Beta-adrenergic-induced cytosolic redistribution of Rap1 in rat parotid acini: role in secretion

Upregulation of Amy1 in the salivary glands of mice exposed to a lunar gravity environment using the multiple artificial gravity research system

Introduction: Space is a unique environment characterized by isolation from community life and exposure to circadian misalignment, microgravity, and space radiation. These multiple differences from those experienced on the earth may cause systemic and local tissue stress. Autonomic nerves, including sympathetic and parasympathetic nerves, regulate functions in multiple organs. Saliva is secreted from the salivary gland, which is regulated by autonomic nerves, and plays several important roles in the oral cavity and digestive processes. The balance of the autonomic nervous system in the seromucous glands, such as the submandibular glands, precisely controls serous and mucous saliva. Psychological stress, radiation damage, and other triggers can cause an imbalance in salivary secretion systems. A previous study reported that amylase is a stress marker in behavioral medicine and space flight crews; however, the detailed mechanisms underlying amylase regulation in the space environment are still unknown. Methods: In this study, we aimed to elucidate how lunar gravity (1/6 g) changes mRNA expression patterns in the salivary gland. Using a multiple artificial gravity research system during space flight in the International Space Station, we studied the effects of two different gravitational levels, lunar and Earth gravity, on the submandibular glands of mice. All mice survived, returned to Earth from space, and their submandibular glands were collected 2 days after landing. Results: We found that lunar gravity induced the expression of the salivary amylase gene Amy1; however, no increase in Aqp5 and Ano1, which regulate water secretion, was observed. In addition, genes involved in the exocrine system, such as vesicle-associated membrane protein 8 (Vamp8) and small G proteins, including Rap1 and Rab families, were upregulated under lunar gravity. Conclusion: These results imply that lunar gravity upregulates salivary amylase secretion via Rap/Rab signaling and exocytosis via Vamp8. Our study highlights Amy1 as a potential candidate marker for stress regulation in salivary glands in the lunar gravity environment.

Longitudinal interrogation of sympathetic neural circuits and hemodynamics in preclinical models

Neurological disorders, including spinal cord injury, result in hemodynamic instability due to the disruption of supraspinal projections to the sympathetic circuits located in the spinal cord. We recently developed a preclinical model that allows the identification of the topology and dynamics through which sympathetic circuits modulate hemodynamics, supporting the development of a neuroprosthetic baroreflex that precisely controls blood pressure in rats, monkeys and humans with spinal cord injuries. Here, we describe the continuous monitoring of arterial blood pressure and sympathetic nerve activity over several months in preclinical models of chronic neurological disorders using commercially available telemetry technologies, as well as optogenetic and neuronal tract-tracing procedures specifically adapted to the sympathetic circuitry. Using a blueprint to construct a negative-pressure chamber, the approach enables the reproduction, in rats, of well-controlled and reproducible episodes of hypotension-mimicking orthostatic challenges already used in humans. Blood pressure variations can thus be directly induced and linked to the molecular, functional and anatomical properties of specific neurons in the brainstem, spinal cord and ganglia. Each procedure can be completed in under 2 h, while the construction of the negative-pressure chamber requires up to 1 week. With training, individuals with a basic understanding of cardiovascular physiology, engineering or neuroscience can collect longitudinal recordings of hemodynamics and sympathetic nerve activity over several months.

VIP and muscarinic synergistic mucin secretion by salivary mucous cells is mediated by enhanced PKC activity via VIP-induced release of an intracellular Ca2+ pool

Mucin secretion by salivary mucous glands is mediated predominantly by parasympathetic acetylcholine activation of cholinergic muscarinic receptors via increased intracellular free calcium ([Ca²⁺]_i) and activation of conventional protein kinase C isozymes (cPKC). However, the parasympathetic co-neurotransmitter, vasoactive intestinal peptide (VIP), also initiates secretion, but to a lesser extent. In the present study, cross talk between VIP- and muscarinic-induced mucin secretion was investigated using isolated rat sublingual tubuloacini. VIP-induced secretion is mediated by cAMP-activated protein kinase A (PKA), independently of increased [Ca²⁺]_i. Synergistic secretion between VIP and the muscarinic agonist, carbachol, was demonstrated but only with submaximal carbachol. Carbachol has no effect on cAMP ± VIP. Instead, PKA activated by VIP releases Ca²⁺ from an intracellular pool maintained by the sarco/endoplasmic reticulum Ca²⁺-ATPase pump. Calcium release was independent of phospholipase C activity. The resultant sustained [Ca²⁺]_i increase is additive to submaximal, but not maximal carbachol-induced [Ca²⁺]_i. Synergistic mucin secretion was mimicked by VIP plus either phorbol 12-myristate 13-acetate or 0.01 μM thapsigargin, and blocked by the PKC inhibitor, Gö6976. VIP-induced Ca²⁺ release also promoted store-operated Ca²⁺ entry. Synergism is therefore driven by VIP-mediated [Ca²⁺]_i augmenting cPKC activity to enhance muscarinic mucin secretion. Additional data suggest ryanodine receptors control VIP/PKA-mediated Ca²⁺ release from a Ca²⁺ pool also responsive to maximal carbachol. A working model of muscarinic and VIP control of mucous cell exocrine secretion is presented. Results are discussed in relation to synergistic mechanisms in other secretory cells, and the physiological and therapeutic significance of VIP/muscarinic synergism controlling salivary mucous cell exocrine secretion.

The structure of Rap1 in complex with RIAM reveals specificity determinants and recruitment mechanism

The small GTPase Rap1 induces integrin activation via an inside-out signaling pathway mediated by the Rap1-interacting adaptor molecule (RIAM). Blocking this pathway may suppress tumor metastasis and other diseases that are related to hyperactive integrins. However, the molecular basis for the specific recognition of RIAM by Rap1 remains largely unknown. Herein we present the crystal structure of an active, GTP-bound GTPase domain of Rap1 in complex with the Ras association (RA)-pleckstrin homology (PH) structural module of RIAM at 1.65 Å. The structure reveals that the recognition of RIAM by Rap1 is governed by side-chain interactions. Several side chains are critical in determining specificity of this recognition, particularly the Lys31 residue in Rap1 that is oppositely charged compared with the Glu31/Asp31 residue in other Ras GTPases. Lys31 forms a salt bridge with RIAM residue Glu212, making it the key specificity determinant of the interaction. We also show that disruption of these interactions results in reduction of Rap1:RIAM association, leading to a loss of co-clustering and cell adhesion. Our findings elucidate the molecular mechanism by which RIAM mediates Rap1-induced integrin activation. The crystal structure also offers new insight into the structural basis for the specific recruitment of RA-PH module-containing effector proteins by their small GTPase partners.

Multiple roles for the actin cytoskeleton during regulated exocytosis

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

The Epac-Rap1 signaling pathway controls cAMP-mediated exocytosis of Weibel-Palade bodies in endothelial cells

Endothelial cells contain specialized storage organelles called Weibel-Palade bodies (WPBs) that release their content into the vascular lumen in response to specific agonists that raise intracellular Ca(2+) or cAMP. We have previously shown that cAMP-mediated WPB release is dependent on protein kinase A (PKA) and involves activation of the small GTPase RalA. Here, we have investigated a possible role for another PKA-independent cAMP-mediated signaling pathway in the regulation of WPB exocytosis, namely the guanine nucleotide exchange factor Epac1 and its substrate, the small GTPase Rap1. Epinephrine stimulation of endothelial cells leads to Rap1 activation in a PKA-independent fashion. siRNA-mediated knockdown of Epac1 abolished epinephrine-induced activation of Rap1 and resulted in decreased epinephrine-induced WPB exocytosis. Down-regulation of Rap1 expression and prevention of Rap1 activation through overexpression of Rap1GAP effectively reduced epinephrine- but not thrombin-induced WPB exocytosis. Taken together, these data uncover a new Epac-Rap1-dependent pathway by which endothelial cells can regulate WPB exocytosis in response to agonists that signal through cAMP.

Optimized amplification and single-cell analysis identify GnRH-mediated activation of Rap1b in primary rat gonadotropes

Identifying the early gene program induced by GnRH would help understand how GnRH-activated signaling pathways modulate gonadotrope secretory response. We previously analyzed GnRH-induced early genes in LβT2 cells, however these lack GnRH self-potentiation, a physiological attribute of gonadotropes. To minimize cellular heterogeneity, rat primary pituitary cultures were enriched for gonadotropes by 40-60% using a sedimentation gradient. Given the limited number of gonadotropes, RNA was amplified prior to microarray analysis. Thirty-three genes were up-regulated 40 min after GnRH stimulation. Real-time PCR confirmed regulation of several transcripts including fosB, c-fos, egr-2 and rap1b, a small GTPase and member of the Ras family. GnRH stimulated rap1b gene expression in gonadotropes, measured by a sensitive single cell assay. Immunocytochemistry revealed increased Rap1 protein in GnRH-stimulated gonadotropes. These data establish rap1b as a novel gene rapidly induced by GnRH and a candidate to modulate gonadotropin secretion in rat gonadotropes.

Rap1-Rac1 circuits potentiate platelet activation

OBJECTIVE

The goal of this study was to investigate the potential crosstalk between Rap1 and Rac1, 2 small GTPases central to platelet activation, particularly downstream of the collagen receptor GPVI.

METHODS AND RESULTS

We compared the activation response of platelets with impaired Rap signaling (double knock-out; deficient in both the guanine nucleotide exchange factor, CalDAG-GEFI, and the Gi-coupled receptor for ADP, P2Y12), to that of wild-type platelets treated with a small-molecule Rac inhibitor, EHT 1864 (wild-type /EHT). We found that Rac1 is sequentially activated downstream of Rap1 on stimulation via GPVI. In return, Rac1 provides important feedback for both CalDAG-GEFI- and P2Y12-dependent activation of Rap1. When analyzing platelet responses controlled by Rac1, we observed (1) impaired lamellipodia formation, clot retraction, and granule release in both double knock-out and EHT 1864-treated wild-type platelets; and (2) reduced calcium store release in EHT 1864-treated wild-type but not double knock-out platelets. Consistent with the latter finding, we identified 2 pools of Rac1, one activated immediately downstream of GPVI and 1 activated downstream of Rap1.

CONCLUSIONS

We demonstrate important crosstalk between Rap1 and Rac1 downstream of GPVI. Whereas Rap1 signaling directly controls sustained Rac1 activation, Rac1 affects CalDAG-GEFI- and P2Y12-dependent Rap1 activation via its role in calcium mobilization and granule/ADP release, respectively.

The tumor suppressor RASSF1A is a novel effector of small G protein Rap1A

Rap1A is a small G protein implicated in a spectrum of biological processes such as cell proliferation, adhesion, differentiation, and embryogenesis. The downstream effectors through which Rap1A mediates its diverse effects are largely unknown. Here we show that Rap1A, but not the related small G proteins Rap2 or Ras, binds the tumor suppressor Ras association domain family 1A (RASSF1A) in a manner that is regulated by phosphorylation of RASSF1A. Interaction with Rap1A is shown to influence the effect of RASSF1A on microtubule behavior.

The contribution of AKAP5 in amylase secretion from mouse parotid acini

A-kinase (PKA) anchoring proteins (AKAPs) are essential for targeting type II PKA to specific locales in the cell to control function. In the present study, AKAP5 (formerly AKAP150) and AKAP6 were identified in mouse parotid acini by type II PKA regulatory subunit (RII) overlay assay and Western blot analysis of mouse parotid cellular fractions, and the role of AKAP5 in mouse parotid acinar cell secretion was determined. Mice were euthanized with CO(2). Immunofluorescence staining of acinar cells localized AKAP5 to the basolateral membrane, whereas AKAP6 was associated with the perinuclear region. In functional studies, amylase secretion from acinar cells of AKAP5 mutant [knockout (KO)] mice treated with the beta-adrenergic agonist, isoproterenol, was reduced overall by 30-40% compared with wild-type (WT) mice. In contrast, amylase secretion in response to the adenylyl cyclase (AC) activator, forskolin, and the cAMP-dependent protein kinase (PKA) activator, N(6)-phenyl-cAMP, was not statistically different in acini from WT and AKAP5 KO mice. Treatment of acini with isoproterenol mimicked the effect of the Epac activator, 8-(4-methoxyphenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate (8-pMeOPT-2'-O-Me-cAMP), in stimulating Rap1. However, in contrast to isoproterenol, treatment of acini with 8-pMeOPT-2'-O-Me-cAMP resulted in stimulation of amylase secretion from both AKAP5 KO and WT acinar cells. As a scaffolding protein, AKAP5 was found to coimmunoprecipitate with AC6, but not AC8. Data suggest that isoproterenol-stimulated amylase secretion occurs via both an AKAP5/AC6/PKA complex and a PKA-independent, Epac pathway in mouse parotid acini.

Rap1, a mercenary among the Ras-like GTPases

The small Ras-like GTPase Rap1 is an evolutionary conserved protein that originally gained interest because of its capacity to revert the morphological phenotype of Ras-transformed fibroblasts. Rap1 is regulated by a large number of stimuli that include growth factors and cytokines, but also physical force and osmotic stress. Downstream of Rap1, a plethora of effector molecules has been proposed on the basis of biochemical studies. Here, we present an overview of genetic studies on Rap1 in various model organisms and relate the observed phenotypes to in vitro studies. The picture that emerges is one in which Rap1 is a versatile regulator of morphogenesis, by regulating diverse processes that include establishment of cellular polarity, cell-matrix interactions and cell-cell adhesion. Surprisingly, genetic experiments indicate that in the various model organisms, Rap1 uses distinct effector molecules that impinge upon the actin cytoskeleton and adhesion molecules.

Epac activates the small G proteins Rap1 and Rab3A to achieve exocytosis

Exocytosis of the acrosome (the acrosome reaction) relies on cAMP production, assembly of a proteinaceous fusion machinery, calcium influx from the extracellular medium, and mobilization from inositol 1,4,5-trisphosphate-sensitive intracellular stores. Addition of cAMP to human sperm suspensions bypasses some of these requirements and elicits exocytosis in a protein kinase A- and extracellular calcium-independent manner. The relevant cAMP target is Epac, a guanine nucleotide exchange factor for the small GTPase Rap. We show here that a soluble adenylyl cyclase synthesizes the cAMP required for the acrosome reaction. Epac stimulates the exchange of GDP for GTP on Rap1, upstream of a phospholipase C. The Epac-selective cAMP analogue 8-pCPT-2'-O-Me-cAMP induces a phospholipase C-dependent calcium mobilization in human sperm suspensions. In addition, our studies identify a novel connection between cAMP and Rab3A, a secretory granule-associated protein, revealing that the latter functions downstream of soluble adenylyl cyclase/cAMP/Epac but not of Rap1. Challenging sperm with calcium or 8-pCPT-2'-O-Me-cAMP boosts the exchange of GDP for GTP on Rab3A. Recombinant Epac does not release GDP from Rab3A in vitro, suggesting that the Rab3A-GEF activation by cAMP/Epac in vivo is indirect. We propose that Epac sits at a critical point during the exocytotic cascade after which the pathway splits into two limbs, one that assembles the fusion machinery into place and another that elicits intracellular calcium release.

Rap1b is critical for glycoprotein VI-mediated but not ADP receptor-mediated alpha2beta1 activation

BACKGROUND

The platelet alpha2beta1 integrin functions as both an adhesion and signaling receptor upon exposure to collagen. Recent studies have indicated that alpha2beta1 function can be activated via inside-out signaling, similar to the prototypical platelet integrin alphaIIbbeta3. However, signaling molecules that regulate alpha2beta1 activation in platelets are not well defined. A strong candidate molecule is the small GTPase Rap1b, the dominant platelet isoform of Rap1, which regulates alphaIIbbeta3 activation.

OBJECTIVES

We hypothesized that Rap1b positively regulates alpha2beta1 during agonist-induced platelet activation.

METHODS

To test whether Rap1b activates alpha2beta1 downstream of glycoprotein (GP)VI or other platelet receptors, we stimulated platelets purified from Rap1b-/- or wild-type mice with diverse agonists and measured alpha2beta1 activation using fluorescein isothiocyanate-labeled monomeric collagen. We also examined the role of Rap1b in outside-in signaling pathways by analyzing adhesion and spreading of Rap1b-/- or wild-type platelets on monomeric, immobilized collagen. Finally, we monitored the activation status of related Rap GTPases to detect changes in signaling pathways potentially associated with Rap1b-mediated events.

RESULTS

Rap1b-/- platelets displayed comparable ADP-induced or thrombin-induced alpha2beta1 activation as wild-type platelets, but reduced convulxin-dependent alpha2beta1 activation. Rap1b-/- platelets exhibited increased spreading on immobilized collagen but similar adhesion to immobilized collagen compared to wild-type platelets. Rap1b-/- platelets also showed Rap1a and Rap2 activation upon agonist stimulation, possibly revealing functional compensation among Rap family members.

CONCLUSIONS

Rap1b is required for maximal GPVI-induced but not ADP-induced activation of alpha2beta1 in murine platelets.

Small G proteins as key regulators of pancreatic digestive enzyme secretion

Small GTP-binding (G) proteins act as molecular switches to regulate a number of cellular processes, including vesicular transport. Emerging evidence indicates that small G proteins regulate a number of steps in the secretion of pancreatic acinar cells. Diverse small G proteins have been localized at discrete compartments along the secretory pathway and particularly on the secretory granule. Rab3D, Rab27B, and Rap1 are present on the granule membrane and play a role in the steps leading up to exocytosis. Whether the function of these G proteins is simply to ensure appropriate targeting or if they are involved as regulatory molecules is discussed. Most evidence suggests that Rab3D and Rab27B play a role in tethering the secretory granule to its target membrane. Other Rabs have been identified on the secretory granule that are associated with different steps in the secretory pathway. The Rho family small G proteins RhoA and Rac1 also regulate secretion through remodeling of the actin cytoskeleton. Possible mechanisms for regulation of these G proteins and their effector molecules are considered.

Redistribution of Rab27-specific effector Slac2-c, but not Slp4-a, after isoproterenol-stimulation in rat parotid acinar cells

Small GTPase Rab27 has been implicated in the regulation of different types of membrane trafficking, including melanosome transport and various regulated secretion events. We have previously shown that Rab27 and its effectors, Slac2-c/MyRIP and Slp4-a/granuphilin-a, are involved in the control of isoproterenol (IPR)-induced amylase release from rat parotid acinar cells. The ability of Rab to interact with the specific effectors is important. However, little is known about the fate of these effectors after beta-adrenergic stimulation in parotid acinar cells. The present study investigated changes in intracellular redistribution of Slac2-c and Slp4-a in parotid acinar cells after IPR treatment. Subcellular fractionation studies detected Slac2-c and Slp4-a in the apical plasma membrane (APM) and secretory granules under resting conditions. After 5min of IPR treatment, Slac2-c was rapidly recruited to the luminal site, but after 30 min, the amount of Slac2-c in the APM fraction was reduced by approximately 80% compared to the increased level after 5 min of IPR treatment. Such reductions in Slac2-c are likely caused by the translocation of Slac2-c from the APM to the cytosol. In addition, we found that Slac2-c in the cytosolic fraction, but not other fractions, disappeared in the presence of Ca(2+). Since Slac2-c contains multiple PEST-like sequences (i.e., potential signals for rapid protein degradation), we suggest that Slac2-c is Ca(2+)-dependently proteolyzed in the cytosol after exocytosis. In contrast, intracellular localization and expression levels of Slp4-a in parotid acinar cells were unaltered even after beta-stimulation, indicating completely different fates for the two Rab27 effectors after beta-stimulation.

Rap1 activation plays a regulatory role in pancreatic amylase secretion

Rap1 is a member of the Ras superfamily of small GTP-binding proteins and is localized on pancreatic zymogen granules. The current study was designed to determine whether GTP-Rap1 is involved in the regulation of amylase secretion. Rap1A/B and the two Rap1 guanine nucleotide exchange factors, Epac1 and CalDAG-GEF III, were identified in mouse pancreatic acini. A fraction of both Rap1 and Epac1 colocalized with amylase in zymogen granules, but only Rap1 was integral to the zymogen granule membranes. Stimulation with cholecystokinin (CCK), carbachol, and vasoactive intestinal peptide all induced Rap1 activation, as did calcium ionophore A23187, phorbol ester, forskolin, 8-bromo-cyclic AMP, and the Epac-specific cAMP analog 8-pCPT-2'-O-Me-cAMP. The phospholipase C inhibitor U-73122 abolished carbachol- but not forskolin-induced Rap1 activation. Co-stimulation with carbachol and 8-pCPT-2'-O-Me-cAMP led to an additive effect on Rap1 activation, whereas a synergistic effect was seen on amylase release. Although the protein kinase A inhibitor H-89 abolished forskolin-stimulated CREB phosphorylation, it did not modify forskolin-induced GTP-Rap1 levels, excluding PKA participation. Overexpression of Rap1 GTPase-activating protein, which blocked Rap1 activation, reduced the effect of 8-bromo-cyclic AMP, 8-pCPT-2'-O-Me-cAMP, and vasoactive intestinal peptide on amylase release by 60% and reduced CCK- as well as carbachol-stimulated pancreatic amylase release by 40%. These findings indicate that GTP-Rap1 is required for pancreatic amylase release. Rap1 activation not only mediates the cAMP-evoked response via Epac1 but is also involved in CCK- and carbachol-induced amylase release, with their action most likely mediated by CalDAG-GEF III.

CalDAG-GEFI and protein kinase C represent alternative pathways leading to activation of integrin alphaIIbbeta3 in platelets

Second messenger-mediated inside-out activation of integrin alphaIIbbeta3 is a key step in platelet aggregation. We recently showed strongly impaired but not absent alphaIIbbeta3-mediated aggregation of CalDAG-GEFI-deficient platelets activated with various agonists. Here we further evaluated the roles of CalDAG-GEFI and protein kinase C (PKC) for alphaIIbbeta3 activation in platelets activated with a PAR4 receptor-specific agonist, GYPGKF (PAR4p). Compared with wild-type controls, platelets treated with the PKC inhibitor Ro31-8220 or CalDAG-GEFI-deficient platelets showed a marked defect in aggregation at low (< 1mM PAR4p) but not high PAR4p concentrations. Blocking of PKC function in CalDAG-GEFI-deficient platelets, how-ever, strongly decreased aggregation at all PAR4p concentrations, demonstrating that CalDAG-GEFI and PKC represent separate, but synergizing, pathways important for alphaIIbbeta3 activation. PAR4p-induced aggregation in the absence of CalDAG-GEFI required cosignaling through the Galphai-coupled receptor for ADP, P2Y12. Independent roles for CalDAG-GEFI and PKC/Galphai signaling were also observed for PAR4p-induced activation of the small GTPase Rap1, with CalDAG-GEFI mediating the rapid but reversible activation of this small GTPase. In summary, our study identifies CalDAG-GEFI and PKC as independent pathways leading to Rap1 and alphaIIbbeta3 activation in mouse platelets activated through the PAR4 receptor.

RasGRP3 mediates phorbol ester-induced, protein kinase C-independent exocytosis

Phorbol esters are involved in neurotransmitter release and hormone secretion via activation of protein kinase C (PKC). In addition, it has been recently reported to enhance neurotransmitter release in a PKC-independent manner. However, the exocytotic machinery is not fully clarified. Nowadays members of the RasGRP family are being identified as novel molecules binding to diacylglycerol and calcium, representing a new class of guanine nucleotide exchange factor that activates small GTPases including Ras and Rap1. In the present study, we demonstrated that RasGRP3 is expressed in endocrine tissues and mediates phorbol ester-induced exocytosis. Furthermore, the effects were partially blocked by PKC inhibitor but not mitogen-activated protein kinase kinase inhibitor, although both significantly suppressed the phorbol ester-induced phosphorylation of extracellular signal-regulated kinase 1/2. These results indicate that RasGRP3 is implicated in phorbol ester-induced, PKC-independent exocytosis.

Evidence for the involvement of cAMP-GEF (Epac) pathway in amylase release from the rat parotid gland

Amylase release from the rat parotid gland is mainly mediated in a cAMP-dependent protein kinase (PKA)-dependent manner. In the present study, amylase release mediated in cAMP-dependent and PKA-independent manners was investigated with a cAMP-regulated guanine nucleotide exchange factor (cAMP-GEF: Epac)-selective cAMP analogue, 8CPT-2Me-cAMP. The Epac was localized in the intracellular and the plasma membrane fractions. PKA activation by 8CPT-2Me-cAMP was 100-fold lower than that by cAMP. The amylase release (% of the total) from the intact parotid acinar cells was 16 and 3.6% by isoproterenol (1microM) and 8CPT-2Me-cAMP (200microM), respectively, and that from the saponin-permeabilized cells was 15 and 3% by cAMP (100microM) and 8CTP-2Me-cAMP (10microM), respectively. H-89 inhibited cAMP-induced amylase release, but did not inhibit 8CPT-2Me-cAMP-induced amylase release. These results indicated that amylase release by beta-adrenergic stimulation is mediated through both the cAMP/PKA and cAMP/Epac signal pathways.

Functional alterations of mesenteric vascular bed, vas deferens and intestinal tracts in a rat hindlimb unloading model of microgravity

1. Prolonged bed rest or exposure to microgravity may cause several alterations in autonomic nervous system response (ANSR). 2. Hindlimb unloading (HU) rats were used as an animal model of simulated microgravity to investigate ANSR changes. The experiments were carried out to investigate the effects of simulated microgravity on the autonomic nervous response of the perfused mesenteric vascular bed (MVB), vas deferens and the colon and duodenum from 2-week HU rats. 3. In MVB preparations of HU rats, the frequency-dependent increases in perfusion pressure with perivascular nerve stimulation (PNS; 8-40 Hz) were inhibited, whereas the noradrenaline (NA) concentration-dependent (1-100 microM) perfusion pressure increases were potentiated. The latter most probably reflected up-regulation of alpha-adrenergic receptor function. Relaxant responses of NA-precontracted MVB to PNS (4-30 Hz) or isoprenaline were not different between control and HU preparations, while vasodilation induced by the endothelial agonist ACh was reduced. 4. Transmural stimulation (2-40 Hz) induced frequency-dependent twitches of the vas deferens which were reduced in vas deferens of HU rats, while the sensitivity to NA-induced contraction was significantly increased. 5. In the gastroenteric system of HU rat, direct contractile responses to carbachol or tachykinin as well as relaxant or contractile responses to nervous stimulation appeared unchanged both in the proximal colon rings and in duodenal longitudinal strips. 6. In conclusion, HU treatment affects peripheral tissues in which the main contractile mediators are the adrenergic ones such as resistance vessels and vas deferens, probably by reducing the release of neuromediator. This study validates NA signalling impairment as a widespread process in microgravity, which may most dramatically result in the clinical phenotype of orthostatic intolerance.

Epac: A new cAMP-binding protein in support of glucagon-like peptide-1 receptor-mediated signal transduction in the pancreatic beta-cell

Recently published studies of islet cell function reveal unexpected features of glucagon-like peptide-1 (GLP-1) receptor-mediated signal transduction in the pancreatic beta-cell. Although GLP-1 is established to be a cAMP-elevating agent, these studies demonstrate that protein kinase A (PKA) is not the only cAMP-binding protein by which GLP-1 acts. Instead, an alternative cAMP signaling mechanism has been described, one in which GLP-1 activates cAMP-binding proteins designated as cAMP-regulated guanine nucleotide exchange factors (cAMPGEFs, also known as Epac). Two variants of Epac (Epac1 and Epac2) are expressed in beta-cells, and downregulation of Epac function diminishes stimulatory effects of GLP-1 on beta-cell Ca(2+) signaling and insulin secretion. Of particular note are new reports demonstrating that Epac couples beta-cell cAMP production to the stimulation of fast Ca(2+)-dependent exocytosis. It is also reported that Epac mediates the cAMP-dependent mobilization of Ca(2+) from intracellular Ca(2+) stores. This is a process of Ca(2+)-induced Ca(2+) release (CICR), and it generates an increase of [Ca(2+)](i) that may serve as a direct stimulus for mitochondrial ATP production and secretory granule exocytosis. This article summarizes new findings concerning GLP-1 receptor-mediated signal transduction and seeks to define the relative importance of Epac and PKA to beta-cell stimulus-secretion coupling.

Rap1A and rap1B ras-family proteins are prominently expressed in the nucleus of squamous carcinomas: nuclear translocation of GTP-bound active form

We recently showed that rap1 regulates growth and proliferation in normal keratinocytes, which provoked us to investigate its expression and regulation in malignant cells. Rap1 is variably expressed in whole cell lysates of squamous cell carcinoma (SCC) cell lines. Immunoblot analysis of nuclear and cytosolic fractions and immunohistochemistry revealed that in addition to cytoplasmic expression, SCC cells also exhibit prominent punctate rap1 expression in the nucleus. This unexpected nuclear distribution was confirmed by the evaluation of human oral cancer specimens by immunohistochemistry, which showed both nuclear and cytoplasmic localization. Cytoplasmic rap1 expression was observed mostly in large differentiated cells, whereas nuclear localization was found in morphologically less differentiated cells. Quantitative reverse transcriptase polymerase chain reaction and Northern blot analysis showed that both rap1A and rap1B are expressed in SCC cell lines although rap1B signals are more prominent. Transfection with enhanced GFP-tagged constitutively active and inactive forms of rap1B demonstrated that the active GTP-bound form translocates to the nucleus whereas inactive rap1B(GDP) is retained in the cytoplasm, much of which is in a perinuclear distribution. Furthermore, growth factors induce nuclear translocation of rap1 in oral cancer cells. This novel discovery that active, GTP-bound rap1 translocates to the nucleus makes it only the second of over 100 small GTP-binding proteins to be identified in the nucleus, and the striking prominence of rap1 expression in the nucleus of SCC cells suggests that activated rap1 plays a role in the malignant process.

RAPL, a Rap1-binding molecule that mediates Rap1-induced adhesion through spatial regulation of LFA-1

The small GTPase Rap1 is a potent activator of leukocyte integrin. However, the regulatory mechanism involved is unknown. Here, we identify the Rap1 effector, RAPL, as an essential regulator in this activation. RAPL was enriched in mouse lymphoid tissues and associated with Rap1 after stimulation by the T cell receptor and with chemokine CXCL12. Human RAPL stimulated lymphocyte polarization and the patch-like redistribution of lymphocyte-function-associated antigen 1 (LFA-1) to the leading edge, resulting in enhanced adhesion to intercellular adhesion molecule 1 (ICAM-1). Triggered by activated Rap1, RAPL associated with LFA-1 and rapidly relocated to the leading edge and accumulated at immunological synapses. Thus, RAPL regulates lymphocyte adhesion through the spatial distribution of LFA-1.

Peripheral vascular responses to heat stress after hindlimb suspension

PURPOSE

The purpose of this study was to determine whether hindlimb suspension (which simulates the effects of microgravity) results in impaired hemodynamic responses to heat stress or alterations in mesenteric small artery sympathetic nerve innervation.

METHODS

Over 28 d, 16 male Sprague-Dawley rats were hindlimb-suspended, and 13 control rats were housed in the same type of cage. After the treatment, mean arterial pressure (MAP), colonic temperature (Tcol), and superior mesenteric and iliac artery resistances (using Doppler flowmetry) were measured during heat stress [exposure to 42 degrees C until the endpoint of 80 mm Hg blood pressure was reached (75 +/- 9 min); endpoint Tcore = 43.6 +/- 0.2] while rats were anesthetized (sodium pentobarbital, 50 mg x kg(-1) BW).

RESULTS

Hindlimb-suspended and control rats exhibited similar increases in Tcol, MAP, and superior mesenteric artery resistance, and similar decreases in iliac resistance during heat stress (endpoint was a fall in MAP below 80 mm Hg). Tyrosine hydroxylase immunostaining indicated similar sympathetic nerve innervation in small mesenteric arteries from both groups.

CONCLUSION

Hindlimb suspension does not alter the hemodynamic or thermoregulatory responses to heat stress in the anesthetized rat or mesenteric sympathetic nerve innervation, suggesting that this sympathetic pathway is intact.

Characterization of the gene EPAC2: structure, chromosomal localization, tissue expression, and identification of the liver-specific isoform

The liver-specific protein cAMP-GEFII (also known as Epac2) belongs to a family of cyclic adenosine monophosphate (cAMP) binding proteins having guanine nucleotide exchange factor (GEF) activity (the cAMP-GEF family). Here we clone the gene EPAC2, encoding cAMP-GEFII, from a human liver cDNA library. Human EPAC2 has at least 31 exons and is mapped to human chromosome 2q31. Analyses by primer extension, reverse transcriptase-polymerase chain reaction, and in situ hybridization revealed the presence of three transcription start sites of liver-specific Epac2: two major sites located in exon 10 and a minor site in intron 9. The same translation start site is used in all three transcripts. Liver-specific cAMP-GEFII protein, which lacks the first cAMP-binding domain and the Dishevelled/Egl-10/Pleckstrin domain, was detected at 79 kDa by immunoblot analysis, confirming the presence of the short form of cAMP-GEFII in the liver. Liver-specific cAMP-GEFII also has GEF activity toward Rap1. These results demonstrate the presence of liver-specific cAMP-GEFII. Together with the previous finding that cAMP-GEFII is responsible for cAMP-dependent exocytosis in secretory cells, our study suggests that cAMP-GEFII may have a distinct role in liver.

Loss of tuberin, the tuberous-sclerosis-complex-2 gene product is associated with angiogenesis

BACKGROUND

Tuberous sclerosis complex (TSC) is an autosomal dominantly inherited disorder associated with an alteration of the TSC2 tumor suppressor gene which encodes for the protein product tuberin. The disease is characterized by the development of hamartomas, e.g. cutaneous angiofibromas which consist of vascular cells, interstitial cells, and normal components of the skin. The Eker rat model, an animal model of inherited cancer, has been shown to carry a mutation of TSC2.

METHODS

Immunohistochemical analyses of human angiofibromas were performed using antibodies directed against tuberin and angiogenic growth factors. Proliferation of human dermal microvascular endothelial cells (HDMEC) was determined after incubation with the supernatants of TSC2 (+/+) and TSC2 (-/-) rat embryonic fibroblasts (REF) that were derived from the Eker strain.

RESULTS

Loss of the expression of tuberin was observed in the interstitial cells of 13 of 39 angiofibromas. The expression of tuberin was retained in the vascular cells. In all analyzed angiofibromas, the angiogenic factors bFGF, PD-ECGF, VEGF and angiogenin were detected in the interstitial cells and/or vascular cells. Expression of PDGF-B and TGF-beta1 was weak. Tissue culture supernatants from TSC2 (-/-) REF stimulated the growth of HDMEC significantly more than supernatants from TSC2 (+/+) REF.

CONCLUSION

A functional loss of tuberin may stimulate vascular growth.

Rap1 signalling: adhering to new models

Ras-like GTPases are ubiquitously expressed, evolutionarily conserved molecular switches that couple extracellular signals to various cellular responses. Rap1, the closest relative of Ras, has attracted much attention because of the possibility that it regulates Ras-mediated signalling. Rap1 is activated by extracellular signals through several regulatory proteins, and it might function in diverse processes, ranging from modulation of growth and differentiation to secretion, integrin-mediated cell adhesion and morphogenesis.

A Rho-related GTPase is involved in Ca(2+)-dependent neurotransmitter exocytosis

Rho, Rac, and Cdc42 monomeric GTPases are well known regulators of the actin cytoskeleton and phosphoinositide metabolism and have been implicated in hormone secretion in endocrine cells. Here, we examine their possible implication in Ca(2+)-dependent exocytosis of neurotransmitters. Using subcellular fractionation procedures, we found that RhoA, RhoB, Rac1, and Cdc42 are present in rat brain synaptosomes; however, only Rac1 was associated with highly purified synaptic vesicles. To determine the synaptic function of these GTPases, toxins that impair Rho-related proteins were microinjected into Aplysia neurons. We used lethal toxin from Clostridium sordellii, which inactivates Rac; toxin B from Clostridium difficile, which inactivates Rho, Rac, and Cdc42; and C3 exoenzyme from Clostridium botulinum and cytotoxic necrotizing factor 1 from Escherichia coli, which mainly affect Rho. Analysis of the toxin effects on evoked acetylcholine release revealed that a member of the Rho family, most likely Rac1, was implicated in the control of neurotransmitter release. Strikingly, blockage of acetylcholine release by lethal toxin and toxin B could be completely removed in <1 s by high frequency stimulation of nerve terminals. Further characterization of the inhibitory action produced by lethal toxin suggests that Rac1 protein regulates a late step in Ca(2+)-dependent neuroexocytosis.

Involvement of Rho GTPases in calcium-regulated exocytosis from adrenal chromaffin cells

The Rho GTPase family, including Rho, Rac and Cdc42 proteins, is implicated in various cell functions requiring the reorganization of actin-based structures. In secretory cells, cytoskeletal rearrangements are a prerequisite for exocytosis. We previously described that, in chromaffin cells, the trimeric granule-bound Go protein controls peripheral actin and prevents exocytosis in resting cells through the regulation of RhoA. To provide further insight into the function of Rho proteins in exocytosis, we focus here on their intracellular distribution in chromaffin cells. By confocal immunofluorescence analysis, we found that Rac1 and Cdc42 are exclusively localized in the subplasmalemmal region in both resting and nicotine-stimulated cells. In contrast, RhoA is associated with the membrane of secretory granules. We then investigated the effects of clostridial toxins, which differentially impair the function of Rho GTPases, on the subplasmalemmal actin network and catecholamine secretion. Clostridium difficile toxin B, which inactivates Rho, Rac and Cdc42, markedly altered the distribution of peripheral actin filaments. Neither Clostridium botulinum C3 toxin, which selectively ADP-ribosylates Rho, nor Clostridium sordellii lethal toxin, which inactivates Rac, affected cortical actin, suggesting that Cdc42 plays a specific role in the organization of subplasmalemmal actin. Indeed, toxin B strongly reduced secretagogue-evoked catecholamine release. This effect on secretion was not observed in cells having their actin cytoskeleton depolymerized by cytochalasin E or Clostridium botulinum C2 toxin, suggesting that the inhibition of secretion by toxin B is entirely linked to the disorganization of actin. C. sordellii lethal toxin also inhibited catecholamine secretion, but this effect was not related to the actin cytoskeleton as seen in cells pretreated with cytochalasin E or C2 toxin. In contrast, C3 exoenzyme did not affect secretion. We propose that Cdc42 plays an active role in exocytosis by coupling the actin cytoskeleton to the sequential steps underlying membrane trafficking at the site of exocytosis.