Hot topics: X-ray crystallographic study defines binding domains for Ca2+ antagonist drugs and their molecular mechanism of action

This year witnessed a tremendous progress in our understanding of the structure-function relationship of voltage-gated Ca²⁺ channels. This is based on the cryo-electron microscopy structure of the rabbit Cav1.1 Ca²⁺ channel complex at a nominal resolution of 3.6 Å ([1] see Hot Topics Sep 20, 2016) which is now nicely complemented by a study defining the binding domains for Ca²⁺ antagonist drugs and their molecular mechanism of action at atomic resolution [2]. The authors took advantage of their elegant previous work solving the structure of bacterial Na+ channels (NavAb) by X-ray crystallography both in a pre-open and inactivated state [3,4]. They also engineered Ca²⁺ selectivity into its selectivity filter (“CavAb”, [5]) and found high affinity inhibition by the different chemical classes of Ca²⁺ antagonist drugs similar to L-type Ca²⁺ channels. Since CavAb assembles as a tetramer, this channel also replicates the four domain structure of the pore-forming subunit of voltage-gated Ca²⁺ and Na+-channels: an excellent model to investigate the drug-channel interaction at atomic resolution was now at hand.
As predicted for L-type Ca²⁺ channel α1-subunits by photoaffinity labeling studies 25 years ago [6], dihydropyridines (DHPs, e.g. amlodipine, nimodipine) bind to an extracellularly exposed intersubunit crevice formed by neighbouring S6 helices and the P-helix of the selectivity filter. Drug binding displaces an endogenous lipid molecule in this site. Interestingly, DHP binding induces a conformational changes which breaks the fourfold symmetry of the channel. As a consequence only one molecule can occupy the channel with high affinity and the Ca²⁺ interaction with the selectivity filter also changes. This results in a higher affinity for Ca²⁺ ions revealing an intriguing mechanism of action for these drugs: rather than directly blocking the pore, they enhance Ca²⁺ affinity for the pore such that Ca²⁺ itself gets stuck in the ion conducting pathway. Therefore DHPs do the opposite of what would be intuitively expected for a “Ca²⁺ antagonist”, namely preventing Ca²⁺ interaction with the channel. Instead, they allosterically enhance Ca²⁺ binding.
In contrast, and also in agreement with photoaffinity labeling and mutational studies phenylalkylamines (PAAs, e.g. Br-verapamil) bind in the central cavity on the intracellular side of the selectivity filter also disrupting fourfold symmetry. Since it is known that PAAs access this site preferentially when the channel opens its intracellular mouth upon activation this nicely explains their known frequency dependent inhibition. Unlike DHPs these drugs bind within the pore and thus must act as pore blockers thus satisfying the term “Ca²⁺ antagonist”.
This work from the Catterall lab must be regarded as a milestone in Ca²⁺ channel research. It not only revealed the mechanism of action of one of the most prescribed classes of cardiovascular drugs but also brings us much closer to predicting structural features of new generations of Ca²⁺ antagonists with high selectivity for different isoforms of voltage-gated Ca²⁺ channels. Within the L-type Ca²⁺ channel family this could be relevant for discovering Cav1.3–selective drugs as potential therapeutics for neuroprotection in Parkinson’s disease and neuropsychiatric disorders, such as autism (7).

[1] Wu et al (2016). Structure of the voltage-gated ²⁺ channel Cav1.1 at 3.6 Å resolution.
Nature 537:191-196. [PMID 27580036]
[2] Tang et al. (2016). Structural basis for inhibition of a voltage-gated Ca²⁺ channel by Ca²⁺ antagonist drugs. Nature 537, 117–121 [PMID 27556947]
[3] Payandeh et al. (2011). The crystal structure of a voltage-gated sodium channel. Nature 475, 353–358 [PMID 21743477]
[4] Payandeh et al. (2012). Crystal structure of a voltage-gated sodium channel in two potentially inactivated states. Nature 486, 135–139 [PMID 22678296]
[5] Tang et al. (2014). Structural basis for Ca²⁺ selectivity of a voltage-gated calcium channel. Nature 505, 56–61 [PMID 24270805]
[6] Catterall and Striessnig (1992). Receptor sites for Ca²⁺ channel antagonists. Trends Pharmacol Sci 13, 256–262 [PMID 1321525]
[7] Ortner and Striessnig (2016). L-type calcium channels as drug targets in CNS disorders. Channels 10: 7–13 [PMID 26039257]

Comments by Jörg Striessnig (Department of Pharmacology and Toxicology – Institute of Pharmacy, Universität Innsbruck)