Occurrence of a transcription factor, signal transducer and activators of transcription 3 (Stat3), in the postsynaptic density of the rat brain.

Distribution of a signal transducer and activators of transcription 3 (Stat3) was examined in the rat brain. Immunoreactivity was distributed in the neurons, as well as glia cells, throughout the rat brain. Western blotting of subcellular fractions showed distribution in the synaptic fractions (synaptosome, synaptic plasma membrane, and postsynaptic density, PSD). The occurrence of Stat3 in the synaptic site, especially in the PSD, was confirmed by immunoelectron microscopic examination. The PSD fraction had an activity that phosphorylated Stat3 at the tyrosine-705 site, which was confirmed by both Western blotting and immunoprecipitation. The PSD fraction also had a janus kinase 2 (Jak2)-like molecule, Jak2, believed to phosphorylate Stat3. These results indicate that the Jak2/Stat3 signaling system, a major pathway of cytokine signaling in the immune response system, may also play a role at the postsynaptic sites.