Cellular excitability is central to many physiological processes such as nervous impulse propagation, cardiac action potential generation and muscular cell excitation and contraction. The key players in these processes are voltage-gated ion channels, membrane proteins which are able to sense changes in the transmembrane potential and conduct ions across the plasma membrane in a voltage-dependent manner. The sodium-selective voltage-gated channels (Nav) are particularly crucial since they are responsible of the rapid rising phase of the action potential through the outward conduction of sodium ions. This is made possible by a fast activation mechanism followed by a relatively fast inactivation mechanism that allows voltage-gated potassium channels (Kv) to then carry out the repolarizing phase of the action potential through the import of potassium ions. The functional cycle of Kv channels is well understood, in large part because high-resolution structures have been available since the early 2000s. On the other hand, the functional cycle of Nav channels is yet mostly uncharacterized. Several reasons can be pinpointed: 1- Eukaryotic Nav channels are heterotetramers (Kv channels are homotetramers), thus increasing the complexity of the mechanisms involved; 2- Nav channels are fast (order of microseconds) and thus more difficult to study using traditional electrophysiology techniques (Kv channels operate on the millisecond timescale); 3- Contrary to Kv channels, Nav channels undergo fast inactivation. While their activation mechanism can be inferred from the one of Kv channels, the structural basis for fast inactivation is largely hypothetical and 4- high resolution structures of eukaryotic Nav channels have only been available since 2017.
In this project, we will test the following hypothesis: The structures of bacterial and eukaryotic Nav channels currently available represent the inactive state. The inactivation mechanism involves a secondary structure change of the pore lining helices similar to what is observed in phylogenetically related TRP channels. The results pave the way to the design of new Nav channel modulators.