Author: Julie Pryor

Two MIT Brain and Cognitive Sciences faculty members earn funding from the G. Harold and Leila Y. Mathers Foundation

by David Orenstein |

Two MIT neuroscientists have received grants from the G. Harold and Leila Y. Mathers Foundation to screen for genes that could help brain cells withstand Parkinson’s disease and to map how gene expression changes in the brain in response to drugs of abuse.

Myriam Heiman, an associate professor in MIT’s Department of Brain and Cognitive Sciences and a core member of the Picower Institute for Learning and Memory and the Broad Institute of MIT and Harvard, and Alan Jasanoff, who is also a professor in biological engineering, brain and cognitive sciences, nuclear science and engineering and an associate investigator at the McGovern Institute for Brain Research, each received three-year awards that formally begin January 1, 2021.

Jasanoff, who also directs MIT’s Center for Neurobiological Engineering, is known for developing sensors that monitor molecular hallmarks of neural activity in the living brain, in real time, via noninvasive MRI brain scanning. One of the MRI-detectable sensors that he has developed is for dopamine, a neuromodulator that is key to learning what behaviors and contexts lead to reward. Addictive drugs artificially drive dopamine release, thereby hijacking the brain’s reward prediction system. Studies have shown that dopamine and drugs of abuse activate gene transcription in specific brain regions, and that this gene expression changes as animals are repeatedly exposed to drugs. Despite the important implications of these neuroplastic changes for the process of addiction, in which drug-seeking behaviors become compulsive, there are no effective tools available to measure gene expression across the brain in real time.

Cerebral vasculature in mouse brain. The Jasanoff lab hopes to develop a method for mapping gene expression the brain with related labeling characteristics .
Image: Alan Jasanoff

With the new Mathers funding, Jasanoff is developing new MRI-detectable sensors for gene expression. With these cutting-edge tools, Jasanoff proposes to make an activity atlas of how the brain responds to drugs of abuse, both upon initial exposure and over repeated doses that simulate the experiences of drug addicted individuals.

“Our studies will relate drug-induced brain activity to longer term changes that reshape the brain in addiction,” says Jasanoff. “We hope these studies will suggest new biomarkers or treatments.”

Dopamine-producing neurons in a brain region called the substantia nigra are known to be especially vulnerable to dying in Parkinson’s disease, leading to the severe motor difficulties experienced during the progression of the incurable, chronic neurodegenerative disorder. The field knows little about what puts specific cells at such dire risk, or what molecular mechanisms might help them resist the disease. In her research on Huntington’s disease, another incurable neurodegenerative disorder in which a specific neuron population in the striatum is especially vulnerable, Heiman has been able to use an innovative method her lab pioneered to discover genes whose expression promotes neuron survival, yielding potential new drug targets. The technique involves conducting an unbiased screen in which her lab knocks out each of the 22,000 genes expressed in the mouse brain one by one in neurons in disease model mice and healthy controls. The technique allows her to determine which genes, when missing, contribute to neuron death amid disease and therefore which genes are particularly needed for survival. The products of those genes can then be evaluated as drug targets. With the new Mathers award, Heiman plans to apply the method to study Parkinson’s disease.

An immunofluorescence image taken in a brain region called the substantia nigra (SN) highlights tyrosine hydroxylase, a protein expressed by dopamine neurons. This type of neuron in the SN is especially vulnerable to neurodegeneration in Parkinson’s disease. Image: Preston Ge/Heiman Lab

“There is currently no molecular explanation for the brain cell loss seen in Parkinson’s disease or a cure for this devastating disease,” Heiman said. “This award will allow us to perform unbiased, genome-wide genetic screens in the brains of mouse models of Parkinson’s disease, probing for genes that allow brain cells to survive the effects of cellular perturbations associated with Parkinson’s disease. I’m extremely grateful for this generous support and recognition of our work from the Mathers Foundation, and hope that our study will elucidate new therapeutic targets for the treatment and even prevention of Parkinson’s disease.”

Sequencing inside cells

by Jennifer Michalowski |

By bringing DNA sequencing out of the sequencer and directly to cells, MIT scientists have revealed an entirely new view of the genome. With a new method for in situ genome sequencing reported December 31, 2020, in the journal Science, researchers can, for the first time, see exactly how DNA sequences are organized and packed inside cells.

The approach, whose development was led by Ed Boyden, the Y. Eva Tan Professor in Neurotechnology at MIT, and Harvard University Stem Cell and Regenerative Biology faculty members Jason Buenrostro and Fei Chen, integrates DNA sequencing technology with microscopy to pinpoint exactly where specific DNA sequences are located inside intact cells.

While alternative methods allow scientists to reconstruct structural information about the genome, this is the first sequencing technology to give scientists a direct look.

The technology creates new opportunities to investigate a broad range of biology, from fundamental questions about how DNA’s three-dimensional organization affects its function to the structural changes and chromosomal rearrangements associated with aging, cancer, brain disorders, and other diseases.

Seeing is believing

“How structure yields function is one of the core themes of biology,” says Boyden, who is also an investigator at the McGovern Institute and the Howard Hughes Medical Institute.“And the history of biology tells us that when you can actually see something, you can make lots of advances.” Seeing how an organism’s genome is packed inside its cells could help explain how different cell types in the brain interpret the genetic code, or reveal structural patterns that mean the difference between health and disease, he says. Additionally, the researchers note, the technique also makes it possible to directly see how proteins and other factors interact with specific parts of the genome.

The new method builds on work underway in Boyden and Chen’s laboratories focused on sequencing RNA inside cells. Buenrostro collaborated with Boyden and Chen, who is also a core member of the Broad Institute, to adapt the technique for use with DNA. “It was clear the technology they had developed would be an extraordinary opportunity to have a new perspective on cells’ genomes,” Boyden says.

Their approach begins by fixing cells onto a glass surface to preserve their structure. Then, after inserting small DNA adapters into the genome, thousands of short segments of DNA—about 20 letters of code apiece—are amplified and sequenced in their original locations inside the cells. Finally, the samples are ground up and put into a sequencer, which sequences all of the cells’ DNA about 300 letters at a time. By finding the location-identified short sequences within those longer segments, the method pinpoints each one’s position within the three-dimensional structure of the cell.

Sequencing inside the cells is done more or less the same way DNA is sequenced inside a standard next-generation sequencer, Boyden explains, by watching under a microscope as a DNA strand is copied using fluorescently labeled building blocks. As in a traditional sequencer, each of DNA’s four building blocks, or nucleotides, is tagged with a different color so that they can be visually identified as they are added to a growing DNA strand.

A collaborative effort

Boyden, Buenrostro, and Chen, who began their collaboration several years ago, say the new technology represents a heroic effort on the part of MIT and Harvard graduate students Andrew Payne, Zachary Chiang, and Paul Reginato, who took the lead in developing and integrating its many technical steps and computational analyses. That involved both recapitulating the methods used in commercial sequencers and introducing several key innovations. “Some advances on the technology side have taken this from impossible to do to being possible,” Chen says.

The team has already used the method to visualize a genome as it reorganizes itself during the earliest moments of life. Brightly colored representations of DNA that they sequenced inside a mouse embryo show how genetic information inherited from each parent remains distinct and compartmentalized immediately after fertilization, then gradually intertwines as development progresses. Their sequencing also reveals how patterns of genome organization, which very early in life vary from cell to cell, are passed on as cells divide, generating a memory of each cell’s developmental origins. Being able to watch these processes unfold across entire cells instead of piecing them together through less direct means offered a dramatic new view of development, the researchers say.

While the team continues to improve the spatial resolution of the technique and adapt it to a broader range of cell types, they have made their method and associated software freely available to other labs. The researchers hope this new approach to DNA sequencing will change the way people think about studying the structure of the genome and will help illuminate patterns and consequences of genome organization across a variety of contexts.

Powered by viruses

by Jennifer Michalowski |

View the interactive version of this story in our Winter 2021 issue of Brain Scan.

Viruses are notoriously adept invaders. The efficiency with which these unseen threats infiltrate tissues, evade immune systems, and occupy the cells of their hosts can be alarming — but it’s exactly why most McGovern neuroscientists keep a stash of viruses in the freezer.

In the hands of neuroscientists, viruses become vital tools for delivering cargo to cells.

With a bit of genetic manipulation, they can instruct neurons to produce proteins that illuminate complex circuitry, report on activity, or place certain cells under scientists’ control. They can even deliver therapies designed to correct genetic defects in patients.

“We rely on the virus to deliver whatever we want,” says McGovern Investigator Guoping Feng. “This is one of the most important technologies in neuroscience.”

Tracing connections

In Ian Wickersham’s lab, researchers are adapting a virus that, in its natural form, is devastating to the mammalian nervous system. Once it gains access to a neuron, the rabies virus spreads to connected cells, killing them within weeks. “That makes it a very dangerous pathogen, but also a very powerful tool for neuroscience,” says Wickersham, a Principal Research Scientist at the Institute.

Taking advantage of its pernicious spread, neuroscientists use a modified version of the rabies virus to introduce a fluorescent protein to infected cells and visualize their connections (above). As a graduate student in Edward Callaway’s lab at the Salk Institute for Biological Studies, Wickersham figured out how to limit the virus’s passage through the nervous system, allowing it to access cells that are directly connected to the neuron it initially infects, but go no further. Rabies virus travels across synapses in the opposite direction of neuronal signals, so researchers can deliver it to a single cell or set of cells, then see exactly where those cells’ inputs are coming from.

Labs around the world use Wickersham’s modified rabies virus to trace neuronal anatomy in the brains of mice. While his team tinkers to make the virus even more powerful, his collaborators have deployed it to map a variety of essential connections, offering clues into how the brain controls movement, detects odors, and retrieves memories.

With the newest tracing tool from the Wickersham lab, moving from anatomical studies to experiments that reveal circuit function is seamless, because the lab has further modified their virus so that it cannot kill cells. Researchers can label connected cells, then proceed to monitor their signals or manipulate their activity in the same animals.

Researchers usually conduct these experiments in genetically modified mice to control the subset of cells that activate the tracing system. It’s the same approach used to restrict most virally-delivered tools to specific neurons, which is crucial, Feng says. When introducing a fluorescent protein for imaging, for example, “we don’t want the gene we deliver to be activated everywhere, otherwise the whole brain will be lighting up,” he says.

Selective targets

In Feng’s lab, research scientist Martin Wienisch is working to make it easier to control this aspect of delivery. Rather than relying on the genetic makeup of an entire animal to determine where a virally-transported gene is switched on, instructions can be programmed directly into the virus, borrowing regulatory sequences that cells already know how to interpret.

Wienisch is scouring the genomes of individual neurons to identify short segments of regulatory DNA called enhancers. He’s focused on those that selectively activate gene expression in just one of hundreds of different neuron types, particularly in animal models that are not very amenable to genetic engineering. “In the real brain, many elements interact to drive cell specific expression. But amazingly sometimes a single enhancer is all we need to get the same effect,” he says.

Researchers are already using enhancers to confine viral tools to select groups of cells, but Wienisch, who is collaborating with Fenna Krienen in Steve McCarroll’s lab at Harvard University, aims to create a comprehensive library. The enhancers they identify will be paired with a variety of genetically-encoded tools and packaged into adeno-associated viruses (AAV), the most widely used vectors in neuroscience. The Feng lab plans to use these tools to better understand the striatum, a part of the primate brain involved in motivation and behavioral choices. “Ideally, we would have a set of AAVs with enhancers that would give us selective access to all the different cell types in the striatum,” Wienisch says.

Enhancers will also be useful for delivering potential gene therapies to patients, Wienisch says. For many years, the Feng lab has been studying how a missing copy of a gene called Shank3 impairs neurons’ ability to communicate, leading to autism and intellectual disability. Now, they are investigating whether they can overcome these deficits by delivering a functional copy of Shank3 to the brain cells that need it. Widespread activation of the therapeutic gene might do more harm than good, but incorporating the right enhancer could ensure it is delivered to the appropriate cells at the right dose, Wienisch says.

Like most gene therapies in development, the therapeutic Shank3, which is currently being tested in animal models, is packaged into an AAV. AAVs safely and efficiently infect human cells, and by selecting the right type, therapies can be directed to specific cells. But AAVs are small viruses, capable of carrying only small genes. Xian Gao, a postdoctoral researcher in the Feng lab, has pared Shank3 down to its most essential components, creating a “minigene” that can be packaged inside the virus, but some things are difficult to fit inside an AAV. Therapies that aim to correct mutations using the CRISPR gene editing system, for example, often exceed the carrying capacity of an AAV.

Expanding options

“There’s been a lot of really phenomenal advances in our gene editing toolkit,” says Victoria Madigan, a postdoctoral researcher in McGovern Investigator Feng Zhang’s lab, where researchers are developing enzymes to more precisely modify DNA. “One of the main limitations of employing these enzymes clinically has been their delivery.”

To open up new options for gene therapy, Zhang and Madigan are working with a group of viruses called densoviruses. Densoviruses and AAVs belong to the same family, but about 50 percent more DNA can be packed inside the outer shell of some densoviruses.

A molecular model of Galleria mellonella densovirus. Image: Victoria Madigan / Zhang Lab

They are an esoteric group of viruses, Madigan says, infecting only insects and crustaceans and perhaps best known for certain members’ ability to devastate shrimp farms. While densoviruses haven’t received a lot of attention from scientists, their similarities to AAV have given the team clues about how to alter their outer capsids to enable them to enter human cells, and even direct them to particular cell types. The fact that they don’t naturally infect people also makes densoviruses promising candidates for clinical use, Madigan says, because patients’ immune systems are unlikely to be primed to reject them. AAV infections, in contrast, are so common that patients are often excluded from clinical trials for AAV-based therapies due to the presence of neutralizing antibodies against the vector.

Ultimately, densoviruses could enable major advances in gene therapy, making it possible to safely deliver sophisticated gene editing systems to patients’ cells, Madigan says — and that’s good reason for scientists to continue exploring the vast diversity in the viral world. “There’s something to be said for looking into viruses that are understudied as new tools,” she says. “There’s a lot of interesting stuff out there — a lot of diversity and thousands of years of evolution.”

New clues to brain changes in Huntington’s disease

by Jill Crittenden |

Huntington’s disease is a fatal inherited disorder that strikes most often in middle age with mood disturbances, uncontrollable limb movements, and cognitive decline. Years before symptom onset, brain imaging shows degeneration of the striatum, a brain region important for the rapid selection of behavioral actions. As the striatal neurons degenerate, their “identity” proteins, the building blocks that give particular cell types their unique function, are gradually turned off.

A new study from the lab of Institute Professor Ann Graybiel has found a surprising exception to this rule. The researchers discovered that in mouse models of Huntington’s disease, the cell identity protein MOR1, named as the Mu type Opioid Receptor, actually becomes more abundant as the striatal neurons degenerate.

“This is one of the most striking immunohistochemical change that I have ever seen in the literature of Huntington’s disease model animals,” says Ryoma Morigaki, a research scientist in the Graybiel laboratory and lead author of the report, who worked with Tomoko Yoshida and others in the Graybiel lab.

Immunohistochemical stainings using anti-mu-opioid receptor antibody. Wild type mouse striatum (left) and Q175 Huntington’s disease model mouse striatum (right) at 19 months old. Image: Ryoma Morigaki

More opioid receptors

MOR1 is a receptor on the surface of neurons that binds to opioids that are produced by the body or those taken for pain relief, such as morphine. The natural opioid in the brain is a small molecule called enkephalin, and it is normally produced by the same striatal neurons that degenerate in the earliest stages of Huntington’s disease.

The research team speculates that the striatum increases the quantity of MOR1 receptors in Huntington’s disease models to compensate for plummeting levels of enkephalin, but they also believe this upregulation may play a role in the perception of reward.

Previous work suggests that MOR1 has distinct signaling mechanisms related to its function in pain perception and its function in drug-seeking. These distinct mechanisms might be related to the fact that MOR1 is produced as multiple “isoforms,” slight variations of a protein that can be read out from the same gene. The MOR1 isoform that is found in the striatum is thought to be more important for drug-seeking behaviors than for pain perception. This in turn means that MOR1 might play a role in a key striatal function, which is to learn what actions are most likely to lead to reward.

“It is now recognized that mood disturbances can pre-date the overt motor abnormalities of Huntington’s patients by many years. These can even be the most disturbing symptoms for patients and their families. The finding that this receptor for opioids becomes so elevated in mood-related sites of the striatum, at least in a mouse model of the disorder, may give a hint to the underlying circuit dysfunction leading to these problems,” says Ann Graybiel.

Clues for treatment

MOR1 is used as a standard to identify subsets of neurons that are located within small clusters of neurons in the striatum that were previously discovered by Ann Graybiel and named striosomes.

“The most exciting point for me is the involvement of striatal compartments [striosomes] in the pathogenesis of Huntington’s disease,” says Morigaki, who has now moved to the University of Fukoshima in Japan and is a practicing neurosurgeon who treats movement disorders.

MOR1-positive striosomal neurons are of high interest in part because they have direct connections to the same dopamine-producing neurons that are thought to degenerate in Parkinson’s disease. Whereas Parkinson’s disease is characterized by a loss of dopamine and loss of movement, Huntington’s disease is characterized by ups and downs in dopamine and excessive movements. In fact, the only drugs that are FDA-approved to treat Huntington’s disease are drugs that minimize dopamine release, thereby working to dampen the abnormal movements. But these treatments come with potentially severe side-effects such as depression and suicide.

This latest discovery might provide mechanistic clues to dopamine fluctuations in Huntington’s disease and provide avenues for more specific treatments.

This research was funded by the CHDI Foundation (A-5552), Broderick Fund for Phytocannabinoid Research at MIT, NIH/NIMH R01 MH060379, the Saks Kavanaugh Foundation, JSPS KAKENHI Grants #16KK0182, 17K10899 and 20K17932 , Dr. Tenley Albright, Kathleen Huber, and Dr. Stephan and Mrs. Anne Kott.

Storytelling brings MIT neuroscience community together

by Julie Pryor |

When the coronavirus pandemic shut down offices, labs, and classrooms across the MIT campus last spring, many members of the MIT community found it challenging to remain connected to one another in meaningful ways. Motivated by a desire to bring the neuroscience community back together, the McGovern Institute hosted a virtual storytelling competition featuring a selection of postdocs, grad students, and staff from across the institute.

“This has been an unprecedented year for us all,” says McGovern Institute Director Robert Desimone. “It has been twenty years since Pat and Lore McGovern founded the McGovern Institute, and despite the challenges this anniversary year has brought to our community, I have been inspired by the strength and perseverance demonstrated by our faculty, postdocs, students and staff. The resilience of this neuroscience community – and MIT as a whole – is indeed something to celebrate.”

The McGovern Institute had initially planned to hold a large 20th anniversary celebration in the atrium of Building 46 in the fall of 2020, but the pandemic made a gathering of this size impossible. The institute instead held a series of virtual events, including the November 12 story slam on the theme of resilience.

A large-scale tool to investigate the function of autism spectrum disorder genes

by Harvard University |

Scientists at Harvard University, the Broad Institute of MIT and Harvard, and MIT have developed a technology to investigate the function of many different genes in many different cell types at once, in a living organism. They applied the large-scale method to study dozens of genes that are associated with autism spectrum disorder, identifying how specific cell types in the developing mouse brain are impacted by mutations.

The “Perturb-Seq” method, published in the journal Science, is an efficient way to identify potential biological mechanisms underlying autism spectrum disorder, which is an important first step toward developing treatments for the complex disease. The method is also broadly applicable to other organs, enabling scientists to better understand a wide range of disease and normal processes.

“For many years, genetic studies have identified a multitude of risk genes that are associated with the development of autism spectrum disorder. The challenge in the field has been to make the connection between knowing what the genes are, to understanding how the genes actually affect cells and ultimately behavior,” said co-senior author Paola Arlotta, the Golub Family Professor of Stem Cell and Regenerative Biology at Harvard. “We applied the Perturb-Seq technology to an intact developing organism for the first time, showing the potential of measuring gene function at scale to better understand a complex disorder.”

The study was also led by co-senior authors Aviv Regev, who was a core member of the Broad Institute during the study and is currently Executive Vice President of Genentech Research and Early Development, and Feng Zhang, a core member of the Broad Institute and an investigator at MIT’s McGovern Institute.

To investigate gene function at a large scale, the researchers combined two powerful genomic technologies. They used CRISPR-Cas9 genome editing to make precise changes, or perturbations, in 35 different genes linked to autism spectrum disorder risk. Then, they analyzed changes in the developing mouse brain using single-cell RNA sequencing, which allowed them to see how gene expression changed in over 40,000 individual cells.

By looking at the level of individual cells, the researchers could compare how the risk genes affected different cell types in the cortex — the part of the brain responsible for complex functions including cognition and sensation. They analyzed networks of risk genes together to find common effects.

“We found that both neurons and glia — the non-neuronal cells in the brain — are directly affected by different sets of these risk genes,” said Xin Jin, lead author of the study and a Junior Fellow of the Harvard Society of Fellows. “Genes and molecules don’t generate cognition per se — they need to impact specific cell types in the brain to do so. We are interested in understanding how these different cell types can contribute to the disorder.”

To get a sense of the model’s potential relevance to the disorder in humans, the researchers compared their results to data from post-mortem human brains. In general, they found that in the post-mortem human brains with autism spectrum disorder, some of the key genes with altered expression were also affected in the Perturb-seq data.

“We now have a really rich dataset that allows us to draw insights, and we’re still learning a lot about it every day,” Jin said. “As we move forward with studying disease mechanisms in more depth, we can focus on the cell types that may be really important.”

“The field has been limited by the sheer time and effort that it takes to make one model at a time to test the function of single genes. Now, we have shown the potential of studying gene function in a developing organism in a scalable way, which is an exciting first step to understanding the mechanisms that lead to autism spectrum disorder and other complex psychiatric conditions, and to eventually develop treatments for these devastating conditions,” said Arlotta, who is also an institute member of the Broad Institute and part of the Broad’s Stanley Center for Psychiatric Research. “Our work also paves the way for Perturb-Seq to be applied to organs beyond the brain, to enable scientists to better understand the development or function of different tissue types, as well as pathological conditions.”

“Through genome sequencing efforts, a very large number of genes have been identified that, when mutated, are associated with human diseases. Traditionally, understanding the role of these genes would involve in-depth studies of each gene individually. By developing Perturb-seq for in vivo applications, we can start to screen all of these genes in animal models in a much more efficient manner, enabling us to understand mechanistically how mutations in these genes can lead to disease,” said Zhang, who is also the James and Patricia Poitras Professor of Neuroscience at MIT and a professor of brain and cognitive sciences and biological engineering at MIT.

This study was funded by the Stanley Center for Psychiatric Research at the Broad Institute, the National Institutes of Health, the Brain and Behavior Research Foundation’s NARSAD Young Investigator Grant, Harvard University’s William F. Milton Fund, the Klarman Cell Observatory, the Howard Hughes Medical Institute, a Center for Cell Circuits grant from the National Human Genome Research Institute’s Centers of Excellence in Genomic Science, the New York Stem Cell Foundation, the Mathers Foundation, the Poitras Center for Psychiatric Disorders Research at MIT, the Hock E. Tan and K. Lisa Yang Center for Autism Research at MIT, and J. and P. Poitras.

Controlling drug activity with light

by Jill Crittenden |

Hormones and nutrients bind to receptors on cell surfaces by a lock-and-key mechanism that triggers intracellular events linked to that specific receptor. Drugs that mimic natural molecules are widely used to control these intracellular signaling mechanisms for therapy and in research.

In a new publication, a team led by McGovern Institute Associate Investigator Polina Anikeeva and Oregon Health & Science University Research Assistant Professor James Frank introduce a microfiber technology to deliver and activate a drug that can be induced to bind its receptor by exposure to light.

“A significant barrier in applying light-controllable drugs to modulate neural circuits in living animals is the lack of hardware which enables simultaneous delivery of both light and drugs to the target brain area,” says Frank, who was previously a postdoctoral associate in Anikeeva’s Bioelectronics group at MIT. “Our work offers an integrated approach for on-demand delivery of light and drugs through a single fiber.”

These devices were used to deliver a “photoswitchable” drug deep into the brain. So-called “photoswitches” are light-sensitive molecules that can be attached to drugs to switch their activity on or off with a flash of light ­– the use of these drugs is called photopharmacology. In the new study, photopharmacology is used to control neuronal activity and behavior in mice.

Creating miniaturized devices from macroscale templates

The lightweight device features two microfluidic channel and an optical waveguide, and can easily be carried by the animal during behavior

To use light to control drug activity, light and drugs must be delivered simultaneously to the targeted cells. This is a major challenge when the target is deep in the body, but Anikeeva’s Bioelectronics group is uniquely equipped to deal with this challenge.  Marc-Joseph (MJ) Antonini, a PhD student in Anikeeva’s Bioelectronics lab and co-first author of the study, specializes in the fabrication of biocompatible multifunctional fibers that house microfluidic channels and waveguides to deliver liquids and transmit light.

The multifunctional fibers used in this study contain a fluidic channel and an optical waveguide and are comprised of many layers of different materials that are fused together to provide flexibility and strength. The original form of the fiber is constructed at a macroscale and then heated and pulled (a process called thermal drawing) to become longer, but nearly 70X smaller in diameter. By this method, 100’s of meters of miniaturized fiber can be created from the original template at a cross-sectional scale of micrometers that minimizes tissue damage.

The device used in this study had an implantable fiber bundle of 480µm × 380µm and weighed only 0.8 g, small enough that a mouse can easily carry it on its head for many weeks.

Synthesis of a new photoswitchable drug

To demonstrate effectiveness of their device for simultaneous delivery of liquids and light, the Anikeeva lab teamed up with Dirk Trauner (Frank’s former PhD advisor) and David Konrad,  pharmacologists who synthesized photoswitchable drugs.

They had previously modified a photoswitchable analog of capsaicin, a molecule found in hot peppers that binds to the TRPV1 receptor on sensory neurons and controls the sensation of heat. This modification allowed the capsaicin analog to be activated by 560 nm wave-length of light (visible green) that is not damaging to tissue compared to the original version of the drug that required ultraviolet light. By adding both the TRPV1 receptor and the new photoswitchable capsaicin analog to neurons, they could be artificially activated with green light.

This new photopharmacology system had been shown by Frank, Konrad and their colleagues to work in cells cultured in a dish, but had never been shown to work in freely-moving animals.

Controlling behavior by photopharmacology

To test whether their system could activate neurons in the brain, Frank and Antonini tested it in mice. They asked whether adding the photoswitchable drug and its receptor to reward-mediating neurons in the mouse brain causes mice to prefer a chamber in which they receive light stimulation.

The multifunctional fiber-inspired neural implant was implanted into a phantom brain (left), and successfully delivered light and a blue dye (right).

The miniaturized multifunctional fiber developed by the team was implanted in the mouse brain’s ventral tegmental area, a deep region rich in dopamine neurons that controls reward-seeking behavior. Through the fluidic channel in the device, the researchers delivered a virus that drives expression of the TRPV1 receptor in the neurons under study.  Several weeks later, the device was then used to deliver both light and the photoswitchable capsaicin analog directly to the same neurons. To control for the specificity of their system, they also tested the effects of delivering a virus that does not express the TRPV1 receptor, and the effects of delivering a wavelength of light that does not switch on the drug.

They found that mice showed a preference only for the chamber where they had previously received all three components required for the photopharmacology to function: the receptor-expressing virus, the photoswitchable receptor ligand and the green light that activates the drug. These results demonstrate the efficacy of this system to control the time and place within the body that a drug is active.

“Using these fibers to enable photopharmacology in vivo is a great example of how our multifunctional platform can be leveraged to improve and expand how we can interact with the brain,” says Antonini. “This combination of technologies allows us to achieve the temporal and spatial resolution of light stimulation with the chemical specificity of drug injection in freely moving animals.”

Therapeutic drugs that are taken orally or by injection often cause unwanted side-effects because they act continuously and throughout the whole body. Many unwanted side effects could be eliminated by targeting a drug to a specific body tissue and activating it only as needed. The new technology described by Anikeeva and colleagues is one step toward this ultimate goal.

“Our next goal is to use these neural implants to deliver other photoswitchable drugs to target receptors which are naturally expressed within these circuits,” says Frank, whose new lab in the Vollum Institute at OHSU is synthesizing new light-controllable molecules. “The hardware presented in this study will be widely applicable for controlling circuits throughout the brain, enabling neuroscientists to manipulate them with enhanced precision.”

Identifying the structure and function of a brain hub

by Jill Crittenden |

Our ability to pay attention, plan, and trouble-shoot involve cognitive processing by the brain’s prefrontal cortex. The balance of activity among excitatory and inhibitory neurons in the cortex, based on local neural circuits and distant inputs, is key to these cognitive functions.

A recent study from the McGovern Institute shows that excitatory inputs from the thalamus activate a local inhibitory circuit in the prefrontal cortex, revealing new insights into how these cognitive circuits may be controlled.

“For the field, systematic identification of these circuits is crucial in understanding behavioral flexibility and interpreting psychiatric disorders in terms of dysfunction of specific microcircuits,” says postdoctoral associate Arghya Mukherjee, lead author on the report.

Hub of activity

The thalamus is located in the center of the brain and is considered a cerebral hub based on its inputs from a diverse array of brain regions and outputs to the striatum, hippocampus, and cerebral cortex. More than 60 thalamic nuclei (cellular regions) have been defined and are broadly divided into “sensory” or “higher-order” thalamic regions based on whether they relay primary sensory inputs or instead have inputs exclusively from the cerebrum.

Considering the fundamental distinction between the input connections of the sensory and higher-order thalamus, Mukherjee, a researcher in the lab of Michael Halassa, the Class of 1958 Career Development Professor in MIT’s Department of Brain and Cognitive Sciences, decided to explore whether there are similarly profound distinctions in their outputs to the cerebral cortex.

He addressed this question in mice by directly comparing the outputs of the medial geniculate body (MGB), a sensory thalamic region, and the mediodorsal thalamus (MD), a higher-order thalamic region. The researchers selected these two regions because the relatively accessible MGB nucleus relays auditory signals to cerebral cortical regions that process sound, and the MD interconnects regions of the prefrontal cortex.

Their study, now available as a preprint in eLife, describes key functional and anatomical differences between these two thalamic circuits. These findings build on Halassa’s previous work showing that outputs from higher-order thalamic nuclei play a central role in cognitive processing.

A side by side comparison of the two microcircuits: (Left) MD receives its primary inputs (black) from the frontal cortex and sends back inhibition dominant outputs to multiple layers of the prefrontal cortex. (Right) MGB receives its primary input (black) from the auditory midbrain and acts as a ‘relay’ by sending excitation dominant outputs specifically to layer 4 of the auditory cortex. Image: Arghya Mukherjee

Circuit analysis

Using cutting-edge stimulation and recording methods, the researchers found that neurons in the prefrontal and auditory cortices have dramatically different responses to activation of their respective MD and MGB inputs.

The researchers stimulated the MD-prefrontal and MGB-auditory cortex circuits using optogenetic technology and recorded the response to this stimulation with custom multi-electrode scaffolds that hold independently movable micro-drives for recording hundreds of neurons in the cortex. When MGB neurons were stimulated with light, there was strong activation of neurons in the auditory cortex. By contrast, MD stimulation caused a suppression of neuron firing in the prefrontal cortex and concurrent activation of local inhibitory interneurons. The separate activation of the two thalamocortical circuits had dramatically different impacts on cortical output, with the sensory thalamus seeming to promote feed-forward activity and the higher-order thalamus stimulating inhibitory microcircuits within the cortical target region.

“The textbook view of the thalamus is an excitatory cortical input, and the fact that turning on a thalamic circuit leads to a net cortical inhibition was quite striking and not something you would have expected based on reading the literature,” says Halassa, who is also an associate investigator at the McGovern Institute. “Arghya and his colleagues did an amazing job following that up with detailed anatomy to explain why might this effect be so.”

Anatomical differences

Using a system called GFP (green fluorescent protein) reconstitution across synaptic partners (mGRASP), the researchers demonstrated that MD and MGB projections target different types of cortical neurons, offering a possible explanation for their differing effects on cortical activity.

With mGRASP, the presynaptic terminal (in this case, MD or MGB) expresses one part of the fluorescent protein and the postsynaptic neuron (in this case, prefrontal or auditory cortex) expresses the other part of the fluorescent protein, which by themselves alone do not fluoresce. Only when there is a close synaptic connection do the two parts of GFP come together to become fluorescent. These experiments showed that MD neurons synapse more frequently onto inhibitory interneurons in the prefrontal cortex whereas MGB neurons synapse onto excitatory neurons with larger synapses, consistent with only MGB being a strong activity driver.

Using fluorescent viral vectors that can cross synapses of interconnected neurons, a technology developed by McGovern principal research scientist Ian Wickersham, the researchers were also able to map the inputs to the MD and MGB thalamic regions. Viruses, like rabies, are well-suited for tracing neural connections because they have evolved to spread from neuron to neuron through synaptic junctions.

The inputs to the targeted higher-order and sensory thalamocortical neurons identified across the brain appeared to arise respectively from forebrain and midbrain sensory regions, as expected. The MGB inputs were consistent with a sensory relay function, arising primarily from the auditory input pathway. By contrast, MD inputs arose from a wide array of cerebral cortical regions and basal ganglia circuits, consistent with MD receiving contextual and motor command information.

Direct comparisons

By directly comparing these microcircuits, the Halassa lab has revealed important clues about the function and anatomy of these sensory and higher-order brain connections. It is only through a systematic understanding of these circuits that we can begin to interpret how their dysfunction may contribute to psychiatric disorders like schizophrenia.

It is this basic scientific inquiry that often fuels their research, says Halassa. “Excitement about science is part of the glue that holds us all together.”

Robert Desimone to receive the Fred Kavli Distinguished Career Contributions Award

by Julie Pryor |

Robert Desimone, the Doris and Don Berkey Professor in Brain and Cognitive Sciences at MIT, has been recognized by the Cognitive Neuroscience Society as this year’s winner of the Fred Kavli Distinguished Career Contributions (DCC) award. Supported annually by the Kavli Foundation, the award honors senior cognitive neuroscientists for their distinguished career, leadership and mentoring in the field of cognitive neuroscience.

Desimone, who is also the director of the McGovern Institute for Brain Research, studies the brain mechanisms underlying attention, and most recently, has been studying animal models for brain disorders.

Desimone will deliver his prize lecture at the annual meeting of the Cognitive Neuroscience Society in March 2021.