Antigen retrieval of loricrin epitopes at desmosomal areas of cornified cell envelopes: an immunoelectron microscopic analysis.

Abstract

Cell envelopes (CEs) are insoluble, chemically and mechanically tough structures formed during terminal differentiation of keratinocytes, providing skin with a protective barrier against the environment. They are 15 to 20 nm thick structures beneath the plasma membrane and continuous with desmosomal attachment plaques. Sequential deposition of several proteins including involucrin and loricrin leads to a gradual increase in envelope thickness and rigidity. Cross-linking of desmosomal components to other CE-proteins has been demonstrated and desmosomes in the cornified cells have been regarded as a part of CEs. Our previous immunoelectron microscopy studies showed that desmosomal areas of granular cells were loricrin-positive, but those in cornified cells were negative. We asked whether this is due to epitope masking and applied trypsin digestion of the electron microscopy sections to retrieve the possibly masked epitopes. Since this treatment made desmosomal structures obscure, one side of the sections was stained with anti-desmoglein antibody as an indicator of desmosomes. Trypsin was applied on the other side followed by immunolabeling with anti-loricrin antibody. Trypsin digestion indeed unmasked the loricrin epitopes in the desmoglein-positive desmosomal areas of CEs. It seems therefore that loricrin is first accumulated at the desmosomes before the CE-assembly and cross-linking of loricrin occurs at the desmosomal areas of CEs as well as at the non-desmosomal areas.