The multifaceted exosome: biogenesis, role in normal and aberrant cellular function, and frontiers for pharmacological and biomarker opportunities

Abstract

Exosomes are bioactive vesicles derived from the cell's endosomal membrane system and secreted into surrounding body fluids. Exosomes contain cell and cell-state specific cargos of protein, mRNA and miRNA. Exosome formation, cargo content, and delivery to surrounding cells is of immense biological interest considering the role that exosomes are believed to play in various pathological conditions. They aid antigen presentation by immune cells and can exhibit either anti-inflammatory or pro-inflammatory properties depending on the parent antigen-presenting cell's conditioning. Viruses can hijack a host cell's exosomal machinery to evade host defense systems aiding in the trans-infection of viruses. Tumor derived exosomes may help establish an oncogenic niche systemically via delivery of protein, mRNA, and miRNA that can aid angiogenesis, cell proliferation, and cell survival. Exosomes have also been implicated in the spread of neurodegenerative diseases. Studies have shown that exosomes are selectively taken up by cells distal from their release. They can reprogram the recipient cells due to their active molecular cargo. Cell-lineage and state-specific exosomes imply that they may therefore harbor body fluid-based biomarkers of unparalleled accuracy, particularly for tissues that are difficult or impossible to access. Exosome-specific membrane proteins provide markers enabling exosome identity and selection, while cell type and cell condition-specific protein, mRNA and miRNA cargo provide a rich potential source of biomarkers. This review serves to provide an overview of the current state of the science in the burgeoning field of exosome biology.

Graphical abstract

Fig. 1

Exosomes contain diverse types of proteins, mRNA and miRNA. This figure shows the exosomal content of exosomes derived from MC9 mouse cells . The exocarta compendium was queried for this exosome study with protein, mRNA and miRNA content recorded from the same samples. Considering the very large number of entities, we considered only those proteins and RNA that are part of the top physiological functional groups. Ingenuity Pathway Analysis was used to cluster the proteins by molecular category, subcellular localization and functional group. In the diagram, the top right panel (a) indicates the subcellular localization of exosomal proteins and the mRNAs (as representatives of proteins they may encode if all the mRNAs are functional and get translated). The symbol and legend are indicated on the figure. The top 10 functional clusters of proteins (b), miRNA (c) and mRNA (c) are graphically depicted. These data represent the top 10 hits from Ingenuity generated canonical pathway analysis for functional protein clusters. According to recent reports exosomes also contain mitochondrial DNA and oncogenic/amplified DNA.

Fig. 2

The biogenesis and molecular relevance of exosomes. (a) Exosomes arise from the cell′s endosomal system. The plasma membrane invaginates and buds off during endocytic internalization. Cargo destined for lysosomal degradation is ubiquitinated. At the early endosome the decision to recycle or be directed to the late endosome relies on specific protein interactions and domain based sorting. The early endosome matures into a late endosomal compartment where inward budding of intralumenal vesicles (ILVs) allows the capture of cytoplasmic cargo. Distinct subsets of ILVs may target cargo for lysosomal degradation (ESCRT dependent, ceramide independent) or release cargo to the cell′s exterior via exosomes. The formation of exosomes is ceramide biosynthesis dependent and relatively ESCRT independent. Exosomes retain similar membrane topology as the plasma membrane due to the two membrane inversion events. Sorting of cargo into exosomes relies on specific interactions with lipids and proteins on membrane sub-domains as well as interactions with the tetraspanin web. (b) Exosomes are internalized by cells close to the donor or originating cell (juxtacrine uptake), some distance from the donor cell (paracrine uptake) or they may spread through systemic circulation and be taken up by distant tissues (endocrine uptake). There is specificity in exosome uptake as a function of recipient cell lineage. Exosome surface markers and host cell surface markers contribute to such selectivity in uptake. Exosomes may begin protein based signaling functions even as they adhere to the host cell. Uptake of exosomes occurs primarily through phago-lysosome intermediates or utilizing specific receptors for exosomal surface proteins or the Tim-4 receptor. Once internalized, exosomal content is released via the phagolysosome or localized in the late endosome. (c) The exosomal lipid, protein, RNA, miRNA and DNA content can alter a recipient cell′s molecular profile by various mechanisms which alter gene expression. Some possibilities for exosomal cargo mediated recipient cell reprogramming may include altered transcriptional or translational profile, transcript or protein modification, regulation of signaling cascades, protein localization and turnaround, key enzymatic reactions and cellular homeostasis. The source and condition of both the donor and recipient cell may determine which of these molecular mechanisms predominate. Intriguingly, exosomes may also have recipient cell type specific functions for the same molecule. EE, early endosome, MVB, multivesicular body, LE, late endosome, ILV, intraluminal vesicle, RE, recycling endosome, Rab, Ras superfamily GTPases, ESCRT, endosomal sorting complex required for transport.

Fig. 3

Exosomes can alter the molecular profile of their microenvironment and recipient cells and can affect tissue health. (a) Exosomes help to establish a metastatic niche to aid tumor growth and tumor metastasis. Utilizing VEGF and cytokine cargo, exosomes enhance recruitment of hematopoietic and endothelial precursor cells to enhance neo-angiogenesis in the tumor. Exosomes have also been suggested to influence planar cell polarity and the extracellular matrix to allow tumor cell mobilization. To aid tumor growth, exosomes are additionally known to trigger apoptotic reduction in anti-tumor immune cells through FasL and TNF-pathways. The growth factor and cytokine cargo helps exosomes to attract disseminated tumor cells from the primary tumor site as well as cells of the myeloid and erythropoietic lineages to the pre-metastatic niche to aid tumor spread. (b) Exosomes share many characteristics with viruses in the molecular machinery of budding from the plasma membrane. Retroviruses like HIV utilize the cell-encoded intercellular vesicle traffic as a means to protect themselves from the host cell's proteolytic system. Segregating into the exosomes during the exosome formation event, the virus is camouflaged and avoids the host surveillance system and proteolysis. The exosome encapsulated virus is furthermore able to disseminate and move between cells utilizing the exosome pathway for entering a host cell without utilizing its own fusion machinery as it searches for optimal target cells. Exosomes thus aid the trans-infection of viruses. (c) Amyloid plaques of the Alzheimer's disease afflicted brain have aggregates of amyloid beta peptide processed from amyloid precursor protein (APP). APP is processed by at least three proteases termed α-, β-, and γ-secretases, which lead to the generation of a number of proteolytic fragments, including Aβ when APP is cleaved by β- and γ-secretases. Exosomes have been found to contain APP, Aβ and other proteolytic fragments of APP, α-, β-, and some components of γ-secretase. It has also been suggested that exosomes from an unhealthy brain cell may cause spread of cellular insults in the form of RNA and protein to surrounding healthy cells and thus aid the spread of neurological disorders. EE, early endosome, AICD, amyloid precursor protein intracellular cytoplasmic/C-terminal domain.