Age-at-injury effects of microglial activation following traumatic brain injury: implications for treatment strategies

Regulation of microglial responses after pediatric traumatic brain injury: exploring the role of SHIP-1

Introduction

Severe traumatic brain injury (TBI) is the world's leading cause of permanent neurological disability in children. TBI-induced neurological deficits may be driven by neuroinflammation post-injury. Abnormal activity of SH2 domain-containing inositol 5' phosphatase-1 (SHIP-1) has been associated with dysregulated immunological responses, but the role of SHIP-1 in the brain remains unclear. The current study investigated the immunoregulatory role of SHIP-1 in a mouse model of moderate-severe pediatric TBI.

Methods

SHIP-1+/- and SHIP-1-/- mice underwent experimental TBI or sham surgery at post-natal day 21. Brain gene expression was examined across a time course, and immunofluorescence staining was evaluated to determine cellular immune responses, alongside peripheral serum cytokine levels by immunoassays. Brain tissue volume loss was measured using volumetric analysis, and behavior changes both acutely and chronically post-injury.

Results

Acutely, inflammatory gene expression was elevated in the injured cortex alongside increased IBA-1 expression and altered microglial morphology; but to a similar extent in SHIP-1-/- mice and littermate SHIP-1+/- control mice. Similarly, the infiltration and activation of CD68-positive macrophages, and reactivity of GFAP-positive astrocytes, was increased after TBI but comparable between genotypes. TBI increased anxiety-like behavior acutely, whereas SHIP-1 deficiency alone reduced general locomotor activity. Chronically, at 12-weeks post-TBI, SHIP-1-/- mice exhibited reduced body weight and increased circulating cytokines. Pro-inflammatory gene expression in the injured hippocampus was also elevated in SHIP-1-/- mice; however, GFAP immunoreactivity at the injury site in TBI mice was lower. TBI induced a comparable loss of cortical and hippocampal tissue in both genotypes, while SHIP-1-/- mice showed reduced general activity and impaired working memory, independent of TBI.

Conclusion

Together, evidence does not support SHIP-1 as an essential regulator of brain microglial morphology, brain immune responses, or the extent of tissue damage after moderate-severe pediatric TBI in mice. However, our data suggest that reduced SHIP-1 activity induces a greater inflammatory response in the hippocampus chronically post-TBI, warranting further investigation.

Early life stress increases vulnerability to the sequelae of pediatric mild traumatic brain injury

Early life stress (ELS) is a risk factor for many psychopathologies that happen later in life. Although stress can occur in cases of child abuse, studies on non-accidental brain injuries in pediatric populations do not consider the possible increase in vulnerability caused by ELS. Hence, we sought to determine whether ELS increases the effects of pediatric mild traumatic brain injury (mTBI) on cognition, hippocampal inflammation, and plasticity. Male rats were subjected to maternal separation for 180 min per day (MS180) or used as controls (CONT) during the first 21 post-natal (P) days. At P21 the rats were anesthetized with isoflurane and subjected to a mild controlled cortical impact or sham injury. At P32 the rats were injected with the cell proliferation marker bromodeoxyuridine (BrdU, 500 mg/kg), then evaluated for spatial learning and memory in a water maze (P35-40) and sacrificed for quantification of Ki67⁺, BrdU⁺ and Iba1⁺ (P42). Neither MS180 nor mTBI impacted cognitive outcome when provided alone but their combination (MS180 + mTBI) decreased spatial learning and memory relative to Sham controls (p < .01). mTBI increased microglial activation and affected BrdU⁺ cell survival in the ipsilateral hippocampus without affecting proliferation rates. However, only MS180 + mTBI increased microglial activation in the area adjacent to the injury and the contralateral CA1 hippocampal subfield, and decreased cell proliferation in the ipsilateral neurogenic niche. Overall, the data show that ELS increases the vulnerability to the sequelae of pediatric mTBI and may be mediated by increased neuroinflammation.

Toll-like Receptor 4 Signaling in Neurons Enhances Calcium-Permeable α-Amino-3-Hydroxy-5-Methyl-4-Isoxazolepropionic Acid Receptor Currents and Drives Post-Traumatic Epileptogenesis

OBJECTIVE

Traumatic brain injury is a major risk factor for acquired epilepsies, and understanding the mechanisms underlying the early pathophysiology could yield viable therapeutic targets. Growing evidence indicates a role for inflammatory signaling in modifying neuronal excitability and promoting epileptogenesis. Here we examined the effect of innate immune receptor Toll-like receptor 4 (TLR4) on excitability of the hippocampal dentate gyrus and epileptogenesis after brain injury.

METHODS

Slice and in vivo electrophysiology and Western blots were conducted in rats subject to fluid percussion brain injury or sham injury.

RESULTS

The studies identify that TLR4 signaling in neurons augments dentate granule cell calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (CP-AMPAR) currents after brain injury. Blocking TLR4 signaling in vivo shortly after brain injury reduced dentate network excitability and seizure susceptibility. When blocking of TLR4 signaling after injury was delayed, however, this treatment failed to reduce postinjury seizure susceptibility. Furthermore, TLR4 signal blocking was less efficacious in limiting seizure susceptibility when AMPAR currents, downstream targets of TLR4 signaling, were transiently enhanced. Paradoxically, blocking TLR4 signaling augmented both network excitability and seizure susceptibility in uninjured controls. Despite the differential effect on seizure susceptibility, TLR4 antagonism suppressed cellular inflammatory responses after injury without impacting sham controls.

INTERPRETATION

These findings demonstrate that independently of glia, the immune receptor TLR4 directly regulates post-traumatic neuronal excitability. Moreover, the TLR4-dependent early increase in dentate excitability is causally associated with epileptogenesis. Identification and selective targeting of the mechanisms underlying the aberrant TLR4-mediated increase in CP-AMPAR signaling after injury may prevent epileptogenesis after brain trauma. ANN NEUROL 2020;87:497-515.

Impact of age on acute post-TBI neuropathology in mice expressing humanized tau: a Chronic Effects of Neurotrauma Consortium Study

OBJECTIVES

We hypothesized that polypathology is more severe in older than younger mice during the acute phase following repetitive mild traumatic brain injury (r-mTBI).

METHODS

Young and aged male and female mice transgenic for human tau (hTau) were exposed to r-mTBI or a sham procedure. Twenty-four hours post-last injury, mouse brain tissue was immunostained for alterations in astrogliosis, microgliosis, tau pathology, and axonal injury.

RESULTS

Quantitative analysis revealed a greater percent distribution of glial fibrillary acid protein and Iba-1 reactivity in the brains of all mice exposed to r-mTBI compared to sham controls. With respect to axonal injury, the number of amyloid precursor protein-positive profiles was increased in young vs aged mice post r-mTBI. An increase in tau immunoreactivity was found in young and aged injured male hTau mice.

CONCLUSIONS

We report the first evidence in our model that r-mTBI precipitates a complex sequelae of events in aged vs young hTau mice at an acute time point, typified by an increase in phosphorylated tau and astroglisosis, and a diminished microgliosis response and axonal injury in aged mice. These findings suggest differential age-dependent effects in TBI pathobiology.

Dynamic changes in growth factor levels over a 7-day period predict the functional outcomes of traumatic brain injury

Traumatic brain injury (TBI) can result in poor functional outcomes and death, and overall outcomes are varied. Growth factors, such as angiopoietin-1 (Ang-1), vascular endothelial growth factor (VEGF), and granulocyte-colony stimulating factor (G-CSF), play important roles in the neurological functions. This study investigated the relationship between serum growth factor levels and long-term outcomes after TBI. Blood samples from 55 patients were collected at 1, 3 and 7 days after TBI. Blood samples from 39 healthy controls were collected as a control group. Serum Ang-1, G-CSF, and VEGF levels were measured using ELISA. Patients were monitored for 3 months using the Glasgow Outcome Scale-Extended (GOSE). Patients having a GOSE score of > 5 at 3 months were categorized as a good outcome, and patients with a GOSE score of 1–5 were categorized as a bad outcome. Our data demonstrated that TBI patients showed significantly increased growth factor levels within 7 days compared with healthy controls. Serum levels of Ang-1 at 1 and 7 days and G-CSF levels at 7 days were significantly higher in patients with good outcomes than in patients with poor outcomes. VEGF levels at 7 days were remarkably higher in patients with poor outcomes than in patients with good outcomes. Receiver operating characteristic analysis showed that the best cut-off points of serum growth factor levels at 7 days to predict functional outcome were 1,333 pg/mL for VEGF, 447.2 pg/mL for G-CSF, and 90.6 ng/mL for Ang-1. These data suggest that patients with elevated levels of serum Ang-1, G-CSF, and decreased VEGF levels had a better prognosis in the acute phase of TBI (within 7 days). This study was registered with the Chinese Clinical Trial Registry (registration number: ChiCTR1800018251) on September 7, 2018.