Intracellular trafficking pathways of Cx43 gap junction channels

Abstract

Gap Junction (GJ) channels, including the most common Connexin 43 (Cx43), have fundamental roles in excitable tissues by facilitating rapid transmission of action potentials between adjacent cells. For instance, synchronization during each heartbeat is regulated by these ion channels at the cardiomyocyte cell-cell border. Cx43 protein has a short half-life, and rapid synthesis and timely delivery of those proteins to particular subdomains are crucial for the cellular organization of gap junctions and maintenance of intracellular coupling. Impairment in gap junction trafficking contributes to dangerous complications in diseased hearts such as the arrhythmias of sudden cardiac death. Of recent interest are the protein-protein interactions with the Cx43 carboxy-terminus. These interactions have significant impact on the full length Cx43 lifecycle and also contribute to trafficking of Cx43 as well as possibly other functions. We are learning that many of the known non-canonical roles of Cx43 can be attributed to the recently identified six endogenous Cx43 truncated isoforms which are produced by internal translation. In general, alternative translation is a new leading edge for proteome expansion and therapeutic drug development. This review highlights recent mechanisms identified in the trafficking of gap junction channels, involvement of other proteins contributing to the delivery of channels to the cell-cell border, and understanding of possible roles of the newly discovered alternatively translated isoforms in Cx43 biology. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Keywords: Actin; Alternative translation initiation; Connexin43; Gap junctions; Ion channel trafficking; Microtubules.

Figure 1. The Cx43 trafficking life cycle and Targeted Delivery to the cell-cell border

(A). The Cx43 lifecycle, comprising anterograde trafficking, lateral diffusion and retrograde trafficking, and highlighting cytoskeletal-based directed targeting of Cx43 to form gap junctions (GJs) at the intercalated disc (ID). Cx43 is targeted to adhesion complexes, containing N-cadherin and catenin-containing regions of the intercalated disc, along EB1-tipped microtubules to form GJ plaques at the ID. Actin is required for GJ localization, but the underlying mechanism is currently unknown. (B). The Cx43 GJ plaque formation at the ID.

Figure 2. One exon of GJA1 mRNA produces seven proteins

Western blot of nonfailing heart probed to monoclonal antibodies against the Cx43 C-terminus. (Western blot is taken from Smyth and Shaw, Cell Reports, 2013).

Figure 3. Prospective mechanism of action for GJA1-20k truncated isoform

The internally translated CJA1-20k isoform is associated with the ER and dynamic vesicles, forming highly organized contacts with F-actin and microtubule networks. GJA1-20k interacts with F-actin, provides stabilization of filamentous actin and microtubules. F-actin disruption by oxidative stress or Latrunculin A impairs growth trajectories of the EB1/microtubule-based trafficking machinery, and decreases GJ localization to the cellular border, which can be rescued by exogenous GJA1-20k.