Results from new research shows that people with Alzheimer’s disease exhibit decreased NMDA receptors in synapses and increased extra-synaptic membranes compared to healthy individuals.
For the study, the team developed a cellular fractionation protocol method that allows for precise analysis of the proteins located in synaptic membranes and in membranes outside the synapses, known as extra-synaptic membranes, in human post-mortem brains.
The process of progressive memory loss in the condition largely depends on synapses, where NMDA receptors play a critical role in learning and memory.
“Most NMDA receptors are found in synapses, where they enhance neuronal connections. However, those located outside the synapse are more associated with processes of toxicity and cell death, which may contribute to disease progression,” explains Inmaculada Cuchillo Ibáñez, study lead.
The team analysed samples from human post-mortem brains, including healthy individuals and patients at various stages of neurodegeneration.
The results show a distinct distribution of NMDA receptors in the cortex of patients with Alzheimer’s disease, where the number of synaptic NMDA receptors is significantly reduced, while extra-synaptic receptors increase compared to healthy individuals.
This imbalance suggests that neuronal toxicity-related activity is favoured in Alzheimer’s disease, as opposed to the primary function of synaptic transmission, likely contributing to disease progression.
A pioneering protocol
The main advancement achieved by the researchers is the optimization of a cellular fractionation protocol that allows for the separation of synaptic membranes from extra-synaptic membranes, a feat that had not been previously accomplished in frozen human post-mortem brains.
“Other studies have measured the total levels of NMDA receptors in the human brain but did not differentiate between those located in synapses and extra-synapses,” highlights Sergio Escamilla, the first author of the article.
“We adapted a protocol designed for fresh mouse brains to human samples, achieving this crucial separation.”
The method is based on the use of detergents that dissolve the lipids in non-synaptic membranes, while synaptic membranes, due to their high protein content, remain mostly intact. Subsequently, centrifugation is used to separate the two membrane types for analysis.
Towards new therapeutic approaches
The findings of this study could open new avenues for Alzheimer’s disease treatment.
“With this protocol, we can accurately determine whether specific agents, such as modulators or blockers, have a greater affinity for synaptic or extra-synaptic receptors—not just NMDA receptors—which has important therapeutic implications,” noted Cuchillo.
The study also utilised transgenic mice to validate the results obtained in humans. Although similar alterations in NMDA receptors were detected, the differences between species underscore the need for studies on human tissue to better understand the disease.
With this pioneering protocol, the researchers are paving the way to explore the molecular basis of Alzheimer’s disease and seek more effective treatments.
In this vein, researcher Javier Sáez Valero, head of Altered Molecular Mechanism in Alzheimer’s Disease and Dementia laboratory at IN UMH-CSIC, emphasises the importance of this type of research due to the role of NMDA receptors in current Alzheimer’s treatments, as memantine, one of the most commonly used drugs for the disease, is an NMDA receptor blocker.
