A gap junction or nexus is a specialized intercellular connection between certain animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells.
One gap junction is composed of two connexons (or hemichannels) which connect across the intercellular space. Gap junctions are analogous to the plasmodesmata that join plant cells.
A notable use of gap junctions is in the electrical synapse found in some neurons.
, gap junction hemichannels are primarily homo
- or hetero
of connexin proteins
gap junctions comprise proteins from the hypothetical innexin family
. However, the recently characterized pannexin
family, functionally similar but genetically distinct from connexins and expressed in both vertebrates and invertebrates, probably encompasses the innexins.
At gap junctions, the intercellular space narrows from 25nm to 3nm and unit connexons in the membrane of each cell are lined up with one another.
Gap junctions formed from two identical hemichannels are called homotypic, while those with differing hemichannels are heterotypic. In turn, hemichannels of uniform connexin composition are called homomeric, while those with differing connexins are heteromeric. Channel composition is thought to influence the function of gap junction channels but it is not yet known how.
Generally, the genes coding for gap junctions are classified in one of three groups, based on sequence similarity: A, B and C (for example, , ). However, genes do not code directly for the expression of gap junctions; genes can only produce the proteins which make up gap junctions (connexins). An alternative naming system based on this protein's molecular weight is also popular (for example: connexin43, connexin30.3).
Levels of organization
- DNA to RNA to Connexin protein.
- One connexin protein has four transmembrane domains
- 6 Connexins create one Connexon (hemichannel). When different connexins join together to form one connexon, it is called a heteromeric connexon
- Two hemichannels, joined together across a cell membrane comprise a Gap Junction.
When two identical connexons come together to form a Gap junction, it is called a homotypic GJ. When one homomeric connexon and one heteromeric connexon come together, it is called a heterotypic gap junction. When two heteromeric connexons join, it is also called a heteromeric Gap Junction.
- Several gap junctions (hundreds) assemble into a macromolecular complex called a plaque.
- Allows for direct electrical communication between cells, although different connexin subunits can impart different single channel conductances, from about 30 pS to 500 pS.
- Allows for chemical communication between cells, through the transmission of small second messengers, such as IP3 and Ca2+, although different connexin subunits can impart different selectivity for particular small molecules.
- Generally allows molecules smaller than 1,000 Daltons to pass through, although different connexin subunits can impart different pore sizes and different charge selectivity. Large biomolecules, for example, nucleic acid and protein, are precluded from cytoplasmic transfer between cells.
- Ensures that molecules and current passing through the gap junction do not leak into the intercellular space.
Up to date, five different functions have been adscribed to gap junction protein:
a) electrical and metabolic coupling between cells
b) Electrical and metabolic exchange through hemichannels
c) Tumor suppressor genes (Cx43, Cx32 and Cx36)
d) Adhessive function independent of conductive gap junction channel (neural migration in neocortex)
e) Role of carboxyl-terminal in signnaling citoplasmatic pathways (Cx43)
Areas of electrical coupling
Gap junctions are particularly important in cardiac muscle
: the signal to contract is passed efficiently through gap junctions, allowing the heart muscle cells to contract in tandem. However, gap junctions are now known to be expressed in virtually all tissues of the body, with the exception of mobile cell types such as sperm
. Several human genetic disorders are now associated with mutations in gap junction genes. Many of those affect the skin because this tissue is heavily dependent upon gap junction communication for the regulation of differentiation
Few locations have been discovered where there is significant coupling between neurons
in the brain. Structures in the brain that have been shown to contain electrically coupled neurons include the vestibular nucleus
, the nucleus of trigeminal nerve
, the inferior olivary nucleus
, and the Ventral Tegmental Area
. There has been some observation of weak neuron to glial cell
coupling in the locus coeruleus
, and in the cerebellum
between Purkinje neurons
and Bergmann glial cells
. It now seems that astrocytes
are strongly coupled by gap junctions. Experimental data show strong gap junction expression in astrocytes. Moreover, mutations in the gap junction genes Cx43 and Cx56.6 cause white matter degeneration similar to that observed in Pelizaeus-Merzbacher disease
and multiple sclerosis
Connexin proteins expressed in neurons include:
Neurons within the retina
show extensive coupling, both within populations of one cell type, and between different cell types.