Extracellular vesicles and why I love them (Part 1)
Sometimes, small is big
Fig: biogenesis of extracellular vesicles in the three domains of life. Vesicle budding indicated with arrows. (a) TEM showing hypervesiculation in the bacterium S. typhimurium. Image kindly provided by Mario F. Feldman (University of Alberta, Canada). (b) SEM showing microvesicles budding from the eukaryote Leishmania donovani. Image reprinted from Silverman et al. (2008). (c) Cryo-TEM of vesicle budding from the archaeon T. kodakaerensis. The protrusion of the S layer can also be observed clearly. (d) TEM of ultrathin cell sections of vesicle budding from T. kodakaerensis. Figures (c) and (d) provided by the authors (Gill, S., Catchpole, R. and Forterre, P., 2019. Extracellular membrane vesicles in the three domains of life and beyond. FEMS microbiology reviews, 43(3), pp.273-303.)
I first encountered the term “extracellular vesicles” during a cell biology lecture wherein the lecturer droned on from Alberts, about various organelles being present inside the cell and I distinctly remember wondering why they’d mention something that is released out of the cell as a cellular organelle. We were quizzed about mitochondria, the powerhouse of the cell, the nucleus – the storage cell of genetic information and a few other ‘important’ organelles and within the pages of my notebook, my brief curiosity for Extracellular vesicles was (briefly) forgotten.
Years later in 2020, while conversing with my advisor, EVs pop up in conversation taking me back to that fleeting classroom memory, making me think about frequent mentions of this obscure ‘organelle’ which none of the great biology textbooks had anything to talk about. A cursory google search following the conversation is what led me to find a tiny niche of tiny vesicles that have remained in obscurity while creating ripples across various microenvironments, species, and genera. A simple structured membrane-bound vesicle packed with tons of bioactive cargo from host cells displaying re-uptake efficiency by other cells simply lies in the scientific blind spot, with a variety of theories and hypotheticals to dig deep but only a handful of facts to lead the way.
Chargaff and West, a scientific duo working on blood coagulation in 1945, were the first to deduce the presence of EV-like agencies, calling them a "breakdown of blood corpuscle” while optimizing a protocol to sieve out clotting factors from cells. Years later, Peter Wolf’s work on human plasma and platelets similar to that of Chargaff and West, came up with the term ‘platelet dust’ to describe what we now know as EVs. With the evolution of microscopy (EM and Cryo) and better visibility into the field, EVs were then observed and studied as ‘matrix vesicles’ that were found embedded in the cell membrane and some of which were also seen to be released out from the luminal spaces within the plasma membrane. The size of these ‘particulate matter’ had also substantially piqued the interest of various researchers keen on unearthing the presence of tumor-causing viruses in body fluids, and briefly, the conundrum of EVs being virus-like particles prevailed due to their size and shape resembling an enveloped virion inside the cell. Most of these discoveries were mere happenstances and lacked cohesiveness to conclusively establish the presence of field; the only common denominator of these discoveries put together was their lack of common functions as ‘particulate fraction’, ‘platelet dust’, ‘matrix vesicles’ all were seen to be responsible for varying aspects of cellular function and communication which points to an obvious conclusion— the range and dynamicity of EVs remain vast and open-ended; a cliff-hanger that could be lead to the cusp of climax only by a more cohesive group of researcher, which is the small EV community we know of today.
For those of my readers who will be reading this from my blog instead of being pleaded to read the file from my laptop, you might find it interesting to know that the filename for the document is ‘What I wish I could tell people who scoff when I talk about EVs’ and this was supposed to be just a journaling exercise for me to yell out unfinished sentences into a Word file in the hopes of minimal sulking resulting from a negative comment from a faculty or a peer often berating me about the “dubiousness” or “insignificance” of the field and how it would be more fruitful for me to channel my energy on other pursuits [such as planning an ostentatious experiment for the sake of prestige]. Thus, although out of spite, the need to gather my own thoughts which aim to highlight the promise this field holds goes further beyond the functional aspect of EVs. Their range of roles varies with cell type, cellular functions, developmental stage, disease pathology, and species. During the 60s and 70s, various articles with pieces of evidence of EVs found them to aid in many cell-specific pathways; EVs had a role in endocrine pathways, intercellular communication, and overall regulation of cellular integrity by maintaining the coding and non-coding RNA pool.The majorly accepted opinion of EVs deemed them as a biologically inactive waste-disposal system containing obsolete cargoes majorly influenced the global research landscape. It was not until the late 90s and early 2000s, when the infamy around EVs being biologically active and responsible for the transfer of bioactive RNA rose – a biological hyperbole of Lazarus rising if you will— and served as a climacteric point in pooling interest in EV research in constructing the versatile community of researchers under the global umbrella of International Society of Extracellular Vesicles (ISEV).
The versatility of EVs does not only limit to their function but spans across species. There are multiple differences separating the three domains of life: Archaea, Bacteria, and Eukarya however EVs remain as common denominators, being produced my members of all three domains. The study of Bacterial EVs (BEVs) was pioneered by Terry Beveridge in the 90s in Pseudomonas aeruginosa while studying virulence factors and their association with EV release. Currently, BEVs are well-studied into types such as LPS containing outer membrane vesicles (OMVs) and outer-inner membrane vesicles’ (O-IMVs). Most BEVs have been studied exclusively in Proteobacteria although various other bacterial genera and species are also being researched. Archaeal EVs were first observed in the thermoacidophilic Sulfolobus islandicus wherein EVs were seen to be enclosed in a cytoplasm-like membrane with a coated S-layer.
A keyword search of EVs would list out their vast presence in eukaryotic animal cells or models wherein most of them have been seen to be studied with a therapeutic approach in mind, the role of EVs in algae and plants is fairly interesting and worth investigating. In algae, the release of EVs is a phenomenon associated with algal stem cells and is thus specific to the developmental stages of some algae with the specific function of endocrine signaling. In plants, EVs have been seen to play a role in their immune system to fend off bacterial or fungal attacks thus aiding the plant in exhibiting antimicrobial/antifungal traits.
The omnipresence of these vesicles indicates their sequence conservation across all domains and thus pushes toward the question of common ancestry. A proteomic analysis of EVs from the three species revealed the presence of proteins homologous to eukaryotic ESCRT-III subunits and to the VPS4 ATPase (key components of EV trafficking and formation) but there is still a lack of consensus on whether these vesicles display homology or not.
Research, for me, is in some way similar to the exercise of finding rabbit traps on a forest floor. You see a suspicious-looking anomaly, dust it off, carefully pull at it and find an immediate string of connections. Often, the best lies in less sparkly and exciting crevices of knowledge, and even though to many this might be an indifferent field with little impact, the possibility of researching extracellular vesicles is no less exciting than diver exploring deep sea creatures. The promise of EVs across a multitude of fields (Immunology, Therapeutics, Cellular Medicine, Cancer Biology, Neurodegeneration, and many more) is enough for me to believe its potential the being the proverbial David ready to battle the Goliath of Science Unknown.
Fig: biogenesis of extracellular vesicles in the three domains of life. Vesicle budding indicated with arrows. (a) TEM showing hypervesiculation in the bacterium S. typhimurium. Image kindly provided by Mario F. Feldman (University of Alberta, Canada). (b) SEM showing microvesicles budding from the eukaryote Leishmania donovani. Image reprinted from Silverman et al. (2008). (c) Cryo-TEM of vesicle budding from the archaeon T. kodakaerensis. The protrusion of the S layer can also be observed clearly. (d) TEM of ultrathin cell sections of vesicle budding from T. kodakaerensis. Figures (c) and (d) provided by the authors (Gill, S., Catchpole, R. and Forterre, P., 2019. Extracellular membrane vesicles in the three domains of life and beyond. FEMS microbiology reviews, 43(3), pp.273-303.)
I first encountered the term “extracellular vesicles” during a cell biology lecture wherein the lecturer droned on from Alberts, about various organelles being present inside the cell and I distinctly remember wondering why they’d mention something that is released out of the cell as a cellular organelle. We were quizzed about mitochondria, the powerhouse of the cell, the nucleus – the storage cell of genetic information and a few other ‘important’ organelles and within the pages of my notebook, my brief curiosity for Extracellular vesicles was (briefly) forgotten.
Years later in 2020, while conversing with my advisor, EVs pop up in conversation taking me back to that fleeting classroom memory, making me think about frequent mentions of this obscure ‘organelle’ which none of the great biology textbooks had anything to talk about. A cursory google search following the conversation is what led me to find a tiny niche of tiny vesicles that have remained in obscurity while creating ripples across various microenvironments, species, and genera. A simple structured membrane-bound vesicle packed with tons of bioactive cargo from host cells displaying re-uptake efficiency by other cells simply lies in the scientific blind spot, with a variety of theories and hypotheticals to dig deep but only a handful of facts to lead the way.
Chargaff and West, a scientific duo working on blood coagulation in 1945, were the first to deduce the presence of EV-like agencies, calling them a "breakdown of blood corpuscle” while optimizing a protocol to sieve out clotting factors from cells. Years later, Peter Wolf’s work on human plasma and platelets similar to that of Chargaff and West, came up with the term ‘platelet dust’ to describe what we now know as EVs. With the evolution of microscopy (EM and Cryo) and better visibility into the field, EVs were then observed and studied as ‘matrix vesicles’ that were found embedded in the cell membrane and some of which were also seen to be released out from the luminal spaces within the plasma membrane. The size of these ‘particulate matter’ had also substantially piqued the interest of various researchers keen on unearthing the presence of tumor-causing viruses in body fluids, and briefly, the conundrum of EVs being virus-like particles prevailed due to their size and shape resembling an enveloped virion inside the cell. Most of these discoveries were mere happenstances and lacked cohesiveness to conclusively establish the presence of field; the only common denominator of these discoveries put together was their lack of common functions as ‘particulate fraction’, ‘platelet dust’, ‘matrix vesicles’ all were seen to be responsible for varying aspects of cellular function and communication which points to an obvious conclusion— the range and dynamicity of EVs remain vast and open-ended; a cliff-hanger that could be lead to the cusp of climax only by a more cohesive group of researcher, which is the small EV community we know of today.
For those of my readers who will be reading this from my blog instead of being pleaded to read the file from my laptop, you might find it interesting to know that the filename for the document is ‘What I wish I could tell people who scoff when I talk about EVs’ and this was supposed to be just a journaling exercise for me to yell out unfinished sentences into a Word file in the hopes of minimal sulking resulting from a negative comment from a faculty or a peer often berating me about the “dubiousness” or “insignificance” of the field and how it would be more fruitful for me to channel my energy on other pursuits [such as planning an ostentatious experiment for the sake of prestige]. Thus, although out of spite, the need to gather my own thoughts which aim to highlight the promise this field holds goes further beyond the functional aspect of EVs. Their range of roles varies with cell type, cellular functions, developmental stage, disease pathology, and species. During the 60s and 70s, various articles with pieces of evidence of EVs found them to aid in many cell-specific pathways; EVs had a role in endocrine pathways, intercellular communication, and overall regulation of cellular integrity by maintaining the coding and non-coding RNA pool.
The majorly accepted opinion of EVs deemed them as a biologically inactive waste-disposal system containing obsolete cargoes majorly influenced the global research landscape. It was not until the late 90s and early 2000s, when the infamy around EVs being biologically active and responsible for the transfer of bioactive RNA rose – a biological hyperbole of Lazarus rising if you will— and served as a climacteric point in pooling interest in EV research in constructing the versatile community of researchers under the global umbrella of International Society of Extracellular Vesicles (ISEV).
The versatility of EVs does not only limit to their function but spans across species. There are multiple differences separating the three domains of life: Archaea, Bacteria, and Eukarya however EVs remain as common denominators, being produced my members of all three domains. The study of Bacterial EVs (BEVs) was pioneered by Terry Beveridge in the 90s in Pseudomonas aeruginosa while studying virulence factors and their association with EV release. Currently, BEVs are well-studied into types such as LPS containing outer membrane vesicles (OMVs) and outer-inner membrane vesicles’ (O-IMVs). Most BEVs have been studied exclusively in Proteobacteria although various other bacterial genera and species are also being researched. Archaeal EVs were first observed in the thermoacidophilic Sulfolobus islandicus wherein EVs were seen to be enclosed in a cytoplasm-like membrane with a coated S-layer.
A keyword search of EVs would list out their vast presence in eukaryotic animal cells or models wherein most of them have been seen to be studied with a therapeutic approach in mind, the role of EVs in algae and plants is fairly interesting and worth investigating. In algae, the release of EVs is a phenomenon associated with algal stem cells and is thus specific to the developmental stages of some algae with the specific function of endocrine signaling. In plants, EVs have been seen to play a role in their immune system to fend off bacterial or fungal attacks thus aiding the plant in exhibiting antimicrobial/antifungal traits.
The omnipresence of these vesicles indicates their sequence conservation across all domains and thus pushes toward the question of common ancestry. A proteomic analysis of EVs from the three species revealed the presence of proteins homologous to eukaryotic ESCRT-III subunits and to the VPS4 ATPase (key components of EV trafficking and formation) but there is still a lack of consensus on whether these vesicles display homology or not.
Research, for me, is in some way similar to the exercise of finding rabbit traps on a forest floor. You see a suspicious-looking anomaly, dust it off, carefully pull at it and find an immediate string of connections. Often, the best lies in less sparkly and exciting crevices of knowledge, and even though to many this might be an indifferent field with little impact, the possibility of researching extracellular vesicles is no less exciting than diver exploring deep sea creatures. The promise of EVs across a multitude of fields (Immunology, Therapeutics, Cellular Medicine, Cancer Biology, Neurodegeneration, and many more) is enough for me to believe its potential the being the proverbial David ready to battle the Goliath of Science Unknown.
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