Recruitment of Actin Modifiers to TrkA Endosomes Governs Retrograde NGF Signaling and Survival

The Solomon H. Snyder Department of Neuroscience and Howard Hughes Medical Institute, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA.
Cell (Impact Factor: 32.24). 08/2011; 146(3):421-34. DOI: 10.1016/j.cell.2011.07.008
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The neurotrophins NGF and NT3 collaborate to support development of sympathetic neurons. Although both promote axonal extension via the TrkA receptor, only NGF activates retrograde transport of TrkA endosomes to support neuronal survival. Here, we report that actin depolymerization is essential for initiation of NGF/TrkA endosome trafficking and that a Rac1-cofilin signaling module associated with TrkA early endosomes supports their maturation to retrograde transport-competent endosomes. These actin-regulatory endosomal components are absent from NT3/TrkA endosomes, explaining the failure of NT3 to support retrograde TrkA transport and survival. The inability of NT3 to activate Rac1-GTP-cofilin signaling is likely due to the labile nature of NT3/TrkA complexes within the acidic environment of TrkA early endosomes. Thus, TrkA endosomes associate with actin-modulatory proteins to promote F-actin disassembly, enabling their maturation into transport-competent signaling endosomes. Differential control of this process explains how NGF but not NT3 supports retrograde survival of sympathetic neurons.

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    • "For example, NGF induced the differentiation of PC12 cells into functional dopaminergic neurons [9]. Mechanistic studies revealed that NGF induced neurite outgrowth and neuronal differentiation through sustained activation of extracellular signal-regulated kinase 1/2 (ERK1/2) by interacting with its specific receptor tyrosine kinase (TrkA) in PC12 cells [10, 11]. "
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    BioMed Research International 06/2014; 2014:306857. DOI:10.1155/2014/306857 · 2.71 Impact Factor
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    • "Biochemical analysis of the actions of NGF upon primary peripheral neurons has often been hampered by the lack of a variety of neuronal cell models responsive to NGF, but do not require it for survival and also because it is difficult to obtain large numbers of sympathetic neurons for in vitro studies [39]. Furthermore, in a primary neuron model the development of sympathetic neurons might be critically regulated by two neurotrophins NT3 and NGF, acting through a common receptor TrkA, as reported earlier [41]. For these reasons, whether the results of the present study using PC12 cells can be extrapolated to primary neurons remains to be investigated. "
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    ABSTRACT: LAMTOR2 (p14), a part of the larger LAMTOR/Ragulator complex, plays a crucial role in EGF-dependent activation of p42/44 mitogen-activated protein kinases (MAPK, ERK1/2). In this study, we investigated the role of LAMTOR2 in nerve growth factor (NGF)-mediated neuronal differentiation. Stimulation of PC12 (rat adrenal pheochromocytoma) cells with NGF is known to activate the MAPK. Pharmacological inhibition of MEK1 as well as siRNA-mediated knockdown of both p42 and p44 MAPK resulted in inhibition of neurite outgrowth. Contrary to expectations, siRNA-mediated knockdown of LAMTOR2 effectively augmented neurite formation and neurite length of PC12 cells. Ectopic expression of a siRNA-resistant LAMTOR2 ortholog reversed this phenotype back to wildtype levels, ruling out nonspecific off-target effects of this LAMTOR2 siRNA approach. Mechanistically, LAMTOR2 siRNA treatment significantly enhanced NGF-dependent MAPK activity, and this effect again was reversed upon expression of the siRNA-resistant LAMTOR2 ortholog. Studies of intracellular trafficking of the NGF receptor TrkA revealed a rapid colocalization with early endosomes, which was modulated by LAMTOR2 siRNA. Inhibition of LAMTOR2 and concomitant destabilization of the remaining members of the LAMTOR complex apparently leads to a faster release of the TrkA/MAPK signaling module and nuclear increase of activated MAPK. These results suggest a modulatory role of the MEK1 adapter protein LAMTOR2 in NGF-mediated MAPK activation required for induction of neurite outgrowth in PC12 cells.
    PLoS ONE 04/2014; 9(4):e95863. DOI:10.1371/journal.pone.0095863 · 3.23 Impact Factor
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    • "These results raise the interesting possibility that endosomal pH influences Akt phosphorylation in addition to TrkA trafficking. Phosphoinositide 3-ki- nase (PI3K)–Akt signaling occurs in endosomes in various cell types (Garcia-Regalado et al., 2008; Schenck et al., 2008; Tsutsumi et al., 2009; Walz et al., 2010; Fujioka et al., 2011; Nazarewicz et al., 2011), including NGF-treated PC12 cells (Lin et al., 2006; Varsano et al., 2006), and endosomal Akt signaling contributes to persistent NGF- TrkA signaling after the ligand–receptor complex is endocytosed (Harrington et al., 2011). Recruitment of not only class II and class III PI3Ks, whose association with endosomes is well recognized, but also class I PI3K to endosomes and their possible activation therein has been suggested (Vanhaesebroeck et al., 2010). "
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    ABSTRACT: Neuronal precursor cells extend multiple neurites during development, one of which extends to form an axon whereas others develop into dendrites. Chemical stimulation of N-methyl D-aspartate (NMDA) receptor in fully-differentiated neurons induces projection of dendritic spines, small spikes protruding from dendrites, thereby establishing another layer of polarity within the dendrite. Neuron-enriched Na(+)/H(+) exchanger NHE5 contributes to both neurite growth and dendritic spine formation. In resting neurons and neuro-endocrine cells, neuron-enriched NHE5 is predominantly associated with recycling endosomes where it colocalizes with nerve growth factor (NGF) receptor TrkA. NHE5 potently acidifies the lumen of TrkA-positive recycling endosomes and regulates cell-surface targeting of TrkA, whereas chemical stimulation of NMDA receptors rapidly recruits NHE5 to dendritic spines, alkalinizes dendrites and down-regulates the dendritic spine formation. Possible roles of NHE5 in neuronal signaling via proton movement in subcellular compartments are discussed.
    Frontiers in Physiology 01/2014; 4:412. DOI:10.3389/fphys.2013.00412 · 3.53 Impact Factor
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