Migrasome, a novel organelle, differs from exosomes

Abstract

Migrasomes, a newly discovered organelle produced by migrating cells, are vesicles with membranous structure that form on the tips and intersections of retraction fibers (RFs). These structures are released into the extracellular environment or taken up by surrounding cells, mediating the release of cytoplasmic contents and intercellular communication. Retractosomes, a new type of small extracellular vesicles generated from broken-off RFs, are closely related to migrasomes in their physical location and origin, but were defined later. Despite their widespread existence in cells and biological organisms, little is known about the regulatory mechanisms underlying their formation and potential function. In this review, we provide an overview of the discovery, biogenesis, distribution, and functions of migrasomes and retractosomes, as well as their differences from exosomes.

Highlights

• •Migrasome is a newly discovered organelle.
• •Both migrasome and retratosome formation are associated with RFs.
• •This paper conducts a review regarding the discovery, biogenesis, distribution and functions of migrasome.

1. Introduction

The migrasome, a newly discovered class of organelles produced by migrating cells, was first reported by Professor Yu et al., in 2014 [1]. When cells migrate, they leave behind filamentous structures called RFs, and the vesicles that form on the tips and intersections of RFs are defined as migrasomes [1]. These structures range in size from 0.5 to 3 μm and exhibit cell migration-dependent characteristics. Cellular contents can be transported to migrasomes via RFs. Migrasomes can be released into the extracellular space via migracytosis [1,2]. As cells migrate and retract RFs, migrasomes are left in situ and fall off from RFs, ultimately mediating the release of cytoplasmic contents and communication between cells [3]. Furthermore, the exfoliation of RFs produces a previously unreported vesicle structure called retractosomes [4]. Recent data suggest that migrasomes are widely distributed in different types of cells, body fluids, and tissues [1,5]. Migrasomes play a critical role in the establishment of signaling molecular regions in early embryonic development, as demonstrated in a Zebrafish experimental model [1,6].

2. The discovery of retraction fibers

In the 1940s, Porter and colleagues [7] observed linear tubular structures in cells using electron microscopy during their studies on tissue culture, describing them as “jagged points” and “finger-like processes”. Subsequent work by Taylor and Robbins [8] used light and transmission electron microscopy to provide a more detailed characterization of these structures. In their study, they documented the appearance of long tubular structures that occur when migrating cells retract from the substratum, which were initially named “retraction fibrils” [8] and later “retract fibers.” Despite their widespread distribution in different cell types, these structures, now known as retraction fibers (RFs), have received little attention.

3. The discovery of migrasomes

In the 2010s, Yu et al. used transmission electron microscopy to observe a “vesicle-like structure similar to an open pomegranate appearing at the cell periphery”. Intrigued by this observation, Professor Yu and colleagues found that some of these extracellular vesicles were empty, while others contained multiple vesicles or vesicles resembling pomegranates. Tentatively, they named these structures “pomegranate-like structures (PLSs)" [1,9]. The study on PLSs was published in Cell Research in 2014.

Yu et al. isolated and purified PLSs by density gradient centrifugation and confirmed their identity by transmission electron microscopy. Further analysis by mass spectrometry identified the quadruple transmembrane protein TSPAN4 as a marker for PLSs, which was verified by constructing plasmids fused with GFP proteins for screening. TSPAN4 localizes to both the cell membrane and the retraction fibers (RFs) protruding from the cell membrane surface, and PLSs form at the tip or bifurcation of the RFs, which retract and break to release the PLSs into the medium. PLSs exist for an average life cycle of about 200–400 min [1,10] before fragmenting and disappearing. Because PLS formation is intimately linked to the cell migration process, they are officially named “migrasomes."

4. The discovery of retractosomes

Upon cell migration, numerous RFs are left behind. However, the fate of these RFs remained unclear until recent studies showed that these filaments break and quickly generate migrasomes on their tips and intersections. Interestingly, TSPAN4 was observed to be evenly distributed along the RFs during this process. As cells continue to move, the RFs eventually detach from the cells, resulting in the appearance of TSPAN4 as small spots along the path occupied by the RFs. Scanning electron microscopy revealed a large number of small, round extracellular vesicles at the distal end of the RFs, which further analysis with transmission electron microscopy confirmed to be true vesicles. These small extracellular vesicles were named retractosomes and were much smaller than migrasomes, ranging from 50 nm to 250 nm in diameter [4].

The overexpression of TSPAN4 significantly promotes the formation of migrasomes and retractosomes, the latter mainly due to the enhanced formation of RFs [1,4]. While migrasomes are assembled from large structural domains rich in cholesterol and four transmembrane proteins, cholesterol depletion inhibits their formation. In contrast, cholesterol consumption promotes the formation of retractosomes, suggesting that the formation of retractosomes depends on TSPAN4 rather than cholesterol, which is different from migrasomes in molecular mechanisms of formation [[11], [12], [13]].

Retractosomes are newly discovered membrane-like structures that are smaller than migrasomes but larger than small extracellular vesicles. Unlike exosomes, retractosomes exhibit low levels of classic exosome markers Tsg101, CD63, and Syntenin-1 protein. Comparative protein analysis showed that the protein composition of retractosomes differed significantly from that of cell bodies, with 1309 proteins being downregulated and 1107 proteins upregulated in retractosomes. However, quantitative mass spectrometry analysis revealed significant similarities between retractosomes and migrasomes in terms of protein composition. Retractosomes are believed to arise from broken RFs during cell migration, but their specific physiological functions are yet to be determined. (Fig. 1).

Fig. 1.

Cell Formation Patterns of Migrasomes, Retractosomes, and Exosomes.

In vitro cultured cells have been observed to continuously produce elongated RFs at the end of their motor domains during migration. These RFs give rise to migrasomes, which are produced at the end and bifurcation of the RFs. Migrasomes contain small extracellular vesicles (EVs) and damaged mitochondria, which are enriched with TSPAN4 and cholesterol. After the breakage of the RFs, the migrasomes leave the cell body and are taken up by other cells. In addition to migrasomes, the breakage of RFs at the ends during cell migration also produces retractosomes, which are bead-like vesicles. On the other hand, exosomes are a minor subpopulation of EVs that are produced by the intracellular vesicle trafficking system. They are involved in early endosomes and the biogenesis of multivesicular bodies (MVBs) after fusion with the plasma membrane. Although exosomes were originally thought to be dispersed in a disorganized manner after leaving the plasma membrane, recent findings indicate that they can also be produced in a migration-independent manner.

5. Mechanism of migrasome formation

5.1. Cell migration pattern regulates the formation of migrasomes

Cell migration is a multi-step process [14] in which cells continuously change shape and polarity [15,16]. To initiate migration, cells must acquire spatial asymmetries that allow them to convert forces generated within the cell into a net translocation. Morphological polarization, which manifests as a clear distinction between the anterior and posterior poles of the cell, is an essential consequence of this asymmetry. The extension of the active membrane, which mainly occurs at the anterior pole of the cell, results in the formation of extended protrusions in the direction of movement, initiating cell migration and stabilizing the process through interactions with the surrounding environment [14]. Migrasome formation is closely related to cell migration, as it occurs on residual bodies (RFs) left behind during the migration process.

Recent studies have suggested that the direction and speed of cell migration can affect migrasome formation. A change in the direction of cell migration leads to a decrease in migrasome formation compared to cells that migrate persistently with a straight trajectory [17]. This decrease may be due to the elongation of the cell body during migration, which results in a narrower tail and an unstable migratory direction, ultimately leading to a reduction in the formation of RFs and migrasomes. Furthermore, Fan et al. [17] found a positive correlation between migration velocity and migrasome formation. With increasing cell migration speed, the length of RFs increases, thereby promoting migrasome formation. However, the removal of vimentin, which is directly related to the maintenance of cell shape, hinders the migration speed and persistence of cells, resulting in reduced migrasome formation. In summary, the above results suggest that cell migration pattern, specifically migration direction and speed, controls migrasome formation by regulating the state of RFs.

5.2. The pairing of integrins and extracellular matrix (ECM) determines the formation of migrasomes

The formation of migrasomes depends on cell migration, which in turn relies on ECM adhesion. Cell migration involves protrusion of the cell front, formation of adhesions, and retraction of the cell tail.

Cell migration is directional and begins with the establishment of cell polarity, which separates the anterior ends from the posterior end of the cell. Polarity is established by a concentration gradient, where specific proteins or signaling molecules are enriched and activated at the anterior end, creating a concentration gradient. After polarity is established, large lamellipodia and thin filopodia form at the cell's front end, with the direction of their formation indicating the direction of cell migration. Following cell protrusion, adhesion to the environment is necessary for stability. Integrins, the adhesion molecule that extensively participates in most cells, play an essential role in this process by mediating attachment to the ECM and forming adhesion complexes. The final step in cell migration is the retraction of the cell tail, which involves depolymerizing the adhesion structure. Integrins mediate cell adhesion, and adhesion disassembly is observed as the cell tail retracts, resulting in integrins remaining behind while the rest of the cell retracts [2,18].

Integrins are transmembrane receptors that mediate cell adhesion to the extracellular matrix (ECM) and exist as αβ heterodimers with large extracellular structural domains [10,19]. Wu et al. [10] reported that integrin α5β1 was concentrated at the base of the migrasome, rather than in focal adhesions, and appeared on retracting fibers prior to migrasome formation. In mammals, the 19 alpha and eight beta integrin subunit genes encode polypeptides that combine to form 25 distinct receptors. The extracellular structural domains of integrins determine binding specificity and recognize multiple matrix ligands, with different integrins binding and adhering to distinct ECM proteins. The formation of migrasomes may require specific pairing of integrins and their extracellular matrix (ECM) chaperone proteins. These proteins exhibit distinct distribution patterns in an organism. For instance, in rat kidney cells (NRK cells) enriched with integrin α5, fibronectin has been shown to be the most effective ECM protein for promoting migrasome formation [10,20]. In contrast, in Chinese hamster ovary cells (CHO cells) enriched with integrin α1, type IV collagen, which exhibits the highest binding affinity for this integrin, significantly enhances migrasome formation, while other ECM-related proteins have no effect [10].

Mechanistically, integrins make cell migration possible when cells migrate. Integrin pairing with ECM chaperones provides adhesion to retracting fibers (RFs). RFs are gradually generated behind migrating cells and eventually form migrasomes. As suggested by a recent study from Lu et al. [20], migrasome formation depends on a combination of cell adhesion and migration. In particular, their study further suggested that cell-ECM adhesion regulates the tension of the RFs and that proper tension of the RFs is a prerequisite for migrasome formation. Thus, these studies indicated that pairing integrins with specific ECM chaperone proteins for proper adhesion is a determinant of migrasome formation. At the same time, integrin enrichment in migrasomes also provides specificity for entry into specific receptor cells, which offers the possibility to answer the questions of “where migrasomes come from” and “where they go,” providing a theoretical basis for the investigation of the biological functions of migrasomes.

5.3. The crucial role of TSPAN4 and cholesterol in migrasome formation through membrane scission

According to Ma et al. [1], TSPAN4 was the most highly enriched protein in migrasomes and was initially used as a marker to identify them. However, TSPAN4's role extends beyond its identification as a protein marker. In fact, of the 33 known mammalian tetraspanins, overexpression of 14 TSPANs has been shown to promote migrasome formation, with 9 of these (TSPAN1, 2, 4, 6, 7, 9, 18, 27, and 28) having strong effects [11,12]. Notably, TSPAN4 is one of the most potent quadruple transmembrane proteins, and knockdown of TSPAN4 impaired migrasome formation in NRK cells and human gastric cancer cells MGC-803, while no impairment was observed in mouse fibroblasts L92, likely due to the presence of other quadruple transmembrane proteins capable of forming migrasomes. Moreover, TSPAN4 is recruited to migrasomes during their formation. Investigation into the biogenesis of migrasomes with TSPAN4-GFP revealed an increase in the average intensity of TSPAN4-GFP on migrasomes, accompanied by a slight decrease on RFs during the initial phase of rapid migrasome growth. Additionally, after reaching the maximum size, the signal of TSPAN4-GFP enters a stable phase in which the expression of TSPAN4-GFP in migrasomes remains unchanged, and enrichment ceases. During migrasome growth, TSPAN4 is recruited from RFs to migrasomes. Once recruited, TSPAN4 cannot migrate out and return to RFs (the mechanism by which TSPAN4 does not return is unclear), suggesting that TSPAN4 may drive migrasome formation. Recent evidence indicates that TSPAN proteins can form tetraspanin-enriched microstructural domains (TEMs) on membranes [13,21].

During migrasome biogenesis, tetraspanin-enriched macrodomains (TEMAs) composed of small TEM components contribute to the formation and growth of both retraction fibers (RFs) and migrasomes. A recent study by Huang et al. [11] employed a modified in vitro migrasome formation system, in which direct force was applied to minor points on the membrane using a glass needle to manually pull the cell membrane and generate RF-like structures. This stretching process induced the assembly of TEMAs from TSPAN4, which eventually expanded spontaneously to form migrasome-like structures. Migrasome-like structures were exclusively observed in TSPAN4-containing RFs and absent in its absence. TSPAN4 concentration in migrasomes was four-fold higher than in RFs, indicating its crucial role in migrasome formation and enrichment by aiding TEMA assembly.

Migrasomes exhibit a high level of enrichment of cholesterol and TSPAN4 [11]. To determine the relative enrichment of TSPAN4-GFP and cholesterol on migrasomes, TSPAN4-GFP-expressing cells were stained with filipin III. The results showed that cholesterol was enriched 10-fold more than TSPAN4. Huang et al. [11] established an in vitro simulation system to form migrasomes and RFs using fluid flow, mechanical stress, and artificial stretching. In this system, TSPAN4 was highly enriched in migrasome-like structures and co-localized with cholesterol. When either cholesterol or TSPAN4 was absent, migrasome-like structures were not observed and both molecules were randomly distributed on RFs. Notably, TSPAN4 did not assemble into TEMAs, indicating its distinct role in migrasome formation. These findings highlight the essential role of cholesterol and TSPAN4 in migrasome formation.

These studies suggest that the assembly of cholesterol and TSPAN4 into TEMAs is a necessary condition for migrasome formation. The in vitro reconstitution system provides a modifiable platform for studying migrasome formation at the molecular level, enabling the measurement and calculation of biophysical parameters to generate data for modeling migrasome formation. Furthermore, the initial formation of TEMAs on the RFs allows the membrane to bend more rigidly than other parts of the RFs, spontaneously expanding to form migrasomes. The experimental parameters are in good agreement with those calculated by modeling, providing a theoretical basis for the mechanism of migrasome genesis and pioneering the investigation of migrasome function with TSPAN4 and TEMAs as the core. Importantly, this study proposes the essential components and fundamental principles of migrasome formation, laying the groundwork for subsequent studies on the regulation of migrasome genesis.

6. Basic distinctions between migrasomes, retractosomes and exosomes

Migrasomes, a newly discovered cellular structure, exhibit significant differences from exosomes, microvesicles, and apoptotic bodies in terms of size, lifecycle, release mechanism, and molecular composition. Exosomes, small extracellular vesicles (sEV) enclosed by a lipid bilayer membrane, are secreted by various cell types [22] and differ substantially from migrasomes (Table 1). The migrasome membrane is characterized by the presence of TSPAN4/7 and integrins α1, α3, α5, and β1, which are critical structural markers [1,10,11]. Additionally, migrasomes are enriched with proteins such as NDST1, PIGK, CPQ, and EOGT, which are either absent or minimally detected in exosomes, and serve as unique protein markers of migrasomes [23].

Table 1.

Basic differences between migrasomes, retractosomes and exosomes.

	Migrasome	Retractosome	Exosome
Origin	During migration, migrasomes are formed at the tip or bifurcation of the RFs	Broken-out retraction fibers	Multivesicular body (MVB)
Essence	Organelle	Extracellular vesicles	Extracellular vesicles
Diameter	500-3000 nm	50–250 nm	30–150 nm
Content	EVs, impaired mitochondria, mRNA, miRNA, Protein, and other components are still being explored	Protein and other components are still being explored	DNA, mRNA, miRNA, LncRNA, Protein, Lipid, MHC-1/2
Protein composition	Scarcely any LAMP1	Scarcely any LAMP1	The LAMP1 can be marked with the label
Markers	TSPAN4, TSPAN7, NDST1, PIGK, CPQ, EOGT, Integrin (α1, α3, α, β1)	PIGK, EOGT, PCCA (Lower enrichment levels compared to the migrasomes)	CD9, CD63, CD81, HSP60, HSP70, HSP90, TSG101
Migration-dependent	Yes	Yes	No
ECM-dependent	Yes	Yes	No

Retractosomes, another recently discovered cellular structure, share significant similarities with migrasomes in terms of protein composition but differ from exosomes [4]. While migrasomes are large vesicle-like structures distributed at the branching points or ends of RFs, retractosomes are small dots arranged in a beaded pattern, which distinguishes them from migrasomes based on morphology and location. However, both migrasomes and retractosomes share migrasome markers such as PIGK, EOGT, and PCCA, although these markers are less enriched in retractosomes compared to migrasomes. Moreover, a recent study has shown that wheat germ agglutinin (WGA) can serve as a rapid and convenient probe for detecting both migrasomes and retractosomes in fixed and live cells [24].

7. Distribution of migrasomes

Migrasomes have been reported to be widely distributed in various in vitro cultured cells and in vivo in different species, including human, mouse, rat, zebrafish, and chicken. Murine or human-derived TSPAN4-GFP plasmids were transfected into twelve in vitro cultured cells, including normal fibroblasts, epithelial cells, and a variety of cancer cells, and migrasome formation was observed in all living cells after 12 h of culture (Table 2). However, the number, size, and length of the RFs varied between cells. Migrasomes have also been found in various tissues and fluids, including ischemic brain tissue, platelets, blood, urine, intestine, eye, placenta, microglia, neutrophils, macrophages, lung, kidney, endoderm, yolk syncytial layer of zebrafish embryos, and the middle layer of the chorioallantoic membrane of chicken embryos. Despite their wide distribution, the precise role of migrasomes in physiological or pathological conditions in the human body remains unclear. Further research is needed to fully understand the function and significance of migrasomes [1,2,5,23,[25], [26], [27], [28], [29], [30], [31], [32], [33]].

Table 2.

Distribution of migrasome.

	Name	References
Cells	NRK (Rat renal tubular epithelial cells)	[1,5,34]
	B16 (Mouse melanoma cells)	[1,5,34]
	N2a (Mouse brain neuroma cells)	[1,5,34]
	MEF and NIH3T3 (mouse embryonal fibroblast cell)	[1,5,34]
	L929 (mouse fibroblast cell)	[1,5,34]
	HaCaT (Human-immortalized keratinocytes)	[1,5,34]
	HCT116 (Human colon cancer cells)	[1,5,34]
	SW480 (Human colon cancer cells)	[1,5,34]
	MGC803 (human gastric carcinoma cell)	[1,5,34]
	SKOV-3 (Human ovarian cancer cells)	[1,5,34]
	MDA-MB-231 (human breast cancer cell)	[1,5,34]
	Human platelets	[28]
	Mouse microglias	[31]
	Mouse neutrophils	[26]
	Mouse macrophages	[2,26,29]
Fluids	Human blood	[23,30,32]
	Human urine	[27,33]
	Mouse urine	[27]
Organs	Human brain (Stroke specimens)	[31]
	Mouse brain (Stroke specimens)	[2,31]
	Mouse intestine	[1,2,5]
	Mouse eyes	[1,2,5]
	Mouse placenta	[1,2,5]
	Rat lung	[1,2,5,34]
	Rat kidney	[1,2,5,34]
	Rat intestine	[1,2,5,34]
	Zebrafish embryos	[2,25]
	Chicken embryo urine capsule	[34]

8. Functions of the migrasome

8.1. Migrasomes coordinate organ morphogenesis through chemotactic factors

In vivo observation of migrasome formation in living animals was first reported in zebrafish embryonic development (Table 3). Jiang et al. [25] detected migrasomes in zebrafish using 3D scanning microscopy. These extracellular vesicles were found in a specialized layer between the endoderm and yolk syncytial layer during gastrulation. Notably, TSPAN4a and TSPAN7, as well as integrin β1b, were identified as regulators of migrasome occurrence in zebrafish embryos. Knockout of TSPAN4a and TSPAN7 led to impaired organogenesis, but exogenous migrasome injection partially reversed the defects, indicating that the lack of migrasomes is responsible for the impaired organogenesis in zebrafish. Migrasomes in zebrafish contain various signaling molecules, such as chemokines, morphogens, growth factors, and cytokines. Quantitative mass spectrometry analysis confirmed this. Of particular importance is the chemokine CXCL12a, which plays a crucial role in organ morphogenesis. Studies have shown that the migrasomes provide a source of CXCL12a, and the spatial and temporal distribution of CXCL12a formed by migrasomes determines the proper localization of dorsal forerunner cells (DFCs) and the correct development of Kupffer's vesicle (KV), ultimately leading to the appropriate development and localization of organs (Fig. 2). Active cell migration occurs during embryonic development, in which the migrasomes exhibit essential functions. In summary, migrasomes in zebrafish embryonic development have confirmed the existence of migrasomes in living animals for the first time, and a novel mechanism has been proposed in which migrasomes function as membrane-encapsulated carriers of signaling molecules, playing essential roles in organ development through regulating the spatial and temporal distribution of signaling molecules [35].

Table 3.

Functions of the migrasomes.

Function	References
Migrasomes coordinate organ morphogenesis through chemokines	[25,35]
Migrosomes mediate the mitochondrial quality control process	[26,36]
Sensitive indicators of early podocyte stress and/or injury	[27,33]
Migrasomes may modify the tumor microenvironment	[37]
Migrasomes may be involved in critical processes of cell development and repair	[37]
Migrasomes may regulate the immune system	[37]

Fig. 2.

Migrasomes significantly contribute to the intricate orchestration of cellular dynamics and the overarching trajectory of biological development.

a. Migrasomes regulate organ formation in zebrafish embryos: Migrasomes serve as chemotactic signals for dorsal forerunner cells (DFCs) in zebrafish embryos, playing a crucial role in the formation of Kupffer's vesicle (KV) within the embryonic shield cavity. This activity ensures the proper development of KV, thus safeguarding the normative progression of organ morphogenesis. b. The absence of migrasomes can precipitate abnormal cell morphogenesis: Predominantly, migrasomes take shape during the gastrulation phase in zebrafish. Interruptions to migrasome generation lead to a marked reduction in their formation in zebrafish, which in turn gives rise to defects in organogenesis. The most conspicuous phenotype manifests as asymmetrical developmental anomalies, encompassing lateral reversals and bilateral duplications of an array of organs (This denotes that organs ordinarily appearing on one side occur on the alternate side, or on both sides concurrently). c. Vascular genesis in chicken embryos: The emergence of the vascular system is paramount to the overall progression of embryonic development. In fact, the vascular system represents the inaugural functional organ within the embryonic structure. The orchestration of angiogenesis is driven by a complex interplay of angiogenic proteins, encapsulating angiogenic growth factors, chemotactic agents, and extracellular matrix proteins. Among these, Vascular Endothelial Growth Factor (VEGF) assumes a commanding role. Mononuclear cells establish a pro-angiogenic microenvironment antecedent to vascular formation by depositing migrasomes enriched with angiogenic factors, including VEGF. This process plays a crucial part in angiogenesis throughout the developmental journey of chicken embryos.

8.2. Exploring the involvement of migrasomes in maintaining mitochondrial function

Damaged mitochondria must be eliminated to maintain a healthy mitochondrial pool (Table 3). Jiao et al. [26] reported “mitocytosis” as a mitochondrial quality control process mediated by migrasomes. Upon exposure to mild mitochondrial stress, damaged mitochondria are selectively transported to the migrasome for disposal, indicating a highly regulated process. In mitochondrial heterogeneity cell experiments, migrasomes isolated from cells with both normal and mutant mitochondrial DNA (mtDNA) demonstrated that most mtDNA in the migrasomes was mutated, suggesting that the migrasome selectively transports functionally damaged mitochondria. Mitocytosis involves KIF5B, Drp 1, and Myo-19. Impaired mitochondria recruit KIF5B to extend them towards the membrane edge, where they bind to the actin cortex of the plasma membrane via Myo-19. The Drp1-mediated mitochondrial fission separates the tips of tubular mitochondria from the actin cortex, enabling the damaged mitochondria to enter the migrasome. Mitocytosis regulates mitochondrial quality control and maintains mitochondrial homeostasis, which can have protective effects. Jiao et al. [26] established an in vivo model using neutrophils to examine the physiological consequences of impaired mitocytosis. The authors found that circulating neutrophils produced numerous migrasomes containing damaged mitochondria. In TSPAN9 gene knockout mice, circulating neutrophils formed fewer migrasomes and had significantly lower membrane potential, leading to reduced viability. These findings demonstrate that mitocytosis is critical for maintaining the survival of circulating neutrophils. Compared to quiescent cells, migratory cells require more energy to support migration, resulting in higher respiration rates, more reactive oxygen species, and higher mitochondrial allostatic load. Migratory cells face greater mitochondrial stress, but mitocytosis helps maintain mitochondrial quality control during migration. This process integrates mitochondrial homeostasis with cell migration, providing an additional mechanism to reduce stress.

Mitocytosis and mitochondrial autophagy are crucial mechanisms for maintaining mitochondrial homeostasis in migratory cells. Mild mitochondrial stress triggers mitocytosis, whereas mitochondrial autophagy is activated under severe damage caused by pathological conditions. Mitocytosis and migration are mostly absent in the presence of mitochondrial oxidative phosphorylation uncoupling agents that induce mitochondrial autophagy. These findings suggest that both processes function as distinct systems in response to different levels of mitochondrial stress, and both are critical for maintaining mitochondrial homeostasis. Understanding their roles may lead to novel therapies for diseases associated with mitochondrial dysfunction [26]. Furthermore, the transport of mitochondria by motor proteins may also be implicated in mitochondrial quality control regulated by motor proteins [36].

8.3. Migrasomes: A promising early indicator of podocyte stress and injury

Podocyte-derived extracellular vesicles (pEVs) have emerged as a topic of intense interest among nephrologists due to their potential as novel biomarkers for kidney disease (Table 3). Podocytes, which control glomerular permeability, are highly motile and release exosomes and migrasomes more readily than other renal cells, with tubular cells secreting fewer migrasomes [27]. These migrasomes, which can be detected in human and mouse urine, have distinct protein and miRNA profiles compared to exosomes released by podocytes. For example, migrasomes isolated from cultured podocytes contain higher levels of PIGK, miR-1303, miR-490–5p, miR-548a, miR 611, and miR-661. In vitro studies have demonstrated that exposure to puromycin aminonucleoside (PAN), lipopolysaccharide (LPS), or high concentrations of glucose (HG) significantly enhance migrasome secretion by podocytes [27]. While the cellular origin of urinary exosomes is complex, migrasomes in urine are predominantly derived from podocytes. Notably, in a mouse model of pan-induced nephropathy, the increase in urinary migrasome levels occurs earlier than the elevation in proteinuria, indicating that migrasomes may serve as a more sensitive biomarker of early podocyte stress and injury [27,33].

The early detection of podocyte stress and injury is crucial for effective management and treatment of nephropathy. Recent research has identified urinary migrasomes, a type of podocyte-derived extracellular vesicles, as more sensitive and reliable indicators of podocyte stress and injury than proteinuria in the early stages of nephropathy. However, despite their potential as biomarkers, the biological function of these migrasomes remains unclear. Advancing our understanding of podocyte biology can improve our knowledge of the mechanisms of the glomerular filtration program and help identify new clinical applications for the diagnosis and treatment of nephropathy. Additionally, it would be interesting to investigate if migrasomes in urine can also serve as indicative biomarkers for other renal diseases. Further research on migrasomes and their role in podocyte biology can significantly enhance our ability to diagnose and treat podocyte-related renal diseases, and may lead to the development of novel therapies and preventive measures for these conditions.

8.4. Migrasomes may modify the tumor microenvironment

The tumor microenvironment (TME) is a complex system where cancer cells can release migrasomes that promote angiogenesis through the transport of pro-angiogenic factors such as VEGF and angiopoietin (Ang-1/2), or inhibitors of adhesion molecules such as E-selectin or VCAM-1 (Table 3). With the continuous proliferation of tumor cells, they become more reliant on blood vessels for nutrients and oxygen, leading to tissue invasion towards the energy source via the release of migrasomes enriched with matrix metalloproteinases (MMPs) or enzymes that regulate extracellular matrix stiffness. Moreover, migrasomes released within the TME may facilitate cancer cell evasion from the immune system by inducing effector T cell apoptosis, suppressing natural killer cell proliferation, or inducing monocyte differentiation into immunosuppressive macrophages [37].

8.5. Migrasomes may be involved in critical processes of cell development and repair

During skeletal muscle development, myogenic progenitor cells release migrasomes to enhance their motility in migrating towards prospective skeletal muscles in the trunk (Table 3). In addition, in cases of chronic myopathies and acute injuries, muscle stem cells may release migrasomes enriched in contractile proteins and muscle growth factors, which contribute to the formation of new muscle fibers and facilitate muscle regeneration after injury. This is consistent with previous findings on other types of extracellular vesicles [37,38].

8.6. Migrasomes may regulate the immune system

The crucial roles played by migrasomes depend on the type of immune population they interact with [37]. For instance, migrasomes released by antigen-presenting cells may modulate, activate, or mobilize other immune cells. Macrophage-derived migrasomes, on the other hand, are filled with processed antigens and can improve the identification of targets. Meanwhile, T cell-derived migrasomes can enhance cytotoxicity or suppression in a manner that is dependent on the environment. These findings underscore the diverse functions of migrasomes in regulating immune responses and highlight their potential as novel therapeutic targets for immune-related diseases (Table 3).

9. Discussion

The formation of migrasomes is an intricate and finely orchestrated process, governed by precise regulatory mechanisms. In the clinical realm, the precise role of migrasomes remains a vast domain that warrants comprehensive investigation and exploration. These enigmatic organelles facilitate the transfer of substances and signals between cells, presenting a promising avenue for novel targets in cell therapy. Moreover, migrasomes play a pivotal role in clinical diagnosis and treatment by upholding mitochondrial homeostasis through a process known as “mitocytosis.” Consequently, migrasomes act as guardians, shielding cells from oxidative stress and apoptosis, and potentially emerging as a novel class of cell protectants. Remarkably, migrasomes also hold immense potential as biomarkers for assessing cellular stress and damage.

Despite these remarkable attributes, the migrasome's true nature remains shrouded in mystery. Several pivotal questions demand elucidation: Which signaling pathways orchestrate migrasome formation beyond the identified molecules? What are the vesicles inside the migrasomes, and how do these vesicles gain entry? Do migrasomes possess cell-autonomous functions, allowing cells to expel unwanted substances, thus maintaining homeostasis and optimal surface molecule levels? Furthermore, while migrasomes appear particularly relevant to embryonic development, their role in other biological processes awaits further clarification.

The annals of scientific history remind us that amidst the known mysteries, lie the “unknown” unknowns—an acknowledgement that nature surpasses our current understanding. Confronting these enigmatic frontiers necessitates the courage to challenge prevailing models and the keen discernment to recognize inexplicable phenomena within existing theoretical frameworks. With such a mindset, fortified by a touch of serendipity, we stand poised to unlock a world teeming with captivating unknowns.

10. Conclusion

Cell migration is a fundamental phenomenon and a basic mechanism that regulates the homeostasis of the body. It is associated with normal cell physiology as well as human diseases, including embryogenesis, trauma healing, immune defense, cardiovascular diseases, ocular diseases, tumor biology, osteoporosis, diabetic nephropathy, and chronic inflammatory diseases such as rheumatoid arthritis and multiple sclerosis [14,32,39,40]. Indeed, migrasomes are intrinsically linked to cell migration. To date, studies have fueled the interest in the biological functions of migrasomes, making it a new area of research in cell biology. In contrast to exosomes, which also have membrane structures, migrasomes are still at the initial stage of research. Therefore, it is necessary to discuss the specific molecular mechanisms of their biogenesis and regulation as soon as possible. In recent years, the physiological and pathological functions of migrasomes or their associated events have been studied in zebrafish developmental models, mouse models of mild mitochondrial stress in neutrophils and macrophages, mouse models of ischemic stroke, pan-induced mouse models of nephropathy, and in vitro cancer cell proliferation [25,27,41]. Considering the prevalence of migration in regulating dynamic homeostasis and disease, we speculate that the functions of migrasomes go far beyond those mentioned above. More basic and clinical studies are needed in the future to further investigate the roles of these fascinating vesicles in physiological and pathological processes.

Funding

This research was supported by The National Natural Science Foundation of China (81802945); The PhD Research Foundation Affiliated Hospital of Jining Medical University (2021-BS-011); Jining Medical University High-level Research Project Cultivation Program (JYGC2022); Jining Key Research and Development Plan (2021YXNS075, 2021YXNS026), China.

Credit author statement

Xun Tan: Writing – original draft, writing – review & editing, drawing – illustration, collecting & organizing literature. Shujin He: Writing – original draft, writing – review & editing, collecting & organizing literature. Wei Wang and Lei Li: Proofreading manuscript, resources, supervision. Fuling Wang: Resources, collecting literature.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

Not applicable.

Declaration of competing interest

The authors declare that they have no competing interests.