UNC-6/Netrin induces neuronal asymmetry and defines the site of axon formation

Abstract

UNC-6/Netrin and its receptor UNC-40/DCC are conserved regulators of growth cone guidance. By directly observing developing neurons in vivo, we show that UNC-6 and UNC-40 also function during axon formation to initiate, maintain and orient asymmetric neuronal growth. The immature HSN neuron of Caenorhabditis elegans breaks spherical symmetry to extend a leading edge toward ventral UNC-6. In unc-6 and unc-40 mutants, leading edge formation fails, the cell remains symmetrical until late in development and the axon that eventually forms is misguided. Thus netrin has two activities: one that breaks neuronal symmetry and one that guides the future axon. As the axon forms, UNC-6, UNC-40 and the lipid modulators AGE-1/phosphoinositide 3-kinase (PI3K) and DAF-18/ PTEN drive the actin-regulatory pleckstrin homology (PH) domain protein MIG-10/lamellipodin ventrally in HSN to promote asymmetric growth. The coupling of a directional netrin cue to sustained asymmetric growth via PI3K signaling is reminiscent of polarization in chemotaxing cells.Early in its development, a neuron extends long neurites tipped by dynamic, actin-rich growth cones. These neurites may become axons or dendrites, depending on the activity of an intrinsic neuronal polarity program that is regulated by the cytoskeleton, PAR polarity proteins, and lipid and protein kinases 1-7 . Although the ability to form long neurites is a defining feature of neurons, it is unclear what causes a neuron to break spherical symmetry and elongate processes. Studies in vitro have suggested that symmetry breaking is an active process that can be stimulated by neurotrophins or extracellular matrix components 6,8 , but little is known about the mechanisms that initiate and orient asymmetric growth in vivo 7 .Scattered studies of axonogenesis in vivo suggest that neurons usually extend their first stable process in the exact direction that will be taken by the eventual axon. Both the initial exploratory filopodia and the first microtubule-containing process of the grasshopper Ti1 neuron emerge from the cell body in the direction of its future axon 9 . Developing vertebrate retinal ganglion cells (RGCs) send their axons directly toward the optic nerve head, a trajectory marked before the growth cone forms by a small thickening of the RGC membrane 10 . These observations suggest that guidance information might direct the earliest stage of asymmetric neuronal growth.Axon formation has conceptual similarities to the polarization of cells during chemotaxis 11, 12 . In both cases, a symmetrical cell generates an asymmetric domain, the growth cone or

The Roles of Multiple UNC-40 (DCC) Receptor-Mediated Signals in Determining Neuronal Asymmetry Induced by the UNC-6 (Netrin) Ligand

“…It is notable that in the unc-6(ev400) and unc-6(rh46) mutants, UNC-40TGFP is not asymmetrically localized but is more equally distributed across the HSN surface ( Figure 3 and Adler et al 2006). In contrast, in the unc-40(ur304);unc-6(ev400) and unc-40(ur304);unc-6(rh46) mutants UNC-40TGFP becomes localized, albeit to random surfaces.…”

Laminin/β1 integrin signal triggers axon formation by promoting microtubule assembly and stabilization

“…In addition to the ECM, a number of extracellular axon-promoting factors have been identified, such as netrin [62], BDNF [14], Sema3a [63], Wnt [57], and TGFβ [15]. Some of these factors interact with integrin signaling during cell proliferation [64], adhesion and migration [65][66][67], neurite outgrowth, and growth cone guidance [68][69][70].…”

Membrane Organization and Dynamics in Cell Polarity

“…For example, PI(3,4,5)P 3 is localized to the leading edge of neutrophils and the growth cone of developing neurons (Servant et al 2000;Li et al 2003;Shi et al 2003;Aoki et al 2005). Deletion of PTEN in Caenorhabditis elegans or overexpression of PTEN in hippocampal neurons results in aberrant polarization (Jiang et al 2005;Adler et al 2006). Similarly, epithelial membrane asymmetry involves phosphoinositides and Cdc42.…”