Functional Abnormalities In The Central Nervous And Cardiovascular Systems Induced By FGF-23 Overexpression Are Phosphorus-Dependent

Fibroblast growth factor (FGF) -23, a new member of polypeptide hormone FGFs, belongs to the FGF-19 subfamily. The FGF-23 protein, a secreted form, contains 251 amino acids including a 24 amino acid hydrophobic amino terminus that is a signal sequence. As one of phosphatonins, FGF-23 overexpression can result in hypophosphatemia. It is well known that inorganic phosphate is an essential element in most physiological processes, and it plays a vital role in a number of other biological processes such as signal transduction, nucleotide metabolism, and enzyme regulation. The decrease in phosphate leads to a depletion in cellular ATP and dysfunction of transmitters, furthermore, chronic severe hypophosphatemia can induce myocardial depression and encephalopathy in humans. On the other hand, FGF-23 can signal to target cells by binding and activating cell-surface tyrosine kinase FGF receptors (FGFRs). Among the seven canonical FGFR isoforms, it has been demonstrated that FGF-23 bound only the c splice isoforms of FGFR1-3, as well as FGFR4. In addition, FGF-23 produces physiological responses in vivo needs klotho protein, an antiaging gene product, as a coreceptor. The localization of FGF-23 mRNA in the brain is found to be preferentially expressed in the ventrolateral thalamic nucleus by in situ hybridization, indicating FGF-23 plays roles in the function of the central nervous system (CNS). Nowadays, researches about FGF-23 focus on the mechenisms of phosphate regulation, however, the effects on the other organs except kidney and bone are still unclear.To assess the roles of FGF-23 overexpression in the CNS and cardiovascular system (CVS), the phenotypes of brain and CVS were analyzed by the comparison of eight-week-old transgenic (TG) mice overexpressing human FGF-23 (R176Q) to their age-matched wild-type (WT) littermates using functional, cellular and molecular approaches.In CNS, TG mice showed spatial memory performance deficit in the Morris water maze, suggesting FGF-23 overexpression can affect the function of CNS. Electrophysiological results showed that excitatory postsynaptic potential slope did not increase after high-frequency stimulation in TG mice, indicating FGF-23 overexpression may impair long-term potentiation in CA1 area of hippocampus. Compared with the WT littermates, the positive percentage of choline acetyltransferase (ChAT) was decreased significantly in basal forebrain in TG mice demonstrated by immunostaining for ChAT. These results suggest that impaired hippocampal synaptic plasticity and decrease of the positive percentage of ChAT in basal forebrain contribute to spatial memory deficit. No abnormality was detected in brains from TG mice by histological observations. The numbers of both the proliferating cell nuclear antigen (PCNA) and caspase-3 positive cells were not altered in TG mice, indicating FGF-23 overexpression does not significantly affect the proliferation and apoptosis of neural cells. The positive percentage of the neuronal specific nuclei protein (NeuN) and the positive area for calbindin D-28k showed no statistic difference between TG and WT mice. These data demonstrate that FGF-23 overexpression does not affect the maturation of neuronal cells and differentiation to association and projection neurons. Similarly, the glial fibrillary acidic protein (GFAP) and the myelin basic protein (MBP) positive cells and fibers were not altered in TG mice, suggesting FGF-23 overexpression does not affect the differentiation of astrocyte and oligodendrocyte, as well as myelin formation.In CVS, systolic blood pressure decreased markedly in TG animals using a noninvasive computerized tail-cuff system, suggesting FGF-23 overexpression also results in a functional abnormality in CVS. Levels of plasma catecholamine and renin-angiotension system (RAS) were up-regulated. The left ventricle was hypertrophic, however, ejection fraction, an indices of cardiac systolic function, was not changed significantly in FGF-23 transgenic mice determined by echocardiography examination. The vascular reactivity of aortic ringes to norepinephrine and angiotension II were decreased significantly, however, the reactivity to KC1 was nornal, in transgenic animals, indicating that FGF-23 overexpression impairs vascular reactivity to vasoconstrictors. The real-time RT-PCR results demonstrated that the levels of the three subtypes of adrenegic receptorα₁ and angiotensin II receptor 1A mRNA were down-regulated significantly in aortas from TG mice relative to those from WT mice, suggesting FGF-23 overexpression results in down-regulation of vasoconstrictive receptors. This study demonstrated that FGF-23 overexpression resulted in hypotension, which was associated to impaired vascular reactivity and down-regulation of vasoconstrictive receptors, whereas the appropriate hypertrophy of left ventricle, up-regulation of plasma catecholamine and RAS activities may resulted from protective reactions against hypotension.To determine whether the functional abnormalities in CNS and CVS occurred in FGF23 transgenic mice are phosphorus-dependent or FGFR-dependent, 3-week-old, sex-matched WT and TG mice were fed on a high phosphate diet (containing 1.25% phosphate in drinking water) for 5 subsequent weeks and the phenotypes of CNS and CVS were analyzed as described above. Results showed the phenotypes of TG mice were rescued by high phosphate diet intake followed improved hypophosphatemia.According to our observations, we can draw conclusions as follows. (1) FGF-23 overexpression can result in functional abnormalities in CNS and CVS including spatial memory deficit and hypotension. (2) The effects of FGF-23 overexpression on CNS and CVS are mediated by hypophosphatemia indirectly, but not by FGFRs in CNS and CVS directly.