Pathophysiology of unilateral ischemia-reperfusion injury: importance of renal counterbalance and implications for the AKI-CKD transition.

Unilateral ischemia-reperfusion (UIR) injury leads to progressive renal atrophy and tubulointerstitial fibrosis (TIF) and is commonly used to investigate the pathogenesis of the acute kidney injury-chronic kidney disease transition. Although it is well known that contralateral nephrectomy (CNX), even 2 wk post-UIR injury, can improve recovery, the physiological mechanisms and tubular signaling pathways mediating such improved recovery remain poorly defined. Here, we examined the renal hemodynamic and tubular signaling pathways associated with UIR injury and its reversal by CNX. Male Sprague-Dawley rats underwent left UIR or sham UIR and 2 wk later CNX or sham CNX. Blood pressure, left renal blood flow (RBF), and total glomerular filtration rate were assessed in conscious rats for 3 days before and over 2 wk after CNX or sham CNX. In the presence of a contralateral uninjured kidney, left RBF was lower (P < 0.05) from 2 to 4 wk following UIR (3.6 ± 0.3 mL/min) versus sham UIR (9.6 ± 0.3 mL/min). Without CNX, extensive renal atrophy, TIF, and tubule dedifferentiation, but minimal pimonidazole and hypoxia-inducible factor-1α positivity in tubules, were present at 4 wk post-UIR injury. Conversely, CNX led (P < 0.05) to sustained increases in left RBF (6.2 ± 0.6 mL/min) that preceded the increases in glomerular filtration rate. The CNX-induced improvement in renal function was associated with renal hypertrophy, more redifferentiated tubules, less TIF, and robust pimonidazole and hypoxia-inducible factor-1α staining in UIR injured kidneys. Thus, contrary to expectations, indexes of hypoxia are not observed with the extensive TIF at 4 wk post-UIR injury in the absence of CNX but are rather associated with the improved recovery of renal function and structure following CNX.

Progression of Chronic Kidney Disease After Acute Kidney Injury: Role of Self-Perpetuating Versus Hemodynamic-Induced Fibrosis.

The relative contribution of self-perpetuating versus hemodynamic-induced fibrosis to the progression of chronic kidney disease (CKD) after acute kidney injury (AKI) is unclear. In the present study, male Sprague-Dawley rats underwent right uninephrectomy and were instrumented with a blood pressure radiotelemeter. Two weeks later, separate groups of rats were subjected to 40 minutes renal ischemia-reperfusion or sham surgery and followed up for 4 or 16 weeks to determine the extent to which glomerulosclerosis and tubulointerstitial fibrosis as a result of the AKI-CKD transition (ie, at 4 weeks post AKI) change over time during the progression of CKD (ie, at 16 weeks post AKI). On average, tubulointerstitial fibrosis was ≈3-fold lower (P<0.05), whereas glomerulosclerosis was ≈6-fold higher (P<0.05) at 16 versus 4 weeks post AKI. At 16 weeks post AKI, marked tubulointerstitial fibrosis was only observed in rats exhibiting marked glomerulosclerosis, proteinuria, and kidney hypertrophy consistent with a hemodynamic pathogenesis of renal injury. Moreover, quantitative analysis between blood pressure and renal injury revealed a clear and modest blood pressure threshold (average 16-week systolic blood pressure of ≈127 mm Hg) for the development of glomerulosclerosis. In summary, modest levels of blood pressure may be playing a substantial role in the progression of renal disease after AKI in settings of preexisting CKD associated with 50% loss of renal mass. In contrast, these data do not support a major role of self-perpetuating tubulointerstitial fibrosis in the progression CKD after AKI in such settings.

Critical blood pressure threshold dependence of hypertensive injury and repair in a malignant nephrosclerosis model.

Most patients with essential hypertension do not exhibit substantial renal damage. Renal autoregulation by preventing glomerular transmission of systemic pressures has been postulated to mediate this resistance. Conversely, malignant nephrosclerosis (MN) has been postulated to develop when severe hypertension exceeds a critical ceiling. If the concept is valid, even modest blood pressure (BP) reductions to below this threshold regardless of antihypertensive class (1) should prevent MN and (2) lead to the healing of the already developed MN lesions. Both predicates were tested using BP radiotelemetry in the stroke-prone spontaneously hypertensive rats receiving 1% NaCl as drinking fluid for 4 weeks. Severe hypertension (final 2 weeks average systolic BP, >200 mm Hg) and MN (histological damage score 36±5; n=27) developed in the untreated stroke-prone spontaneously hypertensive rats but were prevented by all antihypertensive classes (enalapril [n=15], amlodipine [n=13], or a hydralazine/hydrochlorothiazide combination [n=15]) if the final 2-week systolic BP remained <190 mm Hg. More impressively, modest systolic BP reductions to 160 to 180 mm Hg (hydralazine/hydrochlorothiazide regimen) initiated at ≈4 weeks in additional untreated rats after MN had already developed (injury score 35±4 in the right kidney removed before therapy) led to a striking resolution of the vascular and glomerular MN injury over 2 to 3 weeks (post-therapy left kidney injury score 9±2, P<0.0001; n=27). Proteinuria also declined rapidly from 122±9.5 mg/24 hours before therapy to 20.5±3.6 mg 1 week later. These data clearly demonstrate the barotrauma-mediated pathogenesis of MN and the striking capacity for spontaneous and rapid repair of hypertensive kidney damage if new injury is prevented.

PTEN loss defines a TGF-ß-induced tubule phenotype of failed differentiation and JNK signaling during renal fibrosis.

We investigated the signaling basis for tubule pathology during fibrosis after renal injury. Numerous signaling pathways are activated physiologically to direct tubule regeneration after acute kidney injury (AKI) but several persist pathologically after repair. Among these, transforming growth factor (TGF)-ß is particularly important because it controls epithelial differentiation and profibrotic cytokine production. We found that increased TGF-ß signaling after AKI is accompanied by PTEN loss from proximal tubules (PT). With time, subpopulations of regenerating PT with persistent loss of PTEN (phosphate and tension homolog) failed to differentiate, became growth arrested, expressed vimentin, displayed profibrotic JNK activation, and produced PDGF-B. These tubules were surrounded by fibrosis. In contrast, PTEN recovery was associated with epithelial differentiation, normal tubule repair, and less fibrosis. This beneficial outcome was promoted by TGF-ß antagonism. Tubule-specific induction of TGF-ß led to PTEN loss, JNK activation, and fibrosis even without prior AKI. In PT culture, high TGF-ß depleted PTEN, inhibited differentiation, and activated JNK. Conversely, TGF-ß antagonism increased PTEN, promoted differentiation, and decreased JNK activity. Cre-Lox PTEN deletion suppressed differentiation, induced growth arrest, and activated JNK. The low-PTEN state with JNK signaling and fibrosis was ameliorated by contralateral nephrectomy done 2 wk after unilateral ischemia, suggesting reversibility of the low-PTEN dysfunctional tubule phenotype. Vimentin-expressing tubules with low-PTEN and JNK activation were associated with fibrosis also after tubule-selective AKI, and with human chronic kidney diseases of diverse etiology. By preventing tubule differentiation, the low-PTEN state may provide a platform for signals initiated physiologically to persist pathologically and cause fibrosis after injury.