Autophagic clearance of mitochondria in the kidney copes with metabolic acidosis.
Metabolic acidosis, a common complication of CKD, causes mitochondrial stress by undefined mechanisms. Selective autophagy of impaired mitochondria, called mitophagy, contributes toward maintaining cellular homeostasis in various settings. We hypothesized that mitophagy is involved in proximal tubular cell adaptations to chronic metabolic acidosis. In transgenic mice expressing green fluorescent protein-tagged microtubule-associated protein 1 light chain 3 (GFP-LC3), NH4Cl loading increased the number of GFP puncta exclusively in the proximal tubule. In vitro, culture in acidic medium produced similar results in proximal tubular cell lines stably expressing GFP-LC3 and facilitated the degradation of SQSTM1/p62 in wild-type cells, indicating enhanced autophagic flux. Upon acid loading, proximal tubule-specific autophagy-deficient (Atg5-deficient) mice displayed significantly reduced ammonium production and severe metabolic acidosis compared with wild-type mice. In vitro and in vivo, acid loading caused Atg5-deficient proximal tubular cells to exhibit reduced mitochondrial respiratory chain activity, reduced mitochondrial membrane potential, and fragmented morphology with marked swelling in mitochondria. GFP-LC3-tagged autophagosomes colocalized with ubiquitinated mitochondria in proximal tubular cells cultured in acidic medium, suggesting that metabolic acidosis induces mitophagy. Furthermore, restoration of Atg5-intact nuclei in Atg5-deficient proximal tubular cells increased mitochondrial membrane potential and ammoniagenesis. In conclusion, metabolic acidosis induces autophagy in proximal tubular cells, which is indispensable for maintaining proper mitochondrial functions including ammoniagenesis, and thus for adapted urinary acid excretion. Our results provide a rationale for the beneficial effect of alkali supplementation in CKD, a condition in which autophagy may be reduced, and suggest a new therapeutic option for acidosis by modulating autophagy.
Departments of Geriatric Medicine and Nephrology (B6).
Tomoko
T
Namba
Departments of Geriatric Medicine and Nephrology (B6).
Yoshitsugu
Y
Takabatake
Departments of Geriatric Medicine and Nephrology (B6), Department of Molecular Genetics and Microbiology, University of New Mexico, Albuquerque, New Mexico; and.
Tomonori
T
Kimura
Departments of Geriatric Medicine and Nephrology (B6).
Atsushi
A
Takahashi
Departments of Geriatric Medicine and Nephrology (B6).
Takeshi
T
Yamamoto
Departments of Geriatric Medicine and Nephrology (B6).
Jun
J
Matsuda
Departments of Geriatric Medicine and Nephrology (B6).
Harumi
H
Kitamura
Department of Pediatrics, and.
Fumio
F
Niimura
Institute of Medical Science and Department of Internal Medicine, Tokai University School of Medicine, Isehara, Kanagawa, Japan.
Taiji
T
Matsusaka
Departments of Geriatric Medicine and Nephrology (B6).
Hirotsugu
H
Iwatani
Departments of Geriatric Medicine and Nephrology (B6).
Isao
I
Matsui
Advanced Technology for Transplantation.
Junya
J
Kaimori
Medical Biochemistry, and Cardiovascular Medicine, Osaka University Graduate School of Medicine, Suita, Osaka, Japan;
Hidetaka
H
Kioka
Departments of Geriatric Medicine and Nephrology (B6), isaka@kid.med.osaka-u.ac.jp.
Yoshitaka
Y
Isaka
Departments of Geriatric Medicine and Nephrology (B6).
Hiromi
H
Rakugi
D000138:Acidosis / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology*; D064751:Ammonium Compounds / Q000378:metabolism; D000818:Animals; D001343:Autophagy; D002478:Cells, Cultured; D004579:Electron Transport; D005260:Female; D007687:Kidney Tubules, Proximal / Q000378:metabolism / Q000473:pathology / Q000503:physiopathology*; D008297:Male; D053078:Membrane Potential, Mitochondrial; D008822:Mice, Transgenic; D063306:Mitophagy
D064751:Ammonium Compounds