4F2, also termed CD98, is an integral membrane protein consisting of a heavy chain linked to a light chain by disulfide bond. We have generated a monoclonal antibody to the mouse 4F2 light chain and cloned the cDNA. It encodes a mouse counterpart of rat L-type amino acid transporter-1, and induces system L amino acid transport in Xenopus oocytes in the presence of 4F2 heavy chain. Transfection studies in mammalian cells have indicated that the 4F2 heavy chain is expressed on the plasma membrane on its own, whereas the 4F2 light chain can be transported to the surface only in the presence of 4F2 heavy chain. 4F2 heavy chain is expressed diffusely on the surface of fibroblastic L cells, whereas it is localized selectively to the cell-cell adhesion sites in L cells expressing cadherins. These results indicate that the 4F2 heavy chain is associated covalently with an amino acid transporter and controls the cell surface expression as well as the membrane topology of the 4F2 heterodimer. Although 4F2 heavy and light chains are expressed coordinately in most tissues, the light chain is barely detected by the antibody in kidney and intestine, despite the presence of heavy chain in a complex form. The results predict the presence of multiple 4F2 light chains.

Mammalian hairy and Enhancer of split homolog 1 (HES1), a basic helix-loop-helix factor gene, is expressed in retinal progenitor cells, and its expression decreases as differentiation proceeds. Retinal progenitor cells infected with HES1-transducing retrovirus did not differentiate into mature retinal cells, suggesting that persistent expression of HES1 blocks retinal development. In contrast, in the retina of HES1-null mutant mice, differentiation was accelerated, and rod and horizontal cells appeared prematurely and formed abnormal rosette-like structures. Lens and cornea development was also severely disturbed. Furthermore, in the mutant retina, bipolar cells extensively died, and finally disappeared. These studies provide evidence that HES1 regulates differentiation of retinal neurons and is essential for eye morphogenesis.

Whereas vertebrate achaete-scute complex (as-c) and atonal (ato) homologs are required for neurogenesis, their neuronal determination activities in the central nervous system (CNS) are not yet supported by loss-of-function studies, probably because of genetic redundancy. Here, to address this problem, we generated mice double mutant for the as-c homolog Mash1 and the ato homolog Math3. Whereas in Mash1 or Math3 single mutants neurogenesis is only weakly affected, in the double mutants tectal neurons, two longitudinal columns of hindbrain neurons and retinal bipolar cells were missing and, instead, those cells that normally differentiate into neurons adopted the glial fate. These results indicated that Mash1 and Math3 direct neuronal versus glial fate determination in the CNS and raised the possibility that downregulation of these bHLH genes is one of the mechanisms to initiate gliogenesis.

Mice mutant for the bHLH gene Hes1, which is known to keep cells in a proliferative state, mostly lack thymus. Transfer of Hes1-null fetal liver cells into RAG2-null host mice normally reconstitutes B cells but fails to generate mature T cells in the thymus. In the reconstituted thymus, T cell differentiation is arrested at the CD4 − CD8 − double negative (DN) stage. Both the initial T cell receptor (TCR)-independent and the subsequent TCR-dependent selective expansion during the DN stage are severely affected. Thus, Hes1 is essential for the earliest thymocyte expansion in a cell-autonomous manner.

The isthmic organizer, which is located at the midbrain±hindbrain boundary, plays an essential role in development of the midbrain and anterior hindbrain. It has been shown that homeobox genes regulate establishment of the isthmic organizer, but the mechanism by which the organizer is maintained is not well understood. Here, we found that, in mice doubly mutant for the basic helix±loop±helix genes Hes1 and Hes3, the midbrain and anterior hindbrain structures are missing without any signi®cant cell death. In these mutants, the isthmic organizer cells prematurely differentiate into neurons and terminate expression of secreting molecules such as Fgf8 and Wnt1 and the paired box genes Pax2/5, all of which are essential for the isthmic organizer function. These results indicate that Hes1 and Hes3 prevent premature differentiation and maintain the organizer activity of the isthmic cells, thereby regulating the development of the midbrain and anterior hindbrain. Keywords: bHLH/Hes1/Hes3/isthmic organizer/ midbrain±hindbrain boundary IntroductionPattern formation of the neural plate is determined initially by vertical signals from the mesoderm and visceral endoderm and then by planar signals from the local organizers within the neuroectoderm (Lumsden and Krumlauf, 1996;Beddington and Robertson, 1998). These signals regulate the anterior±posterior patterning of gene expression: the homeobox gene Otx2 is expressed in the presumptive forebrain and midbrain, while another homeobox gene, Gbx2, is expressed in the presumptive hindbrain and spinal cord. Recent studies revealed that the boundary region between the Otx2 and Gbx2 expression domains, which is located at the isthmus (the midbrain±hindbrain boundary), constitutes the organizing center, called the isthmic organizer (Joyner et al., 2000;Simeone, 2000;Rhinn and Brand, 2001;Wurst and Bally-Cuif, 2001). Antagonistic regulation between Otx2 and Gbx2 regulates the precise position of the isthmic organizer (Broccoll et al., 1999;Millet et al., 1999;Katahira et al., 2000) while other homeobox genes such as Pax2/5 and En1/2 are required for isthmic organizer activity (Wurst et al., 1994;Schwarz et al., 1997Schwarz et al., , 1999Urba Ânek et al., 1997;Liu and Joyner, 2001). The isthmic organizer expresses secreting molecules such as Fgf8 and Wnt1 and thereby induces the development of the midbrain and anterior hindbrain (McMahon et al., 1992;Bally-Cuif et al., 1995;Crossley and Martin, 1995;Crossley et al., 1996;Lee et al., 1997;Meyers et al., 1998;Liu et al., 1999;Martinez et al., 1999;Xu et al., 2000). Fgf8 expression in the isthmic organizer terminates around embryonic day (E) 13.5, while expression of a related gene, Fgf17, continues until E14.5 in mice (Xu et al., 2000). Similarly, Wnt1 expression continues until E13.5 (Wilkinson et al., 1987). Thus, the isthmic organizer seems to be maintained at least until E14.5 in mice. Although the genes that establish the isthmic organizer, such as homeobox genes, have been characterized extensively, the mechanism by which the organ...

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This study aims to determine the epigenetic mechanism regulating Kiss1 gene expression in the anteroventral periventricular nucleus (AVPV) to understand the mechanism underlying estrogen-positive feedback action on gonadotropin-releasing hormone/gonadotropin surge. We investigated estrogen regulation of the epigenetic status of the mouse AVPV Kiss1 gene locus in comparison with the arcuate nucleus (ARC), in which Kiss1 expression is down-regulated by estrogen. Histone of AVPV Kiss1 promoter region was highly acetylated, and estrogen receptor α was highly recruited at the region by estrogen. In contrast, the histone of ARC Kiss1 promoter region was deacetylated by estrogen. Inhibition of histone deacetylation upregulated in vitro Kiss1 expression in a hypothalamic non-Kiss1-expressing cell line. Gene conformation analysis indicated that estrogen induced formation of a chromatin loop between Kiss1 promoter and the 3′ intergenic region, suggesting that the intergenic region serves to enhance estrogen-dependent Kiss1 expression in the AVPV. This notion was proved, because transgenic reporter mice with a complete Kiss1 locus sequence showed kisspeptin neuronspecific GFP expression in both the AVPV and ARC, but the deletion of the 3′ region resulted in greatly reduced GFP expression only in the AVPV. Taken together, these results demonstrate that estrogen induces recruitment of estrogen receptor α and histone acetylation in the Kiss1 promoter region of the AVPV and consequently enhances chromatin loop formation of Kiss1 promoter and Kiss1 gene enhancer, resulting in an increase in AVPV-specific Kiss1 gene expression. These results indicate that epigenetic regulation of the Kiss1 gene is involved in estrogen-positive feedback to generate the gonadotropin-releasing hormone/gonadotropin surge.

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