Transient neuroprotection by minocycline following traumatic brain injury is associated with attenuated microglial activation but no changes in cell apoptosis or neutrophil infiltration

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Abstract

Cerebral inflammation and apoptotic cell death are two processes implicated in the progressive tissue damage that occurs following traumatic brain injury (TBI), and strategies to inhibit one or both of these pathways are being investigated as potential therapies for TBI patients. The tetracycline derivative minocycline was therapeutically effective in various models of central nervous system injury and disease, via mechanisms involving suppression of inflammation and apoptosis. We therefore investigated the effect of minocycline in TBI using a closed head injury model. Following TBI, mice were treated with minocycline or vehicle, and the effect on neurological outcome, lesion volume, inflammation and apoptosis was evaluated for up to 7 days. Our results show that while minocycline decreases lesion volume and improves neurological outcome at 1 day post-trauma, this response is not maintained at 4 days. The early beneficial effect is likely not due to anti-apoptotic mechanisms, as the density of apoptotic cells is not affected at either time-point. However, protection by minocycline is associated with a selective anti-inflammatory response, in that microglial activation and interleukin-1β expression are reduced, while neutrophil infiltration and expression of multiple cytokines are not affected. These findings demonstrate that further studies on minocycline in TBI are necessary in order to consider it as a novel therapy for brain-injured patients.

Introduction

Brain damage after traumatic brain injury (TBI) results from both the primary mechanical impact and secondary degenerative responses that occur in the minutes to days following trauma. Secondary processes involve diverse pathways, including profound cerebral inflammation, excitatory amino acid and calcium associated cytotoxicity, and ischemic events, all of which may lead to acute and chronic cell death and contribute to functional impairment (McIntosh et al., 1998, Raghupathi, 2004).

The immune responses elicited after TBI involve the activation of resident glial cells, astrocytes and microglia, as well as blood leukocytes accumulated in the injured brain which in concert secrete soluble cytokines (Morganti-Kossmann et al., 2001). Cerebral inflammation can play dual opposing roles, on one hand by supporting processes of repair and on the other hand by exacerbating tissue damage. There is robust evidence that activated glial cells and leukocytes secrete a variety of neurotoxic molecules which likely contribute to progressive neuronal death after TBI. Therefore, therapeutic inhibition of inflammation has been considered to minimise the extent of secondary brain injury and improve neurological recovery.

Minocycline, a tetracycline derivative, is a promising candidate for the treatment of TBI, as it has been shown to be therapeutically effective in various experimental models of central nervous system (CNS) injury and disease that comprise inflammatory and apoptotic components. Minocycline treatment conferred neuroprotection following spinal cord injury (Stirling et al., 2004, Teng et al., 2004), excitotoxicity (Tikka et al., 2001, Tikka and Koistinaho, 2001) and ischemic injury (Arvin et al., 2002, Morimoto et al., 2005, Wang et al., 2003, Xu et al., 2004, Yrjänheikki et al., 1998, Yrjänheikki et al., 1999). It also prevented neurodegeneration in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease (Du et al., 2001, Wu et al., 2002), and delayed mortality in the R6/2 mouse model of Huntington's disease (Chen et al., 2000) as well as in amyotrophic lateral sclerosis mice (Zhu et al., 2002). However, only a single study has been published examining the effect of minocycline treatment following experimental TBI. Sanchez Mejia et al. (2001) found that twice-daily intraperitoneal injections of minocycline after a controlled cortical impact injury resulted in improved neurological function, decreased lesion volume and attenuated production of cerebral interleukin (IL)-1β, compared with saline control mice. However, a few recent reports have shown inconsistent and detrimental effects of minocycline in different models of neurodegeneration (Diguet et al., 2004, Smith et al., 2003, Sriram et al., 2006, Tsuji et al., 2004, Yang et al., 2003), stressing the importance of further testing of minocycline to establish its viability as a potential therapy following TBI.

Although the mechanisms of actions of minocycline treatments have not been fully elucidated, its beneficial effects appear to be associated with inhibition of microglial activation and proliferation (Fan et al., 2005, He et al., 2001, Krady et al., 2005, Tikka et al., 2001, Tikka and Koistinaho, 2001, Wu et al., 2002, Yrjänheikki et al., 1998) and downregulation of inducible nitric oxide synthase transcription (Yrjänheikki et al., 1998). Neuroprotection by minocycline may also be the result of direct anti-apoptotic effects, including inhibition of caspase-1 (Chen et al., 2000, Sanchez Mejia et al., 2001) and caspase-3 (Chen et al., 2000, Krady et al., 2005), and blockade of the permeability transition pore to prevent cytochrome c release from the mitochondria (Teng et al., 2004, Zhu et al., 2002). These findings indicate that minocycline is able to influence multiple cascades of the secondary degenerative responses that occur in the injured brain, suggesting that it may be a valuable therapeutic agent for the treatment of TBI.

In the present study, we investigated whether treatment with minocycline is protective in an experimental model of focal TBI (Chen et al., 1996, Stahel et al., 2000). The volume of tissue damage, as well as the amount of apoptotic cell death, glial/leukocyte activation and the level of cerebral cytokine expression, was assessed for up to 4 days post-injury in relation to functional outcome in minocycline and saline-treated mice.

Section snippets

Mouse model of closed head injury

Unilateral focal brain injury was induced on the left cortex using a mouse model of closed head injury (CHI), performed as previously described (Chen et al., 1996, Stahel et al., 2000). Twelve to 14 week old male C57/BL6 mice were ether-anaesthetised and their skull exposed by a longitudinal incision of the scalp. Focal trauma was induced to the left hemisphere 2 mm lateral to the midline in the midcoronal plane, using an electric weight-drop device with a metal rod of 333 g falling from a

Minocycline does not significantly improve neurological recovery following CHI

The effect of minocycline on functional recovery was assessed by measuring the daily NSS of minocycline- and saline-treated mice for up to 4 days post-CHI and expressing the scores as the improvement of neurological function (ΔNSS) between 1 h post-CHI and at any later time-point (Fig. 1). While at each time point the minocycline mice appeared to display a greater improvement of function reflected by a higher ΔNSS, the difference between the groups did not reach statistical significance,

Discussion

The aim of using anti-inflammatory compounds for the treatment of TBI is to attenuate the deleterious consequences of cerebral inflammation and possibly reduce delayed cell death. In both animal models and patients with brain injuries, a progressive loss of neuronal cells has been demonstrated over a long period of time and reduction of secondary neurotoxic processes is the only approach for improving the morbidity and mortality of patients with severe head trauma. Corticosteroids have been

Acknowledgment

This research was supported by a grant from the Victorian Trauma Foundation.

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