Gap junction

Vertebrate gap junction
Light microscope images do not allow us to see connexons themselves but do let us see the fluorescing dye injected into one cell moving into neighboring cells when gap junctions are known to be present
Annular gap junction cross section in TEM thin section. Gap junctions are usually linear rather than annular in TEM thin sections. It is thought that annular gap junctions result from engulfment by one of the two cells of the membrane plaque to form a vesicle within the cell. This example shows three layers to the junction structure. The membrane from each cell is the dark line with the whiter narrow gap between the two darkly stained membranes. In such electron micrographs there may appear to be up to 7 layers. Two lipid mono-layers in each membrane can stain as 3 layers plus one layer from the gap between them, similar to two stacked bread sandwiches with space between them

Gap junctions are specialized intercellular connections between a multitude of animal cell-types.

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Connexin

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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. This image was prepared by Hanaa Hariri for Dbouk et al., 2009.

Connexins (Cx) (TC# 1.A.24), or gap junction proteins, are structurally related transmembrane proteins that assemble to form vertebrate gap junctions.

Plasmodesma

Plasmodesmata (singular: plasmodesma) are microscopic channels which traverse the cell walls of plant cells and some algal cells, enabling transport and communication between them.

Tobacco mosaic virus movement protein 30 localizes to plasmodesmata

Similar structures, called gap junctions and membrane nanotubes, interconnect animal cells and stromules form between plastids in plant cells.

Cardiac muscle

One of three types of vertebrate muscle tissue, with the other two being skeletal muscle and smooth muscle.

3D rendering showing thick myocardium within the heart wall.
The swirling musculature of the heart ensures effective pumping of blood.
Cardiac muscle
Illustration of a cardiac muscle cell.
Intercalated discs are part of the cardiac muscle cell sarcolemma and they contain gap junctions and desmosomes.
Dog cardiac muscle (400X)

The pacemaker cells are only weakly contractile without sarcomeres, and are connected to neighboring contractile cells via gap junctions.

Connexon

Connexon and connexin structure

In biology, a connexon, also known as a connexin hemichannel, is an assembly of six proteins called connexins that form the pore for a gap junction between the cytoplasm of two adjacent cells.

Astrocyte

Astrocytes (from Ancient Greek ἄστρον, ástron, "star" + κύτος, kútos, "cavity", "cell"), also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord.

An astrocyte from a rat brain grown in tissue culture and stained with antibodies to GFAP (red) and vimentin (green). Both proteins are present in large amounts in the intermediate filaments of this cell, so the cell appears yellow. The blue material shows DNA visualized with DAPI stain, and reveals the nucleus of the astrocyte and of other cells. Image courtesy of EnCor Biotechnology Inc.
Astrocytes (green) in the context of neurons (red) in a mouse cortex cell culture
23-week-old fetal brain culture human astrocyte
Astrocytes (red-yellow) among neurons (green) in the living cerebral cortex
Astrocytes are depicted in red. Cell nuclei are depicted in blue. Astrocytes were obtained from brains of newborn mice.
Metabolic interactions between astrocytes and neurons
Fig. 6 The conjectured switching role of glia in the biological neural detection scheme as suggested by Nossenson et al.

Astrocytes are linked by gap junctions, creating an electrically coupled (functional) syncytium.

Cerebellum

Major feature of the hindbrain of all vertebrates.

Drawing of the human brain, showing cerebellum and pons
View of the cerebellum from above and behind
Schematic representation of the major anatomical subdivisions of the cerebellum. Superior view of an "unrolled" cerebellum, placing the vermis in one plane.
Purkinje cells in the human cerebellum (in orange, from top to bottom 40X, 100X and 200X magnification) stained according to published methods
A mouse Purkinje cell injected with fluorescent dye
Granule cells (GR, bottom), parallel fibers (horizontal lines, top), and Purkinje cells (P, middle) with flattened dendritic trees
Diagram of the layers of the cerebellar cortex showing a glomerulus in the granular layer.
Sagittal cross-section of human cerebellum, showing the dentate nucleus, as well as the pons and inferior olivary nucleus
Schematic illustration of the structure of zones and microzones in the cerebellar cortex
Model of a cerebellar perceptron, as formulated by James Albus
Ultrasound image of the fetal head at 19 weeks of pregnancy in a modified axial section, showing the normal fetal cerebellum and cisterna magna
Cross-section of the brain of a porbeagle shark, with the cerebellum highlighted in blue
Base of the human brain, as drawn by Andreas Vesalius in 1543

Moreover, olivary neurons that send climbing fibers to the same microzone tend to be coupled by gap junctions, which synchronize their activity, causing Purkinje cells within a microzone to show correlated complex spike activity on a millisecond time scale.

GJA1

Protein that in humans is encoded by the GJA1 gene on chromosome 6.

connexin 43 carboxyl terminal domain
Connexin 43 distribution in the rat myocardium (gap junctions between cardiomyocytes)
Predicted secondary structure and sequence conservation of IRES_Cx43

As a connexin, GJA1 is a component of gap junctions, which allow for gap junction intercellular communication (GJIC) between cells to regulate cell death, proliferation, and differentiation.

Cell junction

Cell junctions (or intercellular bridges ) are a class of cellular structures consisting of multiprotein complexes that provide contact or adhesion between neighboring cells or between a cell and the extracellular matrix in animals.

Some examples of cell junctions
This image shows a desmosome junction between cells of the epidermal layer of the skin.
The cartoon of epithelium cells connected by tricellular junctions at the regions where three cells meet.

Cell junctions are also especially important in enabling communication between neighboring cells via specialized protein complexes called communicating (gap) junctions.

Electrical synapse

Diagram of a gap junction

An electrical synapse is a mechanical and electrically conductive link between two neighboring neurons that is formed at a narrow gap between the pre- and postsynaptic neurons known as a gap junction.

Membrane channel

Membrane channels are a family of biological membrane proteins which allow the passive movement of ions (ion channels), water (aquaporins) or other solutes to passively pass through the membrane down their electrochemical gradient.

Cross-sectional view of the structures that can be formed by phospholipids in an aqueous solution

A hemichannel is defined as one-half of a gap junction channel.