Posts tagged synaptic plasticity
Articles and news from the latest research reports.
A critical theory in brain development
Experiments performed in the 1960s showed that rearing young animals with one eye closed dramatically altered brain development such that the parts of the visual cortex that would normally process information from the closed eye instead process information from the open eye. These effects can be induced only within a specific period of time—a ‘critical’ period during which the developing nervous system is particularly sensitive to its environment.
Subsequent work has shown that the onset of the critical period in the primary visual cortex requires the maturation of circuits containing neurons that synthesize and release an inhibitory neurotransmitter called gamma-aminobutyric acid (GABA). Now, Taro Toyoizumi and colleagues from the RIKEN Brain Science Institute have presented a theory that explains how this inhibition triggers the critical period.
The theory is based on a computer model of the primary visual cortex containing neurons that receive and process information from the eyes. The model incorporates spontaneous and visually evoked neuronal activity as reported in earlier studies. The simulation also incorporates an activity-dependent form of synaptic plasticity—the process by which connections between neurons are strengthened or weakened in response to neuronal activity.
During early development, spontaneous activity accounts for the majority of activity in the primary visual cortex. With time, however, spontaneous neuronal activity decreases whereas activity evoked by visual experiences increases. The new theory states that the critical period is triggered by the maturation of inhibitory neuronal circuitry, which suppresses the spontaneous activity in the primary visual cortex relative to the activity driven by incoming visual information.
The researchers turned to mice to find evidence to support the theory. Using electrodes to record primary visual cortex activity in freely moving mice, they showed as predicted by theory that the anti-anxiety drug diazepam, which enhances inhibitory activity, lowered the ratio of spontaneous to visual activity in mutant mice with weak inhibition—those lacking the gene encoding glutamic acid decarboxylase-65, an enzyme for synthesizing GABA. Such mice are known not to enter the critical period even in adulthood, but can be induced to do so by administration of diazepam.
Importantly, the theory explains distinct experience-dependent plasticity that takes place before the onset of the critical period, which has been observed in experiments but not explained by other theories. “In the future,” says Toyoizumi, “it would be useful to be able to control developmental plasticity stages by manipulating spontaneous activity in specific brain areas, which could have therapeutic applications.”
Learning and memory: How neurons activate PP1
A study in The Journal of Cell Biology describes how neurons activate the protein PP1, providing key insights into the biology of learning and memory.
PP1 is known to be a key regulator of synaptic plasticity, the phenomenon in which neurons remodel their synaptic connections in order to store and relay information—the foundation of learning and memory. But how PP1 is controlled has been unclear. Now, a team led by researchers from the LSU Health Science Center describes several mechanisms for PP1 regulation that close some major gaps in our understanding of its role in neuronal signaling.
Among the novel findings, the researchers describe how the neurotransmitter NMDA leads to activation of PP1. They show that, when NMDA activates neuronal synapses, it switches off an enzyme, Cdk5, that would otherwise inhibit PP1. This allows PP1 to activate itself and promote synaptic remodeling. In addition, the researchers suggest that, despite its name, a regulatory protein called inhibitor-2 helps promote PP1 activity in neurons. Together, these findings significantly extend our understanding of how PP1 is regulated in the context of synaptic plasticity.
(Source: eurekalert.org)
Untangling Alzheimer’s Disease
TAU researchers identify specific molecules that could be targeted to treat the disorder
Plaques and tangles made of proteins are believed to contribute to the debilitating progression of Alzheimer’s disease. But proteins also play a positive role in important brain functions, like cell-to-cell communication and immunological response. Molecules called microRNAs regulate both good and bad protein levels in the brain, binding to messenger RNAs to prevent them from developing into proteins.
Now, Dr. Boaz Barak and a team of researchers in the lab of Prof. Uri Ashery of Tel Aviv University’s Department of Neurobiology at the George S. Wise Faculty of Life Sciences and the Sagol School of Neuroscience have identified a specific set of microRNAs that detrimentally regulate protein levels in the brains of mice with Alzheimer’s disease and beneficially regulate protein levels in the brains of other mice living in a stimulating environment.
"We were able to create two lists of microRNAs — those that contribute to brain performance and those that detract — depending on their levels in the brain," says Dr. Barak. "By targeting these molecules, we hope to move closer toward earlier detection and better treatment of Alzheimer’s disease."
Prof. Daniel Michaelson of TAU’s Department of Neurobiology in the George S. Wise Faculty of Life Sciences and the Sagol School of Neuroscience, Dr. Noam Shomron of TAU’s Department of Cell and Developmental Biology and Sagol School of Neuroscience, Dr. Eitan Okun of Bar-Ilan University, and Dr. Mark Mattson of the National Institute on Aging collaborated on the study, published in Translational Psychiatry.
A double-edged sword
Alzheimer’s disease is the most common form of dementia. Currently incurable, it increasingly impairs brain function over time, ultimately leading to death. The TAU researchers became interested in the disease while studying the brains of mice living in an “enriched environment” — an enlarged cage with running wheels, bedding and nesting material, a house, and frequently changing toys. Such environments have been shown to improve and maintain brain function in animals much as intellectual activity and physical fitness do in people.
The researchers ran a series of tests on a part of the mice’s brains called the hippocampus, which plays a major role in memory and spatial navigation and is one of the earliest targets of Alzheimer’s disease in humans. They found that, compared to mice in normal cages, the mice from the enriched environment developed higher levels of good proteins and lower levels of bad proteins. Then, for the first time, they identified the microRNAs responsible for regulating the expression of both good and bad proteins.
Armed with this new information, the researchers analyzed changes in the levels of microRNAs in the hippocampi of young, middle-aged, and old mice with an Alzheimer’s-disease-like condition. They found that some of the microRNAs were expressed in exactly inverse amounts in mice with Alzheimer’s disease as they were in mice from the enriched environment. The results were higher levels of bad proteins and lower levels of good proteins in the hippocampi of old mice with Alzheimer’s disease. The microRNAs the researchers identified had already been shown or predicted to regulate the expression of proteins in ways that contributed to Alzheimer’s disease. Their finding that the microRNAs are inversely regulated in mice from the enriched environment is important, because it suggests the molecules can be targeted by activities or drugs to preserve brain function.
Brain-busting potential
Two findings appear to have particular potential for treating people with Alzheimer’s disease. In the brains of old mice with the disease, microRNA-325 was diminished, leading to higher levels of tomosyn, a protein that is well known to inhibit cellular communication in the brain. The researchers hope that eventually microRNA-325 can be used to create a drug to help Alzheimer’s patients maintain low levels of tomosyn and preserve brain function. Additionally, the researchers found several important microRNAs at low levels starting in the brains of young mice. If the same can be found in humans, these microRNAs could be used as biomarker to detect Alzheimer’s disease at a much earlier age than is now possible — at 30 years of age, for example, instead of 60.
"Our biggest hope is to be able to one day use microRNAs to detect Alzheimer’s disease in people at a young age and begin a tailor-made treatment based on our findings, right away," says Dr. Barak.
(Source: aftau.org)
Made to Order at the Synapse: Dynamics of Protein Synthesis at Neuron Tip is Basis for Memory and Learning
Understanding RNA biology in dendrites may inform neurological and psychiatric illness therapeutics
Protein synthesis in the extensions of nerve cells, called dendrites, underlies long-term memory formation in the brain, among other functions. “Thousands of messenger RNAs reside in dendrites, yet the dynamics of how multiple dendrite messenger RNAs translate into their final proteins remain elusive,” says James Eberwine, PhD, professor of Pharmacology, Perelman School of Medicine at the University of Pennsylvania, and co-director of the Penn Genome Frontiers Institute.
Dendrites, which branch from the cell body of the neuron, play a key role in the communication between cells of the nervous system, allowing for many neurons to connect with each other. Dendrites detect the electrical and chemical signals transmitted to the neuron by the axons of other neurons. The synapse is the neuronal structure where this chemical connection is formed, and investigators surmise that it is here where learning and memory occur.
These structural and chemical changes – called synaptic plasticity — require rapid, new synthesis of proteins. Cells may use different rates of translation in different types of mRNA to produce the right amounts and ratios of required proteins.
Knowing how proteins are made to order – as it were - at the synapse can help researchers better understand how memories are made. Nevertheless, the role of this “local” environment in regulating which messenger RNAs are translated into proteins in a neuron’s periphery is still a mystery.
Eberwine, first author Tae Kyung Kim, PhD, a postdoc in the Eberwine lab, and colleagues including Jai Yoon Sul, PhD, assistant professor in Pharmacology, showed that protein translation of two dendrite mRNAs is complex in space and time, as reported online in Cell Reports this week.
“We needed to look at more than one RNA at the same time to get a better handle on real- world processes, and this is the first study to do that in a live neuron,” Eberwine explains.
At Home in the Hippocampus
“It’s not always one particular RNA that dominates at a translation hotspot versus another type of RNA,” says Eberwine. “Since there are 1,000 to 3,000 different mRNA types present in the dendrite overall, but not 1,000 to 3,000 different translational hot spots, do the mRNAs ‘take turns’ being translated in space and time at the ribosomes at the hotspots?”
The researchers engineered the glutamate receptor RNAs to contain different fluorescent proteins that are independently detectable, as well as a photo-switchable protein to determine when new proteins were being made. In the case of the photo-switchable protein studies, when an mRNA for the glutamate receptor protein is marked green, it means it has already been translated.
When a laser is passed over the green protein, it changes to red as a way of tagging when it has been been translated, and new proteins synthesized at that hotspot would be green, which is visible by the appearance of yellow fluorescence (green + red, as measured by light on the visible spectrum). These tricks of the light allow the team to keep track of newly made proteins over time and space.
“This is the first time this method of protein labeling has been used to measure the act of translation of multiple proteins over space and time in a quantitative way,” says Eberwine. “We call it quantitative functional genomics of live cell translation.”
“Our results suggest that the location of the translational hotspot is a regulator of the simultaneous translation of multiple messenger RNAs in nerve cell dendrites and therefore synaptic plasticity,” says Sul.
Laying the Groundwork
Almost 10 years ago, the Eberwine lab discovered that nerve-cell dendrites have the capacity to splice messenger RNA, a process once believed to take place only in the nucleus of cells. Here, a gene is copied into mRNA, which possesses both exons (mature mRNA regions that code for proteins) and introns (non-coding regions). mRNA splicing works by cutting out introns and merging the remaining exon pieces, resulting in an mRNA capable of being translated into a specific protein.
The vast array of proteins within the human body arises in part from the many ways that mRNAs can be spliced and reconnected. Specifically, splicing removes pieces of intron and exon regions from the RNA. The resulting spliced RNA is made into protein.
If the RNA has different exons spliced in and out of it, then different proteins can be made from this RNA. The Eberwine lab was successful in showing that splicing can occur in dendrites because they used sensitive technologies developed in their lab, which permits them to detect and quantify RNA splicing, as well as the translated protein in single isolated dendrites.
Understanding the dynamics of RNA biology and protein translation in dendrites promises to provide insight into regulatory mechanisms that may be modulated for therapeutic purposes in neurological and psychiatric illnesses. The directed development of therapeutics requires this detailed knowledge, says Eberwine.
Versatile proteins could be new target for Alzheimer’s drugs
NIH-funded discovery began with asking how the brain learns to see
A class of proteins that controls visual system development in the young brain also appears to affect vulnerability to Alzheimer’s disease in the aging brain. The proteins, which are found in humans and mice, join a limited roster of molecules that scientists are studying in hopes of finding an effective drug to slow the disease process.
Image: PirB (red) is heavily concentrated on the surface of growing nerve cells. Courtesy of Dr. Carla Shatz, Stanford.
"People are just beginning to look at what these proteins do in the brain. While more research is needed, these proteins may be a brand new target for Alzheimer’s drugs," said Carla Shatz, Ph.D., the study’s lead investigator. Dr. Shatz is a professor of biology and neurobiology at Stanford University in California, and the director of Stanford’s interdisciplinary biosciences program, BioX.
She and her colleagues report that LilrB2 (pronounced “leer-bee-2”) in humans and PirB (“peer-bee”) in mice can physically partner with beta-amyloid, a protein fragment that accumulates in the brain during Alzheimer’s disease. This in turn triggers a harmful chain reaction in brain cells. In a mouse model of Alzheimer’s, depleting PirB in the brain prevented the chain reaction and reduced memory loss.
The research was funded in part by the National Eye Institute, the National Institute on Aging (NIA), and the National Institute of Neurological Disorders and Stroke (NINDS), all part of the National Institutes of Health. It is reported in the Sept. 20 issue of Science.
"These findings provide valuable insight into Alzheimer’s, a complex disorder involving the abnormal build-up of proteins, inflammation and a host of other cellular changes," said Neil Buckholtz, Ph.D., director of the neuroscience division at NIA. "Our understanding of the various proteins involved, and how these proteins interact with each other, may one day result in effective interventions that delay, treat or even prevent this dreaded disease."
Alzheimer’s disease is the most common cause of dementia in older adults, and affects as many as 5 million Americans. Large clumps—or plaques—of beta-amyloid and other proteins accumulate in the brain during Alzheimer’s, but many researchers believe the disease process starts long before the plaques appear. Even in the absence of plaques, beta-amyloid has been shown to cause damage to brain cells and the delicate connections between them.
Dr. Shatz’s discovery took a unique path. She is a renowned neuroscientist, but Alzheimer’s disease is not her focus area. For decades, she has studied plasticity—the brain’s capacity to learn and adapt—focusing mostly on the visual system.
"Dr. Shatz has always been a leader in the field of plasticity, and now she’s taken yet another innovative step—giving us new insights into the abnormal plasticity that occurs in Alzheimer’s disease," said Michael Steinmetz, Ph.D., a program director at NEI. "These findings rest squarely on basic research into the development of the visual system." NEI has funded Dr. Shatz for more than 35 years.
During development, the eyes compete to connect within a limited territory of the brain—a process known as ocular dominance plasticity. The competition takes place during a limited time in early life. If visual experience through one eye is impaired during that time—for example, by a congenital cataract (present from birth)—it can permanently lose territory to the other eye.
"Ocular dominance is a classic example of how a brain circuit can change with experience," Dr. Shatz said. "We’ve been trying to understand it at a molecular level for a long time."
Her search eventually led to PirB, a protein on the surface of nerve cells in the mouse brain. She discovered that mice without the gene for PirB have an increase in ocular dominance plasticity. In adulthood, when the visual parts of their brains should be mature, the connections there are still flexible. This established PirB as a “brake on plasticity” in the healthy brain, Dr. Shatz said.
It wasn’t long before she began to wonder if PirB might also put a brake on plasticity in Alzheimer’s disease. In the current study, she pursued that question with Taeho Kim, Ph.D., a postdoctoral fellow in her lab, and Christopher M. William, M.D., Ph.D., a neuropathology fellow at Massachusetts General Hospital in Boston. Bradley Hyman, M.D., Ph.D., a professor of neurology at Mass General, was a collaborator on the project.
First, the team repeated the genetic experiment that Dr. Shatz had done in normal mice—but this time, they deleted the PirB gene in the Alzheimer’s mice. By about nine months of age, these mice typically develop learning and memory problems. But that didn’t happen in the absence of PirB.
Next, the researchers began thinking about how PirB might fit into the Alzheimer’s disease process, and particularly how it might interact with beta-amyloid. Dr. Kim theorized that since PirB resides on the surface of nerve cells, it might act as a binding site—or receptor—for beta-amyloid. Indeed, he found that PirB binds tightly to beta-amyloid, especially to tiny clumps of it that are believed to ultimately grow into plaques.
Beta-amyloid is known to weaken synapses—the connections between nerve cells. The researchers found that PirB appears to be an accomplice in this process. Without PirB, synapses in the mouse brain were resistant to the effects of beta-amyloid. Other experiments showed that binding between PirB and beta-amyloid can trigger a cascade of harmful reactions that can lead to the breakdown of synapses.
Although PirB is a mouse protein, humans have a closely related protein called LilrB2. The researchers found that this protein also binds tightly to beta-amyloid. By examining brain tissue from people with Alzheimer’s disease, they also found evidence that LilrB2 may trigger the same harmful reactions that PirB can trigger in the mouse brain.
"These are novel results, and direct interaction between beta-amyloid and PirB-related proteins opens up welcome avenues for investigating new drug targets for Alzheimer’s disease," said Roderick Corriveau, Ph.D., a program director at NINDS.
Dr. Shatz said she hopes to interest other researchers to work on developing drugs to block PirB and LilrB2. Currently, no drugs treat the underlying causes of Alzheimer’s disease. Most of the interventions that have reached clinical testing are designed to clear away beta-amyloid. To date, only two other beta-amyloid receptors (PrP-C and EphB2) have been found and are being pursued as drug targets.
(Source: nei.nih.gov)
Long-term memory in the cortex
Game changing results: Brain uses the cortex for making sensory associations, not the hippocampus
‘Where’ and ‘how’ memories are encoded in a nervous system is one of the most challenging questions in biological research. The formation and recall of associative memories is essential for an independent life. The hippocampus has long been considered a centre in the brain for the long-term storage of spatial associations. Now, Mazahir T. Hasan at the Max Planck Institute for Medical Research and José Maria Delgado-Garcìa at the University Pablo de Olavide of Seville, Spain, were able to provide first experimental evidence that a specific form of memory associations is encoded in the cerebral cortex and is not localized in the hippocampus as described in most Neuroscience textbooks. The new study is a game changer since it strongly suggests that the motor cortical circuits itself, and not the hippocampus, is used as memory storage.
Henry Molaison, known widely as H.M., is a famous name in memory research. Large parts of the American‘s hippocampus – the region of the brain that is a major element in learning and memory processes – were removed in the 1950s in an attempt to cure his epileptic seizures. He subsequently suffered severe memory lapses and was no longer able to remember virtually anything new he had learned. Most scientists thereby concluded that the hippocampus is the site of long-term memory.
However, the extent of H.M.’s brain damage was obviously underestimated, because other regions in addition to the hippocampus were also removed or damaged in the surgical procedure. The researchers from Heidelberg and Seville have therefore investigated the learning behaviour of genetically modified mice in which NMDA receptors are turned off only in the motor cerebral cortex. NMDA receptors bind the neurotransmitter glutamate to the synapses and become active when several signals feed into one synapse at the same time. They are the central molecular elements of learning processes, being involved in increasing or decreasing transmission of the signals to synapses.
As the new study shows, in the motor cortex this so-called synaptic plasticity no longer functions without the NMDA receptors. The scientists were thus able to rule out the hippocampus or other regions as the cause for their observations. Based on the new findings, it is the cerebral cortex, not the hippocampus that is the storage site for some forms of memory.
In behaviour tests, so called eyeblink conditioning, animals with and without NMDA receptors in the primary motor cortex had to learn to link a tone with a subsequent electrical stimulus of the eyelid. This association of two sensory inputs involves the cerebellum which coordinates the necessary movements, as well as the hippocampus and the cerebral cortex, which are important learning and memory centres. “After a learning phase, the animals’ reflex is to close their eye when they hear just the tone. Without NMDA receptors in the primary motor cerebral cortex, the genetically modified mice on the other hand cannot remember the connection between the tone and electrical stimulus, and therefore they keep their eyes open despite the tone”, explains Mazahir T. Hasan of the Max Planck Institute for Medical Research.
The researchers have thus complemented the findings of their Heidelberg-based colleagues that the hippocampus is not the seat of memory. In July 2012, Rolf Sprengel and Peter Seeburg from the Max Planck Institute for Medical Research discovered that mice without NMDA receptors in the hippocampus are still quite capable of learning. “We now think that the hippocampus provides the necessary environmental cues, which are transmitted to the cortex where learning-dependent associations take place. Memories are thus stored at various sites in the cerebral cortex on a long-term basis”, explains Hasan.
The findings of Hasan and Delgado-Garcìa thus represent a paradigm-shift in memory research as they make clear that the cerebral cortex is the brain region where memory associations are linked and stored – not the hippocampus. An advanced and detailed knowledge of the mechanisms for the acquisition, consolidation, and recall of associations in the brain is the prerequisite for a therapeutic treatment of the devastating effects of memory loss in various neurological diseases, such as amnesia, Alzheimer`s disease and dementia.
Neuroscientists identify protein linked to Alzheimer’s-like afflictions
A team of neuroscientists has identified a modification to a protein in laboratory mice linked to conditions associated with Alzheimer’s Disease. Their findings, which appear in the journal Nature Neuroscience, also point to a potential therapeutic intervention for alleviating memory-related disorders.
The research centered on eukaryotic initiation factor 2 alpha (eIF2alpha) and two enzymes that modify it with a phosphate group; this type of modification is termed phosphorylation. The phosphorylation of eIF2alpha, which decreases protein synthesis, was previously found at elevated levels in both humans diagnosed with Alzheimer’s and in Alzheimer’s Disease (AD) model mice.
"These results implicate the improper regulation of this protein in Alzheimer’s-like afflictions and offer new guidance in developing remedies to address the disease," said Eric Klann, a professor in New York University’s Center for Neural Science and the study’s senior author.
The study’s co-authors also included: Douglas Cavener, a professor of biology at Pennsylvania State University; Clarisse Bourbon, Evelina Gatti, and Philippe Pierre of Université de la Méditerranée in Marseille, France; and NYU researchers Tao Ma, Mimi A. Trinh, and Alyse J. Wexler.
It has been known for decades that triggering new protein synthesis is vital to the formation of long-term memories as well as for long-lasting synaptic plasticity — the ability of the neurons to change the collective strength of their connections with other neurons. Learning and memory are widely believed to result from changes in synaptic strength.
In recent years, researchers have found that both humans with Alzheimer’s Disease and AD model mice have relatively high levels of eIF2alpha phosphorylation. But the relationship between this characteristic and AD-related afflictions was unknown.
Klann and his colleagues hypothesized that abnormally high levels of eIF2alpha phosphorylation could become detrimental because, ultimately, protein synthesis would diminish, thereby undermining the ability to form long-term memories.
To explore this question, the researchers examined the neurological impact of two enzymes that phosphorylate eIF2alpha, kinases termed PERK and GCN2, in different populations of AD model mice — all of which expressed genetic mutations akin to those carried by humans with AD. These were: AD model mice; AD model mice that lacked PERK; and AD model mice that lacked GCN2.
Specifically, they looked at eIF2alpha phosphorylation and the regulation of protein synthesis in the mice’s hippocampus region — the part of the brain responsible for the retrieval of old memories and the encoding of new ones. They then compared these levels with those of postmortem human AD patients.
Here, they found both increased levels of phosphorylated eIF2alpha in the hippocampus of both AD patients and the AD model mice. Moreover, in conjunction with these results, they found decreased protein synthesis, known to be required for long-term potentiation — a form of long-lasting synaptic plasticity—and for long-term memory.
To test potential remedies, the researchers examined phosphorylation of eIF2alpha in mice lacking PERK, hypothesizing that removal of this kinase would return protein synthesis to normal levels. As predicted, mice lacking PERK had levels of phosphorylated eIF2alpha and protein synthesis similar to those of normal mice.
They then conducted spatial memory tests in which the mice needed to navigate a series of mazes. Here, the AD model mice lacking PERK were able to successfully maneuver through the mazes at rates achieved by normal mice. By contrast, the other AD model mice lagged significantly in performing these tasks.
The researchers replicated these procedures on AD model mice lacking GCN2. The results here were consistent with those of the AD model mice lacking PERK, demonstrating that removal of both kinases diminished memory deficits associated with Alzheimer’s Disease.
(Source: eurekalert.org)
Re-learning how to see: researchers find crucial on-off switch in visual development
A new discovery by a University of Maryland-led research team offers hope for treating “lazy eye” and other serious visual problems that are usually permanent unless they are corrected in early childhood.
Amblyopia afflicts about three percent of the population, and is a widespread cause of vision loss in children. It occurs when both eyes are structurally normal, but mismatched – either misaligned, or differently focused, or unequally receptive to visual stimuli because of an obstruction such as a cataract in one eye.
During the so-called “critical period” when a young child’s brain is adapting very quickly to new experiences, the brain builds a powerful neural network connecting the stronger eye to the visual cortex. But the weaker eye gets less stimulation and develops fewer synapses, or points of connection between neurons. Over time the brain learns to ignore the weaker eye. Mild forms of amblyopia such as “lazy eye” result in problems with depth perception. In the most severe form, deprivation amblyopia, a cataract blocks light and starves the eye of visual experiences, significantly altering synaptic development and seriously impairing vision.
Because brain plasticity declines rapidly with age, early diagnosis and treatment of amblyopia is vital, said neuroscientist Elizabeth M. Quinlan, an associate professor of biology at UMD. If the underlying cause of amblyopia is resolved early enough, the child’s vision can recover to normal levels. But if the treatment comes after the end of the critical period and the loss of synaptic plasticity, the brain cannot relearn to see with the weaker eye.
“If a child is born with a cataract and it is not removed very early in life, very little can be done to improve vision,” Quinlan said. “The severe amblyopia that results is the most difficult to treat. For that reason, science has the most to gain by a better understanding of the underlying mechanisms.”
Quinlan, who specializes in studying how communication through the brain’s circuits changes over the course of a lifetime, wanted to find out what process controls the timing of the critical period of synaptic plasticity. If researchers could find the neurological on-off switch for the critical period, she reasoned, clinicians could use the information to successfully treat older children and adults.
Researchers in Quinlan’s University of Maryland lab teamed up with the laboratory of Alfredo Kirkwood at Johns Hopkins University to address two questions: What are the age boundaries of the critical period for synaptic plasticity, when it comes to determining eye dominance? And what developmental processes are involved?
Experiments in rodents suggested the timing of the critical period is controlled by a specific class of inhibitory neurons, which come into play after a visual stimulus activates excitatory neurons that link the eye to the visual cortex. The inhibitory neurons act as signal controllers, affecting the interactions between excitatory neurons and synapses.
“The generally accepted view has been that as the inhibitory neurons develop, synaptic plasticity declines, which was thought to occur at about five weeks of age in rodents,” roughly equivalent to five years of age in humans, Quinlan said. But in earlier experiments, Quinlan and Kirkwood found no correlation between the development of these inhibitory neurons and the loss of plasticity. In fact, they found the visual circuitry in rodents was highly adaptable at ages beyond five weeks.
In their latest research the UMD-led team looked “one synapse upstream from these inhibitory neurons,” Quinlan said, studying the control of that synapse by a protein called NARP (Neuronal Activity-Regulated Pentraxin). Working with two sets of mice – one group genetically similar to wild mice and another that lacked the NARP gene - the researchers covered one eye in each animal to simulate conditions that produce amblyopia.
The mice that were genetically similar to wild mice developed amblyopia, with characteristic dominance of the normal eye over the deprived eye. But the mice that lacked NARP did not develop amblyopia, regardless of age or the length of time one eye was deprived of stimulation.
The study, published in the current issue of the peer-reviewed journal Neuron, demonstrated that only one specific class of synapses was affected by the absence of NARP. Without NARP, the mice simply had no critical period in which the brain circuitry was weakened in response to the impaired blocking vision in one eye, Quinlan said. Except for the lack of this plasticity, their vision was normal.
“It’s remarkable how specific the deficit is,” Quinlan said. Without the NARP protein, “these animals develop normal vision. Their brain circuitry just isn’t plastic. We can completely turn off the critical period for plasticity by knocking out this protein.”
Since there are indications that NARP levels vary with age, the discovery raises hope that a treatment targeting NARP levels in humans could allow correction of amblyopia late in life, without affecting other aspects of vision.
When fear factors in
A little bit of learned fear is a good thing, keeping us from making risky, stupid decisions or falling over and over again into the same trap. But new research from neuroscientists and molecular biologists at USC shows that a missing brain protein may be the culprit in cases of severe over-worry, where the fear perseveres even when there’s nothing of which to be afraid.
In a study appearing the week of July 15 in the Proceedings of the National Academy of Sciences, the researchers examined mice without the enzymes monoamine oxidase A and B (MAO A/B), which sit next to each other in a human’s genetic code as well as on that of mice. Prior research has found an association between deficiencies of these enzymes in humans and developmental disabilities along the autism spectrum, such as clinical perseverance, the inability to change or modulate actions along with social context.
“These mice may serve as an interesting model to develop interventions to these neuropsychiatric disorders,” said University Professor and senior author Jean Shih, Boyd & Elsie Welin Professor of Pharmacology and Pharmaceutical Sciences at the USC School of Pharmacy and the Keck School of Medicine of USC. “The severity of the changes in the MAO A/B knockout mice compared to MAO A knockout mice supports the idea that the severity of autistic-like features may be correlated to the amounts of monoamine levels, particularly at early developmental stages.”
Shih is a world leader in understanding the neurobiological and biochemical mechanisms behind such behaviors as aggression and anxiety. In this latest study, Shih and her co-investigators — including lead author Chanpreet Singh, a USC doctoral student at the time of the research who is now at the California Institute of Technology (Caltech), and Richard Thompson, USC University Professor Emeritus and Keck Professor of Psychology and Biological Sciences at the USC Dornsife College of Letters, Arts and Sciences — expanded their past research on MAO A/B, which regulates neurotransmitters known as monoamines, including serotonin, norepinephrine and dopamine.
Comparing mice without MAO A/B with their wild-type littermates, the researchers found significant differences in how the mice without MAO A/B processed fear and other types of learning. Mice without MAO A/B and wild mice were put in a new, neutral environment and given a mild electric shock. All mice showed learned fear the next time they were tested in the same environment, with the MAO A/B knockout mice displaying a greater degree of fear.
But while wild mice continued to explore other new environments freely after the trauma, mice without the MAO A/B enzymes generalized their phobia to other contexts — their fear spilled over onto places where they should have no reason to be afraid.
“The neural substrates processing fear in the brain is very different in these mice,” Singh said. “Enhanced learning in the wrong context is a disorder and is exemplified by these mice. Their brain is not letting them forget. In a survival issue, you need to be able to forget things.”
The mice without MAO A and MAO B also learned eye-blink conditioning much more quickly than wild mice, which has also been noted in autistic patients but not in mice missing only one of these enzymes.
Importantly, the mice without MAO A/B did not display any differences in learning for spatial skills and object recognition, the researchers found, “but in their ability to learn an emotional event, the [MAO A/B knockout mice] are very different than wild types,” Singh said.
He continued: “When both enzymes are missing, it significantly increases the levels of neurotransmitters, which causes developmental changes, which leads to differential expression of receptors that are very important for synaptic plasticity — a measure of learning — and to behavior that is quite similar to what we see along the autism spectrum.”
(Source: news.usc.edu)
Peering into the Protein Pathways of a Cell
As a cell’s central power plant, the mitochondrion is a busy place.
Specially-coded proteins from the nucleus are constantly being ferried across the mitochondrion’s inner membrane, where they help the mighty organelle do its work – producing the cell’s high-energy molecules, carrying out signaling duties, and controlling cell growth.
Scientists have long known that the central channel through which most of these proteins must pass – a critical gatekeeper known as the translocase of the inner mitochondrial membrane 23 or TIM23 for short – requires an electrical field for its gating capabilities to function. But they weren’t quite sure how the whole process worked.
Until now.
Using highly sensitive fluorescent probes, a team of scientists based at UConn has managed to peer deep into the inner workings of a cell, capturing the never-before-seen structural dynamics of the TIM23 channel complex while it functioned in its natural environment.
In doing so, the team, led by Nathan N. Alder, an assistant professor in the Department of Molecular and Cell Biology in the College of Liberal Arts and Sciences, discovered that the TIM23 complex not only opens and closes in response to fluctuations in the energized state of the mitochondrion’s inner membrane, as the scientific community suspected, it also changes its very structure – altering the helical shape of protein segments that line the channel – as the electrical field across the membrane drops.
The research, which appears this week in the peer-reviewed journal Nature Structural & Molecular Biology, explains how the energized state of the membrane drives the structural dynamics of membrane proteins and sheds new light on how cellular transport systems harness energy to perform their work inside the cell. It also shows how fluorescent mapping at the subcellular level may reveal new insights into the underlying causes of neurodegenerative and metabolic disorders associated with mitochondrial function.
In an overview of the research accompanying the paper’s publication, Nikolaus Pfanner of the University of Freiburg, Germany, an international leader in the field of cellular protein trafficking, and several members of his research group, called the study “a major step towards a molecular understanding of a voltage-gated protein translocase.”
“The molecular nature of voltage sensors in membrane proteins is a central question in biochemical research,” Pfanner and his colleagues said. “The study … is not only of fundamental importance for our understanding of mitochondrial biogenesis, but also opens up new perspectives in the search for voltage-responsive elements in membrane proteins.”
Applying a new technique
The fluorescent mapping technique used in the research was a key to the project’s success. Alder says he first realized the application’s potential when he successfully mapped channel proteins in a functioning mitochondrion in 2008. In the current study, he advanced the process further, using probes to capture the behavior of a particular segment of the TIM23 channel complex as it was impacted by voltage changes in the membrane’s electrical field.
“Fluorescent mapping made this possible,” says Alder, who, as a post-doctoral student, worked with protein fluorescent labeling pioneer Arthur E. Johnson of Texas A&M’s Health Science Center. “It allowed us to peer into the functioning dynamics of a protein import channel complex that is responsible for building up the power plant of the cell … What we found was that these protein-trafficking complexes are certainly not static. This is a very, very dynamic channel.”
To monitor the fluorescence probes inside the mitochondria, the research team used advanced spectrofluorimeters equipped with xenon lamps and laser diodes to measure steady-state and time-resolved fluorescence, respectively.
To conduct the study, Alder incorporated cysteine residues modified with a fluorescent probe at specific positions along a transmembrane segment of a TIM23 complex derived from a common species of yeast, Saccharomyces cerevisiae. The team then monitored the probes in real time to observe how the channel’s voltage-gating and structure responded to induced changes in the inner membrane’s electrical field.
“It’s an indirect way of looking at the structure of something, but because we are able to look into an actually functioning mitochondrion, it’s given us a whole world of new information,” says Alder.
“That the magnitude of the voltage gradient across the membrane could play a significant role in defining the structure of these proteins is probably one of the most significant elements of this research,” he adds.
A defining moment
Watching the process was, for Alder, a defining moment in his professional career.
“When I first saw a certain kind of structure that told me I was in the middle of a channel, that was one of the most exciting times in my professional life,” he says. “I knew I was getting insight into a fundamental natural phenomenon, something no one has ever seen before.”
When Alder saw the protein-conducting channel bending and collapsing in response to changes in the membrane’s voltage levels, he was equally thrilled.
“That was one of those rare technical moments in my professional life that showed we were really getting insight into a fundamental process going on inside a cell,” he says. “It’s always been known that you need an energized membrane to make these channels work, but no one had a clue why.”
Joining Alder on the project were UConn graduate students Ketan Malhotra and Murugappan Sathappa and research associate Judith S. Landin. Johnson, Alder’s former mentor at Texas A&M, is also listed as a co-author. The work in the Alder Lab was funded by the National Science Foundation; work done in the Johnson Lab was additionally sponsored by the National Institutes of Health and the Robert A. Welch Foundation.
Alder says the next phase of the research will look toward isolating the TIM23 protein channel complex in an artificial system to see if it continues to respond to voltage fluctuations outside of its natural habitat. The research team is also hoping to identify the particular parts of the protein complex that are acting as voltage sensors.
“Once we start to identify exactly what is the voltage sensor, we will have a better understanding of the translocase process, and ultimately we can apply this knowledge to other kinds of protein transporters whose dysfunction has been implicated in the etiology of diseases such as cardiovascular disease and cancer,” Alder says. “If their function is tied to the energized state of the membrane, we’ll be able to see whether defects in that ability to couple to the membrane might be associated with the pathogenesis of these diseases.”