Mesoderm and neuron induction and activin signaling

激活素属于tgf - β超家族，通过由两种跨膜丝氨酸/苏氨酸激酶（称为I型和II型）组成的受体复合物结合并发出信号。迄今为止，已从哺乳动物中克隆出2个II型（ActR-IIA,-IIB）和2个I型（actr - ia，IB）激活素受体。在本研究中，我们试图阐明I型受体在激活素信号传导中对生长、分化和凋亡的作用，得到了以下结果。(1)通过电穿孔法建立了稳定的过表达acr - ia或acr - ib的HS-72转染物。ActR-IA的过表达抑制激活素诱导的G1期细胞周期阻滞，这是通过诱导p21（一种周期蛋白依赖的激酶抑制剂）抑制视网膜母细胞瘤蛋白磷酸化和随后的细胞凋亡引起的。相比之下，过表达ActR-IB的HS-72克隆显著促进了激活素诱导的细胞凋亡。这些结果表明，ActRA-IA和ActR-IB在激活信号转导方面具有明显的差异性。(2) Follistain（激活素结合蛋白）可促进激活素与细胞表面硫酸肝素的结合。当细胞在卵泡抑素存在的情况下与^<125>I-activin孵育时，观察到激活素的明显降解。这种激活素降解被硫酸肝素、氯喹和溶酶体蛋白酶抑制剂所消除。这些结果表明，细胞相关的卵泡素加速激活素进入细胞的摄取，导致溶酶体酶的降解增加，从而在激活素清除系统中发挥作用。(3)为了了解卵泡素相关蛋白（FRP）在非洲爪蟾胚胎发生中的作用，我们进行了全载原位杂交。在非洲爪蟾胚胎的组织区发现FRP表达始于第10期，表明FRP与卵泡形成素一样起组织因子的作用。从神经期到尾芽期，脊索和底板均有FRP表达，提示其在神经结构诱导中的作用。少

Activin belonging to the TGF-beta superfamily binds to and signals through a receptor complex comprising two transmembrane serine/threonine kinases, called type I and type II.So far, two type II (ActR-IIA,-IIB) and two type I (ActRA-IA,IB) activin receptors have been cloned from mammalian sources. In this study, we attempted to elucidate the role of type I receptors in activin signaling for growth, differentiation and apoptosis, and obtained following results.(1) We established stable HS-72 transfectants overexpressing ActR-IA or ActR-IB by the electroporation procedure. Overexpression of ActR-IA suppressed activin-induced cell-cycle arrest in the G1 phase caused by inhibition of retinoblastoma protein phosphorylation through induction of p21, a cyclin-dependent kinase inhibitor, and subsequent apoptosis. In contrast, HS-72 clones that overexpressed ActR-IB significantly facilitated activin induced apoptosis. These results indicate that ActRA-IA and ActR-IB are distinct from each other … More in activin signal transduction.(2) Follistain (activin-binding protein) was found to promote the binding of activin to cell surface heparan sulfate. When the cells were incubated with ^<125>I-activin in the presence of follistatin, significant degradation of activin was observed.This activin degradation was abolished by heparan sulfate, chloroquine, and lysosomal protease inhibitors. These results indicate that cell-associated follistain accelerates the uptake of activin into cells, leading to increased degradation by lysosomal enzymes, and thus plays a role in the activin clearance system.(3) In order to understand the role of follistain-related protein (FRP) in Xenopus embryogenesis, we carried out whole mount in situ hybridization. FRP expression was found to start at stage 10 of Xenopus embryos in the organizer region, suggesting that like follistatin, FRP functions as a organizer factor. The FRP expression in the notochord and floor plate was observed at the neural stage through the tailbud stage, suggesting its role in induction of neural structures. Less