Schweitzer, J., Löhr, H., Bonkowsky, J.L., Hübscher, K., and Driever, W. (2013) Sim1a and Arnt2 contribute to hypothalamo-spinal axon guidance by regulating Robo2 activity via a Robo3-dependent mechanism. Development (Cambridge, England). 140(1):93-106.
Precise spatiotemporal control of axon guidance factor expression is a prerequisite for formation of functional neuronal connections.
Although Netrin/Dcc- and Robo/Slit-mediated attractive and repulsive guidance of commissural axons have been extensively studied,
little is known about mechanisms controlling mediolateral positioning of longitudinal axons in vertebrates. Here, we use a
genetic approach in zebrafish embryos to study pathfinding mechanisms of dopaminergic and neuroendocrine longitudinal axons
projecting from the hypothalamus into hindbrain and spinal cord. The transcription factors Sim1a and Arnt2 contribute to differentiation
of a defined population of dopaminergic and neuroendocrine neurons. We show that both factors also control aspects of axon
guidance: Sim1a or Arnt2 depletion results in displacement of hypothalamo-spinal longitudinal axons towards the midline. This
phenotype is suppressed in robo3 guidance receptor mutant embryos. In the absence of Sim1a and Arnt2, expression of the robo3 splice isoform robo3a.1 is increased in the hypothalamus, indicating negative control of robo3a.1 transcription by these factors. We further provide evidence that increased Robo3a.1 levels interfere with Robo2-mediated
repulsive axon guidance. Finally, we show that the N-terminal domain unique to Robo3a.1 mediates the block of Robo2 repulsive
activity. Therefore, Sim1a and Arnt2 contribute to control of lateral positioning of longitudinal hypothalamic-spinal axons
by negative regulation of robo3a.1 expression, which in turn attenuates the repulsive activity of Robo2.