Separation of lysosomes and autophagosomes by means of glycyl-phenylalanine-naphthylamide, a lysosome-disrupting cathepsin-C substrate

Initiation of acute pancreatitis in mice is independent of fusion between lysosomes and zymogen granules

The co-localization of the lysosomal protease cathepsin B (CTSB) and the digestive zymogen trypsinogen is a prerequisite for the initiation of acute pancreatitis. However, the exact molecular mechanisms of co-localization are not fully understood. In this study, we investigated the role of lysosomes in the onset of acute pancreatitis by using two different experimental approaches. Using an acinar cell-specific genetic deletion of the ras-related protein Rab7, important for intracellular vesicle trafficking and fusion, we analyzed the subcellular distribution of lysosomal enzymes and the severity of pancreatitis in vivo and ex vivo. Lysosomal permeabilization was performed by the lysosomotropic agent Glycyl-L-phenylalanine 2-naphthylamide (GPN). Acinar cell-specific deletion of Rab7 increased endogenous CTSB activity and despite the lack of re-distribution of CTSB from lysosomes to the secretory vesicles, the activation of CTSB localized in the zymogen compartment still took place leading to trypsinogen activation and pancreatic injury. Disease severity was comparable to controls during the early phase but more severe at later time points. Similarly, GPN did not prevent CTSB activation inside the secretory compartment upon caerulein stimulation, while lysosomal CTSB shifted to the cytosol. Intracellular trypsinogen activation was maintained leading to acute pancreatitis similar to controls. Our results indicate that initiation of acute pancreatitis seems to be independent of the presence of lysosomes and that fusion of lysosomes and zymogen granules is dispensable for the disease onset. Intact lysosomes rather appear to have protective effects at later disease stages.

Lysosomal exocytosis of ATP is coupled to P2Y2 receptor in marginal cells in the stria vascular in neonatal rats

Adenosine triphosphate (ATP) is stored as lysosomal vesicles in marginal cells of the stria vascular in neonatal rats, but the mechanisms of ATP release are unclear. Primary cultures of marginal cells from 1-day-old Sprague-Dawley rats were established. P2Y₂ receptor and inositol 1,4,5-trisphosphate (IP3) receptor were immunolabelled in marginal cells of the stria vascular. We found that 30 μM ATP and 30 μM uridine triphosphate (UTP) evoked comparable significant increases in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in the absence of extracellular Ca²⁺, whereas the response was suppressed by 100 μM suramin, 10 μM 1-(6-(17β-3-methoxyester-1,3,5(10)-trien-17-yl)amino)-hexyl)-1H-pyrrole-2,5-dione(U-73122), 100 μM 2-aminoethoxydiphenyl borate (2-APB) and 5 μM thapsigargin (TG), thus indicating that ATP coupled with the P2Y₂R-PLC-IP3 pathway to evoke Ca²⁺ release from the endoplasmic reticulum (ER). Incubation with 200 μM Gly-Phe-β-naphthylamide (GPN) selectively disrupted lysosomes and caused significant increases in [Ca²⁺]_I; this effect was partly inhibited by P2Y₂R-PLC-IP3 pathway antagonists. After pre-treatment with 5 μM TG, [Ca²⁺]_i was significantly lower than that after treatment with P2Y₂R-PLC-IP3 pathway antagonists under the same conditions, thus indicating that lysosomal Ca²⁺ triggers Ca²⁺ release from ER Ca²⁺ stores. Baseline [Ca²⁺]_i declined after treatment with the Ca²⁺ chelator 50 μM bis-(aminophenolxy) ethane-N,N,N',N'-tetra-acetic acid acetoxyme-thyl ester (BAPTA-AM) and 4 IU/ml apyrase. 30 μM ATP decrease of the number of quinacrine-positive vesicles via lysosome exocytosis, whereas the number of lysosomes did not change. However, lysosome exocytosis was significantly suppressed by pre-treatment with 5 μM vacuolin-1. Release of ATP and β-hexosaminidase both increased after treatment with 200 μM GPN and 5 μM TG, but decreased after incubation with 50 μM BAPTA-AM, 4 IU/ml apyrase and 5 μM vacuolin-1. We suggest that ATP triggers Ca²⁺ release from the ER, thereby contributing to secretion of lysosomal ATP via lysosomal exocytosis. Lysosomal stored Ca²⁺ triggers Ca²⁺ release from the ER directly though the IP3 receptors, and lysosomal ATP evokes Ca²⁺ signals indirectly via the P2Y₂R-PLC-IP3 pathway.

Autophagy: an adaptive physiological countermeasure to cellular senescence and ischaemia/reperfusion-associated cardiac arrhythmias

Oxidative stress placed on tissues that involved in pathogenesis of a disease activates compensatory metabolic changes, such as DNA damage repair that in turn causes intracellular accumulation of detritus and ‘proteotoxic stress’, leading to emergence of ‘senescent’ cellular phenotypes, which express high levels of inflammatory mediators, resulting in degradation of tissue function. Proteotoxic stress resulting from hyperactive inflammation following reperfusion of ischaemic tissue causes accumulation of proteinaceous debris in cells of the heart in ways that cause potentially fatal arrhythmias, in particular ventricular fibrillation (VF). An adaptive response to VF is occurrence of autophagy, an intracellular bulk degradation of damaged macromolecules and organelles that may restore cellular and tissue homoeostasis, improving chances for recovery. Nevertheless, depending on the type and intensity of stressors and inflammatory responses, autophagy may become pathological, resulting in excessive cell death. The present review examines the multilayered defences that cells have evolved to reduce proteotoxic stress by degradation of potentially toxic material beginning with endoplasmic reticulum‐associated degradation, and the unfolded protein response, which are mechanisms for removal from the endoplasmic reticulum of misfolded proteins, and then progressing through the stages of autophagy, including descriptions of autophagosomes and related vesicular structures which process material for degradation and autophagy‐associated proteins including Beclin‐1 and regulatory complexes. The physiological roles of each mode of proteotoxic defence will be examined along with consideration of how emerging understanding of autophagy, along with a newly discovered regulatory cell type called telocytes, may be used to augment existing strategies for the prevention and management of cardiovascular disease.

Ryanodine receptor antagonists adapt NPC1 proteostasis to ameliorate lipid storage in Niemann-Pick type C disease fibroblasts

Niemann-Pick type C disease is a lysosomal storage disorder most often caused by loss-of-function mutations in the NPC1 gene. The encoded multipass transmembrane protein is required for cholesterol efflux from late endosomes and lysosomes. Numerous missense mutations in the NPC1 gene cause disease, including the prevalent I1061T mutation that leads to protein misfolding and degradation. Here, we sought to modulate the cellular proteostasis machinery to achieve functional recovery in primary patient fibroblasts. We demonstrate that targeting endoplasmic reticulum (ER) calcium levels using ryanodine receptor (RyR) antagonists increased steady-state levels of the NPC1 I1061T protein. These compounds also promoted trafficking of mutant NPC1 to late endosomes and lysosomes and rescued the aberrant storage of cholesterol and sphingolipids that is characteristic of disease. Similar rescue was obtained using three distinct RyR antagonists in cells with missense alleles, but not with null alleles, or by over-expressing calnexin, a calcium-dependent ER chaperone. Our work highlights the utility of proteostasis regulators to remodel the protein-folding environment in the ER to recover function in the setting of disease-causing missense alleles.

Galectin 8 targets damaged vesicles for autophagy to defend cells against bacterial invasion

Autophagy defends the mammalian cytosol against bacterial infection.^1-3 Efficient pathogen engulfment is mediated by cargo-selecting autophagy adaptors that rely on unidentified pattern-recognition or danger receptors to label invading pathogens as autophagy cargo, typically by poly-ubiquitin coating.^4-9 Here we show that galectin-8, a cytosolic lectin, is a danger receptor that restricts Salmonella proliferation. Galectin-8 monitors endo-lysosomal integrity and detects bacterial invasion by binding host glycans exposed on damaged Salmonella-containing vacuoles. By recruiting NDP52 galectin-8 activates anti-bacterial autophagy. Galectin-8-dependent recruitment of NDP52 to Salmonella-containing vesicles is transient and followed by ubiquitin-dependent NDP52 recruitment. Since galectin-8 also detects sterile damage to endosomes or lysosomes, as well as invasion by Listeria or Shigella, we suggest galectin-8 serves as a versatile receptor for vesicle-damaging pathogens. Our results illustrate how cells deploy the danger receptor galectin-8 to combat infection by monitoring endo-lysososomal integrity based on the specific lack of complex carbohydrates in the cytosol.

Potential limitations in the use of KillerRed for fluorescence microscopy

KillerRed, a bright red fluorescent protein, is a genetically encoded photosensitizer, which generates radicals and hydrogen peroxide upon green light illumination. The protein is a potentially powerful tool for selective light-induced protein inactivation and cell killing, and can also be used to study downstream effects of locally increased levels of reactive oxygen species. The initial aim of this study was to investigate whether or not KillerRed-mediated reactive oxygen species production inside peroxisomes could trigger the sequestration of these organelles into autophagosomes. Green fluorescent protein-tagged microtubule-associated protein 1 light chain 3 was used as autophagosome marker. We observed that KillerRed also emits weak green fluorescence upon excitation at 480 nm, and this may lead to erroneous data interpretation in conditions where green fluorophores are used. We discuss this potential pitfall of KillerRed for biological imaging and formulate recommendations to avoid misinterpretation of the data.

TRPML: transporters of metals in lysosomes essential for cell survival?

Key aspects of lysosomal function are affected by the ionic content of the lysosomal lumen and, therefore, by the ion permeability in the lysosomal membrane. Such functions include regulation of lysosomal acidification, a critical process in delivery and activation of the lysosomal enzymes, release of metals from lysosomes into the cytoplasm and the Ca(2+)-dependent component of membrane fusion events in the endocytic pathway. While the basic mechanisms of lysosomal acidification have been largely defined, the lysosomal metal transport system is not well understood. TRPML1 is a lysosomal ion channel whose malfunction is implicated in the lysosomal storage disease Mucolipidosis Type IV. Recent evidence suggests that TRPML1 is involved in Fe(2+), Ca(2+) and Zn(2+) transport across the lysosomal membrane, ascribing novel physiological roles to this ion channel, and perhaps to its relatives TRPML2 and TRPML3 and illuminating poorly understood aspects of lysosomal function. Further, alterations in metal transport by the TRPMLs due to mutations or environmental factors may contribute to their role in the disease phenotype and cell death.

Deviant nicotinic acid adenine dinucleotide phosphate (NAADP)-mediated Ca2+ signaling upon lysosome proliferation

Accumulating evidence suggests that the endolysosomal system is a novel intracellular Ca(2+) pool mobilized by the second messenger, nicotinic acid adenine dinucleotide phosphate (NAADP). Although lysosomes in neurons are known to proliferate in numerous neurodegenerative diseases and during the normal course of aging, little is known concerning the effect of lysosomal proliferation on Ca(2+) homeostasis. Here, we induce proliferation of lysosomes in primary cultures of rat hippocampal neurons and PC12 cells through chronic treatment with the cathepsin inhibitor, Z-Phe-Ala-diazomethylketone. We demonstrate that lysosome proliferation increases the size of the lysosomal Ca(2+) pool and enhances Ca(2+) signals in response to direct cellular delivery of NAADP and glutamate, an identified NAADP-producing agonist. Our data suggest that deregulated lysosomal Ca(2+) signaling through NAADP may contribute to neuronal dysfunction and highlight the usefulness of lysosomal hydrolase inhibition in probing NAADP action.

The TRPC6 channel activator hyperforin induces the release of zinc and calcium from mitochondria

Hyperforin, an extract of the medicinal plant hypericum perforatum (also named St John's wort), possesses antidepressant properties. Recent data showed that it elevates the intracellular concentration of Ca(2+) by activating diacylglycerol-sensitive C-class of transient receptor potential (TRPC6) channels without activating the other isoforms (TRPC1, TRPC3, TRPC4, TRPC5, and TRPC7). This study was undertaken to further characterize the cellular neuronal responses induced by hyperforin. Experiments conducted on cortical neurons in primary culture and loaded with fluorescent probes for Ca(2+) (Fluo-4) and Zn(2+) (FluoZin-3) showed that it not only controls the activity of plasma membrane channels but it also mobilizes these two cations from internal pools. Experiments conducted on isolated brain mitochondria indicated that hyperforin, like the inhibitor of oxidative phosphorylation, carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP), collapses the mitochondrial membrane potential. Furthermore, it promotes the release of Ca(2+) and Zn(2+) from these organelles via a ruthenium red-sensitive transporter. In fact, hyperforin exerts complex actions on CNS neurons. This antidepressant not only triggers the entry of cations via plasma membrane TRPC6 channels but it displays protonophore-like properties. As hyperforin is now use to probe the functions of native TRPC6 channels, our data indicate that caution is required when interpreting results obtained with this antidepressant.

Cellular processes essential for African swine fever virus to infect and replicate in primary macrophages

The macrophage (Mø) is an essential immune cell for innate immunity. Such cells are targeted by African swine fever virus (ASFV). The early phases of infection with ASFV have been previously characterized in non-leukocyte cells such as Vero cells. Here, we report on several additional key parameters that ASFV utilizes during the infection of primary Mø. Related to virus infection, we established that receptor-mediated endocytosis of the virus by Mø is not the exclusive means of entry to infect the host cells. Analysis of the ensuing processes identified divalent cation-dependent activities to be particularly important, relating to the virus requirement for microtubule assembly needed for endocytic and endosomal processing. Actin-dependent endocytosis and endocytic flux involving microtubule activity are also implicated, pointing to entry via phagocytosis. Subsequently, the virus avoids terminal degradation by circumventing mature lysosome activities, including autophagosome-lysosome delivery. Nevertheless, the replicative cycle is apparently dependent on certain lysosomal functions, i.e. activities sensitive to propylamine are essential for the virus, whereas vinblastine- and leupeptin-sensitive functions only partially influence viral replication. The present work has identified cellular processes essential for ASFV to infect and replicate in the macrophage. These findings will improve our understanding of the cellular pathways employed by viruses infecting immune scavenger cells.

Extracellular ATP stimulates exocytosis via localized Ca(2+) release from acidic stores in rat pancreatic beta cells

Three different methods, membrane capacitance (C(m)) measurement, amperometry and FM dye labeling were used to investigate the role of extracellular ATP in insulin secretion from rat pancreatic beta cells. We found that extracellular application of ATP mobilized intracellular Ca(2+) stores and synchronously triggered vigorous exocytosis. No influence of ATP on the readily releasable pool of vesicles was observed, which argues against a direct modulation of the secretory machinery at a level downstream of Ca(2+) elevation. The stimulatory effects of ATP were greatly reduced by intracellular perfusion of BAPTA but not EGTA, suggesting a close spatial association of fusion sites with intracellular Ca(2+) releasing sites. ATP-induced Ca(2+) transients and exocytosis were not blocked by thapsigargin (TG), by a ryanodine receptor antagonist or by dissipation of pH in acidic stores by monensin alone, but they were greatly attenuated by IP(3) receptor inhibition as well as ionomycin plus monensin, suggesting involvement of IP(3)-sensitive acidic Ca(2+) stores. Taken together, our data suggest that extracellular ATP triggers exocytosis by mobilizing spatially limited acidic Ca(2+) stores through IP(3) receptors. This mechanism may explain how insulin secretion from the pancreas is coordinated through diffusible ATP that is co-released with insulin.

Lipopolysaccharide and phorbol 12-myristate 13-acetate both impair monocyte differentiation, relating cellular function to virus susceptibility

Both lipopolysaccharide (LPS) and phorbol 12-myristate 13-acetate (PMA) impeded monocyte to macrophage differentiation with respect to typical phenotypic modulation and certain phagocyte-related processes. The down-regulation of the porcine monocyte marker SWC1, and up-regulation of the SWC9 macrophage marker were retarded, but not inhibited, as was the differentiation-associated down-regulation of p53 and myeloperoxidase. Despite this clear impairment of macrophage differentiation, not all cellular functions were equally susceptible. Both agents inhibited phagocytosis, but not low-density lipoprotein receptor-associated endocytosis. Only LPS inhibited tartrate-resistant acid phosphatase up-regulation. In contrast, increase of vacuolar acidification rates was more susceptible to PMA. The activity of certain endosomal/lysosomal enzymes - esterase, nucleotidase, peroxidase and cathepsins - was generally enhanced by both LPS and PMA. This contrasted with autophagosomal activity, detected through the induction of an antiviral state. Disruption of autophagosomes and lysosomes (methionine-O-methyl ester), but not lysosomes alone (glycyl-L-phenylalanine) reversed LPS-induced inhibition of virus replication, without influencing the PMA-induced antiviral effect. Thus, PMA is similar to LPS in inhibiting monocyte to macrophage differentiation, when primary blood monocytes are employed, but not all pathways are equally susceptible. The analyses demonstrate that the pathways modulated during monocyte differentiation function somewhat independently. Moreover, certain functions of monocytic cells are more important with respect to the outcome of virus infection, with autophagosomal activities in particular favouring cell survival.

Functional characterization of thapsigargin and agonist-insensitive acidic Ca2+ stores in Drosophila melanogaster S2 cell lines

The role of acidic intracellular calcium stores in calcium homeostasis was investigated in the Drosophila Schneider cell line 2 (S2) by means of free cytosolic calcium ([Ca2+]i) and intracellular pH (pHi) imaging together with measurements of total calcium concentrations within intracellular compartments. Both a weak base (NH4Cl, 15 mM) and a Na+/H+ ionophore (monensin, 10 microM) evoked cytosolic alkalinization followed by Ca2+ release from acidic intracellular Ca2+ stores. Pretreatment of S2 cells with either thapsigargin (1 microM), an inhibitor of endoplasmic reticulum Ca(2+)-ATPases, or with the Ca2+ ionophore ionomycin (10 microM) was without effect on the amplitude of Ca2+ release evoked by alkalinization. Application of the cholinergic agonist carbamylcholine (100 microM) to transfected S2-DM1 cells expressing a Drosophila muscarinic acetylcholine receptor (DM1) emptied the InsP3-sensitive Ca2+ store but failed to affect the amplitude of alkalinization-evoked Ca2+ release. Glycyl-L-phenylalanine-beta-naphthylamide (200 microM), a weak hydrophobic base known to permeabilize lysosomes by osmotic swelling, triggered Ca2+ release from internal stores, while application of brefeldin A (10 microM), an antibiotic which disperses the Golgi complex, resulted in a smaller increase in [Ca2+]i. These results suggest that the alkali-evoked calcium release is largely attributable to lysosomes, a conclusion that was confirmed by direct measurements of total calcium content of S2 organelles. Lysosomes and endoplasmic reticulum were the only organelles found to have concentrations of total calcium significantly higher than the cytosol. However, NH4Cl (15 mM) reduced the level of total calcium only in lysosomes. Depletion of acidic Ca2+ stores did not elicit depletion-operated Ca2+ entry. They were refilled upon re-exposure of cells to normal saline ([Ca2+]o = 2 mM), but not by thapsigargin-induced [Ca2+]i elevation in Ca(2+)-free saline.

Purification and characterization of autophagosomes from rat hepatocytes

To investigate the properties and intracellular origin of autophagosomes, a procedure for the purification and isolation of these organelles from rat liver has been developed. Isolated hepatocytes were incubated with vinblastine to induce autophagosome accumulation; the cells were then homogenized and treated with the cathepsin C substrate glycyl-l-phenylalanine 2-naphthylamide to cause osmotic disruption of the lysosomes. Nuclei were removed by differential centrifugation, and the postnuclear supernatant was fractionated on a discontinuous Nycodenz density gradient. The autophagosomes, recognized by their content of autophagocytosed lactate dehydrogenase (LDH), could be recovered in an intermediate-density fraction, free from cytosol and mitochondria. Finally, the autophagosomes were separated from the endoplasmic reticulum and other membranous elements by centrifugation in a Percoll colloidal density gradient, followed by flotation in iodixanol to remove the Percoll particles. The final autophagosome preparation represented a 24-fold purification of autophagocytosed LDH relative to intact cells, with a 12% recovery. The purified autophagosomes contained sequestered cytoplasm with a normal ultrastructure, including mitochondria, peroxisomes and endoplasmic reticulum in the same proportions as in intact cells. However, immunoblotting indicated a relative absence of cytoskeletal elements (tubulin, actin and cytokeratin), which may evade autophagic sequestration. The autophagosomes showed no enrichment in protein markers typical of lysosomes (acid phosphatase, cathepsin B, lysosomal glycoprotein of 120 kDa), endosomes (early-endosome-associated protein 1, cation-independent mannose 6-phosphate receptor, asialoglycoprotein receptor) or endoplasmic reticulum (esterase, glucose-regulated protein of 78 kDa, protein disulphide isomerase), suggesting that the sequestering membranes are not derived directly from any of these organelles, but rather represent unique organelles (phagophores).

Isolation and characterization of rat liver amphisomes. Evidence for fusion of autophagosomes with both early and late endosomes

Amphisomes, the autophagic vacuoles (AVs) formed upon fusion between autophagosomes and endosomes, have so far only been characterized in indirect, functional terms. To enable a physical distinction between autophagosomes and amphisomes, the latter were selectively density-shifted in sucrose gradients following fusion with AOM-gold-loaded endosomes (endosomes made dense by asialoorosomucoid-conjugated gold particles, endocytosed by isolated rat hepatocytes prior to subcellular fractionation). Whereas amphisomes, by this criterion, accounted for only a minor fraction of the AVs in control hepatocytes, treatment of the cells with leupeptin (an inhibitor of lysosomal protein degradation) caused an accumulation of amphisomes to about one-half of the AV population. A quantitative electron microscopic study confirmed that leupeptin induced a severalfold increase in the number of hepatocytic amphisomes (recognized by their gold particle contents; otherwise, their ultrastructure was quite similar to autophagosomes). Leupeptin caused, furthermore, a selective retention of endocytosed AOM-gold in the amphisomes at the expense of the lysosomes, consistent with an inhibition of amphisome-lysosome fusion. The electron micrographs suggested that autophagosomes could undergo multiple independent fusions, with multivesicular (late) endosomes to form amphisomes and with small lysosomes to form large autolysosomes. A biochemical comparison between autophagosomes and amphisomes, purified by a novel procedure, showed that the amphisomes were enriched in early endosome markers (the asialoglycoprotein receptor and the early endosome-associated protein 1) as well as in a late endosome marker (the cation-independent mannose 6-phosphate receptor). Amphisomes would thus seem to be capable of receiving inputs both from early and late endosomes.

A novel method for the study of autophagy: destruction of hepatocytic lysosomes, but not autophagosomes, by the photosensitizing porphyrin tetra(4-sulphonatophenyl)porphine

A photoactivatable porphyrin, tetra(4-sulphonatophenyl)porphine (TPPS4), was shown to accumulate in rat hepatocytes as a linear function of dose after intravenous injection, and to localize predominantly in hepatocytic lysosomes. A major fraction of the lysosomal enzymes acid phosphatase and N-acetyl-beta-D-glucosaminidase was inactivated by TPPS4 after 20 h of contact with the drug in vivo in the absence of photoactivation. On exposure of isolated hepatocytes to light, photoactivated TPPS4 caused additional inactivation of the lysosomal enzymes as well as inactivation of intralysosomal lactate dehydrogenase (LDH), a cytosolic enzyme that accumulated in lysosomes as a result of autophagy during a 2 h incubation of hepatocytes at 37 degrees C in the dark (in the presence of the proteinase inhibitor leupeptin to prevent degradation of intralysosomal LDH). Photoactivation of TPPS4 also induced lysosomal rupture, with a loss of lysosomal enzymes, autophagocytosed LDH, endocytosed 125I-tyramine-cellobiose-asialo-orosomucoid and TPPS4 from the lysosomes. However, LDH-containing autophagosomes, accumulated in the presence of vinblastine (a microtubule inhibitor used to prevent the fusion of lysosomes with autophagosomes or endosomes), were not affected by TPPS4. TPPS4 may thus be useful as a selective lysosomal (or endosomal) perturbant in the study of autophagic-endocytic-lysosomal interactions.

The lysosomal Ca2+ pool in MDCK cells can be released by ins(1,4,5)P3-dependent hormones or thapsigargin but does not activate store-operated Ca2+ entry

In several cell types, Ca2+ release from intracellular Ca2+ stores by Ins(1,4,5)P3 elicits Ca2+ influx from the extracellular space into the cytoplasm, termed store-operated Ca2+ entry (SOCE). In MDCK cells, the Ins(1,4,5)P3-sensitive Ca2+ store giving rise to SOCE essentially overlaps with the thapsigargin (TG)-sensitive store. Recent evidence suggests that in MDCK cells lysosomes form a Ca2+ pool that is functionally coupled with the Ins(1,4,5)P3-sensitive Ca2+ store: Ca2+ can be selectively released from lysosomes by glycyl-L-phenylalanine naphthylamide, an agent inducing lysosomal swelling with subsequent and reversible permeabilization of the vesicular membranes. This compartment is also depleted by Ins(1,4,5)P3-dependent agonists or TG, indicating that it is part of a larger, Ins(1,4,5)P3-sensitive Ca2+ pool. Here we show that whereas SOCE is triggered by Ca2+ release from the entire Ins(1,4,5)P3-sensitive Ca2+ pool, selective Ca2+ release from lysosomes alone is unable to trigger SOCE. This finding is consistent with measurements of the store-operated cation current, a direct parameter for store-operated Ca2+ and Na+ entry into MDCK cells. Hence it is proposed that the Ins(1,4,5)P3-sensitive Ca2+ pool is composed of different intracellular compartments that do not uniformly stimulate Ca2+ entry into the cell.

The lysosomal compartment as intracellular calcium store in MDCK cells: a possible involvement in InsP3-mediated Ca2+ release

To test for a possible role of lysosomes in intracellular Ca2+ homeostasis, the effects of glycyl-L-phenylalanine-beta-naphthylamide (GPN), known to permeabilize these organelles by osmotic swelling, were studied in single MDCK cells. Fluorescence of acridine orange, rhodol green dextran, lysotracker green and FITC-dextran indicated that GPN (0.2 mmol/l) elicited a reversible permeabilization of lysosomes. Cytosolic Ca2+ ([Ca2+]i) as determined by Fura-2 fluorescence increased from 60 +/- 11 to 534 +/- 66 nmol/l (n = 41) in the presence of GPN. Whereas only a single intracellular Ca2+ release could be induced by GPN in a Ca(2+)-free perfusate, repetitive release could be evoked in Ca2+ containing solutions suggesting reuptake of Ca2+ into lysosomal stores. GPN-induced Ca2+ release was blunted after pretreatment with thapsigargin (TG), an inhibitor of Ca(2+)-ATPase, or repeated applications of ATP inducing Ca2+ release from inositol trisphosphate (InsP3) sensitive Ca2+ stores. The effect of ATP on Ca2+ release was, however, not abolished by preceding GPN treatment. GPN-induced Ca2+ release from lysosomes was independent of InsP3 formation or Ca(2+)-induced Ca2+ release, since it was unaffected by the phospholipase C inhibitor U-73, 122 or by caffeine and ruthenium red. These results suggest that Ca2+ largely accumulates in lysosomal vesicles. Moreover, these organelles seem to be part or functionally coupled with InsP3-sensitive Ca2+ stores.

Structural aspects of autophagy

As a first step towards isolation of autophagic sequestering membranes (phagophores), we have purified autophagosomes from rat hepatocytes. Lysosomes were selectively destroyed by osmotic rupture, achieved by incubation of hepatocyte homogenates with the cathepsin C substrate glycyl-phenylalanyl-naphthylamide (GPN). Mitochondria and peroxisomes were removed by Nycodenz gradient centrifugation, and cytosol, microsomes and other organelles by rate sedimentation through metrizamide cushions. The purified autophagosomes were bordered by dual or multiple concentric membranes, suggesting that autophagic sequestration might be performed either by single autophagic cisternae or by cisternal stacks. Okadaic acid, a protein phosphatase inhibitor, disrupted the hepatocytic cytokeratin network and inhibited autophagy completely in intact hepatocytes, perhaps suggesting that autophagy might be dependent on intact intermediate filaments. Vinblastine and cytochalasin D, which specifically disrupted microtubules and microfilaments, respectively, had relatively little (25-30%) inhibitory effect on autophagic sequestration. In a cryo-ultrastructural study, the various autophagic-lysosomal vacuoles were immunogold-labelled, using the cytosolic enzyme superoxide dismutase as an autophagic marker, Lgp120 as a lysosomal membrane marker, and bovine serum albumin as an endocytic marker. Vinblastine (50 microM) was found to inhibit both autophagic and endocytic flux into the lysosomes, with a consequent reduction in lysosomal size. Asparagine (20 mM) caused swelling of the lysosomes, probably as a result of the ammonia formation that could be observed at this high asparagine concentration. Autophagosomes and amphisomes (autophagic-endocytic, prelysosomal vacuoles) accumulated in asparagine-treated cells, reflecting an inhibition of autophagic flux that might be a consequence of lysosomal dysfunction.