Distinct roles for neuropilin1 and neuropilin2 during mouse corneal innervation.

Trigeminal sensory innervation of the cornea is critical for protection and synthesis of neuropeptides required for normal vision. Little is known about axon guidance during mammalian corneal innervation. In contrast to the chick where a pericorneal nerve ring forms via Npn/Sema signaling, mouse corneal axons project directly into the presumptive cornea without initial formation of an analogous nerve ring. Here we show that during development of the mouse cornea, Npn1 is strongly expressed by the trigeminal ganglion whereas Npn2 is expressed at low levels. At the same time Sema3A and Sema3F are expressed in distinct patterns in the ocular tissues. Npn1(sema-/-) mutant corneas become precociously and aberrantly innervated by nerve bundles that project further into the corneal stroma. In contrast, stromal innervation was not affected in Npn2(-/-) mutants. The corneal epithelium was prematurely innervated in both Npn1(sema-/-) and Npn2(-/-) mutants. These defects were exacerbated in Npn1(sema-/-);Npn2(-/-) double mutants, which in addition showed ectopic innervation of the region between the optic cup and lens vesicle. Collectively, our data show that Sema3A/Npn1 and Sema3F/Npn2 signaling play distinct roles and both are required for proper innervation of the mouse cornea.