Abstract
Cellular factor XIII (cFXIII, FXIII-A2), a transglutaminase, has been demonstrated in a few cell types. Its main function is to cross-link proteins by isopeptide bonds. Here, we investigated the presence of cFXIII in cells of human cornea. Tissue sections of the cornea were immunostained for FXIII-A in combination with staining for CD34 antigen or isopeptide cross-links. Isolated corneal keratocytes were also evaluated by immunofluorescent microscopy and flow cytometry. FXIII-A in the corneal stroma was quantified by Western blotting. FXIII-A mRNA was detected by RT-qPCR. The cornea of FXIII-A-deficient patients was evaluated by cornea topography. FXIII-A was detected in 68 ± 13% of CD34+ keratocytes. Their distribution in the corneal stroma was unequal; they were most abundant in the subepithelial tertile. cFXIII was of cytoplasmic localization. In the stroma, 3.64 ng cFXIII/mg protein was measured. The synthesis of cFXIII by keratocytes was confirmed by RT-qPCR. Isopeptide cross-links were detected above, but not within the corneal stroma. Slight abnormality of the cornea was detected in six out of nine FXIII-A-deficient patients. The presence of cFXIII in human keratocytes was established for the first time. cFXIII might be involved in maintaining the stability of the cornea and in the corneal wound healing process.
Highlights
The transglutaminase (TG) family consists of nine members, eight of which blood coagulation factor XIII (FXIII) and TGs 1-7 are enzymatically active, while the enzymatically non-active erythrocyte band 4.2 protein belongs to this group on the basis of structural similarity
The potentially active component of plasma FXIII (pFXIII), FXIII-A2 is a cellular enzyme present in a number of cell types. cFXIII was first discovered in platelets [6,7]; it amounts to 3% of the total platelet proteins [8]. cFXIII in resting platelets is of cytoplasmic localization [9,10], activation by strong stimuli, like thrombin+collagen, can translocate part of it to the platelet surface [11]
Cells stained with an antibody specific for FXIII-A were abundant in the corneal stroma but absent in the epithelium
Summary
The transglutaminase (TG) family consists of nine members, eight of which blood coagulation factor XIII (FXIII) and TGs 1-7 are enzymatically active, while the enzymatically non-active erythrocyte band 4.2 protein belongs to this group on the basis of structural similarity. TGs in their enzymatically active form (protein-glutamine:amine γ-glutamyltransferase; EC2.3.2.13) catalyze an acyl transfer reaction in which the carboxamide group of a peptide bound glutamine residue is the acyl donor and a primary amine is the acyl acceptor. As opposed to all other TGs, plasma FXIII (pFXIII) is of tetrameric structure; it consists of two potentially active protransglutaminase A subunits (FXIII-A2) and two inhibitory/protective non-enzymatic B subunits (FXIII-B2). The potentially active component of pFXIII, FXIII-A2 is a cellular enzyme (cFXIII) present in a number of cell types. In subsequent studies cFXIII was demonstrated in monocytes/macrophages [12,13,14], in cells of mineralized tissue, i.e., in osteoblasts, osteoclasts, osteocytes [15,16], and most recently, in preadipocytes [17]
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