investigating-mast-cell-secretory-granules-from-biosynthesis-to

Investigating Mast Cell Secretory Granules; from Biosynthesis to Exocytosis

Mast Cells (MC) are secretory cells of the immune system that accomplish their physiological and pathological functions by releasing pre-formed and newly ...

Sackler Faculty of Medicine, Tel Aviv University

Cincinnati Children's Hospital Medical Center, University of Cincinnati College of Medicine

Exocytosis is a key event in mast cell functions. By this process, mast cells release inflammatory mediators, contained in secretory granules (SGs), which play important roles in immunity and wound healing but also provoke allergic and inflammatory responses. The mechanisms underlying mast cell exocytosis remained poorly understood. An essential step toward deciphering the mechanisms behind exocytosis is the identification of the cellular components that regulate this process. Because Rab GTPases regulate specific trafficking pathways, we screened 44 Rabs for their functional impacts on exocytosis triggered by the FcεRI or combination of Ca ²⁺ ionophore and phorbol ester. Because exocytosis involves the continuous reorganization of the actin cytoskeleton, we also repeated our screen in the presence of cytochalasin D that inhibits actin polymerization. In this paper, we report on the identification of 30 Rabs as regulators of mast cell exocytosis, the involvement of 26 of which has heretofore not been recognized. Unexpectedly, these Rabs regulated exocytosis in a stimulus-dependent fashion, unless the actin skeleton was disrupted. Functional clustering of the identified Rabs suggested their classification as Rabs involved in SGs biogenesis or Rabs that control late steps of exocytosis. The latter could be further divided into Rabs that localize to the SGs and Rabs that regulate transport from the endocytic recycling compartment. Taken together, these findings unveil the Rab networks that control mast cell exocytosis and provide novel insights into their mechanisms of action.

Decoding the regulation of mast cell exocytosis by networks of Rab GTPases.

Secretion of inflammatory mediators prestored in mast cells secretory granules (SGs) enhances immune responses such as in allergy and host defense. However, the mechanisms underlying the biogenesis of the SGs remain largely unresolved. By combining high-resolution live cell imaging and quantitative morphometric analyses, we show that the small GTPase Rab5 controls the SG size and cargo composition by a VAMP8-dependent fusion mechanism. Knockdown of the endogenous Rab5, or expression of constitutively negative mutants, significantly reduces the size of SGs and increases their number. Conversely, expression of constitutively active Rab5 mutants induces few, but giant, SGs. Both the small and giant SGs maintain their exocytosis competence. Finally, we show that Rab5-mediated fusion between Golgi-derived SGs and early endosomes precedes the maturation of the SGs, as reflected by the recruitment of Rab27B, and allows the incorporation of cargo, such as CD63, that traffics through endosomes. Collectively, our results assign Rab5 a key role in mediating mast cell SG fusion during biogenesis, thereby controlling the amount and composition of the SGs content and maintaining the communication between new and pre-existing SGs.

Article	Year
Decoding the regulation of mast cell exocytosis by networks of Rab GTPases. Journal of immunology (Baltimore, Md. : 1950)\| PubMed ID: 22826321	2012
Rab5 is a novel regulator of mast cell secretory granules: impact on size, cargo, and exocytosis. Journal of immunology (Baltimore, Md. : 1950)\| PubMed ID: 24696234	2014

Nurit P. Azouz

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