Therapeutic targets for polycystic kidney disease

Recent Issues in the Development and Application of Targeted Therapies with Respect to Individual Animal Variability

This literature review explores the impact of molecular, genetic, and environmental factors on the efficacy of targeted therapies in veterinary medicine. Relevant studies were identified through systematic searches of PubMed, Web of Science, Scopus, and ScienceDirect using keywords such as "species-specific treatment strategies", "signalling pathways", "epigenetic and paragenetic influences", "targeted therapies", "veterinary medicine", "genetic variation", and "free radicals and oxidative stress". Inclusion criteria included studies focusing on species-specific therapeutic responses, genetic influences, and oxidative stress. To ensure that only the most recent and relevant evidence was included, only peer-reviewed publications from the last two decades were considered. Each study selected for analysis was critically appraised, with a particular emphasis on methodological quality, experimental design, and scientific contribution to the understanding of how environmental and biological factors influence therapeutic outcomes. A special emphasis was placed on studies that used a comparative, cross-species approach to assess variability in therapeutic responses and potential adverse effects. The review synthesises evidence on the role of epigenetic and paragenetic factors and highlights the importance of cross-species studies to understand how environmental and biological factors influence treatment outcomes. By highlighting genetic variation, oxidative stress, and individual species differences, the review argues for personalised and species-specific therapeutic approaches. The review emphasises that such an approach would improve veterinary care and inform future research aimed at optimising targeted therapies, ultimately leading to better animal health and treatment efficacy. A key contribution of the review is its emphasis on the need for more personalised treatment protocols that take into account individual genetic profiles and environmental factors; it also calls for a greater integration of cross-species studies.

Structural basis of tolvaptan binding to the vasopressin V2 receptor

The vasopressin V2 receptor (V2R) is a validated therapeutic target for autosomal dominant polycystic kidney disease (ADPKD), with tolvaptan being the first FDA-approved antagonist. Herein, we used Gaussian accelerated molecular dynamics simulations to investigate the spontaneous binding of tolvaptan to both active and inactive V2R conformations at the atomic-level. Overall, the binding process consists of two stages. Tolvaptan binds initially to extracellular loops 2 and 3 (ECL2/3) before overcoming an energy barrier to enter the pocket. Our simulations result highlighted key residues (e.g., R181, Y205, F287, F178) involved in this process, which were experimentally confirmed by site-directed mutagenesis. This work provides structural insights into tolvaptan-V2R interactions, potentially aiding the design of novel antagonists for V2R and other G protein-coupled receptors.

Role of cyclin-dependent kinase 2 in the progression of mouse juvenile cystic kidney disease

A hallmark of polycystic kidney diseases (PKDs) is aberrant proliferation, which leads to the formation and growth of renal cysts. Proliferation is mediated by cyclin-dependent kinases (Cdks), and the administration of roscovitine (a pan-Cdk inhibitor) attenuates renal cystic disease in juvenile cystic kidney (jck) mice. Cdk2 is a key regulator of cell proliferation, but its specific role in PKD remains unknown. The aim of this study was to test the hypothesis that Cdk2 deficiency reduces renal cyst growth in PKD. Three studies were undertaken: (i) a time course (days 28, 56, and 84) of cyclin and Cdk activity was examined in jck mice and compared with wild-type mice; (ii) the progression was compared in jck mice with or without Cdk2 ablation from birth; and (iii) the effect of sirolimus (an antiproliferative agent) on Cdk2 activity in jck mice was investigated. Renal disease in jck mice was characterized by diffuse tubular cyst growth, interstitial inflammation and fibrosis, and renal impairment, peaking on day 84. Renal cell proliferation peaked during earlier stages of disease (days 28-56), whereas the expression of Cdk2-cyclin partners (A and E) and Cdk1 and 2 activity, was maximal in the later stages of disease (days 56-84). Cdk2 ablation did not attenuate renal disease progression and was associated with persistent Cdk1 activity. In contrast, the postnatal treatment of jck mice with sirolimus reduced both Cdk2 and Cdk1 activity and reduced renal cyst growth. In conclusion, (i) the kinetics of Cdk2 and Cdk2-cyclin partners did not correlate with proliferation in jck mice; and (ii) the absence of Cdk2 did not alter renal cyst growth, most likely due to compensation by Cdk1. Taken together, these data suggest that Cdk2 is dispensable for the proliferation of cystic epithelial cells and progression of PKD.

Detecting Functional and Accessible Folate Receptor Expression in Cancer and Polycystic Kidneys

Folate-based small molecule drug conjugates (SMDCs) are currently under development and have shown promising preclinical and clinical results against various cancers and polycystic kidney disease. Two requisites for response to a folate-based SMDC are (i) folate receptor alpha (FRα) protein is expressed in the diseased tissues, and (ii) FRα in those tissues is accessible and functionally competent to bind systemically administered SMDCs. Here we report on the development of a small molecule reporter conjugate (SMRC), called EC2220, which is composed of a folate ligand for FRα binding, a multilysine containing linker that can cross-link to FRα in the presence of formaldehyde fixation, and a small hapten (fluorescein) used for immunohistochemical detection. Data show that EC2220 produces a far greater IHC signal in FRα-positive tissues over that produced with EC17, a folate-fluorescein SMRC that is released from the formaldehyde-denatured FRα protein. Furthermore, the extent of the EC2220 IHC signal was proportional to the level of FRα expression. This EC2220-based assay was qualified both in vitro and in vivo using normal tissue, cancer tissue, and polycystic kidneys. Overall, EC2220 is a sensitive and effective reagent for evaluating functional and accessible receptor expression in vitro and in vivo.

Comparison of folate-conjugated rapamycin versus unconjugated rapamycin in an orthologous mouse model of polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is a very common genetic disease leading to renal failure. Numerous aberrantly regulated signaling pathways have been identified as promising molecular drug targets for ADPKD therapy. In rodent models, many small-molecule drugs against such targets have proven effective in reducing renal cyst growth. For example, mammalian target of rapamycin (mTOR) inhibition with rapamycin greatly ameliorates renal cystic disease in several rodent models. However, clinical trials with mTOR inhibitors were disappointing largely due to the intolerable extrarenal side effects during long-term treatment with these drugs. Most other potential drug targets in ADPKD are also widely expressed in extrarenal tissues, which makes it likely that untargeted therapies with small-molecule inhibitors against such targets will lead to systemic adverse effects during the necessary long-term treatment of years and decades in ADPKD patients. To overcome this problem, we previously demonstrated that folate-conjugated rapamycin (FC-rapa) targets polycystic kidneys due to the high expression of the folate receptor (FRα) and that treatment of a nonortholgous PKD mouse model leads to inhibition of renal cyst growth. Here we show, in a head-to-head comparison with unconjugated rapamycin, that FCrapa inhibits renal cyst growth, mTOR activation, cell cycling, and fibrosis in an orthologous Pkd1 mouse model. Both unconjugated rapamycin and FC-rapa are similarly effective on polycystic kidneys in this model. However, FC-rapa lacks the extrarenal effects of unconjugated rapamycin, in particular immunosuppressive effects. We conclude that folate-conjugation is a promising avenue for increasing the tissue specificity of small-molecule compounds to facilitate very long-term treatment in ADPKD.