Extension of theCaenorhabditis elegansPharyngeal M1 Neuron Axon Is Regulated by Multiple Mechanisms

Abstract

The guidance of axons to their correct targets is a critical step in development. The C. elegans pharynx presents an attractive system to study neuronal pathfinding in the context of a developing organ. The worm pharynx contains relatively few cells and cell types, but each cell has a known lineage and stereotyped developmental patterns. We found that extension of the M1 pharyngeal axon, which spans the entire length of the pharynx, occurs in two distinct phases. The first proximal phase does not require genes that function in axon extension (unc-34, unc-51, unc-115, and unc-119), whereas the second distal phase does use these genes and is guided in part by the adjacent g1P gland cell projection. unc-34, unc-51, and unc-115 had incompletely penetrant defects and appeared to act in conjunction with the g1P cell for distal outgrowth. Only unc-119 showed fully penetrant defects for the distal phase. Mutations affecting classical neuronal guidance cues (Netrin, Semaphorin, Slit/Robo, Ephrin) or adhesion molecules (cadherin, IgCAM) had, at best, weak effects on the M1 axon. None of the mutations we tested affected the proximal phase of M1 elongation. In a forward genetic screen, we isolated nine mutations in five genes, three of which are novel, showing defects in M1, including axon overextension, truncation, or ectopic branching. One of these mutations appeared to affect the generation or differentiation of the M1 neuron. We conclude that M1 axon extension is a robust process that is not completely dependent on any single guidance mechanism.

Identification of a Peptide Inhibitor of the RPM-1·FSN-1 Ubiquitin Ligase Complex

“…Studies in worms, flies, fish, and mice have identified roles for the PHR proteins in synapse formation (2)(3)(4)(5), axon guidance and extension (6 -11), and axon termination (3,(12)(13)(14).…”

Developmental genetics of the Caenorhabditis elegans pharynx

“…The proximal trajectory of the M2 axons projects straight from the cell body through the isthmus, and is impervious to mutations in growth cones or guidance cues; it must therefore be established in a growth-cone independent manner. 20 , 86 , 89 Consistently, microscopic monitoring of the M2 neuron shows that a growth cone appears only after the axon has elongated to span the nascent isthmus during early embryogenesis. This growth cone emerges within the developing metacorpus and therefore cannot participate in the elongation of the axon through the isthmus.…”

“…The developmental genetics of the M1 neuron has also been investigated in detail. 89 M1 is a unique neuron with a very long rather straight trajectory: its cell body is located within the posterior bulb and its axon extends a proximal trajectory through the isthmus, and a distal portion that projects from the metacorpus and all the way to the anterior end of the pharynx where it terminates into two short posteriorly directed branches ( Figure 1 (a)). M1 innervates the procorpus by forming synapses with the pm1, pm2, and pm3 muscle cells, while receiving input from interneurons in each of the three pharyngeal sectors.…”

“…The M1 straight proximal trajectory, just like that of the M2 neurons, is impervious to mutations in growth cones or guidance cues and must therefore be established in a growth-cone independent manner. 89 Also like the M2 neurons, the developing M1 axon uses an instructive interaction with a specific cell, the gland cell g1P, for the guidance of its growth cone-dependent distal end. 89 Thus, growth cone-independent development of axon trajectories through the isthmus followed by instructive cell–cell interactions in the metacorpus may be a general strategy for the development of many pharyngeal neurons.…”

“…This indicates that sax-3 is important to establish the anterior–posterior polarity of the NSM neurons and to specify their correct position within the primordium. It is interesting to note that while sax-3 provides anterior–posterior information to both the M2 and NSM neurons, these two neuron types interpret the signal to guide axons in opposite directions (M1 is not strongly affected by sax-3 mutation 89 ). Another difference between the M2 and NSM neurons concerns the effect of unc-119 : for M2 this mutation causes abnormally long axons and extra branching but for the NSMs it causes short axons and no excessive branching.…”