Insulin stimulates the cleavage and release of the extracellular domain of Klotho by ADAM10 and ADAM17

Soluble α-Klotho in Liver Cirrhosis and Alcoholism

AIMS AND BACKGROUND

Alpha Klotho is a transmembrane protein that serves as co-receptor for FGF23. Ectodomain of membrane bound α Klotho may be shed by membrane bound proteases (activated, among other factors, by tumor necrosis factor (TNF)-α) generating the soluble form of the protein (sKl) that functions as a hormone by itself. It modulates calcium influx into cells, blunts IGF-1/Insulin signaling, promotes synthesis of antioxidants, generally slows down tumor progression, delays cell senescence, is neuroprotective and promotes oligodendrocyte maturation and myelin synthesis, and muscle rejuvenation. It may be involved in inflammation and exerts antifibrogenic effects. Some of these pathways may become altered in alcoholism or liver cirrhosis, but data are scattered and scarce and an update is required.

METHOD

Literature survey.

RESULTS AND CONCLUSIONS

Alcohol consumption in non-alcoholics is inversely related to sKl, but alcoholic cirrhotics showed higher-than-normal sKl values in association with liver function derangement. In hepatoma cells, the intensity of Klotho staining was related to faster tumor progression and a shortened life span. Among severe alcoholic cirrhotics sKl is directly related to serum TNF-α levels, and, inversely, to brain atrophy. Given the antioxidant, anti-inflammatory, and antifibrogenic effects of Klotho, perhaps the increase in cirrhosis (and in other inflammatory conditions, such as sepsis or cancer) reflects an attempt to regulate increased inflammation, but clinical and experimental research is urgently needed in this field.

Administration of alpha klotho reduces liver and adipose lipid accumulation in obese mice

α-Klotho, a known anti-aging protein, exerts diverse physiological effects including: maintenance of phosphate and calcium homeostasis, modulation of cell proliferation, and enhanced buffering of reactive oxygen species. However, the role of α-Klotho in the regulation of energy metabolism is complex and poorly understood. Here we investigated the effects of 5 weeks peripheral administration of α-Klotho in high fat diet induced obese mice. Food intake, blood glucose, and body weight were measured daily. Energy expenditure was determined with indirect calorimetry and body composition with magnetic resonance imaging. Liver and adipose tissue were collected for lipid content measurements and gene expression analysis. α-Klotho-treated mice experienced reduced adiposity, increased lean mass, and elevated energy expenditure, despite no changes in food intake, body weight, or fed blood glucose levels. Lipid accumulation in liver and adipose tissue was also reduced compared to controls. Furthermore, Real-time quantitative PCR showed reduced expression of key lipogenic genes in α-Klotho treated mice in these organs. Taken together, these data suggest encouraging therapeutic potential of α-Klotho and highlight a need for further research into the specific mechanisms explaining improved body composition, elevated energy expenditure, and reduced lipid content in both liver and adipose tissue in α-Klotho-treated mice.

Soluble delta-like 1 homolog decreases in patients with acromegaly following pituitary surgery: A potential mediator of adipogenesis suppression by growth hormone?

OBJECTIVE

GH excess in acromegaly leads to lower fat mass and insulin resistance; both reverse following pituitary surgery. Soluble delta like-1 homolog (sDlk1) inhibits adipocyte differentiation and may mediate the antiadipogenic effects of GH. It is released into the circulation by ectodomain shedding through 'A Disintegrin And Metalloproteinase domain 17' (ADAM17), which also sheds soluble α-Klotho (sKlotho). Klotho is a transmembrane protein, which influences life span. sKlotho inhibits insulin signalling, and is markedly elevated in acromegaly and decreases after surgery. Therefore, we examined if sDlk1 parallels the course of sKlotho, which could explain the well-known changes in fat mass in patients with acromegaly after surgery.

DESIGN

We measured serum levels of GH, IGF-1, sDlk1 and sKlotho (both by ELISA) in 42 treatment-naïve acromegaly patients (20 females/22 males) before and 1-3 months after transsphenoidal surgery. Data are presented as median(interquartile range).

RESULTS

GH decreased in all patients postoperatively (in 32/42 to <1 ng/ml during oral glucose tolerance testing). Likewise, IGF-1 and sKlotho decreased in all patients, from 587 (432-708) to 195 (133-270) ng/ml, and from 4.0 (2.7-5.9) to 0.7 (0.6-1.2) ng/ml, respectively; sDlk1 fell in 40/42 subjects, from 10.7 (5.8-13.4) to 7.1 (3.7-10.4) ng/ml following pituitary surgery. P < 0.0001 for all parameters.

CONCLUSIONS

sDlk1 declined after pituitary surgery in our patients with acromegaly, but to a lesser extent than sKlotho. It remains to be seen whether this may contribute to the well-known postoperative changes in body composition. Our findings may extend beyond the scope of acromegaly, and thus further elucidate mechanisms in the fields of obesity and anti-ageing.

Lentiviral vector-mediated overexpression of Klotho in the brain improves Alzheimer's disease-like pathology and cognitive deficits in mice

Alzheimer's disease (AD) is the most common type of senile dementia. The antiaging gene Klotho is reported to decline in the brain of patients and animals with AD. However, the role of Klotho in the progression of AD remains elusive. The present study explored the effects and underlying mechanism of Klotho in a mouse model of AD. The upregulation of cerebral Klotho expression was mediated by an intracerebroventricular injection of a lentiviral vector that encoded Klotho (LV-KL) in 7-month-old amyloid precursor protein/presenilin 1 transgenic mice. Three months later, LV-KL significantly induced Klotho overexpression in the brain and effectively ameliorated cognitive deficit and AD-like pathology in amyloid precursor protein/presenilin 1 mice. LV-KL induced autophagy activation and protein kinase B/mammalian target of rapamycin inhibition both in AD mice and BV2 murine microglia. These results suggest that the upregulation of Klotho expression in the brain may promote the autophagic clearance of amyloid beta and protect against cognitive deficits in AD mice. These findings highlight the preventive and therapeutic potential of Klotho for the treatment of AD.

The Biological Role of Klotho Protein in the Development of Cardiovascular Diseases

Klotho is a membrane-bound or soluble antiaging protein, whose protective activity is essential for a proper function of many organs. In 1997, an accidental insertion of a transgene led to creation of transgenic mice with several age-related disorders. In Klotho-deficient mice, the inherited phenotypes closely resemble human aging, while in an animal model of Klotho overexpression, the lifespan is extended. Klotho protein is detected mainly in the kidneys and brain. It is a coreceptor for fibroblast growth factor and hence is involved in maintaining endocrine system homeostasis. Furthermore, an inhibition of insulin/insulin-like growth factor-1 signaling pathway by Klotho regulates oxidative stress and reduces cell death. The association between serum Klotho and the classic risk factors, as well as the clinical history of cardiovascular disease, was also shown. There are a lot of evidences that Klotho deficiency correlates with the occurrence and development of coronary artery disease, atherosclerosis, myocardial infarction, and left ventricular hypertrophy. Therefore, an involvement of Klotho in the signaling pathways and in regulation of a proper cell metabolism could be a crucial factor in the cardiac and vascular protection. It is also well established that Klotho protein enhances the antioxidative response via augmented production of superoxide dismutase and reduced generation of reactive oxygen species. Recent studies have proven an expression of Klotho in cardiomyocytes and its increased expression in stress-related heart injury. Thus, the antioxidative and antiapoptotic activity of Klotho could be considered as the novel protective factor in cardiovascular disease and heart injury.

Role of αKlotho and FGF23 in regulation of type II Na-dependent phosphate co-transporters

Alpha-Klotho is a member of the Klotho family consisting of two other single-pass transmembrane proteins: βKlotho and γKlotho; αKlotho has been shown to circulate in the blood. Fibroblast growth factor (FGF)23 is a member of the FGF superfamily of 22 genes/proteins. αKlotho serves as a co-receptor with FGF receptors (FGFRs) to provide a receptacle for physiological FGF23 signaling including regulation of phosphate metabolism. The extracellular domain of transmembrane αKlotho is shed by secretases and released into blood circulation (soluble αKlotho). Soluble αKlotho has both FGF23-independent and FGF23-dependent roles in phosphate homeostasis by modulating intestinal phosphate absorption, urinary phosphate excretion, and phosphate distribution into bone in concerted interaction with other calciophosphotropic hormones such as PTH and 1,25-(OH)₂D. The direct role of αKlotho and FGF23 in the maintenance of phosphate homeostasis is partly mediated by modulation of type II Na⁺-dependent phosphate co-transporters in target organs. αKlotho and FGF23 are principal phosphotropic hormones, and the manipulation of the αKlotho-FGF23 axis is a novel therapeutic strategy for genetic and acquired phosphate disorders and for conditions with FGF23 excess and αKlotho deficiency such as chronic kidney disease.

JNK and ATF4 as two important platforms for tumor necrosis factor-α-stimulated shedding of receptor for advanced glycation end products

Soluble receptor for advanced glycation end products (sRAGE), shed from cell surfaces, is found in human circulation and has been implicated in cardiovascular disease. Its pathophysiological regulation and underlying mechanisms are scarcely understood. In endothelium-specific human RAGE transgenic mice, human sRAGE was detected in circulation, whereas its level was markedly increased after LPS treatment. That increase was preceded by a rapid rise in TNF-α level. Treatment with TNF-α also significantly increased serum sRAGE. In human microvascular endothelial cells or human umbilical vein endothelial cells with RAGE overexpression, TNF-α markedly induced RAGE shedding, which was dependent on MMP9 and ADAM10. TNF-α-stimulated MMP9 expression was completely dependent on JNK activation, with its inhibition partially effective in suppressing TNF-α-induced RAGE shedding. In contrast, TNF-α transiently induced activation transcription factor (ATF)4, a major component in unfolded protein response (UPR), whereas knockdown of ATF4 abrogated TNF-α-stimulated RAGE shedding. Protein levels of the pro and activated forms of ADAM10 were also decreased by ATF4 knockdown, whereas inhibition of other components of UPR, including XBP1 and ATF6, failed to block TNF-α-stimulated RAGE shedding. Although the endoplasmic reticulum stressors thapsigargin and tunicamycin induced markedly and sustained expression of ATF4 and XBP-1, they did not induce RAGE shedding to the same level as TNF-α, suggesting that ATF4 is necessary but not sufficient alone for TNF-α-mediated RAGE shedding. ATF4 inhibition did not affect TNF-α-stimulated MMP9 expression, whereas inhibition of JNK activity did not influence ADAM10 activation. Thus, inflammatory cascades including TNF-α induced RAGE shedding in endothelial cells in vivo and in vitro. JNK and ATF4 may be 2 platforms for regulation of TNF-α-stimulated RAGE shedding.-Miyoshi, A., Koyama, S., Sasagawa-Monden, M., Kadoya, M., Konishi, K., Shoji, T., Inaba, M., Yamamoto, Y., Koyama, H. JNK and ATF4 as two important platforms for tumor necrosis factor-α-stimulated shedding of receptor for advanced glycation end products.

The Pituitary Is a Candidate Organ That Modulates Circulating Klotho Levels

Context

The antiaging protein Klotho is shed and released into the blood stream (soluble Klotho). Growth hormone (GH) is considered an active Klotho regulator, because growth retardation is described in Klotho-deficient mice. The origin of circulating Klotho is, however, not fully understood.

Objectives

Our objective was to analyze a possible role of the pituitary in regulating soluble Klotho in patients with pituitary adenomas.

Patients, Design, and Setting

We analyzed serum levels of soluble Klotho, GH, and insulin-like growth factor 1 (IGF-1) from 21 consecutive patients in our center with pituitary tumor, 7 with GH-producing adenomas (GHomas), and 14 with non–GH-producing pituitary adenomas (non-GHomas), before and after endoscopic transsphenoidal surgery (eTSS).

Main Outcome Measure

Soluble Klotho levels were determined by ELISA with antihuman Klotho antibodies.

Results

Baseline soluble Klotho levels in all patients, those with GHoma and those with non-GHoma, were 542 (median) (interquartile range: 403, 652), 1083 (425, 1213), and 525 (399, 590), respectively. A drastic reduction in Klotho levels was identified in those with GHoma, accompanied by decreases in GH and IGF-1 levels, after eTSS. Interestingly, patients with non-GHoma had significant declines in soluble Klotho without any significant changes in GH levels. Moreover, an oral glucose tolerance test revealed that soluble Klotho levels decreased, whereas a paradoxical GH peak was observed after glucose intake in a patient with GHoma.

Conclusions

Our data suggest that the pituitary may be a key organ that regulates circulating Klotho concentrations, implying that the pituitary possibly controls circulating Klotho through GH-dependent and/or GH-independent mechanisms.

Renal ADAM10 and 17: Their Physiological and Medical Meanings

A disintegrin and metalloproteinases (ADAMs) are a Zn²⁺-dependent transmembrane and secreted metalloprotease superfamily, so-called “molecular scissors,” and they consist of an N-terminal signal sequence, a prodomain, zinc-binding metalloprotease domain, disintegrin domain, cysteine-rich domain, transmembrane domain and cytoplasmic tail. ADAMs perform proteolytic processing of the ectodomains of diverse transmembrane molecules into bioactive mediators. This review summarizes on their most well-known members, ADAM10 and 17, focusing on the kidneys. ADAM10 is expressed in renal tubular cells and affects the expression of specific brush border genes, and its activation is involved in some renal diseases. ADAM17 is weakly expressed in normal kidneys, but its expression is markedly induced in the tubules, capillaries, glomeruli, and mesangium, and it is involved in interstitial fibrosis and tubular atrophy. So far, the various substrates have been identified in the kidneys. Shedding fragments become released ligands, such as Notch and EGFR ligands, and act as the chemoattractant factors including CXCL16. Their ectodomain shedding is closely correlated with pathological factors, which include inflammation, interstitial fibrosis, and renal injury. Also, the substrates of both ADAMs contain the molecules that play important roles at the plasma membrane, such as meaprin, E-cadherin, Klotho, and CADM1. By being released into urine, the shedding products could be useful for biomarkers of renal diseases, but ADAM10 and 17 per se are also notable as biomarkers. Furthermore, ADAM10 and/or 17 inhibitions based on various strategies such as small molecules, antibodies, and their recombinant prodomains are valuable, because they potentially protect renal tissues and promote renal regeneration. Although temporal and spatial regulations of inhibitors are problems to be solved, their inhibitors could be useful for renal diseases.

The role of klotho in chronic kidney disease

Chronic kidney disease (CKD) is an inherently systemic disease that refers to a long-term loss of kidney function. The progression of CKD has repercussions for other organs, leading to many kinds of extrarenal complications. Intensive studies are now being undertaken to reveal the risk factors and pathophysiological mechanism of this disease. During the past 20 years, increasing evidence from clinical and basic studies has indicated that klotho, which was initially known as an anti-aging gene and is mainly expressed in the kidney, is significantly correlated with the development and progression of CKD and its complications. Here, we discuss in detail the role and pathophysiological implications of klotho in ion disorders, the inflammation response, vascular calcification, mineral bone disorders, and renal fibrosis in CKD. Based on the pathogenic mechanism of klotho deficiency and klotho decline in urine early in CKD stage 2 and even earlier in CKD stage 1, it is not difficult to understand that soluble klotho can serve as an early and sensitive marker of CKD. Moreover, the prevention of klotho decline by several mechanisms can attenuate renal injuries, retard CKD progression, ameliorate extrarenal complications, and improve renal function. In this review, we focus on the functions and pathophysiological implications of klotho in CKD and its extrarenal complications as well as its potential applications as a diagnostic and/or prognostic biomarker for CKD and as a novel treatment strategy to improve and decrease the burden of comorbidity in CKD.

Phosphate as a Signaling Molecule and Its Sensing Mechanism

In mammals, phosphate balance is maintained by influx and efflux via the intestines, kidneys, bone, and soft tissue, which involves multiple sodium/phosphate (Na+/Pi) cotransporters, as well as regulation by several hormones. Alterations in the levels of extracellular phosphate exert effects on both skeletal and extra-skeletal tissues, and accumulating evidence has suggested that phosphate itself evokes signal transduction to regulate gene expression and cell behavior. Several in vitro studies have demonstrated that an elevation in extracellular Piactivates fibroblast growth factor receptor, Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular signal-regulated kinase) pathway and Akt pathway, which might involve the type III Na+/Picotransporter PiT-1. Excessive phosphate loading can lead to various harmful effects by accelerating ectopic calcification, enhancing oxidative stress, and dysregulating signal transduction. The responsiveness of mammalian cells to altered extracellular phosphate levels suggests that they may sense and adapt to phosphate availability, although the precise mechanism for phosphate sensing in mammals remains unclear. Unicellular organisms, such as bacteria and yeast, use some types of Pitransporters and other molecules, such as kinases, to sense the environmental Piavailability. Multicellular animals may need to integrate signals from various organs to sense the phosphate levels as a whole organism, similarly to higher plants. Clarification of the phosphate-sensing mechanism in humans may lead to the development of new therapeutic strategies to prevent and treat diseases caused by phosphate imbalance.

Mechanisms of Dysfunction in the Aging Vasculature and Role in Age-Related Disease

Advancing age promotes cardiovascular disease (CVD), the leading cause of death in the United States and many developed nations. Two major age-related arterial phenotypes, large elastic artery stiffening and endothelial dysfunction, are independent predictors of future CVD diagnosis and likely are responsible for the development of CVD in older adults. Not limited to traditional CVD, these age-related changes in the vasculature also contribute to other age-related diseases that influence mammalian health span and potential life span. This review explores mechanisms that influence age-related large elastic artery stiffening and endothelial dysfunction at the tissue level via inflammation and oxidative stress and at the cellular level via Klotho and energy-sensing pathways (AMPK [AMP-activated protein kinase], SIRT [sirtuins], and mTOR [mammalian target of rapamycin]). We also discuss how long-term calorie restriction-a health span- and life span-extending intervention-can prevent many of these age-related vascular phenotypes through the prevention of deleterious alterations in these mechanisms. Lastly, we discuss emerging novel mechanisms of vascular aging, including senescence and genomic instability within cells of the vasculature. As the population of older adults steadily expands, elucidating the cellular and molecular mechanisms of vascular dysfunction with age is critical to better direct appropriate and measured strategies that use pharmacological and lifestyle interventions to reduce risk of CVD within this population.

High-intensity physical exercise increases serum α-klotho levels in healthy volunteers

The recently discovered klotho proteins have roles in a diverse range of metabolic processes with the oldest protein, α-klotho, implicated in various cellular pathways in energy, glucose, and phosphate metabolism. Circulating soluble klotho (sKl), derived from membrane α-klotho cleavage, not only has effects on ion channels and insulin signaling pathways, but is inversely associated with mortality. Effects of physical exercise on sKl have not been well studied. The effect of a single high-intensity standardized exercise on sKl and serum phosphate (sPi) levels in healthy adults was investigated. A standard Bruce protocol treadmill exercise was undertaken by 10 fasting healthy volunteers. sKl, sPi, and blood glucose levels were measured in samples collected 1-week prior, immediately pre (T_pre), 0 (T_post), 30 (T₃₀), 240 (T₂₄₀) min, and 1-week after exercise. Median (interquartile range) age of participants was 47.5 (44-51) years; five (50%) were male. All study participants achieved at least 90% predicted maximum heart rate (MHR). sKl increased acutely after exercise (T_pre median 448 pg/mL vs. T_post median 576 pg/mL; p < 0.01). There was a nonsignificant sPi decline at T₃₀ (T_pre 0.94 ± 0.12 mmol/L vs. T₃₀ 0.83 ± 0.22 mmol/L). Exercise led to a reduction in blood glucose by T₂₄₀ with median glucose levels at T_pre, T_post, T₃₀, and T₂₄₀ of 6.0, 6.5, 6.3, and 5.7 mmol/L, respectively. In conclusion, a single high-intensity exercise session is associated with a transient increase in sKl, a delayed reduction in blood glucose, and a nonsignificant decrease in sPi levels in healthy adults. The evaluation of long-term effects of cardiovascular fitness programs on sKl and sPi in healthy individuals and disease cohorts are required to identify potential lifestyle modifications to help improve chronic disease management and long-term outcomes.

Klotho, the Key to Healthy Brain Aging?

Brain expression of klotho was first described with the initial discovery of the klotho gene. The prominent age-regulating effects of klotho are attributed to regulation of ion homeostasis through klotho function in the kidney. However, recent advances identified brain functions and cell populations, including adult hippocampal neural progenitors, which require klotho. As well, both human correlational studies and mouse models of disease show that klotho is protective against multiple neurological and psychological disorders. This review focuses on current knowledge as to how the klotho protein effects the brain.

Klotho protein in neurodegenerative disorders

The Klotho protein is a recently discovered protein and its overexpression is associated with life extension. Klotho deficiency or silencing of the Klotho gene in mice leads to an accelerated aging and short life, whereas overexpression of Klotho in mice extends lifespan. Klotho participates in many metabolic pathways and is highly expressed in the kidneys, the choroid plexus and neurons. It plays a key role in the calcium-phosphate metabolism, remyelination, and cognitive processes. The present paper is a short review of the literature on the role of Klotho in neurodegenerative disorders, with special attention paid to multiple sclerosis. The neuroprotective function of Klotho is also reported. It is also important to consider potential clinical applications of Klotho that might be useful in the treatment of many diseases.

Association of the rs495392 Klotho polymorphism with atheromatosis progression in patients with chronic kidney disease

Background

Prevalence of atherosclerotic cardiovascular disease and its rate of progression are higher in patients with chronic kidney disease (CKD) compared with the general population. Mineral metabolism parameters have been shown to be involved in the increased velocity of atheromatosis progression. The aim of this study is to determine the role of 11 single-nucleotide polymorphisms (SNPs) of the Klotho gene on the rate of atherosclerosis progression in CKD.

Methods

This was a multicentre, prospective, observational study of 1439 CKD patients from the NEFRONA cohort. Carotid and femoral ultrasounds were performed at baseline and after 24 months in 10 arterial territories. Progression of atheromatosis was defined as an increase in the number of territories with plaque. Genotyping of 11 SNPs of the Klotho gene was performed and its association with atheromatosis progression was determined by multivariate logistic regression.

Results

Bivariate analysis showed that none of the 11 SNPs was associated with atheroma plaque prevalence, but 3 of them (rs495392, rs562020 and rs567170) showed association with atheromatosis progression. The multivariate analysis revealed that only rs495392 showed a statistically significant association with atheromatosis progression, after adjustment for several parameters known to affect it in CKD patients. Thus, the presence of one allele T was associated with a reduction of 30% of the odds of progression, whereas the presence of the two T alleles was associated with a decrease close to 50%.

Conclusions

The presence of the allele T of the SNP rs495392 of the Klotho gene is associated with a decrease in the odds of progression of atheromatosis in CKD patients.

Antagonistic role of Klotho-derived peptides dynamics in the pancreatic cancer treatment through obstructing WNT-1 and Frizzled binding

Klotho is an anti-aging protein that is engaged in the suppression of canonical WNT signaling. In this study, we investigated the expression pattern of human WNTs and Klotho in the pancreatic cancer. In the cancerous cells, WNT-1 exhibited much higher expression as compared to other WNTs, while no WNT expression was detected in the normal tissue. In contrast, Klotho expression was significantly low in the cancerous tissue. Based on these observations, we intended to explore Klotho binding to WNT-1 and cystein-rich domains (CRDs) of Frizzled (FZD) homologs through molecular docking and dynamics simulation assays. Interestingly, similar region of WNT-1 was detected in binding with Klotho and CRDs of FZD-1/2. FZD-CRDs were grasped by the association of peripheral hydrophobic residues of WNT-1 U-shaped cavity. Subsequently, WNT-1-bound Klotho-peptides were isolated and reevaluated for their binding abilities against WNT-1 and FZD-CRDs., The conformational readjustements of these complexes were deeply analyzed by calculating the size of WNT-1 U-shaped cavity. In comparison to apo-WNT-1, cavity opening was markedly enhanced (8.2 Å to 15.64 Å, 32.89 Å and 35.11 Å) in WNT-1-a, WNT-1-c and WNT-1-e complexes, respectively. Thus Klotho-derived peptides may facilitate distinct conformational changes in the WNT-1-FZD associated region. As a result, aberrant loss of FZD binding may lead to augment WNT signaling. Overall, current study opens up new avenues in the pancreatic cancer therapeutics through antagonizing WNT-1 by Klotho.

Secreted α-Klotho maintains cartilage tissue homeostasis by repressing NOS2 and ZIP8-MMP13 catabolic axis

Progressive loss of tissue homeostasis is a hallmark of numerous age-related pathologies, including osteoarthritis (OA). Accumulation of senescent chondrocytes in joints contributes to the age-dependent cartilage loss of functions through the production of hypertrophy-associated catabolic matrix-remodeling enzymes and pro-inflammatory cytokines. Here, we evaluated the effects of the secreted variant of the anti-aging hormone α-Klotho on cartilage homeostasis during both cartilage formation and OA development. First, we found that α-Klotho expression was detected during mouse limb development, and transiently expressed during in vitro chondrogenic differentiation of bone marrow-derived mesenchymal stem cells. Genome-wide gene array analysis of chondrocytes from OA patients revealed that incubation with recombinant secreted α-Klotho repressed expression of the NOS2 and ZIP8/MMP13 catabolic remodeling axis. Accordingly, α-Klotho expression was reduced in chronically IL1β-treated chondrocytes and in cartilage of an OA mouse model. Finally, in vivo intra-articular secreted α-Kotho gene transfer delays cartilage degradation in the OA mouse model. Altogether, our results reveal a new tissue homeostatic function for this anti-aging hormone in protecting against OA onset and progression.

Soluble Klotho causes hypomineralization in Klotho-deficient mice

The type I transmembrane protein αKlotho (Klotho) serves as a coreceptor for the phosphaturic hormone fibroblast growth factor 23 (FGF23) in kidney, while a truncated form of Klotho (soluble Klotho, sKL) is thought to exhibit multiple activities, including acting as a hormone, but whose mode(s) of action in different organ systems remains to be fully elucidated. FGF23 is expressed primarily in osteoblasts/osteocytes and aberrantly high levels in the circulation acting via signaling through an FGF receptor (FGFR)-Klotho coreceptor complex cause renal phosphate wasting and osteomalacia. We assessed the effects of exogenously added sKL on osteoblasts and bone using Klotho-deficient (kl/kl) mice and cell and organ cultures. sKL induced FGF23 signaling in bone and exacerbated the hypomineralization without exacerbating the hyperphosphatemia, hypercalcemia and hypervitaminosis D in kl/kl mice. The same effects were seen in rodent bone models in vitro, in which we also detected formation of a sKL complex with FGF23-FGFR and decreased Phex (gene responsible for X-linked hypophosphatemic rickets (XLH)/osteomalacia) expression. Further, sKL-FGF23-dependent hypomineralization in vitro was rescued by soluble PHEX. These data suggest that exogenously added sKL directly participates in FGF23 signaling in bone and that PHEX is a downstream effector of the sKL-FGF23-FGFR axis in bone.

Klotho is upregulated in human cardiomyopathy independently of circulating Klotho levels

Klotho is an antiaging protein which exerts known cardioprotection. In kidney, trans-membrane Klotho acts as essential co-receptor of fibroblast growth factor 23 (FGF23). In the heart, soluble Klotho (sKlotho) protects from systolic dysfunction independently of FGF23. Here, we analyzed the association of FGF23 and sKlotho upon progression of chronic heart failure (CHF) and analyzed Klotho expression in human hearts. Serum levels of sKlotho and FGF23 were measured in 287 patients with cardiomyopathy (CMP). Tissue samples from CMP (n = 10) and healthy control hearts (n = 10) were analyzed for Klotho mRNA and protein expression. Individuals in the first FGF23 tertile were 4.1 times more likely of freedom from death, heart transplantation or assist device implantation compared to third tertile. No relationship was found between sKlotho and the combined endpoint. Instead, Klotho mRNA encoding the full-length form was upregulated in human CMP hearts. Immunoblotting confirmed upregulation of sKlotho associated with increased expression of proteases involved in cleavage of Klotho suggesting rather local effects of Klotho in the heart. Therefore, we conclude that in contrast to FGF23, serum sKlotho is not associated with disease severity or progression in CHF. Instead, Klotho is expressed and upregulated in diseased hearts, suggesting local paracrine effects.

Induction of anti-aging gene klotho with a small chemical compound that demethylates CpG islands

Klotho (KL) is described as an anti-aging gene because mutation of Kl gene leads to multiple pre-mature aging phenotypes and shortens lifespan in mice. Growing evidence suggests that an increase in KL expression may be beneficial for age-related diseases such as arteriosclerosis and diabetes. It remains largely unknown, however, how Kl expression could be induced. Here we discovered novel molecular mechanism for induction of Kl expression with a small molecule ‘Compound H’, N-(2-chlorophenyl)-1H-indole-3-caboxamide. Compound H was originally identified through a high-throughput screening of small molecules for identifying Kl inducers. However, how Compound H induces Kl expression has never been investigated. We found that Compound H increased Kl expression via demethylation in CpG islands of the Kl gene. The demethylation was accomplished by activating demethylases rather than inhibiting methylases. Due to demethylation, Compound H enhanced binding of transcription factors, Pax4 and Kid3, to the promoter of the Kl gene. Pax4 and Kid3 regulated Kl promoter activity positively and negatively, respectively. Thus, our results show that demethylation is an important molecular mechanism that mediates Compound H-induced Kl expression. Further investigation is warranted to determine whether Compound H demethylates the Kl gene in vivo and whether it can serve as a therapeutic agent for repressing or delaying the onset of age-related diseases.

Soluble klotho as a marker of renal fibrosis and podocyte injuries in human kidneys

Klotho deficiency is relevant to renal fibrosis and podocyte injury in vivo and in vitro. We examined whether histological findings of renal biopsy specimens were associated with the levels of soluble klotho in humans. We investigated renal biopsy specimens of 67 patients and detailed microscopic findings were reviewed. Soluble serum/urinary klotho and urinary angiotensinogen were assessed by enzyme-linked immunosorbent assays, and tissue klotho expression was assessed by immunohistochemical staining. The median age of the study participants was 35.6 years. High serum klotho levels (≥14 pg/mL) were associated with decreased odds ratios (ORs) of interstitial fibrosis (OR = 0.019, P = 0.003) and segmental sclerosis (OR = 0.190, P = 0.022) in multivariable logistic regression analysis. Patients with a lower urinary klotho-to-creatinine ratio (UKCR) were significantly more likely to have diffuse foot process effacement (OR = 0.450, P = 0.010). The area under the receiver-operating characteristic curve (AUC) of serum klotho for predicting interstitial fibrosis was 0.920 (95% CI, 0.844–0.996), and the best cut-off value of serum klotho was 138.1 pg/mL. The AUC of UKCR for predicting diffuse foot process effacement was 0.754 (95% CI, 0.636–0.872), and the best cut-off value of UKCR was 96.7 pg/mgCr. Urinary angiotensinogen-to-creatinine ratio was not associated with serum klotho, UKCR, or any pathological finding. Our data suggested that soluble serum and urinary klotho levels represent a potential biomarker to predict renal fibrosis and podocyte injury in humans.

Is there a role for newer biomarkers in chronic kidney disease-mineral and bone disorder management?

The current management of Chronic Kidney Disease-Mineral Bone Disorder (CKD-MBD) relies largely on clinical judgement and assessment of biochemical parameters including serum calcium, phosphate and intact parathyroid hormone concentrations. In the past two decades, there has been a leap in the understanding of the pathophysiology of CKD-MBD, leading to the discovery of novel biomarkers. The potential utility of these markers in this clinical setting is an area of intense investigation. In the absence of any guidelines aiding the clinician's understanding and application of these markers, we summarise the current available literature surrounding fibroblast growth factor-23, α-Klotho, sclerostin and serum calcification propensity testing and their respective assays in the context of CKD-MBD management.

Systemic klotho is associated with KLOTHO variation and predicts intrinsic cortical connectivity in healthy human aging

Cognitive decline is a major biomedical challenge as the global population ages. Elevated levels of the longevity factor klotho suppress aging, enhance cognition, and promote synaptic plasticity and neural resilience against aging and Alzheimer’s disease (AD)-related pathogenic proteins. Here, we examined the relationship between human genetic variants of KLOTHO and systemic klotho levels – and assessed neuroanatomic correlates of serum klotho in a cohort of healthy older adults. Serum klotho levels were increased with KL-VS heterozygosity, as anticipated. We report, for the first time, that serum klotho levels were paradoxically decreased with KL-VS homozygosity. Further, we found that higher serum klotho levels were associated with measures of greater intrinsic connectivity in key functional networks of the brain vulnerable to aging and AD such as the fronto-parietal and default mode networks. Our findings suggest that elevated klotho promotes a resilient brain, possibly through increased network connectivity of critical brain regions.

Activation of the Anti-Aging and Cognition-Enhancing Gene Klotho by CRISPR-dCas9 Transcriptional Effector Complex

Multiple lines of evidence show that the anti-aging and cognition-enhancing protein Klotho fosters neuronal survival, increases the anti-oxidative stress defense, and promotes remyelination of demyelinated axons. Thus, upregulation of the Klotho gene can potentially alleviate the symptoms and/or prevent the progression of age-associated neurodegenerative diseases such as Alzheimer's disease and demyelinating diseases such as multiple sclerosis. Here we used a CRISPR-dCas9 complex to investigate single-guide RNA (sgRNA) targeting the Klotho promoter region for efficient transcriptional activation of the Klotho gene. We tested the sgRNAs within the - 1 to - 300 bp of the Klotho promoter region and identified two sgRNAs that can effectively enhance Klotho gene transcription. We examined the transcriptional activation of the Klotho gene using three different systems: a Firefly luciferase (FLuc) and NanoLuc luciferase (NLuc) coincidence reporter system, a NLuc knock-in in Klotho 3'-UTR using CRISPR genomic editing, and two human cell lines: neuronal SY5Y cells and kidney HK-2 cells that express Klotho endogenously. The two sgRNAs enhanced Klotho expression at both the gene and protein levels. Our results show the feasibility of gene therapy for targeting Klotho using CRISPR technology. Enhancing Klotho levels has a therapeutic potential for increasing cognition and treating age-associated neurodegenerative, demyelinating and other diseases, such as chronic kidney disease and cancer.

Protective Effect of Klotho against Ischemic Brain Injury Is Associated with Inhibition of RIG-I/NF-κB Signaling

Aging is the greatest independent risk factor for the occurrence of stroke and poor outcomes, at least partially through progressive increases in oxidative stress and inflammation with advanced age. Klotho is an antiaging gene, the expression of which declines with age. Klotho may protect against neuronal oxidative damage that is induced by glutamate. The present study investigated the effects of Klotho overexpression and knockdown by an intracerebroventricular injection of a lentiviral vector that encoded murine Klotho (LV-KL) or rat Klotho short-hairpin RNA (LV-KL shRNA) on cerebral ischemia injury and the underlying anti-neuroinflammatory mechanism. The overexpression of Klotho induced by LV-KL significantly improved neurobehavioral deficits and increased the number of live neurons in the hippocampal CA1 and caudate putamen subregions 72 h after cerebral hypoperfusion that was induced by transient bilateral common carotid artery occlusion (2VO) in mice. The overexpression of Klotho significantly decreased the immunoreactivity of glial fibrillary acidic protein and ionized calcium binding adaptor molecule-1, the expression of retinoic-acid-inducible gene-I, the nuclear translocation of nuclear factor-κB, and the production of proinflammatory cytokines (tumor necrosis factor α and interleukin-6) in 2VO mice. The knockdown of Klotho mediated by LV-KL shRNA in the brain exacerbated neurological dysfunction and cerebral infarct after 22 h of reperfusion following 2 h middle cerebral artery occlusion in rats. These findings suggest that Klotho itself or enhancers of Klotho may compensate for its aging-related decline, thus providing a promising therapeutic approach for acute ischemic stroke during advanced age.

The role of fibroblast growth factor 23 and Klotho in uremic cardiomyopathy

PURPOSE OF REVIEW

In chronic kidney disease (CKD), multiple factors contribute to the development of cardiac hypertrophy by directly targeting the heart or indirectly by inducing systemic changes such as hypertension, anemia, and inflammation. Furthermore, disturbances in phosphate metabolism have been identified as nonclassical risk factors for cardiovascular mortality in these patients. With declining kidney function, the physiologic regulators of phosphate homeostasis undergo changes in their activity as well as their circulating levels, thus potentially contributing to cardiac hypertrophy once they are out of balance. Recently, two of these phosphate regulators, fibroblast growth factor 23 (FGF23) and Klotho, have been shown to affect cardiac remodeling, thereby unveiling a novel pathomechanism of cardiac hypertrophy in CKD. Here we discuss the potential direct versus indirect effects of FGF23 and the soluble form of Klotho on the heart, and their crosstalk in the regulation of cardiac hypertrophy.

RECENT FINDINGS

In models of CKD, FGF23 can directly target cardiac myocytes via FGF receptor 4 and induce cardiac hypertrophy in a blood pressure-independent manner. Soluble Klotho may directly target the heart via an unknown receptor thereby protecting the myocardium from pathologic stress stimuli that are associated with CKD, such as uremic toxins or FGF23.

SUMMARY

Elevated serum levels of FGF23 and reduced serum levels of soluble Klotho contribute to uremic cardiomyopathy in a synergistic manner.

α-Klotho is a non-enzymatic molecular scaffold for FGF23 hormone signalling

The aging suppressor αKlotho binds to the fibroblast growth factor receptor (FGFR). This commits FGFR to respond to FGF23, a key hormone in the regulation of mineral ion/vitamin D homeostasis. The role and mechanism of this co-receptor are unknown. Here we present the atomic structure of a 1:1:1 ternary complex consisting of the shed extracellular domain of αKlotho, the FGFR1c ligand-binding domain, and FGF23. In this complex, αKlotho simultaneously tethers FGFR1c by its D3 domain and FGF23 by its C-terminal tail, thus implementing FGF23-FGFR1c proximity and conferring stability. The endocrine character of FGF23 notwithstanding, dimerization of the stabilized ternary complexes and receptor activation remain dependent on the binding of heparan sulfate, a mandatory cofactor of paracrine FGF signaling. The structure of αKlotho is incompatible with its purported glycosidase activity. Thus, shed αKlotho functions as an on-demand non-enzymatic scaffold protein that promotes FGF23 signaling.

Role of Fibroblast Growth Factor-23 in Innate Immune Responses

Fibroblast growth factor-23 (FGF-23) is a bone-derived hormone that activates FGFR/α-Klotho binary complexes in the kidney renal tubules to regulate phosphate reabsorption and vitamin D metabolism. The objective of this review is to discuss the emerging data that show that FGF-23 has functions beyond regulation of mineral metabolism, including roles in innate immune and hemodynamic responses. Excess FGF-23 is associated with inflammation and adverse infectious outcomes, as well as increased morbidity and mortality, particularly in patients with chronic kidney disease. Enhancer elements in the FGF-23 promoter have been identified that mediate the effects of inflammatory cytokines to stimulate FGF-23 gene transcription in bone. In addition, inflammation induces ectopic expression of FGF-23 and α-Klotho in macrophages that do not normally express FGF-23 or its binary receptor complexes. These observations suggest that FGF-23 may play an important role in regulating innate immunity through multiple potential mechanisms. Circulating FGF-23 acts as a counter-regulatory hormone to suppress 1,25D production in the proximal tubule of the kidney. Since vitamin D deficiency may predispose infectious and cardiovascular diseases, FGF-23 effects on innate immune responses may be due to suppression of 1,25D production. Alternatively, systemic and locally produced FGF-23 may modulate immune functions through direct interactions with myeloid cells, including macrophages and polymorphonuclear leukocytes to impair immune cell functions. Short-acting small molecules that reversibly inhibit FGF-23 offer the potential to block pro-inflammatory and cardiotoxic effects of FGF-23 with less side effects compared with FGF-23 blocking antibodies that have the potential to cause hyperphosphatemia and soft tissue calcifications in animal models. In conclusion, there are several mechanisms by which FGF-23 impacts the innate immune system and further investigation is critical for the development of therapies to treat diseases associated with elevated FGF-23.

FGF23 Actions on Target Tissues-With and Without Klotho

Fibroblast growth factor (FGF) 23 is a phosphaturic hormone whose physiologic actions on target tissues are mediated by FGF receptors (FGFR) and klotho, which functions as a co-receptor that increases the binding affinity of FGF23 for FGFRs. By stimulating FGFR/klotho complexes in the kidney and parathyroid gland, FGF23 reduces renal phosphate uptake and secretion of parathyroid hormone, respectively, thereby acting as a key regulator of phosphate metabolism. Recently, it has been shown that FGF23 can also target cell types that lack klotho. This unconventional signaling event occurs in an FGFR-dependent manner, but involves other downstream signaling pathways than in "classic" klotho-expressing target organs. It appears that klotho-independent signaling mechanisms are only activated in the presence of high FGF23 concentrations and result in pathologic cellular changes. Therefore, it has been postulated that massive elevations in circulating levels of FGF23, as found in patients with chronic kidney disease, contribute to associated pathologies by targeting cells and tissues that lack klotho. This includes the induction of cardiac hypertrophy and fibrosis, the elevation of inflammatory cytokine expression in the liver, and the inhibition of neutrophil recruitment. Here, we describe the signaling and cellular events that are caused by FGF23 in tissues lacking klotho, and we discuss FGF23's potential role as a hormone with widespread pathologic actions. Since the soluble form of klotho can function as a circulating co-receptor for FGF23, we also discuss the potential inhibitory effects of soluble klotho on FGF23-mediated signaling which might-at least partially-underlie the pleiotropic tissue-protective functions of klotho.

α-klotho and anemia in patients with chronic kidney disease patients: A new perspective

Normocytic normochromic anemia is a common complication of chronic kidney disease (CKD) and is associated with numerous adverse consequences. Certain symptoms previously attributed to CKD are now known to be a consequence of anemia. Anemia contributes to an increased cardiac output, and the development of left ventricular hypertrophy, angina and congestive heart failure, leading to high morbidity and mortality in patients with CKD. The multifunctional α-klotho (KL) protein, which is predominantly expressed in the kidneys, is associated with the occurrence of anemia in patients with CKD. The present review presents current evidence on the potential role of α-KL in renal anemia. Low expression of α-KL appears to improve anemia in patients with CKD, and has been hypothesized to be a compensatory mechanism to attenuate the effects of anemia in patients with CKD. Further understanding of the role of α-KL in renal anemia may offer novel insights into the treatment of patients with CKD complicated with anemia.

Anti-aging factor, serum alpha-Klotho, as a marker of acute physiological stress, and a predictor of ICU mortality, in patients with septic shock

PURPOSE

Genetic deletions decreasing serum alpha-Klotho (alpha-KL) have been associated with rapid aging, multi-organ failure and increased mortality in experimental sepsis. We hypothesized that lower alpha-KL obtained at the onset of septic shock correlates with higher mortality.

MATERIALS AND METHODS

Prospective cohort of 104 adult patients with septic shock. Alpha-KL was measured via ELISA on serum collected on the day of enrollment (within 72h from the onset of shock). Relationship between alpha-KL and clinical outcome measures was evaluated in uni- and multi-variable models.

RESULTS

Median (IQR) alpha-KL was 816 (1020.4) pg/mL and demonstrated a bimodal distribution with two distinct populations, Cohort A [n=97, median alpha-KL 789.3 (767.1)] and Cohort B [n=7, median alpha-KL 4365.1(1374.4), >1.5 IQR greater than Cohort A]. Within Cohort A, ICU non-survivors had significantly higher serum alpha-KL compared to survivors as well as significantly higher APACHE II and SOFA scores, rates of mechanical ventilation, and serum BUN, creatinine, calcium, phosphorus and lactate (all p≤0.05). Serum alpha-KL≥1005, the highest tertile, was an independent predictor of ICU mortality when controlling for co-variates (p=0.028, 95% CI 1.143-11.136).

CONCLUSIONS

Elevated serum alpha-KL in patients with septic shock is independently associated with higher mortality. Further studies are needed to corroborate these findings.

Effects of Klotho on fibrosis and cancer: A renal focus on mechanisms and therapeutic strategies

Klotho is a membrane-bound protein predominantly expressed in the kidney, where it acts as a permissive co-receptor for Fibroblast Growth Factor 23. In its shed form, Klotho exerts anti-fibrotic effects in several tissues. Klotho-deficient mice spontaneously develop fibrosis and Klotho deficiency exacerbates the disease progression in fibrotic animal models. Furthermore, Klotho overexpression or supplementation protects against fibrosis in various models of renal and cardiac fibrotic disease. These effects are mediated at least partially by the direct inhibitory effects of soluble Klotho on TGFβ1 signaling, Wnt signaling, and FGF2 signaling. Soluble Klotho, as present in the circulation, appears to be the primary mediator of anti-fibrotic effects. Similarly, through inhibition of the TGFβ1, Wnt, FGF2, and IGF1 signaling pathways, Klotho also inhibits tumorigenesis. The Klotho promoter gene is generally hypermethylated in cancer, and overexpression or supplementation of Klotho has been found to inhibit tumor growth in various animal models. This review focuses on the protective effects of soluble Klotho in inhibiting renal fibrosis and fibrosis in distant organs secondary to renal Klotho deficiency. We also discuss the structure-function relationships of Klotho domains and biological effects in the context of potential targeted treatment strategies.

Klotho Inhibits Interleukin-8 Secretion from Cystic Fibrosis Airway Epithelia

Chronic inflammation is a hallmark of cystic fibrosis (CF) and associated with increased production of transforming growth factor (TGF) β and interleukin (IL)-8. α-klotho (KL), a transmembrane or soluble protein, functions as a co-receptor for Fibroblast Growth Factor (FGF) 23, a known pro-inflammatory, prognostic marker in chronic kidney disease. KL is downregulated in airways from COPD patients. We hypothesized that both KL and FGF23 signaling modulate TGF β-induced IL-8 secretion in CF bronchial epithelia. Thus, FGF23 and soluble KL levels were measured in plasma from 48 CF patients and in primary CF bronchial epithelial cells (CF-HBEC). CF patients showed increased FGF23 plasma levels, but KL levels were not different. In CF-HBEC, TGF-β increased KL secretion and upregulated FGF receptor (FGFR) 1. Despite increases in KL, TGF-β also increased IL-8 secretion via activation of FGFR1 and Smad 3 signaling. However, KL excess via overexpression or supplementation decreased IL-8 secretion by inhibiting Smad 3 phosphorylation. Here, we identify a novel signaling pathway contributing to IL-8 secretion in the CF bronchial epithelium with KL functioning as an endocrine and local anti-inflammatory mediator that antagonizes pro-inflammatory actions of FGF23 and TGF-β.

Human alternative Klotho mRNA is a nonsense-mediated mRNA decay target inefficiently spliced in renal disease

Klotho is a renal protein involved in phosphate homeostasis, which is downregulated in renal disease. It has long been considered an antiaging factor. Two Klotho gene transcripts are thought to encode membrane-bound and secreted Klotho. Indeed, soluble Klotho is detectable in bodily fluids, but the relative contributions of Klotho secretion and of membrane-bound Klotho shedding are unknown. Recent advances in RNA surveillance reveal that premature termination codons, as present in alternative Klotho mRNA (for secreted Klotho), prime mRNAs for degradation by nonsense-mediated mRNA decay (NMD). Disruption of NMD led to accumulation of alternative Klotho mRNA, indicative of normally continuous degradation. RNA IP for NMD core factor UPF1 resulted in enrichment for alternative Klotho mRNA, which was also not associated with polysomes, indicating no active protein translation. Alternative Klotho mRNA transcripts colocalized with some P bodies, where NMD transcripts are degraded. Moreover, we could not detect secreted Klotho in vitro. These results suggest that soluble Klotho is likely cleaved membrane-bound Klotho only. Furthermore, we found that, especially in acute kidney injury, splicing of the 2 mRNA transcripts is dysregulated, which was recapitulated by various noxious stimuli in vitro. This likely constitutes a novel mechanism resulting in the downregulation of membrane-bound Klotho.

Peripheral Elevation of a Klotho Fragment Enhances Brain Function and Resilience in Young, Aging, and α-Synuclein Transgenic Mice

Cognitive dysfunction and decreased mobility from aging and neurodegenerative conditions, such as Parkinson and Alzheimer diseases, are major biomedical challenges in need of more effective therapies. Increasing brain resilience may represent a new treatment strategy. Klotho, a longevity factor, enhances cognition when genetically and broadly overexpressed in its full, wild-type form over the mouse lifespan. Whether acute klotho treatment can rapidly enhance cognitive and motor functions or induce resilience is a gap in our knowledge of its therapeutic potential. Here, we show that an α-klotho protein fragment (αKL-F), administered peripherally, surprisingly induced cognitive enhancement and neural resilience despite impermeability to the blood-brain barrier in young, aging, and transgenic α-synuclein mice. αKL-F treatment induced cleavage of the NMDAR subunit GluN2B and also enhanced NMDAR-dependent synaptic plasticity. GluN2B blockade abolished αKL-F-mediated effects. Peripheral αKL-F treatment is sufficient to induce neural enhancement and resilience in mice and may prove therapeutic in humans.

Tissue expression and source of circulating αKlotho

αKlotho (Klotho), a type I transmembrane protein and a coreceptor for Fibroblast Growth Factor-23, was initially thought to be expressed only in a limited number of tissues, most importantly the kidney, parathyroid gland and choroid plexus. Emerging data may suggest a more ubiquitous Klotho expression pattern which has prompted reevaluation of the restricted Klotho paradigm. Herein we systematically review the evidence for Klotho expression in various tissues and cell types in humans and other mammals, and discuss potential reasons behind existing conflicting data. Based on current literature and tissue expression atlases, we propose a classification of tissues into high, intermediate and low/absent Klotho expression. The functional relevance of Klotho in organs with low expression levels remain uncertain and there is currently limited data on a role for membrane-bound Klotho outside the kidney. Finally, we review the evidence for the tissue source of soluble Klotho, and conclude that the kidney is likely to be the principal source of circulating Klotho in physiology.

The chronic kidney disease - Mineral bone disorder (CKD-MBD): Advances in pathophysiology

The causes of excess cardiovascular mortality associated with chronic kidney disease (CKD) have been attributed in part to the CKD-mineral bone disorder syndrome (CKD-MBD), wherein, novel cardiovascular risk factors have been identified. New advances in the causes of the CKD-MBD are discussed in this review. They demonstrate that repair and disease processes in the kidneys release factors to the circulation that cause the systemic complications of CKD. The discovery of WNT inhibitors, especially Dickkopf 1 (Dkk1), produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. This lead to the discovery that activin A is a second renal repair factor circulating in increased levels during CKD. Activin A derives from peritubular myofibroblasts of diseased kidneys, wherein it stimulates fibrosis, and decreases tubular klotho expression. Activin A binds to the type 2 activin A receptor, ActRIIA, which is variably affected by CKD in the vasculature. In diabetic/atherosclerotic aortas, specifically in vascular smooth muscle cells (VSMC), ActRIIA signaling is inhibited and contributes to CKD induced VSMC dedifferentiation, osteogenic transition and neointimal atherosclerotic calcification. In nondiabetic/nonatherosclerotic aortas, CKD increases VSMC ActRIIA signaling, and vascular fibroblast signaling causing the latter to undergo osteogenic transition and stimulate vascular calcification. In both vascular situations, a ligand trap for ActRIIA prevented vascular calcification. In the skeleton, activin A is responsible for CKD stimulation of osteoclastogenesis and bone remodeling increasing bone turnover. These studies demonstrate that circulating renal repair and injury factors are causal of the CKD-MBD and CKD associated cardiovascular disease.

The relevance of α-KLOTHO to the central nervous system: Some key questions

α-Klotho is well described as an anti-aging protein, with critical roles in kidney function as a transmembrane co-receptor for FGF23, and as a soluble factor in serum. α-Klotho is also expressed in the choroid plexus, where it is released into the cerebrospinal fluid. Nonetheless, α-Klotho is also expressed in the brain parenchyma. Accumulating evidence indicates that this pool of α-Klotho, which we define as brain α-Klotho, may play important roles as a neuroprotective factor and in promoting myelination, thereby supporting healthy brain aging. Here we summarize what is known about brain α-Klotho before focusing on the outstanding scientific questions related to its function. We believe there is a need for in vitro studies designed to distinguish between brain α-Klotho and other pools of α-Klotho, and for a greater understanding of the basic function of soluble α-Klotho. The mechanism by which the human KL-VS variant affects cognition also requires further elucidation. To help address these questions we suggest some experimental approaches that other laboratories might consider. In short, we hope to stimulate fresh ideas and encourage new research approaches that will allow the importance of α-Klotho for the aging brain to become clear.

FGF23-αKlotho as a paradigm for a kidney-bone network

The vertebrate endoskeleton is not a mere frame for muscle attachment to facilitate locomotion, but is a massive organ integrated with many physiologic functions including mineral and energy metabolism. Mineral balance is maintained by tightly controlled ion fluxes that are external (intestine and kidney) and internal (between bone and other organs), and are regulated and coordinated by many endocrine signals between these organs. The endocrine fibroblast growth factors (FGFs) and Klotho gene families are complex systems that co-evolved with the endoskeleton. In particular, FGF23 and αKlotho which are primarily derived from bone and kidney respectively, are critical in maintaining mineral metabolism where each of these proteins serving highly diverse roles; abound with many unanswered questions regarding their upstream regulation and downstream functions. Genetic lesions of components of this network produce discreet disturbances in many facets of mineral metabolism. One acquired condition with colossal elevations of FGF23 and suppression of αKlotho is chronic kidney disease where multiple organ dysfunction contributes to the morbidity and mortality. However, the single most important group of derangements that encompasses the largest breadth of complications is mineral metabolism disorders. Mineral metabolic disorders in CKD impact negatively and significantly on the progression of renal disease as well as extra-renal complications. Knowledge of the origin, nature, and impact of phosphate, FGF23, and αKlotho derangements is pivotal to understanding the pathophysiology and treatment of CKD.

Potential application of klotho in human chronic kidney disease

The extracellular domain of transmembrane alpha-Klotho (αKlotho, hereinafter simply called Klotho) is cleaved by secretases and released into the circulation as soluble Klotho. Soluble Klotho in the circulation starts to decline early in chronic kidney disease (CKD) stage 2 and urinary Klotho possibly even earlier in CKD stage 1. Therefore soluble Klotho could serve as an early and sensitive marker of kidney function decline. Moreover, preclinical animal data support Klotho deficiency is not just merely a biomarker, but a pathogenic factor for CKD progression and extrarenal CKD complications including cardiovascular disease and disturbed mineral metabolism. Prevention of Klotho decline, re-activation of endogenous Klotho production or supplementation of exogenous Klotho are all associated with attenuation of renal fibrosis, retardation of CKD progression, improvement of mineral metabolism, amelioration of cardiomyopathy, and alleviation of vascular calcification in CKD. Therefore Klotho is not only a diagnostic and/or prognostic marker for CKD, but the treatment of Klotho deficiency may be a promising strategy to prevent, retard, and decrease the burden of comorbidity in CKD.

Acute lung injury complicating acute kidney injury: A model of endogenous αKlotho deficiency and distant organ dysfunction

The lung interfaces with atmospheric oxygen via a large surface area and is perfused by the entire venous return bearing waste products collected from the whole body. It is logical that the lung is endowed with generous anti-oxidative capacity derived both locally and from the circulation. The single-pass pleiotropic alpha-Klotho (αKlotho) protein was discovered when its genetic disruption led to premature multi-organ degeneration and early death. The extracellular domain of αKlotho is cleaved by secretases and released into circulation as endocrine soluble αKlotho protein, exerting wide-ranging cytoprotective effects including anti-oxidation on distant organs including the lung, which exhibits high sensitivity to circulating αKlotho insufficiency. Because circulating αKlotho is derived mainly from the kidney, acute kidney injury (AKI) leads to systemic αKlotho deficiency that in turn increases the risks of pulmonary complications, i.e., edema and inflammation, culminating in the acute respiratory distress syndrome. Exogenous αKlotho increases endogenous anti-oxidative capacity partly via activation of the Nrf2 pathway to protect lungs against injury caused by direct hyperoxia exposure or AKI. This article reviews the current knowledge of αKlotho antioxidation in the lung in the setting of AKI as a model of circulating αKlotho deficiency, an under-recognized condition that weakens innate cytoprotective defenses and contributes to the dysfunction in distant organs.

Association between Soluble Klotho and Change in Kidney Function: The Health Aging and Body Composition Study

CKD appears to be a condition of soluble klotho deficiency. Despite known associations between low soluble klotho levels and conditions that promote kidney damage, such as oxidative stress and fibrosis, little information exists regarding the longitudinal association between soluble klotho levels and change in kidney function. We assayed serum soluble α-klotho in 2496 participants within the Health Aging and Body Composition study, a cohort of older adults. The associations between soluble klotho levels and decline in kidney function (relative decline: eGFR decline ≥30%; absolute decline: eGFR decline >3 ml/min per year) and incident CKD (incident eGFR <60 ml/min per 1.73 m2 and >1 ml/min per year decline) were evaluated. We adjusted models for demographics, baseline eGFR, urine albumin-to-creatinine ratio, comorbidity, and measures of mineral metabolism. Among participants, the mean (SD) age was 75 (3) years, 52% were women, and 38% were black. Median (25th, 75th percentiles) klotho level was 630 (477, 817) pg/ml. In fully adjusted models, each two-fold higher level of klotho associated with lower odds of decline in kidney function (odds ratio, 0.78 [95% confidence interval, 0.66 to 0.93] for 30% decline in eGFR, and 0.85 [95% confidence interval, 0.73 to 0.98] for >3 ml/min per year decline in eGFR), but not of incident CKD (incident rate ratio, 0.90 [95% confidence interval, 0.78 to 1.04]). Overall, a higher soluble klotho level independently associated with a lower risk of decline in kidney function. Future studies should attempt to replicate these results in other cohorts and evaluate the underlying mechanism.

Niche Extracellular Matrix Components and Their Influence on HSC

Maintenance of hematopoietic stem cells (HSC) takes place in a highly specialized microenvironment within the bone marrow. Technological improvements, especially in the field of in vivo imaging, have helped unravel the complexity of the niche microenvironment and have completely changed the classical concept from what was previously believed to be a static supportive platform, to a dynamic microenvironment tightly regulating HSC homeostasis through the complex interplay between diverse cell types, secreted factors, extracellular matrix molecules, and the expression of different transmembrane receptors. To add to the complexity, non-protein based metabolites have also been recognized as a component of the bone marrow niche. The objective of this review is to discuss the current understanding on how the different extracellular matrix components of the niche regulate HSC fate, both during embryonic development and in adulthood. Special attention will be provided to the description of non-protein metabolites, such as lipids and metal ions, which contribute to the regulation of HSC behavior. J. Cell. Biochem. 118: 1984-1993, 2017. © 2017 Wiley Periodicals, Inc.

Biotin-transfer from a trifunctional crosslinker for identification of cell surface receptors of soluble protein ligands

Here we describe a novel crosslinker and its application as a biotin-transfer reagent to identify cell surface receptors of soluble protein ligands on live cells. This crosslinker contains three functional groups: an aldehyde-reactive aminooxy group, a sulfhydryl, and a biotin (ASB). It is readily synthesized via a 3-step addition reaction using standard solid-phase peptide synthesis methods and commercially available intermediates, allowing access to laboratories without specialized synthetic chemistry capabilities. For the biotin-transfer method, ASB is linked to a protein ligand through the sulfhydryl group in a two-step process that allows the introduction of a disulfide bond between the ligand and the crosslinker. Incubation of the labelled ligand with oxidized live cells leads to the formation of crosslinks with aldehyde-containing glycans on the cell surface receptor. Subsequent reduction of the disulfide bond results in biotin transfer from the ligand to the cell surface receptor. Protein biotinylation that is mediated by ligand binding to its receptor is differentiated from background biotinylation events by comparison with a similarly labelled control protein using comparative proteomic mass spectrometry to quantify streptavidin-bound proteins. Using this method, we successfully identified the cell surface receptors of a peptide hormone, a monoclonal antibody, and a single-domain antibody-Fc fusion construct.

Klotho regulates CA1 hippocampal synaptic plasticity

Global klotho overexpression extends lifespan while global klotho-deficiency shortens it. As well, klotho protein manipulations inversely regulate cognitive function. Mice without klotho develop rapid onset cognitive impairment before they are 2months old. Meanwhile, adult mice overexpressing klotho show enhanced cognitive function, particularly in hippocampal-dependent tasks. The cognitive enhancing effects of klotho extend to humans with a klotho polymorphism that increases circulating klotho and executive function. To affect cognitive function, klotho could act in or on the synapse to modulate synaptic transmission or plasticity. However, it is not yet known if klotho is located at synapses, and little is known about its effects on synaptic function. To test this, we fractionated hippocampi and detected klotho expression in both pre and post-synaptic compartments. We find that loss of klotho enhances both pre and post-synaptic measures of CA1 hippocampal synaptic plasticity at 5weeks of age. However, a rapid loss of synaptic enhancement occurs such that by 7weeks, when mice are cognitively impaired, there is no difference from wild-type controls. Klotho overexpressing mice show no early life effects on synaptic plasticity, but decreased CA1 hippocampal long-term potentiation was measured at 6months of age. Together these data suggest that klotho affects cognition, at least in part, by regulating hippocampal synaptic plasticity.