Evidence for Alzheimer’s disease-linked synapse loss and compensation in mouse and human hippocampal CA1 pyramidal neurons

Abstract

Alzheimer’s disease (AD) is associated with alterations in the distribution, number, and size of inputs to hippocampal neurons. Some of these changes are thought to be neurodegenerative, whereas others are conceptualized as compensatory, plasticity-like responses, wherein the remaining inputs reactively innervate vulnerable dendritic regions. Here, we provide evidence that the axospinous synapses of human AD cases and mice harboring AD-linked genetic mutations (the 5XFAD line) exhibit both, in the form of synapse loss and compensatory changes in the synapses that remain. Using array tomography, quantitative conventional electron microscopy, immunogold electron microscopy for AMPARs, and whole-cell patch-clamp physiology, we find that hippocampal CA1 pyramidal neurons in transgenic mice are host to an age-related synapse loss in their distal dendrites, and that the remaining synapses express more AMPA-type glutamate receptors. Moreover, the number of axonal boutons that synapse with multiple spines is significantly reduced in the transgenic mice. Through serial section electron microscopic analyses of human hippocampal tissue, we further show that putative compensatory changes in synapse strength are also detectable in axospinous synapses of proximal and distal dendrites in human AD cases, and that their multiple synapse boutons may be more powerful than those in non-cognitively impaired human cases. Such findings are consistent with the notion that the pathophysiology of AD is a multivariate product of both neurodegenerative and neuroplastic processes, which may produce adaptive and/or maladaptive responses in hippocampal synaptic strength and plasticity.

Three-dimensional synaptic organization of the human hippocampal CA1 field

“…Most synapses presented a simple, macular shape (accounting for 86% of the synapses in all layers of CA1), in agreement with previous reports in different brain areas and species (12,(33)(34)(35)(36).…”

Rac1 activation links tau hyperphosphorylation and Aβ dysmetabolism in Alzheimer’s disease

“…However, it is the synaptic and dendritic loss, which seems to be the best predictor of the clinical symptoms. This dysfunction has been extensively described in early studies [ 45 , 57 , 64 ] and confirmed in more recent years [ 17 , 42 , 59 ]. Reduction in spine number correlates well with the degree of memory loss and cognitive impairment of the patients [ 16 ].…”

Neuronal BIN1 Regulates Presynaptic Neurotransmitter Release and Memory Consolidation

“…Post-embedding immunogold EM Reagents used for EM were from Electron Microscopy Sciences (Hatfield, PA) unless otherwise noted. Tissue was processed as described previously (Neuman et al, 2015). Briefly, mice were perfused transcardially with 4% paraformaldehyde/0.5% glutaraldehyde, after which the hippocampus was dissected and sectioned into 300 mm-thick slices.…”