169 REVIEW

Fusion pore or porosome: structure and dynamics

B P Jena Department of Physiology, Wayne State University School of Medicine, 5239 Scott Hall, 540 E. Canfield Avenue, Detroit, Michigan 48201–4177, USA (Requests for offprints should be addressed toBPJena; Email: [email protected])

Abstract Electrophysiological measurements on live secretory cells microscopy on live exocrine and neuroendocrine cells almost a decade ago suggested the presence of fusion pores demonstrate the presence of such plasma membrane pores, at the cell plasma membrane. Membrane-bound secretory revealing their morphology and dynamics at near nm vesicles were hypothesized to dock and fuse at these sites, resolution and in real time. to release their contents. Our studies using atomic force Journal of Endocrinology (2003) 176, 169–174

Introduction pattern using a xyz piezo tube to scan the surface of the sample (Fig. 1) (Binnig et al. 1986). The deflection of the Hormone release, neurotransmission, and enzyme secre- cantilever measured optically is used to generate an tion are fundamental physiological processes resulting from isoforce relief of the sample (Alexander et al. 1989). Force the fusion of membrane-bound secretory vesicles at the is thus used to image surface profiles of objects by the cell plasma membrane and consequent expulsion of ves- AFM, allowing imaging of live cells and subcellular icular contents. Membrane-bound secretory vesicles dock structures submerged in physiological buffer solutions. To and fuse at specific plasma membrane locations following image live cells, the scanning probe of the AFM operates secretory stimulus. Earlier electrophysiological studies sug- in physiological or near physiological buffers, and may do gested the existence of ‘fusion pores’ at the cell plasma so under two modes: contact or tapping. In the contact membrane. Following stimulation of secretion, the fusion mode, the probe is in direct contact with the sample pores at the plasma membrane become continuous with surface as it scans at a constant vertical force. Although the secretory vesicle membrane (Monck et al. 1995). high-resolution AFM images can be obtained in this mode Using atomic force microscopy (AFM), the existence of of AFM operation, sample height information generated the fusion pore was confirmed, and its structure and may not be accurate since the vertical scanning force may dynamics in both exocrine (Schneider et al. 1997, Cho depress the soft cell. However, information on the visco- et al. 2002d) and neuroendocrine (Cho et al. 2002b,e) cells elastic properties of the cell and the spring constant of the determined at near nm resolution and in real time. cantilever, enables measurement of the cell height. In The resolving power of the light microscope is depen- tapping mode, on the other hand, the cantilever resonates dent on the wavelength of the light used and hence, at and the tip makes brief contacts with the sample, too brief best, 300–400 nm objects can be observed. The recently to allow adhesive forces between probe tip and the sample discovered fusion pore in live secretory cells is cup-shaped, surface. In the tapping mode in fluid, lateral forces are measuring 100–150 nm at its wide end and 15–30 nm in virtually negligible. It is therefore important that informa- relative depth. As a result, it had evaded visual detection tion on the topology of living cells be obtained using both until recently (Schneider et al. 1997, Cho et al. 2002b,d,e). contact and tapping modes of AFM operation. The scan- The development of the AFM (Binnig et al. 1986) has ning rate of the tip over the sample also plays a critical role enabled the imaging of live biological samples in physio- on the quality of the image. Because cells are soft samples, logical buffer at near nm resolution. In AFM, a probe tip a high scanning rate would influence the shape of the cell microfabricated from silicon or silicon nitride and mounted being imaged. Therefore, a slow movement of the tip over on a cantilever spring is used to scan the surface of the the cell would be ideal and results in minimal distortion sample at a constant force (Albrecht et al. 1990). Either the and better resolved images. Rapid cellular events could, probe or the sample can be precisely moved in a raster nonetheless, be monitored using section analysis obtained

Journal of Endocrinology (2003) 176, 169–174 Online version via http://www.endocrinology.org 0022–0795/03/0176–169  2003 Society for Endocrinology Printed in Great Britain

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Figure 1 Schematic diagram depicting key components of an atomic force microscope.

by a rapid line scan of the specific cell surface of interest. 150 nm in diameter (Fig. 2b), and typically 3–4 depres- To examine isolated cells by the AFM, freshly cleaved sions are located within a pit. The basolateral membrane mica coated with Cel-Tak have also been used with great of acinar cells are, however, devoid of both pits and success (Schneider et al. 1997, Cho et al. 2002b,d). The contents of the bathing medium as well as the cell surface to be scanned should be devoid of any large molecular weight proteins or cellular debris. Cellular structures such as the fusion pore and the study of its dynamics could be examined using the AFM at nm resolution and in real time (Binnig et al. 1986, Alexander et al. 1989, Albrecht et al. 1990, Rugard & Hansma 1990, Schneider et al. 1997, Cho et al. 2002b,d,e). AFM-force spectroscopy allows imaging, at nm resolution and in real time, of objects such as live cells, subcellular structures or even single molecules submerged in physiological buffer solutions. The discovery of this new cellular structure and its identification as the fusion pore, reveals a new under- standing of the secretory process and, more importantly, into the workings of a living cell. In this article, the structure and dynamics of the fusion pore in live cells, examined using AFM, is described.

New cellular structures

Pancreatic acinar cells are polarized secretory cells. When stimulated with a secretagog, membrane-bound secretory vesicles dock and fuse at the apical plasma membrane to release vesicular contents. Isolated live pancreatic acinar cells in physiological buffer, when imaged using the AFM (Schneider et al.1997, Cho et al. 2002d), reveal at the Figure 2 AFM micrographs revealing the structure of the fusion apical plasma membrane highly stable and permanent pore at the cell plasma membrane in exocrine and neuroendocrine cells. (a) A pit (white arrowheads) and four circular ‘pits’ (Fig. 2a) measuring 0·4 µm – 1·2 µm in depressions (yellow arrowhead) are identified in a live pancreatic diameter, containing smaller cup-shaped ‘depressions’ acinar cell. (b) Single fusion pore at high resolution. Scale bars = within (Fig. 2b). Each depression measures around 100– 100 nm.

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Figure 3 Dynamics of depressions following stimulation of secretion. The top panel shows a number of depressions within a pit in a live pancreatic acinar cell. The scan line across three depressions in the top panel is represented graphically in the middle panel and defines the diameter and relative depth of the depressions; the middle depression is represented by red arrowheads. The bottom panel represents percentage of total cellular amylase release in the absence (W/O Addn.) or presence of the secretagog Mas 7 (20
M). Notice an increase in the diameter and depth of depressions, correlating with an increase in total cellular amylase release at 5 min after stimulation of secretion. At 30 min after stimulation of secretion, there is a decrease in diameter and depth of depressions, with no further increase in amylase release over the 5 min time point. No significant increases in amylase secretion or depression diameter were observed in resting acini or those exposed to the non-stimulatory mastoparan analog Mas 17. depressions. High resolution AFM images of depressions in in secretagog-induced secretion. Results from these studies live cells further reveal a cone- or cup-shaped morphology suggested that depressions are the fusion pores or poro- (Fig. 2b). The depth of each depression cone measures somes in pancreatic acinar cells. These studies further approximately 15–30 nm. Similarly, both growth hor- demonstrated the involvement of actin in the regulation of mone (GH)-secreting cells of the pituitary gland and the depression structure–function. Similar to the acinar cells, chromaffin cell also possess pits and depression structures depressions in resting GH-secreting cells measure in their plasma membrane (Cho et al. 2002b,e), suggesting 1544·5 nm (meanS.E) in diameter. Exposure of the their universal presence in secretory cells. GH cell to a secretagog resulted in a 40% increase (2154·6 nm; P<0·01) in depression diameter, with no change in pit size. New structures are fusion pores/porosomes Enlargement of depression diameter following exposure of acinar cells to a secretagog correlated with increased When exposed to a secretagog such as mastoparan, pan- secretion. Additionally, actin depolymerizing agents creatic acinar cells show a time-dependent increase (20– known to inhibit secretion (Schneider et al. 1997) resulted 35%) in the diameter of depressions, followed by a return in decreased depression size and accompanied loss in to resting size following completion of the process (Fig. 3). secretion. These studies further suggested that depressions No demonstrable change in pit size is, however, detected were the fusion pores or porosomes. However, a more during this time. Enlargement of depression diameter and direct determination of the function of depressions was an increase in its relative depth following exposure to required. Hence, immuno AFM studies were performed. secretagogs, correlated with increased secretion. Exposure Using gold-conjugated antibody against a specific secreted of pancreatic acinar cells to cytochalasin B, a fungal toxin protein, combined with AFM imaging, provided the that inhibits actin polymerization, results in a 15–20% means to determine if secretion occurs at depressions (Cho decrease in depression size, and a consequent 50–60% loss et al. 2002b,d). The membrane-bound secretory vesicles in www.endocrinology.org Journal of Endocrinology (2003) 176, 169–174

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Figure 4 Fusion pores or porosomes dilate to allow expulsion of vesicular contents. (a and b) AFM micrographs and section analysis of a pit and two out of the four depressions or porosomes, showing enlargement of porosomes following stimulation of secretion. (c) Exposure of live cells to gold conjugated-amylase antibody (Ab) results in specific localization of gold to secretory sites. Note the localization of amylase-specific immunogold at the edge of depressions. (d) AFM micrograph of pits and depressions with immunogold localization is also demonstrated in cells immunolabeled and then fixed. Blue arrowheads point to immunogold clusters and the yellow arrowhead points to a depression or fusion pore.

exocrine pancreas contain the starch digesting enzyme 1990), has recently been shown to directly interact with amylase. Using amylase-specific immunogold AFM SNAREs (Nakano et al. 2001). Recent studies demon- studies, localization of amylase at ‘depressions’ following strate that -fodrin regulates exocytosis through its inter- stimulation of secretion was demonstrated (Fig. 4a) (Cho action with the syntaxin family of proteins (Nakano et al. et al. 2002d). These studies confirm ‘depressions’ to be 2001). The c-terminal coiled coil region of syntaxin the fusion pores or porosomes at the apical plasma mem- interacts with -fodrin, a major component of the sub- brane in pancreatic acinar cells, where membrane-bound membranous cytoskeleton. Similarly, vimentin filaments secretory vesicles dock and fuse to release vesicular interact with SNAP23/25 and control the availability of contents. Similarly, in somatotropes of the pituitary, gold- free SNAP23/25, for assembly of the SNARE complex tagged GH-specific antibody was found to be selectively (Faigle et al. 2000). Additionally, our recent studies localized at depressions following stimulation of secretion confirm earlier findings and further demonstrate a direct (Cho et al. 2002b), again confirming depressions to be the interaction between actin and SNAREs (Jena et al. 2002 ). fusion pores or porosomes. These findings suggest that vimentin, -fodrin, actin and Our studies on the role of actin in the regulation of SNAREs may all be part of the fusion pore or porosome depression structure and dynamics, clearly suggest actin to machinery. Purification and further biochemical charac- be a major component of the porosome complex. Target terization of the fusion pore is required to determine its membrane proteins SNAP25 and syntaxin (t-SNARE) composition. The possible involvement of proteins such as and secretory vesicle-associated membrane protein v-SNARE (VAMP or synaptobrevin), synaptophysin and (v-SNARE), are part of a conserved protein complex myosin, and their interaction when the fusion pore estab- involved in fusion of opposing bilayers (Rothman 1994, lishes continuity with the secretory vesicle membrane, Weber et al. 1998). Since membrane-bound secretory should not be ruled out. The globular tail domain of vesicles dock and fuse at depressions to release vesicular myosin V is its binding site, and VAMP is bound to contents, it is reasonable to suggest that plasma membrane- myosin V in a calcium independent manner (Ohyama associated t-SNAREs are part of the fusion pore complex. et al. 2001). Further interaction of myosin V with syntaxin In the last decade, a number of studies have demonstrated requires calcium and calmodulin. Studies suggest that the involvement of cytoskeletal proteins in exocytosis, VAMP acts as a myosin V receptor on secretory vesicles some directly interacting with SNAREs (Bennett 1990, and regulates formation of the SNARE complex (Ohyama Goodson et al. 1997, Prekereis & Terrian 1997, Schneider et al. 2001). Interaction of VAMP with synaptophysine et al. 1997, Faigle et al. 2000, Miller & Sheetz 2000, and myosin V has further been demonstrated (Prekereis & Nakano et al. 2001, Ohyama et al. 2001). Actin and Terrian 1997, Miller & Sheetz 2000). Our recent studies microtubule-based cytoskeleton have been implicated in reveal the composition of the fusion pore in pancreatic intracellular vesicle traffic (Goodson et al. 1997). Fodrin, acinar cells. Further, structural details of the fusion pore which was previously implicated in exocytosis (Bennett have been confirmed using electron microscopy (Cho

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Figure 5 A schematic diagram of the plasma membrane, showing ‘pit’ (yellow arrowhead) with smaller ‘depressions’ (red arrowhead) within. Depressions are the fusion pores or porosomes where membrane- bound secretory vesicles (SV) dock and fuse to release vesicular contents. et al. 2002c, Jena et al. 2002). The arrangement of (Cho et al. 2002a). Earlier studies on mast cells also t-/v-SNAREs resulting in fusion of opposing bilayers, and demonstrate an increase in the number of spent and the presence of such a composition in the base of the fusion partially spent vesicles following stimulation of secretion, pore complex has been determined (Cho et al. 2002c). without any demonstrable increase in cell size or change in These studies provide a glimpse of the biochemical com- vesicle number (Lawson et al. 1975). Other supporting position, structure, and regulation of the fusion pore or evidence favoring transient fusion is the presence of porosome (Fig. 5). neurotransmitter transporters at the synaptic vesicle mem- brane. These vesicle-associated transporters would be of little use if vesicles were to fuse completely at the plasma Conclusion membrane, to be compensatory endocytosed at a later time. Although the fusion of secretory vesicles at the cell Fusion pores or porosomes in pancreatic acinar or GH- plasma membrane occurs transiently, complete membrane secreting cells are 100–150 nm wide cup-shaped incorporation at the cell plasma membrane results when structures at the plasma membrane. Membrane-bound cells need to incorporate signaling molecules like recep- secretory vesicles ranging in size from 0·2–1·2 µm in tors, second messengers or ion channels, at the plasme diameter dock and fuse at porosomes to release vesicular membrane. contents. Once secretory vesicles dock and fuse at poro- somes, only a 20–35% increase in porosome diameter is Acknowledgements demonstrated. It is therefore reasonable to conclude that secretory vesicles ‘transiently’ dock and fuse at depressions. The author wishes to thank Sang-Joon Cho for help in In contrast to accepted belief, if secretory vesicle mem- preparation of the figures. Studies performed in the branes were to completely incorporate and flatten out and author’s laboratory were supported by grants from the become part of the plasma membrane at depressions, the National Institute of Health (B P J). fusion pore would distend much wider than that which is observed. Furthermore, if secretory vesicles were to com- References pletely fuse at the plasma membrane, there would be a loss in vesicle number following secretion. Examination of Albrecht TH, Akamine S, Carver TE & Quate CF 1990 secretory vesicles within cells before and after secretion, Microfabrication of cantilever styli for the atomic force microscope. demonstrates that the total number of secretory vesicles Journal of Vaccine Science and Technology A8 3386–3396. remains unchanged following secretion, however the Alexander S, Hellemans L, Marti O, Schneir J, ElingsV&Hansma PK 1989 An atomic resolution atomic force microscope number of empty and partially empty vesicles increases implemented using an optical lever. Journal of Applied Physics 65 significantly, supporting the occurrence of transient fusion 164–167. www.endocrinology.org Journal of Endocrinology (2003) 176, 169–174

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