Associations between Serum Tau, Neurological Outcome, and Cognition following Traumatic Brain Injury
Keywords: Cognitive dysfunction, montreal cognitive assessment, serum Tau, traumatic brain injuryKey Message: Serum Tau was significantly higher in patients with TBI. Acute Tau associated with neurological outcomes and cognition may implicate white matter damage and neuronal degeneration. Serum Tau may be used as a reliable biological marker for early diagnosis and cognitive recovery following TBI.
Traumatic brain injury (TBI) is one of the main causes of death and disability in youths and adults in both developing and developed countries. The early assessment of brain function after TBI is of great significance for therapeutic strategy and for predicting prognosis. Clinically, the Glasgow coma scale (GCS) and head CT scans are routinely used for assessment and prediction but due to their own limitations, these methods still have some controversies, such as patients with normal CT scans have the potential neuroendocrinological disorder. Tau protein is a microtubule-associated protein, mainly in the axons and cytoplasm of neurons, which is useful for microtubule assembly, stabilization of microtubules, and maintenance of neuronal function. Moreover, phosphorylation of Tau is closely related to secondary brain damage after TBI.
The epidemiological study has shown that TBI is an independent risk factor for Alzheimer's disease and other neurodegenerative diseases, and phosphorylated Tau is involved in most cases. The alteration of Tau is also found in cognitive dysfunction from chronic traumatic encephalopathy (CTE), ischemic stroke, and aneurysmal subarachnoid hemorrhage (aSAH)., However, it takes a while for TBI patients to undergo cognitive analysis with a battery of cognitive and psychological assessments along with a lack of effective biomarkers. In this study, we investigated the dynamic change of serum Tau in the early stage after TBI and explored its clinical association with severity assessment (GCS), prognosis evaluation (GOS), and cognitive prediction (MoCA).
Around 247 TBI subjects were consecutively admitted from September 2016 to June 2018, excluding 18 cases with death within 14 days after injury or loss to follow-up. Nearly, 229 patients were included in the following study for neurological assessment with GCS and GOS. Of the 229 cases, 153 were male and 76 were females; aged 16–65 years, (mean: 41.3 ± 14.5). In addition, on the basis of the type of injury among the 229 cases, 152 cases of traffic accident injury, 23 cases of bruises, 33 cases of fall injuries, and 21 cases of dropping had been reported. The admitted GCS score was 3 to 8 points in 31 cases (severe TBI group), 9 to 12 points in 23 cases (moderate TBI group), and 13 to 15 points in 175 cases (mild TBI group). We also had 30 healthy age-matched adults in the physical examination in our hospital during this period as the control group, including 18 males and 12 females; aged 17 to 62 years (mean: 40.4 ± 8.2). A remaining 95 TBI patients were enrolled for the cognitive assessment. There were 61 males and 34 females in the age group of 16 to 65 years (mean: 40.7 ± 13.6 years).
Case inclusion criteria: The case inclusion criteria included patients with a clear history of TBI in the age group of 16 to 65 years. The study was approved by the hospital ethics committee, the patient or her family members agreed on the consent form. Case exclusion criteria: Patients with no previous history of TBI, history of cerebrovascular disorder or other intracranial lesions, history of mental illness or drug abuse or alcohol abuse, history of repeated stroke before injury or dementia; combined with severe multiple injuries, having a pituitary or thyroid disease, severe liver and kidney dysfunction, history of diabetes, or a history of systemic diseases such as malignant tumor were excluded from the study.
Serum Tau test: Serum samples were obtained from the TBI patients at 1, 3, 5, 7 and 14 days after TBI. Samples were collected in 10 mL separator tubes and then centrifuged at 1,000 rpm for 15 min at 4°C. Subsequently, the serum samples were frozen and stored at -80°C until further analysis. The Tau protein levels in the serum were measured using the Human Tau Proteins Enzyme-Linked Immunosorbent Assay (ELISA) kit (CSBE12011h; Cusabio Biotech Co., Ltd., Wuhan, China). The minimum detectable concentration of Tau protein was 15.6 pg/mL, according to the manufacturer's protocol. Prior to the analysis of the serum concentration of Tau protein, a standard dilution curve was constructed using standard solutions provided with the ELISA kit. The dilution curve of the serum samples was partial to the standard dilution curve, to ensure the accuracy of the measurements.
Neurological outcome: The neurological outcome was also evaluated with GOS scores at 6 months post-injury. The GOS is a widely utilized measure that classifies outcome into five scales: 5 = good recovery, 4 = moderate disability, 3 = severe disability, 2 = persistent vegetative state, and 1 = death. We grouped the 5 and 4 into the good outcome group and 1–3 into the poor outcome group.
Cognitive assessment: This group of patients was tested for cognitive function by a trained neurosurgeon 6 months after injury. The measurement time started at 2 pm and the maximum time was 30 min. The MoCA scale was used to assess the cognitive function of the patients and the scores of each sub-item were recorded in detail, with a total score of 30, and a score of <26 indicating the presence of cognitive dysfunction.
Statistical analysis. Data were expressed as the mean ± standard deviation and were analyzed using the SPSS version 21.0 software (SPSS, Inc., Chicago, IL, USA). Mean values in two or more groups were analyzed using Student's t-test and protected t-test (One-way analysis of variance [ANOVA]). Pearson analysis was used to determine the relationship between Tau protein level, neurological outcome, and cognitive function. Statistically significant differences were determined by P < 0.05.
Acute changes in serum Tau protein after TBI
The serum Tau content in TBI at each time point was significantly higher than that of the control group (P < 0.05). Serum Tau progressively increased after TBI in the first week, while it began to decline on the seventh day, but it was still remaining higher than the control group (P < 0.05, [Figure 1]).
Relationship between serum Tau and severity degree
The Tau protein increased significantly with the severity of TBI (P < 0.05). See [Table 1] for details. On 1, 3, and 5 days after injury, serum Tau content was significantly negatively correlated with admission GCS scores (r = 0.7348, 0.5232, 0.4402, respectively; P < 0.05).
The relationship between serum Tau and neurological outcome
Serum Tau protein at 1, 3, 5, and 7 days was significantly higher in poor outcome group injury that that in good outcome group (P < 0.05; [Figure 2] and [Table 2]). Serum Tau content at 1, 3, 5, and 7 days after injury had a significant negative correlation with GOS score at 6 months after injury (r = 0.4388, 0.4868, 0.3702, 0.3517; P < 0.05).
Correlation between serum Tau with injury and prognosis in patients with TBI
Pearson analysis was used to assess the correlation between Tau protein at each time point, the GCS score and the 6-month post-injury GOS score. This showed that the Tau protein at 1, 3, and 5 days after injury was statistically correlated with the GCS score (P < 0.05) [Table 3]. The Tau protein at 1, 3, 5, and 7 days after injury were statistically correlated with the GOS score at 6 months after injury (P < 0.05) [Table 4]. It is indicated that the early serum Tau can not only determine the severity of brain injury but also predict the neurological outcome.
For the cognitive assessment, we first analyzed the basic data between the two groups (TBI group vs. control group, [Table 5]). Compared with the age, gender, and education level of two groups, there was no significant difference (P > 0.05)
The occurrence of cognitive dysfunction in TBI patients at 6 months after injury
Of the 95 patients with TBI, 39 had cognitive dysfunction at 6 months after injury, with an incidence of 41%. Cognitive dysfunction mainly lied in visual space and executive function, delayed memory, language, abstract ability, attention, and computational power (P < 0.05). There was no significant difference between the two groups in naming and orientation [Table 6].
Relationship between dynamic changes of serum Tau protein and cognitive impairment in patients with TBI
The serum Tau in the control group was 238.04 ± 75.26 pg/mL. The serum Tau of TBI patients with abnormal cognition increased progressively at 1, 3, and 5 days after injury, and decreased after the seventh day but it was still significantly higher than that of the normal cognition group (P < 0.05, [Figure 3]). Compared with the normal cognition group, the serum Tau protein in the abnormal cognition group was significantly higher at 1, 3, 5, 7, and 14 days after injury (P < 0.05), indicating that the serum Tau protein content increased in the early post-injury group, which was closely related to cognitive dysfunction [Table 7].
Correlation between dynamic changes of serum Tau protein and cognitive dysfunction in patients with TBI
Pearson correlation analysis of the patient's 6-month-old MoCA score and serum Tau protein was negatively correlated with MoCA score at 1, 3, and 5 days after injury (P < 0.05). On the first day after the injury, the correlation coefficient between serum Tau and the cognitive score was the largest with the smallest P value, indicating that the serum Tau protein on the first day after injury could best predict the cognitive function prognosis [Table 8].
Our findings indicate that the serum Tau after acute TBI is obviously increased (P < 0.05), peaked at 5 days, and then gradually decreased, however, it still remained higher than the control group (P < 0.05). This is mainly due to the white matter impairment and blood-brain barrier (BBB) dysfunction after brain injury, with the degradation of Tau proteins released from the central nervous system to the peripheral blood, leading to significantly elevated serum Tau. With the recovery of TBI, serum Tau levels began to decline. This might be because of the repair of BBB and the recovery of white matter. The dynamic change of serum Tau protein is related to the severity of TBI, which can be used as a measure of the degree of injury. In this study, we found that serum Tau was negatively correlated with the degree of TBI injury (P < 0.05), which can potentially reflect the severity of acute TBI damage.
Studies have also reported an increase in cerebrospinal fluid and plasma Tau protein after TBI. Moreover, the degree of this increase is negatively correlated with clinical outcome. Accordingly, we found serum Tau protein at 1, 3, 5, and 7 days after injury was significantly increased in the poor prognosis group (P < 0.05), and serum Tau protein was negatively correlated with the GOS score at 6 months after the injury as well (P < 0.05). This suggests that the serum Tau protein level at the early stage of TBI can predict the neurological outcome.
With the improvement in the treatment and care quality of TBI, the mortality rate in the acute phase of TBI has been reduced to some extent. However, TBI patients often develop persistent dysfunction in the chronic stage, such as cognitive dysfunction, motor function defects, personality changes, and emotional abnormalities. The incidence of cognitive dysfunction after TBI is different in the literature reported based on the severity scale. The incidence of attention and memory impairment is 40% to 60% within 3 months after mild TBI and up to 90% for moderate and severe TBI patients. Clinical studies have found that TBI is an independent risk factor for Alzheimer's disease and non-Alzheimer's dementia. In our study, the MoCA scale was used to evaluate the cognitive function of TBI patients at 6 months after injury. The incidence rate of cognitive dysfunction was 41%, indicating that cognitive dysfunction was widespread in patients with acute TBI, which is worthy of clinical investigation and research. There is currently a lack of relevant biomarkers for early prediction of cognitive impairment after TBI. It was found that serum Tau protein was elevated in patients with cognitive impairment after TBI, and early dynamic changes were associated with cognitive impairment.
The mechanism of cognitive dysfunction after TBI is still not fully understood. There are reports showing cognitive activities such as learning and memory which are related to neurotransmitter abnormalities in the brain, including acetylcholine, dopamine, serotonin, glutamate, etc., Studies have found, increased plasma and CSF Tau protein in patients with cerebral infarction, subarachnoid hemorrhage, and TBI. In our study, the serum Tau in TBI was detected at different time points. The results showed that the serum Tau was higher than that of the control group at 1, 3, 5, 7, and 14 days after injury (P < 0.05). On the fifth day after the injury, the serum Tau protein content peaked and then gradually decreased after the seventh day but still higher than the normal cognition group (P < 0.05).
In a series of neurodegenerative diseases, the most obvious pathology is the hyperphosphorylation of Tau, also known as neurofibrillary tangles., Our study found that serum Tau protein levels were significantly increased on 1, 3, 5, 7, and fourteenth day after injury in patients with cognitive impairment after acute TBI (P < 0.05) compared to those with normal cognition, indicating that the early increase of serum Tau is closely related to cognitive dysfunction. The persistent increase in serum Tau may reflect the excessive release and accumulation of Tau protein in damaged neurons which may cause microtubule instability, axonal transport dysfunction, impaired cellular signaling, and cognitive dysfunction. We previously found that injured rodent brain dissociates the Tau protein from microtubules in an insoluble state, and easily accumulates into NFTs, which cause cognitive dysfunction. The aggregation of Tau has recently been shown to inhibit autophagy and suppressing NMDA receptor expression, to cause memory deficit and neurodegeneration.
In this study, we further linked the MoCA score at 6-months post-injury and the dynamic change of serum Tau protein by Pearson analysis and found that the serum Tau at 1, 3, and 5 days after injury was negatively correlated with the MoCA score (P < 0.05), suggesting that the early serum Tau level is helpful to predict the cognitive function in TBI patients. On the first day after the injury, the correlation coefficient between Tau and MoCA score was the largest, and the P value was the smallest, indicating that the serum Tau protein after 24 h might best reflect the cognitive function. Previous studies have found that serum Tau levels are higher in patients with a severe brain injury than in patients with mild-to-moderate brain injury, and higher Tau levels are associated with poorer neurological outcomes. These findings suggest that the therapeutic strategy can be targeted to reduce the Tau protein in early-stage after brain injury to obtain neuroprotective effects, and to further prevent the development of cognitive disorders. There are currently few reports on Tau based treatment after TBI. A recent paper showed that sodium selenate could promote recovery and improve prognosis in TBI rats by reducing the expression of hyperphosphorylated Tau, suggesting that protecting neural tissue by reducing Tau protein may become a new strategy for neuroprotection in the future.
In conclusion, there is a general cognitive dysfunction in TBI patients at the chronic stage, whose cognitive dysfunction mainly lies in executive function, delayed memory, and attention. The increase of serum Tau in TBI patients might be released from the injured brain matter, which is associated with the occurrence of cognitive impairment. Early monitoring of dynamic Tau expression is helpful in predicting the occurrence of cognitive dysfunction after TBI and assessing the extent of brain injury and neurological outcome. The number of cases in this study is quite small and is based on only one medical center. In the future, it is necessary to further expand clinical samples and multiple centers to study the role of Tau protein in the pathogenesis of cognitive dysfunction after TBI, specifically the underlying fact about how Tau expression affects the specific cognitive aspects.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published, and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
This work was supported by Natural Science Foundation of China (Grant number: 81701231) and Natural Science Foundation of Shanghai (16ZR1431500). The work was also supported by The Featured Clinical Discipline Project of Shanghai Pudong (PWYst2018-01) and Key Discipline Group Construction Project of Shanghai Pudong (PWZxq2017-02).
Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]