Collagen XVIII/endostatin is associated with the epithelial-mesenchymal transformation in the atrioventricular valves during cardiac development.

Type XVIII collagen is a multidomain protein that contains cleavable C-terminal NC1 and endostatin fragments, which have been shown to either induce or inhibit cell migration. Endostatin is being intensely studied because of its anti-angiogenic activity. Three variants of type XVIII collagen have been reported to be distributed in epithelial and endothelial basement membranes in a tissue-specific manner. The single gene encoding collagen XVIII is on chromosome 21 within the region associated with the congenital heart disease phenotype observed in Down's syndrome. In this study, we investigated the expression pattern of collagen XVIII in embryonic mouse hearts during formation of the atrioventricular (AV) valves. We found that collagen XVIII is localized not only in various basement membranes but is also highly expressed throughout the connective tissue core of the endocardial cushions and forming AV valve leaflets. It was closely associated with the epithelial-mesenchymal transformation of endothelial cells into mesenchymal cushion tissue cells and was localized around these cells as they migrated into the cardiac jelly to form the initial connective tissue elements of the valve leaflets. However, after embryonic day 17.5 collagen XVIII expression decreased rapidly in the connective tissue and thereafter remained detectable only in the basement membranes of the endothelial layer covering the leaflets. The staining pattern observed within the AV endocardial cushions suggests that collagen XVIII may have a role in cardiac valve morphogenesis. These results may help us to better understand normal heart development and the aberrant mechanisms that cause cardiac malformations in Down's syndrome.

Structurally altered basement membranes and hydrocephalus in a type XVIII collagen deficient mouse line.

Type XVIII collagen/endostatin is known to be crucial for the eye, as witnessed by severe eye defects in Knobloch syndrome patients with mutations in this collagen and in Col18a1(-/-) mice. We show here that in a specific C57BL background, 20% of the Col18a1(-/-) mice developed hydrocephalus, and dilation of the brain ventricles was observed by MRI in all of the mutant mice. Significant broadening was observed in the epithelial basement membrane (BM) of the choroid plexuses (CP), its width being 86.4+/-10.52 nm, compared with 61.4+/-6.05 nm in wild-type mice. The CP epithelial cell morphology was balloon-shaped rather than cuboidal, and the microvilli of the apical surface of the CP epithelium contained more vacuoles in the null mice than in the wild-type, as also did the CP epithelial cells, which is suggestive of alterations in cerebrospinal fluid production. Analysis of BMs elsewhere in the body revealed a broadened epidermal BM in the Col18a1(-/-) mice, but this did not result in any apparent functional deficiencies. Moreover, markedly broadened BMs were found in the atrioventricular valves of the heart and in the kidney tubules, whereas the glomerular mesangial matrix of the kidneys was expanded in the mutant mice and serum creatinine levels were elevated, indicating alterations in kidney filtration capacity. We thus suggest that type XVIII collagen is a structurally important constituent of BMs, and that its absence can result in a variety of phenotypic alterations.