Loghman‐Adham, Mahmoud

Background Studies have shown that cilia are mechanical fluid shear sensors. We have reported previously that polycystins, ADPKD proteins, play a role in mechanosensation in mouse kidney epithelial cilia. Objective The current study was designed to analyze the functions of polycystins in human cells. Methods Epithelial cells were isolated from normal and ADPKD (both normal‐looking and dilated/cystic tubules) kidneys. These cells were subject to fluid shear analysis. Results Both immortalized and primary cultured renal tubular cells originating from normal human and non‐dilated ADPKD patient kidneys displayed normal ciliary expressions of the polycystins and responded to fluid flow shear stress with a change in cytosolic calcium. In contrast, immortalized and primary cultured cyst‐lining epithelial cells from ADPKD patients with PKD1 mutation or with abnormal ciliary expression of polycystins were not responsive to fluid shear stress. Conclusion Cyst‐lining epithelia in ADPKD kidneys have lost their ciliary mechanosensing ability. Thus, we propose that loss of mechanosensation in the renal tubular epithelia is a feature of the cyst lining epithelia and monitoring cytosolic calcium in this flow assay is a useful functional readout of polycystin function. This work is supported by the NIH and PKD foundation.

We examined whether phosphonoformate (PFA) can cause phosphaturia through its direct action on brush-border membrane (BBM) in vivo. Infusion of PFA or of parathyroid hormone (PTH) to thyroparathyroidectomized rats caused a marked increase in fractional excretion of phosphate without changes in excretion of Na+ or of GFR. The PFA-induced phosphaturia was not accompanied by an increase in urinary adenosine-3',5'-cyclic monophosphate (cAMP); moreover, PFA added in vitro did not influence the PTH-sensitive adenylate cyclase and cAMP-phosphodiesterase in proximal convoluted tubules. In BBM vesicles (BBMV) from rats with PFA-elicited phosphaturia, neither the rate of Na+-Pi symport nor Na+-dependent binding of [14C]PFA on BBMV was changed, whereas in BBMV from PTH-infused rats the Vmax of Na+-Pi symport decreased. PFA is almost completely ultrafiltrable; no metabolic transformation of PFA was detected after [14C]PFA exposure to rat renal cortical slices, homogenate, or to blood. We conclude that PFA causes phosphaturia by direct inhibition of Na+-Pi symport across BBM in proximal tubules, acting from the luminal side. Thus PFA (foscarnet) has a unique direct mechanism of phosphaturic effect, via its action on Pi reabsorption in proximal tubules in vivo.