Mice homozygous for severely hypomorphic alleles of the Klotho gene (klotho mice) exhibit a syndrome resembling human aging, including atherosclerosis, osteoporosis, emphysema, and infertility.
Based on both macroscopic and histological appearance, klotho mice are developmentally normal until 3-4 weeks of age.
Thereafter, they exhibit growth retardation, gradually become inactive, and die prematurely at ~8-9 weeks of age.
The klotho gene encodes a putative type I membrane protein, which consists of an N-terminal signal sequence, an extracellular domain with two internal repeats (KL1 and KL2), a single transmembrane domain, and a short intracellular domain.
The two internal repeats share homology with members of the family 1 glycosidases and exhibit 20-40% sequence identity to β-glucosidases from bacteria and plants as well as mammalian lactase glycosylceramidase.
Analysis of cDNA revealed that the klotho gene also expresses a secreted form, lacking KL2, transmembrane, and intracellular domains, due to alternative RNA splicing.
Thus, klotho may also act as a humoral factor.
Further evidence for this hypothesis includes the observation that klotho mRNA expression was not detectable in many organs in which severe changes occur in klotho mice.
Similarly, transgenic expression of klotho limited to brain and testis was sufficient to rescue all macroscopic features observed in klotho mice.
The mechanism of klotho function remains unclear.
Recent experiments have shown that endothelium-dependent vasodilation of the aorta and arterioles was impaired in heterozygous klotho mice, but could be restored by parabiosis with wild-type mice.
In addition, nitric oxide metabolites (NO2 and NO3) in urine are significantly lower in heterozygous klotho mice.
These results suggest that klotho protein may protect the cardiovascular system through endothelium-derived NO production.
Remarkably, in vivo klotho gene delivery can ameliorate vascular endothelial dysfunction, increase nitric oxide production, reduce elevated blood pressure, and prevent medial hypertrophy and perivascular fibrosis in a rat model with multiple atherogenic risk factors including hypertension, diabetes, obesity, and hyperlipidemia.
Human klotho shows 86% amino acid identity with the mouse protein, and is encoded by a gene that spans over 50 kb on chromosome 13q12.
To date, no premature-aging syndromes have been linked to this region.
Like the mouse Klotho gene, human KLOTHO encodes both membrane-bound and secreted forms; however, in humans the secreted form predominates.
Expression of KLOTHO mRNA occurs primarily in the placenta, kidney, prostate, and small intestine.
Fig. 1. Sequence variation in the KLOTHO locus.
(A) Schematic of mutations detected in KLOTHO.
(B) Amino acid sequence of klotho amino acid positions 346-374 and homologous proteins.
Klotho has two homologous domains, here labeled ''Klotho1'' and ''Klotho2.''
The common allele of klotho is labeled ''Klotho1-wt'' and the rare allele is labeled ''Klotho1-mt.''
Alignment was performed using the CLUSTALW program (MACVECTOR 6.5.1).
Amino acids found in at least 9 of 16 positions are shaded in black.
Gray shading indicates conservation of amino acid character in at least 9 of 16 positions.
The black arrowheads indicate amino acid positions at which mutations in klotho have been detected.
Multiple models of aging invoke accelerated or excessive posttranslational modification of proteins including glycation.
Resultant advanced glycation end-products (AGEs) elicit a wide range of responses that have been proposed to contribute to many age related phenotypes, including atherosclerosis, Alzheimer's disease, diabetic complications, and microvascular changes.
It is possible that the proposed glycosidase activity of klotho retards the accumulation of AGEs.
Alternatively, klotho may contribute to or regulate signaling cascades with downstream targets that influence aging.
Based on observations in klotho deficient mice, these targets may include factors involved in NO production and maintenance of proper endothelial function.
These hypotheses remain to be tested in humans or mouse models of normal or accelerated aging.