During brain development neurons send out axons, long thin processes. Axons grow towards a target area, where they eventually form synaptic connections with target cells to form functional neuronal circuit. Axons grow out in a defined order. The first ones are called “pioneers” and establish the main axon tracts. Later outgrowing follower axons can use the pioneer axon for navigation by adhering to them and extending along them. We are interested in identifying and studying the genes that control navigation of pioneers and followers.
We focus on axons navigating in the ventral nerve cord (VNC) of C. elegans. The VNC is the largest longitudinal axon bundle and contains interneuron and motor neuron axons that form the core of the motor circuit controlling movement of the animal. The order of axon outgrowth is known. The axon of the AVG neuron pioneer the right VNC axon tract and the PVPL axon pioneers the left axon tract. Motor neuron axons grow out next and finally interneuron axon extend into the VNC from the brain. We use genetic screens and a candidate gene approach to identify the genes controlling navigation of pioneer and follower axons in the VNC. Some recent projects are briefly described below.
Screens for axon guidance mutants
In genetic screens animals are mutagenized, which randomly destroys genes. Progeny with defective axon navigation can be identified in live animals when neurons are labelled with fluorescent markers like the green fluorescent protein (GFP). We collected a number of mutants in novel genes with defects ranging from subtle fasciculation defects within the ventral cord to severe defects affecting axon navigation in various parts of the nervous system (see Figure).
We recently identified COL-99, a transmembrane collagen, as important for VNC follower navigation, mostly notably interneuron axons and one class of motor neuron axons (Mol Cell Neurosci. 89:9-19 (2018)). col-99 is expressed in the epidermis, but likely cleaved from the cell surface. Independently, we identified two putative collagen receptors, ddr-1 and ddr-2 as having a similar role in axon guidance (Dev Biol. 374(1):142-52 (2013)). col-99 acts genetically in the same pathway as ddr-1 and ddr-2, which are expressed by neurons affected in col-99 mutants. This suggests that DDR-1 and DDR-2 may function as receptors for COL-99.
Pioneer and follower navigation
Pioneer axons provide crucial navigation information for later outgrowing axons. We and others found that in the absence of the AVG pioneer neuron, later outgrowing axons indeed commit navigation errors (see Figure), but they are not completely lost and many of them still find their way to their targets. In other words, pioneer neurons do not seem to be absolutely essential, but provide one (or more) of many different navigation cues for later outgrowing axons. We found that the flamingo-type cadherin FMI-1 is required for the correct navigation of follower axons, which normally extend along the axons of earlier outgrowing pioneers (Development 137(21):3663-73 (2010)). FMI-1 is expressed in both pioneer and follower axons, suggesting that it acts as adhesion molecule allowing the follower axon to adhere to the pioneer axon. We are currently characterizing a putative co-receptor of FMI-1 also involved in this process.
In genetic screens we recently identified a number of mutants required for AVG pioneer axon navigation. One of them is an allele of aex-3, which regulates vesicle trafficking in neurons. AEX-3 potentially is required for trafficking of the axon guidance receptor UNC-5 (Genetics 203(3):1235-47 (2016) .
