Background:  The Vif protein of HIV-1 has been shown to interact with members of the APOBEC family of cytidine deaminases, particularly APOBEC3G /F. The distribution of APOBEC3G in the brain is currently unknown.

Methods:  In this study, we isolated RNA from 12 regions of the brain from 2 pig-tailed macaques that had been exsanguinated and perfused with saline. Aliquots of each region were used for immunoblot analysis using a monoclonal antibody against APOBEC3G and for extraction of RNA for use in real-time polymerase chain reaction (RT-PCR) to detect the presence of mRNA. Whole brain sections were used in immunohistochemistry to examine the cell types that expressed APOBEC3G.

Results:  Our results RT-PCR analyses indicate that APOBEC3G was detected in all regions of the brain analyzed. Immunoblot analysis using lysates prepared from these same regions of the brain and a monoclonal antibody to APOBEC3G confirmed the RT-PCR findings. To determine which cell types express APOBEC3G, immunohistochemical studies were performed using this monoclonal antibody on whole brain sections. Our results clearly show that the pyramidal neurons within the gray matter of cerebral and cerebellar cortices express APOBEC3G. However, APOBEC3G expression in the pyramidal neurons appeared to be nuclear or associated with nuclei. In contrast to our findings in the cerebral cortex, immunohistochemical analysis of the spleen and kidney tissues revealed that APOBEC3G expression in cells of these tissues was predominantly cytoplasmic. We further investigated the expression of APOBEC3G in astrocytes. Immunohistochemical staining of serial sections was performed using antibodies to glial fibrillary acidic protein (GFAP) and APOBEC3G. As expected, the cortical and cerebellar white matter showed extensive immunostaining of astrocytes with the antibody against GFAP but a lack of reactivity to the antibody to APOBEC3G. Additionally, Western blot analysis of lysates prepared from primary human fetal astrocytes revealed a lack of APOBEC3G expression. 

Conclusions:  These results indicate that APOBEC3G expression is restricted to neurons in the brain and that astrocytes and microglia either do not express this protein or express it at levels undetectable by immunohistochemistry. These finding have implications for the brain as a potential reservoir for Vif-defective viruses.