Blockade of Hedgehog Signaling Attenuates Biliary Cystogenesis in the Polycystic Kidney (PCK) Rat

Pkd1l1-deficiency drives biliary atresia through ciliary dysfunction in biliary epithelial cells

BACKGROUND & AIMS

Syndromic biliary atresia is a cholangiopathy characterized by fibro-obliterative changes in the extrahepatic bile duct (EHBD) and congenital malformations including laterality defects. The etiology remains elusive and faithful animal models are lacking. Genetic syndromes provide important clues regarding the pathogenic mechanisms underlying the disease. We investigated the role of the gene Pkd1l1 in the pathophysiology of syndromic biliary atresia.

METHODS

Constitutive and conditional Pkd1l1 knockout mice were generated to explore genetic pathology as a cause of syndromic biliary atresia. We investigated congenital malformations, EHBD and liver pathology, EHBD gene expression, and biliary epithelial cell turnover. Biliary drainage was functionally assessed with cholangiography. Histology and serum chemistries were assessed after DDC (3,5-diethoxycarbony l-1,4-dihydrocollidine) diet treatment and inhibition of the ciliary signaling effector GLI1.

RESULTS

Pkd1l1-deficient mice exhibited congenital anomalies including malrotation and heterotaxy. Pkd1l1-deficient EHBDs were hypertrophic and fibrotic. Pkd1l1-deficient EHBDs were patent but displayed delayed biliary drainage. Pkd1l1-deficient livers exhibited ductular reaction and periportal fibrosis. After DDC treatment, Pkd1l1-deficient mice exhibited EHBD obstruction and advanced liver fibrosis. Pkd1l1-deficient mice had increased expression of fibrosis and extracellular matrix remodeling genes (Tgfα, Cdkn1a, Hb-egf, Fgfr3, Pdgfc, Mmp12, and Mmp15) and decreased expression of genes mediating ciliary signaling (Gli1, Gli2, Ptch1, and Ptch2). Primary cilia were reduced on biliary epithelial cells and altered expression of ciliogenesis genes occurred in Pkd1l1-deficient mice. Small molecule inhibition of the ciliary signaling effector GLI1 with Gant61 recapitulated Pkd1l1-deficiency.

CONCLUSIONS

Pkd1l1 loss causes both laterality defects and fibro-proliferative EHBD transformation through disrupted ciliary signaling, phenocopying syndromic biliary atresia. Pkd1l1-deficient mice function as an authentic genetic model for study of the pathogenesis of biliary atresia.

IMPACT AND IMPLICATIONS

The syndromic form of biliary atresia is characterized by fibro-obliteration of extrahepatic bile ducts and is often accompanied by laterality defects. The etiology is unknown, but Pkd1l1 was identified as a potential genetic candidate for syndromic biliary atresia. We found that loss of the ciliary gene Pkd1l1 contributes to hepatobiliary pathology in biliary atresia, exhibited by bile duct hypertrophy, reduced biliary drainage, and liver fibrosis in Pkd1l1-deficient mice. Pkd1l1-deficient mice serve as a genetic model of biliary atresia and reveal ciliopathy as an etiology of biliary atresia. This model will help scientists uncover new therapeutic approaches for patients with biliary atresia, while pediatric hepatologists should validate the diagnostic utility of PKD1L1 variants.

Primary cilia and hypoxia-associated signaling in developmental odontogenic cysts in relation to autosomal dominant polycystic kidney disease - A novel insight

Developmental cysts are pathological epithelial-lined cavities arising in various organs as a result of systemic or hereditary diseases. Molecular mechanisms involved in the formation of developmental odontogenic cysts (OCs) are not fully understood yet; the cystogenesis of renal cysts originating from the autosomal dominant polycystic kidney disease (ADPKD) has been, however, explored in much greater detail. This narrative review aimed i) to summarize molecular and cellular processes involved in the formation and growth of developmental OCs, especially dentigerous cysts (DCs) and odontogenic keratocysts (OKCs), ii) to find if there are any similarities in their cystogenesis to ADPKD cysts, and, based on that, iii) to suggest potential factors, candidate molecules, and mechanisms that could be involved in the DC formation, thus proposing further research directions. Here we suggest a possible association of developmental OCs with primary cilia disruption and with hypoxia, which have been previously linked with cyst formation in ADPKD patients. This is illustrated on the imagery of tissues from an ADPKD patient (renal cyst) and from developmental OCs, supporting the similarities in cell proliferation, apoptosis, and primary cilia distribution in DC/OKC/ADPKD tissues. Based on all that, we propose a novel hypothesis of OCs formation suggesting a crucial role of mutations associated with the signaling pathways of primary cilia (in particular, Sonic Hedgehog). These can lead to excessive proliferation and formation of cell agglomerates, which is followed by hypoxia-driven apoptosis in the centers of such agglomerates (controlled by molecules such as Hypoxia-inducible factor-1 alpha), leading to cavity formation and, finally, the OCs development. Based on this, we propose future perspectives in the investigation of OC pathogenesis.

Ppia is the most stable housekeeping gene for qRT-PCR normalization in kidneys of three Pkd1-deficient mouse models

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is the most common inherited renal disorder, characterized by renal cyst development leading to end-stage renal disease. Although the appropriate choice of suitable reference is critical for quantitative RNA analysis, no comparison of frequently used “housekeeping” genes is available. Here, we determined the validity of 7 candidate housekeeping genes (Actb, Actg1, B2m, Gapdh, Hprt, Pgam1 and Ppia) in kidney tissues from mouse models orthologous to ADPKD, including a cystic mice (CY) 10–12 weeks old (Pkd1^flox/flox:Nestin^cre/Pkd1^flox/−:Nestin^cre, n = 10) and non-cystic (NC) controls (Pkd1^flox/^flox/Pkd1^flox/^-, n = 10), Pkd1-haploinsufficient (HT) mice (Pkd1^+/−, n = 6) and wild-type (WT) controls (Pkd1^+/+, n = 6) and a severely cystic (SC) mice 15 days old (Pkd1^V/V, n = 7) and their controls (CO, n = 5). Gene expression data were analyzed using six distinct statistical softwares. The estimation of the ideal number of genes suggested the use of Ppia alone as sufficient, although not ideal, to analyze groups altogether. Actb, Hprt and Ppia expression profiles were correlated in all samples. Ppia was identified as the most stable housekeeping gene, while Gapdh was the least stable for all kidney samples. Stat3 expression level was consistent with upregulation in SC compared to CO when normalized by Ppia expression. In conclusion, present findings identified Ppia as the best housekeeping gene for CY + NC and SC + CO groups, while Hprt was the best for the HT + WT group.

Notch-Hes1 signaling activation in Caroli disease and polycystic liver disease

The Notch signaling pathway plays a key role in the morphogenesis of the biliary tree, but its involvement in cystic biliary diseases, such as Caroli disease (CD) and polycystic liver disease (PLD), has yet to be determined. Immunostaining was performed using liver sections of CD and PLD, and the results were compared with those of congenital hepatic fibrosis (CHF) and von Meyenburg complex (VMC). The expression of Notch receptor 1 (Notch1) was increased in the nuclei of biliary epithelial cells in all cases of CD and PLD, whereas it remained at a low level in CHF and VMC. In addition, Notch2 and Notch3 were preferably expressed in the nuclei of biliary epithelial cells of PLD. Accordingly, the Notch effector Hes1 was highly expressed in biliary epithelial cells of CD and PLD, and the cell proliferative activity was significantly higher in CD and PLD. The expression of the Notch ligand Delta-like 1 was significantly increased in biliary epithelial cells of CD and PLD, which may be causally associated with the nuclear overexpression of Notch1 and Hes1. These results indicate that aberrant activation of the Notch-Hes1 signaling pathway may be responsible for the progression of biliary cystogenesis in CD and PLD.

The cellular pathways and potential therapeutics of Polycystic Kidney Disease

Polycystic Kidney Disease (PKD) refers to a group of disorders, driven by the formation of cysts in renal tubular cells and is currently one of the leading causes of end-stage renal disease. The range of symptoms observed in PKD is due to mutations in cilia-localising genes, resulting in changes in cellular signalling. As such, compounds that are currently in preclinical and clinical trials target some of these signalling pathways that are dysregulated in PKD. In this review, we highlight these pathways including cAMP, EGF and AMPK signalling and drugs that target them and may show promise in lessening the disease burden of PKD patients. At present, tolvaptan is the only approved therapy for ADPKD, however, it carries several adverse side effects whilst comparatively, no pharmacological drug is approved for ARPKD treatment. Aside from this, drugs that have been the subject of multiple clinical trials such as metformin, which targets AMPK signalling and somatostatins, which target cAMP signalling have shown great promise in reducing cyst formation and cellular proliferation. This review also discusses other potential and novel targets that can be used for future interventions, such as β-catenin and TAZ, where research has shown that a reduction in the overexpression of these signalling components results in amelioration of disease phenotype. Thus, it becomes apparent that well-designed preclinical investigations and future clinical trials into these pathways and other potential signalling targets are crucial in bettering disease prognosis for PKD patients and could lead to personalised therapy approaches.

Cell-Autonomous Hedgehog Signaling Is Not Required for Cyst Formation in Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

PKD1 or PKD2, the two main causal genes for autosomal dominant polycystic kidney disease (ADPKD), encode the multipass transmembrane proteins polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Polycystins localize to the primary cilium, an organelle essential for cell signaling, including signal transduction of the Hedgehog pathway. Mutations in ciliary genes that build and maintain the cilium also cause renal cystic disease through unknown pathways. Although recent studies have found alterations in Hedgehog signaling in ADPKD-related models and tissues, the relationship between Hedgehog and polycystic kidney disease is not known.

METHODS

To examine the potential role of cell-autonomous Hedgehog signaling in regulating kidney cyst formation in vivo in both early- and adult-onset mouse models of ADPKD, we used conditional inactivation of Pkd1 combined with conditional modulation of Hedgehog signaling components in renal epithelial cells, where mutations in Pkd1 initiate cyst formation. After increasing or decreasing levels of Hedgehog signaling in cells that underwent inactivation of Pkd1, we evaluated the effects of these genetic manipulations on quantitative parameters of polycystic kidney disease severity.

RESULTS

We found that in Pkd1 conditional mutant mouse kidneys, neither downregulation nor activation of the Hedgehog pathway in epithelial cells along the nephron significantly influenced the severity of the polycystic kidney phenotype in mouse models of developmental or adult-onset of ADPKD.

CONCLUSIONS

These data suggest that loss of Pkd1 function results in kidney cysts through pathways that are not affected by the activity of the Hedgehog pathway.