Extracellular vesicles are small, bag-like structures secreted by cells, and they exist in multiple subtypes. These subtypes are distinguished based on their size, biological properties, biogenesis mechanisms, and functions. Below, we will explain the main classifications of extracellular vesicles.
Exosomes
Exosomes are extracellular vesicles with a diameter of approximately 30-150 nm, and they are secreted from multivesicular bodies within the cytoplasm. Exosomes contain proteins, lipids, and nucleic acids (such as mRNA, miRNA, lncRNA), and they facilitate intercellular communication by transporting these biological molecules to target cells. Exosomes are involved in various biological processes, including immune responses, neuronal signaling, formation of the tumor microenvironment, cellular aging, and apoptosis.
Ectosomes
Ectosomes are small membrane-covered particles released by cells into the environment. These particles are usually formed through “blebbing” or “pinching” of the cell membrane, where a portion of the cell membrane separates to form new small particles. Ectosomes can carry various molecules from the original cell and, as a result, can transmit information to other cells.
Ectosomes are widely recognized to be involved in intercellular communication. They contain biologically important substances such as proteins, lipids, RNA, and DNA. The composition of these components can vary depending on how ectosomes are formed and from which cells they are released, allowing ectosomes to function as “packages” for specific information to specific cells.
Ectosomes also have the potential to be useful in the diagnosis and treatment of diseases. For example, certain cancer cells are known to release ectosomes that contain specific proteins. By detecting these ectosomes, doctors may be able to identify the presence and progression of cancer. Furthermore, ectosomes may serve as “vectors” or “carriers” for delivering drugs to specific cells, potentially offering a new way of delivering drugs.
Exomeres
Exomeres are a type of smaller extracellular particles compared to exosomes and microvesicles. This term has emerged from recent studies aiming to understand the diversity and complexity of extracellular particles.
Exomeres have a diameter of approximately 35 nanometers, considerably smaller than exosomes (typically 40-100 nanometers in diameter) and microvesicles (hundreds of nanometers in diameter).
Studies have shown that exomeres carry a unique set of biomolecules, including proteins, lipids, and nucleic acids. The distinct composition of exomeres sets them apart from other types of extracellular particles, suggesting that they may have their own biological functions.
Microvesicles
Microvesicles are extracellular vesicles with a diameter of approximately 100-1000 nm, and they are generated by budding directly from the cell membrane. Microvesicles contain biological molecules such as lipids, proteins, and nucleic acids, and they are involved in intercellular communication similar to exosomes. Microvesicles play roles in diverse biological processes, including immune responses, coagulation reactions, and the proliferation, invasion, and metastasis of tumor cells.
Aptopoietic Bodies
Aptopoietic bodies are extracellular vesicles with a diameter of approximately 500-5000 nm, and they are generated during the process of apoptosis (cell self-destruction). Aptopoietic bodies contain proteins and nucleic acids involved in apoptosis and facilitate the engulfment of apoptotic cell remnants by neighboring cells. This enables efficient removal of apoptotic cell remnants and suppression of inflammatory reactions. Aptopoietic bodies play important roles in maintaining tissue homeostasis and regulating the immune system.
Oncosomes
Oncosomes are large extracellular vesicles derived from cancer cells, with a diameter ranging from approximately 1000-10000 nm. Oncosomes contain proteins, nucleic acids, lipids, and other molecules involved in the proliferation, invasion, and metastasis of cancer cells. These biological molecules may influence surrounding normal cells and immune cells, potentially promoting the progression of cancer.
Exophers
Exophers are membrane-bound extracellular vesicles (EVs) released from cells into the extracellular space. Exophers have attracted attention due to their size, averaging approximately 4 micrometers and being capable of shedding complete organelles such as mitochondria and lysosomes as cargo. Exophers have been observed to be released from neuronal and muscle cells in nematodes and even from mouse cardiac myocytes. Exophers may remain connected to the cells that produce them by membrane-like filaments reminiscent of tunneling nanotubes. While exophers share similarities with large oncosomes, they differ in that they are produced by physiologically normal cells rather than abnormal cells associated with tumors.
The production of exophers is thought to be a mechanism by which cells maintain homeostasis. Exophers are produced in response to protein aggregation, reactive oxygen species (ROS), high-temperature, high osmolarity environments, starvation, and even space flight. The production of exophers has been found to depend on extracellular receptor signaling. Specifically, the production of exophers in nematodes involves two MAPK pathways: epidermal growth factor (EGF) and fibroblast growth factor (FGF) signaling. Additionally, the clearance of exophers by phagocytosis in mouse cardiac tissue requires the cell surface receptor MERTK expressed by resident macrophages.
Exophers may be associated with diseases. In the hearts of mice, removing macrophages or blocking the ability to incorporate exophers causes inflammation and dysregulation of ventricular function. Furthermore, exophers have the potential to facilitate the spreading of pathological protein aggregates in neurodegenerative diseases by carrying aggregated proteins, including human huntingtin protein, outside neurons.
Other Extracellular Vesicles
Extracellular vesicles are classified into various subtypes based on their size and biological properties, and as research progresses, new subtypes may be discovered. Additionally, extracellular vesicles can exhibit different characteristics depending on the cell type and state, and tissue-specific and disease-specific extracellular vesicles have been reported.
Future Perspectives
In the latest version of MISEV2018, it is recommended to classify extracellular vesicles based on size, using the terms “small Extracellular Vesicles” (sEVs) for those below 200 nm and “medium/large EVs” (m/l EVs) for those above 200 nm, rather than using the terms “exosomes” or “microvesicles”.
Furthermore, regarding the tetraspanin family of transmembrane proteins (CD9, CD63, CD81), which are widely recognized as exosome markers, it is recommended to add additional notation such as “(CD63+/CD81+-EVs)” due to variations in their expression levels depending on cell type, isolation method, and size.
The classification of extracellular vesicles is expected to become more detailed as research advances. The establishment of standardized classification and isolation/purification methods for extracellular vesicles is expected to improve the reproducibility and comparability of research results and deepen our understanding of the biological properties and functions of extracellular vesicles. Additionally, extracellular vesicles have the potential to contribute to the development of diagnostic and therapeutic approaches and to the elucidation of the onset and progression mechanisms of diseases.
References
- Raposo, G., & Stoorvogel, W. (2013). Extracellular vesicles: Exosomes, microvesicles, and friends. The Journal of Cell Biology, 200(4), 373-383.
- Tkach, M., & Théry, C. (2016). Communication by extracellular vesicles: Where we are and where we need to go. Cell, 164(6), 1226-1232.
- Colombo, M., Raposo, G., & Théry, C. (2014). Biogenesis, secretion, and intercellular interactions of exosomes and other extracellular vesicles. Annual review of cell and developmental biology, 30, 255-289.
- Maas, S. L., Breakefield, X. O., & Weaver, A. M. (2017). Extracellular vesicles: Unique intercellular delivery vehicles. Trends in cell biology, 27(3), 172-188.
