GENE-TRANSFER STRATEGY IN MOUSE BRAINS TURNSMEMORY CELLS OFF, THEN ON AGAIN
By David N. LeffScience Editor
One cerebrally challenged breed of transgenic rodent can besmartened up by a common antibiotic.
These remarkable animals are the progeny, as it were, ofneurophysiologist Eric Kandel and his co-workers at the Columbia University College of Physicians &Surgeons in New York. They constructed these mice as geneticmodels of memory acquisition, storage, loss and recovery.
From the 1960s until recent years, Kandel analyzed memory in alower form of life, an ocean-going mollusk called the sea hare,(Aplysia).
"Our original goal," Kandel told BioWorld Today, "was to look atlearning and memory in very simple animals, using very simple formsof learning. Now we've taken advantage of the fact that we can studygene modifications in mice, to explore the molecular mechanism oflearning and memory in more complex, mammalian, animals. That'sour long-range goal."
Kandel is senior author of an article in the current issue of Science,dated Dec. 6, 1996, titled: "Control of memory formation throughregulated expression of a CaMKII transgene."
"This is a methodology paper," Kandel said, "to show that one canexpress a gene in an animal that will make it dumb. That raises aquestion," he continued: "Is the transgenic mouse dumb because youinterfered with the mature functioning of its synapse, or because youproduced a developmental alteration that interferes with the normalformation of connections in the brain?"
To model the answer in vivo, Kandel's co-authors inserted mutantgenes into the pre-embryonic cells of future mice. The sequenceswere designed to knock out two specific brain regions, thehippocampus and the amygdala, involved in memory learning andstorage.
Those transgenic mice were dumb at birth, and remained so inadulthood. Specifically, they lacked the brain cells for two basicforms of memory, explicit and implicit.
In mice, as in humans, Kandel explained, "Explicit memory ismemory for people, places and things. If they have lesions in thehippocampus, they can't find their way around places, they don'trecognize familiar objects; people don't remember new names.
"By contrast," he continued, "implicit memory is unconsciousmemory of motor [muscle-activating] strategies, motor skills, fearresponses."
A classic way to measure explicit _ spatial _ memory in mice is tosee how well they remember using the relationship of visual cuessurrounding a brightly lit circular maze, with 40 holes around the rim.Mice hate bright light, so they seek out the one hole leading to adarkened escape tunnel.
A hippocampally intact animal soon learns to line up a particularpattern of markers with the escape hole. Not so in Kandel'stransgenic mice, with mutant genes expressed in their hippocampi.After training for 40 consecutive days, they still didn't get it.
Flunked Mice Won New Lease On Memory
Now for the beauty part: Mice with this memory deficit recoveredfully after getting doxycycline, a wide-spectrum, tetracycline-derivedantibiotic, in their drinking water for 40 days.
Similar results attended the implicit memory tests. Here, the normalmurine reaction to a fearful stimulus is to freeze. The transgenicmice, frightened by a suddenly intruding rat _ failed to recall thisfear-inspiring event. The doxycycline deus ex machina restored theirability to do so, by nullifying the effect of the mutant gene implantedin their amygdala.
"The gene that we mutated," Kandel said, "calcium-calmodulinkinase II, [CaMKII], is a key component of long-term potentiation,[LTP], which is thought to be a cellular mechanism for memorystorage in the mammalian brain." In other words, LTP is a memoryprocess by which neurons change at their synapses, as a result of theirexperience. (See BioWorld Today, Aug. 19, 1994, p. 1.)
The evidence for this, he added, "is that, if you knock out theCaMKII gene, and thus LTP, you interfere with learning and memoryin the brain."
The transgene's expressed product, Kandel observed, "is a serine-threonine enzyme that phosphorylates receptors and ion channelsthought to be important for synaptic transmission in the brain. It caneven phosphorylate itself."
On/Off Ability Is Key
From studying their transgenic mice, the team "began to realize itwould be wonderful to have a methodology whereby we could turnthese transgenes off, and see whether or not those animals wouldrecover their intelligence and their normal LTP. That's what thisScience paper is all about."
He added: "The nice thing is that it gives you, for the first time in thebrain, both temporally regulated and regional expression of atransgene _ two of the really high-order requirements for studyingthe role of genes in memory storage. We were the first ones who wereable to switch genes off and on in the brain."
Defects in implicit and explicit memory characterize senile dementiasin people. Kandel opined that "the ability to turn genes on and off is avery useful technology to have in studies of any drugs that might beof value in memory storage. Columbia University," he observed, "isinterested in developing patents around it."
He and his group "have not yet started to use the technology in anyspecific way to screen for drugs." Nor does he envisage moving uponce again to a higher form of preclinical life. "Mice are terrific forthese kinds of things," Kandel said. "I think they are going to proveextremely valuable for assays of drugs."
He and his co-workers are now "interested in how cells in thehippocampus that record space are handled by animals that expressthis transgene. We find that in fact those so-called place field cellshave a severe defect in how they retain information about space. Andwe have a nice paper on that topic coming out in the Dec. 20 issue ofCell. n
(c) 1997 American Health Consultants. All rights reserved.