Induction and alternative splicing of delta isoform of Ca(2+)/calmodulin-dependent protein kinase II during neural differentiation of P19 embryonal carcinoma cells and during brain development

Hsc70 phosphorylation patterns and calmodulin regulate AP2 Clathrin-Coated-Vesicle life span for cell adhesion protein transport

AP2 forms AP2 CCV with clathrin and over 60 additional coat proteins. Due to this complexity, we have a limited understanding of CCV life cycle regulation. Synapses contain canonical AP2 CCV, canCCV, and more stable, thereby longer lived, AP2 CCV. The more stable AP2 CCV can be distinguished from canCCV due to the stable binding of Hsc70 to clathrin. The AP1/σ1B complex knockout leads to impaired synaptic vesicle recycling and altered endosomal protein sorting. This causes as a secondary phenotype the twofold upregulation of endocytosis by canCCV and by more stable AP2 CCV. These stable CCV are more stabilized than their wt counterpart, hence stCCV. They have less of the uncoating proteins synaptojanin1 and Hsc70, and more of the coat stabilizing AAK1. Hsc70 clathrin dissociation activity is regulated by complex phosphorylation patterns. Two major groups of hyper- and of hypo-phosphorylated Hsc70 proteins are formed. The latter are enriched in wt stable CCV and stabilized stCCV. Hsc70 T265 phosphorylation regulates binding of CaM/Ca2+. CaM/Ca2+ binding to the T265 domain blocks Hsc70 homodimerization and its concentration in stCCV required for clathrin disassembly. Kinases DYRK1A and CaMK-IIδ can phosphorylate T265 preventing CaM/Ca2+ binding. Their and the levels of STK38L and STK39/Cab39, which are able to phosphorylate additional Hsc70 residues are reduced in stCCV. The stCCV pathway sorts specifically the cell adhesion proteins CHL1 and Neurocan, supporting our model of that the stCCV pathway fulfills specific functions in synaptic plasticity.

Role of Ca2+/calmodulin-dependent kinase II-IRAK1 interaction in LMP1-induced NF-κB activation

We have previously reported that interleukin-1 (IL-1) receptor-associated kinase (IRAK1) is essential for Epstein-Barr virus (EBV) latent infection membrane protein 1 (LMP1)-induced p65/RelA serine 536 phosphorylation and NF-κB activation but not for IκB kinase α (IKKα) or IKKβ activation (Y. J. Song, K. Y. Jen, V. Soni, E. Kieff, and E. Cahir-McFarland, Proc. Natl. Acad. Sci. U. S. A. 103:2689-2694, 2006, doi:10.1073/pnas.0511096103). Since the kinase activity of IRAK1 is not required for LMP1-induced NF-κB activation, IRAK1 is proposed to function as a scaffold protein to recruit a p65/RelA serine 536 kinase(s) to enhance NF-κB-dependent transcriptional activity. We now report that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) interacts with IRAK1 and is critical for LMP1-induced p65/RelA serine 536 phosphorylation and NF-κB activation. CaMKII bound the death domain of IRAK1 and directly phosphorylated p65/RelA at serine 536 in vitro. Downregulation of CaMKII activity or expression significantly reduced LMP1-induced p65/RelA serine 536 phosphorylation and NF-κB activation. Furthermore, LMP1-induced CaMKII activation and p65/RelA serine 536 phosphorylation were significantly reduced in IRAK1 knockout (KO) mouse embryonic fibroblasts (MEFs). Thus, IRAK1 may recruit and activate CaMKII, which phosphorylates p65/RelA serine 536 to enhance the transactivation potential of NF-κB in LMP1-induced NF-κB activation pathway.

Calcium-calmodulin-dependent protein kinase II isoforms differentially impact the dynamics and structure of the actin cytoskeleton

Calcium-calmodulin-dependent protein kinase II (CaMKII) has been implicated in a wide variety of cellular processes, which include a critical regulatory role in actin cytoskeletal assembly. CaMKII is ubiquitous in cells, expressed as one of four isoforms termed α, β, γ, and δ. Characterization of the CaMKII-actin interaction has mainly focused on the β isoform, which has been shown to bundle actin filaments and sequester actin monomers in an activity-dependent manner. Much less is known about the interactions of other CaMKII isoforms with actin. In this work, isoform specific interactions of CaMKII with actin are described and reveal that the δ isoform of CaMKII bundles F-actin filaments like the β isoform while the γ isoform induces a novel layered structure in filaments. Using electron tomography, CaMKII holoenzymes are clearly identified in the complexes bridging the actin filaments, allowing direct visualization of the interactions between CaMKII isoforms and actin. In addition, we determined the isoform specificity of CaMKII-mediated inhibition of actin polymerization and discovered that all isoforms inhibit polymerization to varying degrees: β > γ ≈ δ > α (from most to least effective). Ca(2+)/CaM activation of all kinase isoforms produced a robust increase in actin polymerization that surpassed the rates of polymerization in the absence of kinase inhibition. These results indicate that diversity exists between the types of CaMKII-actin interactions mediated by the different isoforms and that the CaMKII isoform composition differentially impacts the formation and maintenance of the actin cytoskeleton.

Muscleblind-like 1 knockout mice reveal novel splicing defects in the myotonic dystrophy brain

Myotonic dystrophy type 1 (DM1) is a multi-systemic disorder caused by a CTG trinucleotide repeat expansion (CTG(exp)) in the DMPK gene. In skeletal muscle, nuclear sequestration of the alternative splicing factor muscleblind-like 1 (MBNL1) explains the majority of the alternative splicing defects observed in the HSA(LR) transgenic mouse model which expresses a pathogenic range CTG(exp). In the present study, we addressed the possibility that MBNL1 sequestration by CUG(exp) RNA also contributes to splicing defects in the mammalian brain. We examined RNA from the brains of homozygous Mbnl1(ΔE3/ΔE3) knockout mice using splicing-sensitive microarrays. We used RT-PCR to validate a subset of alternative cassette exons identified by microarray analysis with brain tissues from Mbnl1(ΔE3/ΔE3) knockout mice and post-mortem DM1 patients. Surprisingly, splicing-sensitive microarray analysis of Mbnl1(ΔE3/ΔE3) brains yielded only 14 candidates for mis-spliced exons. While we confirmed that several of these splicing events are perturbed in both Mbnl1 knockout and DM1 brains, the extent of splicing mis-regulation in the mouse model was significantly less than observed in DM1. Additionally, several alternative exons, including Grin1 exon 4, App exon 7 and Mapt exons 3 and 9, which have previously been reported to be aberrantly spliced in human DM1 brain, were spliced normally in the Mbnl1 knockout brain. The sequestration of MBNL1 by CUG(exp) RNA results in some of the aberrant splicing events in the DM1 brain. However, we conclude that other factors, possibly other MBNL proteins, likely contribute to splicing mis-regulation in the DM1 brain.

Increased expression of calcium/calmodulin-dependent protein kinase type II subunit δ after rat traumatic brain injury

Many cellular responses to Ca(2+) signals are mediated by Ca(2+)/calmodulin-dependent enzymes, among which is the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). CaMKII was originally described in rat brain tissue. In rat brain, four different subunits of the kinase have been identified: α, β, γ, and δ. This study aims to investigate changes of CaMKIIδ after traumatic brain injury and its possible role. Rat traumatic brain injury (TBI) model was established by controlled cortical injury system. In the present study, we mainly investigated the expression and cellular localization of CaMKIIδ after traumatic brain injury. Western blot analysis revealed that CaMKIIδ was present in normal rat brain cortex. It gradually increased, reached a peak at the third day after TBI, and then decreased. Importantly, more CaMKIIδ was colocalized with neuron. In addition, Western blot detection showed that the third day postinjury was also the apoptosis peak indicated by the elevated expression of caspase-3.Importantly, immunohistochemistry analysis revealed that injury-induced expression of CaMKIIδ was colabeled by caspase-3 (apoptosis cells marker). Moreover, pretreatment with the CaMKII inhibitor (KN62) reduced the injury-induced activation of caspase-3. Noticeably, the CaMKII inhibitor KN-62 could reduce TBI-induced cell injury assessed with lesion volume and attenuate behavioral outcome evaluated by motor test. These data suggested that CaMKIIδ may be implicated in the apoptosis of neuron and the recovery of neurological outcomes. However, the inherent mechanisms remained unknown. Further studies are needed to confirm the exact role of CaMKIIδ after brain injury.

Vitamin A metabolite, all-trans-retinoic acid, mediates alternative splicing of protein kinase C deltaVIII (PKCdeltaVIII) isoform via splicing factor SC35

Vitamin A metabolite, all-trans-retinoic acid (RA), induces cell growth, differentiation, and apoptosis and has an emerging role in gene regulation and alternative splicing events. Protein kinase Cdelta (PKCdelta), a serine/threonine kinase, has a role in cell proliferation, differentiation, and apoptosis. We reported an alternatively spliced variant of human PKCdelta, PKCdeltaVIII that functions as a pro-survival protein (1). RA regulates the splicing and expression of PKCdeltaVIII via utilization of a downstream 5' splice site of exon 10 on PKCdelta pre-mRNA. Here, we further elucidate the molecular mechanisms involved in RA regulation of alternative splicing of PKCdeltaVIII mRNA. Overexpression and knockdown of the splicing factor SC35 (i.e. SRp30b) indicated that it is involved in PKCdeltaVIII alternative splicing. To identify the cis-elements involved in 5' splice site selection we cloned a minigene, which included PKCdelta exon 10 and its flanking introns in the pSPL3 splicing vector. Alternative 5' splice site utilization in the minigene was promoted by RA. Further, co-transfection of SC35 with PKCdelta minigene promoted selection of 5' splice site II. Mutation of the SC35 binding site in the PKCdelta minigene abolished RA-mediated utilization of 5' splice splice II. RNA binding assays demonstrated that the enhancer element downstream of PKCdelta exon 10 is a SC35 cis-element. We conclude that SC35 is pivotal in RA-mediated PKCdelta pre-mRNA alternative splicing. This study demonstrates how a nutrient, vitamin A, via its metabolite RA, regulates alternative splicing and thereby gene expression of the pro-survival protein PKCdeltaVIII.

Molecular Mechanism of Learning and Memory Based on the Research for Ca2+/Calmodulin-dependent Protein Kinase II

In the central nervous system (CNS), the synapse is a specialized junctional complex by which axons and dendrites emerging from different neuron intercommunicates. Changes in the efficiency of synaptic transmission are important for a number of aspects of neural function. Much has been learned about the activity-dependent synaptic modifications that are thought to underlie memory storage, but the mechanism by which these modifications are stored remains unclear. Thus, it is important to find and characterize "memory molecules," and "memory apparatus or memory forming apparatus." A good candidate for the storage mechanism is Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase II). CaM kinase II is one of the most prominent protein kinases, present in essentially every tissue but most concentrated in the brain. Neuronal CaM kinase II regulates important neuronal functions, including neurotransmitter synthesis, neurotransmitter release, modulation of ion channel activity, cellular transport, cell morphology and neurite extension, synaptic plasticity, learning and memory, and gene expression. Studies concerning this kinase open a door of the molecular basis of nerve function, especially learning and memory, and indicate one direction for the studies in the field of neuroscience. This review presents molecular structure, properties and functions of CaM kinase II, as a major component of neuron, which are mainly developed in our laboratory.

A Differential Phosphoproteomic Analysis of Retinoic Acid-Treated P19 Cells

External stimuli trigger internal signaling events within a cell that may represent either a temporary or permanent shift in the phosphorylation state of its proteome. Numerous reports have elucidated phosphorylation sites from a variety of biological samples and more recent studies have monitored the temporal dynamics of protein phosphorylation as a given system is perturbed. Understanding which proteins are phosphorylated as well as when they are phosphorylated may indicate novel functional roles within a system and allow new therapeutic avenues to be explored. To elucidate the dynamics of protein phosphorylation within differentiating murine P19 embryonal carcinoma cells, we induced P19 cells to differentiate using all-trans-retinoic acid and developed a strategy that combines isotopically labeled methyl esterification, immobilized metal affinity chromatography, mass spectrometric analysis, and a rigorous and unique data evaluation approach. We present the largest differential phosphoproteomic analysis using isotopically labeled methyl esterification to date, identifying a total of 472 phosphorylation sites on 151 proteins; 56 of these proteins had altered abundances following treatment with retinoic acid and approximately one-third of these have been previously associated with cellular differentiation. A series of bioinformatic tools were used to extract information from the data and explore the implications of our findings. This study represents the first global gel-free analysis that elucidates protein phosphorylation dynamics during cellular differentiation.

Rat leucine-rich protein binds and activates the promoter of the beta isoform of Ca2+/calmodulin-dependent protein kinase II gene

We previously found the neuronal cell-type specific promoter and binding partner of the beta isoform of Ca(2+)/calmodulin-dependent protein kinase II (beta CaM kinase II) in rat brain [Donai, H., Morinaga, H., Yamauchi, T., 2001. Genomic organization and neuronal cell type specific promoter activity of beta isoform of Ca(2+)/calmodulin-dependent protein kinase II of rat brain. Mol. Brain Res. 94, 35-47]. In the present study, we purified a protein that binds specifically a promoter region of beta CaM kinase II gene from a nuclear extract of the rat cerebellum using DEAE-cellulose column chromatography, ammonium sulfate fractionation, gel filtration and polyacrylamide gel electrophoresis. The purified protein was identified as rat leucine-rich protein 157 (rLRP157) using tandem mass spectrometry. Then, we prepared its cDNA by reverse transcriptase-polymerase chain reaction (RT-PCR) from poly(A)(+)RNA of rat cerebellum. The rLRP157 cDNA was introduced into mouse neuroblastomaxrat glioma hybrid NG108-15 cells, and cells stably expressing rLRP157 (NG/LRP cells) were isolated. Binding of rLRP157 with the promoter sequence was confirmed by electrophoretic mobility shift assay using nuclear extract of NG/LRP cells. A luciferase reporter gene containing a promoter of beta CaM kinase II was transiently expressed in NG/LRP cells. Under the conditions, the promoter activity was enhanced about 2.6-fold in NG/LRP cells as compared with wild-type cells. The expression of rLRP157 mRNA was paralleled with that of beta CaM kinase II in the adult and embryo rat brain detected by in situ hybridization. Nuclear localization of rLRP157 was confirmed using GFP-rLRP157 fusion protein investigated under a confocal microscope. These results indicate that rLRP157 is one of the proteins binding to, and regulating the activity of, the promoter of beta CaM kinase II.

Laminin activates CaMK-II to stabilize nascent embryonic axons

In neurons, the interaction of laminin with its receptor, beta1 integrin, is accompanied by an increase in cytosolic Ca2+. Neuronal behavior is influenced by CaMK-II, the type II Ca2+/calmodulin-dependent protein kinase, which is enriched in axons of mouse embryonic neurons. In this study, we sought to determine whether CaMK-II is activated by laminin, and if so, how CaMK-II influences axonal growth and stability. Axons grew up to 200 microm within 1 day of plating P19 embryoid bodies on laminin-1 (EHS laminin). Activated CaMK-II was found enriched along the axon and in the growth cone as detected using a phospho-Thr(287) specific CaMK-II antibody. beta1 integrin was found in a similar pattern along the axon and in the growth cone. Direct inhibition of CaMK-II in 1-day-old neurons immediately froze growth cone dynamics, disorganized F-actin and ultimately led to axon retraction. Collapsed axonal remnants exhibited diminished phospho-CaMK-II levels. Treatment of 1-day neurons with a beta1 integrin-blocking antibody (CD29) also reduced axon length and phospho-CaMK-II levels and, like CaMK-II inhibitors, decreased CaMK-II activation. Among several CaMK-II variants detected in these cultures, the 52-kDa delta variant preferentially associated with actin and beta 3 tubulin as determined by reciprocal immunoprecipitation. Our findings indicate that persistent activation of delta CaMK-II by laminin stabilizes nascent embryonic axons through its influence on the actin cytoskeleton.

Study on the Regulation of Synaptic Function by Ca2+/Calmodulin-dependent Protein Kinase II

Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is one of the most abundant protein kinases in the mammalian brain, especially in the hippocampus. Neuronal CaMKII is a multifunctional mediator of activity dependent on an increase in the Ca(2+) level in excitable cells. It plays an important role in synaptic plasticity, including learning and memory, and is recognized as a "memory molecule." The expression of the kinase increases most rapidly during the most active phase in the formation of synapses in the postnatal brain and remains at a high level after synaptic maturation, indicating that the kinase is carefully regulated in the space-temporal gene expression. It is accumulated in the postsynaptic density (PSD), which is central in synaptic transmission. This review presents the gene expression and alternative splicing of CaMKII during neural differentiation, molecular constituents of PSD, and regulation of CaMKII by activity-regulated cytoskeleton-associated protein (Arc) mainly developed in our study.

Increased expression of deltaCaMKII isoforms in skeletal muscle regeneration: Implications in dystrophic muscle disease

The expression of delta isoforms of calcium-calmodulin/dependent protein kinase II (CaMKII) has been reported in mammalian skeletal muscle; however, their functions in this tissue are largely unknown. This study was conducted to determine if deltaCaMKII expression was altered during regeneration of skeletal muscle fibers in two distinct models. In the first model, necrosis and regeneration were induced in quadriceps of normal mice by intramuscular administration of 50% glycerol. Immunostaining and confocal microscopy revealed that deltaCaMKII expression was clearly enhanced in fibers showing centralized nuclei. The second model was the mdx mouse, which undergoes enhanced muscle necrosis and regeneration due to a mutation in the dystrophin gene. sern blot analysis of hind leg extracts from 4 to 6 week old mdx mice revealed that deltaCaMKII content was decreased when compared to age-matched control mice. This loss in delta kinase content was seen in myofibrillar and membrane fractions and was in contrast to unchanged deltaCaMKII levels in cardiac and brain extracts from dystrophic mice. Confocal microscopy of mdx quadriceps and tibialis muscle showed that deltaCaMKII expression was uniformly decreased in most fibers from dystrophic mice; however, enhanced kinase expression was observed in regenerating muscle fibers. These data support a fundamental role for deltaCaMKII in the regeneration process of muscle fibers in normal and mdx skeletal muscle and may have important implications in the reparative process following muscle death.

Neuronal Ca2+/calmodulin-dependent protein kinase II--discovery, progress in a quarter of a century, and perspective: implication for learning and memory

Much has been learned about the activity-dependent synaptic modifications that are thought to underlie memory storage, but the mechanism by which these modifications are stored remains unclear. A good candidate for the storage mechanism is Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). CaM kinase II is one of the most prominent protein kinases, present in essentially every tissue but most concentrated in brain. Although it has been about a quarter of a century since the finding, CaM kinase II has been of the major interest in the region of brain science. It plays a multifunctional role in many intracellular events, and the expression of the enzyme is carefully regulated in brain regions and during brain development. Neuronal CaM kinase II regulates important neuronal functions, including neurotransmitter synthesis, neurotransmitter release, modulation of ion channel activity, cellular transport, cell morphology and neurite extension, synaptic plasticity, learning and memory, and gene expression. Studies concerning this kinase have provided insight into the molecular basis of nerve functions, especially learning and memory, and indicate one direction for studies in the field of neuroscience. This review presents the molecular structure, properties and functions of CaM kinase II, as a major component of neurons, based mainly developed on findings made in our laboratory.

Neuronal cell type-specific promoter of the α CaM kinase II gene is activated by Zic2, a Zic family zinc finger protein

To understand the neuronal cell type-specific expression of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), we investigated binding proteins that specifically activated the promoter of the alpha isoform of CaM kinase II (alpha CaM kinase II). Proteins that bind the promoter sequence were found in rat brain nuclear extract by electrophoretic mobility shift assay. Then, we screened for binding proteins in a mouse brain cDNA library using the yeast one-hybrid system. Zic2, a Zic family zinc finger transcription factor, was identified as one of the binding proteins. To investigate the effect of Zic2 on the promoter activity, Zic2 cDNA was expressed with a luciferase reporter gene containing a neuronal cell type-specific promoter of alpha CaM kinase II in neuronal and non-neuronal cells. The promoter activity of alpha CaM kinase II was enhanced 1.3-5-fold in cultured neuronal cells by Zic2. The activation was varied among neuronal cell types. Zic2 also increased the promoter activity in non-neuronal cells, although the relative luciferase activites in non-neuronal cells were lower than those in neuronal cell lines. These results indicated that Zic2 was one of the proteins binding to, and regulating the activity of, the promoter of alpha CaM kinase II.

CaMK-II oligomerization potential determined using CFP/YFP FRET

Members of the Ca(2+)/calmodulin-dependent protein kinase II (CaMK-II) family are encoded throughout the animal kingdom by up to four genes (alpha, beta, gamma, and delta). Over three dozen known CaMK-II splice variants assemble into approximately 12-subunit oligomers with catalytic domains facing out from a central core. In this study, the catalytic domain of alpha, beta, and delta CaMK-IIs was replaced with cyan (CFP) or yellow fluorescent protein (YFP) for fluorescence resonance energy transfer (FRET) studies. FRET, when normalized to total CFP and YFP, reproducibly yielded values which reflected oligomerization preference, inter-subunit spacing, and localization. FRET occurred when individual CFP and YFP-linked CaMK-IIs were co-expressed, but not when they were expressed separately and then mixed. All hetero-oligomers exhibited FRET values that were averages of their homo-oligomeric parents, indicating no oligomeric preference or restriction. FRET for CaMK-II homo-oligomers was inversely proportional to the variable region length. FPs were monomerized (Leu221 to Lys221) for this study, thus eliminating any potential artifact caused by FP-CaMK-II aggregates. Our results indicate that alpha, beta, and delta CaMK-IIs can freely hetero-oligomerize and that increased variable region lengths place amino termini further apart, potentially influencing the rate of inter-subunit autophosphorylation.

Organization and evolution of multifunctional Ca2+/CaM-dependent protein kinase genes

The "multi-functional" Ca(2+) and calmodulin-dependent protein kinase, type II (CaMK-II) is an evolutionarily conserved protein. It has been found as a single gene in the horseshoe crab, marine sponge, sea urchin, nematode, and fruit fly, whereas most vertebrates possess four genes (alpha, beta, gamma, and delta). Species from fruit flies to humans encode alternative splice variants which are differentially targeted to phosphorylate diverse downstream targets of Ca(2+) signaling. By comparing known CaMK-II protein and nucleotide sequences, we have now provided evidence for the evolutionary relatedness of CaMK-IIs. Parsimony analyses unambiguously indicate that the four vertebrate CaMK-II genes arose via repeated duplications. Nucleotide phylogenies show consistent but moderate support for the placement of the vertebrate delta CaMK-II as the earliest diverging vertebrate gene. delta CaMK-II is the only gene with both central and C-terminal variable domains and has three to four times more intronic sequence than the other three genes. beta and gamma CaMK-II genes show strong sequence similarity and have comparable exon and intron organization and utilization. alpha CaMK-II is absent from amphibians (Xenopus laevis) and has the most restricted tissue specificity in mammals, whereas beta, gamma, and delta CaMK-IIs are expressed in most tissues. All 38 known mammalian CaMK-II splice variants were compiled with their tissue specificity and exon usage. Some of these variants use alternative 5' and 3' donors within a single exon as well as alternative promoters. These findings serve as an important benchmark for future phylogenetic, developmental, or biochemical studies on this important, conserved, and highly regulated gene family.

Nuclear CaMKII inhibits neuronal differentiation of PC12 cells without affecting MAPK or CREB activation

Ca(2+)/calmodulin-regulated protein kinase II (CaMKII) mediates many cellular events. The four CaMKII isoforms have numerous splice variants, three of which contain nuclear localization signals. Little is known about the role of nuclear localized CaMKII in neuronal development. To study this process, PC12 cells were transfected to produce CaMKII targeted to either the cytoplasm or the nucleus and then treated with nerve growth factor (NGF). NGF triggers a signaling cascade (MAPK) that results in the differentiation of PC12 cells into a neuronal phenotype, marked by neurite outgrowth. The present study found that cells expressing nuclear targeted CaMKII failed to grow neurites, whereas cells expressing cytoplasmic CaMKII readily produced neurites. Inhibition of neuronal differentiation by nuclear CaMKII was independent of MAPK signaling, as sustained Erk phosphorylation was not affected. Phosphorylation of CREB was also unaffected. Thus nuclear CaMKII modifies neuronal differentiation by a mechanism independent of MAPK and CREB activation.

Axonal localization of delta Ca2+/calmodulin-dependent protein kinase II in developing P19 neurons

Ca(2+)/calmodulin-dependent protein kinase, type II (CaMK-II) is an enzyme encoded by four genes (alpha, beta, gamma and delta) and traditionally associated with synaptic function in the adult central nervous system, but also believed to play a role during neuronal development. P19 mouse embryonic cells are a model system for neurogenesis and primarily express isozymes of delta CaMK-II. It is not yet known whether or where delta CaMK-II is expressed in P19 neurons. Using an antibody specific for the delta CaMK-II C-terminal tail, we detected a 20-fold increase in levels of delta CaMK-II along axons after 8 days of development. This coincides with increased mRNA and protein levels of delta(C) CaMK-II, which contains the alternative tail. This follows the initial stages of neurite outgrowth and beta(3) tubulin expression, which occur after 4 days. delta CaMK-II co-localizes with the axonal protein GAP-43, but not the dendritic microtubule-associated protein MAP-2, a known substrate of alpha CaMK-II. Like delta CaMK-II, GAP-43 shows increased expression after 8 days. These findings demonstrate developmental regulation of the alternative C-terminal delta CaMK-II exon and implicate endogenous delta CaMK-II in axonal development in embryonic cells.

Cytosolic targeting domains of gamma and delta calmodulin-dependent protein kinase II

Ca(2+)/calmodulin-dependent protein kinase II (CaMK-II) isozyme variability is the result of alternative usage of variable domain sequences. Isozyme expression is cell type-specific to transduce the appropriate Ca(2+) signals. We have determined the subcellular targeting domain of delta(E) CaMK-II, an isozyme that induces neurite outgrowth, and of a structurally similar isozyme, gamma(C) CaMK-II, which does not induce neurite outgrowth. delta(E) CaMK-II co-localizes with filamentous actin in the perinuclear region and in cellular extensions. In contrast, gamma(C) CaMK-II is uniformly cytosolic. Constitutively active delta(E) CaMK-II induces F-actin-rich extensions, thereby supporting a functional role for its localization. C-terminal constructs, which lack central variable domain sequences, can oligomerize and localize like full-length delta(E) and gamma(C) CaMK-II. Central variable domains themselves are monomeric and have no targeting capability. The C-terminal 95 residues of delta CaMK-II also has no targeting capability but can efficiently oligomerize. These findings define a targeting domain for gamma and delta CaMK-IIs that is in between the central variable and association domains. This domain is responsible for the subcellular targeting differences between gamma and delta CaMK-IIs.

Role of beta isoform-specific insertions of Ca2+/calmodulin-dependent protein kinase II

Alpha and beta isoforms of Ca2+/calmodulin-dependent protein kinase II (alpha and beta CaM kinase II, respectively) are highly conserved except for beta-specific insertions 1 and 2, located at amino acids 316-340 and 354-392, respectively. To investigate the role of these beta-specific insertions, we prepared the deletion mutants betaDelta1, betaDelta2 and betaDelta1/2, which lacked insertions 1, 2 and both, respectively. These mutant DNAs were expressed in neuroblastoma cells and compared with the wild-type enzyme. Green fluorescent protein tagged CaM kinase II was used to further explore the distribution of the kinase in living cells. Most (80%) of wild-type beta and mutant betaDelta1 were located in the particulate fraction, and distributed in the cell body and neurites, forming punctate or spot-like structures in the neurites. Mutants betaDelta2 and betaDelta1/2 were distributed in almost equal amounts in the soluble and particulate fractions. They were concentrated in the base of neurites and only partlially distributed throughout neurites, indicating that their transport to neurites was impaired. Beta(1-410), a deletion mutant of the association domain with a monomeric form, was located primarily in the soluble fraction. These results indicate that insertion 2, the association domain, and the oligomeric form of beta CaM kinase II play an important role in the cellular distribution of beta CaM kinase II.