Cholangiociliopathies: genetics, molecular mechanisms and potential therapies

Inhibition of Cdc25A suppresses hepato-renal cystogenesis in rodent models of polycystic kidney and liver disease

BACKGROUND & AIMS

In polycystic kidney disease and polycystic liver disease (PLD), the normally nonproliferative hepato-renal epithelia acquire a proliferative, cystic phenotype that is linked to overexpression of cell division cycle 25 (Cdc25)A phosphatase and cell-cycle deregulation. We investigated the effects of Cdc25A inhibition in mice and rats via genetic and pharmacologic approaches.

METHODS

Cdc25A(+/-) mice (which have reduced levels of Cdc25A) were cross-bred with polycystic kidney and hepatic disease 1 (Pkhd1(del2/del2)) mice (which have increased levels of Cdc25A and develop hepatic cysts). Cdc25A expression was analyzed in livers of control and polycystic kidney (PCK) rats, control and polycystic kidney 2 (Pkd2(ws25/-)) mice, healthy individuals, and patients with PLD. We examined effects of pharmacologic inhibition of Cdc25A with vitamin K3 (VK3) on the cell cycle, proliferation, and cyst expansion in vitro; hepato-renal cystogenesis in PCK rats and Pkd2(ws25/-)mice; and expression of Cdc25A and the cell-cycle proteins regulated by Cdc25A. We also examined the effects of the Cdc25A inhibitor PM-20 on hepato-renal cystogenesis in Pkd2(ws25/-) mice.

RESULTS

Liver weights and hepatic and fibrotic areas were decreased by 32%-52% in Cdc25A(+/-):Pkhd1(del2/del2) mice, compared with Pkhd1(del2/del2) mice. VK3 altered the cell cycle and reduced proliferation of cultured cholangiocytes by 32%-83% and decreased growth of cultured cysts by 23%-67%. In PCK rats and Pkd2(ws25/-) mice, VK3 reduced liver and kidney weights and hepato-renal cystic and fibrotic areas by 18%-34%. PM-20 decreased hepato-renal cystogenesis in Pkd2(ws25/-) mice by 15%.

CONCLUSIONS

Cdc25A inhibitors block cell-cycle progression and proliferation, reduce liver and kidney weights and cyst growth in animal models of polycystic kidney disease and PLD, and might be developed as therapeutics for these diseases.

Congenital hepatic fibrosis and portal hypertension in autosomal dominant polycystic kidney disease

OBJECTIVES

Autosomal dominant (ADPKD) and recessive (ARPKD) polycystic kidney diseases are the most common hepatorenal fibrocystic diseases (ciliopathies). Characteristics of liver disease of these disorders are quite different. All of the patients with ARPKD have congenital hepatic fibrosis (CHF) often complicated by portal hypertension. In contrast, typical liver involvement in ADPKD is polycystic liver disease, although rare atypical cases with CHF are reported. Our goal was to describe the characteristics of CHF in ADPKD.

PATIENTS AND METHODS

As a part of an intramural study of the National Institutes of Health on ciliopathies (www.clinicaltrials.gov, trial NCT00068224), we evaluated 8 patients from 3 ADPKD families with CHF. We present their clinical, biochemical, imaging, and PKD1 and PKHD1 sequencing results. In addition, we tabulate the characteristics of 15 previously reported patients with ADPKD-CHF from 11 families.

RESULTS

In all of the 19 patients with ADPKD-CHF (9 boys, 10 girls), portal hypertension was the main manifestation of CHF; hepatocelllular function was preserved and liver enzymes were largely normal. In all of the 14 families, CHF was not inherited vertically, that is the parents of the index cases had PKD but did not have CHF-suggesting modifier gene(s). Our 3 families had pathogenic mutations in PKD1; sequencing of the PKHD1 gene as a potential modifier did not reveal any mutations.

CONCLUSIONS

Characteristics of CHF in ADPKD are similar to CHF in ARPKD. ADPKD-CHF is caused by PKD1 mutations, with probable contribution from modifying gene(s). Given that both boys and girls are affected, these modifier(s) are likely located on autosomal chromosome(s) and less likely X-linked.

Caroli's Disease: Current Knowledge of Its Biliary Pathogenesis Obtained from an Orthologous Rat Model

Caroli's disease belongs to a group of hepatic fibropolycystic diseases and is a hepatic manifestation of autosomal recessive polycystic kidney disease (ARPKD). It is a congenital disorder characterized by segmental saccular dilatations of the large intrahepatic bile duct and is frequently associated with congenital hepatic fibrosis (CHF). The most viable theory explaining its pathogenesis suggests that it is related to ductal plate malformation. The development of the polycystic kidney (PCK) rat, an orthologous rodent model of Caroli's disease with CHF as well as ARPKD, has allowed the molecular pathogenesis of the disease and the therapeutic options for its treatment to be examined. The relevance of the findings of studies using PCK rats and/or the cholangiocyte cell line derived from them to the pathogenesis of human Caroli's disease is currently being analyzed. Fibrocystin/polyductin, the gene product responsible for ARPKD, is normally localized to primary cilia, and defects in the fibrocystin from primary cilia are observed in PCK cholangiocytes. Ciliopathies involving PCK cholangiocytes (cholangiociliopathies) appear to be associated with decreased intracellular calcium levels and increased cAMP concentrations, causing cholangiocyte hyperproliferation, abnormal cell matrix interactions, and altered fluid secretion, which ultimately result in bile duct dilatation. This article reviews the current knowledge about the pathogenesis of Caroli's disease with CHF, particularly focusing on studies of the mechanism responsible for the biliary dysgenesis observed in PCK rats.

Systematic review: the pathophysiology and management of polycystic liver disease

BACKGROUND

Polycystic liver diseases (PCLD) represent a group of genetic disorders in which cysts occur solely in the liver, or together with renal cysts. Most of the patients with PCLD are asymptomatic, however, in some patients, expansion of liver cysts causes invalidating abdominal symptoms.

AIM

To provide a systemic review on the pathophysiology and management of PCLD.

METHODS

A PubMed search was undertaken to identify relevant literature using search terms including polycystic liver disease, pathophysiology, surgical and medical management.

RESULTS

The most common complication in patients with PCLD is extensive hepatomegaly, which may lead to malnutrition and can be lethal. Conservative surgical approaches are only partially effective and do not change the natural course of the disease. Liver transplantation has been successfully performed in PCLD, however, in an era of organ shortage, medical management needs to be evaluated. A better understanding of the pathophysiology and the availability of animal models have already identified promising drugs. Abnormalities in cholangiocyte proliferation/apoptosis and enhanced fluid secretion are key factors in the pathophysiology. It has been demonstrated in rodents and in humans that somatostatin analogues diminish liver volume. The role of the inhibitors of the mammalian target of rapamycin (mTOR) in the management of PCLD is still under investigation.

CONCLUSIONS

The exact pathophysiology of polycystic liver disease still remains unclear. In symptomatic patients, none of the currently available surgical options except liver transplantation have been shown to change the natural course of the disease. The use of somatostatin analogues has been shown to diminish liver volume.

The role of cilia in the regulation of bile flow

Cholangiocytes, the epithelial cells lining intrahepatic bile ducts, are ciliated cells. Each cholangiocyte has a primary cilium consisting of (i) a microtubule-based axoneme and (ii) the basal body, centriole-derived, microtubule-organizing center from which the axoneme emerges. Primary cilia in cholangiocytes were described decades ago, but their physiological and pathophysiological significance remained unclear until recently. We now recognize that cholangiocyte cilia extend from the apical plasma membrane into the bile duct lumen and, as such, are ideally positioned to detect changes in bile flow, bile composition and bile osmolality. These sensory organelles act as cellular antennae that can detect and transmit signals that influence cholangiocyte function. Indeed, recent data show that cholangiocyte primary cilia can activate intracellular signaling pathways when they sense modifications in the flow, molecular constituents and osmolarity of bile. Their ability to sense and transmit signals depends on the participation of a growing number of specific ciliary-associated proteins that act as receptors, channels and transporters. Cholangiocyte cilia, in addition to being important in normal biliary physiology, likely contribute to the cholangiopathies when their normal structure or function is disturbed. Indeed, the polycystic liver diseases that occur in combination with autosomal dominant and recessive polycystic kidney disease (i.e. ADPKD and ARPKD) are two important examples of such conditions. Recent insights into the role of cholangiocyte cilia in cystic liver disease using in vitro and animal models have already resulted in clinical trials that have influenced the management of cystic liver disease.

Regulation of biliary proliferation by neuroendocrine factors: implications for the pathogenesis of cholestatic liver diseases

The proliferation of cholangiocytes occurs during the progression of cholestatic liver diseases and is critical for the maintenance and/or restoration of biliary mass during bile duct damage. The ability of cholangiocytes to proliferate is important in many different human pathologic conditions. Recent studies have brought to light the concept that proliferating cholangiocytes serve as a unique neuroendocrine compartment in the liver. During extrahepatic cholestasis and other pathologic conditions that trigger ductular reaction, proliferating cholangiocytes acquire a neuroendocrine phenotype. Cholangiocytes have the capacity to secrete and respond to a variety of hormones, neuropeptides, and neurotransmitters, regulating their surrounding cell functions and proliferative activity. In this review, we discuss the regulation of cholangiocyte growth by neuroendocrine factors in animal models of cholestasis and liver injury, which includes a discussion of the acquisition of neuroendocrine phenotypes by proliferating cholangiocytes and how this relates to cholangiopathies. We also review what is currently known about the neuroendocrine phenotypes of cholangiocytes in human cholestatic liver diseases (ie, cholangiopathies) that are characterized by ductular reaction.

Dicer regulates the development of nephrogenic and ureteric compartments in the mammalian kidney

MicroRNAs (miRNAs) are a large and growing class of small, non-coding, regulatory RNAs that control gene expression predominantly at the post-transcriptional level. The production of most functional miRNAs depends on the enzymatic activity of Dicer, an RNase III class enzyme. To address the potential action of Dicer-dependent miRNAs in mammalian kidney development, we conditionally ablated Dicer function within cells of nephron lineage and the ureteric bud-derived collecting duct system. Six2Cre-mediated removal of Dicer activity from the progenitors of the nephron epithelium led to elevated apoptosis and premature termination of nephrogenesis. Thus, Dicer action is important for maintaining the viability of this critical self-renewing progenitor pool and, consequently, development of a normal nephron complement. HoxB7Cre-mediated removal of Dicer function from the ureteric bud epithelium led to the development of renal cysts. This was preceded by excessive cell proliferation and apoptosis, and accompanied by disrupted ciliogenesis within the ureteric bud epithelium. Dicer removal also disrupted branching morphogenesis with the phenotype correlating with downregulation of Wnt11 and c-Ret expression at ureteric tips. Thus Dicer, and by inference Dicer-dependent miRNA activity, have distinct regulatory roles within different components of the developing mouse kidney. Furthermore, an understanding of miRNA action may provide new insights into the etiology and pathogenesis of renal cyst-based kidney disease.

Pathogenesis of cholestatic liver disease and therapeutic approaches

Cholestatic liver disorders are caused by genetic defects, mechanical aberrations, toxins, or dysregulations in the immune system that damage the bile ducts and cause accumulation of bile and liver tissue damage. They have common clinical manifestations and pathogenic features that include the responses of cholangiocytes and hepatocytes to injury. We review the features of bile acid transport, tissue repair and regulation, apoptosis, vascular supply, immune regulation, and cholangiocytes that are associated with cholestatic liver disorders. We now have a greater understanding of the physiology of cholangiocytes at the cellular and molecular levels, as well as genetic factors, repair pathways, and autoimmunity mechanisms involved in the pathogenesis of disease. These discoveries will hopefully lead to new therapeutic approaches for patients with cholestatic liver disease.

Fibrocystic disease of liver and pancreas; under-recognized features of the X-linked ciliopathy oral-facial-digital syndrome type 1 (OFD I)

OFD I is an X-linked dominant male-lethal ciliopathy characterized by prominent external features including oral clefts, hamartomas or cysts of the tongue, and digital anomalies. Although these external features are easy to recognize and often lead to diagnosis in early childhood, visceral findings in OFD I, especially the fibrocystic liver and pancreas disease, are under-recognized. In addition, while the occurrence of polycystic kidney disease (PKD) in OFD I is well known, few patients are evaluated and monitored for this complication. We report on two adult females diagnosed with OFD I in infancy, but not evaluated for visceral involvement. In adulthood, they were incidentally found to have severe hypertension and chronic renal insufficiency due to undiagnosed PKD. A pancreatic cystic lesion, also discovered incidentally, was thought to be malignant and led to consideration of major surgery. We present NIH evaluations, including documentation of OFD I mutations, extreme beading of the intrahepatic bile ducts, pancreatic cysts, and tabulate features of reported OFD I cases having hepatic, pancreatic, and renal cystic disease. Liver and pancreas are not routinely evaluated in OFD I patients. Increased awareness and lifelong monitoring of visceral complications, particularly involving the liver, pancreas, and kidney, are essential for timely and accurate treatment.

Polycystic liver diseases

Polycystic liver diseases (PCLDs) are genetic disorders with heterogeneous etiologies and a range of phenotypic presentations. PCLD exhibits both autosomal or recessive dominant pattern of inheritance and is characterized by the progressive development of multiple cysts, isolated or associated with polycystic kidney disease, that appear more extensive in women. Cholangiocytes have primary cilia, functionally important organelles (act as mechanosensors) that are involved in both normal developmental and pathological processes. The absence of polycystin-1, 2, and fibrocystin/polyductin, normally localized to primary cilia, represent a potential mechanism leading to cyst formation, associated with increased cell proliferation and apoptosis, enhanced fluid secretion, abnormal cell-matrix interactions, and alterations in cell polarity. Proliferative and secretive activities of cystic epithelium can be regulated by estrogens either directly or by synergizing growth factors including nerve growth factor, IGF1, FSH and VEGF. The abnormalities of primary cilia and the sensitivity to proliferative effects of estrogens and different growth factors in PCLD cystic epithelium provide the morpho-functional basis for future treatment targets, based on the possible modulation of the formation and progression of hepatic cysts.

Organogenesis and development of the liver

Embryonic development of the liver has been studied intensely, yielding insights that impact diverse areas of developmental and cell biology. Understanding the fundamental mechanisms that control hepatogenesis has also laid the basis for the rational differentiation of stem cells into cells that display many hepatic functions. Here, we review the basic molecular mechanisms that control the formation of the liver as an organ.

Isolated polycystic liver disease

Isolated polycystic liver disease (PCLD) is an autosomal dominant disease with genetic and clinical heterogeneity. Apart from liver cysts, it exhibits few extrahepatic manifestations, and the majority of patients with this condition are asymptomatic or subclinical. However, a small fraction of these patients develop acute liver cyst-related complications and/or massive cystic liver enlargement, causing morbidity and mortality. Currently, the management for symptomatic PCLD is centered on palliating symptoms and treating complications.

Hepato-renal pathology in pkd2ws25/- mice, an animal model of autosomal dominant polycystic kidney disease

Polycystic liver diseases, the most important of which are autosomal dominant and autosomal recessive polycystic kidney diseases, are incurable pathological conditions. Animal models that resemble human pathology in these diseases provide an opportunity to study the mechanisms of cystogenesis and to test potential treatments. Here we demonstrate that Pkd2ws25/- mice, an animal model of autosomal dominant polycystic kidney disease, developed hepatic cysts. As assessed by micro-computed tomography scanning of intact livers and by light microscopy of hepatic tissue, hepatic cystic volumes increased from 12.82+/-3.16% (5- to 8-month-old mice) to 21.58+/-4.81% (9- to 12-month-old mice). Renal cystogenesis was more severe at early stages of disease: in 5- to 7-month-old mice, cystic volumes represented 40.67+/-5.48% of kidney parenchyma, whereas in older mice cysts occupied 31.04+/-1.88% of kidney parenchyma. Mild fibrosis occurred only in liver, and its degree was unchanged with age. Hepatic cysts were lined by single or multiple layers of squamous cholangiocytes. Cystic cholangiocyte cilia were short and malformed, whereas in renal cysts they appeared normal. In Pkd2ws25/- mice, mitotic and apoptotic indices in both kidney and liver were increased compared with wild-type mice. In conclusion, Pkd2ws25/- mice exhibit hepatorenal pathology resembling human autosomal dominant polycystic kidney disease and represent a useful model to study mechanisms of cystogenesis and to evaluate treatment options.