Lysosomes of the renal cortex: heterogeneity and role in protein handling

Functional differentiation and scalable production of renal proximal tubular epithelial cells from human pluripotent stem cells in a dynamic culture system

OBJECTIVE

To provide a standardized protocol for large-scale production of proximal tubular epithelial cells (PTEC) generated from human pluripotent stem cells (hPSC).

METHODS

The hPSC were expanded and differentiated into PTEC on matrix-coated alginate beads in an automated levitating fluidic platform bioLevitator. Differentiation efficacy was evaluated by immunofluorescence staining and flow cytometry, ultrastructure visualized by electron microscopy. Active reabsorption by PTEC was investigated by glucose, albumin, organic anions and cations uptake assays. Finally, the response to cisplatin-treatment was assessed to check the potential use of PTEC to model drug-induced nephrotoxicity.

RESULTS

hPSC expansion and PTEC differentiation could be performed directly on matrix-coated alginate beads in suspension bioreactors. Renal precursors arose 4 days post hPSC differentiation and PTEC after 8 days with 80% efficiency, with a 10-fold expansion from hPSC in 24 days. PTEC on beads, exhibited microvilli and clear apico-basal localization of markers. Functionality of PTECs was confirmed by uptake of glucose, albumin, organic anions and cations and expression of KIM-1 after Cisplatin treatment.

CONCLUSION

We demonstrate the efficient expansion of hPSC, controlled differentiation to renal progenitors and further specification to polarized tubular epithelial cells. This is the first report employing biolevitation and matrix-coated beads in a completely defined medium for the scalable and potentially automatable production of functional human PTEC.

Chapter 5 Role of lysosomes in cell injury

Lysosomes are acidic intracellular vacuoles of heterogeneous shape, size, and content. Lysosomes contain hydrolytic enzymes that degrade proteins, lipids, carbohydrates, and nucleic acids derived from intracellular (through autophagy) and extracellular (through heterophagy) sources. Lysosomal degradation regulates several physiological cell functions. These include turnover of cellular organelles and extracellular constituents; amino acid and glucose homeostasis; processing of proteins; lipid metabolism; cell growth, differentiation, and involution; host defenses against microorganisms and other pathogens; and removal of necrotic and foreign material from the circulation and from tissues.

Lysosomal degradation also plays an important role in the pathophysiology of acute and chronic cell injury, inflammation and repair, and tumor growth and metastasis. The participation of the lysosomes in the specific types of cell injury we have discussed is due to altered regulation of one or more of the following processes: turnover of cellular organelles by autophagic degradation; levels and activities of lysosomal hydrolases; levels of intracellular and extracellular lysosomal hydrolase inhibitors; transport of degradation products from the lysosomal matrix to the cytosol; permeability of the lysosomal membrane to hydrolases; lysosomal vacuolar acidification; transport of degradable substrates and of pathogens to the lysosomes; transport and processing of secretory proteins and lysosomal hydrolases during biogenesis; traffic and fusion of lysosomal vacuoles and vesicles; secretion of lysosomal hydrolases; and accumulation of metals, particularly iron, acidotropic agents, and undegraded and/or undegradable materials in lysosomes.

An ultrastructural histochemistry and light microscopy study of the early development of renal proximal tubular vacuolation after a single administration of the contrast enhancement medium "Iotrolan"

The time course of contrast media (CM)-induced renal proximal tubular vacuolation was investigated in rats by light microscopy, transmission electron microscopy (TEM), and ultrastructural histochemistry for acid phosphate activity. Young adult male rats were treated with a single dose of 3.0 g I/kg Iotrolan (Isovist 300 mg I/ml) and sacrificed at 0 min, 5-min, 15-min, 15-min, 2-hr, and 24-hr intervals. Light microscopy of vibratome sections of freshly excised tissue of cryostat and paraffin sections was also performed to allow comparison of the appearance of the vacuoles in the fresh state with light and electron microscopy. The sequence of events seen to occur can be summarized as follows. CM-induced vacuolation occurred at a low level a soon as 5 min after compound administration. The vacuolation was observed by TEM but could not be detected by light microscopy. This was followed by an increase in size and numbers of vacuoles up to the 24-hr timepoint with a sequential increase in the staining for acid phosphatase activity of the vacuoles, most marked at the 24-hr timepoint. At timepoints less than 24 hr there appeared to be no marked increased in the normal complement by lysosomes or in the components of the Golgi-endoplasmic reticulum-lysosome pathway. At 24 hr, the vast majority, but not all, of the CM-induced vacuoles were positive for acid phosphatase activity. The intensity of staining varied, and there was evidence of infusion of small lysosomes with CM-induced vacuoles. These results suggest that formation of CM-induced vacuoles is a 2-stage process, following a normal pathway for the handling of endogenous and exogenous substances.

Binding of lanthanides to cell membranes in the presence of ligands

The effect of a series of ligands on the binding of the lanthanide, europium (Eu), to rabbit intestinal cell membranes was investigated in vitro. When tested as Eu-ligand complexes (ratio of Eu:ligand, 1:2) of intermediate stability (log stability constant, log K1, for the reaction Eu + L = EuL, of about 7-12) such as Eu-citrate and Eu-nitrilotriacetate (NTA), Eu was available for uptake in a soluble form by intestinal brush-border membrane vesicles (BBMV) in phosphate- and bicarbonate-free solutions at pH 7.2. Ligands with lower log K1 did not maintain Eu in solution whilst those of higher affinity did not donate it to membranes. Generally, there was a clear relationship between log K1 of the Eu-ligand complex and the binding of Eu to BBMV. This relationship identifies ligands that can effectively donate Eu to vesicles under these conditions. BBMV uptake of Eu was due to binding at two sites. Binding to the diethylenetriaminepentaacetate (DTPA)-sensitive site predominated at 20 degrees C and uptake by the DTPA-insensitive site was enhanced at 37 degrees C. Only trace amounts of the bound Eu appeared to be internalized within the vesicles. In the presence of physiological concentrations of phosphate and bicarbonate in cell culture medium, Eu was precipitated from most complexes (at 1:2 and 1:5 Eu:ligand ratio) except DTPA and albumin. Eu precipitation could be prevented by increasing the ligand:Eu ratio. When isolated hepatocytes in cell culture medium were incubated with EuCl3, about 60% of Eu was bound to the cells; Eu-albumin was not bound by hepatocytes.

Biochemical and morphological effects of contrast media on the kidney

The intravenous use of roentgen contrast media (CM) is associated with a low incidence of renal impairment. This paper considers the intravascular handling and retention of CM in relation to effects on renal function - specifically the ability of the kidney to reabsorb and catabolise low molecular weight proteins. Renal morphology following experimental administration of a high dose of an isotonic dimeric CM (iodixanol at 3 g I/kg) in rats showed numerous, large, protein-containing vacuoles or droplets in the cells of the proximal convoluted tubule. These were fully formed within 3.5 hours. The process of vacuole-formation involving the uptake of CM appears to be analogous to dextran uptake that occurs via fluid phase endocytosis. These vacuoles or CM droplets are abundant for 7 days but then slowly decline over several weeks. The quantitative recovery of (14)C iodixanol (3g I/kg) from the kidneys between 3.5 hours to 7 days after administration was about 1% of the dose, with some 0.2% of the original dose still present at 28 days. Subcellular analysis to determine the site of the radiolabel showed that the (14)C was associated with lysosomal marker enzymes. The CM-induced vacuoles/droplets are most probably giant lysosomes, which contain the intracellularly retained CM. Co-administration of tracer doses of (125)I-labelled cytochrome C with iodixanol showed some impairment of low molecular weight protein reabsorption, but remarkably this process was not effected when the vacuoles were fully formed. The conspicuous morphology of the vacuoles, the CM retention and the transient proteinuria and enzymuria cannot presently be associated with any functionally significant impairment of tubular or cellular processes.

Accumulation and degradation of the protein moiety of cadmium-metallothionein (CdMT) in the mouse kidney

Of major concern in Cd toxicity is its ability to produce renal damage after chronic exposure in humans and experimental animals. Renal injury affects predominantly the proximal tubules and more specifically the first segments of these tubules. Similar toxic effects to the kidneys are observed after administration of cadmium bound to metallothionein (CdMT). Therefore, CdMT was used in this study as a model to understand the mechanism(s) of Cd nephrotoxicity. It has been recently demonstrated that Cd from CdMT was preferentially taken up by the proximal convoluted tubules. Therefore, the purpose of these studies was to determine if the organic portion of the complex was also accumulated in these tubules. [35S]CdMT prepared from rat liver was administered intravenously to mice at a nonnephrotoxic dose (0.1 mg Cd/kg). The radioactivity in the kidney showed maximum level (80% of the dose) 15 min after the injection. This preferential renal uptake was also observed after administration of various doses of [35S]CdMT. In contrast to the earlier observed persistency of 109Cd in the kidney after 109CdMT administration, 35S disappeared rapidly (with a half-life of approximately 2 hr), and 24 hr after injection of [35S]CdMT, there was very little 35S left in the kidneys. These observations indicate that the protein portion of CdMT is rapidly degraded after renal uptake of CdMT and the released Cd is retained in the kidney. Within the kidney, 35S distributed mainly to the cortex. Light microscopic autoradiography showed that [35S]CdMT preferentially distributed to the proximal convoluted tubule (S1 and S2), which is the site of nephrotoxicity. Within the S1 and S2 segments, a greater distribution of 35S to the apical portion of the cells was observed after administration of both a nonnephrotoxic (0.1 mg Cd/kg) and a nephrotoxic (0.3 mg Cd/kg) dose. 109Cd administered as 109CdMT also distributed to the apical portion of the S1 and S2 cells. Therefore, both the organic (35S) and inorganic (109Cd) portions of CdMT are rapidly and efficiently taken up by the S1 and S2 cells of the proximal tubules, the site of nephrotoxicity. These observations support the concept that CdMT is readily taken up by the proximal tubular cells as a complex, and then its protein portion is rapidly degraded to release Cd that binds permanently to intracellular sites and produces nephrotoxicity.

Luminal and basolateral uptake and degradation of insulin in the proximal tubules of the dog kidney

In order to determine the major routes of insulin degradation in the body, insulin was labelled with a 'trapped' or 'residualizing' label: [125I]tyramine-cellobiose ([125I]TC) and injected intravenously in dogs. In contrast to conventional iodine-labelled insulin (131I-insulin), the [125I]TC-insulin allows measurements of total uptake in specific organs in vivo because the radioactive degradation products do not leave the cells. One h after the injection of trace doses, the amount of radioactivity recovered in the kidney from [125I]TC-insulin was nine times higher than when conventional [131I]insulin was used. In the blood, the amount of acid-precipitable radioactivity was the same for both labelled preparations, indicating similar clearance rates. A comparison of the uptake of insulin in filtering vs. non-filtering (ureter-occluded) kidneys indicated that the uptake of insulin is twice as high through the luminal than through the basolateral cell membrane; after 60 min, 8.9 +/- 0.8% of the injected [125I]TC-insulin dose remained in the filtering kidney and 3.2 +/- 0.2% of the dose was accumulated in the contralateral kidney, with occluded ureter but normal blood perfusion. In both filtering and non-filtering (ureter-occluded) kidneys, the subcellular distributions of [125I]TC-insulin were studied after various times by isopycnic sedimentation in sucrose gradients. No difference between peritubular and tubular uptake was discernible. The intracellular transport was rapid, leading to accumulation of radioactive label in dense lysosomes within 10 min.

Intracellular transport of ovalbumin afterin vivo endocytosis in rainbow trout liver

The intracellular handing of a mannose-terminated glycoprotein taken up in rainbow trout liver cells by receptor-mediated endocytosis has been studied. The intracellular transport and degradation of ovalbumin (OA) were studied by means of subcellular fractionation in Nycodenz gradients and by differential centrifugation following intravenous injection of the ligand. By using OA labelled with(125)I-tyramine cellobiose ((125)I-TC), the subcellular distribution of labelled degradation products could be studied, since they are trapped intracellularly in the organelle where the degradation takes place. (125)I-TC-OA was shortly after injection (45 min) localized in a homogenous population of endosomes. Labelled degradation products firs appeared in an organelle with the same density distribution as the endosomes. In livers homogenized 2h after injection the degradation products appeared in organelles with increasing size and density. After 24h, the degradation products were recovered in at least two populations of lysosomes with a distribution profile which coincided with that of the lysosomal enzyme β-acetylglucosaminidase.The heterogeneous distribution of the late degradation products seemed not to be due to uptake of ligand in different liver cell types as only the parenchymal liver cells took up labelled OA after intravenous injection of the ligand.

Primary cultures of rabbit renal proximal tubule cells: I. Growth and biochemical characteristics

Before the usefulness of a new in vitro model can be ascertained, the model must be properly defined and characterized. This study presents the growth rate and biochemical characteristics of rabbit renal proximal tubule cells in primary culture over a 2-wk culture period. When grown in a hormonally defined, antibiotic-free medium these cells form confluent monolayer cultures within 7 d after plating. Multicellular dome formation, an indicator of transepithelial solute transport, was expressed after confluent cultures were formed. The activity of the cytosolic enzyme, lactate dehydrogenase, and the lysosomal enzyme, N-acetyl-glucosaminidase, increased 14- and 2-fold during the first 8 d of culture, respectively. In contrast, the activity of a brush border enzyme, alkaline phosphatase, decreased 85% within the first 8 d of culture. Release of these enzyme markers into the culture medium, which are routinely used to measure cytotoxicity, stabilized after 8 d in culture. The ratio of cellular protein to DNA changed according to the state of cellular growth. Values rose from 0.035 mg protein/micrograms DNA in preconfluent cultures to 0.059 mg protein/micrograms DNA in confluent cultures. These results document the characteristics of a primary proximal tubule cell culture system for future studies in in vitro toxicology.

Effect of fasting on lysosomes in kidney cortex of glomerulonephritic rats

The effect of food restriction (FR) on the kidney cortex lysosomes prepared by rate and isopycnic zonal centrifugation was studied in rats with passive Heymann glomerulonephritis (PHN). FR reduced the renal mass by 41%, but the capacity for handling of labelled endocytosed proteins by the lysosomes was not different from fed PHN rats. While PHN with heavy proteinuria increased the recovery of lysosomal enzymes in the large lysosomes located in the proximal tubule, no changes were observed in FR-PHN rats in spite of significant proteinuria. The density of the small lysosomes was significantly shifted/reduced (from 1,200 and 1,235 g/ml to 1,185 and 1,225 g/ml, respectively) in both fed and FR-PHN rats, suggesting that the handling of extra loads of protein may enhance the absorptive function of small lysosomes found in the lower part of the nephron. FR reduced the mechanical fragility of lysosomes in the kidney cortex of PHN-rats. The highly increased urinary excretion of lysosomal enzymes in fed PHN rats was not observed in FR-PHN rats. As a conclusion, FR reduces both the fragility of lysosomes and the proportion of digestive enzymes in fragile lysosomes. These lysosomal enzymes may be of pathogenic importance in PHN causing cell damage when liberated from disrupted lysosomes.

Tissue distribution of 125I-labelled bovine superoxide dismutase (SOD) in the rat

Bovine copper/zinc superoxide dismutase (SOD) was labelled with 125I using the chloramine-T method. The tissue distribution of 125I-SOD (dose of SOD 5 mg/kg) was studied with whole-body and microautoradiography at various times after an intravenous injection. The distribution of 125I-SOD showed a remarkable organ specificity in that the localization of the enzyme to the kidneys and the urinary tract completely dominated the autoradiograms. The time pattern of localization of 125I-SOD also gives a clear picture of the renal handling of the enzyme in that, as a consequence of the renal elimination, the enzyme rapidly disappears from the circulation with an elimination half time of about 6 min. Up to 20 min. after the injection, there were high concentrations of 125I-SOD in the renal pelvis, ureter and urinary bladder showing that in addition to renal uptake there was an initial substantial urinary excretion of the enzyme. From the microautoradiography it is clear that the grains were exclusively localized over proximal tubular cells and tended to be concentrated at the luminal rather than the peritubular side of tubule. This would be compatible with renal uptake secondary to glomerular filtration of 125I-SOD, which is what one would expect from the renal handling of a protein with a molecular weight around 31,000 and an isoelectric point around pH 5.4. Pretreatment with a large dose of SOD (88 mg/kg) tended to competitively decrease the renal uptake of labelled SOD after 5 min. and apparently further increase its renal excretion. However, a noticeable renal uptake of 125I-SOD was still apparent.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in lysosome populations in the rat kidney cortex induced by experimental proteinuria

1. Experimental proteinuria (262.9 mg protein/24 hr urine) was induced in rats by repeated intraperitoneal injections of BSA. 2. Hypertrophy of the kidney cortex was significant 8 days after the start of the BSA injections, and the activities of lysosomal enzymes in kidney cortex and urine were significantly higher in proteinuric compared to nonproteinuric rats. 3. Lysosome populations in the kidney cortex were examined by rate sedimentation of the homogenate and by rate zonal and isopycnic centrifugation of the lysosome-rich ML fraction. 4. The activity of lysosomal enzymes in the kidney cortex increased slightly, essentially in the large, fragile lysosomes mainly recovered from the proximal tubule. 5. Proteinuria induced a shift/reduction in the density of small lysosomes from 1.235 and 1.20 g/ml to 1.225 and 1.185 g/ml, respectively. 6. Proteinuria induced a new population of small lysosomes (density 1.185 g/ml) enriched in cathepsin D.

Latency of acid hydrolases in rat kidney cortex

1. Some lysosomal populations in the rat kidney cortex appear to be mechanically weak and are readily disrupted by gentle homogenization, while other populations remain intact even after repeated homogenization. 2. Lysosomes in the rat kidney cortex appear to be resistant to hypertonic media but are readily disrupted under hypotonic conditions. 3. Lysosomes in rat kidney cortex are readily disrupted when incubated in isotonic sucrose at 37 degrees C. 4. Measurement of total and free activity of three acid hydrolases: N-acetyl-beta-D-glucosaminidase (NAG), acid beta-galactosidase and acid beta-glycerophosphatase, indicates that the latency of these enzymes is relatively low in the homogenate (10-29%) and the ML-fraction (14-42%), but high (60-95%) in the purified large lysosomes (protein droplets). 5. The latency of purified small lysosomes is relatively lower (30-60%) than that of large lysosomes, suggesting that small lysosome populations are relatively permeable to the acid hydrolase substrates. 6. Latency variations of acid hydrolases amongst subcellular fractions appear to reflect the heterogeneity of lysosomal populations present in the kidney cortical homogenate.