New biomarker of Parkinson's disease can be found in urine

Sergey Syrov, XXII CENTURY, based on the materials of the University of Alabama at Birmingham: Parkinson's disease biomarker found in patient urine samples

For more than five years, urine and cerebrospinal fluid samples of patients with Parkinson's disease have been collected in the freezers of the National Institute of Neurological Disorders and Stroke (NINDS) so that one day they could serve as research material shedding light on the hitherto hidden mechanism of development of this slowly progressing neurodegenerative disease.

The work of Andrew West and his colleagues from the University of Alabama at Birmingham shows that the collected samples contain a new type of biomarker that correlates with the presence and severity of Parkinson's disease – phosphorylated proteins. West and his colleagues continue to work with samples, exploring the possibility of using a new biomarker in the development of clinical methods of treating the disease, as well as to monitor the effectiveness of drugs and procedures in real time during treatment.

"No one thought that we could measure the activity of this important LRRK2 protein in biological fluids, since it is usually located inside neurons in the brain," says West. "New biochemical markers like those that we found, together with new neuroimaging approaches, will be the key to a successful fight against Parkinson's disease. I think that the days of blind testing of new treatments for complex diseases such as Parkinson's disease, without active feedback between the target drug and its effectiveness for the patient, are fortunately coming to an end."

Biomarkers help doctors predict, diagnose and control the development of the disease. The biomarker indicates the presence or risk of the disease, and its level may change as the disease progresses. Confirmed biomarkers can help in conducting both preclinical laboratory tests and clinical trials of drugs for the treatment of Parkinson's disease. West and his colleagues are currently developing a drug that should prevent neuroinflammation and neurodegeneration in an animal model of the disease, the detection of a biomarker of Parkinson's disease is only part of their work.

Articles about the new biomarker have been published in two scientific journals – Neurology (March 2016) and Movement Disorders (June 2016).

The LRRK2 protein has been shown to play an important role in the hereditary form of Parkinson's disease, and is most often associated with the G2019S mutation. It is known that the mutation leads to excessive phosphorylation of proteins, but why this leads to Parkinson's disease has not yet been clarified.

Reversible phosphorylation of amino acid side chains is a widespread way of regulating the activity of key cell proteins, including enzymes and proteins of signaling pathways. It is believed that about a third of all eukaryotic proteins are subject to phosphorylation.

The protein phosphorylation reaction is understood as the addition of a phosphate group via a phosphoester bond (O-phosphorylation) to the hydroxyl group of the side chain of a serine, threonine or tyrosine residue, with ATP being the phosphate donor. In the vast majority of cases, phosphorylation occurs precisely on these three amino acid residues. However, phosphorylation of histidine and arginine residues (N-phosphorylation), aspartate and glutamate (A-phosphorylation) is also found in nature. Phosphoric acid esters formed during phosphorylation are very stable, therefore, special enzymes – protein phosphatases - are needed for their destruction. This creates grounds for fine regulation of the level of protein phosphorylation by controlling the level of the corresponding protein kinases and protein phosphatases.

The key to the discovery of the new biomarker was the understanding that LRRK2 can be found in exosomes in all human biological fluids, including urine and saliva. The cells of the body constantly secrete exosomes that contain a mixture of proteins, RNA and DNA. West and his colleagues were able to isolate exosomes from 90-120 ml of urine, and then measure the level of phosphorylated LRRK2 in these exosomes.

An article published in Neurology reports that an increased level of phosphorylated LRRK2 predicts the risk of Parkinson's disease in people with mutations in the gene encoding this protein (such patients are only about 2-3% of all patients with Parkinson's disease). This was shown during a small preliminary study of urine samples from 14 people with Parkinson's disease. This was followed by a larger study – 72 samples from bio-storage facilities were used.

A subsequent publication in the journal Movement Disorders reports on the study of urine samples taken from patients without the LRRK2 mutation, most of these patients. A study of 158 urine samples from both patients with Parkinson's disease and healthy people showed that about 20% of people without the LRRK2 mutation, but with Parkinson's disease, also have elevated levels of phosphorylated LRRK2, which is not observed in healthy people. It can be assumed that people with elevated levels of phosphorylated LRRK2 can be helped by drugs being developed that will reduce the phosphorylation of LRRK2.

The study by West and his colleagues raises many questions. In particular, it is necessary to find out what is the source of the found exosomes. Given the supposed role of inflammation in the development of Parkinson's disease, it is important that the expression of LRRK2 is high in the cells of the immune system. A possible explanation for phosphorylated LRRK2 in patients with a more severe course of the disease may be an increase in inflammation.

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