Endogenous expression of inhibitor of apoptosis proteins in facial motoneurons of neonatal and adult rats following axotomy

Abstract

The inhibitor of apoptosis protein family members inhibit cell death resulting from a variety of apoptotic stimuli. However, the endogenous expression of neuronal inhibitor of apoptosis proteins following axonal injury has not been thoroughly examined. Neonatal facial motoneurons are highly susceptible to axotomy-induced apoptosis, whereas adult facial motoneurons survive axotomy. We hypothesized that the endogenous expression of inhibitor of apoptosis proteins may be involved in the differential susceptibility of adult and neonatal facial motoneurons to axonal injury. In this study, we examined the expression of two endogenous inhibitor of apoptosis proteins, neuronal apoptosis inhibitory protein and x-linked inhibitory apoptosis protein, in adult and neonatal rat facial motoneurons following axotomy. Analyses using reverse-transcription polymerase chain reaction and in situ hybridization indicated that neuronal apoptosis inhibitory protein mRNA was increased in neonatal facial nuclei 24 h post axotomy. In the adult, neuronal apoptosis inhibitory protein mRNA expression increased at 1, 3, 7 and 14 days post axotomy, while little change in the expression of X-linked inhibitory apoptosis protein mRNA was detected at any age or time point time point analyzed. Interestingly, immunohistochemistry using antibodies for neuronal apoptosis inhibitory protein and X-linked inhibitory apoptosis protein, revealed the level of these proteins was higher in the neonatal motoneurons when compared with the adult. Furthermore, immunohistochemistry and western blot for neuronal apoptosis inhibitory protein revealed, in contrast to the observed increase in neuronal apoptosis inhibitory protein mRNA, a decline in the expression of neuronal apoptosis inhibitory protein following axotomy in the adult, whereas no change in neuronal apoptosis inhibitory protein was detected in neonatal facial motoneurons. X-linked inhibitory apoptosis protein, as analyzed by immunohistochemistry and western blot, remained unchanged by axotomy in neonatal motoneurons and adult motoneurons. These results indicate differential expression and/or turnover of inhibitor of apoptosis proteins in neonatal versus adult facial motoneurons, and suggest the level of inhibitor of apoptosis protein expression alone is not an indicator of cell fate following axotomy.