Section on Molecular Morphogenesis, Program in Cellular Regulation and Metabolism (PCRM), Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), National Institutes of Health (NIH), Bethesda, Maryland, United States of America.
PLoS One. 2012;7(5):e36707. doi: 10.1371/journal.pone.0036707. Epub 2012 May 31.
Frog metamorphosis is totally dependent on thyroid hormone (T3) and mimics the postembryonic period around birth in mammals. It is an excellent model to study the molecular basis of postembryonic development in vertebrate. We and others have shown that many, if not all, matrix metalloproteinases (MMPs), which cleave proteins of the extracellular matrix as well as other substrates, are induced by T3 and important for metamorphosis. MMP activity can be inhibited by tissue inhibitors of metalloproteinase (TIMPs). There are 4 TIMPs in vertebrates and their roles in postembryonic development are poorly studied.
METHODOLOGY/PRINCIPAL FINDINGS: We analyzed the TIMP2 genes in Xenopus laevis and the highly related species Xenopus tropicalis and discovered that TIMP2 is a single copy gene in Xenopus tropicalis as in mammals but is duplicated in Xenopus laevis. Furthermore, the TIMP2 locus in Xenopus tropicalis genome is different from that in human, suggesting an evolutionary reorganization of the locus. More importantly, we found that the duplicated TIMP2 genes were similarly regulated in the developing limb, remodeling intestine, resorbing tail during metamorphosis. Unexpectedly, like its MMP target genes, the TIMP2 genes were upregulated by T3 during both natural and T3-induced metamorphosis.
CONCLUSIONS/SIGNIFICANCE: Our results indicate that TIMP2 is highly conserved among vertebrates and that the TIMP2 locus underwent a chromosomal reorganization during evolution. Furthermore, the unexpected upregulation of TIMP2 genes during metamorphosis suggests that proper balance of MMP activity is important for metamorphosis.
青蛙变态完全依赖于甲状腺激素（T3），模拟了哺乳动物出生后的胚胎后时期。它是研究脊椎动物胚胎后发育分子基础的极佳模型。我们和其他人已经表明，许多（如果不是全部）基质金属蛋白酶（MMPs），它们可以切割细胞外基质的蛋白质以及其他底物，被 T3 诱导，对变态很重要。MMP 活性可以被金属蛋白酶组织抑制剂（TIMPs）抑制。脊椎动物中有 4 种 TIMPs，但其在胚胎后发育中的作用研究甚少。
方法/主要发现：我们分析了非洲爪蟾（Xenopus laevis）和高度相关的物种非洲爪蟾（Xenopus tropicalis）的 TIMP2 基因，发现 TIMP2 在非洲爪蟾中是一个单拷贝基因，就像在哺乳动物中一样，但在非洲爪蟾中是复制的。此外，非洲爪蟾基因组中的 TIMP2 基因座与人类的不同，这表明该基因座在进化上发生了重新组织。更重要的是，我们发现复制的 TIMP2 基因在发育中的肢体、重塑的肠道和变态过程中尾巴的吸收中以类似的方式被调节。出乎意料的是，与它的 MMP 靶基因一样，TIMP2 基因在自然和 T3 诱导的变态过程中均被 T3 上调。
结论/意义：我们的研究结果表明，TIMP2 在脊椎动物中高度保守，并且在进化过程中 TIMP2 基因座发生了染色体重排。此外，TIMP2 基因在变态过程中的意外上调表明，MMP 活性的适当平衡对变态很重要。
