Role of Collagen Binding Receptors in Glomerulosclerosis

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Abstract Glomerulosclerosis is one of the hallmarks of end stage kidney disease and it is characterized by the replacement of the glomerular tissue with extracellular matrix components (mainly collagens) leading to the loss of functioning glomeruli. The goal of this grant is to investigate the molecular mechanisms that underlie the modulation of collagen turnover in injured glomeruli to devise more effective therapies to prevent glomerulosclerosis. Although many pathways have been implicated in both initiation and progression to glomerular fibrosis, we focus on the collagen binding receptor integrin α1β1 (Itgα1β1). This receptor plays an anti-fibrotic action by recruiting and activating the tyrosine phosphatase TCPTP thus downregulating the phosphorylation of pro-fibrotic receptors, including the EGF receptor (EGFR). Moreover, Itgα1β1 negatively regulates collagen levels at both transcriptional and translational levels. Recently, we started to investigate the mechanisms whereby Itgα1β1 controls collagen synthesis at the nuclear level. As nuclear translocation and activation of many transcription factors and/or ribonucleoproteins are regulated by tyrosine phosphorylation, we analyzed the levels of tyrosine phosphorylated nuclear proteins in wild type and Itgα1KO mesangial cells to identify proteins tyrosine phosphorylated only in Itgα1KO cells. We identified the ribonucleoprotein Fused in Sarcoma (FUS) as a candidate. We show that in Itgα1KO mesangial cells, increased levels of total and tyrosine phosphorylated nuclear FUS are associated with increased collagen production and downregulation of FUS decreases collagen synthesis. Interestingly, FUS contains two tyrosines that can be phosphorylated by EGFR and dephosphorylated by TCPTP and the levels of nuclear FUS are associated with levels of activated EGFR. Based on these observations and the finding that FUS is upregulated in injured human and mouse kidneys, we propose that FUS is a positive regulator of collagen synthesis and plays a pro-fibrotic action in the course of glomerulosclerosis. We hypothesize that Itgα1β1 negatively regulates FUS tyrosine phosphorylation and function in an EGFR-dependent and - independent manner. Thus, Itgα1β1-mediated dephosphorylation of FUS represents an important, but previously undescribed mechanism to selectively reduce FUS activation and consequent progression to fibrosis. The aims of this grant are designed to define the contribution of FUS to glomerular disease and to determine whether blocking its function is beneficial for the treatment of glomerulosclerosis. In Aim 1 we will determine in vitro the mechanisms whereby FUS transcriptionally controls collagen production and determine whether inhibiting FUS ameliorates collagen synthesis. In Aim 2 we will determine the role of FUS in the progression to glomerular injury using a genetic and pharmacological approach. We will investigate the response of wild type and Itgα1KO mice crossed with global FUSKO mice, as well as mice overexpressing wild type FUS or mutated FUS no longer able to translocate to the nucleus to glomerular injury. We will then translate the relevance of these findings to a more clinically relevant setting, by investigating the response to glomerular injury in wild type and Itgα1KO mice untreated or treated with newly generated cell-penetrating peptides able to prevent FUS nuclear translocation. Understanding how the ribonucleoprotein FUS controls collagen production in glomerulosclerosis and exploring the consequences of its inhibition based on innovative delivery of nuclear transport modifiers will offer an entirely novel approach for the treatment and, ideally, prevention of glomerulosclerosis.