neurotrophic factor (BDNF) plays an important role in synaptic plasticity but the underlying signaling mechanisms remain unknown. and plasticity (Lu 2003 In cultured hippocampal or cortical neurons application of BDNF elicits a rapid potentiation of excitatory synaptic transmission primarily by enhancing presynaptic transmitter MYO7A release (Lessmann 1998 Takei et al. 1998 In slices BDNF facilitates hippocampal long-term potentiation (LTP) and enhances synaptic response WHI-P 154 to LTP-inducing tetanus (Figurov et al. 1996 Patterson et al. 1996 Both in vitro and in vivo studies demonstrate that BDNF induces complex effects on dendritic arborization of pyramidal neurons (McAllister et al. 1995 Despite rapid progress in this area the molecular mechanisms remain ill defined (Lu 2003 All the functions of BDNF are mediated by TrkB a receptor tyrosine kinase (RTK; Kaplan and Miller 2000 Binding of BDNF rapidly activates its tyrosine kinase which in turn WHI-P 154 triggers multiple intracellular signaling pathways. Downstream pathways include MAPK phosphatidylinositol 3-kinase (PI3-K) and PLCγ. A critical yet poorly understood issue is how signals from this receptor are transduced to mediate diverse biological functions in CNS neurons. One WHI-P 154 idea for specific signal-function coupling is that different signaling pathways may be transduced in different subcellular compartments. More specifically it has been proposed that cholesterol/sphingolipid-rich microdomains called lipid rafts make a specialized signaling platform in the plasma membrane and therefore can transduce signals different from those in the nonraft membrane (Simons and Toomre 2000 Anderson and Jacobson 2002 Because both lipid components are resistant to solubilization with nonionic detergents lipid rafts can be biochemically isolated as detergent-resistant membrane fractions. Raft fractions prepared from brain tissues are enriched in proteins that carry lipid modifications such as glycosylphosphatidylinositol (GPI)-anchored proteins as well as palmitylated or myristoylated WHI-P 154 proteins such as Src-family kinases and trimeric or small G proteins suggesting a crucial role of lipid rafts in signal transduction in the CNS (Paratcha and Ibanez 2002 Recently lipid rafts have been shown to serve as organizing platforms for chemotrophic guidance of nerve growth cones (Guirland et al. 2004 Transmembrane RTKs including EGF receptor (Mineo et al. 1999 and FGF receptor (Citores et al. 1999 are associated with rafts. The localization of certain signaling molecules in the rafts allows them to interact with each other more efficiently and prevents them from interacting with the proteins WHI-P 154 outside rafts (Simons and Toomre 2000 Thus entering and exiting lipid rafts of RTKs represent a unique mechanism that transduces differential signals at the subcellular levels. In the present study we used brain tissues slices and dissociated cultures to examine whether TrkB receptor is localized in lipid rafts of the plasma membrane and if so how the localization is regulated and what the functional roles are. Our results reveal a BDNF-induced TrkB translocation into the lipid rafts and such translocation is important for BDNF-induced synaptic modulation in CNS neurons. Results BDNF-induced translocation of TrkB into lipid rafts Lipid raft fraction was prepared from tissues or primary cultures of cerebral cortex according to the method of Kawabuchi et al. (2000)(Fig. S1A available at http://www.jcb.org/cgi/content/full/jcb.200404106/DC1). We first..