Molecular dissection of laminin α5 in vivo reveals separable domain-specific roles in embryonic development and kidney function

Laminins are a family of basement membrane proteins with diverse roles in fundamental developmental processes such as epiblast polarization and gastrulation, as well as in organ development and function. We have focused on the laminin α1 and α5 chains, the ancestral laminin α chains required for development. To elucidate the unique functions of laminin α1 and α5 and their COOH-terminal LG domains, we have produced a collection of laminin knockout and transgenic mice expressing full length and chimeric laminin α5/α1 chains. Crossing the transgenes onto the Lama5-/- background generates "pseudo-knockins", so called because endogenous laminin α5 is replaced by transgene-encoded proteins. Expression of a chimera with the entire α5LG domain replaced by α1LG had minimal ameliorative effects on the defects observed in Lama5-/- embryos. In contrast, high level expression of a chimera with only the α5LG3-5 tandem replaced by α1LG3-5 completely rescued defects in digit septation, neural tube closure, placental labyrinth morphology, lung lobe septation, hair growth, and vascularization of kidney glomeruli. These mice were viable for several months, but they developed a lethal nephrotic syndrome. Our results show that: (1) the laminin α5LG1-2 tandem plays an essential role during development and harbors the great majority of the functionality of the α5LG domain; and (2) the α5LG3-5 tandem serves as a novel determinant required for the kidney's glomerular filtration barrier to plasma protein.