Researchers have discovered a new class of proteins that protect synapses from being destroyed. Synapses are the structures where electrical impulses pass from one neuron to another.

The discovery, published in the journal Nature Neuroscience, has implications for Alzheimer's disease and schizophrenia. If proven, increasing the number of these protective proteins could be a novel therapy for the management of those diseases, researchers said.

In Alzheimer's disease, loss of synapses leads to memory problems and other clinical symptoms. In schizophrenia, synapse losses during development predispose an individual to the disorder.

"We are studying an immune system pathway in the brain that is responsible for eliminating excess synapses; this is called the complement system," said the senior author of the research.

"Complement system proteins are deposited onto synapses," the author explained. "They act as signals that invite immune cells called macrophages to come and eat excess synapses during development. We discovered proteins that inhibit this function and essentially act as 'don't eat me' signals to protect synapses from elimination."

During development, synapses are overproduced. Humans have the most synapses at the ages of 12 to 16, and from then to about age 20, there is net synapse elimination that is a normal part of the brain's maturation. This process requires the complement system.

In adults, synapse numbers are stable, as synapse elimination and formation balance out. But in certain neurological diseases, the brain somehow is injured and begins to overproduce complement proteins, which leads to excessive synapse loss.

"This occurs most notably in Alzheimer's disease," the author said. In mouse models of Alzheimer's disease, researchers have found that the removal of complement proteins from the brain protects it from neurodegeneration, the author said.

"We've known about the complement proteins, but there was no data to show that there were actually any complement inhibitors in the brain," the author said. "We discovered for the first time that there are, that they affect complement activation in the brain, and that they protect synapses against complement activation."

The researchers focused on a neuronal complement inhibitor called SRPX2. The studies are being conducted in mice that lack the SRPX2 gene, that demonstrate complement system overactivation and that exhibit excessive synapse loss.

The authors show that the sushi domain protein SRPX2 is a neuronally expressed complement inhibitor that regulates complement-dependent synapse elimination. SRPX2 directly binds to C1q and blocks its activity, and SRPX2^−/Y mice show increased C3 deposition and microglial synapse engulfment. They also show a transient decrease in synapse numbers and increase in retinogeniculate axon segregation in the lateral geniculate nucleus.

In the somatosensory cortex, SRPX2^−/Y mice show decreased thalamocortical synapse numbers and increased spine pruning. C3^−/−;SRPX2^−/Y double-knockout mice exhibit phenotypes associated with C3^−/− mice rather than SRPX2^−/Y mice, which indicates that C3 is necessary for the effect of SRPX2 on synapse elimination. 

https://news.uthscsa.edu/synapse-saving-class-of-proteins-discovered/

https://www.nature.com/articles/s41593-020-0672-0