Limited protection of TAT-Bcl-XL against pneumolysin-induced neuronal cell death

Objectives: The severe brain damage that patients with pneumococcal meninigits often suffer is in part caused by the cytosolic pneumococcal protein pneumolysin. The devastating effect of this neurotoxin might be alleviated by interfering with the death pathways that it sets in motion. An important player in these death pathways is Bcl-XL, an antiapoptotic protein of the Bcl-2 family, which is neuroprotective in various in vitro and in vivo systems. We investigated whether its membrane-permeable form, the TAT-Bcl-XL fusion protein, is capable of protecting human SH-SY5Y neuroblastoma cells against pneumolysin-induced cell death.

Materials and Methods: SH-SY5Y neuroblastoma cells were treated with different concentrations of pneumolysin (0.5, 1, 2, and 4 microgram/ml) and TAT-Bcl-XL (100 nM and 300 nM) for different time periods. Cell viability was determined using the WST-1 Cell Proliferation Reagent (Roche Applied Science). Data were analyzed by Student's t-test or ANOVA followed by Bonferroni's multiple comparison test to correct for repeated testing. P values <0.05 were considered statistically significant.

Results: Pneumolysin induced cell death in SH-SY5Y in a time- and dose-dependent manner. Under mild pneumolysin-induced neuronal injury (<30% of cells), pretreatment with TAT-Bcl-XL increased cell viability by approximately 40% (82.7 + 16.1% versus 70.0 + 8.2%; p=0.04). When the cells were exposed to a more rigorous pneumolysin treatment, either by application of higher doses of pneumolysin or treatment for a longer time period, TAT-Bcl-XL had no protective effect.

Conclusion: The moderate protective effect of Bcl-XL in pneumolysin-induced cell death suggests the involvement of additional neuronal death pathways not controlled by Bcl-XL in this model. Therefore, Bcl-XL, although being a promising therapeutic candidate for ischemia and neurodegenerative diseases, is only of limited efficacy in preventing the direct neurotoxicity of pneumolysin.