Membrane-anchored proteases in endothelial cell biology

Abstract

Purpose of review: The endothelial cell plasma membrane is a metabolically active, dynamic, and fluid microenvironment where pericellular proteolysis plays a critical role. Membrane-anchored proteases may be expressed by endothelial cells as well as mural cells and leukocytes with distribution both inside and outside of the vascular system. Here, we will review the recent advances in our understanding of the direct and indirect roles of membrane-anchored proteases in vascular biology and the possible conservation of their extravascular functions in endothelial cell biology.

Recent findings: Membrane-anchored proteases belonging to the serine or metalloprotease families contain amino-terminal or carboxy-terminal domains, which serve to tether their extracellular protease domains directly at the plasma membrane. This architecture enables protease function and substrate repertoire to be regulated through dynamic localization in distinct areas of the cell membrane. These proteases are proving to be key components of the cell machinery for regulating vascular permeability, generation of vasoactive peptides, receptor tyrosine kinase transactivation, extracellular matrix proteolysis, and angiogenesis.

Summary: A complex picture of the interdependence between membrane-anchored protease localization and function is emerging that may provide a mechanism for precise coordination of extracellular signals and intracellular responses through communication with the cytoskeleton and with cellular signaling molecules.

Figure 1. Domain structures of the membrane-anchored serine proteases and metalloproteases associated with endothelial and vascular biology

The serine-type proteases are comprised of the membrane-anchored serine proteases, the proprotein convertases and the prolyl oligopeptidases. The metalloproteases consist of the membrane-type MMPs (MT-MMPs), the ADAMs (a disintegrin and metalloproteinase) and the angiotensin-converting enzymes (ACE). The catalytic domains, which function to hydrolyze the peptide bonds in protein substrates, are all located extracellularly. The catalytic domains of the membrane-anchored serine proteases are either chymotrypsin-like (S1 family), bacterial subtilisin-like (S8 family) or related to the prolyl oligopeptidases (S9 family). They all share the conserved catalytic triad amino acid residues His, Asp and Ser, albeit in different configurations. There are a total of 17 human type II transmembrane serine proteases, synthesized with a unique stem region adjacent to the plasma membrane, containing diverse protein-protein interaction domains such as scavenger receptor cysteine-rich (SRCR), CUB, SEA and Frizzled domains [46]. There are two human GPI-anchored serine proteases, testisin and prostasin. The proprotein convertases are all Type I transmembrane serine proteases containing a cysteine rich region and conserved P-domains which are essential for folding and activation. The prolyl oligopeptidases are synthesized as type II transmembrane homodimeric glycoproteins. The membrane-anchored metalloproteases differ from the membrane-anchored serine proteases in that they utilize a zinc ion to create a nucleophile for protease catalytic activity. The 6 MT-MMPs expressed in humans are either Type I (MT1-, MT2-, MT3- and MT5-MMP) or GPI-anchored (MT4- and MT6-MMP). All of the MT-MMPs share a similar hinge region and hemopexin domain, and are synthesized with an amino-terminal signal sequence and pro-domain which is cleaved by proprotein convertases during transport from the ER/Golgi to the surface, allowing the MT-MMPs to be expressed on the cell surface as active proteases [47]. The ADAM metalloproteinases share a complex domain structure containing the pro-domain followed by the metalloprotease catalytic domain, a disintegrin-like domain enabling cell-cell interactions via integrins, a cysteine-rich domain and an epidermal growth factor-like domain [48]. The ACE family of metalloproteases includes both ACE and ACE2 transmembrane proteases, which consist of an amino-terminal signal domain followed by a large extracellular domain that consists of either two independent (ACE) or one (ACE2) catalytic domains containing zinc binding motifs. The cytoplasmic extensions vary in length for these membrane-anchored proteases and are involved in membrane trafficking, and localization, with some participating in cell signaling.