A Review of the Roles of Specialized Extracellular Vesicles, Migrasomes, and Exosomes in Normal Cell Physiology and Disease

Abstract

Migrasomes are newly-discovered cellular organelle which are generated during cell migration and released from cells as extracellular vesicles (EVs), first described in 2015. Cellular contents are actively transported to migrasomes and released into extracellular space, then are taken up by other cells. Thus, migrasomes are proposed as a new mechanism for cell–cell communications, which show remarkable resemblance to exosomes, another classic EVs. The properties of exosomes in regulating intracellular communication have advanced their potential value in the therapeutic control of multiple diseases such as neurodegenerative conditions and cancer. Moreover, acting as potential biomarkers of various diseases, exosomes can be potentially valuable for diagnosis and assessment of the prognosis of patients with cancer or other diseases. Migrasomes are similar to exosomes in many characteristics. For instance, migrasomes can also mediate the lateral or horizontal transfer of materials among cells. On the other hand, although it is poorly understood, migrasomes show their own properties in normal cell physiology and disease. This review primarily summarizes recent advances in our understanding of the similarities and differences of migrasomes and exosomes in biogenesis, contents, and physiological and pathological effects on organisms, which may help us to have a better understanding of various types of EVs. This article aims to review of the roles of the specialized extracellular vesicles, migrasomes, and exosomes in normal cell physiology and disease.

Background

Extracellular vesicles (EVs) are nanometer-ranged structures delimited by a lipid membrane, released by all cells under both normal and pathological conditions [1]. EVs are heterogeneous in origin, size, and cargo composition [1]. For instance, ectosomes are EVs that pinch off the surface of the plasma membrane via outward budding and are released directly from the plasma membrane [2]. Exosomes are another classic type of EVs, with an average diameter of ~100 nanometers with an endosomal origin [2]. It is formed by inward budding of the endosomal system and released after the multivesicular bodies (MVBs) fusion with the plasma membrane [3]. Exosomes are involved in a wide variety of physiological and pathological processes such as aging, immunity, and cancer [2]. Intercellular communication is an essential property of multicellular organisms. Exosomes also play important roles in communication among several organ systems [4] and can mediate intercellular communication by serving as vehicles for intercellular transfer of membrane and cytosolic proteins, lipids, and RNAs [5].

Recently, a new exosomelike organelle, named ‘migrasome’, has been attracting a great deal of attention [6]. The migrasomes, with diameters from 0.5 μm to 3 μm, contain numerous smaller vesicles with diameters of about 50–100 nm, which resemble opened pomegranates [6]. They derive from retraction fibers at the rear of migrating cells, and share some characteristics with EVs [6]. Migrasomes can also mediate the lateral or horizontal transfer of materials such as RNAs and proteins among cells [7]. Herein, we review the similarities and differences of migrasomes and exosomes in biogenesis, contents, intercellular communication, and as biomarkers for diseases, which may help us to have a better understanding of various types of EVs. This article aims to review of the roles of migrasomes and exosomes in normal cell physiology and disease.

Exosomes

Exosomes are classic EVs typically 40–150 nm in diameter, generated in intracellular MVBs which contain intraluminal vesicles in the endosomal system [3]. The endosomal system is composed of primary endocytic vesicles, early endosomes (EEs), and late endosomes (LEs), which derive from EEs by stepwise maturation, and MVBs [8]. Upon formation, primary endocytic vesicles are transferred to EEs, which themselves mature into LEs. Then, the cargo can be targeted to MVBs, a type of LEs, which can fuse with plasma membrane to form exosomes [3,9,10] (Figure 1A). Exosomes can be secreted in vitro and in vivo by nearly all cell types and exist in almost all tissues, organs, and body fluids [11–14].

Figure 1.

The biogenesis of migrasomes and exosomes. (A) The biogenesis of exosomes. The endocytosed components can be delivered to the endocytic pathway and form endocytic vesicles, and then can be transferred into early endosome (EE) (a), early endosomal membranes can form intraluminal vesicles (ILVs) which package the cargo and deliver to late endosomes (LE) (b) and lysosomes (c). ILVs in multivesicular(MVB) endosomes can be released extracellularly as exosomes (d), which package various cellular contents such as proteins and RNAs. Software (name: PowerPoint; version: 15.31 (170216); manufacturer: Microsoft) was used for creation of the figure. (B) The biogenesis of migrasomes. When a cell moves, retraction fibers are formed at the edge of the migrating cell. Migrasomes form at the tips of, or at the intersections between, these retraction fibers. The migrasomes contain numerous smaller vesicles, and package cytosol, including proteins and RNAs. Software (name: PowerPoint; version: 15.31 (170216); manufacturer: Microsoft) was used for creation of the figure.

As a consequence of endosomal origin, nearly all exosomes contain proteins involved in endosome maturation (eg, Annexins, flotillin), MVBs biogenesis (eg, Alix and Tumor susceptibility gene 101 protein (TSG101)), heat-shock proteins, tetraspanins, and integrins [15]. Proteins of the tetraspanin family, characterized by containing 4 transmembrane domains, have 33 members [16]. Several members of the tetraspanin family have been identified in all kinds of exosomes independently of cell types, such as CD9, CD63, CD81, and CD82 [15], thus frequently serving as the surface markers of exosomes [17]. Exosomes also contain cell-type-specific proteins and lipids due to different cellular ancestries. For instance, exosomes derived from oligodendrocytes carry major myelin proteins [18]; the neural adhesion protein is only expressed in neural-derived exosomes [19].

Exosomes Mediate Intercellular Communication

Exosomes are important mediators of intercellular communication by delivering functional substances, including proteins, genetic cargo, RNAs, and lipids to recipient cells [2]. Exosomes are potential tools for cells to remove waste or toxic species such as prions, amyloid β, tau, and α-synuclein, which are involved in Creutzfeldt-Jacob disease, Alzheimer’s disease (AD), and Parkinson’s disease (PD) respectively [20–22]. Moreover, those toxic forms of aggregated proteins have been suggested to be spread from one cell to another through exosomes, causing an inflammatory cascade [20–22]. In addition, genetic cargo can be transferred between populations of tumor cells via exosomes, promoting drug resistance and progression of cancer [23–25]. The exosomes derived from THP1 cells carry miR-150 and deliver miR-150 into human microvascular endothelial cell lines (HMEC-1 cells). Then, these miR-150 significantly reduced c-Myb expression in HMEC-1 cells and promote cell migration [26].

Exosomes May be Used as Biomarkers

Exosomes may act as biomarkers for the diagnosis of various diseases. For instance, central nervous system-derived exosomal α-synuclein in plasma is substantially higher in PD patients, serving as a PD biomarker with high sensitivity and specificity [27]; exosome-derived biomarkers such as tau phosphorylated at r-181, cathepsin D, and Lysosome-associated membrane protein (LAMP-1) show high value in the diagnosis of AD and mild cognitive impairment [19,28]. Exosomes containing tumor-specific RNAs or proteins released from tumor cells can be potentially used for cancer diagnosis [29,30]. Exosomes from amniotic fluids may be used as a potent tool for prenatal diagnosis [31], and salivary exosomes may be useful in detecting Sjogren’s syndrome [32].

Migrasomes

Migrasomes are newly-discovered organelles produced by migrating cells [7]. Migracytosis is a new migration-dependent release mechanism for releasing cellular contents [6]. Cell migration is a complex process involving the coordinated action of signaling networks, cytoskeletal dynamics, cell adhesion, and membrane trafficking [33–35]. During migration, long tubular cytoplasmic extensions are pulled out from the rear end, which are called retraction fibers [36]. Then, membrane shedding can occur at the tips of, or at the intersections between, these thin retraction fibers, forming bulblike vesicles named “migrasomes” [6]. When retraction fibers break up, migrasomes are released from cells and exist in a package of unilamellar vesicles [6] (Figure 1B). Interestingly, migrasomes can release their internal vesicles to produce EV-like nanoparticles through self-rupture and via a process similar to cell plasma membrane budding [37]. Migrasome generation is strictly migration-dependent and is left in the tracks of migrating cells. Moreover, the number of migrasomes is related to cell migratory behavior [38].

Similar to exosomes, migrasomes are present in multiple cell types, such as human and mouse cell lines, and are even found in macrophages, primary neurons, and embryonic stem cells [6,7,39]. In vivo, migrasome-like structures were found in various organs such as the lungs, eyes, and intestines [6], and even in postmortem brain tissue of stroke patients [40]. Migrasomes also been isolated from body fluid, such as serum and urine [41,42]. Before leaving the cell, migrasomes can carry out some cellular functions [6] (Table 1).

Table 1.

Comparison of physiological features between exosomes and migrasomes.

	Exosomes	Migrasomes	Reference
Diameter	40–150 nm	0.5–3 um	[4,6]
Membrane structure	The double-layer membrane structure	Unilamellar vesicles	[4,6]
Morphology	Cup-shaped morphology	Pomegranate	[3,6]
Major RNAs	miRNAs	mRNA	[54]
Biogenesis	Endosomal origin	Not entirely Clear	[4,6]
Migration-dependent	No	Yes	[3,6]
Markers	Alix, Tsg101, CD9, CD63	NDST1, CPQ, PIGK, EOGT	[41,42,54]
Existence form	Extracellular vesicles	Before left cell, migrasomes are organelles	[3,6]
Diffusion regularity	No fixed direction	Along the migratory path	[3,4,6]

Tsg101 – tumor susceptibility gene 101 protein; NDST1 – bifunc tional heparan sulfate N-deacetylase/N-sulfotransferase 1; CPQ – carboxypeptidase Q; PIGK – phosphatidylinositol glycan anchor biosynthesis, class K; EOGT – EGF domain-specific O-linked N-acetylglucosaminetransferase.

Compared to cell bodies, numerous proteins are enriched or depleted in migrasomes [43]. Similar to exosomes, migrasomal contents may be particularly associated with their biogenesis. The proteins enriched in migrasomes are involved in cell substrate adhesion, cell migration, lipid catabolic processes, protein glycosylation, and glycoprotein metabolic processes [41]. Moreover, as a migrating organelle, one feature of migrasomes is their enrichment in the contractile proteins actin and myosin, annexin proteins, and cytoskeleton [6,44].

The Contents of Migrasomes Compared to Exosomes

Different proteins, metabolites, and nucleic acids in migrasomes and exosomes may lead to a crucial difference in their physiological and pathological functions. There are similarities and differences of contents in migrasomes and exosomes. For instance, the tetraspanin family is enriched both in exosomes and migrasomes [45]. Huang et al observed that overexpression of 14 kinds of tetraspanins (including CD81 and CD82, which are specifically enriched in exosomes) enhanced migrasome formation in a dose-dependent manner [46]. However, tetraspanin CD9 and CD63, another 2 exosomal markers, showed no significant effect on migrasome formation [46]. Proteins of the tetraspanin family can segregate in the membrane plane into tetraspanin-enriched microdomains (TEMs) [47]. Cholesterol, another key component of TEMs, is also abundant on both migrasomes and exosomes [46,48]. Similar to tetraspanin, cholesterol is necessary for migrasome formation [46]. On the other hand, tetraspanin and cholesterol are sufficient for migrasome formation [46].

Integrins, a large family of transmembrane receptors, are composed of α and β subunits, and connect cells to the extracellular matrix (ECM) [49]. The integrin α5 and β1 are enriched in migrasomes, and can pair with their respective ECM partner to hold migrasomes on the ECM, emerging as an important principle for determining migrasomes formation and specific spatial and temporal distribution [39]. Although exosomes are always dissociated from cells and exist alone, they also contain integrins, including α5 and β1 [50]. For instance, exosomal integrin α5β1/asparaginyl endopeptidase complex, derived from epithelial ovarian cancer cells, plays an important role in mesothelial cell proliferation and migration, thus promoting peritoneal metastasis [50].

Interestingly, exosome release can affect the production of migrasomes. When Rab27a plasma membrane docking is inhibited, the migration of neural crest cells becomes slow and directionless [51]. The migration speed and persistence are positively correlated with migrasome formation by regulating retraction fibers [52].

Although there are many critical proteins in both migrasomes and exosomes, quantitative mass spectrometry suggests that the 2 structures share only 27% of their proteins [41]. In addition to tetraspanin 4 (TSPAN4) [53], Zhao et al identified 4 migrasome-specific potential biomarkers – NDST1 (bifunctional heparan sulfate N-deacetylase/N-sulfotransferase 1), CPQ (carboxypeptidase Q),PIGK (phosphatidylinositol glycan anchor biosynthesis, class K), and EOGT (EGF domain- specific O-linked N-acetylglucosaminetransferase) – which are abundant in migrasomes but are not present in exosomes [41]. On the other hand, exosomal markers such as Alix, Tsg101 and CD63 are found in small amounts in migrasomes [42,54,55] (Table 1).

In addition to proteins, RNAs are also present in migrasomes. In contrast to exosomal RNAs that mostly belong to small RNA species such as miRNAs, migrasomal RNAs are mainly composed of mRNA species that are long (>200 nt) [7,54] (Table 1). In addition, the expression profile of miRNAs in migrasomes also differs from that in exosomes. For instance, podocyte migrasomes contain more miR-1303, miR-548a, miR-490-5p, miR- 611, and miR-661 than podocyte exosomes, while exosomes have larger amounts of miR-221-3p, miR-144-3p, and miR-4286 than do migrasomes [42].

The Role of Migrasomes in Cell–Cell Communications

Cellular contents can be released through both exosomes and migrasomes. As described above, migrasomes can be released into the extracellular environment when retraction fibers break up [6]. Jiao et al suggested that damaged mitochondria can be transported into migrasomes and subsequently be cleared from migrating cells, thus maintaining the quality of the mitochondrial pool [56]. Similar to exosomes, intact migrasomes can be engulfed by surrounding cells, mediating release and transfer of cytoplasmic contents during cell migration [6]. Thus, migrasomes may be a newly-discovered mechanism for communication and transportation of cellular contents between cells [54]. Zhu et al found that full-length Pten mRNA can be transferred from migrasomes to recipient cells, then are translated into Pten proteins, which can modulate the pAKT level in recipient cells and inhibit the proliferation of recipient cells [54]. Migrasomal Pten protein itself can also be transferred into recipient cells, with functional consequence [54].

Normal Physiology of Migrasomes

Suppression of migrasome formation through knockdown of tspan4a and tspan7 in zebrafish gastrulas leads to deficiency of organ morphogenesis, including morphological deformation or reduction of size in many organs, which can be successfully rescued by injection of exogenous migrasomes [43]. Migrasomes contain various signaling molecules such as Cxcl12, which has been suggested to play an important role in organ morphogenesis [57]. Jiang et al. suggested that migrasomes can deliver Cxcl12a for signaling during gastrulation, and Cxcl12a (ligand)–Cxcr4b (receptor) signaling axis is necessary for zebrafish organ morphogenesis [43]. Thus, migrasomes may carry out their physiological function by transfer of signaling molecules to modulate the behaviors of surrounding cells [43]. The chemoattractant stromal cell-derived factor 1 (SDF-1) was detected in mesenchymal stromal cell–associated migrasomes, which enabled migrasomes to attract leukemic KG-1a cells and primary CD34+ hematopoietic progenitors in co-cultures [55]. Moreover, those migrasomes can be selectively taken up by migrating leukemic cells [55]. The monocyte migrasomes are enriched in pro-angiogenic factors, which are pivotal regulators of angiogenesis [58]. Monocytes promote capillary and endothelial cell tube formation by migrasomal pro-angiogenic factors vascular endothelial growth factor (VEGFA) and Cxcl12 [58].

Migrasomes in Diseases

Sodium chloride has been shown to induce marked migrasome formation of microglia/macrophages in an acute ischemic stroke model [40], and the extent of migrasome formation is correlated with the number of shrunk neurons [40]. Moreover, neurons are found in close vicinity to migrasomes, and double immunofluorescent staining revealed co-localization of microglia migrasomes and neuron fragments, suggesting a potential interaction between the migrasomes and surrounding neurons [40]. Migrasomes can be internalized by retinal pigmented epithelium (RPE), promoting RPE activation and proliferative vitreoretinopathy (PVR) progression by increasing the migration and proliferation ability of RPE [59]. TSPAN4 is abundant in the migrasome membrane and is one of the most effective tetraspanins for migrasome induction [46], stabilizing the migrasome structure and acting as a migrasome marker [6,60]. The expressionof migrasome-related genes such as pigk, itgb1, and tspan4 are significantly up-regulated in a variety of tumor types [61–63]. Moreover, expression levels of multiple migrasome-related genes were associated with a poorer prognosis and drug sensitivity [61]. Using single-cell sequencing analysis, Yue Zheng et al showed that the migrasome-related gene tspan4 was highly correlated with the atherosclerosis regression-associated macrophages, intraplaque hemorrhage, and ruptured plaques [62].

Migrasomes Act as Potential Biomarkers

Recent works by Liu et al discovered that podocytes can release migrasomes during migration [42]. Podocyte injury can trigger its motility [64,65], thus significantly promoting the release of migrasomes [42]. In vivo, higher levels of podocytes-derived migrasomes was detected in urine in a mouse nephropathy model than those in the control group, which was validated in human patients with kidney injury [42]. The increase of migrasomes occurs before the appearance of proteinuria [42]. Thus, levels of urinary migrasomes may serve as a potential diagnostic biomarker for early development of podocyte-based nephropathies.

Conclusions

The migrasomes and exosomes are similar in many aspects, and also have their own characteristics, and even show functional complementation. For instance, the originating cell source of exosomes in urine is complex, while urinary migrasomes are mainly derived from podocytes [42]. Therefore, it seems that urinary migrasome is more suitable as a noninvasive renal disease marker [42].

Future Directions

It is poorly understood how migrasomes travel so far, and much more research is needed to explore the physiological functions and pathological significance of migrasomes. For instance, the mechanisms underlying migrasome biogenesis and the pathways controlling migrasome formation are largely unclear. Moreover, it is unknown whether migrasomes carry disease-related proteins similar to exosomes, and if so, their role in various diseases. In addition, it is unclear whether migrasomes transfer toxic forms of aggregated proteins or genetic cargo between cells in diseases such as neurodegenerative disease and cancer. It is also unknown whether production of migrasomes from tumor cells is the major part of tumor metastasis. Exosomes may be engineered to deliver various therapeutic payloads, including chemotherapeutic agents, immune modulators, and short interfering RNAs, with an ability to direct their delivery to a desired target.

Much more research is needed to determine whether migrasomes have the potential to be engineered to deliver drugs, similar to exosomes.