The Regulation of Exosome Generation and Function in Physiological and Pathological Processes

Exosomes, a type of extracellular vesicle with a diameter of approximately 100 nm that is secreted by all cells, regulate the phenotype and function of recipient cells by carrying molecules such as proteins, nucleic acids, and lipids and are important mediators of intercellular communication. Exosomes are involved in various physiological and pathological processes such as immunomodulation, angiogenesis, tumorigenesis, metastasis, and chemoresistance. Due to their excellent properties, exosomes have shown their potential application in the clinical diagnosis and treatment of disease. The functions of exosomes depend on their biogenesis, uptake, and composition. Thus, a deeper understanding of these processes and regulatory mechanisms can help to find new targets for disease diagnosis and therapy. Therefore, this review summarizes and integrates the recent advances in the regulatory mechanisms of the entire biological process of exosomes, starting from the formation of early-sorting endosomes (ESCs) by plasma membrane invagination to the release of exosomes by fusion of multivesicular bodies (MVBs) with the plasma membrane, as well as the regulatory process of the interactions between exosomes and recipient cells. We also describe and discuss the regulatory mechanisms of exosome production in tumor cells and the potential of exosomes used in cancer diagnosis and therapy.

Metabolic labelling of a subpopulation of small extracellular vesicles using a fluorescent palmitic acid analogue

Exosomes are among the most puzzling vehicles of intercellular communication, but several crucial aspects of their biogenesis remain elusive, primarily due to the difficulty in purifying vesicles with similar sizes and densities. Here we report an effective methodology for labelling small extracellular vesicles (sEV) using Bodipy FL C16, a fluorescent palmitic acid analogue. In this study, we present compelling evidence that the fluorescent sEV population derived from Bodipy C16-labelled cells represents a discrete subpopulation of small exosomes following an intracellular pathway. Rapid cellular uptake and metabolism of Bodipy C16 resulted in the incorporation of fluorescent phospholipids into intracellular organelles specifically excluding the plasma membrane and ultimately becoming part of the exosomal membrane. Importantly, our fluorescence labelling method facilitated accurate quantification and characterization of exosomes, overcoming the limitations of nonspecific dye incorporation into heterogeneous vesicle populations. The characterization of Bodipy-labelled exosomes reveals their enrichment in tetraspanin markers, particularly CD63 and CD81, and in minor proportion CD9. Moreover, we employed nanoFACS sorting and electron microscopy to confirm the exosomal nature of Bodipy-labelled vesicles. This innovative metabolic labelling approach, based on the fate of a fatty acid, offers new avenues for investigating exosome biogenesis and functional properties in various physiological and pathological contexts.

Exomap1 mouse: a transgenic model for in vivo studies of exosome biology

Exosomes are small extracellular vesicles (sEVs) of ~30-150 nm in diameter that have the same topology as the cell, are enriched in selected exosome cargo proteins, and play important roles in health and disease. To address large unanswered questions regarding exosome biology in vivo, we created the exomap1 transgenic mouse model. In response to Cre recombinase, exomap1 mice express HsCD81mNG, a fusion protein between human CD81, the most highly enriched exosome protein yet described, and the bright green fluorescent protein mNeonGreen. As expected, cell type-specific expression of Cre induced the cell type-specific expression of HsCD81mNG in diverse cell types, correctly localized HsCD81mNG to the plasma membrane, and selectively loaded HsCD81mNG into secreted vesicles that have the size (~80 nm), topology (outside out), and content (presence of mouse exosome markers) of exosomes. Furthermore, mouse cells expressing HsCD81mNG released HsCD81mNG-marked exosomes into blood and other biofluids. Using high-resolution, single-exosome analysis by quantitative single molecule localization microscopy, we show here that that hepatocytes contribute ~15% of the blood exosome population whereas neurons contribute <1% of blood exosomes. These estimates of cell type-specific contributions to blood EV population are consistent with the porosity of liver sinusoidal endothelial cells to particles of ~50-300 nm in diameter, as well as with the impermeability of blood-brain and blood-neuron barriers to particles >5 nm in size. Taken together, these results establish the exomap1 mouse as a useful tool for in vivo studies of exosome biology, and for mapping cell type-specific contributions to biofluid exosome populations. In addition, our data confirm that CD81 is a highly-specific marker for exosomes and is not enriched in the larger microvesicle class of EVs.

GPR143 controls ESCRT-dependent exosome biogenesis and promotes cancer metastasis

Exosomes transport a variety of macromolecules and modulate intercellular communication in physiology and disease. However, the regulation mechanisms that determine exosome contents during exosome biogenesis remain poorly understood. Here, we find that GPR143, an atypical GPCR, controls the endosomal sorting complex required for the transport (ESCRT)-dependent exosome biogenesis pathway. GPR143 interacts with HRS (an ESCRT-0 Subunit) and promotes its association to cargo proteins, such as EGFR, which subsequently enables selective protein sorting into intraluminal vesicles (ILVs) in multivesicular bodies (MVBs). GPR143 is elevated in multiple cancers, and quantitative proteomic and RNA profiling of exosomes in human cancer cell lines showed that the GPR143-ESCRT pathway promotes secretion of exosomes that carry unique cargo, including integrins signaling proteins. Through gain- and loss-of-function studies in mice, we show that GPR143 promotes metastasis by secreting exosomes and increasing cancer cell motility/invasion through the integrin/FAK/Src pathway. These findings provide a mechanism for regulating the exosomal proteome and demonstrate its ability to promote cancer cell motility.

The Machinery of Exosomes: Biogenesis, Release, and Uptake

Exosomes are a subtype of membrane-contained vesicles 40-200 nm in diameter that are secreted by cells into their surroundings. By transporting proteins, lipids, mRNA, miRNA, lncRNA, and DNA, exosomes are able to perform such vital functions as maintaining cellular homeostasis, removing cellular debris, and facilitating intercellular and interorgan communication. Exosomes travel in all body fluids and deliver their molecular messages in autocrine, paracrine as well as endocrine manners. In recent years, there has been an increased interest in studying exosomes as diagnostic markers and therapeutic targets, since in many disease conditions this machinery becomes dysregulated or hijacked by pathological processes. Additionally, delivery of exosomes and exosomal miRNA has already been shown to improve systemic metabolism and inhibit progression of cancer development in mice. However, the subcellular machinery of exosomes, including their biogenesis, release and uptake, remains largely unknown. This review will bring molecular details of these processes up to date with the goal of expanding the knowledge basis for designing impactful exosome experiments in the future.

Type 2 diabetes mellitus stimulated pulmonary vascular inflammation and exosome biogenesis in rats

It has been shown that type 2 Diabetes Mellitus (T2DM) changes the paracrine activity of several cell types. Whether the biogenesis of exosomes is changed during diabetic conditions is the subject of debate. Here, we investigated the effect of T2M on exosome biogenesis in rat pulmonary tissue. Rats received a high-fat diet regime and a single low dose of Streptozocin to mimic the T2DM-like condition. A total of 8 weeks after induction of T2DM, rats were subjected to several analyses. Besides histological examination, vascular cell adhesion molecule 1 (VCAM-1) levels were detected using immunohistochemistry (IHC) staining. Transcription of several genes such as IL-1β, Alix, and Rab27b was calculated by real-time polymerase chain reaction assay. Using western blot analysis, intracellular CD63 levels were measured. The morphology and exosome secretion activity were assessed using acetylcholinesterase (AChE) assay and scanning electron microscopy, respectively. Histological results exhibited a moderate-to-high rate of interstitial pneumonia with emphysematous changes. IHC staining showed an increased VCAM-1 expression in the diabetic lungs compared with the normal conditions (p < .05). Likewise, we found the induction of IL-1β, and exosome-related genes Alix and Rab27b under diabetic conditions compared with the control group (p < .05). Along with these changes, protein levels of CD63 and AChE activity were induced upon the initiation of T2DM, indicating accelerated exosome biogenesis. Taken together, current data indicated the induction of exosome biogenesis in rat pulmonary tissue affected by T2DM. It seems that the induction of inflammatory niche is touted as a stimulatory factor to accelerate exosome secretion.

Exocyst controls exosome biogenesis via Rab11a

Tumor cells actively release large quantities of exosomes, which pivotally participate in the regulation of cancer biology, including head and neck cancer (HNC). Exosome biogenesis and release are complex and elaborate processes that are considered to be similar to the process of exocyst-mediated vesicle delivery. By analyzing the expression of exocyst subunits and their role in patients with HNC, we aimed to identify exocyst and its functions in exosome biogenesis and investigate the molecular mechanisms underlying the regulation of exosome transport in HNC cells. We observed that exocysts were highly expressed in HNC cells and could promote exosome secretion in these cells. In addition, downregulation of exocyst expression inhibited HN4 cell proliferation by reducing exosome secretion. Interestingly, immunofluorescence and electron microscopy revealed the accumulation of multivesicular bodies (MVBs) after the knockdown of exocyst. Autophagy, the major pathway of exosome degradation, is not activated by this intracellular accumulation of MVBs, but these MVBs are consumed when autophagy is activated under the condition of cell starvation. Rab11a, a small GTPase that is involved in MVB fusion, also interacted with the exocyst. These findings suggest that the exocyst can regulate exosome biogenesis and participate in the malignant behavior of tumor cells.

An Examination of the Putative Role of Melatonin in Exosome Biogenesis

During the last two decades, melatonin has been found to have pleiotropic effects via different mechanisms on its target cells. Data are abundant for some aspects of the signaling pathways within cells while other casual mechanisms have not been adequately addressed. From an evolutionary perspective, eukaryotic cells are equipped with a set of interrelated endomembrane systems consisting of intracellular organelles and secretory vesicles. Of these, exosomes are touted as cargo-laden secretory vesicles that originate from the endosomal multivesicular machinery which participate in a mutual cross-talk at different cellular interfaces. It has been documented that cells transfer various biomolecules and genetic elements through exosomes to sites remote from the original cell in a paracrine manner. Findings related to the molecular mechanisms between melatonin and exosomal biogenesis and cargo sorting are the subject of the current review. The clarification of the interplay between melatonin and exosome biogenesis and cargo sorting at the molecular level will help to define a cell's secretion capacity. This review precisely addresses the role and potential significance of melatonin in determining the efflux capacity of cells via the exosomal pathway. Certain cells, for example, stem cells actively increase exosome efflux in response to melatonin treatment which accelerates tissue regeneration after transplantation into the injured sites.

On-Chip Biogenesis of Circulating NK Cell-Derived Exosomes in Non-Small Cell Lung Cancer Exhibits Antitumoral Activity

As the recognition between natural killer (NK) cells and cancer cells does not require antigen presentation, NK cells are being actively studied for use in adoptive cell therapies in the rapidly evolving armamentarium of cancer immunotherapy. In addition to utilizing NK cells, recent studies have shown that exosomes derived from NK cells also exhibit antitumor properties. Furthermore, these NK cell-derived exosomes exhibit higher stability, greater modification potentials and less immunogenicity compared to NK cells. Therefore, technologies that allow highly sensitive and specific isolation of NK cells and NK cell-derived exosomes can enable personalized NK-mediated cancer therapeutics in the future. Here, a novel microfluidic system to collect patient-specific NK cells and on-chip biogenesis of NK-exosomes is proposed. In a small cohort of non-small cell lung cancer (NSCLC) patients, both NK cells and circulating tumor cells (CTCs) were isolated, and it is found NSCLC patients have high numbers of NK and NK-exosomes compared with healthy donors, and these concentrations show a trend of positive and negative correlations with bloodborne CTC numbers, respectively. It is further demonstrated that the NK-exosomes harvested from NK-graphene oxide chip exhibit cytotoxic effect on CTCs. This versatile system is expected to be used for patient-specific NK-based immunotherapies along with CTCs for potential prognostic/diagnostic applications.

Improvement, scaling-up, and downstream analysis of exosome production

Exosomes are the most researched extracellular vesicles. In many biological, physiological, and pathological studies, they have been identified as suitable candidates for treatment and diagnosis of diseases by acting as the carriers of both drugs and genes. Considerable success has been achieved regarding the use of exosomes for tissue regeneration, cancer diagnosis, and targeted drug/gene delivery to specific tissues. While major progress has been made in exosome extraction and purification, extraction of large quantities of exosomes is still a major challenge. This issue limits the scope of both exosome-based research and therapeutic development. In this review, we have aimed to summarize experimental studies focused at increasing the number of exosomes. Biotechnological studies aimed at identifying the pathways of exosome biogenesis to manipulate some genes in order to increase the production of exosomes. Generally, two major strategies are employed to increase the production of exosomes. First, oogenesis pathways are genetically manipulated to overexpress activator genes of exosome biogenesis and downregulate the genes involved in exosome recycling pathways. Second, manipulation of the cell culture medium, treatment with specific drugs, and limiting certain conditions can force the cell to produce more exosomes. In this study, we have reviewed and categorized these strategies. It is hoped that the information presented in this review will provide a better understanding for expanding biotechnological approaches in exosome-based therapeutic development.

Syntenin: PDZ Protein Regulating Signaling Pathways and Cellular Functions

Syntenin is an adaptor-like molecule that has two adjacent tandem postsynaptic density protein 95/Discs large protein/Zonula occludens 1 (PDZ) domains. The PDZ domains of syntenin recognize multiple peptide motifs with low to moderate affinity. Many reports have indicated interactions between syntenin and a plethora of proteins. Through interactions with various proteins, syntenin regulates the architecture of the cell membrane. As a result, increases in syntenin levels induce the metastasis of tumor cells, protrusion along the neurite in neuronal cells, and exosome biogenesis in various cell types. Here, we review the updated data that support various roles for syntenin in the regulation of neuronal synapses, tumor cell invasion, and exosome control.

Emerging Role of Genetic Alterations Affecting Exosome Biology in Neurodegenerative Diseases

The abnormal deposition of proteins in brain tissue is a common feature of neurodegenerative diseases (NDs) often accompanied by the spread of mutated proteins, causing neuronal toxicity. Exosomes play a fundamental role on their releasing in extracellular space after endosomal pathway activation, allowing to remove protein aggregates by lysosomal degradation or their inclusion into multivesicular bodies (MVBs), besides promoting cellular cross-talk. The emerging evidence of pathogenic mutations associated to ND susceptibility, leading to impairment of exosome production and secretion, opens a new perspective on the mechanisms involved in neurodegeneration. Recent findings suggest to investigate the genetic mechanisms regulating the different exosome functions in central nervous system (CNS), to understand their role in the pathogenesis of NDs, addressing the identification of diagnostic and pharmacological targets. This review aims to summarize the mechanisms underlying exosome biogenesis, their molecular composition and functions in CNS, with a specific focus on the recent findings invoking a defective exosome biogenesis as a common biological feature of the major NDs, caused by genetic alterations. Further definition of the consequences of specific genetic mutations on exosome biogenesis and release will improve diagnostic and pharmacological studies in NDs.

MicroRNA Shuttle from Cell-To-Cell by Exosomes and Its Impact in Cancer

The identification of exosomes, their link to multivesicular bodies and their potential role as a messenger vehicle between cancer and healthy cells opens up a new approach to the study of intercellular signaling. Furthermore, the fact that their main cargo is likely to be microRNAs (miRNAs) provides the possibility of the transfer of such molecules to control activities in the recipient cells. This review concerns a brief overview of the biogenesis of both exosomes and miRNAs together with the movement of such structures between cells. The possible roles of miRNAs in the development and progression of breast, ovarian and prostate cancers are discussed.

Progress and potential of exosome analysis for early pancreatic cancer detection

INTRODUCTION

Pancreatic cancer (PaCa) is the most deadly malignancy, due to late diagnosis prohibiting surgery. Thus, strong efforts are taken improving early diagnosis via biomarkers recovered in the serum of PaCa patients.

AREAS COVERED

One promising option are PaCa-derived exosomes in patients' sera. Exosomes, small vesicles delivered by live cells and recovered in all body fluids, are a powerful diagnostic tool due to relative stability and composition covering the whole range of cancer-related biomarkers including proteins, metabolites, DNA, DNA modifications, coding and noncoding RNA. We discuss the mechanisms accounting for the condensed packaging of biomarkers, refer to studies using PaCa serum-exosomes for diagnosis. Based on an extensive literature search, we outline questions that answers may help establishing a serum-exosome-based screening for early PaCa detection. Expert commentary: Improved proteomic and genomic characterization and progress in the biogenesis of exosomes will allow for optimized and unified screening panels for PaCa diagnosis via TEX in body fluids.