A Meta-Analysis on Presynaptic Changes in Alzheimer’s Disease

Background: A key aspect of synaptic dysfunction in Alzheimer’s disease (AD) is loss of synaptic proteins. Previous publications showed that the presynaptic machinery is more strongly affected than postsynaptic proteins. However, it has also been reported that presynaptic protein loss is highly variable and shows region- and protein-specificity. Objective: The objective of this meta-analysis was to provide an update on the available literature and to further characterize patterns of presynaptic protein loss in AD. Methods: Systematic literature search was conducted for studies published between 2015–2022 which quantified presynaptic proteins in postmortem tissue from AD patients and healthy controls. Three-level random effects meta-analyses of twenty-two identified studies was performed to characterize overall presynaptic protein loss and changes in specific regions, proteins, protein families, and functional categories. Results: Meta-analysis confirmed overall loss of presynaptic proteins in AD patients. Subgroup analysis revealed region specificity of protein loss, with largest effects in temporal and frontal cortex. Results concerning different groups of proteins were also highly variable. Strongest and most consistently affected was the family of synaptosome associated proteins, especially SNAP25. Among the most severely affected were proteins regulating dense core vesicle exocytosis and the synaptic vesicle cycle. Conclusions: Results confirm previous literature related to presynaptic protein loss in AD patients and provide further in-depth characterization of most affected proteins and presynaptic functions.

Overall outcome of meta-analysis when altering specific analysis parameters.Top: When using different correlation coefficients (rho) for effect size correlation within studies in the three-level meta-analysis model, the overall outcome did not change substantially.Bottom: Clustering on the level of research team instead of study also did not change results.Removing one study with a very large negative effect size, however, did change results: the presynaptic protein loss in AD and heterogeneity on all levels was lower.Significances of meta-analysis result is indicated by p; significance of Q-test for heterogeneity is indicated by pQ.SMD, standardized mean difference; CI, confidence interval.Supplementary Figure 2. Syntaxin proteins unaltered in AD.Forest plot of three-level random effects meta-analysis on proteins in syntaxin family.Syntaxins showed no significant decrease in AD (p = 0.21).To allow visualization of large amount of data included in analysis, ES were aggregated to one value per study to generate the forest plot.Size of effect size symbol represents its weight.Sample sizes represent the maximum n per group contributing to the analysis for each study.Dashed line depicts no difference between control and AD cohort.AD, Alzheimer's disease, ES, effect size; C, Control; SMD, standardized mean difference; CI, confidence interval

Yamazaki et al., 2019 [22] d
reported in the selected publications, they were predominantly shown in graphs.Numerical values were rarely presented.The Table therefore lists the means by which we extracted the 'Protein data' from each publication under consideration including further demographic information for AD and control groups (Age, PMI, % male subjects, selected cohorts for this meta-analysis from the available study cohorts) and notes on the protein extraction and analysis methods implemented by each author.Also shown are the number of effect sizes the contributed to this meta-analysis.+ fully available for each group, (+) partly available, -not available.AD, Alzheimer's disease; CA, cornu ammonis; DEM, dementia; DG, dentate gyrus; DLB, dementia with Lewy bodies; ELISA: enzyme-linked immunosorbent assay; ERC, Entorhinal cortex; ES, effect size; IHC, immunohistochemistry; MCI, mild cognitive impairment; NCI, no cognitive impairment; PDD, Parkinson's disease with dementia; PMI, post-mortem interval; SD, Standard deviation; SEM, Standard error of the mean; SLC: solute carrier family; STG, Superior temporal gyrus; STXBP: syntaxin-binding protein; WB, western blot; WBD, WebPlotDigitizer.aData was not available from all subjects for each protein/ area.Demographic characteristics for each group overall.bPMI reported as range.cDemographic Data only available for whole sample.dDemographic data reported as median and range.eWhole-tissue proteomic/ transcriptomic studies with secondary validation method meeting inclusion criteria.Supplementary

Tiwari et al., 2015 [19]
for 22 studies, including source of brain samples (Brain Bank), proteins (given in abbreviated form only: for full names of relevant proteins, see main text) and areas analysed, method of protein quantification and demographic characteristics (Age, % Men, PMI) of control and AD groups where available.For western blotting studies, loading control is given in brackets.Demographic data is mean (SD) or range.AD, Alzheimer's disease; BDRN, Brains for Dementia Research Network; CA, cornu ammonis; DG, dentate gyrus; ELISA, enzyme-linked immunosorbent assay; IHC, immunohistochemistry; IP-MS, immunoprecipitation mass spectrometry; LC-MS3, liquid chromatography mass spectrometry with quantification at MS3 level; LC-MS/MS, liquid chromatography with tandem mass spectrometry; MAP, Rush Memory and Aging Project; MRC: Medical research council; MSBB, Mount Sinai Brain Bank; PMI, postmortem interval; SD, standard deviation; ROS, Religious Order Study; WB, western blot.a Samples were pooled for protein quantification according to group and APOE status, resulting in n=2 per group for meta-analysis.
b Demographic characteristics only available for whole sample.Supplementary
Figure1.SNAP loss in AD.Forest plot of three-level random effects meta-analysis on proteins in the SNAP family.SNAPs showed a significant lowering in AD (p < 0.001).To allow visualization of large amounts of data included in analysis, effect sizes were aggregated to one value per study to generate the Forest plot.Sample sizes represent the maximum n per group contributing to the analysis for each study.Dashed line depicts no difference between control and AD cohort.SNAP, synaptosome associated protein; AD, Alzheimer's disease, ES, effect size; C, Control; SMD, standardized mean difference; CI, confidence interval