Written by Carrie A. Sheeler Edited by Dr. Marija Cvetanovic
Group 1 p21-associated kinases (PAKs) present a new avenue for SCA1 research.
Spinocerebellar ataxia type 1 (SCA1) is caused by a specific mutation in the Ataxin1 gene, which causes the protein that’s made from that gene (also called Ataxin1) to have an abnormally elongated polyglutamine (polyQ) tract. This leads to dysfunction and death in the affected cells of the brain (predominantly Purkinje neurons in the cerebellum), which causes symptoms in patients that include a progressive worsening of coordination and balance. While there is currently no cure for SCA1, several studies suggest that lowering the amount of Ataxin1 protein in the brain may delay the onset of the disease and decrease the severity of symptoms. This leads us to an important question: how do we most effectively decrease the amount of Ataxin1 in SCA1 patients? One paper recently published by Bondar and colleagues suggests that a multi-pronged approach could be the most effective means of reducing this toxic protein.
The amount of any specific protein in the body can be altered by either decreasing the amount of protein produced or increasing the rate at which cells break those proteins down. Proteins are made using messenger RNA (mRNA), which is created following specific instructions found in DNA. Decreasing the production or stability of mRNA decreases the amount of corresponding protein made. One way to target the mRNA that causes production of a specific protein is with antisense oligonucleotides (ASOs). ASOs are designed to target specific mRNA sequences by binding to them directly. Binding of ASOs to mRNA causes those molecules to be marked for destruction within the cell. Proteins in the body are also regularly recycled, but without the blueprints to build a new protein, cells cannot replenish the protein supply it loses over time. So, if Ataxin1 mRNAs are targeted and destroyed by ASO treatment, the amount of Ataxin1 in our cells would theoretically decrease.
Some proteins can also be altered by other proteins, creating another way that their stability, shape, and function can be regulated. This leads us to the other way we can alter the amount of a specific protein in a cell: regulating the regulators. In terms of SCA1, this could mean removing a protein that helps stabilize Ataxin1 or increasing the production of a protein that breaks Ataxin1 down. Previous research has identified several proteins of interest that regulate Ataxin1 protein stability, including several kinases. Kinases are a class of proteins that transfer a phosphate group from adenosine triphosphate (ATP) to another protein in the cell. The addition of this phosphate group acts as an energy source to the receiving protein, altering its stability or how it interacts with other molecules in the cell (usually by causing it to change its shape). Recently, Bondar and colleagues have identified a new potential regulator of Ataxin1: a group of proteins known as p21-activated kinases (PAKs) (Bondar et al 2018).
PAKs have previously been noted as proteins of interest in neurological diseases like Fragile X Syndrome and Huntington’s Disease. These disorders, like SCA1, result from a genetic mutation in which the normal gene expands so much that the resulting protein does not work properly. Previous research has shown that increased production of PAK family proteins worsens the symptoms of Huntington’s Disease in cell culture (a way of studying disease that uses cells grown in a dish to model the disease) (Luo et al 2008). To determine whether PAK family proteins were relevant to SCA1 pathology in a living organism, researchers started with a fruit fly model of SCA1. Fruit flies with an expanded ATXN1 gene have flight problems and eye defects. When the amount of group 1 PAK proteins was decreased in flies, researchers saw a subsequent decrease in Ataxin1 protein in the flies. This decrease in Ataxin1 protein also seemed to lessen the severity of the eye and flight problems. Together, this evidence suggested that PAK could be a therapeutic target of interest.
While these results are promising, research in flies is only a starting point when studying human disease. A follow up study was done in cell culture using mouse neurons. Decreasing the amount of PAK in these neurons reduced Ataxin1 protein levels in the same cells. Thus, the effect is not restricted to fly brain cells. While this suggests that the hypothesis applies to mammals, this is not enough to conclude that decreasing PAK1 would decrease Ataxin1 in humans. To test this directly, the researchers decreased PAK protein in two different human cell lines. Reducing group 1 PAK protein levels effectively decreased the amounts of both mutant and normal Ataxin1 protein in both types of human cells. Building on the results from animal models, this new data further suggests that PAK proteins could be a therapeutic target for human SCA1. The next step was to determine how PAK proteins interact with Ataxin1.
As we stated earlier, scientists have already identified ways to decrease the amount of Ataxin1 protein in cells. So why does it matter that scientists have now found and tested one more? The key is that group 1 PAKs target the Ataxin1 protein in a different manner from previously identified proteins. As stated before, PAKs are kinases and function by adding phosphate groups to other proteins (a process called “phosphorylation”). However, when the phosphorylation site on Ataxin1 that affects its stability is mutated so that a phosphate group cannot be added, PAK reduction still reduces the amount of Ataxin1 present. Further experiments suggest that PAK proteins do not phosphorylate Ataxin1 nor do PAK proteins bind directly to Ataxin1 or two of its known interactors (capicua and 14-3-3). Interestingly blocking the process by which proteins are tagged for destruction and subsequently broken down (known as the proteasome pathway), prevents the reduction of Ataxin1 when PAK is targeted. While this leaves much of the “how” up in the air, it suggests that reducing the amount of PAK protein in the cell can decrease the amount of Ataxin1 without competing with other currently known methods of reduction. This provides an opportunity to decrease the amount of Ataxin1 in the cell through a multi-pronged or “combinatorial” approach: reduce Ataxin1 by targeting the proteins that interact directly with it and PAK proteins, which don’t.
Bondar and colleagues conclude their study by testing three PAK-targeting drugs. Two were assessed in cell cultures of mouse neurons, while the third was tested in a living mouse that was genetically engineered to model SCA1. Of the drugs tested, all three decreased Ataxin1 protein levels in a dose dependent manner (meaning that higher concentrations of drug caused greater losses of Ataxin1). They then assessed the effect of a combined treatment – reducing PAK and MEK protein levels – in human neurons. MEK is one of the regulator proteins that does cause a direct Ataxin1 interaction at its phosphorylation site (therefore, stabilizing the Ataxin1 protein). When both MEK and PAK are reduced, Ataxin1 is decreased beyond what is seen when either drug was given separately. This suggests that a combination treatment could be more effective than a single drug. Targeting Ataxin1 in this way could more effectively decrease the amount of Ataxin1 present in cells, avoiding increases in dosage that, for a single drug, could become toxic.
On the whole, this is an exciting discovery with a strong therapeutic potential. The development of combination therapies has been highly effective for the treatment of certain cancers and could be a useful technique for SCA1 and other neurodegenerative disorders. It is worth noting, though, that the researchers did not test the effect of their potential drugs of interest on symptoms of SCA1 at this stage. Decreased Ataxin1 has previously been shown to lessen symptom severity, but the changes demonstrated in the figures seem small and, on their own, may not be enough to cause an effect. It’s likely that future work from this laboratory will assess SCA1 symptoms in animal models treated with PAK-targeting drugs alone and in combination with other drug targets.
Key Terms
Polyglutamine tract: a section of protein made up of many of the same type of amino acid (in this case, glutamine). In some neurological disorders, including SCA1, this repeating glutamine section has mutated and increased beyond what cells can handle, causing disease.
Messenger RNA (mRNA): A single strand of nucleic acids that is created following a DNA template. After formation, mRNA leave the nucleus of the cell and serve as the template from which proteins are made.
Kinase: a protein which transfers a phosphate group from a molecule of ATP to another protein, often changing the shape and/or activity of the target protein.
Adenosine Triphosphate (ATP): the primary energy molecule of the cell.
Phosphorylation: the process of adding a phosphate group.
Phosphorylate: to add a phosphate group
MAPK/ERK kinase (MEK): a type of kinase, or protein that is responsible for phosphorylating a number of other proteins
Conflict of Interest Statement
The writer, Carrie Sheeler, works in the laboratory of Harry T. Orr, one of the authors on this paper.
Citation of Article Reviewed
Bondar V V., Adamski CJ, Onur TS, et al. PAK1 regulates ATXN1 levels providing an opportunity to modify its toxicity in spinocerebellar ataxia type 1. Hum Mol Genet. 2018. doi:10.1093/hmg/ddy200
Luo S, Mizuta H, Rubinsztein DC. p21-activated kinase 1 promotes soluble mutant huntingtin self-interaction and enhances toxicity. Hum Mol Genet. 2008. doi:10.1093/hmg/ddm362
