Proteolytic Mechanisms of Traumatic Brain Injury

Traumatic brain injury (TBI) is the leading cause of mortality and morbidity among young adults. While recent studies in humans and experimental models of TBI have well established a role for necrosis in traumatic cell damage, accumulating evidence now suggests that apoptosis may also be a prominent contributor. However, the precise mechanisms leading to neuronal and glial degeneration after TBI are still poorly understood. Interestingly, the activation of two classes of cysteine proteases, the calpains (i.e., calcium activated neutral proteases), and the caspases (i.e., cysteinyl aspartate specific proteases) has been inferred in mediating necrotic and/or apoptotic cell death after injury in vitro and in vivo. Micro ()-clapain and milli (m)-calpain have been shown to be activated after TBI. However, the exact role of these calpain isoforms in necrotic and/or apoptotic cell degeneration after TBI in vivo has yet not been investigated. Importantly, although suggested by recent in vitro data, it is unclear whether -clapain and/or m- calpain, solely or via the modulation of caspase-activation, may contribute to apoptosis after TBI in vivo. Caspases can be divided into two main groups: initiator caspases (such as caspase-2, -6, -8, and -9) and effector caspases, such as caspase-3. It is believed that initiator caspases process and activate effector caspases which then cleave specific proteins, ultimately leading to apoptotic cell degeneration. Although recent studies have provided some evidence that the effector caspase-3 is activated after experimental and human TBI, no study to date has investigated the interaction between initiator and effector caspases after TBI in vivo. In addition, the mitochondria have recently been shown to play a crucial role in mediating caspase-induced apoptotic degeneration of CNS cells in vitro. However, no studies to date have elucidated the role of the mitochondrion for apoptosis after TBI in vivo. Therefore, this study will examine four major aims: Aim 1 will investigate the interaction between initiator caspases-2, -6, -8 and -9, and the effector caspase-3, including the role of mitochondria in mediating neuronal and glial apoptotic cell death after TBI in vivo. Aim 2 will test the hypothesis whether -calpain and/or m-calpain are putative mediators of neuronal and glial apoptotic cell death following TBI. Aim 3 will examine the interactions between -calpain or m-calpain and the initiator caspases-2, -8 and -9, and the effector caspase-3 following TBI in vivo. Aim 4 will study the effects of a calpain inhibitor on the activation of the initiator caspases-2, -6, -8 and -9, and the effector caspase-3, including the effects of the inhibitor on the temporal and spatial evolution of necrotic and apoptotic cell death, and on functional outcome after TBI.

Traumatic brain injury still is a leading cause of morbidity and mortality among young adults. Recent studies in experimental models of traumatic brain injury and in humans reported on a potential role for apoptosis in neuronal and glial cell degeneration after head trauma. Two major apoptotic pathways have been identified: The extrinsic pathway involves cell surface receptors such as Fas, the intrinsic apoptotic pathway is initiated by the release of cytochrome c into the cytosol. During the study period of the research project P15308-B05 funded by the Austrian Science Fund from November 2001 to November 2005 the Innsbruck Neurotrauma Research Laboratory has generated data linking these two distinct death pathways following traumatic brain injury in vivo. For example, studies of the funded research project have provided first evidence of the cleavage of certain proapoptotic proteins in the traumatized cortex in neurons and glial cells. As this event is strategically located upstream of mitochondria and effector protease processing, it may represent an attractive therapeutic target for CNS diseases in which apoptotic cell degeneration is prominent. In addition, studies on protease degradation products, such as breakdown products of axonal proteins (namely amyloid beta), suggested that these "biomarkers" may be supportive predictors for clinical outcome after traumatic brain injury. Due to its implication for the care of patients with severe traumatic brain injury this work was awarded the Prize of the Tyrolean Medical Association in 2005. Importantly, results of the research project P15308 representing a logical refinement of a previously funded project, have indeed advanced the understanding of neuronal and glial degeneration after experimental traumatic brain injury and therefore have the capacity to improve the management of the traumatically injured patient and to minimize morbidity and mortality and eventually costs associated with CNS injury. References: Franz et al., Neurology 2003;60:1457-1461; Franz et al., J Cereb Blood Flow Metab 2002;22:951-958; Blasko et al., J Neural Transm 2004;111:523-536.