Retrograde communication from axonal targets to neuronal cell bodies is critical for both development and function of the nervous system. Much progress has been made in recent years linking long-distance, retrograde signaling to a signaling endosome, yet the mechanisms governing the trafficking and signaling of these endosomes remain mainly uncharacterized. Here we report that in mouse sympathetic neurons the target-derived NGF-TrkA signaling endosome, upon arrival at the cell body, induces the expression and recruitment of a novel effector protein known as Coronin-1. In the absence of Coronin-1, the NGF-TrkA signaling endosome fuses to lysosomes 6–10 fold faster than when Coronin-1 is intact. We also define a novel Coronin-1-dependent trafficking event where signaling endosomes recycle and re-internalize upon arrival at the cell body. Beyond influencing endosomal trafficking, Coronin-1 is also required for several NGF-TrkA dependent-signaling events including calcium release, calcineurin activation, and CREB phosphorylation. These results establish Coronin-1 as an essential component of a novel feedback loop mediating NGF-TrkA endosome stability, recycling, and signaling as a critical mechanism governing developmental competition for survival.

The retrograde transport of Trk containing endosomes from the axon to the cell body by cytoplasmic dynein is necessary for axonal and neuronal survival. We investigated the recruitment of dynein to signaling endosomes in rat embryonic neurons and PC12 cells. We identified a novel phospho-serine on the dynein intermediate chains (IC) and we observed a time-dependent neurotrophin-stimulated increase in intermediate chain phosphorylation on this site in both cell types. Pharmacological studies, over-expression of constitutively active MEK, and an in vitro assay with recombinant proteins demonstrated that the intermediate chains are phosphorylated by the MAP kinase ERK1/2, extracellular-signal-regulated kinase, a major downstream effector of Trk. Live cell imaging with fluorescently-tagged IC mutants demonstrated that the dephospho-mimic mutants had significantly reduced co-localization with Trk and Rab7, but not a mitochondrial marker. The phosphorylated intermediate chains were enriched on immuno-affinity purified Trk containing organelles. Inhibition of ERK reduced the amount of phospho-IC and the total amount of dynein that co-purified with the signaling endosomes. In addition, inhibition of ERK1/2 reduced the motility of Rab7 and TrkB containing endosomes and the extent of their co-localization with dynein in axons. NGF-dependent survival of sympathetic neurons was significantly reduced by the over-expression of the dephospho-mimic mutant IC-1B-S80A, but not WT IC-1B, further demonstrating the functional significance of phosphorylation on this site. These results demonstrate that neurotrophin binding to Trk initiates the recruitment of cytoplasmic dynein to signaling endosomes through ERK1/2 phosphorylation of intermediate chains for their subsequent retrograde transport in axons.

NO plays an important role in the shared signalling pathway of the P- and Fe-deficiency-induced formation of cluster roots in white lupin.

Development of a functional peripheral nervous system requires axons to rapidly innervate and arborize into final target organs and then slow but not halt their growth to establish stable connections while keeping pace with organ growth. Here we examine the role of the NGF-TrkA effector protein, Coronin-1, on postganglionic sympathetic neuron final target innervation. In the absence of Coronin-1 we find that NGF-TrkA-PI3K signaling drives robust axon growth and branching in part by suppressing GSK3␤. In contrast, the presence of Coronin-1 (wild-type neurons) suppresses but does not halt NGF-TrkA-dependent growth and branching. This relative suppression in axon growth behaviors is due to Coronin-1-dependent calcium release via PLC-␥1 signaling, which releases PI3K-dependent suppression of GSK3␤. Finally, we demonstrate that Coro1a Ϫ/Ϫ mice display sympathetic axon overgrowth and overbranching phenotypes in the developing heart. Together with previous work demonstrating the Coronin-1 expression is NGF dependent, this work suggests that periods before and after NGF-TrkA-induced Coronin-1 expression (and likely other factors) defines two distinct axon growth states, which are critical for proper circuit formation in the sympathetic nervous system.

Formation of cluster roots is a typical morphological response to phosphorus (P) deficiency in white lupin (Lupinus albus), but its physiological and molecular mechanisms are still unclear. We investigated the role of auxin in the initiation of cluster roots by distinguishing the sources of auxin, measuring the longitudinal distribution patterns of free indole-3-acetic acid (IAA) along the root and the related gene expressions responsible for polar auxin transport (PAT) in different developmental stages of cluster roots. We found that removal of shoot apex or primary root apex and application of auxin-influx or -efflux transport inhibitors, 3-chloro-4-hydroxyphenylacetic acid, N-1-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid, to the stem did not affect the number of cluster roots and the free-IAA concentration in the roots of P-deficient plants, but when these inhibitors were applied directly to the growth media, the cluster-root formation was greatly suppressed, suggesting the fundamental role of root-derived IAA in cluster-root formation. The concentration of free IAA in the roots was higher in P-deficient plants than in P-adequate ones, and the highest in the lateral-root apex and the lowest in the mature cluster roots. Meanwhile the expression patterns of LaAUX1, LaPIN1 and LaPIN3 transcripts related to PAT was consistent with concentrations of free IAA along the lateral root, indicating the contribution of IAA redistribution in the cluster-root development. We proposed that root-derived IAA plays a direct and important role in the P-deficiency-induced formation of cluster roots.