During development of the vertebrate forebrain, regional patterns of gene expression contribute to the functional differentiation of parallel sensory pathways. In a microarray screen comparing visual and auditory regions of the perinatal murine thalamus, two genes in the Zic family of transcription factors, Zic1 and Zic4, were identified as strongly enriched in the lateral geniculate nucleus (LGN), a relay and processing target for incoming retinal afferents. These results were confirmed with RT-PCR and in situ hybridization. In the mouse, Zic1 and Zic4 have been shown to be required for proper cerebellar and dorsal spinal cord morphology, while other members of the Zic family, Zic2 and Zic3, have been implicated in the axonal pathfinding of retinal ganglion cells. In order to correlate Zic1 and Zic4 to various time points in visual pathway development, we characterized Zic1 and Zic4 expression in the developing retina and thalamus at different ages. Both Zic1 and Zic4 are expressed in a nasal/low to temporal/high gradient in the P0 retina and in a ventral-lateral/low to dorsal-medial/high gradient in the LGN from E15.5 through P5. Zic4 expression in the LGN is absent by P16, while Zic1 expression decreases by P28 and persists at a low level into adulthood. To elucidate a possible role in functional differentiation of the LGN, we examined cell-type specificity of Zic1 and Zic4 expressing cells using glial, differentiated neuronal, and excitatory and inhibitory cell markers. In order to examine potential downstream pathways of Zic1 and Zic4 regulation, we performed a transcription factor binding site analysis probing for consensus Zic1 DNA binding sites among promoter regions of putative target genes. This analysis identified several Eph receptor genes as potential targets of Zic regulation. In situ hybridizations for these genes will be performed on thalamic sections of control and Zic1, Zic4 and Zic1/4 +/- and -/- mice to test for potential regulation by Zic1 or Zic4. Preliminary experiments using intraocular CTB injections to test for retinogeniculate patterning defects in control and Zic4 knock-out mice showed no gross differences. Further experiments to test the role of Zic1 and Zic4 in the patterning of geniculocortical projections will be pursued.

Supported by T32 EY13935-04