Regulating small G protein signaling to coordinate axon adhesion and repulsion

Two distinct mechanisms of Plexin A function in Drosophila optic lobe lamination and morphogenesis

Visual circuit development is characterized by subdivision of neuropils into layers that house distinct sets of synaptic connections. We find that, in the Drosophila medulla, this layered organization depends on the axon guidance regulator Plexin A. In Plexin A null mutants, synaptic layers of the medulla neuropil and arborizations of individual neurons are wider and less distinct than in controls. Analysis of semaphorin function indicates that Semaphorin 1a, acting in a subset of medulla neurons, is the primary partner for Plexin A in medulla lamination. Removal of the cytoplasmic domain of endogenous Plexin A has little effect on the formation of medulla layers; however, both null and cytoplasmic domain deletion mutations of Plexin A result in an altered overall shape of the medulla neuropil. These data suggest that Plexin A acts as a receptor to mediate morphogenesis of the medulla neuropil, and as a ligand for Semaphorin 1a to subdivide it into layers. Its two independent functions illustrate how a few guidance molecules can organize complex brain structures by each playing multiple roles.

14-3-3ε: a protein with complex physiology function but promising therapeutic potential in cancer

Over the past decade, the role of the 14-3-3 protein has received increasing interest. Seven subtypes of 14-3-3 proteins exhibit high homology; however, each subtype maintains its specificity. The 14-3-3ε protein is involved in various physiological processes, including signal transduction, cell proliferation, apoptosis, autophagy, cell cycle regulation, repolarization of cardiac action, cardiac development, intracellular electrolyte homeostasis, neurodevelopment, and innate immunity. It also plays a significant role in the development and progression of various diseases, such as cardiovascular diseases, inflammatory diseases, neurodegenerative disorders, and cancer. These immense and various involvements of 14-3-3ε in diverse processes makes it a promising target for drug development. Although extensive research has been conducted on 14-3-3 dimers, studies on 14-3-3 monomers are limited. This review aimed to provide an overview of recent reports on the molecular mechanisms involved in the regulation of binding partners by 14-3-3ε, focusing on issues that could help advance the frontiers of this field. Video Abstract.

Manipulating oligodendrocyte intrinsic regeneration mechanism to promote remyelination

In demyelinated lesions, astrocytes, activated microglia and infiltrating macrophages secrete several factors regulating oligodendrocyte precursor cells' behaviour. What appears to be the initiation of an intrinsic mechanism of myelin repair is only leading to partial recovery and inefficient remyelination, a process worsening over the course of the disease. This failure is largely due to the concomitant accumulation of inhibitory cues in and around the lesion sites opposing to growth promoting factors. Here starts a complex game of interactions between the signalling pathways controlling oligodendrocytes migration or differentiation. Receptors of positive or negative cues are modulating Ras, PI3K or RhoGTPases pathways acting on oligodendrocyte cytoskeleton remodelling. From the description of this intricate signalling network, this review addresses the extent to which the modulation of the global response to inhibitory cues may pave the route towards novel therapeutic approaches for myelin repair.

Retinal Pigment Epithelium and Neural Retinal Progenitors Interact via Semaphorin 6D to Facilitate Optic Cup Morphogenesis

Cell movement propels embryonic tissues to acquire shapes required for mature function. The movements are driven both by acto-myosin signaling and by cells interacting with the extracellular matrix (ECM). Unknown is whether cell-cell interactions within a tissue are also required, and the molecular mechanisms by which such communication might occur. Here, we use the developing visual system of zebrafish as a model to understand the role cell-cell communication plays in tissue morphogenesis in the embryonic nervous system. We identify that cell-cell-mediated contact between two distinct cell populations, progenitors of the neural retina and retinal pigment epithelium (RPE), facilitates epithelial flow to produce the mature cupped retina. We identify for the first time the need in eye morphogenesis for distinct populations of progenitors to interact, and suggest a novel role for a member of a key developmental signaling family, the transmembrane Semaphorin6d, as mediating communication between distinct cell types to control tissue morphogenesis.

Phosphoproteomic Analysis as an Approach for Understanding Molecular Mechanisms of cAMP-Dependent Actions

In recent years, highly sensitive mass spectrometry-based phosphoproteomic analysis is beginning to be applied to identification of protein kinase substrates altered downstream of increased cAMP. Such studies identify a very large number of phosphorylation sites regulated in response to increased cAMP. Therefore, we now are tasked with the challenge of determining how many of these altered phosphorylation sites are relevant to regulation of function in the cell. This minireview describes the use of phosphoproteomic analysis to monitor the effects of cyclic nucleotide phosphodiesterase (PDE) inhibitors on cAMP-dependent phosphorylation events. More specifically, it describes two examples of this approach carried out in the authors' laboratories using the selective PDE inhibitor approach. After a short discussion of several likely conclusions suggested by these analyses of cAMP function in steroid hormone-producing cells and also in T-cells, it expands into a discussion about some newer and more speculative interpretations of the data. These include the idea that multiple phosphorylation sites and not a single rate-limiting step likely regulate these and, by analogy, many other cAMP-dependent pathways. In addition, the idea that meaningful regulation requires a high stoichiometry of phosphorylation to be important is discussed and suggested to be untrue in many instances. These new interpretations have important implications for drug design, especially for targeting pathway agonists. SIGNIFICANCE STATEMENT: Phosphoproteomic analyses identify thousands of altered phosphorylation sites upon drug treatment, providing many possible regulatory targets but also highlighting questions about which phosphosites are functionally important. These data imply that multistep processes are regulated by phosphorylation at not one but rather many sites. Most previous studies assumed a single step or very few rate-limiting steps were changed by phosphorylation. This concept should be changed. Previous interpretations also assumed substoichiometric phosphorylation was not of regulatory importance. This assumption also should be changed.

14-3-3β Is necessary in the regulation of polarization and directional migration of alveolar myofibroblasts by lipopolysaccharide

Aims:Bronchopulmonary dysplasia (BPD) is characterized by alveolarization arrest. During alveolarization, alveolar myofibroblasts are thought to migrate into the septal tips and elongate secondary septa. Lipopolysaccharide (LPS) exposure has been reported to disrupt directional migration and final location of alveolar myofibroblasts in a rat model of BPD induced by intra-amniotic injection of LPS. However, molecular mechanisms that control directional migration of alveolar myofibroblasts have not so far been investigated clearly. Materials and Methods: We assessed the polarization of myofibroblast using scrape wounding assays combined with Golgi tracking. Transwell migration assay was used to detect the directional migration of myofibroblasts. Pull-down assays were performed to isolate the active GTP-bound form using the RhoA activation assay kits. Western blotting analysis was performed to evaluate the changes in protein expression. Functional analysis was performed via siRNA interference. Results: Here, we showed that LPS might affect the directional migration of myofibroblasts by disturbing the polarization of myofibroblasts. In addition, as a main member of RhoGTPases family which plays a vital role in establishing and maintaining cell polarity, RhoA activity was significantly upregulated in myofibroblasts treated with LPS, while activity of epidermal growth factor receptor (EGFR) was upregulated and overexpression of its ligand, TGF-α, in myofibroblasts by LPS treatment. AG1478, an EGFR inhibitor, could abrogate the upregulated RhoA activity of myofibroblasts by LPS and rhTGF-α. Moreover, if we knock down 14-3-3β, LPS and rhTGF-α could not activate RhoA and disturb myofibroblasts polarization. Conclusions: Taken together, our findings suggest that LPS exposure may increase RhoA activity of myofibroblasts by TGF-α/EGFR/14-3-3β signaling pathway, and then disturb myofibroblasts polarization and directional migration.

Semaphorin 6A-Plexin A2/A4 Interactions with Radial Glia Regulate Migration Termination of Superficial Layer Cortical Neurons

Precise regulation of neuronal migration termination is crucial for the establishment of brain cytoarchitectures. However, little is known about how neurons terminate migration. Here we focused on interactions between migrating cortical neurons and their substrates, radial glial (RG) cells, and analyzed the role of Plexin A2 and A4 (PlxnA2/A4) receptors and their repulsive ligand, Semaphorin 6A (Sema6A), for this process. In both PlxnA2/A4 double-knockout and Sema6A mutant mice, the outermost cortical plate neurons ectopically invade layer 1 at a stage when they should reach their destinations. PlxnA2/A4 proteins are abundantly expressed on their leading processes, whereas Sema6A mRNA is enriched in RG cell somata. Cell-targeted gene expression and conditional knockouts indicate critical roles for these molecules. We hypothesize that the timely appearance of repulsive signaling mediated by Sema6A-PlxnA2/A4 weakens migrating neuron-RG cell interactions, leading to migration termination.

MicroRNA-30b regulates the polarity of retinal ganglion cells by inhibiting semaphorin-3A

Purpose

Retinal ganglion cell (RGC) polarity plays an important role in optic nerve regeneration. This study was designed to investigate whether semaphorin-3A (Sema3A) is involved in the regulation of RGC polarity and Sema3A protein expression.

Methods

Cultured primary RGCs were treated with Fc-Sema3A or Sema3A siRNA or transfected with purified miR-30b recombinant adenoassociated virus (rAAV). The polarity of the RGCs was observed with immunofluorescence. A western blot analysis of phosphorylated protein kinase A (p-PKA), the downstream effector molecule phosphorylated glycogen synthase kinase 3 beta (GSK-3β), and collapsing response mediator protein 2 (CRMP2) was performed.

Results

We found that Sema3A could statistically significantly promote dendritic branching while inhibiting the growth of axons in RGCs. miR-30b overexpression and Sema3A siRNA could statistically significantly promote the growth of axons while inhibiting the growth of dendrites from RGCs. Additionally, miR-30b could restrain the expression of Sema3A protein and its downstream PKA/GSK-3β/CRMP2 signaling pathways.

Conclusions

The results indicate that Sema3A promotes dendritic growth and inhibits axonal growth, which is not conducive to the early repair of optic nerve injury. The overexpression of miR-30b can overcome this problem, and may represent a new target for the treatment of nerve injury and regeneration in the future.

PKC induces release of a functional ectodomain of the guidance cue semaphorin6A

Semaphorins (Semas) are a family of secreted and transmembrane proteins that play critical roles in development. Interestingly, several vertebrate transmembrane Sema classes are capable of producing functional soluble ectodomains. However, little is known of soluble Sema6 ectodomains in the nervous system. Herein, we show that the soluble Sema6A ectodomain, sSema6A, exhibits natural and protein kinase C (PKC)-induced release. We show that PKC mediates Sema6A phosphorylation at specific sites and while this phosphorylation is not the primary mechanism regulating sSema6A production, we found that the intracellular domain confers resistance to ectodomain release. Finally, sSema6A is functional as it promotes the cohesion of zebrafish early eye field explants. This suggests that in addition to its canonical contact-mediated functions, Sema6A may have regulated, long-range, forward-signaling capacity.

Semaphorins and their Signaling Mechanisms

Semaphorins are extracellular signaling proteins that are essential for the development and maintenance of many organs and tissues. The more than 20-member semaphorin protein family includes secreted, transmembrane and cell surface-attached proteins with diverse structures, each characterized by a single cysteine-rich extracellular sema domain, the defining feature of the family. Early studies revealed that semaphorins function as axon guidance molecules, but it is now understood that semaphorins are key regulators of morphology and motility in many different cell types including those that make up the nervous, cardiovascular, immune, endocrine, hepatic, renal, reproductive, respiratory and musculoskeletal systems, as well as in cancer cells. Semaphorin signaling occurs predominantly through Plexin receptors and results in changes to the cytoskeletal and adhesive machinery that regulate cellular morphology. While much remains to be learned about the mechanisms underlying the effects of semaphorins, exciting work has begun to reveal how semaphorin signaling is fine-tuned through different receptor complexes and other mechanisms to achieve specific outcomes in various cellular contexts and physiological systems. These and future studies will lead to a more complete understanding of semaphorin-mediated development and to a greater understanding of how these proteins function in human disease.

Fyn-dependent phosphorylation of PlexinA1 and PlexinA2 at conserved tyrosines is essential for zebrafish eye development

Plexins (Plxns) are semaphorin (Sema) receptors that play important signaling roles, particularly in the developing nervous system and vasculature. Sema-Plxn signaling regulates cellular processes such as cytoskeletal dynamics, proliferation, and differentiation. However, the receptor-proximal signaling mechanisms driving Sema-Plxn signal transduction are only partially understood. Plxn tyrosine phosphorylation is thought to play an important role in these signaling events as receptor and nonreceptor tyrosine kinases have been shown to interact with Plxn receptors. The Src family kinase Fyn can induce the tyrosine phosphorylation of PlxnA1 and PlxnA2. However, the Fyn-dependent phosphorylation sites on these receptors have not been identified. Here, using mass spectrometry-based approaches, we have identified highly conserved, Fyn-induced PlexinA (PlxnA) tyrosine phosphorylation sites. Mutation of these sites to phenylalanine results in significantly decreased Fyn-dependent PlxnA tyrosine phosphorylation. Furthermore, in contrast to wild-type human PLXNA2 mRNA, mRNA harboring these point mutations cannot rescue eye developmental defects when coinjected with a plxnA2 morpholino in zebrafish embryos. Together these data suggest that Fyn-dependent phosphorylation at two critical tyrosines is a key feature of vertebrate PlxnA1 and PlxnA2 signal transduction.

Characterizing PKA-Mediated Phosphorylation of Plexin Using Purified Proteins

Protein phosphorylation is one of the widely used posttranslational modifications that alter protein function in vivo. We recently showed phosphorylation of Drosophila Plexin A by cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and subsequent inhibition of plexin-mediated repulsive guidance. This phosphorylation occurs in the active site of the plexin GTPase-activating protein (GAP) domain, which in turn inhibits endogenous GAP activity toward Ras/Rap family small GTP-binding proteins by recruiting the phospho-serine/threonine-binding protein 14-3-3ε. Here we describe how phosphorylation of Plexin A can be detected and quantitated using an in vitro kinase assay and radioactive [γ-P32] adenosine 5'-triphosphate (ATP).

Switching responses: spatial and temporal regulators of axon guidance

The ability of the axonal growth cone to switch between attraction and repulsion in response to guidance cues in the extracellular environment during nervous system development is fundamental to the precise wiring of complex neural circuits. Regulation of cell-surface receptors by means of transcriptional control, local translation, trafficking and proteolytic processing are powerful mechanisms to regulate the response of the growth cone. Important work has also revealed how intracellular signalling pathways, including calcium and cyclic nucleotide signalling, can alter the directional response elicited by a particular cue. Here, we describe how these multiple regulatory mechanisms influence growth cone turning behaviour. We focus on recent evidence that suggests a significant role for 14-3-3 adaptor proteins in modifying growth cone turning behaviour and mediating directional polarity switches during development. Characterizing how 14-3-3 s regulate growth cone signalling will provide invaluable insight into nervous system development and may facilitate the identification of novel targets for promoting nerve regeneration following injury.

What axons tell each other: axon-axon signaling in nerve and circuit assembly

A remarkable feature of nervous system development is the ability of axons emerging from newly formed neurons to traverse, by cellular scale, colossal distances to appropriate targets. The earliest axons achieve this in an essentially axon-free environment, but the vast majority of axons eventually grow along a scaffold of nerve tracts created by earlier extending axons. Signal exchange between sequentially or simultaneously extending axons may well represent the predominant mode of axonal navigation, but proportionally few efforts have so far been directed at deciphering the underlying mechanisms. This review intends to provide a conceptual update on the cellular and molecular principles driving axon-axon interactions, with emphasis on those contributing to the fidelity of axonal navigation, sorting and connectivity during nerve and circuit assembly.