Background
Nephrotic syndrome (NS) is associated with kidney podocyte injury and may occur as part of thyroid autoimmunity such as Graves' disease. Therefore, the present study was designed to ascertain if and how podocytes respond to and regulate the input of biologically active thyroid hormone (TH), 3,5,3'-triiodothyronine (T3); and also to decipher the pathophysiological role of type 3 deiodinase (D3), a membrane-bound selenoenzyme that inactivates TH, in kidney disease.
Methods
To study D3 function in healthy and injured (PAN, puromycin aminonucleoside and LPS, Lipopolysaccharide-mediated) podocytes, immunofluorescence, qPCR and podocyte-specific D3 knockout mouse were used. Surface plasmon resonance (SPR), co-immunoprecipitation and Proximity Ligation Assay (PLA) were used for the interaction studies. Findings: Healthy podocytes expressed D3 as the predominant deiodinase isoform. Upon podocyte injury, levels of Dio3 transcript and D3 protein were dramatically reduced both in vitro and in the LPS mouse model of podocyte damage. D3 was no longer directed to the cell membrane, it accumulated in the Golgi and nucleus instead. Further, depleting D3 from the mouse podocytes resulted in foot process effacement and proteinuria. Treatment of mouse podocytes with T3 phenocopied the absence of D3 and elicited activation of αvβ3 integrin signaling, which led to podocyte injury. We also confirmed presence of an active thyroid stimulating hormone receptor (TSH-R) on mouse podocytes, engagement and activation of which resulted in podocyte injury. Interpretation: The study provided a mechanistic insight into how D3-αvβ3 integrin interaction can minimize T3-dependent integrin activation, illustrating how D3 could act as a renoprotective thyrostat in podocytes. Further, injury caused by binding of TSH-R with TSH-R antibody, as found in patients with Graves' disease, explained a plausible link between thyroid disorder and NS. Funding: This work was supported by American Thyroid Association (ATA-2018-050.R1).