Abstract：
Pax3 and Pax7 transcription factors are paralogs within the Pax gene family that that are expressed in early embryos in partially overlapping expression domains and have distinct functions. Significantly, mammalian development is largely unaffected by Pax7 systemic deletion but systemic Pax3 deletion results in defects in neural tube closure, neural crest emigration, cardiac outflow tract septation, muscle hypoplasia and in utero lethality by E14. However, we previously demonstrated that Pax3 hypomorphs expressing only 20% functional Pax3 protein levels exhibit normal neural tube and heart development, but myogenesis is selectively impaired. To determine why only some Pax3-expressing cell lineages are affected and to further titrate Pax3 threshold levels required for neural tube and heart development, we generated hypomorphs containing both a hypomorphic and a null Pax3 allele. This resulted in mutants only expressing 10% functional Pax3 protein with exacerbated neural tube, neural crest and muscle defects, but still a normal heart. To examine why the cardiac neural crest appears resistant to very low Pax3 levels, we examined its paralog Pax7. Significantly, Pax7 expression is both ectopically expressed in Pax3-expressing dorsal neural tube cells and is also upregulated in the Pax3-expressing lineages. To test whether this compensatory Pax7 expression is functional, we deleted Pax7 both systemically and lineage-specifically in hypomorphs expressing only 10% Pax3. Removal of one Pax7 allele resulted in partial outflow tract defects, and complete loss of Pax7 resulted in full penetrance outflow tract defects and in utero lethality. Moreover, combinatorial loss of Pax3 and Pax7 resulted in severe craniofacial defects and a total block of neural crest cell emigration from the neural tube. Pax7Cre lineage mapping revealed ectopic labeling of Pax3-derived neural crest tissues and within the outflow tract of the heart, experimentally confirming the observation of ectopic activation of Pax7 in 10% Pax3 hypomorphs. Finally, genetic cell ablation of Pax7Cre-marked cells is sufficient to cause outflow tract defects in hypomorphs expressing only 10% Pax3, confirming that ectopic and induced Pax7 can play an overlapping functional genetic compensational role in both cardiac neural crest lineage and during craniofacial development, which is normally masked by the dominant role of Pax3.
摘要：
Pax3和Pax7转录因子是Pax基因家族中的旁系同源物,其在早期胚胎中在部分重叠的表达结构域中表达并且具有不同的功能。重要的是,哺乳动物的发育在很大程度上不受Pax7系统性缺失的影响，但系统性Pax3缺失会导致神经管闭合缺陷，神经嵴迁移，心脏流出道间隔,E14引起的肌肉发育不全和子宫内致死性。然而,我们先前证明，仅表达20％功能性Pax3蛋白水平的Pax3次形态表现出正常的神经管和心脏发育，但是肌肉发生是选择性受损的。为了确定为什么只有一些表达Pax3的细胞系受到影响，并进一步滴定神经管和心脏发育所需的Pax3阈值水平，我们产生了同时包含低态和空Pax3等位基因的低态。这导致突变体仅表达10％的功能性Pax3蛋白，加剧了神经管，神经嵴和肌肉缺陷,但还是一颗正常的心.为了检查为什么心脏神经峰对非常低的Pax3水平有抵抗力，我们检查了它的paralogPax7。重要的是,Pax7表达既在表达Pax3的背侧神经管细胞中异位表达，也在表达Pax3的谱系中上调。为了测试这种补偿性Pax7表达是否有功能，我们在仅表达10％Pax3的次形态中系统和谱系特异性地删除了Pax7。去除一个Pax7等位基因导致部分流出道缺陷，Pax7的完全丧失导致完全外显流出道缺陷和子宫内致死性。此外，Pax3和Pax7的组合丢失导致严重的颅面缺损和神经c细胞从神经管迁移的完全阻滞。Pax7Cre谱系图显示了Pax3衍生的神经c组织和心脏流出道内的异位标记，实验证实了在10％Pax3副形态中观察到Pax7的异位激活。最后,Pax7Cre标记细胞的遗传细胞消融足以在仅表达10％Pax3的下形态中引起流出道缺陷，证实异位和诱导的Pax7可以在心脏神经rest谱系和颅面发育中发挥重叠的功能遗传补偿作用，这通常被Pax3的主导作用所掩盖。
