Irx3 and Pax6 establish differential competence for Shh-mediated induction of GABAergic and glutamatergic neurons of the thalamus

Abstract

During embryonic development, the presumptive GABAergic rostral thalamus (rTh) and glutamatergic caudal thalamus (cTh) are induced by Sonic hedgehog (Shh) signaling from the zona limitans intrathalamica (ZLI) at the rostral border of the thalamic primordium. We found that these inductions are limited to the neuroepithelium between the ZLI and the forebrain-midbrain boundary, suggesting a prepattern that limits thalamic competence. We hypothesized that this prepattern is established by the overlapping expression of two transcription factors: Iroquois-related homeobox gene 3 (Irx3) posterior to the ZLI, and paired box gene 6 (Pax6) anterior to the forebrain-midbrain boundary. Consistent with this assumption, we show that misexpression of Irx3 in the prethalamus or telencephalon results in ectopic induction of thalamic markers in response to Shh, that it functions as a transcriptional repressor in this context, and that antagonizing its function in the diencephalon attenuates thalamic specification. Similarly, misexpression of Pax6 in the midbrain together with Shh pathway activation results in ectopic induction of cTh markers in clusters of cells that fail to integrate into tectal layers and of atypical long-range projections, whereas antagonizing Pax6 function in the thalamus disrupts cTh formation. However, rTh markers are negatively regulated by Pax6, which itself is down-regulated by Shh from the ZLI in this area. Our results demonstrate that the combinatorial expression of Irx3 and Pax6 endows cells with the competence for cTh formation, whereas Shh-mediated down-regulation of Pax6 is required for rTh formation. Thus, thalamus induction and patterning depends both on a prepattern of Irx3 and Pax6 expression that establishes differential cellular competence and on Shh signaling from the ZLI organizer.cell signaling | neural development

The Shark Alar Hypothalamus: Molecular Characterization of Prosomeric Subdivisions and Evolutionary Trends

“…Pax6 has been described in the alar hypothalamus of amniotes (Li et al, 1994; Stoykova and Gruss, 1994; Stoykova et al, 1996; Puelles et al, 2000; Moreno et al, 2012; Robertshaw et al, 2013 and their supplemental material) but not in Xenopus (Bachy et al, 2002; Moreno et al, 2008). The expression of Pax6 in shark (Ferreiro-Galve et al, 2008; present results) supports an early acquisition in the alar hypothalamus of basal gnathostomes.…”

“…Members of the neurogenin family have been reported in the forebrain of different vertebrate groups, including lampreys (Wullimann et al, 2005; Guérin et al, 2009; Nieber et al, 2009; Osório et al, 2010; Robertshaw et al, 2013). Neurog2 has been described in the pallium, prethalamic eminence and Pa territory of amniotes (see “supraoptic/paraventricular” and anterior hypothalamus in Medina et al, 2004; Osório et al, 2010; see also Puelles et al, 2012).…”

Crosstalk among electrical activity, trophic factors and morphogenetic proteins in the regulation of neurotransmitter phenotype specification

“…In the chick embryonic diencephalon, the thalamus develops into a rostral population of GABAergic neurons and a caudal region of glutamatergic thalamic neurons. These are patterned by different Shh levels through the regulatory control of Pax6 and Irx3 expression; Pax6 is necessary for the expression of the glutamatergic phenotype and inhibits the GABAergic phenotype (Robertshaw et al, 2013). In the dorsal spinal cord, Tlx1 and Tlx3, transcription factors upregulated by Wnt signaling (Kondo et al, 2011), act as selectors of the glutamatergic over the GABAergic phenotype in chick and mouse embryos (Cheng et al, 2004), while Lbx1 inverts the selection inducing the specification of the GABAergic phenotype in these spinal neurons (Cheng et al, 2005).…”

The conserved barH-like homeobox-2 gene barhl2 acts downstream of orthodentricle-2 and together with iroquois-3 in establishment of the caudal forebrain signaling center induced by Sonic Hedgehog

“…During the early steps of neural induction, an underlying prepattern emerges in the neural plate, partly encoded by transcription factors (TFs). These early patterning cues contribute to the specification of forebrain (diencephalon and telencephalon) territories and influence the way neighboring cell populations respond differentially to similar morphogens (Gomez-Skarmeta et al, 2003;Kobayashi et al, 2002;Robertshaw et al, 2013). Based on the expression patterns of TFs, the diencephalic primordium is divided into three transverse segments called prosomeres (p) that generate three distinct histogenic fields (reviewed in Puelles and Rubenstein, 2003): p3 corresponds to the prethalamus; p2 gives rise to the epithalamus and the thalamus; and p1 generates the presumptive pretectum (reviewed in Figdor and Stern, 1993;Martinez-Ferre and Martinez, 2012;Puelles and Rubenstein, 2003).…”