Chronic kidney disease (CKD) remains one of the leading causes of death in the developed world and acute kidney injury (AKI) is now recognized as a major risk factor in its development. Understanding the factors leading to CKD after acute injury are limited by current animal models of AKI which concurrently target various kidney cell types such as epithelial, endothelial and inflammatory cells. Here we developed a mouse model of kidney injury using the Six2-Cre-LoxP technology to selectively activate expression of the simian diphtheria toxin receptor in renal epithelia derived from the metanephric mesenchyme. By adjusting the timing and dose of diphtheria toxin a highly selective model of tubular injury was created to define the acute and chronic consequences of isolated epithelial injury. The diphtheria toxin-induced sublethal tubular epithelial injury was confined to the S1 and S2 segments of the proximal tubule rather than being widespread in the metanephric mesenchyme derived epithelial lineage. Acute injury was promptly followed by inflammatory cell infiltration and robust tubular cell proliferation leading to complete recovery after a single toxin insult. In striking contrast, three insults to renal epithelial cells at one week intervals resulted in maladaptive repair with interstitial capillary loss, fibrosis and glomerulosclerosis which was highly correlated with the degree of interstitial fibrosis. Thus, selective epithelial injury can drive the formation of interstitial fibrosis, capillary rarefaction and potentially glomerulosclerosis, substantiating a direct role for damaged tubule epithelium in the pathogenesis of CKD.

Currently, no blood biomarker that specifically indicates injury to the proximal tubule of the kidney has been identified. Kidney injury molecule-1 (KIM-1) is highly upregulated in proximal tubular cells following kidney injury. The ectodomain of KIM-1 is shed into the lumen, and serves as a urinary biomarker of kidney injury. We report that shed KIM-1 also serves as a blood biomarker of kidney injury. Sensitive assays to measure plasma and serum KIM-1 in mice, rats, and humans were developed and validated in the current study. Plasma KIM-1 levels increased with increasing periods of ischemia (10, 20, or 30 minutes) in mice, as early as 3 hours after reperfusion; after unilateral ureteral obstruction (day 7) in mice; and after gentamicin treatment (50 or 200 mg/kg for 10 days) in rats. In humans, plasma KIM-1 levels were higher in patients with AKI than in healthy controls or post-cardiac surgery patients without AKI (area under the curve, 0.96). In patients undergoing cardiopulmonary bypass, plasma KIM-1 levels increased within 2 days after surgery only in patients who developed AKI (P,0.01). Blood KIM-1 levels were also elevated in patients with CKD of varous etiologies. In a cohort of patients with type 1 diabetes and proteinuria, serum KIM-1 level at baseline strongly predicted rate of eGFR loss and risk of ESRD during 5-15 years of follow-up, after adjustment for baseline urinary albuminto-creatinine ratio, eGFR, and Hb1Ac. These results identify KIM-1 as a blood biomarker that specifically reflects acute and chronic kidney injury.

Urinary biomarkers, such as albumin and other markers of kidney injury, are frequently reported as a normalized ratio to urinary creatinine (UCr) concentration [UCr] to control for variations in urine flow rate. The implicit assumption is that UCr excretion is constant across and within individuals, such that changes in the ratio will reflect changes in biomarker excretion. Using computer simulations of creatinine kinetics, we found that normalized levels of a biomarker reflecting tubular injury can be influenced by dynamic changes in the UCr excretion rate when the glomerular filtration rate changes. Actual timed urine collections from hospitalized patients with changing glomerular filtration rates and/or critical illness exhibited variability in UCr excretion rates across and within individuals. Normalization by [UCr] may, therefore, result in an underestimation or overestimation of the biomarker excretion rate depending on the clinical context. Lower creatinine excretion in the setting of acute kidney injury or poor renal allograft function may amplify a tubular injury biomarker signal, thereby increasing its clinical utility. The variability of creatinine excretion, however, will complicate the determination of a threshold value for normalized biomarkers of acute or chronic kidney disease, including albumin. Thus, we suggest that the most accurate method to quantify biomarkers requires the collection of timed urine specimens to estimate the actual excretion rate, provided that the biomarker is stable over the period of collection. This ideal must be balanced, however, against practical considerations.