Zinc inhibits the voltage-gated proton channel HCNL1.

Voltage-gated ion channels allow ion flux across biological membranes in response to changes in the membrane potential. HCNL1 is a recently discovered voltage-gated ion channel that selectively conducts protons through its voltage-sensing domain (VSD), reminiscent of the well-studied depolarization-activated Hv1 proton channel. However, HCNL1 is activated by hyperpolarization, allowing the influx of protons, which leads to an intracellular acidification in zebrafish sperm. Zinc ions (Zn2+) are important cofactors in many proteins and essential for sperm physiology. Proton channels such as Hv1 and Otopetrin1 are inhibited by Zn2+. We investigated the effect of Zn2+ on heterologously expressed HCNL1 channels using electrophysiological and fluorometric techniques. Extracellular Zn2+ inhibits HCNL1 currents with an apparent half-maximal inhibition (IC50) of 26 µM. Zn2+ slows voltage-dependent current kinetics, shifts the voltage-dependent activation to more negative potentials, and alters hyperpolarization-induced conformational changes of the voltage sensor. Our data suggest that extracellular Zn2+ inhibits HCNL1 currents by multiple mechanisms, including modulation of channel gating. Two histidine residues located at the extracellular side of the VSD might weakly contribute to Zn2+ coordination: mutants with either histidine replaced with alanine show modest shifts of the IC50 values to higher concentrations. Interestingly, Zn2+ inhibits HCNL1 at even lower concentrations from the intracellular side (IC50 ≈ 0.5 µM). A histidine residue at the intracellular end of S1 (position 50) is important for Zn2+ binding: much higher Zn2+ concentrations are required to inhibit the mutant HCNL1-H50A (IC50 ≈ 106 µM). We anticipate that Zn2+ will be a useful ion to study the structure-function relationship of HCNL1 as well as the physiological role of HCNL1 in zebrafish sperm.

Popeye domain containing proteins modulate the voltage-gated cardiac sodium channel Nav1.5.

Popeye domain containing (POPDC) proteins are predominantly expressed in the heart and skeletal muscle, modulating the K2P potassium channel TREK-1 in a cAMP-dependent manner. POPDC1 and POPDC2 variants cause cardiac conduction disorders with or without muscular dystrophy. Searching for POPDC2-modulated ion channels using a functional co-expression screen in Xenopus oocytes, we found POPDC proteins to modulate the cardiac sodium channel Nav1.5. POPDC proteins downregulate Nav1.5 currents in a cAMP-dependent manner by reducing the surface expression of the channel. POPDC2 and Nav1.5 are both expressed in different regions of the murine heart and consistently POPDC2 co-immunoprecipitates with Nav1.5 from native cardiac tissue. Strikingly, the knock-down of popdc2 in embryonic zebrafish caused an increased upstroke velocity and overshoot of cardiac action potentials. The POPDC modulation of Nav1.5 provides a new mechanism to regulate cardiac sodium channel densities under sympathetic stimulation, which is likely to have a functional impact on cardiac physiology and inherited arrhythmias.