Extracellular signal-regulated kinase inhibition slows disease progression in mice with polycystic kidney disease

Mediation of angiotensin II-induced Ca2+ signaling by polycystin 2 in glomerular mesangial cells

Ca(+) influx across the plasma membrane is a major component of mesangial cell (MC) response to vasoconstrictors. Polycystin 2 (PC2), the protein product of the gene mutated in type 2 autosomal dominant polycystic kidney disease, has been shown to function as a nonselective cation channel in a variety of cell types. The present study was performed to test the hypothesis that PC2 and its binding partners constitute a Ca(2+)-permeable channel and contribute to ANG II-induced Ca(2+) signaling in MCs. Western blot and immunocytochemistry showed PC2 expression in cultured human MCs. The existence of PC2 in MCs was further confirmed by immunohistochemsitry in rat kidney sections. Coimmunoprecipitation displayed a selective interaction of PC2 with canonical transient receptor potential (TRPC) proteins TRPC1 and TRPC4. Cell-attached patch-clamp experiments revealed that ANG II-induced membrane currents were enhanced by overexpression of pkd2 but significantly inhibited by knock down of pkd2, 30 microM Gd(3+) (a PC2 channel blocker), and dominant-negative pkd2 mutant (pkd2-D511V). Corresponding to the increase in channel currents, ANG II stimulation increased expression of PC2 on the cell surface of MCs and interaction with TRPC1 and TRPC4. Furthermore, ANG II-induced MC contraction was significantly reduced in pkd2-knocked down MCs. These data suggest that PC2 selectively assembles with TRPC1 and TRPC4 to form channel complexes mediating ANG II-induced Ca(2+) responses in MCs.

Tracking kidney volume in mice with polycystic kidney disease by magnetic resonance imaging

Polycystic kidney disease is characterized by the progressive enlargement of kidneys due to expanding fluid-filled cysts with the rate of renal volume increase held to be a marker of disease progression. Magnetic resonance imaging (MRI) is used to monitor changes in renal volume in patients with polycystic kidney disease; however, it has not been effectively used in mice to monitor changes in kidney volume during drug treatment studies. We used a powerful 9.4-T horizontal bore magnetic resonance scanner to track changes in kidney volume in pcy/pcy mice, an ortholog of nephronophthisis type 3. Mice were sequentially scanned from 4 to 30 weeks of age and kidney volumes determined from high-resolution images. Kidney volume and maximal cross-sectional surface area correlated positively with kidney weight and the histologic determination of surface area. The increase in kidney volume was exponential up to 20 weeks of age, after which there was a plateau consistent with the replacement of normal parenchyma by fibrotic tissue. Our study demonstrates that MRI accurately determines the rate of kidney volume increase in mice with polycystic kidney disease and hence may be useful in assessing the effectiveness of therapeutic agents to slow disease progression.

Autosomal dominant polycystic kidney disease: recent advances in pathogenesis and treatment

Autosomal dominant polycystic kidney disease (ADPKD) is a common genetic disorder affecting 1 in 1,000 people in the general population and accounts for up to 10% of all patients on renal replacement therapy. Numerous fluid-filled epithelial cysts arise from different nephron segments as spherical dilatations or small out-pouchings, enlarge progressively and eventually become disconnected from the rest of the renal tubule. The development of cysts is accompanied by destruction of the renal parenchyma, interstitial fibrosis, cellular infiltration and loss of functional nephrons. ADPKD is not only a kidney disease but also a systemic disorder associated with intracranial arterial aneurysms, cardiac valvular defects, colonic diverticulosis and cyst formation in other organs such as the liver, spleen and pancreas. The identification of PKD1 and PKD2 together with the drive to elucidate the functions of their encoded proteins, polycystin-1 (PC1) and polycystin-2 (PC2), has led to an explosion of clinical and scientific interest in this common disorder. The aim of this review is to highlight recent advances in our understanding of ADPKD pathogenesis which are leading to exciting new treatment strategies.

Acute downregulation of ENaC by EGF involves the PY motif and putative ERK phosphorylation site

The epithelial sodium channel (ENaC) is expressed in a variety of tissues, including the renal collecting duct, where it constitutes the rate-limiting step for sodium reabsorption. Liddle's syndrome is caused by gain-of-function mutations in the β and γ subunits of ENaC, resulting in enhanced Na reabsorption and hypertension. Epidermal growth factor (EGF) causes acute inhibition of Na absorption in collecting duct principal cells via an extracellular signal–regulated kinase (ERK)–dependent mechanism. In experiments with primary cultures of collecting duct cells derived from a mouse model of Liddle's disease (β-ENaC truncation), it was found that EGF inhibited short-circuit current (Isc) by 24 ± 5% in wild-type cells but only by 6 ± 3% in homozygous mutant cells. In order to elucidate the role of specific regions of the β-ENaC C terminus, Madin-Darby canine kidney (MDCK) cell lines that express β-ENaC with mutation of the PY motif (P616L), the ERK phosphorylation site (T613A), and C terminus truncation (R564stop) were created using the Phoenix retroviral system. All three mutants exhibited significant attenuation of the EGF-induced inhibition of sodium current. In MDCK cells with wild-type β-ENaC, EGF-induced inhibition of Isc (<30 min) was fully reversed by exposure to an ERK kinase inhibitor and occurred with no change in ENaC surface expression, indicative of an effect on channel open probability (P_o). At later times (>30 min), EGF-induced inhibition of Isc was not reversed by an ERK kinase inhibitor and was accompanied by a decrease in ENaC surface expression. Our results are consistent with an ERK-mediated decrease in ENaC open probability and enhanced retrieval of sodium channels from the apical membrane.

When cilia go bad: cilia defects and ciliopathies

Defects in the function of cellular organelles such as peroxisomes, lysosomes and mitochondria are well-known causes of human diseases. Recently, another organelle has also been added to this list. Cilia--tiny hair-like organelles attached to the cell surface--are located on almost all polarized cell types of the human body and have been adapted as versatile tools for various cellular functions, explaining why cilia-related disorders can affect many organ systems. Several molecular mechanisms involved in cilia-related disorders have been identified that affect the structure and function of distinct cilia types.

The management of CKD: a look into the future

The increasing global prevalence of chronic kidney disease (CKD) and end-stage renal disease with the associated spiraling cost has profound public health and economic implications. This has made slowing the progression of CKD, a major health-care priority. CKD is invariably characterized by progressive kidney fibrosis and at present, treatment aiming to slow the progression of CKD is limited to aggressive blood pressure control, with few therapies targeting the fibrotic process itself. In this review, we explore the potential of experimental therapeutic strategies, based on preventing or reversing the pathophysiologic steps of kidney remodeling that lead to fibrosis.

Autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease is the most prevalent, potentially lethal, monogenic disorder. It is associated with large interfamilial and intrafamilial variability, which can be explained to a large extent by its genetic heterogeneity and modifier genes. An increased understanding of the disorder's underlying genetic, molecular, and cellular mechanisms and a better appreciation of its progression and systemic manifestations have laid out the foundation for the development of clinical trials and potentially effective treatments.

Long-lasting arrest of murine polycystic kidney disease with CDK inhibitor roscovitine

Polycystic kidney diseases (PKDs) are primarily characterized by the growth of fluid-filled cysts in renal tubules leading to end-stage renal disease. Mutations in the PKD1 or PKD2 genes lead to autosomal dominant PKD (ADPKD), a slowly developing adult form. Autosomal recessive polycystic kidney disease results from mutations in the PKHD1 gene, affects newborn infants and progresses very rapidly. No effective treatment is currently available for PKD. A previously unrecognized site of subcellular localization was recently discovered for all proteins known to be disrupted in PKD: primary cilia. Because ciliary functions seem to be involved in cell cycle regulation, disruption of proteins associated with cilia or centrioles may directly affect the cell cycle and proliferation, resulting in cystic disease. We therefore reasoned that the dysregulated cell cycle may be the most proximal cause of cystogenesis, and that intervention targeted at this point could provide significant therapeutic benefit for PKD. Here we show that treatment with the cyclin-dependent kinase (CDK) inhibitor (R)-roscovitine does indeed yield effective arrest of cystic disease in jck and cpk mouse models of PKD. Continuous daily administration of the drug is not required to achieve efficacy; pulse treatment provides a robust, long-lasting effect, indicating potential clinical benefits for a lifelong therapy. Molecular studies of the mechanism of action reveal effective cell-cycle arrest, transcriptional inhibition and attenuation of apoptosis. We found that roscovitine is active against cysts originating from different parts of the nephron, a desirable feature for the treatment of ADPKD, in which cysts form in multiple nephron segments. Our results indicate that inhibition of CDK is a new and effective approach to the treatment of PKD.