Not all experiences form long-term memories, so how do our brains decide which events to remember and which to forget? Scientists led by Dr. Kausik Si from Stowers Institute for Medical Research have identified the proteins essential for creating and maintaining long-term memories. Moreover, they have discovered some proteins act in a manner similar to prions while others function as regulators within our neural circuitry. Their new research which sheds light on the molecular processes underlying recollection appears in PLOS Biology. “At its most fundamental level, a memory is an increase in synaptic strength that persists over time,” Richard Robinson noted in an accompanying synopsis published in the journal. “That persistence requires synthesis of a specific set of synaptic proteins, a process regulated by so-called Cytoplasmic Polyadenylation Element Binding (CPEB) proteins.”
Proteins and Prions
A protein is a large biological molecule made up of a chain of amino acids. The sequence of amino acids determines how an individual protein is folded, and in turn, this folded structure influences the protein’s activity. The word prion derives from the words protein and infectious. Prions, then, are infectious agents composed of a protein in an abnormally folded form, and they cause damage to the structure of neural tissue. In fact, they spur normally-folding proteins to misfold, and for this reason, they are notorious for their destructive power, interfering with cellular function as they spread without control. Mad cow disease, for example, is a prion disease.
It is astonishing, then, for CPEB to possess prion-like abilities that appear to be essential to forming long-term memory though such abilities do not cause damage to the brain. "This protein is not toxic," Si explained in a press release. He noted that, for this protein to be both effective and safe, it would somehow have to limit memory creation to the appropriate neural circuits. It only made sense, then, that the protein be regulated. "We know that all experiences do not form long-term memory — somehow the nervous system has a way to discriminate,” Si said. “So if prion-formation is the biochemical basis of memory, it must be regulated."
What is regulating CPEB so that this protein only acts prion-like in response to specific stimuli? Si and his team of researchers set to work designing experiments to answer this question.
The reason researchers often experiment on fruit flies is that, although their brain structure is much simpler and has far fewer neurons than a human brain, the mushroom shape of their brains is similar to our own peripheral cortex. This similarity, then, allows researchers to gain insights into how memories are acquired, stored, and retrieved. In fruit flies, for instance, the protein known as CPEB in humans is called Orb2 and it occurs in two forms: Orb2A and Orb2B. One of Orb2A’s binding partners is Tob, another protein. Studying the fruit fly, the researchers learned that Tob doubled the half-life of Orb2A and so increased its abundance within their brains. Yet such a relatively slight boost in stability didn’t seem to be enough to increase synaptic strength and so ensure the persistence of memory. In other words, although Tob seemed a likely candidate for regulating Orb2A, but further proof was needed to show that it was somehow influencing both the timing and location of Orb2A’s activity.
So the team designed an experiment based on a well-known fact that demonstrates the fruit fly's ability to create a memory: When males are repeatedly rejected by the same female, they eventually stop courting her. To see if Tob had an effect on memory, then, the scientists simply blocked Tob production in certain male fruit flies. What happened? Contrary to their normal behavior, these males continued to court females who had repeatedly given them the cold shoulder. Clearly, Tob played some role in stabilizing memories.
Investigating further, the scientists learned that, only in response to an incoming nerve signal, would TOB attach to Orb2A. In turn, this triggered phosphates, a type of chemical tag, to bind to both of the proteins. And, once phosphorylated, the TOB-Orb2A complex falls apart and Orb2A becomes more stable, with a new half-life of 24 hours. They also discovered the enzyme placing the phosphate tag on Orb2A is Lim kinase, activated at the synapse. (Kinases, generally, modify the behavior of proteins.) By investigating the precise chemical processes, then, the scientists showed how Orb2's conversion to the prion state can be confined in both time and space by Tob. And when all of these biochemical changes occur, in simple response to a stimulation of the nerves, a new memory is preserved.
Si noted that corresponding proteins to Orb2 and TOB have been identified in the brains of mice and humans while in a particular sea snail, scientists have proven the conversion to a prion-like state facilitates long-term change in synaptic strength. "This basic mechanism appears to be conserved across species," Si concluded.
Source: White-Grindley E, Li L, Khan RM, Ren F, Saraf A, Florens L, Si K. Contribution of Orb2A Stability in Regulated Amyloid-Like Oligomerization of Drosophila Orb2. PLOS Biology. 2014.