Ultrastructural localization of Fras1 in the sublamina densa of embryonic epithelial basement membranes.

Fras1 is the first identified member of a protein family comprising Fras1 and the related extracellular matrix proteins Frem1, Frem2 and Frem3. Mutations in Fras1, Frem1 and Frem2 have been associated with the bleb phenotype in mouse, whereas mutations in the human orthologs FRAS1 and FREM2 have been implicated in the pathogenesis of the human Fraser syndrome. Bleb mutant mice are characterized by embryonic sub-epidermal blistering, unilateral or bilateral renal agenesis or dysgenesis, cutaneous syndactyly and fused eyelids. As revealed by immunofluorescence, Fras1 co-localizes with the markers of epithelial basement membranes and is ultrastructurally detected underneath the lamina densa of embryonic mouse epithelia. Since the loss of Fras1 mainly affects the cohesiveness of the embryonic skin basement membrane with its underlying mesenchyme, we compared here the ultrastructural localization of Fras1 in the dermal-epidermal junction and in the basement membrane of other embryonic epithelia that do not show any overt phenotype using preembedding immunocytochemistry. Fras1 immunoreactivity was detected in all epithelia examined, within the sublamina densa adjacent to stromal tissue, as clustered gold/silver enhanced depositions, usually attached to anchoring fibrils. Interestingly, clusters corresponding to Fras1 were frequently detected in close proximity to mesenchymal cells, indicating that Fras1 could serve as a direct link between the sublamina densa and mesenchyme. The localization of Fras1 is consistent with previous results indicating that Fras1 exerts its function below the lamina densa and that Fras1 displays the same localization pattern in all epithelial basement membranes.