Recent Advances of mTOR Inhibitors Use in Autosomal Dominant Polycystic Kidney Disease: Is the Road Still Open?

Inspiring Tactics with the Improvement of Mitophagy and Redox Balance for the Development of Innovative Treatment against Polycystic Kidney Disease

Polycystic kidney disease (PKD) is the most common genetic form of chronic kidney disease (CKD), and it involves the development of multiple kidney cysts. Not enough medical breakthroughs have been made against PKD, a condition which features regional hypoxia and activation of the hypoxia-inducible factor (HIF) pathway. The following pathology of CKD can severely instigate kidney damage and/or renal failure. Significant evidence verifies an imperative role for mitophagy in normal kidney physiology and the pathology of CKD and/or PKD. Mitophagy serves as important component of mitochondrial quality control by removing impaired/dysfunctional mitochondria from the cell to warrant redox homeostasis and sustain cell viability. Interestingly, treatment with the peroxisome proliferator-activated receptor-α (PPAR-α) agonist could reduce the pathology of PDK and might improve the renal function of the disease via the modulation of mitophagy, as well as the condition of gut microbiome. Suitable modulation of mitophagy might be a favorable tactic for the prevention and/or treatment of kidney diseases such as PKD and CKD.

Calcium signaling in polycystic kidney disease- cell death and survival

Polycystic kidney disease is typified by cysts in the kidney and extra-renal manifestations including hypertension and heart failure. The main genetic underpinning this disease are loss-of function mutations to the two polycystin proteins, polycystin 1 and polycystin 2. Molecularly, the disease is characterized by changes in multiple signaling pathways including down regulation of calcium signaling, which, in part, is contributed by the calcium permeant properties of polycystin 2. These signaling pathways enable the cystic cells to survive and avoid cell death. This review focuses on the studies that have emerged in the past 5 years describing how the structural insights gained from PC-1 and PC-2 inform the calcium dependent molecular pathways of autophagy and the unfolded protein response that are regulated by the polycystin proteins and how it leads to cell survival and/or cell death.

Metabolomics Profiling of Cystic Renal Disease towards Biomarker Discovery

Simple Summary

Cystic renal disease (CRD) is a group of diseases characterized by abnormal sacs, or cysts, in the kidneys. CRD can be detected using certain imaging modalities (i.e., ultrasound). Patients with CRD might be symptoms-free, while others can show symptoms long after cysts development. Although these cysts represent structural changes, we hypothesized that they have an underlying biochemical alteration. If so, this would open the floor for potential biomarker discovery, which would aid in CRD diagnosis and, possibly, treatment. On that basis, this study focuses on identifying biomarkers for CRD. To achieve that, we employed a metabolomics-based approach to identify intermediate molecules inside the cells that are byproducts of biochemical reactions. We used dry blood spots and serum samples of CRD patients and healthy controls to study the differences in their metabolomic profile. Our results suggest that certain metabolites, including uridine diphosphate, cystine-5-diphosphate, and morpholine, are potential biomarkers for CRD. The affected biochemical pathways in CRD include aminoacyl-tRNA biosynthesis, purine, pyrimidine, glutathione, TCA cycle, and some amino acid metabolism. These preliminary results could be the starting point for possible diagnostic and therapeutic approaches for CRD in the future.

Abstract

Cystic renal disease (CRD) comprises a heterogeneous group of genetic and acquired disorders. The cystic lesions are detected through imaging, either incidentally or after symptoms develop, due to an underlying disease process. In this study, we aim to study the metabolomic profiles of CRD patients for potential disease-specific biomarkers using unlabeled and labeled metabolomics using low and high-resolution mass spectrometry (MS), respectively. Dried-blood spot (DBS) and serum samples, collected from CRD patients and healthy controls, were analyzed using the unlabeled and labeled method. The metabolomics profiles for both sets of samples and groups were collected, and their data were processed using the lab’s standard protocol. The univariate analysis showed (FDR p < 0.05 and fold change 2) was significant to show a group of potential biomarkers for CRD discovery, including uridine diphosphate, cystine-5-diphosphate, and morpholine. Several pathways were involved in CRD patients based on the metabolic profile, including aminoacyl-tRNA biosynthesis, purine and pyrimidine, glutathione, TCA cycle, and some amino acid metabolism (alanine, aspartate and glutamate, arginine and tryptophan), which have the most impact. In conclusion, early CRD detection and treatment is possible using a metabolomics approach that targets alanine, aspartate, and glutamate pathway metabolites.

Effect of Reducing Ataxia-Telangiectasia Mutated (ATM) in Experimental Autosomal Dominant Polycystic Kidney Disease

The DNA damage response (DDR) pathway is upregulated in autosomal dominant polycystic kidney disease (ADPKD) but its functional role is not known. The ataxia-telangiectasia mutated (ATM) and AT and Rad3-related (ATR) protein kinases are key proximal transducers of the DDR. This study hypothesized that reducing either ATM or ATR attenuates kidney cyst formation and growth in experimental ADPKD. In vitro, pharmacological ATM inhibition by AZD0156 reduced three-dimensional cyst growth in MDCK and human ADPKD cells by up to 4.4- and 4.1-fold, respectively. In contrast, the ATR inhibitor, VE-821, reduced in vitro MDCK cyst growth but caused dysplastic changes. In vivo, treatment with AZD0156 by oral gavage for 10 days reduced renal cell proliferation and increased p53 expression in Pkd1^RC/RC mice (a murine genetic ortholog of ADPKD). However, the progression of cystic kidney disease in Pkd1^RC/RC mice was not altered by genetic ablation of ATM from birth, in either heterozygous (Pkd1^RC/RC/Atm^+/−) or homozygous (Pkd1^RC/RC/Atm^−/−) mutant mice at 3 months. In conclusion, despite short-term effects on reducing renal cell proliferation, chronic progression was not altered by reducing ATM in vivo, suggesting that this DDR kinase is dispensable for kidney cyst formation in ADPKD.

Autophagy in kidney homeostasis and disease

Autophagy is a conserved lysosomal pathway for the degradation of cytoplasmic components. Basal autophagy in kidney cells is essential for the maintenance of kidney homeostasis, structure and function. Under stress conditions, autophagy is altered as part of the adaptive response of kidney cells, in a process that is tightly regulated by signalling pathways that can modulate the cellular autophagic flux - mammalian target of rapamycin, AMP-activated protein kinase and sirtuins are key regulators of autophagy. Dysregulated autophagy contributes to the pathogenesis of acute kidney injury, to incomplete kidney repair after acute kidney injury and to chronic kidney disease of varied aetiologies, including diabetic kidney disease, focal segmental glomerulosclerosis and polycystic kidney disease. Autophagy also has a role in kidney ageing. However, questions remain about whether autophagy has a protective or a pathological role in kidney fibrosis, and about the precise mechanisms and signalling pathways underlying the autophagy response in different types of kidney cells and across the spectrum of kidney diseases. Further research is needed to gain insights into the regulation of autophagy in the kidneys and to enable the discovery of pathway-specific and kidney-selective therapies for kidney diseases and anti-ageing strategies.