A Chinese hamster ovary cell line with a temperature-conditional defect in receptor recycling is pleiotropically defective in lysosome biogenesis

An arrested late endosome-lysosome intermediate aggregate observed in a Chinese hamster ovary cell mutant isolated by novel three-step screening

Chinese hamster ovary cell mutants defective in the post-uptake degradation of low-density lipoprotein (LDL) in lysosomes were selected from mutagenized cells by novel three-step screening. First, in the presence of LDL, clones sensitive to an inhibitor of the rate-limiting enzyme of the cholesterol biosynthetic pathway, 3-hydroxy-3-methylglutaryl-CoA reductase, were isolated. Second, from the selected clones, those lacking in the degradation of a constituent of a fluorescent LDL were qualitatively screened by microscopy. Third, the clones were further screened by previously established quantitative analytical flow cytometry that detects the early-phase disintegration of LDL by lysosomal acid hydrolases. One of the isolated mutant clones, LEX1 (Lysosome-Endosome X 1), was a recessive mutant, and exhibited a specific disorder in the late endocytic pathway. LEX1 cells showed an unusual perinuclear aggregate of vesicles, heterogeneously positive for lysosomal glycoprotein-B/cathepsin D and rab7, yet negative for the cation-independent mannose 6-phosphate receptor. The aggregate was formed around the microtubule organizing center, and was disrupted by nocodazole treatment. Internalized octadecyl rhodamine B-labeled LDL (R18-LDL) was accumulated in the perinuclear rab7-positive vesicles. In a Percoll density gradient, neither internalized R18-LDL nor internalized horseradish peroxidase was efficiently chased into heavy lysosomal fractions positive for beta-hexosaminidase. LEX1 cells showed differences in the activity and subcellular distribution of lysosomal enzymes. These characteristics of LEX1 cells are consistent with the ideas that the perinuclear vesicle aggregate is an arrested intermediate of direct fusion or divergence between lysosomes and rab7-positive, cation-independent mannose 6-phosphate receptor-negative late endosomes, and that equilibrium between the lysosomes and the late endosomes is shifted towards the late endosomes in LEX1 cells. Such fusion or divergence between the late endosomes and the lysosomes would determine an appropriate equilibrium between them, and might thereby play an important role for proper lysosomal digestive functions. LEX1 mutant cells would be helpful for the dissection of the as yet unrevealed details of the late endocytic membrane dynamics and for the identification of factors involved in the process arrested by the mutation.

The Niemann-Pick C1 protein resides in a vesicular compartment linked to retrograde transport of multiple lysosomal cargo

Niemann-Pick C disease (NP-C) is a neurovisceral lysosomal storage disorder. A variety of studies have highlighted defective sterol trafficking from lysosomes in NP-C cells. However, the heterogeneous nature of additional accumulating metabolites suggests that the cellular lesion may involve a more generalized block in retrograde lysosomal trafficking. Immunocytochemical studies in fibroblasts reveal that the NPC1 gene product resides in a novel set of lysosome-associated membrane protein-2 (LAMP2)(+)/mannose 6-phosphate receptor(-) vesicles that can be distinguished from cholesterol-enriched LAMP2(+) lysosomes. Drugs that block sterol transport out of lysosomes also redistribute NPC1 to cholesterol-laden lysosomes. Sterol relocation from lysosomes in cultured human fibroblasts can be blocked at 21 degrees C, consistent with vesicle-mediated transfer. These findings suggest that NPC1(+) vesicles may transiently interact with lysosomes to facilitate sterol relocation. Independent of defective sterol trafficking, NP-C fibroblasts are also deficient in vesicle-mediated clearance of endocytosed [14C]sucrose. Compartmental modeling of the observed [14C]sucrose clearance data targets the trafficking defect caused by mutations in NPC1 to an endocytic compartment proximal to lysosomes. Low density lipoprotein uptake by normal cells retards retrograde transport of [14C]sucrose through this same kinetic compartment, further suggesting that it may contain the sterol-sensing NPC1 protein. We conclude that a distinctive organelle containing NPC1 mediates retrograde lysosomal transport of endocytosed cargo that is not restricted to sterol.

Wortmannin retards the movement of the mannose 6-phosphate/insulin-like growth factor II receptor and its ligand out of endosomes

The effect of wortmannin on the trafficking of the mannose 6-phosphate/insulin-like growth factor II receptor (Man-6-P/IGF-II receptor) and its ligand beta-glucuronidase has been determined in murine L cells and normal rat kidney cells. The drug induced a 90% decrease in the steady-state level of the Man-6-P/IGF-II receptor at the plasma membrane without affecting the rate of internalization, indicating that the return of receptor from endosomes to the plasma membrane is retarded. Wortmannin also slowed the movement of receptor from endosomes to the trans-Golgi network by about 60%. Such a kinetic block would dramatically reduce the number of Man-6-P/IGF-II receptors in the trans-Golgi network, which could account for the previously described hypersecretion of procathepsin D induced by wortmannin. In addition, the drug slowed delivery of endocytosed beta-glucuronidase from endosomes to dense lysosomes. These data, taken together with the published reports of others, indicate that wortmannin inhibits membrane trafficking out of multiple compartments of the endosomal system and suggest a role for phosphatidylinositol 3-kinase in regulating these processes.

Human hepatoma cell mutant defective in cell surface protein trafficking

To isolate a mutant liver cell defective in the endocytic pathway, a selection strategy using toxic ligands for two distinct membrane receptors was devised. Ovalbumin-gelonin and asialoorosomucoid (ASOR)-gelonin were incubated with mutagenized HuH-7 cells, and a rare survivor termed trafficking mutant 1 (Trf1) was isolated. Trf1 cells were stably 3-fold more resistant than the parental HuH-7 to both toxic conjugates. The anterograde steps of intracellular endocytic processing of ASOR, including internalization, endosomal acidification, and ligand degradation, were unaltered in Trf1 cells. In contrast, retrograde diacytosis of asialoglycoprotein receptor (ASGR).ASOR complex back to the cell surface was enhanced by about 250%. Selective labeling revealed an approximately 46% reduction in cell surface-associated ASGR in Trf1 cells, although their total cellular ASGR content was essentially equivalent to that in HuH-7. Similar results were obtained with the transferrin receptor. Binding of 125I-ASOR and 125I-transferrin was reduced in Trf1 cells to 49 +/- 2.5% and 30 +/- 2%, respectively, of HuH-7 cells. The methionine transporter was also reduced in Trf1 cells, as revealed by a 2-fold reduction in Vmax with no change in apparent Km. Pretreatment with monensin, sodium azide, or colchicine reduced surface binding of 125I-ASOR in HuH-7 cells by 50% but had no effect on binding to Trf1 cells. This result is predicted for a cell that expresses only State 1 ASGRs, which are resistant to modulation by metabolic and cytoskeletal inhibitors in contrast to State 2, which are responsive to these agents (Weigel, P. H., and Oka, J. A. (1984) J. Biol. Chem. 259, 1150-1154). The Trf1 mutant, having lost the ability to express State 2 receptors, provides genetic evidence for the existence of these two receptor subpopulations and an approach to identifying the biochemical mechanism by which they are generated.

Ceramide as a modulator of endocytosis

The effects of ceramide analogs on the uptake of markers for fluid-phase (horseradish peroxidase, HRP) and receptor-mediated (low density lipoprotein, LDL) endocytosis were studied in Chinese hamster fibroblasts. N-Hexanoyl-D-erythro-sphingosine (C6-Cer) decreased the uptake of HRP in a dose-dependent manner. Internalization was inhibited > 40% with 25 microM C6-Cer, relative to controls, and was apparent within 5 min. Internalization of HRP was also inhibited by other Cer analogs and by treatment with agents that raise levels of endogenous Cer (sphingomyelinase or the glycosphingolipid synthesis inhibitor, 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP)), but not by N-hexanoyl-D-erythrosphinganine (C6-dihydro-Cer) or sphingosine. Internalization of LDL was also inhibited by C6-Cer in a concentration-dependent manner, but was less pronounced than the effect on HRP internalization (10% versus 40% inhibition with 25 microM C6-Cer), suggesting that ceramide might affect fluid-phase and receptor-mediated endocytosis to different extents. C6-Cer also slowed HRP and LDL transport from endosomes to lysosomes as studied by analysis of endocytic vesicles on Percoll density gradients and induced a redistribution of endocytic organelles as determined by fluorescence microscopy of intact cells using appropriate markers. This resulted in decreased degradation of 125I-labeled LDL in the presence of C6-Cer. These results suggest that endogenous ceramide may modulate endocytosis.

Maturation of early endosomes and vesicular traffic to lysosomes in relation to membrane recycling

The controversy whether endocytic processing occurs by organellar maturation or by vesicular traffic has not been resolved. It is also not clear whether maturation continues to the stage of lysosomes, to what extent it involves a decrease in organellar fusogenicity, and how it relates to membrane recycling. Maturation and vesicular traffic imply distinct kinetics for the intermingling of endocytic markers after sequential endocytic uptake. We have studied the kinetics of intermingling of fluid-phase markers (fluorescein-labelled dextran and horseradish peroxidase) and cell surface-derived membrane (labelled by galactosylation) in organelles at early and late stages of the endocytic pathway in macrophage-like P388D1 cells. Intermingling declined by sigmoid kinetics, indicating that endosomes matured within about 3 minutes to become non-fusogenic towards early endosomes. During maturation about 60% of internalized membrane was recycled with T1/2 approximately 2 minutes. Whereas matured endosomes were non-fusogenic towards early endosomes and towards each other, a second phase of intermingling was observed upon delivery to lysosomes. This intermingling occurred by a first-order process (T1/2 approximately 4 minutes), concurrent with recycling of the remaining 40% of internalized membrane marker. These kinetic observations suggest a model for endocytic processing which reconciles maturation of early endosomes with the known function of carrier vesicles: Endocytic carrier vesicles do not bud off from permanent early endosomes as proposed for vesicular traffic, but are derived, together with recycling vesicles, from the maturation of early endosomes which are consumed by this process; these carrier vesicles subsequently mediate delivery to lysosomes by vesicular traffic during which the remaining surface-derived membrane is recycled.

The receptor-recycling and lysosome biogenesis mutant TfT1.11 belongs to a new complementation group, End6

We have previously isolated a Chinese hamster ovary (CHO) mutant with a temperature-dependent pleiotropic defect in receptor recycling. This mutant, TfT1.11, has also been shown to have defects in fluid-phase endocytosis and lysosome biogenesis. Previously isolated CHO cell mutants with defects in endocytosis have been assigned to five recessive complementation groups (End1-End5). We have performed complementation analysis using polyethylene glycol-induced fusion of genetically marked sublines of TfT1.11 with representative mutants from each of the End groups. Complementation of the receptor trapping and lysosome biogenesis defects as well as temperature lethality was observed with all groups, demonstrating that TfT1.11 defines a new complementation group, End6.