The lipid rafts, Also known by their English name "Lipid Rafts", they are microdomains or localized regions of the plasma membrane, rich in complex lipids and cholesterol. They are very small, although they can comprise between 30 and 40% of the membrane.
These microdomains are also found in other cell membranes and in the Golgi complex. In general, they carry out a great variety of cellular processes such as the regulation and translation of signals, endocytosis, exocytosis and cell mobility, among others..
It is known that lipid rafts, being involved in various cellular processes, are related to diseases such as Parkinson's disease, Alzheimer's, asthma and many others..
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For several years it was thought that cell membrane proteins and lipids had a random or random distribution on the plasma membrane. The fluid mosaic model, proposed by Singer-Nicolson, in 1972, indicated this.
In later years, new evidence began to emerge from research related to membrane lipid clusters and X-ray diffraction, among others, thus leading to the formulation of new hypotheses regarding the ordering and location of lipids..
In 1982, Karnovsky and colleagues formalized the concept of lipid domains in the membrane. Thanks to this research, they were able to demonstrate the existence of heterogeneity in the disintegration of the life of DPH, also known as 1,6-diphenyl-1,3,5-hexatriene..
The finding by Karnovsky et al. Meant the discovery that there were multiple phases in the lipid environment of the membrane..
In addition to this study, in 1988 the scientists Simons and van Meer, in a refocusing on lipid domains or rafts, proposed that these domains were composed of an important variety of lipids such as cholesterol and other complex compounds.
The knowledge of these regions does not belong to a single author, but to the accumulation of knowledge about them. This is because the existence of membrane microdomains or lipid rafts had been postulated in 1970, before the Singer-Nicolson model, using biophysical approaches that could later be corroborated..
In more recent years, knowledge of lipid rafts has grown enormously. Several investigations have revealed their size, shape, the role they play in the cell, and also their functions and other aspects of these microdomains.
Lipid rafts are characterized by being microdomains of about 10 to 300 nanometers (nm). Although they are small in size, they generally make up a large part of the plasma membrane. They have about 3 to 5 times the amount of cholesterol found in the surrounding bilayer.
As described above, the rafts are enriched with complex lipids such as sphingolipids and sphingomyelin. Unsaturated fatty acids are hardly present in them, and they are insoluble in low concentrations of non-ionic detergents..
These microdomains are called rafts, because they form a denser lipid phase than the molecules of the phospholipid groups. These constitute particular areas in the plasma membrane that resemble suspended or floating bags among the rest of the lipids..
Not all lipid rafts are identical to each other. These can also be composed of an important variety of proteins anchored to kinases, synthases, proteins linked to glycosylphosphatidylinositol (GPI), caveolins, and also flotilins, among others..
Regarding the location of the common or typical lipids of the rafts (cholesterol, sphingomyelin and also glycosphingolipids), these are usually found in the exofacial valve of the membrane.
On the other hand, glycerophospholipids tend to show a certain preference for the cytofacial region of the plasma membrane..
In mammalian spermatozoa, lipid rafts are found over the entire surface and are not restricted by particular domains.
The lipid rafts or lipid rafts are formed, in mammals, in the Golgi complex. Scientists, even knowing where they are formed, do not know for sure how this process is carried out, and how later the rafts remain preserved as an independent entity.
So far, two types of lipid rafts have been discovered: caveolae, and flat.
Also known as non-caveolae or glycolipid lipid rafts. They are not invaginated rafts; that is, they are continuous to the plane of the plasma membrane. No further information is known about its morphology or structure..
They are lipid rafts represented as invaginations of the plasma membrane with sizes ranging from 50 to 100 nm. They are rich in proteins and lipids such as cholesterol and enfingomyelins. Its biogenesis and maintenance depend on integral proteins called caveolins.
The main function of lipid rafts is transduction, that is, converting or transforming signals into other responses or specific signals. They do this thanks to the presence of molecules involved in signaling and the heterogeneity of their composition..
A wide variety of functions performed by lipid rafts are known. Next we will see some of the most important.
Some research suggests an active participation of lipid rafts in immune system responses. The different rafts are associated for transduction, which in the case of the immune system activates T lymphocytes, finally provoking the response.
The opposite case occurs when said rafts break their association through physical separation, which results in the absence of the activating signal, ending the immune response. In this process, the rafts not only fulfill the function of transduction but also of regulation of the activity.
The exocytosis process consists of the fusion of vesicles loaded with intracellular fluid with the plasma membrane, to release the vesicular content in the extracellular medium and also incorporate vesicular proteins and lipids to the membrane..
Various studies reveal that cholesterol, sphingolipids and lipid rafts play a crucial role in this process. Lipid rafts, in exocytosis, concentrate proteins in specific places on the plasma membrane and also play a regulatory role in the process.
Today, it is known that lipid rafts can serve as an entry point for different types of external agents, such as microorganisms, viruses and toxins. However, the reason why these cellular components fulfill this function is not yet known.
What is known is that lipid rafts are used by various microorganisms to protect themselves from the immune system and thus be able to spread throughout the body..
The migration or movement of rafts towards the contact points of various pathogens and the dependence on cholesterol have been observed, so that the entry and even the exit of the external agent occurs.
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