CELL COMMUNICATION: TYPES, IMPORTANCE, EXAMPLES - BIOLOGY - 2025

The cellular communication, also called intercellular communication, is the transmission of extracellular signal molecules. These molecules start from a signal-generating cell and bind to receptors on the target cell, producing a specific response.

The signal molecule can be a small molecule (example: an amino acid), a peptide, or a protein. Therefore, communication, which is chemical, is a characteristic of unicellular and multicellular organisms.

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In bacteria, the signal molecules are bacterial pheromones. These are necessary for functions such as horizontal gene transfer, bioluminescence, biofilm formation, and the production of antibiotics and pathogenic factors.

In multicellular organisms, cellular communication can take place between cells that are adjacent, or between cells that are separate. In the latter case, the signal molecules must diffuse and travel long distances. Among the functions of the signals are changes in gene expression, morphology and cell movement.

Cell communication can also be carried out by extracellular vesicles (EV), called ectosomes and exosomes. Some functions of EVs are: modulation of lymphocytes and macrophages; control of synaptic function; in blood vessels and the heart, coagulation and angiogenesis; and RNA exchange.

Types (systems / mechanisms)

In bacteria, there is a type of cellular communication called quorum sensing, which consists of behaviors that occur only when the density of the bacterial population is high. Quorum sensing involves the production, release, and subsequent detection of high concentrations of signal molecules, termed autoinducers.

In unicellular eukaryotes, such as T. brucei, there is also quorum sensing. In yeasts, sexual behavior and cell differentiation occurs in response to pheromone communication and environmental changes.

In plants and animals, the use of extracellular signal molecules, such as hormones, neurotransmitters, growth factors or gases, is an important type of communication that involves the synthesis of the signal molecule, its release, its transport to the target cell, detection signal and specific response.

In relation to the transport of the signal molecule in animals, the action distance of the molecule determines two types of signals: 1) autocrine and paracrine, which act, respectively, on the same cell and on nearby cells; and 2) endocrine, which acts on a distant target cell, being transported by the bloodstream.

Cellular communication by extracellular vesicles is an important type of cellular communication in eukaryotic organisms and Archaea.

As the single-celled eukaryotic or bacterial population grows, it reaches a sufficient number of cells, or quorum, that produces the concentration of inducer capable of producing an effect in the cells. This constitutes a census-taking mechanism.

Three types of quorum sensing systems are known in bacteria: one is gram-negative; another in gram-positive; and another on the gram negative Vibrio harveyi.

In gram-negative bacteria, the autoinducer is acylated homoserine lactone. This substance is synthesized by the LuxI-type enzyme and passively diffuses through the membrane, accumulating in the extracellular and intracellular space. When the stimulating concentration is reached, the transcription of genes regulated by QS is activated.

In gram-negative bacteria, autoinducers are modified peptides, which are exported to the extracellular space, where they interact together with membrane proteins. A phosphorylation cascade occurs that activates proteins, which bind to DNA and control the transcription of target genes.

Vibrio harveyi produces two autoinducers, designated HAI-1 and A1-2. HAI-1 is acylated lactone homoserine, but its synthesis is not dependent on LuxI. A1-2 is furanosyl borate diester. Both substances act through a phosphorylation cascade similar to that of other gram-negative bacteria. This type of QS controls bioluminescence.

Chemical communication

Specific binding of the signal molecule, or ligand, to the receptor protein produces a specific cellular response. Each type of cell has certain types of receptors. Although a certain type of receptor can also be found in different types of cells, and produce different responses to the same ligand.

The nature of the signal molecule determines the pathway that will be used to enter the cell. For example, hydrophobic hormones, such as steroids, diffuse through the lipid bilayer and bind to receptors to form complexes that regulate the expression of specific genes.

Gases such as nitric oxide and carbon monoxide diffuse through the membrane and generally activate cyclic GMP-producing guanylyl cyclase. Most of the signal molecules are hydrophilic.

Its receptors are found on the cell surface. The receptors act as signal translators that alter the behavior of the target cell.

Cell surface receptors are divided into: a) G protein coupled receptors; b) receptors with enzyme activity, such as tyrosine kinase; and c) ion channel receptors.

Characteristics of G protein-coupled receptors

G protein-coupled receptors are found in all eukaryotes. In general, they are receptors with seven domains that cross the membrane, with the N-terminal region towards the outside of the cell and the C-terminal towards the inside of the cell. These receptors associate with a G protein that translates the signals.

When the ligand binds to the receptor, the G protein becomes activated. This in turn activates an effector enzyme that produces a second intracellular messenger, which can be cyclic adenosine monophosphate (cAMP), arachidonic acid, diacylglycerol, or inositol-3-phosphate, which acts as a signal amplifier. initial.

Protein G has three subunits: alpha, beta and gamma. Activation of the G protein involves the dissociation of GDP from the G protein and the binding of GTP to the alpha subunit. In the G _alpha -GTP complex they dissociate from the beta and gamma subunits, interacting specifically with effector proteins, activating them.

The cAMP pathway can be activated by beta-adrenergic receptors. CAMP is produced by adenylyl cyclase. The phosphoinositol pathway is activated by muscarinic acetylcholine receptors. They activate phospholipase C. The arachidonic acid pathway is activated by the histamine receptor. Activates phospholipase A2.

CAMP pathway

Binding of the ligand to the receptor, the stimulatory protein G (G _s), bound to GDP, causes the exchange of GDP for GTP, and the dissociation of the alpha subunit of G _s from the beta and gamma subunits. The G _alpha -GTP complex associates with a domain of adenyl cyclase, activating the enzyme, and producing cAMP from ATP.

CAMP binds to the regulatory subunits of the cAMP-dependent protein kinase. Releases catalytic subunits, which phosphorylate proteins that regulate cellular responses. This pathway is regulated by two types of enzymes, namely phosphodiesterases, and protein phosphatases.

Phosphoinositol pathway

Binding of the ligand to the receptor activates the G protein (G _q), which activates phospholipase C (PLC). This enzyme breaks phosphatidyl inositol 1,4,5-bisphosphate (PIP ₂) into two second messengers, inositol 1,4,5-triphosphate (IP ₃) and diacylglycerol (DAG).

IP ₃ diffuses into the cytoplasm and binds to receptors in the endoplasmic reticulum, causing the release of Ca ⁺² from within. DAG remains in the membrane and activates protein kinase C (PKC). Some isoforms of PKC require Ca ⁺².

Arachidonic acid pathway

Binding of the ligand to the receptor causes the beta and gamma subunits of the G protein to activate phospholipase A ₂ (PLA ₂). This enzyme hydrolyzes phosphatidylinositol (PI) in the plasma membrane, releasing arachidonic acid, which is metabolized by different pathways, such as 5 and 12-lipoxygenase and cyclooxygenase.

Characteristics of receptor tyrosine kinase

Receptor tyrosine kinase (RTK) have extracellular regulatory domains and intracellular catalytic domains. Unlike the G protein-coupled receptor, the polypeptide chain of receptor tyrosine kinase crosses the plasma membrane only once.

The binding of the ligand, which is a hormone or growth factor, to the regulatory domain causes the two receptor subunits to associate. This allows for autophosphorylation of the receptor on a tyrosine residue, and activation of protein phosphorylation cascades.

Phosphorylated tyrosine residues of the receptor tyrosine kinase (RTK) interact with adapter proteins, which link the activated receptor to components of the signal transduction pathway. Adapter proteins serve to form multiprotein signaling complexes.

RTK binds to different peptides, such as: epidermal growth factor; fibroblast growth factors; brain growth factors; nerve growth factor; and insulin.

General characteristics of the receivers

Activation of surface receptors produces changes in protein phosphorylation by activating two types of protein kinases: tyrosine kinase and serine and threonine kinases.

The serine and threonine kinases are: cAMP-dependent protein kinase; cGMP-dependent protein kinase; protein kinase C; and the Ca ⁺² / Calmodulin- dependent protein. In these protein kinases, with the exception of the cAMP-dependent kinase, the catalytic and regulatory domain is found on the same polypeptide chain.

The second messenger binds to these serine and threonine kinases, activating them.

Characteristics of receptors that are ion channels

Ion channel receptors have the following characteristics: a) they conduct ions; b) recognize and select specific ions; c) open and close in response to chemical, electrical or mechanical signals.

Ion channel receptors can be a monomer, or they can be heteroligomers or homoligomers, whose regions of the polypeptide chain cross the plasma membrane. There are three families of ion channels: a) ligand gate channels; b) gap junction channels; and c) Na ⁺ -dependent voltage channels.

Some examples of ion channel receptors are the neuromuscular junction acetylcholine receptors and the ionotropic glutamate receptors, NMDA and non-NMDA, in the central nervous system.

Communication via extracellular vesicles

Extracellular vesicles (EV) are a mixture of ectosomes and exosomes, which are responsible for transmitting biological information (RNA, enzymes, reactive oxygen species, etc.) between cell and cell. The origin of both vesicles is different.

Ectosomes are vesicles produced by sprouting from the plasma membrane, followed by their separation and release into the extracellular space.

First, the clustering of membrane proteins into discrete domains occurs. Then the protein lipid anchors accumulate cytosolic proteins and RNA in the lumen, thereby growing the bud.

Exosomes are vesicles that form from multivesicular bodies (MVB) and are released by exocytosis into the extracellular space. MVBs are late endosomes, in which there are intraluminal vesicles (ILVs). MVBs can fuse to lysosomes and continue the degradative pathway, or release ILVS as exosomes through exocytosis.

EVs interact with the target cell in different ways: 1) vanishing of the EV membrane and release of the active factors within it; 2) EVs establish contact with the surface of the target cell, which they fuse, releasing their content in the cytosol; and 3) EVs are captured entirely by macropinocytosis and phagocytosis.

Importance

The wide variety of functions of intercellular communication alone indicates its importance. Some examples illustrate the importance of different types of cellular communication.

- Importance of quorum sensing. QS regulates various processes such as virulence within a species, or microorganisms of different species or genera. For example, one strain of Staphylococcus aureus uses a signal molecule in the quorum sensing to infect the host, and inhibits other strains of S. aureus from doing so.

- Importance of chemical communication. Chemical marking is necessary for the survival and reproductive success of multicellular organisms.

For example, programmed cell death, which regulates multicellular development, removes entire structures and allows the development of specific tissues. All of this is mediated by trophic factors.

- Importance of EVs. They play an important role in diabetes, inflammation, and neurodegenerative and cardiovascular diseases. The EVs of normal cells and cancer cells differ greatly. EVs can carry factors that promote or suppress the cancer phenotype in target cells.

References

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