Chronic complement dysregulation drives neuroinflammation after traumatic brain injury: a transcriptomic study

Amer Toutonji,Stephen Tomlinson,Mamatha Mandava,Silvia Guglietta

doi:10.1186/s40478-021-01226-2

Abstract

Activation of the complement system propagates neuroinflammation and brain damage early and chronically after traumatic brain injury (TBI). The complement system is complex and comprises more than 50 components, many of which remain to be characterized in the normal and injured brain. Moreover, complement therapeutic studies have focused on a limited number of histopathological outcomes, which while informative, do not assess the effect of complement inhibition on neuroprotection and inflammation in a comprehensive manner. Using high throughput gene expression technology (NanoString), we simultaneously analyzed complement gene expression profiles with other neuroinflammatory pathway genes at different time points after TBI. We additionally assessed the effects of complement inhibition on neuropathological processes. Analyses of neuroinflammatory genes were performed at days 3, 7, and 28 post injury in male C57BL/6 mice following a controlled cortical impact injury. We also characterized the expression of 59 complement genes at similar time points, and also at 1- and 2-years post injury. Overall, TBI upregulated the expression of markers of astrogliosis, immune cell activation, and cellular stress, and downregulated the expression of neuronal and synaptic markers from day 3 through 28 post injury. Moreover, TBI upregulated gene expression across most complement activation and effector pathways, with an early emphasis on classical pathway genes and with continued upregulation of C2, C3 and C4 expression 2 years post injury. Treatment using the targeted complement inhibitor, CR2-Crry, significantly ameliorated TBI-induced transcriptomic changes at all time points. Nevertheless, some immune and synaptic genes remained dysregulated with CR2-Crry treatment, suggesting adjuvant anti-inflammatory and neurotropic therapy may confer additional neuroprotection. In addition to characterizing complement gene expression in the normal and aging brain, our results demonstrate broad and chronic dysregulation of the complement system after TBI, and strengthen the view that the complement system is an attractive target for TBI therapy.

Highlights

Neuroinflammation is a major component of secondary injury that occurs after traumatic brain injury (TBI) and contributes to continuous tissue damage long after the initial mechanical insult
Patterns of differential gene expression in TBI To investigate the inflammatory changes occurring over time in the brain after a traumatic brain injury (TBI), we used the NanoString Neuroinflammation panel to quantify the expression of 757 genes in the injured hemispheres of mice subjected to a unilateral controlled cortical impact (CCI)
Brains were processed on days 3, 7, and 28 post injury, and gene expression was compared to uninjured brains

Summary

Introduction

Neuroinflammation is a major component of secondary injury that occurs after traumatic brain injury (TBI) and contributes to continuous tissue damage long after the initial mechanical insult. Toutonji et al acta neuropathol commun (2021) 9:126 very few anti-inflammatory drugs have been investigated in clinical TBI, with three clinical trials assessing the effects of cyclosporine and minocycline [1,2,3] These studies had small sample sizes, and while they reported no life-threatening events requiring termination of the studies, they did show increased incidence of adverse events with cyclosporine treatment and increased levels of injury markers with minocycline treatment, even though biomarker evaluation indicated decreased microglial activation [3]. Proteomic data showed complement component C3 to be a hub protein in cortical protein–protein interaction networks early after a weight-drop open-head TBI [10], while gene expression data showed chronic upregulation in the brain at 6 months after injury of the complement genes C1q, C1s, C2, and C3 in a midline fluid percussion injury [7].

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