Innexons

Innexons are the hexameric hemichannels formed by innexin proteins in invertebrate species that function as gap junction channels when two hemichannels dock from adjacent cells. They are the invertebrate counterpart to vertebrate connexons, which are built from connexins. An innexon consists of six innexin subunits arranged to create a central pore that permits passage of ions and small signaling molecules, typically under about 1 kilodalton in size. When two innexons align across a cell–cell interface, they dock to form a complete gap junction channel that enables direct electrical and metabolic coupling between neighboring cells.

Molecular structure and regulation. Innexins generally display four transmembrane domains with cytoplasmic N- and C- termini. The extracellular loops mediate docking between hemichannels, while cytoplasmic regions interact with cytoskeletal and signaling proteins. Channel gating is influenced by parameters such as transmembrane voltage, intracellular calcium concentration, and pH. Subunit composition can affect conductance and selectivity, allowing assembly into homomeric or heteromeric innexons with differing permeability profiles.

Genetics and distribution. Multiple innexin genes exist across invertebrates; expression patterns vary by tissue and developmental stage. In Caenorhabditis elegans, well-studied innexins include UNC-7, UNC-9 and INX-1, while Drosophila melanogaster features shakB as a prominent innexin gene. These genes underpin electrical synapses in neural circuits as well as gap junctions in non-neural tissues, supporting rapid, bidirectional communication.

Function and significance. Innexons mediate rapid electrical coupling that coordinates neuronal activity and tissue function. They contribute to synchronized locomotion, development, and homeostasis in invertebrates, and are a focus of comparative studies with vertebrate gap junctions to understand the evolution of intercellular communication.

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