Neuroscientists from Johns Hopkins have conducted a quantitative proteomic study on brain synapses and have identified a set of long-lasting, stable proteins. These synaptic proteins can outlast neighboring proteins by weeks and months, providing key insight in the molecular machinery that governs mammalian long-term memory and learning, as well as memory loss. The team’s recent findings are published in PNAS.
“We knew that the overall structure of synapses tends to be very stable, lasting at least a year in the brains of rats. So, we reasoned, there must be proteins in those synapses that are long-lasting, too, and we believe we have found a lot of them,” explained study senior author Richard Huganir.
Huganir and his team performed an in vivo metabolic pulse-chase labeling experiment by feeding mice isotope-labeled food. Mice first ate food with heavy amino acid isotopes for seven weeks. During this period, long-lasting proteins tended to incorporate the heavy tracer at a slower rate than the shorter-lived proteins. For another seven weeks, the mice were then fed light isotope-containing food. Because long-lived proteins turnover less frequently, such stable proteins are expected to retain a greater ratio of heavy to normal isotope.
Brain tissue from mice after the first seven weeks and after another seven weeks were then extracted and analyzed by mass spectrometry. The team recorded the isotope ratio of 2,272 synaptic proteins and found most of them to be short-lived, lasting one to two days. However, 164 proteins were found to be long-lasting, with some estimated to last from months to years. A number of these proteins were found to be linked to cellular scaffolding, and one, in particular, belonged to the RAS signaling pathway. Additional in vitro work with neuronal cultures also showed that these proteins were susceptible to pharmacological manipulations.
Overall, the team’s findings demonstrate the presence of long-lived proteins in brain synapses, highlighting their potential role in brain plasticity, memory, and learning. In addition, because these “old” proteins may accumulate damage over time, the team also aims to determine any connections between these proteins and signs of human age-related cognitive decline.