Up‐regulation of Bax Protein in Degenerating Retinal Ganglion Cells Precedes Apoptotic Cell Death after Optic Nerve Lesion in the Rat

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Abstract
Retrograde degeneration of retinal ganglion cells as a consequence of optic nerve lesion has been shown to fulfil the criteria of apoptosis. In the present study, we investigated the time course of ganglion cell apoptosis following intraorbital crushing of the optic nerve in adult rats using morphological criteria and applying a terminal transferase technique (TUNEL) for in situ detection of DNA strand breaks. In addition, we examined expression patterns of the anti‐apoptotic proteins Bcl‐2 and Bcl‐X and the cell death‐promoting protein Bax in retinae after crushing the optic nerve. Apoptotic nuclei were detected in the ganglion cell layer in the first 3 weeks after optic nerve crush, with a peak after 6 days. Bcl‐2 and Bcl‐X proteins were expressed in ganglion cells at low levels. Expression of Bcl‐2 decreased further during the days following crush. Bcl‐X expression was initially increased, followed by a decline over the following days. In contrast, Bax protein, which was expressed in most ganglion cells at moderate baseline levels, was sharply increased as early as 30 min after crush, reached peak levels after 3 days, and remained up‐regulated for at least 1 week thereafter. Double labelling for Bax and TUNEL in retinal sections, however, did not reveal colocalization of the two signals in individual retinal ganglion cells, consistent with the idea that increases in Bax precede apoptosis after optic nerve lesion. Thus, retinal ganglion cell death might be prevented by ablation of Bax protein in these cells, or by up‐regulation of Bax‐antagonists such as Bcl‐2 or Bcl‐X.