The Russian physiologist Ivan Petrovich Pavlov (1849-1936) is revered as the father of conditional and learned reflexes. His best-remembered experiments focused on gastric-juice secretions in dogs.
“Pavlov rang a bell whenever he offered meat to a dog as the unconditioned stimulus,” recounted molecular biologist Jerry Yin, at Cold Spring Harbor Laboratory in New York. “So when he paired the bell and the meat a number of times, pretty soon the dogs, when they heard the bell, expected the meat, and start salivating getting ready to eat. And their stomachs duly secreted gastric juice.”
In his own study of nervous reflexes, Yin’s in vivo model is not a dog, a mouse, a rat or any other mammal. Rather, he conducts his Pavlovian-like experiments on the common fruit fly Drosophila melanogaster. The gap is not all that distant.
Yin is senior author of a paper in the April 2002 issue of Nature Neuroscience. Its title: “Memory enhancement and formation by atypical PKM activity in Drosophila melanogaster.” It’s accompanied by a briefer communication in the same journal, titled: “Protein kinase Mz is necessary and sufficient for LTP [long-term potentiation] maintenance.” Its senior author is neurologist Todd Sacktor, at SUNY Downstate Medical Center in Brooklyn, N.Y.
“The most important finding is in these two papers, one from our laboratory and one from Todd’s,” Yin told BioWorld Today. “Both labs manipulate the same molecule in the same way. Our analysis was in Drosophila at the level of behavior. In our paper, the novel aspects are that we can either enhance or disrupt memory. It’s quite common to be able to disrupt the process, much more difficult to enhance it.
“The analysis in Todd’s lab,” Yin went on, “was done in rat hippocampal slices, at the level of neurophysiology. In both cases, what we see is longer persistence or elongation of memory prolonging the time that the memory or the potentiation lasts.”
Yin continued: “The overall importance of our two articles is that this PKM molecule, and the mechanisms that it participates in, must be highly conserved to be easily seen both at the level of behavior in fruit flies, and physiology in the brains of rats. The molecule, which we’re both manipulating,” Yin added, “is virtually identical from flies on up to humans.
“Most of our paper reports utilizing the gene from mice placed randomly into the fly, although the fly has its own gene. So at the very end of the paper we also manipulate the fly’s gene, and see the same results. The work in Todd’s lab mostly made protein from the mouse gene, and then put it into rats.”
Some Things Flies Don’t Forget Quickly
Yin addressed the question of whether Drosophila in a state of nature has recourse to memory; does it remember things?
“It clearly does,” he rejoined. “Buzzing around the fruit bowl, the fly primarily uses olfactory odor-based cues. The Drosophila world is probably dominated to a large degree by odor. We did olfactory avoidance tests, in which flies are first exposed to a single odor that is presented in the paired presence of electric shock. Next a second odor that’s not paired with anything. Then they’re put in a T-maze setup, where they get both odors. If they remember the association of the first smell with electric shock they all go crawling, not flying, to the arm of the T-maze recalling the second odor.
“We define learning as test ’em in a single training trial in the T-maze setup immediately after the initial training. If we don’t test right after training, they remember out to about 24 hours, then distribute their odor preference 50-50. In order for them to remember this for a longer period of time, we have to give them repeated trials 24 hours, four days, seven days out to a more distant time point.
“Now it turns out,” Yin observed, “that in the early ’70s, people found mutations that affected learning and memory in the fly. Some of those early mutants also affect the second behavior studied courtship.
“In this memory-related behavior,” Yin explained, “what essentially happens is if one puts a male fly together with a virgin female, the male will court and mate the female. But a male put together with a female that’s not a virgin that’s already mated the female rebuffs the male’s advances. The normal male figures this out pretty quickly and goes somewhere else. This is real life,” Yin remarked, “something we all can identify with.
“However, if the male has a mutation in one of its genes, discovered in an olfactory learning paradigm an artificial lab setup those males never learn that the female is not interested, and they keep trying to court her. So if one looks at salient behavior in the life of a fly, namely mating, we find that those mutants that have been isolated in the lab have very dramatic effects. That is, these molecules in the normal life of a fly do have important components requiring learning and memory formation.”
Drug Discovery On Front Burner
“Since our recent reports,” Yin said, “that key molecule is now called PKM protein kinase M. This is a protein with the function of adding phosphate groups to other proteins. It modifies the activity or the nature of other proteins. So it may either arrange or break and disrupt certain cellular interactions. All of these proteins exist in two forms, one of which is a so-called M form, a truncated form. We and Todd’s group both mimicked that short protein artificially. It’s the thing that enhances memory, whereas the full-length protein does not.
“Finally, “ Yin summed up, “we’re interested in commercializing drug discovery. I would do a drug screen for small molecules that affect the activity of this atypical M. I would look for both enhancers and blockers of activity. Enhancers because ultimately we’re interested in supplementing and enriching the function of patients who have memory formation defects in Alzheimer’s or other neurodegenerative diseases or normal aging. On the flip side, we’re interested in inhibitors to supplement psychotherapy behavioral therapeutic approaches that currently exist.
“Preliminary patent applications have been filed,” Yin noted, “covering enhancement or interference with memory formation in normal individuals and psychiatric dysfunctions. We’re beginning to make contact with companies, but haven’t made any decisions yet.” He concluded: “We’re interested in talking to people.”