TOPICAL REVIEW Regulated exocytosis in immune function: are SNARE-proteins involved? A. S HUKLA*, L. BERGLUND { , L. P. N IELSEN* , S. N IELSEN { , H. J. H OFFMANN* AND R. D AHL* *Department of Respiratory Diseases, Aarhus University Hospital, Aarhus, { Protein Chemistry Laboratory and { Department of Cell Biology, Institute of Anatomy, University of Aarhus, Aarhus, Denmark INTRODUCTION In£ammation is an important feature in the pathogenesis of most chronic lung diseases. It is characterized by tissue in¢ltration with various in£ammatory cells, including eosinophils, mast cells, basophils, macrophages, neutro- phils, T- and B-lymphocytes, and dendritic cells (1). In tissues granulocytes release their toxic granule proteins after being stimulated by soluble mediators released by other in£ammatory cells (2). Therefore, it is important to characterize the intracellular mechanisms regulating the transport of granule contents in in£ammatory cells. Intracellular vesicle-tra⁄c in mammalian cells is mediated by transport vesicles that emerge from donor compartments and they are speci¢cally targeted to acceptor compartments where they deliver their content after membrane fusion (3).This tra⁄c leads to three types of fusion results: vesicle ^ intracellular membranes, vesi- cle ^ vesicle or vesicle ^ plasma membrane. The process leading to fusion of vesicle ^ plasma membrane is called exocytosis, and it delivers proteins to the cell surface (re- ceptors, e.g. CD11b, CD18) and export soluble molecules (mediators, e.g. ECP) from the cell. A number of key pro- teins involved in regulated exocytosis have been identi¢ed from in£ammatory cells.This review is a brief summary of these proteins and it includes recent results from studies on regulated exocytosis in in£ammatory cells. CELLULAR SECRETORY PATHWAYS Protein secretion from mammalian cells can be either constitutive or regulated (4,5) (Table 1). While both pathways exist in most cells, the stimuli required for and products released by regulated exocytosis are speci¢c for di¡erentiated cells. THE SNARE-HYPOTHESIS Regulated exocytosis occurs through a pathway consist- ing of two steps: interaction of vesicles with the plasma membrane (docking) and Ca 2+ - triggered membrane fusion [Fig.1(a)]. Most of the proteins participating in regulated exocy- tosis were ¢rst biochemically characterized and cloned from neurons (6,7). The physical contact between the synaptic vesicles and the plasma membrane is thought to be mediated by speci¢c protein ^ protein interaction. The SNARE (soluble N-ethylmaleimide sensitive factor attachment receptor)-hypothesis attempts to explain this interaction (8,9). The original SNARE-hypothesis postulates that vesicular tra⁄cking in a mammalian cell involve a unique vesicle-bound ligand (v-SNARE) that speci¢cally recognizes and interacts with two unique receptor target molecules (t-SNAREs) found in the plasma membrane. In synapses a vesicle-associated membrane protein called VAMP-1 (or synaptobrevin-1) has been identi¢ed as a v-SNARE, which interacts with the t-SNAREs syntaxin-1 and SNAP-25 (synaptosomal- associated protein of 25 kDa) in the plasma membrane. These three proteins form the SNARE-complex. The essential role of these SNARE-proteins in regu- lated exocytosis has been established by their sensitivity to speci¢c proteolytic cleavage by the clostridial neuro- toxins tetanus toxin and botulin toxin (10). Synaptotagmin, a vesicular integral membrane pro- tein, interacts transiently with the SNARE-complex via SNAP-25 (11). According to the hypothesis, an increase of intracellular Ca 2+ -concentration triggers the association of the cytosolic proteins a- and Correspondence should be addressed to: Alok Shukla, MSc, PhD, Department of Respiratory Diseases, Aarhus University Hospital, DK- 8000 Aarhus C, Denmark. Fax: +45 89492110; E-mail: ashukla@biobase.dk Vol. 95 (2001) 773^780  c 2001 Harcourt Publishers Ltd doi:10.1053/rmed.2001.1167, available online at http://www.idealibrary.com on