Polycystin-1 dependent regulation of polycystin-2 via GRP94, a member of HSP90 family that resides in the endoplasmic reticulum

The structural and functional dynamics of BiP and Grp94: opportunities for therapeutic discovery

Binding immunoglobulin protein (BiP) and glucose-regulated protein 94 (Grp94) are endoplasmic reticulum (ER)-localized molecular chaperones that ensure proper protein folding and maintain protein homeostasis. However, overexpression of these chaperones during ER stress can contribute to disease progression in numerous pathologies. Although these chaperones represent promising therapeutic targets, their inhibition has been challenged by gaps in understanding of targetable chaperone features and their complex biology. To overcome these challenges, a new assay has been developed to selectively target BiP, and compounds that exploit subtle conformational changes of Grp94 have been designed. This review summarizes recent advances in elucidating structural and functional dynamics of BiP and Grp94. We explore leveraging this information to develop novel therapeutic interventions. Finally, given the recent advances in computing, we discuss how machine learning methods can be used to accelerate drug discovery efforts.

Histone deacetylases and their inhibitors in kidney diseases

Histone deacetylases (HDACs) have emerged as key regulators in the pathogenesis of various kidney diseases. This review explores recent advancements in HDAC research, focusing on their role in kidney development and their critical involvement in the progression of chronic kidney disease (CKD), acute kidney injury (AKI), autosomal dominant polycystic kidney disease (ADPKD), and diabetic kidney disease (DKD). It also discusses the therapeutic potential of HDAC inhibitors in treating these conditions. Various HDAC inhibitors have shown promise by targeting specific HDAC isoforms and modulating a range of biological pathways. Their protective effects include modulation of apoptosis, autophagy, inflammation, and fibrosis, underscoring their broad therapeutic potential for kidney diseases. However, further research is essential to improve the selectivity of HDAC inhibitors, minimize toxicity, overcome drug resistance, and enhance their pharmacokinetic properties. This review offers insights to guide future research and prevention strategies for kidney disease management.

Alisol B 23-Acetate Down-Regulated GRP94 to Restore Endoplasmic Reticulum Homeostasis on Non-Alcoholic Steatohepatitis

Non-alcoholic steatohepatitis (NASH) poses a serious threat to human health. Alisol B 23-Acetate (AB23A) has shown beneficial effects on NASH, but its mechanism of action remains unclear. We conducted in vitro experiments by inducing L02 cell damage with free fatty acids (FFA) and administering various concentrations of AB23A. We found that AB23A intervention reduced triglyceride (TG) levels in FFA-induced L02 cells and improved cellular steatosis. Transcriptomic analysis revealed that AB23A intervention significantly downregulated glucose-regulated protein 94 (Grp94), indicating that AB23A primarily regulates the protein processing pathway in the endoplasmic reticulum. Within this pathway, AB23A intervention also significantly downregulated endoplasmic reticulum stress (ERS)-related genes (PERK, eIF2α, ATF4) and ER-associated degradation (ERAD)-related genes (FBXO2, DERL, HSP90AA1). When we silenced GRP94, the regulatory effects of AB23A on TG levels, cellular steatosis, ERS-related proteins (p-PERK/PERK, p-eIF2α/eIF2α, ATF4), and ERAD-related proteins (FBXO2, DERL, HSP90α) disappeared. In vivo, AB23A intervention promoted recovery of the liver index in NASH mice, reduced hepatic inflammatory infiltration and lipid deposition, improved serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities, and reduced liver TG levels. RT-qPCR and Western blot results demonstrated that AB23A intervention dose-dependently downregulated the gene and protein expression of GRP94 and ERS- and ERAD-related factors. There was no significant difference between the effects of high-dose AB23A intervention and PPC intervention. This study demonstrated, through both in vitro and in vivo experiments, that AB23A improves hepatic steatosis. This effect may be related to the downregulation of GRP94, which suppresses ERS and ERAD, thereby restoring ER homeostasis.

Identification of polycystin 2 missense mutants targeted for endoplasmic reticulum-associated degradation

Autosomal dominant polycystic kidney disease (ADPKD) is a common genetic disorder leading to end-stage renal disease. ADPKD arises from mutations in the PKD1 and PKD2 genes, which encode polycystin 1 (PC1) and polycystin 2 (PC2), respectively. PC2 is a nonselective cation channel, and disease-linked mutations disrupt normal cellular processes, including signaling and fluid secretion. In this study, we investigate whether disease-causing missense mutations compromise PC2 folding, an event that can lead to endoplasmic reticulum-associated degradation (ERAD). To this end, we first developed a new yeast PC2 expression system. We show that the yeast system provides a tractable model to investigate PC2 biogenesis and that a disease-associated PC2 mutant, D511V, exhibits increased polyubiquitination and accelerated proteasome-dependent degradation compared with wild-type PC2. In contrast to wild-type PC2, the PC2 D511V variant also failed to improve the growth of yeast strains that lack endogenous potassium transporters, highlighting a loss of channel function at the cell surface and a new assay for loss-of-function PKD2 variants. In HEK293 cells, both D511V along with another disease-associated mutant, R322Q, were targeted for ERAD. Consistent with defects in protein folding, the surface localization of these PC2 variants was increased by incubation at low-temperature in HEK293 cells, underscoring the potential to pharmacologically rescue these and perhaps other misfolded PC2 alleles. Together, our study supports the hypothesis that select PC2 missense variants are degraded by ERAD, the potential for screening PKD2 alleles in a new genetic system, and the possibility that chemical chaperone-based therapeutic interventions might be used to treat ADPKD.NEW & NOTEWORTHY This study indicates that select missense mutations in PC2, a protein that when mutated leads to ADPKD, result in protein misfolding and degradation via the ERAD pathway. Our work leveraged a new yeast model and an HEK293 cell model to discover the mechanism underlying PC2 instability and demonstrates the potential for pharmacological rescue. We also suggest that targeting the protein misfolding phenotype with chemical chaperones may offer new therapeutic strategies to manage ADPKD-related protein dysfunction.

Role of the histone deacetylase family in lipid metabolism: Structural specificity and functional diversity

Lipids play crucial roles in signal transduction. Lipid metabolism is associated with several transcriptional regulators, including peroxisome proliferator activated receptor γ, sterol regulatory element-binding protein 1, and acetyl-CoA carboxylase. In recent years, increasing evidence has suggested that members of the histone deacetylase (HDAC) family play key roles in lipid metabolism. However, the mechanisms by which each member of this family regulates lipid metabolism remain unclear. This review discusses the latest research on the roles played by HDACs in fat metabolism. The role of HDACs in obesity, diabetes, and atherosclerosis has also been discussed. In addition, the interaction of HDACs with the gut microbiome and circadian rhythm has been reviewed, and the future development trend in HDACs has been predicted, which may potentiate therapeutic application of targeted HDACs in related metabolic diseases.

Glis2 is an early effector of polycystin signaling and a target for therapy in polycystic kidney disease

Mouse models of autosomal dominant polycystic kidney disease (ADPKD) show that intact primary cilia are required for cyst growth following the inactivation of polycystin-1. The signaling pathways underlying this process, termed cilia-dependent cyst activation (CDCA), remain unknown. Using translating ribosome affinity purification RNASeq on mouse kidneys with polycystin-1 and cilia inactivation before cyst formation, we identify the differential 'CDCA pattern' translatome specifically dysregulated in kidney tubule cells destined to form cysts. From this, Glis2 emerges as a candidate functional effector of polycystin signaling and CDCA. In vitro changes in Glis2 expression mirror the polycystin- and cilia-dependent changes observed in kidney tissue, validating Glis2 as a cell culture-based indicator of polycystin function related to cyst formation. Inactivation of Glis2 suppresses polycystic kidney disease in mouse models of ADPKD, and pharmacological targeting of Glis2 with antisense oligonucleotides slows disease progression. Glis2 transcript and protein is a functional target of CDCA and a potential therapeutic target for treating ADPKD.

Clinical manifestation, epidemiology, genetic basis, potential molecular targets, and current treatment of polycystic liver disease

Polycystic liver disease (PLD) is a rare condition observed in three genetic diseases, including autosomal dominant polycystic liver disease (ADPLD), autosomal dominant polycystic kidney disease (ADPKD), and autosomal recessive polycystic kidney disease (ARPKD). PLD usually does not impair liver function, and advanced PLD becomes symptomatic when the enlarged liver compresses adjacent organs or increases intra-abdominal pressure. Currently, the diagnosis of PLD is mainly based on imaging, and genetic testing is not required except for complex cases. Besides, genetic testing may help predict patients' prognosis, classify patients for genetic intervention, and conduct early treatment. Although the underlying genetic causes and mechanisms are not fully understood, previous studies refer to primary ciliopathy or impaired ciliogenesis as the main culprit. Primarily, PLD occurs due to defective ciliogenesis and ineffective endoplasmic reticulum quality control. Specifically, loss of function mutations of genes that are directly involved in ciliogenesis, such as Pkd1, Pkd2, Pkhd1, and Dzip1l, can lead to both hepatic and renal cystogenesis in ADPKD and ARPKD. In addition, loss of function mutations of genes that are involved in endoplasmic reticulum quality control and protein folding, trafficking, and maturation, such as PRKCSH, Sec63, ALG8, ALG9, GANAB, and SEC61B, can impair the production and function of polycystin1 (PC1) and polycystin 2 (PC2) or facilitate their degradation and indirectly promote isolated hepatic cystogenesis or concurrent hepatic and renal cystogenesis. Recently, it was shown that mutations of LRP5, which impairs canonical Wnt signaling, can lead to hepatic cystogenesis. PLD is currently treated by somatostatin analogs, percutaneous intervention, surgical fenestration, resection, and liver transplantation. In addition, based on the underlying molecular mechanisms and signaling pathways, several investigational treatments have been used in preclinical studies, some of which have shown promising results. This review discusses the clinical manifestation, complications, prevalence, genetic basis, and treatment of PLD and explains the investigational methods of treatment and future research direction, which can be beneficial for researchers and clinicians interested in PLD.

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.

A SEC61A1 variant is associated with autosomal dominant polycystic liver disease

BACKGROUND AND AIMS

Autosomal dominant polycystic liver and kidney disease is a spectrum of hereditary diseases, which display disturbed function of primary cilia leading to cyst formation. In autosomal dominant polycystic kidney disease a genetic cause can be determined in almost all cases. However, in isolated polycystic liver disease (PLD) about half of all cases remain genetically unsolved, suggesting more, so far unidentified genes to be implicated in this disease.

METHODS

Customized next-generation sequencing was used to identify the underlying pathogenesis in two related patients with PLD. A variant identified in SEC61A1 was further analysed in immortalized patients' urine sediment cells and in an epithelial cell model.

RESULTS

In both patients, a heterozygous missense change (c.706C>T/p.Arg236Cys) was found in SEC61A1, which encodes for a subunit of the translocation machinery of protein biosynthesis at the endoplasmic reticulum (ER). While kidney disease is absent in the proposita, her mother displays an atypical polycystic kidney phenotype with severe renal failure. In immortalized urine sediment cells, mutant SEC61A1 is expressed at reduced levels, resulting in decreased levels of polycystin-2 (PC2). In an epithelial cell culture model, we found the proteasomal degradation of mutant SEC61A1 to be increased, whereas its localization to the ER is not affected.

CONCLUSIONS

Our data expand the allelic and clinical spectrum for SEC61A1, adding PLD as a new and the major phenotypic trait in the family described. We further demonstrate that mutant SEC61A1 results in enhanced proteasomal degradation and impaired biosynthesis of PC2.

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.