established that serum phosphate levels decreased in FGF23 knockout mice following Erlotinib exogenous FGF23 administration, but not in klotho knockout mice identifying the obligate role of klotho.[16] The site of klotho expression in the nephron and actions related to signal transduction relationships remain controversial. While FGF23 has been found to bind to multiple FGFR,[84] signalling by FGF23 was seen only with FGFR-1c, 3c and -4 in cell lines that co-express klotho.[15] In rats, members of the FGFR family are expressed in the kidney at specific locations. FGFR-3 is

expressed in both proximal and distal tubules whilst FGFR-1 and-4 are only seen in distal tubules.[85] In vitro studies support FGFR-3 as the physiologically relevant receptor in FGF23 downstream signalling because phosphate transport occurs in the proximal tubules. Interestingly, a study by Liu et al. concluded that FGFR-3 and FGFR-4 did not mediate renal effects of FGF23 and instead, FGFR-1 was seen to co-localize with klotho in mice in the

distal tubules.[85] While this study suggests distal tubule distribution, more recent studies have reported convincing proximal tubular distribution, which is more physiologically likely since this is where most phosphate transport occurs.[7, 21, 22, 76] Nevertheless, a distal Navitoclax in vivo tubule response manifests itself early either directly or indirectly via proximal tubule signalling,[86] and exact human mechanisms are yet to be validated. Andrukhova et al. have established that FGF23 directly acts in a murine proximal tubule cell culture model to downregulate NaPi-IIa through extracellular signal-regulated kinases (ERK)-1/2 and serum/glucocorticoid-regulated kinase-1 (SGK1) signalling and this pathway is dependent on the presence of klotho at physiological FGF23 levels.[22] Through its enzymatic

activity, sKl can act directly to reduce recycling of NaPi-IIa, thereby itself inducing next a phosphaturic response.[76] It is plausible that while mKl is abundant within the distal tubules, proximal tubule distribution of mKl facilitates some degree of phosphate excretion and cleaved sKl through paracrine action promotes a response in the proximal tubule despite being cleaved downstream. These exact mechanisms are still unclear. One of the earliest abnormalities that develops as kidney function declines is a sustained reduction in tubular phosphate reabsorption resulting from the phosphaturic actions of FGF23 and PTH.[2, 87] As the remaining nephrons attempt to preserve phosphate balance, the amount of phosphate excreted per nephron increases, with increases in single-nephron glomerular filtration and fractional excretion of phosphate (FEPi) measurements.[87, 88] Ultimately serum phosphate levels are affected when the declining kidney is no longer able to compensate for the reduction in total phosphate excretion.[88] This is usually observed when the glomerular filtration rate (GFR) falls below 30 mL/min.