Hippocampal 'zipper' slice studies reveal a necessary role for calcineurin in the increased activity of L-type Ca(2+) channels with aging

Development of a monoclonal antibody specific for a calpain-generated ∆48 kDa calcineurin fragment, a marker of distressed astrocytes

BACKGROUND

Calcineurin (CN) is a Ca2+/calmodulin-dependent protein phosphatase. In healthy tissue, CN exists mainly as a full-length (∼60 kDa) highly-regulated protein phosphatase involved in essential cellular functions. However, in diseased or injured tissue, CN is proteolytically converted to a constitutively active fragment that has been causatively-linked to numerous pathophysiologic processes. These calpain-cleaved CN fragments (∆CN) appear at high levels in human brain at early stages of cognitive decline associated with Alzheimer's disease (AD).

NEW METHOD

We developed a monoclonal antibody to ∆CN, using an immunizing peptide corresponding to the C-terminal end of the ∆CN fragment.

RESULTS

We obtained a mouse monoclonal antibody, designated 26A6, that selectively detects ∆CN in Western analysis of calpain-cleaved recombinant human CN. Using this antibody, we screened both pathological and normal human brain sections provided by the University of Kentucky's Alzheimer's Disease Research Center. 26A6 showed low reactivity towards normal brain tissue, but detected astrocytes both surrounding AD amyloid plaques and throughout AD brain tissue. In brain tissue with infarcts, there was considerable concentration of 26A6-positive astrocytes within/around infarcts, suggesting a link with anoxic/ischemia pathways.

COMPARISON WITH EXISTING METHOD

The results obtained with the new monoclonal are similar to those obtained with a polyclonal we had previously developed. However, the monoclonal is an abundant tool available to the dementia research community.

CONCLUSIONS

The new monoclonal 26A6 antibody is highly selective for the ∆CN proteolytic fragment and labels a subset of astrocytes, and could be a useful tool for marking insidious brain pathology and identifying novel astrocyte phenotypes.

Effect of ladostigil treatment of aging rats on gene expression in four brain areas associated with regulation of memory

Episodic and spatial memory decline in aging and are controlled by the hippocampus, perirhinal, frontal and parietal cortices and the connections between them. Ladostigil, a drug with antioxidant and anti-inflammatory activity, was shown to prevent the loss of episodic and spatial memory in aging rats. To better understand the molecular effects of aging and ladostigil on these brain regions we characterized the changes in gene expression using RNA-sequencing technology in rats aged 6 and 22 months. We found that the changes induced by aging and chronic ladostigil treatment were brain region specific. In the hippocampus, frontal and perirhinal cortex, ladostigil decreased the overexpression of genes regulating calcium homeostasis, ion channels and those adversely affecting synaptic function. In the parietal cortex, ladostigil increased the expression of several genes that provide neurotrophic support, while reducing that of pro-apoptotic genes and those encoding pro-inflammatory cytokines and their receptors. Ladostigil also decreased the expression of axonal growth inhibitors and those impairing mitochondrial function. Together, these actions could explain the protection by ladostigil against age-related memory decline.

Presynaptic L-Type Ca2+ Channels Increase Glutamate Release Probability and Excitatory Strength in the Hippocampus during Chronic Neuroinflammation

Neuroinflammation is involved in the pathogenesis of several neurologic disorders, including epilepsy. Both changes in the input/output functions of synaptic circuits and cell Ca²⁺ dysregulation participate in neuroinflammation, but their impact on neuron function in epilepsy is still poorly understood. Lipopolysaccharide (LPS), a toxic byproduct of bacterial lysis, has been extensively used to stimulate inflammatory responses both in vivo and in vitro LPS stimulates Toll-like receptor 4, an important mediator of the brain innate immune response that contributes to neuroinflammation processes. Although we report that Toll-like receptor 4 is expressed in both excitatory and inhibitory mouse hippocampal neurons (both sexes), its chronic stimulation by LPS induces a selective increase in the excitatory synaptic strength, characterized by enhanced synchronous and asynchronous glutamate release mechanisms. This effect is accompanied by a change in short-term plasticity with decreased facilitation, decreased post-tetanic potentiation, and increased depression. Quantal analysis demonstrated that the effects of LPS on excitatory transmission are attributable to an increase in the probability of release associated with an overall increased expression of L-type voltage-gated Ca²⁺ channels that, at presynaptic terminals, abnormally contributes to evoked glutamate release. Overall, these changes contribute to the excitatory/inhibitory imbalance that scales up neuronal network activity under inflammatory conditions. These results provide new molecular clues for treating hyperexcitability of hippocampal circuits associated with neuroinflammation in epilepsy and other neurologic disorders.SIGNIFICANCE STATEMENT Neuroinflammation is thought to have a pathogenetic role in epilepsy, a disorder characterized by an imbalance between excitation/inhibition. Fine adjustment of network excitability and regulation of synaptic strength are both implicated in the homeostatic maintenance of physiological levels of neuronal activity. Here, we focused on the effects of chronic neuroinflammation induced by lipopolysaccharides on hippocampal glutamatergic and GABAergic synaptic transmission. Our results show that, on chronic stimulation with lipopolysaccharides, glutamatergic, but not GABAergic, neurons exhibit an enhanced synaptic strength and changes in short-term plasticity because of an increased glutamate release that results from an anomalous contribution of L-type Ca²⁺ channels to neurotransmitter release.

Interactions with PDZ proteins diversify voltage-gated calcium channel signaling

Voltage-gated Ca²⁺ (Ca_V ) channels are crucial for neuronal excitability and synaptic transmission upon depolarization. Their properties in vivo are modulated by their interaction with a variety of scaffolding proteins. Such interactions can influence the function and localization of Ca_V channels, as well as their coupling to intracellular second messengers and regulatory pathways, thus amplifying their signaling potential. Among these scaffolding proteins, a subset of PDZ (postsynaptic density-95, Drosophila discs-large, and zona occludens)-domain containing proteins play diverse roles in modulating Ca_V channel properties. At the presynaptic terminal, PDZ proteins enrich Ca_V channels in the active zone, enabling neurotransmitter release by maintaining a tight and vital link between channels and vesicles. In the postsynaptic density, these interactions are essential in regulating dendritic spine morphology and postsynaptic signaling cascades. In this review, we highlight the studies that demonstrate dynamic regulations of neuronal Ca_V channels by PDZ proteins. We discuss the role of PDZ proteins in controlling channel activity, regulating channel cell surface density, and influencing channel-mediated downstream signaling events. We highlight the importance of PDZ protein regulations of Ca_V channels and evaluate the link between this regulatory effect and human disease.

Altered function of neuronal L-type calcium channels in ageing and neuroinflammation: Implications in age-related synaptic dysfunction and cognitive decline

The rapid developments in science have led to an increase in human life expectancy and thus, ageing and age-related disorders/diseases have become one of the greatest concerns in the 21st century. Cognitive abilities tend to decline as we get older. This age-related cognitive decline is mainly attributed to aberrant changes in synaptic plasticity and neuronal connections. Recent studies show that alterations in Ca²⁺ homeostasis underlie the increased vulnerability of neurons to age-related processes like cognitive decline and synaptic dysfunctions. Dysregulation of Ca²⁺ can lead to dramatic changes in neuronal functions. We discuss in this review, the recent advances on the potential role of dysregulated Ca²⁺ homeostasis through altered function of L-type voltage gated Ca²⁺ channels (LTCC) in ageing, with an emphasis on cognitive decline. This review therefore focuses on age-related changes mainly in the hippocampus, and with mention of other brain areas, that are important for learning and memory. This review also highlights age-related memory deficits via synaptic alterations and neuroinflammation. An understanding of these mechanisms will help us formulate strategies to reverse or ameliorate age-related disorders like cognitive decline.

A Novel Small Molecule Modulator of Amyloid Pathology

Because traditional approaches to drug development for Alzheimer's disease are becoming increasingly expensive and in many cases disappointingly unsuccessful, alternative approaches are required to shift the paradigm. Following leads from investigations of dihydropyridine calcium channel blockers, we observed unique properties from a class of functionalized naphthyridines and sought to develop these as novel therapeutics that minimize amyloid pathology without the adverse effects associated with current therapeutics. Our data show methyl 2,4-dimethyl-5-oxo-5,6-dihydrobenzo[c][2,7]naphthyridine-1-carboxylate (BNC-1) significantly decreases amyloid burden in a well-established mouse model of amyloid pathology through a unique mechanism mediated by Elk-1, a transcriptional repressor of presenilin-1. Additionally, BNC-1 treatment leads to increased levels of synaptophysin and synapsin, markers of synaptic integrity, but does not adversely impact presenilin-2 or processing of Notch-1, thus avoiding negative off target effects associated with pan-gamma secretase inhibition. Overall, our data show BNC-1 significantly decreases amyloid burden and improves markers of synaptic integrity in a well-established mouse model of amyloid deposition by promoting phosphorylation and activation of Elk-1, a transcriptional repressor of presenilin-1 but not presenilin-2. These data suggest BNC-1 might be a novel, disease-modifying therapeutic that will alter the pathogenesis of Alzheimer's disease.

Danger: High Voltage-The Role of Voltage-Gated Calcium Channels in Central Nervous System Pathology

Voltage-gated calcium channels (VGCCs) are widely distributed within the central nervous system (CNS) and presumed to play an important role in the pathophysiology of a broad spectrum of CNS disorders including Alzheimer's and Parkinson's disease as well as multiple sclerosis. Several calcium channel blockers have been in clinical practice for many years so that their toxicity and side effects are well studied. However, these drugs are primarily used for the treatment of cardiovascular diseases and most if not all effects on brain functions are secondary to peripheral effects on blood pressure and circulation. While the use of calcium channel antagonists for the treatment of CNS diseases therefore still heavily depends on the development of novel strategies to specifically target different channels and channel subunits, this review is meant to provide an impulse to further emphasize the importance of future research towards this goal.

Calcineurin proteolysis in astrocytes: Implications for impaired synaptic function

Mounting evidence suggests that astrocyte activation, found in most forms of neural injury and disease, is linked to the hyperactivation of the protein phosphatase calcineurin. In many tissues and cell types, calcineurin hyperactivity is the direct result of limited proteolysis. However, little is known about the proteolytic status of calcineurin in activated astrocytes. Here, we developed a polyclonal antibody to a high activity calcineurin proteolytic fragment in the 45-48kDa range (ΔCN) for use in immunohistochemical applications. When applied to postmortem human brain sections, the ΔCN antibody intensely labeled cell clusters in close juxtaposition to amyloid deposits and microinfarcts. Many of these cells exhibited clear activated astrocyte morphology. The expression of ΔCN in astrocytes near areas of pathology was further confirmed using confocal microscopy. Multiple NeuN-positive cells, particularly those within microinfarct core regions, also labeled positively for ΔCN. This observation suggests that calcineurin proteolysis can also occur within damaged or dying neurons, as reported in other studies. When a similar ΔCN fragment was selectively expressed in hippocampal astrocytes of intact rats (using adeno-associated virus), we observed a significant reduction in the strength of CA3-CA1 excitatory synapses, indicating that the hyperactivation of astrocytic calcineurin is sufficient for disrupting synaptic function. Together, these results suggest that proteolytic activation of calcineurin in activated astrocytes may be a central mechanism for driving and/or exacerbating neural dysfunction during neurodegenerative disease and injury.

Pycnogenol protects CA3-CA1 synaptic function in a rat model of traumatic brain injury

Pycnogenol (PYC) is a patented mix of bioflavonoids with potent anti-oxidant and anti-inflammatory properties. Previously, we showed that PYC administration to rats within hours after a controlled cortical impact (CCI) injury significantly protects against the loss of several synaptic proteins in the hippocampus. Here, we investigated the effects of PYC on CA3-CA1 synaptic function following CCI. Adult Sprague-Dawley rats received an ipsilateral CCI injury followed 15 min later by intravenous injection of saline vehicle or PYC (10 mg/kg). Hippocampal slices from the injured (ipsilateral) and uninjured (contralateral) hemispheres were prepared at seven and fourteen days post-CCI for electrophysiological analyses of CA3-CA1 synaptic function and induction of long-term depression (LTD). Basal synaptic strength was impaired in slices from the ipsilateral, relative to the contralateral, hemisphere at seven days post-CCI and susceptibility to LTD was enhanced in the ipsilateral hemisphere at both post-injury timepoints. No interhemispheric differences in basal synaptic strength or LTD induction were observed in rats treated with PYC. The results show that PYC preserves synaptic function after CCI and provides further rationale for investigating the use of PYC as a therapeutic in humans suffering from neurotrauma.

Calcineurin and glial signaling: neuroinflammation and beyond

Similar to peripheral immune/inflammatory cells, neuroglial cells appear to rely on calcineurin (CN) signaling pathways to regulate cytokine production and cellular activation. Several studies suggest that harmful immune/inflammatory responses may be the most impactful consequence of aberrant CN activity in glial cells. However, newly identified roles for CN in glutamate uptake, gap junction regulation, Ca2+ dyshomeostasis, and amyloid production suggest that CN's influence in glia may extend well beyond neuroinflammation. The following review will discuss the various actions of CN in glial cells, with particular emphasis on astrocytes, and consider the implications for neurologic dysfunction arising with aging, injury, and/or neurodegenerative disease.

Early exposure to sevoflurane inhibits Ca(2+) channels activity in hippocampal CA1 pyramidal neurons of developing rats

Sevoflurane is one of inhalation anesthetics and has been commonly used in obstetric and pediatric anesthesia. The widespread use of sevoflurane in newborns and infants has made its safety a health issue of concern. Voltage-gated Ca(2+) channels (VGCCs) play an important role in neuronal excitability and are essential for normal brain development. However, the role of sevoflurane on regulating Ca(2+) channels during the period of rapid brain development is still not well understood. The aim of this study is to explore the effects of sevoflurane on voltage-gated Ca(2+) channels for hippocampal CA1 pyramidal neurons during the period of rapid brain development. 1-week-old Sprague-Dawley rats were randomly divided into 3 groups: control group, 2.1% sevoflurane group (exposed to 2.1% sevoflurane for 6h) and 3% sevoflurane group (exposed to 3% sevoflurane for 6h). Whole-cell patch clamp technique was used. I-V curve, steady-state activation and inactivation curves of Ca(2+) channels were studied in rats of the both 3 treated groups at 5 different ages (1 week, 2 weeks, 3 weeks, 4 and 5 weeks old). After anesthesia with sevoflurane at 1-week-old rats, Ca(2+) channels current density was significantly decreased at week 1 and week 2 (p<0.01). And 3% sevoflurane exposure resulted in a rightward shift in steady-state activation curve at week 1 and week 2, as well as the inactivation curve from week 1 to week 3. However, the 2.1% sevoflurane-induced rightward shift was only found in steady-state inactivation curve of Ca(2+) channels at week 1 and week 2. Both the slope factor (k) of Ca(2+) channels activation and inactivation curves increased by 3% sevoflurane at week 1 (p<0.05). Therefore, early exposure to sevoflurane persistently inhibits Ca(2+) channels activity in hippocampal CA1 pyramidal neurons of developing rats but the development of Ca(2+) channels recovers to normal level at juvenile age. Moreover, the inhibition of 3% sevoflurane on VGCCs is greater than that of 2.1% sevoflurane.

Methods for the discovery of new anti-aging products--targeted approaches

INTRODUCTION

Aging is considered to be one of the most complicated and heterogeneous phenomena and is the main risk factor for most chronic diseases, disabilities and declining health. Aging cells cease to divide and drive the progression of illness through various pathways. Over the years, a number of anti-aging medicines of natural and synthetic origin have been introduced. Indeed, some studies have identified senescent cells as potential therapeutic targets in the treatment of aging and age-related diseases.

AREAS COVERED

In this review, the authors highlight and critically review the possible mechanisms of the aging process and related illnesses. The authors give particular attention to illnesses, including Alzheimer's disease, Parkinson's disease, skin aging and cardiovascular diseases.

EXPERT OPINION

Several reports have highlighted that mitochondria are a key factor in the progression of aging and neurodegenerative illnesses. This is due to their production of extra amounts of reactive oxygen species, which leads into progressive caspase-dependent apoptosis and cell death. Therefore, strategies to prevent/reduce oxidative stress-mediated aging, whether environmental, nutritional and pharmacological, need to be taken into account. Presently, Drosophila melanogaster and Caenorhabditis elegans, which focus on the evolutionary and genetic foundations of aging, have helped to establish the screening of several synthetic and natural compounds with large cohorts in a quick manner. However, there is yet to be any efficient experimental evidence to prove the exact role of senescent cells in age-related dysfunction and further studies are required to better understand these processes.

Surface L-type Ca2+ channel expression levels are increased in aged hippocampus

Age-related increase in L-type Ca²⁺ channel (LTCC) expression in hippocampal pyramidal neurons has been hypothesized to underlie the increased Ca²⁺ influx and subsequent reduced intrinsic neuronal excitability of these neurons that lead to age-related cognitive deficits. Here, using specific antibodies against Ca_v1.2 and Ca_v1.3 subunits of LTCCs, we systematically re-examined the expression of these proteins in the hippocampus from young (3 to 4 month old) and aged (30 to 32 month old) F344xBN rats. Western blot analysis of the total expression levels revealed significant reductions in both Ca_v1.2 and Ca_v1.3 subunits from all three major hippocampal regions of aged rats. Despite the decreases in total expression levels, surface biotinylation experiments revealed significantly higher proportion of expression on the plasma membrane of Ca_v1.2 in the CA1 and CA3 regions and of Ca_v1.3 in the CA3 region from aged rats. Furthermore, the surface biotinylation results were supported by immunohistochemical analysis that revealed significant increases in Ca_v1.2 immunoreactivity in the CA1 and CA3 regions of aged hippocampal pyramidal neurons. In addition, we found a significant increase in the level of phosphorylated Ca_v1.2 on the plasma membrane in the dentate gyrus of aged rats. Taken together, our present findings strongly suggest that age-related cognitive deficits cannot be attributed to a global change in L-type channel expression nor to the level of phosphorylation of Ca_v1.2 on the plasma membrane of hippocampal neurons. Rather, increased expression and density of LTCCs on the plasma membrane may underlie the age-related increase in L-type Ca²⁺ channel activity in CA1 pyramidal neurons.

FK506-binding protein 1b/12.6: a key to aging-related hippocampal Ca2+ dysregulation?

It has been recognized for some time that the Ca(2+)-dependent slow afterhyperpolarization (sAHP) is larger in hippocampal neurons of aged compared with young animals. In addition, extensive studies since have shown that other Ca(2+)-mediated electrophysiological responses are increased in hippocampus with aging, including Ca(2+) transients, L-type voltage-gated Ca(2+) channel activity, Ca(2+) spike duration and action potential accommodation. Elevated Ca(2+)-induced Ca(2+) release from ryanodine receptors (RyRs) appears to drive amplification of the Ca(2+) responses. Components of this Ca(2+) dysregulation phenotype correlate with deficits in cognitive function and plasticity, indicating they may play critical roles in aging-related impairment of brain function. However, the molecular mechanisms underlying aging-related Ca(2+) dysregulation are not well understood. FK506-binding proteins 1a and 1b (FKBP1a/1b, also known as FKBP12/12.6) are immunophilin proteins that bind the immunosuppressant drugs FK506 and rapamycin. In muscle cells, FKBP1a/1b also bind RyRs and inhibits Ca(2+)-induced Ca(2+) release, but it is not clear whether FKBPs act similarly in brain cells. Recently, we found that selectively disrupting hippocampal FKBP1b function in young rats, either by microinjecting adeno-associated viral vectors expressing siRNA, or by treatment with rapamycin, increases the sAHP and recapitulates much of the hippocampal Ca(2+) dysregulation phenotype. Moreover, in microarray studies, we found FKBP1b gene expression was downregulated in hippocampus of aging rats and early-stage Alzheimer's disease subjects. These results suggest the novel hypothesis that declining FKBP function is a key factor in aging-related Ca(2+) dysregulation in the brain and point to potential new therapeutic targets for counteracting unhealthy brain aging.

Calcium dysregulation and neuroinflammation: discrete and integrated mechanisms for age-related synaptic dysfunction

Some of the best biomarkers of age-related cognitive decline are closely linked to synaptic function and plasticity. This review highlights several age-related synaptic alterations as they relate to Ca(2+) dyshomeostasis, through elevation of intracellular Ca(2+), and neuroinflammation, through production of pro-inflammatory cytokines including interleukin-1 beta (IL-1β) and tumor necrosis factor-alpha (TNF-α). Though distinct in many ways, Ca(2+) and neuroinflammatory signaling mechanisms exhibit extensive cross-talk and bidirectional interactions. For instance, cytokine production in glial cells is strongly dependent on the Ca(2+) dependent protein phosphatase calcineurin, which shows elevated activity in animal models of aging and disease. In turn, pro-inflammatory cytokines, such as TNF, can augment the expression/activity of L-type voltage sensitive Ca(2+) channels in neurons, leading to Ca(2+) dysregulation, hyperactive calcineurin activity, and synaptic depression. Thus, in addition to discussing unique contributions of Ca(2+) dyshomeostasis and neuroinflammation, this review emphasizes how these processes interact to hasten age-related synaptic changes.

Targeting astrocytes ameliorates neurologic changes in a mouse model of Alzheimer's disease

Astrocytes are the most abundant cell type in the brain and play a critical role in maintaining healthy nervous tissue. In Alzheimer's disease (AD) and most other neurodegenerative disorders, many astrocytes convert to a chronically "activated" phenotype characterized by morphologic and biochemical changes that appear to compromise protective properties and/or promote harmful neuroinflammatory processes. Activated astrocytes emerge early in the course of AD and become increasingly prominent as clinical and pathological symptoms progress, but few studies have tested the potential of astrocyte-targeted therapeutics in an intact animal model of AD. Here, we used adeno-associated virus (AAV) vectors containing the astrocyte-specific Gfa2 promoter to target hippocampal astrocytes in APP/PS1 mice. AAV-Gfa2 vectors drove the expression of VIVIT, a peptide that interferes with the immune/inflammatory calcineurin/NFAT (nuclear factor of activated T-cells) signaling pathway, shown by our laboratory and others to orchestrate biochemical cascades leading to astrocyte activation. After several months of treatment with Gfa2-VIVIT, APP/PS1 mice exhibited improved cognitive and synaptic function, reduced glial activation, and lower amyloid levels. The results confirm a deleterious role for activated astrocytes in AD and lay the groundwork for exploration of other novel astrocyte-based therapies.

A role for calcineurin in Alzheimer's disease

Alzheimer's disease (AD) is an incurable age-related neurodegenerative disorder characterized by profound memory dysfunction. This bellwether symptom suggests involvement of the hippocampus -- a brain region responsible for memory formation -- and coincidentally an area heavily burdened by hyperphosphorylated tau and neuritic plaques of amyloid beta (Aβ). Recent evidence suggests that pre-fibrillar soluble Aβ underlies an early, progressive loss of synapses that is a hallmark of AD. One of the downstream effects of soluble Aβ aggregates is the activation of the phosphatase calcineurin (CaN). This review details the evidence of CaN hyperactivity in 'normal' aging, models of AD, and actual disease pathogenesis; elaborates on how this could manifest as memory impairment, neuroinflammation, hyperphosphorylated tau, and neuronal death.

Inhibition of soluble tumor necrosis factor ameliorates synaptic alterations and Ca2+ dysregulation in aged rats

The role of tumor necrosis factor α (TNF) in neural function has been investigated extensively in several neurodegenerative conditions, but rarely in brain aging, where cognitive and physiologic changes are milder and more variable. Here, we show that protein levels for TNF receptor 1 (TNFR1) are significantly elevated in the hippocampus relative to TNF receptor 2 (TNFR2) in aged (22 months) but not young adult (6 months) Fischer 344 rats. To determine if altered TNF/TNFR1 interactions contribute to key brain aging biomarkers, aged rats received chronic (4–6 week) intracranial infusions of XPro1595: a soluble dominant negative TNF that preferentially inhibits TNFR1 signaling. Aged rats treated with XPro1595 showed improved Morris Water Maze performance, reduced microglial activation, reduced susceptibility to hippocampal long-term depression, increased protein levels for the GluR1 type glutamate receptor, and lower L-type voltage sensitive Ca²⁺ channel (VSCC) activity in hippocampal CA1 neurons. The results suggest that diverse functional changes associated with brain aging may arise, in part, from selective alterations in TNF signaling.

Hypoxia enhances high-voltage-activated calcium currents in rat primary cortical neurons via calcineurin

Hypoxia regulates neuronal ion channels, sometimes resulting in seizures. We evaluated the effects of brief sustained hypoxia (1% O(2), 4h) on voltage-gated calcium channels (VGCCs) in cultured rat primary cortical neurons. High-voltage activated (HVA) Ca(2+) currents were acquired immediately after hypoxic exposure or after 48h recovery in 95% air/5% CO(2). Maximal Ca(2+) current density increased 1.5-fold immediately after hypoxia, but reverted to baseline after 48h normoxia. This enhancement was primarily due to an increase in L-type VGCC activity, since nimodipine-insensitive residual Ca(2+) currents were unchanged. The half-maximal potentials of activation and steady-state inactivation were unchanged. The calcineurin inhibitors FK-506 (in the recording pipette) or cyclosporine A (during hypoxia) prevented the post-hypoxic increase in HVA Ca(2+) currents, while rapamycin and okadaic acid did not. L-type VGCCs were the source of Ca(2+) for calcineurin activation, as nimodipine during hypoxia prevented post-hypoxic enhancement. Hypoxia transiently potentiated L-type VGCC currents via calcineurin, suggesting a positive feedback loop to amplify neuronal calcium signaling that may contribute to seizure generation.

Reduction in neuronal L-type calcium channel activity in a double knock-in mouse model of Alzheimer's disease

Increased function of neuronal L-type voltage-sensitive Ca(2+) channels (L-VSCCs) is strongly linked to impaired memory and altered hippocampal synaptic plasticity in aged rats. However, no studies have directly assessed L-VSCC function in any of the common mouse models of Alzheimer's disease where neurologic deficits are typically more robust. Here, we used cell-attached patch-clamp recording techniques to measure L-VSCC activity in CA1 pyramidal neurons of partially dissociated hippocampal "zipper" slices prepared from 14-month-old wild-type mice and memory-impaired APP/PS1 double knock-in mice. Surprisingly, the functional channel density of L-VSCCs was significantly reduced in the APP/PS1 group. No differences in voltage dependency and unitary conductance of L-VSCCs were observed. The results suggest that mechanisms for Ca(2+) dysregulation can differ substantially between animal models of normal aging and models of pathological aging.

Calcium binding protein-mediated regulation of voltage-gated calcium channels linked to human diseases

Calcium ion entry through voltage-gated calcium channels is essential for cellular signalling in a wide variety of cells and multiple physiological processes. Perturbations of voltage-gated calcium channel function can lead to pathophysiological consequences. Calcium binding proteins serve as calcium sensors and regulate the calcium channel properties via feedback mechanisms. This review highlights the current evidences of calcium binding protein-mediated channel regulation in human diseases.

Preparation of acute hippocampal slices from rats and transgenic mice for the study of synaptic alterations during aging and amyloid pathology

The rodent hippocampal slice preparation is perhaps the most broadly used tool for investigating mammalian synaptic function and plasticity. The hippocampus can be extracted quickly and easily from rats and mice and slices remain viable for hours in oxygenated artificial cerebrospinal fluid. Moreover, basic electrophysisologic techniques are easily applied to the investigation of synaptic function in hippocampal slices and have provided some of the best biomarkers for cognitive impairments. The hippocampal slice is especially popular for the study of synaptic plasticity mechanisms involved in learning and memory. Changes in the induction of long-term potentiation and depression (LTP and LTD) of synaptic efficacy in hippocampal slices (or lack thereof) are frequently used to describe the neurologic phenotype of cognitively-impaired animals and/or to evaluate the mechanism of action of nootropic compounds. This article outlines the procedures we use for preparing hippocampal slices from rats and transgenic mice for the study of synaptic alterations associated with brain aging and Alzheimer's disease (AD)(1-3). Use of aged rats and AD model mice can present a unique set of challenges to researchers accustomed to using younger rats and/or mice in their research. Aged rats have thicker skulls and tougher connective tissue than younger rats and mice, which can delay brain extraction and/or dissection and consequently negate or exaggerate real age-differences in synaptic function and plasticity. Aging and amyloid pathology may also exacerbate hippocampal damage sustained during the dissection procedure, again complicating any inferences drawn from physiologic assessment. Here, we discuss the steps taken during the dissection procedure to minimize these problems. Examples of synaptic responses acquired in "healthy" and "unhealthy" slices from rats and mice are provided, as well as representative synaptic plasticity experiments. The possible impact of other methodological factors on synaptic function in these animal models (e.g. recording solution components, stimulation parameters) are also discussed. While the focus of this article is on the use of aged rats and transgenic mice, novices to slice physiology should find enough detail here to get started on their own studies, using a variety of rodent models.

Proteolysis of calcineurin is increased in human hippocampus during mild cognitive impairment and is stimulated by oligomeric Abeta in primary cell culture

Recent reports demonstrate that the activation and interaction of the protease calpain (CP) and the protein phosphatase calcineurin (CN) are elevated in the late stages of Alzheimer's disease (AD). However, the extent to which CPs and CN interact during earlier stages of disease progression remains unknown. Here, we investigated CP and CN protein levels in cytosolic, nuclear, and membrane fractions prepared from human postmortem hippocampal tissue from aged non-demented subjects, and subjects diagnosed with mild cognitive impairment (MCI). The results revealed a parallel increase in CP I and the 48 kDa CN-Aα (ΔCN-Aα48) proteolytic fragment in cytosolic fractions during MCI. In primary rat hippocampal cultures, CP-dependent proteolysis and activation of CN was stimulated by application of oligomeric Aβ((1-42)) peptides. Deleterious effects of Aβ on neuronal morphology were reduced by blockade of either CP or CN. NMDA-type glutamate receptors, which help regulate cognition and neuronal viability, and are modulated by CPs and CN, were also investigated in human hippocampus. Relative to controls, MCI subjects showed significantly greater proteolytic levels of the NR2B subunit. Within subjects, the extent of NR2B proteolysis was strongly correlated with the generation of ΔCN-Aα48 in the cytosol. A similar proteolytic pattern for NR2B was also observed in primary rat hippocampal cultures treated with oligomeric Aβ and prevented by inhibition of CP or CN. Together, the results demonstrate that the activation and interaction of CPs and CN are increased early in cognitive decline associated with AD and may help drive other pathologic processes during disease progression.

Neuroimmunomodulation by calcineurin in aging and Alzheimer's disease

Alzheimer's disease (AD) is a progressive, age-related neurodegenerative disorder which first manifests as profound memory dysfunction. The majority of cases are idiopathic, although advanced age is the greatest risk factor for AD. Recent evidence suggests that pre-fibrillar soluble amyloid-beta (Aβ) underlies an early, progressive loss of synapses that is a hallmark of AD. One of the downstream effects mediated by soluble Aβ aggregates is the hyperactivation of the phosphatase calcineurin (CaN). This important phosphatase is abundant in the nervous system and intimately involved in the mechanisms of memory as well as the immune response. Such a duality places CaN at the crux of neuroimmunomodulation processes. In the present review, we briefly summarize the role of CaN in physiological aging and discuss how CaN hyperactivity could cause the memory impairment, neuroinflammation, and neuronal death that are pathological mechanisms of AD.

BDNF upregulation rescues synaptic plasticity in middle-aged ovariectomized rats

Brain-derived neurotrophic factor (BDNF) has emerged as a possible broad-spectrum treatment for the plasticity losses found in rodent models of human conditions associated with memory and cognitive deficits. We have tested this strategy in the particular case of ovariectomy. The actin polymerization in spines normally found after patterned afferent stimulation was greatly reduced, along with the stabilization of long-term potentiation, in hippocampal slices prepared from middle-aged ovariectomized rats. Both effects were fully restored by a 60-minute infusion of 2 nM BDNF. Comparable rescue results were obtained after elevating endogenous BDNF protein levels in hippocampus with 4 daily injections of a short half-life ampakine (positive modulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate [AMPA]-type glutamate receptors). These results provide the first evidence that minimally invasive, mechanism-based drug treatments can ameliorate defects in spine plasticity caused by depressed estrogen levels.

Recovery of afferent function and synaptic strength in hippocampal CA1 following traumatic brain injury

Cortical contusion injury can result in the partial loss of ipsilateral CA3 neurons within 48 h, leading to a proportional reduction in the number of afferent fibers to CA1 stratum radiatum. While the loss of afferent input to CA1 exhibits a remarkable, albeit incomplete, recovery over the next few weeks, little is known about the functional status of presynaptic afferents during the depletion and recovery phases following injury. Here, we prepared hippocampal slices from adult Sprague Dawley rats at 2, 7, and 14 days after lateral cortical contusion injury and measured fiber volley (FV) amplitudes extracellularly in CA1 stratum radiatum. Field excitatory post-synaptic potentials (EPSPs) were also measured and plotted as a function of FV amplitude to assess relative synaptic strength of residual and/or regenerated synaptic contacts. At 2 days post-injury, FV amplitude and synaptic strength were markedly reduced in the ipsilateral, relative to the contralateral, hippocampus. FV amplitude in ipsilateral CA1 showed a complete recovery by 7 days, indicative of a post-injury sprouting response. Synaptic strength in ipsilateral CA1 also showed a dramatic recovery over this time; however, EPSP-to-FV curves remained slightly suppressed at both the 7 and 14 day time points. Despite these deficits, ipsilateral slices retained the capacity to express long-term potentiation, indicating that at least some mechanisms for synaptic plasticity remain intact, or are compensated for. These results are in agreement with anatomical evidence showing a profound deafferentation, followed by a remarkable re-enervation, of ipsilateral CA1 in the first few weeks after traumatic brain injury. Although plasticity mechanisms appear to remain intact, synaptic strength deficits in CA1 could limit information throughput in the hippocampus, leading to persistent memory dysfunction.

Intracellular redox state alters NMDA receptor response during aging through Ca2+/calmodulin-dependent protein kinase II

The contribution of the NMDA receptors (NMDARs) to synaptic plasticity declines during aging, and the decline is thought to contribute to memory deficits. Here, we demonstrate that an age-related shift in intracellular redox state contributes to the decline in NMDAR responses through Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). The oxidizing agent xanthine/xanthine oxidase (X/XO) decreased the NMDAR-mediated synaptic responses at hippocampal CA3-CA1 synapses in slices from young (3-8 months) but not aged (20-25 months) rats. Conversely, the reducing agent dithiothreitol (DTT) selectively enhanced NMDAR response to a greater extent in aged hippocampal slices. The enhancement of NMDAR responses facilitated induction of long-term potentiation in aged but not young animals. The DTT-mediated growth in the NMDAR response was not observed for the AMPA receptor-mediated synaptic responses. A similar increase was observed by intracellular application of the membrane-impermeable reducing agent, L-glutathione (L-GSH), through the intracellular recording pipette, indicating that the increased NMDAR response was dependent on intracellular redox state. DTT enhancement of the NMDAR response was dependent on CaMKII activity and was blocked by the CaMKII inhibitor--myristoylated autocamtide-2-related inhibitory peptide (myr-AIP)--but not by inhibition of the activity of protein phosphatases--PP1 and calcineurin (CaN/PP2B) or protein kinase C. CaMKII activity assays established that DTT increased CaMKII activity in CA1 cytosolic extracts in aged but not in young animals. These findings indicate a link between oxidation of CaMKII during aging, a decline in NMDAR responses, and altered synaptic plasticity.

Cognitive decline in Alzheimer's disease is associated with selective changes in calcineurin/NFAT signaling

Upon activation by calcineurin, the nuclear factor of activated T-cells (NFAT) translocates to the nucleus and guides the transcription of numerous molecules involved in inflammation and Ca(2+) dysregulation, both of which are prominent features of Alzheimer's disease (AD). However, NFAT signaling in AD remains relatively uninvestigated. Using isolated cytosolic and nuclear fractions prepared from rapid-autopsy postmortem human brain tissue, we show that NFATs 1 and 3 shifted to nuclear compartments in the hippocampus at different stages of neuropathology and cognitive decline, whereas NFAT2 remained unchanged. NFAT1 exhibited greater association with isolated nuclear fractions in subjects with mild cognitive impairment (MCI), whereas NFAT3 showed a strong nuclear bias in subjects with severe dementia and AD. Similar to NFAT1, calcineurin-Aalpha also exhibited a nuclear bias in the early stages of cognitive decline. But, unlike NFAT1 and similar to NFAT3, the nuclear bias for calcineurin became more pronounced as cognition worsened. Changes in calcineurin/NFAT3 were directly correlated to soluble amyloid-beta (Abeta((1-42))) levels in postmortem hippocampus, and oligomeric Abeta, in particular, robustly stimulated NFAT activation in primary rat astrocyte cultures. Oligomeric Abeta also caused a significant reduction in excitatory amino acid transporter 2 (EAAT2) protein levels in astrocyte cultures, which was blocked by NFAT inhibition. Moreover, inhibition of astrocytic NFAT activity in mixed cultures ameliorated Abeta-dependent elevations in glutamate and neuronal death. The results suggest that NFAT signaling is selectively altered in AD and may play an important role in driving Abeta-mediated neurodegeneration.

Estradiol reverses a calcium-related biomarker of brain aging in female rats

An increase in L-type voltage-gated calcium channel (LTCC) current is a prominent biomarker of brain aging and is believed to contribute to cognitive decline and vulnerability to neuropathologies. Studies examining age-related changes in LTCCs have focused primarily on males, although estrogen (17beta-estradiol, E2) affects calcium-dependent activities associated with cognition. Therefore, to better understand brain aging in females, the effects of chronic E2 replacement on LTCC current activity in hippocampal neurons of young and aged ovariectomized rats were determined. The zipper slice preparation was used to expose cornu ammonis 1 (CA1) pyramidal neurons for recording LTCC currents using the cell-attached patch-clamp technique. We found that an age-related increase in LTCC current in neurons from control animals was prevented by E2 treatment. In addition, in situ hybridization revealed that within stratum pyramidale of the CA1 area, mRNA expression of the Ca(v)1.2 LTCC subunit, but not the Ca(v)1.3 subunit, was decreased in aged E2-treated rats. Thus, the reported benefits of E2 on cognition and neuronal health may be attributed, at least in part, to its age-related decrease in LTCC current.