Multifunctional Ca2+/calmodulin-dependent protein kinase is necessary for nuclear envelope breakdown

Interplay of Ca2+ and K+ signals in cell physiology and cancer

The cytoplasmic Ca2+ concentration and the activity of K+ channels on the plasma membrane regulate cellular processes ranging from mitosis to oriented migration. The interplay between Ca2+ and K+ signals is intricate, and different cell types rely on peculiar cellular mechanisms. Derangement of these mechanisms accompanies the neoplastic progression. The calcium signals modulated by voltage-gated (KV) and calcium-dependent (KCa) K+ channel activity regulate progression of the cell division cycle, the release of growth factors, apoptosis, cell motility and migration. Moreover, KV channels regulate the cell response to the local microenvironment by assembling with cell adhesion and growth factor receptors. This chapter summarizes the pathophysiological roles of Ca2+ and K+ fluxes in normal and cancer cells, by concentrating on several biological systems in which these functions have been studied in depth, such as early embryos, mammalian cell lines, T lymphocytes, gliomas and colorectal cancer cells. A full understanding of the underlying mechanisms will offer a comprehensive view of the ion channel implication in cancer biology and suggest potential pharmacological targets for novel therapeutic approaches in oncology.

Calcium, a Cell Cycle Commander, Drives Colon Cancer Cell Diffpoptosis

The story of the cell commonder, calcium, reaches into all corners of the cell and controls cell proliferation, differentiation, function, and even death. The calcium-driven eukaryotic revolution is one of the great turning points in the life history, happened about two billion years later when it was converted from a dangerous killer that had to be kept out of cell into the cell master which drives the cell. This review article will take the readers to a tour of tissues chosen to best show the calcium's many faces (proliferator, differentiator, and killer). The reader will first see calcium and its many helpers, such as the calcium-binding signaler protein calmodulin, directing the key events of the cell cycle. Then the tour will move onto the colon to show calcium driving the proliferation of progenitor cells, then the differentiation and ultimately the programmed death of their progeny. Moreover, the reader will learn of the striking disabling and bypassing of calcium-dependent control mechanisms during carcinogenesis. Finally, recommendations should be taken from the underlying mechanisms through which calcium masters the presistance, progression, and even apoptosis of colorectal cancer cells. Thus, this could be of great interest for designing of chemoprevention protocols.

A novel crosstalk between calcium/calmodulin kinases II and IV regulates cell proliferation in myeloid leukemia cells

CaMKs link transient increases in intracellular Ca(2+) with biological processes. In myeloid leukemia cells, CaMKII, activated by the bcr-abl oncogene, promotes cell proliferation. Inhibition of CaMKII activity restricts cell proliferation, and correlates with growth arrest and differentiation. The mechanism by which the inhibition of CaMKII results in growth arrest and differentiation in myeloid leukemia cells is still unknown. We report that inhibition of CaMKII activity results in an upregulation of CaMKIV mRNA and protein in leukemia cell lines. Conversely, expression of CaMKIV inhibits autophosphorylation and activation of CaMKII, and elicits G0/G1cell cycle arrest,impairing cell proliferation. Furthermore, U937 cells expressing CaMKIV show elevated levels of Cdk inhibitors p27(kip1) and p16(ink4a) and reduced levels of cyclins A, B1 and D1. These findings were also confirmed in the K562 leukemic cell line. The relationship between CaMKII and CaMKIV is also observed in primary acute myeloid leukemia (AML) cells, and it correlates with their immunophenotypic profile. Indeed, immature MO/M1 AML showed increased CaMKIV expression and decreased pCaMKII, whereas highly differentiated M4/M5 AML showed decreased CaMKIV expression and increased pCaMKII levels. Our data reveal a novel cross-talk between CaMKII and CaMKIV and suggest that CaMKII suppresses the expression of CaMKIV to promote leukemia cell proliferation.

Ca(2+) signaling, genes and the cell cycle

Changes in the concentration and spatial distribution of Ca(2+) ions in the cytoplasm constitute a ubiquitous intracellular signaling module in cellular physiology. With the advent of Ca(2+) dyes that allow direct visualization of Ca(2+) transients, combined with powerful experimental tools such as electrophysiological recordings, intracellular Ca(2+) transients have been implicated in practically every aspect of cellular physiology, including cellular proliferation. Ca(2+) signals are associated with different phases of the cell cycle and interfering with Ca(2+) signaling or downstream pathways often disrupts progression of the cell cycle. Although there exists a dependence between Ca(2+) signals and the cell cycle the mechanisms involved are not well defined and given the cross-talk between Ca(2+) and other signaling modules, it is difficult to assess the exact role of Ca(2+) signals in cell cycle progression. Two exceptions however, include fertilization and T-cell activation, where well-defined roles for Ca(2+) signals in mediating progression through specific stages of the cell cycle have been clearly established. In the case of T-cell activation Ca(2+) regulates entry into the cell cycle through the induction of gene transcription.

Regulation of store-operated Ca2+ entry during the cell cycle

Cytoplasmic Ca(2+) signals are central to numerous cell physiological processes, including cellular proliferation. Historically, much of the research effort in this area has focused on the role of Ca(2+) signals in cell-cycle progression. It is becoming clear, however, that the relationship between Ca(2+) signaling and the cell cycle is a 'two-way street'. Specifically, Ca(2+)-signaling pathways are remodeled during M phase, leading to altered Ca(2+) dynamics. Such remodeling probably better serves the large variety of functions that cells must perform during cell division compared with during interphase. This is clearly the case during oocyte meiosis, because remodeling of Ca(2+) signals partially defines the competence of the egg to activate at fertilization. Store-operated Ca(2+) entry (SOCE) is a ubiquitous Ca(2+)-signaling pathway that is regulated during M phase. In this Commentary, we discuss the latest advances in our understanding of how SOCE is regulated during cell division.

cdc2-cyclin B regulates eEF2 kinase activity in a cell cycle- and amino acid-dependent manner

The calcium/calmodulin-dependent kinase that phosphorylates and inactivates eukaryotic elongation factor 2 (eEF2 kinase; eEF2K) is subject to multisite phosphorylation, which regulates its activity. Phosphorylation at Ser359 inhibits eEF2K activity even at high calcium concentrations. To identify the kinase that phosphorylates Ser359 in eEF2K, we developed an extensive purification protocol. Tryptic mass fingerprint analysis identified it as cdc2 (cyclin-dependent kinase 1). cdc2 co-purifies with Ser359 kinase activity and cdc2-cyclin B complexes phosphorylate eEF2K at Ser359. We demonstrate that cdc2 contributes to controlling eEF2 phosphorylation in cells. cdc2 is activated early in mitosis. Kinase activity against Ser359 in eEF2K also peaks at this stage of the cell cycle and eEF2 phosphorylation is low in mitotic cells. Inactivation of eEF2K by cdc2 may serve to keep eEF2 active during mitosis (where calcium levels rise) and thereby permit protein synthesis to proceed in mitotic cells. Amino-acid starvation decreases cdc2's activity against eEF2K, whereas loss of TSC2 (a negative regulator of mammalian target of rapamycin complex 1(mTORC1)) increases it. These data closely match the control of Ser359 phosphorylation and indicate that cdc2 may be regulated by mTORC1.

Ca2+ homeostasis regulates Xenopus oocyte maturation

In contrast to the well-defined role of Ca2+ signals during mitosis, the contribution of Ca2+ signaling to meiosis progression is controversial, despite several decades of investigating the role of Ca2+ and its effectors in vertebrate oocyte maturation. We have previously shown that during Xenopus oocyte maturation, Ca2+ signals are dispensable for entry into meiosis and for germinal vesicle breakdown. However, normal Ca2+ homeostasis is essential for completion of meiosis I and extrusion of the first polar body. In this study, we test the contribution of several downstream effectors in mediating the Ca2+ effects during oocyte maturation. We show that calmodulin and calcium-calmodulin-dependent protein kinase II (CAMK2) are not critical downstream Ca2+ effectors during meiotic maturation. In contrast, accumulation of Aurora kinase A (AURKA) protein is disrupted in cells deprived of Ca2+ signals. Since AURKA is required for bipolar spindle formation, failure to accumulate AURKA may contribute to the defective spindle phenotype following Ca2+ deprivation. These findings argue that Ca2+ homeostasis is important in establishing the oocyte's competence to undergo maturation in preparation for fertilization and embryonic development.

Dual mechanism of a natural CaMKII inhibitor

Ca(2+)/calmodulin (CaM)-dependent protein kinase II (CaMKII) is a major mediator of cellular Ca(2+) signaling. Several inhibitors are commonly used to study CaMKII function, but these inhibitors all lack specificity. CaM-KIIN is a natural, specific CaMKII inhibitor protein. CN21 (derived from CaM-KIIN amino acids 43-63) showed full specificity and potency of CaMKII inhibition. CNs completely blocked Ca(2+)-stimulated and autonomous substrate phosphorylation by CaMKII and autophosphorylation at T305. However, T286 autophosphorylation (the autophosphorylation generating autonomous activity) was only mildly affected. Two mechanisms can explain this unusual differential inhibitor effect. First, CNs inhibited activity by interacting with the CaMKII T-site (and thereby also interfered with NMDA-type glutamate receptor binding to the T-site). Because of this, the CaMKII region surrounding T286 competed with CNs for T-site interaction, whereas other substrates did not. Second, the intersubunit T286 autophosphorylation requires CaM binding both to the "kinase" and the "substrate" subunit. CNs dramatically decreased CaM dissociation, thus facilitating the ability of CaM to make T286 accessible for phosphorylation. Tat-fusion made CN21 cell penetrating, as demonstrated by a strong inhibition of filopodia motility in neurons and insulin secrection from isolated Langerhans' islets. These results reveal the inhibitory mechanism of CaM-KIIN and establish a powerful new tool for dissecting CaMKII function.

alpha-CaMKII controls the growth of human osteosarcoma by regulating cell cycle progression

Osteosarcoma is the most frequent type of primary bone cancer in children and adolescents. These malignant osteoid forming tumors are characterized by their uncontrolled hyperproliferation. Here, we investigate the role of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) in the growth of human osteosarcoma. We show that alpha-CaMKII is expressed in human osteosarcoma cell lines and in primary osteosarcoma tissue derived from patients. The pharmacologic inhibition of CaMKII in MG-63 and 143B human osteosarcoma cells by KN-93 resulted in an 80 and 70% decrease in proliferation, respectively, and induced cell cycle arrest in the G(0)/G(1) phase. The in vivo administration of KN-93 to mice xenografted with human osteosarcoma cells significantly decreased intratibial and subcutaneous tumor growth. Mechanistically, KN-93 and alpha-CaMKII siRNA increased p21((CIP/KIP)) gene expression, protein levels, and decreased the phosphorylation of retinoblastoma protein and E2F transactivation. Furthermore, the inhibition of CaMKII decreased membrane-bound Tiam1 and GTP-bound Rac1, which are known to be involved in p21 expression and tumor growth in a variety of solid malignant neoplasms. Our results suggest that CaMKII plays a critical role in the growth of osteosarcoma, and its inhibition could be an attractive therapeutic target to combat conventional high-grade osteosarcoma in children.

Two phases of calcium requirement during starfish meiotic maturation

During meiosis in oocytes of the starfish, Asterina pectinifera, a Ca(2+) transient has been observed. To clarify the role of Ca(2+) during oocyte maturation in starfish, an intracellular Ca(2+) blocker, TMB-8, was applied. The oocyte maturation induced by 1-methyladenine (1-MA) was blocked by 100 microM TMB-8. Reinitiation of meiosis with germinal vesicle breakdown (GVBD) and the following chromosome condensation did not take place. Maturation-promoting factor (MPF) activity did not increase and GVBD and chromosome condensation did not occur. Ca(2+) transient observed immediately after 1-MA application in control oocytes was also blocked by TMB-8. When calyculin A, which activate the MPF directly, was applied to the oocytes instead of 1-MA in seawater containing 100 microM TMB-8, GVBD and chromosome condensation were blocked. Cytoplasmic transplantation studies confirmed that MPF was activated, although TMB-8 blocked GVBD. These results show that TMB-8 blocked the increase of MPF activity induced by 1-MA and the process of active MPF inducing GVBD and subsequent chromosome condensation. Together with the above phenomena, it is conceivable that there are two phases of Ca(2+) requirement during starfish oocyte maturation. These are the activation of MPF, moreover, GVBD, and the subsequent chromosome condensation.

Differential expression of CaMK-II genes during early zebrafish embryogenesis

CaMK-II is a highly conserved Ca(2+)/calmodulin-dependent protein kinase expressed throughout the lifespan of all vertebrates. During early development, CaMK-II regulates cell cycle progression and "non-canonical" Wnt-dependent convergent extension. In the zebrafish, Danio rerio, CaMK-II activity rises within 2 hr after fertilization. At the time of somite formation, zygotic expression from six genes (camk2a1, camk2b1, camk2g1, camk2g2, camk2d1, camk2d2) results in a second phase of increased activity. Zebrafish CaMK-II genes are 92-95% identical to their human counterparts in the non-variable regions. During the first three days of development, alternative splicing yields at least 20 splice variants, many of which are unique. Whole-mount in situ hybridization reveals that camk2g1 comprises the majority of maternal expression. All six genes are expressed strongly in ventral regions at the 18-somite stage. Later, camk2a1 is expressed in anterior somites, heart, and then forebrain. Camk2b1 is expressed in somites, mid- and forebrain, gut, retina, and pectoral fins. Camk2g1 appears strongly along the midline and then in brain, gut, and pectoral fins. Camk2g2 is expressed early in the midbrain and trunk and exhibits the earliest retinal expression. Camk2d1 is elevated early at somite boundaries, then epidermal tissue, while camk2d2 is expressed in discrete anterior locations, steadily increasing along either side of the dorsal midline and then throughout the brain, including the retina. These findings reveal a complex pattern of CaMK-II gene expression consistent with pleiotropic roles during development.

Manipulation of protein kinases reveals different mechanisms for upregulation of heat shock proteins in motor neurons and non-neuronal cells

Motor neurons have a high threshold for induction of heat shock proteins (Hsps) in response to stress, a property associated with impaired ability to activate heat shock transcription factor 1 (Hsf1). Hyperphosphorylation of Hsf1 has been established as a requirement for transactivation of heat shock genes. This study demonstrated that the impaired heat shock response in motor neurons is not due to altered phosphorylation of Hsf1 by kinases previously shown to affect activation of Hsf1 in other cells (PKC, GSK3beta, ERK1, CaMKIIalpha). However, a constitutively active form of CaMKIV induced robust expression of Hsp70, as well as transcription of a GFP reporter gene driven by the human inducible Hsp70 promoter in unstressed motor neurons, but not in mouse embryonic fibroblasts. The results point to novel mechanisms of activation of heat shock genes in motor neurons that have relevance to exploitation of endogenous stress responses therapeutically.

Calcium microdomains and cell cycle control

The cell division cycle comprises successive rounds of genome replication and segregation that are never error-free. A complex signalling network chaperones cell cycle events to ensure that cell cycle progression does not occur until any errors detected are put right. The signalling network consists of cell cycle control proteins that are phosphorylated and dephosphorylated, synthesized and degraded interactively to generate a set of sensors and molecular switches that are thrown at appropriate times to permit or trigger cell cycle progression. In early embryos, discrete calcium signals have been shown to be a key component of the molecular switch mechanism. In somatic cells in contrast, the participation of calcium signals in cell cycle control is far from clear. Recent experiments in syncytial Drosophila embryos have shown that localised calcium signals in the nucleus and mitotic spindle can be detected. It appears that the nucleus comprises a calcium signalling microdomain bounded by endoplasmic reticulum that isolates the nucleus and spindle. These findings offer a possible explanation for the apparent absence of calcium signals in somatic cells during mitosis.

The sea urchin kinome: a first look

This paper reports a preliminary in silico analysis of the sea urchin kinome. The predicted protein kinases in the sea urchin genome were identified, annotated and classified, according to both function and kinase domain taxonomy. The results show that the sea urchin kinome, consisting of 353 protein kinases, is closer to the Drosophila kinome (239) than the human kinome (518) with respect to total kinase number. However, the diversity of sea urchin kinases is surprisingly similar to humans, since the urchin kinome is missing only 4 of 186 human subfamilies, while Drosophila lacks 24. Thus, the sea urchin kinome combines the simplicity of a non-duplicated genome with the diversity of function and signaling previously considered to be vertebrate-specific. More than half of the sea urchin kinases are involved with signal transduction, and approximately 88% of the signaling kinases are expressed in the developing embryo. These results support the strength of this nonchordate deuterostome as a pivotal developmental and evolutionary model organism.

A functional genomic and proteomic perspective of sea urchin calcium signaling and egg activation

The sea urchin egg has a rich history of contributions to our understanding of fundamental questions of egg activation at fertilization. Within seconds of sperm-egg interaction, calcium is released from the egg endoplasmic reticulum, launching the zygote into the mitotic cell cycle and the developmental program. The sequence of the Strongylocentrotus purpuratus genome offers unique opportunities to apply functional genomic and proteomic approaches to investigate the repertoire and regulation of Ca(2+) signaling and homeostasis modules present in the egg and zygote. The sea urchin "calcium toolkit" as predicted by the genome is described. Emphasis is on the Ca(2+) signaling modules operating during egg activation, but the Ca(2+) signaling repertoire has ramifications for later developmental events and adult physiology as well. Presented here are the mechanisms that control the initial release of Ca(2+) at fertilization and additional signaling components predicted by the genome and found to be expressed and operating in eggs at fertilization. The initial release of Ca(2+) serves to coordinate egg activation, which is largely a phenomenon of post-translational modifications, especially dynamic protein phosphorylation. Functional proteomics can now be used to identify the phosphoproteome in general and specific kinase targets in particular. This approach is described along with findings to date. Key outstanding questions regarding the activation of the developmental program are framed in the context of what has been learned from the genome and how this knowledge can be applied to functional studies.

Extracellular tau is toxic to neuronal cells

The degeneration of neurons in disorders such as Alzheimer's disease has an immediate consequence, the release of intracellular proteins into the extracellular space. One of these proteins, tau, has proven to be toxic when added to cultured neuronal cells. This toxicity varies according to the degree of protein aggregation. The addition of tau to cultured neuroblastoma cells provoked an increase in the levels of intracellular calcium, which is followed by cell death. We suggest that this phenomenon may be mediated by the interaction of tau with muscarinic receptors, which promotes the liberation of calcium from intracellular stores.

When a sperm meets an egg: block to polyspermy

Embryonic development is initiated after the fertilizing spermatozoon enters the egg and triggers a series of events known as egg activation. Activation results in an increase in intracellular calcium concentration, cortical granule exocytosis (CGE), cell cycle resumption and recruitment of maternal mRNA. CGE is an evolutionary developed mechanism that causes modification of the zona pellucida to prevent penetration of additional spermatozoa, ensuring successful egg activation and embryo development. The egg CGE is a unique and convenient mammalian model for studying the different proteins participating at the membrane fusion cascade, which, unlike other secretory cells, occurs only once in the egg's lifespan. This article highlights a number of proteins, ascribed to participate in CGE and thus the block to polyspermy. CGE can be triggered either by a calcium dependent pathway, or via protein kinase C (PKC) activation that requires a very low calcium concentration. In a recent study, we suggested that the filamentous actin (F-actin) at the egg's cortex is a dynamic network. It can be maneuvered towards allowing CGE by activated actin associated proteins and/or by activated PKC and its down stream proteins, such as myristoylated alanine-rich C kinase substrate (MARCKS). MARCKS, a protein known to cross-link F-actin in other cell types, was found to be expressed and colocalized with actin in non-activated MII eggs. We further demonstrated MARCKS dissociation from actin after activation by ionomycin, a process that can lead to the breakdown of the actin network, thus allowing CGE. The more we know of the intricate process of CGE and of the proteins participating in it, the more the assisted reproductive procedures might benefit from that knowledge.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

Microdomains bounded by endoplasmic reticulum segregate cell cycle calcium transients in syncytial Drosophila embryos

Cell cycle calcium signals are generated by the inositol trisphosphate (InsP₃)–mediated release of calcium from internal stores (Ciapa, B., D. Pesando, M. Wilding, and M. Whitaker. 1994. Nature. 368:875–878; Groigno, L., and M. Whitaker. 1998. Cell. 92:193–204). The major internal calcium store is the endoplasmic reticulum (ER); thus, the spatial organization of the ER during mitosis may be important in shaping and defining calcium signals. In early Drosophila melanogaster embryos, ER surrounds the nucleus and mitotic spindle during mitosis, offering an opportunity to determine whether perinuclear localization of ER conditions calcium signaling during mitosis. We establish that the nuclear divisions in syncytial Drosophila embryos are accompanied by both cortical and nuclear localized calcium transients. Constructs that chelate InsP₃ also prevent nuclear division. An analysis of nuclear calcium concentrations demonstrates that they are differentially regulated. These observations demonstrate that mitotic calcium signals in Drosophila embryos are confined to mitotic microdomains and offer an explanation for the apparent absence of detectable global calcium signals during mitosis in some cell types.

An increase in [Ca2+]i is sufficient but not necessary for driving mitosis in early mouse embryos

An increase in intracellular Ca2+ concentration ([Ca2+]i) has been shown to drive sea-urchin embryos and some fibroblasts through nuclear-envelope breakdown (NEBD) and the metaphase-to-anaphase transition. Mitotic Ca2+ transients can be pan-cellular global events or localized to the perinuclear region. It is not known whether Ca2+ is a universal regulator of mitosis or whether its role is confined to specific cell types. To test the hypothesis that Ca2+ is a universal regulator of mitosis, we have investigated the role of Ca2+ in mitosis in one-cell mouse embryos. Fertilized embryos generate Ca2+ transients during the first mitotic division. Imposing a Ca2+ transient by photorelease of inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] resulted in acceleration of mitosis entry, suggesting that a [Ca2+]i increase is capable of triggering mitosis. Mitotic Ca2+ transients were inhibited using three independent approaches: injection of intracellular Ca2+ buffers; downregulation of Ins(1,4,5)P3 receptors; and removal of extracellular Ca2+. None of the interventions had any effects on the timing of NEBD or cytokinesis. The possibility that NEBD is driven by localized perinuclear Ca2+ transients was examined using two-photon microscopy but no Ca2+-dependent increases in fluorescence were found to precede NEBD. Finally, the second mitotic division took place in the absence of any detectable [Ca2+]i increase. Thus, although an induced [Ca2+]i increase can accelerate mitosis entry, neither cytosolic nor perinuclear [Ca2+] increases appear to be necessary for progression through mitosis in mouse embryos.

Inhibiting MAP kinase activity prevents calcium transients and mitosis entry in early sea urchin embryos

A transient calcium increase triggers nuclear envelope breakdown (mitosis entry) in sea urchin embryos. Cdk1/cyclin B kinase activation is also known to be required for mitosis entry. More recently, MAP kinase activity has also been shown to increase during mitosis. In sea urchin embryos, both kinases show a similar activation profile, peaking at the time of mitosis entry. We tested whether the activity of both kinases is required for mitosis entry and whether either kinase controls mitotic calcium signals. We found that reducing the activity of either mitotic kinase prevents nuclear envelope breakdown, despite the presence of a calcium transient, when cdk1/cyclin B kinase activity is alone inhibited. When MAP kinase activity alone was inhibited, the calcium signal was absent, suggesting that MAP kinase activity is required to generate the calcium transient that triggers nuclear envelope breakdown. However, increasing intracellular free calcium by microinjection of calcium buffers or InsP(3) while MAP kinase was inhibited did not itself induce nuclear envelope breakdown, indicating that additional MAP kinase-regulated events are necessary. After MAP kinase inhibition early in the cell cycle, the early events of the cell cycle (pronuclear migration/fusion and DNA synthesis) were unaffected, but chromosome condensation and spindle assembly are prevented. These data indicate that in sea urchin embryos, MAP kinase activity is part of a signaling complex alongside two components previously shown to be essential for entry into mitosis: the calcium transient and the increase in cdk1/cyclinB kinase activity.

Cell signalling during sea urchin development: a model for assessing toxicity of environmental contaminants

The early development of sea urchins has been thoroughly studied since the beginning of the 20th century thanks to the particular features of the model involving cell signalling, making it easy to follow the complex cell-to-cell interactions that lead to development. In this chapter, the prominent role of cell-to-cell communication in developmental events is discussed, as well as the role of intracellular ion changes that are in turn regulated by signal molecules belonging to the cholinergic system. The results seem to indicate that the zygote stage is the most suitable to study the role of the cholinergic system, as at this stage, a calcium spike can be evoked by exposure to acetylcholine (ACh) or to muscarinic drugs, at any time before the nuclear breakdown. The described outcomes also open a path to a new way of considering biomarkers. In fact, most environmental factors have the capacity to interfere with the cholinergic system: stress, wounds, inflammation and pollution in general. In particular, this offers a way to investigate the presence in the environment and the degree of aggressiveness of neurotoxic contaminants, such as organophosphate and carbamate pesticides, largely used in European countries for many purposes, including agricultural pest control and medical treatment. These drugs exert their function by interfering with the regulation of the cholinergic system and the consequent electrical events. Thus, the sea urchin zygote could represent a reliable model to be used in biosensors with the capacity to translate the effect of neurotoxic pesticides, and generally of stress-inducing contaminants, in living cell responses, such as electrical responses.

Ca(2+)(cyt) negatively regulates the initiation of oocyte maturation

Ca²⁺ is a ubiquitous intracellular messenger that is important for cell cycle progression. Genetic and biochemical evidence support a role for Ca²⁺ in mitosis. In contrast, there has been a long-standing debate as to whether Ca²⁺ signals are required for oocyte meiosis. Here, we show that cytoplasmic Ca²⁺ (Ca²⁺_cyt) plays a dual role during Xenopus oocyte maturation. Ca²⁺ signals are dispensable for meiosis entry (germinal vesicle breakdown and chromosome condensation), but are required for the completion of meiosis I. Interestingly, in the absence of Ca²⁺_cyt signals oocytes enter meiosis more rapidly due to faster activation of the MAPK-maturation promoting factor (MPF) kinase cascade. This Ca²⁺-dependent negative regulation of the cell cycle machinery (MAPK-MPF cascade) is due to Ca²⁺_cyt acting downstream of protein kinase A but upstream of Mos (a MAPK kinase kinase). Therefore, high Ca²⁺_cyt delays meiosis entry by negatively regulating the initiation of the MAPK-MPF cascade. These results show that Ca²⁺ modulates both the cell cycle machinery and nuclear maturation during meiosis.

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.

Molecular cloning and characterization of a novel calcium/calmodulin-dependent protein kinase II inhibitor from human dendritic cells

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a widely distributed protein kinase that regulates numerous physiological functions. Inhibitors of CaMKII are useful tools for investigating the CaMKII functions. Here we identified a novel CaMKII inhibitor protein (CaM-KIIN) from the human dendritic cell cDNA library by large-scale random sequencing. Human CaM-KIIN contains 79 amino acids, which shares 98% identity and 98% positives with rat CaMKII inhibitor protein beta and 65% identity and 78% positives with rat CaMKII inhibitor alpha. Human CaM-KIIN mRNA expression was detectable in various tissues and cell lines by Northern blot and RT-PCR. To investigate its biological functions, full-length human CaM-KIIN was overexpressed in colon adenocarcinoma LoVo cells. When expressed in LoVo cells, it could inhibit cell proliferation, block cell growth, and decrease the viable cell number. These results characterize a potential cellular inhibitor protein of CaMKII that plays an important role in the regulation of cell growth.

Calcium and mitosis

Calcium is thought to be involved in regulating mitotic transitions. The basis for this view is set out. Recent data from experiments on sea urchin embryos is discussed. The relative simplicity of the embryonic cell cycle and the relative ease with which cell physiology can be done in sea urchin embryos has allowed the clear demonstration that the phosphoinositide-calcium-calmodulin signalling pathway is required for and regulates mitosis entry and anaphase onset. The relevance of the sea urchin work to mitosis in other cell types is briefly discussed.

Breakdown of cytoskeletal proteins during meiosis of starfish oocytes and proteolysis induced by calpain

Meiosis reinitiation in starfish oocytes is characterized by Ca(2+) transients in the cytosol and in the nucleus and is accompanied by the disassembly of the nuclear envelope, a process which is likely to be mediated by the cleavage of selected proteins. We have used mass spectrometry analysis (mass profile fingerprinting) on 2D polyacrylamide gels of extracts of oocytes in which meiosis resumption was induced by 1-methyladenine and have identified five proteins that were specifically degraded: alpha-tubulin, lamin B, dynamin, and two kinds of actin. They are all components of the cytoskeleton or associated with it. We then investigated whether calpain, which is activated by the increase in cell Ca(2+), could cleave the same proteins that became degraded under the influence of 1-methyladenine and thus be involved in nuclear membrane breakdown. The investigation was prompted by the finding that microinjection of calpain into the nuclei of prophase arrested oocytes induced meiosis in the absence of 1-methyladenine. Incubation of prophase arrested (disrupted) oocytes with calpain produced a 2D gel protein pattern in which some of the degradation products coincided with those seen in oocytes challenged with 1-methyladenine.

Inhibition of calcium/calmodulin kinase II alpha subunit expression results in epileptiform activity in cultured hippocampal neurons

Several models that develop epileptiform discharges and epilepsy have been associated with a decrease in the activity of calmodulin-dependent kinase II. However, none of these studies has demonstrated a causal relationship between a decrease in calcium/calmodulin kinase II activity and the development of seizure activity. The present study was conducted to determine the effect of directly reducing calcium/calmodulin-dependent kinase activity on the development of epileptiform discharges in hippocampal neurons in culture. Complimentary oligonucleotides specific for the alpha subunit of the calcium/calmodulin kinase were used to decrease the expression of the enzyme. Reduction in kinase expression was confirmed by Western analysis, immunocytochemistry, and exogenous substrate phosphorylation. Increased neuronal excitability and frank epileptiform discharges were observed after a significant reduction in calmodulin kinase II expression. The epileptiform activity was a synchronous event and was not caused by random neuronal firing. Furthermore, the magnitude of decreased kinase expression correlated with the increased neuronal excitability. The data suggest that decreased calmodulin kinase II activity may play a role in epileptogenesis and the long-term plasticity changes associated with the development of pathological seizure activity and epilepsy.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.

Calcium/calmodulin-dependent protein kinase II containing a nuclear localizing signal is altered in retinal neurons exposed to N-methyl-D-aspartate

This study investigated N-methyl-D-aspartate (NMDA) mediated cell death and its possible regulation by calcium/calmodulin-dependent protein kinase II (CaMKII) in the adult rat retina. To investigate cell death, the terminal deoxyribonucleotidyltransferase (TdT)-mediated biotin-16-dUTP nick-end labelling (TUNEL) method was used to detect fragmented DNA in fixed tissue sections of rat retina. The TUNEL assay confirmed that apoptosis occurs in the inner nuclear layer (INL) and ganglion cell layer (GCL) following NMDA injection. The level of antibody binding to CaMKII-alpha, the activity of CaMKII, and the mRNA level for the alpha(B) subunit of CaMKII were found to be elevated for short time periods (30 min, 2 h) after a single intravitreal injection of NMDA. In contrast to this, there was a decrease in CaMKII activity and in the CaMKII-alpha(B) mRNA levels at longer time periods (24 h) following injection of NMDA. These effects were specific for the mRNA for the alpha(B) subunit, an alternatively spliced product of the CaMKII-alpha gene, that contains a nuclear localizing signal (NLS) known to target this protein to the nucleus. It is suggested that regulated expression of CaMKII-alpha(B) could be involved in the NMDA-mediated cell death in retinal neurons.

NMDA receptor-mediated control of protein synthesis at developing synapses

We demonstrate a rapid and complex effect of N-methyl-d-aspartate receptor (NMDAR) activation on synaptic protein synthesis in the superior colliculi of young rats. Within minutes of receptor activation, translation of alpha Ca2+/calmodulin dependent kinase II (alphaCamK II) was increased, whereas total protein synthesis was reduced. NMDAR activation also increased phosphorylation of eukaryotic elongation factor 2 (eEF2), a process known to inhibit protein translation by reducing peptide chain elongation. Low doses of cycloheximide, which reduce elongation rate independently of eEF2 phosphorylation, decreased overall protein synthesis but increased alphaCaMK II synthesis. These observations suggest that regulation of peptide elongation via eEF2 phosphorylation can link NMDAR activation to local increases in the synthesis of specific proteins during activity-dependent synaptic change.

Timing of calcium and protein synthesis requirements for the first mitotic cell cycle in fertilised Xenopus eggs

Mitosis is governed by the activity of the M-phase promoting factor (MPF). In some systems, particularly early embryos, transient increases in calcium concentration have been shown to be necessary for mitosis and regulate its timing. By microinjection of the calcium buffer, dibromoBAPTA, into fertilised Xenopus eggs, we have assessed whether calcium events are required to initiate MPF activation and inactivation. Since initial experiments showed that this buffer inhibited protein synthesis, we measured when mitosis and cleavage became independent of translation. We found that, after a period of protein synthesis essential for cleavage, there was a phase during which continued translation affected the timing of cleavage, but was not essential for its occurrence. Measurement of MPF activity in single embryos injected with calcium buffer at different times in the first cell cycle, showed that there were two sensitive periods. The first period of sensitivity blocked MPF activation and coincided with the time at which cleavage became completely independent of protein synthesis. The second sensitive period occurred just before histone kinase activity peaked, and was necessary for kinase inactivation. Preventing inactivation in this way arrested egg extracts in mitosis. These results support the view that transient increases in free calcium concentration contribute to mitotic progression by first triggering MPF activation and subsequently, with elevated MPF activity, inducing its inactivation.

Downregulation of delta CaM kinase II in human tumor cells

Over two dozen alternative splice variants of CaMK-II, the type II Ca(2+)/CaM-dependent protein kinase, are encoded from four genes (alpha, beta, gamma and delta) in mammalian cells. Isozymes of alpha and beta CaMK-II are well characterized in brain; however, an understanding of the relative endogenous levels of CaMK-II isozymes in a wide variety of non-neuronal cells has not yet been described. In this study, we have demonstrated that CaMK-II consists primarily of the 54 kDa delta CaMK-II (delta(2) or delta(C)) isozyme in rodent fibroblasts. beta and gamma CaMK-II isozymes are minor and alpha CaMK-II was not expressed. The primary delta CaMK-II in human fibroblasts and the MCF10A mammary epithelial cell line was the 52 kDa delta(4) CaMK-II, an isozyme identical to delta(2) except for a missing 21-amino-acid C-terminal tail. delta CaMK-II levels were diminished in both human and rodent fibroblasts after SV40 transformation and in the mammary adenocarcinoma MCF7 cell line when compared to MCF10A cells. In fact, most tumor cells exhibited CaMK-II specific activities which were two- to tenfold lower than in untransformed fibroblasts. We conducted complementary CaMK-II studies on the NGF-induced differentiation of rat PC-12 cells. Although no new synthesis of CaMK-II occurs, neurite outgrowth in these cells is accompanied by a preferential activation of delta CaMK-II. Endogenous delta CaMK-II has a perinuclear distribution in fibroblasts and extends along neurites in PC-12 cells. These findings point to a role for delta CaMK-II isozymes in cellular differentiation.

Structural examination of autoregulation of multifunctional calcium/calmodulin-dependent protein kinase II

Regulation of Ca(2+)/calmodulin-dependent protein kinase II is likely based on an auto-inhibitory mechanism in which a segment of the kinase occupies the catalytic site in the absence of calmodulin. We analyze potential auto-inhibitory associations by employing charge reversal and hydrophobic-to-charged residue mutagenesis. We identify interacting amino acid pairs by using double mutants to test which modification in the catalytic domain complements a given change in the auto-inhibitory domain. Our studies identify the core pseudosubstrate sequence (residues 297-300) but reveal that distinct sequences centered about the autophosphorylation site at Thr-286 are involved in the critical auto-inhibitory interactions. Individual changes in any of the residues Arg-274, His-282, Arg-283, Lys-291, Arg-297, Phe-293, and Asn-294 in the auto-inhibitory domain or their interacting partners in the catalytic domain produces an enhanced affinity for calmodulin or generates a constitutively active enzyme. A structural model of Ca(2+)/calmodulin-dependent protein kinase II that incorporates these interactions shows that Thr-286 is oriented inwardly into a hydrophobic channel. The model explains why calmodulin must bind to the auto-inhibitory domain in order for Thr-286 in that domain to be phosphorylated and why introduction of phospho-Thr-286 produces the important Ca(2+)-independent state of the enzyme.

Xenon induces metaphase arrest in rat astrocytes

The use of xenon as an almost ideal anesthetic with very little side effects is gaining clinical acceptance, yet its effects on the cellular level are still unclear. It affects intracellular Ca2+-homeostasis but up to now no cellular event or Ca2+-signaling system has been described to be specifically sensitive to xenon. Here we report for the first time a specific effect of xenon on astroglial cells not found with another volatile anesthetic, isoflurane, nor with helium nor with N2: treatment of primary astroglial cells from embryonic rat brain with xenon induces, apart from a slight retardation of the cell cycle, a block at metaphase. Upon removal of xenon cells arrested at metaphase complete their mitosis normally. Even under continuous exposure to xenon, cells can be rescued from metaphase arrest by a small and transient increase in intracellular Ca2+; cells enter anaphase despite the presence of xenon and complete cell division, exhibiting normal rate of chromosome movement and normal chromosome separation. These results suggest that xenon interferes with some Ca2+-dependent regulatory system required for the metaphase-anaphase transition; taking into account its anesthetic effects, xenon may be also involved in the control of glia-mediated signaling transfer.

Functional implications of the subunit composition of neuronal CaM kinase II

The assembly of 6-12 subunits of Ca(2+)/calmodulin-dependent kinase II (CaM kinase II) into holoenzymes is an important structural feature of the enzyme and its postulated role as a molecular detector of Ca(2+) oscillations. Using single cell reverse transcriptase-polymerase chain reaction, we show that alpha- and beta-CaM kinase II mRNAs are simultaneously present in the majority of hippocampal neurons examined and that co-assembly of their protein products into heteromers is therefore possible. The subunit composition of CaM kinase II holoenzymes was analyzed by immunoprecipitation with subunit-specific monoclonal antibodies. Rat forebrain CaM kinase II consists of heteromers composed of alpha and beta subunits at a ratio of 2:1 and homomers composed of only alpha subunits. We examined the functional effect of the heteromeric assembly by analyzing the calmodulin dependence of autophosphorylation. Recombinant homomers of alpha- or beta-CaM kinase II, as well as of alternatively spliced beta isoforms, have distinct calmodulin dependences for autophosphorylation based on differences in their calmodulin affinities. Half-maximal autophosphorylation of alpha is achieved at 130 nM calmodulin, while that for beta occurs at 15 nM calmodulin. In CaM kinase II isolated from rat forebrain, however, the calmodulin dependence for autophosphorylation of the beta subunits is shifted toward that of alpha homomers. This suggests that Thr(287) in beta subunits is phosphorylated by alpha subunits present in the same holoenzyme. Once autophosphorylated, beta-CaM kinase II traps calmodulin by reducing the rate of calmodulin dissociation.

Comparative biology of calcium signaling during fertilization and egg activation in animals

During animal fertilizations, each oocyte or egg must produce a proper intracellular calcium signal for development to proceed normally. As a supplement to recent synopses of fertilization-induced calcium responses in mammals, this paper reviews the spatiotemporal properties of calcium signaling during fertilization and egg activation in marine invertebrates and compares these patterns with what has been reported for other animals. Based on the current database, fertilization causes most oocytes or eggs to generate multiple wavelike calcium oscillations that arise at least in part from the release of internal calcium stores sensitive to inositol 1,4,5-trisphosphate (IP3). Such calcium waves are modulated by upstream pathways involving oolemmal receptors and/or soluble sperm factors and in turn regulate calcium-sensitive targets required for subsequent development. Both "protostome" animals (e.g., mollusks, annelids, and arthropods) and "deuterostomes" (e.g., echinoderms and chordates) display fertilization-induced calcium waves, IP3-mediated calcium signaling, and the ability to use a combination of external calcium influx and internal calcium release. Such findings fail to support the dichotomy in calcium signaling modes that had previously been proposed for protostomes vs deuterostomes and instead suggest that various features of fertilization-induced calcium signals are widely shared throughout the animal kingdom.

Reconstitution of regulated exocytosis in cell-free systems: a critical appraisal

Regulated exocytosis involves the tightly controlled fusion of a transport vesicle with the plasma membrane. It includes processes as diverse as the release of neurotransmitters from presynaptic nerve endings and the sperm-triggered deposition of a barrier preventing polyspermy in oocytes. Cell-free model systems have been developed for studying the biochemical events underlying exocytosis. They range from semi-intact permeabilized cells to the reconstitution of membrane fusion from isolated secretory vesicles and their target plasma membranes. Interest in such cell-free systems has recently been reinvigorated by new evidence suggesting that membrane fusion is mediated by a basic mechanism common to all intracellular fusion events. In this chapter, we review some of the literature in the light of these new developments and attempt to provide a critical discussion of the strengths and limitations of the various cell-free systems.

Calcium/calmodulin-dependent phosphorylation and activation of human Cdc25-C at the G2/M phase transition in HeLa cells

The human tyrosine phosphatase (p54(cdc25-c)) is activated by phosphorylation at mitosis entry. The phosphorylated p54(cdc25-c) in turn activates the p34-cyclin B protein kinase and triggers mitosis. Although the active p34-cyclin B protein kinase can itself phosphorylate and activate p54(cdc25-c), we have investigated the possibility that other kinases may initially trigger the phosphorylation and activation of p54(cdc25-c). We have examined the effects of the calcium/calmodulin-dependent protein kinase (CaM kinase II) on p54(cdc25-c). Our in vitro experiments show that CaM kinase II can phosphorylate p54(cdc25-c) and increase its phosphatase activity by 2.5-3-fold. Treatment of a synchronous population of HeLa cells with KN-93 (a water-soluble inhibitor of CaM kinase II) or the microinjection of AC3-I (a specific peptide inhibitor of CaM kinase II) results in a cell cycle block in G2 phase. In the KN-93-arrested cells, p54(cdc25-c) is not phosphorylated, p34(cdc2) remains tyrosine phosphorylated, and there is no increase in histone H1 kinase activity. Our data suggest that a calcium-calmodulin-dependent step may be involved in the initial activation of p54(cdc25-c).

Calcium/calmodulin-dependent protein kinase II is associated with the N-methyl-D-aspartate receptor

The molecular basis of long-term potentiation (LTP), a long-lasting change in synaptic transmission, is of fundamental interest because of its implication in learning. Usually LTP depends on Ca2+ influx through postsynaptic N-methyl-D-aspartate (NMDA)-type glutamate receptors and subsequent activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII). For a molecular understanding of LTP it is crucial to know how CaMKII is localized to its postsynaptic targets because protein kinases often are targeted to their substrates by adapter proteins. Here we show that CaMKII directly binds to the NMDA receptor subunits NR1 and NR2B. Moreover, activation of CaMKIIalpha by stimulation of NMDA receptors in forebrain slices increase this association. This interaction places CaMKII not only proximal to a major source of Ca2+ influx but also close to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-type glutamate receptors, which become phosphorylated upon stimulation of NMDA receptors in these forebrain slices. Identification of the postsynaptic adapter for CaMKII fills a critical gap in the understanding of LTP because CaMKII-mediated phosphorylation of AMPA receptors is an important step during LTP.

Sequential PKC- and Cdc2-mediated phosphorylation events elicit zebrafish nuclear envelope disassembly

Molecular markers of the zebrafish inner nuclear membrane (NEP55) and nuclear lamina (L68) were identified, partially characterized and used to demonstrate that disassembly of the zebrafish nuclear envelope requires sequential phosphorylation events by first PKC, then Cdc2 kinase. NEP55 and L68 are immunologically and functionally related to human LAP2beta and lamin B, respectively. Exposure of zebrafish nuclei to meiotic cytosol elicits rapid phosphorylation of NEP55 and L68, and disassembly of both proteins. L68 phosphorylation is completely inhibited by simultaneous inhibition of Cdc2 and PKC and only partially blocked by inhibition of either kinase. NEP55 phosphorylation is completely prevented by inhibition or immunodepletion of cytosolic Cdc2. Inhibition of cAMP-dependent kinase, MEK or CaM kinase II does not affect NEP55 or L68 phosphorylation. In vitro, nuclear envelope disassembly requires phosphorylation of NEP55 and L68 by both mammalian PKC and Cdc2. Inhibition of either kinase is sufficient to abolish NE disassembly. Furthermore, novel two-step phosphorylation assays in cytosol and in vitro indicate that PKC-mediated phosphorylation of L68 prior to Cdc2-mediated phosphorylation of L68 and NEP55 is essential to elicit nuclear envelope breakdown. Phosphorylation elicited by Cdc2 prior to PKC prevents nuclear envelope disassembly even though NEP55 is phosphorylated. The results indicate that sequential phosphorylation events elicited by PKC, followed by Cdc2, are required for zebrafish nuclear disassembly. They also argue that phosphorylation of inner nuclear membrane integral proteins is not sufficient to promote nuclear envelope breakdown, and suggest a multiple-level regulation of disassembly of nuclear envelope components during meiosis and at mitosis.

Chimeric calcium/calmodulin-dependent protein kinase in tobacco: differential regulation by calmodulin isoforms

cDNA clones of chimeric Ca2+/calmodulin-dependent protein kinase (CCaMK) from tobacco (TCCaMK-1 and TCCaMK-2) were isolated and characterized. The polypeptides encoded by TCCaMK-1 and TCCaMK-2 have 15 different amino acid substitutions, yet they both contain a total of 517 amino acids. Northern analysis revealed that CCaMK is expressed in a stage-specific manner during anther development. Messenger RNA was detected when tobacco bud sizes were between 0.5 cm and 1.0 cm. The appearance of mRNA coincided with meiosis and became undetectable at later stages of anther development. The reverse polymerase chain reaction (RT-PCR) amplification assay using isoform-specific primers showed that both of the CCaMK mRNAs were expressed in anther with similar expression patterns. The CCaMK protein expressed in Escherichia coli showed Ca2+-dependent autophosphorylation and Ca2+/calmodulin-dependent substrate phosphorylation. Calmodulin isoforms (PCM1 and PCM6) had differential effects on the regulation of autophosphorylation and substrate phosphorylation of tobacco CCaMK, but not lily CCaMK. The evolutionary tree of plant serine/threonine protein kinases revealed that calmodulin-dependent kinases form one subgroup that is distinctly different from Ca2+-dependent protein kinases (CDPKs) and other serine/threonine kinases in plants.

Effect of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, on the first mitotic divisions of the fertilized sea urchin egg

We have reported earlier that the polyphosphoinositide messenger system may control mitosis in sea urchin eggs. Besides phospholipase C activation and its second messengers, phosphatidylinositol (PI) 3-kinase has been proposed to affect a wide variety of cellular processes in other cellular systems. Therefore, we have investigated whether PI 3-kinase could play a role in regulating the sea urchin early embryonic development. Our data presented here suggest that PI 3-kinase is present in sea urchin eggs. We found that wortmannin, an inhibitor of PI 3-kinase, led to arrest of the cell cycle. Chromosome condensation, nuclear envelope breakdown, microtubular aster polymerization, protein and DNA synthesis were not affected when fertilization was performed in the presence of the drug. However, maturation-promoting factor (MPF) activation was inhibited and centrosome duplication was perturbed preventing the formation of a bipolar mitotic spindle in wortmannin treated eggs. We discuss how PI 3-kinase might be involved in the cascade of events leading to the first mitotic divisions of the fertilized sea urchin egg.

Phosphorylation at the nuclear localization signal of Ca2+/calmodulin-dependent protein kinase II blocks its nuclear targeting

Translocation of protein kinases with broad substrate specificities between different subcellular compartments by activation of signaling pathways is an established mechanism to direct the activity of these enzymes toward particular substrates. Recently, we identified two isoforms of Ca2+/calmodulin-dependent protein kinase II (CaM kinase II), which are targeted to the nucleus by an alternatively spliced nuclear localization signal (NLS). Here we report that cotransfection with constitutively active mutants of CaM kinase I or CaM kinase IV specifically blocks nuclear targeting of CaM kinase II as a result of phosphorylation of a Ser immediately adjacent to the NLS of CaM kinase II. Both CaM kinase I and CaM kinase IV are able to phosphorylate this Ser residue in vitro, and mutagenesis studies suggest that this phosphorylation is both necessary and sufficient to block nuclear targeting. Furthermore, we provide experimental evidence that introduction of a negatively charged residue at this phosphorylation site reduces binding of the kinase to an NLS receptor in vitro, thus providing a mechanism that may explain the blockade of nuclear targeting that we have observed in situ.