<i>Gbx1</i> and <i>Gbx2</i> Are Essential for Normal Patterning and Development of Interneurons and Motor Neurons in the Embryonic Spinal Cord

The molecular mechanisms regulating neurogenesis involve the control of gene expression by transcription factors. <i>Gbx1</i> and <i>Gbx2</i>, two members of the Gbx family of homeodomain-containing transcription factors, are known for their essential roles in central nervous system development. The expression domains of mouse <i>Gbx1</i> and <i>Gbx2</i> include regions of the forebrain, anterior hindbrain, and spinal cord. In the spinal cord, <i>Gbx1</i> and <i>Gbx2</i> are expressed in PAX2⁺ interneurons of the dorsal horn and ventral motor neuron progenitors. Based on their shared domains of expression and instances of overlap, we investigated the functional relationship between <i>Gbx</i> family members in the developing spinal cord using <i>Gbx1</i>^−/−, <i>Gbx2</i>^−/−, and <i>Gbx1</i>^−/−/<i>Gbx2</i>^−/− embryos. In situ hybridization analyses of embryonic spinal cords show upregulation of <i>Gbx2</i> expression in <i>Gbx1</i>^−/− embryos and upregulation of <i>Gbx1</i> expression in <i>Gbx2</i>^−/− embryos. Additionally, our data demonstrate that <i>Gbx</i> genes regulate development of a subset of PAX2⁺ dorsal inhibitory interneurons. While we observe no difference in overall proliferative status of the developing ependymal layer, expansion of proliferative cells into the anatomically defined mantle zone occurs in <i>Gbx</i> mutants. Lastly, our data shows a marked increase in apoptotic cell death in the ventral spinal cord of <i>Gbx</i> mutants during mid-embryonic stages. While our studies reveal that both members of the <i>Gbx</i> gene family are involved in development of subsets of PAX2⁺ dorsal interneurons and survival of ventral motor neurons, <i>Gbx1</i> and <i>Gbx2</i> are not sufficient to genetically compensate for the loss of one another. Thus, our studies provide novel insight to the relationship harbored between <i>Gbx1</i> and <i>Gbx2</i> in spinal cord development.