Less than 5 Netrin-1 molecules initiate attraction but 200 Sema3A molecules are necessary for repulsion

Beyond a Transmission Cable-New Technologies to Reveal the Richness in Axonal Electrophysiology

The axon is a neuronal structure capable of processing, encoding, and transmitting information. This assessment contrasts with a limiting, but deeply rooted, perspective where the axon functions solely as a transmission cable of somatodendritic activity, sending signals in the form of stereotypical action potentials. This perspective arose, at least partially, because of the technical difficulties in probing axons: their extreme length-to-diameter ratio and intricate growth paths preclude the study of their dynamics through traditional techniques. Recent findings are challenging this view and revealing a much larger repertoire of axonal computations. Axons display complex signaling processes and structure-function relationships, which can be modulated via diverse activity-dependent mechanisms. Additionally, axons can exhibit patterns of activity that are dramatically different from those of their corresponding soma. Not surprisingly, many of these recent discoveries have been driven by novel technology developments, which allow for in vitro axon electrophysiology with unprecedented spatiotemporal resolution and signal-to-noise ratio. In this review, we outline the state-of-the-art in vitro toolset for axonal electrophysiology and summarize the recent discoveries in axon function it has enabled. We also review the increasing repertoire of microtechnologies for controlling axon guidance which, in combination with the available cutting-edge electrophysiology and imaging approaches, have the potential for more controlled and high-throughput in vitro studies. We anticipate that a larger adoption of these new technologies by the neuroscience community will drive a new era of experimental opportunities in the study of axon physiology and consequently, neuronal function.

Optical tweezers across scales in cell biology

Optical tweezers (OT) provide a noninvasive approach for delivering minute physical forces to targeted objects. Controlling such forces in living cells or in vitro preparations allows for the measurement and manipulation of numerous processes relevant to the form and function of cells. As such, OT have made important contributions to our understanding of the structures of proteins and nucleic acids, the interactions that occur between microscopic structures within cells, the choreography of complex processes such as mitosis, and the ways in which cells interact with each other. In this review, we highlight recent contributions made to the field of cell biology using OT and provide basic descriptions of the physics, the methods, and the equipment that made these studies possible.

Mechanotransduction in hippocampal neurons operates under localized low picoNewton forces

There is growing evidence suggesting that mechanical properties of CNS neurons may play an important regulatory role in cellular processes. Here, we employ an oscillatory optical tweezers (OOT) to exert a local indentation with forces in the range of 5–50 pN. We found that single local indentation above a threshold of 13 ± 1 pN evokes a transient intracellular calcium change, whereas repeated mechanical stimulations induce a more sustained and variable calcium response. Importantly, neurons were able to differentiate the magnitude of mechanical stimuli. Chemical perturbation and whole-cell patch clamp recordings suggest that mechanically evoked response requires the influx of extracellular calcium through transmembrane ion channels. Moreover, we observed a mechanically evoked activation of the CAMKII and small G protein RhoA. These results all together suggest that mechanical signaling among developed neurons fully operates in neuronal networks under physiological conditions.

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Hippocampal neurons are mechanically adapted to forces in the picoNewton range

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Localized compressive mechanical stimuli in the range of 13–50 pN lead to Ca²⁺ influx

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The evoked Ca²⁺ neuronal response is differentiated by the magnitude of the force

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The subsequent mechanotransduction pathway involves the activation of CAMKII and RhoA

Developmental genes controlling neural circuit formation are expressed in the early postnatal hypothalamus and cellular lining of the third ventricle

The arcuate nucleus of the hypothalamus is central in the regulation of body weight homeostasis through its ability to sense peripheral metabolic signals and relay them, through neural circuits, to other brain areas, ultimately affecting physiological and behavioural changes. The early postnatal development of these neural circuits is critical for normal body weight homeostasis, such that perturbations during this critical period can lead to obesity. The role for peripheral regulators of body weight homeostasis, including leptin, insulin and ghrelin, in this postnatal development is well described, yet some of the fundamental processes underpinning axonal and dendritic growth remain unclear. Here, we hypothesised that molecules known to regulate axonal and dendritic growth processes in other areas of the developing brain would be expressed in the postnatal arcuate nucleus and/or target nuclei where they would function to mediate the development of this circuitry. Using state-of-the-art RNAscope^® technology, we have revealed the expression patterns of genes encoding Dcc/Netrin-1, Robo1/Slit1 and Fzd5/Wnt5a receptor/ligand pairs in the early postnatal mouse hypothalamus. We found that individual genes had unique expression patterns across developmental time in the arcuate nucleus, paraventricular nucleus of the hypothalamus, ventromedial nucleus of the hypothalamus, dorsomedial nucleus of the hypothalamus, median eminence and, somewhat unexpectedly, the third ventricle epithelium. These observations indicate a number of new molecular players in the development of neural circuits regulating body weight homeostasis, as well as novel molecular markers of tanycyte heterogeneity.

Understanding, engineering, and modulating the growth of neural networks: An interdisciplinary approach

A deeper understanding of the brain and its function remains one of the most significant scientific challenges. It not only is required to find cures for a plethora of brain-related diseases and injuries but also opens up possibilities for achieving technological wonders, such as brain-machine interface and highly energy-efficient computing devices. Central to the brain's function is its basic functioning unit (i.e., the neuron). There has been a tremendous effort to understand the underlying mechanisms of neuronal growth on both biochemical and biophysical levels. In the past decade, this increased understanding has led to the possibility of controlling and modulating neuronal growth in vitro through external chemical and physical methods. We provide a detailed overview of the most fundamental aspects of neuronal growth and discuss how researchers are using interdisciplinary ideas to engineer neuronal networks in vitro. We first discuss the biochemical and biophysical mechanisms of neuronal growth as we stress the fact that the biochemical or biophysical processes during neuronal growth are not independent of each other but, rather, are complementary. Next, we discuss how utilizing these fundamental mechanisms can enable control over neuronal growth for advanced neuroengineering and biomedical applications. At the end of this review, we discuss some of the open questions and our perspectives on the challenges and possibilities related to controlling and engineering the growth of neuronal networks, specifically in relation to the materials, substrates, model systems, modulation techniques, data science, and artificial intelligence.

Optical Tweezers Exploring Neuroscience

Over the past decade, optical tweezers (OT) have been increasingly used in neuroscience for studies of molecules and neuronal dynamics, as well as for the study of model organisms as a whole. Compared to other areas of biology, it has taken much longer for OT to become an established tool in neuroscience. This is, in part, due to the complexity of the brain and the inherent difficulties in trapping individual molecules or manipulating cells located deep within biological tissue. Recent advances in OT, as well as parallel developments in imaging and adaptive optics, have significantly extended the capabilities of OT. In this review, we describe how OT became an established tool in neuroscience and we elaborate on possible future directions for the field. Rather than covering all applications of OT to neurons or related proteins and molecules, we focus our discussions on studies that provide crucial information to neuroscience, such as neuron dynamics, growth, and communication, as these studies have revealed meaningful information and provide direction for the field into the future.

Copper Binding Regulates Cellular Prion Protein Function

The cellular prion protein (PrP^C), mainly known for its role in neurodegenerative diseases, is involved in several physiological processes including neuritogenesis. In addition, its ability to bind copper or zinc has been suggested for its role in metal homeostasis. Although PrP^C has been known as a copper-binding molecule, little is known about how copper can affect PrP^C physiological functions. By combining genomic approaches, cellular assays, and focal stimulation technique, we found that PrP^C neuritogenesis function is directly influenced by N-terminal copper-binding amino acids. Several recombinant mouse PrP (recMoPrP) mutants at N-terminal copper-binding sites were produced, and primary hippocampal cultures were treated either in bulk or exposed near the hippocampal growth cones (GC) of single neurons in local stimulation manner. While focal stimulation of GC with wild-type recMoPrP induced neurite outgrowth and rapid GC turning toward the source, N-terminal mutants fail to support this effect. Indeed, disrupting all the copper-binding sites at the N-terminus of PrP^C was toxic to neurons indicating that these regions are crucial for the protein function. Mutants at both octarepeat and non-octarepeat region abolished the neuritogenesis effect. Altogether, our findings indicate the crucial role of copper-binding sites in maintaining the neuritogenesis function in PrP, suggesting a potential link between loss-of-function of the protein and disease initiation.

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.

The binding of DCC-P3 motif and FAK-FAT domain mediates the initial step of netrin-1/DCC signaling for axon attraction

Netrin-1 plays a key role in axon guidance through binding to its receptor, Deleted in Colorectal Cancer (DCC). The initial step of signaling inside the cell after netrin-1/DCC ligation is the binding of DCC cytoplasmic P3 motif to focal adhesion targeting (FAT) domain of focal adhesion kinase (FAK). Here we report the crystal structure of P3/FAT complex. The helical P3 peptide interacts with a helix-swapped FAT dimer in a 2:2 ratio. Dimeric FAT binding is P3-specific and stabilized by a calcium ion. Biochemical studies showed that DCC-P3 motif and calcium ion could facilitate FAT dimerization in solution. Axon guidance assays confirm that the DCC/FAK complex is essential for netrin-1-induced chemoattraction. We propose that netrin-1/DCC engagement creates a small cluster of P3/FAT for FAK recruitment close to the cell membrane, which exerts a concerted effect with PIP2 for FAK signaling. We also compare P3/FAT binding with paxillin/FAT binding and discuss their distinct recognition specificity on a common FAT domain for axon attraction versus integrin signaling, respectively.

Distinct functions for netrin 1 in chicken and murine semicircular canal morphogenesis

The vestibular system of the inner ear detects head position using three orthogonally oriented semicircular canals; even slight changes in their shape and orientation can cause debilitating behavioral defects. During development, the canals are sculpted from pouches that protrude from the otic vesicle, the embryonic anlage of the inner ear. In the center of each pouch, a fusion plate forms where cells lose their epithelial morphology and the basement membrane breaks down. Cells in the fusing epithelia intercalate and are removed, creating a canal. In mice, fusion depends on the secreted protein netrin 1 (Ntn1), which is necessary for basement membrane breakdown, although the underlying molecular mechanism is unknown. Using gain-of-function approaches, we found that overexpression of Ntn1 in the chick otic vesicle prevented canal fusion by inhibiting apoptosis. In contrast, ectopic expression of the same chicken Ntn1 in the mouse otic vesicle, where apoptosis is less prominent, resulted in canal truncation. These findings highlight the importance of apoptosis for tissue morphogenesis and suggest that Ntn1 may play divergent cellular roles despite its conserved expression during canal morphogenesis in chicken and mouse.

Combining FRET and optical tweezers to study RhoGTPases spatio-temporal dynamics upon local stimulation

Local stimulation with optical tweezers has been used to mimic natural stimuli that occur in biological processes such as cell migration or differentiation. Carriers (beads and lipid vesicles) with sizes down to 30 nm can be manipulated with a high spatial and temporal resolution: they are positioned with a sub-micrometric precision on a specific cell compartment and the beginning of the stimulation can be triggered with millisecond precision. RhoGTPases are a Ras-related family of proteins that regulate many different functions including cell polarity, microtubule dynamics and membrane transport pathways. Here we combine local stimulation with FRET microscopy to study RhoGTPases spatial and temporal activation following guidance cue local stimulation. We used two different vectors for local delivery: silica micro-beads and micro-sized lipid vesicles. The experimental methods associated with neuronal growth cone local stimulation are discussed in detail, as well as the analysis methods. Here we present a protocol that enables to study neuronal growth cone cytoskeleton rearrangements in response to a gradient of molecules in a way that better mimics physiological conditions, and it can be similarly applied to each secreted molecule involved in cell signaling.

Characterization of prion protein function by focal neurite stimulation

The cellular prion protein (PrP^C), encoded by the PRNP gene, is a ubiquitous glycoprotein, which is highly expressed in the brain. This protein, mainly known for its role in neurodegenerative diseases, is involved in several physiological processes including neurite outgrowth. By using a novel focal stimulation technique, we explored the potential function of PrP^C, in its soluble form, as a signaling molecule. Thus, soluble recombinant prion proteins (recPrP) encapsulated in micro-vesicles were released by photolysis near the hippocampal growth cones. Local stimulation of wild-type growth cones with full-length recPrP induced neurite outgrowth and rapid growth cone turning towards the source. This effect was shown to be concentration dependent. Notably, PrP^C-knockout growth cones were insensitive to recPrP stimulation, but this property was rescued in PrP-knockout growth cones expressing GFP-PrP. Taken together, our findings indicate that recPrP functions as a signaling molecule, and that its homophilic interaction with membrane-anchored PrP^C might promote neurite outgrowth and facilitate growth cone guidance.

A new approach to follow a single extracellular vesicle-cell interaction using optical tweezers

Extracellular vesicles (EVs) are spherical membrane structures released by most cells. These highly conserved mediators of intercellular communication carry proteins, lipids, and nucleic acids, and transfer these cellular components between cells by different mechanisms, such as endocytosis, macropinocytosis, or fusion. However, the temporal and spatial dynamics of vesicle-cell interactions still remain largely unexplored. Here we used optical tweezers to drive single EVs produced by microglial cells onto the surface of astrocytes or microglia in primary culture. By visualizing single EV-cell contacts, we observed that microglial vesicles displayed different motilities on the surface of astrocytes compared with microglia. After contact, EVs positioned on astrocytes displayed some minor oscillatory motion around the point of adhesion, while vesicles dragged to microglia displayed quite regular directional movement on the plasma membrane. Both the adhesion and motion of vesicles on glial cells were strongly reduced by cloaking phosphatidylserine (PS) residues, which are externalized on the vesicle membrane and act as determinants for vesicle recognition by target cells. These data identify optical manipulation as a powerful tool to monitor in vitro vesicle-cell dynamics with high temporal and spatial resolution and to determine in a quantitative manner the contribution of surface receptors/extracellular protein ligands to the contact.

Phenotypic analysis of mice completely lacking netrin 1

Netrin 1 (Ntn1) is a multifunctional guidance cue expressed in the ventricular zone and floor plate of the embryonic neural tube. Although Ntn1 is best known for acting as an axon guidance cue through Dcc and neogenin receptors, it is also thought to regulate neuronal survival and blood vessel development through Unc5 family receptors. However, the Ntn1 gene trap mutant mouse does not display all the phenotypes predicted from in vitro assays or analyses of mice lacking predicted receptors. Since the gene trap strain still produces wild-type Ntn1 protein, it is unclear whether the absence of phenotypes reflects the activity of alternative cues or of residual Ntn1. To resolve the full contribution of Ntn1 to development, we generated a null allele of Ntn1 and re-examined tissues exhibiting phenotypic discrepancies between receptor mutants and Ntn1 hypomorphs. We found that in Ntn1 null animals commissural axons rarely cross the midline, resulting in a strongly enhanced phenotype relative to Ntn1 hypomorphs, which retain many axons with normal trajectories. Thus, low levels of Ntn1 can account for persistent attraction to the midline in hypomorphs. By contrast, Ntn1 null mice do not show all of the phenotypes reported for Unc5 receptor mutants, indicating that Ntn1 is not necessarily the dominant ligand for Unc5 family members in vivo and ruling out primary roles in survival or angiogenesis.

Summary: Complete deletion of the axon guidance cue netrin 1 leads to severe defects in midline crossing of spinal cord axons. However, the mutants did not fully phenocopy loss of the netrin 1 receptor Unc5.

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.

Subrepellent doses of Slit1 promote Netrin-1 chemotactic responses in subsets of axons

Background

Axon pathfinding is controlled by guidance cues that elicit specific attractive or repulsive responses in growth cones. It has now become clear that some cues such as Netrin-1 can trigger either attraction or repulsion in a context-dependent manner. In particular, it was recently found that the repellent Slit1 enables the attractive response of rostral thalamic axons to Netrin-1. This finding raised the intriguing possibility that Netrin-1 and Slit1, two essential guidance cues, may act more generally in an unexpected combinatorial manner to orient specific axonal populations. To address this major issue, we have used an innovative microfluidic device compatible not only with dissociated neuronal cultures but also with explant cultures to systematically and quantitatively characterize the combinatorial activity of Slit1 and Netrin-1 on rostral thalamic axons as well as on hippocampal neurons.

Results

We found that on rostral thalamic axons, only a subthreshold concentration of the repellent Slit1 triggered an attractive response to a gradient of Netrin-1. On hippocampal neurons, we similarly found that Slit1 alone is repulsive and a subthreshold concentration of Slit1 triggered a potent attractive or repulsive behavioral response to a gradient of Netrin-1, depending on the nature of the substrate.

Conclusions

Our study reveals that at subthreshold repulsive levels, Slit1 acts as a potent promoter of both Netrin-1 attractive and repulsive activities on distinct neuronal cell types, thereby opening novel perspectives on the role of combinations of cues in brain wiring.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-015-0036-8) contains supplementary material, which is available to authorized users.

The crystal structure of netrin-1 in complex with DCC reveals the bifunctionality of netrin-1 as a guidance cue

Netrin-1 is a guidance cue that can trigger either attraction or repulsion effects on migrating axons of neurons, depending on the repertoire of receptors available on the growth cone. How a single chemotropic molecule can act in such contradictory ways has long been a puzzle at the molecular level. Here we present the crystal structure of netrin-1 in complex with the Deleted in Colorectal Cancer (DCC) receptor. We show that one netrin-1 molecule can simultaneously bind to two DCC molecules through a DCC-specific site and through a unique generic receptor binding site, where sulfate ions staple together positively charged patches on both DCC and netrin-1. Furthermore, we demonstrate that UNC5A can replace DCC on the generic receptor binding site to switch the response from attraction to repulsion. We propose that the modularity of binding allows for the association of other netrin receptors at the generic binding site, eliciting alternative turning responses.

Investigating axonal guidance with microdevice-based approaches

The precise wiring of the nervous system relies on processes by which axons navigate in a complex environment and are guided by a concerted action of attractive and repulsive factors to reach their target. Investigating these guidance processes depends critically on our ability to control in space and time the microenvironment of neurons. The implementation of microfabrication techniques in cell biology now enables a precise control of the extracellular physical and chemical environment of cultured cells. However, microtechnology is only beginning to be applied in the field of axon guidance due to specific requirements of neuronal cultures. Here we review microdevices specifically designed to study axonal guidance and compare them with the conventional assays used to probe gradient sensing in cell biology. We also discuss how innovative microdevice-based approaches will enable the investigation of important systems-level questions on the gradient sensing properties of nerve cells, such as the sensitivity and robustness in the detection of directional signals or the combinatorial response to multiple cues.

Cell signaling experiments driven by optical manipulation

Cell signaling involves complex transduction mechanisms in which information released by nearby cells or extracellular cues are transmitted to the cell, regulating fundamental cellular activities. Understanding such mechanisms requires cell stimulation with precise control of low numbers of active molecules at high spatial and temporal resolution under physiological conditions. Optical manipulation techniques, such as optical tweezing, mechanical stress probing or nano-ablation, allow handling of probes and sub-cellular elements with nanometric and millisecond resolution. PicoNewton forces, such as those involved in cell motility or intracellular activity, can be measured with femtoNewton sensitivity while controlling the biochemical environment. Recent technical achievements in optical manipulation have new potentials, such as exploring the actions of individual molecules within living cells. Here, we review the progress in optical manipulation techniques for single-cell experiments, with a focus on force probing, cell mechanical stimulation and the local delivery of active molecules using optically manipulated micro-vectors and laser dissection.