Yan C, Costa R H, Darnell J E, Chen J D, Van Dyke T A
Department of Biological Sciences, University of Pittsburgh, PA 15260.
EMBO J. 1990 Mar;9(3):869-78. doi: 10.1002/j.1460-2075.1990.tb08184.x.
Transthyretin (TTR) is a thyroid hormone transport protein that is secreted by hepatocytes into the serum and by the choroid plexus epithelium into the cerebral spinal fluid. The protein is not made elsewhere in adult animals in significant amounts. We find that the start site for mRNA synthesis is the same in both cell types. The sequences required for mouse TTR expression in cultured hepatocytes include an enhancer at -1.86 to -1.96 kbp and a promoter-proximal region at -70 to -200 bp relative to the mRNA cap site. We demonstrate that in transgenic mice these regulatory regions (approximately 300 bp) are sufficient for quantitatively normal expression of a TTR minigene in hepatocytes, but not for restricted expression in the choroid plexus cells of the brain. Instead, they direct aberrant widespread expression in regions of the brain outside the choroid plexus. With 3 kbp of upstream sequence the TTR minigene is expressed specifically in the choroid plexus as well as in the liver, demonstrating the normal cell type specificity for TTR. These results suggest that 3 kbp of upstream sequence contains positive element(s) required for choroid plexus expression which are distinct from those utilized in the hepatocyte, and may also contain negative element(s) that function to suppress transcription in other brain cell types.
转甲状腺素蛋白（TTR）是一种甲状腺激素转运蛋白，由肝细胞分泌到血清中，并由脉络丛上皮细胞分泌到脑脊液中。在成年动物的其他部位，该蛋白的合成量很少。我们发现，两种细胞类型中mRNA合成的起始位点相同。在培养的肝细胞中，小鼠TTR表达所需的序列包括一个位于-1.86至-1.96 kbp的增强子和一个相对于mRNA帽位点位于-70至-200 bp的启动子近端区域。我们证明，在转基因小鼠中，这些调控区域（约300 bp）足以使TTR小基因在肝细胞中实现定量正常表达，但不足以使其在脑脉络丛细胞中特异性表达。相反，它们会导致在脉络丛以外的脑区出现异常的广泛表达。当有3 kbp的上游序列时，TTR小基因在脉络丛以及肝脏中特异性表达，证明了TTR正常的细胞类型特异性。这些结果表明，3 kbp的上游序列包含脉络丛表达所需的正向元件，这些元件与肝细胞中利用的元件不同，并且可能还包含负向元件，其作用是抑制其他脑细胞类型中的转录。
