Tubular protein uptake pattern in the frog model (Rana temporaria): The effect of previous protein loading

Abstract

The effect of increasing protein load on subsequent receptor-mediated protein uptake was studied in the kidney of the common frog Rana temporaria L. Results of in vivo experiments were analyzed in fixed kidney sections using fluorescent or confocal microscopy and immunohistochemistry. Lysozyme was used for daily tubular loading in short-term experiments. Reabsorption of yellow fluorescent protein (YFP) in the proximal tubule (PT) was tested 60 min after introduction into the dorsal lymphatic sac. YFP uptake decreased progressively with increasing duration of lysozyme preload from 2 to 4 days. Lysozyme loading and single protein injections did not change the morphological characteristics of frog glomeruli and PTs, as shown by light and electron microscopy and morphometric analysis. Cessation of loading led to a decrease in the amount of lysozyme accumulated in PT cells. Reduced YFP uptake gradually recovered after cessation of the 4-day load. Restoration of YFP reabsorption was accompanied by increasing expression of endocytic receptors, megalin and cubilin. Based on the data obtained, the frog model can be successfully used for studying both morphological and functional changes in the nephron caused by tubular or glomerular proteinuria and molecular mechanisms involved in the process of renal protein reabsorption.