No abstract

Abstract. Acute renal failure (ARF) secondary to ischemic injury remains a common and potentially devastating problem. A transcriptome-wide interrogation strategy was used to identify renal genes that are induced very early after renal ischemia, whose protein products might serve as novel biomarkers for ARF. Seven genes that are upregulated Ͼ10-fold were identified, one of which (Cyr61) has recently been reported to be induced after renal ischemia. Unexpectedly, the induction of the other six transcripts was novel to the ARF field. In this study, one of these previously unrecognized genes was further characterized, namely neutrophil gelatinase-associated lipocalin (NGAL), because it is a small secreted polypeptide that is protease resistant and consequently might be readily detected in the urine. The marked upregulation of NGAL mRNA and protein levels in the early postischemic mouse kidney was confirmed. NGAL protein expression was detected predominantly in proliferating cell nuclear antigen-positive proximal tubule cells, in a punctate cytoplasmic distribution that colocalized with markers of late endosomes. NGAL was easily detected in the urine in the very first urine output after ischemia in both mouse and rat models of ARF. The appearance of NGAL in the urine was related to the dose and duration of renal ischemia and preceded the appearance of other urinary markers such as N-acetyl-␤-D-glucosaminidase and ␤2-microglobulin. The origin of NGAL from tubule cells was confirmed in cultured human proximal tubule cells subjected to in vitro ischemic injury, where NGAL mRNA was rapidly induced in the cells and NGAL protein was readily detectable in the culture medium within 1 h of mild ATP depletion. NGAL was also easily detectable in the urine of mice with cisplatininduced nephrotoxicity, again preceding the appearance of N-acetyl-␤-D-glucosaminidase and ␤2-microglobulin. The results indicate that NGAL may represent an early, sensitive, noninvasive urinary biomarker for ischemic and nephrotoxic renal injury.

Our understanding of kidney disease pathogenesis is limited by an incomplete molecular characterization of the cell types responsible for the organ’s multiple homeostatic functions. To help fill this knowledge gap, we characterized 57,979 cells from healthy mouse kidneys using unbiased single-cell RNA sequencing. Based on gene expression patterns, we infer that inherited kidney diseases that arise from distinct genetic mutations but have similar phenotypic manifestations share the same cell of origin. We also found that the kidney collecting duct in adult mice generates a spectrum of cell types via a newly identified transitional cell. Computational cell trajectory analysis and in vivo lineage tracing revealed that intercalated cells and principal cells undergo transitions mediated by the Notch signaling pathway. In mouse and human kidney disease, these transitions were shifted toward a principal cell fate and were associated with metabolic acidosis.

Neutrophil gelatinase-associated lipocalin (NGAL) is expressed and secreted by immune cells, hepatocytes, and renal tubular cells in various pathologic states. NGAL exerts bacteriostatic effects, which are explained by its ability to capture and deplete siderophores, small iron-binding molecules that are synthesized by certain bacteria as a means of iron acquisition. Consistently, NGAL deficiency in genetically modified mice leads to an increased growth of bacteria. However, growing evidence suggests effects of the protein beyond fighting microorganisms. NGAL acts as a growth and differentiation factor in multiple cell types, including developing and mature renal epithelia, and some of this activity is enhanced in the presence of siderophore:iron complexes. This has led to the hypothesis that eukaryotes might synthesize siderophore-like molecules that bind NGAL. Accordingly, NGAL-mediated iron shuttling between the extracellular and intracellular spaces may explain some of the biologic activities of the protein. Interest in NGAL has been sparked by the observation that NGAL is massively upregulated after renal tubular injury and may participate in limiting kidney damage. This review summarizes the current knowledge about the dual effects of NGAL as a siderophore:iron-binding protein and as a growth factor and examines the role of these effects in renal injury.

Neutrophil gelatinase-associated lipocalin (Ngal), also known as siderocalin, forms a complex with ironbinding siderophores (Ngal:siderophore:Fe). This complex converts renal progenitors into epithelial tubules.In this study, we tested the hypothesis that Ngal:siderophore:Fe protects adult kidney epithelial cells or accelerates their recovery from damage. Using a mouse model of severe renal failure, ischemia-reperfusion injury, we show that a single dose of Ngal (10 μg), introduced during the initial phase of the disease, dramatically protects the kidney and mitigates azotemia. Ngal activity depends on delivery of the protein and its siderophore to the proximal tubule. Iron must also be delivered, since blockade of the siderophore with gallium inhibits the rescue from ischemia. The Ngal:siderophore:Fe complex upregulates heme oxygenase-1, a protective enzyme, preserves proximal tubule N-cadherin, and inhibits cell death. Because mouse urine contains an Ngaldependent siderophore-like activity, endogenous Ngal might also play a protective role. Indeed, Ngal is highly accumulated in the human kidney cortical tubules and in the blood and urine after nephrotoxic and ischemic injury. We reveal what we believe to be a novel pathway of iron traffic that is activated in human and mouse renal diseases, and it provides a unique method for their treatment.