This is a landmark study because it brings together and adds to previous research and points to a potentially significant treatment.
Coalition researcher Dr. Michael R. Hayden teamed up with Dr. Stuart Lipton,
developer of the glutamate stabilizer memantine, to test its effects in cell and mouse models of Huntington's Disease. Working with a cell model of the disease, they found that normal synaptic activity (where neurons communicate with each other through electrical impulses) protects the brain from the misfolded HD protein but excessive extrasynaptic activity enhances the HD protein's toxic effect. This is the first time researchers have linked the electric activity of the synapses to protein folding.
They then tested a low dose and a high dose of memantine in the HD mouse. Low doses reduced the extrasynaptic activity but not the normal synaptic activity and treated the disease. High doses actually increased pathology because they blocked the protective, normal synaptic activity as well as the extrasynaptic activity.
Where does this extrasynaptic activity come from, both in the cell culture and in the brain? There is a pool of glutamate within the cell which is used in metabolism. They speculate that neurons infected with the HD protein may be leaking the glutamate. Further, glutamate released from astrocytes is known to cause extrasynaptic activity.
The mechanisms that were found to cause the neuroprotective effect and the toxic effect link the new research to previous studies. The cumulative research answers some key questions about how the HD protein does its damage.
They found that normal synaptic activity induces aggregation through a T complex-1 (TCP-1) ring complex (TRiC)-dependent mechanism. The soluable HD protein is more toxic than the aggregates.
The extrasynaptic activity impairs the normally protective CREB-PGC1-alpha complex. The Cyclic AMP response element-binding protein (CREB), along with the CREB-binding protein (CBP) which binds with phosphorylated CREB, triggers the upregulation of the neuroprotective PGC1-alpha pathway. They also found that RNAi knockdown of TCP will cause the same result. Low doses of memantine restored PGC1-alpha.
In 2000, Dr. Leslie Thompson and colleagues showed that the HD protein interacts with CBP and represses gene transcription. In 2001, Dr. Christopher Ross and colleagues found the CBP was dislocated from its normal nuclear location and that overexpression rescued cells from neurotoxicity. In 2006, he found that it was the depletion of CBP rather than its inclusion in aggregates that directly caused neurotoxicity.
In 2006, research by Dr. Weydt, Dr. Albert LaSpada, and other colleagues showed that the PGC-1 alpha gene is downregulated in the brains of Huntington's patients. They were led to look at the gene after discovering that the R6/2 HD mice have a below normal body temperature which continues to drop as the disease progresses. Working independently, another team of researchers from Massachusetts General and New York University Medical School lead by Dr. Dimitri Krainc found that transcription of PGC-1 is repressed by the HD protein which leads to mitochondrial dysfunction. Delivery of PGC-1 to transgenic HD mice results is neuroprotective while crossing the HD mice with PGC-1 null mice results in more severe symptoms of Huntington's.
Drs Hayden and Lipton also found a connection with another known HD pathology. They showed that extrasynaptic activity increases the amount of rhes, a protein found mainly in the striatum, while low doses of memantine decrease it. This is important because a recent study conducted by Dr. Solomon Snyder and colleagues at Johns Hopkins found that rhes binds to the HD protein and causes toxicity.
The mechanism by which this occurs is sumoylation. SUMO is a Small Ubiquitin-like Modifying protein. The SUMO protein is attached or detached to another protein as part of a post-translational process which modifies the protein's function. Sumoylation is known to contribute to HD pathology.
In 2004, Dr. Joan Steffan and colleagues found that sumoylation decreases aggregation of the HD protein (the soluble HD protein is more toxic than the aggregates), masks a signal for the HD protein to stay in the cytoplasm, and promotes the dysregulation of gene transcription in the nucleus of the cell.
Research like this is very important because it links various pathological mechanisms which have been known to occur in Huntington's Disease and because it identifies a major therapeutic target for which there is already an FDA approved treatment. International clinical trials of memantine are now being planned.
Huntington's disease is caused by an expanded CAG repeat in the gene encoding huntingtin (HTT), resulting in loss of striatal and cortical neurons. Given that the gene product is widely expressed, it remains unclear why neurons are selectively targeted. Here we show the relationship between synaptic and extrasynaptic activity, inclusion formation of mutant huntingtin protein (mtHtt) and neuronal survival. Synaptic N-methyl-D-aspartate-type glutamate receptor (NMDAR) activity induces mtHtt inclusions via a T complex-1 (TCP-1) ring complex (TRiC)-dependent mechanism, rendering neurons more resistant to mtHtt-mediated cell death. In contrast, stimulation of extrasynaptic NMDARs increases the vulnerability of mtHtt-containing neurons to cell death by impairing the neuroprotective cyclic AMP response element-binding protein (CREB)-peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) cascade and increasing the level of the small guanine nucleotide-binding protein Rhes, which is known to sumoylate and disaggregate mtHtt. Treatment of transgenic mice expressing a yeast artificial chromosome containing 128 CAG repeats (YAC128) with low-dose memantine blocks extrasynaptic (but not synaptic) NMDARs and ameliorates neuropathological and behavioral manifestations. By contrast, high-dose memantine, which blocks both extrasynaptic and synaptic NMDAR activity, decreases neuronal inclusions and worsens these outcomes. Our findings offer a rational therapeutic approach for protecting susceptible neurons in Huntington's disease.
Investigators at Burnham Institute for Medical Research (Burnham), the University of British Columbia's Centre for Molecular Medicine and Therapeutics and the University of California, San Diego have found that normal synaptic activity in nerve cells (the electrical activity in the brain that allows nerve cells to communicate with one another) protects the brain from the misfolded proteins associated with Huntington's disease. In contrast, excessive extrasynaptic activity (aberrant electrical activity in the brain, usually not associated with communication between nerve cells) enhances the misfolded proteins' deadly effects. Researchers also found that the drug Memantine, which is approved to treat Alzheimer's disease, successfully treated Huntington's disease in a mouse model by preserving normal synaptic electrical activity and suppressing excessive extrasynaptic electrical activity. The research was published in the journal Nature Medicine on November 15.
Huntington's disease is a hereditary condition caused by a mutated huntingtin gene that creates a misfolded, and therefore dysfunctional, protein. The new research shows that normal synaptic receptor activity makes nerve cells more resistant to the mutant proteins. However, excessive extrasynaptic activity contributed to increased nerve cell death. The research team found that low doses of Memantine reduce extrasynaptic activity without impairing protective synaptic activity. The work was led by Stuart A. Lipton, M.D., Ph.D., director of the Del E. Webb Center for Neuroscience, Aging and Stem Cell Research at Burnham and professor in the department of Neurosciences and attending neurologist at the University of California, San Diego and Michael R. Hayden, M.D., Ph.D., University Killam professor in the department of Medical Genetics at UBC and director of the Centre for Molecular Medicine and Therapeutics at the Child & Family Research Institute.
"Chronic neurodegenerative diseases like Huntington's, Alzheimer's and Parkinson's are all related to protein misfolding," said Dr. Lipton. "We show here, for the first time, that electrical activity controls protein folding, and if you have a drug that can adjust the electrical activity to the correct levels, you can protect against misfolding. Also, this verifies that appropriate electrical activity is protective, supporting the 'use it or lose it theory' of brain activity at the molecular level. For example, this finding may explain why epidemiologists have found that 'using' your brain by performing crossword puzzles and other games can stave off cognitive decline in diseases like Alzheimer's."
In the new study, researchers initially tested nerve cell cultures transfected with mutant Huntingtin protein and found that reducing excessive NMDA-type glutamate receptor activity with Memantine and other antagonists protected the nerve cells (glutamate receptors are the main trigger of excitatory electrical activity in the brain but in excess can cause nerve cell death, a process called excitotoxicity). They also found that normal synaptic activity was protective. Subsequently, they treated Huntington's disease model mice with both high and low doses of Memantine and found that the low doses were protective by blocking pathological extrasynaptic activity, while high-dose Memantine encouraged disease progression because it also blocked the protective synaptic NMDA receptor activity.
"For a long time it's been known that excitotoxicity is an early marker of Huntington's disease," said Dr. Hayden. "However, now we have dissected the mechanism by which this happens, particularly focusing on NMDA receptors outside the synapse. This creates novel therapeutic opportunities to modulate these receptors with potential protective effects on nerve cells."
A small human clinical trial of Memantine for Huntington's disease has also recently shown positive effects. Larger, international clinical trials are now being planned.
Dr. Lipton is the named inventor on worldwide patents for the use of Memantine (marketed in the USA under the name Namenda®) in neurodegenerative disorders, including Alzheimer's and Huntington's disease. He is credited with the groundbreaking discovery more than ten years ago of how Memantine works in the brain and for spearheading early human clinical trials with the drug.
About Burnham Institute for Medical Research
Burnham Institute for Medical Research is dedicated to discovering the fundamental molecular causes of disease and devising the innovative therapies of tomorrow. Burnham, with operations in California and Florida, is one of the fastest-growing research institutes in the country. The institute ranks among the top four institutions nationally for NIH grant funding and among the top organizations worldwide for its research impact. For the past decade (1999-2009), Burnham ranked first worldwide in the fields of biology and biochemistry for the impact of its research publications (defined by citations per publication), according to the Institute for Scientific Information.
Burnham utilizes a unique, collaborative approach to medical research and has established major research programs in cancer, neurodegeneration, diabetes, and infectious, inflammatory, and childhood diseases. The Institute is especially known for its world-class capabilities in stem cell research and drug discovery technologies. Burnham is a nonprofit public benefit corporation. For more information, please visit www.burnham.org.
About the Centre for Molecular Medicine and Therapeutics:
CMMT is a premier genetics science research centre in Canada and the world, dedicated to unraveling and solving the many genetic questions surrounding human illness and well being. It is a unique collaboration of committed scientists and researchers, who participate in multidisciplinary teams with the optimal combination of expertise, to find new approaches to treatment and prevention that can overcome the causes of illness. Affiliated with the University of British Columbia and the Child & Family Research Institute, CMMT conducts discovery research and translates that research into novel and effective clinical and therapeutic strategies to promote health. For more information, visit http://www.cmmt.ubc.ca.
About the Child & Family Research Institute:
CFRI conducts discovery, clinical and applied research to benefit the health of children and families. It is the largest institute of its kind in Western Canada. CFRI works in close partnership with UBC, BC Children's and Sunny Hill Health Centre for Children, BC Women's, PHSA, and BC Children's Hospital Foundation. CFRI has additional important relationships with BC's five regional health authorities and with BC academic institutions Simon Fraser University, the University of Victoria, the University of Northern British Columbia, and the British Columbia Institute of Technology. For more information, visit www.cfri.ca.
About the University of British Columbia:
UBC is one of Canada's largest and most prestigious public research and teaching institutions and consistently ranks among the top 40 universities in the world. It offers a range of innovative undergraduate, graduate and professional programs in the arts, sciences, medicine, law, commerce and other faculties. UBC has particular strengths in biotechnology, ranks in the top 10 universities in North America and number one in Canada for commercializing research and for its patent activity in the life sciences. For more information, visit www.ubc.ca.