Matrix metalloproteinases are required for retinal ganglion cell axon guidance at select decision points

Axons receive guidance information from extrinsic cues in their environment in order to reach their targets. In the frog Xenopus laevis, retinal ganglion cell (RGC) axons make three key guidance decisions en route through the brain. First, they cross to the contralateral side of the brain at the optic chiasm. Second, they turn caudally in the mid-diencephalon. Finally, they must recognize the optic tectum as their target. The matrix metalloproteinase (MMP) and a disintegrin and metalloproteinase (ADAM) families are zinc (Zn)-dependent proteolytic enzymes. The latter functions in axon guidance, but a similar role has not yet been identified for the MMP family. Our previous work implicated metalloproteinases in the guidance decisions made by Xenopus RGC axons. To test specifically the importance of MMPs, we used two different in vivo exposed brain preparations in which RGC axons were exposed to an MMP-specific pharmacological inhibitor (SB-3CT), either as they reached the optic chiasm or as they extended through the diencephalon en route to the optic tectum. Interestingly, SB-3CT affected only two of the guidance decisions, with misrouting defects at the optic chiasm and tectum. Only at higher concentrations was RGC axon extension also impaired. These data implicate MMPs in the guidance of vertebrate axons, and suggest that different metalloproteinases function to regulate axon behaviour at distinct choice points: an MMP is important in guidance at the optic chiasm and the target, while either a different MMP or an ADAM is required for axons to make the turn in the mid-diencephalon.