2014
PLCE1 Controls the TGF-B1 Signalling Response in Podocytes
May, C, Saleem, M, Welsh, G.
Academic Renal Unit, University of Bristol
Introduction
Mutations in PLCE1can cause Diffuse Mesangial Sclerosis (DMS) and Focal Segmental Glomerulosclerosis (FSGS) by preventing normal glomerular development. However, little is known about the function of PLCE1, particularly in the target cell of this disease, the podocyte.
A conditionally immortalised human podocyte cell line was generated from the explant of a DMS patient. This patientcarried a homozygous SNP at 321. The SNP 321 C>T changes the codon from CGA (arginine) to TGA (stop). This premature stop codon leads to the truncation of the mutant PLCε. This truncation is so severe that all catalytic domains of the protein are lost. This ability to generate podocyte cell lines from patient tissue allows for the detailed study of disease relevant mutations. Mutant cells demonstrated a pronounced mesenchymal phenotype. Injured podocytes in FSGS overexpress the pleiotropic cytokine TGF-B1, which is a regulator of Epithelial-Mesenchymal-Transition (EMT)and fibrosis/sclerosis.Hence the responses of PLCE1 mutant podocytes to TGF-B1 were studied.
Methods
Both wild-type and PLCE1 mutant podocytes were cultured and treated with human recombinant TGF-B1. The responses of both cell lines were measured using western blotting and motility assays. After establishing the baseline response of each cell line, siRNA knockdown of PLCE1 in the wild-type was performed in order to replicate the mutant phenotype.
Results
SMAD2 and SMAD3 are receptor mediated SMADs that are crucial in TGF-B1 signalling. The PLCE1 mutant podocytes do not demonstrate SMAD2 phosphorylation following TGF-B1 treatment unlike their wild-type counterparts. The wild-type podocytes exhibit significantly reduced expression of epithelial markers following 24 hour exposure to doses of TGF-B1 of 2 and 10 ng/ml, there is no such trend in the PLCE1 mutant podocytes. Additionally the PLCE1 mutant podocytes do not show increased motility or loss of viability following treatment with TGF-B1. The PLCE1 mutant podocytes are protected from TGF-B1 mediated epithelial-mesenchymal-transition, loss of viability and hypermotility. Despite the lack of SMAD2 phosphorylation in response to TGF-B1 treatment, the PLCE1 mutant podocytes do express SMAD2. Interestingly, however, is the altered SMAD2/SMAD3 ratio in the PLCE1 mutant podocytes relative to the wild-type podocytes. This altered ratio could be reproduced in the wild-type podocytes by transient knockdown of PLCE1 using siRNA technology.
Conclusion
The SMAD2/3 ratio is 2:1 in the wild-type podocytes and 1:1 in the PLCE1 mutants. The mutant podocytes do not phosphorylate SMAD2 in response to TGF-B1 treatment suggesting that SMAD3 can out compete SMAD for phosphorylation at the level of the receptor. Lack of SMAD2 phosphorylation in the PLCE1 mutant podocyte renders the SMAD2 mediated TGF-B1 response inactive and upregulates the SMAD3 mediated TGF-B1 response. SMAD3 controls the pro-fibrotic response to TGF-B1. Hence the altered ratio could sensitise the mutant podocytes to fibrosis. Knockdown of PLCE1 by siRNA replicated the mutant ratio of 1:1 between SMAD2 and 3 in the wild type cells. This suggests that PLCE1 expression can affect the SMAD2/3 ratio by some unknown mechanism to influence the fibrotic response of the podocyte.