Podocytes are key components of the glomerular filtration barrier, and their interaction with the glomerular basement membrane (GBM) composition is crucial to maintain the permeability of this barrier. Although the GBM is primarily composed by laminin and Collagen type IV, the de novo production of the ?1 chain of collagen type I (Col I) has been observed in mouse models of Alport Syndrome (AS). Discoidin domain receptor 1 (DDR1) is a unique receptor tyrosine kinase that is activated by collagens. Interestingly, deletion of the DDR1 in Col4a3 knockout (Col4a3KO) mice, a mouse model for AS, was shown to improve survival and renal function, suggesting that DDR1 may represent a potential pharmaceutical target for AS. However, if and how DDR1 activation by aberrant collagen production in AS contributes to podocyte injury and the development of proteinuria is poorly understood. Our preliminary data show that DDR1 activity is increased in kidney cortex from Col4a3KO mice. In addition, we show that the degree of DDR1 activation correlates with blood urine nitrogen (BUN). In vitro, we demonstrate that DDR1 is activated by collagen type I (50g/mL, 18hr) in cultured human podocytes. In other preliminary data, we show for the first time that both collagen I and genetic activation of DDR1 are associated with cellular lipotoxicity, free fatty acid (FFA) uptake and intracellular lipid droplet deposition, suggesting that collagen I-induced/DDR1-mediated lipotoxicity may represent a novel mechanism leading to podocyte injury in AS. We propose to investigate this novel mechanism using a combined in vitro and in vivo approach.
In specific aim 1, we will determine if DDR1-induced podocyte injury in AS is due to increased in FFA uptake and triglyceride accumulation.
Specific aim 2 will test if DDR1-induced accumulation of triglycerides in podocytes is due to CD36 mediated FFA uptake. Lipid accumulation will be assessed using Oil Red O staining. FFA uptake will be measured by using fluorescent labeled FFAs. In vivo, we will test if genetic deletion of CD36 in Col4a3KO mice will protect from podocyte lipotoxicity and the development of proteinuria when compared to Col4a3KO mice. Findings derived from this application may demonstrate that DDR1 activation in AS induces podocyte triglyceride accumulation and apoptosis mediated through CD36 dependent fatty acid uptake. Results obtained from this research will set the basis for the development of novel drugs for the treatment of AS targeting DDR1 or CD36.
Alport Syndrome (AS) is an inherited disease that affects the kidney, the inner ear and eye; some patients with AS will develop end-stage renal disease (ESRD). We have identified a protein named discoidin domain receptor 1 (DDR1) that plays an important role in the progression of AS; we believe that when this gene is activated by collagen in AS, it causes injury to kidney cells called podocytes by increasing the uptake of free fatty acid. We propose that DDR1 might serve as a novel drug target to prevent kidney disease in AS.