Lysosome-related organelles: a view from immunity and pigmentation

The melano-macrophage: The black leukocyte of fish immunity

Melanin and the process of melanin synthesis or melanogenesis have central roles in the immune system of insects, and production of melanin-synthesizing enzymes from their haemocytes may be induced following activation through danger signals. Melanin-containing macrophage-like cells have been extensively studied in amphibians and they are also present in reptiles. In fish, melano-macrophages are especially recognized with respect to melano-macrophage centres (MMCs), hypothesized to be analogues of germinal centres in secondary lymphoid organs of mammals and some birds. Melano-macrophages are in addition present in several inflammatory conditions, in particular melanised focal changes, or black spots, in the musculature of farmed Atlantic salmon, Salmo salar. Melanins are complex compounds that may be divided into different forms which all have the ability to absorb and scatter light. Other functions include the quenching of free radicals and a direct effect on the immune system. According to the common view held in the pigment cell community, vertebrate melanin synthesis with melanosome formation may only occur in cells of ectodermal origin. However, abundant information suggests that also myeloid cells of ectothermic vertebrates may be classified as melanocytes. Here, we discuss these opposing views and review relevant literature. Finally, we review the current status on the research concerning melanised focal muscle changes that represent the most severe quality problem in Norwegian salmon production, but also other diseases where melano-macrophages play important roles.

Collaboration between a cis-interacting natural killer cell receptor and membrane sphingolipid is critical for the phagocyte function

Inhibitory natural killer (NK) cell receptors recognize MHC class I (MHC-I) in trans on target cells and suppress cytotoxicity. Some NK cell receptors recognize MHC-I in cis, but the role of this interaction is uncertain. Ly49Q, an atypical Ly49 receptor expressed in non-NK cells, binds MHC-I in cis and mediates chemotaxis of neutrophils and type I interferon production by plasmacytoid dendritic cells. We identified a lipid-binding motif in the juxtamembrane region of Ly49Q and found that Ly49Q organized functional membrane domains comprising sphingolipids via sulfatide binding. Ly49Q recruited actin-remodeling molecules to an immunoreceptor tyrosine-based inhibitory motif, which enabled the sphingolipid-enriched membrane domain to mediate complicated actin remodeling at the lamellipodia and phagosome membranes during phagocytosis. Thus, Ly49Q facilitates integrative regulation of proteins and lipid species to construct a cell type-specific membrane platform. Other Ly49 members possess lipid binding motifs; therefore, membrane platform organization may be a primary role of some NK cell receptors.

The Innate Immune Response to Infection by Polyascus gregaria in the Male Chinese Mitten Crab (Eriocheir sinensis), Revealed by Proteomic Analysis

The Chinese mitten crab (Eriocheir sinensis) is a representative catadromous invertebrate of the Yangtze River and a commercial species widely cultivated in China. Both cultivated and wild crabs suffer from a variety of parasites and pathogens, which can result in catastrophic economic losses in aquaculture revenue. Polyascus gregaria, a parasitic barnacle with a highly derived morphology, is specialized in invading these crabs. This study examines the immunological mechanism in E. sinensis infected with P. gregaria. Tandem mass tags (TMT), a specialized method of mass-spectrometry, was used to analyze the infection by P. gregaria resistance at the protein level. In the hepatopancreas of infected crabs, 598 proteins differentially expressed relating to physiological change, of which, 352 were upregulated and 246 were downregulated. Based on this differential protein expression, 104 GO terms and 13 KEGG pathways were significantly enriched. Differentially expressed proteins, such as ATG, cathepsin, serpin, iron-related protein, Rab family, integrin, and lectin, are associated with the lysosome GO term and the autophagy-animal KEGG pathways, both of which likely relate to the immune response to the parasitic P. gregaria infection. These results show the benefit of taking a detailed, protein-level approach to understanding the innate immune response of aquatic invertebrates to macroparasite infection.

Melanogenesis Connection with Innate Immunity and Toll-Like Receptors

The epidermis is located in the outermost layer of the living body and is the place where external stimuli such as ultraviolet rays and microorganisms first come into contact. Melanocytes and melanin play a wide range of roles such as adsorption of metals, thermoregulation, and protection from foreign enemies by camouflage. Pigmentary disorders are observed in diseases associated with immunodeficiency such as Griscelli syndrome, indicating molecular sharing between immune systems and the machineries of pigment formation. Melanocytes express functional toll-like receptors (TLRs), and innate immune stimulation via TLRs affects melanin synthesis and melanosome transport to modulate skin pigmentation. TLR2 enhances melanogenetic gene expression to augment melanogenesis. In contrast, TLR3 increases melanosome transport to transfer to keratinocytes through Rab27A, the responsible molecule of Griscelli syndrome. TLR4 and TLR9 enhance tyrosinase expression and melanogenesis through p38 MAPK (mitogen-activated protein kinase) and NFκB signaling pathway, respectively. TLR7 suppresses microphthalmia-associated transcription factor (MITF), and MITF reduction leads to melanocyte apoptosis. Accumulating knowledge of the TLRs function of melanocytes has enlightened the link between melanogenesis and innate immune system.

Ultrastructural evidence of the evolutional process in malakoplakia

CONTEXT

Malakoplakia can be caused by incomplete digestion of Escherichia coli by lysosomes, leading to recurrent urinary tract infections and consequential mass-forming events that mimic tumors.

OBJECTIVES

By using ultrastructural findings, we aimed to specify the process of phagolysosome to evoke malakoplakia.

DESIGN

We observed a series of processes to form a peculiar Michaelis-Gutmann (MG) body in three patients with malakoplakia and compared with xanthogranulomatous pyelonephritis.

RESULTS

The ultrastructural findings were realigned according to the sequence of events as pre-phagosomal, phagosomal, and post-phagosomal stages. For the mature MG body, numerous lysosomal aggregates targeting pathogens and subsequent incomplete digestion are prerequisite factors for the pre-phagosomal stage. Scattered lamellated residue is late evidence of the pre-phagosomal stage. Phagosomes can be formed by the fusion of multiple pathogens and multiple lysosomes. We utilized transmission and scanning electron microscopy to speculate on the process of phagolysosomal formation.

CONCLUSION

The recognition of E. coli captured by phagosomes or partially damaged by lysosomal attack within the cell was recorded for the first time. Furthermore, SEM observation was performed on human tissue.

Lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a group of over 70 diseases that are characterized by lysosomal dysfunction, most of which are inherited as autosomal recessive traits. These disorders are individually rare but collectively affect 1 in 5,000 live births. LSDs typically present in infancy and childhood, although adult-onset forms also occur. Most LSDs have a progressive neurodegenerative clinical course, although symptoms in other organ systems are frequent. LSD-associated genes encode different lysosomal proteins, including lysosomal enzymes and lysosomal membrane proteins. The lysosome is the key cellular hub for macromolecule catabolism, recycling and signalling, and defects that impair any of these functions cause the accumulation of undigested or partially digested macromolecules in lysosomes (that is, 'storage') or impair the transport of molecules, which can result in cellular damage. Consequently, the cellular pathogenesis of these diseases is complex and is currently incompletely understood. Several LSDs can be treated with approved, disease-specific therapies that are mostly based on enzyme replacement. However, small-molecule therapies, including substrate reduction and chaperone therapies, have also been developed and are approved for some LSDs, whereas gene therapy and genome editing are at advanced preclinical stages and, for a few disorders, have already progressed to the clinic.

Lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a group of over 70 diseases that are characterized by lysosomal dysfunction, most of which are inherited as autosomal recessive traits. These disorders are individually rare but collectively affect 1 in 5,000 live births. LSDs typically present in infancy and childhood, although adult-onset forms also occur. Most LSDs have a progressive neurodegenerative clinical course, although symptoms in other organ systems are frequent. LSD-associated genes encode different lysosomal proteins, including lysosomal enzymes and lysosomal membrane proteins. The lysosome is the key cellular hub for macromolecule catabolism, recycling and signalling, and defects that impair any of these functions cause the accumulation of undigested or partially digested macromolecules in lysosomes (that is, 'storage') or impair the transport of molecules, which can result in cellular damage. Consequently, the cellular pathogenesis of these diseases is complex and is currently incompletely understood. Several LSDs can be treated with approved, disease-specific therapies that are mostly based on enzyme replacement. However, small-molecule therapies, including substrate reduction and chaperone therapies, have also been developed and are approved for some LSDs, whereas gene therapy and genome editing are at advanced preclinical stages and, for a few disorders, have already progressed to the clinic.

Imaging FITC-dextran as a Reporter for Regulated Exocytosis

Regulated exocytosis is a process by which cargo, which is stored in secretory granules (SGs), is released in response to a secretory trigger. Regulated exocytosis is fundamental for intercellular communication and is a key mechanism for the secretion of neurotransmitters, hormones, inflammatory mediators, and other compounds, by a variety of cells. At least three distinct mechanisms are known for regulated exocytosis: full exocytosis, where a single SG fully fuses with the plasma membrane, kiss-and-run exocytosis, where a single SG transiently fuses with the plasma membrane, and compound exocytosis, where several SGs fuse with each other, prior to or after SG fusion with the plasma membrane. The type of regulated exocytosis undertaken by a cell is often dictated by the type of secretory trigger. However, in many cells, a single secretory trigger can activate multiple modes of regulated exocytosis simultaneously. Despite their abundance and importance across cell types and species, the mechanisms that determine the different modes of secretion are largely unresolved. One of the main challenges in investigating the different modes of regulated exocytosis, is the difficulty in distinguishing between them as well as exploring them separately. Here we describe the use of fluorescein isothiocyanate (FITC)-dextran as an exocytosis reporter, and live cell imaging, to differentiate between the different pathways of regulated exocytosis, focusing on compound exocytosis, based on the robustness and duration of the exocytic events.

Effects of a non-cyclodextrin cyclic carbohydrate on mouse melanoma cells: Characterization of a new type of hypopigmenting sugar

Cyclic nigerosyl nigerose (CNN) is a cyclic tetrasaccharide that exhibits properties distinct from other conventional cyclodextrins. Herein, we demonstrate that treatment of B16 melanoma with CNN results in a dose-dependent decrease in melanin synthesis, even under conditions that stimulate melanin synthesis, without significant cytotoxity. The effects of CNN were prolonged for more than 27 days, and were gradually reversed following removal of CNN. Undigested CNN was found to accumulate within B16 cells at relatively high levels. Further, CNN showed a weak but significant direct inhibitory effect on the enzymatic activity of tyrosinase, suggesting one possible mechanism of hypopigmentation. While a slight reduction in tyrosinase expression was observed, tyrosinase expression was maintained at significant levels, processed into a mature form, and transported to late-stage melanosomes. Immunocytochemical analysis demonstrated that CNN treatment induced drastic morphological changes of Pmel17-positive and LAMP-1-positive organelles within B16 cells, suggesting that CNN is a potent organelle modulator. Colocalization of both tyrosinase-positive and LAMP-1-positive regions in CNN-treated cells indicated possible degradation of tyrosinase in LAMP-1-positive organelles; however, that possibility was ruled out by subsequent inhibition experiments. Taken together, this study opens a new paradigm of functional oligosaccharides, and offers CNN as a novel hypopigmenting molecule and organelle modulator.

Preferential secretion of inducible HSP70 by vitiligo melanocytes under stress

Inducible HSP70 (HSP70i) chaperones peptides from stressed cells, protecting them from apoptosis. Upon extracellular release, HSP70i serves an adjuvant function, enhancing immune responses to bound peptides. We questioned whether HSP70i differentially protects control and vitiligo melanocytes from stress and subsequent immune responses. We compared expression of HSP70i in skin samples, evaluated the viability of primary vitiligo and control melanocytes exposed to bleaching phenols, and measured secreted HSP70i. We determined whether HSP70i traffics to melanosomes to contact immunogenic proteins by cell fractionation, western blotting, electron microscopy, and confocal microscopy. Viability of vitiligo and control melanocytes was equally affected under stress. However, vitiligo melanocytes secreted increased amounts of HSP70i in response to MBEH, corroborating with aberrant HSP70i expression in patient skin. Intracellular HSP70i colocalized with melanosomes, and more so in response to MBEH in vitiligo melanocytes. Thus, whereas either agent is cytotoxic to melanocytes, MBEH preferentially induces immune responses to melanocytes.

Reprogramming of lysosomal gene expression by interleukin-4 and Stat6

Background

Lysosomes play important roles in multiple aspects of physiology, but the problem of how the transcription of lysosomal genes is coordinated remains incompletely understood. The goal of this study was to illuminate the physiological contexts in which lysosomal genes are coordinately regulated and to identify transcription factors involved in this control.

Results

As transcription factors and their target genes are often co-regulated, we performed meta-analyses of array-based expression data to identify regulators whose mRNA profiles are highly correlated with those of a core set of lysosomal genes. Among the ~50 transcription factors that rank highest by this measure, 65% are involved in differentiation or development, and 22% have been implicated in interferon signaling. The most strongly correlated candidate was Stat6, a factor commonly activated by interleukin-4 (IL-4) or IL-13. Publicly available chromatin immunoprecipitation (ChIP) data from alternatively activated mouse macrophages show that lysosomal genes are overrepresented among Stat6-bound targets. Quantification of RNA from wild-type and Stat6-deficient cells indicates that Stat6 promotes the expression of over 100 lysosomal genes, including hydrolases, subunits of the vacuolar H⁺ ATPase and trafficking factors. While IL-4 inhibits and activates different sets of lysosomal genes, Stat6 mediates only the activating effects of IL-4, by promoting increased expression and by neutralizing undefined inhibitory signals induced by IL-4.

Conclusions

The current data establish Stat6 as a broadly acting regulator of lysosomal gene expression in mouse macrophages. Other regulators whose expression correlates with lysosomal genes suggest that lysosome function is frequently re-programmed during differentiation, development and interferon signaling.

WASH is required for lysosomal recycling and efficient autophagic and phagocytic digestion

Wiskott-Aldrich syndrome protein and SCAR homologue (WASH) is an important regulator of vesicle trafficking. By generating actin on the surface of intracellular vesicles, WASH is able to directly regulate endosomal sorting and maturation. We report that, in Dictyostelium, WASH is also required for the lysosomal digestion of both phagocytic and autophagic cargo. Consequently, Dictyostelium cells lacking WASH are unable to grow on many bacteria or to digest their own cytoplasm to survive starvation. WASH is required for efficient phagosomal proteolysis, and proteomic analysis demonstrates that this is due to reduced delivery of lysosomal hydrolases. Both protease and lipase delivery are disrupted, and lipid catabolism is also perturbed. Starvation-induced autophagy therefore leads to phospholipid accumulation within WASH-null lysosomes. This causes the formation of multilamellar bodies typical of many lysosomal storage diseases. Mechanistically, we show that, in cells lacking WASH, cathepsin D becomes trapped in a late endosomal compartment, unable to be recycled to nascent phagosomes and autophagosomes. WASH is therefore required for the maturation of lysosomes to a stage at which hydrolases can be retrieved and reused.

Transcription of the tyrosinase gene family in an Atlantic salmon leukocyte cell line (SHK-1) is influenced by temperature, but not by virus infection or bacterin stimulation

The present study was performed to address putative links between the immune and pigmentary systems. A pigment-producing leukocyte-like cell-line (SHK-1 cells) of Atlantic salmon (Salmo salar L.) was exposed to different temperatures, poly I:C, bacterin or infected with virus (infectious pancreatic necrosis virus or infectious salmon anaemia virus). The effect of this stimulation regarding the transcription-pattern of the tyrosinase gene family (melanin genes) and the immune-related genes MHC class II and IFN-1 was analysed using real-time RT-qPCR. At 10°C cultivation, tyrosinase and dopachrome tautomerase remained unregulated. At 15°C, a moderate up-regulation was induced, while at 20°C, these genes were up-regulated in an exponential manner over time. Temperature did not affect the transcription of the immune-related genes. Virus infections, poly I:C or bacterin had no influence on the transcription of the melanogenesis-related genes, but triggered the immune-related genes. Our findings revealed no connections between the pigmentary and immune systems, but demonstrated a hereto undiscovered temperature-effect on the tyrosinase gene family.

The intracellular Ca²⁺ channels of membrane traffic

Regulation of organellar fusion and fission by Ca ( 2+) has emerged as a central paradigm in intracellular membrane traffic. Originally formulated for Ca ( 2+) -driven SNARE-mediated exocytosis in the presynaptic terminals, it was later expanded to explain membrane traffic in other exocytic events within the endo-lysosomal system. The list of processes and conditions that depend on the intracellular membrane traffic includes aging, antigen and lipid processing, growth factor signaling and enzyme secretion. Characterization of the ion channels that regulate intracellular membrane fusion and fission promises novel pharmacological approaches in these processes when their function becomes aberrant. The recent identification of Ca ( 2+) permeability through the intracellular ion channels comprising the mucolipin (TRPMLs) and the two-pore channels (TPCs) families pinpoints the candidates for the Ca ( 2+) channel that drive intracellular membrane traffic. The present review summarizes the recent developments and the current questions relevant to this topic.

Pigment-producing granulomatous myopathy in Atlantic salmon: a novel inflammatory response

Melanin comprises a complex group of pigmented polymers whose primary function is ascribed to dermal solar protection, but may also have an interesting role in innate immunity. In ectothermic vertebrates, melanogenesis is reported in leukocyte populations, but it is not known if this occurs in connection with inflammatory reactions. Melanin accumulations in ectopic locations, in particular muscle, represent a serious quality problem in salmon production. Here, we investigated such changes for the expression of dopachrome tautomerase and tyrosinase as well as some important immune genes and pathogens. Furthermore, the nature of the pathological changes was addressed by morphological methods. Gene transcripts encoding key enzymes in melanogenesis, suggesting a de novo melanin synthesis in pigmented muscle, were found. MHC class II transcripts were up-regulated and there was no indication of bacterial or viral infection. The histological examination revealed granulomatous inflammation with distribution of MHC class II positive cells and T cells, analogous to the pattern found in mammals. Importantly, in contrast to mammals pigmented cells were contributing in the inflammation. We demonstrate that melanin production occurs in granulomatous inflammation in salmon, revealing a close and hitherto unreported link between the pigmentary and immune systems.

Extracellular vesicles and their convergence with viral pathways

Extracellular vesicles (microvesicles), such as exosomes and shed microvesicles, contain a variety of molecules including proteins, lipids, and nucleic acids. Microvesicles appear mostly to originate from multivesicular bodies or to bud from the plasma membrane. Here, we review the convergence of microvesicle biogenesis and aspects of viral assembly and release pathways. Herpesviruses and retroviruses, amongst others, recruit several elements from the microvesicle biogenesis pathways for functional virus release. In addition, noninfectious pleiotropic virus-like vesicles can be released, containing viral and cellular components. We highlight the heterogeneity of microvesicle function during viral infection, addressing microvesicles that can either block or enhance infection, or cause immune dysregulation through bystander action in the immune system. Finally, endogenous retrovirus and retrotransposon elements deposited in our genomes millions of years ago can be released from cells within microvesicles, suggestive of a viral origin of the microvesicle system or perhaps of an evolutionary conserved system of virus-vesicle codependence. More research is needed to further elucidate the complex function of the various microvesicles produced during viral infection, possibly revealing new therapeutic intervention strategies.

Linking exocytosis and endocytosis during phagocytosis

Phagocytosis is used by macrophages, dendritic cells and neutrophils to capture and destroy pathogens and particulate antigens. Although localized assembly of actin filaments is the driving force for particle internalization, exocytosis of intracellular compartments, and in particular endocytic compartments, has been shown recently to be required for the early steps of phagosome formation. Here we report on the different compartments undergoing exocytosis during phagocytosis, with a special focus on late endosomes. We then compare this process with secretion from lysosomes or lysosome-related organelles in specialized cells. Finally, we discuss how some of the molecular mechanisms responsible for lysosome-related organelle secretion could also be implicated in phagosome formation.

Molecular mechanisms of endolysosomal Ca2+ signalling in health and disease

Endosomes, lysosomes and lysosome-related organelles are emerging as important Ca2+ storage cellular compartments with a central role in intracellular Ca2+ signalling. Endocytosis at the plasma membrane forms endosomal vesicles which mature to late endosomes and culminate in lysosomal biogenesis. During this process, acquisition of different ion channels and transporters progressively changes the endolysosomal luminal ionic environment (e.g. pH and Ca2+) to regulate enzyme activities, membrane fusion/fission and organellar ion fluxes, and defects in these can result in disease. In the present review we focus on the physiology of the inter-related transport mechanisms of Ca2+ and H+ across endolysosomal membranes. In particular, we discuss the role of the Ca2+-mobilizing messenger NAADP (nicotinic acid adenine dinucleotide phosphate) as a major regulator of Ca2+ release from endolysosomes, and the recent discovery of an endolysosomal channel family, the TPCs (two-pore channels), as its principal intracellular targets. Recent molecular studies of endolysosomal Ca2+ physiology and its regulation by NAADP-gated TPCs are providing exciting new insights into the mechanisms of Ca2+-signal initiation that control a wide range of cellular processes and play a role in disease. These developments underscore a new central role for the endolysosomal system in cellular Ca2+ regulation and signalling.

Role of AP1 and Gadkin in the traffic of secretory endo-lysosomes

Whereas lysosome-related organelles (LRO) of specialized cells display both exocytic and endocytic features, lysosomes in nonspecialized cells can also acquire the property to fuse with the plasma membrane upon an acute rise in cytosolic calcium. Here, we characterize this unconventional secretory pathway in fibroblast-like cells, by monitoring the appearance of Lamp1 on the plasma membrane and the release of lysosomal enzymes into the medium. After sequential ablation of endocytic compartments in living cells, we find that donor membranes primarily derive from a late compartment, but that an early compartment is also involved. Strikingly, this endo-secretory process is not affected by treatments that inhibit endosome dynamics (microtubule depolymerization, cholesterol accumulation, overexpression of Rab7 or its effector Rab-interacting lysosomal protein [RILP], overexpression of Rab5 mutants), but depends on Rab27a, a GTPase involved in LRO secretion, and is controlled by F-actin. Moreover, we find that this unconventional endo-secretory pathway requires the adaptor protein complexes AP1, Gadkin (which recruits AP1 by binding to the γ1 subunit), and AP2, but not AP3. We conclude that a specific fraction of the AP2-derived endocytic pathway is dedicated to secretory purposes under the control of AP1 and Gadkin.

Small molecule Toll-like receptor 7 agonists localize to the MHC class II loading compartment of human plasmacytoid dendritic cells

TLR7 and TLR8 are intracellular sensors activated by single-stranded RNA species generated during viral infections. Various synthetic small molecules can also activate TLR7 or TLR8 or both through an unknown mechanism. Notably, direct interaction between small molecules and TLR7 or TLR8 has never been shown. To shed light on how small molecule agonists target TLRs, we labeled 2 imidazoquinolines, resiquimod and imiquimod, and one adenine-based compound, SM360320, with 2 different fluorophores [5(6) carboxytetramethylrhodamine and Alexa Fluor 488] and monitored their intracellular localization in human plasmacytoid dendritic cells (pDCs). All fluorescent compounds induced the production of IFN-α, TNF-α, and IL-6 and the up-regulation of CD80 and CD86 by pDCs showing they retained TLR7-stimulating activity. Confocal imaging of pDCs showed that, similar to CpG-B, all compounds concentrated in the MHC class II loading compartment (MIIC), identified as lysosome-associated membrane protein 1+, CD63, and HLA-DR+ endosomes. Treatment of pDCs with bafilomycin A, an antagonist of the vacuolar-type proton ATPase controlling endosomal acidification, prevented the accumulation of small molecule TLR7 agonists, but not of CpG-B, in the MIIC. These results indicate that a pH-driven concentration of small molecule TLR7 agonists in the MIIC is required for pDC activation.

Rab36 regulates the spatial distribution of late endosomes and lysosomes through a similar mechanism to Rab34

Small GTPase Rab36 is homologous to Rab34 with 56% amino acid sequence identity. Rab34 was characterized as a Golgi-associated Rab protein and regulates lysosomal positioning through interaction with RILP; however, the properties and functions of Rab36 have not been investigated. To investigate Rab36, we constructed EGFP-Rab36 wild type, the active GTP-bound mutant EGFP-Rab36Q116L and negative GDP-bound mutant EGFP-Rab36T71N. Expression of EGFP-Rab36 wild type revealed that Rab36 co-localized with Golgi markers GM130, Syntaxin 5 and TGN46 in Hela cells, indicating Rab36 is associated with Golgi apparatus. Over-expression of Rab36 induced late endosome and lysosome clustering around the Golgi apparatus, marked by LBPA, CD63, Lamp1 and Lamp2, without effects on early endosomal compartment marked by EEA1. GST-pulldown assay demonstrated that Rab36 can also interact with RILP. In addition, the binding region for Rab36 is in the C-terminal region (aa199-401) of RILP. Our data suggested that Rab36 may regulate the spatial distribution of late endosomes and lysosomes through a similar mechanism to Rab34.

Proteomics analysis of A33 immunoaffinity-purified exosomes released from the human colon tumor cell line LIM1215 reveals a tissue-specific protein signature

Exosomes are 40-100-nm-diameter nanovesicles of endocytic origin that are released from diverse cell types. To better understand the biological role of exosomes and to avoid confounding data arising from proteinaceous contaminants, it is important to work with highly purified material. Here, we describe an immunoaffinity capture method using the colon epithelial cell-specific A33 antibody to purify colorectal cancer cell (LIM1215)-derived exosomes. LC-MS/MS revealed 394 unique exosomal proteins of which 112 proteins (28%) contained signal peptides and a significant enrichment of proteins containing coiled coil, RAS, and MIRO domains. A comparative protein profiling analysis of LIM1215-, murine mast cell-, and human urine-derived exosomes revealed a subset of proteins common to all exosomes such as endosomal sorting complex required for transport (ESCRT) proteins, tetraspanins, signaling, trafficking, and cytoskeletal proteins. A conspicuous finding of this comparative analysis was the presence of host cell-specific (LIM1215 exosome) proteins such as A33, cadherin-17, carcinoembryonic antigen, epithelial cell surface antigen (EpCAM), proliferating cell nuclear antigen, epidermal growth factor receptor, mucin 13, misshapen-like kinase 1, keratin 18, mitogen-activated protein kinase 4, claudins (1, 3, and 7), centrosomal protein 55 kDa, and ephrin-B1 and -B2. Furthermore, we report the presence of the enzyme phospholipid scramblase implicated in transbilayer lipid distribution membrane remodeling. The LIM1215-specific exosomal proteins identified in this study may provide insights into colon cancer biology and potential diagnostic biomarkers.

Involvement of ABC transporters in melanogenesis and the development of multidrug resistance of melanoma

Because melanomas are intrinsically resistant to conventional radiotherapy and chemotherapy, many alternative treatment approaches have been developed such as biochemotherapy and immunotherapy. The most common cause of multidrug resistance (MDR) in human cancers is the expression and function of one or more ATP-binding cassette (ABC) transporters that efflux anticancer drugs from cells. Melanoma cells express a group of ABC transporters (such as ABCA9, ABCB1, ABCB5, ABCB8, ABCC1, ABCC2, and ABCD1) that may be associated with the resistance of melanoma cells to a broad range of anticancer drugs and/or of melanocytes to toxic melanin intermediates and metabolites. In this review, we propose a model (termed the ABC-M model) in which the intrinsic MDR of melanoma cells is at least in part because of the transporter systems that may also play a critical role in reducing the cytotoxicity of the melanogenic pathway in melanocytes. The ABC-M model suggests molecular strategies to reverse MDR function in the context of the melanogenic pathway, which could open therapeutic avenues towards the ultimate goal of circumventing clinical MDR in patients with melanoma.

GpnmbR150X allele must be present in bone marrow derived cells to mediate DBA/2J glaucoma

BACKGROUND

The Gpnmb gene encodes a transmembrane protein whose function(s) remain largely unknown. Here, we assess if a mutant allele of Gpnmb confers susceptibility to glaucoma by altering immune functions. DBA/2J mice have a mutant Gpnmb gene and they develop a form of glaucoma preceded by a pigment dispersing iris disease and abnormalities of the immunosuppressive ocular microenvironment.

RESULTS

We find that the Gpnmb genotype of bone-marrow derived cell lineages significantly influences the iris disease and the elevation of intraocular pressure. GPNMB localizes to multiple cell types, including pigment producing cells, bone marrow derived F4/80 positive antigen-presenting cells (APCs) of the iris and dendritic cells. We show that APCs of DBA/2J mice fail to induce antigen induced immune deviation (a form of tolerance) when treated with TGFbeta2. This demonstrates that some of the immune abnormalities previously identified in DBA/2J mice result from intrinsic defects in APCs. However, the tested APC defects are not dependent on a mutant Gpnmb gene. Finally, we show that the Gpnmb mediated iris disease does not require elevated IL18 or mature B or T lymphocytes.

CONCLUSION

These results establish a role for Gpnmb in bone marrow derived lineages. They suggest that affects of Gpnmb on innate immunity influence susceptibility to glaucoma in DBA/2J mice.

The yeast protein sorting pathway as an experimental model for lysosomal trafficking

Lysosomes are conserved organelles that are present in all eukaryotic cells. They are part of a complicated network of intracellular trafficking routes - the lysosomal transport system. Lysosomes are necessary for the maintenance of cellular homeostasis and for many specialized functions, including the activity of many components of the mammalian immune system. Dysfunctions of the lysosomal system are associated with numerous diseases, such as storage disorders, neuro- and myopathies, cancer and some types of albinism and immunological deficiencies. High conservation of the processes of lysosomal biogenesis and transport enables the use of yeast as a model for studying the mechanisms that underlie these diseases. In this review, we discuss several examples of such models in an attempt to present an overview of the most important experimental methods available in yeast research.

The Drosophila Pigmentation Gene pink (p) Encodes a Homologue of Human Hermansky-Pudlak Syndrome 5 (HPS5)

Lysosome-related organelles comprise a group of specialized intracellular compartments that include melanosomes and platelet dense granules (in mammals) and eye pigment granules (in insects). In humans, the biogenesis of these organelles is defective in genetic disorders collectively known as Hermansky-Pudlak syndrome (HPS). Patients with HPS-2, and two murine HPS models, carry mutations in genes encoding subunits of adaptor protein (AP)-3. Other genes mutated in rodent models include those encoding VPS33A and Rab38. Orthologs of all of these genes in Drosophila melanogaster belong to the 'granule group' of eye pigmentation genes. Other genes associated with HPS encode subunits of three complexes of unknown function, named biogenesis of lysosome-related organelles complex (BLOC)-1, -2 and -3, for which the Drosophila counterparts had not been characterized. Here, we report that the gene encoding the Drosophila ortholog of the HPS5 subunit of BLOC-2 is identical to the granule group gene pink (p), which was first studied in 1910 but had not been identified at the molecular level. The phenotype of pink mutants was exacerbated by mutations in AP-3 subunits or in the orthologs of VPS33A and Rab38. These results validate D. melanogaster as a genetic model to study the function of the BLOCs.

TRPML cation channels regulate the specialized lysosomal compartment of vertebrate B-lymphocytes

B-lymphocytes possess a specialized lysosomal compartment, the regulated transformation of which has been implicated in B-cell antigen presentation. Members of the mucolipin (TRPML) family of cation channels have been implicated in regulated vesicular transport in several tissues, but a role for TRPML function in lymphocyte vesicular transport physiology has not been previously described. To address the role of TRPML proteins in lymphocyte vesicular transport, we analyzed the lysosomal compartment in cultured B-lymphocytes engineered to lack TRPML1 or after expression of N- or C-terminal GFP fusion proteins of TRPML1 or TRPML2. Consistent with previous analyses of lymphocytes derived from human patients with mutations in TRPML1, we were not able to detect abnormalities in the lysosomes of TRPML1-deficient DT40 B-lymphocytes. However, while N-terminal GFP fusions of TRPML2 localized to normal appearing lysosomes, C-terminal GFP fusions of either TRPML1 or TRPML2 acted to antagonize endogenous TRPML function, localizing to large vesicular structures, the histological properties of which were indistinguishable from the enlarged lysosomes observed in affected tissues of TRPML1-deficient humans. Endocytosed B-cell receptors were delivered to these enlarged lysosomes, demonstrating that a TRPML-dependent process is required for normal regulation of the specialized lysosome compartment of vertebrate B-lymphocytes.

The zebrafish mutant vps18 as a model for vesicle-traffic related hypopigmentation diseases

Hypopigmentation is a characteristic of several diseases associated with vesicle traffic defects, like the Hermansky-Pudlak, Chediak-Higashi, and Griscelli syndromes. Hypopigmentation is also a characteristic of the zebrafish mutant vps18(hi2499A), which is affected in the gene vps18, a component of the homotypic fusion and protein sorting complex that is involved in tethering during vesicular traffic. Vps18, as part of this complex, participates in the formation of early endosomes, late endosomes, and lysosomes. Here, we show that Vps18 is also involved in the formation of melanosomes. In the zebrafish mutant vps18(hi2499A) the retroviral insertion located at exon 4 of vps18, leads to the formation of two abnormal splicing variants lacking the coding sequence for the clathrin repeat and the RING finger conserved domains. A deficiency of Vps18 in zebrafish larvae results in hepatomegaly and skin hypopigmentation. We also observed a drastic reduction in the number of melanosomes in the eye's retinal pigmented epithelium along with the accumulation of immature melanosomes. A significant reduction in the vps18(hi2499A) larvae visual system capacity was found using the optokinetic response assay. We propose that the insertional mutant vps18(hi2499A) can be used as a model for studying hypopigmentation diseases in which vesicle traffic problems exist.

Prostasome-derived proteins capable of eliciting an immune response in prostate cancer patients

Prostate cancer consistently remains a difficult clinical enigma. Therefore, the development of novel strategies for diagnosis and treatment (e.g. immunotherapy) of prostate cancer is essential. We tried to identify the prostasome-derived proteins that were immunogenic in prostate cancer patients. Prostate cancer patients' sera (n = 44) with high enzyme-linked immunosorbent assay (ELISA) titers against prostasomes were selected for immunoblotting against purified seminal prostasomes. The SDS-PAGE and immunoblotting experiments were performed with Bio-Rad systems. Twenty-five of the recognized proteins were isolated and analyzed by means of mass spectrometry. Out of 44 patients' sera, 31 (70%) demonstrated in immunoblotting experiments reactivity against several prostasomal protein bands in the molecular weight range of 10-200 kDa. Some of the bands (55, 70 and 170 kDa) were more frequently recognized by the patients' sera. Concomitantly run control sera generated only very weak or no bands at all. The most frequently occurring prostasomal proteins were identified as heat shock proteins (HSP 70, 71) and clusterin. This study identified the most important molecular targets of autoantibodies against prostasomes generated in connection with the development of prostate cancer in man. These immunogenic prostasomal proteins could be appropriate target molecules for specific immunotherapy of prostate cancer patients.

BLOC-1 interacts with BLOC-2 and the AP-3 complex to facilitate protein trafficking on endosomes

The adaptor protein (AP)-3 complex is a component of the cellular machinery that controls protein sorting from endosomes to lysosomes and specialized related organelles such as melanosomes. Mutations in an AP-3 subunit underlie a form of Hermansky-Pudlak syndrome (HPS), a disorder characterized by abnormalities in lysosome-related organelles. HPS in humans can also be caused by mutations in genes encoding subunits of three complexes of unclear function, named biogenesis of lysosome-related organelles complex (BLOC)-1, -2, and -3. Here, we report that BLOC-1 interacts physically and functionally with AP-3 to facilitate the trafficking of a known AP-3 cargo, CD63, and of tyrosinase-related protein 1 (Tyrp1), a melanosomal membrane protein previously thought to traffic only independently of AP-3. BLOC-1 also interacts with BLOC-2 to facilitate Tyrp1 trafficking by a mechanism apparently independent of AP-3 function. Both BLOC-1 and -2 localize mainly to early endosome-associated tubules as determined by immunoelectron microscopy. These findings support the idea that BLOC-1 and -2 represent hitherto unknown components of the endosomal protein trafficking machinery.

Isolation of the Atlantic salmon tyrosinase gene family reveals heterogenous transcripts in a leukocyte cell line

In ectothermic vertebrates, visceral organs harbor melanin-containing cells. Their ability as pigment producers is nevertheless disputed. To address expression of the key genes for melanogenesis in Atlantic salmon (Salmo salar), a tyrosinase-positive leukocyte cell line (SHK-1) and skin were used to obtain full-length tyrosinase (Tyr), tyrosinase-like protein-1 (Tyrp1), and dopachrome tautomerase (Dct) mRNA transcripts. In the SHK-1 cells, two different Tyrp1 transcripts were identified, one lacking exon 1. However, only the full-length version of Tyrp1 was identified in the skin. Sequencing of Tyrp1 genomic region revealed that the two Tyrp1 transcripts might originate from two different loci, possibly a result of pseudo-tetraploidity of the Atlantic salmon genome. Expression of Tyr, Tyrp1 and Dct was investigated by quantitative real-time reverse transcriptase polymerase cain reaction showing highest expression in the SHK-1 cell line and skin, intermediate in pronephros, and negligible or absent in liver and muscle. Histological approaches were used to demonstrate melanin and revealed presence of melanized cells in skin, kidney and liver, and absence of such cells in muscle. In addition to verify melanin synthesis abilities of visceral-located cells, our results indicate loci-specific transcription differences between populations of melanin-producing cells in Atlantic salmon.

Melanogenesis and evidence for melanosome transport to the plasma membrane in a CD83 teleost leukocyte cell line

Visceral organs of ectothermic vertebrates harbour melanin-containing leukocytes termed melanomacrophages. These cells are thought to participate in immune reactions and free-radical trapping. In teleosts, the melanin-producing ability of melanomacrophages has hitherto not been confirmed by molecular techniques. Here, a leukocyte marker and the apparatus for melanosome production and transport were investigated in an Atlantic salmon (Salmo salar) pronephros-derived mononuclear leukocyte (SHK-1) cell line. The SHK-1 cells expressed transcripts specific for a mammalian CD83 homologue, a standard surface marker for activated or differentiated dendritic cells, and dopachrome tautomerase/tyrosinase-related protein-2, a melanocyte specific enzyme essential for melanin production. Reduction potential of melanin or its precursors was demonstrated histochemically after prolonged cultivation. Ultrastructural investigations revealed tyrosinase and acid phosphate activity in identical organelles and BSA-gold co-localized with multilamellar melanosomes after 2 h internalization. Apparently, melanosomes were transported and released through periodically occurring tubules fusing with the plasma membrane. Video monitoring revealed filopodia and macropinocytosis. These results showed that the SHK-1 cell line is capable of melanogenesis and melanosome secretion. Melanin-producing cells in teleost pronephros may represent a distinct CD83(+) leukocyte population consisting of phylogenetically relict multifunctional cells. This is the first report of a melanin-producing leukocyte cell-line.

Subcellular proteomics reveals neuromelanin granules to be a lysosome-related organelle

The powerful combination of subcellular fractionation and protein identification by electrospray ionization tandem mass spectrometry (ESI-MS/MS) pioneered the molecular elucidation of neuromelanin (NM) granules. We recently isolated NM granules from the human brain and succeeded in the establishment of the first protein profile of this compartment. NM granules are pigmented organelles, which are mainly found in the catecholaminergic neurons of the human substantia nigra (SN) pars compacta and the locus coeruleus. These granules contain the insoluble pigment NM, which is regarded as the most important iron storage system in these neurons. A global examination of NM granules, however, has so far been hampered due to the lack of a pigmented brain stem in rodents, the absence of an appropriate experimental system and their scarcity in the human brain. 'Subcellular proteomics', which increasingly emerges as the method of choice to characterize cellular compartments and to elucidate their biogenesis, has recently been shown to be an adequate approach to tackle a thorough description of NM granules. Thereby, NM granules could be described as a 'lysosome-related organelle'. This indicates a genetic program underlying a biogenesis of NM rather than its autoxidative formation.

P-Selectin and CD63 Use Different Mechanisms for Delivery to Weibel-Palade Bodies

The biogenesis of endothelial-specific Weibel-Palade bodies (WPB) is poorly understood, despite their key role in both haemostasis and inflammation. Biogenesis of specialized organelles of haemopoietic cells is often adaptor protein complex 3-dependent (AP-3-dependent), and AP-3 has previously been shown to play a role in the trafficking of both WPB membrane proteins, P-selectin and CD63. However, WPB are thought to form at the trans Golgi network (TGN), which is inconsistent with a role for AP-3, which operates in post-Golgi trafficking. We have therefore investigated in detail the mechanisms of delivery of these two membrane proteins to WPB. We find that P-selectin is recruited to forming WPB in the trans-Golgi by AP-3-independent mechanisms that use sorting information within both the cytoplasmic tail and the lumenal domain of the receptor. In contrast, CD63 is recruited to already-budded WPB by an AP-3-dependent route. These different mechanisms of recruitment lead to the presence of distinct immature and mature populations of WPB in human umbilical vein endothelial cells (HUVEC).

A modified method for purifying amelanotic melanocytes from human hair follicles

We describe a modified method for establishing long-term pure cultures of amelanotic melanocytes (AMMC) derived from human hair follicles. Normal human corpse scalp (just after death, 1 h) was transected 1 mm below the epidermis, and hair follicles in the remaining dermis were isolated by a two-step enzyme treatment. Hair follicle cell suspensions were prepared by 0.50% trypsin treatment for 30 min and cultured in an optimized melanoblast proliferation nature mitogen medium. Cells attached to the substratum were mostly amelanotic melanocytic in character with small, bipolar shapes in the early stage; only a few keratinocytes and rare fibroblasts were observed. Keratinocytes were easily removed by differential trypsinization. After the third passage, the proliferating cells were all amelanotic melanocytes as confirmed by immunostaining with polyclonal antibodies to alphaPEP7h, which recognized the tyrosinase protein located on melanosomes and NKI/beteb, which is a pre-melanosomal antigen against synthetic peptides corresponding to the carboxyl termini of human melanosomal protein GP100. Cultured AMMC were highly positive to L-dopa reactivity after the addition of IBMX to the culture medium for 7 days. Many stage I and II melanosomes and occasional stage III melanosomes without stage IV melanosomes were found in the cytoplasm by transmission electron microscope. This modified technique is potentially more suitable for cultivating amelanotic melanocytes. The availability of pure cultures of hair-follicle amelanotic melanocytes will facilitate investigations of the roles of those cells in migration and differentiation during treatment of vitiligo.

The cell biology of Hermansky-Pudlak syndrome: recent advances

Hermansky-Pudlak syndrome (HPS) defines a group of at least seven autosomal recessive disorders characterized by albinism and prolonged bleeding. These manifestations arise from defects in the biogenesis of lysosome-related organelles, including melanosomes and platelet dense granules. Most genes associated with HPS in humans and rodent models of the disease encode components of multisubunit protein complexes that are expressed ubiquitously and play roles in intracellular protein trafficking and/or organelle distribution. A small GTPase of the Rab family, Rab38, is also implicated in the pathogenesis of the disease. This article reviews recent progress toward elucidating the cellular functions of these proteins.

Conformational Variation of Surface Class II MHC Proteins during Myeloid Dendritic Cell Differentiation Accompanies Structural Changes in Lysosomal MIIC

Dendritic cells (DC), uniquely among APC, express an open/empty conformation of MHC class II (MHC-II) proteins (correctly folded molecules lacking bound peptides). Generation and trafficking of empty HLA-DR during DC differentiation are investigated here. HLA-DR did not fold as an empty molecule in the endoplasmic reticulum/trans-Golgi network, did not derived from MHC/Ii complexes trafficking to the cell surface, but was generated after invariant chain degradation within lysosomal-like MHC-II rich compartments (MIIC). In pre-DC, generated from monocytes cultured in the presence of GM-CSF, Lamp-1(+)MHC-II(+) compartments are predominantly electron dense and, in these cells, empty MHC-II molecules accounts for as much as 20% of total surface HLA-DR. In immature DC, generated in presence of GM-CSF and IL-4, empty HLA-DR reside in multilamellar MIIC, but are scarcely observed at the cell surface. Thus, the morphology/composition of lysosomal MIIC at different DC maturational stages appear important for surface egression or intracellular retention of empty HLA-DR. Ag loading can be achieved for the fraction of empty HLA-DR present in the "peptide-receptive" form. Finally, in vivo, APC-expressing surface empty HLA-DR were found in T cell areas of secondary lymphoid organs.

Leishmania salvage and remodelling of host sphingolipids in amastigote survival and acidocalcisome biogenesis

Sphingolipids (SLs) play essential roles in most eukaryotes, but in the trypanosomatid protozoan Leishmania major their functions differ significantly. Previously we showed that null mutants defective in de novo sphingoid base synthesis (spt2-) lacked SLs but grew well and retained lipid rafts while replicating as promastigotes in vitro. However, they experienced catastrophic defects in membrane trafficking on entry into stationary phase, and failed to differentiate to the infective metacyclic form. Here we showed this mutant retained the ability to enter macrophages silently and inhibit activation, although as expected most parasites were destroyed. However, in mouse infections, after a delay rapidly progressive lesions appeared, and purified amastigotes were fully virulent to macrophages and mice. Mass spectrometry of spt2- amastigote lipids revealed the presence of high levels of parasite-specific inositol phosphorylceramides (IPCs) not synthesized by the mammalian hosts. Inhibitor studies showed that salvage occurs at the level of complex SLs, suggesting that parasites carry out 'headgroup' remodelling. Additionally, we describe a new defect of the spt2- promastigotes involving 'empty' acidocalcisomes (ACs), which may point to the origin of this organelle from the lysosome-related organelle/multivesicular body biogenesis pathway. However, ACs in spt2- amastigotes appeared quantitatively and morphologically normal. Thus salvage of SLs and other molecules by intracellular amastigotes play key roles in AC biogenesis and parasite survival in the host.

Vaccine-associated granulomatous inflammation and melanin accumulation in Atlantic salmon, Salmo salar L., white muscle

The purpose of this study was to investigate the nature of variably sized pigmented foci encountered in fillets of farmed Atlantic salmon, Salmo salar L. The material was sampled on the fillet production line and on salmon farms from fish with an average size of 3 kg from various producers. The fish had been routinely vaccinated by injection. Gross pathology, histology, immunohistochemistry using antisera against major histocompatibility complex (MHC) class II beta chain and transmission electron microscopy (TEM) were used to characterize the changes. Macroscopically, melanized foci were seen penetrating from the peritoneum deep into the abdominal wall, sometimes right through to the skin, and also embedded in the caudal musculature. Histological investigation revealed muscle degeneration and necrosis, fibrosis and granulomatous inflammation containing varying numbers of melano-macrophages. Vacuoles, either empty or containing heterogeneous material, were frequently seen. The presence of abundant MHC class II+ cells indicated an active inflammatory condition. TEM showed large extracellular vacuoles and leucocytes containing homogeneous material of lipid-like appearance. The results showed that the melanized foci in Atlantic salmon fillet resulted from an inflammatory condition probably induced by vaccination. The described condition is not known in wild salmon and in farmed salmon where injection vaccination is not applied.

Exosomes: A Bubble Ride for Prions?

In certain cell types, endosomal multivesicular bodies may fuse with the cell surface in an exocytic manner. During this process, the small 50-90-nm-diameter vesicles contained in their lumen are released into the extracellular environment. The released vesicles are called exosomes. Exosome secretion can be used by cells to eject molecules targeted to intraluminal vesicles of multivesicular bodies, but particular cell types exploit exosomes as intercellular communication devices for transfer of proteins and lipids between cells. The molecular composition of exosomes is determined by sorting events within endosomes that occur concomitantly with the generation of intraluminal vesicles. As other raft-associated components, the glycosylphosphatidylinositol-linked prion protein transits through multivesicular bodies. Recent findings in non-neuronal cell models indicate prion protein association with secreted exosomes. Thus, exosomes could constitute vehicles for transmission of the infectious prion protein, bypassing cell-cell contact in the dissemination of prions.

Melanin pigmentation in mammalian skin and its hormonal regulation

Cutaneous melanin pigment plays a critical role in camouflage, mimicry, social communication, and protection against harmful effects of solar radiation. Melanogenesis is under complex regulatory control by multiple agents interacting via pathways activated by receptor-dependent and -independent mechanisms, in hormonal, auto-, para-, or intracrine fashion. Because of the multidirectional nature and heterogeneous character of the melanogenesis modifying agents, its controlling factors are not organized into simple linear sequences, but they interphase instead in a multidimensional network, with extensive functional overlapping with connections arranged both in series and in parallel. The most important positive regulator of melanogenesis is the MC1 receptor with its ligands melanocortins and ACTH, whereas among the negative regulators agouti protein stands out, determining intensity of melanogenesis and also the type of melanin synthesized. Within the context of the skin as a stress organ, melanogenic activity serves as a unique molecular sensor and transducer of noxious signals and as regulator of local homeostasis. In keeping with these multiple roles, melanogenesis is controlled by a highly structured system, active since early embryogenesis and capable of superselective functional regulation that may reach down to the cellular level represented by single melanocytes. Indeed, the significance of melanogenesis extends beyond the mere assignment of a color trait.

Caveolin-1 and MAL are located on prostasomes secreted by the prostate cancer PC-3 cell line

MAL, BENE and MAL2 are raft-associated integral membrane proteins of the MAL family of proteins involved in membrane trafficking processes. We show here that the human prostate carcinoma PC-3 cell line expresses the transcripts for the three proteins simultaneously. MAL, BENE and MAL2 co-fractionated with caveolin-1 in the raft fraction of PC-3 cells, and immunofluorescence analysis showed colocalization of these proteins with caveolin-1 in a multivesicular intracellular compartment. Markers of the Golgi apparatus, early and recycling endosomes and lipid droplets were excluded from this compartment. Prostate epithelial cells contain vesicular organelles enriched in raft components named prostasomes that are secreted in the prostate fluid. Interestingly, the prostasome fraction isolated from the culture supernatant of PC-3 cells consisted mainly of 30-130 nm cup-shaped vesicles that were positive for MAL, caveolin-1 and CD59, a glycosylphosphatidylinositol-anchored protein previously found in prostasomes. CD63, an integral membrane protein found in multivesicular bodies/lysosomes and secretory granules was also found in PC-3 cell-derived prostasomes. Prostasome secretion was not inhibited by brefeldin A, a compound that blocks the conventional secretory pathway. However, wortmannin, an inhibitor of phosphatidylinositol-3 kinase, reduced the secretion of prostasomes in PC-3 cells. Our results suggest that MAL family proteins are associated with caveolin-1 in a multivesicular compartment that may be involved in prostasomal secretion in PC-3 cells.

Identification of protein associations in organelles, using mass spectrometry-based proteomics

Recent literature that highlights the power of using mass spectrometry (MS) for protein identification from preparations of highly purified organelles and other large subcellular structures is covered in this review with an emphasis on techniques that preserve the integrity of the functional protein complexes. Recent advances in distinguishing contaminant proteins from "bonafide" organelle-localized proteins and the affinity capture of protein complexes are reviewed, as well as bioinformatic strategies to predict protein organellar localization and to integrate protein-protein interaction maps obtained from MS-affinity capture methods with data obtained from other techniques. Those developments demonstrate that a revolution in cellular biology, fueled by technical advances in MS-based proteomic techniques, is well underway.

Endosomes, exosomes and Trojan viruses

Retroviruses are enveloped viruses that are generally assumed to bud at the plasma membrane of infected cells. Recently it has become apparent that some of these viruses use the endocytic pathway to coordinate their assembly and release. In addition, these and some other enveloped viruses exploit the machinery that generates the internal membranes of multivesicular bodies (MVB). These observations and others have led to the suggestion that retroviruses be regarded as "viral exosomes". Here we discuss this concept and the emerging evidence that compartments of the endocytic pathway play important roles in the biogenesis of both the internal vesicles of MVB and viruses.

Cells release prions in association with exosomes

Prion diseases are infectious neurodegenerative disorders linked to the accumulation in the central nervous system of the abnormally folded prion protein (PrP) scrapie (PrPsc), which is thought to be the infectious agent. Once present, PrPsc catalyzes the conversion of naturally occurring cellular PrP (PrPc) to PrPsc. Prion infection is usually initiated in peripheral organs, but the mechanisms involved in infectious spread to the brain are unclear. We found that both PrPc and PrPsc were actively released into the extracellular environment by PrP-expressing cells before and after infection with sheep prions, respectively. Based on Western blot with specific markers, MS, and morphological analysis, our data revealed that PrPc and PrPsc in the medium are associated with exosomes, membranous vesicles that are secreted upon fusion of multivesicular endosomes with the plasma membrane. Furthermore, we found that exosomes bearing PrPsc are infectious. Our data suggest that exosomes may contribute to intercellular membrane exchange and the spread of prions throughout the organism.