Tubular IKKß Deletion Alleviates Acute Ischemic Kidney Injury and Facilitates Tissue Regeneration.

Acute kidney injury (AKI) is a common renal injury leading to relevant morbidity and mortality worldwide. Most of the clinical cases of AKI are caused by ischemia reperfusion (I/R) injury with renal ischemia injury followed by reperfusion injury and activation of the innate immune response converging to NF-ĸB pathway induction. Despite the clear role of NF-ĸB in inflammation, it has recently been acknowledged that NF-ĸB may impact other cell functions. To identify NF-ĸB function with respect to metabolism, vascular function and oxidative stress after I/R injury and to decipher in detail the underlying mechanism, we generated a transgenic mouse model with targeted deletion of IKKß along the tubule and applied I/R injury followed by its analysis after 2 and 14 days after I/R injury. Tubular IKKß deletion ameliorated renal function and reduced tissue damage. RNAseq data together with immunohistochemical, biochemical and morphometric analysis demonstrated an ameliorated vascular organization and mRNA expression profile for increased angiogenesis in mice with tubular IKKß deletion at 2 days after I/R injury. RNAseq and protein analysis indicate an ameliorated metabolism, oxidative species handling and timely-adapted cell proliferation and apoptosis as well as reduced fibrosis in mice with tubular IKKß deletion at 14 days after I/R injury. In conclusion, mice with tubular IKKß deletion upon I/R injury display improved renal function and reduced tissue damage and fibrosis in association with improved vascularization, metabolism, reactive species disposal and fine-tuned cell proliferation.

Megalin Orchestrates FcRn Endocytosis and Trafficking.

The neonatal Fc receptor (FcRn) is highly expressed in the renal proximal tubule and is important for the reclamation of albumin by cellular transcytosis to prevent its loss in the urine. The initial event of this transcellular transport mechanism is the endocytosis of albumin by the apical scavenger receptors megalin and cubilin. An interaction of megalin and FcRn was postulated, however, evidence is still missing. Similarly, the intracellular trafficking of FcRn remains unknown and shall be identified in our study. Using a Venus-based bimolecular fluorescence complementation system, we detected an interaction between megalin and FcRn in the endosomal compartment, which significantly increased with the induction of endocytosis using albumin or lactoglobulin as a ligand. The interaction between megalin and FcRn occurred at a neutral and acidic pH between the extracellular domains of both proteins. Amnionless, another transmembrane acceptor of cubilin, revealed no interaction with FcRn. With the induction of endocytosis by albumin or lactoglobulin, super resolution microscopy demonstrated a redistribution of megalin and FcRn into clathrin vesicles and early endosomes. This trafficking into clathrin vesicles was impaired in megalin-deficient cells upon albumin-induced endocytosis, supporting the role of megalin in FcRn redistribution. Our results indicate that megalin and FcRn specifically bind and interact within their extracellular domains. The availability of megalin is necessary for the redistribution of FcRn. Megalin, therefore, orchestrates FcRn endocytosis and intracellular trafficking as an early event intranscytosis.