Immunocytochemical localization of cathepsin H in rat kidney. Light and electron microscopic study

Spatiotemporal Landscape of Kidney Tubular Responses to Glomerular Proteinuria

Key Points

Background

Large increases in glomerular protein filtration induce major changes in body homeostasis and are associated with a higher risk of kidney functional decline and cardiovascular disease. We investigated how elevated protein exposure modifies the landscape of tubular function along the entire nephron, to understand the cellular changes that mediate these important clinical phenomena.

Methods

We conducted single-nucleus RNA sequencing, functional intravital imaging, and antibody staining to spatially map transport processes along the mouse kidney tubule. We then delineated how these were altered in a transgenic mouse model of inducible glomerular proteinuria (POD-ATTAC) at 7 and 28 days.

Results

Glomerular proteinuria activated large-scale and pleiotropic changes in gene expression in all major nephron sections. Extension of protein uptake from early (S1) to later (S2) parts of the proximal tubule initially triggered dramatic expansion of a hybrid S1/2 population, followed by injury and failed repair, with the cumulative effect of loss of canonical S2 functions. Proteinuria also induced acute injury in S3. Meanwhile, overflow of luminal proteins to the distal tubule caused transcriptional convergence between specialized regions and generalized dedifferentiation.

Conclusions

Proteinuria modulated cell signaling in tubular epithelia and caused distinct patterns of remodeling and injury in a segment-specific manner.

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Spatiotemporal organisation of protein processing in the kidney

The kidney regulates plasma protein levels by eliminating them from the circulation. Proteins filtered by glomeruli are endocytosed and degraded in the proximal tubule and defects in this process result in tubular proteinuria, an important clinical biomarker. However, the spatiotemporal organization of renal protein metabolism in vivo was previously unclear. Here, using functional probes and intravital microscopy, we track the fate of filtered proteins in real time in living mice, and map specialized processing to tubular structures with singular value decomposition analysis and three-dimensional electron microscopy. We reveal that degradation of proteins requires sequential, coordinated activity of distinct tubular sub-segments, each adapted to specific tasks. Moreover, we leverage this approach to pinpoint the nature of endo-lysosomal disorders in disease models, and show that compensatory uptake in later regions of the proximal tubule limits urinary protein loss. This means that measurement of proteinuria likely underestimates severity of endocytotic defects in patients.

Polesel et al. visualize plasma protein filtration, uptake and metabolism in the kidneys of living mice in real-time. They reveal coordinated activity of different specialized tubular segments, with major compensatory adaptations occurring in disease states.

Axial differences in endocytosis along the kidney proximal tubule

The proximal tubule (PT) reabsorbs filtered proteins via receptor-mediated endocytosis to prevent energetically inefficient wasting in the urine. Recent intravital imaging studies have suggested that protein reabsorption occurs in early (S1) segments, which have a very high capacity. In contrast, uptake of fluid phase substrates also occurs in distal (S2) segments. In this article, we will review these findings and their implications for understanding integrated proximal tubular function, patterns of damage caused by endocytosed toxins, and the origins of proteinuria. We will also discuss whether compensatory downstream increases in protein uptake might occur in disease states, and the environmental factors that could drive these changes.

Combined Structural and Functional Imaging of the Kidney Reveals Major Axial Differences in Proximal Tubule Endocytosis

BACKGROUND

The kidney proximal convoluted tubule (PCT) reabsorbs filtered macromolecules via receptor-mediated endocytosis (RME) or nonspecific fluid phase endocytosis (FPE); endocytosis is also an entry route for disease-causing toxins. PCT cells express the protein ligand receptor megalin and have a highly developed endolysosomal system (ELS). Two PCT segments (S1 and S2) display subtle differences in cellular ultrastructure; whether these translate into differences in endocytotic function has been unknown.

METHODS

To investigate potential differences in endocytic function in S1 and S2, we quantified ELS protein expression in mouse kidney PCTs using real-time quantitative polymerase chain reaction and immunostaining. We also used multiphoton microscopy to visualize uptake of fluorescently labeled ligands in both living animals and tissue cleared using a modified CLARITY approach.

RESULTS

Uptake of proteins by RME occurs almost exclusively in S1. In contrast, dextran uptake by FPE takes place in both S1 and S2, suggesting that RME and FPE are discrete processes. Expression of key ELS proteins, but not megalin, showed a bimodal distribution; levels were far higher in S1, where intracellular distribution was also more polarized. Tissue clearing permitted imaging of ligand uptake at single-organelle resolution in large sections of kidney cortex. Analysis of segmented tubules confirmed that, compared with protein uptake, dextran uptake occurred over a much greater length of the PCT, although individual PCTs show marked heterogeneity in solute uptake length and three-dimensional morphology.

CONCLUSIONS

Striking axial differences in ligand uptake and ELS function exist along the PCT, independent of megalin expression. These differences have important implications for understanding topographic patterns of kidney diseases and the origins of proteinuria.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Biosynthetic processing of cathepsins and lysosomal degradation are abolished in asparaginyl endopeptidase-deficient mice

Asparaginyl endopeptidase (AEP)/legumain, an asparagine-specific cysteine proteinase in animals, is an ortholog of plant vacuolar processing enzyme (VPE), which processes the exposed asparagine residues of various vacuolar proteins. In search for its physiological role in mammals, here we generated and characterized AEP-deficient mice. Although their body weights were significantly reduced, they were normally born and fertile. In the wild-type kidney where the expression of AEP was exceedingly high among various organs, the localization of AEP was mainly found in the lamp-2-positive late endosomes in the apical region of the proximal tubule cells. In these cells of AEP-deficient mice, the lamp-2-positive membrane structures were found to be greatly enlarged. These aberrant lysosomes, merged with the late endosomes, accumulated electron-dense and membranous materials. Furthermore, the processing of the lysosomal proteases, cathepsins B, H, and L, from the single-chain forms into the two-chain forms was completely defected in the deficient mice. Thus, the AEP deficiency caused the accumulation of macromolecules in the lysosomes, highlighting a pivotal role of AEP in the endosomal/lysosomal degradation system.

Membrane receptors for endocytosis in the renal proximal tubule

The renal proximal tubule exhibits a very extensive apical endocytic apparatus consisting of an elaborate network of coated pits and small coated and noncoated endosomes. In addition, the cells contain a large number of late endosomes/prelysosomes, lysosomes, and so-called dense apical tubules involved in receptor recycling from the endosomes to the apical plasma membrane. This endocytic apparatus is involved in the reabsorption of molecules filtered in the glomeruli. The process is very effective as demonstrated by the fact that although several grams of protein are filtered daily in the human glomeruli, human urine is virtually devoid of proteins under physiological conditions. Several key receptors appear to be involved in this function, which serves not only to conserve protein as such for the organism but also to reabsorb vital substances such as different vitamins in complex with their binding proteins. Recent research has established megalin, a 600-kDa protein belonging to the LDL receptor family, as probably the most important receptor in this process in the proximal tubule mediating endocytosis of a large variety of ligands and therefore classifying it as a scavenger receptor. More specific receptors like the folate receptor, IGF-II/Man-6-P receptor, and gp280/IFR, identical to the intrinsic factor receptor, are also functioning in the apical endocytic pathway of renal proximal tubules. A better understanding of these receptors will give us new insight into these very important processes for the organism.

Immunelectron microscopic localization of cathepsin B in human exocrine glands

The distribution of the lysosomal enzymes cathepsin B, lysozyme, chymotrypsin, and neutrophil elastase was examined in eccrine, apocrine, and sebaceous glands using a postembedding immunogold labeling procedure. Various amounts of cathepsin B were detected in all glands. Lysozyme, however, was detected in apocrine glands only. The other two lysosomal enzymes were not detectable immunologically. In apocrine and eccrine glands, anti-cathepsin B antibody labeled all secretory granules. In sebaceous glands, only the peripheral layer of cells showed immunological activity for cathepsin B. In apocrine glands, granules containing remnants of cristae were more intensively labeled than those lacking cristae which supports the assumption that both granules are derived from mitochondria by acquiring lysosomal enzymes. The enzymes convert mitochondria to granules with cristae and finally to granules without cristae. Thus the difference in morphology is part of a spectrum of the degradation of mitochondria to granules.

Purification and partial characterization of a thiol proteinase activating prokallikrein from the rat kidney cortex

The rat kidney cortex contains at least three kinds of prokallikrein-activating proteinase, and among these the one with the highest molecular weight was purified by a procedure including chromatography on CM-cellulose, concanavalin A-Sepharose, organomercurial-Sepharose 4B and Sephadex G-100. The resulting preparation was apparently homogeneous, as assessed by SDS-polyacrylamide gel electrophoresis. The molecular weight was estimated to be 57,000. The optimal pH for the activation of prokallikrein by the preparation, termed activator I, was around 4.5. Activator I was inhibited by E-64, iodoacetate and leupeptin, but not by PMSF and phosphoramidon. In immunodiffusion analysis, the antiserum to activator I formed an immunoprecipitin arc with the extract from the kidney cortex or submandibular gland, but not with that from the pancreas. These results indicate that activator I is a thiol proteinase with a molecular weight of 57,000. A proteinase immunologically identical with activator I appears to be present in the submandibular gland.

A new method of the immunohistochemical detection of cellular antigens for light and electron microscopy

A new immunohistochemical method for light and electron microscopy of tissue- and cell-specific antigens by using ferric colloid-labeled antibody is presented. The antibodies labeled with the cationic cacodylate ferric colloid are stable and bind specifically to the target antigens to show clearly the site of antigens in tissue sections and on free cells by Prussian blue reaction for light microscopy and by the specific figure of electron opaque ferric colloid particles for electron microscopy. The staining procedure is very simple and it gives clear picture. So the method will be of beneficial for general laboratory use in immunohistochemical researches.

Involvement of cathepsins B and H in lysosomal degradation of horseradish peroxidase endocytosed by the proximal tubule cells of the rat kidney: I. Histochemical and immunohistochemical studies

Immunohistochemical localizations of horseradish peroxidase (HRP) and cathepsins B and H in the lysosomal system of the rat kidney proximal tubule cells were investigated during the internalization and degradation of HRP by these cells. At fixed intervals after intravenous injection of HRP, the kidneys were fixed by perfusion. Five to fifteen minutes after this injection, endocytosed HRP was detected in fine granules beneath the brush border. Thirty minutes later, it accumulated into large aggregates of granules (phagosomes), where both enzymatic and antigenic activities appeared to cause HRP to be lost rapidly. After 1 hour, the aggregates of phagosomes increased in number and size but soon began to decrease gradually. In the early stage, cathepsins B and H were not detected in the apical cytoplasmic granules. However, they were present in the lysosomal compartments, together with the HRP. The large aggregates of phagosomes showed a patchy reaction, especially approximately 30 minutes after injection, whereas other medium-sized phagosomes were stained heavily. Aggregates of phagosomes were present only in the S1 segment. These results suggest strongly that cathepsins B and H are involved in the cellular degradation of endocytosed HRP.

The heterogeneity of rat kidney lysosomes revealed by immunoelectron microscopic staining for cathepsins B and H

The heterogeneity of lysosomes was studied by analyzing the immunostaining behavior of cathepsins B and H in rat kidney proximal tubule cells. Rat kidneys were fixed by perfusion and embedded in Lowicryl K4M. A protein A-gold technique was applied to serial sections and a double labeling technique to conventional sections. By analyzing the immunostaining behavior of cathepsins B and H in the same lysosomes which were cut into separate sections, four types of lysosomes were found: Type 1 positive for both proteinases; type 2 strongly positive for cathepsin B, but weakly or negative for cathepsin H; type 3 strongly positive for cathepsin H, but weakly or negative for cathepsin B; and type 4 negative for both proteinases. The double labeling by two different sizes of the protein A-gold probes showed these four types of lysosomes. The results indicate that there exists the lysosomal heterogeneity of the proteinase content in the kidney proximal tubule cells.

Localization of cathepsin L in rat kidney revealed by immunoenzyme and immunogold techniques

Localization of cathepsin L in rat kidney was investigated by immunocytochemical techniques. Kidneys were fixed by perfusion and embedded in Epon or Lowicryl K4M without postosmication. For light microscopy (LM), semi-thin sections of the Epon-embedded material were stained by the immunoenzyme technique after removal of epoxy resin. For electron microscopy (EM), ultra-thin sections of Lowicryl K4M-embedded material were stained by the protein A-gold technique. By LM, reaction deposits for cathepsin L were present in the cytoplasmic granules of proximal tubule cells, but little or no reaction product was noted in distal tubule, collecting tubule, and most of urinary tubules in the medulla. By EM, heavy gold label for cathepsin L was confined exclusively to lysosomes of the proximal tubule cells, but little or no label to those of the other segments. In immunocytochemical control sections, no reaction was observed. These results indicate that a main container of cathepsin L is lysosomes of the proximal tubule and suggest that the enzyme plays a role in the degradation of endocytosed proteins.

Immunocytochemical localization of cathepsins B and H in rat liver

Light and electron microscopic localization of cathepsins B and H in rat liver was investigated by immunoenzyme and protein A-gold techniques. For light microscopy (LM), semi-thin sections of the Epon-embedded material were stained by the immunoenzyme technique after removal of epoxy resin. For electron microscopy (EM), ultra-thin sections of the Lowicryl K4M-embedded material were stained by the protein A-gold technique. By LM, reaction deposits for cathepsins B and H were present in the cytoplasmic granules of parenchymal cells and endothelial cells, and Kupffer cells. The sinus-lining cells and the parenchymal cells showed the similar staining intensity. By EM, gold particles were present exclusively in lysosomes of all the cell types cited above. The same results were obtained from quantitative analysis. In addition, Golgi complexes themselves were mostly negative but some small vesicles on the trans side of them were labeled for these proteinases. The results indicate that cathepsins B and H are present in the lysosomes of rat liver and that these enzymes seem to be transported by small vesicles from endoplasmic reticulum to lysosomes via tubuloreticular network of the trans Golgi region.