Laminin {alpha}1 globular domains 4-5 induce fetal development but are not vital for embryonic basement membrane assembly.

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 {alpha}1 chain contains five Lm globular (LG) domains. In vitro, {alpha}1LG1–3 domains bind integrins, and {alpha}1LG4 binds dystroglycan, heparin, and sulfatides. A prevailing hypothesis is that {alpha}1LG4 is crucial as a structural domain for BM assembly, whereas integrin-binding sites conduct signaling. The in vivo role of {alpha}1LG4–5 (also called E3) has not been studied. Mice lacking {alpha}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 {alpha}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 {alpha}1LG4–5-null embryonic stem cells. Yet, stem cells could not develop into polarized epiblast cells. Thus, {alpha}1LG4–5 provides vital signals for the conversion of stem cells to polarized epithelium.