Research ArticleIon Channels

mTORC1 controls lysosomal Ca2+ release through the two-pore channel TPC2

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Sci. Signal.  10 Apr 2018:
Vol. 11, Issue 525, eaao5775
DOI: 10.1126/scisignal.aao5775

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Channeling lysosomal Ca2+

Inhibition of the multiprotein complex mTORC1 alleviates pulmonary hypertension in animal models. Using mouse pulmonary arterial myocytes and transfected cells, Ogunbayo et al. identified mTORC1 inhibition as a mechanism for activation of the ion channel TPC2 that led to the mobilization of Ca2+ from lysosomes. Furthermore, they showed that TPC2 may be regulated by mTORC1 inhibition and the ligand NAADP through a common pathway. The authors suggest that modulating TPC2 activity could be a promising therapeutic strategy for pulmonary hypertension, which currently lacks effective treatments.

Abstract

Two-pore segment channel 2 (TPC2) is a ubiquitously expressed, lysosomally targeted ion channel that aids in terminating autophagy and is inhibited upon its association with mechanistic target of rapamycin (mTOR). It is controversial whether TPC2 mediates lysosomal Ca2+ release or selectively conducts Na+ and whether the binding of nicotinic acid adenine dinucleotide phosphate (NAADP) or phosphatidylinositol 3,5-bisphosphate [PI(3,5)P2] is required for the activity of this ion channel. We show that TPC2 is required for intracellular Ca2+ signaling in response to NAADP or to mTOR inhibition by rapamycin. In pulmonary arterial myocytes, rapamycin and NAADP evoked global Ca2+ transients that were blocked by depletion of lysosomal Ca2+ stores. Preincubation of cells with high concentrations of rapamycin resulted in desensitization and blocked NAADP-evoked Ca2+ signals. Moreover, rapamycin and NAADP did not evoke discernable Ca2+ transients in myocytes derived from Tpcn2 knockout mice, which showed normal responses to other Ca2+-mobilizing signals. In HEK293 cells stably overexpressing human TPC2, shRNA-mediated knockdown of mTOR blocked rapamycin- and NAADP-evoked Ca2+ signals. Confocal imaging of a genetically encoded Ca2+ indicator fused to TPC2 demonstrated that rapamycin-evoked Ca2+ signals localized to lysosomes and were in close proximity to TPC2. Therefore, inactivation of mTOR may activate TPC2 and consequently lysosomal Ca2+ release.

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