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New Technology Opens Evolutionary Window into Brain Development

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DNA runs across the top and bottom. Skulls show the evolution of man from left to right

One of the great mysteries in biology is how we humans ended up with such large, complex brains. In search of clues, researchers have spent years studying the protein-coding genes activated during neurodevelopment. But some answers may also be hiding in non-coding regions of the human genome, where sequences called regulatory elements increase or decrease the activity of genes.


A fascinating example involves a type of regulatory element called a human accelerated region (HAR). Although “human” is part of this element’s name, it turns out that the genomes of all vertebrates—not just humans—contain the DNA segments now designated as HARs.

In most organisms, HARs show a relatively low rate of mutation, which means these regulatory elements have been highly conserved across species throughout evolutionary time [1]. The big exception is Homo sapiens, in which HARs have exhibited a much higher rate of mutations.

The accelerated rate of HARs mutations observed in humans suggest that, over the course of very long periods of time, these genomic changes might have provided our species with some sort of evolutionary advantage. What might that be? Many have speculated the advantage might involve the brain because HARs are often associated with genes involved in neurodevelopment. Now, in a paper published in the journal Neuron, an NIH-supported team confirms that’s indeed the case [2].

In the new work, researchers found that about half of the HARs in the human genome influence the activity, or expression, of protein-coding genes in neural cells and tissues during the brain’s development [3]. The researchers say their study—the most comprehensive to date of the 3,171 HARs in the human genome—firmly establishes that this type of regulatory element helps to drive patterns of neurodevelopmental gene activity specific to humans.

Yet to be determined is precisely how HARs affect the development of the human brain. The quest to uncover these details will no doubt shed new light on fundamental questions about the brain, its billions of neurons, and their trillions of interconnections. For example, why does human neural development span decades, longer than the life spans of most primates and other mammals? Answering such questions could also reveal new clues into a range of cognitive and behavioral disorders. In fact, early research has already made tentative links between HARs and neurodevelopmental conditions such as autism spectrum disorder and schizophrenia [3].

The latest work was led by Kelly Girskis, Andrew Stergachis, and Ellen DeGennaro, all of whom were in the lab of Christopher Walsh while working on the project. An NIH grantee, Walsh is director of the Allen Discovery Center for Brain Evolution at Boston Children’s Hospital and Harvard Medical School, which is supported by the Paul G. Allen Foundation Frontiers Group, and is an Investigator of the Howard Hughes Medical Institute.

Though HARs have been studied since 2006, one of the big challenges in systematically assessing them has been technological. The average length of a HAR is about 269 bases of DNA, but current technologies for assessing function can only easily analyze DNA molecules that span 150 bases or less.

Ryan Doan, who was then in the Walsh Lab, and his colleagues solved the problem by creating a new machine called CaptureMPRA. (MPRA is short for “massively parallel reporter assays.”) This technological advance cleverly barcodes HARs and, more importantly, makes it possible to analyze HARs up to about 500 bases in length.

Using CaptureMPRA technology in tandem with cell culture studies, researchers rolled up their sleeves and conducted comprehensive, full-sequence analyses of more than 3,000 HARs. In their initial studies, primarily in neural cells, they found nearly half of human HARs are active to drive gene expression in cell culture. Of those, 42 percent proved to have increased ability to enhance gene expression compared to their orthologues, or counterparts, in chimpanzees.

Next, the team integrated these data with an existing epigenetic dataset derived from developing human brain cells, as well as additional datasets generated from sorted brain cell types. They found that many HARs appeared to have the ability to increase the activity of protein-coding genes, while a smaller—but very significant—subset of the HARs appeared to be enhancing gene expression specifically in neural progenitor cells, which are responsible for making various neural cell types.

The data suggest that as the human HAR sequences mutated and diverged from other mammals, they increased their ability to enhance or sometimes suppress the activity of certain genes in neural cells. To illustrate this point, the researchers focused on two HARs that appear to interact specifically with a gene referred to as R17. This gene can have highly variable gene expression patterns not only in different human cell types, but also in cells from other vertebrates and non-vertebrates.

In the human cerebral cortex, the outermost part of the brain that’s responsible for complex behaviors, R17 is expressed only in neural progenitor cells and only at specific time points. The researchers found that R17 slows the progression of neural progenitor cells through the cell cycle. That might seem strange, given the billions of neurons that need to be made in the cortex. But it’s consistent with the biology. In the human, it takes more than 130 days for the cortex to complete development, compared to about seven days in the mouse.

Clearly, to learn more about how the human brain evolved, researchers will need to look for clues in many parts of the genome at once, including its non-coding regions. To help researchers navigate this challenging terrain, the Walsh team has created an online resource displaying their comprehensive HAR data. It will appear soon, under the name HAR Hub, on the University of California Santa Cruz Genome Browser.

References:

[1] An RNA gene expressed during cortical development evolved rapidly in humans. Pollard KS, Salama SR, Lambert N, Lambot MA, Coppens S, Pedersen JS, Katzman S, King B, Onodera C, Siepel A, Kern AD, Dehay C, Igel H, Ares M Jr, Vanderhaeghen P, Haussler D. Nature. 2006 Sep 14;443(7108):167-72.

[2] Rewiring of human neurodevelopmental gene regulatory programs by human accelerated regions. Girskis KM, Stergachis AB, DeGennaro EM, Doan RN, Qian X, Johnson MB, Wang PP, Sejourne GM, Nagy MA, Pollina EA, Sousa AMM, Shin T, Kenny CJ, Scotellaro JL, Debo BM, Gonzalez DM, Rento LM, Yeh RC, Song JHT, Beaudin M, Fan J, Kharchenko PV, Sestan N, Greenberg ME, Walsh CA. Neuron. 2021 Aug 25:S0896-6273(21)00580-8.

[3] Mutations in human accelerated regions disrupt cognition and social behavior. Doan RN, Bae BI, Cubelos B, Chang C, Hossain AA, Al-Saad S, Mukaddes NM, Oner O, Al-Saffar M, Balkhy S, Gascon GG; Homozygosity Mapping Consortium for Autism, Nieto M, Walsh CA. Cell. 2016 Oct 6;167(2):341-354.

Links:

Christopher Walsh Laboratory (Boston Children’s Hospital and Harvard Medical School)

The Paul G. Allen Foundation Frontiers Group (Seattle)

NIH Support: National Institute of Neurological Disorders and Stroke; National Institute of Mental Health; National Institute of General Medical Sciences; National Cancer Institute


First Comprehensive Census of Cell Types in Brain Area Controlling Movement

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Credit: SciePro/Shutterstock; BRAIN Initiative Cell Census Network, Nature, 2021

The primary motor cortex is the part of the brain that enables most of our skilled movements, whether it’s walking, texting on our phones, strumming a guitar, or even spiking a volleyball. The region remains a major research focus, and that’s why NIH’s Brain Research Through Advancing Innovative Neurotechnologies® (BRAIN) Initiative – Cell Census Network (BICCN) has just unveiled two groundbreaking resources: a complete census of cell types present in the mammalian primary motor cortex, along with the first detailed atlas of the region, located along the back of the frontal lobe in humans (purple stripe above).

This remarkably comprehensive work, detailed in a flagship paper and more than a dozen associated articles published in the journal Nature, promises to vastly expand our understanding of the primary motor cortex and how it works to keep us moving [1]. The papers also represent the collaborative efforts of more than 250 BICCN scientists from around the world, teaming up over many years.

Started in 2013, the BRAIN Initiative is an ambitious project with a range of groundbreaking goals, including the creation of an open-access reference atlas that catalogues all of the brain’s many billions of cells. The primary motor cortex was one of the best places to get started on assembling an atlas because it is known to be well conserved across mammalian species, from mouse to human. There’s also a rich body of work to aid understanding of more precise cell-type information.

Taking advantage of recent technological advances in single-cell analysis, the researchers categorized into different types the millions of neurons and other cells in this brain region. They did so on the basis of morphology, or shape, of the cells, as well as their locations and connections to other cells. The researchers went even further to characterize and sort cells based on: their complex patterns of gene expression, the presence or absence of chemical (or epigenetic) marks on their DNA, the way their chromosomes are packaged into chromatin, and their electrical properties.

The new data and analyses offer compelling evidence that neural cells do indeed fall into distinct types, with a high degree of correspondence across their molecular genetic, anatomical, and physiological features. These findings support the notion that neural cells can be classified into molecularly defined types that are also highly conserved or shared across mammalian species.

So, how many cell types are there? While that’s an obvious question, it doesn’t have an easy answer. The number varies depending upon the method used for sorting them. The researchers report that they have identified about 25 classes of cells, including 16 different neuronal classes and nine non-neuronal classes, each composed of multiple subtypes of cells.

These 25 classes were determined by their genetic profiles, their locations, and other characteristics. They also showed up consistently across species and using different experimental approaches, suggesting that they have important roles in the neural circuitry and function of the motor cortex in mammals.

Still, many precise features of the cells don’t fall neatly into these categories. In fact, by focusing on gene expression within single cells of the motor cortex, the researchers identified more potentially important cell subtypes, which fall into roughly 100 different clusters, or distinct groups. As scientists continue to examine this brain region and others using the latest new methods and approaches, it’s likely that the precise number of recognized cell types will continue to grow and evolve a bit.

This resource will now serve as a springboard for future research into the structure and function of the brain, both within and across species. The datasets already have been organized and made publicly available for scientists around the world.

The atlas also now provides a foundation for more in-depth study of cell types in other parts of the mammalian brain. The BICCN is already engaged in an effort to generate a brain-wide cell atlas in the mouse, and is working to expand coverage in the atlas for other parts of the human brain.

The cell census and atlas of the primary motor cortex are important scientific advances with major implications for medicine. Strokes commonly affect this region of the brain, leading to partial or complete paralysis of the opposite side of the body.

By considering how well cell census information aligns across species, scientists also can make more informed choices about the best models to use for deepening our understanding of brain disorders. Ultimately, these efforts and others underway will help to enable precise targeting of specific cell types and to treat a wide range of brain disorders that affect thinking, memory, mood, and movement.

Reference:

[1] A multimodal cell census and atlas of the mammalian primary motor cortex. BRAIN Initiative Cell Census Network (BICCN). Nature. Oct 6, 2021.

Links:

NIH Brain Research Through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)

BRAIN Initiative – Cell Census Network (BICCN) (NIH)

NIH Support: National Institute of Mental Health; National Institute of Neurological Disorders and Stroke


New Microscope Technique Provides Real-Time 3D Views

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Most of the “cool” videos shared on my blog are borne of countless hours behind a microscope. Researchers must move a biological sample through a microscope’s focus, slowly acquiring hundreds of high-res 2D snapshots, one painstaking snap at a time. Afterwards, sophisticated computer software takes this ordered “stack” of images, calculates how the object would look from different perspectives, and later displays them as 3D views of life that can be streamed as short videos.

But this video is different. It was created by what’s called a multi-angle projection imaging system. This new optical device requires just a few camera snapshots and two mirrors to image a biological sample from multiple angles at once. Because the device eliminates the time-consuming process of acquiring individual image slices, it’s up to 100 times faster than current technologies and doesn’t require computer software to construct the movie. The kicker is that the video can be displayed in real time, which isn’t possible with existing image-stacking methods.

The video here shows two human melanoma cells, rotating several times between overhead and side views. You can see large amounts of the protein PI3K (brighter orange hues indicate higher concentrations), which helps some cancer cells divide and move around. Near the cell’s perimeter are small, dynamic surface protrusions. PI3K in these “blebs” is thought to help tumor cells navigate and survive in foreign tissues as the tumor spreads to other organs, a process known as metastasis.

The new multi-angle projection imaging system optical device was described in a paper published recently in the journal Nature Methods [1]. It was created by Reto Fiolka and Kevin Dean at the University of Texas Southwestern Medical Center, Dallas.

Like most technology, this device is complicated. Rather than the microscope and camera doing all the work, as is customary, two mirrors within the microscope play a starring role. During a camera exposure, these mirrors rotate ever so slightly and warp the acquired image in such a way that successive, unique perspectives of the sample magically come into view. By changing the amount of warp, the sample appears to rotate in real-time. As such, each view shown in the video requires only one camera snapshot, instead of acquiring hundreds of slices in a conventional scheme.

The concept traces to computer science and an algorithm called the shear warp transform method. It’s used to observe 3D objects from different perspectives on a 2D computer monitor. Fiolka, Dean, and team found they could implement a similar algorithm optically for use with a microscope. What’s more, their multi-angle projection imaging system is easy-to-use, inexpensive, and can be converted for use on any camera-based microscope.

The researchers have used the device to view samples spanning a range of sizes: from mitochondria and other tiny organelles inside cells to the beating heart of a young zebrafish. And, as the video shows, it has been applied to study cancer and other human diseases.

In a neat, but also scientifically valuable twist, the new optical method can generate a virtual reality view of a sample. Any microscope user wearing the appropriately colored 3D glasses immediately sees the objects.

While virtual reality viewing of cellular life might sound like a gimmick, Fiolka and Dean believe that it will help researchers use their current microscopes to see any sample in 3D—offering the chance to find rare and potentially important biological events much faster than is possible with even the most advanced microscopes today.

Fiolka, Dean, and team are still just getting started. Because the method analyzes tissue very quickly within a single image frame, they say it will enable scientists to observe the fastest events in biology, such as the movement of calcium throughout a neuron—or even a whole bundle of neurons at once. For neuroscientists trying to understand the brain, that’s a movie they will really want to see.

Reference:

[1] Real-time multi-angle projection imaging of biological dynamics. Chang BJ, Manton JD, Sapoznik E, Pohlkamp T, Terrones TS, Welf ES, Murali VS, Roudot P, Hake K, Whitehead L, York AG, Dean KM, Fiolka R. Nat Methods. 2021 Jul;18(7):829-834.

Links:

Metastatic Cancer: When Cancer Spreads (National Cancer Institute)

Fiolka Lab (University of Texas Southwestern Medical Center, Dallas)

Dean Lab (University of Texas Southwestern)

Microscopy Innovation Lab (University of Texas Southwestern)

NIH Support: National Cancer Institute; National Institute of General Medical Sciences


The Amazing Brain: Motor Neurons of the Cervical Spine

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Today, you may have opened a jar, done an upper body workout, played a guitar or a piano, texted a friend, or maybe even jotted down a grocery list longhand. All of these “skilled” arm, wrist, and hand movements are made possible by the bundled nerves, or circuits, running through a part of the central nervous system in the neck area called the cervical spine.

This video, which combines sophisticated imaging and computation with animation, shows the density of three types of nerve cells in the mouse cervical spine. There are the V1 interneurons (red), which sit between sensory and motor neurons; motor neurons associated with controlling the movement of the bicep (blue); and motor neurons associated with controlling the tricep (green).

At 4 seconds, the 3D animation morphs to show all the colors and cells intermixed as they are naturally in the cervical spine. At 8 seconds, the animation highlights the density of these three cells types. Notice in the bottom left corner, a light icon appears indicating the different imaging perspectives. What’s unique here is the frontal, or rostral, view of the cervical spine. The cervical spine is typically imaged from a lateral, or side, perspective.

Starting at 16 seconds, the animation highlights the location and density of each of the individual neurons. For the grand finale, viewers zoom off on a brief fly-through of the cervical spine and a flurry of reds, blues, and greens.

The video comes from Jamie Anne Mortel, a research assistant in the lab of Samuel Pfaff, Salk Institute, La Jolla, CA. Mortel is part of a team supported by the NIH-led Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative that’s developing a comprehensive atlas of the circuitry within the cervical spine that controls how mice control their forelimb movements, such as reaching and grasping.

This basic research will provide a better understanding of how the mammalian brain and spinal cord work together to produce movement. More than that, this research may provide valuable clues into better treating paralysis to arms, wrists, and/or hands caused by neurological diseases and spinal cord injuries.

As a part of this project, the Pfaff lab has been busy developing a software tool to take their imaging data from different parts of the cervical spine and present it in 3D. Mortel, who likes to make cute cartoon animations in her spare time, noticed that the software lacked animation capability. So she took the initiative and spent the next three weeks working after hours to produce this video—her first attempt at scientific animation. No doubt she must have been using a lot of wrist and hand movements!

With a positive response from her Salk labmates, Mortel decided to enter her scientific animation debut in the 2021 Show Us BRAINs! Photo and Video Contest. To her great surprise and delight, Mortel won third place in the video competition. Congratulations, and continued success for you and the team in producing this much-needed atlas to define the circuitry underlying skilled arm, wrist, and hand movements.

Links:

Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)

Spinal Cord Injury Information Page (National Institute of Neurological Disorders and Stroke/NIH)

Samuel Pfaff (Salk Institute, La Jolla, CA)

Show Us Your BRAINs! Photo and Video Contest (Brain Initiative/NIH)

NIH Support: National Institute of Neurological Disorders and Stroke


The Amazing Brain: Tracking Molecular Events with Calling Cards

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In days mostly gone by, it was fashionable in some circles for people to hand out calling cards to mark their arrival at special social events. This genteel human tradition is now being adapted to the lab to allow certain benign viruses to issue their own high-tech calling cards and mark their arrival at precise locations in the genome. These special locations show where there’s activity involving transcription factors, specialized proteins that switch genes on and off and help determine cell fate.

The idea is that myriad, well-placed calling cards can track brain development over time in mice and detect changes in transcription factor activity associated with certain neuropsychiatric disorders. This colorful image, which won first place in this year’s Show Us Your BRAINs! Photo and Video contest, provides a striking display of these calling cards in action in living brain tissue.

The image comes from Allen Yen, a PhD candidate in the lab of Joseph Dougherty, collaborating with the nearby lab of Rob Mitra. Both labs are located in the Washington University School of Medicine, St. Louis.

Yen and colleagues zoomed in on this section of mouse brain tissue under a microscope to capture dozens of detailed images that they then stitched together to create this high-resolution overview. The image shows neural cells (red) and cell nuclei (blue). But focus in on the neural cells (green) concentrated in the brain’s outer cortex (top) and hippocampus (two lobes in the upper center). They’ve been labelled with calling cards that were dropped off by adeno-associated virus [1].

Once dropped off, a calling card doesn’t bear a pretentious name or title. Rather, the calling card, is a small mobile snippet of DNA called a transposon. It gets dropped off with the other essential component of the technology: a specialized enzyme called a transposase, which the researchers fuse to one of many specific transcription factors of interest.

Each time one of these transcription factors of interest binds DNA to help turn a gene on or off, the attached transposase “grabs” a transposon calling card and inserts it into the genome. As a result, it leaves behind a permanent record of the interaction.

What’s also nice is the calling cards are programmed to give away their general locations. That’s because they encode a fluorescent marker (in this image, it’s a green fluorescent protein). In fact, Yen and colleagues could look under a microscope and tell from all the green that their calling card technology was in place and working as intended.
The final step, though, was to find out precisely where in the genome those calling cards had been left. For this, the researchers used next-generation sequencing to produce a cumulative history and map of each and every calling card dropped off in the genome.

These comprehensive maps allow them to identify important DNA-protein binding events well after the fact. This innovative technology also enables scientists to attribute past molecular interactions with observable developmental outcomes in a way that isn’t otherwise possible.

While the Mitra and Dougherty labs continue to improve upon this technology, it’s already readily adaptable to answer many important questions about the brain and brain disorders. In fact, Yen is now applying the technology to study neurodevelopment in mouse models of neuropsychiatric disorders, specifically autism spectrum disorder (ASD) [2]. This calling card technology also is available for any lab to deploy for studying a transcription factor of interest.

This research is supported by the Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative. One of the major goals of BRAIN Initiative is to accelerate the development and application of innovative technologies to gain new understanding of the brain. This award-winning image is certainly a prime example of striving to meet this goal. I’ll look forward to what these calling cards will tell us in the future about ASD and other important neurodevelopmental conditions affecting the brain.

References:

[1] A viral toolkit for recording transcription factor-DNA interactions in live mouse tissues. Cammack AJ, Moudgil A, Chen J, Vasek MJ, Shabsovich M, McCullough K, Yen A, Lagunas T, Maloney SE, He J, Chen X, Hooda M, Wilkinson MN, Miller TM, Mitra RD, Dougherty JD. Proc Natl Acad Sci U S A. 2020 May 5;117(18):10003-10014.

[2] A MYT1L Syndrome mouse model recapitulates patient phenotypes and reveals altered brain development due to disrupted neuronal maturation. Jiayang Chen, Mary E. Lambo, Xia Ge, Joshua T. Dearborn, Yating Liu, Katherine B. McCullough, Raylynn G. Swift, Dora R. Tabachnick, Lucy Tian, Kevin Noguchi, Joel R. Garbow, John N. Constantino. bioRxiv. May 27, 2021.

Links:

Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)

Autism Spectrum Disorder (National Institute of Mental Health/NIH)

Dougherty Lab (Washington University School of Medicine, St. Louis)

Mitra Lab (Washington University School of Medicine)

Show Us Your BRAINs! Photo and Video Contest (BRAIN Initiative/NIH)

NIH Support: National Institute of Neurological Disorders and Stroke; National Institute of Mental Health; National Center for Advancing Translational Sciences; National Human Genome Research Institute; National Institute of General Medical Sciences


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