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Abstract: FR-OR037

Characterization of a New Mechanism of Proteinuria

Session Information

Category: Anemia and Iron Metabolism

  • 201 Anemia and Iron Metabolism: Basic

Authors

  • Beenken, Andrew, New York Presbyterian/Columbia University, New York, New York, United States
  • Qiu, Andong, Tongji University, Shanghai, SHANGHAI, China
  • Jin, Guangchun, Columbia University, New York, New York, United States
  • Shen, Tian, Columbia University, New York, New York, United States
  • Levitman, Abraham D., Columbia University, New York, New York, United States
  • Barasch, Jonathan M., Columbia Presbyterian, New York, New York, United States
Background

A central problem in biology is the mechanism by which cells regulate iron transport. This problem arises because the common oxidized form of iron (ferric-Fe3+) precipitates at physiologic pH, while the reduced form of iron (ferrous-Fe2+) is unstable in oxygenated atmospheres, and produces toxic radicals. To investigate iron transport in the kidney, we performed homology searches with yeast iron transporters and identified Spin, a novel transporter of ferrous iron.

Methods

Using in vivo LacZ reporters and by raising antibodies we found that Spin is specific to the endosomes and lysosomes of the proximal tubule. We constructed a conditional knockout of Spin driven by Megalin-Cre and found iron deposits and intense ferritin staining in the cytosol indicating defective iron localization in the KO.

Results

We saw proximal tubule dysfunction in the KO as measured by proteinuria. Western blot revealed leakage of NGAL and transferrin in the urine, and silver stain showed albuminuria. We investigated this phenotype by intraperitoneally injecting FITC-labeled dextran and found impaired endocytosis in the KO. While megalin expression was unchanged between KO and wild type as seen on confocal microscopy, autophagy appeared to be inhibited in the KO. Electron microscopy showed increased size and number of vesicles in the proximal tubules, and Western blot showed increased abundance of LC3B-II in the homogenate of kidney cortex, suggesting that the Spin KO may hinder autophagosome fusion with the lysosome. The endocytic phenotype appears to be regulated by mislocalized iron because iron deficiency can rescue endocytosis.

Conclusion

In sum, we have found a critical iron transporter that regulates the endocytic functions of the proximal tubule.

Funding

  • NIDDK Support