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

Abstract

The polarization of post-mitotic neurons is poorly understood. Preexisting spatially asymmetric cues, distributed within the neuron or as extracellular gradients, could be required for neurons to polarize. Alternatively, neurons might have the intrinsic ability to polarize without any preestablished asymmetric cues. In Caenorhabditis elegans, the UNC-40 (DCC) receptor mediates responses to the extracellular UNC-6 (netrin) guidance cue. For the HSN neuron, an UNC-6 ventral-dorsal gradient asymmetrically localizes UNC-40 to the ventral HSN surface. There an axon forms, which is ventrally directed by UNC-6. In the absence of UNC-6, UNC-40 is equally distributed and the HSN axon travels anteriorly in response to other cues. However, we find that a single amino acid change in the UNC-40 ectodomain causes randomly oriented asymmetric UNC-40 localization and a wandering axon phenotype. With UNC-6, there is normal UNC-40 localization and axon migration. A single UNC-6 amino acid substitution enhances the mutant phenotypes, whereas UNC-6 second-site amino acid substitutions suppress the phenotypes. We propose that UNC-40 mediates multiple signals to polarize and orient asymmetry. One signal triggers the intrinsic ability of HSN to polarize and causes randomly oriented asymmetry. Concurrently, another signal biases the orientation of the asymmetry relative to the UNC-6 gradient. The UNC-40 ectodomain mutation activates the polarization signal, whereas different forms of the UNC-6 ligand produce UNC-40 conformational changes that allow or prohibit the orientation signal.

UNC-6/netrin and its receptors UNC-5 and UNC-40/DCC modulate growth cone protrusion in vivo in C. elegans

“…7G). These results are consistent with previous results showing that UNC-6 localizes UNC-40 to the ventral surface of the HSN and, in an unc-6 mutant, UNC-40 localizes uniformly around the cell body and protrusions extend from around the cell body (Xu et al, 2009). These results indicate that UNC-5, UNC-6 and UNC-40 control the polarity of filopodial protrusion from growth cones that are repelled (VDs) or attracted (HSNs) to UNC-6/netrin.…”

“…We next determined whether orientation of filopodial protrusion was altered in unc-5, unc-6 and unc-40 mutants. Previous studies indicated that UNC-6 was required for polarized distribution of UNC-40 and MIG-10/lamellipodin on the ventral region of the HSN neuron and that unc-6 and unc-40 mutant HSN neurons displayed a loss of polarized ventral protrusion (Adler et al, 2006;Quinn et al, 2006;Xu et al, 2009). To quantify polarity of filopodial protrusion, we divided the growth cones into four quadrants along the AP and DV axes, based upon the orientation of the animal, regardless of the direction that the growth cone was navigating (some VD growth cones in mutants were misguided and were migrating laterally or even ventrally; and many HSN growth cones protruded laterally rather than ventrally).…”

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

“…This model was proposed from genetic evidence that indicates that conformational changes to the UNC-40 receptor can trigger independent signals (Xu et al 2009). One signal causes the UNC-40 receptor to polarize to one side of the neuron, whereas another acts to orient the localization relative to the gradient of the UNC-6 ligand.…”

“…UNC-40 self-organizing polarization is a stochastic process (Xu et al 2009;Kulkarni et al 2013). The UNC-40 receptor will asymmetrically localize to one surface with some probability.…”

“…Although in some cases UNC-6 appears to trigger UNC-40 self-organizing polarization, the process does not require the UNC-6 ligand, and UNC-40 asymmetric localization does not require UNC-6 (Xu et al 2009;Kulkarni et al 2013). UNC-40-mediated signaling is also required for Q neuroblast migration, muscle arm extension, postsynaptic organization, anchor cell membrane extension, and the movement of intestinal cells (Honigberg and Kenyon 2000;Alexander et al 2009;Tu et al 2015;Asan et al 2016).…”