The Proteome of the Dentate Terminal Zone of the Perforant Path Indicates Presynaptic Impairment in Alzheimer Disease*Presynaptic Impairment in Alzheimer Disease
Synaptic dysfunction is an early pathogenic event in Alzheimer disease (AD) that contributes to network disturbances and cognitive decline. Some synapses are more vulnerable than others, including the synapses of the perforant path, which provides the main excitatory input to the hippocampus. To elucidate the molecular mechanisms underlying the dysfunction of these synapses, we performed an explorative proteomic study of the dentate terminal zone of the perforant path. The outer two-thirds of the molecular layer of the dentate gyrus, where the perforant path synapses are located, was microdissected from five subjects with AD and five controls. The microdissected tissues were dissolved and digested by trypsin. Peptides from each sample were labeled with different isobaric tags, pooled together and pre-fractionated into 72 fractions by high-resolution isoelectric focusing. Each fraction was then analyzed by liquid chromatography-mass spectrometry. We quantified the relative expression levels of 7322 proteins, whereof 724 showed significantly altered levels in AD. Our comprehensive data analysis using enrichment and pathway analyses strongly indicated that presynaptic signaling, such as exocytosis and synaptic vesicle cycle processes, is severely disturbed in this area in AD, whereas postsynaptic proteins remained unchanged. Among the significantly altered proteins, we selected three of the most downregulated synaptic proteins; complexin-1, complexin-2 and synaptogyrin-1, for further validation, using a new cohort consisting of six AD and eight control cases. Semi-quantitative analysis of immunohistochemical staining confirmed decreased levels of complexin-1, complexin-2 and synaptogyrin-1 in the outer two-thirds of the molecular layer of the dentate gyrus in AD. Our in-depth proteomic analysis provides extensive knowledge on the potential molecular mechanism underlying synaptic dysfunction related to AD and supports that presynaptic alterations are more important than postsynaptic changes in early stages of the disease. The specific synaptic proteins identified could potentially be targeted to halt synaptic dysfunction in AD.
Laser microdissection of highly vulnerable hippocampal region.
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Proteomic analysis of postmortem human brain tissue of AD and control cases.
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Decreased levels of presynaptic proteins, but not postsynaptic proteins, in AD.
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Immunohistochemistry verifies decreased levels of selected presynaptic proteins.
Alzheimer's disease
mass spectrometry
clinical proteomics
immunohistochemistry
pathway analysis
neurodegenerative diseases
molecular layer of the dentate gyrus
perforant path
postmortem human brain
presynaptic impairment
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Author contributions: H.H., G.M., and S.M.N. performed research; H.H., G.M., C.E., L.M.O., L.O.T., and S.F. analyzed data; H.H., G.M., and S.F. wrote the paper; S.L.C. and A.-C.G. contributed new reagents/analytic tools; N.B., L.M.O., and S.F. designed research; B.W. and A.-C.G. input on the design and writing of the manuscript; N.B. input on writing of the manuscript; G.B. helped with interpretation of the results; L.M.O. and L.O.T. contributed to the writing of the manuscript.
This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 676144 (Synaptic Dysfunction in Alzheimer Disease, SyDAD). The project was also supported by grants from Stiftelsen för gamla tjänarinnor, O.E. och Edla Johanssons Vetenskapliga Stiftelse, Demensfonden, Gun och Bertil Stohnes stiftelse, Margaretha af Ugglas Stiftelse, and Alzheimerfonden. The authors declare that they have no conflicts of interest with the contents of this article.
