Biomarkers can be broadly defined as qualitative or quantitative measurements that convey information on the physiopathological state of a subject at a certain time point or disease state. Biomarkers can indicate health, pathology, or response to treatment, including unwanted side effects. When used as outcomes in clinical trials, biomarkers act as surrogates or substitutes for clinically meaningful endpoints. Biomarkers of disease can be diagnostic (the identification of the nature and cause of a condition) or prognostic (predicting the likelihood of a person’s survival or outcome of a disease). In addition, genetic biomarkers can be used to quantify the risk of developing a certain disease. In the specific case of traumatic brain injury, surrogate blood biomarkers of imaging can improve the standard of care and reduce the costs of diagnosis. In addition, a prognostic role for biomarkers has been suggested in the case of post-traumatic epilepsy. Given the extensive literature on clinical biomarkers, we will focus herein on biomarkers which are present in peripheral body fluids such as saliva and blood. In particular, blood biomarkers, such as glial fibrillary acidic protein and salivary/blood S100B, will be discussed together with the use of nucleic acids (eg, DNA) collected from peripheral cells.

BackgroundBlood biomarkers of neurovascular damage are used clinically to diagnose the presence severity or absence of neurological diseases, but data interpretation is confounded by a limited understanding of their dependence on variables other than the disease condition itself. These include half-life in blood, molecular weight, and marker-specific biophysical properties, as well as the effects of glomerular filtration, age, gender, and ethnicity. To study these factors, and to provide a method for markers’ analyses, we developed a kinetic model that allows the integrated interpretation of these properties.MethodsThe pharmacokinetic behaviors of S100B (monomer and homodimer), Glial Fibrillary Acidic Protein and Ubiquitin C-Terminal Hydrolase L1 were modeled using relevant chemical and physical properties; modeling results were validated by comparison with data obtained from healthy subjects or individuals affected by neurological diseases. Brain imaging data were used to model passage of biomarkers across the blood–brain barrier.ResultsOur results show the following: (1) changes in biomarker serum levels due to age or disease progression are accounted for by differences in kidney filtration; (2) a significant change in the brain-to-blood volumetric ratio, which is characteristic of infant and adult development, contributes to variation in blood concentration of biomarkers; (3) the effects of extracranial contribution at steady-state are predicted in our model to be less important than suspected, while the contribution of blood–brain barrier disruption is confirmed as a significant factor in controlling markers’ appearance in blood, where the biomarkers are typically detected; (4) the contribution of skin to the marker S100B blood levels depends on a direct correlation with pigmentation and not ethnicity; the contribution of extracranial sources for other markers requires further investigation.ConclusionsWe developed a multi-compartment, pharmacokinetic model that integrates the biophysical properties of a given brain molecule and predicts its time-dependent concentration in blood, for populations of varying physical and anatomical characteristics. This model emphasizes the importance of the blood–brain barrier as a gatekeeper for markers’ blood appearance and, ultimately, for rational clinical use of peripherally-detected brain protein.Electronic supplementary materialThe online version of this article (doi:10.1186/s12987-016-0045-y) contains supplementary material, which is available to authorized users.

The brain is a complex system that requires continual regulation of parenchymal pressure, osmolarity, and waste removal for optimal function; despite this, human brain lacks any obvious extension of lymphatic circulation for moderating fluid and waste regulation. We recapitulate herein a recent analysis of proteinaceous waste deposition in the human brain, its observed route of clearance, and the implications of abnormal accumulation along this clearance pathway as a potential mechanism of neurological diseases. This study uncovered an analogous staining pattern of cerebral phosphorylated tau in temporal lobe epilepsy (TLE) and chronic traumatic encephalopathy (CTE). Regardless of the underlying physiopathology, p-tau elimination occurred via circulation through the perivenous space, as predicted by a glymphatic route of clearance. Remarkably, we demonstrated that p-tau is associated with a neurological disease that can develop independent of head trauma, since in both CTE and TLE: 1) Extracellular p-tau followed unidirectional, fluid-driven pathways that led toward the space bordering large (>100 μm diameter) blood vessels; 2) Tau-positive staining occurred within astroglial cells adjacent to blood vessels, which signified transcellular transport of p-tau as a potential secondary efflux route; 3) P-tau frequently appeared clustered within the perivenous space. This waste aggregation bears significant implications in the disruption of interstitial fluid circulation, which may contribute to exacerbation of disease states. A better understanding of waste elimination in the human brain may prove significant as a therapeutic target to improve parenchymal fluid circulation, and consequently, mitigate the hydrostatic, osmotic and oncotic imbalances that often cause or exacerbate brain diseases.

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