Regulation by phosphorylation of reversible association of a myristoylated protein kinase C substrate with the plasma membrane

Modulation of phosphoinositide-protein kinase C signal transduction by omega-3 fatty acids: implications for the pathophysiology and treatment of recurrent neuropsychiatric illness

The phosphoinositide (PI)-protein kinase C (PKC) signal transduction pathway is initiated by pre- and postsynaptic Galphaq-coupled receptors, and regulates several clinically relevant neurochemical events, including neurotransmitter release efficacy, monoamine receptor function and trafficking, monoamine transporter function and trafficking, axonal myelination, and gene expression. Mounting evidence for PI-PKC signaling hyperactivity in the peripheral (platelets) and central (premortem and postmortem brain) tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, coupled with evidence that PI-PKC signal transduction is down-regulated in rat brain following chronic, but not acute, treatment with antipsychotic, mood-stabilizer, and antidepressant medications, suggest that PI-PKC hyperactivity is central to an underlying pathophysiology. Evidence that membrane omega-3 fatty acids act as endogenous antagonists of the PI-PKC signal transduction pathway, coupled with evidence that omega-3 fatty acid deficiency is observed in peripheral and central tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, support the hypothesis that omega-3 fatty acid deficiency may contribute to elevated PI-PKC activity in these illnesses. The data reviewed in this paper outline a potential molecular mechanism by which omega-3 fatty acids could contribute to the pathophysiology and treatment of recurrent neuropsychiatric illness.

Myristoylated, alanine-rich C-kinase substrate phosphorylation regulates growth cone adhesion and pathfinding

Repellents evoke growth cone turning by eliciting asymmetric, localized loss of actin cytoskeleton together with changes in substratum attachment. We have demonstrated that semaphorin-3A (Sema3A)-induced growth cone detachment and collapse require eicosanoid-mediated activation of protein kinase C epsilon (PKC epsilon) and that the major PKC epsilon target is the myristoylated, alanine-rich C-kinase substrate (MARCKS). Here, we show that PKC activation is necessary for growth cone turning and that MARCKS, while at the membrane, colocalizes with alpha3-integrin in a peripheral adhesive zone of the growth cone. Phosphorylation of MARCKS causes its translocation from the membrane to the cytosol. Silencing MARCKS expression dramatically reduces growth cone spread, whereas overexpression of wild-type MARCKS inhibits growth cone collapse triggered by PKC activation. Expression of phosphorylation-deficient, mutant MARCKS greatly expands growth cone adhesion, and this is characterized by extensive colocalization of MARCKS and alpha3-integrin, resistance to eicosanoid-triggered detachment and collapse, and reversal of Sema3A-induced repulsion into attraction. We conclude that MARCKS is involved in regulating growth cone adhesion as follows: its nonphosphorylated form stabilizes integrin-mediated adhesions, and its phosphorylation-triggered release from adhesions causes localized growth cone detachment critical for turning and collapse.

Phosphorylation of Rho-associated kinase (Rho-kinase/ROCK/ROK) substrates by protein kinases A and C

Rho-associated kinase (Rho-kinase/ROCK/ROK) is a serine/threonine kinase and plays an important role in various cellular functions. The cAMP-dependent protein kinase (protein kinase A/PKA) and protein kinase C (PKC) are also serine/threonine kinases, and directly and/or indirectly take part in the signal transduction pathways of Rho-kinase. They have similar phosphorylation site motifs, RXXS/T and RXS/T. The purpose of this study was to identify whether sites phosphorylated by Rho-kinase could be targets for PKA and PKC and to find peptide substrates that are specific to Rho-kinase, i.e., with no phosphorylation by PKA and PKC. A total of 18 substrates for Rho-kinase were tested for phosphorylation by PKA and PKC. Twelve of these sites were easily phosphorylated. These results mean that Rho-kinase substrates can be good substrates for PKA and/or PKC. On the other hand, six Rho-kinase substrates showing no or very low phosphorylation efficiency (<20%) for PKA and PKC were identified. Kinetic parameters (K(m) and k(cat)) showed that two of these peptides could be useful as substrates specific to Rho-kinase phosphorylation.

Analysis of a bacterial lipopolysaccharide-activated serine kinase that phosphorylates p65/L-plastin in macrophages

We previously identified p65/L-plastin as a phosphorylated protein in LPS-stimulated macrophages and determined its phosphorylation site. In vitro kinase assay using peptide substrates revealed that LPS-stimulated kinase activity selectively phosphorylated their serine-5 (Ser-5) residue. Kinase inhibitors for cAMP-dependent kinase such as H-89 inhibited the Ser-5 phosphorylation, but cAMP was not essential for the kinase activity. The LPS-stimulated kinase activity in cytosol fractions of macrophages was recovered as a sharp peak by anion exchange chromatography. These findings suggest that an as yet unknown H-89-sensitive serine kinase is rapidly activated by LPS stimulation and then phosphorylates p65/L-plastin, playing a vital role in macrophage activation.

Trif-related adapter molecule is phosphorylated by PKC{epsilon} during Toll-like receptor 4 signaling

PKCepsilon has been shown to play a key role in the effect of the Gram-negative bacterial product LPS; however, the target for PKCepsilon in LPS signaling is unknown. LPS signaling is mediated by Toll-like receptor 4, which uses four adapter proteins, MyD88, MyD88 adapter-like (Mal), Toll/IL-1R domain-containing adapter inducing IFN-beta (Trif), and Trif-related adapter molecule (TRAM). Here we show that TRAM is transiently phosphorylated by PKCepsilon on serine-16 in an LPS-dependent manner. Activation of IFN regulatory factor 3 and induction of the chemokine RANTES, which are both TRAM-dependent, were attenuated in PKCepsilon-deficient cells. TRAMS16A is inactive when overexpressed and is attenuated in its ability to reconstitute signaling in TRAM-deficient cells. We have therefore uncovered a key process in Toll-like receptor 4 signaling, identifying TRAM as the target for PKCepsilon.

The myristoylation of TRIF-related adaptor molecule is essential for Toll-like receptor 4 signal transduction

TRIF-related adaptor molecule (TRAM) is the fourth Toll/IL-1 resistance domain-containing adaptor to be described that participates in Toll-like receptor (TLR) signaling. TRAM functions exclusively in the TLR4 pathway. Here we show by confocal microscopy that TRAM is localized in the plasma membrane and the Golgi apparatus, where it colocalizes with TLR4. Membrane localization of TRAM is the result of myristoylation because mutation of a predicted myristoylation site in TRAM (TRAM-G2A) brought about dissociation of TRAM from the membrane and its relocation to the cytosol. Further, TRAM, but not TRAM-G2A, was radiolabeled with [3H]myristate in vivo. Unlike wild-type TRAM, overexpression of TRAM-G2A failed to elicit either IFN regulatory factor 3 or NF-kappaB signaling. Moreover, TRAM-G2A was unable to reconstitute LPS responses in bone marrow-derived macrophages from TRAM-deficient mice. These observations provide clear evidence that the myristoylation of TRAM targets it to the plasma membrane, where it is essential for LPS responses through the TLR4 signal transduction pathway, and suggest a hitherto unappreciated manner in which LPS responses can be regulated.

A peptide against the N-terminus of myristoylated alanine-rich C kinase substrate inhibits degranulation of human leukocytes in vitro

Leukocytes synthesize a variety of inflammatory mediators that are packaged and stored in the cytoplasm within membrane-bound granules. Upon stimulation, the cells secrete the granule contents via an exocytotic process whereby the granules translocate to the cell periphery, the granule membranes fuse with the plasma membrane, and the granule contents are released extracellularly. We have reported previously that another exocytotic process, release of mucin by secretory cells of the airway epithelium, is regulated by the myristoylated alanine-rich C kinase substrate (MARCKS) (Li Y, Martin LD, Spizz G, Adler KB. MARCKS protein is a key molecule regulating mucin secretion by human airway epithelial cells in vitro. J Biol Chem 2001;276:40982-40990; Singer M, Martin LD, Vargaftig BB, Park J, Gruber AD, Li Y, Adler KB. A MARCKS-related peptide blocks mucus hypersecretion in a mouse model of asthma. Nat Med 2004;10:193-196). In those studies, mucin secretion in vitro and in vivo was attenuated by a synthetic peptide identical to the N-terminus of MARCKS, named the MANS peptide (Li and colleagues, 2001). In this study, we used the MANS peptide to investigate possible involvement of MARCKS in secretion of leukocyte granule proteins. In neutrophils isolated from human blood, phorbol 12-myristate 13-acetate-induced myeloperoxidase release was attenuated in a concentration-dependent manner by MANS but not by equal concentrations of a missense control peptide. In additional studies using human leukocyte cell lines, secretion of eosinophil peroxidase from the eosinophil-like cell line HL-60 clone 15, lysozyme from the monocytic leukemia cell line U937, and granzyme from the lymphocyte natural killer cell line NK-92 were attenuated by preincubation of the cells with MANS but not with the missense control peptide. The results indicate that MARCKS protein may play an important role in the secretion of membrane-bound granules from different leukocytes. MARCKS may be an important component of secretory pathways associated with release of granules by different cell types.

Myristoylated alanine rich C kinase substrate (MARCKS) heterozygous mutant mice exhibit deficits in hippocampal mossy fiber-CA3 long-term potentiation

The myristoylated alanine-rich C kinase substrate (MARCKS) is a primary protein kinase C (PKC) substrate in brain thought to transduce PKC signaling into alterations in the filamentous (F) actin cytoskeleton. Within the adult hippocampus, MARCKS is highly expressed in the dentate gyrus (DG)-CA3 mossy fiber pathway, but is expressed at low levels in the CA3-CA1 Schaffer collateral-CA1 pathway. We have previously demonstrated that 50% reductions in MARCKS expression in heterozygous Marcks mutant mice produce robust deficits in spatial reversal learning, but not contextual fear conditioning, suggesting that only specific aspects of hippocampal function are impaired by reduction in MARCKS expression. To further elucidate the role of MARCKS in hippocampal synaptic plasticity, in the present study we examined basal synaptic transmission, paired-pulse facilitation, post-tetanic potentiation, and long-term potentiation (LTP) in the hippocampal mossy fiber-CA3 and Schaffer collateral-CA1 pathways of heterozygous Marcks mutant and wild-type mice. We found that LTP is significantly impaired in the mossy fiber-CA3 pathway, but not in the Schaffer collateral-CA1 pathway, in heterozygous Marcks mutant mice, whereas basal synaptic transmission, paired-pulse facilitation, and post-tetanic potentiation are unaffected in both pathways. These findings indicate that a 50% reduction in MARCKS expression impairs processes required for long-term, but not short-term, synaptic plasticity in the mossy fiber-CA3 pathway. The implications of these findings for the role of the mossy fiber-CA3 pathway in hippocampus-dependent learning processes are discussed.

Alternative mechanisms of structuring biomembranes: self-assembly versus self-organization

We study two mechanisms for the formation of protein patterns near membranes of living cells by mathematical modelling. Self-assembly of protein domains by electrostatic lipid-protein interactions is contrasted with self-organization due to a nonequilibrium biochemical reaction cycle of proteins near the membrane. While both processes lead eventually to quite similar patterns, their evolution occurs on very different length and time scales. Self-assembly produces periodic protein patterns on a spatial scale below 0.1 microm in a few seconds followed by extremely slow coarsening, whereas self-organization results in a pattern wavelength comparable to the typical cell size of 100 microm within a few minutes suggesting different biological functions for the two processes.

Expression of MARCKS Effector Domain Mutants Alters Phospholipase D Activity and Cytoskeletal Morphology of SK-N-MC Neuroblastoma Cells

Stable overexpression of myristoylated alanine-rich C-kinase substrate (MARCKS) is known to enhance phorbol ester stimulation of phospholipase D (PLD) activity and protein kinase Calpha (PKCalpha) levels in SK-N-MC neuroblastoma cells. In contrast, expression of MARCKS mutants (S152A or S156A) lacking key PKC phosphorylation sites within the central basic effector domain (ED) had no significant effect on PLD activity or PKCalpha levels relative to vector control cells. Like control cells, those expressing wild type MARCKS were elongated and possessed longitudinally oriented stress fibers, although these cells were more prone to detach from the substratum and undergo cell death upon phorbol ester treatment. However, cells expressing MARCKS ED mutants were irregularly shaped and stress fibers were either shorter or less abundant, and cell adhesion and viability were not affected. These results suggest that intact phosphorylation sites within the MARCKS ED are required for PLD activation and influence both membrane-cytoskeletal organization and cell viability.

Two N-myristoyltransferase isozymes play unique roles in protein myristoylation, proliferation, and apoptosis

N-myristoyltransferases (NMT) add myristate to the NH(2) termini of certain proteins, thereby regulating their localization and/or biological function. Using RNA interference, this study functionally characterizes the two NMT isozymes in human cells. Unique small interfering RNAs (siRNA) for each isozyme were designed and shown to decrease NMT1 or NMT2 protein levels by at least 90%. Ablation of NMT1 inhibited cell replication associated with a loss of activation of c-Src and its target FAK as well as reduction of signaling through the c-Raf/mitogen-activated protein kinase/extracellular signal-regulated kinase kinase/extracellular signal-regulated kinase pathway. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assays showed that depletion of either NMT isozyme induced apoptosis, with NMT2 having a 2.5-fold greater effect than NMT1. Western blot analyses revealed that loss of NMT2 shifted the expression of the BCL family of proteins toward apoptosis. Finally, intratumoral injection of siRNA for NMT1 or for both NMT1 and NMT2 inhibited tumor growth in vivo, whereas the same treatment with siRNA for NMT2 or negative control siRNA did not. Overall, the data indicate that NMT1 and NMT2 have only partially overlapping functions and that NMT1 is critical for tumor cell proliferation.

Protein kinase C isoforms as specific targets for modulation of vascular smooth muscle function in hypertension

Vascular contraction is an important determinant of the peripheral vascular resistance and blood pressure. The mechanisms underlying vascular smooth muscle (VSM) contraction and the pathological changes that occur in hypertension have been the subject of numerous studies and interpretations. Activation of VSM by vasoconstrictor stimuli at the cell surface causes an increase in [Ca(2+)](i), Ca(2+)-dependent activation of myosin light chain (MLC) kinase, MLC phosphorylation, actin-myosin interaction and VSM contraction. Additional signaling pathways involving Rho-kinase and protein kinase C (PKC) may increase the myofilament force sensitivity to [Ca(2+)](i) and MLC phosphorylation, and thereby maintain vascular contraction. PKC is a particularly intriguing protein kinase as it comprises a family of Ca(2+)-dependent and Ca(2+)-independent isoforms, which have different tissue and subcellular distribution, and undergo differential translocation during cell activation. PKC translocation to the cell surface may trigger a cascade of protein kinases, such as mitogen-activated protein kinase (MAPK) and MAPK kinase (MEK) that ultimately interact with the contractile myofilaments and cause VSM contraction. Also, PKC translocation to the nucleus may promote VSM growth and proliferation. Increased PKC expression and activity have been identified in several forms of hypertension. The subcellular location of PKC may determine the state of VSM activity, and may be useful in the diagnosis/prognosis of hypertension. Vascular PKC isoforms may represent specific targets for modulation of VSM hyperactivity, and isoform-specific PKC inhibitors may be useful in treatment of Ca(2+) antagonist-resistant forms of hypertension.

Protein kinase C and the regulation of the actin cytoskeleton

Protein kinase C (PKC) isoforms are central components in intracellular networks that regulate a vast number of cellular processes. It has long been known that in most cell types, one or more PKC isoforms influences the morphology of the F-actin cytoskeleton and thereby regulates processes that are affected by remodelling of the microfilaments. These include cellular migration and neurite outgrowth. This review focuses on the role of classical and novel PKC isoforms in migration and neurite outgrowth, and highlights some regulatory steps that may be of importance in the regulation by PKC of migration and neurite outgrowth. Many studies indicate that integrins are crucial mediators both upstream and downstream of PKC in inducing morphological changes. Furthermore, a number of PKC substrates, directly associated with the microfilaments, such as MARCKS, GAP43, adducin, fascin, ERM proteins and others have been identified. Their potential role in PKC effects on the cytoskeleton is discussed.

Retrodifferentiation and reversibility of aging: forever young?

Maturation of stem cells or precursor cells is associated with the acquisition of certain properties finally resulting in specifically functional cell types within the diverse tissues. This maturation process requires distinct steps of differentiation and is accompanied by a constantly increasing process of aging paralleled by a progressively reduced proliferative capacity. The eventually growth arrested and terminally differentiated cells perform their appropriate specific functions associated with developing senescence by STASIS (stress or aberrant signaling‐inducing senescence) and/or by replicative senescence. Finally, elimination via apoptosis concludes their life span. However, nature also provides a surprise within this concept of life: Sometimes, differentiation and aging steps are reversible. A biological phenomenon of completely reversible differentiation events has been characterized as retrodifferentiation rather than dedifferentiation. Thus, all morphological and functional properties of retrodifferentiated and previously more undifferentiated cells are indistinguishable. Consequently, reversible differentiation may simultaneously be associated with a reversibility of the aging process and therefore, contributes to longevity and rejuvenation. Tissue renewals or regenerative potential for tissue‐specific requirements, if not sufficiently compensated by the appropriate stem cells, may necessitate the generation of undifferentiated precursors by retrodifferentiation followed by a subsequent transdifferentiation process with the consequence of cell type conversion which also includes the risk for tumor development. This interference with the normal biological clock mediated by threshold effects in certain individual cells, raises important questions: What signals trigger retrodifferentiation and what would be the finite life span of cells with a retrodifferentiation capacity?

Travelling lipid domains in a dynamic model for protein-induced pattern formation in biomembranes

Cell membranes are composed of a mixture of lipids. Many biological processes require the formation of spatial domains in the lipid distribution of the plasma membrane. We have developed a mathematical model that describes the dynamic spatial distribution of acidic lipids in response to the presence of GMC proteins and regulating enzymes. The model encompasses diffusion of lipids and GMC proteins, electrostatic attraction between acidic lipids and GMC proteins as well as the kinetics of membrane attachment/detachment of GMC proteins. If the lipid-protein interaction is strong enough, phase separation occurs in the membrane as a result of free energy minimization and protein/lipid domains are formed. The picture is changed if a constant activity of enzymes is included into the model. We chose the myristoyl-electrostatic switch as a regulatory module. It consists of a protein kinase C that phosphorylates and removes the GMC proteins from the membrane and a phosphatase that dephosphorylates the proteins and enables them to rebind to the membrane. For sufficiently high enzymatic activity, the phase separation is replaced by travelling domains of acidic lipids and proteins. The latter active process is typical for nonequilibrium systems. It allows for a faster restructuring and polarization of the membrane since it acts on a larger length scale than the passive phase separation. The travelling domains can be pinned by spatial gradients in the activity; thus the membrane is able to detect spatial clues and can adapt its polarity dynamically to changes in the environment.

cGMP-dependent phosphorylation and degradation of myristoylated alanine-rich C-kinase substrate

In the mammalian brain, nitric oxide (NO) has been implicated in neuronal signal transmissions. NO stimulates guanylate cyclase to increase intracellular cGMP, which in turn activates cGMP-dependent protein kinases (PKG), but the targets of PKG in the brain have not fully been understood. In this study, we examined cGMP-dependent phosphorylation of proteins in rat brain and found that one of the possible substrates was myristoylated alanine-rich C-kinase substrate (MARCKS), an actin-binding membrane-associated protein that regulates cell adhesion. In addition, possible degradation products of MARCKS were observed after transfection of PKG or stimulation with 8pCPT-cGMP. Western blot analysis showed that the MARCKS protein levels were decreased when the cells were stimulated with 8pCPT-cGMP. These results suggest that MARCKS is a target of PKG, and PKG-dependent phosphorylation of MARCKS results in its degradation to reduce its protein levels in the cells.

Northern blot and in situ hybridization analyses for the development of myristoylated alanine-rich c-kinase substrate mRNA in the monkey cerebral cortex

Myristoylated alanine-rich C-kinase substrate (MARCKS) is a major neuron-specific substrate for protein kinase C, and is involved in both neurite outgrowth and synaptic plasticity. Using both Northern blot and in situ hybridization techniques, we investigated whether the expression of MARCKS mRNA in the monkey cerebral neocortex and hippocampus changed during the developmental period. In each of four neocortical areas examined, i.e. the prefrontal area (area FD of [Illinois Monographs in the Medical Sciences (1947) 1]), the temporal association area (TE), the primary somatosensory area (PB), and the primary visual area (OC), the Northern blot analysis showed that the amount of MARCKS mRNA was high during the fetal and early postnatal periods, and decreased sharply between postnatal day 70 and postnatal month 6. The in situ hybridization experiments showed that the expression of MARCKS mRNA was decreased in every layer of neocortical areas at postnatal month 6 or later. In the primary sensory areas (areas PB and OC), the degree of decrease was higher in the supragranular layers (layers II and III) than in the infragranular layers (layers V and VI). In the hippocampus, the developmental change in the amount of MARCKS mRNA was small, but the in situ hybridization revealed a prominent decrease in Ammon's horn in monkeys on postnatal month 8 and later. These findings indicate that region-specific expression of MARCKS mRNA is established around postnatal month 6. We suggest that the extensive expression of MARCKS mRNA is one of the molecular bases of high plasticity in the infant cerebral cortex.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Myristoylated alanine-rich C kinase substrate (MARCKS) is involved in myoblast fusion through its regulation by protein kinase Calpha and calpain proteolytic cleavage

MARCKS (myristoylated alanine-rich C kinase substrate) is a major cytoskeletal protein substrate of PKC (protein kinase C) whose cellular functions are still unclear. However numerous studies have implicated MARCKS in the stabilization of cytoskeletal structures during cell differentiation. The present study was performed to investigate the potential role of Ca(2+)-dependent proteinases (calpains) during myogenesis via proteolysis of MARCKS. It was first demonstrated that MARCKS is a calpain substrate in vitro. Then, the subcellular expression of MARCKS was examined during the myogenesis process. Under such conditions, there was a significant decrease in MARCKS expression associated with the appearance of a 55 kDa proteolytic fragment at the time of intense fusion. The addition of calpastatin peptide, a specific calpain inhibitor, induced a significant decrease in the appearance of this fragment. Interestingly, MARCKS proteolysis was dependent of its phosphorylation by the conventional PKCalpha. Finally, ectopic expression of MARCKS significantly decreased the myoblast fusion process, while reduced expression of the protein with antisense oligonucleotides increased the fusion. Altogether, these data demonstrate that MARCKS proteolysis is necessary for the fusion of myoblasts and that cleavage of the protein by calpains is involved in this regulation.

The bi-directional translocation of MARCKS between membrane and cytosol regulates integrin-mediated muscle cell spreading

The regulation of the cytoskeleton is critical to normal cell function during tissue morphogenesis. Cell-matrix interactions mediated by integrins regulate cytoskeletal dynamics, but the signaling cascades that control these processes remain largely unknown. Here we show that myristoylated alanine-rich C-kinase substrate (MARCKS) a specific substrate of protein kinase C (PKC), is regulated by alpha5beta1 integrin-mediated activation of PKC and is critical to the regulation of actin stress fiber formation during muscle cell spreading. Using MARCKS mutants that are defective in membrane association or responsiveness to PKC-dependent phosphorylation, we demonstrate that the translocation of MARCKS from the membrane to the cytosol in a PKC-dependent manner permits the initial phases of cell adhesion. The dephosphorylation of MARCKS and its translocation back to the membrane permits the later stages of cell spreading during the polymerization and cross-linking of actin and the maturation of the cytoskeleton. All of these processes are directly dependent on the binding of alpha5beta1 integrin to its extracellular matrix receptor, fibronectin. These results demonstrate a direct biochemical pathway linking alpha5beta1 integrin signaling to cytoskeletal dynamics and involving bi-directional translocation of MARCKS during the dramatic changes in cellular morphology that occur during cell migration and tissue morphogenesis.

Spatiotemporal interactions of myristoylated alanine-rich C kinase substrate (MARCKS) protein with the actin cytoskeleton and exocytosis of oxytocin upon prostaglandin F2alpha stimulation of bovine luteal cells

In the bovine corpus luteum (CL) phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) protein in response to prostaglandin F2alpha (PGF2alpha) is correlated with the secretion of oxytocin. The present study was conducted to 1) examine the intracellular translocation characteristics of wild-type and mutant forms of a green fluorescent protein (GFP)-conjugated MARCKS (MARCKS-GFP) after PGF2alpha treatment and 2) evaluate PGF2alpha-induced temporal changes in MARCKS-GFP and actin cortex associated with exocytosis of oxytocin. In experiment 1, cells of the bovine CL were cultured on coverslips overnight. Then, wild-type and mutant MARCKS-GFP constructs were transfected separately into cells and expression was detected through fluorescence microscopy. Forty-eight hours after transfection, cells were treated with vehicle, PGF2alpha (56 nM), or a phorbol ester (12-O-tetradecanoylphorbol-13-acetate [TPA], 1 microM). Treatment of cells expressing wild-type MARCKS-GFP with PGF2alpha and TPA resulted in translocation of MARCKS from the plasma membrane to the cytoplasm within 2.5 min. Phosphorylation mutant MARCKS-GFP (m3) protein was localized on the plasma membrane, and treatments did not cause its translocation to the cytoplasm. Myristoylation mutant MARCKS-GFP (G2A) was observed solely in the cytoplasm, and no changes were detected in the intracellular location of this mutant MARCKS after treatment. In experiment 2, luteal cells were transfected with one of the three MARCKS-GFP constructs. Cells were then fixed and probed sequentially for oxytocin and filamentous actin. Results revealed that only wild-type MARCKS-GFP transfected large luteal cells contained advanced signs of exocytosis (peripheral movement of oxytocin vesicles; shorter actin filaments) with translocation of MARCKS-GFP from membrane to cytoplasm in response to PGF2alpha treatment. These data demonstrate that phosphorylation of membrane-bound MARCKS protein is requisite for exocytosis of oxytocin to occur in bovine large luteal cells.

Chronic carbamazepine treatment increases myristoylated alanine-rich C kinase substrate phosphorylation in the rat cerebral cortex via down-regulation of calcineurin Aα

Carbamazepine (CBZ) is generally used as a mood-stabilizing drug for the treatment of bipolar disorders. However, little is known about the molecular mechanisms of CBZ actions in the brain, which account for this therapeutic profile. In the present study, we examined the effects of chronic CBZ treatment on the protein kinase C (PKC) pathway. Male Wistar rats received injections of CBZ once daily for 3-5 weeks. The protein levels of PKC isozymes, calcineurin Aalpha subunit (CaN-Aalpha) and myristoylated alanine-rich C kinase substrate (MARCKS), and phosphorylation of MARCKS in the rat cerebral cortex were determined by immunoblot analysis. The content of CaN-Aalpha mRNA was determined by Northern blot analysis. Nomicr; significant changes were observed in PKC alpha, beta, gamma, delta and epsilon in the cytosol and membrane fractions after 5 weeks of CBZ treatment. There were no significant changes in the actin-binding PKCepsilon. Interestingly, phosphorylation of MARCKS was increased more than twofold, while no significant changes were observed in MARCKS protein level in the cytosol fraction. Furthermore, CaN-Aalpha was significantly decreased at both the protein and mRNA levels. The level of MARCKS phosphorylation is reportedly regulated by the balance between PKC-mediated phosphorylation and CaN-mediated dephosphorylation. Our results indicate that chronic CBZ treatment increases MARCKS phosphorylation via decreasing the content of CaN-Aalpha. Phosphorylation of MARCKS has been reported to play an important role in the release of neurotransmitters, such as noradrenaline and serotonin. Therefore, the increase in phosphorylation of MARCKS observed only after chronic CBZ treatment may be related to the mood-stabilizing effects of CBZ.

Altered expression and phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) in postmortem brain of suicide victims with or without depression

Myristoylated alanine-rich C kinase substrate (MARCKS), an acidic, heat-stable protein, is involved in important physiological functions such as neurotransmitter release and re-uptake. It is also a substrate for phosphorylation by protein kinase C (PKC) and has been shown to play a role in the pathophysiology of mood disorders. In this study, protein and mRNA expression of MARCKS as well as phosphorylation of MARCKS were determined in the prefrontal cortex (PFC) and hippocampus of postmortem brain obtained from suicide victims, with or without depression, and normal control subjects. There were no significant differences in mRNA and protein levels of MARCKS between suicide subjects and controls. However, protein levels of MARCKS were significantly increased in the membrane but not in cytosol fraction of PFC and hippocampus obtained from depressed suicide subjects as compared to normal controls. When PKC-mediated MARCKS phosphorylation was determined, it was observed that MARCKS phosphorylation was significantly decreased in the membrane fraction of PFC and hippocampus obtained from total suicide subjects as well as depressed and non-depressed suicide subjects compared with control population. Although the mechanism of such alterations in MARCKS in depressed and non-depressed suicide subjects is not clear, results of the present study indicate that an increase in membrane MARCKS is associated with depressed suicide victims and a decrease in MARCKS phosphorylation may be a common feature of suicide victims independent of diagnosis.

Protein kinase Calpha negatively regulates cell spreading and motility in MDA-MB-231 human breast cancer cells downstream of epidermal growth factor receptor

Previous work has shown that phorbol esters modulate chemotaxis. Here, we demonstrate that PKC activation via phorbol 12-myristate 13-acetate (PMA) treatment of MDA-MB-231 cells inhibits EGF-induced cell spreading, the initial event of motility and chemotaxis. Of five PKC isoforms (alpha,iota,lambda,delta,and epsilon) identified in this cell line, PMA treatment only induced PKCalpha translocation from the cytosol to the membrane, an event that correlated with the development of the rounded morphology. Cell recovery was linked to PKCalpha downregulation in part via the proteasome pathway since treatment with MG101 in the presence of PMA did not lead to PKCalpha degradation and cell recovery. Co-immunoprecipitation and immunolocalization demonstrated that EGF co-localized with PKCalpha and EGFR, however, PMA did not abrogate EGFR transactivation. This work suggests that PKCalpha is the primary target of PMA acting as a transient negative regulator of cell spreading and motility in MDA-MB-231 breast cancer cells.

The difference between sleep and anaesthesia is in the intracellular signal: propofol and GABA use different subtypes of the GABA(A) receptor beta subunit and vary in their interaction with actin

BACKGROUND

Propofol is known to interact with the gamma-aminobutyric acidA (GABA(A)) receptor, however, activating the receptor alone is not sufficient for producing anaesthesia.

METHODS

To compare propofol and GABA, their interaction with the GABAA receptor beta subunit and actin were studied in three cellular fractions of cultured rat neurons using Western blot technique.

RESULTS

Propofol tyrosine phosphorylated the GABA(A) receptor beta2 (MW 54 and 56 kDa) and beta3 (MW 57 kDa) subtypes. The increase was shown in both the cytoskeleton (beta2(54) and beta2(56) subtypes) and the cell membrane (beta2(54) and beta3 subtypes). Concurrently the 56 kDa beta2 subtype was reduced in the cytosol. Propofol, but not GABA, also tyrosine phosphorylated actin in the cell membrane and cytoskeletal fraction. Without extracellular calcium available, the amount of actin decreased in the cytoskeleton, but tyrosine phosphorylation was unchanged. GABA caused increased tyrosine phosphorylation of beta2(56) and beta3 subtypes in the membrane and both beta2 subtypes in the cytoskeleton but no cytosolic tyrosine phosphorylation.

CONCLUSION

The difference between propofol and GABA at the GABA(A) receptor was shown to take place in the membrane, where the beta2(54) was increased by propofol and instead the beta2(56) subtype was increased by GABA. Only propofol also tyrosine phosphorylated actin in the cell membrane and cytoskeletal fraction. This interaction between the GABAA receptor and actin might explain the difference between anaesthesia and physiological neuronal inhibition.

Involvement of protein phosphatase-1-mediated MARCKS translocation in myogenic differentiation of embryonic muscle cells

Myristoylated alanine-rich C kinase substrate (MARCKS) translocates from the cytosol to the plasma membrane while mononucleated myoblasts fuse to form multinucleated myotubes. Here, we show that protein phosphatase-1-mediated dephosphorylation of MARCKS largely influences its subcellular localization and the fusion process. Treatment with okadaic acid or tautomycin, which are potent inhibitors of protein phosphatases and cell fusion, was found to reversibly block the MARCKS translocation. Moreover, the dephosphorylating activity against MARCKS markedly increased during myogenesis, and this increase was closely correlated with the membrane fusion of the cells. In addition, protein phosphatase-1 was identified as a major enzyme that is responsible for dephosphorylation of MARCKS. Furthermore, a mutation preventing MARCKS phosphorylation and thus facilitating MARCKS translocation resulted in promotion of the cell fusion. In contrast, overexpression of MARCKS carrying a mutation that blocks myristoylation and thus prevents the MARCKS translocation impaired the myoblast fusion. Together with the fact that MARCKS regulates the cytoskeleton dynamics by crosslinking the actin filaments in the plasma membrane and that myoblast fusion accompanies massive cytoskeleton reorganization, these results suggest that protein phosphatase-1-mediated MARCKS localization at the membrane is required for the fusion of embryonic muscle cells.

Interaction of the C-terminal region of the rat serotonin transporter with MacMARCKS modulates 5-HT uptake regulation by protein kinase C

The serotonin transporter (SERT) mediates the re-uptake of released serotonin into presynaptic nerve terminals. Its activity is regulated by different mechanisms including protein kinase C (PKC) triggered internalization. Here, we used yeast 2-hybrid screening and cotransfection into 293 cells to identify a homologue of the myristoylated alanine-rich C kinase substrate (MARCKS), MacMARCKS, as a C-terminally interacting protein of SERT. Upon cotransfection with SERT, MacMARCKS caused a reduction in the maximal rate of [(3)H]serotonin uptake and reduced its down-regulation elicited by activation of PKC. Our data are consistent with MARCKS proteins regulating the plasma membrane dynamics of neurotransmitter transporters.

Cross-talk unfolded: MARCKS proteins

The proteins of the MARCKS (myristoylated alanine-rich C kinase substrate) family were first identified as prominent substrates of protein kinase C (PKC). Since then, these proteins have been implicated in the regulation of brain development and postnatal survival, cellular migration and adhesion, as well as endo-, exo- and phago-cytosis, and neurosecretion. The effector domain of MARCKS proteins is phosphorylated by PKC, binds to calmodulin and contributes to membrane binding. This multitude of mutually exclusive interactions allows cross-talk between the signal transduction pathways involving PKC and calmodulin. This review focuses on recent, mostly biophysical and biochemical results renewing interest in this protein family. MARCKS membrane binding is now understood at the molecular level. From a structural point of view, there is a consensus emerging that MARCKS proteins are "natively unfolded". Interestingly, domains similar to the effector domain have been discovered in other proteins. Furthermore, since the effector domain enhances the polymerization of actin in vitro, MARCKS proteins have been proposed to mediate regulation of the actin cytoskeleton. However, the recent observations that MARCKS might serve to sequester phosphatidylinositol 4,5-bisphosphate in the plasma membrane of unstimulated cells suggest an alternative model for the control of the actin cytoskeleton. While myristoylation is classically considered to be a co-translational, irreversible event, new reports on MARCKS proteins suggest a more dynamic picture of this protein modification. Finally, studies with mice lacking MARCKS proteins have investigated the functions of these proteins during embryonic development in the intact organism.

Prostaglandin F2alpha-activated protein kinase Calpha phosphorylates myristoylated alanine-rich C kinase substrate protein in bovine luteal cells

Prostaglandin F2alpha (PGF2alpha)-induced secretion of oxytocin by the bovine corpus luteum involves the phosphorylation of a unique protein kinase C (PKC) substrate, myristoylated alanine-rich C kinase substrate (MARCKS) protein. This study was conducted to determine the specific PKC isoform engaged in phosphorylation of MARCKS protein in bovine luteal cells. In experiment 1, dispersed luteal cells recovered from the corpus luteum on d 8 of the estrous cycle were preincubated with [32P] orthophosphate and then exposed to PGF2alpha alone or in combination with PKC inhibitors. Autoradiography and densitometry of Western blots revealed that MARCKS protein was phosphorylated by a conventional PKC (cPKC) isoform. Experiment 2 was conducted to identify the specific cPKC isoform that phosphorylates MARCKS protein in luteal cells. Corpora lutea were removed from control and PGF2alpha-treated heifers on d 8 of the cycle, and PKC isoforms associated with membrane and cytosolic fractions were determined. Treatment with PGF2alpha increased membrane concentrations of PKCalpha within 5 min after treatment (p < 0.005). Collectively, these data suggest that phosphorylation of MARCKS protein coinciding with oxytocin secretion is mediated by PKCalpha.

Overexpression of the myristoylated alanine-rich C-kinase substrate inhibits cell adhesion to extracellular matrix components

Mice lacking the myristoylated alanine-rich C-kinase substrate, or MARCKS protein, exhibit abnormalities consistent with a defect in the ability of neurons to migrate appropriately during forebrain development. To investigate the possibility that this phenotype could be due to disruption of normal cellular adhesion to extracellular matrix, an assay was developed in which 293 cells co-expressing MARCKS and green fluorescent protein were tested for their adhesion competence on various substrates. Fluorescence-activated cell sorting of adherent and non-adherent green fluorescent protein-expressing cells demonstrated that wild-type MARCKS inhibited adhesion of cells to fibronectin, whereas a non-myristoylated mutant did not inhibit adhesion of cells to a variety of substrates. The fibronectin competitive inhibitor RGD peptide inhibited adhesion of cells expressing all MARCKS variants equally. Cytochalasin D inhibited the adhesion of cells expressing non-myristoylated MARCKS, but did not further decrease the adhesion of cells expressing adhesion-inhibitory proteins. Confocal microscopy demonstrated the presence of inhibitory, myristoylated MARCKS at the plasma membrane, suggesting that localization at this region might be important for MARCKS to inhibit cellular adhesion. These data suggest a possible myristoylation-dependent function of MARCKS to inhibit cellular adhesion to extracellular matrix proteins, indicating a potential mechanism for the cell migration defects seen in the MARCKS-deficient mice.

Amyloid beta protein activates PKC-delta and induces translocation of myristoylated alanine-rich C kinase substrate (MARCKS) in microglia

The increased accumulation of activated microglia containing amyloid beta protein (Abeta) around senile plaques is a common pathological feature in subjects with Alzheimer's disease (AD). Much less is known, however, of intracellular signal transduction pathways for microglial activation in response to Abeta. We investigated intracellular signaling in response to Abeta stimulation in primary cultured rat microglia. We found that the kinase activity of PKC-delta but not that of PKC-alpha or -epsilon is increased by stimulation of microglia with Abeta, with a striking tyrosine phosphorylation of PKC-delta. In microglia stimulated with Abeta, tyrosine phosphorylation of PKC-delta was evident at the membrane fraction without an overt translocation of PKC-delta. PKC-delta co-immunoprecipitated with MARCKS from microglia stimulated with Abeta. Abeta induced translocation of MARCKS from the membrane fraction to the cytosolic fraction. Immunocytochemical analysis revealed that phosphorylated MARCKS accumulated in the cytoplasm, particularly at the perinuclear region in microglia treated with Abeta. Taken together with our previous observations that Abeta-induced phosphorylation of MARCKS and chemotaxis of microglia are inhibited by either tyrosine kinase or PKC inhibitors, our results provide evidence that Abeta induces phosphorylation and translocation of MARCKS through the tyrosine kinase-PKC-delta signaling pathway in microglia.

Tob-mediated cross-talk between MARCKS phosphorylation and ErbB-2 activation

The biochemical path for the activation of ErbB-2 by PKC activator was investigated in MDA-MB-231 human breast cancer cells. We found that PMA-induced phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) increased its binding with Tob that exerts an anti-proliferative effect through the binding with ErbB-2. The phosphorylation site domain (PSD) of MARCKS was relevant to its interaction with Tob. Decreased binding of Tob with ErbB-2 and subsequent activation of ErbB-2 were observed in MDA-MB-231 cells in response to PMA treatment. The present study proposes that MARCKS phosphorylation by PKC removes Tob from ErbB-2 by increasing its binding affinity with Tob, and thereby activates the ErbB-2 mediated signal transduction.

Apical accumulation of MARCKS in neural plate cells during neurulation in the chick embryo

BACKGROUND

The neural tube is formed by morphogenetic movements largely dependent on cytoskeletal dynamics. Actin and many of its associated proteins have been proposed as important mediators of neurulation. For instance, mice deficient in MARCKS, an actin cross-linking membrane-associated protein that is regulated by PKC and other kinases, present severe developmental defects, including failure of cranial neural tube closure.

RESULTS

To determine the distribution of MARCKS, and its possible relationships with actin during neurulation, chick embryos were transversely sectioned and double labeled with an anti-MARCKS polyclonal antibody and phalloidin. In the neural plate, MARCKS was found ubiquitously distributed at the periphery of the cells, being conspicuously accumulated in the apical cell region, in close proximity to the apical actin meshwork. This asymmetric distribution was particularly noticeable during the bending process. After the closure of the neural tube, the apically accumulated MARCKS disappeared, and this cell region became analogous to the other peripheral cell zones in its MARCKS content. Actin did not display analogous variations, remaining highly concentrated at the cell subapical territory. The transient apical accumulation of MARCKS was found throughout the neural tube axis. The analysis of another epithelial bending movement, during the formation of the lens vesicle, revealed an identical phenomenon.

CONCLUSIONS

MARCKS is transiently accumulated at the apical region of neural plate and lens placode cells during processes of bending. This asymmetric subcellular distribution of MARCKS starts before the onset of neural plate bending. These results suggest possible upstream regulatory actions of MARCKS on some functions of the actin subapical meshwork.

Regulation of intercellular tight junctions by zonula occludens toxin and its eukaryotic analogue zonulin

The intestinal epithelium represents the largest interface between the external environment and the internal host milieu and constitutes the major barrier through which molecules can either be absorbed or secreted. There is now substantial evidence that tight junctions (tj) play a major role in regulating epithelial permeability by influencing paracellular flow of fluid and solutes. Tj are one of the hallmarks of absorptive and secretory epithelia. Evidence now exists that tj are dynamic rather than static structures and readily adapt to a variety of developmental, physiological, and pathological circumstances. These adaptive mechanisms are still incompletely understood. Activation of PKC either by Zonula occludens toxin (Zot) or by phorbol esters increases paracellular permeability. Alteration of epithelial tj is a recently described property for infectious agents. Clostridium difficile toxin A and B and influenza and vesicular stomatitis viruses have been shown to loosen tj in tissue culture monolayers. Unlike what occurs after the Zot stimulus, these changes appear to be irreversible and are associated with destruction of the tj complex. On the basis of this observation, we postulated that Zot may mimic the effect of a functionally and immunologically related endogenous modulator of epithelial tj. We were able to identify an intestinal Zot analogue, which we named zonulin. It is conceivable that the zonulins participate in the physiological regulation of intercellular tj not only in the small intestine, but also throughout a wide range of extraintestinal epithelia as well as the ubiquitous vascular endothelium, including the blood-brain barrier. Disregulation of this hypothetical zonulin model may contribute to disease states that involve disordered intercellular communication, including developmental and intestinal disorders, tissue inflammation, malignant transformation, and metastasis.

Biphasic modulation of protein kinase C and enhanced cell toxicity by amyloid beta peptide and anoxia in neuronal cultures

A major feature of Alzheimer's disease is the deposition of the amyloid beta peptide (Abeta) in the brain by mechanisms which remain unclear. One hypothesis suggests that oxidative stress and Abeta aggregation are interrelated processes. Protein kinase C, a major neuronal regulatory protein is activated after oxidative stress and is also altered in the Alzheimer's disease brain. Therefore, we examined the effects of Abeta(1-40) peptide on the protein kinase C cascade and cell death in primary neuronal cultures following anoxic conditions. Treatment with Abeta(1-40) for 48 h caused a significant increase in the content and activity of Ca2+ dependent and Ca2+ independent protein kinase C isoforms. By 72 h various protein kinase C isoforms were down-regulated. Following 90 min anoxia and 6 h normoxia, a decrease in protein kinase C isoforms was noticed, independent of Abeta(1-40) treatment. A combination of Abeta(1-40) and 30-min anoxia enhanced cytotoxicity as noticed by a marked loss in the mitochondrial ability to convert 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide and by enhanced 4',6-diamidino-2-phenylindole nuclear staining. Phosphorylation of two downstream protein kinase C substrates of apparent molecular mass 80 and 43 kDa, tentatively identified as the myristoyl alanine-rich C-kinase substrate (MARCKS), were gradually elevated up to 72 h upon incubation with Abeta(1-40). Anoxia followed by 30 min normoxia enhanced MARCKS phosphorylation in the membrane but not in the cytosolic fraction. In the presence of Abeta(1-40), phosphorylation of MARCKS was reduced. After 6 h normoxia, MARCKS phosphorylatability was diminished possibly because of protein kinase C down-regulation. The data suggest that a biphasic modulation of protein kinase C and MARCKS by Abeta(1-40) combined with anoxic stress may play a role in Alzheimer's disease pathology.

Aberrant expression and localization of the cytoskeleton-binding pp52 (LSP1) protein in hairy cell leukemia

Non-retractable cell surface projections and cytoskeleton-mediated functional defects are distinguishing features of both hairy cell leukemia (HCL) and neutrophil actin dysfunction (NAD). These defects in NAD neutrophils are attributed to moderate over-expression of pp52 (LSP1), the F-actin-binding, leukocyte-specific phosphoprotein. Here we report that pp52 is similarly elevated in HCL patient PBMCs. Established HCL cell lines exhibited characteristic morphological features like those of fresh HCL cells and showed elevated pp52 levels. The excess pp52 in these HCL cell lines was selectively associated with the F-actin-rich cytoskeletal arrays in surface projections. Treatments producing radical changes in HCL cell shape also altered pp52 expression and intracellular distribution. Alpha interferon (IFNalpha, used to treat HCL) reduced pp52 levels, normalized intracellular pp52 distribution and reverted HCL cells to rounded B cell morphology. Phorbol ester stimulation rapidly generated hyper-phosphorylated pp52 isoforms which translocated from the cytoskeleton to the cytosol prior to the further elongation of surface spikes. This indicates a direct role for phosphorylation in controlling pp52 interactions with the cytoskeleton. Overall, these findings strongly suggest that elevated pp52 expression and/or selective cytoskeletal association contributes to the distinctive morphology of HCL cells.

A newly characterized human endometrial adenocarcinoma cell line (CAC-1) differentiates in response to retinoic acid treatment

A new cell line of poorly differentiated human endometrial adenocarcinoma cells termed "CAC-1" cells has been established. These cells are epithelial, as indicated by positive cytokeratin and negative vimentin staining. They are rounded and possess a high nuclear-to-cytoplasmic ratio, desmosomes, surface microvilli, intercelular lumens, and pleomorphic nuclei containing multiple nucleoli. These cells have been in long-term culture for 2 years. Our previous studies demonstrated that moderately differentiated (RL95-2) cells differentiated in response to retinoic acid treatment, illustrated by their reorganization of actin filaments and cell enlargement (Carter et al., 1996; Anticancer Res. 16, 17-24). CAC-1 cells exhibited a similar response because they also organized actin filaments and enlarged in response to retinoic acid treatment. Concurrently, retinoic acid treatment caused a 40% decrease in cell detachment in an in vitro detachment assay compared to controls. A slight lag in cell growth was observed when CAC-1 cells were treated with 1 microM 13-cis or all-trans retinoic acid during a 12-day growth curve. In addition, we examined the effects of retinoic acid on protein kinase C-alpha (PKC-alpha) and myristoylated alanine-rich C-kinase substrate (MARCKS). Treatment with retinoic acid caused cytoplasmic PKC-alpha to increase concomitant with a decrease in PKC-alpha in the membrane. In contrast, MARCKS increased in the membrane in response to retinoic acid treatment. These data indicate that retinoid treatment causes inactivation of PKC-alpha, allowing MARCKS to relocalize to the membrane, where it can cross-link actin filaments. CAC-1 cells represent an ideal model for investigating the effects of retinoids on differentiation induction concomitant with actin reorganization.

A novel N-terminal domain directs membrane localization of mouse testis-specific calpastatin

Multiple isoforms of calpastatin have been identified with unique N-terminal regions followed by identical calpain inhibitory domains (II-IV). In many instances the isoforms are cell-type specific, although the precise functional differences among these N-terminal regions are largely unknown. Here we report a germ cell-specific isoform of calpastatin (tCAST) that consists of a novel N-terminal peptide of 40 amino acids (domain T) followed by domains II to IV of somatic calpastatin (sCAST). Domain T is responsible for membrane association of tCAST through a protein modification by myristylation. Mutation of the myristylation site eliminates membrane targeting. Unlike most of the isoforms of calpastatin that are generated through alternative RNA splicing or post-translational proteolysis, the testis-specific isoform is transcribed from an intronic promoter in haploid germ cells of the testis. The intronic promoter directs specific expression of a reporter transgene in developing germ cells of the mouse testis.

The adhesion signaling molecule p190 RhoGAP is required for morphogenetic processes in neural development

Rho GTPases direct actin rearrangements in response to a variety of extracellular signals. P190 RhoGAP (GTPase activating protein) is a potent Rho regulator that mediates integrin-dependent adhesion signaling in cultured cells. We have determined that p190 RhoGAP is specifically expressed at high levels throughout the developing nervous system. Mice lacking functional p190 RhoGAP exhibit several defects in neural development that are reminiscent of those described in mice lacking certain mediators of neural cell adhesion. The defects reflect aberrant tissue morphogenesis and include abnormalities in forebrain hemisphere fusion, ventricle shape, optic cup formation, neural tube closure, and layering of the cerebral cortex. In cells of the neural tube floor plate of p190 RhoGAP mutant mice, polymerized actin accumulates excessively, suggesting a role for p190 RhoGAP in the regulation of +Rho-mediated actin assembly within the neuroepithelium. Significantly, several of the observed tissue fusion defects seen in the mutant mice are also found in mice lacking MARCKS, the major substrate of protein kinase C (PKC), and we have found that p190 RhoGAP is also a PKC substrate in vivo. Upon either direct activation of PKC or in response to integrin engagement, p190 RhoGAP is rapidly translocated to regions of membrane ruffling, where it colocalizes with polymerized actin. Together, these results suggest that upon activation of neural adhesion molecules, the action of PKC and p190 RhoGAP leads to a modulation of Rho GTPase activity to direct several actin-dependent morphogenetic processes required for normal neural development.

Role of MARCKS in regulating endothelial cell proliferation

Myristoylated alanine-rich C kinase substrate (MARCKS), as a specific protein kinase C (PKC) substrate, mediates PKC signaling through its phosphorylation and subsequent modification of its association with filamentous actin (F-actin) and calmodulin (CaM). PKC has long been implicated in cell proliferation, and recent studies have suggested that MARCKS may function as a cell growth suppressor. Therefore, in the present study, we investigated MARCKS protein expression, distribution, and phosphorylation in preconfluent and confluent bovine pulmonary microvascular endothelial cells (BPMEC) in the presence or absence of the vascular endothelial growth factor (VEGF). In addition, we examined functional alterations of MARCKS in these cells by studying the association of MARCKS with F-actin and CaM-dependent myosin light chain (MLC) phosphorylation. Our results indicate that MARCKS protein is downregulated during BPMEC proliferation. Decreased MARCKS association with F-actin, increased actin polymerization, and CaM-dependent MLC phosphorylation appear to mediate cell shape changes and motility during BPMEC growth. In contrast, VEGF stimulated MARCKS phosphorylation without alteration of protein expression during BPMEC proliferation, which may result in reduced interaction between MARCKS and actin or CaM, leading to actin reorganization and MLC phosphorylation. Our data suggest a regulatory role of MARCKS during endothelial cell proliferation.

Endotoxin causes phosphorylation of MARCKS in pulmonary vascular endothelial cells

Protein kinase C (PKC) has been implicated in lipopolysaccharide (LPS)-induced endothelial cell (EC) monolayer permeability. Myristoylated alanine-rich C kinase substrate (MARCKS), as a specific PKC substrate, appears to mediate PKC signaling by PKC-dependent phosphorylation of MARCKS and subsequent modification of the association of MARCKS with filamentous actin and calmodulin (CaM). Therefore, in the present study, we investigated LPS-induced MARCKS phosphorylation in bovine pulmonary artery EC (BPAEC). LPS potentiated MARCKS phosphorylation in BPAEC in a time- and dose-dependent manner. The PKC inhibitor, calphostin C, significantly decreased LPS-induced phosphorylation of MARCKS. In addition, downregulation of PKC with phorbol 12-myristate 13-acetate (PMA) did not affect the LPS-induced MARCKS phosphorylation, suggesting that LPS and PMA activate different isoforms of PKC. Pretreatment with SB203580, a specific inhibitor of p38 MAP kinase, or genistein, a tyrosine kinase inhibitor, prevented LPS-induced MARCKS phosphorylation. Phosphorylation at appropriate sites will induce translocation of MARCKS from the cell membrane to the cytosol. However, LPS, in contrast to PMA, did not generate MARCKS translocation in BPAEC, suggesting that MARCKS translocation may not play a role in LPS-induced actin rearrangement and EC permeability. LPS also enhanced both thrombin- and PMA-induced phosphorylation of MARCKS, suggesting that LPS was able to prime these signaling pathways in BPAEC. Because the CaM-dependent phosphorylation of myosin light chains (MLC) results in EC contraction, we studied the effect of LPS on MLC phosphorylation in BPAEC. LPS induced diphosphorylation of MLC in a time-dependent manner, which occurred at lower doses of LPS, than those required to induce MARCKS phosphorylation. In addition, there was no synergism between LPS and thrombin in the induction of MLC phosphorylation. These data indicate that MLC phosphorylation is independent of MARCKS phosphorylation. In conclusion, LPS stimulated MARCKS phosphorylation in BPAEC. This phosphorylation appears to involve activation of PKC, p38 MAP kinase, and tyrosine kinases. Further studies are needed to explore the role of MARCKS phosphorylation in LPS-induced actin rearrangement and EC permeability.

MARCKS-related protein binds to actin without significantly affecting actin polymerization or network structure. Myristoylated alanine-rich C kinase substrate

Actinis a 42-kDa protein which, due to its ability to polymerize into filaments (F-actin), is one of the major constituents of the cytoskeleton. It has been proposed that MARCKS (an acronym for myristoylated alanine-rich C kinase substrate) proteins play an important role in regulating the structure and mechanical properties of the actin cytoskeleton by cross-linking actin filaments. We have recently reported that peptides corresponding to the effector domain of MARCKS proteins promote actin polymerization and cause massive bundling of actin filaments. We now investigate the effect of MARCKS-related protein, a 20-kDa member of the MARCKS family, on both filament structure and the kinetics of actin polymerization in vitro. Our experiments document that MRP binds to F-actin with micromolar affinity and that the myristoyl chain at the N-terminus of MRP is not required for this interaction. In marked contrast to the effector peptide, binding of MRP is not accompanied by an acceleration of actin polymerization kinetics, and we also could not reliably observe an actin cross-linking activity of MRP.

Importance of protein kinase C targeting for the phosphorylation of its substrate, myristoylated alanine-rich C-kinase substrate

We visualized the translocation of myristoylated alanine-rich protein kinase C substrate (MARCKS) in living Chinese hamster ovary-K1 cells using MARCKS tagged to green fluorescent protein (MARCKS-GFP). MARCKS-GFP was rapidly translocated from the plasma membrane to the cytoplasm after the treatment with phorbol ester, which translocates protein kinase C (PKC) to the plasma membrane. In contrast, PKC activation by hydrogen peroxide, which was not accompanied by PKC translocation, did not alter the intracellular localization of MARCKS-GFP. Non-myristoylated mutant of MARCKS-GFP was distributed throughout the cytoplasm, including the nucleoplasm, and was not translocated by phorbol ester or by hydrogen peroxide. Phosphorylation of wild-type MARCKS-GFP was observed in cells treated with phorbol ester but not with hydrogen peroxide, whereas non-myristoylated mutant of MARCKS-GFP was phosphorylated in cells treated with hydrogen peroxide but not with phorbol ester. Phosphorylation of both MARCKS-GFPs reduced the amount of F-actin. These findings revealed that PKC targeting to the plasma membrane is required for the phosphorylation of membrane-associated MARCKS and that a mutant MARCKS existing in the cytoplasm can be phosphorylated by PKC activated in the cytoplasm without translocation but not by PKC targeted to the membrane.

Membrane binding of MARCKS-related protein studied by tryptophan fluorescence spectroscopy

MARCKS-related protein (MRP) is a peripheral membrane protein whose binding to membranes is mediated by the N-terminal myristoyl moiety and a central, highly basic effector domain. MRP mediates cross-talk between protein kinase C and calmodulin and is thought to link the actin cytoskeleton to the plasma membrane. Since MRP contains no tryptophan residues, we mutated a phenylalanine in the effector domain to tryptophan (MRP F93W) and used fluorescence spectroscopy to monitor binding of the protein to phospholipid vesicles. We report in detail the evaluation procedure necessary to extract quantitative information from the raw data. The spectra of MRP F93W obtained in the presence of increasing amounts of lipid crossed at an isosbestic point, indicating a simple transition between two states: free and membrane-bound protein. The change in fluorescence toward values typical of a more hydrophobic environment was used to quantify membrane binding. The partition coefficient agreed well with values obtained previously by other methods. To study the interaction of the N-terminus of MRP with membranes, a tryptophan residue was also introduced at position 4 (MRP S4W). Our data suggest that only the myristoylated N-terminus interacted with liposomes. These results demonstrate the versatility of site-directed incorporation of tryptophan residues to study protein-membrane interactions.

Myristoylation-dependent N-terminal cleavage of the myristoylated alanine-rich C kinase substrate (MARCKS) by cellular extracts

The myristoylated alanine-rich C kinase substrate (MARCKS) has been proposed to regulate the plasticity of the actin cytoskeleton at its site of attachment to membranes. In macrophages, MARCKS is implicated in various cellular events including motility, adhesion and phagocytosis. In this report we show that macrophage extracts contain a protease which specifically cleaves human MARCKS, expressed in a cell-free system or in E. coli, between Lys-6 and Thr-7. Cleavage of MARCKS decreases its affinity for macrophage membranes by ca. one order of magnitude, highlighting the contribution of the myristoyl moiety of MARCKS to membrane binding. Importantly, cleavage requires myristoylation of MARCKS. Furthermore, MARCKS-related protein (MRP), the second member of the MARCKS family, is not digested. Since Thr-7 is lacking in MRP this suggests that Thr-7 at the P1 position is important for the recognition of lipid-modified substrates. A different product is observed when MARCKS is incubated with a calf brain cytosolic extract. This product can be remyristoylated in the presence of myristoyl-CoA and N-myristoyl transferase, demonstrating that cycles of myristoylation/demyristoylation of MARCKS can be achieved in vitro. Although the physiological relevance of these enzymes still needs to be demonstrated, our results reveal the presence of a new class of cleaving enzymes recognizing lipid-modified protein substrates.