Distinct transcriptional regulatory mechanisms underlie left-right asymmetric expression of lefty-1 and lefty-2

Abstract

Both lefty-1 and lefty-2 genes are expressed on the left side of developing mouse embryos and are implicated in left-right (L-R) axis formation. With the use of transgenic analysis, the transcriptional regulatory regions of these genes responsible for their L-R asymmetric expression have now been investigated. The 9.5-kb upstream region of lefty-1 and the 5.5-kb upstream region of lefty-2 reproduced the expression pattern of the corresponding gene. Examination of deletion constructs revealed the presence of a left side-specific enhancer (ASE) that is essential and sufficient for lefty-2 asymmetric expression. In contrast, the asymmetric expression of lefty-1 was shown to be determined by a combination of bilateral enhancers and a right side-specific silencer (RSS). The 9.5-kb region of lefty-1 and the 5.5-kb region of lefty-2 responded to iv and inv, upstream genes of lefty-1 and lefty-2. The regulation of lefty-2 by iv and inv was mediated by ASE. These results suggest that, in spite of the similarities between lefty-1 and lefty-2, different regulatory mechanisms underlie their asymmetric expression.[Key Words: Left-right asymmetry; lefty; transcriptional regulation] Received November 13, 1998; revised version accepted December 22, 1998.Vertebrates exhibit numerous left-right (L-R) asymmetries, such as the positions of the heart and spleen on the left side of the body. In rodents, L-R asymmetry appears to be determined at the presomite stage (Brown et al. 1991; Fujinaga and Baden 1991a,b). One of the earliest morphological indicators of laterality is the looping of the cardiac tube toward the right side. This event is accompanied by a clockwise axial rotation referred to as embryonic turning, and is followed, much later, by the asymmetric orientation of various visceral organs.The molecular mechanism that underlies L-R axis formation has remained largely unknown until recently, when insights into this important process have been gained (for review, see King and Brown 1997;Varlet and Robertson 1997;Levin and Mercola 1998). The entire process by which L-R asymmetry is established can be divided into three phases: (1) the initial determination of L-R polarity, (2) L-R asymmetric expression of signaling molecules, and (3) L-R asymmetric morphogenesis induced by these signaling molecules. The mechanism of the initial determination of L-R polarity remains unknown, although several models have been proposed (Brown and Wolpert 1990;Yost 1992). In mice, several mutations are known to affect L-R patterning (Hummel and Chapman 1959; Yokoyama et al. 1991;Heymer et al. 1997; Melloy et al. 1997), two of the more extensively studied of which are iv and inv. L-R specification is randomized in iv/iv mice, whereas it is reversed in inv/inv mice. These phenotypes suggest that the iv and inv genes likely contribute to the initial (early) determination of L-R polarity. The iv and inv genes have been shown to encode a protein related to axonemal dynein (Supp et al. 1997), and a novel protein with ankyrin repeats (Mochizuki et al. 19...

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TGF-beta Family Signaling in Embryonic Stem Cells

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