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Wiring the Brain: How Viruses Can Be Engineered to Trace Neural Connections
The fact that the rabies virus can spread from an infected neuron to other neurons connected to it makes it an almost perfect vector for tracing connections in the brain.
Dr. Kevin Mitchell is a geneticist who studies neural development. In this guest post, he explains how viruses can be used to map connections in the brain.
What's the Big Idea?
It is hard to overstate the complexity of the brain. Not only are there tens of billions of individual nerve cells, or neurons, which make literally trillions ofconnections between each other. The complexity really lies in the fact that there are hundreds or maybe even thousands of different types of neurons, which are arranged in highly-organised patterns, and which connect to each other in very specific ways. Now, a new technique is emerging that enables neuroscientists to trace these kinds of connections by enlisting help from an unlikely source – viruses.
To understand why this approach is so exciting, we need to consider the scope of the problem it can help solve. This begins with individual neurons themselves. Neurons are polarised – they have an end for inputs and an end for outputs. Each of these may be branched to give thousands of independent sites of input and output. For any given neuron, there are other neurons that connect to it (information flows from all those neurons into our subject neuron) and other neurons that it connects to (information flows from our subject neuron out to all these neurons).
But neurons are not all the same. The most obvious and perhaps most important difference between neurons is that some are excitatory and some inhibitory. When an excitatory neuron is activated, it releases neurotransmitter at the connections it makes with its output neurons – this neurotransmitter tends to make those other cells electrically active. The exact opposite happens when an inhibitory cell is activated – it releases a different neurotransmitter onto its target neurons, which makes them less electrically active.
Neurons That Fire Together Wire Together
At any given moment, each single neuron is integrating the incoming information from typically hundreds of excitatory and inhibitory neurons – the balance between these and their precise timing will determine whether it becomes active enough to “fire” a signal to its own output neurons. (Without the inhibitory neurons, any electrical activity would rapidly spread through the entire brain in an epileptic-like firestorm).
There are hundreds of subtypes of excitatory and inhibitory neurons, all with different jobs to do. The way in which these different cell types are interconnected determines the functional properties of each little microcircuit in the brain – the type of information that comes into the system, how it filters and transforms that information, how long a neuron will be active before it's shut off, whether it will fire with a rhythm and at what frequency, etc.
These parameters determine the computations that the circuit can perform. While some of the details of such microcircuits have been worked out, for the most part we remain unblissfully ignorant of this crucial information. But we cannot hope to understand how the brain functions without knowing how the basic units of computation are put together.
How to solve this problem? In looking for ways to map the connections of neurons, scientists are turning to viruses – in particular, those that have evolved to infect neurons and to spread from one to another through the synaptic connections between them. One of these is the deadly rabies virus – a specialist in infecting neurons.
Neural connections in the brain
Rabies is typically transmitted from one infected animal to another through saliva, often via a bite, which releases viral particles that infect peripheral neurons. From there, it spreads backwards into the spinal cord and brain, passing from the initially infected neurons into every neuron that connects to them. This continues in the next neurons, resulting in the rapid spread of infection throughout the enture nervous system. This has a striking effect onthe host – among other symptoms, increasing aggression and attack behaviors, along with salivation, resulting in spread to a new host.
The fact that the virus can spread from an infected neuron to other neuronsconnected to it makes it an almost perfect vector for tracing these connections in the brains of experimental animals. To make it perfect required some modifications.
Modifying the Virus
First, using the tools of molecular biology, researchers have modified the genomeof the rabies virus, so that, as well as its own genes, the virus now carries so-called marker proteins, like the well-known green fluorescent protein from jellyfish. When ultraviolet light is shone on this protein, it fluoresces, giving off avivid green light. Neurons infected with the virus (either directly or via synaptic connections) can thus be beautifully visualised.
A major problem, however, is the rabies virus is too efficient – it continues to spread to all the neurons connected to each of the neurons connected to the first neuron, obscuring the pattern we are interested in. To get around this, the virus had to be crippled by removing one of the genes it needs to spread.
The sequence of the rabies virus genome - which is made from RNA, not DNA - encodes five proteins that are essential for the virus to replicate and be transmitted from one cell to another. When a rabies virus infects a neuron, it starts to express these proteins and replicate its genome. These replicates are packaged into a viral particle, which includes an envelope of host cell membrane studded with viral proteins.
One of these is the rabies glycoprotein, encoded by the G gene. This protein is essential for the viral particle to infect another neuron – it accomplishes this by binding to receptor proteins on the surface ofthese neurons, which leads to it being sucked into the cell (tricksy!).
A Solution: Delete the Genes
Even more tricksy is how scientists have taken advantage of this system to control which cells can be infected and to limit the spread of virus to only one synapse. This involves modifying both the virus and the cells of the host animal (usually a transgenic mouse). First, the G gene can be deleted from the viral genome – these viral particles can infect neurons when the virus is grown ina dish with the G protein added. But after they infect one neuron they cannotspread to others because they can’t make more G protein. The trick is to makethe infected host cell supply that protein – this can be achieved by adding DNA encoding this protein directly into a single cell.
When that single cell is infected, it leads to replication of the virus and transfer to only those cells with directinputs on the primary neuron. Scientists can thus visualise all the inputs ontoa single neuron, and painstakingly reconstruct the connectivity of an entiremicrocircuit.
What's the Significance?
Since the development of these techniques there have been a whole range of further modifications, allowing infection of specific cell types, the tracing of connections in the other direction (to all the output neurons) using different viruses, and the transfer of other cargo proteins to allow all kinds of additional analyses of the functions of these microcircuits.
In combination with tremendous advances in microscopy, these approaches are leading to a resurgence of “descriptive” neuroanatomy – just the kinds of studies that fueled the insights of pioneering neuroscientists like Ramon y Cajal 100 years ago.
Those insights derived from the application of new technologies forvisualising single neurons. These new techniques take us the next step to see themicrocircuits that make up the computational units of the brain.
A man's skeleton, found facedown with his hands bound, was unearthed near an ancient ceremonial circle during a high speed rail excavation project.
- A skeleton representing a man who was tossed face down into a ditch nearly 2,500 years ago with his hands bound in front of his hips was dug up during an excavation outside of London.
- The discovery was made during a high speed rail project that has been a bonanza for archaeology, as the area is home to more than 60 ancient sites along the planned route.
- An ornate grave of a high status individual from the Roman period and an ancient ceremonial circle were also discovered during the excavations.
Foul play?<p>A skeleton representing a man who was tossed face down into a ditch nearly 2,500 years ago with his hands bound in front of his hips was dug up during a high speed rail excavation.</p><p>The positioning of the remains have led archaeologists to suspect that the man may have been a victim of an ancient murder or execution. Though any bindings have since decomposed, his hands were positioned together and pinned under his pelvis. There was also no sign of a grave or coffin. </p><p>"He seems to have had his hands tied, and he was face-down in the bottom of the ditch," <a href="https://www.livescience.com/iron-age-murder-victim-england.html" target="_blank">said archaeologist Rachel Wood</a>, who led the excavation. "There are not many ways that you end up that way."</p><p>Currently, archaeologists are examining the skeleton to uncover more information about the circumstances of the man's death. Fragments of pottery found in the ditch may offer some clues as to exactly when the man died. </p><p>"If he was struck across the head with a heavy object, you could find a mark of that on the back of the skull," Wood said to <a href="https://www.livescience.com/iron-age-murder-victim-england.html" target="_blank">Live Science</a>. "If he was stabbed, you could find blade marks on the ribs. So we're hoping to find something like that, to tell us how he died."</p>
Other discoveries at Wellwick Farm<p>The grim discovery was made at Wellwick Farm near Wendover. That is about 15 miles north-west of the outskirts of London, where <a href="https://www.hs2.org.uk/building-hs2/hs2-green-corridor/" target="_blank">a tunnel</a> is going to be built as part of a HS2 high-speed rail project due to open between London and several northern cities sometime after 2028. The infrastructure project has been something of a bonanza for archaeology as the area is home to more than 60 ancient sites along the planned route that are now being excavated before construction begins. </p><p>The farm sits less than a mile away from the ancient highway <a href="http://web.stanford.edu/group/texttechnologies/cgi-bin/stanfordnottingham/places/?icknield" target="_blank">Icknield Way</a> that runs along the tops of the Chiltern Hills. The route (now mostly trails) has been used since prehistoric times. Evidence at Wellwick Farm indicates that from the Neolithic to the Medieval eras, humans have occupied the region for more than 4,000 years, making it a rich area for archaeological finds. </p><p>Wood and her colleagues found some evidence of an ancient village occupied from the late Bronze Age (more than 3,000 years ago) until the Roman Empire's invasion of southern England about 2,000 years ago. At the site were the remains of animal pens, pits for disposing food, and a roundhouse — a standard British dwelling during the Bronze Age constructed with a circular plan made of stone or wood topped with a conical thatched roof.</p>
Ceremonial burial site<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yMzUzMTk0Ni9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTY0NDgwNTIyMX0.I49n1-j8WVhKjIZS_wVWZissnk3W1583yYXB7qaGtN8/img.jpg?width=1245&coordinates=0%2C82%2C0%2C83&height=700" id="44da7" class="rm-shortcode" data-rm-shortcode-id="46cfc8ca1c64fc404b32014542221275" data-rm-shortcode-name="rebelmouse-image" alt="top down view of coffin" data-width="1245" data-height="700" />
A high status burial in a lead-lined coffin dating back to Roman times.
Photo Credit: HS2<p>While these ancient people moved away from Wellwick Farm before the Romans invaded, a large portion of the area was still used for ritual burials for high-status members of society, Wood told Live Science. The ceremonial burial site included a circular ditch (about 60 feet across) at the center, and was a bit of a distance away from the ditch where the (suspected) murder victim was uncovered. Additionally, archaeologists found an ornately detailed grave near the sacred burial site that dates back to the Roman period, hundreds of years later when the original Bronze Age burial site would have been overgrown.</p><p>The newer grave from the Roman period encapsulated an adult skeleton contained in a lead-lined coffin. It's likely that the outer coffin had been made of wood that rotted away. Since it was clearly an ornate burial, the occupant of the grave was probably a person of high status who could afford such a lavish burial. However, according to Wood, no treasures or tokens had been discovered. </p>
Sacred timber circle<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yMzUzMTk0Ny9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTY2MDAwOTQ4Mn0.eVJAUcD0uBUkVMFuMOPSgH8EssGkfLf_MjwUv0zGCI8/img.jpg?width=1245&coordinates=0%2C149%2C0%2C149&height=700" id="9de6a" class="rm-shortcode" data-rm-shortcode-id="ee66520d470b26f5c055eaef0b95ec06" data-rm-shortcode-name="rebelmouse-image" alt="An aerial view of the sacred circular monument." data-width="1245" data-height="700" />
An aerial view of the sacred circular monument.
Photo Credit: HS2<p>One of the most compelling archaeological discoveries at Wellwick Farm are the indications of a huge ceremonial circle once circumscribed by timber posts lying south of the Bronze Age burial site. Though the wooden posts have rotted away, signs of the post holes remain. It's thought to date from the Neolithic period to 5,000 years ago, according to Wood.</p><p>This circle would have had a diameter stretching 210 feet across and consisted of two rings of hundreds of posts. There would have been an entry gap to the south-west. Five posts in the very center of the circle aligned with that same gap, which, according to Wood, appeared to have been in the direction of the rising sun on the day of the midwinter solstice. </p><p>Similar Neolithic timber circles have been discovered around Great Britain, such as one near <a href="https://bigthink.com/culture-religion/stonehenge-sarsens" target="_blank">Stonehenge</a> that is considered to date back to around the same time. </p>
Research reveals a new evolutionary feature that separates humans from other primates.
- Researchers find a new feature of human evolution.
- Humans have evolved to use less water per day than other primates.
- The nose is one of the factors that allows humans to be water efficient.
A model of water turnover for humans and chimpanzees who have similar fat free mass and body water pools.
Credit: Current Biology
Being skeptical isn't just about being contrarian. It's about asking the right questions of ourselves and others to gain understanding.
- It's not always easy to tell the difference between objective truth and what we believe to be true. Separating facts from opinions, according to skeptic Michael Shermer, theoretical physicist Lawrence Krauss, and others, requires research, self-reflection, and time.
- Recognizing your own biases and those of others, avoiding echo chambers, actively seeking out opposing voices, and asking smart, testable questions are a few of the ways that skepticism can be a useful tool for learning and growth.
- As Derren Brown points out, being "skeptical of skepticism" can also lead to interesting revelations and teach us new things about ourselves and our psychology.