Lysosome-endosome fusion and lysosome biogenesis

Controlled elimination of clathrin heavy-chain expression in DT40 lymphocytes

We exploited the high rate of homologous recombination shown by the chicken B cell line DT40 to inactivate the endogenous alleles for clathrin heavy chain and replace them with human clathrin complementary DNA under the control of a tetracycline-regulatable promoter. Clathrin repression perturbed the activities of Akt-mediated and mitogen-activated protein kinase-mediated signaling pathways and induced apoptosis; this finding suggests that in DT40 cells clathrin helps to maintain the integrity of antiapoptotic survival pathways. We also describe a variant cell line in which these signaling pathways were unaffected by clathrin down-regulation. This variant cell line did not undergo apoptosis in the absence of clathrin and was used to examine the effects of clathrin depletion on membrane-trafficking pathways. Receptor-mediated and fluid-phase endocytosis were both substantially inhibited, and transferrin-receptor recycling was modestly inhibited. Surprisingly, clathrin removal did not affect the morphology or biochemical composition of lysosomes.

Ile (476), a constituent of di-leucine-based motif of a major lysosomal membrane protein, LGP85/LIMP II, is important for its proper distribution in late endosomes and lysosomes

Lysosomal membrane glycoprotein termed LGP85 or LIMP II extends a COOH-terminal cytoplasmic tail of R459GQGSMDEGTADERAPLIRT478, in which an L475 I476 sequence lies as a di-leucine-based motif for lysosomal targeting. In the present study, we explored the role of the I476 residue in the localization of LGP85 to the endocytic organelles using two substitution mutants called I476A and I476L in which alanine and leucine are replaced at I476, respectively, and I476R477T478-deleted LGP85 called Delta 476-478. Immunofluorescence analyses showed that I476A and I476L are largely colocalized in intracellular organelles with an endogenous late endosomal and lysosomal marker, LAMP-1, but there were some granules in which staining for the LGP85 mutants was prominent, while Delta 476-478 is detected in LAMP-1-positive and LAMP-1-negative intracellular organelles, and on the cell surface. The subcellular fractionation studies revealed that I476A, I476L, and Delta 476-478 are different from wild-type LGP85 in the distribution of early endosomes, late endosomes, and lysosomes. I476A and I476L are present more in late endosomes than in the densest lysosomes, whereas wild-type LGP85 is mainly lysosomal. Substitution of I476 for A and L differentially modified the ratios of late endosomal to lysosomal LGP85. A major portion of Delta 476-478 resided in the light buoyant density fraction containing plasma membrane and early endosomes. Taken together, these results indicate that the existence of the 476th amino acid residue is essential for localization of LGP85 to late endocytic compartments. The fact that isoleucine but not leucine is in the 476th position is especially of importance in the proper distribution of LGP85 in late endosomes and lysosomes.

The dark side of lysosome-related organelles: specialization of the endocytic pathway for melanosome biogenesis

Melanosomes are lysosome-related organelles within which melanin pigments are synthesized and stored in melanocytes and retinal pigment epithelial cells. Early ultrastructural studies of pigment cells revealed that melanosomes consist of a complex series of organelles; more recently, these structures have been correlated with cargo constituents. By studying the fate of melanosomal and endosomal cargo in melanocytic cells, the effects of disease-related mutations on melanosomal morphology, and the genes affected by these mutations, we are beginning to gain novel insights into the biogenesis of these complex organelles and their relationship to the endocytic pathway. These insights demonstrate how specialized cells integrate unique and ubiquitous molecular mechanisms in subverting the endosomal system to generate cell-type specific structures and their associated functions. Further dissection of the melanosomal system will likely shed light not only on the biogenesis of lysosome-related organelles but also on general aspects of vesicular transport in the endosomal system.

Role of adaptor complex AP-3 in targeting wild-type and mutated CD63 to lysosomes

CD63 is a lysosomal membrane protein that belongs to the tetraspanin family. Its carboxyterminal cytoplasmic tail sequence contains the lysosomal targeting motif GYEVM. Strong, tyrosine-dependent interaction of the wild-type carboxyterminal tail of CD63 with the AP-3 adaptor subunit mu 3 was observed using a yeast two-hybrid system. The strength of interaction of mutated tail sequences with mu 3 correlated with the degree of lysosomal localization of similarly mutated human CD63 molecules in stably transfected normal rat kidney cells. Mutated CD63 containing the cytosolic tail sequence GYEVI, which interacted strongly with mu 3 but not at all with mu 2 in the yeast two-hybrid system, localized to lysosomes in transfected normal rat kidney and NIH-3T3 cells. In contrast, it localized to the cell surface in transfected cells of pearl and mocha mice, which have genetic defects in genes encoding subunits of AP-3, but to lysosomes in functionally rescued mocha cells expressing the delta subunit of AP-3. Thus, AP-3 is absolutely required for the delivery of this mutated CD63 to lysosomes. Using this AP-3-dependent mutant of CD63, we have shown that AP-3 functions in membrane traffic from the trans-Golgi network to lysosomes via an intracellular route that appears to bypass early endosomes.

Hantaan virus enters cells by clathrin-dependent receptor-mediated endocytosis

The cellular entry of Hantaan virus (HTN) occurs through interactions with beta(3) integrins as cellular receptors. However, the process of HTN infection following attachment to the cell surface is not well understood. Our data indicate that overexpression of a dominant-negative mutant dynamin inhibits HTN internalization and that compounds that block clathrin- but not caveolae-dependent endocytosis also reduce HTN infectivity. In addition, we show that HTN colocalizes with the clathrin heavy chain but not with caveolae. At the early phase of infection HTN colocalizes with EEA-1, an early endosome marker, and later, HTN colocalizes with LAMP-1, a lysosome marker. Cells treated with lysosomotropic agents are largely resistant to infection, suggesting that a low-pH-dependent step is required for HTN infection. These findings demonstrate that HTN enters cells via the clathrin-coated pit pathway and uses low-pH-dependent intracellular compartments for infectious entry.

Dictyostelium LvsB mutants model the lysosomal defects associated with Chediak-Higashi syndrome

Chediak-Higashi syndrome is a genetic disorder caused by mutations in a gene encoding a protein named LYST in humans ("lysosomal trafficking regulator") or Beige in mice. A prominent feature of this disease is the accumulation of enlarged lysosome-related granules in a variety of cells. The genome of Dictyostelium discoideum contains six genes encoding proteins that are related to LYST/Beige in amino acid sequence, and disruption of one of these genes, lvsA (large volume sphere), results in profound defects in cytokinesis. To better understand the function of this family of proteins in membrane trafficking, we have analyzed mutants disrupted in lvsA, lvsB, lvsC, lvsD, lvsE, and lvsF. Of all these, only lvsA and lvsB mutants displayed interesting phenotypes in our assays. lvsA-null cells exhibited defects in phagocytosis and contained abnormal looking contractile vacuole membranes. Loss of LvsB, the Dictyostelium protein most similar to LYST/Beige, resulted in the formation of enlarged vesicles that by multiple criteria appeared to be acidic lysosomes. The rates of endocytosis, phagocytosis, and fluid phase exocytosis were normal in lvsB-null cells. Also, the rates of processing and the efficiency of targeting of lysosomal alpha-mannosidase were normal, although lvsB mutants inefficiently retained alpha-mannosidase, as well as two other lysosomal cysteine proteinases. Finally, results of pulse-chase experiments indicated that an increase in fusion rates accounted for the enlarged lysosomes in lvsB-null cells, suggesting that LvsB acts as a negative regulator of fusion. Our results support the notion that LvsB/LYST/Beige function in a similar manner to regulate lysosome biogenesis.

A subset of yeast vacuolar protein sorting mutants is blocked in one branch of the exocytic pathway

Exocytic vesicles that accumulate in a temperature-sensitive sec6 mutant at a restrictive temperature can be separated into at least two populations with different buoyant densities and unique cargo molecules. Using a sec6 mutant background to isolate vesicles, we have found that vacuolar protein sorting mutants that block an endosome-mediated route to the vacuole, including vps1, pep12, vps4, and a temperature-sensitive clathrin mutant, missort cargo normally transported by dense exocytic vesicles, such as invertase, into light exocytic vesicles, whereas transport of cargo specific to the light exocytic vesicles appears unaffected. Immunoisolation experiments confirm that missorting, rather than a changed property of the normally dense vesicles, is responsible for the altered density gradient fractionation profile. The vps41Delta and apl6Delta mutants, which block transport of only the subset of vacuolar proteins that bypasses endosomes, sort exocytic cargo normally. Furthermore, a vps10Delta sec6 mutant, which lacks the sorting receptor for carboxypeptidase Y (CPY), accumulates both invertase and CPY in dense vesicles. These results suggest that at least one branch of the yeast exocytic pathway transits through endosomes before reaching the cell surface. Consistent with this possibility, we show that immunoisolated clathrin-coated vesicles contain invertase.

Actin filaments and myosin I alpha cooperate with microtubules for the movement of lysosomes

An earlier report suggested that actin and myosin I alpha (MMIalpha), a myosin associated with endosomes and lysosomes, were involved in the delivery of internalized molecules to lysosomes. To determine whether actin and MMIalpha were involved in the movement of lysosomes, we analyzed by time-lapse video microscopy the dynamic of lysosomes in living mouse hepatoma cells (BWTG3 cells), producing green fluorescent protein actin or a nonfunctional domain of MMIalpha. In GFP-actin cells, lysosomes displayed a combination of rapid long-range directional movements dependent on microtubules, short random movements, and pauses, sometimes on actin filaments. We showed that the inhibition of the dynamics of actin filaments by cytochalasin D increased pauses of lysosomes on actin structures, while depolymerization of actin filaments using latrunculin A increased the mobility of lysosomes but impaired the directionality of their long-range movements. The production of a nonfunctional domain of MMIalpha impaired the intracellular distribution of lysosomes and the directionality of their long-range movements. Altogether, our observations indicate for the first time that both actin filaments and MMIalpha contribute to the movement of lysosomes in cooperation with microtubules and their associated molecular motors.

Evidence for distinct membrane traffic pathways to melanosomes and lysosomes in melanocytes

We report here morphologic and biochemical evidence that melanosomes are distinct from lysosomes. Immunofluorescence analysis revealed that TRP-1, a melanosomal membrane protein, did not colocalize with lysosomal membrane proteins LAMP1 and LGP85 in melan-a cells. Wortmannin treatment of melanocytes enhanced the distinct compartmentalization of these melanosomal/lysosomal membrane proteins by the swelling of the endosomal-lysosomal systems. The heavily melanized melanosomes did not have an altered shape, which suggests a lesser degree of membrane dynamics of stage IV melanosomes. Terminal lysosomes loaded with TR-dextran are also distinct from melanosomes, thus indicating that melanosomes are isolated from the endocytic pathway that is a representative route to lysosomes. Because AP-3 mutation leads to mistargeting of both melanosome and lysosome membrane proteins, we propose that there is a late sorting step for melanosomes and lysosomes in melanocytes after AP-3 sorting.

The melanosome: membrane dynamics in black and white

Melanosomes are morphologically and functionally unique organelles within which melanin pigments are synthesized and stored. Melanosomes share some characteristics with lysosomes, but can be distinguished from them in many ways. The biogenesis and intracellular movement of melanosomes and related organelles are disrupted in several genetic disorders in mice and humans. The recent characterization of genes defective in these diseases has reinvigorated interest in the melanosome as a model system for understanding the molecular mechanisms that underlie intracellular membrane dynamics.

Evidence for the Involvement of annexin 6 in the trafficking between the endocytic compartment and lysosomes

Annexins are a family of calcium-dependent phospholipid-binding proteins, which have been implicated in a variety of biological processes including membrane trafficking. The annexin 6/lgp120 prelysosomal compartment of NRK cells was loaded with low-density lipoprotein (LDL) and then its transport from this endocytic compartment and its degradation in lysosomes were studied. NRK cells were microinjected with the mutated annexin 6 (anx6(1-175)), to assess the possible involvement of annexin 6 in the transport of LDL from the prelysosomal compartment. The results indicated that microinjection of mutated annexin 6, in NRK cells, showed the accumulation of LDL in larger endocytic structures, denoting retention of LDL in the prelysosomal compartment. To confirm the involvement of annexin 6 in the trafficking and the degradation of LDL we used CHO cells transfected with mutated annexin 6(1-175). Thus, in agreement with NRK cells the results obtained in CHO cells demonstrated a significant inhibition of LDL degradation in CHO cells expressing the mutated form of annexin 6 compared to controls overexpressing wild-type annexin 6. Therefore, we conclude that annexin 6 is involved in the trafficking events leading to LDL degradation.

The relationship between lumenal and limiting membranes in swollen late endocytic compartments formed after wortmannin treatment or sucrose accumulation

Immunofluorescence and electron microscopy were used to evaluate the formation of swollen endosomes in NRK cells after treatment with wortmannin or sucrose and to study the relationship between lumenal and limiting membrane. Both treatments resulted in the formation of two populations of swollen late endocytic vacuoles, positive for lysosomal glycoproteins or cation-independent mannose 6-phosphate receptors, but those induced by wortmannin were characterised by time-dependent accumulation of lumenal vesicles, whereas those induced by sucrose uptake did not accumulate lumenal vesicles. In both cases, the distribution of the late endosomal marker, lysobisphosphatidic acid, remained unchanged and was present within the lumen of the swollen vacuoles. Consumption of plasma membrane and peripheral early endosomes, and the appearance of transferrin receptors in swollen late endosomes, indicated that continued membrane influx from early endocytic compartments, together with inhibition of membrane traffic out of the swollen compartments, is sufficient to account for the observed phenotype of cells treated with wortmannin. The accumulation of organelles with the characteristic morphology of endocytic carrier vesicles in cells that have taken up sucrose offers an explanation for the paucity of lumenal vesicles in swollen sucrosomes. Our data suggest that in fibroblast cells the swollen endosome phenotype induced by wortmannin is a consequence of endocytic membrane influx, coupled with the failure to recycle membrane to other cellular destinations, and not the inhibition of multivesicular body biogenesis.

Late endosomes: sorting and partitioning in multivesicular bodies

Late endosomes, which have the morphological characteristics of multivesicular bodies, have received relatively little attention in comparison with early endosomes and lysosomes. Recent work in mammalian and yeast cells has given insights into their structure and function, including the generation of their multivesicular morphology. Lipid partitioning to create microdomains enriched in specific lipids is observed in late endosomes, with some lumenal vesicles enriched in lysobisphosphatidic acid and others in phosphatidylinositol 3-phosphate. Sorting of membrane proteins into the lumenal vesicles may occur because of the properties of their trans-membrane domains, or as a result of tagging with ubiquitin. Yeast class E Vps proteins and their mammalian orthologs are the best candidates to make up the protein machinery that controls inward budding, a process that starts in early endosomes. Late endosomes are able to undergo homotypic fusion events and also heterotypic fusion with lysosomes, a process that delivers endocytosed macromolecules for proteolytic degradation.

Prolonged antigen persistence within nonterminal late endocytic compartments of antigen-specific B lymphocytes

Although Ag-specific B lymphocytes can process Ag and express peptide-class II complexes as little as 1 h after Ag exposure, it requires 3-5 days for the immune system to develop a population of Ag-specific effector CD4 T lymphocytes to interact with these complexes. Presently, it is unclear how B cells maintain the expression of cell surface antigenic peptide-class II complexes until effector CD4 T lymphocytes become available. Therefore, we investigated B cell receptor (BCR)-mediated Ag processing and presentation by normal B lymphocytes to determine whether these cells have a mechanism to prolong the cell surface expression of peptide-class II complexes derived from the processing of cognate AG: Interestingly, after transit of early endocytic compartments, internalized Ag-BCR complexes are delivered to nonterminal late endosomes where they persist for a prolonged period of time. In contrast, Ags internalized via fluid phase endocytosis are rapidly delivered to terminal lysosomes and degraded. Moreover, persisting Ag-BCR complexes within nonterminal late endosomes exhibit a higher degree of colocalization with the class II chaperone HLA-DM/H2-M than with the HLA-DM/H2-M regulator HLA-DO/H2-O. Finally, B cells harboring persistent Ag-BCR complexes exhibit prolonged cell surface expression of antigenic peptide-class II complexes. These results demonstrate that B lymphocytes possess a mechanism for prolonging the intracellular persistence of Ag-BCR complexes within nonterminal late endosomes and suggest that this intracellular Ag persistence allows for the prolonged cell surface expression of peptide-class II complexes derived from the processing of specific AG:

Human Vam6p promotes lysosome clustering and fusion in vivo

Regulated fusion of mammalian lysosomes is critical to their ability to acquire both internalized and biosynthetic materials. Here, we report the identification of a novel human protein, hVam6p, that promotes lysosome clustering and fusion in vivo. Although hVam6p exhibits homology to the Saccharomyces cerevisiae vacuolar protein sorting gene product Vam6p/Vps39p, the presence of a citron homology (CNH) domain at the NH(2) terminus is unique to the human protein. Overexpression of hVam6p results in massive clustering and fusion of lysosomes and late endosomes into large (2-3 microm) juxtanuclear structures. This effect is reminiscent of that caused by expression of a constitutively activated Rab7. However, hVam6p exerts its effect even in the presence of a dominant-negative Rab7, suggesting that it functions either downstream of, or in parallel to, Rab7. Data from gradient fractionation, two-hybrid, and coimmunoprecipitation analyses suggest that hVam6p is a homooligomer, and that its self-assembly is mediated by a clathrin heavy chain repeat domain in the middle of the protein. Both the CNH and clathrin heavy chain repeat domains are required for induction of lysosome clustering and fusion. This study implicates hVam6p as a mammalian tethering/docking factor characterized with intrinsic ability to promote lysosome fusion in vivo.

CHMP1 functions as a member of a newly defined family of vesicle trafficking proteins

A multivesicular body is a vesicle-filled endosome that targets proteins to the interior of lysosomes. We have identified a conserved eukaryotic protein, human CHMP1, which is strongly implicated in multivesicular body formation. Immunocytochemistry and biochemical fractionation localize CHMP1 to early endosomes and CHMP1 physically interacts with SKD1/VPS4, a highly conserved protein directly linked to multivesicular body sorting in yeast. Similar to the action of a mutant SKD1 protein, overexpression of a fusion derivative of human CHMP1 dilates endosomal compartments and disrupts the normal distribution of several endosomal markers. Genetic studies in Saccharomyces cerevisiae further support a conserved role of CHMP1 in vesicle trafficking. Deletion of CHM1, the budding yeast homolog of CHMP1, results in defective sorting of carboxypeptidases S and Y and produces abnormal, multi-lamellar prevacuolar compartments. This phenotype classifies CHM1 as a member of the class E vacuolar protein sorting genes. Yeast Chm1p belongs to a structurally-related, but rather divergent family of proteins, including Vps24p and Snf7p and three novel proteins, Chm2p, Chm5p and Chm6p, which are all essential for multivesicular body sorting. These observations identify the conserved CHMP/Chmp family as a set of proteins fundamental to understanding multivesicular body sorting in eukaryotic organisms.

AP-3 mediates tyrosinase but not TRP-1 trafficking in human melanocytes

Patients with Hermansky-Pudlak syndrome type 2 (HPS-2) have mutations in the beta 3A subunit of adaptor complex-3 (AP-3) and functional deficiency of this complex. AP-3 serves as a coat protein in the formation of new vesicles, including, apparently, the platelet's dense body and the melanocyte's melanosome. We used HPS-2 melanocytes in culture to determine the role of AP-3 in the trafficking of the melanogenic proteins tyrosinase and tyrosinase-related protein-1 (TRP-1). TRP-1 displayed a typical melanosomal pattern in both normal and HPS-2 melanocytes. In contrast, tyrosinase exhibited a melanosomal (i.e., perinuclear and dendritic) pattern in normal cells but only a perinuclear pattern in the HPS-2 melanocytes. In addition, tyrosinase exhibited a normal pattern of expression in HPS-2 melanocytes transfected with a cDNA encoding the beta 3A subunit of the AP-3 complex. This suggests a role for AP-3 in the normal trafficking of tyrosinase to premelanosomes, consistent with the presence of a dileucine recognition signal in the C-terminal portion of the tyrosinase molecule. In the AP-3-deficient cells, tyrosinase was also present in structures resembling late endosomes or multivesicular bodies; these vesicles contained exvaginations devoid of tyrosinase. This suggests that, under normal circumstances, AP-3 may act on multivesicular bodies to form tyrosinase-containing vesicles destined to fuse with premelanosomes. Finally, our studies demonstrate that tyrosinase and TRP-1 use different mechanisms to reach their premelanosomal destination.

The autophagosomal-lysosomal compartment in programmed cell death

In the last decade a tremendous progress has been achieved in understanding the control of apoptosis by survival and death factors as well as the molecular mechanisms of preparation and execution of the cell's suicide. However, accumulating evidence suggests that programmed cell death (PCD) is not confined to apoptosis but that cells use different pathways for active self-destruction as reflected by different morphology: condensation prominent, type I or apoptosis; autophagy prominent, type II; etc. Autophagic PCD appears to be a phylogenetically old phenomenon, it may occur in physiological and disease states. Recently, distinct biochemical and molecular features have been be assigned to this type of PCD. However, autophagic and apoptotic PCD should not be considered as mutually exclusive phenomena. Rather, they appear to reflect a high degree of flexibility in a cell's response to changes of environmental conditions, both physiological or pathological. Furthermore, recent data suggest that diverse or relatively unspecific signals such as photodamage or lysosomotropic agents may be mediated by lysosomal cysteine proteases (cathepsins) to caspases and thus, apoptosis. The present paper reviews morphological, functional and biochemical/molecular data suggesting the participation of the autophagosomal-lysosomal compartment in programmed cell death.

Regeneration of a lytic central vacuole and of neutral peripheral vacuoles can be visualized by green fluorescent proteins targeted to either type of vacuoles

Protein trafficking to two different types of vacuoles was investigated in tobacco (Nicotiana tabacum cv SR1) mesophyll protoplasts using two different vacuolar green fluorescent proteins (GFPs). One GFP is targeted to a pH-neutral vacuole by the C-terminal vacuolar sorting determinant of tobacco chitinase A, whereas the other GFP is targeted to an acidic lytic vacuole by the N-terminal propeptide of barley aleurain, which contains a sequence-specific vacuolar sorting determinant. The trafficking and final accumulation in the central vacuole (CV) or in smaller peripheral vacuoles differed for the two reporter proteins, depending on the cell type. Within 2 d, evacuolated (mini-) protoplasts regenerate a large CV. Expression of the two vacuolar GFPs in miniprotoplasts indicated that the newly formed CV was a lytic vacuole, whereas neutral vacuoles always remained peripheral. Only later, once the regeneration of the CV was completed, the content of peripheral storage vacuoles could be seen to appear in the CV of a third of the cells, apparently by heterotypic fusion.

Lipid domains in the endocytic pathway

Whereas endosomes connect with both exocytic and endocytic organelle via extensive lipid and protein traffic, each endosome has a distinct lipid and protein composition. Recent observations suggest that different lipid membrane domains exist even in the same endosome. These lipid domains, together with low pH milieu, may present a variety of micro-environments to cargo molecules. Evidence is accumulating which suggests that the alteration of these lipid microdomains may be involved in a number of pathological conditions.

The molecular machinery for lysosome biogenesis

The lysosome serves as a site for delivery of materials targeted for removal from the eukaryotic cell. The mechanisms underlying the biogenesis of this organelle are currently the subject of renewed interest due to advances in our understanding of the protein sorting machinery. Genetic model systems such as yeast and Drosophila have been instrumental in identifying both protein and lipid components of this machinery. Importantly, many of these components, as well as the processes in which they are involved, are proving conserved in mammals. Other recently identified components, however, appear to be unique to higher eukaryotes. BioEssays 23:333-343, 2001. Published 2001 John Wiley & Sons, Inc.

Dynamic movements of organelles containing Niemann-Pick C1 protein: NPC1 involvement in late endocytic events

People homozygous for mutations in the Niemann-Pick type C1 (NPC1) gene have physiological defects, including excess accumulation of intracellular cholesterol and other lipids, that lead to drastic neural and liver degeneration. The NPC1 multipass transmembrane protein is resident in late endosomes and lysosomes, but its functions are unknown. We find that organelles containing functional NPC1-fluorescent protein fusions undergo dramatic movements, some in association with extending strands of endoplasmic reticulum. In NPC1 mutant cells the NPC1-bearing organelles that normally move at high speed between perinuclear regions and the periphery of the cell are largely absent. Pulse-chase experiments with dialkylindocarbocyanine low-density lipoprotein showed that NPC1 organelles function late in the endocytic pathway; NPC1 protein may aid the partitioning of endocytic and lysosomal compartments. The close connection between NPC1 and the drug U18666A, which causes NPC1-like organelle defects, was established by rescuing drug-treated cells with overproduced NPC1. U18666A inhibits outward movements of NPC1 organelles, trapping membranes and cholesterol in perinuclear organelles similar to those in NPC1 mutant cells, even when cells are grown in lipoprotein-depleted serum. We conclude that NPC1 protein promotes the creation and/or movement of particular late endosomes, which rapidly transport materials to and from the cell periphery.

Distinct protein sorting and localization to premelanosomes, melanosomes, and lysosomes in pigmented melanocytic cells

Melanosomes and premelanosomes are lysosome-related organelles with a unique structure and cohort of resident proteins. We have positioned these organelles relative to endosomes and lysosomes in pigmented melanoma cells and melanocytes. Melanosome resident proteins Pmel17 and TRP1 localized to separate vesicular structures that were distinct from those enriched in lysosomal proteins. In immunogold-labeled ultrathin cryosections, Pmel17 was most enriched along the intralumenal striations of premelanosomes. Increased pigmentation was accompanied by a decrease in Pmel17 and by an increase in TRP1 in the limiting membrane. Both proteins were largely excluded from lysosomal compartments enriched in LAMP1 and cathepsin D. By kinetic analysis of fluid phase uptake and immunogold labeling, premelanosomal proteins segregated from endocytic markers within an unusual endosomal compartment. This compartment contained Pmel17, was accessed by BSA-gold after 15 min, was acidic, and displayed a cytoplasmic planar coat that contained clathrin. Our results indicate that premelanosomes and melanosomes represent a distinct lineage of organelles, separable from conventional endosomes and lysosomes within pigmented cells. Furthermore, they implicate an unusual clathrin-coated endosomal compartment as a site from which proteins destined for premelanosomes and lysosomes are sorted.

Colocalization of Fc gamma RI-targeted antigen with class I MHC: implications for antigen processing

The high-affinity receptor for IgG (CD64 or FcgammaRI) is constitutively expressed exclusively on professional APCs (monocytes, macrophages, and dendritic cells). When Ag is targeted specifically to FcgammaRI, Ag presentation is markedly enhanced, although the mechanism of this enhancement is unknown. In an effort to elucidate the pathways involved in FcgammaRI targeting, we developed a model targeted Ag using enhanced green fluorescent protein (eGFP). This molecule, wH22xeGFP, consists of the entire humanized anti-FcgammaRI mAb H22 with eGFP genetically fused to the C-terminal end of each CH3 domain. wH22xeGFP binds within the ligand-binding region by its Fc end, as well as outside the ligand-binding region by its Fab ends, thereby cross-linking FcgammaRI. Confocal microscopy studies revealed that wH22xeGFP was rapidly internalized by the high-FcgammaRI-expressing cell line U937 10.6, but did not associate with intracellular proteins Rab4, Rab5a, or Lamp-1, suggesting that the targeted fusion protein was not localized in early endosomes, recycling vesicles, or lysosomes. Interestingly, wH22xeGFP was found colocalized with intracellular MHC class I, suggesting that FcgammaRI-targeted Ags may converge upon a class I processing pathway. These data are in agreement with studies in the mouse showing that FcgammaRI targeting can lead to Ag-specific activation of cytotoxic T cells. Data obtained from these studies should lead to a better understanding of how Ags targeted to FcgammaRI are processed and under what conditions they lead to presentation of antigenic peptides in MHC class I, as a foundation for the use of FcgammaRI-targeted Ags as vaccines.

Loss of cation-independent mannose 6-phosphate receptor expression promotes the accumulation of lysobisphosphatidic acid in multilamellar bodies

A number of recent studies have highlighted the importance of lipid domains within endocytic organelles in the sorting and movement of integral membrane proteins. In particular, considerable attention has become focussed upon the role of the unusual phospholipid lysobisphosphatidic acid (LBPA). This lipid appears to be directly involved in the trafficking of cholesterol and glycosphingolipids, and accumulates in a number of lysosomal storage disorders. Antibody-mediated disruption of LBPA function also leads to mis-sorting of cation-independent mannose 6-phosphate receptors. We now report that the converse is also true, and that spontaneous loss of cation-independent mannose 6-phosphate receptors from a rat fibroblast cell line led to the formation of aberrant late endocytic structures enriched in LBPA. Accumulation of LBPA was directly dependent upon the loss of the receptors, and could be reversed by expression of bovine cation-independent mannose 6-phosphate receptors in the mutant cell line. Ultrastructural analysis indicated that the abnormal organelles were electron-dense, had a multi-lamellar structure, accumulated endocytosed probes, and were distinct from dense-core lysosomes present within the same cells. The late endocytic structures present at steady state within any particular cell likely reflect the balance of membrane traffic through the endocytic pathway of that cell, and the rate of maturation of individual endocytic organelles. Moreover, there is considerable evidence which suggests that cargo receptors also play a direct mechanistic role in membrane trafficking events. Therefore, loss of such a protein may disturb the overall equilibrium of the pathway, and hence cause the accumulation of aberrant organelles. We propose that this mechanism underlies the phenotype of the mutant cell line, and that the formation of inclusion bodies in many lysosomal storage diseases is also due to an imbalance in membrane trafficking within the endocytic pathway.

Exosome: from internal vesicle of the multivesicular body to intercellular signaling device

Exosomes are small membrane vesicles that are secreted by a multitude of cell types as a consequence of fusion of multivesicular late endosomes/lysosomes with the plasma membrane. Depending on their origin, exosomes can play roles in different physiological processes. Maturing reticulocytes externalize obsolete membrane proteins such as the transferrin receptor by means of exosomes, whereas activated platelets release exosomes whose function is not yet known. Exosomes are also secreted by cytotoxic T cells, and these might ensure specific and efficient targeting of cytolytic substances to target cells. Antigen presenting cells, such as B lymphocytes and dendritic cells, secrete MHC class-I- and class-II-carrying exosomes that stimulate T cell proliferation in vitro. In addition, dendritic-cell-derived exosomes, when used as a cell-free vaccine, can eradicate established murine tumors. Although the precise physiological target(s) and functions of exosomes remain largely to be resolved, follicular dendritic cells (accessory cells in the germinal centers of secondary lymphoid organs) have recently been shown to bind B-lymphocyte-derived exosomes at their cell surface, which supports the notion that exosomes play an immunoregulatory role. Finally, since exosomes are derived from multivesicular bodies, their molecular composition might provide clues to the mechanism of protein and lipid sorting in endosomes.

Syntaxin 7 is localized to late endosome compartments, associates with Vamp 8, and Is required for late endosome-lysosome fusion

Protein traffic from the cell surface or the trans-Golgi network reaches the lysosome via a series of endosomal compartments. One of the last steps in the endocytic pathway is the fusion of late endosomes with lysosomes. This process has been reconstituted in vitro and has been shown to require NSF, alpha and gamma SNAP, and a Rab GTPase based on inhibition by Rab GDI. In Saccharomyces cerevisiae, fusion events to the lysosome-like vacuole are mediated by the syntaxin protein Vam3p, which is localized to the vacuolar membrane. In an effort to identify the molecular machinery that controls fusion events to the lysosome, we searched for mammalian homologues of Vam3p. One such candidate is syntaxin 7. Here we show that syntaxin 7 is concentrated in late endosomes and lysosomes. Coimmunoprecipitation experiments show that syntaxin 7 is associated with the endosomal v-SNARE Vamp 8, which partially colocalizes with syntaxin 7. Importantly, we show that syntaxin 7 is specifically required for the fusion of late endosomes with lysosomes in vitro, resulting in a hybrid organelle. Together, these data identify a SNARE complex that functions in the late endocytic system of animal cells.

Biogenesis of the sorting endosome: the role of Rab5

Rab5 is a regulatory guanosine triphosphatase that is associated with the sorting endosome and participates in endosomal membrane fusion reactions. Recent experiments have provided insights into Rab5 function by demonstrating direct links between Rab5-interacting proteins and components of the membrane fusion apparatus. In addition, a realisation that Rab5 has additional functions in endosome biogenesis is emerging. These advances may be profoundly important in changing the way that we view the sorting endosome and in developing models that properly reflect the dynamic qualities of the endocytic pathway.