Distinct effect of actin cytoskeleton disassembly on exo- and endocytic events in a membrane patch of rat melanotrophs

Multiple Roles of Actin in Exo- and Endocytosis

Cytoskeletal filamentous actin (F-actin) has long been considered a molecule that may regulate exo- and endocytosis. However, its exact roles remained elusive. Recent studies shed new light on many crucial roles of F-actin in regulating exo- and endocytosis. Here, this progress is reviewed from studies of secretory cells, particularly neurons and endocrine cells. These studies reveal that F-actin is involved in mediating all kinetically distinguishable forms of endocytosis, including ultrafast, fast, slow, bulk, and overshoot endocytosis, likely via membrane pit formation. F-actin promotes vesicle replenishment to the readily releasable pool most likely via active zone clearance, which may sustain synaptic transmission and overcome short-term depression of synaptic transmission during repetitive firing. By enhancing plasma membrane tension, F-actin promotes fusion pore expansion, vesicular content release, and a fusion mode called shrink fusion involving fusing vesicle shrinking. Not only F-actin, but also the F-actin assembly pathway, including ATP hydrolysis, N-WASH, and formin, are involved in mediating these roles of exo- and endocytosis. Neurological disorders, including spinocerebellar ataxia 13 caused by Kv3.3 channel mutation, may involve impairment of F-actin and its assembly pathway, leading in turn to impairment of exo- and endocytosis at synapses that may contribute to neurological disorders.

Exocytotic fusion pore under stress

Exocytosis is a universal process of eukaryotic cells, consisting of fusion between the vesicle and the plasma membranes, leading to the formation of a fusion pore, a channel through which vesicle cargo exits into the extracellular space. In 1986, Rand and Parsegian proposed several stages to explain the nature of membrane fusion. Following stimulation, it starts with focused stress destabilization of membranes in contact, followed by the coalescence of two membrane surfaces. In the next fraction of a millisecond, restabilization of fused membranes is considered to occur to maintain the cell's integrity. This view predicted that once a fusion pore is formed, it must widen abruptly, irreversibly and fully, whereby the vesicle membrane completely integrates with and collapses into the plasma membrane (full fusion exocytosis). However, recent experimental evidence has revealed that once the fusion pore opens, it may also reversibly close (transient or kiss-and-run exocytosis). Here, we present a historical perspective on understanding the mechanisms that initiate the membrane merger and fusion pore formation. Next, post-fusion mechanisms that regulate fusion pore stability are considered, reflecting the state in which the forces of widening and constriction of fusion pores are balanced. Although the mechanisms generating these forces are unclear, they may involve lipids and proteins, including SNAREs, which play a role not only in the pre-fusion but also post-fusion stages of exocytosis. How molecules stabilize the fusion pore in the open state is key for a better understanding of fusion pore physiology in health and disease.

Nestin affects fusion pore dynamics in mouse astrocytes

AIM

Astrocytes play a homeostatic role in the central nervous system and influence numerous aspects of neurophysiology via intracellular trafficking of vesicles. Intermediate filaments (IFs), also known as nanofilaments, regulate a number of cellular processes including organelle trafficking and adult hippocampal neurogenesis. We have recently demonstrated that the IF protein nestin, a marker of neural stem cells and immature and reactive astrocytes, is also expressed in some astrocytes in the unchallenged hippocampus and regulates neurogenesis through Notch signalling from astrocytes to neural stem cells, possibly via altered trafficking of vesicles containing the Notch ligand Jagged-1.

METHODS

We thus investigated whether nestin affects vesicle dynamics in astrocytes by examining single vesicle interactions with the plasmalemma and vesicle trafficking with high-resolution cell-attached membrane capacitance measurements and confocal microscopy. We used cell cultures of astrocytes from nestin-deficient (Nes^-/- ) and wild-type (wt) mice, and fluorescent dextran and Fluo-2 to examine vesicle mobility and intracellular Ca²⁺ concentration respectively.

RESULTS

Nes^-/- astrocytes exhibited altered sizes of vesicles undergoing full fission and transient fusion, altered vesicle fusion pore geometry and kinetics, decreased spontaneous vesicle mobility and altered ATP-evoked mobility. Purinergic stimulation evoked Ca²⁺ signalling that was slightly attenuated in Nes^-/- astrocytes, which exhibited more oscillatory Ca²⁺ responses than wt astrocytes.

CONCLUSION

These results demonstrate at the single vesicle level that nestin regulates vesicle interactions with the plasmalemma and vesicle trafficking, indicating its potential role in astrocyte vesicle-based communication.

Fascin expression in cholesteatoma: correlation with destruction of the ossicular chain and extent of disease

OBJECTIVE

Fascin is an actin-bundling protein found in cell membrane protrusions and increases cell motility. The expression of fascin in epithelial neoplasms has been described only recently. No data are available concerning the role of this protein in invasive cholesteatoma. Thus, we investigated the expression of fascin in cholesteatoma tissue and the relationship between fascin expression and intraoperative evaluation of the destruction of the ossicular chain and extent of disease.

METHOD

Cholesteatoma specimens of 28 patients and external auditory canal (EAC) skin specimens of the same patients (as the control group) were collected from mastoidectomies. Immunohistochemical technique was used to investigate the fascin expression in all cholesteatoma tissues and EAC skin specimens. Immunohistochemical staining was assessed semiquantitatively based on the thickness of epithelium. SPSS software version 16.0 (SPSS Inc., Chicago, IL, USA) was performed to statistically analyse the relationships between fascin expression and intraoperative evaluation destruction of ossicular chain and extent of the disease.

RESULTS

Immunohistochemically, there was no or very low fascin expression observed in normal epithelial cells of EAC skin, while expressed in cholesteatoma tissue. Also, fascin expression in cholesteatoma tissues was significantly correlated with destruction of ossicular chain and extent of the disease.

CONCLUSIONS

Fascin expression is usually found in cholesteatoma epithelium and is correlated with destruction of the ossicular chain and extent of disease. Considering all of the correlations between the clinical and histopathological findings, 'fascin immunoexpression scoring' may be used for histological grading of cholesteatoma.

The platelet actin cytoskeleton associates with SNAREs and participates in alpha-granule secretion

Following platelet activation, platelets undergo a dramatic shape change mediated by the actin cytoskeleton and accompanied by secretion of granule contents. While the actin cytoskeleton is thought to influence platelet granule secretion, the mechanism for this putative regulation is not known. We found that disruption of the actin cytoskeleton by latrunculin A inhibited alpha-granule secretion induced by several different platelet agonists without significantly affecting activation-induced platelet aggregation. In a cell-free secretory system, platelet cytosol was required for alpha-granule secretion. Inhibition of actin polymerization prevented alpha-granule secretion in this system, and purified platelet actin could substitute for platelet cytosol to support alpha-granule secretion. To determine whether SNAREs physically associate with the actin cytoskeleton, we isolated the Triton X-100 insoluble actin cytoskeleton from platelets. VAMP-8 and syntaxin-2 associated only with actin cytoskeletons of activated platelets. Syntaxin-4 and SNAP-23 associated with cytoskeletons isolated from either resting or activated platelets. When syntaxin-4 and SNAP-23 were tested for actin binding in a purified protein system, only syntaxin-4 associated directly with polymerized platelet actin. These data show that the platelet cytoskeleton interacts with select SNAREs and that actin polymerization facilitates alpha-granule release.

Regulated exocytosis and vesicle trafficking in astrocytes

Astrocytes are increasingly viewed as crucial cells supporting and integrating brain functions. It is thought that the release of gliotransmitters into the extracellular space by regulated exocytosis supports a significant part of communication between astrocytes and neurons. Prior to exocytosis, the membrane-bound vesicles are transported through the astrocyte cytoplasm. Our recent studies have revealed new insights into vesicle trafficking in the cytoplasm of astrocytes and are reviewed in this article. The prefusion mobility of fluorescently labeled peptidergic vesicles was studied in cultured rat and mouse astrocytes. Vesicle delivery to the plasma membrane involved an interaction with the cytoskeleton, in particular with microtubules and actin filaments. Interestingly, vesicle mobility in mouse astrocytes deficient in intermediate filaments show impaired directionality of peptidergic vesicle mobility. To explore whether stimuli that increase the concentration of free calcium ions in the cytoplasm triggered vesicular ATP release from astrocytes, human embryonic kidney-293T cells transfected with a P2X(3) receptor were used as sniffers to detect ATP release. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small, transient, inward currents in sniffer cells, reminiscent of postsynaptic quantal events observed at synapses. Some of the membrane-bound vesicles are retrieved from the plasma membrane to be recycled back into the cytosol. Trafficking velocity of postfusion (recycling) atrial natriuretic peptide vesicles was one order of magnitude slower in comparison to the mobility of prefusion vesicles. However, transport of all vesicle types studied required an intact cytoskeleton.

Low-noise recording of single-vesicle capacitance steps in cell-attached patches

Capacitance recording provides a readout of membrane area that can be used to monitor exo- and endocytosis in neurons and secretory cells in real time. By interfacing a lock-in amplifier to a patch-clamp amplifier, the capacitance of cell-attached membrane patches can be measured with sufficient sensitivity to reveal the fusion and retrieval of single vesicles as unitary stepwise changes in capacitance. The small size of many secretory vesicles, especially of synaptic vesicles, places a premium on the reduction of noise in a capacitance recording. With care, the capacitance noise in cell-attached patches can be reduced to below 10 aF root-mean-square (rms), thus bringing into view steps resulting from the fusion of vesicles as small as about 18 nm in diameter. Thus, the lowest achievable noise level enables the resolution of changes in capacitance associated with the smallest secretory vesicles. This chapter presents the method of capacitance recording from cell-attached patches with an emphasis on noise reduction. It also addresses the closely related issue of extracting fusion pore properties from these recordings.

Stimulation inhibits the mobility of recycling peptidergic vesicles in astrocytes

Astrocytes are increasingly viewed as playing many roles in the integration of brain function. These cells store among other gliotransmitters also neuroactive peptides in membrane bound vesicles, the trafficking and release of which, may be changed in altered conditions, therefore affecting the physiological status of neurons. In general, peptidergic membrane-bound secretory vesicles fuse with the plasma membrane in the process of exocytosis. Some of them are retrieved from the plasma membrane to be recycled back into the cytosol. The mobility of retrieving vesicles in astrocytes was not studied yet, however, understanding the mechanisms of such trafficking would highlight the communication paths between astrocytes and neurons. We labeled vesicles with antibodies against the vesicle atrial natriuretic peptide (ANP), which is stored inside secretory vesicles. ANP-vesicles in astrocytes have been proposed to enter Ca2+-dependent secretion and here we show that they are associated with synaptotagmin IV (SytIV), a regulator of exocytosis in astrocytes. Moreover, the results show that recycling ANP-vesicles are to a significant extent acidic. Their velocity (0.06+/-0.001 microm/s) is one order of magnitude lower than the velocity of vesicles trafficking to the plasma membrane (Potokar et al. (2005) Biochem Biophys Res Commun 329:678-683; Potokar et al. (2007) Traffic 8:12-20). Interestingly, ionomycin or ATP application further attenuated ANP-vesicle mobility to 0.02+/-0.002 and to 0.03+/-0.001 microm/s, respectively. In summary, the mobility of recycling peptidergic vesicles appears to be slower than the vesicle traffic to the plasma membrane and it requires an intact cytoskeleton. Physiological implications of attenuated traffic of ANP-vesicles are considered in the discussion.

The Ins and Outs of Secretion from Pancreatic β-Cells: Control of Single-Vesicle Exo- and Endocytosis

Exocytosis of insulin-containing secretory vesicles in pancreatic β-cells is crucial to maintenance of plasma glucose levels. They fuse with the plasma membrane in a regulated manner to release their contents and are subsequently recaptured either intact or through conventional clathrin-mediated endocytosis. Here, we discuss these mechanisms in β-cells at the single-vesicle level.

Calcium increases endocytotic vesicle size and accelerates membrane fission in insulin-secreting INS-1 cells

In many cells, endocytotic membrane retrieval is accelerated by Ca2+. The effect of Ca2+ on single endocytotic vesicles and fission pore kinetics was examined by measuring capacitance and conductance changes in small membrane patches of insulin-secreting INS-1 cells. In intact cells, elevation of Ca2+ by glucose stimulation induced a 1.8-fold increase in membrane internalisation. This surprisingly resulted from an increased unitary capacitance of endocytotic vesicles whereas the frequency of endocytosis was unaltered. This effect of glucose was prevented by inhibition of L- or R-type Ca2+ channels. Extracellular (pipette) Ca2+ was found to regulate endocytotic vesicle capacitance in a bimodal manner. Vesicle capacitance was increased at intermediate Ca2+ (2.6 mM), but not at high Ca2+ (10 mM). Similar results were obtained upon direct application of 100 nM and 0.5 mM Ca2+ to the intracellular surface of inside-out excised membrane patches, and in these experiments the increase in vesicle capacitance was prevented by the calcineurin inhibitor deltamethrin. Endocytotic fission pore kinetics were accelerated by Ca2+ in both the intact cells and isolated membrane patches; however, the effect in this case was neither bimodal nor deltamethrin sensitive. Membrane retrieval can therefore be upregulated by a Ca2+-dependent increase in endocytotic vesicle size and acceleration of membrane fission in insulin-secreting INS-1 cells.

Binary bacterial toxins: biochemistry, biology, and applications of common Clostridium and Bacillus proteins

Certain pathogenic species of Bacillus and Clostridium have developed unique methods for intoxicating cells that employ the classic enzymatic "A-B" paradigm for protein toxins. The binary toxins produced by B. anthracis, B. cereus, C. botulinum, C. difficile, C. perfringens, and C. spiroforme consist of components not physically associated in solution that are linked to various diseases in humans, animals, or insects. The "B" components are synthesized as precursors that are subsequently activated by serine-type proteases on the targeted cell surface and/or in solution. Following release of a 20-kDa N-terminal peptide, the activated "B" components form homoheptameric rings that subsequently dock with an "A" component(s) on the cell surface. By following an acidified endosomal route and translocation into the cytosol, "A" molecules disable a cell (and host organism) via disruption of the actin cytoskeleton, increasing intracellular levels of cyclic AMP, or inactivation of signaling pathways linked to mitogen-activated protein kinase kinases. Recently, B. anthracis has gleaned much notoriety as a biowarfare/bioterrorism agent, and of primary interest has been the edema and lethal toxins, their role in anthrax, as well as the development of efficacious vaccines and therapeutics targeting these virulence factors and ultimately B. anthracis. This review comprehensively surveys the literature and discusses the similarities, as well as distinct differences, between each Clostridium and Bacillus binary toxin in terms of their biochemistry, biology, genetics, structure, and applications in science and medicine. The information may foster future studies that aid novel vaccine and drug development, as well as a better understanding of a conserved intoxication process utilized by various gram-positive, spore-forming bacteria.

Hyperosmolarity enhances the lung capillary barrier

Although capillary barrier deterioration underlies major inflammatory lung pathology, barrier-enhancing strategies are not available. To consider hyperosmolar therapy as a possible strategy, we gave 15-minute infusions of hyperosmolar sucrose in lung venular capillaries imaged in real time. Surprisingly, this treatment enhanced the capillary barrier, as indicated by quantification of the capillary hydraulic conductivity. The barrier enhancement was sufficient to block the injurious effects of thrombin, TNF-alpha, and H2O2 in single capillaries, and of intratracheal acid instillation in the whole lung. Capillary immunofluorescence indicated that the hyperosmolar infusion markedly augmented actin filament formation and E-cadherin expression at the endothelial cell periphery. The actin-depolymerizing agent latrunculin B abrogated the hyperosmolar barrier enhancement as well as the actin filament formation, suggesting a role for actin in the barrier response. Furthermore, hyperosmolar infusion blocked TNF-alpha-induced P-selectin expression in an actin-dependent manner. Our results provide the first evidence to our knowledge that in lung capillaries, hyperosmolarity remodels the endothelial barrier and the actin cytoskeleton to enhance barrier properties and block proinflammatory secretory processes. Hyperosmolar therapy may be beneficial in lung inflammatory disease.

Facilitation of Ca(2+)-dependent exocytosis by Rac1-GTPase in bovine chromaffin cells

Rho family GTPases are primary mediators of cytoskeletal reorganization, although they have also been reported to regulate cell secretion. Yet, the extent to which Rho family GTPases are activated by secretory stimuli in neural and neuroendocrine cells remains unknown. In bovine adrenal chromaffin cells, we found Rac1, but not Cdc42, to be rapidly and selectively activated by secretory stimuli using an assay selective for the activated GTPases. To examine effects of activated Rac1 on secretion, constitutively active mutants of Rac1 (Rac1-V12, Rac1-L61) were transiently expressed in adrenal chromaffin cells. These mutants facilitated secretory responses elicited from populations of intact and digitonin-permeabilized cells as well as from cells under whole cell patch clamp. A dominant negative Rac1 mutant (Rac1-N17) produced no effect on secretion. Expression of RhoGDI, a negative regulator of Rac1, inhibited secretory responses while overexpression of effectors of Rac1, notably, p21-activated kinase (Pak1) and actin depolymerization factor (ADF) promoted evoked secretion. In addition, expression of effector domain mutants of Rac1-V12 that exhibit reduced activation of the cytoskeletal regulators Pak1 and Partner of Rac1 (POR1) resulted in a loss of Rac1-V12-mediated enhancement of evoked secretion. These findings suggest that Rac1, in part, functions to modulate secretion through actions on the cytoskeleton. Consistent with this hypothesis, the actin modifying drugs phalloidin and jasplakinolide enhanced secretion, while latrunculin-A inhibited secretion and eliminated the secretory effects of Rac1-V12. In summary, Rac1 was activated by secretory stimuli and modulated the secretory pathway downstream of Ca2+ influx, partly through regulation of cytoskeletal organization.