ADCK4 Deficiency Destabilizes the Coenzyme Q Complex, Which Is Rescued by 2,4-Dihydroxybenzoic Acid Treatment
Authors: Widmeier Eugen , Yu Seyoung , Nag Anish , Chung Youn Wook , Nakayama Makiko , Fernández-del-Río Lucía , Hugo Hannah , Schapiro David , Buerger Florian , Choi Won-Il , Helmstädter Martin , Kim Jae-woo , Ryu Ji-Hwan , Lee Min Goo , Clarke Catherine F. , Hildebrandt Friedhelm , Gee Heon Yung 
Published: May 2020

Significance Statement

ADCK4 mutations generally manifest as steroid-resistant nephrotic syndrome, and cause coenzyme Q10 (CoQ10) deficiency. However, ADCK4’s function remains obscure. Using mouse and cell models, the authors demonstrated that podocyte-specific Adck4 deletion in mice significantly reduced survival and caused severe FSGS, effects that were prevented by treatment with 2,4-dihydroxybenzoic acid (2,4-diHB), a CoQ10 precursor analogue. ADCK4-knockout podocytes exhibited a significantly reduced CoQ10 level and defects in mitochondrial function that were rescued by 2,4-diHB treatment, thus these phenotypes were attributed to decreased CoQ10 levels. The authors also found that ADCK4 interacted with mitochondrial proteins, including COQ5, and that ADCK4 knockout decreased COQ complex levels. These findings reveal that ADCK4 is required for CoQ10 biosynthesis and mitochondrial function in podocytes, and suggests a treatment strategy for nephrotic syndrome caused by ADCK4 mutations.

Background
Mutations in ADCK4 (aarF domain containing kinase 4) generally manifest as steroid-resistant nephrotic syndrome and induce coenzyme Q10 (CoQ10) deficiency. However, the molecular mechanisms underlying steroid-resistant nephrotic syndrome resulting from ADCK4 mutations are not well understood, largely because the function of ADCK4 remains unknown.

Methods
To elucidate the ADCK4’s function in podocytes, we generated a podocyte-specific, Adck4-knockout mouse model and a human podocyte cell line featuring knockout of ADCK4. These knockout mice and podocytes were then treated with 2,4-dihydroxybenzoic acid (2,4-diHB), a CoQ10 precursor analogue, or with a vehicle only. We also performed proteomic mass spectrometry analysis to further elucidate ADCK4’s function.

Results
Absence of Adck4 in mouse podocytes caused FSGS and albuminuria, recapitulating features of nephrotic syndrome caused by ADCK4 mutations. In vitro studies revealed that ADCK4-knockout podocytes had significantly reduced CoQ10 concentration, respiratory chain activity, and mitochondrial potential, and subsequently displayed an increase in the number of dysmorphic mitochondria. However, treatment of 3-month-old knockout mice or ADCK4-knockout cells with 2,4-diHB prevented the development of renal dysfunction and reversed mitochondrial dysfunction in podocytes. Moreover, ADCK4 interacted with mitochondrial proteins such as COQ5, as well as cytoplasmic proteins such as myosin and heat shock proteins. Thus, ADCK4 knockout decreased the COQ complex level, but overexpression of ADCK4 in ADCK4-knockout podocytes transfected with wild-type ADCK4 rescued the COQ5 level.

Conclusions
Our study shows that ADCK4 is required for CoQ10 biosynthesis and mitochondrial function in podocytes, and suggests that ADCK4 in podocytes stabilizes proteins in complex Q in podocytes. Our study also suggests a potential treatment strategy for nephrotic syndrome resulting from ADCK4 mutations.