The neurotrophin receptor TrkB is essential for normal function of the mammalian brain1,2,3. It is expressed in three splice variants. Full-length receptors (TrkBFL) possess an intracellular tyrosine kinase domain and are considered as those TrkB receptors that mediate the crucial effects of brain-derived neurotrophic factor (BDNF) or neurotrophin 4/5 (NT-4/5). By contrast, truncated receptors (TrkB-T1 and TrkB-T2) lack tyrosine kinase activity and have not been reported to elicit rapid intracellular signalling4. Here we show that astrocytes predominately express TrkB-T1 and respond to brief application of BDNF by releasing calcium from intracellular stores. The calcium transients are insensitive to the tyrosine kinase blocker K-252a and persist in mutant mice lacking TrkBFL. By contrast, neurons produce rapid BDNF-evoked signals through TrkBFL and the Nav1.9 channel5,6. Expression of antisense TrkB messenger RNA strongly reduces BDNF-evoked calcium signals in glia. Thus, our results show that, unexpectedly, TrkB-T1 has a direct signalling role in mediating inositol-1,4,5-trisphosphate-dependent calcium release; in addition, they identify a previously unknown mechanism of neurotrophin action in the brain.
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We thank H. Thoenen for critically discussing the manuscript; T. Hunter for the TrkB cDNA isoforms; J.-i. Miyazaki for pCAGGS and pCAGGS–eGFP; M. Meyer for help with the knockout mice; and I. Schneider, R. Maul and I. Mühlhahn for technical assistance. This work was supported by grants from the Deutsche Forschungsgemeinschaft (to C.R.R. and A.K.).
The authors declare that they have no competing financial interests.
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