The relationship between autophagy and the intracellular degradation of asialoglycoproteins in cultured rat hepatocytes

Autophagy failure in Alzheimer's disease and the role of defective lysosomal acidification

Autophagy is a lysosomal degradative process which recycles cellular waste and eliminates potentially toxic damaged organelles and protein aggregates. The important cytoprotective functions of autophagy are demonstrated by the diverse pathogenic consequences that may stem from autophagy dysregulation in a growing number of neurodegenerative disorders. In many of the diseases associated with autophagy anomalies, it is the final stage of autophagy-lysosomal degradation that is disrupted. In several disorders, including Alzheimer's disease (AD), defective lysosomal acidification contributes to this proteolytic failure. The complex regulation of lysosomal pH makes this process vulnerable to disruption by many factors, and reliable lysosomal pH measurements have become increasingly important in investigations of disease mechanisms. Although various reagents for pH quantification have been developed over several decades, they are not all equally well suited for measuring the pH of lysosomes. Here, we evaluate the most commonly used pH probes for sensitivity and localisation, and identify LysoSensor yellow/blue-dextran, among currently used probes, as having the optimal profile of properties for measuring lysosomal pH. In addition, we review evidence that lysosomal acidification is defective in AD and extend our original findings, of elevated lysosomal pH in presenilin 1 (PS1)-deficient blastocysts and neurons, to additional cell models of PS1 and PS1/2 deficiency, to fibroblasts from AD patients with PS1 mutations, and to neurons in the PS/APP mouse model of AD.

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

In density-gradient analyses of autophagic vacuoles from isolated rat hepatocytes, autophagosomes could be recognized by the presence of an autophagically sequestered cytosolic enzyme, lactate dehydrogenase (LDH). Lysosomes were identified by marker enzymes such as acid phosphatase, or by degradation products from 125I-tyramine-cellobiose-asialoorosomucoid (125I-TC-AOM) loaded into the lysosomes by an intravenous injection in vivo 18 h prior to cell isolation. Autophagosomes and lysosomes showed similar, largely overlapping, density distributions both in hypertonic sucrose gradients and in isotonic Nycodenz gradients. As a step towards the purification of autophagosomes, we investigated the possibility of using lysosomal enzyme substrates to achieve selective destruction of lysosomes by swelling. Hepatocytes were first incubated for 2 h at 37 degrees C with vinblastine (50 microM) to obtain an accumulation of autophagosomes (to 3-5-times above the control level). The cells were then electrodisrupted and the disruptates incubated with a variety of substrates for lysosomal enzymes. Among these, glycyl-phenylalanine-2-naphthylamide (GPN), a cathepsin-C substrate, and methionine-O-methylester (MetOMe), an esterase substrate, turned out to induce extensive rupture of lysosomes, as measured by a strongly reduced sedimentability of acid phosphatase and a nearly complete loss of 125I-TC-AOM sedimentability in substrate-treated preparations from control or vinblastine-treated cells. The lysosomes of cells treated with leupeptin or asparagine were largely resistant to the action of GPN, probably as a result of interference with cathepsin-C activity or lysosomal function in general. Autophagosomes were partially destroyed by MetOMe, as indicated by a reduction in sedimentable LDH, but GPN had no effect on either autophagosomes or mitochondria. The ability of GPN to selectively destroy lysosomes without affecting the autophagosomes of vinblastine-treated cells should make GPN treatment a useful aid in the purification of rat liver autophagosomes.

An inhibitory effect of a protein kinase inhibitor, staurosporine, on delivery of endocytosed asialoglycoprotein to lysosome in monolayer culture of rat hepatocytes

An inhibitor of protein kinases, staurosporine (ssp), was found to affect the endocytic pathway of asialoglycoproteins subsequent to endocytosis in monolayer cultures of rat hepatocytes. The effect of 5 or 10 microM staurosporine on the internalization of a synthetic ligand (galBSA-HRP: bovine serum albumin exposing galactose, horseradish peroxidase conjugates) prebound to the cell surface was minimal. The presence of 5, 7, or 10 microM ssp during a 1-h chase period resulted in the ligand remaining in a low density (1.04-1.05 g/ml), nonlysosomal subcellular fraction in a Percoll gradient. The ligand, arrested by 7 microM ssp, was further processed to the lysosome during subsequent incubation in the absence of ssp. Cells maintained the ability to internalize ligand at 37 degrees C for 1 h in the presence of these concentrations of ssp. During a 1-h continuous uptake of 0-50 micrograms/ml nonlabeled ligand, the presence of 7 microM ssp did not cause any decrease in the amount of asialoglycoprotein receptor at the cell surface, which indicates receptor recycling occurred normally. These results suggest a possible involvement of protein kinase(s), which can be inhibited by ssp, in the delivery of endocytosed ligand to the lysosome, but not in ligand endocytosis and receptor recycling.

Lysosomal movements during heterophagy and autophagy: with special reference to nematolysosome and wrapping lysosome

Recent studies on lysosomal movements during heterophagy and autophagy performed in our laboratory for the past several years were reviewed; methods for the investigation of lysosomes and the cytoskeleton in these studies mainly involved electron microscopic cytochemistry. Lysosomal movements during heterophagy were observed in cultured rat alveolar macrophages taking up horseradish peroxidase (HRP) and rat peroxidase-antiperoxidase (PAP) by fluid-phase pinocytosis and adsorptive pinocytosis, respectively. A characteristic lysosomal change which was induced by the pinocytosis was the appearance of long, threadlike lysosomes (nematolysosomes) in the cytoplasm. The effects of actin filament destabilizer and antimicrotubular drug on lysosomal changes revealed that the appearance of nematolysosomes was dependent on the presence of both actin filaments and microtubules. The close morphological relationship between lysosomes and cytoskeletal elements, such as actin filaments and microtubules in the alveolar macrophages, supports the participation of the cytoskeletal system in the regulatory mechanism of lysosomal movements. In the study of the lysosomal wrapping mechanism (LWM), which is one type of lysosomal movement that occurs during autophagy, it was found that the occurrence of LWM was dependent on energy--namely, the supply of ATP--and on the presence of actin filaments. However, deconstruction of microtubules induced or favored the occurrence of LWM. It is conceivable that the LWM is also related to the cytoskeletal system. We conclude that intracellular dynamics of lysosomes during heterophagy and autophagy are largely a consequence of complicated modulation by the cytoskeletal system.

The cyanogen bromide fragment I of asialoorosomucoid is transported more efficiently than asialoorosomucoid in rat hepatocytes

Cultured rat hepatocytes internalized and degraded 125I-labeled asialoorosomucoid (125I-ASOR) through asialoglycoprotein receptor at rates about half that of its cyanogen bromide fragment I (125I-ASCNBr-I). Reduction and carboxymethylation of the fragment resulted in decreased rates of internalization and degradation which were still greater than those of 125I-ASOR. In the presence of 5 microM colchicine, degradation of all three ligands was inhibited. However, the intracellular level of 125I-ASOR at steady state remained unchanged, while those of the fragments increased continuously. Study of the binding of these ligands to hepatocytes at 4 degrees C indicated that there was no significant difference in binding parameters between ASOR, ASCNBr-I and RC-ASCNBr-I (reduced and carboxymet ASCNBr-I). Studies of the fate of these ligands preloaded in the cell at 37 degrees C indicated that a higher fraction of the internalized ASOR than of the fragments was released by diacytosis. In contrast to ASOR, diacytosis of the fragments was not enhanced by colchicine. Studies of the distribution of intracellular ligands by Percoll density gradient centrifugation indicated that they were internalized initially into two early endosomal compartments of d = 1.037 g/ml and d = 1.045 g/ml. In the presence of colchicine, accumulation of the ligands in a third endosomal compartment of d = 1.08-1.095 g/ml was revealed, while in the presence of leupeptin accumulation of the ligands in lysosomes was observed. The results of a kinetic analysis indicated that both cyanogen bromide fragments were transported to all these compartments more rapidly than was ASOR. It appears that they are internalized and degraded more rapidly than ASOR due to a more efficient sorting of the internalized ligand into the pathway of lysosomal degradation.