Glutamate is a neurotransmitter, a chemical used to transmit signals between neurons. Its dynamics in the brain are highly complex and it is sensed by a variety of receptors, including metabotropic glutamate receptor 5 (mGluR5). mGluR5 is of particular interest due to its recently-uncovered role in Alzheimer’s disease. The receptor can interact with short strings (“oligomers”) of amyloid-beta, a toxic protein implicated in the pathology of Alzheimer’s. Multiple studies show that loss or inhibition of mGluR5 can alleviate Alzheimer’s symptoms in animal models.
An important question remaining to be answered is exactly how mGluR5 contributes to Alzheimer’s. One hypothesis is that the receptor’s interactions with amyloid-beta oligomers trigger a pathogenic signaling cascade. Another possibility is that amyloid-beta is not involved, and instead the dysregulated glutamate signaling is to blame.
A recent study published in Cell Reports attempted to solve this dilemma. Researchers from Yale University used a silent allosteric modulation (SAM) drug to target mGluR5. In humans, complete inhibition of mGluR5 would be deadly, since the receptor is necessary to maintain proper glutamate signaling in the central nervous system. To avoid this problem, the SAM drug was carefully designed so that it blocked the ability of mGluR5 to interact with amyloid-beta oligomers, but still allowed it to function normally in glutamate signaling.
The researchers then administered the drug to mice that have a mutation causing them to develop Alzheimer’s disease. After four weeks of treatment, the mice underwent a battery of tests designed to test memory and cognition. Normally, the mice with Alzheimer’s disease perform very poorly on these tests. However, after treatment with the drug, the Alzheimer’s mice performed as well as the non-Alzheimer’s mice. This result is striking, because most drug candidates for Alzheimer’s disease are only able to stop the cognitive decline from getting any worse. It is rare for a treatment to actually reverse the memory deficits.
The scientists took it a step further by examining what was going on inside the mice’s brains at the cellular level. They found that levels of amyloid-beta plaques and damage to glial cells were unchanged by the drug. This is surprising, because these two factors are often considered to be among the main driving forces of Alzheimer’s disease. In contrast, they observed a dramatic change in the mice’s synapses, the junctions where neurons send signals to each other. Mice with Alzheimer’s disease typically have fewer synapses than normal mice. However, those receiving the treatment showed recovery of synapses, suggesting that modulation of synapses could be how the drug reverses memory decline.
An important limitation of the mice used in these experiments is that they only develop the amyloid-beta pathology of Alzheimer’s disease. In humans, there are many other toxic proteins involved, including a particularly important one called tau. To address this problem, the researchers also administered the drug to a different mouse strain, which expressed both amyloid-beta and tau. They saw that levels of tau were alleviated in the mice receiving the treatment.
This study helps to solve an important dilemma, demonstrating that mGluR5’s contributions to Alzheimer’s disease are solely due to its interactions with amyloid-beta, and not due to abnormalities in glutamate signaling. Thus by developing human versions of the SAM drug used in this study, it might be possible to stop or even reverse memory decline in Alzheimer’s patients. However, it’s important not to get too excited just yet. We’ve seen time and time again that the vast majority of drug candidates that have encouraging results in mice end up failing to treat the disease in humans. Only time will tell whether these results could have clinical applications.