Gap Junctions Signaling

Gap junctions - molecular structure and components

In vertebrates, gap junctions are hexameric intercellular channels formed by proteins of the connexin family. Six connexin subunits assemble into a hemichannel, or connexon, which docks with another connexon on an adjacent cell to create a continuous pore that bridges the cytoplasm of the two cells.

Connexon composition is highly variable. Channels may be homomeric, composed of a single connexin type, or heteromeric, containing a mix of different connexins. These combinations result in channels with distinct electrical conductance, gating properties and permeability profiles. Nearly all cell types express connexins, although the specific isoforms vary by tissue, giving rise to specialized channel functions.

Mechanism of gap junction signaling

Gap junction signaling follows a series of steps beginning with connexin biosynthesis followed by channel formation, culminating in the direct exchange of ions and small molecules between neighboring cells.

Connexin formation

Connexins are a family of tetraspan transmembrane proteins that form the basic building blocks of gap junction channels. Individual connexins are synthesized in the endoplasmic reticulum, where they undergo folding, quality control and oligomerization. Six connexin subunits assemble into a hemichannel, also known as a connexon, within the Golgi apparatus.

Channel formation

Once assembled, connexons are trafficked to the plasma membrane, where they insert as hemichannels. Functional intercellular channels form when a connexon in one cell docks with a connexon in an adjacent cell, aligning their central pores to create a continuous channel across the intercellular gap. Clusters of these channels aggregate into gap junction plaques, which can expand, shrink or remodel in response to cellular conditions. The gating of these channels is regulated by voltage, pH, phosphorylation and intracellular calcium levels, allowing cells to dynamically open or close communication pathways.

Molecular exchange

Through these channels, cells exchange many different small molecules, typically under 1 kDa in size. These molecules include inorganic ions such as potassium and calcium, metabolic intermediates like ATP and glucose-6-phosphate and signaling molecules such as cyclic AMP (cAMP) and inositol trisphosphate (IP3). The direct passage of these molecules enables cells to synchronize electrical activity, coordinate metabolic states and propagate second messenger signals rapidly across tissues. By bypassing extracellular receptors and diffusion through the interstitial space, gap junctions provide one of the most efficient means of intercellular signaling in multicellular organisms.

Regulation of gap junctions

Multiple signaling pathways dynamically regulate the assembly, gating and turnover of gap junctions. The mechanisms include:

• Growth factor signaling
  Growth factors such as EGF and TGF-β activate intracellular signaling cascades that regulate connexin phosphorylation and trafficking in response to growth signals.
• Phosphorylation and kinase signaling
  Connexins such as Cx43 are extensively regulated by phosphorylation. Kinases including PKC, CK1, MAPKs, and Src-family kinases phosphorylate connexin residues, which directly impacts channel opening, trafficking and degradation.
• Calcium and pH sensitivity
  Intracellular calcium elevation or acidification closes gap junction channels, serving as a protective mechanism that isolates injured or stressed cells from their neighbors.
• Voltage gating
  Connexin isoforms show different sensitivity to transjunctional voltage. Some connexins close in response to voltage gradients, while others remain open. These different responses help tailor electrical coupling to tissue-specific needs such as rapid conduction in cardiac muscle or synchronization in neurons.
• Connexin turnover
  Connexins are highly dynamic proteins with half-lives of only a few hours. Gap junction plaques are removed from the plasma membrane through internalization into double-membrane vesicles called annular junctions, which are then targeted for lysosomal or proteasomal degradation. This turnover is tightly controlled by post-translational modifications such as ubiquitination and phosphorylation.

Gap junction biological functions

Gap junctions support a wide variety of biological functions that extend beyond simple molecular exchange, enabling cells to act in coordinated networks rather than as isolated units. Their roles span development, tissue repair, electrophysiological synchronization and mechanosensing.

Tissue development and regeneration

During embryonic development, gap junctions ensure coordinated differentiation and organ formation by facilitating the spread of morphogen signals and synchronizing gene expression patterns across groups of cells. Later in life, they remain essential for tissue repair, helping transmit calcium and ATP signals that direct cell migration and proliferation during wound healing and regeneration.

Electrophysiological signaling

Gap junctions enable rapid and coordinated signaling in excitable tissues. For example, within the nervous system, neuronal gap junctions form electrical synapses that synchronize activity across networks. In cardiac muscle, where the atrial and ventricular chambers need to contract coordinatedly, connexins such as Cx43 and Cx45 assemble into channels that provide low-resistance conduits for depolarizing currents.

Mechanical sensing

Gap junctions also function as mechanosensors that respond to physical forces. In endothelial and epithelial tissues, connexins detect changes in shear stress or stretch and relay these signals to neighboring cells through changes in calcium level and second messengers.

Bone homeostasis

In the skeletal system, gap junctions connect osteoblasts, osteoclasts and osteocytes into an integrated signaling network that regulates bone remodeling. Connexin 43 (Cx43) is particularly abundant in osteocytes, where it facilitates transmission of mechanical strain signals to bone-forming osteoblasts, helping to maintain skeletal integrity throughout an individual’s lifetime.

Clinical relevance of connexin dysfunction

Mutations in connexin genes are associated with a group of disorders oftentimes referred to as “channelopathies” or “communication-opathies”. For example, mutations in GJB2 (encoding connexin 26) which disrupt potassium levels and impair electrical signals in the cochlea are the most common cause of hereditary deafness.

Connexin mutations are also associated with neuropathies such as Charcot–Marie–Tooth disease, and in cardiac arrhythmias where disrupted gap junction signaling impairs conduction. Dysregulated connexin expression has additionally been observed in cancer, where altered intercellular communication influences tumor progression and metastasis.

Pharmacological modulators of connexins, including mimetic peptides and small molecules, are being explored as therapeutic agents for diseases linked to connexin dysfunction.

References

1. Goodenough DA, Paul DL. Gap junctions. Cold Spring Harb Perspect Biol. 2009;1(1):a002576.
2. Nielsen MS, Axelsen LN, Sorgen PL, Verma V, Delmar M, Holstein-Rathlou NH. Gap junctions. Compr Physiol. 2012;2(3):1981–2035.
3. Laird DW. Life cycle of connexins in health and disease. Biochem J. 2006;394(Pt 3):527–543.