FLIM FRET Visualization of Cdc42 Activation by Netrin-1 in Embryonic Spinal Commissural Neuron Growth Cones.

Netrin-1 is an essential extracellular chemoattractant that signals through its receptor DCC to guide commissural axon extension in the embryonic spinal cord. DCC directs the organization of F-actin in growth cones by activating an intracellular protein complex that includes the Rho GTPase Cdc42, a critical regulator of cell polarity and directional migration. To address the spatial distribution of signaling events downstream of netrin-1, we expressed the FRET biosensor Raichu-Cdc42 in cultured embryonic rat spinal commissural neurons. Using FLIM-FRET imaging we detected rapid activation of Cdc42 in neuronal growth cones following application of netrin-1. Investigating the signaling mechanisms that control Cdc42 activation by netrin-1, we demonstrate that netrin-1 rapidly enriches DCC at the leading edge of commissural neuron growth cones and that netrin-1 induced activation of Cdc42 in the growth cone is blocked by inhibiting src family kinase signaling. These findings reveal the activation of Cdc42 in embryonic spinal commissural axon growth cones and support the conclusion that src family kinase activation downstream of DCC is required for Cdc42 activation by netrin-1.

Tuning cell-surface affinity to direct cell specific responses to patterned proteins.

Interactions with local extracellular cues direct cell migration. A versatile method to study cell response to a protein consists of patterning the protein cue on a substrate and quantifying the distribution of cells between patterned and non-patterned areas. Here, we define the concepts of (i) cell-surface affinity to describe cell choices, and of (ii) reference surface (RS) to clarify that the choice is made relative to a reference. Furthermore, we report a method to systematically tune the RS and show that it can dominate the experimental cell response to a protein cue. The cell response to a cue can be switched from strong preference to strong aversion by only changing the RS. Using microcontact printing, we patterned the extracellular matrix proteins fibronectin or netrin-1 adjacent to a series of RSs with different ratios of poly-D-lysine (PDL) and polyethylene glycol (PEG), which are of high affinity and of low-affinity for cells, respectively. C2C12 myoblasts or primary neurons seeded on substrates with a high affinity RS (high % PDL) did not respond to a printed protein of interest, and conversely on RSs of low affinity (high % PEG) the cells preferred the printed protein even in the absence of a specific interaction. However, when testing cell response to a standardized series of RSs varying from high to low affinity, a specific response curve was obtained that was unique to each cell type-protein pair. Importantly, for intermediate RSs with moderate affinity, the cell response to the cue was dependent on the activation of biologically relevant protein-specific biochemical signal transduction pathways. Our results establish that choices made by cells in response to a surface-bound cue must take into account, and be interpreted in the context of, the RS. The use of a series of RSs with varying cell-surface affinity reveals specific response curves of cells to a cue that can be compared quantitatively and that may help gain new insights into cellular responses to extracellular proteins.

Netrins: versatile extracellular cues with diverse functions.

Netrins are secreted proteins that were first identified as guidance cues, directing cell and axon migration during neural development. Subsequent findings have demonstrated that netrins can influence the formation of multiple tissues, including the vasculature, lung, pancreas, muscle and mammary gland, by mediating cell migration, cell-cell interactions and cell-extracellular matrix adhesion. Recent evidence also implicates the ongoing expression of netrins and netrin receptors in the maintenance of cell-cell organisation in mature tissues. Here, we review the mechanisms involved in netrin signalling in vertebrate and invertebrate systems and discuss the functions of netrin signalling during the development of neural and non-neural tissues.

Patterning protein concentration using laser-assisted adsorption by photobleaching, LAPAP.

The study of cellular responses to changes in the spatial distribution of molecules in development, immunology and cancer, requires reliable methods to reproduce in vitro the precise distributions of proteins found in vivo. Here we present a straightforward method for generating substrate-bound protein patterns which has the simplicity required to be implemented in typical life science laboratories. The method exploits photobleaching of fluorescently tagged molecules to generate patterns and concentration gradients of protein with sub-micron spatial resolution. We provide an extensive characterization of the technique and demonstrate, as proof of principle, axon guidance by gradients of substrate-bound laminin peptide generated in vitro using LAPAP.

Rho inhibition recruits DCC to the neuronal plasma membrane and enhances axon chemoattraction to netrin 1.

Molecular cues, such as netrin 1, guide axons by influencing growth cone motility. Rho GTPases are a family of intracellular proteins that regulate the cytoskeleton, substrate adhesion and vesicle trafficking. Activation of the RhoA subfamily of Rho GTPases is essential for chemorepellent axon guidance; however, their role during axonal chemoattraction is unclear. Here, we show that netrin 1, through its receptor DCC, inhibits RhoA in embryonic spinal commissural neurons. To determine whether netrin 1-mediated chemoattraction requires Rho function, we inhibited Rho signaling and assayed axon outgrowth and turning towards netrin 1. Additionally, we examined two important mechanisms that influence the guidance of axons to netrin 1: substrate adhesion and transport of the netrin receptor DCC to the plasma membrane. We found that inhibiting Rho signaling increased plasma membrane DCC and adhesion to substrate-bound netrin 1, and also enhanced netrin 1-mediated axon outgrowth and chemoattractive axon turning. Conversely, overexpression of RhoA or constitutively active RhoA inhibited axonal responses to netrin 1. These findings provide evidence that Rho signaling reduces axonal chemoattraction to netrin 1 by limiting the amount of plasma membrane DCC at the growth cone, and suggest that netrin 1-mediated inhibition of RhoA activates a positive-feedback mechanism that facilitates chemoattraction to netrin 1. Notably, these findings also have relevance for CNS regeneration research. Inhibiting RhoA promotes axon regeneration by disrupting inhibitory responses to myelin and the glial scar. By contrast, we demonstrate that axon chemoattraction to netrin 1 is not only maintained but enhanced, suggesting that this might facilitate directing regenerating axons to appropriate targets.