Identification of cleavage sites leading to the shed form of the anti-aging protein klotho

Fibroblast growth factor-23 and Alpha-Klotho concentrations in dogs with canine Leishmaniasis

This study aimed to assess the concentrations of Fibroblast Growth Factor-23 (FGF-23) and α-Klotho in healthy dogs and dogs at different stages of Canine Leishmaniasis (CanL), and investigate the changes of these parameters in relation to renal function and calcium‑phosphorus metabolism. A total of 74 dogs (22 healthy and 52 with CanL) of varying ages, sexes, and medium-sized breeds were included. Dogs with CanL were categorized into different stages (Stage I-IV) based on Leishvet recommendations. In addition to routine hematological parameters, plasma FGF-23, serum α-Klotho, urea, creatinine, phosphorus, calcium, parathormone, vitamin D concentrations, and urine protein/creatinine ratio were measured. Data from healthy dogs were compared to dogs with CanL overall and by stage. Dogs with CanL exhibited higher concentrations of FGF-23 (p < 0.05), α-Klotho, and parathormone (p < 0.001), as well as lower concentrations of vitamin D and calcium (p < 0.001). FGF-23 concentration was particularly elevated in Stage IV compared to other stages. However, no significant differences in α-Klotho levels were observed among the stages. FGF-23 levels showed a weak positive correlation with urea and creatinine concentrations and a moderate positive correlation with urine protein/creatinine ratio. This study demonstrated increased levels of FGF-23 and α-Klotho in dogs with CanL for the first time. The increase in FGF-23 levels was more prominent in advanced stages of the disease and correlated with higher urea and creatinine concentrations. These findings may serve as a basis for future diagnostic and therapeutic investigations, contributing to the understanding of the pathophysiology of kidney disease in CanL.

Involvement of α-klotho in growth hormone (GH) signaling

Growth hormone (GH) exerts multiple effects on different organs directly or via its main mediator, insulin-like growth factor1 (IGF1). In this study, we focused on the novel relationship between GH action and the antiaging hormone α-klotho. Immunofluorescent staining of α-klotho was observed in the renal distal tubules and pituitary glands of somatostatin- and GH-positive cells in wild-type (WT) mice. Treatment of 4-week-old WT mice with GH increased IGF1 mRNA expression in the pituitary gland, liver, heart, kidney, and bone but increased α-klotho mRNA expression only in the pituitary gland, kidney, and bone. Increased α-klotho protein levels were observed in the kidney but not in the pituitary gland. No induction of α-klotho RNA expression by GH was observed in juvenile mice with kidney disease, indicating GH resistance. Furthermore, GH and α-klotho supplementation in HEK293 cells transfected with GHR increased Janus kinase 2 mRNA (a GH downstream signal) expression compared to supplementation with GH alone. In conclusion, we suggest that 1) the kidney is the main source of secreted α-klotho, which is detected in blood by the downstream action of GH, 2) α-klotho induction by GH is resistant in kidney disease, and 3) α-klotho might be an enhanced regulator of GH signaling.

Overexpression of α-Klotho isoforms promotes distinct Effects on BDNF-Induced Alterations in Dendritic Morphology

α-Klotho (α-Kl) is a modulator of aging, neuroprotection, and cognition. Transcription of the Klotho gene produces two splice variants-a membrane protein (mKl), which can be cleaved and released into the extracellular milieu, and a truncated secreted form (sKl). Despite mounting evidence supporting a role for α-Kl in brain function, the specific roles of α-Kl isoforms in neuronal development remain elusive. Here, we examined α-Kl protein levels in rat brain and observed region-specific expression in the adult that differs between isoforms. In the developing hippocampus, levels of isoforms decrease after the third postnatal week, marking the end of the critical period for development. We overexpressed α-Kl isoforms in primary cultures of rat cortical neurons and evaluated effects on brain-derived neurotrophic factor (BDNF) signaling. Overexpression of either isoform attenuated BDNF-mediated signaling and reduced intracellular Ca2+ levels, with mKl promoting a greater effect. mKl or sKl overexpression in hippocampal neurons resulted in a partially overlapping reduction in secondary dendrite branching. Moreover, mKl overexpression increased primary dendrite number. BDNF treatment of neurons overexpressing sKl resulted in a dendrite branching phenotype similar to control neurons. In neurons overexpressing mKl, BDNF treatment restored branching of secondary and higher order dendrites close, but not distal, to the soma. Taken together, the data presented support the idea that sKl and mKl play distinct roles in neuronal development, and specifically, in dendrite morphogenesis.

Klotho and Clinical Outcomes in CKD: Findings From the Chronic Renal Insufficiency Cohort (CRIC) Study

RATIONALE & OBJECTIVE

Klotho deficiency may affect clinical outcomes in chronic kidney disease (CKD) through fibroblast growth factor-23 (FGF23)-dependent and -independent pathways. However, the association between circulating Klotho and clinical outcomes in CKD remains unresolved and was the focus of this study.

STUDY DESIGN

Prospective observational study.

SETTING & PARTICIPANTS

1,088 participants in the Chronic Renal Insufficiency Cohort (CRIC) Study with an estimated glomerular filtration rate (eGFR) of 20-70mL/min/1.73m2.

EXPOSURE

Plasma Klotho level at the year-1 study visit.

OUTCOMES

5-year risks of all-cause mortality, heart failure hospitalization, atherosclerotic cardiovascular events, and a composite kidney end point that comprised a sustained 50% decrease in eGFR, dialysis, kidney transplant, or eGFR<15mL/min/1.73m2.

ANALYTICAL APPROACH

We divided Klotho into 6 groups to account for its nonnormal distribution. We used Cox proportional hazards regression and subdistribution hazards models to compare survival and clinical outcomes, respectively, between Klotho groups. We sequentially adjusted for demographic characteristics, kidney function, cardiovascular risk factors, sample age, and FGF23.

RESULTS

Mean eGFR was 42mL/min/1.73m2, and median Klotho concentration was 0.31ng/mL (IQR, 0.10-3.27ng/mL). When compared with the lowest Klotho group, survival (HR, 0.77; 95% CI, 0.32-1.89), heart failure hospitalization (HR, 1.10; 95% CI, 0.38-3.17), atherosclerotic cardiovascular events (HR, 1.19; 95% CI, 0.57-2.52), and CKD progression (HR, 1.05; 95% CI, 0.58-1.91) did not differ in the high Klotho group. In contrast, FGF23 was significantly associated with mortality and heart failure hospitalization independent of Klotho levels.

LIMITATIONS

Despite adjustments, we cannot exclude the potential influence of residual confounding or sample storage on the results. A single measurement of plasma Klotho concentration may not capture Klotho patterns over time.

CONCLUSIONS

In a large, diverse, well-characterized CKD cohort, Klotho was not associated with clinical outcomes, and Klotho deficiency did not confound the association of FGF23 with mortality or heart failure hospitalization.

PLAIN-LANGUAGE SUMMARY

Klotho is a protein that is vital to mineral metabolism and aging and may protect against cardiovascular disease. Klotho levels decrease in chronic kidney disease (CKD), but the association between Klotho and clinical outcomes in CKD remains uncertain. In a prospective cohort study of more than 1,000 people with CKD, circulating Klotho levels were not associated with kidney disease progression, cardiovascular outcomes, or mortality. These results suggest that the decrease in circulating Klotho levels in CKD does not play a prominent role in the development of poor clinical outcomes.

Soluble klotho induces the heat shock factor 1 through EGR1 expression

Klotho is an antiaging protein that has multiple functions. The purpose of this study is to investigate whether soluble klotho plays a role in cellular stress response pathways. We found that klotho deficiency (kl-/-) largely decreased HSF1 levels and impaired heat shock protein expression. Interestingly, recombinant soluble klotho-induced HSF1 and HSPs such as HSP90, HSP70, and HSP27 in kl-/- mouse embryonic fibroblasts (MEFs). Soluble Klotho treatment also induced cell proliferation and HSF1 promoter activity in MEF kl-/- cells in a concentration-dependent manner. Furthermore, using point mutagenesis, we identified regulatory/binding sites of transcription factors EGR1 regulated by soluble klotho in the HSF1 promoter. Taken together, our findings unravel the molecular basis of klotho and provide molecular evidence supporting a direct interaction between soluble klotho and HSF1-mediated stress response pathway.

Klotho-derived peptide KP1 ameliorates SARS-CoV-2-associated acute kidney injury

Introduction: The severe cases of COVID-19, a disease caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), often present with acute kidney injury (AKI). Although old age and preexisting medical conditions have been identified as principal risk factors for COVID-19-associated AKI, the molecular basis behind such a connection remains unknown. In this study, we investigated the pathogenic role of Klotho deficiency in COVID-19-associated AKI and explored the therapeutic potential of Klotho-derived peptide 1 (KP1). Methods: We assessed the susceptibility of Klotho deficient Kl/Kl mice to developing AKI after expression of SARS-CoV-2 N protein. The role of KP1 in ameliorating tubular injury was investigated by using cultured proximal tubular cells (HK-2) in vitro and mouse model of ischemia-reperfusion injury (IRI) in vivo. Results: Renal Klotho expression was markedly downregulated in various chronic kidney disease (CKD) models and in aged mice. Compared to wild-type counterparts, mutant KL/KL mice were susceptible to overexpression of SARS-CoV-2 N protein and developed kidney lesions resembling AKI. In vitro, expression of N protein alone induced HK-2 cells to express markers of tubular injury, cellular senescence, apoptosis and epithelial-mesenchymal transition, whereas both KP1 and Klotho abolished these lesions. Furthermore, KP1 mitigated kidney dysfunction, alleviated tubular injury and inhibited apoptosis in AKI model induced by IRI and N protein. Conclusion: These findings suggest that Klotho deficiency is a key determinant of developing COVID-19-associated AKI. As such, KP1, a small peptide recapitulating Klotho function, could be an effective therapeutic for alleviating AKI in COVID-19 patients.

Relationship between klotho and physical function in healthy aging

Epidemiological studies have reported a strong association between circulating Klotho and physical function; however, the cohorts were comprised of older adults with multiple comorbidities. Herein, we examined the relationship between Klotho and physical function in a community-based cohort of healthy adults. In this cross-sectional study, serum Klotho was measured in 80 adults who visited the Musculoskeletal Function, Imaging, and Tissue Resource Core of the Indiana Center for Musculoskeletal Health. Participants (n = 20, 10 [50%] men per group) were chosen into four age groups: 20-34, 35-49, 50-64, and ≥ 65 years, and were further grouped based on performance (low vs. high) on grip strength and chair stand tests. Klotho levels were lower in the ≥ 65 years group (703.0 [189.3] pg/mL; p = 0.022) and the 50-64 years group (722.6 [190.5] pg/mL; p = 0.045) compared to 20-34 years (916.1 [284.8] pg/mL). No differences were observed in Klotho between the low and high performers. The ≥ 65 years group walked a shorter distance during the 6-min walk test (6MWT) compared to 20-34 years (p = 0.005). Klotho was correlated with age (p < 0.001), body fat (p = 0.037), and 6MWT distance (p = 0.022). Klotho levels decline as early as the fifth decade of life, potentially before the onset of age-related impairment in exercise capacity.

Potential role of soluble CD40 receptor in chronic inflammatory diseases

The CD40 receptor and its ligand CD154 are widely expressed in various immune-competent cells. Interaction of CD154 with CD40 is essential for B-cell growth, differentiation, and immunoglobulin class switching. Many other immune-competent cells involved in innate and adaptive immunity communicate through this co-stimulatory ligand-receptor dyad. CD40-CD154 interaction is involved in the pathogenesis of numerous inflammatory and autoimmune diseases. While CD40 and CD154 are membrane-bound proteins, their soluble counterparts are generated by proteolytic cleavage or alternative splicing. This review summarises current knowledge about the impact of single nucleotide polymorphisms in the human CD40 gene and compensatory changes in the plasma level of the soluble CD40 receptor (sCD40) isoform in related pro-inflammatory diseases. It discusses regulation patterns of the disintegrin metalloprotease ADAM17 function leading to ectodomain shedding of transmembrane proteins, such as pro-inflammatory adhesion molecules or CD40. The role of sCD40 as a potential biomarker for chronic inflammatory diseases will also be discussed.

The Importance of α-Klotho in Depression and Cognitive Impairment and Its Connection to Glutamate Neurotransmission-An Up-to-Date Review

Depression is a serious neuropsychiatric disease affecting an increasing number of people worldwide. Cognitive deficits (including inattention, poor memory, and decision-making difficulties) are common in the clinical picture of depression. Cognitive impairment has been hypothesized to be one of the most important components of major depressive disorder (MDD; referred to as clinical depression), although typical cognitive symptoms are less frequent in people with depression than in people with schizophrenia or bipolar disorder (BD; sometimes referred to as manic-depressive disorder). The importance of α-Klotho in the aging process has been well-documented. Growing evidence points to the role of α-Klotho in regulating other biological functions, including responses to oxidative stress and the modulation of synaptic plasticity. It has been proven that a Klotho deficit may contribute to the development of various nervous system pathologies, such as behavioral disorders or neurodegeneration. Given the growing evidence of the role of α-Klotho in depression and cognitive impairment, it is assumed that this protein may be a molecular link between them. Here, we provide a research review of the role of α-Klotho in depression and cognitive impairment. Furthermore, we propose potential mechanisms (related to oxidative stress and glutamatergic transmission) that may be important in α-Klotho-mediated regulation of mental and cognitive function.

Age-dependent changes on fractalkine forms and their contribution to neurodegenerative diseases

The chemokine fractalkine (FKN, CX3CL1), a member of the CX3C subfamily, contributes to neuron-glia interaction and the regulation of microglial cell activation. Fractalkine is expressed by neurons as a membrane-bound protein (mCX3CL1) that can be cleaved by extracellular proteases generating several sCX3CL1 forms. sCX3CL1, containing the chemokine domain, and mCX3CL1 have high affinity by their unique receptor (CX3CR1) which, physiologically, is only found in microglia, a resident immune cell of the CNS. The activation of CX3CR1contributes to survival and maturation of the neural network during development, glutamatergic synaptic transmission, synaptic plasticity, cognition, neuropathic pain, and inflammatory regulation in the adult brain. Indeed, the various CX3CL1 forms appear in some cases to serve an anti-inflammatory role of microglia, whereas in others, they have a pro-inflammatory role, aggravating neurological disorders. In the last decade, evidence points to the fact that sCX3CL1 and mCX3CL1 exhibit selective and differential effects on their targets. Thus, the balance in their level and activity will impact on neuron-microglia interaction. This review is focused on the description of factors determining the emergence of distinct fractalkine forms, their age-dependent changes, and how they contribute to neuroinflammation and neurodegenerative diseases. Changes in the balance among various fractalkine forms may be one of the mechanisms on which converge aging, chronic CNS inflammation, and neurodegeneration.

Association of Klotho with Coronary Artery Disease in Subjects with Type 2 Diabetes Mellitus and Preserved Kidney Function: A Case-Control Study

Circulating Klotho levels are significantly reduced in subjects with type 2 diabetes mellitus (T2DM) and in kidney disease patients. In this work, the relationship between Klotho levels and the coronary artery disease (CAD) burden in subjects with T2DM and preserved kidney function was analyzed. For this, we performed a cross-sectional case-control study involving 133 subjects with T2DM and 200 age-, sex- and CAD-incidence-matched, non-diabetic patients undergoing non-emergency diagnostic coronary angiography. All of them were non-albuminuric and with normal glomerular filtration rates. The concentrations of serum Klotho, fibroblast growth factor 23, and inflammatory markers were also measured. As expected, the serum Klotho concentration was lower in the T2DM group (12.3% lower, p = 0.04). However, within the group of patients with T2DM, those subjects with CAD presented significantly higher Klotho levels than those without significant coronary stenosis (314.5 (6.15-562.81) vs. 458.97 (275.2-667.2) pg/mL; p = 0.02). Multiple regression analysis revealed that serum Klotho was positively related with stenosis values exclusively in subjects with T2DM (adjusted R2 = 0.153, p < 0.01). Moreover, logistic regression analysis showed that Klotho was positively associated with the presence of significant CAD in the group of T2DM patients (OR: 1.001; p = 0.041). Our data suggest that higher levels of circulating Klotho in subjects with T2DM and preserved kidney function are associated with the presence of significant CAD.

Time-Dependent Changes of Klotho and FGF-23 Levels after Kidney Transplantation: Role of Cold Ischemia Time, Renal Function and Graft Inflammation

We investigated the evolution of serum klotho (s-Kl) and FGF-23 during the first two years post-kidney transplantation (KT), considering the cold ischemia time (CIT), glomerular filtration rate (GFR) and graft subclinical inflammation (SCI). We undertook a prospective, cohort, multicenter study of consecutive patients between April 2018 and January 2021 (with follow-up at 24 months). Subgroups were analyzed according to the median CIT (<14 vs. ≥14 h), the median GFR (≤40 vs. >40 mL/min/1.73 m²) and the presence of SCI at month 3. A total of 147 patients were included. s-Kl and fibroblast growth factor-23 (FGF-23) levels were measured at baseline and at months 3, 12 and 24. Graft biopsies (n = 96) were performed at month 3. All patients had low s-Kl levels at month 3. Patients with CIT < 14 h exhibited a significant increase in s-Kl at month 24. In patients with CIT ≥ 14 h, s-Kl at month 3 fell and lower s-Kl levels were seen at month 24. Patients with a GFR > 40 had a lesser decrease in s-Kl at month 3. FGF-23 fell significantly at months 3 and 12 in both GFR groups, a reduction maintained during follow-up. There were significant inter-group differences in s-Kl from months 3 to 24. CIT, GFR at 3 months and SCI were significantly associated with s-KI at month 3. A reduction in s-Kl at month 3 post-KT could be explained by longer CIT and delayed graft function as well as by impaired graft function. Early SCI may regulate s-Kl increase post-KT.

Fibroblast growth factor 23, klotho and heparin

PURPOSE OF REVIEW

Fibroblast growth factor (FGF) 23 is a bone-derived hormone that regulates phosphate and vitamin D metabolism by targeting the kidney. When highly elevated, such as in chronic kidney disease (CKD), FGF23 can also target the heart and induce pathologic remodeling. Here we discuss the mechanisms that underlie the physiologic and pathologic actions of FGF23, with focus on its FGF receptors (FGFR) and co-receptors.

RECENT FINDINGS

Klotho is a transmembrane protein that acts as an FGFR co-receptor for FGF23 on physiologic target cells. Klotho also exists as a circulating variant, and recent studies suggested that soluble klotho (sKL) can mediate FGF23 effects in cells that do not express klotho. Furthermore, it has been assumed that the actions of FGF23 do not require heparan sulfate (HS), a proteoglycan that acts as a co-receptor for other FGF isoforms. However, recent studies revealed that HS can be part of the FGF23:FGFR signaling complex and modulate FGF23-induced effects.

SUMMARY

sKL and HS have appeared as circulating FGFR co-receptors that modulate the actions of FGF23. Experimental studies suggest that sKL protects from and HS accelerates CKD-associated heart injury. However, the in vivo relevance of these findings is still speculative.

Upstream and downstream regulators of Klotho expression in chronic kidney disease

Klotho is a critical protein that protects the kidney. Klotho is severely downregulated in chronic kidney disease (CKD), and its deficiency is implicated in the pathogenesis and progression of CKD. Conversely, an increase in Klotho levels results in improved kidney function and delays CKD progression, supporting the notion that modulating Klotho levels could represent a possible therapeutic strategy for CKD treatment. Nevertheless, the regulatory mechanisms responsible for the loss of Klotho remain elusive. Previous studies have demonstrated that oxidative stress, inflammation, and epigenetic modifications can modulate Klotho levels. These mechanisms result in a decrease in Klotho mRNA transcript levels and reduced translation, thus can be grouped together as upstream regulatory mechanisms. However, therapeutic strategies that aim to rescue Klotho levels by targeting these upstream mechanisms do not always result in increased Klotho, indicating the involvement of other regulatory mechanisms. Emerging evidence has shown that endoplasmic reticulum (ER) stress, the unfolded protein response, and ER-associated degradation also affect the modification, translocation, and degradation of Klotho, and thus are proposed to be downstream regulatory mechanisms. Here, we discuss the current understanding of upstream and downstream regulatory mechanisms of Klotho and examine potential therapeutic strategies to upregulate Klotho expression for CKD treatment.

Differential toxicity profile of secreted and processed α-Klotho expression over mineral metabolism and bone microstructure

The aging-protective gene α-Klotho (KL) produces two main transcripts. The full-length mRNA generates a transmembrane protein that after proteolytic ectodomain shedding can be detected in serum as processed Klotho (p-KL), and a shorter transcript which codes for a putatively secreted protein (s-KL). Both isoforms exhibit potent pleiotropic beneficial properties, although previous reports showed negative side effects on mineral homeostasis after increasing p-KL concentration exogenously. Here, we expressed independently both isoforms using gene transfer vectors, to assess s-KL effects on mineral metabolism. While mice treated with p-KL presented altered expression of several kidney ion channels, as well as altered levels of P_i and Ca²⁺ in blood, s-KL treated mice had levels comparable to Null-treated control mice. Besides, bone gene expression of Fgf23 showed a fourfold increase after p-KL treatment, effects not observed with the s-KL isoform. Similarly, bone microstructure parameters of p-KL-treated mice were significantly worse than in control animals, while this was not observed for s-KL, which showed an unexpected increase in trabecular thickness and cortical mineral density. As a conclusion, s-KL (but not p-KL) is a safe therapeutic strategy to exploit KL anti-aging protective effects, presenting no apparent negative effects over mineral metabolism and bone microstructure.

The expression of anti-aging protein Klotho is increased during neural differentiation of bone marrow-derived mesenchymal stem cells

Klotho, as an antiaging protein, is involved in the maintenance and differentiation of neuronal or glial cells and, therefore, has been noticed as a potential therapeutic target for neurodegenerative disorders. Expression of Klotho has been examined in different cells and organs, however, our information about the developmental pattern of this protein during differentiation of mesenchymal stem cells (MSCs) into neuron-like cells is limited. In this study, we conducted neural differentiation of mouse bone marrow-derived-MSCs and monitored the expression of Klotho together with selected neuron-specific genes at messenger RNA (mRNA) on days 7 and 14 of differentiation using quantitative real-time PCR. In addition, Klotho status at protein level was evaluated by immunocytochemistry. The results showed a significant change in the morphology of MSCs towards neuron-like cells. These changes were observed with progressive growth and formation of cell connections towards the formation of a chain of neuron-like cells which occurred in the second week of differentiation. Morphological changes were associated with a significant increase in the expression of neuron-specific genes like pax-6, neuN and, neurofilaments (NfL). Likewise, there was an increased expression of Klotho mRNA, and accumulation of Klotho protein in neuronal cell bodies, during the cellular differentiation of MSCs. These findings provided new evidence that neuronal differentiation from the MSCs is associated with increased expression of Klotho. These data may provide insight into the importance of Klotho protein in stem cell differentiation and regeneration in response to cell death in the central nervous system.

The impact of SGLT2 inhibitors on αKlotho in renal MDCK and HK-2 cells

αKlotho is a transmembrane protein predominantly expressed in the kidney serving as a co-receptor for phosphate homeostasis-regulating hormone FGF23 and has an extracellular domain that can be cleaved off and is a hormone. αKlotho deficiency results in accelerated aging and early onset of aging-associated diseases while its overexpression strongly expands the lifespan of mice. Moreover, αKlotho exerts health-beneficial anti-inflammatory, anti-neoplastic, anti-fibrotic, and anti-oxidant effects. Higher αKlotho levels are associated with better outcomes in renal and cardiovascular diseases. SGLT2 inhibitors are novel drugs in the treatment of diabetes by inhibiting renal glucose transport and have additional nephro- and cardioprotective effects. We explored whether SGLT2 inhibitors affect αKlotho gene expression and protein secretion. Experiments were performed in renal MDCK and HK-2 cells, and αKlotho transcripts were determined by qRT-PCR and Klotho protein by ELISA. SGLT2 inhibitors canagliflozin, sotagliflozin, and dapagliflozin enhanced whereas empagliflozin reduced αKlotho gene expression in MDCK cells. By the same token, canagliflozin, sotagliflozin, dapagliflozin, but not empagliflozin down-regulated p65 subunit of pro-inflammatory NFκB. In HK-2 cells, all SGLT2 inhibitors reduced αKlotho transcripts. Canagliflozin and sotagliflozin, however, increased Klotho protein concentration in the cell culture supernatant, an effect paralleled by up-regulation of ADAM17. Taken together, our investigations demonstrate complex effects of different SGLT2 inhibitors on αKlotho gene expression and protein secretion in renal MDCK and HK-2 cells.

Role of Klotho Protein in Neuropsychiatric Disorders: A Narrative Review

Neuropsychiatric disorders are comprised of diseases having both the neurological and psychiatric manifestations. The increasing burden of the disease on the population worldwide makes it necessary to adopt measures to decrease the prevalence. The Klotho is a single pass transmembrane protein that decreases with age, has been associated with various pathological diseases, like reduced bone mineral density, cardiac problems and cognitive impairment. However, multiple studies have explored its role in different neuropsychiatric disorders. A comprehensive search was undertaken in the Pubmed database for articles with the keywords "Klotho" and "neuropsychiatric disorders". The available literature, based on the above search strategy, has been compiled in this brief narrative review to describe the emerging role of Klotho in various neuropsychiatric disorders. The Klotho levels were decreased in various neuropsychiatric disorders except for bipolar disorder. A suppressed Klotho protein levels induced oxidative stress and incited pro-inflammatory conditions significantly contributing to the pathophysiology of neuropsychiatric disorder. The increasing evidence of altered Klotho protein levels in cognition-decrement-related disorders warrants its consideration as a biomarker in various neuropsychiatric diseases. However, further evidence is required to understand its role as a therapeutic target.

Klotho's impact on diabetic nephropathy and its emerging connection to diabetic retinopathy

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease worldwide and is a significant burden on healthcare systems. α-klotho (klotho) is a protein known for its anti-aging properties and has been shown to delay the onset of age-related diseases. Soluble klotho is produced by cleavage of the full-length transmembrane protein by a disintegrin and metalloproteases, and it exerts various physiological effects by circulating throughout the body. In type 2 diabetes and its complications DN, a significant decrease in klotho expression has been observed. This reduction in klotho levels may indicate the progression of DN and suggest that klotho may be involved in multiple pathological mechanisms that contribute to the onset and development of DN. This article examines the potential of soluble klotho as a therapeutic agent for DN, with a focus on its ability to impact multiple pathways. These pathways include anti-inflammatory and oxidative stress, anti-fibrotic, endothelial protection, prevention of vascular calcification, regulation of metabolism, maintenance of calcium and phosphate homeostasis, and regulation of cell fate through modulation of autophagy, apoptosis, and pyroptosis pathways. Diabetic retinopathy shares similar pathological mechanisms with DN, and targeting klotho may offer new insights into the prevention and treatment of both conditions. Finally, this review assesses the potential of various drugs used in clinical practice to modulate klotho levels through different mechanisms and their potential to improve DN by impacting klotho levels.

Aging-suppressor Klotho: Prospects in diagnostics and therapeutics

INTRODUCTION

The protein Klotho (KL) was first discovered in KL-deficient mice, which developed a syndrome similar to premature aging in humans. Since then, KL has been implicated in multiple molecular signaling pathways and diseases. KL has been shown to have anti-aging, healthspan and lifespan extending, cognitive enhancing, anti-oxidative, anti-inflammatory, and anti-tumor properties. KL levels decrease with age and in many diseases. Therefore, it has been of great interest to develop a KL-boosting or restoring drug, or to supplement endogenous Klotho with exogenous Klotho genetic material or recombinant Klotho protein, and to use KL levels in the body as a marker for the efficacy of such drugs and as a biomarker for the diagnosis and management of diseases.

OBJECTIVE

The goal of this study was to provide a comprehensive review of KL levels across age groups in individuals who are healthy or have certain health conditions, using four sources: blood, cerebrospinal fluid, urine, and whole biopsy/necropsy tissue. By doing so, baseline KL levels can be identified across the lifespan, in the absence or presence of disease. In turn, these findings can be used to guide the development of future KL-based therapeutics and biomarkers, which will heavily rely on an individual's baseline KL range to be efficacious.

METHODS

A total of 65 studies were collected primarily using the PubMed database. Research articles that were published up to April 2022 were included. Statistical analysis was conducted using RStudio.

RESULTS

Mean and median blood KL levels in healthy individuals, mean blood KL levels in individuals with renal conditions, and mean blood KL levels in individuals with metabolic or endocrine conditions were shown to decrease with age. Similarly, CSF KL levels in patients with AD also declined compared with age-matched controls.

CONCLUSIONS

The present study confirms the trend that KL levels in blood decrease with age in humans, among those who are healthy, and even further among those with renal and endocrine/metabolic illnesses. Further, by drawing this trend from multiple published works, we were able to provide a general idea of baseline KL ranges, specifically in blood in these populations. These data add to the current knowledge on normal KL levels in the body and how they change with time and in disease, and can potentially support efforts to create KL-based treatments and screening tools to better manage aging, renal, and metabolic/endocrine diseases.

Protein-Labeling Fluorescent Probe Reveals Ectodomain Shedding of Transmembrane Carbonic Anhydrases

Ectodomain shedding is a form of limited proteolysis in which a protease cleaves a transmembrane protein, releasing the extracellular domain from the cell surface. Cells use this process to regulate a wide variety of biological events. Typically, immunological detection methods are employed for the analysis of ectodomains secreted into the cultured media. In this paper, we describe a new strategy using an affinity-based protein-labeling fluorescent probe to study ectodomain shedding. We analyzed the ectodomain shedding of cell surface carbonic anhydrases (CAIX and CAXII), which are important biomarkers for tumor hypoxia. Using both chemical and genetic approaches, we identified that the ADAM17 metalloprotease is responsible for the shedding of carbonic anhydrases. Compared to current immunological methods, this protein-labeling approach not only detects ectodomain released into the culture media but also allows real-time living cell tracking and quantitative analysis of remnant proteins on the cell surface, thereby providing a more detailed insight into the mechanism of ectodomain shedding as well as protein lifetime on the cell surface.

Impact of cytotoxic agents or apoptosis stimulants on αklotho in MDCK, NRK-52E and HK2 kidney cells

αKlotho is a transmembrane protein acting as a co-receptor for FGF23, a bone hormone regulating renal phosphate and vitamin D metabolism. αKlotho expression is controlled by PPARγ. Soluble αklotho (sKL) regulates cellular signaling impacting stress resistance and death. αKlotho deficiency causes early onset of aging-associated diseases while its overexpression markedly increases lifespan. Cellular stress due to cytotoxic therapeutics or apoptosis induction through caspase activation or serum deficiency may result in cell death. Owing to αklotho's role in cellular stress and aging, this study explored the effect of cytotoxic agents or apoptosis stimulants on cellular αklotho expression. Experiments were performed in renal MDCK, NRK-52E and HK-2 cells. Gene expression was determined by qRT-PCR, sKL by ELISA, apoptosis and necrosis by annexin V binding and a fluorescent DNA dye, and cell viability by MTT assay. Cytostatic drugs cisplatin, paclitaxel, and doxorubicin as well as apoptosis induction with caspase 3 activator PAC-1 and serum deprivation induced αklotho and PPARG gene expression while decreasing viability and proliferation and inducing apoptosis of MDCK and NRK-52E cells to a variable extent. PPARγ antagonism attenuated up-regulation of αklotho in MDCK cells. In HK-2 cells, αklotho gene expression and sKL protein were down-regulated by chemotherapeutics. SKL serum levels in patients following chemotherapy were not significantly changed. In summary, potentially fatal stress results in up-regulation of αKlotho gene expression in MDCK and NRK-52E cells and down-regulation in HK-2 cells. These results indicate that different renal cell lines may exhibit completely different regulation of αklotho.

Soluble α-klotho and heparin modulate the pathologic cardiac actions of fibroblast growth factor 23 in chronic kidney disease

Fibroblast growth factor (FGF) 23 is a phosphate-regulating hormone that is elevated in patients with chronic kidney disease and associated with cardiovascular mortality. Experimental studies showed that elevated FGF23 levels induce cardiac hypertrophy by targeting cardiac myocytes via FGF receptor isoform 4 (FGFR4). A recent structural analysis revealed that the complex of FGF23 and FGFR1, the physiologic FGF23 receptor in the kidney, includes soluble α-klotho (klotho) and heparin, which both act as co-factors for FGF23/FGFR1 signaling. Here, we investigated whether soluble klotho, a circulating protein with cardio-protective properties, and heparin, a factor that is routinely infused into patients with kidney failure during the hemodialysis procedure, regulate FGF23/FGFR4 signaling and effects in cardiac myocytes. We developed a plate-based binding assay to quantify affinities of specific FGF23/FGFR interactions and found that soluble klotho and heparin mediate FGF23 binding to distinct FGFR isoforms. Heparin specifically mediated FGF23 binding to FGFR4 and increased FGF23 stimulatory effects on hypertrophic growth and contractility in isolated cardiac myocytes. When repetitively injected into two different mouse models with elevated serum FGF23 levels, heparin aggravated cardiac hypertrophy. We also developed a novel procedure for the synthesis and purification of recombinant soluble klotho, which showed anti-hypertrophic effects in FGF23-treated cardiac myocytes. Thus, soluble klotho and heparin act as independent FGF23 co-receptors with opposite effects on the pathologic actions of FGF23, with soluble klotho reducing and heparin increasing FGF23-induced cardiac hypertrophy. Hence, whether heparin injections during hemodialysis in patients with extremely high serum FGF23 levels contribute to their high rates of cardiovascular events and mortality remains to be studied.

Klotho-derived peptide 6 ameliorates diabetic kidney disease by targeting Wnt/β-catenin signaling

Diabetic kidney disease (DKD) is one of the most common and devastating complications of diabetic mellitus, and its prevalence is rising worldwide. Klotho, an anti-aging protein, is kidney protective in DKD. However, its large size, prohibitive cost and structural complexity hamper its potential utility in clinics. Here we report that Klotho-derived peptide 6 (KP6) mimics Klotho function and ameliorates DKD. In either an accelerated model of DKD induced by streptozotocin and advanced oxidation protein products in unilateral nephrectomized mice or db/db mice genetically prone to diabetes, chronic infusion of KP6 reversed established proteinuria, attenuated glomerular hypertrophy, mitigated podocyte damage, and ameliorated glomerulosclerosis and interstitial fibrotic lesions, but did not affect serum phosphorus and calcium levels. KP6 inhibited β-catenin activation in vivo and blocked the expression of its downstream target genes in glomerular podocytes and tubular epithelial cells. In vitro, KP6 prevented podocyte injury and inhibited β-catenin activation induced by high glucose without affecting Wnt expression. Co-immunoprecipitation revealed that KP6 bound to Wnt ligands and disrupted the engagement of Wnts with low density lipoprotein receptor-related protein 6, thereby interrupting Wnt/β-catenin signaling. Mutated KP6 with a scrambled amino acid sequence failed to bind Wnts and did not alleviate DKD in db/db mice. Thus, our studies identified KP6 as a novel Klotho-derived peptide that ameliorated DKD by blocking Wnt/β-catenin. Hence, our findings also suggest a new therapeutic strategy for the treatment of patients with DKD.

The Role of Alterations in Alpha-Klotho and FGF-23 in Kidney Transplantation and Kidney Donation

Cardiovascular disease and mineral bone disorders are major contributors to morbidity and mortality among patients with chronic kidney disease and often persist after renal transplantation. Ongoing hormonal imbalances after kidney transplant (KT) are associated with loss of graft function and poor outcomes. Fibroblast growth factor 23 (FGF-23) and its co-receptor, α-Klotho, are key factors in the underlying mechanisms that integrate accelerated atherosclerosis, vascular calcification, mineral disorders, and osteodystrophy. On the other hand, kidney donation is also associated with endocrine and metabolic adaptations that include transient increases in circulating FGF-23 and decreases in α-Klotho levels. However, the long-term impact of these alterations and their clinical relevance have not yet been determined. This manuscript aims to review and summarize current data on the role of FGF-23 and α-Klotho in the endocrine response to KT and living kidney donation, and importantly, underscore specific areas of research that may enhance diagnostics and therapeutics in the growing population of KT recipients and kidney donors.

Impact of klotho on the expression of SRGAP2a in podocytes in diabetic nephropathy

Background

Diabetic nephropathy (DN) is the major cause of kidney failure, and glomerular podocytes play critical roles in the pathogenesis of DN by maintaining the glomerular structure and filtration barrier. Klotho and Slit-Robo GTP activating protein 2a (SRGAP2a) have been indicated to play protective roles in reducing kidney injury, but whether there is an internal relationship between these two factors is unclear.

Methods

In this study, we cultured differentiated rat podocytes in vitro and measured the SRGAP2a expressions by immunofluorescence staining, quantitative real-time PCR (qRT-PCR) and western blotting, after siRNA-mediated transforming growth factor β1 (TGF-β1) silencing, TGF-β1 overexpression and in the presence of a reactive oxygen species (ROS) inhibitor. And we detected the expressions of SRGAP2a, small mother against decapentaplegic (Smad)2/3, phosphorylated-Smad2/3 (p-Smad2/3), Smad7, and NAD(P)H oxidase 4 (NOX4), ROS levels and podocyte cytoskeletal remodelling under high glucose (HG) and exogenous klotho conditions. In addition, we performed haematoxylin–eosin (HE) staining and immunohistochemistry with diabetic rat models to confirm the in vitro results.

Results

The results indicated that SRGAP2a expression was significantly upregulated under siRNA-mediated TGF-β1 silencing conditions or after adding a ROS inhibitor, but significantly downregulated with TGF-β1 overexpression, in the presence of HG. The supplementation of exogenous klotho under HG conditions significantly increased the SRGAP2a expression, remodelled the actin cytoskeleton and altered the expressions of Smad2/3, p-Smad2/3, Smad7 and NOX4 and reduced the ROS generation in podocytes. Moreover, klotho administration protected kidney injury in DN rats.

Conclusions

This study indicated that klotho may modulate the expression of SRGAP2a by regulating the ROS and TGF-β1 signalling pathways and provided theoretical support for klotho protein as a novel therapeutic strategy for treating DN patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12882-022-02765-z.

Klotho: A Possible Role in the Pathophysiology of Nephrotic Syndrome

Klotho, encoded by the klotho gene, is associated with phosphate homeostasis. Klotho acts as a co-receptor for FGF23 for binding to its receptors. With FGF23, klotho regulates the systemic mineral homeostasis by regulation of vitamin D and parathyroid hormone. The anti-inflammatory, antifibrotic and antioxidant properties of klotho give it a cardinal role in the development of various renal diseases. The protective effect of klotho has been evident in different types of nephropathy, including diabetic nephropathy, cyclosporine A-induced nephropathy, Calcineurin inhibitors-induced nephropathy, and renal ischemic-reperfusion injury. Nephrotic syndrome is distinguished by hypoproteinemia, proteinuria, and hypercholesterolemia as a result of the aberration of the glomerular filtration barrier. The various factors and pathways associated with the pathophysiology of the nephrotic syndrome have similarities with other types of nephropathy. Despite these similarities, the role of klotho in the pathology of nephrotic syndrome remains still unexplored. This minireview builds the case for the possible role of klotho in nephrotic syndrome. The review explores the possible pathways where klotho can play a major role by identifying the similarities in the pathophysiology of nephrotic syndrome and other types of nephropathy.

AAV-mediated expression of secreted and transmembrane αKlotho isoforms rescues relevant aging hallmarks in senescent SAMP8 mice

Senescence represents a stage in life associated with elevated incidence of morbidity and increased risk of mortality due to the accumulation of molecular alterations and tissue dysfunction, promoting a decrease in the organism's protective systems. Thus, aging presents molecular and biological hallmarks, which include chronic inflammation, epigenetic alterations, neuronal dysfunction, and worsening of physical status. In this context, we explored the AAV9-mediated expression of the two main isoforms of the aging-protective factor Klotho (KL) as a strategy to prevent these general age-related features using the senescence-accelerated mouse prone 8 (SAMP8) model. Both secreted and transmembrane KL isoforms improved cognitive performance, physical state parameters, and different molecular variables associated with aging. Epigenetic landscape was recovered for the analyzed global markers DNA methylation (5-mC), hydroxymethylation (5-hmC), and restoration occurred in the acetylation levels of H3 and H4. Gene expression of pro- and anti-inflammatory mediators in central nervous system such as TNF-α and IL-10, respectively, had improved levels, which were comparable to the senescence-accelerated-mouse resistant 1 (SAMR1) healthy control. Additionally, this improvement in neuroinflammation was supported by changes in the histological markers Iba1, GFAP, and SA β-gal. Furthermore, bone tissue structural variables, especially altered during senescence, recovered in SAMP8 mice to SAMR1 control values after treatment with both KL isoforms. This work presents evidence of the beneficial pleiotropic role of Klotho as an anti-aging therapy as well as new specific functions of the KL isoforms for the epigenetic regulation and aged bone structure alteration in an aging mouse model.

Undetected αKlotho in serum is associated with the most aggressive phenotype of breast cancer

Klotho, a cellular anti-senescence protein, is related to antitumor actions, growth regulation, proliferation and invasiveness in several types of tumor, including breast cancer. The present study aimed to analyze the serum levels of αKlotho in patients with breast cancer according to histopathological and immunohistochemical variables. A total of 74 patients and 60 healthy controls were recruited. Peripheral blood samples were collected and serum levels were assessed by sandwich ELISA. Clinical and diagnostic data were obtained from medical records and databases of the Clinical Hospital of the Federal University of Uberlândia (Uberlândia, Brazil). The results indicated no difference in the levels of αKlotho between patients and controls (P=0.068); however, the number of patients with breast cancer with undetectable αKlotho was high (n=52). Thus, the variables that were associated with the lowest survival rates were analyzed, relating them to undetectable αKlotho. Among cases of metastatic tumors or tumors with poor differentiation, positive lymph node status and triple-negative status, patients with undetectable αKlotho predominated and had unfavorable overall survival. Due to the significant results obtained in triple-negative patients, an in vitro analysis was performed to determine whether estrogen receptors (ERs) have a role in αKlotho production. Treatment of MCF-7 cells with ER agonists, estradiol (E2) and diarylpropionitrile (DPN), resulted in increases in αKlotho expression and supernatant levels of both agonists, demonstrating a direct association between the ER and Klotho production; of note, the ERβ-specific agonist DPN tripled αKlotho expression when compared to E2 (P=0.078). These data suggested that undetectable αKlotho in the serum of patients with breast cancer is related to unfavorable histopathological variables and poor prognosis and ERs possibly have an important role in maintaining adequate quantities of αKlotho.

Investigation of the Role of Induced Overexpression of the Isolated Secreted Klotho on the A-172 Human Glioblastoma Cells

Klotho gene, identified in 1997 as an anti-aging gene, can manufacture two protein products: transmembrane and secreted forms. The later research revealed the involvement of klotho in carcinogenesis. However, little is known about the action of different Klotho forms on antitumor effects is still. The purpose of this article is to evaluate the effect of isolated secreted Klotho overexpression on the growth features of human glioblastoma cell line A-172.A-172 was transfected by a plasmid vector incorporating secreted Klotho sequence by the liposomal method. Overexpression assay was carried out quantitatively on both mRNA and protein using RT-qPCR and ELISA, correspondingly. It was shown that the relative expression of secreted Klotho in the experimental group was significantly higher than in the untransfected group by both methods (p < 0.001). At the same time, the growth curves and MTT proliferation assay demonstrated significantly decreased values under induced overexpression (p < 0.01). The increased amount of cells with activated caspases and annexin V (p < 0.001) corresponded with the expression of secreted Klotho. This mechanism, as suggested, maybe causative of the observed effects.

Klotho Level as a Marker of Low Bone Mineral Density in Egyptian Sickle Cell Disease Patients

Bone involvement of sickle cell disease (SCD) patients varies from acute clinical manifestations of painful vaso-occlusive crises or osteomyelitis to more chronic affection of bone mineral density (BMD) and debilitating osteonecrosis and osteoporosis. Secreted klotho protein is involved in calcium (Ca) reabsorption in the kidney. This study aimed to measure serum klotho levels in children with SCD to determine the possibility of using it as a marker of low BMD in children with SCD in correlation with a dual-energy radiograph absorptiometry scan. This study included 60 sickle disease patients and 30 age-matched and sex-matched control participants without SCD. A highly statistically significant difference was found between patients with normal BMD and those with low BMD, with serum Ca and klotho levels being lower in the latter group. Klotho serum level correlated positively with both serum Ca and BMD. Serum klotho level showed 94.9% sensitivity and 95.2% specificity in the detection of low BMD. Both serum Ca and klotho serum levels may be useful markers for detection of low BMD related to SCD with high sensitivity and specificity; however, klotho may be a better indicator as it is less affected by the nutritional and endocrinal status of patients or by intake of Ca supplements.

Klotho and Mesenchymal Stem Cells: A Review on Cell and Gene Therapy for Chronic Kidney Disease and Acute Kidney Disease

Chronic kidney disease (CKD) and acute kidney injury (AKI) are public health problems, and their prevalence rates have increased with the aging of the population. They are associated with the presence of comorbidities, in particular diabetes mellitus and hypertension, resulting in a high financial burden for the health system. Studies have indicated Klotho as a promising therapeutic approach for these conditions. Klotho reduces inflammation, oxidative stress and fibrosis and counter-regulates the renin-angiotensin-aldosterone system. In CKD and AKI, Klotho expression is downregulated from early stages and correlates with disease progression. Therefore, the restoration of its levels, through exogenous or endogenous pathways, has renoprotective effects. An important strategy for administering Klotho is through mesenchymal stem cells (MSCs). In summary, this review comprises in vitro and in vivo studies on the therapeutic potential of Klotho for the treatment of CKD and AKI through the administration of MSCs.

Physiologic Regulation of Systemic Klotho Levels by Renal CaSR Signaling in Response to CaSR Ligands and pH o

BACKGROUND

The kidney is the source of sKlotho and kidney-specific loss of Klotho leads to a phenotype resembling the premature multiorgan failure phenotype in Klotho-hypomorphic mice ( kl/kl mice). Klotho and the Ca-sensing receptor (CaSR) are highly expressed in the distal convoluted tubule (DCT). The physiologic mechanisms that regulate sKlotho levels are unknown.

METHODS

We measured sKlotho in WT and tubule-specific CaSR -/- (TS-CaSR -/- ) mice treated with calcimimetics, alkali, or acid, and Klotho shed from minced mouse kidneys, and from HEK-293 cells expressing the CaSR and Klotho, in response to calcimimetics, calcilytics, alkalotic and acidic pH, and ADAM protease inhibitors. The CaSR, Klotho, and ADAM10 were imaged in mouse kidneys and cell expression systems using confocal microscopy.

RESULTS

The CaSR, Klotho, and ADAM10 colocalize on the basolateral membrane of the DCT. Calcimimetics and HCO 3 increase serum sKlotho levels in WT but not in CaSR -/- mice, and acidic pH suppresses sKlotho levels in WT mice. In minced kidneys and cultured cells, CaSR activation with high Ca, calcimimetics, or alkali increase shed Klotho levels via ADAM10, as demonstrated using the ADAM10 inhibitor GI254023X and siRNA. In cultured cells, the CaSR, Klotho, and ADAM10 form cell surface aggregates that disperse after CaSR activation.

CONCLUSIONS

We identify a novel physiologic mechanism for regulation of sKlotho levels by the renal CaSR-ADAM10-Klotho pathway. We show that CaSR activators, including alkali, increase renal CaSR-stimulated Klotho shedding and predict that this mechanism is relevant to the effects of acidosis and alkali therapy on CKD progression.

Generation and Release of Neurogranin, Vimentin, and MBP Proteolytic Peptides, Following Traumatic Brain Injury

Traumatic brain injury (TBI) is a major neurological disorder without FDA-approved therapies. In this study, we have examined the concept that TBI might trigger global brain proteolysis in the acute post-injury phase. Thus, we conducted a systemic proteolytic peptidomics analysis using acute cerebrospinal fluid (CSF) samples from TBI patients and normal control samples. We employed ultrafiltration-based low molecular weight (LMW; < 10 kDa) peptide enrichment, coupled with nano-reversed-phase liquid chromatography/tandem mass spectrometry analysis, followed with orthogonal quantitative immunoblotting-based protein degradation analysis. We indeed identified novel patterns of injury-dependent proteolytic peptides derived from neuronal components (pre- and post-synaptic terminal, dendrites, axons), extracellular matrix, oligodendrocytes, microglial cells, and astrocytes. Among these, post-synaptic protein neurogranin was identified for the first time converted to neurogranin peptides including neurogranin peptide (aa 16-64) that is phosphorylated at Ser-36/48 (P-NG-fragment) in acute human TBI CSF samples vs. normal control with a receiver operating characteristic area under the curve of 0.957. We also identified detailed processing of astroglia protein (vimentin) and oligodendrocyte protein (MBP and Golli-MBP) to protein breakdown products (BDPs) and/or LMW proteolytic peptides after TBI. In addition, using MS/MS selected reaction monitoring method, two C-terminally released MBP peptides TQDENPVVHFF and TQDENPVVHF were found to be elevated in acute and subacute TBI CSF samples as compared to their normal control counterparts. These findings imply that future therapeutic strategies might be placed on the suppression of brain proteolysis as a target. The endogenous proteolytic peptides discovered in human TBI biofluid could represent useful diagnostic and monitoring tools for TBI.

Klotho inhibits neuronal senescence in human brain organoids

Aging is a major risk factor for many neurodegenerative diseases. Klotho (KL) is a glycosylated transmembrane protein that is expressed in the choroid plexus and neurons of the brain. KL exerts potent anti-aging effects on multiple cell types in the body but its role in human brain cells remains largely unclear. Here we show that human cortical neurons, derived from human pluripotent stem cells in 2D cultures or in cortical organoids, develop the typical hallmarks of senescent cells when maintained in vitro for prolonged periods of time, and that moderate upregulation or repression of endogenous KL expression in cortical organoids inhibits and accelerates senescence, respectively. We further demonstrate that KL expression alters the expression of senescence-associated genes including, extracellular matrix genes, and proteoglycans, and can act in a paracrine fashion to inhibit neuronal senescence. In summary, our results establish an important role for KL in the regulation of human neuronal senescence and offer new mechanistic insight into its role in human brain aging.

Post-kidney transplant soluble Klotho levels are determined by pretransplant soluble Klotho levels in both living donors and recipients

BACKGROUND

Soluble Klotho (sKl), the free form of membrane-bound Klotho predominantly expressed in the kidney, is detectable in serum and may have multiple pleiotropic effects. Patients with end-stage kidney disease are possibly sKl deficient, and kidney transplantation is the treatment of choice in these patients; however, little is known about changes in posttransplant sKl level and the factors influencing these changes.

METHODS

We conducted a prospective longitudinal study to examine changes in posttransplant sKl level in recipients for 12 months after living-donor kidney transplantation and analyzed correlations between posttransplant changes in sKl levels and various influencing factors in both recipients and donors.

RESULTS

29 kidney transplant recipients and their living donors were included for analysis. The results showed that sKl levels transiently decreased at 1 week posttransplant but progressively increased thereafter for 12 months. Multivariable linear regression analysis showed that body surface area-adjusted donor sKl levels were associated with posttransplant increases in recipient sKl levels at 12 months. In addition, pretransplant recipient sKl levels and body surface area-adjusted donor sKl levels were identified as an independent predictor of 12-month posttransplant sKl levels.

CONCLUSION

Pretransplant sKl levels in both kidney recipients and living donors are a strong determinant of sKl levels after kidney transplantation.

Exploiting the neuroprotective effects of α-klotho to tackle ageing- and neurodegeneration-related cognitive dysfunction

Cognitive dysfunction is a key symptom of ageing and neurodegenerative disorders, such as Alzheimer's disease (AD). Strategies to enhance cognition would impact the quality of life for a significant proportion of the ageing population. The α-klotho protein may protect against cognitive decline through multiple mechanisms: such as promoting optimal synaptic function via activation of N-methyl-d-aspartate (NMDA) receptor signalling; stimulating the antioxidant defence system; reducing inflammation; promoting autophagy and enhancing clearance of amyloid-β. However, the molecular and cellular pathways by which α-klotho mediates these neuroprotective functions have yet to be fully elucidated. Key questions remain unanswered: which form of α-klotho (transmembrane, soluble or secreted) mediates its cognitive enhancing properties; what is the neuronal receptor for α-klotho and which signalling pathways are activated by α-klotho in the brain to enhance cognition; how does peripherally administered α-klotho mediate neuroprotection; and what is the molecular basis for the beneficial effect of the VS variant of α-klotho? In this review, we summarise the recent research on neuronal α-klotho and discuss how the neuroprotective properties of α-klotho could be exploited to tackle age- and neurodegeneration-associated cognitive dysfunction.

In search of alternatively spliced alpha-Klotho Kl1 protein in mouse brain

Alpha‐Klotho is a multi‐functional protein essential for maintenance of a myriad of cell functions. αKlotho is a single transmembrane protein with a large extracellular segment consisting of two domains (termed Kl1 and Kl2) which is shed into the extracellular fluid by proteolytic cleavage to furnish circulating soluble αKlotho. Based on cDNA sequence, an alternatively spliced mRNA is predicted to translate to a putative soluble αKlotho protein in mouse and human with only the Kl1 domain that represents a “spliced αKlotho Kl1” (spKl1) and is released from the cell without membrane targeting or cleavage. The existence of this protein remains in silico for two decades. We generated a novel antibody (anti‐spE15) against the 15 amino acid epitope (E15; VSPLTKPSVGLLLPH) which is not present in Kl1 or full‐length αKlotho and validated its specific reactivity against spKl1 in vitro. Using anti‐spE15 and two well‐established anti‐αKlotho monoclonal antibodies, we performed immunoblots, immunoprecipitation, and immunohistochemistry to investigate for expression of spKl1 in the mouse brain. We found anti‐spE15 labeling in mouse brain but were not able to see co‐labelling of Kl1 and spE15 epitopes on the same protein, which is the pre‐requisite for the existence of a spKl1 polypeptide, indicating that anti‐spE15 likely binds to another protein other than the putative spKl1. In isolated choroid plexus from mouse brain, we found strong staining with anti‐spE15, but did not find the spliced αKlotho transcript. We conclude that using reliable reagents and inclusion of proper controls, there is no evidence of the spKl1 protein in the mouse brain.

Triiodothyronine (T3) enhances lifespan and protects against oxidative stress via activation of Klotho in Caenorhabditis elegans

Age predisposes individuals to significant diseases, and the biological processes contributing to aging are currently under intense investigation. Klotho is an anti-aging protein with multifaceted roles and is an essential component of the endocrine fibroblast growth factor. In Caenorhabditis elegans (C. elegans), there are two prospective orthologs of α-Klotho, C50F7.10, and E02H9.5, identified. The two orthologs' products are homologous to the highly conserved KL1 domain of human and mouse Klotho protein. Considering the endocrine system's major involvement in an organism's homeostasis and that thyroid disorders increase with advancing age, the molecular mechanisms underlying its impact on different endocrine components during the aging process remain poorly characterized. In this study, we sought to determine the regulatory role of Triiodothyronine (T3) on homologs genes of klotho and its impact on different parameters of aging in the C. elegans model organism. We showed that T3 could increase the mRNA expressions of the klotho homologous genes in C. elegans. Moreover, T3 could also extend a worm lifespan and modulate oxidative stress resistance and aging biomarkers significantly and positively. Further investigations employing different mutant and transgenic strains reveal that these observed effects are mediated through the EGL-17/EGL-15 pathway via Klotho activation along with the involvement of transcription factor DAF-16. In conclusion, these findings have revealed an unexpected link between T3 and Klotho and how this link can modulate the aging process in C. elegans via activation of klotho. This study will help understand the crosstalk and regulations of different endocrine components and their consequences on the aging process in multiple species.

Roles for fibroblast growth factor-23 and α-Klotho in acute kidney injury

Acute kidney injury is a global disease with high morbidity and mortality. Recent studies have revealed that the fibroblast growth factor-23-α-Klotho axis is closely related to chronic kidney disease, and has multiple biological functions beyond bone-mineral metabolism. However, although dysregulation of fibroblast growth factor-23-α-Klotho has been observed in acute kidney injury, the role of fibroblast growth factor-23-α-Klotho in the pathophysiology of acute kidney injury remains largely unknown. In this review, we describe recent findings regarding fibroblast growth factor-23-α-Klotho, which is mainly involved in inflammation, oxidative stress, and hemodynamic disorders. Further, based on these recent results, we put forth novel insights regarding the relationship between the fibroblast growth factor-23-α-Klotho axis and acute kidney injury, which may provide new therapeutic targets for treating acute kidney injury.

Shedding of Klotho: Functional Implications in Chronic Kidney Disease and Associated Vascular Disease

α-Klotho (Klotho) exists in two different forms, a membrane-bound and soluble form, which are highly expressed in the kidney. Both forms play an important role in various physiological and pathophysiological processes. Recently, it has been identified that soluble Klotho arises exclusively from shedding or proteolytic cleavage. In this review, we will highlight the mechanisms underlying the shedding of Klotho and the functional effects of soluble Klotho, especially in CKD and the associated cardiovascular complications. Klotho can be cleaved by a process called shedding, releasing the ectodomain of the transmembrane protein. A disintegrin and metalloproteases ADAM10 and ADAM17 have been demonstrated to be mainly responsible for this shedding, resulting in either full-length fragments or sub-fragments called KL1 and KL2. Reduced levels of soluble Klotho have been associated with kidney disease, especially chronic kidney disease (CKD). In line with a protective effect of soluble Klotho in vascular function and calcification, CKD and the reduced levels of soluble Klotho herein are associated with cardiovascular complications. Interestingly, although it has been demonstrated that soluble Klotho has a multitude of effects its direct impact on vascular cells and the exact underlying mechanisms remain largely unknown and should therefore be a major focus of further research. Moreover, functional implications of the cleavage process resulting in KL1 and KL2 fragments remain to be elucidated.

Klotho in Clinical Nephrology: Diagnostic and Therapeutic Implications

αKlotho (called Klotho here) is a membrane protein that serves as the coreceptor for the circulating hormone fibroblast growth factor 23 (FGF23). Klotho is also cleaved and released as a circulating substance originating primarily from the kidney and exerts a myriad of housekeeping functions in just about every organ. The vital role of Klotho is shown by the multiorgan failure with genetic deletion in rodents, with certain features reminiscent of human disease. The most common causes of systemic Klotho deficiency are AKI and CKD. Preclinical data on Klotho biology have advanced considerably and demonstrated its potential diagnostic and therapeutic value; however, multiple knowledge gaps exist in the regulation of Klotho expression, release, and metabolism; its target organs; and mechanisms of action. In the translational and clinical fronts, progress has been more modest. Nonetheless, Klotho has potential clinical applications in the diagnosis of AKI and CKD, in prognosis of progression and extrarenal complications, and finally, as replacement therapy for systemic Klotho deficiency. The overall effect of Klotho in clinical nephrology requires further technical advances and additional large prospective human studies.

Association of Soluble Klotho Level with Adverse Outcomes in Patients on Maintenance Hemodialysis

Background

The predictive value of soluble Klotho (sKlotho) for adverse outcomes in patients on maintenance hemodialysis (MHD) is controversial. In this study, we aimed to clarify the potential association of sKlotho levels with adverse outcomes in this patient population.

Materials

A total of 211 patients on MHD were identified and stratified according to the median sKlotho level. Patients were followed up for adverse outcomes including cardiovascular (CV) morbidity and all-cause mortality.

Results

During the 36-month follow-up, 75 patients [51 CV events (including 16 CV deaths) and 40 deaths] experienced adverse outcomes. After stratification according to median sKlotho level, patients with a lower sKlotho level had a greater risk of CV events (38.2% vs. 19.5%, p = 0.006), all-cause mortality (28.4% vs. 11.6%, p = 0.003), and combined adverse outcomes (51.0% vs. 24.2%, p < 0.001). Similar observations were made from analyses using Kaplan-Meier survival curves. Cox regression analysis showed that a low sKlotho level was strongly correlated with CV morbidity [1.942 (1.030–3.661), p = 0.040)], all-cause mortality [2.073 (1.023–4.203), p = 0.043], and combined adverse outcomes [1.818 (1.092–3.026), p = 0.021] in fully adjusted models.

Conclusions

The sKlotho level was an independent predictive factor of adverse outcomes including CV morbidity and mortality in patients on MHD.

Investigation on urinary and serum alpha klotho in dogs with chronic kidney disease

Background

As a co-receptor for fibroblast growth factor 23, klotho plays a pivotal role in phosphate metabolism. The kidney is known to be the main source of soluble alpha-klotho and the principal regulator of its concentration. Previous studies in human participants showed that the concentration of soluble alpha-klotho in serum and urine decreased in chronic kidney disease (CKD) patients. However, no previous study has assessed soluble alpha-klotho levels in dogs. This study aimed to measure serum and urinary alpha-klotho levels in CKD dogs and identify their associations with International Renal Interest Society (IRIS) CKD stages and other parameters known to be associated with CKD.

Results

Serum and urinary alpha klotho concentrations were measured by a commercially available canine-specific sandwich enzyme-linked immunosorbent assay kit and compared between groups by a nonparametric Kruskal–Wallis test. Spearman’s correlation coefficient was used to evaluate the relationships between variables. A stepwise multiple regression analysis was performed to estimate the effects of independent predictors on klotho concentrations. The urine klotho-to-creatinine ratio (UrKl/Cr) was significantly lower in stage 3 dogs than the control group and was significantly lower in dogs with stage 3 and 4 CKD than in those with stage 1 and 2 disease. UrKl/Cr was negatively correlated with serum symmetric dimethylarginine (sSDMA), blood urea nitrogen (BUN), creatinine, and phosphorus concentration. Serum alpha-klotho concentration in dogs with stages 2 and 3 CKD was significantly lower than those in the control group. There was no significant correlation between serum alpha-klotho and BUN, creatinine, and phosphorus concentrations. No statistically significant differences were observed in UrKl/Cr and serum alpha-klotho concentration between groups based on sex, age, urine protein-to-creatinine ratio (UPC), or blood pressure.

Conclusions

UrKl/Cr decreased in dogs with advanced CKD, and it was negatively correlated with sSDMA, BUN, creatinine, and phosphorus concentrations. Thus, klotho is associated with CKD and its clinical consequences, including CKD-mineral bone disorder, in dogs. Although serum klotho concentration was negatively correlated with sSDMA levels, it was not apparently related to IRIS CKD stage or other parameters known to be associated with CKD.

Uremic Toxins and Vascular Dysfunction

Vascular dysfunction is an essential element found in many cardiovascular pathologies and in pathologies that have a cardiovascular impact such as chronic kidney disease (CKD). Alteration of vasomotricity is due to an imbalance between the production of relaxing and contracting factors. In addition to becoming a determining factor in pathophysiological alterations, vascular dysfunction constitutes the first step in the development of atherosclerosis plaques or vascular calcifications. In patients with CKD, alteration of vasomotricity tends to emerge as being a new, less conventional, risk factor. CKD is characterized by the accumulation of uremic toxins (UTs) such as phosphate, para-cresyl sulfate, indoxyl sulfate, and FGF23 and, consequently, the deleterious role of UTs on vascular dysfunction has been explored. This accumulation of UTs is associated with systemic alterations including inflammation, oxidative stress, and the decrease of nitric oxide production. The present review proposes to summarize our current knowledge of the mechanisms by which UTs induce vascular dysfunction.

Clinical Potential of Targeting Fibroblast Growth Factor-23 and αKlotho in the Treatment of Uremic Cardiomyopathy

Chronic kidney disease is highly prevalent, affecting 10% to 15% of the adult population worldwide and is associated with increased cardiovascular morbidity and mortality. As chronic kidney disease worsens, a unique cardiovascular phenotype develops characterized by heart muscle disease, increased arterial stiffness, atherosclerosis, and hypertension. Cardiovascular risk is multifaceted, but most cardiovascular deaths in patients with advanced chronic kidney disease are caused by heart failure and sudden cardiac death. While the exact drivers of these deaths are unknown, they are believed to be caused by uremic cardiomyopathy: a specific pattern of myocardial hypertrophy, fibrosis, with both diastolic and systolic dysfunction. Although the pathogenesis of uremic cardiomyopathy is likely to be multifactorial, accumulating evidence suggests increased production of fibroblast growth factor-23 and αKlotho deficiency as potential major drivers of cardiac remodeling in patients with uremic cardiomyopathy. In this article we review the increasing understanding of the physiology and clinical aspects of uremic cardiomyopathy and the rapidly increasing knowledge of the biology of both fibroblast growth factor-23 and αKlotho. Finally, we discuss how dissection of these pathological processes is aiding the development of therapeutic options, including small molecules and antibodies, directly aimed at improving the cardiovascular outcomes of patients with chronic kidney disease and end-stage renal disease.

Identification of the cleavage sites leading to the shed forms of human and mouse anti-aging and cognition-enhancing protein Klotho

Klotho is an age-extending, cognition-enhancing protein found to be down-regulated in aged mammals when age-related diseases start to appear. Low levels of Klotho occur in neurodegenerative diseases, kidney disease and many cancers. Many normal and pathologic processes involve the proteolytic shedding of membrane proteins. Transmembrane (TM) Klotho contains two homologous domains, KL1 and KL2 with homology to glycosidases. After shedding by ADAM 10 and 17, a shed Klotho isoform is released into serum and urine by the kidney, and into the CSF by the choroid plexus. We previously reported that human Klotho contains two major cleavage sites. However, the exact cleavage site responsible for the cleavage between the KL1 and KL2 domains remains unknown for the human Klotho, and both sites are unknown for mouse Klotho. In this study, we aimed to identify the cleavage sites leading to the shed forms of human and mouse Klotho. Mutations in the region close to the TM domain of mouse Klotho result in the reduced shedding of the 130 kD (KL1+KL2) and 70 kD (KL1) fragments, suggesting that the cleavage site lies within the mutated region. We further identified the cleavage sites responsible for the cleavage between KL1 and KL2 of human and mouse Klotho. Moreover, mutated Klotho proteins have similar subcellular localization patterns as wild type Klotho. Finally, in an FGF23 functional assay, all Klotho mutants with a nine amino acid deletion can also function as an FGFR1 co-receptor for FGF23 signaling, however, the signaling activity was greatly reduced. The study provides new and important information on Klotho shedding, and paves the way for studies aimed to distinguish between the distinct roles of the various isoforms of Klotho.

Protective potential of klotho protein on diabetic retinopathy: Evidence from clinical and in vitro studies

AIMS/INTRODUCTION

The purpose of the present study was to observe the relationship between serum α-klotho (KL) protein level and diabetic retinopathy (DR), and to further examine the effects of KL protein on apoptosis induced by palmitic acid (PA) in human retinal endothelial cells.

MATERIALS AND METHODS

A total of 17 healthy people and 60 type 2 diabetes patients were included. According to the results from fundus fluorescein angiography, the diabetes patients were divided into three subgroups: without DR, non-proliferative DR and proliferative DR. Serum KL level was measured by enzyme-linked immunosorbent assay. In vitro, human retinal endothelial cells were exposed to PA with or without KL protein. Apoptosis rates were analyzed by flow cytometry analysis. Apoptotic-related protein expressions were detected by western blotting analysis.

RESULTS

Serum KL level was lower in diabetes patients than that in healthy participants (P = 0.007), and was gradually decreased among the without DR, non-proliferative DR and proliferative DR subgroups (P = 0.045). A logistic regression analysis showed that after adjusting for the other confounding factors, serum KL level was independently and negatively related with DR (P = 0.049). Furthermore, the increased apoptosis rates induced by PA were inhibited with the addition of KL protein. Consistently, KL protein reversed the expression levels of the increased pro-apoptotic protein Bax and the decreased anti-apoptotic protein Bcl-2 induced by PA. However, the anti-apoptotic effect of KL protein was attenuated by LY294002 through the phosphatidylinositol 3 kinase-serine∕threonine kinase pathway.

CONCLUSIONS

The data suggested that KL protein was probably a potential protective factor against retinopathy in type 2 diabetes patients.

Klotho regulation by albuminuria is dependent on ATF3 and endoplasmic reticulum stress

Proteinuria is associated with renal function decline and cardiovascular mortality. This association may be attributed in part to alterations of Klotho expression induced by albuminuria, yet the underlying mechanisms are unclear. The presence of albumin decreased Klotho expression in the POD-ATTAC mouse model of proteinuric kidney disease as well as in kidney epithelial cell lines. This downregulation was related to both decreased Klotho transcription and diminished protein half-life, whereas cleavage by ADAM proteases was not modified. The regulation was albumin specific since it was neither observed in the analbuminemic Col4α3^-/- Alport mice nor induced by exposure of kidney epithelial cells to purified immunoglobulins. Albumin induced features of ER stress in renal tubular cells with ATF3/ATF4 activation. ATF3 and ATF4 induction downregulated Klotho through altered transcription mediated by their binding on the Klotho promoter. Inhibiting ER stress with 4-PBA decreased the effect of albumin on Klotho protein levels without altering mRNA levels, thus mainly abrogating the increased protein degradation. Taken together, albuminuria decreases Klotho expression through increased protein degradation and decreased transcription mediated by ER stress induction. This implies that modulating ER stress may improve proteinuria-induced alterations of Klotho expression, and hence renal and extrarenal complications associated with Klotho loss.

αKlotho-FGF23 interactions and their role in kidney disease: a molecular insight

Following the serendipitous discovery of the ageing suppressor, αKlotho (αKl), several decades ago, a growing body of evidence has defined a pivotal role for its various forms in multiple aspects of vertebrate physiology and pathology. The transmembrane form of αKl serves as a co-receptor for the osteocyte-derived mineral regulator, fibroblast growth factor (FGF)23, principally in the renal tubules. However, compelling data also suggest that circulating soluble forms of αKl, derived from the same source, may have independent homeostatic functions either as a hormone, glycan-cleaving enzyme or lectin. Chronic kidney disease (CKD) is of particular interest as disruption of the FGF23-αKl axis is an early and common feature of disease manifesting in markedly deficient αKl expression, but FGF23 excess. Here we critically discuss recent findings in αKl biology that conflict with the view that soluble αKl has substantive functions independent of FGF23 signalling. Although the issue of whether soluble αKl can act without FGF23 has yet to be resolved, we explore the potential significance of these contrary findings in the context of CKD and highlight how this endocrine pathway represents a promising target for novel anti-ageing therapeutics.