We explain how cells talk to each other.
The cells of multicellular organisms do not perform their functions independently of one another.
Fine coordination between various tissues and cell types is needed for the body to function properly. This coordination comes thanks to signals that come from outside the cell.
To detect this type of signals, which influence the behavior of cells at various levels, it is necessary that the cells present within them and in the membrane that surrounds them sufficient structures to detect these signals and produce changes in the cell accordingly. . This function is carried out by so-called cell receptors.
In this article we will discuss the function and different types of cell receptors, to better understand how cells communicate in our body.
What is a cell receiver?
There is great diversity in terms of forms and functions of cell receptors, but they all work in a similar way: they need to bind to a ligand in order to carry out their effect . If the cell receiver were an antenna that is ready to read a signal, the ligand is the signal to be read. They are molecules that will bind to the receptor, triggering different effects depending on each case.
Most receptors are very specific, being able to recognize one (or a few) different types of ligands . This is also the case for ligands, which can usually be recognized by only one or a small group of receptors. When these come together, they initiate cellular changes that help cells respond to their environment appropriately.
There are various types of cell receptors and ligands that bind to them, in order to cover a certain number of possible responses to stimuli by the cell. These receptors can be divided in many ways, but in this article we will look at their distinctions at two main levels, depending on where they are in the cell and depending on the functions and morphology of the receptors themselves.
Types of cell receptor according to their position in the cell
The vast majority of cell receptors are found in the cell membrane, since most of the signals to be detected will travel through the extracellular space towards the different cells . Even so, we may be surprised to learn that there are also cell receptors within the cytosolic space, that is, inside the cell.
1. Intracellular (cytosolic) receptors
As paradoxical as it may initially seem, cytosolic receptors also respond to signals that come from outside the cell. Unlike the receptors that are expressed on the membrane, these are found floating freely inside it, until it detects its corresponding ligand.
We may wonder how these ligands that interact with cytosolic receptors can interact with elements behind the cytoplasmic membrane. This is so because this type of ligands, for the most part, represent hydrophobic molecules, capable of crossing the lipid membrane . Some notable examples of these ligands are hormones, such as testosterone or estradiol.
The cytosolic receptors, once they have contacted the appropriate ligand, will reach the interior nucleus of the cell, where they will act directly on its genetic expression, causing the pertinent changes.
2. Membrane receptors
Membrane cell receptors, also known as surface receptors, are molecules that are attached to the cell membrane. They generally have at least one part outside the cell that will bind the ligand in the extracellular space itself.
In addition, this type of receptor has a domain (the different parts of the receptors can be called domains) within the membrane itself and another domain in the cytosol, which is usually the part of the receptor that will be the one that transmits the signal to the rest of the cells. the cell. Membrane receptors are more frequent and varied, so we will see different general examples to learn more about them .
- It might interest you: “Dopamine: what it is, and functions of this neurotransmitter”
Types of cell receptor according to morphology and function
There are many types of cell surface receptors, but in this article we will study only 4 of them , with different morphologies and functions, to simplify this topic:
- G protein-coupled receptors
- ligand-mediated ion channels
- receptor tyrosine kinase ligands
- integrins
1. Receptor tyrosine kinase
Tyrosine kinase receptors belong to a family of membrane receptors, which have an intracellular domain that is associated with an enzyme or directly has an enzymatic function. It is usually this enzyme that, after detection of the extracellular ligand by the extracellular part of the receptor, will initiate the response to the ligand .
Receptor tyrosine kinases usually function in pairs. The ligands bind to two adjacent receptor tyrosine kinases, which then form a dimer (bind). Following this binding, they phosphorylate their intracellular domains mutually. This structure is capable of binding to specific cytosolic proteins, which will initiate cascading biochemical reactions within the cell, effecting the changes.
The function of these receptors is extremely important. They are related, for example, to growth factors, molecules that regulate cell proliferation, maturation and specialization processes . Disruption of this type of receptor contributes, for example, to tumor formation.
2. Ligand-mediated ion channels
The control of the passage of ions through the cell membrane is essential for their proper functioning, especially when we talk about the nervous system. There are several receptors that work by altering the permeability of the membrane for certain ions , temporarily allowing an altered flow of these.
In general, ion channels are transmembrane proteins that are usually impermeable to ions. When a ligand, such as a neurotransmitter, comes into contact with the ion channel, it changes its shape, temporarily allowing the passage of a specific type of ion. This change in the ionic balance of the cell can affect other proteins or molecules.
After some time, the ion channel closes, but enters a state where it will not respond to the presence of the ligand . We call channels in this state “desensitized,” a state that slowly resolves back to normal.
3. G protein-linked receptors
G protein-coupled receptors have a very similar structure and represent one of the largest families of cell surface receptors . This structure has an end (N-terminal) outside the cell, followed by seven transmembrane protein helices that end in an intracellular domain (C-terminal).
This intracellular end is bound to cytoplasmic proteins (G proteins) capable of binding to GTP (Guanosine triphosphate, an energetic molecule) to use the energy obtained from converting GTP into GDP (Guanosine diphosphate, the same molecule as GTP but with a phosphate less, so it is also less energetic). This energy expenditure occurs only if the extracellular part of the receptor is bound to the ligand.
The activation of G proteins starts a cascade of enzymatic reactions, which cause different changes in the cells. An important function of this type of receptors is to act as olfactory receptors .
4. Integrins
Integrins are also receptors found on the surface of the cell. These are responsible for attaching cells to the extracellular matrix, which is necessary for cells to proliferate and survive . In addition, they are capable of mediating certain mechanical and chemical signals.
These types of receptors recognize extracellular matrix proteins and are responsible for regulating other cell receptors, such as growth factor receptors or ion channel receptors, which in turn are responsible for mediating functions such as apoptosis, cell proliferation or differentiation.
Bibliographic references
- Uings, IJ, & Farrow, S.N. (2000). Cell receptors and cell signalling. Molecular pathology: MP, 53(6), 295–299. doi:10.1136/mp.53.6.295.
- Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Functions of Cell Surface Receptors. Available at: https://www.ncbi.nlm.nih.gov/books/NBK9866/.
To the classic question “what do you do?” I always answer “basically I am a psychologist”. In fact, my academic training has revolved around the psychology of development, education and community, a field of study influenced my volunteer activities, as well as my first work experiences in personal services.