Focal adhesion kinase modulates Cdc42 activity downstream of positive and negative axon guidance cues

Selective Suppression of Integrin-Ligand Binding by Single Molecular Tension Probes Mediates Directional Cell Migration

Cell migration interacting with continuously changing microenvironment, is one of the most essential cellular functions, participating in embryonic development, wound repair, immune response, and cancer metastasis. The migration process is finely tuned by integrin-mediated binding to ligand molecules. Although numerous biochemical pathways orchestrating cell adhesion and motility are identified, how subcellular forces between the cell and extracellular matrix regulate intracellular signaling for cell migration remains unclear. Here, it is showed that a molecular binding force across integrin subunits determines directional migration by regulating tension-dependent focal contact formation and focal adhesion kinase phosphorylation. Molecular binding strength between integrin αvβ3 and fibronectin is precisely manipulated by developing molecular tension probes that control the mechanical tolerance applied to cell-substrate interfaces. This data reveals that integrin-mediated molecular binding force reduction suppresses cell spreading and focal adhesion formation, attenuating the focal adhesion kinase (FAK) phosphorylation that regulates the persistence of cell migration. These results further demonstrate that manipulating subcellular binding forces at the molecular level can recapitulate differential cell migration in response to changes of substrate rigidity that determines the physical condition of extracellular microenvironment. Novel insights is provided into the subcellular mechanics behind global mechanical adaptation of the cell to surrounding tissue environments featuring distinct biophysical signatures.

Delving into the Genetic Causes of Language Impairment in a Case of Partial Deletion of NRXN1

Introduction

Copy-number variations (CNVs) impacting on small DNA stretches and associated with language deficits provide a unique window to the role played by specific genes in language function.

Methods

We report in detail on the cognitive, language, and genetic features of a girl bearing a small deletion (0.186 Mb) in the 2p16.3 region, arr[hg19] 2p16.3(50761778_50947729)×1, affecting exons 3-7 of NRXN1, a neurexin-coding gene previously related to schizophrenia, autism (ASD), attention deficit hyperactivity disorder (ADHD), mood disorder, and intellectual disability (ID).

Results

The proband exhibits many of the features commonly found in subjects with deletions of NRXN1, like ASD-like traits (including ritualized behaviors, disordered sensory aspects, social disturbances, and impaired theory of mind), ADHD symptoms, moderate ID, and impaired speech and language. Regarding this latter aspect, we observed altered speech production, underdeveloped phonological awareness, minimal syntax, serious shortage of active vocabulary, impaired receptive language, and inappropriate pragmatic behavior (including lack of metapragmatic awareness and communicative use of gaze). Microarray analyses point to the dysregulation of several genes important for language function in the girl compared to her healthy parents.

Discussion

Although some basic cognitive deficit - such as the impairment of executive function - might contribute to the language problems exhibited by the proband, molecular evidence suggests that they might result, to a great extent, from the abnormal expression of genes directly related to language.

Cell-Cell Interaction-Mediated Signaling in the Testis Induces Reproductive Dysfunction—Lesson from the Toxicant/Pharmaceutical Models

Emerging evidence has shown that cell-cell interactions between testicular cells, in particular at the Sertoli cell-cell and Sertoli-germ cell interface, are crucial to support spermatogenesis. The unique ultrastructures that support cell-cell interactions in the testis are the basal ES (ectoplasmic specialization) and the apical ES. The basal ES is found between adjacent Sertoli cells near the basement membrane that also constitute the blood-testis barrier (BTB). The apical ES is restrictively expressed at the Sertoli-spermatid contact site in the apical (adluminal) compartment of the seminiferous epithelium. These ultrastructures are present in both rodent and human testes, but the majority of studies found in the literature were done in rodent testes. As such, our discussion herein, unless otherwise specified, is focused on studies in testes of adult rats. Studies have shown that the testicular cell-cell interactions crucial to support spermatogenesis are mediated through distinctive signaling proteins and pathways, most notably involving FAK, Akt1/2 and Cdc42 GTPase. Thus, manipulation of some of these signaling proteins, such as FAK, through the use of phosphomimetic mutants for overexpression in Sertoli cell epithelium in vitro or in the testis in vivo, making FAK either constitutively active or inactive, we can modify the outcome of spermatogenesis. For instance, using the toxicant-induced Sertoli cell or testis injury in rats as study models, we can either block or rescue toxicant-induced infertility through overexpression of p-FAK-Y397 or p-FAK-Y407 (and their mutants), including the use of specific activator(s) of the involved signaling proteins against pAkt1/2. These findings thus illustrate that a potential therapeutic approach can be developed to manage toxicant-induced male reproductive dysfunction. In this review, we critically evaluate these recent findings, highlighting the direction for future investigations by bringing the laboratory-based research through a translation path to clinical investigations.

Selective axonal translation of the mRNA isoform encoding prenylated Cdc42 supports axon growth

The small Rho-family GTPase Cdc42 has long been known to have a role in cell motility and axon growth. The eukaryotic Ccd42 gene is alternatively spliced to generate mRNAs with two different 3' untranslated regions (UTRs) that encode proteins with distinct C-termini. The C-termini of these Cdc42 proteins include CaaX and CCaX motifs for post-translational prenylation and palmitoylation, respectively. Palmitoyl-Cdc42 protein was previously shown to contribute to dendrite maturation, while the prenyl-Cdc42 protein contributes to axon specification and its mRNA was detected in neurites. Here, we show that the mRNA encoding prenyl-Cdc42 isoform preferentially localizes into PNS axons and this localization selectively increases in vivo during peripheral nervous system (PNS) axon regeneration. Functional studies indicate that prenyl-Cdc42 increases axon length in a manner that requires axonal targeting of its mRNA, which, in turn, needs an intact C-terminal CaaX motif that can drive prenylation of the encoded protein. In contrast, palmitoyl-Cdc42 has no effect on axon growth but selectively increases dendrite length. Together, these data show that alternative splicing of the Cdc42 gene product generates an axon growth promoting, locally synthesized prenyl-Cdc42 protein. This article has an associated First Person interview with one of the co-first authors of the paper.

The effects of the M2a macrophage-induced axonal regeneration of neurons by arginase 1

BACKGROUND

Spinal cord injury (SCI) is a challenge worldwide, but there are no effective treatments or therapeutic methods in the clinic. Recent studies have shown that type I arginase (Arginase1, Arg1) is closely associated with the treatment of SCI. The classical treatment for SCI involves filling the local area of SCI with activated M2a macrophages to allow the repair and regeneration of some synapses, but the specific mechanism of action of Arg1 is not clear.

METHOD

In the present study, we first induced the polarization of RAW264.7 macrophages to M2a-type cells using IL-4 and constructed an Arg1 knockout cell line through the use of shRNA; we used these cells to treat a rat model of SCI. Finally, the present study explored the mechanism and pathway by which Arginase 1 regulates spinal repair by immunoblotting and immunohistochemistry.

RESULT

Suspended M2a (Arg1-/+) macrophages were transplanted into the injury site in a rat model of contusion SCI. Compared with the model group and the shArg1 group, the shScramble (shSc) group exhibited higher Basso, Beattie, Bresnahan motor function scores, more compact structures and more Nissl bodies. Immunohistochemical results showed that the shSc group expressed higher levels of NeuN (a neuronal marker) and tau (an axonal marker), as well as the up-regulation of Cdc42, N-WASP, Arp2/3 and tau, as determined by Western blot.

CONCLUSION

The study found that the polarization of M2a macrophages promoted the expression of Arginase 1, which restored axonal regeneration, promoted axonal regeneration, and promoted the structural and functional recovery of the contused spinal cord.

The neurorepellent, Slit2, prevents macrophage lipid loading by inhibiting CD36-dependent binding and internalization of oxidized low-density lipoprotein

Atherosclerosis is characterized by retention of modified lipoproteins, especially oxidized low density lipoprotein (oxLDL) within the sub-endothelial space of affected blood vessels. Recruited monocyte-derived and tissue-resident macrophages subsequently ingest oxLDL by binding and internalizing oxLDL via scavenger receptors, particularly CD36. The secreted neurorepellent, Slit2, acting through its transmembrane receptor, Roundabout-1 (Robo-1), was previously shown to inhibit recruitment of monocytes into nascent atherosclerotic lesions. The effects of Slit2 on oxLDL uptake by macrophages have not been explored. We report here that Slit2 inhibits uptake of oxLDL by human and murine macrophages, and the resulting formation of foam cells, in a Rac1-dependent and CD36-dependent manner. Exposure of macrophages to Slit2 prevented binding of oxLDL to the surface of cells. Using super-resolution microscopy, we observed that exposure of macrophages to Slit2 induced profound cytoskeletal remodeling with formation of a thick ring of cortical actin within which clusters of CD36 could not aggregate, thereby attenuating binding of oxLDL to the surface of cells. By inhibiting recruitment of monocytes into early atherosclerotic lesions, and the subsequent binding and internalization of oxLDL by macrophages, Slit2 could represent a potent new tool to combat individual steps that collectively result in progression of atherosclerosis.

Language Impairment with a Partial Duplication of DOCK8

Duplications of the distal region of the short arm of chromosome 9 are rare, but are associated with learning disabilities and behavioral disturbances. We report in detail the cognitive and language features of a child with a duplication in the 9p24.3 region, arr[hg19] 9p24.3(266,045-459,076)×3. The proband exhibits marked expressive and receptive problems, which affect both structural and functional aspects of language. These problems might result from a severe underlying deficit in working memory. Regarding the molecular causes of the observed symptoms, they might result from the altered expression of selected genes involved in procedural learning, particularly some of components of the SLIT/ROBO/FOXP2 network, strongly related to the development and evolution of language. Dysregulation of specific components of this network can result in turn from an altered interaction between DOCK8, affected by the microduplication, and CDC42, acting as the hub component of the network encompassing language-related genes.

Manipulation of Axonal Outgrowth via Exogenous Low Forces

Neurons are mechanosensitive cells. The role of mechanical force in the process of neurite initiation, elongation and sprouting; nerve fasciculation; and neuron maturation continues to attract considerable interest among scientists. Force is an endogenous signal that stimulates all these processes in vivo. The axon is able to sense force, generate force and, ultimately, transduce the force in a signal for growth. This opens up fascinating scenarios. How are forces generated and sensed in vivo? Which molecular mechanisms are responsible for this mechanotransduction signal? Can we exploit exogenously applied forces to mimic and control this process? How can these extremely low forces be generated in vivo in a non-invasive manner? Can these methodologies for force generation be used in regenerative therapies? This review addresses these questions, providing a general overview of current knowledge on the applications of exogenous forces to manipulate axonal outgrowth, with a special focus on forces whose magnitude is similar to those generated in vivo. We also review the principal methodologies for applying these forces, providing new inspiration and insights into the potential of this approach for future regenerative therapies.

The potential of Slit2 as a therapeutic target for central nervous system disorders

Introduction: Slit2 is an extracellular matrix protein that regulates migration of developing axons during central nervous system (CNS) development. Roundabout (Robo) receptors expressed by various cell types in the CNS, mediate intracellular signal transduction pathways for Slit2. Recent studies indicate that Slit2 plays important protective roles in a myriad of processes such as cell migration, immune response, vascular permeability, and angiogenesis in CNS pathologies. Areas covered: This review provides an overview of the diverse functions of Slit2 in CNS disorders and discusses the potential of Slit2 as a therapeutic target. We reviewed preclinical studies reporting the role of Slit2 in various CNS disease models, transgenic animal research, and rodent models that utilized Slit2 as a therapy. Expert opinion: Slit2 exerts a wide array of beneficial effects ranging from anti-migration, blood-brain barrier (BBB) protection, inhibition of peripheral immune cell infiltration, and anti-apoptosis in various disease models. However, a dual role of Slit2 in endothelial permeability has been observed in transgenic animals. Further research on Slit2 will be crucial including key issues such as effects of transgenic overexpression versus exogenous Slit2, function of Slit2 dependent on cellular expression of Robo receptors and the underlying pathology for potential clinical translation.

Activation of FAK/Rac1/Cdc42-GTPase signaling ameliorates impaired microglial migration response to Aβ42 in triggering receptor expressed on myeloid cells 2 loss-of-function murine models

Mutation of Triggering receptor expressed on myeloid cells 2 (TREM2) impairs the response of microglia to amyloid-β (Aβ) pathology in Alzheimer's disease (AD), although the mechanism governing TREM2-regulated microglia recruitment to Aβ plaques remains unresolved. Here, we confirm that TREM2 mutation attenuates microglial migration. Then, using Trem2^-/- mice and an R47H variant mouse model for AD generated for this study, we show that TREM2 deficiency or the AD-associated R47H mutation results in inhibition of FAK and Rac1/Cdc42-GTPase signaling critical for cell migration. Intriguingly, treatment with CN04, a Rac1/Cdc42-GTPase activator, partially enhances microglial migration in response to oligomeric Aβ₄₂ in Trem2^-/- or R47H microglia both in vitro and in vivo. Our study shows that the dysfunction of microglial migration in the AD-associated TREM2 R47H variant is caused by FAK/Rac1/Cdc42 signaling disruption, and that activation of this signaling ameliorates impaired microglial migration response to Aβ₄₂ , suggesting a therapeutic target for R47H-bearing patients with high risk of AD.

Surface Roughness Gradients Reveal Topography-Specific Mechanosensitive Responses in Human Mesenchymal Stem Cells

The topographic features of an implant, which mechanically regulate cell behaviors and functions, are critical for the clinical success in tissue regeneration. How cells sense and respond to the topographical cues, e.g., interfacial roughness, is yet to be fully understood and even debatable. Here, the mechanotransduction and fate determination of human mesenchymal stem cells (MSCs) on surface roughness gradients are systematically studied. The broad range of topographical scales and high-throughput imaging is achieved based on a catecholic polyglycerol coating fabricated by a one-step-tilted dip-coating approach. It is revealed that the adhesion of MSCs is biphasically regulated by interfacial roughness. The cell mechanotransduction is investigated from focal adhesion to transcriptional activity, which explains that cellular response to interfacial roughness undergoes a direct force-dependent mechanism. Moreover, the optimized roughness for promoting cell fate specification is explored.

Wnt5b integrates Fak1a to mediate gastrulation cell movements via Rac1 and Cdc42

Focal adhesion kinase (FAK) mediates vital cellular pathways during development. Despite its necessity, how FAK regulates and integrates with other signals during early embryogenesis remains poorly understood. We found that the loss of Fak1a impaired epiboly, convergent extension and hypoblast cell migration in zebrafish embryos. We also observed a clear disturbance in cortical actin at the blastoderm margin and distribution of yolk syncytial nuclei. In addition, we investigated a possible link between Fak1a and a well-known gastrulation regulator, Wnt5b, and revealed that the overexpression of fak1a or wnt5b could cross-rescue convergence defects induced by a wnt5b or fak1a antisense morpholino (MO), respectively. Wnt5b and Fak1a were shown to converge in regulating Rac1 and Cdc42, which could synergistically rescue wnt5b and fak1a morphant phenotypes. Furthermore, we generated several alleles of fak1a mutants using CRISPR/Cas9, but those mutants only revealed mild gastrulation defects. However, injection of a subthreshold level of the wnt5b MO induced severe gastrulation defects in fak1a mutants, which suggested that the upregulated expression of wnt5b might complement the loss of Fak1a. Collectively, we demonstrated that a functional interaction between Wnt and FAK signalling mediates gastrulation cell movements via the possible regulation of Rac1 and Cdc42 and subsequent actin dynamics.

On the relationship of YAP and FAK in hMSCs and osteosarcoma cells: Discrimination of FAK modulation by nuclear YAP depletion or YAP silencing

The HIPPO pathway effector YAP has been shown to be regulated by FAK-signaling. However, the existence of an inverse relationship between YAP and FAK is unknown. Here we demonstrate in hMSCs and in the human osteosarcoma derived cell line Saos that Verteporfin- or RNAi-dependent YAP depletion has opposing influence on FAK. While Verteporfin strikingly reduced cellular FAK protein and phosphorylation, RNAi led to an increase of both molecules and point on a generalizable aspect of the YAP/FAK interrelationship. YAP depletion also caused down-regulation of osteogenic genes in hMSCs, irrespective from the YAP intervention mode. Verteporfin induced topological changes in conjunction with reduced protein levels of β1 integrin, paxillin, and zyxin of focal adhesions (FAs) in hMSCs, suggesting FAK-decrease-related alterations in FAs, which seems to be a FAK-dependent mechanism. On the cell behavioral level, YAP-FAK-interrelation involves proliferation and senescence, as indicated by proliferation inhibition and increase of β-Galactosidase-activity in hMSCs. Our findings, derived from this dual strategy of YAP intervention, reveal a YAP-FAK relationship in conjunction with molecular and cell behavioral consequences. Moreover, they deepen the current scientific knowledge on YAP from a different scientific point of view, since this inverse YAP/FAK-relationship seems to be transferrable to other cell types, including cell entities with pathological background.

Spatiotemporal Regulation of Rho GTPases in Neuronal Migration

Neuronal migration is essential for the orchestration of brain development and involves several contiguous steps: interkinetic nuclear movement (INM), multipolar–bipolar transition, locomotion, and translocation. Growing evidence suggests that Rho GTPases, including RhoA, Rac, Cdc42, and the atypical Rnd members, play critical roles in neuronal migration by regulating both actin and microtubule cytoskeletal components. This review focuses on the spatiotemporal-specific regulation of Rho GTPases as well as their regulators and effectors in distinct steps during the neuronal migration process. Their roles in bridging extracellular signals and cytoskeletal dynamics to provide optimal structural support to the migrating neurons will also be discussed.

Opposing functions of F-BAR proteins in neuronal membrane protrusion, tubule formation, and neurite outgrowth

Neurite formation is a fundamental antecedent to axon and dendrite formation, but the mechanisms that underlie this important process are poorly characterized. Here, we demonstrate that two F-BAR proteins, CIP4 and FBP17, have opposing functions in early cortical neuron development.

The F-BAR family of proteins play important roles in many cellular processes by regulating both membrane and actin dynamics. The CIP4 family of F-BAR proteins is widely recognized to function in endocytosis by elongating endocytosing vesicles. However, in primary cortical neurons, CIP4 concentrates at the tips of extending lamellipodia and filopodia and inhibits neurite outgrowth. Here, we report that the highly homologous CIP4 family member, FBP17, induces tubular structures in primary cortical neurons and results in precocious neurite formation. Through domain swapping and deletion experiments, we demonstrate that a novel polybasic region between the F-BAR and HR1 domains is required for membrane bending. Moreover, the presence of a poly-PxxP region in longer splice isoforms of CIP4 and FBP17 largely reverses the localization and function of these proteins. Thus, CIP4 and FBP17 function as an antagonistic pair to fine-tune membrane protrusion, endocytosis, and neurite formation during early neuronal development.

Biological activity of laminin/polylaminin-coated poly-ℇ-caprolactone filaments on the regeneration and tissue replacement of the rat sciatic nerve

Unlike the central nervous system, peripheral nerves can regenerate after injury. However, depending on the size of the lesion, the endogenous regenerative potential is not enough to replace the lost nerve tissue. Many strategies have been used to generate biomaterials capable of restoring nerve functions. Here, we set out to investigate whether adsorbing the extracellular matrix protein, laminin (LM), to poly-ℇ-caprolactone (PCL) filaments would enhance functional nerve regeneration. Initial in vitro studies showed that explants of dorsal root ganglia (DRGs) of P1 neonate mice exhibited stronger neuritogenesis on a substrate of LM that had been previously polymerized (polylaminin [polyLM]) than on ordinary LM. On the other hand, when silicone tubes filled with PCL filaments were used to bridge a 10-mm sciatic nerve gap in rats, only filaments coated with LM improved tissue replacement beyond that obtained with empty tubes. Motor function recovery correlated with tissue replacement as only LM-coated filaments consistently improved motor skills. Finally, analysis of the lateral gastrocnemius muscle revealed that the LM group presented twice the amount of α-bungarotixin–labeled motor plates. In conclusion, although polyLM was more effective in stimulating growth of sensory fibers out of DRGs in vitro, LM adsorbed to PCL filaments exhibited the best regenerative properties in inducing functional motor recovery after peripheral injury in vivo.

Inhibitory Effect of Slit2-N on VEGF165-induced proliferation of vascular endothelia via Slit2-N-Robo4-Akt pathway in choroidal neovascularization

Researches have been focusing on the role of Slit2 in angiogenesis, specifically in cell migration and vessel permeability. Nevertheless, the role of Slit2-N, the bioactive fragment of Slit2, in the proliferation of vascular endothelia in choroidal neovascularization and some related mechanisms have not been studied yet. Thus, our study aimed to explore the role of Slit2-N in proliferation of vascular endothelia and the related mechanisms in choroidal neovascularization. Fluorescein isothiocyanate perfusion and HE staining were performed to evaluate volumes of choroidal neovascularization lesions. The effect of Slit2-N on VEGF165-induced cell proliferation and some related mechanisms were detected by CCK8 assay, flow cytometry, siRNA transfection, and western blotting. We found that Slit2-N reduced volumes of laser-induced choroidal neovascularization networks in vivo. Results of the in vitro study showed Slit2-N reduced VEGF165-induced cell proliferation of both human umbilical vascular endothelial cells and human microvascular endothelial cells possibly via activation of AKT rather than that of ERK1/2. Additionally, Robo4, one of the receptors binding to Slit2-N, was involved in the inhibitory effect of Slit2-N. Generally, our findings revealed the inhibitory role of Slit2-N in proliferation of vascular endothelia and some related mechanisms, and presented some potential targets, molecules along Slit2-N-Robo4-AKT axis, to choroidal neovascularization therapy.

A Subtle Network Mediating Axon Guidance: Intrinsic Dynamic Structure of Growth Cone, Attractive and Repulsive Molecular Cues, and the Intermediate Role of Signaling Pathways

A fundamental feature of both early nervous system development and axon regeneration is the guidance of axonal projections to their targets in order to assemble neural circuits that control behavior. In the navigation process where the nerves grow toward their targets, the growth cones, which locate at the tips of axons, sense the environment surrounding them, including varies of attractive or repulsive molecular cues, then make directional decisions to adjust their navigation journey. The turning ability of a growth cone largely depends on its highly dynamic skeleton, where actin filaments and microtubules play a very important role in its motility. In this review, we summarize some possible mechanisms underlying growth cone motility, relevant molecular cues, and signaling pathways in axon guidance of previous studies and discuss some questions regarding directions for further studies.

Filopodia and focal adhesions: An integrated system driving branching morphogenesis in neuronal pathfinding and angiogenesis

Single cell branching during development in vertebrates is typified by neuronal branching to form neurites and vascular branches formed by sprouting angiogenesis. Neurons and endothelial tip cells possess subcellular protrusions that share many common features from the morphological to the molecular level. Both systems utilize filopodia as their cellular protrusion organelles and depend on specific integrin-mediated adhesions to the local extracellular matrix for guidance in their pathfinding. We discuss the similar molecular machineries involved in these two types of cell branch formation and use their analogy to propose a new mechanism for angiogenic filopodia function, namely as adhesion assembly sites. In support of this model we provide primary data of angiogenesis in zebrafish in vivo showing that the actin assembly factor VASP participates in both filopodia formation and adhesion assembly at the base of the filopodia, enabling forward progress of the tip cell. The use of filopodia and their associated adhesions provide a common mechanism for neuronal and endothelial pathfinding during development in response to extracellular matrix cues.

MARCKS regulates neuritogenesis and interacts with a CDC42 signaling network

Through the process of neuronal differentiation, newly born neurons change from simple, spherical cells to complex, sprawling cells with many highly branched processes. One of the first stages in this process is neurite initiation, wherein cytoskeletal modifications facilitate membrane protrusion and extension from the cell body. Hundreds of actin modulators and microtubule-binding proteins are known to be involved in this process, but relatively little is known about how upstream regulators bring these complex networks together at discrete locations to produce neurites. Here, we show that Myristoylated alanine-rich C kinase substrate (MARCKS) participates in this process. Marcks^−/− cortical neurons extend fewer neurites and have less complex neurite arborization patterns. We use an in vitro proteomics screen to identify MARCKS interactors in developing neurites and characterize an interaction between MARCKS and a CDC42-centered network. While the presence of MARCKS does not affect whole brain levels of activated or total CDC42, we propose that MARCKS is uniquely positioned to regulate CDC42 localization and interactions within specialized cellular compartments, such as nascent neurites.

Nano-topography: Quicksand for cell cycle progression?

The 3-D spatial and mechanical features of nano-topography can create alternative environments, which influence cellular response. In this paper, murine fibroblast cells were grown on surfaces characterized by protruding nanotubes. Cells cultured on such nano-structured surface exhibit stronger cellular adhesion compared to control groups, but despite the fact that stronger adhesion is generally believed to promote cell cycle progression, the time cells spend in G1 phase is doubled. This apparent contradiction is solved by confocal microscopy analysis, which shows that the nano-topography inhibits actin stress fiber formation. In turn, this impairs RhoA activation, which is required to suppress the inhibition of cell cycle progression imposed by p21/p27. This finding suggests that the generation of stress fibers, required to impose the homeostatic intracellular tension, rather than cell adhesion/spreading is the limiting factor for cell cycle progression. Indeed, nano-topography could represent a unique tool to inhibit proliferation in adherent well-spread cells.

Inhibition of cell migration by focal adhesion kinase: Time-dependent difference in integrin-induced signaling between endothelial and hepatoblastoma cells

Angiogenesis plays an important role in the development and progression of tumors, and it involves a series of signaling pathways contributing to the migration of endothelial cells for vascularization and to the invasion of cancer cells for secondary tumor formation. Among these pathways, the focal adhesion kinase (FAK) signaling cascade has been implicated in a variety of human cancers in connection with cell adhesion and migration events leading to tumor angiogenesis, metastasis and invasion. Therefore, the inhibition of FAK in endothelial and/or cancer cells is a potential target for anti-angiogenic therapy. In the present study, a small-molecule FAK inhibitor, 1,2,4,5-benzenetetramine tetrahydrochloride (Y15), was used to study the effects of FAK inhibition on the adhesion and migration behaviors of vascular endothelial cells (VECs) and human hepatoblastoma cells. Furthermore, the time-dependent differences in proteins associated with the integrin-mediated FAK/Rho GTPases signaling pathway within 2 h were examined. The results indicated that the inhibition of FAK significantly decreased the migration ability of VECs and human hepatoblastoma cells in a dose-dependent manner. Inhibition of FAK promoted cell detachment by decreasing the expression of focal adhesion components, and blocked cell motility by reducing the level of Rho GTPases. However, the expression of crucial proteins involved in integrin-induced signaling in two cell lines exhibited a time-dependent difference with increased duration of FAK inhibitor treatment, suggesting different mechanisms of FAK-mediated cell migration behavior. These results suggest that the mechanism underlying FAK-mediated adhesion and migration behavior differs among various cells, which is expected to provide evidence for future FAK therapy targeted against tumor angiogenesis.

Protocadherin-αC2 is required for diffuse projections of serotonergic axons

Serotonergic axons extend diffuse projections throughout various brain areas, and serotonergic system disruption causes neuropsychiatric diseases. Loss of the cytoplasmic region of protocadherin-α (Pcdh-α) family proteins, products of the diverse clustered Pcdh genes, causes unbalanced distributions (densification and sparsification) of serotonergic axons in various target regions. However, which Pcdh-α member(s) are responsible for the phenotype is unknown. Here we demonstrated that Pcdh-αC2 (αC2), a Pcdh-α isoform, was highly expressed in serotonergic neurons, and was required for normal diffusion in single-axon-level analyses of serotonergic axons. The loss of αC2 from serotonergic neurons, but not from their target brain regions, led to unbalanced distributions of serotonergic axons. Our results suggest that αC2 expressed in serotonergic neurons is required for serotonergic axon diffusion in various brain areas. The αC2 extracellular domain displays homophilic binding activity, suggesting that its homophilic interaction between serotonergic axons regulates axonal density via αC2′s cytoplasmic domain.

A nanoporous titanium surface promotes the maturation of focal adhesions and formation of filopodia with distinctive nanoscale protrusions by osteogenic cells

While topography is a key determinant of the cellular response to biomaterials, the mechanisms implicated in the cell-surface interactions are complex and still not fully elucidated. In this context, we have examined the effect of nanoscale topography on the formation of filopodia, focal adhesions, and gene expression of proteins associated with cell adhesion and sensing. Commercially pure titanium discs were treated by oxidative nanopatterning with a solution of H₂SO₄/H₂O₂ 50:50 (v/v). Scanning electron microscopy and atomic force microscopy characterizations showed that this facile chemical treatment efficiently creates a unique nanoporous surface with a root-mean-square roughness of 11.5nm and pore diameter of 20±5nm. Osteogenic cells were cultured on polished (control) and nanotextured discs for periods of 6, 24, and 72h. Immunofluorescence analysis revealed increases in the adhesion formation per cell area, focal adhesion length, and maturity on the nanoporous surface. Gene expression for various focal adhesion markers, including paxillin and talin, and different integrins (e.g. α1, β1, and α5) was also significantly increased. Scanning electron microscopy revealed the presence of more filopodia on cells grown on the nanoporous surface. These cell extensions displayed abundant and distinctive nanoscale lateral protrusions of 10-15nm diameter that molded the nanopore walls. Together the increase in the focal adhesions and abundance of filopodia and associated protrusions could contribute to strengthening the adhesive interaction of cells with the surface, and thereby, alter the nanoscale biomechanical relationships that trigger cellular cascades that regulate cell behavior.

STATEMENT OF SIGNIFICANCE

Oxidative patterning was exploited to create a unique three-dimensional network of nanopores on titanium surfaces. Our study illustrates how a facile chemical treatment can be advantageously used to modulate cellular behavior. The nanoscale lateral protrusions on filopodia elicited by this surface are novel adhesive structures. Altogether, the increases in focal adhesion, length, maturity, and filopodia with distinctive lateral protrusions could substantially increase the contact area and adhesion strength of cells, thereby promoting the activation of cellular signaling cascades that may explain the positive osteogenic outcomes previously achieved with this surface. Such physicochemical cueing offers a simple attractive alternative to the use of bioactive agents for guiding tissue repair/regeneration around implantable metals.

TRIM9-dependent ubiquitination of DCC constrains kinase signaling, exocytosis, and axon branching

In the presence of netrin, tripartite motif protein 9 (TRIM9) promotes deleted in colorectal cancer (DCC) clustering, but TRIM9-dependent ubiquitination of DCC is reduced. Loss of ubiquitination promotes an interaction between DCC and FAK and FAK activation. FAK activation is required for the progression from SNARE assembly to exocytic vesicle fusion, which supplies membrane material for axon branching.

Extracellular netrin-1 and its receptor deleted in colorectal cancer (DCC) promote axon branching in developing cortical neurons. Netrin-dependent morphogenesis is preceded by multimerization of DCC, activation of FAK and Src family kinases, and increases in exocytic vesicle fusion, yet how these occurrences are linked is unknown. Here we demonstrate that tripartite motif protein 9 (TRIM9)-dependent ubiquitination of DCC blocks the interaction with and phosphorylation of FAK. Upon netrin-1 stimulation TRIM9 promotes DCC multimerization, but TRIM9-dependent ubiquitination of DCC is reduced, which promotes an interaction with FAK and subsequent FAK activation. We found that inhibition of FAK activity blocks elevated frequencies of exocytosis in vitro and elevated axon branching in vitro and in vivo. Although FAK inhibition decreased soluble N-ethylmaleimide attachment protein receptor (SNARE)-mediated exocytosis, assembled SNARE complexes and vesicles adjacent to the plasma membrane increased, suggesting a novel role for FAK in the progression from assembled SNARE complexes to vesicle fusion in developing murine neurons.

Cyclic Nucleotide Control of Microtubule Dynamics for Axon Guidance

Graded distribution of intracellular second messengers, such as Ca(2+) and cyclic nucleotides, mediates directional cell migration, including axon navigational responses to extracellular guidance cues, in the developing nervous system. Elevated concentrations of cAMP or cGMP on one side of the neuronal growth cone induce its attractive or repulsive turning, respectively. Although effector processes downstream of Ca(2+) have been extensively studied, very little is known about the mechanisms that enable cyclic nucleotides to steer migrating cells. Here, we show that asymmetric cyclic nucleotide signaling across the growth cone mediates axon guidance via modulating microtubule dynamics and membrane organelle transport. In embryonic chick dorsal root ganglion neurons in culture, contact of an extending microtubule with the growth cone leading edge induces localized membrane protrusion at the site of microtubule contact. Such a contact-induced protrusion requires exocytosis of vesicle-associated membrane protein 7 (VAMP7)-positive vesicles that have been transported centrifugally along the microtubule. We found that the two cyclic nucleotides counteractively regulate the frequency of microtubule contacts and targeted delivery of VAMP7 vesicles: cAMP stimulates and cGMP inhibits these events, thereby steering the growth cone in the opposite directions. By contrast, Ca(2+) signals elicit no detectable change in either microtubule contacts or VAMP7 vesicle delivery during Ca(2+)-induced growth cone turning. Our findings clearly demonstrate growth cone steering machinery downstream of cyclic nucleotide signaling and highlight a crucial role of dynamic microtubules in leading-edge protrusion for cell chemotaxis.

SIGNIFICANCE STATEMENT

Developing neurons can extend long axons toward their postsynaptic targets. The tip of each axon, called the growth cone, recognizes extracellular guidance cues and navigates the axon along the correct path. Here we show that asymmetric cyclic nucleotide signaling across the growth cone mediates axon guidance through localized regulation of microtubule dynamics and resulting recruitment of specific populations of membrane vesicles to the growth cone's leading edge. Remarkably, cAMP stimulates microtubule growth and membrane protrusion, whereas cGMP promotes microtubule retraction and membrane senescence, explaining the opposite directional polarities of growth cone turning induced by these cyclic nucleotides. This study reveals a novel microtubule-based mechanism through which cyclic nucleotides polarize the growth cone steering machinery for bidirectional axon guidance.

Xenopus laevis as a model system to study cytoskeletal dynamics during axon pathfinding

The model system, Xenopus laevis, has been used in innumerable research studies and has contributed to the understanding of multiple cytoskeletal components, including actin, microtubules, and neurofilaments, during axon pathfinding. Xenopus developmental stages have been widely characterized, and the Xenopus genome has been sequenced, allowing gene expression modifications through exogenous molecules. Xenopus cell cultures are ideal for long periods of live imaging because they are easily obtained and maintained, and they do not require special culture conditions. In addition, Xenopus have relatively large growth cones, compared to other vertebrates, thus providing a suitable system for imaging cytoskeletal components. Therefore, X. laevis is an ideal model organism in which to study cytoskeletal dynamics during axon pathfinding.

Urinary bladder extracellular matrix hydrogels and matrix-bound vesicles differentially regulate central nervous system neuron viability and axon growth and branching

Central nervous system neurons often degenerate after trauma due to the inflammatory innate immune response to injury, which can lead to neuronal cell death, scarring, and permanently lost neurologic function. Extracellular matrix bioscaffolds, derived by decellularizing healthy tissues, have been widely used in both preclinical and clinical studies to promote positive tissue remodeling, including neurogenesis, in numerous tissues, with extracellular matrix from homologous tissues often inducing more positive responses. Extracellular matrix hydrogels are liquid at room temperature and enable minimally invasive extracellular matrix injections into central nervous system tissues, before gelation at 37℃. However, few studies have analyzed how extracellular matrix hydrogels influence primary central nervous system neuron survival and growth, and whether central nervous system and non-central nervous system extracellular matrix specificity is critical to neuronal responses. Urinary bladder extracellular matrix hydrogels increase both primary hippocampal neuron survival and neurite growth to similar or even greater extents, suggesting extracellular matrix from non-homologous tissue sources, such as urinary bladder matrix-extracellular matrix, may be a more economical and safer alternative to developing central nervous system extracellular matrices for central nervous system applications. Additionally, we show matrix-bound vesicles derived from urinary bladder extracellular matrix are endocytosed by hippocampal neurons and positively regulate primary hippocampal neuron neurite growth. Matrix-bound vesicles carry protein and RNA cargos, including noncoding RNAs and miRNAs that map to the human genome and are known to regulate cellular processes. Thus, urinary bladder matrix-bound vesicles provide natural and transfectable cargoes which offer new experimental tools and therapeutic applications to study and treat central nervous system neuron injury.

The microtubule plus-end-tracking protein TACC3 promotes persistent axon outgrowth and mediates responses to axon guidance signals during development

Background

Formation of precise neuronal connections requires proper axon guidance. Microtubules (MTs) of the growth cone provide a critical driving force during navigation of the growing ends of axons. Pioneer MTs and their plus-end tracking proteins (+TIPs) are thought to play integrative roles during this navigation. TACC3 is a + TIP that we have previously implicated in regulating MT dynamics within axons. However, the role of TACC3 in axon guidance has not been previously explored.

Results

Here, we show that TACC3 is required to promote persistent axon outgrowth and prevent spontaneous axon retractions in embryonic Xenopus laevis neurons. We also show that overexpressing TACC3 can counteract the depolymerizing effect of low doses of nocodazole, and that TACC3 interacts with MT polymerase XMAP215 to promote axon outgrowth. Moreover, we demonstrate that manipulation of TACC3 levels interferes with the growth cone response to the axon guidance cue Slit2 ex vivo, and that ablation of TACC3 causes pathfinding defects in axons of developing spinal neurons in vivo.

Conclusion

Together, our results suggest that by mediating MT dynamics, the + TIP TACC3 is involved in axon outgrowth and pathfinding decisions of neurons during embryonic development.

Calpain-Mediated Proteolysis of Talin and FAK Regulates Adhesion Dynamics Necessary for Axon Guidance

Guidance of axons to their proper synaptic target sites requires spatially and temporally precise modulation of biochemical signals within growth cones. Ionic calcium (Ca²⁺) is an essential signal for axon guidance that mediates opposing effects on growth cone motility. The diverse effects of Ca²⁺ arise from the precise localization of Ca²⁺ signals into microdomains containing specific Ca²⁺ effectors. For example, differences in the mechanical and chemical composition of the underlying substrata elicit local Ca²⁺ signals within growth cone filopodia that regulate axon guidance through activation of the protease calpain. However, how calpain regulates growth cone motility remains unclear. Here, we identify the adhesion proteins talin and focal adhesion kinase (FAK) as proteolytic targets of calpain in Xenopus laevis spinal cord neurons both in vivo and in vitro Inhibition of calpain increases the localization of endogenous adhesion signaling to growth cone filopodia. Using live cell microscopy and specific calpain-resistant point-mutants of talin (L432G) and FAK (V744G), we find that calpain inhibits paxillin-based adhesion assembly through cleavage of talin and FAK, and adhesion disassembly through cleavage of FAK. Blocking calpain cleavage of talin and FAK inhibits repulsive turning from focal uncaging of Ca²⁺ within filopodia. In addition, blocking calpain cleavage of talin and FAK in vivo promotes Rohon-Beard peripheral axon extension into the skin. These data demonstrate that filopodial Ca²⁺ signals regulate axon outgrowth and guidance through calpain regulation of adhesion dynamics through specific cleavage of talin and FAK.SIGNIFICANCE STATEMENT The proper formation of neuronal networks requires accurate guidance of axons and dendrites during development by motile structures known as growth cones. Understanding the intracellular signaling mechanisms that govern growth cone motility will clarify how the nervous system develops and regenerates, and may identify areas of therapeutic intervention in disease or injury. One important signal that controls growth cones is that of local Ca²⁺ transients, which control the rate and direction of axon outgrowth. We demonstrate here that Ca²⁺-dependent inhibition axon outgrowth and guidance is mediated by calpain proteolysis of the adhesion proteins talin and focal adhesion kinase. Our findings provide mechanistic insight into Ca²⁺/calpain regulation of growth cone motility and axon guidance during neuronal development.

Soft matrices downregulate FAK activity to promote growth of tumor-repopulating cells

Tumor-repopulating cells (TRCs) are a tumorigenic sub-population of cancer cells that drives tumorigenesis. We have recently reported that soft fibrin matrices maintain TRC growth by promoting histone 3 lysine 9 (H3K9) demethylation and Sox2 expression and that Cdc42 expression influences H3K9 methylation. However, the underlying mechanisms of how soft matrices induce H3K9 demethylation remain elusive. Here we find that TRCs exhibit lower focal adhesion kinase (FAK) and H3K9 methylation levels in soft fibrin matrices than control melanoma cells on 2D rigid substrates. Silencing FAK in control melanoma cells decreases H3K9 methylation, whereas overexpressing FAK in tumor-repopulating cells enhances H3K9 methylation. Overexpressing Cdc42 or RhoA in the presence of FAK knockdown restores H3K9 methylation levels. Importantly, silencing FAK, Cdc42, or RhoA promotes Sox2 expression and proliferation of control melanoma cells in stiff fibrin matrices, whereas overexpressing each gene suppresses Sox2 expression and reduces growth of TRCs in soft but not in stiff fibrin matrices. Our findings suggest that low FAK mediated by soft fibrin matrices downregulates H3K9 methylation through reduction of Cdc42 and RhoA and promotes TRC growth.

Guidance of Axons by Local Coupling of Retrograde Flow to Point Contact Adhesions

Growth cones interact with the extracellular matrix (ECM) through integrin receptors at adhesion sites termed point contacts. Point contact adhesions link ECM proteins to the actin cytoskeleton through numerous adaptor and signaling proteins. One presumed function of growth cone point contacts is to restrain or "clutch" myosin-II-based filamentous actin (F-actin) retrograde flow (RF) to promote leading edge membrane protrusion. In motile non-neuronal cells, myosin-II binds and exerts force upon actin filaments at the leading edge, where clutching forces occur. However, in growth cones, it is unclear whether similar F-actin-clutching forces affect axon outgrowth and guidance. Here, we show in Xenopus spinal neurons that RF is reduced in rapidly migrating growth cones on laminin (LN) compared with non-integrin-binding poly-d-lysine (PDL). Moreover, acute stimulation with LN accelerates axon outgrowth over a time course that correlates with point contact formation and reduced RF. These results suggest that RF is restricted by the assembly of point contacts, which we show occurs locally by two-channel imaging of RF and paxillin. Further, using micropatterns of PDL and LN, we demonstrate that individual growth cones have differential RF rates while interacting with two distinct substrata. Opposing effects on RF rates were also observed in growth cones treated with chemoattractive and chemorepulsive axon guidance cues that influence point contact adhesions. Finally, we show that RF is significantly attenuated in vivo, suggesting that it is restrained by molecular clutching forces within the spinal cord. Together, our results suggest that local clutching of RF can control axon guidance on ECM proteins downstream of axon guidance cues.

SIGNIFICANCE STATEMENT

Here, we correlate point contact adhesions directly with clutching of filamentous actin retrograde flow (RF), which our findings strongly suggest guides developing axons. Acute assembly of new point contact adhesions is temporally and spatially linked to attenuation of RF at sites of forward membrane protrusion. Importantly, clutching of RF is modulated by extracellular matrix (ECM) proteins and soluble axon guidance cues, suggesting that it may regulate axon guidance in vivo. Consistent with this notion, we found that RF rates of spinal neuron growth cones were slower in vivo than what was observed in vitro. Together, our study provides the best evidence that growth cone-ECM adhesions clutch RF locally to guide axons in vivo.

Cellular and molecular mechanisms regulating neuronal growth by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) and its receptors TrkB and p75 regulate dendritic and axonal growth during development and maintenance of the mature nervous system; however, the cellular and molecular mechanisms underlying this process are not fully understood. In recent years, several advances have shed new light on the processes behind the regulation of BDNF-mediated structural plasticity including control of neuronal transcription, local translation of proteins, and regulation of cytoskeleton and membrane dynamics. In this review, we summarize recent advances in the field of BDNF signaling in neurons to induce neuronal growth. © 2016 Wiley Periodicals, Inc.

Downregulation of YAP-dependent Nupr1 promotes tumor-repopulating cell growth in soft matrices

Despite decades of significant progress in understanding the molecular mechanisms of malignant tumorigenic cells, it remains elusive what these tumorigenic cells are and what controls the growth of these malignant cells. Recently, we have mechanically selected and grown highly malignant and tumorigenic tumor-repopulating cells (TRCs), a small sub-population of cancer cells, by culturing single cancer cells in soft fibrin matrices. However, it is unclear what regulates TRC growth besides Sox2. Here we show that nuclear protein 1 (Nupr1), a protein independent of Sox2, is downregulated in TRCs of melanoma, ovarian cancer and breast cancer cultured in soft fibrin matrices. Nupr1 expression depends on nuclear translocation of YAP that is enriched at the Nupr1 promoter sites; YAP is controlled by Cdc42-mediated F-actin and Lats1 interactions. Nupr1 regulates tumor-suppressor p53 and negatively regulates Nestin and Tert that are independent of Sox2 and promote TRC growth. Silencing Nupr1 increases TRC growth and Nupr1 overexpression inhibits TRC growth in culture and in immune-competent mice. Our results suggest that Nupr1 is a suppressor of growth of highly tumorigenic TRCs and may have a critical role in cancer progression.

Increased expression of Slit2 and its receptors Robo1 and Robo4 in reactive astrocytes of the rat hippocampus after transient forebrain ischemia

Slit2 is a secreted glycoprotein that was originally identified as a chemorepulsive factor in the developing brain; however, it was recently reported that Slit2 is associated with adult neuronal function including a variety of pathophysiological processes. To elucidate whether Slit2 is implicated in the pathophysiology of ischemic injury, we investigated the temporal changes and cellular localization of Slit2 and its predominant receptors, Robo1 and Robo4, for 28 days after transient forebrain ischemia. Slit2 and its receptors had similar overall expression patterns in the control and ischemic hippocampi. The ligand and receptors were constitutively expressed in hippocampal neurons in control animals; however, in animals with ischemic injury, their upregulation was detected in reactive astrocytes, but not in neurons or activated microglia, in the CA1 region. Astroglial induction of Slit2 and its receptors occurred by day 3 after reperfusion, and appeared to increase progressively until the final time point on day 28. Their temporal expression patterns overlapped with the time period in which reactive astrocytes undergo dynamic structural changes and appear hypertrophic in the ischemic hippocampus. The immunohistochemical data were consistent with the results of the immunoblot analyses, indicating that the expression of Slit2 and Robo increased progressively over the relatively long period of 28 days examined here. Collectively, these results suggest that Slit2/Robo signaling may be involved in regulating the astroglial reaction via autocrine or paracrine mechanisms in post-ischemic processes. Moreover, this may contribute to the dynamic morphological changes that occur in astrocytes in response to ischemic injury.

Targeting the Metastasis Suppressor, N-Myc Downstream Regulated Gene-1, with Novel Di-2-Pyridylketone Thiosemicarbazones: Suppression of Tumor Cell Migration and Cell-Collagen Adhesion by Inhibiting Focal Adhesion Kinase/Paxillin Signaling

Metastasis is a complex process that is regulated by multiple signaling pathways, with the focal adhesion kinase (FAK)/paxillin pathway playing a major role in the formation of focal adhesions and cell motility. N-myc downstream regulated gene-1 (NDRG1) is a potent metastasis suppressor in many solid tumor types, including prostate and colon cancer. Considering the antimetastatic effect of NDRG1 and the crucial involvement of the FAK/paxillin pathway in cellular migration and cell-matrix adhesion, we assessed the effects of NDRG1 on this important oncogenic pathway. In the present study, NDRG1 overexpression and silencing models of HT29 colon cancer and DU145 prostate cancer cells were used to examine the activation of FAK/paxillin signaling and the formation of focal adhesions. The expression of NDRG1 resulted in a marked and significant decrease in the activating phosphorylation of FAK and paxillin, whereas silencing of NDRG1 resulted in an opposite effect. The expression of NDRG1 also inhibited the formation of focal adhesions as well as cell migration and cell-collagen adhesion. Incubation of cells with novel thiosemicarbazones, namely di-2-pyridylketone 4,4-dimethyl-3-thiosemicarbazone and di-2-pyridylketone 4-cyclohexyl-4-methyl-3-thiosemicarbazone, that upregulate NDRG1 also resulted in decreased phosphorylation of FAK and paxillin. The ability of these thiosemicarbazones to inhibit cell migration and metastasis could be mediated, at least in part, through the FAK/paxillin pathway.

Deregulation of SLIT2-mediated Cdc42 activity is associated with esophageal cancer metastasis and poor prognosis

INTRODUCTION

SLIT2, a secreted protein, has been found to inactivate Cdc42 GTPase to modulate neural cell migration. However, alteration of SLIT2-mediated Cdc42 in terms of migration regulation remains undefined in esophageal squamous cell carcinoma (ESCC).

METHODS

We report here in ESCC cell, animal, and clinical models that SLIT2 acts as a migration suppressor and serves as a prognostic biomarker.

RESULTS

The immunohistochemistry data indicated that 31.8% (49 of 154) of tumors from ESCC patients showed low expression of SLIT2 protein which correlated with poor overall survival and disease-free survival. DNA methylation analysis suggested that promoter hypermethylation is responsible for low expression of SLIT2 in ESCC. Knockdown of SLIT2 increased ESCC cell migration, while SLIT2 stable overexpression reduced cell migration. ESCC cells treated with conditioned media from cells overexpressing SLIT2 also suppressed cell migration. Importantly, silencing of SLIT2 decreased the complex formation, and thus induced Cdc42 activity and promoted membrane localization of focal adhesion kinase and Paxillin. Anti-metastatic effect of SLIT2 was confirmed in an experimental metastasis model of SLIT2 knockdown ESCC cells.

CONCLUSION

Our results provide novel evidence that low expression of SLIT2 correlates with poor prognosis and promotes metastasis in ESCC, which may be regulated by the Cdc42-mediated pathways.

Cdc42 and RhoA reveal different spatio-temporal dynamics upon local stimulation with Semaphorin-3A

Small RhoGTPases, such as Cdc42 and RhoA, are key players in integrating external cues and intracellular signaling pathways that regulate growth cone (GC) motility. Indeed, Cdc42 is involved in actin polymerization and filopodia formation, whereas RhoA induces GC collapse and neurite retraction through actomyosin contraction. In this study we employed Förster Resonance Energy Transfer (FRET) microscopy to study the spatio-temporal dynamics of Cdc42 and RhoA in GCs in response to local Semaphorin-3A (Sema3A) stimulation obtained with lipid vesicles filled with Sema3A and positioned near the selected GC using optical tweezers. We found that Cdc42 and RhoA were activated at the leading edge of NG108-15 neuroblastoma cells during spontaneous cycles of protrusion and retraction, respectively. The release of Sema3A brought to a progressive activation of RhoA within 30 s from the stimulus in the central region of the GC that collapsed and retracted. In contrast, the same stimulation evoked waves of Cdc42 activation propagating away from the stimulated region. A more localized stimulation obtained with Sema3A coated beads placed on the GC, led to Cdc42 active waves that propagated in a retrograde manner with a mean period of 70 s, and followed by GC retraction. Therefore, Sema3A activates both Cdc42 and RhoA with a complex and different spatial-temporal dynamics.

Spatiotemporal dynamics of traction forces show three contraction centers in migratory neurons

Traction force microscopy provides a comprehensive description of the spatiotemporal dynamics of contractile activities and their regulation by guidance molecules in migrating neurons, as well as the underlying molecular mechanisms.

Traction force against the substrate is required for neuronal migration, but how it is generated and regulated remains controversial. Using traction force microscopy, we showed in cultured granule cells the coexistence of three distinct contraction centers (CCs) that are located at the distal and proximal regions of the leading process as well as at the trailing process, regions exhibiting high-level myosin-II activities. The CC activities depended on myosin-II, actin filaments, and microtubules, as well as substrate adhesion, and exhibited apparently independent fluctuation. The difference of strain energies associated with CC activities between leading versus trailing processes tightly correlated with the displacement of the soma at any given time. Application of brain-derived neurotrophic factor (BDNF) and Slit2, factors known to guide neuronal migration, at the leading process altered CC activities by regulating the small GTPases Cdc42 and RhoA, respectively, leading to forward and rearward soma translocation. These results delineate the multiple origins and spatiotemporal dynamics of the traction force underlying neuronal migration.

Mechanochemical regulation of growth cone motility

Neuronal growth cones are exquisite sensory-motor machines capable of transducing features contacted in their local extracellular environment into guided process extension during development. Extensive research has shown that chemical ligands activate cell surface receptors on growth cones leading to intracellular signals that direct cytoskeletal changes. However, the environment also provides mechanical support for growth cone adhesion and traction forces that stabilize leading edge protrusions. Interestingly, recent work suggests that both the mechanical properties of the environment and mechanical forces generated within growth cones influence axon guidance. In this review we discuss novel molecular mechanisms involved in growth cone force production and detection, and speculate how these processes may be necessary for the development of proper neuronal morphogenesis.

A bibenzyl from Dendrobium ellipsophyllum inhibits migration in lung cancer cells

Metastatic cancer cells have been shown to have aggressive behaviors accounting for the high incidence of chemotherapeutic failure and mortality. Because migration and invasion are crucial behaviors for cancer cell dissemination, promising compounds exhibiting potential antimigration effects are of interest for metastasis-based therapeutic approaches. This study aimed to evaluate the activity of a bibenzyl, 4,5,4'-trihydroxy-3,3'-dimethoxybibenzyl (TDB), isolated from Dendrobium ellipsophyllum Tang and Wang, in the suppression of migration in human lung cancer cells. TDB at nontoxic concentrations (1 and 5 µM) significantly inhibited the motility of lung cancer cells in scratch-wound assay. Chemotaxis-induced migration and invasion assays also revealed that the cell motility dramatically diminished in the cells treated with 1-5 µM TDB. Western blot analysis provided the underlying molecular mechanism, showing that TDB reduced such cell migration and invasion by decreasing migration-regulating proteins, including integrins αv, α4, β1, β3 and β5, as well as downstream signaling proteins, such as activated focal adhesion kinase (pFAK), activated Ras-related C3 botulinum toxin substrate 1 (Rac1-GTP) and cell division control protein 42 (Cdc42). As the presence of cellular protrusion, called filopodia, has been indicated as a hallmark of migrating cells, we showed that the reduction of the mentioned proteins correlated well with the disappearance of filopodia. In summary, this study demonstrates the promising activity of TDB and its mechanism in the inhibition of lung cancer cell migration, which might be useful for encouraging the development of this compound for antimetastatic approaches.

Transcriptome sequencing and genome-wide association analyses reveal lysosomal function and actin cytoskeleton remodeling in schizophrenia and bipolar disorder

Schizophrenia (SCZ) and bipolar disorder (BPD) are severe mental disorders with high heritability. Clinicians have long noticed the similarities of clinic symptoms between these disorders. In recent years, accumulating evidence indicates some shared genetic liabilities. However, what is shared remains elusive. In this study, we conducted whole transcriptome analysis of post-mortem brain tissues (cingulate cortex) from SCZ, BPD and control subjects, and identified differentially expressed genes in these disorders. We found 105 and 153 genes differentially expressed in SCZ and BPD, respectively. By comparing the t-test scores, we found that many of the genes differentially expressed in SCZ and BPD are concordant in their expression level (q⩽0.01, 53 genes; q⩽0.05, 213 genes; q⩽0.1, 885 genes). Using genome-wide association data from the Psychiatric Genomics Consortium, we found that these differentially and concordantly expressed genes were enriched in association signals for both SCZ (P<10(-7)) and BPD (P=0.029). To our knowledge, this is the first time that a substantially large number of genes show concordant expression and association for both SCZ and BPD. Pathway analyses of these genes indicated that they are involved in the lysosome, Fc gamma receptor-mediated phagocytosis, regulation of actin cytoskeleton pathways, along with several cancer pathways. Functional analyses of these genes revealed an interconnected pathway network centered on lysosomal function and the regulation of actin cytoskeleton. These pathways and their interacting network were principally confirmed by an independent transcriptome sequencing data set of the hippocampus. Dysregulation of lysosomal function and cytoskeleton remodeling has direct impacts on endocytosis, phagocytosis, exocytosis, vesicle trafficking, neuronal maturation and migration, neurite outgrowth and synaptic density and plasticity, and different aspects of these processes have been implicated in SCZ and BPD.

Globularity and language-readiness: generating new predictions by expanding the set of genes of interest

This study builds on the hypothesis put forth in Boeckx and Benítez-Burraco (2014), according to which the developmental changes expressed at the levels of brain morphology and neural connectivity that resulted in a more globular braincase in our species were crucial to understand the origins of our language-ready brain. Specifically, this paper explores the links between two well-known 'language-related' genes like FOXP2 and ROBO1 implicated in vocal learning and the initial set of genes of interest put forth in Boeckx and Benítez-Burraco (2014), with RUNX2 as focal point. Relying on the existing literature, we uncover potential molecular links that could be of interest to future experimental inquiries into the biological foundations of language and the testing of our initial hypothesis. Our discussion could also be relevant for clinical linguistics and for the interpretation of results from paleogenomics.

FAK is required for tension-dependent organization of collective cell movements in Xenopus mesendoderm

Collective cell movements are integral to biological processes such as embryonic development and wound healing and also have a prominent role in some metastatic cancers. In migrating Xenopus mesendoderm, traction forces are generated by cells through integrin-based adhesions and tension transmitted across cadherin adhesions. This is accompanied by assembly of a mechanoresponsive cadherin adhesion complex containing keratin intermediate filaments and the catenin-family member plakoglobin. We demonstrate that focal adhesion kinase (FAK), a major component of integrin adhesion complexes, is required for normal morphogenesis at gastrulation, closure of the anterior neural tube, axial elongation and somitogenesis. Depletion of zygotically expressed FAK results in disruption of mesendoderm tissue polarity similar to that observed when expression of keratin or plakoglobin is inhibited. Both individual and collective migrations of mesendoderm cells from FAK depleted embryos are slowed, cell protrusions are disordered, and cell spreading and traction forces are decreased. Additionally, keratin filaments fail to organize at the rear of cells in the tissue and association of plakoglobin with cadherin is diminished. These findings suggest that FAK is required for the tension-dependent assembly of the cadherin adhesion complex that guides collective mesendoderm migration, perhaps by modulating the dynamic balance of substrate traction forces and cell cohesion needed to establish cell polarity.

Integrin signalling and traffic during axon growth and regeneration

Adult corticospinal tract axons do not regenerate because they have low intrinsic growth ability, and are exposed to inhibitory molecules after injury. PNS axons have a better regenerative capacity, mediated in part by integrins (extracellular matrix receptors). These are subject to complex regulation by signalling and trafficking. Recent studies have found that integrin mediated axon growth relies on signalling via focal adhesion molecules, and that integrins are inactivated by inhibitory molecules in the CNS. Forced activation of integrins can overcome inhibition and increase axon regeneration, however integrins are not transported into some CNS axons. Studies of PNS integrin traffic have identified molecules that can be manipulated to increase axonal integrin expression, suggesting strategies for repairing the injured spinal cord.

Involvement of Rho-family GTPases in axon branching

Development of the nervous system requires efficient extension and guidance of axons and dendrites culminating in synapse formation. Axonal growth and navigation during embryogenesis are controlled by extracellular cues. Many of the same extracellular signals also regulate axonal branching. The emergence of collateral branches from the axon augments the complexity of nervous system innervation and provides an additional mechanism for target selection. Rho-family GTPases play an important role in regulating intracellular cytoskeletal and signaling pathways that facilitate axonal morphological changes. RhoA/G and Rac1 GTPase functions are complex and they can induce or inhibit branch formation, depending on neuronal type, cell context or signaling mechanisms. Evidence of a role of Cdc42 in axon branching is mostly lacking. In contrast, Rac3 has thus far been implicated in the regulation of axon branching. Future analysis of the upstream regulators and downstream effectors mediating the effects of Rho-family GTPase will provide insights into the cellular processes effected, and shed light on the sometimes opposing roles of these GTPases in the regulation of axon branching.

Teneurin-4 promotes cellular protrusion formation and neurite outgrowth through focal adhesion kinase signaling

Teneurin-4 (Ten-4), a transmembrane protein, is highly expressed in the central nervous system; however, its cellular and molecular function in neuronal differentiation remains unknown. In this study, we aimed to elucidate the function of Ten-4 in neurite outgrowth. Ten-4 expression was induced during neurite outgrowth of the neuroblastoma cell line Neuro-2a. Ten-4 protein was localized at the neurite growth cones. Knockdown of Ten-4 expression in Neuro-2a cells decreased the formation of the filopodia-like protrusions and the length of individual neurites. Conversely, overexpression of Ten-4 promoted filopodia-like protrusion formation. In addition, knockdown and overexpression of Ten-4 reduced and elevated the activation of focal adhesion kinase (FAK) and Rho-family small GTPases, Cdc42 and Rac1, key molecules for the membranous protrusion formation downstream of FAK, respectively. Inhibition of the activation of FAK and neural Wiskott-Aldrich syndrome protein (N-WASP), which is a downstream regulator of FAK and Cdc42, blocked protrusion formation by Ten-4 overexpression. Further, Ten-4 colocalized with phosphorylated FAK in the filopodia-like protrusion regions. Together, our findings show that Ten-4 is a novel positive regulator of cellular protrusion formation and neurite outgrowth through the FAK signaling pathway.