Megalin Expression in the Kidney throughout Gestation: A Role for Urinary Retinol-Binding Protein As a Marker of Renal Tubular Maturity in Premature Infants
The renal proximal tubule is the primary site for reabsorption of bioactive substances and thus has been identified as a potential source of proteinuria in premature neonates. Megalin, a well-known endocytic transmembrane receptor, is localized to the proximal tubular epithelial cells and binds a wide variety of ligands including retinol-binding protein (RBP). RBP transports and stabilizes vitamin A, a critical participant in organogenesis of both the lungs and kidneys in fetal and neonatal development. The expression of megalin in the proximal tubule throughout gestation has not been determined in humans and it is unclear if the immature proximal tubules of premature neonates are capable of reabsorbing RBP.
To quantify the amount of RBP present in the urine of living human premature neonates and to determine the expression and presence of megalin in the proximal tubules of post-mortem fetal and neonatal kidney samples from 20 to 40 weeks of gestation.
Urine samples were obtained from neonates of increasing gestational ages (28-40 weeks) throughout their NICU course until a post menstrual age of 40 weeks. The urine was analyzed with a commercially available RBP ELISA kit (Detect X, Arbor AssaysŪ) with values normalized to urine creatinine values. Separately, post-mortem kidney samples from 33 fetal and premature neonates (20-40 weeks gestation) were stained with antibodies to megalin, the proximal tubule (Lotus tetragonolobus agglutinin) and RBP. Imaging software was used to calculate the area of megalin and proximal tubule present in the nephrogenic zone under a 40x microscopic view.
Urinary RBP (uRBP) concentration was significantly higher in the 28-32 week group (<32 weeks: log(uRBP)=4.2) when compared at birth to the 33-35 week group (33-35 weeks: log(uRBP)=3.5, p=0.02) and the 38-40 week group (38-40 weeks: log (uRBP)=2.4, p=0.02). In contrast, megalin was present in only small amounts at the earliest gestational ages in kidney samples of post-mortem neonates and increased linearly as gestational age increased (R= 0.99, p < 0.01, Figure 1). Megalin and RBP co-localized with the proximal tubule under fluorescent light microscopy (Figure 2).
Megalin is present in the proximal tubule as early as 20 weeks of gestation and expression increases throughout gestation. In contrast, urinary RBP is elevated in the more premature neonates and decreases as they mature. Urinary RBP may serve as a biomarker for proximal tubular maturation and megalin expression in premature human neonates. Because urinary losses of RBP may lead to a compromised ability to deliver vitamin A to the developing kidneys and lungs, future studies should be focused on the potential that premature neonates may require supplementation of megalin specific ligands.