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Beta-actin mRNA molecules
This movie shows beta-actin mRNA molecules traveling within
the dendrites of a cultured live hippocampal mouse neuron.
Credit: Credit: Hye Yoon Park, Ph.D, Albert Einstein College of Medicine
Molecules morph into memories - the movie
A breakthrough allows us to see how memories form. It was made possible by a technological tour de force - a mouse developed with memory making molecules fluorescently "tagged" so they can be observed traveling in real time in living brain cells.
Efforts to discover how neurons make memories have a major roadblock: neurons are extremely sensitive. Any kind of disruption, such as probing their innermost workings, interrups the molecular processes that culminate in memories. To peer deep into neurons without harming them, Albert Einstein College of Medicine researchers developed a model mouse with fluorescently tagged messenger RNA (mRNA) molecules. These molecules code for beta-actin protein – a structural protein found in large amounts in brain neurons and considered essential in making memories.
mRNA is a family of RNA molecules that copy DNA's genetic information, translating it into the proteins that make life possible.
"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, Ph.D., the senior author of both papers and professor and co-chair of Einstein College of Medicine's department of anatomy & structural biology and co-director of the Gruss Lipper Biophotonics Center at Einstein. He also holds the Harold and Muriel Block Chair in Anatomy & Structural Biology at Einstein.
In research described in two Science journal papers, Einstein researchers stimulated neurons in the mouse hippocampus, where memories are made and stored. Now they were able to watch the fluorescent glow from beta-actin mRNA molecules forming in the nuclei of neurons and traveling within dendrites - projections from the neurons.
They discovered that mRNA in neurons is regulated through a novel process described as "masking" and "unmasking," which allows beta-actin protein to be synthesized at specific times, places and amounts.
"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," said Dr. Singer. "Attaching green fluorescent protein to mRNA molecules did not affect the mice, which were able to reproduce."
Neurons join at synapses, where slender dendritic "spines" of neurons grasp each other, much as fingers of one hand grasp those of another.
Evidence supports that repeated neural stimulation increases the strength of synaptic connections by changing the shape of the interlocking dendrite "fingers."
Beta-actin protein appears to strengthen these synaptic connections by altering the shape of dendritic spines. Memories are thought to be encoded when stable, long-lasting synaptic connections form between neurons in contact with each other.
The first Science* paper describes the work of Hye Yoon Park, Ph.D., a postdoctoral student in Dr. Singer's lab at the time and now an instructor at Einstein College of Medicine. Her research was instrumental in developing the mice containing fluorescent beta-actin mRNA—a process that took about three years.
Dr. Park stimulated individual hippocampal neurons of the mouse and observed newly formed beta-actin mRNA molecules within 10 to 15 minutes, indicating that nerve stimulation had caused rapid transcription of the beta-actin gene.
Further observations suggested that these beta-actin mRNA molecules continuously assemble and disassemble into large and small particles, respectively. These mRNA particles were seen traveling to their destinations in dendrites where beta-actin protein would be synthesized.
In the second Science• paper, lead author and graduate student Adina Buxbaum of Dr. Singer's lab showed that neurons may be unique among cells in how they control the synthesis of beta-actin protein.
Dr. Singer: "Having a long, attenuated structure means that neurons face a logistical problem. Their beta-actin mRNA molecules must travel throughout the cell, but neurons need to control mRNA so that it makes beta-actin protein only in certain regions at the base of dendritic spines."
Dr. Buxbaum's research revealed the novel mechanism by which brain neurons handle this challenge. She found that as soon as beta-actin mRNA molecules form in the nucleus of hippocampal neurons and travel out to the cytoplasm, the mRNAs are packaged into granules and so become inaccessible for making protein. She then saw that stimulating the neuron caused these granules to fall apart, so that mRNA molecules became unmasked and available for synthesizing beta-actin protein.
But that observation raised a question: How do neurons prevent newly liberated mRNAs from making more beta-actin protein than is desirable?
Dr. Singer: "Ms. Buxbaum made the remarkable observation that mRNA's availability in neurons is a transient phenomenon. She saw that after the mRNA molecules make beta-actin protein for just a few minutes, they suddenly repackage and once again become masked. In other words, the default condition for mRNA in neurons is to be packaged and inaccessible."
These findings suggest that neurons have an ingenious strategy for controlling how memory-making proteins do their job.
"The observation that neurons selectively activate protein synthesis and then shut it off — fits perfectly with how we think memories are made. 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 a synapse."
Robert Singer, Ph.D., senior author of both papers, professor and co-chair of Einstein College of Medicine's department of anatomy & structural biology and co-director of the Gruss Lipper Biophotonics Center at Einstein.
To gain further insight into memory's molecular basis, the Singer lab is developing technologies for imaging neurons in the intact brains of living mice in collaboration with another Einstein faculty member in the same department, Vladislav Verkhusha, Ph.D. Since the hippocampus resides deep in the brain, they hope to develop infrared fluorescent proteins that emit light that can pass through tissue. Another possibility is a fiberoptic device that can be inserted into the brain to observe memory-making hippocampal neurons.
The two Einstein papers appearing in the January 24 issue of the journal Science are:
*"Visualization of Dynamics of Single Endogenous mRNA as Labeled in Live Mouse"
The transcription and transport of messenger RNA (mRNA) are critical steps in regulating the spatial and temporal components of gene expression, but it has not been possible to observe the dynamics of endogenous mRNA in primary mammalian tissues. We have developed a transgenic mouse in which all β-actin mRNA is fluorescently labeled. We found that β-actin mRNA in primary fibroblasts localizes predominantly by diffusion and trapping as single mRNAs. In cultured neurons and acute brain slices, we found that multiple β-actin mRNAs can assemble together, travel by active transport, and disassemble upon depolarization by potassium chloride. Imaging of brain slices revealed immediate early induction of β-actin transcription after depolarization. Studying endogenous mRNA in live mouse tissues provides insight into its dynamic regulation within the context of the cellular and tissue microenvironment.
Received for publication 16 April 2013.
Accepted for publication 15 October 2013.
•"Single Beta-actin mRNA Detection in Neurons Reveals a Mechanism for Regulating Its Translatability."
The physical manifestation of learning and memory formation in the brain can be expressed by strengthening or weakening of synaptic connections through morphological changes. Local actin remodeling underlies some forms of plasticity and may be facilitated by local β-actin synthesis, but dynamic information is lacking. In this work, we use single-molecule in situ hybridization to demonstrate that dendritic β-actin messenger RNA (mRNA) and ribosomes are in a masked, neuron-specific form. Chemically induced long-term potentiation prompts transient mRNA unmasking, which depends on factors active during synaptic activity. Ribosomes and single β-actin mRNA motility increase after stimulation, indicative of release from complexes. Hence, the single-molecule assays we developed allow for the quantification of activity-induced unmasking and availability for active translation. Further, our work demonstrates that β-actin mRNA and ribosomes are in a masked state that is alleviated by stimulation.
Received for publication 9 July 2013.
Accepted for publication 4 December 2013.
In addition to the authors noted above, other Einstein authors of these papers were Bin Wu, Ph.D., Young J. Yoon, Ph.D., Melissa Lopez-Jones and Xiuhua Meng. Additional contributors are Antonia Follenzi, M.D., at Università del Piemonte orientale “Amedeo Avogadro,” Vercelli, Italy; Chiso Nwokafor and Hyungsik Lim, both at Hunter College of The City University of New York, New York, NY.
The Park study was supported by NIH grants from the National Institute of Neurological Diseases and Stroke (NS083085-19) the National Institute of General Medical Sciences (GM 084364) and the National Institute of Biomedical Imaging and Bioengineering (EB13571), a National Research Service Award (GM87122), and Einstein's Integrated Imaging Program. The Buxbaum study was supported by grant (NS083085-19) and the Weisman Family Foundation.
About Albert Einstein College of Medicine of Yeshiva University
Albert Einstein College of Medicine of Yeshiva University is one of the nation's premier centers for research, medical education and clinical investigation. During the 2013-2014 academic year, Einstein is home to 734 M.D. students, 236 Ph.D. students, 106 students in the combined M.D./Ph.D. program, and 353 postdoctoral research fellows. The College of Medicine has more than 2,000 full-time faculty members located on the main campus and at its clinical affiliates. In 2013, Einstein received more than $155 million in awards from the NIH. This includes the funding of major research centers at Einstein in diabetes, cancer, liver disease, and AIDS. Other areas where the College of Medicine is concentrating its efforts include developmental brain research, neuroscience, cardiac disease, and initiatives to reduce and eliminate ethnic and racial health disparities. Its partnership with Montefiore Medical Center, the University Hospital and academic medical center for Einstein, advances clinical and translational research to accelerate the pace at which new discoveries become the treatments and therapies that benefit patients. Through its extensive affiliation network involving Montefiore, Jacobi Medical Center–Einstein's founding hospital, and five other hospital systems in the Bronx, Manhattan, Long Island and Brooklyn, Einstein runs one of the largest residency and fellowship training programs in the medical and dental professions in the United States. For more information, please visit http://www.einstein.yu.edu and follow us on Twitter @EinsteinMed.