Me Or Your Own Eyes: RNA-Seq and the Kidney

Rationale and design of a prospective, clinical study of kidney biopsies in people with type 2 diabetes and severely increased albuminuria (the PRIMETIME 2 study)

INTRODUCTION

Diabetic kidney disease is a severe complication of diabetes. The diagnosis is based on clinical characteristics such as persistently elevated albuminuria, hypertension and decline in kidney function, although this definition is not specific to kidney disease caused by diabetes. The only way to establish an accurate diagnosis-diabetic nephropathy-is by performing a kidney biopsy. The histological presentation of diabetic nephropathy can be associated with a heterogeneous range of histological features with many pathophysiological factors involved demonstrating the complexity of the condition. Current treatment strategies aim to slow disease progression and are not specific to the underlying pathological processes.This study will investigate the prevalence of diabetic nephropathy in individuals with type 2 diabetes (T2D) and severely elevated albuminuria. The deep molecular characterisation of the kidney biopsy and biological specimens may pave the way for improved diagnostic accuracy and a better understanding of the pathological processes involved and may also reveal new targets for individualised treatment.

METHODS AND ANALYSIS

In the PRecIsion MEdicine based on kidney TIssue Molecular interrogation in diabetic nEphropathy 2 study, research kidney biopsies will be performed in 300 participants with T2D, urine albumin/creatinine ratio ≥700 mg/g and estimated glomerular filtration ratio >30 mL/min/1.73 m2. Cutting-edge molecular technologies will be applied to the kidney, blood, urine, faeces and saliva samples for comprehensive multi-omics profiling. The associated disease course and clinical outcomes will be assessed by annual follow-up for 20 years.

ETHICS AND DISSEMINATION

The Danish Regional Committee on Health Research Ethics and the Knowledge Center on Data Protection (in the Capital Region of Denmark) have granted approval for the study. The results will be published in peer-reviewed journals.

TRIAL REGISTRATION NUMBER

NCT04916132.

Combined Kelch-like 3 and Cullin 3 Degradation is a Central Mechanism in Familial Hyperkalemic Hypertension in Mice

BACKGROUND

Mutations in the ubiquitin ligase scaffold protein Cullin 3 (CUL3) gene cause the disease familial hyperkalemic hypertension (FHHt). In the kidney, mutant CUL3 (CUL3-Δ9) increases abundance of With-No-Lysine (K) Kinase 4 (WNK4), inappropriately activating sterile 20/SPS-1-related proline/alanine-rich kinase (SPAK), which then phosphorylates and hyperactivates the Na⁺Cl^- cotransporter (NCC). The precise mechanism by which CUL3-Δ9 causes FHHt is unclear. We tested the hypothesis that reduced abundance of CUL3 and of Kelch-like 3 (KLHL3), the CUL3 substrate adaptor for WNK4, is mechanistically important. Because JAB1, an enzyme that inhibits CUL3 activity by removing the ubiquitin-like protein NEDD8, cannot interact with CUL3-Δ9, we also determined whether Jab1 disruption mimicked the effects of CUL3-Δ9 expression.

METHODS

We used an inducible renal tubule-specific system to generate several mouse models expressing CUL3-Δ9, mice heterozygous for both CUL3 and KLHL3 (Cul3^+/-/Klhl3^+/- ), and mice with short-term Jab1 disruption (to avoid renal injury associated with long-term disruption).

RESULTS

Renal KLHL3 was higher in Cul3^-/- mice, but lower in Cul3^-/-/Δ9 mice and in the Cul3^+/-/Δ9 FHHt model, suggesting KLHL3 is a target for both WT and mutant CUL3. Cul3^+/-/Klhl3^+/- mice displayed increased WNK4-SPAK activation and phospho-NCC abundance and an FHHt-like phenotype with increased plasma [K⁺] and salt-sensitive blood pressure. Short-term Jab1 disruption in mice lowered the abundance of CUL3 and KLHL3 and increased the abundance of WNK4 and phospho-NCC.

CONCLUSIONS

Jab1^-/- mice and Cul3^+/-/Klhl3^+/- mice recapitulated the effects of CUL3-Δ9 expression on WNK4-SPAK-NCC. Our data suggest degradation of both KLHL3 and CUL3 plays a central mechanistic role in CUL3-Δ9-mediated FHHt.