A facilitatory effect on the induction of long-term potentiation in vivo by chronic administration of antisense oligodeoxynucleotides against catalytic subunits of calcineurin

Kinase and Phosphatase Engagement Is Dissociated Between Memory Formation and Extinction

Associative long-term memories (LTMs) support long-lasting behavioral changes resulting from sensory experiences. Retrieval of a stable LTM by means of a large number of conditioned stimulus (CS) alone presentations produces inhibition of the original memory through extinction. Currently, there are two opposing hypotheses to account for the neural mechanisms supporting extinction. The unlearning hypothesis posits that extinction affects the original memory trace by reverting the synaptic changes supporting LTM. On the contrary, the new learning hypothesis proposes that extinction is simply the formation of a new associative memory that inhibits the expression of the original one. We propose that detailed analysis of extinction-associated molecular mechanisms could help distinguish between these hypotheses. Here we will review experimental evidence regarding the role of protein kinases and phosphatases (K&P) on LTM formation and extinction. Even though K&P regulate both memory processes, their participation appears to be dissociated. LTM formation recruits kinases, but is constrained by phosphatases. Memory extinction presents a more diverse molecular landscape, requiring phosphatases and some kinases, but also being constrained by kinase activity. Based on the available evidence, we propose a new theoretical model for memory extinction: a neuronal segregation of K&P supports a combination of time-dependent reversible inhibition of the original memory [CS-unconditioned stimulus (US)], with establishment of a new associative memory trace (CS-noUS).

Effects Following Intracerebroventricular Injection of Immunosuppressant Cyclosporine A On Inhibitory Avoidance Learning and Memory in Mice

Background:

Protein phosphatase-2B or calcineurin (CN) is the main phosphatase and a critical regulator of cellular pathways for learning, memory, and plasticity. Cyclosporine A(CyA), a phosphatase and peptidyl-prolyl cis/trans isomerase inhibitor, is a common immune suppressant extensively used in tissue transplantation. To further clarify the role of CN in different stages of learning and memory, the aim of the present study was to evaluate the role of CyA in an inhibitory avoidance (IA) model in mice.

Materials and Methods:

Using intracerebroventricular (ICV) injection of different doses of CyA (0.5, 5, and 50 nM) at different periods (pre-/post-training and pre-test), the effect of the drug was evaluated in a step-down IA paradigm. The latency of step-down (sec) was considered a criterion for memory performance.

Results:

The pre-training injections of CyA (0.5, 5 nM), however not of 50 nM, impaired IA learning acquisition. The post-training injection of high-dose CyA (50 nM) impaired memory consolidation. The pre-test ICV CyA injection did not impair memory retrieval; the ICV injection of CyA caused no change in locomotion.

Conclusion:

These findings suggest that CyA selectively interferes with acquisition, retention, but not retrieval, of information processing in mice. Given the crucial role of CN in common signaling pathway of memory performance and cognition, it could be a probable therapeutic target in the treatment of a wide variety of neurological conditions involving memory.

Pin1 mediates Aβ42-induced dendritic spine loss

Early-stage Alzheimer's disease is characterized by the loss of dendritic spines in the neocortex of the brain. This phenomenon precedes tau pathology, plaque formation, and neurodegeneration and likely contributes to synaptic loss, memory impairment, and behavioral changes in patients. Studies suggest that dendritic spine loss is induced by soluble, multimeric amyloid-β (Aβ₄₂), which, through postsynaptic signaling, activates the protein phosphatase calcineurin. We investigated how calcineurin caused spine pathology and found that the cis-trans prolyl isomerase Pin1 was a critical downstream target of Aβ₄₂-calcineurin signaling. In dendritic spines, Pin1 interacted with and was dephosphorylated by calcineurin, which rapidly suppressed its isomerase activity. Knockout of Pin1 or exposure to Aβ₄₂ induced the loss of mature dendritic spines, which was prevented by exogenous Pin1. The calcineurin inhibitor FK506 blocked dendritic spine loss in Aβ₄₂-treated wild-type cells but had no effect on Pin1-null neurons. These data implicate Pin1 in dendritic spine maintenance and synaptic loss in early Alzheimer's disease.

Reconsolidation and extinction are dissociable and mutually exclusive processes: behavioral and molecular evidence

Memory persistence is critically influenced by retrieval. In rats, a single presentation of a conditioned fear stimulus induces memory reconsolidation and fear memory persistence, while repeated fear cue presentations result in loss of fear through extinction. These two opposite behavioral outcomes are operationally linked by the number of cue presentations at memory retrieval. However, the behavioral properties and mechanistic determinants of the transition have not yet been explored; in particular, whether reconsolidation and extinction processes coexist or are mutually exclusive, depending on the exposure to non-reinforced retrieval events. We characterized both behaviorally and molecularly the transition from reconsolidation to extinction of conditioned fear and showed that an increase in calcineurin (CaN) in the basolateral amygdala (BLA) supports the shift from fear maintenance to fear inhibition. Gradually increasing the extent of retrieval induces a gradual decrease in freezing responses to the conditioned stimulus and a gradual increase in amygdala CaN level. This newly synthesized CaN is required for the extinction, but not the reconsolidation, of conditioned fear. During the transition from reconsolidation to extinction, we have revealed an insensitive state of the fear memory where NMDA-type glutamate receptor agonist and antagonist drugs are unable either to modulate CaN levels in the BLA or alter the reconsolidation or extinction processes. Together, our data indicate both that reconsolidation and extinction are mutually exclusive processes and also reveal the presence of a transitional, or "limbo," state of the original memory between these two alternative outcomes of fear memory retrieval, when neither process is engaged.

Neural functions of calcineurin in synaptic plasticity and memory

Major brain functions depend on neuronal processes that favor the plasticity of neuronal circuits while at the same time maintaining their stability. The mechanisms that regulate brain plasticity are complex and engage multiple cascades of molecular components that modulate synaptic efficacy. Protein kinases (PKs) and phosphatases (PPs) are among the most important of these components that act as positive and negative regulators of neuronal signaling and plasticity, respectively. In these cascades, the PP protein phosphatase 2B or calcineurin (CaN) is of particular interest because it is the only Ca(2+)-activated PP in the brain and a major regulator of key proteins essential for synaptic transmission and neuronal excitability. This review describes the primary properties of CaN and illustrates its functions and modes of action by focusing on several representative targets, in particular glutamate receptors, striatal enriched protein phosphatase (STEP), and neuromodulin (GAP43), and their functional significance for synaptic plasticity and memory.

Role of calcineurin in inhibiting disadvantageous associations

Calcineurin is an important calcium-dependent phosphatase that is evolutionarily conserved in all studied species, and has been implicated in the consolidation and maintenance of new memories. However, recent evidence has extended the role of calcineurin. In contrast to learning tasks that require behavioral acquisition, extinction tasks that require behavioral inhibition have been shown to be reliant on calcineurin. In the present study, using a Morris water maze, we have demonstrated that pharmacological inhibition of calcineurin causes augmentation of spatial learning and perseveration of spatial reversal-learning in a dose-dependent manner. Direct infusions of a specific calcineurin inhibitor, cyclosporine A, into the dorsal hippocampi bilaterally, prior to spatial learning, led to increased learning, whereas similar injections of cyclosporine A following a spatial learning task and prior to a spatial reversal-learning task resulted in perseveration of reversal-learning. Our results indicate that injections of cyclosporin A resulted in decreased calcineurin activity in the dorsal hippocampus and increased difficulty in switching to new task demands, in a dose-dependent manner, despite evidence indicating no deficit in ability to learn new information. Therefore, calcineurin activity contributes to the inhibition of previously learned but unwanted behavioral responses during competitive spatial learning. Involvement of calcineurin in extinction of fear memory has recently been demonstrated. Our results also indicate that calcineurin activity plays a role in memory extinction in spatial memory tasks, and therefore, suggest that calcineurin might be an important molecule in mediating behavioral flexibility in general.

Calcium-dependent phosphorylation regulates neuronal stability and plasticity in a highly precise pacemaker nucleus

Specific types of neurons show stable, predictable excitability properties, while other neurons show transient adaptive plasticity of their excitability. However, little attention has been paid to how the cellular pathways underlying adaptive plasticity interact with those that maintain neuronal stability. We addressed this question in the pacemaker neurons from a weakly electric fish because these neurons show a highly stable spontaneous firing rate as well as an N-methyl-D-aspartate (NMDA) receptor-dependent form of plasticity. We found that basal firing rates were regulated by a serial interaction of conventional and atypical PKC isoforms and that this interaction establishes individual differences within the species. We observed that NMDA receptor-dependent plasticity is achieved by further activation of these kinases. Importantly, the PKC pathway is maintained in an unsaturated baseline state to allow further Ca(2+)-dependent activation during plasticity. On the other hand, the Ca(2+)/calmodulin-dependent phosphatase calcineurin does not regulate baseline firing but is recruited to control the duration of the NMDA receptor-dependent plasticity and return the pacemaker firing rate back to baseline. This work illustrates how neuronal plasticity can be realized by biasing ongoing mechanisms of stability (e.g., PKC) and terminated by recruiting alternative mechanisms (e.g., calcineurin) that constrain excitability. We propose this as a general model for regulating activity-dependent change in neuronal excitability.

Activin plays a key role in the maintenance of long-term memory and late-LTP

A recent study has revealed that fear memory may be vulnerable following retrieval, and is then reconsolidated in a protein synthesis-dependent manner. However, little is known about the molecular mechanisms of these processes. Activin betaA, a member of the TGF-beta superfamily, is increased in activated neuronal circuits and regulates dendritic spine morphology. To clarify the role of activin in the synaptic plasticity of the adult brain, we examined the effect of inhibiting or enhancing activin function on hippocampal long-term potentiation (LTP). We found that follistatin, a specific inhibitor of activin, blocked the maintenance of late LTP (L-LTP) in the hippocampus. In contrast, administration of activin facilitated the maintenance of early LTP (E-LTP). We generated forebrain-specific activin- or follistatin-transgenic mice in which transgene expression is under the control of the Tet-OFF system. Maintenance of hippocampal L-LTP was blocked in the follistatin-transgenic mice. In the contextual fear-conditioning test, we found that follistatin blocked the formation of long-term memory (LTM) without affecting short-term memory (STM). Furthermore, consolidated memory was selectively weakened by the expression of follistatin during retrieval, but not during the maintenance phase. On the other hand, the maintenance of memory was also influenced by activin overexpression during the retrieval phase. Thus, the level of activin in the brain during the retrieval phase plays a key role in the maintenance of long-term memory.

Reciprocal repression between microRNA-133 and calcineurin regulates cardiac hypertrophy: a novel mechanism for progressive cardiac hypertrophy

Cardiac hypertrophy involves a remodeling process of the heart in response to diverse pathological stimuli. Both calcineurin/nuclear factor of activated T cells pathway and microRNA-133 (miR-133) have been shown to play a critical role in cardiac hypertrophy. It has been recognized that the expression and activity of calcineurin increases and miR-133 expression decreases in the hypertrophic heart, and inhibition of calcineurin or increase of miR-133 expression protects against cardiac hypertrophy. Here we tested the interaction between miR-133 and calcineurin in cardiac hypertrophy. Cardiac hypertrophy in vivo and in vitro was induced by transverse aortic constriction and phenylephrine treatment. mRNA levels were measured by using real-time PCR methods. Luciferase assays showed that transfection of miR-133 in HEK293 cells downregulated calcineurin expression, which was reversed by cotransfection with the miR-133-specific 2'-O-methyl antisense inhibitory oligoribonucleotides. These results were confirmed in cultured primary cardiomyocytes. miR-133 expression was downregulated, and calcineurin activity was enhanced in both in vivo and in vitro cardiac hypertrophy models. Treatment of cells and animals with cyclosporin A, an inhibitor of calcineurin, prevented miR-133 downregulation. Moreover, the antisense oligodeoxynucleotides against the catalytic subunits of calcineurin Abeta and the decoy oligodeoxynucleotides targeting nuclear factor of activated T cells transcription factor, a calcineurin downstream effector, increased miR-133 expression in cultured primary cardiomyocytes. Our data show that reciprocal repression between miR-133 and calcineurin regulates cardiac hypertrophy.

The molecular and cellular biology of enhanced cognition

Most molecular and cellular studies of cognitive function have focused on either normal or pathological states, but recent research with transgenic mice has started to address the mechanisms of enhanced cognition. These results point to key synaptic and nuclear signalling events that can be manipulated to facilitate the induction or increase the stability of synaptic plasticity, and therefore enhance the acquisition or retention of information. Here, we review these surprising findings and explore their implications to both mechanisms of learning and memory and to ongoing efforts to develop treatments for cognitive disorders. These findings represent the beginning of a fundamental new approach in the study of enhanced cognition.

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.

Novel effects on memory observed following unilateral intracranial administration of okadaic acid, cyclosporin A, FK506 and [MeVal4]CyA

The involvement of protein phosphatases and peptidyl-prolyl cis/trans isomerases (PPIases) in memory formation in the chick has previously been investigated using a single-trial learning task. In these studies, inhibitory agents were administered bilaterally directly to a critical area of the chick brain. These studies are now extended to investigate whether similar effects are obtained if the drugs are administered unilaterally. All of the effects reported previously following bilateral administration of okadaic acid (OA), cyclosporin A (CyA), FK506 and [MeVal(4)]CyA can be attributed to their action in just one hemisphere. OA, at a concentration known to selectively inhibit PP2A in vitro (0.5 nM) results in permanent memory loss from 30-40 min post-training when injected in the left hemisphere, but has no effect when injected in the right hemisphere. A higher concentration of OA (100 nM), which inhibits both PP2A and PP1 in vitro, has a similar effect in the left hemisphere but causes a transient period of memory loss from 10-20 min post-training when injected in the right hemisphere. CyA (5 nM and 20 nM), which inhibits both PP2B and PPIase activity, causes permanent memory loss from 60 min post-training when injected into the left hemisphere, an effect also observed following administration of FK506 (20 nM), which also inhibits PP2B and PPIase activity, and [MeVal(4)]CyA (5 nM), which inhibits PPIase activity but not PP2B activity. Administration of CyA (20 nM) and FK506, but not [MeVal(4)]CyA, in the right hemisphere leads to a transient period of memory loss from 10-20 min post-training. These results confirm significant roles for phosphatases and PPIases in memory processing but challenge previous conclusions drawn on the basis of bilateral drug administration protocols.

Different phosphatase-dependent mechanisms mediate long-term depression and depotentiation of long-term potentiation in mouse hippocampal CA1 area

Two types of synaptic depression have been described in the hippocampus, long-term depression and depotentiation of long-term potentiation known to recruit the serine/threonine protein phosphatases PP1, PP2A and PP2B (calcineurin). The contribution of each of these protein phosphatases is controversial. To examine the role of the Ca2+/calmodulin-dependent protein phosphatase calcineurin in long-term depression and depotentiation, we analysed the effect of genetically inhibiting calcineurin reversibly in the hippocampus, using the doxycycline-dependent rtTA system in transgenic mice. We show that reducing calcineurin activity has no effect on long-term depression but reversibly affects depotentiation. Consistently, the calcineurin inhibitor FK-506 reproduces the depotentiation impairment observed in the mutant mice but does not affect long-term depression in control animals. In contrast, the PP1/PP2A inhibitor okadaic acid fully blocks both long-term depression and depotentiation. These data demonstrate that the nature of signalling cascades induced by synaptic activity depends on the initial synaptic state. While depression of potentiated synaptic responses requires activation of PP1/PP2A and/or calcineurin, depression of basal synaptic responses depends only on PP1/PP2A activation.

Inhibition of calcineurin facilitates the induction of memory for sensitization in Aplysia: requirement of mitogen-activated protein kinase

The induction of both synaptic plasticity and memory is thought to depend on the balance between opposing molecular regulatory factors, such as protein kinases and phosphatases. Here we show that inhibition of protein phosphatase 2B (calcineurin, CaN) facilitates the induction of intermediate-term memory (ITM) and long-term memory (LTM) for tail shock-induced sensitization in Aplysia without any effect on short-term memory. To identify the molecular cascade underlying the improvement of memory by inhibition of CaN, we examined the role of extracellular signal-regulated kinase 1/2/mitogen-activated protein kinase (MAPK). Molecular experiments revealed that one pulse of serotonin, which by itself does not activate MAPK, leads to significant MAPK activation in the sensory neurons of the pleural ganglia when CaN is inhibited. Extending these observations, behavioral experiments showed that the facilitated induction of ITM and LTM produced by CaN inhibition depends on MAPK activity. These results demonstrate: (i) that CaN acts as an inhibitory constraint in the formation of long-lasting phases of memory, and (ii) that facilitated induction of ITM and LTM by CaN inhibition requires MAPK activity.

Long-term depression in the adult hippocampus in vivo involves activation of extracellular signal-regulated kinase and phosphorylation of Elk-1

Protein kinase cascades likely play a critical role in the signaling events that underlie synaptic plasticity and memory. The extracellular signal-regulated kinase (ERK) cascade is suited well for such a role because its targets include regulators of gene expression. Here we report that the ERK cascade is recruited during long-term depression (LTD) of synaptic strength in area CA1 of the adult hippocampus in vivo and selectively impacts on phosphorylation of the nuclear transcription factor Elk-1. Using a combination of in vivo electrophysiology, biochemistry, pharmacology, and immunohistochemistry, we found the following: (1) ERK phosphorylation, including phosphorylation of nuclear ERK, and ERK phosphotransferase activity are increased markedly, albeit transiently, after the induction of NMDA receptor-dependent LTD at the commissural input to area CA1 pyramidal cells in the hippocampus of anesthetized adult rats; (2) LTD-inducing paired-pulse stimulation fails to produce lasting LTD in the presence of the ERK kinase inhibitor SL327, which suggests that ERK activation is necessary for the persistence of LTD; and (3) ERK activation during LTD results in increased phosphorylation of Elk-1 but not of the transcription factor cAMP response element-binding protein. Our findings indicate that the ERK cascade transduces signals from the synapse to the nucleus during LTD in hippocampal area CA1 in vivo, as it does during long-term potentiation in area CA1, but that the pattern of coupling of the ERK cascade to transcriptional regulators differs between the two forms of synaptic plasticity.

Cyclosporin A, FK506 and rapamycin produce multiple, temporally distinct, effects on memory following single-trial, passive avoidance training in the chick

Few studies have used a pharmaco-behavioural methodology to directly investigate roles for the calcium-dependent protein phosphatase calcineurin (CaN) in memory formation, due partly to the absence of specific inhibitory agents. A number of drugs with different inhibitory profiles were used to examine this issue in groups of chicks trained on a single-trial, passive-avoidance task. Bilateral intracranial administration of the immunosuppressants FK506 and cyclosporin A (CyA) led to two temporally distinct effects, distinguished by the concentration of drug required and the effective time of administration relative to training. In addition to inhibiting CaN, CyA and FK506 inhibit distinct classes of peptidyl prolyl-cis/trans-isomerases (PPIases). Other agents known to inhibit these enzymes, including the Map kinase inhibitor Rapamycin, also induced memory deficits in a complex, dose- and time-of-administration-dependent, manner. The data fail to conclusively implicate CaN in memory formation, but are consistent with proposals that a phosphatase cascade may participate in an early stage of information storage. PPIases may be required at a later stage to catalyse the folding of new or translocated proteins, the synthesis of which is required for formation of long-term memory, although other possible explanations for the data remain to be investigated.

Long-term depression: a cascade of induction and expression mechanisms

The aims of this paper are to provide a comprehensive and up to date review of the mechanisms of induction and expression of long-term depression (LTD) of synaptic transmission. The review will focus largely on homosynaptic LTD and other forms of LTD will be considered only where appropriate for a fuller understanding of LTD mechanisms. We shall concentrate on what are felt to be some of the most interesting recent findings concerning LTD in the central nervous system. Wherever possible we shall try to consider some of the disparities in results and possible reasons for these. Finally, we shall briefly consider some of the possible functional consequences of LTD for normal physiological function.

FK506 ameliorates the discrimination learning impairment due to preventing the rarefaction of white matter induced by chronic cerebral hypoperfusion in rats

We examined the effects of the immunosuppressant tacrolimus (FK506) on the discrimination learning impairment induced by chronic cerebral hypoperfusion in rats. Chronic cerebral hypoperfusion was prepared by permanent ligation of bilateral common carotid arteries for male Wistar rats aged 9 weeks. FK506 (0.05 mg/kg, s.c.) recovered the learning impairment and also prevented the rarefaction of white matter and striatal neuronal cell damage. Our findings suggest that FK506 ameliorates the learning impairment mainly due to preventing neuropathological alterations.

Memory mechanisms: The yin and yang of protein phosphorylation

Protein phosphorylation has long been known to play a key role in triggering the synaptic changes underlying learning and memory. Recent studies highlight the importance of tightly regulated dephosphorylation as a mechanism controlling the induction of long-term synaptic change and lasting memory.

Inducible and reversible enhancement of learning, memory, and long-term potentiation by genetic inhibition of calcineurin

The threshold for hippocampal-dependent synaptic plasticity and memory storage is thought to be determined by the balance between protein phosphorylation and dephosphorylation mediated by the kinase PKA and the phosphatase calcineurin. To establish whether endogenous calcineurin acts as an inhibitory constraint in this balance, we examined the effect of genetically inhibiting calcineurin on plasticity and memory. Using the doxycycline-dependent rtTA system to express a calcineurin inhibitor reversibly in the mouse brain, we find that the transient reduction of calcineurin activity facilitates LTP in vitro and in vivo. This facilitation is PKA dependent and persists over several days in vivo. It is accompanied by enhanced learning and strengthened short- and long-term memory in several hippocampal-dependent spatial and nonspatial tasks. The LTP and memory improvements are reversed fully by suppression of transgene expression. These results demonstrate that endogenous calcineurin constrains LTP and memory.

Antisense DNA against calcineurin facilitates memory in contextual fear conditioning by lowering the threshold for hippocampal long-term potentiation induction

In the previous study, we demonstrated that the antisense oligodeoxynucleotides against calcineurin Aalpha and Abeta, catalytic subunits of Ca(2+)/calmodulin-dependent protein phosphatase, produce a facilitatory effect on long-term potentiation induction in the hippocampal CA1 region in rats anesthetized with urethane. Here, we have studied how animals, in which the hippocampal long-term potentiation induction is enhanced by antisense oligodeoxynucleotides against calcineurin, perform in learning tasks that depend on hippocampal function. The rats received antisense oligodeoxynucleotides by bilateral ventricular administration via miniosmotic pumps. We tested four groups of rats, three infused with either antisense oligodeoxynucleotides, scramble oligodeoxynucleotides, or saline, and untreated rats, for two types of hippocampus-dependent learning, water maze and contextual fear conditioning. After the behavioral tests, we conducted a long-term potentiation induction test to determine whether long-term potentiation induction was enhanced. In contextual fear conditioning, rats in which long-term potentiation induction was enhanced by antisense oligodeoxynucleotides displayed significantly more conditioned freezing response than control rats. Rats with enhanced long-term potentiation induction showed no differences in shock sensitivity, general activity, or light-dark choice from control rats. In contrast with contextual fear conditioning, rats with enhanced long-term potentiation induction showed no difference in spatial learning performance on the water maze compared with control rats. These results demonstrate that an enhancement in long-term potentiation induction produced by the inhibition of calcineurin leads to an increase in memory strength in specific forms of hippocampus-dependent learning.

Potential role of calcineurin for brain ischemia and traumatic injury

Calcineurin belongs to the family of Ca2+/calmodulin-dependent protein phosphatase, protein phosphatase 2B. Calcineurin is the only protein phosphatase which is regulated by a second messenger, Ca2+. Furthermore, calcineurin is highly localized in the central nervous system, especially in those neurons vulnerable to ischemic and traumatic insults. For these reasons, calcineurin is considered to play important roles in neuron-specific functions. Recently, on the basis of the finding that FK506 and cyclosporin A serve as calcineurin-specific inhibitors, this enzyme has become the subject of much study. It is clear that calcineurin is involved in many neuronal (or non-neuronal) functions such as neurotransmitter release, regulation of receptor functions, signal transduction systems, neurite outgrowth, gene expression and neuronal cell death. In this review, we describe the calcineurin functions, functions of the substrates, and the pathogenesis of traumatic and ischemic insults, and we discuss the potential role of calcineurin. There are many similarities in traumatic and ischemic pathogenesis of the brain in which the release of excessive glutamate is followed by an intracellular Ca2+ increase. However, the intracellular cascade which leads to neuronal cell death after the release of excess Ca2+ is unclear. Although calcineurin is thought to be a key toxic enzyme on the basis of studies using immunosuppressants (FK506 or cyclosporin A), many of the functions of the substrates for calcineurin protect against neuronal cell death. We concluded that calcineurin is a bi-directional enzyme for neuronal cell death, having protective and toxic actions, and the balance of the bi-directional effects may be important in ischemic and traumatic pathogenesis.

Modulation of protein kinases and protein phosphatases by reactive oxygen species: implications for hippocampal synaptic plasticity

1. Reactive oxygen species are known for their role in neurotoxicity. However, recent studies indicate that reactive oxygen species also play a role in cell function under physiological conditions. 2. Both superoxide and hydrogen peroxide alter the activity of various protein kinases and protein phosphatases, some of which are involved in hippocampal synaptic plasticity. Specifically, the activity of protein kinase C, extracellular-regulated kinase 2, and a protein tyrosine kinase(s) is increased in the presence of these reactive oxygen species, whereas the activity of protein phosphatases 2A and 2B, and a protein tyrosine phosphatase(s) is decreased. 3. Protein kinase C, extracellular-regulated kinase 2, and protein tyrosine kinases critically participate in the induction and/or early expression of long-term potentiation at glutamatergic synapses in hippocampus. Protein phosphatases 2A and 2B participate in the induction and/or early expression of long-term depression at these synapses. 4. Treatment of hippocampal slices with scavengers of either superoxide or hydrogen peroxide prevents the full expression of long-term potentiation. Long-term potentiation in hippocampus also is attenuated in transgenic mice that overexpress Cu/Zn superoxide dismutase. 5. The link between reactive oxygen species and long-term potentiation may be the activating effect on protein kinases. The inhibiting effect of reactive oxygen species on protein phosphatases may also contribute to long-term potentiation. 6. The authors hypothesize that reactive oxygen species play a critical role in hippocampal long-term potentiation by favoring the activation of a protein kinase over a protein phosphatase signaling cascade.

A calcineurin inhibitor, FK506, blocks voltage-gated calcium channel-dependent LTP in the hippocampus

The effects of FK506, an immunosuppressant and protein phosphatase 2B (calcineurin) inhibitor, on the voltage-gated calcium channel (VGCC)-dependent long-term potentiation (LTP) were investigated in the CA1 region of mice hippocampal slices. VGCC-dependent LTP was induced either by a brief application of a potassium channel blocker tetraethyleneanmonium (TEA), or by a strong tetanic stimulation under the blockade of NMDA-receptors. FK506 (1-50 microM) produced dose-dependent inhibition on TEA-induced LTP. Cyclosporin A (CysA 50 microM), another calcineurin inhibitor, showed a similar inhibitory effect on TEA-induced LTP. FK506 (10 microM) also blocked the strong tetanus-induced LTP, but had no effect on the post-tetanic potentiation. By using a subthreshold weak tetanic stimulation protocol, we also found that low concentration of FK506 (1 microM) produced neither inhibition nor potentiation on VGCC-dependent LTP. These results showed FK506 and CysA exerted inhibitory effects on VGCC-dependent LTP, and suggest that calcineurin is involved in the processes of this kind of synaptic plasticity.

Preconditioning in vivo ischemia inhibits anoxic long-term potentiation and functionally protects CA1 neurons in the gerbil

Preconditioning with sublethal ischemia induces tolerance to subsequent lethal ischemia in neurons. We investigated electrophysiologic aspects of the ischemic tolerance phenomenon in the gerbil hippocampus. Gerbils were subjected to 2 minutes of forebrain ischemia (preconditioning ischemia). Some of them were subjected to a subsequent 5 minutes of forebrain ischemia 2 to 3 days after the preconditioning ischemia (double ischemia). Hippocampal slices were prepared from these gerbils subjected to the preconditioning or double ischemia, and field excitatory postsynaptic potentials were recorded from CA1 pyramidal neurons. Capacity for long-term potentiation triggered by tetanic stimulation (tetanic LTP) was transiently inhibited 1 to 2 days after the double ischemia but then recovered. Latency of anoxic depolarization was not significantly different between slices from preconditioned gerbils and those from sham-operated gerbils when these slices were subjected to in vitro anoxia. Postanoxic potentiation of N-methyl-D-aspartate (NMDA) receptor-mediated transmission (anoxic LTP) was inhibited in slices from gerbils 2 to 3 days after the preconditioning ischemia, whereas it was observed in slices from sham-operated gerbils and gerbils 9 days after the preconditioning ischemia. These results suggest that protection by induced tolerance is (1) not only morphologic but also functional, and (2) expressed in inhibiting postischemic overactivation of NMDA receptor-mediated synaptic responses.

Regulation of neuronal plasticity in the central nervous system by phosphorylation and dephosphorylation

Neuronal plasticity can be defined as adaptive changes in structure and function of the nervous system, an obvious example of which is the capacity to remember and learn. Long-term potentiation and long-term depression are the experimental models of memory in the central nervous system (CNS), and have been frequently utilized for the analysis of the molecular mechanisms of memory formation. Extensive studies have demonstrated that various kinases and phosphatases regulate neuronal plasticity by phosphorylating and dephosphorylating proteins essential to the basic processes of adaptive changes in the CNS. These proteins include receptors, ion channels, synaptic vesicle proteins, and nuclear proteins. Multifunctional kinases (cAMP-dependent protein kinase, Ca2+/phospholipid-dependent protein kinase, and Ca2+/calmodulin-dependent protein kinases) and phosphatases (calcineurin, protein phosphatases 1, and 2A) that specifically modulate the phosphorylation status of neuronal-signaling proteins have been shown to be required for neuronal plasticity. In general, kinases are involved in upregulation of the activity of target substrates, and phosphatases downregulate them. Although this rule is applicable in most of the cases studied, there are also a number of exceptions. A variety of regulation mechanisms via phosphorylation and dephosphorylation mediated by multiple kinases and phosphatases are discussed.

FK506 and the role of immunophilins in nerve regeneration

FK506 is a new FDA-approved immunosuppressant used for prevention of allograft rejection in, for example, liver and kidney transplantations. FK506 is inactive by itself and requires binding to an FK506 binding protein-12 (FKBP-12), or immunophilin, for activation. In this regard, FK506 is analogous to cyclosporin A, which must bind to its immunophilin (cyclophilin A) to display activity. This FK506-FKBP complex inhibits the activity of the serine/threonine protein phosphatase 2B (calcineurin), the basis for the immunosuppressant action of FK506. The discovery that immunophilins are also present in the nervous system introduces a new level of complexity in the regulation of neuronal function. Two important calcineurin targets in brain are the growth-associated protein GAP-43 and nitric oxide (NO) synthase (NOS). This review focuses on studies showing that systemic administration of FK506 dose-dependently speeds nerve regeneration and functional recovery in rats following a sciatic-nerve crush injury. The effect appears to result from an increased rate of axonal regeneration. The nerve regenerative property of this class of agents is separate from their immunosuppressant action because FK506-related compounds that bind to FKBP-12 but do not inhibit calcineurin are also able to increase nerve regeneration. Thus, FK506's ability to increase nerve regeneration arises via a calcineurin-independent mechanism (i.e., one not involving an increase in GAP-43 phosphorylation). Possible mechanisms of action are discussed in relation to known actions of FKBPs: the interaction of FKBP-12 with two Ca2+ release-channels (the ryanodine and inositol 1,4,5-triphosphate receptors) which is disrupted by FK506, thereby increasing Ca2+ flux; the type 1 receptor for the transforming growth factor-beta (TGF-beta 1), which stimulates nerve growth factor (NGF) synthesis by glial cells, and is a natural ligand for FKBP-12; and the immunophilin FKBP-52/FKBP-59, which has also been identified as a heat-shock protein (HSP-56) and is a component of the nontransformed glucocorticoid receptor. Taken together, studies of FK506 indicate broad functional roles for the immunophilins in the nervous system. Both calcineurin-dependent (e.g., neuroprotection via reduced NO formation) and calcineurin-independent mechanisms (i.e., nerve regeneration) need to be invoked to explain the many different neuronal effects of FK506. This suggests that multiple immunophilins mediate FK506's neuronal effects. Novel, nonimmunosuppressant ligands for FKBPs may represent important new drugs for the treatment of a variety of neurological disorders.

Metaplasticity: a new vista across the field of synaptic plasticity

Over the past 20 years there has been an increasing understanding of the properties and mechanisms underlying long-term potentiation (LTP) and long-term depression (LTD) of synaptic efficacy, putative learning and memory mechanisms in the mammalian brain. More recently, however, it has become apparent that synaptic activity can also elicit persistent neuronal responses not manifest as changes in synaptic strength. Some of these changes may nonetheless modify the ability of synapses to undergo strength changes in response to subsequent episodes of synaptic activity. This kind of activity-dependent modulatory plasticity we have termed "metaplasticity". Metaplasticity has been observed physiologically as an inhibition of LTP and concomitant facilitation of LTD by prior N-methyl-D-aspartate receptor activation or, conversely, a facilitation of LTP induction by prior metabotropic glutamate receptor activation. The examples of metaplasticity described to date are input specific, and last as long as several hours. The mechanisms underlying such phenomena remain to be fully characterized, although some likely possibilities are an altered N-methyl-D-aspartate receptor function, altered calcium buffering, altered states of kinases or phosphatases, and a priming of protein synthesis machinery. While some details vary, experimentally observed metaplasticity bears some similarity to the "sliding threshold" feature of the Bienenstock, Cooper and Munro model of experience-dependent synaptic plasticity. Metaplasticity may serve several functions including (1) providing a way for synapses to integrate a response across temporally spaced episodes of synaptic activity and (2) keeping synapses within a dynamic functional range, and thus preventing them from entering states of saturated LTP or LTD.

Calcineurin regulation of synaptic function: from ion channels to transmitter release and gene transcription

Calcineurin is a calcium (Ca2+)/calmodulin (CaM)-dependent protein phosphatase that has been shown to regulate the activity of ion channels, neurotransmitter and hormone release, synaptic plasticity and gene transcription. At glutamatergic synapses, the inhibition of calcineurin with immunosuppressant drugs has been reported to enhance both the presynaptic release of glutamate and postsynaptic responsiveness. Several other ligand- and voltage-gated ion channels are negatively regulated by calcineurin. Hormone release in insulin-secreting pancreatic beta cells and pituitary corticotrope tumour (AtT20) cells is also negatively regulated by calcineurin. In this article, Jerrel Yakel discusses the evidence that calcineurin plays a vital role in regulating neuronal excitability and hormone release.