Gap junction


Gap junctions are membrane channels between adjacent cells that allow the direct exchange of cytoplasmic substances, such as small molecules, substrates, and metabolites. Gap junctions were first described as close appositions alongside tight junctions, but later electron microscopy studies saw them renamed as gap junctions to distinguished them from tight junctions. They bridge a 2-4 nm gap between cell membranes.
Gap junctions use protein complexes known as connexons, composed of connexin proteins to connect one cell to another. Gap junction proteins include the more than 26 types of connexin, as well as at least 12 non-connexin components that make up the gap junction complex or nexus, including the tight junction protein ZO-1—a protein that holds membrane content together and adds structural clarity to a cell, sodium channels, and aquaporin.
More gap junction proteins have become known due to the development of next-generation sequencing. Connexins were found to be structurally homologous between vertebrates and invertebrates but different in sequence. As a result, the term innexin is used to differentiate invertebrate connexins. There are more than 20 known innexins, along with unnexins in parasites and vinnexins in viruses.
An electrical synapse is a gap junction that can transmit action potentials between neurons. Such synapses create bidirectional continuous-time electrical coupling between neurons. Connexon pairs act as generalized regulated gates for ions and smaller molecules between cells. Hemichannel connexons form channels to the extracellular environment.
A gap junction is different from an ephaptic coupling that involves electrical signals external to the cells.

Structure

In vertebrates, gap junction hemichannels are primarily homo- or hetero-hexamers of connexin proteins. Hetero-hexamers at gap junction plaques, help form a uniform intercellular space of 2-4 nm. In this way hemichannels in the membrane of each cell are aligned with one another forming an intercellular communication path.
Invertebrate gap junctions comprise proteins from the innexin family. Innexins have no significant sequence homology with connexins. Though differing in sequence to connexins, innexins are similar enough to connexins to form gap junctions in vivo in the same way connexins do.
The more recently characterized pannexin family, which was originally thought to form intercellular channels in fact functions as a single-membrane channel that communicates with the extracellular environment and has been shown to pass calcium and ATP. This has led to the idea that pannexins may not form intercellular junctions in the same way connexins and innexins do and therefore should not use the same hemi-channel/channel naming. Others have presented evidence based on genetic sequencing and overall functioning in tissues, that pannexins should still be considered part of the gap junction family of proteins despite structural differences. These researchers also note that there are still more groups of connexin orthologs to be discovered.
Gap junction channels formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. In turn, hemichannels of uniform protein composition are called homomeric, while those with differing proteins are heteromeric. Channel composition influences the function of gap junction channels, and different connexins will not necessarily form heterotypic with all others.
Before innexins and connexins were well characterized, the genes coding for the connexin gap junction channels were classified in one of three groups, based on gene mapping and sequence similarity. However, connexin genes do not code directly for the expression of gap junction channels; genes can produce only the proteins that make up gap junction channels. An alternative naming system based on the protein's molecular weight is the most widely used.

Levels of organization

In vertebrates, two pairs of six connexin proteins form a connexon. In invertebrates, six innexin proteins form an innexon. Otherwise, the structures are similar.
  1. The connexin genes are transcribed to RNA, which is then translated to produce a connexin.
  2. One connexin protein has four transmembrane domains
  3. Six connexin proteins create one connexon channel a hemichannel. When identical connexin proteins join to form one connexon, it is called a homomeric connexon. When different connexin proteins join to form one connexon, it is called a heteromeric connexon.
  4. Two connexons, joined across a cell membrane, comprise a gap junction channel.
When two identical connexons come together to form a gap junction channel, it is called a homotypic channel. When one homomeric connexon and one heteromeric connexon come together, it is called a heterotypic gap junction channel. When two heteromeric connexons join, it is also called a heterotypic gap junction channel.
  1. Tens to thousands of gap junction channels cluster in areas to enable connexon pairs to form. The macromolecular complex is called a gap junction plaque. Molecules other than connexins are involved in gap junction plaques including tight junction protein 1 and sodium channels.

    Properties of connexon pairs

A connexon or innexon channel pair:
  1. Allows for direct electrical communication between cells, although different hemichannel subunits can impart different single channel conductances, from about 30 pS to 500 pS.
  2. Allows for chemical communication between cells through the transmission of small second messengers, such as inositol triphosphate and calcium, although different hemichannel subunits can impart different selectivities for particular molecules.
  3. Generally allows transmembrane movement of molecules smaller than 485 daltons, although different hemichannel subunits may impart different pore sizes and different charge selectivity. Large biomolecules, including nucleic acids and proteins, are precluded from cytoplasmic transfer between cells through gap junction hemichannel pairs.
  4. Ensures that molecules and current passing through the gap junction do not leak into the intercellular space.

    Properties of connexons as hemichannels

Unpaired connexons or innexons can act as hemichannels in a single membrane, allowing the cell to exchange molecules directly with the exterior of the cell. It has been shown that connexons would be available to do this prior to being incorporated into the gap junction plaques. Some of the properties of these unpaired connexons are listed below:
  1. Pore or transmembrane channel size is highly variable, in the range of approximately 8-20Å in diameter.
  2. They connect the cytoplasm of the cell to the cell exterior and are thought to be in a closed state by default in order to prevent leakage from the cell.
  3. Some connexons respond to external factors by opening up. Mechanical shear and various diseases can cause this to happen.
Establishing further connexon properties different to those of connexon pairs, proves difficult due to separating their effects experimentally in organisms.

Occurrence and distribution

Gap junctions have been found in nearly all animal cells that touch each other. Techniques such as confocal microscopy allow more rapid surveys of large areas of tissue. Tissues that were traditionally considered to have isolated cells such as in bone were shown to have cells that were still connected with gap junctions, however tenuously. Exceptions to this are cells not normally in contact with neighboring cells, such as blood cells suspended in blood plasma. Adult skeletal muscle is a possible exception to the rule though their large size makes it difficult to be certain of this. An argument used against skeletal muscle gap junctions is that if they were present gap junctions may propagate contractions in an arbitrary way through cells making up the muscle. However, other muscle types do have gap junctions which do not cause arbitrary contractions. Sometimes the number of gap junctions are reduced or absent in diseased tissues such as cancers or the aging process.
Since the discovery of innexins, pannexins and unnexins, gaps in our knowledge of intercellular communication are becoming more defined. Innexins look and behave similarly to connexins and can be seen to fill a similar role to connexins in invertebrates. Pannexins also look individually similar to connexins though they do not appear to easily form gap junctions. Of the over 20 metazoan groups connexins have been found only in vertebrata and tunicata. Innexins and pannexins are far more widespread including innexin homologues in vertebrates. The unicellular Trypanosomatidae parasites presumably have unnexin genes to aid in their infection of animals including humans. The even smaller adenovirus has its own vinnexin, apparently derived from an innexin, to aid its transmission between the virus's insect hosts.
A gap junction cannot be defined by a single protein or family of proteins with a specific function. For example, gap junction structures are found in sponges, without pannexins, but may prove to indicate intercellular communications pathways.

Functions

At least five discrete functions have been ascribed to gap junction proteins:
  1. Electrical and metabolic coupling between cells
  2. Electrical and metabolic exchange through hemichannels
  3. Tumor suppressor genes
  4. Adhesive function independent of conductive gap junction channel
  5. Role of carboxyl-terminal in signaling cytoplasmic pathways
In a more general sense, gap junctions may be seen to function at the simplest level as a direct cell to cell pathway for electrical currents, small molecules and ions. The control of this communication allows complex downstream effects on multicellular organisms.

Embryonic, organ and tissue development

In the 1980s, more subtle roles of gap junctions in communication have been investigated. It was discovered that gap junction communication could be disrupted by adding anti-connexin antibodies into embryonic cells. Embryos with areas of blocked gap junctions failed to develop normally. The mechanism by which antibodies blocked the gap junctions was unclear; systematic studies were undertaken to elucidate the mechanism. Refinement of these studies suggested that gap junctions were key in the development of cell polarity and the left-right symmetry in animals. While signaling that determines the position of body organs appears to rely on gap junctions, so does the more fundamental differentiation of cells at later stages of embryonic development.
Gap junctions were found to be responsible for the transmission of signals required for drugs to have an effect. Conversely, some drugs were shown to block gap junction channels.