neurology
Defining Neurons in Technicolor
Posted on by Dr. Francis Collins
Can you identify a familiar pattern in this image’s square grid? Yes, it’s the outline of the periodic table! But instead of organizing chemical elements, this periodic table sorts 46 different types of neurons present in the visual cortex of a mouse brain.
Scientists, led by Hongkui Zeng at the Allen Institute for Brain Science, Seattle, constructed this periodic table by assigning colors to their neuronal discoveries based upon their main cell functions [1]. Cells in pinks, violets, reds, and oranges have inhibitory electrical activity, while those in greens and blues have excitatory electrical activity.
For any given cell, the darker colors indicate dendrites, which receive signals from other neurons. The lighter colors indicate axons, which transmit signals. Examples of electrical properties—the number and intensity of their “spikes”—appear along the edges of the table near the bottom.
To create this visually arresting image, Zeng’s NIH-supported team injected dye-containing probes into neurons. The probes are engineered to carry genes that make certain types of neurons glow bright colors under the microscope.
This allowed the researchers to examine a tiny slice of brain tissue and view each colored neuron’s shape, as well as measure its electrical response. They followed up with computational tools to combine these two characteristics and classify cell types based on their shape and electrical activity. Zeng’s team could then sort the cells into clusters using a computer algorithm to avoid potential human bias from visually interpreting the data.
Why compile such a detailed atlas of neuronal subtypes? Although scientists have been surveying cells since the invention of the microscope centuries ago, there is still no consensus on what a “cell type” is. Large, rich datasets like this atlas contain massive amounts of information to characterize individual cells well beyond their appearance under a microscope, helping to explain factors that make cells similar or dissimilar. Those differences may not be apparent to the naked eye.
Just last year, Allen Institute researchers conducted similar work by categorizing nearly 24,000 cells from the brain’s visual and motor cortex into different types based upon their gene activity [2]. The latest research lines up well with the cell subclasses and types categorized in the previous gene-activity work. As a result, the scientists have more evidence that each of the 46 cell types is actually distinct from the others and likely drives a particular function within the visual cortex.
Publicly available resources, like this database of cell types, fuel much more discovery. Scientists all over the world can look at this table (and soon, more atlases from other parts of the brain) to see where a cell type fits into a region of interest and how it might behave in a range of brain conditions.
References:
[1] Classification of electrophysiological and morphological neuron types in the mouse visual cortex. N Gouwens NW, et al. Neurosci. 2019 Jul;22(7):1182-1195.
[2] Shared and distinct transcriptomic cell types across neocortical areas. Tasic B, et al. Nature. 2018 Nov;563(7729):72-78.
Links:
Brain Basics: The Life and Death of a Neuron (National Institute of Neurological Disorders and Stroke/NIH)
Cell Types: Overview of the Data (Allen Brain Atlas/Allen Institute for Brain Science, Seattle)
Hongkui Zeng (Allen Institute)
NIH Support: National Institute of Mental Health; Eunice Kennedy Shriver National Institute of Child Health & Human Development
Multiplex Rainbow Technology Offers New View of the Brain
Posted on by Dr. Francis Collins
The NIH-led Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative is revolutionizing our understanding of how the brain works through its creation of new imaging tools. One of the latest advances—used to produce this rainbow of images—makes it possible to view dozens of proteins in rapid succession in a single tissue sample containing thousands of neural connections, or synapses.
Apart from their colors, most of these images look nearly identical at first glance. But, upon closer inspection, you’ll see some subtle differences among them in both intensity and pattern. That’s because the images capture different proteins within the complex network of synapses—and those proteins may be present in that network in different amounts and locations. Such findings may shed light on key differences among synapses, as well as provide new clues into the roles that synaptic proteins may play in schizophrenia and various other neurological disorders.
Synapses contain hundreds of proteins that regulate the release of chemicals called neurotransmitters, which allow neurons to communicate. Each synaptic protein has its own specific job in the process. But there have been longstanding technical difficulties in observing synaptic proteins at work. Conventional fluorescence microscopy can visualize at most four proteins in a synapse.
As described in Nature Communications [1], researchers led by Mark Bathe, Massachusetts Institute of Technology (MIT), Cambridge, and Jeffrey Cottrell, Broad Institute of MIT and Harvard, Cambridge, have just upped this number considerably while delivering high quality images. They did it by adapting an existing imaging method called DNA PAINT [2]. The researchers call their adapted method PRISM. It is short for: Probe-based Imaging for Sequential Multiplexing.
Here’s how it works: First, researchers label proteins or other molecules of interest using antibodies that recognize those proteins. Those antibodies include a unique DNA probe that helps with the next important step: making the proteins visible under a microscope.
To do it, they deliver short snippets of complementary fluorescent DNA, which bind the DNA-antibody probes. While each protein of interest is imaged separately, researchers can easily wash the probes from a sample to allow a series of images to be generated, each capturing a different protein of interest.
In the original DNA PAINT, the DNA strands bind and unbind periodically to create a blinking fluorescence that can be captured using super-resolution microscopy. But that makes the process slow, requiring about half an hour for each protein.
To speed things up with PRISM, Bathe and his colleagues altered the fluorescent DNA probes. They used synthetic DNA that’s specially designed to bind more tightly or “lock” to the DNA-antibody. This gives a much brighter signal without the blinking effect. As a result, the imaging can be done faster, though at slightly lower resolution.
Though the team now captures images of 12 proteins within a sample in about an hour, this is just a start. As more DNA-antibody probes are developed for synaptic proteins, the team can readily ramp up this number to 30 protein targets.
Thanks to the BRAIN Initiative, researchers now possess a powerful new tool to study neurons. PRISM will help them learn more mechanistically about the inner workings of synapses and how they contribute to a range of neurological conditions.
References:
[1] Multiplexed and high-throughput neuronal fluorescence imaging with diffusible probes. Guo SM, Veneziano R, Gordonov S, Li L, Danielson E, Perez de Arce K, Park D, Kulesa AB, Wamhoff EC, Blainey PC, Boyden ES, Cottrell JR, Bathe M. Nat Commun. 2019 Sep 26;10(1):4377.
[2] Super-resolution microscopy with DNA-PAINT. Schnitzbauer J, Strauss MT, Schlichthaerle T, Schueder F, Jungmann R. Nat Protoc. 2017 Jun;12(6):1198-1228.
Links:
Schizophrenia (National Institute of Mental Health)
Mark Bathe (Massachusetts Institute of Technology, Cambridge)
Jeffrey Cottrell (Broad Institute of MIT and Harvard, Cambridge)
Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)
NIH Support: National Institute of Mental Health; National Human Genome Research Institute; National Institute of Neurological Disorders and Stroke; National Institute of Environmental Health Sciences
The Amazing Brain: Shining a Spotlight on Individual Neurons
Posted on by Dr. Francis Collins
A major aim of the NIH-led Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative is to develop new technologies that allow us to look at the brain in many different ways on many different scales. So, I’m especially pleased to highlight this winner of the initiative’s recent “Show Us Your Brain!” contest.
Here you get a close-up look at pyramidal neurons located in the hippocampus, a region of the mammalian brain involved in memory. While this tiny sample of mouse brain is densely packed with many pyramidal neurons, researchers used new ExLLSM technology to zero in on just three. This super-resolution, 3D view reveals the intricacies of each cell’s structure and branching patterns.
The group that created this award-winning visual includes the labs of X. William Yang at the University of California, Los Angeles, and Kwanghun Chung at the Massachusetts Institute of Technology, Cambridge. Chung’s team also produced another quite different “Show Us Your Brain!” winner, a colorful video featuring hundreds of neural cells and connections in a part of the brain essential to movement.
Pyramidal neurons in the hippocampus come in many different varieties. Some important differences in their functional roles may be related to differences in their physical shapes, in ways that aren’t yet well understood. So, BRAIN-supported researchers are now applying a variety of new tools and approaches in a more detailed effort to identify and characterize these neurons and their subtypes.
The video featured here took advantage of Chung’s new method for preserving brain tissue samples [1]. Another secret to its powerful imagery was a novel suite of mouse models developed in the Yang lab. With some sophisticated genetics, these models make it possible to label, at random, just 1 to 5 percent of a given neuronal cell type, illuminating their full morphology in the brain [2]. The result was this unprecedented view of three pyramidal neurons in exquisite 3D detail.
Ultimately, the goal of these and other BRAIN Initiative researchers is to produce a dynamic picture of the brain that, for the first time, shows how individual cells and complex neural circuits interact in both time and space. I look forward to their continued progress, which promises to revolutionize our understanding of how the human brain functions in both health and disease.
References:
[1] Protection of tissue physicochemical properties using polyfunctional crosslinkers. Park YG, Sohn CH, Chen R, McCue M, Yun DH, Drummond GT, Ku T, Evans NB, Oak HC, Trieu W, Choi H, Jin X, Lilascharoen V, Wang J, Truttmann MC, Qi HW, Ploegh HL, Golub TR, Chen SC, Frosch MP, Kulik HJ, Lim BK, Chung K. Nat Biotechnol. 2018 Dec 17.
[2] Genetically-directed Sparse Neuronal Labeling in BAC Transgenic Mice through Mononucleotide Repeat Frameshift. Lu XH, Yang XW. Sci Rep. 2017 Mar 8;7:43915.
Links:
Chung Lab (Massachusetts Institute of Technology, Cambridge)
Yang Lab (University of California, Los Angeles)
Show Us Your Brain! (BRAIN Initiative/NIH)
Brain Research through Advancing Innovative Neurotechnologies® (BRAIN) Initiative (NIH)
NIH Support: National Institute of Mental Health; National Institute of Neurological Disorders and Stroke; National Institute of Biomedical Imaging and Bioengineering
Making Personalized Blood-Brain Barriers in a Dish
Posted on by Dr. Francis Collins
The blood-brain barrier, or BBB, is a dense sheet of cells that surrounds most of the brain’s blood vessels. The BBB’s tiny gaps let vital small molecules, such as oxygen and water, diffuse from the bloodstream into the brain while helping to keep out larger, impermeable foreign substances that don’t belong there.
But in people with certain neurological disorders—such as amyotrophic lateral sclerosis (ALS) and Huntington’s disease—abnormalities in this barrier may block the entry of biomolecules essential to healthy brain activity. The BBB also makes it difficult for needed therapies to reach their target in the brain.
To help look for solutions to these and other problems, researchers can now grow human blood-brain barriers on a chip like the one pictured above. The high-magnification image reveals some of the BBB’s cellular parts. There are endothelial-like cells (magenta), which are similar to those that line the small vessels surrounding the brain. In close association are supportive brain cells known as astrocytes (green), which help to regulate blood flow.
While similar organ chips have been created before, what sets apart this new BBB chip is its use of induced pluripotent stem cell (iPSC) technology combined with advanced chip engineering. The iPSCs, derived in this case from blood samples, make it possible to produce a living model of anyone’s unique BBB on demand.
The researchers, led by Clive Svendsen, Cedars-Sinai, Los Angeles, first use a biochemical recipe to coax a person’s white blood cells to become iPSCs. At this point, the iPSCs are capable of producing any other cell type. But the Svendsen team follows two different recipes to direct those iPSCs to differentiate into endothelial and neural cells needed to model the BBB.
Also making this BBB platform unique is its use of a sophisticated microfluidic chip, produced by Boston-based Emulate, Inc. The chip mimics conditions inside the human body, allowing the blood-brain barrier to function much as it would in a person.
The channels enable researchers to flow cerebral spinal fluid (CSF) through one side and blood through the other to create the fully functional model tissue. The BBB chips also show electrical resistance and permeability just as would be expected in a person. The model BBBs are even able to block the entry of certain drugs!
As described in Cell Stem Cell, the researchers have already created BBB chips using iPSCs from a person with Huntington’s disease and another from an individual with a rare congenital disorder called Allan-Herndon-Dudley syndrome, an inherited disorder of brain development.
In the near term, his team has plans to model ALS and Parkinson’s disease on the BBB chips. Because these chips hold the promise of modeling the human BBB more precisely than animal models, they may accelerate studies of potentially promising new drugs. Svendsen suggests that individuals with neurological conditions might one day have their own BBB chips made on demand to help in selecting the best-available therapeutic options for them. Now that’s a future we’d all like to see.
Reference:
[1] Human iPSC-Derived Blood-Brain Barrier Chips Enable Disease Modeling and Personalized Medicine Applications. Vatine GD, Barrile R, Workman MJ, Sances S, Barriga BK, Rahnama M, Barthakur S, Kasendra M, Lucchesi C, Kerns J, Wen N, Spivia WR, Chen Z, Van Eyk J, Svendsen CN. Cell Stem Cell. 2019 Jun 6;24(6):995-1005.e6.
Links:
Tissue Chip for Drug Screening (National Center for Advancing Translational Sciences/NIH)
Stem Cell Information (NIH)
Svendsen Lab (Cedars-Sinai, Los Angeles)
NIH Support: National Institute of Neurological Disorders and Stroke; National Center for Advancing Translational Sciences
NIH Big Read 2019
Posted on by Dr. Francis Collins
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