Molecular sensor detects early signs of multiple sclerosis

For some, the disease multiple sclerosis (MS) attacks its victims slowly and progressively over a period of many years. For others, it strikes without warning in fits and starts. But all patients share one thing in common: the disease had long been present in their nervous systems, hiding under the radar from even the most sophisticated detection methods. But now, scientists at the Gladstone Institutes have devised a new molecular sensor that can detect MS at its earliest stages—even before the onset of physical signs.

In a new study from the laboratory of Gladstone Investigator Katerina Akassoglou, PhD, scientists reveal in animal models that the heightened activity of a protein called thrombin in the brain could serve as an early indicator of MS. By developing a fluorescently labeled probe specifically designed to track thrombin, the team found that active thrombin could be detected at the earliest phases of MS—and that this active thrombin correlates with disease severity. These findings, reported online in Annals of Neurology, could spur the development of a much-needed early-detection method for this devastating disease.

MS, which afflicts millions of people worldwide, develops when the body’s immune system attacks the protective myelin sheath that surrounds nerve cells. This attack damages the nerve cells, leading to a host of symptoms that include numbness, fatigue, difficulty walking, paralysis and loss of vision. While some drugs can delay these symptoms, they do not treat the disease’s underlying causes—causes that researchers are only just beginning to understand.

Last year, Dr. Akassoglou and her team found that a key step in the progression of MS is the disruption of the blood brain barrier (BBB). This barrier physically separates the brain from the blood circulation and if it breaks down, a blood protein called fibrinogen seeps into the brain. When this happens, thrombin responds by converting fibrinogen into fibrin—a protein that should normally not be present in the brain. As fibrin builds up in the brain, it triggers an immune response that leads to the degradation of the nerve cells’ myelin sheath, over time contributing to the progression of MS.

"We already knew that the buildup of fibrin appears early in the development of MS—both in animal models and in human patients, so we wondered whether thrombin activity could in turn serve as an early marker of disease." said Dr. Akassoglou, who directs the Gladstone Center for In Vivo Imaging Research (CIVIR). She is also a professor of neurology at the University of California, San Francisco, with which Gladstone is affiliated. "In fact, we were able to detect thrombin activity even in our animal models—before they exhibited any of the disease’s neurological signs."

New compound for slowing the aging process can lead to novel treatments for brain diseases

Drug Reduces Brain Changes, Motor Deficits Associated With Huntington’s Disease

A drug that acts like a growth-promoting protein in the brain reduces degeneration and motor deficits associated with Huntington’s disease in two mouse models of the disorder, according to a study appearing November 27 in The Journal of Neuroscience. The findings add to a growing body of evidence that protecting or boosting neurotrophins — the molecules that support the survival and function of nerve cells — may slow the progression of Huntington’s disease and other neurodegenerative disorders.

Huntington’s disease is a brain disorder characterized by the emergence of decreased motor, cognitive, and psychiatric abilities, most commonly appearing in the mid-30s and 40s. The disease is caused by a genetic mutation that leads to abnormal clumps of protein in the brain, eventually resulting in the atrophy and death of nerve cells. While there are drugs to alleviate some symptoms of the disease, there are currently no therapies to delay the onset or slow its progression.

Previous studies of people with Huntington’s disease point to a link between low levels of a neurotrophin called brain-derived neurotrophic factor (BDNF) and symptoms of the disorder. In the current study, Frank Longo, MD, PhD, and others at Stanford University, tested LM22A-4, a drug that specifically binds to and activates the BDNF receptor TrkB on nerve cells, in mice that model the disorder. They found LM22A-4 reduces abnormal protein accumulation, delays nerve cell degeneration, and improves motor skills in the animals. The findings support other recent rodent studies that showed drugs that enhance the action of BDNF can reduce brain changes and symptoms of Huntington’s disease.

“These results strongly suggest that drugs that act, in part, like BDNF could be effective therapeutics for treating Huntington’s disease and other neurodegenerative conditions,” Longo said. 

How quickly the symptoms of Huntington’s disease progress in people vary greatly. Longo’s group examined the effects of LM22A-4 treatment in mice that were predisposed to develop symptoms of Huntington’s disease rapidly (within weeks) or gradually (within months). LM22A-4 treatment reduced the accumulation of abnormal proteins in the striatum and cortex — brain regions affected in Huntington’s disease. Motor behaviors (downward climbing and grip strength) also improved in the mice that received LM22A-4 treatments daily. “The search for treatments that slow the progression of neurodegenerative diseases has gradually shifted from ameliorating symptoms to finding agents that reduce the progression of the disease,” said Gary Lynch, PhD, who studies neurodegeneration at the University of California, Irvine, and was not involved with this study. “Given that this drug is clinically plausible, these results open up exciting possibilities for treating a devastating neurodegenerative disease,” he added.

Brain imaging differences in infants at genetic risk for Alzheimer’s

Researchers from Brown University and Banner Alzheimer’s Institute have found that infants who carry a gene associated with increased risk for Alzheimer’s disease tend to have differences in brain development compared to children without the gene. The study, published in JAMA Neurology, demonstrates some of the earliest developmental differences associated with a gene variant called APOE ε4, a common genotype and a known risk factor for late-onset Alzheimer’s.

The researchers imaged the brains of 162 healthy infants between the ages of two months and 25 months. All of the infants had DNA tests to see which variant of the APOE gene they carried. Sixty of them had the ε4 variant that has been linked to an increased risk of Alzheimer’s. Using a specialized MRI technique, the researchers compared the brains of ε4 carriers with non-carriers. They found that children who carry the APOE ε4 gene tended to have increased brain growth in areas in the frontal lobe, and decreased growth in areas in several areas in the middle and rear of the brain. The decreased growth was found in areas that tend to be affected in elderly patients who have Alzheimer’s disease.

Researchers emphasized that the findings do not mean that any of the children in the study are destined to develop Alzheimer’s or that the brain changes detected are the first clinical signs of the disease. What the findings do suggest, however, is that brains of APOE ε4 carriers tend to develop differently from those of non-ε4 carriers beginning very early in life. It is possible that these early changes provide a “foothold” for the later pathologies that lead to Alzheimer’s symptoms, the researchers say. Information from this study may be an important step toward understanding how this gene confers risk for Alzheimer’s, something that is not currently well understood.

“This work is about understanding how this gene influences brain development,” said Sean Deoni, who oversees Brown University’s Advanced Baby Imaging Lab and was one of the study’s senior authors. “These results do not establish a direct link to the changes seen in Alzheimer’s patients, but with more research they may tell us something about how the gene contributes to Alzheimer’s risk later in life.”

The APOE ε4 variant linked to Alzheimer’s is present in about 25 percent of the U.S. population. Not everyone who carries the gene gets Alzheimer’s, but 60 percent of people who develop the disease have at least one copy of the ε4 gene.

The gene is thought to have several different roles in the blood and brain, some of which remain to be clarified. For instance, it has been shown to participate in regulation of cholesterol, a molecule that is involved in the development of gray matter and white matter brain cells. It has also been shown to participate in the regulation of amyloid, a brain protein that accumulates in Alzheimer’s and is now being targeted by investigational treatments. Studies are needed to clarify the ways in which APOE, human development, aging and other risk factors may conspire to produce the brain changes involved in Alzheimer’s disease.

The researchers used an MRI technique developed at Brown’s Advanced Baby Imaging Lab. The technique quiets the MRI machine to a whisper, enabling the brains of healthy babies to be imaged while they sleep without medication. The technique also enables imaging of both gray matter — the part of the brain that contains neurons and nerve fibers — and white matter, which contains the fatty material that insulates the nerve fibers. Both gray and white matter are thought to have a role in Alzheimer’s. White matter growth begins shortly after birth and is an important measure of brain development.

“We’re in a good spot to be able to investigate how this gene influences development in healthy infants,” said Deoni, assistant professor of engineering at Brown. “These infants are not medicated and not showing any cognitive decline — quite the opposite, actually; they’re developing normally.”

There is no reason to believe that the children won’t continue to develop normally, Deoni said. There is no consistent evidence to suggest that ε4 carriers suffer any cognitive problems or developmental delay. And the areas of increased growth raise the possibility that the gene might actually confer some advantages to infants early on. Utimately the researchers hope the findings could lead to new strategies for preventing a disease that currently affects more than 5.2 million people in the U.S. alone.

“It may sound scary that we could detect these brain differences in infants,” said Dr. Eric Reiman, executive director of the Banner Alzheimer’s Institute in Arizona and another senior author on the paper. “But it is our sincere hope that an understanding of the earliest brain changes involved in the predisposition to Alzheimer’s will help researchers find treatments to prevent the clinical onset of Alzheimer’s disease — and do so long before these children become senior citizens.”

Breaking the Brain Clock Predisposes Nerve Cells to Neurodegeneration

As we age, our body rhythms lose time before they finally stop. Breaking the body clock by genetically disrupting a core clock gene, Bmal1, in mice has long been known to accelerate aging , causing arthritis, hair loss, cataracts, and premature death.

New research now reveals that the nerve cells of these mice with broken clocks show signs of deterioration before the externally visible signs of aging are apparent, raising the possibility of novel approaches to staving off or delaying neurodegeneration – hallmarks of Parkinson’s and Alzheimer’s diseases.

Erik Musiek, M.D., Ph.D., who was a postdoctoral fellow in the lab of Garret FitzGerald, M.D., director of the Institute of Translational Medicine and Therapeutics, Perelman School of Medicine, University of Pennsylvania, took on this project four years ago. Musiek, now an assistant professor at Washington University, completed this line of research over the last two years in the lab of David Holtzman, M.D., also at WashU.

The Penn-WashU team found that the expression of certain clock genes, including Bmal1, plays a fundamental role in delaying emergence of age-related signs of decay in the brain. The clock proteins appear to do this by protecting the brain against oxidative stress – a process akin to rusting – that is normally controlled by enzymes that degrade harmful forms of oxygen generated in the course of normal metabolism. Their findings appear this week in the Journal of Clinical Investigation.

 “I had lunch with Garret four years ago when I was a resident in neurology at Penn and this led me to work in his lab,” recalls Musiek. “He had studied oxidative stress in cells and the lab was actively pursuing the role of the molecular clock in cardiovascular and metabolic function. However, he hadn’t studied the brain nor the role of the clock as a regulator of oxidative stress. Others had connected the clock to signs of aging, but hadn’t focused on the brain - it seemed like an opportunity to pursue.”

They found, to their surprise, that inflammation – reflected by activation of astrocytes – brain cells involved in this type of response, among other functions — was marked in young mice in which the clock was broken by deleting Bmal1. This anticipated even more marked changes in brain pathology as the mice aged, including declines in how parts of the brain connected to each other and degenerative features in nerve-cell anatomy – all characteristic of Parkinsons and Alzheimer’s disease in humans.

“When we saw this, we knew we were on to something,” notes Musiek.

Further experiments revealed that these effects were not restricted to disrupting the function of Bmal1, but also occurred when genes – Clock and Npas2 – with which Bmal1 works in tandem, were both removed. By contrast, deletion of other genes in the clock apparatus had no such effect.

As for mechanism, the exaggerated rusting, or oxidation, was key. Expression of several antioxidant enzymes, which normally keep oxidant stress in check are themselves controlled by clock proteins, and thus were depleted when the clock was broken. Musiek and his colleagues found evidence that inflammation and the attendant oxidant stress were both increased in the brains of the mutant mice.

Experimental drugs are beginning to emerge that may retain waning rhythms driven by the molecular clock. “Erik’s studies raise the intriguing possibility of novel therapeutic approaches to delaying the progress of age-related diseases, perhaps not only those related to the brain, as suggested by the present studies, but also in other systems, such as cardiometabolic function,” says FitzGerald.

In a final twist, the Penn-WashU team pinned the neuroprotective role of the body clock to clock genes in neurons and astrocytes, rather than changes in whole-animal circadian rhythms. By selectively deleting Bmal1 in these cell types, they found that the inflammatory aspects of astrocytes, neurodegeneration, and hallmarks of oxidative stress and inflammation seen when Bmal1 was missing in all cells of the body was recapitulated.

 “Our findings indicate that the protein complex of BMAL1 with CLOCK or NPAS2, in addition to, or perhaps intrinsic to the complex’s internal body-clock function, regulates protection of the brain from inflammation and oxygen free-radical induced damage. This dynamic system connects impaired clock-gene function to neurodegeneration for the first time,” says Musiek.

Common brain cell plays key role in shaping neural circuit

Stanford University School of Medicine neuroscientists have discovered a new role played by a common but mysterious class of brain cells.

Their findings, published online Nov. 24 in Nature, show that these cells, called astrocytes because of their star-like shape, actively refine nerve-cell circuits by selectively eliminating synapses — contact points through which nerve cells, or neurons, convey impulses to one another — much as a sculptor chisels away excess bits of rock to create an artwork.

“This was an entirely unknown function of astrocytes,” said Ben Barres, MD, PhD, professor and chair of neurobiology and the study’s senior author. The lead author was Won-Suk Chung, PhD, a postdoctoral scholar in Barres’ lab. More than one-third of all the cells in the human brain are astrocytes. But until quite recently, their role in the brain has remained obscure.

The study was performed on brain tissue from mice, but it is likely to apply to people as well, Barres said.

The discovery adds to a growing body of evidence that substantial remodeling of brain circuits takes place in the adult brain and that astrocytes are master sculptors of its constantly evolving synaptic architecture. The findings also raise the question of whether deficits and excesses in this astrocytic function could underlie, respectively, the loss of this remodeling capacity in old age or the wholesale destruction of synapses that erupts in neurodegenerative disorders, such as Alzheimer’s and Parkinson’s disease.

“Astrocytes are in the driver’s seat when it comes to synapse formation, function and elimination,” Barres said. In previous studies, he and his colleagues have shown that astrocytes play a critical role in determining exactly where and when new synapses are generated.

The new study showed that astrocytes’ synapse-gobbling behavior persists into adulthood and is triggered by activity in the neurons, suggesting astrocytes may be central to the constant fine-tuning and reconfiguring of brain circuits occurring throughout our lives in response to experiences such as learning, recollection, emotion and motion. While a healthy brain’s neurons remain intact for much a person’s lifetime, the connections between them — the synapses — are constantly forming, strengthening, weakening or dying.

The Barres team also has previously implicated another brain cell type, collectively known as microglia, in synaptic pruning in early development, when the young brain undergoes ongoing episodes of circuit remodeling. The role of astrocytes in synaptic refining, the new study shows, differs from that of microglia both in timing and mechanism.

Barres’ team began to suspect astrocytes’ participation in the pruning process when, having developed methods for isolating exceptionally pure populations of different types of brain cells, they saw that the genes for two separate biochemical pathways were active in astrocytes. Both of these pathways are involved in phagocytosis, the trash-collection process by which specialized cells in the body engulf, ingest and digest dead cells; foreign materials, including bacteria; debris from wounds; and so forth. At the leading end of the two pathways were two phagocytic receptors, MERKTK and MEGF10, which in other cell types have been shown to bind to particular proteins on targeted cells or materials, triggering the ensuing engulfment, ingestion and digestion of the targets.

It’s known that much of an astrocyte’s surface membrane is typically in close contact with neurons. In fact, a single astrocyte may ensheathe thousands of synapses. It was only natural, Barres said, to wonder whether astrocytes play some role in eliminating synapses.

The researchers first demonstrated that both MERKTK and MEGF10, along with their entire tool kits of cooperating proteins, are present in living astrocytes in the mouse brain. (In unpublished work, they have since confirmed this using human astrocytes.) Next, they showed that mouse astrocytes in a lab dish eagerly gobbled up synapses and dispatched them to their lysosomes, highly acidic internal garbage disposals found in most cells in the body. But this engulfment was dependent on astrocytes having functional MEGF10 and MERTK. Disabling one or the other receptor’s function cut in half astrocytes’ ability to engorge themselves on synapses; knocking out both receptors lowered the synapse-eating activity by about 90 percent.

To see if this happens in real life, Chung, Barres and their associates turned to a familiar experimental model: a brain area called the lateral geniculate nucleus, which is a critical component of the brain’s vision-processing system. The LGN receives inputs from neurons just a couple of steps downstream from the photoreceptors in the retina. In early development, neurons in the LGN are innervated by inputs from both eyes. But at a critical point in development, a highly selective synaptic-pruning process kicks in, resulting in each neuron from one side of the LGN being contacted pretty much only by neurons from a single eye. This pruning process in the LGN is dependent on the transmission of waves of spontaneous neuronal impulses originating in the retina.

Experimenting with mice that had entered the critical period for synaptic pruning in the LGN, the investigators labeled the incoming neurons in this system with different-colored stains so their synaptic regions could be identified within astrocytes if the astrocytes ate them up. And sure enough, a lot of this label turned up inside astrocytes’ lysosomes, indicating that astrocytes were actively ingesting synapses. Knocking out one or another or, especially, both of the two phagocytic receptors greatly reduced the amount of labeled synaptic material found in astrocytes. Impairing astrocytic MERKTK and MEGF10 function also caused a failure of LGN neurons to restrict their inputs to only neurons from just one eye, clearly implicating astrocytes in that process. Electrophysiology experiments proved that the LGN neurons in the MERKTK- and MEGF10-knockout mice retained an excessive number of synapses, demonstrating that astrocytes play an active role in pruning synapses during development.

Importantly, injection of a drug blocking the transmission of spontaneous waves of electrical impulses originating in the retina severely impaired astrocytes’ ability to eat synapses, showing that the synapse-pruning propensity is linked to neuronal activity. Other tests showed that astrocytic phagocytosis of synapses continues into adulthood.

Barres said this raises the question of whether astrocytes function throughout life to continually restructure our neuronal circuitry in response to experientially induced brain activity. If astrocytes’ synaptic snacking slows with aging, as that of other phagocytic cell types is known to do, it could reduce the aging brain’s capacity to adapt to new experiences, he said. “Maybe you need the astrocytes to gobble up old synapses to make room for new ones.”

If so, it may be possible someday to design drugs to keep astrocytes’ phagocytic process from slowing, Barres added. Such drugs might prevent the accumulation in aging brains of past-their-prime synapses, which are vulnerable to degeneration in Alzheimer’s, Parkinson’s and other neurodegenerative disease characterized by massive synapse loss.

(Image credit)