Studies on the regulatory domain of Ca2+/calmodulin-dependent protein kinase II by expression of mutated cDNAs in Escherichia coli

Methods optimization for the expression and purification of human calcium calmodulin-dependent protein kinase II alpha

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a complex multifunctional kinase that is highly expressed in central nervous tissues and plays a key regulatory role in the calcium signaling pathway. Despite over 30 years of recombinant expression and characterization studies, CaMKII continues to be investigated for its impact on signaling cooperativity and its ability to bind multiple substrates through its multimeric hub domain. Here we compare and optimize protocols for the generation of full-length wild-type human calcium/calmodulin-dependent protein kinase II alpha (CaMKIIα). Side-by-side comparison of expression and purification in both insect and bacterial systems shows that the insect expression method provides superior yields of the desired autoinhibited CaMKIIα holoenzymes. Utilizing baculovirus insect expression system tools, our results demonstrate a high yield method to produce homogenous, monodisperse CaMKII in its autoinhibited state suitable for biophysical analysis. Advantages and disadvantages of these two expression systems (baculovirus insect cell versus Escherichia coli expression) are discussed, as well as purification optimizations to maximize the enrichment of full-length CaMKII.

Ca2+/Calmodulin-Dependent Protein Kinase II in Vascular Smooth Muscle

Ca²⁺-dependent signaling pathways are central regulators of differentiated vascular smooth muscle (VSM) contractile function. In addition, Ca²⁺ signals regulate VSM gene transcription, proliferation, and migration of dedifferentiated or "synthetic" phenotype VSM cells. Synthetic phenotype VSM growth and hyperplasia are hallmarks of pervasive vascular diseases including hypertension, atherosclerosis, postangioplasty/in-stent restenosis, and vein graft failure. The serine/threonine protein kinase Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) is a ubiquitous mediator of intracellular Ca²⁺ signals. Its multifunctional nature, structural complexity, diversity of isoforms, and splice variants all characterize this protein kinase and make study of its activity and function challenging. The kinase has unique autoregulatory mechanisms, and emerging studies suggest that it can function to integrate Ca²⁺ and reactive oxygen/nitrogen species signaling. Differentiated VSM expresses primarily CaMKIIγ and -δ isoforms. CaMKIIγ isoform expression correlates closely with the differentiated phenotype, and some studies link its function to regulation of contractile activity and Ca²⁺ homeostasis. Conversely, synthetic phenotype VSM cells primarily express CaMKIIδ and substantial evidence links it to regulation of gene transcription, proliferation, and migration of VSM in vitro, and vascular hypertrophic and hyperplastic remodeling in vivo. CaMKIIδ and -γ isoforms have opposing functions at the level of cell cycle regulation, proliferation, and VSM hyperplasia in vivo. Isoform switching following vascular injury is a key step in promoting vascular remodeling. Recent availability of genetically engineered mice with smooth muscle deletion of specific isoforms and transgenics expressing an endogenous inhibitor protein (CAMK2N) has enabled a better understanding of CaMKII function in VSM and should facilitate future studies.

Calmodulin kinase II activation is required for the maintenance of basal activity of L-type Ca2+ channels in guinea-pig ventricular myocytes

The roles of calmodulin (CaM)-dependent protein kinase II (CaMKII) in the maintenance of basal activity and the reversion of run-down of L-type Ca2+ channels were studied in guinea-pig ventricular myocytes by the patch-clamp technique. In the cell-attached configuration, the Ca2+-channel activity was inhibited to 82% - 26% by 1-10 microM KN-93 and to 92% - 66% by 0.1-1 microM autocamtide-2-related inhibitory peptide (AIP) myristoylated. In the inside-out configuration, the bovine cardiac cytoplasm recovered Ca2+-channel activity to 87% of that recorded in the cell-attached configuration, while the CaMKII inhibitor 281-301 at 10 microM reduced the recovery effect to 19%. CaM + ATP recovered the channel activity to 93% and 28% of that recorded in the cell-attached configuration when applied at 1 and 5 min after run-down, respectively, showing a time-dependent attenuation. However, in the presence of 0.33 microM CaMKII, this attenuation was abolished, showing 85% and 75% recovery when applied at 1 and 5 min after run-down, respectively. This recovery effect was suppressed by 10 microM AIP, applied at 5 min, but not at 1 min after run-down. We concluded that CaMKII activation is required in the maintenance of basal activity of L-type Ca2+ channels.

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.

High level expression and preparation of autonomous Ca2+/calmodulin-dependent protein kinase II in Escherichia coli

The chymotryptic fragment of Ca2+/calmodulin-dependent protein kinase II (30K-CaMKII) is a constitutively active enzyme that phosphorylates a variety of protein substrates in vitro. Although 30K-CaMKII is an often used and powerful tool for protein phosphorylation, the efficient production of catalytically active 30K-CaMKII in Escherichia coli has not yet been successfully realized, probably due to its toxicity in host cells. In this study, we found that a high-level expression of 30K-CaMKII as an insoluble form was attained when the N-terminal 43 amino acid residues of Xenopus CaMKI were fused to the N-terminal end of 30K-CaMKII (CX-30K-CaMKII). The inactive CX-30K-CaMKII thus expressed in E. coli was reactivated by simple denaturation/renaturation processes and purified on a Ni2+-chelating column. The renatured CX-30K-CaMKII exhibited specific activity similar to that of rat brain CaMKII, and phosphorylated various proteins such as histones, myosin light chain, myelin basic protein, and synapsin I, as in case of 30K-CaMKII or purified CaMKII. Thus, CX-30K-CaMKII, an autonomous CaMKII, can be obtained with a simple procedure using E. coli expression system.

Interaction of peptide substrate outside the active site influences catalysis by CaMKII

The interaction of calcium/calmodulin-dependent protein kinase II (CaMKII) with the NR2B subunit of N-methyl-D-aspartate-type glutamate receptor is thought to be one of the important events leading to synaptic plasticity. CaMKII binds NR2B by its catalytic site and by the autophosphorylation site binding pocket (APBP), a non-catalytic site. Mutagenesis of Glu-236, a residue in the APBP of CaMKII that is likely to be interacting with NR2B, influences phosphorylation of NR2B. The phosphorylation of syntide-2, a classical catalytic site substrate of CaMKII, is influenced to a much lesser extent by this mutation. Taken together these results indicate that interaction of NR2B at the non-catalytic site of CaMKII influences catalysis. Our data suggest that kinetic models of peptide substrate phosphorylation by CaMKII should incorporate the non-catalytic mode of binding of peptides that is dependent on the sequence of the peptide.

Immunohistochemical localization of Ca(2+)/calmodulin-dependent protein kinase kinase beta in the rat central nervous system

We examined regional and intracellular distribution of Ca(2+)/calmodulin-dependent protein kinase kinase beta (CaM-KK beta), which activated Ca(2+)/calmodulin-dependent protein kinase I and IV (CaM-K I and IV) immunohistochemically in the central nervous system of the rat by light and electron microscopy. Although most neurons in the brain and spinal cord exhibited the immunoreactivity, no labeled neurons were observed in the globus pallidus or entopeduncular nucleus, and only a small number of neurons showed weak immunoreactivity in the substantia nigra pars reticulata. In general, the immunoreactivity was observed both in the cytoplasm and cellular nucleus, although the immunoreactivity was not found in the cellular nucleus in some large neurons such as in the mesencephalic trigeminal nucleus, lateral vestibular nucleus or gigant cellular reticular formation. As to motoneurons of the cranial nerve nuclei and the anterior horn of the spinal cord, they revealed the immunoreactivity both in the cytoplasm and nucleus. The reaction product appeared as fine granules in the cytoplasm and nucleus under light microscopy. Electron microscopic observations confirmed that the reaction product was localized mainly on the Golgi apparatus or on the nuclear chromatin. Immunolabeling for antibody against CaM-KK beta was discussed with the distribution of CaM-K I, IV and another CaM-KK, CaM-KK alpha, in the central nervous system.

Calbrain, a novel two EF-hand calcium-binding protein that suppresses Ca2+/calmodulin-dependent protein kinase II activity in the brain

A cDNA clone that encodes a novel Ca2+-binding protein was isolated from a human brain cDNA library. The gene for this clone, termed calbrain, encodes a 70-amino acid polypeptide with a predicted molecular mass of 8.06 kDa. The analysis of deduced amino acid sequence revealed that calbrain contains two putative EF-hand motifs that show significantly high homology to those of the calmodulin (CaM) family rather than two EF-hand protein families. By Northern hybridization analysis, an approximate 1.5-kilobase pair transcript of calbrain was detected exclusively in the brain, and in situ hybridization study revealed its abundant expression in the hippocampus, habenular area in the epithalamus, and in the cerebellum. A recombinant calbrain protein showed a Ca2+ binding capacity, suggesting the functional potency as a regulator of Ca2+-mediated cellular processes. Ca2+/calmodulin-dependent kinase II, the most abundant protein kinase in the hippocampus and strongly implicated in the basic neuronal functions, was used to evaluate the physiological roles of calbrain. Studies in vitro revealed that calbrain competitively inhibited CaM binding to Ca2+/calmodulin-dependent kinase II (Ki = 129 nM) and reduced its kinase activity and autophosphorylation.

Regulatory segments of Ca2+/calmodulin-dependent protein kinases

Catalytic cores of skeletal and smooth muscle myosin light chain kinases and Ca2+/calmodulin-dependent protein kinase II are regulated intrasterically by different regulatory segments containing autoinhibitory and calmodulin-binding sequences. The functional properties of these regulatory segments were examined in chimeric kinases containing either the catalytic core of skeletal muscle myosin light chain kinase or Ca2+/calmodulin-dependent protein kinase II with different regulatory segments. Recognition of protein substrates by the catalytic core of skeletal muscle myosin light chain kinase was altered with the regulatory segment of protein kinase II but not with smooth muscle myosin light chain kinase. Similarly, the catalytic properties of the protein kinase II were altered with regulatory segments from either myosin light chain kinase. All chimeric kinases were dependent on Ca2+/calmodulin for activity. The apparent Ca2+/calmodulin activation constant was similarly low with all chimeras containing the skeletal muscle catalytic core. The activation constant was greater with chimeric kinases containing the catalytic core of Ca2+/calmodulin-dependent protein kinase II with its endogenous or myosin light chain kinase regulatory segments. Thus, heterologous regulatory segments affect substrate recognition and kinase activity. Furthermore, the sensitivity to calmodulin activation is determined primarily by the respective catalytic cores, not the calmodulin-binding sequences.

Evidence that autophosphorylation at Thr-286/Thr-287 is required for full activation of calmodulin-dependent protein kinase II

Calmodulin-dependent protein kinase II (CaM-kinase II) undergoes a very rapid autophosphorylation at Thr-286/Thr-287 in the presence of Ca2+/calmodulin and ATP/Mg2+, and this has greatly hampered studies on the role of the autophosphorylation in the regulation of the enzyme activity, because it has been difficult to measure the activity of the non-autophosphorylated enzyme in the presence of Ca2+/calmodulin. In the present study, this difficulty was overcome by using adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) in place of ATP. When the enzyme was assayed with 2 microM ATP gamma S and 200 microM syntide-2 at 5 degrees C in the presence of Ca2+/calmodulin, the linear reaction rate of thiophosphorylation of syntide-2 was much slower than that of the enzyme which had previously undergone autothiophosphorylation. Under the limiting assay conditions, thiophosphorylation of the enzyme did not occur significantly during assay. Using this assay condition, activation by autothiophosphorylation was examined. When CaM-kinase II was autothiophosphorylated at 5 degrees C, the concomitant stimulation of both activities in the presence and absence of Ca2+/calmodulin was observed. The activity of the recombinant wild-type enzyme in the presence of Ca2+/calmodulin as well as in its absence was also markedly activated upon autothiophosphorylation, whereas those of the recombinant mutated enzyme, whose Thr-287 was replaced by Ala, was not activated at all. These results provide strong support for the contention that CaM-kinase II initially possesses a basal low level of the total activity and that the initial rapid autophosphorylation on Thr-286/Thr-287 results in full activation of the enzyme.

Calmodulin and the regulation of smooth muscle contraction

Calmodulin, the ubiquitous and multifunctional Ca(2+)-binding protein, mediates many of the regulatory effects of Ca2+, including the contractile state of smooth muscle. The principal function of calmodulin in smooth muscle is to activate crossbridge cycling and the development of force in response to a [Ca2+]i transient via the activation of myosin light-chain kinase and phosphorylation of myosin. A distinct calmodulin-dependent kinase, Ca2+/calmodulin-dependent protein kinase II, has been implicated in modulation of smooth-muscle contraction. This kinase phosphorylates myosin light-chain kinase, resulting in an increase in the calmodulin concentration required for half-maximal activation of myosin light-chain kinase, and may account for desensitization of the contractile response to Ca2+. In addition, the thin filament-associated proteins, caldesmon and calponin, which inhibit the actin-activated MgATPase activity of smooth-muscle myosin (the cross-bridge cycling rate), appear to be regulated by calmodulin, either by the direct binding of Ca2+/calmodulin or indirectly by phosphorylation catalysed by Ca2+/calmodulin-dependent protein kinase II. Another level at which calmodulin can regulate smooth-muscle contraction involves proteins which control the movement of Ca2+ across the sarcolemmal and sarcoplasmic reticulum membranes and which are regulated by Ca2+/calmodulin, e.g. the sarcolemmal Ca2+ pump and the ryanodine receptor/Ca2+ release channel, and other proteins which indirectly regulate [Ca2+]i via cyclic nucleotide synthesis and breakdown, e.g. NO synthase and cyclic nucleotide phosphodiesterase. The interplay of such regulatory mechanisms provides the flexibility and adaptability required for the normal functioning of smooth-muscle tissues.

The multifunctional Ca2+/calmodulin-dependent protein kinases

Ca2+ mediates the effect of many hormones, neurotransmitters and growth factors on contractility and motility, carbohydrate metabolism, cell cycle, gene expression and neuronal plasticity. Multifunctional Ca2+/calmodulin-dependent (CaM) kinase, CaM kinase Ia, CaM kinase Ib and CaM kinase IV are four of the kinases that mediate Ca(2+)-signaling pathways. Recent studies have clarified our understanding of their structure, regulation and function.

Multifunctional Ca2+/calmodulin-dependent protein kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is a prominent mediator of neurotransmitters which elevate Ca2+. It coordinates cellular responses to external stimuli by phosphorylating proteins involved in neurotransmitter synthesis, neurotransmitter release, carbohydrate metabolism, ion flux and neuronal plasticity. Structure/function studies of CaM kinase have provided insights into how it decodes Ca2+ signals. The kinase is kept relatively inactive in its basal state by the presence of an autoinhibitory domain. Binding of Ca2+/calmodulin eliminates this inhibitory constraint and allows the kinase to phosphorylate its substrates, as well as itself. This autophosphorylation significantly slows dissociation of calmodulin, thereby trapping calmodulin even when Ca2+ levels are subthreshold. The kinase may respond particularly well to multiple Ca2+ spikes since trapping may enable a spike frequency-dependent recruitment of calmodulin with each successive Ca2+ spike leading to increased activation of the kinase. Once calmodulin dissociates, CaM kinase remains partially active until it is dephosphorylated, providing for an additional period in which its response to brief Ca2+ transients is potentiated.

Developmental changes in the levels of Ca2+/calmodulin-dependent protein kinase II alpha and beta proteins in soluble and particulate fractions of the rat brain

Developmental changes in Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) have been immunochemically examined in the forebrain, cerebellum and brainstem of the rat using antibodies against its alpha or beta protein. The concentration of alpha and beta proteins varied markedly in different brain regions at increasing postnatal ages. In early postnatal brain, the concentration of the alpha and beta proteins was low, and a large increase was observed between postnatal days 10 and 30. The maximum expression of the alpha protein was in the order of 6.01, 2.33 and 0.168 micrograms/mg of forebrain, brainstem and cerebellum proteins respectively, in the soluble or particulate fraction. On the other hand, that of the beta protein was in the order of 1.81, 0.495 and 0.291 micrograms/mg of forebrain, cerebellum or brainstem protein. The ratio of alpha and beta proteins also differed in the soluble and particulate fractions. The maximum expression of the alpha protein was observed at day 30 in soluble and particulate fractions of forebrain, and at day 20 in those of the brainstem. The major alpha protein peak was observed on or after day 30 in particulate and soluble fractions from cerebellum, respectively. The maximum expression of the beta protein was observed at day 20 in soluble and particulate fractions of the forebrain as well as in soluble fraction of the cerebellum, and was observed at day 30 in the particulate fraction of cerebellum. The expression of the alpha and beta proteins roughly correlated with the CaM kinase II activity from forebrain and brainstem.

Decoding calcium signals by multifunctional CaM kinase

Multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) is one of the three major protein kinases coordinating cellular responses to hormones and neurotransmitters. It mediates the action of Ca2+ on neurotransmitter synthesis and release, on carbohydrate metabolism and on the cytoskeleton. CaM kinase has structural/functional properties that facilitate its response to distinctive attributes of Ca2+ signals which often involve transient increases that span a narrow concentration range and increases that are pulsatile rather than persistent. The kinase responds to the narrow working range of Ca2+ signals by the use of calmodulin as the Ca2+ sensor. It is activated by the binding of calmodulin to an autoinhibitory domain that keeps the kinase inactive in the basal state. The transient nature of the signal is accommodated by autophosphorylation of this autoinhibitory domain which allows the kinase to remain partially active after calmodulin dissociates and thereby switches it to a Ca(2+)-independent species. The pulsatile nature of Ca2+ signals may also be decoded by CaM kinase. Autophosphorylation traps calmodulin on autophosphorylated subunits by greatly reducing its off-rate. At high frequency of stimulation, calmodulin would remain trapped during the brief interval between Ca2+ oscillations and each successive rise in Ca2+ would recruit more calmodulin. This may enable a stimulus frequency dependent activation of CaM kinase.