Tears – More to Them Than Meets the Eye: Why Tears Are A Good Source of Biomarkers in Parkinson'S Disease

Abstract

Tears are a known source of biomarkers for both ocular and systemic diseases with particular advantages; specifically, the noninvasiveness of sample collection and a unique and increasingly better-defined protein composition. Here, we discuss our rationale for use of tears for discovery of biomarkers for Parkinson's disease (PD). These reasons include literature supporting changes in tear flow and composition in PD, and the interconnections between the ocular surface system and neurons affected in PD. We highlight recent data on the identification of tear biomarkers including oligomeric α-synuclein, associated with neuronal degeneration in PD, in tears of PD patients and discuss possible sources for its release into tears. Challenges and next steps for advancing such biomarkers to clinical usage are highlighted.

A Pilot Study of Changes in the Level of Catecholamines and the Activity of α-2-Macroglobulin in the Tear Fluid of Patients with Parkinson’s Disease and Parkinsonian Mice

“…However, data published by various authors are often contradictory [ 14 , 17 , 38 , 39 , 40 , 41 , 42 , 43 , 46 , 47 , 48 ]. This is probably due to the fact that the concentration of analytes in the blood and, to a lesser extent, in the cerebrospinal fluid is an integral index of a wide range of metabolic processes associated with degeneration and plasticity of central and peripheral neurons, as well as desympathization of the internal organs [ 8 , 20 ]. In this context, the use of the TF for searching PD biomarkers has certain advantages over the use of the cerebrospinal fluid and blood.…”

“…In this context, the use of the TF for searching PD biomarkers has certain advantages over the use of the cerebrospinal fluid and blood. These include non-invasive atraumatic collection and a smaller spectrum of substances contained in the TF [ 20 , 49 ].…”

“…Until recently, the development of a differential and early diagnosis of PD has been based on the search for changes in the cerebrospinal fluid and blood in PD patients at an early clinical stage [ 4 , 17 ]. However, in recent years, the search for changes in the TF has also been considered promising for the development of the PD diagnosis [ 18 , 19 , 20 ]. The attractiveness of using the TF is explained by the fact that its collection is a non-invasive, easily reproducible procedure without negative consequences [ 20 ].…”

“…However, in recent years, the search for changes in the TF has also been considered promising for the development of the PD diagnosis [ 18 , 19 , 20 ]. The attractiveness of using the TF is explained by the fact that its collection is a non-invasive, easily reproducible procedure without negative consequences [ 20 ].…”

“…This is probably due to the fact that central and peripheral catecholaminergic neurons, degenerating in PD, play a key role not only in the regulation of motor function, but also in the regulation of the eye [ 30 , 31 ]. In fact, the eye, including the conjunctiva and cornea, and surrounding tissues have extensive sympathetic innervation [ 20 , 32 , 33 ], and catecholamines have been found in the TF [ 15 , 34 , 35 , 36 ]. This idea is supported by our previous observation of the degradation of the catecholaminergic systems of the eye in parkinsonian mice [ 37 ].…”