Bidirectional Ca2+ signalling occurs between the endoplasmic reticulum and acidic organelles

The endoplasmic reticulum (ER) and acidic organelles (endo-lysosomes) act as separate Ca2+ stores that release Ca2+ in response to the second messengers IP3 and cADPR (ER) or NAADP (acidic vesicles). Typically, trigger-Ca2+ rleased from acidic organelles by NAADP subsequently recruits IP3 (or ryanodine) receptors on the EF, and anterograde signal important for amplification and Ca2+ oscillations/waves. We therefore investigated whether the ER can signal back to acidic organelles, using organelle pH as a reporter of NAADP action. We show that Ca2+ released from the ER can activate the NAADP pathway in two ways: first, by stimulating CA2+-dependent NAADP synthesis; second, by activating NAADP-regulated channels. Moreoever, the differential effects of EGTA and BAPTA (slow and fast Ca2+ chelators respectively) suggest that the acidic organelles are preferentially activated by local microdomains of high Ca2+ at junctions between the ER and acidic organelles. Biodirectional organelle communication may have wider implications for endo-lysosomal function as well as the generation of Ca2+ oscillations and waves.