Formation of apical tubules from large endocytic vacuoles in kidney proximal tubule cells during absorption of horseradish peroxidase

Abstract

Using horseradish peroxidase (HRP) as a tracer, we have investigated if the so-called apical tubules (AT) in the kidney proximal tubule cells are directly involved in the endocytic process by carrying the tracer into the cells, or if they are derived from the intracellular membrane compartments. Rat kidney was fixed by vascular perfusion at different time intervals after intravenous injection of HRP and prepared for electron microscopy. An analysis revealed that 0.5 min after injection, invaginations of the plasma membrane and small apical endocytic vesicles, including coated vesicles, were labelled with reaction product, whereas almost all large apical endocytic vacuoles and the AT were negative. The endocytic vacuoles and about 18% of the AT were labelled 1 min after injection. The reaction product in the large endocytic vacuoles was usually seen along the luminal surface of the vacuoles. The AT with reaction product appeared as a branched network, and were frequently connected with the labelled endocytic vacuoles. Three min after injection, reaction product was detected in about 38% of the AT, and thereafter, the percentage increased to about 74% after 7 min. No reaction product was detected in the Golgi complex at any time after HRP-injection. These findings indicate that the AT are probably formed by budding off from the large endocytic vacuoles, rather than being directly involved in the endocytic process.

Kidney-Specific Inactivation of the Megalin Gene Impairs Trafficking of Renal Inorganic Sodium Phosphate Cotransporter (NaPi-IIa)

“…In particular, the anastomosing network of DAT, an organelle that has a characteristic spinelike protein coat covering its inner surface and mostly lacks an external clathrin coat (8), was absent (Figure 4, a and a'). HRP, a marker of fluid phase endocytosis, was intravenously applied to study alterations of this process with respect to megalin deficiency.…”

Electron-microscopic demonstration of carbohydrates and proteins in the apical tubules of kidney proximal tubule cells.

“…It has been suggested based on ultrastructural tracer studies that the AT originate, by budding off, from the large endocytic vacuoles and transport endocytosed membrane material back to the apical cell surface (membrane recycling) (2,11,12,15,18). The view that the AT play a role in membrane recycling has been supported by enzyme cytochemistry that the reaction products for acid and alkaline phosphatase are present in both the AT and the apical cell surface of the absorptive cells of rat yolk sac endoderm (42).…”

“…Recent ultrastructural tracer studies have suggested that the AT of these epithelia have a role in recycling of the apical plasma membrane during the process of endocytosis (2,11,12,15,18). The AT originate, by budding off, from the large endocytic vacuoles and transport endocytosed membrane material back to the apical cell surface (membrane recycling).…”

Preserved tissue structure of efferent ductules in aromatase-deficient mice

“…Three types of tubule-vesicular components were observed by transmission electron microscopy. One is a component belonging to an endocytic apparatus including intermicrovillous membrane invaginations that are decorated with clathrin coat lining over the cytoplasmic surface of the invaginating region, clathrin-coated vesicles, and vacuolar early sorting endosomes (Hatae et al 1986a). This component was observed in the WT, Ar K/K , and Esr1 K/K efferent ductules (Fig.…”