To navigate successfully to their targets, developing axons must respond to relevant guidance signals in their environment, while ignoring competing and potentially distracting signals. Bonanomi et al. now show that an intracellular signalling molecule, p190RhoGAP (p190), acts as a context-dependent filter to suppress responses to inappropriate guidance cues in mouse motor neurons.
During their development, motor neuron axons travel from the ventral spinal cord to the ventral root exit point, cross the basal lamina and finally navigate to their target muscles. Throughout their journey, the axons must rapidly modulate their responsiveness to various guidance cues to allow the correct pathfinding ‘decisions’ to be made. Adding to the complexity of this challenge, the route followed by the neurons overlaps with and/or crosses the routes of axons that have very different final destinations (and thus respond to different sets of guidance cues). Thus, pathfinding axons must employ mechanisms to avoid erroneous responses to inappropriate cues.
To identify the molecules involved in such a control system, the authors conducted a forward genetic screen for mutations that produce defects in motor axon guidance in mice. They identified a mutation that mapped to the gene that encodes p190 and caused motor axon guidance defects at a subset of choice points encountered during pathfinding. Mice carrying the mutation or in which the gene encoding p190 was knocked out exhibited disrupted exiting at the ventral root exit point (with misrouted axons projecting close to the pial surface) and abnormal muscle targeting, indicating a cell-intrinsic function for p190 in these two important stages of motor axon pathfinding.
p190 is involved in the intracellular signalling pathways downstream of many axon guidance receptors. The classical guidance molecule netrin 1 is known to be involved in axon pathfinding in spinal motor neurons and the authors observed a colocalization of p190 and the netrin receptor DCC in motor axon growth cones. In an explant assay, they found that motor axons lacking p190 grew towards a source of netrin 1, whereas those taken from control mice were unresponsive to netrin 1.
This finding suggested that p190 silences netrin 1 attraction in motor neurons to ensure accurate axon pathfinding. The authors hypothesized that the loss of p190 may cause motor axons to be inappropriately attracted to netrin 1 expressed at the pial surface (where it acts to guide commissural axons) and thus disrupt spinal cord exiting. In support of this idea, mice lacking both p190 and netrin 1 exhibited normal spinal exit. The distal muscle-targeting deficits observed in mice lacking p190 were not rescued by the removal of netrin 1, however, indicating that an alternative function of p190 is involved in this stage of pathfinding. Interestingly, in vivo structure —function analyses revealed that, whereas guidance within the limb was dependent on p190’s classical RhoGAP function, a ‘non-canonical’ RhoGAP-independent activity of p190 regulated motor axon exiting.
Next, the authors considered the mechanism by which p190 might silence netrin 1 attraction during spinal exit. They identified a specific interaction between p190 and the intracellular domain of DCC that restricts the recruitment of the molecular effectors of DCC signalling to the receptor complex, suggesting that p190 might act to prevent signalling downstream of netrin 1–DCC binding.
“pathfinding axons must employ mechanisms to avoid erroneous responses to inappropriate cues”
This study highlights the importance of regulatory mechanisms that suppress responses to irrelevant signals, or ‘noise’, in a cell’s environment. In the case of axon pathfinding, such suppression must be temporary and reversible, to allow the responsiveness of axons to different cues to change as they progress toward their destination; further work is required to identify the mechanisms that gate p190’s effects in a time and context-dependent manner.
Bonanomi, D. et al. p190RhoGAP filters competing signals to resolve axon guidance conflicts. Neuron https://doi.org/10.1016/j.neuron.2019.02.034 (2019)