Sickle cell disease up-regulates vasopressin, aquaporin 2, urea transporter A1, Na-K-Cl cotransporter 2, and epithelial Na channels in the mouse kidney medulla despite compromising urinary concentration ability.

Sickle cell disease (SCD)-induced urinary concentration defect has been proposed as caused by impaired ability of the occluded vasa recta due to red blood cell sickling to serve as countercurrent exchangers and renal tubules to absorb water and solutes. However, the exact molecular mechanisms remain largely unknown. The present studies were undertaken to determine the effects of SCD on vasopressin, aquaporin2 (AQP2), urea transporter A1 (UTA1), Na-K-Cl co-transporter 2 (NKCC2), epithelial Na channels (ENaC), aquaporin1 (AQP1), nuclear factor of activated T cells 5 (NFAT5) and Src homology region-2 domain-containing phosphatase-1 (SHP-1), an important regulator of NFAT5, in the Berkeley SCD mouse kidney medulla. Under water repletion, SCD only induced a minor urinary concentration defect associated with increased urinary vasopressin level alone with the well-known effects of vasopressin: protein abundance of AQP2, UTA1 and ENaC-β and apical targeting of AQP2 as compared with non-SCD. SCD did not significantly affect AQP1 protein level. Water restriction had no further significant effect on SCD urinary vasopressin. NFAT5 is also critical to urinary concentration. Instead, water restriction-activated NFAT5 associated with inhibition of SHP-1 in the SCD mice. Yet, water restriction only elevated urinary osmolality by 28% in these mice as opposed to 104% in non-SCD mice despite similar degree increases of protein abundance of AQP2, NKCC2 and AQP2-S256-P. Water-restriction had no significant effect on protein abundance of ENaC or AQP1 in either strain. In conclusion, under water repletion SCD, only induces a minor defect in urinary concentration because of compensation from the up-regulated vasopressin system. However, under water restriction, SCD mice struggle to concentrate urine despite activating NFAT5. SCD-induced urinary concentration defect appears to be resulted from the poor blood flow in vasa recta rather than the renal tubules' ability to absorb water and solutes.