Given that receptor:ligand binding events are critical for intercellular signaling, proteolytic processing of transmembrane receptors and ligands can have a dramatic impact on signaling-mediated cellular responses. Proteolysis of eukaryotic proteins is performed by members of the metzincin superfamily, including matrix metalloproteinases (MMPs), A disintegrin and metalloproteinases (ADAMs), and ADAM-thrombospondins (ADAM-TS). Although all active metzincin proteases contain zinc-binding motifs within their protease domains, the presence of a transmembrane domain is characteristic of the ADAMs family. Thus, studies examining the processing of receptors and ligands expressed on cell surfaces have focused on ADAMs, which perform ectodomain shedding as well as regulated intramembrane proteolysis (RIP) of transmembrane proteins. Shedding extracellular domains of membrane-anchored proteins releases soluble fragments into extracellular space. This event can down-regulate signaling events that require transmembrane receptor ex
A Disintegrin and metalloproteinases (ADAMs) regulate cell signaling pathways by cleaving the extracellular domains of membrane-bound receptors and ligands. Consequently, these proteins serve as initiators for signaling pathways that require regulated intramembrane proteolysis (RIP) of receptor:ligand complexes. The shedding of membrane anchored proteins releases soluble fragments into the extracellular milieu that can subsequently modulate signaling events. Although ectodomain shedding is thought to occur constitutively, RIP requires the binding of ligands expressed on adjacent cells. Numerous receptors including Notch undergo regulated proteolysis of the receptor: ligand complex to release their intracellular domains (ICD), that subsequently translocates to the nucleus and alters gene ex
Fig. 1 ADAM structure. ADAMs are composed of multiple domains, as depicted in this figure.
The prototypical ADAM contains an inhibitory pro-domain, a highly conserved metalloprotease domain, a disintegrin domain conferring substrate specificity, a cysteine-rich region, a transmembrane portion, and a cytoplasmic tail capable of binding SH3 domains. Among the 38 ADAMs identified to date, the most studied are ADAMs 8, 9, 10, 12, 15, 17, and 33, that contain the conserved zinc-binding consensus motif which confers proteolytic activity to the protease domain. Within this subset, ADAM10 has emerged as an important mediator of ectodomain shedding and RIP of multiple substrates including epidermal growth factor (EGF), Fas-ligand, CD23, and most notably Notch. This proteolytic processing is critical for appropriate cellular processes and its dysregulation results in the pathogenesis of multiple disease states, including Alzheimer’s, cancer, and inflammation. Thus, there is growing interest in elucidating ADAM10 as well as its inhibition for pharmacologic treatment. However, determination of the physiologic consequences of ADAM10-mediated cleavage events has been limited by lethality of ADAM10-null murine embryos. These same studies highlight the important role of ADAM10 in hematopoietic cell development mediated via regulation of Notch signaling.
Many studies have indicated the importance of Notch signaling in lymphocyte development. The Notch signaling pathway is highly conserved, consisting of four families of receptors (Notch1-4) that interact with ligands (Jagged and Delta) expressed by neighboring cells. Following ribosomal synthesis, the Notch receptor undergoes a furin-mediated maturation at site 1 (S1) in the Golgi apparatus prior to trafficking to the cell surface. At the surface, Notch is expressed as an integral membrane protein, consisting of both extracellular (NEXT) and intracellular domains (NICD). Once engaged with its ligand, the extracellular domain undergoes an ADAM10-mediated cleavage at site 2 (S2). This event generates a substrate for the γ-secretase complex to perform a final cleavage of Notch at site 3 (S3), releasing the transcriptionally active NICD. Several studies have reported the accumulation of intact receptor and the S2 product as a result of ADAM10 and γ-secretase blockade, respectively(2, 4)- CITATION_IS_EMPTY. Although inhibition of both enzymes prevents NICD activation, the consequences of accumulation of these different cleaved products on hematopoiesis remains to be determined.