Co-localization of activating transcription factor 3 and phosphorylated c-Jun in axotomized facial motoneurons

Activating transcription factor 3 (ATF3) and c-Jun play key roles in either cell death or cell survival, depending on the cellular background. To evaluate the functional significance of ATF3/c-Jun in the peripheral nervous system, we examined neuronal cell death, activation of ATF3/c-Jun, and microglial responses in facial motor nuclei up to 24 weeks after an extracranial facial nerve axotomy in adult rats. Following the axotomy, neuronal survival rate was progressively but significantly reduced to 79.1% at 16 weeks post-lesion (wpl) and to 65.2% at 24 wpl. ATF3 and phosphorylated c-Jun (pc-Jun) were detected in the majority of ipsilateral facial motoneurons with normal size and morphology during the early stage of degeneration (1-2 wpl). Thereafter, the number of facial motoneurons decreased gradually, and both ATF3 and pc-Jun were identified in degenerating neurons only. ATF3 and pc-Jun were co-localized in most cases. Additionally, a large number of activated microglia, recognized by OX6 (rat MHC II marker) and ED1 (phagocytic marker), gathered in the ipsilateral facial motor nuclei. Importantly, numerous OX6- and ED1-positive, phagocytic microglia closely surrounded and ingested pc-Jun-positive, degenerating neurons. Taken together, our results indicate that long-lasting co-localization of ATF3 and pc-Jun in axotomized facial motoneurons may be related to degenerative cascades provoked by an extracranial facial nerve axotomy.