Phosphorylation of Syntaphilin by cAMP-dependent Protein Kinase Modulates Its Interaction with Syntaxin-1 and Annuls Its Inhibitory Effect on Vesicle Exocytosis

The amyloid precursor protein and its derived fragments concomitantly contribute to the alterations of mitochondrial transport machinery in Alzheimer's disease

Mitochondria dysfunctions and mitophagy failure have been associated with several Alzheimer's disease (AD) related molecular actors including amyloid beta (Aβ) and recently the amyloid precursor protein-C terminal fragments (APP-CTFs). The efficacy of the mitophagy process in neurons relies on regulated mitochondrial transport along axons involving a complex molecular machinery. The contribution of the amyloid precursor protein (APP) and its derived fragments to the mitochondrial transport machinery alterations in AD have not been investigated before. We report herein a change of the expression of mitochondrial transport proteins (SNPH and Miro1), motor adapters (TRANK1 and TRAK2), and components of the dynein and kinesin motors (i.e., IC1,2 and Kif5 (A, B, C) isoforms) by endogenous APP and by overexpression of APP carrying the familial Swedish mutation (APPswe). We show that APP-CTFs and Aβ concomitantly regulate the expression of a set of transport proteins as demonstrated in APPswe cells treated with β- and γ-secretase inhibitors and in cells Knock-down for presenilin 1 and 2. We further report the impact of APP-CTFs on the expression of transport proteins in AAV-injected C99 mice brains. Our data also indicate that both Aβ oligomers (Aβo) and APP-CTFs impair the colocalization of mitochondria and transport proteins. This has been demonstrated in differentiated SH-SY5Y naive cells treated with Aβo and in differentiated SH-SY5Y and murine primary neurons expressing APPswe and treated with the γ-secretase inhibitor. Importantly, we uncover that the expression of a set of transport proteins is modulated in a disease-dependent manner in 3xTgAD mice and in human sporadic AD brains. This study highlights molecular mechanisms underlying mitochondrial transport defects in AD that likely contribute to mitophagy failure and disease progression.

All-optical presynaptic plasticity induction by photoactivated adenylyl cyclase targeted to axon terminals

Intracellular signaling plays essential roles in various cell types. In the central nervous system, signaling cascades are strictly regulated in a spatiotemporally specific manner to govern brain function; for example, presynaptic cyclic adenosine monophosphate (cAMP) can enhance the probability of neurotransmitter release. In the last decade, channelrhodopsin-2 has been engineered for subcellular targeting using localization tags, but optogenetic tools for intracellular signaling are not well developed. Therefore, we engineered a selective presynaptic fusion tag for photoactivated adenylyl cyclase (bPAC-Syn1a) and found its high localization at presynaptic terminals. Furthermore, an all-optical electrophysiological method revealed rapid and robust short-term potentiation by bPAC-Syn1a at brain stem-amygdala synapses in acute brain slices. Additionally, bPAC-Syn1a modulated mouse immobility behavior. These results indicate that bPAC-Syn1a can manipulate presynaptic cAMP signaling in vitro and in vivo. The all-optical manipulation technique developed in this study can help further elucidate the dynamic regulation of various cellular functions.

Synaptic retrograde regulation of the PKA-induced SNAP-25 and Synapsin-1 phosphorylation

BACKGROUND

Bidirectional communication between presynaptic and postsynaptic components contribute to the homeostasis of the synapse. In the neuromuscular synapse, the arrival of the nerve impulse at the presynaptic terminal triggers the molecular mechanisms associated with ACh release, which can be retrogradely regulated by the resulting muscle contraction. This retrograde regulation, however, has been poorly studied. At the neuromuscular junction (NMJ), protein kinase A (PKA) enhances neurotransmitter release, and the phosphorylation of the molecules of the release machinery including synaptosomal associated protein of 25 kDa (SNAP-25) and Synapsin-1 could be involved.

METHODS

Accordingly, to study the effect of synaptic retrograde regulation of the PKA subunits and its activity, we stimulated the rat phrenic nerve (1 Hz, 30 min) resulting or not in contraction (abolished by µ-conotoxin GIIIB). Changes in protein levels and phosphorylation were detected by western blotting and cytosol/membrane translocation by subcellular fractionation. Synapsin-1 was localized in the levator auris longus (LAL) muscle by immunohistochemistry.

RESULTS

Here we show that synaptic PKA Cβ subunit regulated by RIIβ or RIIα subunits controls activity-dependent phosphorylation of SNAP-25 and Synapsin-1, respectively. Muscle contraction retrogradely downregulates presynaptic activity-induced pSynapsin-1 S9 while that enhances pSNAP-25 T138. Both actions could coordinately contribute to decreasing the neurotransmitter release at the NMJ.

CONCLUSION

This provides a molecular mechanism of the bidirectional communication between nerve terminals and muscle cells to balance the accurate process of ACh release, which could be important to characterize molecules as a therapy for neuromuscular diseases in which neuromuscular crosstalk is impaired.

Functional regulation of syntaxin-1: An underlying mechanism mediating exocytosis in neuroendocrine cells

The fusion of the secretory vesicle with the plasma membrane requires the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein complexes formed by synaptobrevin, syntaxin-1, and SNAP-25. Within the pathway leading to exocytosis, the transitions between the "open" and "closed" conformations of syntaxin-1 function as a switch for the fusion of vesicles with the plasma membranes; rapid assembly and disassembly of syntaxin-1 clusters on the plasma membrane provide docking and fusion sites for secretory vesicles in neuroendocrine cells; and the fully zippered trans-SNARE complex, which requires the orderly, rapid and accurate binding of syntaxin-1 to other SNARE proteins, play key roles in triggering fusion. All of these reactions that affect exocytosis under physiological conditions are tightly regulated by multiple factors. Here, we review the current evidence for the involvement of syntaxin-1 in the mechanism of neuroendocrine cell exocytosis, discuss the roles of multiple factors such as proteins, lipids, protein kinases, drugs, and toxins in SNARE complex-mediated membrane fusion, and present an overview of syntaxin-1 mutation-associated diseases with a view to developing novel mechanistic therapeutic targets for the treatment of neuroendocrine disorders.

The M2 muscarinic receptor, in association to M1 , regulates the neuromuscular PKA molecular dynamics

Muscarinic acetylcholine receptor 1 subtype (M₁ ) and muscarinic acetylcholine receptor 2 subtype (M₂ ) presynaptic muscarinic receptor subtypes increase and decrease, respectively, neurotransmitter release at neuromuscular junctions. M₂ involves protein kinase A (PKA), although the muscarinic regulation to form and inactivate the PKA holoenzyme is unknown. Here, we show that M₂ signaling inhibits PKA by downregulating Cβ subunit, upregulating RIIα/β and liberating RIβ and RIIα to the cytosol. This promotes PKA holoenzyme formation and reduces the phosphorylation of the transmitter release target synaptosome-associated protein 25 and the gene regulator cAMP response element binding. Instead, M₁ signaling, which is downregulated by M₂ , opposes to M₂ by recruiting R subunits to the membrane. The M₁ and M₂ reciprocal actions are performed through the anchoring protein A kinase anchor protein 150 as a common node. Interestingly, M₂ modulation on protein expression needs M₁ signaling. Altogether, these results describe the dynamics of PKA subunits upon M₂ muscarinic signaling in basal and under presynaptic nerve activity, uncover a specific involvement of the M₁ receptor and reveal the M₁ /M₂ balance to activate PKA to regulate neurotransmission. This provides a molecular mechanism to the PKA holoenzyme formation and inactivation which could be general to other synapses and cellular models.

Shaping mitochondrial dynamics: The role of cAMP signalling

In recent years, our idea of mitochondria evolved from "mere" energy and metabolite producers to key regulators of many cellular functions. In order to preserve and protect their functional status, these organelles engage a number of dynamic processes that allow them to decrease accumulated burden and maintain their homeostasis. Indeed, mitochondria can unite (fusion), divide (fission), position themselves strategically in the cell (motility/trafficking) and if irreversibly damaged or dysfunctional eliminated (mitophagy). These dynamic processes can be controlled both by mitochondrial and cellular signalling pathways, hence allowing mitochondria to tune their function to the cellular needs. Among the regulatory mechanisms, reversible phosphorylation downstream the cyclic AMP (cAMP) signalling cascade was shown to deeply influence mitochondrial dynamics. This review explores the emerging evidence suggesting that cAMP is a key player in the orchestration of mitochondrial fusion/fission, motility and mitophagy, extending the repertoire of this second messenger, which is now recognised as a major regulator of mitochondrial homeostasis.

Potentiation of Schaffer-Collateral CA1 Synaptic Transmission by eEF2K and p38 MAPK Mediated Mechanisms

The elongation factor 2 kinase (eEF2K), likewise known as CaMKIII, has been demonstrated to be involved in antidepressant responses of NMDA receptor antagonists. Even so, it remains open whether direct inhibition of eEF2K without altering up-stream or other signaling pathways affects hippocampal synaptic transmission and neuronal network synchrony. Inhibition of eEF2K by the selective and potent eEF2K inhibitor A-484954 induced a fast pre-synaptically mediated enhancement of synaptic transmission and synchronization of neural network activity. The eEF2K-inhibition mediated potentiation of synaptic transmission of hippocampal CA1 neurons is most notably independent of protein synthesis and does not rely on protein kinase C, protein kinase A or mitogen-activated protein kinase (MAPK)/extracellular signal-regulated protein kinase 1/2. Moreover, the strengthening of synaptic transmission in the response to the inhibition of eEF2K was strongly attenuated by the inhibition of p38 MAPK. In addition, we show the involvement of barium-sensitive and more specific the TWIK-related potassium-1 (TREK-1) channels in the eEF2K-inhibition mediated potentiation of synaptic transmission. These findings reveal a novel pathway of eEF2K mediated regulation of hippocampal synaptic transmission. Further research is required to study whether such compounds could be beneficial for the development of mood disorder treatments with a fast-acting antidepressant response.

Differential role of SNAP-25 phosphorylation by protein kinases A and C in the regulation of SNARE complex formation and exocytosis in PC12 cells

The final step of regulated exocytosis, membrane fusion, is mediated by formation of the SNARE complex by syntaxin, SNAP-25 (synaptosomal-associated protein of 25 kDa), and VAMP (vesicle-associated membrane protein). Phosphorylation of SNARE and accessory proteins contributes to regulation of exocytosis. We previously identified residues of SNAP-25 phosphorylated by protein kinase A (PKA) and PKC. However, the physiological role of SNAP-25 phosphorylation in exocytosis, in particular with regard to SNARE complex formation, has remained elusive. SNARE complex formation by purified recombinant SNAP-25, syntaxin-1, and VAMP-2 in vitro was inhibited or promoted as a result of the phosphorylation at Thr(138) by PKA or at Ser(187) by PKC, respectively. SNARE complex formation in intact PC12 cells was similarly inhibited by forskolin (activator of PKA) and promoted by phorbol 12-myristate 13-acetate (PMA, activator of PKC). Noradrenaline secretion from PC12 cells induced by a high K(+) concentration was enhanced by forskolin or PMA. Stable depletion of SNAP-25 inhibited high-K(+)-induced noradrenaline secretion. Forced expression of WT SNAP-25 restored the secretory response of the SNAP-25-depleted cells to high-K(+), and this response was enhanced by forskolin or PMA. Expression of the nonphosphorylatable T138A or S187A mutants of SNAP-25 similarly rescued the secretory response to high-K(+), but the augmentation of this response by forskolin was more pronounced in the cells expressing SNAP-25 (T138A) than in those expressing SNAP-25 (WT), whereas that by PMA was less pronounced in those expressing SNAP-25 (S187A). Our results thus suggest that SNAP-25 phosphorylation by PKA or PKC contributes differentially to the control of exocytosis in PC12 cells by regulating SNARE complex formation.

Multiple roles for the actin cytoskeleton during regulated exocytosis

Regulated exocytosis is the main mechanism utilized by specialized secretory cells to deliver molecules to the cell surface by virtue of membranous containers (i.e., secretory vesicles). The process involves a series of highly coordinated and sequential steps, which include the biogenesis of the vesicles, their delivery to the cell periphery, their fusion with the plasma membrane, and the release of their content into the extracellular space. Each of these steps is regulated by the actin cytoskeleton. In this review, we summarize the current knowledge regarding the involvement of actin and its associated molecules during each of the exocytic steps in vertebrates, and suggest that the overall role of the actin cytoskeleton during regulated exocytosis is linked to the architecture and the physiology of the secretory cells under examination. Specifically, in neurons, neuroendocrine, endocrine, and hematopoietic cells, which contain small secretory vesicles that undergo rapid exocytosis (on the order of milliseconds), the actin cytoskeleton plays a role in pre-fusion events, where it acts primarily as a functional barrier and facilitates docking. In exocrine and other secretory cells, which contain large secretory vesicles that undergo slow exocytosis (seconds to minutes), the actin cytoskeleton plays a role in post-fusion events, where it regulates the dynamics of the fusion pore, facilitates the integration of the vesicles into the plasma membrane, provides structural support, and promotes the expulsion of large cargo molecules.

Copy number variants and infantile spasms: evidence for abnormalities in ventral forebrain development and pathways of synaptic function

Infantile spasms (ISS) are an epilepsy disorder frequently associated with severe developmental outcome and have diverse genetic etiologies. We ascertained 11 subjects with ISS and novel copy number variants (CNVs) and combined these with a new cohort with deletion 1p36 and ISS, and additional published patients with ISS and other chromosomal abnormalities. Using bioinformatics tools, we analyzed the gene content of these CNVs for enrichment in pathways of pathogenesis. Several important findings emerged. First, the gene content was enriched for the gene regulatory network involved in ventral forebrain development. Second, genes in pathways of synaptic function were overrepresented, significantly those involved in synaptic vesicle transport. Evidence also suggested roles for GABAergic synapses and the postsynaptic density. Third, we confirm the association of ISS with duplication of 14q12 and maternally inherited duplication of 15q11q13, and report the association with duplication of 21q21. We also present a patient with ISS and deletion 7q11.3 not involving MAGI2. Finally, we provide evidence that ISS in deletion 1p36 may be associated with deletion of KLHL17 and expand the epilepsy phenotype in that syndrome to include early infantile epileptic encephalopathy. Several of the identified pathways share functional links, and abnormalities of forebrain synaptic growth and function may form a common biologic mechanism underlying both ISS and autism. This study demonstrates a novel approach to the study of gene content in subjects with ISS and copy number variation, and contributes further evidence to support specific pathways of pathogenesis.

Ethanol-enhanced GABA release: a focus on G protein-coupled receptors

While research on the actions of ethanol at the GABAergic synapse has focused on postsynaptic mechanisms, recent data have demonstrated that ethanol also facilitates GABA release from presynaptic terminals in many, but not all, brain regions. The ability of ethanol to increase GABA release can be regulated by different G protein-coupled receptors (GPCRs), such as the cannabinoid-1 receptor, corticotropin-releasing factor 1 receptor, GABA(B) receptor, and the 5-hydroxytryptamine 2C receptor. The intracellular messengers linked to these GPCRs, including the calcium that is released from internal stores, also play a role in ethanol-enhanced GABA release. Hypotheses are proposed to explain how ethanol interacts with the GPCR pathways to increase GABA release and how this interaction contributes to the brain region specificity of ethanol-enhanced GABA release. Defining the mechanism of ethanol-facilitated GABA release will further our understanding of the GABAergic profile of ethanol and increase our knowledge of how GABAergic neurotransmission may contribute to the intoxicating effects of alcohol and to alcohol dependence.

Rescuing the subprime meltdown in insulin exocytosis in diabetes

Neuroendocrine pancreatic islet beta-cells secrete the hormone insulin in response to glucose stimulation and adapt efficiently to increased demand by peripheral tissues to maintain glucose homeostasis. Insulin is packed within dense-core granules, which traffic and dock onto the plasma membrane whereby a Ca(2+) stimulus evokes exocytosis by soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE), complex-mediated, membrane fusion. Recent studies have unveiled postdocking steps mediated by "priming" factors that influence SNARE complex assembly to confer fusion readiness to the docked granules. This review will summarize recent insights into the priming role for Munc13 in the exocytosis of insulin granules. We present evidence for the interaction of Munc13-1 with exocytotic substrates involved in cAMP-mediated potentiation of insulin release, the latter we show to mediate enhanced granule-to-granule fusion events underlying compound exocytosis. We thus also further review the current understanding of granule-to-granule fusion. As agents acting on cAMP signaling are clinically used to augment insulin release in diabetes, this better understanding of priming steps may reveal additional novel therapeutic strategies to increase the capacity for insulin release to improve the treatment of diabetes.

Positioning mitochondrial plasticity within cellular signaling cascades

Mitochondria evolved from alpha-proteobacteria captured within a host between two and three billion years ago. This origin resulted in the formation of a double-layered organelle resulting in four distinct sub-compartments: the outer membrane, the intermembrane space, the inner membrane and the matrix. The inner membrane is organized in cristae, harboring the respiratory chain and ATP synthase complexes responsible of the oxidative phosphorylation, the main energy-generating system of the cell. It is generally considered that the ultrastructure of the inner membrane provides a large variety of morphologies that facilitate metabolic output. This classical view of mitochondria as bean-shaped organelles was static until in the last decade when new imaging studies and genetic screens provided a more accurate description of a dynamic mitochondrial reticulum that fuse and divide continuously. Since then significant findings have been made in the study of machineries responsible for fusion, fission and motility, however the mechanisms and signals that regulate mitochondrial dynamics are only beginning to emerge. A growing body of evidence indicates that metabolic and cellular signals influence mitochondrial dynamics, leading to a new understanding of how changes in mitochondrial shape can have a profound impact on the functional output of the organelle. The mechanisms that regulate mitochondrial morphology are incompletely understood, but evidence to date suggests that the morphology machinery is modulated through the use of post-translational modifications, including nucleotide-binding proteins, phosphorylation, ubiquitination, SUMOylation, and changes in the lipid environment. This review focuses on the molecular switches that control mitochondrial dynamics and the integration of mitochondrial morphology within cellular signaling cascades.

cAMP-dependent protein kinase A and the dynamics of epithelial cell surface domains: moving membranes to keep in shape

Cyclic adenosine monophosphate (cAMP) and cAMP-dependent protein kinase A (PKA) are evolutionary conserved molecules with a well-established position in the complex network of signal transduction pathways. cAMP/PKA-mediated signaling pathways are implicated in many biological processes that cooperate in organ development including the motility, survival, proliferation and differentiation of epithelial cells. Cell surface polarity, here defined as the anisotropic organisation of cellular membranes, is a critical parameter for most of these processes. Changes in the activity of cAMP/PKA elicit a variety of effects on intracellular membrane dynamics, including membrane sorting and trafficking. One of the most intriguing aspects of cAMP/PKA signaling is its evolutionary conserved abundance on the one hand and its precise spatial-temporal actions on the other. Here, we review recent developments with regard to the role of cAMP/PKA in the regulation of intracellular membrane trafficking in relation to the dynamics of epithelial surface domains.

Presynaptic signaling by heterotrimeric G-proteins

G-proteins (guanine nucleotide-binding proteins) are membrane-attached proteins composed of three subunits, alpha, beta, and gamma. They transduce signals from G-protein coupled receptors (GPCRs) to target effector proteins. The agonistactivated receptor induces a conformational change in the G-protein trimer so that the alpha-subunit binds GTP in exchange for GDP and alpha-GTP, and betagamma-subunits separate to interact with the target effector. Effector-interaction is terminated by the alpha-subunit GTPase activity, whereby bound GTP is hydrolyzed to GDP. This is accelerated in situ by RGS proteins, acting as GTPase-activating proteins (GAPs). Galpha-GDP and Gbetagamma then reassociate to form the Galphabetagamma trimer. G-proteins primarily involved in the modulation of neurotransmitter release are G(o), G(q) and G(s). G(o) mediates the widespread presynaptic auto-inhibitory effect of many neurotransmitters (e.g., via M2/M4 muscarinic receptors, alpha(2) adrenoreceptors, micro/delta opioid receptors, GABAB receptors). The G(o) betagamma-subunit acts in two ways: first, and most ubiquitously, by direct binding to CaV2 Ca(2+) channels, resulting in a reduced sensitivity to membrane depolarization and reduced Ca(2+) influx during the terminal action potential; and second, through a direct inhibitory effect on the transmitter release machinery, by binding to proteins of the SNARE complex. G(s) and G(q) are mainly responsible for receptor-mediated facilitatory effects, through activation of target enzymes (adenylate cyclase, AC and phospholipase-C, PLC respectively) by the GTP-bound alpha-subunits.

Presynaptic lonotropic receptors

The release of transmitters through vesicle exocytosis from nerve terminals is not constant but is subject to modulation by various mechanisms, including prior activity at the synapse and the presence of neurotransmitters or neuromodulators in the synapse. Instantaneous responses of postsynaptic cells to released transmitters are mediated by ionotropic receptors. In contrast to metabotropic receptors, ionotropic receptors mediate the actions of agonists in a transient manner within milliseconds to seconds. Nevertheless, transmitters can control vesicle exocytosis not only via slowly acting metabotropic, but also via fast acting ionotropic receptors located at the presynaptic nerve terminals. In fact, members of the following subfamilies of ionotropic receptors have been found to control transmitter release: ATP P2X, nicotinic acetylcholine, GABA(A), ionotropic glutamate, glycine, 5-HT(3), andvanilloid receptors. As these receptors display greatly diverging structural and functional features, a variety of different mechanisms are involved in the regulation of transmitter release via presynaptic ionotropic receptors. This text gives an overview of presynaptic ionotropic receptors and briefly summarizes the events involved in transmitter release to finally delineate the most important signaling mechanisms that mediate the effects of presynaptic ionotropic receptor activation. Finally, a few examples are presented to exemplify the physiological and pharmacological relevance of presynaptic ionotropic receptors.

Interaction between Munc13-1 and RIM is critical for glucagon-like peptide-1 mediated rescue of exocytotic defects in Munc13-1 deficient pancreatic beta-cells

OBJECTIVE

Glucagon-like peptide-1 (GLP-1) rescues insulin secretory deficiency in type 2 diabetes partly via cAMP actions on exchange protein directly activated by cAMP (Epac2) and protein kinase A (PKA)-activated Rab3A-interacting molecule 2 (Rim2). We had reported that haplodeficient Munc13-1(+/-) mouse islet beta-cells exhibited reduced insulin secretion, causing glucose intolerance. Munc13-1 binds Epac2 and Rim2, but their functional interactions remain unclear.

RESEARCH DESIGN AND METHODS

We used Munc13-1(+/-) islet beta-cells to examine the functional interactions between Munc13-1 and Epac2 and PKA. GLP-1 stimulation of Munc13-1(+/-) islets normalized the reduced biphasic insulin secretion by its actions on intact islet cAMP production and normal Epac2 and Rim2 levels.

RESULTS

To determine which exocytotic steps caused by Munc13-1 deficiency are rescued by Epac2 and PKA, we used patch-clamp capacitance measurements, showing that 1) cAMP restored the reduced readily releasable pool (RRP) and partially restored refilling of a releasable pool of vesicles in Munc13-1(+/-) beta-cells, 2) Epac-selective agonist [8-(4-chloro-phenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate] partially restored the reduced RRP and refilling of a releasable pool of vesicles, and 3) PKA blockade by H89 (leaving Epac intact) impaired cAMP ability to restore the RRP and refilling of a releasable pool of vesicles. Conversely, PKA-selective agonist (N(6)-benzoyladenosine-cAMP) completely restored RRP and partially restored refilling of a releasable pool of vesicles. To determine specific contributions within Epac-Rim2-Munc13-1 interaction sites accounting for cAMP rescue of exocytosis caused by Munc13-1 deficiency, we found that blockade of Rim2-Munc13-1 interaction with Rim-Munc13-1-binding domain peptide abolished cAMP rescue, whereas blockade of Epac-Rim2 interaction with Rim2-PDZ peptide only moderately reduced refilling with little effect on RRP.

CONCLUSIONS

cAMP rescue of priming defects caused by Munc13-1 deficiency via Epac and PKA signaling pathways requires downstream Munc13-1-Rim2 interaction.

The effects of disruption of A kinase anchoring protein-protein kinase A association on protein kinase A signalling in neuroendocrine melanotroph cells of Xenopus laevis

The secretory activity of melanotroph cells from Xenopus laevis is regulated by multiple neurotransmitters that act through adenylyl cyclase. Cyclic adenosine monophosphate (cAMP), acting on protein kinase A (PKA), stimulates the frequency of intracellular Ca(2+) oscillations and the secretory activity of the melanotroph cell. Anchoring of PKA near target proteins is essential for many PKA-regulated processes, and the family of A kinase anchoring proteins (AKAPs) is involved in the compartmentalisation of PKA type II (PKA II) regulatory subunits. In the present study, we determined to what degree cAMP signalling in Xenopus melanotrophs depends on compartmentalised PKA II. For this purpose, a membrane-permeable stearated form of Ht31 (St-Ht31), which dislodges PKA II from AKAP (thus disrupting PKA II signalling), was used. The effect of St-Ht31 on both secretion of radiolabelled peptides and intracellular Ca(2+) signalling by superfused Xenopus melanotrophs was assessed. St-Ht31 stimulated secretion but had no effect on Ca(2+) signalling. We conclude Xenopus melanotrophs possess a St-Ht31-sensitive PKA II that is associated with the exocytosis machinery and, furthermore, that Ca(2+) signalling is regulated by an AKAP-independent signalling system. Moreover, our results support a recent proposal that AKAP participates in regulating PKA activity independently from cAMP.

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.

Role of efficient neurotransmitter release in barrel map development

Cortical maps are remarkably precise, with organized arrays of thalamocortical afferents (TCAs) that project into distinct neuronal modules. Here, we present evidence for the involvement of efficient neurotransmitter release in mouse cortical barrel map development using barrelless mice, a loss-of-function mutant of calcium/calmodulin-activated adenylyl cyclase I (AC1), and mice with a mutation in Rab3-interacting molecule 1alpha (RIM1alpha), an active zone protein that regulates neurotransmitter release. We demonstrate that release efficacy is substantially decreased in barrelless TCAs. We identify RIMs as important phosphorylation targets for AC1 in the presynaptic terminal. We further show that RIM1alpha mutant mice have reduced TCA neurotransmitter release efficacy and barrel map deficits, although not as severe as those found in barrelless mice. This supports the role of RIM proteins in mediating, in part, AC1 signaling in barrel map development. Finally, we present a model to show how inadequacies in presynaptic function can interfere with activity-dependent processes in neuronal circuit formation. These results demonstrate how efficient synaptic transmission mediated by AC1 function contributes to the development of cortical barrel maps.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

PKA-catalyzed phosphorylation of tomosyn and its implication in Ca2+-dependent exocytosis of neurotransmitter

Neurotransmitter is released from nerve terminals by Ca2+-dependent exocytosis through many steps. SNARE proteins are key components at the priming and fusion steps, and the priming step is modulated by cAMP-dependent protein kinase (PKA), which causes synaptic plasticity. We show that the SNARE regulatory protein tomosyn is directly phosphorylated by PKA, which reduces its interaction with syntaxin-1 (a component of SNAREs) and enhances the formation of the SNARE complex. Electrophysiological studies using cultured superior cervical ganglion (SCG) neurons revealed that this enhanced formation of the SNARE complex by the PKA-catalyzed phosphorylation of tomosyn increased the fusion-competent readily releasable pool of synaptic vesicles and, thereby, enhanced neurotransmitter release. This mechanism was indeed involved in the facilitation of neurotransmitter release that was induced by a potent biological mediator, the pituitary adenylate cyclase-activating polypeptide, in SCG neurons. We describe the roles and modes of action of PKA and tomosyn in Ca2+-dependent neurotransmitter release.

Modulation of neurotransmitter release by the second messenger-activated protein kinases: implications for presynaptic plasticity

Activity-dependent modulation of synaptic function and structure is emerging as one of the key mechanisms underlying synaptic plasticity. Whereas over the past decade considerable progress has been made in identifying postsynaptic mechanisms for synaptic plasticity, the presynaptic mechanisms involved have remained largely elusive. Recent evidence implicates that second messenger regulation of the protein interactions in synaptic vesicle release machinery is one mechanism by which cellular events modulate synaptic transmission. Thus, identifying protein kinases and their targets in nerve terminals, particularly those functionally regulated by synaptic activity or intracellular [Ca2+], is critical to the elucidation of the molecular mechanisms underlying modulation of neurotransmitter release and presynaptic plasticity. The phosphorylation and dephosphorylation states of synaptic proteins that mediate vesicle exocytosis could regulate the biochemical pathways leading from synaptic vesicle docking to fusion. However, functional evaluation of the activity-dependent phosphorylation events for modulating presynaptic functions still represents a considerable challenge. Here, we present a brief overview of the data on the newly identified candidate targets of the second messenger-activated protein kinases in the presynaptic release machinery and discuss the potential impact of these phosphorylation events in synaptic strength and presynaptic plasticity.

GABA Transporter Currents Activated by Protein Kinase A Excite Midbrain Neurons during Opioid Withdrawal

Adaptations in neurons of the midbrain periaqueductal gray (PAG) induced by chronic morphine treatment mediate expression of many signs of opioid withdrawal. The abnormally elevated action potential rate of opioid-sensitive PAG neurons is a likely cellular mechanism for withdrawal expression. We report here that opioid withdrawal in vitro induced an opioid-sensitive cation current that was mediated by the GABA transporter-1 (GAT-1) and required activation of protein kinase A (PKA) for its expression. Inhibition of GAT-1 or PKA also prevented withdrawal-induced hyperexcitation of PAG neurons. Our findings indicate that GAT-1 currents can directly increase the action potential rates of neurons and that GAT-1 may be a target for therapy to alleviate opioid-withdrawal symptoms.