Genetic Dissection of BDNF and TrkB Expression in Glial Cells.

The brain-derived neurotrophic factor (BDNF) and its high-affinity receptor tropomyosin-related kinase receptor B (TrkB) are widely expressed in the central nervous system. It is well documented that neurons express BDNF and full-length TrkB (TrkB.FL) as well as a lower level of truncated TrkB (TrkB.T). However, there are conflicting reports regarding the expression of BDNF and TrkB in glial cells, particularly microglia. In this study, we employed a sensitive and reliable genetic method to characterize the expression of BDNF and TrkB in glial cells in the mouse brain. We utilized three Cre mouse strains in which Cre recombinase is expressed in the same cells as BDNF, TrkB.FL, or all TrkB isoforms, and crossed them to Cre-dependent reporter mice to label BDNF- or TrkB-expressing cells with soma-localized EGFP. We performed immunohistochemistry with glial cell markers to examine the expression of BDNF and TrkB in microglia, astrocytes, and oligodendrocytes. Surprisingly, we found no BDNF- or TrkB-expressing microglia in examined CNS regions, including the somatomotor cortex, hippocampal CA1, and spinal cord. Consistent with previous studies, most astrocytes only express TrkB.T in the hippocampus of adult brains. Moreover, there are a small number of astrocytes and oligodendrocytes that express BDNF in the hippocampus, the function of which is to be determined. We also found that oligodendrocyte precursor cells, but not mature oligodendrocytes, express both TrkB.FL and TrkB.T in the hippocampus of adult mice. These results not only clarify the expression of BDNF and TrkB in glial cells but also open opportunities to investigate previously unidentified roles of BDNF and TrkB in astrocytes and oligodendrocytes.