Connexins: a myriad of functions extending beyond assembly of gap junction channels

Abstract

Connexins constitute a large family of trans-membrane proteins that allow intercellular communication and the transfer of ions and small signaling molecules between cells. Recent studies have revealed complex translational and post-translational mechanisms that regulate connexin synthesis, maturation, membrane transport and degradation that in turn modulate gap junction intercellular communication. With the growing myriad of connexin interacting proteins, including cytoskeletal elements, junctional proteins, and enzymes, gap junctions are now perceived, not only as channels between neighboring cells, but as signaling complexes that regulate cell function and transformation. Connexins have also been shown to form functional hemichannels and have roles altogether independent of channel functions, where they exert their effects on proliferation and other aspects of life and death of the cell through mostly-undefined mechanisms. This review provides an updated overview of current knowledge of connexins and their interacting proteins, and it describes connexin modulation in disease and tumorigenesis.

Figure 1

Life cycle and protein associations of connexins. Connexins are synthesized on ER-bound ribosomes and inserted into the ER cotranslationally. This is followed by oligomerization between the ER and trans-Golgi network (depending on the connexin type) into connexons, which are then delivered to the membrane via the actin or microtubule networks. Connexons may also be delivered to the plasma membrane by direct transfer from the rough ER. Upon insertion into the membrane, connexons may remain as hemichannels or they dock with compatible connexons on adjacent cells to form gap junctions. Newly delivered connexons are added to the periphery of pre-formed gap junctions, while the central "older" gap junction fragment are degraded by internalization of a double-membrane structure called an annular junction into one of the two cells, where subsequent lysosomal or proteasomal degradation occurs, or in some cases the connexons are recycled to the membrane (indicated by dashed arrow). During their life cycle, connexins associate with different proteins, including (1) cytoskeletal components as microtubules, actin, and actin-binding proteins α-spectrin and drebrin, (2) junctional molecules including adherens junction components such as cadherins, α-catenin, and β-catenin, as well as tight junction components such as ZO-1 and ZO-2, (3) enzymes such as kinases and phosphatases which regulate the assembly, function, and degradation, and (4) other proteins such as caveolin.