Directing traffic in neural cells: determinants of receptor tyrosine kinase localization and cellular responses

Abstract

The trafficking of receptor tyrosine kinases (RTKs) to distinct subcellular locations is essential for the specificity and fidelity of signal transduction and biological responses. This is particularly important in the PNS and CNS in which RTKs mediate key events in the development and maintenance of neurons and glia through a wide range of neural processes, including survival, proliferation, differentiation, neurite outgrowth, and synaptogenesis. The mechanisms that regulate the targeting of RTKs to their subcellular destinations for appropriate signal transduction, however, are still elusive. In this review, we discuss evidence for the spatial organization of signaling machinery into distinct subcellular compartments, as well as the role for ligand specificity, receptor sorting signals, and lipid raft microdomains in RTK targeting and the resultant cellular responses in neural cells.

Fig. 1

Schematic of receptor trafficking. See text for description. ee, early endosome; le, late endosome; se, sorting endosome; ly, lysosome; erc, endosomal recycling compartment.

Fig. 2

TrkC but not the IGF-IR is down-regulated following ligand stimulation. (a) TrkC phosphorylation and protein levels are down-regulated in response to continuous exposure to NT-3 in oligodendrocyte progenitor cells (OPCs). Late OPCs were treated with or without NT-3 and isolated protein was analyzed for phospho-Trk and total Trk C receptor. NT-3 was replaced in the media 15 min prior to protein isolation at 10 h. Blots of phosphorylated-Trk and total Trk C receptor are shown after 15 min, 2 h, and 10 h treatments (top panel). Left graph shows quantitation of band density expressed as the percentage of Trk phosphorylation. Quantitation of total Trk C (right graph) is expressed as percentage of the 15 min control levels. *p < 0.03 versus 10 h CTRL and **p < 0.002 versus CTRL. (b) IGF-IRβ phosphorylation and protein levels are maintained during continuous exposure to IGF-I through 48 h. Late OPCs were treated with or without IGF-I and isolated protein was analyzed for IGF-IRβ expression. Blots were stripped and used for analysis of β-actin as a control for equal protein loading. Top panel shows a representative blot of total IGF-IRβ after 22 or 48 h treatment. Left graph shows quantitation of band density expressed as the fold increase in IGF-IR phosphorylation compared with 22 h control. Quantification of total IGF-IR (right graph) is expressed as percentage of 22 h control levels. *p < 0.001 versus CTRL and **p < 0.05 versus 48 h IGF-I¹. [¹Reprinted from Mol. Cellular Neurosci. 20, J. K. Ness and T. L. Wood, Insulin-like growth factor I, but not neurotrophin-3, sustains Akt activation and provides long-term protection of immature oligodendrocytes from glutamate-mediated apoptosis, 476–488, 2002, with permission from Elsevier.]

Fig. 3

IGF-IR trafficking in oligodendrocyte progenitors. (a) IGF-I alters IGF-IR surface availability. Cells were serum starved and treated with 1 nM IGF-I or basal media for indicated times. Cells were labeled with 0.3 mg/mL sulfo-NHS-biotin. Total and surface precipitated lysates were processed for sodium dodecyl sulfate–polyacrylamide gel electrophoresis and western blot analysis for IGF-IRβ subunit immunoreactivity. Top panel shows representative western blot. Surface availability of IGF-IRβ is represented as a ratio of surface to total IGF-IRβ and as a percentage of t0 (bottom panel). *p < 0.05 versus t0 and **p < 0.004 versus t0. Data shown represent the mean ± SEM (n = 3).² (b) Model for IGF-IR trafficking in OPCs based on empirical data and mathematical modeling. IGF-I ligand binding results in phosphorylation and internalization of the IGF-IR. IGF-IR internalization is required for phosphorylation of Akt. The IGF-IR is targeted through Rab11-positive recycling endosomes to the cell surface. The model includes a surface state of the IGF-IR that is unavailable for ligand binding following receptor recycling. For details, see Romanelli et al. (2007). [²Reprinted from J. Biol. Chem. 282, R. J. Romanelli, A. P. Lebeau, C. G. Fulmer, D. A. Lazzaarino, A. Hochberg, T. L. Wood, Insulin-like growth factor type I receptor internalization and recycling mediate the sustained phosphorylation of Akt, 22513–22524, 2007.]

Fig. 4

Comparison of Trk juxtamembrane recycling domain. Sequence alignment of the juxtamebrane regions of the human TrkA, TrkB, and TrkC (highlighted). TrkA recycling domain is not present in the homologous sequence of TrkB or TrkC.

Fig. 5

Lipid rafts promote the formation of receptor-signaling complexes. Saturated phospholipids preferentially interact with cholesterol and modified hydrophobic proteins (including acylated and glycophosphatidylinositol-anchored proteins), as well as various kinases and phosphatases in microdomains termed lipid rafts. Upon activation, RTKs can translocate into lipid rafts, promoting the activation of various signal transduction pathways through the formation of receptor-signaling complexes.