A membrane receptor long suspected of triggering excitotoxicity and neurodegeneration has been working with an accomplice all along. This revelation may explain why attempts to keep a lid on the membrane receptor have failed. It also suggests a better means of neuroprotection—keeping the accomplices apart.
According to investigators at Heidelberg University, the membrane receptor, specifically, the N-methyl-D-aspartate receptor (NMDAR), may even turn out to be something of a patsy. We won’t know until all the mechanistic details emerge. But the investigators have already been demonstrated that preventing NMDAR from physically interacting with its accomplice, a transient receptor potential channel known as TRPM4, can eliminate excitotoxicity in vitro and in vivo.
If TRPM4 is the new villain in excitotoxicity, the new heroes are compounds 8 and 19. These modestly named breaker upperers are small molecules that were selected on the basis of structure-based computational drug screens, internal strain calculations, and docking scores. Compounds 8 and 19, the Heidelberg University scientists assert, represent a new class of “interface inhibitors.” What’s more, the scientists say that interface inhibitors like compounds 8 and 19 may mitigate currently untreatable human neurodegenerative diseases.
“[Interface inhibitors] spare NMDAR-induced calcium signaling but disrupt the NMDAR/TRPM4 complex,” wrote the article’s authors. “[They] strongly reduce NMDA-triggered toxicity and mitochondrial dysfunction, abolish cyclic adenosine monophosphate–responsive element–binding protein (CREB) shutoff, boost gene induction, and reduce neuronal loss in mouse models of stroke and retinal degeneration.”
These findings came as something of a surprise. The ability of NMDARs to induce excitotoxicity was thought to be intimately linked to high intracellular calcium load. Instead, NMDARs induce excitotoxicity by physically coupling with TRPM4. This turned out to be a good surprise. If interface inhibitors are deployed, NMDAR-mediated toxicity can be eliminated without affecting NMDAR-induced calcium signaling.
NMDAR, an ion channel protein that is activated by the neurotransmitter glutamate, allows calcium to flow into the cell. The calcium signal sets in motion plasticity processes in the synapse but also propagates into the cell nucleus, where it activates a protective genetic program. Glutamate-activated NMDA receptors located in the junctions of the nerve cells have a key function in the brain, contributing to learning and memory processes as well as neuroprotection. But the same receptors are also found outside of synapses. These extrasynaptic NMDA receptors pose a threat because their activation can lead to cell death. Normally, however, efficient cellular uptake systems for glutamate make sure that these receptors are not activated and nerve cells remain undamaged.
This situation can change dramatically in the presence of disease. If, for example, parts of the brain are not supplied with sufficient oxygen after a stroke, disruptions in circulation negate the glutamate uptake systems. The glutamate level outside synapses increases, thereby activating the extrasynaptic NMDA receptors. The result is nerve cell damage and death accompanied by restrictions in brain function. Increased glutamate levels outside the synapses occur not only during circulatory disturbances of the brain.
“The evidence suggests that the toxic properties of extrasynaptic NMDA receptors play a central role in a number of neurodegenerative diseases,” explained Heidelberg University’s Hilmar Bading, PhD, the corresponding author of the current study. According to Bading, these diseases include Alzheimer’s disease, amyotrophic lateral sclerosis, and possibly even brain damage after infections with viruses or parasites.
While glutamate-activated NMDA receptors inside neuronal junctions help build up a protective shield, outside synapses, they change from Dr. Jekyll into Mr. Hyde. “Understanding why extrasynaptic NMDA receptors lead to nerve cell death is the key to developing neuroprotective therapies,” continued Bading. That is precisely where the Heidelberg researchers are focusing their efforts.
In their experiments on mouse models, they were able to demonstrate that the NMDA receptors found outside synapses form a type of “death complex” with TRPM4, which has a variety of functions in the body, with roles in the cardiovascular system and immune responses. And now it appears that TRPM4 also confers toxic properties on extrasynaptic NMDA receptors.
Using molecular and protein biochemical methods, the scientists identified the contact surfaces of the two interacting proteins. With this knowledge, they used a structure-based search to identify substances that might disrupt this very bond, thereby dismantling and inactivating the “death complex.” These substances proved to be extremely effective protectors of nerve cells.
“We’re working with a completely new principle for therapeutic agents here,” declared Bading. “The interface inhibitors give us a tool that can selectively remove the toxic properties of extrasynaptic NMDA receptors.”
Bading and his team were already able to demonstrate the efficacy of the new inhibitors in mouse models of stroke or retinal degeneration. According to the Heidelberg researcher, there is good reason to hope that such interface inhibitors—administered orally as broad-spectrum neuroprotectants—offer treatment options for currently untreatable neurodegenerative diseases.
“Their possible approval as pharmaceutical drugs for human use will take several more years,” he cautioned. “The new substances must first successfully pass through a number of preclinical and clinical testing phases.”
