Spatial control of exocytosis

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Abstract

During many key biological processes, exocytosis is confined to distinct regions of the plasma membrane. Spatial control of exocytosis correlates with altered membrane skeleton dynamics and assembly of local membrane microdomains. These domains act as local stages for the assembly and the regulation of molecular complexes (targeting patches) that mediate vesicle–membrane fusion. Furthermore, local activation of signaling pathways reinforces formation of these patches and might effect global repositioning of the secretory pathway toward sites of localized exocytosis.

Introduction

Exocytosis, the fusion of intracellular membrane vesicles with the plasma membrane, occurs constitutively in all eukaryotic cells and is upregulated in some cell types in response to extrinsic stimuli (e.g. neurotransmitters and hormones). Membrane fusion is mediated by soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptors (SNAREs) found on transport vesicles (v-SNAREs) and their target membrane (t-SNAREs) [1]. In addition to the soluble proteins α-SNAP and NSF, which are responsible for the dissociation of SNARE complexes, several signaling molecules, cytoskeletal components and plasma-membrane-associated proteins and lipids have been implicated in the control of membrane fusion [2].

Spatial control of exocytosis underlies many key biological processes (Figure 1). In neurons, exocytosis of secretory granules and vesicles is confined to the synaptic cleft and takes place within a millisecond time frame [2]. During immune surveillance, contact between an antigen-presenting cell (APC) and a cytotoxic T cell leads to formation of the immunological synapse, and vesicle transport of soluble agents and membrane proteins is confined within this zone of cell–cell contact [3]. During cell migration and wound healing, exocytosis is tightly coupled to local changes in plasma membrane tension and membrane-cytoskeleton dynamics [4]. During cell division, localized exocytosis is thought to mediate ingression of the cleavage furrow 5., 6.. Development of cell polarity is accompanied by directed exocytosis at specialized sites of membrane growth [7].

Here, we review recent evidence and dissect the spatial control of exocytosis as a process that relies on a number of factors: molecular cues that define membrane domains for localized exocytosis; local assembly and regulation of molecular complexes (targeting patches) that mediate membrane fusion; and cytoskeleton tracks and membrane carriers that connect the trans-Golgi network (TGN) with targeting patches of the plasma membrane.

Section snippets

Molecular cues and membrane domains regulating spatial control of exocytosis

Spatial control of exocytosis implies that vesicle fusion does not occur randomly on the plasma membrane. With the advent of evanescent wave microscopy, also termed ‘total internal reflection fluorescence microscopy’ (TIR-FM), single vesicle fusion events have been imaged and analyzed for constitutive and regulated exocytosis, which are considered to either require or not require a specific signal for vesicle fusion, respectively. Although constitutive exocytosis in COS-1 cells appears to occur

Assembly of targeting patches: local machineries and global regulators

According to the SNARE hypothesis, specificity of membrane fusion is determined by physicochemical properties of SNARE proteins (v-/t-SNARE pairing), which can be further modulated by their distribution within distinct subcellular compartments and by their interaction with regulatory proteins [36]. In addition, exocytosis can be spatially controlled by differential distribution of SNARE proteins within distinct domains of the plasma membrane. Evidence for this type of mechanism comes from

Cytoskeleton tracks and membrane carriers make the connection

While a targeting patch marks the site of exocytic membrane fusion, transport vesicles that depart the Golgi complex must be directionally oriented toward the plasma membrane and sites of restricted exocytosis. Although vesicles could slowly reach their final destination by random diffusion, imaging of post-Golgi traffic indicates that vesicles travel towards the plasma membrane along curvilinear paths that overlap with microtubules [53]. In addition, proteins that are destined to different

Conclusions

Spatial control of exocytosis correlates with altered membrane-skeleton dynamics and assembly of local membrane microdomains of distinct structural and functional organization. These domains act as local stages for the activation of signaling pathways, which contribute to the formation of a targeting patch and could effect global repositioning of the secretory apparatus. Most of this model derives from studies in tissue culture cells and from paradigms of regulated exocytosis. Thus, it remains

Update

The role of cytoskeleton tracks in constitutive exocytosis was demonstrated recently by a study showing that disruption of microtubules not only alters the shape and directionality of membrane vesicles but also reduces the efficiency of the fusion process (an increase in the number of incomplete fusion events was observed as more fusion events were required for the discharge of cargo proteins into the plasma membrane) [70]. According to the same study, disruption of the actin cytoskeleton

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • of special interest

  • ••

    of outstanding interest

Acknowledgements

We apologize to our colleagues for having to omit many references that detail the studies reported in this review, owing to space constraints. Work from the Nelson laboratory is supported by National Institutes of Health grant GM35527 to WJ Nelson. ET Spiliotis is a fellow of the Jane Coffin Childs Memorial Fund for Medical Research.

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