Cell-autonomous reduction of CYFIP2 is insufficient to induce Alzheimer's disease-like pathologies in the hippocampal CA1 pyramidal neurons of aged mice

Abstract

Cytoplasmic FMR1-interacting protein 2 (CYFIP2) is an evolutionarily conserved multifunctional protein that regulates the neuronal actin cytoskeleton, mRNA translation and transport, and mitochondrial morphology and function. Supporting its critical roles in proper neuronal development and function, human genetic studies have repeatedly identified variants of the CYFIP2 gene in individuals diagnosed with neurodevelopmental disorders. Notably, a few recent studies have also suggested a mechanistic link between reduced CYFIP2 level and Alzheimer's disease (AD). Specifically, in the hippocampus of 12-month-old Cyfip2 heterozygous mice, several AD-like pathologies were identified, including increased levels of Tau phosphorylation and gliosis, and loss of dendritic spines in CA1 pyramidal neurons. However, detailed pathogenic mechanisms, such as cell types and their circuits where the pathologies originate, remain unknown for AD-like pathologies caused by CYFIP2 reduction. In this study, we aimed to address this issue by examining whether the cell-autonomous reduction of CYFIP2 in CA1 excitatory pyramidal neurons is sufficient to induce AD-like phenotypes in the hippocampus. We performed immunohistochemical, morphological, and biochemical analyses in 12-month-old Cyfip2 conditional knock-out mice, which have postnatally reduced CYFIP2 expression level in CA1, but not in CA3, excitatory pyramidal neurons of the hippocampus. Unexpectedly, we could not find any significant AD-like phenotype, suggesting that the CA1 excitatory neuron-specific reduction of CYFIP2 level is insufficient to lead to AD-like pathologies in the hippocampus. Therefore, we propose that CYFIP2 reduction in other neurons and/or their synaptic connections with CA1 pyramidal neurons may be critically involved in the hippocampal AD-like phenotypes of Cyfip2 heterozygous mice.

Cell-autonomous reduction of CYFIP2 changes dendrite length, dendritic protrusion morphology, and inhibitory synapse density in the hippocampal CA1 pyramidal neurons of 17-month-old mice

“…Before investigating detailed neuronal morphologies, we first validated the downregulation of CYFIP2 levels in the hippocampal CA1 region of 17-month-old Cyfip2 cKO mice compared to control mice through immunohistochemistry ( Figure 1 A). Similar to findings from 12-month-old Cyfip2 cKO mice (Ma et al 2023 ), CYFIP2 levels decreased by approximately 50% in cKO mice compared to control mice. The remaining CYFIP2 signals detected in cKO mice might arise from the expression of CYFIP2 in inhibitory neurons (Lee SH et al 2020 ) and/or technical background noise in the immunostaining process.…”

“…The Cyfip2 cKO ( Cyfip2 floxed/floxed ; CaMKIIα-Cre T29-1) mice used in this study were previously described (Lee SH et al 2020 ; Zhang et al 2020 ; Ma et al 2023 ). Thy1-YFP [B6.Cg-Tg(Thy1-YFP)HJrs/J] mice were obtained from the Jackson Laboratory and bred with Cyfip2 cKO mice as previously described (Zhang et al 2020 ).…”

“…Indeed, some studies have shown morphological and functional changes in excitatory and inhibitory synapses, as well as alterations in dendritic structures, when the expression of CYFIP1 is either decreased or increased in mouse brains or cultured neurons (Pathania et al 2014 ; Oguro-Ando et al 2015 ; Davenport et al 2019 ). However, when it comes to CYFIP2, such phenotypes have been explored to a lesser extent, although there are reports suggesting alterations in the morphology of dendritic spines (the dendritic protrusions where excitatory synapses are formed) in Cyfip2 mutant mice (Han K et al 2015 ; Lee SH et al 2020 ; Zhang et al 2020 ; Ma et al 2023 ).…”

“…Mechanistically, the diminished expression of CYFIP2 leads to aberrant mRNA translation of various AD-related proteins at the synaptic compartment, resulting in the overproduction of Aβ and hyperphosphorylation of Tau in Cyfip2 heterozygous mice. Intriguingly, a recent study showed that 12-month-old Cyfip2 conditional knock-out (cKO, Cyfip2 floxed/floxed ; CaMKIIα-Cre T29-1 (Tsien et al 1996 )) mice, which specifically reduce CYFIP2 protein levels in CA1, but not CA3, pyramidal neurons of the hippocampus, do not exhibit AD-like phenotypes (Ma et al 2023 ). These cKO mice displayed normal levels of phospho-Tau, gliosis, and dendritic spines (with a subtle reduction in thin spines of basal dendrites) in the hippocampus when compared to age-matched wild-type mice.…”

“…Nevertheless, there is the possibility that AD-like abnormalities, such as dendritic spine loss, could be merely delayed in Cyfip2 cKO mice compared to Cyfip2 heterozygous mice (Ma et al 2023 ). Consequently, these defects may become apparent in cKO mice beyond the age of 12 months.…”