The presence and functions of calmodulin in the postsynaptic density

Role of CA2+/calmodulin on ethanol neurobehavioral effects

RATIONALE

The cAMP-dependent protein kinase A (PKA) signaling transduction pathway has been shown to play an important role in the modulation of several ethanol-induced behaviors. Different studies have demonstrated intracellular calcium (Ca(2+))-dependent activation of the PKA cascade after ethanol administration. Thus, the cAMP cascade mediator Ca(2+)-dependent calmodulin (CaM) has been strongly implicated in the central effects of ethanol.

OBJECTIVES

In this study, we assessed the role of the CaM inhibitor W7 on ethanol-induced stimulation, ethanol intake, and ethanol-induced activation of PKA.

METHODS

Swiss mice were pretreated with W7 (0-10 mg/kg) 30 min before ethanol (0-3.75 g/kg) administration. Immediately, animals were placed during 20 min in an open-field chamber. Ethanol (10 %, v/v) intake in 2 h was assessed using a limited access paradigm. Experiments with caffeine (0-15 mg/kg), cocaine (0-4 mg/kg), and saccharine (0.1 %, w/v) were designed to compare their results to those obtained with ethanol. Western blot was assayed 45 min after ethanol administration.

RESULTS

Results showed that pretreatment with W7, reduced selectively in a dose-dependent fashion ethanol-induced locomotor stimulation and ethanol intake. The ethanol-induced activation of PKA was also prevented by W7 administration.

CONCLUSIONS

These results demonstrate that CaM inhibition resulted in a selective reduction of ethanol-stimulating effects and ethanol intake. The PKA activation induced by ethanol was blocked after the CaM blockade with W7. These results provide further evidence of the key role of cellular Ca(2+)-dependent pathways on the central effects of ethanol.

Neurogranin targets calmodulin and lowers the threshold for the induction of long-term potentiation

Calcium entry and the subsequent activation of CaMKII trigger synaptic plasticity in many brain regions. The induction of long-term potentiation (LTP) in the CA1 region of the hippocampus requires a relatively high amount of calcium-calmodulin. This requirement is usually explained, based on in vitro and theoretical studies, by the low affinity of CaMKII for calmodulin. An untested hypothesis, however, is that calmodulin is not randomly distributed within the spine and its targeting within the spine regulates LTP. We have previously shown that overexpression of neurogranin enhances synaptic strength in a calmodulin-dependent manner. Here, using post-embedding immunogold labeling, we show that calmodulin is not randomly distributed, but spatially organized in the spine. Moreover, neurogranin regulates calmodulin distribution such that its overexpression concentrates calmodulin closer to the plasma membrane, where a high level of CaMKII immunogold labeling is also found. Interestingly, the targeting of calmodulin by neurogranin results in lowering the threshold for LTP induction. These findings highlight the significance of calmodulin targeting within the spine in synaptic plasticity.

The Ca2+/calmodulin system in neuronal hyperexcitability

Calmodulin (CaM) is a major Ca2+-binding protein in the brain, where it plays an important role in the neuronal response to changes in the intracellular Ca2+ concentration. Calmodulin modulates numerous Ca2+-dependent enzymes and participates in relevant cellular functions. Among the different CaM-binding proteins, the Ca2+/CaM dependent protein kinase II and the phosphatase calcineurin are especially important in the brain because of their abundance and their participation in numerous neuronal functions. Therefore, the role of the Ca2+/CaM signalling system in different neurotoxicological or neuropathological conditions associated to alterations in the intracellular Ca2+ concentration is a subject of interest. We here report different evidences showing the involvement of CaM and the CaM-binding proteins above mentioned in situations of neuronal hyperexcitability induced by convulsant agents. Signal transduction pathways mediated by specific CaM binding proteins warrant future study as potential targets in the development of new drugs to inhibit convulsant responses or to prevent or attenuate the alterations in neuronal function associated to the deleterious increases in the intracellular Ca2+ levels described in different pathological situations.

The Ca2+/calmodulin signaling system in the neural response to excitability. Involvement of neuronal and glial cells

Ca2+ plays a critical role in the normal function of the central nervous system. However, it can also be involved in the development of different neuropathological and neurotoxicological processes. The processing of a Ca2+ signal requires its union with specific intracellular proteins. Calmodulin is a major Ca(2+)-binding protein in the brain, where it modulates numerous Ca(2+)-dependent enzymes and participates in relevant cellular functions. Among the different calmodulin-binding proteins, the Ca2+/calmodulin-dependent protein kinase II and the phosphatase calcineurin are especially important in the brain because of their abundance and their participation in numerous neuronal functions. We present an overview on different works aimed at the study of the Ca2+/calmodulin signalling system in the neural response to convulsant agents. Ca2+ and calmodulin antagonists inhibit the seizures induced by different convulsant agents, showing that the Ca2+/calmodulin signalling system plays a role in the development of the seizures induced by these agents. Processes occurring in association with seizures, such as activation of c-fos, are not always sensitive to calmodulin, but depend on the convulsant agent considered. We characterized the pattern of expression of the three calmodulin genes in the brain of control mice and detected alterations in specific areas after inducing seizures. The results obtained are in favour of a differential regulation of these genes. We also observed alterations in the expression of the Ca2+/calmodulin-dependent protein kinase II and calcineurin after inducing seizures. In addition, we found that reactive microglial cells increase the expression of calmodulin and Ca2+/calmodulin-dependent protein kinase II in the brain after seizures.

Brain myosin-V, a calmodulin-carrying myosin, binds to calmodulin-dependent protein kinase II and activates its kinase activity

Myosin-V, an unconventional myosin, has two notable structural features: (i) a regulatory neck domain having six IQ motifs that bind calmodulin and light chains, and (ii) a structurally distinct tail domain likely responsible for its specific intracellular interactions. Myosin-V copurifies with synaptic vesicles via its tail domain, which also is a substrate for calmodulin-dependent protein kinase II. We demonstrate here that myosin-V coimmunoprecipitates with CaM-kinase II from a Triton X-100-solubilized fraction of isolated nerve terminals. The purified proteins also coimmunoprecipitate from dilute solutions and bind in overlay experiments on Western blots. The binding region on myosin-V was mapped to its proximal and medial tail domains. Autophosphorylated CaM-kinase II binds to the tail domain of myosin-V with an apparent Kd of 7.7 nM. Surprisingly, myosin-V activates CaM-kinase II activity in a Ca2+-dependent manner, without the need for additional CaM. The apparent activation constants for the autophosphorylation of CaM-kinase II were 10 and 26 nM, respectively, for myosin-V versus CaM. The maximum incorporation of 32P into CaM-kinase II activated by myosin-V was twice that for CaM, suggesting that myosin-V binding to CaM-kinase II entails alterations in kinetic and/or phosphorylation site parameters. These data suggest that myosin-V, a calmodulin-carrying myosin, binds to and delivers CaM to CaM-kinase II, a calmodulin-dependent enzyme.

NMDA-R1 subunit of the cerebral cortex co-localizes with neuronal nitric oxide synthase at pre- and postsynaptic sites and in spines

The majority of nitric oxide's (NO) physiologic and pathologic actions in the brain has been linked to NMDA receptor activation. In order to determine how the NO-synthesizing enzyme within brain, neuronal NO synthase (nNOS), and NMDA receptors are functionally linked, previous studies have used in situ hybridization techniques in combination with light microscopic immunocytochemistry to show that the two are expressed within single neurons. However, this light microscopic finding does not guarantee that NMDA receptors are distributed sufficiently close to nNOS within single neurons to allow direct interaction of the two. Thus, in this study, dual immuno-electron microscopy was performed to determine whether nNOS and NMDA receptors co-exist within fine neuronal processes. We show that nNOS and the obligatory subunit of functional NMDA receptors, i.e. the NMDA-R1, co-exist within dendritic shafts, spines and terminals of the adult rat visual cortex. Axon terminals form asymmetric synaptic junctions with the dually labeled dendrites, suggesting that the presynaptic terminals release glutamate. Axons and dendrites expressing one without the other also are detected. These results indicate that it is possible for the generation of NO to be temporally coordinated with glutamatergic synaptic transmission at axo-dendritic and axo-axonic junctions and that NO may be generated independently of glutamatergic synaptic transmission. Together, our observations point to a greater complexity than previously recognized for glutamatergic neurotransmission, based on the joint versus independent actions of NO relative to NMDA receptors at pre- versus postsynaptic sites.

Developmental expression of calmodulin mRNA and protein in regions of the postnatal rat brain

The expression of calmodulin (CaM) protein and mRNA was analyzed in specific regions of the rat brain during postnatal development. CaM levels in the adult brain were more abundant in the cerebral hemispheres and thalamus compared to brain stem and superior plus inferior colliculus. All brain regions contained higher CaM protein and mRNA levels than in non-neural tissues such as the kidney. During postnatal development of the brain, maximal levels of CaM protein and CaM I mRNAs were attained at day 10 or 15. Protein levels declined thereafter in the adult in all regions except the thalamus. With respect to products of the rat CaM I gene, the 4.0 kb neural transcript demonstrated a pronounced increase during postnatal development, whereas the 1.8 kb message showed little change.

Effect of hexachlorocyclohexane isomers on calmodulin mRNA expression in the central nervous system

Three different calmodulin genes that encode the same protein have been found in the brain of all mammalian species so far examined. Little is known about the factors involved in regulating the expression of this gene family in the central nervous system. We have investigated the possibility of differential expression of two calmodulin genes, CaM I and CaM II, which are expressed strongly in neuronal cells in the adult rat brain, after treatment with the gamma (lindane) and the delta isomers of the hexachlorocyclohexane (HCH). In this study a decrease of CaM I mRNA (mainly in the 4.0 kb transcript) was found in the cortex of the rats after 24 h of isomer administration. CaM I expression seemed to be more sensitive to delta isomer action, whereas the gamma isomer acted mainly at CaM II level. The levels of mRNA of calmodulin CaM II gene were also found to decrease after lindane administration; delta-HCH produced an increase of this transcript. These results were obtained by Northern blot analysis and confirmed by means of in situ hybridization. Our results suggest that levels of neuronal calmodulin mRNA species are modified in response to changes in neuronal activity.

Plasticity of complex terminals in lamina II in partially deafferented spinal cord: the cat spared root preparation

Projections to the dorsal horn change in adult mammals in response to complete or partial deafferentation. The number of synaptic terminals remains constant after complete lumbosacral deafferentation, indicating replacement of lost dorsal root terminals by newly formed terminals from spared intrinsic systems. The density of a spared central projection of a dorsal root is increased in dorsal horn after partial deafferentation, consistent with sprouting by the axons in the spared root. In this study, we have used electron microscopy to study morphological changes in a specific class of terminals in the dorsal horn induced by partial deafferentation. Complex terminals (CTs) in the dorsal horn originate exclusively from dorsal roots and are readily distinguished morphologically. The CTs and the postsynaptic densities (PSDs) associated with CTs were measured in lamina II at L5 and L6 in cats subjected to unilateral spared root (L6) dorsal rhizotomies and compared to CTs in the control side. Acutely following partial deafferentation, the number of CT profiles decreased. At more chronic survivals, the number of CT profiles were restored to normal levels, and both the number and the length of PSDs were increased. The changes in CTs and PSDs suggest sprouting and synaptogenesis by the spared dorsal root fibers that produce changes in the postsynaptic neuron. Spared root deafferentation thus elicits compensatory changes in presynaptic terminals of the spared root and also in their postsynaptic target neurons.

Modulation of a neuronal calmodulin mRNA species in the rat brain stem by reserpine

Reserpine evokes transsynaptic impulse activity by depleting catecholaminergic neurotransmitters in the rat brain. Previous studies suggest a relationship between catecholaminergic activity and calmodulin concentration. In this report we employ Northern blot analysis to examine the effect of a single subcutaneous injection of reserpine on levels of calmodulin mRNA species which are preferentially expressed in neurons of the rat brain. Regional differences in mRNA levels were also investigated by in situ hybridization and drug-induced changes were noted particularly in specific regions of the rat brain stem. The riboprobe used in the in situ hybridization study recognized a 4.0 kilobase neuronal calmodulin mRNA species (NGB1), which was derived from the rat CaM1 gene. A calmodulin radio-immunoassay was utilized to demonstrate a drug-induced increased in calmodulin protein levels in a region which included the brain stem.

Developmental expression of neuronal calmodulin mRNA species in the rat brain analyzed by in situ hybridization

The temporal and spatial distribution of calmodulin mRNAs which are preferentially expressed in neurons was determined during postnatal development of rat central nervous system. Expression of these mRNAs was strongly detected in the developing neocortex, hippocampus, and cerebellum. Differences in the pattern of expression of a 1.8 and 4.0 kb neuronal calmodulin mRNA species were identified in the developing cerebellum. Expression of the smaller mRNA appeared to correlate with proliferating and developing cerebellar granule neurons while the larger mRNA was present in the mature granule neuron population. A transient elevation in the neuronal calmodulin mRNA species was observed in the superior and inferior colliculus and in the thalamus at postnatal days 5 and 10.

Molecular cloning of calmodulin mRNA species which are preferentially expressed in neurons in the rat brain

A cDNA clone designated NGB, which was isolated from a rat brain expression library, detected two mRNA species of 1.8 and 4.0 kb which are highly enriched in brain tissue. cDNAs NGB1 and NGB2 corresponding to these two mRNAs have been isolated and characterized. Sequence data showed that both mRNA species contain the same open reading frames but differ in their 3' untranslated regions. The open reading frame encodes a calmodulin protein of 148 amino acids. Both mRNA species are derived from the rat CaMI gene by utilization of different polyadenylation addition sites. Analysis of the 3' untranslated sequence which is unique to the larger mRNA species revealed a putative AU-rich 'destabilizer' sequence which is thought to be involved in mechanisms of selective mRNA breakdown. In situ hybridization studies revealed that the two calmodulin mRNAs are expressed strongly in neuronal cells in the adult rat brain. Levels of the two mRNA species increased during early postnatal development.

Anatomical localization of calmodulin mRNA in the rat brain with cloned cDNA and synthetic oligonucleotide probes

Calmodulin is a small, acidic calcium-binding protein that regulates a number of calcium-dependent enzyme activities and is thought to be involved in neurotransmission. To begin to explore further the regulation of this important protein in the brain, we have cloned a rat calmodulin cDNA and designed an oligonucleotide probe based on this sequence. Both the cDNA and oligonucleotide probes revealed a markedly heterogeneous distribution of hybridization signal for calmodulin mRNA in the rat brain. The greatest apparent abundance of mRNA for calmodulin was seen in the hippocampus and cerebral cortex, whereas many brain regions showed relatively low hybridization signal, including the striatum and portions of the hypothalamus and brainstem.

Independent protein kinases associated with the rat cerebral synaptic junction: comparison with cyclic AMP-dependent and Ca2+/calmodulin-dependent protein kinases in the synaptic junction

Independent protein kinases in the synaptic junction (SJ) isolated from rat cerebrum were characterized. SJ showed a protein kinase activity, phosphorylating intrinsic proteins, even in the absence of cyclic AMP or Ca2+ plus calmodulin (CaM) exogenously added. The activity was affected neither by Ca2+ concentrations in the physiological fluctuation range nor by the addition of specific ligands such as glutamate, aspartate, acetylcholine, and concanavalin A. The activity was not due to cyclic AMP-dependent protein kinase in SJ, since the activity was not inhibited by an inhibitor protein for cyclic AMP-dependent protein kinase, and since synapsin I was not specifically phosphorylated whereas cyclic AMP-dependent kinase appeared to phosphorylate selectively the protein in SJ. Phosphorylation of SJ proteins by the independent kinases was about one-third of that of the Ca2+/CaM-dependent protein kinase intrinsic to SJ. The apparent Km for ATP was estimated to be 700 microM. Proteins of 16K Mr and 117K Mr were specifically phosphorylated under the basic condition (in the absence of the substances known to activate specifically protein kinases), as well as six other proteins both under the basic conditions and in the presence of Ca2+ and CaM. The phosphorylation of 150K Mr, 60K Mr, 51K Mr, and 16K Mr SJ proteins was enhanced after prephosphorylation of SJ proteins by intrinsic kinase in the presence of Ca2+ and CaM.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of the catalytic subunit of adenylate cyclase in vertebrates: state of the art in 1987

After 30 years of effort, the mammalian adenylate cyclase catalytic subunit has now been purified. It is a glycoprotein of 155 kDa, representing less than 0.01% of synaptosomal membrane protein. As measured in the presence of forskolin, its specific activity is 10-20 mumol of cAMP X mg-1 X min-1. The enzyme obtained is completely devoid of Gs alpha subunits, and is calmodulin-dependent. The purification procedures involve an affinity chromatography step, either with calmodulin, or with forskolin, or both. If gel filtration precedes the affinity chromatography, two different fractions with high specific enzyme activity are obtained. One contains the 155 kDa protein as the sole component. The other contains, as its major component, a 105 kDa protein. The relationship between the 2 proteins remains to be defined.

Molecular mechanisms of neuronal excitability: possible involvement of CaM kinase II in seizure activity

A type II calmodulin-dependent protein kinase (CaM kinase II) has been characterized in the synaptic region and may mediate some of the effects of Ca2+ on neuronal excitability. The activity of CaM kinase II is inhibited by anticonvulsant compounds and may be the molecular basis of their neuro-modulatory effects. The direct injection of purified CaM kinase II into invertebrate neurons has demonstrated that this kinase can directly alter specific ion conductances and neuronal activity. A long-lasting decrease in CaM kinase II activity is associated with septal kindling, an experimental model of epilepsy and long-term memory. In summary, CaM kinase II appears to be a central mediator of the effects of Ca2+ on neuronal function. Further investigation of this enzyme and its effects on neuronal activity may provide a molecular insight into an endogenous mechanism for modulating some of the effects of Ca2+ on neuronal excitability and may increase our understanding of the complex regulatory mechanisms that underlie the pathogenesis of seizure discharge and its regulation by anticonvulsant compounds.

Calmodulin regulation of the cholinergic receptor in the rat heart during ontogeny and senescence

The contents of calmodulin and cholinergic muscarinic receptor binding sites in the hearts of fetal, adult and aged rats have been examined. A biphasic pattern of calmodulin development was observed. A relatively high level of calmodulin appeared on gestational days 14-15 followed by a steady but significant decrease at birth and during the first week of postnatal life. The level of calmodulin then increased significantly during the first month followed by a decrease at 6 and 26 months of age. Calmodulin contents were significantly higher in the atrium than in the ventricle in the age groups of 1-26 months. The number of [3H]QNB binding sites showed a steady increase during the gestational periods studied, reaching a peak at 9 days after birth and followed by a significant (P less than 0.05) decline at 6 and 26 months of age. A good correlation between the levels of [3H]QNB binding and calmodulin was observed from day 9 of the postnatal period to 26 months of age. In the presence of calcium, calmodulin induced a dose-dependent receptor binding loss in the hearts of postnatal, young adult and aged rats under phosphorylating conditions. These findings support the suggestion that calmodulin may regulate cholinergic functions during ontogeny and senescence.

Characterization of Ca2+/calmodulin-dependent protein kinase associated with rat cerebral synaptic junction: substrate specificity and effect of autophosphorylation

The Ca2+/calmodulin (CaM)-dependent protein kinase associated with rat cerebral synaptic junction (SJ) was characterized, using the SJ fraction as the enzyme preparation, to clarify the functional significance of the enzyme in situ. The protein kinase was greatly activated in the presence of micromolar concentrations of both Ca2+ and calmodulin (EC50 for Ca2+, 1.0 microM; that for CaM, 100 nM). The Km for ATP was 150 microM. SJ proteins were phosphorylated without a lag time, and the phosphorylation reached its maximum within 2-10 min at 25 degrees C. The endogenous substrates consisted of four major (160K, 120K, 60K, and 51K Mr) and 10 minor proteins. Compared with the endogenous substrate phosphorylation, the phosphorylation of exogenously added proteins (myosin light chains from chicken muscle, casein, arginine-rich histone, microtubule-associated protein-2, tau-protein, and tubulin) was weak, although they are expected to be good substrates for the soluble form of the Ca2+/CaM-dependent protein kinase. Autophosphorylation of the enzyme in SJ inhibited its activity and did not alter the subcellular distribution of the enzyme.

Postsynaptic inhibitory effects of phenothiazines at cholinergic synapses may not involve calmodulin

Treatment of frog cutaneous-pectoris nerve-muscle preparations with calmidazolium (R 24571), a calmodulin-inhibitor, at concentrations of 2 X 10(-7) mol/l and 5 X 10(-7) mol/l had no discernable effect on MEPP amplitude. 10(-6) mol/l calmidazolium caused a small (10-35%) increase in MEPP amplitude in most preparations. The phenothiazine calmodulin-inhibitor chlorpromazine (5 X 10(-6) mol/l) caused a clear reduction in MEPP amplitude (20%) after 30 min treatment. Similar experiments carried out with chlorpromazine sulphoxide (a derivative of chlorpromazine that is 60 X less potent in inhibition of calmodulin-activated enzymes) produced data that were very similar to those obtained with chlorpromazine. It is concluded that the postsynaptic inhibitory effect of phenothiazines at cholinergic synapses is unlikely to involve calmodulin.

Quantitative analysis of ultrastructural changes in synapses of the rat hippocampal field CA3 in vitro in different functional states

Transverse slices (250-350 microns) of the rat hippocampus were used for estimation of quantitative correlations between the ultrastructure and function of giant spinous synapses localized in stratum lucidum of the field CA3. Spontaneous and evoked spike discharges were used to determine the following five functional states of the neurons: "control"; "depletion" was achieved by long-term continuous stimulation (30-50 Hz for 1 h and longer); "recovery" when the slices rested after "depletion" till the evoked response was recovered; long-term potentiation I was achieved by short-term tetanic stimulation (5-15 s, 50-70 Hz); long-term potentiation II was achieved by a similar tetanic stimulation as for long-term potentiation I after the "recovery". For quantitative analysis of ultrastructural changes in the giant spinous synapses the following parameters were used: density of presynaptic vesicles determined as a ratio between the number of vesicles located within the giant bouton and the area of the latter (number of vesicles per 1 micron2); vesicle diameter distribution; area and length of the postsynaptic densities. A correlation of these parameters with the functional state of CA3 neurons was found. The area and length of postsynaptic densities are the most statistically significant parameters of the giant spinous synapses in different functional states. In contrast to other states, an increase in the length and the area of postsynaptic densities in long-term potentiation was found. A hypothesis on postsynaptic densities' role in long-term potentiation formation is suggested. The role of presynaptic and postsynaptic ultrastructural rearrangements is discussed as a possible mechanism determining the efficiency of synaptic transmission.

Calcium-binding proteins in whole brain and synaptic subfractions

At least 19 calcium-binding proteins were detected in avian brain subfraction using 45Ca2+ binding to proteins immobilized in polyacrylamide gels. Half of the 45Ca2+ binding proteins were observed in presynaptic cytoplasm. Two-dimensional gel electrophoresis of this material revealed at least 14 45Ca2+ binding polypeptides besides calmodulin. These proteins may be important in brain and nerve terminal function.

Calmodulin-dependent protein phosphatase: immunocytochemical localization in chick retina

Calmodulin-dependent protein phosphatase, previously called CaM-BP80 or calcineurin, is present in high concentrations in the central nervous system. The level of the phosphatase has been shown by radioimmunoassay to increase during development in the retinas of embryonic and hatching chicks (Tallant, E.A., and W.Y. Cheung, 1983, Biochemistry, 22:3630-3635). The aims of this study are to immunocytochemically localize the phosphatase in developing and mature retinas and to determine if the phosphatase is present in fractions of retinal synaptic membranes and synaptic junctions. Vibratome slices of fixed chick retina and Western blots of detergent-solubilized retinal fractions are both treated sequentially with rabbit primary antisera and goat anti-rabbit Fab fragments conjugated to peroxidase, and then reacted with hydrogen peroxide and diaminobenzidine. The tissue slices are further processed for electron microscopy. This paper demonstrates the presence of peroxidase reaction product in the retina just before synapse formation. In the outer plexiform layer the product is confined to photoreceptor synaptic terminals, whereas in the inner plexiform layer it is present in synaptic terminals of bipolar cells and in dendrites of ganglion cells. In this latter site the product is present postsynaptically at bipolar and amacrine synapses. The phosphatase is detected in Western blots of both synaptic plasma membrane and synaptic junction fractions.

Assay of brain calmodulin levels using high-performance liquid chromatography

A simple, sensitive, and efficient HPLC method for the determination of calmodulin levels in brain tissue extracts is described. The assay is linear with respect to both calmodulin and protein concentrations. The specificity and validity of this assay for calmodulin is demonstrated by parallel radioimmunoassay determinations which give equivalent results. Determination of calmodulin levels in various brain regions revealed a high concentration of this protein in the hypothalamus, by comparison to other areas examined.

Calmodulin binding proteins of the cholinergic electromotor synapse: synaptosomes, synaptic vesicles, receptor-enriched membranes, and cytoskeleton

Calmodulin binding proteins (CBPs) have been identified using a gel overlay technique for fractions isolated from Torpedo electromotor nerve endings. Different fractions possessed characteristic patterns of CBPs. Synaptosomes showed five major CBPs--Mr 220,000, 160,000, 125,000, 55,000, and 51,000. Polypeptides of Mr 55,000 and 51,000 were found in the cytoplasm and the others are membrane-associated. The Triton X-100-insoluble cytoskeleton of synaptosomes was isolated in the presence or absence of calcium. The major CBPs had Mr of 19,000, 18,000, and 16,000. In the presence of calcium, no other CBPs were seen. In the absence of calcium, an Mr 160,000 polypeptide was present in the Triton cytoskeleton. Synaptic vesicles showed CBPs of Mr 160,000, 25,000, and 20,000. Membrane fragments enriched in acetylcholine receptors contained two major CBPs, Mr 160,000 and 125,000, together with a less prominent protein at Mr 26,000. A protein of Mr similar to that of fodrin was present in synaptosomes and acetylcholine receptor membrane fragments, but only in small amounts relative to the other polypeptides observed. The heavy and light chains of clathrin-coated vesicles from pig brain did not bind calmodulin, although strong labelling of an Mr 47,000 polypeptide was found. Results showed that calelectrin does not bind calmodulin. The possible identity of the calmodulin binding proteins is discussed.

Particulate cyclic 3',5'-nucleotide phosphodiesterase and calmodulin of cardiac muscle

Cyclic AMP and cyclic GMP phosphodiesterase and calmodulin were measured in purified subcellular fractions of cardiac muscle. Phosphodiesterase activity solubilized by sonication of the nuclear fraction yielded a major 6.6 S form which was calcium-sensitive and cyclic GMP-specific. Phosphodiesterase activity occurring in the nuclear fraction could be further enriched by subfractionation on sucrose density gradients in the presence of MgCl2.

Calmodulin systems in neuronal excitability: a molecular approach to epilepsy

Calmodulin is a major Ca2+ -binding protein that may mediate many Ca2+ -regulated processes in neuronal function. Calmodulin is present in the presynaptic nerve terminal in association with synaptic vesicles and in postsynaptic density fractions. Several calmodulin-regulated synaptic biochemical processes have been identified. These results indicate that calmodulin may modulate some aspects of neuronal excitability. Phenytoin, carbamazepine, and the benzodiazepines inhibit Ca2+ -calmodulin-regulated protein phosphorylation and neurotransmitter release by synaptic vesicles. A saturable, stereospecific membrane binding site has been identified for the benzodiazepines. The potency of the benzodiazepines to bind to these sites correlates with their ability to inhibit maximal electroshock-induced seizures. Phenytoin and carbamazepine can displace benzodiazepine binding from these binding sites. Binding to these "anticonvulsant" sites regulates Ca2+ -calmodulin-stimulated membrane protein phosphorylation and depolarization-dependent Ca2+ uptake in intact synaptosome preparations. These results provide evidence that major anticonvulsant drugs regulate Ca2+ -calmodulin systems at the synapse. Kindling alters Ca2+ -calmodulin protein phosphorylation in brain membrane. In addition, alterations in Ca2+ -calmodulin kinase systems have been associated with some strains of seizure-susceptible mice. Thus, evidence from multiple sources suggests that calmodulin-mediated processes may play a role in the development of altered neuronal excitability and in some forms of seizure disorders.

Surface antigens of brain synapses: identification of minor proteins using polyclonal antisera

Antigenic proteins of brain synaptic plasma membranes (SPM) and postsynaptic densities (PSD) were characterized using antisera raised against SPM. Immunostaining of brain sections showed that the antigens were restricted to synapses, and electron microscopy revealed staining at both presynaptic terminals and PSDs. In primary brain cell cultures the antisera were also neuron-specific but the antigens were distributed throughout the entire neuronal plasma membrane, suggesting that some restrictive influence present in whole tissue is absent when neurons are grown dispersed. The antigenic proteins with which these antisera react were identified using SDS gel immunoblots. SPM and PSD differed from one another in their characteristic antigenic proteins. Comparison with amido-black stained gel blots showed that in both cases most of these did not correspond to known abundant proteins of SPM or PSDs revealed by conventional biochemical techniques. None of the antigens revealed by the polyclonal antisera were detected by any of a large series of monoclonal antibodies against SPM.

Biochemical and immunochemical evidence that the "major postsynaptic density protein" is a subunit of a calmodulin-dependent protein kinase

By three criteria, two biochemical and one immunochemical, the major postsynaptic density protein (mPSDp) is indistinguishable from the 50-kilodalton (kDa) alpha subunit of a brain calmodulin-dependent protein kinase. First, the two proteins comigrate on NaDodSO4/polyacrylamide gels. Second, iodinated tryptic peptide maps of the two are identical. Finally, a monoclonal antibody (6G9) that was raised against the protein kinase binds on immunoblots to a single 50 kDa band in crude brain homogenates and to both the alpha subunit of the purified kinase and the mPSDp from postsynaptic density fractions. The purified kinase holoenzyme also contains a 60-kDa subunit termed beta. A comparison of the peptide map of beta with the maps of 60-kDa proteins from the postsynaptic density fraction suggests that beta is present there but is not the only protein present in this molecular weight range. These results indicate that the calmodulin-dependent protein kinase is a major constituent of the postsynaptic density fraction and thus may be a component of type I postsynaptic densities.

Calcium in long-term potentiation as a model for memory

The granule, CA1 and CA3 cells of the hippocampus have been much investigated during the last decade because there is superimposed on the standard features of synaptic transmission a very prolonged potentiation lasting for weeks that is called long-term potentiation. Evidently long-term potentiation is a promising candidate in the construction of a model for memory. The thesis here developed is that the influx of calcium ions across the membrane of the granule pyramidal cells plays the key role in the generation of long-term potentiation. The proposal makes it possible to account for the necessity of strong repetitive synaptic stimulation, preferably in bursts so as to optimize the conditions for the calcium influx. Studies on hippocampal slices with variations in the synaptic inputs in the granule cells give evidence of cooperativity, which is interpreted in relation to the threshold membrane depolarization for calcium influx. It is conjectured that the large increase of calcium in the granule and pyramidal cells results in the combination with the specific protein, calmodulin, to form a second messenger system, which produces metabolic changes leading to an increase in receptors of the postsynaptic membrane of the spine synapses, i.e. the postsynaptic densities, to the synaptic transmitter, glutamate. For example, Ca2+ could activate calcium-dependent kinases in the postsynaptic density resulting in the modification of protein components by phosphorylation. Other postsynaptic factors contributing to long-term potentiation are presumed to be protein synthesis with spine swelling and increased transport up the dendritic microtubules. There is discussion of the evidence for the alternative hypothesis that long-term potentiation is primarily presynaptic, being due to an increased output of transmitter. A unifying hypothesis is formulated, namely, that the primary event in long-term potentiation is in the increased sensitivity of the postsynaptic densities to the transmitter, and that, secondarily, this induces an increased output of transmitter from the presynaptic terminals by a trophic action across the synaptic cleft. It is shown how the proposed combination of calcium with calmodulin will account for the hypothesis of Marr that cognitive memory is due to conjunction potentiation. Furthermore, the Marr-Albus hypothesis for cerebellar learning is accounted for if the calcium-calmodulin messenger system causes the observed depression of the transmitter sensitivity of the spine synapses on Purkynĕ cells.

The calmodulin hypothesis of neurotransmission

Ca2+ plays a major role in neurotransmission and synaptic modulation. Evidence is presented to support the calmodulin hypothesis of neurotransmission developed in this laboratory stating that calmodulin, a major Ca2+ binding protein in brain, mediates the effects of Ca2+ on neurotransmission. Calmodulin was isolated from highly enriched preparations of synaptic vesicles and nerve terminal cytoplasm. Ca2+ and calmodulin were shown to regulate several synaptic processes in isolated and intact preparations, including endogenous synaptic Ca2+-calmodulin protein kinase activity, neurotransmitter release, and synaptic vesicle and synaptic membrane interactions. Ca2+ and calmodulin were shown to activate a synaptic tubulin kinase system which was shown to be a distinct enzyme system from the cyclic AMP protein kinase. Ca2+ and calmodulin stimulated phosphorylation of tubulin altered the properties of tubulin, forming insoluble tubulin fibrils. Evidence for the role of Ca2+-calmodulin kinase activity, especially the calmodulin-tubulin kinase, in neurotransmission are presented. The effects of several neuroactive drugs on the synaptic calmodulin system are presented. The results support the hypothesis that calmodulin mediates many of calcium's actions at the synapse, and that the effects of Ca2+ on synaptic protein phosphorylation, especially synaptic tubulin, may provide a biochemical mechanism for converting the Ca2+ signal into a motor force in the process of neurotransmission.

Regulation of acetylcholine receptor phosphorylation by calcium and calmodulin

Acetylcholine receptor-enriched membranes prepared from frozen electric organ of Torpedo californica by differential centrifugation and density step gradient centrifugation were assayed for endogenous phosphorylation in the absence and presence of calmodulin and calcium. Each of the membrane fractions exhibited a 3- to 6-fold stimulation of endogenous phosphorylation by calcium and calmodulin. Both calcium and calmodulin were needed for maximal stimulation although calcium alone afforded a small, reproducible stimulation of endogenous phosphorylation. In the presence of fluoride, a phosphatase inhibitor, the calmodulin plus calcium stimulation was increased an additional 3-fold. The phosphorylation reaction was rapid, and maximal phosphorylation was achieved in 2 min. Stimulation of phosphorylation by calcium and calmodulin was completely inhibited by 25 microM trifluoperazine; at 50 microM it inhibited basal phosphorylation by 60%, suggesting that most of the basal phosphorylation may be due to the endogenous calmodulin present in our membrane preparation. NaDodSO4/polyacrylamide gel electrophoresis revealed that at least three of the phosphorylated species (both in the presence and in the absence of calcium and calmodulin) correspond to subunits of the purified acetylcholine receptor from T. californica (i.e., 65,000, 58,000, and 50,000 daltons) which are the beta, gamma, and delta subunits of the receptor.

Presynaptic localization of phosphoprotein B-50

A major phosphoprotein of synaptic membranes, the phosphorylation of which is stimulated by Ca2+ and inhibited by ACTh, appears to be identical with protein B-50 described by Zwiers, Schotman and Gispen [40]. We have investigated its subsynaptic localization by means of a variety of subfractionation techniques and compared it with that of a number of other phosphoproteins found in synaptic membranes. It appears to be predominantly, if not exclusively, associated with presynaptic membranes of low bouyant density. This localization pattern is similar to, but somewhat more extreme than that exhibited by Protein I, as a brain specific phosphoprotein studied by Greengard and his collaborators [11].

Function of calmodulin in postsynaptic densities. I. Presence of a calmodulin-activatable cyclic nucleotide phosphodiesterase activity

The postsynaptic density (PSD) fraction from canine cerebra cortex was found to contain an endogenous cyclic nucleotide-phosphodiesterase activity that was independent on Mn2+ and/or Mg2+ but not on Ca2+. Maximal activity was obtained at 1 micrometer Mn2+. This cyclic nucleotide phosphodiesterase activity was not decreased upon removal of the calmodulin from the PSD fraction, nor was it increased by the addition of calmodulin to a postsynaptic density fraction deficient in calmodulin. The enzymatic activity could be extracted by sonication, with the soluble enzyme having properties similar to those found in the native structure. Two peaks of cyclic nucleotide phosphodiesterase activities could be obtained after S-300 Sephacryl column chromatography of this soluble fraction: fraction I (excluded peak) and fraction II (215,000 mol wt). The fraction I activity preferred cyclic AMP over cyclic GMP and was not activated by calmodulin. The fraction II activity has an approximately fourfold lower Km for cyclic GMP over cyclic AMP. This fraction II activity was activatable by calmodulin, which increased the Vmax and decreased the Km in the case of both cyclic nucleotides. We conclude that two activities are present in the PSD, one activatable, and one not activatable, by calmodulin.

Function of a calmodulin in postsynaptic densities. II. Presence of a calmodulin-activatable protein kinase activity

Because the calmodulin in postsynaptic densities (PSDs) activates a cyclic nucleotide phosphodiesterase, we decided to explore the possibility that the PSD also contains a calmodulin-activatable protein kinase activity. As seen by autoradiographic analysis of coomassie blue-stained SDS polyacrylamide gels, many proteins in a native PSD preparation were phosphorylated in the presence of [gamma-(32)P]ATP and Mg(2+) alone. Addition of Ca(2+) alone to the native PSD preparation had little or no effect on phosphorylation. However, upon addition of exogenous calmodulin there was a general increase in background phosphorylation with a statistically significant increase in the phosphorylation of two protein regions: 51,000 and 62,000 M(r). Similar results were also obtained in sonicated or freeze thawed native PSD preparations by addition of Ca(2+) alone without exogenous calmodulin, indicating that the calmodulin in the PSD can activate the kinase present under certain conditions. The calmodulin dependency of the reaction was further strengthened by the observed inhibition of the calmodulin-activatable phosphorylation, but not of the Mg(2+)-dependent activity, by the Ca(2+) chelator, EGTA, which also removes the calmodulin from the structure (26), and by the binding to calmodulin of the antipsychotic drug chlorpromazine in the presence of Ca(2+). In addition, when a calmodulin-deficient PSD preparation was prepared (26), sonicated, and incubated with [gamma-(32)P]ATP, Mg(2+) and Ca(2+), one could not induce a Ca(2+)-stimulation of protein kinase activity unless exogenous calmodulin was added back to the system, indicating a reconstitution of calmodulin into the PSD. We have also attempted to identify the two major phosphorylated proteins. Based on SDS polyacrylamide gel electrophoresis, it appears that the major 51,000 M(r) PSD protein is the one that is phosphorylated and not the 51,000 M(r) component of brain intermediate filaments, which is a known PSD contaminant. In addition, papain digestion of the 51,000 M(r) protein revealed multiple phosphorylation sites different from those phosphorylated by the Mg(2+)-dependent kinase(s). Finally, although the calmodulin-activatable protein kinase may phosphorylate proteins I(a) and I(b), the cyclic AMP-dependent protein kinase, which definitely does phosphorylate protein I(a) and I(b) and is present in the PSD, does not phosphorylate the 51,000 and 62,000 M(r) proteins, because specific inhibition of this kinase has no effect on the levels of the phosphorylation of these latter two proteins.

Function of a calmodulin in postsynaptic densities. III. Calmodulin-binding proteins of the postsynaptic density

A method has been developed for binding calmodulin, radioiodinated by the lactoperoxidase method, to denaturing gels and has been used to attempt to identify the calmodulin-binding proteins of cerebral cortex postsynaptic densities (PSDs). Calmodulin primarily bound to the major 51,000 Mr protein in a saturatable manner; secondarily bound to the 60,000 Mr region, 140,000 Mr region, and 230,000 Mr protein; and bound in lesser amounts to a number of other proteins. The major 51,000 Mr calmodulin-binding protein is one of unknown identity. Binding of iodinated calmodulin to these proteins was blocked by EDTA, EGTA, chlorpromazine, and preincubation with unlabeled calmodulin. Calmodulin iodinated by the chloramine-T method, which inactivates calmodulin did not bind to the PSD but bound nonspecifically to histone. Calmodulin did not bind to proteins from a variety of sources for which calmodulin interactions have not been found. Except for three proteins, all of the proteins of synaptic membranes that bind calmodulin could be accounted for by proteins of the PSD which are a part of the synaptic membrane fraction. The major 51,000 M, protein and the corresponding iodinated calmodulin binding were greatly reduced in cerebellar PSDs and this difference between cerebral cortex and cerebellar PSDs is discussed in light of the possible function of calmodulin in synaptic excitatory responses.