Exosomes in urine: who would have thought...?

Ultrafast and Controlled Capturing, Loading, and Release of Extracellular Vesicles by a Portable Microstructured Electrochemical Fluidic Device

Extracellular vesicles (EVs) are secreted by all living cells and are found in body fluids. They exert numerous physiological and pathological functions and serve as cargo shuttles. Due to their safety and inherent bioactivity, they have emerged as versatile therapeutic agents, biomarkers, and potential drug carriers. Despite the growing interest in EVs, current progress in this field is, in part, limited by relatively inefficient isolation techniques. Conventional methods are indeed slow, laborious, require specialized laboratory equipment, and may result in low yield and purity. This work describes an electrochemically controlled "all-in-one" device enabling capturing, loading, and releasing of EVs. The device is composed of a fluidic channel confined within antibody-coated microstructured electrodes. It rapidly isolates EVs with a high level of purity from various biofluids. As a proof of principle, the device is applied to isolate EVs from skin wounds of healthy and diabetic mice. Strikingly, it is found that EVs from healing wounds of diabetic mice are enriched in mitochondrial proteins compared to those of healthy mice. Additionally, the device improves the loading protocol of EVs with polyplexes, and may therefore find applications in nucleic acid delivery. Overall, the electrochemical device can greatly facilitate the development of EVs-based technologies.

Exosomal mitochondrial tRNAs and miRNAs as potential predictors of inflammation in renal proximal tubular epithelial cells

Exosomes have emerged as a valuable repository of novel biomarkers for human diseases such as chronic kidney disease (CKD). From a healthy control group, we performed microRNA (miRNA) profiling of urinary exosomes and compared it with a cell culture model of renal proximal tubular epithelial cells (RPTECs). Thereby, a large fraction of abundant urinary exosomal miRNAs could also be detected in exosomes derived from RPTECs, indicating them as a suitable model system for investigation of CKD. We subsequently analyzed exosomes from RPTECs in pro-inflammatory and pro-fibrotic states, mimicking some aspects of CKD. Following cytokine treatment, we observed a significant increase in exosome release and identified 30 dysregulated exosomal miRNAs, predominantly associated with the regulation of pro-inflammatory and pro-fibrotic-related pathways. In addition to miRNAs, we also identified 16 dysregulated exosomal mitochondrial RNAs, highlighting a pivotal role of mitochondria in sensing renal inflammation. Inhibitors of exosome biogenesis and release significantly altered the abundance of selected candidate miRNAs and mitochondrial RNAs, thus suggesting distinct sorting mechanisms of different non-coding RNA (ncRNA) species into exosomes. Hence, these two exosomal ncRNA species might be employed as potential indicators for predicting the pathogenesis of CKD and also might enable effective monitoring of the efficacy of CKD treatment.

Urinary Extracellular Vesicles for Renal Tubular Transporters Expression in Patients With Gitelman Syndrome

Background: The utility of urinary extracellular vesicles (uEVs) to faithfully represent the changes of renal tubular protein expression remains unclear. We aimed to evaluate renal tubular sodium (Na⁺) or potassium (K⁺) associated transporters expression from uEVs and kidney tissues in patients with Gitelman syndrome (GS) caused by inactivating mutations in SLC12A3.

Methods: uEVs were isolated by ultracentrifugation from 10 genetically-confirmed GS patients. Membrane transporters including Na⁺-hydrogen exchanger 3 (NHE3), Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2), NaCl cotransporter (NCC), phosphorylated NCC (p-NCC), epithelial Na⁺ channel β (ENaCβ), pendrin, renal outer medullary K1 channel (ROMK), and large-conductance, voltage-activated and Ca²⁺-sensitive K⁺ channel (Maxi-K) were examined by immunoblotting of uEVs and immunofluorescence of biopsied kidney tissues. Healthy and disease (bulimic patients) controls were also enrolled.

Results: Characterization of uEVs was confirmed by nanoparticle tracking analysis, transmission electron microscopy, and immunoblotting. Compared with healthy controls, uEVs from GS patients showed NCC and p-NCC abundance were markedly attenuated but NHE3, ENaCβ, and pendrin abundance significantly increased. ROMK and Maxi-K abundance were also significantly accentuated. Immunofluorescence of the representative kidney tissues from GS patients also demonstrated the similar findings to uEVs. uEVs from bulimic patients showed an increased abundance of NCC and p-NCC as well as NHE3, NKCC2, ENaCβ, pendrin, ROMK and Maxi-K, akin to that in immunofluorescence of their kidney tissues.

Conclusion: uEVs could be a non-invasive tool to diagnose and evaluate renal tubular transporter adaptation in patients with GS and may be applied to other renal tubular diseases.

Analysis of urinary exosomes applications for rare kidney disorders

Introduction: Exosomes are nanovesicles that play important functions in a variety of physiological and pathological conditions. They are powerful cell-to-cell communication tool thanks to the protein, mRNA, miRNA, and lipid cargoes they carry. They are also emerging as valuable diagnostic and prognostic biomarker sources. Urinary exosomes carry information from all the cells of the urinary tract, downstream of the podocyte. Rare kidney diseases are a subset of an inherited diseases whose genetic diagnosis can be unclear, and presentation can vary due to genetic, epigenetic, and environmental factors. Areas covered: In this review, we focus on a group of rare and often neglected kidney diseases, for which we have sufficient available literature data on urinary exosomes. The analysis of their content can help to comprehend pathological mechanisms and to identify biomarkers for diagnosis, prognosis, and therapeutic targets. Expert opinion: The foreseeable large-scale application of system biology approach to the profiling of exosomal proteins as a source of renal disease biomarkers will be also useful to stratify patients with rare kidney diseases whose penetrance, phenotypic presentation, and age of onset vary sensibly. This can ameliorate the clinical management.

Urinary exosome proteomic profiling defines stage-specific rapid progression of autosomal dominant polycystic kidney disease and tolvaptan efficacy

ADPKD is the most common genetic disease of the kidney leading to end-stage renal disease necessitating renal replacement therapy at any time between the 1^st and 8^th decades of life due to widely variable rates of disease progression. This presents significant patient anxiety and a significant prognostic and therapeutic challenge. Tolvaptan is the only approved drug licensed to slow ADPKD progression by reducing renal cystic expansion but side-effects can limit its efficacy. To address the need to identify new biomarkers to monitor progression of ADPKD and to evaluate the therapeutic effects of Tolvaptan, proteomic analysis was conducted on defined (40-100nm) urinary exosomes isolated from ADPKD patients phenotyped and clinically monitored over a 10-year period. Comparative Gene Ontology analysis of Tandem Mass Tag labelled mass spectrometry-derived protein profiles from urinary exosomes from ADPKD patients with rapid (>10ml/min/5 years decline in estimated glomerular filtration rate) versus slow progression showed distinctive patterns of pathway up-regulation. Clear discrimination between rapid and slowly-progressive profiles were seen in all stages functional decline in ADPKD patients whether with mild (>70ml/min), moderate (50-69ml/min) or severe (<49ml/min) disease at onset. Discriminatory pathways and proteins included Notch-, integrin- and growth factor-signalling; microtubular kinase, vesicular proteins and epidermal growth factor substrates. Confocal microscopy of fluorescently-labelled normal versus ADPKD epithelial cell-derived exosomes in vitro also identified ADPKD-dependent abnormalities in intracellular vesicular trafficking and implicated changes in ADPKD-dependent exosome secretion and target cell uptake as factors underlying urinary exosome excretion biomarker properties. Comparative proteomic analysis of urinary exosomal proteins in individual patients before and after treatment with Tolvaptan for 4 years also identified distinct patterns of pathway modification dependent on the degree of effectiveness of the therapeutic response. Up-regulation of Wnt-pathway and vesicular proteins were characteristic of urinary exosomes from ADPKD patients with good responses to Tolvaptan while upregulation of angiogenesis pathways and additional molecular forms of vasopressin receptor AVPR2 were characteristic in urinary exosomes of ADPKD patients with poor responses. Taken together, these studies conclude that proteomic profiling of urinary exosome biomarkers provides a specific, sensitive and practical non-invasive method to identify and monitor the rate of disease progression and the effects of Tolvaptan therapy in individual ADPKD patients. This provides a means to identify those patients most likely to benefit maximally from therapy and to progress towards a personalization of ADPKD prognosis and management.

Isolation and Characterization of Exosomes from Mouse Feces

Exosomes secreted by colonic epithelial cells are present in feces and contain valuable epigenetic information, such as miRNAs, proteins, and metabolites. An in-depth study of this information is conducive to the diagnosis or treatment of relevant diseases. A crucial prerequisite of such a study is to establish an efficient isolation method, through which we can obtain a relatively more significant amount of exosomes from feces. This protocol is designed to effectively isolate a large number of exosomes from contaminants and other particles in feces by a combined method with fast filtration and sucrose density gradient ultracentrifugation. Exosomes generated by this method are suitable for further RNA, protein, and lipid analysis.

Extracellular vesicle-coupled miRNA profiles in follicular fluid of cows with divergent post-calving metabolic status

Most high-yielding dairy cows enter a state of negative energy balance (NEB) during early lactation. This, in turn, results in changes in the level of various metabolites in the blood and follicular fluid microenvironment which contributes to disturbed fertility. Extracellular vesicles (EVs) are evolutionarily conserved communicasomes that transport cargo of miRNA, proteins and lipids. EV-coupled miRNAs have been reported in follicular fluid. However, the association between postpartum NEB and EV-coupled miRNA signatures in follicular fluid is not yet known. Energy balance analysis in lactating cows shortly after post-calving revealed that the majority of the cows exhibited transiently negative energy balance levels, whereas the remaining cows exhibited either consistently negative or consistently positive energy levels. Metabolic status was associated with EV-coupled miRNA composition in the follicular fluid. Cows experiencing NEB showed reduced expression of a large number of miRNAs while cows with positive energy balances primarily exhibited elevated expression of EV-coupled miRNAs. The miRNAs that were suppressed under NEB were found to be involved in various metabolic pathways. This is the first study to reveal the presence of an association between EV-coupled miRNA in follicular fluid and metabolic stress in dairy cows. The involvement of differentially expressed miRNAs in various pathways associated with follicular growth and oocyte maturation suggest the potential involvement of specific follicular miRNAs in oocyte developmental competence, which may partially explain reduced fertility in cows due to post-calving metabolic stress.

Hydrophobic sand is a viable method of urine collection from the rat for extracellular vesicle biomarker analysis

Previously we have shown in rats a new method of urine collection, hydrophobic sand, to be an acceptable alternate in place of the traditional method using metabolic cages. Hydrophobic sand is non-toxic, induces similar or lower levels of stress in the rat, and does not contaminate clinical urine markers nor metal concentrations in collected samples (Hoffman et al., 2017 and 2018). Urine is often used in humans and many animal models as a readily-attainable biosample which contains proteins and microRNAs (miRNAs) within extracellular vesicles (EVs) that can be isolated to indicate changes in health. In order to ensure hydrophobic sand did not in any way contaminate or disrupt the extraction and analysis of these EVs and miRNAs, we used urine samples from the same 8 rats in the within-subjects crossover experiment comparing hydrophobic sand and metabolic cage collection methods. We isolated EVs and miRNAs from the urine set and examined their quantity and quality between the urine collection methods. We found no significant differences in particle size, particle concentration, total RNA, or the type and abundance of miRNAs contained within the urine EVs due to urine collection method, suggesting hydrophobic sand represents an easy-to-use, non-invasive method to collect rodent urine for EVs and biomarker studies.

Biogenesis, Stabilization, and Transport of microRNAs in Kidney Health and Disease

The kidneys play key roles in the maintenance of homeostasis, including fluid balance, blood filtration, erythropoiesis and hormone production. Disease-driven perturbation of renal function therefore has profound pathological effects, and chronic kidney disease is a leading cause of morbidity and mortality worldwide. Successive annual increases in global chronic kidney disease patient numbers in part reflect upward trends for predisposing factors, including diabetes, obesity, hypertension, cardiovascular disease and population age. Each kidney typically possesses more than one million functional units called nephrons, and each nephron is divided into several discrete domains with distinct cellular and functional characteristics. A number of recent analyses have suggested that signaling between these nephron regions may be mediated by microRNAs. For this to be the case, several conditions must be fulfilled: (i) microRNAs must be released by upstream cells into the ultrafiltrate; (ii) these microRNAs must be packaged protectively to reach downstream cells intact; (iii) these packaged microRNAs must be taken up by downstream recipient cells without functional inhibition. This review will examine the evidence for each of these hypotheses and discuss the possibility that this signaling process might mediate pathological effects.

A Non-invasive Liquid Biopsy Screening of Urine-Derived Exosomes for miRNAs as Biomarkers in Endometrial Cancer Patients

Exosomes have great potential to serve as a source of diagnostic and prognostic biomarkers for endometrial cancer (EC). Urine-derived exosomes from patients with EC and patients with symptoms of EC, but without established EC, were used to evaluate a unique miRNA expression profile. Of the 84 miRNA studied, 57 were amplified in qPCR, suggesting the differential packaging of miRNA in exosomes. Further, hsa-miR-200c-3p was identified to be enriched the most. Various bioinformatics and in silico tools were used to evaluate the biological significance of hsa-miR-200c-3p in EC. We conclude that differential miRNA in exosomes can be utilized for discovery of biomarker signatures and EC diagnosis; hsa-miR-200c-3p is one such candidate. Urine-derived exosomes pave the way for the development of non-invasive biomarkers.

Extracellular Vesicles as Markers and Mediators in Sepsis

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. It remains a highly lethal condition in which current tools for early diagnosis and therapeutic decision-making are far from ideal. Extracellular vesicles (EVs), 30 nm to several micrometers in size, are released from cells upon activation and apoptosis and express membrane epitopes specific for their parental cells. Since their discovery two decades ago, their role as biomarkers and mediators in various diseases has been intensively studied. However, their potential importance in the sepsis syndrome has gained attention only recently. Sepsis and EVs are both complex fields in which standardization has long been overdue. In this review, several topics are discussed. First, we review current studies on EVs in septic patients with emphasis on their variable quality and clinical utility. Second, we discuss the diagnostic and therapeutic potential of EVs as well as their role as facilitators of cell communication via micro RNA and the relevance of micro-organism-derived EVs. Third, we give an overview over the potential beneficial but also detrimental roles of EVs in sepsis. Finally, we focus on the role of EVs in selected intensive care scenarios such as coagulopathy, mechanical ventilation and blood transfusion. Overall, the prospect for EV use in septic patients is bright, ranging from rapid and precise (point-of-care) diagnostics, prevention of harmful iatrogenic interventions, to using EVs as guides of individualized therapy. Before the above is achieved, however, the EV research field requires reliable standardization of the current methods and development of new analytical procedures that can close the existing technological gaps.

Proteomic Analysis of Exosomes and Its Application in HIV-1 Infection

Exosomes are 30-100 nm extracellular vesicles secreted from late endosomes by various types of cells. Numerous studies have suggested that exosomes play significant roles in human immunodeficiency virus 1 (HIV-1) biogenesis. Proteomics coupled with exosome fractionation has been successfully used to identify various exosomal proteins and helped to uncover the interactions between exosomes and HIV-1. To inform the current progress in the intersection of exosome, proteomics, and HIV-1, this review is focused on: i) analyzing different exosome isolation, purification methods, and their implications in HIV-1 studies; ii) evaluating the roles of various proteomic techniques in defining exosomal contents; iii) discussing the research and clinical applications of proteomics and exosome in HIV-1 biology.

Expression and immunolocalization of metallothioneins MT1, MT2 and MT3 in rat nephron

Rodent kidneys exhibit three isoforms of metallothioneins (MTs), MT1, MT2 and MT3, with poorly characterized localization along the nephron. Here we studied in adult male Wistar rats the renal expression of MTs mRNA by end-point RT-PCR and MT proteins by immunochemical methods The expression pattern of MT1 mRNA was cortex (CO)>outer stripe (OS)=inner stripe (IS)=inner medulla (IM), of MT2 mRNA was IM>CO>IS=OS, and of MT3 mRNA was IM>CO=OS=IM. MT1/2-antibody stained with heterogeneous intensity the cell cytoplasm and nuclei in proximal tubule (PT) and thin ascending limb, whereas MT3-antibody stained weakly the cell cytoplasm in various cortical tubules and strongly the nuclei in all nephron segments. However, the isolated nuclei exhibited an absence of MT1/2 and presence of MT3 protein. In MT1/2-positive PT cells, the intracellular staining appeared diffuse or bipolar, but the isolated brush-border, basolateral and endosomal membranes were devoid of MT1/2 proteins. In the lumen of some PT profiles, the heterogeneously sized MT1/2-rich vesicles were observed, with the limiting membrane positive for NHE3, but negative for V-ATPase, CAIV, and megalin, whereas their interior was positive for CAII and negative for cytoskeleton. They seem to be pinched off from the luminal membrane of MT1/2-rich cells, as also indicated by transmission electron microscopy. We conclude that in male rats, MTs are heterogeneously abundant in the cell cytoplasm and/or nuclei along the nephron. The MT1/2-rich vesicles in the tubule lumen may represent a source of urine MT and membranous material, whereas MT3 in nuclei may handle zink and locally-produced reactive oxygen species.

Proteomic analysis of exosomal cargo: the challenge of high purity vesicle isolation

The re-discovery of exosomes as intercellular messengers with high potential for diagnostic and therapeutic utility has led to them becoming a popular topic of research in recent years. One of the essential research areas in this field is the characterization of exosomal cargo, which includes numerous non-randomly packed proteins and nucleic acids. Unexpectedly, a very challenging aspect of exploration of extracellular vesicles has turned out to be their effective and selective isolation. The plurality of developed protocols leads to qualitative and quantitative variability in terms of the obtained exosomes, which significantly affects the results of downstream analyses and makes them difficult to compare, reproduce and interpret between research groups. Currently, there is a general consensus among the exosome-oriented community concerning the urgent need for the optimization and standardization of methods employed for the purification of these vesicles. Hence, we review here several strategies for exosome preparation including ultracentrifugation, chemical precipitation, affinity capturing and filtration techniques. The advantages and disadvantages of different approaches are discussed with special emphasis being placed on their adequacy for proteomics applications, which are particularly sensitive to sample quality. We conclude that certain methods, exemplified by ultracentrifugation combined with iodixanol density gradient centrifugation or gel filtration, although labor-intensive, provide superior quality exosome preparations suitable for reliable analysis by mass spectrometry.

Extracellular Vesicles in Renal Pathophysiology

Extracellular vesicles are a heterogeneous population of microparticles released by virtually all living cells which have been recently widely investigated in different biological fields. They are typically composed of two primary types (exosomes and microvesicles) and are recently commanding increasing attention as mediators of cellular signaling. Indeed, these vesicles can affect recipient cells by carrying and delivering complex cargos of biomolecules (including proteins, lipids and nucleic acids), protected from enzymatic degradation in the environment. Their importance has been demonstrated in the pathophysiology of several organs, in particular in kidney, where different cell types secrete extracellular vesicles that mediate their communication with downstream urinary tract cells. Over the past few years, evidence has been shown that vesicles participate in kidney development and normal physiology. Moreover, EVs are widely demonstrated to be implicated in cellular signaling during renal regenerative and pathological processes. Although many EV mechanisms are still poorly understood, in particular in kidney, the discovery of their role could help to shed light on renal biological processes which are so far elusive. Lastly, extracellular vesicles secreted by renal cells gather in urine, thus becoming a great resource for disease or recovery markers and a promising non-invasive diagnostic instrument for renal disease. In the present review, we discuss the most recent findings on the role of extracellular vesicles in renal physiopathology and their potential implication in diagnosis and therapy.

Urinary exosomal activating transcriptional factor 3 as the early diagnostic biomarker for sepsis-induced acute kidney injury

Background

An early sepsis-induced acute kidney injury (sepsis-AKI) biomarker is currently in needed. Urinary neutrophil gelatinase-associated lipocalin (uNGAL) is a candidate of sepsis-AKI biomarker but with different cut-point values. Urinary exosomal activating transcriptional factor 3 (uATF3) has been mentioned as an interesting biomarker.

Methods

We conducted experiments in mice and a prospective, multicenter study in patients as a proof of concept that urine exosome is an interesting biomarker. An early expression of ATF3 in kidney of CD-1 mice at 6 h after cecal ligation and puncture implied the possibility of uATF3 as an early sepsis-AKI biomarker. Increase serum creatinine (Scr) ≥0.3 mg/dL from the baseline was used as an AKI diagnosis and urine was analyzed for uATF3 and uNGAL. Patients with baseline Scr at admission ≥1.5 mg/dL were excluded.

Results

The analysis showed higher Scr, uNGAL and uATF3 in patients with sepsis-AKI in comparison with patients with sepsis-non-AKI and healthy volunteers. A fair correlation, r² = 0.47, between uATF3 and uNGAL was showed in sepsis-AKI group with Scr ≥2 mg/dL. To see if uATF3 could be an early sepsis-AKI biomarker, urine sample was collected daily during the first week of the admission. In sepsis-AKI and sepsis-non-AKI groups, uNGAL were 367 ± 43 ng/mL and 183 ± 23 ng/mL, respectively; and uATF3 were 19 ± 4 ng/mL and 1.4 ± 0.8 ng/mL, respectively. With the mean value of uNGAL and uATF3 in sepsis AKI as a cut-off level, AUROC of uNGAL and uATF3 were 64% (95% CI 0.54 to 0.74) and 84% (95% CI 0.77 to 0.91), respectively.

Conclusions

Urine exosome is an interesting source of urine biomarker and uATF3 is an interesting sepsis-AKI biomarker.

Comprehensive Metaproteomic Analyses of Urine in the Presence and Absence of Neutrophil-Associated Inflammation in the Urinary Tract

Inflammation in the urinary tract results in a urinary proteome characterized by a high dynamic range of protein concentrations and high variability in protein content. This proteome encompasses plasma proteins not resorbed by renal tubular uptake, renal secretion products, proteins of immune cells and erythrocytes derived from trans-urothelial migration and vascular leakage, respectively, and exfoliating urothelial and squamous epithelial cells. We examined how such proteins partition into soluble urine (SU) and urinary pellet (UP) fractions by analyzing 33 urine specimens 12 of which were associated with a urinary tract infection (UTI). Using mass spectrometry-based metaproteomic approaches, we identified 5,327 non-redundant human proteins, 2,638 and 4,379 of which were associated with SU and UP fractions, respectively, and 1,206 non-redundant protein orthology groups derived from pathogenic and commensal organisms of the urogenital tract. Differences between the SU and UP proteomes were influenced by local inflammation, supported by respective comparisons with 12 healthy control urine proteomes. Clustering analyses showed that SU and UP fractions had proteomic signatures discerning UTIs, vascular injury, and epithelial cell exfoliation from the control group to varying degrees. Cases of UTI revealed clusters of proteins produced by activated neutrophils. Network analysis supported the central role of neutrophil effector proteins in the defense against invading pathogens associated with subsequent coagulation and wound repair processes. Our study expands the existing knowledge of the urinary proteome under perturbed conditions, and should be useful as reference dataset in the search of biomarkers.

Residual urinary extracellular vesicles in ultracentrifugation supernatants after hydrostatic filtration dialysis enrichment

Urinary extracellular vesicles (UEVs) appear an ideal source of biomarkers for kidney and urogenital diseases. The majority of protocols designed for their isolation are based on differential centrifugation steps. However, little is still known of the type and amount of vesicles left in the supernatant. Here we used an isolation protocol for UEVs which uses hydrostatic filtration dialysis as first pre-enrichment step, followed by differential centrifugation. Transmission electron microscopy (TEM), mass spectrometry (MS), western blot, ELISA assays and tuneable resistive pulse sensing (TRPS) were used to characterise and quantify UEVs in the ultracentrifugation supernatant. TEM showed the presence of a variety of small size vesicles in the supernatant while protein identification by MS matched accurately with the protein list available in Vesiclepedia. Screening and relative quantification for specific vesicle markers showed that the supernatant was preferentially positive for CD9 and TSG101. ELISA tests for quantification of exosome revealed that 14%, was left in the supernatant with a particle diameter of 110 nm and concentration of 1.54 × 10¹⁰/ml. Here we show a comprehensive characterisation of exosomes and other small size urinary vesicles which the conventional differential centrifugation protocol may lose.

Specific renal parenchymal-derived urinary extracellular vesicles identify age-associated structural changes in living donor kidneys

Non-invasive tests to identify age and early disease-associated pathology within the kidney are needed. Specific populations of urinary extracellular vesicles (EVs) could potentially be used for such a diagnostic test. Random urine samples were obtained from age- and sex-stratified living kidney donors before kidney donation. A biopsy of the donor kidney was obtained at the time of transplantation to identify nephron hypertrophy (larger glomerular volume, cortex per glomerulus and mean profile tubular area) and nephrosclerosis (% fibrosis, % glomerulosclerosis and arteriosclerosis). Renal parenchymal-derived EVs in cell-free urine were quantified by digital flow cytometry. The relationship between these EV populations and structural pathology on the kidney biopsy was assessed. Clinical characteristics of the kidney donors (n=138, age range: 20–70 years, 50% women) were within the normative range. Overall, urine from women contained more EVs than that from men. The number of exosomes, juxtaglomerular cells and podocyte marker–positive EVs decreased (p<0.05) with increasing age. There were fewer total EVs as well as EVs positive for mesangial cell, parietal cell, descending limb of Henle's loop (simple squamous epithelium), collecting tubule-intercalated cell and monocyte chemoattractant protein-1 markers (p<0.05) in persons with nephron hypertrophy. The number of EVs positive for intercellular adhesion molecule-1, juxtaglomerular cell, podocyte, parietal cell, proximal tubular epithelial cell, distal tubular epithelial cell and collecting duct cells were fewer (p<0.05) in persons with nephrosclerosis. EVs carrying markers of cells from the renal pelvis epithelium did not associate with any indices of nephron hypertrophy or nephrosclerosis. Therefore, specific populations of EVs derived from cells of the glomerulus and nephron associate with underlying kidney structural changes. Further validation of these findings in other cohorts is needed to determine their clinical utility.

Exosomes as renal inductive signals in health and disease, and their application as diagnostic markers and therapeutic agents

Cells secrete around 30–1000 nm membrane-enclosed vesicles, of which members of the subgroup between 30 and 100 nm are termed exosomes (EXs). EXs are released into the extracellular space and are widely present in body fluids and incorporated mRNA, miRNA, proteins, and signaling molecules. Increasing amounts of evidence suggest that EXs play an important role not only in cell-to-cell communication but also in various physiological and disease processes. EXs secreted by kidney cells control nephron function and are involved in kidney diseases and cancers. This makes them potential targets for diagnostic and therapeutic applications such as non-invasive biomarkers and cell-free vaccines and for use as drug delivery vehicles. This review provides an overview on the known roles of EXs in kidney development and diseases, including renal cancer. Additionally, it covers recent findings on their significance as diagnostic markers and on therapeutic applications to renal diseases and cancers. The intention is to promote an awareness of how many questions still remain open but are certainly worth investigating.

Defining nephrotic syndrome from an integrative genomics perspective

Nephrotic syndrome (NS) is a clinical condition with a high degree of morbidity and mortality, caused by failure of the glomerular filtration barrier, resulting in massive proteinuria. Our current diagnostic, prognostic and therapeutic decisions in NS are largely based upon clinical or histological patterns such as "focal segmental glomerulosclerosis" or "steroid sensitive". Yet these descriptive classifications lack the precision to explain the physiologic origins and clinical heterogeneity observed in this syndrome. A more precise definition of NS is required to identify mechanisms of disease and capture various clinical trajectories. An integrative genomics approach to NS applies bioinformatics and computational methods to comprehensive experimental, molecular and clinical data for holistic disease definition. A unique aspect is analysis of data together to discover NS-associated molecules, pathways, and networks. Integrating multidimensional datasets from the outset highlights how molecular lesions impact the entire individual. Data sets integrated range from genetic variation to gene expression, to histologic changes, to progression of chronic kidney disease (CKD). This review will introduce the tenets of integrative genomics and suggest how it can increase our understanding of NS from molecular and pathophysiological perspectives. A diverse group of genome-scale experiments are presented that have sought to define molecular signatures of NS. Finally, the Nephrotic Syndrome Study Network (NEPTUNE) will be introduced as an international, prospective cohort study of patients with NS that utilizes an integrated systems genomics approach from the outset. A major NEPTUNE goal is to achieve comprehensive disease definition from a genomics perspective and identify shared molecular drivers of disease.

Increased DJ-1 in urine exosome of Korean males with Parkinson's disease

Parkinson's disease (PD) is a difficult disease to diagnose although it is the second most common neurodegenerative disease. Recent studies show that exosome isolated from urine contains LRRK2 or DJ-1, proteins whose mutations cause PD. To investigate a potential use for urine exosomes as a tool for PD diagnosis, we compared levels of LRRK2, α-synuclein, and DJ-1 in urine exosomes isolated from Korean PD patients and non-PD controls. LRRK2 and DJ-1, but not α-synuclein, were detected in the urine exosome samples, as reported previously. We initially could not detect any significant difference in these protein levels between the patient and the control groups. However, when age, disease duration, L-dopa daily dose, and gender were considered as analytical parameters, LRRK2 and DJ-1 protein levels showed clear gender-dependent differences. In addition, DJ-1 level was significantly higher (1.7-fold) in male patients with PD than that in male non-PD controls and increased in an age-dependent manner in male patients with PD. Our observation might provide a clue to lead to a novel biomarker for PD diagnosis, at least in males.

Kidney: polycystic kidney disease

Polycystic kidney disease (PKD) is a life-threatening genetic disorder characterized by the presence of fluid-filled cysts primarily in the kidneys. PKD can be inherited as autosomal recessive (ARPKD) or autosomal dominant (ADPKD) traits. Mutations in either the PKD1 or PKD2 genes, which encode polycystin 1 and polycystin 2, are the underlying cause of ADPKD. Progressive cyst formation and renal enlargement lead to renal insufficiency in these patients, which need to be managed by lifelong dialysis or renal transplantation. While characteristic features of PKD are abnormalities in epithelial cell proliferation, fluid secretion, extracellular matrix and differentiation, the molecular mechanisms underlying these events are not understood. Here we review the progress that has been made in defining the function of the polycystins, and how disruption of these functions may be involved in cystogenesis.

Subfractionation, characterization, and in-depth proteomic analysis of glomerular membrane vesicles in human urine

Urinary exosome-like vesicles (ELVs) are a heterogenous mixture (diameter 40–200nm) containing vesicles shed from all segments of the nephron including glomerular podocytes. Contamination with Tamm Horsfall protein (THP) oligomers has hampered their isolation and proteomic analysis. Here we improved ELV isolation protocols employing density centrifugation to remove THP and albumin, and isolated a glomerular membranous vesicle (GMV) enriched subfraction from 7 individuals identifying 1830 proteins and in 3 patients with glomerular disease identifying 5657 unique proteins. The GMV fraction was composed of podocin/podocalyxin positive irregularly shaped membranous vesicles and podocin/podocalyxin negative classical exosomes. Ingenuity pathway analysis identified integrin, actin cytoskeleton and RhoGDI signaling in the top three canonical represented signaling pathways and 19 other proteins associated with inherited glomerular diseases. The GMVs are of podocyte origin and the density gradient technique allowed isolation in a reproducible manner. We show many nephrotic syndrome proteins, proteases and complement proteins involved in glomerular disease are in GMVs and some were shed in the disease state (nephrin, TRPC6 and INF2 and PLA2R). We calculated sample sizes required to identify new glomerular disease biomarkers, expand the ELV proteome and provide a reference proteome in a database that may prove useful in the search for biomarkers of glomerular disease.

Exosome release from infected dendritic cells: a clue for a fast spread of prions in the periphery?

Prion diseases are incurable transmissible neurological disorders. In many natural and experimental prion diseases, infectious prions can be detected in the lymphoreticular system (LRS) long before they reach the brain where they cause a fatal rapidly progressive degeneration. Although major cell types that contribute to prion accumulation have been identified, the mode of prion dissemination in the LRS remains elusive. Recent evidence of a remarkably fast splenic prion accumulation after peripheral infection of mice, resulting in high prion titers in dendritic cells (DCs) and a release of prions from infected DCs via exosomes suggest that intercellular dissemination may contribute to rapid prion colonization in the LRS. A vast body of evidence from retroviral infections shows that DCs and other antigen-presenting cells (APCs) share viral antigens by intercellular transfer to warrant immunity against viruses if APCs remain uninfected. Evolved to adapt the immune response to evading pathogens, these pathways may constitute a portal for unimpeded prion dissemination owing to the tolerance of the immune system against host-encoded prion protein. In this review we summarize current paradigms for antigen-sharing pathways which may be relevant to better understand dissemination of rogue neurotoxic proteins.

Analysis of endoscopic pancreatic function test (ePFT)-collected pancreatic fluid proteins precipitated via ultracentrifugation

CONTEXT

We have shown previously that trichloroacetic acid precipitation is an effective method of protein extraction from pancreatic fluid for downstream biomarker discovery, compared to other common extraction methods tested.

OBJECTIVE

We aim to assess the utility of ultracentrifugation as an alternative method of protein extraction from pancreatic fluid.

DESIGN

Proteins extracted from trichloroacetic acid- and ultracentrifugation-precipitated pancreatic fluid were identified using mass spectrometry techniques (in-gel tryptic digestion followed by liquid chromatography-tandem mass spectrometry; GeLC-MS/MS). Data were analyzed using Proteome Discoverer and Scaffold 3.

SETTING

This is a proteomic analysis experiment of endoscopically collected fluid in an academic center.

PATIENTS

The study population included adult patients referred to the Center for Pancreatic Disease at Brigham and Women's Hospital, Boston, MA, USA for the evaluation of abdominal pain and gastrointestinal symptoms.

INTERVENTIONS

Secretin-stimulated pancreatic fluid was collected as standard of care for the evaluation of abdominal pain and gastrointestinal symptoms.

MAIN OUTCOME MEASURES

We compared proteins identified via standard trichloroacetic acid precipitation and this alternative ultracentrifugation strategy.

RESULTS

A subset of pancreatic fluid proteins was identified via the ultracentrifugation method. Of these proteins, similar numbers were obtained from fully tryptic or semi-tryptic database searching. Proteins identified in the ultracentrifugation-precipitated samples included previously identified biomarker candidates of chronic pancreatitis.

CONCLUSIONS

This alternative ultracentrifugation strategy requires less time and fewer handling procedures than standard trichloroacetic acid precipitation, at the expense of higher sample volume. As such, this method is well suited for targeted assays (i.e., dot blotting or targeted mass spectrometry) if the protein of interest is among those readily identified by ultracentrifugation-promoted precipitation.

Purification of exosome-like vesicles from urine

Urinary exosome-like vesicles (ELVs), 20-200nm membrane-bound particles shed by renal epithelium, have been shown to interact with the primary cilia of distant epithelial cells of the nephron. These ELVs are emerging as an important source of protein, mRNA, and miRNA biomarkers to monitor renal disease. However, purification of ELVs is compromised by the presence of large amounts of the urinary protein Tamm-Horsfall Protein (THP). THP molecules oligomerize into long, double-helical strands several microns long. These linear assemblies form a three-dimensional gel which traps and sequesters ELVs in any centrifugation-based protocol. Here, we present a purification protocol that separates ELVs from THP and divides urinary ELVs into three distinct populations.

Comparative and targeted proteomic analyses of urinary microparticles from bladder cancer and hernia patients

Bladder cancer is a common urologic cancer whose incidence continues to rise annually. Urinary microparticles are an attractive material for noninvasive bladder cancer biomarker discovery. In this study, we applied isotopic dimethylation labeling coupled with liquid chromatography-tandem mass spectrometry (LC-MS/MS) to discover bladder cancer biomarkers in urinary microparticles isolated from hernia (control) and bladder cancer patients. This approach identified 2964 proteins based on more than two distinct peptides, of which 2058 had not previously been reported as constituents of human urine exosomes/microparticles. A total of 107 differentially expressed proteins were identified as candidate biomarkers. Differences in the concentrations of 29 proteins (41 signature peptides) were precisely quantified by LC-MRM/MS in 48 urine samples of bladder cancer, hernia, and urinary tract infection/hematuria. Concentrations of 24 proteins changed significantly (p<0.05) between bladder cancer (n=28) and hernia (n=12), with area-under-the-curve values ranging from 0.702 to 0.896. Finally, we quantified tumor-associated calcium-signal transducer 2 (TACSTD2) in raw urine specimens (n=221) using a commercial ELISA and confirmed its potential value for diagnosis of bladder cancer. Our study reveals a strong association of TACSTD2 with bladder cancer and highlights the potential of human urinary microparticles in the noninvasive diagnosis of bladder cancer.

Application of systems biology principles to protein biomarker discovery: urinary exosomal proteome in renal transplantation

PURPOSE

In mass spectrometry (MS)-based studies to discover urinary protein biomarkers, an important question is how to analyze the data to find the most promising potential biomarkers to be advanced to large-scale validation studies. Here, we describe a "systems biology-based" approach to address this question.

EXPERIMENTAL DESIGN

We analyzed large-scale liquid chromatography-tandem mass spectrometry (LC-MS/MS) data of urinary exosomes from renal allograft recipients with biopsy-proven evidence of immunological rejection or tubular injury (TI). We asked whether bioinformatic analysis of urinary exosomal proteins can identify biological-process based protein groups that correlate with biopsy findings and whether the protein groups fit with general knowledge of the pathophysiological mechanisms involved.

RESULTS

LC-MS/MS analysis of urinary exosomal proteomes identified more than 1000 proteins in each pathologic group. These protein lists were analyzed computationally to identify the Biological Process and Kyoto Encyclopedia of Genes and Genomes (KEGG) Pathway terms that are significantly associated with each pathological group. Among the most informative terms for each group were: "sodium ion transport" for TI; "immune response" for all rejection; "epithelial cell differentiation" for cell-mediated rejection; and "acute inflammatory response" for antibody-mediated rejection. Based on these terms, candidate biomarkers were identified using a novel strategy to allow a dichotomous classification between different pathologic categories.

CONCLUSIONS AND CLINICAL RELEVANCE

The terms and candidate biomarkers identified make rational connections to pathophysiological mechanisms, suggesting that the described bioinformatic approach will be useful in advancing large-scale biomarker identification studies toward a validation phase.

Role of biomarkers in the diagnosis and prognosis of acute kidney injury in patients with cardiorenal syndrome

Cardiac and renal disease frequently coexist but have long been difficult to diagnose in a timely manner and treat effectively. Noninvasive and cost-effective biomarkers are needed to help identify cardiac patients who are at risk of acute kidney injury early in the course of disease. Biomarkers can provide insights into underlying mechanisms and lead to a better understanding of complex disease states such as the cardiorenal syndrome, which can lead to better therapies and, ultimately, to improved patient outcomes. The natriuretic peptides are established biomarkers in heart failure and have set the standard for how a well-validated biomarker can be useful for diagnosis/prognosis, monitoring response to therapy and chronic disease management. For patients with acute kidney injury in the setting of cardiac disease, new biomarkers such as neutrophil gelatinase-associated lipocalin, cystatin C, kidney injury molecule-1 and IL-18 are emerging as early signals of renal dysfunction prior to any elevations in serum creatinine. Other promising candidate biomarkers for the early diagnosis of acute kidney injury include osteopontin, N-acetyl-b-d-glucosaminidase, stromal cell-derived factor-1 and exosomes. More research with all of these novel biomarkers is needed; however, the early results are very promising.

Differences in immunoglobulin light chain species found in urinary exosomes in light chain amyloidosis (Al)

Renal involvement is a frequent consequence of plasma cell dyscrasias. The most common entities are light chain amyloidosis, monoclonal immunoglobulin deposition disease and myeloma cast nephropathy. Despite a common origin, each condition has its own unique histologic and pathophysiologic characteristic which requires a renal biopsy to distinguish. Recent studies have shown urinary exosomes containing kidney-derived membrane and cytosolic proteins that can be used to probe the proteomics of the entire urinary system from the glomerulus to the bladder. In this study, we analyzed urine exosomes to determine the differences between exosomes from patients with light chain amyloidosis, multiple myeloma, monoclonal gammopathy of undetermined significance, and non-paraproteinemia related kidney disease controls. In patients with light chain amyloidosis, multiple myeloma and monoclonal gammopathy of undetermined significance, immunoreactive proteins corresponding to monomeric light chains were found in exosomes by western blot. In all of the amyloidosis samples with active disease, high molecular weight immunoreactive species corresponding to a decamer were found which were not found in exosomes from the other diseases or in amyloidosis exosomes from patients in remission. Few or no light chains monomeric bands were found in non-paraproteinemia related kidney disease controls. Our results showed that urinary exosomes may have tremendous potential in furthering our understanding of the pathophysiology and diagnosis of plasma cell dyscrasia related kidney diseases.

Plasmacytoid dendritic cells sequester high prion titres at early stages of prion infection

In most transmissible spongiform encephalopathies prions accumulate in the lymphoreticular system (LRS) long before they are detectable in the central nervous system. While a considerable body of evidence showed that B lymphocytes and follicular dendritic cells play a major role in prion colonization of lymphoid organs, the contribution of various other cell types, including antigen-presenting cells, to the accumulation and the spread of prions in the LRS are not well understood. A comprehensive study to compare prion titers of candidate cell types has not been performed to date, mainly due to limitations in the scope of animal bioassays where prohibitively large numbers of mice would be required to obtain sufficiently accurate data. By taking advantage of quantitative in vitro prion determination and magnetic-activated cell sorting, we studied the kinetics of prion accumulation in various splenic cell types at early stages of prion infection. Robust estimates for infectious titers were obtained by statistical modelling using a generalized linear model. Whilst prions were detectable in B and T lymphocytes and in antigen-presenting cells like dendritic cells and macrophages, highest infectious titers were determined in two cell types that have previously not been associated with prion pathogenesis, plasmacytoid dendritic (pDC) and natural killer (NK) cells. At 30 days after infection, NK cells were more than twice, and pDCs about seven-fold, as infectious as lymphocytes respectively. This result was unexpected since, in accordance to previous reports prion protein, an obligate requirement for prion replication, was undetectable in pDCs. This underscores the importance of prion sequestration and dissemination by antigen-presenting cells which are among the first cells of the immune system to encounter pathogens. We furthermore report the first evidence for a release of prions from lymphocytes and DCs of scrapie-infected mice ex vivo, a process that is associated with a release of exosome-like membrane vesicles.

Exosomes and the kidney: prospects for diagnosis and therapy of renal diseases

Exosomes are 40-100 nm membrane vesicles secreted into the extracellular space by numerous cell types. These structures can be isolated from body fluids including urine and plasma. Exosomes contain proteins, mRNAs, miRNAs, and signaling molecules that reflect the physiological state of their cells of origin and consequently provide a rich source of potential biomarker molecules. Aside from diagnostic uses, exosome-mediated transfer of proteins, mRNAs, miRNAs, and signaling molecules offer the promise that they may be used for therapeutic purposes. In this review, we integrate new knowledge about exosomes from outside the field of nephrology with recent progress by renal researchers in order to provide a basis for speculation about how the study of exosomes may affect the fields of nephrology and renal physiology in the next few years.

Endometrial secretions: creating a stimulatory microenvironment within the human early placenta and implications for the aetiopathogenesis of preeclampsia

Endometrial glands represent an important source of nutrients for the conceptus during the first trimester. Their secretions are enriched with carbohydrates, and glycogen accumulates within the syncytiotrophoblast of the placenta. It has been assumed that fetal and placental metabolism follow adult pathways, although it is now appreciated that early development occurs in a low-oxygen environment. In past decades, a novel family of putative insulin mediators, inositol phosphoglycans (IPGs), was discovered. These molecules act as allosteric activators and/or inhibitors of enzymes and transduction proteins involved in the control of cell signalling and metabolic pathways, and determine the specificity of responses after activation of the insulin receptor. One member, IPG P-type, activates pyruvate dehydrogenase phosphatase (PDH-Pase), glycogen synthase phosphatase, and glycerol-3-phosphate acyltransferase. Activation of key phosphatases play a major role in the regulation of glucose disposal by oxidative metabolism via PDH, and the non-oxidative storage by glycogen synthesis, both pathways classically known to be regulated by insulin. High concentrations of IPG P-type in amniotic fluid suggest a role in the regulation of carbohydrate metabolism in the fetal-placental unit. Glycogen accumulation in the syncytiotrophoblast also occurs in preeclamptic pregnancies, and is consistently associated with higher placental levels of IPG P-type. Here, we explore the relationship between nutrients provided by the endometrial glands during early pregnancy, IPG P-type and fetal metabolic requirements. We also discuss whether a disconnect between the placental/fetal metabolic state and oxygen tension could lead to a preeclamptic-type syndrome via leakage of Warburg/IPG mediators into the maternal circulation.

Comparison of three methods for isolation of urinary microvesicles to identify biomarkers of nephrotic syndrome

Urinary microvesicles, such as 40-100 nm exosomes and 100-1000 nm microparticles, contain many proteins that may serve as biomarkers of renal disease. Microvesicles have been isolated by ultracentrifugation or nanomembrane ultrafiltration from normal urine; however, little is known about the efficiency of these methods in isolating microvesicles from patients with nephrotic-range proteinuria. Here we compared three techniques to isolate microvesicles from nephrotic urine: nanomembrane ultrafiltration, ultracentrifugation, and ultracentrifugation followed by size-exclusion chromatography (UC-SEC). Highly abundant urinary proteins were still present in sufficient quantity after ultrafiltration or ultracentrifugation to blunt detection of less abundant microvesicular proteins by MALDI-TOF-TOF mass spectrometry. The microvesicular markers neprilysin, aquaporin-2, and podocalyxin were highly enriched following UC-SEC compared with preparations by ultrafiltration or ultracentrifugation alone. Electron microscopy of the UC-SEC fractions found microvesicles of varying size, compatible with the presence of both exosomes and microparticles. Thus, UC-SEC following ultracentrifugation to further enrich and purify microparticles facilitates the search for prognostic biomarkers that might be used to predict the clinical course of nephrotic syndrome.

Role of metallothionein in cadmium traffic and toxicity in kidneys and other mammalian organs

Metallothioneins are cysteine-rich, small metal-binding proteins present in various mammalian tissues. Of the four common metallothioneins, MT-1 and MT-2 (MTs) are expressed in most tissues, MT-3 is predominantly present in brain, whereas MT-4 is restricted to the squamous epithelia. The expression of MT-1 and MT-2 in some organs exhibits sex, age, and strain differences, and inducibility with a variety of stimuli. In adult mammals, MTs have been localized largely in the cell cytoplasm, but also in lysosomes, mitochondria and nuclei. The major physiological functions of MTs include homeostasis of essential metals Zn and Cu, protection against cytotoxicity of Cd and other toxic metals, and scavenging free radicals generated in oxidative stress. The role of MTs in Cd-induced acute and chronic toxicity, particularly in liver and kidneys, is reviewed in more details. In acute toxicity, liver is the primary target, whereas in chronic toxicity, kidneys are major targets of Cd. The intracellular MTs bind Cd ions and form CdMT. In chronic intoxication, Cd stimulates de novo synthesis of MTs; it is assumed that toxicity in the cells starts when loading with Cd ions exceeds the buffering capacity of intracellular MTs. CdMT, released from the Cd-injured organs, or when applied parenterally for experimental purposes, reaches the kidneys via circulation, where it is filtered, endocytosed in the proximal tubule cells, and degraded in lysosomes. Liberated Cd can immediately affect the cell structures and functions. The resulting proteinuria and CdMT in the urine can be used as biomarkers of tubular injury.

The water channel aquaporin-1 partitions into exosomes during reticulocyte maturation: implication for the regulation of cell volume

Aquaporin-1 (AQP-1), the universal water channel, is responsible for rapid response of cell volume to changes in plasma tonicity. In the membrane of the red cell the concentration of the protein is tightly controlled. Here, we show that AQP-1 is partially lost during in vitro maturation of mouse reticulocytes and that it is associated with exosomes, released throughout this process. AQP-1 in young reticulocytes localizes to the plasma membrane and also in endosomal compartments and exosomes, formed both in vitro and in vivo. During maturation a part of the total pool of AQP-1 is differentially sorted and released via the exosomal pathway. A proteasome inhibitor, MG132, suppresses secretion of AQP-1, implying that ubiquitination is a sorting signal for its release. We further show that modulation of medium tonicity in vitro regulates the secretion of AQP-1, thus showing that extracellular osmotic conditions can drive sorting of selected proteins by the exosomal pathway. These results lead us to suggest that AQP-1 sorting into exosomes may be the mechanism by which the reticulocyte adapts to environmental changes during its maturation.

Proteomic analysis of human parotid gland exosomes by multidimensional protein identification technology (MudPIT)

Human ductal saliva contributes over a thousand unique proteins to whole oral fluids. The mechanism by which most of these proteins are secreted by salivary glands remains to be determined. The present study used a mass spectrometry-based, shotgun proteomics approach to explore the possibility that a subset of the proteins found in saliva are derived from exosomes, membrane-bound vesicles of endosomal origin within multivesicular endosomes. Using MudPIT (multidimensional protein identification technology) mass spectrometry, we catalogued 491 proteins in the exosome fraction of human parotid saliva. Many of these proteins were previously observed in ductal saliva from parotid glands (265 proteins). Furthermore, 72 of the proteins in parotid exosomes overlap with those previously identified as urinary exosome proteins, proteins which are also frequently associated with exosomes from other tissues and cell types. Gene Ontology (GO) and KEGG pathway analyses found that cytosolic proteins comprise the largest category of proteins in parotid exosomes (43%), involved in such processes as phosphatidylinositol signaling system, calcium signaling pathway, inositol metabolism, protein export, and signal transduction, among others; whereas the integral plasma membrane proteins and associated/peripheral plasma membrane proteins (26%) were associated with extracellular matrix-receptor interaction, epithelial cell signaling, T-cell and B-cell receptor signaling, cytokine receptor interaction, and antigen processing and presentation, among other biological functions. In addition, these putative saliva exosomal proteins were linked to specific diseases (e.g., neurodegenerative disorders, prion disease, cancers, type I and II diabetes). Consequently, parotid glands secrete exosomes that reflect the metabolic and functional status of the gland and may also carry informative protein markers useful in the diagnosis and treatment of systemic diseases.