UNC-119 suppresses axon branching

Typically, mature neurons are polarized cells composed of a dendritic arbor and a single axon (reviewed by Craig and Banker, 1994; Higgins et al., 1997). This polarity becomes apparent early during development when the neuron extends a growth cone to form a single unbranched axon (reviewed by Goodman, 1996; Goodman and Tessier-Lavigne, 1997; Mueller, 1999). In some neuronal populations collateral branches project from axons after the growth cone has reached its primary target (O’Leary and Terashima, 1988; O’Leary, 1992; Bastmeyer and O’Leary, 1996). The growth cones of other neurons bifurcate during extension (Bray, 1973; Bunge, 1973). Once the growth cone reaches its target, functional synapses are established at the tip of the axon (reviewed by Sanes and Scheller, 1997; Sanes and Lichtman, 1999) and the differentiated neuron stops growing. The stability of the differentiated nervous system can be perturbed by nerve injury or disease, resulting in abnormal axon branching (Lankford et al., 1998; Stoll and Muller, 1999). Similarly, the induction of epileptic seizures in experimental animals results in overproduction of axon collateral sprouts (reviewed by McNamara, 1999). While a variety of molecules stimulate branching during regeneration (Ernfors et al., 1991; Bendotti et al., 1993; Aguayo et al., 1996; Caroni, 1998), the molecular mechanisms inhibiting branching are largely unknown. We have discovered that a new molecule, UNC-119, suppresses abnormal axon branching. unc-119 was originally identified as a mutation affecting nematode locomotion (Maduro and Pilgrim, 1995). The unc-119 transcript is expressed primarily in neurons early in development and throughout adulthood (Maduro and Pilgrim, 1995). UNC-119 does not contain any well-defined structural motifs, although several proteins that are similar to UNC-119 have been identified in C. elegans, Drosophila melanogaster and vertebrates (Maduro et al., 2000). Two related vertebrate proteins, HRG4 (human UNC119) and RRG4 (rat UNC119), were recently identified based on their high level of gene expression in the retina (Higashide, 1996; Swanson et al., 1998). Specifically, these proteins are localized to the presynaptic zone of the retinal ribbon synapses (Higashide, 1998). The human homolog, HRG4 was recently mapped to 17q11.2 (Swanson et al., 1998; Higashide and Inana, 1999). The function of the UNC-119 protein family members has not yet been determined. Here we demonstrate that GABA motor neuron axons are severely branched in adult unc-119 mutants and synapses are inappropriately localized. Axon branching in unc-119 mutants could result from defects that occur before, during or after outgrowth. Time-lapse analyses of unc-119 growth cones demonstrated that, despite timing defects, the behaviors exhibited by GABA motor neuron axons during migration are normal. Instead, motor axon branching occurred after axon outgrowth and was the result of supernumerary growth cone activity. Transient expression of UNC-119 protein after outgrowth rescued the unc-119 phenotype. Together these results suggest that the UNC-119 protein maintains the differentiated morphology of the neuron by suppressing supernumerary axon branching and restricting the distribution of synapses. 4079 Development 128, 4079-4092 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV8759