The expression and function of netrin-4 in murine ocular tissues

Netrin-4, a member of the netrin family, is a potent regulator of embryonic development. It promotes neurite extension and regulates pulmonary airway branching, vasculogenesis patterning, and endothelial proliferation in pathological angiogenesis. The initial characterization of netrin-4 expression was focused on epithelial-derived organs (kidney, lung and salivary gland) and the central nervous system. Ocular development is an ideal system to study netrin-4 expression and function, as it involves both ectodermal (cornea, lens and retina) and mesodermal (sclera and choroid) derivatives and has an extensive and well-characterized angiogenic process. Netrin-4 is expressed in all ocular tissues. It is a prominent component of the basement membranes of the lens and cornea, as well as all three basement membranes of the retina: the inner limiting membrane, vascular basement membranes, and Bruch's membrane. Netrin-4 is differentially deposited in vascular basement membranes, with more intense anti-netrin-4 reactivity on the arterial side. The retinal microcirculation also expresses netrin-4. In order to test the function of netrin-4 in vivo, we generated a conventional mouse lacking Ntn4 expression. Basement membrane formation in the cornea, lens and retina is undisrupted by netrin-4 deletion, demonstrating that netrin-4 is not a major structural component of these basement membranes. In the Ntn4 homozygous null (Ntn4-/-) cornea, the overall morphology of the cornea, as well as the epithelial, stromal and endothelial stratification are normal; however, epithelial cell proliferation is increased. In the Ntn4-/- retina, neurogenesis appears to proceed normally, as does retinal lamination. In the Ntn4-/- retina, retinal ganglion cell targeting is intact, although there are minor defects in axon fasciculation. In the retinal vasculature of the Ntn4-/- retina, the distribution patterns of astrocytes and the vasculature are largely normal, with the possible exception of increased branching in the deep capillary plexus, suggesting that netrin-4 may act as a negative regulator of angiogenesis. These data, taken together, suggest that netrin-4 is a negative regulator of corneal epithelial cell proliferation and retinal vascular branching in vivo, whereas netrin-4 may be redundant with other members of the netrin family in other ocular tissue development. Ntn4-/- mice may serve as a good model in which to study the role of netrins in vivo of the pathobiologic vascular remodeling in the retina and cornea.