Klotho is an
enzyme that in humans is encoded by the KLgene.[5] The three subfamilies of klotho are α-klotho, β-klotho, and γ-klotho.[6] α-klotho activates
FGF23, and β-klotho activates
FGF19 and
FGF21.[7] When the subfamily is not specified, the word "klotho" typically refers to the α-klotho subfamily, because α-klotho was discovered before the other members.[8][7]
α-klotho is highly
expressed in the brain, liver and kidney.[9] β-klotho is predominantly expressed in the liver.[10][9] γ-klotho is expressed in the skin.[9]
Klotho can exist in a membrane-bound form or a (hormonal) soluble, circulating form.[11]Proteases can convert the membrane-bound form into the circulating form.[12]
The KL gene encodes a type-I single-pass transmembrane protein[7] that is related to
β-glucuronidases. Reduced production of this protein has been observed in patients with chronic
kidney failure (CKF), and this may be one of the factors underlying degenerative processes (e.g.,
arteriosclerosis,
osteoporosis, and
skin atrophy) seen in CKF. Mutations within the family have been associated with ageing, bone loss and alcohol consumption.[13][14] Transgenic mice that overexpress Klotho live longer than wild-type mice.[15]
Structure
The α-klotho gene is located on chromosome 13, and is translated into a single-pass
integral membrane protein.[9] The intracellular portion of the α-klotho protein is short (11
amino acids), whereas the extracellular portion is long (980 amino acids).[9] The transmembrane portion is also comparatively short (21 amino acids).[9] The extracellular portion contains two repeat sequences, termed the KL1 (about 450 amino acids) and KL2 (about 430 amino acids) domains.[9][7] In the
kidney and the
choroid plexus of the brain, the transmembrane protein can be
proteolytically cleaved to produce a 130-
Kilo-Dalton, soluble form of α-klotho protein, released into the
circulation and
cerebrospinal fluid, respectively.[9] In humans, the secreted form of klotho is more dominant than the membrane form.[16]
The β-Klotho gene is located on chromosome 4. The protein shares homology (43.1% identity and 60.1% similarity) with α-klotho. [17]
It should not be confused with the alpha-cut and beta-cut of alpha-klotho, which releases KL1+KL2 and KL2 domain, respectively.
Function
Klotho is a
transmembrane protein that, in addition to other effects, provides some control over the sensitivity of the organism to
insulin and appears to be involved in
ageing. Its discovery was documented in 1997 by Makoto Kuro-o et al.[18] The name of the gene comes from Klotho or
Clotho, one of the
Moirai, or Fates, in
Greek mythology, who spins the thread of human life.[7]
The klotho protein is a novel
β-glucuronidase (
EC number 3.2.1.31) capable of hydrolyzing
steroidβ-glucuronides. Genetic variants in KLOTHO have been associated with human aging,[19] and klotho protein has been shown to be a circulating factor detectable in
serum that declines with age.[20]
α-klotho, which binds to the endocrine FGF FGF23 changes cellular calcium homeostasis, by both increasing the expression and activity of
TRPV5 (decreasing phosphate reabsorption in the kidney) and decreasing that of
TRPC6 (decreasing phosphate absorption from the intestine).[24] α-klotho increases kidney calcium reabsorption by stabilizing TPRV5.[25] About 95% to 98% of Ca2+ filtered from the blood by the kidney is normally reabsorbed by the kidney's
renal tubule, which is mediated by TRPV5.[26]
β-klotho activation of
FGF21 protein has a protective effect on
heart muscle cells.[28]Obesity is characterized by FGF21 resistance, believed to be caused by the inhibition of β-klotho by the inflammatory cell signalling protein (
cytokine)
tumor necrosis factor alpha,[28] but there is evidence against this mechanism.[16]
Klotho is required for
oligodendrocyte maturation,
myelin integrity, and can protect neurons from toxic effects.[29] Mice deficient in klotho have a reduced number of
synapses and cognitive deficits, whereas mice overexpressing klotho have enhanced learning and memory.[30] Research with injections of klotho in primates demonstrates a positive effect on memory that lasts for as long as two weeks.[31]
It has been found that the decreased klotho expression may be due to DNA hypermethylation, which may have been induced by the overexpression of DNMT3a.[33] Klotho may be a reliable gene for early detection of methylation changes in oral tissues, and can be used as a target for therapeutic modification in oral cancer during the early stages.
Klotho-deficient mice manifest a
syndrome resembling accelerated human ageing and display extensive and accelerated
arteriosclerosis. Additionally, they exhibit impaired
endothelium dependent
vasodilation and impaired
angiogenesis, suggesting that klotho protein may protect the cardiovascular system through endothelium-derived
nitric oxide production.[16]
Research with injections of α-klotho in primates suggests a positive effect on memory that could have implications for research with humans.[31] Interestingly the cognitive effects in rhesus monkeys were observed even with
subcutaneous injection despite previous results showing that klotho protein fails to cross the
blood–brain barrier.[36]
Effects on aging
Reduced α-klotho or FGF23 can result in impaired
phosphate excretion from the kidney, leading to
hyperphosphatemia.[7] In mice, this leads to a
phenotype characteristic of premature aging, which can be mitigated by feeding the mice a low phosphate diet.[7]
The plasma (soluble) form of α-klotho is most easily measured, and has been shown to decrease after 40 years of age in humans.[37] Lower plasma levels of α-klotho in older adults is associated with increased
frailty and
all-cause mortality.[37] Physical activity has been shown to increase plasma α-klotho.[37]
Mice lacking either
fibroblast growth factor 23 or the α-klotho enzyme display premature aging due to
hyperphosphatemia.[24] Many of these symptoms can be alleviated by feeding the mice a low phosphate diet.[7]
Although the majority of research has explored klotho's absence, it was demonstrated that klotho over-expression in
mice extended their
average life span between 19% and 31% compared to normal mice.[15] In addition, variations in the Klotho gene (SNP Rs9536314) are associated with both life extension and increased cognition in human populations and mice, but only if the gene expression was
heterozygous, not
homozygous.[38][9] The cognitive benefits of α-klotho are primarily seen late in life.[9]
Klotho increases membrane expression of the inward rectifier
ATP-dependent
potassium channelROMK.[24] Klotho-deficient mice show increased production of vitamin D, and altered mineral-ion homeostasis is suggested to cause premature aging‑like phenotypes, because reduced vitamin D activity from dietary restriction reverses the premature aging‑like phenotypes and prolongs survival in these mutants. These results suggest that aging‑like phenotypes were due to klotho-associated vitamin D metabolic abnormalities (hypervitaminosis).[39][40][41][42]
Klotho is an antagonist of the
Wnt signaling pathway, and chronic Wnt stimulation can lead to
stem cell depletion and aging.[43] Klotho inhibition of Wnt signaling can inhibit
cancer.[32] The anti-aging effects of klotho are also a consequence of increased resistance to
inflammation and
oxidative stress.[16]
Extracellular vesicles (EV) in young mice carried more copies of klotho-producing mRNA than those from old mice. Transfusing young EVs into older mice helped rebuild their muscles.[44]
The presence of
senescent cells decreases α-klotho levels.
Senolytic drugs reduce the level of these cells, allowing α-klotho levels to increase.[45]
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^"Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
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^Adhikari BR, Uehara O, Matsuoka H, Takai R, Harada F, Utsunomiya M, et al. (September 2017). "Immunohistochemical evaluation of Klotho and DNA methyltransferase 3a in oral squamous cell carcinomas". Medical Molecular Morphology. 50 (3): 155–160.
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