Functional Classification of Cell Junctions
In vertebrates, tissues are attached to each other via cell junctions. Invertebrates and plants also have some type of cell junctions. These all have been discussed in detail below.
Basically three types of cell junctions are there.
1. Tight junctions (vertebrates only) These are also known as zonula(e) occludens.
Tight junctions are found only in vertebrates.
These have branching network of sealing strands. More the number of strands, greater is the tightness in the junction.
Major proteins which form these strands are “occludins” and “claudin”. These are transmembrane proteins. These proteins are joined to peripheral protein (e.g. ZO-1) —which further—-joins to actin in the interior of the cell.
They form virtually impermeable barriers between two cells. Through these barriers fluid cannot pass at all. Barrier can be protective or functional.
Important: Maintains polarity of cell. Polarity means two surface are there: apical and basolateral. Apical surface faces towards the lumen of the intestine and the basal surface is away from the lumen. Basolateral means (basal + lateral) surface. As a result of polarity, receptor mediated endocytosis occurs at the apical surface, and exocytosis occurs at the basal surface. This is called as transcellular transport; and this type of barrier is known as protective barrier.
Tight junctions disallow movement of integral proteins between apical and basolateral surface. So, only active transport and diffusion is possible. This helps in maintaining osmotic balance of the cell. Because of this, tight junction is called as functional barrier.
Extent of tight junction will be found out by freeze-fracture method.
Found in tight epithelia of bile ducts, collecting duct and DCT. These internal structures have one to two layers of cells. Also found in skin (within many layers).
Absent in leaky epithelia of PCT.
Septate junctions (invertebrates mainly)– In invertebrates, the corresponding occluding junctions are called as septate junctions.
Appear ladder like
ANCHORING JUNCTIONS- cells convert into tissue and organs by anchoring to each other. Cells are anchored to each other and to contents of Extracellular matrix.
Actin filament attachment sites- when the cell junctions joins to actin filaments at the inside of the cell.
1. Cell-cell junctions (Adherens junctions): contain Cadherins
Important to note: Cadherins are always important in cell-cell junctions. If they are not there, then cell-cell binding will be hampered. This may lead to diseases like pemphigus vulgaris (in which, epidermal cells lose adhesion, form blisters and fall off).
CADHERINS: single pass transmembrane glyco-proteins 700-750 amino acids long present as dimers or oligomers in plasma membrane. Because they are transmembrane proteins so they will have three domains: extracellular, intermediate and intracellular. The extracellular domain further has 5 domains. Each domain is bound by calcium. They bind two cells together via homophilic binding.
Also called as Zonula adherens
Cells are linked together via linker protein. In these junctions, linker protein is calcium dependent protein known as Cadherins.
Cadherins are calcium-dependent transmembrane proteins. Like all transmembrane proteins, these proteins have three parts: outer N-terminal domain, transmembrane domain and inner C-terminal domain. The outer domain is different in different cadherins. One type of cadherin (E, P and N) binds to same type of cadherin always.
The inner C-terminal domain binds to proteins known as alpha, beta and gamma catenin. These further binds to actin.
This binding of actin of first cell———Catenins of first cell——–Cadherin of first cell——– Cadherin of second cell——- catenins of second cell——–actin of second cell; forms a strong binding between two cells, therefore, called as adherens junctions.
Present on apical surface just below tight junctions; known as adhesion belt.
2. Cell-matrix junctions (focal adhesions)
These are mediated by the aid of a special class of proteins known as Integrins.
INTEGRINS: They are principle cell-matrix adhesion molecules. Exception: Some integrins mediate cell-cell binding by heterophilic binding but this is rare.
They are trans-membrane proteins,
Extra-cellular domain: has two glycoprotein subunits: alpha and beta. These two proteins binds to proteins like fibronectin, fibrinogen, collagen etc. of ECM. Binding depends on Ca and Mg.
Intracellular domain: This portion binds to adaptor proteins like talin, vinculin, alpha-actinin. These further bind to actin and intermediate filaments.
Intermediate filament attachment sites-when the cell junction joins to intermediate filaments at the inside of the cell.
1. Cell-cell junctions (Desmosomes): contain cadherins
Also called as maculla adherens.
The linker proteins are three calcium dependent cadherin proteins, desmocollin I, desmocollin II and desmoglein. Each protein has four extracellular domains; just like cadherins.
Cytoplasmic domains linked to intermediate filaments through a cytoplasmic structure known as plague.
Cytoplasmic Plague: a mixture of proteins plakoglobin and desmoplakin.
Button-like point of attachment between two cells.
Cadherin extracellular domain—–(on outer side)——bind to plague—(inner side of plaque)—-keratin in epithelial cells.
In heart, desmin filaments are found instead of keratin.
2. Cell-matrix junctions (Hemi-desmosomes)
Half desmosomes because they resemble desmosomes.
Connect basal surface of epithelial cells to basal lamina. In other words, they connect epithelial cells to connective tissue.
Transmembrane protein is present between epithelial cell and connective cell.
All these protein are linker protein. Here, linker protein is integrins.
COMMUNICATING JUNCTIONS: allow communication between cells.
1. Gap junctions:
Also called as nexus or macula communicans.
Direct connection between cytoplasm of adjacent cells.
They act as channels.
Cells are about 3-4nm away from each other.
Allow inorganic ions and small water soluble molecules of 1000Da to pass through them.
Couple the cells electrically and metabolically.
Present in all tissues of the body; except fully developed (please note) skeletal muscle and mobile cells like RBCs and sperms. In lower invertebrates like sponges, slime molds these are absent.
Linker protein is Connexin in vertebrates and Innexin in invertebrates. Recently, Pannexins have been discovered.
Six Connexin couple to form a cylinder which is a pore in the center, known as Connexon. A connexon is a hemi-channel (hemi means half). Two hemi-channels bind to form a continuous channel.
2. Chemical synapses
In this the cells are at about 20-40nm from each other.
Through these neuronic signals are sent to other neurons or non-neuronic cells like muscles and glands.
One neuron contains neurotransmitter (in a vesicle). On receiving a stimulus this neurotransmitter is released into the space between the two neurons. The second neuron contains a receptor. To this receptor, the neurotransmitter binds, giving the stimulus to the second neuron. Since the connection is because of a chemical, i.e. a neurotransmitter, therefore, it is called as chemical synapse.
Adult human brain contain about 100 to 500 trillion (1012) synapses.
3. Plasmodesmata (plants only):
Gap junctions of plants. Please note that they are structurally unrelated to gap junctions of animals.
50-60 nm in diameter.
A typical plant cell can have about 103 to 105
Made up of three layers:
Plasma membrane: outer most layer of plasmodesmata. Plasma membrane of one cell is connected to plasma membrane of second cell. At this point cell wall is not there.
Cytoplasmic sleeve: this is an extension of cytosol. The cytosol present in the junction point is called as cytoplasmic sleeve. Because of this cytoplasm, movement of ions is possible.
Desmotubule: Flattened Smooth Endoplasmic reticulum, which passes through this pore. Through this channel, some protein molecules pass. Mainly molecules pass through cytoplasmic sleeve only.