Sequential activation of individual PKC isozymes in integrin-mediated muscle cell spreading: a role for MARCKS in an integrin signaling pathway

To understand how muscle cell spreading and survival are mediated by integrins, we studied the signaling events initiated by the attachment of muscle cells to fibronectin (FN). We have previously demonstrated that muscle cell spreading on FN is mediated by alpha5beta1 integrin, is associated with rapid phosphorylation of focal adhesion kinase and is dependent on activation of protein kinase C (PKC). Here we investigated the role of individual PKC isozymes in these cellular processes. We show that alpha, delta and epsilonPKC are expressed in muscle cells and are activated upon integrin engagement with different kinetics - epsilonPKC was activated early, whereas alpha and deltaPKC were activated later. Using isozyme-specific inhibitors, we found that the activation of epsilonPKC was necessary for cell attachment to FN. However, using isozyme-specific activators, we found that activation of each of three isozymes was sufficient to promote the spreading of alpha5-integrin-deficient cells on FN. To investigate further the mechanism by which integrin signaling and PKC activation mediate cell spreading, we studied the effects of these processes on MARCKS, a substrate of PKC and a protein known to regulate actin dynamics. We found that MARCKS was localized to focal adhesion sites soon after cell adhesion and that MARCKS translocated from the membrane to the cytosol during the process of cell spreading. This translocation correlated with different phases of PKC activation and with reorganization of the actin cytoskeleton. Using MARCKS-antisense cDNA, we show that alpha5-expressing cells in which MARCKS expression is inhibited fail to spread on FN, providing evidence for the crucial role of MARCKS in muscle cell spreading. Together, the data suggest a model in which early activation of epsilonPKC is necessary for cell attachment; the later activation of alpha or deltaPKC may be necessary for the progression from attachment to spreading. The mechanism of PKC-mediated cell spreading may be via the phosphorylation of signaling proteins, such as MARCKS, that are involved in the reorganization of the actin cytoskeleton.

Northern blot and in situ hybridization analyses of MARCKS mRNA expression in the cerebral cortex of the macaque monkey

Myristoylated alanine-rich C-kinase substrate (MARCKS) is a major substrate for protein kinase C, and is involved in synaptic plasticity. Using both Northern blot and in situ hybridization techniques, we investigated whether MARCKS expression varied according to the cerebral region, including the hippocampal formation, or according to the type of neuron. Northern blot analysis showed that the MARCKS mRNA level was higher in the association areas than in the primary sensory and motor areas of the cerebral neocortex. MARCKS mRNA levels in the hippocampus and the amygdala were as high as those in the association areas. The in situ hybridization experiments confirmed the Northern blot results and showed the distribution and characteristics of MARCKS mRNA-positive neurons. In the association areas of the neocortex, prominent signals were observed in neurons in layers II-VI. In the primary areas, prominent signals were restricted to neurons in layers IV-VI. In the hippocampus, the most intense hybridization signals were observed in neurons in the granule cell layer of the dentate gyrus. The observed region-specific expression might reflect functional specialization for plasticity in individual regions of the monkey cerebral cortex.

Myristoylated alanine-rich C kinase substrate (MARCKS) sequesters spin-labeled phosphatidylinositol 4,5-bisphosphate in lipid bilayers

The myristoylated alanine-rich protein kinase C substrate (MARCKS) may function to sequester phosphoinositides within the plane of the bilayer. To characterize this interaction with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), a novel spin-labeled derivative, proxyl-PIP(2), was synthesized and characterized. In the presence of molecules known to bind PI(4,5)P(2) the EPR spectrum of this label exhibits an increase in line width because of a decrease in label dynamics, and titration of this probe with neomycin yields the expected 1:1 stoichiometry. Thus, this probe can be used to quantitate the interactions made by the PI(4,5)P(2) head group within the bilayer. In the presence of a peptide comprising the effector domain of MARCKS the EPR spectrum broadens, but the changes in line shape are modulated by both changes in label correlation time and spin-spin interactions. This result indicates that at least some proxyl-PIP(2) are in close proximity when bound to MARCKS and that MARCKS associates with multiple PI(4,5)P(2) molecules. Titration of the proxyl-PIP(2) EPR signal by the MARCKS-derived peptide also suggests that multiple PI(4,5)P(2) molecules interact with MARCKS. Site-directed spin labeling of this peptide shows that the position and conformation of this protein segment at the membrane interface are not altered significantly by binding to PI(4,5)P(2). These data are consistent with the hypothesis that MARCKS functions to sequester multiple PI(4,5)P(2) molecules within the plane of the membrane as a result of interactions that are driven by electrostatic forces.

Inhibition of rho-kinase-induced myristoylated alanine-rich C kinase substrate (MARCKS) phosphorylation in human neuronal cells by H-1152, a novel and specific Rho-kinase inhibitor

The functions of small G protein Rho-associated kinase (Rho-kinase) have been determined in muscle and non-muscle cells, but, particularly in neuronal cells, its effector(s) has not been well known. Recently, we preliminarily reported that Rho-kinase phosphorylates the Ser159 residue in myristoylated alanine-rich C kinase substrate (MARCKS) in vitro, but it remains obscure in vivo. To further clarify this point, we developed an isoquinolinesulfonamide derivative, H-1152, that is a more specific, stronger and membrane-permeable inhibitor of Rho-kinase with a Ki value of 1.6 nM, but poor inhibitor of other serine/threonine kinases. H-1152 dose-dependently inhibited the phosphorylation of MARCKS in human neuroteratoma (NT-2) cells stimulated by Rho-activator lysophosphatidic acid (LPA), which was determined by phosphorylation site-specific antibody against phospho-Ser159 in MARCKS, whereas it hardly inhibited the phosphorylation stimulated by phorbol-12,13-dibutyrate (PDBu). In contrast, two other Rho-kinase inhibitors, HA-1077 at 30 microM and Y-27632 at 10-30 microM, inhibited the phosphorylation of MARCKS in the cells stimulated by LPA and PDBu. A PKC inhibitor Ro-31-8220 selectively inhibited PDBu-induced phosphorylation of MARCKS. Taken together with our previous results, the present findings strongly suggest that Rho/Rho-kinase phosphorylates MARCKS at Ser159 residue in neuronal cells in response to LPA stimulation and that H-1152 is a useful tool to confirm Rho-kinase function(s) in cells and tissues.

Nanomolar amyloid beta protein activates a specific PKC isoform mediating phosphorylation of MARCKS in Neuro2A cells

Myristoylated alanine-rich C kinase substrate (MARCKS), a protein associated with cell growth, neurosecretion and macrophage activation, is activated by protein kinase C (PKC) phosphorylation. We reported that amyloid beta protein (Abeta) activated MARCKS through a tyrosine kinase and PKC-delta in rat cultured microglia. Here we report that Abeta signaling pathway through a specific PKC isoform is involved in the phosphorylation of MARCKS in Neuro2A cells. Selective PKC inhibitors but not tyrosine kinase inhibitors significantly inhibited the phosphorylation of MARCKS induced by Abeta. Abeta selectively activated PKC-alpha among the four PKC isoforms localized in Neuro2A cells. PKC-alpha activated by Abeta directly phosphorylated a recombinant MARCKS in vitro, Translocation of PKC-alpha from the cytoplasm to the membrane and accumulation of phospho-MARCKS in the cytoplasm were induced by Abeta. These results suggest involvement of a phosphoinositide signaling system through PKC-alpha in the phosphorylation of MARCKS in neurons, an event which may be associated with mechanisms underlying neurotrophic and neurotoxic effects of Abeta.

Protein kinase C and phospholipase C activity and expression of their specific isozymes is decreased and expression of MARCKS is increased in platelets of bipolar but not in unipolar patients

Phospholipase C (PLC) and protein kinase C (PKC) are important components of the phosphoinositide (PI) signaling system. To examine if the abnormalities observed in the PI signaling system of patients with affective disorders, reported in previous studies, are related to abnormalities in one or more of its components, we studied PKC, PI-PLC activity, the expression of their specific isozymes, and expression of myristoylated alanine-rich C-kinase substrate (MARCKS) in platelets obtained from 15 drug-free hospitalized patients with bipolar disorder and 15 with major depressive disorder (unipolar) and from 15 nonhospitalized normal control subjects. We observed a significant decrease in PI-PLC and PKC activity and the expression of selective PKC alpha, betaI, betaII, and PLC delta(1) isozymes in membrane and cytosol fraction of platelets from bipolar but not unipolar patients. On the other hand, the level of MARCKS was significantly increased in membrane and cytosol fraction of platelets from patients with bipolar but not unipolar disorders. These results suggest that alterations in PKC, PLC, and MARCKS may be involved in the pathophysiology of bipolar illness.

Obligatory role of protein kinase Cbeta and MARCKS in vesicular trafficking in living neurons

Neurotransmitter release from neurons involves both vesicular trafficking and subsequent fusion of synaptic vesicles with the plasma membrane. The mechanisms involving the formation and fusion of vesicles that allow the exocytotic release of transmitters are understood well. Little is known, however, about the signaling mechanism involved in the trafficking of vesicles along the neurites. In this study, we used real-time confocal microscopy to search for evidence that vesicular trafficking in neurons requires the activation of protein kinase Cbeta (PKCbeta) and the myristoylated alanine-rich C kinase substrate (MARCKS) signaling pathway. Dopamine-beta-hydroxylase fused to green fluorescent protein has been used to trace vesicular movement. Angiotensin II, an established neuromodulatory hormone, stimulates translocation of green fluorescent protein-dopamine-beta-hydroxylase vesicles from the cell body to neurites. This translocation was blocked by an antisense oligonucleotide to PKCbeta and MARCKS. Stimulation of PKC by other means, such as phorbol-12-myristate-13-acetate or carbachol, also resulted in the redistribution of fluorescence in a manner similar to that observed for angiotensin II. These observations demonstrate that PKCbeta-MARCKS signaling may be a general mechanism for the stimulation of vesicular trafficking in brain neurons.

Multicolor imaging of Ca(2+) and protein kinase C signals using novel epifluorescence microscopy

Dynamic changes in intracellular free Ca(2+) concentrations ([Ca(2+)](i)s) control many important cellular events, including binding of Ca(2+)-calmodulin (Ca(2+)-CaM) and phosphorylation by protein kinase C (PKC). The two signals compete for the same domains in certain substrates, such as myristoylated alanine-rich PKC-substrate (MARCKS). To observe the convergence and relative time of arrival of CaM and PKC signals at their shared domain of MARCKS, we need to image cells that are loaded with more than two fluorescent dyes at a reasonable speed. We have developed a simple and powerful multicolor imaging system using conventional fluorescence microscopy. The epifluorescence configuration uses a glass reflector and rotating filter wheels for excitation and emission paths. As it is free of dichroic (multichroic) mirrors, multiple fluorescence images can be acquired rapidly regardless of the colors of fluorophores. We visualized Ca(2+)-CaM and PKC together with the dynamics of their common target, MARCKS, in single live cells. Receptor-activation resulted in translocation of MARCKS from the plasma membrane to cytosol through its phosphorylation by PKC. By observing fluorescence resonance energy transfer, we also obtained direct evidence that Ca(2+)-CaM binds MARCKS to drag it away from the membrane in circumstances when Ca(2+)-mobilization predominates over PKC activation.

The novel and specific Rho-kinase inhibitor (S)-(+)-2-methyl-1-[(4-methyl-5-isoquinoline)sulfonyl]-homopiperazine as a probing molecule for Rho-kinase-involved pathway

We have developed several kinds of protein kinase inhibitors, which are classified as isoquinolinesulfonamides and characterized as ATP competitive inhibitors of Ser/Thr protein kinases. These include H9, H89, KN62, and 1-(5-isoquinolinesulfonyl)-homopiperazine (HA-1077) against protein kinase C (PKC), protein kinase A, Ca(2+)/calmodulin-dependent protein kinase II, and Rho-kinase, respectively, and they have been used widely to confirm the involvement of the target protein kinase in biological or physiological reaction(s). In some cases, inhibitors have predicted the involvement of the target protein kinase in cell or tissue before its precise mechanism or its effector was defined. On a clinical level, we developed the Rho-kinase inhibitor HA-1077 as an anti-spastic that effectively suppresses the spasm of cerebral arteries after subarachnoid hemorrhage. We have improved HA-1077 to obtain (S)-(+)-2-methyl-1-[(4-methyl-5-isoquinoline)sulfonyl]-homopiperazine (H-1152P), which is a more selective inhibitor of Rho-kinase, with a K(i) value of 1.6 nM for Rho-kinase, 630 nM for protein kinase A, and 9270 nM for PKC. This inhibitor suppressed the phosphorylation of myristoylated alanine-rich C-kinase substance (MARCKS) in neuronal cells stimulated with lysophosphatidic acid, whose phosphorylation site was confirmed to be the Ser159 residue, using a phosphorylation site-specific antibody. In contrast, phorbol 12-myristate 13-acetate-induced phosphorylation of MARCKS was scarcely inhibited by H-1152P. Furthermore, lysophosphatidic acid-stimulated phosphorylation in neuronal cells was characterized as a C3 toxin-sensitive event. Our results show that the Rho-kinase inhibitor targets a protein with a well-known function, MARCKS in neuronal cells. Although MARCKS is widely recognized as a substrate of PKC, our results raise the possibility that MARCKS is a target protein of Rho-kinase in neuronal cells. In this review, we address the possible role of Rho-kinase in neuronal functions, using the Rho-kinase specific inhibitor H-1152P.

Vacuolar system-associated protein-60: a protein characterized from bovine granulosa and luteal cells that is associated with intracellular vesicles and related to human 80K-H and murine beta-glucosidase II

It has been suggested that proteins of molecular size 56-58 kDa play an important role in bovine ovarian follicular development and oocyte maturation. A polyclonal antibody was raised against a 56- to 58-kDa protein band purified from bovine granulosa cells and was used to screen granulosa or luteal cell cDNA expression libraries. This work resulted in the identification of a cDNA encoding for a protein of 60.1 kDa with a signal peptide of 13 residues. The bovine 60.1-kDa protein shared an overall 86.7% and 81.8% identity with, respectively, the human 80K-H protein and the mouse putative beta subunit of glucosidase II (beta-GII), and was named vacuolar system-associated protein-60 (VASAP-60). Marked differences in sequence identity were noted in a putative molecular adapter domain containing a tandem D and E amino acid stretch flanked by proline-rich sequences presenting the minimal PXXP SH3 motif. VASAP-60 was shown to be unglycosylated using endoglycosidase H treatment and was found mainly in a cellular membrane fraction of bovine corpus luteum. VASAP-60 was localized in a rat hepatic Golgi/endosome fraction and in wheat germ agglutinin (WGA) affinity chromatographic eluates, thereby suggesting the presence of interactions with membrane glycoproteins. A polyclonal antibody was raised against the putative adapter domain of the recombinant VASAP-60; this was shown to recognize a major 88-kDa and two minor 58-kDa and 50-kDa proteins, suggesting that the major 88-kDa protein band represents the complete VASAP-60 protein whereas the 58-kDa and the 50-kDa bands represent its proteolytic fragments. Northern blot analysis demonstrated the presence of a single 2.3-kilobase transcript in all the bovine tissues analyzed with variation in the steady state level between tissues. Immunohistochemical observations showed that VASAP-60 was widely distributed in bovine tissues and was localized in pericytoplasmic and perinuclear membranes. In epithelial cells, the staining presented a basolateral or apical polarity associated with intracellular vacuoles. In conclusion, we have characterized a novel acidic membrane protein, associated with organelles of the vacuolar system, that is widely and histospecifically expressed in bovine tissues. VASAP-60 represents either the bovine ortholog or a new family member of the previously characterized human 80K-H and murine beta-GII proteins. Our results suggest that VASAP-60 presents characteristics of a molecular adaptor protein with functions in membrane-trafficking events.

The SUN family of Saccharomyces cerevisiae: the double knock-out of UTH1 and SIM1 promotes defects in nucleus migration and increased drug sensitivity

UTH1 and SIM1 are two of four 'SUN' genes (SIM1, UTH1, NCA3 and SUN4/SCW3) whose products are involved in different cellular processes such as DNA replication, lifespan, mitochondrial biogenesis or cell septation. UTH1 or SIM1 inactivation did not affect cell growth, shape or nuclear migration, whereas the double null mutant presented phenotypes of numerous binucleate cells and benomyl sensitivity, suggesting that microtubule function could be altered; the uth1Deltasim1Delta strain also presented defects which could be related to the Ras/cAMP pathway: pet phenotype, heat shock sensitivity, inability to store glycogen, sensitivity to starvation and failure of spores to germinate. These observations suggested that Uth1p could be involved as a connection step between pathways controlling growth and those controlling division.

Fractionation of beta-glucosidases and related extracellular enzymes from Aspergillus niger

Industrial concentrates from Aspergillus niger culture filtrates were fractionated by ion-exchange and adsorption chromatography. Several other types of hydrolases were completely removed. Eight partially purified components were obtained. Using specific activity as an estimate of purification, one aryl-beta-glucosidase was purified 35-fold. Another component showed 147-fold purification using a viscosimetric assay with carboxymethylcellulose as substrate. The aryl-beta-glucosidase was distinctly more thermolabile than the carboxymethylcellulase.

Distinctive properties of beta-glucosidases and related enzymes derived from a commercial Aspergillus niger cellulase

Eight highly purified beta-glucosidases from Aspergillus niger were compared enzymatically, chemically, and immunologically. Ultraviolet spectra, pH-activity responses, substrate specificities, thermal stabilities, kinetic changes in the viscosity of substrate, Michaelis-Menten parameters, adsorption characteristics on cellulose, and exclusion characteristics on dextran gels were determined. The data indicate that the several components represent distinctly different enzymes in terms of mode of attack on substrate. The concept of partial denaturation of a single enzyme precursor is unable to explain the heterogeneity observed. Comparison of the effect of pH on hydrolysis of carboxymethylcellulose and cellohexaose suggests that a negative charge center on the substrate has a pronounced inhibitory effect on the enzymes.

Novel recruitment of Shc, Grb2, and Sos by fibroblast growth factor receptor-1 in v-Src-transformed cells

In response to fibroblast growth factor (FGF), FGF receptor-1 (FGFR-1) (flg) becomes tyrosine phosphorylated and associates with phospholipase C gamma (PLC gamma) and a 90 kDa protein. We report here that in cells transformed by v-Src, FGFR-1 becomes phosphorylated on tyrosine; however, neither PLC gamma nor p90 was found to be associated with tyrosine-phosphorylated FGFR-1. Instead, there was a strong constitutive association of FGFR-1 with the adaptor proteins Shc and Grb2 and the Ras guanine nucleotide exchange factor Sos. Association with Shc and Grb2 and Sos was not observed in response to FGF. Suramin did not prevent either tyrosine phosphorylation or Shc/Grb2/Sos association, indicating a non-autocrine mechanism. Thus, in cells transformed by v-Src, tyrosine phosphorylation of FGFR-1 results not in the expected association with PLC gamma and p90, but rather in the recruitment of the Ras activating Shc/Grb2/Sos complex. These data suggest a mechanism for Ras activation by v-Src involving phosphorylation of novel tyrosine(s) on FGFR-1.

Identification of the active site nucleophile in the thermostable beta-glycosidase from the archaeon Sulfolobus solfataricus expressed in Escherichia coli

Sulfolobus solfataricus beta-glycosidase expressed in Escherichia coli was fully inactivated at 65 degrees C, according to pseudo-first-order kinetics, by [3H]conduritol B epoxide (DL-1,2 anhydro-myo-inositol) synthesized as the active site directed inhibitor by a slight modification of Legler's procedure [Legler, G. (1977) Methods Enzymol. 46, 368-381]. The determination of kinetic constants for the inactivation showed that the process took place through the formation of a stabilized inhibitor-enzyme intermediate. Inactivation and reactivation studies suggested that the inhibitor-enzyme intermediate complex was formed more rapidly and hydrolyzed at a lower rate than it was for other glycosidases. Moreover, the stoichiometry of the binding, determined by electrospray mass spectrometric analysis, revealed that one molecule of the inhibitor was covalently bound to each enzyme subunit. The binding site for [3H]conduritol B epoxide was identified by the isolation and partial sequence analysis of the radioactive peptide obtained by cyanogen bromide and pepsin digests. Electrospray tandem mass analysis of the labeled peptide showed that the inhibitor was covalently bound to E387. This result, in agreement with data obtained from sequence alignments of S. solfataricus beta-glycosidase with other gluco- and galactosidases of the glycosyl hydrolase family 1 [Henrissat, B. (1991) Biochem. J. 280, 309-316], indicates that the conserved E387 is the nucleophilic amino acid residue in the active site of the enzyme.