Gene expression studies of pregastrulation development: the basement membrane is essential for cell differentiation

Sammanfattning: Basement membranes (BMs) are sheet-like structures of extracellular matrix. They act as a supporting structure but can also significantly influence cellular behavior in development, tissue homeostasis and disease. Laminins, a major BM component, are multidomain proteins, consisting of three polypeptide chains (α, β and γ). During pregastrulation development, stem cells convert and epithelial tissues are formed. This process is faithfully mimicked in vitro by embryoid body (EB) cultures. Fibroblast growth factor (FGF) signaling is crucial when the step-like process of EB development is initiated with the formation of an endoderm. A subendodermal BM is formed, in which the globular domains LG4-5 of the laminin α1 chain (α1LG4-5) are responsible for the induction of the epiblast
EBs derived form embryonic stem (ES) cells, modified to repress FGF receptor signaling, have been described before. However, a full-scale analysis of the transcriptome was missing. We therefore analysed these EBs at four time points during differentiation by the use of microarray technique. An extensive catalogue of affected genes was reported. A majority of the genes directed by FGF signalling were encoding BM and endodermal proteins. In addition, we also analysed the expression profile of wild type EBs. In both these studies, we found interesting genes not previously described in early development or identified as FGF targets. Hopefully, our gene catalogue will be a valuable source for the scientific community interested in FGF signaling, developmental biology and stem cell research. Furthermore, a gene expression study was set up to get a better insight of epiblast inducement by α1LG4-5. EBs derived form ES cells with a targeted deletion of the α1LG4-5 domains were analysed. To our surprise, we found several indications of an incomplete differentiation of the visceral endoderm. We therefore hypothesize a novel autocrine mechanism for α1LG4-5 in regulating the developing endoderm.
We also suggest novel roles for laminin LG4-5 in the neuromuscular system. Using laminin α2 chain deficient mice overexpressing laminin α1 chain lacking the LG4-5 domains, we show that these domains, and consequently binding to the receptor dystroglycan are not crucial in diaphragm and heart, but essential in the peripheral nervous system.