Optic nerve pathfinding is a useful model for investigating neural network formation in the central nervous system (CNS). Understanding the molecular mechanism underlying optic nerve pathfinding will lead to progress in regenerative
therapy for acquired CNS damage such as
glaucoma and
spinal cord injury in humans. To investigate why retinal ganglion cells extend their axons toward the brain, we focused on the role of
proteoglycans in optic nerve guidance. Immunohistochemical analyses showed intense upregulation in expression of
proteoglycans in the inner
retinal layers during eye development. We found that
proteoglycans inhibited neurite outgrowth of retinal ganglion cells in culture. Subsequently, we disrupted the gene for Ext 1, an essential
enzyme for
glycosaminoglycan synthesis of all the
heparan sulfate proteoglycans. The Ext 1 mutant mice in which Ext 1 was selectively disrupted in the CNS exhibited severe guidance errors in optic nerve and brain commissural axons when the axons crossed the midline. When the optic nerve crossed the midline at the chiasm, the vast majority of axons projected ectopically into the contralateral optic nerve. Generation of Slit2 and Ext 1 compound mutants caused disturbed activity of Slit
proteins,
heparin/
heparan sulfate-binding chemorepulsive guidance factors. The data suggest that
heparan sulfate proteoglycans in optic nerves probably modulate the activity of Slit during the optic chiasm formation. Therefore, to examine whether the interaction between
heparan sulfate and
heparin-binding molecules is also critical for other ocular developmental events, we selectively disrupted
heparan sulfate in the neural crest cells constituent of the anterior ocular segment in mice.
Heparan sulfate deficiency in neural crest cells caused anterior chamber angle dysgenesis, including corneal endothelium defects, corneal stroma hypoplasia, and iridocorneal dysgenesis. The anomalies are comparable to Peters anomaly, a type of developmental
glaucoma in humans. Loss of
heparan sulfate in neural crest cells disturbed TGFbeta2 signaling such as impaired TGFbeta2-dependent cell proliferation and reduced activity of TGFbeta2-downstream molecules. Furthermore, impaired interaction between
heparan sulfate and TGFbeta2 caused developmental
glaucoma, which was manifested as elevated intraocular pressure caused by iridocorneal angle dysgenesis. These developmental animal models revealed that
heparan sulfate proteoglycans have an essential role in regulation of
heparin-binding molecules in vivo.