Podocyte stress and detachment measured in urine are related to mean arterial pressure in healthy humans

Abstract

Hypertension-associated progressive glomerulosclerosis is a significant driver of both de novo and all-cause chronic kidney disease leading to end-stage kidney failure. The progression of glomerular disease proceeds via continuing depletion of podocytes from the glomeruli into the ultrafiltrate. To non-invasively assess injury patterns associated with mean arterial pressure (MAP), we conducted an observational study of 87 healthy normotensive individuals who were cleared for living kidney donation. Urine pellet podocin and aquaporin2 mRNAs normalized to the urine creatinine concentration (UPod:Creat ratio and UAqp2:Creat ratio) were used as markers of podocyte detachment and tubular injury, respectively. The ratio of two podocyte mRNA markers, podocin to nephrin (UPod:Neph) as well as the ratio of podocin to the tubular marker aquaporin2 (UPod:Aqp2) estimated the relative rates of podocyte stress and glomerular vs. tubular injury. The MAP was positively correlated with the UPod:Neph and UPod:Aqp2, thereby confirming the relationship of MAP with podocyte stress and the preferential targeting of the glomerulus by higher MAP. In multivariable linear regression analysis, both UPod:Neph and UPod:Creat, but not UAqp2:Creat or proteinuria, were both significantly related to a range of normal MAP (70 to 110 mm Hg). Systolic, as opposed to diastolic or pulse pressure was associated with UPod:Creat. Thus, higher podocyte stress and detachment into the urine are associated with MAP even in a relatively "normal" range of MAP. Hence, urine pellet mRNA monitoring can potentially identify progression risk before the onset of overt hypertension, proteinuria or chronic kidney disease.

Keywords: glomerulosclerosis; hypertension; podocyte.

Figure 1.. Unadjusted urine pellet marker data plotted against MAP.

Panel A: The ratio of a glomerular podocyte mRNA marker (podocin) divided by a tubular mRNA marker (aquaporin2) provides a readout of relative glomerular versus tubular injury. Even in unadjusted data the relationship was highly significant (P=0.003) demonstrating preferential glomerular versus tubular injury in relation to MAP. Panel B: The ratio of two podocyte-specific mRNA markers (podocin and nephrin shown as the podocin: nephrin mRNA ratio) in which relative downregulation of the nephrin mRNA versus podocin mRNA expression provides a readout for podocyte stress. Even in unadjusted data the relationship with MAP reached statistical significance (P=0.04). Panels C-E: Urine pellet mRNAs (podocin, nephrin and aquaporin2) shown in relation to urine creatinine concentration analogous to the urine protein: creatinine ratio shown in Panel F. None of these relationships reached statistical significance.

Figure 2.. Urine pellet markers data adjusted for clinical variables shown in Table 2 plotted against MAP.

Panel A: The ratio of a glomerular podocyte mRNA marker (podocin) divided by a tubular mRNA marker (aquaporin2) provides a readout of relative glomerular versus tubular injury. A highly significant relationship with MAP was observed (P<0.0001) indicating preferential glomerular versus tubular injury in relation to MAP. Panel B: The ratio of two podocyte-specific mRNA markers (podocin and nephrin shown as the podocin: nephrin mRNA ratio) in which relative downregulation of the nephrin mRNA versus podocin mRNA expression provides a readout for podocyte stress. A highly significant relationship between podocyte stress and MAP was observed (P=0.002). Panels C-E: Urine pellet mRNAs (podocin, nephrin and aquaporin2) shown in relation to urine creatinine concentration analogous to the urine protein: creatinine ratio shown in Panel F. Only the urine podocin mRNA: creatinine ratio was significantly related to MAP (P=0.04). The urine nephrin mRNA: creatinine ratio showed a downward trend with increasing MAP that did not reach statistical significance. No relationship of MAP to either the tubular marker (aquaporin2) or proteinuria was observed. Note: Predictive margins were calculated after the linear regression command to test the effect of increments in MAP on urine marker of interest keeping all other covariates constant.