Gene expression of C1q A-chain in the rat facial nucleus after axotomy

Differential hybridization screening of the cDNA libraries derived from the rat facial nucleus was performed, and a number of cDNA clones were isolated and found to be upregulated after facial nerve axotomy. One of the isolated cDNA clones encoded the A-chain of C1q (C1q-A) of the rat, whose cloning has not been reported in the literature. Sequence analysis showed that C1q-A is well conserved in the rat, mouse and human on the DNA and protein levels. In situ hybridization demonstrated that there were strong signals for C1q-A mRNA within the rat facial nucleus damaged by axotomy, and the signals were localized in activated microglia. Immunohistochemistry showed strong immunoreactivity for C1q protein in the activated microglia of the rat facial nucleus damaged by axotomy. C1q-immunoreactivity was also found in the extracellular space of the damaged facial nucleus. No significant C1q-A mRNA signals and C1q-immunoreactivity were observed in the normal brain tissue. These results suggest that microglia markedly up-regulate the C1q-A gene to produce and secrete C1q in response to the neuronal damage caused by axotomy.