Specific postsynaptic density proteins bind tubulin and calmodulin-dependent protein kinase type II

Cytoskeletal disrupting agents prevent calmodulin kinase, IQ domain and voltage-dependent facilitation of L-type Ca2+ channels

A calmodulin (CaM) binding 'IQ' domain on the L-type Ca(2+) channel (LTCC) C terminus and calmodulin kinase II (CaMK) both signal increases in LTCC opening probability (P(o)) by shifting LTCCs into a gating mode (mode 2) with long openings through a process called facilitation. However, the mechanism whereby CaMK and the IQ domain are targeted to LTCCs is unknown. Endogenous CaMK is targeted to LTCCs in excised cell membrane patches because LTCC P(o) increased significantly in CaM-enriched (20 microM) bath solution and this effect was prevented by a specific CaMK inhibitory peptide, but not by an inactive control peptide. Pre-exposure of myocytes to the cytoskeletal disrupting agents nocodazole (microtubule specific) or cytochalasin D (microfilament specific) prevented the effects of CaM-dependent increases in P(o) of LTCCs in excised membrane patches. Neither cytochalasin D nor nocodazole altered the distribution of LTCC gating modes under basal conditions in on-cell mode or excised cell membrane patches, but each of these agents occluded the response of LTCCs to exogenous, constitutively active CaMK and to an IQ-mimetic peptide (IQmp). Cytochalasin D and nocodazole pretreatment also prevented LTCC facilitation that followed a cell membrane depolarizing prepulse. In contrast, cytochalasin D and nocodazole did not affect the increase in LTCC P(o) or prevent the shift to mode 2 gating in response to protein kinase A, indicating that cytoskeletal disruption specifically prevents prepulse, CaMK and IQ-dependent LTCC facilitation.

Reduced level of calmodulin in PC12 cells induced by stable expression of calmodulin antisense RNA inhibits cell proliferation and induces neurite outgrowth

The role calmodulin plays in the growth and differentiation of nerve cells was assessed by altering the levels of calmodulin in the PC12 rat pheochromocytoma cell line and determining the effects of altering these levels on cellular proliferation and differentiation. Calmodulin levels in the PC12 cells were increased or decreased by transfecting the cells with a mammalian expression vector into which the rat calmodulin gene I had been cloned in the sense or antisense orientation, respectively. The cells transfected with the calmodulin sense gene showed increased levels of calmodulin immunoreactivity and increased levels of calmodulin messenger RNA as ascertained by immunocytochemistry and slot-blot analysis, respectively. Cells transfected with the calmodulin antisense construct showed reduced levels of calmodulin immunoreactivity. Reducing the levels of calmodulin by expression of antisense calmodulin messenger RNA resulted in a marked inhibition of cell growth, whereas increasing the levels of calmodulin by overexpressing calmodulin messenger RNA resulted in an acceleration of cell growth. Transfected PC12 cells having reduced levels of calmodulin immunoreactivity exhibited spontaneous outgrowth of long, stable and highly branched neuritic processes. PC12 cells in which calmodulin was overexpressed showed no apparent changes in cell morphology, but did show an altered response to the addition of nerve growth factor. While nerve growth factor slowed cellular proliferation and induced extensive neurite outgrowth, in parental PC12 cells nerve growth factor induced little or no neurite outgrowth and little inhibition of cell proliferation in transfected cells overexpressing calmodulin. These results indicate that calmodulin is essential for the proliferation of nerve cells and for the morphological changes that nerve cells undergo during differentiation. The study also suggests the possibility that a calmodulin antisense approach may be used to inhibit the proliferation of neuronal tumors.

Existence of a putative specific postsynaptic density protein produced during Purkinje cell spine maturation

This study identified a 140 kDa polypeptide as a putative specific component of Purkinje cell spines' postsynaptic densities and which began to appear during the critical period of cerebellar cortex synaptogenesis. Mouse cerebellar cortices at postnatal days 5, 7, 9, 11, 15 and young adult, between days 30 and 40, were used to purify subcellular fractions of synaptosomes, synaptic membranes and postsynaptic densities. The purity of the subcellular fractions was assessed by electron microscopy and the protein composition of the different fractions was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polypeptides of apparent molecular weights of 25, 26, 27, 30, 33, 37, 43, 45, 52, 64, 74, 85, 94, 110, 125, 130, 165 and 174 kDa were found in the synaptosomal fractions of all the ages studied, even before the critical period of synaptogenesis, at postnatal day 7, when the postsynaptic densities were still nonexistent, indicating that the polypeptides are nonspecific constituents of these structures. On the other hand, a 140 kDa polypeptide was detected in the postsynaptic density fractions at postnatal day 11, immediately after postsynaptic structures began to appear, suggesting the possibility that this protein is a specific component of the cerebellar cortex postsynaptic densities. The 140 kDa polypeptide was electroeluted from the gel and analysed for its amino acid composition by reverse-phase high-pressure liquid chromatography. The analysis showed that this protein has a high content of nonpolar amino acid residues, such as leucine, isoleucine, glycine, phenylalanine and valine. A hypothetical model relative to the participation of the 140 kDa protein in the molecular organization of the postsynaptic density is suggested which may contribute to the understanding of the role played by this structure in synaptic function.

Onset of expression of the alpha subunit of Ca2+/calmodulin-dependent protein kinase II and a novel related protein in the developing retina

Calcium-calmodulin-dependent protein kinase II is an abundant protein in the nervous system and has been associated with many aspects of neuronal function, including events related to synaptic transmission. The purpose of this study is to correlate the onset of expression of this kinase with a specific developmental event in retinal morphogenesis using a monoclonal antibody to the 50-kDa alpha-subunit. Microscopy showed the antigen to be associated with the plexiform layers of the retina. Western blots demonstrated that the onset of expression of the alpha-subunit coincided in time with the initial formation of the plexiform layers. However, the onset of expression of the 50-kDa alpha-subunit was preceded by the earlier embryonic appearance of a related 82.5-kDa antigen that was recognized by the antibody. The amount of this latter protein declined as the amount of the alpha-subunit increased in retinal homogenates. Although this related 82.5 kDa protein disappeared from blots of retinal homogenates after embryonic d 14, it could be detected in concentrated supernatant fractions isolated from the retinae of hatched chicks. Microscopy showed that a subset of retinal cells and their processes contained this antigen in early embryonic chicks. Finally, the 50 kDa alpha-subunit of kinase II and the 82.5 kDa novel antigen were shown to be separable by differential centrifugation.

Interaction of autophosphorylated Ca2+/calmodulin-dependent protein kinase II with neuronal cytoskeletal proteins. Characterization of binding to a 190-kDa postsynaptic density protein

Subcellular localization of Ca2+/calmodulin-dependent protein kinase II (CaMKII) by interaction with specific anchoring proteins may be an important mechanism contributing to the regulation of CaMKII. Proteins capable of binding CaMKII were identified by the use of a gel overlay assay with recombinant mouse CaMKII alpha (mCaMKII alpha) or Xenopus CaMKII beta (xCaMKII beta) 32P-autophosphorylated at Thr286/287 as a probe. Numerous [32P]CaMKII-binding proteins were identified in various whole rat tissue extracts, but binding was most prominent to forebrain proteins of 190 kDa (p190) and 140 kDa (p140). Fractionation of forebrain extracts localized p190 and p140 to a crude particulate/cytoskeletal fraction and isolated postsynaptic densities. [32P]m-CaMKII alpha-bound to p190 with an apparent Kd of 609 nM (subunit concentration) and a Bmax of 7.0 pmol of mCaMKII alpha subunit bound per mg of P2 protein, as measured using the overlay assay. Binding of 100 nM [32P]m-CaMKII alpha to p190 was competed by nonradioactive mCaMKII alpha autophosphorylated on Thr286 (EC50% = 200 nM), but to a much lesser extent by nonradioactive mCaMKII alpha autophosphorylated on Thr306 (EC50% > 2000 nM). In addition, nonphosphorylated mCaMKII alpha was a poor competitor for [32P]mCaMKII alpha binding to p190. The competition data indicate that Ca2+/CaM-dependent autophosphorylation at Thr286 promotes binding to p190, whereas, Ca2+/CaM-independent autophosphorylation at Thr306 does not enhance binding. Therefore, CaMKII may become localized to postsynaptic densities by p190 following its activation by an increase of dendritic Ca2+ concentration.

Subcellular localization of the F5 protein to the neuronal membrane-associated cytoskeleton

F5 was identified originally as an interleukin-2-regulated gene in the murine helper T-lymphocyte clone L2. Subsequent studies demonstrated high levels of F5 mRNA and protein in mature neurons in adult mouse central and peripheral nervous systems. The F5 protein was present in dendrites and perikarya but not in axons. In the present studies, the intracellular localization of the F5 protein in adult mouse brain was determined by subcellular fractionation and Western blotting. Although the deduced F5 sequence predicts a soluble protein, virtually no F5 immunoreactivity was found in the cytosol. The F5 protein was restricted to the P2 crude mitochondrial and P3 crude microsomal particulate fractions. Within the P2 fraction, F5 protein was enriched in the P2B synaptosomal subfraction. The results of temperature-dependent phase separation with Triton X-114 and alkaline extraction with sodium carbonate of the P2 and P3 fractions were consistent with the F5 protein being an extrinsic membrane-associated protein. Although essentially all of the F5 protein in the P3 fraction was membrane-associated, a substantial proportion of P2-associated F5 protein and nearly all of the synaptosomal F5 protein was detergent-insoluble. Direct isolation and subfractionation of brain cytoskeleton confirmed colocalization of F5 immunoreactivity with the membrane-associated cytoskeleton and postsynaptic densities. These studies suggest that the F5 protein, which has a large number of potential phosphorylation sites, plays a role in membrane-cytoskeletal interactions and in dynamic aspects of synaptic structure or function.

Regulation and role of brain calcium/calmodulin-dependent protein kinase II

Ca2+/calmodulin-dependent protein kinase II (CaMKII) exhibits a broad substrate specificity and regulates diverse responses to physiological changes of intracellular Ca2+ concentrations. Five isozymic subunits of the highly abundant brain kinase are encoded by four distinct genes. Expression of each gene is tightly regulated in a cell-specific and developmental manner. CaMKII immunoreactivity is broadly distributed within neurons but is discretely associated with a number of subcellular structures. The unique regulatory properties of CaMKII have attracted a lot of attention. Ca2+/calmodulin-dependent autophosphorylation of a specific threonine residue (alpha-Thr286) within the autoinhibitory domain generates partially Ca(2+)-independent CaMKII activity. Phosphorylation of this threonine in CaMKII is modulated by changes in intracellular Ca2+ concentrations in a variety of cells, and may prolong physiological responses to transient increases in Ca2+. Additional residues within the calmodulin-binding domain are autophosphorylated in the presence of Ca2+ chelators and block activation by Ca2+/calmodulin. This Ca(2+)-independent autophosphorylation is very rapid following prior Ca2+/calmodulin-dependent autophosphorylation at alpha-Thr286 and generates constitutively active, Ca2+/calmodulin-insensitive CaMKII activity. Ca(2+)-independent autophosphorylation of CaMKII also occurs at a slower rate when alpha-Thr286 is not autophosphorylated and results in inactivation of CaMKII. Thus, Ca(2+)-independent autophosphorylation of CaMKII generates a form of the kinase that is refractory to activation by Ca2+/calmodulin. CaMKII phosphorylates a wide range of neuronal proteins in vitro, presumably reflecting its involvement in the regulation of diverse functions such as postsynaptic responses (e.g. long-term potentiation), neurotransmitter synthesis and exocytosis, cytoskeletal interactions and gene transcription. Recent evidence indicates that the levels of CaMKII are altered in pathological states such as Alzheimer's disease and also following ischemia.

Regional and subcellular distribution of [125I]endothelin binding sites in rat brain

The binding of [125I]endothelin-1 (125I-ET-1) to membranes from whole rat brain, from individual brain regions, and derived from subcellular fractionation of whole rat brain was investigated. 125I-ET-1 binding to whole rat brain membranes was rapid, concentration-dependent, saturable, and characterized as irreversible because it was not displaced by unlabeled endothelin-1 (ET-1) and different concentrations of ligand produced, with time, a similar magnitude of binding. The maximum binding site capacity and second-order forward rate association constant of binding were 1,946 +/- 147 fm/mg protein and 5.53 +/- 1.72 x 10(6) M-1 s-1. Removal of either extramembranal calcium or membrane-bound calcium and calcium binding proteins did not affect the binding of 125I-ET-1 to whole rat brain membranes. The brain stem and cerebellum contained the highest levels of 125I-ET-1 binding sites, whereas the cerebral cortex, striatum, and hippocampus contained binding site levels three- to fourfold less. Subcellular fractionation of whole rat brain and subsequent analyses of the distribution of 125I-ET-1 binding demonstrated a twofold enrichment of binding sites in the synaptosomal fraction compared to the homogenate. The myelin fraction contained a similar density of binding sites compared to the homogenate, while the mitochondrial and microsomal fractions contained considerably less binding sites. The ribosomal fraction did not contain any 125I-ET-1 binding sites. The subcellular distribution of 125I-ET-1 binding sites did not correlate with the distribution of 5'-nucleotidase, cytochrome-C oxidase, phosphodiesterase, and alkaline phosphatase. Depletion of extracellular calcium increased 125I-ET-1 binding in the synaptosomal fraction but not in the myelin and mitochondrial fractions.(ABSTRACT TRUNCATED AT 250 WORDS)

Calmodulin-dependent protein kinase II. Multifunctional roles in neuronal differentiation and synaptic plasticity

One of the most important mechanisms for regulating neuronal functions is through second messenger cascades that control protein kinases and the subsequent phosphorylation of substrate proteins. Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II) is the most abundant protein kinase in mammalian brain tissues, and the alpha-subunit of this kinase is the major protein and enzymatic molecule of synaptic junctions in many brain regions. CaM-kinase II regulates itself through a complex autophosphorylation mechanism whereby it becomes calcium-independent following its initial activation. This property has implicated CaM-kinase II as a potential molecular switch at central nervous system (CNS) synapses. Recent studies have suggested that CaM-kinase II is involved in many diverse phenomena such as epilepsy, sensory deprivation, ischemia, synapse formation, synaptic transmission, long-term potentiation, learning, and memory. During brain development, the expression of CaM-kinase II at both protein and mRNA levels coincides with the active periods of synapse formation and, therefore, factors regulating the genes encoding kinase subunits may play a role in the cell-to-cell recognition events that underlie neuronal differentiation and the establishment of mature synaptic functions. Recent findings have demonstrated that the mRNA encoding the alpha-subunit of CaM-kinase II is localized in neuronal dendrites. Current speculation suggests that the localized translation of dendritic mRNAs encoding specific synaptic proteins may be responsible for producing synapse-specific changes associated with the processing, storage, and retrieval of information in neural networks.

Calmodulin, cell growth and gene expression

Calmodulin is thought to regulate a number of intracellular processes, including cell proliferation. Previous studies using drugs that antagonize calmodulin function have indicated that calmodulin is required for progression at specific points in the eukaryotic cell cycle. However, interpretation of these results has previously been difficult due to the lack of specificity of these inhibitors in living cells. Recent studies have used a combination of molecular biological and genetic analysis techniques to approach the study of calmodulin-dependent cell cycle control with greater precision and specificity. These studies have confirmed that calmodulin is an important regulator of the cell cycle, and provide new ways in which to examine the cellular mechanisms involved in calmodulin-dependent cell cycle control.

Characteristics of microtubules at the different stages of neuronal differentiation and maturation

The developing nervous system has proved to be a very powerful tool to analyze how MT are involved in basic biological processes such as cell proliferation, cell migration, cell shaping, and transport. A better knowledge of the basic events occurring during neurogenesis also affords us the possibility of establishing the basis of experiments and trying to solve unanswered and important questions. Despite the considerable value of cell culture, we need to use more discrete regions of the developing brain in situ in order to analyze the MT and their modifications into cells developing their "natural" environment. One major problem remains the question of the mode of assembly and disassembly, that is, the behavior of MT in living cells. Dynamic instability and/or treadmilling are accurate interpretations of the dynamics of MT at least in vitro or in cell culture, but we do need more information on what happens in situ and in vitro. One of the main tasks of cell biologists is to devise satisfactory tests to approach this fundamental question. In this view, pharmacological manipulation of embryos treated in whole-embryo culture systems might be a possible way. Microtubules are ubiquitous cell components. However, the extensive heterogeneity of MAP and tubulin in the CNS confers on the neurons a wide range of capabilities of assembly of these proteins and suggests that the neuron has a unique potential of a relation between MT composition and cell function. We have seen that each major event during neurogenesis is related to a specific series of modifications of the MT components. It remains to be determined if there is a causal or just a correlative relationship between the appearance of specific isotypes and the occurrence of specific events and/or functions. We have also to determine the exact spatial and temporal relations among the different isotypes of MT proteins, tubulin, and MAP. Is there a close correspondence between a tubulin and a MAP isotype? Can the appearance of one isotype of tubulin influence the appearance and the assembly of a specific MAP, or vice versa? Recent results obtained with the Tyr- and Glu-MT shed light on these questions and suggest a whole series of possibilities for cells to modulate the structure, behavior, and function of MT in specific domains of the neuron or in specific regions of the brain, by only a minute modification of the molecule of tubulin. Microtubule protein heterogeneity raises also a number of questions.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium, calmodulin and cell proliferation

Calcium and calmodulin have been proposed to be regulatory factors in cell cycle progression. Clonal mouse cell lines harboring episomally-carried genes have been prepared to address this question. Some lines produce extra calmodulin, others express antisense RNA to decrease calmodulin, while others produce the Ca2+-buffering protein parvalbumin. The results show that calmodulin acts at two points in the cell cycle--the G1/S boundary and metaphase transition. An additional Ca2+ event that is calmodulin-independent occurs at mitotic prophase. The elevated (or depressed) level of intracellular Ca2+ binding protein does not markedly affect gene expression. In cells containing excess calmodulin, the synthesis mechanisms that normally control the level of calmodulin post-transcriptionally are overridden. Genes normally expressed in G1 whose products are involved in growth control show increases in calmodulin over producing cell lines. Elevated calmodulin decreases tubulin mRNA presumably due to its effect on microtubule stability. The availability of cell lines in which calmodulin can be inducibly increased or decreased should provide tools to elucidate the molecular mechanisms that govern the regulatory roles for this protein in cell cycle progression.

Identification of zinc-binding sites of proteins: zinc binds to the amino-terminal region of tubulin

The discovery that certain proteins may require zinc for their activity, and the fact that several of them cannot be purified in large amounts, has led us to develop a rapid, sensitive method to detect these proteins in samples. This method is based on the fractionation of the proteins by gel electrophoresis, blotting onto nitrocellulose paper, and overlaying with 65Zn. We have tested the procedure with well-characterized zinc-binding proteins. In the case of tubulin, we have used this method to localize its zinc-binding site. It was found that zinc binds to the first 150 amino acids of both alpha- and beta-tubulin subunits.

Two types of brain calmodulin-dependent protein kinase II: morphological, biochemical and immunochemical properties

Two forms of the soluble calmodulin-dependent protein kinase type II can be isolated from rat brain: one oligomeric enzyme complex contains the alpha and beta subunits of the enzyme, whereas the other oligomeric species is comprised of a constant ratio of the subunits of the kinase and tubulin in the presence of several other minor polypeptides. The unassociated enzyme oligomer does not detectably exchange with the tubulin-containing form, and both forms rechromatograph by ion-exchange to their respective positions. In the molecular complex of proteins eluting at high ionic strength, the ratio of kinase subunits to tubulin remains constant throughout sedimentation velocity centrifugation and gel permeation chromatography. Furthermore, a similar complex of proteins is coprecipitated by the anti-kinase monoclonal antibody. Hydrodynamic parameters demonstrate that the tubulin-associated enzyme is larger than the unassociated enzyme, and displays heterodisperse behavior as well. Electron microscopic examination of negatively stained enzyme preparations reveals that the free enzyme constitutes uniform 10-20 nm diameter oligomers in contrast to the tubulin-associated kinase which forms elongated structures with varying morphology. Interestingly, enzyme purified through the calmodulin-Sepharose step can also form 'polymers' featuring ultrastructural similarities to postsynaptic densities and brain microsomal cytoskeletal preparations. We discuss the relevance of these observations to the ability of the type II calmodulin-dependent protein kinase to interact with other polypeptides and to form cytoskeletal structures such as the postsynaptic density.

Developmental changes in calmodulin-kinase II activity at brain synaptic junctions: alterations in holoenzyme composition

Synaptic junctions (SJs) from rat forebrain were isolated at increasing postnatal ages and examined for endogenous protein kinase activities. Our studies focused on the postnatal maturation of the multifunctional protein kinase designated Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II). This kinase is comprised of a major 50-kilodalton (kDa) and a minor 60-kDa subunit. Experiments examined the developmental properties of CaM-kinase II associated with synaptic plasma membranes (SPMs) and synaptic junctions (SJs), as well as the holoenzyme purified from cytosolic extracts. Large developmental increases in CaM-kinase II activity of SJ fractions were observed between postnatal days 6 and 20; developmental changes were examined for a number of properties including (a) autophosphorylation, (b) endogenous substrate phosphorylation, (c) exogenous substrate phosphorylation, and (d) immunoreactivity. Results demonstrated that forebrain CaM-kinase II undergoes a striking age-dependent change in subunit composition. In early postnatal forebrain the 60-kDa subunit constitutes the major catalytic and immunoreactive subunit of the holoenzyme. The major peak of CaM-kinase II activity in SJ fractions occurred at approximately postnatal day 20, a time near the end of the most active period of in vivo synapse formation. Following this developmental age, CaM-kinase II continued to accumulate at SJs; however, its activity was not as highly activated by Ca2+ plus calmodulin.

The effects of chemically and electrically-induced convulsions on [3H]nitrendipine binding in mouse brain

The effects of chemically and electrically-evoked seizures on [3H]nitrendipine binding to voltage-dependent calcium channels in mouse brain were determined 30 and 60 min following the initiation of convulsions. While maximal electroconvulsive shock, pentylenetetrazol and strychnine exhibited either no or marginal effects, Ro 5-4864 produced a decrease (14%) in the Bmax of [3H]nitrendipine at 30 min but not 60 min. The convulsant dihydropyridine calcium channel activator, BAY K 8644, produced a significant increase in the Kd (31%) of [3H]nitrendipine at 30 min, and a significant increase in both the Bmax (21%) and Kd (28%) of [3H]nitrendipine 60 min following the initiation of convulsions. While maximal electroconvulsive shock, pentylenetetrazol and strychnine exhibited either no or marginal effects, Ro 5-4864 produced a decrease (14%) in the Bmax of [3H]nitrendipine at 30 min but not 60 min following the initiation of convulsions. These findings indicate that modulation of voltage-dependent calcium channels by certain convulsants may be important in the genesis of seizures or in post-ictal compensatory processes.

Amino acid sequence analysis of the neuronal type II calmodulin-dependent protein kinase by tandem mass spectrometry

The primary structure of the neuronal Type II calmodulin-dependent protein kinase has been examined by protein sequence analysis and compared to cDNA-derived sequence. Tandem mass spectroscopic analysis was used for the sequence determination. Comparison with published cDNA sequence data for the alpha subunit revealed that the difference between the alpha- and beta-subunits lay in two insertions into the sequence for the alpha-subunit and a short alpha-specific sequence. The N-terminal amino acid of the alpha subunit which is blocked to Edman degradation has been tentatively identified as N-acetyl-alanine.