Paul T Brinkkötter - CAP 1

The terminally differentiated podocyte is a critical component of the glomerular filtration barrier. Loss of podocytes contributes to the development of kidney diseases resulting in proteinuria and glomerulosclerosis. In this project we investigated the role of prohibitins in glomerular podocytes, characterized a link between insulin signaling and prohibitin function and identified an extra-mitochondrial function of prohibitin-1/-2 at the kidney filtration barrier.

Mitochondrial dysfunction and alterations in energy metabolism have been implicated in a variety of human diseases. Mitochondrial fusion is essential for maintenance of mitochondrial function and requires the prohibitin ring complex subunit prohibitin-2 (PHB2) at the mitochondrial inner membrane. We generated podocyte-specific Phb2 knockout mice (Phb2^pko) using a conditional Phb2 allele as a model of dysfunctional mitochondriae. Mice of all genotypes were born in predicted Mendelian frequency. Phb2^pko mutants appeared as healthy as their littermate controls at birth. By 3 weeks of age proteinuria and growth restriction became evident in Phb2^pko mice. By 4-5 weeks of age, complete destruction of the glomerular tuft was observed accompanied by progressive proteinuria, glomerulosclerosis and endstage renal failure. Mitochondria appeared disorganized with multiple swollen cristae structure.

We provided a link between PHB2 deficiency and hyperactive insulin/IGF-1 signaling. Deletion of PHB2 in podocytes resulted in hyper-phosphorylation of S6 ribosomal protein (S6RP), a known mediator of the mTOR signaling pathway. Inhibition of the insulin/IGF-1 signaling system through genetic deletion of insulin receptor alone or in combination with the IGF-1 receptor or treatment with rapamycin prevented hyper-phosphorylation of S6RP alleviated renal disease and delayed the onset of kidney failure in PHB2 deficient animals without affecting the mitochondrial structural defect. Of note, despite its beneficial role on animal survival and renal function, inhibition of the insulin/IGF-1 signaling cascade did not prevent the onset and magnitude of proteinuria. Hence, perturbation of insulin/IGF-1 receptor signaling contributes to tissue damage in mitochondrial disease, which may allow therapeutic intervention against a wide spectrum of glomerular diseases.

Additional immunogold labeling and EM studies in mice as well as studies in the nematode C. elegans suggest additional, non-mitochondrial function of PHB-2 in podocytes that could explain the severely damaged filtration barrier.

Clinical and experimental evidence emphasizes the mitochondrial impact on podocyte function. Within this research project we will provide further mechanistic insights into the role of mitochondria with respect to oxidative phosphorylation (OXPHOS) and generation of ATP as well as mitochondrial signaling events in states of health and disease. We will further study the link between mitochondrial function and the insulin signaling pathway. The long-term perspective of this project is to determine the mitochondrial contribution to podocyte-driven glomerular disease and potentially characterize novel, drugable signaling pathways. We expect that our results will not only lead to a better understanding of glomerular diseases but also to important insights into mitochondrial physiology and insulin signaling in general.