Change in a specific phosphoprotein band following associative learning in Hermissenda

Binding of Gd(3+) to the neuronal signalling protein calexcitin identifies an exchangeable Ca(2+)-binding site

Calexcitin was first identified in the marine snail Hermissenda crassicornis as a neuronal-specific protein that becomes upregulated and phosphorylated in associative learning. Calexcitin possesses four EF-hand motifs, but only the first three (EF-1 to EF-3) are involved in binding metal ions. Past work has indicated that under physiological conditions EF-1 and EF-2 bind Mg(2+) and Ca(2+), while EF-3 is likely to bind only Ca(2+). The fourth EF-hand is nonfunctional owing to a lack of key metal-binding residues. The aim of this study was to use a crystallographic approach to determine which of the three metal-binding sites of calexcitin is most readily replaced by exogenous metal ions, potentially shedding light on which of the EF-hands play a `sensory' role in neuronal calcium signalling. By co-crystallizing recombinant calexcitin with equimolar Gd(3+) in the presence of trace Ca(2+), EF-1 was shown to become fully occupied by Gd(3+) ions, while the other two sites remain fully occupied by Ca(2+). The structure of the Gd(3+)-calexcitin complex has been refined to an R factor of 21.5% and an Rfree of 30.4% at 2.2 Å resolution. These findings suggest that EF-1 of calexcitin is the Ca(2+)-binding site with the lowest selectivity for Ca(2+), and the implications of this finding for calcium sensing in neuronal signalling pathways are discussed.

Proteomic analysis of short- and intermediate-term memory in Hermissenda

Changes in cellular and synaptic plasticity related to learning and memory are accompanied by both upregulation and downregulation of the expression levels of proteins. Both de novo protein synthesis and post-translational modification of existing proteins have been proposed to support the induction and maintenance of memory underlying learning. However, little is known regarding the identity of proteins regulated by learning that are associated with the early stages supporting the formation of memory over time. In this study we have examined changes in protein abundance at two different times following one-trial in vitro conditioning of Hermissenda using two-dimensional difference gel electrophoresis (2D-DIGE), quantification of differences in protein abundance between conditioned and unpaired controls, and protein identification with tandem mass spectrometry. Significant regulation of protein abundance following one-trial in vitro conditioning was detected 30 min and 3 h post-conditioning. Proteins were identified that exhibited statistically significant increased or decreased abundance at both 30 min and 3 h post-conditioning. Proteins were also identified that exhibited a significant increase in abundance only at 30 min, or only at 3 h post-conditioning. A few proteins were identified that expressed a significant decrease in abundance detected at both 30 min and 3 h post-conditioning, or a significant decrease in abundance only at 3 h post-conditioning. The proteomic analysis indicates that proteins involved in diverse cellular functions such as translational regulation, cell signaling, cytoskeletal regulation, metabolic activity, and protein degradation contribute to the formation of memory produced by one-trial in vitro conditioning. These findings support the view that changes in protein abundance over time following one-trial in vitro conditioning involve dynamic and complex interactions of the proteome.

Nucleoside triphosphate diphosphohydrolases role in the pathophysiology of cognitive impairment induced by seizure in early age

Studies have shown that seizures in young animals lead to later cognitive deficits. There is evidence that long-term potentiation (LTP) and long-term depression (LTD) might contribute to the neural basis for learning and memory mechanism and might be modulated by ATP and/or its dephosphorylated product adenosine produced by a cascade of cell-surface transmembrane enzymes, such as E-NTPDases (ecto-nucleoside triphosphate diphosphohydrolases) and ecto-5'-nucleotidase. Thus, we have investigated if hippocampal ecto-nucleotidase activities are altered at different time periods after one episode of seizure induced by kainic acid (KA) in 7 days old rats. We also have evaluated if 90 day-old rats previously submitted to seizure induced by KA at 7 days of age presented cognitive impairment in Y-maze behavior task. Our results have shown memory impairment of adult rats (Postnatal day 90) previously submitted to one single seizure episode in neonatal period (Postnatal day 7), which is accompanied by an increased ATP hydrolysis in hippocampal synaptosomes. The metabolism of ATP evaluated by HPLC confirmed that ATP hydrolysis was faster in adult rats treated with KA in neonatal period than in controls. Surprisingly, the mRNA and protein levels as seen by PCR and Western blot, respectively, were not altered by the KA administration in early age. Since we have found an augmented hydrolysis of ATP and this nucleotide seems to be important to LTP induction, we could assume that impairment of memory and learning observed in adult rats which have experienced a convulsive episode in postnatal period may be a consequence of the increased ATP hydrolysis. These findings correlate the purinergic signaling to the cognitive deficits induced by neonatal seizures and contribute to a better understanding about the mechanisms of seizure-induced memory dysfunction.

Proteomic analysis of post-translational modifications in conditioned Hermissenda

Post-translational modifications of proteins are a major determinant of biological function. Phosphorylation of proteins involved in signal transduction contributes to the induction and maintenance of several examples of cellular and synaptic plasticity. In this study we have identified phosphoproteins regulated by Pavlovian conditioning in lysates of Hermissenda nervous systems using two-dimensional electrophoresis (2DE) in conjunction with (32)P labeling, fluorescence based phosphoprotein in-gel staining, and mass spectrometry. Modification of protein phosphorylation regulated by conditioning was first assessed by densitometric analysis of (32)P labeled proteins resolved by 2DE from lysates of conditioned and pseudorandom control nervous systems. An independent assessment of phosphorylation regulated by conditioning was obtained from an examination of 2D gels stained with Pro-Q Diamond phosphoprotein dye. Mass spectrometric analysis of protein digests from phosphoprotein stained analytical gels or Coomassie Blue stained preparative gels provided for the identification of phosphoproteins that exhibited statistically significant increased phosphorylation in conditioned groups as compared to pseudorandom controls. A previously identified cytoskeletal related protein, Csp24 (24 kDa conditioned stimulus pathway phosphoprotein), involved in intermediate-term memory exhibited significantly increased phosphorylation detected 24 h post-conditioning. Our results show that proteins involved in diverse cellular functions such as transcriptional regulation, cell signaling, cytoskeletal regulation, metabolic activity, and protein degradation contribute to long-term post-translational modifications associated with Pavlovian conditioning.

Guanidinoacetate administration increases acetylcholinesterase activity in striatum of rats and impairs retention of an inhibitory avoidance task

Guanidinoacetate methyltransferase deficiency (GAMT-deficiency) is an inborn error of metabolism biochemically characterized by accumulation of guanidinoacetate (GAA) and depletion of creatine; the pathogenesis of brain dysfunction in this disorder is not yet established. In the present study we investigated the effect of intrastriatal administration of GAA on acetylcholinesterase (AChE) activity and on memory acquisition, consolidation and retrieval of step-down inhibitory avoidance task in rat. Results showed that GAA significantly increased AChE activity in rat striatum 30 min (50%) and 3 h (25%), but not 6 h after drug administration. GAA impaired test session performance when applied 30 min before training or after training, and before testing sessions, i.e., impaired memory acquisition, consolidation and retrieval. When injected with a 6 hour interval, GAA affected only memory retrieval. Although the mechanisms of action of GAA on AChE activity and on memory are unclear, these findings suggest that the accumulation of GAA found in patients with GAMT-deficiency may be one of the mechanisms involved in neural dysfunction. Further studies are necessary to evaluate these mechanisms.

PKC signaling deficits: a mechanistic hypothesis for the origins of Alzheimer's disease

There is strong evidence that protein kinase C (PKC) isozyme signaling pathways are causally involved in associative memory storage. Other observations have indicated that PKC signaling pathways regulate important molecular events in the neurodegenerative pathophysiology of Alzheimer's disease (AD), which is a progressive dementia that is characterized by loss of recent memory. This parallel involvement of PKC signaling in both memory and neurodegeneration indicates a common basis for the origins of both the symptoms and the pathology of AD. Here, we discuss this conceptual framework as a basis for an autopsy-validated peripheral biomarker--and for AD drug design targeting drugs (bryostatin and bryologs) that activate PKC isozymes--that has already demonstrated significant promise for treating both AD neurodegeneration and its symptomatic memory loss.

Subcellular, cellular, and circuit mechanisms underlying classical conditioning in Hermissenda crassicornis

A breakthrough for studying the neuronal basis of learning emerged when invertebrates with simple nervous systems, such as the sea slug Hermissenda crassicornis, were shown to exhibit classical conditioning. Hermissenda learns to associate light with turbulence: prior to learning, naive animals move toward light (phototaxis) and contract their foot in response to turbulence; after learning, conditioned animals delay phototaxis in response to light. The photoreceptors of the eye, which receive monosynaptic inputs from statocyst hair cells, are both sensory neurons and the first site of sensory convergence. The memory of light associated with turbulence is stored as changes in intrinsic and synaptic currents in these photoreceptors. The subcellular mechanisms producing these changes include activation of protein kinase C and MAP kinase, which act as coincidence detectors because they are activated by convergent signaling pathways. Pathways of interneurons and motorneurons, where additional changes in excitability and synaptic connections are found, contribute to delayed phototaxis. Bursting activity recorded at several points suggest the existence of small networks that produce complex spatiotemporal firing patterns. Thus, the change in behavior may be produced by a nonlinear transformation of spatiotemporal firing patterns caused by plasticity of synaptic and intrinsic channels. The change in currents and the activation of PKC and MAPK produced by associative learning are similar to those observed in hippocampal and cerebellar neurons after rabbit classical conditioning, suggesting that these represent general mechanisms of memory storage. Thus, the knowledge gained from further study of Hermissenda will continue to illuminate mechanisms of mammalian learning.

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

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

Pavlovian conditioning of Hermissenda: current cellular, molecular, and circuit perspectives

The less-complex central nervous system of many invertebrates make them attractive for not only the molecular analysis of the associative learning and memory, but also in determining how neural circuits are modified by learning to generate changes in behavior. The nudibranch mollusk Hermissenda crassicornis is a preparation that has contributed to an understanding of cellular and molecular mechanisms of Pavlovian conditioning. Identified neurons in the conditioned stimulus (CS) pathway have been studied in detail using biophysical, biochemical, and molecular techniques. These studies have resulted in the identification and characterization of specific membrane conductances contributing to enhanced excitability and synaptic facilitation in the CS pathway of conditioned animals. Second-messenger systems activated by the CS and US have been examined, and proteins that are regulated by one-trial and multi-trial Pavlovian conditioning have been identified in the CS pathway. The recent progress that has been made in the identification of the neural circuitry supporting the unconditioned response (UR) and conditioned response (CR) now provides for the opportunity to understand how Pavlovian conditioning is expressed in behavior.

Training in inhibitory avoidance causes a reduction of Na+,K+-ATPase activity in rat hippocampus

Compelling evidence has indicated the involvement of Na(+),K(+)-ATPase in the mechanisms of synaptic plasticity. In the present study, we investigated the effect of inhibitory avoidance training on Na(+),K(+)-ATPase activity, at different times after training, in the rat hippocampus. Male adult Wistar rats were trained in a step-down inhibitory avoidance task and compared to those submitted to isolated footshock (0.4 mA) or placed directly onto the platform. Na(+),K(+)-ATPase activity decreased, by 60%, in hippocampus of rats sacrificed immediately after the isolated footshock, as well as immediately (0 min) and 6 h after training; this effect was not present 24 h after training. We also verified that enzyme activity was not altered in rats killed after just being on the platform. These findings suggest that Na(+),K(+)-ATPase activity may be involved in the memory consolidation of step-down inhibitory avoidance in the hippocampus.

Inhibition of conditioned stimulus pathway phosphoprotein 24 expression blocks the development of intermediate-term memory in Hermissenda

Studies of memory consolidation have identified multiple phases or stages in the formation of memories. The multiple components of memory can be broadly divided into the three phases; short-term, intermediate-term, and long-term. Although molecular changes underlying short- and long-term memory have been examined extensively, the molecular mechanisms supporting the formation of intermediate-term memory are poorly understood. In several examples of cellular and synaptic plasticity, intermediate memory depends on translation but not transcription. One-trial conditioning in Hermissenda results in the development of intermediate memory that is associated with enhanced cellular excitability and the phosphorylation of a 24 kDa protein referred to as conditioned stimulus pathway phosphoprotein (Csp24). Using amino acid sequences derived from Csp24 peptide fragments, a full-length cDNA was cloned and shown to contain multiple beta-thymosin-like domains. The expression of Csp24 and the development of enhanced excitability, a characteristic of intermediate memory, were blocked by antisense oligonucleotide-mediated downregulation of Csp24 without affecting the induction of immediate enhanced excitability, a characteristic of short-term memory. These results demonstrate that the synthesis of Csp24 is required for the development and maintenance of intermediate memory.

Protein synthesis-dependent and mRNA synthesis-independent intermediate phase of memory in Hermissenda

The conditioned stimulus pathway in Hermissenda has been used to examine the time-dependent mechanisms of memory consolidation following one-trial conditioning. Here we report an intermediate phase of memory consolidation following one-trial conditioning that requires protein synthesis, but not mRNA synthesis. In conditioned animals, enhanced excitability normally expressed during an intermediate phase of memory was reversed by the protein synthesis inhibitor anisomycin, but not by the mRNA synthesis inhibitor 5, 6-dichloro-1-beta-D-ribobenzimidazole (DRB). Associated with the intermediate phase of memory is an increase in the phosphorylation of a 24-kDa protein. Anisomycin present during the intermediate phase blocked the increased phosphorylation of the 24-kDa phosphoprotein, but did not block the increased phosphorylation of other proteins associated with conditioning or significantly change their baseline phosphorylation. DRB did not reverse enhanced excitability or decrease protein phosphorylation expressed during the intermediate phase of memory formation, but it did reverse enhanced excitability 3.5 h after conditioning. Phosphorylation of the 24-kDa protein may support enhanced excitability during the intermediate phase, in the transition period between short- and long-term memory.

Time domains of neuronal Ca2+ signaling and associative memory: steps through a calexcitin, ryanodine receptor, K+ channel cascade

Synaptic changes that underlie associative learning and memory begin with temporally related activity of two or more independent synaptic inputs to common postsynaptic targets. In turn, temporally related molecular events regulate cytosolic Ca2+ during progressively longer-lasting time domains. Associative learning behaviors of living animals have been correlated with changes of neuronal voltage-dependent K+ currents, protein kinase C-mediated phosphorylation and synthesis of the Ca2+ and GTP-binding protein, calexcitin (CE),and increased expression of the Ca2+-releasing ryanodine receptor (type II). These molecular events, some of which have been found to be dysfunctional in Alzheimer's disease, provide means of altering dendritic excitability and thus synaptic efficacy during induction, consolidation and storage of associative memory. Apparently, such stages of behavioral learning correspond to sequential differences of Ca2+ signaling that could occur in spatially segregated dendritic compartments distributed across brain structures, such as the hippocampus.

Ubiquitous molecular substrates for associative learning and activity-dependent neuronal facilitation

Recent evidence suggests that many of the molecular cascades and substrates that contribute to learning-related forms of neuronal plasticity may be conserved across ostensibly disparate model systems. Notably, the facilitation of neuronal excitability and synaptic transmission that contribute to associative learning in Aplysia and Hermissenda, as well as associative LTP in hippocampal CA1 cells, all require (or are enhanced by) the convergence of a transient elevation in intracellular Ca2+ with transmitter binding to metabotropic cell-surface receptors. This temporal convergence of Ca2+ and G-protein-stimulated second-messenger cascades synergistically stimulates several classes of serine/threonine protein kinases, which in turn modulate receptor function or cell excitability through the phosphorylation of ion channels. We present a summary of the biophysical and molecular constituents of neuronal and synaptic facilitation in each of these three model systems. Although specific components of the underlying molecular cascades differ across these three systems, fundamental aspects of these cascades are widely conserved, leading to the conclusion that the conceptual semblance of these superficially disparate systems is far greater than is generally acknowledged. We suggest that the elucidation of mechanistic similarities between different systems will ultimately fulfill the goal of the model systems approach, that is, the description of critical and ubiquitous features of neuronal and synaptic events that contribute to memory induction.

Inhibitory avoidance learning inhibits ectonucleotidases activities in hippocampal synaptosomes of adult rats

Several lines of evidence indicate that ATP may play an important role in Long-Term Potentiation. In this investigation we evaluated the effect of a memory task (step-down inhibitory avoidance) on the synaptosomal ecto-enzymes (ATP diphosphohydrolase and 5'-nucleotidase) involved in the degradation of ATP to adenosine. After the training session, a decrease in the ATPase (40%) and ADPase (29%) activities of ATP diphosphohydrolase as well as was a decrease in 5'-nucleotidase activity (31%) was observed in hippocampal synaptosomes of rats trained and killed immediately after training. In synaptosomes of rats killed 30 minutes after training, a decrease in ATPase activity (28%) was observed. In the test session, no significant changes were observed in the enzyme activities studied. These results provide new information about the activity of ecto-enzymes involved in nucleotide degradation and their possible participation in mechanisms of acquisition and modulation of memory processing.

Phosphorylation of mitogen-activated protein kinase by one-trial and multi-trial classical conditioning

The pathway supporting the conditioned stimulus (CS) is one site of plasticity that has been studied extensively in conditioned Hermissenda. Several signal transduction pathways have been implicated in classical conditioning of this preparation, although the major emphasis has been on protein kinase C. Here we provide evidence for the activation and phosphorylation of a mitogen-activated protein kinase (MAPK) pathway by one-trial and multi-trial conditioning. A one-trial in vitro conditioning procedure consisting of light (CS) paired with the application of 5-HT results in the increased incorporation of 32PO4 into proteins detected with two-dimensional gel electrophoresis. Two of the phosphoproteins have molecular weights of 44 and 42 kDa, consistent with extracellular signal-regulated protein kinases (ERK1 and ERK2). Phosphorylation of the 44 and 42 kDa proteins by one-trial conditioning was inhibited by pretreatment with PD098059, A MEK1 (ERK-Activating kinase) inhibitor. Assays of ERK activity with brain myelin basic protein as a substrate revealed greater ERK activity for the group that received one-trial conditioning compared with an unpaired control group. Western blot analysis of phosphorylated ERK using antibodies recognizing the dually phosphorylated forms of ERK1 and ERK2 showed an increase in phosphorylation after one-trial conditioning compared with unpaired controls. The increased phosphorylation of ERK after one-trial conditioning was blocked by pretreatment with PD098059. Hermissenda that received 10 or 15 conditioning trials showed significant behavioral suppression compared with pseudo-random controls. After conditioning and behavioral testing, the conditioned animals showed significantly greater phosphorylation of ERK compared with the pseudo-random controls. These results show that the ERK-MAPK signaling pathway is activated in Pavlovian conditioning of Hermissenda.

Protein synthesis-dependent memory and neuronal enhancement in Hermissenda are contingent on parameters of training and retention

Following contiguous pairings of light and rotation, light alone elicits a conditioned contraction of Hermissenda's foot, indicative of an associative memory. After a 5-min retention interval, this conditioned response was evident following two or nine (but not one) conditioning trials but persisted for 90 min only after nine trials. In vivo incubation of animals in the protein synthesis inhibitor anisomycin (ANI; 1 microM) did not affect the conditioned response at the 5-min retention interval but significantly attenuated conditioned responding at the 90-min interval even following nine training trials. Deacetylanisomycin (DANI; 1 microM; an inactive form of anisomycin) had no effect on either 5- or 90-min retention. In a companion procedure, groups of isolated nervous systems were exposed to comparable light and rotation pairings, and the B photoreceptors (considered a site of storage for the associative memory) underwent electrophysiological analysis. An increase in neuronal excitability (indexed by depolarizing voltage responses to injected current) in the B photoreceptors paralleled the expression of conditioned responding in intact animals, that is, two training trials produced a short-term increase in excitability that dissipated within 45 min, whereas nine trials produced a persistent (at least 90-min) increase in excitability. In a fmal experiment, isolated nervous systems were exposed to nine training trials, and ANI or DANI was either present in the bathing medium before and during training or was introduced 5 min after training. Following training in ANI, a short-term (5- to 45-min) but not persistent (90-min) increase in excitability in the B photoreceptors was observed. ANI had no effect on either the short-term or persistent increase in excitability if the drug was applied 5 min after the last (ninth) training trial, and DANI had no effect on training-induced increases in excitability at any retention intervals. These results suggest that short-term retention in Hermissenda is protein synthesis independent but that new protein synthesis initiated during or shortly after the training event is necessary for even 90-min retention. Moreover, these results indicate that under some conditions, a critical threshold of training must be exceeded to initiate protein synthesis-dependent retention.

Secondary structure and Ca2+-induced conformational change of calexcitin, a learning-associated protein

Calexcitin/cp20 is a low molecular weight GTP- and Ca2+-binding protein, which is phosphorylated by protein kinase C during associative learning, and reproduces many of the cellular effects of learning, such as the reduction of potassium currents in neurons. Here, the secondary structure of cloned squid calexcitin was determined by circular dichroism in aqueous solution and by Fourier transform infrared spectroscopy both in solution and on dried films. The results obtained with the two techniques are in agreement with each other and coincide with the secondary structure computed from the amino acid sequence. In solution, calexcitin is one-third in alpha-helix and one-fifth in beta-sheet. The conformation of the protein in solid state depends on the concentration of the starting solution, suggesting the occurrence of surface aggregation. The secondary structure also depends on the binding of calcium, which causes an increase in alpha-helix and a decrease in beta-sheet, as estimated by circular dichroism. The conformation of calexcitin is independent of ionic strength, and the calcium-induced structural transition is slightly inhibited by Mg2+ and low pH, while favored by high pH. The switch of calexcitin's secondary structure upon calcium binding, which was confirmed by intrinsic fluorescence spectroscopy and nondenaturing gel electrophoresis, is reversible and occurs in a physiologically meaningful range of Ca2+ concentration. The calcium-bound form is more globular than the apoprotein. Unlike other EF-hand proteins, calexcitin's overall lipophilicity is not affected by calcium binding, as assessed by hydrophobic liquid chromatography. Preliminary results from patch-clamp experiments indicated that calcium is necessary for calexcitin to inhibit potassium channels and thus to increase membrane excitability. Therefore the calcium-dependent conformational equilibrium of calexcitin could serve as a molecular switch for the short term modulation of neuronal activity following associative conditioning.

B-50/GAP-43 phosphorylation and PKC activity are increased in rat hippocampal synaptosomal membranes after an inhibitory avoidance training

Several lines of evidence indicate that protein kinase C (PKC) is involved in long-term potentiation (LTP) and in certain forms of learning. Recently, we found a learning-specific, time-dependent increase in [3H]phorbol dibutyrate binding to membrane-associated PKC in the hippocampus of rats subjected to an inhibitory avoidance task. Here we confirm and extend this observation, describing that a one trial inhibitory avoidance learning was associated with rapid and specific increases in B-50/GAP-43 phosphorylation in vitro and in PKC activity in hippocampal synaptosomal membranes. The increased phosphorylation of B-50/GAP-43, was seen at 30 min (+35% relative to naive or shocked control groups), but not at 10 or 60 min after training. This learning-associated increase in the phosphorylation of B-50/GAP-43 is mainly due to an increase in the activity of PKC. This is based on three different sets of data: 1) PKC activity increased by 24% in hippocampal synaptosomal membranes of rats sacrificed 30 min after training; 2) B-50/GAP-43 immunoblots revealed no changes in the amount of this protein among the different experimental groups; 3) phosphorylation assays, performed in the presence of bovine purified PKC or in the presence of the selective PKC inhibitor CGP 41231, exhibited no differences in B-50/GAP-43 phosphorylation between naive and trained animals. In conclusion, these results support the contention that hippocampal PKC participates in the early neural events of memory formation of an aversively-motivated learning task.

Calexcitin: a signaling protein that binds calcium and GTP, inhibits potassium channels, and enhances membrane excitability

A previously uncharacterized 22-kDa Ca(2+)-binding protein that also binds guanosine nucleotides was characterized, cloned, and analyzed by electrophysiological techniques. The cloned protein, calexcitin, contains two EF-hands and also has homology with GTP-binding proteins in the ADP ribosylation factor family. In addition to binding two molecules of Ca2+, calexcitin bound GTP and possessed GTPase activity. Calexictin is also a high affinity substrate for protein kinase C. Application of calexcitin to the inner surface of inside-out patches of human fibroblast membranes, in the presence of Ca2+ and the absence of endogenous Ca2+/calmodulin kinase type II or protein kinase C activity, reduced the mean open time and mean open probability of 115 +/- 6 pS K+ channels. Calexcitin thus appears to directly regulate K+ channels. When microinjected into molluscan neurons or rabbit cerebellar Purkinje cell dendrites, calexcitin was highly effective in enhancing membrane excitability. Because calexcitin translocates to the cell membrane after phosphorylation, calexcitin could serve as a Ca(2+)-activated signaling molecule that increases cellular excitability, which would in turn increase Ca2+ influx through the membrane. This is also the first known instance of a GTP-binding protein that binds Ca2+.

Surface protein phosphorylation by ecto-protein kinase is required for the maintenance of hippocampal long-term potentiation

During the induction of long-term potentiation (LTP) in hippocampal slices adenosine triphosphate (ATP) is secreted into the synaptic cleft, and a 48 kDa/50 kDa protein duplex becomes phosphorylated by extracellular ATP. All the criteria required as evidence that these two proteins serve as principal substrates of ecto-protein kinase activity on the surface of hippocampal pyramidal neurons have been fulfilled. This phosphorylation activity was detected on the surface of pyramidal neurons assayed after synaptogenesis, but not in immature neurons nor in glial cells. Addition to the extracellular medium of a monoclonal antibody termed mAb 1.9, directed to the catalytic domain of protein kinase C (PKC), inhibited selectively this surface protein phosphorylation activity and blocked the stabilization of LTP induced by high frequency stimulation (HFS) in hippocampal slices. This antibody did not interfere with routine synaptic transmission nor prevent the initial enhancement of synaptic responses observed during the 1-5 min period immediately after the application of HFS (the induction phase of LTP). However, the initial increase in the slope of excitatory postsynaptic potentials, as well as the elevated amplitude of the population spike induced by HFS, both declined gradually and returned to prestimulus values within 30-40 min after HFS was applied in the presence of mAb 1.9. A control antibody that binds to PKC but does not inhibit its activity had no effect on LTP. The selective inhibitory effects observed with mAb 1.9 provide the first direct evidence of a causal role for ecto-PK in the maintenance of stable LTP, an event implicated in the process of learning and the formation of memory in the brain.

Alzheimer and beta-amyloid-treated fibroblasts demonstrate a decrease in a memory-associated GTP-binding protein, Cp20

The two proteins most consistently identified in the brains of patients with Alzheimer disease (AD) have been beta-amyloid and tau, whose roles in the physiology or pathophysiology of brain cells are not fully understood. To identify other protein(s) involved in AD that have been implicated in physiological contexts, we undertook to analyze a specific memory-associated protein, Cp20, in fibroblasts from AD and control donors. Cp20, a GTP-binding protein that is a member of the ADP-ribosylation factor family, was significantly decreased in fibroblasts from AD patients. Normal control fibroblasts exposed to 10 nM beta-amyloid, the same concentration that induced AD-like K+ changes in control fibroblasts, showed a similar decrease in Cp20. Since it has been previously demonstrated that Cp20 is a potent regulator of K+ channels, these findings suggest that changes in this memory-associated protein may explain previously observed differences in AD K+ channels and suggest a pathophysiologic involvement linked to soluble beta-amyloid metabolism that could contribute to the characteristic memory loss of AD.

Characterization of a GTP-binding protein implicated in both memory storage and interorganelle vesicle transport

The phosphorylation state of cp20, a low molecular weight GTP-binding protein that is a high-affinity substrate for protein kinase C, was previously shown to change after associative conditioning of molluscs and mammals and to induce many of the biophysical and structural modifications that accompany memory retention. Here, cp20 was purified from squid optic lobes and biochemically characterized. A monoclonal antibody prepared against squid cp20 reacted with Hermissenda cp20 and a 20-kDa protein in rabbit hippocampus, while a polyclonal antibody also cross-reacted with Sar1p and ADP-ribosylation factor (ARF). A partial peptide sequence of squid cp20 was 50% identical (23/46 amino acids) with Sar1p, a yeast GTP-binding protein involved in vesicle transport, indicating that cp20 is probably a new member of the ARF family. This classification is consistent with our recent demonstration that cp20 affects retrograde movement of intraaxonal organelles or particles and suggests a possible role for particle traffic between intraneuronal organelles in memory acquisition.

The origin of synaptic neuroplasticity: crucial molecules or a dynamical cascade?

What is neuroplasticity and what are its origins? These questions have been the subject of intense theoretical and experimental research in the neurosciences for decades. Basically, the term neuroplasticity refers to the ability of neurons to alter some functional property in response to alterations in input. Traditional definitions, however, are often imprecise and restricted to particular 'model' neural systems. In the present article we will consider several of the most widely studied models of synaptic-level neuroplasticity including alterations in response properties of two types of invertebrate sensory neurons, long-term potentiation (LTP) in mammalian hippocampus and cortex, and ocular dominance shifts in cat visual cortex. While many other forms of neuroplasticity have been studied, these examples typify the diversity of the subject, as well as illustrate our contention that no unitary model of the phenomena is possible for all conditions. This last point is of particular importance for the mammalian literature, since many hypotheses concerning the mechanism(s) underlying the initiation of neuroplasticity have proposed a single crucial molecular element as the primary causal agent. A closer examination of these various hypotheses, in concert to several examples from the invertebrate literature, leads, however, to the conclusion that synaptic neuroplasticity must arise from a series of inter-related molecular events of a particular form, a cascade, in which individual elements may differ radically from system to system. We next provide an overview of our studies of age-dependent regulation of excitatory and inhibitory ionotropic neurotransmitter receptor populations in cortex in response to agonist and depolarizing stimulation. We provide evidence that such regulation for ionotropic receptors is under the control of ionically driven receptor kinase and phosphatase activity which is also age-dependent in function. These data provide the basis for a cascade model of receptor regulation. Based on this qualitative model, we describe a quantitative computer simulation of certain age-dependent stages in the receptor regulatory cascade which may interact to produce LTP-like effects. While such a model is not exclusive, it nevertheless provides a demonstration that elements in the proposed cascade may comprise the necessary and sufficient conditions for some forms of neuroplasticity. We also propose some of the principles underlying our model as a means of unifying much of the diverse phenomenology reported in the literature. Finally, we make a series of explicit predictions which are testable with current experimental techniques.(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstruction of ionic currents in a molluscan photoreceptor

Two-microelectrode voltage-clamp measurements were made to determine the kinetics and voltage dependence of ionic currents across the soma membrane of the Hermissenda type B photoreceptor. The voltage-dependent outward potassium currents, IA and ICa(2+)-K+, the inward voltage-dependent calcium current, ICa2+ and the light-induced current, IIgt, were then described with Hodgkin-Huxley-type equations. The fast-activating and inactivating potassium current, IA, was described by the equation; IA(t) = gA(max)(ma infinity[1-exp(-t/tau ma)])3 x (ha infinity [1-exp(-t/tau ha)] + exp(-t/tau ha)) (Vm-EK), where the parameters ma infinity, ha infinity, tau ma, and tau ha are functions of membrane potential, Vm, and ma infinity and ha infinity are steady-state activation and inactivation parameters. Similarly, the calcium-dependent outward potassium current, ICa(2+)-K+, was described by the equation, ICa(2+)-K+ (t) = gc(max)(mc infinity(VC)(1-exp[-t/tau mc (VC)]))pc (hc infinity(VC) [1-exp(-t/tau hc)] + exp(-t/tau hc(VC)])pc(VC-EK). In high external potassium, ICa(2+)-K+ could be measured in approximate isolation from other currents as a voltage-dependent inward tail current following a depolarizing command pulse from a holding potential of -60 mV. A voltage-dependent inward calcium current across the type B soma membrane, ICa2+, activated rapidly, showed little inactivation, and was described by the equation: ICa2+ = gCa(max) [1 + exp](-Vm-5)/7]-1 (Vm-ECa), where gCa(max) was 0.5 microS. The light-induced current with both fast and slow phases was described by: IIgt(t) = IIgt1 + IIgt2 + IIgt3, IIgti = gIgti [1-exp(- ton/tau mi)] exp(-ton/tau hi)(Vm-EIgti) (i = 1, 2). For i = 3, /Igt(t) = gigt3m33h3(Vm - Eigt3)exp(-ton/Ton) x exp(-tfoff/t Off). Based on these reconstructions of ionic currents, learning-induced enhancement of the long lasting depolarization (LLD) of the photoreceptor'slight response was shown to arise from progressive inactivation of /A, lca2+ -K+, and lCa2+.

Cell specificity of molecular changes during memory storage

The aeolid nudibranch, Hermissenda crassicornis, exhibits Pavlovian conditioning to paired light and rotational stimuli and it has been suggested that protein kinase C(PKC) may play a critical role in the cellular mechanism for this conditioned behavioral response in the B-cell photoreceptor. The present study was designed to further examine learning-specific PKC involvement in identified cellular areas, particularly those in the visual-vestibular network, of the Hermissenda nervous system after Pavlovian conditioning. As used in previous vertebrate studies, the highly specific PKC radioligand, [3H]phorbol-12,13-dibutyrate ([3H]-PDBU), was used to determine the binding characteristics of the molluscan protein receptor considered to be PKC. The binding was specific, saturable, and could be displaced by a soluble diacylglycerol analogue. The binding activity was distributed evenly between the cytosol and the membrane. All of these analyses suggest that [3H]PDBU binds primarily to PKC in Hermissenda as it does in many other systems. Computerized grain image analysis was then used to determine the cellular localization of PKC as a function of Pavlovian conditioning. The medial and intermediate B photoreceptor and the optic ganglion showed significantly increased [3H]PDBU binding in conditioned animals. The present results provide the first report of an associative learning change of a key signal transduction component in identified neurons.

Classical conditioning-induced changes in low-molecular-weight GTP-binding proteins in rabbit hippocampus

Classical conditioning of Hermissenda, involving paired light-rotation events, results in a 30-35% decrease in the levels of a 20-kDa G protein (cp20). To test whether a similar protein exists in vertebrates, rabbits were trained to associate a tone with periorbital electrical stimulation and G proteins were analyzed by photoaffinity labeling with [alpha-32P]GTP-azidoanilide. A 20-kDa G protein similar to cp20 decreased by 36% in the hippocampus of rabbits subjected to paired tone and electrical stimulation, but not in unpaired controls. Learning-specific decreases were also found in the amount of ras protein.

Outgrowths from Hermissenda photoreceptor somata are associated with activation of protein kinase C

We have found changes in the morphology of photoreceptor somata from the mollusc Hermissenda that are produced by application of 12,13-phorbol dibutyrate (PDBU), an activator of PKC, in combination with elevated intracellular Ca2+ levels. The changes in morphology were expressed as rapid and repetitive outgrowths and additionally as more general changes in shape of the soma. Application of 4 alpha-PMA, a phorbol ester which does not activate PKC, did not produce these changes. The functional integrity of the photoreceptors in these dissociated eye preparations was maintained throughout the period of incubation with PDBU according to standard electrophysiological criteria. It has previously been shown that classical conditioning produced a reduction of dendritic volume in the type B photoreceptor of Hermissenda, a specific locus for associative memory storage. These changes in dendritic morphology were correlated with increased resistance across the cell membrane caused by learning-induced reductions of outward somatic K+ currents. Such conditioning-specific reductions of somatic K+ currents appear to depend on the phosphorylation of a 20-kDa G-protein (CP20) mediated by the Ca2+ and phospholipid-dependent kinase, protein kinase C (PKC). Thus PKC activity may be important in structural changes of the synaptic region of specific neurons involved in associative memory. The results of the present study suggest that the effects of PKC activation may also include structural changes in the soma of these same neurons.

Isolation of a G protein that is modified by learning and reduces potassium currents in Hermissenda

In Hermissenda crassicornis conditioned to associate light and rotation, type B photoreceptor neurons exhibit pairing-specific decreases in the potassium currents IA and IK-Ca, which account for many of the behavioral changes elicited by associative conditioning. To determine which proteins are involved in storage of this memory, high-performance liquid chromatography was used to examine proteins from Hermissenda eyes. Conditioning-specific changes in four phosphoproteins were observed 24 hours after conditioning. One of these proteins, cp20, was purified to apparent homogeneity and found to be a G protein. When injected back into Hermissenda type B cells, cp20 reduced IK and IK-Ca in a manner indistinguishable from the reduction caused by conditioning, suggesting that this protein may play a crucial role in memory acquisition or retention.

Isolation of a G protein that is modified by learning and reduces potassium currents in Hermissenda

In Hermissenda crassicornis conditioned to associate light and rotation, type B photoreceptor neurons exhibit pairing-specific decreases in the potassium currents IA and IK-Ca, which account for many of the behavioral changes elicited by associative conditioning. To determine which proteins are involved in storage of this memory, high-performance liquid chromatography was used to examine proteins from Hermissenda eyes. Conditioning-specific changes in four phosphoproteins were observed 24 hours after conditioning. One of these proteins, cp20, was purified to apparent homogeneity and found to be a G protein. When injected back into Hermissenda type B cells, cp20 reduced IK and IK-Ca in a manner indistinguishable from the reduction caused by conditioning, suggesting that this protein may play a crucial role in memory acquisition or retention.

Contraction of neuronal branching volume: an anatomic correlate of Pavlovian conditioning

Associative memory of the mollusc Hermissenda crassicornis, previously correlated with changes of specific K+ currents, protein phosphorylation, and increased synthesis of mRNA and specific proteins, is here shown to be accompanied by macroscopic alteration in the structure of a single identified neuron, the medial type B photoreceptor cell. Four to five days after training, terminal arborizations of B cells iontophoretically injected with Ni2+ ions and then treated with rubeanic acid were measured with charge-coupled device (CCD)-digitized pseudocolor images of optical sections under "blind" conditions. Boundary volumes enclosing medial-type B-cell arborizations from classically conditioned animals were unequivocally reduced compared with volumes for naive animals or those trained with unpaired stimuli. Branch volume magnitude was correlated with input resistance of the medial type B-cell soma. Such associative learning-induced structural changes may share function with "synapse elimination" described in developmental contexts.

Properties of a protein kinase C activity in synaptic plasma membrane and postsynaptic density fractions isolated from canine cerebral cortex

Protein kinase C (PKC) activity (phosphorylation increased by addition of Ca2+/phosphatidylserine or Ca2+/phosphatidylserine/phorbol ester) was found in both a synaptic plasma membrane (SPM) and a postsynaptic density (PSD) fraction. The SPM fraction had as endogenous substrates 87K-, 60K-, 50K-, and 20K-Mr proteins, whereas the PSD fraction had only the 20K-Mr protein. The PKC activity was also detected using histone III-S as a substrate, in SPM but much less in PSD. Phosphorylations of histone and the endogenous substrates of PKC, assayed in the absence of Ca2+, were enhanced in the SPM prepared after treatment of brain homogenate with phorbol 12-myristate 13-acetate (TPA), but very little enhancement was found in PSD after such treatment. The SPM PKC activity (both for endogenous substrate proteins and for histone), which was enhanced by TPA treatment of brain homogenate, was inhibited by calcium (IC50, 3 x 10(-7) M). The phosphorylations of the 20K-Mr protein in PSD, and in SPM prepared with and without TPA treatment, were all inhibited by H-7. The 20K-Mr protein in the PSD fraction is also phosphorylated by a PSD Ca2+/calmodulin-dependent protein kinase II. The evidence indicates that both SPM and PSD fractions contain a PKC activity. Detergent treatment of SPM, to produce a purified PSD fraction, results in a PSD fraction that has lost most of the endogenous substrates, lost the TPA-induced enhanced activity assayed in the absence of Ca2+, and lost the inhibitory effect of low Ca2+ concentration.

Serotonin-stimulated biochemical events in the procerebrum of Limax

1. The procerebrum (PC) of the terrestrial slug Limax maximus is of interest as a potential site of olfactory information processing (Gelperin et al., 1989). The neuromodulator serotonin is present in the procerebrum and can elicit action potentials from cultured procerebral neurons. We have investigated the effects of serotonin on second-messenger signaling systems and protein phosphorylation as a prelude to studies on long-term synaptic plasticity in the Limax procerebral lobe. 2. We found that several biochemical changes are triggered within 20 min of adding serotonin to the isolated procerebral lobe: adenylate cyclase is activated, protein phosphorylation and synthesis are modulated, and phosphatidylinositol-metabolism is stimulated. 3. Serotonin causes a rapid synthesis of cAMP, reaching a 20- to 30-fold increase within 1 min. Serotonin affects the rate of phosphorylation of several proteins, detected after a brief (20-min) incubation of the procerebral lobe in [32P]phosphate-containing medium. The level of synthesis of several proteins is altered by serotonin, as determined by alterations in [35S]methionine incorporation during a 20-min incubation. Serotonin also causes a slow accumulation of inositoltrisphosphate. 4. Our study shows that within a short time (less than 20 min) serotonin can influence several second-messenger signaling systems and the functional state and abundance of proteins in the procerebral lobe. These serotonin-stimulated events should have direct consequences for intercellular communication in the odor-processing network of the procerebral lobe.

Diazepam inhibits calcium, calmodulin-dependent protein kinase in primary astrocyte cultures

The effect of the anticonvulsants diazepam, phenytoin, and valproic acid on calcium, calmodulin-dependent protein phosphorylation in astrocytes was investigated. We found that diazepam inhibited calcium, calmodulin-stimulated phosphorylation in both supernatant and membrane fractions from primary cultures of rat astrocytes, whereas phenytoin and valproic acid (50-500 microM) had little to no effect. Phosphate incorporation in several protein bands, including the major substrates of 59 and 53 kDa, was inhibited by diazepam. A decrease in phosphate incorporation in these crude preparations was observed at 25 microM diazepam and 50% inhibition was attained at about 100 microM. Dibutyryl cyclic AMP-treated astrocytes were enriched in the 59 kDa phosphoprotein; this band was particularly sensitive to diazepam in these cells. These results indicate that diazepam is capable of inhibiting calcium, calmodulin-dependent protein kinase activity in astrocytes, thereby suggesting a possible site of diazepam action and a potential mechanism for a role of astrocytes in epileptogenesis.

Learning-induced activation of protein kinase C. A molecular memory trace

PKC activation has been shown to mimic the biophysical consequences of classical conditioning in both rabbit hippocampus and Hermissenda type B cells. Furthermore, conditioning in rabbits results in the 24 h translocation of PKC from cytosol to membrane, which is probably responsible for mediating the biophysical consequences of conditioning. A model has been presented that suggests that long-term translocation of PKC occurs via the synergistic activation of a DG dependent pathway that activates PKC and a calcium dependent pathway that activates CaM kinase. Translocation of PKC to the plasma membrane, by altering ion channel properties, could subserve memory lasting for days, whereas translocation to the nuclear membrane could induce cellular change, by genomic regulation, lasting beyond days. We are, therefore, suggesting that protein kinase C may play a critical role in the formation of short, intermediate, and long-term associative memory.

Specific protein changes during memory acquisition and storage

Changes in several distinct types of neuronal proteins are now known to be associated with learning. In this review, we will summarize the properties of these proteins and relate these properties to prominent theories of the biochemical basis of memory.

Prolonged RNA changes in the Hermissenda eye induced by classical conditioning

The incorporation of 32P into mRNA and the total amount of mRNA were increased 3- to 4-fold in eyes isolated from Hermissenda crassicornis trained to associate light with rotation on a turntable compared with animals trained with equal numbers of light and rotation events presented randomly and with naive animals. Incorporation of 32P into poly(A)- RNA was reduced by as much as 60%. The RNA changes were strongly correlated with the degree of learning and could not be accounted for by changes in [32P]ATP content. The RNA changes were maximal at 24 hr and were still detectable after 4 days, indicating that associative conditioning produces a period of increased DNA transcription that could be an intermediate step in memory consolidation. The RNA changes may in part account for recently observed conditioning-specific changes in the synthesis rates of specific proteins.

Regulation of Hermissenda K+ channels by cytoplasmic and membrane-associated C-kinase

Pharmacologic activation of endogenous protein kinase C (PKC) together with elevation of the intracellular Ca2+ level was previously shown to cause reduction of two voltage-dependent K+ currents (IA and ICa2+-K+) across the soma membrane of the type B photoreceptor within the eye of the mollusc Hermissenda crassicornis. Similar effects were also found to persist for days after acquisition of a classically conditioned response. Also, the state of phosphorylation of a low-molecular-weight protein was changed only within the eyes of conditioned Hermissenda. To examine the role of PKC in causing K+ current changes as well as changes of phosphorylation during conditioning (and possibly other physiologic contexts), we studied here the effects of endogenous PKC activation and exogenous PKC injection on phosphorylation and K+ channel function. Several phosphoproteins (20, 25, 56, and 165 kilodaltons) showed differences in phosphorylation in response to PKC activators applied to intact nervous systems or to isolated eyes. Specific differences were observed for membrane and cytosolic fractions in response to both the phorbol ester 12-deoxyphorbol 13-isobutyrate 20-acetate (DPBA) or exogenous PKC in the presence of Ca2+ and phosphatidylserine/diacylglycerol. Type B cells pretreated with DPBA responded to PKC injection with a persistent reduction of K+ currents. In the absence of DPBA, PKC injection also caused K+ current reduction only following Ca2+ loading conditions. However, the direct effect of PKC injection in the absence of DPBA was only to increase ICa2+-K+. According to a proposed model, the amplitude of the K+ currents would depend on the steady-state balance of effects mediated by PKC within the cytoplasm and membrane-associated PKC. The model further specifies that the effects on K+ currents of cytoplasmic PKC require an intervening proteolytic step. Such a model predicts that increasing the concentration of cytoplasmic protease, e.g., with trypsin, will increase K+ currents, whereas blocking endogenous protease, e.g., with leupeptin, will decrease K+ currents. These effects should be opposed by preexposure of the cells to DPBA. Furthermore, prior injection of leupeptin should block or reverse the effects of subsequent injection of PKC into the type B cell. All of these predictions were confirmed by results reported here. Taken together, the results of this and previous studies suggest that PKC regulation of membrane excitability critically depends on its cellular locus. The implications of such function for long-term physiologic transformations are discussed.

Classical conditioning induces long-term translocation of protein kinase C in rabbit hippocampal CA1 cells

The role of the Ca2+/phospholipid-dependent, diacylglycerol-activated enzyme protein kinase C (PKC) in rabbit eyelid conditioning was examined. PKC was partially purified from the CA1 region of hippocampal slices from naive, pseudoconditioned, and conditioned rabbits 24 hr after the rabbits were well conditioned. Crude membrane and cytosol fractions were prepared. In conditioned rabbits, significantly more PKC activity (63.3%) was associated with the membrane fraction (and significantly less with the cytosol fraction) compared to naive (42.0%) and pseudoconditioned (44.7%) animals. These differences in distribution of enzyme activity were paralleled by differences in stimulation of enzyme activity by Ca2+, phospholipid, and diacylglycerol. There were no between-group differences in basal protein kinase activity. These results suggest that there is a long-term translocation of PKC from cytosol to membrane as a result of conditioning. Autoradiographic binding of radioactive phorbol 12,13-dibutyrate to PKC demonstrated that almost all specific binding was in the stratum radiatum, a region containing the proximal apical dendrites of CA1 pyramidal neurons. Therefore, this may be the site of the conditioning-specific PKC translocation, a locus well-suited to underlie the biophysical effects of conditioning.

Transient and persistent depolarization-induced changes of protein phosphorylation in a molluscan nervous system

Phosphoproteins in the CNS of the nudibranch mollusc, Hermissenda crassicornis, were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. After preincubation in artificial sea-water containing 32P, nervous systems were exposed to elevation of external K+ (100 or 300 mM) for a period (e.g., 30 min) approximating a period of depolarization which occurs during classical conditioning. Elevated external K+ was found to change the state of phosphorylation of three distinct proteins (Mr 56,000, 25,000, and 20,000) in three distinct ways without consistently changing that of any other proteins. Phosphorylation of an Mr 56,000 protein was increased by high K+ about twofold only in the presence of external Ca2+ [( Ca2+]o). Phosphorylation of Mr 25,000 protein, on the other hand, was decreased up to 10-fold by high K+, irrespective of the level of [Ca2+]o. The effect of depolarization on Mr 25,000 protein phosphorylation most likely represents dephosphorylation rather than proteolysis. This interpretation is consistent with the observations that (a) reappearance of the Mr 25,000 protein occurred in the presence of the protein synthesis inhibitors cycloheximide, puromycin, or anisomycin, and (b) the Hermissenda nervous system apparently contains a NaF- and EDTA-sensitive protein phosphatase capable of dephosphorylating Mr 25,000 protein. High K+ also reduced Mr 20,000 protein phosphorylation which was dependent on [Ca2+]o even in normal low K+ (10 mM) medium. Removal of [Ca2+]o enhanced reduction of Mr 20,000 phosphorylation due to the high K+ treatment. Interestingly, reduction of the Mr 25,000 protein phosphorylation was long-lasting, i.e., its phosphorylation did not fully recover to a control level for at least 30 min after the high K+ conditions had been removed.(ABSTRACT TRUNCATED AT 250 WORDS)

A spatial-temporal model of cell activation

A spatial-temporal model of calcium messenger function is proposed to account for sustained cellular responses to sustained stimuli, as well as for the persistent enhancement of cell responsiveness after removal of a stimulus, that is, cellular memory. According to this model, spatial separation of calcium function contributes to temporal separation of distinct phases of the cellular response. At different cellular sites, within successive temporal domains, the calcium messenger is generated by different mechanisms and has distinct molecular targets. In particular, prolonged cell activation is brought about by the interaction of calcium with another spatially confined messenger, diacylglycerol, to cause the association of protein kinase C with the plasma membrane. Activity of the membrane-associated protein kinase C is controlled by the rate of calcium cycling across the plasma membrane. In some instances, a single stimulus induces both protein kinase C activation and calcium cycling and thus causes prolonged activation; but in others, a close temporal association of distinct stimuli brings about cell activation via interaction of these intracellular messengers. Persistent enhancement of cell responsiveness after removal of stimuli is suggested to be due to the continued association, or anchoring, of protein kinase C to the membrane.

Hyperammonemia causes altered protein phosphorylation in astrocytes

Treatment of primary astrocyte cultures with ammonium chloride for one day prior to phosphoprotein labeling resulted in a reduction in phosphate incorporation in a 66-kDa protein. Increasing ammonium chloride concentrations (2, 5, and 10 mM) led to greater reductions in phosphate incorporation in this band. The specificity of the effect was indicated by the lack of change in phosphate incorporation in 7 other protein bands. These results indicate that protein phosphorylation can be affected by pathophysiological concentrations of ammonia and suggest that altered protein phosphorylation may be related to the pathogenesis of disorders such as hepatic encephalopathy and Reye's syndrome where ammonia has been implicated as an important etiological factor.