The link between circadian rhythms and aging

Human sleeping and waking patterns are largely governed by an internal circadian clock that corresponds closely with the 24-hour cycle of light and darkness. This circadian clock also controls other body functions, such as metabolism and temperature regulation.

Studies in animals have found that when that rhythm gets thrown off, health problems including obesity and metabolic disorders such as diabetes can arise. Studies of people who work night shifts have also revealed an increased susceptibility to diabetes.

A new study from MIT shows that a gene called SIRT1, previously shown to protect against diseases of aging, plays a key role in controlling these circadian rhythms. The researchers found that circadian function decays with aging in normal mice, and that boosting their SIRT1 levels in the brain could prevent this decay. Conversely, loss of SIRT1 function impairs circadian control in young mice, mimicking what happens in normal aging. 

Since the SIRT1 protein itself was found to decline with aging in the normal mice, the findings suggest that drugs that enhance SIRT1 activity in humans could have widespread health benefits, says Leonard Guarente, the Novartis Professor of Biology at MIT and senior author of a paper describing the findings in the June 20 issue of Cell.

“If we could keep SIRT1 as active as possible as we get older, then we’d be able to retard aging in the central clock in the brain, and health benefits would radiate from that,” Guarente says.

Staying on schedule

In humans and animals, circadian patterns follow a roughly 24-hour cycle, directed by the circadian control center of the brain, called the suprachiasmatic nucleus (SCN), located in the hypothalamus.

“Just about everything that takes place physiologically is really staged along the circadian cycle,” Guarente says. “What’s now emerging is the idea that maintaining the circadian cycle is quite important in health maintenance, and if it gets broken, there’s a penalty to be paid in health and perhaps in aging.”

Last year, Guarente found that a robust circadian period correlated with longer lifespan in mice. That got him wondering what role SIRT1, which has been shown to prolong lifespan in many animals, might play in that phenomenon. SIRT1, which Guarente first linked with aging more than 15 years ago, is a master regulator of cell responses to stress, coordinating a variety of hormone networks, proteins and genes to help keep cells alive and healthy.

To investigate SIRT1’s role in circadian control, Guarente and his colleagues created genetically engineered mice that produce different amounts of SIRT1 in the brain. One group of mice had normal SIRT1 levels, another had no SIRT1, and two groups had extra SIRT1 — either twice or 10 times as much as normal.

Mice lacking SIRT1 had slightly longer circadian cycles (23.9 hours) than normal mice (23.6 hours), and mice with a 10-fold increase in SIRT1 had shorter cycles (23.1 hours).

In mice with normal SIRT1 levels, the researchers confirmed previous findings that when the 12-hour light/dark cycle is interrupted, younger mice readjust their circadian cycles much more easily than older ones. However, they showed for the first time that mice with extra SIRT1 do not suffer the same decline in circadian control as they age.

The researchers also found that SIRT1 exerts this control by regulating the genes BMAL and CLOCK, the two major keepers of the central circadian clock.

Enhancing circadian function

A growing body of evidence suggests that being able to respond to large or small disruptions of the light/dark cycle is important to maintaining healthy metabolic function, Guarente says.

“Essentially we experience a mini jet lag every day because the light cycle is constantly changing. The critical thing for us is to be able to adapt smoothly to these jolts,” Guarente says. “Many studies in mice say that while young mice do this perfectly well, it’s the old mice that have the problem. So that could well be true in humans.”

If so, it could be possible to treat or prevent diseases of aging by enhancing circadian function — either by delivering SIRT1 activators in the brain or developing drugs that enhance another part of the circadian control system, Guarente says.

“I think we should look at every aspect of the machinery of the circadian clock in the brain, and any intervention that can maintain that machinery with aging ought to be good,” he says. “One entry point would be SIRT1, because we’ve shown in mice that genetic maintenance of SIRT1 helps maintain circadian function.”

Some SIRT1 activators are now being tested against diabetes, inflammation and other diseases, but they are not designed to cross the blood-brain barrier and would likely not be able to reach the SCN. However, Guarente believes it could be possible to design SIRT1 activators that can get into the brain.

Roman Kondratov, an associate professor of biology at Cleveland State University, says the study raises several exciting questions regarding the potential to delay or reverse age-related changes in the brain through rejuvenation of the circadian clock with SIRT1 enhancement.

“The importance of this study is that it has both basic and potentially translational applications, taking into account the fact that pharmacological modulators of SIRT1 are currently under active study,” Kondratov says.

Researchers in Guarente’s lab are now investigating the relationship between health, circadian function and diet. They suspect that high-fat diets might throw the circadian clock out of whack, which could be counteracted by increased SIRT1 activation.

(Image: Wikimedia Commons)

"BigBrain" Study Provides Most Detailed 3-D Map of the Brain Yet

A landmark three-dimensional digital reconstruction of a complete human brain, called the BigBrain, shows the brain anatomy in microscopic detail at a spatial resolution of 20 micrometers—smaller than the size of one fine strand of hair.

The reconstruction, published in the 21 June issue of the journal Science, exceeds the resolution of all existing reference brains presently in the public domain, and will be made freely available to the broader scientific community.

The fine-grained anatomical resolution of the BigBrain will allow scientists who use it to gain insights into the neurobiological basis of cognition, language, emotions and other processes, according to the study. The anatomical tool yielded by the researchers will serve as an atlas for neurosurgery and provide a framework for research in many directions, including enhanced understanding of brain diseases, such as Alzheimer’s disease.

"It is a common basis for scientific discussions because everybody can work with this brain model," said Science co-author Karl Zilles, senior professor of the Jülich Aachen Research Alliance.

The new reference brain, which is part of the European Human Brain Project, “redefines traditional maps from the beginning of the 20th century,” explained lead author Katrin Amunts from the Research Centre Jülich. Amunts serves as director of the Cecile and Oskar Vogt Institute for Brain Research at the Heinrich Heine University Düsseldorf in Germany.

"The authors pushed the limits of current technology," said Science Senior Editor Peter Stern. Existing reference brains do not probe further than the macroscopic, or visible, components of the brain. The BigBrain provides a resolution much finer than the typical 1 millimeter resolution from MRI studies. "The spatial resolution the researchers achieved exceeds that of presently available reference brains by a factor of 50," said Stern.

"Of course, we would love to have spatial resolution going down to 1 micrometer," said Amunts in a 19 June press teleconference. However, "there are simply no computers at this moment which would be capable to process such data, to visualize this or to analyze it."

To create the detailed brain atlas, Amunts and colleagues took advantage of new advances in computing capacities and image analysis. Using a special tool called a microtome, they carefully cut the paraffin-covered brain of a 65-year-old female into 20 micrometer-thick sections.

The project was “a tour-de-force to assemble images of over 7400 individual histological sections, each with its own distortions, rips and tears, into a coherent 3-D volume,” said Science co-author Alan Evans, a professor at the Montreal Neurological Institute at McGill University in Montreal, Canada.

The sections were mounted on slides, stained to detect cell structures and finally digitized with a high-resolution flatbed scanner so researchers could reconstruct the high-resolution 3-D brain model. It took approximately 1000 hours to collect the data.

The researchers’ future plans for using the map include extracting measurements of cortical thickness to gain insights into aging and neurodegenerative disorders. Eventually, Amunts and colleagues hope to build a brain model at the resolution of 1 micron to capture details of single cell morphology. Detailed brain maps can aid researchers who are exploring the full set of neural connections and real-time brain activity, as scientists discussed recently in a Capitol Hill briefing sponsored by AAAS.

The creation of such a detailed brain map, offering a gateway to unprecedented insights into the brain’s anatomy and organization, was long in the works. “It was a dream for almost 20 years,” Amunts said. “The dream came true because of an interdisciplinary and intercontinental collaboration spanning from Europe to Canada and from neuroanatomy to supercomputing .”

Though not directly related to the BRAIN Initiative announced by President Barack Obama earlier this year, the work by Amunts and colleagues supports the Initiative’s goal of giving scientists the best possible tools with which to obtain a dynamic picture of the brain.

Scientists Discover Key Signaling Pathway that Makes Young Neurons Connect

Neuroscientists at The Scripps Research Institute (TSRI) have filled in a significant gap in the scientific understanding of how neurons mature, pointing to a better understanding of some developmental brain disorders.

In the new study, the researchers identified a molecular program that controls an essential step in the fast-growing brains of young mammals. The researchers found that this signaling pathway spurs the growth of neuronal output connections by a mechanism called “mitochondrial capture,” which has never been described before.

“Mutations that may affect this signaling pathway already have been found in some autism cases,” said TSRI Professor Franck Polleux, who led the research, published June 20, 2013 in the journal Cell.

Branching Out

Polleux’s laboratory is focused on identifying the signaling pathways that drive neural development, with special attention to the neocortex—a recently evolved structure that handles the “higher” cognitive functions in the mammalian brain and is highly developed in humans.

In a widely cited study published in 2007, Polleux’s team identified a trigger of an early step in the development of the most important class of neocortical neurons. As these neurons develop following asymmetric division of neural stem cells, they migrate to their proper place in the developing brain. Meanwhile they start to sprout a root-like mesh of input branches called dendrites from one end, and, from the other end, a long output stalk called an axon. Polleux and his colleagues found that the kinase LKB1 provides a key signal for the initiation of axon growth in these immature cortical neurons.

In the new study, Polleux’s team followed up this discovery and found that LKB1 also is crucially important for a later stage of these neurons’ development: the branching of the end of the axon onto the dendrites of other neurons.

“In experiments with mice, we knocked the LKB1 gene out of immature cortical neurons that had already begun growing an axon, and the most striking effect was a drastic reduction in terminal branching,” said Julien Courchet, a research associate in the Polleux laboratory who was a lead co-author of the study. “We saw this also in lab dish experiments, and when we overexpressed the LKB1 gene, the result was a dramatic increase in axon branching.”

Further experiments by Courchet showed that LKB1 drives axonal branching by activating another kinase, NUAK1. The next step was to try to understand how this newly identified LKB1-NUAK1 signaling pathway induced the growth of new axon branches.

Stopping the Train in Its Tracks

Following a thin trail of clues, the researchers decided to look at the dynamics of microtubules. These tiny railway-like tracks are laid down within axons for the efficient transport of molecular cargoes and are altered and extended during axonal branching. Although they could find no major change in microtubule dynamics within immature axons lacking LKB1 or NUAK1, the team did discover one striking abnormality in the transport of cargoes along these microtubules. Tiny oxygen-reactors called mitochondria, which are the principal sources of chemical energy in cells, were transported along axons much more actively—and by contrast, became almost immobile when LKB1 and NUAK1 were overexpressed.

But the LKB1-NUAK1 signals weren’t just immobilizing mitochondria randomly. They were effectively inducing their capture at points on the axons where axons form synaptic connections with other neurons. “When we removed LKB1 or NUAK1 in cortical neurons, the mitochondria were no longer captured at these points,” said Tommy Lewis, Jr., a research associate in the Polleux Laboratory who was co-lead author of the study.

“We argue that there must be an active ‘homing factor’ that specifies where these mitochondria stop moving,” said Polleux. “And we think that this is essentially what the LKB1-NAUK1 signaling pathway does here.”

Looking Ahead

Precisely how the capture of mitochondria at nascent synapses promotes axonal branching is the object of a further line of investigation in the Polleux laboratory. “We think that we have uncovered something very interesting about mitochondrial function at synapses,” Polleux said.

In addition to its basic scientific importance, the work is likely to be highly relevant medically. Developmentally related brain disorders such as epilepsy, autism and schizophrenia typically involve abnormalities in neuronal connectivity. Recent genetic surveys have found NUAK1-related gene mutations in some children with autism, for example. “Our study is the first one to identify that NUAK1 plays a crucial role during the establishment of cortical connectivity and therefore suggests why this gene might play a role in autistic disorder,” Polleux says.

He notes, too, that declines in normal mitochondrial transport within axons have been observed in neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases. “In the light of our findings, we wonder if the decreased mitochondrial mobility observed in these cases might be due not to a transport defect, but instead to a defect in mitochondrial capture in aging neurons,” he said. “We’re eager to start doing experiments to test such possibilities.”

(Image: Shutterstock)

Animal study shows promising path to prevent epilepsy

Duke Medicine researchers have identified a receptor in the nervous system that may be key to preventing epilepsy following a prolonged period of seizures.

Their findings from studies in mice, published online in the journal Neuron on June 20, 2013, provide a molecular target for developing drugs to prevent the onset of epilepsy, not just manage the disease’s symptoms.

"Unfortunately, there are no preventive therapies for any common disorder of the human nervous system – Alzheimer’s, Parkinson’s, schizophrenia, epilepsy – with the exception of blood pressure-lowering drugs to reduce the likelihood of stroke," said study author James O. McNamara, M.D., professor of neurobiology at Duke Medicine.

Epilepsy is a serious neurological disorder marked by recurring seizures. Temporal lobe epilepsy – where seizures occur in the region of the brain where memories are stored and language, emotions and senses are processed – is the most common form, and can be devastating. Because afflicted individuals have seizures that impair their awareness and may have associated behavioral problems, they may have difficulty with everyday activities, including holding a job or obtaining a driver’s license.

Conventional therapies to treat epilepsy address the disease’s symptoms by trying to reduce the likelihood of having a seizure. However, many people with temporal lobe epilepsy still have seizures despite taking these drugs.

"This study opens a promising new avenue of research into treatments that may prevent the development of epilepsy," said Vicky Whittemore, PhD, a program director at the National Institute of Neurological Disorders and Stroke, who oversees the grants that funded this study.

Retrospective studies of people with severe temporal lobe epilepsy reveal that many of them initially have an episode of prolonged seizures, known as status epilepticus. Status epilepticus is often followed by a period of seizure-free recovery before people start to experience recurring temporal lobe seizures.

In animal studies, inducing status epilepticus in an otherwise healthy animal can cause them to become epileptic. The prolonged seizures in status epilepticus are therefore thought to cause or importantly contribute to the development of epilepsy in humans.

"An important goal of this field has been to identify the molecular mechanism by which status epilepticus transforms a brain from normal to epileptic," said McNamara. "Understanding that mechanism in molecular terms would provide a target with which one could intervene pharmacologically, perhaps to prevent an individual from becoming epileptic."

Earlier research in epilepsy flagged a receptor in the nervous system called TrkB as a key player in transforming the brain from normal to epileptic. In the current study, McNamara and his colleagues sought to confirm if TrkB was important for status epilepticus-induced epilepsy.

Using an approach combining chemistry and genetic analyses, the researchers studied normal and genetically altered mice. The genetically altered mice were unique in that a drug, 1NMPP1, inhibited TrkB in their brains. If the drug stopped the genetically altered mice from becoming epileptic, this genetic approach would prove that inhibiting TrkB prevents the onset of epilepsy.

When the researchers caused status epilepticus in the animals, both the normal and genetically modified mice developed epilepsy. However, treatment with 1NMPP1 after the prolonged period of seizures prevented epilepsy in the genetically altered but not the normal mice.

"This demonstrated that it is possible to intervene following status epilepticus and prevent the animal from becoming epileptic," McNamara said.

Importantly, the researchers only administered treatment with 1NMPP1 for two weeks, which was sufficient to prevent epilepsy from developing in the mice when tested many weeks later. The results suggest that a preventive therapy may only need to be given for a limited period of time following the initial bout of prolonged seizures, not an individual’s entire life, which could prevent unnecessary side effects that come with long-term use of drugs.

In future studies, the researchers hope to determine the exact time window in which TrkB signaling needs to be repressed to prevent the onset of epilepsy. Long term, this research provides a molecular target for developing the first drugs to prevent epilepsy.

"This study provides a strong rationale for the development of selective inhibitors of TrkB signaling," said McNamara.