Lox10, a member of the NK-2 homeobox gene class, is expressed in a segmental pattern in the endoderm and in the cephalic nervous system of the leech Helobdella

A novel leech homeobox gene, Lox10, is shown to encode a homeodomain sequence characteristic of a phyletically widespread NK-2 homeobox gene class. Lox10 expression was examined in leech embryos of various ages by in situ hybridization. In the unsegmented cephalic region, Lox10 RNA is expressed in a subset of the cells descended from the a' and b' micromeres, including a small cluster of cells, believed to be postmitotic neurons, within the supraesophageal ganglion of the central nervous system. Hybridization signal was not detected in either the mesoderm or ectoderm of the trunk segments, and the apparent restriction of Lox10 ectodermal expression to the nonsegmented cephalic domain resembles the restricted forebrain expression pattern of its mammalian homologues. Lox10 is also expressed within the endodermal tissues of the leech midgut, which arises by cellularization from a polynucleate syncytium. Endodermal expression is organized into a pattern of transverse stripes and spots which are aligned with the intersegmental septa, and which prefigure the pattern of gut wall constrictions observed at later stages of development. Lox10 is the first molecular marker of segmentally periodic endoderm differentiation reported for any animal species.

XlHbox 8: a novel Xenopus homeo protein restricted to a narrow band of endoderm

We report the isolation of a new homeobox gene from Xenopus laevis genomic DNA. The homeodomain sequence is highly diverged from the prototype Antennapedia sequence, and contains a unique histidine residue in the helix that binds to DNA. The homeodomain is followed by a 65 amino acid carboxyterminal domain, the longest found to date in any vertebrate homeobox gene. We have raised specific antibodies against an XlHbox 8-beta-gal fusion protein to determine the spatial and temporal expression of this gene. The nuclear protein first appears in a narrow band of the endoderm at stage 33 and develops into expression within the epithelial cells of the pancreatic anlagen and duodenum. Expression within the pancreatic epithelium persists into the adult frog. This unprecedented restriction to an anteroposterior band of the endoderm suggests that vertebrate homeobox genes might be involved in specifying positional information not only in the neuroectoderm and mesoderm, but also in the endoderm. Our data suggest that XlHbox 8 may therefore represent the first member of a new class of position-dependent transcription factors affecting endodermal differentiation.