Fibroblast growth factors and receptors are intimately connected to the extracellular matrix by their affinity to heparan sulfate proteoglycans. They mediate multiple processes during embryonic development and adult life. In this study, embryonic stem cell–derived embryoid bodies were used to model fibroblast growth factor signaling during early epithelial morphogenesis. To avoid redundancy caused by multiple receptors, we employed a dominant negative mutation of Fgfr2. Mutant-derived embryoid bodies failed to form endoderm, ectoderm, and basement membrane and did not cavitate. However, in mixed cultures they displayed complete differentiation induced by extracellular products of the normal cell. Evidence will be presented here that at least one of these products is the basement membrane or factors connected to it. It will be shown that in the mutant, collagen IV and laminin-1 synthesis is coordinately suppressed. We will demonstrate that the basement membrane is required for embryoid body differentiation by rescuing columnar ectoderm differentiation and cavitation in the mutant by externally added basement membrane proteins. This treatment induced transcription of Eomesodermin, an early developmental gene, suggesting that purified basement membrane proteins can activate inherent developmental programs. Our results provide a new paradigm for the role of fibroblast growth factor signaling in basement membrane formation and epithelial differentiation.

The members of the laminin family of heterotrimers are major constituents of all basement membranes, sheet-like extracellular structures, present in almost all organs. The laminins bind to cell surface receptors and thereby tightly connect the basement membrane to the adjacent cell layer. This provides for the specific basement membrane functions to stabilize cellular structures, to serve as effective physical barriers, and furthermore, to govern cell fate by inducing intracellular signalling cascades. Many different types of diseases involve basement membranes and laminins. Metastasizing solid tumors must pass through basement membranes to reach the vascular system, and various microbes and viruses enter the cells through direct interaction with laminins. Furthermore, whereas mutations in one specific laminin chain lead to a muscular disorder, mutations of other laminin chains cause skin blistering and kidney defects, respectively. This review summarizes recent progress concerning the molecular mechanisms of laminins in development and disease. The current knowledge may lead to clinical treatment of lamininopathies and may include stem-cell approaches as well as gene therapy.

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During early mouse embryogenesis, each laminin (Lm) chain of the first described Lm, a heterotrimer of ␣1, ␤1, and ␥1 chains (Lm-1), is essential for basement membrane (BM) assembly, which is required for pregastrulation development. Individual domains may have other functions, not necessarily structural. The cell binding C terminus of Lm ␣1 chain contains five Lm globular (LG) domains. In vitro, ␣1LG1-3 domains bind integrins, and ␣1LG4 binds dystroglycan, heparin, and sulfatides. A prevailing hypothesis is that ␣1LG4 is crucial as a structural domain for BM assembly, whereas integrin-binding sites conduct signaling. The in vivo role of ␣1LG4 -5 (also called E3) has not been studied. Mice lacking ␣1LG4 -5 were therefore made. Null embryos implanted, but presumptive epiblast cells failed to polarize and did not survive past day 6.5. BM components including truncated Lm ␣1 were detected in Reichert's membrane. Surprisingly, embryonic BM assembly between visceral endoderm and stem cells was normal in null embryos and in embryoid bodies of ␣1LG4 -5-null embryonic stem cells. Yet, stem cells could not develop into polarized epiblast cells. Thus, ␣1LG4 -5 provides vital signals for the conversion of stem cells to polarized epithelium.epiblast ͉ epithelial polarity ͉ stem cells ͉ mouse development