Role of Exosomes/Microvesicles in the Nervous System and Use in Emerging Therapies
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REVIEW ARTICLE published: 27 June 2012 doi: 10.3389/fphys.2012.00228 Role of exosomes/microvesicles in the nervous system and use in emerging therapies Charles Pin-Kuang Lai 1,2 and Xandra Owen Breakefield 1,2* 1 Department of Neurology, Neuroscience Center, Massachusetts General Hospital and Program in Neuroscience, Harvard Medical School, Boston, MA, USA 2 Department of Radiology, Center for Molecular Imaging Research, Massachusetts General Hospital, Boston, MA, USA Edited by: Extracellular membrane vesicles (EMVs) are nanometer sized vesicles, including exosomes Claudia Verderio, CNR Institute of and microvesicles capable of transferring DNAs, mRNAs, microRNAs, non-coding RNAs, Neuroscience, Italy proteins, and lipids among cells without direct cell-to-cell contact, thereby representing Reviewed by: Patrizia Rosa, Italian National a novel form of intercellular communication. Many cells in the nervous system have Research Council, Italy been shown to release EMVs, implicating their active roles in development, function, and Rachel Susan Kraut, Nanyang pathologies of this system. While substantial progress has been made in understanding Technological University, Singapore the biogenesis, biophysical properties, and involvement of EMVs in diseases, relatively less Stefano Pluchino, University of Cambridge, UK information is known about their biological function in the normal nervous system. In addi- Mikael Simons, Max-Planck-Institut tion, since EMVs are endogenous vehicles with low immunogenicity, they have also been für Experimentelle Medizin, Germany actively investigated for the delivery of therapeutic genes/molecules in treatment of cancer *Correspondence: and neurological diseases.The present review summarizes current knowledge about EMV Xandra Owen Breakefield, Molecular functions in the nervous system under both physiological and pathological conditions, as Neurogenetics Unit, Massachusetts General Hospital-East, 13th Street, well as emerging EMV-based therapies that could be applied to the nervous system in the Building 149, Charlestown, MA foreseeable future. 02129, USA. e-mail: breakefi[email protected] Keywords: microvesicles, exosomes, neuron, neuroregeneration, neurodegeneration, development, cancer, therapy INTRODUCTION ILVs), into the endosomes where they accumulate with subse- Ligand-receptor interaction and direct cell–cell contacts via spe- quent maturation of the complex into large multivesicular bodies cialized physical conduits, such as gap junctions and membrane (MVBs; Denzer et al., 2000). At this stage, MVBs may be trafficked nanotubes, have long been considered as the predominant means to lysosomes for degradation (“degradative MVBs”) or, instead, of intercellular communication (Davis and Sowinski, 2008; Good- fuse with the plasma membrane (“exocytic MVBs”) for the release enough and Paul, 2009). Yet, a novel method of cell-to-cell of ILVs into the extracellular space, where upon they are referred communication has recently emerged from groundbreaking dis- to as exosomes (Mathivanan et al., 2010). A study on oligodendro- coveries in the past few years on nucleic acid content of extra- cytes suggested that ILV release is ESCRT-independent and relies cellular membrane vesicles (EMVs). EMVs have been demon- on the distribution of sphingolipid ceramide in MVBs, which strated to facilitate horizontal transfer of mRNAs, microRNAs directs the extracellular release of ILVs as exosomes (Trajkovic (miRNAs), and proteins between cells without direct cell-to-cell et al., 2008). Additional investigations are needed to determine contact (Bergsmedh et al., 2001; Ratajczak et al., 2006a; Valadi if distinct MVB or ILV populations destined for degradation or et al., 2007; Al-Nedawi et al., 2008; Skog et al., 2008; Balaj et al., exocytic release are present, as well as whether a common exoso- 2011; Ramachandran and Palanisamy, 2011; Turchinovich et al., mal trafficking mechanism exists in all cell types (Mathivanan 2011; Chen et al., 2012). There are several EMV categories known et al., 2010). Understanding the biogenesis and trafficking of to-date, which are included under the general terms exosomes, exosomes will provide insight into how cells employ these extracel- microvesicles (MVs), and apoptotic blebs (ABs). lular organelles for intercellular communication. In some studies, Exosomes are the smallest EMVs (40–100 nm in diameter), and release of exosomes appears to depend of Rab27 (Ostrowski et al., homogenous in shape (cup-shaped after fixation under electron 2010) and Rab 35 (Hsu et al., 2010), and can be blocked with an microscopy with a buoyant density of 1.13–1.19 g/cm3 (Théry inhibitor of neutral sphingomyelinase (Trajkovic et al., 2008). In 2C 2C et al., 2001; Hristov et al., 2004). Unlike other types of EMVs that addition, elevated [Ca ]i, following Ca and ionophore A23187 are directly shed/released from the plasma membrane, exosomes treatment was found to induce exosome and microvesicle release are formed by a series of processes beginning with inward invagi- from erythrocytes (Allan et al., 1980; Salzer et al., 2002), further nation of clathrin-coated microdomains on the plasma membrane supporting a role of EMVs in response to different stimuli. (Denzer et al., 2000). Once these vacuoles have entered the cell, the Microvesicles (MVs) are irregularly shaped, larger EMVs with a Endosomal Sorting Complex Required for Transport (ESCRT) 100–1,000 nm diameter (Pilzer et al., 2005; Cocucci et al., 2009). A facilitates the development of the invaginated vacuoles carrying defined buoyant density of MVs has not yet been determined, ubiquitinated cargos into early endosomes. This is followed by a but it may overlap that of exosomes (Théry et al., 2009; van secondary invagination of vesicles (termed intraluminal vesicles, Dommelen et al., 2011). In contrast to the endocytotic origin of www.frontiersin.org June 2012 | Volume 3 | Article 228 | 1 Lai and Breakefield Extracellular vesicles in the nervous system and therapies exosomes, release of MVs results from outward budding at the the purpose of this review, EMVs will be used to encompass these plasma membrane followed by fission of their connecting mem- extracellular vesicle types. brane stalks (Kobayashi et al., 1984; Dolo et al., 2000; Cocucci et al., 2007; Piccin et al., 2007). While MV biogenesis remains BIOPHYSICAL PROPERTIES AND LIPID COMPOSITION to be defined, microdomains on the plasma membrane contain- Aside from the different biophysical properties (i.e., size, shape, ing a high cholesterol level and signaling complexes, or lipid buoyant density) mentioned above for exosomes, MVs, and ABs rafts, have been suggested to selectively sequester lipids for MV (Table 1), different types of EMVs also have different lipid generation (Del Conde et al., 2005). Work by Gould and collabo- compositions. By using liquid chromatography and mass spec- rators indicates that MV release may be triggered by oligomer- trometry, a variety of lipid components constituting EMVs iso- izing proteins on the cell surface and may share mechanistic lated from different cells have been identified, including phos- elements with release of enveloped viruses (Gould et al., 2003; phatidylcholine, phosphatidylethanolamine, phosphatidylserine Shen et al., 2011). MV production is observed in a variety of (PS), lyso-bisphosphatidic acid, ceramide, cholesterol, and spin- cells in a resting state, but can be significantly elevated under gomyelin (Chu et al., 2005; Subra et al., 2007). The particular lipid 2C various stimulations, including increased [Ca ]i, cellular stress composition of each EMV type likely contributes to its biophysical (e.g., DNA damage), decreased cholesterol levels, cytokine expo- properties. Indeed, Parolini et al.(2009) recently reported that dif- sure, and anticancer drug treatment (Salzer et al., 2002; Shedden ferent lipid compositions, namely those containing sphingomyelin et al., 2003; Yu et al., 2006; Llorente et al., 2007; Lehmann et al., and N -acetylneuraminyl-galactosylglucosylceramide (GM3), are 2008; Bianco et al., 2009). Notably, even larger EMVs (1–5 mm in directly related to rigidity and delivery efficiency of exosomes diameter) are released from tumor cells, especially in response to to other cells. In addition, the level of PS exposed on the outer EGF stimulation (Di Vizio et al., 2009). Altogether, these findings leaflet of exosomes appears to be lower than that of MVs and ABs suggest an active physiological role of MVs under different cellular (Mathivanan et al., 2011). This observation may correlate with conditions. the different biogenesis of EMV populations wherein exosomes Apoptotic blebs are 50–4,000 nm in diameter with a buoyant are of endocytic origin, and MVs and ABs are derived from out- density of 1.16–1.28 g/cm3 (Hristov et al., 2004; Simak and Gel- ward budding from the plasma membrane. PS is displayed on the derman, 2006). Similar to MVs, ABs are also irregularly shaped, outer exosome leaflet through floppase, flippase, and scramblase making them difficult to discern from one another based on their activities, and appears to mediate docking of proteins involved in morphology. ABs, as its name suggests, are released from con- signaling and fusion to the plasma membrane (Piccin et al., 2007). densed and fragmented apoptotic cells during late stages of cell Therefore, the varying level of PS may affect communication func- death (Henson et al., 2001; Hristov et al., 2004). ABs retain DNA tions of EMVs. Furthermore, ongoing studies indicate that EMVs fragments