HDAC5 mediated epigenetic regulation of Mef2 transcription factor in ADPKD pathogenesis
Autosomal-dominant polycystic kidney disease (ADPKD) is a monogenic renal disease characterized by progressive development of bilateral renal cysts, decline in renal function and many extrarenal manifestations. ADPKD has a very high worldwide prevalence rate of 1:400 to 1:1000 and it typically results in end-stage renal disease. ADPKD is caused by mutations in one of the genes, PKD1 and PKD2. PKD1 encodes polycystin-1 (PC1), which is a large G-protein coupled receptor-like protein and PKD2 encodes polycystin-2 (PC2), which is a TRP calcium channel.
Fluid flow-induced HDAC5 nuclear export in cultured kidney cells (HDAC5 is shown in green, cell nuclei in blue.)
PC1 can interact with PC2 to form a complex on the primary cilia, bilateral and basal membrane of the renal epithelium cells. PC1-PC2 complex can act as a calcium permeable mechanosensor. Loss of polycystins has been shown to disrupt flow-dependent and steady-state intracellular calcium signaling further confirming that the PC1-PC2 complex acts as a mechanosensor. Fluid flow on cultured renal epithelium cells stimulates calcium influx, which activates kinases like protein kinase C. Our lab has recently shown that these kinases then directly or indirectly phosphorylate the 14-3-3 binding sites on histone deacetylase 5 (HDAC5), a member of class IIa HDACs, which function as negative regulator of myocyte-specific enhancer factor 2C (MEF2C) target genes. Phosphorylation of the 14-3-3 binding site allows 14-3-3 proteins to bind and HDAC5 to be exported from the nucleus. With HDAC5 repression relieved, the transcription factor MEF2C is free to interact with histone acetyl transferases (HAT) p300/CREB binding protein (CBP). The acetylation of nucleosomal histones surrounding MEF2C-binding sites by HATs leads to the activation of MEF2C target genes. Similar signaling mechanism has also been shown in muscle gene expression.
It has been shown that Trichostatin A (TSA), a class I & class II selectiveHDAC inhibitor, can reduce cyst formation in PKD2-/- mouse embryos. Also, TSA caused elevated expression of missing in metastasis (MIM), a MEF2C target gene that regulates the actin cytoskeleton. These data support the important role HDAC5 plays in ADPKD pathogenesis and thus in maintaining the normal organization of epithelial cells in kidney tubules. The potential clinical implication of using HDAC inhibitors as drugs for ADPKD has led to extensive study to find HDAC inhibitors that can reduce the progression of renal cyst and slow the decline in renal function. As a class IIa HDACs, HDAC5 does not posses intrinsic enzymatic activity making it difficult to design specific inhibitors. However, Class II HDACs are known to form a complex with HDAC3, an enzymatically active class I HDAC in HeLa and 293 cells. My aim is to identify the active HDAC5 complex in kidney collecting duct cells. Also, I will perform a screen using a small pool of FDA approved compounds with well characterized targets involved in diverse signaling pathways using flow-induced HDAC5 export as a paradigm. The overall goal of my research is to dissect the mechanism of epigenetic regulation of cyst formation in ADPKD and to find novel therapeutic targets to treat PKD pathogenesis.