Gradient steepness influences the pathfinding decisions of neuronal growth cones in vivo

MACC1-induced migration in tumors: Current state and perspective

Malignant tumors are still a global, heavy health burden. Many tumor types cannot be treated curatively, underlining the need for new treatment targets. In recent years, metastasis associated in colon cancer 1 (MACC1) was identified as a promising biomarker and drug target, as it is promoting tumor migration, initiation, proliferation, and others in a multitude of solid cancers. Here, we will summarize the current knowledge about MACC1-induced tumor cell migration with a special focus on the cytoskeletal and adhesive systems. In addition, a brief overview of several in vitro models used for the analysis of cell migration is given. In this context, we will point to issues with the currently most prevalent models used to study MACC1-dependent migration. Lastly, open questions about MACC1-dependent effects on tumor cell migration will be addressed.

Neuro-taxis: Neuronal movement in gradients of chemical and physical environments

Environmental chemical and physical cues dynamically interact with migrating neurons and sprouting axons, and in particular, the gradients of environmental cues are regarded as one of the factors intimately involved in the neuronal movement. Since a growth cone was first described by Cajal, more than one century ago, chemical gradients have been suggested as one of the mechanisms by which the neurons determine proper paths and destinations. However, the gradients of physical cues, such as stiffness and topography, which also interact constantly with the neurons and their axons as a component of the extracellular environments, have rarely been noted regarding the guidance of neurons, despite their gradually increasingly reported influences in the case of nonneuronal-cell migration. In this review, we discuss chemical (i.e., chemo- and hapto-) and physical (i.e., duro-) taxis phenomena on the movement of neurons including axonal elongation. In addition, we suggest topotaxis, the most recently proposed physical-taxis phenomenon, as another potential mechanism in the neuronal movement, based on the reports of neuronal recognition of and responses to nanotopography.

Determining Neurotrophin Gradients in Vitro To Direct Axonal Outgrowth Following Spinal Cord Injury

A spinal cord injury can damage neuronal connections required for both motor and sensory function. Barriers to regeneration within the central nervous system, including an absence of neurotrophic stimulation, impair the ability of injured neurons to reestablish their original circuitry. Exogenous neurotrophin administration has been shown to promote axonal regeneration and outgrowth following injury. The neurotrophins possess chemotrophic properties that guide axons toward the region of highest concentration. These growth factors have demonstrated potential to be used as a therapeutic intervention for orienting axonal growth beyond the injury lesion, toward denervated targets. However, the success of this approach is dependent on the appropriate spatiotemporal distribution of these molecules to ensure detection and navigation by the axonal growth cone. A number of in vitro gradient-based assays have been employed to investigate axonal response to neurotrophic gradients. Such platforms have helped elucidate the potential of applying a concentration gradient of neurotrophins to promote directed axonal regeneration toward a functionally significant target. Here, we review these techniques and the principles of gradient detection in axonal guidance, with particular focus on the use of neurotrophins to orient the trajectory of regenerating axons.

Neuronal Innervation of the Subgenual Organ Complex and the Tibial Campaniform Sensilla in the Stick Insect Midleg

Mechanosensory organs in legs play are crucial receptors in the feedback control of walking and in the detection of substrate-borne vibrations. Stick insects serve as a model for the physiological role of chordotonal organs and campaniform sensilla. This study documents, by axonal tracing, the neural innervation of the complex chordotonal organs and groups of campaniform sensilla in the proximal tibia of the midleg in Sipyloidea sipylus. In total, 6 nerve branches innervate the different sensory structures, and the innervation pattern associates different sensilla types by their position. Sensilla on the anterior and posterior tibia are innervated from distinct nerve branches. In addition, the variation in innervation is studied for five anatomical branching points. The most common variation is the innervation of the subgenual organ sensilla by two nerve branches rather than a single one. The fusion of commonly separated nerve branches also occurred. However, a common innervation pattern can be demonstrated, which is found in >75% of preparations. The variation did not include crossings of nerves between the anterior and posterior side of the leg. The study corrects the innervation of the posterior subgenual organ reported previously. The sensory neuroanatomy and innervation pattern can guide further physiological studies of mechanoreceptor organs and allow evolutionary comparisons to related insect groups.

Characterization of plexinA and two distinct semaphorin1a transcripts in the developing and adult cricket Gryllus bimaculatus

Guidance cues act during development to guide growth cones to their proper targets in both the central and peripheral nervous systems. Experiments in many species indicate that guidance molecules also play important roles after development, though less is understood about their functions in the adult. The Semaphorin family of guidance cues, signaling through Plexin receptors, influences the development of both axons and dendrites in invertebrates. Semaphorin functions have been extensively explored in Drosophila melanogaster and some other Dipteran species, but little is known about their function in hemimetabolous insects. Here, we characterize sema1a and plexA in the cricket Gryllus bimaculatus. In fact, we found two distinct predicted Sema1a proteins in this species, Sema1a.1 and Sema1a.2, which shared only 48% identity at the amino acid level. We include a phylogenetic analysis that predicted that many other insect species, both holometabolous and hemimetabolous, express two Sema1a proteins as well. Finally, we used in situ hybridization to show that sema1a.1 and sema1a.2 expression patterns were spatially distinct in the embryo, and both roughly overlap with plexA. All three transcripts were also expressed in the adult brain, mainly in the mushroom bodies, though sema1a.2 was expressed most robustly. sema1a.2 was also expressed strongly in the adult thoracic ganglia while sema1a.1 was only weakly expressed and plexA was undetectable.

High-throughput three-dimensional chemotactic assays reveal steepness-dependent complexity in neuronal sensation to molecular gradients

Many cellular programs of neural development are under combinatorial regulation by different chemoattractive or chemorepulsive factors. Here, we describe a microfluidic platform that utilizes well-controlled three-dimensional (3D) diffusion to generate molecular gradients of varied steepness in a large array of hydrogel cylinders, allowing high-throughput 3D chemotactic assays for mechanistic dissection of steepness-dependent neuronal chemotaxis. Using this platform, we examine neuronal sensitivity to the steepness of gradient composed of netrin-1, nerve growth factor, or semaphorin3A (Sema3A) proteins, and reveal dramatic diversity and complexity in the associated chemotactic regulation of neuronal development. Particularly for Sema3A, we find that serine/threonine kinase-11 and glycogen synthase kinase-3 signaling pathways are differentially involved in steepness-dependent chemotactic regulation of coordinated neurite repellence and neuronal migration. These results provide insights to the critical role of gradient steepness in neuronal chemotaxis, and also prove the technique as an expandable platform for studying other chemoresponsive cellular systems.

Merging orthogonal microfluidic flows to generate multi-profile concentration gradients

This work describes a novel microfluidic device capable of generating multi-profile gradients that include sigmoidal, parabolic, and exponential concentration variations across its main channel.

A simple generative model of the mouse mesoscale connectome

Recent technological advances now allow for the collection of vast data sets detailing the intricate neural connectivity patterns of various organisms. Oh et al. (2014) recently published the most complete description of the mouse mesoscale connectome acquired to date. Here we give an in-depth characterization of this connectome and propose a generative network model which utilizes two elemental organizational principles: proximal attachment ‒ outgoing connections are more likely to attach to nearby nodes than to distant ones, and source growth ‒ nodes with many outgoing connections are likely to form new outgoing connections. We show that this model captures essential principles governing network organization at the mesoscale level in the mouse brain and is consistent with biologically plausible developmental processes.

Co-immobilization of semaphorin3A and nerve growth factor to guide and pattern axons

Immobilization of axon guidance cues offers a powerful tissue regenerative strategy to control the presentation and spatial location of these biomolecules. We use our previously developed immobilization strategy to specifically tether recombinant biotinylated nerve growth factor (bNGF) and biotinylated semaphorin3A (bSema3A) to chitosan films as an outgrowth and guidance platform. DRG neurite length and number for a range of single cues of immobilized bNGF or bSema3A were examined to determine a concentration response. Next single and dual cues of bNGF and bSema3A were immobilized and DRG guidance was assessed in response to a step concentration change from zero. Overall, immobilized groups caused axon extension, retraction and turning depending on the ratio of bNGF and bSema3A immobilized in the encountered region. This response indicated the exquisite sensitivity of DRG axons to both attractive and repulsive tethered cues. bSema3A concentrations of 0.10 and 0.49 ng/mm(2), when co-immobilized with bNGF (at 0.86 and 0.43 ng/mm(2) respectively), caused axons to turn away from the co-immobilized region. Immunocytochemical analysis showed that at these bSema3A concentrations, axons inside the co-immobilized region display microtubule degradation and breakdown of actin filaments. At the lowest bSema3A concentration (0.01 ng/mm(2)) co-immobilized with a higher bNGF concentration (2.16 ng/mm(2)), neurite lengths are shorter in the immobilized area, but bNGF dominates the guidance mechanism as neurites are directed toward the immobilized region. Future applications can pattern these cues in various geometries and gradients in order to better modulate axon guidance in terms of polarity, extension and branching.

STATEMENT OF SIGNIFICANCE

Nervous system formation and regeneration requires key molecules for guiding the growth cone and nervous system patterning. In vivo these molecules work in conjunction with one another to modulate axon guidance, and often they are tethered to limit spatial distribution. The novelty of this research is that we provide a specific attachment method to immobilize an attractive signal, nerve growth factor, along with an inhibitory cue, semaphorin3A, to a substrate in order to analyze the interplay of these proteins on axon guidance responses. The scientific impact of this manuscript is that we show that dual-cued platforms are necessary in order to finetune and tailor specific axon responses for varying neuronal regenerative purposes.

Microfluidic analysis of extracellular matrix-bFGF crosstalk on primary human myoblast chemoproliferation, chemokinesis, and chemotaxis

Exposing myoblasts to basic fibroblast growth factor (bFGF), which is released after muscle injury, results in receptor phosphorylation, faster migration, and increased proliferation. These effects occur on time scales that extend across three orders of magnitude (10(0)-10(3) minutes). Finite element modeling of Transwell assays, which are traditionally used to assess chemotaxis, revealed that the bFGF gradient formed across the membrane pore is short-lived and diminishes 45% within the first minute. Thus, to evaluate bFGF-induced migration over 10(2) minutes, we employed a microfluidic assay capable of producing a stable, linear concentration gradient to perform single-cell analyses of chemokinesis and chemotaxis. We hypothesized that the composition of the underlying extracellular matrix (ECM) may affect the behavioral response of myoblasts to soluble bFGF, as previous work with other cell types has suggested crosstalk between integrin and fibroblast growth factor (FGF) receptors. Consistent with this notion, we found that bFGF significantly reduced the doubling time of myoblasts cultured on laminin but not fibronectin or collagen. Laminin also promoted significantly faster migration speeds (13.4 μm h(-1)) than either fibronectin (10.6 μm h(-1)) or collagen (7.6 μm h(-1)) without bFGF stimulation. Chemokinesis driven by bFGF further increased migration speed in a strictly additive manner, resulting in an average increase of 2.3 μm h(-1) across all ECMs tested. We observed relatively mild chemoattraction (∼67% of myoblast population) in response to bFGF gradients of 3.2 ng mL(-1) mm(-1) regardless of ECM identity. Thus, while ECM-bFGF crosstalk did impact chemoproliferation, it did not have a significant effect on chemokinesis or chemotaxis. These data suggest that the main physiological effect of bFGF on myoblast migration is chemokinesis and that changes in the surrounding ECM, resulting from aging and/or disease may impact muscle regeneration by altering myoblast migration and proliferation.

Quantifying biased response of axon to chemical gradient steepness in a microfluidic device

Axons are very sensitive to molecular gradients and can discriminate extremely small differences in gradient steepness. Microfluidic devices capable of generating chemical gradients and adjusting their steepness could be used to quantify the sensitivity of axonal response. Here, we present a versatile and robust microfluidic device that can generate substrate-bound molecular gradients with evenly varying steepness on a single chip to precisely quantify axonal response. In this device, two solutions are perfused into a central channel via two inlets while partially flowing into two peripheral channels through interconnecting grooves, which gradually decrease the fluid velocity along the central channel. Molecular gradients with evenly and gradually decreased steepness can therefore be generated with a high resolution that is less than 0.05%/mm. In addition, the overall distribution range and resolution of the gradient steepness can be highly and flexibly controlled by adjusting various parameters of the device. Using this device, we quantified the hippocampal axonal response to substrate-bound laminin and ephrin-A5 gradients with varying steepnesses. Our results provided more detailed information on how and to what extent different steepnesses guide hippocampal neuron development during the initial outgrowth. Furthermore, our results show that axons can sensitively respond to very shallow laminin and ephrin-A5 gradients, which could effectively initiate biased differentiation of hippocampal neurons in the steepness range investigated in this study.

Theoretical and experimental quantification of the role of diffusive chemogradients on neuritogenesis within three-dimensional collagen scaffolds

A critical challenge to regenerating close mimics of native axonal pathways under chronic neurodegenerative disease or injury conditions is the inability to stimulate, sustain and steer neurite outgrowth over a long distance, until they reach their intended targets. Understanding neurite outgrowth necessitates quantitative determination of the role of molecular gradients on growth cone navigation under dynamic physiological conditions. High-fidelity biomimetic platforms are needed to computationally and experimentally acquire and analyze spatiotemporal molecular gradient evolution and the growth cone response across multiple conditions along this gradient pathway. In this study, we utilized a simple microfluidic platform in which diffusive gradients were generated within a 3-D porous scaffold in a defined and reproducible manner. The platform's characteristics (spatiotemporal gradient, steepness, diffusion time, etc.) were precisely quantified at every specified location within the scaffold. Using this platform, we show that the cortical neurite response within 3-D collagen scaffolds, at both the cellular and molecular level, is extremely sensitive to subtle changes in localized concentration and gradient steepness of IGF-1 within that region. This platform could also be used to study other biological processes such as morphogenesis and cancer metastasis, where chemogradients are expected to significantly regulate the outcomes. Results from this study might be of tremendous use in designing biomaterial scaffolds for neural tissue engineering, axonal pathway regeneration under injury or disease, and in formulating targeted drug-delivery strategies.

Synergistic effects of 3D ECM and chemogradients on neurite outgrowth and guidance: a simple modeling and microfluidic framework

During nervous system development, numerous cues within the extracellular matrix microenvironment (ECM) guide the growing neurites along specific pathways to reach their intended targets. Neurite motility is controlled by extracellular signal sensing through the growth cone at the neurite tip, including chemoattractive and repulsive cues. However, it is difficult to regenerate and restore neurite tracts, lost or degraded due to an injury or disease, in the adult central nervous system. Thus, it is important to evaluate the dynamic interplay between ECM and the concentration gradients of these cues, which would elicit robust neuritogenesis. Such information is critical in understanding the processes involved in developmental biology, and in developing high-fidelity neurite regenerative strategies post-injury, and in drug discovery and targeted therapeutics for neurodegenerative conditions. Here, we quantitatively investigated this relationship using a combination of mathematical modeling and in vitro experiments, and determined the synergistic role of guidance cues and ECM on neurite outgrowth and turning. Using a biomimetic microfluidic system, we have shown that cortical neurite outgrowth and turning under chemogradients (IGF-1 or BDNF) within 3D scaffolds is highly regulated by the source concentration of the guidance cue and the physical characteristics of the scaffold. A mechanistic-driven partial differential equation model of neurite outgrowth has been proposed, which could also be used prospectively as a predictive tool. The parameters for the chemotaxis term in the model are determined from the experimental data using our microfluidic assay. Resulting model simulations demonstrate how neurite outgrowth was critically influenced by the experimental variables, which was further supported by experimental data on cell-surface-receptor expressions. The model results are in excellent agreement with the experimental findings. This integrated approach represents a framework for further elucidation of biological mechanisms underlying neuronal responses of specialized cell types, during various stages of development, and under healthy or diseased conditions.

Simultaneous generation of gradients with gradually changed slope in a microfluidic device for quantifying axon response

Over the past decades, various microfluidic devices have been developed to investigate the role of the molecular gradient in axonal development; however, there are very few devices providing quantitative information about the response of axons to molecular gradients with different slopes. Here, we propose a novel laminar-based microfluidic device enabling simultaneous generation of multiple gradients with gradually changed slope on a single chip. This device, with two asymmetrically designed peripheral channels and opposite flow direction, could generate gradients with gradually changed slope in the center channel, enabling us to investigate simultaneously the response of axons to multiple slope gradients with the same batch of neurons. We quantitatively investigated the response of axon growth rate and growth direction to substrate-bound laminin gradients with different slopes using this single-layer chip. Furthermore, we compartmented this gradient generation chip and a cell culture chip by a porous membrane to investigate quantitatively the response of axon growth rate to the gradient of soluble factor netrin-1. The results suggested that contacting with a molecular gradient would effectively accelerate neurites growth and enhance axonal formation, and the axon guidance ratio obviously increased with the increase of gradient slope in a proper range. The capability of generating a molecular gradient with continuously variable slopes on a single chip would open up opportunities for obtaining quantitative information about the sensitivity of axons and other types of cells in response to gradients of various proteins.

Optimality and saturation in axonal chemotaxis

Chemotaxis (detecting and following chemical gradients) plays a crucial role in the function of many biological systems. In particular, gradient following by neuronal growth cones is important for the correct wiring of the nervous system. There is increasing interest in the constraints that determine how small chemotacting devices respond to gradients, but little quantitative information is available in this regard for neuronal growth cones. Mortimer et al. ( 2009 ) and Mortimer, Dayan, Burrage, and Goodhill ( 2011 ) proposed a Bayesian ideal observer model that predicts chemotactic performance for shallow gradients. Here we investigated two important aspects of this model. First, we found by numerical simulation that although the analytical predictions of the model assume shallow gradients, these predictions remain remarkably robust to large deviations in gradient steepness. Second, we found experimentally that the chemotactic response increased linearly with gradient steepness for very shallow gradients as predicted by the model; however, the response saturated for steeper gradients. This saturation could be reproduced in simulations of a growth rate modulation response mechanism. Together these results illuminate the domain of validity of the Bayesian model and give further insight into the biological mechanisms of axonal chemotaxis.

Chemistry with spatial control using particles and streams()

Spatial control of chemical reactions, with micro- and nanometer scale resolution, has important consequences for one pot synthesis, engineering complex reactions, developmental biology, cellular biochemistry and emergent behavior. We review synthetic methods to engineer this spatial control using chemical diffusion from spherical particles, shells and polyhedra. We discuss systems that enable both isotropic and anisotropic chemical release from isolated and arrayed particles to create inhomogeneous and spatially patterned chemical fields. In addition to such finite chemical sources, we also discuss spatial control enabled with laminar flow in 2D and 3D microfluidic networks. Throughout the paper, we highlight applications of spatially controlled chemistry in chemical kinetics, reaction-diffusion systems, chemotaxis and morphogenesis.

Calcium and cAMP levels interact to determine attraction versus repulsion in axon guidance

Correct guidance of axons to their targets depends on an intricate network of signaling molecules in the growth cone. Calcium and cAMP are two key regulators of whether axons are attracted or repelled by molecular gradients, but how these molecules interact to determine guidance responses remains unclear. Here, we constructed a mathematical model for the relevant signaling network, which explained a large range of previous biological data and made predictions for when axons will be attracted or repelled. We then confirmed these predictions experimentally, in particular showing that while small increases in cAMP levels promote attraction large increases do not, and that under some circumstances reducing cAMP levels promotes attraction. Together, these results show that a relatively simple mathematical model can quantitatively predict guidance decisions across a wide range of conditions, and that calcium and cAMP levels play a more complex role in these decisions than previously determined.

Gradient biomaterials and their influences on cell migration

Cell migration participates in a variety of physiological and pathological processes such as embryonic development, cancer metastasis, blood vessel formation and remoulding, tissue regeneration, immune surveillance and inflammation. The cells specifically migrate to destiny sites induced by the gradually varying concentration (gradient) of soluble signal factors and the ligands bound with the extracellular matrix in the body during a wound healing process. Therefore, regulation of the cell migration behaviours is of paramount importance in regenerative medicine. One important way is to create a microenvironment that mimics the in vivo cellular and tissue complexity by incorporating physical, chemical and biological signal gradients into engineered biomaterials. In this review, the gradients existing in vivo and their influences on cell migration are briefly described. Recent developments in the fabrication of gradient biomaterials for controlling cellular behaviours, especially the cell migration, are summarized, highlighting the importance of the intrinsic driving mechanism for tissue regeneration and the design principle of complicated and advanced tissue regenerative materials. The potential uses of the gradient biomaterials in regenerative medicine are introduced. The current and future trends in gradient biomaterials and programmed cell migration in terms of the long-term goals of tissue regeneration are prospected.

Delivery of chemotropic proteins and improvement of dopaminergic neuron outgrowth through a thixotropic hybrid nano-gel

Chemotropic proteins guide neuronal projections to their final target during embryo development and are useful to guide axons of neurons used in transplantation therapies. Site-specific delivery of the proteins however is needed for their application in the brain to avoid degradation and pleiotropic affects. In the present study we report the use of Poly (ethylene glycol)-Silica (PEG-Si) nanocomposite gel with thixotropic properties that make it injectable and suitable for delivery of the chemotropic protein semaphorin 3A. PEG-Si gel forms a functional gradient of semaphorin that enhances axon outgrowth of dopaminergic neurons from rat embryos or differentiated from stem cells in culture. It is not cytotoxic and its properties allowed its injection into the striatum without inflammatory response in the short term. Long term implantation however led to an increase in macrophages and glial cells. The inflammatory response could have resulted from non-degraded silica particles, as observed in biodegradation assays.

Cyclic nucleotide-dependent switching of mammalian axon guidance depends on gradient steepness

Correct wiring of the nervous system during development requires axons to respond appropriately to gradients of attractive and repulsive guidance cues. However, the steepness and concentration of these gradients vary in vivo, for instance, with distance from the target. Understanding how these changing conditions affect the navigation strategies used by developing axons is important for understanding how they are guided over long distances. Previous work has shown that cyclic nucleotide levels determine whether axons are attracted or repelled by steep gradients of the same guidance cue, but it is unknown whether this is also true for shallow gradients. We therefore investigated the guidance responses of rat superior cervical ganglion (SCG) axons in both steep and shallow gradients of nerve growth factor (NGF). In steep gradients we found that cyclic nucleotide-dependent switching occurred, consistent with previous reports. Surprisingly however, we found that in shallow NGF gradients, cyclic nucleotide-dependent switching did not occur. These results suggest that there may be substantial differences in the way axons respond to gradient-based guidance cues depending on where they are within the gradient.

Positional cues in the Drosophila nerve cord: semaphorins pattern the dorso-ventral axis

During the development of neural circuitry, neurons of different kinds establish specific synaptic connections by selecting appropriate targets from large numbers of alternatives. The range of alternative targets is reduced by well organised patterns of growth, termination, and branching that deliver the terminals of appropriate pre- and postsynaptic partners to restricted volumes of the developing nervous system. We use the axons of embryonic Drosophila sensory neurons as a model system in which to study the way in which growing neurons are guided to terminate in specific volumes of the developing nervous system. The mediolateral positions of sensory arbors are controlled by the response of Robo receptors to a Slit gradient. Here we make a genetic analysis of factors regulating position in the dorso-ventral axis. We find that dorso-ventral layers of neuropile contain different levels and combinations of Semaphorins. We demonstrate the existence of a central to dorsal and central to ventral gradient of Sema 2a, perpendicular to the Slit gradient. We show that a combination of Plexin A (Plex A) and Plexin B (Plex B) receptors specifies the ventral projection of sensory neurons by responding to high concentrations of Semaphorin 1a (Sema 1a) and Semaphorin 2a (Sema 2a). Together our findings support the idea that axons are delivered to particular regions of the neuropile by their responses to systems of positional cues in each dimension.

Spatially patterned gene expression for guided neurite extension

Axon pathfinding by localized expression of guidance molecules is critical for the proper development of the nervous system. In this report, we present a well-defined spatially patterned gene expression system to investigate neurite guidance in vitro. Nonviral gene delivery was patterned by combining substrate-mediated gene delivery with soft lithography techniques, and the amount of protein produced at the region of localized expression was varied by altering the vector concentration and the width of the pattern, highlighting the flexibility of the system. A neuronal coculture model was used to investigate responses to spatial patterns of nerve growth factor (NGF) expression. The soluble NGF gradient elicited a guidance cue, and the degree of guidance was governed by the distance a neuron was cultured from the pattern and the time between accessory cell and neuron seedings. A portion of the diffusible NGF bound to the culture surface in the extracellular space, and the surface-associated NGF supported neuron survival and neurite outgrowth. However, the surface-bound NGF gradient alone did not elicit a guidance signal, and in fact masked the guidance cue by soluble NGF gradients. Mathematical modeling of NGF diffusion was used to predict the concentration gradients, and both the absolute and fractional gradients capable of guiding neurites produced by patterned gene expression differed substantially from the values obtained with existing engineered protein gradients. Spatially patterned gene expression provides a versatile tool to investigate the factors that may promote neurite guidance.

Reverse leukocyte migration can be attractive or repulsive

The directional migration of cells within multicellular organisms is governed by gradients of both chemical attractants and repellents in diverse processes, including leukocyte trafficking and neuronal pathfinding in vivo. These complex extracellular environments direct the orchestrated bidirectional trafficking of leukocytes between the vasculature and tissues. Substantial progress has been made in dissecting the molecular mechanisms involved in orchestrating the directed movement of leukocytes into host tissues; however, less is known about the reverse migration of leukocytes from the tissues to the vasculature. In this article, we discuss the functional interplay between chemoattraction and chemorepulsion in the bidirectional movement of cells in complex in vivo environments, and we describe how these mechanisms influence both normal physiology and human disease.

Biomolecular gradients in cell culture systems

Biomolecule gradients have been shown to play roles in a wide range of biological processes including development, inflammation, wound healing, and cancer metastasis. Elucidation of these phenomena requires the ability to expose cells to biomolecule gradients that are quantifiable, controllable, and mimic those that are present in vivo. Here we review the major biological phenomena in which biomolecule gradients are employed, traditional in vitro gradient-generating methods developed over the past 50 years, and new microfluidic devices for generating gradients. Microfluidic gradient generators offer greater levels of precision, quantitation, and spatiotemporal gradient control than traditional methods, and may greatly enhance our understanding of many biological phenomena. For each method, we outline the salient features, capabilities, and applications.

Bound attractant at the leading vs. the trailing edge determines chemotactic prowess

We have analyzed chemotaxis of neutrophil-differentiated HL60 cells in microfluidic devices that create exponential gradients of the chemoattractant, f-Met-Leu-Phe (fMLP). Such gradients expose each cell to a difference in fMLP concentration (DeltaC) across its diameter that is directly proportional to the ambient concentration (C) at that cell's position in the gradient, so the ratio DeltaC/C is constant everywhere. Cells exposed to ambient fMLP concentrations near the constant of dissociation (K(d)) for fMLP binding to its receptor ( approximately 10 nM) crawl much less frequently when DeltaC/C is 0.05 than when it is 0.09 or 0.13. Hence, cells can detect the gradient across their diameter without moving and, thus, without experiencing temporal changes in attractant concentration. At all DeltaC/C ratios tested, the average chemotactic prowess of individual cells (indicated by the distance a cell traveled in the correct direction divided by the length of its migration path) is maximal for cells that start migrating at concentrations near the K(d) and progressively decreases at higher or lower starting concentrations.

The PLEXIN PLX-2 and the ephrin EFN-4 have distinct roles in MAB-20/Semaphorin 2A signaling in Caenorhabditis elegans morphogenesis

Semaphorins are extracellular proteins that regulate axon guidance and morphogenesis by interacting with a variety of cell surface receptors. Most semaphorins interact with plexin-containing receptor complexes, although some interact with non-plexin receptors. Class 2 semaphorins are secreted molecules that control axon guidance and epidermal morphogenesis in Drosophila and Caenorhabditis elegans. We show that the C. elegans class 2 semaphorin MAB-20 binds the plexin PLX-2. plx-2 mutations enhance the phenotypes of hypomorphic mab-20 alleles but not those of mab-20 null alleles, indicating that plx-2 and mab-20 act in a common pathway. Both mab-20 and plx-2 mutations affect epidermal morphogenesis during embryonic and in postembryonic development. In both contexts, plx-2 null mutant phenotypes are much less severe than mab-20 null phenotypes, indicating that PLX-2 is not essential for MAB-20 signaling. Mutations in the ephrin efn-4 do not synergize with mab-20, indicating that EFN-4 may act in MAB-20 signaling. EFN-4 and PLX-2 are coexpressed in the late embryonic epidermis where they play redundant roles in MAB-20-dependent cell sorting.

Bias in the gradient-sensing response of chemotactic cells

We apply linear stability theory and perform perturbation studies to better characterize, and to generate new experimental predictions from, a model of chemotactic gradient sensing in eukaryotic cells. The model uses reaction-diffusion equations to describe 3(') phosphoinositide signaling and its regulation at the plasma membrane. It demonstrates a range of possible gradient-sensing mechanisms and captures such characteristic behaviors as strong polarization in response to static gradients, adaptation to differing mean levels of stimulus, and plasticity in response to changing gradients. An analysis of the stability of polarized steady-state solutions indicates that the model is most sensitive to off-axis perturbations. This biased sensitivity is also reflected in responses to localized external stimuli, and leads to a clear experimental prediction, namely, that a cell which is polarized in a background gradient will be most sensitive to transient point-source stimuli lying within a range of angles that are oblique with respect to the polarization axis. Stimuli at angles below this range will elicit responses whose directions overshoot the stimulus angle, while responses to stimuli applied at larger angles will undershoot the stimulus angle. We argue that such a bias is likely to be a general feature of gradient sensing in highly motile cells, particularly if they are optimized to respond to small gradients. Finally, an angular bias in gradient sensing might lead to preferred turn angles and zigzag movements of cells moving up chemotactic gradients, as has been noted under certain experimental conditions.

MAPK-mediated bimodal gene expression and adaptive gradient sensing in yeast

The mating pathway in Saccharomyces cerevisiae has been the focus of considerable research effort, yet many quantitative aspects of its regulation still remain unknown. Using an integrated approach involving experiments in microfluidic chips and computational modelling, we studied gene expression and phenotypic changes associated with the mating response under well-defined pheromone gradients. Here we report a combination of switch-like and graded pathway responses leading to stochastic phenotype determination in a specific range of pheromone concentrations. Furthermore, we show that these responses are critically dependent on mitogen-activated protein kinase (MAPK)-mediated regulation of the activity of the pheromone-response-specific transcription factor, Ste12, as well as on the autoregulatory feedback of Ste12. In particular, both the switch-like characteristics and sensitivity of gene expression in shmooing cells to pheromone concentration were significantly diminished in cells lacking Kss1, one of the MAP kinases activated in the mating pathway. In addition, the dynamic range of gradient sensing of Kss1-deficient cells was reduced compared with wild type. We thus provide unsuspected functional significance for this kinase in regulation of the mating response.

Semaphorin 2a secreted by oenocytes signals through plexin B and plexin A to guide sensory axons in the Drosophila embryo

The semaphorin gene family has been shown to play important roles in axonal guidance in both vertebrates and invertebrates. Both transmembrane (Sema1a, Sema1b, Sema5c) and secreted (Sema2a, Sema2b) forms of semaphorins exist in Drosophila. Two Sema receptors, plexins (Plex) A and B, have also been identified. Many questions remain concerning the axon guidance functions of the secreted semaphorins, including the identity of their receptors. We have used the well-characterized sensory system of the Drosophila embryo to address these problems. We find novel sensory axon defects in sema2a loss-of-function mutants in which particular axons misproject and follow inappropriate pathways to the CNS. plexB loss-of-function mutants show similar phenotypes to sema2a mutants and sema2a interacts genetically with plexB, supporting the hypothesis that Sema2a signals through PlexB receptors. Sema2a protein is expressed by larval oenocytes, a cluster of secretory cells in the lateral region of the embryo and the sema2a mutant phenotype can be rescued by driving Sema2a in these cells. Ablation of oenocytes results in sensory axon defects similar to the sema2a mutant phenotype. These data support a model in which Sema2a, while being secreted from oenocytes, acts in a highly localized fashion: It represses axon extension from the sensory neuron cell body, but only in regions in direct contact with oenocytes.

Neutrophil chemorepulsion in defined interleukin-8 gradients in vitro and in vivo

We report for the first time that primary human neutrophils can undergo persistent, directionally biased movement away from a chemokine in vitro and in vivo, termed chemorepulsion or fugetaxis. Robust neutrophil chemorepulsion in microfluidic gradients of interleukin-8 (IL-8; CXC chemokine ligand 8) was dependent on the absolute concentration of chemokine, CXC chemokine receptor 2 (CXCR2), and was associated with polarization of cytoskeletal elements and signaling molecules involved in chemotaxis and leading edge formation. Like chemoattraction, chemorepulsion was pertussis toxin-sensitive and dependent on phosphoinositide-3 kinase, RhoGTPases, and associated proteins. Perturbation of neutrophil intracytoplasmic cyclic adenosine monophosphate concentrations and the activity of protein kinase C isoforms modulated directional bias and persistence of motility and could convert a chemorepellent to a chemoattractant response. Neutrophil chemorepulsion to an IL-8 ortholog was also demonstrated and quantified in a rat model of inflammation. The finding that neutrophils undergo chemorepulsion in response to continuous chemokine gradients expands the paradigm by which neutrophil migration is understood and may reveal a novel approach to our understanding of the homeostatic regulation of inflammation.

Distinguishing modes of eukaryotic gradient sensing

We develop a mathematical model of phosphoinositide-mediated gradient sensing that can be applied to chemotactic behavior in highly motile eukaryotic cells such as Dictyostelium and neutrophils. We generate four variants of our model by adjusting parameters that control the strengths of coupled positive feedbacks and the importance of molecules that translocate from the cytosol to the membrane. Each variant exhibits a qualitatively different mode of gradient sensing. Simulations of characteristic behaviors suggest that differences between the variants are most evident at transitions between efficient gradient detection and failure. Based on these results, we propose criteria to distinguish between possible modes of gradient sensing in real cells, where many biochemical parameters may be unknown. We also identify constraints on parameters required for efficient gradient detection. Finally, our analysis suggests how a cell might transition between responsiveness and nonresponsiveness, and between different modes of gradient sensing, by adjusting its biochemical parameters.

Growth cones turn and migrate up an immobilized gradient of the laminin IKVAV peptide

Growth cone navigation is guided by extrinsic environmental proteins, called guidance cues. Many in vitro studies have characterized growth cone turning up and down gradients of soluble guidance cues. Although previous studies have shown that axonal elongation rates can be regulated by gradients of surface-bound molecules, there are no convincing demonstrations of growth cones turning to migrate up a surface-bound gradient of an adhesive ligand or guidance cue. In order to test this mode of axonal guidance, we used a photo-immobilization technique to create grids and gradients of an adhesive laminin peptide on polystyrene culture dish surfaces. Chick embryo dorsal root ganglia (DRGs) were placed on peptide grid patterns containing surface-bound gradients of the IKVAV-containing peptide. DRG growth cones followed a path of surface-bound peptide to the middle of a perpendicularly oriented gradient with a 25% concentration difference across 30 microm. The majority of growth cones turned and migrated up the gradient, turning until they were oriented directly up the gradient. Growth cones slowed their migration when they encountered the gradient, but growth cone velocity returned to the previous rate after turning up or down the gradient. This resembles in vivo situations where growth cones slow at a choice point before changing the direction of axonal extension. Thus, these results support the hypothesis that mechanisms of axonal guidance include growth cone orientation by gradients of surface-bound adhesive molecules and guidance cues.

The tibial-1 pioneer pathway: an in vivo model for neuronal outgrowth and guidance

As neurons extend axons to their targets during development, growth cones must reorient their direction of migration in response to extracellular guidance cues. A variety of model systems have been employed in order to dissect the cellular and molecular mechanisms that underlie this complex process. One preparation, the developing grasshopper limb bud, has proved to offer a number of advantages in which to examine mechanisms of growth cone guidance and motility in vivo. First, the relatively large size of the embryonic nervous system allows for straightforward imaging of both fixed and live neurons in vivo. Second, the peripheral nerves generated in the limb bud are highly stereotyped. Third, intact embryos can be cultured for a period of days, allowing for fairly easy perturbations at precise developmental stages. Fourth, due to the ease of dissection, numerous cell biological and molecular techniques can be utilized in the limb bud. Finally, axon guidance molecules and mechanisms are conserved between grasshoppers and other organism, including vertebrates.