Effect of alterations in the size of the vacuolar compartment on pinocytosis in J774.2 macrophages

Regional differences in the ultrastructure of mucosal macrophages in the rat large intestine

We previously clarified the histological characteristics of macrophages in the rat small intestine using serial block-face scanning electron microscopy (SBF-SEM). However, the regional differences in the characteristics of macrophages throughout the large intestine remain unknown. Here, we performed a pilot study to explore the regional differences in the ultrastructure of mucosal macrophages in the large intestine by using SBF-SEM analysis. SBF-SEM analysis conducted on the luminal side of the cecum and descending colon revealed macrophages as amorphous cells possessing abundant lysosomes and vacuoles. Macrophages in the cecum exhibited a higher abundance of lysosomes and a lower abundance of vacuoles than those in the descending colon. Macrophages with many intraepithelial cellular processes were observed beneath the intestinal superficial epithelium in the descending colon. Moreover, macrophages in contact with nerve fibers were more prevalent in the cecum than in the descending colon, and a subset of them surrounded a nerve bundle only in the cecum. In conclusion, the present pilot study suggested that the quantity of some organelles (lysosomes and vacuoles) in macrophages differed between the cecum and the descending colon and that there were some region-specific subsets of macrophages like nerve-associated macrophages in the cecum.

SLC12A9 is a lysosome-detoxifying ammonium - chloride co-transporter

Ammonia is a ubiquitous, toxic by-product of cell metabolism. Its high membrane permeability and proton affinity causes ammonia to accumulate inside acidic lysosomes in its poorly membrane-permeant form: ammonium (NH ₄ ⁺ ). Ammonium buildup compromises lysosomal function, suggesting the existence of mechanisms that protect cells from ammonium toxicity. Here, we identified SLC12A9 as a lysosomal ammonium exporter that preserves lysosomal homeostasis. SLC12A9 knockout cells showed grossly enlarged lysosomes and elevated ammonium content. These phenotypes were reversed upon removal of the metabolic source of ammonium or dissipation of the lysosomal pH gradient. Lysosomal chloride increased in SLC12A9 knockout cells and chloride binding by SLC12A9 was required for ammonium transport. Our data indicate that SLC12A9 is a chloride-driven ammonium co-transporter that is central in an unappreciated, fundamental mechanism of lysosomal physiology that may have special relevance in tissues with elevated ammonia, such as tumors.

Sugar transporter Slc37a2 regulates bone metabolism in mice via a tubular lysosomal network in osteoclasts

Osteoclasts are giant bone-digesting cells that harbor specialized lysosome-related organelles termed secretory lysosomes (SLs). SLs store cathepsin K and serve as a membrane precursor to the ruffled border, the osteoclast's 'resorptive apparatus'. Yet, the molecular composition and spatiotemporal organization of SLs remains incompletely understood. Here, using organelle-resolution proteomics, we identify member a2 of the solute carrier 37 family (Slc37a2) as a SL sugar transporter. We demonstrate in mice that Slc37a2 localizes to the SL limiting membrane and that these organelles adopt a hitherto unnoticed but dynamic tubular network in living osteoclasts that is required for bone digestion. Accordingly, mice lacking Slc37a2 accrue high bone mass owing to uncoupled bone metabolism and disturbances in SL export of monosaccharide sugars, a prerequisite for SL delivery to the bone-lining osteoclast plasma membrane. Thus, Slc37a2 is a physiological component of the osteoclast's unique secretory organelle and a potential therapeutic target for metabolic bone diseases.

Detection and quantification of the vacuolar H+ATPase using the Legionella effector protein SidK

Acidification of secretory and endocytic organelles is required for proper receptor recycling, membrane traffic, protein degradation, and solute transport. Proton-pumping vacuolar H+ ATPases (V-ATPases) are responsible for this luminal acidification, which increases progressively as secretory and endocytic vesicles mature. An increasing density of V-ATPase complexes is thought to account for the gradual decrease in pH, but available reagents have not been sufficiently sensitive or specific to test this hypothesis. We introduce a new probe to localize and quantify V-ATPases. The probe is derived from SidK, a Legionella pneumophila effector protein that binds to the V-ATPase A subunit. We generated plasmids encoding fluorescent chimeras of SidK1-278, and labeled recombinant SidK1-278 with Alexa Fluor 568 to visualize and quantify V-ATPases with high specificity in live and fixed cells, respectively. We show that V-ATPases are acquired progressively during phagosome maturation, that they distribute in discrete membrane subdomains, and that their density in lysosomes depends on their subcellular localization.

Endomembrane Tension and Trafficking

Eukaryotic cells employ diverse uptake mechanisms depending on their specialized functions. While such mechanisms vary widely in their defining criteria: scale, molecular machinery utilized, cargo selection, and cargo destination, to name a few, they all result in the internalization of extracellular solutes and fluid into membrane-bound endosomes. Upon scission from the plasma membrane, this compartment is immediately subjected to extensive remodeling which involves tubulation and vesiculation/budding of the limiting endomembrane. This is followed by a maturation process involving concomitant retrograde transport by microtubule-based motors and graded fusion with late endosomes and lysosomes, organelles that support the degradation of the internalized content. Here we review an important determinant for sorting and trafficking in early endosomes and in lysosomes; the control of tension on the endomembrane. Remodeling of endomembranes is opposed by high tension (caused by high hydrostatic pressure) and supported by the relief of tension. We describe how the timely and coordinated efflux of major solutes along the endocytic pathway affords the cell control over such tension. The channels and transporters that expel the smallest components of the ingested medium from the early endocytic fluid are described in detail as these systems are thought to enable endomembrane deformation by curvature-sensing/generating coat proteins. We also review similar considerations for the lysosome where resident hydrolases liberate building blocks from luminal macromolecules and transporters flux these organic solutes to orchestrate trafficking events. How the cell directs organellar trafficking based on the luminal contents of organelles of the endocytic pathway is not well-understood, however, we propose that the control over membrane tension by solute transport constitutes one means for this to ensue.

Macropinosomes as units of signal transduction

Macropinosome formation occurs as a localized sequence of biochemical activities and associated morphological changes, which may be considered a form of signal transduction leading to the construction of an organelle. Macropinocytosis may also convey information about the availability of extracellular nutrients to intracellular regulators of metabolism. Consistent with this idea, activation of the metabolic regulator mechanistic target of rapamycin complex-1 (mTORC1) in response to acute stimulation by growth factors and extracellular amino acids requires internalization of amino acids by macropinocytosis. This suggests that macropinocytosis is necessary for mTORC1-dependent growth of metazoan cells, both as a route for delivery of amino acids to sensors associated with lysosomes and as a platform for growth factor-dependent signalling to mTORC1 via phosphatidylinositol 3-kinase (PI3K) and the Akt pathway. Because the biochemical signals required for the construction of macropinosomes are also required for cell growth, and inhibition of macropinocytosis inhibits growth factor signalling to mTORC1, we propose that signalling by growth factor receptors is organized into stochastic, structure-dependent cascades of chemical reactions that both build a macropinosome and stimulate mTORC1. More generally, as discrete units of signal transduction, macropinosomes may be subject to feedback regulation by metabolism and cell dimensions. This article is part of the Theo Murphy meeting issue 'Macropinocytosis'.

Architectural remodeling of the tonoplast during fluid-phase endocytosis

During fluid phase endocytosis (FPE) in plant storage cells, the vacuole receives a considerable amount of membrane and fluid contents. If allowed to accumulate over a period of time, the enlarging tonoplast and increase in fluids would invariably disrupt the structural equilibrium of the mature cells. Therefore, a membrane retrieval process must exist that will guarantee membrane homeostasis in light of tonoplast expansion by membrane addition during FPE. We examined the morphological changes to the vacuolar structure during endocytosis in red beet hypocotyl tissue using scanning laser confocal microscopy and immunohistochemistry. The heavily pigmented storage vacuole allowed us to visualize all architectural transformations during treatment. When red beet tissue was incubated in 200 mM sucrose, a portion of the sucrose accumulated entered the cell by means of FPE. The accumulation process was accompanied by the development of vacuole-derived vesicles which transiently counterbalanced the addition of surplus endocytic membrane during rapid rates of endocytosis. Topographic fluorescent confocal micrographs showed an ensuing reduction in the size of the vacuole-derived vesicles and further suggest their reincorporation into the vacuole to maintain vacuolar unity and solute concentration.

Maturation of the Coxiella burnetii parasitophorous vacuole requires bacterial protein synthesis but not replication

This study examined whether protein synthesis and replication are required for maturation and fusogenicity of the lysosomal-like, large and spacious parasitophorous vacuole (PV) of Coxiella burnetii, an obligate intracellular bacterium. Large and spacious PV with multiple non-replicating C. burnetii were observed by phase microscopy in Vero cells infected at a multiplicity of infection of ten and treated with a bacteriostatic concentration of nalidixic acid or carbenicillin, antimicrobics that inhibit DNA and cell wall biosynthesis respectively. Conversely, large and spacious PV were not observed in cells treated with a bacteriostatic concentration of the protein synthesis inhibitor chloramphenicol. Rather, fluorescence microscopy of individual cells revealed multiple, acidic PV harbouring a single organism tightly bounded by a LAMP-1 positive vacuolar membrane. These vacuoles homotypically fused to form a large and spacious PV upon removal of the drug. Chloramphenicol also inhibited trafficking of latex beads to large and spacious PV and caused mature PV to collapse. Collectively, these results demonstrate that C. burnetii protein synthesis, but not replication, is required for fusion between nascent C. burnetii PV and latex bead phagosomes, and also for formation and maintenance of large and spacious, replicative PV. However, transit of nascent PV through the endocytic pathway to ultimately acquire lysosomal markers appears to occur irrespective of Coxiella protein synthesis.

Susceptibility of lysosomes to rupture is a determinant for plasma membrane disruption in tumor necrosis factor alpha-induced cell death

Since a release of intracellular contents can induce local inflammatory responses, mechanisms that lead to loss of plasma membrane integrity in cell death are important to know. We showed previously that deficiency of the plasma membrane Ca2+ ATPase 4 (PMCA4) in L929 cells impaired tumor necrosis factor alpha (TNF-alpha)-induced enlargement of lysosomes and reduced cell death. The lysosomal changes can be determined by measuring the total volume of intracellular acidic compartments per cell (VAC), and we show here that inhibition of the increase in VAC due to PMCA4 deficiency not only reduced cell death but also converted TNF-alpha-induced cell death from a process involving disruption of the plasma membrane to a cell demise with a nearly intact plasma membrane. The importance of the size of lysosomes in determining plasma membrane integrity during cell death was supported by the observations that chemical inhibitors that reduce VAC also reduced the plasma membrane disruption induced by TNF-alpha in wild-type L929 cells, while increases in VAC due to genetic mutation, senescence, cell culture conditions, and chemical inhibitors all changed the morphology of cell death from one with an originally nearly intact plasma membrane to one with membrane disruption in a number of different cells. Moreover, the ATP depletion-mediated change from apoptosis to necrosis is also associated with the increases of VAC. The increase in lysosomal size may due to intracellular self-digestion of dying cells. Big lysosomes are easy to rupture, and the release of hydrolytic enzymes from ruptured lysosomes can cause plasma membrane disruption.

Resistance to tumor necrosis factor-induced cell death mediated by PMCA4 deficiency

We used retrovirus insertion-mediated random mutagenesis to generate tumor necrosis factor (TNF)-resistant lines from L929 cells. Using this approach, we discovered that the plasma membrane calcium ATPase 4 (PMCA4) is required for TNF-induced cell death in L929 cells. Under basal conditions, PMCA4-deficient (PMCA(mut)) cells have a normal phenotype. However, stimulation with TNF induces an abnormal increase in the intracellular calcium concentration ([Ca(2+)](i)). The substantially elevated [Ca(2+)](i) caused resistance to TNF-induced cell death. We found that an increase in the total volume of acidic compartments (VAC), mainly constituted by lysosomes, is a common event in cell death caused by a variety of agonists. The increased [Ca(2+)](i) in PMCA(mut) cells promoted lysosome exocytosis, which, at least in part, accounted for the inhibition of TNF-induced increase in VAC and cell death. Promoting lysosome exocytosis by calcium inhibited TNF-induced cell death in wild-type L929 cells, while inhibition of lysosome exocytosis or increase of VAC by sucrose restored the sensitivity of PMCA(mut) cells to TNF-induced cell death. Thus, increase of the volume of acidic compartment is a part of the cell death process, and the antideath effect of calcium is mediated, at least in part, by inhibition of the TNF-induced increase in VAC.

Cholera toxin and Escherichia coli enterotoxin B-subunits inhibit macrophage-mediated antigen processing and presentation: evidence for antigen persistence in non-acidic recycling endosomal compartments

Cholera toxin (Ctx) and the closely related Escherichia coli heat-labile enterotoxin (Etx) not only act as mediators of diarrhoeal disease but also exert potent immunomodulatory properties on mammalian immune systems. The toxins normally exert their diarrhoeagenic effects by initiating receptor-mediated uptake into vesicles that enter a retrograde trafficking pathway, circumventing degradative compartments and targeting them to the trans-Golgi network (TGN) and endoplasmic reticulum. Here, we examine whether receptor-mediated binding and cellular entry by the toxin B-subunits also lead to concomitant changes in uptake and trafficking of exogenous antigens that could contribute to the potent immunomodulatory properties of these toxins. Treatment of the macrophage (J774.2) cell line with Etx B-subunit (EtxB) resulted in EtxB transport to the TGN and also led to the formation of large, translucent, non-acidic, EtxB-devoid vacuoles. When exogenous antigens were added, EtxB-treated cells were found to be proficient in both internalization of ovalbumin (OVA) and phagocytosis of bacterial particles. However, the internalized OVA, instead of trafficking along a lysosome-directed endocytic pathway via acidified endosomes, persisted in a non-acidic, light-density compartment that was distinct from the translucent vacuoles. The rerouted OVA did not co-localize with the endosomal markers rab5 or rab11, nor with EtxB, but was retained in a transferrin receptor-positive compartment. The failure of OVA to enter the late endosomal/lysosomal compartments correlated with a striking inhibition of OVA peptide processing and presentation to OVA-responsive CD4+ T-cells. CtxB also modulated OVA trafficking and inhibited antigen presentation. These findings demonstrate that the B-subunits of Ctx and Etx alter the progression of exogenous antigens along the endocytic processing pathway, and prevent or delay efficient epitope presentation and T-cell stimulation. The formation of such 'antigen depots' could contribute to the immunomodulatory properties of these bacterial virulence determinants.

Dense core lysosomes can fuse with late endosomes and are re-formed from the resultant hybrid organelles

Electron microscopy was used to evaluate the function and formation of dense core lysosomes. Lysosomes were preloaded with bovine serum albumin (BSA)-gold conjugates by fluid phase endocytosis using a pulse-chase protocol. The gold particles present in dense core lysosomes and late endosomes were flocculated, consistent with proteolytic degradation of the BSA. A second pulse of BSA-gold also accumulated in the pre-loaded dense core lysosomes at 37 degrees C, but accumulation was reversibly blocked by incubation at 20 degrees C. Time course experiments indicated that mixing of the two BSA-gold conjugates initially occurred upon fusion of mannose 6-phosphate receptor-positive/lysosomal glycoprotein-positive late endosomes with dense core lysosomes. Treatment for 5 hours with wortmannin, a phosphatidyl inositide 3-kinase inhibitor, caused a reduction in number of dense core lysosomes preloaded with BSA-gold and prevented a second pulse of BSA-gold accumulating in them. After wortmannin treatment the two BSA-gold conjugates were mixed in swollen late endosomal structures. Incubation of NRK cells with 0.03 M sucrose resulted in the formation of swollen sucrosomes which were morphologically distinct from preloaded dense core lysosomes and were identified as late endosomes and hybrid endosome-lysosome structures. Subsequent endocytosis of invertase resulted in digestion of the sucrose and re-formation of dense core lysosomes. These observations suggest that dense core lysosomes are biologically active storage granules of lysosomal proteases which can fuse with late endosomes and be re-formed from the resultant hybrid organelles prior to subsequent cycles of fusion and re-formation.

Lipofuscin accumulation in cultured retinal pigment epithelial cells causes enhanced sensitivity to blue light irradiation

Lipofuscin accumulates with age within secondary lysosomes of retinal pigment epithelial (RPE) cells of humans and many animals. The autofluorescent lipofuscin pigment has an excitation maximum within the range of visible blue light, while it is emitting in the yellow-orange area. This physico-chemical property of the pigment indicates that it may have a photo-oxidative capacity and, consequently, then should destabilize lysosomal membranes of blue-light exposed RPE. To test this hypothesis, being of relevance to the understanding of age-related macular degeneration, cultures of heavily lipofuscin-loaded RPE cells were blue-light-irradiated and compared with respect to lysosomal stability and cell viability to relevant controls. To rapidly convert primary cultures of RPE, obtained from neonatal rabbits, into aged, lipofuscin-loaded cells, they were allowed to phagocytize artificial lipofuscin that was prepared from outer segments of bovine rods and cones. Following blue-light irradiation, lysosomal membrane stability was measured by vital staining with the lysosomotropic weak base, and metachromatic fluorochrome, acridine orange (AO). Quantifying red (high AO concentration within intact lysosomes with preserved proton gradient over their membranes) and green fluorescence (low AO concentration in nuclei, damaged lysosomes with decreased or lost proton gradients, and in the cytosol) allowed an estimation of the lysosomal membrane stability after blue-light irradiation. Cellular viability was estimated with the delayed trypan blue dye exclusion test. Lipofuscin-loaded blue-light-exposed RPE cells showed a considerably enhanced loss of both lysosomal stability and viability when compared to control cells. It is concluded that the accumulation of lipofuscin within secondary lysosomes of RPE sensitizes these cells to blue light by inducing photo-oxidative alterations of their lysosomal membranes resulting in a presumed leakage of lysosomal contents to the cytosol with ensuing cellular degeneration of apoptotic type. The suggested mechanism may have bearings on the development of age-related macular degeneration.

Evidence for retrograde traffic between terminal lysosomes and the prelysosomal/late endosome compartment

We have investigated the interactions occurring between the prelysosomal compartment, PLC/late endosome, and terminal lysosomes using an approach that allowed us to internalize and deliver specific tracers to these compartments, and look for evidence of their meeting. After internalization of sucrose, the lysosomes (sucrosomes), but not the PLC/late endosomes, became significantly swollen and visible in the light microscope. If invertase is then added to the medium it reaches the lysosomes where it cleaves sucrose into fructose and glucose. These sugars, unlike sucrose, can be transported into the cytosol, resulting in the disappearance of the sucrosomes. We previously showed that phagocytosed latex beads are delivered specifically to, and reside in, the PLC/late endosome, a stage earlier than the lysosomes in the endocytic pathway (Rabinowitz et al. (1992) J. Cell Biol. 116, 95–112). In the present study, we demonstrate that invertase conjugated to the latex beads, and thus immobilized in late endosomes, has access to the sucrose present in the more distal lysosomes. Experiments using nocodazole indicate that this retrograde fusion event requires the presence of an intact microtubule network. The simplest interpretation of our results is that the two compartments fuse, allowing for a retrograde transport of sucrose from the lysosomes to the PLC/late endosomes.

Characterization of HeLa cell vacuoles induced by Helicobacter pylori broth culture supernatant

Helicobacter pylori broth culture supernatants induce eukaryotic cell vacuolation in vitro, a phenomenon that has been attributed to cytotoxic activity. We sought to characterize further the vacuolation of HeLa cells that occurs in response to H pylori culture supernatant. Nascent vacuoles were detectable by electron microscopy after 90 minutes of incubation with H pylori supernatant and were not associated with any identifiable organelle. After 6 days of incubation with H pylori supernatant, vacuoles were membrane-bound structures filled with electron-dense debris, which resembled secondary lysosomes. Acid phosphatase activity was detected within the vacuoles. The vacuoles induced by H pylori supernatant were then compared with vacuoles induced by trimethylamine, a weak base known to induce lysosomal swelling. Neutral red dye rapidly entered the vacuoles induced by either H pylori supernatant or trimethylamine, and both types of vacuoles were reversible. Compared with trimethylamine-induced vacuoles, the vacuoles induced by H pylori supernatant were larger and typically lacked a limiting membrane. In the early stages of formation, vacuoles induced by trimethylamine were labeled by lucifer yellow, a pinocytotic marker, whereas H pylori cytotoxin-induced vacuoles were not. These data suggest that trimethylamine-induced vacuoles arise directly from endocytic compartments, whereas H pylori cytotoxin induces vacuole formation via an autophagic mechanism.

Cellular dimensions affecting the nucleocytoplasmic volume ratio

Although it has long been appreciated that larger eukaryotic cells have larger nuclei, little is known about how this size relationship is maintained. Here we describe a method for measuring the aqueous volume ratio of nucleus to cytoplasm, two compartments which are interconnected via the pores in the nuclear envelope. We then use that method to identify proportional cellular dimensions in variously treated cells and in different cell types. Cells were scrape loaded with a mixture of fluorescent dextrans: Texas red dextran, average mol wt = 10,000 (TRDx10), and fluorescein isothiocyanate dextran, average mol wt = 70,000 (FDx70). After introduction into the cytoplasmic space, the TRDx10 distributed into both the nucleus and cytoplasm, whereas the FDx70 was restricted to cytoplasm, due to size exclusion by the nuclear pores. The aqueous nucleocytoplasmic volume ratio (RN/C) was determined by measuring, from fluorescence images of spread cells, total cellular fluorescence of each of the two probes and the fluorescence ratio of those probes in the cytoplasm. RN/C was unaffected by the measurement procedure or by varying temperatures between 23 degrees and 37 degrees C. Loading excess unlabeled dextrans had little effect on RN/C, with the single exception that high concentrations of large dextrans could lower RN/C in endothelial cells. Expanding intracellular membranous compartments of macrophages by phagocytosis of latex beads decreased RN/C. Expanding the same compartment by pinocytosis of sucrose, which nearly doubled total cell volume, had little effect on RN/C, indicating that nuclear volume was more closely linked to the cytoplasmic volume, exclusive of vesicular organelles, than to total cell volume. RN/C was the same in mononucleate and binucleate endothelial cells. Finally, measurements of RN/C in murine bone marrow-derived macrophages, bovine aortic endothelial cells, Swiss 3T3 fibroblasts, PtK2 cells, and CV-1 cells revealed that nuclear volume scaled allometrically with cell volume. The allometric relationship indicated that cell volume was proportional to nuclear surface area.

The effects of acetylated low-density lipoproteins on fluid-phase pinocytosis by macrophages

A simple method has been set up to measure the rate of fluid-phase pinocytosis in resident mouse peritoneal macrophages in culture. The method uses 125I-labelled polyvinylpyrrolidone as a nondegradable marker of fluid-phase pinocytosis. The accumulation of 125I-labelled polyvinylpyrrolidone by the cells was directly proportional to its concentration in the culture medium up to at least 200 micrograms/ml. The estimates of the rate of fluid-phase pinocytosis were reproducible within each experiment (coefficient of variation 8.5%) but varied between individual experiments. Fluid-phase pinocytosis was undetectable at 4 degrees C and reduced greatly at 37 degrees C by metabolic inhibitors and 1 mM ZnSO4. High concentrations of human acetylated low-density lipoproteins, which are taken up rapidly by macrophages, decreased the rate of fluid-phase pinocytosis by up to about 70%. The inhibition was seen after only 2 h of incubation. Unmodified low-density lipoproteins, which are taken up only slowly by macrophages, did not usually inhibit fluid-phase pinocytosis (in fact, they sometimes increased it). Modified low-density lipoprotein uptake, leading to massive lipid accumulation in macrophages in the arterial wall, has been postulated to be involved in the pathogenesis of atherosclerosis. This study raises the possibility that the rate of fluid-phase pinocytosis in these lipid-laden arterial macrophages may be reduced.

Tubular lysosomes accompany stimulated pinocytosis in macrophages

A network of tubular lysosomes extends through the cytoplasm of J774.2 macrophages and phorbol ester-treated mouse peritoneal macrophages. The presence of this network is dependent upon the integrity of cytoplasmic microtubules and correlates with high cellular rates of accumulation of Lucifer Yellow (LY), a marker of fluid phase pinocytosis. We tested the hypothesis that the efficiency of LY transfer between the pinosomal and lysosomal compartments is increased in the presence of tubular lysosomes by asking how conditions that deplete the tubular lysosome network affect pinocytic accumulation of LY. Tubular lysosomes were disassembled in cells treated with microtubule-depolymerizing drugs or in cells that had phagocytosed latex beads. In unstimulated peritoneal macrophages, which normally contain few tubular lysosomes and which exhibit relatively inefficient transfer of pinocytosed LY to lysosomes, such treatments had little effect on pinocytosis. However, in J774 macrophages and phorbol ester-stimulated peritoneal macrophages, these treatments markedly reduced the efficiency of pinocytic accumulation of LY. We conclude that a basal level of solute accumulation via pinocytosis proceeds independently of the tubular lysosomes, and that an extended tubular lysosomal network contributes to the elevated rates of solute accumulation that accompany macrophage stimulation. Moreover, we suggest that the transformed mouse macrophage cell line J774 exhibits this stimulated pinocytosis constitutively.

Tubular lysosome morphology and distribution within macrophages depend on the integrity of cytoplasmic microtubules

Pinocytosis of the fluorescent dye lucifer yellow labels elongated, membrane-bound tubular organelles in several cell types, including cultured human monocytes, thioglycolate-elicited mouse peritoneal macrophages, and the macrophage-like cell line J774.2. These tubular structures can be identified as lysosomes by acid phosphatase histochemistry and immunofluorescence localization of cathepsin L. The abundance of tubular lysosomes is markedly increased by treatment with phorbol 12-myristate 13-acetate (10 ng/ml). When labeled by pinocytosis of microperoxidase and examined by electron microscopic histochemistry, the tubular lysosomes have an outside diameter of approximately 75 nm and a length of several micrometers; they radiate from the cell's centrosphere in alignment with cytoplasmic microtubules and intermediate filaments. Incubation of phorbol myristate acetate-treated macrophages at 4 degrees C or in medium containing 5 microM colchicine or nocodazole at 37 degrees C leads to disassembly of microtubules and fragmentation of the tubular lysosomes. Return of the cultures to 37 degrees C or removal of nocodazole from the medium leads to reassembly of microtubules and the reappearance of tubular lysosomes within 10-20 min. We conclude that microtubules are essential for the maintenance of tubular lysosome morphology and that, in macrophages, a significant proportion of the lysosomal compartment is contained within these tubular structures.