Title

Distinct Regulatory Functions of Gbx2 and FGF Signaling in Midbrain-Hindbrain Lineage Restriction Border Formation and Stability

Date of Completion

January 2012

Keywords

Biology, Genetics|Biology, Neurobiology|Health Sciences, Human Development

Degree

Ph.D.

Abstract

The prospective midbrain-hindbrain border (MHB) in the developing vertebrate brain is demarcated at the end of gastrulation by the common expression border of Gbx2 and Otx2, two homeobox genes. Researchers have shown that a compartment boundary exists at the MHB, but the precise timing and mechanism of its formation are still unclear. By genetic inducible fate-mapping in a Gbx2CreER knock-in mouse line, we demonstrate that descendants of Gbx2+ cells as early as embryonic day (E) 7.5 do not cross the MHB. Gbx2 is required to specify hindbrain fate. In its absence, hindbrain lineage cells abnormally populate the midbrain. In chimaeric embryos that mimic the nascent brain, Gbx2+ and Otx2+ cells segregate from each other, suggesting that the differential expression of these two genes leads to cell sorting, which initiates MHB formation. Fgf8, the midbrain-hindbrain organiser gene, is required to maintain the lineage-restriction boundary after E7.5. In our studies, even partial deletion of Fgf8 impairs lineage separation. Activation of FGF pathways has a cell-autonomous effect on cell sorting in midbrain progenitor cells. Our findings suggest that Fgf8-activated FGF signalling induces distinct cell surface properties or cell-cell communication, which is an important step in restricting cell movement across the MHB. Overall, our work highlights the distinct temporal and spatial requirements for Gbx2 and Fgf8 in establishing a lineage restriction border at the MHB and reveals a new role for Fgf8 in maintaining the MHB lineage border. ^