The go-between : a Sox2-regulated gene expression program in relay neurons of the visual thalamus as a possible contributor to inherited vision disease

The visual thalamus (dorsolateral geniculate nucleus, dLGN) receives visual input from the retina via the optic nerve, and in turn connects, via its relay neurons, with the visual cerebral cortex. The development of vision requires the precise assembly of a correct neuronal connectivity between retina and brain, with the dLGN as a key intermediary. dLGN disfunction can thus contribute to visual impairment. Heterozygous mutations in the human SOX2 gene, encoding a transcription factor, lead to severe eye and vision defects. By Cre-mediated, thalamus-specific conditional deletion of Sox2 in the mouse, we uncovered a novel requirement for Sox2 in the development of the dLGN and its correct connectivity with the retina and cortex; unexpectedly, in the dLGN, Sox2 (well-known as a “stem cell gene”) is expressed and required within differentiated projection neurons (Mercurio et al, iScience 2019, 15:257). We undertook a search for genes differentially expressed in the dLGN of thalamic Sox2 mutants, versus controls, by RNAseq, and identified several hundreds of genes downregulated, or upregulated, in Sox2 mutants. We similarly identified genes differentially expressed, in the dLGN, following thalamic deletion of Nr2f1, a gene whose mutation in man causes vision defects in Bosch-Boonstra-Schaaf syndrome, and whose thalamic deletion in mouse causes dLGN defects similar to those caused by Sox2 deletion. We found a significant overlap between genes down- or up-regulated in the dLGN of Sox2 and Nr2f1 mutants, pointing to a gene regulatory network controlled by both transcription factors. By the sensitive CUT&RUN technique, we determined a genome-wide binding profile for SOX2 in the dLGN, that we are now correlating with the RNAseq data to identify genes directly regulated by SOX2. To this end, we are taking advantage of genome-wide maps of long-range interactions we identified in neural cells chromatin, connecting gene promoters with distant enhancers; here, SOX2 preferentially binds to distant enhancers, to control the maintenance and function of this interaction network (Bertolini et al, Cell Stem Cell 2019, 24:462). Gene products differentially expressed in the dLGN of our mutants are enriched in signaling molecules and receptors involved in axon guidance, as well as synapsis development and function. They thus provide an opportunity to shed light on the mechanisms of eye-brain connectivity development, and possibly of its amelioration in a therapy perspective.