Vesicle formation within endosomes: An ESCRT marks the spot

The science of exosomes: Understanding their formation, capture, and role in cellular communication

Extracellular vesicles (EVs) serve as a crucial method for transferring information among cells, which is vital in multicellular organisms. Among these vesicles, exosomes are notable for their small size, ranging from 20 to 150 nm, and their role in cell-to-cell communication. They carry lipids, proteins, and nucleic acids between cells. The creation of exosomes begins with the inward budding of the cell membrane, which then encapsulates various macromolecules as cargo. Once filled, exosomes are released into the extracellular space and taken up by target cells via endocytosis and similar processes. The composition of exosomal cargo varies, encompassing diverse macromolecules with specific functions. Because of their significant roles, exosomes have been isolated from various cell types, including cancer cells, endothelial cells, macrophages, and mesenchymal cells, with the aim of harnessing them for therapeutic applications. Exosomes influence cellular metabolism, and regulate lipid, glucose, and glutamine pathways. Their role in pathogenesis is determined by their cargo, which can manipulate processes such as apoptosis, proliferation, inflammation, migration, and other molecular pathways in recipient cells. Non-coding RNA transcripts, a common type of cargo, play a pivotal role in regulating disease progression. Exosomes are implicated in numerous biological and pathological processes, including inflammation, cancer, cardiovascular diseases, diabetes, wound healing, and ischemic-reperfusion injury. As a result, they hold significant potential in the treatment of both cancerous and non-cancerous conditions.

Intervening in hnRNPA2B1-mediated exosomal transfer of tumor-suppressive miR-184-3p for tumor microenvironment regulation and cancer therapy

BACKGROUND

Despite being a common malignant tumor, the molecular mechanism underlying the initiation and progression of triple-negative breast cancers (TNBCs) remain unclear. Tumor-associated macrophages (TAMs) are often polarized into a pro-tumor phenotype and are associated with a poor prognosis of TNBCs. Exosomes, important mediators of cell-cell communication, can be actively secreted by donor cells to reprogram recipient cells. The functions and molecular mechanisms of tumor cell-derived exosomes in TNBCs progression and TAMs reprogramming urgently need to be further explored.

RESULTS

We demonstrated that tumor cell-derived exosomes enriched with miR-184-3p were taken up by macrophages to inhibit JNK signaling pathway by targeting EGR1, thereby inducing M2 polarization of macrophages and synergistically promoting tumor progression. Nanoparticles loaded with oncogene c-Myc inhibitor JQ1 could suppress the polarization process by reducing Rac1-related exosome uptake by macrophage. More importantly, it was found for the first time that tumor-suppressive miR-184-3p was actively sorted into exosomes by binding to RNA-binding protein heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1), thus facilitating tumor cell proliferation and metastasis by relieving the inhibitory effect of miR-184-3p on Mastermind-like 1 (MAML1). Overexpressing miR-184-3p in tumor cells and simultaneously knocking down hnRNPA2B1 to block its secretion through exosomes could effectively inhibit tumor growth and metastasis.

CONCLUSIONS

Our study revealed that hnRNPA2B1-mediated exosomal transfer of tumor-suppressive miR-184-3p from breast cancer cells to macrophages was an important mediator of TNBCs progression, providing new insights into TNBCs pathogenesis and therapeutic strategies.

Non-targeted effects of radiation: a personal perspective on the role of exosomes in an evolving paradigm

PURPOSE

Radiation-induced non-targeted effects (NTE) have implications in a variety of areas relevant to radiation biology. Here we evaluate the various cargo associated with exosomal signalling and how they work synergistically to initiate and propagate the non-targeted effects including Genomic Instability and Bystander Effects.

CONCLUSIONS

Extra cellular vesicles, in particular exosomes, have been shown to carry bystander signals. Exosome cargo may contain nucleic acids, both DNA and RNA, as well as proteins, lipids and metabolites. These cargo molecules have all been considered as potential mediators of NTE. A review of current literature shows mounting evidence of a role for ionizing radiation in modulating both the numbers of exosomes released from affected cells as well as the content of their cargo, and that these exosomes can instigate functional changes in recipient cells. However, there are significant gaps in our understanding, particularly regarding modified exosome cargo after radiation exposure and the functional changes induced in recipient cells.

The Emerging World of Membrane Vesicles: Functional Relevance, Theranostic Avenues and Tools for Investigating Membrane Function

Lipids are essential components of cell membranes and govern various membrane functions. Lipid organization within membrane plane dictates recruitment of specific proteins and lipids into distinct nanoclusters that initiate cellular signaling while modulating protein and lipid functions. In addition, one of the most versatile function of lipids is the formation of diverse lipid membrane vesicles for regulating various cellular processes including intracellular trafficking of molecular cargo. In this review, we focus on the various kinds of membrane vesicles in eukaryotes and bacteria, their biogenesis, and their multifaceted functional roles in cellular communication, host-pathogen interactions and biotechnological applications. We elaborate on how their distinct lipid composition of membrane vesicles compared to parent cells enables early and non-invasive diagnosis of cancer and tuberculosis, while inspiring vaccine development and drug delivery platforms. Finally, we discuss the use of membrane vesicles as excellent tools for investigating membrane lateral organization and protein sorting, which is otherwise challenging but extremely crucial for normal cellular functioning. We present current limitations in this field and how the same could be addressed to propel a fundamental and technology-oriented future for extracellular membrane vesicles.

Stem cell-derived exosomes for wound healing: current status and promising directions

Wound healing, especially of chronic wounds, is still an unmet therapeutic area since assessment and management are extremely complicated. Although many efforts have been made to treat wounds, all strategies have achieved limited results for chronic wounds. Stem cell-based therapy is considered a promising approach for complex wounds such as those occurring in diabetics. Mesenchymal stem cell transplantation significantly improves wound closure, angiogenesis and wound healing. However, cell therapy is complex, expensive and time-consuming. Recent studies have shown that stem cell-derived exosomes can be an exciting approach to treat wounds. Exosomes derived from mesenchymal stem cells can induce benefit in almost all stages of wound healing, including control of immune responses, inhibition of inflammation, promoting cell proliferation and angiogenesis, while reducing scar formation during the wound healing process. This review aimed at offering an updated overview of the use of exosomes in biological applications, such as wound healing, and addresses not only current applications but also new directions for this next-generation approach in wound healing.

Tropism-facilitated delivery of CRISPR/Cas9 system with chimeric antigen receptor-extracellular vesicles against B-cell malignancies

The CRISPR/Cas9 system is an efficient genome-editing system that has been successfully applied in the field of gene therapy. However, clinical applications of the CRISPR/Cas9 system are limited by the delivery method and safety concerns. Extracellular Vesicles (EVs) can be released from almost every type of cell, and they act as shuttles to convey molecules between cells. Here, we used EVs derived from epithelial cells as a biosafety delivery platform for the CRISPR/Cas9 system and modified the EVs with a chimeric-antigen receptor (CAR) to give them selective tropism to tumors. Compared to normal EVs, CAR-EVs accumulated in cancer tumors rapidly and released the CRISPR/Cas9 system targeting the MYC oncogene efficiently, both in vitro and in vivo. Taken together, the combination of EV and CAR was confirmed to be a novel strategy facilitating the use of natural gene therapy platforms in cancer treatment in this proof-of-concept research.

Exosomes: roles and therapeutic potential in osteoarthritis

Exosomes participate in many physiological and pathological processes by regulating cell-cell communication, which are involved in numerous diseases, including osteoarthritis (OA). Exosomes are detectable in the human articular cavity and were observed to change with OA progression. Several joint cells, including chondrocytes, synovial fibroblasts, osteoblasts, and tenocytes, can produce and secrete exosomes that influence the biological effects of targeted cells. In addition, exosomes from stem cells can protect the OA joint from damage by promoting cartilage repair, inhibiting synovitis, and mediating subchondral bone remodeling. This review summarizes the roles and therapeutic potential of exosomes in OA and discusses the perspectives and challenges related to exosome-based treatment for OA patients in the future.

Unravelling the Biology of Adult Cardiac Stem Cell-Derived Exosomes to Foster Endogenous Cardiac Regeneration and Repair

Cardiac remuscularization has been the stated goal of the field of regenerative cardiology since its inception. Along with the refreshment of lost and dysfunctional cardiac muscle cells, the field of cell therapy has expanded in scope encompassing also the potential of the injected cells as cardioprotective and cardio-reparative agents for cardiovascular diseases. The latter has been the result of the findings that cell therapies so far tested in clinical trials exert their beneficial effects through paracrine mechanisms acting on the endogenous myocardial reparative/regenerative potential. The endogenous regenerative potential of the adult heart is still highly debated. While it has been widely accepted that adult cardiomyocytes (CMs) are renewed throughout life either in response to wear and tear and after injury, the rate and origin of this phenomenon are yet to be clarified. The adult heart harbors resident cardiac/stem progenitor cells (CSCs/CPCs), whose discovery and characterization were initially sufficient to explain CM renewal in response to physiological and pathological stresses, when also considering that adult CMs are terminally differentiated cells. The role of CSCs in CM formation in the adult heart has been however questioned by some recent genetic fate map studies, which have been proved to have serious limitations. Nevertheless, uncontested evidence shows that clonal CSCs are effective transplantable regenerative agents either for their direct myogenic differentiation and for their paracrine effects in the allogeneic setting. In particular, the paracrine potential of CSCs has been the focus of the recent investigation, whereby CSC-derived exosomes appear to harbor relevant regenerative and reparative signals underlying the beneficial effects of CSC transplantation. This review focuses on recent advances in our knowledge about the biological role of exosomes in heart tissue homeostasis and repair with the idea to use them as tools for new therapeutic biotechnologies for "cell-less" effective cardiac regeneration approaches.

Exosomes in intercellular communication and implications for osteoarthritis

Osteoarthritis (OA) is the most prevalent of the musculoskeletal conditions and represents a significant public health burden. While degeneration of articular cartilage is a key feature, it is now increasingly recognized as a complex condition affecting the whole joint, with synovial inflammation present in a significant proportion of patients. As a secretory tissue, the OA synovium is a rich source of both soluble inflammatory mediators and extracellular vesicles, including exosomes, which have been implicated in cell-cell communication. Exosome cargo has been found to include proteins, lipids and various RNA subtypes such as mRNA and miRNA, potentially capable of regulating gene expression in target cells and tissues. Profiling of exosome cargo and understanding effects on cartilage could elucidate novel regulatory mechanisms within the joint, providing insight for targeted treatment. The aim of this article is to review current literature on exosome biology, highlighting the relevance and application for OA pathogenesis.

Molecular Simulation of Mechanical Properties and Membrane Activities of the ESCRT-III Complexes

The endosomal sorting complex required for transport (ESCRT) machinery carries out the membrane scission reactions that are required for many biological processes throughout cells. How ESCRTs bind and deform cellular membranes and ultimately produce vesicles has been a matter of active research in recent years. In this study, we use fully atomistic molecular dynamics simulations to scrutinize the structural details of a filament composed of Vps32 protomers, a major component of ESCRT-III complexes. The simulations show that both hydrophobic and electrostatic interactions between monomers help maintain the structural stability of the filament, which exhibits an intrinsic bend and twist. Our findings suggest that the accumulation of bending and twisting stresses as the filament elongates on the membrane surface likely contributes to the driving force for membrane invagination. The filament exposes a large cationic surface that senses the negatively charged lipids in the membrane, and the N-terminal amphipathic helix of the monomers not only acts as a membrane anchor but also generates significant positive membrane curvature. Taking all results together, we discuss a plausible mechanism for membrane invagination driven by ESCRT-III.

The mysteries of insect RNAi: A focus on dsRNA uptake and transport

RNA interference (RNAi) is becoming a practical tool to control insect pests. Many mysteries of how double-stranded RNA (dsRNA) is transported into, within, and between cells to generate an efficient RNAi response in insects are still to be unraveled. This review provides an overview of the evidence that supports a key role of endocytosis in the uptake of dsRNA on both cellular and tissue levels. Additionally, other components of cellular membrane transport and their impact on the efficiency of RNAi in insects are explored. It is now evident that the membrane transport and potentially dsRNA release from the endosome may comprise some of the limiting factors in insects that are recalcitrant to dsRNA. This review concludes with the apparent connection between gene products that are necessary for cellular trafficking of dsRNA and highly lethal RNAi targets.

Exosomes in Nasopharyngeal Carcinoma

Exosomes are nanosized (30-100nm) membrane microvesicles secreted through a complex cellular process. Exosomes contain a variety of bioactive molecules, such as proteins, microRNAs(miRNAs or miRs) and long non-coding RNAs (lncRNAs), playing an important role in the cell-to-cell substance transportation and signal transduction. Nasopharyngeal carcinoma-related exosomes (NPC-Exo) have been identified in circulating blood and contribute to tumor cell proliferation, angiopoiesis, and immune tolerance through remodeling of tumor microenvironment (TME). Nasopharyngeal carcinoma-related exosomes may also induce epithelial-mesenchymal transition (EMT), thus promoting tumor metastasis and chemoradioresistance. Clinically, the exosomes may serve as novel biomarkers for diagnosis and targeted therapies of nasopharyngeal carcinoma. This review article updates the understanding of exosomes in nasopharyngeal carcinoma(NPC).

Tumor susceptibility gene 101 regulates predisposition to apoptosis via ESCRT machinery accessory proteins

ESCRT proteins are implicated in myriad cellular processes, including endosome formation, fusion of autophagosomes/amphisomes with lysosomes, and apoptosis. The role played by these proteins in either facilitating or protecting against apoptosis is unclear. In this study, while trying to understand how deficiency of Mahogunin RING finger 1 (MGRN1) affects cell viability, we uncovered a novel role for its interactor, the ESCRT-I protein TSG101: it directly participates in mitigating ER stress-mediated apoptosis. The association of TSG101 with ALIX prevents predisposition to apoptosis, whereas ALIX-ALG-2 interaction favors a death phenotype. Altered Ca2+ homeostasis in cells and a simultaneous increase in the protein levels of ALIX and ALG-2 are required to elicit apoptosis by activating ER stress-associated caspase 4/12. We further demonstrate that in the presence of membrane-associated, disease-causing prion protein CtmPrP, increased ALIX and ALG-2 levels are detected along with ER stress markers and associated caspases in transgenic brain lysates and cells. These effects were rescued by overexpression of TSG101. This is significant because MGRN1 deficiency is closely associated with neurodegeneration and prenatal and neonatal mortality, which could be due to excess cell death in selected brain regions or myocardial apoptosis during embryonic development.

Exosomes: Fundamental Biology and Roles in Cardiovascular Physiology

Exosomes are nanosized membrane particles that are secreted by cells that transmit information from cell to cell. The information within exosomes prominently includes their protein and RNA payloads. Exosomal microRNAs in particular can potently and fundamentally alter the transcriptome of recipient cells. Here we summarize what is known about exosome biogenesis, content, and transmission, with a focus on cardiovascular physiology and pathophysiology. We also highlight some of the questions currently under active investigation regarding these extracellular membrane vesicles and their potential in diagnostic and therapeutic applications.

Bud-neck scaffolding as a possible driving force in ESCRT-induced membrane budding

Membrane budding is essential for processes such as protein sorting and transport. Recent experimental results with ESCRT proteins reveal a novel budding mechanism, with proteins emerging in bud necks but separated from the entire bud surface. Using an elastic model, we show that ESCRT protein shapes are sufficient to spontaneously create experimentally observed structures, with protein-membrane interactions leading to protein scaffolds in bud-neck regions. Furthermore, the model reproduces experimentally observed budding directions and bud sizes. Finally, our results reveal that membrane-mediated sorting has the capability of creating structures more complicated than previously assumed.

Hrs and STAM function synergistically to bind ubiquitin-modified cargoes in vitro

The turnover of integral membrane proteins requires a specialized transport pathway mediated by components of the endosomal sorting complex required for transport (ESCRT) machinery. In most cases, entry into this pathway requires that cargoes undergo ubiquitin-modification, thereby facilitating their sequestration on endosomal membranes by specific, ubiquitin-binding ESCRT subunits. However, requirements underlying initial cargo recognition of mono-ubiquitinated cargos remain poorly defined. In this study, we determine the capability of each ESCRT complex that harbors a ubiquitin-binding domain to bind a reconstituted integral membrane cargo (VAMP2), which has been covalently linked to mono-ubiquitin. We demonstrate that ESCRT-0, but not ESCRT-I or ESCRT-II, is able to associate stably with the mono-ubiquitinated cargo within a lipid bilayer. Moreover, we show that the ubiquitin-binding domains in both Hrs and STAM must be intact to enable cargo binding. These results indicate that the two subunits of ESCRT-0 function together to bind and sequester cargoes for downstream sorting into intralumenal vesicles.

Signaling by exosomal microRNAs in cancer

A class of small non-coding RNAs, the microRNAs (miRNAs), have recently attracted great attention in cancer research since they play a central role in regulation of gene-expression and miRNA aberrant expression is found in almost all types of human cancer. The discovery of circulating miRNAs in body fluids and the finding that they are often tumor specific and can be detected early in tumorigenesis has soon led to the evaluation of their possible use as cancer biomarkers and treatment-response predictors. The evidence that tumor cells communicate via the secretion and delivery of miRNAs packed into tumor-released microvesicles has prompted to investigate miRNA contribution as signaling molecules to the establishment and maintenance of the tumor microenvironment and the metastatic niche in cancer. In this review we highlight the recent advances on the role of exosomal miRNAs as mediators of cancer cell-to-cell communication.

MicroRNAs transported by exosomes in body fluids as mediators of intercellular communication in cancer

Cancer-cell communication is an important and complex process, achieved through a diversity of mechanisms that allows tumor cells to mold and influence their environment. In recent years, evidence has accumulated indicating that cells communicate via the release and delivery of microRNAs (miRNAs) packed into tumor-released (TR) exosomes. Understanding the role and mode of action of miRNAs from TR exosomes is of paramount importance in the field of cancer biomarker discovery and for the development of new biomedical applications for cancer therapeutics. In this review, we focus on miRNAs secreted via TR exosomes, which by acting in a paracrine or endocrine manner, facilitate a diversity of signaling mechanisms between cancer cells. We address their contribution as signaling molecules, to the establishment, maintenance, and enhancement of the tumor microenvironment and the metastatic niche in cancer. Finally, we address the potential role of these molecules as biomarkers in cancer diagnosis and prognosis and their impact as a biomedical tool in cancer therapeutics.

Molecular mechanisms of the membrane sculpting ESCRT pathway

The endosomal sorting complexes required for transport (ESCRT) drive multivesicular body (MVB) biogenesis and cytokinetic abscission. Originally identified through genetics and cell biology, more recent work has begun to elucidate the molecular mechanisms of ESCRT-mediated membrane remodeling, with special focus on the ESCRT-III complex. In particular, several light and electron microscopic studies provide high-resolution imaging of ESCRT-III rings and spirals that purportedly drive MVB morphogenesis and abscission. These studies highlight unifying principles to ESCRT-III function, in particular: (1) the ordered assembly of the ESCRT-III monomers into a heteropolymer, (2) ESCRT-III as a dynamic complex, and (3) the role of the AAA ATPase Vps4 as a contributing factor in membrane scission. Mechanistic comparisons of ESCRT-III function in MVB morphogenesis and cytokinesis suggest common mechanisms in membrane remodeling.

Worming our way in and out of the Caenorhabditis elegans germline and developing embryo

The germline and embryo of the nematode Caenorhabditis elegans have emerged as powerful model systems to study membrane dynamics in an intact, developing animal. In large part, this is due to the architecture of the reproductive system, which necessitates de novo membrane and organelle biogenesis within the stem cell niche to drive compartmentalization throughout the gonad syncytium. Additionally, membrane reorganization events during oocyte maturation and fertilization have been demonstrated to be highly stereotypic, facilitating the development of quantitative assays to measure the impact of perturbations on protein transport. This review will focus on regulatory mechanisms that govern protein trafficking, which have been elucidated using a combination of C. elegans genetics, biochemistry and high-resolution microscopy. Collectively, studies using the simple worm highlight an important niche that the organism holds to define new pathways that regulate vesicle transport, many of which appear to be absent in unicellular systems but remain highly conserved in mammals.

The endolysosomal system in cell death and survival

The endocytic pathway is a system specialized for the uptake of compounds from the cell microenvironment for their degradation. It contains an arsenal of hydrolases, including proteases, which are normally enclosed in membrane-bound organelles, but if released to the cytosol can initiate apoptosis signaling pathways. Endogenous and exogenous compounds have been identified that can mediate destabilization of lysosomal membranes, and it was shown that lysosomal proteases are not only able to initiate apoptotic signaling but can also amplify the apoptotic pathways initiated in other cellular compartments. The endocytic pathway also receives cargo destined for degradation via the autophagic pathway. By recycling energy and biosynthetic substrates, and by degrading damaged organelles and molecules, the endocytic system assists the autophagic system in resisting apoptotic stimuli. Steps leading to lysosomal membrane permeabilization and subsequent triggering of cell death as well as the therapeutic potential of intervention in lysosomal membrane permeabilization will be discussed.

Exosomes and microvesicles: extracellular vesicles for genetic information transfer and gene therapy

Exosomes and microvesicles are extracellular nanovesicles released by most but not all cells. They are specifically equipped to mediate intercellular communication via the transfer of genetic information, including the transfer of both coding and non-coding RNAs, to recipient cells. As a result, both exosomes and microvesicles play a fundamental biological role in the regulation of normal physiological as well as aberrant pathological processes, via altered gene regulatory networks and/or via epigenetic programming. For example, microvesicle-mediated genetic transfer can regulate the maintenance of stem cell plasticity and induce beneficial cell phenotype modulation. Alternatively, such vesicles play a role in tumor pathogenesis and the spread of neurodegenerative diseases via the transfer of specific microRNAs and pathogenic proteins. Given this natural property for genetic information transfer, the possibility of exploiting these vesicles for therapeutic purposes is now being investigated. Stem cell-derived microvesicles appear to be naturally equipped to mediate tissue regeneration under certain conditions, while recent evidence suggests that exosomes might be harnessed for the targeted delivery of human genetic therapies via the introduction of exogenous genetic cargoes such as siRNA. Thus, extracellular vesicles are emerging as potent genetic information transfer agents underpinning a range of biological processes and with therapeutic potential.

Endosomal sorting complex required for transport (ESCRT) complexes induce phase-separated microdomains in supported lipid bilayers

The endosomal sorting complex required for transport (ESCRT) system traffics ubiquitinated cargo to lysosomes via an unusual membrane budding reaction that is directed away from the cytosol. Here, we show that human ESCRT-II self-assembles into clusters of 10-100 molecules on supported lipid bilayers. The ESCRT-II clusters are functional in that they bind to ubiquitin and the ESCRT-III subunit VPS20 at nanomolar concentrations on membranes with the same stoichiometries observed in solution and in crystals. The clusters only form when cholesterol is included in the lipid mixture at >10 mol %. The clusters induce the formation of ordered membrane domains that exclude the dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbo-cyanine perchlorate. These results show that ESCRT complexes are capable of inducing lateral lipid phase separation under conditions where the lipids themselves do not spontaneously phase-separate. This property could facilitate ESCRT-mediated membrane budding.