Onuma Yasuko, Yeo Chang-Yeol, Whitman Malcolm
Department of Developmental Biology, Harvard School of Dental Medicine, 188 Longwood Avenue, Boston, MA 02115, USA.
Development. 2006 Jan;133(2):237-50. doi: 10.1242/dev.02188. Epub 2005 Dec 8.
Members of the EGF-CFC family facilitate signaling by a subset of TGFbeta superfamily ligands that includes the nodal-related factors and GDF1/VG1. Studies in mouse, zebrafish, and chick point to an essential role for EGF-CFC proteins in the action of nodal/GDF1 signals in the early establishment of the mesendoderm and later visceral left-right patterning. Antisense knockdown of the only known frog EGF-CFC factor (FRL1), however, has argued against an essential role for this factor in nodal/GDF1 signaling. To address this apparent paradox, we have identified two additional Xenopus EGF-CFC family members. The three Xenopus EGF-CFC factors show distinct patterns of expression. We have examined the role of XCR2, the only Xenopus EGF-CFC factor expressed in post-gastrula embryos, in embryogenesis. Antisense morpholino oligonucleotide-mediated depletion of XCR2 disrupts left-right asymmetry of the heart and gut. Although XCR2 is expressed bilaterally at neurula stage, XCR2 is required on the left side, but not the right side, for normal left-right patterning. Left-side expression of XNR1 in the lateral plate mesoderm depends on XCR2, whereas posterior bilateral expression of XNR1 does not, suggesting that distinct mechanisms maintain XNR1 expression in different regions of neurula-tailbud embryos. Ectopic XCR2 on the right side initiates premature right-side expression of XNR1 and XATV, and can reverse visceral patterning. This activity of XCR2 depends on its co-receptor function. These observations indicate that XCR2 has a crucial limiting role in maintaining a bistable asymmetry in nodal family signaling across the left-right axis.
EGF-CFC家族成员可促进转化生长因子β（TGFβ）超家族中一部分配体的信号传导，这些配体包括与节点相关的因子以及生长分化因子1（GDF1）/ Vg1。对小鼠、斑马鱼和鸡的研究表明，EGF-CFC蛋白在中内胚层早期形成以及后期内脏左右模式形成过程中，对于节点/GDF1信号的作用至关重要。然而，对唯一已知的蛙类EGF-CFC因子（FRL1）进行反义敲低实验，却对该因子在节点/GDF1信号传导中的关键作用提出了质疑。为了解决这一明显的矛盾，我们又鉴定出另外两个非洲爪蟾EGF-CFC家族成员。这三个非洲爪蟾EGF-CFC因子呈现出不同的表达模式。我们研究了XCR2（非洲爪蟾中唯一在原肠胚后期胚胎中表达的EGF-CFC因子）在胚胎发育中的作用。反义吗啉代寡核苷酸介导的XCR2缺失会破坏心脏和肠道的左右不对称性。尽管XCR2在神经胚阶段双侧表达，但正常的左右模式形成只需要左侧而非右侧表达XCR2。侧板中胚层中XNR1的左侧表达依赖于XCR2，而XNR1的后侧双侧表达则不依赖于XCR2，这表明在神经胚-尾芽胚胎的不同区域，维持XNR1表达的机制不同。右侧异位表达XCR2会引发XNR1和XATV的右侧过早表达，并可逆转内脏模式。XCR2的这种活性依赖于其共受体功能。这些观察结果表明，XCR2在维持节点家族信号在左右轴上的双稳态不对称性方面具有关键的限制作用。
