Aβ synaptoxicity is Dkk1-dependent12,24 and also appears to be APP-dependent25.
The Wnt-PCP signalling pathway acts through RhoA and ROCK26, and can be inhibited pharmacologically, for example, with fasudil24, a ROCK inhibitor that is in clinical use for the treatment of cerebral vasospasm.
Fasudil also reversed the stimulatory effect of Vangl2 and Dkk1 on Aβ production in the same cells as determined by ELISA based quantification of Aβ1–40 levels in the paired culture media from the same experiments (Fig. 3d).
Animals receiving fasudil had significantly lower soluble Aβ1–40 levels than controls (Fig. 4b).
APP has previously been shown to interact with Vangl2, a Wnt-PCP-specific co-receptor protein, and independently has been shown to bind LRP6, a receptor component of the Wnt-β-catenin pathway21.
In cells expressing either wild-type or Swedish APP, the amount of Aβ produced was reduced in cells stimulated with Wnt3a, which promotes Wnt-βcatenin signalling, whereas Aβ production was enhanced in cells stimulated with Wnt5a, which promotes Wnt-PCP signalling (Fig. 2c).
Comparison of signalling activity and Aβ production under each of the different conditions showed clearly that Aβ production was negatively correlated with Wnt-β-catenin signalling activity (Fig. 2d) and positively correlated with Wnt-PCP signalling (Fig. 2e).
APP did though potentiate both Wnt3a-driven Wnt-β-catenin signalling and Wnt5a-driven Wnt-PCP signalling.
In contrast with wild-type APP which, as before, potentiated both canonical and non-canonical Wnt signalling, the Swedish mutant form of APP antagonised canonical Wnt signalling (Fig. 2a), and potentiated non-canonical Wnt signalling to a greater degree than wild-type APP (APPWT) (Fig. 2b).
Overexpression of APP enhanced the inhibitory effects of Dkk1 on Wnt3a induced Wnt-β-catenin signalling, counteracting the enhanced activity resulting from APP overexpression and reducing the IC50 of Dkk1 to 122ng/mL from 173ng/mL in the absence of APP (Fig. 2f) .
In contrast, the stimulatory effects of Dkk1 on WntPCP signalling induced by Wnt5a were enhanced by APP overexpression, decreasing the EC50 of Dkk1 to 599ng/ mL from 1405ng/mL (Fig. 2g).
Dkk1 resulted in substantial loss of dendritic spines, which was blocked by 10µM fasudil treatment (Fig. 3e, f).
Notably, in parallel with the protective effect of fasudil on synapses (Fig. 3e, f), treatment with fasudil reversed the stimulatory effects of Dkk1 on Aβ production (Fig. 3g).
In addition to causing a significant reduction in the numbers of dendritic spines, Dkk1 treatment also resulted a substantial increase in levels of all three Aβ species (Fig. 3g).
As previously reported, Dkk1-treatment markedly reduced the number of dendritic spines on APP+/+ neurons (Fig. 3a, b). In contrast, APP-deficiency protected neurons against the synaptotoxic activity of Dkk1 (Fig. 3a, b).
The ability of APP to potentiate Wnt-β-catenin signalling in response to Wnt3a was enhanced by the overexpression of LRP6, and attenuated by Vangl2 (Fig. 1g).
Conversely, the ability of APP to potentiate Wnt-PCP signalling in response to Wnt5a was enhanced by Vangl2 and attenuated by LRP6 (Fig. 1h).
Co-expression of LRP6 with APP reduced the production of Aβ, while co-expression of Vangl2 with APP led to increased Aβ production (Fig. 2c).
As expected, cells expressing the Swedish mutant form of APP695 produced much more Aβ than the control wildtype-expressing cells.
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If you find BEL Commons useful in your work, please consider citing: Hoyt, C. T., Domingo-Fernández, D., & Hofmann-Apitius, M. (2018). BEL Commons: an environment for exploration and analysis of networks encoded in Biological Expression Language. Database, 2018(3), 1–11.