Epitope mapping of the laminin molecule in murine skin basement membrane zone: demonstration of spatial differences in ultrastructural localization

Results of studies performed to date with polyclonal antilaminin antibodies have been conflicting as to the ultrastructural localization of this glycoprotein in skin basement membrane zone (BMZ). Whereas initial reports suggested its presence solely within the lamina lucida (LL), others have suggested that laminin is instead an exclusive component of the lamina densa (LD). In an attempt to more critically address this issue, we have examined both intact and partially separated (via 1 M NaCl) murine skin BMZ by indirect immunoelectron microscopy via a two-step immunoperoxidase technique on unfixed cryopreserved tissue, utilizing nine well-characterized monoclonal antibodies with binding specificity for laminin. Localization of the sites of the epitopes recognized by these antibodies on isolated laminin molecules was previously determined by rotary shadowing and by biochemical analyses on enzymatic fragments of laminin. Whereas at least faint immunoreactants were detected in both regions with eight of nine antibodies, predominant staining was noted within the LL with three of eight and within (and even sparsely below) the LD in three of eight. One antibody bound solely to the LL; another bound equally within both regions. Although some overlap was noted, it appears that the epitope on the distal portion of the long arm of the laminin molecule resides primarily within the skin LD, whereas epitopes on more central portions of the short arms are present within the LL or within both LL and LD. The findings of stratification of laminin epitopes within skin BMZ supports a similar recent observation in mouse kidney and suggests that portions of the laminin molecule span both LD and LL, and that there may be a non-random spatial orientation for the laminin molecule within murine skin BMZ.