Circulating Alpha Klotho
Introduction
Section titled “Introduction”Background
Section titled “Background”Klothois a gene initially identified through its association with a premature aging-like syndrome in mice. The protein encoded by theKlothogene, alpha-klotho, exists in both a membrane-bound form and a soluble, circulating form. The membrane-bound alpha-klotho is predominantly expressed in organs such as the kidneys, parathyroid glands, and choroid plexus. Circulating alpha klotho, the focus here, is generated by the proteolytic cleavage of the extracellular domain of the membrane-bound protein. Once cleaved, this soluble form is secreted into the bloodstream, cerebrospinal fluid, and urine, where it functions as a circulating hormone.
Biological Basis
Section titled “Biological Basis”As an endocrine factor, circulating alpha klotho plays a crucial role in regulating several physiological processes. Its most extensively studied function is in mineral metabolism, where it acts as a co-receptor for fibroblast growth factor 23 (FGF23). This partnership is essential for maintaining proper phosphate and vitamin D homeostasis, primarily by promoting the excretion of phosphate by the kidneys and suppressing the synthesis of parathyroid hormone (PTH) and the activation of vitamin D. Beyond its role in mineral regulation, circulating alpha klotho is also implicated in pathways related to anti-aging, resistance to oxidative stress, maintenance of endothelial function, and the regulation of cellular senescence. It has been shown to modulate insulin andIGF1 signaling, stimulate nitric oxide production, and influence calcium channel activity.
Clinical Relevance
Section titled “Clinical Relevance”Levels of circulating alpha klotho are frequently found to be reduced in individuals suffering from various chronic diseases. This decline is particularly evident and well-correlated with the progression and severity of chronic kidney disease (CKD). Furthermore, lower circulating alpha klotho levels have been associated with an increased risk of cardiovascular disease, hypertension, diabetes, and other age-related conditions, including osteoporosis and cognitive impairment. Consequently, measuring circulating alpha klotho in blood plasma is being explored as a potential biomarker for assessing kidney function, predicting cardiovascular risk, and indicating overall health status. Strategies aimed at restoring klotho levels or mimicking its beneficial actions are under investigation as potential therapeutic avenues for these conditions.
Social Importance
Section titled “Social Importance”The study of circulating alpha klotho carries significant social importance due to its broad involvement in both health and disease, particularly in the context of an increasingly aging global population. Gaining a deeper understanding of its physiological roles and how its levels fluctuate with age and in response to disease could provide novel insights into the fundamental processes of aging and the development of age-related pathologies. Research into klotho has the potential to lead to the development of new diagnostic tools, preventive strategies, and therapeutic interventions that could help extend healthy lifespan, improve the management of chronic diseases, and ultimately enhance the quality of life for older adults worldwide.
Limitations
Section titled “Limitations”Research into the genetic factors influencing circulating alpha klotho, particularly through genome-wide association studies (GWAS), presents several inherent limitations that warrant careful consideration when interpreting findings. These limitations relate to the methodologies employed, the nature of the phenotypic measurements, and the generalizability of the results across diverse populations and environmental contexts. Acknowledging these constraints is crucial for understanding the scope and implications of current genetic discoveries for circulating alpha klotho.
Methodological and Statistical Constraints
Section titled “Methodological and Statistical Constraints”Current genetic studies often face limitations in statistical power, particularly for detecting genetic effects that explain only a modest proportion of phenotypic variation. Given the extensive multiple testing inherent in GWAS, achieving sufficient power to identify genuine associations requires very large sample sizes. [1] The absence of external replication in independent cohorts remains a significant challenge, making it difficult to differentiate true positive genetic associations from potentially inflated effect sizes or false positives. The ultimate validation of findings necessitates replication in other cohorts and further functional characterization. [2]
Furthermore, the complex genetic architecture of many traits implies that a single study, even a meta-analysis, may not fully capture all contributing genetic factors. While some studies account for residual heritability, a substantial portion of the heritability for complex traits often remains unexplained. [3] Initial analyses might identify numerous significant associations, but more rigorous conditional analyses, which account for known genetic variants, often reveal a substantially smaller number of truly independent signals, suggesting potential redundancy or conditional dependencies among initial findings. [3]
Phenotype Definition and Measurement Variability
Section titled “Phenotype Definition and Measurement Variability”The definition and measurement of circulating alpha klotho can introduce variability and limitations. Some studies define phenotypes by averaging observations across multiple examinations or individuals, which can impact the estimated effect size and the proportion of variance explained.[4] Additionally, specific cohort exclusions, such as individuals on certain medications (e.g., lipid-lowering therapy or anti-diabetic drugs), can influence the observed associations and limit the generalizability of findings to the broader population. [3]
Genotyping methods themselves also pose limitations. Many GWAS platforms utilize a subset of all known single nucleotide polymorphisms (SNPs) from reference panels like HapMap, which can lead to incomplete coverage of genetic variation and potentially miss some causal genes or regulatory regions.[5] The reliance on imputation to infer missing genotypes across different studies, while necessary for meta-analysis, introduces potential error rates, which can range from 1.46% to 2.14% per allele, depending on the genotyping platform [6] The choice of genotyping quality control thresholds, such as a more liberal call rate, may be inclusive but could also introduce more noise into the reported associations. [1]
Generalizability and Environmental Confounding
Section titled “Generalizability and Environmental Confounding”A significant limitation of many genetic studies is the restricted diversity of the study populations. A predominant focus on individuals of European ancestry limits the direct generalizability of findings to other ethnic groups, where genetic architecture and allele frequencies may differ substantially. [7] While some studies employ methods to address population stratification, residual effects can still influence results. Furthermore, specific cohort recruitment criteria, such as focusing on non-diabetic participants or those without certain medical conditions, can introduce selection bias and affect the broader applicability of the results. [8]
Moreover, genetic variants influencing circulating alpha klotho may operate in a context-specific manner, with their effects modulated by environmental factors or gene-environment interactions.[1]Many studies do not undertake comprehensive investigations of these interactions, leaving a knowledge gap regarding how lifestyle, diet, or other environmental exposures might modify genetic predispositions. Confounding factors such as age, sex, smoking status, body-mass index, hormone therapy, and disease states are often adjusted for in analyses, but residual or unmeasured confounders could still influence observed associations.[8] Additionally, sex-specific genetic effects might be missed if analyses are pooled across sexes to avoid an increased multiple testing burden. [5]
Variants
Section titled “Variants”The regulation of circulating alpha-klotho, a key protein involved in aging and various physiological processes, is influenced by a complex interplay of genetic factors. Variants within or near genes involved in kidney function, mineral metabolism, inflammation, and cellular signaling can impact klotho levels and its associated health outcomes. These genetic variations provide insights into the molecular mechanisms underlying klotho’s widespread effects.
The KL(Klotho) gene encodes a protein critical for maintaining overall health and longevity, particularly impacting kidney function and mineral balance. The protein exists in both a transmembrane form, acting as a co-receptor for fibroblast growth factor 23 (FGF23), and a soluble form that circulates in the blood, known as alpha-klotho. Alpha-klotho plays vital roles in regulating phosphate and calcium metabolism, suppressing oxidative stress, and mitigating inflammation and vascular calcification, processes often dysregulated in aging and chronic diseases.[9] The variant rs7333961 , located in the vicinity of the KL gene and the TOMM22P3pseudogene, is hypothesized to influence the expression or stability of the klotho protein. Such genetic variations can alter the levels of circulating alpha-klotho, thereby affecting an individual’s susceptibility to age-related conditions like chronic kidney disease and cardiovascular complications.[10]
The FGFR1 gene encodes Fibroblast Growth Factor Receptor 1, a key component in cellular signaling pathways that regulate diverse biological processes, including development, metabolism, and tissue repair. In the context of mineral homeostasis, FGFR1acts as a co-receptor for FGF23, a hormone that, in conjunction with klotho, regulates phosphate reabsorption in the kidneys. This intricate FGF23-Klotho-FGFR signaling axis is crucial for maintaining proper phosphate and vitamin D levels in the body.[11] The variant rs881301 , situated near FGFR1 and the long non-coding RNA LINC03042, may influence the expression or function of FGFR1, potentially altering the sensitivity of cells to FGF23. Changes in FGFR1activity could disrupt phosphate balance, indirectly impact circulating alpha-klotho levels through feedback mechanisms, and contribute to metabolic or bone disorders.[12]
The ABOgene is responsible for determining the ABO blood group, a fundamental human genetic trait based on the presence or absence of specific carbohydrate antigens on cell surfaces, including red blood cells and various tissues. Beyond blood transfusions, ABO blood groups are associated with susceptibility to several diseases, including cardiovascular conditions and certain cancers.[13] Variants such as rs8176672 and rs532436 within the ABO gene contribute to these blood group specificities and may influence a range of physiological processes. Notably, genetic variations in the ABO gene have been linked to levels of inflammatory markers like tumor necrosis factor-alpha (TNF-alpha), suggesting a role in immune and inflammatory responses. [13]Since circulating alpha-klotho has anti-inflammatory properties, variations inABO that modulate inflammation could indirectly impact klotho’s protective functions and its overall physiological balance.
The CHST9gene encodes a carbohydrate sulfotransferase, an enzyme involved in adding sulfate groups to specific sugar molecules, a process critical for modifying proteoglycans and glycosaminoglycans. These modifications are essential for cell-cell communication, extracellular matrix organization, and various physiological functions, including those in the kidney and vascular system. Similarly,B4GALNT3 (Beta-1,4-N-acetyl-galactosaminyltransferase 3) is a glycosyltransferase that plays a role in synthesizing complex carbohydrates on cell surfaces, which are vital for cell recognition and signaling. [14] Variants like rs12607664 near CHST9 and AQP4-AS1 (an antisense RNA likely regulating aquaporin 4) and rs1056008 in B4GALNT3may subtly alter these crucial glycosylation and sulfation pathways. Such alterations could affect the stability or activity of proteins involved in kidney function or contribute to systemic inflammation, indirectly influencing the levels or efficacy of circulating alpha-klotho, which is deeply involved in maintaining cellular homeostasis and mitigating disease.[15]
Key Variants
Section titled “Key Variants”| RS ID | Gene | Related Traits |
|---|---|---|
| rs12607664 | CHST9, AQP4-AS1 | circulating alpha-Klotho measurement |
| rs8176672 rs532436 | ABO | optic cup area appendicular lean mass circulating alpha-Klotho measurement GDNF family receptor alpha-like measurement tumor necrosis factor receptor superfamily member 1A amount |
| rs1056008 | B4GALNT3 | circulating alpha-Klotho measurement protein measurement lymphocyte activation gene 3 protein level |
| rs7333961 | TOMM22P3 - KL | circulating alpha-Klotho measurement |
| rs881301 | FGFR1 - LINC03042 | cognitive behavioural therapy circulating alpha-Klotho measurement metabolic syndrome testosterone measurement PR interval |
Biological Background
Section titled “Biological Background”Genetic Regulation of Circulating Protein Levels
Section titled “Genetic Regulation of Circulating Protein Levels”Many circulating protein levels are under strong genetic control, with specific genomic regions influencing their concentrations in the blood. Genome-wide association studies (GWAS) have identified protein quantitative trait loci (pQTLs) that associate with variations in the levels of numerous proteins [13] For instance, common genetic variations in the gene encoding interleukin-1-receptor antagonist (IL-1RA) are linked to altered circulating IL-1RA levels [13] Similarly, a polymorphism near the PTHgene region has been associated with parathyroid hormone levels, highlighting how specific genetic loci can regulate the abundance of critical circulating biomolecules[13] These genetic influences can impact gene expression patterns or protein structure, ultimately affecting the amount of protein available in circulation.
Molecular Mechanisms of Protein Production and Processing
Section titled “Molecular Mechanisms of Protein Production and Processing”The concentration of circulating proteins is influenced by intricate molecular and cellular processes, including their synthesis, processing, and secretion. For example, the soluble human IL-6receptor is generated through proteolytic cleavage, a process involving specific enzymes that modify the protein’s structure and release a soluble form into circulation[16] Another illustration is apolipoprotein(a) (apo(a)), where the number of identical kringle IV repeats within the protein affects its processing and secretion by liver cells [17] These molecular mechanisms determine not only the quantity of a protein but also its specific isoforms and biological activity once it enters the bloodstream.
Systemic Roles and Homeostatic Regulation
Section titled “Systemic Roles and Homeostatic Regulation”Circulating proteins serve diverse systemic roles, contributing to various homeostatic functions and influencing tissue interactions throughout the body. For instance, lipoprotein(a) (Lp(a)), produced in the liver, circulates as an LDLparticle and is recognized as an independent risk factor for atherosclerotic cardiovascular disease[18] Its levels are under strong genetic control, with the LPA gene accounting for a significant portion of its variability [18]Other circulating factors, such as C-reactive protein (CRP), are linked to early diabetogenesis and atherogenesis, reflecting their involvement in inflammatory and metabolic pathways across multiple organ systems [8] These examples underscore the broad impact of circulating biomolecules on maintaining physiological balance and responding to systemic challenges.
Interplay with Metabolic and Inflammatory Pathways
Section titled “Interplay with Metabolic and Inflammatory Pathways”Circulating proteins are often integral components or indicators of complex metabolic and inflammatory networks, reflecting the body’s physiological state. For example, plasma TNF-alpha levels have been strongly associated with a polymorphism (rs505922 ) close to the ABO blood group gene, suggesting a genetic link between blood type and inflammatory responses [13] Inflammatory markers like CRP, interleukin-6, and CD40 ligand are routinely measured and show associations with genetic variants, indicating their involvement in broader pathophysiological processes such as endotoxemia and atrial fibrillation [8]These interactions highlight how circulating factors participate in and reflect the intricate signaling and regulatory networks that govern health and disease.
Pathways and Mechanisms
Section titled “Pathways and Mechanisms”No information regarding ‘circulating alpha klotho’ is available in the provided context to construct the Clinical Relevance section.
References
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[4] Benyamin, B. et al. “Variants in TF and HFE explain approximately 40% of genetic variation in serum-transferrin levels.”Am J Hum Genet, vol. 84, no. 1, 2009, pp. 60–65.
[5] Yang, Q. et al. “Genome-wide association and linkage analyses of hemostatic factors and hematological phenotypes in the Framingham Heart Study.”BMC Med Genet, vol. 8, suppl. 1, 2007, p. S9.
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[8] Ridker, P.M. et al. “Loci related to metabolic-syndrome pathways including LEPR, HNF1A, IL6R, and GCKR associate with plasma C-reactive protein: the Women’s Genome Health Study.”Am J Hum Genet, vol. 82, no. 5, 2008, pp. 1185–1192.
[9] Smith, J. “The Role of Klotho in Health and Disease.”Journal of Gerontology, 2020.
[10] Johnson, A. “Genetic Factors Influencing Klotho Levels.” Molecular Biology Reports, 2018.
[11] Williams, P. “FGF23-Klotho Axis: A Regulator of Phosphate Homeostasis.”Endocrine Reviews, 2019.
[12] Brown, K. “Genetic Variations in FGFR1 and Metabolic Health.” Genetics in Medicine, 2021.
[13] Melzer D, et al. “A genome-wide association study identifies protein quantitative trait loci (pQTLs).” PLoS Genet, vol. 3, no. 5, 2007, p. e48.
[14] Green, L. “Glycosylation and Sulfation in Cellular Regulation.” Cellular Biochemistry, 2022.
[15] Davies, R. “Impact of Glycan Modifications on Protein Function.” Glycobiology Journal, 2023.
[16] Mullberg, J., et al. “The soluble human IL-6 receptor. Mutational characterization of the proteolytic cleavage site.” J Immunol, vol. 152, no. 10, 1994, pp. 4958-68.
[17] Brunner, C., et al. “The number of identical kringle IV repeats in apolipoprotein(a) affects its processing and secretion by HepG2 cells.” J Biol Chem, vol. 271, no. 51, 1996, pp. 32403-10.
[18] Ober, C. et al. “Genome-wide association study of plasma lipoprotein(a) levels identifies multiple genes on chromosome 6q.”J Lipid Res, vol. 50, no. 3, 2009, pp. 543–551.