The Molecular Zip Code Research Yields a Drug Target

HD Lighthouse Contributing Editor's Comment:

This is breakthrough research!

The HDlighthouse has been following the molecular zipcode research (see http://www.hdlighthouse.org/research/brain/updates/1365protein.php ) so it is exciting to see how it has been advancing, yielding a new drug target for an upstream pathology as well as new insights into the normal function of the huntingtin protein.

The research team of Dr. Ray Truant has found that the huntingtin protein is associated with the endoplasmic reticulum (ER). The ER is a structure within the neuron (and other cells too) where proteins and lipids are synthesized and processed for use inside and outside the cells. It's also where protein quality control is maintained. If a protein isn't folding right, then the ER will try to refold it properly so it can be used or degrade it so it won't do any damage.

The huntingtin protein associates with the ER through its first 18 amino terminals. They reversibly target the ER as well as vesicles in the cell and the process is dependent on ATP, cellular energy.

They also found that the normal function of the huntingtin protein is to respond to ER stress by communicating with the nucleus, the control center of the cell. The huntingtin protein shuttles back and forth between the ER and the cell nucleus. Previously, it was thought that the normal huntingtin protein was a cytoplasmic protein which did not go into the nucleus of the cell, but through the use of their live cell model technology, the researchers could see that it does. It's just that it enters and exits very quickly.

However, the HD version of the protein goes into the nucleus of the cell and has trouble exiting. It builds up and is toxic to the cell.

The association of the huntingtin protein with the ER sheds some light on some earlier research results. A team of Japanese researchers found that using RNA interference to knock down the normal mouse huntingtin protein led to malformation of the ER -- but not other organelles and structures in the cell.

The good news for HD families is in the press release and not in the article itself. First, there is a kinase (an enzyme which works by transfering phosphate groups from high energy donors such as ATP) which directs the huntingtin protein into the nucleus of the cell. Keeping the HD protein out of the nucleus would save the cell. They are working on developing a drug to inhibit this kinase and have already determined the structure of the kinase, a necessary step in the process.

Second, even if the kinase inhibitor does not prove to be a treatment, other drugs can be tested in this live cell system and the researchers will be able to tell what they are doing in the cell in real time.

Basic research into HD pathology has been following several valuable lines of investigation and plans for drug development and clinical trials focus on different HD pathologies. Because so much has been going on, it can be hard for HD families to see just how research findings have been building on each other and how much progress has been made toward understanding and treating the disease.

If you visit the Huntington Society of Canada's website through the link below, you can see Dr. Truant's original proposal in 2001 to investigate this line of research. http://www.huntingtonsociety.ca/english/truant.htm

You can follow Dr. Truant's team's progress through research articles by doing a search on Pub Med. Go here http://www.ncbi.nlm.nih.gov/sites/entrez?db=PubMed and search under 'Huntington's Disease Truant R'. Funding for the research is now paying off in terms of understanding the function of the huntingtin protein and suggesting a promising new drug target.

Whether or not a kinase inhibitor proves to be a treatment, this study represents a major advance in our knowledge of the disease and will certainly contribute to the development of a drug to treat Huntington's Disease.

References

Omi K, Hachiya N, Tokunaga K, and Kaneko K, "siRNA-mediated inhibition of endogenous Huntington disease gene expression induces an aberrant configuration of the ER network in vitro." Biochemical and Biophysical Research Communications 2005 December;338(2):1229-1235

Wu JC, Liang ZQ, and Quin ZH, "Quality control system of the endoplasmic reticulum and related diseases." Acta biochimica et biophysica Sinica (Shanghai) 2006 Apr;38(4):219-26.

-- Marsha L. Miller, Ph.D.
Posted to the HDL: 21 Aug 2007

Ray Truant, associate professor, Department of Biochemistry and Biomedical Sciences.

A new molecular zip code, and a new drug target for Huntington's disease: McMaster University researchers have first insight into how Huntington's disease is triggered

McMaster University researchers have first insight into how Huntington's disease (HD) is triggered. The research will be published online in the British Journal, Human Molecular Genetics, on Monday, August 20.

"These are exciting results by the McMaster team,” said Dr. Rémi Quirion, Scientific Director at the Canadian Institutes of Health Research, Institute of Neuroscience, Mental Health and Addiction. Even if the huntingtin protein has been known for almost 20 years, the cause of Huntington’s disease is still not clear. Data reported here shed new lights on this aspect and possibly leading to new therapeutic potential in the future."

Ray Truant, professor in the Department of Biochemistry and Biomedical Sciences, has been studying the biological role of the huntingtin protein and the sequences in the protein that tell it where to go within a brain cell.

Huntington disease (HD) is a neurological disorder resulting from degeneration of brain cells. The degeneration causes uncontrolled limb movements and loss of intellectual faculties, eventually leading to death. There is no treatment. HD is a familial disease, passed from parent to child through a mutation in the normal gene. The disorder is estimated to affect about one in every 10,000 persons.

Truant and PhD candidate graduate student, Randy Singh Atwal, have discovered a small protein sequence in huntingtin that allows it to locate to the part of the cell critical for protein quality control. Similar findings have been seen to be very important for other neurodegenerative diseases such as Parkinson’s and Alzheimer’s diseases.

Huntingtin protein is essential for normal development in all mammals, and is found in all cells, yet its function was unknown. It appears that huntingtin is crucial for a brain cell’s response to stress, and moves from the endoplasmic reticulum into the nucleus, the control centre of the cell. When mutant huntingtin is expressed however, it enters the nucleus as it should in response to stress, but it cannot exit properly, piling up in the nucleus and leading to brain cell death in HD.

“What is important to Huntington disease research is that in the learning of the basic cell biology of this protein, we have also uncovered a new drug target for the disease,” says Atwal.

Atwal additionally found that huntingtin can be sent to the nucleus by protein modifying enzymes called kinases, and he has determined the three-dimensional shape of this sequence.

Truant and Atwal’s work indicates that if mutant huntingtin is prevented from entering the nucleus, it cannot kill a brain cell. This means that a kinase inhibitor drug may be effective for Huntington's disease. Kinase inhibitors form the largest number of successful new generation drugs that are coming to market for a plethora of diseases including stroke, arthritis and cancer.

“This is most exciting to us, because we immediately have all the tools and support in hand at McMaster to quickly hunt this kinase down, and find potential new drugs for Huntington’s disease in ways that are similar or better than a large pharmaceutical company”, says Truant. Truant’s lab is also collaborating in the US with the Cure Huntington’s Disease Initiative (CHDI) a novel, non-profit virtual pharmaceutical company focused on HD.

A large portion of this work was completed in the new McMaster biophotonics facility (www.macbiophotonics.ca), and additional research will be done in McMaster’s unique high throughput screening lab (hts.mcmaster.ca) and other new labs being established at the University.

“We can actually watch huntingtin protein move inside of a single live brain cell in real time in response to stress, and we can watch mutant huntingtin kill that cell, even over days,” says Truant. “Using molecular tools, computer software and sophisticated laser microscopy techniques which we’ve been developing at McMaster over the last seven years, researchers can now use these methods to hopefully watch a drug stop this from happening.”

Truant’s laboratory is supported by grants from the United States High Q Foundation, the Canadian Institutes of Health Research, the Huntington Society of Canada and the Canada Foundation for Innovation.

“This discovery reflects Dr. Truant’s growing contribution to the international campaign to create a world free from Huntington disease,” says Don Lamont, CEO & Executive Director of the Huntington Society of Canada – Canada’s only organization focused on research, education and support in the HD field.

“Our families live on a ‘tightrope’ waiting for an effective treatment or a cure for HD”, says Lamont. “The discovery provides hope for the Huntington community – most of all, hope that their children will not have to suffer the devastation of this inherited disease.”

The Journal Abstract

Huntington's disease (HD) is caused by an expanded polyglutamine tract in huntingtin protein, leading to accumulation of huntingtin in the nuclei of striatal neurons. The 18 amino-acid amino-terminus of huntingtin is an amphipathic alpha helical membrane binding domain that can reversibly target to vesicles and the endoplasmic reticulum (ER). The association of huntingtin to the ER is affected by ER stress. A single point mutation in huntingtin 1-18 predicted to disrupt this helical structure displayed striking phenotypes of complete inhibition of polyglutamine-mediated aggregation, increased huntingtin nuclear accumulation, and greatly increased mutant huntingtin toxicity in a striatal-derived mouse cell line. Huntingtin vesicular interaction mediated by 1-18 is specific to late endosomes and autophagic vesicles. We propose that huntingtin has a normal biological function as an ER-associated protein that can translocate to the nucleus and back out in response to ER stress or other events. The increased nuclear entry of mutant huntingtin due to loss of ER-targeting results in increased toxicity.

Source: Human Molecular Genetics Advance Access published on August 18, 2007

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