Biomedical subjects

K Losos

Publications and source records attributed to K Losos.

4 recordsLinked to original sources

PubMed1999

A role for Gbx2 in repression of Otx2 and positioning the mid/hindbrain organizer.

The mid/hindbrain (MHB) junction can act as an organizer to direct the development of the midbrain and anterior hindbrain. In mice, Otx2 is expressed in the forebrain and midbrain and Gbx2 is expressed in the anterior hindbrain, with a shared border at the level of the MHB organizer. Here we show that, in Gbx2-/- mutants, the earliest phenotype is a posterior expansion of the Otx2 domain during early somite stages. Furthermore, organizer genes are expressed at the shifted Otx2 border, but not in a normal spatial relationship. To test whether Gbx2 is sufficient to position the MHB organizer, we transiently expressed Gbx2 in the caudal Otx2 domain and found that the Otx2 caudal border was indeed shifted rostrally and a normal appearing organizer formed at this new Otx2 border. Transgenic embryos then showed an expanded hindbrain and a reduced midbrain at embryonic day 9.5-10. We propose that formation of a normal MHB organizer depends on a sharp Otx2 caudal border and that Gbx2 is required to position and sharpen this border.

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PubMed1999

FGF8 can activate Gbx2 and transform regions of the rostral mouse brain into a hindbrain fate.

The mid/hindbrain junction region, which expresses Fgf8, can act as an organizer to transform caudal forebrain or hindbrain tissue into midbrain or cerebellar structures, respectively. FGF8-soaked beads placed in the chick forebrain can similarly induce ectopic expression of mid/hindbrain genes and development of midbrain structures (Crossley, P. H., Martinez, S. and Martin, G. R. (1996) Nature 380, 66-68). In contrast, ectopic expression of Fgf8a in the mouse midbrain and caudal forebrain using a Wnt1 regulatory element produced no apparent patterning defects in the embryos examined (Lee, S. M., Danielian, P. S., Fritzsch, B. and McMahon, A. P. (1997) Development 124, 959-969). We show here that FGF8b-soaked beads can not only induce expression of the mid/hindbrain genes En1, En2 and Pax5 in mouse embryonic day 9.5 (E9.5) caudal forebrain explants, but also can induce the hindbrain gene Gbx2 and alter the expression of Wnt1 in both midbrain and caudal forebrain explants. We also show that FGF8b-soaked beads can repress Otx2 in midbrain explants. Furthermore, Wnt1-Fgf8b transgenic embryos in which the same Wnt1 regulatory element is used to express Fgf8b, have ectopic expression of En1, En2, Pax5 and Gbx2 in the dorsal hindbrain and spinal cord at E10.5, as well as exencephaly and abnormal spinal cord morphology. More strikingly, Fgf8b expression in more rostral brain regions appears to transform the midbrain and caudal forebrain into an anterior hindbrain fate through expansion of the Gbx2 domain and repression of Otx2 as early as the 7-somite stage. These findings suggest that normal Fgf8 expression in the anterior hindbrain not only functions to maintain development of the entire mid/hindbrain by regulating genes like En1, En2 and Pax5, but also might function to maintain a metencephalic identity by regulating Gbx2 and Otx2 expression.

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PubMed1998

Multiple developmental stage-specific enhancers regulate CD8 expression in developing thymocytes and in thymus-independent T cells.

We and others have recently identified a CD8 locus enhancer (E8) that directs expression in mature CD8 single-positive thymocytes and peripheral CD8+ T cells and in extrathymically derived intestinal intraepithelial lymphocytes (IEL). In this study, we show that deletion of E8, by homologous recombination results in reduced CD8alphaalpha homodimer expression on IEL. Since CD8 expression on thymus-derived T cells was normal, other enhancers regulate CD8 expression in these cells. By exploiting a transgenic reporter expression assay, we identified three additional enhancers that directed expression in diverse thymocyte subsets and mature T cells but not in CD8alphaalpha+ IEL. The results suggest that CD8alpha expression is primarily regulated by E8, in IEL and by the novel enhancers in the thymus-dependent lineages.

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PubMed1997

An enhancer that directs lineage-specific expression of CD8 in positively selected thymocytes and mature T cells.

Positive selection of CD4+CD8+ T cells to the CD4+CD8- helper and CD4- CD8+ cytotoxic lineages is a multistep process that involves complex regulation of coreceptor gene expression. By analyzing expression of a reporter gene in transgenic mice, we have identified a DNA segment, located between the murine CD8beta and CD8alpha genes, that has enhancer activity restricted to CD8 lineage cells. Remarkably, this enhancer functions in thymocytes undergoing positive selection to the CD4-CD8+ phenotype but not in immature double-positive thymocytes. The enhancer also functions in gut intraepithelial lymphocytes that express CD8alpha but not CD8beta, suggesting that it is specific for CD8alpha expression. The tight correlation between activation of this enhancer and the final step in positive selection has important implications for understanding the mechanism of lineage commitment in thymocytes.

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