Barhl2 Determines the Early Patterning of the Diencephalon by Regulating Shh

The diencephalon is the primary relay network transmitting sensory information to the anterior forebrain. During development, distinct progenitor domains in the diencephalon give rise to the pretectum (p1), the thalamus and epithalamus (p2), and the prethalamus (p3), respectively. Shh plays a significant role in establishing the progenitor domains. However, the upstream events influencing the expression of Shh are largely unknown. Here, we show that Barhl2 homeobox gene is expressed in the p1 and p2 progenitor domains and the in zona limitans intrathalamica (ZLI) and regulates the acquisition of identity of progenitor cells in the developing diencephalon. Targeted deletion of Barhl2 results in the ablation of Shh expression in the dorsal portion of ZLI and causes thalamic p2 progenitors to take the fate of p1 progenitors and form pretectal neurons. Moreover, loss of Barhl2 leads to the absence of thalamocortical axon projections, the loss of habenular afferents and efferents, and a gross diminution of the pineal gland. Thus, by acting upstream of Shh signaling pathway, Barhl2 plays a crucial role in patterning the progenitor domains and establishing the positional identities of progenitor cells in the diencephalon.