Cross-talk between acidic compartments and the endoplasmic reticulum Ca2+ store triggered by Ca2+ release via two-pore segment channels.

Recent studies have revealed the importance of two-pore segment channels(TPCs) in mediating NAADP-evoked Ca2þrelease from acidic organelles.This Ca2þsignal initiated from the acidic stores can recruit additional Ca2þre-lease from endoplasmic reticulum (ER) via Ca2þ-induced Ca2þrelease (CICR).Both humans and mice express two TPCs, TPC1 and TPC2 whereas many othervertebrates express also TPC3. We have shown that each TPC subtype may betargeted to different acidic organelles, with TPC1 predominantly localized toa subpopulation of endosomes while TPC2 almost exclusively expressed in ly-sosomes. Consistent with these expression patterns and the fact that endosomesare smaller and less clustered than lysosomes, TPC1, when overexpressed inHEK293 cells, mediated Ca2þrelease in response to 10 nM NAADP thatwas spatially restricted in nature and did not trigger global Ca2þtransient.However, application of 10 nM NAADP in cells overexpressing TPC2 evokeda biphasic Ca2þresponse with a pacemaking phase followed by a large second-ary and global Ca2þtransient. The responses to NAADP in both TPC1 andTPC2 expressing cells were abolished upon depletion of acidic Ca2þstoreswith bafilomycin. By contrast, Ca2þtransients evoked in TPC2-, but notTPC1-, expressing cells were attenuated by prior depletion of ER Ca2þstoreswith thapsigargin. Therefore, only TPC2 appears to couple to the ER by CICR.Interestingly, the expression of chicken and rabbit TPC3 in HEK293 cellsyielded distinct subcellular localizations and functional data are consistentwith differential organelle targeting. Our data suggest that cross-talk betweenacidic stores and ER is governed by the capacity of local Ca2þsignals fromacidic Ca2þstores to summate, and in a manner that may determine whetherthe threshold for CICR from ER is breached.