Ray Truant and Colleagues Publish
Could Phosphorylation Research Lead to a
Cumulative research into phosphorylation of the HD protein may be bringing us closer to a potential therapy for Huntington’s disease. Dr. Ray Truant, a recent Ph.D. graduate Dr. Randy Singh Atwal, and colleagues have screened compounds in a cell model and found a class of drugs which will restore impaired phosphorylation of the HD protein in the N-terminal 17 (NT17) domain. Phosphorylation is a post-translational (ie, after the protein is made) modification of a protein in which a phosphate group is added to a serine, threonine, or tyrosine residue (parts of amino acids used in protein synthesis) by a kinase (an enzyme that transfers phosphate groups). Proteins are commonly regulated through phosphorylation. Their findings were published yesterday (5/30/11) in Nature Chemical Biology.
Last year we reported that research by Dr. Joan Steffan and colleagues showed that phosphorylation of the HD protein at serines 13 and 16 was impaired compared to the process with the normal protein. Research by Dr. William Yang and colleagues showed that making genetic changes in the BAC HD mouse model to mimic permanent phosphorylation of those two sites prevented the disease from developing in the mice in the twelve months that they were studied.
The first seventeen amino acids at the front of the huntingtin protein appear to be very important in the regulation of the protein and have been widely studied. This N-terminal 17 region precedes the polyglutamine region (the CAG repeats) on the huntingtin protein. Past research By Dr. Truant, Dr. Steffan and others has shown that this region is involved in subcellular localization, stability, cellular toxicity, and aggregation of the HD protein. Working with a full-length, endogenous huntingtin cell line derived from a genetic knock-in mouse model of Huntington’s disease, Drs. Truant and Atwal and colleagues used a combination of methods including mutating residues in the NT17 region and high throughput screening to discover the signaling pathways involved in the phosphorylation process. They have learned more about the normal behavior of the huntingtin protein and the consequences of reduced phosphorylation in this region and identified a potential target for treatment.
Under conditions of stress such as those associated with aging, the normal huntingtin’s protein is phosphorylated at serines 13 and 16.
The phosphorylation affects the conformation (shape) of the NT17 region and targets full-length huntingtin to chromatin-dependent subregions of the nucleus, the mitotic spindle and cleavage furrow during cell division.
But in the disease version of the protein, with its elongated polyglutamine stretch, these serines are not properly phosphorylated. The HD protein doesn’t go to the right target within the cell’s nucleus and it interacts inappropriately with transcription factors. “This is one explanation as to why it takes until middle age for Huntington’s to develop in most patients, because the role of the huntingtin protein is more critical as a person ages,” said Truant.
The researchers used a high content screening in live cells to discover drugs which would enhance the phosphorylation process. Dr. Yang’s research was encouraging in principle but for treatment purposes, a drug that would cause phosphorylation of serines 13 and 16 to take place would need to be discovered or developed. Drs. Truant and Atwal discovered that some drugs that fall within the class of IKK beta kinase inhibitors restore phosphorylation. The next step is to look at existing drugs in that class to find one which will cross the blood brain barrier, be effective and have a good safety and tolerabiloity profile.
Randy Singh Atwal, Carly R. Desmond, Nicholas Caron, Tamara Maiuri, Jianrun Xia, Simonetta Siplone, and Ray Truant. “Kinase inhibitors modulate huntingtin cell localization and toxicity.’ Nature Chemical Biology Published online: 29 May 2011.
MacMaster’s University Press Release.
– Marsha L. Miller, Ph.D., May 31, 2011