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  • Fibrocystin is required for normal branching morphogenesis

    2019-07-24

    Fibrocystin is required for normal branching morphogenesis of the ureteric bud during embryonic renal development [8,13]. In ARPKD kidneys, cystic dilation is restricted to the ureteric bud-derived collecting tubules (CTs) and is associated with increased epithelial cell proliferation and luminal fluid secretion [14], as well as abnormalities in apoptosis [15,16], epithelial cell polarity [17] and cell-matrix interactions [18]. Fibrocystin is also localised in primary cilia protruding from the apical surface of CT Dihexa (PNB-0408) [19,20], together with Polycystins (PC)-1 and -2 [21], the protein products of the Autosomal Dominant (AD)PKD-causative genes PKD1 and PKD-2 [3,4]. In the kidney, primary cilia are non-motile sensory organelles that act as signal transducers involved in cell signalling pathways [22]. Both canonical Wnt and non-canonical Wnt/PCP pathways are relevant to ciliary signalling and kidney development [23]. Loss of Fibrocystin function causes shorter cilia in the bile ducts of a mouse model with no functional Fibrocystin [24]. Canonical Wnt signalling is initiated when Wnt ligands bind to Frizzled (Fz) receptors in the presence of LRP5 or 6 [25]. This results in Dishevelled (Dvl) activation and stabilisation of β-catenin, which then translocates to the nucleus and initiates transcriptional activation of Wnt target genes. The other key downstream Wnt signalling pathway in the kidney is the non-canonical Wnt/PCP pathway. The binding of non-canonical Wnt ligands here results in the recruitment of Fz and Dvl to the membrane, culminating in cytoskeletal rearrangements that affect cellular organisation, shape and migration. In the kidney, interaction of Inversin with Dvl is hypothesised to trigger non-canonical Wnt/PCP pathway activation [26]. Many core and effector PCP proteins have been implicated in kidney development and disease and one of these is Atmin, a transcription factor with diverse roles in DNA damage repair and ciliogenesis [[27], [28], [29]]. Kidney development was demonstrated to be greatly impaired in Atmin deficient mice, with homozygous mutant kidneys displaying reduced numbers of differentiated ureteric buds and renal vesicles [30]. Loss of Atmin resulted in changes in expression of non-canonical Wnt components (Wnt9b, Wnt11, Vangl2, Daam2) that affected oriented cell division and the cytoskeletal organisation in the renal epithelial cells of the kidney. The significant role of Wnt signalling is gaining recognition in cystic renal disease. Disrupted Wnt signalling components, including Wnt9b, Fat4, Vangl2 and Inversin are associated with polycystic kidney disease [[31], [32], [33], [34], [35], [36]] and frizzled-related-protein-4 is upregulated in human ADPKD and in ADPKD mouse models [37]. Furthermore, DKK3, a β-catenin antagonist, is a potential modifier of ADPKD [38] and Wnt ligands bind to the PKD1-encoded PC-1 extracellular domain and activate the PC-1/PC-2 channel [39]. While defective Wnt signalling has been implicated in ADPKD and mis-oriented cell division was detected in Pck rats [40], the role of non-canonical Wnt/PCP signalling has not been investigated in detail in human ARPKD. Hence an examination of novel Wnt pathways implicated in ARPKD was conducted in mice, cell lines and human kidneys, providing a unified approach into understanding paediatric polycystic kidney disease mechanisms.
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    Introduction Deletion of the nuclear factor kappa-B (NF-κB) essential modulator (NEMO) also known as NF-κB inhibitor of kinase subunit gamma (IKK-γ) in liver parenchymal cells (LPC) causes spontaneous development of increased hepatocyte apoptosis and compensatory regeneration leading to a sequence of steatohepatitis, fibrosis and, ultimately, liver cancer [1], making this a well-suited mouse model to study mechanisms of inflammation-driven hepatocarcinogenesis [2]. In hepatocytes, NF-κB is activated in response to stress signals like TNF-α via transforming growth factor beta-activated kinase 1 (TAK1) and NEMO, preventing cleavage and activation of caspase 8 [3]. In contrast, in hepatocytes where NEMO is absent, NF-κB signaling is blocked leading to cleavage of caspase 8 and 3 and subsequently to hepatocyte apoptosis [1,3]. Hepatocyte death then triggers a chronic inflammatory and a regenerative response in the liver finally leading to the development of hepatocellular carcinoma [4]. While these hepatocyte-intrinsic pathways have been widely explored in the model of hepatocyte-specific NEMO knock-out (NEMOLPC-KO) mice, the immune responses driving inflammation and fibrosis are less well understood.