Pincher-mediated macroendocytosis underlies retrograde signaling by neurotrophin receptors

Syntaxin-1 is necessary for UNC5A-C/Netrin-1-dependent macropinocytosis and chemorepulsion

Introduction

Brain connectivity requires correct axonal guidance to drive axons to their appropriate targets. This process is orchestrated by guidance cues that exert attraction or repulsion to developing axons. However, the intricacies of the cellular machinery responsible for the correct response of growth cones are just being unveiled. Netrin-1 is a bifunctional molecule involved in axon pathfinding and cell migration that induces repulsion during postnatal cerebellar development. This process is mediated by UNC5 homolog receptors located on external granule layer (EGL) tracts.

Methods

Biochemical, imaging and cell biology techniques, as well as syntaxin-1A/B (Stx1A/B) knock-out mice were used in primary cultures and brain explants.

Results and discussion

Here, we demonstrate that this response is characterized by enhanced membrane internalization through macropinocytosis, but not clathrin-mediated endocytosis. We show that UNC5A, UNC5B, and UNC5C receptors form a protein complex with the t-SNARE syntaxin-1. By combining botulinum neurotoxins, an shRNA knock-down strategy and Stx1 knock-out mice, we demonstrate that this SNARE protein is required for Netrin1-induced macropinocytosis and chemorepulsion, suggesting that Stx1 is crucial in regulating Netrin-1-mediated axonal guidance.

The Neuron Navigators: Structure, function, and evolutionary history

Neuron navigators (Navigators) are cytoskeletal-associated proteins important for neuron migration, neurite growth, and axon guidance, but they also function more widely in other tissues. Recent studies have revealed novel cellular functions of Navigators such as macropinocytosis, and have implicated Navigators in human disorders of axon growth. Navigators are present in most or all bilaterian animals: vertebrates have three Navigators (NAV1-3), Drosophila has one (Sickie), and Caenorhabditis elegans has one (Unc-53). Structurally, Navigators have conserved N- and C-terminal regions each containing specific domains. The N-terminal region contains a calponin homology (CH) domain and one or more SxIP motifs, thought to interact with the actin cytoskeleton and mediate localization to microtubule plus-end binding proteins, respectively. The C-terminal region contains two coiled-coil domains, followed by a AAA+ family nucleoside triphosphatase domain of unknown activity. The Navigators appear to have evolved by fusion of N- and C-terminal region homologs present in simpler organisms. Overall, Navigators participate in the cytoskeletal response to extracellular cues via microtubules and actin filaments, in conjunction with membrane trafficking. We propose that uptake of fluid-phase cues and nutrients and/or downregulation of cell surface receptors could represent general mechanisms that explain Navigator functions. Future studies developing new models, such as conditional knockout mice or human cerebral organoids may reveal new insights into Navigator function. Importantly, further biochemical studies are needed to define the activities of the Navigator AAA+ domain, and to study potential interactions among different Navigators and their binding partners.

Cryo-electron tomography reveals structural insights into the membrane remodeling mode of dynamin-like EHD filaments

Eps15-homology domain containing proteins (EHDs) are eukaryotic, dynamin-related ATPases involved in cellular membrane trafficking. They oligomerize on membranes into filaments that induce membrane tubulation. While EHD crystal structures in open and closed conformations were previously reported, little structural information is available for the membrane-bound oligomeric form. Consequently, mechanistic insights into the membrane remodeling mechanism have remained sparse. Here, by using cryo-electron tomography and subtomogram averaging, we determined structures of nucleotide-bound EHD4 filaments on membrane tubes of various diameters at an average resolution of 7.6 Å. Assembly of EHD4 is mediated via interfaces in the G-domain and the helical domain. The oligomerized EHD4 structure resembles the closed conformation, where the tips of the helical domains protrude into the membrane. The variation in filament geometry and tube radius suggests a spontaneous filament curvature of approximately 1/70 nm^-1. Combining the available structural and functional data, we suggest a model for EHD-mediated membrane remodeling.

Growth cone macropinocytosis of neurotrophin receptor and neuritogenesis are regulated by neuron navigator 1

Neuron navigator 1 (Nav1) is a cytoskeleton-associated protein expressed during brain development that is necessary for proper neuritogenesis, but the underlying mechanisms are poorly understood. Here we show that Nav1 is present in elongating axon tracts during mouse brain embryogenesis. We found that depletion of Nav1 in cultured neurons disrupts growth cone morphology and neurotrophin-stimulated neuritogenesis. In addition to regulating both F-actin and microtubule properties, Nav1 promotes actin-rich membrane ruffles in the growth cone and promotes macropinocytosis at those membrane ruffles, including internalization of the TrkB receptor for the neurotrophin brain-derived neurotropic factor (BDNF). Growth cone macropinocytosis is important for downstream signaling, neurite targeting, and membrane recycling, implicating Nav1 in one or more of these processes. Depletion of Nav1 also induces transient membrane blebbing via disruption of signaling in the Rho GTPase signaling pathway, supporting the novel role of Nav1 in dynamic actin-based membrane regulation at the cell periphery. These data demonstrate that Nav1 works at the interface of microtubules, actin, and plasma membrane to organize the cell periphery and promote uptake of growth and guidance cues to facilitate neural morphogenesis during development.

Mechanisms for Regulating and Organizing Receptor Signaling by Endocytosis

Intricate relationships between endocytosis and cellular signaling, first recognized nearly 40 years ago through the study of tyrosine kinase growth factor receptors, are now known to exist for multiple receptor classes and to affect myriad physiological and developmental processes. This review summarizes our present understanding of how endocytosis orchestrates cellular signaling networks, with an emphasis on mechanistic underpinnings and focusing on two receptor classes-tyrosine kinase and G protein-coupled receptors-that have been investigated in particular detail. Together, these examples provide a useful survey of the current consensus, uncertainties, and controversies in this rapidly advancing area of cell biology.

TrkA-mediated endocytosis of p75-CTF prevents cholinergic neuron death upon γ-secretase inhibition

The findings shed light into the adverse effects of GSIs observed in the Alzheimer’s field and explain, at least in part, the unexpected worsening in cognition observed in the semagacestat Phase 3 trial.

γ-secretase inhibitors (GSI) were developed to reduce the generation of Aβ peptide to find new Alzheimer’s disease treatments. Clinical trials on Alzheimer’s disease patients, however, showed several side effects that worsened the cognitive symptoms of the treated patients. The observed side effects were partially attributed to Notch signaling. However, the effect on other γ-secretase substrates, such as the p75 neurotrophin receptor (p75NTR) has not been studied in detail. p75NTR is highly expressed in the basal forebrain cholinergic neurons (BFCNs) during all life. Here, we show that GSI treatment induces the oligomerization of p75CTF leading to the cell death of BFCNs, and that this event is dependent on TrkA activity. The oligomerization of p75CTF requires an intact cholesterol recognition sequence (CRAC) and the constitutive binding of TRAF6, which activates the JNK and p38 pathways. Remarkably, TrkA rescues from cell death by a mechanism involving the endocytosis of p75CTF. These results suggest that the inhibition of γ-secretase activity in aged patients, where the expression of TrkA in the BFCNs is already reduced, could accelerate cholinergic dysfunction and promote neurodegeneration.

NGF Signaling in Endosomes

During the development of the nervous system, neurons respond to diffusible cues secreted by target cells. Because such target-derived factors regulate development, maturation, and maintenance of axons as well as somatodendritic compartments, signals initiated at distal axons must be retrogradely transmitted toward cell bodies. Neurotrophins, including the nerve growth factor (NGF), provide one of the best-known examples of target-derived growth factors. The cell biological processes of endocytosis and retrograde trafficking of their Trk receptors from growth cones to cell bodies are key mechanisms by which target-derived neurotrophins influence neurons. Evidence accumulated over the past several decades has begun to uncover the molecular mechanisms of formation, transport, and biological functions of these specialized endosomes called "signaling endosomes."

Cellular functions and intrinsic attributes of the ATP-binding Eps15 homology domain-containing proteins

Several cellular processes rely on a cohort of dedicated proteins that manage tubulation, fission, and fusion of membranes. A notably large number of them belong to the dynamin superfamily of proteins. Among them is the evolutionarily conserved group of ATP-binding Eps15-homology domain-containing proteins (EHDs). In the two decades since their discovery, EHDs have been linked to a range of cellular processes that require remodeling or maintenance of specific membrane shapes such as during endocytic recycling, caveolar biogenesis, ciliogenesis, formation of T-tubules in skeletal muscles, and membrane resealing after rupture. Recent work has shed light on their structure and the unique attributes they possess in linking ATP hydrolysis to membrane remodeling. This review summarizes some of these recent developments and reconciles intrinsic protein functions to their cellular roles.

EHD1 and RUSC2 Control Basal Epidermal Growth Factor Receptor Cell Surface Expression and Recycling

Epidermal growth factor receptor (EGFR) is a prototype receptor tyrosine kinase and an oncoprotein in many solid tumors. Cell surface display of EGFR is essential for cellular responses to its ligands. While postactivation endocytic trafficking of EGFR has been well elucidated, little is known about mechanisms of basal/preactivation surface display of EGFR. Here, we identify a novel role of the endocytic regulator EHD1 and a potential EHD1 partner, RUSC2, in cell surface display of EGFR. EHD1 and RUSC2 colocalize with EGFR in vesicular/tubular structures and at the Golgi compartment. Inducible EHD1 knockdown reduced the cell surface EGFR expression with accumulation at the Golgi compartment, a phenotype rescued by exogenous EHD1. RUSC2 knockdown phenocopied the EHD1 depletion effects. EHD1 or RUSC2 depletion impaired the EGF-induced cell proliferation, demonstrating that the novel, EHD1- and RUSC2-dependent transport of unstimulated EGFR from the Golgi compartment to the cell surface that we describe is functionally important, with implications for physiologic and oncogenic roles of EGFR and targeted cancer therapies.

Fibroblast Growth Factor 2-Mediated Regulation of Neuronal Exosome Release Depends on VAMP3/Cellubrevin in Hippocampal Neurons

Extracellular vesicles (EVs) are endogenous membrane-derived vesicles that shuttle bioactive molecules between glia and neurons, thereby promoting neuronal survival and plasticity in the central nervous system (CNS) and contributing to neurodegenerative conditions. Although EVs hold great potential as CNS theranostic nanocarriers, the specific molecular factors that regulate neuronal EV uptake and release are currently unknown. A combination of patch-clamp electrophysiology and pH-sensitive dye imaging is used to examine stimulus-evoked EV release in individual neurons in real time. Whereas spontaneous electrical activity and the application of a high-frequency stimulus induce a slow and prolonged fusion of multivesicular bodies (MVBs) with the plasma membrane (PM) in a subset of cells, the neurotrophic factor basic fibroblast growth factor (bFGF) greatly increases the rate of stimulus-evoked MVB-PM fusion events and, consequently, the abundance of EVs in the culture medium. Proteomic analysis of neuronal EVs demonstrates bFGF increases the abundance of the v-SNARE vesicle-associated membrane protein 3 (VAMP3, cellubrevin) on EVs. Conversely, knocking-down VAMP3 in cultured neurons attenuates the effect of bFGF on EV release. The results determine the temporal characteristics of MVB-PM fusion in hippocampal neurons and reveal a new function for bFGF signaling in controlling neuronal EV release.

Localization of phosphotyrosine adaptor protein ShcD/SHC4 in the adult rat central nervous system

Background

Mammalian Shc (Src homology and collagen) proteins comprise a family of four phosphotyrosine adaptor molecules which exhibit varied spatiotemporal expression and signaling functions. ShcD is the most recently discovered homologue and it is highly expressed in the developing central nervous system (CNS) and adult brain. Presently however, its localization within specific cell types of mature neural structures has yet to be characterized.

Results

In the current study, we examine the expression profile of ShcD in the adult rat CNS using immunohistochemistry, and compare with those of the neuronally enriched ShcB and ShcC proteins. ShcD shows relatively widespread distribution in the adult brain and spinal cord, with prominent levels of staining throughout the olfactory bulb, as well as in sub-structures of the cerebellum and hippocampus, including the subgranular zone. Co-localization studies confirm the expression of ShcD in mature neurons and progenitor cells. ShcD immunoreactivity is primarily localized to axons and somata, consistent with the function of ShcD as a cytoplasmic adaptor. Regional differences in expression are observed among neural Shc proteins, with ShcC predominating in the hippocampus, cerebellum, and some fiber tracts. Interestingly, ShcD is uniquely expressed in the olfactory nerve layer and in glomeruli of the main olfactory bulb.

Conclusions

Together our findings suggest that ShcD may provide a distinct signaling contribution within the olfactory system, and that overlapping expression of ShcD with other Shc proteins may allow compensatory functions in the brain.

Pharmacologically diverse antidepressants facilitate TRKB receptor activation by disrupting its interaction with the endocytic adaptor complex AP-2

Several antidepressant drugs activate tropomyosin-related kinase B (TRKB) receptor, but it remains unclear whether these compounds employ a common mechanism for TRKB activation. Here, using MS, we found that a single intraperitoneal injection of fluoxetine disrupts the interaction of several proteins with TRKB in the hippocampus of mice. These proteins included members of adaptor protein complex-2 (AP-2) involved in vesicular endocytosis. The interaction of TRKB with the cargo-docking μ subunit of the AP-2 complex (AP2M) was confirmed to be disrupted by both acute and repeated fluoxetine treatments. Of note, fluoxetine disrupted the coupling between full-length TRKB and AP2M, but not the interaction between AP2M and the TRKB C-terminal region, indicating that the fluoxetine-binding site in TRKB lies outside the TRKB:AP2M interface. ELISA experiments revealed that in addition to fluoxetine, other chemically diverse antidepressants, such as imipramine, rolipram, phenelzine, ketamine, and its metabolite 2R,6R-hydroxynorketamine, also decreased the interaction between TRKB and AP2M in vitro Silencing the expression of AP2M in a TRKB-expressing mouse fibroblast cell line (MG87.TRKB) increased cell-surface expression of TRKB and facilitated its activation by brain-derived neurotrophic factor (BDNF), observed as levels of phosphorylated TRKB. Moreover, animals haploinsufficient for the Ap2m1 gene displayed increased levels of active TRKB, along with enhanced cell-surface expression of the receptor in cultured hippocampal neurons. Taken together, our results suggest that disruption of the TRKB:AP2M interaction is a common mechanism underlying TRKB activation by several chemically diverse antidepressants.

SIPA1L2 controls trafficking and local signaling of TrkB-containing amphisomes at presynaptic terminals

Amphisomes are organelles of the autophagy pathway that result from the fusion of autophagosomes with late endosomes. While biogenesis of autophagosomes and late endosomes occurs continuously at axon terminals, non-degradative roles of autophagy at boutons are barely described. Here, we show that in neurons BDNF/TrkB traffick in amphisomes that signal locally at presynaptic boutons during retrograde transport to the soma. This is orchestrated by the Rap GTPase-activating (RapGAP) protein SIPA1L2, which connects TrkB amphisomes to a dynein motor. The autophagosomal protein LC3 regulates RapGAP activity of SIPA1L2 and controls retrograde trafficking and local signaling of TrkB. Following induction of presynaptic plasticity, amphisomes dissociate from dynein at boutons enabling local signaling and promoting transmitter release. Accordingly, sipa1l2 knockout mice show impaired BDNF-dependent presynaptic plasticity. Taken together, the data suggest that in hippocampal neurons, TrkB-signaling endosomes are in fact amphisomes that during retrograde transport have local signaling capacity in the context of presynaptic plasticity.

There is growing evidence that autophagy might serve specialized functions in neurons besides its role in protein homeostasis. In this study, authors demonstrate that axonal retrograde transport of BDNF/TrkB in neuronal amphisomes is involved in plasticity-relevant local signaling at presynaptic boutons and that SIPA1L2, a member of the SIPA1L family of neuronal RapGAPs, associates via LC3b to TrkB-containing amphisomes to regulate its motility and signaling at the axon terminals

Retrograde transport of Akt by a neuronal Rab5-APPL1 endosome

Long-distance axonal trafficking plays a critical role in neuronal function and transport defects have been linked to neurodegenerative disorders. Various lines of evidence suggest that the small GTPase Rab5 plays a role in neuronal signaling via early endosomal transport. Here, we characterized the motility of Rab5 endosomes in primary cultures of mouse hippocampal pyramidal cells by live-cell imaging and showed that they exhibit bi-directional long-range motility in axons, with a strong bias toward retrograde transport. Characterization of key Rab5 effectors revealed that endogenous Rabankyrin-5, Rabenosyn-5 and APPL1 are all present in axons. Further analysis of APPL1-positive endosomes showed that, similar to Rab5-endosomes, they display more frequent long-range retrograde than anterograde movement, with the endosomal levels of APPL1 correlated with faster retrograde movement. Interestingly, APPL1-endosomes transport the neurotrophin receptor TrkB and mediate retrograde axonal transport of the kinase Akt1. FRET analysis revealed that APPL1 and Akt1 interact in an endocytosis-dependent manner. We conclude that Rab5-APPL1 endosomes exhibit the hallmarks of axonal signaling endosomes to transport Akt1 in hippocampal pyramidal cells.

The many disguises of the signalling endosome

Neurons are highly complex and polarised cells that must overcome a series of logistic challenges to maintain homeostasis across their morphological domains. A very clear example is the propagation of neurotrophic signalling from distal axons, where target-released neurotrophins bind to their receptors and initiate signalling, towards the cell body, where nuclear and cytosolic responses are integrated. The mechanisms of propagation of neurotrophic signalling have been extensively studied and, eventually, the model of a 'signalling endosome', transporting activated receptors and associated complexes, has emerged. Nevertheless, the exact nature of this organelle remains elusive. In this Review, we examine the evidence for the retrograde transport of neurotrophins and their receptors in endosomes, outline some of their diverse physiological and pathological roles, and discuss the main interactors, morphological features and trafficking destinations of a highly flexible endosomal signalling organelle with multiple molecular signatures.

Tropomyosin-Related Kinase B (TrkB) Regulates Neurite Outgrowth via a Novel Interaction with Suppressor of Cytokine Signalling 2 (SOCS2)

Brain-derived neurotrophic factor (BDNF) is highly expressed in the hippocampus, where it can initiate signalling pathways leading to neurite outgrowth, neuron survival, spine maturation and increased synapse strength. Although suppressor of cytokine signalling 2 (SOCS2) is primarily known to negatively regulate cytokine signalling, it is also highly expressed in the hippocampus and exerts neuron-specific functions in the brain, effecting the length and architecture of neurons. However, little is known about the role of SOCS2 in the hippocampus. In this study, we hypothesised that SOCS2 may have a regulatory role in BDNF-dependent neurite growth and hippocampal neuronal function. Here our data demonstrate that SOCS2 interacts with the kinase domain of the BDNF receptor TrkB. Germline overexpression of SOCS2 results in a BDNF-dependent increase in hippocampal neurite outgrowth, whereas deletion of SOCS2 results in shorter neurite outgrowth. Expression of SOCS2 also results in increased ubiquitination of the juxtamembrane region of TrkB, and alters the trafficking of TrkB into recycling endosomes. Collectively, our data suggest a novel role for SOCS2 in interacting with and regulating the trafficking of TrkB, leading to increased neurite outgrowth in hippocampus neurons.

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 neurotrophin receptor signaling endosome: Where trafficking meets signaling

Neurons are the largest cells in the body and form subcellular compartments such as axons and dendrites. During both development and adulthood building blocks must be continually trafficked long distances to maintain the different regions of the neuron. Beyond building blocks, signaling complexes are also transported, allowing for example, axons to communicate with the soma. The critical roles of signaling via ligand-receptor complexes is perhaps best illustrated in the context of development, where they are known to regulate polarization, survival, axon outgrowth, dendrite development, and synapse formation. However, knowing 'when' and 'how much' signaling is occurring does not provide the complete story. The location of signaling has a significant impact on the functional outcomes. There are therefore complex and functionally important trafficking mechanisms in place to control the precise spatial and temporal aspects of many signal transduction events. In turn, many of these signaling events affect trafficking mechanisms, setting up an intricate connection between trafficking and signaling. In this review we will use neurotrophin receptors, specifically TrkA and TrkB, to illustrate the cell biology underlying the links between trafficking and signaling. Briefly, we will discuss the concepts of how trafficking and signaling are intimately linked for functional and diverse signaling outputs, and how the same protein can play different roles for the same receptor depending on its localization. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.

Expression and Purification of Neurotrophin-Elastin-Like Peptide Fusion Proteins for Neural Regeneration

BACKGROUND

Neural injuries such as spinal cord injuries, traumatic brain injuries, or nerve transection injuries pose a major health problem. Neurotrophins such as nerve growth factor (NGF) or brain-derived neurotrophic factor (BDNF) have been shown to improve the outcome of neural injuries in several pre-clinical models, but their use in clinics is limited by the lack of a robust delivery system that enhances their bioavailability and half-life.

OBJECTIVES

We describe two fusion proteins comprising NGF or BDNF fused with elastin-like peptides (ELPs). The aim of this study was to investigate the biological activity of neurotrophin-ELP (N-ELP) fusion proteins via in vitro culture models.

METHODS

NGF and BDNF were cloned in front of an elastin-like polypeptide sequence V40C2. These proteins were expressed in bacteria as inclusion bodies. These fusion proteins underwent solubilization via 8 M urea and purification via inverse transition cycling (ITC). We measured the particle size and the effect of temperature on precipitated particles using dynamic light scattering (DLS). We used western blot analysis to confirm the specificity of NGF-ELP to tropomyosin receptor kinase A (TrkA) antibody and to confirm the specificity of BDNF-ELP to TrkB antibody. PC12 cells were used to perform a neurite outgrowth assay to determine the biological activity of NGF-ELP. Bioactivity of BDNF-ELP was ascertained via transfecting human epithelial kidney (HEK 293-T) cells to express the TrkB receptor.

RESULTS

The proteins were successfully purified to high homogeneity by exploiting the phase transition property of ELPs and urea, which solubilize inclusion bodies. Using PC12 neurite outgrowth assay, we further demonstrated that the biological activity of NGF was retained in the fusion. Similarly, BDNF-ELP phosphorylated the TrkB receptor, suggesting the biological activity of BDNF was also retained in the fusion. We further show that owing to the phase transition property of ELPs in the fusion, these proteins self-assembled into nanoparticles at their respective transition temperatures.

CONCLUSION

These fusion proteins are useful for neural regeneration, as they not only retain the biological activity of the neurotrophin but also self-assemble into nanoparticles, thereby simultaneously serving as drug-delivery vehicles. These nanoparticles can serve as drug depots and will increase bioavailability by limiting neurotrophin loss due to diffusion, thereby allowing controlled spatio-temporal delivery of the neurotrophin.

Unravelling the Mechanism of TrkA-Induced Cell Death by Macropinocytosis in Medulloblastoma Daoy Cells

Macropinocytosis is a normal cellular process by which cells internalize extracellular fluids and nutrients from their environment and is one strategy that Ras-transformed pancreatic cancer cells use to increase uptake of amino acids to meet the needs of rapid growth. Paradoxically, in non-Ras transformed medulloblastoma brain tumors, we have shown that expression and activation of the receptor tyrosine kinase TrkA overactivates macropinocytosis, resulting in the catastrophic disintegration of the cell membrane and in tumor cell death. The molecular basis of this uncontrolled form of macropinocytosis has not been previously understood. Here, we demonstrate that the overactivation of macropinocytosis is caused by the simultaneous activation of two TrkA-mediated pathways: (i) inhibition of RhoB via phosphorylation at Ser(185) by casein kinase 1, which relieves actin stress fibers, and (ii) FRS2-scaffolded Src and H-Ras activation of RhoA, which stimulate actin reorganization and the formation of lamellipodia. Since catastrophic macropinocytosis results in brain tumor cell death, improved understanding of the mechanisms involved will facilitate future efforts to reprogram tumors, even those resistant to apoptosis, to die.

Retrolinkin recruits the WAVE1 protein complex to facilitate BDNF-induced TrkB endocytosis and dendrite outgrowth

Retrolinkin, a neuronal membrane protein, coordinates with endophilin A1 and mediates early endocytic trafficking and signal transduction of the ligand-receptor complex formed between brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin-related kinase B (TrkB), in dendrites of CNS neurons. Here we report that retrolinkin interacts with the CYFIP1/2 subunit of the WAVE1 complex, a member of the WASP/WAVE family of nucleation-promoting factors that binds and activates the Arp2/3 complex to promote branched actin polymerization. WAVE1, not N-WASP, is required for BDNF-induced TrkB endocytosis and dendrite outgrowth. Disruption of the interaction between retrolinkin and CYFIP1/2 impairs recruitment of WAVE1 to neuronal plasma membrane upon BDNF addition and blocks internalization of activated TrkB. We also show that WAVE1-mediated endocytosis of BDNF-activated TrkB is actin dependent and clathrin independent. These results not only reveal the mechanistic role of retrolinkin in BDNF-TrkB endocytosis, but also indicate that WASP/WAVE-dependent actin polymerization during endocytosis is regulated by cell type-specific and cargo-specific modulators.

BDNF trafficking and signaling impairment during early neurodegeneration is prevented by moderate physical activity

Physical exercise can attenuate the effects of aging on the central nervous system by increasing the expression of neurotrophins such as brain-derived neurotrophic factor (BDNF), which promotes dendritic branching and enhances synaptic machinery, through interaction with its receptor TrkB. TrkB receptors are synthesized in the cell body and are transported to the axonal terminals and anchored to plasma membrane, through SLP1, CRMP2 and Rab27B, associated with KIF1B. Retrograde trafficking is made by EDH-4 together with dynactin and dynein molecular motors. In the present study it was found that early neurodegeneration is accompanied by decrease in BDNF signaling, in the absence of hyperphosphorylated tau aggregation, in hippocampus of 11 months old Lewis rats exposed to rotenone. It was also demonstrated that moderate physical activity (treadmill running, during 6 weeks, concomitant to rotenone exposure) prevents the impairment of BDNF system in aged rats, which may contribute to delay neurodegeneration. In conclusion, decrease in BDNF and TrkB vesicles occurs before large aggregate-like p-Tau are formed and physical activity applied during early neurodegeneration may be of relevance to prevent BDNF system decay.

Loss of the transcription factor Meis1 prevents sympathetic neurons target-field innervation and increases susceptibility to sudden cardiac death

Although cardio-vascular incidents and sudden cardiac death (SCD) are among the leading causes of premature death in the general population, the origins remain unidentified in many cases. Genome-wide association studies have identified Meis1 as a risk factor for SCD. We report that Meis1 inactivation in the mouse neural crest leads to an altered sympatho-vagal regulation of cardiac rhythmicity in adults characterized by a chronotropic incompetence and cardiac conduction defects, thus increasing the susceptibility to SCD. We demonstrated that Meis1 is a major regulator of sympathetic target-field innervation and that Meis1 deficient sympathetic neurons die by apoptosis from early embryonic stages to perinatal stages. In addition, we showed that Meis1 regulates the transcription of key molecules necessary for the endosomal machinery. Accordingly, the traffic of Rab5⁺ endosomes is severely altered in Meis1-inactivated sympathetic neurons. These results suggest that Meis1 interacts with various trophic factors signaling pathways during postmitotic neurons differentiation.

DOI:http://dx.doi.org/10.7554/eLife.11627.001

Nerve cells called sympathetic neurons can control the activity of almost all of our organs without any conscious thought on our part. For example, these nerve cells are responsible for accelerating the heart rate during exercise.

In a developing embryo, there are initially more of these neurons than are needed, and only those that develop correctly and form a connection with a target cell will survive. This is because the target cells provide the growing neurons with vital molecules called neurotrophins, which are trafficked back along the nerve fiber and into the main body of the nerve cell to ensure its survival. However, it is largely unknown which proteins or genes are also involved in this developmental process.

Now, Bouilloux, Thireau et al. show that if a gene called Meis1 is inactivated in mice, the sympathetic neurons start to develop and grow nerve fibers, but then fail to establish connections with their target cells and finally die. The Meis1 gene encodes a transcription factor, which is a protein that regulates gene activity. Therefore, Bouilloux, Thireau et al. looked for the genes that are regulated by this transcription factor in sympathetic neurons. This search uncovered several genes that are involved in the packaging and trafficking of molecules within cells.

Other experiments then revealed that the trafficking of molecules back along the nerve fiber was altered in mutant neurons in which the Meis1 gene had been inactivated. Furthermore, Meis1 mutant mice had problems with their heart rate, especially during exercise, and an increased risk of dying from a sudden cardiac arrest.

These findings reveal a transcription factor that helps to establish a connection between a neuron and its target, and that activates a pattern of gene expression that works alongside the neurotrophin-based signals. Since all neurons undergo similar processes during development, future work could ask if comparable patterns of gene expression exist in other types of neurons, and if problems with such processes contribute to some neurodegenerative diseases.

DOI:http://dx.doi.org/10.7554/eLife.11627.002

Post-CNS-inflammation expression of CXCL12 promotes the endogenous myelin/neuronal repair capacity following spontaneous recovery from multiple sclerosis-like disease

BACKGROUND

Demyelination and axonal degeneration, hallmarks of multiple sclerosis (MS), are associated with the central nervous system (CNS) inflammation facilitated by C-X-C motif chemokine 12 (CXCL12) chemokine. Both in MS and in experimental autoimmune encephalomyelitis (EAE), the deleterious CNS inflammation has been associated with upregulation of CXCL12 expression in the CNS. We investigated the expression dynamics of CXCL12 in the CNS with progression of clinical EAE and following spontaneous recovery, with a focus on CXCL12 expression in the hippocampal neurogenic dentate gyrus (DG) and in the corpus callosum (CC) of spontaneously recovered mice, and its potential role in promoting the endogenous myelin/neuronal repair capacity.

METHODS

CNS tissue sections from mice with different clinical EAE phases or following spontaneous recovery and in vitro differentiated adult neural stem cell cultures were analyzed by immunofluorescent staining and confocal imaging for detecting and enumerating neuronal progenitor cells (NPCs) and oligodendrocyte precursor cells (OPCs) and for expression of CXCL12.

RESULTS

Our expression dynamics analysis of CXCL12 in the CNS with EAE progression revealed elevated CXCL12 expression in the DG and CC, which persistently increases following spontaneous recovery even though CNS inflammation has subsided. Correspondingly, the numbers of NPCs and OPCs in the DG and CC, respectively, of EAE-recovered mice increased compared to that of naïve mice (NPCs, p < 0.0001; OPCs, p < 0.00001) or mice with active disease (OPCs, p < 0.0005). Notably, about 30 % of the NPCs and unexpectedly also OPCs (~50 %) express CXCL12, and their numbers in DG and CC, respectively, are higher in EAE-recovered mice compared with naïve mice and also compared with mice with ongoing clinical EAE (CXCL12(+) NPCs, p < 0.005; CXCL12(+) OPCs, p < 0.0005). Moreover, a significant proportion (>20 %) of the CXCL12(+) NPCs and OPCs co-express the CXCL12 receptor, CXCR4, and their numbers significantly increase with recovery from EAE not only relative to naïve mice (p < 0.0002) but also to mice with ongoing EAE (p < 0.004).

CONCLUSIONS

These data link CXCL12 expression in the DG and CC of EAE-recovering mice to the promotion of neuro/oligodendrogenesis generating CXCR4(+) CXCL12(+) neuronal and oligodendrocyte progenitor cells endowed with intrinsic neuro/oligondendroglial differentiation potential. These findings highlight the post-CNS-inflammation role of CXCL12 in augmenting the endogenous myelin/neuronal repair capacity in MS-like disease, likely via CXCL12/CXCR4 autocrine signaling.

Membrane turnover and receptor trafficking in regenerating axons

Peripheral axonal regeneration requires surface-expanding membrane addition. The continuous incorporation of new membranes into the axolemma allows the pushing force of elongating microtubules to drive axonal growth cones forwards. Hence, a constant supply of membranes and cytoskeletal building blocks is required, often for many weeks. In human peripheral nerves, axonal tips may be more than 1 m away from the neuronal cell body. Therefore, in the initial phase of regeneration, membranes are derived from pre-existing vesicles or synthesised locally. Only later stages of axonal regeneration are supported by membranes and proteins synthesised in neuronal cell bodies, considering that the fastest anterograde transport mechanisms deliver cargo at 20 cm/day. Whereas endocytosis and exocytosis of membrane vesicles are balanced in intact axons, membrane incorporation exceeds membrane retrieval during regeneration to compensate for the loss of membranes distal to the lesion site. Physiological membrane turnover rates will not be established before the completion of target reinnervation. In this review, the current knowledge on membrane traffic in axonal outgrowth is summarised, with a focus on endosomal vesicles as the providers of membranes and carriers of growth factor receptors required for initiating signalling pathways to promote the elongation and branching of regenerating axons in lesioned peripheral nerves.

Increased cortical synaptic activation of TrkB and downstream signaling markers in a mouse model of Down Syndrome

Down Syndrome (DS), trisomy 21, is characterized by synaptic abnormalities and cognitive deficits throughout the lifespan and with development of Alzheimer's disease (AD) neuropathology and progressive cognitive decline in adults. Synaptic abnormalities are also present in the Ts65Dn mouse model of DS, but which synapses are affected and the mechanisms underlying synaptic dysfunction are unknown. Here we show marked increases in the levels and activation status of TrkB and associated signaling proteins in cortical synapses in Ts65Dn mice. Proteomic analysis at the single synapse level of resolution using array tomography (AT) uncovered increased colocalization of activated TrkB with signaling endosome related proteins, and demonstrated increased TrkB signaling. The extent of increases in TrkB signaling differed in each of the cortical layers examined and with respect to the type of synapse, with the most marked increases seen in inhibitory synapses. These findings are evidence of markedly abnormal TrkB-mediated signaling in synapses. They raise the possibility that dysregulated TrkB signaling contributes to synaptic dysfunction and cognitive deficits in DS.

The deubiquitinating enzyme UBPy/USP8 interacts with TrkA and inhibits neuronal differentiation in PC12 cells

The tropomyosin-related kinase (Trk) family of receptor tyrosine kinases controls synaptic function, plasticity and sustains differentiation, morphology, and neuronal cell survival. Understanding Trk receptors down-regulation and recycling is a crucial step to point out sympathetic and sensory neuron function and survival. PC12 cells derived from pheochromocytoma of the rat adrenal medulla have been widely used as a model system for studies of neuronal differentiation as they respond to nerve growth factor (NGF) with a dramatic change in phenotype and acquire a number of properties characteristic of sympathetic neurons. In this study we demonstrated that in PC12 cells the TrkA receptor interacts with the deubiquitinating enzyme USP8/UBPy in a NGF-dependent manner and that it is deubiquitinated in vivo and in vitro by USP8. USP8 overexpression blocked NGF-induced neurites outgrowth while the overexpression of the catalytically inactive mutant USP8/UBPy(C748A) caused a marked increase of cell differentiation. Localization and biochemical experiments have point out that USP8 and TrkA partially co-localize in endosomes after NGF stimulation. Finally we have studied the role played by USP8 on TrkA turnover; using specific siRNA for USP8 we found that USP8 knockdown increases TrkA half-life, suggesting that the deubiquitinating activity of USP8 promotes TrkA degradation.

Biogenesis and function of the NGF/TrkA signaling endosome

Target-derived neurotrophin nerve growth factor (NGF) and its receptor TrkA are well known for retrograde signaling to promote survival and innervation of sympathetic and sensory neurons. In recent years, the signaling endosome model has been used to describe the sustained NGF/TrkA retrograde signaling as a process of endocytosis and retrograde transport of NGF/TrkA-containing endosomes from the axon terminal to the cell body for activation of NGF-inducible gene expression responsible for neuronal survival and development. Here, we review the biogenesis and function of NGF, TrkA, and the signaling endosome and discuss possible roles of Rab GTPases in the biogenesis and trafficking of signaling endosomes.

Delivery of CSF-1R to the lumen of macropinosomes promotes its destruction in macrophages

Activation of the macrophage colony stimulating factor-1 receptor (CSF-1R) by CSF-1 stimulates pronounced macropinocytosis and drives proliferation of macrophages. Although the role of macropinocytosis in CSF-1R signaling remains unknown, we show here that, despite internalizing large quantities of plasma membrane, macropinosomes contribute little to the internalization of the CSF-1-CSF-1R complex. Rather, internalization of the CSF-1R in small endocytic vesicles that are sensitive to clathrin disruption, outcompetes macropinosomes for CSF-1R endocytosis. Following internalization, small vesicles carrying the CSF-1R underwent homotypic fusion and then trafficked to newly formed macropinosomes bearing Rab5. As these macropinosomes matured, acquiring Rab7, the CSF-1R was transported into their lumen and degraded. Inhibition of macropinocytosis delayed receptor degradation despite no disruption to CSF-1R endocytosis. These data indicate that CSF-1-stimulated macropinosomes are sites of multivesicular body formation and accelerate CSF-1R degradation. Furthermore, we demonstrate that macropinocytosis and cell growth have a matching dose dependence on CSF-1, suggesting that macropinosomes might be a central mechanism coupling CSF-1R signaling and macrophage growth.

Formation of α-synuclein Lewy neurite-like aggregates in axons impedes the transport of distinct endosomes

Aggregates of α-synuclein (α-syn) accumulate in neurons in Parkinson's disease and other synucleinopathies. These inclusions predominantly localize to axons even in the early stages of the disease, but their affect on axon function has remained unknown. Previously we established a model in which the addition of preformed α-syn fibrils to primary neurons seeds formation of insoluble α-syn inclusions built from endogenously expressed α-syn that closely recapitulate the neuropathological phenotypes of Lewy neurites found in human diseased brains. Here we show, using live-cell imaging, that immobile α-syn inclusions accumulate in axons from the recruitment of α-syn located on mobile α-syn-positive vesicles. Ultrastructural analyses and live imaging demonstrate that α-syn accumulations do not cause a generalized defect in axonal transport; the inclusions do not fill the axonal cytoplasm, disrupt the microtubule cytoskeleton, or affect the transport of synaptophysin or mitochondria. However, the α-syn aggregates impair the transport of Rab7 and TrkB receptor-containing endosomes, as well as autophagosomes. In addition, the TrkB receptor-associated signaling molecule pERK5 accumulates in α-syn aggregate-bearing neurons. Thus α-syn pathology impairs axonal transport of signaling and degradative organelles. These early effects of α-syn accumulations may predict points of intervention in the neurodegenerative process.

The role of rab proteins in neuronal cells and in the trafficking of neurotrophin receptors

Neurotrophins are a family of proteins that are important for neuronal development, neuronal survival and neuronal functions. Neurotrophins exert their role by binding to their receptors, the Trk family of receptor tyrosine kinases (TrkA, TrkB, and TrkC) and p75NTR, a member of the tumor necrosis factor (TNF) receptor superfamily. Binding of neurotrophins to receptors triggers a complex series of signal transduction events, which are able to induce neuronal differentiation but are also responsible for neuronal maintenance and neuronal functions. Rab proteins are small GTPases localized to the cytosolic surface of specific intracellular compartments and are involved in controlling vesicular transport. Rab proteins, acting as master regulators of the membrane trafficking network, play a central role in both trafficking and signaling pathways of neurotrophin receptors. Axonal transport represents the Achilles' heel of neurons, due to the long-range distance that molecules, organelles and, in particular, neurotrophin-receptor complexes have to cover. Indeed, alterations of axonal transport and, specifically, of axonal trafficking of neurotrophin receptors are responsible for several human neurodegenerative diseases, such as Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis and some forms of Charcot-Marie-Tooth disease. In this review, we will discuss the link between Rab proteins and neurotrophin receptor trafficking and their influence on downstream signaling pathways.

Syntaxin 8 modulates the post-synthetic trafficking of the TrkA receptor and inflammatory pain transmission

Nerve growth factor (NGF) promotes the survival, maintenance, and neurite outgrowth of sensory and sympathetic neurons, and the effects are mediated by TrkA receptor signaling. Thus, the cell surface location of the TrkA receptor is crucial for NGF-mediated functions. However, the regulatory mechanism underlying TrkA cell surface levels remains incompletely understood. In this study, we identified syntaxin 8 (STX8), a Q-SNARE protein, as a novel TrkA-binding protein. Overexpression and knockdown studies showed that STX8 facilitates TrkA transport from the Golgi to the plasma membrane and regulates the surface levels of TrkA but not TrkB receptors. Furthermore, STX8 modulates downstream NGF-induced TrkA signaling and, consequently, the survival of NGF-dependent dorsal root ganglia neurons. Finally, knockdown of STX8 in rat dorsal root ganglia by recombinant adeno-associated virus serotype 6-mediated RNA interference led to analgesic effects on formalin-induced inflammatory pain. These findings demonstrate that STX8 is a modulator of TrkA cell surface levels and biological functions.

Regulation of neurotrophin receptor (Trk) signaling: suppressor of cytokine signaling 2 (SOCS2) is a new player

The classic neurotrophins Nerve Growth Factor (NGF), Brain Derived Neurotrophic Factor (BDNF) and Neurotrophins NT-3 and NT-4 are well known to regulate various aspects of neuronal differentiation, survival and growth. They do this by binding to their cognate receptors, members of the Tropomyosin-related kinase (Trk) receptor tyrosine kinase family, namely TrkA, TrkB, and TrkC. These receptors are then internalized and localized to different cellular compartments, where signal transduction occurs. Conversely, members of the suppressor of cytokine signaling (SOCS) family are best known as negative regulators of signaling via the JAK/STAT pathway. Some members of the family, and in particular SOCS2, have roles in the nervous system that at least partially overlap with that of neurotrophins, namely neuronal differentiation and neurite outgrowth. Recent evidence suggests that SOCS2 is a novel regulator of NGF signaling, altering TrkA cellular localization and downstream signaling to affect neurite growth but not neuronal survival. This review first discusses regulation of Trk receptor signaling, followed by the role of SOCS2 in the nervous system and finishes with a discussion of possible mechanisms by which SOCS2 may regulate TrkA function.

Local versus long-range neurotrophin receptor signalling: endosomes are not just carriers for axonal transport

Neurotrophins play a critical role in neuronal development and survival, as well as maintenance of the adult nervous system. Neurotrophins can mediate their effects by signalling locally at the nerve terminal, or signalling retrogradely from the axonal terminal to the cell soma to regulate gene expression. Given that the axon terminals of many nerve cells can be up to a metre away from their soma, neurons have evolved specialized long-range signalling platforms that depend on a highly regulated network of intracellular membrane compartments termed "signalling endosomes". Endosomal trafficking of activated receptors controls not only the axonal retrograde signals but also local receptor recycling and degradation. Endosomal trafficking involving the sorting and compartmentalizing of different signals, which are subsequently distributed to the appropriate cellular destination, can at least partially explain how neurotrophins generate a diverse array of signalling outcomes. Although signalling endosomes provide a useful model for understanding how different cell surface receptor-mediated signals are generated and transported, the precise role, identity and functional definition of a signalling endosome remains unclear. In this review we will discuss the regulation of local versus long-range neurotrophin signalling, with a specific focus on recent developments in the role of endosomes in regulating the fate of Trk receptors.

Neuronal signaling through endocytosis

The distinctive morphology of neurons, with complex dendritic arbors and extensive axons, presents spatial challenges for intracellular signal transduction. The endosomal system provides mechanisms that enable signaling molecules initiated by extracellular cues to be trafficked throughout the expanse of the neuron, allowing intracellular signals to be sustained over long distances. Therefore endosomes are critical for many aspects of neuronal signaling that regulate cell survival, axonal growth and guidance, dendritic branching, and cell migration. An intriguing characteristic of neuronal signal transduction is that endosomal trafficking enables physiological responses that vary based on the subcellular location of signal initiation. In this review, we will discuss the specialized mechanisms and the functional significance of endosomal signaling in neurons, both during normal development and in disease.

Coronin-1 is a neurotrophin endosomal effector that is required for developmental competition for survival

Retrograde communication from axonal targets to neuronal cell bodies is critical for both the 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 mostly uncharacterized. Here we report that in mouse sympathetic neurons, the target-derived nerve growth factor (NGF)-tropomyosin-related kinase type 1 (TrkA, also called Ntrk1) signaling endosome, on arrival at the cell body, induces the expression and recruitment of a new effector protein known as Coronin-1 (also called Coro1a). In the absence of Coronin-1, the NGF-TrkA signaling endosome fuses to lysosomes sixfold to tenfold faster than when Coronin-1 is intact. We also define a new Coronin-1-dependent trafficking event in which signaling endosomes recycle and re-internalize on 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 phosphorylation of cAMP responsive element binding protein (CREB). These results establish Coronin-1 as an essential component of a feedback loop that mediates NGF-TrkA endosome stability, recycling and signaling as a critical mechanism governing developmental competition for survival.

Spatiotemporal intracellular dynamics of neurotrophin and its receptors. Implications for neurotrophin signaling and neuronal function

Neurons possess a polarized morphology specialized to contribute to neuronal networks, and this morphology imposes an important challenge for neuronal signaling and communication. The physiology of the network is regulated by neurotrophic factors that are secreted in an activity-dependent manner modulating neuronal connectivity. Neurotrophins are a well-known family of neurotrophic factors that, together with their cognate receptors, the Trks and the p75 neurotrophin receptor, regulate neuronal plasticity and survival and determine the neuronal phenotype in healthy and regenerating neurons. Is it now becoming clear that neurotrophin signaling and vesicular transport are coordinated to modify neuronal function because disturbances of vesicular transport mechanisms lead to disturbed neurotrophin signaling and to diseases of the nervous system. This chapter summarizes our current understanding of how the regulated secretion of neurotrophin, the distribution of neurotrophin receptors in different locations of neurons, and the intracellular transport of neurotrophin-induced signaling in distal processes are achieved to allow coordinated neurotrophin signaling in the cell body and axons.

Regulation of thalamocortical axon branching by BDNF and synaptic vesicle cycling

During development, axons form branches in response to extracellular molecules. Little is known about the underlying molecular mechanisms. Here, we investigate how neurotrophin-induced axon branching is related to synaptic vesicle cycling for thalamocortical axons. The exogenous application of brain-derived neurotrophic factor (BDNF) markedly increased axon branching in thalamocortical co-cultures, while removal of endogenous BDNF reduced branching. Over-expression of a C-terminal fragment of AP180 that inhibits clathrin-mediated endocytosis affected the laminar distribution and the number of branch points. A dominant-negative synaptotagmin mutant that selectively targets synaptic vesicle cycling, strongly suppressed axon branching. Moreover, axons expressing the mutant synaptotagmin were resistant to the branch-promoting effect of BDNF. These results suggest that synaptic vesicle cycling might regulate BDNF induced branching during the development of the axonal arbor.

Mutant Huntingtin alters retrograde transport of TrkB receptors in striatal dendrites

Huntingtin (HTT), the protein mutated in Huntington's disease (HD), controls transport of the neurotrophin, brain-derived neurotrophic factor (BDNF), within corticostriatal neurons. Transport and delivery of BDNF to the striatum are reduced in disease, which contributes to striatal neuron degeneration. BDNF released by cortical neurons activates TrkB receptors at striatal dendrites to promote striatum survival. However, it remains to be determined whether transport of TrkB, the BDNF receptor, depends on HTT and whether such transport is altered in mutant situation. Here we show that TrkB binds to and colocalizes with HTT and dynein. Silencing HTT reduces vesicular transport of TrkB in striatal neurons. In HD, the polyQ expansion in HTT alters the binding of TrkB-containing vesicles to microtubules and reduces transport. Using a combination of microfluidic devices that isolate dendrites from cell bodies and BDNF coupled to quantum dots, we selectively analyzed TrkB retrograde transport in response to BDNF stimulation at dendrite terminals. We show that the retrograde transport of TrkB vesicles within striatal dendrites and the BDNF/TrkB-induced signaling through ERK phosphorylation and c-fos induction are decreased in neurons from an HD mouse model. Together, our findings demonstrate that HTT is a crucial regulator of TrkB trafficking. Transport defects in HD are not restricted to BDNF transport in cortical neurons but also affect trafficking of its ligand-bound receptor in the striatal neurons. This transport alteration may further impair BDNF-TrkB survival signaling within the corticostriatal connection that is most affected in HD.

Short multiwall carbon nanotubes promote neuronal differentiation of PC12 cells via up-regulation of the neurotrophin signaling pathway

Numerous unique properties of carbon nanotubes make them attractive for applications in neurobiology such as drug delivery, tissue regeneration, and as scaffolds for neuronal growth. In this study, the critical roles of the length of multiwall carbon nanotubes (MWCNTs) on a neuronal-like model cell line PC12 cells are investiaged. Incubation of PC12 cells with carboxylated MWCNTs did not significantly affect cellular morphology and viability at lower concentrations. Short MWCNTs show higher cellular uptake and more obvious removal compared to longer ones, which can result in higher ability to promote PC12 cell differentiation. Pre-incubation of short MWCNTs can up-regulate the expression of neurotrophin signaling pathway-associated TrkA/p75 receptors and Pincher/Gap43/TH proteins, which might be the underlying mechanism for the improved differentiation in PC12 cells. The current results provide insight for future applications of MWCNTs in neuron drug delivery and neurodegenerative disease treatment.

β-Amyloid (Aβ) oligomers impair brain-derived neurotrophic factor retrograde trafficking by down-regulating ubiquitin C-terminal hydrolase, UCH-L1

We previously found that BDNF-dependent retrograde trafficking is impaired in AD transgenic mouse neurons. Utilizing a novel microfluidic culture chamber, we demonstrate that Aβ oligomers compromise BDNF-mediated retrograde transport by impairing endosomal vesicle velocities, resulting in impaired downstream signaling driven by BDNF/TrkB, including ERK5 activation, and CREB-dependent gene regulation. Our data suggest that a key mechanism mediating the deficit involves ubiquitin C-terminal hydrolase L1 (UCH-L1), a deubiquitinating enzyme that functions to regulate cellular ubiquitin. Aβ-induced deficits in BDNF trafficking and signaling are mimicked by LDN (an inhibitor of UCH-L1) and can be reversed by increasing cellular UCH-L1 levels, demonstrated here using a transducible TAT-UCH-L1 strategy. Finally, our data reveal that UCH-L1 mRNA levels are decreased in the hippocampi of AD brains. Taken together, our data implicate that UCH-L1 is important for regulating neurotrophin receptor sorting to signaling endosomes and supporting retrograde transport. Further, our results support the idea that in AD, Aβ may down-regulate UCH-L1 in the AD brain, which in turn impairs BDNF/TrkB-mediated retrograde signaling, compromising synaptic plasticity and neuronal survival.

Identification and analysis of expressed genes using a cDNA library from rat thymus during regeneration following cyclophosphamide-induced T cell depletion

Understanding the mechanisms of thymus regeneration is necessary for designing strategies to enhance host immunity when immune function is suppressed due to T cell depletion. In this study, expressed sequence tag (EST) analysis was performed following generation of a regenerating thymus cDNA library to identify genes expressed in thymus regeneration. A total of 1,000 ESTs were analyzed, of which 770 (77%) matched to known genes, 178 matched to unknown genes (17.8%) and 52 (5.2%) did not match any known sequences. The ESTs matched to known genes were grouped into eight functional categories: gene/protein synthesis (28%), metabolism (24%), cell signaling and communication (17%), cell structure and motility (6%), cell/organism defense and homeostasis (6%), cell division (3%), cell death/apoptosis (2%), and unclassified genes (14%). Based on the data of RT-PCR analysis, the expression of TLP, E2IG2, pincher, Paip2, TGF-β1, 4-1BB and laminin α3 genes was increased during thymus regeneration. These results provide extensive molecular information, for the first time, on thymus regeneration indicating that the regenerating thymus cDNA library may be a useful source for identifying various genes expressed during thymus regeneration.

Trk activation of the ERK1/2 kinase pathway stimulates intermediate chain phosphorylation and recruits cytoplasmic dynein to signaling endosomes for retrograde axonal transport

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 phosphoserine on the dynein intermediate chains (ICs), and we observed a time-dependent neurotrophin-stimulated increase in intermediate chain phosphorylation on this site in both cell types. Pharmacological studies, overexpression of constitutively active MAP kinase kinase, 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 dephosphomimic mutants had significantly reduced colocalization with Trk and Rab7, but not a mitochondrial marker. The phosphorylated intermediate chains were enriched on immunoaffinity-purified Trk-containing organelles. Inhibition of ERK reduced the amount of phospho-IC and the total amount of dynein that copurified with the signaling endosomes. In addition, inhibition of ERK1/2 reduced the motility of Rab7- and TrkB-containing endosomes and the extent of their colocalization with dynein in axons. NGF-dependent survival of sympathetic neurons was significantly reduced by the overexpression of the dephosphomimic 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.

Anterograde trafficking of neurotrophin-3 in the adult olfactory system in vivo

The olfactory system continuously incorporates new neurons into functional circuits throughout life. Axons from olfactory sensory neurons (OSNs) in the nasal cavity synapse on mitral, tufted and periglomerular (PG) cells in the main olfactory bulb, and low levels of turnover within the OSN population results in ingrowth of new axons under normal physiological conditions. Subpopulations of bulb interneurons are continually eliminated by apoptosis, and are replaced by new neurons derived from progenitors in the adult forebrain subventricular zone. Integration of new neurons, including PG cells that are contacted by sensory axons, leads to ongoing reorganization of adult olfactory bulb circuits. The mechanisms regulating this adaptive structural plasticity are not all known, but the process is reminiscent of early nervous system development. Neurotrophic factors have well-established roles in controlling neuronal survival and connectivity during development, leading to speculation that trophic interactions between OSNs and their target bulb neurons may mediate some of these same processes in adults. A number of different trophic factors and their cognate receptors are expressed in the adult olfactory pathway. Neurotrophin-3 (NT3) is among these, as reflected by beta-galactosidase expression in transgenic reporter mice expressing lacZ under the NT3 promoter. Using a combination of approaches, including immunocytochemistry, real-time PCR of laser-captured RNA, and adenovirus-mediated gene transfer of NT3 fusion peptides in vivo, we demonstrate that OSNs express and anterogradely transport NT3 to the olfactory bulb. We additionally observe that in mice treated with adenovirus encoding NT3 tagged with hemagglutinin (HA), a subset of bulb neurons expressing the TrkC neurotrophin receptor are immunoreactive for HA, suggesting their acquisition of the fusion peptide from infected sensory neurons. Our results therefore provide evidence that OSNs may serve as an afferent source of trophic signals for the adult mouse olfactory bulb.

Numb and Alzheimer's disease: the current picture

Twenty-three years ago, numb was identified as a critical regulator in Drosophila sensory organ precursor cell asymmetric divisions. Beyond the recently recognized role in carcinogenesis, Numb seems to be important in Alzheimer’s disease. This assertion comes from the involvement in various processes such as synapse morphogenesis, amyloid precursor protein trafficking, notch signaling, and neurogenesis. The purpose of the present mini-review is to provide the current picture of Numb’s participation in mechanisms underlying Alzheimer’s disease pathogenesis and emphasize potential aspects for future research.