Severe defects in dorsal thalamic development in low-density lipoprotein receptor-related protein-6 mutants.

Mice with mutations in the Wnt coreceptor low-density lipoprotein receptor-related protein-6 (LRP6) have a smaller and severely disorganized dorsal thalamus and lack thalamocortical projections. Using molecular markers, we showed that most dorsal thalamic and epithalamic neurons were missing, and most of the major dorsal thalamic nuclei were not identifiable. However, the ventral thalamus was essentially unaffected, although the dorsal thalamic defect leads to rostral displacement of portions of the ventral thalamus. Analysis of younger embryos showed that epithalamic and dorsal thalamic neurons were not produced at early stages of development, whereas ventral thalamic neurons were still produced. These defects were accompanied by improper formation of the boundary between dorsal and ventral thalamus, the zona limitans interthalamica (ZLI). Furthermore, the expression of an early marker of posterior forebrain development that marks the compartment from the midbrain-hindbrain junction to the ZLI (including the future dorsal thalamus, pretectum, and midbrain) was disrupted, supporting the idea that diencephalic development is abnormal from very early in embryogenesis. This study provides compelling in vivo evidence that thalamic development requires normal activity of the LRP6-mediated canonical Wnt signaling pathway.

A role for Wnt/beta-catenin signaling in lens epithelial differentiation.

The differentiation of epithelial cells and fiber cells from the anterior and posterior compartments of the lens vesicle, respectively, give the mammalian lens its distinctive polarity. While much progress has been made in understanding the molecular basis of fiber differentiation, little is known about factors that govern the differentiation of the epithelium. Members of the Wnt growth factor family appear to be key regulators of epithelial differentiation in various organ systems. Wnts are ligands for Frizzled receptors and can activate several signaling pathways, of which the best understood is the Wnt/beta-catenin pathway. The presence of LDL-related protein coreceptors (LRPs) 5 or 6 has been shown to be a requirement for Wnt signaling through the beta-catenin pathway. To access the role of this signaling pathway in the lens, we analyzed mice with a null mutation of lrp6. These mice had small eyes and aberrant lenses, characterized by an incompletely formed anterior epithelium resulting in extrusion of the lens fibers into the overlying corneal stroma. We also showed that multiple Wnts, including 5a, 5b, 7a, 7b, 8a, 8b, and Frizzled receptors 1, 2, 3, 4, and 6, were detected in the lens. Expression of these molecules was generally present throughout the lens epithelium and extended into the transitional zone, where early fiber elongation occurs. In addition to both LRP5 and LRP6, we also showed the expression of other molecules involved in Wnt signaling and its regulation, including Dishevelleds, Dickkopfs, and secreted Frizzled-related proteins. Taken together, these results indicate a role for Wnt signaling in regulating the differentiation and behavior of lens cells.

Villin: A marker for development of the epithelial pyloric border.

In the adult gastrointestinal tract, the morphologic borders between esophagus and stomach and between stomach and small intestine are literally one cell thick. The patterning mechanisms that underlie the development of these sharp regional divisions from a once continuous endodermal tube are still obscure. In the embryonic endoderm of the developing gut, region-specific expression of certain genes (e.g., intestine-specific expression of the actin bundling protein villin) can be detected as early as 9.0 days post coitum, although the morphologic differentiation of the gut epithelium proper does not begin until 4 to 5 days later. By using a mouse model in which a beta-galactosidase marker has been inserted into the endogenous villin locus, we examined the development of the stomach/intestinal (pyloric) border during gut organogenesis. The data indicate that the border is not sharp from the outset. Rather, the initial border region is characterized by a decreasing gradient of villin/beta-galactosidase expression that extends into the distal stomach. A sharp epithelial border of villin/beta-galactosidase expression appears abruptly at day 16 and is further refined over the next 3 weeks to form the distinct one-cell-thick border characteristic of the adult. These results indicate that an important previously unrecognized patterning event occurs in the gut epithelium at 16 days; this event may define an epithelial compartment boundary between the stomach and the intestine. The villin/beta-galactosidase mouse model characterized here provides an excellent substrate with which to further dissect the mechanisms involved in this patterning process.