Die endocytose-phase der eiwei\resorption im proximalen nierentubulus: Untersuchungen am Ferritin-resorbierenden Einzeltubulus der Rattenniere

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.

Endocytosis in proximal tubule cells involves a two-phase membrane-recycling pathway

The aims of the present study were to examine the initial events of endocytosis with respect to migration, internalization, and recycling of protein binding sites on the luminal plasma membrane of genuine renal proximal tubule cells (not cultured cells), and in particular the sequential involvement of dense apical tubules (DAT). Isolated rabbit proximal tubules were first perfused at low temperature (9 degrees C) with insulin-gold to label the binding sites and then with disuccinimidyl suberate (DSS) for 20 min to covalently cross-link insulin-gold and binding sites. The tubules were subsequently perfused for 4-30 min at 25 or 37 degrees C before fixation to follow the pathways of the binding sites. The present experimental approach allowed a temporal separation of the subcellular processes involved in endocytosis. The identity and organization of the different organelles were certified by serial sectioning of the experimental tubules. In tubules fixed directly at low temperature after cross-linking the label was located on the microvilli membranes and in plasma membrane invaginations. The labeling of microvilli decreased with time of perfusion at 25 degrees C in parallel with a simultaneous increase of labeling of invaginations, suggesting lateral migration of the binding sites. The invaginations pinched off from the surface to form endocytic vesicles. The binding sites were subsequently trafficked either to 1) tubular elongations from endocytic vesicles or small vacuoles, which again form DAT, or 2) in a later sequence trafficked to larger vacuoles, from which tubular elongations and DAT also form. The elongations formed the DAT that transported binding sites back to the luminal plasma membrane. Lysosomes and Golgi apparatus were not involved in membrane recycling. Thus the present study provides evidence for a two-phase recycling pathway of membrane proteins involved in endocytosis.

Freeze-fracture features of epithelioid cells, multinucleated giant cells, and phagocytic macrophages. Investigations using the model of experimental autoimmune (anti-TBM) tubulo-interstitial nephritis

The freeze-fracture morphology of epithelioid cells, multinucleated giant cells (Langhans' type), and phagocytic macrophages was investigated. The intensely folded and interdigitating surface membranes of epithelioid cells and multinucleated giant cells displayed no specialized areas of cell contact. The size of the intramembranous particles (IMP) and the fact that the area density of IMPs was higher in the cytoplasmic (P) faces than in the external (E) faces of the cell membranes agreed with observations in other eukaryotic cells. The area densities of the IMPs suggest lower transport rates of molecules across the cell membranes of granuloma cells than of certain epithelial cells. Small pits were detected in the surface membranes of the granuloma cells but an extrusion of granules was not observed. The cytoplasmic granules displayed very different sizes and shapes ranging from spherical to rod-shaped. The latter type of granules (probably primary lysosomes) dominated in multinucleated giant cells. The granule membranes were studded with IMPs whose area densities increased with the granule size. Multilamellar bodies with smooth (lipid) fracture faces were found only in phagocytic macrophages. The nuclear pores of the granuloma cells were distributed over the entire surfaces of the nuclei and displayed moderate clustering. The values of the area densities of the nuclear pores were in keeping with the values observed in mammalian and human epithelial or mesenchymal cells, indicating similar exchange rates of molecules between the nucleoplasm and the cytoplasm in these different cell types. In a single phagocytic macrophage the E-face of the inner membrane of the nuclear envelope displayed a network of fine filaments whose nature is at present unknown.

Phagocytosis of bacteria by proximal tubular epithelium in experimental pyelonephritis

Acute pyelonephritis was induced in rats by temporary unilateral ureteric obstruction and the intravenous injection of Escherichia coli. Animals were sacrificed 48 h after infection and changes in renal cortical tubules due to the presence of bacteria were studied. Bacteria appeared and multiplied in the tubular lumina and proximal tubular epithelial cells endocytosed the microorganisms in large numbers. Coalescence of phagosomes with lysosomes resulted in the surrounding of engulfed bacteria with acid phosphatase. However, the lysosomal apparatus of the cells did not eliminate Escherichia coli since the bacteria multiplied within phagosomes and destroyed the normal cell architecture. The peritubular interstitial inflammatory infiltrate caused ischemia of tubules, enhancing bacterial damage to the proximal tubules. The cytoplasm of the injured tubular cells was sometimes detached from the basement membrane. Cells of the distal tubules and collecting ducts did not show significant endocytosis or bacterial tubular damage.

Inhibition by insulin of cellular autophagy in proximal tubular cells of rat kidney

Adult male Sprague-Dawley rats were injected intraperitoneally with 5 U insulin/kg body wt (45 animals). As determined by quantitative electron microscopy, the volume fraction and the numerical density of autophagic vacuoles (AV) in proximal tubular cells decreased within 10 min by 46 and 26%, respectively. A partial recovery of the AV volume fraction was observed 20 and 30 min after the injection contrary to our previous findings with liver (J. Cell Biol. 78: 152-167, 1978). In an additional experiment (12 animals) it was shown that an insulin dose of 0.5 U but not of 0.05 U/kg body wt reduced the AV volume fraction to an extent similar to that of 5 U. To eliminate possible secondary effects, Ringer solution containing 0.8 microM insulin was dropped intravitally for 15 min to one pole of the decapsulated kidney and Ringer solution without additions to the other pole (8 animals). After intravital fixation, the AV volume fraction and numerical density in proximal tubular cells was found to be reduced under the influence of insulin by 22 and 36%, respectively. This data shows that insulin inhibits the process of cellular autophagy in proximal tubular cells of the kidney.

Characterization of cells involved in the formation of granuloma. An ultrastructural study on macrophages, epitheloid cells, and giant cells in experimental tubulo-interstitial nephritis

In experimental tubulo-interstitial (anti-basement membrane) nephritis of the rat, granulomatous inflammation develops around immunologically altered tubular basement membranes. The present light- and electron microscopic studies indicate that in the course of the granulomatous reaction, tissue monocytes evolve from blood monocytes and pursue two independent pathways of differentation. On the one hand they may differentiate into macrophages ("distant from tubules") or, alternatively, into epitheloid cells ("adjacent to tubules"). The latter, through cell fusion, develop into multinucleated giant cells of the Langhans' type. The cytoplasmic components of the epitheloid cells and the multinucleated giant cells should be interpreted as an activation of cellular biosynthesis. Its products, upon being secreted at the immunodefective basement membrane, will obviously serve immune defense mechanisms.