Molecular control of organogenesis : role of laminin γ2 and γ2*, type XVIII collagen and Wnt2b
1University of Oulu, Biocenter Oulu
2University of Oulu, Faculty of Science, Department of Biochemistry
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|Persistent link:|| http://urn.fi/urn:isbn:9514265661
|Publish Date:|| 2001-11-15
|Thesis type:||Doctoral Dissertation
|Defence Note:||Academic Dissertation to be presented with the assent of the Faculty of Medicine, University of Oulu, for public discussion in Kuusamonsali (Auditorium YB210), Linnanmaa, on December 7th, 2001, at 12 noon.
Doctor Veli-Matti Kähäri
Doctor Päivi Miettinen
How cell and tissue interactions lead to complex structures and differentiated cell types during organogenesis is still one of the most fundermental questions in modern molecular biology. Laminin appears to have a role in branching morphogenesis during organ development. Laminin5 (α3β3γ2) is an epithelium-specific isoform of laminin and previous report has shown that two alternative transcripts for the γ2 chain, the longer γ2 and the shorter γ2*, result from alternative use of the last exon in the human LAMC2 gene. But the transcription of murine laminin γ2 and γ2* and their biological significance have remained unclear. Type XVIII collagen is a newly identified member of the collagen family. It may be involved in the Wnt signaling pathway, since its longest N-terminal variant contains a frizzled domain, which is part of the Wnt receptor and could antagonize Wnt signaling when secreted. Wnt2b is a new member of the Wnt family. Also its function in organogenesis is unknown. In this study, we have investigated the expressions of laminin γ2 and γ2*, type XVIII collagen and Wnt2b during mouse organogenesis. The function of type XVIII collagen in developing lung, kidney and a recombination of ureteric bud and lung mesenchyme tissue and the function of the Wnt-2b gene during kidney organogenesis were studied by using the combined methods of traditional experimental embryology and modern molecular biology.
Two alternative laminin γ2 transcripts were demonstrated in mouse. In the developing kidney, the shorter γ2* form was localized in the mesenchyme, whereas the longer γ2 form was only present in the epithelium of the Wolffian duct and in the ureteric bud, indicating different functions for the γ2 variants. Type XVIII collagen was expressed throughout the respective epithelial bud at the initiation of lung and kidney organogenesis. It becomed localized to the epithelial tips in the early-stage lung, while it was confined to the epithelial stalk region and was absent from the nearly formed ureteric tips in the kidney. In recombinants of ureteric bud and lung mesenchyme, the type XVIII collagen expression pattern in the ureteric bud shifted from the kidney to the lung type, accompanied by a shift in epithelial Sonic Hedgehog expression. The lung mesenchyme was also sufficient to induce ectopic lung Surfactant Protein C expression in the ureteric bud. A blocking antibody for the type XVIII collagen reduced the number of epithelial tips in the lung and completely blocked ureteric development with lung mesenchyme, which was associated with a notable reduction in the expression of Wnt2. The shift in type XVIII collagen expression in ureteric bud and lung mesenchyme tissue recombinant was also accompanied by the significant morphological changes in the branching pattern in ureteric bud development. Wnt2b was expressed in numerous developing organs in the mouse embryo, but it was typically localized in the perinephric mesenchymal cells in the region that partly overlaps the presumptive renal stroma at E11.5. Functional studies of the kidney demonstrated that 3T3 cells expressing Wnt2b were not capable of inducing tubule formation but rather stimulated ureteric development. Recombination of ureteric bud treated with cells expressing Wnt2b and isolated kidney mesenchyme resulted in recovery of the expression of epithelial marker genes and better reconstituted organogenesis. Lithium, a known activator of Wnt signaling, was also sufficient to promote ureteric branching in reconstituted kidney in a manner comparable to Wnt2b signaling.
Our data suggest that different organ morphogenesis is regulated by an intraorgan patterning process that involves coordination between inductive signals and matrix molecules, such as type XVIII collagen. In the mouse kidney, Wnt2b may act as an early mesenchymal signal controlling morphogenesis of epithelial tissue, and the Wnt pathway may regulate ureteric branching directly.
Acta Universitatis Ouluensis. D, Medica
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