Playing fluorescent ‘tag’ to study the biology of memory

Never-before-seen preclinical fluorescence imaging has broken new ground in the understanding of how memories are formed by nerve cells in the brain. 

The Albert Einstein College of Medicine in The Bronx, N.Y., announced two new studies published Jan. 24 in Science. The first living brain research into memory formation was conducted on mouse models that underwent molecular imaging with fluorescent markers that tagged nerve cell messenger RNA (mRNA) that relay the protein beta-actin, thought vital to the making of memories. 

“It’s noteworthy that we were able to develop this mouse without having to use an artificial gene or other interventions that might have disrupted neurons and called our findings into question,” said Robert Singer, PhD, lead author of both papers and professor and co-chair of Einstein’s department of anatomy and structural biology, in a release.  

In these studies, conducted during the course of three years, researchers watched the molecules of fluorescent-tagged beta-actin mRNA flicker when neurons in the mouse’s hippocampus were stimulated. They were able to watch as the molecules were produced in neuron nuclei and then moved within dendrites as they were masked and unmasked. This function of neural masking allows the production of beta-actin proteins to be regimented into set volumes, times and locations.

“We know the beta-actin mRNA we observed in these two papers was ‘normal’ RNA, transcribed from the mouse’s naturally occurring beta-actin gene,” remarked Singer. “And attaching green fluorescent protein to mRNA molecules did not affect the mice, which were healthy and able to reproduce.”

Beta-actin synthesis appears to have an important effect of strengthening synaptic connections between neuronal dendrites. The researchers stimulated singular neurons within the hippocampus and watched as beta-actin mRNA molecules arrived on the scene within minutes. This proved that stimulation of the nerve cells prompted fast-action genetic transcription. In fact, beta-actin mRNA molecules go through continuous cycles of assembly and disassembly from large to small particles and moving along dendrites toward areas of beta-actin protein synthesis. One of the studies noted how nerve cells differ from other cells in the way they synthesize this protein.

“Having a long, attenuated structure means that neurons face a logistical problem,” said Singer. “Their beta-actin mRNA molecules must travel throughout the cell, but neurons need to control their mRNA so that it makes beta-actin protein only in certain regions at the base of dendritic spines.”

The research eventually revealed how the mRNA molecules are packed up into granules in cell cytoplasm and are no longer able to synthesize further protein until the neurons are stimulated and unbound, or unmasked, and begin production of beta-actin again. One of the major discoveries is that beta-actin synthesis is a transient affair and that, for the most part, these mRNA molecules are masked and not producing new proteins.

“This observation that neurons selectively activate protein synthesis and then shut it off fits perfectly with how we think memories are made,” added Singer. “Frequent stimulation of the neuron would make mRNA available in frequent, controlled bursts, causing beta-actin protein to accumulate precisely where it’s needed to strengthen the synapse.”

The team is now working to develop technologies to image infrared fluorescent proteins through layers of skin or a fiberoptic imaging system that can be planted into the brain to observe hippocampal neurons make new memories.