Marco Terenzio

Signalling endosomes in axonal transport: travel updates on the molecular highway.

Neurons are highly polarised cells. They make contact with their targets through long axons, along which a steady flux of proteins, lipids, nucleic acids and organelles is constantly maintained. This process is crucial to the development and maintenance of the nervous system, as proven by the many neurodegenerative disorders associated with defective axonal transport. Specific pools of endocytic organelles, which travel along the axon towards the cell body, have assumed a growing importance by virtue of their transported signals. These organelles, named signalling endosomes, vehicle growth factors, such as neurotrophins, and their signalling receptors all the way from the axon terminals to the neuronal cell body. Due to the central importance of neurotrophins in neuronal development and survival, significant efforts have gone over the years into the study of long-range neutrophin trafficking and signalling. Recent evidence has pointed to a role of signalling endosomes in the axonal retrograde transport of many morphogenetic and survival factors, increasing their importance even further. In light of these findings, signalling endosomes have shown potential for integration of different growth factors signals and the ability to decode them by differential sorting in the neuronal cell body. In this review we aim to discuss the state of the field regarding the nature and dynamics of signalling endosomes, their signalling capabilities, their energy requirements for axonal transport and last but not least, their importance in health and disease.

Bicaudal-D1 regulates the intracellular sorting and signalling of neurotrophin receptors

We have identified a new function for the dynein adaptor Bicaudal D homolog 1 (BICD1) by screening a siRNA library for genes affecting the dynamics of neurotrophin receptor‐containing endosomes in motor neurons (MNs). Depleting BICD1 increased the intracellular accumulation of brain‐derived neurotrophic factor (BDNF)‐activated TrkB and p75 neurotrophin receptor (p75NTR) by disrupting the endosomal sorting, reducing lysosomal degradation and increasing the co‐localisation of these neurotrophin receptors with retromer‐associated sorting nexin 1. The resulting re‐routing of active receptors increased their recycling to the plasma membrane and altered the repertoire of signalling‐competent TrkB isoforms and p75NTR available for ligand binding on the neuronal surface. This resulted in attenuated, but more sustained, AKT activation in response to BDNF stimulation. These data, together with our observation that Bicd1 expression is restricted to the developing nervous system when neurotrophin receptor expression peaks, indicate that BICD1 regulates neurotrophin signalling by modulating the endosomal sorting of internalised ligand‐activated receptors.

Growth control mechanisms in neuronal regeneration

Neurons grow during development and extend long axons to make contact with their targets with the help of an intrinsic program of axonal growth as well as a range of extrinsic cues and a permissive milieu. Injury events in adulthood induce some neuron types to revert to a regenerative state in the peripheral nervous system (PNS). Neurons from the central nervous system (CNS), however, reveal a much lower capacity for regenerative growth. A number of intrinsic regeneration‐promoting mechanisms have been described, including priming by calcium waves, epigenetic modifications, local mRNA translation, and dynein‐driven retrograde transport of transcription factors (TFs) or signaling complexes that lead to TF activation and nuclear translocation. Differences in the availability or recruitment of these mechanisms may partially explain the limited response of CNS neurons to injury.

Activated leukocyte cell adhesion molecule modulates neurotrophin signaling

J. Neurochem. (2012) 121, 575–586.

Nucleolin-Mediated RNA Localization Regulates Neuron Growth and Cycling Cell Size

Summary How can cells sense their own size to coordinate biosynthesis and metabolism with their growth needs? We recently proposed a motor-dependent bidirectional transport mechanism for axon length and cell size sensing, but the nature of the motor-transported size signals remained elusive. Here, we show that motor-dependent mRNA localization regulates neuronal growth and cycling cell size. We found that the RNA-binding protein nucleolin is associated with importin β1 mRNA in axons. Perturbation of nucleolin association with kinesins reduces its levels in axons, with a concomitant reduction in axonal importin β1 mRNA and protein levels. Strikingly, subcellular sequestration of nucleolin or importin β1 enhances axonal growth and causes a subcellular shift in protein synthesis. Similar findings were obtained in fibroblasts. Thus, subcellular mRNA localization regulates size and growth in both neurons and cycling cells.

Locally translated mTOR controls axonal local translation in nerve injury

Local control of localized protein synthesis Localized protein synthesis provides spatiotemporal precision for injury responses and growth decisions at remote positions in nerve axons. Terenzio et al. show that this process is controlled by local translation of preexisting axonal mRNA encoding the master regulator mTOR (see the Perspective by Riccio). mTOR controls both its own synthesis and that of most newly synthesized proteins at axonal injury sites, thereby determining the subsequent survival and growth of the injured neuron. Science, this issue p. 1416; see also p. 1331 Axonal localization of mTOR mRNA enables subcellular regulation of local protein synthesis in injured nerves. How is protein synthesis initiated locally in neurons? We found that mTOR (mechanistic target of rapamycin) was activated and then up-regulated in injured axons, owing to local translation of mTOR messenger RNA (mRNA). This mRNA was transported into axons by the cell size–regulating RNA-binding protein nucleolin. Furthermore, mTOR controlled local translation in injured axons. This included regulation of its own translation and that of retrograde injury signaling molecules such as importin β1 and STAT3 (signal transducer and activator of transcription 3). Deletion of the mTOR 3′ untranslated region (3′UTR) in mice reduced mTOR in axons and decreased local translation after nerve injury. Both pharmacological inhibition of mTOR in axons and deletion of the mTOR 3′UTR decreased proprioceptive neuronal survival after nerve injury. Thus, mRNA localization enables spatiotemporal control of mTOR pathways regulating local translation and long-range intracellular signaling.

Compartmentalized Signaling in Neurons: From Cell Biology to Neuroscience

Neurons are the largest known cells, with complex and highly polarized morphologies. As such, neuronal signaling is highly compartmentalized, requiring sophisticated transfer mechanisms to convey and integrate information within and between sub-neuronal compartments. Here, we survey different modes of compartmentalized signaling in neurons, highlighting examples wherein the fundamental cell biological processes of protein synthesis and degradation, membrane trafficking, and organelle transport are employed to enable the encoding and integration of information, locally and globally within a neuron. Comparisons to other cell types indicate that neurons accentuate widely shared mechanisms, providing invaluable models for the compartmentalization and transfer mechanisms required and used by most eukaryotic cells.

The more, the better: the BICD family gets bigger.

Balancing the flow of organelles and molecular complexes to and from the cell periphery is very important, and this is especially true for neurons, which are an extreme example of polarised cells. Neurons need to precisely regulate anterograde and retrograde transport of many cargoes such as mRNA, mitochondria and signalling molecules in time and space. Hence, there is a great deal of interest in identifying adaptor proteins controlling the association of molecular motors with different cargoes. One such class of adaptors is the Bicaudal‐D (BICD) protein family. This issue of the EMBO Journal presents a study by Schlager et al , which describes two new members of the BICD family, Bicaudal‐D‐related protein‐1 and 2 (BICDR‐1 and BICDR‐2), and identifies an essential role for BICDR‐1 in neurons. Bicaudal‐D ( BICD ) (meaning ‘two tails’) was first identified in Drosophila and named after the striking phenotype of its mutant, where embryos have an anterior‐to‐posterior transformation. BICD is an adaptor for dynein‐dependent transport along microtubules. Only one BICD gene is present …

siRNA screen of ES cell-derived motor neurons identifies novel regulators of tetanus toxin and neurotrophin receptor trafficking

Neurons rely on the long-range transport of several signaling molecules such as neurotrophins and their receptors, which are required for neuronal development, function and survival. However, the nature of the machinery controlling the trafficking of signaling endosomes containing activated neurotrophin receptors has not yet been completely elucidated. We aimed to identify new players involved in the dynamics of neurotrophin signaling endosomes using a medium-throughput unbiased siRNA screening approach to quantify the intracellular accumulation of two fluorescently tagged reporters: the binding fragment of tetanus neurotoxin (HCT), and an antibody directed against the neurotrophin receptor p75NTR. This screen performed in motor neurons differentiated from mouse embryonic stem (ES) cells identified a number of candidate genes encoding molecular motors and motor adaptor proteins involved in regulating the intracellular trafficking of these probes. Bicaudal D homolog 1 (BICD1), a molecular motor adaptor with pleiotropic roles in intracellular trafficking, was selected for further analyses, which revealed that BICD1 regulates the intracellular trafficking of HCT and neurotrophin receptors and likely plays an important role in nervous system development and function.

Translating regeneration: Local protein synthesis in the neuronal injury response

Neurons convey signals over long distances, for example motor neurons and sensory neurons project axons up to a meter long in humans. To this end, a sophisticated network of long-range signaling mechanisms enables communication between neuronal processes and somata. These mechanisms are activated during axonal injury and have essential roles both for sensing the injury and regulating subsequent regeneration. Here we survey the role of one such mechanism, axonal translation, which contributes to both retrograde injury signaling and as a source of proteins for regenerating axons. The nature of the axonal synthesis machinery has become progressively clearer over the past decade. A large number of axonally localized mRNAs have been identified, which cover a wide spectrum of protein families; and axonal ribosomes have been detected, even though their origin is still subject to debate. Various kinase pathways, most prominently mTOR, have been implicated in driving local translation in axons. Finally, new technologies are becoming available to visualize axonal translation and enable proteomic analyses. These technological improvements offer new avenues towards comprehensive characterization of the axonal translational machinery.