αKlotho decreases after reduced weight-bearing from both spaceflight and hindlimb unloading

Alpha(α)Klotho, a soluble transmembrane protein, facilitates calcium-phosphorus homeostasis through feedback between bone and kidney and is a potential systemic biomarker for bone-kidney health during spaceflight. We determined if: (1) plasma αKlotho was reduced after both spaceflight aboard the ISS and hindlimb unloading (HU); and (2) deficiency could be reversed with exercise. Both spaceflight and HU lowered circulating plasma αKlotho: plasma αKlotho recovered with exercise after HU.