Segment-Specific Cyst Index (SSCI): A Quantitative Analysis of Segment-Specific Responses during Cystogenesis

Background: Polycystic kidney disease (PKD) is a genetic disease in which loss of polycystin-1 or 2 causes tubular segments to undergo proliferation and dilation leading to progressive renal dysfunction. We developed SSCI to better quantify segment-specific responses during cystogenesis Methods: Immunofluorescence microscopy using Lotus tetragonolobuslectin to identify proximal tubule (PT); uromodulin for thick ascending limb (TAL); calbindin for distal convoluted tubule (DCT); and Dolichos biflorusagglutinin for collecting duct (CD) was performed on cortex images from Pkd1fl/fl;Pax8-rtTA;TetO-Cre mice (treated ± doxycycline from 4-6 weeks) at 8 and 12 weeks. Lumen and tubular area quantification for each cortical tubule segment was determined by manual masking in 5-7 randomly assigned boxes. The SSCI was expressed as ratio of lumen to tubular area percentage. Results: Uninduced (UI) control kidneys showed a slight increase in SSCI from 8 to 12 weeks in all segments likely related to growth-induced GFR increases.Pkd1 tubule knock-out (PkdTKO) led to progressive dilation of all segments. Consistent with previous observations, CD exhibited the greatest SSCI (46% at 8 weeks and 62% at 12 weeks), with PT, TAL, and DCT showing less lumen dilation (10%, 11%, and 21% at 8 weeks, and 34%, 30%, and 37% at 12 weeks, respectively). Because CD had a significantly higher baseline SSCI in UI kidneys than all other segments, the fold-increase in SSCI following Pkd1 loss was the least in CD compared to the other 3 segments (3.7-fold for CD vs 7.4-fold for PT; 4.4-fold for TAL; and 7.3-fold for DCT). Conclusion: SSCI reveals that Pkd1 loss caused expansion of all segments at 8 and 12 weeks, with the greatest lumen area found in CD. However, CD exhibited the least fold increase in SSCI following loss of Pkd1, suggesting that the CD, despite being the most cystic, may respond less to the loss of Pkd1 than other segments Funding: Other U.S. Government Support8-week images of (A) LTL for cPT, UMOD for cTAL; calbindin for cDCT and DBA for cCD (B) SSCI for each segment; dots signify the mean value calculated from one kidney *P<0.05, **P<0.01, ***P<0.001 and ****P<0.0001