Abstract
Nephron loss initiates compensatory hemodynamic and cellular effects on the remaining nephrons. Increases in single nephron glomerular filtration rate and tubular flow rate exert higher fluid shear stress (FSS) on tubules. In principal cell (PC) culture models FSS induces ERK, and ERK is implicated in the regulation of transepithelial sodium (Na) transport, as well as, proliferation. Thus, we hypothesize that high tubular flow and FSS mediate ERK activation in the cortical collecting duct (CCD) of solitary kidney which regulates amiloride sensitive Na transport and affects CCD cell number. Immunoblotting of whole kidney protein lysate was performed to determine phospho-ERK (pERK) expression. Next, sham and unilateral nephrectomized mice were stained with anti-pERK antibodies, and dolichos biflorus agglutinin (DBA) to identify PCs with pERK. Murine PCs (mpkCCD) were grown on semi-permeable supports under static, FSS, and FSS with U0126 (a MEK1/2 inhibitor) conditions to measure the effects of FSS and ERK inhibition on amiloride sensitive Na short circuit current (Isc). pERK abundance was greater in kidney lysate of unilateral vs. sham nephrectomies. The total number of cells in CCD and pERK positive PCs increased in nephrectomized mice (9.3 ± 0.4 vs. 6.1 ± 0.2 and 5.1 ± 0.5 vs. 3.6 ± 0.3 cell per CCD nephrectomy vs. sham, respectively, n > 6 per group, p < 0.05). However, Ki67, a marker of proliferation, did not differ by immunoblot or immunohistochemistry in nephrectomy samples at 1 month compared to sham. Next, amiloride sensitive Isc in static mpkCCD cells was 25.3 ± 1.7 μA/cm2 (n = 21), but after exposure to 24 h of FSS the Isc increased to 41.4 ± 2.8 μA/cm2 (n = 22; p < 0.01) and returned to 19.1 ± 2.1 μA/cm2 (n = 18, p < 0.01) upon treatment with U0126. Though FSS did not alter α- or γ-ENaC expression in mpkCCD cells, γ-ENaC was reduced in U0126 treated cells. In conclusion, pERK increases in whole kidney and, specifically, CCD cells after nephrectomy, but pERK was not associated with active proliferation at 1-month post-nephrectomy. In vitro studies suggest high tubular flow induces ERK dependent ENaC Na absorption and may play a critical role in Na balance post-nephrectomy.
Highlights
Nephron loss is accompanied by compensatory changes in the remaining nephrons to maintain (1) glomerular filtration rate (GFR), (2) ion and water balance, and (3) acid-base homeostasis
Unilateral nephrectomy is accompanied by compensatory increases in tubular flow rate which leads to augmented fluid shear stress (FSS) and tubular stretch
Though background pERK signal in proximal tubule (PT) was noted in sham (Figure 2A) and unilateral nephrectomy (Figure 2B) kidneys, heterogenous expression, where a single cell expressing pERK was adjacent to a cell lacking pERK, appeared to morphologically represent collecting duct (CCD), where two distinct cell populations reside (Figures 2A,B)
Summary
Nephron loss is accompanied by compensatory changes in the remaining nephrons to maintain (1) glomerular filtration rate (GFR), (2) ion and water balance, and (3) acid-base homeostasis. A “non-pathologic” model of nephron loss is unilateral nephrectomy which is accompanied by compensatory changes in the solitary kidney. Human studies show an increase in single kidney blood flow rate of ∼42% and single kidney GFR of ∼40% after a median of follow up time of 6.3 years; all of these changes occur during the first year post-nephrectomy (Lenihan et al, 2015). The rise in single kidney GFR is principally to due to proportionate increases in the whole kidney ultrafiltration coefficient (Kf) which reflects the filtration surface area (Lenihan et al, 2015). The literature suggests most of these adaptive fluctuations in cation transport occur at the level of the distal tubule and collecting duct, but the biological mechanisms by which these changes are induced are not clear
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