Stem cells are unspecialized cells with long-lasting self-renewal potential. After differentiation they can become specialized cells with new physiological functions Bongso and Lee, 2005. In recent years, stem cells have been discovered to exhibit other useful functions, including secretion of cytokines Kilroy et al., 2007, i.e. growth factors which help stimulate tissue regeneration Boomsma and Geenen, 2012, and notably immune modulation of mesenchymal stem cells Abdi et al., 2008da Silva Meirelles et al., 2009Prockop and Oh, 2012Yanez et al., 2006. To date, stem cells (particularly MSCs) have been discovered to have at least 3 functionst: homing and differentiation into tissue specialized cells, production of cytokines and growth factors, and immune modulation.

Given these functions, stem cells have been tested for various diseases; in fact over the last 50 years they have been evaluated in more than 50 different diseases Ginn et al., 2013Squillaro et al., 2016Van Pham, 2016. Hematopoietic stem cell transplantation has been widely used to treat hematopoietic malignancies, and MSC therapy has already been approved as routine treatment for a number of diseases in Canada, Japan, Korea, China, India and Vietnam. Compared to HSCs, MSCs play many more functions in vivo and possess unique characteristics, such as immune modulation and tissue healing via secreted factors Chase and Vemuri, 2012.

One of the most well-known secreted factors is extracellular vesicles (EVs). EVs have distinct physiological characteristics and have been studied for disease applications for the past 5 years. This review aims to summarize the characteristics of EVs and their applications in the clinic.

Extracellular vesicles are nano-sized particles produced from the live cells during their lifespan. EVs can be classified into two main kinds based on their size: exosomes and microvesicles. Exosomes are about 40-150 nm in diameter whereas microvesicles are about 50nm-2000 nm in diameter. They differ in the way they are produced and, thus, exosomes and microvesicles exhibit different properties.

Generally, exosomes are produced from secretory mechanisms and are regulated by endosomal sorting complex mechanisms, which are associated with transport proteins (e.g. ESCRT), Rab proteins, tumor protein p53 pathway, tumor suppressor-activated pathway 6, and ceramide/neutral sphingomyelinase Lespagnol et al., 2008Ostrowski et al., 2010Rak, 2013. Moreover, exosomes are rich in tetraspanins (CD63, CD81, CD9, etc.), gangliosides, sphingomyelin, and saturated lipids. Exosomes generally have a more rigid membrane than that of microvesicles which allow them to be more resistant to degradation and thus more stable Pols and Klumperman, 2009Raposo and Stoorvogel, 2013Stoorvogel et al., 2002.

Microvesicles are directly produced from the plasma cellular membrane and therefore they contain some cytoplasmic components. Unlike exosomes, microvesicles contain markers of the original cells, such as common proteins of the cellular membrane like integrins, glycoprotein Ib (GPIb), and P-selectin Kastelowitz and Yin, 2014Raposo and Stoorvogel, 2013.

Besides exosomes and microvesicles, apoptotic bodies and oncosomes can also be found in EVs Crescitelli et al., 2013Meehan et al., 2016. Apoptotic bodies are products of the apoptotic process while oncosomes are larger vesicles produced from cancer cells.

EVs are comprised of exosomes, microvesicles, apoptotic bodies and oncosomes, and play important physiological roles, especially in cellular communication. They are important not only in the normal physiological processes but also in pathological conditions. The roles that EVs play are dependent on the content they cargo. It was initially discovered that EVs contain siRNA molecules Eirin et al., 2014Kumar et al., 2015Lai et al., 2015Vallabhaneni et al., 2015. Nowadays, it is known that they carry many more forms of “information”. In fact, exosomes have been described as “information cargos” for their transport of siRNA, DNA, peptides, and proteins Baglio et al., 2012Biancone et al., 2012Camussi et al., 2010Lai et al., 2015Rani et al., 2015Yu et al., 2014. All of these aforementioned molecules help regulate cell targeting by modulating gene expression and gene regulation in target cells at the level of post-transcription and translation Camussi et al., 2011Collino et al., 2010Mokarizadeh et al., 2012Zhang et al., 2015b.

It has been known for over a decade that MSCs can produce EVs. MSC-derived EVs (MSC-EVs) contain at least 2 components: exosomes and microvesicles. Both exosomes and microvesicles express tetraspanin molecules and MSC markers on their surface; these include CD9, CD63, CD81 and CD107, and CD29, CD73, CD44 and CD105, respectively Lai et al., 2015Yu et al., 2014.

The components inside EVs have been the focus of many research studies Baglio et al., 2012Lo Sicco et al., 2017Wang et al., 2017Yuan et al., 2017. The main components found inside MSC-EVs are miRNAs De Luca et al., 2016Fafian-Labora et al., 2017 Livingston and Wei. Notably, MSC-EVs have been found to contain miR-223, miR-564, and miR-451 De Luca et al., 2016Nawaz et al., 2016. These miRNAs play the important roles in cell survival, cell differentiation, and immune regulation Yanez-Mo et al., 2015). Besides miRNAs, other RNAs can be found in MSC-EVs Borger et al., 2017. Such RNAs include transcription factor CP2/clock homolog (which regulates transcription), retinoblastoma-like 1 (which also regulates transcription), small ubiquitin-related modifier 1 (which regulates cell proliferation), and interleukin-1 receptor antagonist (which regulates immune responses) Tomasoni et al., 2013.

MSC-EVs are generally comprised of 3 main groups of proteins/molecules, including surface receptors, signaling molecules, and cell adhesion molecules. These proteins were demonstrated to regulate cell self-renewal and differentiation. Surface receptors and cell adhesion molecules are present on the surface of EVs, and likely originated from cell membrane. Conversely, signaling molecules are usually found within EVs, and likely originated from secretory processes. Some common surface receptors found in MSC-EVs include platelet-derived growth factor receptor, epidermal growth factor receptor, and plasminogen activator urokinase receptor. Some common cell adhesion proteins include fibronectin, ezrin, IQ motif containing GTPase activating protein 1, CD47, integrins, lectin galactose binding soluble 1 (LGALS1), and lectin galactose binding soluble 3 (LGALS3) ( Figure 1 ).

Other important molecules to be found within MSC-EVs include RAS-related protein/neuroblastoma RAS, mitogen-activated protein kinase 1 (MAPK1), guanine nucleotide-binding protein subunit 13/G protein subunit 12 (GNA13/ GNG12), cell division control protein 42 homolog, Vav guanine nucleotide exchange factor 2, transforming growth factor beta, mitogen-activated protein kinase, and peroxisome proliferator-activated receptor Baglio et al., 2012. Given these important aforementioned components, MSC-EVs represent a cellular therapeutic approach with great potential in regenerative medicine ( Figure 1 ).

EVs from MSCs contain some biological components such as DNA, RNA and proteins. These molecules help EVs exhibit particular physiological activities and functions, similar to those of MSCs, such as stimulation of tissue regeneration and immune modulation. Therefore, EVs from MSCs have become increasingly popular to study in recent years. Importantly, primary investigations have indicated the promise of EVs in applications of regenerative medicine.

ARF: Acute renal failure

EB: Epidermolysis bullosa

EVs: Extracellular vesicles

GVHD: Graft versus host disease

HSCT: Hematopoietic stem cell transplantation

MSCs: Mesenchymal stem cells

SCI: Spinal cord injury

T1D: Diabetes type 1