Induced NG2 expressing microglia in the facial motor nucleus after facial nerve axotomy

Chondroitin sulfate proteoglycan (NG2) expressing cells, ubiquitously distributed in the CNS respond to injured or diseased neurons; however, their behaviors toward injured neurons have remained to be fully explored. In the present study, along with astrocytic and microglial responses, NG2 expressing cells reacted swiftly and robustly in the facial motor nucleus (FMN) subjected to axotomy. With time, hypertrophic NG2 expressing cells gradually adhered to and enwrapped the axotomized motoneurons. Tight encapsulations around axotomized motoneurons were eventually formed at 7, 14, and 28 days after axotomy. NG2 positive processes appeared to interpose between synapsin-1 immunoreactive nerve terminals and surfaces of axotomized motoneurons. Double labeling results showed that NG2 expressing cells encapsulating axotomized facial motoneurons were mainly microglia marked by OX42 and lectin; only a few of them were positive to platelet-derived growth factor-alpha receptor and none of them positive to ED-1. No Rhodamine particle was detected in the FMN ipsilateral to axotomy after venous injection of the particles. The results suggest that activated microglia in lesioned FMN were induced to express NG2 molecules. It is concluded that axotomized FMN showed two types of NG2 expressing cells namely constitutive NG2 cells and induced-NG2 expressing microglia.