Interplay between adhesion turnover and cytoskeleton dynamics in the control of growth cone migration

Syk kinases are required for spinal commissural axon repulsion at the midline via the ephrin/Eph pathway

In the hematopoietic system, Syk family tyrosine kinases are essential components of immunoreceptor ITAM-based signaling. While there is increasing data indicating the involvement of immunoreceptors in neural functions, the contribution of Syk kinases remains obscure. Previously, we identified phosphorylated forms of Syk kinases in specialized populations of migrating neurons or projecting axons. Moreover, we identified ephrin/Eph as guidance molecules utilizing the ITAM-bearing CD3zeta (Cd247) and associated Syk kinases for the growth cone collapse response induced in vitro Here, we show that in the developing spinal cord, Syk is phosphorylated in navigating commissural axons. By analyzing axon trajectories in open-book preparations of Syk(-/-); Zap70(-/-) mouse embryos, we show that Syk kinases are dispensable for attraction towards the midline but confer growth cone responsiveness to repulsive signals that expel commissural axons from the midline. Known to serve a repulsive function at the midline, ephrin B3/EphB2 are obvious candidates for driving the Syk-dependent repulsive response. Indeed, Syk kinases were found to be required for ephrin B3-induced growth cone collapse in cultured commissural neurons. In fragments of commissural neuron-enriched tissues, Syk is in a constitutively phosphorylated state and ephrin B3 decreased its level of phosphorylation. Direct pharmacological inhibition of Syk kinase activity was sufficient to induce growth cone collapse. In conclusion, Syk kinases act as a molecular switch of growth cone adhesive and repulsive responses.

Two-tiered coupling between flowing actin and immobilized N-cadherin/catenin complexes in neuronal growth cones

Neuronal growth cones move forward by dynamically connecting actin-based motility to substrate adhesion, but the mechanisms at the individual molecular level remain unclear. We cultured primary neurons on N-cadherin-coated micropatterned substrates, and imaged adhesion and cytoskeletal proteins at the ventral surface of growth cones using single particle tracking combined to photoactivated localization microscopy (sptPALM). We demonstrate transient interactions in the second time scale between flowing actin filaments and immobilized N-cadherin/catenin complexes, translating into a local reduction of the actin retrograde flow. Normal actin flow on micropatterns was rescued by expression of a dominant negative N-cadherin construct competing for the coupling between actin and endogenous N-cadherin. Fluorescence recovery after photobleaching (FRAP) experiments confirmed the differential kinetics of actin and N-cadherin, and further revealed a 20% actin population confined at N-cadherin micropatterns, contributing to local actin accumulation. Computer simulations with relevant kinetic parameters modeled N-cadherin and actin turnover well, validating this mechanism. Such a combination of short- and long-lived interactions between the motile actin network and spatially restricted adhesive complexes represents a two-tiered clutch mechanism likely to sustain dynamic environment sensing and provide the force necessary for growth cone migration.

β1-Integrin cytoskeletal signaling regulates sensory neuron response to matrix dimensionality

Neuronal differentiation, pathfinding and morphology are directed by biochemical cues that in vivo are presented in a complex scaffold of extracellular matrix. This microenvironment is three-dimensional (3D) and heterogeneous. Therefore, it is not surprising that more physiologically-relevant cellular responses are found in 3D culture environments rather than on two-dimensional (2D) flat substrates. One key difference between 2D and 3D environments is the spatial arrangement of cell-matrix interactions. Integrins and other receptor proteins link the various molecules presented in the extracellular environment to intracellular signaling cascades and thus influence a number of neuronal responses including the availability and activation of integrins themselves. We have previously reported that a 3D substrate induces an important morphological transformation of embryonic mouse dorsal root ganglion (DRG) neurons. Here, we investigate the hypothesis that β1-integrin signaling via focal adhesion kinase (FAK) and the RhoGTPases Rac and Rho influences neuronal morphology in 2D vs 3D environments. We report that β1-integrin activity and FAK phosphorylation at tyrosine 397 (FAKpY397) are linked to neuronal polarization as well as neurite outgrowth and branching. Rac and Rho expression are decreased in 3D vs 2D culture but not correlated with β1-integrin function. These results suggest that proper β1-integrin activity is required for the elaboration of physiologic DRG morphology and that 3D culture provides a more appropriate milieu to the mimic in vivo scenario. We propose that neuronal morphology may be directed during development and regeneration by factors that influence how β1-integrin, FAK and RhoGTPase molecules integrate substrate signals in the 3D microenvironment.

Mechanoregulation of cytoskeletal dynamics by TRP channels

The ability of cells to respond to mechanical stimulation is crucial to a variety of biological processes, including cell migration, axonal outgrowth, perception of pain, cardiovascular responses and kidney physiology. The translation of mechanical cues into cellular responses, a process known as mechanotransduction, typically takes place in specialized multiprotein structures such as cilia, cell-cell or cell-matrix adhesions. Within these structures, mechanical forces such as shear stress and membrane stretch activate mechanosensitive proteins, which set off a series of events that lead to altered cell behavior. Members of the transient receptor potential (TRP) family of cation channels are emerging as important players in mechanotransductory pathways. Localized within mechanosensory structures, they are activated by mechanical stimuli and trigger fast as well as sustained cytoskeletal responses. In this review, we will provide an overview of how TRP channels affect cytoskeletal dynamics in various mechano-regulated processes.

Growth cone travel in space and time: the cellular ensemble of cytoskeleton, adhesion, and membrane

Growth cones, found at the tip of axonal projections, are the sensory and motile organelles of developing neurons that enable axon pathfinding and target recognition for precise wiring of the neural circuitry. To date, many families of conserved guidance molecules and their corresponding receptors have been identified that work in space and time to ensure billions of axons to reach their targets. Research in the past two decades has also gained significant insight into the ways in which growth cones translate extracellular signals into directional migration. This review aims to examine new progress toward understanding the cellular mechanisms underlying directional motility of the growth cone and to discuss questions that remain to be addressed. Specifically, we will focus on the cellular ensemble of cytoskeleton, adhesion, and membrane and examine how the intricate interplay between these processes orchestrates the directed movement of growth cones.

The formation of actin waves during regeneration after axonal lesion is enhanced by BDNF

During development, axons of neurons in the mammalian central nervous system lose their ability to regenerate. To study the regeneration process, axons of mouse hippocampal neurons were partially damaged by an UVA laser dissector system. The possibility to deliver very low average power to the sample reduced the collateral thermal damage and allowed studying axonal regeneration of mouse neurons during early days in vitro. Force spectroscopy measurements were performed during and after axon ablation with a bead attached to the axonal membrane and held in an optical trap. With this approach, we quantified the adhesion of the axon to the substrate and the viscoelastic properties of the membrane during regeneration. The reorganization and regeneration of the axon was documented by long-term live imaging. Here we demonstrate that BDNF regulates neuronal adhesion and favors the formation of actin waves during regeneration after axonal lesion.

Neuritogenesis: the prion protein controls β1 integrin signaling activity

Cytoskeleton modifications are required for neuronal stem cells to acquire neuronal polarization. Little is known, however, about mechanisms that orchestrate cytoskeleton remodeling along neuritogenesis. Here, we show that the silencing of the cellular prion protein (PrP(C)) impairs the initial sprouting of neurites upon induction of differentiation of the 1C11 neuroectodermal cell line, indicating that PrP(C) is necessary to neuritogenesis. Such PrP(C) function relies on its capacity to negatively regulate the clustering, activation, and signaling activity of β1 integrins at the plasma membrane. β1 Integrin aggregation caused by PrP(C) depletion triggers overactivation of the RhoA-Rho kinase-LIMK-cofilin pathway, which, in turn, alters the turnover of focal adhesions, increases the stability of actin microfilaments, and in fine impairs neurite formation. Inhibition of Rho kinases is sufficient to compensate for the lack of PrP(C) and to restore neurite sprouting. We also observe an increased secretion of fibronectin in the surrounding milieu of PrP(C)-depleted 1C11 cells, which likely self-sustains β1 integrin signaling overactivation and contributes to neuritogenesis defect. Our overall data reveal that PrP(C) contributes to the acquisition of neuronal polarization by modulating β1 integrin activity, cell interaction with fibronectin, and cytoskeleton dynamics.

Trafficking guidance receptors

Wiring of the brain relies initially on the correct outgrowth of axons to reach the appropriate target area for innervation. A large number of guidance receptors present in the plasma membrane of axonal growth cones and elsewhere on the neuron read and execute directional cues present in the extracellular environment of the navigating growth cone. The exact timing, levels, and localization of expression of the guidance receptors in the plasma membrane therefore determine the outcome of guidance decisions. Many guidance receptors are localized in exquisitely precise spatial and temporal patterns. The cellular mechanisms ensuring these localization patterns include spatially accurate sorting after synthesis in the secretory pathway, retrieval of inappropriately expressed receptors by endocytosis followed by degradation or recycling, and restriction of diffusion. This article will discuss the machinery and regulation underlying the restricted distribution of membrane receptors, focusing on the currently best-studied example, the L1 cell adhesion molecule. In addition to the long-range mechanisms ensuring appropriate localization, the same mechanisms can act locally to adjust levels and localization of receptors. These local mechanisms are regulated by ligand binding and subsequent activation of local signaling cascades. It is likely that the localization of all guidance receptors is regulated by a combination of sorting, retrieval, recycling and retention, similar to the ones we discuss here for L1.