2 Cystinosis (ctns) zebrafish mutant shows pronephric glomerular and tubular dysfunction 27 ctns‒/‒ zebrafish pronephros shows enlarged lysosomes in proximal tubular cells and partial podocyte foot process effacement Analysis by light microscopy showed no apparent glomerular or tubular abnormalities compared to wt (Fig. 5a, b). Analysis by block face scanning electron microscopy revealed however that proximal tubular epithelial cells (PTECs) in ctns‒/‒ pronephros had numerous and enlarged lysosomes compared to wt (Fig. 5c, d). We calculated the numbers and average surface area of lysosomes in complete cut-sections of wt and ctns‒/‒ larvae at the level of proximal tubules (n= 10 each). Per cut-section lysosomal numbers were higher in the proximal tubules of ctns‒/‒ larvae (68.4±4.7) compared to wt larvae (24.5±3.8), P<0.001. Average lysosomal surface area was also higher in the ctns‒/‒ larvae (1.38±0.1 µm2) compared to the wt (0.51±0.03 µm2), P<0.001 (Fig.5e). On the other hand, cystinotic PTECs did not show cystine crystal accumulation or brush border flattening. The ultrastructural analysis of podocytes of ctns‒/‒ larvae showed partial foot process effacement and narrowed slit diaphragmatic spaces when compared to wt larvae, while glomerular basement membrane appeared to be of normal thickness (Fig. 5f, g). To assess podocyte foot process effacement in a quantitative manner, average podocyte foot process width (FPW) was measured. A previously described formula was used to perform this analysis35. ctns‒/‒larvae showed higher podocyte FPW (0.62±0.09 µm) when compared to wt larvae (0.51±0.05 µm), P=0.033 (Fig.5h). Thus, at the cellular level the ctns‒/‒ larvae also showed some of the human pathological features of the disease.
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