In Alzheimer’s disease (AD), deposition of pathological tau and amyloid-β (Aβ) is thought to drive synaptic loss and cognitive decline. Major questions in the field center around possi-ble connections between the two pathologies and what molecular mechanisms can impact tau pathology progression. We aimed to determine, if different proteins reported to modify tau pathology could modify tau phosphorylation in vitro and tau spreading in vivo.
Two kinases, Pyk2 (Ptk2b) and Fyn, are part of a signaling cascade activated by Aß oli-gomers binding to the cellular prion protein receptor (PrPC) and have also been reported to interact with GSK3ß, one of the best characterized tau kinases. We hypothesized that Aß might induce tau phosphorylation through PrpC-mGluR5-Fyn-Pyk2-GSK3ß signaling. We first studied the interactions of the three kinases and tau in different in vitro model systems, in-cluding HEK-293T over-expression system and iPSC-derived neurons. In HEK-293T cells, Pyk2 and Fyn worked in synergy to increase GSK3ß phosphorylation and subsequent tau phosphor-ylation. Furthermore, GSK3ß co-immunoprecipitated with Pyk2, and Fyn kinase inhibition reduced Fyn’s proximity to tau as well as reduced tau spreading in mouse cortical neuron cultures seeded with human tau. Thus, initial data supported a role for the three kinases in contributing to tau hyperphosphorylation and spreading. Contrary to these results, GSK3ß phosphorylation remained unchanged in human iPSC-derived neurons upon inhibition of Pyk2 and Fyn kinases. Surprisingly, tau phosphorylation on some epitopes was even increased up-on Pyk2 inhibition. In addition, iPSC-derived neurons were incubated either short- or long-term with synthetic Aßo. Upon long-term treatment, neurons showed decreases in synaptic density as expected. In contrast to expectations, short-term treatment with sAßo did not ac-tivate Pyk2 and Fyn kinases and showed no effect on GSK3ß, tau or other downstream tar-gets.
To study modulation of tau spreading by different factors, misfolded tau aggregates ex-tracted from human AD brains were injected into WT and transgenic mice to drive templat-ed spreading of tau pathology. We assessed the impact of Aβ co-pathology, of deleting loci known to modify AD risk (Ptk2b, Grn, and Tmem106b) and of pharmacological intervention with a Fyn kinase inhibitor on tau spreading after injection of AD tau extracts. The density and spreading of tau inclusions triggered by human tau seed were unaltered in the hippo-campus and cortex of APPswe/PSEN1ΔE9 transgenic and AppNL-F/NL-F knock-in mice. In mice with human tau sequence replacing mouse tau, template matching enhanced neuritic tau burden. Human AD brain tau-enriched preparations contained aggregated Aβ, and the Aβ co-injection caused a redistribution of Aβ aggregates in mutant AD model mice. The injection-induced Aβ phenotype was spatially distinct from tau accumulation and could be ameliorat-ed by depleting Aβ from tau extracts. These data suggest that Aβ and tau pathologies prop-agate by largely independent mechanisms after their initial formation. Altering the activity of the Fyn and Pyk2 kinases involved in Aβ-oligomer–induced signaling, or deleting expres-sion of the progranulin and TMEM106B lysosomal proteins, did not alter the somatic tau in-clusion burden or spreading. However, mouse aging had a prominent effect to increase the accumulation of neuritic tau after injection of human AD tau seeds into WT mice.
Overall, these results suggest that observations from over-expression non-neuronal model studies of Pyk2 and Fyn acting on GSK3ß and tau do not translate faithfully into neu-ronal model systems. Further, these studies refined our knowledge of factors (in-)capable of modulating tau spreading and lend evidence to the hypothesis of a more complex role of tau phosphorylation regulation by Fyn and Pyk2 in different model systems
