The pathophysiology of repetitive concussive traumatic brain injury in experimental models; new developments and open questions

Abstract

In recent years, there has been an increasing interest in the pathophysiology of repetitive concussive traumatic brain injury (rcTBI) in large part due to the association with dramatic cases of progressive neurological deterioration in professional athletes, military personnel, and others. However, our understanding of the pathophysiology of rcTBI is less advanced than for more severe brain injuries. Most prominently, the mechanisms underlying traumatic axonal injury, microglial activation, amyloid-beta accumulation, and progressive tau pathology are not yet known. In addition, the role of injury to dendritic spine cytoskeletal structures, vascular reactivity impairments, and microthrombi are intriguing and subjects of ongoing inquiry. Methods for quantitative analysis of axonal injury, dendritic injury, and synaptic loss need to be refined for the field to move forward in a rigorous fashion. We and others are attempting to develop translational approaches to assess these specific pathophysiological events in both animals and humans to facilitate clinically relevant pharmacodynamic assessments of candidate therapeutics. In this article, we review and discuss several of the recent experimental results from our lab and others. We include new initial data describing the difficulty in modeling progressive tau pathology in experimental rcTBI, and results demonstrating that sertraline can alleviate social interaction deficits and depressive-like behaviors following experimental rcTBI plus foot shock stress. Furthermore, we propose a discrete set of open, experimentally tractable questions that may serve as a framework for future investigations. In addition, we also raise several important questions that are less experimentally tractable at this time, in hopes that they may stimulate future methodological developments to address them. This article is part of a Special Issue entitled "Traumatic Brain Injury".

Keywords: Amyloid-beta; Concussion; Depression; Microglia; Mouse; Sertraline; Social behavior; Tau; Traumatic axonal injury; Traumatic brain injury.

Figure 1. Diffusion tensor imaging in a mouse model of pericontusional traumatic axonal injury

Mice were injured with controlled cortical impact at 3 different severities (1.0 mm, 1.5 or 2.5 mm impact depth), scanned with DTI 24 hours later, and then sacrificed for quantitative histological assessment of axonal injury using stereological counting of APP stained axonal varicosities. Methods were otherwise as previously described (Mac Donald et al., 2007a; Mac Donald et al., 2007b). Notably, relative anisotropy was reduced in the least severely injured mice (1.0 mm) even though essentially no dilated, APP-immunoreactive axons were observed. Subsequent work has demonstrated the presence of non-dilated, APP-negative injured axons following less severe injuries. (Adapted from Brody et al, “Current and Future Diagnostic Tools for Traumatic Brain Injury: CT, Conventional MRI, and Diffusion Tensor Imaging” Handbook of Neurology, 2015 in press)

Figure 2. No effects of repetitive concussive TBI on tau immunostaining in hTau mice

6-8 week old hTau mice or tau knockout littermates were injured with 4 concussive impacts 24 hours apart, then sacrificed 7 days later. A. The injuries caused extensive silver staining in the cortex, corpus callosum (C.C.), external capsule (E.C.) and thalamus comparable to the results previously shown for 2 concussive impacts (Shitaka et al., 2011). B. Quantification of silver staining in the white matter by blinded observers by densitometry revealed substantially increased staining in injured mice compared with shams (p<0.001, 2-way ANOVA followed by Tukey post-hoc test. However, there was no difference in silver staining between the injured hTau mice and identically injured tau knockout littermates (A.U.: arbitrary units). C. Tau immunohistochemistry using CP13, a monoclonal antibody recognizing phosphorylated tau. No difference was apparent in the dentate gyrus between sham and injured hTau mice. D. Quantitative unbiased, blinded stereological analysis of CP13 positive cells in the subgranular layer of the dentate gyrus. No difference was found between groups (p=0.19, Student's t-test).

Figure 3. Sertraline treatment improved depression-like behavior and social interaction following repetitive concussive TBI plus foot shock stress

6-8 week old male C56Bl6 mice were subjected to 2 concussive TBIs 24 hours apart, then foot shock stress plus extinction training (Klemenhagen et al., 2013). Seven days later, mice were randomly assigned to sertraline vs. vehicle administered in the drinking water daily for 3 weeks prior to behavioral testing. A. Sertraline dose dependently reduced immobility time in the tail suspension test, as scored by a blinded observer. B. No effect of sertraline and little effect of injury plus shock stress in the sucrose preference test. C. Sertraline dose dependently increased social interaction in the social recognition test during both the initial 9 trials and the final trial with a second novel mouse, as scored by a blinded observer. D. No effect of injury plus stress or sertraline on olfactory function as measured using the buried cookie test.