Exosomes are 30 – 90 nm vesicles secreted by mammalian cells and would be the result of late endosomes or multi vesicular bodies (MVB) coalescing with the cell membrane. According to scientists, the initial even in the formation of exosomes is the spontaneous invagination of the cell membrane which would give rise to the formation of intracellular vesicles. It would then be broken down into multiple small vesicles, which would be known as endosomes. A collection of endosomes would be expelled from the cell by a process known as exocytosis and once expelled, these vesicles would be known as exosomes.
What is the structure of an exosome?
As mentioned before, an exosome would have a diameter of around 30 – 90 nm and would differ in its composition depending on the cell of origin. Thus, many cell types would secrete exosomes while each one of them would have its own uniqueness. However, there can be many similarities in the protein composition of exosomes and this is clearly illustrated by the exosomes secreted by mast cells, colorectal cancer cells as well as by the cells lining the urinary system, which would have around 31 common proteins. Apart from this, proteins such as Hsc70 have been recognized in most exosomes.
According to researchers, there are several more similarities between exosomes secreted by different cells. These include, the lipid-bilayer, which it inherits from the cell membrane along with the ligands associated with such cells. At the same time, the density of sucrose inside each exosome would also be similar and according to scientists, it ranges between 1.13 – 1.19 g/ml.
Along with many types of proteins, each exosome will contain mRNA and miRNA, which would be known as ‘exosomal shuttle RNA’. These RNAs will be able to play an influential role in the genetic composition of the cell, which absorbs the exosomes following its release.
What is the role played by exosomes in human cells?
At the time of its discovery, around 25 years ago, the scientists believed that exocytosis, the process in which exosomes are released, was a method to clean-up obsolete or damaged particles within the cell by means of externalizing the same. However, at present, they realize it is not only a clean-up job but exosomes perform many other functions, which are deemed necessary for effective cellular functioning.
According to scientists, some of the main functions performed by exosomes include, cellular communication, modulation of the immune system to counteract immune challenges, as well as progression of a tumor through distributing mutated RNAs as ‘exosomal shuttle RNA’ to the normal cells in the vicinity of the tumor.
Some researchers have shed light to another clinical importance of exosomes as it can be present in elevated amounts in certain diseases. Some of these diseases include ovarian cancers, lung cancer, and melanoma. Thus, assessing the exosome load and its variations would provide the clinicians with a non-invasive technique to detect such diseases early as well as to monitor its progression and the response towards certain therapies. Therefore, exosomes would be facilitating a blood-based biomarker for solid tumors, which would avoid the necessity to perform biopsies to obtain cellular samples.
Furthermore, exosomes are being investigated as the basis for producing vaccines for certain cancers. However, it needs many years of research before it becomes viable as a cost effective and an efficient mode of therapy.
Presence of Exosomes in the urine would be another useful diagnostic tool for clinicians as it can provide valuable information regarding certain diseases such as prostate cancer. Thus, it can help in making an early detection of such diseases and could allow clinicians a method of keeping a tab on the disease progression.
Reference:
ExoCarta – Exosome proteins and RNA database
Kristin Denzer et al, Exosome: from internal vesicle of the multivesicular body to intercellular signaling device, Journal of Cell Science 113, 3365-3374 (2000)
G. Mignot et al, Prospects for exosomes in immunotherapy of cancer, Journal of Cellular and Molecular Medicine, Volume 10, Issue 2, pages 376–388, April 2006
Sascha Keller et al, Exosomes: From biogenesis and secretion to biological function, Letters, Issue 2, 15 November 2006, Pages 102-108
