Localization and function of soluble N-ethylmaleimide-sensitive factor attachment protein-25 and vesicle-associated membrane protein-2 in functioning gastric parietal cells

Electrophysiological Techniques on the Study of Endolysosomal Ion Channels

Endolysosomal ion channels are a group of ion channel proteins that are functionally expressed on the membrane of endolysosomal vesicles. The electrophysiological properties of these ion channels in the intracellular organelle membrane cannot be observed using conventional electrophysiological techniques. This section compiles the different electrophysiological techniques utilized in recent years to study endolysosomal ion channels and describes their methodological characteristics, emphasizing the most widely used technique for whole endolysosome recordings to date. This includes the use of different pharmacological tools and genetic tools for the application of patch-clamping techniques for specific stages of endolysosomes, allowing the recording of ion channel activity in different organelles, such as recycling endosomes, early endosomes, late endosomes, and lysosomes. These electrophysiological techniques are not only cutting-edge technologies that help to investigate the biophysical properties of known and unknown intracellular ion channels but also help us to investigate the physiopathological role of these ion channels in the distribution of dynamic vesicles and to identify new therapeutic targets for precision medicine and drug screening.

Regulation of Transporters and Channels by Membrane-Trafficking Complexes in Epithelial Cells

The vectorial secretion and absorption of fluid and solutes by epithelial cells is dependent on the polarized expression of membrane solute transporters and channels at the apical and basolateral membranes. The establishment and maintenance of this polarized expression of transporters and channels are affected by divers protein-trafficking complexes. Moreover, regulation of the magnitude of transport is often under control of physiological stimuli, again through the interaction of transporters and channels with protein-trafficking complexes. This review highlights the value in utilizing transporters and channels as cargo to characterize core trafficking machinery by which epithelial cells establish and maintain their polarized expression, and how this machinery regulates fluid and solute transport in response to physiological stimuli.

MST4 kinase phosphorylates ACAP4 protein to orchestrate apical membrane remodeling during gastric acid secretion

Digestion in the stomach depends on acidification of the lumen. Histamine-elicited acid secretion is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. Our recent study revealed the functional role of PKA-MST4-ezrin signaling axis in histamine-elicited acid secretion. However, it remains uncharacterized how the PKA-MST4-ezrin signaling axis operates the insertion of H,K-ATPases into the apical plasma membranes of gastric parietal cells. Here we show that MST4 phosphorylates ACAP4, an ARF6 GTPase-activating protein, at Thr⁵⁴⁵ Histamine stimulation activates MST4 and promotes MST4 interaction with ACAP4. ACAP4 physically interacts with MST4 and is a cognate substrate of MST4 during parietal cell activation. The phosphorylation site of ACAP4 by MST4 was mapped to Thr⁵⁴⁵ by mass spectrometric analyses. Importantly, phosphorylation of Thr⁵⁴⁵ is essential for acid secretion in parietal cells because either suppression of ACAP4 or overexpression of non-phosphorylatable ACAP4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, persistent overexpression of MST4 phosphorylation-deficient ACAP4 results in inhibition of gastric acid secretion and blockage of tubulovesicle fusion to the apical membranes. Significantly, phosphorylation of Thr⁵⁴⁵ enables ACAP4 to interact with ezrin. Given the location of Thr⁵⁴⁵ between the GTPase-activating protein domain and the first ankyrin repeat, we reason that MST4 phosphorylation elicits a conformational change that enables ezrin-ACAP4 interaction. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ACAP4 signaling cascade to polarized acid secretion in gastric parietal cells.

Cell Polarity Kinase MST4 Cooperates with cAMP-dependent Kinase to Orchestrate Histamine-stimulated Acid Secretion in Gastric Parietal Cells

The digestive function of the stomach depends on acidification of the gastric lumen. Acid secretion into the lumen is triggered by activation of the PKA cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. A coupling protein is ezrin, whose phosphorylation at Ser-66 by PKA is required for parietal cell activation. However, little is known regarding the molecular mechanism(s) by which this signaling pathway operates in gastric acid secretion. Here we show that PKA cooperates with MST4 to orchestrate histamine-elicited acid secretion by phosphorylating ezrin at Ser-66 and Thr-567. Histamine stimulation activates PKA, which phosphorylates MST4 at Thr-178 and then promotes MST4 kinase activity. Interestingly, activated MST4 then phosphorylates ezrin prephosphorylated by PKA. Importantly, MST4 is important for acid secretion in parietal cells because either suppression of MST4 or overexpression of non-phosphorylatable MST4 prevents the apical membrane reorganization and proton pump translocation elicited by histamine stimulation. In addition, overexpressing MST4 phosphorylation-deficient ezrin results in an inhibition of gastric acid secretion. Taken together, these results define a novel molecular mechanism linking the PKA-MST4-ezrin signaling cascade to polarized epithelial secretion in gastric parietal cells.

Rab11a regulates syntaxin 3 localization and microvillus assembly in enterocytes

Rab11a is a key component of the apical recycling endosome that aids in the trafficking of proteins to the luminal surface in polarized epithelial cells. Utilizing conditional Rab11a-knockout specific to intestinal epithelial cells, and human colonic epithelial CaCo2-BBE cells with stable Rab11a knockdown, we examined the molecular and pathological impact of Rab11a deficiency on the establishment of apical cell polarity and microvillus morphogenesis. We demonstrate that loss of Rab11a induced alterations in enterocyte polarity, shortened microvillar length and affected the formation of microvilli along the lateral membranes. Rab11a deficiency in enterocytes altered the apical localization of syntaxin 3. These data affirm the role of Rab11a in apical membrane trafficking and the maintenance of apical microvilli in enterocytes.

Spatial control of proton pump H,K-ATPase docking at the apical membrane by phosphorylation-coupled ezrin-syntaxin 3 interaction

The digestive function of the stomach depends on acidification of the gastric lumen. Acid secretion into the lumen is triggered by activation of a cAMP-dependent protein kinase (PKA) cascade, which ultimately results in the insertion of gastric H,K-ATPases into the apical plasma membranes of parietal cells. A coupling protein is ezrin whose phosphorylation at Ser-66 by PKA is required for parietal cell activation. However, little is known regarding the molecular mechanism(s) by which ezrin operates in gastric acid secretion. Here we show that phosphorylation of Ser-66 induces a conformational change of ezrin that enables its association with syntaxin 3 (Stx3) and provides a spatial cue for H,K-ATPase trafficking. This conformation-dependent association is specific for Stx3, and the binding interface is mapped to the N-terminal region. Biochemical analyses show that inhibition of ezrin phosphorylation at Ser-66 prevents ezrin-Stx3 association and insertion of H,K-ATPase into the apical plasma membrane of parietal cells. Using atomic force microscopic analyses, our study revealed that phosphorylation of Ser-66 induces unfolding of ezrin molecule to allow Stx3 binding to its N terminus. Given the essential role of Stx3 in polarized secretion, our study presents the first evidence in which phosphorylation-induced conformational rearrangement of the ezrin molecule provides a spatial cue for polarized membrane trafficking in epithelial cells.

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Role of membrane traffic in the generation of epithelial cell asymmetry

Epithelial cells have an apical-basolateral axis of polarity, which is required for epithelial functions including barrier formation, vectorial ion transport and sensory perception. Here we review what is known about the sorting signals, machineries and pathways that maintain this asymmetry, and how polarity proteins interface with membrane-trafficking pathways to generate membrane domains de novo. It is becoming apparent that membrane traffic does not simply reinforce polarity, but is critical for the generation of cortical epithelial cell asymmetry.

A role for the Ca2+ channel TRPML1 in gastric acid secretion, based on analysis of knockout mice

BACKGROUND & AIMS

Mutations in TRPML1, a lysosomal Ca(2+)-permeable TRP channel, lead to mucolipidosis type IV, a neurodegenerative lysosomal storage disease. An unusual feature of mucolipidosis type IV is constitutive achlorhydria. We produced Trpml1(-/-) (null) mice to investigate the requirement for this protein in gastric acid secretion.

METHODS

Trpml1-null mice were generated by gene targeting. The expression of Trpml1 and its role in acid secretion by gastric parietal cells were analyzed using biochemical, histologic, and ultrastructural approaches.

RESULTS

Trpml1 is expressed by parietal cells and localizes predominantly to the lysosomes; it was dynamically palmitoylated and dephosphorylated in vivo following histamine stimulation of acid secretion. Trpml1-null mice had significant impairments in basal and histamine-stimulated gastric acid secretion and markedly reduced levels of the gastric proton pump. Histologic and ultrastructural analyses revealed that Trpml1(-/-) parietal cells were enlarged, had multivesicular and multi-lamellated lysosomes, and maintained an abnormal intracellular canalicular membrane. The intralysosomal Ca(2+) content and receptor-mediated Ca(2+) signaling were, however, unaffected in Trpml1(-/-) gastric glands, indicating that Trpml1 does not function in the regulation of lysosomal Ca(2+).

CONCLUSIONS

Loss of Trpml1 causes reduced levels and mislocalization of the gastric proton pump and alters the secretory canaliculi, causing hypochlorhydria and hypergastrinemia. The lysosomal enlargement and defective intracellular canaliculi formation observed in Trpml1(-/-) parietal cells indicate that Trpml1 functions in the formation and trafficking of the tubulovesicles. This study provides direct evidence for the regulation of gastric acid secretion by a TRP channel; TRPML1 is an important protein in parietal cell apical membrane trafficking.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

Localization, trafficking, and significance for acid secretion of parietal cell Kir4.1 and KCNQ1 K+ channels

BACKGROUND & AIMS

K(+) recycling at the apical membrane of gastric parietal cells is a prerequisite for gastric acid secretion. Two K(+) channels are currently being considered for this function, namely KCNQ1 and inwardly rectifying K(+) channels (Kir). This study addresses the subcellular localization, trafficking, and potential functional significance of KCNQ1 and Kir4.1 channels during stimulated acid secretion.

METHODS

The effect of pharmacologic KCNQ1 blockade on acid secretion was studied in cultured rat and rabbit parietal cells and in isolated mouse gastric mucosa. The subcellular localization of KCNQ1 and Kir4.1 was determined in highly purified membrane fractions by Western blot analysis as well as in fixed and living cells by confocal microscopy.

RESULTS

In cultured parietal cells and in isolated gastric mucosa, a robust acid secretory response was seen after complete pharmacologic blockade of KCNQ1. Both biochemical and morphologic data demonstrate that Kir4.1 and KCNQ1 colocalize with the H(+)/K(+)-ATPase but do so in different tubulovesicular pools. All Kir4.1 translocates to the apical membrane after stimulation in contrast to only a fraction of KCNQ1, which mostly remains cytoplasmic.

CONCLUSIONS

Acid secretion can be stimulated after complete pharmacologic blockade of KCNQ1 activity, suggesting that additional apical K(+) channels regulate gastric acid secretion. The close association of Kir4.1 channels with H(+)/K(+)-ATPase in the resting and stimulated membrane suggests a possible role for Kir4.1 channels during the acid secretory cycle.

A mechanism of Munc18b-syntaxin 3-SNAP25 complex assembly in regulated epithelial secretion

Syntaxin and Munc18 are essential for regulated exocytosis in all eukaryotes. It was shown that Munc18 inhibition of neuronal syntaxin 1 can be overcome by CDK5 phosphorylation, indicating that structural change disrupts the syntaxin-Munc18 interaction. Here, we show that this phosphorylation promotes the assembly of Munc18b-syntaxin 3-SNAP25 tripartite complex and membrane fusion machinery SNARE. Using siRNAs to screen for genes required for regulated epithelial secretion, we identified the requirements of CDK5 and Munc18b in cAMP-dependent gastric acid secretion. Biochemical characterization revealed that Munc18b bears a syntaxin 3-selective binding site located at its most C-terminal 53 amino acids. Significantly, the phosphorylation of Thr572 by CDK5 attenuates Munc18b-syntaxin 3 interaction and promotes formation of Munc18b-syntaxin 3-SNAP25 tripartite complex, leading to an assembly of functional Munc18b-syntaxin 3-SNAP25-VAMP2 membrane fusion machinery. Thus, our studies suggest a novel regulatory mechanism in which phosphorylation of Munc18b operates vesicle docking and fusion in regulated exocytosis.

CytLEK1 is a regulator of plasma membrane recycling through its interaction with SNAP-25

SNAP-25 is a component of the SNARE complex that is involved in membrane docking and fusion. Using a yeast two-hybrid screen, we identify a novel interaction between SNAP-25 and cytoplasmic Lek1 (cytLEK1), a protein previously demonstrated to associate with the microtubule network. The binding domains within each protein were defined by yeast two-hybrid, coimmunoprecipitation, and colocalization studies. Confocal analyses reveal a high degree of colocalization between the proteins. In addition, the endogenous proteins can be isolated as a complex by immunoprecipitation. Further analyses demonstrate that cytLEK1 and SNAP-25 colocalize and coprecipitate with Rab11a, myosin Vb, VAMP2, and syntaxin 4, components of the plasma membrane recycling pathway. Overexpression of the SNAP-25-binding domain of cytLEK1, and depletion of endogenous Lek1 alters transferrin trafficking, consistent with a function in vesicle recycling. Taken together, our studies indicate that cytLEK1 is a link between recycling vesicles and the microtubule network through its association with SNAP-25. This interaction may play a key role in the regulation of the recycling endosome pathway.

The KCNE2 potassium channel ancillary subunit is essential for gastric acid secretion

Genes in the KCNE family encode single transmembrane domain ancillary subunits that co-assemble with voltage-gated potassium (Kv) channel alpha subunits to alter their function. KCNE2 (also known as MiRP1) is expressed in the heart, is associated with human cardiac arrhythmia, and modulates cardiac Kv alpha subunits hERG and KCNQ1 in vitro. KCNE2 and KCNQ1 are also expressed in parietal cells, leading to speculation they form a native channel complex there. Here, we disrupted the murine kcne2 gene and found that kcne2 (-/-) mice have a severe gastric phenotype with profoundly reduced parietal cell proton secretion, abnormal parietal cell morphology, achlorhydria, hypergastrinemia, and striking gastric glandular hyperplasia arising from an increase in the number of non-acid secretory cells. KCNQ1 exhibited abnormal distribution in gastric glands from kcne2 (-/-) mice, with increased expression in non-acid secretory cells. Parietal cells from kcne2 (+/-) mice exhibited normal architecture but reduced proton secretion, and kcne2 (+/-) mice were hypochlorhydric, indicating a gene-dose effect and a primary defect in gastric acid secretion. These data demonstrate that KCNE2 is essential for gastric acid secretion, the first genetic evidence that a member of the KCNE gene family is required for normal gastrointestinal function.

Identification of soluble N-ethylmaleimide-sensitive factor attachment protein receptor exocytotic machinery in human plasma cells: SNAP-23 is essential for antibody secretion

Plasma cells (PC) are B-lymphocytes terminally differentiated in a postmitotic state, with the unique purpose of manufacturing and exporting Igs. Despite the importance of this process in the survival of vertebrates, no studies have been made to understand the molecular events that regulate Ig exocytosis by PC. The present study explores the possible presence of the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) system in human PC, and examines its functional role in Ig secretion. Syntaxin-2, Syntaxin-3, Syntaxin-4, vesicle-associated membrane protein (VAMP)-2, VAMP-3, and synaptosome-associated protein (SNAP)-23 could be readily detected in normal human PC obtained from intestinal lamina propria and blood, as well as in human PC lines. Because SNAP-23 plays a central role in SNAREs complex formation, it was chosen to examine possible functional implications of the SNARE system in PC Ig secretion. When recombinant SNAP-23 fusion protein was introduced into the cells, a complete abolishment of Ig production was observed in the culture supernatants of PC lines, as well as in those of normal PC. These results provide insights, for the first time, into the molecular machinery of constitutive vesicular trafficking in human PC Ig secretion and present evidence indicating that at least SNAP-23 is essential for Ab production.

cAMP increases surface expression of NKCC2 in rat thick ascending limbs: role of VAMP

NaCl absorption by the thick ascending limb of Henle's loop (TAL) is mediated by the apical Na-K-2Cl cotransporter NKCC2. cAMP increases NaCl absorption in the TAL by stimulating NKCC2. In oocytes, cAMP increases NKCC2 activity by regulating its trafficking. However, the mechanism by which cAMP stimulates NKCC2 in TALs is not clear. We hypothesized that cAMP increases surface expression of NKCC2 and NaCl absorption in TALs and that vesicle-associated membrane protein (VAMP) is involved in this mechanism. We used surface biotinylation of rat medullary TALs (mTAL) to examine surface and total NKCC2 levels. When mTAL suspensions were treated with dibutyryl cAMP (db-cAMP) or forskolin plus IBMX for 20 min, surface NKCC2 expression increased by 126 +/- 23 and 92 +/- 17% above basal, respectively (P < 0.03). No changes in total NKCC2 expression were observed, suggesting that cAMP increased translocation of NKCC2. We studied the role of VAMP in NKCC2 translocation and found that incubating mTALs with tetanus toxin (30 nM), which inhibits vesicle trafficking by inactivating VAMP-2 and -3, completely blocked the stimulatory effect of db-cAMP on surface NKCC2 expression (tetanus toxin = 100% vs. tetanus toxin + db-cAMP = 102 +/- 21% of control; not significant). We studied VAMP-2 and -3 expression and localization in isolated perfused TALs by confocal microscopy and found that both of them were located in the subapical space of the TAL. Finally, in isolated perfused mTALs, db-cAMP increased net Cl absorption by 95.0 +/- 34.8% (P < 0.03), and pretreatment of TALs with tetanus toxin blocked the stimulation of Cl absorption (from 110.9 +/- 15.9 to 109.7 +/- 15.6 pmol.min(-1).mm(-1); not significant). We concluded that cAMP increases NKCC2 surface expression by a mechanism involving VAMP and that NKCC2 trafficking to the apical membrane is involved in the stimulation of TAL NaCl absorption by cAMP.

Cellular Localization and Stimulation-Associated Distribution Dynamics of Syntaxin-1 and Syntaxin-3 in Gastric Parietal Cells

Syntaxins are differentially localized in polarized cells and play an important role in vesicle trafficking and membrane fusion. These soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are believed to be involved in tubulovesicle trafficking and membrane fusion during the secretory cycle of the gastric parietal cell. We examined the cellular localization and distribution of syntaxin-1 and syntaxin-3 in rabbit parietal cells. Fractionation of gastric epithelial cell membranes showed that syntaxin-1 was more abundant in a fraction enriched in apical plasma membranes, whereas syntaxin-3 was found predominantly in the H,K-ATPase-rich tubulovesicle fraction. We also examined the cellular localization of syntaxins in cultured parietal cells. Parietal cells were infected with CFP-syntaxin-1 and CFP-syntaxin-3 adenoviral constructs. Fluorescence microscopy of live and fixed cells demonstrated that syntaxin-1 was primarily on the apical membrane vacuoles of infected cells, but there was also the expression of syntaxin-1 in a subadjacent cytoplasmic compartment. In resting, non-secreting parietal cells, syntaxin-3 was distributed throughout the cytoplasmic compartment; after stimulation, syntaxin-3 translocated to the apical membrane vacuoles, there co-localizing with H,K-ATPase, syntaxin-1 and F-actin. The differential location of these syntaxin isoforms in gastric parietal cells suggests that these proteins may be critical for maintaining membrane compartment identity and that they may play important, but somewhat different, roles in the membrane recruitment processes associated with secretory activation.

Osmotic regulation of renal betaine transport: transcription and beyond

Cells in the kidney inner medulla are routinely exposed to high extracellular osmolarity during normal operation of the urinary concentrating mechanism. One adaptation critical for survival in this environment is the intracellular accumulation of organic osmolytes to balance the osmotic stress. Betaine is an important osmolyte that is accumulated via the betaine/gamma-aminobutyric acid transporter (BGT1) in the basolateral plasma membrane of medullary epithelial cells. In response to hypertonic stress, there is transcriptional activation of the BGT1 gene, followed by trafficking and membrane insertion of BGT1 protein. Transcriptional activation, triggered by changes in ionic strength and water content, is an early response that is a key regulatory step and has been studied in detail. Recent studies suggest there are additional post-transcriptional regulatory steps in the pathway leading to upregulation of BGT1 transport, and that additional proteins are required for membrane insertion. Reversal of this adaptive process, upon removal of hypertonic stress, involves a rapid efflux of betaine through specific release pathways, a reduction in betaine influx, and a slower downregulation of BGT1 protein abundance. There is much more to be learned about many of these steps in BGT1 regulation.

The mitochondrial uncoupling-protein homologues

Uncoupling protein(UCP)1 is an integral membrane protein that is located in the mitochondrial inner membrane of brown adipocytes. Its physiological role is to mediate a regulated, thermogenic proton leak. UCP2 and UCP3 are recently identified UCP1 homologues. They also mediate regulated proton leak, and might function to control the production of superoxide and other downstream reactive oxygen species. However, their role in normal physiology remains unknown. Recent studies have shown that UCP2 has an important part in the pathogenesis of type-2 diabetes. The obscure roles of the UCP homologues in normal physiology, together with their emerging role in pathophysiology, provide exciting potential for further investigation.

Agonist-induced polarized trafficking and surface expression of the adenosine 2b receptor in intestinal epithelial cells: role of SNARE proteins

Adenosine, acting through the A2b receptor, induces vectorial chloride and IL-6 secretion in intestinal epithelia and may play an important role in intestinal inflammation. We have previously shown that apical or basolateral adenosine receptor stimulation results in the recruitment of the A2b receptor to the plasma membrane. In this study, we examined domain specificity of recruitment and the role of soluble N-ethylmaleimide (NEM) attachment receptor (SNARE) proteins in the agonist-mediated recruitment of the A2b receptor to the membrane. The colonic epithelial cell line T84 was used because it only expresses the A2b-subtype adenosine receptor. Cell fractionation, biotinylation, and electron microscopic studies showed that the A2b receptor is intracellular at rest and that apical or basolateral adenosine stimulation resulted in the recruitment of the receptor to the apical membrane. Upon agonist stimulation, the A2b receptor is enriched in the vesicle fraction containing vesicle-associated membrane protein (VAMP)-2. Furthermore, in cells stimulated with apical or basolateral adenosine, we demonstrate a complex consisting of VAMP-2, soluble NEM-sensitive factor attachment protein (SNAP)-23, and A2b receptor that is coimmunoprecipitated in cells stimulated with adenosine within 5 min and is no longer detected within 15 min. Inhibition of trafficking with NEM or nocodazole inhibits cAMP synthesis induced by apical or basolateral adenosine by 98 and 90%, respectively. cAMP synthesis induced by foskolin was not affected, suggesting that generalized signaling is not affected under these conditions. Collectively, our data suggest that 1) the A2b receptor is intracellular at rest; 2) apical or basolateral agonist stimulation induces recruitment of the A2b receptor to the apical membrane; 3) the SNARE proteins, VAMP-2 and SNAP-23, participate in the recruitment of the A2b receptor; and 4) the SNARE-mediated recruitment of the A2b receptor may be required for its signaling.

Membrane fusion correlates with surface charge in exocytic vesicles

Stimulation of gastric parietal cells results in exocytic recruitment of the proton pump (H(+),K(+)-ATPase) from a pool of intracellular membranes (tubulovesicles) to the apical plasma membrane. We have previously reconstituted a step in this process, the homotypic fusion of tubulovesicles, and shown that they also fuse with liposomes in a protein-dependent manner [Duman, J. G., Singh, G., Lee, G. Y., Machen, T. E., and Forte, J. G. (2002) Traffic 3, 203-17]. Further, the lipid composition of the liposomes affects their ability to undergo fusion with tubulovesicles. In the present study, we investigated the lipid requirements for tubulovesicular membrane fusion using a fluorescent probe relaxation assay as well as transfer of protein between tubulovesicles and liposomes of defined composition. Initially, we tested the ability of tubulovesicles to undergo fusion with a panel of synthetic phosphatidylcholine-based liposomes containing a variety of common membrane lipids of various shapes and charges. We found that anionic lipids such as phosphatidylserine, phosphatidic acid, and phosphoinositides were best able to enhance tubulovesicle-liposome fusion and that they did it in a dose-dependent, apparently saturable manner. Next, we altered the lipid compositions of actual tubulovesicles and observed that addition of anionic lipids was able to enhance tubulovesicle-tubulovesicle fusion in vitro; thus, we hypothesized that the charge imparted by the lipids, per se, was responsible for the enhancement of membrane fusion. Accordingly, addition of negative charges to one of two pools of tubulovesicles in a fusion assay using anionic detergents increased membrane fusion; whereas, addition of positively charged cationic detergent decreased membrane fusion and could be used to back-titrate the anionic effects. Surprisingly, when both pools of fusing membranes were loaded with anionic detergents, fusion was markedly increased. The ability of anionic charges to enhance fusion was diminished as the ionic strength of the fusion medium was increased, suggesting that the mechanism of fusion enhancement depends on the surface charge of the membranes. Finally, the fusion reaction was highly dependent on temperature, and anionic charge appears to lower the activation energy of the fusion reaction. Taken together, these data suggest that (1) tubulovesicular fusion is enhanced by an increase in membrane surface negative charge associated with a lower activation energy and (2) neutralization or reversal of the surface charge prevents tubulovesicular fusion.

What is the role of SNARE proteins in membrane fusion?

Soluble N-ethylmaleimide-sensitive factor activating protein receptor (SNARE) proteins have been at the fore-front of research on biological membrane fusion for some time. The subcellular localization of SNAREs and their ability to form the so-called SNARE complex may be integral to determining the specificity of intracellular fusion (the SNARE hypothesis) and/or serving as the minimal fusion machinery. Both the SNARE hypothesis and the idea of the minimal fusion machinery have been challenged by a number of experimental observations in various model systems, suggesting that SNAREs may have other functions. Considering recent advances in the SNARE literature, it appears that SNAREs may actually function as part of a complex fusion "machine." Their role in the machinery could be any one or a combination of roles, including establishing tight membrane contact, formation of a scaffolding on which to build the machine, binding of lipid surfaces, and many others. It is also possible that complexations other than the classic SNARE complex participate in membrane fusion.