Insignificant effect of secretin in rodent models of polycystic kidney and liver disease

Enteroendocrine cells and gut hormones as potential targets in the crossroad of the gut-kidney axis communication

Recent studies suggest that disruptions in intestinal homeostasis, such as changes in gut microbiota composition, infection, and inflammatory-related gut diseases, can be associated with kidney diseases. For instance, genomic investigations highlight how susceptibility genes linked to IgA nephropathy are also correlated with the risk of inflammatory bowel disease. Conversely, investigations demonstrate that the use of short-chain fatty acids, produced through fermentation by intestinal bacteria, protects kidney function in models of acute and chronic kidney diseases. Thus, the dialogue between the gut and kidney seems to be crucial in maintaining their proper function, although the factors governing this crosstalk are still emerging as the field evolves. In recent years, a series of studies have highlighted the significance of enteroendocrine cells (EECs) which are part of the secretory lineage of the gut epithelial cells, as important components in gut-kidney crosstalk. EECs are distributed throughout the epithelial layer and release more than 20 hormones in response to microenvironment stimuli. Interestingly, some of these hormones and/or their pathways such as Glucagon-Like Peptide 1 (GLP-1), GLP-2, gastrin, and somatostatin have been shown to exert renoprotective effects. Therefore, the present review explores the role of EECs and their hormones as regulators of gut-kidney crosstalk and their potential impact on kidney diseases. This comprehensive exploration underscores the substantial contribution of EEC hormones in mediating gut-kidney communication and their promising potential for the treatment of kidney diseases.

Emerging therapies for autosomal dominant polycystic kidney disease with a focus on cAMP signaling

Autosomal dominant polycystic kidney disease (ADPKD), with an estimated genetic prevalence between 1:400 and 1:1,000 individuals, is the third most common cause of end stage kidney disease after diabetes mellitus and hypertension. Over the last 3 decades there has been great progress in understanding its pathogenesis. This allows the stratification of therapeutic targets into four levels, gene mutation and polycystin disruption, proximal mechanisms directly caused by disruption of polycystin function, downstream regulatory and signaling pathways, and non-specific pathophysiologic processes shared by many other diseases. Dysfunction of the polycystins, encoded by the PKD genes, is closely associated with disruption of calcium and upregulation of cyclic AMP and protein kinase A (PKA) signaling, affecting most downstream regulatory, signaling, and pathophysiologic pathways altered in this disease. Interventions acting on G protein coupled receptors to inhibit of 3',5'-cyclic adenosine monophosphate (cAMP) production have been effective in preclinical trials and have led to the first approved treatment for ADPKD. However, completely blocking cAMP mediated PKA activation is not feasible and PKA activation independently from cAMP can also occur in ADPKD. Therefore, targeting the cAMP/PKA/CREB pathway beyond cAMP production makes sense. Redundancy of mechanisms, numerous positive and negative feedback loops, and possibly counteracting effects may limit the effectiveness of targeting downstream pathways. Nevertheless, interventions targeting important regulatory, signaling and pathophysiologic pathways downstream from cAMP/PKA activation may provide additive or synergistic value and build on a strategy that has already had success. The purpose of this manuscript is to review the role of cAMP and PKA signaling and their multiple downstream pathways as potential targets for emergent therapies for ADPKD.

Genetics, pathobiology and therapeutic opportunities of polycystic liver disease

Polycystic liver diseases (PLDs) are inherited genetic disorders characterized by progressive development of intrahepatic, fluid-filled biliary cysts (more than ten), which constitute the main cause of morbidity and markedly affect the quality of life. Liver cysts arise in patients with autosomal dominant PLD (ADPLD) or in co-occurrence with renal cysts in patients with autosomal dominant or autosomal recessive polycystic kidney disease (ADPKD and ARPKD, respectively). Hepatic cystogenesis is a heterogeneous process, with several risk factors increasing the odds of developing larger cysts. Depending on the causative gene, PLDs can arise exclusively in the liver or in parallel with renal cysts. Current therapeutic strategies, mainly based on surgical procedures and/or chronic administration of somatostatin analogues, show modest benefits, with liver transplantation as the only potentially curative option. Increasing research has shed light on the genetic landscape of PLDs and consequent cholangiocyte abnormalities, which can pave the way for discovering new targets for therapy and the design of novel potential treatments for patients. Herein, we provide a critical and comprehensive overview of the latest advances in the field of PLDs, mainly focusing on genetics, pathobiology, risk factors and next-generation therapeutic strategies, highlighting future directions in basic, translational and clinical research.

Modulation of polycystic kidney disease by G-protein coupled receptors and cyclic AMP signaling

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a systemic disorder associated with polycystic liver disease (PLD) and other extrarenal manifestations, the most common monogenic cause of end-stage kidney disease, and a major burden for public health. Many studies have shown that alterations in G-protein and cAMP signaling play a central role in its pathogenesis. As for many other diseases (35% of all approved drugs target G-protein coupled receptors (GPCRs) or proteins functioning upstream or downstream from GPCRs), treatments targeting GPCR have shown effectiveness in slowing the rate of progression of ADPKD. Tolvaptan, a vasopressin V2 receptor antagonist is the first drug approved by regulatory agencies to treat rapidly progressive ADPKD. Long-acting somatostatin analogs have also been effective in slowing the rates of growth of polycystic kidneys and liver. Although no treatment has so far been able to prevent the development or stop the progression of the disease, these encouraging advances point to G-protein and cAMP signaling as a promising avenue of investigation that may lead to more effective and safe treatments. This will require a better understanding of the relevant GPCRs, G-proteins, cAMP effectors, and of the enzymes and A-kinase anchoring proteins controlling the compartmentalization of cAMP signaling. The purpose of this review is to provide an overview of general GPCR signaling; the function of polycystin-1 (PC1) as a putative atypical adhesion GPCR (aGPCR); the roles of PC1, polycystin-2 (PC2) and the PC1-PC2 complex in the regulation of calcium and cAMP signaling; the cross-talk of calcium and cAMP signaling in PKD; and GPCRs, adenylyl cyclases, cyclic nucleotide phosphodiesterases, and protein kinase A as therapeutic targets in ADPKD.

Oxidative Stress and Mitochondrial Abnormalities Contribute to Decreased Endothelial Nitric Oxide Synthase Expression and Renal Disease Progression in Early Experimental Polycystic Kidney Disease

Vascular abnormalities are the most important non-cystic complications in Polycystic Kidney Disease (PKD) and contribute to renal disease progression. Endothelial dysfunction and oxidative stress are evident in patients with ADPKD, preserved renal function, and controlled hypertension. The underlying biological mechanisms remain unknown. We hypothesized that in early ADPKD, the reactive oxygen species (ROS)-producing nicotinamide adenine dinucleotide phosphate hydrogen (NAD(P)H)-oxidase complex-4 (NOX4), a major source of ROS in renal tubular epithelial cells (TECs) and endothelial cells (ECs), induces EC mitochondrial abnormalities, contributing to endothelial dysfunction, vascular abnormalities, and renal disease progression. Renal oxidative stress, mitochondrial morphology (electron microscopy), and NOX4 expression were assessed in 4- and 12-week-old PCK and Sprague-Dawley (wild-type, WT) control rats (n = 8 males and 8 females each). Endothelial function was assessed by renal expression of endothelial nitric oxide synthase (eNOS). Peritubular capillaries were counted in hematoxylin-eosin (H&E)-stained slides and correlated with the cystic index. The enlarged cystic kidneys of PCK rats exhibited significant accumulation of 8-hydroxyguanosine (8-OHdG) as early as 4 weeks of age, which became more pronounced at 12 weeks. Mitochondria of TECs lining cysts and ECs exhibited loss of cristae but remained preserved in non-cystic TECs. Renal expression of NOX4 was upregulated in TECs and ECs of PCK rats at 4 weeks of age and further increased at 12 weeks. Contrarily, eNOS immunoreactivity was lower in PCK vs. WT rats at 4 weeks and further decreased at 12 weeks. The peritubular capillary index was lower in PCK vs. WT rats at 12 weeks and correlated inversely with the cystic index. Early PKD is associated with NOX4-induced oxidative stress and mitochondrial abnormalities predominantly in ECs and TECs lining cysts. Endothelial dysfunction precedes capillary loss, and the latter correlates with worsening of renal disease. These observations position NOX4 and EC mitochondria as potential therapeutic targets in PKD.

Therapeutic Targets in Polycystic Liver Disease

Polycystic liver diseases (PLD) are a group of genetic disorders initiated by mutations in several PLD-related genes and characterized by the presence of multiple cholangiocyte-derived hepatic cysts that progressively replace liver tissue. PLD co-exists with Autosomal Dominant Polycystic Kidney Disease (ADPKD) and Autosomal Recessive PKD as well as occurs alone (i.e., Autosomal Dominant Polycystic Liver Disease [ADPLD]). PLD associated with ADPKD and ARPKD belong to a group of disorders known as cholangiociliopathies since many disease-causative and disease-related proteins are expressed in primary cilia of cholangiocytes. Aberrant expression of these proteins in primary cilia affects their structures and functions promoting cystogenesis. Current medical therapies for PLD include symptomatic management and surgical interventions. To date, the only available drug treatment for PLD patients that halt disease progression and improve quality of life are somatostatin analogs. However, the modest clinical benefits, need for long-term maintenance therapy, and the high cost of treatment justify the necessity for more effective treatment options. Substantial evidence suggests that experimental manipulations with components of the signaling pathways that influence cyst development (e.g., cAMP, intracellular calcium, receptor tyrosine kinase, transient receptor potential cation channel subfamily V member 4 (TRPV4) channel, mechanistic target of rapamycin (mTOR), histone deacetylase (HDAC6), Cdc25A phosphatase, miRNAs and metalloproteinases) attenuate growth of hepatic cysts. Many of these targets have been evaluated in pre-clinical trials suggesting their value as potential new therapies. This review outlines the current clinical and preclinical treatment strategies for PLD.

Rationale for early treatment of polycystic kidney disease

In hereditary cystic disorders, renal injury begins with the formation of the first cyst. Renal injury may manifest as large kidneys, abdominal pain, hypertension and hematuria in children and young adults with autosomal dominant polycystic kidney disease (ADPKD). In autosomal recessive PKD (ARPKD) and ADPKD, cysts form primarily in collecting ducts and expand progressively. Collecting duct cysts that block urine flow have the potential to block urine formation in large numbers of upstream nephrons. In an ARPKD rat congenitally lacking vasopressin, only a few cysts developed until exogenous arginine vasopressin (AVP) was administered. AVP elevates cyclic AMP in vulnerable tubule cells to stimulate mitogenesis and fluid secretion, thereby causing cysts to form and enlarge indefinitely. The administration of an AVP-V2 receptor inhibitor or the consumption of sufficient water to persistently lower plasma AVP levels will ameliorate disease progression. Renal volume measurements provide the most reliable way to forecast long-term outcome in individual children and adult patients with ADPKD. Many drugs that have demonstrated efficacy in small clinical trials, preclinical trials and cell-based studies are in the treatment pipeline. Counseling, regular exercise, limitation of dietary calories, salt, protein and fat, increased fluid intake throughout the day and treatment of hypertension are components of a rational treatment program that can be offered at an early age to those with, or at risk for developing PKD.

Tolvaptan plus pasireotide shows enhanced efficacy in a PKD1 model

Autosomal dominant polycystic kidney disease (ADPKD) is a leading cause of ESRD. A central defect associated with ADPKD pathology is elevated levels of 3', 5'-cyclic AMP (cAMP). Compounds such as tolvaptan and pasireotide, which indirectly reduce adenylyl cyclase 6 (AC6) activity, have hence proven effective in slowing cyst progression. Here, we tested the efficacy of these compounds individually and in combination in a hypomorphic PKD1 model, Pkd1(R3277C/R3277C) (Pkd1(RC/RC)), in a 5-month preclinical trial. Initially, the Pkd1(RC/RC) model was inbred into the C57BL/6 background, minimizing disease variability, and the pathogenic effect of elevating cAMP was confirmed by treatment with the AC6 stimulant desmopressin. Treatment with tolvaptan or pasireotide alone markedly reduced cyst progression and in combination showed a clear additive effect. Furthermore, combination treatment significantly reduced cystic and fibrotic volume and decreased cAMP to wild-type levels. We also showed that Pkd1(RC/RC) mice experience hepatic hypertrophy that can be corrected by pasireotide. The observed additive effect reinforces the central role of AC6 and cAMP in ADPKD pathogenesis and highlights the likely benefit of combination therapy for patients with ADPKD.

Polycystic liver diseases: advanced insights into the molecular mechanisms

Polycystic liver diseases are genetic disorders characterized by progressive bile duct dilatation and/or cyst development. The large volume of hepatic cysts causes different symptoms and complications such as abdominal distension, local pressure with back pain, hypertension, gastro-oesophageal reflux and dyspnea as well as bleeding, infection and rupture of the cysts. Current therapeutic strategies are based on surgical procedures and pharmacological management, which partially prevent or ameliorate the disease. However, as these treatments only show short-term and/or modest beneficial effects, liver transplantation is the only definitive therapy. Therefore, interest in understanding the molecular mechanisms involved in disease pathogenesis is increasing so that new targets for therapy can be identified. In this Review, the genetic mechanisms underlying polycystic liver diseases and the most relevant molecular pathways of hepatic cystogenesis are discussed. Moreover, the main clinical and preclinical studies are highlighted and future directions in basic as well as clinical research are indicated.

The physiological roles of secretin and its receptor

Secretin is secreted by S cells in the small intestine and affects the function of a number of organ systems. Secretin receptors (SR) are expressed in the basolateral domain of several cell types. In addition to regulating the secretion of a number of epithelia (e.g., in the pancreas and biliary epithelium in the liver), secretin exerts trophic effects in several cell types. In this article, we will provide a comprehensive review on the multiple roles of secretin and SR signaling in the regulation of epithelial functions in various organ systems with particular emphasis in the liver. We will discuss the role of secretin and its receptor in health and biliary disease pathogenesis. Finally, we propose future areas of research for the further evaluation of the secretin/secretin receptor axis in liver pathophysiology.

An mTOR anti-sense oligonucleotide decreases polycystic kidney disease in mice with a targeted mutation in Pkd2

Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening hereditary disease in the USA. In human ADPKD studies, sirolimus, a mammalian target of rapamycin complex 1 (mTORC1) inhibitor, had little therapeutic effect. While sirolimus robustly inhibits mTORC1, it has a minimal effect on mTOR complex 2 (mTORC2). Polycystic kidneys of Pkd2WS25/- mice, an orthologous model of human ADPKD caused by a mutation in the Pkd2 gene, had an early increase in pS6 (marker of mTORC1) and pAktSer(473) (marker of mTORC2). To investigate the effect of combined mTORC1 and 2 inhibition, Pkd2WS25/- mice were treated with an mTOR anti-sense oligonucleotide (ASO) that blocks mTOR expression thus inhibiting both mTORC1 and 2. The mTOR ASO resulted in a significant decrease in mTOR protein, pS6 and pAktSer(473). Pkd2WS25/- mice treated with the ASO had a normalization of kidney weights and kidney function and a marked decrease in cyst volume. The mTOR ASO resulted in a significant decrease in proliferation and apoptosis of tubular epithelial cells. To demonstrate the role of mTORC2 on cyst growth, Rictor, the functional component of mTORC2, was silenced in Madin-Darby canine kidney cell cysts grown in 3D cultures. Silencing Rictor significantly decreased cyst volume and expression of pAktSer(473). The decreased cyst size in the Rictor silenced cells was reversed by introduction of a constitutively active Akt1. In vitro, combined mTORC1 and 2 inhibition reduced cyst growth more than inhibition of mTORC1 or 2 alone. In conclusion, combined mTORC1 and 2 inhibition has therapeutic potential in ADPKD.

Combination of secretin and fluvastatin ameliorates the polyuria associated with X-linked nephrogenic diabetes insipidus in mice

X-linked nephrogenic diabetes insipidus (X-NDI) is a disease caused by inactivating mutations of the vasopressin (AVP) type 2 receptor (V2R) gene. Loss of V2R function prevents plasma membrane expression of the AQP2 water channel in the kidney collecting duct cells and impairs the kidney concentration ability. In an attempt to develop strategies to bypass V2R signaling in X-NDI, we evaluated the effects of secretin and fluvastatin, either alone or in combination, on kidney function in a mouse model of X-NDI. The secretin receptor was found to be functionally expressed in the kidney collecting duct cells. Based on this, X-NDI mice were infused with secretin for 14 days but urinary parameters were not altered by the infusion. Interestingly, secretin significantly increased AQP2 levels in the collecting duct but the protein primarily accumulated in the cytosol. Since we previously reported that fluvastatin treatment increased AQP2 plasma membrane expression in wild-type mice, secretin-infused X-NDI mice received a single injection of fluvastatin. Interestingly, urine production by X-NDI mice treated with secretin plus fluvastatin was reduced by nearly 90% and the urine osmolality was doubled. Immunostaining showed that secretin increased intracellular stores of AQP2 and the addition of fluvastatin promoted AQP2 trafficking to the plasma membrane. Taken together, these findings open new perspectives for the pharmacological treatment of X-NDI.

Osmoregulation, vasopressin, and cAMP signaling in autosomal dominant polycystic kidney disease

PURPOSE OF REVIEW

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent inherited nephropathy. This review will focus on the vasopressin and 3'-5'-cyclic adenosine monophosphate (cAMP) signaling pathways in ADPKD and will discuss how these insights offer new possibilities for the follow-up and treatment of the disease.

RECENT FINDINGS

Defective osmoregulation is an early manifestation of ADPKD and originates from both peripheral (renal effect of vasopressin) and central (release of vasopressin) components. Copeptin, which is released from the vasopressin precursor, may identify ADPKD patients at risk for rapid disease progression. Increased levels of cAMP in tubular cells, reflecting modifications in intracellular calcium homeostasis and abnormal stimulation of the vasopressin V2 receptor (V2R), play a central role in cystogenesis. Blocking the V2R lowers cAMP in cystic tissues, slows renal cystic progression and improves renal function in preclinical models. A phase III clinical trial investigating the effect of the V2R antagonist tolvaptan in ADPKD patients has shown that this treatment blunts kidney growth, reduces associated symptoms and slows kidney function decline when given over 3 years.

SUMMARY

These advances open perspectives for the understanding of cystogenesis in ADPKD, the mechanisms of osmoregulation, the role of polycystins in the brain, and the pleiotropic action of vasopressin.

Recent advances in the morphological and functional heterogeneity of the biliary epithelium

This review focuses on the recent advances related to the heterogeneity of different-sized bile ducts with regard to the morphological and phenotypical characteristics, and the differential secretory, apoptotic and proliferative responses of small and large cholangiocytes to gastrointestinal hormones/peptides, neuropeptides and toxins. We describe several in vivo and in vitro models used for evaluating biliary heterogeneity. Subsequently, we discuss the heterogeneous proliferative and apoptotic responses of small and large cholangiocytes to liver injury and the mechanisms regulating the differentiation of small into large (more differentiated) cholangiocytes. Following a discussion on the heterogeneity of stem/progenitor cells in the biliary epithelium, we outline the heterogeneity of bile ducts in human cholangiopathies. After a summary section, we discuss the future perspectives that will further advance the field of the functional heterogeneity of the biliary epithelium.