Towards a Deeper Understanding of the Diverse Roles of the CaV1.2 S6

Abstract

The CaV1.2 L-type channel is responsible for the regulated entry of calcium into most excitable cells and is extensively expressed in heart and brain. To achieve the precise tuning necessary for its diverse functions, CaV1.2 is subject to multiple forms of regulation. These include voltage-dependent channel activation, controlled membrane trafficking, and two major forms of feedback inhibition, Ca2+- dependent inactivation (CDI) and voltage-dependent inactivation (VDI). The S6 region plays an important role in this regulation and is known to be particularly important for channel activation. Furthermore, previous work predicts that manipulations at this locus which upset channel activation will consequently alter CDI. This was demonstrated in the first mutations described in the CaV1.2 IS6 region which underlie Timothy Syndrome (TS), a multisystem disorder characterized by autism, severe long-QT syndrome and developmental delays. In addition to disrupting voltage-dependent activation, these TS mutations also perturbed both VDI and CDI. Since these first mutations, an increasing number of deleterious mutations have been identified, many of which cluster near an S6 region of the channel. Here we show that some of these mutations are capable of distinctly and independently disrupting either form of channel inactivation (CDI or VDI) without altering channel activation. This represents a remarkable departure from what is expected for mutations at the S6 and specifically from the predicted connectivity between CDI and activation at this locus. In addition, we find that some mutations at the S6 also modify channel trafficking to the membrane, potentially amplifying the effects of mutations on cellular function by augmenting the membrane load of mutant channels. Together, these results illustrate important new insights into the complex and diverse roles of the S6 region in channel function and activity.