Sheddases cleave membrane proteins at the cell surface, releasing soluble ectodomains with altered location and function. Some sheddases are membrane proteins themselves that belong to metalloprotease (ADAM and MMP) or aspartic protease (BACE) families. Their activity can be constitutive or regulated through various processes such as PKC activation, Ca2+ influx, and lipid rafts.1-3
A single sheddase may cleave a variety of substrates. A classic example in this category, ADAM17, was initially identified as TNF-alpha-converting Enzyme (TACE), and is known to shed a variety of growth factors, receptors, and adhesion molecules.1,4 This suggests that overall conformations of the substrates are more important than primary amino acid sequences in determining the accessibility for cleavage by sheddases.
Multiple sheddases can cleave the same substrate. Under certain circumstances this may result in different consequences. ADAM17, ADAM10, and MMP-14/MT1-MMP are all known to shed CD44, an adhesion molecule that interacts with hyaluronic acid in the ECM.2 Additionally, amyloid precursor protein (APP) processing by alpha, or by beta- and gamma-secretases has differential effects on the production of alpha beta peptide, a major plaque component found in brains of Alzheimer’s disease patients. Cleavage of APP by beta-secretase (BACE-1 and -2) creates a substrate for gamma-secretase, resulting in alpha beta peptide production. In contrast, alpha-secretase (ADAM10, 17 and 9) cleavage between the beta- and gamma-secretase sites prevents alpha beta peptide production.5 Juxtamembrane cleavage that creates a substrate for further processing that results in the release of a cytoplasmic domain has been termed regulated intramembrane proteolysis (RIP). Notch processing by ADAM10 RIP liberates the Notch intracellular domain, allowing it to translocate to the nucleus and activate the transcription of target genes.6
|Figure 1. Diagram illustrating how sheddases alter ligand (left) or receptor (right) location and function.|
Sheddases can act as “thermostats” that either up- or down-regulate the activity of their substrates. Using a transmembrane ligand/receptor as an example, shedding may remove/terminate the molecule locally, yield a decoy that sequesters soluble counterparts, or transduce a signal in conjunction with RIP as described above (Figure 1). The SARS-CoV receptor, also known as ACE-2, is shed by ADAM17 and the soluble ACE-2 is able to block cell binding by SARS-CoV spike protein.7 Similarly, several soluble cytokine receptors, such as sIL-15 R,8 compete with membrane-bound receptors, while others including sIL-6 R,9 remain agonistic when cytokine is bound.
Sheddases can also act as “travel agents,” regulating cell adhesion and migration. For example, CD44-dependent cell migration is proposed to occur by ADAM17-mediated CD44 ectodomain shedding at the leading edge of the cell. Extension of the lamellipod triggers Ca2+ influx, and CD44 shedding by Ca2+ -activated ADAM10 at the trailing edge facilitates detachment. RIP creates a cytoplasmic CD44 fragment that promotes new CD44 synthesis. This link between CD44 proteolysis and new transcription results in rapid turnover of CD44, facilitating efficient cell migration.2 Similar models also fit with currently available data for L-selectin,4 E-Cadherin/beta-catenin,10 NCAM-L1,11 and other adhesion molecules. Soluble adhesion ectodomains can be functional; sL-selectin directs migration of activated leukocytes,12 while sE-cadherin causes scattering of epithelial cells and induction of invasion.10 Association of transmembrane Ephrins and their Eph receptors on opposing cells often results in cell-cell repulsion. New data on ADAM10 shedding of Ephrin/Eph complex has explained this paradox.13 Associated with Eph on one cell surface, ADAM10 cleaves Ephrin within the Ephrin/Eph complex formed between two cell surfaces. When ephrin is freed from the opposing cell, the entire Ephrin/Eph complex is endocytosed. This shedding in trans had not been previously shown, but may well be involved in other shedding events.