Objectives: 

Release of Ca2+ from acidic intracellular storage compartments such as lysosomes and endolysosomes, can be triggered by nicotinic acid adenine dinucleotide phosphate (NAADP). Two-pore channels (TPCs) are localized on acidic stores and there is evidence that this family of ion-channel may act as the pathway for the release of Ca2+. We have recently shown that the specialized gating and conduction properties of human two-pore channel type 2 (TPC2) enable it to function as a lysosomal NAADP-sensitive Ca2+-release channel. We now investigate if this is also true for TPC1.

Materials: 

We have reconstituted purified human TPC1 into artificial membranes under voltage-clamp conditions to investigate the fundamental gating and conducting properties of this isoform.

Results: 

Our results demonstrate that TPC1 has a similar Ca2+ conductance to TPC2 (19±4 pS (TPC2±15 pS) ; 10mM cis/50 mM trans Ca2+;SD; n=3) but a lower K+ conductance compared to TPC2 (87±3 pS (TPC2±300 pS); symmetrical 210 mM K+; SD; n=3). Similar to TPC2, there is no evidence for anion permeability (in a 210mM trans: 510mM cis KCl gradient, the reversal potential coincides with the calculated value for a channel ideally selective for cations (Erev=-25mV)). We find that TPC1 is dependent on voltage such that the channels are always more open at positive than at negative potentials. Even at low potentials, the voltage-dependency could be clearly observed (Mean average current: 8.9±5.3 pA at +10 mV and 1.3±0.7 pA at -10 mV (SD, n=3).

Conclusions: 

In conclusion, while our results provide evidence that TPC1 could also act as a Ca2+-release channel, we demonstrate that TPC1 and TPC2 exhibit significant functional differences. These differences will provide flexibility to this tightly controlled Ca2+-release system, enabling the possibility of wide variations in the type of Ca2+ fluxes that can be triggered from acidic stores.

Supported by the BHF