Calcium and Klotho: the secret of immortality? Clinical relevance The life expectancy of humans is continuously rising and consequently ageing-related disorders increasingly challenge the quality of life of the elderly. Ageing is tightly associated with a negative calcium (Ca

2+) balance leading

to among others osteoporosis, arteriosclerosis and ectopic calcification. Recently, klotho (named after the Greek goddess who spins the thread of life) was identified as new “anti-ageing” hormone being instrumental in the process of age- related adaptations in the Ca

2+ balance.

Background Klotho is a type I membrane glycoprotein which shares homology to β-glucosidase enzymes. Interestingly, klotho gene ablation in mice resulted in a syndrome closely resembling human ageing, including short life span, bone aberrations, skin atrophy and a disturbed Ca

2+ balance together with high serum vitamin D levels. In addition,

polymorphisms in the klotho gene have been linked to reduced bone mineral density in humans. However, the molecular function of klotho and the down-stream targets of this anti- ageing hormone remain to be identified. Recently, we showed that klotho is predominantly expressed in kidney, where it is secreted in the pro-urine. Furthermore, we demonstrated that the epithelial Ca

2+ channel (TRPV5), which is the gate-keeper in the process of

Ca2+

active reabsorption, is stimulated by klotho via a novel extracellular activation mechanism. We hypothesize that klotho hydrolyses via its β-glucuronidase activity the extracellular TRPV5 sugar residues, entrapping the channel in the plasma membrane to maintain durable Ca

2+ transport activity.

Goals The general aim of this project is to delineate in detail the function of klotho in the regulation of the Ca

2+ balance

providing new insights into the molecular mechanism of ageing. To this end the following key objectives will be addressed: i) Molecular mechanism of klotho-mediated TRPV5 channel activation. We will investigate the modification of the extracellular sugar residues of TRPV5 by klotho, the molecular identity of the klotho-mediated signaling cascade and finally the effect of klotho on the plasma membrane recycling process of TRPV5. ii) The ‘in vivo’ function of klotho on renal Ca

2+ handling. The mechanism and hormonal regulation of the klotho

secretion into the pro-urine will be studied in animal models. Subsequently, the action of klotho on TRPV5, Ca2+

reabsorption and ultimately Ca

2+ balance will be investigated.

iii) Functional correlation of klotho and Ca2+

homeostasis in humans during ageing. The association between the urinary klotho expression and the Ca

2+ balance during ageing will be determined in subjects of The Nijmegen

Biomedical Study. Subsequently, the functional consequences of the recently identified non-synonymous SNPs in klotho on the Ca

2+ balance and TRPV5 channel activity will be studied.

We offer: The possibility to perform and present high-quality clinically-oriented research in a professional, multicultural and highly-motivating working environment with about 35 colleagues in a well-equipped department. The student will learn how to perform basic molecular biology techniques including cloning, quatitative real-time PCR analysis, patient DNA sequencing, immunohistochemistry, confocal laser scanning microscopy and various biochemical assays including blood ion determinations. Contact: Department: Physiology Contact person: Tim Schreuder Telephone number: 0243614222 Email address: tim.schreuder@radboudumc.nl Website: www.physiomics.eu