Dopamine Related Signaling Pathways on Generation of Projection Pattern at the Mouse Chiasm

Ocular albinism is an inherited genetic disorder characterized by lack of expression of the melanogenic enzymes in the eyes. Albinism may lead to underdevelopment of the fovea, reduced retinal cell numbers and misrouted ipsilateral axon projections at the optic chiasm. Tyrosinase, the rate-limiting enzyme in melanin synthesis, oxidizes L-tyrosine to L-DOPA. And then, L-DOPA is decarboxylated via aromatic L-amino acid decarboxylase (DDC) into dopamine. OA1 is the protein product of the ocular albinism type 1 gene, whose ligand L-DOPA is recently identified. The OA1 knock-out mice or mutations in the oa1 gene lead to reduced size of the uncrossed pathway and abnormality in melanosome distribution. Administration of L-DOPA to retinal cell cultures or in vitro whole ocular preparations results in reduction of mitosis in the albino mammals. A mutation of tyrosine hydroxylase produces a decrease in the number of uncrossed projection axons, while ectopic expression of tyrosine hydroxylase in mouse albino retinal pigment epithelial cells could not correct the uncrossed projection.;In this project, we will examine whether dopamine related signaling pathways are involved in the projection pattern at the mouse chiasm. And then we examine the possible function of OA1 and D1A receptors in the developing mouse optic pathway to search for novel strategies approaching visual dysfunctions, ocular albinism.;We have found most of L-DOPA positive cells are Vimentin positive Muller glia at E13. From E14, L-DOPA is expressed on retinal ganglion cells (RGCs). In E13-E15 retina, OA1 protein is mainly localized in retinal ganglion cells and Muller glial cells. In the ventral diencephalon, OA1 and L-DOPA are expressed strongly in the optic chiasm and optic tract, but weakly expressed in the optic stalk. From E13 to E15, dopamine and D1A staining is mainly expressed in the radially arranged cells and later to the neuronal population in the inner retinal layer. In the ventral diencephalon, dopamine and D1A are both strongly expressed in the optic chiasm, optic tract, and optic stalk.;Furthermore, we found that L-DOPA could significantly inhibit retinal axon outgrowth from both DN and VT retinal explants. Dopamine inhibits retinal axon outgrowth and this inhibition is abolished by the presence of D1A antagonist SCH23390.;Brain slice culture experiments show L-DOPA could significantly inhibit axon crossing at the optic chiasm of mouse embryos at E13, and this effect was abolished by dopamine, suggesting dopamine could function as an OA1 antagonist. At E15, L-DOPA had no effect on the uncrossed projection. The albino mice (ICR) showed an increase in crossed axons compared with C57 pigmented mice at E13, but there is no statistical difference. L-DOPA could also inhibit the axon crossing at the optic chiasm of albino mice. At E15, the albino mice show a reduced in ipsilateral retinal projections compared with normal C57 pigmented mice. L-DOPA and dopamine appear to increase the uncrossed projection in albino mice; however this increase has no statistically significance. We finally find transcription factor Zic2 could upregulate the expression of OA1.;As a conclusion, L-DOPA and dopamine are negative regulators to optic axon growth, probably through binding to OA1 and D1A, respectively. These dopamine signaling related molecules may play a role on axon crossing during early development of the pathway, but not on bilateral routing that develops at later stages of development.