Molecular cloning of an isoform of Doc2 having two C2-like domains

Phosphorylation of Doc2 by EphB2 modulates Munc13-mediated SNARE complex assembly and neurotransmitter release

At the synapse, presynaptic neurotransmitter release is tightly controlled by release machinery, involving the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins and Munc13. The Ca2+ sensor Doc2 cooperates with Munc13 to regulate neurotransmitter release, but the underlying mechanisms remain unclear. In our study, we have characterized the binding mode between Doc2 and Munc13 and found that Doc2 originally occludes Munc13 to inhibit SNARE complex assembly. Moreover, our investigation unveiled that EphB2, a presynaptic adhesion molecule (SAM) with inherent tyrosine kinase functionality, exhibits the capacity to phosphorylate Doc2. This phosphorylation attenuates Doc2 block on Munc13 to promote SNARE complex assembly, which functionally induces spontaneous release and synaptic augmentation. Consistently, application of a Doc2 peptide that interrupts Doc2-Munc13 interplay impairs excitatory synaptic transmission and leads to dysfunction in spatial learning and memory. These data provide evidence that SAMs modulate neurotransmitter release by controlling SNARE complex assembly.

16p11.2 CNV gene Doc2α functions in neurodevelopment and social behaviors through interaction with Secretagogin

Copy-number variations (CNVs) of the human 16p11.2 genetic locus are associated with neurodevelopmental disorders, including autism spectrum disorders (ASDs) and schizophrenia. However, it remains largely unclear how this locus is involved in the disease pathogenesis. Doc2α is localized within this locus. Here, using in vivo and ex vivo electrophysiological and morphological approaches, we show that Doc2α-deficient mice have neuronal morphological abnormalities and defects in neural activity. Moreover, the Doc2α-deficient mice exhibit social and repetitive behavioral deficits. Furthermore, we demonstrate that Doc2α functions in behavioral and neural phenotypes through interaction with Secretagogin (SCGN). Finally, we demonstrate that SCGN functions in social/repetitive behaviors, glutamate release, and neuronal morphology of the mice through its Doc2α-interacting activity. Therefore, Doc2α likely contributes to neurodevelopmental disorders through its interaction with SCGN.

Synaptotagmin-7-mediated activation of spontaneous NMDAR currents is disrupted in bipolar disorder susceptibility variants

Synaptotagmin-7 (Syt7) plays direct or redundant Ca2+ sensor roles in multiple forms of vesicle exocytosis in synapses. Here, we show that Syt7 is a redundant Ca2+ sensor with Syt1/Doc2 to drive spontaneous glutamate release, which functions uniquely to activate the postsynaptic GluN2B-containing NMDARs that significantly contribute to mental illness. In mouse hippocampal neurons lacking Syt1/Doc2, Syt7 inactivation largely diminishes spontaneous release. Using 2 approaches, including measuring Ca2+ dose response and substituting extracellular Ca2+ with Sr2+, we detect that Syt7 directly triggers spontaneous release via its Ca2+ binding motif to activate GluN2B-NMDARs. Furthermore, modifying the localization of Syt7 in the active zone still allows Syt7 to drive spontaneous release, but the GluN2B-NMDAR activity is abolished. Finally, Syt7 SNPs identified in bipolar disorder patients destroy the function of Syt7 in spontaneous release in patient iPSC-derived and mouse hippocampal neurons. Therefore, Syt7 could contribute to neuropsychiatric disorders through driving spontaneous glutamate release.

Ca2+ sensor proteins in spontaneous release and synaptic plasticity: Limited contribution of Doc2c, rabphilin-3a and synaptotagmin 7 in hippocampal glutamatergic neurons

Presynaptic neurotransmitter release is strictly regulated by SNARE proteins, Ca2+ and a number of Ca2+ sensors including synaptotagmins (Syts) and Double C2 domain proteins (Doc2s). More than seventy years after the original description of spontaneous release, the mechanism that regulates this process is still poorly understood. Syt-1, Syt7 and Doc2 proteins contribute predominantly, but not exclusively, to synchronous, asynchronous and spontaneous phases of release. The proteins share a conserved tandem C2 domain architecture, but are functionally diverse in their subcellular location, Ca2+-binding properties and protein interactions. In absence of Syt-1, Doc2a and -b, neurons still exhibit spontaneous vesicle fusion which remains Ca2+-sensitive, suggesting the existence of additional sensors. Here, we selected Doc2c, rabphilin-3a and Syt-7 as three potential Ca2+ sensors for their sequence homology with Syt-1 and Doc2b. We genetically ablated each candidate gene in absence of Doc2a and -b and investigated spontaneous and evoked release in glutamatergic hippocampal neurons, cultured either in networks or on microglial islands (autapses). The removal of Doc2c had no effect on spontaneous or evoked release. Syt-7 removal also did not affect spontaneous release, although it altered short-term plasticity by accentuating short-term depression. The removal of rabphilin caused an increased spontaneous release frequency in network cultures, an effect that was not observed in autapses. Taken together, we conclude that Doc2c and Syt-7 do not affect spontaneous release of glutamate in hippocampal neurons, while our results suggest a possible regulatory role of rabphilin-3a in neuronal networks. These findings importantly narrow down the repertoire of synaptic Ca2+ sensors that may be implicated in the spontaneous release of glutamate.

Metastatic suppression by DOC2B is mediated by inhibition of epithelial-mesenchymal transition and induction of senescence

Senescence induction and epithelial-mesenchymal transition (EMT) events are the opposite sides of the spectrum of cancer phenotypes. The key molecules involved in these processes may get influenced or altered by genetic and epigenetic changes during tumor progression. Double C2-like domain beta (DOC2B), an intracellular vesicle trafficking protein of the double C2 protein family, plays a critical role in exocytosis, neurotransmitter release, and intracellular vesicle trafficking. DOC2B is repressed by DNA promoter hypermethylation and functions as a tumor growth regulator in cervical cancer. To date, the molecular mechanisms of DOC2B in cervical cancer progression and metastasis is elusive. Herein, the biological functions and molecular mechanisms regulated by DOC2B and its impact on senescence and EMT are described. DOC2B inhibition promotes proliferation, growth, and migration by relieving G0/G1-S arrest, actin remodeling, and anoikis resistance in Cal27 cells. It enhanced tumor growth and liver metastasis in nude mice with the concomitant increase in metastasis-associated CD55 and CD61 expression. Inhibition of EMT and promotion of senescence by DOC2B is a calcium-dependent process and accompanied by calcium-mediated interaction between DOC2B and CDH1. In addition, we have identified several EMT and senescence regulators as targets of DOC2B. We show that DOC2B may act as a metastatic suppressor by inhibiting EMT through induction of senescence via DOC2B-calcium-EMT-senescence axis.

Conventional and Unconventional Mechanisms by which Exocytosis Proteins Oversee β-cell Function and Protection

Type 2 diabetes (T2D) is one of the prominent causes of morbidity and mortality in the United States and beyond, reaching global pandemic proportions. One hallmark of T2D is dysfunctional glucose-stimulated insulin secretion from the pancreatic β-cell. Insulin is secreted via the recruitment of insulin secretory granules to the plasma membrane, where the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and SNARE regulators work together to dock the secretory granules and release insulin into the circulation. SNARE proteins and their regulators include the Syntaxins, SNAPs, Sec1/Munc18, VAMPs, and double C2-domain proteins. Recent studies using genomics, proteomics, and biochemical approaches have linked deficiencies of exocytosis proteins with the onset and progression of T2D. Promising results are also emerging wherein restoration or enhancement of certain exocytosis proteins to β-cells improves whole-body glucose homeostasis, enhances β-cell function, and surprisingly, protection of β-cell mass. Intriguingly, overexpression and knockout studies have revealed novel functions of certain exocytosis proteins, like Syntaxin 4, suggesting that exocytosis proteins can impact a variety of pathways, including inflammatory signaling and aging. In this review, we present the conventional and unconventional functions of β-cell exocytosis proteins in normal physiology and T2D and describe how these insights might improve clinical care for T2D.

Doc2 Proteins Are Not Required for the Increased Spontaneous Release Rate in Synaptotagmin-1-Deficient Neurons

Regulated secretion is controlled by Ca²⁺ sensors with different affinities and subcellular distributions. Inactivation of Syt1 (synaptotagmin-1), the main Ca²⁺ sensor for synchronous neurotransmission in many neurons, enhances asynchronous and spontaneous release rates, suggesting that Syt1 inhibits other sensors with higher Ca²⁺ affinities and/or lower cooperativities. Such sensors could include Doc2a and Doc2b, which have been implicated in spontaneous and asynchronous neurotransmitter release and compete with Syt1 for binding SNARE complexes. Here, we tested this hypothesis using triple-knock-out mice. Inactivation of Doc2a and Doc2b in Syt1-deficient neurons did not reduce the high spontaneous release rate. Overexpression of Doc2b variants in triple-knock-out neurons reduced spontaneous release but did not rescue synchronous release. A chimeric construct in which the C2AB domain of Syt1 was substituted by that of Doc2b did not support synchronous release either. Conversely, the soluble C2AB domain of Syt1 did not affect spontaneous release. We conclude that the high spontaneous release rate in synaptotagmin-deficient neurons does not involve the binding of Doc2 proteins to Syt1 binding sites in the SNARE complex. Instead, our results suggest that the C2AB domains of Syt1 and Doc2b specifically support synchronous and spontaneous release by separate mechanisms. (Both male and female neurons were studied without sex determination.)SIGNIFICANCE STATEMENT Neurotransmission in the brain is regulated by presynaptic Ca²⁺ concentrations. Multiple Ca²⁺ sensor proteins contribute to synchronous (Syt1, Syt2), asynchronous (Syt7), and spontaneous (Doc2a/Doc2b) phases of neurotransmitter release. Genetic ablation of synchronous release was previously shown to affect other release phases, suggesting that multiple sensors may compete for similar release sites, together encoding stimulus-secretion coupling over a large range of synaptic Ca²⁺ concentrations. Here, we investigated the extent of functional overlap between Syt1, Doc2a, and Doc2b by reintroducing wild-type and mutant proteins in triple-knock-out neurons, and conclude that the sensors are highly specialized for different phases of release.

Protein kinase C iota facilitates insulin-induced glucose transport by phosphorylation of soluble nSF attachment protein receptor regulator (SNARE) double C2 domain protein b

AIMS/INTRODUCTION

Double C2 domain protein b (DOC2b), one of the synaptotagmins, has been shown to translocate to the plasma membrane, and to initiate membrane-fusion processes of vesicles containing glucose transporter 4 proteins on insulin stimulation. However, the mechanism by which DOC2b is regulated remains unclear. Herein, we identified the upstream regulatory factors of DOC2b in insulin signal transduction. We also examined the role of DOC2b on systemic homeostasis using DOC2b knockout (KO) mice.

MATERIALS AND METHODS

We first identified DOC2b binding proteins by immunoprecipitation and mutagenesis experiments. Then, DOC2b KO mice were generated by disrupting the first exon of the DOC2b gene. In addition to the histological examination, glucose metabolism was assessed by measuring parameters on glucose/insulin tolerance tests. Insulin-stimulated glucose uptake was also measured using isolated soleus muscle and epididymal adipose tissue.

RESULTS

We identified an isoform of atypical protein kinase C (protein kinase C iota) that can bind to DOC2b and phosphorylates one of the serine residues of DOC2b (S34). This phosphorylation is essential for DOC2b translocation. DOC2b KO mice showed insulin resistance and impaired oral glucose tolerance on insulin and glucose tolerance tests, respectively. Insulin-stimulated glucose uptake was impaired in isolated soleus muscle and epididymal adipose tissues from DOC2b KO mice.

CONCLUSIONS

We propose a novel insulin signaling mechanism by which protein kinase C iota phosphorylates DOC2b, leading to glucose transporter 4 vesicle translocation, fusion and facilitation of glucose uptake in response to insulin. The present results also showed DOC2b to play important roles in systemic glucose homeostasis.

DOC2B promotes insulin sensitivity in mice via a novel KLC1-dependent mechanism in skeletal muscle

AIMS/HYPOTHESIS

Skeletal muscle accounts for >80% of insulin-stimulated glucose uptake; dysfunction of this process underlies insulin resistance and type 2 diabetes. Insulin sensitivity is impaired in mice deficient in the double C2 domain β (DOC2B) protein, while whole-body overexpression of DOC2B enhances insulin sensitivity. Whether insulin sensitivity in the skeletal muscle is affected directly by DOC2B or is secondary to an effect on other tissues is unknown; the underlying molecular mechanisms also remain unclear.

METHODS

Human skeletal muscle samples from non-diabetic or type 2 diabetic donors were evaluated for loss of DOC2B during diabetes development. For in vivo analysis, new doxycycline-inducible skeletal-muscle-specific Doc2b-overexpressing mice fed standard or high-fat diets were evaluated for insulin and glucose tolerance, and insulin-stimulated GLUT4 accumulation at the plasma membrane (PM). For in vitro analyses, a DOC2B-overexpressing L6-GLUT4-myc myoblast/myotube culture system was coupled with an insulin resistance paradigm. Biochemical and molecular biology methods such as site-directed mutagenesis, co-immunoprecipitation and mass spectrometry were used to identify the molecular mechanisms linking insulin stimulation to DOC2B.

RESULTS

We identified loss of DOC2B (55% reduction in RNA and 40% reduction in protein) in the skeletal muscle of human donors with type 2 diabetes. Furthermore, inducible enrichment of DOC2B in skeletal muscle of transgenic mice enhanced whole-body glucose tolerance (AUC decreased by 25% for female mice) and peripheral insulin sensitivity (area over the curve increased by 20% and 26% for female and male mice, respectively) in vivo, underpinned by enhanced insulin-stimulated GLUT4 accumulation at the PM. Moreover, DOC2B enrichment in skeletal muscle protected mice from high-fat-diet-induced peripheral insulin resistance, despite the persistence of obesity. In L6-GLUT4-myc myoblasts, DOC2B enrichment was sufficient to preserve normal insulin-stimulated GLUT4 accumulation at the PM in cells exposed to diabetogenic stimuli. We further identified that DOC2B is phosphorylated on insulin stimulation, enhancing its interaction with a microtubule motor protein, kinesin light chain 1 (KLC1). Mutation of Y301 in DOC2B blocked the insulin-stimulated phosphorylation of DOC2B and interaction with KLC1, and it blunted the ability of DOC2B to enhance insulin-stimulated GLUT4 accumulation at the PM.

CONCLUSIONS/INTERPRETATION

These results suggest that DOC2B collaborates with KLC1 to regulate insulin-stimulated GLUT4 accumulation at the PM and regulates insulin sensitivity. Our observation provides a basis for pursuing DOC2B as a novel drug target in the muscle to prevent/treat type 2 diabetes.

Molecular underpinnings of synaptic vesicle pool heterogeneity

Neuronal communication relies on chemical synaptic transmission for information transfer and processing. Chemical neurotransmission is initiated by synaptic vesicle fusion with the presynaptic active zone resulting in release of neurotransmitters. Classical models have assumed that all synaptic vesicles within a synapse have the same potential to fuse under different functional contexts. In this model, functional differences among synaptic vesicle populations are ascribed to their spatial distribution in the synapse with respect to the active zone. Emerging evidence suggests, however, that synaptic vesicles are not a homogenous population of organelles, and they possess intrinsic molecular differences and differential interaction partners. Recent studies have reported a diverse array of synaptic molecules that selectively regulate synaptic vesicles' ability to fuse synchronously and asynchronously in response to action potentials or spontaneously irrespective of action potentials. Here we discuss these molecular mediators of vesicle pool heterogeneity that are found on the synaptic vesicle membrane, on the presynaptic plasma membrane, or within the cytosol and consider some of the functional consequences of this diversity. This emerging molecular framework presents novel avenues to probe synaptic function and uncover how synaptic vesicle pools impact neuronal signaling.

Structural elements that underlie Doc2β function during asynchronous synaptic transmission

Double C2-like domain-containing proteins alpha and beta (Doc2α and Doc2β) are tandem C2-domain proteins proposed to function as Ca(2+) sensors for asynchronous neurotransmitter release. Here, we systematically analyze each of the negatively charged residues that mediate binding of Ca(2+) to the β isoform. The Ca(2+) ligands in the C2A domain were dispensable for Ca(2+)-dependent translocation to the plasma membrane, with one exception: neutralization of D220 resulted in constitutive translocation. In contrast, three of the five Ca(2+) ligands in the C2B domain are required for translocation. Importantly, translocation was correlated with the ability of the mutants to enhance asynchronous release when overexpressed in neurons. Finally, replacement of specific Ca(2+)/lipid-binding loops of synaptotagmin 1, a Ca(2+) sensor for synchronous release, with corresponding loops from Doc2β, resulted in chimeras that yielded slower kinetics in vitro and slower excitatory postsynaptic current decays in neurons. Together, these data reveal the key determinants of Doc2β that underlie its function during the slow phase of synaptic transmission.

DOC2 isoforms play dual roles in insulin secretion and insulin-stimulated glucose uptake

AIMS/HYPOTHESIS

Glucose-stimulated insulin secretion (GSIS) and insulin-stimulated glucose uptake are processes that rely on regulated intracellular vesicle transport and vesicle fusion with the plasma membrane. DOC2A and DOC2B are calcium-sensitive proteins that were identified as key components of vesicle exocytosis in neurons. Our aim was to investigate the role of DOC2 isoforms in glucose homeostasis, insulin secretion and insulin action.

METHODS

DOC2 expression was measured by RT-PCR and western blotting. Body weight, glucose tolerance, insulin action and GSIS were assessed in wild-type (WT), Doc2a (-/-) (Doc2aKO), Doc2b (-/-) (Doc2bKO) and Doc2a (-/-)/Doc2b (-/-) (Doc2a/Doc2bKO) mice in vivo. In vitro GSIS and glucose uptake were assessed in isolated tissues, and exocytotic proteins measured by western blotting. GLUT4 translocation was assessed by epifluorescence microscopy.

RESULTS

Doc2b mRNA was detected in all tissues tested, whereas Doc2a was only detected in islets and the brain. Doc2aKO and Doc2bKO mice had minor glucose intolerance, while Doc2a/Doc2bKO mice showed pronounced glucose intolerance. GSIS was markedly impaired in Doc2a/Doc2bKO mice in vivo, and in isolated Doc2a/Doc2bKO islets in vitro. In contrast, Doc2bKO mice had only subtle defects in insulin secretion in vivo. Insulin action was impaired to a similar degree in both Doc2bKO and Doc2a/Doc2bKO mice. In vitro insulin-stimulated glucose transport and GLUT4 vesicle fusion were defective in adipocytes derived from Doc2bKO mice. Surprisingly, insulin action was not altered in muscle isolated from DOC2-null mice.

CONCLUSIONS/INTERPRETATION

Our study identifies a critical role for DOC2B in insulin-stimulated glucose uptake in adipocytes, and for the synergistic regulation of GSIS by DOC2A and DOC2B in beta cells.

DNA promoter methylation-dependent transcription of the double C2-like domain β (DOC2B) gene regulates tumor growth in human cervical cancer

Double C2-like domain β (DOC2B) gene encodes for a calcium-binding protein, which is involved in neurotransmitter release, sorting, and exocytosis. We have identified the promoter region of the DOC2B gene as hypermethylated in pre-malignant, malignant cervical tissues, and cervical cancer cell lines by methylation-sensitive dimethyl sulfoxide-polymerase chain reaction and bisulfite genome sequencing; whereas, it was unmethylated in normal cervical tissues (p < 0.05). The promoter hypermethylation was inversely associated with mRNA expression in SiHa, CaSki, and HeLa cells and treatment with demethylating agent 5-aza-2-deoxycytidine restored DOC2B expression. The region -630 to +25 bp of the DOC2B gene showed robust promoter activity by a luciferase reporter assay and was inhibited by in vitro artificial methylation with Sss1 methylase prior to transient transfections. Overexpression of the DOC2B gene in SiHa cells when compared with controls showed significantly reduced colony formation, cell proliferation, induced cell cycle arrest, and repressed cell migration and invasion (p < 0.05). Ectopic expression of DOC2B resulted in anoikis-mediated cell death and repressed tumor growth in a nude mice xenograft model (p < 0.05). DOC2B expressing cells showed a significant increase in intracellular calcium level (p < 0.05), impaired AKT1 and ERK1/2 signaling, and induced actin cytoskeleton remodeling. Our results show that promoter hypermethylation and silencing of the DOC2B gene is an early and frequent event during cervical carcinogenesis and whose reduced expression due to DNA promoter methylation may lead to selective cervical tumor growth.

Molecular mechanisms for synchronous, asynchronous, and spontaneous neurotransmitter release

Most neuronal communication relies upon the synchronous release of neurotransmitters, which occurs through synaptic vesicle exocytosis triggered by action potential invasion of a presynaptic bouton. However, neurotransmitters are also released asynchronously with a longer, variable delay following an action potential or spontaneously in the absence of action potentials. A compelling body of research has identified roles and mechanisms for synchronous release, but asynchronous release and spontaneous release are less well understood. In this review, we analyze how the mechanisms of the three release modes overlap and what molecular pathways underlie asynchronous and spontaneous release. We conclude that the modes of release have key fusion processes in common but may differ in the source of and necessity for Ca(2+) to trigger release and in the identity of the Ca(2+) sensor for release.

Distinct Initial SNARE Configurations Underlying the Diversity of Exocytosis

The dynamics of exocytosis are diverse and have been optimized for the functions of synapses and a wide variety of cell types. For example, the kinetics of exocytosis varies by more than five orders of magnitude between ultrafast exocytosis in synaptic vesicles and slow exocytosis in large dense-core vesicles. However, in all cases, exocytosis is mediated by the same fundamental mechanism, i.e., the assembly of soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. It is often assumed that vesicles need to be docked at the plasma membrane and SNARE proteins must be preassembled before exocytosis is triggered. However, this model cannot account for the dynamics of exocytosis recently reported in synapses and other cells. For example, vesicles undergo exocytosis without prestimulus docking during tonic exocytosis of synaptic vesicles in the active zone. In addition, epithelial and hematopoietic cells utilize cAMP and kinases to trigger slow exocytosis of nondocked vesicles. In this review, we summarize the manner in which the diversity of exocytosis reflects the initial configurations of SNARE assembly, including trans-SNARE, binary-SNARE, unitary-SNARE, and cis-SNARE configurations. The initial SNARE configurations depend on the particular SNARE subtype (syntaxin, SNAP25, or VAMP), priming proteins (Munc18, Munc13, CAPS, complexin, or snapin), triggering proteins (synaptotagmins, Doc2, and various protein kinases), and the submembraneous cytomatrix, and they are the key to determining the kinetics of subsequent exocytosis. These distinct initial configurations will help us clarify the common SNARE assembly processes underlying exocytosis and membrane trafficking in eukaryotic cells.

Calcium sensing in exocytosis

Neurotransmitters, neuropeptides and hormones are released through regulated exocytosis of synaptic vesicles and large dense core vesicles. This complex and highly regulated process is orchestrated by SNAREs and their associated proteins. The triggering signal for regulated exocytosis is usually an increase in intracellular calcium levels. Besides the triggering role, calcium signaling modulates the precise amount and kinetics of vesicle release. Thus, it is a central question to understand the molecular machineries responsible for calcium sensing in exocytosis. Here we provide an overview of our current understanding of calcium sensing in neurotransmitter release and hormone secretion.

Doc2 is a Ca2+ sensor required for asynchronous neurotransmitter release

Synaptic transmission involves a fast synchronous phase and a slower asynchronous phase of neurotransmitter release that are regulated by distinct Ca(2+) sensors. Though the Ca(2+) sensor for rapid exocytosis, synaptotagmin I, has been studied in depth, the sensor for asynchronous release remains unknown. In a screen for neuronal Ca(2+) sensors that respond to changes in [Ca(2+)] with markedly slower kinetics than synaptotagmin I, we observed that Doc2--another Ca(2+), SNARE, and lipid-binding protein--operates on timescales consistent with asynchronous release. Moreover, up- and downregulation of Doc2 expression levels in hippocampal neurons increased or decreased, respectively, the slow phase of synaptic transmission. Synchronous release, when triggered by single action potentials, was unaffected by manipulation of Doc2 but was enhanced during repetitive stimulation in Doc2 knockdown neurons, potentially due to greater vesicle availability. In summary, we propose that Doc2 is a Ca(2+) sensor that is kinetically tuned to regulate asynchronous neurotransmitter release.

Doc2 supports spontaneous synaptic transmission by a Ca(2+)-independent mechanism

Two families of Ca(2+)-binding proteins have been proposed as Ca(2+) sensors for spontaneous release: synaptotagmins and Doc2s, with the intriguing possibility that Doc2s may represent high-affinity Ca(2+) sensors that are activated by deletion of synaptotagmins, thereby accounting for the increased spontaneous release in synaptotagmin-deficient synapses. Here, we use an shRNA-dependent quadruple knockdown of all four Ca(2+)-binding proteins of the Doc2 family to confirm that Doc2-deficient synapses exhibit a marked decrease in the frequency of spontaneous release events. Knockdown of Doc2s in synaptotagmin-1-deficient synapses, however, failed to reduce either the increased spontaneous release or the decreased evoked release of these synapses, suggesting that Doc2s do not constitute Ca(2+) sensors for asynchronous release. Moreover, rescue experiments revealed that the decrease in spontaneous release induced by the Doc2 knockdown in wild-type synapses is fully reversed by mutant Doc2B lacking Ca(2+)-binding sites. Thus, our data suggest that Doc2s are modulators of spontaneous synaptic transmission that act by a Ca(2+)-independent mechanism.

In silico ascription of gene expression differences to tumor and stromal cells in a model to study impact on breast cancer outcome

Breast tumors consist of several different tissue components. Despite the heterogeneity, most gene expression analyses have traditionally been performed without prior microdissection of the tissue sample. Thus, the gene expression profiles obtained reflect the mRNA contribution from the various tissue components. We utilized histopathological estimations of area fractions of tumor and stromal tissue components in 198 fresh-frozen breast tumor tissue samples for a cell type-associated gene expression analysis associated with distant metastasis. Sets of differentially expressed gene-probes were identified in tumors from patients who developed distant metastasis compared with those who did not, by weighing the contribution from each tumor with the relative content of stromal and tumor epithelial cells in their individual tumor specimen. The analyses were performed under various assumptions of mRNA transcription level from tumor epithelial cells compared with stromal cells. A set of 30 differentially expressed gene-probes was ascribed solely to carcinoma cells. Furthermore, two sets of 38 and five differentially expressed gene-probes were mostly associated to tumor epithelial and stromal cells, respectively. Finally, a set of 26 differentially expressed gene-probes was identified independently of cell type focus. The differentially expressed genes were validated in independent gene expression data from a set of laser capture microdissected invasive ductal carcinomas. We present a method for identifying and ascribing differentially expressed genes to tumor epithelial and/or stromal cells, by utilizing pathologic information and weighted t-statistics. Although a transcriptional contribution from the stromal cell fraction is detectable in microarray experiments performed on bulk tumor, the gene expression differences between the distant metastasis and no distant metastasis group were mostly ascribed to the tumor epithelial cells of the primary breast tumors. However, the gene PIP5K2A was found significantly elevated in stroma cells in distant metastasis group, compared to stroma in no distant metastasis group. These findings were confirmed in gene expression data from the representative compartments from microdissected breast tissue. The method described was also found to be robust to different histopathological procedures.

Role of the polybasic sequence in the Doc2alpha C2B domain in dense-core vesicle exocytosis in PC12 cells

The double C2 (Doc2) family is characterized by an N-terminal Munc13-1-interacting domain and C-terminal tandem C2 domains, and it comprises three isoforms, Doc2alpha, Doc2beta, and Doc2gamma, in humans and mice. Doc2alpha, the best-characterized, brain-specific isoform, exhibits Ca(2+)-dependent phospholipid-binding activity through its C2A domain, and the Ca(2+)-binding activity is thought to be important for the regulation of Ca(2+)-dependent exocytosis. In contrast to the C2A domain, however, nothing is known about the physiological functions of the C2B domain in regulated exocytosis. In this study, we demonstrated by a mutation analysis that the polybasic sequence in the C2B domain of Doc2alpha (306 KKSKHKTCVKKK 317) is required for binding of syntaxin-1a/synaptosome-associated protein of 25 kDa (SNAP-25) heterodimer. We also investigated the effect of Lys-to-Gln (named KQ) mutations in the polybasic sequence of the C2B domain on vesicle dynamics by total internal reflection fluorescence microscopy in PC12 cells. A Doc2alpha(KQ) mutant, which lacks binding activity toward syntaxin-1a/SNAP-25 heterodimer, significantly decreased the number of plasma membrane-docked vesicles before stimulation and strongly inhibited high-KCl-induced exocytosis from the plasma membrane-docked vesicles. These results indicate that the polybasic sequence in the C2B domain functions as a binding site for syntaxin-1a/SNAP-25 heterodimer and controls the number of 'readily releasable' vesicles in neuroendocrine cells.

DOC2B, C2 domains, and calcium: A tale of intricate interactions

Ca(+2)-dependent exocytosis involves vesicle docking, priming, fusion, and recycling. This process is performed and regulated by a vast number of synaptic proteins and depends on proper protein-protein and protein-lipid interactions. Double C2 domain (DOC2) is a protein family of three isoforms found while screening DNA libraries with a C2 probe. DOC2 has three domains: the Munc13-interacting domain and tandem C2s (designated C2A and C2B) connected by a short polar linker. The C2 domain binds phospholipids in a Ca(2+)-dependent manner. This review focuses on the ubiquitously expressed isoform DOC2B. Sequence alignment of the tandem C2 protein family in mouse revealed high homology (81%) between rabphilin-3A and DOC2B proteins. We created a structural model of DOC2B's C2A based on the crystal structure of rabphilin-3A with and without calcium and found that the calcium-binding loops of DOC2B move upon calcium binding, enabling efficient plasma membrane penetration of its C2A. Here, we discuss the potential relation between the DOC2B bioinformatical model and its function and suggest a possible working model for its interaction with other proteins of the exocytotic machinery, including Munc13, Munc18, and syntaxin.

Calcium-sensing beyond neurotransmitters: functions of synaptotagmins in neuroendocrine and endocrine secretion

Neurotransmitters, neuropeptides and hormones are released through the regulated exocytosis of SVs (synaptic vesicles) and LDCVs (large dense-core vesicles), a process that is controlled by calcium. Synaptotagmins are a family of type 1 membrane proteins that share a common domain structure. Most synaptotagmins are located in brain and endocrine cells, and some of these synaptotagmins bind to phospholipids and calcium at levels that trigger regulated exocytosis of SVs and LDCVs. This led to the proposed synaptotagmin-calcium-sensor paradigm, that is, members of the synaptotagmin family function as calcium sensors for the regulated exocytosis of neurotransmitters, neuropeptides and hormones. Here, we provide an overview of the synaptotagmin family, and review the recent mouse genetic studies aimed at understanding the functions of synaptotagmins in neurotransmission and endocrine-hormone secretion. Also, we discuss potential roles of synaptotagmins in non-traditional endocrine systems.

Doc2beta is a novel Munc18c-interacting partner and positive effector of syntaxin 4-mediated exocytosis

The widely expressed Sec/Munc18 (SM) protein Munc18c is required for SNARE-mediated insulin granule exocytosis from islet beta cells and GLUT4 vesicle exocytosis in skeletal muscle and adipocytes. Although Munc18c function is known to involve binding to the t-SNARE Syntaxin 4, a paucity of Munc18c-binding proteins has restricted elucidation of the mechanism by which it facilitates these exocytosis events. Toward this end, we have identified the double C2 domain protein Doc2beta as a new binding partner for Munc18c. Unlike its granule/vesicle localization in neuronal cells, Doc2beta was found principally in the plasma membrane compartment in islet beta cells and adipocytes. Moreover, co-immunoprecipitation and GST interaction assays showed Doc2beta-Munc18c binding to be direct and complexes to be devoid of Syntaxin 4. Supporting the notion of Munc18c binding with Syntaxin 4 and Doc2beta in mutually exclusive complexes, in vitro competition with Syntaxin 4 effectively displaced Munc18c from binding to Doc2beta. The second C2 domain (C2B) of Doc2beta and an N-terminal region of Munc18c were sufficient to confer complex formation. Disruption of endogenous Munc18c-Doc2beta complexes by addition of the Doc2beta binding domain of Munc18c (residues 173-255) was found to selectively inhibit glucose-stimulated insulin release. Moreover, increased expression of Doc2beta enhanced glucose-stimulated insulin secretion by approximately 40%, whereas siRNA-mediated depletion of Doc2beta attenuated insulin release. All changes in secretion correlated with parallel alterations in VAMP2 granule docking with Syntaxin 4. Taken together, these data support a model wherein Munc18c transiently switches from association with Syntaxin 4 to association with Doc2beta at the plasma membrane to facilitate exocytosis.

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.

Calcium-dependent regulation of exocytosis

A rapid increase in intracellular calcium directly triggers regulated exocytosis. In addition, changes in intracellular calcium concentration can adjust the extent of exocytosis (quantal content) or the magnitude of individual release events (quantal size) in both the short- and long-term. It is generally agreed that calcium achieves this regulation via an interaction with a number of different molecular targets located at or near to the site of membrane fusion. We review here the synaptic proteins with defined calcium-binding domains and protein kinases activated by calcium, summarize what is known about their function in membrane fusion and the experimental evidence in support of their involvement in synaptic plasticity.

Association of ABCA2 expression with determinants of Alzheimer's disease

With the use of a novel method for detecting differential gene expression, alterations in functional gene clusters related to transport or oxidative stress response and beta-amyloid (Abeta) peptide metabolism were identified in a HEK293 cell line engineered to overexpress the human ATP binding cassette transporter ABCA2. These included fatty acid binding protein, phospholipid binding protein, phospholipid synthesis protein, transporter cofactors, seladin-1, Abeta precursor protein (APP), vimentin, and low-density lipoprotein receptor-related protein. ABCA2 was highly expressed in neuroblastoma cells and colocalized with Abeta and APP. Additionally, increased APP protein levels were detected within ABCA2/APP double-transfected cells, and increased Abeta was detected in the media of ABCA2-transfected cells relative to controls. The transporter was abundant in the temporal and frontal regions of both normal and Alzheimer's disease (AD) brain but was detected at lower concentrations in the parietal, occipital, and cerebellar regions. The ABCA2 transfected cell line expressed resistance to a free radical initiator, confirming involvement in protection against reactive oxygen species and suggesting a further possible link to AD.

Ca(2+)-induced recruitment of the secretory vesicle protein DOC2B to the target membrane

Ca(2+)-dependent fusion of transport vesicles at their target can be enhanced by intracellular Ca2+ and diacylglycerol. Diacylglycerol induces translocation of the vesicle priming factor Munc13 and association of the secretory vesicle protein DOC2B to the membrane. Here we demonstrate that a rise in intracellular Ca2+ is sufficient for a Munc13-independent recruitment of DOC2B to the target membrane. This novel mechanism occurred readily in the absence of Munc13 and was not influenced by DOC2B mutations that abolish Munc13 binding. Purified DOC2B (expressed as a bacterial fusion protein) bound phospholipids in a Ca(2+)-dependent way, suggesting that the translocation is the result of a C2 domain activation mechanism. Ca(2+)-induced translocation was also observed in cultured neurons expressing DOC2B-enhanced green fluorescent protein. In this case, however, various degrees of membrane association occurred under resting conditions, suggesting that physiological Ca2+ concentrations modulate DOC2B localization. Depolarization of the neurons induced a complete translocation of DOC2B-enhanced green fluorescent protein to the target membrane within 5 s. We hypothesize that this novel Ca(2+)-induced activity of DOC2B functions synergistically with diacylglycerol-induced Munc13 binding to enhance exocytosis during episodes of high secretory activity.

The C2A domain of synaptotagmin-like protein 3 (Slp3) is an atypical calcium-dependent phospholipid-binding machine: comparison with the C2A domain of synaptotagmin I

The synaptotagmin-like protein (Slp) family consists of an N-terminal Rab27-binding domain and C-terminal tandem C2 motifs, and although it has been suggested to regulate Rab27-dependent membrane trafficking, such as Ca2+-regulated granule exocytosis in T-lymphocytes [Kuroda, Fukuda, Ariga and Mikoshiba (2002) J. Biol. Chem. 277, 9212-9218], little is known about the Ca2+-binding property of the Slp family. In this study, I demonstrated that the C2A domain of Slp3 exhibits Ca(2+)-dependent phospholipid-binding activity similar to that of the C2A domain of synaptotagmin I (Syt I) with regard to phospholipid selectivity, bivalent cation selectivity and effect of ionic strength. This finding was surprising because the C2A domains of other C-terminal-type (C-type) tandem C2 proteins require five conserved acidic residues in the putative Ca2+-binding loops 1 and 3 on the top of the beta-sandwich structure for their Ca2+-/phospholipid-binding activity, whereas the C2A domain of Slp3 contains only one conserved acidic residue in the putative Ca2+-binding loop 1. Site-directed mutagenesis and chimaeric analysis of the C2A domains of Syt I and Slp3 showed that Glu-336 and Glu-337 in the putative Ca2+-binding loop 1 and polybasic sequence (Lys-359, Lys-360 and Lys-361) in the beta-4 strand of the C2 structure are crucial for Ca2+-dependent phospholipid-binding activity of the Slp3 C2A domain, whereas the similar polybasic sequence in the C2A domain of Syt I is dispensable for Ca2+-dependent phospholipid-binding activity. These results indicate that the C2A domain of Slp3 is an atypical Ca2+-/phospholipid-binding machine, compared with other C-type tandem C2 proteins.

The Slp homology domain of synaptotagmin-like proteins 1-4 and Slac2 functions as a novel Rab27A binding domain

rab27A, which encodes a small GTP-binding protein, was recently identified as a gene in which mutations caused human hemophagocytic syndrome (Griscelli syndrome) and ashen mice, which exhibit defects in melanosome transport as well as in regulated granule exocytosis in cytotoxic T lymphocytes. However, little is known about the molecular mechanism of Rab27A-dependent membrane trafficking or the specific effector molecules of Rab27A. In this study, we discovered that the Slp (synaptotagmin-like protein) homology domain (SHD) of Slp1--3 and Slac2-a/b specifically and directly binds the GTP-bound form of Rab27A both in vitro and in intact cells but not of the other Rabs tested (Rab1, Rab2, Rab3A, Rab4, Rab5A, Rab6A, Rab7, Rab8, Rab9, Rab10, Rab11A, Rab17, Rab18, Rab20, Rab22, Rab23, Rab25, Rab28, and Rab37). Immunocytochemical analysis revealed that Slp2 (or Slp1) colocalized with Rab27A in the melanosomes of melanoma cells. Slp2 and Rab27A were distributed to the periphery of the cells (especially at the dendritic tips) in the wild-type melanoma cells, whereas they accumulated in the perinuclear region in the melanosome transport-defective cells (S91/Cloudman). These results strongly indicated that the SHD of Slp1--3 and Slac2 functions as an in vivo Rab27A binding domain.

The N-terminal cysteine cluster is essential for membrane targeting of B/K protein

B/K protein belongs to a family of C-terminal-type (C-type) tandem C2 proteins that contain two C2 Ca(2+)-binding motifs at the C-terminus. Although other C-type tandem C2 proteins have been found to have a unique N-terminal domain that is involved in membrane anchoring (e.g. synaptotagmin) or specific ligand binding (e.g. rabphilin-3A and Doc2), no research has been conducted on the function of the N-terminal domain of B/K protein. In this study we showed that despite lacking a transmembrane domain, both native and recombinant B/K proteins are tightly bound to the membrane fraction, which was completely resistant to 0.1 M Na(2)CO(3), pH 11, or 1 M NaCl treatment. Deletion and mutation analyses indicated that the cysteine cluster at the N-terminal domain (consisting of seven cysteine residues, Cys-19, Cys-23, Cys-26, Cys-27, Cys-30, Cys-35 and Cys-36) is essential for the membrane localization of B/K protein. When wild-type B/K was expressed in PC12 cells, B/K proteins were localized mainly in the perinuclear region (trans-Golgi network), whereas mutant B/K proteins carrying Cys-to-Ala substitutions were present in the cytosol. Based on our findings, we propose that the N-terminal domain of B/K protein contains a novel cysteine-based protein motif that may allow B/K protein to localize in the trans-Golgi network.

The C2A domain of double C2 protein gamma contains a functional nuclear localization signal

The C2 domain was originally defined as a homologous domain to the C2 regulatory region of Ca2+ -dependent protein kinase C and has been identified in more than 50 different signaling molecules. The original C2 domain of protein kinase Calpha functions as a Ca2+ binding module, and the Ca2+ binding to the C2 domain allows translocation of proteins to phospholipid membranes. By contrast, however, some C2 domains do not exhibit Ca2+ binding activity because of amino acid substitutions at Ca2+ -binding sites, and their physiological meanings remain largely unknown. In this study, we discovered an unexpected function of the Ca2+ -independent C2A domain of double C2 protein gamma (Doc2gamma) in nuclear localization. Deletion and mutation analyses revealed that the putative Ca2+ binding loop 3 of Doc2gamma contains six Arg residues ((177)RLRRRRR(183)) and that this basic cluster is both necessary and sufficient for nuclear localization of Doc2gamma. Because of the presence of the basic cluster, the C2A domain of Doc2gamma did not show Ca2+ -dependent phospholipid binding activity. Our findings indicate that by changing the nature of the putative Ca2+ binding loops the C2 domain has more diversified function in cellular signaling than a simple Ca2+ binding motif.

Novel splicing isoforms of synaptotagmin-like proteins 2 and 3: identification of the Slp homology domain

Slp1-3 (synaptotagmin-like protein 1-3) is a new family of carboxyl-terminal-type (C-type) tandem C2 proteins that show higher sequence similarity to the C2 domains of granuphilin-a/Slp-4 than those of other C-type tandem C2 proteins (e.g., synaptotagmin and the Doc2 family). However, the amino (N)-terminal domains of the original Slp1-3 do not contain any known protein motifs and do not show any sequence similarities to each other. We report four alternative splicing isoforms of Slp2 (designated Slp2-a-d, with the original Slp2 renamed Slp2-c) and two alternative splicing isoforms of Slp3 (Slp3-a and Slp3-b, the original Slp3). These isoforms share the same C-terminal tandem C2 structures, but their N-terminal nucleotide sequences are completely different due to the alternate use of different exons. Sequence alignment of the Slp1, Slp2-a, Slp3-a, and Slp4 amino terminal domains reveals the presence of two conserved regions among the Slp family, designated SHD1 (Slp homology domain 1) and SHD2, which may function as protein interaction sites. The SHD1 and SHD2 of Slp3-a and Slp4 are separated by a putative Zn(2+)-binding sequence, whereas Slp1 and Slp2 lack such Zn(2+)-binding sequences and their SHD1 and SHD2 are linked together. In addition, we show that the Slp2-a/c/d mRNAs are differentially distributed in different mouse tissues and at different stages of development, suggesting that these transcripts may be regulated by different promoters.

Synaptotagmin-like protein 1-3: a novel family of C-terminal-type tandem C2 proteins

Synaptotagmins (Syt), rabphilin-3A, and Doc2 belong to a family of carboxyl terminal type (C-type) tandem C2 proteins and are thought to be involved in vesicular trafficking. We have cloned and characterized a novel family of C-type tandem C2 proteins, designated Slp1-3 (synaptotagmin-like protein 1-3). The Slp1-3 C2 domains show high homology to granuphilin-a C2 domains, but the amino-terminal domain of Slp1-3 does not contain any known protein motifs or a transmembrane domain. A subcellular fractionation study indicated that Slp1-3 proteins are peripheral membrane proteins. Phospholipid binding experiments indicated that Slp3 is a Ca(2+)-dependent isoform, but Slp1 and Slp2 are Ca(2+)-independent isoforms, because only the Slp3 C2A domain showed Ca(2+)-dependent phospholipid binding activity. The C-terminus of Slp1-3 also bound neurexin Ialpha in vitro, in the same manner as Syt family proteins, which may be important for the membrane association of Slp1-3. In addition, Slp family proteins are differentially distributed in different mouse tissues and at different developmental stages.

Assignment of the gene for a new hereditary nail disorder, isolated congenital nail dysplasia, to chromosome 17p13

Isolated congenital nail dysplasia is an autosomal dominant disorder recently observed in a large family from southern Germany. The disorder is characterized by longitudinal streaks, thinning, and impaired formation of the nail plates leading to increased vulnerability of the free nail margins. In most cases, all fingernails and toenails are similarly involved with some accentuation of the thumb and great toenails. Histologic changes include hypergranulosis of the nail matrix and epithelial outgrowths from the nail bed. Patients do not show any alterations of hair growth and dentition, no malfunction of sweat glands and sensory organs, and no skeletal abnormalities. Isolated congenital nail dysplasia manifests from the first year of life with variable expressivity. In order to localize chromosomally the gene underlying isolated congenital nail dysplasia, linkage to the known keratin gene cluster regions on chromosomes 12q12 and 17q21 was ruled out first. The analysis of 150 microsatellite markers on various chromosomes mapped the isolated congenital nail dysplasia gene to the 6 cM interval between markers at D17S926 and D17S1528 on chromosome 17p13. Markers at D17S849, D17S 1840, and D17S1529 co-segregated completely with the isolated congenital nail dysplasia locus. The maximum two-point LOD score was found for the marker at D17S 1840 (Zmax = 6.72 at Thetamax = 0.00). The identified region harbors no currently known genes involved in skin or nail abnormalities. Isolated congenital nail dysplasia probably represents a novel isolated defect of nail development. The localization of this gene is, therefore, the first step towards the identification of a new factor in nail formation.

Is double C2 protein (DOC2) expressed in bovine adrenal medulla? A commercial anti-DOC2 monoclonal antibody recognizes a major bovine mitochondrial antigen

We have examined the expression in bovine adrenal medulla of double C2 protein (DOC2), a vesicular protein which associates with intracellular phospholipid and Ca(2+) and is implicated in the modulation of regulated exocytosis. Extensive reverse transcription-PCR, Northern blot analyses and in vitro translation reactions have been combined with immunological studies to provide data to suggest that neither DOC2alpha nor DOC2beta is expressed at detectable levels in bovine adrenal chromaffin cells, and that a widely used monoclonal antibody directed against DOC2 also recognizes mitochondrial complex III core protein 2.

Doc2gamma, a third isoform of double C2 protein, lacking calcium-dependent phospholipid binding activity

The Doc2 (double C2) family consists of two isoforms (Doc2alpha and Doc2beta) characterized by an N-terminal Munc13-1 interacting domain (Mid) and two C2 domains that interact with Ca(2+) and phospholipid at the C-terminus. This Ca(2+)-binding property is thought to be important to the regulation of neurotransmitter release. In this paper, we report a third isoform of mouse Doc2, named Doc2gamma. Doc2gamma also contains a putative Mid domain and two C2 domains, and it is 45.6 and 43.2% identical to mouse Doc2alpha and Doc2beta, respectively, at the amino acid level. In contrast to the other Doc2 isoforms, the C2 domains of Doc2gamma impair Ca(2+)-dependent phospholipid binding activity. The highest expression of Doc2gamma mRNA was found in the heart, but occurs ubiquitously, the same as Doc2beta. These findings indicate that Doc2gamma may also function as an effector for Munc13-1 and that it may be involved in the regulation of vesicular trafficking.

Double C2 protein. A review

Concerted effort has led to the identification of dozens of synaptic proteins and has thereby opened the door for the characterisation of the molecular mechanisms underlying regulated exocytosis. Calcium is known to play a number of roles in regulated exocytosis, acting as the trigger for fast synaptic transmission and also acting at some of the steps preceding vesicle fusion. Investigators have therefore focussed considerable attention on possible calcium sensors. What many of the candidate proteins have in common is a C2 domain, one of the four conserved domains originally described in protein kinase C. Such domains have been shown to bind calcium and phospholipid in a large number of intracellular proteins. Synaptotagmin, a C2-domain protein, is a very strong candidate for the protein involved in triggering fast calcium-dependent vesicle fusion. Recent attention has also concerned the other calcium sensors, which may play roles in the 'priming' or transport of vesicles. This review concerns one of these tentative calcium-binding proteins, double C2 or DOC2. DOC2 was originally isolated from nervous tissue but subsequently has been found to be more widely expressed. DOC2 is a vesicular protein that may be involved in the early stages of preparing vesicles for exocytosis.

Different spatiotemporal expression of DOC2 genes in the developing rat brain argues for an additional, nonsynaptic role of DOC2B in early development

DOC2A and DOC2B are two homologous genes implicated in synaptic vesicle exocytosis. Their complementary, nonoverlapping expression patterns in adult rat brain suggest that they exert similar functions in different neurons. We have analysed the expression pattern of the two genes in the developing rat brain by in situ hybridization. Unexpectedly, we found no parallel expression of the two genes during development. DOC2B mRNA was highly expressed as early as embryonic day 12 (E12) throughout the neuroepithelium, long before synaptic transmission is functional, and the expression remained abundant from E12 onwards. In contrast, faint expression of DOC2A transcripts was first detected at E17 in ventral brain areas, and it extended gradually to other brain structures in the sequence of their ontology, i.e. structures that had formed first also expressed DOC2A first. At postnatal day 3, both genes were highly expressed throughout the brain. This overlapping expression diverged to the complementary distribution of the adult brain. The temporal and spatial differences in expression point to a functional divergence between these homologous genes during brain development: the pattern of DOC2A is consistent with its proposed synaptic function, whereas that of DOC2B suggests an additional, nonsynaptic role in proliferating cells.

Doc2alpha is an activity-dependent modulator of excitatory synaptic transmission

Doc2alpha is a synaptic vesicle-associated Ca2 + -binding protein. To study the role of Doc2alpha in synaptic transmission and modulation, we generated homozygous null Doc2alpha mutant mice. In the CA1 region of hippocampal slices in the mutant mice, excitatory synaptic responses evoked with prolonged 5 Hz stimulation showed a significantly larger frequency facilitation followed by a steeper depression than those in wild-type mice, whereas there was no difference in synaptic transmission at lower frequencies or in paired-pulse facilitation. These results suggest that Doc2alpha regulates synaptic transmission when high Ca2 + concentrations in the presynaptic terminal are sustained. Furthermore, the mutant mice showed impairment in long-term potentiation and passive avoidance task. Thus, Doc2alpha may regulate transmitter release during repetitive synaptic activation, thereby contributing to memory formation.

Rabphilin knock-out mice reveal that rabphilin is not required for rab3 function in regulating neurotransmitter release

Rab3A and rab3C are GTP-binding proteins of synaptic vesicles that regulate vesicle exocytosis. Rabphilin is a candidate rab3 effector at the synapse because it binds to rab3s in a GTP-dependent manner, it is co-localized with rab3s on synaptic vesicles, and it dissociates with rab3s from the vesicles during exocytosis. Rabphilin contains two C(2) domains, which could function as Ca(2+) sensors in exocytosis and is phosphorylated as a function of stimulation. However, it is unknown what essential function, if any, rabphilin performs. One controversial question regards the respective roles of rab3s and rabphilin in localizing each other to synaptic vesicles: although rabphilin is mislocalized in rab3A knock-out mice, purified synaptic vesicles were shown to require rabphilin for binding of rab3A but not rab3A for binding of rabphilin. To test whether rabphilin is involved in localizing rab3s to synaptic vesicles and to explore the functions of rabphilin in regulating exocytosis, we have now analyzed knock-out mice for rabphilin. Mice that lack rabphilin are viable and fertile without obvious physiological impairments. In rabphilin-deficient mice, rab3A is targeted to synaptic vesicles normally, whereas in rab3A-deficient mice, rabphilin transport to synapses is impaired. These results show that rabphilin binds to vesicles via rab3s, consistent with an effector function of rabphilin for a synaptic rab3-signal. Surprisingly, however, no abnormalities in synaptic transmission or plasticity were observed in rabphilin-deficient mice; synaptic properties that are impaired in rab3A knock-out mice were unchanged in rabphilin knock-out mice. Our data thus demonstrate that rabphilin is endowed with the properties of a rab3 effector but is not essential for the regulatory functions of rab3 in synaptic transmission.

Co-expression in Xenopus neurons and neuroendocrine cells of messenger RNA homologues of exocytosis proteins DOC2 and munc18-1

The proteins munc18-1 and DOC2 are assumed to play a role in docking of synaptic vesicles in neurotransmitter exocytosis at the presynaptic junction. As the proteins are known to interact, they should co-exist within neurons. We have tested this hypothesis for exocytosis of both classical and peptidergic messengers, by investigating the distribution of the messenger RNAs of munc 18-1 and DOC2 homologues in the brain and pituitary gland of the clawed toad Xenopus laevis, using in situ hybridization. For this purpose we cloned a partial complementary DNA encoding Xenopus unc18 (xunc18) and used a corresponding RNA probe, together with an RNA probe for Xenopus DOC2. At the messenger RNA level DOC2 and xunc18 were found to be expressed throughout the Xenopus brain. All brain nuclei expressing DOC2-messenger RNA showed xunc18-messenger RNA expression as well. Co-expression was shown at the individual cell level in consecutive sections of large-sized neurons. A strong expression was demonstrated in the suprachiasmatic and magnocellular nuclei and in peptidergic endocrine cells in the intermediate and anterior lobes of the pituitary gland, suggesting roles of DOC2 and xunc18 in messenger release from peptidergic secretory systems. Combined in situ hybridization and immunocytochemical analyses show that neuropeptide Y-containing cells in the suprachiasmatic nucleus also express DOC2 and xunc18 messenger RNAs. Since these cells have a high secretory activity, controlling the activity of the pituitary pars intermedia, the levels of expression of DOC2 and xunc18 may be indicators for neuronal secretory activity. The present data represent the first evidence for the co-existence of DOC2 and munc18-1 and suggest co-ordinate action of these proteins at the level of brain nuclei, individual neurons and endocrine cells.

Ca2(+)-dependent interaction of N-copine, a member of the two C2 domain protein family, with OS-9, the product of a gene frequently amplified in osteosarcoma

N-copine is a novel two C2 domain protein that shows Ca2(+)-dependent phospholipid binding and membrane association. By using yeast two-hybrid assays, we identified OS-9 as a protein capable of interacting with N-copine. We further revealed that the second C2 domain of N-copine bound with the carboxy-terminal region of OS-9. Their interaction in vivo was also confirmed by co-immunoprecipitation from 293E cells co-expressing transfected N-copine and OS-9. In vitro binding assays showed that this interaction was Ca2(+)-dependent. By Northern blot analysis, N-copine and OS-9 were co-expressed in the same regions of human brain. These results reveal that OS-9 is a potential target of N-copine.

Structure of the Janus-faced C2B domain of rabphilin

C2 domains are widespread protein modules that often occur as tandem repeats in many membrane-trafficking proteins such as synaptotagmin and rabphilin. The first and second C2 domains (C2A and C2B, respectively) have a high degree of homology but also specific differences. The structure of the C2A domain of synaptotagmin I has been extensively studied but little is known about the C2B domains. We have used NMR spectroscopy to determine the solution structure of the C2B domain of rabphilin. The overall structure of the C2B domain is very similar to that of other C2 domains, with a rigid beta-sandwich core and loops at the top (where Ca2+ binds) and the bottom. Surprisingly, a relatively long alpha-helix is inserted at the bottom of the domain and is conserved in all C2B domains. Our results, together with the Ca(2+)-independent interactions observed for C2B domains, indicate that these domains have a Janus-faced nature, with a Ca(2+)-binding top surface and a Ca(2+)-independent bottom surface.

Localization and subcellular distribution of N-copine in mouse brain

N-Copine is a novel protein with two C2 domains. Its expression is brain specific and up-regulated by neuronal activity such as kainate stimulation and tetanus stimulation evoking hippocampal CA1 long-term potentiation. We examined the localization and subcellular distribution of N-copine in mouse brain. In situ hybridization analysis showed that N-copine mRNA was expressed exclusively in neurons of the hippocampus and in the main and accessory olfactory bulb, where various forms of synaptic plasticity and memory formation are known to occur. In immunohistochemical analyses, N-copine was detected mainly in the cell bodies and dendrites in the neurons, whereas presynaptic proteins such as synaptotagmin I and rab3A were detected in the regions where axons pass through. In fractionation experiments of brain homogenate, N-copine was associated with the membrane fraction in the presence of Ca2+ but not in its absence. As a GST-fusion protein with the second C2 domain of N-copine showed Ca2+-dependent binding to phosphatidylserine, this domain was considered to be responsible for the Ca2+-dependent association of N-copine with the membrane. Thus, N-copine may have a role as a Ca2+ sensor in postsynaptic events, in contrast to the known roles of "double C2 domain-containing proteins," including synaptotagmin I, in presynaptic events.

Interaction of Doc2 with tctex-1, a light chain of cytoplasmic dynein. Implication in dynein-dependent vesicle transport

Doc2 has one Munc13-interacting domain at the N-terminal region and two C2-like domains interacting with Ca2+ and phospholipid at the C-terminal region. Doc2 consists of two isoforms, Doc2alpha and -beta. Doc2alpha is specifically expressed in neuronal cells and implicated in Ca2+-dependent neurotransmitter release, whereas Doc2beta is ubiquitously expressed and its function is unknown. We show here that both Doc2alpha and -beta interact with rat tctex-1, a light chain of cytoplasmic dynein, in both cell-free and intact cell systems. Overexpression of the N-terminal fragment of Doc2 containing the tctex-1-interacting domain induces changes in the intracellular localization of cation-independent mannose 6-phosphate receptor and its ligand, cathepsin D, which are transported from trans-Golgi network to late endosomes. Overexpression of the C-terminal fragment containing two C2-like domains shows the similar effect, but to a lesser extent, whereas overexpression of full-length Doc2 or the C-terminal fragment of rabphilin3 containing two C2-like domains does not show this effect. Because dynein is a minus-end-directed microtubule-based motor protein, these results suggest that Doc2, especially Doc2beta, plays a role in dynein-dependent intracellular vesicle transport.

Role of the Doc2 alpha-Munc13-1 interaction in the neurotransmitter release process

Doc2alpha and Munc13-1 proteins are highly concentrated on synaptic vesicles and the presynaptic plasma membrane, respectively, and have been implicated in Ca2+-dependent neurotransmitter release. Doc2alpha interacts with Munc13-1 through the N-terminal region of Doc2alpha (the Mid domain; amino acid residues 13-37). Here we examine whether the interaction between Doc2alpha and Munc13-1 is required for Ca2+-dependent neurotransmitter release from intact neuron. A synthetic Mid peptide (the Mid peptide), but not a control mutated Mid peptide or a scrambled Mid peptide, inhibited the interaction between Doc2alpha and Munc13-1 in vitro. Introduction of the Mid peptide into presynaptic neurons of cholinergic synapses, formed between rat superior cervical ganglion neurons, reversibly inhibited synaptic transmission evoked by action potentials. In contrast, the control peptides did not inhibit synaptic transmission. This inhibitory effect depended on the presynaptic activity and was affected by extracellular Ca2+ concentrations. The onset of the Mid peptide effect was shortened when the neuron was stimulated at a higher frequency, and the inhibition was more potent at 1 mM Ca2+ than at 5.1 mM Ca2+. These results suggest that the Doc2alpha-Munc13-1 interaction plays a role in a step before the final fusion step of synaptic vesicles with the presynaptic plasma membrane in the evoked neurotransmitter release process.