Overview and Update on Extracellular Vesicles: Considerations on Exosomes and Their Application in Modern Medicine

Abstract

In recent years, there has been a rapid growth in the knowledge of cell-secreted extracellular vesicle functions. They are membrane enclosed and loaded with proteins, nucleic acids, lipids, and other biomolecules. After being released into the extracellular environment, some of these vesicles are delivered to recipient cells; consequently, the target cell may undergo physiological or pathological changes. Thus, extracellular vesicles as biological nano-carriers, have a pivotal role in facilitating long-distance intercellular communication. Understanding the mechanisms that mediate this communication process is important not only for basic science but also in medicine. Indeed, extracellular vesicles are currently seen with immense interest in nanomedicine and precision medicine for their potential use in diagnostic, prognostic, and therapeutic applications. This paper aims to summarize the latest advances in the study of the smallest subtype among extracellular vesicles, the exosomes. The article is divided into several sections, focusing on exosomes' nature, characteristics, and commonly used strategies and methodologies for their separation, characterization, and visualization. By searching an extended portion of the relevant literature, this work aims to give a quick outline of advances in exosomes' extensive nanomedical applications. Moreover, considerations that require further investigations before translating them to clinical applications are summarized.

Keywords: drug delivery systems; exosomes characterization; extracellular vesicles; nanomedicine.

Figure 1

Deployment of extracellular vesicles to exchange biological molecules contained within a membrane boundary. (i) Budding of microvesicles off the plasma membrane (ectosomes or microvesicles); (ii) release of exosomes through the fusion of multivesicular bodies (MVBs) with the plasma membrane; (iii) blebbing off of larger vesicles, especially from apoptotic cells (apoptotic bodies). In the upper side there are representative transmission electron images: (A) microvesicles liberation (arrowheads), and (B) an apoptotic cell. In the bottom side: (C) inward invaginations of the MVB indicate the beginning of exosomes biogenesis, (D) MVB enclosing a lot of exosomes, and (E) MVB near to fuse with the membrane. n = nucleus.

Figure 2

Molecular composition of exosomes. These vesicles contain different cell surface proteins, membrane transport and fusion, antigen presentation, signal transduction, targeting, and adhesion proteins. Several molecules of nucleic acids and lipids are present.

Figure 3

Schematic representation of the exosome uptake: exosomes released into the extracellular space from donor cell enter target cells in three ways, which are direct fusion with the plasma membrane, endocytosis, and protein–receptor interactions.

Figure 4

Steps of differential ultracentrifugation for exosome isolation from cell culture medium.

Figure 5

Some of the major uses of exosomes in clinical development. Exosomes affect various aspects of cell biology and can find employment in different fields of nanomedicine.

Figure 6

Main strategies for loading exosomes with non-native cargos: cellular modifications or direct vesicle modifications can generate enhanced exosomes to be used as delivery vehicles.

Figure 7

Electron microscopic view of individual exosomes. Whole-mount exosome preparations purified with ultracentrifugation and negative stained from plants (A), and different cell lines grown in culture (B–D). The cup-shaped morphology is clearly visible.

Figure 8

Transmission electron micrographs of exosome samples collected from cell culture and visualized after negative staining. In (A) the preparation is a population of large and small vesicles; the morphology is not uniform, some vesicles appear with a cup-like structure some with a deformed shape. Slight clumping can also be observed. In (B) vesicles are surrounded by microparticles, precipitates, and impurities generated in the stain or during preparation.

Figure 9

Immuno-electron detection and negative staining used to reveal the presence of exosome membrane protein labeled with gold particles.

Figure 10

Isolation and characterization techniques for exosomal samples.

Figure 11

Different stages and challenges in the development of EV-based therapeutics. From the industrial-scale production of clinical-grade exosomes to the loading approaches and preservation of exosome biocompatibility, these bottlenecks need great efforts to accelerate the road to clinic.