Transcriptional control of early tract formation in the embryonic chick midbrain

Abstract

The earliest step in establishing the complex neuronal networks in the vertebrate brain is the formation of a scaffold of axon tracts. How the formation of the early axon scaffold is controlled at the molecular level is unclear. Forming part of the scaffold, neurons located at the ventral midbrain-forebrain border (MFB) give rise to the medial longitudinal fascicle(mlf) and the posterior commissure (pc). We demonstrate that the homeobox genes Sax1, Six3, Emx2 and Pax6 are expressed in distinct domains in this area, suggesting that the specification of mlf and pc neurons might be controlled by the combinatorial activity of these transcription factors. We have tested this hypothesis by analysing the function of Sax1 in the embryonic chick brain. Gain-of-function experiments with Sax1 result in alterations to the early axon scaffold, most prominently an enlargement of the mlf at the expense of the pc. Ectopic expression of Sax1 also affects the expression of other ventral homeobox genes, particularly Six3 and Emx2. Our results indicate that the specification of neurons forming the early axon scaffold is governed by a homeobox code, thus resembling the mechanism of neuronal specification in the spinal cord.

PAX6 Immunoreactivity in the Diencephalon and Midbrain of <i>Alligator</i> during Early Development

“…6 A and 7 A), shark [Ferreiro-Galve et al, 2008], zebrafish [Wullimann and Rink, 2001], chick [Agarwala et al, 2001;Sanders et al, 2002;Schubert and Lumsden, 2005;Ferran et al, 2007Ferran et al, , 2008 and mouse [Stoykova and Gruss, 1994]. Of these studies, only the observations in chick [Schubert and Lumsden, 2005;Ferran et al, 2007] examined a time period that overlapped the developmental interval of the present observations.…”

“…PAX6 was chosen for basal plate identification during this time period because it was expressed in the basal plate of the most caudal part of the chick diencephalon [Sanders et al, 2002;Schubert and Lumsden, 2005;Ferran et al, 2007] over a time period that partly overlapped that which we investigated in Alligator . In the process of this analysis, additional data on PAX6 expression were documented for alar areas of the diencephalon and for the midbrain.…”

Expression of chick Fgf19 and mouse Fgf15 orthologs is regulated in the developing brain by Fgf8 and Shh

“…This parabasal region seems further characterized by the expression of Pax6, Mgn, Mash, Sulf1, Olig2, and PLP-Dm20 in the neuroepithelium (Figs. 4, 7), as well as by acetylcholinesterase histochemistry and by the expression of other homeobox genes (Sanders et al, 2002;Schubert and Lumsden, 2005). Some of these Fgf19-expressing cells coexpress Nkx6.1, which has been proposed as a marker of red nucleus cells (Barth and Wilson, 1995;Puelles et al, 2004).…”

Early expression of axon guidance molecules in the embryonic chick mesencephalon and pretectum

“…In situ hybridisation and immunohistochemistry followed previously described procedures (Schubert and Lumsden, 2005). Briefly, following bleaching and detergent treatment the embryos were prehybridised in hybridisation buffer (50% formamide/5x SSC/2% SDS/2% Boehringer blocking reagent/250 μg*ml -1 RNA/100 μg*ml -1 heparin), before adding the Digoxigenin-labelled probe.…”

“…The expression of Unc5H3 overlaps the nMLF (Fig. 6D), but the ventrally located neurons projecting into the tract of the PC are intermingled with the MLF neurons (Schubert and Lumsden, 2005), and thus might also express this receptor. Hence, repulsion by Netrin2 could be involved in the positioning of the PC to the caudal pretectum.…”