An enzyme and synaptic plasticity: Study reveals novel role for the Pin1

Synapses are “dynamic” things: they can regulate their action in neural processes related to learning, for example, but also as a consequence of diseases. A research team –led by SISSA– has demonstrated the role of a small enzyme (Pin1) in synaptic plasticity. The study has just been published in the journal Nature Communications.

A small, “empty” space teeming with activity: a synapse is a complex structure where the neural (electrical) signal from the presynaptic neuron, as it travels towards its target –a muscle, a gland or another neuron– turns into a chemical signal capable of crossing the synaptic space before becoming electrical again once on the other side. A synapse is a “dynamic” space not only because of the endless work that goes on there, but also for its ability to change its action over time (synaptic plasticity) as a result of either normal physiological processes (e.g., during learning) or because of disorders due to pathological conditions. A study, mainly carried out by SISSA researchers (which also involved the University of Zurich, LNCIB in Trieste, and EBRI in Rome), showed that a small enzyme (Pin1, peptidylprolyl isomerase) that plays a mediating role in signal transmission has an effect on synaptic plasticity.

The synapse we studied is of the inhibitory kind. The signal it transmits hinders activation of the postsynaptic neuron, making it less likely for it to become activated and emit its action potential”, explains Paola Zacchi, a SISSA researcher who coordinated the study. “When Pin1 is absent from the synapse, signal transmission occurs “at full strength”, but also without control. Instead, when it is present, it regulates signal strength, making it weaker. We observed that Pin1 is able to modify the number of postsynaptic receptors”. The larger the number of receptors capable of binding to the neurotransmitter, the stronger the signal that reaches the postsynaptic membrane. “This also means that Pin1 plays a role in plasticity” explains Zacchi.

How does a synapse work? “A chemical synapse, the most common in vertebrates, is a small gap between nerve cells where the passage of a neural signal occurs”, explains Zacchi. In chemical synapses the two neurons are not in contact but they are separated by a distance of about 20 nanometres. For this reason, the electrical signal travelling along the presynaptic nerve ending is interrupted before resuming on the neuron on the other side of the gap. In between the two nerve cells the electrical signal is translated into a chemical signal (which then becomes electrical again).

“Arrival of the action potential on the presynaptic button causes release, into the interneural space, of molecules of neurotransmitter, which are picked up by receptors on the postsynaptic membrane”, says Zacchi. “If the synapse is excitatory, this leads to postsynaptic activation which, if sufficiently intense, triggers another action potential. If the synapse is inhibitory, as in our studies, the signal suppresses postsynaptic activation and inhibits firing of the electrical potential. In the process of neurotransmitter release and binding, other molecules come into play, such as scaffold proteins, which assemble receptors at the right place on the membrane in front of the neurotransmitter release sites, and neuroligins which act as bridges between the two ends of the synapse as well as interacting with the scaffold proteins. Pin1, the enzyme in the study, interacts with both neuroligins and scaffold proteins.

The Pin1 enzyme has long been known for its role in cancer and the development of neurodegenerative diseases such as Alzheimer’s and Parkinson’s (whereas neuroligins seem to be involved in autism). “Studies like this enhance our understanding of the biochemical mechanisms of synaptic plasticity, extending our knowledge of healthy mechanisms, but also helping those who are trying to understand what can be done in a wide range of pathological conditions”.

Neuroscientists use snail research to help explain “chemo brain”

It is estimated that as many as half of patients taking cancer drugs experience a decrease in mental sharpness. While there have been many theories, what causes “chemo brain” has eluded scientists.

In an effort to solve this mystery, neuroscientists at The University of Texas Health Science Center at Houston (UTHealth) conducted an experiment in an animal memory model and their results point to a possible explanation. Findings appeared in The Journal of Neuroscience.

In the study involving a sea snail that shares many of the same memory mechanisms as humans and a drug used to treat a variety of cancers, the scientists identified memory mechanisms blocked by the drug. Then, they were able to counteract or unblock the mechanisms by administering another agent.

“Our research has implications in the care of people given to cognitive deficits following drug treatment for cancer,” said John H. “Jack” Byrne, Ph.D., senior author, holder of the June and Virgil Waggoner Chair and chairman of the Department of Neurobiology and Anatomy at the UTHealth Medical School. “There is no satisfactory treatment at this time.”

While much work remains, Byrne, who runs the university’s Neuroscience Research Center, said understanding how these drugs impact the brain is an important first step in alleviating this condition characterized by forgetfulness, trouble concentrating and difficulty multitasking.

Byrne’s laboratory is known for its use of a large snail called Aplysia californica to further the understanding of the biochemical signaling among nerve cells (neurons). The snails have large neurons that relay information much like those in humans.

When Byrne’s team compared cell cultures taken from normal snails to those administered a dose of a cancer drug called doxorubicin, the investigators pinpointed a neuronal pathway that was no longer passing along information properly.

With the aid of an experimental drug, the scientists were able to reopen the pathway. Unfortunately, this drug would not be appropriate for humans, Byrne said. “We want to identify other drugs that can rescue these memory mechanisms,” he added.

The scientists confirmed their findings in tests on the nerve cells of rats.

“The big picture is to determine if this cancer drug acts in the same way in humans,” Byrne said.

A Gene Linked to Disease Found to Play a Critical Role in Normal Memory Development

It has been more than 20 years since scientists discovered that mutations in the gene huntingtin cause the devastating progressive neurological condition Huntington’s disease, which involves involuntary movements, emotional disturbance and cognitive impairment. Surprisingly little, however, has been known about the gene’s role in normal brain activity.

Now, a study from The Scripps Research Institute’s (TSRI’s) Florida campus and Columbia University shows it plays a critical role in long-term memory.

“We found that huntingtin expression levels are necessary for what is known as long-term synaptic plasticity—the ability of the synapses to grow and change—which is critical to the formation of long-term memory,” said TSRI Assistant Professor Sathyanarayanan V. Puthanveettil, who led the study with Nobel laureate Eric Kandel of Columbia University.

In the study, published recently by the journal PLOS ONE, the team identified an equivalent of the human huntingtin protein in the marine snail Aplysia, a widely used animal model in genetic studies, and found that, just like its human counterpart, the protein in Aplysia is widely expressed in neurons throughout the central nervous system.

Using cellular models, the scientists studied what is known as the sensory-to-motor neuron synapse of Aplysia—in this case, gill withdrawal, a defensive move that occurs when the animal is disturbed.

The study found that the expression of messenger RNAs of huntingtin—messenger RNAs are used to produce proteins from instructions coded in genes—is increased by serotonin, a neurotransmitter released during learning in Aplysia. After knocking down production of the huntingtin protein, neurons failed to function normally.

“During the learning, production of the huntingtin mRNAs is increased both in pre- and post-synaptic neurons—that is a new finding,” Puthanveettil said. “And if you block production of the protein either in pre- or post-synaptic neuron, you block formation of memory.”

The findings could have implications for the development of future treatments of Huntington’s disease. While the full biological functions of the huntingtin protein are not yet fully understood, the results caution against a therapeutic approach that attempts to eliminate the protein entirely.

Choice bias: A quirky byproduct of learning from reward

The price of learning from rewarding choices may be just a touch of self-delusion, according to a new study in Neuron.

The research by Brown University brain scientists links a fundamental problem in neuroscience called “credit assignment” — how the brain reinforces learning only in the exact circuits that caused the rewarding choice — to an oft-observed quirk of behavior called “choice bias” – we value the rewards we choose more than equivalent rewards we don’t choose. The researchers used computational modeling and behavioral and genetic experiments to discover evidence that choice bias is essentially a byproduct of credit assignment.

“We weren’t looking to explain anything about choice bias to start off with,” said lead author Jeffrey Cockburn, a graduate student in the research group of senior author Michael Frank, associate professor of cognitive, linguistic, and psychological sciences. “This just happened to be the behavioral phenomenon we thought would emerge out of this credit assignment model.”

So the next time a friend raves about the movie he chose and is less enthusiastic about the just-as-good one that you chose, you might be able to chalk it up to his basic learning circuitry and a genetic difference that affects it.

Modeled mechanism

The model, developed by Frank, Cockburn, and co-author Anne Collins, a postdoctoral researcher, was based on prior research on the function of the striatum, a part of the brain’s basal ganglia (BG) that is principally involved in representing reward values of actions and picking one. “An interaction between three key BG regions moderates that decision-making process. When a rewarding choice has been made, the substantia nigra pars compacta (SNc) releases dopamine into the striatum to reinforce connections between cortex and striatum, so that rewarded actions are more likely to be repeated. But how does the SNc reinforce just the circuits that made the right call? The authors proposed a mechanism by which another part of the subtantia nigra, the SNr, detects when actions are worth choosing and then simultaneously amplifies any dopamine signal coming from the SNc.”

“The novel part here is that we have proposed a mechanism by which the BG can detect when it has selected an action and should therefore amplify the dopamine reinforcing event specifically at that time,” Frank said. “When the SNr decides that striatal valuation signals are strong enough for one action, it releases the brakes not only on downstream structures that allow actions to be executed, but also on the SNc dopamine system, so any unexpected rewards are amplified.”

Specifically, dopamine provides reinforcement by enhancing the responsiveness of connections between cells so that a circuit can more easily repeat its rewarding behavior in the future. But along with that process of reinforcing the action of choosing, the value placed on the resulting reward becomes elevated compared to rewards not experienced this way.

Experimental evidence

That prediction seemed intriguing, but it still had to be tested. The authors identified both behavioral and genetic tests that would be telling.

They recruited 80 people at Brown and elsewhere in Providence to play a behavioral game and to donate some saliva for genetic testing.

The game first presented the subjects pictures of arbitrary Japanese characters that would have different probabilities of rewards if chosen ranging from a 20 percent to 80 percent chance of winning a point or losing a point. For some characters, the player could choose a character to discover its resulting reward or penalty, whereas for others, its result was simply given to them. After that learning phase, the subjects were then presented the characters in pairs and instructed to pick the one they thought had the highest chance of winning based on what they had learned.

The researchers built the game so that for every character a player could choose, there was an equally rewarding one that had merely been given to them. On average, players showed a clear choice bias in that they were more likely to prefer rewarding characters that they had chosen over equally rewarding characters they had been given.

Notably, they exhibited no choice bias between unrewarding characters suggesting that choice bias emerges only in relation to reward, one of the key predictions of their model. But they wanted to test further whether the impact of reward on choice bias was related to the proposed biological mechanism, that striatal dopaminergic learning is enhanced to chosen rewards.

The genetic tests focused on single-letter differences in a gene called DARPP-32, which governs how well cells in the striatum respond to the reinforcing influence of dopamine.

People with one version of the gene have been shown in previous research to be less able to learn from rewards, while people with other versions were less driven by reward in learning.

“The reason why this gene is interesting is because we know something about the biology of what it does and where it is expressed in the brain,” Frank said. “It’s predominant in the striatum and specifically affects synaptic plasticity induced by dopamine signaling. It’s related to the imbalance by which you learn from really good things or not so good things.

“The logic was if the mechanism that we think describes this choice bias and credit assignment problem is accurate then that gene should predict the impact of how good something was on this choice bias phenomenon,” he said.

Indeed, that’s what the data showed. People with the form of the gene that predisposed them to be responsive to big rewards also showed more choice bias from the most strongly rewarded characters. Interestingly, the other people also showed choice bias, but more strongly for those characters that were more mediocre. This pattern was mirrored by the authors’ model when it simulated the effects of DARPP-32 on reward learning imbalances from positive vs. negative outcomes.

For some people, the plums are sweeter if they picked them.

Blame it on the astrocytes

In the brains of all vertebrates, information is transmitted through synapses, a mechanism that allows an electric or chemical signal to be passed from one brain cell to another. Chemical synapses, which are the most abundant type of synapse, can be either excitatory or inhibitory. Synapse formation is crucial for learning, memory, perception and cognition, and the balance between excitatory and inhibitory synapses critical for brain function. For instance, every time we learn something, the new information is transformed into memory through synaptic plasticity, a process in which synapses are strengthened and become more responsive to different stimuli or environmental cues. Synapses may change their shape or function in a matter of seconds or over an entire lifetime. In humans, a number of disorders are associated with dysfunctional synapses, including autism, epilepsy, substance abuse and depression.

Astrocytes, named for their star-like shape, are ubiquitous brain cells known for regulating excitatory synapse formation through cells. Recent studies have shown that astrocytes also play a role in forming inhibitory synapses, but the key players and underlying mechanisms have remained unknown until now.

A new study just published in the journal Glia and available online on July 11th, details the newly discovered mechanism by which astrocytes are involved in inhibitory synapse formation and presents strong evidence that Transforming Growth Factor Beta 1 (TGF β1), a protein produced by many cell types (including astrocytes) is a key player in this process. The team led by Flávia Gomes of the Rio de Janeiro Institute of Biomedical Sciences at the Federal University of Rio de Janeiro investigated the process in both mouse and human tissues, first in test tubes, then in living brain cells.

Previous evidence has shown that TGF β1, a molecule associated with essential functions in nervous system development and repair, modulates other components responsible for normal brain function. In this study, the authors were able to show that TGF β1 triggers N-methyl-D-aspartate receptor (NMDA), a molecule controlling memory formation and maintenance through synaptic plasticity. In the study, the group also shows that TGF β1-induction of inhibitory synapses depends on activation of another molecule - Ca2+/calmodulin-dependent protein kinase II (CaMK2)-, which works as a mediator for learning and memory. “Our study is the first to associate this complex pathway of molecules, of which TGF β1 seems to be a key player, to astrocytes’ ability to modulate inhibitory synapses”, says Flávia Gomes.

The idea that the balance between excitatory and inhibitory inputs depends on astrocyte signals gains strong support with this new study and suggests a pivotal role for astrocytes in the development of neurological disorders involving impaired inhibitory synapse transmission. Knowing the players and mechanisms underlying inhibitory synapses may improve our understanding of synaptic plasticity and cognitive processes and may help develop new drugs for treating these diseases.

(Image credit)

Bad learning

University of Iowa researchers have discovered a new form of neurotransmission that influences the long-lasting memory created by addictive drugs, like cocaine and opioids, and the subsequent craving for these drugs of abuse. Loss of this type of neurotransmission creates changes in brains cells that resemble the changes caused by drug addiction.

The findings, published June 22 in the journal Nature Neuroscience, suggest that targeting this type of neurotransmission might lead to new therapies for treating drug addiction.

“Molecular therapies for drug addiction are pretty much non-existent,” says Collin Kreple, UI graduate student and co-first author of the study. “I think this finding at least provides the possibility of a new molecular target.”

The new form of neurotransmission involves proteins called acid-sensing ion channels (ASICs), which have previously been shown to promote learning and memory, and which are abundant in a part of the brain that is involved in drug addiction. The researchers, led by John Wemmie, professor of psychiatry in the UI Carver College of Medicine, reasoned that disrupting ASIC activity in this brain region (the nucleus accumbens) should reduce learned addiction-related behaviors. However, their experiments showed that loss of ASIC signaling actually increases learned drug-seeking in mice.

When mice learned to associate one side of a chamber with receiving cocaine, animals that lacked the ASIC protein developed an even stronger preference for the “cocaine side” than control mice, suggesting that loss of ASIC had increased addiction behavior. The same result was seen for morphine, another drug of abuse, which has a different mechanism of action than cocaine.

"Always before, the data suggested that when you get rid of ASICs, learning and memory are impaired," Wemmie says. "So we expected the same trend when we studied reward-related learning and behavior and we were surprised to find the opposite."

In a second experiment, rats learned to press a lever to self-administer cocaine. Blocking or removing ASIC in the rat brains caused the animals to self-administer more cocaine than control animals. Conversely, increasing the amount of ASIC by over-expressing the protein seemed to decrease the animals’ craving for cocaine.

"There are many forms of addiction," says Wemmie, who also holds appointments in the UI Departments of Molecular Physiology and Biophysics and Neurosurgery, and with the Iowa City VA Medical Center. "We’d like to see if these mechanisms also apply to other addictions besides cocaine and morphine. And, we want to move forward to see if this pathway can be used to target addiction."

Novel neurotransmission

As the name suggests, acid-sensing ion channels are activated by acid, in the form of protons. This research and a second UI study recently published in PNAS show that protons and ASICs form a previously unrecognized neurotransmitter pair that helps neurons communicate in a novel way; and appear to influence several forms of learning and memory, including fear, as well as addiction.

Manipulating the activity of ASICs or the level of protons (acidity) may provide a new way to treat addiction.

"We are still a long way from using these findings to create a therapy," notes Yuan Lu, co-first author and UI postdoctoral scholar. "The key significance of this study is that we have found new, different targets [that might allow us to inhibit the addiction behavior].”

Drugs change the brain

Previous research has shown that drug abuse and addiction physically alter the connections between neurons (synapses) that are important for the creation and storage of memories. Although normal learning requires synapses to be dynamic and plastic, exposure to addictive drugs abnormally increases synaptic plasticity in a way that is thought to underlie drug-related learning and addiction behaviors. The UI study found that absence of ASIC-proton mediated neurotransmission also increased synaptic plasticity in a way that resembled the changes created by addiction and drug withdrawal.

"It seemed like everything we looked at (physiology and structural changes) really paralleled what you would see in an animal undergoing drug withdrawal, even though these animals missing ASIC had never been exposed to drugs," Kreple says.

Overall the study findings suggest that ASIC-related neurotransmission in the nucleus accumbens may play a role in reducing synaptic plasticity and appropriately stabilizing synapses.

(Image caption: Dendrite of an amygdala principal neuron with dendritic spines (white). Inhibitory synaptic contacts are shown in red. Credit: © MPI f. Brain Research/ J. Letzkus)

Learning brakes in the brain

A brain capable of learning is important for survival: only those who learn can endure in the natural world. When it learns, the brain stores new information by changing the strength of the junctions that connect its nerve cells. This process is referred to as synaptic plasticity. Scientists at the Max-Planck Institute for Brain Research in Frankfurt, working with researchers from Basel, have demonstrated for the first time that inhibitory neurons need to be at least partly blocked during learning. This disinhibition is a bit like taking the foot off the brake in a car: if the inhibitory neurons are less active, learning is accelerated.

Learning is often a matter of timing: different stimuli become strongly associated if they occur in close succession. The Max Planck scientists made use of this phenomenon in conditioning experiments in which mice learned to react to a tone. For this learning effect to occur, the synapses of the so-called principal neurons in the amygdala need to become more sensitive. The researchers concentrated on two types of inhibitory neurons which produce the proteins parvalbumin and somatostatin and inhibit the principal neurons of the amygdala.

The results obtained by the Max Planck researchers show that both cell types are inhibited during different phases of the learning process. This disinhibition enhances the activation of the principal neurons. Moreover, the scientists were able to control the learning behaviour of the mice through the use of optogenetics. In these experiments, they equipped both types of inhibitory neurons in the amygdala with light-sensitive ion channels, allowing them to use light to switch the neurons on or off as required. “When we prevent disinhibition, the mice learn less well. In contrast, enhancing the disinhibition leads to intensified learning”, says Johannes Letzkus from the Max Planck Institute for Brain Research. Next, the scientists aim to identify the nerve pathways which are involved in disinhibition.