Traumatic brain injury reduces soluble extracellular amyloid-β in mice: A methodologically novel combined microdialysis-controlled cortical impact study

Abstract

Acute amyloid-β peptide (Aβ) deposition has been observed in young traumatic brain injury (TBI) patients, leading to the hypothesis that elevated extracellular Aβ levels could underlie the increased risk of dementia following TBI. However, a recent microdialysis-based study in human brain injury patients found that extracellular Aβ dynamics correlate with changes in neurological status. Because neurological status is generally diminished following injury, this correlation suggested the alternative hypothesis that soluble extracellular Aβ levels may instead be reduced after TBI relative to baseline. We have developed a methodologically novel mouse model that combines experimental controlled cortical impact TBI with intracerebral microdialysis. In this model, we found that Aβ levels in microdialysates were immediately decreased by 25–50% in the ipsilateral hippocampus following TBI. This result was found in PDAPP, Tg2576, and Tg2576-ApoE2 transgenic mice producing human Aβ plus wild-type animals. Changes were not due to altered probe function, edema, changes in APP levels, or Aβ deposition. Similar decreases in Aβ were observed in phosphate buffered saline-soluble tissue extracts. Hippocampal electroencephalographic activity was also decreased up to 40% following TBI, and correlated with reduced microdialysate Aβ levels. These results support the alternative hypothesis that post-injury extracellular soluble Aβ levels are acutely decreased relative to baseline. Reduced neuronal activity may contribute, though the underlying mechanisms have not been definitively determined. Further work will be needed to assess the dynamics of insoluble and oligomeric Aβ after TBI.

Introduction

Moderate to severe TBI is a well-documented environmental risk factor for the later development of dementia of the Alzheimer type (Bazarian et al., 2009, Guo et al., 2000, Plassman et al., 2000, Van Den Heuvel et al., 2007). The amyloid-β peptide (Aβ) is believed to play a central role in both familial and late-onset Alzheimer's disease (AD), and may also be involved in TBI-related dementia. Histologically apparent Aβ deposits have been detected in young TBI patients as early as 2–4 h after injury (Ikonomovic et al., 2004, Roberts et al., 1994). However, deposits occur only in 20–30% of human TBI patients coming to autopsy or requiring decompressive surgery (Ikonomovic et al., 2004, Roberts et al., 1994).

In contrast to neuropathological studies, intracerebral microdialysis permits dynamic sampling of soluble, extracellular Aβ in the interstitial fluid (ISF) (Brody et al., 2008, Cirrito et al., 2008, Cirrito et al., 2003, Cirrito et al., 2005, Elvang et al., 2009, Kang et al., 2007, Kang et al., 2009, Marklund et al., 2009). In the brains of awake, behaving mice, microdialysis studies have uncovered a clear relationship between neuronal activity and ISF Aβ concentrations (Cirrito et al., 2005). In a subsequent study, ISF Aβ levels were shown to depend in large part on synaptically coupled endocytosis (Cirrito et al., 2008). Physiological modulations of neuronal activity have been shown to similarly affect Aβ levels (Kang et al., 2007, Kang et al., 2009).

Recently, our group measured the dynamics of Aβ by intracerebral microdialysis in acutely brain-injured patients (Brody et al., 2008). We found that ISF Aβ levels generally rose over time, and that these changes were positively correlated with changes in neurological status as assessed by the Glasgow Coma Score (GCS). Because we could not measure pre-injury levels in our human subjects, the true relationship of post-injury to pre-injury levels was unknown (Suppl. Fig. S1A). Additionally, the relationship of ISF Aβ to levels in other tissue compartments could not be assessed in the human study; this is an important consideration, as the extent of equilibration between pools of Aβ (Suppl. Fig. S1B) is largely unknown.

To address these gaps, we developed a novel mouse model that combined a standardized experimental traumatic brain injury (Brody et al., 2007) with intracerebral microdialysis in awake, behaving mice (Fig. 1). While similar methods have been used in rats (Bell et al., 1998, Krishnappa et al., 1999, Palmer et al., 1993, Rose et al., 2002) this mouse model allowed the study of both wild-type and transgenic animals expressing human-sequence Aβ.

Using this model, we found that Aβ levels were reduced immediately after TBI in 4 genotypes of mice and in a dose-of-injury dependent fashion. There was a quantitative correlation between the extent of reductions in ISF Aβ levels and in local electroencephalographic (EEG) activity after injury. This supports the hypothesis that ISF Aβ levels are reduced acutely following TBI, but leaves unresolved the question of why TBI increases the later risk of dementia of the Alzheimer type.