Abraham D. Levitman

Key Points: Transferrin receptor 1 is critical for perinatal nephron growth and maturation; its deletion results in widespread cystic hypodysplasia. Nontransferrin bound iron traffic is a physiologic source of iron in the midgestation embryo and complements transferrin receptor 1. Cystic hypodysplasia can be rescued by exogenous iron or by the activation of systemic iron traffic with hypoxia inducible factor activators. Background: Periconceptual maternal iron deficiency (FeD) is a worldwide cause of premature births and low birth weights. Yet, it is unknown whether FeD affects all developing tissues equally or rather target- s pecific lineages. In addition, since FeD restricts both transferrin bound and nontransferrin bound iron species, their unique contributions to organogenesis are indeterminant. Methods: To address questions of iron traffic and kidney development, we examined the deletion of the singular transferrin receptor ( TfR1−/− ), created green flourescent protein-labeled TfR1−/− embryonic stem cells for inoculation into wild blastocysts, and created TfR1 -floxed mice to generate cell autonomous deletions of TfR1 in mesenchymal, ureteric, and stromal derivatives. Finally, we created a model of global FeD with iron poor diets, for comparison with cell autonomous TfR1 deletions. Results: Transferrin receptor deletions only modestly suppressed tubulogenesis, had little, if any effect on the growth of the ureteric bud and no gross effects on kidney stroma at mid gestation. By contrast, nutritional FeD nearly abolished kidney development, highlighting the limited phenotypes induced by transferrin receptor deletion. Yet, in the second postnatal week, the critical function of TfR1 became evident by the growth of residual TfR1+ cells that had escaped Cre-mediated deletion and by tubular segment-specific polycystic transformation. Timed treatment with iron or systemic activators of iron trafficking prevented both cystic dysplasia and the terminal loss of kidney function, reversing extensive malformations of the kidney. Conclusions: TfR1 is the critical iron species targeting postnatal tubulogenesis, but in the embryo, TfR1 must be complemented by alternative iron species called nontransferrin bound iron. Iron-deficient kidney disease is reversible postnatally.

The current strategy to detect acute injury of kidney tubular cells relies on changes in serum levels of creatinine. Yet serum creatinine (sCr) is a marker of both functional and pathological processes and does not adequately assay tubular injury. In addition, sCr may require days to reach diagnostic thresholds, yet tubular cells respond with programs of damage and repair within minutes or hours. To detect acute responses to clinically relevant stimuli, we created mice expressing Rosa26-floxed-stop uracil phosphoribosyltransferase ( Uprt ) and inoculated 4-thiouracil (4-TU) to tag nascent RNA at selected time points. Cre-driven 4-TU–tagged RNA was isolated from intact kidneys and demonstrated that volume depletion and ischemia induced different genetic programs in collecting ducts and intercalated cells. Even lineage-related cell types expressed different genes in response to the 2 stressors. TU tagging also demonstrated the transient nature of the responses. Because we placed Uprt in the ubiquitously active Rosa26 locus, nascent RNAs from many cell types can be tagged in vivo and their roles interrogated under various conditions. In short, 4-TU labeling identifies stimulus-specific, cell-specific, and time-dependent acute responses that are otherwise difficult to detect with other technologies and are entirely obscured when sCr is the sole metric of kidney damage.