Expression of the Pro-Apoptotic Bcl-2 Family Member Bim Is Regulated by Multiple Signaling Pathways During Retinal Development and Degeneration

Abstract

Purpose: : Apoptosis is essential for retinal development but is also a central component of many retinal dystrophies The purpose of this study was to investigate the signaling pathways involved in the suppression of the pro-apoptotic Bcl-2 family member Bim following developmental cell death in the retina. Additionally, the signals leading to its re-expression during apoptosis induced by several death stimuli are also explored.

Methods: : Cell death was analyzed by TUNEL during postnatal development in wild type and Bim^-/- mice and in retinal explant sections under various conditions. Protein expression of components of the JNK, PI3-K and MAPK transduction cascades were examined by western blot. Inhibitor studies using SP600125, LY294002 and UO126 were carried out in retinal explant cultures.

Results: : We report that increased phosphatidylinositol (PI) activity during development results in decreased levels of Bim_EL transcript, most likely through inhibiting the transcriptional activity of members of the forkhead family. Additionally, we show that as the retina matures activation of the ERK1/2 pathway is necessary to promote Bim_EL phosphorylation, an event that leads to an increase in the turnover of Bim_EL protein. Finally, in relation to retinal degeneration we demonstrate that cell death in explants is accompanied by the activation of c-jun N-terminal kinase (JNK) and increased phosphorylation/activation of the transcription factor c-jun. Moreover, treatment of explants with the selective JNK inhibitor SP600125 prevented these events and afforded significant protection against cell death.

Conclusions: : These results show that the activation of multiple signaling pathways is required to silence Bim once developmental apoptosis is complete. Our findings also indicate that Bim acts as a sensor to many apoptotic stimuli and is re-expressed under conditions of stress. This induction of Bim is essential for apoptosis and requires JNK activation indicating a critical role for the JNK signaling pathway in mediating retinal apoptosis.

Keywords: apoptosis/cell death • retina • retinal development