Here’s a recent JBC article I wrote about for my biochemistry course. It’s about Parkinson’s Disease (PD). I’ll give you some background here. PD is a neurodegenerative disease involving dopaminergic neurons. What does that mean? Well, dopaminergic neurons produce the neurotransmitter dopamine, which is involved in mood, reward, and inhibiting or stopping motion. People with PD are deficient in dopamine, and so they experience tremors.
There is extensive evidence of damage to a dopaminergic region called the substantia nigra. One sign is that protein buildups (proteosomes), known as Lewy Bodies, have been found bound to the membranes of surviving neurons. The major protein involved in this is called α-synuclein, or α-syn. Another common observation in PD brains is that almost 90% of the α-syn is phosphorylated at serine residue 129 (Ser-129) whereas only about 5% is phosphorylated (Ser(P)-129 α-syn) in a healthy brain.
What Visanji and colleagues thought was that perhaps the phosphorylation is involved in binding to the synaptic membrane, which in turn affects its structure and function. Other hypotheses they had were that phosphorylation was an intermediate in binding to the membrane.
Well, here’s some evidence of what’s not happening. In cell-free assays (utilizing synaptosomes extracted from mice and α-syn filled cytosol extractions), they determined that α-syn can be found both in the cytosol and in the membrane. They also found that phosphorylation had no apparent effect on binding. Also, in binding and dissociation, they found that some of the phosphorylated proteins are lost (not dephosphorylated, lost). They concluded that phosphorylation is probably not an intermediate step, and that the protein can be in either cytosolic or membrane portions.
What next? Well they wanted to know what would happen with endogenously expressed proteins when you played with phosphorylation. So, they went to tinker with the genes of mice. They got knockout mice that did not express murine α-syn, but did express one of three human forms of the protein. Then they used vectors to overexpress or not express a phosphorylating enzyme (kinase) in neural progenitor cells. Well, they still got data saying that its effect was essentially crap-all in terms of membrane binding.
Honestly, I don’t have time to go through all the stuff they did essentially gave a “nothin’ goin’ on here” result.
BUT WAIT! This isn’t the end for the Ser(P)-129 α-syn Story! There is some significant stuff happening!
There are two mutations of α-syn implicated in hereditary forms of PD. A30P an A53T missense mutations are the two forms that were tested. The third was the wild-type (non-mutant) protein.
Well, what happens when we accutely, or temporarily, phosphorylate the proteins NOT using kinases? Well, the researchers used epoxomycin, which is normally a proteasome inhibitor. Well, in the presence of this chemical, wild type proteins were not effected. The mutants, however, binded to the membrane like crazy.
The researchers suggested that phosphorylation could also be involved in protein-protein interactions. This means that once they’ve stuck to the membrane, the phosphorylation could be making other proteins stick to them, resulting in these big Lewy Bodies messing up the synapse. The missense mutations make the resulting proteins more likely to bind and form these damages, thus they result in the hereditary forms of PD.
I found this pretty interesting, and I especially liked the way they ruled out a crap-ton (that’s a metric measurement btw) of other possibilities before going into what actually gave significant results. This is really good science: “here’s what’s probably happening, and here’s a bunch of stuff that’s almost definitely not happening.”
Sorry for all the words.
Visanji, N.P., Wislet-Gendebien, S., Oschipok, L.W., Zhang, G., Aubert, I., Fraser, P.E. and Tandon, A. (2011). Effect of Ser-129 Phosphorylation on Interaction of α-Synuclein with Synaptic and Cellular Membranes. Journal of Biological Chemistry 286, 35863-35873.
PS: I’ll try to do something else here soon that isn’t just summarizing a primary research paper, or that isn’t so molecule-focused. I just have molecules on the mind.