CXCR5 Regulates Neuronal Polarity Development and Migration in the Embryonic Stage via F-Actin Homeostasis and Results in Epilepsy-Related Behavior

Epilepsy is a common, chronic neurological disorder that has been associated with impaired neurodevelopment and immunity. The chemokine receptor CXCR5 is involved in seizures via an unknown mechanism. Here, we first determined the expression pattern and distribution of the CXCR5 gene in the mouse brain during different stages of development and the brain tissue of patients with epilepsy. Subsequently, we found that the knockdown of CXCR5 increased the susceptibility of mice to pentylenetetrazol- and kainic acid-induced seizures, whereas CXCR5 overexpression had the opposite effect. CXCR5 knockdown in mouse embryos via viral vector electrotransfer negatively influenced the motility and multipolar-to-bipolar transition of migratory neurons. Using a human-derived induced an in vitro multipotential stem cell neurodevelopmental model, we determined that CXCR5 regulates neuronal migration and polarization by stabilizing the actin cytoskeleton during various stages of neurodevelopment. Electrophysiological experiments demonstrated that the knockdown of CXCR5 induced neuronal hyperexcitability, resulting in an increased number of seizures. Finally, our results suggested that CXCR5 deficiency triggers seizure-related electrical activity through a previously unknown mechanism, namely, the disruption of neuronal polarity.

Ciliated sensory neurons can regenerate axons after complete axon removal

Axon regeneration helps maintain lifelong function of neurons in many animals. Depending on the site of injury, new axons can grow either from the axon stump (after distal injury) or from the tip of a dendrite (after proximal injury). However, some neuron types do not have dendrites to be converted to a regenerating axon after proximal injury. For example, many sensory neurons receive information from a specialized sensory cilium rather than a branched dendrite arbor. We hypothesized that the lack of traditional dendrites would limit the ability of ciliated sensory neurons to respond to proximal axon injury. We tested this hypothesis by performing laser microsurgery on ciliated lch1 neurons in Drosophila larvae and tracking cells over time. These cells survived proximal axon injury as well as distal axon injury, and, like many other neurons, initiated growth from the axon stump after distal injury. After proximal injury, neurites regrew in a surprisingly flexible manner. Most cells initiated outgrowth directly from the cell body, but neurite growth could also emerge from the short axon stump or base of the cilium. New neurites were often branched. Although outgrowth after proximal axotomy was variable, it depended on the core DLK axon injury signaling pathway. Moreover, each cell had at least one new neurite specified as an axon based on microtubule polarity and accumulation of the endoplasmic reticulum. We conclude that ciliated sensory neurons are not intrinsically limited in their ability to grow a new axon after proximal axon removal.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

Autosomal dominant lateral temporal epilepsy in a family exhibiting a rare heterozygous mutation and deletion in the leucine-rich glioma inactivated 1 gene

Autosomal dominant lateral temporal epilepsy (ADLTE) is an inherited syndrome caused by mutations in the leucine-rich glioma inactivated 1 (LGI1) gene. In a family with six ADLTE patients spanning four generations, our linkage and exome sequencing investigations revealed a rare frameshift heterozygous mutation in LGI1 (c.1494del(p.Phe498LeufsTer15)). Gene cloning methods were used to create plasmids with wild-type and mutant LGI1 alleles. Through transfection of HEK293 cells and primary neurons, they were utilized to assess the subcellular location of wild-type and mutant LGI1. Moreover, the plasmid-transfected primary neurons were analyzed for neuronal complexity and density of dendritic spines. According to our results. the mutation decreased LGI1 secretion in transfected HEK293 cells. In primary neurons, mutant LGI1 affected neuronal polarity and complexity. Our findings have broadened the phenotypic spectrum of LGI1 mutations and provided evidence regarding the pathogenicity of this mutation. In addition, we discovered new information about the role of LGI1 in the development of temporal lobe epilepsy, along with a possible link between neuronal polarity disorder and ADLTE.

Microtubule polarity is instructive for many aspects of neuronal polarity

Many neurons in bilaterian animals are polarized with functionally distinct axons and dendrites. Microtubule polarity, microtubule stability, and the axon initial segment (AIS) have all been shown to influence polarized transport in neurons. Each of these cytoskeletal cues could act independently to control axon and dendrite identity, or there could be a hierarchy in which one acts upstream of the others. Here we test the hypothesis that microtubule polarity acts as a master regulator of neuronal polarity by using a Drosophila genetic background in which some dendrites have normal minus-end-out microtubule polarity and others have the axonal plus-end-out polarity. In these mosaic dendrite arbors, we found that ribosomes, which are more abundant in dendrites than axons, were reduced in plus-end-out dendrites, while an axonal cargo was increased. In addition, we determined that microtubule stability was different in plus-end-out and minus-end-out dendrites, with plus-end-out ones having more stable microtubules like axons. Similarly, we found that ectopic diffusion barriers, like those at the AIS, formed at the base of dendrites with plus-end-out regions. Thus, changes in microtubule polarity were sufficient to rearrange other cytoskeletal features associated with neuronal polarization. However, overall neuron shape was maintained with only subtle changes in branching in mosaic arbors. We conclude that microtubule polarity can act upstream of many aspects of intracellular neuronal polarization, but shape is relatively resilient to changes in microtubule polarity in vivo.

GPR3 accelerates neurite outgrowth and neuronal polarity formation via PI3 kinase-mediating signaling pathway in cultured primary neurons

During neuronal development, immature neurons extend neurites and subsequently polarize to form an axon and dendrites. We have previously reported that G protein-coupled receptor 3 (GPR3) levels increase during neuronal development, and that GPR3 has functions in neurite outgrowth and neuronal differentiation in cerebellar granular neurons. Moreover, GPR3 is transported and concentrated at the tips of neurite, thereby contributing to the local activation of protein kinase A (PKA). However, the signaling pathways for GPR3-mediated neurite outgrowth and its subsequent effects on neuronal polarization have not yet been elucidated. We therefore analyzed the signaling pathways related to GPR3-mediated neurite outgrowth, and also focused on the possible roles of GPR3 in axon polarization. We demonstrated that, in cerebellar granular neurons, GPR3-mediated neurite outgrowth was mediated by multiple signaling pathways, including those of PKA, extracellular signal-regulated kinases (ERKs), and most strongly phosphatidylinositol 3-kinase (PI3K). In addition, the GPR3-mediated activation of neurite outgrowth was associated with G protein-coupled receptor kinase 2 (GRK2)-mediated signaling and phosphorylation of the C-terminus serine/threonine residues of GPR3, which affected downstream protein kinase B (Akt) signaling. We further demonstrated that GPR3 was transiently increased early in the development of rodent hippocampal neurons. It was subsequently concentrated at the tip of the longest neurite, and was thus associated with accelerated polarity formation in a PI3K-dependent manner in rat hippocampal neurons. In addition, GPR3 knockout in mouse hippocampal neurons led to delayed neuronal polarity formation, thereby affecting the dephosphorylation of collapsing response mediator protein 2 (CRMP2), which is downstream of the PI3K signaling pathway. Taken together, these findings suggest that the intrinsic expression of GPR3 in differentiated neurons constitutively activates PI3K-mediated signaling pathway predominantly, thus accelerating neurite outgrowth and further augmenting polarity formation in primary cultured neurons.

Inheritance and flexibility of cell polarity: a clue for understanding human brain development and evolution

Cell polarity is fundamentally important for understanding brain development. Here, we hypothesize that the inheritance and flexibility of cell polarity during neocortex development could be implicated in neocortical evolutionary expansion. Molecular and morphological features of cell polarity may be inherited from one type of progenitor cell to the other and finally transmitted to neurons. Furthermore, key cell types, such as basal progenitors and neurons, exhibit a highly flexible polarity. We suggest that both inheritance and flexibility of cell polarity are implicated in the amplification of basal progenitors and tangential dispersion of neurons, which are key features of the evolutionary expansion of the neocortex.

Summary: We suggest that the inheritance and flexibility of cell polarity are implicated in the evolutionary expansion of the developing neocortex by promoting the amplification of neural progenitors and tangential migration of neurons.

Dual Color, Live Imaging of Vesicular Transport in Axons of Cultured Sensory Neurons

The function of neurons in afferent reception, integration, and generation of electrical activity relies on their strikingly polarized organization, characterized by distinct membrane domains. These domains have different compositions resulting from a combination of selective targeting and retention of membrane proteins. In neurons, most proteins are delivered from their site of synthesis in the soma to the axon via anterograde vesicular transport and undergo retrograde transport for redistribution and/or lysosomal degradation. A key question is whether proteins destined for the same domain are transported in separate vesicles for local assembly or whether these proteins are pre-assembled and co-transported in the same vesicles for delivery to their cognate domains. To assess the content of transport vesicles, one strategy relies on staining of sciatic nerves after ligation, which drives the accumulation of anterogradely and retrogradely transported vesicles on the proximal and distal side of the ligature, respectively. This approach may not permit confident assessment of the nature of the intracellular vesicles identified by staining, and analysis is limited to the availability of suitable antibodies. Here, we use dual color live imaging of proteins labeled with different fluorescent tags, visualizing anterograde and retrograde axonal transport of several proteins simultaneously. These proteins were expressed in rat dorsal root ganglion (DRG) neurons cultured alone or with Schwann cells under myelinating conditions to assess whether glial cells modify the patterns of axonal transport. Advantages of this protocol are the dynamic identification of transport vesicles and characterization of their content for various proteins that is not limited by available antibodies.

Microtubule organization of vertebrate sensory neurons in vivo

Dorsal root ganglion (DRG) neurons are the predominant cell type that innervates the vertebrate skin. They are typically described as pseudounipolar cells that have central and peripheral axons branching from a single root exiting the cell body. The peripheral axon travels within a nerve to the skin, where free sensory endings can emerge and branch into an arbor that receives and integrates information. In some immature vertebrates, DRG neurons are preceded by Rohon-Beard (RB) neurons. While the sensory endings of RB and DRG neurons function like dendrites, we use live imaging in zebrafish to show that they have axonal plus-end-out microtubule polarity at all stages of maturity. Moreover, we show both cell types have central and peripheral axons with plus-end-out polarity. Surprisingly, in DRG neurons these emerge separately from the cell body, and most cells never acquire the signature pseudounipolar morphology. Like another recently characterized cell type that has multiple plus-end-out neurites, ganglion cells in Nematostella, RB and DRG neurons maintain a somatic microtubule organizing center even when mature. In summary, we characterize key cellular and subcellular features of vertebrate sensory neurons as a foundation for understanding their function and maintenance.

Identification of Neuronal Polarity by Node-Based Machine Learning

Identifying the direction of signal flows in neural networks is important for understanding the intricate information dynamics of a living brain. Using a dataset of 213 projection neurons distributed in more than 15 neuropils of a Drosophila brain, we develop a powerful machine learning algorithm: node-based polarity identifier of neurons (NPIN). The proposed model is trained only by information specific to nodes, the branch points on the skeleton, and includes both Soma Features (which contain spatial information from a given node to a soma) and Local Features (which contain morphological information of a given node). After including the spatial correlations between nodal polarities, our NPIN provided extremely high accuracy (>96.0%) for the classification of neuronal polarity, even for complex neurons with more than two dendrite/axon clusters. Finally, we further apply NPIN to classify the neuronal polarity of neurons in other species (Blowfly and Moth), which have much less neuronal data available. Our results demonstrate the potential of NPIN as a powerful tool to identify the neuronal polarity of insects and to map out the signal flows in the brain’s neural networks if more training data become available in the future.

Centella asiatica promotes early differentiation, axodendritic maturation and synaptic formation in primary hippocampal neurons

BACKGROUND

Centella asiatica is a 'medhya-rasayana (nootrophic or memory booster)' herb that has been indicated in Ayurveda for improving memory function and treating dementia disorders. Although the neuroprotective effects of C. asiatica have been reported in earlier studies, the information on whether this nootropic herb could promote early differentiation and development of axon and dendrites in primary hippocampal neurons is currently limited.

THE AIM OF THE STUDY

To investigate the effects of C. asiatica and asiatic acid, one of the principal active constituents of C. asiatica, on the various stages of neuronal polarity, including early neuronal differentiation, axonal outgrowth, dendritic arborization, axonal maturation, and synaptic formation.

MATERIALS AND METHODS

Embryonic rat hippocampal neurons were incubated with C. asiatica leaf extract (CAE) or asiatic acid. After an indicated time, neurons were fixed and immunolabeled to visualize the neuronal morphology. Morphometric analyses for early neuronal differentiation, axonal and dendritic maturation and synaptogenesis were performed using Image J software. Neuronal viability was determined using trypan blue exclusion assay.

RESULTS

CAE at varying concentrations ranging from 3.75 to 15 μg/mL enhanced neurite outgrowth with the highest optimal concentration of 7.5 μg/mL. The effects of CAE commenced immediately after cell seeding, as indicated by its accelerating effect on neuronal differentiation. Subsequently, CAE significantly elaborated dendritic and axonal morphology and facilitated synapse formation. Asiatic acid also facilitated neurite outgrowth, but to a lesser extent than CAE.

CONCLUSION

These findings revealed that CAE exerted its modulatory effects in every stage of neuronal development, supporting its previously claimed neurotrophic function and suggest that this natural nootropic and its active component asiatic acid can be further investigated to explore a promising solution for degenerative brain disorders and injuries.

Cytoskeletal and synaptic polarity of LWamide-like+ ganglion neurons in the sea anemone Nematostella vectensis

The centralized nervous systems of bilaterian animals rely on directional signaling facilitated by polarized neurons with specialized axons and dendrites. It is not known whether axo-dendritic polarity is exclusive to bilaterians or was already present in early metazoans. We therefore examined neurite polarity in the starlet sea anemone Nematostella vectensis (Cnidaria). Cnidarians form a sister clade to bilaterians and share many neuronal building blocks characteristic of bilaterians, including channels, receptors and synaptic proteins, but their nervous systems comprise a comparatively simple net distributed throughout the body. We developed a tool kit of fluorescent polarity markers for live imaging analysis of polarity in an identified neuron type, large ganglion cells of the body column nerve net that express the LWamide-like neuropeptide. Microtubule polarity differs in bilaterian axons and dendrites, and this in part underlies polarized distribution of cargo to the two types of processes. However, in LWamide-like+ neurons, all neurites had axon-like microtubule polarity suggesting that they may have similar contents. Indeed, presynaptic and postsynaptic markers trafficked to all neurites and accumulated at varicosities where neurites from different neurons often crossed, suggesting the presence of bidirectional synaptic contacts. Furthermore, we could not identify a diffusion barrier in the plasma membrane of any of the neurites like the axon initial segment barrier that separates the axonal and somatodendritic compartments in bilaterian neurons. We conclude that at least one type of neuron in Nematostella vectensis lacks the axo-dendritic polarity characteristic of bilaterian neurons.

Neurons survive simultaneous injury to axons and dendrites and regrow both types of processes in vivo

Neurons extend dendrites and axons to receive and send signals. If either type of process is removed, the cell cannot function. Rather than undergoing cell death, some neurons can regrow axons and dendrites. Axon and dendrite regeneration have been examined separately and require sensing the injury and reinitiating the correct growth program. Whether neurons in vivo can sense and respond to simultaneous axon and dendrite injury with polarized regeneration has not been explored. To investigate the outcome of simultaneous axon and dendrite damage, we used a Drosophila model system in which neuronal polarity, axon regeneration, and dendrite regeneration have been characterized. After removal of the axon and all but one dendrite, the remaining dendrite was converted to a process that had a long unbranched region that extended over long distances and a region where shorter branched processes were added. These observations suggested axons and dendrites could regrow at the same time. To further test the capacity of neurons to implement polarized regeneration after axon and dendrite damage, we removed all neurites from mature neurons. In this case a long unbranched neurite and short branched neurites were regrown from the stripped cell body. Moreover, the long neurite had axonal plus-end-out microtubule polarity and the shorter neurites had mixed polarity consistent with dendrite identity. The long process also accumulated endoplasmic reticulum at its tip like regenerating axons. We conclude that neurons in vivo can respond to simultaneous axon and dendrite injury by initiating growth of a new axon and new dendrites.

Non-radial tortuous migration with cell polarity alterations of newly generated granule neurons in the neonatal rat dentate gyrus

To establish functional neuronal circuits, newborn neurons generally migrate from the ventricular germinal zones to their final positions during embryonic periods. However, most excitatory neurons of the hippocampal dentate gyrus are born postnatally in the hilus, far from the lateral ventricle. Newly generated granule neurons must then migrate to the surrounding granule cell layer (GCL), which suggests that newborn granule cells may migrate by unique cellular mechanisms. In the present study, we describe the migratory behaviors of postnatally generated granule neurons using combined retroviral labeling and time-lapse imaging analysis. Our results show that whereas half of the newly generated neurons undergo radial migration, the remainder engages in more complex migratory patterns with veering and turning movements accompanied by process formation and cell polarity alterations. These data reveal a previously unappreciated diversity of mechanisms by which granule neurons distribute throughout the GCL to contribute to hippocampal circuitry.

The intriguing nature of dorsal root ganglion neurons: Linking structure with polarity and function

Dorsal root ganglion (DRG) neurons are the first neurons of the sensory pathway. They are activated by a variety of sensory stimuli that are then transmitted to the central nervous system. An important feature of DRG neurons is their unique morphology where a single process -the stem axon- bifurcates into a peripheral and a central axonal branch, with different functions and cellular properties. Distinctive structural aspects of the two DRG neuron branches may have important implications for their function in health and disease. However, the link between DRG axonal branch structure, polarity and function has been largely neglected in the field, and relevant information is rather scattered across the literature. In particular, ultrastructural differences between the two axonal branches are likely to account for the higher transport and regenerative ability of the peripheral DRG neuron axon when compared to the central one. Nevertheless, the cell intrinsic factors contributing to this central-peripheral asymmetry are still unknown. Here we critically review the factors that may underlie the functional asymmetry between the peripheral and central DRG axonal branches. Also, we discuss the hypothesis that DRG neurons may assemble a structure resembling the axon initial segment that may be responsible, at least in part, for their polarity and electrophysiological features. Ultimately, we suggest that the clarification of the axonal ultrastructure of DRG neurons using state-of-the-art techniques will be crucial to understand the physiology of this peculiar cell type.

Trafficking mechanisms of synaptogenic cell adhesion molecules

Nearly every aspect of neuronal function, from wiring to information processing, critically depends on the highly polarized architecture of neurons. Establishing and maintaining the distinct molecular composition of axonal and dendritic compartments requires precise control over the trafficking of the proteins that make up these cellular domains. Synaptic cell adhesion molecules (CAMs), membrane proteins with a critical role in the formation, differentiation and plasticity of synapses, require targeting to the correct pre- or postsynaptic compartment for proper functioning of neural circuits. However, the mechanisms that control the polarized trafficking, synaptic targeting, and synaptic abundance of CAMs are poorly understood. Here, we summarize current knowledge about the sequential trafficking events along the secretory pathway that control the polarized surface distribution of synaptic CAMs, and discuss how their synaptic targeting and abundance is additionally influenced by post-secretory determinants. The identification of trafficking-impairing mutations in CAMs associated with various neurodevelopmental disorders underscores the importance of correct protein trafficking for normal brain function.

Perturbation of canonical and non-canonical BMP signaling affects migration, polarity and dendritogenesis of mouse cortical neurons

Bone morphogenetic protein (BMP) signaling has been implicated in the regulation of patterning of the forebrain and as a regulator of neurogenesis and gliogenesis in the mammalian cortex. However, its role in other aspects of cortical development in vivo remains unexplored. We hypothesized that BMP signaling might regulate additional processes during the development of cortical neurons after observing active BMP signaling in a spatiotemporally dynamic pattern in the mouse cortex. Our investigation revealed that BMP signaling specifically regulates the migration, polarity and the dendritic morphology of upper layer cortical neurons born at E15.5. On further dissection of the role of canonical and non-canonical BMP signaling in each of these processes, we found that migration of these neurons is regulated by both pathways. Their polarity, however, appears to be affected more strongly by canonical BMP signaling, whereas dendritic branch formation appears to be somewhat more strongly affected by LIMK-mediated non-canonical BMP signaling.

MicroRNA let-7f-5p regulates neuronal differentiation of rat bone marrow mesenchymal stem cells by targeting Par6α

Par6α (partitioning defective 6 homologue alpha), a component of the Par3/Par6/aPKC complex, was recently shown to be essential for axon specification during neuronal development. However, the biological functions and regulatory mechanisms of Par6α in the mesenchymal stem cell (MSC) differentiation process have not been investigated. In this study, we found that the expression of let-7f-5p was downregulated during differentiation of bone marrow-derived MSCs to neuron-like cells. Interestingly, Par6α was predicted to be a target gene of let-7f-5p by computerized analysis and the luciferase reporter assay. Using gain- and loss-of-function approaches, we found that expression of Par6α was inversely correlated with let-7f-5p levels during differentiation (p < 0.05). By silencing Par6α using siRNAs, we demonstrated that Par6α was necessary for MSC neuronal differentiation. Altogether, our studies proved that inhibition of let-7f-5p facilitates induction of MSCs into neuron-like cells by directly targeting Par6α.

Neuronal polarization: From spatiotemporal signaling to cytoskeletal dynamics

Neuronal polarization establishes distinct molecular structures to generate a single axon and multiple dendrites. Studies over the past years indicate that this efficient separation is brought about by a network of feedback loops. Axonal growth seems to play a major role in fueling those feedback loops and thereby stabilizing neuronal polarity. Indeed, various effectors involved in feedback loops are pivotal for axonal growth by ultimately acting on the actin and microtubule cytoskeleton. These effectors have key roles in interconnecting actin and microtubule dynamics - a mechanism crucial to commanding the growth of axons. We propose a model connecting signaling with cytoskeletal dynamics and neurite growth to better describe the underlying processes involved in neuronal polarization. We will discuss the current views on feedback loops and highlight the current limits of our understanding.

Acetylated tau destabilizes the cytoskeleton in the axon initial segment and is mislocalized to the somatodendritic compartment

BACKGROUND

Neurons are highly polarized cells in which asymmetric axonal-dendritic distribution of proteins is crucial for neuronal function. Loss of polarized distribution of the axonal protein tau is an early sign of Alzheimer's disease (AD) and other neurodegenerative disorders. The cytoskeletal network in the axon initial segment (AIS) forms a barrier between the axon and the somatodentritic compartment, contributing to axonal retention of tau. Although perturbation of the AIS cytoskeleton has been implicated in neurological disorders, the molecular triggers and functional consequence of AIS perturbation are incompletely understood.

RESULTS

Here we report that tau acetylation and consequent destabilization of the AIS cytoskeleton promote the somatodendritic mislocalization of tau. AIS cytoskeletal proteins, including ankyrin G and βIV-spectrin, were downregulated in AD brains and negatively correlated with an increase in tau acetylated at K274 and K281. AIS proteins were also diminished in transgenic mice expressing tauK274/281Q, a tau mutant that mimics K274 and K281 acetylation. In primary neuronal cultures, the tauK274/281Q mutant caused hyperdynamic microtubules (MTs) in the AIS, shown by live-imaging of MT mobility and fluorescence recovery after photobleaching. Using photoconvertible tau constructs, we found that axonal tauK274/281Q was missorted into the somatodendritic compartment. Stabilizing MTs with epothilone D to restore the cytoskeletal barrier in the AIS prevented tau mislocalization in primary neuronal cultures.

CONCLUSIONS

Together, these findings demonstrate that tau acetylation contributes to the pathogenesis of neurodegenerative disease by compromising the cytoskeletal sorting machinery in the AIS.

Neuronal polarization

Neurons are highly polarized cells with structurally and functionally distinct processes called axons and dendrites. This polarization underlies the directional flow of information in the central nervous system, so the establishment and maintenance of neuronal polarization is crucial for correct development and function. Great progress in our understanding of how neurons establish their polarity has been made through the use of cultured hippocampal neurons, while recent technological advances have enabled in vivo analysis of axon specification and elongation. This short review and accompanying poster highlight recent advances in this fascinating field, with an emphasis on the signaling mechanisms underlying axon and dendrite specification in vitro and in vivo.

Analyzing kinesin motor domain translocation in cultured hippocampal neurons

Neuronal microtubules are subject to extensive posttranslational modifications and are bound by MAPs, tip-binding proteins, and other accessory proteins. All of these features, which are difficult to replicate in vitro, are likely to influence the translocation of kinesin motors. Here we describe assays for evaluating the translocation of a population of fluorescently labeled kinesin motor domains, based on their accumulation in regions of the cell enriched in microtubule plus ends. Neurons lend themselves to these experiments because of their microtubule organization. In axons, microtubules are oriented with their plus ends out; dendrites contain a mixed population of microtubules, but those near the tips are also plus end out. The assays involve the expression of constitutively active kinesins that can walk processively, but that lack the autoinhibitory domain in the tail that normally prevents their binding to microtubules until they attach to vesicles. The degree to which such motor domains accumulate at neurite tips serves as a measure of the efficiency of their translocation. Although these assays cannot provide the kind of quantitative kinetic information obtained from in vitro assays, they offer a simple way to examine kinesin translocation in living neurons. They can be used to compare the translocation efficiency of different kinesin motors and to evaluate how mutations or posttranslational modifications within the motor domain influence kinesin translocation. Changes to motor domain accumulation in these assays can also serve as readout for changes in the microtubule cytoskeleton that affect kinesin translocation.

elegans Ror kinase CAM-1

Wnts are a conserved family of secreted glycoproteins that regulate various developmental processes in metazoans. Three of the five Caenorhabditis elegans Wnts, CWN-1, CWN-2 and EGL-20, and the sole Wnt receptor of the Ror kinase family, CAM-1, are known to regulate the anterior polarization of the mechanosensory neuron ALM. Here we show that CAM-1 and the Frizzled receptor MOM-5 act in parallel pathways to control ALM polarity. We also show that CAM-1 has two functions in this process: an autonomous signaling function that promotes anterior polarization and a nonautonomous Wnt-antagonistic function that inhibits anterior polarization. These antagonistic activities can account for the weak ALM phenotypes displayed by cam-1 mutants. Our observations suggest that CAM-1 could function as a Wnt receptor in many developmental processes, but the analysis of cam-1 mutants may fail to reveal CAM-1's role as a receptor in these processes because of its Wnt-antagonistic activity. In this model, loss of CAM-1 results in increased levels of Wnts that act through other Wnt receptors, masking CAM-1's autonomous role as a Wnt receptor.

Functional redundancy of protein kinase D1 and protein kinase D2 in neuronal polarity

Mammalian protein kinase D (PKD) isoforms have been proposed to regulate diverse biological processes, including the establishment and maintenance of neuronal polarity. To investigate the function of PKD in neuronal polarization in vivo, we generated PKD knockout (KO) mice. Here, we show that the brain, particularly the hippocampus, of both PKD1 KO and PKD2 KO mice was similar to that of control animals. Neurite length in cultured PKD1 KO and PKD2 KO hippocampal neurons was similar to that of wild-type neurons. However, hippocampal neurons deficient in both PKD1 and PKD2 genes showed a reduction in axonal elongation and an increase in the percentage of neurons with multiple axons relative to control neurons. These results reveal that whereas PKD1 and PKD2 are essential for neuronal polarity, there exists a functional redundancy between the two proteins.

Regulation of neuronal polarity

The distinctive polarized morphology of neuronal cells is essential for the proper wiring of the nervous system. The rodent hippocampal neuron culture established about three decades ago has provided an amenable in vitro system to uncover the molecular mechanisms underlying neuronal polarization, a process relying on highly regulated cytoskeletal dynamics, membrane traffic and localized protein degradation. More recent research in vivo has highlighted the importance of the extracellular environment and cell-cell interactions in neuronal polarity. Here, I will review some key signaling pathways regulating neuronal polarization and provide some insights on the complexity of this process gained from in vivo studies.

RalA promotes a direct exocyst-Par6 interaction to regulate polarity in neuronal development

Cell polarization is essential for neuronal development in both the embryonic and postnatal brain. Here, using primary cultures, in vivo postnatal electroporation and conditional genetic ablation, we show that the Ras-like small GTPase RalA and its effector, the exocyst, regulate the morphology and polarized migration of neural progenitors derived from the subventricular zone, a major neurogenic niche in the postnatal brain. Active RalA promotes the direct binding between the exocyst subunit Exo84 and the PDZ domain of Par6 through a non-canonical PDZ-binding motif. Blocking the Exo84-Par6 interaction impairs polarization in postnatal neural progenitors and cultured embryonic neurons. Our results provide the first in vivo characterization of RalA function in the mammalian brain and highlight a novel molecular mechanism for cell polarization. Given that the exocyst and the Par complex are conserved in many tissues, the functional significance of their interaction and its regulation by RalA are likely to be important in a wide range of polarization events.

The conserved transmembrane RING finger protein PLR-1 downregulates Wnt signaling by reducing Frizzled, Ror and Ryk cell-surface levels in C. elegans

Wnts control a wide range of essential developmental processes, including cell fate specification, axon guidance and anteroposterior neuronal polarization. We identified a conserved transmembrane RING finger protein, PLR-1, that governs the response to Wnts by lowering cell-surface levels of the Frizzled family of Wnt receptors in Caenorhabditis elegans. Loss of PLR-1 activity in the neuron AVG causes its anteroposterior polarity to be symmetric or reversed because signaling by the Wnts CWN-1 and CWN-2 are inappropriately activated, whereas ectopic PLR-1 expression blocks Wnt signaling and target gene expression. Frizzleds are enriched at the cell surface; however, when PLR-1 and Frizzled are co-expressed, Frizzled is not detected at the surface but instead is colocalized with PLR-1 in endosomes. The Frizzled cysteine-rich domain (CRD) and invariant second intracellular loop lysine are crucial for PLR-1 downregulation. The PLR-1 RING finger and protease-associated (PA) domain are essential for activity. In a Frizzled-dependent manner, PLR-1 reduces surface levels of the Wnt receptors CAM-1/Ror and LIN-18/Ryk. PLR-1 is a homolog of the mammalian transmembrane E3 ubiquitin ligases RNF43 and ZNRF3, which control Frizzled surface levels in an R-spondin-sensitive manner. We propose that PLR-1 downregulates Wnt receptor surface levels via lysine ubiquitylation of Frizzled to coordinate spatial and temporal responses to Wnts during neuronal development.

Hsp90 activity is necessary to acquire a proper neuronal polarization

Chaperones are critical for the folding and regulation of a wide array of cellular proteins. Heat Shock Proteins (Hsps) are the most representative group of chaperones. Hsp90 represents up to 1-2% of soluble protein. Although the Hsp90 role is being studied in neurodegenerative diseases, its role in neuronal differentiation remains mostly unknown. Since neuronal polarity mechanisms depend on local stability and degradation, we asked whether Hsp90 could be a regulator of axonal polarity and growth. Thus, we studied the role of Hsp90 activity in a well established model of cultured hippocampal neurons using an Hsp90 specific inhibitor, 17-AAG. Our present data shows that Hsp90 inhibition at different developmental stages disturbs neuronal polarity formation or axonal elongation. Hsp90 inhibition during the first 3h in culture promotes multiple axon morphology, while this inhibition after 3h slows down axonal elongation. Hsp90 inhibition was accompanied by decreased Akt and GSK3 expression, as well as, a reduced Akt activity. In parallel, we detected an alteration of kinesin-1 subcellular distribution. Moreover, these effects were seconded by changes in Hsp70/Hsc70 subcellular localization that seem to compensate the lack of Hsp90 activity. In conclusion, our data strongly suggests that Hsp90 activity is necessary to control the expression, activity or location of specific kinases and motor proteins during the axon specification and axon elongation processes. Even more, our data demonstrate the existence of a "time-window" for axon specification in this model of cultured neurons after which the inhibition of Hsp90 only affects axonal elongation mechanisms.