Translocation of the Serine Protease Omi/HtrA2 from Mitochondria into the Cytosol Upon Seizure-Induced Hippocampal Injury in the Neonatal Rat Brain

Abstract

Omi/HtrA2 is a pro-apoptotic mitochondrial serine protease involved in caspase-dependent as well as caspase-independent cell death upon various brain injuries. However, the role of Omi/HtrA2 in neuronal death induced by status epilepticus (SE) in the immature brain has not been reported. In this study, we analyzed the contribution of serine protease Omi/HtrA2, its substrate X-linked inhibitor of apoptosis protein (XIAP) and the caspase-3 activation to damage of hippocamplal CA1 cells following lithium-pilocarpine SE in P14 rat pups. Status epilepticus in the immature brain significantly induced translocation of Omi/HtrA2 from mitochondria into the cytosol, increased cytosolic accumulation of Omi/HtrA2, induced appearance of XIAP-breakdown products and enhanced caspase-3 activity in the selectively vulnerable hippocampal CA1-subfield. Taken together, these results demonstrate for the first time that SE in the immature brain results in Omi/HtrA2 accumulation in the cytosol, where it probably promotes neuronal death by neutralizing and cleaving XIAP, one of the most potent endogenous inhibitors of apoptosis.

Specific Alterations of the HtrA2/HAX-1 Ratio in the Penumbra Upon Focal Cerebral Ischemia in Mice

“…In contrast, we found that neurons located in the ischemic core display necrotic cell death. We have reported in our previous study that Omi/ HtrA2 was specifically upregulated in the penumbra and translocated from mitochondria to cytosol after cerebral ischemia as well as after epilepsy in rats [25,26]. In the present study we demonstrated an up-regulation of HtrA2 in the penumbra after focal ischemia which was accompanied by a parallel down-regulation of the antiapoptotic protein HAX-1.…”

“…We have previously reported that focal ischemia results in Omi/HtrA2 accumulation in the cytosol, where it probably promotes neuronal death by neutralizing and cleaving XIAP, one of the most potent endogenous inhibitors of apoptosis [25,26]. In the present study, we sought to determine whether (1) HAX-1 expression is altered in brains subjected to focal cerebral ischemia; (2) and if so, to investigate whether this alteration correlated to HtrA2-redistribution and execution of neuronal death.…”

Alterations in the Expression of the Anti-Apoptotic Factor HAX-1 upon Seizures-Induced Hippocampal Injury in the Neonatal Rat Brain

“…In addition, we should keep in mind that HAX-1 is not the only antiapoptotic protein that is cleaved and inactivated by HtrA2 during apoptotic cell death. For instance XIAP, the most potent endogenous inhibitor of apoptosis, can itself undergo protease-mediated breakdown after SE [15]. As matter of fact, it has been recently shown that the interaction between HAX-1 and XIAP blocks polyubiquitination of XIAP [26].…”

“…This suggests that upon apoptosis stimulation, the formation of HAX-1-XIAP complex increases the stability of XIAP by suppressing its ubiquitination, and thus the stabilized complex attenuates apoptosis by inhibition of caspase-9 activation. However, seeing that XIAP is also cleaved by HtrA2 following SE and cerebral ischemia [15,27], we can assume that the HAX-1-XIAP complex does not function correctly. In this regard, a very interesting question is whether HAX-1 is involved in caspase-independent cell death in either pathophysiological or physiological conditions.…”

“…We have previously reported that status epilepticus induced translocation of Omi/HtrA2 from mitochondria into the cytosol, increased cytosolic accumulation of Omi/ HtrA2, induced appearance of XIAP-breakdown products and enhanced caspase-3 activity in the selectively vulnerable hippocampal CA1-subfield [15]. In this study, we sought to determine whether (1) HAX-1 is accumulated in the cytosol of hippocampal neurons in brains subjected to SE; (2) and if so, to investigate whether this accumulation correlated to HtrA2-redistribution and execution of neuronal death.…”