Molecular Organization and Regulation of the Mammalian Synapse by the Post-Translational Modification SUMOylation

Neurotransmission occurs within highly specialized compartments forming the active synapse where the complex organization and dynamics of the interactions are tightly orchestrated both in time and space. Post-translational modifications (PTMs) are central to these spatiotemporal regulations to ensure an efficient synaptic transmission. SUMOylation is a dynamic PTM that modulates the interactions between proteins and consequently regulates the conformation, the distribution and the trafficking of the SUMO-target proteins. SUMOylation plays a crucial role in synapse formation and stabilization, as well as in the regulation of synaptic transmission and plasticity. In this review, we summarize the molecular consequences of this protein modification in the structural organization and function of the mammalian synapse. We also outline novel activity-dependent regulation and consequences of the SUMO process and explore how this protein modification can functionally participate in the compartmentalization of both pre- and post-synaptic sites.

Unlocking Therapeutic Synergy: Tailoring Drugs for Comorbidities such as Depression and Diabetes through Identical Molecular Targets in Different Cell Types

Research in the field of pharmacology aims to generate new treatments for pathologies. Nowadays, there are an increased number of chronic disorders that severely and durably handicap many patients. Among the most widespread pathologies, obesity, which is often associated with diabetes, is constantly increasing in incidence, and in parallel, neurodegenerative and mood disorders are increasingly affecting many people. For years, these pathologies have been so frequently observed in the population in a concomitant way that they are considered as comorbidities. In fact, common mechanisms are certainly at work in the etiology of these pathologies. The main purpose of this review is to show the value of anticipating the effect of baseline treatment of a condition on its comorbidity in order to obtain concomitant positive actions. One of the implications would be that by understanding and targeting shared molecular mechanisms underlying these conditions, it may be possible to tailor drugs that address both simultaneously. To this end, we firstly remind readers of the close link existing between depression and diabetes and secondly address the potential benefit of the pleiotropic actions of two major active molecules used to treat central and peripheral disorders, first a serotonin reuptake inhibitor (Prozac ®) and then GLP-1R agonists. In the second part, by discussing the therapeutic potential of new experimental antidepressant molecules, we will support the concept that a better understanding of the intracellular signaling pathways targeted by pharmacological agents could lead to future synergistic treatments targeting solely positive effects for comorbidities.

Sortilin-derived peptides promote pancreatic beta-cell survival through CREB signaling pathway

Deterioration of insulin secretion and pancreatic beta-cell mass by inflammatory attacks is one of the main pathophysiological features of type 2 diabetes (T2D). Therefore, preserving beta-cell mass and stimulating insulin secretion only in response to glucose for avoiding the hypoglycemia risks, are the most state-of-the-art option for the treatment of T2D. In this study we tested two correlated hypothesis that 1/ the endogenous peptide released from sortilin, known as PE, that stimulates insulin secretion only in response to glucose, protects beta-cells against death induced by cytokines, and 2/ Spadin and Mini-Spadin, two synthetic peptides derived from PE, that mimic the effects of PE in insulin secretion, also provide beneficial effect on beta-cells survival. We show that PE and its derivatives by inducing a rise of intracellular calcium concentration by depolarizing the membrane protect beta-cells against death induced by Interleukin-1β. Using biochemical, confocal imaging and cell biology techniques, we reveal that the protective effects of PE and its derivatives rely on the activation of the CaM-Kinase pathway, and on the phosphorylation and activation of the transcription factor CREB. In addition, Mini-Spadin promotes beta-cell proliferation, suggesting its possible regenerative effect. This study highlights new possible roles of PE in pancreatic beta-cell survival and its derivatives as pharmacological tools against diabetes.

Therapeutic potential of prenylated stilbenoid macasiamenene F through its anti-inflammatory and cytoprotective effects on LPS-challenged monocytes and microglia

ETHNOPHARMACOLOGICAL RELEVANCE

Macaranga Thou. (Euphorbiaceae) is a large genus that comprises over 300 species distributed between Western Africa and the islands of the South Pacific. Plants of this genus have a long-standing history of use in traditional medicine for different purposes, including the treatment of inflammation. Fresh and dried leaves of certain Macaranga species (e.g. M. tanarius (L.) Müll.Arg.), have been used to treat cuts, bruises, boils, swellings, sores and covering of wounds in general. Several reports described Macaranga spp. being a rich source of polyphenols, such as prenylated stilbenoids and flavonoids, mostly responsible for its biological activity. Similarly, an abundant content of prenylated stilbenes was also described in M. siamensis S.J.Davies, species recently identified (2001) in Thailand. While the respective biological activity of the prenylated stilbenes from M. siamensis was poorly investigated to date, our recent study pointed out the interest as the natural source of several novel anti-inflammatory stilbenoids isolated from this species.

AIM OF THE STUDY

This work investigated the potential anti-inflammatory effects of the stilbenoid macasiamenene F (MF) isolated from M. siamensis S.J.Davies (Euphorbiaceae) on the lipopolysaccharide (LPS)-induced inflammation-like response of monocytes and microglia, major cells involved in the peripheral and central inflammatory response, respectively.

MATERIALS AND METHODS

LPS-induced stimulation of TLR4 signaling led to the activation of inflammatory pathways in in vitro models of THP-1 and THP-1-XBlue™-MD2-CD14 human monocytes, BV-2 mouse microglia, and an ex vivo model of brain-sorted mouse microglia. The ability of the stilbenoid MF to intervene in the IкB/NF-кB and MAPKs/AP-1 inflammatory cascade was investigated. The gene and protein expressions of the pro-inflammatory cytokines IL-1β and TNF-α were evaluated at the transcription and translation levels. The protective effect of MF against LPS-triggered microglial loss was assessed by cell counting and the LDH assay.

RESULTS

MF demonstrated beneficial effects, reducing both monocyte and microglial inflammation as assessed in vitro. It efficiently inhibited the degradation of IкBα, thereby reducing the NF-кB activity and TNF-α expression in human monocytes. Furthermore, the LPS-induced expression of IL-1β and TNF-α in microglia was dampened by pre-, co-, or post-treatment with MF. In addition to its anti-inflammatory effect, MF demonstrated a cytoprotective effect against the LPS-induced death of BV-2 microglia.

CONCLUSION

Our research into anti-inflammatory and protective effects of MF has shown that it is a promising candidate for further in vitro and in vivo investigations of MF interventions with respect to acute and chronic inflammation, including potentially beneficial effects on the inflammatory component of brain diseases such as stroke and Alzheimer's disease.

Bridging the Gap Between Diabetes and Stroke in Search of High Clinical Relevance Therapeutic Targets

Diabetes affects more than 425 million people worldwide, a scale approaching pandemic proportion. Diabetes represents a major risk factor for stroke, and therefore is actively addressed for stroke prevention. However, how diabetes affects stroke severity has not yet been extensively considered, which is surprising given the evident but understudied common mechanistic features of both pathologies. The increase in number of diabetic people, incidence of stroke in the presence of this specific risk factor, and the exacerbation of ischemic brain damage in diabetic conditions (at least in animal models) warrants the need to integrate this comorbidity in preclinical studies of brain ischemia to develop novel therapeutic approaches. Therefore, a better understanding of the commonalties involved in the course of both diseases would offer the promise of discovering novel neuroprotective pathways that would be more appropriated to clinical scenarios. In this article, we will review the relevant mechanisms that have been identified as common traits of both pathologies and that could be, to our knowledge, potential targets in both pathologies.

Sortilin in Glucose Homeostasis: From Accessory Protein to Key Player?

The pharmacological properties and physiological roles of the type I receptor sortilin, also called neurotensin receptor-3, are various and complex. Sortilin is involved in important biological functions from neurotensin and pro-Nerve Growth Factor signaling in the central nervous system to regulation of glucose and lipid homeostasis in the periphery. The peripheral functions of sortilin being less extensively addressed, the focus of the current review is to discuss recent works describing sortilin-induced molecular mechanisms regulating blood glucose homeostasis and insulin signaling. Thus, an overview of several roles ascribed to sortilin in diabetes and other metabolic diseases are presented. Investigations on crucial cellular pathways involved in the protective effect of sortilin receptor on beta cells, including recent discoveries about regulation of cell fate, are also detailed. In addition, we provide a special focus on insulin secretion regulation involving complexes between sortilin and neurotensin receptors. The last section comments on the future research areas which should be developed to address the function of new effectors of the sortilin system in the endocrine apparatus.

Focal Adhesion Kinase-Dependent Role of the Soluble Form of Neurotensin Receptor-3/Sortilin in Colorectal Cancer Cell Dissociation

The aim of the present review is to unravel the mechanisms of action of the soluble form of the neurotensin (NT) receptor-3 (NTSR3), also called Sortilin, in numerous physiopathological processes including cancer development, cardiovascular diseases and depression. Sortilin/NTSR3 is a transmembrane protein thought to exert multiple functions both intracellularly and at the level of the plasma membrane. The Sortilin/NTSR3 extracellular domain is released by shedding from all the cells expressing the protein. Although the existence of the soluble form of Sortilin/NTSR3 (sSortilin/NTSR3) has been evidenced for more than 10 years, the studies focusing on the role of this soluble protein at the mechanistic level remain rare. Numerous cancer cells, including colonic cancer cells, express the receptor family of neurotensin (NT), and particularly Sortilin/NTSR3. This review aims to summarize the functional role of sSortilin/NTSR3 characterized in the colonic cancer cell line HT29. This includes mechanisms involving signaling cascades through focal adhesion kinase (FAK), a key pathway leading to the weakening of cell-cell and cell-extracellular matrix adhesions, a series of events which could be responsible for cancer metastasis. Finally, some future approaches targeting the release of sNTSR3 through the inhibition of matrix metalloproteases (MMPs) are suggested.

Sortilin derived propeptide regulation during adipocyte differentiation and inflammation

In this work, we aimed to correlate the expression of sortilin with the production of sortilin-derived propeptide (PE) during adipocyte differentiation, insulin resistance and inflammation. We also investigated the effect of spadin, a shorter analogue of PE that exerts a potent antidepressant in mice, on adipocyte functions. During adipogenesis, insulin resistance and inflammation, we measured the mRNA and protein expression of sortilin, by quantitative PCR and Western-blot, and quantified the expression of PE by a specific dosing method. We observed that the production of PE was correlated with the sortilin expression during adipogenesis. Immunostaining experiments allowed to visualize the co-localization of sortilin, PE and VAMP2 in 3T3-L1 adipocytes. TNFα treatment induced insulin resistance and a decrease of sortilin expression (mRNA and protein), correlated with the decrease of the PE production. By contrast, treatment with dexamethasone, which also induced insulin resistance, was without effect on sortilin expression and PE production. As a putative bioactive peptide, we have evaluated its autocrine effect by the use of spadin on 3T3-L1 adipocytes by performing glucose uptake and signalling experiments. Any effect was measured on adipocytes indicating that the use of spadin as an antidepressant would have no side effects on adipocyte physiology.

Potentiation of Calcium Influx and Insulin Secretion in Pancreatic Beta Cell by the Specific TREK-1 Blocker Spadin

Inhibition of the potassium channels TREK-1 by spadin (SPA) is currently thought to be a promising therapeutic target for the treatment of depression. Since these channels are expressed in pancreatic β-cells, we investigated their role in the control of insulin secretion and glucose homeostasis. In this study, we confirmed the expression of TREK-1 channels in the insulin secreting MIN6-B1 β-cell line and in mouse islets. We found that their blockade by SPA potentiated insulin secretion induced by potassium chloride dependent membrane depolarization. Inhibition of TREK-1 by SPA induced a decrease of the resting membrane potential (ΔV_m ~ 12 mV) and increased the cytosolic calcium concentration. In mice, administration of SPA enhanced the plasma insulin level stimulated by glucose, confirming its secretagogue effect observed in vitro. Taken together, this work identifies SPA as a novel potential pharmacological agent able to control insulin secretion and glucose homeostasis.

Impairement of HT29 Cancer Cells Cohesion by the Soluble Form of Neurotensin Receptor-3

The neurotensin (NT) receptor-3 (NTSR3), also called sortilin is a multifunctional protein localized at the intracellular and plasma membrane level. The extracellular domain of NTSR3 (sNTSR3) is released by shedding from several cell lines including colonic cancer cells. This soluble protein acts as an active ligand through its ability to bind, to be internalized in the human adenocarcinoma epithelial HT29 cells and to stimulate the PI3 kinase pathway. The aim of this study was to investigate cellular responses induced by sNTSR3 in HT29 cells. The cellular functions of sNTSR3 were monitored by immunofluocytochemistry, electron microscopy and quantitative PCR in order to characterize the cell shape and the expression of adhesion proteins. We evidenced that sNTSR3 significantly regulates the cellular morphology as well as the cell-cell and the cell-matrix adherens properties by decreasing the expession of several integrins and by modifying the structure of desmosomes. Altogether, these properties lead to an increase of cell detachment upon sNTSR3 treatment on HT29, HCT116 and SW620 cancer cells. Our results indicate that sNTSR3 may induce the first phase of a process which weaken HT29 epithelial properties including desmosome architecture, cell spreading, and initiation of cell separation, all events which could be responsible for cancer metastasis.

The anti-apoptotic role of neurotensin

The neuropeptide, neurotensin, exerts numerous biological functions, including an efficient anti-apoptotic role, both in the central nervous system and in the periphery. This review summarizes studies that clearly evidenced the protective effect of neurotensin through its three known receptors. The pivotal involvement of the neurotensin receptor-3, also called sortilin, in the molecular mechanisms of the anti-apoptotic action of neurotensin has been analyzed in neuronal cell death, in cancer cell growth and in pancreatic beta cell protection. The relationships between the anti-apoptotic role of neurotensin and important physiological and pathological contexts are discussed in this review.

Neurotensin and its receptors in the control of glucose homeostasis

The pharmacological roles of the neuropeptide neurotensin through its three known receptors are various and complex. Neurotensin is involved in several important biological functions including analgesia and hypothermia in the central nervous system and also food intake and glucose homeostasis in the periphery. This review focuses on recent works dealing with molecular mechanisms regulating blood glucose level and insulin secretion upon neurotensin action. Investigations on crucial cellular components involved in the protective effect of the peptide on beta cells are also detailed. The role of xenin, a neurotensin-related peptide, on the regulation of insulin release by glucose-dependent insulinotropic polypeptide is summarized. The last section comments on the future research areas which should be developed to address the function of new effectors of the neurotensinergic system in the endocrine pancreas.

Neurotensin-induced Erk1/2 phosphorylation and growth of human colonic cancer cells are independent from growth factors receptors activation

Neurotensin (NT) promotes the proliferation of human colonic cancer cells by undefined mechanisms. We already demonstrated that, in the human colon adenocarcinoma cell line HT29, the effects of NT were mediated by a complex formed between the NT receptor-1 (NTSR1) and-3 (NTSR3). Here we examined cellular mechanisms that led to NT-induced MAP kinase phosphorylation and growth factors receptors transactivation in colonic cancer cells and proliferation in HT29 cells. With the aim to identify upstream signaling involved in NT-elicited MAP kinase activation, we found that the stimulatory effects of the peptide were totally independent from the activation of the epidermal growth factor receptor (EGFR) both in the HT29 and the HCT116 cells. NT was unable to promote phosphorylation of EGFR and to compete with EGF for its binding to the receptor. Pharmacological approaches allowed us to differentiate EGF and NT signaling in HT29 cells since only NT activation of Erk1/2 was shown to be sensitive to PKC inhibitors and since only NT increased the intracellular level of calcium. We also observed that NT was not able to transactivate Insulin-like growth factor receptor. Our findings indicate that, in the HT29 and HCT116 cell lines, NT stimulates MAP kinase phosphorylation and cell growth by a pathway which does not involve EGF system but rather NT receptors which transduce their own intracellular effectors. These results indicate that depending on the cell line used, blocking EGFR is not the general rule to inhibit NT-induced cancer cell proliferation.

ICER induced by hyperglycemia represses the expression of genes essential for insulin exocytosis

The GTPases Rab3a and Rab27a and their effectors Granuphilin/Slp4 and Noc2 are essential regulators of neuroendocrine secretion. Chronic exposure of pancreatic beta-cells to supraphysiological glucose levels decreased selectively the expression of these proteins. This glucotoxic effect was mimicked by cAMP-raising agents and blocked by PKA inhibitors. We demonstrate that the transcriptional repressor ICER, which is induced in a PKA-dependent manner by chronic hyperglycemia and cAMP-raising agents, is responsible for the decline of the four genes. ICER overexpression diminished the level of Granuphilin, Noc2, Rab3a and Rab27a by binding to cAMP responsive elements located in the promoters of these genes and inhibited exocytosis of beta-cells in response to secretagogues. Moreover, the loss in the expression of the genes of the secretory machinery caused by glucose and cAMP-raising agents was prevented by an antisense construct that reduces ICER levels. We propose that induction of inappropriate ICER levels lead to defects in the secretory process of pancreatic beta-cells possibly contributing, in conjunction with other known deleterious effects of hyperglycemia, to defective insulin release in type 2 diabetes.

Functional assays for the investigation of the role of Rab GTPase effectors in dense core granule release

Rab3 and Rab27 GTPases control late events in the secretory pathway of mammalian cells including docking and fusion of secretory vesicles with the plasma membrane. The action of Rab3 and Rab27 on the exocytotic process is exerted through the activation of specific effectors. Several proteins with the capacity to interact in a GTP-dependent manner with Rab3 and Rab27 have been identified. However, for most of these potential Rab effectors a precise function in the secretory process has not yet been attributed. In this chapter we describe a series of approaches that can be applied to assess the properties of Rab3 and Rab27 effectors and their potential role in the regulation of dense core granule release.

Complexin I regulates glucose-induced secretion in pancreatic β-cells

The neuronal-specific protein complexin I (CPX I) plays an important role in controlling the Ca2+-dependent neurotransmitter release. Since insulin exocytosis and neurotransmitter release rely on similar molecular mechanisms and that pancreatic β-cells and neuronal cells share the expression of many restricted genes, we investigated the potential role of CPX I in insulin-secreting cells. We found that pancreatic islets and several insulin-secreting cell lines express high levels of CPX I. The β-cell expression of CPX I is mediated by the presence of a neuron restrictive silencer element located within the regulatory region of the gene. This element bound the transcriptional repressor REST, which is found in most cell types with the exception of mature neuronal cells and β-cells. Overexpression of CPX I or silencing of the CPX I gene (Cplx1) by RNA interference led to strong impairment in β-cell secretion in response to nutrients such as glucose, leucine and KCl. This effect was detected both in the early and the sustained secretory phases but was much more pronounced in the early phase. We conclude that CPX I plays a critical role in β-cells in the control of the stimulated-exocytosis of insulin.

The Rab-binding protein Noc2 is associated with insulin-containing secretory granules and is essential for pancreatic beta-cell exocytosis

The small GTPases Rab3 and Rab27 are associated with secretory granules of pancreatic beta-cells and regulate insulin exocytosis. In this study, we investigated the role of Noc2, a potential partner of these two GTPases, in insulin secretion. In the beta-cell line INS-1E wild-type Noc2, Noc265E, and Noc258A, a mutant capable of interacting with Rab27 but not Rab3, colocalized with insulin-containing vesicles. In contrast, two mutants (Noc2138S,141S and Noc2154A,155A,156A) that bind neither Rab3 nor Rab27 did not associate with secretory granules and were uniformly distributed throughout the cell cytoplasm. Overexpression of wild-type Noc2, Noc265E, or Noc258A inhibited hormone secretion elicited by insulin secretagogues. In contrast, overexpression of the mutants not targeted to secretory granules was without effect. Silencing of the Noc2 gene by RNA interference led to a strong impairment in the capacity of INS-1E cells to respond to insulin secretagogues, indicating that appropriate levels of Noc2 are essential for pancreatic beta-cell exocytosis. The defect was already detectable in the early secretory phase (0-10 min) but was particularly evident during the sustained release phase (10-45 min). Protein-protein binding studies revealed that Noc2 is a potential partner of Munc13, a component of the machinery that controls vesicle priming and insulin exocytosis. These data suggest that Noc2 is involved in the recruitment of secretory granules at the plasma membrane possibly via the interaction with Munc13.

Involvement of the Rab27 binding protein Slac2c/MyRIP in insulin exocytosis

Rab27a is a GTPase associated with insulin-containing secretory granules of pancreatic beta-cells. Selective reduction of Rab27a expression by RNA interference did not alter granule distribution and basal secretion but impaired exocytosis triggered by insulin secretagogues. Screening for potential effectors of the GTPase revealed that the Rab27a-binding protein Slac2c/MyRIP is associated with secretory granules of beta-cells. Attenuation of Slac2c/MyRIP expression by RNA interference did not modify basal secretion but severely impaired hormone release in response to secretagogues. Although beta-cells express Myosin-Va, a potential partner of Slac2c/MyRIP, no functional link between the two proteins could be demonstrated. In fact, overexpression of the Myosin-Va binding domain of Slac2c/MyRIP did not affect granule localization and hormone exocytosis. In contrast, overexpression of the actin-binding domain of Slac2c/MyRIP led to a potent inhibition of exocytosis without detectable alteration in granule distribution. This effect was prevented by point mutations that abolish actin binding. Taken together our data suggest that Rab27a and Slac2c/MyRIP are part of a complex mediating the interaction of secretory granules with cortical actin cytoskeleton and participate to the regulation of the final steps of insulin exocytosis.

Pancreatic beta-cell protein granuphilin binds Rab3 and Munc-18 and controls exocytosis

Granuphilin/Slp-4 is a member of the synaptotagmin-like protein family expressed in pancreatic beta-cells and in the pituitary gland. We show by confocal microscopy that both granuphilin-a and -b colocalize with insulin-containing secretory granules positioned at the periphery of pancreatic beta-cells. Overexpression of granuphilins in insulin-secreting cell lines caused a profound inhibition of stimulus-induced exocytosis. Granuphilins were found to bind to two components of the secretory machinery of pancreatic beta-cells, the small GTP-binding protein Rab3 and the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-binding protein Munc-18. The interaction with Rab3 occurred only with the GTP-bound form of the protein and was prevented by a point mutation in the effector domain of the GTPase. Structure-function studies using granuphilin-b mutants revealed that complete loss of Rab3 binding is associated with a reduction in the capacity to inhibit exocytosis. However, the granuphilin/Rab3 complex alone is not sufficient to mediate the decrease of exocytosis, suggesting the existence of additional binding partners. Taken together, our observations indicate that granuphilins play an important role in pancreatic beta-cell exocytosis. In view of the postulated role of Munc-18 in secretory vesicle docking, our data suggest that granuphilins may also be involved in this process.

The death domain of Rab3 guanine nucleotide exchange protein in GDP/GTP exchange activity in living cells

Rab3 GTPases regulate exocytosis of neurons, endocrine and exocrine cells. In the present paper, we report a system to measure the guanine nucleotide status of Rab3 proteins in living cells. The assay is based on the ability of the Rab3 interacting molecule RIM to extract selectively the GTP-bound form of Rab3. Using this system, we found that approx. 20% of wild-type Rab3A, -B, -C or -D transfected in the insulin-secreting cell line HIT-T15 is in the GTP-bound conformation. The pool of activated Rab3 is decreased under conditions that stimulate exocytosis or by co-expression of the Rab3 GTPase-activating protein. In contrast, co-expression of Mss4 or Rab3-GEP (guanine nucleotide exchange protein) increases by approx. 3-fold the GTP-bound pool of Rab3 isoforms. Rab3-GEP is very similar to MADD, a death domain-containing protein that associates with the type 1 tumour necrosis factor receptor. We observed that the death domain of Rab3-GEP is involved in intramolecular interactions and that deletions or mutations that affect this domain of the protein impair the nucleotide exchange activity towards Rab3. We propose that the death domain of Rab3-GEP acts as a molecular switch and co-ordinates multiple functions of the protein by exchanging its binding partners.

Involvement of Rho GTPases and their effectors in the secretory process of PC12 cells

We investigated the involvement of Rho GTPases in the secretory process of PC12 cells. Overexpression of wild-type RhoA, Rac1, or Cdc42 did affect exocytosis. In contrast, secretion elicited by depolarizing K(+) concentrations was enhanced by the dominant negative mutants RhoA(N19), Rac1(N17), and Cdc42(N17) and was diminished by the constitutively active mutants RhoA(V14), Rac1(V12), and Cdc42(V12). The inhibition observed in the presence of RhoA(V14) was likely a result of the activation of ROK(alpha), since the catalytic domain of this kinase was able to mimic both the reorganization of the actin cytoskeleton and the decrease in exocytosis induced by the RhoA mutant. Part of the effect of Rac1(V12) may be due to POR1 activation. Thus, overexpression of full-length POR1 diminished K(+)-stimulated exocytosis, and a point mutation in the effector domain of Rac1(V12) that prevents the interaction with POR1 abolished the inhibitory effect of the GTPase. We also searched for the Cdc42(V12) target but overexpression of the Cdc42 effector WASP did not mimic the inhibition of exocytosis observed in cells transfected with the activated GTPase. Our findings indicate that different signaling cascades resulting in the activation of RhoA, Rac1, or Cdc42 can modulate the exocytotic process of neuroendocrine cells.

Direct interaction of the Rab3 effector RIM with Ca2+ channels, SNAP-25, and synaptotagmin

To define the role of the Rab3-interacting molecule RIM in exocytosis we searched for additional binding partners of the protein. We found that the two C(2) domains of RIM display properties analogous to those of the C(2)B domain of synaptotagmin-I. Thus, RIM-C(2)A and RIM-C(2)B bind in a Ca(2+)-independent manner to alpha1B, the pore-forming subunit of N-type Ca(2+) channels (EC(50) = approximately 20 nm). They also weakly interact with the alpha1C but not the alpha1D subunit of L-type Ca(2+) channels. In addition, the C(2) domains of RIM associate with SNAP-25 and synaptotagmin-I. The binding affinities for these two proteins are 203 and 24 nm, respectively, for RIM-C(2)A and 224 and 16 nm for RIM-C(2)B. The interactions of the C(2) domains of RIM with SNAP-25 and synaptotagmin-I are modulated by Ca(2+). Thus, in the presence of Ca(2+) (EC(50) = approximately 75 microm) the interaction with synaptotagmin-I is increased, whereas SNAP-25 binding is reduced. Synaptotagmin-I binding is abolished by mutations in two positively charged amino acids in the C(2) domains of RIM and by the addition of inositol polyphosphates. We propose that the Rab3 effector RIM is a scaffold protein that participates through its multiple binding partners in the docking and fusion of secretory vesicles at the release sites.

Rabphilin dissociated from Rab3 promotes endocytosis through interaction with Rabaptin-5

Rabphilin is a secretory vesicle protein that interacts with the GTP-bound form of the small GTPase Rab3. We investigated the involvement of Rabphilin in endocytosis using different point mutants of the protein. Overexpression of wild-type Rabphilin in the insulin-secreting cell line HIT-T15 did not affect receptor-mediated transferrin endocytosis. By contrast, Rabphilin V61A, a mutant that is unable to interact with Rab3, enhanced the rate of transferrin internalization. The effect of Rabphilin V61A was not mimicked by Rabphilin L83A, another mutant with impaired Rab3 binding. Careful analysis of the properties of the two mutants revealed that Rabphilin V61A and Rabphilin L83A are both targeted to secretory vesicles, have stimulatory activity on exocytosis, and bind equally well to (α)-actinin. However, Rabphilin L83A fails to interact with Rabaptin-5, an important component of the endocytotic machinery. These results indicate that Rabphilin promotes receptor-mediated endocytosis and that its action is negatively modulated by Rab3. We propose that the hydrolysis of GTP that is coupled to the exocytotic event disrupts the Rabphilin-Rab3 complex and permits the recruitment of Rabaptin-5 at the fusion site. Our data show that immediately after internalization the transferrin receptor and VAMP-2 colocalize on the same vesicular structures, suggesting that Rabphilin favors the rapid recycling of the components of the secretory vesicle.

Cysteine string proteins are associated with cortical granules of Xenopus laevis oocytes

Cysteine string proteins (csps) are associated with secretory organelles in a wide range of eukaryotic cells. Functional studies of these proteins indicate that they subserve one or more vital steps in the pathway of regulated exocytosis. Here, we document the presence of csps in fully grown (stage VI) oocytes of the frog, Xenopus laevis. Both Northern and immunoblot data support the conclusion that csps are expressed in these cells. In addition, immunoreactive csp is seen even at the earliest stage of oocyte development, namely, in stage I oocytes. Finally, immunoblot and immunocytochemical results indicate that csps are associated with cortical granules of stage II-VI oocytes. These observations suggest that csps participate in the cortical reaction that underlies the sustained block to polyspermy in Xenopus eggs. Moreover, because of the relative ease of manipulating cells as large as Xenopus oocytes, this system harbors considerable promise as a model for studying the role of csps and other proteins in exocytotic events.

VAMP2, but not VAMP3/cellubrevin, mediates insulin-dependent incorporation of GLUT4 into the plasma membrane of L6 myoblasts

Like neuronal synaptic vesicles, intracellular GLUT4-containing vesicles must dock and fuse with the plasma membrane, thereby facilitating insulin-regulated glucose uptake into muscle and fat cells. GLUT4 colocalizes in part with the vesicle SNAREs VAMP2 and VAMP3. In this study, we used a single-cell fluorescence-based assay to compare the functional involvement of VAMP2 and VAMP3 in GLUT4 translocation. Transient transfection of proteolytically active tetanus toxin light chain cleaved both VAMP2 and VAMP3 proteins in L6 myoblasts stably expressing exofacially myc-tagged GLUT4 protein and inhibited insulin-stimulated GLUT4 translocation. Tetanus toxin also caused accumulation of the remaining C-terminal VAMP2 and VAMP3 portions in Golgi elements. This behavior was exclusive to these proteins, because the localization of intracellular myc-tagged GLUT4 protein was not affected by the toxin. Upon cotransfection of tetanus toxin with individual vesicle SNARE constructs, only toxin-resistant VAMP2 rescued the inhibition of insulin-dependent GLUT4 translocation by tetanus toxin. Moreover, insulin caused a cortical actin filament reorganization in which GLUT4 and VAMP2, but not VAMP3, were clustered. We propose that VAMP2 is a resident protein of the insulin-sensitive GLUT4 compartment and that the integrity of this protein is required for GLUT4 vesicle incorporation into the cell surface in response to insulin.

Disruption of Rab3-calmodulin interaction, but not other effector interactions, prevents Rab3 inhibition of exocytosis

Rab GTPases regulate membrane traffic between the cellular compartments of eukaryotic cells. Rab3 is associated with secretory vesicles of neuronal and endocrine cells and controls the Ca(2+)-triggered release of neurotransmitters and hormones. To clarify the mode of action of Rab3 we generated mutants of the GTPase that do not interact efficiently with its putative effectors Rabphilin and RIM. Surprisingly, these mutants transfected in PC12 cells were still capable of inhibiting Ca(2+)-evoked secretion. Rab3 was shown previously to bind to calmodulin in a Ca(2+)-dependent manner. By replacing two arginines conserved between Rab3 isoforms, we generated a mutant with a reduced affinity for calmodulin. This mutant retained the capacity to interact with the Rab3 regulatory proteins, Rabphilin, RIM, Mss4 and RabGDI, and was correctly targeted to dense-core secretory granules. However, replacement of the two arginines abolished the ability of the GTP-bound form of Rab3 to inhibit exocytosis of catecholamine- and insulin-secreting cells. We propose that a Rab3-calmodulin complex generated by elevated Ca(2+) concentrations mediated at least some of the effects of the GTPase and limited the number of exocytotic events that occurred in response to secretory stimuli.

High affinity Rab3 binding is dispensable for Rabphilin-dependent potentiation of stimulated secretion

Rabphilin is a protein that associates with the GTP-bound form of Rab3, a small GTPase that controls a late step in Ca(2+)-triggered exocytosis. Rabphilin is found only in neuroendocrine cells where it co-localises with Rab3A on the secretory vesicle membrane. The Rab3 binding domain (residues 45 to 170), located in the N-terminal part of Rabphilin, includes a cysteine-rich region with two zinc finger motifs that are required for efficient interaction with the small GTPase. To determine whether binding to Rab3A is necessary for the subcellular localisation of Rabphilin, we synthesised point mutants within the Rab3-binding domain. We found that two unique mutations (V61A and L83A) within an amphipathic alpha-helix of this region abolish detectable binding to endogenous Rab3, but only partially impair the targetting of the protein to secretory vesicles in PC12 and pancreatic HIT-T15 cells. Furthermore, both mutants transfected in the HIT-T15 beta cell line stimulate Ca(2+)-regulated exocytosis to the same extent as wild-type Rabphilin. Surprisingly, another Rabphilin mutant, R60A, which possesses a wild-type affinity for Rab3, and targets efficiently to membranes, does not potentiate regulated secretion. High affinity binding to Rab3 is therefore dispensable for the targetting of Rabphilin to secretory vesicles and for the potentiation of Ca(2+)-regulated secretion. The effects of Rabphilin on secretion may be mediated through interaction with another, unknown, factor that recognizes the Rab3 binding domain.

Structure, functional expression, and cerebral localization of the levocabastine-sensitive neurotensin/neuromedin N receptor from mouse brain

This work describes the cloning and expression of the levocabastine-sensitive neurotensin (NT) receptor from mouse brain. The receptor protein comprises 417 amino acids and bears the characteristics of G-protein-coupled receptors. This new NT receptor (NTR) type is 39% homologous to, but pharmacologically distinct from, the only other NTR cloned to date from the rat brain and the human HT29 cell line. When the receptor is expressed in Xenopus laevis oocytes, the H1 antihistaminic drug levocabastine, like NT and neuromedin N, triggers an inward current. The pharmacological properties of this receptor correspond to those of the low-affinity, levocabastine-sensitive NT binding site described initially in membranes prepared from rat and mouse brain. It is expressed maximally in the cerebellum, hippocampus, piriform cortex, and neocortex of adult mouse brain.

Widespread expression of human cysteine string proteins

We present the nucleotide and deduced amino acid sequence of a human systeine string protein (csp) and a unique truncated csp variant that derives from the retention of an exonic sequence that introduces a premature, in-frame stop codon. Low stringency Southern analysis is compatible with the presence of a single human csp gene. Northern analysis reveals that human csp mRNA has both a more heterogeneous size distribution and a more widespread tissue distribution than previously reported for other csps. Exemplifying this is the fact that csp immunoreactivity is detected in Triton X-114 extracts of human blood, an observation which may facilitate blood-based diagnostic assays of csp status in man.

Expression cloning of a protective Leishmania antigen

Parasite-specific CD4+ T cells have been shown to transfer protection against Leishmania major in susceptible BALB/c mice. An epitope-tagged expression library was used to identify the antigen recognized by a protective CD4+ T cell clone. The expression library allowed recombinant proteins made in bacteria to be captured by macrophages for presentation to T cells restricted to major histocompatibility complex class II. A conserved 36-kilodalton member of the tryptophan-aspartic acid repeat family of proteins was identified that was expressed in both stages of the parasite life cycle. A 24-kilodalton portion of this antigen protected susceptible mice when administered as a vaccine with interleukin-12 before infection.

Cloning provides evidence for a family of inward rectifier and G-protein coupled K+ channels in the brain

MbIRK3, mbGIRK2 and mbGIRK3 K+ channels cDNAs have been cloned from adult mouse brain. These cDNAs encode polypeptides of 445, 414 and 376 amino acids, respectively, which display the hallmarks of inward rectifier K+ channels, i.e. two hydrophobic membrane-spanning domains M1 and M2 and a pore-forming domain H5. MbIRK3 shows around 65% amino acid identity with IRK1 and rbIRK2 and only 50% with ROMK1 and GIRK1. On the other hand, mbGIRK2 and mbGIRK3 are more similar to GIRK1 (60%) than to ROMK1 and IRK1 (50%). Northern blot analysis reveals that these three novel clones are mainly expressed in the brain. Xenopus oocytes injected with mbIRK3 and mbGIRK2 cRNAs display inward rectifier K(+)-selective currents very similar to IRK1 and GIRK1, respectively. As expected from the sequence homology, mbGIRK2 cRNA directs the expression of G-protein coupled inward rectifier K+ channels which has been observed through their functional coupling with co-expressed delta-opioid receptors. These results provide the first evidence that the GIRK family, as the IRK family, is composed of multiple genes with members specifically expressed in the nervous system.

Molecular cloning of a murine N-type calcium channel alpha 1 subunit. Evidence for isoforms, brain distribution, and chromosomal localization

A cDNA encoding a N-type Ca2+ channel has been cloned from the murine neuroblastoma cell line N1A103. The open reading frame encodes a protein of 2,289 amino acids (257 kDa). Analysis of different clones provided evidence for the existence of distinct isoforms of N-type channels. High levels of mRNA were found in the pyramidal cell layers CA1, CA2 and CA3 of the hippocampus, in the dentate gyrus, in the cortex layers 2 and 4, in the subiculum and the habenula. The N-type Ca2+ channel gene has been localized on the chromosome 2, band A.

Characterization and partial purification from pheochromocytoma cells of an endogenous equivalent of scyllatoxin, a scorpion toxin which blocks small conductance Ca(2+)-activated K+ channels

This work describes the partial purification of a heat-stable peptide which has the same properties as the scorpion toxin, scyllatoxin, a specific blocker of one class of Ca(2+)-activated K+ channels: (i) it competes with [125I]apamin for binding to the same site, (ii) like apamin and scyllatoxin, it blocks the after-potential hyperpolarization in skeletal muscle cells in culture, (iii) like apamin and scyllatoxin, it contracts guinea-pig taenia coli relaxed by epinephrine, (iv) it cross-reacts with antibodies raised against scyllatoxin but not with antibodies raised against apamin.

[Anesthesia, awareness and wakefulness]

Drugs used in anesthesia do not always produce the expected neurophysiological results. By studying those cases in which the reappearance of intraoperative awareness is most common, an attempt was made to estimate how many and which levels of cerebral function could be identified. Four levels were recognised: awareness with or without memory, and wakefulness with or without dreams. These conditions are related to greater or lesser cortical activity. They can be identified using Tunstall's technique and by carefully interviewing the patient immediately after the operation.

Complete sequence of a cDNA encoding an active rat choline acetyltransferase: a tool to investigate the plasticity of cholinergic phenotype expression

A cDNA clone encoding the complete sequence of an active rat choline acetyltransferase (ChoAcTase; acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6) has been isolated. Analysis of the deduced amino acid sequence reveals 85% and 31% identity with the porcine and Drosophila melanogaster enzymes, respectively. To further elucidate the molecular basis of neurotransmitter-related phenotypic plasticity, the expression of ChoAcTase mRNA was compared with that of tyrosine hydroxylase [TH; tyrosine 3-monooxygenase, L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2], in neurons from superior cervical ganglia grown in the following conditions: 1) normal medium, 2) high K+ medium, and 3) normal medium supplemented with 50% muscle-conditioned medium (CM). TH mRNA was expressed in all three media; its level rose in high K+ and decreased strikingly in the presence of CM. ChoAcTase mRNA could be visualized in CM, but fell to undetectable levels in normal and high K+ media. These results suggest that translational or post-translational mechanisms do not play a major role for the modulation of neurotransmitter-associated phenotype.

The calcium channel antagonists receptor from rabbit skeletal muscle. Reconstitution after purification and subunit characterization

The Ca2+ channel antagonists receptor from rabbit skeletal muscle was purified to homogeneity. Following reconstitution into phosphatidylcholine vesicles, binding experiments with (+)[3H]PN 200-110, (-)[3H]D888 and d-cis-[3H]diltiazem demonstrated that receptor sites for the three most common Ca2+ channel markers copurified with binding stoichiometries close to 1:1:1. Sodium dodecyl sulfate gel analysis of the purified receptor showed that it is composed of only one protein of Mr 170,000 under non-reducing conditions and of two polypeptides of Mr 140,000 and 32,000 under disulfide-reducing conditions. Iodination of the protein of Mr 170,000 and immunoblots experiments with antisera directed against the different components demonstrated that the Ca2+ channel antagonists receptor is a complex of Mr 170,000 composed of a polypeptide chain of Mr 140,000 associated to one polypeptide chain of Mr 32,000 by disulfide bridges. One of the problems concerning this subunit structure of the putative Ca2+ channel was the presence of smaller polypeptide chains of Mr 29,000 and 25,000. Peptide mapping of these polypeptide chains and analysis of their cross-reactivity with sera directed against the proteins of Mr 170,000 and 32,000 demonstrated that they were degradative products of the Mr 32,000 component. Both the large (140 kDa) and the small (32 kDa) component of the putative Ca2+ channel are heavily glycosylated. At least 20-22% of their mass were removed by enzymatic deglycosylation. Finally the possibility that both the 140-kDa and 32-kDa components originate from a single polypeptide chain of Mr 170,000 which is cleaved by proteolysis upon purification is discussed.

Antibodies reveal the cytolocalization and subunit structure of the 1,4-dihydropyridine component of the neuronal Ca2+ channel

Rabbit brain synaptosomes bind the 1,4-dihydropyridine derivative (+)[3H]-PN 200-110 with an equilibrium dissociation constant of 0.04 nM and a maximal binding capacity of 400 fmol/mg of protein. Using polyclonal antibodies raised against the different components of the skeletal muscle 1,4-dihydropyridine receptor, we have demonstrated that the brain and muscle receptors share the same subunit composition comprising a large polypeptide chain of Mr 140,000 associated by disulfide bridges with a smaller peptide of Mr 32,000. These antibodies have been used in immunofluorescence staining of brain sections. They reveal a distribution of the Ca2+ channel protein similar to that of 1,4-dihydropyridine binding sites with (+)[3H]PN 200-110 by the autoradiographic technique.

Immunochemical analysis of subunit structures of 1,4-dihydropyridine receptors associated with voltage-dependent Ca2+ channels in skeletal, cardiac, and smooth muscles

Previous purification studies of the 1,4-dihydropyridine receptor associated with the calcium channel of rabbit skeletal muscle had shown that it is composed of a large glycoprotein of Mr 140,000-145,000 associated with a smaller component of Mr 32,000-34,000. Specific antisera have now been prepared against the larger component (anti-140 serum) and the smaller one (anti-32 serum). The specificity of these two antisera has been analyzed by immunoblot assays with microsomal preparations of rabbit skeletal muscle. Under disulfide-reducing conditions the anti-140 serum specifically labeled a polypeptide of Mr 140,000 while the anti-32 serum labeled three polypeptides of Mr 32,000, 29,000, and 26,000. Under nonreducing conditions both the anti-140 and the anti-32 sera specifically recognized a single large polypeptide of Mr 170,000. The same type of approach showed that the dihydropyridine receptor in cardiac and smooth muscles had a polypeptide composition similar to that found in skeletal muscle with a large polypeptide of Mr 170,000-176,000 made of two different chains of about Mr 140,000 and 34,000-32,000 associated by disulfide bridges.