TrkB is expressed by human astrocytes in vivo and in vitro and up-regulated in MS lesions. (A and B) TrkB and p75NTR protein expression in normal control white matter (A) or MS chronic inactive lesions (B). Left panels show immunostainings with an antibody directed to the TrkB-ECD. In the first column, immunohistochemistry demonstrates brown DAB staining on astrocyte-like cells. Double immunofluorescence and confocal imaging for TrkB-ECD and GFAP are shown in the second column. Insets depict single stainings. Third and fourth columns show immunostainings for TrkB isoforms: TrkB-T1 isoform (brown DAB staining) in the third column and full-length TrkB (TrkB-TK) in the fourth column. The fifth column shows the absence of p75NTR immunoreactivity in GFAP-positive astrocytes. Representative stainings are shown. DAPI was used to stain for nuclei. (C and D) Quantitative PCR assessment of messenger RNA (mRNA) transcripts encoding full-length TrkB-TK and the truncated T1 or Shc isoforms in normal control white matter (control) and MS lesions (MS; C) and in human primary astrocytes (D). Data are expressed as the percentage of the housekeeping gene PPIA. Red lines represent mean values for each group. Error bars indicate SD. *, P < 0.05. (E) Cytofluorimetric analysis of TrkB surface expression in human cultured astrocytes. (F) Confocal microscopy for TrkB-T1 isoform, GFAP, and DAPI in cultured astrocytes. Bars, 50 µm.