Three-dimensional synaptic organization of the CA1 field in the normal human hippocampus and in alzheimer´s disease

Abstract

The hippocampus plays a crucial role in spatial orientation, learning and memory, and many pathological conditions (e.g., epilepsy and Alzheimer’s disease (AD)) are closely associated with synaptic alterations in this region. Numerous studies have described the ultrastructural characteristics of hippocampal synapses in experimental animals. However, there is very little information about the synaptic organization of the human hippocampus and the brain in general, which is a major problem when it comes to ap-propriately interpreting alterations in disease. A major goal in neuroscience is to di-rectly study the human brain. In the present study, we started to address this issue by analyzing the synaptic organization of the hippocampus, focusing on the CA1 field. The CA1 field receives and integrates a massive amount of information in a laminar-specific manner, and sends projections mainly to the subiculum and to subcortical nuclei and polymodal association cortices. This region is severely affected by amyloid and tau pa-thologies in AD, which are related to neuronal degeneration and synaptic alterations that seem to constitute the major neurobiological basis of cognitive dysfunction in AD. Studying the human brain via electron microscopic techniques presents certain prob-lems and the scarcity of human brain tissue suitable for ultrastructural studies is one of the most important issues to overcome. Recently, we have shown that Focused Ion Beam/Scanning Electron Microscopy (FIB/SEM) technology can be applied to study in detail the 3D synaptic organization of the human brain obtained from autopsies, yield-ing excellent results. A variety of synaptic parameters can be studied, including synap-tic density, type and spatial distribution, postsynaptic targets and the shape and size of the synaptic junctions. Using this technology, 33,705 synapses were fully reconstructed in the neuropil in all CA1 layers in five control cases with no apparent neurological al-teration and in CA1 stratum pyramidale and radiatum in five AD cases with different stages of the disease. In controls, we found that most synapses are excitatory, targeting dendritic spines and displaying a macular shape, regardless of the layer examined. However, remarkable differences were observed between layers. In AD, cases with ear-ly stages of the disease had morphologically normal-looking synapses and we observed no significant differences with regard to many synaptic parameters, including synaptic density. However, differences in the distribution of postsynaptic targets and synaptic shapes, and a lower proportion of larger excitatory synapses were found in AD cases. Individuals in late stages of the disease suffered the most severe synaptic alterations, including a decrease in synaptic density and morphological alterations of the remaining synapses. Since AD cases showed cortical atrophy, our data indicate a reduction in the total number (but not the density) of synapses at early stages of the disease, with this reduction being much more accentuated in subjects with late stages of AD. The ob-served synaptic alterations may represent a structural basis for the progressive learn-ing and memory dysfunctions seen in AD cases. Taken together, these data constitute the first extensive description of the synaptic organization of the neuropil of the human CA1 field and its possible alterations in AD, which is a necessary step for better under-standing its functional organization in both health and disease