Functions of Intracellular Regulators of Notch Signaling

Mastermind （Mam）是Notch信号的进化保守元件之一。果蝇的遗传分析暗示它是该途径的一个重要的正调节因子。我们鉴定了哺乳动物Mam蛋白家族的3个成员（Mam-1、Mam-2和Mam-3），并阐明了Mam蛋白的生化作用机制。所有三种人类和果蝇的Mam稳定并参与CSL DNA结合蛋白的DNA结合复合体和Notch细胞内结构域，作为信号传导的中间体。所有的Mam蛋白都通过Notch信号增强了靶启动子的转录激活。为了阐明Mam在哺乳动物体内的功能，我们通过同源重组产生了缺乏Mam-1和Mam-2的小鼠品系。mam -1缺陷小鼠表现为生长迟缓。这种表型在围产期很明显，并在产后加剧。这些小鼠在断奶前就死亡了，这表明Mam-1具有非冗余功能，是一个必不可少的基因。相反，我们没有发现mam -2缺陷小鼠的主要发育缺陷。此外，mam -2缺陷小鼠的造血系统包括T淋巴细胞和B淋巴细胞均未发现缺陷。这些结果表明，Mam基因座在脊椎动物系统中具有独特的功能。先天性糖基化失调（cdgiic）是一种以生长迟缓、智力迟钝和严重免疫缺陷为特征的隐性综合征。最近，研究人员发现负责cdgiic的基因编码一种聚焦于GDP的转运蛋白。众所周知，GDP聚焦对于n链聚糖的聚焦化和O-聚焦化是必不可少的，这两种聚焦修饰都存在于Notch上。我们发现哺乳动物Gfr是哺乳动物培养细胞中Notch信号传递所必需的。因此，Notch信号的减少与cdgic的病理有关。

Mastermind (Mam) is one of the evolutionarily conserved elements of Notch signaling. Genetic analyses in Drosophila implicated it as an essential positive regulator of the pathway. We had identified mammalian Mam family of proteins that consists of three members (Mam-1,Mam-2 and Mam-3), and elucidated biochemical mechanism of action of the Mam proteins. All three human and Drosophila Mam stabilize and participate in the DNA binding complex of a CSL DNA-binding protein and the Notch intracellular domains that serve as intermediates of the signaling. All the Mam proteins enhanced the activation of transcription from target promoters by Notch signaling.In order to elucidate the in vivo function of the Mam in a mammalian system, we have generated mouse strains deficient in Mam-1 and Mam-2 by homologous recombination. Mam-1-deficient mice exhibit growth retardation. This phenotype is apparent at the perinatal stage and exacerbated in their postnatal period. The mice succumb before weaning, indicating that Mam-1 has a non-redundant function and is an essential gene. In contrast, we have not identified major developmental defect in Mam-2-deficient mice. Furthermore, no defect has been found in hematopoietic system including T and B lymphocytes of the Mam-2-deficient mice. These results indicate that the Mam loci have distinct function in the vertebrate system.Congenital disorder of glycosylation IIc (CDG IIc) is a recessive syndrome characterized by slowed growth, mental retardation, and severe immunodeficiency. Recently, the gene responsible for CDG IIc was found to encode a GDP-fucose transporter. GDP-fucose is known to be essential for the fucosylation of N-linked glycans and for O-fucosylation, and both fucose modifications are present on Notch. We found that mammalian Gfr is required for Notch signaling in mammalian cultured cells. Therefore, reduced Notch signaling is implicated in the pathology of CDG IIc.