The cellular mechanisms that maintain neuronal polarity

Axon initial segment structure and function in health and disease

At the simplest level, neurons are structured to integrate synaptic input and perform computational transforms on that input, converting it into an action potential (AP) code. This process, converting synaptic input into AP output, typically occurs in a specialized region of the axon termed the axon initial segment (AIS). The AIS, as its name implies, is often contained to the first section of axon abutted to the soma and is home to a dizzying array of ion channels, attendant scaffolding proteins, intracellular organelles, extracellular proteins, and, in some cases, synapses. The AIS serves multiple roles as the final arbiter for determining if inputs are sufficient to evoke APs, as a gatekeeper that physically separates the somatodendritic domain from the axon proper, and as a regulator of overall neuronal excitability, dynamically tuning its size to best suit the needs of parent neurons. These complex roles have received considerable attention from experimentalists and theoreticians alike. Here, we review recent advances in our understanding of the AIS and its role in neuronal integration and polarity in health and disease.

Cryo-ET suggests tubulin chaperones form a subset of microtubule lumenal particles with a role in maintaining neuronal microtubules

The functional architecture of the long-lived neuronal microtubule (MT) cytoskeleton is maintained by various MT-associated proteins (MAPs), most of which are known to bind to the MT outer surface. However, electron microscopy (EM) has long ago revealed the presence of particles inside the lumens of neuronal MTs, of yet unknown identity and function. Here, we use cryogenic electron tomography (cryo-ET) to analyze the three-dimensional (3D) organization and structures of MT lumenal particles in primary hippocampal neurons, human induced pluripotent stem cell-derived neurons, and pluripotent and differentiated P19 cells. We obtain in situ density maps of several lumenal particles from the respective cells and detect common structural features underscoring their potential overarching functions. Mass spectrometry-based proteomics combined with structural modeling suggest that a subset of lumenal particles could be tubulin-binding cofactors (TBCs) bound to tubulin monomers. A different subset of smaller particles, which remains unidentified, exhibits densities that bridge across the MT protofilaments. We show that increased lumenal particle concentration within MTs is concomitant with neuronal differentiation and correlates with higher MT curvatures. Enrichment of lumenal particles around MT lattice defects and at freshly polymerized MT open-ends suggests a MT protective role. Together with the identified structural resemblance of a subset of particles to TBCs, these results hint at a role in local tubulin proteostasis for the maintenance of long-lived neuronal MTs.

Glia detect and transiently protect against dendrite substructure disruption in C. elegans

Glia assess axon structure to modulate myelination and axon repair. Whether glia similarly detect dendrites and their substructures is not well understood. Here we show that glia monitor the integrity of dendrite substructures and transiently protect them against perturbations. We demonstrate that disruption of C. elegans sensory neuron dendrite cilia elicits acute glial responses, including increased accumulation of glia-derived extracellular matrix around cilia, changes in gene expression, and alteration of secreted protein repertoire. DGS-1, a 7-transmembrane domain neuronal protein, and FIG-1, a multifunctional thrombospondin-domain glial protein, are required for glial detection of cilia integrity, physically interact, and exhibit mutually-dependent localization to and around cilia, respectively. Glial responses to dendrite cilia disruption transiently protect against damage. Thus, our studies uncover a homeostatic, protective, dendrite-glia signaling interaction regulating dendrite substructure integrity.

Unraveling Axonal Transcriptional Landscapes: Insights from iPSC-Derived Cortical Neurons and Implications for Motor Neuron Degeneration

Neuronal function and pathology are deeply influenced by the distinct molecular profiles of the axon and soma. Traditional studies have often overlooked these differences due to the technical challenges of compartment specific analysis. In this study, we employ a robust RNA-sequencing (RNA-seq) approach, using microfluidic devices, to generate high-quality axonal transcriptomes from iPSC-derived cortical neurons (CNs). We achieve high specificity of axonal fractions, ensuring sample purity without contamination. Comparative analysis revealed a unique and specific transcriptional landscape in axonal compartments, characterized by diverse transcript types, including protein-coding mRNAs, RNAs encoding ribosomal proteins (RPs), mitochondrial-encoded RNAs, and long non-coding RNAs (lncRNAs). Previous works have reported the existence of transcription factors (TFs) in the axon. Here, we detect a set of TFs specific to the axon and indicative of their active participation in transcriptional regulation. To investigate transcripts and pathways essential for central motor neuron (MN) degeneration and maintenance we analyzed KIF1C-knockout (KO) CNs, modeling hereditary spastic paraplegia (HSP), a disorder associated with prominent length-dependent degeneration of central MN axons. We found that several key factors crucial for survival and health were absent in KIF1C-KO axons, highlighting a possible role of these also in other neurodegenerative diseases. Taken together, this study underscores the utility of microfluidic devices in studying compartment-specific transcriptomics in human neuronal models and reveals complex molecular dynamics of axonal biology. The impact of KIF1C on the axonal transcriptome not only deepens our understanding of MN diseases but also presents a promising avenue for exploration of compartment specific disease mechanisms.

The intracellular C-terminus confers compartment-specific targeting of voltage-gated calcium channels

To achieve the functional polarization that underlies brain computation, neurons sort protein material into distinct compartments. Ion channel composition, for example, differs between axons and dendrites, but the molecular determinants for their polarized trafficking remain obscure. Here, we identify mechanisms that target voltage-gated Ca2+ channels (CaVs) to distinct subcellular compartments. In hippocampal neurons, CaV2s trigger neurotransmitter release at the presynaptic active zone, and CaV1s localize somatodendritically. After knockout of all three CaV2s, expression of CaV2.1, but not CaV1.3, restores neurotransmitter release. We find that chimeric CaV1.3s with CaV2.1 intracellular C-termini localize to the active zone, mediate synaptic vesicle exocytosis, and render release sensitive to CaV1 blockers. This dominant targeting function of the CaV2.1 C-terminus requires the first EF hand in its proximal segment, and replacement of the CaV2.1 C-terminus with that of CaV1.3 abolishes CaV2.1 active zone localization and function. We conclude that CaV intracellular C-termini mediate compartment-specific targeting.

Rab6-Mediated Polarized Transport of Synaptic Vesicle Precursors Is Essential for the Establishment of Neuronal Polarity and Brain Formation

Neurons are highly polarized cells that are composed of a single axon and multiple dendrites. Axon-dendrite polarity is essential for proper tissue formation and brain functions. Intracellular protein transport plays an important role in the establishment of neuronal polarity. However, the regulatory mechanism of polarized transport remains unclear. Here, we show that Rab6, a small GTPase that acts on the regulation of intracellular vesicular trafficking, plays key roles in neuronal polarization and brain development. Central nervous system-specific Rab6a/b double knock-out (Rab6 DKO) mice of both sexes exhibit severe dysplasia of the neocortex and the cerebellum. In the Rab6 DKO neocortex, impaired axonal extension of neurons results in hypoplasia of the intermediate zone. In vitro, deletion of Rab6a and Rab6b in cultured neurons from both sexes causes the abnormal accumulation of synaptic vesicle precursors (SVPs) adjacent to the Golgi apparatus, which leads to defects in axonal extension and the loss of axon-dendrite polarity. Moreover, Rab6 DKO causes significant expansion of lysosomes in the soma in neurons. Overall, our results reveal that Rab6-mediated polarized transport of SVPs is crucial for neuronal polarization and subsequent brain formation.

A D-π-A-type ratiometric fluorescent probe to detect polarity changes and inhibition effect during ferroptosis

To thoroughly understand ferroptosis's biological functions in living cells, it is crucial to investigate the polarity variations that occur during this unique Fe(II)-facilitated oxidative type of cell death. In this work, we report the development of a ratiometric probe (Po-P) to visualize the polarity changes in living cells and the inhibition effect during ferroptosis. The polarity-responsive fluorophore utilized by Po-P has a D-π-A-type structure. Based on theoretical calculations, ICT was proposed as the basis for Po-P's polarity-responsive mechanism. According to cell imaging results, Po-P had a desirable capacity for monitoring polarity fluctuations and erastin-induced ferroptosis. Furthermore, inhibition imaging revealed that dihydrolipoic acid (DHLA) could potentially prevent polarity changes that occur during erastin-induced ferroptosis, just as vitamin E (VE). We anticipate that the probe Po-P could be a valuable tool to quickly monitor polarity fluctuations and inhibition effects during ferroptosis and create new medications for treating disorders related to ferroptosis.

Kinesin Regulation in the Proximal Axon is Essential for Dendrite-selective Transport

Neurons are polarized and typically extend multiple dendrites and one axon. To maintain polarity, vesicles carrying dendritic proteins are arrested upon entering the axon. To determine whether kinesin regulation is required for terminating anterograde axonal transport, we overexpressed the dendrite-selective kinesin KIF13A. This caused mistargeting of dendrite-selective vesicles to the axon and a loss of dendritic polarity. Polarity was not disrupted if the kinase MARK2/Par1b was coexpressed. MARK2/Par1b is concentrated in the proximal axon, where it maintains dendritic polarity-likely by phosphorylating S1371 of KIF13A, which lies in a canonical 14-3-3 binding motif. We probed for interactions of KIF13A with 14-3-3 isoforms and found that 14-3-3β and 14-3-3ζ bound KIF13A. Disruption of MARK2 or 14-3-3 activity by small molecule inhibitors caused a loss of dendritic polarity. These data show that kinesin regulation is integral for dendrite-selective transport. We propose a new model in which KIF13A that moves dendrite-selective vesicles in the proximal axon is phosphorylated by MARK2. Phosphorylated KIF13A is then recognized by 14-3-3, which causes dissociation of KIF13A from the vesicle and termination of transport. These findings define a new paradigm for the regulation of vesicle transport by localized kinesin tail phosphorylation, to restrict dendrite-selective vesicles from entering the axon.

MARK2 phosphorylates KIF13A at a 14-3-3 binding site to polarize vesicular transport of transferrin receptor within dendrites

Neurons regulate the microtubule-based transport of certain vesicles selectively into axons or dendrites to ensure proper polarization of function. The mechanism of this polarized vesicle transport is still not fully elucidated, though it is known to involve kinesins, which drive anterograde transport on microtubules. Here, we explore how the kinesin-3 family member KIF13A is regulated such that vesicles containing transferrin receptor (TfR) travel only to dendrites. In experiments involving live-cell imaging, knockout of KIF13A, BioID assay, we found that the kinase MARK2 phosphorylates KIF13A at a 14-3-3 binding motif, strengthening interaction of KIF13A with 14-3-3 such that it dissociates from TfR-containing vesicles, which therefore cannot enter axons. Overexpression of KIF13A or knockout of MARK2 leads to axonal transport of TfR-containing vesicles. These results suggest a unique kinesin-based mechanism for polarized transport of vesicles to dendrites.

Polarized transport requires AP-1-mediated recruitment of KIF13A and KIF13B at the trans-Golgi

Neurons are polarized cells that require accurate membrane trafficking to maintain distinct protein complements at dendritic and axonal membranes. The Kinesin-3 family members KIF13A and KIF13B are thought to mediate dendrite-selective transport, but the mechanism by which they are recruited to polarized vesicles and the differences in the specific trafficking role of each KIF13 have not been defined. We performed live-cell imaging in cultured hippocampal neurons and found that KIF13A is a dedicated dendrite-selective kinesin. KIF13B confers two different transport modes, dendrite- and axon-selective transport. Both KIF13s are maintained at the trans-Golgi network by interactions with the heterotetrameric adaptor protein complex AP-1. Interference with KIF13 binding to AP-1 resulted in disruptions to both dendrite- and axon-selective trafficking. We propose that AP-1 is the molecular link between the sorting of polarized cargoes into vesicles and the recruitment of kinesins that confer polarized transport.

Location of the axon initial segment assembly can be predicted from neuronal shape

The axon initial segment (AIS) is located at the proximal axon demarcating the boundary between axonal and somatodendritic compartments. The AIS facilitates the generation of action potentials and maintenance of neuronal polarity. In this study, we show that the location of AIS assembly, as marked by Ankyrin G, corresponds to the nodal plane of the lowest-order harmonic of the Laplace-Beltrami operator solved over the neuronal shape. This correlation establishes a coupling between location of AIS assembly and neuronal cell morphology. We validate this correlation for neurons with atypical morphology and neurons containing multiple AnkG clusters on distinct neurites, where the nodal plane selects the appropriate axon showing enriched Tau. Based on our findings, we propose that Turing patterning systems are candidates for dynamically governing AIS location. Overall, this study highlights the importance of neuronal cell morphology in determining the precise localization of the AIS within the proximal axon.

Expressing Optogenetic Actuators Fused to N-terminal Mucin Motifs Delivers Targets to Specific Subcellular Compartments in Polarized Cells

Optogenetics is a powerful approach in neuroscience research. However, other tissues of the body may benefit from controlled ion currents and neuroscience may benefit from more precise optogenetic expression. The present work constructs three subcellularly-targeted optogenetic actuators based on the channelrhodopsin ChR2-XXL, utilizing 5, 10, or 15 tandem repeats (TR) from mucin as N-terminal targeting motifs and evaluates expression in several polarized and non-polarized cell types. The modified channelrhodopsin maintains its electrophysiological properties, which can be used to produce continuous membrane depolarization, despite the expected size of the repeats. This work then shows that these actuators are subcellularly localized in polarized cells. In polarized epithelial cells, all three actuators localize to just the lateral membrane. The TR-tagged constructs also express subcellularly in cortical neurons, where TR5-ChR2XXL and TR10-ChR2XXL mainly target the somatodendrites. Moreover, the transfection efficiencies are shown to be dependent on cell type and tandem repeat length. Overall, this work verifies that the targeting motifs from epithelial cells can be used to localize optogenetic actuators in both epithelia and neurons, opening epithelia processes to optogenetic manipulation and providing new possibilities to target optogenetic tools.

Cell polarity changes in cancer initiation and progression

Cell polarity, which consists of the morphological, structural, and functional organization of cells along a defined axis, is a feature of healthy cells and tissues. In contrast, abnormal polarity is a hallmark of cancer cells. At the molecular level, key evolutionarily conserved proteins that control polarity establishment and maintenance in various contexts are frequently altered in cancer, but the relevance of these molecular alterations in the oncogenic processes is not always clear. Here, we summarize the recent findings, shedding new light on the involvement of polarity players in cancer development, and discuss the possibility of harnessing cell polarity changes to better predict, diagnose, and cure cancers.

The intracellular C-terminus confers compartment-specific targeting of voltage-gated Ca2+ channels

To achieve the functional polarization that underlies brain computation, neurons sort protein material into distinct compartments. Ion channel composition, for example, differs between axons and dendrites, but the molecular determinants for their polarized trafficking remain obscure. Here, we identify the mechanisms that target voltage-gated Ca2+ channels (CaVs) to distinct subcellular compartments. In hippocampal neurons, CaV2s trigger neurotransmitter release at the presynaptic active zone, and CaV1s localize somatodendritically. After knockout of all three CaV2s, expression of CaV2.1, but not of CaV1.3, restores neurotransmitter release. Chimeric CaV1.3 channels with CaV2.1 intracellular C-termini localize to the active zone, mediate synaptic vesicle exocytosis, and render release fully sensitive to blockade of CaV1 channels. This dominant targeting function of the CaV2.1 C-terminus requires an EF hand in its proximal segment, and replacement of the CaV2.1 C-terminus with that of CaV1.3 abolishes CaV2.1 active zone localization. We conclude that the intracellular C-termini mediate compartment-specific CaV targeting.

MARK2 phosphorylates KIF13A at a 14-3-3 binding site to polarize vesicular transport of transferrin receptor within dendrites

Neurons regulate the microtubule-based transport of certain vesicles selectively into axons or dendrites to ensure proper polarization of function. The mechanism of this polarized vesicle transport is still not fully elucidated, though it is known to involve kinesins, which drive anterograde transport on microtubules. Here we explore how the kinesin-3 family member KIF13A is regulated such that vesicles containing transferrin receptor (TfR) travel only to dendrites. In experiments involving live-cell imaging, knockout of KIF13A, BioID assay, we found that the kinase MARK2 phosphorylates KIF13A at a 14-3-3 binding motif, strengthening interaction of KIF13A with 14-3-3 such that it dissociates from TfR-containing vesicles, which therefore cannot enter axons. Overexpression of KIF13A or knockout of MARK2 leads to axonal transport of TfR-containing vesicles. These results suggest a novel kinesin-based mechanism for polarized transport of vesicles to dendrites.

Significance

Our findings suggest that at least one type of vesicles, those containing transferrin receptor, travel exclusively to dendrites and are excluded from axons because the kinase MARK2 phosphorylates the kinesin KIF13A to promote its separation from vesicles at the proximal axon, preventing vesicle transport into axons, such that they travel only to dendrites. Future studies should explore how this mechanism of polarized vesicle transport supports neuronal function.

Glia detect and mount a protective response to loss of dendrite substructure integrity in C. elegans

Neurons have elaborate structures that determine their connectivity and functions. Changes in neuronal structure accompany learning and memory formation and are hallmarks of neurological disease. Here we show that glia monitor dendrite structure and respond to dendrite perturbation. In C. elegans mutants with defective sensory-organ dendrite cilia, adjacent glia accumulate extracellular matrix-laden vesicles, secrete excess matrix around cilia, alter gene expression, and change their secreted protein repertoire. Inducible cilia disruption reveals that this response is acute. DGS-1, a 7-transmembrane domain neuronal protein, and FIG-1, a multifunctional thrombospondin-domain glial protein, are required for glial detection of cilia integrity, and exhibit mutually-dependent localization to and around cilia, respectively. While inhibiting glial secretion disrupts dendritic cilia properties, hyperactivating the glial response protects against dendrite damage. Our studies uncover a homeostatic protective dendrite-glia interaction and suggest that similar signaling occurs at other sensory structures and at synapses, which resemble sensory organs in architecture and molecules.

Intracellular traffic and polarity in brain development

Neurons forming the human brain are generated during embryonic development by neural stem and progenitor cells via a process called neurogenesis. A crucial feature contributing to neural stem cell morphological and functional heterogeneity is cell polarity, defined as asymmetric distribution of cellular components. Cell polarity is built and maintained thanks to the interplay between polarity proteins and polarity-generating organelles, such as the endoplasmic reticulum (ER) and the Golgi apparatus (GA). ER and GA affect the distribution of membrane components and work as a hub where glycans are added to nascent proteins and lipids. In the last decades our knowledge on the role of polarity in neural stem and progenitor cells have increased tremendously. However, the role of traffic and associated glycosylation in neural stem and progenitor cells is still relatively underexplored. In this review, we discuss the link between cell polarity, architecture, identity and intracellular traffic, and highlight how studies on neurons have shaped our knowledge and conceptual framework on traffic and polarity. We will then conclude by discussing how a group of rare diseases, called congenital disorders of glycosylation (CDG) offers the unique opportunity to study the contribution of traffic and glycosylation in the context of neurodevelopment.

Synaptic vesicle proteins are selectively delivered to axons in mammalian neurons

Neurotransmitter-filled synaptic vesicles (SVs) mediate synaptic transmission and are a hallmark specialization in neuronal axons. Yet, how SV proteins are sorted to presynaptic nerve terminals remains the subject of debate. The leading model posits that these proteins are randomly trafficked throughout neurons and are selectively retained in presynaptic boutons. Here, we used the RUSH (retention using selective hooks) system, in conjunction with HaloTag labeling approaches, to study the egress of two distinct transmembrane SV proteins, synaptotagmin 1 and synaptobrevin 2, from the soma of mature cultured rat and mouse neurons. For these studies, the SV reporter constructs were expressed at carefully controlled, very low levels. In sharp contrast to the selective retention model, both proteins selectively and specifically entered axons with minimal entry into dendrites. However, even moderate overexpression resulted in the spillover of SV proteins into dendrites, potentially explaining the origin of previous non-polarized transport models, revealing the limited, saturable nature of the direct axonal trafficking pathway. Moreover, we observed that SV constituents were first delivered to the presynaptic plasma membrane before incorporation into SVs. These experiments reveal a new-found membrane trafficking pathway, for SV proteins, in classically polarized mammalian neurons and provide a glimpse at the first steps of SV biogenesis.

KAP is the neuronal organelle adaptor for Kinesin-2 KIF3AB and KIF3AC

Kinesin-driven organelle transport is crucial for neuron development and maintenance, yet the mechanisms by which kinesins specifically bind their organelle cargoes remain undefined. In contrast to other transport kinesins, the neuronal function and specific organelle adaptors of heterodimeric Kinesin-2 family members KIF3AB and KIF3AC remain unknown. We developed a novel microscopy-based assay to define protein-protein interactions in intact neurons. The experiments found that both KIF3AB and KIF3AC bind kinesin-associated protein (KAP). These interactions are mediated by the distal C-terminal tail regions and not the coiled-coil domain. We used live-cell imaging in cultured hippocampal neurons to define the localization and trafficking parameters of KIF3AB and KIF3AC organelle populations. We discovered that KIF3AB/KAP and KIF3AC/KAP bind the same organelle populations and defined their transport parameters in axons and dendrites. The results also show that ∼12% of KIF3 organelles contain the RNA-binding protein adenomatous polyposis coli. These data point toward a model in which KIF3AB and KIF3AC use KAP as their neuronal organelle adaptor and that these kinesins mediate transport of a range of organelles.

Propofol attenuates kinesin-mediated axonal vesicle transport and fusion

Propofol is a widely used general anesthetic, yet the understanding of its cellular effects is fragmentary. General anesthetics are not as innocuous as once believed and have a wide range of molecular targets that include kinesin motors. Propofol, ketamine, and etomidate reduce the distances that Kinesin-1 KIF5 and Kinesin-2 KIF3 travel along microtubules in vitro. These transport kinesins are highly expressed in the CNS, and their dysfunction leads to a range of human pathologies including neurodevelopmental and neurodegenerative diseases. While in vitro data suggest that general anesthetics may disrupt kinesin transport in neurons, this hypothesis remains untested. Here we find that propofol treatment of hippocampal neurons decreased vesicle transport mediated by Kinesin-1 KIF5 and Kinesin-3 KIF1A ∼25-60%. Propofol treatment delayed delivery of the KIF5 cargo NgCAM to the distal axon. Because KIF1A participates in axonal transport of presynaptic vesicles, we tested whether prolonged propofol treatment affects synaptic vesicle fusion mediated by VAMP2. The data show that propofol-induced transport delay causes a significant decrease in vesicle fusion in distal axons. These results are the first to link a propofol-induced delay in neuronal trafficking to a decrease in axonal vesicle fusion, which may alter physiological function during and after anesthesia.

The kinesin motor KIF1C is a putative transporter of the exon junction complex in neuronal cells

Neurons critically depend on regulated RNA localization and tight control of spatio-temporal gene expression to maintain their morphological and functional integrity. Mutations in the kinesin motor protein gene KIF1C cause Hereditary Spastic Paraplegia, an autosomal recessive disease leading to predominant degeneration of the long axons of central motoneurons. In this study we aimed to gain insight into the molecular function of KIF1C and understand how KIF1C dysfunction contributes to motoneuron degeneration. We used affinity proteomics in neuronally differentiated neuroblastoma cells (SH-SY5Y) to identify the protein complex associated with KIF1C in neuronal cells; candidate interactions were then validated by immunoprecipitation and mislocalization of putative KIF1C cargoes was studied by immunostainings. We found KIF1C to interact with all core components of the exon junction complex (EJC); expression of mutant KIF1C in neuronal cells leads to loss of the typical localization distally in neurites. Instead, EJC core components accumulate in the pericentrosomal region, here co-localizing with mutant KIF1C. These findings suggest KIF1C as a neuronal transporter of the EJC. Interestingly, the binding of KIF1C to the EJC is RNA-mediated, as treatment with RNAse prior to immunoprecipitation almost completely abolishes the interaction. Silica-based solid-phase extraction of UV-crosslinked RNA-protein complexes furthermore supports direct interaction of KIF1C with RNA, as recently also demonstrated for kinesin heavy chain. Taken together, our findings are consistent with a model where KIF1C transports mRNA in an EJC-bound and therefore transcriptionally silenced state along neurites, thus providing the missing link between the EJC and mRNA localization in neurons.

Dynamics of axonal β-actin mRNA in live hippocampal neurons

Localization of mRNA facilitates spatiotemporally controlled protein expression in neurons. In axons, mRNA transport followed by local protein synthesis plays a critical role in axonal growth and guidance. However, it is not yet clearly understood how mRNA is transported to axonal subcellular sites and what regulates axonal mRNA localization. Using a transgenic mouse model in which endogenous β-actin mRNA is fluorescently labeled, we investigated β-actin mRNA movement in axons of hippocampal neurons. We cultured neurons in microfluidic devices to separate axons from dendrites and performed single-particle tracking of axonal β-actin mRNA. Compared with dendritic β-actin mRNA, axonal β-actin mRNA showed less directed motion and exhibited mostly subdiffusive motion, especially near filopodia and boutons in mature dissociated hippocampal neurons. We found that axonal β-actin mRNA was likely to colocalize with actin patches (APs), regions that have a high density of filamentous actin (F-actin) and are known to have a role in branch initiation. Moreover, simultaneous imaging of F-actin and axonal β-actin mRNA in live neurons revealed that moving β-actin mRNA tended to be docked in the APs. Our findings reveal that axonal β-actin mRNA localization is facilitated by actin networks and suggest that localized β-actin mRNA plays a potential role in axon branch formation.

Endocytosis in the axon initial segment maintains neuronal polarity

Neurons are highly polarized cells that face the fundamental challenge of compartmentalizing a vast and diverse repertoire of proteins in order to function properly1. The axon initial segment (AIS) is a specialized domain that separates a neuron's morphologically, biochemically and functionally distinct axon and dendrite compartments2,3. How the AIS maintains polarity between these compartments is not fully understood. Here we find that in Caenorhabditis elegans, mouse, rat and human neurons, dendritically and axonally polarized transmembrane proteins are recognized by endocytic machinery in the AIS, robustly endocytosed and targeted to late endosomes for degradation. Forcing receptor interaction with the AIS master organizer, ankyrinG, antagonizes receptor endocytosis in the AIS, causes receptor accumulation in the AIS, and leads to polarity deficits with subsequent morphological and behavioural defects. Therefore, endocytic removal of polarized receptors that diffuse into the AIS serves as a membrane-clearance mechanism that is likely to work in conjunction with the known AIS diffusion-barrier mechanism to maintain neuronal polarity on the plasma membrane. Our results reveal a conserved endocytic clearance mechanism in the AIS to maintain neuronal polarity by reinforcing axonal and dendritic compartment membrane boundaries.

Transcytosis and trans-synaptic retention by postsynaptic ErbB4 underlie axonal accumulation of NRG3

Neuregulins (NRGs) are EGF-like ligands associated with cognitive disorders. Unprocessed proNRG3 is cleaved by BACE1 to generate the mature membrane-bound NRG3 ligand, but the subcellular site of proNRG3 cleavage, mechanisms underlying its transport into axons, and presynaptic accumulation remain unknown. Using an optogenetic proNRG3 cleavage reporter (LA143-NRG3), we investigate the spatial-temporal dynamics of NRG3 processing and sorting in neurons. In dark conditions, unprocessed LA143-NRG3 is retained in the trans-Golgi network but, upon photoactivation, is cleaved by BACE1 and released from the TGN. Mature NRG3 then emerges on the somatodendritic plasma membrane from where it is re-endocytosed and anterogradely transported on Rab4+ vesicles into axons via transcytosis. By contrast, the BACE1 substrate APP is sorted into axons on Rab11+ vesicles. Lastly, by a mechanism we denote "trans-synaptic retention," NRG3 accumulates at presynaptic terminals by stable interaction with its receptor ErbB4 on postsynaptic GABAergic interneurons. We propose that trans-synaptic retention may account for polarized expression of other neuronal transmembrane ligands and receptors.

Axon-dendrite and apical-basolateral sorting in a single neuron

Cells are highly organized machines with functionally specialized compartments. For example, membrane proteins are localized to axons or dendrites in neurons and to apical or basolateral surfaces in epithelial cells. Interestingly, many sensory cells-including vertebrate photoreceptors and olfactory neurons-exhibit both neuronal and epithelial features. Here, we show that Caenorhabditis elegans amphid neurons simultaneously exhibit axon-dendrite sorting like a neuron and apical-basolateral sorting like an epithelial cell. The distal ∼5-10 µm of the dendrite is apical, while the remainder of the dendrite, soma, and axon are basolateral. To determine how proteins are sorted among these compartments, we studied the localization of the conserved adhesion molecule SAX-7/L1CAM. Using minimal synthetic transmembrane proteins, we found that the 91-aa cytoplasmic tail of SAX-7 is necessary and sufficient to direct basolateral localization. Basolateral localization can be fully recapitulated using either of 2 short (10-aa or 19-aa) tail sequences that, respectively, resemble dileucine and Tyr-based motifs known to mediate sorting in mammalian epithelia. The Tyr-based motif is conserved in human L1CAM but had not previously been assigned a function. Disrupting key residues in either sequence leads to apical localization, while "improving" them to match epithelial sorting motifs leads to axon-only localization. Indeed, changing only 2 residues in a short motif is sufficient to redirect the protein between apical, basolateral, and axonal localization. Our results demonstrate that axon-dendrite and apical-basolateral sorting pathways can coexist in a single cell, and suggest that subtle changes to short sequence motifs are sufficient to redirect proteins between these pathways.

BARS Influences Neuronal Development by Regulation of Post-Golgi Trafficking

Neurons are highly polarized cells requiring precise regulation of trafficking and targeting of membrane proteins to generate and maintain different and specialized compartments, such as axons and dendrites. Disruption of the Golgi apparatus (GA) secretory pathway in developing neurons alters axon/dendritic formation. Therefore, detailed knowledge of the mechanisms underlying vesicles exiting from the GA is crucial for understanding neuronal polarity. In this study, we analyzed the role of Brefeldin A-Ribosylated Substrate (CtBP1-S/BARS), a member of the C-terminal-binding protein family, in the regulation of neuronal morphological polarization and the exit of membrane proteins from the Trans Golgi Network. Here, we show that BARS is expressed during neuronal development in vitro and that RNAi suppression of BARS inhibits axonal and dendritic elongation in hippocampal neuronal cultures as well as largely perturbed neuronal migration and multipolar-to-bipolar transition during cortical development in situ. In addition, using plasma membrane (PM) proteins fused to GFP and engineered with reversible aggregation domains, we observed that expression of fission dominant-negative BARS delays the exit of dendritic and axonal membrane protein-containing carriers from the GA. Taken together, these data provide the first set of evidence suggesting a role for BARS in neuronal development by regulating post-Golgi membrane trafficking.

Visualizing Vesicle-Bound Kinesins in Cultured Hippocampal Neurons

Eukaryotic cells use microtubule-based vesicle transport to exchange molecules between compartments. Kinesin family members mediate all microtubule plus end-directed vesicle transport. Of the 45 kinesins expressed in humans, some 20 mediate microtubule plus-end directed vesicle transport. Here we describe a technique to visualize vesicle-bound kinesins in cultured hippocampal neurons. The method involves the expression of the vesicle-binding tail domain while minimizing the cytoplasmic pool. Using this approach drastically improves vesicle labeling compared to full-length kinesins. This tool is useful for systematically comparing the localization of different kinesins in the same cell type and for identifying cargo proteins that reside in vesicles moved by a specific kinesin family member. While we describe the assay in cultured hippocampal neurons, we expect it to be easily transferable to other eukaryotic cell types.

Imaging Diversity in Slow Axonal Transport

The polarized morphology of neurons necessitates the delivery of proteins synthesized in the soma along the length of the axon to distal synapses; critical for sustaining communication between neurons. This constitutive and dynamic process of protein transport along axons termed "axonal transport" was initially characterized by classic pulse-chase radiolabeling studies which identified two major rate components: a fast component and a slow component. Early radiolabeling studies indicated "cohesive co-transport" of slow transport cargos. However, this approach could not be used to visualize or provide mechanistic insights on this highly dynamic process. The advent of fluorescent and photoactivatable imaging probes have now enabled real-time imaging of axonal transport. Conventional fluorescent probes have helped visualize and characterize the molecular mechanisms of transport of vesicular proteins. These proteins typically move in the fast component of axonal transport and appear as "punctate structures" along axons. However, a large majority of transported proteins that move in the slow component of transport, typically show a "uniform diffusive glow" along axons when tagged to conventional fluorescent probes. This makes it challenging to unequivocally track them in real time. Our lab has used photoactivatable fluorescent probes to tag three individual cytosolic proteins moving in the slow component of axonal transport, and identified three distinct modes of transport along axons. Our data from these experiments argue against the prevailing hypothesis based on classic radiolabeling studies, which suggested that all slow-transport proteins may move along the axon as one large macromolecular protein complex. Although other labs have started using photoactivation to study axonal transport of cytosolic proteins, this technique remains largely under-utilized. Here, we describe the detailed protocols to image and analyze axonal transport of three typical slow-component cargoes along axons of cultured hippocampal neurons.

NgCAM and VAMP2 reveal that direct delivery and dendritic degradation maintain axonal polarity

Neurons are polarized cells of extreme scale and compartmentalization. To fulfill their role in electrochemical signaling, axons must maintain a specific complement of membrane proteins. Despite being the subject of considerable attention, the trafficking pathway of axonal membrane proteins is not well understood. Two pathways, direct delivery and transcytosis, have been proposed. Previous studies reached contradictory conclusions about which of these mediates delivery of axonal membrane proteins to their destination, in part because they evaluated long-term distribution changes and not vesicle transport. We developed a novel strategy to selectively label vesicles in different trafficking pathways and determined the trafficking of two canonical axonal membrane proteins, neuron-glia cell adhesion molecule and vesicle-associated membrane protein-2. Results from detailed quantitative analyses of transporting vesicles differed substantially from previous studies and found that axonal membrane proteins overwhelmingly undergo direct delivery. Transcytosis plays only a minor role in axonal delivery of these proteins. In addition, we identified a novel pathway by which wayward axonal proteins that reach the dendritic plasma membrane are targeted to lysosomes. These results redefine how axonal proteins achieve their polarized distribution, a crucial requirement for elucidating the underlying molecular mechanisms.

Microtubule dynamics influence the retrograde biased motility of kinesin-4 motor teams in neuronal dendrites

Microtubules establish the directionality of intracellular transport by kinesins and dynein through polarized assembly, but it remains unclear how directed transport occurs along microtubules organized with mixed polarity. We investigated the ability of the plus end–directed kinesin-4 motor KIF21B to navigate mixed polarity microtubules in mammalian dendrites. Reconstitution assays with recombinant KIF21B and engineered microtubule bundles or extracted neuronal cytoskeletons indicate that nucleotide-independent microtubule-binding regions of KIF21B modulate microtubule dynamics and promote directional switching on antiparallel microtubules. Optogenetic recruitment of KIF21B to organelles in live neurons induces unidirectional transport in axons but bidirectional transport with a net retrograde bias in dendrites. Removal of the secondary microtubule-binding regions of KIF21B or dampening of microtubule dynamics with low concentrations of nocodazole eliminates retrograde bias in live dendrites. Further exploration of the contribution of microtubule dynamics in dendrites to directionality revealed plus end–out microtubules to be more dynamic than plus end–in microtubules, with nocodazole preferentially stabilizing the plus end–out population. We propose a model in which both nucleotide-sensitive and -insensitive microtubule-binding sites of KIF21B motors contribute to the search and selection of stable plus end–in microtubules within the mixed polarity microtubule arrays characteristic of mammalian dendrites to achieve net retrograde movement of KIF21B-bound cargoes.

The vSNAREs VAMP2 and VAMP4 control recycling and intracellular sorting of post-synaptic receptors in neuronal dendrites

The endosomal recycling system dynamically tunes synaptic strength, which underlies synaptic plasticity. Exocytosis is involved in the expression of long-term potentiation (LTP), as postsynaptic cleavage of the SNARE (soluble NSF-attachment protein receptor) protein VAMP2 by tetanus toxin blocks LTP. Moreover, induction of LTP increases the exocytosis of transferrin receptors (TfRs) and markers of recycling endosomes (REs), as well as post-synaptic AMPA type receptors (AMPARs). However, the interplay between AMPAR and TfR exocytosis remains unclear. Here, we identify VAMP4 as the vesicular SNARE that mediates most dendritic RE exocytosis. In contrast, VAMP2 plays a minor role in RE exocytosis. LTP induction increases the exocytosis of both VAMP2- and VAMP4-labeled organelles. Knock down (KD) of VAMP4 decreases TfR recycling but increases AMPAR recycling. Moreover, VAMP4 KD increases AMPAR-mediated synaptic transmission, which consequently occludes LTP expression. The opposing changes in AMPAR and TfR recycling upon VAMP4 KD reveal their sorting into separate endosomal populations.

Inducible manipulation of motor-cargo interaction using engineered kinesin motors

Molecular motors drive long-range intracellular transport of various vesicles and other cargoes within a cell. Identifying which kinesin motors interact with which type of transport vesicles has been challenging, especially in complex neuronal cells. Here, we present a highly adaptable toolbox of engineered kinesin motors to control and interrogate the selectivity and regulation of cargo transport with acute chemical induction. Selectivity of cargo-motor interaction can be addressed by systematic screening of a library of kinesin tails and neuronal cargoes. Additionally, our toolbox can be used to study kinesin-cargo regulatory mechanisms, and we found that cargo trafficking by KIF16B is regulated by its PX domain. Furthermore, our toolbox enables acute manipulation of polarized trafficking in living neurons by steering transport into axons or dendrites. Engineering kinesin motors provides a powerful tool to map the specificity of interactions between kinesin and cargoes, manipulate polarized transport and investigate cargo-motor interaction modes.

Quantitative mapping of dense microtubule arrays in mammalian neurons

The neuronal microtubule cytoskeleton underlies the polarization and proper functioning of neurons, amongst others by providing tracks for motor proteins that drive intracellular transport. Different subsets of neuronal microtubules, varying in composition, stability, and motor preference, are known to exist, but the high density of microtubules has so far precluded mapping their relative abundance and three-dimensional organization. Here, we use different super-resolution techniques (STED, Expansion Microscopy) to explore the nanoscale organization of the neuronal microtubule network in rat hippocampal neurons. This revealed that in dendrites acetylated microtubules are enriched in the core of the dendritic shaft, while tyrosinated microtubules are enriched near the plasma membrane, thus forming a shell around the acetylated microtubules. Moreover, using a novel analysis pipeline we quantified the absolute number of acetylated and tyrosinated microtubules within dendrites and found that they account for 65-75% and ~20-30% of all microtubules, respectively, leaving only few microtubules that do not fall in either category. Because these different microtubule subtypes facilitate different motor proteins, these novel insights help to understand the spatial regulation of intracellular transport.

ER - lysosome contacts at a pre-axonal region regulate axonal lysosome availability

Neuronal function relies on careful coordination of organelle organization and transport. Kinesin-1 mediates transport of the endoplasmic reticulum (ER) and lysosomes into the axon and it is increasingly recognized that contacts between the ER and lysosomes influence organelle organization. However, it is unclear how organelle organization, inter-organelle communication and transport are linked and how this contributes to local organelle availability in neurons. Here, we show that somatic ER tubules are required for proper lysosome transport into the axon. Somatic ER tubule disruption causes accumulation of enlarged and less motile lysosomes at the soma. ER tubules regulate lysosome size and axonal translocation by promoting lysosome homo-fission. ER tubule - lysosome contacts often occur at a somatic pre-axonal region, where the kinesin-1-binding ER-protein P180 binds microtubules to promote kinesin-1-powered lysosome fission and subsequent axonal translocation. We propose that ER tubule - lysosome contacts at a pre-axonal region finely orchestrate axonal lysosome availability for proper neuronal function.

Specific KIF1A-adaptor interactions control selective cargo recognition

Intracellular transport in neurons is driven by molecular motors that carry many different cargos along cytoskeletal tracks in axons and dendrites. Identifying how motors interact with specific types of transport vesicles has been challenging. Here, we use engineered motors and cargo adaptors to systematically investigate the selectivity and regulation of kinesin-3 family member KIF1A–driven transport of dense core vesicles (DCVs), lysosomes, and synaptic vesicles (SVs). We dissect the role of KIF1A domains in motor activity and show that CC1 regulates autoinhibition, CC2 regulates motor dimerization, and CC3 and PH mediate cargo binding. Furthermore, we identify that phosphorylation of KIF1A is critical for binding to vesicles. Cargo specificity is achieved by specific KIF1A adaptors; MADD/Rab3GEP links KIF1A to SVs, and Arf-like GTPase Arl8A mediates interactions with DCVs and lysosomes. We propose a model where motor dimerization, posttranslational modifications, and specific adaptors regulate selective KIF1A cargo trafficking.

The functions of kinesin and kinesin-related proteins in eukaryotes

Kinesins constitute a superfamily of ATP-driven microtubule motor enzymes that convert the chemical energy of ATP hydrolysis into mechanical work along microtubule tracks. Kinesins are found in all eukaryotic organisms and are essential to all eukaryotic cells, involved in diverse cellular functions such as microtubule dynamics and morphogenesis, chromosome segregation, spindle formation and elongation and transport of organelles. In this review, we explore recently reported functions of kinesins in eukaryotes and compare their specific cargoes in both plant and animal kingdoms to understand the possible roles of uncharacterized motors in a kingdom based on their reported functions in other kingdoms.

EFA6 in Axon Regeneration, as a Microtubule Regulator and as a Guanine Nucleotide Exchange Factor

Axon regeneration after injury is a conserved biological process that involves a large number of molecular pathways, including rapid calcium influx at injury sites, retrograde injury signaling, epigenetic transition, transcriptional reprogramming, polarized transport, and cytoskeleton reorganization. Despite the numerous efforts devoted to understanding the underlying cellular and molecular mechanisms of axon regeneration, the search continues for effective target molecules for improving axon regeneration. Although there have been significant historical efforts towards characterizing pro-regenerative factors involved in axon regeneration, the pursuit of intrinsic inhibitors is relatively recent. EFA6 (exchange factor for ARF6) has been demonstrated to inhibit axon regeneration in different organisms. EFA6 inhibition could be a promising therapeutic strategy to promote axon regeneration and functional recovery after axon injury. This review summarizes the inhibitory role on axon regeneration through regulating microtubule dynamics and through affecting ARF6 (ADP-ribosylation factor 6) GTPase-mediated integrin transport.

Leep1 interacts with PIP3 and the Scar/WAVE complex to regulate cell migration and macropinocytosis

Polarity is essential for diverse functions in many cell types. Establishing polarity requires targeting a network of specific signaling and cytoskeleton molecules to different subregions of the cell, yet the full complement of polarity regulators and how their activities are integrated over space and time to form morphologically and functionally distinct domains remain to be uncovered. Here, by using the model system Dictyostelium and exploiting the characteristic chemoattractant-stimulated translocation of polarly distributed molecules, we developed a proteomic screening approach, through which we identified a leucine-rich repeat domain–containing protein we named Leep1 as a novel polarity regulator. We combined imaging, biochemical, and phenotypic analyses to demonstrate that Leep1 localizes selectively at the leading edge of cells by binding to PIP₃, where it modulates pseudopod and macropinocytic cup dynamics by negatively regulating the Scar/WAVE complex. The spatiotemporal coordination of PIP₃ signaling, Leep1, and the Scar/WAVE complex provides a cellular mechanism for organizing protrusive structures at the leading edge.

Combined kinesin-1 and kinesin-3 activity drives axonal trafficking of TrkB receptors in Rab6 carriers

Neurons depend on proper localization of neurotrophic receptors in their distal processes for their function. The Trk family of neurotrophin receptors controls neuronal survival, differentiation, and remodeling and are well known to function as retrograde signal carriers transported from the distal axon toward the cell body. However, the mechanism driving anterograde trafficking of Trk receptors into the axon is not well established. We used microfluidic compartmental devices and inducible secretion assay to systematically investigate the retrograde and anterograde trafficking routes of TrkB receptor along the axon in rat hippocampal neurons. We show that newly synthesized TrkB receptors traffic through the secretory pathway and are directly delivered into axon. We found that these TrkB carriers associate and are regulated by Rab6. Furthermore, the combined activity of kinesin-1 and kinesin-3 is needed for the formation of axon-bound TrkB secretory carriers and their effective entry and processive anterograde transport beyond the proximal axon.

Mechanistic basis of propofol-induced disruption of kinesin processivity

Propofol is a widely used general anesthetic to induce and maintain anesthesia, and its effects are thought to occur through impact on the ligand-gated channels including the GABA_A receptor. Propofol also interacts with a large number of proteins including molecular motors and inhibits kinesin processivity, resulting in significant decrease in the run length for conventional kinesin-1 and kinesin-2. However, the molecular mechanism by which propofol achieves this outcome is not known. The structural transition in the kinesin neck-linker region is crucial for its processivity. In this study, we analyzed the effect of propofol and its fluorine derivative (fropofol) on the transition in the neck-linker region of kinesin. Propofol binds at two crucial surfaces in the leading head: one at the microtubule-binding interface and the other in the neck-linker region. We observed in both the cases the order-disorder transition of the neck-linker was disrupted and kinesin lost its signal for forward movement. In contrast, there was not an effect on the neck-linker transition with propofol binding at the trailing head. Free-energy calculations show that propofol at the microtubule-binding surface significantly reduces the microtubule-binding affinity of the kinesin head. While propofol makes pi-pi stacking and H-bond interactions with the propofol binding cavity, fropofol is unable to make a suitable interaction at this binding surface. Therefore, the binding affinity of fropofol is much lower compared to propofol. Hence, this study provides a mechanism by which propofol disrupts kinesin processivity and identifies transitions in the ATPase stepping cycle likely affected.

GRDN-1/Girdin regulates dendrite morphogenesis and cilium position in two specialized sensory neuron types in C. elegans

Primary cilia are located at the dendritic tips of sensory neurons and house the molecular machinery necessary for detection and transduction of sensory stimuli. The mechanisms that coordinate dendrite extension with cilium position during sensory neuron development are not well understood. Here, we show that GRDN-1, the Caenorhabditis elegans ortholog of the highly conserved scaffold and signaling protein Girdin/GIV, regulates both cilium position and dendrite extension in the postembryonic AQR and PQR gas-sensing neurons. Mutations in grdn-1 disrupt dendrite outgrowth and mislocalize cilia to the soma or proximal axonal segments in AQR, and to a lesser extent, in PQR. GRDN-1 is localized to the basal body and regulates localization of HMR-1/Cadherin to the distal AQR dendrite. However, knockdown of HMR-1 and/or loss of SAX-7/LICAM, molecules previously implicated in sensory dendrite development in C. elegans, do not alter AQR dendrite morphology or cilium position. We find that GRDN-1 localization in AQR is regulated by UNC-116/Kinesin-1, and that correspondingly, unc-116 mutants exhibit severe AQR dendrite outgrowth and cilium positioning defects. In contrast, GRDN-1 and cilium localization in PQR is modulated by LIN-44/Wnt signaling. Together, these findings identify upstream regulators of GRDN-1, and describe new cell-specific roles for this multifunctional protein in sensory neuron development.

KIF3A accelerates KIF3C within the kinesin-2 heterodimer to generate symmetrical phosphate release rates for each processive step

Heterodimeric KIF3AC is a mammalian kinesin-2 that is highly expressed in the central nervous system and associated with vesicles in neurons. KIF3AC is an intriguing member of the kinesin-2 family because the intrinsic kinetics of KIF3A and KIF3C when expressed as homodimers and analyzed in vitro are distinctively different from each other. For example, the single-molecule velocities of the engineered homodimers KIF3AA and KIF3CC are 293 and 7.5 nm/s, respectively, whereas KIF3AC has a velocity of 186 nm/s. These results led us to hypothesize that heterodimerization alters the intrinsic catalytic properties of the two heads, and an earlier computational analysis predicted that processive steps would alternate between a fast step for KIF3A followed by a slow step for KIF3C resulting in asymmetric stepping. To test this hypothesis directly, we measured the presteady-state kinetics of phosphate release for KIF3AC, KIF3AA, and KIF3CC followed by computational modeling of the KIF3AC phosphate release transients. The results reveal that KIF3A and KIF3C retain their intrinsic ATP-binding and hydrolysis kinetics. Yet within KIF3AC, KIF3A activates the rate of phosphate release for KIF3C such that the coupled steps of phosphate release and dissociation from the microtubule become more similar for KIF3A and KIF3C. These coupled steps are the rate-limiting transition for the ATPase cycle suggesting that within KIF3AC, the stepping kinetics are similar for each head during the processive run. Future work will be directed to define how these properties enable KIF3AC to achieve its physiological functions.

New insights on epigenetic mechanisms supporting axonal development: histone marks and miRNAs

Mechanisms supporting axon growth and the establishment of neuronal polarity have remained largely disconnected from their genetic and epigenetic fundamentals. Recently, post-transcriptional modifications of histones involved in chromatin folding and transcription, and microRNAs controlling translation have emerged as regulators of axonal specification, growth, and guidance. In this article, we review novel evidence supporting the concept that epigenetic mechanisms work at both transcriptional and post-transcriptional levels to shape axons. We also discuss the role of splicing on axonal growth, as one of the most (if not the most) powerful post-transcriptional mechanism to diversify genetic information. Overall, we think exploring the gap between epigenetics and axonal growth raises new questions and perspectives to the development of axons in physiological and pathological contexts.

Mechanisms of melanocyte polarity and differentiation: What can we learn from other neuroectoderm-derived lineages?

Melanocytes are neuroectoderm-derived pigment-producing cells with highly polarized dendritic morphology. They protect the skin against ultraviolet radiation by providing melanin to neighbouring keratinocytes. However, the mechanisms underlying melanocyte polarization and its relevance for diseases remain mostly elusive. Numerous studies have instead revealed roles for polarity regulators in other neuroectoderm-derived lineages including different neuronal cell types. Considering the shared ontogeny and morphological similarities, these lineages may be used as reference models for the exploration of melanocyte polarity, for example, regarding dendrite formation, spine morphogenesis and polarized organelle transport. In this review, we summarize and compare the latest progress in understanding polarity regulation in neuronal cells and melanocytes and project key open questions for future work.

A novel labeling strategy reveals that myosin Va and myosin Vb bind the same dendritically polarized vesicle population

Neurons are specialized cells with a polarized geometry and several distinct subdomains that require specific complements of proteins. Delivery of transmembrane proteins requires vesicle transport, which is mediated by molecular motor proteins. The myosin V family of motor proteins mediates transport to the barbed end of actin filaments, and little is known about the vesicles bound by myosin V in neurons. We developed a novel strategy to visualize myosin V-labeled vesicles in cultured hippocampal neurons and systematically characterized the vesicle populations labeled by myosin Va and Vb. We find that both myosins bind vesicles that are polarized to the somatodendritic domain where they undergo bidirectional long-range transport. A series of two-color imaging experiments showed that myosin V specifically colocalized with two different vesicle populations: vesicles labeled with the transferrin receptor and vesicles labeled by low-density lipoprotein receptor. Finally, coexpression with Kinesin-3 family members found that myosin V binds vesicles concurrently with KIF13A or KIF13B, supporting the hypothesis that coregulation of kinesins and myosin V on vesicles is likely to play an important role in neuronal vesicle transport. We anticipate that this new assay will be applicable in a broad range of cell types to determine the function of myosin V motor proteins.

Anterograde Axonal Transport in Neuronal Homeostasis and Disease

Neurons are highly polarized cells with an elongated axon that extends far away from the cell body. To maintain their homeostasis, neurons rely extensively on axonal transport of membranous organelles and other molecular complexes. Axonal transport allows for spatio-temporal activation and modulation of numerous molecular cascades, thus playing a central role in the establishment of neuronal polarity, axonal growth and stabilization, and synapses formation. Anterograde and retrograde axonal transport are supported by various molecular motors, such as kinesins and dynein, and a complex microtubule network. In this review article, we will primarily discuss the molecular mechanisms underlying anterograde axonal transport and its role in neuronal development and maturation, including the establishment of functional synaptic connections. We will then provide an overview of the molecular and cellular perturbations that affect axonal transport and are often associated with axonal degeneration. Lastly, we will relate our current understanding of the role of axonal trafficking concerning anterograde trafficking of mRNA and its involvement in the maintenance of the axonal compartment and disease.

The Spatiotemporal Construction of the Axon Initial Segment via KIF3/KAP3/TRIM46 Transport under MARK2 Signaling

The axon initial segment (AIS) is a compartment that serves as a molecular barrier to achieve axon-dendrite differentiation. Distribution of specific proteins during early neuronal development has been proposed to be critical for AIS construction. However, it remains unknown how these proteins are specifically targeted to the proximal axon within this limited time period. Here, we reveal spatiotemporal regulation driven by the microtubule (MT)-based motor KIF3A/B/KAP3 that transports TRIM46, influenced by a specific MARK2 phosphorylation cascade. In the proximal part of the future axon under low MARK2 activity, the KIF3/KAP3 motor recognizes TRIM46 as cargo and transports it to the future AIS. In contrast, in the somatodendritic area under high MARK2 activity, KAP3 phosphorylated at serine 60 by MARK2 cannot bind with TRIM46 and be transported. This spatiotemporal regulation between KIF3/KAP3 and TRIM46 under specific MARK2 activity underlies the specific transport needed for axonal differentiation.