The regulation of neurotransmitter secretion by protein kinase C

Interplay of G-proteins and Serotonin in the Neuroimmunoinflammatory Model of Chronic Stress and Depression: A Narrative Review

INTRODUCTION

This narrative review addresses the clinical challenges in stress-related disorders such as depression, focusing on the interplay between neuron-specific and pro-inflammatory mechanisms at the cellular, cerebral, and systemic levels.

OBJECTIVE

We aim to elucidate the molecular mechanisms linking chronic psychological stress with low-grade neuroinflammation in key brain regions, particularly focusing on the roles of G proteins and serotonin (5-HT) receptors.

METHODS

This comprehensive review of the literature employs systematic, narrative, and scoping review methodologies, combined with systemic approaches to general pathology. It synthesizes current research on shared signaling pathways involved in stress responses and neuroinflammation, including calcium-dependent mechanisms, mitogen-activated protein kinases, and key transcription factors like NF-κB and p53. The review also focuses on the role of G protein-coupled neurotransmitter receptors (GPCRs) in immune and pro-inflammatory responses, with a detailed analysis of how 13 of 14 types of human 5-HT receptors contribute to depression and neuroinflammation.

RESULTS

The review reveals a complex interaction between neurotransmitter signals and immunoinflammatory responses in stress-related pathologies. It highlights the role of GPCRs and canonical inflammatory mediators in influencing both pathological and physiological processes in nervous tissue.

CONCLUSION

The proposed Neuroimmunoinflammatory Stress Model (NIIS Model) suggests that proinflammatory signaling pathways, mediated by metabotropic and ionotropic neurotransmitter receptors, are crucial for maintaining neuronal homeostasis. Chronic mental stress can disrupt this balance, leading to increased pro-inflammatory states in the brain and contributing to neuropsychiatric and psychosomatic disorders, including depression. This model integrates traditional theories on depression pathogenesis, offering a comprehensive understanding of the multifaceted nature of the condition.

The Effect of Isosaponarin Derived from Wasabi Leaves on Glutamate Release in Rat Synaptosomes and Its Underlying Mechanism

Excessive glutamate release is known to be involved in the pathogenesis of neurological diseases, and suppression of glutamate release from nerve terminals is considered to be a treatment strategy. In this study, we investigated whether isosaponarin, a flavone glycoside isolated from wasabi leaves, could affect glutamate release in rat cerebral cortex nerve terminals (synaptosomes). The release of glutamate was evoked by the K⁺ channel blocker 4-aminopyridine (4-AP) and measured by an online enzyme-coupled fluorimetric assay. Isosaponarin produced a concentration-dependent inhibition of 4-AP-evoked glutamate release with a half-maximum inhibition of release value of 22 μM. The inhibition caused by isosaponarin was prevented by eliminating extracellular Ca²⁺ or by using bafilomycin A1, an inhibitor of synaptic vesicle exocytosis. Isosaponarin decreased intrasynaptosomal rises in Ca²⁺ levels that were induced by 4-AP, without affecting the synaptosomal membrane potential. The isosaponarin-induced inhibition of glutamate release was significantly prevented in synaptosomes that were pretreated with a combination of the calcium channel blockers ω-conotoxin GVIA (N-type) and ω-agatoxin IVA (P/Q-types). The protein kinase C (PKC) pan-inhibitor GF109203X and the Ca²⁺-dependent PKC inhibitor Go6976 abolished the inhibition of glutamate release by isosaponarin, while the Ca²⁺-independent PKC inhibitor rottlerin did not show any effect. The results from immunoblotting assays also showed that isosaponarin lowered PKC, PKCα, synaptosomal-associated protein of 25 kDa (SNAP-25), and myristoylated alanine-rich C-kinase substrate (MARCKS) phosphorylation induced by 4-AP. In addition, FM1-43-labeled synaptic vesicles in synaptosomes showed that treatment with isosaponarin resulted in an attenuation of the 4-AP-induced decrease in fluorescence intensity that is consistent with glutamate release. Transmission electron microscopy of synaptosomes also provided evidence that isosaponarin altered the number of synaptic vesicles. These results indicate that isosaponarin suppresses the Ca²⁺-dependent PKC/SNAP-25 and MARCKS pathways in synaptosomes, causing a decrease in the number of available synaptic vesicles, which inhibits vesicular glutamate release from synaptosomes.

Neuroprotective Role of the B Vitamins in the Modulation of the Central Glutamatergic Neurotransmission

BACKGROUND

Regulation of glutamate release is crucial for maintaining normal brain function, but excess glutamate release is implicated in many neuropathological conditions. Therefore, the minimum glutamate release from presynaptic nerve terminals is an important neuroprotective mechanism.

OBJECTIVE

In this mini-review, we analyze the three B vitamins, namely vitamin B2 (riboflavin), vitamin B6 (pyridoxine), and vitamin B12 (cyanocobalamin), that affect the 4-aminopyridine (4- AP)-evoked glutamate release from presynaptic nerve terminal in rat and discuss their neuroprotective role.

METHODS

In this study, the measurements include glutamate release, DiSC3(5), and Fura-2.

RESULTS

The riboflavin, pyridoxine, and cyanocobalamin produced significant inhibitory effects on 4-aminopyridine-evoked glutamate release from rat cerebrocortical nerve terminals (synaptosomes) in a dose-dependent relationship. These presynaptic inhibitory actions of glutamate release are attributed to inhibition of physiologic Ca2+-dependent vesicular exocytosis but not Ca2+-independent nonvesicular release. These effects also did not affect membrane excitability, while diminished cytosolic (Ca2+)c through a reduction of direct Ca2+ influx via Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels, rather than through indirect Ca2+induced Ca2+ release from ryanodine-sensitive intracellular stores. Furthermore, their effects were attenuated by GF109203X and Ro318220, two protein kinase C (PKC) inhibitors, suggesting suppression of PKC activity. Taken together, these results suggest that riboflavin, pyridoxine, and cyanocobalamin inhibit presynaptic vesicular glutamate release from rat cerebrocortical synaptosomes, through the depression Ca2+ influx via voltage- dependent Cav2.2 (N-type) and Cav2.1 (P/Q-type) Ca2+ channels, and PKC signaling cascade.

CONCLUSION

Therefore, these B vitamins may reduce the strength of glutamatergic synaptic transmission and is of considerable importance as potential targets for therapeutic agents in glutamate- induced excitation-related diseases.

Protein kinase C theta (Prkcq) affects nerve degeneration and regeneration through the c-fos and c-jun pathways in injured rat sciatic nerves

BACKGROUND

Previous finding using DNA microarray and bioinformatics analysis, we have reported some key factors which regulated gene expression and signaling pathways in injured sciatic nerve during Wallerian Degeneration (WD). This research is focused on protein kinase C theta (Prkcq) participates in the regulation of the WD process.

METHODS

In this study, we explored the molecular mechanism by which Prkcq in Schwann cells (SCs) affects nerve degeneration and regeneration in vivo and in vitro after rat sciatic nerve injury.

RESULTS

Study of the cross-sectional model showed that Prkcq expression decreased significantly during sciatic nerve repair. Functional analysis showed that upregulation and downregulation of Prkcq could affect the proliferation, migration and apoptosis of Schwann cells and lead to the expression of related factors through the activation of the β-catenin, c-fos, and p-c-jun/c-jun pathways.

CONCLUSION

The study provides insights into the role of Prkcq in early WD during peripheral nerve degeneration and/or regeneration.

The Molecular and Pathophysiological Functions of Members of the LNX/PDZRN E3 Ubiquitin Ligase Family

The ligand of Numb protein-X (LNX) family, also known as the PDZRN family, is composed of four discrete RING-type E3 ubiquitin ligases (LNX1, LNX2, LNX3, and LNX4), and LNX5 which may not act as an E3 ubiquitin ligase owing to the lack of the RING domain. As the name implies, LNX1 and LNX2 were initially studied for exerting E3 ubiquitin ligase activity on their substrate Numb protein, whose stability was negatively regulated by LNX1 and LNX2 via the ubiquitin-proteasome pathway. LNX proteins may have versatile molecular, cellular, and developmental functions, considering the fact that besides these proteins, none of the E3 ubiquitin ligases have multiple PDZ (PSD95, DLGA, ZO-1) domains, which are regarded as important protein-interacting modules. Thus far, various proteins have been isolated as LNX-interacting proteins. Evidence from studies performed over the last two decades have suggested that members of the LNX family play various pathophysiological roles primarily by modulating the function of substrate proteins involved in several different intracellular or intercellular signaling cascades. As the binding partners of RING-type E3s, a large number of substrates of LNX proteins undergo degradation through ubiquitin-proteasome system (UPS) dependent or lysosomal pathways, potentially altering key signaling pathways. In this review, we highlight recent and relevant findings on the molecular and cellular functions of the members of the LNX family and discuss the role of the erroneous regulation of these proteins in disease progression.

E3 ubiquitin ligases LNX1 and LNX2 are major regulators of the presynaptic glycine transporter GlyT2

The neuronal glycine transporter GlyT2 is an essential regulator of glycinergic neurotransmission that recaptures glycine in presynaptic terminals to facilitate transmitter packaging in synaptic vesicles. Alterations in GlyT2 expression or activity result in lower cytosolic glycine levels, emptying glycinergic synaptic vesicles and impairing neurotransmission. Lack of glycinergic neurotransmission caused by GlyT2 loss-of-function mutations results in Hyperekplexia, a rare neurological disease characterized by generalized stiffness and motor alterations that may cause sudden infant death. Although the importance of GlyT2 in pathology is known, how this transporter is regulated at the molecular level is poorly understood, limiting current therapeutic strategies. Guided by an unbiased screening, we discovered that E3 ubiquitin ligase Ligand of Numb proteins X1/2 (LNX1/2) modulate the ubiquitination status of GlyT2. The N-terminal RING-finger domain of LNX1/2 ubiquitinates a cytoplasmic C-terminal lysine cluster in GlyT2 (K751, K773, K787 and K791), and this process regulates the expression levels and transport activity of GlyT2. The genetic deletion of endogenous LNX2 in spinal cord primary neurons causes an increase in GlyT2 expression and we find that LNX2 is required for PKC-mediated control of GlyT2 transport. This work identifies, to our knowledge, the first E3 ubiquitin-ligases acting on GlyT2, revealing a novel molecular mechanism that controls presynaptic glycine availability. Providing a better understanding of the molecular regulation of GlyT2 may help future investigations into the molecular basis of human disease states caused by dysfunctional glycinergic neurotransmission, such as hyperekplexia and chronic pain.

Proteolytic shedding of the prion protein via activation of metallopeptidase ADAM10 reduces cellular binding and toxicity of amyloid-β oligomers

The cellular prion protein (PrP^C) is a key neuronal receptor for β-amyloid oligomers (AβO), mediating their neurotoxicity, which contributes to the neurodegeneration in Alzheimer's disease (AD). Similarly to the amyloid precursor protein (APP), PrP^C is proteolytically cleaved from the cell surface by a disintegrin and metalloprotease, ADAM10. We hypothesized that ADAM10-modulated PrP^C shedding would alter the cellular binding and cytotoxicity of AβO. Here, we found that in human neuroblastoma cells, activation of ADAM10 with the muscarinic agonist carbachol promotes PrP^C shedding and reduces the binding of AβO to the cell surface, which could be blocked with an ADAM10 inhibitor. Conversely, siRNA-mediated ADAM10 knockdown reduced PrP^C shedding and increased AβO binding, which was blocked by the PrP^C-specific antibody 6D11. The retinoic acid receptor analog acitretin, which up-regulates ADAM10, also promoted PrP^C shedding and decreased AβO binding in the neuroblastoma cells and in human induced pluripotent stem cell (iPSC)-derived cortical neurons. Pretreatment with acitretin abolished activation of Fyn kinase and prevented an increase in reactive oxygen species caused by AβO binding to PrP^C Besides blocking AβO binding and toxicity, acitretin also increased the nonamyloidogenic processing of APP. However, in the iPSC-derived neurons, Aβ and other amyloidogenic processing products did not exhibit a reciprocal decrease upon acitretin treatment. These results indicate that by promoting the shedding of PrP^C in human neurons, ADAM10 activation prevents the binding and cytotoxicity of AβO, revealing a potential therapeutic benefit of ADAM10 activation in AD.

BDNF-TrkB Signaling Coupled to nPKCε and cPKCβI Modulate the Phosphorylation of the Exocytotic Protein Munc18-1 During Synaptic Activity at the Neuromuscular Junction

Munc18-1, a neuron-specific member of the Sec1/Munc18 family, is involved in neurotransmitter release by binding tightly to syntaxin. Munc18-1 is phosphorylated by PKC on Ser-306 and Ser-313 in vitro which reduces the amount of Munc18-1 able to bind syntaxin. We have previously identified that PKC is involved in neurotransmitter release when continuous electrical stimulation imposes a moderate activity on the NMJ and that muscle contraction through TrkB has an important impact on presynaptic PKC isoforms levels, specifically cPKCβI and nPKCε. Therefore, the present study was designed to understand how Munc18-1 phosphorylation is affected by (1) synaptic activity at the neuromuscular junction, (2) nPKCε and cPKCβI isoforms activity, (3) muscle contraction per se, and (4) the BDNF/TrkB signaling in a neuromuscular activity-dependent manner. We performed immunohistochemistry and confocal techniques to evidence the presynaptic location of Munc18-1 in the rat diaphragm muscle. To study synaptic activity, we stimulated the phrenic nerve (1 Hz, 30 min) with or without contraction (abolished by μ-conotoxin GIIIB). Specific inhibitory reagents were used to block nPKCε and cPKCβI activity and to modulate the tropomyosin receptor kinase B (TrkB). Main results obtained from Western blot experiments showed that phosphorylation of Munc18-1 at Ser-313 increases in response to a signaling mechanism initiated by synaptic activity and directly mediated by nPKCε. Otherwise, cPKCβI and TrkB activities work together to prevent this synaptic activity-induced Munc18-1 phosphorylation by a negative regulation of cPKCβI over nPKCε. Therefore, a balance between the activities of these PKC isoforms could be a relevant cue in the regulation of the exocytotic apparatus. The results also demonstrate that muscle contraction prevents the synaptic activity-induced Munc18-1 phosphorylation through a mechanism that opposes the TrkB/cPKCβI/nPKCε signaling.

Regulation of insulin exocytosis by calcium-dependent protein kinase C in beta cells

The control of insulin release from pancreatic beta cells helps ensure proper blood glucose level, which is critical for human health. Protein kinase C has been shown to be one key control mechanism for this process. After glucose stimulation, calcium influx into beta cells triggers exocytosis of insulin-containing dense-core granules and activates protein kinase C via calcium-dependent phospholipase C-mediated generation of diacylglycerol. Activated protein kinase C potentiates insulin release by enhancing the calcium sensitivity of exocytosis, likely by affecting two main pathways that could be linked: (1) the reorganization of the cortical actin network, and (2) the direct phosphorylation of critical exocytotic proteins such as munc18, SNAP25, and synaptotagmin. Here, we review what is currently known about the molecular mechanisms of protein kinase C action on each of these pathways and how these effects relate to the control of insulin release by exocytosis. We identify remaining challenges in the field and suggest how these challenges might be addressed to advance our understanding of the regulation of insulin release in health and disease.

Echinacoside Inhibits Glutamate Release by Suppressing Voltage-Dependent Ca(2+) Entry and Protein Kinase C in Rat Cerebrocortical Nerve Terminals

The glutamatergic system may be involved in the effects of neuroprotectant therapies. Echinacoside, a phenylethanoid glycoside extracted from the medicinal Chinese herb HerbaCistanche, has neuroprotective effects. This study investigated the effects of echinacoside on 4-aminopyridine-evoked glutamate release in rat cerebrocortical nerve terminals (synaptosomes). Echinacoside inhibited Ca²⁺-dependent, but not Ca²⁺-independent, 4-aminopyridine-evoked glutamate release in a concentration-dependent manner. Echinacoside also reduced the 4-aminopyridine-evoked increase in cytoplasmic free Ca²⁺ concentration but did not alter the synaptosomal membrane potential. The inhibitory effect of echinacoside on 4-aminopyridine-evoked glutamate release was prevented by ω-conotoxin MVIIC, a wide-spectrum blocker of Cav2.2 (N-type) and Cav2.1 (P/Q-type) channels, but was insensitive to the intracellular Ca²⁺ release-inhibitors dantrolene and 7-chloro-5-(2-chloropheny)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157). Furthermore, echinacoside decreased the 4-aminopyridine-induced phosphorylation of protein kinase C, and protein kinase C inhibitors abolished the effect of echinacoside on glutamate release. According to these results, we suggest that the inhibitory effect of echinacoside on evoked glutamate release is associated with reduced voltage-dependent Ca²⁺ entry and subsequent suppression of protein kinase C activity.

Hesperidin inhibits glutamate release and exerts neuroprotection against excitotoxicity induced by kainic acid in the hippocampus of rats

The citrus flavonoid hesperidin exerts neuroprotective effects and could cross the blood-brain barrier. Given the involvement of glutamate neurotoxicity in the pathogenesis of neurodegenerative disorders, this study was conducted to evaluate the potential role of hesperidin in glutamate release and glutamate neurotoxicity in the hippocampus of rats. In rat hippocampal nerve terminals (synaptosomes), hesperidin inhibited the release of glutamate and elevation of cytosolic free Ca(2+) concentration evoked by 4-aminopyridine (4-AP), but did not alter 4-AP-mediated depolarization. The inhibitory effect of hesperidin on evoked glutamate release was prevented by chelating the extracellular Ca(2+) ions and blocking the activity of Cav2.2 (N-type) and Cav2.1 (P/Q-type) channels or protein kinase C. In hippocampal slice preparations, whole-cell patch clamp experiments showed that hesperidin reduced the frequency of spontaneous excitatory postsynaptic currents without affecting their amplitude, indicating the involvement of a presynaptic mechanism. In addition, intraperitoneal (i.p.) injection of kainic acid (KA, 15 mg/kg) elevated the extracellular glutamate levels and caused considerable neuronal loss in the hippocampal CA3 area. These KA-induced alterations were attenuated by pretreatment with hesperidin (10 or 50 mg/kg, i.p.) before administering the KA. These results demonstrate that hesperidin inhibits evoked glutamate release in vitro and attenuates in vivo KA-induced neuronal death in the hippocampus. Our findings indicate that hesperidin may be a promising candidate for preventing or treating glutamate excitotoxicity related brain disorders such as neurodegenerative diseases.

Acute onset of generalized tremors, mild ataxia, and hyperesthesia in a young dog after presumptive ingestion of a giant sea hare belonging to the genus Aplysia

OBJECTIVE

To describe an unusual case of canine toxicosis from presumptive sea hare ingestion, its management, and outcome.

CASE SUMMARY

A young dog was treated for acute toxicosis after partially ingesting a giant sea hare washed up on a beach. The primary symptoms of generalized tremors, mild ataxia, and hyperesthesia occurred rapidly within 20 minutes of ingestion. Decontamination procedures were performed early and the dog made a full recovery within an approximate period of 6 hours.

NEW OR UNIQUE INFORMATION PROVIDED

Sea hare toxicosis should be considered as a potential cause of acute onset of tremors, ataxia, and hyperesthesia in previously healthy dogs living in coastal areas.

Dimebon, an antihistamine drug, inhibits glutamate release in rat cerebrocortical nerve terminals

The excessive release of glutamate is a critical element in the neuropathology of acute and chronic brain disorders. The purpose of the present study was to investigate the effect and possible mechanism of dimebon, an antihistamine with a neuroprotective profile, on endogenous glutamate release in the nerve terminals (synaptosomes) of the rat cerebral cortex. Dimebon inhibited the release of glutamate that was evoked by exposing the synaptosomes to the K(+) channel blocker 4-aminopyridine, and this effect was prevented by chelating extracellular Ca(2+) ions, and the vesicular transporter inhibitor bafilomycin A1. Dimebon inhibited depolarization-evoked increase in cytosolic free Ca(2+) concentration, and the dimebon-mediated inhibition of glutamate release was prevented by the Cav2.2 (N-type) and Cav2.1 (P/Q-type) channel blocker ω-conotoxin MVIIC. The inhibitory action of dimebon on glutamate release was not due to its decreasing synaptosomal excitability, because dimebon did not alter the resting synaptosomal membrane potential or 4-aminopyridine-mediated depolarization. Furthemore, the dimebon effect on 4-aminopyridine-evoked glutamate release was prevented by the protein kinase C inhibitor, and dimebon substantially reduced the 4-AP-induced phosphorylation of protein kinase C. However, the dimebon-mediated inhibition of glutamate release was unaffected by the N-methyl-d-aspartate receptor agonist or antagonist. These results suggest that dimebon inhibits glutamate release from rat cortical synaptosomes by suppressing presynaptic voltage-dependent Ca(2+) entry and protein kinase C activity. This implies that the inhibition of glutamate release is an additional pharmacological activity of dimebon that may play a critical role in the apparent clinical efficacy of this compound.

PMA increases M3 muscarinic receptor levels and decreases retinal cells proliferation through a change in the levels of cell-cycle regulatory proteins

Protein kinase C (PKC) pathway plays important roles in different phenomena in nervous system development. Our previous data demonstrated that phorbol 12-myristate 13-acetate (PMA) treatment, a PKC activator, for 48 h decreases retinal cells proliferation by a mechanism mediated by muscarinic receptor activation, involving a decrease in M1 receptors levels. The aim of this work was to analyze how PMA interferes in the levels of cell cycle control proteins p53, p21 and cyclin D1 and also to investigate its influence on M3 receptor levels. Our results show that PMA (50 ng/mL) produces a significant increase in p21 and p53 levels, decreases cyclin D1 levels, and also enhances M3 receptors levels in cell cultures. Evaluating the postnatal retinal tissue development until 30 days, we observed that tissue differentiation is accompanied by an increase in M3 and p21 levels. Based on our results we suggest that PMA treatment is promoting a change in muscarinic receptors expression mimicking the pattern observed during tissue differentiation, indicating that PMA is probably accelerating the cholinergic differentiation in rat retinal cell cultures.

Protein kinase C controls vesicular transport and secretion of apolipoprotein E from primary human macrophages

Macrophage-specific apolipoprotein E (apoE) secretion plays an important protective role in atherosclerosis. However, the precise signaling mechanisms regulating apoE secretion from primary human monocyte-derived macrophages (HMDMs) remain unclear. Here we investigate the role of protein kinase C (PKC) in regulating basal and stimulated apoE secretion from HMDMs. Treatment of HMDMs with structurally distinct pan-PKC inhibitors (calphostin C, Ro-31-8220, Go6976) and a PKC inhibitory peptide all significantly decreased apoE secretion without significantly affecting apoE mRNA or apoE protein levels. The PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated apoE secretion, and both PMA-induced and apoAI-induced apoE secretion were inhibited by PKC inhibitors. PKC regulation of apoE secretion was found to be independent of the ATP binding cassette transporter ABCA1. Live cell imaging demonstrated that PKC inhibitors inhibited vesicular transport of apoE to the plasma membrane. Pharmacological or peptide inhibitor and knockdown studies indicate that classical isoforms PKCα/β and not PKCδ, -ε, -θ, or -ι/ζ isoforms regulate apoE secretion from HMDMs. The activity of myristoylated alanine-rich protein kinase C substrate (MARCKS) correlated with modulation of PKC activity in these cells, and direct peptide inhibition of MARCKS inhibited apoE secretion, implicating MARCKS as a downstream effector of PKC in apoE secretion. Comparison with other secreted proteins indicated that PKC similarly regulated secretion of matrix metalloproteinase 9 and chitinase-3-like-1 protein but differentially affected the secretion of other proteins. In conclusion, PKC regulates the secretion of apoE from primary human macrophages.

Putative depolarisation-induced retrograde signalling accelerates the repeated hypoxic depression of excitatory synaptic transmission in area CA1 of rat hippocampus via group I metabotropic glutamate receptors

Excitatory synaptic transmission in area CA1 of the mammalian hippocampus is rapidly depressed during hypoxia. The depression is largely attributable to an increase in extracellular adenosine and activation of inhibitory adenosine A(1) receptors on presynaptic glutamatergic terminals. However, sequential exposure to hypoxia results in a slower subsequent hypoxic depression of excitatory synaptic transmission, a phenomenon we have previously ascribed to a reduction in the release of extracellular adenosine. In the present study we show that this delayed depression of excitatory postsynaptic currents (EPSCs) to repeated hypoxia can be reversed by a period of postsynaptic depolarisation delivered to an individual CA1 neuron, under whole-cell voltage clamp, between two periods of hypoxia. The depolarisation-induced acceleration of the hypoxic depression of the EPSC is dependent upon postsynaptic Ca(2+) influx, the activation of PKC and is blocked by intracellular application of GDP-β-S and N-ethylmaleimide (NEM), inhibitors of membrane fusion events. In addition, the acceleration of the hypoxic depression of the EPSC was prevented by the GI mGluR antagonist AIDA, but not by the CB1 cannabinoid receptor antagonist AM251. Our results suggest a process initiated in the postsynaptic cell that can influence glutamate release during subsequent metabolic stress. This may reflect a novel neuroprotective strategy potentially involving retrograde release of adenosine and/or glutamate.

Regulation of catecholamine release in human adrenal chromaffin cells by β-adrenoceptors

The adrenal gland plays a fundamental role in the response to a variety of stress situations. After a stress condition, adrenal medullary chromaffin cells release, by exocytosis, high quantities of catecholamine (epinephrine, EP; norepinephrine, NE), especially EP. Once in the blood stream, catecholamines reach different target organs, and induce their biological actions through the activation of different adrenoceptors. Adrenal gland cells may also be activated by catecholamines, through hormonal, paracrine and/or autocrine system. The presence of functional adrenoceptors on human adrenal medulla and their involvement on catecholamines secretion was not previously evaluated. In the present study we investigated the role of β(1)-, β(2)- and β(3)-adrenoceptors on catecholamine release from human adrenal chromaffin cells in culture. We observed that the β-adrenoceptor agonist (isoproterenol) and β(2)-adrenoceptor agonist (salbutamol) stimulated catecholamine (NE and EP) release from human adrenal chromaffin cells. Furthermore, the β(2)-adrenoceptor antagonist (ICI 118,551; 100 nM) and β(3)-adrenoceptor antagonist (SR 59230A; 100 nM) inhibited the catecholamine release stimulated by isoproterenol and nicotine in chromaffin cells. The β(1)-adrenoceptor antagonist (atenolol; 100 nM) did not change the isoproterenol- neither the nicotine-evoked catecholamine release from human adrenal chromaffin cells. Moreover, our results show that the protein kinase A (PKA), protein kinase C (PKC), mitogen-activated protein kinase (MAPK) and phospholipase C (PLC) are intracellular mechanisms involved in the catecholamine release evoked by salbutamol. In conclusion, our data suggest that the activation of β(2)- and β(3)-adrenoceptors modulate the basal and evoked catecholamine release, NE and EP, via an autocrine positive feedback loop in human adrenal chromaffin cells.

Brain GLP-1 signaling regulates femoral artery blood flow and insulin sensitivity through hypothalamic PKC-δ

OBJECTIVE

Glucagon-like peptide 1 (GLP-1) is a gut-brain hormone that regulates food intake, energy metabolism, and cardiovascular functions. In the brain, through a currently unknown molecular mechanism, it simultaneously reduces femoral artery blood flow and muscle glucose uptake. By analogy to pancreatic β-cells where GLP-1 activates protein kinase C (PKC) to stimulate insulin secretion, we postulated that PKC enzymes would be molecular targets of brain GLP-1 signaling that regulate metabolic and vascular function.

RESEARCH DESIGN AND METHODS

We used both genetic and pharmacological approaches to investigate the role of PKC isoforms in brain GLP-1 signaling in the conscious, free-moving mouse simultaneous with metabolic and vascular measurements.

RESULTS

In normal wild-type (WT) mouse brain, the GLP-1 receptor (GLP-1R) agonist exendin-4 selectively promotes translocation of PKC-δ (but not -βII, -α, or -ε) to the plasma membrane. This translocation is blocked in Glp1r(-/-) mice and in WT mice infused in the brain with exendin-9, an antagonist of the GLP-1R. This mechanism coordinates both blood flow in the femoral artery and whole-body insulin sensitivity. Consequently, in hyperglycemic, high-fat diet-fed diabetic mice, hypothalamic PKC-δ activity was increased and its pharmacological inhibition improved both insulin-sensitive metabolic and vascular phenotypes.

CONCLUSIONS

Our studies show that brain GLP-1 signaling activates hypothalamic glucose-dependent PKC-δ to regulate femoral artery blood flow and insulin sensitivity. This mechanism is attenuated during the development of experimental hyperglycemia and may contribute to the pathophysiology of type 2 diabetes.

Idebenone inhibition of glutamate release from rat cerebral cortex nerve endings by suppression of voltage-dependent calcium influx and protein kinase A

The present study was aimed at investigating the effect and the possible mechanism of idebenone on endogenous glutamate release in nerve terminals of rat cerebral cortex (synaptosomes). Idebenone inhibited the release of glutamate that was evoked by exposing synaptosomes to the K(+) channel blocker 4-aminopyridine (4-AP), and this phenomenon was concentration dependent. Inhibition of glutamate release by idebenone was prevented by chelating extracellular Ca(2+), or by the vesicular transporter inhibitor bafilomycin A1, but was insensitive to DL-threo-beta-benzyl-oxyaspartate, a glutamate transporter inhibitor. Idebenone decreased the depolarization-induced increase in the cytosolic free Ca(2+) concentration ([Ca(2+)](C)),whereas it did not alter the resting synaptosomal membrane potential or 4-AP-mediated depolarization. The inhibitory effect of idebenone on evoked glutamate release was prevented by blocking the Ca(v)2.2 (N-type) and Ca(v)2.1 (P/Q-type) channels, but not by blocking intracellular Ca(2+) release or Na(+)/Ca(2+) exchange. Furthermore, the idebenone effect on 4-AP-evoked Ca(2+) influx and glutamate release was completely abolished by the protein kinase A (PKA) inhibitors, H89 and KT5720. On the basis of these results, it was concluded that idebenone inhibits glutamate release from rat cortical synaptosomes and this effect is linked to a decrease in [Ca(2+)](C) contributed by Ca(2+) entry through presynaptic voltage-dependent Ca(2+) channels and to the suppression of PKA signaling cascade.

Dynamics of peptidergic secretory granule transport are regulated by neuronal stimulation

Background

Peptidergic neurons store and secrete the contents of large dense core vesicles (LDCVs) from axon terminals and from dendrites. Secretion of peptides requires a highly regulated exocytotic mechanism, plus coordinated synthesis and transport of LDCVs to their sites of release. Although these trafficking events are critical to function, little is known regarding the dynamic behavior of LDCVs and the mechanisms by which their transport is regulated. Sensory neurons also package opiate receptors in peptide-containing LDCVs, which is thought to be important in pain sensation. Since peptide granules cannot be refilled locally after their contents are secreted, it is particularly important to understand how neurons support regulated release of peptides.

Results

A vector encoding soluble peptidylglycine α-hydroxylating monooxygenase fused to green fluorescent protein was constructed to address these questions in cultured primary peptidergic neurons of the trigeminal ganglion using time lapse confocal microscopy. The time course of release differs with secretagogue; the secretory response to depolarization with K⁺ is rapid and terminates within 15 minutes, while phorbol ester stimulation of secretion is maintained over a longer period. The data demonstrate fundamental differences between LDCV dynamics in axons and growth cones under basal conditions.

Conclusions

Under basal conditions, LDCVs move faster away from the soma than toward the soma, but fewer LDCVs travel anterograde than retrograde. Stimulation decreased average anterograde velocity and increases granule pausing. Data from antibody uptake, quantification of enzyme secretion and appearance of pHluorin fluorescence demonstrate distributed release of peptides all along the axon, not just at terminals.

Facilitation of glutamate release from rat cerebrocortical glutamatergic nerve terminals (synaptosomes) by phosphatidylserine and phosphatidylcholine

Phosphatidylserine (PS) and phosphatidylcholine (PC) have been shown to enhance cognitive function. Considering that brain glutamatergic system is thought to participate in cognitive processing, our objective was to determine the effect of PS and PC on glutamate release from the nerve terminal (synaptosome) freshly isolated from rat cerebral cortex. Data showed that both PS and PC potently facilitate 4-aminopyridine (4-AP)-evoked Ca(2+)-dependent and Ca(2+)-independent glutamate release. Facilitation of glutamate release by PS or PC was associated with an increase of 4-AP-evoked depolarization and downstream elevation of cytoplasmic free calcium concentration ([Ca(2+)](c)). In addition, glutamate release elicited by direct Ca(2+)-entry with Ca(2+)-ionophore (ionomycin) was also facilitated by PS or PC. Furthermore, PS- or PC-mediated facilitation of 4-AP-evoked glutamate release was superseded or suppressed by protein kinase C (PKC) activator and inhibitor, respectively. Together, these results suggest that PS or PC effects a facilitation of glutamate exocytosis by increasing nerve terminal excitability and Ca(2+) influx into cerebrocortical nerve terminals through a signaling cascade involving PKC.

Cyanocobalamin, vitamin B12, depresses glutamate release through inhibition of voltage-dependent Ca2+ influx in rat cerebrocortical nerve terminals (synaptosomes)

The effect of cyanocobalamin, vitamin B12, on glutamate release in isolated nerve terminals (synaptosomes) prepared from rat prefrontal cortex was examined. Cyanocobalamin inhibited the release of glutamate evoked by 4-aminopyridine in a concentration-dependent manner. The inhibitory action of cyanocobalamin was blocked by the vesicular transporter inhibitor bafilomycin A1, not by the glutamate transporter inhibitor L-transpyrrolidine-2,4-dicarboxylic acid or the nontransportable glutamate inhibitor DL-threo-beta-benzyloxyaspartate, indicating that this release inhibition results from a reduction of vesicular exocytosis and not from an inhibition of Ca(2+)-independent efflux via glutamate transporter. Examination of the effect of cyanocobalamin on cytosolic free Ca(2+) concentration revealed that the inhibition of glutamate release could be attributed to a reduction in voltage-dependent Ca(2+) influx. Consistent with this, the N- and P/Q-type Ca(2+) channel blocker omega-conotoxin MVIIC, largely attenuated the inhibitory effect of cyanocobalamin on 4-aminopyridine-evoked glutamate release, but the Ca(2+) release inhibitor dantrolene had no effect. Cyanocobalamin did not alter the resting synaptosomal membrane potential or 4-aminopyridine-mediated depolarization; thus, the inhibition of 4-aminopyridine-evoked Ca(2+) influx and glutamate release produced by cyanocobalamin was not due to its decreasing synaptosomal excitability. In addition, cyanocobalamin-mediated inhibition of 4-aminopyridine-evoked Ca(2+) influx and glutamate release was significantly attenuated by protein kinase C inhibitors GF109203X and Ro318220. Furthermore, 4-aminopyridine-induced phosphorylation of protein kinase C was significantly reduced by cyanocobalamin. These results suggest that cyanocobalamin effects a decrease in protein kinase C activation, which subsequently reduces the Ca(2+) entry through voltage-dependent N- and P/Q-type Ca(2+) channels to cause a decrease in evoked glutamate release.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Ginsenoside Rg1 and Rb1 enhance glutamate exocytosis from rat cortical nerve terminals by affecting vesicle mobilization through the activation of protein kinase C

Our previous study showed that ginsenoside Rg1 and Rb1, the active ingredients of ginseng, facilitated depolarization-evoked glutamate release from isolated rat cortical nerve terminals by increasing voltage-dependent Ca(2+) entry. However, it remains to be clarified whether ginsenoside Rg1 and Rb1 possess an action at the exocytotic machinery itself, downstream of a Ca(2+) influx. We now show that ionomycin-induced glutamate release and KCl-evoked FM1-43 dye release were facilitated by ginsenoside Rg1 and Rb1 in rat cortical nerve terminals (synaptosomes), suggesting that some steps after Ca(2+) entry are regulated by ginsenoside Rg1 or Rb1. The activation of protein kinase C is essential for ginsenoside Rg1 or Rb1 action, since protein kinase C inhibitors GF109203X and Ro318220 abolished the facilitatory effect of ginsenoside Rg1 or Rb1 on ionomycin-evoked glutamate release. Furthermore, ginsenoside Rg1 or Rb1 increased ionomycin-evoked protein kinase C or its presynaptic target myristoylated alanine-rich C kinase substrate (MARCKS) phosphorylation. Additionally, ginsenoside Rg1 or Rb1-mediated facilitation of ionomycin-evoked glutamate release was occluded by cytochalasin D, a membrane-permeant inhibitor of actin polymerization. Together, these results suggest that ginsenoside Rg1 or Rb1-mediated facilitation of glutamate release involves the modulation of some exocytotic steps, possibly through the activation of protein kinase C and disassembly of cytoskeleton, resulting in an increase in synaptic vesicle availability.

Heterotrimeric G proteins and the single-transmembrane domain IGF-II/M6P receptor: functional interaction and relevance to cell signaling

The G protein-coupled receptor (GPCR) family represents the largest and most versatile group of cell surface receptors. Classical GPCR signaling constitutes ligand binding to a seven-transmembrane domain receptor, receptor interaction with a heterotrimeric G protein, and the subsequent activation or inhibition of downstream intracellular effectors to mediate a cellular response. However, recent reports on direct, receptor-independent G protein activation, G protein-independent signaling by GPCRs, and signaling of nonheptahelical receptors via trimeric G proteins have highlighted the intrinsic complexities of G protein signaling mechanisms. The insulin-like growth factor-II/mannose-6 phosphate (IGF-II/M6P) receptor is a single-transmembrane glycoprotein whose principal function is the intracellular transport of lysosomal enzymes. In addition, the receptor also mediates some biological effects in response to IGF-II binding in both neuronal and nonneuronal systems. Multidisciplinary efforts to elucidate the intracellular signaling pathways that underlie these effects have generated data to suggest that the IGF-II/M6P receptor might mediate transmembrane signaling via a G protein-coupled mechanism. The purpose of this review is to outline the characteristics of traditional and nontraditional GPCRs, to relate the IGF-II/M6P receptor's structure with its role in G protein-coupled signaling and to summarize evidence gathered over the years regarding the putative signaling of the IGF-II/M6P receptor mediated by a G protein.

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.

Presynaptic adenosine and P2Y receptors

Adenine-based purines, such as adenosine and ATP, are ubiquitous molecules that, in addition to their roles in metabolism, act as modulators of neurotransmitter release through activation of presynaptic P1 purinoceptors or adenosine receptors (activated by adenosine) and P2 receptors (activated by nucleotides). Of the latter, the P2Y receptors are G protein-coupled, whereas the P2X receptors are ligand-gated ion channels and not covered in this review.

(-)-Epigallocatechin gallate, the most active polyphenolic catechin in green tea, presynaptically facilitates Ca2+-dependent glutamate release via activation of protein kinase C in rat cerebral cortex

(-)-Epigallocatechin gallate (EGCG), the main polyphenolic constituent of green tea, has been reported to improve cognitive decline. Considering the central glutamatergic activity is crucial to cognitive function, the objective of this study was to investigate the effect of EGCG on the release of endogenous glutamate using nerve terminals purified from rat cerebral cortex. Results showed that the release of glutamate evoked by 4-aminopyridine (4AP) was facilitated by EGCG in a concentration-dependent manner, and this effect resulted from an enhancement of vesicular exocytosis and not from an increase in Ca2+-independent efflux via glutamate transporter. Examination of the effect of EGCG on cytoplasmic free Ca2+ concentration ([Ca2+]c) revealed that the facilitation of glutamate release could be attributed to an increase in Ca2+ influx through N- and P/Q-type voltage-dependent Ca2+ channels. Consistent with this, the EGCG-mediated facilitation of 4AP-evoked glutamate release was significantly prevented in synaptosomes pretreated with a combination of the N- and P/Q-type Ca2+ channel blockers. Additionally, inhibition of protein kinase C (PKC) by treatment with Ro318220 significantly reduced the facilitatory effect of EGCG on 4AP-evoked glutamate release and phosphorylation of PKC or its presynaptic target myristoylated alanine-rich C kinase substrate (MARCKS). These results suggest that EGCG effects a facilitation of glutamate release from glutamatergic terminals by positively modulating N- and P/Q-type Ca2+ channel activation through a signaling cascade involving PKC. In this EGCG/PKC signaling cascade facilitating glutamate release, the regulation of cytoskeleton dynamics was also indicated to be involved by disruption of cytoskeleton organization with cytochalasin D occluded the EGCG-mediated facilitation of 4AP-evoked glutamate release.

Chronic ethanol exposure inhibits dopamine release via effects on the presynaptic actin cytoskeleton in PC12 cells

An increase in nucleus accumbens dopamine release appears to mediate the "rewarding" properties of drugs of abuse. Using PC12 cells, it has been shown that chronic ethanol exposure can significantly reduce nicotine-stimulated dopamine release. Here, a novel mechanism of ethanol in regulating presynaptic dopamine release is demonstrated. In neuronal cells, a layer of filamentous actin beneath the cell surface regulates the movement and release of synaptic vesicles. Upon stimulation, there is a protein kinase C (PKC)-dependent breakdown of this actin cytoskeleton, allowing vesicles to move near the nerve terminal membrane for release. Chronic ethanol alters PKC signaling, thus the hypothesis that chronic ethanol inhibits presynaptic actin cytoskeleton breakdown in response to stimulation was tested. PC12 cells were chronically exposed to ethanol and then acutely exposed to multiple forms of stimulation (nicotine, sucrose, potassium, and ionophore). In ethanol-treated cells, dopamine release was inhibited following stimulation by forms of release shown to be PKC-dependent (nicotine, sucrose, and potassium). In contrast, dopamine release was not altered following stimulation by PKC-independent forms of release (ionophore). Actin cytoskeleton breakdown was also inhibited following stimulation with PKC-dependent forms of stimulated release but not by PKC-independent (ionophore) forms. Further, cytochalasin B, an agent which depolymerizes actin, reversed the effects of chronic ethanol on both actin depolymerization and dopamine release. These data suggest that chronic ethanol inhibits presynaptic actin breakdown, likely resulting in decreased neurotransmitter release.

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.

Role of omega-3 fatty acids in brain development and function: potential implications for the pathogenesis and prevention of psychopathology

The principle omega-3 fatty acid in brain, docosahexaenoic acid (DHA), accumulates in the brain during perinatal cortical expansion and maturation. Animal studies have demonstrated that reductions in perinatal brain DHA accrual are associated with deficits in neuronal arborization, multiple indices of synaptic pathology including deficits in serotonin and mesocorticolimbic dopamine neurotransmission, neurocognitive deficits, and elevated behavioral indices of anxiety, aggression, and depression. In primates and humans, preterm delivery is associated with deficits in fetal cortical DHA accrual, and children/adolescents born preterm exhibit deficits in cortical gray matter maturation, neurocognitive deficits particularly in the realm of attention, and increased risk for attention-deficit/hyperactivity disorder (ADHD) and schizophrenia. Individuals diagnosed with ADHD or schizophrenia exhibit deficits in cortical gray matter maturation, and medications found to be efficacious in the treatment of these disorders increase cortical and striatal dopamine neurotransmission. These associations in conjunction with intervention trials showing enhanced cortical visual acuity and cognitive outcomes in preterm and term infants fed DHA, suggest that perinatal deficits in brain DHA accrual may represent a preventable neurodevelopmental risk factor for the subsequent emergence of psychopathology.

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.

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.

Temperature-sensitive paralytic mutants: insights into the synaptic vesicle cycle

Forward genetic screens have identified numerous proteins with critical roles in neurotransmission. One particularly fruitful screening target in Drosophila has been TS (temperature-sensitive) paralytic mutants, which have revealed proteins acutely required in neuronal signalling. In the present paper, we review recent insights and current questions from one recently cloned TS paralytic mutant, rbo (rolling blackout). The rbo mutant identifies a putative integral lipase of the pre-synaptic plasma membrane that is required for the SV (synaptic vesicle) cycle. Identification of this mutant adds to a growing body of evidence that lipid-modifying enzymes locally control specialized lipid microenvironments and lipid signalling pathways with key functions regulating neurotransmission strength. The RBO protein is absolutely required for phospholipase C signalling in phototransduction. We posit that RBO might be required to regulate the availability of fusogenic lipids such as phosphatidylinositol 4,5-bisphosphate and diacylglycerol that may directly modify membrane properties and/or activate lipid-binding fusogenic proteins mediating SV exocytosis.

Single transmembrane domain insulin-like growth factor-II/mannose-6-phosphate receptor regulates central cholinergic function by activating a G-protein-sensitive, protein kinase C-dependent pathway

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is a single-pass transmembrane glycoprotein that plays an important role in the intracellular trafficking of lysosomal enzymes and endocytosis-mediated degradation of IGF-II. However, its role in signal transduction after IGF-II binding remains unclear. In the present study, we report that IGF-II/M6P receptor in the rat brain is coupled to a G-protein and that its activation by Leu27IGF-II, an analog that binds rather selectively to the IGF-II/M6P receptor, potentiates endogenous acetylcholine release from the rat hippocampal formation. This effect is mediated by a pertussis toxin (PTX)-sensitive GTP-binding protein and is dependent on protein kinase Calpha (PKCalpha)-induced phosphorylation of downstream substrates, myristoylated alanine-rich C kinase substrate, and growth associated protein-43. Additionally, treatment with Leu27IGF-II causes a reduction in whole-cell currents and depolarization of cholinergic basal forebrain neurons. This effect, which is blocked by an antibody against the IGF-II/M6P receptor, is also sensitive to PTX and is mediated via activation of a PKC-dependent pathway. These results together revealed for the first time that the single transmembrane domain IGF-II/M6P receptor expressed in the brain is G-protein coupled and is involved in the regulation of central cholinergic function via the activation of specific intracellular signaling cascades.

Essential role of the LIM domain in the formation of the PKCɛ–ENH–N-type Ca2+ channel complex

A LIM domain is a specialized double-zinc finger motif found in a variety of proteins. LIM domains are thought to function as molecular modules, mediating specific protein-protein interactions in cellular signaling. In a recent study, we have demonstrated that ENH, which has three consecutive LIM domains, acts as an adaptor protein for the formation of a functional PKCepsilon-ENH-N-type Ca2+ channel complex in neurons. Formation of this complex selectively recruits PKCepsilon to its specific substrate, N-type Ca2+ channels, and is critical for rapid and efficient potentiation of the Ca2+ channel activity by PKC in neurons. However, it is not clear whether changes in the local Ca2+ concentrations near the channel mouth may affect the formation of the triprotein complex. Furthermore, the molecular determinants for the interactions among these three proteins remain unknown. Biochemical studies were performed to address these questions. Within the physiological Ca2+ concentration range (0-300 microM), binding of ENH to the channel C-terminus was significantly increased by Ca2+, whereas increased Ca2+ levels led to dissociation of PKCepsilon from ENH. Mutagenesis studies revealed that the second LIM domain in ENH was primarily responsible for Ca2+-dependent binding of ENH to both the Ca2+ channel C-terminus and PKCepsilon. ENH existed as a dimer in vivo. PKCepsilon translocation inhibition peptide, which blocks the translocation of PKCepsilon from the cytosol to the membrane, inhibited the interaction between PKCepsilon and ENH. These results provide a molecular mechanism for how the PKCepsilon-ENH-N-type Ca2+ channel complex is formed and regulated, as well as potential drug targets to selectively disrupt the PKC signaling complex.

Hyposmolarity evokes norepinephrine efflux from synaptosomes by a depolarization- and Ca2+ -dependent exocytotic mechanism

Osmolarity reduction (20%) elicited 3H-norepinephrine (NE) efflux from rat cortical synaptosomes. The hyposmotic NE release resulted from the following events: (i) a Na+-dependent and La3+-, Gd3+- and ruthenium red-sensitive depolarization; (ii) a cytosolic Ca2+ ([Ca2+]i) rise with contributions from external Ca2+ influx and internal Ca2+ release, probably through the mitochondrial Na+-Ca2+ exchanger; and (iii) activation of a [Ca2+]i-evoked, tetanus toxin (TeTX)-sensitive, PKC-modulated NE efflux mechanism. This sequence was established from results showing a drop in the hyposmotic [Ca2+]i rise by preventing depolarization with La3+, and by the inhibitory effects of Ca2+-free medium (EGTA; 50%), CGP37157 (the mitochondrial Na+-Ca2+ exchanger blocker; 48%), EGTA + CGP37157 or by EGTA-AM (> 95% in both cases). In close correspondence with these effects, NE efflux was 92% decreased by Na+ omission, 75% by La3+, 47% by EGTA, 50% by CGP37157, 90% by EGTA + CGP37157 and 88% by EGTA-AM. PKC influenced the intracellular Ca2+ release and, mainly through this action, modulated NE efflux. TeTX suppressed NE efflux. The K+-stimulated NE release, studied in parallel, was unaffected by Na+ omission, or by La3+, Gd3+ or ruthenium red. It was fully dependent on external Ca2+, insensitive to CGP37157 and abolished by TeTX. These results suggest that the hyposmotic events, although different from the K+-evoked depolarization and [Ca2+]i rise mechanisms, are able to trigger a depolarization-dependent, Ca2+-dependent and TeTX-sensitive mechanism for neurotransmitter release.

A protein phosphatase 2calpha-Ca2+ channel complex for dephosphorylation of neuronal Ca2+ channels phosphorylated by protein kinase C

Phosphorylation and dephosphorylation are primary means for rapid regulation of a variety of neuronal functions, such as membrane excitability, neurotransmitter release, and gene expression. Voltage-gated Ca2+ channels are targets for phosphorylation by a variety of second messengers through activation of different types of protein kinases (PKs). Protein phosphatases (PPs), like PKs, are equally important in regulating Ca2+ channels in neurons. However, much less is understood about whether and how a particular type of PP contributes to regulating neuronal Ca2+ channel activities. This is primarily because of the lack of specific inhibitors/activators for different types of PPs, particularly the PP2c family. The functional roles of PP2c and its substrates in the brain remain virtually unknown. During our yeast two-hybrid screening, PP2calpha was pulled out by both N- and P/Q-type Ca2+ channel C termini. This raised the possibility that PP2calpha might be associated with voltage-gated Ca2+ channels for regulation of the Ca(2+) channel activity. Biochemical studies show that PP2calpha binds directly to neuronal Ca2+ channels forming a functional protein complex in vivo. PP2calpha, unlike PP1, PP2a and PP2b, is more effective in dephosphorylation of neuronal Ca2+ channels after their phosphorylation by PKC. In hippocampal neurons, disruption of the PP2calpha-Ca2+ channel interaction significantly enhances the response of Ca2+ channels to modulation by PKC. Thus, the PP2calpha-Ca2+ channel complex is responsible for rapid dephosphorylation of Ca2+ channels and may contribute to regulation of synaptic transmission in neurons.

Effect of myristoylated alanine-rich C kinase substrate (MARCKS) overexpression on hippocampus-dependent learning and hippocampal synaptic plasticity in MARCKS transgenic mice

The myristoylated alanine-rich C kinase substrate (MARCKS) is a primary substrate of protein kinase C (PKC) thought to regulate membrane-filamentous actin cytoskeletal plasticity in response to PKC activity in the regulation of synaptic efficacy. We have recently reported that MARCKS expression is significantly elevated (45%) in the hippocampus of DBA/2J mice, which exhibit impaired hippocampus-dependent learning and hippocampal long-term potentiation (LTP), compared with C57BL/6J mice. The latter finding led us to hypothesize that elevations in MARCKS expression are detrimental to hippocampal plasticity and function. To assess this more directly, we examined hippocampal (CA1) paired-pulse facilitation and LTP, and hippocampus-dependent learning in mice overexpressing MARCKS through the expression of a human MARCKS transgene (Tg+). The human MARCKS protein was confirmed to be expressed in the hippocampus of Tg+ mice but not in Tg- mice. Schaffer collateral paired-pulse facilitation, input-output responses, and LTP did not differ between Tg+ and Tg- mice, indicating that neurotransmitter release, short-term, and long-term synaptic plasticity are not impaired by MARCKS overexpression. In the Morris water maze, Tg+ mice exhibited a mild but significant spatial learning impairment during initial acquisition, and a more severe impairment during reversal training. Tg+ did not exhibit impaired swim speed or visible platform performance relative to Tg- mice, indicating the absence of gross sensorimotor deficits. Fear conditioning to either context or cue was not impaired in Tg+ mice. Behavioral deficits could not be attributed to differences in hippocampal PKC isozyme (alpha beta(II), gamma, epsilon, zeta) or calmodulin expression, or alterations in hippocampal cytoarchitecture or infrapyramidal mossy fiber limb length. Collectively, these results indicate that elevations in MARCKS expression are detrimental to specific aspects of hippocampal function.

MARCKS in advanced stages of neural retina histogenesis

Myristoylated alanine-rich kinase C substrate (MARCKS), an actin-binding protein, is involved in several signal transduction pathways. It is susceptible to be phosphorylated by protein kinases as protein kinase C and some proline-directed kinases. These phosphorylations differently modulate its functions. We previously showed that a phosphorylation at its Ser25 (S25p-MARCKS) in chickens is a signature of this ubiquitous protein in neuron differentiation. To gain insight into the possible involvement of MARCKS in late retinal histogenesis, we compared the developmental expression patterns of the total protein and its S25p variants. Here we show that the most outstanding modifications occur at the outer retina, where S25p disappears at the end of embryonic development and where MARCKS is missing in adults. These results suggest diverse functional specializations in the different retinal layers.

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.

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.

Protease-activated receptor-1 activation of endothelial cells induces protein kinase Calpha-dependent phosphorylation of syntaxin 4 and Munc18c: role in signaling p-selectin expression

Endothelial cells exhibit regulated exocytosis in response to inflammatory mediators such as thrombin and histamine. The exocytosis of Weibel-Palade bodies (WPBs) containing von Willebrand factor, P-selectin, and interleukin-8 within minutes after stimulation is important for vascular homeostasis. SNARE proteins are key components of the exocytic machinery in neurons and some secretory cells, but their role in regulating exocytosis in endothelial cells is not well understood. We examined the function of SNARE proteins in mediating exocytosis of WPBs in endothelial cells. We identified the presence of syntaxin 4, syntaxin 3, and the high affinity syntaxin 4-regulatory protein Munc18c in human lung microvascular endothelial cells. Small interfering RNA-induced knockdown of syntaxin 4 (but not of syntaxin 3) inhibited exocytosis of WPBs as detected by the reduction in thrombin-induced cell surface P-selectin expression. Thrombin ligation of protease-activated receptor-1 activated the phosphorylation of syntaxin 4 and Munc18c, which, in turn, disrupted the interaction between syntaxin 4 and Munc18. Protein kinase Calpha activation was required for the phosphorylation of syntaxin 4 and Munc18c as well as the cell surface expression of P-selectin. We also observed that syntaxin 4 knockdown inhibited the adhesion of neutrophils to thrombin-activated endothelial cells, demonstrating the functional role of syntaxin 4 in promoting endothelial adhesivity. Thus, protease-activated receptor-1-induced protein kinase Calpha activation and phosphorylation of syntaxin 4 and Munc18c are required for the cell surface expression of P-selectin and the consequent binding of neutrophils to endothelial cells.