[7] The klotho knockout mouse is now an established animal model of ageing, allowing further study of well-accepted processes that occur with ageing, such as arteriosclerosis, arterial calcification LEE011 purchase and osteoporosis, and other less well-studied processes such as angiogenesis.[7, 11, 12] The klotho gene encodes a 1012 amino acid long single-pass transmembrane protein,[7] commonly referred

to as α-klotho, to differentiate it from two subsequently discovered members of the klotho family; β-klotho and γ-klotho. All three are single-pass transmembrane proteins of different lengths, which not only share a substantial degree of homology, but function as obligate co-receptors to endocrine FGF.[13] Within the extensive superfamily of FGF, only the FGF19 subfamily consisting of FGF19, FGF21 and FGF23 are endocrine FGF while the other FGF function as paracrine/autocrine factors.[13, 14] FGF receptors (FGFR) are detected ubiquitously while klotho expression is limited to certain tissues, thereby determining tissue specificity for the endocrine action of their respective FGF.[15] α-klotho is an obligate co-receptor for physiological FGF23

signalling and appears essential for FGF23-mediated phosphate regulation MK-2206 in animal models.[15-17] It is now also evident that klotho proteins play a role in a range of other metabolic processes.[7, 8, 15, 18-20] β-klotho, that augments FGF19 and FGF21 signalling, is found in liver, gall bladder, pancreas, colon and adipose tissue and participates in bile acid metabolic pathways.[19, 20] γ-klotho is coupled to FGF19 and is found in the eye, adipose and kidney and its function remains cryptic.[13] The remainder of this review focuses on α-klotho and will henceforth be referred to as klotho. Klotho exists in two forms – membrane-bound klotho (mKl) and soluble klotho (sKl). mKl is variably expressed in different tissues including parathyroid, brain, heart and testis with low-level expression Oxymatrine also detected in the aorta.[7, 21] Klotho is most abundantly described in the kidney with earlier reports focused on distal convoluted tubule expression,[7] though more recently

proximal tubule expression of mKl has been reported.[22] sKl, on the other hand, is produced in two ways. The first is a result of ectodomain cleavage of mKl (∼130 kDa) although factors regulating ectodomain shedding remain poorly characterized. A number of proteases have been implicated, most notably a disintegrin and metalloproteinase (ADAM) 10/17, which is also expressed in the distal convoluted tubule. The second is a product of alternative splicing leading to a shorter form of sKl (∼70 kDa). Proteomic analysis of various extracellular fluids suggests that the longer form of sKl, generated by cleavage is the major circulating species in humans.[23-25] The actions of mKl and sKl differ, with mKl predominantly supporting FGF23 in regulating phosphate.