Scientists have discovered biological mechanisms that may link Parkinson's disease to Alzheimer's disease, according to a study published in the journal Acta Neuropathologica.
Researchers from the University of Florida say their findings could lead to targets for new treatment that combats both Alzheimer's and Parkinson's disease, as well as many other neurological disorders.
For their study, the researchers investigated a protein called tau. They explain that tau is a soluble protein in the brain that binds to microtubules - components that play an important part in cell processes - helping to support neuronal function.
The normal function of tau is supported by phosphorylation - a process that switches protein enzymes on and off and regulates their function and activity.
However, the researchers say that in some diseased brains, the tau protein can become abnormally phosphorylated and cause "clumps" or "tangles." These tangles, called tauopathies, are linked to cognitive impairment found in some neurodegenerative diseases, such as Alzheimer's, and are also linked to around 20% of Parkinson's cases.
LRRK2 'phosphorylates tau protein'
Parkinson's patients can develop tangles of the tau protein as a result of a mutated enzyme called LRRK2, the researchers say.
From conducting a series of mammalian studies, the researchers discovered that standard LRRK2 adds groups of phosphate to tau protein, while mutated LRRK2 adds significantly more phosphates to tau protein.
Using these findings, the research team were able to uncover two sites on the tau protein - T149 and T153 - where the mutated LRRK2 was able to add phosphate groups, which they linked to tangle formations.
The researchers then went on to test their findings on human brain tissue. From this, they discovered that the two sites previously identified on the tau protein that were targeted by LRRK2, were altered the same way in human tissue with LRRK2 mutation.
When testing this finding on human brain tissue that was affected by Alzheimer's disease, and other neurodegenerative diseases that form tangles, they found these alterations were the same.
Explaining their findings, the researchers say:
"Our data, in aggregate, demonstrate that LRRK2 directly phosphorylates tau at T149 and T153 in vitro and the ability of LRRK2 to phosphorylate tau at these sites may underlie its ability to promote tauopathy in our novel mouse model.
Our current in vivo studies are the first of their kind and provide compelling evidence that LRRK2 and tau interact in a disease-relevant manner."
Potential for therapeutic targets
The researchers add that their findings have the potential to provide new therapeutic targets for both Alzheimer's and Parkinson's disease.
"Until now, nobody has really understood what the overlap between Alzheimer's and Parkinson's disease was, or if it were important," says Jada Lewis, associate professor of neuroscience at the Centre for Translational Research in Neurodegenerative Disease at the University of Florida. "Our study ties these diseases together in a unique way."
The researchers conclude that their findings show that LRRK2 genetic studies in human tauopathies may be warranted.
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