Changes in subpodocyte space parameters are correlated with diabetic nephropathy progression.
Dong, Alice   (University of Washington)
Mentor: Najafian, Behzad
Diabetic nephropathy (DN) is the leading cause of End Stage Kidney Disease (ESKD) in the US. The progression of DN is gradual, with patients accumulating structural lesions for many years before clinical findings such as albuminuria and declining GFR are evident. Early prediction of DN progression is necessary for successful intervention before irreversible injury is established in the kidney. However, the currently available structural functional relationship (SFR) models developed based on classical lesions of DN do not accurately predict progression of the disease early on. This may be related to structural variables affecting kidney function but are not included in the SFR models thus far. The subpodocyte space (SPS) is defined as the space confined between interdigitating foot processes covering the glomerular basement membrane (GBM) and the bottom surface of podocyte cell bodies. While SPS can create substantial resistance to flow across the glomerular filtration barrier and the filtration rate is very sensitive to changes in SPS parameters (etc. increased SPS height correlates with decreased filtration resistance), SPS changes in kidney diseases (including DN) is not known. We hypothesized that there will be changes in SPS correlating with DN progression. In this study, we aim to develop unbiased morphometric techniques to quantify SPS parameters and explore the relationships between SPS changes and DN progression. We studied kidney needle biopsies in 4 groups of patients (4 living kidney donors as controls, 3 T2D w/normoalbuminuria, 3 T2D w/microalbuminuria and 4 T2D w/ macroalbuminuria). Complete profiles of glomeruli were built using transmission electron microscopy (TEM) images at ~8000X. The average SPS height and the SPS number were examined in the glomeruli. We accomplished this by segmenting the SPS and other urinary space compartments (i.e. interpodocyte (IPS) and peripheral urinary (PUS) spaces using Adobe Photoshop, counting the number of SPS per glomerular profile (corrected for the profile area) via ImageJ cell counter and measuring the average SPS height via the orthogonal intercept method. The results show reduced SPS number and increased SPS height predominantly in macroalbuminuric T2D patients. In addition, there was a negative correlation between the number of SPS and the albumin: creatinine ratio (ACR) (N=11, R=-0.88, p=<0.01) and a positive correlation between SPS height and ACR (N=6, R=0.87, p=0.024). There is no relationship noted with GFR. This demonstrates that as a patient’s DN progresses, the SPS number decreases and the SPS height increases. Both of these changes are predicted to reduce SPS resistance against filtration and thus may represent compensatory mechanisms to preserve GFR in DN. This preliminary data is limited by its small sample size and characterizing SPS/IPS/PUS is an ongoing project for our lab. Ultimately, we hope to create novel fluid dynamic models that incorporates the parameters of the SPS/IPS/PUS and be able to better understand the structural changes of DN and renal function, particularly in the earlier stages.