Calmodulin-Cork Model of Gap Junction Channel Gating-One Molecule, Two Mechanisms.

Camillo Peracchia

doi:10.3390/ijms21144938

Abstract

The Calmodulin-Cork gating model is based on evidence for the direct role of calmodulin (CaM) in channel gating. Indeed, chemical gating of cell-to-cell channels is sensitive to nanomolar cytosolic calcium concentrations [Ca2+]i. Calmodulin inhibitors and inhibition of CaM expression prevent chemical gating. CaMCC, a CaM mutant with higher Ca2+-sensitivity greatly increases chemical gating sensitivity (in CaMCC the NH2-terminal EF-hand pair (res. 9–76) is replaced by the COOH-terminal pair (res. 82–148). Calmodulin colocalizes with connexins. Connexins have high-affinity CaM binding sites. Several connexin mutants paired to wild-type connexins have a high gating sensitivity that is eliminated by inhibition of CaM expression. Repeated transjunctional voltage (Vj) pulses slowly and progressively close a large number of channels by the chemical/slow gate (CaM lobe). At the single-channel level, the chemical/slow gate closes and opens slowly with on-off fluctuations. The model proposes two types of CaM-driven gating: “Ca-CaM-Cork” and “CaM-Cork”. In the first, gating involves Ca2+-induced CaM-activation. In the second, gating takes place without [Ca2+]i rise. The Ca-CaM-Cork gating is only reversed by a return of [Ca2+]i to resting values, while the CaM-Cork gating is reversed by Vj positive at the gated side.

Highlights

In most tissues, cells in contact with each other exchange cytosolic molecules of low molecular weight via channels aggregated at gap junctions
A gap junction channel is made of the interaction of two hemichannels that form a hydrophilic pathway across the two apposed plasma membranes and the extracellular space
Each connexon/innexon is an oligomer of six proteins that span the plasma membrane and create a hydrophilic pore insulated from lipid bilayer and extracellular medium

Summary

Introduction

Cells in contact with each other exchange cytosolic molecules of low molecular weight via channels aggregated at gap junctions. A gap junction channel is made of the interaction of two hemichannels (connexons/innexons) that form a hydrophilic pathway across the two apposed plasma membranes and the extracellular space (gap). Gap junction channels have been thought to possess as many as four types of gates: fast transjunctional voltage (Vj) gate, slow Vj-gate, chemical gate and gate sensitive to membrane potential (Vm). Since the behavior of the slow Vj-gate and the Vm-sensitive is the same as that of the chemical gate, most likely these gates are the same. The other, “CaM-Cork”, proposes a physical blockage of the channel’s mouth by a CaM lobe, but without Ca2+-activation. The first is only reversed by the return of intracellular Ca2+ concentration ([Ca2+]i) to resting values The latter is reversed by Vj positive at the gated side

Why Calmodulin?

CaM Inhibitors Prevent Chemical Gating

Inhibition of CaM Expression Prevents Chemical Gating

Potential Role of CaM-Activated Enzymes in Chemical Gating?

Findings

Behavior of Heterotypic Mutant-Wildtype Channels