Tuning voltage-gated channel activity and cellular excitability with a sphingomyelinase

Abstract

Voltage-gated ion channels generate action potentials in excitable cells and help set the resting membrane potential in nonexcitable cells like lymphocytes. It has been difficult to investigate what kinds of phospholipids interact with these membrane proteins in their native environments and what functional impacts such interactions create. This problem might be circumvented if we could modify specific lipid types in situ. Using certain voltage-gated K+ (KV) channels heterologously expressed in Xenopus laevis oocytes as a model, our group has shown previously that sphingomyelinase (SMase) D may serve this purpose. SMase D is known to remove the choline group from sphingomyelin, a phospholipid primarily present in the outer leaflet of plasma membranes. This SMase D action lowers the energy required for voltage sensors of a KV channel to enter the activated state, causing a hyperpolarizing shift of the Q-V and G-V curves and thus activating them at more hyperpolarized potentials. Here, we find that this SMase D effect vanishes after removing most of the voltage-sensor paddle sequence, a finding supporting the notion that SMase D modification of sphingomyelin molecules alters these lipids' interactions with voltage sensors. Then, using SMase D to probe lipid-channel interactions, we find that SMase D not only similarly stimulates voltage- gated Na+ (NaV) and Ca2+ channels but also markedly slows NaV channel inactivation. However, the latter effect is not observed in tested mammalian cells, an observation highlighting the profound impact of the membrane environment on channel function. Finally, we directly demonstrate that SMase D stimulates both native KV1.3 in nonexcitable human T lymphocytes at their typical resting membrane potential and native NaV channels in excitable cells, such that it shifts the action potential threshold in the hyperpolarized direction. These proof-of-concept studies illustrate that the voltage-gated channel activity in both excitable and nonexcitable cells can be tuned by enzymatically modifying lipid head groups. © 2013 Combs et al.