An Introduction into EV Markers

Extracellular vesicles (EVs) are tiny, spherical structures that are released by cells into the extracellular environment. They are usually composed of a lipid bilayer membrane and contain various biomolecules, such as proteins, lipids, and RNA. EVs play a critical role in intercellular communication, as they are capable of transferring functional molecules and information between cells. This allows cells to coordinate their activities, promote tissue repair, and modulate immune responses, among other functions.

Biogenesis and secretion of EVs are carefully controlled, yet can be adjusted by a range of external and internal triggers. Notably, their involvement in the advancement of medical conditions such as cancer, neurological problems, and cardiovascular diseases is well-known. With cancer, tumor-derived EVs may exacerbate the growth and metastasis of malignant cells by transferring oncogenic messages and impeding immune responses. On the other hand, EVs can also serve as diagnostic and therapeutic tools. For example, circulating exosomes have been proposed as minimally invasive biomarkers for cancer and other diseases, as they contain specific molecular signatures that can be used to monitor disease progression and response to treatment.

There are three main types of EVs: exosomes, microvesicles, and apoptotic bodies. Exosomes are small (30-100 nm in diameter) and are generated by the inward budding of intracellular membranes. They contain a unique set of proteins and RNA species, which are selected and packaged according to the origin and state of the producing cell. Microvesicles are larger (100-1,000 nm in diameter) and are generated by the outward budding of the plasma membrane. They contain a more heterogeneous set of biomolecules, reflecting the parent cell's surface properties. Apoptotic bodies, on the other hand, are formed during programmed cell death and are the largest of the three types of EVs (1-5 μm in diameter).

Exosomes are a type of EVs that play a crucial role in intracellular communication. They are small (30-100 nm in diameter) spherical structures composed of a lipid bilayer membrane that contain various biomolecules, including proteins, lipids, and RNA. Exosomes are generated by the inward budding of intracellular membranes, and they contain a unique set of biomolecules that are selected and packaged according to the origin and state of the producing cell.

Exosomes have been implicated in a variety of physiological and pathological processes, including the regulation of immune responses, the progression of cancer, and the development of neurodegenerative diseases. They can transfer functional molecules and information between cells, leading to the modulation of cell behavior, such as the promotion of tumor growth and the suppression of the immune response.

Microvesicles are miniature, lipoprotein-enclosed sacs released from the cell membrane surface. These nanometer-sized vesicles contain a mixture of proteins and nucleic acids wrapped in a lipid bilayer for protection against external environments. Microvesicles are generated through a process called budding, in which a portion of the cell membrane pinches off to form a separate vesicle. They play important roles in intercellular communication, as they can transfer information from one cell to another.

Apoptotic bodies play an important role in maintaining cellular and tissue homeostasis, as they help to prevent the spread of damaged or infected cells. Not only do they rid cells of debris and recycle cellular components, the contents from apoptotic bodies can also send messages to adjacent cells while triggering either inflammation or tissue regeneration.

The formation of apoptotic bodies is strictly overseen and managed by multiple signaling pathways as well as molecular constituents, such as caspases, Bcl-2 family members, and other proteins. The disruption of the apoptosis process, such as a lack or overabundance of apoptotic bodies, has been tied to several medical conditions including cancer and autoimmune diseases.

One of the foremost impediments to EV research lies in the lack of consistent protocols for isolating and purifying EVs from biological samples. With disparate methods yielding distinct populations of EVs, it becomes a challenge to compare outcomes across studies. Furthermore, EVs are often present in low numbers and are highly heterogeneous in size and composition, making them difficult to isolate and analyze in a reliable and reproducible manner.

Despite accumulating evidence that EVs are crucial for intercellular communication, we still have little comprehension of the biological importance of the EV's content and related mechanisms. As such, further research is needed to elucidate these pathways in order to fully understand EV functionality. Furthermore, it is not yet clear how EVs from different cell types and in different biological conditions compare, and how their contents may change in response to different stimuli.

Lastly, new and advanced analytical methods are needed to accurately characterize EVs and their contents. Currently, the most common approaches utilized such as flow cytometry, electron microscopy, and mass spectrometry have restrictions in terms of sensitivity, specificity and throughput. The development of new, more advanced techniques is critical for advancing the field and improving our understanding of the biological significance of EVs.

EVs have the capability to uncover biomarkers through offering a distinctive set of biomolecules that echo the state of their producing cells. They can be easily obtained from blood, urine, cerebrospinal and other bodily fluids. Their accessibility, stability and specificity make them ideal for clinical applications, potentially revolutionizing our knowledge on disease movement and treatment response.

EVs have been proposed as biomarkers for a variety of diseases, including cancer, neurological disorders, and cardiovascular diseases. For example, circulating exosomes have been proposed as minimally invasive biomarkers for cancer, as they contain unique molecular profiles that reflect the state of the producing tumor cells. In addition, the study of EVs in neurodegenerative diseases has led to the identification of novel biomarkers for early diagnosis and disease progression.

MiRNAs have been increasingly recognized as promising biomarkers for various diseases, and their presence in EVs, including exosomes, has been proposed as a source of disease-specific biomarkers. MiRNAs in EVs are packaged and protected by the vesicular membrane, allowing them to remain stable and functional in circulation. They can be easily isolated from various body fluids and analyzed using well-established methods to determine the miRNA signature.