Full Length ArticleLong-term cognitive deficits after traumatic brain injury associated with microglia activation
Introduction
Microglia are the central nervous system (CNS) resident innate immune cells that play critical physiological roles in the healthy and injured brain. They detect and rapidly respond to any disruption in the status quo of the CNS, including infections or tissue injury, and often act to remove cellular debris [1]. The activation of microglia from their resting surveillance state occurs within minutes of the injury and is critical for recovery. However, prolonged activation may be detrimental and contribute to secondary damage. When the microglia are activated in response to any disruption, this activation alters their gene expression and morphology [2]. Although they have distinctive lineage, microglia resemble blood-derived macrophages that infiltrate the CNS from the periphery in response to tissue damage [3]. Microglia are usually classified into the classical M1 phenotype that is considered proinflammatory, and the M2 anti-inflammatory is thought to be involved in neural repair [4]. However, more recent data suggest that microglia's activation status is on a continuum between anti- and pro-inflammation rather than a rigid dichotomous phenotype [5].
Chronic neuroinflammatory response to an acquired brain insult such as a TBI contributes to the injury and lengthens or halts recovery [6]. In chronic neuroinflammation, microglia remain activated for an extended period during which the production of repair mediators is sustained longer than usual [7]. Of note, chronic microglial activation has also been linked to most, if not all, neurodegenerative diseases like Alzheimer's disease, amyotrophic lateral sclerosis, and Parkinson's disease [[8], [9], [10]]. In humans with traumatic brain injury (TBI), microglial activation has been reported as early as 72 h after injury [11], and it can persist for months after injury [12].
TBI is the most prevalent of all CNS injuries and results in chronic neurologic and cognitive deficits [13]. TBI causes cell death and neurologic dysfunction through secondary injury mechanisms characterized by edema, neuronal cell death, glial activation, and infiltration of peripheral immune cells [14]. Extensive research has been conducted investigating the role of microglia after head injury and its interaction with the neural microenvironment suggesting a role for microglia in the injury process as well as in neurotransmission and maintenance of synaptic integrity [15]. TBI initiates a neuroinflammatory cascade that persists over time and may lead to prolonged cognitive deficits. The regional distribution of microglial activation is yet to be determined. Some studies found chronic activation in the thalamus, others in the hippocampus in addition to the injury site [16]. Despite evidence showing a significant role for microglia in the pathogenesis of TBI, no studies to date have examined the spatiotemporal microglial activation response after injury and explored its potential correlation with cognitive deficits [[17], [18], [19]].
Here we hypothesized that TBI would result in phenotypic changes in the microglial cell population that persists for a long time after the injury. We also hypothesized that chronic microglial activation is associated with cognitive deficits. We analyzed the spatiotemporal course of microglia changes isolated from the injured brain up to 35 days after controlled cortical impact (CCI) in mice. We used conventional flow cytometry techniques followed by bioinformatics-based multi-parametric methods that are not constrained by assumptions or bias. We observed heterogeneity in microglia phenotypes and temporal changes in microglia subpopulations following TBI. A particular subpopulation that we called hyperactivated microglia was correlated with chronic deficits in learning and spatial memory after injury.
Section snippets
Animals
The Institutional Animal Care and Utilization Committee (IACUC) of the American University of Beirut (AUB) approved this study. The study is reported in accordance with ARRIVE guidelines [64]. C57BL/6 mice were obtained from the Animal Care Facility of the American University of Beirut and housed in a controlled environment (12 h reverse light/dark cycles, 22 ± 2 °C). All efforts were made to reduce the number of animals used and their suffering. All animals were handled and fed regular chow
Traumatic brain injury affects motor and sensory performance
We evaluated the performance of mice on the pole climbing test. At 48 h, mice with mild and severe TBI performed significantly worse than sham-injured mice as indicated by the total time taken by the animals to descend the pole (mild:11.7 ± 0.6 and severe: 21.4 ± 3.6 vs. 6.4 ± 0.8 s, P < 0.001 and P < 0.0001 respectively). Less time is indicative of better motor coordination and balance. Both TBI groups showed recovered motor performance at 7- and 35-days post-injury (dpi) in comparison to
Discussion
In the present study, we investigated whether CCI traumatic brain injury results in a spatiotemporal phenotypic change in microglia and if these changes are linked to a neurologic and cognitive deficit in a mouse model. Microglia are the brain immune cells, and they are implicated in almost all physiological processes in the CNS and play an important role in several inflammatory and neurodegenerative diseases. Chronic changes in microglia phenotype and function, support the notion that chronic
Author contribution
Conceptualization: SJK, HD, FK
Data curation, Formal analysis, Investigation, Methodology: ESS, MK, LN, HD, SJK
Writing - original draft, Writing - review & editing.: ESS, HD, SJK
Funding acquisition: SJK, HD
Funding information
This study was funded by the Office of Naval Research (ONR), [ONRG - NICOP - N62909-17-1 2059] AWARD. The funders had no role in study design, data collection, analysis, decision to publish, or manuscript preparation.
Availability of data and materials
All data generated or analyzed during this study are included in this published article.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Esber S Saba and Mona Karout contributed equally to this work as first authors.