Modulation of calcium-evoked [3H]noradrenaline release from permeabilized cerebrocortical synaptosomes by the MARCKS protein, calmodulin and the actin cytoskeleton

Alterations of the myristoylated, alanine-rich C kinase substrate (MARCKS) in prefrontal cortex in schizophrenia

Abnormal synaptic plasticity has been implicated in the cognitive deficits seen in schizophrenia, where alterations have been found in neurotransmission, signaling and dendritic dynamics. Rapid rearrangement of the actin cytoskeleton is critical for plasticity and abnormalities of molecular regulators of this process are candidates for understanding mechanisms underlying these changes in schizophrenia. The myristoylated, alanine-rich C-kinase substrate (MARCKS) is crucial for many roles associated with synaptic plasticity, including facilitation of neurotransmission, dendritic branching and in turn cognitive function. Accordingly, we hypothesized that this protein is abnormally expressed or regulated in schizophrenia. We measured protein expression of MARCKS by Western blot analysis in postmortem samples of dorsolateral prefrontal cortex (DLPFC) from elderly schizophrenia patients (N=16) and a comparison group (N=20). We also assayed phosphorylated-MARCKS (pMARCKS), given the role of phosphorylation in reversing membrane association by MARCKS. We found decreased expression of both MARCKS and pMARCKS in schizophrenia. Altered myristoylation may be a mechanism that explains this down-regulation of MARCKS, so we also assayed expression of the two isoforms of the key myristoylation enzyme, NMT, and an enzymatic inhibitor of this enzyme, NMT-inhibitor protein (NIP71) by Western blotting in these same subjects. Expression did not change between groups for these proteins, suggesting a mechanism other than myristoylation is responsible for decreased MARCKS expression in schizophrenia. These data suggest a potential mechanism underlying aspects of altered synaptic plasticity observed in schizophrenia.

MARCKS regulation of mucin secretion by airway epithelium in vitro: interaction with chaperones

We have reported previously that myristoylated alanine-rich C kinase substrate (MARCKS) is a key regulatory molecule controlling mucin secretion by airway epithelial cells in vitro and in vivo. The results of those studies supported a mechanism whereby MARCKS, upon phosphorylation by protein kinase C (PKC), translocates from plasma membrane to cytoplasm, where its binding to membranes of intracellular mucin granules is a key component of the secretory pathway. It remains unknown how MARCKS is targeted to and/or preferentially attaches to mucin granule membranes. We hypothesized that the chaperone cysteine string protein (CSP) may play an important role in this process. CSP was shown to associate with membranes of intracellular mucin granules in well-differentiated normal human bronchial epithelial (NHBE) cells in vitro, as determined by ultrastructural immunohistochemistry and Western blotting of isolated granule membranes. CSP in these cells complexed with MARCKS, as shown by co-immunoprecipitation. Given reported associations between CSP and a second chaperone, heat shock protein 70 (HSP70), a role for HSP70 in the MARCKS-dependent secretory mechanism also was investigated. HSP70 appeared to form a trimeric complex with MARCKS and CSP associated with mucin granule membranes within airway epithelial cells. Transfection of the HBE1 human bronchial epithelial cell line with siRNAs targeting sequences of MARCKS, CSP, or HSP70 resulted, in each case, in significant knockdown of expression of these proteins and subsequent attenuation of mucin secretion. The results provide the first evidence that CSP and HSP70, and their interactions with MARCKS, are involved in mucin secretion.

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.

Myristoylated Alanine-rich C Kinase Substrate-mediated Neurotensin Release via Protein Kinase C-δ Downstream of the Rho/ROK Pathway

Myristoylated alanine-rich protein kinase C substrate (MARCKS) is a cellular substrate for protein kinase C (PKC). Recently, we have shown that PKC isoforms-alpha and -delta, as well as the Rho/Rho kinase (ROK) pathway, play a role in phorbol 12-myristate 13-acetate (PMA)-mediated secretion of the gut peptide neurotensin (NT) in the BON human endocrine cell line. Here, we demonstrate that activation of MARCKS protein is important for PMA- and bombesin (BBS)-mediated NT secretion in BON cells. Small interfering RNA (siRNA) to MARCKS significantly inhibited, whereas overexpression of wild-type MARCKS significantly increased PMA-mediated NT secretion. Endogenous MARCKS and green fluorescent protein-tagged wild-type MARCKS were translocated from membrane to cytosol upon PMA treatment, further confirming MARCKS activation. MARCKS phosphorylation was inhibited by PKC-delta siRNA, ROKalpha siRNA, and C3 toxin (a Rho protein inhibitor), suggesting that the PKC-delta and the Rho/ROK pathways are necessary for MARCKS activation. The phosphorylation of PKC-delta was inhibited by C3 toxin, demonstrating that the role of MARCKS in NT secretion was regulated by PKC-delta downstream of the Rho/ROK pathway. BON cell clones stably transfected with the receptor for gastrin releasing peptide, a physiologic stimulant of NT, and treated with BBS, the amphibian equivalent of gastrin releasing peptide, demonstrated a similar MARCKS phosphorylation as noted with PMA. BBS-mediated NT secretion was attenuated by MARCKS siRNA. Collectively, these findings provide evidence for novel signaling pathways, including the sequential regulation of MARCKS activity by Rho/ROK and PKC-delta proteins, in stimulated gut peptide secretion.

Involvement of membrane GABA transporter in α-latrotoxin-stimulated [3H]GABA release

Alpha-latrotoxin evokes massive [3H]GABA release from rat brain synaptosomes by stimulating exocytosis and outflow from non-vesicular pool. In the present study, GABA transporter-mediated [3H]GABA release was shown to be involved in alpha-latrotoxin-triggered release of [3H]GABA from non-vesicular pool. The following agents have been exploited as tools: (1) a protonophore carbonyl cyanide-p-trifluoromethoxyphenyl-hydrazon (FCCP) and bafilomycin A1 for evoking depletion of synaptic vesicle [3H]GABA and enlargement of non-vesicular pool; (2) a non-substrate high-affinity GABA transport blocker NO-711 for determining participation of GABA carrier in the toxin-stimulated GABA release; (3) a competitive inhibitor of GABA reuptake nipecotic acid for heteroexchange [3H]GABA release. As shown by the experiments with nipecotic acid, FCCP and bafilomycin A1 considerably increase the content of non-vesicular [3H]GABA. The treatment of the synaptosomes with these agents modified the response to alpha-latrotoxin, particularly to its subnanomolar concentrations: the lack or substantial lowering of the toxin-evoked release during the first 2 min after the toxin addition and substantial enhancement of release up to the 5th minute were observed. Only the step of enhanced release was sensitive to GABA transporter blocker NO-711. Distinct sensitivity to NO-711 was shown to be characteristic for different steps of alpha-latrotoxin-stimulated [3H]GABA release from the control, untreated synaptosomes: lack of any effect of NO-711 during the first 2 min and powerful inhibition in 10 min after the toxin application. Taken together these data appear to indicate that the toxin non-simultaneously from vesicular and non-vesicular origins releases the neurotransmitter, the first rapid step reflects exocytosis stimulation, and the second tardy step is at least in part due to the release mediated by GABA transporters. The incomplete inhibition with NO-711 of the tardy step of the release evoked by nanomolar toxin concentrations suggests the participation not only of the GABA transporters.

Analysis of differential gene expression supports a role for amyloid precursor protein and a protein kinase C substrate (MARCKS) in long-term memory

Previous work has identified the intermediate and medial part of the hyperstriatum ventrale (IMHV) as a region of the chick brain storing information acquired through the learning process of imprinting. We have examined in this brain region changes in expression of candidate genes involved in memory. Chicks were exposed to a rotating red box and the strength of their preference for it, a measure of learning, determined. Brain samples were removed approximately 24 h after training. Candidate genes whose expressions were different in IMHV samples derived from strongly imprinted chicks relative to those from chicks showing little or no learning were identified using subtractive hybridization. The translation products of two candidate genes were investigated further in samples from the left and right IMHV and from two other brain regions not previously implicated in imprinting, the left and right posterior neostriatum. One of the proteins was the amyloid precursor protein (APP), the other was myristoylated alanine rich C kinase substrate (MARCKS). In the left IMHV the levels of the two proteins increased with the strength of learning. The effects in the right IMHV were not significantly different from those in the left. There were no effects of learning in the posterior neostriatum. This is the first study to relate changes in the amounts of MARCKS and APP proteins to the strength of learning in a brain region known to be a memory store and demonstrates that the systematic identification of protein molecules involved in memory formation is possible.