Extracellular vesicles (EVs) are defined as a heterogenic group of lipid bilayer vesicular structures with a size in the range of 30–4000 nm that are released by all types of cultured cells. EVs derived from platelets, mononuclears, endothelial cells, and adipose tissue cells significantly increase in several cardiovascular diseases, including in atrial fibrillation (AF). EVs are engaged in cell-to-cell cooperation, endothelium integrity, inflammation, and immune response and are a cargo for several active molecules, such as regulatory peptides, receptors, growth factors, hormones, and lipids. Being transductors of the intercellular communication, EVs regulate angiogenesis, neovascularization, coagulation, and maintain tissue reparation.
1. Introduction
Atrial fibrillation (AF) is the most common form of cardiac arrhythmia amongst older people and patients with cardiovascular (CV) diseases (CVD) and continues to demonstrate steady growth in the general population
[1]. The prevalence of AF is increasing at epidemic proportions in both developed and developing countries, regardless of the presence of conventional CV risk factors
[2,3][2][3]. Indeed, the current prevalence of AF in the European Union is 7.3%, and it will increase to 89% by 2060
[1,3][1][3]. Moreover, at least 65% of senior citizens in the European Union will have AF in 2060, and paroxysmal, persistent, and permanent forms of AF are projected to be diagnosed in approximately 5,989,000, approximately 2,833,000, and approximately 5,579,000 of older people, respectively
[3]. Electroanatomic and adverse cardiac remodeling resulting in natural CVD evolution and AF persistence is considered to be a substrate for the development of cardiac dysfunction, heart failure (HF) occurrence, and thromboembolic complications, which sufficiently reduce life span duration and quality of life in patients affected by this condition
[4]. In addition, AF is independently associated with both high rates of morbidity, and mortality is a common cause of premature disability in CVD patients
[5]. Overall, the risk of thromboembolism in these patients is not just associated with AF, but it also varies widely depending on coexisting comorbidities (HF, chronic kidney disease, chronic obstructive pulmonary disease, diabetes mellitus, obesity, and cardiomyopathy), low physical activity, patient age and gender, and methods of cardioversion and anticoagulation
[6,7,8][6][7][8]. Despite the numerous benefits of CVD prevention therapy and its wide implementation in routine practice, there remains an unacceptably high risk of potentially devastating complications associated with catheter pulmonary vein isolation (PVI), such as stroke/transient ischemic attack and systemic thromboembolism during persistent AF or the occurrence of its permanent form
[9]. The number of one-year recurrent AF episodes has remained high (20%)
[10], whereas single-procedural 1-year and 5-year arrhythmia-free survival is 66% and 44%, respectively
[11,12][11][12]. However, the prevention of thromboembolic complications remains the focus of pragmatic strategy development for AF therapy
[13,14][13][14].
The extracellular vesicles (EVs) are vesicular structures that are secreted from numerous cells and supply several biologically active molecules (growth factors, active peptides, regulatory proteins, pro-inflammatory cytokines, micro-RNAs (miR)) that are involved in cell-to-cell communication, including in the modulation of tissue repair, inflammation, angiogenesis/neovascularization, immune response, extracellular matrix accumulation, and vascular integrity enhancement
[15,16,17][15][16][17]. Moreover, EVs that originate from the activated or apoptotic cells exert variable effects that are dependent on a spectrum of encapsulated cytokines, proteome, lipidome, and miRs and have an epigenetic impact on target cells
[16]. There is strong evidence of the fact that EVs that are derived from a large spectrum of circulating blood cells, including platelets, mononuclear cells/leucocytes, endothelial cells, and even adipose tissue cells and antigen-presenting cells, are a cargo for pro-coagulant phospholipids, mainly phosphatidylserine, active molecules, non-coding RNAs, and other components, that regulate coagulation cascade and play a crucial role in the incidence of AF-related thromboembolism
[15,16,17][15][16][17].
2. Extracellular Vesicles: Definition, Nomenclature, Biological Function
According to the International Society on Extracellular Vesicles (ISEV), EVs are defined as a heterogenic group of lipid bilayer vesicular structures with a size in the range of 30–4000 nm that are released by all types of cultured cells and are found in abundance in body fluids (blood, lymph, saliva, urine, bile, synovial fluid, cerebrospinal fluid)
[18]. EVs consist of three subpopulations: exosomes, microvesicles (MVs), and apoptotic bodies (ABs), that can be distinguished from each other based on their size, immune phenotypes, origin, biogenesis, and mechanism of release and component delivery
[19,20,21][19][20][21].
Table 1 contains the main characteristics of EVs. Although a universal definition of EVs subpopulations remain elusive, the ISEV recommends not using other criteria apart from the size of the EVs to classify them because there is overlap among the different phenotypes of these vesicular structures
[22]. Biological pathways of generation, shedding, and the release of EVs exert remarkable diversity. For instance, exosomes appear following endocytosis from endosomes, whereas MVs are synthesized after blebbing from the plasma membrane and being released in biological fluid and contain all of the antigens that are widely expressed on the surface of the mother cells
[23]. In addition, MVs and exosomes are not only produced by cellular activation, but they are also produced via pro-inflammatory stimulation, shear stress, and the influence of pro-thrombotic and pro-apoptotic substances. ABs occur as the result of the shrinkage and blebbing of apoptotic cells, but there is alternative pathway that relates to lysosome vesicle secretion and secretory autophagy. Therefore, mechanical activation and hemolysis, which are common features in patients with CVD (valve stenosis, prosthetic valves, AF, HF) as well as non-CVD (infections, sepsis, and eclampsia), are strongly associated with apoptotic cell breakdown
[24,25,26,27][24][25][26][27]. In fact, an evaluation of the EVs secretome demonstrated that different sub-populations of EVs had either overlapping protein components or coincidentally similar protein arrangements, and this was a consequence of the type of stimulation that caused the EVs to release and the type of the mother cells
[28].
Table 1.
Characteristics of EVs.
Characteristics |
Exosomes |
Microvesicles |
Apoptotic Bodies |
Diameter, nm |
30–150 |
100–1000 |
500–4000 |
Sedimentation, g |
100,000 |
20,000 |
16,000 |
Pathway for biogenesis |
Endocytosis from endosomes and exocytosis of late endosomes/MVBs |
Blebbing from plasma membranes |
Shrinkage and blebbing of apoptotic cells |
Unconventional secretion pathway |
Cellular activation |
Early apoptosis |
Lysosome vesicle secretion and secretory autophagy |
Delivery contents |
Alix, chaperones, Rab proteins, Rab GTPases, SNAREs, lipid rafts, proteins (flotillin), myokines, inflammatory cytokines, growth factors, miRs. |
Arachidonic acid, cytokines, chemokine RANTES/CCL5, P-selectin, lipids, signaling proteins, miRNA, and microRNA, membrane-anchored receptors (PPARγ) and adhesion molecules |
Organelles and/or nuclear content including chromatin, DNA, miRNAs, microRNAs, histones, oncogenes. |
Membrane-specific antigens |
Tetraspanins (CD9, CD81, CD63), TSG101, |
Integrins, selectins, membrane proteins of parental cells |
Annexin-V(+) |