Flux of signalling endosomes undergoing axonal retrograde transport is encoded by presynaptic activity and TrkB

Neural Tracing Protein-Functionalized Nanoparticles Capable of Fast Retrograde Axonal Transport in Live Neurons

Neural tracing proteins like horseradish peroxidase-conjugated wheat germ agglutinin (WGA-HRP) can target the central nervous system (CNS) through anatomic retrograde transport without crossing the blood-brain barrier (BBB). Conjugating WGA-HRP to nanoparticles may enable the creation of BBB-bypassing nanomedicine. Microfluidics and two-photon confocal microscopy is applied to screen nanocarriers for transport efficacy and gain mechanistic insights into their interactions with neurons. Protein modification of gold nanoparticles alters their cellular uptake at the axonal terminal and activates fast retrograde transport. Trajectory analysis of individual endosomes carrying the nanoparticles reveals a run-and-pause pattern along the axon with endosomes carrying WGA-HRP-conjugated gold nanoparticles exhibiting longer run duration and faster instantaneous velocity than those carrying nonconjugated nanoparticles. The results offer a mechanistic explanation of the different axonal transport dynamics as well as a cell-based functional assay of neuron-targeted nanoparticles with the goal of developing BBB-bypassing nanomedicine for the treatment of nervous system disorders.

PLC-γ-Ca2+ pathway regulates axonal TrkB endocytosis and is required for long-distance propagation of BDNF signaling

Brain-derived neurotrophic factor (BDNF) and its tropomyosin receptor kinase B (TrkB) are important signaling proteins that regulate dendritic growth and maintenance in the central nervous system (CNS). After binding of BDNF, TrkB is endocytosed into endosomes and continues signaling within the cell soma, dendrites, and axon. In previous studies, we showed that BDNF signaling initiated in axons triggers long-distance signaling, inducing dendritic arborization in a CREB-dependent manner in cell bodies, processes that depend on axonal dynein and TrkB activities. The binding of BDNF to TrkB triggers the activation of different signaling pathways, including the ERK, PLC-γ and PI3K-mTOR pathways, to induce dendritic growth and synaptic plasticity. How TrkB downstream pathways regulate long-distance signaling is unclear. Here, we studied the role of PLC-γ-Ca2+ in BDNF-induced long-distance signaling using compartmentalized microfluidic cultures. We found that dendritic branching and CREB phosphorylation induced by axonal BDNF stimulation require the activation of PLC-γ in the axons of cortical neurons. Locally, in axons, BDNF increases PLC-γ phosphorylation and induces intracellular Ca2+ waves in a PLC-γ-dependent manner. In parallel, we observed that BDNF-containing signaling endosomes transport to the cell body was dependent on PLC-γ activity and intracellular Ca2+ stores. Furthermore, the activity of PLC-γ is required for BDNF-dependent TrkB endocytosis, suggesting a role for the TrkB/PLC-γ signaling pathway in axonal signaling endosome formation.

Activin is a neural inducer of a male-specific muscle in Drosophila

Drosophila melanogaster has a pair of male-specific muscles called the muscle of Lawrence (MOL) in abdominal segment 5 (A5) of adult flies. The MOL is produced only when its innervating motoneuron expresses FruitlessM (FruM) neural masculinizing proteins. We show that MOL induction is hampered by: (1) silencing electrical activities in the motoneuron, (2) blocking vesicular release from the motoneuron, and (3) knocking down Activin ß (Actß) in the motoneuron or knocking down Actß signaling pathway components in the myoblasts. Our timelapse live imaging of the developing neuromuscular system reveals that, upon contact with the presumptive MOL, the motoneuronal axon retracts concomitant with the progression of MOL degeneration resulting from neural silencing. We conclude that MOL formation depends on the bidirectional trophic interactions between pre- and postsynaptic cells, with motoneuron-derived Actß playing an inducing role in MOL formation.

Location, location, location: Protein kinase nanoclustering for optimised signalling output

Protein kinases (PKs) are proteins at the core of cellular signalling and are thereby responsible for most cellular physiological processes and their regulations. As for all intracellular proteins, PKs are subjected to Brownian thermal energy that tends to homogenise their distribution throughout the volume of the cell. To access their substrates and perform their critical functions, PK localisation is therefore tightly regulated in space and time, relying upon a range of clustering mechanisms. These include post-translational modifications, protein-protein and protein-lipid interactions, as well as liquid-liquid phase separation, allowing spatial restriction and ultimately regulating access to their substrates. In this review, we will focus on key mechanisms mediating PK nanoclustering in physiological and pathophysiological processes. We propose that PK nanoclusters act as a cellular quantal unit of signalling output capable of integration and regulation in space and time. We will specifically outline the various super-resolution microscopy approaches currently used to elucidate the composition and mechanisms driving PK nanoscale clustering and explore the pathological consequences of altered kinase clustering in the context of neurodegenerative disorders, inflammation, and cancer.

Multiple Labeling of Compartmentalized Cortical Neurons in Microfluidic Chambers

Neurons are complex cells with two distinct compartments: the somatodendritic and the axonal domains. Because of their polarized morphology, it is challenging to study the differential cellular and molecular mechanisms that occur in axons and impact the soma and dendrites using conventional in vitro culture systems. Compartmentalized cultures offer a solution by physically and chemically separating the axonal from the somatodendritic domain of neurons. The microfluidic chamber model presented in this work is valuable for studying these mechanisms in primary cortical cultures derived from rat and mouse. In addition, this chamber model is compatible with various microscopy methods, such as phase contrast, and fluorescence imaging of living and fixed cells. Key features • Preparation and attachment of PDMS microfluidic chambers to glass coverslips. • Primary culture of cortical neurons and plating cortical neurons in microfluidic chamber. • Confirmation of compartmentalization using the retrograde transport of the fluorescently labeled form of cholera toxin subunit B (f-Ctb). • Immunofluorescence and multilabeling of compartmentalized cortical neurons. • Retrograde transport of fluorescently labeled BDNF.

When Super-Resolution Microscopy Meets Microfluidics: Enhanced Biological Imaging and Analysis with Unprecedented Resolution

Super-resolution microscopy is rapidly developed in recent years, allowing biologists to extract more quantitative information on subcellular processes in live cells that is usually not accessible with conventional techniques. However, super-resolution imaging is not fully exploited because of the lack of an appropriate and multifunctional experimental platform. As an important tool in life sciences, microfluidics is capable of cell manipulation and the regulation of the cellular environment because of its superior flexibility and biocompatibility. The combination of microfluidics and super-resolution microscopy revolutionizes the study of complex cellular properties and dynamics, providing valuable insights into cellular structure and biological functions at the single-molecule level. In this perspective, an overview of the main advantages of microfluidic technology that are essential to the performance of super-resolution microscopy are offered. The main benefits of performing super-resolution imaging with microfluidic devices are highlighted and perspectives on the diverse applications that are facilitated by combining these two powerful techniques are provided.

Hericerin derivatives activates a pan-neurotrophic pathway in central hippocampal neurons converging to ERK1/2 signaling enhancing spatial memory

The traditional medicinal mushroom Hericium erinaceus is known for enhancing peripheral nerve regeneration through targeting nerve growth factor (NGF) neurotrophic activity. Here, we purified and identified biologically new active compounds from H. erinaceus, based on their ability to promote neurite outgrowth in hippocampal neurons. N-de phenylethyl isohericerin (NDPIH), an isoindoline compound from this mushroom, together with its hydrophobic derivative hericene A, were highly potent in promoting extensive axon outgrowth and neurite branching in cultured hippocampal neurons even in the absence of serum, demonstrating potent neurotrophic activity. Pharmacological inhibition of tropomyosin receptor kinase B (TrkB) by ANA-12 only partly prevented the NDPIH-induced neurotrophic activity, suggesting a potential link with BDNF signaling. However, we found that NDPIH activated ERK1/2 signaling in the absence of TrkB in HEK-293T cells, an effect that was not sensitive to ANA-12 in the presence of TrkB. Our results demonstrate that NDPIH acts via a complementary neurotrophic pathway independent of TrkB with converging downstream ERK1/2 activation. Mice fed with H. erinaceus crude extract and hericene A also exhibited increased neurotrophin expression and downstream signaling, resulting in significantly enhanced hippocampal memory. Hericene A therefore acts through a novel pan-neurotrophic signaling pathway, leading to improved cognitive performance.

BDNF-dependent modulation of axonal transport is selectively impaired in ALS

Axonal transport ensures long-range delivery of essential cargoes between proximal and distal compartments, and is needed for neuronal development, function, and survival. Deficits in axonal transport have been detected at pre-symptomatic stages in the SOD1^G93A and TDP-43^M337V mouse models of amyotrophic lateral sclerosis (ALS), suggesting that impairments in this critical process are fundamental for disease pathogenesis. Strikingly, in ALS, fast motor neurons (FMNs) degenerate first whereas slow motor neurons (SMNs) are more resistant, and this is a currently unexplained phenomenon. The main aim of this investigation was to determine the effects of brain-derived neurotrophic factor (BDNF) on in vivo axonal transport in different α-motor neuron (MN) subtypes in wild-type (WT) and SOD1^G93A mice. We report that despite displaying similar basal transport speeds, stimulation of wild-type MNs with BDNF enhances in vivo trafficking of signalling endosomes specifically in FMNs. This BDNF-mediated enhancement of transport was also observed in primary ventral horn neuronal cultures. However, FMNs display selective impairment of axonal transport in vivo in symptomatic SOD1^G93A mice, and are refractory to BDNF stimulation, a phenotype that was also observed in primary embryonic SOD1^G93A neurons. Furthermore, symptomatic SOD1^G93A mice display upregulation of the classical non-pro-survival truncated TrkB and p75^NTR receptors in muscles, sciatic nerves, and Schwann cells. Altogether, these data indicate that cell- and non-cell autonomous BDNF signalling is impaired in SOD1^G93A MNs, thus identifying a new key deficit in ALS.

The Rab11-regulated endocytic pathway and BDNF/TrkB signaling: Roles in plasticity changes and neurodegenerative diseases

Neurons are highly polarized cells that rely on the intracellular transport of organelles. This process is regulated by molecular motors such as dynein and kinesins and the Rab family of monomeric GTPases that together help move cargo along microtubules in dendrites, somas, and axons. Rab5-Rab11 GTPases regulate receptor trafficking along early-recycling endosomes, which is a process that determines the intracellular signaling output of different signaling pathways, including those triggered by BDNF binding to its tyrosine kinase receptor TrkB. BDNF is a well-recognized neurotrophic factor that regulates experience-dependent plasticity in different circuits in the brain. The internalization of the BDNF/TrkB complex results in signaling endosomes that allow local signaling in dendrites and presynaptic terminals, nuclear signaling in somas and dynein-mediated long-distance signaling from axons to cell bodies. In this review, we briefly discuss the organization of the endocytic pathway and how Rab11-recycling endosomes interact with other endomembrane systems. We further expand upon the roles of the Rab11-recycling pathway in neuronal plasticity. Then, we discuss the BDNF/TrkB signaling pathways and their functional relationships with the postendocytic trafficking of BDNF, including axonal transport, emphasizing the role of BDNF signaling endosomes, particularly Rab5-Rab11 endosomes, in neuronal plasticity. Finally, we discuss the evidence indicating that the dysfunction of the early-recycling pathway impairs BDNF signaling, contributing to several neurodegenerative diseases.

Calcineurin and huntingtin form a calcium-sensing machinery that directs neurotrophic signals to the nucleus

When a neurotrophin binds at the presynapse, it sends survival signals all the way to the nucleus on signaling endosomes. These endosomes fuel their own journey with on-board glycolysis—but how is that journey initiated and maintained? Using microfluidic devices and mice, we find that the calcium released upon brain-derived neurotrophic factor (BDNF) binding to its receptor, tropomyosin receptor kinase B (TrkB), is sensed by calcineurin on the cytosolic face of the endosome. Calcineurin dephosphorylates huntingtin, the BDNF scaffold, which sets the endosome moving in a retrograde direction. In an in vitro reconstituted microtubule transport system, controlled calcium uncaging prompts purified vesicles to move to the microtubule minus end. We observed similar retrograde waves of TrkA- and epidermal growth factor receptor (EGFR)-bearing endosomes. Signaling endosomes in neurons thus carry not only their own fuel, but their own navigational system.

Live-Cell Superresolution Imaging of Retrograde Axonal Trafficking Using Pulse-Chase Labeling in Cultured Hippocampal Neurons

The entanglement of long axons found in cultured dissociated hippocampal neurons restricts the analysis of the machinery underlying directed axonal trafficking. Further, hippocampal neurons exhibit "en passant" presynapses that may confound the analysis of long-range retrograde axonal transport. To solve these issues, we and others have developed microfluid-based methods to specifically follow the fates of the retrograde axonal cargoes following pulse-chase labeling by super-resolution live-cell imaging, and automatically tracking their directed transport and analyzing their kinetical properties. These methods have allowed us to visualize the trafficking of fluorescently tagged signaling endosomes and autophagosomes derived from axonal terminals and resolve their localizations and movements with high spatial and temporal accuracy. In this chapter, we describe how to use a commercially available microfluidic device to enable the labeling and tracking of retrograde axonal carriers, including (1) how to culture and transfect rat hippocampal neurons in the microfluidic device; (2) how to perform pulse-chase to label specific populations of retrograde axonal carriers; and (3) how to conduct the automatic tracking and data analysis using open-source software.

In Vivo Live Imaging of Axonal Transport in Developing Zebrafish Axons

Axonal transport is crucial for neuronal homeostasis, survival, and development. Indeed, axonal transport needs to be precisely regulated for developing axons to swiftly and accurately respond to their complex and evolving environment in space and time. A growing number of studies have started to unravel the diversity of regulatory and adaptor proteins required to orchestrate the axonal transport machinery. Despite some discrepancies between in vitro and in vivo axonal transport studies, most analyses aiming at deciphering these regulatory complexes, as well as their mode of action, were carried out in vitro in primary cultures of neurons, and mainly focused on their impact on axon specification and elongation, but rarely on axon navigation per se. Given the clear influence of the in vivo environment on axonal transport, including chemical and physical interactions with neighboring cells, it is essential to develop in vivo models to identify and characterize the molecular complexes involved in this key process. Here, we describe an experimental system to monitor axonal transport in vivo in developing axons of live zebrafish embryos with high spatial and temporal resolution. Due to its optical transparency and easy genetic manipulation, the zebrafish embryo is ideally suited to study such cellular dynamics at a single axon scale. Using this approach, we were able to unravel the key role of Fidgetin-like 1 in the regulation of bidirectional axonal transport required for motor axon targeting. Moreover, this protocol can be easily adapted to characterize a wide range of axonal transport regulators and components in physiological conditions and may additionally be used to screen new therapeutic compounds based on their ability to recue axonal transport defects in pathological conditions.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

The axonal radial contractility: Structural basis underlying a new form of neural plasticity

Axons are the longest cellular structure reaching over a meter in the case of human motor axons. They have a relatively small diameter and contain several cytoskeletal elements that mediate both material and information exchange within neurons. Recently, a novel type of axonal plasticity, termed axonal radial contractility, has been unveiled. It is represented by dynamic and transient diameter changes of the axon shaft to accommodate the passages of large organelles. Mechanisms underpinning this plasticity are not fully understood. Here, we first summarised recent evidence of the functional relevance for axon radial contractility, then discussed the underlying structural basis, reviewing nanoscopic evidence of the subtle changes. Two models are proposed to explain how actomyosin rings are organised. Possible roles of non-muscle myosin II (NM-II) in axon degeneration are discussed. Finally, we discuss the concept of periodic functional nanodomains, which could sense extracellular cues and coordinate the axonal responses. Also see the video abstract here: https://youtu.be/ojCnrJ8RCRc.

Do Changes in Synaptic Autophagy Underlie the Cognitive Impairments in Huntington's Disease?

Although Huntington's disease (HD) is classically considered from the perspective of the motor syndrome, the cognitive changes in HD are prominent and often an early manifestation of disease. As such, investigating the underlying pathophysiology of cognitive changes may give insight into important and early neurodegenerative events. In this review, we first discuss evidence from both HD patients and animal models that cognitive changes correlate with early pathological changes at the synapse, an observation that is similarly made in other neurodegenerative conditions that primarily affect cognition. We then describe how autophagy plays a critical role supporting synaptic maintenance in the healthy brain, and how autophagy dysfunction in HD may thereby lead to impaired synaptic maintenance and thus early manifestations of disease.

In Vivo Imaging of Anterograde and Retrograde Axonal Transport in Rodent Peripheral Nerves

Axonal transport, which is the process mediating the active shuttling of a variety cargoes from one end of an axon to the other, is essential for the development, function, and survival of neurons. Impairments in this dynamic process are linked to diverse nervous system diseases and advanced ageing. It is thus essential that we quantitatively study the kinetics of axonal transport to gain an improved understanding of neuropathology as well as the molecular and cellular mechanisms regulating cargo trafficking. One of the best ways to achieve this goal is by imaging individual, fluorescent cargoes in live systems and analyzing the kinetic properties of their progression along the axon. We have therefore developed an intravital technique to visualize different organelles, such as signaling endosomes and mitochondria, being actively transported in the axons of both motor and sensory neurons in live, anesthetized rodents. In this chapter, we provide step-by-step instructions on how to deliver specific organelle-targeting, fluorescent probes using several routes of administration to image individual cargoes being bidirectionally transported along axons within the exposed sciatic nerve. This method can provide detailed, physiologically relevant information on axonal transport, and is thus poised to elucidate mechanisms regulating this process in both health and disease.

Current molecular approaches to investigate pre-synaptic dysfunction

Over the course of the last few decades it has become clear that many neurodevelopmental and neurodegenerative disorders have a synaptic defect, which contributes to pathogenicity. A rise in new techniques, and in particular '-omics'-based methods providing large datasets, has led to an increase in potential proteins and pathways implicated in synaptic function and related disorders. Additionally, advancements in imaging techniques have led to the recent discovery of alternative modes of synaptic vesicle recycling. This has resulted in a lack of clarity over the precise role of different pathways in maintaining synaptic function and whether these new pathways are dysfunctional in neurodevelopmental and neurodegenerative disorders. A greater understanding of the molecular detail of pre-synaptic function in health and disease is key to targeting new proteins and pathways for novel treatments and the variety of new techniques currently available provides an ideal opportunity to investigate these functions. This review focuses on techniques to interrogate pre-synaptic function, concentrating mainly on synaptic vesicle recycling. It further examines techniques to determine the underlying molecular mechanism of pre-synaptic dysfunction and discusses methods to identify molecular targets, along with protein-protein interactions and cellular localization. In combination, these techniques will provide an expanding and more complete picture of pre-synaptic function. With the application of recent technological advances, we are able to resolve events with higher spatial and temporal resolution, leading research towards a greater understanding of dysfunction at the presynapse and the role it plays in pathogenicity.

Autophagy and the endolysosomal system in presynaptic function

The complex morphology of neurons, the specific requirements of synaptic neurotransmission and the accompanying metabolic demands create a unique challenge for proteostasis. The main machineries for neuronal protein synthesis and degradation are localized in the soma, while synaptic junctions are found at vast distances from the cell body. Sophisticated mechanisms must, therefore, ensure efficient delivery of newly synthesized proteins and removal of faulty proteins. These requirements are exacerbated at presynaptic sites, where the demands for protein turnover are especially high due to synaptic vesicle release and recycling that induces protein damage in an intricate molecular machinery, and where replacement of material is hampered by the extreme length of the axon. In this review, we will discuss the contribution of the two major pathways in place, autophagy and the endolysosomal system, to presynaptic protein turnover and presynaptic function. Although clearly different in their biogenesis, both pathways are characterized by cargo collection and transport into distinct membrane-bound organelles that eventually fuse with lysosomes for cargo degradation. We summarize the available evidence with regard to their degradative function, their regulation by presynaptic machinery and the cargo for each pathway. Finally, we will discuss the interplay of both pathways in neurons and very recent findings that suggest non-canonical functions of degradative organelles in synaptic signalling and plasticity.

Loss of MUNC18-1 leads to retrograde transport defects in neurons

Loss of the exocytic Sec1/MUNC18 protein MUNC18-1 or its target-SNARE partners SNAP25 and syntaxin-1 results in rapid, cell-autonomous and unexplained neurodegeneration, which is independent of their known role in synaptic vesicle exocytosis. cis-Golgi abnormalities are the earliest cellular phenotypes before degeneration occurs. Here, we investigated whether loss of MUNC18-1 causes defects in intracellular membrane transport pathways in primary murine neurons that may explain neurodegeneration. Electron, confocal and super resolution microscopy confirmed that loss of MUNC18-1 expression results in a smaller cis-Golgi. In addition, we now show that medial-Golgi and the trans-Golgi Network are also affected. However, stacking and cisternae ultrastructure of the Golgi were normal. Overall, ultrastructure of null mutant neurons was remarkably normal just hours before cell death occurred. By synchronizing protein trafficking by conditional cargo retention in the endoplasmic reticulum using selective hooks (RUSH) and immunocytochemistry, we show that anterograde Endoplasmic Reticulum-to-Golgi and Golgi exit of endogenous and exogenous proteins were normal. In contrast, loss of MUNC18-1 caused reduced retrograde Cholera Toxin B-subunit transport from the plasma membrane to the Golgi. In addition, MUNC18-1-deficiency resulted in abnormalities in retrograde TrkB trafficking in an antibody uptake assay. We conclude that MUNC18-1 deficient neurons have normal anterograde but reduced retrograde transport to the Golgi. The impairments in retrograde pathways suggest a role of MUNC18-1 in endosomal SNARE-dependent fusion and provide a plausible explanation for the observed Golgi abnormalities and cell death in MUNC18-1 deficient neurons.

Combining Single Molecule Super-Resolution Imaging Techniques to Unravel the Nanoscale Organization of the Presynapse

The fusion of synaptic vesicles with the plasma membrane underpins neurotransmission. A number of presynaptic proteins play a critical role in overcoming the energy barrier inherent to the fusion of the negatively charged vesicular and plasma membranes. Emerging concepts suggest that this process is hierarchical and dependent on rapid and transient reorganization of proteins in and out of small nanoclusters located in the active zones of nerve terminals. Examining the nanoscale organization of presynaptic molecules requires super-resolution microscopy to overcome the limits of conventional light microscopy. In this chapter, we describe three super-resolution techniques that allow for the examination of the nanoscale organization of proteins within live hippocampal nerve terminals. We used (1) single-particle tracking photoactivated localization microscopy (sptPALM) to resolve the mobility and clustering of syntaxin1A (STX1A), (2) universal Point Accumulation Imaging in Nanoscale Topography (uPAINT) to study the mobility of a pool of vesicular-associated membrane protein 2 (VAMP2) transiting on the plasma membrane, and (3) subdiffractional Tracking of Internalized Molecules (sdTIM) to track VAMP2-positive recycling synaptic vesicles in conjunction with Cholera Toxin subunit B (CTB), which has recently been shown to be trafficked retrogradely from the presynapse to the cell body via signaling endosomes.

Synaptic vesicle traffic is supported by transient actin filaments and regulated by PKA and NO

Synaptic vesicles (SVs) can be pooled across multiple synapses, prompting questions about their dynamic allocation for neurotransmission and plasticity. We find that the axonal traffic of recycling vesicles is not supported by ubiquitous microtubule-based motility but relies on actin instead. Vesicles freed from synaptic clusters undergo ~1 µm bouts of active transport, initiated by nearby elongation of actin filaments. Long distance translocation arises when successive bouts of active transport were linked by periods of free diffusion. The availability of SVs for active transport can be promptly increased by protein kinase A, a key player in neuromodulation. Vesicle motion is in turn impeded by shutting off axonal actin polymerization, mediated by nitric oxide-cyclic GMP signaling leading to inhibition of RhoA. These findings provide a potential framework for coordinating post-and pre-synaptic strength, using retrograde regulation of axonal actin dynamics to mobilize and recruit presynaptic SV resources.

Activity-Dependent Global Downscaling of Evoked Neurotransmitter Release across Glutamatergic Inputs in Drosophila

Within mammalian brain circuits, activity-dependent synaptic adaptations, such as synaptic scaling, stabilize neuronal activity in the face of perturbations. Stability afforded through synaptic scaling involves uniform scaling of quantal amplitudes across all synaptic inputs formed on neurons, as well as on the postsynaptic side. It remains unclear whether activity-dependent uniform scaling also operates within peripheral circuits. We tested for such scaling in a Drosophila larval neuromuscular circuit, where the muscle receives synaptic inputs from different motoneurons. We used motoneuron-specific genetic manipulations to increase the activity of only one motoneuron and recordings of postsynaptic currents from inputs formed by the different motoneurons. We discovered an adaptation which caused uniform downscaling of evoked neurotransmitter release across all inputs through decreases in release probabilities. This "presynaptic downscaling" maintained the relative differences in neurotransmitter release across all inputs around a homeostatic set point, caused a compensatory decrease in synaptic drive to the muscle affording robust and stable muscle activity, and was induced within hours. Presynaptic downscaling was associated with an activity-dependent increase in Drosophila vesicular glutamate transporter expression. Activity-dependent uniform scaling can therefore manifest also on the presynaptic side to produce robust and stable circuit outputs. Within brain circuits, uniform downscaling on the postsynaptic side is implicated in sleep- and memory-related processes. Our results suggest that evaluation of such processes might be broadened to include uniform downscaling on the presynaptic side.SIGNIFICANCE STATEMENT To date, compensatory adaptations which stabilise target cell activity through activity-dependent global scaling have been observed only within central circuits, and on the postsynaptic side. Considering that maintenance of stable activity is imperative for the robust function of the nervous system as a whole, we tested whether activity-dependent global scaling could also manifest within peripheral circuits. We uncovered a compensatory adaptation which causes global scaling within a peripheral circuit and on the presynaptic side through uniform downscaling of evoked neurotransmitter release. Unlike in central circuits, uniform scaling maintains functionality over a wide, rather than a narrow, operational range, affording robust and stable activity. Activity-dependent global scaling therefore operates on both the presynaptic and postsynaptic sides to maintain target cell activity.

Altered Sensory Neuron Development in CMT2D Mice Is Site-Specific and Linked to Increased GlyRS Levels

Dominant, missense mutations in the widely and constitutively expressed GARS1 gene cause peripheral neuropathy that usually begins in adolescence and principally impacts the upper limbs. Caused by a toxic gain-of-function in the encoded glycyl-tRNA synthetase (GlyRS) enzyme, the neuropathology appears to be independent of the canonical role of GlyRS in aminoacylation. Patients display progressive, life-long weakness and wasting of muscles in hands followed by feet, with frequently associated deficits in sensation. When dysfunction is observed in motor and sensory nerves, there is a diagnosis of Charcot-Marie-Tooth disease type 2D (CMT2D), or distal hereditary motor neuropathy type V if the symptoms are purely motor. The cause of this varied sensory involvement remains unresolved, as are the pathomechanisms underlying the selective neurodegeneration characteristic of the disease. We have previously identified in CMT2D mice that neuropathy-causing Gars mutations perturb sensory neuron fate and permit mutant GlyRS to aberrantly interact with neurotrophin receptors (Trks). Here, we extend this work by interrogating further the anatomy and function of the CMT2D sensory nervous system in mutant Gars mice, obtaining several key results: (1) sensory pathology is restricted to neurons innervating the hindlimbs; (2) perturbation of sensory development is not common to all mouse models of neuromuscular disease; (3) in vitro axonal transport of signaling endosomes is not impaired in afferent neurons of all CMT2D mouse models; and (4) Gars expression is selectively elevated in a subset of sensory neurons and linked to sensory developmental defects. These findings highlight the importance of comparative neurological assessment in mouse models of disease and shed light on key proposed neuropathogenic mechanisms in GARS1-linked neuropathy.

Developmental demands contribute to early neuromuscular degeneration in CMT2D mice

Dominantly inherited, missense mutations in the widely expressed housekeeping gene, GARS1, cause Charcot-Marie-Tooth type 2D (CMT2D), a peripheral neuropathy characterised by muscle weakness and wasting in limb extremities. Mice modelling CMT2D display early and selective neuromuscular junction (NMJ) pathology, epitomised by disturbed maturation and neurotransmission, leading to denervation. Indeed, the NMJ disruption has been reported in several different muscles; however, a systematic comparison of neuromuscular synapses from distinct body locations has yet to be performed. We therefore analysed NMJ development and degeneration across five different wholemount muscles to identify key synaptic features contributing to the distinct pattern of neurodegeneration in CMT2D mice. Denervation was found to occur along a distal-to-proximal gradient, providing a cellular explanation for the greater weakness observed in mutant Gars hindlimbs compared with forelimbs. Nonetheless, muscles from similar locations and innervated by axons of equivalent length showed significant differences in neuropathology, suggestive of additional factors impacting on site-specific neuromuscular degeneration. Defective NMJ development preceded and associated with degeneration, but was not linked to a delay of wild-type NMJ maturation processes. Correlation analyses indicate that muscle fibre type nor synaptic architecture explain the differential denervation of CMT2D NMJs, rather it is the extent of post-natal synaptic growth that predisposes to neurodegeneration. Together, this work improves our understanding of the mechanisms driving synaptic vulnerability in CMT2D and hints at pertinent pathogenic pathways.

Graphene Promotes Axon Elongation through Local Stall of Nerve Growth Factor Signaling Endosomes

Several works reported increased differentiation of neuronal cells grown on graphene; however, the molecular mechanism driving axon elongation on this material has remained elusive. Here, we study the axonal transport of nerve growth factor (NGF), the neurotrophin supporting development of peripheral neurons, as a key player in the time course of axonal elongation of dorsal root ganglion neurons on graphene. We find that graphene drastically reduces the number of retrogradely transported NGF vesicles in favor of a stalled population in the first 2 days of culture, in which the boost of axon elongation is observed. This correlates with a mutual charge redistribution, observed via Raman spectroscopy and electrophysiological recordings. Furthermore, ultrastructural analysis indicates a reduced microtubule distance and an elongated axonal topology. Thus, both electrophysiological and structural effects can account for graphene action on neuron development. Unraveling the molecular players underneath this interplay may open new avenues for axon regeneration applications.

Radial contractility of actomyosin rings facilitates axonal trafficking and structural stability

Most mammalian neurons have a narrow axon, which constrains the passage of large cargoes such as autophagosomes that can be larger than the axon diameter. Radial axonal expansion must therefore occur to ensure efficient axonal trafficking. In this study, we reveal that the speed of various large cargoes undergoing axonal transport is significantly slower than that of small ones and that the transit of diverse-sized cargoes causes an acute, albeit transient, axonal radial expansion, which is immediately restored by constitutive axonal contractility. Using live super-resolution microscopy, we demonstrate that actomyosin-II controls axonal radial contractility and local expansion, and that NM-II filaments associate with periodic F-actin rings via their head domains. Pharmacological inhibition of NM-II activity significantly increases axon diameter by detaching the NM-II from F-actin and impacts the trafficking speed, directionality, and overall efficiency of long-range retrograde trafficking. Consequently, prolonged NM-II inactivation leads to disruption of periodic actin rings and formation of focal axonal swellings, a hallmark of axonal degeneration.

Endocytosis, trafficking and exocytosis of intact full-length botulinum neurotoxin type a in cultured rat neurons

Botulinum toxin A (BoNT/A) is a potent neurotoxin that acts primarily by silencing synaptic transmission by blocking neurotransmitter release. BoNT/A comprises a light chain (LC/A) intracellular protease and a heavy chain (HC/A) composed of a receptor binding domain (HCC/A) and a translocation domain (HCN/A) that mediates cell entry. Following entry into the neuron, the disulphide bond linking the two peptide chains is reduced to release the LC/A. To gain better insight into the trafficking and fate of BoNT/A before dissociation we have used a catalytically inactive, non-toxic full-length BoNT/A(0) mutant. Our data confirm that BoNT/A(0) enters cortical neurons both in an activity-dependent manner and via a pathway dependent on fibroblast growth factor receptor 3 (Fgfr3) signalling. We demonstrate that both dynamin-dependent endocytosis and lipid rafts are involved in BoNT/A internalisation and that full-length BoNT/A(0) traffics to early endosomes. Furthermore, while a proportion of BoNT/A remains stable in neurons for 3 days, BoNT/A degradation is primarily mediated by the proteasome. Finally, we demonstrate that a fraction of the endocytosed full-length BoNT/A(0) is capable of exiting the cell to intoxicate other neurons. Together, our data shed new light on the entry routes, trafficking and degradation of BoNT/A, and confirm that trafficking properties previously described for the isolated HCC/A receptor binding domain of are also applicable to the intact, full-length toxin.

Retrograde signaling via axonal transport through signaling endosomes

Neurons extend axons far from cell bodies, and retrograde communications from distal axons to cell bodies and/or dendrites play critical roles in the development and maintenance of neuronal circuits. In neurotrophin signaling, the retrograde axonal transport of endosomes containing active ligand-receptor complexes from distal axons to somatodendrite compartments mediates retrograde signaling. However, the generality and specificity of these endosome-based transportations called "signaling endosomes" remain to be elucidated. Here, I summarize the discovery of semaphorin3A signaling endosomes, the first example other than neurotrophins to regulate dendritic development via AMPA receptor GluA2 localization in dendrites. The molecular components of Sema3A and neurotrophin signaling endosomes are distinct, but partially overlap to regulate specific and common cellular events. Because receptors are transported back to the cell bodies, neurons must replenish receptors on the growth cone surface to ensure continued response to the target-derived ligands. Recent findings have demonstrated that retrograde signaling endosomes also induce anterograde delivery of nascent receptors in neurotrophin signaling. The coupling between anterograde and retrograde axonal transport via signaling endosomes therefore plays a critical role in regulating proper neuronal network formation.

Frequency-dependent lipid raft uptake at rat diaphragm muscle axon terminals

INTRODUCTION

In motor neurons, cholera toxin B (CTB) binds to the cell-surface ganglioside GM1 and is internalized and transported via structurally unique components of plasma membranes (lipid rafts).

METHODS

Lipid raft uptake by axon terminals adjoining type-identified rat diaphragm muscle fibers was investigated using CTB and confocal imaging.

RESULTS

Lipid raft uptake increased significantly at higher frequency stimulation (80 Hz), compared with lower frequency (20 Hz) and unstimulated (0 Hz) conditions. The fraction of axon terminal occupied by CTB was ∼45% at 0- or 20-Hz stimulation, and increased to ∼65% at 80 Hz. Total CTB fluorescence intensity also increased (∼20%) after 80-Hz stimulation compared with 0 Hz.

DISCUSSION

Evidence of increased lipid raft uptake at high stimulation frequencies supports an important role for lipid raft signaling at rat diaphragm muscle axon terminals, primarily for motor units physiologically activated at the higher frequencies. Muscle Nerve 59:611-611, 2019.

Emerging Concepts and Functions of Autophagy as a Regulator of Synaptic Components and Plasticity

Protein homeostasis (proteostasis) is crucial to the maintenance of neuronal integrity and function. As the contact sites between neurons, synapses rely heavily on precisely regulated protein-protein interactions to support synaptic transmission and plasticity processes. Autophagy is an effective degradative pathway that can digest cellular components and maintain cellular proteostasis. Perturbations of autophagy have been implicated in aging and neurodegeneration due to a failure to remove damaged proteins and defective organelles. Recent evidence has demonstrated that autophagosome formation is prominent at synaptic terminals and neuronal autophagy is regulated in a compartment-specific fashion. Moreover, synaptic components including synaptic proteins and vesicles, postsynaptic receptors and synaptic mitochondria are known to be degraded by autophagy, thereby contributing to the remodeling of synapses. Indeed, emerging studies indicate that modulation of autophagy may be required for different forms of synaptic plasticity and memory formation. In this review, I will discuss our current understanding of the important role of neuronal/synaptic autophagy in maintaining neuronal function by degrading synaptic components and try to propose a conceptual framework of how the degradation of synaptic components via autophagy might impact synaptic function and contribute to synaptic plasticity.

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.

Vti1a/b regulate synaptic vesicle and dense core vesicle secretion via protein sorting at the Golgi

The SNAREs Vti1a/1b are implicated in regulated secretion, but their role relative to canonical exocytic SNAREs remains elusive. Here, we show that synaptic vesicle and dense-core vesicle (DCV) secretion is indeed severely impaired in Vti1a/b-deficient neurons. The synaptic levels of proteins that mediate secretion were reduced, down to 50% for the exocytic SNARE SNAP25. The delivery of SNAP25 and DCV-cargo into axons was decreased and these molecules accumulated in the Golgi. These defects were rescued by either Vti1a or Vti1b expression. Distended Golgi cisternae and clear vacuoles were observed in Vti1a/b-deficient neurons. The normal non-homogeneous distribution of DCV-cargo inside the Golgi was lost. Cargo trafficking out of, but not into the Golgi, was impaired. Finally, retrograde Cholera Toxin trafficking, but not Sortilin/Sorcs1 distribution, was compromised. We conclude that Vti1a/b support regulated secretion by sorting secretory cargo and synaptic secretion machinery components at the Golgi.

Cholinergic activity is essential for maintaining the anterograde transport of Choline Acetyltransferase in Drosophila

Cholinergic activity is essential for cognitive functions and neuronal homeostasis. Choline Acetyltransferase (ChAT), a soluble protein that synthesizes acetylcholine at the presynaptic compartment, is transported in bulk in the axons by the heterotrimeric Kinesin-2 motor. Axonal transport of soluble proteins is described as a constitutive process assisted by occasional, non-specific interactions with moving vesicles and motor proteins. Here, we report that an increase in the influx of Kinesin-2 motor and association between ChAT and the motor during a specific developmental period enhances the axonal entry, as well as the anterograde flow of the protein, in the sensory neurons of intact Drosophila nervous system. Loss of cholinergic activity due to  Hemicholinium and Bungarotoxin treatments, respectively, disrupts the interaction between ChAT and Kinesin-2 in the axon, and the episodic enhancement of axonal influx of the protein. Altogether, these observations highlight a phenomenon of synaptic activity-dependent, feedback regulation of a soluble protein transport in vivo, which could potentially define the quantum of its pre-synaptic influx.

Mechanisms of neurotrophin trafficking via Trk receptors

In neurons, long-distance communication between axon terminals and cell bodies is a critical determinant in establishing and maintaining neural circuits. Neurotrophins are soluble factors secreted by post-synaptic target tissues that retrogradely control axon and dendrite growth, survival, and synaptogenesis of innervating neurons. Neurotrophins bind Trk receptor tyrosine kinases in axon terminals to promote endocytosis of ligand-bound phosphorylated receptors into signaling endosomes. Trk-harboring endosomes function locally in axons to acutely promote growth events, and can also be retrogradely transported long-distances to remote cell bodies and dendrites to stimulate cytoplasmic and transcriptional signaling necessary for neuron survival, morphogenesis, and maturation. Neuronal responsiveness to target-derived neurotrophins also requires the precise axonal targeting of newly synthesized Trk receptors. Recent studies suggest that anterograde delivery of Trk receptors is regulated by retrograde neurotrophin signaling. In this review, we summarize current knowledge on the functions and mechanisms of retrograde trafficking of Trk signaling endosomes, and highlight recent discoveries on the forward trafficking of nascent receptors.

Visualizing endocytic recycling and trafficking in live neurons by subdiffractional tracking of internalized molecules

Our understanding of endocytic pathway dynamics is restricted by the diffraction limit of light microscopy. Although super-resolution techniques can overcome this issue, highly crowded cellular environments, such as nerve terminals, can also dramatically limit the tracking of multiple endocytic vesicles such as synaptic vesicles (SVs), which in turn restricts the analytical dissection of their discrete diffusional and transport states. We recently introduced a pulse-chase technique for subdiffractional tracking of internalized molecules (sdTIM) that allows the visualization of fluorescently tagged molecules trapped in individual signaling endosomes and SVs in presynapses or axons with 30- to 50-nm localization precision. We originally developed this approach for tracking single molecules of botulinum neurotoxin type A, which undergoes activity-dependent internalization and retrograde transport in autophagosomes. This method was then adapted to localize the signaling endosomes containing cholera toxin subunit-B that undergo retrograde transport in axons and to track SVs in the crowded environment of hippocampal presynapses. We describe (i) the construction of a custom-made microfluidic device that enables control over neuronal orientation; (ii) the 3D printing of a perfusion system for sdTIM experiments performed on glass-bottom dishes; (iii) the dissection, culturing and transfection of hippocampal neurons in microfluidic devices; and (iv) guidance on how to perform the pulse-chase experiments and data analysis. In addition, we describe the use of single-molecule-tracking analytical tools to reveal the average and the heterogeneous single-molecule mobility behaviors. We also discuss alternative reagents and equipment that can, in principle, be used for sdTIM experiments and describe how to adapt sdTIM to image nanocluster formation and/or tubulation in early endosomes during sorting events. The procedures described in this protocol take ∼1 week.

The impact of cytoskeletal organization on the local regulation of neuronal transport

Neurons are akin to modern cities in that both are dependent on robust transport mechanisms. Like the best mass transit systems, trafficking in neurons must be tailored to respond to local requirements. Neurons depend on both high-speed, long-distance transport and localized dynamics to correctly deliver cargoes and to tune synaptic responses. Here, we focus on the mechanisms that provide localized regulation of the transport machinery, including the cytoskeleton and molecular motors, to yield compartment-specific trafficking in the axon initial segment, axon terminal, dendrites and spines. The synthesis of these mechanisms provides a sophisticated and responsive transit system for the cell.

The SAC1 domain in synaptojanin is required for autophagosome maturation at presynaptic terminals

Presynaptic terminals are metabolically active and accrue damage through continuous vesicle cycling. How synapses locally regulate protein homeostasis is poorly understood. We show that the presynaptic lipid phosphatase synaptojanin is required for macroautophagy, and this role is inhibited by the Parkinson's disease mutation R258Q. Synaptojanin drives synaptic endocytosis by dephosphorylating PI(4,5)P₂, but this function appears normal in Synaptojanin^RQ knock-in flies. Instead, R258Q affects the synaptojanin SAC1 domain that dephosphorylates PI(3)P and PI(3,5)P₂, two lipids found in autophagosomal membranes. Using advanced imaging, we show that Synaptojanin^RQ mutants accumulate the PI(3)P/PI(3,5)P₂-binding protein Atg18a on nascent synaptic autophagosomes, blocking autophagosome maturation at fly synapses and in neurites of human patient induced pluripotent stem cell-derived neurons. Additionally, we observe neurodegeneration, including dopaminergic neuron loss, in Synaptojanin^RQ flies. Thus, synaptojanin is essential for macroautophagy within presynaptic terminals, coupling protein turnover with synaptic vesicle cycling and linking presynaptic-specific autophagy defects to Parkinson's disease.

Methodological advances in imaging intravital axonal transport

Axonal transport is the active process whereby neurons transport cargoes such as organelles and proteins anterogradely from the cell body to the axon terminal and retrogradely in the opposite direction. Bi-directional transport in axons is absolutely essential for the functioning and survival of neurons and appears to be negatively impacted by both aging and diseases of the nervous system, such as Alzheimer’s disease and amyotrophic lateral sclerosis. The movement of individual cargoes along axons has been studied in vitro in live neurons and tissue explants for a number of years; however, it is currently unclear as to whether these systems faithfully and consistently replicate the in vivo situation. A number of intravital techniques originally developed for studying diverse biological events have recently been adapted to monitor axonal transport in real-time in a range of live organisms and are providing novel insight into this dynamic process. Here, we highlight these methodological advances in intravital imaging of axonal transport, outlining key strengths and limitations while discussing findings, possible improvements, and outstanding questions.

Retrograde apoptotic signaling by the p75 neurotrophin receptor

Neurotrophins are target-derived factors necessary for mammalian nervous system development and maintenance. They are typically produced by neuronal target tissues and interact with their receptors at axonal endings. Therefore, locally generated neurotrophin signals must be conveyed from the axon back to the cell soma. Retrograde survival signaling by neurotrophin binding to Trk receptors has been extensively studied. However, neurotrophins also bind to the p75 receptor, which can induce apoptosis in a variety of contexts. Selective activation of p75 at distal axon ends has been shown to generate a retrograde apoptotic signal, although the mechanisms involved are poorly understood. The present review summarizes the available evidence for retrograde proapoptotic signaling in general and the role of the p75 receptor in particular, with discussion of unanswered questions in the field. In-depth knowledge of the mechanisms of retrograde apoptotic signaling is essential for understanding the etiology of neurodegeneration in many diseases and injuries.