An Endosomal NAADP-Sensitive Two-Pore Ca2+ Channel Regulates ER-Endosome Membrane Contact Sites to Control Growth Factor Signaling

摘要

class="head no_bottom_margin" id="sec1title">IntroductionHow organelles communicate is a fundamental question that arises given the compartmentalized nature of eukaryotic cell function. Although vesicular traffic is an established means of information transfer, it is becoming clear that traffic also proceeds by non-vesicular means. In particular, membrane contact sites have emerged as potential platforms for both Ca²⁺ signaling and lipid transfer (, , , ). Membrane contact sites are regions of close apposition between membranes that are stabilized by tethering complexes. The endoplasmic reticulum (ER) forms multiple classes of contacts with both the plasma membrane and organelles such as endosomes, lysosomes, and mitochondria. Endosome-ER contacts have been implicated in endosome positioning (, ), dephosphorylation of internalized receptors, and components of the endosomal sorting complex required for transport (ESCRT) machinery (, , ), endosome fission (), actin nucleation and retromer-dependent budding (), and cholesterol transport (). We have identified multiple populations of contact sites that form between the ER and different endocytic organelles (), which include those dependent on VAPs (). Notably, contact sites between the ER and EGF receptor-containing endosomes require annexin-A1 and its Ca²⁺-dependent binding partner S100A11 (), raising the possibility that Ca²⁺ fluxes may regulate contact.Ca²⁺ is a widespread signaling ion regulating a range of cellular processes including aspects of vesicle formation, fusion, and traffic (href="#bib2" rid="bib2" class=" bibr popnode">Berridge et al., 2003). Ca²⁺ signals often invade the cell entirety (global) but they can also be spatially restricted (local), as exemplified by signals generated by the Ca²⁺-mobilizing messenger, nicotinic acid adenine dinucleotide phosphate (NAADP) (href="#bib14" rid="bib14" class=" bibr popnode">Galione, 2015). NAADP is unusual in mediating Ca²⁺ release from the endo-lysosomal system, an acidic Ca²⁺ store filled by Ca²⁺/H⁺ exchange (href="#bib6" rid="bib6" class=" bibr popnode">Churchill et al., 2002, href="#bib31" rid="bib31" class=" bibr popnode">Patel and Muallem, 2011, href="#bib27" rid="bib27" class=" bibr popnode">Melchionda et al., 2016). It does so by activating two-pore channels (TPCs) (href="#bib5" rid="bib5" class=" bibr popnode">Calcraft et al., 2009, href="#bib3" rid="bib3" class=" bibr popnode">Brailoiu et al., 2009, href="#bib30" rid="bib30" class=" bibr popnode">Patel, 2015). Local NAADP-mediated Ca²⁺ release events from acidic organelles are amplified by Ca²⁺ channels on canonical Ca²⁺ stores of the ER to generate global signals (href="#bib14" rid="bib14" class=" bibr popnode">Galione, 2015). This occurs during signaling by external cues such as hormones and neurotransmitters (href="#bib44" rid="bib44" class=" bibr popnode">Yamasaki et al., 2005, href="#bib29" rid="bib29" class=" bibr popnode">Pandey et al., 2009). However, it is also evident that local TPC-mediated Ca²⁺ release events function in a constitutive manner. For instance, NAADP/TPC signaling regulates several membrane trafficking events, including retrograde traffic from endosomes to the Golgi (href="#bib39" rid="bib39" class=" bibr popnode">Ruas et al., 2010, href="#bib40" rid="bib40" class=" bibr popnode">Ruas et al., 2014) and the trafficking of cholesterol, receptors, and viruses (href="#bib16" rid="bib16" class=" bibr popnode">Grimm et al., 2014, href="#bib40" rid="bib40" class=" bibr popnode">Ruas et al., 2014, href="#bib41" rid="bib41" class=" bibr popnode">Sakurai et al., 2015). This pathway also regulates endo-lysosomal morphology (href="#bib24" rid="bib24" class=" bibr popnode">Lin-Moshier et al., 2014, href="#bib18" rid="bib18" class=" bibr popnode">Hockey et al., 2015, href="#bib30" rid="bib30" class=" bibr popnode">Patel, 2015), likely through Ca²⁺-dependent vesicular fusion/fission events (href="#bib33" rid="bib33" class=" bibr popnode">Pryor et al., 2000, href="#bib25" rid="bib25" class=" bibr popnode">Luzio et al., 2007, href="#bib26" rid="bib26" class=" bibr popnode">Marchant and Patel, 2015). However, what role TPCs play in non-vesicular trafficking is unexplored (href="#bib4" rid="bib4" class=" bibr popnode">Burgoyne et al., 2015).Here, we reveal an essential requirement for NAADP and TPC1 in regulating membrane contact site formation between endosomes and the ER to control growth factor signaling.