Cullin-Ring ubiquitin ligases in kidney health and disease

A novel approach to explore metabolic diseases: Neddylation

Protein post translational modification (PTM) is the main regulatory mechanism for eukaryotic cell function, among which ubiquitination is based on the reversible degradation of proteins by the ubiquitin proteasome system to regulate cell homeostasis. The neural precursor cell expressed developmental downregulated gene 8 (NEDD8) is a kind of ubiquitin like protein that shares 80 % homology and 60 % identity with ubiquitin. The PTM process by covalently binding NEDD8 to lysine residues in proteins is called neddylation. The neddylation reaction could be regulated by NEDD8, its precursors, substrates, E1 activating enzymes, E2 binding enzymes, E3 ligases, de-neddylases, and its inhibitors, such as MLN4924. NEDD8 is widely expressed in the whole cell structure of multiple tissues and species, and neddylation related factors are highly expressed in metabolism related adrenal glands, thyroid glands, parathyroid glands, skeletal muscles, myocardium, and adipose tissues, related to metabolic cardiovascular, cerebrovascular and liver diseases, adipogenic and osteogenic differentiation, as well as tumor glycolysis and glucose metabolism resulting from angiogenesis and endothelial disfunction, hepatotoxicity, adipogenesis, osteogenesis, Warburg effect, and insulin function. This review provides researchers with a new approach to explore metabolic diseases via searching and analyzing the histological, cytological, and subcellular localization of neddylation specific molecules in databases, and exploring specific mechanism neddylation mediating metabolic diseases by searching for neddylation related terms with the development of pre-clinical neddylation pharmacological inhibitors.

Deletion of KS-WNK1 promotes NCC activation by increasing WNK1/4 abundance

Dietary potassium deficiency causes stimulation of sodium reabsorption leading to an increased risk in blood pressure elevation. The distal convoluted tubule (DCT) is the main rheostat linking plasma K+ levels to the activity of the Na-Cl cotransporter (NCC). This occurs through basolateral membrane potential sensing by inwardly rectifying K+ channels (Kir4.1/5.1); decrease in intracellular Cl-; activation of WNK4 and interaction and phosphorylation of STE20/SPS1-related proline/alanine-rich kinase (SPAK); binding of calcium-binding protein 39 (cab39) adaptor protein to SPAK, leading to its trafficking to the apical membrane; and SPAK binding, phosphorylation, and activation of NCC. As kidney-specific with-no-lysine kinase 1 (WNK1) isoform (KS-WNK1) is another participant in this pathway, we examined its function in NCC regulation. We eliminated KS-WNK1 specifically in the DCT and demonstrated increased expression of WNK4 and long WNK1 (L-WNK1) and increased phosphorylation of NCC. As in other KS-WNK1 models, the mice were not hyperkalemic. Although wild-type mice under low-dietary K+ conditions demonstrated increased NCC phosphorylation, the phosphorylation levels of the transporter, already high in KS-WNK1, did not change under the low-K+ diet. Thus, in the absence of KS-WNK1, the transporter lost its sensitivity to low plasma K+. We also show that under low K+ conditions, in the absence of KS-WNK1, there was no formation of WNK bodies. These bodies were observed in adjacent segments, not affected by the targeting of KS-WNK1. As our data are overall consistent with those of the global KS-WNK1 knockout, they indicate that the DCT is the predominant segment affecting the salt transport regulated by KS-WNK1.NEW & NOTEWORTHY In this paper, we show that KS-WNK1 is a critical component of the distal convoluted tubule (DCT) K+ switch pathway. Its deletion results in an inability of the DCT to sense changes in plasma potassium. Absence of KS-WNK1 leads to abnormally high levels of WNK4 and L-WNK1 in the DCT, resulting in increased Na-Cl phosphorylation and function. Our data are consistent with KS-WNK1 targeting WNK4 and L-WNK1 to degradation.

Hyperuricemia Subtypes Classified According to Renal Uric Acid Handling Manifesting Distinct Phenotypic and Genetic Profiles in People With Gout

OBJECTIVE

Hyperuricemia can be stratified into four subtypes according to renal uric acid handling. The aim of this study was to comprehensively describe the biologic characteristics (including genetic background) of clinically defined hyperuricemia subtypes in two large geographically independent gout cohorts.

METHODS

Hyperuricemia subtype was defined as renal uric acid overload (ROL), renal uric acid underexcretion (RUE), combined, or renal normal. Twenty single nucleotide polymorphisms (SNPs) previously identified as gout risk loci or associated with serum urate (SU) concentration in the East Asian population were genotyped. Weighted polygenic risk scores were calculated to assess the cumulative effect of genetic risks on the subtypes.

RESULTS

Of the 4,873 participants, 8.8% had an ROL subtype, 60.9% RUE subtype, 23.1% combined subtype, and 7.2% normal subtype. The ROL subtype was independently associated with older age at onset, lower SU, tophi, and diabetes mellitus; RUE was associated with lower body mass index (BMI) and non-diabetes mellitus; the combined subtype was associated with younger age at onset, higher BMI, SU, estimated glomerular filtration rate (eGFR), and smoking; and the normal subtype was independently associated with older age at onset, lower SU, and eGFR. Thirteen SNPs were associated with gout with 6 shared loci and subtype-dependent risk loci patterns. High polygenic risk scores were associated with ROL subtype (odds ratio [OR] = 9.63, 95% confidence interval [95% CI] 4.53-15.12), RUE subtype (OR = 2.18, 95% CI 1.57-3.03), and combined subtype (OR = 6.32, 95% CI 4.22-9.48) compared with low polygenic risk scores.

CONCLUSION

Hyperuricemia subtypes classified according to renal uric acid handling have subtype-specific clinical and genetic features, suggesting subtype-unique pathophysiologic mechanisms.

TRIM65 promotes renal cell carcinoma through ubiquitination and degradation of BTG3

As a typical E3 ligase, TRIM65 (tripartite motif containing 65) is involved in the regulation of antiviral innate immunity and the pathogenesis of certain tumors. However, the role of TRIM65 in renal cell carcinoma (RCC) and the underlying mechanism has not been determined yet. In this study, we identified TRIM65 as a novel oncogene in RCC, which enhanced the tumor cell proliferation and anchorage-independent growth abilities both in vitro and in vivo. Moreover, we found that TRIM65-regulated RCC proliferation mainly via direct interaction with BTG3 (BTG anti-proliferation factor 3), which in turn induced the K48-linked ubiquitination and subsequent degradation through K41 amino acid. Furthermore, TRIM65 relieved G2/M phase cell cycle arrest via degradation of BTG3 and regulated downstream factors. Further studies revealed that TRIM65 acts through TRIM65-BTG3-CyclinD1 axis and clinical sample IHC chip data indicated a negative correction between TRIM65 and BTG3. Taken together, our findings demonstrated that TRIM65 promotes RCC cell proliferation via regulation of the cell cycle through degradation of BTG3, suggesting that TRIM65 may be a promising target for RCC therapy.

Regulation of the water channel aquaporin-2 by cullin E3 ubiquitin ligases

Aquaporin 2 (AQP2) is a vasopressin (VP)-regulated water channel in the renal collecting duct. Phosphorylation and ubiquitylation of AQP2 play an essential role in controlling the cellular abundance of AQP2 and its accumulation on the plasma membrane in response to VP. Cullin-RING ubiquitin ligases (CRLs) are multisubunit E3 ligases involved in ubiquitylation and degradation of their target proteins, eight of which are expressed in the collecting duct. Here, we used an established cell model of the collecting duct (mpkCCD14 cells) to study the role of cullins in modulating AQP2. Western blotting identified Cul-1 to Cul-5 in mpkCCD14 cells. Treatment of cells for 4 h with a pan-cullin inhibitor (MLN4924) decreased AQP2 abundance, prevented a VP-induced reduction in AQP2 Ser261 phosphorylation, and attenuated VP-induced plasma membrane accumulation of AQP2 relative to the vehicle. AQP2 ubiquitylation levels were significantly higher after MLN4924 treatment compared with controls, and they remained higher despite VP treatment. Cullin inhibition increased ERK1/2 activity, a kinase that regulates AQP2 Ser261 phosphorylation, and VP-induced reductions in ERK1/2 phosphorylation were absent during MLN4924 treatment. Furthermore, the greater Ser261 phosphorylation and reduction in AQP2 abundance during MLN4924 treatment were attenuated during ERK1/2 inhibition. MLN4924 increased intracellular calcium levels via calcium release-activated calcium channels, inhibition of which abolished MLN4924 effects on Ser261 phosphorylation and AQP2 abundance. In conclusion, CRLs play a vital role in mediating some of the effects of VP to increase AQP2 plasma membrane accumulation and AQP2 abundance. Whether modulation of cullin activity can contribute to body water homeostasis requires further studies.NEW & NOTEWORTHY Aquaporin 2 (AQP2) is essential for body water homeostasis and is regulated by the antidiuretic hormone vasopressin. The posttranslational modification ubiquitylation is a key regulator of AQP2 abundance and plasma membrane localization. Here we demonstrate that cullin-RING E3 ligases play a vital role in mediating some of the effects of vasopressin to increase AQP2 abundance and plasma membrane accumulation. The results suggest that manipulating cullin activity could be a novel strategy to alter kidney water handling.

Kidney Fibrosis and Oxidative Stress: From Molecular Pathways to New Pharmacological Opportunities

Kidney fibrosis, diffused into the interstitium, vessels, and glomerulus, is the main pathologic feature associated with loss of renal function and chronic kidney disease (CKD). Fibrosis may be triggered in kidney diseases by different genetic and molecular insults. However, several studies have shown that fibrosis can be linked to oxidative stress and mitochondrial dysfunction in CKD. In this review, we will focus on three pathways that link oxidative stress and kidney fibrosis, namely: (i) hyperglycemia and mitochondrial energy imbalance, (ii) the mineralocorticoid signaling pathway, and (iii) the hypoxia-inducible factor (HIF) pathway. We selected these pathways because they are targeted by available medications capable of reducing kidney fibrosis, such as sodium-glucose cotransporter-2 (SGLT2) inhibitors, non-steroidal mineralocorticoid receptor antagonists (MRAs), and HIF-1alpha-prolyl hydroxylase inhibitors. These drugs have shown a reduction in oxidative stress in the kidney and a reduced collagen deposition across different CKD subtypes. However, there is still a long and winding road to a clear understanding of the anti-fibrotic effects of these compounds in humans, due to the inherent practical and ethical difficulties in obtaining sequential kidney biopsies and the lack of specific fibrosis biomarkers measurable in easily accessible matrices like urine. In this narrative review, we will describe these three pathways, their interconnections, and their link to and activity in oxidative stress and kidney fibrosis.

The emerging insight into E3 ligases as the potential therapeutic target for diabetic kidney disease

Diabetic kidney disease (DKD) is a major diabetic complication and global health concern, occurring in nearly 30 % to 40 % of people with diabetes. Importantly, several therapeutic strategies are being used against DKD; however, available treatments are not uniformly effective and the continuous rise in the prevalence of DKD demands more potential therapeutic approaches or targets. Epigenetic modifiers are regarded for their potential therapeutic effects against DKD. E3 ligases are such epigenetic modifier that regulates the target gene expression by attaching ubiquitin to the histone protein. In recent years, the E3 ligases came up as a potential therapeutic target as it selectively attaches ubiquitin to the substrate proteins in the ubiquitination cascade and modulates cellular homeostasis. The E3 ligases are also actively involved in DKD by regulating the expression of several proteins involved in the proinflammatory and profibrotic pathways. Burgeoning reports suggest that several E3 ligases such as TRIM18 (tripartite motif 18), Smurf1 (Smad ubiquitination regulatory factor 1), and NEDD4-2 (neural precursor cell-expressed developmentally downregulated gene 4-2) are involved in kidney epithelial-mesenchymal transition, inflammation, and fibrosis by regulating respective signaling pathways. However, the various signaling pathways that are regulated by different E3 ligases in the progression of DKD are poorly understood. In this review, we have discussed E3 ligases as potential therapeutic target for DKD. Moreover, different signaling pathways regulated by E3 ligases in the progression of DKD have also been discussed.

COP9 signalosome deletion promotes renal injury and distal convoluted tubule remodeling

Cullin-RING ligases are a family of E3 ubiquitin ligases that control cellular processes through regulated degradation. Cullin 3 targets with-no-lysine kinase 4 (WNK4), a kinase that activates the Na⁺-Cl^- cotransporter (NCC), the main pathway for Na⁺ reabsorption in the distal convoluted tubule (DCT). Mutations in the cullin 3 gene lead to familial hyperkalemic hypertension by increasing WNK4 abundance. The constitutive photomorphogenesis 9 (COP9) signalosome (CSN) regulates the activity of cullin-RING ligases by removing the ubiquitin-like protein neural precursor cell expressed developmentally downregulated protein 8. Genetic deletion of the catalytically active CSN subunit, Jab1, along the nephron in mice (KS-Jab1^-/-) led to increased WNK4 abundance; however, NCC abundance was substantially reduced. We hypothesized that the reduction in NCC resulted from a cortical injury that led to hypoplasia of the segment, which counteracted WNK4 activation of NCC. To test this, we studied KS-Jab1^-/- mice at weekly intervals over a period of 3 wk. The results showed that NCC abundance was unchanged until 3 wk after Jab1 deletion, at which time other DCT-specific proteins were also reduced. The kidney injury markers kidney injury molecule-1 and neutrophil gelatinase-associated lipocalin demonstrated kidney injury immediately after Jab1 deletion; however, the damage was initially limited to the medulla. The injury progressed and expanded into the cortex 3 wk after Jab1 deletion coinciding with loss of the DCT. The data indicate that nephron-specific disruption of the cullin-RING ligase system results in a complex progression of tubule injury that leads to hypoplasia of the DCT.NEW & NOTEWORTHY Cullin 3 (CUL3) targets with-no-lysine-kinase 4 (WNK4), which activates Na⁺-Cl^- cotransporter (NCC) in the distal convoluted tubule (DCT) of the kidney. Renal-specific genetic deletion of the constitutive photomorphogenesis 9 signalosome, an upstream regulator of CUL3, resulted in a reduction of NCC due to DCT hypoplasia, which coincided with cortical kidney injury. The data indicate that nephron-specific disruption of the cullin-RING ligase system results in a complex progression of tubule injury leading to hypoplasia of the DCT.

LZTR1: A promising adaptor of the CUL3 family

The study of the disorders of ubiquitin-mediated proteasomal degradation may unravel the molecular basis of human diseases, such as cancer (prostate cancer, lung cancer and liver cancer, etc.) and nervous system disease (Parkinson's disease, Alzheimer's disease and Huntington's disease, etc.) and help in the design of new therapeutic methods. Leucine zipper-like transcription regulator 1 (LZTR1) is an important substrate recognition subunit of cullin-RING E3 ligase that plays an important role in the regulation of cellular functions. Mutations in LZTR1 and dysregulation of associated downstream signaling pathways contribute to the pathogenesis of Noonan syndrome (NS), glioblastoma and chronic myeloid leukemia. Understanding the molecular mechanism of the normal function of LZTR1 is thus critical for its eventual therapeutic targeting. In the present review, the structure and function of LZTR1 are described. Moreover, recent advances in the current knowledge of the functions of LZTR1 in NS, glioblastoma (GBM), chronic myeloid leukemia (CML) and schwannomatosis and the influence of LZTR1 mutations are also discussed, providing insight into how LZTR1 may be targeted for therapeutic purposes.

NaCl cotransporter activity and Mg2+ handling by the distal convoluted tubule

The genetic disease Gitelman syndrome, knockout mice, and pharmacological blockade with thiazide diuretics have revealed that reduced activity of the NaCl cotransporter (NCC) promotes renal Mg²⁺ wasting. NCC is expressed along the distal convoluted tubule (DCT), and its activity determines Mg²⁺ entry into DCT cells through transient receptor potential channel subfamily M member 6 (TRPM6). Several other genetic forms of hypomagnesemia lower the drive for Mg²⁺ entry by inhibiting activity of basolateral Na⁺-K⁺-ATPase, and reduced NCC activity may do the same. Lower intracellular Mg²⁺ may promote further Mg²⁺ loss by directly decreasing activity of Na⁺-K⁺-ATPase. Lower intracellular Mg²⁺ may also lower Na⁺-K⁺-ATPase indirectly by downregulating NCC. Lower NCC activity also induces atrophy of DCT cells, decreasing the available number of TRPM6 channels. Conversely, a mouse model with increased NCC activity was recently shown to display normal Mg²⁺ handling. Moreover, recent studies have identified calcineurin and uromodulin (UMOD) as regulators of both NCC and Mg²⁺ handling by the DCT. Calcineurin inhibitors paradoxically cause hypomagnesemia in a state of NCC activation, but this may be related to direct effects on TRPM6 gene expression. In Umod^-/- mice, the cause of hypomagnesemia may be partly due to both decreased NCC expression and lower TRPM6 expression on the cell surface. This mini-review discusses these new findings and the possible role of altered Na⁺ flux through NCC and ultimately Na⁺-K⁺-ATPase in Mg²⁺ reabsorption by the DCT.

Hypertension-causing cullin 3 mutations disrupt COP9 signalosome binding

The discovery of new genetic mutations that cause hypertension has illuminated previously unrecognized physiological pathways. One such regulatory pathway was identified when mutations in with no lysine kinase (WNK)4, Kelch-like 3 (KLHL3), and cullin 3 (CUL3) were shown to cause the disease familial hyperkalemic hypertension (FHHt). Mutations in all three genes upregulate the NaCl cotransporter (NCC) due to an impaired ability to degrade WNK protein through the cullin-RING-ligase (CRL) ubiquitin-proteasome system. The CUL3 FHHt mutations cause the most severe phenotype, yet the precise mechanism by which these mutations cause the disease has not been established and current proposed models are controversial. New data have identified a possible novel mechanism involving dysregulation of CUL3 activity by the COP9 signalosome (CSN). The CSN interaction with mutant CUL3 is diminished, causing hyperneddylation of the CRL. Recent work has shown that direct renal CSN impairment mimics some aspects of the CUL3 mutation, including lower KLHL3 abundance and activation of the WNK-NCC pathway. Furthermore, in vitro and in vivo studies of CSN inhibition have shown selective degradation of CRL substrate adaptors via auto-ubiquitination, allowing substrate accumulation. In this review, we will focus on recent research that highlights the role of the CSN role in CUL3 mutations that cause FHHt. We will also highlight how these results inform other recent studies of CSN dysfunction.