Domain IV of mouse laminin beta1 and beta2 chains

Laminin-10 is crucial for hair morphogenesis

The role of the extracellular matrix in cutaneous morphogenesis is poorly understood. Here, we describe the essential role of laminin-10 (alpha5beta1gamma1) in hair follicle development. Laminin-10 was present in the basement membrane of elongating hair germs, when other laminins were downregulated, suggesting a role for laminin-10 in hair development. Treatment of human scalp xenografts with antibodies to laminin-10, or its receptor beta1 integrin, produced alopecia. E16.5 Lama5 -/- mouse skin, lacking laminin-10, contained fewer hair germs compared with controls, and after transplantation, Lama5 -/- skin showed a failure of hair germ elongation followed by complete hair follicle regression. Lama5 -/- skin showed defective basement membrane assembly, without measurable increases in anoikis. Instead, Lama5 -/- skin showed decreased expression of early hair markers including sonic hedgehog and Gli1, implicating laminin-10 in developmental signaling. Intriguingly, treatment of Lama5 -/- skin with purified laminin-10 corrected basement membrane defects and restored hair follicle development. We conclude that laminin-10 is required for hair follicle development and report the first use of exogenous protein to correct a cutaneous developmental defect.

Mesangial cells organize the glomerular capillaries by adhering to the G domain of laminin alpha5 in the glomerular basement membrane

In developing glomeruli, laminin alpha5 replaces laminin alpha1 in the glomerular basement membrane (GBM) at the capillary loop stage, a transition required for glomerulogenesis. To investigate domain-specific functions of laminin alpha5 during glomerulogenesis, we produced transgenic mice that express a chimeric laminin composed of laminin alpha5 domains VI through I fused to the human laminin alpha1 globular (G) domain, designated Mr51. Transgene-derived protein accumulated in many basement membranes, including the developing GBM. When bred onto the Lama5 -/- background, Mr51 supported GBM formation, preventing the breakdown that normally occurs in Lama5 -/- glomeruli. In addition, podocytes exhibited their typical arrangement in a single cell layer epithelium adjacent to the GBM, but convolution of glomerular capillaries did not occur. Instead, capillaries were distended and exhibited a ballooned appearance, a phenotype similar to that observed in the total absence of mesangial cells. However, here the phenotype could be attributed to the lack of mesangial cell adhesion to the GBM, suggesting that the G domain of laminin alpha5 is essential for this adhesion. Analysis of an additional chimeric transgene allowed us to narrow the region of the alpha5 G domain essential for mesangial cell adhesion to alpha5LG3-5. Finally, in vitro studies showed that integrin alpha3beta1 and the Lutheran glycoprotein mediate adhesion of mesangial cells to laminin alpha5. Our results elucidate a mechanism whereby mesangial cells organize the glomerular capillaries by adhering to the G domain of laminin alpha5 in the GBM.

Expression and biological role of laminin-1

Of the approximately 15 laminin trimers described in mammals, laminin-1 expression seems to be largely limited to epithelial basement membranes. It appears early during epithelial morphogenesis in most tissues of the embryo, and remains present as a major epithelial laminin in some adult tissues. Previous organ culture studies with embryonic tissues have suggested that laminin-1 is important for epithelial development. Recent data using genetically manipulated embryonic stem (ES) cells grown as embryoid bodies provide strong support for the view of a specific role of laminin-1 in epithelial morphogenesis. One common consequence of genetic ablation of FGF signaling, beta1-integrin or laminin gamma1 chain expression in ES cells is the absence of laminin-1, which correlates with failure of BM assembly and epiblast differentiation. Partial but distinct rescue of epiblast differentiation has been achieved in all three mutants by exogenously added laminin-1. Laminin-1 contains several biologically active modules, but several are found in beta1 or gamma1 chains shared by at least 11 laminins. However, the carboxytermini of the alpha chains contain five laminin globular (LG) modules, distinct for each alpha chain. There is increasing evidence for a particular role of alpha1LG4 binding to its receptors for epithelial tubulogenesis. The biological roles of this and other domains of laminin-1 are currently being explored by genetic means. The pathways controlling laminin-1 synthesis have remained largely unknown, but recent advances raise the possibility that laminin-1 and collagen IV synthesis can be regulated by pro-survival kinases of the protein kinase B/Akt family.

Evidence of nidogen-2 compensation for nidogen-1 deficiency in transgenic mice

Previous studies have shown that inhibition of nidogen-laminin binding interferes with basement membrane stabilization in various mouse organ cultures while no overt phenotype has been observed following inactivation of the nidogen-1 gene in mice. We have now used recombinant mouse nidogen-1 and nidogen-2 in order to evaluate a possible compensation between the two isoforms in the knock-out mice. Essentially, a comparable in vitro binding of nidogens-1 and -2 to the same laminin gamma1 chain structure and to several other basement membrane proteins has been revealed. Quantitative radioimmuno-assays have demonstrated high concentrations of nidogen-1 exceeding those of laminin gamma1 and nidogen-2 by factors of 5 and 20-50, respectively, in tissue extracts of wild-type mice. A three- to sevenfold increase in nidogen-2 was observed in heart and muscle of mice with nidogen-1 deficiency and confirmed by a similar increase in the intensity of immunogold staining of these tissues. However, a few of the tissues from mice with the gene knock-out still contained some nidogen-1-like immunoreactivity (1% of wild-type). Furthermore, both nidogen isoforms showed a similar distribution in various organs during embryonic development which, however, as shown previously, changed in some adult tissues. The data support the nidogen-2 compensation hypothesis to explain the limited phenotype observed following elimination of the nidogen-1 gene.

The mouse Kreisler (Krml1/MafB) segmentation gene is required for differentiation of glomerular visceral epithelial cells

Molecular components of the glomerular filtration mechanism play critical roles in renal diseases. Many of these components are produced during the final stages of differentiation of glomerular visceral epithelial cells, also known as podocytes. While basic domain leucine zipper (bZip) transcription factors of the Maf subfamily have been implicated in cellular differentiation processes, Kreisler (Krml1/MafB), the gene affected in the mouse kreisler (kr) mutation, is known for its role in hindbrain patterning. Here we show that mice homozygous for the kr(enu) mutation develop renal disease and that Kreisler is essential for cellular differentiation of podocytes. Consistent with abnormal podocyte differentiation, kr(enu) homozygotes show proteinuria, and fusion and effacement of podocyte foot processes, which are also observed in the nephrotic syndrome. Kreisler acts during the final stages of glomerular development-the transition between the capillary loop and mature stages-and downstream of the Pod1 basic domain helix-loop-helix transcription factor. The levels of Podocin, the gene mutated in autosomal recessive steroid-resistant nephrotic syndrome (NPHS2), and Nephrin, the gene mutated in congenital nephrotic syndrome of the Finnish type (NPHS1), are slightly reduced in kr(enu)/kr(enu) podocytes. However, these observations alone are unlikely to account for the aberrant podocyte foot process formation. Thus, Kreisler must regulate other unknown genes required for podocyte function and with possible roles in kidney disease.

A mutant form of the Wilms' tumor suppressor gene WT1 observed in Denys-Drash syndrome interferes with glomerular capillary development

The Wilms' tumor suppressor gene WT1 encodes a zinc finger protein that is required for urogenital development. In the kidney, WT1 is most highly expressed in glomerular epithelial cells or podocytes, which are an essential component of the filtering system. Human subjects heterozygous for point mutations in the WT1 gene develop renal failure because of the formation of scar tissue within glomeruli. The relationship between WT1 expression in podocytes during development and glomerular scarring is not well understood. In this study, transgenic mice that expressed a mutant form of WT1 in podocytes were derived. The capillaries within transgenic glomeruli were dilated, indicating that WT1 might regulate the expression of growth factors that affect capillary development. Platelet endothelial cell adhesion molecule-1 expression was greatly reduced on glomerular endothelial cells of transgenic kidneys. These results suggest that WT1 controls the expression of growth factors that regulate glomerular capillary development and that abnormal capillary development might lead to glomerular disease.