Immunolabelling of the cytoplasm and processes of apoptotic facial motoneurons following axotomy in the neonatal rat

A polyclonal antibody intended to recognize c-Jun (Oncogene Science, c-jun/AP-1, Ab-2) has previously been shown to recognize an apparently novel “apoptosis-specific protein” (ASP) in the cytoplasm of cells undergoing apoptotic cell death in vitro. We have investigated whether this antibody would also serve as a reliable marker for apoptotic motoneurons in vivo. Following transection of the left facial nerve in anesthetized neonatal rat pups, which results in over 90% death of the facial motoneurons, we performed immunohistochemistry on frozen brain stem sections with Oncogene Science Ab-1 and Ab-2 antibodies which are raised against different peptide fragments of c-Jun. While Ab-1/c-Jun labelling was seen in the nuclei of the majority of axotomized motoneurons, Ab-2/ASP immunoreactivity was present only in scattered cells, all of which had characteristic apoptotic morphology. Furthermore, Ab-2/ASP immunoreactivity was cytoplasmic and frequently included the dendrites and axons of dying neurons. Some cerebellar granule cells undergoing postnatal developmental cell death were also Ab-2/ASP positive. The time course of the number of Ab-2/ASP-labelled motoneurons corresponded relatively closely with our previous data on DNA fragmentation in these cells, as assessed by an in situ end labelling (ISEL) technique. When facial nerve axotomy was performed at 7 and 14 days postnatum, resulting in reduced cell death, the number of Ab-2/ASP immunoreactive cells decreased correspondingly. Although the exact identity of the epitope recognized by Ab-2 is unclear, we conclude that, by labelling the cytoplasmic and neuritic components of apoptotic motoneurons, Ab-2/ASP immunohistochemistry is a valuable complementary technique to existing in situ methods based on the detection of fragmented DNA in the cell nucleus.