Synaptic mechanisms and calcium binding proteins in the aged rat brain

Carnosine research in relation to aging brain and neurodegeneration: A blessing for geriatrics and their neuronal disorders

Carnosine, an endogenous dipeptide (β-Ala-l-His), is enriched in prefrontal cortex and olfactory bulb of the brain, blood and also in muscle. It has mainly antioxidant and antiglycating properties which makes this molecule unique. Its content reduces during aging and aging-induced neurodegenerative diseases. Aging is a progressive biological process that leads to develop the risk factors of diseases and death. During aging the morphological, biochemical, cellular and molecular changes occur in brain and blood including other tissues. The objective of this review is to combine the updated information from the existing literature about the aging-induced neurodegeneration and carnosine research to meet the lacuna of mechanism of carnosine. The grey matter and white matter loses its normal ratio in aging, and hence the brain volume and weight. Different aging related neurodegenerative disorders arise due to loss of neurons, and synapses as a result of proteinopathies in some cases. Carnosine, being an endogenous biomolecule and having antioxidant, antiglycating properties has shown its potency to counteract erroneous protein biosynthesis, stress, activated microglial and astrocyte activity, and different neurodegenerative disorders. It (carnosine) can also inhibit the metal ion-induced degeneration by acting as a metal chelator. In this review the trends in carnosine research in relation to aging brain and neurodegeneration have been discussed with a view to its (carnosine) eligibility (including its mechanism of action) to be used as a promising neurotherapeutic for the betterment of elderly populations of our society at the national and international levels in near future.

Detrimental effects of a high fat/high cholesterol diet on memory and hippocampal markers in aged rats

High fat diets have detrimental effects on cognitive performance, and can increase oxidative stress and inflammation in the brain. The aging brain provides a vulnerable environment to which a high fat diet could cause more damage. We investigated the effects of a high fat/high cholesterol (HFHC) diet on cognitive performance, neuroinflammation markers, and phosphorylated Tau (p-Tau) pathological markers in the hippocampus of Young (4-month old) versus Aged (14-month old) male rats. Young and Aged male Fisher 344 rats were fed a HFHC diet or a normal control diet for 6 months. All animals underwent cognitive testing for 12days in a water radial arm maze to assess spatial and working reference memory. Hippocampal tissue was analyzed by immunohistochemistry for structural changes and inflammation, and Western blot analysis. Young and Aged rats fed the HFHC diet exhibited worse performance on a spatial working memory task. They also exhibited significant reduction of NeuN and calbindin-D28k immunoreactivity as well as an increased activation of microglial cells in the hippocampal formation. Western blot analysis of the hippocampus showed higher levels of p-Tau S202/T205 and T231 in Aged HFHC rats, suggesting abnormal phosphorylation of Tau protein following the HFHC diet exposure. This work demonstrates HFHC diet-induced cognitive impairment with aging and a link between high fat diet consumption and pathological markers of Alzheimer's disease.

Pretreatment with apoaequorin protects hippocampal CA1 neurons from oxygen-glucose deprivation

Ischemic stroke affects ∼795,000 people each year in the U.S., which results in an estimated annual cost of $73.7 billion. Calcium is pivotal in a variety of neuronal signaling cascades, however, during ischemia, excess calcium influx can trigger excitotoxic cell death. Calcium binding proteins help neurons regulate/buffer intracellular calcium levels during ischemia. Aequorin is a calcium binding protein isolated from the jellyfish Aequorea victoria, and has been used for years as a calcium indicator, but little is known about its neuroprotective properties. The present study used an in vitro rat brain slice preparation to test the hypothesis that an intra-hippocampal infusion of apoaequorin (the calcium binding component of aequorin) protects neurons from ischemic cell death. Bilaterally cannulated rats received an apoaequorin infusion in one hemisphere and vehicle control in the other. Hippocampal slices were then prepared and subjected to 5 minutes of oxygen-glucose deprivation (OGD), and cell death was assayed by trypan blue exclusion. Apoaequorin dose-dependently protected neurons from OGD--doses of 1% and 4% (but not 0.4%) significantly decreased the number of trypan blue-labeled neurons. This effect was also time dependent, lasting up to 48 hours. This time dependent effect was paralleled by changes in cytokine and chemokine expression, indicating that apoaequorin may protect neurons via a neuroimmunomodulatory mechanism. These data support the hypothesis that pretreatment with apoaequorin protects neurons against ischemic cell death, and may be an effective neurotherapeutic.

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.

Aging-related changes in calcium-binding proteins in rat perirhinal cortex

Dysregulation of intracellular calcium homeostasis has been linked to neuropathological symptoms observed in aging and age-related disease. Alterations in the distribution and relative frequency of calcium-binding proteins (CaBPs), which are important in regulating intracellular calcium levels, may contribute to disruption of calcium homeostasis. Here we examined the laminar distribution of three CaBPs in rat perirhinal cortex (PR) as a function of aging. Calbindin-D28k (CB), parvalbumin (PV), and calretinin (CR) were compared in adult (4 mo.), middle-aged (13 mo.) and aged (26 mo.) rats. Results show an aging-related and layer-specific decrease in the number of CB-immunoreactive (-ir) neurons, beginning in middle-aged animals. Dual labeling suggests that the age-related decrease in CB reflects a decrease in neurons that are not immunoreactive for the inhibitory neurotransmitter GABA. In contrast, no aging-related differences in PV- or CR-immunoreactivity were observed. These data suggest that selective alterations in CB-ir neurons may contribute to aging-related learning and memory deficits in tasks that depend upon PR circuitry.

Learning and aging related changes in intrinsic neuronal excitability

A goal of many laboratories that study aging is to find a key cellular change(s) that can be manipulated and restored to a young-like state, and thus, reverse the age-related cognitive deficits. We have chosen to focus our efforts on the alteration of intrinsic excitability (as reflected by the postburst afterhyperpolarization, AHP) during the learning process in hippocampal pyramidal neurons. We have consistently found that the postburst AHP is significantly reduced in hippocampal pyramidal neurons from young adults that have successfully learned a hippocampus-dependent task. In the context of aging, the baseline intrinsic excitability of hippocampal neurons is decreased and therefore cognitive learning is impaired. In aging animals that are able to learn, neuron changes in excitability similar to those seen in young neurons during learning occur. Our challenge, then, is to understand how and why excitability changes occur in neurons from aging brains and cause age-associated learning impairments. After understanding the changes, we should be able to formulate strategies for reversing them, thus making old neurons function more as they did when they were young. Such a reversal should rescue the age-related cognitive deficits.

Age-dependent changes in calretinin immunoreactivity and its protein level in the gerbil hippocampus

Calretinin (CR)-immunoreactive interneurons are well known as the interneuron specific interneurons in the hippocampus. CR-immunoreactive neurons form cellular network and regulate the activity of other GABAergic inhibitory interneurons in the hippocampus. In the present study, we investigated age-related changes in CR-immunoreactive neurons and protein levels in the gerbil hippocampus during normal aging. In all subregions of the gerbil hippocampus, the number of CR-immunoreactive neurons was significantly decreased in the postnatal month 6 (PM 6) group compared to that in the PM 1 group. Thereafter, CR-immunoreactive neurons were decreased with age. In addition, the number of CR-immunoreactive cells in the subgranular zone were significantly decreased in the PM 6 group. We also observed that CR protein levels were decreased gradually with age. These results indicate that both CR immunoreactivity and its protein level were decreased with age in the gerbil hippocampus during normal aging.

The excitatory thalamo-"cortical" projection within the song control system of zebra finches is formed by calbindin-expressing neurons

The learning and production of vocalizations in songbirds are controlled by a system of interconnected brain nuclei organized into a direct vocal motor pathway and an anterior forebrain (pallium-basal ganglia-thalamo-pallial) loop. Here we show that the thalamo-pallial ("thalamo-cortical") projection (from the medial part of the dorsolateral thalamic nucleus to the lateral magnocellular nucleus of the anterior nidopallium--DLM to LMAN) within the anterior forebrain loop is composed of cells positive for the calcium-binding protein calbindin. We show that the vast majority of cells within DLM express calbindin, based both on immunocytochemistry (ICC) for calbindin protein and in situ hybridization for calb mRNA. Using a combination of tract-tracing and ICC we show that the neurons that participate in the DLM-to-LMAN projection are calbindin-positive. We also demonstrate that DLM is devoid of cells expressing mRNA for the GABAergic marker zGAD65. This observation confirms that the calbindin-expressing cells in DLM are not GABAergic, in accordance with previous electrophysiological data indicating that the DLM-to-LMAN projection is excitatory. Furthermore, we use ICC to determine the trajectory of the fibers within the DLM-to-LMAN projection, and to demonstrate a sex difference in calbindin expression levels in the fibers of the DLM-to-LMAN projection. Our findings provide a clear-cut neurochemical signature for a critical projection in the songbird vocal control pathways that enable song learning.

Calcium buffering systems and calcium signaling in aged rat basal forebrain neurons

Disturbances of neuronal Ca2+ homeostasis are considered to be important determinants of age-related cognitive impairment. Cholinergic neurons of the basal forebrain (BF) are principal targets of decline associated with aging and dementia. During the last several years, we have attempted to link these concepts in a rat model of 'normal' aging. In this review, we will describe some changes that we have observed in Ca2+ signaling of aged BF neurons and the reversal of one of these changes by dietary caloric restriction. Our evidence supports a scenario in which subtle changes in the properties of voltage-gated Ca2+ channels result in increased Ca2+ influx during aging. This increased Ca2+, in turn, triggers an increase in rapid Ca2+ buffering in the somatic compartment of aged BF neurons. However, this nominal 'compensation', along with other changes in Ca2+ handling machinery (notably mitochondria) alters the Ca2+ signal with age in a way that is dependent on the magnitude of the Ca2+ load. By combining whole-cell patch clamp electrophysiology, ratiometric Ca2+-sensitive microfluorimetry and single-cell reverse transcription-polymerase chain reaction, we have determined that age-related rapid buffering changes are present in identified cholinergic BF neurons and that these changes can be prevented by a caloric restriction dietary regimen. Because caloric restriction extends lifespan and retards the progression of age-related dysfunction, these findings suggest that increased Ca2+ buffering in cholinergic neurons may be relevant to cognitive decline during normal aging. Importantly, calcium homeostatic mechanisms of BF cholinergic neurons are amenable to dietary interventions that could promote cognitive health during aging.

Selective age-related loss of calbindin-D28k from basal forebrain cholinergic neurons in the common marmoset (Callithrix jacchus)

A significant number of the cholinergic neurons in the basal forebrain of the primate, but not the rodent brain contain the calcium binding protein calbindin-D28k (CB). Previous experiments in our laboratory have demonstrated a substantial age-related loss of CB from the human basal forebrain cholinergic neurons (BFCN). The present study investigated the possible age-related loss of CB from the BFCN in a non-human primate species, the common marmoset (Callithrix jacchus). Quantitative analysis of matching sections as well as unbiased stereological determination of neuronal number were used in 16 adult marmosets ranging in age between 2 and 15 years. No significant changes were observed in the number of choline acetyltransferase-positive BFCN when a group of young animals (< or =4 years) was compared with a 6-8-year-old group and a 9-15-year-old group. Similarly, no age-related changes were observed in Nissl-stained magnocellular basal forebrain (putatively cholinergic) neurons. In contrast, the BFCN of the two older groups of animals displayed a significant loss of CB. The age-related loss of CB occurred in all sectors of the BFCN, but was greatest in the anterior sector of this cell group. The CB loss was neurochemically specific since the BFCN in the older groups of animals continued to express other markers such as high and low affinity neurotrophin receptors. The age-related loss of CB from the marmoset BFCN was also regionally selective as CB positive neurons in other structures, such as the cerebral cortex and the striatum displayed no apparent age-related changes. These results indicate that the marmoset BFCN display a significant and selective age-related loss of CB reminiscent of that observed in the human. Therefore, the common marmoset represents an appropriate animal model in which the consequences of BFCN CB loss can be investigated in depth. Loss of CB from the aged BFCN is likely to reduce the capacity of these neurons to buffer intracellular calcium and to leave them vulnerable to insults which can result in increased calcium levels. The vulnerability of the CB-negative BFCN in the aged marmoset to various insults which disturb calcium homeostasis remains to be investigated.

Cholinergic system regulation of spatial representation by the hippocampus

The role of the basal forebrain cholinergic system in hippocampal spatial representation was explored by examining the effects of immunotoxic lesions of the septo-hippocampal cholinergic neurons on the firing patterns of hippocampal place cells as rats explored familiar and novel environments. In a highly familiar environment, the basic qualities and stability of place fields were unaffected by the lesion. When first exposed to a set of novel environmental cues without otherwise disorienting the animals, place cells in both normal and lesioned animals responded with similar alterations in their firing patterns. Upon subsequent repetitive exposures to the new environment, place cells of normal rats developed a spatial representation distinct from that of the familiar environment. By contrast, place cells of lesioned animals reconverged in the direction of the representation associated with the familiar environment. These results suggest that cholinergic input may determine whether new visual information or a stored representation of the current environment will be actively processed in the hippocampal network.

Molecular mechanisms of calcium and magnesium binding to parvalbumin

Molecular dynamics simulations have been used to investigate the relationship between the coordinating residues of the EF-hand calcium binding loop of parvalbumin and the overall plasticity and flexibility of the protein. The first simulation modeled the transition from Ca(2+) to Mg(2+) coordination by varying the van der Waals parameters for the bound metal ions. The glutamate at position 12 could be accurately and reversibly seen to be a source of selective bidentate ligation of Ca(2+) in the simulations. A second simulation correlated well with the experimental observation that an E101D substitution at EF loop position 12 results in a dramatically less tightly bound monodentate Ca(2+) coordination by aspartate. A final set of simulations investigated Ca(2+) binding in the E101D mutant loop in the presence of applied external forces designed to impose bidentate coordination. The results of these simulations illustrate that the aspartate is capable of attaining a suitable orientation for bidentate coordination, thus implying that it is the inherent rigidity of the loop that prevents bidentate coordination in the parvalbumin E101D mutant.

Modification of ion channels and calcium homeostasis of basal forebrain neurons during aging

In this paper we review the last several years of work from our lab with attention to changes in the properties of basal forebrain neurons during aging. These neurons play a central role in behavioral functions, such as: attention, arousal, cognition and autonomic activity, and these functions can be adversely affected during aging. Therefore, it is fundamental to define the cellular mechanisms of aging in order to understand the basal forebrain and to correct deficits associated with aging. We have examined changes in the physiological properties of basal forebrain neurons during aging with whole-cell and single-channel patch-clamp, as well as, microfluorimetric measurements of intracellular calcium concentrations. These studies contribute to the understanding of integration within the basal forebrain and to the identification of age-related changes within central mammalian neurons. Although extensive functional/behavioral decline is often assumed to occur during aging, our data support an interpretation of compensatory increases in function for excitatory amino acid receptors, GABA(A) receptors, voltage-gated calcium currents and calcium homeostatic mechanisms. We believe that these changes occur to compensate for decrements accruing with age, such as decreased synaptic contacts, ion imbalances or neuronal loss. The basal forebrain must retain functionality into late aging if senescence is to be productive. Thus, it is critical to recognize the potential cellular and subcellular targets for therapeutic interventions intended to correct age-related behavioral deficits.

Metal-ion affinity and specificity in EF-hand proteins: coordination geometry and domain plasticity in parvalbumin

BACKGROUND

The EF-hand family is a large set of Ca(2+)-binding proteins that contain characteristic helix-loop-helix binding motifs that are highly conserved in sequence. Members of this family include parvalbumin and many prominent regulatory proteins such as calmodulin and troponin C. EF-hand proteins are involved in a variety of physiological processes including cell-cycle regulation, second messenger production, muscle contraction, microtubule organization and vision.

RESULTS

We have determined the structures of parvalbumin mutants designed to explore the role of the last coordinating residue of the Ca(2+)-binding loop. An E101D substitution has been made in the parvalbumin EF site. The substitution decreases the Ca(2+)-binding affinity 100-fold and increases the Mg(2+)-binding affinity 10-fold. Both the Ca(2+)- and Mg(2+)-bound structures have been determined, and a structural basis has been proposed for the metal-ion-binding properties.

CONCLUSIONS

The E101D mutation does not affect the Mg(2+) coordination geometry of the binding loop, but it does pull the F helix 1.1 A towards the loop. The E101D-Ca(2+) structure reveals that this mutant cannot obtain the sevenfold coordination preferred by Ca(2+), presumably because of strain limits imposed by tertiary structure. Analysis of these results relative to previously reported structural information supports a model wherein the characteristics of the last coordinating residue and the plasticity of the Ca(2+)-binding loop delimit the allowable geometries for the coordinating sphere.

Contribution of cholinergic and gabaergic functions to memory processes in BALB/cANnCrlBR mice

Several lines of evidence indicate that glucose influences on memory depend on interactions between glucose, glucoregulation and hippocampal cholinergic function. We previously demonstrated that glucose and scopolamine differentially affected memory consolidation for an operant bar pressing task in two closely-related BALB/c mouse strains. Whereas glucose normally improves memory in several animal strains, memory consolidation was not effected by systemic glucose injections in BALB/cANnCrlBR mice. Moreover, these mice were relatively insensitive to the normally observed amnestic effects of scopolamine. We therefore sought to determine whether cholinergic mechanisms in the dorsal hippocampus were involved in such atypical drug effects on memory processing in that strain of mice. In Experiment 1, we examined whether post-training oxotremorine would also atypically influence memory consolidation for an appetitively reinforced operant bar pressing task following microinjection in the dorsal hippocampus. In Experiment 2, we examined the effects of intrahippocampal GABAA drugs on memory consolidation. The non-selective muscarinic agonist, oxotremorine, dose-dependently impaired memory and the GABAA antagonist, bicuculline, improved retention in BALB/cANnCrlBR mice. It was concluded that GABA-mediated influences on hippocampal pyramidal output in BALB/cANnCrlBR mice and other strains are similar; but the amnestic effects of oxotremorine from the dorsal hippocampus were opposite to facilitating effects normally observed in other animal strains. Results are discussed relative to possible altered septo-hippocampal cholinergic neurotransmission in BALB/cANnCrlBR mice.

Increased calcium buffering in basal forebrain neurons during aging

Increased calcium buffering in basal forebrain neurons during aging. J. Neurophysiol. 80: 350-364, 1998. Alterations of neuronal calcium (Ca2+) homeostasis are thought to underlie many age-related changes in the nervous system. Basal forebrain neurons are susceptible to changes associated with aging and to related dysfunctions such as Alzheimer's disease. It recently was shown that neurons from the medial septum and nucleus of the diagonal band (MS/nDB) of aged (24-27 mo) F344 rats have an increased current influx through voltage-gated Ca2+ channels (VGCCs) relative to those of young (1-4. 5 mo) rats. Possible age-related changes in Ca2+ buffering in these neurons have been investigated using conventional whole cell and perforated-patch voltage clamp combined with fura-2 microfluorimetric techniques. Basal intracellular Ca2+ concentrations ([Ca2+]i), Ca2+ influx, Ca2+ transients (Delta[Ca2+]i), and time course of Delta[Ca2+]i were quantitated, and rapid Ca2+ buffering values were calculated in MS/nDB neurons from young and aged rats. The involvement of the smooth endoplasmic reticulum (SER) was examined with the SER Ca2+ uptake blocker, thapsigargin. An age-related increase in rapid Ca2+ buffering and Delta[Ca2+]i time course was observed, although basal [Ca2+]i was unchanged with age. The SER and endogenous diffusible buffering mechanisms were found to have roles in Ca2+ buffering, but they did not mediate the age-related changes. These findings suggest a model in which some aging central neurons could compensate for increased Ca2+ influx with greater Ca2+ buffering.

Effects of chronic nimodipine on working memory of old rats in relation to defects in synaptosomal calcium homeostasis

The present study was designed to investigate whether chronic (from 12 to 23 months of age) dietary treatment with the L-type Ca2+ channel blocker nimodipine (30 mg/kg body weight) enhances the cognitive behavior of aged animals and whether such a treatment would have long-term effects on the mechanisms of Ca2+ regulation in synaptic terminals from the aged rat brain. Cognitive behavior was evaluated in an 8-arm radial maze in 6 test series comprising a total of 105 test sessions, with intervals of no training between series. Nimodipine-treated rats performed better than vehicle-treated, aged-matched controls in all the test series, making more correct choices every time a new series was initiated. However, differences between nimodipine- and vehicle-treated rats were most remarkable in the last three test series, when the rats were 19 to 22 months. In these series 74% of the nimodipine-treated rats were able to perform the task in 4 to 9 test sessions whereas only 12%, 14% or none of the control rats learned the task. To study Ca2+ regulation in synaptosomes derived from cerebral cortex and hippocampus, we analyzed 45Ca2+ accumulation as well as the levels of the Ca2+-binding proteins calbindin-D28K and calreticulin by Western blotting. Nimodipine administration had no effect on hippocampal synaptosomes but increased the levels of calbindin-D28K and calreticulin in cerebral cortex preparations. These results indicate that chronic nimodipine treatment from 12 to 23 months of age prevents age-induced learning deficits without showing any signs of toxicity, and that these effects are associated with a small increase in the levels of synaptosomal Ca2+-binding proteins from cerebral cortex. The up-regulation of these proteins might provide a link between the long-term effects of nimodipine on gene expression and learning ability in old rats.

Calcium and neuronal ageing

Brain ageing is associated with a marked decline in mental faculties. One hypothesis postulates that sustained changes in the regulation of intracellular Ca2+ concentration, [Ca2+]i, are the major cause of neuronal degeneration. This 'calcium hypothesis' is supported by demonstrations of the impairment in aged neurones of molecular cascades that regulate [Ca2+]i. However, the number of direct measurements of [Ca2+]i in senescent neurones is limited, and the hypothesis cannot be regarded as fully confirmed. Furthermore, physiological brain ageing, at least in certain regions, need not necessarily be a degenerative process accompanied by neuronal loss. Pharmacological manipulation of Ca2+ entry has been shown to be effective in increasing some aspects of cognitive function of the aged brain. Therefore, further exploration of Ca2+ homeostasis and signalling might reveal the mechanisms involved in the age-dependent decline in neuronal performance, and might aid the search for new therapeutic treatments.