Chapter 2 30 independent experiments, P=0.01 and 0.032. These results emphasize the similarity between zebrafish and human cystinosis patients. ctns‒/‒ zebrafish pronephros show impaired proximal tubular function impaired endocytosis of low molecular weight dextran Another functional aspect that we investigated was the tubular reabsorption. Here we carried out a histological evaluation of the number of dextran droplets in the proximal tubular cell wall of fixed larvae after the injection of a fluorescent labelled low molecular weight (LMW) dextran (4-kDa) into the vascular system of 72 hpf larvae in both ctns‒/‒ and wt (N=10 of each genotype). The low molecular weight dextran freely passes the glomerular filtration barrier and is efficiently reabsorbed by the proximal tubular endosomal machinery. Interestingly, the fluorescence over the proximal tubule of ctns‒/‒ larvae was virtually absent compared to the injected wt larvae (P<0.001, Fig. 6), suggesting that proximal tubular reabsorption was defective in the ctns‒/‒ larvae. altered abundance and localization of the endocytic receptor megalin Altered apical abundance of the multi-ligand receptor megalin has been linked with the abnormal endocytosis and defective function of cystinotic PTECs in both mice and human cells11,37. We evaluated the abundance and localization of megalin in the proximal tubules in both 5 dpf ctns‒/‒ and wt larvae, as described previously38. The overall level of megalin present in ctns‒/‒ larvae was about half of that in the wt (Fig. 7a-e); however, the most striking feature was the altered distribution of megalin in the cystinotic PTECs where it accumulated in sub-apical punctate rings or cytoplasmic vacuoles when compared to the wt, where it was more evenly distributed in the apical brush border. This altered localization denotes a defective recycling of megalin from apical endosomes, which may explain, at least partially, the disturbed endocytosis in cystinotic larvae. We also evaluated the overall megalin gene (lrp2a) expression in homogenized 3 dpf and 6 dpf ctns‒/‒ and wt larvae. At the RNA level, there was no statistical significant difference detected between both genotypes(Fig. 7f) similar to zebrafish models of other genetic disorders with impaired proximal tubular endocytosis, such as Lowe syndrome38. The reduced abundance of megalin protein at the proximal tubular brush border in the absence of decreased transcript levels is consistent with the abnormal recycling of the protein rather than a decreased transcription and is similar to the findings in humans37,39.
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