Calcineurin enhances L-type Ca(2+) channel activity in hippocampal neurons: increased effect with age in culture

Molecular activity underlying working memory

The prefrontal cortex is necessary for directing thought and planning action. Working memory, the active, transient maintenance of information in mind for subsequent monitoring and manipulation, lies at the core of many simple, as well as high-level, cognitive functions. Working memory has been shown to be compromised in a number of neurological and psychiatric conditions and may contribute to the behavioral and cognitive deficits associated with these disorders. It has been theorized that working memory depends upon reverberating circuits within the prefrontal cortex and other cortical areas. However, recent work indicates that intracellular signals and protein dephosphorylation are critical for working memory. The present article will review recent research into the involvement of the modulatory neurotransmitters and their receptors in working memory. The intracellular signaling pathways activated by these receptors and evidence that indicates a role for G(q)-initiated PI-PLC and calcium-dependent protein phosphatase calcineurin activity in working memory will be discussed. Additionally, the negative influence of calcium- and cAMP-dependent protein kinase (i.e., calcium/calmodulin-dependent protein kinase II (CaMKII), calcium/diacylglycerol-activated protein kinase C (PKC), and cAMP-dependent protein kinase A (PKA)) activities on working memory will be reviewed. The implications of these experimental findings on the observed inverted-U relationship between D(1) receptor stimulation and working memory, as well as age-associated working memory dysfunction, will be presented. Finally, we will discuss considerations for the development of clinical treatments for working memory disorders.

Contribution of downregulation of L-type calcium currents to delayed neuronal death in rat hippocampus after global cerebral ischemia and reperfusion

Transient forebrain ischemia induces delayed, selective neuronal death in the CA1 region of the hippocampus. The underlying molecular mechanisms are as yet unclear, but it is known that activation of L-type Ca2+ channels specifically increases the expression of a group of genes required for neuronal survival. Accordingly, we examined temporal changes in L-type calcium-channel activity in CA1 and CA3 pyramidal neurons of rat hippocampus after transient forebrain ischemia by patch-clamp techniques. In vulnerable CA1 neurons, L-type Ca2+-channel activity was persistently downregulated after ischemic insult, whereas in invulnerable CA3 neurons, no change occurred. Downregulation of L-type calcium channels was partially caused by oxidation modulation in postischemic channels. Furthermore, L-type but neither N-type nor P/Q-type Ca2+-channel antagonists alone significantly inhibited the survival of cultured hippocampal neurons. In contrast, specific L-type calcium-channel agonist remarkably reduced neuronal cell death and restored the inhibited channels induced by nitric oxide donor. More importantly, L-type calcium-channel agonist applied after reoxygenation or reperfusion significantly decreased neuronal injury in in vitro oxygen-glucose deprivation ischemic model and in animals subjected to forebrain ischemia-reperfusion. Together, the present results suggest that ischemia-induced inhibition of L-type calcium currents may give rise to delayed death of neurons in the CA1 region, possibly via oxidation mechanisms. Our findings may lead to a new perspective on neuronal death after ischemic insult and suggest that a novel therapeutic approach, activation of L-type calcium channels, could be tested at late stages of reperfusion for stroke treatment.

Calcineurin-independent inhibition of the delayed rectifier K+ current by the immunosuppressant FK506 in rat hippocampal neurons

The immunosuppressant drug FK506 was found to be a potent neuroprotective agent in animal models of brain ischemia. However, the mechanisms underlying the action remain to be elucidated. The delayed rectifier K(+) channel has been implicated in ischemic injury and neuronal death in the brain. The aim of the present study is to investigate whether the neuroprotective action of FK506 results from blocking the K(+) channel. In acutely dissociated CA1 pyramidal neurons of rat hippocampus, superfusion of FK506 (0.01-100 microM) selectively inhibited the delayed rectifier K(+) current (I(K)) with an IC(50) value of 13.2+/-4.9 microM. The inhibition of I(K) by FK506 (10 microM) had a rapid onset, and then gradually reached a steady-state level. The inhibition was voltage-dependent, became more potent when the currents were elicited by strong depolarization. Moreover, FK506 (10 microM) caused marked negative shifts of the steady-state activation and inactivation curves of I(K), and accelerated its recovery from inactivation. Intracellular dialysis of FK506 (30 microM) was ineffective. The inhibition of I(K) by FK506 (10 microM) persisted under the low-Ca(2+) conditions that blocked the basal activity of protein phosphatase 2B (calcineurin). Rapamycin did not antagonize FK506 but mimicked it. Cyclosporin A inhibited I(K) only at 30 and 100 microM. Taken together, the results suggest that FK506 exert a direct inhibition on the delayed rectifier K(+) channel without involvement of calcineurin.

Calcineurin-independent inhibition of KV1.3 by FK-506 (tacrolimus): a novel pharmacological property

The interaction of FK-506 with KV1.3, stably expressed in Chinese hamster ovary cells, was investigated with the whole cell patch-clamp technique. FK-506 inhibited KV1.3 in a reversible, concentration-dependent manner with an IC50of 5.6 μM. Rapamycin, another immunosuppressant, produced effects that were similar to those of FK-506 (IC50= 6.7 μM). Other calcineurin inhibitors (cypermethrin or calcineurin autoinhibitory peptide) alone had no effect on the amplitude or kinetics of KV1.3. In addition, the inhibitory action of FK-506 continued, even after the inhibition of calcineurin activity. The inhibition produced by FK-506 was voltage dependent, increasing in the voltage range for channel activation. At potentials positive to 0 mV (where maximal conductance is reached), however, no voltage-dependent inhibition was found. FK-506 exhibited a strong use-dependent inhibition of KV1.3. FK-506 shifted the steady-state inactivation curves of KV1.3 in the hyperpolarizing direction in a concentration-dependent manner. The apparent dissociation constant for FK-506 to inhibit KV1.3 in the inactivated state was estimated from the concentration-dependent shift in the steady-state inactivation curve and was calculated to be 0.37 μM. Moreover, the rate of recovery from inactivation of KV1.3 was decreased. In inside-out patches, FK-506 not only reduced the current amplitude but also accelerated the rate of inactivation during depolarization. FK-506 also inhibited KV1.5 and KV4.3 in a concentration-dependent manner with IC50of 4.6 and 53.9 μM, respectively. The present results indicate that FK-506 inhibits KV1.3 directly and that this effect is not mediated via the inhibition of the phosphatase activity of calcineurin.

Calcineurin triggers reactive/inflammatory processes in astrocytes and is upregulated in aging and Alzheimer's models

Astrocyte reactivity (i.e., activation) and associated neuroinflammation are increasingly thought to contribute to neurodegenerative disease. However, the mechanisms that trigger astrocyte activation are poorly understood. Here, we studied the Ca2+-dependent phosphatase calcineurin, which regulates inflammatory signaling pathways in immune cells, for a role in astrogliosis and brain neuroinflammation. Adenoviral transfer of activated calcineurin to primary rat hippocampal cultures resulted in pronounced thickening of astrocyte somata and processes compared with uninfected or virus control cultures, closely mimicking the activated hypertrophic phenotype. This effect was blocked by the calcineurin inhibitor cyclosporin A. Parallel microarray studies, validated by extensive statistical analyses, showed that calcineurin overexpression also induced genes and cellular pathways representing most major markers associated with astrocyte activation and recapitulated numerous changes in gene expression found previously in the hippocampus of normally aging rats or in Alzheimer's disease (AD). No genomic or morphologic evidence of apoptosis or damage to neurons was seen, indicating that the calcineurin effect was mediated by direct actions on astrocytes. Moreover, immunocytochemical studies of the hippocampus/neocortex in normal aging and AD model mice revealed intense calcineurin immunostaining that was highly selective for activated astrocytes. Together, these studies show that calcineurin overexpression is sufficient to trigger essentially the full genomic and phenotypic profiles associated with astrocyte activation and that hypertrophic astrocytes in aging and AD models exhibit dramatic upregulation of calcineurin. Thus, the data identify calcineurin upregulation in astrocytes as a novel candidate for an intracellular trigger of astrogliosis, particularly in aging and AD brain.

Caloric restriction prevents aging-associated changes in spike-mediated Ca2+ accumulation and the slow afterhyperpolarization in hippocampal CA1 pyramidal neurons

In hippocampal pyramidal neurons from aged animals voltage-gated Ca2+ entry and the slow, post-burst afterhyperpolarization are enhanced. As a result, there is a decrease in neuronal excitability and, in turn, an alteration in synaptic plasticity. Restricting the caloric intake of a rodent is a well-known paradigm for increasing lifespan and ameliorating a number of neurodegenerative features of aging, including deficits in synaptic plasticity and cognition. Here we show in rat CA1 pyramidal neurons from aged animals (18-20 months old) that a restricted diet prevents the enhancement of dendritic spike-mediated Ca2+ accumulation. In contrast, no significant changes in the rates of Ca2+ recovery were observed suggesting that Ca2+ clearance mechanisms are not affected by aging or caloric restriction. Lastly, we found that caloric restriction also prevented the aging-associated increase in the slow, post-burst afterhyperpolarization. Our results suggest that caloric restriction-sensitive changes in Ca2+ accumulation and membrane excitability may in part account for the protective effects of dietary restriction on synaptic plasticity and learning deficits in aged animals.

Protective effects of Ca2+ handling drugs against abnormal Ca2+ homeostasis and cell damage in myopathic skeletal muscle cells

Deficiency of delta-sarcoglycan (delta-SG), a component of the dystrophin-glycoprotein complex (DGC), causes skeletal muscular dystrophy and cardiomyopathy in BIO14.6 hamsters. Here, we studied the involvement of abnormal Ca2+ homeostasis in muscle degeneration and the protective effect of drugs against Ca2+ handling proteins in vivo as well as in vitro. First, we characterized the properties of cultured myotubes from muscles of normal and BIO14.6 hamsters (30-60 days old). While there were no apparent differences in the levels of expression of various Ca2+ handling proteins (L-type Ca2+ channel, ryanodine receptor, SR-Ca2+ ATPase, and Na+/Ca2+ exchanger), muscle-specific proteins (contractile actin and acetylcholine receptor), or DGC member proteins except SGs, BIO14.6 myotubes showed a high degree of susceptibility to mechanical stressors, such as cyclic stretching and hypo-osmotic stress as compared to normal myotubes, as evidenced by marked increases in creatine phosphokinase (CK) release and bleb formation. BIO14.6 myotubes showed abnormal Ca2+ homeostasis characterized by elevated cytosolic Ca2+ concentration, frequent Ca2+ oscillation, and increased 45Ca2+ uptake. These abnormal Ca2+ events and CK release were significantly prevented by Ca2+ handling drugs, tranilast, diltiazem, and FK506. The calpain inhibitor E64 prevented CK release, but not 45Ca2+ uptake. Some of these drugs (tranilast, diltiazem, and FK506) also exerted a significant protective effect for muscle degeneration in BIO14.6 hamsters and mdx mice in vivo. These observations suggest that elevated Ca2+ entry through sarcolemmal Ca2+ channels predominantly contributes to muscle degeneration and that the drugs tested here may have novel therapeutic potential against muscular dystrophy.

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.

Electrical remodeling in cardiac hypertrophy

In recent years, electrical remodeling has emerged as an important pathophysiologic mechanism in many types of cardiac pathology. Because clinical heart disease often involves both hypertrophic and failure phenotypes, identification of disease-specific mechanisms is essential. This review focuses on mechanisms of electrical remodeling in cardiac hypertrophy, emphasizing transmembrane Ca2+ fluxes and Ca(2+)-responsive signaling pathways. Where information is available, the remodeling of hypertrophy is contrasted with what is known about heart failure.

Calcineurin in memory and bidirectional plasticity

The molecular mechanisms of learning and memory, and the underlying bidirectional changes in synaptic plasticity that sustain them largely implicate protein kinases and phosphatases. Specifically, Ca(2+)-dependent kinases and phosphatases actively control neuronal processing by forming a tightly regulated balance in which they oppose each other. In this balance, calcineurin (PP2B) is a critical protein phosphatase whose main function is to negatively modulate learning, memory, and plasticity. It acts by dephosphorylating numerous substrates in different neuronal compartments. This review outlines some of CN neuronal targets and their implication in synaptic functions, and describes the role of CN in the acquisition, storage, retrieval, and extinction of memory, as well as in bidirectional plasticity.

Calcineurin regulation of neuronal plasticity

From the most basic of nervous systems to the intricate circuits found within the human brain, a fundamental requirement of neuronal function is that it be malleable, altering its output based upon experience. A host of cellular proteins are recruited for this purpose, which themselves are regulated by protein phosphorylation. Over the past several decades, research has demonstrated that the Ca(2+) and calmodulin-dependent protein phosphatase calcineurin (protein phosphatase 2B) is a critical regulator of a diverse array of proteins, leading to both short- and long-term effects on neuronal excitability and function. This review describes many of the influences of calcineurin on a variety of proteins, including ion channels, neurotransmitter receptors, enzymes, and transcription factors. Intriguingly, due to the bi-directional influences of Ca(2+) and calmodulin on calcineurin activity, the strength and duration of particular stimulations may cause apparently antagonistic functions of calcineurin to work in concert.

Enhanced long-term potentiation during aging is masked by processes involving intracellular calcium stores

The contribution of Ca(2+) release from intracellular Ca(2+) stores (ICS) for regulation of synaptic plasticity thresholds during aging was investigated in hippocampal slices of old (22-24 mo) and young adult (5-8 mo) male Fischer 344 rats. Inhibition of Ca(2+)-induced Ca(2+) release by thapsigargin, cyclopiazonic acid (CPA), or ryanodine during pattern stimulation near the threshold for synaptic modification (5 Hz, 900 pulses) selectively induced long-term potentiation (LTP) to CA1 Schaffer collateral synapses of old rats. Increased synaptic strength was specific to test pathways and blocked by AP-5. Intracellular recordings demonstrated that ICS plays a role in the augmentation of the afterhyperpolarization (AHP) in old rats. The decrease in the AHP by ICS inhibition was reversed by the L-channel agonist, Bay K8644. Under conditions of ICS inhibition and a Bay K8644-mediated enhancement of the AHP, pattern stimulation failed to induce LTP, consistent with the idea that the AHP amplitude shapes the threshold for LTP induction. Finally, ICS inhibition was associated with an increase in the N-methyl-d-aspartate (NMDA) receptor component of synaptic transmission in old animals. This increase in the synaptic response was blocked by the calcineurin inhibitor FK506. The results reveal an age-related increase in susceptibility to LTP-induction that is normally inhibited by ICS and suggest that the age-related shift in Ca(2+) regulation and Ca(2+)-dependent synaptic plasticity is coupled to changes in cell excitability and NMDA receptor function through ICS.

Okadaic acid mediates tau phosphorylation via sustained activation of the L-voltage-sensitive calcium channel

Accumulation of phosphorylated isoforms of the microtubule-associated protein tau is one hallmark of affected neurons in Alzheimer's disease (AD). This increase has been attributed to increased kinase or decreased phosphatase activity. Prior studies indicate that one of the kinases that phosphorylates tau (mitogen-activated protein kinase, or MAP kinase) does so at least in part indirectly within intact neuronal cells by phosphorylating and activating the L-voltage-sensitive calcium channel. Resultant calcium influx then fosters tau phosphorylation via one or more calcium-activated kinases. We demonstrate herein that treatment of differentiated SH-SY-5Y human neuroblastoma with the phosphatase inhibitor okadaic acid (OA) similarly may increase tau phosphorylation via sustained activation of the L-voltage-sensitive calcium channel. OA increased phospho-tau as indicated by increased immunoreactivity towards an antibody (PHF-1) directed against paired helical filaments from AD brain. This increase was blocked by co-treatment with the channel antagonist nimodipine. OA treatment increased channel phosphorylation. The increases in calcium influx, PHF-1 immunoreactivity and channel phosphorylation were all attenuated by co-treatment with PD98059, which inhibits MAP kinase activity, suggesting that OA mediates these effects at least in part via sustained activation of MAP kinase. These findings underscore that divergent and convergent kinase and phosphatase activities regulate tau phosphorylation.

Calcium channel and NMDA receptor activities differentially regulate nuclear C/EBPbeta levels to control neuronal survival

Insulin-like growth factor-1 (IGF-1) promotes the survival of cerebellar granule neurons by enhancing calcium influx through L-type calcium channels, whereas NMDA receptor-mediated calcium influx can lead to excitotoxic death. Here we demonstrate that L and NMDA receptor channel activities differentially regulate the transcription factor C/EBPbeta to control neuronal survival. Specifically, we show that L channel-dependent calcium influx results in increased CaMKIV activity, which acts to decrease nuclear C/EBPbeta levels. Conversely, NMDA receptor-mediated influx rapidly elevates nuclear C/EBPbeta and induces excitotoxic death via activation of the calcium-dependent phosphatase, calcineurin. Moderate levels of AMPA receptor activity stimulate L channels to improve survival, whereas higher levels stimulate NMDA receptors and reduce neuronal survival, suggesting differential synaptic effects. Finally, N-type calcium channel activity reduces survival, potentially by increasing glutamate release. Together, these results show that the L-type calcium channel-dependent survival and NMDA receptor death pathways converge to regulate nuclear C/EBPbeta levels, which appears to be pivotal in these mechanisms.

Hydrogen peroxide modulation of synaptic plasticity

Unlike the proposed role of reactive oxygen species in neurodegeneration, acute effects of reactive oxygen on synaptic plasticity are poorly understood. Using rat hippocampal slices, we found that exposure to a high concentration (0.5-5 mm) of H(2)O(2) reduces EPSPs in both potentiated and nonpotentiated synapses. Exposure of the slices to 20 microm H(2)O(2) did not affect expression of preestablished long-term potentiation (LTP) but prevented induction of new LTP and enhanced long-term depression (LTD). Surprisingly, 1 microm H(2)O(2) caused a twofold increase in LTP compared with controls, and it further enhanced NMDA-independent LTP. A low concentration of H(2)O(2) also suppressed LTD. Nifedipine, an L-type calcium channel blocker, did not affect control LTP but blocked effects of both 1 and 20 microm H(2)O(2). Calcineurin inhibitors [FK506 (FR900506) and cyclosporin A but not rapamycin] acted similarly and also restored LTP in the presence of 20 microm H(2)O(2). These results suggest that H(2)O(2) alters NMDA-independent, voltage-gated calcium channel-mediated LTP by activating calcineurin.

Hydrogen peroxide modulation of synaptic plasticity

Unlike the proposed role of reactive oxygen species in neurodegeneration, acute effects of reactive oxygen on synaptic plasticity are poorly understood. Using rat hippocampal slices, we found that exposure to a high concentration (0.5-5 mm) of H(2)O(2) reduces EPSPs in both potentiated and nonpotentiated synapses. Exposure of the slices to 20 microm H(2)O(2) did not affect expression of preestablished long-term potentiation (LTP) but prevented induction of new LTP and enhanced long-term depression (LTD). Surprisingly, 1 microm H(2)O(2) caused a twofold increase in LTP compared with controls, and it further enhanced NMDA-independent LTP. A low concentration of H(2)O(2) also suppressed LTD. Nifedipine, an L-type calcium channel blocker, did not affect control LTP but blocked effects of both 1 and 20 microm H(2)O(2). Calcineurin inhibitors [FK506 (FR900506) and cyclosporin A but not rapamycin] acted similarly and also restored LTP in the presence of 20 microm H(2)O(2). These results suggest that H(2)O(2) alters NMDA-independent, voltage-gated calcium channel-mediated LTP by activating calcineurin.

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.