Previous studies of knock-out mouse embryos have shown that the Wilms' tumor suppressor gene (Wt1) is indispensable for the development of kidneys, gonads, heart, adrenals and spleen. Using OPT (Optical Projection Tomography) we have found a new role for Wt1 in mouse liver development. In the absence of Wt1, the liver is reduced in size, and shows lobing abnormalities. In normal embryos, coelomic cells expressing Wt1, GATA-4, RALDH2 and RXRalpha delaminate from the surface of the liver, intermingle with the hepatoblasts and incorporate to the sinusoidal walls. Some of these cells express desmin, suggesting a contribution to the stellate cell population. Other cells, keeping high levels of RXRalpha immunoreactivity, are negative for stellate or smooth muscle cell markers. However, coelomic cells lining the liver of Wt1-null embryos show decreased or absent RALDH2 expression, the population of cells expressing high levels of RXRalpha is much reduced and the proliferation of hepatoblasts and RXRalpha-positive cells is significantly decreased. On the other hand, the expression of smooth muscle cell specific alpha-actin increases throughout the liver, suggesting an accelerated and probably anomalous differentiation of stellate cell progenitors. We describe a similar retardation of liver growth in RXRalpha-null mice as well as in chick embryos after inhibition of retinoic acid synthesis. We propose that Wt1 expression in cells delaminating from the coelomic epithelium is essential for the expansion of the progenitor population of liver stellate cells and for liver morphogenesis. Mechanistically, at least part of this effect is mediated via the retinoic acid signaling pathway.

The pufferfish Fugu rubripes has a genome Ϸ7.5 times smaller than that of mammals but with a similar number of genes. Although conserved synteny has been demonstrated between pufferfish and mammals across some regions of the genome, there is some controversy as to what extent Fugu will be a useful model for the human genome, e.g., [Gilley, J., Armes, N. & Fried, M. (1997) Nature (London) 385, 305-306]. We report extensive conservation of synteny between a 1.5-Mb region of human chromosome 11 and <100 kb of the Fugu genome in three overlapping cosmids. Our findings support the idea that the majority of DNA in the region of human chromosome 11p13 is intergenic. Comparative analysis of three unrelated genes with quite different roles, WT1, RCN1, and PAX6, has revealed differences in their structural evolution. Whereas the human WT1 gene can generate 16 protein isoforms via a combination of alternative splicing, RNA editing, and alternative start site usage, our data predict that Fugu WT1 is capable of generating only two isoforms. This raises the question of the extent to which the evolution of WT1 isoforms is related to the evolution of the mammalian genitourinary system. In addition, this region of the Fugu genome shows a much greater overall compaction than usual but with significant noncoding homology observed at the PAX6 locus, implying that comparative genomics has identified regulatory elements associated with this gene.

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Detailed studies of lipoprotein A+ formation during AKH action have been made in Locusta migratoria L. using gel filtration combined with the use of radiolabelled haemolymph protein probes. In addition, we have assessed the quantitative contribution of the CL-proteins to A+ formation by direct measurement of the changes in concentration of free CL-proteins and report some properties of the C-I and C-I1 proteins: they appear to be glycoproteins of 20,000 and 16,000 MW respectively, but do not bind to concanavalin A. We have confirmed earlier observations (using different techniques) which showed that liproprotein Ayellow is not involved per se in A+ formation during the first 15 min of AKH action. In contrast, the (two) CL-proteins take part in A+ formation without any apparent delay after hormone injection. Our observations show that A+ formation is essentially complete within 30 min of AKH injection, although further CL-protein binding and lipid-loading do occur subsequently. After 30 min there is no further decrease in the Ayellow titre. It is argued that much, if not all, CL-protein is located at the surface of the A+ particle. From the changes in titres which occur in Ayellow and CL-proteins during AKH action we estimate that A+ is formed from 1 mole of Ayellc,w and approximately 28 moles of CL-proteins. Using these figures we calculate an apparent molecular weight for A+ within the range of 1.65-2 . 1 2~ lo6, which is in reasonable agreement with estimates derived from gel exclusion chromatography data. These studies emphasize the dynamic and fully reversible nature of lipoprotein A+ formation and highlight the complex nature of the lipoprotein transformations occurring during hormone-stimulated lipid transport in locusts.