Posts tagged stem cells
Posts tagged stem cells
Researchers from the University of Bonn use reprogrammed patient neurons for drug testing
Why do certain Alzheimer medications work in animal models but not in clinical trials in humans? A research team from the University of Bonn and the biomedical enterprise LIFE & BRAIN GmbH has been able to show that results of established test methods with animal models and cell lines used up until now can hardly be translated to the processes in the human brain. Drug testing should therefore be conducted with human nerve cells, conclude the scientists. The results are published by Cell Press in the journal “Stem Cell Reports”.
In the brains of Alzheimer patients, deposits form that consist essentially of beta-amyloid and are harmful to nerve cells. Scientists are therefore searching for pharmaceutical compounds that prevent the formation of these dangerous aggregates. In animal models, certain non-steroidal anti-inflammatory drugs (NSAIDs) were found to a reduced formation of harmful beta-amyloid variants. Yet, in subsequent clinical studies, these NSAIDs failed to elicit any beneficial effects.
"The reasons for these negative results have remained unclear for a long time", says Prof. Dr. Oliver Brüstle, Director of the Institute for Reconstructive Neurobiology of the University of Bonn and CEO of LIFE & BRAIN GmbH. "Remarkably, these compounds were never tested directly on the actual target cells – the human neuron", adds lead author Dr. Jerome Mertens of Prof. Brüstle’s team, who now works at the Laboratory of Genetics in La Jolla (USA). This is because, so far, living human neurons have been extremely difficult to obtain. However, with the recent advances in stem cell research it has become possible to derive limitless numbers of brain cells from a small skin biopsy or other adult cell types.
Scientists transform skin cells into nerve cells
Now a research team from the Institute for Reconstructive Neurobiology and the Department of Neurology of the Bonn University Medical Center together with colleagues from the LIFE & BRAIN GmbH and the University of Leuven (Belgium) has obtained such nerve cells from humans. The researchers used skin cells from two patients with a familial form of Alzheimer’s Disease to produce so-called induced pluripotent stem cells (iPS cells), by reprogramming the body’s cells into a quasi-embryonic stage. They then transformed the resulting so-called “jack-of-all-trades cells” into nerve cells.
Using these human neurons, the scientists tested several compounds in the group of non-steroidal anti-inflammatory drugs. As control, the researchers used nerve cells they had obtained from iPS cells of donors who did not have the disease. Both in the nerve cells obtained from the Alzheimer patients and in the control cells, the NSAIDs that had previously tested positive in the animal models and cell lines typically used for drug screening had practically no effect: The values for the harmful beta-amyloid variants that form the feared aggregates in the brain remained unaffected when the cells were treated with clinically relevant dosages of these compounds.
Metabolic processes in animal models differ from humans
"In order to predict the efficacy of Alzheimer drugs, such tests have to be performed directly on the affected human nerve cells", concludes Prof. Brüstle’s colleague Dr. Philipp Koch, who led the study. Why do NSAIDs decrease the risk of aggregate formation in animal experiments and cell lines but not in human neurons? The scientists explain this with differences in metabolic processes between these different cell types. "The results are simply not transferable", says Dr. Koch.
The scientists now hope that in the future, testing of potential drugs for the treatment of Alzheimer’s disease will be increasingly conducted using neurons obtained from iPS cells of patients. “The development of a single drug takes an average of ten years”, says Prof. Brüstle. “By using patient-specific nerve cells as a test system, investments by pharmaceutical companies and the tedious search for urgently needed Alzheimer medications could be greatly streamlined”.
A team of researchers has brought new clarity to the picture of how gene-environmental interactions can kill nerve cells that make dopamine. Dopamine is the neurotransmitter that sends messages to the part of the brain that controls movement and coordination. Their discoveries, described in a paper published online in Cell today, include identification of a molecule that protects neurons from pesticide damage.
"For the first time, we have used human stem cells derived from Parkinson’s disease patients to show that a genetic mutation combined with exposure to pesticides creates a ‘double hit’ scenario, producing free radicals in neurons that disable specific molecular pathways that cause nerve-cell death," said Stuart Lipton, M.D., Ph.D., professor and director of Sanford-Burnham Medical Research Institute’s Del E. Webb Center for Neuroscience, Aging, and Stem Cell Research and senior author of the study.
Until now, the link between pesticides and Parkinson’s disease was based mainly on animal studies and epidemiological research that demonstrated an increased risk of disease among farmers, rural populations, and others exposed to agricultural chemicals.
In the new study, Lipton, along with Rajesh Ambasudhan, Ph.D., research assistant professor in the Del E. Webb Center, and Rudolf Jaenisch, M.D., founding member of Whitehead Institute for Biomedical Research and professor of biology at the Massachusetts Institute of Technology, used skin cells from Parkinson’s patients that had a mutation in the gene encoding a protein called alpha-synuclein. Alpha-synuclein is the primary protein found in Lewy bodies—protein clumps that are the pathological hallmark of Parkinson’s disease.
Using patient skin cells, the researchers created human induced pluripotent stem cells (hiPSCs) containing the mutation, and then “corrected” the alpha-synuclein mutation in other cells. Next, they reprogrammed all of these cells to become the specific type of nerve cell that is damaged in Parkinson’s disease, called A9 dopamine-containing neurons—thus creating two sets of neurons—identical in every respect except for the alpha-synuclein mutation.
"Exposing both normal and mutant neurons to pesticides—including paraquat, maneb, and rotenone—created excessive free radicals in cells with the mutation, causing damage to dopamine-containing neurons that led to cell death," said Frank Soldner, M.D., research scientist in Jaenisch’s lab and co-author of the study.
"In fact, we observed the detrimental effects of these pesticides with short exposures to doses well below EPA-accepted levels," said Scott Ryan, Ph.D., researcher in the Del E. Webb Center and lead author of the paper.
Having access to genetically matched neurons with the exception of a single mutation simplified the interpretation of the genetic contribution to pesticide-induced neuronal death. In this case, the researchers were able to pinpoint how cells with the mutation, when exposed to pesticides, disrupt a key mitochondrial pathway—called MEF2C-PGC1alpha—that normally protects neurons that contain dopamine. The free radicals attacked the MEF2C protein, leading to the loss of function of this pathway that would otherwise have protected the nerve cells from the pesticides.
"Once we understood the pathway and the molecules that were altered by the pesticides, we used high-throughput screening to identify molecules that could inhibit the effect of free radicals on the pathway," said Lipton. "One molecule we identified was isoxazole, which protected mutant neurons from cell death induced by the tested pesticides. Since several FDA-approved drugs contain derivatives of isoxazole, our findings may have potential clinical implications for repurposing these drugs to treat Parkinson’s."
While the study clearly shows the relationship between a mutation, the environment, and the damage done to dopamine-containing neurons, it does not exclude other mutations and pathways from being important as well. The team plans to explore additional molecular mechanisms that demonstrate how genes and the environment interact to contribute to Parkinson’s and other neurodegenerative diseases, such as Alzheimer’s and ALS.
"In the future, we anticipate using the knowledge of mutations that predispose an individual to these diseases in order to predict who should avoid a particular environmental exposure. Moreover, we will be able to screen for patients who may benefit from a specific therapy that can prevent, treat, or possibly cure these diseases," Lipton said.
A specialized type of brain cell that tamps down stem cell activity ironically, perhaps, encourages the survival of the stem cells’ progeny, Johns Hopkins researchers report. Understanding how these new brain cells “decide” whether to live or die and how to behave is of special interest because changes in their activity are linked to neurodegenerative diseases such as Alzheimer’s, mental illness and aging.
"We’ve identified a critical mechanism for keeping newborn neurons, or new brain cells, alive," says Hongjun Song, Ph.D., professor of neurology and director of Johns Hopkins Medicine’s Institute for Cell Engineering’s Stem Cell Program. "Not only can this help us understand the underlying causes of some diseases, it may also be a step toward overcoming barriers to therapeutic cell transplantation."
Working with a group led by Guo-li Ming, M.D., Ph.D., a professor of neurology in the Institute for Cell Engineering, and other collaborators, Song’s research team first reported last year that brain cells known as parvalbumin-expressing interneurons instruct nearby stem cells not to divide by releasing a chemical signal called GABA.
In their new study, as reported Nov. 10 online in Nature Neuroscience, Song and Ming wanted to find out how GABA from surrounding neurons affects the newborn neurons that stem cells produce. Many of these newborn neurons naturally die soon after their “birth,” Song says; if they do survive, the new cells migrate to a permanent home in the brain and forge connections called synapses with other cells.
To learn whether GABA is a factor in the newborn neurons’ survival and behavior, the research team tagged newborn neurons from mouse brains with a fluorescent protein, then watched their response to GABA. “We didn’t expect these immature neurons to form synapses, so we were surprised to see that they had built synapses from surrounding interneurons and that GABA was getting to them that way,” Song says. In the earlier study, the team had found that GABA was getting to the synapse-less stem cells by a less direct route, drifting across the spaces between cells.
To confirm the finding, the team engineered the interneurons to be either stimulated or suppressed by light. When stimulated, the cells would indeed activate nearby newborn neurons, the researchers found. They next tried the light-stimulation trick in live mice, and found that when the specialized interneurons were stimulated and gave off more GABA, the mice’s newborn neurons survived in greater numbers than otherwise. This was in contrast to the response of the stem cells, which go dormant when they detect GABA.
"This appears to be a very efficient system for tuning the brain’s response to its environment," says Song. "When you have a high level of brain activity, you need more newborn neurons, and when you don’t have high activity, you don’t need newborn neurons, but you need to prepare yourself by keeping the stem cells active. It’s all regulated by the same signal."
Song notes that parvalbumin-expressing interneurons have been found by others to behave abnormally in neurodegenerative diseases such as Alzheimer’s and mental illnesses such as schizophrenia. “Now we want to see what the role of these interneurons is in the newborn neurons’ next steps: migrating to the right place and integrating into the existing circuitry,” he says. “That may be the key to their role in disease.” The team is also interested in investigating whether the GABA mechanism can be used to help keep transplanted cells alive without affecting other brain processes as a side effect.
A stem cell therapy previously shown to reduce inflammation in the critical time window after traumatic brain injury also promotes lasting cognitive improvement, according to preclinical research led by Charles Cox, M.D., at The University of Texas Health Science Center at Houston (UTHealth) Medical School.
The research was published in today’s issue of STEM CELLS Translational Medicine.
Cellular damage in the brain after traumatic injury can cause severe, ongoing neurological impairment and inflammation. Few pharmaceutical options exist to treat the problem. About half of patients with severe head injuries need surgery to remove or repair ruptured blood vessels or bruised brain tissue.
A stem cell treatment known as multipotent adult progenitor cell (MAPC) therapy has been found to reduce inflammation in mice immediately after traumatic brain injury, but no one had been able to gauge its usefulness over time.
The research team led by Cox, the Children’s Fund, Inc. Distinguished Professor of Pediatric Surgery at the UTHealth Medical School, injected two groups of brain-injured mice with MAPCs two hours after the mice were injured and again 24 hours later. One group received a dose of 2 million cells per kilogram and the other a dose five times stronger.
After four months, the mice receiving the stronger dose not only continued to have less inflammation—they also made significant gains in cognitive function. A laboratory examination of the rodents’ brains confirmed that those receiving the higher dose of MAPCs had better brain function than those receiving the lower dose.
“Based on our data, we saw improved spatial learning, improved motor deficits and fewer active antibodies in the mice that were given the stronger concentration of MAPCs,” Cox said.
The study indicates that intravenous injection of MAPCs may in the future become a viable treatment for people with traumatic brain injury, he said.
It was once thought that each cell in a person’s body possesses the same DNA code and that the particular way the genome is read imparts cell function and defines the individual. For many cell types in our bodies, however, that is an oversimplification. Studies of neuronal genomes published in the past decade have turned up extra or missing chromosomes, or pieces of DNA that can copy and paste themselves throughout the genomes.
The only way to know for sure that neurons from the same person harbor unique DNA is by profiling the genomes of single cells instead of bulk cell populations, the latter of which produce an average. Now, using single-cell sequencing, Salk Institute researchers and their collaborators have shown that the genomic structures of individual neurons differ from each other even more than expected. The findings were published November 1, 2013, in Science.
"Contrary to what we once thought, the genetic makeup of neurons in the brain aren’t identical, but are made up of a patchwork of DNA," says corresponding author Fred Gage, Salk’s Vi and John Adler Chair for Research on Age-Related Neurodegenerative Disease.
In the study, led by Mike McConnell, a former junior fellow in the Crick-Jacobs Center for Theoretical and Computational Biology at the Salk, researchers isolated about 100 neurons from three people posthumously. The scientists took a high-level view of the entire genome—looking for large deletions and duplications of DNA called copy number variations or CNVs—and found that as many as 41 percent of neurons had at least one unique, massive CNV that arose spontaneously, meaning it wasn’t passed down from a parent. The CNVs are spread throughout the genome, the team found.
The miniscule amount of DNA in a single cell has to be chemically amplified many times before it can be sequenced. This process is technically challenging, so the team spent a year ruling out potential sources of error in the process.
"A good bit of our study was doing control experiments to show that this is not an artifact," says Gage. "We had to do that because this was such a surprise—finding out that individual neurons in your brain have different DNA content."
The group found a similar amount of variability in CNVs within individual neurons derived from the skin cells of three healthy people. Scientists routinely use such induced pluripotent stem cells (iPSCs) to study living neurons in a culture dish. Because iPSCs are derived from single skin cells, one might expect their genomes to be the same.
"The surprising thing is that they’re not," says Gage. "There are quite a few unique deletions and amplifications in the genomes of neurons derived from one iPSC line."
Interestingly, the skin cells themselves are genetically different, though not nearly as much as the neurons. This finding, along with the fact that the neurons had unique CNVs, suggests that the genetic changes occur later in development and are not inherited from parents or passed to offspring.
It makes sense that neurons have more diverse genomes than skin cells do, says McConnell, who is now an assistant professor of biochemistry and molecular genetics at the University of Virginia School of Medicine in Charlottesville. “The thing about neurons is that, unlike skin cells, they don’t turn over, and they interact with each other,” he says. “They form these big complex circuits, where one cell that has CNVs that make it different can potentially have network-wide influence in a brain.”
Spontaneously occurring CNVs have also been linked to risk for brain disorders such as schizophrenia and autism, but those studies usually pool many blood cells. As a result, the CNVs uncovered in those studies affect many if not all cells, which suggests that they arise early in development.
The purpose of CNVs in the healthy brain is still unclear, but researchers have some ideas. The modifications might help people adapt to new surroundings encountered over a lifetime, or they might help us survive a massive viral infection. The scientists are working out ways to alter genomic variability in iPSC-derived neurons and challenge them in specific ways in the culture dish.
Cells with different genomes probably produce unique RNA and then proteins. However, for now, only one sequencing technology can be applied to a single cell.
"If and when more than one method can be applied to a cell, we will be able to see whether cells with different genomes have different transcriptomes (the collection of all the RNA in a cell) in predictable ways," says McConnell.
In addition, it will be necessary to sequence many more cells, and in particular, more cell types, notes corresponding author Ira Hall, an associate professor of biochemistry and molecular genetics at the University of Virginia. “There’s a lot more work to do to really understand to what level we think the things we’ve found are neuron-specific or associated with different parameters like age or genotype,” he says.
A group of Brigham and Women’s Hospital, and Harvard Stem Cell Institute researchers, and collaborators at MIT and Massachusetts General Hospital have found a way to use stem cells as drug delivery vehicles.
The researchers inserted modified strands of messenger RNA into connective tissue stem cells—called mesenchymal stem cells—which stimulated the cells to produce adhesive surface proteins and secrete interleukin-10, an anti-inflammatory molecule. When injected into the bloodstream of a mouse, these modified human stem cells were able to target and stick to sites of inflammation and release biological agents that successfully reduced the swelling.
“If you think of a cell as a drug factory, what we’re doing is targeting cell-based, drug factories to damaged or diseased tissues, where the cells can produce drugs at high enough levels to have a therapeutic effect,” said research leader Jeffrey Karp, PhD, a Harvard Stem Cell Institute principal faculty member and Associate Professor at the Brigham and Women’s Hospital, Harvard Medical School, and Affiliate faculty at MIT.
Karp’s proof of concept study, published in the journal Blood, is drawing early interest from biopharmaceutical companies for its potential to target biological drugs to disease sites. While ranked as the top sellers in the drug industry, biological drugs are still challenging to use, and Karp’s approach may improve their clinical application as well as improve the historically mixed, clinical trial results of mesenchymal stem cell-based treatments.
Mesenchymal stem cells have become cell therapy researchers’ tool of choice because they can evade the immune system, and thus are safe to use even if they are derived from another person. To modify the cells with messenger RNA, the researchers used the RNA delivery and cell programming technique that was previously developed in the MIT laboratory of Mehmet Fatih Yanik, PhD. This RNA technique to program cells is harmless, as it does not modify the cells’ genome, which can be a problem when DNA is used (via viruses) to manipulate gene expression.
“This opens the door to thinking of messenger RNA transfection of cell populations as next generation therapeutics in the clinic, as they get around some of the delivery challenges that have been encountered with biological agents,” said Oren Levy, PhD, co-lead author of the study and Instructor of Medicine in Karp’s lab. The study was also co-led by Weian Zhao, PhD, at University of California, Irvine who was previously a postdoctoral fellow in Karp’s lab.
One such challenge with using mesenchymal stem cells is they have a “hit-and-run” effect, since they are rapidly cleared after entering the bloodstream, typically within a few hours or days. The Harvard/MIT team demonstrated that rapid targeting of the cells to the inflamed tissue produced a therapeutic effect despite the cells being rapidly cleared. The scientists want to extend cell lifespan even further and are experimenting with how to use messenger RNA to make the stem cells produce pro-survival factors.
“We’re interested to explore the platform nature of this approach and see what potential limitations it may have or how far we can actually push it,” Zhao said. “Potentially, we can simultaneously deliver proteins that have synergistic therapeutic impacts.”
University of South Florida researchers have suggested a new view of how stem cells may help repair the brain following trauma. In a series of preclinical experiments, they report that transplanted cells appear to build a “biobridge” that links an uninjured brain site where new neural stem cells are born with the damaged region of the brain.
Their findings were recently reported online in the peer-reviewed journal PLOS ONE.
“The transplanted stem cells serve as migratory cues for the brain’s own neurogenic cells, guiding the exodus of these newly formed host cells from their neurogenic niche towards the injured brain tissue,” said principal investigator Cesar Borlongan, PhD, professor and director of the USF Center for Aging and Brain Repair.
Based in part on the data reported by the USF researchers in this preclinical study, the U.S. Food and Drug Administration recently approved a limited clinical trial to transplant SanBio Inc’s SB632 cells (an adult stem cell therapy) in patients with traumatic brain injury.
Stem cells are undifferentiated, or blank, cells with the potential to give rise to many different cell types that carry out different functions. While the stem cells in adult bone marrow or umbilical cord blood tend to develop into the cells that make up the organ system from which they originated, these multipotent stem cells can be manipulated to take on the characteristics of neural cells.
To date, there have been two widely-held views on how stem cells may work to provide potential treatments for brain damage caused by injury or neurodegenerative disorders. One school of thought is that stem cells implanted into the brain directly replace dead or dying cells. The other, more recent view is that transplanted stem cells secrete growth factors that indirectly rescue the injured tissue.
The USF study presents evidence for a third concept of stem-cell mediated brain repair.
The researchers randomly assigned rats with traumatic brain injury and confirmed neurological impairment to one of two groups. One group received transplants of bone marrow-derived stem cells (SB632 cells) into the region of the brain affected by traumatic injury. The other (control group) received a sham procedure in which solution alone was infused into the brain with no implantation of stem cells.
At one and three months post-TBI, the rats receiving stem cell transplants showed significantly better motor and neurological function and reduced brain tissue damage compared to rats receiving no stem cells. These robust improvements were observed even though survival of the transplanted cells was modest and diminished over time.
The researchers then conducted a series of experiments to examine the host brain tissue.
At three months post-traumatic brain injury, the brains of transplanted rats showed massive cell proliferation and differentiation of stem cells into neuron-like cells in the area of injury, the researchers found. This was accompanied by a solid stream of stem cells migrating from the brain’s uninjured subventricular zone — a region where many new stem cells are formed – to the brain’s site of injury.
In contrast, the rats receiving solution alone showed limited proliferation and neural-commitment of stem cells, with only scattered migration to the site of brain injury and virtually no expression of newly formed cells in the subventricular zone. Without the addition of transplanted stem cells, the brain’s self-repair process appeared insufficient to mount a defense against the cascade of traumatic brain injury-induced cell death.
The researchers conclude that the transplanted stem cells create a neurovascular matrix that bridges the long-distance gap between the region in the brain where host neural stem cells arise and the site of injury. This pathway, or “biobridge,” ferries the newly emerging host cells to the specific place in the brain in need of repair, helping promote functional recovery from traumatic brain injury.
A new experimental approach to treating a type of brain cancer called medulloblastoma has been developed by researchers at Sanford-Burnham. The method targets cancer stem cells—the cells that are critical for maintaining tumor growth—and halts their ability to proliferate by inhibiting enzymes that are essential for tumor progression. The process destroys the ability of the cancer cells to grow and divide, paving the way for a new type of treatment for patients with this disease.
The research team, led by Robert Wechsler-Reya, Ph.D., professor in Sanford-Burnham’s NCI-Designated Cancer Center and director of the Tumor Initiation and Maintenance Program, discovered that the medulloblastoma cancer cells responsible for tumor growth and progression (called cancer stem cells or tumor-propagating cells—TPCs) divide more quickly than normal cells. Correspondingly, they have higher levels of certain enzymes that regulate the cell cycle (Aurora and Polo-like kinases). By using small-molecule inhibitors to stop the action of these enzymes, the researchers were able to block the growth of tumor cells from mice as well as humans. The research findings are described in an online paper published today by Cancer Research.
“One tumor can have many different types of cells in it, and they can grow at different rates. By targeting fast-growing TPCs with cell-cycle inhibitors, we have developed a new route to assault medulloblastoma. In this study, we have shown that cell-cycle inhibitors essentially block medulloblastoma tumor progression by halting TPC expansion, and have opened the window to preventing cancer recurrence,” said Wechsler-Reya.
The team’s first set of experiments used a mouse model for medulloblastoma. In-vitro studies of mouse tumor cells showed that cell-cycle inhibitors caused tumor cell death. In vivo, mice that were treated with the inhibitor had smaller tumors that weighed less compared to mice that were not treated, essentially halting the progression of the tumor.
The second set of experiments used human medulloblastoma cells. When the researchers treated these human tumor cells with cell-cycle inhibitors, they also observed a significant reduction in tumor growth and progression.
Finally, when the scientists combined cell-cycle inhibitors with treatments currently used for medulloblastoma, they found that the combination worked together to produce results that were greater than either inhibitor alone.
“These results strongly support an approach to treatment that combines current therapies with cell-cycle inhibitors to treat medulloblastoma. Our hope is that the combination of these inhibitors will prevent tumor progression and drug resistance, and improve the overall effectiveness of current treatment options. We look forward to clinical studies in human medulloblastoma patients as well as other cancers that are suitable for this approach,” Wechsler-Reya said.
The learning and physical disabilities that affect people with Down syndrome may be due at least in part to defective stem cell regulation throughout the body, according to researchers at the Stanford University School of Medicine. The defects in stem cell growth and self-renewal observed by the researchers can be alleviated by reducing the expression of just one gene on chromosome 21, they found.
The finding marks the first time Down syndrome has been linked to stem cells, and addresses some long-standing mysteries about the disorder. Although the gene, called Usp16, is unlikely to be the only contributor to the disease, the finding raises the possibility of an eventual therapy based on reducing its expression.
“There appear to be defects in the stem cells in all the tissues that we tested, including the brain,” said Michael Clarke, MD, Stanford’s Karel H. and Avice N. Beekhuis Professor in Cancer Biology. The researchers conducted their studies in both mouse and human cells. “We believe Usp16 overexpression is a major contributor to the neurological deficits seen in Down syndrome.”
Clarke is the senior author of the research, published Sept. 11 in Nature. Postdoctoral scholar Maddalena Adorno, PhD, is the lead author.
“Conceptually, this study suggests that drug-based strategies to slow the rate of stem cell use could have profound effects on cognitive function, aging and risk for Alzheimer’s disease in people with Down syndrome,” said co-author Craig Garner, PhD, who is the co-director of Stanford’s Center for Research and Treatment of Down Syndrome and a professor of psychiatry and behavioral sciences.
Down syndrome, which is caused by an extra copy of chromosome 21, affects about 400,000 people in the United States and 6 million worldwide. It causes both physical and cognitive problems. While many of the physical issues, such as vulnerability to heart problems, can now be treated, no treatments exist for poor cognitive function.
The new study’s findings suggest answers to many long-standing mysteries about the condition, including why people with Down syndrome appear to age faster and exhibit early Alzheimer’s disease.
“This study is the first to provide a possible explanation for these tendencies,” said Garner. The fact that people with Down syndrome have three copies of chromosome 21 and the Usp16 gene “accelerates the rate at which stem cells are used during early development, which likely exhausts stem cell pools and impairs tissue regeneration in adults with Down syndrome. As a result, their brains age faster and are susceptible to early onset neurodegenerative disorders.”
The researchers didn’t confine their studies to laboratory mice. They also investigated the effect of Usp16 overexpression in human cells. Adorno and colleagues in the laboratory of co-author Samuel Cheshier, MD, assistant professor of neurosurgery, found that the presence of excess Usp16 caused skin cells from unaffected people to grow more slowly. Furthermore, neural progenitor cells (those self-renewing cellular factories responsible for the development and maintenance of many of the cell types in the brain) were less able to form balls of cells called neurospheres — a laboratory test that reflects the number and robustness of nerve stem cells in a culture. Conversely, reducing Usp16 expression in skin and nerve-progenitor cells from people with Down syndrome allowed the cells, which usually proliferate slowly, to assume normal growth patterns.
“This gene is clearly regulating processes that are central to aging in mice and humans,” said Clarke, “and stem cells are severely compromised. Reducing Usp16 expression gives an unambiguous rescue at the stem cell level. The fact that it’s also involved in this human disorder highlights how critical stem cells are to our well-being.”
Adorno and Clarke didn’t set out to study Down syndrome. Clarke’s past research has focused on how normal stem cells and cancer stem cells regenerate themselves, and Adorno was searching for genes that could inhibit a specific molecular pathway involved in the self-renewal of these cells. Understanding how normal stem cells regenerate themselves could help to repair tissue and organ damage from disease, and understanding how cancer stem cells maintain themselves could help explain why they are unusually resistant to chemotherapy or radiation therapy — often resulting in a patient’s relapse after seemingly successful treatment. Usp16 seemed to fit the bill; it plays a critical role in a self-renewal pathway previously identified by Clarke and his colleagues.
But Adorno and Clarke soon realized that Usp16 had another interesting property: in humans, it is found on chromosome 21.
They turned to Garner and Cheshier to help them evaluate a possible link to Down syndrome. Garner supplied two strains of mice commonly used to study the condition. One, Ts65Dn, has three copies of 132 genes found on human chromosome 21 — including Usp16. The second, Ts1Cje, has three copies of 79 genes from the chromosome, but only two copies of Usp16. Although both mice display some symptoms of the disorder, Ts65Dn more closely mimics the craniofacial structure and learning and memory disabilities seen in affected humans.
Colleagues in the Cheshier laboratory found that neural stem cells from the more-severely affected Ts65Dn mice were less able to self-renew and grow normally than were cells from the Ts1Cje mice. Reducing the expression of Usp16 in the cells from the Ts65Dn mice to more normal levels largely corrected these functional defects.
“We demonstrated that central nervous system stem cells in Down syndrome mice were defective in their ability to self-renew — the process by which stem cells regenerate themselves upon cell division. Blocking Usp16 expression in these cells restored this ability,” said Cheshier. “We hope in the future that correcting this Usp16 defect can lead to therapeutics that will ameliorate the central nervous system defects seen in patients with Down syndrome.”
Finally, the researchers created a new, Ts65Dn-derived mouse strain in which one of the three copies of Usp16 was mutated. This normalized the level of expression of that gene, without affecting the overexpression of the other 131 triplicated genes in these mice. Nerve progenitor cells from these mice were equally able as normal cells to form neurospheres. The researchers are now continuing their studies of these mice.
“We are really interested in learning how other genes in this chromosomal region may be affecting stem cell renewal,” said Clarke. “We also want to understand how much we’re able to rescue the neurological defect by normalizing the expression of Usp16 in this mouse model. How does this compare to what is happening in humans? We’re sure it plays some significant role.”
Rodent research suggests feasibility of restoring neuron function
Research from the School of Medicine at The University of Texas Health Science Center at San Antonio suggests the exciting possibility of using cell transplants to treat schizophrenia.
Cells called “interneurons” inhibit activity within brain regions, but this braking or governing function is impaired in schizophrenia. Consequently, a group of nerve cells called the dopamine system go into overdrive. Different branches of the dopamine system are involved in cognition, movement and emotions.
“Since these cells are not functioning properly, our idea is to replace them,” said study senior author Daniel Lodge, Ph.D., assistant professor of pharmacology in the School of Medicine.
Transplant restored normal function
Dr. Lodge and lead author Stephanie Perez, graduate student in his laboratory, biopsied tissue from rat fetuses, isolated cells from the tissue and injected the cells into a brain center called the hippocampus. This center regulates the dopamine system and plays a role in learning, memory and executive functions such as decision making. Rats treated with the transplanted cells have restored hippocampal and dopamine function.
Stem cells are able to become different types of cells, and in this case interneurons were selected. “We put in a lot of cells and not all survived, but a significant portion did and restored hippocampal and dopamine function back to normal,” Dr. Lodge said.
‘You can essentially fix the problem’
Unlike traditional approaches to treating schizophrenia, such as medications and deep-brain stimulation, transplantation of interneurons potentially can produce a permanent solution. “You can essentially fix the problem,” Dr. Lodge said. “Ultimately, if this is translated to humans, we want to reprogram a patient’s own cells and use them.”
After meeting with other students, Perez brought the research idea to Dr. Lodge. “The students have journal club, and somebody had done a similar experiment to restore motor deficits and had good results,” Perez said. “We thought, why can’t we use it for schizophrenia and have good results, and so far we have.”
The study is in Molecular Psychiatry.