Aging-related organ degeneration is driven by multiple factors including the cell maintenance mechanisms of autophagy, the cytoprotective protein αKlotho, and the lesser known effects of excess phosphate (Pi), or phosphotoxicity. To examine the interplay between Pi, autophagy, and αKlotho, we used the BK/BK mouse (homozygous for mutant Becn1 F121A ) with increased autophagic flux, and αKlothohypomorphic mouse (kl/kl) with impaired urinary Pi excretion, low autophagy, and premature organ dysfunction. BK/BK mice live longer than WT littermates, and have heightened phosphaturia from downregulation of two key NaPi cotransporters in the kidney. The multi-organ failure in kl/kl mice was rescued in the double-mutant BK/ BK;kl/kl mice exhibiting lower plasma Pi, improved weight gain, restored plasma and renal αKlotho levels, decreased pathology of multiple organs, and improved fertility compared to kl/kl mice. The beneficial effects of heightened autophagy from Becn1 F121A was abolished by chronic high-Pi diet which also shortened life span in the BK/BK;kl/kl mice. Pi promoted beclin 1 binding to its negative regulator BCL2, which impairs autophagy flux. Pi downregulated αKlotho, which also independently impaired autophagy. In conclusion, Pi, αKlotho, and autophagy interact intricately 3130 | SHI et al.

Klotho- and beclin 1-driven autophagy extends life. We examined the role of beclin 1 in modifying acute kidney injury (AKI) and whether beclin 1 mediates Klotho’s known renoprotective action in AKI. AKI was induced by ischemia-reperfusion injury in mice with different levels of autophagy activity by genetic manipulation: wild-type (WT) mice with normal beclin 1 expression and function, mice with normal beclin 1 levels but high activity through knockin of gain-of-function mutant beclin 1 ( Becn1F121A), mice with low beclin 1 levels and activity caused by heterozygous global deletion of beclin 1 ( Becn1+/−), or mice with extremely low beclin 1 activity from knockin of the mutant constitutively active beclin 1 inhibitor Bcl-2 ( Bcl2AAA). Klotho was increased by transgenic overexpression ( Tg-Kl) or recombinant Klotho protein administration. After ischemia-reperfusion injury, Becn1F121A mice (high autophagy) had milder AKI and Becn1+/− and Bcl2AAA mice (low autophagy) had more severe AKI than WT mice. Tg-Kl mice had milder AKI, but its renoprotection was partially attenuated in Becn1+/− ;Tg-Kl mice and was significantly reduced, although not completely abolished, in Bcl2AAA;Tg-Kl mice. Recombinant Klotho protein conferred more renoprotection from AKI in WT mice than in Becn1+/− or Bcl2AAA mice. Klotho reduced beclin 1/Bcl-2 protein complexes and increased autophagy activity, but this effect was less prominent in mice or cells with Bcl2AAA. Transfected Bcl2AAA or Becn1F123A decreased or increased autophagy activity and rendered cells more susceptible or more resistant to oxidative cytotoxicity, respectively. In conclusion, beclin 1 confers renoprotection by activating autophagy. Klotho protects the kidney partially via disruption of beclin 1/Bcl-2 interactions and enhancement of autophagy activity.