Exiting Ca²⁺ inactivates polycystin-2L1

Polycystin-2L1, a ciliary Ca²⁺-permeable channel related to polycystin-2, is uniquely inactivated by high intracellular Ca²⁺ concentrations. New research by Paul DeCaen and colleagues shows that exiting Ca²⁺ inactivates polycystin-2L1 through a high-affinity interaction with the Asp523 residue.

Although the cilioplasm and cytoplasm are connected, ciliary levels of Ca²⁺ are higher than cytoplasmic levels; this difference might be related to Ca²⁺-induced inactivation of polycystin-2L1. “We wanted to understand the structural determinants of this Ca²⁺-dependent inactivation, a potential feedback inhibition of polycystin-2L1, to shed light on ciliary Ca²⁺ signalling,” explains DeCaen.

The researchers measured ionic currents of heterologously expressed human polycystin-2L1 on the plasma membrane using patch clamp electrophysiology, and modulated internal Ca²⁺ levels by photo-uncaging. “This reductionist approach allowed us to mutate polycystin-2L1 and determine the position of residues responsible for the Ca²⁺-dependent inactivation of the channel,” says DeCaen.

removal of the C-terminal EF hands ... or the coiled-coil domain ... of polycystin-2L1 did not alter its inactivation

Surprisingly, removal of the C-terminal EF hands (Ca²⁺ coordination sites that commonly regulate ion channels) or the coiled-coil domain (which might facilitate subunit oligomerization) of polycystin-2L1 did not alter its inactivation by high intracellular Ca²⁺. This inactivation was mediated by the high-affinity binding of exiting Ca²⁺ to the selectivity-filter site Asp523. “Our findings suggest that the polycystin-2L1 channel forms a one-way passage by which Ca²⁺ can move across the ciliary membrane — a feature potentially unique to polycystin-2-related channels,” notes DeCaen.

Several mutations in PKD2, which encodes polycystin-2, are associated with autosomal dominant polycystic kidney disease (ADPKD). “Future work will determine if polycystins set the resting Ca²⁺ levels in primary cilia and if ADPKD-causing mutations alter the Ca²⁺-dependent inactivation of polycystin channels,” concludes DeCaen.