Plexin A3 and turnout regulate motor axonal branch morphogenesis in zebrafish

Area of Science:

• Developmental neurobiology
• Molecular genetics
• Cell biology

Background:

• Motor axons form complex branches at target muscles during embryogenesis.
• Mechanisms controlling axonal branching timing and location remain unclear.
• Previous studies lack detailed understanding of early axonal branching events.

Purpose of the Study:

• To investigate the timing and spatial determination of axonal branching morphogenesis.
• To identify genetic factors regulating motor axon branching.
• To elucidate the roles of turnout and plexin A3 in axonal branching.

Main Methods:

• Live cell imaging of zebrafish motor axons.
• Forward genetic screen to identify mutants.
• Molecular genetic mapping and sequence analysis.
• Chimeric analysis and single-cell labeling.
• Time-lapse analysis of axonal protrusions.

Main Results:

• First axonal branches form via growth cone bifurcation at the ventral myotome.
• Subsequent branches arise from collateral branching in the synaptic target field.
• Turnout mutants exhibit precocious collateral branching along the proximal axon shaft.
• Plexin A3 null mutants show identical branching defects.
• Mutant branching defects result from increased stability of axonal protrusions.

Conclusions:

• Turnout and plexin A3 are crucial for suppressing collateral branch formation before target field navigation.
• Turnout acts extrinsically, while plexin A3 acts intrinsically in motor axon guidance and branching.
• These mechanisms ensure precise spatial and temporal control of motor axon branching morphogenesis.