Fates of Neurotrophins after Retrograde Axonal Transport: Phosphorylation of p75NTR Is a Sorting Signal for Delayed Degradation

Abstract

Neurotrophins can mediate survival or death of neurons. Opposing functions of neurotrophins are based on binding of these ligands to two distinct types of receptors: trk receptors and p75NTR. Previous work showed that target-derived NGF induces cell death, whereas BDNF and NT-3 enhance survival of neurons in the isthmo-optic nucleus of avian embryos. To determine the fate of retrogradely transported neurotrophins and test whether their sorting differs between neurotrophins mediating survival-or death-signaling pathways, we traced receptor-binding, sorting, and degradation kinetics of target-applied radiolabeled neurotrophins that bind in this system to trk receptors (BDNF, NT-3) or only to p75NTR (NGF). At the ultrastructural level, the p75NTR-bound NGF accumulates with a significant delay in multivesicular bodies and organelles of the degradation pathway on arrival in the cell body when compared with trk-bound BDNF or NT-3. This delayed lysosomal accumulation was restricted to target-derived NGF, but was not seen when NGF was supplied to the soma in vitro. The kinase inhibitors K252a and Gö6976 alter the kinetics of organelle accumulation: phosphorylation of p75NTR is a sorting signal for delayed sequestering of p75NTR-bound NGF in multivesicular bodies and delayed degradation in lysosomes when compared with trk-bound neurotrophins. Mutagenesis and mass spectrometry studies indicate that p75NTR is phosphorylated by conventional protein kinase C on serine 266. We conclude that, in addition to the known phosphorylation of trks, the phosphorylation of p75NTR can also significantly affect neuronal survival in vivo by changing the intracellular sorting and degradation kinetics of its ligands and thus signaling duration.

Retrograde apoptotic signaling by the p75 neurotrophin receptor

“…After ligand-induced internalization in PC12 cells and sympathetic neurons, it was reported that p75 avoided the lysosome and was trafficked to Rab11 + recycling endosomes or multivesicular bodies (MVBs), where some of the receptor was exocytosed [55]. It is interesting that, in the chick ION, retrograde p75-mediated cell death correlated with accumulation of NGF in the neuronal soma in MVBs [57], suggesting that retrograde transport of p75 occurs in MVBs. In contrast, in motor neurons p75 is first trafficked into Rab5 + early endosomes, then into Rab7 + endosomes for retrograde transport [56,58].…”

Multivesicular bodies in neurons: Distribution, protein content, and trafficking functions

“…Different categories of endogenous molecules have been identified and localized in neuronal MVBs, including growth factors/trophic factors (Fig. 2A-D) (Butowt and von Bartheld, 2001, 2009; Claude et al ., 1982a; Claude et al ., 1982b; Rind et al ., 2005; Schwab and Thoenen, 1977; Schwab and Thoenen, 1976; von Bartheld et al ., 1996) and their receptors (Pioro et al ., 1991; Valdez et al ., 2005). Another important category is comprised by receptors for neurotransmitters, ion channels, and G-protein-coupled receptors (Bloch et al ., 2003; Grimes and Miettinen, 2003; Sorkin and von Zastrow, 2002, 2009; Vitalis et al ., 2008), and their ligands, such as the neuropeptides neurotensin, calcitonin gene-related peptide (CGRP), substance P and small cations, for example calcium (Table 3).…”

“…MVBs keep the cellular machinery functional, reduce inventory, and help with “just-in-time” delivery of cargo where it is needed. As shown in Table 3, numerous proteins have been localized in neuronal MVBs, but only few studies have examined sorting/recycling in neurons (Butowt and von Bartheld, 2009; Cooney et al ., 2002; Grimes and Miettinen, 2003; Rind et al ., 2005; Valdez et al ., 2005). Nevertheless, this fundamental MVB function is believed to be applicable to neurons as it is to other cell types, as mentioned above.…”

Nanoparticle-mediated signaling endosome localization regulates growth cone motility and neurite growth

“…Neurotrophins, like brain-derived neurotrophic factor (BDNF), activate tropomyosin-related kinase B (TrkB) receptors and are endocytosed by clathrin-dependent and -independent mechanisms into signaling endosomes (10,11). These signaling endosomes signal persistently during retrograde (12) and anterograde (13,14) transport in axons or dendrites (15,16) directing neurite growth, survival, and cell migration (17,18). Signaling endosomes are critical long-range communication links used by neurons in the central and peripheral nervous system during development and regeneration (17), whose dysfunction is linked to nervous system disorders (19)(20)(21).…”