Transforming growth factor alpha and epidermal growth factor expression in experimental murine polycystic kidney disease

Pathophysiology of childhood polycystic kidney diseases: new insights into disease-specific therapy

Autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD) are significant causes of morbidity and mortality in children and young adults. ADPKD, with an incidence of 1:400 to 1:1,000, affects more than 13 million individuals worldwide and is a major cause of end-stage renal disease in adults. However, symptomatic disease is increasingly recognized in children. ARPKD is a dual-organ hepatorenal disease with an incidence of 1:20,000 to 1:40,000 and a heterozygote carrier rate of 1 in 70. Currently, no clinically significant disease-specific therapy exists for ADPKD or ARPKD. The genetic basis of both ADPKD and ARPKD have been identified, and delineation of the basic molecular and cellular pathophysiology has led to the discovery that abnormal ADPKD and ARPKD gene products interact to create "polycystin complexes" located at multiple sites within affected cells. The extracellular matrix and vessels produce a variety of soluble factors that affect the biology of adjacent cells in many dynamic ways. This review will focus on the molecular and cellular bases of the abnormal cystic phenotype and discuss the clinical translation of such basic data into new therapies that promise to alter the natural history of disease for children with genetic PKDs.

Apoptosis in polycystic kidney disease

Apoptosis is the process of programmed cell death. It is a ubiquitous, controlled process consuming cellular energy and designed to avoid cytokine release despite activation of local immune cells, which clear the cell fragments. The process occurs during organ development and in maintenance of homeostasis. Abnormalities in any step of the apoptotic process are associated with autoimmune diseases and malignancies. Polycystic kidney disease (PKD) is the most common inherited kidney disease leading to end-stage renal disease (ESRD). Cyst formation requires multiple mechanisms and apoptosis is considered one of them. Abnormalities in apoptotic processes have been described in various murine and rodent models of PKD as well as in human PKD kidneys. The purpose of this review is to outline the role of apoptosis in progression of PKD as well as to describe the mechanisms involved. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Mouse models of polycystic kidney disease

Polycystic kidney disease (PKD) is a diverse group of human monogenic lethal conditions inherited as autosomal dominant (AD) or recessive (AR) traits. Recent development of genetically engineered mouse models of ADPKD, ARPKD, and nephronophthisis/medullary cystic disease (NPHP) are providing additional insights into the molecular mechanisms governing of these disease processes as well as the developmental differentiation of the normal kidney. Genotypic and phenotypic mouse models are discussed and provide evidence for the fundamental involvement of cell-matrix, cell-cell, and primary cilia-lumen interactions, as well as epithelial proliferation, apoptosis, and polarization. Structure/function relationships between the PKD1, PKD2, PKHD1, and NPHP genes and proteins support the notion of a regulatory multiprotein cystic complex with a mechanosensory function that integrates signals from the extracellular environment. The plethora of intracellular signaling cascades that can impact renal cystic development suggest an exquisitely sensitive requirement for integrated downstream transduction and provide potential targets for therapeutic intervention. Appropriate genocopy models that faithfully recapitulate the phenotypic characteristics of the disease will be invaluable tools to analyze the effects of modifier genes and small molecule inhibitor therapies.

Murine models of polycystic kidney disease: molecular and therapeutic insights

Numerous murine (mouse and rat) models of polycystic kidney disease (PKD) have been described in which the mutant phenotype results from a spontaneous mutation or engineering via chemical mutagenesis, transgenic technologies, or gene-specific targeting in mouse orthologs of human PKD genes. These murine phenotypes closely resemble human PKD, with common abnormalities observed in tubular epithelia, the interstitial compartment, and the extracellular matrix of cystic kidneys. In both human and murine PKD, genetic background appears to modulate the renal cystic phenotype. In murine models, these putative modifying effects have been dissected into discrete factors called quantitative trait loci and genetically mapped. Several lines of experimental evidence support the hypothesis that PKD genes and their modifiers may define pathways involved in cystogenesis and PKD progression. Among the various pathway abnormalities described in murine PKD, recent provocative data indicate that structural and/or functional defects in the primary apical cilia of tubular epithelia may play a key role in PKD pathogenesis. This review describes the most widely studied murine models; highlights the data regarding specific gene defects and genetic modifiers; summarizes the data from these models that have advanced our understanding of PKD pathogenesis; and examines the effect of various therapeutic interventions in murine PKD.

A new in vitro bioassay for cyst formation by renal cells from an autosomal dominant rat model of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most frequent human inherited diseases. The main feature of the disease is the development of renal cysts, first occurring in the proximal tubules, and with time, dominating all segments of the nephron, leading to end-stage renal disease in 50% of the patients in their fifth decade of life. A therapy for polycystic kidney disease (PKD) has not yet been developed. Patients coming to end-stage ADPKD require long-term dialysis and/or transplantation. A suitable animal model to study ADPKD is the spontaneously mutated Han:SPRD (cy/+) rat, but a method to cultivate Han:SPRD (cy/+) derived renal cells which preserves their ability to form cyst-like structures in vitro has previously not been reported. Based on this well-characterized animal model, we developed a cell culture model of renal cyst formation in vitro. When renal cells of the Han:SPRD (cy/+) rat were isolated and cultured under conditions that prevent cell-substratum adhesion, large amounts of cyst-like structures were formed de novo from Han:SPRD (cy/+) derived renal cells, but only a few from control rat renal cells. In contrast, when cultivated on plastic as monolayer cultures, Han:SPRD (cy/+)-derived and control rat-derived renal cells were indistinguishable and did not form cyst-like structures. Immunohistochemical characterization of the cyst-like structures suggests tubular epithelial origin of the cyst-forming cells. The amount of cysts formed from Han:SPRD (cy/+)-derived renal cells grown in a stationary suspension culture is susceptible to modulation by different conditions. Human cyst fluid and epidermal growth factor both stimulated the formation of cysts from Han:SPRD (cy/+)-derived renal cells whereas taxol inhibited cystogenesis. In contrast, neither human cyst fluid nor epidermal growth factor affected the amount of cysts formed by control rat renal cells. As the culture model reported here allows not only the distinction of PKD-derived tubular epithelium from its normal counterpart, but also the modulation of cyst formation especially by Han:SPRD (cy/+)-derived renal cells, it might be a useful prescreening protocol for potential treatments for PKD and thus reduce the need for animal experiments.

New insights into polycystic kidney disease and its treatment

Major advances in the understanding of the genetics and pathogenesis of autosomal dominant polycystic kidney disease have occurred within the past year. The proteins encoded by the PKD1 and PKD2 genes, polycystin 1 and polycystin 2, are membrane proteins, capable of interacting physically in vitro, and are likely components of a complex signalling pathway. The majority of PKD1 and PKD2 mutations so far identified are unique inactivating mutations dispersed over the entire genes. Immunohistochemical studies have shown that polycystin 1 and polycystin 2 are developmentally regulated and are overexpressed in polycystic kidneys. The cysts probably result from clonal expansions of single cells. The demonstration of loss of heterozygosity for PKD1 and the absence of immunoreactive polycystin 1 in approximately 20% of the cysts supports a two-hit tumor suppressor gene model of cystogenesis. Regardless of the nature of the initial pathogenic mechanism, the cysts in autosomal dominant polycystic kidney disease are accompanied by partial dedifferentiation of the epithelial cells, disregulation of epithelial cell proliferation, expression of a secretory phenotype, and disarray of cell matrix interactions which leads to interstitial inflammation and matrix accumulation. Recent observations in animal models of inherited polycystic kidney disease have implicated oxidative stress in its pathogenesis. These downstream pathogenetic events have been targeted for intervention, and an increasing number of studies have demonstrated that the course of polycystic kidney disease in rodents can be altered by environmental and pharmacological interventions. Nevertheless, these experimental observations cannot be extrapolated to human autosomal dominant polycystic kidney disease. The recent generation of mice with PKD1 or PKD2 targeted mutations will help to bridge this gap.