Extracellular vesicles: exosomes, microparticles, their parts, and their targets to enable their biomanufacturing and clinical applications
Graphical abstract
Introduction
Extracellular vesicles (EVs) are generated by most if not all mammalian cells [1, 2, 3] and carry RNAs, proteins, and lipids from their parent cells during EV generation, which takes place frequently under cellular activation or stress [2]. Among EVs, the submicron-size microparticles/microvesicles (MPs/MVs; also known as ectosomes) are the larger ones ranging from 100 to 1000 nm in size. They bud off the cytoplasmic membrane of the parent cell under normal physiological or pathophysiological conditions, including coagulation, inflammation, tumorigenesis, and differentiation [2]. Exosomes (Exos), distinct from MPs, are nano-size particle (<100 nm) which originate from multivesicular bodies through exocytosis [2,4]. Besides mammalian cells, outer membrane vesicles (OMVs), derived from bacteria (especially Gram-negative bacteria) are involved in stress response, promoting survival, pathogenesis, and interaction between bacteria in a community [5].
Cells use EVs to communicate with other cells by delivering signals through their content [2]. As reviewed (e.g. Refs. [6, 7, 8]), over the last few years, EVs have emerged as important mediators of intercellular communication regulating an ever-expanding range of biological processes, both on normophysiology and pathophysiology. The former includes enhancing and accelerating native developmental programs in immunology, vascular repair, and angiogenesis, while the latter includes carcinogenesis and cancer metastasis, neurodegenerative disorders, and infectious and cardiovascular diseases. As such, EVs are suitable for a broad range of applications, from minimally invasive diagnostic applications to therapeutic interventions, including cell therapies and macromolecular drug delivery. In order to pursue such applications involving EVs, better EV characterization, as well as better understanding of the mechanisms of cell targeting and methods for EV biomanufacturing are needed (Figure 1).
Section snippets
How do EVs recognize and deliver cargo to target cells?
Understanding how do EVs target and are taken up by cells is crucial for their applications [9,10]. The interaction typically starts with a ligand–receptor mediated binding, adhesion, or docking of EVs to target cells (Figure 1). The ligands and receptors involved are EV and target-cell specific, and, in some cases, this ligand–receptor recognition step is sufficient to alter the fate of target cells [11, 12, 13]. Yet, in most cases, EVs exert their biological effect through transferring of
EVs alter the biology and fate of target cells through diverse mechanisms
Uptake of EVs enables delivery of EV cargo to recipient cells, thus triggering a broad spectrum of biological phenotypes. Tumor-derived EVs regulate the tumor microenvironment and impart an invasive effect in cancer progression and angiogenesis [24,25]. Exosomes derived from hepatocellular carcinoma cells delivered both SMAD Family Member 3 (SMAD3) protein and mRNA to circulating hepatocellular carcinoma cells, enhanced their adhesive ability, and supported their metastasis [26••]. EVs also
Translational applications of native EVs
As discussed, native EVs (MP and exosomes) have a good potential as therapeutic agents in translational applications. Below, we review the target specificity and the nature of the native cargo of EVs derived from a few select cell types. These and additional reports are summarized in Table 1.
Engineering using EVs or their components for cargo delivery
EVs exhibit desirable native characteristics that makes them suitable as vehicles for cargo delivery (Figure 3). As summarized in Figure 3, all EVs components (their native cargo, surface proteins, and membranes), can be engineered for various applications. Loading of synthetic cargo is an important first engineering goal. EVs can be also engineered to enhance target specificity. Finally, their native cargo can be used for diagnostic, discovery or therapeutic applications.
Mechanisms of EV biogenesis
Key to the biomanufacturing process is the mechanism by which one can induce EV formation from various cell types. EVs are produced typically under physiological or pathophysiological stress or stimulation. Biogenesis of two major type of EVs (exosomes or MPs) is quite distinct. For exosome generation, several stimuli such as cellular stress, irradiation, hypoxia, or starvation have been shown to increase exosome production. Details regarding the mechanisms of exosomes biogenesis at a molecular
Biomanufacturing of EVs: exosomes and MPs
EVs are currently pursued by several startup and larger companies for a broad range of applications, from minimally invasive diagnostic applications to therapeutic interventions, including cell therapies and cargo delivery. To achieve large-scale EV production, it is necessary to develop EV manufacturing using Good Manufacturing Practices (GMP). Currently, several approaches have been reported for GMP-grade manufacturing of exosomes either from MSCs [58••,72] or from cardiac progenitor cells [57
Future developments
Understanding the mechanisms of EV-to-target recognition and its specificity is crucial for developing more effective technologies for EV-based therapies or for using parts of the EVs or their parent cells to construct semi-synthetic delivery systems. Are there any reasons as to why some EV has exquisite target specificity while other do not? Can we determine if a cell’s EVs will have some specific targets? Can we use modular engineering of one EV type to specifically target different cell
Conflict of interest statement
The two authors are listed as inventors on a pending US/PCT patent application (publication US20170058262A1, Application No. 15/308,221, PCT No. PCT/US15/31388) on the MkMP-based technologies. The rights to the pending patent belong to the three inventors of the patent.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgement
This work was supported by a grant (CBET-1804741) by the US National Science Foundation.
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