The white arrow highlights the primary neuronal cilium, a hair-like structure on nerve cells. The neuron on the right has no cilium because of the loss of a protein linked to intellectual disability in humans. Credit: YOSHIHO IKEUCHI

Intellectual disability linked to nerve cells that lose their ‘antennae’

An odd and little-known feature of nerve cells may be linked to several forms of inherited intellectual disability, researchers at Washington University School of Medicine in St. Louis have learned.

The scientists report that a genetic mutation that causes intellectual disability also blocks formation of the neuronal primary cilium, a hair-like structure that protrudes from the bodies of nerve cells.

"The primary cilium acts as a kind of antenna for nerve cells,” said first author Yoshiho Ikeuchi, PhD, a staff scientist. “It’s covered in receptors that monitor environmental conditions outside the cell and may influence the cell’s functions.”

Learning more about how the mutation sabotages production of the nerve cell cilium eventually will help scientists develop drugs to treat intellectual disability, according to senior author Azad Bonni, MD, PhD, the Edison Professor and chairman of the Department of Anatomy and Neurobiology.

"Intellectual disability—sometimes known as mental retardation—affects 1 to 2 percent of the general population, and researchers have identified more than 100 genes on the X chromosome that can cause these conditions,” Bonni said. “But we don’t know what most of these genes do, and that information is essential for new treatments.”

The research appears online Aug. 29 in Cell Reports.

Nearly every cell in the mammalian body has a primary cilium—a structure that acts as an environmental sensor. Some cells have many cilia that move together in waves. Problems with cilia are associated with disorders throughout the body, including illnesses of the kidneys, eyes and reproductive organs.

"Some of the X-linked intellectual disorders are syndromes that not only hamper brain development but also cause problems elsewhere in the body,” Bonni said. “That makes sense in the context of this new connection we’ve identified between intellectual disability and the primary cilium.”

Scientists only recently have recognized the potential of a primary cilium malfunction to impair nerve cell development and function. Studies have suggested that the primary cilium may be where nerve cells receive the growth signals that allow them to extend branches to each other and form circuits. Other research has shown that blocking of signal receptors on the primary cilium leads to memory problems in mice.

Bonni’s path to the primary cilium led through the nucleus, the command center that contains a cell’s DNA. Proteins found inside a cell’s nucleus often regulate the turning on or off of other genes, making them influential in orchestrating the responses and functions of cells.

Bonni and his colleagues scanned the literature on X chromosome genes linked to intellectual disability to learn which genes produce proteins found in the nucleus. When they disabled 15 such genes in individual nerve cells, they found that the loss of the gene for polyglutamine-binding protein 1 (PQBP1) produced the most dramatic effect, leaving nerve cells with shortened primary cilia or no cilia at all.

In other cell types outside the brain, PQBP1 is typically found only in the nucleus. But the new results show that in neurons the protein is present both in the nucleus and, surprisingly, at the base of the primary cilium.

The scientists learned PQBP1 binds to another protein outside the nucleus that suppresses growth of the primary cilium. By binding to the suppressor, PQBP1 gets that suppressor out of the way, allowing cilium formation to proceed normally.

Scientists may one day try to imitate this effect with drugs, potentially allowing the brain to develop more normally when PQBP1 is mutated. For now, the researchers want to learn more about the suppressor protein and also are investigating the possibility that PQBP1 may continue to influence the functions of the primary cilium after it is formed.

Researchers discover a potential cause of autism

Key enzymes are found to have a ‘profound effect’ across dozens of genes linked to autism. The insight could help illuminate environmental factors behind autism spectrum disorder and contribute to a unified theory of how the disorder develops.

Problems with a key group of enzymes called topoisomerases can have profound effects on the genetic machinery behind brain development and potentially lead to autism spectrum disorder (ASD), according to research announced today in the journal Nature. Scientists at the University of North Carolina School of Medicine have described a finding that represents a significant advance in the hunt for environmental factors behind autism and lends new insights into the disorder’s genetic causes.

“Our study shows the magnitude of what can happen if topoisomerases are impaired,” said senior study author Mark Zylka, PhD, associate professor in the Neuroscience Center and the Department of Cell Biology and Physiology at UNC. “Inhibiting these enzymes has the potential to profoundly affect neurodevelopment — perhaps even more so than having a mutation in any one of the genes that have been linked to autism.”

The study could have important implications for ASD detection and prevention.

“This could point to an environmental component to autism,” said Zylka. “A temporary exposure to a topoisomerase inhibitor in utero has the potential to have a long-lasting effect on the brain, by affecting critical periods of brain development. ”

This study could also explain why some people with mutations in topoisomerases develop autism and other neurodevelopmental disorders.

Topiosomerases are enzymes found in all human cells. Their main function is to untangle DNA when it becomes overwound, a common occurrence that can interfere with key biological processes.

Most of the known topoisomerase-inhibiting chemicals are used as chemotherapy drugs. Zylka said his team is searching for other compounds that have similar effects in nerve cells. “If there are additional compounds like this in the environment, then it becomes important to identify them,” said Zylka. “That’s really motivating us to move quickly to identify other drugs or environmental compounds that have similar effects — so that pregnant women can avoid being exposed to these compounds.”

Zylka and his colleagues stumbled upon the discovery quite by accident while studying topotecan, a topoisomerase-inhibiting drug that is used in chemotherapy. Investigating the drug’s effects in mouse and human-derived nerve cells, they noticed that the drug tended to interfere with the proper functioning of genes that were exceptionally long — composed of many DNA base pairs. The group then made the serendipitous connection that many autism-linked genes are extremely long.

“That’s when we had the ‘Eureka moment,’” said Zylka. “We realized that a lot of the genes that were suppressed were incredibly long autism genes.”

Of the more than 300 genes that are linked to autism, nearly 50 were suppressed by topotecan. Suppressing that many genes across the board — even to a small extent — means a person who is exposed to a topoisomerase inhibitor during brain development could experience neurological effects equivalent to those seen in a person who gets ASD because of a single faulty gene.

The study’s findings could also help lead to a unified theory of how autism-linked genes work. About 20 percent of such genes are connected to synapses — the connections between brain cells. Another 20 percent are related to gene transcription — the process of translating genetic information into biological functions. Zylka said this study bridges those two groups, because it shows that having problems transcribing long synapse genes could impair a person’s ability to construct synapses.

“Our discovery has the potential to unite these two classes of genes — synaptic genes and transcriptional regulators,” said Zylka. “It could ultimately explain the biological mechanisms behind a large number of autism cases.”

Nicotine exposure gives baby rats addictive personalities

Results suggest explanation for why people exposed to nicotine in the womb are more likely to become smokers.

Exposure to nicotine in the womb increases the production of brain cells that stimulate appetite, leading to overconsumption of nicotine, alcohol and fatty foods in later life, according to a new study in rats.

Smoking during pregnancy is known to alter fetal brain development and increase the risk of premature birth, low birth weight and miscarriage. Prenatal exposure to nicotine also increases the likelihood of tobacco use and nicotine addiction in later life, but exactly how is unclear.

To understand the mechanisms behind this effect, Sarah Leibowitz, a behavioural neurobiologist at the Rockefeller University in New York, and her colleagues injected pregnant rats with small doses of nicotine — which the researchers say are comparable to the amount a pregnant woman would get from smoking one cigarette a day — and then examined the brains and behaviour of the offspring.   

In a paper published in Journal of Neuroscience, they found that nicotine increased the production of specific types of neurons in the amygdala and hypothalamus. These cells produce orexin, enkephalin and melanin-concentrating hormone, neuropeptides that stimulate appetite and increase food intake.

Rats exposed to nicotine in the womb had more of these cells and produced more of the neuropeptides than those that were not, and this had long-term consequences on their behaviour. As adolescents, they not only self-administered more nicotine, but also ate more fat-rich food and drank more alcohol.

“These peptide systems stimulate food intake,” says Leibowitz, “but we found that they similarly increase the consumption of drugs and stimulate the brain’s reward mechanisms that promote addiction and substance abuse.”

Leibowitz notes that children whose mothers smoked during pregnancy are more likely to smoke themselves during adolescence and adulthood. Her team’s findings suggest a possible mechanism for that.

The use of nicotine patches or e-cigarettes during pregnancy could have a similar effect. “Whether given subcutaneously, as in our study, or via smoking or patches, the same amount of nicotine would still get into the brain to affect neuronal development and function,” Leibowitz says.

The results highlight the toxic effects of nicotine exposure on brain development, says George Koob, a neurobiologist at the Scripps Research Institute in La Jolla, California. He also adds that the study casts new light on the role of these neuropeptides in reward and motivation.

In earlier work, Leibowitz and her colleagues showed that rats exposed to fat and alcohol in the womb likewise overconsume these substances as adolescents. “Our studies make it very clear that neuronal development in utero is highly sensitive to these substances,” she says, “with each promoting their overconsumption and addictive-like behaviour in the offspring.” 

She and her collaborators are now comparing the effects of nicotine, fat and alcohol to learn more about how this promotion occurs. They are also exploring ways to reverse the effects of prenatal exposure to these substances, thus preventing their overconsumption in later life, which could lead to addiction and obesity.

Researchers find caffeine during pregnancy negatively impacts mice brains

A team of European researchers has found that mice who consume caffeine while pregnant give birth to pups with negative changes to their brains. In their paper published in the journal Science Translational Medicine, the team reports on their findings after examining the brains of mice pups whose mothers were given caffeine during pregnancy.

Medical researchers have shown that drugs such as cocaine, heroin or even marijuana can have a negative impact on fetal development—in contrast most believe that moderate amounts of caffeine consumption during pregnancy is “safe” meaning it has little or no adverse impact on fetal development. This new study doesn’t change that view, but it does suggest that perhaps more research needs to be done.

In their study, the researchers administered the equivalent of 4 or 5 cups of coffee a day to pregnant mice—afterwards they studied the brains of the pups that were born. In so doing, they found that GABA neurons didn’t migrate during brain development to their proper location in the Hippocampus at the same rate as untreated mice. GABA neurons are responsible for controlling the flow of information in the brain. Subsequent tests found the treated pups to be more susceptible to seizures.

The team also found that if they allowed the treated pups to grow to adulthood, they tended to demonstrate problems with memory—instead of playing with new objects placed in their cages, for example, they were satisfied with playing with objects they already knew—a trait that is uncommon for mice. Autopsies of adult brains also showed fewer neurons in the Hippocampus.

The researchers point out that their results in mice are not necessarily applicable to humans and to reinforce that point another team of researchers also published a Focus piece in the same journal pointing out that there are significant differences in the developmental process of humans and mice fetuses and thus the study with mice has no real bearing on whether caffeine may or may not cause developmental problems with human babies.

Still, the results do indicate that perhaps more research should be done to find out if caffeine does indeed have an unknown negative impact on human fetal development.

Scientists Find Key Signal that Guides Brain Development

Scientists at The Scripps Research Institute (TSRI) have decoded an important molecular signal that guides the development of a key region of the brain known as the neocortex. The largest and most recently evolved region of the brain, the neocortex is particularly well developed in humans and is responsible for sensory processing, long-term memory, reasoning, complex muscle actions, consciousness and other functions.

“The mammalian neocortex has a distinctive structure featuring six layers of neurons, and our finding helps explain how this layered structure is generated in early life,” said Ulrich Mueller, chair of TSRI’s Department of Molecular and Cellular Neuroscience and director of the Dorris Neuroscience Center at TSRI.

The discovery, which appears in the August 7,2013 issue of Neuron, also is likely to aid research on autism, schizophrenia and other psychiatric conditions. “With studies such as this one, we’re starting to understand the normal functions of molecules whose disruption by gene mutations can cause developmental brain disorders,” Mueller said.

Finding Their Proper Place

The signal uncovered by Mueller’s team is one that helps guide the migration of baby neurons through the developing neocortex. Such neurons are born from stem-like cells at the bottom of the neocortex, where it wraps around a large, fluid-filled space in the brain called ventricle. The newborn neurons then migrate upward, or radially away from the ventricle, being directed to their proper places in the neocortex’s six-layered, columnar structure by—among others—special guide cells called Cajal-Retzius (CR) cells.

Decades ago, scientists discovered a key signaling protein, reelin, which CR cells secrete and baby neocortical neurons must detect to migrate properly. (Mutant mice that lack a functional form of the protein show, among other abnormalities, a reeling gait—thus the name.) There have been hints since then that CR cells and baby neocortical neurons exchange other molecular signals, too. “But in many years of study, no one has been able to find these other signals,” said Mueller.

However, in a study published in 2011, Mueller and his laboratory colleagues found a significant clue. Reelin, they discovered, guides neuronal migration at least in part by boosting baby neurons’ expression of a generic cell-adhesion molecule, cadherin2 (Cdh2). Since Cdh2 can be expressed by almost any cell type in the developing neocortex, the team then began to look for other factors that would account for the specificity of the interaction between CR cells and migrating baby neurons.

An Interesting Pattern

One set of candidates were the nectins—cell-adhesion proteins known to work with cadherins in other contexts. Lead author Cristina Gil-Sanz, a senior research associate in the Mueller laboratory, mapped the expression levels of the four known types of mammalian nectin proteins in the developing mouse cortex and found an interesting pattern. “We observed that nectin1 is expressed specifically by CR cells and nectin3 by migrating neurons,” said Gil-Sanz. “At the same time, we knew from previous research that nectin1 and nectin3 are preferred binding partners.”

Gil-Sanz and her colleagues followed up with other experiments and soon confirmed that the hookup of nectin1 on CR cells with nectin3 on baby neurons is essential for proper neuronal migration. “This showed for the first time the importance of direct contacts between CR cells and migrating neurons,” Gil-Sanz said.

The experiments also showed that this direct nectin-to-nectin connection is effectively part of the reelin signaling pathway, since reelin’s promotion of Cdh2’s function in migrating neurons turns out to work largely via nectin3. “This helps explain how the interaction occurs specifically between neurons and CR cells, and doesn’t involve other nearby cells that also express Cdh2,” she said.

New Possibilities

The finding points to the possibility of other cell-specific pairings that work via generic Cdh2-to-Cdh2 adhesions in brain development. “We know that there are four nectin proteins, plus a slew of nectin-like molecules,” said Mueller. “We think that there are others that do this as well, and we’re hoping to find them.”

The new study represents a big step toward the full scientific understanding of neuronal migration in the neocortex, and it is likely to be relevant to the study of developmental brain diseases too. Reelin-signaling abnormalities in humans have been linked to autism, depression, schizophrenia and even Alzheimer’s, and, in recent years, cadherin protein mutations also have been linked to disorders including schizophrenia and autism. “Studies like ours provide insight into such findings, by showing that these molecules, in cooperation with nectins, regulate key developmental processes such as the positioning of neurons in the neocortex,” said Mueller.

Baby owls sleep like baby humans

Researchers at the Max Planck Institute for Ornithology and the University of Lausanne have discovered that the sleeping patterns of baby birds are similar to that of baby mammals. What is more, the sleep of baby birds appears to change in the same way as it does in humans. Studying barn owls in the wild, the researchers discovered that this change in sleep is strongly correlated with the expression of a gene involved in producing dark, melanic feather spots, a trait known to covary with behavioral and physiological traits in adult owls. These findings raise the intriguing possibility that sleep-related developmental processes in the brain contribute to the link between melanism and other traits observed in adult barn owls and other animals.

Sleep in mammals and birds consists of two phases, REM sleep (“Rapid Eye Movement Sleep”) and non-REM sleep. We experience our most vivid dreams during REM sleep, a paradoxical state characterized by awake-like brain activity. Despite extensive research, REM sleep’s purpose remains a mystery. One of the most salient features of REM sleep is its preponderance early in life. A variety of mammals spend far more time in REM sleep during early life than when they are adults. For example, as newborns, half of our time asleep is spent in REM sleep, whereas last night REM sleep probably encompassed only 20-25% percent of your time snoozing.Although birds are the only non-mammalian group known to clearly engage in REM sleep, it has been unclear whether sleep develops in the same manner in baby birds. Consequently, Niels Rattenborg of the MPIO, Alexandre Roulin of Unil, and their PhD student Madeleine Scriba, reexamined this question in a population of wild barn owls. They used an electroencephalogram (EEG) and movement data logger in conjunction with minimally invasive EEG sensors designed for use in humans, to record sleep in 66 owlets of varying age. During the recordings, the owlets remained in their nest box and were fed normally by their parents. After having their sleep patterns recorded for up to five days, the logger was removed. All of the owlets subsequently fledged and returned at normal rates to breed in the following year, indicating that there were no long-term adverse effects of eves-dropping on their sleeping brains.

Despite lacking significant eye movements (a trait common to owls), the owlets spent large amounts of time in REM sleep. “During this sleep phase, the owlets’ EEG showed awake-like activity, their eyes remained closed, and their heads nodded slowly”, reports Madeleine Scriba from the University of Lausanne (see video). Importantly, the researchers discovered that just as in baby humans, the time spent in REM sleep declined as the owlets aged.

In addition, the team examined the relationship between sleep and the expression of a gene in the feather follicles involved in producing dark, melanic feather spots. “As in several other avian and mammalian species, we have found that melanic spotting in owls covaries with a variety of behavioral and physiological traits, many of which also have links to sleep, such as immune system function and energy regulation”, notes Alexander Roulin from the University of Lausanne. Indeed, the team found that owlets expressing higher levels of the gene involved in melanism had less REM sleep than expected for their age, suggesting that their brains were developing faster than in owlets expressing lower levels of this gene. In line with this interpretation, the enzyme encoded by this gene also plays a role in producing hormones (thyroid and insulin) involved in brain development.

Although additional research is needed to determine exactly how sleep, brain development, and pigmentation are interrelated, these findings nonetheless raise several intriguing questions. Does variation in sleep during brain development influence adult brain organization? If so, does this contribute to the link between behavioral and physiological traits and melanism observed in adult owls? Do sleep and pigmentation covary in adult owls, and if so how does this influence their behavior and physiology? Finally, Niels Rattenborg from the Max Planck Institute for Ornithology in Seewiesen hopes that “this naturally occurring variation in REM sleep during a period of brain development can be used to reveal exactly what REM sleep does for the developing brain in baby owls, as well as humans.”

Stray prenatal gene network suspected in schizophrenia

Researchers have reverse-engineered the outlines of a disrupted prenatal gene network in schizophrenia, by tracing spontaneous mutations to where and when they likely cause damage in the brain. Some people with the brain disorder may suffer from impaired birth of new neurons, or neurogenesis, in the front of their brain during prenatal development, suggests the study, which was funded by the National Institutes of Health.

“Processes critical for the brain’s development can be revealed by the mutations that disrupt them,” explained Mary-Claire King, Ph.D., University of Washington (UW), Seattle, a grantee of NIH’s National Institute of Mental Health (NIMH). “Mutations can lead to loss of integrity of a whole pathway, not just of a single gene. Our results implicate networked genes underlying a pathway responsible for orchestrating neurogenesis in the prefrontal cortex in schizophrenia.”

King, and collaborators at UW and seven other research centers participating in the NIMH genetics repository, report on their discovery Aug. 1, 2013 in the journal Cell.

“By linking genomic findings to functional measures, this approach gives us additional insight into how early development differs in the brain of someone who will eventually manifest the symptoms of psychosis,” said NIMH Director Thomas R. Insel, M.D.

Earlier studies had linked spontaneous mutations to non-familial schizophrenia and traced them broadly to genes involved in brain development, but little was known about convergent effects on pathways. King and colleagues set out to explore causes of schizophrenia by integrating genomic data with newly available online transcriptome resources that show where in the brain and when in development genes turn on. They compared spontaneous mutations in 105 people with schizophrenia with those in 84 unaffected siblings, in families without previous histories of the illness.

Unlike most other genes, expression levels of many of the 50 mutation-containing genes that form the suspected network were highest early in fetal development, tapered off by childhood, but conspicuously increased again in early adulthood – just when schizophrenia symptoms typically first develop. This adds to evidence supporting the prevailing neurodevelopmental model of schizophrenia. The implicated genes play important roles in migration of cells in the developing brain, communication between brain cells, regulation of gene expression, and related intracellular workings.

Having an older father increased the likelihood of spontaneous mutations for both affected and unaffected siblings. Yet affected siblings were modestly more likely to have mutations predicted to damage protein function. Such damaging mutations were estimated to account for 21 percent of schizophrenia cases in the study sample. The mutations tend to be individually rare; only one gene harboring damaging mutations turned up in more than one of the cases, and several patients had damaging mutations in more than one gene.

The networks formed by genes harboring these damaging mutations were found to vary in connectivity, based on the extent to which their proteins are co-expressed and interact. The network formed by genes harboring damaging mutations in schizophrenia had significantly more nodes, or points of connection, than networks modeled from unaffected siblings. By contrast, the network of genes harboring non-damaging mutations in affected siblings had no more nodes than similar networks in unaffected siblings.

When the researchers compared such network connectivity across different brain tissues and different periods of development, they discovered a notable difference between affected and unaffected siblings: Genes harboring damaging mutations that are expressed together in the fetal prefrontal cortex of people with schizophrenia formed a network with significantly greater connectivity than networks modeled from genes harboring similar mutations in their unaffected siblings at that time in development.

The study results are consistent with several lines of evidence implicating the prefrontal cortex in schizophrenia. The prefrontal cortex organizes information from other brain regions to coordinate executive functions like thinking, planning, attention span, working memory, problem-solving, and self-regulation. The findings suggest that impairments in such functions — often beginning before the onset of symptoms in early adulthood, when the prefrontal cortex fully matures – appear to be early signs of the illness.

The study demonstrates how integrating genomic data and transcriptome analysis can help to pinpoint disease mechanisms and identify potential treatment targets. For example, the mutant genes in the patients studied suggest the possible efficacy of medications targeting glutamate and calcium channel pathways, say the researchers.

"These results are striking, as they show that the genetic architecture of schizophrenia cannot be understood without an appreciation of how genes work in temporal and spatial networks during neurodevelopment," said Thomas Lehner, Ph.D., chief of the NIMH Genomics Research Branch.

FASD impacts brain development throughout childhood and adolescence not just at birth

Medical researchers at the University of Alberta recently published findings showing that brain development is delayed throughout childhood and adolescence for people born with Fetal Alcohol Spectrum Disorder (FASD).

Christian Beaulieu and Carmen Rasmussen, the two primary investigators in the research study, recently published the results of their work in the peer-reviewed journal, The Journal of Neuroscience. Their team scanned 17 people with FASD, and 27 people without the disorder, who were between 5 and 15 years old. Each participant underwent two to three scans, with each scan taking place two to four years apart. This is the first research study involving multiple scans of the same FASD study participants.

Researchers used an advanced MRI method that examines white matter in the brain. White matter forms connections between various regions of the brain and usually develops significantly during childhood and adolescence. Those who took part in the study were imaged multiple times, to see what kinds of changes occurred in brain development as the participants aged. Those without the disorder had marked increases in brain volume and white matter – growth that was lacking in those with FASD. However, the advanced MRI method revealed greater changes in the brain wiring of white matter in the FASD group, which the authors suggest may reflect compensation for delays in development earlier in childhood.

“These findings may suggest that significant brain changes happened earlier in the study participants who didn’t have FASD,” says the study’s first author, Sarah Treit, who is a student in the Centre for Neuroscience at the U of A. “This study suggests alcohol-induced injury with FASD isn’t static – those with FASD have altered brain development, they aren’t developing at the same rate as those without the disorder. And our research showed those with FASD consistently scored lower on all cognitive measures in the study.”

Treit said the research team also made other important observations. Children with FASD who demonstrated the greatest changes in white matter development also made the greatest gains in reading ability – “so the connection seems relevant.” And those with the most severe FASD showed the greatest changes in white matter brain wiring. Scans also confirmed those with FASD have less overall brain volume – this issue neither rectified itself nor worsened throughout the course of the study.

Beaulieu is a researcher in the Department of Biomedical Engineering, while Rasmussen works in the Department of Pediatrics. Their research was funded by the Canadian Institutes of Health Research.

The team is continuing their research in this area, in hopes of finding a biomarker for FASD, and to examine how the brain changes from adolescence into adulthood in those with the disorder. The advanced MRI imaging the team used can pinpoint brain damage present in those with FASD, and could one day guide medical interventions for those with the disorder, which affects one in every 100 Canadians.