Polarity proteins in axon specification and synaptogenesis

Epithelial UNC-23 limits mechanical stress to maintain glia-neuron architecture in C. elegans

For an organ to maintain correct architecture and function, its diverse cellular components must coordinate their size and shape. Although cell-intrinsic mechanisms driving homotypic cell-cell coordination are known, it is unclear how cell shape is regulated across heterotypic cells. We find that epithelial cells maintain the shape of neighboring sense-organ glia-neuron units in adult Caenorhabditis elegans (C. elegans). Hsp co-chaperone UNC-23/BAG2 prevents epithelial cell shape from deforming, and its loss causes head epithelia to stretch aberrantly during animal movement. In the sense-organ glia, amphid sheath (AMsh), this causes progressive fibroblast growth factor receptor (FGFR)-dependent disruption of the glial apical cytoskeleton. Resultant glial cell shape alteration causes concomitant shape change in glia-associated neuron endings. Epithelial UNC-23 maintenance of glia-neuron shape is specific both spatially, within a defined anatomical zone, and temporally, in a developmentally critical period. As all molecular components uncovered are broadly conserved across central and peripheral nervous systems, we posit that epithelia may similarly regulate glia-neuron architecture cross-species.

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.

A Fluorescent Vector of Carbon Dot to Deliver Rab13 and Rab14 Plasmids for Promoting Neurite Outgrowth

The complex processes of neuron differentiation and neuron repair are critical for treating nervous system injuries and neurodegenerative diseases. Neurite outgrowth plays a crucial role in these processes by enabling the formation of connections between neurons and the generation of neuroplasticity to restore the function of the nervous system. In this study, we fabricated functionalized carbon dots (CDs) with distinctive photoluminescence and low cytotoxicity for use as fluorescence imaging probes and nanocarriers to deliver plasmid DNAs to neurons effectively for inducing neurite outgrowth. CDs were prepared through a reflux process in nitric acid solution, and their surface was then modified using polyethylenimine (PEI) to obtain positively charged CDs for increasing the absorption of plasmid DNAs and the efficiency of cell uptake. Experimental results indicated that the fabricated CDs maintained a low cytotoxicity and exhibited a high neuron uptake of up to 97%. An improvement in the plasmid DNA ingestion of neurons resulted in enhanced expression of Rab13-Q67L and Rab14 proteins, which considerably promoted neurite sprouting and elongation. After the fabricated PEI-modified CDs were used to deliver the Rab13-Q67L and Rab14 plasmids, more than 56% of the neurons had a neurite length that was greater than twice the size of their soma. Thus, DNA delivery through functionalized CDs has a high potential for use in gene therapy for neuronal injuries and diseases.

Genetic regulation of central synapse formation and organization in Drosophila melanogaster

A goal of modern neuroscience involves understanding how connections in the brain form and function. Such a knowledge is essential to inform how defects in the exquisite complexity of nervous system growth influence neurological disease. Studies of the nervous system in the fruit fly Drosophila melanogaster enabled the discovery of a wealth of molecular and genetic mechanisms underlying development of synapses-the specialized cell-to-cell connections that comprise the essential substrate for information flow and processing in the nervous system. For years, the major driver of knowledge was the neuromuscular junction due to its ease of examination. Analogous studies in the central nervous system lagged due to a lack of genetic accessibility of specific neuron classes, synaptic labels compatible with cell-type-specific access, and high resolution, quantitative imaging strategies. However, understanding how central synapses form remains a prerequisite to understanding brain development. In the last decade, a host of new tools and techniques extended genetic studies of synapse organization into central circuits to enhance our understanding of synapse formation, organization, and maturation. In this review, we consider the current state-of-the-field. We first discuss the tools, technologies, and strategies developed to visualize and quantify synapses in vivo in genetically identifiable neurons of the Drosophila central nervous system. Second, we explore how these tools enabled a clearer understanding of synaptic development and organization in the fly brain and the underlying molecular mechanisms of synapse formation. These studies establish the fly as a powerful in vivo genetic model that offers novel insights into neural development.

From the Cover: BDE-47 and BDE-49 Inhibit Axonal Growth in Primary Rat Hippocampal Neuron-Glia Co-Cultures via Ryanodine Receptor-Dependent Mechanisms

Polybrominated diphenyl ethers (PBDEs) are widespread environmental contaminants associated with adverse neurodevelopmental outcomes in children and preclinical models; however, the mechanisms by which PBDEs cause developmental neurotoxicity remain speculative. The structural similarity between PBDEs and nondioxin-like (NDL) polychlorinated biphenyls (PCBs) suggests shared toxicological properties. Consistent with this, both NDL PCBs and PBDEs have been shown to stabilize ryanodine receptors (RyRs) in the open configuration. NDL PCB effects on RyR activity are causally linked to increased dendritic arborization, but whether PBDEs similarly enhance dendritic growth is not known. In this study, we quantified the effects of individual PBDE congeners on not only dendritic but also axonal growth since both are regulated by RyR-dependent mechanisms, and both are critical determinants of neuronal connectivity. Neuronal-glial co-cultures dissociated from the neonatal rat hippocampus were exposed to BDE-47 or BDE-49 in the culture medium. At concentrations ranging from 20 pM to 2 µM, neither PBDE congener altered dendritic arborization. In contrast, at concentrations ≥ 200 pM, both congeners delayed neuronal polarization resulting in significant inhibition of axonal outgrowth during the first few days in vitro. The axon inhibitory effects of these PBDE congeners occurred independent of cytotoxicity, and were blocked by pharmacological antagonism of RyR or siRNA knockdown of RyR2. These results demonstrate that the molecular and cellular mechanisms by which PBDEs interfere with neurodevelopment overlap with but are distinct from those of NDL PCBs, and suggest that altered patterns of neuronal connectivity may contribute to the developmental neurotoxicity of PBDEs.

Regulation of plasma membrane expansion during axon formation

Here, will review current evidence regarding the signaling pathways and mechanisms underlying membrane addition at sites of active growth during axon formation. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 170-180, 2018.

The functions of Reelin in membrane trafficking and cytoskeletal dynamics: implications for neuronal migration, polarization and differentiation

Reelin is a large extracellular matrix protein with relevant roles in mammalian central nervous system including neurogenesis, neuronal polarization and migration during development; and synaptic plasticity with its implications in learning and memory, in the adult. Dysfunctions in reelin signaling are associated with brain lamination defects such as lissencephaly, but also with neuropsychiatric diseases like autism, schizophrenia and depression as well with neurodegeneration. Reelin signaling involves a core pathway that activates upon reelin binding to its receptors, particularly ApoER2 (apolipoprotein E receptor 2)/LRP8 (low-density lipoprotein receptor-related protein 8) and very low-density lipoprotein receptor, followed by Src/Fyn-mediated phosphorylation of the adaptor protein Dab1 (Disabled-1). Phosphorylated Dab1 (pDab1) is a hub in the signaling cascade, from which several other downstream pathways diverge reflecting the different roles of reelin. Many of these pathways affect the dynamics of the actin and microtubular cytoskeleton, as well as membrane trafficking through the regulation of the activity of small GTPases, including the Rho and Rap families and molecules involved in cell polarity. The complexity of reelin functions is reflected by the fact that, even now, the precise mode of action of this signaling cascade in vivo at the cellular and molecular levels remains unclear. This review addresses and discusses in detail the participation of reelin in the processes underlying neurogenesis, neuronal migration in the cerebral cortex and the hippocampus; and the polarization, differentiation and maturation processes that neurons experiment in order to be functional in the adult brain. In vivo and in vitro evidence is presented in order to facilitate a better understanding of this fascinating system.

GIT1 promotes lung cancer cell metastasis through modulating Rac1/Cdc42 activity and is associated with poor prognosis

G-protein-coupled receptor kinase interacting protein 1 (GIT1) is participated in cell movement activation, which is a fundamental process during tissue development and cancer progression. GIT1/PIX forming a functional protein complex that contributes to Rac1/Cdc42 activation, resulting in increasing cell mobility. Although the importance of Rac1/Cdc42 activation is well documented in cancer aggressiveness, the clinical importance of GIT1 remains largely unknown. Here, we investigated the clinical significance of GIT1 expression in non-small-cell lung cancer (NSCLC) and also verified the importance of GIT1-Rac1/Cdc42 axis in stimulating NSCLC cell mobility. The result indicated higher GIT1 expression patients had significantly poorer prognoses in disease-free survival (DFS) and overall survival (OS) compared with lower GIT1 expression patients. Higher GIT1 expression was an independent prognostic factor by multivariate analysis and associated with migration/invasion of NSCLC cells in transwell assay. In vivo studies indicated that GIT1 promotes metastasis of NSCLC cells. Finally, GIT1 was found to stimulate migration/invasion by altering the activity of Rac1/Cdc42 in NSCLC cells. Together, the GIT1 expression is associated with poor prognosis in patients with NSCLC. GIT1 is critical for the invasiveness of NSCLC cells through stimulating the activity of Rac1/Cdc42.

NGL-2 Is a New Partner of PAR Complex in Axon Differentiation

Neuronal polarization is pivotal for neural network formation during brain development. Axon differentiation is a hallmark of initial neuronal polarization. Here, we report that the leucine-rich repeat-containing protein netrin-G ligand-2 (NGL-2) as a polarity regulator that localizes asymmetrically in rat hippocampal neurons and is required for differentiation of the future axon. NGL-2 was associated with PAR complex, and this interaction resulted in local stabilization of axonal microtubules. Further study showed that the C terminal of NGL-2 binds to the PDZ domain of PAR6, and NGL-2 interacts with PAR3 and atypical PKCζ (aPKCζ), with PAR6 acting as a bridge or modifier. Then, NGL-2 regulates the local stabilization of microtubules and promotes axon differentiation by the aPKCζ/microtubule affinity-regulating kinase 2 pathway. These findings reveal the critical role of NGL-2 in regulating axon differentiation in rat hippocampal neurons and reveal a novel partner of the PAR complex.

Phosphorylation of LKB1/Par-4 establishes Schwann cell polarity to initiate and control myelin extent

The Schwann cell (SC)-axon interface represents a membrane specialization that integrates axonal signals to coordinate cytoskeletal dynamics resulting in myelination. Here we show that LKB1/Par-4 is asymmetrically localized to the SC-axon interface and colocalizes with the polarity protein Par-3. Using purified SCs and myelinating cocultures, we demonstrate that localization is dependent on the phosphorylation of LKB1 at serine-431. SC-specific deletion of LKB1 significantly attenuates developmental myelination, delaying the initiation and altering the myelin extent into adulthood, resulting in a 30% reduction in the conduction velocity along adult sciatic nerves. Phosphorylation of LKB1 by protein kinase A is essential to establish the asymmetric localization of LKB1 and Par-3 and rescues the delay in myelination observed in the SC-specific knockout of LKB1. Our findings suggest that SC polarity may coordinate multiple signaling complexes that couple SC-axon contact to the redistribution of specific membrane components necessary to initiate and control myelin extent.

Methylmercury induces caspase-dependent apoptosis and autophagy in human neural stem cells

Methylmercury (MeHg) is a well-known human neurotoxic agent whose exposure sources are mainly environmental and aquatic-derived food. MeHg is reported to induce central nervous system disability. However, the exact mechanism of MeHg-induced neurotoxicity is still unknown. In this study, to investigate which cell death signaling pathway is related with MeHg-induced cytotoxicity, the effects of MeHg on apoptosis and autophagy were evaluated in HB1.F3 human neural stem cells (NSCs). Human NSCs were treated with 1 μM of MeHg for 48 hr and the effect of MeHg on cell signaling pathway was elucidated. MeHg inhibited Akt1/mTOR signaling that led to induction of caspase-dependent apoptosis and autophagy in the NSCs. Furthermore, retinoic acid (RA)-induced neuronal differentiation was inhibited by MeHg. Taken together, these results suggest that MeHg inhibits the differentiation of human NSCs by induction of caspase-dependent apoptosis and autophagy.

Second messenger signaling for neuronal polarization: cell mechanics-dependent pattern formation

Neuronal polarization is a critical step in the neuronal morphogenesis. Despite the identification of several evolutionarily conserved factors for neural polarization, the exact mechanisms by which cells initiate and maintain polarity remain to be characterized. Here, we review the recent progress on the roles of second messengers, specifically the cyclic nucleotides and membrane-associated phospholipids, in the initiation, propagation, and integration of polarization signals, and propose an inhibitor-free model for neural polarization. The characteristic features of neuron polarization include the formation of single axon and multiple dendrites. These features involve chemical and mechanical mechanisms such as reaction-diffusion and tug-of-war, by which second messengers can act in concert to initiate and stabilize the cellular asymmetry. Nevertheless, biochemical factors eliciting the long-range inhibition remain ambiguous. Thus, we provide a simple, inhibitor-free model that can incorporate known cytochemical and cytomechanical factors, and produce features of neuronal polarization in environments provided with minimized extracellular regulators.

Regulatory mechanisms underlying the differential growth of dendrites and axons

A typical neuron is comprised of an information input compartment, or the dendrites, and an output compartment, known as the axon. These two compartments are the structural basis for functional neural circuits. However, little is known about how dendritic and axonal growth are differentially regulated. Recent studies have uncovered two distinct types of regulatory mechanisms that differentiate dendritic and axonal growth: dedicated mechanisms and bimodal mechanisms. Dedicated mechanisms regulate either dendritespecific or axon-specific growth; in contrast, bimodal mechanisms direct dendritic and axonal development in opposite manners. Here, we review the dedicated and bimodal regulators identified by recent Drosophila and mammalian studies. The knowledge of these underlying molecular mechanisms not only expands our understanding about how neural circuits are wired, but also provides insights that will aid in the rational design of therapies for neurological diseases.

Peptide binding properties of the three PDZ domains of Bazooka (Drosophila Par-3)

The Par complex is a conserved cell polarity regulator. Bazooka/Par-3 is scaffold for the complex and contains three PDZ domains in tandem. PDZ domains can act singly or synergistically to bind the C-termini of interacting proteins. Sequence comparisons among Drosophila Baz and its human and C. elegans Par-3 counterparts indicate a divergence of the peptide binding pocket of PDZ1 and greater conservation for the pockets of PDZ2 and PDZ3. However, it is unclear whether the domains from different species share peptide binding preferences, or if their tandem organization affects their peptide binding properties. To investigate these questions, we first used phage display screens to identify unique peptide binding profiles for each single PDZ domain of Baz. Comparisons with published phage display screens indicate that Baz and C. elegans PDZ2 bind to similar peptides, and that the peptide binding preferences of Baz PDZ3 are more similar to C. elegans versus human PDZ3. Next we quantified the peptide binding preferences of each Baz PDZ domain using single identified peptides in surface plasmon resonance assays. In these direct binding studies, each peptide had a binding preference for a single PDZ domain (although the peptide binding of PDZ2 was weakest and the least specific). PDZ1 and PDZ3 bound their peptides with dissociation constants in the nM range, whereas PDZ2-peptide binding was in the µM range. To test whether tandem PDZ domain organization affects peptide binding, we examined a fusion protein containing all three PDZ domains and their normal linker regions. The binding strengths of the PDZ-specific peptides to single PDZ domains and to the PDZ domain tandem were indistinguishable. Thus, the peptide binding pockets of each PDZ domain in Baz are not obviously affected by the presence of neighbouring PDZ domains, but act as isolated modules with specific in vitro peptide binding preferences.

Distinct temporal hierarchies in membrane and cytoskeleton dynamics precede the morphological polarization of developing neurons

Final morphological polarization of neurons, with the development of a distinct axon and of several dendrites, is preceded by phases of non-polarized architecture. The earliest of these phases is that of the round neuron arising from the last mitosis. A second non polarized stage corresponds to the bipolar neuron, with two morphologically identical neurites. Both phases have their distinctive relevance in the establishment of neuronal polarity. During the round cell stage a decision is made as to where from the cell periphery a first neurite will form, thus creating the first sign of asymmetry. At the bipolar stage a decision is made as to which of the two neurites becomes the axon in neurons polarizing in vitro and the leading edge in neurons in situ. In this study we analysed cytoskeletal and membrane dynamics in cells at these two “pre-polarity” stages. By mean of time lapse imaging in dissociated hippocampal neurons and ex vivo cortical slices we show that both stages are characterized by polarized intracellular arrangements, however with distinct temporal hierarchies: polarized actin dynamics marks the site of first polarization in round cells, whereas polarized membrane dynamics precedes asymmetric growth in the bipolar stage.

Competing molecular interactions of aPKC isoforms regulate neuronal polarity

Atypical protein kinase C (aPKC) isoforms ζ and λ interact with polarity complex protein Par3 and are evolutionarily conserved regulators of cell polarity. Prkcz encodes aPKC-ζ and PKM-ζ, a truncated, neuron-specific alternative transcript, and Prkcl encodes aPKC-λ. Here we show that, in embryonic hippocampal neurons, two aPKC isoforms, aPKC-λ and PKM-ζ, are expressed. The localization of these isoforms is spatially distinct in a polarized neuron. aPKC-λ, as well as Par3, localizes at the presumptive axon, whereas PKM-ζ and Par3 are distributed at non-axon-forming neurites. PKM-ζ competes with aPKC-λ for binding to Par3 and disrupts the aPKC-λ-Par3 complex. Silencing of PKM-ζ or overexpression of aPKC-λ in hippocampal neurons alters neuronal polarity, resulting in neurons with supernumerary axons. In contrast, the overexpression of PKM-ζ prevents axon specification. Our studies suggest a molecular model wherein mutually antagonistic intermolecular competition between aPKC isoforms directs the establishment of neuronal polarity.

Regulation of neurite growth by inorganic pyrophosphatase 1 via JNK dephosphorylation

Neural cell differentiation during development is controlled by multiple signaling pathways, in which protein phosphorylation and dephosphorylation play an important role. In this study, we examined the role of pyrophosphatase1 (PPA1) in neuronal differentiation using the loss and gain of function analysis. Neuronal differentiation induced by external factors was studied using a mouse neuroblastoma cell line (N1E115). The neuronal like differentiation in N1E115 cells was determined by morphological analysis based on neurite growth length. In order to analyze the loss of the PPA1 function in N1E115, si-RNA specifically targeting PPA1 was generated. To study the effect of PPA1 overexpression, an adenoviral gene vector containing the PPA1 gene was utilized to infect N1E115 cells. To address the need for pyrophosphatase activity in PPA1, D117A PPA1, which has inactive pyrophosphatase, was overexpressed in N1E115 cells. We used valproic acid (VPA) as a neuronal differentiator to examine the effect of PPA1 in actively differentiated N1E115 cells. Si-PPA1 treatment reduced the PPA1 protein level and led to enhanced neurite growth in N1E115 cells. In contrast, PPA1 overexpression suppressed neurite growth in N1E115 cells treated with VPA, whereas this effect was abolished in D117A PPA1. PPA1 knockdown enhanced the JNK phosphorylation level, and PPA1 overexpression suppressed it in N1E115 cells. It seems that recombinant PPA1 can dephosphorylate JNK while no alteration of JNK phosphorylation level was seen after treatment with recombinant PPA1 D117A. Enhanced neurite growth by PPA1 knockdown was also observed in rat cortical neurons. Thus, PPA1 may play a role in neuronal differentiation via JNK dephosphorylation.

Preliminary crystallographic analysis of the kinase domain of SAD-1, a protein essential for presynaptic differentiation in Caenorhabditis elegans

SAD-1 is a serine/threonine kinase which plays an important role in the regulation of both neuronal polarity and synapse formation in Caenorhabditis elegans. The kinase domain of SAD-1 from C. elegans was overexpressed in Escherichia coli BL21 (DE3) cells and purified to homogeneity using nickel-nitrilotriacetic acid metal-affinity, ion-exchange and gel-filtration chromatography. Diffraction-quality crystals were grown using the sitting-drop vapour-diffusion technique from a condition consisting of 1 M CAPSO pH 9.6, 10%(w/v) polyethylene glycol 3350. The crystals belonged to the monoclinic space group C2, with unit-cell parameters a = 205.4, b = 57.1, c = 71.7 Å, β = 106.1°. X-ray diffraction data were recorded to 3.0 Å resolution from a single crystal using synchrotron radiation.

Plasticity-related gene 3 promotes neurite shaft protrusion

Background

Recently, we and others proposed plasticity-related gene 3 (PRG3) as a novel molecule in neuritogenesis based on PRG3 overexpression experiments in neuronal and non-neuronal cell lines. However, direct information on PRG3 effects in neuronal development and, in particular, its putative spatio-temporal distribution and conditions of action, is sparse.

Results

We demonstrate here that PRG3 induces filopodia formation in HEK293 cells depending on its N-glycosylation status. The PRG3 protein was strongly expressed during mouse brain development in vivo from embryonic day 16 to postnatal day 5 (E16 – P5). From P5 on, expression declined. Furthermore, in early, not yet polarized hippocampal cultured neurons, PRG3 was expressed along the neurite shaft. Knock-down of PRG3 in these neurons led to a decreased number of neurites. This phenotype is rescued by expression of an shRNA-resistant PRG3 construct in PRG3 knock-down neurons. After polarization, endogenous PRG3 expression shifted mainly to axons, specifically to the plasma membrane along the neurite shaft. These PRG3 pattern changes appeared temporally and spatially related to ongoing synaptogenesis. Therefore we tested (i) whether dendritic PRG3 re-enhancement influences synaptic currents and (ii) whether synaptic inputs contribute to the PRG3 shift. Our results rendered both scenarios unlikely: (i) PRG3 over-expression had no influence on miniature excitatory postsynaptic currents (mEPSC) and (ii) blocking of incoming signals did not alter PRG3 distribution dynamics. In addition, PRG3 levels did not interfere with intrinsic neuronal properties.

Conclusion

Taken together, our data indicate that endogenous PRG3 promotes neurite shaft protrusion and therefore contributes to regulating filopodia formation in immature neurons. PRG3 expression in more mature neurons, however, is predominantly localized in the axon. Changes in PRG3 levels did not influence intrinsic or synaptic neuronal properties.

Construction of a polarized neuron

Aside from rare counterexamples (e.g. the starburst amacrine cell in retina), neurons are polarized into two compartments, dendrites and axon, which are linked at the cell body. This structural polarization carries an underlying molecular definition and maps into a general functional polarization whereby inputs are collected by the dendrites and cell body, and output is distributed via the axon. Explanations of how the polarized structure arises invariably coalesce around somatic polarity, defined by the roving location of the microtubule organizing centre, or centrosome, the Golgi apparatus, associated endosomes and the nucleus during early development. In some neurons, proper positioning of these structures can determine the sites for axon and dendrite elongation, and support processes that underlie cell migration. We briefly review these events as a basis to propose a new role for polarized arrangement of somatic organelles as a potential determinant for patterned innervation of the cell body membrane. We cite an example from preliminary studies of synaptogenesis at the calyx of Held, a large nerve terminal that selectively innervates the cell body of its postsynaptic partner, and suggest other neural systems in which polarity mechanisms may guide initial synapse formation onto the somatic surface.

The ability to induce microtubule acetylation is a general feature of formin proteins

Cytoplasmic microtubules exist as distinct dynamic and stable populations within the cell. Stable microtubules direct and maintain cell polarity and it is thought that their stabilization is dependent on coordinative organization between the microtubule network and the actin cytoskeleton. A growing body of work suggests that some members of the formin family of actin remodeling proteins also regulate microtubule organization and stability. For example, we showed previously that expression of the novel formin INF1 is sufficient to induce microtubule stabilization and tubulin acetylation, but not tubulin detyrosination. An important issue with respect to the relationship between formins and microtubules is the determination of which formin domains mediate microtubule stabilization. INF1 has a distinct microtubule-binding domain at its C-terminus and the endogenous INF1 protein is associated with the microtubule network. Surprisingly, the INF1 microtubule-binding domain is not essential for INF1-induced microtubule acetylation. We show here that expression of the isolated FH1 + FH2 functional unit of INF1 is sufficient to induce microtubule acetylation independent of the INF1 microtubule-binding domain. It is not yet clear whether or not microtubule stabilization is a general property of all mammalian formins; therefore we expressed constitutively active derivatives of thirteen of the fifteen mammalian formin proteins in HeLa and NIH3T3 cells and measured their effects on stress fiber formation, MT organization and MT acetylation. We found that expression of the FH1 + FH2 unit of the majority of mammalian formins is sufficient to induce microtubule acetylation. Our results suggest that the regulation of microtubule acetylation is likely a general formin activity and that the FH2 should be thought of as a dual-function domain capable of regulating both actin and microtubule networks.

Spatial confinement instigates environmental determination of neuronal polarity

Here we used patterned self-assembled monolayer (SAM) chemistry to explore the role of spatial confinement on the growth and proliferation of a developing neuron. Despite extensive previous work on the molecular mechanisms controlling these processes, classical biological approaches have not been able to clearly distinguish whether differentiation is predetermined or environmentally determined.

The role of small GTPases in neuronal morphogenesis and polarity

The highly dynamic remodeling and cross talk of the microtubule and actin cytoskeleton support neuronal morphogenesis. Small RhoGTPases family members have emerged as crucial regulators of cytoskeletal dynamics. In this review we will comprehensively analyze findings that support the participation of RhoA, Rac, Cdc42, and TC10 in different neuronal morphogenetic events ranging from migration to synaptic plasticity. We will specifically address the contribution of these GTPases to support neuronal polarity and axonal elongation.

Induction of axon growth arrest without growth cone collapse through the N-terminal region of four-transmembrane glycoprotein M6a

During development, axons elongate vigorously, carefully controlling their speed, to connect with their targets. In general, rapid axon growth is correlated with active growth cones driven by dynamic actin filaments. For example, when the actin-driven tip is collapsed by repulsive guidance molecules, axon growth is severely impaired. In this study, we report that axon growth can be suppressed, without destroying the actin-based structure or motility of the growth cones, when antibodies bind to the four-transmembrane glycoprotein M6a concentrated on the growth cone edge. Surprisingly, M6a-deficient axons grow actively but are not growth suppressed by the antibodies, arguing for an inductive action of the antibody. The binding of antibodies clusters and displaces M6a protein from the growth cone edge membrane, suggesting that the spatial rearrangement of this protein might underlie the unique growth cone behavior triggered by the antibodies. Molecular dissection of M6a suggested involvement for the N-terminal intracellular domain in this antibody-induced growth cone arrest.

N-cadherin specifies first asymmetry in developing neurons

The precise polarization and orientation of developing neurons is essential for the correct wiring of the brain. In pyramidal excitatory neurons, polarization begins with the sprouting of opposite neurites, which later define directed migration and axo-dendritic domains. We here show that endogenous N-cadherin concentrates at one pole of the newborn neuron, from where the first neurite subsequently emerges. Ectopic N-cadherin is sufficient to favour the place of appearance of the first neurite. The Golgi and centrosome move towards this newly formed morphological pole in a second step, which is regulated by PI3K and the actin/microtubule cytoskeleton. Moreover, loss of function experiments in vivo showed that developing neurons with a non-functional N-cadherin misorient their cell axis. These results show that polarization of N-cadherin in the immediate post-mitotic stage is an early and crucial mechanism in neuronal polarity.

Assembly of Bazooka polarity landmarks through a multifaceted membrane-association mechanism

Epithelial cell polarity is essential for animal development. The scaffold protein Bazooka (Baz/PAR-3) forms apical polarity landmarks to organize epithelial cells. However, it is unclear how Baz is recruited to the plasma membrane and how this is coupled with downstream effects. Baz contains an oligomerization domain, three PDZ domains, and binding regions for the protein kinase aPKC and phosphoinositide lipids. With a structure-function approach, we dissected the roles of these domains in the localization and function of Baz in the Drosophila embryonic ectoderm. We found that a multifaceted membrane association mechanism localizes Baz to the apical circumference. Although none of the Baz protein domains are essential for cortical localization, we determined that each contributes to cortical anchorage in a specific manner. We propose that the redundancies involved might provide plasticity and robustness to Baz polarity landmarks. We also identified specific downstream effects, including the promotion of epithelial structure, a positive-feedback loop that recruits aPKC, PAR-6 and Crumbs, and a negative-feedback loop that regulates Baz.

WIP is a negative regulator of neuronal maturation and synaptic activity

Wiskott-Aldrich syndrome protein (WASP) -interacting protein (WIP) is an actin-binding protein involved in the regulation of actin polymerization in cells, such as fibroblasts and lymphocytes. Despite its recognized function in non-neuronal cells, the role of WIP in the central nervous system has not been examined previously. We used WIP-deficient mice to examine WIP function both in vivo and in vitro. We report here that WIP(-)(/-) hippocampal neurons exhibit enlargement of somas as well as overgrowth of neuritic and dendritic branches that are more evident in early developmental stages. Dendritic arborization and synaptogenesis, which includes generation of postsynaptic dendritic spines, are actin-dependent processes that occur in parallel at later stages. WIP deficiency also increases the amplitude and frequency of miniature excitatory postsynaptic currents, suggesting that WIP(-)(/-) neurons have more mature synapses than wild-type neurons. These findings reveal WIP as a previously unreported regulator of neuronal maturation and synaptic activity.

The long and the short of SAD-1 kinase

The Ser/Thr SAD kinases are evolutionarily conserved, critical regulators of neural development. Exciting findings in recent years have significantly advanced our understanding of the mechanism through which SAD kinases regulate neural development. Mammalian SAD-A and SAD-B, activated by a master kinase LKB1, regulate microtubule dynamics and polarize neurons. In C. elegans, the sad-1 gene encodes two isoforms, namely the long and the short, which exhibit overlapping and yet distinct functions in neuronal polarity and synaptic organization. Surprisingly, our most recent findings in C. elegans revealed a SAD-1-independent LKB1 activity in neuronal polarity. We also found that the long SAD-1 isoform directly interacts with a STRADalpha pseudokinase, STRD-1, to regulate neuronal polarity and synaptic organization. We elaborate here a working model of SAD-1 in which the two isoforms dimer/oligomerize to form a functional complex, and STRD-1 clusters and localizes the SAD-1 complex to synapses. While the mechanistic difference between the vertebrate and invertebrate SAD kinases may be puzzling, a recent discovery of the functionally distinct SAD-B isoforms predicts that the difference likely arises from our incomplete understanding of the SAD kinase mechanism and may eventually be reconciled as the revelation continues.

Axon selection: From a polarized cytoplasm to a migrating neuron

The shape of a neuron supplies valuable clues as to its function. Neurons typically extend a single long, thin axon, which will transmit signals and several shorter and thicker dendrites, which will receive signals. The understanding of the means by which neurons acquire a polarized morphology is a fundamental issue in developmental neurobiology. The current view suggests that axon selection involves a stochastic mechanism. However, new data suggest that a polarized cytoplasm not only determines the position of neurite emergence, but also sets the conditions for morphological polarization. In vertebrates, neurons migrate before establishing their final morphology. Recent work shows that the polarized cytoplasm also determines how neurons migrate. Thus, neuronal migration might influence the processes by which neurons form an axon.

Differential regulation of dendritic and axonal development by the novel Krüppel-like factor Dar1

Dendrites and axons are two major neuronal compartments with differences that are critical for neuronal functions. To learn about the differential regulation of dendritic and axonal development, we conducted a genetic screen in Drosophila and isolated the dendritic arbor reduction 1 (dar1) mutants, which display defects in dendritic but not axonal growth. The dar1 gene encodes a novel transcription regulator in the Krüppel-like factor family. Neurons lacking dar1 function have severely reduced growth of microtubule- but not F-actin-based dendritic branches. In contrast, overexpression of Dar1 dramatically increased the growth of microtubule-based dendritic branches. Our results suggest that Dar1 promotes dendrite growth in part by suppressing the expression of the microtubule-severing protein Spastin. Our study thus uncovers a novel transcriptional program for microtubule regulation that preferentially controls dendrite growth.

Decreased Lin7b expression in layer 5 pyramidal neurons may contribute to impaired corticostriatal connectivity in huntington disease

Motor dysfunction, cognitive impairment, and regional cortical atrophy indicate cerebral cortical involvement in Huntington disease (HD). To address the hypothesis that abnormal corticostriatal connectivity arises from polyglutamine-related alterations in cortical gene expression, we isolated layer 5 cortical neurons by laser-capture microdissection and analyzed transcriptome-wide mRNA changes in them. Enrichment of transcription factor mRNAs including foxp2, tbr1, and neuroD6, and neurotransmission- and plasticity-related RNAs including sema5A, pclo, ntrk2, cntn1, and Lin7b were observed. Layer 5 motor cortex neurons of transgenic R6/2 HD mice also demonstrated numerous transcriptomic changes, including decreased expression of mRNAs encoding the Lin7 homolog b ([Lin7b] also known as veli-2 and mals2). Decreases in LIN7B and CNTN1 RNAs were also detected in human HD layer 5 motor cortex neurons. Lin7 homolog b, a scaffold protein implicated in synaptic plasticity, neurite outgrowth, and cellular polarity, was decreased at the protein level in layer 5 cortical neurons in R6/2 mice and human HD brains. Decreases in Lin7b and Lin7a mRNAs were detected in R6/2 cortex as early as 6 weeks of age, suggesting that this is an early pathogenetic event. Thus, decreased cortical LIN7 expression may contribute to abnormal corticostriatal connectivity in HD.

Centrosome motility is essential for initial axon formation in the neocortex

The mechanisms underlying the normal development of neuronal morphology remain a fundamental question in neurobiology. Studies in cultured neurons have suggested that the position of the centrosome and the Golgi may predict the site of axon outgrowth. During neuronal migration in the developing cortex, however, the centrosome and Golgi are oriented toward the cortical plate at a time when axons grow toward the ventricular zone. In the current work, we use in situ live imaging to demonstrate that the centrosome and the accompanying polarized cytoplasm exhibit apical translocation in newborn cortical neurons preceding initial axon outgrowth. Disruption of centrosomal activity or downregulation of the centriolar satellite protein PCM-1 affects axon formation. We further show that downregulation of the centrosomal protein Cep120 impairs microtubule organization, resulting in increased centrosome motility. Decreased centrosome motility resulting from microtubule stabilization causes an aberrant centrosomal localization, leading to misplaced axonal outgrowth. Our results reveal the dynamic nature of the centrosome in developing cortical neurons, and implicate centrosome translocation and microtubule organization during the multipolar stage as important determinants of axon formation.

The PAR complex regulates pulsed actomyosin contractions during amnioserosa apical constriction in Drosophila

Apical constriction is a major mechanism underlying tissue internalization during development. This cell constriction typically requires actomyosin contractility. Thus, understanding apical constriction requires characterization of the mechanics and regulation of actomyosin assemblies. We have analyzed the relationship between myosin and the polarity regulators Par-6, aPKC and Bazooka (Par-3) (the PAR complex) during amnioserosa apical constriction at Drosophila dorsal closure. The PAR complex and myosin accumulate at the apical surface domain of amnioserosa cells at dorsal closure, the PAR complex forming a patch of puncta and myosin forming an associated network. Genetic interactions indicate that the PAR complex supports myosin activity during dorsal closure, as well as during other steps of embryogenesis. We find that actomyosin contractility in amnioserosa cells is based on the repeated assembly and disassembly of apical actomyosin networks, with each assembly event driving constriction of the apical domain. As the networks assemble they translocate across the apical patch of PAR proteins, which persist at the apical domain. Through loss- and gain-of-function studies, we find that different PAR complex components regulate distinct phases of the actomyosin assembly/disassembly cycle: Bazooka promotes the duration of actomyosin pulses and Par-6/aPKC promotes the lull time between pulses. These results identify the mechanics of actomyosin contractility that drive amnioserosa apical constriction and how specific steps of the contractile mechanism are regulated by the PAR complex.

NOS1AP associates with Scribble and regulates dendritic spine development

The formation and function of the neuronal synapse is dependent on the asymmetric distribution of proteins both presynaptically and postsynaptically. Recently, proteins important in establishing cellular polarity have been implicated in the synapse. We therefore performed a proteomic screen with known polarity proteins and identified novel complexes involved in synaptic function. Specifically, we show that the tumor suppressor protein, Scribble, associates with neuronal nitric oxide synthase (nNOS) adaptor protein (NOS1AP) [also known as C-terminal PDZ ligand of nNOS (CAPON)] and is found both presynaptically and postsynaptically. The Scribble-NOS1AP association is direct and is mediated through the phosphotyrosine-binding (PTB) domain of NOS1AP and the fourth PDZ domain of Scribble. Further, we show that Scribble bridges NOS1AP to a beta-Pix [beta-p21-activated kinase (PAK)-interacting exchange factor]/Git1 (G-protein-coupled receptor kinase-interacting protein)/PAK complex. The overexpression of NOS1AP leads to an increase in dendritic protrusions, in a fashion that depends on the NOS1AP PTB domain. Consistent with these observations, both full-length NOS1AP and the NOS1AP PTB domain influence Rac activity. Together these data suggest that NOS1AP plays an important role in the mammalian synapse.

C. elegans STRADalpha and SAD cooperatively regulate neuronal polarity and synaptic organization

Neurons are polarized cells with morphologically and functionally distinct axons and dendrites. The SAD kinases are crucial for establishing the axon-dendrite identity across species. Previous studies suggest that a tumour suppressor kinase, LKB1, in the presence of a pseudokinase, STRADalpha, initiates axonal differentiation and growth through activating the SAD kinases in vertebrate neurons. STRADalpha was implicated in the localization, stabilization and activation of LKB1 in various cell culture studies. Its in vivo functions, however, have not been examined. In our present study, we analyzed the neuronal phenotypes of the first loss-of-function mutants for STRADalpha and examined their genetic interactions with LKB1 and SAD in C. elegans. Unexpectedly, only the C. elegans STRADalpha, STRD-1, functions exclusively through the SAD kinase, SAD-1, to regulate neuronal polarity and synaptic organization. Moreover, STRD-1 tightly associates with SAD-1 to coordinate its synaptic localizations. By contrast, the C. elegans LKB1, PAR-4, also functions in an additional genetic pathway independently of SAD-1 and STRD-1 to regulate neuronal polarity. We propose that STRD-1 establishes neuronal polarity and organizes synaptic proteins in a complex with the SAD-1 kinase. Our findings suggest that instead of a single, linear genetic pathway, STRADalpha and LKB1 regulate neuronal development through multiple effectors that are shared in some cellular contexts but distinct in others.

The neurodevelopmental implications of PI3K signaling

Our understanding of the mechanisms involved in the formation of the complex arrangement of neurons and their interconnections within the brain has made significant progress in recent years. Current research has uncovered a network of intracellular signaling events that provide precise coordination of a diverse array of cellular responses, including trafficking events, cytoskeletal remodeling, gene transcription, and protein ubiquitination and translation. This chapter considers the specific cellular responses controlled by the phosphatidylinositol 3-kinase (PI3K) signaling pathway, which is instructive with regard to a number of important steps involved in the development of the brain. These range from the mediation of extrinsic signals - such as growth factors, axon guidance cues, and extracellular matrix components - to intrinsic effectors, such as downstream signaling components that act, for example, at the translation level. PI3K signaling is, consequently, at the heart of controlling neuronal migration and neuronal morphogenesis, as well as dendrite and synapse development. Many neurobehavioral disorders arise as a consequence of subtle developmental abnormalities. Unsurprisingly, therefore, aberrant PI3K signaling has been indicated by many studies to be a contributing factor to the pathophysiology of disorders such as schizophrenia and autism. In this chapter, we will focus on the specific, yet divergent, cellular processes that are achieved through PI3K signaling in neurons and are key to brain development.

Rab13 regulates neurite outgrowth in PC12 cells through its effector protein, JRAB/MICAL-L2

Neurite outgrowth is the first step in the processes of neuronal differentiation and regeneration and leads to synaptic polarization and plasticity. Rab13 small G protein shows an increased mRNA expression level during neuronal regeneration; it is therefore thought to be involved in this process. We previously identified JRAB (junctional Rab13-binding protein)/MICAL-L2 (molecules interacting with CasL-like 2) as a novel Rab13 effector protein. Here, we show that Rab13 regulates neurite outgrowth in the rat pheochromocytoma cell line PC12 through an interaction with JRAB/MICAL-L2. The expression of JRAB/MICAL-L2 alone inhibits neurite outgrowth, whereas coexpression of the dominant active form of Rab13 rescues this effect. We also demonstrate an intramolecular interaction between the N-terminal calponin-homology (CH) and LIM domains of JRAB/MICAL-L2 and the C-terminal coiled-coil domain. Finally, we show that the binding of Rab13 to JRAB/MICAL-L2 stimulates the interaction of JRAB/MICAL-L2 with actinin-4, an actin-binding protein, which localizes to the cell body and the tips of the neurites in PC12 cells. These results suggest that Rab13 and JRAB/MICAL-L2 may act to transfer actinin-4 from the cell body to the tips of neurites, where actinin-4 is involved in the reorganization of the actin cytoskeleton which results in neurite outgrowth.

A modifier screen in the Drosophila eye reveals that aPKC interacts with Glued during central synapse formation

BACKGROUND

The Glued gene of Drosophila melanogaster encodes the homologue of the vertebrate p150Glued subunit of dynactin. The Glued1 mutation compromises the dynein-dynactin retrograde motor complex and causes disruptions to the adult eye and the CNS, including sensory neurons and the formation of the giant fiber system neural circuit.

RESULTS

We performed a 2-stage genetic screen to identify mutations that modified phenotypes caused by over-expression of a dominant-negative Glued protein. We screened over 34,000 flies and isolated 41 mutations that enhanced or suppressed an eye phenotype. Of these, 12 were assayed for interactions in the giant fiber system by which they altered a giant fiber morphological phenotype and/or altered synaptic function between the giant fiber and the tergotrochanteral muscle motorneuron. Six showed interactions including a new allele of atypical protein kinase C (aPKC). We show that this cell polarity regulator interacts with Glued during central synapse formation. We have mapped the five other interacting mutations to discrete chromosomal regions.

CONCLUSION

Our results show that an efficient way to screen for genes involved in central synapse formation is to use a two-step strategy in which a screen for altered eye morphology precedes the analysis of central synaptogenesis. This has highlighted a role for aPKC in the formation of an identified central synapse.

Microtubule assembly, organization and dynamics in axons and dendrites

During the past decade enormous advances have been made in our understanding of the basic molecular machinery that is involved in the development of neuronal polarity. Far from being mere structural elements, microtubules are emerging as key determinants of neuronal polarity. Here we review the current understanding of the regulation of microtubule assembly, organization and dynamics in axons and dendrites. These studies provide new insight into microtubules' function in neuronal development and their potential contribution to plasticity.

Myosin-II negatively regulates minor process extension and the temporal development of neuronal polarity

The earliest stage in the development of neuronal polarity is characterized by extension of undifferentiated "minor processes" (MPs), which subsequently differentiate into the axon and dendrites. We investigated the role of the myosin II motor protein in MP extension using forebrain and hippocampal neuron cultures. Chronic treatment of neurons with the myosin II ATPase inhibitor blebbistatin increased MP length, which was also seen in myosin IIB knockouts. Through live-cell imaging, we demonstrate that myosin II inhibition triggers rapid minor process extension to a maximum length range. Myosin II activity is determined by phosphorylation of its regulatory light chains (rMLC) and mediated by myosin light chain kinase (MLCK) or RhoA-kinase (ROCK). Pharmacological inhibition of MLCK or ROCK increased MP length moderately, with combined inhibition of these kinases resulting in an additive increase in MP length similar to the effect of direct inhibition of myosin II. Selective inhibition of RhoA signaling upstream of ROCK, with cell-permeable C3 transferase, increased both the length and number of MPs. To determine whether myosin II affected development of neuronal polarity, MP differentiation was examined in cultures treated with direct or indirect myosin II inhibitors. Significantly, inhibition of myosin II, MLCK, or ROCK accelerated the development of neuronal polarity. Increased myosin II activity, through constitutively active MLCK or RhoA, decreased both the length and number of MPs and, consequently, delayed or abolished the development of neuronal polarity. Together, these data indicate that myosin II negatively regulates MP extension, and the developmental time course for axonogenesis.

Genetically engineered nanofiber-like viruses for tissue regenerating materials

Controlling structural organization and signaling motif display of biomimetic matrices at the nanometer scale is of great importance to the functional design of tissue regenerating materials. We have genetically engineered M13 bacteriophage (phage), naturally occurring nanofiber-like viruses, to display a high density of cell-signaling peptides on their major coat proteins. Structural orientation of these phage building blocks can be achieved due to their long-rod shape and monodispersity, which lead them to self-assemble into directionally organized liquid crystalline-like materials. We showed that the constructed viral nanofiber scaffolds were able to support neural progenitor cell proliferation and differentiation as well as direct orientation of their growth in three dimensions. Such functionalized and structurally aligned phage matrices offer promising opportunities for therapies that address challenging medical problems, such as nerve tissue regeneration after spinal cord injuries, or as in vitro model systems for studying complicated cell signaling environments.

Molecular mechanisms of presynaptic differentiation

Information processing in the nervous system relies on properly localized and organized synaptic structures at the correct locations. The formation of synapses is a long and intricate process involving multiple interrelated steps. Decades of research have identified a large number of molecular components of the presynaptic compartment. In addition to neurotransmitter-containing synaptic vesicles, presynaptic terminals are defined by cytoskeletal and membrane specializations that allow highly regulated exo- and endocytosis of synaptic vesicles and that maintain precise registration with postsynaptic targets. Functional studies at multiple levels have revealed complex interactions between the transport of vesicular intermediates, the presynaptic cytoskeleton, growth cone navigation, and synaptic targets. With the advent of finer anatomical, physiological, and molecular tools, great insights have been gained toward the mechanistic dissection of functionally redundant processes controlling the specificity and dynamics of synapses. This review highlights the recent findings pertaining to the cellular and molecular regulation of presynaptic differentiation.

Beyond polymer polarity: how the cytoskeleton builds a polarized cell

Cell polarity relies on the asymmetric organization of cellular components and structures. Actin and microtubules are well suited to provide the structural basis for cell polarization because of their inherent structural polarity along the polymer lattices and intrinsic dynamics that allow them to respond rapidly to polarity cues. In general, the actin cytoskeleton drives the symmetry-breaking process that enables the establishment of a polarized distribution of regulatory molecules, whereas microtubules build on this asymmetry and maintain the stability of the polarized organization. Crosstalk coordinates the functions of the two cytoskeletal systems.