Characterization of the Gbx1−/− Mouse Mutant: A Requirement for Gbx1 in Normal Locomotion and Sensorimotor Circuit Development

The Gbx class of homeobox genes encodes DNA binding transcription factors involved in regulation of embryonic central nervous system (CNS) development. Gbx1 is dynamically expressed within spinal neuron progenitor pools and becomes restricted to the dorsal mantle zone by embryonic day (E) 12.5. Here, we provide the first functional analysis of Gbx1. We generated mice containing a conditional Gbx1 allele in which exon 2 that contains the functional homeodomain is flanked with loxP sites (Gbx1flox); Cre-mediated recombination of this allele results in a Gbx1 null allele. In contrast to mice homozygous for a loss-of-function allele of Gbx2, mice homozygous for the Gbx1 null allele, Gbx1−/−, are viable and reproductively competent. However, Gbx1−/− mice display a gross locomotive defect that specifically affects hindlimb gait. Analysis of embryos homozygous for the Gbx1 null allele reveals disrupted assembly of the proprioceptive sensorimotor circuit within the spinal cord, and a reduction in ISL1+ ventral motor neurons. These data suggest a functional requirement for Gbx1 in normal development of the neural networks that contribute to locomotion. The generation of this null allele has enabled us to functionally characterize a novel role for Gbx1 in development of the spinal cord.

同源框基因（homeobox gene）家族中的Gbx亚家族，编码DNA结合转录因子，参与胚胎中枢神经系统（central nervous system，CNS）的发育调控。Gbx1在脊髓神经元祖细胞库中呈动态表达模式，并于胚胎第12.5天（embryonic day，E12.5）时局限于背侧套层区域。本研究首次开展了Gbx1的功能分析：我们构建了携带条件性Gbx1等位基因的小鼠，该等位基因中包含功能同源结构域的外显子2（exon 2）的两侧连有loxP位点（记为Gbx1flox）；经Cre重组酶介导的等位基因重组后，可获得Gbx1敲除等位基因。与携带Gbx2功能丧失等位基因的纯合小鼠不同，Gbx1敲除等位基因的纯合小鼠（Gbx1−/−）可正常存活并具备生殖能力。但Gbx1−/−小鼠存在显著的运动缺陷，该缺陷特异性影响其后肢步态。对Gbx1敲除等位基因纯合胚胎的分析显示，其脊髓内本体感觉感觉运动环路的组装过程受到破坏，且ISL1阳性腹侧运动神经元的数量减少。上述数据表明，Gbx1对于运动相关神经网络的正常发育具有功能性必要作用。该敲除等位基因的构建，使我们得以阐明Gbx1在脊髓发育中此前未被报道的全新功能。