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Growing evidence suggests that cells undergo apoptosis after spinal cord injury (SCI). However, little is known about the early events that trigger apoptosis in the contused cord. The BH3-only subfamily of pro-apoptotic regulators (e.g., bim, bad, and dp5) is recognized as initiators of the apoptotic cascade, and is subject to stringent control, both at the transcriptional and post-translational level. In the current study, we studied upstream events regulating trauma-induced apoptosis in the spinal cord. Within 1 h after SCI in rats, DP5 was induced, while Bim and Bad levels remained unchanged. In parallel, SCI also activated the stress-induced c-Jun N-terminal kinase (JNK), leading to the phosphorylation of c-Jun, with a similar temporal profile. Immunohistochemical analysis revealed that p-JNK and DP5 colocalized to neurons and oligodendrocytes undergoing apoptosis in the injured cord, but were absent in uninjured spinal cord. Furthermore, inhibition of JNK activity with in vivo delivery of SP600125 or a jnk1 antisense oligodeoxynucleotide (ODN) attenuated DP5 induction and caspase-3 activation. These results suggest that JNK activation contributes to trauma-induced DP5 expression and subsequent apoptosis in SCI.

We have explored the molecular mechanism underlying amyloid beta-peptide (Aβ)-mediated cytotoxicity in vitro. Exposure of murine cerebral endothelial cells (CECs) or C6 glioma cells to Aβ25-35 resulted in dose-dependent cell death. Ceramide is a pro-apoptotic lipid mediator. Forced elevation of cellular ceramide levels, either by application of an exogenous C2 ceramide analogue or bacterial sphingomyelinase that induces endogenous ceramide release from sphingomyelin, mimicked Aβ25–35 cytotoxicity in both CECs and C6 glioma cells. Aβ25–35-induced synthesis of ceramide was selectively mediated by activation of neutral sphingomyelinase (nSMase), but not acidic sphingomyelinase (aSMase) or ceramide synthase. Both 3-O-Me-SM and N-acetyl-l-cysteine, the selective and nonselective pharmacological inhibitors of nSMase, respectively, suppressed nSMase activation, ceramide production, and cytotoxic action induced by Aβ25–35 in CECs. Furthermore, genetic knockdown of nSMase by an antisense strategy rendered C6 glioma cells specifically resistant to Aβ25–35 cytotoxicity without affecting their vulnerability to serum deprivation. Together, nSMase activation with subsequent ceramide production may contribute, at least partially, to Aβ25–35 cytotoxicity in cell types with cerebral endothelial and glial lineage.