biomolecules Article The Role of Exosomes in Lysosomal Storage Disorders Adenrele M. Gleason, Elizabeth G. Woo , Cindy McKinney and Ellen Sidransky * Medical Genetics Branch, National Human Genome Research Institute, National Institutes of Health, Bethesda, MD 20892, USA; [email protected] (A.M.G.); [email protected] (E.G.W.); [email protected] (C.M.) * Correspondence: [email protected] Abstract: Exosomes, small membrane-bound organelles formed from endosomal membranes, rep- resent a heterogenous source of biological and pathological biomarkers capturing the metabolic status of a cell. Exosomal cargo, including lipids, proteins, mRNAs, and miRNAs, can either act as inter-cellular messengers or are shuttled for autophagic/lysosomal degradation. Most cell types in the central nervous system (CNS) release exosomes, which serve as long and short distance communi- cators between neurons, astrocytes, oligodendrocytes, and microglia. Lysosomal storage disorders are diseases characterized by the accumulation of partially or undigested cellular waste. The exosomal content in these diseases is intrinsic to each individual disorder. Emerging research indicates that lysosomal dysfunction enhances exocytosis, and hence, in lysosomal disorders, exosomal secretion may play a role in disease pathogenesis. Furthermore, the unique properties of exosomes and their ability to carry cargo between adjacent cells and organs, and across the blood–brain barrier, make them attractive candidates for use as therapeutic delivery vehicles. Thus, understanding exosomal content and function may have utility in the treatment of specific lysosomal storage disorders. Since lysosomal dysfunction and the deficiency of at least one lysosomal enzyme, glucocerebrosidase, is associated with the development of parkinsonism, the study and use of exosomes may contribute to an improved understanding of Parkinson disease, potentially leading to new therapeutics. Citation: Gleason, A.M.; Woo, E.G.; McKinney, C.; Sidransky, E. The Role Keywords: exosomes; endocytic pathways; neurodegenerative disease; Gaucher disease; Parkinson of Exosomes in Lysosomal Storage disease; lysosomes; lysosomal storage disorder Disorders. Biomolecules 2021, 11, 576. https://doi.org/10.3390/biom11040576 Academic Editor: Enrico Moro 1. Introduction Cells have several different means of conveying intercellular bioinformation. This Received: 17 March 2021 may occur by direct cell-to-cell contact, through signaling via secreted soluble molecules, Accepted: 12 April 2021 and by small vesicle-packaged molecules [1]. These different vesicles (see AppendixA for Published: 15 April 2021 the definitions of terms shown in bold) include exosomes, ectosomes, apoptotic bodies, and autophagosomes. Exosomes, the subject of this review, [2,3] were initially described as Publisher’s Note: MDPI stays neutral waste material, encapsulated in small lipid vesicles, captured from the cytoplasm of matur- with regard to jurisdictional claims in ing reticulocytes. However, it subsequently became clear that exosome cargos represent a published maps and institutional affil- iations. heterogeneous source of normal or pathological biomarkers that capture a cell’s metabolic state at a given point in time. Exosomes are formed from endosomal membranes and their evolution intersects with steps in autophagy-lysosomal pathways (ALPs). As they mature, exosomes become filled with a selected set of sorted biomolecules. These may include lipids, metabolites, mRNAs, miRNAs, and proteins. While many important aspects of Copyright: © 2021 by the authors. exosome biology in health and disease remain to be defined, exosomal cargos have at Licensee MDPI, Basel, Switzerland. least two fates. They may act as intercellular messengers (e.g., paracrine signaling) and/or This article is an open access article inter-organ communicators (e.g., metastatic disease). They also carry defined intracellular distributed under the terms and conditions of the Creative Commons cargo from interconnected autophagic/lysosomal degradation pathways in the cell. De- Attribution (CC BY) license (https:// pending on the cell of origin and the cargo source, the exosomes contribute to homeostatic creativecommons.org/licenses/by/ or metabolic regulation, as well as to disease processes, including viral infection, cancer 4.0/). metastasis, and neurodegenerative diseases [4]. While a better understanding of the role of Biomolecules 2021, 11, 576. https://doi.org/10.3390/biom11040576 https://www.mdpi.com/journal/biomolecules Biomolecules 2021, 11, x FOR PEER REVIEW 2 of 15 exosomes contribute to homeostatic or metabolic regulation, as well as to disease pro- cesses, including viral infection, cancer metastasis, and neurodegenerative diseases [4]. While a better understanding of the role of exosomes in normal physiology is just now emerging, their role in pathological conditions, such as neurodegeneration, has been more Biomolecules 2021, 11, 576 2 of 14 rigorously examined. 2. The Biogenesis of Exosomes: A Subclass of Extracellular Vesicles (EVs). exosomes in normal physiology is just now emerging, their role in pathological conditions, suchAll as neurodegeneration, cells release exosomes, has been 30– more150 nm rigorously membrane examined.-bound vesicles that derive from the invagination of endosomal membranes. Exosomes acquire their content both from biosyn- thetic2. The routes Biogenesis and ofby Exosomes: endocytosis A Subclass, a form ofof Extracellularintracellular Vesiclestrafficking. (EVs) Endocytosis begins withAll the cells invagination release exosomes, of the plasma 30–150 membrane nm membrane-bound (PM), which vesicles can be that clathrin derive- fromcoated the. Upon internalization,invagination of endosomal these vesicles membranes. first fuse Exosomes with the acquire early theirendosome content (EE), both fromalso biosyn-described as thethetic so routesrting endosome. and by endocytosis Here, major, a form decisions of intracellular on the next trafficking. trafficking Endocytosis route occur. begins Classi- withcally, the cargo invagination destined of for the degradation plasma membrane enter (PM),the endosomal which can-lysosomal be clathrin-coated system,. Upon where the EEsinternalization, mature and these begin vesicles to invaginate first fuse with parts the ofearly their endosome membranes.(EE), These also described intraluminal as the vesi- clessorting (ILVs) endosome. then become Here, major part decisions of the multivesicular on the next trafficking bodies route (MVB), occur. organelles Classically, cargocomprised ofdestined vacuoles for degradationdelimited by enter a single the endosomal-lysosomal membrane [5]. The system, MVBs where host the multiple EEs mature vesicles, and and begin to invaginate parts of their membranes. These intraluminal vesicles (ILVs) then become depending on the invagination size, these vesicles range from 50-150 nm in diameter [6]. part of the multivesicular bodies (MVB), organelles comprised of vacuoles delimited by a Tsinglewo trafficking membrane [routes5]. The can MVBs occur host at multiple the MVB. vesicles, Membrane and depending components on the invaginationand other macro- moleculessize, these vesicles encapsulated range from in these 50-150 vesicles nm in diameter can be [hydrolyzed6]. Two trafficking after fusing routes canwith occur lysosomes. at Enzymesthe MVB. Membranewithin the components lysosomes andthen other degrade macromolecules the delivered encapsulated carbohydrates, in these proteins, vesicles fats, ancand be other hydrolyzed cellular after products fusing into with smaller lysosomes. and Enzymes simpler withincomponents the lysosomes that are then then degrade recycled as buildingthe delivered blocks carbohydrates, for new molecules proteins, (Figure fats, and 1). otherAlternatively, cellular products these vesicles into smaller become and future exosomessimpler components when released that are from then cells recycled, through as building a process blocks called for new secretion. molecules In (Figurethis trafficking1) . Alternatively,step, MVBs fuse these with vesicles the becomeplasma futuremembrane exosomes and when release released the ILVs from as cells, exosomes through [7]. a The processexosome called membrane secretion. composition In this trafficking is similar step, MVBsto that fuse of the with plasma the plasma membrane membrane and and contains insertedrelease the and ILVs captured as exosomes cytoplasmic [7]. The exosome proteins. membrane The internal composition cargo within is similar small to that vesicles of is the plasma membrane and contains inserted and captured cytoplasmic proteins. The internal sorted, sometimes as part of protein complexes that assist in moving and/or sorting. This cargo within small vesicles is sorted, sometimes as part of protein complexes that assist complex of proteins is also known as endosomal sorting complex required for transport in moving and/or sorting. This complex of proteins is also known as endosomal sorting (complexESCRT) required-associated for transportpathways (ESCRT) [8]. -associated pathways [8]. Figure 1. Intracellular biogenesis of exosomes. Exosomes can be formed through the endocytic trafficking pathway. These Figurevesicles 1. areIntracellular generated biogenesis by limited inwardof exosomes budding. Exosomes of the late can endosome be formed to formthrough the intraluminalthe endocytic vesicles trafficking (ILVs). pathway ILVs are. These vesiclescomponents are generated of multivesicular by limited