Jaskoll Tina, Witcher Dan, Toreno Leo, Bringas Pablo, Moon Anne M, Melnick Michael
Laboratory for Developmental Genetics, University of Southern California, Los Angeles, CA 90089-0641, USA.
Dev Biol. 2004 Apr 15;268(2):457-69. doi: 10.1016/j.ydbio.2004.01.004.
FGF8 has been shown to play important morphoregulatory roles during embryonic development. The observation that craniofacial, cardiovascular, pharyngeal, and neural phenotypes vary with Fgf8 gene dosage suggests that FGF8 signaling induces differences in downstream responses in a dose-dependent manner. In this study, we investigated if FGF8 plays a dose-dependent regulatory role during embryonic submandibular salivary gland (SMG) morphogenesis. We evaluated SMG phenotypes of Fgf8 hypomorphic mice, which have decreased Fgf8 gene function throughout embryogenesis. We also evaluated SMG phenotypes of Fgf8 conditional mutants in which Fgf8 function has been completely ablated in its expression domain in the first pharyngeal arch ectoderm from the time of arch formation. Fgf8 hypomorphs have hypoplastic SMGs, whereas conditional mutant SMGs exhibit ontogenic arrest followed by involution and are absent by E18.5. SMG aplasia in Fgf8 ectoderm conditional mutants indicates that FGF8 signaling is essential for the morphogenesis and survival of Pseudoglandular Stage and older SMGs. Equally important, the presence of an initial SMG bud in Fgf8 conditional mutants indicates that initial bud formation is FGF8 independent. Mice heterozygous for either the Fgf8 null allele (Fgf8(+/N)) or the hypomorphic allele (Fgf8(+/H)) have SMGs that are indistinguishable from wild-type (Fgf8(+/+)) mice which suggest that there is not only an FGF8 dose-dependent phenotypic response, but a nonlinear, threshold-like, epistatic response as well. We also found that enhanced FGF8 signaling induced, and abrogated FGF8 signaling decreased, SMG branching morphogenesis in vitro. Furthermore, since FGF10 and Shh expression is modulated by Fgf8 levels, we postulated that exogenous FGF10, Shh, or FGF10 + Shh peptide supplementation in vitro would largely "rescue" the abnormal SMG phenotype associated with decreased FGF8 signaling. This is as expected, though there is no synergistic effect with FGF10 + Shh peptide supplementation. These in vitro experiments model the principle that mutations have different effects in the context of different epigenotypes.
FGF8已被证明在胚胎发育过程中发挥重要的形态调节作用。颅面、心血管、咽部和神经表型随Fgf8基因剂量变化的观察结果表明，FGF8信号以剂量依赖的方式诱导下游反应的差异。在本研究中，我们调查了FGF8在胚胎下颌下唾液腺（SMG）形态发生过程中是否发挥剂量依赖性调节作用。我们评估了Fgf8低表达小鼠的SMG表型，这些小鼠在整个胚胎发育过程中Fgf8基因功能降低。我们还评估了Fgf8条件性突变体的SMG表型，其中Fgf8功能在第一鳃弓外胚层的表达域中从鳃弓形成时起就已完全缺失。Fgf8低表达小鼠的SMG发育不全，而条件性突变体的SMG表现出发育停滞，随后退化，在E18.5时消失。Fgf8外胚层条件性突变体中的SMG发育不全表明，FGF8信号对于假腺期及更晚期SMG的形态发生和存活至关重要。同样重要的是，Fgf8条件性突变体中存在初始SMG芽表明初始芽的形成不依赖FGF8。Fgf8无效等位基因（Fgf8(+/N)）或低表达等位基因（Fgf8(+/H)）的杂合小鼠的SMG与野生型（Fgf8(+/+)）小鼠无法区分，这表明不仅存在FGF8剂量依赖性表型反应，还存在非线性、阈值样的上位性反应。我们还发现，增强的FGF8信号诱导体外SMG分支形态发生，而消除FGF8信号则降低体外SMG分支形态发生。此外，由于FGF10和Shh的表达受Fgf8水平调节，我们推测体外补充外源性FGF10、Shh或FGF10 + Shh肽将在很大程度上“挽救”与FGF8信号降低相关的异常SMG表型。正如预期的那样，尽管FGF10 + Shh肽补充没有协同效应。这些体外实验模拟了这样一个原理，即突变在不同表观基因型的背景下具有不同的影响。
