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First published online 15 August 2007
doi: 10.1242/dev.004267

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Novel mutations affecting axon guidance in zebrafish and a role for plexin signalling in the guidance of trigeminal and facial nerve axons

Hideomi Tanaka^1,2,*, Ryu Maeda^1,2,, Wataru Shoji^2,3, Hironori Wada^1,, Ichiro Masai^4,, Toshiyuki Shiraki^1,5, Megumi Kobayashi^1,2, Ryoko Nakayama^1,5 and Hitoshi Okamoto^1,2,¶

¹ Laboratory for Developmental Gene Regulation, Brain Science Institute, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
² Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Corporation (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan.
³ Department of Cell Biology, Institute of Development, Aging and Cancer, Tohoku University, Sendai, 980-8575, Japan.
⁴ Masai Initiative Research Unit, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan.
⁵ Research Resources Center, Brain Science Institute, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama, 351-0198, Japan.

^¶ Author for correspondence (e-mail: hitoshi{at}brain.riken.jp)

Accepted 12 June 2007

In zebrafish embryos, the axons of the posterior trigeminal (Vp) and facial (VII) motoneurons project stereotypically to a small number of target muscles derived from the first and second branchial arches (BA1, BA2). Use of the Islet1 (Isl1)-GFP transgenic line enabled precise real-time observations of the growth cone behaviour of the Vp and VII motoneurons within BA1 and BA2. Screening for N-ethyl-N-nitrosourea-induced mutants identified seven distinct mutations affecting different steps in the axonal pathfinding of these motoneurons. The class 1 mutations caused severe defasciculation and abnormal pathfinding in both Vp and VII motor axons before they reached their target muscles in BA1. The class 2 mutations caused impaired axonal outgrowth of the Vp motoneurons at the BA1-BA2 boundary. The class 3 mutation caused impaired axonal outgrowth of the Vp motoneurons within the target muscles derived from BA1 and BA2. The class 4 mutation caused retraction of the Vp motor axons in BA1 and abnormal invasion of the VII motor axons in BA1 beyond the BA1-BA2 boundary. Time-lapse observations of the class 1 mutant, vermicelli (vmc), which has a defect in the plexin A3 (plxna3) gene, revealed that Plxna3 acts with its ligand Sema3a1 for fasciculation and correct target selection of the Vp and VII motor axons after separation from the common pathways shared with the sensory axons in BA1 and BA2, and for the proper exit and outgrowth of the axons of the primary motoneurons from the spinal cord.

Key words: Zebrafish, Trigeminal motoneuron, Facial motoneuron, Plexin A3 mutant, Axon pathfinding

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