The antiaging protein Klotho is encoded by the Klotho gene first identified as an ‘aging suppressor’, in mice. Klotho deficiency is involved in premature aging and early death, while its overexpression is related to longevity. Klotho is mostly expressed in the kidney, but also in the brain, and in other organs. Two forms of Klotho, the cell membrane and secreted form, have pleiotropic activities that include regulation of general metabolism, oxidative stress, and mineral metabolism that correlates with its effect on accelerating aging. Membrane Klotho serves as an obligate co-receptor for the fibroblast growth factor (FGF), while secreted Klotho plays its role as a humoral factor. Klotho protein participates in the regulation of several biological activities, including regulation of calcium-phosphate homeostasis and PTH as well as vitamin D metabolism. The active form of vitamin D, 1,25(OH)D (1,25-dihydroxy-vitamin D3 = calcitriol), acts as a neurosteroid that participates in the regulation of multiple brain functions. It provides neuroprotection and suppresses oxidative stress, inhibits inflammation and inflammatory mediators, and stimulates various neurotrophins. Calcitriol is involved in many brain-related diseases, including multiple sclerosis, Alzheimer´s disease, Parkinson´s disease, and schizophrenia. This review covers the most recent advances in Klotho research and discusses Klotho-dependent roles of calcitriol in neuro-psycho-pathophysiology.
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