Renal protein absorption into sub-cellular particles. I. Studies with intact kidneys and fractionated homogenates

Immunocytochemical investigation of the in vivo endocytosis by renal tubular epithelial cells

The internalization and degradation of glomerular filtered serum proteins by the proximal tubular epithelium has been extensively studied by microperfusion methods. By using a cationic probe that easily traverses the glomerular wall into the urinary space, we have performed a morpho-cytochemical and quantitative study of the in vivo endocytotic activity of the proximal tubular epithelial cell. Bovine serum albumin (BSA) was tagged with dinitrophenol (DNP) and cationized to pI over 8. It was introduced into the circulation of normal mice for 5, 10, and 30 minutes and the distribution of the labeling was determined by protein A-gold immunocytochemistry, using specific antiDNP antibodies on tissue sections of routinely aldehyde-fixed, osmiumpostfixed, and Epon-embedded kidneys. Cationic BSA-DNP was detected at the endothelial and epithelial sides of the glomerular basement membrane, and over capillary and tubular basement membranes. In the proximal tubular epithelial cell, labeling was present over microvilli as well as over endosomal and lysosomal compartments, with labeling intensities varying from one compartment to the other. Morphometric evaluations of the labeling demonstrated a progressive incorporation of the probe from microvilli and endocytic compartments at 5 minutes to endocytic and lysosomal compartments at 10 and then 30 minutes. When considering labeling densities, no significant differences were found on microvilli and basolateral membranes between times of circulation; however, the labeling density over endosomal and lysosomal compartments was very intense at 10 minutes compared with 5 minutes, decreasing at 30 minutes. Results from this study validate the cationic albumin tagged with DNP as a tool in the study of the quantitative aspects of protein endocytosis at the ultrastructural level, in the kidney tubular epithelium.

Covalent linkage of ribonuclease S-peptide to microinjected proteins causes their intracellular degradation to be enhanced during serum withdrawal

The amino-terminal 20 amino acids are required for microinjected ribonuclease A (RNase A) to be taken up by lysosomes and degraded at an enhanced rate during serum withdrawal. We used water-soluble carbodiimides to covalently attach the RNase S-peptide (residues 1-20) to [3H]RNase S-protein (residues 21-124) at unspecified locations. We then measured catabolism of the [3H]S-protein-S-peptide conjugate after its microinjection into human diploid fibroblasts. The attached S-peptide caused the degradation of S-protein to be enhanced 2-fold in the absence of serum. Control experiments showed that degradation of [3H]RNase S-protein remained unresponsive to serum after conjugation with the inactive fragment, RNase S-peptide (residues 1-10). Covalent attachment of RNase S-peptide had a similar effect on the catabolism of two other proteins. Degradation rates of microinjected 125I-labeled lysozyme and 125I-labeled insulin A chain are normally unresponsive to serum withdrawal. However, breakdown rates of microinjected 125I-labeled lysozyme-S-peptide and 125I-labeled insulin A chain-S-peptide conjugates were increased 2-fold during serum deprivation. We suggest that RNase S-peptide acts as a "single sequence" that directs cytosolic proteins to lysosomes through a pathway that is activated by deprivation conditions.

Sex different cytochrome-c uptake in the proximal tubule of the rat kidney

The present study deals with the dose- and time-dependent uptake of cytochrome c (CYT c) in the proximal tubule of the rat kidney, and shows that there are segment and sex differences in the reabsorption of CYT c. Rats of both sexes were intravenously injected with different doses of CYT c (0.75-9.0 mg per 100 g body weight), and the kidneys were investigated by light and electron microscopy at different times (3 min, 10 min, and 2 h) after the injection. After 3 and 10 min, CYT c was demonstrated in apical vacuoles of different sizes and in some lysosomes of the S1 and S2 segments, whereas after 2 h, CYT c was found only in lysosomes of all three segments of the proximal tubule. At these times, the S1 segment contained more CYT c than the S2 and S3 segments. However, 2 h after the injection of 6 or 9 mg CYT c, the differences between the S1 and S2 segments disappeared almost completely, due to a strong lysosomal accumulation of CYT c in the S2 segment. At all studied times and CYT-c doses, the S3 segment contained less CYT c than the S1 and S2 segments. On the whole, different levels of CYT-c reabsorption were found in the different segments of the proximal tubule, which was saturable with increasing CYT-c doses, i.e. firstly in the proximal and then in the distal parts of the proximal tubule. Two hours after the injection of CYT c, a difference between males and females was observed, with the lysosomes of the S1 and S2 segments of females containing more CYT c than those of males. Thus, more CYT c was reabsorbed in the proximal tubule of females than in that of males.

Regulation of catabolism of microinjected ribonuclease A requires the amino-terminal 20 amino acids

RNase A introduced into the cytoplasm of IMR-90 human diploid fibroblasts by erythrocyte-mediated microinjection is degraded with a half-life of approximately equal to 75 hr in the presence of fetal bovine serum. In response to serum deprivation the degradative rate of microinjected RNase A is enhanced 2-fold. RNase S protein (amino acids 21-124) is degraded with a half-life similar to that of RNase A in the presence of serum, but its catabolism is not increased during serum withdrawal. Reconstitution of RNase S protein with RNase S peptide (amino acids 1-20) restored full enzymatic activity to the S protein as well as the ability of fibroblasts to increase its catabolism during serum deprivation. Finally, RNase S peptide microinjected alone shows the full 2-fold increase in degradative rate during serum withdrawal. These results show that recognition of RNase A for enhanced breakdown during serum deprivation is based on some feature of its amino-terminal 20 amino acids. Furthermore, our results indicate that the enhanced protein catabolism during serum deprivation can be highly selective.

Degradation of proteins microinjected into IMR-90 human diploid fibroblasts

Erythrocyte ghosts loaded with 125I-labeled proteins were fused with confluent monolayers of IMR-90 fibroblasts using polyethylene glycol. Erythrocyte-mediated microinjection of 125I-proteins did not seriously perturb the metabolism of the recipient fibroblasts as assessed by measurements of rates of protein synthesis, rates of protein degradation, or rates of cellular growth after addition of fresh serum. A mixture of cytosolic proteins was degraded after microinjection according to expected characteristics established for catabolism of endogenous cytosolic proteins. Furthermore, withdrawal of serum, insulin, fibroblast growth factor, and dexamethasone from the culture medium increased the degradative rates of microinjected cytosolic proteins, and catabolism of long-lived proteins was preferentially enhanced with little or no effect on degradation of short-lived proteins. Six specific polypeptides were degraded after microinjection with markedly different half-lives ranging from 20 to 320 h. Degradative rates of certain purified proteins (but not others) were also increased in the absence of serum, insulin, fibroblast growth factor, and dexamethasone. The results suggest that erythrocyte-mediated microinjection is a valid approach for analysis of intracellular protein degradation. However, one potential limitation is that some microinjected proteins are structurally altered by the procedures required for labeling proteins to high specific radioactivities. Of the four purified proteins examined in this regard, only ribonuclease A consistently showed unaltered enzymatic activity and unaltered susceptibility to proteolytic attack in vitro after iodination.

Uptake of iodinated human kidney alpha-D-mannosidase by rat liver- Association with membrane elements and stability in vivo and in vitro

1. Human kidney alpha-D-mannosidase (form A) was labelled with 125I to a specific radio-activity of approx. 2250muCi/mg of protein, essentially without loss of enzymic activity. The enzymic activity and radioactivity of the iodinated material also co-migrated in gel filtration and gel electrophoresis. 2. The binding of 125I-labelled mannosidase in vitro to particulate material in liver and kidney homogenates was of the other of 2 pg/mg of particulate material in liver and kidney homogenates was of the order of 2pg/mg of particulate protein withing 16h at 37 degrees C, and essentially zero in intervals of up to 60 min. The degradation in vitro of labelled exogenous mannosidase was of the order of 10-20pg/ 16th per mg of protein in postnuclear supernatant, and it was saturated entirely within 1h at 37 degrees C. 3. The binding of labelled mannosidase in vivo to particulate elements of liver homogenates 60 min after intravenous injection was at least 10 times higher in terms of specific radioactivity than the highest value attainable in vitro. Virtually all exogenous enzyme bound to liver particulate material could be recovered in macromolecular form after disruption of membranes by detergents. 4. The radioactive enzyme bound to liver particulate material could be detached almost completely by shearing, repeated freezing and thawing, and exposure to strong detergents under conditions that do not eliminate rough-endoplasmic-membrane structure. It could bot be released, however, by high salt concentration (0.5M-KC1) or by exposure to weak detergents such as Tween 80. The particle-bound enzyme should thus be associated with plasma membranes and lysosome-like elements. 5. Of the rat tissues studied, only liver could approach, within 60 min after the injection, the concentration of exogenous mannosidase found in the blood serum. The activity per g tissue weight fell progressively from liver (60% of serum value) to kidney (16% of serum value), lung (8% of serum vlaue), spleen (6% of serum value) and brain (0.9% of serum value). Most of the radioactive enzyme found in tissues other than liver appeared to be present in a free form, whereas in liver more than 50% of the labelled enzyme was associated with membrane elements.

Rapid protein uptake and digestion in proximal tubule lysosomes

The aim of the present study was to determine the initial time course for the handling of protein by the proximal tubule cells in rat kidney, using 125I-labeled cytochrome C as a tracer. The uptake of the protein was followed by electron microscope autoradiography in kidneys from rats fixed by perfusion after i.v. injection of cytochrome C. Protein degradation was studied by incubating cortical slices, taken from rats, injected with the same label, and measuring the release of trichloroacetic acid (TCA)-soluble radioactivity from the slices. It was shown by autoradiography that lysosomes in proximal tubule cells start to accumulate labeled cytochrome C within 3 min after injection of the protein, and that the concentration of label in lysosomes increases during the first 30 min after injection, whereas it decreases in endocytic vacuoles. The catabolism of protein as measured in cortical slices began within 13 min after the i.v. injection. Other experiments showed that the accumulation of cytochrome C in the kidneys is very fast. The maximum accumulation, 37% of the injected dose, was reached seven minutes after injection. The results show that protein uptake in proximal tubule cells, transport into lysosomes and the digestion of protein is a more rapid process than previously reported in ultrastructural studies.

Renal and hepatic lysosomal catabolism of luteinizing hormone

Following an intravenous injection of tritiated ovine lutenizing hormone (LH) into mature male rats, the liver and kidneys accumulate a significant portion of the non-excreted hormone. The subcellular distribution of total radioactivity in both tissues was found to be similar to that of beta-galactosidase, a lysosomal enzyme marker. Moreover, the subcellular fraction with the highest relative specific activity of beta-galactosidase exhibited the highest degradation rate of endogenous hormone under in vitro conditions. Based on these and other observations, it is concluded that the intracellular catabolism of LH by these tissues is due to lysosomal enzymes. An analysis of the radioactive degradation products produced by a lysosomal-rich subcellular fraction showed the presence of free amino acids and oligopeptides. Thus, the uptake and degradation of the hormone by these tissues appear to occur by endocytosis followed by lysosomal catabolism. This phenomenon may represent a regulatory role in the control of (circulating) hormone concenttrations.

Purification and properties of human acid-thermostable ribonucleases, and diagnosis of childhood pancreatic fibrosis

Acid-thermostable ribonucleases were isolated from human pancreas, duodenal contents, liver, spleen, serum and urine, and purified 15--1000-fold. The pH optima, ionic requirements, and some of the specificity requirements, of these enzymes were investigated. The isolated enzymes formed two distinct groups: (a) The ribonucleases of the pancreas, duodenal contents and fraction A of serum and urine exhibit a pH optimum of 8.5, are inhibited by An2+ and Cu2+, and relatively rapidly hydrolyze the synthetic substrate uridine 3'-(alpha-naphthylphosphate); (b) the ribonucleases of the liver and spleen, and of fractions B of the serum and urine, with a pH optimum of 7, are less sensitive to An2+ and Cu2+, and exhibit negligible activity versus uridine 3'-(alpha-naphthylphosphate). Determination of the serum level of pancreatic-type ribonuclease activity, with the use of uridine 3'-(alpha-naphthylphosphate) or RNA as substrates, appears to be a valid diagnostic tool for pancreatic fibrosis in children.

The metabolism of luteinizing hormone. Plasma clearance, urinary excretion, and tissue uptake

The kinetics of plasma clearance, tissue uptake, and urinary excretion of tritiated ovine pituitary luteinizing hormone in adult male rats are reported. Most of the intravenously injected tritiated gonadotropin is cleared from circulation with a half-life of five minutes, and this is independent of the injected amount of hormone over a wide dose range. It was found that the hormone is rapidly removed from circulation by the kidneys, probably by glomerular filtration, and excreted in the urine. The radioactivity present in the urine is associated with material of the same molecular size as the native hormone and, moreover, the urinary hormone retains a significant amount of biological activity. A small amount of the hormone is catabolized by the kidney and liver, and our data suggest that this occurs in the cortex and hepatocytes, respectively.

Metal-ion effects on proteolysis and stability in secondary lysosomes of mouse kidney

In previous studies, in vitro digestion of [1 2 5-I] ribonuclease by lysosomes of mouse kidney was limited because breakdown, which was rapid at first, slowed markedly so that most of the labeled protein escaped degradation. We now describe incubation conditions which allow digestion to proceed until approximately 70% of the exogenous protein label is released in acid-soluble form, after 30-45 min at 37 degrees C. Such activity is seen with either the addition of EDTA or incubation of concentrated cell particle suspensions. EDTA is effective in low concentrations and shows the same stimulation of digestion over a range of approximately 10-minus 6--10-minus 3 M. Other chelating agents have similar effects; dipyridyl and hydroxquinoline are as effective as EDTA, o-phenanthroline and diethyldithiocarbamate are slightly less effective. When the incubation medium had been treated with a chelating resin, Chelex 100, dilute suspensions of lysosomes were as active as those in EDTA. These results lead to the conclusion that metal ions, present as contaminants in very small concentrations, inhibit the activity of mouse kidney lysosomes. The effect of the metal ions is to diminish lysosomal stability, leading to release of intact labeled ribonuclease in non-sedimentable form. Interaction between lysosomes and metal, leading to inhibition of digestion upon heating, occurs at low temperature, but breakdown requires incubation at 37 degrees C and may be autolytic. In contrast to chelators, mercaptoethanol is without marked effect on stability; the stimulation in digestion rate caused by this agent is due either to a direct effect on the lysosomal enzymes or to a non-destructive influence on the lysosomal structure.

A thermally induced alteration in lysosome membranes: salt permeability at 0 and 37 degrees C

Preparations of radioactive lysosomes were obtained from mouse kidney after injection of radioactive iodine-labeled bovine ribonuclease. Stability of these lysosomes in various media was estimated from measurements of proteolytic activity towards the ribonuclease, and of ribonuclease retention in particles. The lysosomes were stable at 37 degrees C in isotonic, sucrose-free solutions of KCl, NaCl, and potassium acetate, and in mixtures of these with MgCl2, showing that these salts are relatively impermeant through the lysosomal membranes. The membranes were less permeable to Na+ than to K+. Both KCl and NaCl exerted their optimal protective effects over a broad concentration range above 0.125 M in 0.025 M acetate buffer. Mg2+ enhanced the protective effect of both K4 and Na+; the osmotic effect of 0.075 M NaC1-0.05 M MgCl2 was indistinguishable during the entire course of ribonuclease digestion from that of isotonic sucrose. Osmotic protection by KC1-MgC12 was demonstrated over the H range5.5-7.0. A marked alteration in membrane properties occurs at lower temperatures in 0.11 M KC1-0.01 M MgCl2 such that, at 0 degrees C, K+ permeability is much higher than at 37 degrees C, as shown by a several-fold decrease in stability at the lower temperature.