reticulum (ER) and suppresses B-cell antigen receptor (BCR)-mediated [Ca(2+)](i) rise. The predictions were validated in DT40 B lymphocytes of heterozygous NCLX knockout (NCLX(+/-)). In NCLX(+/-) cells, mitochondrial Ca(2+) efflux via NCX(mit) was strongly decelerated, suggesting NCLX is a gene responsible for NCX(mit) in B lymphocytes. Consistent with the predictions, ER Ca(2+) content declined and [Ca(2+)](i) hardly rose upon BCR activation in NCLX(+/-) cells. ER Ca(2+) uptake was reduced to ∼58% of the wild-type (WT), while it was comparable to WT when mitochondrial respiration was disturbed. Essentially the same results were obtained by a pharmacological inhibition or knockdown of NCLX by siRNA in A20 B lymphocytes. Unexpectedly, ER Ca(2+) leak was augmented and co-localization of mitochondria with ER was lower in NCLX(+/-) and NCLX silenced cells. Taken together, we concluded that NCLX is a key Ca(2+) provider to ER, and that NCLX-mediated Ca(2+) recycling between mitochondria and ER is pivotal in B cell responses to antigen.

Epileptic seizures in infants and children with mitochondrial diseases

The purpose of this study is to describe the characteristics of epileptic seizures in infants and children with mitochondrial diseases. From 1984 to December 2010, data from 46 of 76 patients diagnosed as having mitochondrial diseases with epileptic seizures were reviewed. Age at seizure onset, epileptic phenotypes, electroencephalogram findings, magnetic resonance imaging features, and treatment outcome in patients with syndromic or nonsyndromic mitochondrial diseases were analyzed. Thirty (65%) of 46 patients manifested seizures before the age of 1 year; 43% had Leigh syndrome and 53% had nonsyndromic mitochondrial diseases. Twenty-eight (61%) of 46 patients exhibited seizures as the manifesting complaint. Nineteen (68%) of 28 patients had nonsyndromic mitochondrial diseases. The most frequently observed electroencephalogram finding was background slow activity (28/46; 61%) in both groups. The most common cortical abnormality relevant to clinical seizures was diffuse brain atrophy on the brain magnetic resonance imaging (26/45; 58%), which was commonly observed in patients with nonsyndromic mitochondrial diseases (16/26; 62%). Despite treatment, 49% of patients experienced less than 50% seizure reduction rate, 77% of whom had nonsyndromic mitochondrial diseases. Leigh syndrome and nonsyndromic mitochondrial diseases often manifest as infantile seizures. Epileptic seizure as the initial complaint, diffuse brain atrophy, and refractory epilepsy were more common in patients with nonsyndromic mitochondrial diseases.

TAT fusion protein transduction into isolated mitochondria is accelerated by sodium channel inhibitors

Stringent control of ion and protein transport across the mitochondrial membranes is required to maintain mitochondrial function and biogenesis. In particular, the inner mitochondrial membrane is generally impermeable to proteins entering the matrix except via tightly regulated protein import mechanisms. Recently, cell penetrant peptides have been shown to move across the inner mitochondrial membrane in a manner suggesting an independent mechanism. HIV-1 transactivator of transcription (TAT) is an arginine-rich cell penetrant peptide, 47YGRKKRRQRRR57, which can transduce full-length proteins not only across the cell membrane but also into intracellular organelles. In this study, we investigated the ability of a TAT-containing protein to move into the mitochondrial matrix. Using a novel FACS assay for isolated, purified mitochondria, we show that TAT can deliver a modified fluorescent protein, mMDH-GFP, to the matrix of mitochondria and it is subsequently processed by the matrix peptidases. In addition, transduction of TAT-mMDH-GFP into mitochondria is independent of canonical protein import pathways as well as mitochondrial membrane potential. In direct contrast to published reports regarding the cell membrane where the sodium channel inhibitor, amiloride, blocks endocytosis and inhibits TAT transduction, TAT transduction into mitochondria is markedly increased by this same sodium channel inhibitor. These results confirm that the cell penetrant peptide, TAT, can readily transduce a protein cargo into the mitochondrial matrix. These results also demonstrate a novel role for mitochondrial sodium channels in mediating TAT transduction into mitochondria that is independent of endocytotic mechanisms. The mechanism of TAT transduction into mitochondria therefore is distinctly different from transduction across the cell membrane.

Calcium dysregulation and homeostasis of neural calcium in the molecular mechanisms of neurodegenerative diseases provide multiple targets for neuroprotection

The intracellular free calcium concentration subserves complex signaling roles in brain. Calcium cations (Ca(2+)) regulate neuronal plasticity underlying learning and memory and neuronal survival. Homo- and heterocellular control of Ca(2+) homeostasis supports brain physiology maintaining neural integrity. Ca(2+) fluxes across the plasma membrane and between intracellular organelles and compartments integrate diverse cellular functions. A vast array of checkpoints controls Ca(2+), like G protein-coupled receptors, ion channels, Ca(2+) binding proteins, transcriptional networks, and ion exchangers, in both the plasma membrane and the membranes of mitochondria and endoplasmic reticulum. Interactions between Ca(2+) and reactive oxygen species signaling coordinate signaling, which can be either beneficial or detrimental. In neurodegenerative disorders, cellular Ca(2+)-regulating systems are compromised. Oxidative stress, perturbed energy metabolism, and alterations of disease-related proteins result in Ca(2+)-dependent synaptic dysfunction, impaired plasticity, and neuronal demise. We review Ca(2+) control processes relevant for physiological and pathophysiological conditions in brain tissue. Dysregulation of Ca(2+) is decisive for brain cell death and degeneration after ischemic stroke, long-term neurodegeneration in Alzheimer's disease, Parkinson's disease, Huntington's disease, inflammatory processes, such as in multiple sclerosis, epileptic sclerosis, and leucodystrophies. Understanding the underlying molecular processes is of critical importance for the development of novel therapeutic strategies to prevent neurodegeneration and confer neuroprotection.

Modes of Neuronal Calcium Entry and Homeostasis following Cerebral Ischemia

One of the major instigators leading to neuronal cell death and brain damage following cerebral ischemia is calcium dysregulation. The neuron's inability to maintain calcium homeostasis is believed to be a result of increased calcium influx and impaired calcium extrusion across the plasma membrane. The need to better understand the cellular and biochemical mechanisms of calcium dysregulation contributing to neuronal loss following stroke/cerebral ischemia is essential for the development of new treatments in order to reduce ischemic brain injury. The aim of this paper is to provide a concise overview of the various calcium influx pathways in response to ischemia and how neuronal cells attempts to overcome this calcium overload.

Hyperexcitable substantia nigra dopamine neurons in PINK1- and HtrA2/Omi-deficient mice

The electrophysiological properties of substantia nigra pars compacta (SNC) dopamine neurons can influence their susceptibility to degeneration in toxin-based models of Parkinson's disease (PD), suggesting that excitotoxic and/or hypoactive mechanisms may be engaged during the early stages of the disease. It is unclear, however, whether the electrophysiological properties of SNC dopamine neurons are affected by genetic susceptibility to PD. Here we show that deletion of PD-associated genes, PINK1 or HtrA2/Omi, leads to a functional reduction in the activity of small-conductance Ca(2+)-activated potassium channels. This reduction causes SNC dopamine neurons to fire action potentials in an irregular pattern and enhances burst firing in brain slices and in vivo. In contrast, PINK1 deletion does not affect firing regularity in ventral tegmental area dopamine neurons or substantia nigra pars reticulata GABAergic neurons. These findings suggest that changes in SNC dopamine neuron excitability may play a role in their selective vulnerability in PD.

Mechanisms of axonal injury: internodal nanocomplexes and calcium deregulation

Axonal degeneration causes morbidity in many neurological conditions including stroke, neurotrauma and multiple sclerosis. The limited ability of central nervous system (CNS) neurons to regenerate, combined with the observation that axonal damage causes clinical disability, has spurred efforts to investigate the mechanisms of axonal degeneration. Ca influx from outside the axon is a key mediator of injury. More recently, substantial pools of intra-axonal Ca sequestered in the 'axoplasmic reticulum' have been reported. These Ca stores are under the control of multimolecular 'nanocomplexes' located along the internodes under the myelin. The overactivation of these complexes during disease can lead to a lethal release of Ca from intra-axonal stores. Rich receptor pharmacology offers tantalizing therapeutic options targeting these nanocomplexes in the many diseases where axonal degeneration is prominent.

The ups and downs of mitochondrial calcium signalling in the heart

Regulation of intramitochondrial free calcium ([Ca2+]m) is critical in both physiological and pathological functioning of the heart. The full extent and importance of the role of [Ca2+]m is becoming apparent as evidenced by the increasing interest and work in this area over the last two decades. However, controversies remain, such as the existence of beat-to-beat mitochondrial Ca2+ transients; the role of [Ca2+]m in modulating whole-cell Ca2+ signalling; whether or not an increase in [Ca2+]m is essential to couple ATP supply and demand; and the role of [Ca2+]m in cell death by both necrosis and apoptosis, especially in formation of the mitochondrial permeability transition pore. The role of [Ca2+]m in heart failure is an area that has also recently been highlighted. [Ca2+]m can now be measured reasonably specifically in intact cells and hearts thanks to developments in fluorescent indicators and targeted proteins and more sensitive imaging technology. This has revealed interactions of the mitochondrial Ca2+ transporters with those of the sarcolemma and sarcoplasmic reticulum, and has gone a long way to bringing the mitochondrial Ca2+ transporters to the forefront of cardiac research. Mitochondrial Ca2+ uptake occurs via the ruthenium red sensitive Ca2+ uniporter (mCU), and efflux via an Na+/Ca2+ exchanger (mNCX). The purification and cloning of the transporters, and development of more specific inhibitors, would produce a step-change in our understanding of the role of these apparently critical but still elusive proteins. In this article we will summarise the key physiological roles of [Ca2+]m in ATP production and cell Ca2+ signalling in both adult and neonatal hearts, as well as highlighting some of the controversies in these areas. We will also briefly discuss recent ideas on the interactions of nitric oxide with [Ca2+]m.

Measurement of selected ions related to oxidative stress and energy metabolism in Saudi autistic children

OBJECTIVES

Autism is a developmental disorder characterized by social and emotional deficits, language impairments and stereotyped behaviors that manifest in early postnatal life. This study aims to clarify the role of selected ions related to energy metabolism as a consequence of oxidative stress in the deterioration accompanied autism.

MATERIALS AND METHODS

Malonaldehyde as measure of lipid peroxidation, Na(+)/K(+) ion pump (ATPase), together with the concentrations of Na(+), K(+), Mg(2+), Ca(2+) and Pb(2+) were determined in plasma of 30 Saudi autistic patients and compared to 30 age-matching control samples.

RESULTS

The obtained data recorded that Saudi autistic patients have a remarkable higher activities of Na(+)/K(+) ATPase and high levels of lipid peroxidation compared to control. In addition, they have significantly elevated levels of K(+) and Pb(2+) while Ca(2+) recorded a significantly lower level compared to age-matching control subjects. On the other hand both Mg(2+) and Na(+) were non-significantly changed in autistic patients.

CONCLUSION

Alteration of the selected measured ions confirms that oxidative stress and defective mitochondrial energy production could represent the primary causative factor in the pathogenesis of autism.

Short-term effects of zinc on acetylcholine metabolism and viability of SN56 cholinergic neuroblastoma cells

Excessive accumulation of zinc in the brain is one of putative factors involved in pathomechanism of cholinergic encephalopathies. The aim of this work was to investigate whether short-term increase of zinc concentration in the extracellular space may affect energy and acetylcholine metabolism in SN56 cholinergic cells of septal origin. Short 30 min exposition of SN56 cells to increasing zinc levels caused greater loss of viability of differentiated (DC, [EC(0.4)] 0.09 mM) than nondifferentiated cells (NC, [EC(0.4)] 0.14 mM). Concentration-dependent accumulation of zinc displayed exponential non-saturable kinetics. Zinc accumulation caused the decrease of calcium accumulation in mitochondria and its increase in cytoplasmic compartment of SN56 cells. Significant inverse and direct correlations were found between zinc accumulation and calcium levels in mitochondrial (r=-0.96, p=0.028) and cytoplasmic (r=0.97, p=0.028) compartments of DC, respectively. Zinc exerted similar inhibition of pyruvate dehydrogenase, aconitase and isocitrate dehydrogenase both in NC and DC homogenates, at Ki values equal to about 0.07, 0.08 and 0.005 mM, respectively. On the other hand, ketoglutarate dehydrogenase activity in DC was inhibited by zinc (Ki 0.0005 mM) 8 times stronger that in NC (Ki 0.004 mM). Also zinc-evoked decreases in acetylcholine content and its release were significantly greater in DC than in NC. Same conditions caused suppression of cytoplasmic and mitochondrial content of acetyl-CoA, that positively correlated with inhibition of transmitter functions (r=0.995, p=005) and loss of cell viability (r=0.990, p=0.0006), respectively. Significant correlations were also found in zinc-challenged cells between pyruvate dehydrogenase activity and both mitochondrial acetyl-CoA content and cell viability. These data indicate that pyruvate dehydrogenase-dependent acetyl-CoA synthesis in neuronal mitochondria may be a primary target for short-term neurotoxic effects of zinc. In consequence, shortages of acetyl-CoA in the mitochondrial compartment would cause fast loss of functional and structural integrity of cholinergic neurons.

Regulation of plasma membrane calcium fluxes by mitochondria

The role of mitochondria in cell signaling is becoming increasingly apparent, to an extent that the signaling role of mitochondria appears to have stolen the spotlight from their primary function as energy producers. In this chapter, we will review the ionic basis of calcium handling by mitochondria and discuss the mechanisms that these organelles use to regulate the activity of plasma membrane calcium channels and transporters.