Alzheimer's disease is the most common neurodegenerative disease characterized by memory loss and learning impairments. Past studies suggest that synaptic loss is a key factor for cognitive decline in early stages of Alzheimer's disease [1,2,3]. These findings highlight the need for more studies to investigate how synapses are affected in Alzheimer's disease, which may provide invaluable insights for developing therapeutic approaches to prevent or treat the disease.
To understand how synapses are changed, it is important to have a method that is clear to identify the structure of synapses at the ultrastructural level. For many years, electron microscope (EM) has been the gold standard as it produces high-resolution images of presynaptic vesicles and postsynaptic density, thereby allowing the clear visualization of synaptic connections [4].
The remarkable advance of three-dimensional (3D)-EM technique, one of which is called FIB/SEM, has significantly enhanced this capability. This highly automated technique uses a focused gallium ion beam (FIB) to mill the sample of the brain tissue into ultrathin layers with nanometer precision, followed by scanning electron microscopy (SEM) to image the exposed surface and obtain three-dimensional data subsequently. FIB/SEM is particularly valuable in studying synaptic changes by enabling the reconstruction of 3D models from brain tissue samples [5].
Despite its strengths, there are also some limitations of utilizing EM as a tool to identify synapses. EM is only used on post-mortem tissues; therefore, it is not possible to observe synaptic changes in disease progression. Also, 3D EM is undoubtedly time-consuming and highly technical, requiring specialized skills to produce and analyze the data [4].
Findings from 3D EM Techniques
The relationship between synaptic loss and neurodegeneration remains with lots of questions to be answered. Recent 3D EM studies have shown that synaptic loss is more of a consequence rather than a cause of neuronal loss in most analyzed brain regions. The results suggest that synapses have varying degrees of vulnerability in different layers or regions of the brain. For instance, synapse density of post-mortem Alzheimer’s brain at layers II and III of the entorhinal cortex was reduced, suggesting that they are more vulnerable than those in different regions, such as in the transentorhinal cortex and CA1 stratum pyramidale (hippocamus) and CA1 stratum radiatum (hippocamus) [6-9]. These findings raise important questions about what makes some synapses in specific brain regions more resilient than other regions.
When it comes to which type of synapses are more prone to degenerate in Alzheimer's disease, 3D EM studies reveal that both excitatory and inhibitory synapses are vulnerable in Alzheimer’s disease, with distinct changes in morphology and location observed across brain regions [7-11]. Excitatory synapses show increased fragmentation of postsynaptic and a shift in synaptic targeting from dendritic spines to shafts, while inhibitory synapses appear less affected [7,8,10]. These findings highlight synapse-specific changes in Alzheimer’s pathology, but further research is essential to clarify whether these patterns are consistent across all brain regions, particularly those where synapse loss yields neuronal death. Understanding these dynamics may provide insights into early disease mechanisms and potential intervention points.
The development of EM techniques, such as 3D EM analysis, has enabled more detailed studies of synaptic synaptic degeneration in neurodegenerative diseases such as Alzheimer’s disease. However, to reach a meaningful conclusion, it is necessary to have more studies in a broader range of brain regions to further investigate synaptic changes.
References
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2. Serrano-Pozo, A., Frosch, M. P., Masliah, E., & Hyman, B. T. (2011). Neuropathological alterations in Alzheimer disease. Cold Spring Harbor perspectives in medicine, 1(1), a006189. https://doi.org/10.1101/cshperspect.a0061894.
3. DeKosky, S. T., & Scheff, S. W. (1990). Synapse loss in frontal cortex biopsies in Alzheimer's disease: correlation with cognitive severity. Annals of neurology, 27(5), 457–464. https://doi.org/10.1002/ana.410270502
4. Martínez-Serra, R., Alonso-Nanclares, L., Cho, K., & Giese, K. P. (2022). Emerging insights into synapse dysregulation in Alzheimer's disease. Brain communications, 4(2), fcac083. https://doi.org/10.1093/braincomms/fcac083
5. Merchán-Pérez A, Rodriguez JR, Alonso-Nanclares L, Schertel A, Defelipe J. Counting Synapses Using FIB/SEM Microscopy: A True Revolution for Ultrastructural Volume Reconstruction. Front Neuroanat. 2009 Oct 5;3:18. doi: 10.3389/neuro.05.018.2009. PMID: 19949485; PMCID: PMC2784681.
6. Scheff, S. W., Price, D. A., Schmitt, F. A., DeKosky, S. T., & Mufson, E. J. (2007). Synaptic alterations in CA1 in mild Alzheimer disease and mild cognitive impairment. Neurology, 68(18), 1501–1508. https://doi.org/10.1212/01.wnl.0000260698.46517.8f
7. Domínguez-Álvaro, M., Montero-Crespo, M., Blazquez-Llorca, L., Plaza-Alonso, S., Cano-Astorga, N., DeFelipe, J., & Alonso-Nanclares, L. (2021). 3D Analysis of the Synaptic Organization in the Entorhinal Cortex in Alzheimer's Disease. eNeuro, 8(3), ENEURO.0504-20.2021. https://doi.org/10.1523/ENEURO.0504-20.2021
8. Domínguez-Álvaro, M., Montero-Crespo, M., Blazquez-Llorca, L., Insausti, R., DeFelipe, J., & Alonso-Nanclares, L. (2018). Three-dimensional analysis of synapses in the transentorhinal cortex of Alzheimer's disease patients. Acta neuropathologica communications, 6(1), 20. https://doi.org/10.1186/s40478-018-0520-6
9. Montero-Crespo, M., Domínguez-Álvaro, M., Alonso-Nanclares, L., DeFelipe, J., & Blazquez-Llorca, L. (2021). Three-dimensional analysis of synaptic organization in the hippocampal CA1 field in Alzheimer's disease. Brain: a journal of neurology, 144(2), 553–573. https://doi.org/10.1093/brain/awaa406
10. Domínguez-Álvaro, M., Montero-Crespo, M., Blazquez-Llorca, L., DeFelipe, J., & Alonso-Nanclares, L. (2019). 3D Electron Microscopy Study of Synaptic Organization of the Normal Human Transentorhinal Cortex and Its Possible Alterations in Alzheimer's Disease. eNeuro, 6(4), ENEURO.0140-19.2019. https://doi.org/10.1523/ENEURO.0140-19.2019
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