Posts tagged Huntington’s disease

Scientists have described a way to convert human skin cells directly into a specific type of brain cell affected by Huntington’s disease, an ultimately fatal neurodegenerative disorder. Unlike other techniques that turn one cell type into another, this new process does not pass through a stem cell phase, avoiding the production of multiple cell types, the study’s authors report.

(Image caption: Human skin cells (top) can be converted into medium spiny neurons (bottom) with exposure to the right combination of microRNAs and transcription factors, according to work by Andrew Yoo and his research team)

The researchers, at Washington University School of Medicine in St. Louis, demonstrated that these converted cells survived at least six months after injection into the brains of mice and behaved similarly to native cells in the brain.

“Not only did these transplanted cells survive in the mouse brain, they showed functional properties similar to those of native cells,” said senior author Andrew S. Yoo, PhD, assistant professor of developmental biology. “These cells are known to extend projections into certain brain regions. And we found the human transplanted cells also connected to these distant targets in the mouse brain. That’s a landmark point about this paper.”

The work appears Oct. 22 in the journal Neuron.

The investigators produced a specific type of brain cell called medium spiny neurons, which are important for controlling movement. They are the primary cells affected in Huntington’s disease, an inherited genetic disorder that causes involuntary muscle movements and cognitive decline usually beginning in middle-adulthood. Patients with the condition live about 20 years following the onset of symptoms, which steadily worsen over time.

The research involved adult human skin cells, rather than more commonly studied mouse cells or even human cells at an earlier stage of development. In regard to potential future therapies, the ability to convert adult human cells presents the possibility of using a patient’s own skin cells, which are easily accessible and won’t be rejected by the immune system.

To reprogram these cells, Yoo and his colleagues put the skin cells in an environment that closely mimics the environment of brain cells. They knew from past work that exposure to two small molecules of RNA, a close chemical cousin of DNA, could turn skin cells into a mix of different types of neurons.

In a skin cell, the DNA instructions for how to be a brain cell, or any other type of cell, is neatly packed away, unused. In past research published in Nature, Yoo and his colleagues showed that exposure to two microRNAs called miR-9 and miR-124 altered the machinery that governs packaging of DNA. Though the investigators still are unraveling the details of this complex process, these microRNAs appear to be opening up the tightly packaged sections of DNA important for brain cells, allowing expression of genes governing development and function of neurons.

Knowing exposure to these microRNAs alone could change skin cells into a mix of neurons, the researchers then started to fine tune the chemical signals, exposing the cells to additional molecules called transcription factors that they knew were present in the part of the brain where medium spiny neurons are common.

“We think that the microRNAs are really doing the heavy lifting,” said co-first author Matheus B. Victor, a graduate student in neuroscience. “They are priming the skin cells to become neurons. The transcription factors we add then guide the skin cells to become a specific subtype, in this case medium spiny neurons. We think we could produce different types of neurons by switching out different transcription factors.”

Yoo also explained that the microRNAs, but not the transcription factors, are important components for the general reprogramming of human skin cells directly to neurons. His team, including co-first author Michelle C. Richner, senior research technician, showed that when the skin cells were exposed to the transcription factors alone, without the microRNAs, the conversion into neurons wasn’t successful.

The researchers performed extensive tests to demonstrate that these newly converted brain cells did indeed look and behave like native medium spiny neurons. The converted cells expressed genes specific to native human medium spiny neurons and did not express genes for other types of neurons. When transplanted into the mouse brain, the converted cells showed morphological and functional properties similar to native neurons.

To study the cellular properties associated with the disease, the investigators now are taking skin cells from patients with Huntington’s disease and reprogramming them into medium spiny neurons using the approach described in the new paper. They also plan to inject healthy reprogrammed human cells into mice with a model of Huntington’s disease to see if this has any effect on the symptoms.

(Source: news.wustl.edu)

Creating induced pluripotent stem cells or iPS cells allows researchers to establish “disease in a dish” models of conditions ranging from Alzheimer’s disease to diabetes. Scientists at Yerkes National Primate Research Center, Emory University have now applied the technology to a model of Huntington’s disease (HD) in transgenic nonhuman primates, allowing them to conveniently assess the efficacy of potential therapies on neuronal cells in the laboratory.

(Image caption: Neural progenitor cells derived from transgenic rhesus macaque iPS cells show features of Huntington’s disease pathology, making them a useful tool for therapeutic discovery.)

The results were published this week in Stem Cell Reports.

"A highlight of our model is that our progenitor cells and neurons developed cellular features of HD such as intranuclear inclusions of mutant Huntingtin protein, which most of the currently available cell models do not present," says senior author Anthony Chan, PhD, DVM, associate professor of human genetics at Emory University School of Medicine and Yerkes National Primate Research Center. "We could use these features as a readout for therapy using drugs or a genetic manipulation."

Chan and his colleagues were the first in the world to establish a transgenic nonhuman primate model of HD. HD is an inherited neurodegenerative disorder that leads to the appearance of uncontrolled movements and cognitive impairments, usually in adulthood. It is caused by a mutation that introduces an expanded region where one amino acid (glutamine) is repeated dozens of times in the huntingtin protein.

The non-human primate model has extra copies of the huntingtin gene that contains the expanded glutamine repeats. In the non-human primate model, motor and cognitive deficits appear more quickly than in most cases of Huntington’s disease in humans, becoming noticeable within the first two years of the monkeys’ development.

First author Richard Carter, PhD, a graduate of Emory’s Genetics and Molecular Biology doctoral program, and his colleagues created iPS cells from the transgenic monkeys by reprogramming cells derived from the skin or dental pulp. This technique uses retroviruses to introduce reprogramming factors into somatic cells and induces a fraction of them to become pluripotent stem cells. Pluripotent stem cells are able to differentiate into any type of cell in the body, under the right conditions.

Carter and colleagues induced the iPS cells to become neural progenitor cells and then differentiated neurons. The iPS-derived neural cells developed intracellular and intranuclear aggregates of the mutant huntingtin protein, a classic sign of Huntington’s pathology, as well as an increased sensitivity to oxidative stress.

The sensitivity to oxidative stress was a useful indicator; it could be ameliorated in cell culture, either by a RNA-based gene knockdown approach, or the drug memantine, which is currently being investigated for Huntington’s disease in a human clinical trial.

"We tested two known experimental interventions, but our findings are a proof of principle that this system could be a valuable tool for the discovery and evaluation of other therapies," Chan says.

(Source: news.emory.edu)

The protein that is mutated in Huntington’s disease is critical for wiring the brain in early life, according to a new Duke University study.

(Image caption: The protein associated with Huntington’s disease, Htt, is critical in early brain development. Brains of 5-week-old mice whose Htt was deleted show signs of cellular stress — reactive astrocytes (green) and microglia (white and red) and faulty connections — in brain circuits that have already been linked to the disease. Credit: Spencer McKinstry)

Huntington’s disease is a progressive neurodegenerative disorder that causes a wide variety of symptoms, such as uncontrolled movements, inability to focus or remember, depression and aggression. By the time these symptoms appear, usually in middle age, the disease has already ravaged the brain.

The new findings, published July 9 in the Journal of Neuroscience, add to growing evidence that Huntington’s and other neurodegenerative disorders, such as Alzheimer’s disease, may take root during development, said lead author Cagla Eroglu, an assistant professor of cell biology in the Duke University Medical School, and member of the Duke Institute for Brain Sciences.

“The study is exciting because it means that, if we understand what these developmental errors are, we may be able to interfere with the first stage of the disease, before it shows itself,” Eroglu said.

Several years ago, Eroglu and her team were looking for molecular players involved in the formation of new connections, or synapses, in early brain development in mice when their studies unexpectedly hit on the huntingtin (Htt) protein, which is present throughout the body and which forms clumps in the brain cells of people with Huntington’s disease.

“(Htt) had been implicated in certain cellular functions and synaptic dysfunction in Huntington’s, but the possibility that Htt is playing a direct role in synapse formation was not explored,” Eroglu said.

To understand the protein’s role as synapses form, the scientists created mice in which Htt is deleted only in the cortex, a part of the brain that is implicated in the disease and that controls perception, memory and thought.

At three weeks of age (roughly similar to the first two years of human life), a time when a mouse begins to take in its surroundings through its eyes and ears, the synapses of the mutant mice formed more rapidly compared with those of healthy mice, the scientists found.

 But by five weeks, when some synapses typically strengthen while others weaken in a normal process called pruning, the synapses had completely deteriorated in the mutant mice. In collaboration with another Duke researcher, Henry Yin, an assistant professor in psychology & neuroscience, the team also investigated the changes in synaptic function in these mutant mice and found severe alterations of the synaptic physiology.

Not only did the researchers see faulty circuits in the mice missing cortical Htt, they also saw signs of cellular stress in the brain, in the exact spot within the cortex that projects to the striatum, another brain area targeted by Huntington’s disease in people. “There’s something about that particular circuit that is vulnerable to changes in Htt,” Eroglu said.

The researchers also examined what happens in early brain development in a mouse model of Huntington’s disease. Similar to people with the disease, these animals have one normal copy of the Htt gene, and one mutated copy, which produces a protein that is present in cells but in expanded form.

The researchers found the same pattern: the Huntington’s disease model animals have synapses that initially mature much faster than normal in the cortex and then die off.

The new results also suggest that missing Htt for a prolonged period may not only affect the development but also the maintenance of healthy synapses, Eroglu said.

That’s especially relevant to a current strategy for treating Huntington’s disease: dialing down Htt levels in the brain using gene therapy or small-molecule inhibitors. But it has been a challenge to target the mutated copy of the gene, not the normal copy. Interested in the implications of lowering overall Htt levels, the group plans to delete Htt in the mouse brain later in life and measure the number of its synapses.

Other mouse models of the disease are also likely to have these faulty circuits. “We think this is probably a common thing, but that’s something we’re working on: whether we can detect early signs of faulty connections, correct it before the disease starts, and make these mice better,” Eroglu said.

(Source: today.duke.edu)

Researchers believe they have learned how mutations in the gene that causes Huntington’s disease kill brain cells, a finding that could open new opportunities for treating the fatal disorder. Scientists first linked the gene to the inherited disease more than 20 years ago.

Huntington’s disease affects five to seven people out of every 100,000. Symptoms, which typically begin in middle age, include involuntary jerking movements, disrupted coordination and cognitive problems such as dementia. Drugs cannot slow or stop the progressive decline caused by the disorder, which leaves patients unable to walk, talk or eat.

Lead author Hiroko Yano, PhD, of Washington University School of Medicine in St. Louis, found in mice and in mouse brain cell cultures that the disease impairs the transfer of proteins to energy-making factories inside brain cells. The factories, known as mitochondria, need these proteins to maintain their function. When disruption of the supply line disables the mitochondria, brain cells die.

“We showed the problem could be fixed by making cells overproduce the proteins that make this transfer possible,” said Yano, assistant professor of neurological surgery, neurology and genetics. “We don’t know if this will work in humans, but it’s exciting to have a solid new lead on how this condition kills brain cells.”

The findings are available online in Nature Neuroscience.

Huntington’s disease is caused by a defect in the huntingtin gene, which makes the huntingtin protein. Life expectancy after initial onset is about 20 years.

Scientists have known for some time that the mutated form of the huntingtin protein impairs mitochondria and that this disruption kills brain cells. But they have had difficulty understanding specifically how the gene harms the mitochondria.

For the new study, Yano and collaborators at the University of Pittsburgh worked with mice that were genetically modified to simulate the early stages of the disorder.

Yano and her colleagues found that the mutated huntingtin protein binds to a group of proteins called TIM23. This protein complex normally helps transfer essential proteins and other supplies to the mitochondria. The researchers discovered that the mutated huntingtin protein impairs that process.

The problem seems to be specific to brain cells early in the disease. At the same point in the disease process, the scientists found no evidence of impairment in liver cells, which also produce the mutated huntingtin protein.

The researchers speculated that brain cells might be particularly reliant on their mitochondria to power the production and recycling of the chemical signals they use to transmit information. This reliance could make the cells vulnerable to disruption of the mitochondria.

Other neurodegenerative conditions, including Alzheimer’s disease and amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease, have been linked to problems with mitochondria. Scientists may be able to build upon these new findings to better understand these disorders.

(Source: news.wustl.edu)

Scientists at the University of Pittsburgh School of Medicine have identified for the first time a key molecular mechanism by which the abnormal protein found in Huntington’s disease can cause brain cell death. The results of these studies, published today in Nature Neuroscience, could one day lead to ways to prevent the progressive neurological deterioration that characterizes the condition.

Huntington’s disease patients inherit from a parent a gene that contains too many repeats of a certain DNA sequence, which results in the production of an abnormal form of a protein called huntingtin (HTT), explained senior investigator Robert Friedlander, M.D., UPMC Professor of Neurosurgery and Neurobiology and chair, Department of Neurological Surgery, Pitt School of Medicine. But until now, studies have not suggested how HTT could cause disease.

“This study connects the dots for the first time and shows how huntingtin can cause problems for the mitochondria that lead to the death of neurons,” Dr. Friedlander said. “If we can disrupt the pathway, we may be able to identify new treatments for this devastating disease.”

Examination of brain tissue samples from both mice and human patients affected by Huntington’s disease showed that mutant HTT collects in the mitochondria, which are the energy suppliers of the cell. Using several biochemical approaches in follow-up mouse studies, the research team identified the mitochondrial proteins that bind to mutant HTT, noting its particular affinity for TIM23, a protein complex that transports other proteins from the rest of the cell into the mitochondria.

Further investigation revealed that mutant HTT inhibited TIM23’s ability to transport proteins across the mitochondrial membrane, slowing metabolic activity and ultimately triggering cell-suicide pathways. The team also found that mutant HTT-induced mitochondrial dysfunction occurred more often near the synapses, or junctions, of neurons, likely impairing the neuron’s ability to communicate or signal its neighbors.

To verify the findings, the researchers showed that producing more TIM23 could overcome the protein transport deficiency and prevent cell death.

“We learned also that these events occur very early in the disease process, not as the result of some other mutant HTT-induced changes,” Dr. Friedlander said. “This means that if we can find ways to intervene at this point, we may be able to prevent neurological damage.”

The team’s next steps include identifying exact binding sites and agents that can influence the interactions of HTT and TIM23.

(Source: upmc.com)

Working with genetically engineered mice, Johns Hopkins neuroscientists report they have identified what they believe is the cause of the vast disintegration of a part of the brain called the corpus striatum in rodents and people with Huntington’s disease: loss of the ability to make the amino acid cysteine. They also found that disease progression slowed in mice that were fed a diet rich in cysteine, which is found in foods such as wheat germ and whey protein.

Their results suggest further investigation into cysteine supplementation as a candidate therapeutic in people with the disease.

Up to 90 percent of the human corpus striatum, a brain structure that moderates mood, movement and cognition, degenerates in people with Huntington’s disease, a condition marked by widespread motor and intellectual disability. And while the genetic mutation underlying Huntington’s disease has long been known, the precise cause of that degeneration has remained a mystery.

In a report on their discovery in the advanced online publication of Nature on March 26, the Johns Hopkins researchers, led by Solomon Snyder, M.D., tracked the degenerative process to the absence of an enzyme, cystathionine gamma lyase, or CSE.

"Usually it’s very hard, if not impossible, to develop straightforward mechanisms that explain what’s going on in a disease. What’s even harder is even if you can find a mechanism that causes a tissue to rot, usually there’s nothing you can do about it,” says Snyder, a professor of neuroscience at the Johns Hopkins University School of Medicine. “In this case, there is."

Huntington’s disease, an inherited disorder, does its damage because of abnormal DNA coding for the amino acid glutamine. Healthy individuals have some 15 to 20 DNA “repeats” in that part of their genetic code, while Huntington’s disease gene carriers have more than 36 — and often upward of 100. Children born to a parent carrier have a 50/50 chance of inheriting the disorder, and the greater the number of repeats, the earlier the age of onset of the incurable disorder.

Bindu Diana Paul, Ph.D., a molecular neuroscientist and faculty instructor in Snyder’s laboratory, was studying mice lacking CSE, which helps make the amino acid cysteine and hydrogen sulfide that moderate blood pressure and heart function. Paul, who had previous research experience with Huntington’s disease, says she was startled to observe that her mutant mice also behaved a lot like those with the disease.

When a normal mouse is dangled upside down from its tail, it will twist and turn and try to bite the offending hand, she explains. But her CSE-knockout mice stayed relatively still and clasped their paws together — the same behavior she’d observed in mice with the rodent equivalent of Huntington’s disease. “It looked like there was a neurological deficit,” Paul says. “But nobody had looked at CSE in the brain.”

Paul and Snyder began monitoring CSE in mouse and human brain tissues and found considerably less CSE in all diseased tissues. All people carry some normal huntingtin protein made by the Huntington’s disease gene, although the protein’s function remains elusive. But people with Huntington’s disease also carry mutant huntingtin proteins. Snyder and his team saw that the mutant proteins were attaching themselves to a crucial protein responsible for turning the CSE gene on or off, which ultimately led the diseased rodent and human brain tissues to be deprived of cysteine.

To see if loss of cysteine was directly responsible for the symptoms associated with Huntington’s disease, the Johns Hopkins team turned to readily available sources of the substance in everyday foods and fed mice a cysteine-rich diet.

The results, Paul says, were striking. When those mice were dangled from their tails, they resumed struggling, although with a bit less vigor than their healthy peers. They were able to grip an object with greater strength, and they took longer to fall off a balancing apparatus than CSE-knockout mice. Their life expectancies increased one to two weeks.

Snyder and Paul say they are cautiously optimistic about the results, noting that although they suggest a possible treatment for Huntington’s disease, it’s clear that a high cysteine diet merely slows rather than halts the progression of the disease. Moreover, the results in live mice may not occur in humans.

(Source: hopkinsmedicine.org)

Researchers at Lund University have succeeded in preventing very early symptoms of Huntington’s disease, depression and anxiety, by deactivating the mutated huntingtin protein in the brains of mice.

“We are the first to show that it is possible to prevent the depression symptoms of Huntington’s disease by deactivating the diseased protein in nerve cell populations in the hypothalamus in the brain. This is hugely exciting and bears out our previous hypotheses”, explains Åsa Petersén, Associate Professor of Neuroscience at Lund University.

Huntington’s is a debilitating disease for which there is still neither cure nor sufficient treatment. The dance-like movements that characterise the disease have long been the focus for researchers, but the emotional problems affect the patient earlier than the motor symptoms. These are now believed to stem from a different part of the brain – the small emotional centre called the hypothalamus.

“Now that we have been able to show in animal experiments that depression and anxiety occur very early in Huntington’s disease, we want to identify more specifically which nerve cells in the hypothalamus are critical in the development of these symptoms. In the long run, this gives us better opportunities to develop more accurate treatments that can attack the mutated huntingtin where it does the most damage”, says Åsa Petersén.

As the role of the hypothalamus in Huntington’s disease is gradually mapped, knowledge might be gained from drug research for other psychiatric diseases. It is likely that similar mechanisms control different types of depression, according to Åsa Petersén.

Publication:
Hypothalamic expression of mutant huntingtin contributes to the development of depressive-like behavior in the BAC transgenic mouse model of Huntington’s disease
Human Molecular Genetics
Sofia Hult Lundh, Nathalie Nilsson, Rana Soylu, Deniz Kirik and Åsa Petersén

(Source: lunduniversity.lu.se)