Binding of target-derived neurotrophins to Trk receptors at nerve terminals are required to stimulate neuronal survival, differentiation, innervation and synaptic plasticity. The distance between the soma and nerve terminal is tremendous, making efficient anterograde Trk transport critical for their synaptic translocation and signaling. The mechanism responsible for this trafficking remains poorly understood. Here we show that the sorting receptor sortilin interacts with TrkA, -B, and –C and enables their anterograde axonal transport, thereby enhancing neurotrophin signaling. Cultured DRG neurons lacking sortilin exhibit blunted MAPK signaling and reduced neurite outgrowth upon stimulation with NGF. Moreover, deficiency for sortilin considerably aggravates TrkA, -B- and –C phenotypes present in p75NTR knockouts, and results in increased embryonic lethality and sympathetic neuropathy in mice heterozygous for TrkA. Our findings demonstrate a novel and unexpected role for sortilin as an anterograde trafficking receptor for Trk and a positive modulator of neurotrophin-induced neuronal survival.

Transit of proteins through the endosomal organelle following endocytosis is critical for regulating the homeostasis of cell-surface proteins and controlling signal transduction pathways. However, the mechanisms that control these membrane-transport processes are poorly understood. The Phox-homology (PX) domain-containing proteins sorting nexin (SNX) 17, SNX27, and SNX31 have emerged recently as key regulators of endosomal recycling and bind conserved Asn-Pro-Xaa-Tyr-sorting signals in transmembrane cargos via an atypical band, 4.1/ezrin/radixin/moesin (FERM) domain. Here we present the crystal structure of the SNX17 FERM domain bound to the sorting motif of the P-selectin adhesion protein, revealing both the architecture of the atypical FERM domain and the molecular basis for recognition of these essential sorting sequences. We further show that the PX-FERM proteins share a promiscuous ability to bind a wide array of putative cargo molecules, including receptor tyrosine kinases, and propose a model for their coordinated molecular interactions with membrane, cargo, and regulatory proteins.endosome | protein crystallography | X-ray scattering | membrane trafficking T he cell-surface levels of signaling and adhesion receptors, nutrient transporters, ion channels, and many other proteins are tightly regulated by opposing endocytic, exocytic, and endosomal recycling transport pathways. The selective sorting of these transmembrane proteins is the consequence of their interaction with essential adaptor proteins via conserved motifs present in their cytosolic tails, generally based on short linear amino acid sequences such as the Yxxϕ, DxxLL, and [DE]xxxL [LI] motifs (where ϕ is any bulky hydrophobic side-chain, and x is any residue) recognized by clathrin adaptors (1-3). The first identified sorting signal was the Asn-Pro-Xaa-Tyr (NPxY) motif, initially described in the LDL receptor (LDLR) isolated from patients suffering from familial hypercholesterolemia, where mutation of the sequence results in defective internalization and cholesterol uptake (4). The recent structure of the autosomal recessive hypercholesterolemia protein (ARH) phosphotyrosinebinding (PTB) domain in complex with the LDLR intracellular domain (ICD) provides the molecular basis of LDLR recognition and internalization within clathrin-coated pits by endocytic adaptor proteins (5).Although little is known about the sorting sequences and mechanisms required for competing endosome-to-cell surface recycling pathways, emerging evidence shows that the NPxY motif not only is vital to internalization but also aids cargo recycling via organelle-specific recognition by the endosomal protein, sorting nexin 17 (SNX17). SNX17 is a member of the Phox-homology (PX) domain-containing protein family and has been shown to be a critical regulator of endosomal sorting and cell-surface recycling of several essential cargo molecules. These include P-selectin (6-8), amyloid precursor protein (APP) (9, 10), integrins (11,12), and members of the LDL receptor family (13-17) ...

Balancing trophic and apoptotic cues is critical for development and regeneration of neuronal circuits. Here we identify SorCS2 as a proneurotrophin (proNT) receptor, mediating both trophic and apoptotic signals in conjunction with p75(NTR). CNS neurons, but not glia, express SorCS2 as a single-chain protein that is essential for proBDNF-induced growth cone collapse in developing dopaminergic processes. SorCS2- or p75(NTR)-deficient in mice caused reduced dopamine levels and metabolism and dopaminergic hyperinnervation of the frontal cortex. Accordingly, both knockout models displayed a paradoxical behavioral response to amphetamine reminiscent of ADHD. Contrary, in PNS glia, but not in neurons, proteolytic processing produced a two-chain SorCS2 isoform that mediated proNT-dependent Schwann cell apoptosis. Sciatic nerve injury triggered generation of two-chain SorCS2 in p75(NTR)-positive dying Schwann cells, with apoptosis being profoundly attenuated in Sorcs2(-/-) mice. In conclusion, we have demonstrated that two-chain processing of SorCS2 enables neurons and glia to respond differently to proneurotrophins.