Calcium homeostasis in aged neurones

Recent Neurotherapeutic Strategies to Promote Healthy Brain Aging: Are we there yet?

Owing to the global exponential increase in population ageing, there is an urgent unmet need to develop reliable strategies to slow down and delay the ageing process. Age-related neurodegenerative diseases are among the main causes of morbidity and mortality in our contemporary society and represent a major socio-economic burden. There are several controversial factors that are thought to play a causal role in brain ageing which are continuously being examined in experimental models. Among them are oxidative stress and brain inflammation which are empirical to brain ageing. Although some candidate drugs have been developed which reduce the ageing phenotype, their clinical translation is limited. There are several strategies currently in development to improve brain ageing. These include strategies such as caloric restriction, ketogenic diet, promotion of cellular nicotinamide adenine dinucleotide (NAD+) levels, removal of senescent cells, 'young blood' transfusions, enhancement of adult neurogenesis, stem cell therapy, vascular risk reduction, and non-pharmacological lifestyle strategies. Several studies have shown that these strategies can not only improve brain ageing by attenuating age-related neurodegenerative disease mechanisms, but also maintain cognitive function in a variety of pre-clinical experimental murine models. However, clinical evidence is limited and many of these strategies are awaiting findings from large-scale clinical trials which are nascent in the current literature. Further studies are needed to determine their long-term efficacy and lack of adverse effects in various tissues and organs to gain a greater understanding of their potential beneficial effects on brain ageing and health span in humans.

The aging mouse brain: cognition, connectivity and calcium

Aging is a complex process that differentially impacts multiple cognitive, sensory, neuronal and molecular processes. Technological innovations now allow for parallel investigation of neuronal circuit function, structure and molecular composition in the brain of awake behaving adult mice. Thus, mice have become a critical tool to better understand how aging impacts the brain. However, a more granular systems-based approach, which considers the impact of age on key features relating to neural processing, is required. Here, we review evidence probing the impact of age on the mouse brain. We focus on a range of processes relating to neuronal function, including cognitive abilities, sensory systems, synaptic plasticity and calcium regulation. Across many systems, we find evidence for prominent age-related dysregulation even before 12 months of age, suggesting that emerging age-related alterations can manifest by late adulthood. However, we also find reports suggesting that some processes are remarkably resilient to aging. The evidence suggests that aging does not drive a parallel, linear dysregulation of all systems, but instead impacts some processes earlier, and more severely, than others. We propose that capturing the more fine-scale emerging features of age-related vulnerability and resilience may provide better opportunities for the rejuvenation of the aged brain.

Memory and Learning Deficits Are Associated With Ca2+ Dyshomeostasis in Normal Aging

Neuronal intracellular Ca2+ homeostasis is critical to the normal physiological functions of neurons and neuronal Ca2+ dyshomeostasis has been associated with the age-related decline of cognitive functions. Accumulated evidence indicates that the underlying mechanism for this is that abnormal intracellular Ca2+ levels stimulate the dysregulation of intracellular signaling, which subsequently induces neuronal cell death. We examined intracellular Ca2+ homeostasis in cortical (in vivo) and hippocampal (in vitro) neurons from young (3-months), middle-age (12-months), and aged (24-months) wild type C57BL6J mice. We found a progressive age-related elevation of intracellular resting calcium ([Ca2+]r) in cortical (in vivo) and hippocampal (in vitro) neurons associated with increased hippocampal neuronal calpain activity and reduced cell viability. In vitro, removal of extracellular Ca2+ or treatment with SAR7334 or dantrolene reduced [Ca2+]r in all age groups and dantrolene treatment lowered calpain activity and increased cell viability. In vivo, both middle-aged and aged mice showed cognitive deficits compared to young mice, which improved after dantrolene treatment. These findings support the hypothesis that intracellular Ca2+ dyshomeostasis is a major mechanism underlying the cognitive deficits seen in both normal aging and degenerative neurologic diseases.

The Puzzling Role of Neuron-Specific PMCA Isoforms in the Aging Process

The aging process is a physiological phenomenon associated with progressive changes in metabolism, genes expression, and cellular resistance to stress. In neurons, one of the hallmarks of senescence is a disturbance of calcium homeostasis that may have far-reaching detrimental consequences on neuronal physiology and function. Among several proteins involved in calcium handling, plasma membrane Ca²⁺-ATPase (PMCA) is the most sensitive calcium detector controlling calcium homeostasis. PMCA exists in four main isoforms and PMCA2 and PMCA3 are highly expressed in the brain. The overall effects of impaired calcium extrusion due to age-dependent decline of PMCA function seem to accumulate with age, increasing the susceptibility to neurotoxic insults. To analyze the PMCA role in neuronal cells, we have developed stable transfected differentiated PC12 lines with down-regulated PMCA2 or PMCA3 isoforms to mimic age-related changes. The resting Ca²⁺ increased in both PMCA-deficient lines affecting the expression of several Ca²⁺-associated proteins, i.e., sarco/endoplasmic Ca²⁺-ATPase (SERCA), calmodulin, calcineurin, GAP43, CCR5, IP₃Rs, and certain types of voltage-gated Ca²⁺ channels (VGCCs). Functional studies also demonstrated profound changes in intracellular pH regulation and mitochondrial metabolism. Moreover, modification of PMCAs membrane composition triggered some adaptive processes to counterbalance calcium overload, but the reduction of PMCA2 appeared to be more detrimental to the cells than PMCA3.

Regulation of intrinsic excitability: Roles for learning and memory, aging and Alzheimer's disease, and genetic diversity

Plasticity of intrinsic neuronal excitability facilitates learning and memory across multiple species, with aberrant modulation of this process being linked to the development of neurological symptoms in models of cognitive aging and Alzheimer's disease. Learning-related increases in intrinsic excitability of neurons occurs in a variety of brain regions, and is generally thought to promote information processing and storage through enhancement of synaptic throughput and induction of synaptic plasticity. Experience-dependent changes in intrinsic neuronal excitability rely on activity-dependent gene expression patterns, which can be influenced by genetic and environmental factors, aging, and disease. Reductions in baseline intrinsic excitability, as well as aberrant plasticity of intrinsic neuronal excitability and in some cases pathological hyperexcitability, have been associated with cognitive deficits in animal models of both normal cognitive aging and Alzheimer's disease. Genetic factors that modulate plasticity of intrinsic excitability likely underlie individual differences in cognitive function and susceptibility to cognitive decline. Thus, targeting molecular mediators that either control baseline intrinsic neuronal excitability, subserve learning-related intrinsic neuronal plasticity, and/or promote resilience may be a promising therapeutic strategy for maintaining cognitive function in aging and disease. In this review, we discuss the complementary relationship between intrinsic excitability and learning, with a particular focus on how this relationship varies as a function of age, disease state, and genetic make-up, and how targeting these factors may help to further elucidate our understanding of the role of intrinsic excitability in cognitive function and cognitive decline.

Vitamin D, reactive oxygen species and calcium signalling in ageing and disease

Vitamin D is a hormone that maintains healthy cells. It functions by regulating the low resting levels of cell signalling components such as Ca(2+) and reactive oxygen species (ROS). Its role in maintaining phenotypic stability of these signalling pathways depends on the ability of vitamin D to control the expression of those components that act to reduce the levels of both Ca(2+) and ROS. This regulatory role of vitamin D is supported by both Klotho and Nrf2. A decline in the vitamin D/Klotho/Nrf2 regulatory network may enhance the ageing process, and this is well illustrated by the age-related decline in cognition in rats that can be reversed by administering vitamin D. A deficiency in vitamin D has also been linked to two of the major diseases in man: heart disease and Alzheimer's disease (AD). In cardiac cells, this deficiency alters the Ca(2+) transients to activate the gene transcriptional events leading to cardiac hypertrophy and the failing heart. In the case of AD, it is argued that vitamin D deficiency results in the Ca(2+) landscape that initiates amyloid formation, which then elevates the resting level of Ca(2+) to drive the memory loss that progresses to neuronal cell death and dementia.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Intrinsic Hippocampal Excitability Changes of Opposite Signs and Different Origins in CA1 and CA3 Pyramidal Neurons Underlie Aging-Related Cognitive Deficits

Aging-related cognitive deficits have been attributed to dysfunction of neurons due to failures at synaptic or intrinsic loci, or both. Given the importance of the hippocampus for successful encoding of memory and that the main output of the hippocampus is via the CA1 pyramidal neurons, much of the research has been focused on identifying the aging-related changes of these CA1 pyramidal neurons. We and others have discovered that the postburst afterhyperpolarization (AHP) following a train of action potentials is greatly enlarged in CA1 pyramidal neurons of aged animals. This enlarged postburst AHP is a significant factor in reducing the intrinsic excitability of these neurons, and thus limiting their activity in the neural network during learning. Based on these data, it has largely been thought that aging-related cognitive deficits are attributable to reduced activity of pyramidal neurons. However, recent in vivo and ex vivo studies provide compelling evidence that aging-related deficits could also be due to a converse change in CA3 pyramidal neurons, which show increased activity with aging. In this review, we will incorporate these recent findings and posit that an interdependent dynamic dysfunctional change occurs within the hippocampal network, largely due to altered intrinsic excitability in CA1 and CA3 hippocampal pyramidal neurons, which ultimately leads to the aging-related cognitive deficits.

Reduced expressions of calmodulin genes and protein and reduced ability of calmodulin to activate plasma membrane Ca(2+)-ATPase in the brain of protein undernourished rats: modulatory roles of selenium and zinc supplementation

The roles of protein undernutrition as well as selenium (Se) and zinc (Zn) supplementation on the ability of calmodulin (CaM) to activate erythrocyte ghost membrane (EGM) Ca(2+)-ATPase and the calmodulin genes and protein expressions in rat's cortex and cerebellum were investigated. Rats on adequate protein diet and protein-undernourished (PU) rats were fed with diet containing 16% and 5% casein, respectively, for a period of 10 weeks. The rats were then supplemented with Se and Zn at a concentration of 0.15 and 227 mg l(-1), respectively, in drinking water for 3 weeks. The results obtained from the study showed significant reductions in synaptosomal plasma membrane Ca(2+)-ATPase (PMCA) activity, Ca(2+)/CaM activated EGM Ca(2+) ATPase activity and calmodulin genes and protein expressions in PU rats. Se or Zn supplementation improved the ability of Ca(2+)/CaM to activate EGM Ca(2+)-ATPase and protein expressions. Se or Zn supplementation improved gene expression in the cerebellum but not in the cortex. Also, the activity of PMCA was significantly improved by Zn. In conclusion, it is postulated that Se and Zn might be beneficial antioxidants in protecting against neuronal dysfunction resulting from reduced level of calmodulin such as present in protein undernutrition.

A switch in the mode of the sodium/calcium exchanger underlies an age-related increase in the slow afterhyperpolarization

During aging, the Ca(2+)-sensitive slow afterhyperpolarization (sAHP) of hippocampal neurons is known to increase in duration. This change has also been observed in the serotonergic cerebral giant cells (CGCs) of the pond snail Lymnaea stagnalis, but has yet to be characterized. In this article, we confirm that there is a reduction in firing rate, an increase in the duration of the sAHP, and an alteration in the strength and speed of spike frequency adaptation in the CGCs during aging, a finding that is compatible with an increase in the sAHP current. We go on to show that age-related changes in the kinetics of spike frequency adaptation are consistent with a reduction in Ca(2+) clearance from the cell, which we confirm with Ca(2+) imaging and pharmacological manipulation of the sodium calcium exchanger. These experiments suggest that the sodium calcium exchanger may be switching to a reverse-mode configuration in the CGCs during aging.

Inhibition of PMCA activity by tau as a function of aging and Alzheimer's neuropathology

Ca2+-ATPases are plasma membrane and intracellular membrane transporters that use the energy of ATP hydrolysis to pump cytosolic Ca2+ out of the cell (PMCA) or into internal stores. These pumps are the main high-affinity Ca2+ systems involved in the maintenance of intracellular free Ca2+ at the properly low level in eukaryotic cells. The failure of neurons to keep optimal intracellular Ca2+ concentrations is a common feature of neurodegeneration by aging and aging-linked neuropathologies, such as Alzheimer's disease (AD). This disease is characterized by the accumulation of β-amyloid senile plaques and neurofibrillary tangles of tau, a protein that plays a key role in axonal transport. Here we show a novel inhibition of PMCA activity by tau which is concentration-dependent. The extent of inhibition significantly decreases with aging in mice and control human brain membranes, but inhibition profiles were similar in AD-affected brain membrane preparations, independently of age. No significant changes in PMCA expression and localization with aging or neuropathology were found. These results point out a link between Ca2+-transporters, aging and neurodegeneration mediated by tau protein.

Calcium signaling alterations, oxidative stress, and autophagy in aging

SIGNIFICANCE

Aging is a multi-factorial process that may be associated with several functional and structural deficits which can evolve into degenerative diseases. In this review, we present data that may depict an expanded view of molecular aging theories, beginning with the idea that reactive oxygen species (ROS) are the major effectors in this process. In addition, we have correlated the importance of autophagy as a neuroprotective mechanism and discussed a link between age-related molecules, Ca(2+) signaling, and oxidative stress.

RECENT ADVANCES

There is evidence suggesting that alterations in Ca(2+) homeostasis, including mitochondrial Ca(2+) overload and alterations in electron transport chain (ETC) complexes, which increase cell vulnerability, are linked to oxidative stress in aging. As much as Ca(2+) signaling is altered in aged cells, excess ROS can be produced due to an ineffective coupling of mitochondrial respiration. Damaged mitochondria might not be removed by the macroautophagic system, which is hampered in aging by lipofuscin accumulation, boosting ROS generation, damaging DNA, and, ultimately, leading to apoptosis.

CRITICAL ISSUES

This process can lead to altered protein expression (such as p53, Sirt1, and IGF-1) and progress to cell death. This cycle can lead to increased cell vulnerability in aging and contribute to an increased susceptibility to degenerative processes.

FUTURE DIRECTIONS

A better understanding of Ca(2+) signaling and molecular aging alterations is important for preventing apoptosis in age-related diseases. In addition, caloric restriction, resveratrol and autophagy modulation appear to be predominantly cytoprotective, and further studies of this process are promising in age-related disease therapeutics.

The timing of cognitive plasticity in physiological aging: a tDCS study of naming

This study aimed to explore the effects of transcranial direct current stimulation (tDCS) on physiologically aging adults performing a naming task. tDCS is a method that modulates human cortical excitability. Neuroplasticity is considered to have its foundation in cortical excitability as a property that adjusts the connection strength between neurons in the brain. Language efficiency, as all functions, relies on integration of information (i.e., effectiveness of connectivity) through neurons in the brain. So the use of tDCS, to modulate cortical excitability, can help to define the state of cognitive plasticity in the aging brain. Based on Hebb's rule, an increase in synaptic efficacy does not rely only on the increase of excitability but also on the timing of activation. Therefore, a key issue in this study is the timing of tDCS application in relation to a task: When to deliver tDCS to induce modulatory effects on task execution to facilitate naming. Anodal tDCS was applied to the left dorsolateral prefrontal cortex of older and young adults before and during a naming task. In older adults, tDCS improved naming performance and decreased the verbal reaction times only if it was applied during the task execution, whereas in young subjects both stimulation conditions improved naming performance. These findings highlight that in healthy aging adults, the cerebral network dedicated to lexical retrieval processing may be facilitated only if stimulation is applied to an "active" neural network. We hypothesize that this change is due to the neuronal synaptic changes, in the aging brain, which reduce the window of when cortical excitability can facilitate synaptic efficacy and therefore plasticity.

Reciprocal disruption of neuronal signaling and Aβ production mediated by extrasynaptic NMDA receptors: a downward spiral

It is becoming increasingly clear that aberrant neuronal activity can be the cause and the result of amyloid beta production. Synaptic activation facilitates non-amyloidogenic processing of amyloid precursor protein (APP) and cell survival, primarily through synaptic NMDA receptors (NMDARs) and perhaps specifically those containing GluN2A-subunits. In contrast, extrasynaptic and GluN2B-containing NMDARs promote beta-secretase cleavage of APP into amyloid-beta (Aβ). The opposing nature of these NMDAR populations is reflected in their control over cell survival and death pathways. Subtle changes in glutamate homeostasis may shift the balance between these pathways and could play a role in Alzheimer's disease (AD). Indeed, Aβ production, regional loss of brain connectivity and neurodegeneration correlate with neuronal activity in AD patients. From another perspective, Aβ oligomers (Aβo) alter neuronal signaling through several mechanisms involving NMDARs and intracellular calcium mishandling. While Aβo affect multiple receptors, GluN2B-NMDARs have emerged as primary mediators of altered synaptic plasticity and neurotoxicity. Memantine and its successor, NitroMemantine, are efficient at blocking or reversing the deleterious actions of Aβo largely due to their selectivity for extrasynaptic NMDARs. Recently, Aβo were shown to trigger astrocytic release of glutamate to the extrasynaptic space where it activates NMDARs to promote further Aβ production and synaptic depression. Combined with the reciprocal regulation between neuronal activity and Aβ production, extrasynaptic glutamate release adds to a maladaptive model and ultimately results in synaptotoxicity and neurodegeneration of AD. Extrasynaptic NMDAR antagonists remain as a promising therapeutic avenue by interfering with this cascade.

Altered calcium metabolism in aging CA1 hippocampal pyramidal neurons

Altered neuronal calcium homeostasis is widely hypothesized to underlie cognitive deficits in normal aging subjects, but the mechanisms that underlie this change are unknown, possibly due to a paucity of direct measurements from aging neurons. Using CCD and two-photon calcium imaging techniques on CA1 pyramidal neurons from young and aged rats, we show that calcium influx across the plasma membrane increases with aging, and that this change is countered by increased intracellular calcium buffering. The additional buffer in aging neurons balances the increased calcium influx following a small number (<3) action potentials, but is overwhelmed during sustained or theta-like activity which leads to a greater rise in intracellular calcium concentration in aging than that in young neurons. Our results demonstrate that calcium overload occurs regularly in aging CA1 pyramidal neurons under physiological conditions. This overload may be a critical factor in age-related decline in hippocampus-dependent cognitive function.

Insights into CNS ageing from animal models of senescence

In recent years, novel model systems have made significant contributions to our understanding of the processes that control the ageing of whole organisms. However, there are limited data to show that the mechanisms that gerontologists have identified as having a role in organismal ageing contribute significantly to the ageing of the central nervous system. Two recent discoveries illustrate this particularly well. The first is the consistent failure of researchers to demonstrate a simple relationship between organismal ageing and oxidative stress--a mechanism often assumed to have a primary role in brain ageing. The second is the demonstration that senescent cells play a causal part in organismal ageing but remain essentially unstudied in a CNS context. We argue that the animal models now available (including rodents, flies, molluscs and worms), if properly applied, will allow a paradigm shift in our current understanding of the normal processes of brain ageing.

The absence of the calcium-buffering protein calbindin is associated with faster age-related decline in hippocampal metabolism

Although reductions in the expression of the calcium-buffering proteins calbindin D-28K (CB) and parvalbumin (PV) have been observed in the aging brain, it is unknown whether these changes contribute to age-related hippocampal dysfunction. To address this issue, we measured basal hippocampal metabolism and hippocampal structure across the lifespan of C57BL/6J, calbindin D-28k knockout (CBKO) and parvalbumin knockout (PVKO) mice. Basal metabolism was estimated using steady state relative cerebral blood volume (rCBV), which is a variant of fMRI that provides the highest spatial resolution, optimal for the analysis of individual subregions of the hippocampal formation. We found that like primates, normal aging in C57BL/6J mice is characterized by an age-dependent decline in rCBV-estimated dentate gyrus (DG) metabolism. Although abnormal hippocampal fMRI signals were observed in CBKO and PVKO mice, only CBKO mice showed accelerated age-dependent decline of rCBV-estimated metabolism in the DG. We also found age-independent structural changes in CBKO mice, which included an enlarged hippocampus and neocortex as well as global brain hypertrophy. These metabolic and structural changes in CBKO mice correlated with a deficit in hippocampus-dependent learning in the active place avoidance task. Our results suggest that the decrease in CB that occurs during normal aging is involved in age-related hippocampal metabolic decline. Our findings also illustrate the value of using multiple MRI techniques in transgenic mice to investigate mechanisms involved in the functional and structural changes that occur during aging.

Altered calcium signaling following traumatic brain injury

Cell death and dysfunction after traumatic brain injury (TBI) is caused by a primary phase, related to direct mechanical disruption of the brain, and a secondary phase which consists of delayed events initiated at the time of the physical insult. Arguably, the calcium ion contributes greatly to the delayed cell damage and death after TBI. A large, sustained influx of calcium into cells can initiate cell death signaling cascades, through activation of several degradative enzymes, such as proteases and endonucleases. However, a sustained level of intracellular free calcium is not necessarily lethal, but the specific route of calcium entry may couple calcium directly to cell death pathways. Other sources of calcium, such as intracellular calcium stores, can also contribute to cell damage. In addition, calcium-mediated signal transduction pathways in neurons may be perturbed following injury. These latter types of alterations may contribute to abnormal physiology in neurons that do not necessarily die after a traumatic episode. This review provides an overview of experimental evidence that has led to our current understanding of the role of calcium signaling in death and dysfunction following TBI.

Mitochondrial dysfunction in long-term neuronal cultures mimics changes with aging

Background

Aging is a highly complex process that affects various tissues and systems in the body. Senescent changes are relatively more prevalent and severe in the postmitotic cells. Mitochondria play an important role in the aging process. Recently, cell cultures have been widely used as an in vitro model to study aging. The present study was designed to investigate mitochondrial dysfunction associated with aging in a long-term cell culture system.

Material/Methods

Rat hippocampal neurons were maintained in culture in serum-free medium for 30 days in vitro (DIV). The morphology and development of hippocampal neurons was observed by phase contrast microscope. The levels of cellular senescence were evaluated by cytochemical staining of senescence-associated β-galactosidase (SA-β-Gal) at DIV 5, 10, 15, 20, 25 and 30. In addition, we investigated the changes in mitochondrial membrane potential (Δψm) and intracellular reactive oxygen species (ROS) generation of hippocampal neurons by flow cytometry at different ages.

Results

The proportion of the senescent cells steadily increased with age in neuron cultures. Δψm decreased gradually with age in long-term culture, while ROS generation increased.

Conclusions

This study indicates an age-related decrease in mitochondrial function in long-term hippocampal neuronal culture and suggests that DIV 25 neurons could possibly serve as a platform for the future study of anti-aging from the perspective of mitochondrial function.

IP(3) Receptors, Mitochondria, and Ca Signaling: Implications for Aging

The tight interplay between endoplasmic-reticulum-(ER-) and mitochondria-mediated Ca(2+) signaling is a key determinant of cellular health and cellular fate through the control of apoptosis and autophagy. Proteins that prevent or promote apoptosis and autophagy can affect intracellular Ca(2+) dynamics and homeostasis through binding and modulation of the intracellular Ca(2+)-release and Ca(2+)-uptake mechanisms. During aging, oxidative stress becomes an additional factor that affects ER and mitochondrial function and thus their role in Ca(2+) signaling. Importantly, mitochondrial dysfunction and sustained mitochondrial damage are likely to underlie part of the aging process. In this paper, we will discuss the different mechanisms that control intracellular Ca(2+) signaling with respect to apoptosis and autophagy and review how these processes are affected during aging through accumulation of reactive oxygen species.

Cultured cerebellar granule neurons as an in vitro aging model: topoisomerase IIβ as an additional biomarker in DNA repair and aging

Aging in the brain is a multicellular process manifesting as neurodegeneration and associated functional impairment. In the present study, we report that cerebellar granule neurons (CGNs) in culture show senescence-mediated molecular changes indicating establishment of aging processes in vitro. CGNs were viable for 5 weeks followed by cellular degeneration. Molecular changes correlated with cellular senescence and aging include the elevation of senescence-mediated beta galactosidase (SA-β-gal) activity and intracellular Ca(2+) levels. Decreased base excision repair (BER) as well as non-homologous end joining (NHEJ) activities in CGNs were also observed upon aging in vitro. The decrease in NHEJ activity was shown correlated with corresponding decrease in the levels of topoisomerase IIβ (topo IIβ), Ku 70 and Ku 80 suggesting a crucial role for topo IIβ in repair capacity of CGNs. These studies, besides establishing that CGNs would serve as a good in vitro model for analysis of aging phenomena, also brought out that topo IIβ, by virtue of its significant role in controlling NHEJ activity, would serve as an additional biomarker for studying aging process.

Influence of aging on the calbindin-D-28k immunoreactive positive dopaminergic neurons in the substantia nigra pars compacta of rats

We studied the relationship between aging and the vulnerability of substantia nigra pars compacta (SNc) calbindin-D-28k immunoreactive positive (CB+) dopaminergic (DA) neurons. Immunohistochemistry and cell counting were used to determine the number of CB+ DA neuron in aged rats (24 mon) compared to adult rats (5 mon). Furthermore, the expression of CB mRNA and protein levels in SN was studied by semi-quantitative RT-PCR and Western blotting. An 11% loss of CB+ DA neurons was detected in both the rostral (8.9%) and caudal (1.7%) segments but not in the intermedial segment of SNc in aged rats compared to adult rats (P<0.05). No difference was detected in CB mRNA and protein levels between aged and adult rats (P>0.05). These data suggest that expression levels of CB mRNA and protein may increase in the existing SNc DA neurons, which may compensate for the partial age dependent loss of CB+ DA neurons in the SNc.

The endoplasmic reticulum stress response and aging

The endoplasmic reticulum (ER) is a multifunctional organelle which co-ordinates protein folding, lipid biosynthesis, calcium storage and release. Perturbations that disrupt ER homeostasis lead to ER stress and upregulation of a signaling pathway called the unfolded protein response (UPR). The UPR while robust in young animals appears to be compromised with aging; many of the components of the UPR have decreased expression and activity with age. There is also considerable evidence of oxidative damage. There are suggestions that an impaired UPR may contribute to the acceleration of neurodegenerative disorders.

In vivo calcium imaging of the aging and diseased brain

PURPOSE

Over the last decade, in vivo calcium imaging became a powerful tool for studying brain function. With the use of two-photon microscopy and modern labelling techniques, it allows functional studies of individual living cells, their processes and their interactions within neuronal networks. In vivo calcium imaging is even more important for studying the aged brain, which is hard to investigate in situ due to the fragility of neuronal tissue.

METHODS

In this article, we give a brief overview of the techniques applicable to image aged rodent brain at cellular resolution.

RESULTS

We use multicolor imaging to visualize specific cell types (neurons, astrocytes, microglia) as well as the autofluorescence of the "aging pigment" lipofuscin.

CONCLUSIONS

Further, we illustrate an approach for simultaneous imaging of cortical cells and senile plaques in mouse models of Alzheimer's disease.

Increased intraneuronal resting [Ca2+] in adult Alzheimer's disease mice

Neurodegeneration in Alzheimer's disease (AD) has been linked to intracellular accumulation of misfolded proteins and dysregulation of intracellular Ca2+. In the current work, we determined the contribution of specific Ca2+ pathways to an alteration in Ca2+ homeostasis in primary cortical neurons from an adult triple transgenic (3xTg-AD) mouse model of AD that exhibits intraneuronal accumulation of beta-amyloid proteins. Resting free Ca2+ concentration ([Ca2+](i)), as measured with Ca2+-selective microelectrodes, was greatly elevated in neurons from 3xTg-AD and APP(SWE) mouse strains when compared with their respective non-transgenic neurons, while there was no alteration in the resting membrane potential. In the absence of the extracellular Ca2+, the [Ca2+](i) returned to near normal levels in 3xTg-AD neurons, demonstrating that extracellular Ca2+contributed to elevated [Ca2+](i). Application of nifedipine, or a non-L-type channel blocker, SKF-96365, partially reduced [Ca2+](i). Blocking the ryanodine receptors, with ryanodine or FLA-365 had no effect, suggesting that these channels do not contribute to the elevated [Ca2+](i). Conversely, inhibition of inositol trisphosphate receptors with xestospongin C produced a partial reduction in [Ca2+](i). These results demonstrate that an elevation in resting [Ca2+](i), contributed by aberrant Ca2+entry and release pathways, should be considered a major component of the abnormal Ca2+ homeostasis associated with AD.

Expansion of the calcium hypothesis of brain aging and Alzheimer's disease: minding the store

Evidence accumulated over more than two decades has implicated Ca²⁺ dysregulation in brain aging and Alzheimer's disease (AD), giving rise to the Ca²⁺ hypothesis of brain aging and dementia. Electrophysiological, imaging, and behavioral studies in hippocampal or cortical neurons of rodents and rabbits have revealed aging-related increases in the slow afterhyperpolarization, Ca²⁺ spikes and currents, Ca²⁺ transients, and L-type voltage-gated Ca²⁺ channel (L-VGCC) activity. Several of these changes have been associated with age-related deficits in learning or memory. Consequently, one version of the Ca²⁺ hypothesis has been that increased L-VGCC activity drives many of the other Ca²⁺-related biomarkers of hippocampal aging. In addition, other studies have reported aging- or AD model-related alterations in Ca²⁺ release from ryanodine receptors (RyR) on intracellular stores. The Ca²⁺-sensitive RyR channels amplify plasmalemmal Ca²⁺ influx by the mechanism of Ca²⁺-induced Ca²⁺ release (CICR). Considerable evidence indicates that a preferred functional link is present between L-VGCCs and RyRs which operate in series in heart and some brain cells. Here, we review studies implicating RyRs in altered Ca²⁺ regulation in cell toxicity, aging, and AD. A recent study from our laboratory showed that increased CICR plays a necessary role in the emergence of Ca²⁺-related biomarkers of aging. Consequently, we propose an expanded L-VGCC/Ca²⁺ hypothesis, in which aging/pathological changes occur in both L-type Ca²⁺ channels and RyRs, and interact to abnormally amplify Ca²⁺ transients. In turn, the increased transients result in dysregulation of multiple Ca²⁺-dependent processes and, through somewhat different pathways, in accelerated functional decline during aging and AD.

Age-dependent changes in Ca2+ homeostasis in peripheral neurones: implications for changes in function

Calcium ions represent universal second messengers within neuronal cells integrating multiple cellular functions, such as release of neurotransmitters, gene expression, proliferation, excitability, and regulation of cell death or apoptotic pathways. The magnitude, duration and shape of stimulation-evoked intracellular calcium ([Ca2+]i) transients are determined by a complex interplay of mechanisms that modulate stimulation-evoked rises in [Ca2+]i that occur with normal neuronal function. Disruption of any of these mechanisms may have implications for the function and health of peripheral neurones during the aging process. This review focuses on the impact of advancing age on the overall function of peripheral adrenergic neurones and how these changes in function may be linked to age-related changes in modulation of [Ca2+]i regulation. The data in this review suggest that normal aging in peripheral autonomic neurones is a subtle process and does not always result in dramatic deterioration in their function. We present studies that support the idea that in order to maintain cell viability peripheral neurones are able to compensate for an age-related decline in the function of at least one of the neuronal calcium-buffering systems, smooth endoplasmic reticulum calcium ATPases, by increased function of other calcium-buffering systems, namely, the mitochondria and plasmalemma calcium extrusion. Increased mitochondrial calcium uptake may represent a 'weak point' in cellular compensation as this over time may contribute to cell death. In addition, we present more recent studies on [Ca2+]i regulation in the form of the modulation of release of calcium from smooth endoplasmic reticulum calcium stores. These studies suggest that the contribution of the release of calcium from smooth endoplasmic reticulum calcium stores is altered with age through a combination of altered ryanodine receptor levels and modulation of these receptors by neuronal nitric oxide containing neurones.

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.

Nitrosative/oxidative modifications and ageing

We present here a brief description of the relationships among metals, nitric oxide metabolism, and ageing. In particular, we will discuss the interactions occurring between redox (copper, iron) and non-redox (zinc) metals and nitric oxide, the metal- and nitric oxide-catalyzed formation of thiol adducts (nitrosothiols, mixed disulphides) and the possible involvement of these species in the ageing process.

The redox-sensitive DNA binding sites responsible for age-related downregulation of SMP30 by ERK pathway and reversal by calorie restriction

It was recently found that age-related changes in SMP30 expression can be modulated by antioxidative action. In the current study, the modulation of SMP30 gene expression was explored by (a) antioxidative calorie restriction (CR), (b) proinflammatory lipopolysaccharide (LPS), in aged rat, (c) oxidative stress promoter, tert-butylhydroperoxide (t-BHP)-injected mouse, and (d) t-BHP-treated Ac2F cells. Utilizing EMSA, particular attention was given to the binding activity of unidentified transcription factor in sites 3 and 5 that are located in -800 bp of the SMP30 promoter. Results showed that CR prevented the age-related decrease in SMP30 expression, and also showed that SMP30 gene expression and binding activities of sites 3 and 5 decreased with treatments of t-BHP or LPS. These findings were confirmed by the antioxidant NAC and ERK-specific inhibitor PD098059 that blunted decreased SMP30 gene expression and binding activity of sites 3 and 5 by t-BHP in Ac2F cell system. Our data strongly indicate that the SMP30 transcriptional process is redox-sensitive and its modulation occurs at DNA binding sites 3 and 5 in the promoter region. Perhaps a more significant finding of the present study is that the downregulation of SMP30 is likely involved in ERK signal pathway.

Calcium chelation improves spatial learning and synaptic plasticity in aged rats

Impaired regulation of intracellular calcium is thought to adversely affect synaptic plasticity and cognition in the aged brain. Comparing young (2-3 months) and aged (23-26 months) Fisher 344 rats, stratum radiatum-evoked CA1 field EPSPs were smaller and long-term potentiation (LTP) was diminished in aged hippocampal slices. Resting calcium, in presynaptic axonal terminals in the CA1 stratum radiatum area, was elevated in aged slices. Loading the slice with the calcium chelator, BAPTA-AM, depressed LTP in young slices, but enhanced this plasticity in old slices. Forty-five minutes following LTP-inducing high frequency stimulation, resting calcium levels were significantly increased in both young and old presynaptic terminals, and significantly reduced by pretreatment with BAPTA-AM. In vivo, intraperitoneal administration of BAPTA-AM prior to training in the reference memory version of the Morris water maze test, significantly improved the acquisition of spatial learning in aged animals, without a significant effect in young rats. These results support the hypothesis that increasing intracellular neuronal buffering power for calcium in aged rats ameliorates age-related impaired synaptic plasticity and learning.

Physiology and Pathophysiology of the Calcium Store in the Endoplasmic Reticulum of Neurons

The endoplasmic reticulum (ER) is the largest single intracellular organelle, which is present in all types of nerve cells. The ER is an interconnected, internally continuous system of tubules and cisterns, which extends from the nuclear envelope to axons and presynaptic terminals, as well as to dendrites and dendritic spines. Ca2+release channels and Ca2+pumps residing in the ER membrane provide for its excitability. Regulated ER Ca2+release controls many neuronal functions, from plasmalemmal excitability to synaptic plasticity. Enzymatic cascades dependent on the Ca2+concentration in the ER lumen integrate rapid Ca2+signaling with long-lasting adaptive responses through modifications in protein synthesis and processing. Disruptions of ER Ca2+homeostasis are critically involved in various forms of neuropathology.

Ca2+ regulation and gene expression in normal brain aging

Understanding the cellular mechanisms that characterize the functional changes of the aged brain is an ongoing and formidable challenge for the neuroscience community. Evidence now links changes in Ca(2+) influx and homeostasis with perturbations induced by the aging process in the function of the main intracellular organelles involved in Ca(2+) regulation: the endoplasmic reticulum and mitochondria. New perspectives are also offered by recent gene microarray studies, illustrating the multifactorial nature of the aging process.

[Neurotransmitters, calcium signalling and neuronal communication]

In this article we show some recent findings that constitute a great progress in the molecular knowledge of synaptic dynamics. To communicate, neurons use a code that includes electrical (action potentials) and chemical signals (neurotransmitters, neuromodulators). At the moment a great variety of molecules are known, whose neurotransmitter function in brain and the peripheral nervous system are out of question. Monoamines like acetylcholine, dopamine, noradrenaline, adrenaline, histamine, serotonin, glutamate, aspartate, glycine, ATP and GABA are good examples. Opioid neuropeptides, vasoactive intestinal peptide (VIP), neurokinines (substance P), somatostatin, neurotensin, neuropeptide Y, cholecystokinine, vasopressin or oxitocin have been related to the control of the stress response, sexual behaviour, food intake, pain, learning and memory, qualities that are also related to nitric oxide (NO). A great part of the molecular structure of the secretory machinery is known to be responsible for fast neurotransmitter release at the synapse, in response to action potentials. Proteins like sinaptobrevin (located in the membrane of the synaptic vesicle), sintaxin and SNAP-25 (both located at the presynaptic plasma membrane) constitute a trimeric complex which is responsible of the vesicular docking at the active sites for exocytosis. From this strategic location, vesicles release their neurotransmitter within few milliseconds, when the action potential invades the nerve terminal and activates the opening of the different subtypes of voltage-dependent Ca2+ channels. The asymmetric geographical distribution of each type of channel, in different neurons, rose the hypothesis that Ca2+ that enters through each subtype of channel is compartmentalised, thus favouring the generation of Ca2+ microdomains, in the cytosol and the nucleus, involved in different cellular functions. This great biochemical synaptic heterogeneity is facilitating the selection of many biological targets to develop drugs with potential therapeutic applications in neuropsychiatric diseases i.e. Alzheimer's, Parkinson, epilepsies, stroke, vascular dementia, depression, schizophrenia, anxiety and so on.

Mitochondrial polarisation status and [Ca2+]i signalling in rat cerebellar granule neurones aged in vitro

Mitochondrial membrane potential is a major factor that controls, ultimately, the cellular energy supply. By use of a mitochondrial membrane potential dye (rhodamine 123, R123) and image analysis we show that during long-term (>3 weeks) culture of primary neurones (cerebellar granule neurones) there is a gradual and time-dependent depolarisation of neuronal mitochondria. This process was demonstrated by analysing the changes in the heterogeneity of the cytosolic rhodamine 123 fluorescent signal as a function of the age in culture and by measuring the amplitude of the rhodamine 123 fluorescence evoked by the addition of a mitochondrial protonophore (FCCP). The relationship between cytosolic [Ca(2+)](i) and mitochondrial membrane potential was assessed by recording both parameters simultaneously, in neurones loaded with fura-2 and rhodamine 123. Neuronal stimulation (KCl-evoked depolarisation) induced a mitochondrial depolarisation response resulting from the entry of cytosolic Ca(2+) into mitochondria. In young cultures (10 DIV), the mitochondrial membrane potential recovered fully within 30s from the start of the stimulation, despite the continuous presence of the depolarisation stimulus and the maintained cytosolic [Ca(2+)](i) signal. In contrast, in older neurones (DIV 22), the mitochondrial response was of smaller amplitude and displayed a much longer repolarization period. Also, in these older neurones, the threshold [Ca(2+)](i) level required for the initiation of the mitochondrial depolarisation response was increased by 50%. Thus, the present results indicate that neuronal maturation and ageing in conditions of long-term in vitro culture determine significant changes in the mitochondrial polarisation status that are manifest both in resting conditions and during stimulation and could explain some of the reported changes in neuronal homeostasis in long-term neuronal cultures.

Endoplasmic reticulum Ca(2+) homeostasis and neuronal death

The endoplasmic reticulum (ER) is a universal signalling organelle, which regulates a wide range of neuronal functional responses. Calcium release from the ER underlies various forms of intracellular Ca(2+) signalling by either amplifying Ca(2+) entry through voltage-gated Ca(2+) channels by Ca(2+)-induced Ca(2+) release (CICR) or by producing local or global cytosolic calcium fluctuations following stimulation of metabotropic receptors through inositol-1,4,5-trisphosphate-induced Ca(2+) release (IICR). The ER Ca(2+) store emerges as a single interconnected pool, thus allowing for a long-range Ca(2+) signalling via intra-ER tunnels. The fluctuations of intra-ER free Ca(2+) concentration regulate the activity of numerous ER resident proteins responsible for post-translational protein folding and modification. Disruption of ER Ca(2+) homeostasis results in the developing of ER stress response, which in turn controls neuronal survival. Altered ER Ca(2+) handling may be involved in pathogenesis of various neurodegenerative diseases including brain ischemia and Alzheimer dementia.

Neurodegeneration: a key factor in the ageing gut

Many individuals experience gastrointestinal (GI) dysfunction more frequently as they age, and the segment of the human population that is growing the most rapidly is the 'oldest old', who are >/= 80 years old. There has recently been renewed interest in the age-related changes intrinsic to the gut, and these investigations may help physicians understand the 'normal' aged GI tract, as distinct from disordered bowel function that is the result of comorbid conditions and/or GI side effects of medications used to treat those conditions. In this concise review we summarize recent data that suggest age-related neurodegenerative changes in the enteric nervous system (ENS) are key to functional changes observed with advanced age. Morphological studies are reviewed that demonstrate clearly the loss of enteric neurones in both submucosal and myenteric plexuses in humans and in rodents. Recent studies that indicate selective preservation of nitrergic, but not cholinergic, neurones are reviewed, as are preliminary findings that intrinsic sensory neurones may be among the most 'age-labile' subpopulations of the ENS. Caloric restriction remains the only intervention known that prevents neurodegeneration of ageing in the ENS, and mechanisms involved in this phenomenon are discussed. The field of ageing research in enteric neurobiology is ripe for rapid progression from phenomenology of age-related losses of neurones and associated functional changes to discovery of therapeutic approaches that may help ameliorate deterioration of bowel function and thereby contribute significantly to improved quality of life in advanced age.

Intraneuronal amyloid-β1-42 production triggered by sustained increase of cytosolic calcium concentration induces neuronal death

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by the presence in the brain of senile plaques which contain an amyloid core made of beta-amyloid peptide (Abeta). Abeta is produced by the cleavage of the amyloid precursor protein (APP). Since impairment of neuronal calcium signalling has been causally implicated in ageing and AD, we have investigated the influence of an influx of extracellular calcium on the metabolism of human APP in rat cortical neurones. We report that a high cytosolic calcium concentration, induced by neuronal depolarization, inhibits the alpha-secretase cleavage of APP and triggers the accumulation of intraneuronal C-terminal fragments produced by the beta-cleavage of the protein (CTFbeta). Increase in cytosolic calcium concentration specifically induces the production of large amounts of intraneuronal Abeta1-42, which is inhibited by nimodipine, a specific antagonist of l-type calcium channels. Moreover, calcium release from endoplasmic reticulum is not sufficient to induce the production of intraneuronal Abeta, which requires influx of extracellular calcium mediated by the capacitative calcium entry mechanism. Therefore, a sustained high concentration of cytosolic calcium is needed to induce the production of intraneuronal Abeta1-42 from human APP. Our results show that this accumulation of intraneuronal Abeta1-42 induces neuronal death, which is prevented by a functional gamma-secretase inhibitor.

Neuronal ageing from an intraneuronal perspective: roles of endoplasmic reticulum and mitochondria

The nature of brain ageing and the age-dependent decline in cognitive functions remains poorly understood. Physiological brain ageing is characterised by mild mental dysfunctions, whereas age-dependent neurodegeneration, as illustrated by Alzheimer disease (AD), results rapidly in severe dementia. These two states of the aged brain, the physiological and the pathological, are fundamentally different as the latter stems from significant neuronal loss, whereas the former develops without significant neuronal demise. In this paper, we review the changes in neuronal Ca(2+) homeostasis that occur during brain ageing, and conclude that normal, physiological ageing is characterised mainly by a decrease of neuronal homeostatic reserve, defined as the capacity to respond effectively to functional and metabolic stressors, but does not reach the trigger required to induce neuronal death. In contrast, during neurodegenerative states, Ca(2+) homeostasis is affected early during the pathological process and result in significant neuronal demise. We also review recent evidence suggesting that the endoplasmic reticulum (ER) might play an important role in controlling the balance between healthy and pathological neuronal ageing.

Changes in mitochondrial status associated with altered Ca2+ homeostasis in aged cerebellar granule neurons in brain slices

In the present work, we investigated the relationship between mitochondrial function and Ca2+ homeostasis in brain slices obtained from mice that aged normally. In acute preparations, the cerebellar neurons had similar values for intracellular free Ca2+ ([Ca2+]i) regardless of their age (range, 6 weeks to 24 months). However, compared with the young slices, the aged neurons (20-24 months) showed an enhanced rate of [Ca2+]i increases as a function of the time the slices were maintained in vitro. When slices were stimulated (KCl depolarization), there were significant differences in the patterns of [Ca2+]i signal displayed by the young and old cerebellar granule neurons. More importantly, the aged neurons showed a significant delay in their capacity to recover the resting [Ca2+]i. The relationship between [Ca2+]i and mitochondrial membrane potential was assessed by recording both parameters simultaneously, using fura-2 and rhodamine-123. In both young and aged neurons, the cytosolic [Ca2+]i signal was associated with a mitochondrial depolarization response. In the aged neurons, the mitochondria had a significantly longer repolarization response, and quantitative analysis showed a direct correlation between the delays in mitochondrial repolarization and [Ca2+]i recovery, indicating the causal relationship between the two parameters. Thus, the present results show that the reported changes in Ca2+ homeostasis associated with aging, which manifest principally in a decreased capacity of maintaining a stable resting [Ca2+]i or recovering the resting [Ca2+]i values after stimulation, are primarily attributable to a metabolic dysfunction in which the mitochondrial impairment plays an important role.

Sustained Ca2+-induced Ca2+-release underlies the post-glutamate lethal Ca2+ plateau in older cultured hippocampal neurons

Several studies have shown that a prolonged Ca(2+) elevation follows a glutamate-mediated excitotoxic insult in cultured neurons, and may be associated with impending cell death. Recently, we showed that the prolonged Ca(2+) elevation that emerges as neurons age in culture is specifically linked to an age-related increase in excitotoxic vulnerability. However, the multiple sources of Ca(2+) that contribute to Ca(2+) elevation during and after glutamate exposure are not well understood. Here, we examined the Ca(2+) sources of the age-related prolonged Ca(2+) elevation in cultured hippocampal neurons. Studies with caffeine showed that the ryanodine receptor-dependent releasable pool of Ca(2+) from intracellular stores was similar in older and younger neurons. Thapsigargin, which inhibits intracellular store refilling, did not mimic the age-related prolonged Ca(2+) elevation and, in fact, partially reduced it. Ryanodine, which blocks Ca(2+)-induced Ca(2+)-release (CICR) from stores, completely blocked the age-related prolonged Ca(2+) elevation following glutamate exposure but did not alter maximal Ca(2+) elevation during the glutamate exposure. Thus, we conclude that sustained CICR plays a selective and key role in generating the lethal, age-related, prolonged Ca(2+) elevation, and is the likely mechanism underlying age-related, enhanced vulnerability to excitotoxicity in neurons.

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.

Oxidative stress and protein aggregation during biological aging

Biological aging is a fundamental process that represents the major risk factor with respect to the development of cancer, neurodegenerative, and cardiovascular diseases in vertebrates. It is, therefore, evident that the molecular mechanisms of aging are fundamental to understand many disease processes. In this regard, the oxidation and nitration of intracellular proteins and the formation of protein aggregates have been suggested to underlie the loss of cellular function and the reduced ability of senescent animals to withstand physiological stresses. Since oxidatively modified proteins are thermodynamically unstable and assume partially unfolded tertiary structures that readily form aggregates, it is likely that oxidized proteins are intermediates in the formation of amyloid fibrils. It is, therefore, of interest to identify oxidatively sensitive protein targets that may play a protective role through their ability to down-regulate energy metabolism and the consequent generation of reactive oxygen species (ROS). In this respect, the maintenance of cellular calcium gradients represents a major energetic expense, which links alterations in intracellular calcium levels to ATP utilization and the associated generation of ROS through respiratory control mechanisms. The selective oxidation or nitration of the calcium regulatory proteins calmodulin and Ca-ATPase that occurs in vivo during aging and under conditions of oxidative stress may represent an adaptive response to oxidative stress that functions to down-regulate energy metabolism and the associated generation of ROS. Since these calcium regulatory proteins are also preferentially oxidized or nitrated under in vitro conditions, these results suggest an enhanced sensitivity of these critical calcium regulatory proteins, which modulate signal transduction processes and intracellular energy metabolism, to conditions of oxidative stress. Thus, the selective oxidation of critical signal transduction proteins probably represents a regulatory mechanism that functions to minimize the generation of ROS through respiratory control mechanisms. The reduction of the rate of ROS generation, in turn, will promote cellular survival under conditions of oxidative stress, when reactive oxygen and nitrogen species overwhelm cellular antioxidant defense systems, by minimizing the non-selective oxidation of a range of biomolecules. Since protein aggregation occurs if protein repair and degradative systems are unable to act upon oxidized proteins and restore cellular function, the reduction of the oxidative load on the cell by the down-regulation of the electron transport chain functions to minimize protein aggregation. Thus, ROS function as signaling molecules that fine-tune cellular metabolism through the selective oxidation or nitration of calcium regulatory proteins in order to minimize wide-spread oxidative damage and protein aggregation. Oxidative damage to cellular proteins, the loss of calcium homeostasis and protein aggregation contribute to the formation of amyloid deposits that accumulate during biological aging. Critical to understand the relationship between these processes and biological aging is the identification of oxidatively sensitive proteins that modulate energy utilization and the associated generation of ROS. In this latter respect, oxidative modifications to the calcium regulatory proteins calmodulin (CaM) and the sarco/endoplasmic reticulum Ca-ATPase (SERCA) function to down-regulate ATP utilization and the associated generation of ROS associated with replenishing intracellular ATP through oxidative phosphorylation. Reductions in the rate of ROS generation, in turn, will minimize protein oxidation and facilitate intracellular repair and degradative systems that function to eliminate damaged and partially unfolded proteins. Since the rates of protein repair or degradation compete with the rate of protein aggregation, the modulation of intracellular calcium concentrations and energy metabolism through the selective oxidation or nitration of critical signal transduction proteins (i.e. CaM or SERCA) is thought to maintain cellular function by minimizing protein aggregation and amyloid formation. Age-dependent increases in the rate of ROS generation or declines in cellular repair or degradation mechanisms will increase the oxidative load on the cell, resulting in corresponding increases in the concentrations of oxidized proteins and the associated formation of amyloid.

Dependence of vital cell function on endoplasmic reticulum calcium levels: implications for the mechanisms underlying neuronal cell injury in different pathological states

The endoplasmic reticulum (ER) is a subcellular compartment playing a pivotal role in the control of vital calcium-related cell functions, including calcium storage and signalling. In addition, newly synthesized membrane and secretory proteins are folded and processed in the ER, reactions which are strictly calcium dependent. The ER calcium activity is therefore high, being several orders of magnitude above that of the cytoplasm. Depletion of ER calcium stores causes an accumulation of unfolded proteins in the ER lumen, a pathological situation which induces the activation of two highly conserved stress responses, the ER overload response (EOR) and the unfolded protein response (UPR). EOR triggers activation of the transcription factor NF kappa B, which, in turn, activates the expression of target genes. UPR triggers two downstream processes: it activates the expression of genes coding for ER-resident stress proteins, and it causes a suppression of the initiation of protein synthesis. A similar stress response is activated in pathological states of the brain including cerebral ischaemia, implying common underlying mechanisms. Depending on the extent and duration of the disturbance, an isolated impairment of ER function is sufficient to induce cell injury. In this review, evidence is presented that ER function is indeed disturbed in various diseases of the brain, including acute pathological states (e.g. cerebral ischaemia) and degenerative diseases (e.g. Alzheimer's disease). A body of evidence suggests that disturbances of ER function could be a global pathomechanism underlying neuronal cell injury in various acute and chronic disorders of the central nervous system. If that is true, restoration of ER function or attenuation of secondary disturbances induced by ER dysfunction could present a highly promising new avenue for pharmacological intervention to minimize neuronal cell injury in different pathological states of the brain.

Neuronal ageing in long-term cultures: alterations of Ca2+ homeostasis

Deficiencies of Ca2+ homeostasis are proposed to play an important role in neuronal ageing and/or neurodegeneration. The aim of this study was to investigate, in a defined neuronal population, primary cerebellar granule neuron culture, the time-dependent changes in Ca2+ homeostasis and compare them with data obtained in cerebellar brain slices from aged rats. In neurons aged in culture (DIV 23), a small decrease in the resting [Ca2+]i was associated with a decrease in the maximal rate of [Ca2+]i increase upon KCl-induced depolarization and in the amplitude of the [Ca2+]i response, when compared with mature neurons (DIV 9). The most significant change of [Ca2+]i signal parameters was a 50% decrease in the rate of [Ca2+]i recovery after the stimulation. These results were similar to those obtained in aged brain slices, and indicate that primary neuronal cultures could serve as a model for studying the age-related changes in Ca2+ homeostasis.