Rapid Nuclear Responses to Target-Derived Neurotrophins Require Retrograde Transport of Ligand–Receptor Complex

Abstract

Target-derived neurotrophins initiate signals that begin at nerve terminals and cross long distances to reach the cell bodies and regulate gene expression. Neurotrophin receptors, Trks, themselves serve as retrograde signal carriers. However, it is not yet known whether the retrograde propagation of Trk activation reflects movement of Trk receptors from neurites to cell bodies or reflects serial activation of stationary Trk molecules. Here, we show that neurotrophins selectively applied to distal neurites of sensory neurons rapidly induce phosphorylation of the transcription factor cAMP response element-binding protein (CREB) and also cause a slower increase in Fos protein expression. Both nuclear responses require activation of neurotrophin receptors (Trks) at distal nerve endings and retrograde propagation of Trk activation to the nerve cell bodies. Using photobleach and recovery techniques to follow biologically active, green fluorescent protein (GFP)-tagged BDNF receptors (TrkB-GFP) in live cells during retrograde signaling, we show that TrkB-GFP moves rapidly from neurites to the cell bodies. This rapid movement requires ligand binding, Trk kinase activity, and intact axonal microtubules. When they reach the cell bodies, the activated TrkB receptors are in a complex with ligand. Thus, the retrograde propagation of activated TrkB from neurites to cell bodies, although rapid, reflects microtubule-dependent transport of phosphorylated Trk-ligand complexes. Moreover, the relocation of activated Trk receptors from nerve endings to cell bodies is required for nuclear signaling responses. Together, these data support a model of retrograde signaling whereby rapid vesicular transport of ligand-receptor complex from the neurites to the cell bodies mediates the nuclear responses.

Nerve growth factor as a paradigm of neurotrophins related to Alzheimer's disease

“…Moreover, since tau controls the bidirectionality of axonal motor-driven transport in a concentration-dependent manner and differentially modulates the kinesin and dynein activity along microtubule tracks (Dixit et al, 2008), defective intracellular trafficking of cargoes, including NTFs, could be due to an increased expression level of this protein (Ebneth et al, 1998;Stamer et al, 2002;Mandelkow et al, 2003) or to its altered intracellular localization (Thies et al, 2007) or hyperphosphorylation (Tatebayashi et al, 2004;Alonso et al, 1997). To this regard, the finding that the retrograde transport of I-125-NGF and activated Trk receptors is inhibited by colchicine-a drug that interferes with the polymerization of microtubules (Watson et al, 1999;Sandow et al, 2000)-suggests that an altered function of tau protein may account for age-related deficiency of long-range NTF signaling in cholinergic neurons. Actually, the hypothesis that the failure of tau-mediated axonal transport might be responsible for the lack of trophic support in aged or AD brains (Salehi et al, 2004;Niewiadomska et al, 2006a;Schindowski et al, 2008) is supported by several evidences.…”

“…Actually, the hypothesis that the failure of tau-mediated axonal transport might be responsible for the lack of trophic support in aged or AD brains (Salehi et al, 2004;Niewiadomska et al, 2006a;Schindowski et al, 2008) is supported by several evidences. An endosome-containing activated TrkA following NGF binding is transported from the presynaptic terminals axons of the BFCNs to the cell body via a microtubule-dependent mechanisms (Watson et al, 1999;Delcroix et al, 2004; Figure 1 Scheme representing the NGF-dependent hippocampal neuronal model. Hippocampal neurons were prepared from embryonic days 17/18 (E17/E18) embryos from timed pregnant Wistar rats (Charles River).…”

Clinically Relevant Doses of Fluoxetine and Reboxetine Induce Changes in the TrkB Content of Central Excitatory Synapses

“…This increased BDNF in the synaptic cleft would bind to and activate TrkB, a proportion of which is in PSDs. Binding of neurotrophins to their receptors induces the endocytosis of the neurotrophin-receptor complex (Grimes and Miettinen, 2003;Howe et al, 2001;Huang and Reichardt, 2003;Watson et al, 1999;York et al, 2000). The TrkB/BDNF complex should then, at least in part, be retrogradely transported from the spines to the cell body, where it enhances further BDNF synthesis.…”

How does an axon grow?

“…These signaling endosomes carry with them the machinery of the ras-raf-MAP kinase signaling cascades, and can continue to activate these pathways once inside the cell (Howe et al 2001). Neurotrophin signaling endosomes may then be retrogradely transported back to the cell body along microtubules, where they can activate transcriptional regulators and induce new gene expression (Riccio et al 1997;Watson et al 1999 In these experiments, a wall separates the culture dish into two or more compartments, and although the axons can grow under the wall into the distal compartment, the contents of the culture media do not diffuse from one side to the other. In this way, factors can be added exclusively to the axons or cell bodies to the exclusion of the other.…”