Transmigration of leukocytes from the blood
stream through the endothelial cell layer is essential for proper immune
responses to fight invading pathogens. Several adhesion molecules on leukocytes
and endothelial cells, such as CD99, have been proposed to significantly
trigger the transmigration process during inflammation. CD99, a type-I-transmembrane
protein, is expressed in most hematopoietic cells and was shown to accumulate at
endothelial cell-cell contacts. In a mass spectrometry based proteomics
approach, CD99 has been identified as putative substrate for the
metalloprotease meprin β, a dimeric membrane bound enzyme involved in collagen
assembly, neurodegeneration and inflammation. Meprin β is associated with
pulmonary hypertension (PH) characterized by lung fibrosis and inflammation,
and we hypothesized that dysregulation of the protease leads to altered CD99
levels. Indeed, CD99 levels were decreased in Fra2 transgenic mice which serve
as a model for PH and that show strong upregulation of meprin β expression in
the lung. Interestingly, this effect was reverted in lung tissue of meprin β
knockout mice where a significant accumulation of CD99 was detected. In vitro assays and cell culture
experiments confirmed meprin β-mediated cleavage resulting in ectodomain
shedding of CD99. Confocal microscopy experiments showed strong colocalisation of
meprin β and CD99 in overexpressing Cos7 cells. Additionally, recombinant meprin
β has been shown to influence the permeability of monolayers of endothelial
cells in vitro, indicating decreased
cell-adhesiveness. Experiments using the γ-secretase inhibitor DAPT in cell
culture revealed CD99 to be modulated at the cell surface by regulated
intramembrane proteolysis (RIP) initiated by meprin β. Subsequently this might
lead to the induction of downstream signaling events by the release of the CD99
intracellular domain from the membrane, which may be of importance for
regulation of cell adhesion and apoptosis during inflammatory and fibrotic
diseases.