Fibroblast growth factor 23 signaling in hippocampal cells: impact on neuronal morphology and synaptic density

Abstract

Endocrine fibroblast growth factor 23 (FGF23) is predominantly secreted by osteocytes and facilitates renal phosphate excretion. However, FGF23 is also present in cerebrospinal fluid. In chronic kidney disease, FGF23 serum levels are excessively elevated and associated with learning and memory deficits. Structural plasticity of the hippocampus such as formation of new synapses or an altered dendritic arborization comprises a cellular and morphological correlate of memory formation. Therefore, we hypothesize that FGF23 alters hippocampal neuron morphology and synapses. To address this, we prepared primary murine hippocampal cultures and incubated them with recombinant FGF23 alone or together with a soluble isoform of its co-receptor a-Klotho. Neuronal expression of a fluorescent reporter allowed for a detailed evaluation of the neuronal morphology by Sholl analysis. Additionally, we evaluated synaptic density, identified by stainings, for synaptic markers. We show an enhanced number of primary neurites combined with a reduced arborization, resulting in a less complex morphology of neurons treated with FGF23. Moreover, FGF23 enhances the synaptic density in a FGF-receptor (FGF-R) dependent manner. Finally, we addressed the corresponding signaling events downstream of FGF-R employing a combination of western blots and quantitative immunofluorescence. Interestingly, FGF23 induces phospholipase Cc activity in primary hippocampal neurons. Co-application of soluble a-Klotho leads to activation of the Akt-pathway and modifies FGF23-impact on neuronal morphology and synaptic density. Compared with other FGFs, this alternative signaling pattern is a possible reason for differential effects of FGF23 on hippocampal neurons and may thereby contribute to learning and memory deficits in chronic kidney disease patients.

Rostral ventrolateral medulla neuron activity is suppressed by Klotho and stimulated by FGF23 in newborn Wistar rats

“…Moreover, a previous study shows that the high serum FGF23 levels cause hypertension in patients with CKD (Li et al, 2018). Recently, FGF23 was found to be present in the cerebrospinal fluid (CSF) (Hensel et al, 2016) with studies suggesting increased CSF levels of FGF23 among patients with CKD (Haffner and Leifheit-Nestler, 2017;Marebwa et al, 2018). Based on the aforementioned results, we hypothesized that the high levels of FGF23 among patients with CKD would directly stimulate RVLM neurons.…”

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

“…In this paper, mouse hippocampal neurons were reported to display altered neuronal morphology and changes in cell signaling in response to recombinant FGF23. Curiously, the observed effects appear to be modulated by soluble α-Klotho, indicating that the lack of m-KL in the brain does not necessarily preclude a role for brain α-Klotho in FGF23 signaling (Hensel et al, 2016). But in any case, the combined data point towards a primary requirement for brain α-Klotho as s-KL, and suggest that this role may be especially important more developed brains of humans.…”

“…As we describe in the next section, m-KL functions as a co-receptor for FGF23 in the kidney, and it is interesting to note that a related protein, β-Klotho, acts as a transmembrane co-receptor for FGF21 in the suprachiasmatic nucleus (Bookout et al, 2013). However, m-KL is highly selective for FGF23, and there is little evidence that FGF23 is active in the brain, beyond the intriguing study of Hensel and colleagues (Hensel et al, 2016). In this paper, mouse hippocampal neurons were reported to display altered neuronal morphology and changes in cell signaling in response to recombinant FGF23.…”

Involvement of Bone in Systemic Endocrine Regulation

“…In addition to systemic circulation, FGF23 is present in the cerebrovascular fluid. In the course of CKD, the FGF23 is taken up in the hippocampus, where it alters the neuronal morphology and causes memory insufficiency (Hensel et al 2016). Interestingly, FGF23 and Klotho α have also been found increased in cirrhotic alcoholics.…”