Mapping the spatial proteomic signature of dorsal and ventral hippocampus in a mouse model of early Alzheimer’s disease: changes in synaptic plasticity-related proteins associated with sexual dimorphism

Abstract

Background: An initial neuropathological hallmark of Alzheimer's disease (AD) is the hippocampal dysfunction caused by amyloid-β (Aβ) peptides accumulation. Soluble oligomeric forms of Aβ shift synaptic plasticity induction threshold leading to memory deficits in male and female mice in early amyloidosis models. Some protein changes underlying those deficits have been previously studied, but the spatial distribution within the hippocampus, as well as the potential sex differences, remain unknown. Since each hippocampal region (dorsal vs. ventral) has clearly distinct functionality and connectivity, we postulated that some protein changes may be unique to each and might also be sex dependent. Methods: An innovative spatial proteomics study was performed to map whole hippocampal proteome distribution using matrix-assisted laser desorption/ionization (MALDI) imaging mass spectrometry, which allows protein detection with spatial resolution directly on tissue sections. Brains from sixteen adult male and female mice intracerebroventricularly injected with Aβ1-42 oligomers or vehicle were sectioned. MALDI imaging was performed using a RapifleXTM MALDI TissuetyperTM TOF/TOF mass spectrometer followed by protein identification by traditional tandem mass spectrometry (MS/MS) directly on the tissue. To precisely delineate both dorsal and ventral hippocampus, a Nissl staining was performed on succeeding tissue sections. Results: Of the 234 detected peptides, significant differences in expression levels were found in 34 proteins, due to treatment, sex, or hippocampal location. Moreover, a significant protein-protein interaction (PPI) was observed, showing a relationship to long-term potentiation (LTP), the functional basis of memory. Accordingly, 14 proteins related to synaptic plasticity and/or AD were selected to further study. Results showed many of the altered protein to modulate glycogen synthase kinase-3β (GSK-3β), a protein widely involved in the regulation of synaptic plasticity induction threshold. In fact, hippocampal GSK-3β was found overactivated suggesting a facilitated long-term depression (LTD) instead of LTP in AD models. Conclusions: This study offers for the first time the specific protein changes in dorsal/ventral hippocampus in both male and female mice, that modulate GSK-3 β activity, providing new insight in the pathogenesis of early AD and valuable potential biomarkers for early diagnosis and therapeutic targets.