Monoubiquitination and Cellular Distribution of XIAP in Neurons after Traumatic Brain Injury

XIAP is a member of the inhibitor of apoptosis (IAP) gene family that, in addition to suppressing cell death by inhibition and polyubiquitination of caspases, is involved in an increasing number of signaling cascades. Moreover, the function and regulation of XIAP in the central nervous system (CNS) is poorly understood. In this study, the authors investigated the cell-type expression, the subcellular distribution, ubiquitination of XIAP, and levels of Smac/DIABLO in the normal adult rat brain and in brains subjected to moderate traumatic brain injury (TBI). In the normal brain, XIAP was predominantly expressed in the perinuclear region of neurons. Traumatized brains showed dramatic alterations in cellular and regional expression of XIAP early after injury. Stereologic analyses of the number of XIAP-positive cells within the hippocampus of both hemispheres showed a biphasic response. Immunoprecipitation and immunoblots of extracts derived from different brain regions demonstrated that a single ubiquitin modifies XIAP. Normal cortex contained significantly higher levels of monoubiquitinated XIAP than hippocampus. TBI induced alterations in levels of monoubiquitinated XIAP that correlated with changes in XIAP distribution and immunoreactivity, suggesting that monoubiquitination of XIAP may be a regulator of XIAP location or activity. Similar levels of Smac/DIABLO were present in lysates of normal and traumatized brains. These data demonstrate for the first time a region-specific regulation of XIAP monoubiquitination in the normal adult rat brain, and after TBI, that may be a key event in the regulation of XIAP function contributing to the pathogenesis following injury.