Transporters and tubule crystals in the insect Malpighian tubule

Abstract

The insect renal (Malpighian) tubules are functionally homologous to the mammalian kidney. Accumulating evidence indicates that renal tubule crystals form in a manner similar to mammalian kidney stones. In Drosophila melanogaster, crystals can be induced by diet, toxic substances, or genetic mutations that reflect circumstances influencing or eliciting kidney stones in mammals. Incredibly, many mammalian proteins have distinct homologs in Drosophila, and the function of most homologs have been demonstrated to recapitulate their mammalian and human counterparts. Here, we discuss the present literature establishing Drosophila as a nephrolithiasis model. This insect model may be used to investigate and understand the etiology of kidney stone diseases, especially with regard to calcium oxalate, calcium phosphate and xanthine or urate crystallization.

Figure 1. Calcium oxalate (CaOx) crystallization in Drosophila renal tubules.

Crystals in Drosophila Malpighian tubules are birefringent and easily visualized under polarized light microscopy: (a) Calcium ions that are secreted into the lumen may form precipitates that evolve into crystals. (b) Diagram depicting how polarized light passes through molecular crystal lattice. (c) Model of CaOx crystal formation. Oxalate can be transported into the principal cells from the hemolymph and/or produced from hydroxyproline or ethylene glycol precursors. Both hydroxyproline and ethylene glycol are degraded into glyoxylates in multistep pathways with hydroxyproline degradation also producing pyruvate. Glyoxylate is converted to oxalate by a proposed CG31676 protein or diverted to glycine by dAgxt. Oxalate is transported across the principal cell, apical membrane by dPrestin Cl⁻ exchange. Cl⁻ ions that drive oxalate export are supplied from stellate cell transporters, Clc-a (basolateral) and secCl (apical). Oxalate in the MT lumen precipitates with Ca²⁺ (secreted from the blind ends of the tubule) to form CaOx crystals. Water transport by Prip, Drip, Eglp2, and Eglp4 may influence solute saturation and precipitation. (d) CaOx tubuloliths in anterior MT exhibiting birefringence due to polarized light. Yellow arrows indicate crystals, scale bar = 200 μm.

Figure 2. Xanthine and Urate Crystallization in Drosophila renal tubules.

(a) Model of xanthine and urate crystal formation. Purine degradation leads to the production of hypoxanthine. Xanthinuria dehydrogenase/oxidase (Xdh) converts hypoxanthine to xanthine, and then to urate. Urate is then oxidized by urate oxidase (i.e. uricase) to allantoin. Hypoxanthine, xanthine, urate, and allantoin are transported into the lumen by unknown transporter(s) on the apical membrane of principal cells. Allantoin is converted to urea in A. aegypti but this pathway has not yet been identified in other insects. Crystals can form from hypoxanthine, xanthine, or urate with the presence of Zn²⁺, Ca²⁺ or Mg²⁺. Xdh is inhibited by Allopurinol. Crystal formation is inhibited by increased pH from NHE2 and/or Sip-1. Water transport by Prip, Drip, Eglp2, and Eglp4 may influence solute saturation and precipitation. (b) Urate tubuloliths in anterior MT of Sip1^(−/−) mutants and xanthine tubuloliths in rosy^(−/−) mutants exhibit birefringence due to polarized light (reproduced with permission from Ref. [42^•]). Yellow arrows indicate the tubuloliths; black arrowheads mark MT lumen.

Figure 3. Calcium phosphate crystallization in Drosophila renal tubules.

(a) Model of calcium phosphate crystal formation. Phosphate is transported from the hemolymph to the tubule lumen by a basolateral NaP_i-T and an unknown apical phosphate transporter. Phosphate transport may be regulated by binding of fibroblast growth factor, breathless (btl), to a receptor complex, such as the branchless (bnl) + klotho dimer. Water transport by Prip, Drip, Eglp2, and Eglp4 may influence solute saturation and precipitation. (b) Paraffin sections of phosphate-fed Drosophila are stained with Von Kossa stain (phosphate, black precipitate; top) or Alizarin Red stain (calcium, red stain; bottom). Arrows indicate positive staining tubuloliths in anterior MT (reproduced with permission from Ref. [31^••]).