Dysregulated Cytoplasmic FRMP interacting protein 1/2 in a model of Alzheimer’s Disease

Abstract

Alzheimer’s disease (AD) is pathologically hallmarked by amyloid-β (Aβ) plaques and phosphorylated tau neurofibrillary tangles. It is still unclear about how these neuropathological manifestations are linked to synapse dysfunction and neurodegeneration. Local dendritic protein synthesis is a crucial mechanism for maintaining and regulating synapse function, because this mechanism enables the precise spatial and temporal control of synaptic protein expression. As regulators of the local dendritic protein synthesis, cytoplasmic FMR1 interacting proteins 1 and 2(CYFIP1/2) were suggested to be a potential hub for mediating the adverse effects of Aβ on synaptic functions. Previous work showed that in post-mortem tissues the protein level of CYFIP1/2 are significantly reduced in severe AD as well as in aged Tg2576 mice a mouse model of AD (12-month-old). Knockdown of CYFIP2 results in elevated APP protein expression and Aβ production without affecting APP mRNA levels in mice. Further, Cyfip2 knockdown leads to the emergence of age- dependent tau pathology, synapse loss and memory impairment. All these data suggest that loss of CYFIP2 contributes to neurodegeneration, whereas the evidence showing the impact of Aβ on CYFIP1/2 expression remain inconclusive. Hence, the first aim of this work is to investigate the link between Aβ exposure and CYFIP1/2 reduction. In primary cortical neurons, it was found that oligomeric Aβ treatment resulted in significant reduction of CYFIP2 and to a lesser extent, CYFIP1. This reduced expression was rescued by inhibition of the kinase MNK. These findings highlight Aβ as the causative factor for CYFIP2 reduction and also propose MNK activation as the primary mediator for Aβ-induced CYFIP1/2 reduction. The second aim of this work was to validate the above findings in 5xFAD mice, a more disease-relevant context. Using immunoblotting and immunohistochemistry, the changes in the protein level of CYFIP2 was analysed. CYFIP2, but not CYFIP1 expression was found to be reduced in young (3 months-old) 5xFAD mice, a time point when the mice displays having defined Aβ pathology, but neither tau tangle pathology nor neuronal or synapse loss. The reduction of CYFIP2 correlated with the activation of MNK and was most prominently affected in the dentate gyrus. Findings in the 5xFAD mice reinforced the notion that Aβ induce CYFIP2 reduction, mediated by MNK, and that CYFIP1 and CYFIP2 react differently to Aβ stimulation. Moreover, in Braak Stage VI AD human hippocampus, CYFIP2 was severely reduced when compared to the control group, whereas a reduction in CYFIP1 expression correlated with synapse loss. Finally, through examination of post-mortem hippocampus samples from different types of dementia, it was revealed that reduction of CYFIP1/2 is a specific feature for AD. As a conclusion, this body of work proposed a pathological cascade: 1) Aβ triggers the reduction of CYFIP1/2, particularly CYFIP2 through activation of MNK. 2) In parallel with loss of CYFIP2, dysregulation of dendritic protein synthesis is also noted after Aβ exposure. Such dysregulation may give rise to the upregulation of APP and tau kinases. 3) Upregulated APP and tau kinase further fuelled Aβ production and tau hyperphosphorylation, which in turn aggravate the AD pathology. Furthermore, reduction of CYFIP2 also shows the potential to be a biomarker specific for AD. Taken together, the current work proposes a central role of CYFIP2 in AD.

Date of Award	1 Mar 2022
Original language	English
Awarding Institution	King's College London
Supervisor	Peter Giese (Supervisor) & Gerald Finnerty (Supervisor)