Ca2+ and mitochondria as substrates for deficits in synaptic plasticity in normal brain ageing

Normal brain ageing is associated with a degree of functional impairment of neuronal activity that results in a reduction in memory and cognitive functions. One mechanism proposed to explain the age-dependent changes was the "Ca(2+) hypothesis of ageing" but data accumulated in the last decade revealed a number of inconsistencies. Two important questions were raised: (a) which are, if any, the most reliable age-associated change in neuronal Ca(2+) homeostasis and (b) are these changes primary, and thus determinant of the ageing phenotype, or are they secondary to other changes in the physiology of the aged neurones. After a brief review of the evidence accumulated for the age-induced changes in synaptic plasticity, we assess the proposal that these changes are, ultimately, determined by changes in the metabolic state of the aged neurones, that are manifest particularly after neuronal stimulation. In this context, it appears that the changes in mitochondrial status and function are of primary importance.     

Calcium ions in neuronal degeneration

Neuronal Ca(2+) homeostasis and Ca(2+) signaling regulate multiple neuronal functions, including synaptic transmission, plasticity, and cell survival. Therefore disturbances in Ca(2+) homeostasis can affect the well-being of the neuron in different ways and to various degrees. Ca(2+) homeostasis undergoes subtle dysregulation in the physiological ageing. Products of energy metabolism accumulating with age together with oxidative stress gradually impair Ca(2+) homeostasis, making neurons more vulnerable to additional stress which, in turn, can lead to neuronal degeneration. Neurodegenerative diseases related to aging, such as Alzheimer's disease, Parkinson's disease, or Huntington's disease, develop slowly and are characterized by the positive feedback between Ca(2+) dyshomeostasis and the aggregation of disease-related proteins such as amyloid beta, alfa-synuclein, or huntingtin. Ca(2+) dyshomeostasis escalates with time eventually leading to neuronal loss. Ca(2+) dyshomeostasis in these chronic pathologies comprises mitochondrial and endoplasmic reticulum dysfunction, Ca(2+) buffering impairment, glutamate excitotoxicity and alterations in Ca(2+) entry routes into neurons. Similar changes have been described in a group of multifactorial diseases not related to ageing, such as epilepsy, schizophrenia, amyotrophic lateral sclerosis, or glaucoma. Dysregulation of Ca(2+) homeostasis caused by HIV infection or by sudden accidents, such as brain stroke or traumatic brain injury, leads to rapid neuronal death. The differences between the distinct types of Ca(2+) dyshomeostasis underlying neuronal degeneration in various types of pathologies are not clear. Questions that should be addressed concern the sequence of pathogenic events in an affected neuron and the pattern of progressive degeneration in the brain itself. Moreover, elucidation of the selective vulnerability of various types of neurons affected in the diseases described here will require identification of differences in the types of Ca(2+) homeostasis and signaling among these neurons. This information will be required for improved targeting of Ca(2+) homeostasis and signaling components in future therapeutic strategies, since no effective treatment is currently available to prevent neuronal degeneration in any of the pathologies described here.     

Selective age-related changes in the PKC-sensitive, calmodulin-binding protein, neurogranin, in the mouse brain

Brain ageing is associated with a dysregulation of intracellular calcium (Ca(2+)) homeostasis which leads to deficits in Ca(2+)-dependent signalling pathways and altered neuronal functions. Given the crucial role of neurogranin/RC3 (Ng) in the post-synaptic regulation of Ca(2+) and calmodulin levels, age-dependent changes in the levels of Ng mRNA and protein expression were analysed in 3, 12, 24 and 31-month-old mouse brains. Ageing produced significant decreases in Ng mRNA expression in the dorsal hippocampal subfields, retrosplenial and primary motor cortices, whereas no reliable changes were seen in any other cortical regions examined. Western blot indicated that Ng protein expression was also down-regulated in the ageing mouse brain. Analysis of Ng immunoreactivity in both hippocampal CA1 and retrosplenial areas indicated that Ng protein in aged mice decreased predominantly in the dendritic segments of pyramidal neurones. These data suggest that age-related changes of post-synaptic Ng in selected brain areas, and particularly in hippocampus, may contribute to altered Ca(2+)/calmodulin-signalling pathways and to region-specific impairments of synaptic plasticity and cognitive decline.     

Plasma membrane Ca-ATPases in the nervous system during development and ageing

Calcium signaling is used by neurons to control a variety of functions, including cellular differentiation, synaptic maturation, neurotransmitter release, intracellular signaling and cell death. This review focuses on one of the most important Ca(2+) regulators in the cell, the plasma membrane Ca(2+)-ATPase (PMCA), which has a high affinity for Ca(2+) and is widely expressed in brain. The ontogeny of PMCA isoforms, linked to specific requirements of Ca(2+) during development of different brain areas, is addressed, as well as their function in the adult tissue. This is based on the high diversity of variants in the PMCA family in brain, which show particular kinetic differences possibly related to specific localizations and functions of the cell. Conversely, alterations in the activity of PMCAs could lead to changes in Ca(2+) homeostasis and, consequently, to neural dysfunction. The involvement of PMCA isoforms in certain neuropathologies and in brain ageing is also discussed.     

Nitric oxide-induced calcium release via ryanodine receptors regulates neuronal function

Mobilization of intracellular Ca(2+) stores regulates a multitude of cellular functions, but the role of intracellular Ca(2+) release via the ryanodine receptor (RyR) in the brain remains incompletely understood. We found that nitric oxide (NO) directly activates RyRs, which induce Ca(2+) release from intracellular stores of central neurons, and thereby promote prolonged Ca(2+) signalling in the brain. Reversible S-nitrosylation of type 1 RyR (RyR1) triggers this Ca(2+) release. NO-induced Ca(2+) release (NICR) is evoked by type 1 NO synthase-dependent NO production during neural firing, and is essential for cerebellar synaptic plasticity. NO production has also been implicated in pathological conditions including ischaemic brain injury, and our results suggest that NICR is involved in NO-induced neuronal cell death. These findings suggest that NICR via RyR1 plays a regulatory role in the physiological and pathophysiological functions of the brain.     

Calpain-cleaved type 1 inositol 1,4,5-trisphosphate receptor impairs ER Ca(2+) buffering and causes neurodegeneration in primary cortical neurons

Disruption of neuronal Ca(2+) homeostasis plays a well-established role in cell death in a number of neurodegenerative disorders. Recent evidence suggests that proteolysis of the type 1 inositol 1,4,5-trisphosphate receptor (InsP(3) R1), a Ca(2+) release channel on the endoplasmic reticulum, generates a dysregulated channel, which may contribute to aberrant Ca(2+) signaling and neurodegeneration in disease states. However, the specific effects of InsP(3) R1 proteolysis on neuronal Ca(2+) homeostasis are unknown, as are the functional contributions of this pathway to neuronal death. This study evaluates the consequences of calpain-mediated InsP(3) R1 proteolysis on neuronal Ca(2+) signaling and survival using adeno-associated viruses to express a recombinant cleaved form of the channel (capn-InsP(3) R1) in rat primary cortical neurons. Here, we demonstrate that expression of capn-InsP(3) R1 in cortical cultures reduced cellular viability. This effect was associated with increased resting cytoplasmic Ca(2+) concentration ([Ca(2+) ](i) ), increased [Ca(2+) ](i) response to glutamate, and enhanced sensitivity to excitotoxic stimuli. Together, our results demonstrate that InsP(3) R1 proteolysis disrupts neuronal Ca(2+) homeostasis, and potentially acts as a feed-forward pathway to initiate or execute neuronal death.     

Calpain and synaptic function

Proteolysis by calpain is a unique posttranslational modification that can change integrity, localization, and activity of endogenous proteins. Two ubiquitous calpains, mu-calpain and m-calpain, are highly expressed in the central nervous system, and calpain substrates such as membrane receptors, postsynaptic density proteins, kinases, and phosphatases are localized to the synaptic compartments of neurons. By selective cleavage of synaptically localized molecules, calpains may play pivotal roles in the regulation of synaptic processes not only in physiological states but also during various pathological conditions. Activation of calpains during sustained synaptic activity is crucial for Ca2+-dependent neuronal functions, such as neurotransmitter release, synaptic plasticity, vesicular trafficking, and structural stabilization. Overactivation of calpain following dysregulation of Ca2+ homeostasis can lead to neuronal damage in response to events such as epilepsy, stroke, and brain trauma. Calpain may also provide a neuroprotective effect from axotomy and some forms of glutamate receptor overactivation. This article focuses on recent findings on the role of calpain-mediated proteolytic processes in potentially regulating synaptic substrates in physiological and pathophysiological events in the nervous system.     

Glial cells in (patho)physiology

Neuroglial cells define brain homeostasis and mount defense against pathological insults. Astroglia regulate neurogenesis and development of brain circuits. In the adult brain, astrocytes enter into intimate dynamic relationship with neurons, especially at synaptic sites where they functionally form the tripartite synapse. At these sites, astrocytes regulate ion and neurotransmitter homeostasis, metabolically support neurons and monitor synaptic activity; one of the readouts of the latter manifests in astrocytic intracellular Ca(2+) signals. This form of astrocytic excitability can lead to release of chemical transmitters via Ca(2+) -dependent exocytosis. Once in the extracellular space, gliotransmitters can modulate synaptic plasticity and cause changes in behavior. Besides these physiological tasks, astrocytes are fundamental for progression and outcome of neurological diseases. In Alzheimer's disease, for example, astrocytes may contribute to the etiology of this disorder. Highly lethal glial-derived tumors use signaling trickery to coerce normal brain cells to assist tumor invasiveness. This review not only sheds new light on the brain operation in health and disease, but also points to many unknowns.     

Sensing change: the emerging role of calcium sensors in neuronal disease

Calcium (Ca(2+)) is a fundamental intracellular signalling molecule in neurons. Therefore, significant interest has been expressed in understanding how the dysregulation of Ca(2+) signals might impact on neuronal function and the progression of different disease states. Many previous studies have examined the role of Ca(2+) in neuronal excitotoxicity and some have started to understand how Ca(2+) dysregulation might be a cause or consequence of neurodegeneration. This review will therefore focus on the significance of Ca(2+) sensors, proteins that transduce Ca(2+) signals, in neuronal function and dysfunction. Finally, we will assess their potential role in neurodegenerative processes, such as Alzheimer's disease (AD), arguing that they could serve as potential therapeutic targets.     

Mitochondrial Ca2+ homeostasis in health and disease

Although it has long been known that mitochondria possess a complex molecular repertoire for accumulating and releasing Ca2+, only in recent years has a large body of data demonstrated that these organelles promptly respond to Ca(2+)-mediated cell stimulations. In this contribution, we will review the principles of mitochondrial Ca2+ homeostasis and its signaling role in different physiological and pathological conditions.     

Normal brain ageing: models and mechanisms

Normal ageing is associated with a degree of decline in a number of cognitive functions. Apart from the issues raised by the current attempts to expand the lifespan, understanding the mechanisms and the detailed metabolic interactions involved in the process of normal neuronal ageing continues to be a challenge. One model, supported by a significant amount of experimental evidence, views the cellular ageing as a metabolic state characterized by an altered function of the metabolic triad: mitochondria-reactive oxygen species (ROS)-intracellular Ca2+. The perturbation in the relationship between the members of this metabolic triad generate a state of decreased homeostatic reserve, in which the aged neurons could maintain adequate function during normal activity, as demonstrated by the fact that normal ageing is not associated with widespread neuronal loss, but become increasingly vulnerable to the effects of excessive metabolic loads, usually associated with trauma, ischaemia or neurodegenerative processes. This review will concentrate on some of the evidence showing altered mitochondrial function with ageing and also discuss some of the functional consequences that would result from such events, such as alterations in mitochondrial Ca2+ homeostasis, ATP production and generation of ROS.     

Mitochondrial Ca(2+) and neurodegeneration

Mitochondria are essential for ensuring numerous fundamental physiological processes such as cellular energy, redox balance, modulation of Ca(2+) signaling and important biosynthetic pathways. They also govern the cell fate by participating in the apoptosis pathway. The mitochondrial shape, volume, number and distribution within the cells are strictly controlled. The regulation of these parameters has an impact on mitochondrial function, especially in the central nervous system, where trafficking of mitochondria is critical to their strategic intracellular distribution, presumably according to local energy demands. Thus, the maintenance of a healthy mitochondrial population is essential to avoid the impairment of the processes they regulate: for this purpose, cells have developed mechanisms involving a complex system of quality control to remove damaged mitochondria, or to renew them. Defects of these processes impair mitochondrial function and lead to disordered cell function, i.e., to a disease condition. Given the standard role of mitochondria in all cells, it might be expected that their dysfunction would give rise to similar defects in all tissues. However, damaged mitochondrial function has pleiotropic effects in multicellular organisms, resulting in diverse pathological conditions, ranging from cardiac and brain ischemia, to skeletal muscle myopathies to neurodegenerative diseases. In this review, we will focus on the relationship between mitochondrial (and cellular) derangements and Ca(2+) dysregulation in neurodegenerative diseases, emphasizing the evidence obtained in genetic models. Common patterns, that recognize the derangement of Ca(2+) and energy control as a causative factor, have been identified: advances in the understanding of the molecular regulation of Ca(2+) homeostasis, and on the ways in which it could become perturbed in neurological disorders, may lead to the development of therapeutic strategies that modulate neuronal Ca(2+) signaling.     

Mitochondrial uncoupling protein-4 regulates calcium homeostasis and sensitivity to store depletion-induced apoptosis in neural cells

An increase in the cytoplasmic-free Ca(2+) concentration mediates cellular responses to environmental signals that influence a range of processes, including gene expression, motility, secretion of hormones and neurotransmitters, changes in energy metabolism, and apoptosis. Mitochondria play important roles in cellular Ca(2+) homeostasis and signaling, but the roles of specific mitochondrial proteins in these processes are unknown. Uncoupling proteins (UCPs) are a family of proteins located in the inner mitochondrial membrane that can dissociate oxidative phosphorylation from respiration, thereby promoting heat production and decreasing oxyradical production. Here we show that UCP4, a neuronal UCP, influences store-operated Ca(2+) entry, a process in which depletion of endoplasmic reticulum Ca(2+) stores triggers Ca(2+) influx through plasma membrane "store-operated" channels. PC12 neural cells expressing human UCP4 exhibit reduced Ca(2+) entry in response to thapsigargin-induced endoplasmic reticulum Ca(2+) store depletion. The elevations of cytoplasmic and intramitochondrial Ca(2+) concentrations and mitochondrial oxidative stress induced by thapsigargin were attenuated in cells expressing UCP4. The stabilization of Ca(2+) homeostasis and preservation of mitochondrial function by UCP4 was correlated with reduced mitochondrial reactive oxygen species generation, oxidative stress, and Gadd153 up-regulation and increased resistance of the cells to death. Reduced Ca(2+)-dependent cytosolic phospholipase A2 activation and oxidative metabolism of arachidonic acid also contributed to the stabilization of mitochondrial function in cells expressing human UCP4. These findings demonstrate that UCP4 can regulate cellular Ca(2+) homeostasis, suggesting that UCPs may play roles in modulating Ca(2+) signaling in physiological and pathological conditions.     

Calcium and glial cell death

Calcium (Ca2+) homeostasis is crucial for development and survival of virtually all types of cells including glia of the central nervous system (CNS). Astrocytes, oligodendrocytes and microglia, the major glial cell types in the CNS, are endowed with a rather sophisticated array of Ca2+-permeable receptors and channels, as well as store-operated channels and pumps, all of which determine Ca2+ homeostasis. In addition, glial cells detect functional activity in neighbouring neurons and respond to it by means of Ca2+ signals that can modulate synaptic interactions. Like in neurons, Ca2+ overload resulting from dysregulation of channels and pumps can be deleterious to glia. In this review, we summarize recent advances in the understanding Ca2+ homeostasis in glial cells, the consequences of its alteration in cell demise as well as in neurological and psychiatric disorders that experience glial cell loss.     

The importance of being subtle: small changes in calcium homeostasis control cognitive decline in normal aging

Aging is a complex, multifactorial process. One of the features of normal aging of the brain is a decline in cognitive functions and much experimental attention has been devoted to understanding this process. Evidence accumulated in the last decade indicates that such functional changes are not due to gross morphological alterations, but to subtle functional modification of synaptic connectivity and intracellular signalling and metabolism. Such synaptic modifications are compatible with a normal level of activity and allow the maintenance of a certain degree of functional reserve. This is in contrast to the changes in various neurodegenerative diseases, characterized by significant neuronal loss and dramatic and irreversible functional deficit. This whole special issue has been initiated with the intention of focusing on the processes of normal brain aging. In this review, we present data that shows how subtle changes in Ca(2+) homeostasis or in the state of various Ca(2+)-dependent processes or molecules, which occur in aging can have significant functional consequences.     

Cations in the human blood serum

The review summarizes data (more than 450 references) on concentration of human serum cations (Na+, K+, Ca2+, and Mg2+) and human blood serum osmolality depending on age, diverse physiological and pathological states, and action of physiologically active substances. There are summarized data of many thousand measurements of physicochemical parameters of the blood serum, the mean values of osmolality and cation concentrations in healthy people are calculated. The values are kept at a stable level throughtout the entire life since the moment of birth; in many cases they are maintained by regulatory systems within the normal limits and during various physiological and pathological states. There are formulated the main types of the states characterized by deviations from norm of physicochemical parameters of the internal medium fluids.     

Regucalcin (RGN/SMP30) alters agonist- and thapsigargin-induced cytosolic [Ca2+] transients in cells by increasing SERCA Ca(2+)ATPase levels

Regucalcin (RGN), also reported as senescence marker protein-30 (SMP30), plays a role in Ca(2+) homeostasis by modulating a number of Ca(2+)-dependent proteins. RGN also plays a cyto-protective role and its decrease is linked to age-related diseases and cell death. This study shows that RGN reduces agonist (histamine)-induced Ca(2+) transients in RGN(+) transfected COS-7 cells (RGN(+)) and also increases their Ca(2+) storage capacity. These observations are explained by RGN(+) cells having increased mRNA and protein expression levels of sarco/endoplasmic reticulum Ca(2+)-ATPase (SERCA). Therefore down-regulation of RGN expression may contribute to characteristics of age-dependent Ca(2+) homeostasis dis-regulation, by decreasing SERCA levels.     

Endoplasmic reticulum dysfunction – a common denominator for cell injury in acute and degenerative diseases of the brain?

Various physiological, biochemical and molecular biological disturbances have been put forward as mediators of neuronal cell injury in acute and chronic pathological states of the brain such as ischemia, epileptic seizures and Alzheimer's or Parkinson's disease. These include over-activation of glutamate receptors, a rise in cytoplasmic calcium activity and mitochondrial dysfunction. The possible involvement of the endoplasmic reticulum (ER) dysfunction in this process has been largely neglected until recently, although the ER plays a central role in important cell functions. Not only is the ER involved in the control of cellular calcium homeostasis, it is also the subcellular compartment in which the folding and processing of membrane and secretory proteins takes place. The fact that blocking of these processes is sufficient to cause cell damage indicates that they are crucial for normal cell functioning. This review presents evidence that ER function is disturbed in many acute and chronic diseases of the brain. The complex processes taken place in this subcellular compartment are however, affected in different ways in various disorders; whereas the ER-associated degradation of misfolded proteins is affected in Parkinson's disease, it is the unfolded protein response which is down-regulated in Alzheimer's disease and the ER calcium homeostasis that is disturbed in ischemia. Studying the consequences of the observed deteriorations of ER function and identifying the mechanisms causing ER dysfunction in these pathological states of the brain will help to elucidate whether neurodegeneration is indeed caused by these disturbances, and will help to facilitate the search for drugs capable of blocking the pathological process directly at an early stage.