The Functional Role of Six1 in Renal Fibrosis

Renal fibrosis is a common pathway leading to end-stage renal failure. Therapies for renal fibrosis, however, remain very limited, non-specific, and non-effective. TGF-β/Smad signaling is a key pathway which mediates renal fibrosis. However, because of the anti-inflammatory properties of TGF-β, directly targeting TGF-β may cause adverse effects on the immune system. Thus, a better approach for combating chronic kidney disease is the identification and targeting of the TGF-β/Smad-specific effectors related to fibrosis.

A novel and potentially important preliminary finding is that Six1, an important gene for fetal kidney development, was upregulated in diseased kidneys from hypertensive patients and ligated kidneys from a mouse model of obstructive nephropathy. Renal Six1 expression was strongly correlated with renal fibrosis and activation of TGF-β signaling. Importantly, delivery of a Six1 overexpression plasmid into ligated kidneys further enhanced renal fibrosis in mice. In vitro studies also confirmed that overexpression of Six1 promoted TGF-β1-induced fibrosis in tubular epithelial cells and knocking down the Six1 transcripts attenuated this induction. In addition, both in vitro and in vivo studies showed that Six1 expression was positively correlated with TGF-β receptor 1 expression. These new findings strongly indicate that Six1 may participate in TGF-β-induced renal fibrosis.

Based on these findings, we hypothesize that Six1 may enhance renal fibrosis by activating TGF-β signaling. This hypothesis will be firstly tested by determining the functional role of Six1 and its cofactor, Eya1, in renal fibrosis (Specific Aim 1). We will then investigate the mechanism of how Six1 promotes TGF-β-induced renal fibrosis by in vitro studies (Specific Aim 2). Finally, we will determine the therapeutic potential of anti-Six1 approach for renal fibrosis in a mouse model of obstructive nephropathy by delivering Six1 knockdown plasmids using an ultrasound-microbubble-mediated gene delivery system (Specific Aim 3).

Determining the pathogenic role of Six1 in renal fibrosis will uncover a new mechanism of kidney diseases. Notably, suppression of Six1 expression in the kidney will inhibit renal fibrosis. Although recent advances have led to a much better understanding of these processes, there is as yet no successful strategy which suppresses renal fibrosis. Therefore, therapeutic strategies to combat renal fibrosis are urgently needed. The proposed studies will have a specific, novel and highly effective clinical impact in terms of the prevention and treatment of the progression of chronic kidney disease.