The Role of Bone Morphogenetic Proteins in Chick Eye Patterning and Axonal Pathfinding

Teri Belecky-Adams, Scott Carlson, Brandon Anderson, and Julie Whitsett

Department of Biology and the Center for Regenerative Biology and Medicine
Indiana University-Purdue University Indianapolis

Hypothesis: The topographical projection patterns from the retina to target tectal neurons are reliant upon the correct spatiotemporal patterning of retinal ganglion cells. Several secreted signaling factors are known to regulate patterning of the developing optic cup, including sonic hedgehog, retinoic acid, and bone morphogenetic proteins (BMPs). The spatiotemporal expression patterns of the various BMPs and their receptors have led us to hypothesize that while BMP4 is essential for determining the dorsal identity of retinal stem cells, BMP7 is, in conjunction with other factors, necessary for the ventral identity of retinal stem cells.

Materials and Methods: To address the above hypothesis, chick embryos were microinjected with chicken-specific retroviruses that expressed a control alkaline phosphatase (AP) or BMP binding proteins noggin or follistatin. Embryos were fixed and either reacted for wholemount immunocytochemistry or in situ hybridization, or were sectioned and immunolabeled.

Results: Embryos infected with the control AP showed normal dorso-ventral patterning as well as retinal ganglion cell axonal pathfinding. Embryos infected with noggin or follistatin both appeared to have enlarged optic nerve heads that encroached into the ventral optic cup accompanied by abnormalities in axonal pathfinding of various degrees. In addition to defects seen in the region of the optic nerve head, embryos infected with the noggin retrovirus also had many defects in the dorso-ventral patterning.

Conclusions: Noggin binds BMPs 2, 4 and 7, and follistatin binds BMPs 5-7, and activins. The only protein that is bound by both noggin and follistatin is BMP7, hence a comparison of the phenotypes obtained following the injection of each retrovirus should indicate the contribution of BMP7 to retinal patterning. Both the noggin and follistatin injected embryos shared defects in which optic nerve head and optic stalk patterning and ventral retina were defective, leading to enlarged optic stalk at the expense of ventral retina. These results are consistent with our hypothesis that BMP7 is involved in the patterning of the ventral optic cup. This work is supported by a March of Dimes Basil O’Connor award, the National Eye Institute, and the American Cancer Society.