HOME

TheInfoList



OR:

"Intercellular communication" refers to the varying ways and structures biological cells use to communicate with each other directly or through their environment. Not all cells use all of the proteins or mechanisms and there are likely to be more. Components of each type of intercellular communication may be involved in more than one type of communication making attempts at clearly separating the types of communication listed somewhat futile. The sections are loosely compiled from various areas of research rather than by a systematic attempt of classification by functional or structural characteristics.


Signaling within an organism


Cell signalling


Simple cell signaling

Single celled organisms will sense their environment to seek food and may send out signals to other cells to behave symbiotically or reproduce. A classic example of this is the
slime mold Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mul ...
. The slim mold shows how intercellular communication with a small molecule eg.
cyclic AMP Cyclic adenosine monophosphate (cAMP, cyclic AMP, or 3',5'-cyclic adenosine monophosphate) is a second messenger important in many biological processes. cAMP is a derivative of adenosine triphosphate (ATP) and used for intracellular signal tra ...
allows a simple organism to form from an organized aggregation of single cells. Research into cell signalling investigated a receptor specific to each signal or multiple receptors potentially being activated by a single signal. It is not only the presence or absence of a signal that is important but also the strength. Using a chemical gradient to coordinate cell growth and differentiation continues to be important as multicellular animals and plants become more complex. This type of intercellular communication within an organism is commonly referred to as cell signalling. This type of intercellular communication is typified by a small signalling molecule diffusing through the spaces around cells, often relying on a diffusion gradient forming part of the signalling response.


Cell junctions

Complex organisms may have molecules to hold the cells together which can also be involved in intercellular communication. Some binding molecules are termed the
extracellular matrix In biology, the extracellular matrix (ECM), also called intercellular matrix, is a three-dimensional network consisting of extracellular macromolecules and minerals, such as collagen, enzymes, glycoproteins and hydroxyapatite that provide s ...
and may involve longer molecules like
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...
for the cell wall in plants or collagen in animals. When the membranes of two cells are close they may form special types of cell junction which come in five broad types, adherens, desmosomes, gap, tight and tricellular junctions. Adherens, desmosomes, tight and tricellular junctions, serve structural roles. The structures they form also form parts of complex protein signaling pathways. In one respect tight junctions play a generic role in cell signaling in that they may form a tight zip around cells forming an barrier to stop even small unwanted signalling molecules getting between cells. Otherwise signalling molecules may spread to another group of cells which are not requiring the signal or allow signalling molecules escape to quickly from where they are needed.


Pannexins, connexins, innexins

Pannexins,
connexin Connexins (Cx)TC# 1.A.24, or gap junction proteins, are structurally related transmembrane proteins that assemble to form vertebrate gap junctions. An entirely different family of proteins, the innexins, form gap junctions in invertebrates. Ea ...
s, and innexins are
transmembrane protein A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequent ...
s that are all named after the Latin term nexus, meaning to connect. They are grouped as they all share a similar structure of 4 transmembrane domains crossing the cell membrane in a similar way but they do not all share enough sequence homology to allow them to be considered directly related. Earlier investigations involving the connexins demonstrated cells forming a direct connection with each other using groups of connexins but not connections with the cell exterior. As such they were not considered to participate in the extracellular cell signalling at the time. Later studies made it apparent connexins could connect directly to the cell exterior meaning they are a conduit for the release an uptake of signalling molecules from the environment external to the cell. Furthermore, pannexins appear to do this to such an extent they may rarely if ever participate in direct cell to cell coupling. As indicated on the pannexin/innexin/connexin tree on the right many animals do not appear to have pannexins/innexins/connexins perhaps indicating there may be other similar proteins still to be discovered that serve to aid intercellular communication in these animals.


Direct links between cells through intercellular cytoplasmic connection (ICC)


Septal pores

In fungi, pores crossing their cell walls that separate cellular compartments act as an ICC for the movement of molecules to their neighboring compartments. Most red algae may have pores in the cell septum that partitions a cell/filament called a pit connection. As a left over of the mitotic division it may be plugged up by the cell. There are also similar connections between neighboring cells/filaments that may allowing sharing of nutrients. Cells of a different species may initiate and form a pit connection with the host algae.


Plasmodesmata in plants

Plant cells usually have thick cell walls which need to be crossed if neighboring cells are to communicate directly. Plasmodesmata form a pipe through the cell wall forming an ICC. The pipe has another smaller membranous pipe concentric to it connecting the endoplasmic reticulum of the two cells via a tube called the desmotubule. The larger pipe also contains cytoskeletal and other elements. It is presumed viruses use plasmodesmata as a route through the cell walls to spread through the plant.


Gap junctions in animals

Gap junctions can form intercellular links, effectively a tiny direct regulated "pipe" called a
connexon 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. This channel allows for bidirectional flow of ions and ...
pair between the cytoplasms of the two cells that form the junction. 6 connexins make a connexon, 2 connexons make a connexon pair so 12 connexin proteins build each tiny ICC. This ICC allows two cells to communicate directly while being sealed from the outside world. Cells may form one or thousands of these tiny ICCs between them and their other neighbors, potentially forming large networks of directly linked cells. The connexon pairs form ICCs that can transport water, many other molecules up to around 1000 atoms in size and can be very rapidly signaled to turn on and off as required. These ICCs are also communicating electrical signals that can be rapidly turned on and off. To add to their versatility there are a range of these ICC types due to their being over 20 different connexins with different properties that can combine with each other in a variety of ways. The variety of potential signaling combinations that results is enormous. A much studied example of gap junctions electrical signalling abilities is in the
electrical synapse Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by ...
s found on nerves. In heart muscle gap junctions function to coordinate the beating of the heart. Adding even further to their versatility gap junctions can also function to form a direct connection to the exterior of a cell paralleling the functioning of the protein cousin the pannexins which are explained elsewhere.


Intercellular bridge

Intercellular bridges are larger than gap junction ICCs so are able to allow the movement of not only small signaling molecules but also large DNA molecules or even whole cell organelles. They are maintained between two cells allowing them to exchange cytoplasmic contents and are frequently observed when cells need intimate communication such as when they are reproducing. They are found in
Prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...
s for exchanging DNA, small organisms such as
Pinnularia ''Pinnularia'' is a genus of fresh water algae, more specifically a type of diatom. Habitat ''Pinnularia'' is a predominantly fresh-water algae usually found in ponds and moist soil. They can also be found in springs, estuaries, sediments, and ...
,
Valonia ventricosa ''Valonia ventricosa'', also known as bubble algae, sea grape or sailor's eyeballs is a species of algae found in oceans throughout the world in tropical and subtropical regions, within the phylum Chlorophyta. It is one of the largest known unice ...
,
Volvox ''Volvox'' is a polyphyletic genus of chlorophyte green algae in the family Volvocaceae. It forms spherical colonies of up to 50,000 cells. They live in a variety of freshwater habitats, and were first reported by Antonie van Leeuwenhoek in 170 ...
,C elegans and mitosis generally (
Cytokinesis Cytokinesis () is the part of the cell division process during which the cytoplasm of a single eukaryotic cell divides into two daughter cells. Cytoplasmic division begins during or after the late stages of nuclear division in mitosis and mei ...
),
Blepharisma ''Blepharisma'' is a genus of unicellular ciliate protists found in fresh and salt water. The group includes about 40 accepted species, and many sub-varieties and strains. While species vary considerably in size and shape, most are easily iden ...
for sexual reproduction and during
Meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately r ...
including
Spermatocytogenesis Spermatocytogenesis is the male form of gametocytogenesis and involves stem cells dividing to replace themselves and to produce a population of cells destined to become mature sperm. The stem cells involved are called spermatogonia and are a s ...
to synchronise development of germ cells and oogenesis in larger organisms. Bridges have shown to assist in cell migration as shown in the adjacent picture. Cytoplasmic bridges can also be used to attack another cell as in the case of Vampirococcus.


Vesicles

Lipid membrane bound vesicles of a large range of sizes are found inside and outside of cells, containing a huge variety of things ranging from food to invading organisms, water to signaling molecules. Using an electrical nerve impulse from a
neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...
of a neuromuscular junction to stimulate a muscle to contract is an example of very small (about 0.05μm) vesicles being directly involved in regulating intercellular communication. The neuron produces thousands of tiny vesicles, each containing thousands of signalling molecules. One vesicle is released close to the muscle every second or so when resting. When activated by a nerve impulse more than 100 vesicles will be released at once, hundreds of thousands of signalling molecules, causing a significant contraction of the muscle fiber. All this happens in a small fraction of a second. Generally small vesicles used to transport signalling molecules released from the cell are termed exosomes or simply extracellular vesicles, and in addition to their importance to the organism are also important for
biosensor A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector. The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell rece ...
s. Examples of larger vesicles are in regulatory secretary pathways in endocrine,
exocrine Exocrine glands are glands that secrete substances on to an epithelial surface by way of a duct. Examples of exocrine glands include sweat, salivary, mammary, ceruminous, lacrimal, sebaceous, prostate and mucous. Exocrine glands are one of t ...
tissues,
transcytosis Transcytosis (also known as cytopempsis) is a type of transcellular transport in which various macromolecules are transported across the interior of a cell. Macromolecules are captured in vesicles on one side of the cell, drawn across the cell, and ...
and the
vesiculo-vacuolar organelle A vesiculo-vacuolar organelle (VVO) is an organelle that contributes to endothelial cell permeability. VVOs are found in the endothelium of normal blood vessels and vessels associated with tumors or allergic inflammation. VVOs actively transpor ...
(VVO) in endothelial and perhaps other cell types. Another form of transfer of pieces of membrane around junctions is called trans-endocytosis. Some large intercellular vesicles also appear to stay intact as they transport their contents from one part of a tissue to another and involve gap junction plaques.


Ecosystem intercellular communication

The definition of biological communication is not simple. In the field of cell biology early research was at a cellular to organism level. How the individual cells in one organism could affect those in another was difficult to trace and not of primary concern. If intercellular communication includes one cell transmitting a signal to another to elicit a response, intercellular communication is not restricted to the cells within a single organism. Over short distances interkingdom communication in plants is reported. In-water reproduction often involves vast synchronized release of
gametes A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...
called
spawn Spawn or spawning may refer to: * Spawn (biology), the eggs and sperm of aquatic animals Arts, entertainment, and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** '' Spawn: ...
ing. Over large distances cells in one plant will communicate with cells in another plant of the same species and other species by releasing signals into the air such as green leaf volatiles that can, among other things, pre-warn neighbors of herbivores or in the case of ethylene gas the signal triggers
ripening Ripening is a process in fruits that causes them to become more palatable. In general, fruit becomes sweeter, less green, and softer as it ripens. Even though the acidity of fruit increases as it ripens, the higher acidity level does not make th ...
in fruits. Intercellular signalling in plants can also happen below ground with the
mycorrhizal network A Mycorrhizal network (also known as a common mycorrhizal network or CMN) is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects indivi ...
which can link large areas of plants via fungal networks allowing the redistribution of environmental resources. Looking at insect colonies such as bees and
ant Ants are eusocial insects of the family Formicidae and, along with the related wasps and bees, belong to the order Hymenoptera. Ants evolved from vespoid wasp ancestors in the Cretaceous period. More than 13,800 of an estimated total of ...
s we have discovered the pheromones released from one organism's cells to another organism's cells can coordinate colonies in a way reminiscent of
slime mold Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms with a life cycle that includes a free-living single-celled stage and the formation of spores. Spores are often produced in macroscopic mul ...
s. Pheromones have been shown to act across species. Cell to cell signalling using pheromones is also recognized in complex vertebrate animals including between species. The idea that intercellular communication is so similar among cells within an organism as well as cells between different organisms, even prey, is demonstrated by
vinnexin Vinnexin is a transmembrane protein whose DNA code is held in a virus genome. When the virus genome is expressed in a cell the vinnexin gene from the virus is made into a functioning protein by the infected cell. The vinnexin protein is then inco ...
. This protein is a modified form of an innexin protein found in a caterpillar. That is, the vinnexin is very similar to the caterpillar's own innexin, and could only have been derived from a non-viral innexin in some way that is unclear. The caterpillar innexin forms normal intercellular connections inside the caterpillar as part of the caterpillar's immune response to an egg implanted by a parasitic wasp. The innexin helps ensure the wasp egg is neutralized, saving the caterpillar from the parasite. So what does the vinnexin do and how? Evolution has led to a virus that communicates with the wasp in a way that evades the wasps antiviral responses, allowing the virus to live and replicate in the wasps ovaries. When the wasp injects its egg into the caterpillar host many virus from the wasp's ovary are also injected. The virus particles do not replicate in the caterpillar cells but rather communicate with the caterpillars genetic machinery to produce vinnexin protein. The vinnexin protein incorporates itself into the caterpillar's cells altering the communication in the caterpillar so the caterpillar goes on living but with an altered immune response. Vinnexins are able to mix with normal innexins to alter communication within the caterpillar and probably do. The altered communication within the caterpillar prevents the caterpillar's defenses rejecting the wasps egg. As a result, the wasp egg hatches, consumes the caterpillar and the virus the wasp larva's mother used, and repeats the cycle. It can be seen the virus and wasp are essential to each other and communicate well with each other to allow the virus to live and replicate, but only in a non-destructive way inside the wasp ovary. The virus is injected into a caterpillar by the wasp, but the virus does not replicate in the caterpillar, the virus only communicates with the caterpillar to modify it in a non-lethal way. The wasp larvae will then slowly eat the caterpillar without being stopped while communicating with the virus again to ensure that the wasp has a place in its ovary for it to again replicate. Connexins/innexins/vinnexins, once thought to only participate in providing a path for signaling molecules or electrical signals have now been shown to act as a signaling molecule itself.


References

{{Reflist, 2 Cell biology Cell communication Cell anatomy Cell signaling Systems biology