Barhl2 Determines the Early Patterning of the Diencephalon by Regulating Shh

Abstract

Summary 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.

Expression of a Barhl1a reporter in subsets of retinal ganglion cells and commissural neurons of the developing zebrafish brain

“…Prior to Barhl1a reporter activity in the retina, Barhl1a:GFP positive cells can be seen starting from 10-14 somites in the forebrain, in locations corresponding to the previously reported expression of barhl1 mRNA in diencephalic domains of mammals, Xenopus and fish 1,3,4,19,35 (Fig. 5A, B Finally, imaging of the forebrain of Tg(barhl1a:GFP) embryos additionally revealed previously not reported barhl1a expression in few cells of the pituitary (Fig.…”

An Evolutionarily Conserved Network Mediates Development of the zona limitans intrathalamica, a Sonic Hedgehog-Secreting Caudal Forebrain Signaling Center

“…In X. laevis embryos depleted for Barhl2, development of the zli is abolished [ 12 ]. Similarly, in barhl2 −/− mice, shh expression within the zli is significantly reduced [ 67 , 71 ]. Barhl2-depleted X. laevis embryos resemble zebrafish Otx1l/2-depleted embryos: although most forebrain markers are unaltered, the embryos exhibit defects in shh expression in the zli and in the formation of the mid-diencephalic furrow [ 12 , 71 ].…”

“…Similarly, in barhl2 −/− mice, shh expression within the zli is significantly reduced [ 67 , 71 ]. Barhl2-depleted X. laevis embryos resemble zebrafish Otx1l/2-depleted embryos: although most forebrain markers are unaltered, the embryos exhibit defects in shh expression in the zli and in the formation of the mid-diencephalic furrow [ 12 , 71 ]. Noteworthy, in zebrafish loss of Otx2 induces a loss of barhl2 expression while otx2 expression is maintained in Barhl2-depleted X. laevis .…”

“…Indeed, depletion of otx genes impairs forebrain development (reviewed in [ 57 ]). In both frog and mouse, targeted-depletion of barhl2 generates thalamic developmental defects [ 12 , 71 ]. In barhl2 −/− mice, p2 thalamic progenitors acquire a pretectal fate and there is an absence of thalamo-cortical axon projections [ 71 ].…”

“…In both frog and mouse, targeted-depletion of barhl2 generates thalamic developmental defects [ 12 , 71 ]. In barhl2 −/− mice, p2 thalamic progenitors acquire a pretectal fate and there is an absence of thalamo-cortical axon projections [ 71 ]. Finally, irx gene depletion in frog, chicken, and zebrafish disrupts thalamus development [ 3 , 12 , 16 , 17 , 46 , 109 , 131 ].…”