NAADP Activates Two-Pore Channels on T Cell Cytolytic Granules to Stimulate Exocytosis and Killing

Summary

A cytotoxic T lymphocyte (CTL) kills an infected or tumorigenic cell by Ca ²⁺-dependent exocytosis of cytolytic granules at the immunological synapse formed between the two cells. Although inositol 1,4,5-trisphosphate (IP ₃)-mediated Ca ²⁺ release from the endoplasmic reticulum activates the store-operated Ca ²⁺-influx pathway that is necessary for exocytosis, it is not a sufficient stimulus [ 1–4]. Here we identify the Ca ²⁺-mobilizing messenger nicotinic acid adenine dinucleotide phosphate (NAADP) and its recently identified molecular target, two-pore channels (TPCs) [ 5–7], as being important for T cell receptor signaling in CTLs. We demonstrate that cytolytic granules are not only reservoirs of cytolytic proteins but are also the acidic Ca ²⁺ stores mobilized by NAADP via TPC channels on the granules themselves, so that TPCs migrate to the immunological synapse upon CTL activation. Moreover, NAADP activates TPCs to drive exocytosis in a way that is not mimicked by global Ca ²⁺ signals induced by IP ₃ or ionomycin, suggesting that critical, local Ca ²⁺ nanodomains around TPCs stimulate granule exocytosis. Hence, by virtue of the NAADP/TPC pathway, cytolytic granules generate Ca ²⁺ signals that lead to their own exocytosis and to cell killing. This study highlights a selective role for NAADP in stimulating exocytosis crucial for immune cell function and may impact on stimulus-secretion coupling in wider cellular contexts.

Abstract

Graphical Abstract

Highlights

► The T cell receptor mobilizes acidic Ca ²⁺ stores via NAADP/two-pore channels (TPCs) ► Exocytotic granules are themselves acidic Ca ²⁺ stores decorated with TPCs ► TPCs move to the immunological synapse upon TCR activation ► NAADP stimulates exocytosis more efficiently than does IP ₃/Ca ²⁺ influx alone