Podocyte glycogen synthase kinase 3 (GSK3) is an evolutionarily conserved master regulator of glomerular/excretory function controlling podocyte differentiation and cell cycling.
Authors: Hurcombe, J. et al.
Start date: 19 June 2017
Introduction: Glycogen synthase kinase 3 (GSK3) is a multi-functional enzyme existing as two structurally related isoforms (α and β) in mammals coded by different genes; but exists as a single gene in Drosophila called Shaggy. There are several recent publications suggesting that inhibiting mammalian podocyte GSK3 is beneficial in a number of disease settings. They suggest these effects are through the β isoform, however there are no GSK3β specific pharmacological inhibitors and they all also inhibit GSK3α.
This study set out to define the evolutionary importance of GSK3 in the mammalian podocyte and drosophila nephrocyte and to elucidate the mechanisms through which this enzyme works.
Objectives: This study set out to define the evolutionary importance of GSK3 in the mammalian podocyte and drosophila nephrocyte and to elucidate the mechanisms through which this enzyme works.
Methods: We have studied the role of GSK3 in the glomerular podocyte using transgenic mice and additionally a Drosophila nephrocyte-specific gene knockout model. We have also developed an in vitro system using conditionally immortalised podocytes derived from GSK3α/β floxed mice transduced with a lentivirus expressing cre recombinase to induce gene excision.
Results: Developmental deletion of both GSK3 isoforms in the mouse podocyte results in death at postnatal day 10-16 with massive albuminuria, renal failure and acidosis. However, deleting 3 out of 4 alleles of GSK3α/β causes no phenotype demonstrating a high level of compensation within this system. The evolutionary importance of GSK3 in podocyte development is further supported with the observation that nephrocyte-specific loss of the Drosophila GSK3 orthologue, Shaggy, results in a complete lack of nephrocytes, the closest invertebrate model of mammalian podocytes. Mechanistically we thought that this detrimental phenotype would be driven by wnt signalling through activation of β-catenin. However this was not the case as contemporaneous transgenic podocyte-specific deletion of β-catenin together with GSK3α/β did not improve the phenotype of these mice. Non-biased proteomic analysis of ex vivo GSK3α/β knockout podocytes revealed that mechanistically these cells enter re-enter the cell cycle but do not proliferate. Instead they progress to mitotic catastrophe and lose their differentiation markers.
Conclusion: Podocyte GSK3 is a critical enzyme for renal function and inhibiting both isoforms too much is highly detrimental.
Care must therefore be taken when considering therapeutic pharmacological inhibition of this enzyme.