Klotho, a membrane protein mainly expressed in parathyroid glands, kidney, and 
choroid plexus, counteracts aging and increases the life span. Accordingly, life 
span is significantly shorter in Klotho-deficient mice (klotho(-/-)) than in 
their wild-type littermates (klotho(+/+)). The pleotropic effects of Klotho 
include inhibition of 1,25-dihydroxyvitamin D(3)(1,25(OH)(2)D(3)) formation. 
Vitamin D-deficient diet reverses the shortening of life span in klotho(-/-) 
mice. In a variety of cells, 1,25(OH)(2)D(3) stimulates Ca(2+) entry. In 
erythrocytes, increased Ca(2+) entry stimulates suicidal erythrocyte death, 
which is characterized by cell shrinkage and phosphatidylserine exposure at the 
erythrocyte surface. The present study explored the putative impact of Klotho on 
eryptosis. According to Fluo3 fluorescence, cytosolic Ca(2+) concentration was 
significantly larger in klotho(-/-) erythrocytes as compared to klotho(+/+) 
erythrocytes. According to annexin V-binding, phosphatidylserine exposure was 
significantly enhanced, and according to forward scatter, cell volume 
significantly decreased in klotho(-/-) erythrocytes as compared to klotho(+/+) 
erythrocytes. Energy depletion (13 h glucose depletion) and oxidative stress (35 
min 1 mM tert-butyl-hydroxyl-peroxide [tert-BOOH]) increased phosphatidylserine 
exposure to values again significantly larger in klotho(-/-) erythrocytes as 
compared to klotho(+/+) erythrocytes. Reticulocyte number was significantly 
increased in klotho (-/-) mice, pointing to enhanced erythrocyte turnover. 
Vitamin D-deficient diet reversed the enhanced Ca(2+) entry and annexin 
V-binding of klotho(-/-) erythrocytes. The present observations reveal a novel 
function of Klotho, i.e., the at least partially vitamin D-dependent regulation 
of cytosolic Ca(2+) activity in and suicidal death of erythrocytes.