Roles of calmodulin and calmodulin-binding proteins in synaptic vesicle recycling during regulated exocytosis at submicromolar Ca2+ concentrations

Dopamine D3 receptor modulates D2 receptor effects on cAMP and GABA release at striatopallidal terminals-Modulation by the Ca2+-Calmodulin-CaMKII system

Dopamine D2 receptor (D2R) is expressed in striatopallidal neurons and decreases forskolin-stimulated cyclic adenine monophosphate (cAMP) accumulation and gamma-aminobutyric acid (GABA) release. Dopamine D3 receptor (D3R) mRNA is expressed in a population of striatal D2R-expressing neurons. Also, D3R protein and binding have been reported in the neuropil of globus pallidus. We explore whether D2R and D3R colocalize in striatopallidal terminals and whether D3R modulates the D2R effect on forskolin-stimulated [3H]cAMP accumulation in pallidal synaptosomes and high K+ stimulated-[3H]GABA release in pallidal slices. Previous reports in heterologous systems indicate that calmodulin (CaM) and CaMKII modulate D2R and D3R functions; thus, we study whether this system regulates its functional interaction. D2R immunoprecipitates with CaM, and pretreatment with ophiobolin A or depolarization of synaptosomes with 15 mM of K+ decreases it. Both treatments increase the D2R inhibition of forskolin-stimulated [3H]cAMP accumulation when activated with quinpirole, indicating a negative modulation of CaM on D2R function. Quinpirole also activates D3R, potentiating D2R inhibition of cAMP accumulation in the ophiobolin A-treated synaptosomes. D2R and D3R immunoprecipitate in pallidal synaptosomes and decrease after the kainic acid striatal lesion, indicating the striatal origin of the presynaptic receptors. CaM-kinase II alfa (CaMKIIα) immunoprecipitates with D3R and increases after high K+ depolarization. In the presence of KN62, a CaMKIIα blocker, D3R potentiates D2R effects on cAMP accumulation in depolarized synaptosomes and GABA release in pallidal slices, indicating D3R function regulation by CaMKIIα. Our data indicate that D3R potentiates the D2R effect on cAMP accumulation and GABA release at pallidal terminals, an interaction regulated by the CaM-CaMKIIα system.

Compensatory endocytosis occurs after cortical granule exocytosis in mouse eggs

Compensatory endocytosis (CE) is one of the primary mechanisms through which cells maintain their surface area after exocytosis. Considering that in eggs massive exocytosis of cortical granules (CG) takes place after fertilization, the aim of this study was to evaluate the occurrence of CE following cortical exocytosis in mouse eggs. For this purpose, we developed a pulse-chase assay to detect CG membrane internalization. Results showed internalized labeling in SrCl₂ -activated and fertilized eggs when chasing at 37°C, but not at a nonpermissive temperature (4°C). The use of kinase and calcineurin inhibitors led us to conclude that this internal labeling corresponded to CE. Further experiments showed that CE in mouse eggs is dependent on actin dynamics and dynamin activity, and could be associated with a transient exposure of phosphatidylserine. Finally, CE was impaired in A23187 ionophore-activated eggs, highlighting once again the mechanistic differences between the activation methods. Altogether, these results demonstrate for the first time that egg activation triggers CE in mouse eggs after exocytosis of CG, probably as a plasma membrane homeostasis mechanism.

Proteomic identification of the molecular basis of mammalian CNS growth cones

The growth cone, which is a unique structure with high motility that forms at the tips of extending axons and dendrites, is crucial to neuronal network formation. Axonal growth of the mammalian CNS is most likely achieved by the complicated coordination of cytoskeletal rearrangement and vesicular trafficking via many proteins. Before recent advances, no methods to identify numerous proteins existed; however, proteomics revolutionarily resolved such problems. In this review, I summarize the profiles of the mammalian growth cone proteins revealed by proteomics as the molecular basis of the growth cone functions, with molecular mapping. These results should be used as a basis for understanding the mechanisms of the complex mammalian CNS developmental process.

Synaptic vesicle recycling: steps and principles

Synaptic vesicle recycling is one of the best-studied cellular pathways. Many of the proteins involved are known, and their interactions are becoming increasingly clear. However, as for many other pathways, it is still difficult to understand synaptic vesicle recycling as a whole. While it is generally possible to point out how synaptic reactions take place, it is not always easy to understand what triggers or controls them. Also, it is often difficult to understand how the availability of the reaction partners is controlled: how the reaction partners manage to find each other in the right place, at the right time. I present here an overview of synaptic vesicle recycling, discussing the mechanisms that trigger different reactions, and those that ensure the availability of reaction partners. A central argument is that synaptic vesicles bind soluble cofactor proteins, with low affinity, and thus control their availability in the synapse, forming a buffer for cofactor proteins. The availability of cofactor proteins, in turn, regulates the different synaptic reactions. Similar mechanisms, in which one of the reaction partners buffers another, may apply to many other processes, from the biogenesis to the degradation of the synaptic vesicle.

Point mutation in syntaxin-1A causes abnormal vesicle recycling, behaviors, and short term plasticity

Syntaxin-1A is a t-SNARE that is involved in vesicle docking and vesicle fusion; it is important in presynaptic exocytosis in neurons because it interacts with many regulatory proteins. Previously, we found the following: 1) that autophosphorylated Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), an important modulator of neural plasticity, interacts with syntaxin-1A to regulate exocytosis, and 2) that a syntaxin missense mutation (R151G) attenuated this interaction. To determine more precisely the physiological importance of this interaction between CaMKII and syntaxin, we generated mice with a knock-in (KI) syntaxin-1A (R151G) mutation. Complexin is a molecular clamp involved in exocytosis, and in the KI mice, recruitment of complexin to the SNARE complex was reduced because of an abnormal CaMKII/syntaxin interaction. Nevertheless, SNARE complex formation was not inhibited, and consequently, basal neurotransmission was normal. However, the KI mice did exhibit more enhanced presynaptic plasticity than wild-type littermates; this enhanced plasticity could be associated with synaptic response than did wild-type littermates; this pronounced response included several behavioral abnormalities. Notably, the R151G phenotypes were generally similar to previously reported CaMKII mutant phenotypes. Additionally, synaptic recycling in these KI mice was delayed, and the density of synaptic vesicles was reduced. Taken together, our results indicated that this single point mutation in syntaxin-1A causes abnormal regulation of neuronal plasticity and vesicle recycling and that the affected syntaxin-1A/CaMKII interaction is essential for normal brain and synaptic functions in vivo.

Multidirectional effects of calmodulin kinase II on transmitter release in mature and newly formed mouse motor synapses

Calmodulin inhibitor W-7 did not cause changes in the quantal content of postsynaptic end-plate potentials (EPP) in newly formed synapses, but prevented facilitation of acetylcholine secretion induced by L-type Ca(2+)channels blocker nitrendipine. CaMKII inhibitor KN-62 produced similar effect and suppressed the increase in EPP quantal content caused by blockade of L-type Ca(2+)channels. Phosphatase PP2A inhibitor okadaic acid significantly facilitated secretion in newly formed synapses; the effect was completely blocked by KN-62. In mature synapses, okadaic acid had no effect on transmitter secretion. KN-62 increased EPP quantal content. We hypothesize that CaMKII produced different effects on acetylcholine secretion in mature and immature synapses depending on specificity of calcium signaling and PP2A phosphatase activity.

Molecular determinants of modulation of CaV2.1 channels by visinin-like protein 2

CaV2.1 channels, which conduct P/Q-type Ca2+ currents, initiate synaptic transmission at most synapses in the central nervous system. Ca2+/calmodulin-dependent facilitation and inactivation of these channels contributes to short-term facilitation and depression of synaptic transmission, respectively. Other calcium sensor proteins displace calmodulin (CaM) from its binding site, differentially regulate CaV2.1 channels, and contribute to the diversity of short-term synaptic plasticity. The neuronal calcium sensor protein visinin-like protein 2 (VILIP-2) inhibits inactivation and enhances facilitation of CaV2.1 channels. Here we examine the molecular determinants for differential regulation of CaV2.1 channels by VILIP-2 and CaM by construction and functional analysis of chimeras in which the functional domains of VILIP-2 are substituted in CaM. Our results show that the N-terminal domain, including its myristoylation site, the central α-helix, and the C-terminal lobe containing EF-hands 3 and 4 of VILIP-2 are sufficient to transfer its regulatory properties to CaM. This regulation by VILIP-2 requires binding to the IQ-like domain of CaV2.1 channels. Our results identify the essential molecular determinants of differential regulation of CaV2.1 channels by VILIP-2 and define the molecular code that these proteins use to control short-term synaptic plasticity.

Presynaptic roles of intracellular Ca(2+) stores in signalling and exocytosis

The signalling roles of Ca(2+)(ic) (intracellular Ca(2+)) stores are well established in non-neuronal and neuronal cells. In neurons, although Ca(2+)(ic) stores have been assigned a pivotal role in postsynaptic responses to G(q)-coupled receptors, or secondarily to extracellular Ca(2+) influx, the functions of dynamic Ca(2+)(ic) stores in presynaptic terminals remain to be fully elucidated. In the present paper, we review some of the recent evidence supporting an involvement of Ca(2+)(ic) in presynaptic function, and discuss loci at which this source of Ca(2+) may impinge. Nerve terminal preparations provide good models for functionally examining putative Ca(2+)(ic) stores under physiological and pathophysiological stimulation paradigms, using Ca(2+)-dependent activation of resident protein kinases as sensors for fine changes in intracellular Ca(2+) levels. We conclude that intraterminal Ca(2+)(ic) stores may, directly or indirectly, enhance neurotransmitter release following nerve terminal depolarization and/or G-protein-coupled receptor activation. During conditions that prevail following neuronal ischaemia, increased glutamate release instigated by Ca(2+)(ic) store activation may thereby contribute to excitotoxicity and eventual synaptopathy.

Characterisation of Plasmodium invasive organelles; an ookinete microneme proteome

Secretion of microneme proteins is essential to Plasmodium invasion but the molecular composition of these secretory organelles remains poorly defined. Here, we describe the first Plasmodium microneme proteome. Purification of micronemes by subcellular fractionation from cultured ookinetes was confirmed by enrichment of known micronemal proteins and electron microscopy. Quantitation of electron micrographs showed >14-fold microneme enrichment compared to the intact ookinete, such that micronemes comprised 85% of the identifiable organelles in the fraction. Gel LC-MS/MS of the most abundant protein constituents of the fraction identified three known micronemal proteins chitinase, CTRP, SOAP, together with protein disulphide isomerase (PDI) and HSP70. Highly sensitive MudPIT shotgun proteomics described a total of 345 proteins in the fraction. M1 aminopeptidase and PDI, the former a recognised target of drug development, were both shown to have a micronemal location by IFA. We further identified numerous proteins with established vesicle trafficking and signaling functions consistent with micronemes being part of a regulated secretory pathway. Previously uncharacterised proteins comprise the largest functional group of the microneme proteome and will include secreted proteins important to invasion.

Calcium fingerprints induced by calmodulin interactors in eukaryotic cells

Calcium (Ca2+) is a ubiquitous second messenger which promotes cell responses through transient changes in intracellular concentrations. The prominent role of Ca2+ in cell physiology is mediated by a whole set of proteins constituting a Ca2+-signalling toolkit involved in Ca2+-signal generation, deciphering and arrest. The different Ca2+-signalosomes deliver Ca2+-signals with spatial and temporal dynamics to control the function of specific cell types. Among the intracellular proteins involved in Ca2+-signal deciphering, calmodulin (CaM) plays a pivotal role in controlling Ca2+-homeostasis and downstream Ca2+-based signalling events. Due to its ubiquitous expression in eukaryotic cells and the variety of proteins it interacts with, CaM is central in Ca2+-signalling networks. For these reasons, it is expected that disrupting or modifying CaM interactions with its target proteins will affect Ca2+-homeostasis and cellular responses. The resulting calcium response will vary depending on which interactions between CaM and target proteins are altered by the molecules and on the specific Ca2+-toolkit expressed in a given cell, even in the resting state. In the present paper, the effect of six classical CaM interactors (W5, W7, W12, W13, bifonazole and calmidazolium) was studied on Ca2+-signalling in tumor initiating cells isolated from human glioblastoma (TG1) and tobacco cells (BY-2) using the fluorescent Ca2+-sensitive Indo-1 dye and aequorin, respectively. Various Ca2+-fingerprints were obtained depending both on the CaM interactor used and the cell type investigated. These data demonstrate that interaction between the antagonists and CaM results in a differential inhibition of CaM-dependent proteins involved in Ca2+-signal regulation. In addition, the distinct Ca2+-fingerprints in tobacco and human tumor initiating glioblastoma cells induced by a given CaM interactor highlight the specificity of the Ca2+-signalosome in eukaryotic cells.

Oomycete RXLR effectors: delivery, functional redundancy and durable disease resistance

To manipulate host defences, plant pathogenic oomycetes secrete and translocate RXLR effectors into plant cells. Recent reports have indicated that RXLR effectors are translocated from the extrahaustorial matrix during the biotrophic phase of infection and that they are able to suppress PAMP-triggered immunity. Oomycete genomes contain potentially hundreds of highly diverse RXLR effector genes, providing the potential for considerable functional redundancy and the consequent ability to readily shed effectors that are recognised by plant surveillance systems without compromising pathogenic fitness. Understanding how these effectors are translocated, their precise roles in virulence, and the extent to which functional redundancy exists in oomycete RXLR effector complements, are major challenges for the coming years.