New Insights Into The Molecular Basis Of Memory

by Kandis Elliot, Mo Fayyaz, University of Wisconsin, Madison

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The Red Hot Debate about Transmissible Alzheimer’s

In the 25 years that John Collinge has studied neurology, he has seen hundreds of human brains. But the ones he was looking at under the microscope in January 2015 were like nothing he had seen before.

He and his team of pathologists were examining the autopsied brains of four people who had once received injections of growth hormone derived from human cadavers. It turned out that some of the preparations were contaminated with a misfolded protein—a prion—that causes a rare and deadly condition called Creutzfeldt–Jakob disease (CJD), and all four had died in their 40s or 50s as a result. But for Collinge, the reason that these brains looked extraordinary was not the damage wrought by prion disease; it was that they were scarred in another way. “It was very clear that something was there beyond what you’d expect,” he says. The brains were spotted with the whitish plaques typical of people with Alzheimer’s disease. They looked, in other words, like young people with an old person’s disease.

For Collinge, this led to a worrying conclusion: that the plaques might have been transmitted, alongside the prions, in the injections of growth hormone—the first evidence that Alzheimer’s could be transmitted from one person to another. If true, that could have far-reaching implications: the possibility that ‘seeds’ of the amyloid-β protein involved in Alzheimer’s could be transferred during other procedures in which fluid or tissues from one person are introduced into another, such as blood transfusions, organ transplants and other common medical procedures.

Collinge felt a duty to inform the public quickly. And that’s what he did, publishing the study inNature in September, to headlines around the world. “Can you CATCH Alzheimer’s?” asked Britain’s Daily Mail, about the “potentially explosive new study”. Collinge has been careful to temper the alarm. “Our study does not mean that Alzheimer’s is actually contagious,” he stresses. Carers won’t catch it on the job, nor family members, however close. “But it raises concern that some medical procedures could be inadvertently transferring amyloid-β seeds.”

Since then, the headlines have died away, but the academic work and discussion have taken off. Could seeds of amyloid-β proteins really be transmitted and, if so, are they harmless or do they cause disease? And could seeds of other related diseases involving misfolded proteins be transmitted in a similar way? In the past decade or so, evidence has been mounting for a controversial theory that rogue proteins, known collectively as amyloids and associated with diverse neurodegenerative diseases—from Alzheimer’s to Parkinson’s and Huntington's—might share some properties of prions, including their transmissibility. Collinge’s data bolstered that theory.

Urgent though these questions are, it could take years to find answers. The paper by Collinge and his colleagues has sparked a worldwide hunt for similar amyloid pathology in autopsied brains, and a small study published in January 2016 revealed a handful of related cases. Researchers are also trying to work out what the putative amyloid seeds look like, and whether different 'strains’ of amyloids exist that are particularly damaging.

Some researchers say that it is much too early to be alarmed. They point out that the number of patients in Collinge’s study was tiny, that none had displayed symptoms of Alzheimer’s disease before their death and that another toxic protein called tau also seems to be required to cause the condition. “We have to remember that there is no conclusive evidence that seeds of amyloids can transmit actual disease or that amyloids spread in the brain in a prion-like way,” says Pierluigi Nicotera, scientific director of the German Centre for Neurodegenerative Diseases in Bonn. “There may be other biological explanations.”

Right now, there are few solid answers, but plenty of concerns. The sceptics worry that they might one day find themselves working under tight biosecurity regulations to handle proteins that they view as relatively innocuous. Others feel that the dangers may have been underestimated, and that scientists have a duty to investigate this as quickly as they can. “In my opinion, all amyloids should be considered dangerous until proven safe,” says prion and amyloid researcher Adriano Aguzzi at the University Hospital Zurich in Switzerland.

DANGEROUS FOLDS

A few decades ago, it was almost inconceivable that a protein, which has no genetic material or any other obvious way to self-replicate, could cause infectious disease. But that changed in 1982, when Stanley Prusiner, now at the University of California, San Francisco, introduced evidence for disease-causing prions, coining the term from the words 'proteinacious’ and 'infectious’. Prusiner showed that prion proteins (PrP) exist in a normal cellular form, and in a misfolded infectious form. The misfolded form causes the normal protein to also misfold, creating a cascade that overwhelms and kills cells. It cause animal brains to turn into a spongy mess in scrapie, a disease of sheep, and in bovine spongiform encephalopathy (BSE or 'mad cow disease’), as well as in human prion diseases such as CJD.

Prusiner and others also investigated how prions could spread. They showed that injecting brain extracts containing infectious prions into healthy animals seeds disease. These prions can be so aggressive that in some cases, simply eating infected brains is sufficient to transmit disease. For example, many cases of variant CJD (vCJD) are now thought to have arisen in the United Kingdom in the 1990s after people ate meat from cattle that were infected with BSE.

Since then, scientists have come to appreciate that many proteins associated with neurodegenerative diseases—including amyloid-β and tau in Alzheimer’s disease and α-synuclein in Parkinson’s disease—misfold catastrophically. Structural biologists call the entire family of misfolded proteins (including PrP) amyloids. Amyloid-β clumps into whitish plaques, tau forms ribbons called tangles and α-synuclein creates fibrous deposits called inclusions.

A decade ago, these similarities prompted neuroscientist Mathias Jucker at the University of Tübingen in Germany to test whether injecting brain extracts containing misfolded amyloid-β into mice could seed an abnormal build-up of amyloid in the animals’ brains. He found that it could, and that it also worked if he injected amyloids into the muscles. “We saw no reason not to believe that if amyloid seeds entered the human brain, they would also cause amyloid pathology in the same way,” says Jucker.

This didn’t cause alarm at the time, because it wasn’t clear how an amyloid seed from the brain of someone with Alzheimer’s could be transferred into another person’s body and find its way to their brain. To investigate that, what was needed was a group of people who had been injected with material from another person, and the opportunity to examine their brains in great detail, preferably when they were still relatively young and before they might have spontaneously developed early signs of Alzheimer’s.

The CJD brains provided just that opportunity. Between 1958 and 1985, around 30,000 people worldwide received injections of growth hormone derived from the adrenal glands of cadavers to treat growth problems. Some of the preparations were contaminated with the prion that causes CJD. Like all prion diseases, CJD has a very long incubation period, but once it gets going it rages through the brain, destroying all tissue in its wake and typically killing people from their late 40s onwards. According to 2012 statistics, 226 people around the world have died from CJD as a result of prion-contaminated growth-hormone preparations.

Collinge had not set out to find a link with Alzheimer's—it emerged as part of routine work at the National Prion Clinic in London, which he heads, and where around 70% of all people in the United Kingdom who die from prion-related causes are now autopsied. The clinic routinely looks for signs of all amyloid proteins in these brains to distinguish prion disease from other conditions. It was thanks to this routine work that the cluster of unusual cases emerged of people who had clearly died of CJD, but who also had obvious signs of amyloid pathology in their grey matter and cerebral blood vessels.

As soon as he saw these brains, Collinge knew that he could get into stormy waters. Keen to strike a balance between warning of a possible public-health risk and causing unwarranted panic, he sketched a carefully worded press release that would go out from the National Prion Centre and set up hotlines for people who had been treated with growth hormone in the past. But no panic occurred: apart from one or two overwrought headlines, the news stories were fairly measured, he says. Only around ten people called the hotlines.

For scientists, however, the paper was a red flag. “As soon as the paper came out we realized the health implications and started collecting slides and paraffin blocks from patients,” says Jiri Safar, director of the National Prion Disease Pathology Surveillance Center at Case Western Reserve University in Cleveland, Ohio. Like other pathologists in countries where people had died of CJD associated with medical procedures, he rushed to check the centre’s archives of autopsied brains to see if any of them contained the ominous amyloid deposits.

The answers are not yet in. Safar says that it has not proved easy to trace brain samples in the United States, but that he is working to do so with the National Institutes of Health and the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia. Charles Duyckaerts at the Pitié-Salpêtrière Hospital in Paris, France, has now examined brain tissues from around 24 patients and is likely to report the results later this year.

A further 228 cases of CJD were caused by transplantation of prion-contaminated dura mater—the membrane surrounding the brain and spinal cord—prepared from cadavers around the world. Dura-mater preparations were regularly used in brain surgery as repair patches until the late 1990s. For the study published in January, Herbert Budka at the National Prion Diseases Reference Center at University Hospital Zurich and his colleagues examined the brains of seven such patients from Switzerland and Austria, and found that five had amyloid deposits in grey matter and blood vessels. In Japan, dementia researcher Masahito Yamada at Kanazawa University is making his way through a large number of such autopsy specimens and says that the 16 brains he has examined so far show signs of unusually high levels of amyloid deposition in cerebral blood vessels.

Yet such case studies can only ever provide circumstantial evidence that seeds of amyloid-β were transferred during the treatments. And they cannot entirely rule out the possibility that the treatments themselves—or the patients’ original medical conditions—caused the amyloid pathology. More-conclusive evidence would come from checking whether the original growth hormone and dura-mater preparations contained infectious amyloid seeds, by injecting them into animals and seeing whether this triggers disease. Most of these preparations, however, have long since disappeared. Collinge has access to some original samples of growth hormone stored by the UK Department of Health, and he is planning to analyse them for the presence of amyloid seeds and then inject them into mice. That work will take a couple of years to complete, he says.

SEEDS OF DOUBT

There is another hitch: no one knows for sure what size and shape the amyloid seeds might be. Jucker is hunting for them in an unusual source of human brain tissue that has nothing to do with CJD. A team in Bonn has collected frozen samples from more than 700 people with epilepsy who were operated on over the past 25 years to remove tissue that was driving their seizures. “It is the best source of fresh human brain tissue available at the moment,” says Jucker, who plans to scrutinize it carefully under the microscope for anything that might resemble tiny clumps or seeds of amyloid-β. The team also has records of the patients’ cognitive skills, such as language and memory skills, before and at regular intervals after the operations. This should allow Jucker’s team to correlate the presence of any amyloid-β seeds it finds with changes in the cognitive function of individual patients over time.

Scientists have shown that tau and α-synuclein can also seed pathological features in mice. In two studies, from 2012, scientists injected fibrils of α-synuclein into the brains of mice already engineered to develop some of the characteristics of Parkinson’s disease. This triggered the early onset of some of the signs and symptoms of Parkinson’s, and eventually killed the animals. A third study showed that similar injections into normal mice caused some of the neurodegeneration typical of Parkinson’s disease and the mice became less agile. In humans, α-synuclein would not necessarily turn out to be equally aggressive—mouse models of neurodegenerative diseases do not mimic human disease very closely—but scientists are taking the possibility seriously.

If the transmissibility hypothesis proves true, the implications could be severe. Amyloids stick like glue to metal surgical instruments, and normal sterilization does not remove them, so amyloid seeds might possibly be transferred during surgery. The seeds might sit in the body for years or decades before spreading into plaques, and perhaps enabling the other pathological changes needed to induce Alzheimer’s disease. Having amyloid plaques in cerebral blood vessels could be dangerous in another way, because they increase the risk that the vessel walls might break, leading to small strokes.

But if common medical procedures really increased the risk of neurodegenerative disorders, then wouldn’t that already have been detected? Not necessarily, says epidemiologist Roy Anderson at Imperial College London. “The proper epidemiological studies have not been done yet,” he says. They require very large and carefully curated databases of people with Alzheimer’s disease, which include information about the development of symptoms and autopsy data. He and his team are now studying the handful of reliable databases that exist to tease out a signal that might associate medical procedures with Alzheimer’s progression. The number of patients currently available may turn out to be too small to draw conclusions, he says, but a more definitive answer could emerge as the databases grow.

Faced with so much uncertainty, some researchers and public-health agencies have adopted a wait-and-see approach. “We are right at the beginning of this story,” says Nicotera, “and if there is one message to come out right now it is that we need more work to see if this is a relevant mechanism.” The CDC and the European Centre for Disease Prevention and Control in Solna, Sweden, say that they are keeping a cautious eye on the issue.

If further research does confirm that common neurodegenerative diseases are transmissible, what then? One immediate priority would be rigorous sterilization procedures for medical and surgical instruments that would destroy amyloids, in the way that extremely high temperatures and harsh chemicals destroy prions. Aguzzi says that funding agencies should put out calls now to researchers to develop cheap and simple sterilization methods. “It’s not very sexy science, but it is urgently needed,” he says. He also worries about the safety of researchers working with amyloids—particularly α-synuclein. “I have nightmares that someone in my lab may catch Parkinson’s,” he says. “While the story is in flux, our first duty is to protect lab workers.”

STRAIN SEEKERS

The similarities between prions and other amyloids is throwing open other avenues of research. Prions can exist as distinct strains—proteins that have the same sequence of amino acids but misfold in different ways and have distinct biological behaviours, much as different strains of a pathogenic virus can be aggressive or weak. The outbreak of vCJD in the United Kingdom in the 1990s was traced to BSE-contaminated meat because the prion strain was the same in both.

Over the past few years, research in animals has shown that different strains of amyloid-β and α-synuclein exist. And a landmark paper in 2013 reported that strains of amyloid-β with different 3D structures were associated with different disease progression in two people with Alzheimer’s. Structural biologist Robert Tycko, who led the work at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Maryland, is now looking at many more brain samples from such patients.

Knowing the structures of pathological forms of amyloid seeds should help to design small molecules that bind to them and stop them doing damage, says biophysicist Ronald Melki at the Paris-Saclay Institute of Neuroscience, who works on α-synuclein strains. His lab is designing small peptides that target the seeds and mimic regions of 'chaperone’ molecules, which usually bind to proteins and help them to fold correctly. Melki’s small peptides mimic these binding regions, sticking to the amyloid proteins to stop them from aggregating further.

In the research community, much of the agitation in response to Collinge’s paper boils down to semantics. Some scientists do not like to use the word 'prion’ in connection with the amyloids associated with common neurodegenerative diseases, or to describe any of their properties as 'prion-like'—because of its connotation of infectious, deadly disease. “The public has this perception of the word 'prion’,” says Alzheimer’s researcher Brad Hyman at Harvard Medical School in Boston, Massachusetts, and this matters, even if their ideas are wrong. “One of my patients told me that she wasn’t getting any hugs any more from her husband who had read about the case in the media—that made me sad,” he says.

Others, however, feel that it is helpful to consider prions and other amyloids as being part of a single spectrum of conditions involving proteins that misfold and misbehave. It means that researchers studying prion diseases and neurodegenerative diseases, who until recently had considered their disciplines to be separate, now find themselves tackling shared questions.

Both fields are wary of raising premature alarm, even though they wonder what the future will bring. Jucker, only half-jokingly, says he could imagine a future in which people would go into hospital every ten years or so and get the amyloid seeds cleared out of their brains with antibodies. “You’d be good then to go for another decade.”

Image 1 Credit: ©iStock.com

Image 2 Credit:  Juan Gaertner/Shutterstock

Source: Scientific American (By Alison Abbott, Nature magazine)

A New Way to Cross the Blood–Brain Barrier -  A Mental Unblock

The brain presents a unique challenge for medical treatment: it is locked away behind an impenetrable layer of tightly packed cells. Although the blood-brain barrier prevents harmful chemicals and bacteria from reaching our control center, it also blocks roughly 95 percent of medicine delivered orally or intravenously. As a result, doctors who treat patients with neurodegenerative diseases, such as Parkinson’s, often have to inject drugs directly into the brain, an invasive approach that requires drilling into the skull.

Some scientists have had minor successes getting intravenous drugs past the barrier with the help of ultrasound or in the form of nanoparticles, but those methods can target only small areas. Now neuroscientist Viviana Gradinaru and her colleagues at the California Institute of Technology show that a harmless virus can pass through the barricade and deliver treatment throughout the brain.

Gradinaru’s team turned to viruses because the infective agents are small and adept at entering cells and hijacking the DNA within. They also have protein shells that can hold beneficial deliveries, such as drugs or genetic therapies. To find a suitable virus to enter the brain, the researchers engineered a strain of an adeno-associated virus into millions of variants with slightly different shell structures. They then injected these variants into a mouse and, after a week, recovered the strains that made it into the brain. A virus named AAV-PHP.B most reliably crossed the barrier.

Next the team tested to see if AAV-PHP.B could work as a potential vector for gene therapy, a technique that treats diseases by introducing new genes into cells or by replacing or inactivating genes already there. The scientists injected the virus into the bloodstream of a mouse. In this case, the virus was carrying genes that encoded green fluorescent proteins. So if the virus made it to the brain and the new DNA was incorporated in neurons, the success rate could be tracked via a green glow on dissection. Indeed, the researchers observed that the virus infiltrated most brain cells and that the glowing effects lasted as long as one year. The results were recently published in Nature Biotechnology.

In the future, this approach could be used to treat a range of neurological diseases. “The ability to deliver genes to the brain without invasive methods will be extremely useful as a research tool. It has tremendous potential in the clinic as well,” says Anthony Zador, a neuroscientist who studies brain wiring at Cold Spring Harbor Laboratory. Gradinaru also thinks the method is a good candidate for targeting areas other than the brain, such as the peripheral nervous system. The sheer number of peripheral nerves has made pain treatment for neuropathy difficult, and a virus could infiltrate them all.

Image Credit: Thomas Fuchs

Source: Scientific American (By Monique Brouillette)

Quote from #JaneGoodall primatologist and anthropologist. More quotes like this to inspire you in my new journal I Love Science, in stores March but ready for preorder now! #womeninscience #ilovescience #anthropology #scientificliteracy

Antibiotic resistance discovered in the guts of ancient mummies

The gut bacteria inside 1000-year-old mummies from the Inca Empire are resistant to most of today’s antibiotics, even though we only discovered these drugs within the last 100 years.

“At first we were very surprised,” Tasha Santiago-Rodriguez of California Polytechnic State University in San Louis Opisbo, told the Annual Meeting of the American Society for Microbiology last month.

Her team studied the DNA within the guts of three Incan mummies dating back to between the 10th and 14thcenturies and six mummified people from Italy, from between the 15th and 18th centuries. They found an array of genes that have the potential to resist almost all modern antibiotics, including penicillin, vancomycin and tetracycline.

These ancient genes were largely in microbes whose resistance is problematic today, including Enteroccocus bacteria that can infect wounds and cause urinary tract infections. But they found that many other species, including some harmless ones, carried some of these resistant genes too.

Enterococcus enigma

“When you think about it, almost all these antibiotics are naturally produced, so it makes sense to find antibiotic genes as well,” says Santiago-Rodriguez.

Their finding shows that genes that can confer resistance to antibiotics were relatively widespread hundreds of years before Alexander Fleming discovered penicillin in 1928. “It’s ridiculous to think evolution of antibiotic resistance began when penicillin was discovered,” said team-member Raul Cano, also at California Polytechnic State University, at the meeting while discussing the findings. “It’s been going on for 2 billion years.”

These genes existed long before antibiotics became common, but it is our overuse of these drugs in both people and livestock that caused the superbug resistance to explode worldwide, said Cano.

“This is exciting data,” says Adam Roberts, who studies antibiotic resistance genes at University College London. While it is already well known that antibiotic resistance occurred naturally before people started using antibiotics, this study shows that resistance genes were already within the human gut long before we started using these drugs, he says.

“It begs the question of what was selecting for these genes at this time? Was it the natural production of antibiotics by other bacteria, or were there other, as yet unknown forces at play?” asks Roberts.

Happy Valentine’s Day! Hope everybody gets their share of dopamine and oxytocin today. #lovefeelings #scientificliteracy #braininlove #brainfeels

“Sixth sense” may be more than just a feeling

With the help of two young patients with a unique neurological disorder, an initial study by scientists at the National Institutes of Health suggests that a gene called PIEZO2 controls specific aspects of human touch and proprioception, a “sixth sense” describing awareness of one’s body in space. Mutations in the gene caused the two to have movement and balance problems and the loss of some forms of touch. Despite their difficulties, they both appeared to cope with these challenges by relying heavily on vision and other senses.

“Our study highlights the critical importance of PIEZO2 and the senses it controls in our daily lives,” said Carsten G. Bönnemann, M.D., senior investigator at the NIH’s National Institute of Neurological Disorders and Stroke (NINDS) and a co-leader of the study published in the New England Journal of Medicine. “The results establish that PIEZO2 is a touch and proprioception gene in humans. Understanding its role in these senses may provide clues to a variety of neurological disorders.”

Dr. Bönnemann’s team uses cutting edge genetic techniques to help diagnose children around the world who have disorders that are difficult to characterize. The two patients in this study are unrelated, one nine and the other 19 years old. They have difficulties walking; hip, finger and foot deformities; and abnormally curved spines diagnosed as progressive scoliosis.

Working with the laboratory of Alexander T. Chesler, Ph.D., investigator at NIH’s National Center for Complementary and Integrative Health (NCCIH), the researchers discovered that the patients have mutations in the PIEZO2 gene that appear to block the normal production or activity of Piezo2 proteins in their cells. Piezo2 is what scientists call a mechanosensitive protein because it generates electrical nerve signals in response to changes in cell shape, such as when skin cells and neurons of the hand are pressed against a table. Studies in mice suggest that Piezo2 is found in the neurons that control touch and proprioception.

“As someone who studies Piezo2 in mice, working with these patients was humbling,” said Dr. Chesler. “Our results suggest they are touch-blind. The patient’s version of Piezo2 may not work, so their neurons cannot detect touch or limb movements.”

Further examinations at the NIH Clinical Center suggested the young patients lack body awareness. Blindfolding them made walking extremely difficult, causing them to stagger and stumble from side to side while assistants prevented them from falling. When the researchers compared the two patients with unaffected volunteers, they found that blindfolding the young patients made it harder for them to reliably reach for an object in front of their faces than it was for the volunteers. Without looking, the patients could not guess the direction their joints were being moved as well as the control subjects could.

The patients were also less sensitive to certain forms of touch. They could not feel vibrations from a buzzing tuning fork as well as the control subjects could. Nor could they tell the difference between one or two small ends of a caliper pressed firmly against their palms. Brain scans of one patient showed no response when the palm of her hand was brushed.

Nevertheless, the patients could feel other forms of touch. Stroking or brushing hairy skin is normally perceived as pleasant. Although they both felt the brushing of hairy skin, one claimed it felt prickly instead of the pleasant sensation reported by unaffected volunteers. Brain scans showed different activity patterns in response to brushing between unaffected volunteers and the patient who felt prickliness.

Despite these differences, the patients’ nervous systems appeared to be developing normally. They were able to feel pain, itch, and temperature normally; the nerves in their limbs conducted electricity rapidly; and their brains and cognitive abilities were similar to the control subjects of their age.

“What’s remarkable about these patients is how much their nervous systems compensate for their lack of touch and body awareness,” said Dr. Bönnemann. “It suggests the nervous system may have several alternate pathways that we can tap into when designing new therapies.”

Previous studies found that mutations in PIEZO2 may have various effects on the Piezo2 protein that may result in genetic musculoskeletal disorders, including distal arthrogryposis type 5, Gordon Syndrome, and Marden-Walker Syndrome. Drs. Bönnemann and Chesler concluded that the scoliosis and joint problems of the patients in this study suggest that Piezo2 is either directly required for the normal growth and alignment of the skeletal system or that touch and proprioception indirectly guide skeletal development.

“Our study demonstrates that bench and bedside research are connected by a two-way street,” said Dr. Chesler. “Results from basic laboratory research guided our examination of the children. Now we can take that knowledge back to the lab and use it to design future experiments investigating the role of PIEZO2 in nervous system and musculoskeletal development.”

Today is World Anaesthesia Day! Here’s a look at the chemistry behind a range of anaesthetics. More info here and here.

(Image caption: The synapses of pyramid cells in the cerebral cortex form functional groups. Some of the related synapses are shown in green in the reconstruction. Credit: © MPI of Neurobiology / Scheuss)

Neurons form synapse clusters

The cerebral cortex resembles a vast switchboard. Countless lines carrying information about the environment, for example from the sensory organs, converge in the cerebral cortex. In order to direct the flow of data into meaningful pathways, the individual pyramidal cells of the cerebral cortex act like miniature switchboard operators. Each cell receives information from several thousand lines. If the signals make sense, the line is opened, and the information is relayed onward. Scientists at the Max Planck Institute of Neurobiology in Martinsried have now shown for the first time that contact points between specific neuron types are clustered in groups on the target neuron. It is probable that signals are coordinated with each other in this way to make them more “convincing”.

The cells of the cerebral cortex have a lot to do. They process various types of information depending on the area in which they are located. For example, signals from the retina arrive in the visual cortex, where, among other things, the motion of objects is detected. The pyramidal cells of the cerebral cortex receive information from other cells through thousands of contact points called synapses. Depending on where, how many and how often synapses are activated, the cell relays the signal onward – or not.

Information is passed on in the form of electrical signals. The neurobiologists were able to measure these signals at various contact points of the neuron. “The exciting thing is that the signals that a cell receives from, say, ten simultaneously active synapses can be greater than the sum of the signals from the ten individual synapses,” says Volker Scheuss, summarizing the basis of his recently published study. “However, until now it was unclear whether this phenomenon can be explained by a specific arrangement of synapses on pyramidal cells.”

By combining modern methods, the neurobiologists in Tobias Bonhoeffer’s Department have analysed the arrangement of synapses. They were able to selectively activate a specific type of pyramid cell in brain slices from mice using optogenetics. Thanks to simultaneous “calcium imaging”, they were then able to observe and record the activity of individual synapses under a two-photon microscope. In this way, they succeeded in showing for the first time how synapses are arranged with respect to each other.

The result of such synapse mapping analysed with a newly developed algorithm was clear: The synapses of pyramidal cells form clusters consisting of 4 to 14 synapses arranged within an area of less than 30 micrometres along the dendrite. “The existence of these clusters suggests that the synapses interact with each other to control the strength of the combined signal,” explains Onur Gökçe, author of the study. This is the first anatomical explanation for the disproportionate strength of clustered synapse signals in comparison to the individual signals – a finding known from activity measurements. The observation in layer 5 pyramidal cells was of particular interest, as the activity of these cells oscillates synchronously. “This rhythmic activity, which probably influences the processing of visual information, could synchronously activate synapse clusters, thus boosting the overall signal received,” says Scheuss.