Mitochondria Matter: A Critical Role of ADCK4 in Stabilizing the CoQ Complex in Podocytes in Steroid-Resistant Nephrotic Syndrome

Podocytes are a key component of the glomerular filtration barrier, and their dysfunction is central to the underlying pathophysiology of glomerular diseases. In humans, the mutations associated with steroid-resistant nephrotic syndrome (SRNS) or FSGS affect podocyte actin cytoskeleton proteins and the slit diaphragm,¹ supporting the view that podocytes are causally related to disease development.

Interestingly, a group of mutations in genes involved in coenzyme Q₁₀ (CoQ₁₀; ubiquinone) biosynthesis, such as COQ₆; COQ₂; prenyl diphosphate synthase, subunit 2 (PDSS2); and ADCK4 (COQ8B), have also been associated with childhood-onset FSGS and SRNS. CoQ₁₀ is a component of the mitochondrial inner membrane and plays important roles in supporting electron transport of oxidative phosphorylation (OXPHOS), protection from oxidative stress, and activation of mitochondrial enzymes required in metabolic pathways, including pyrimidine synthesis.² Disruption of CoQ₁₀ biosynthesis in podocytes supports the view that mitochondrial function is crucial for the maintenance and function of the glomerular filtration barrier.³

Mitochondria are critical for cellular metabolism, homeostasis, and initiation of apoptosis. Beyond ATP production, mitochondria maintain ion homeostasis; produce precursors for macromolecules, such as lipids, proteins, and DNA; and generate as well as sequester potentially damaging metabolic byproducts such as ammonia and reactive oxygen species (ROS). They also play active roles in integrating signaling pathways and responses to stressors, and are dynamic, with these functions being tightly linked to their form, fission and fusion, motility, and positioning. Considering that podocytes harbor an actin-based cytoskeleton with a contractile machinery that allows for rapid cell shape remodeling and movement, as well as maintaining a very high surface area,⁴ it would suggest that podocytes need high metabolic activity for function under constant physical forces. However, a recent study indicates that anaerobic glycolysis represents the predominant energy source of podocytes, independent from mitochondrial energy sources under physiologic conditions.⁵ Hence, the perceived critical role of mitochondria as an energy source for podocyte function is currently debated.

In this issue of JASN, a study by Widmeier et al.⁶ demonstrates that Adck4 function is required for podocyte maintenance and homeostasis in mice by stabilizing the CoQ complex (Figure 1). Using whole-exome sequencing in patients with SRNS, this group was first to identify homozygous loss-of-function mutations in ADCK4 as disease causative.⁷ ADCK4 encodes the aarF domain containing kinase 4, which localizes specifically to mitochondria within foot processes of rat podocytes. In this study, the authors generated a podocyte-specific Adck4 knockout mice (Adck4ΔPodocyte) to examine the pathogenic mechanisms involved. These mice developed albuminuria at 4 months. By 10 months, the kidneys presented abnormal glomeruli with significant fibrosis, disturbed podocyte morphology with severe foot process effacement, and disorganization of the filtration slit. As the mice aged, abnormal mitochondria characterized by hyperproliferation and increased size were identified in podocytes. Overall, the glomerular phenotype of Adck4ΔPodocyte mice recapitulated aspects of the pathology of FSGS in humans resulting from ADCK4 mutations.

Figure 1.

Adck4 function is required for podocyte maintenance and homeostasis by stabilizing the CoQ complex. Podocyte specific Adck4 ablation in mice resulted in disturbed podocyte morphology with severe foot process effacement, disorganization of the filtration slit, abnormally large and dysfunctional mitochondria and reduced CoQ₁₀. 2,4-diHB treatment restored CoQ₁₀, mitochondrial function, improved podocytes morphology and prevented foot process effacement. 

Levels of CoQ₁₀ were decreased in individuals with ADCK4 mutations,⁷ as well as in Adck4ΔPodocyte mice, suggesting that ADCK4 is indeed involved in CoQ₁₀ biosynthesis. In the absence of Adck4, podocytes had decreased OXPHOS complex II-III proteins and membrane potential, generating increased ROS, making podocytes more susceptible to mitochondrial injury compared with other kidney derived cells. Next, the authors discovered that Adck4 interacted with mainly mitochondrial proteins, including CoQ₅, and that Adck4 ablation in podocytes destabilized the CoQ complex and the cytoskeleton. The investigators used 2,4-dihydroxybenzoic acid (2,4-diHB) to bypass defects in the penultimate step of CoQ biosynthesis, which is mediated by CoQ₇ hydroxylase.⁸

Treatment with 2,4-diHB improved survival and, despite persistence of proteinuria, reduced sclerotic glomeruli and expression of fibrotic markers, and improved plasma albumin level and renal function. Podocytes increased expression of nephrin and synaptopodin, preserving slit morphology without foot process effacement. The work highlights the importance of diagnosing mutations in mitochondrial proteins. The authors suggest the intriguing possibility that early intervention bypassing dysfunctional CoQ₁₀ biosynthesis could be an effective and safe treatment for patients with ADCK4 mutations.

The work by Widmeier et al.⁶ emphasizes the effect of impaired electron transport in podocytes in the maintenance of the glomerular filtration barrier. These findings are in agreement with recent research demonstrating that the loss-of-function mutation of the complex IV assembly cofactor heme A:farnesyltransferase (COX10) in cells of the developing nephrons was sufficient to cause FSGS.⁹ COX10 is a critical complex in mitochondrial respiratory ATP production, and deletion leads to mitochondrial dysfunction in podocytes. However, diverse and unexpected functions of podocyte mitochondria under physiologic and stress conditions continue to emerge. For instance, podocyte-specific deletion of inner mitochondrial membrane protein Mpv17, involved in mitochondrial DNA maintenance, or deletion of ABCA1, which regulates the cholesterol efflux and cardiolipin content, results in mitochondrial dysfunction and severe glomerular disease phenotype under conditions of stress.^10,11 Although these proteins do not affect the OXPHOS machinery directly, they protect podocytes against oxidative stress–induced injury. Importantly, when clearance of defective podocyte mitochondria is impaired, FSGS ensues, with enlarged dysfunctional mitochondria and increased ROS.¹²

Mechanistic insight obtained from patients with rare forms of monogenic disease such as ADCK4 provide an invaluable opportunity to dissect the underlying molecular mechanisms of more common disease conditions, such CKD, where the patients have lower plasma concentrations of CoQ₁₀,¹³ and where supplementation or bypassing dysfunctional CoQ₁₀ biosynthesis may improve mitochondrial function, decrease oxidative stress, and slow progression. Also, as genetic testing becomes more prevalent and affordable, we can expect that opportunities for a “precision medicine” approach will become available for patients with nephrotic syndrome.

Unanswered questions remain about metabolic state and mitochondrial energetics of podocytes from mice or patients with ADCK4 mutations. Future work could provide resolution of the response by other glomerular cells, because dysfunctioning podocytes likely signal to neighboring endothelial cells,¹⁴ which in this case would also harbor the ADCK4 mutation, specifying reparative or pathologic responses. Because mutations of mitochondrial proteins cause disease in a limited range of organs, new approaches to better understand mitochondrial contributions in specific cells and tissues, in homeostasis and disease, is warranted.

Disclosures

Dr. Daehn has a consultancy agreement with CAMP4.

Funding

This study was supported by National Institutes of Health grant R01DK097253 (to Dr. Daehn).