In Search of the Memory Molecule, Researchers Discover Key Protein Complex

In Search of the Memory Molecule, Researchers Discover Key Protein Complex

  9:40:00 AM    Science Lover

---

Have a tough time remembering where you put your keys, learning a new language or recalling names at a cocktail party? New research from the Lisman Laboratory at Brandeis University points to a molecule that is central to the process by which memories are stored in the brain.

The CaMKII molecule has 12 lobes (6 are shown here), 
each of which has enzymatic activity. This molecule can 
bind to the NMDA receptor, forming a complex. The 
number of such complexes at the synapse may increase 
the amount of memory that can be stored. 
(Credit: Neal Waxham)

A paper published in the June 22 issue of the Journal of Neuroscience describes the new findings.

The brain is composed of neurons that communicate with each other through structures called synapses, the contact point between neurons. Synapses convey electrical signals from the "sender" neuron to the "receiver" neuron. Importantly, a synapse can vary in strength; a strong synapse has a large effect on its target cell, a weak synapse has little effect.

New research by John Lisman, professor of biology and the Zalman Abraham Kekst chair in neuroscience, helps explain how memories are stored at synapses. His work builds on previous studies showing that changes in the strength of these synapses are critical in the process of learning and memory.

"It is now quite clear that memory is encoded not by the change in the number of cells in the brain, but rather by changes in the strength of synapses," Lisman says. "You can actually now see that when learning occurs, some synapses become stronger and others become weaker."

But what is it that controls the strength of a synapse?

Lisman and others have previously shown that a particular molecule called Ca/calmodulin-dependent protein kinase II (CaMKII) is required for synapses to change their strength. Lisman's team is now showing that synaptic strength is controlled by the complex of CaMKII with another molecule called the NMDAR-type glutamate receptor (NMDAR). His lab has discovered that the amount of this molecular complex (called the CaMKII/NMDAR complex) actually determines how strong a synapse is, and, most likely, how well a memory is stored.

"We're claiming that if you looked at a weak synapse you'd find a small number of these complexes, maybe one," says Lisman. "But at a strong synapse you might find many of these complexes."

A key finding in their experiment used a procedure that reduced the amount of this complex. When the complex was reduced, the synapse became weaker. This weakening was persistent, indicating that the memory stored at that synapse was erased.



The experiments were done using small slices of rat hippocampus, the part of the brain crucial for memory storage.

"We can artificially induce learning-like changes in the strength of synapses because we know the firing pattern that occurs during actual learning in an animal," Lisman says.

To prove their hypothesis, he explained, his team first strengthened the synapse, eventually saturating it to the point where no more learning or memory could take place. They then added a chemical called CN-19 to the synapse, which they suspected would dissolve the CaMKII/NMDAR complex. As predicted, it did in fact make the synapse weaker, suggesting the loss of memory.

A final experiment, says Lisman, was the most exciting: They started out by making the synapse so strong that it was "saturated," as indicated by the fact that no further strengthening could be induced. They then "erased" the memory with the chemical CN-19. If the "memory" was really erased, the synapse should no longer be saturated. To test this hypothesis, Lisman's team again stimulated the synapse and found that it could once again "learn." Taken together, these results demonstrated the ability of CN19 to erase the memory of a synapse -- a critical criterion for establishing that the CaMKII/NMDAR complex is the long sought memory storage molecule in the brain.

Lisman's team used CN19 due to previous studies, which indicate that the chemical could affect the CaMKII/NMDAR complex. Lisman's team wanted to show that CN19 would decrease the complex in living cells. Several key control experiments proved this to be the case.

"Most people accept that the change in the synapses that you can see under the microscope is the mechanism that actually occurs during learning," says Lisman. "So this paper will have a lot of impact -- but in science you still have to prove things, so the next step would be to try this in an actual animal and see if we can make it forget something it has previously learned."

Lisman says that if memory is understood at the biochemical level, the impact will be enormous.

"You have to understand how memory works before you can understand the diseases of memory."

Lisman assembled a large team to undertake this complex research. A key collaborator was Magdalena Sanhueza, who once worked with Dr. Lisman at Brandeis, and her student, German Fernandez-Villalobos, both now of the University of Chile, Department of Biology and Ulli Bayer of the University of Colorado Denver School of Medicine, Department of Pharmacology, who developed CN19, a particular form that could actually enter neurons.

Others involved include Nikolai Otmakhov and Peng Zhang from Brandeis and Gyulnara Kasumova, who worked in the Lisman laboratory for several years as an undergraduate. An additional group contributing to the work was that of Johannes Hell, Professor of Pharmacology at the UC Davis School of Medicine. He and his student, Ivar S. Stein, used immunoprecipitation methods to actually show that the CN19 had dissolved the CaMKII/NMDAR complex.

Read More

Early Light Refines Brain's Circuitry for Vision: Studies Show Importance of Visual Stimulation in Wiring Up Species' Brains to See

Early Light Refines Brain's Circuitry for Vision: Studies Show Importance of Visual Stimulation in Wiring Up Species' Brains to See

  10:08:00 AM    Science Lover

---

Any parent knows that newborns still have a lot of neurological work to do to attain fully acute vision. In a wide variety of nascent animals, genes provide them with only a rough wiring plan and then leave it to the developing nervous system to do its own finish work. Two studies by Brown University researchers provide new evidence of a role for exposure to light in the environment as mouse pups and tadpoles organize and refine the circuitry of their vision systems.

Light and sight: connected at the beginning Because 
the retinal layer of rods and cones is not connected 
early in mice, neuroscientists had no reason to suspect 
that light helps develop neural connections for vision. 
David Berson, right, with Jordan Renna, has shown 
that photosensitive cells he discovered a decade ago are 
connected and do help with neural development. 
(Credit: Mike Cohea/Brown University)

"Through a combination of light-independent and light-dependent processes, the visual system is getting tuned up over time," said David Berson, professor of neuroscience.

His new work, published in advance online June 5 in Nature Neuroscience, offers the surprising result that light exposure can enhance how well mice can organize the nerve endings from their left eye and their right eye in an area of the brain where they start out somewhat jumbled. Neuroscientists had thought that mammals were unable to see at this stage, but a new type of light-sensitive cell that Berson discovered a decade ago turns out to let in the light.

Meanwhile, Berson's colleague Carlos Aizenman, assistant professor of neuroscience, co-authored a paper online May 31 in the Journal of Neuroscience showing that newborn tadpoles depend on light to coordinate and improve the response speed, strength and reliability of a network of neurons in a vision-processing region of their brains.

"This is how activity is allowing visual circuits to refine and sort themselves out," said Aizenman. "Activity is fine-tuning all these connections. It's making the circuit function in a much more efficient, synchronous way."

Not completely blind mice
Berson, postdoctoral scholar Jordan Renna, and former postdoctoral researcher Shijun Weng conducted several experiments in newborn mice to see whether light influences the process by which the mice rewire to distinguish between their eyes.

"For certain functions, the brain wants to keep track of which eye is which," Berson said. Among those functions are the perception of depth and distance.

At a circuit level, the brain keeps signals from the two eyes distinct by segregating their nerve endings into separate regions in the dorsal lateral geniculate nucleus (dLGN), a key waystation on the path to the visual cortex and conscious visual perception. Scientists have long known this sorting-out process depends on waves of activity that spontaneously excite cells in the inner retina. They did not know until now that the waves are influenced by a light-sensitive type of cell called intrinsically photosensitive retinal ganglion cells (ipRGCs).

About a decade ago, a team Berson led at Brown discovered the ipRGCs, which are the first light-sensitive cells to develop in the eye. They reside in the inner retina, the home of retinal cells that send visual information directly to the brain. The outer retina is where the more familiar rods and cones sense light. Early in life, when the brain is segregating nerve endings into distinct regions in the dLGN, the two retinal layers are not connected, so until ipRGCs were discovered there was no reason to believe that light would affect the sorting process.

The new research doesn't say anything definitive about the consequences of light exposure at this stage for eyesight in adults, especially given that some mammals (such as monkeys) experience this developmental stage in utero.

"Whether different animals in nature are exposed to enough light to induce a change in segregation patterns is unclear," Renna said.

But the research shows that light exposure does improve how well the sorting goes, Berson said, and the work advances neuroscientists' understanding of the eye-distinction process, which is widely studied as a model of "activity-driven" neural development.

To assess the effect of light on retinal waves, Renna used electrodes to record the activity of cells in the inner retinas of newborn mice, first recording in the dark, then in the light, and then again in the dark. In every case retinas experienced waves, but when the retinas were exposed to light, the waves lasted about 50 percent longer.

Renna then tested whether the light-sensitive cells were really creating this wave-lengthening effect by repeating the study in "knock-out" mice in which the ability of the ipRGCs to sense light had been genetically abolished. With the cells disabled, exposure to light no longer made any difference in the duration of the waves.

Finally, to assess the effect of light on the left-right sorting process in the dLGN, Renna examined the tissues from normal mice and the mice whose ipRGCs couldn't sense light. In each case he fluorescently labeled the nerve endings from one eye red and the other green. A computer comparison of the tissues showed that the normal mice developed a higher degree of segregation between red and green than the knockout mice. In other words, the ability of ipRGCs to sense light improved sorting out one eye from another in the dLGN.

Twinkling tadpoles

In his study, Aizenman collaborated with Arto Nurmikko, professor of engineering and physics, to investigate the function of in the optic tectum of tadpole brains. They flooded the tectal neurons in live tadpoles with a molecule that makes calcium ions fluoresce. As whole networks of neurons became active, they'd take in the ions and glow. The researchers recorded the tadpoles with a high-resolution, high-speed camera that could capture the millisecond-to-millisecond activity of the neurons.

Led in the lab by engineering graduate student Heng Xu, the lead author, and postdoctoral researcher Arseny Khakhalin, the team reared some young tadpoles under normal conditions of 12 hours of light and 12 hours of darkness during the crucial days of development when the tectum is developing. They reared others in the dark, and still others with a chemical that blocks the activity of NMDA receptors, a subtype of receptor to the neurotransmitter glutamate, that is known to promote neural rewiring.

Then they exposed all the tadpoles, however they were reared, to blue LED light flashes delivered via a fiber optic cable mounted next to the eye.

What they found over the course of several experiments was that the neural networks in the tectums of tadpoles reared under normal conditions developed a faster, more cohesive, and stronger response (in terms of the number of neurons) to light.

The tectal neural networks of tadpoles kept in the dark during development failed to progress at all. Those whose NMDA receptors were blocked occupied a middle ground, showing more progress than dark-reared tadpoles but less than normal tadpoles. Tadpoles, they found, train their brains with the light they see.

Aizenman said he hopes the calcium ion imaging technique will prove useful in a wide variety of other neuroscience experiments, including studying how tadpoles neurally encode behaviors such as fleeing when they see certain stimuli.

In the meantime, his team and Berson's have added to the understanding scientists have been building of how creatures turn the somewhat mushy approximations of their brains at birth into high-functioning animal minds.

"That's what everybody is after," Aizenman said. "How do you get this fine-tuned, finely wired brain in the first place?"

Berson and Renna's work was funded by the National Institutes of Health. Aizemnan and Nurmikko's research received support from the National Science Foundation, the NIH's National Eye Institute, and the Whitehall Foundation.

Read More