Figure 4.

Time course and cellular localization of translocator protein (TSPO) and matrix metalloproteinase 9 (MMP-9) expression after focal cerebral ischemia. Histologic validation of micro positron emission tomography (μPET) data of the same animal as depicted in Figure 2. Images were taken from infarcted areas indicated by the white cross in Figure 2. MMP-9 immunohistochemistry was taken from the infarct core, and TSPO immunohistochemistry from a core-to-periinfarct region, since the massive expression of TSPO in the infarct core did not allow to distinguish single cells. TSPO expression within the infarct is time dependent: 24 to 48 hours after transient middle cerebral artery occlusion (tMCAo) the density of TSPO expressing cells is relatively low, compared with later time points. TSPO was also localized surrounding some larger vessels in the ipsilateral hemisphere at this time. At 7 and 14 days, TSPO staining increased in density with colocalization with microglia cells as defined by Iba-1 staining and morphology. The TSPO signal correlated to the imaging data derived from [¹⁸F]DPA-714 μPET. No TSPO immunoreactivity was detected in the contralateral hemisphere, except at 21 days after tMCAo, where faintly TSPO-positive endothelial cells could be found. In correlation with the [¹⁸F]BR-351 μPET data, MMP-9 staining was found in the infarct area at 24 to 48 hours after tMCAo in association with the vasculature and microglia (Iba-1), and after 7 days in activated microglia and apoptotic cells. At later time points (14 and 21 days after tMCAo), MMP-9 was mainly found in apoptotic cells. All images were taken with the same magnification. Scale bar: 100 μm.