Cut loose and run: The complex role of ADAM proteases during neural crest cell development

Abstract

ADAM metalloproteases have been shown to play critical roles during development. In this review, we will describe functional evidence that implicates ADAM proteins during the genesis, migration and differentiation of neural crest cells. We will restrict our analysis to the transmembrane ADAMs as other reviews have addressed the role of extracellular metalloproteases (Christian et al. [2013] Critical Reviews in Biochemistry and Molecular Biology 48:544-560). This review will describe advances that have been obtained mainly through the use of two vertebrate model systems, the frog, and avian embryos. The role of the principal substrates of ADAMs, the cadherins, has been extensively described in other reviews, most recently in (Cousin [1997] Mechanisms of Development 148:79-88; Taneyhill and Schiffmacher [2017] Genesis, 55). The function of ADAMs in the migration of other cell types, including the immune system, wound healing and cancer has been described previously in (Dreymueller et al. [2017] Mediators of Inflammation 2017: 9621724). Our goal is to illustrate both the importance of ADAMs in controlling neural crest behavior and how neural crest cells have helped us understand the molecular interactions, substrates, and functions of ADAM proteins in vivo.

Keywords: Xenopus; avian; development; disintegrin; metalloprotease; neural crest.

Figure 1. ADAM13 processing in neural crest cells

ADAMs are proteins that contain a Disintegrin (D) and Metalloprotease (M) domain. They are synthesized as pro-enzymes that get cleaved by Furin during the transit to the cell surface (elimination of the pro-domain not presented here). At the surface, ADAM13 cleaves both transmembrane proteins such as Cadherins, Protocadherins and Ephrin-B as well as extracellular matrix proteins such as fibronectin (FN). ADAM13 undergoes additional processing steps including self-proteolysis (right) within the cysteine (C) rich domain that releases the metalloprotease (M) domain associated with the disintegrin (D) and part of the cysteine (C) rich domain in the extracellular compartment. This protein remains proteolytically active and can bind to FN and Laminin (LN). The EGF repeat (E) remains associated with the transmembrane and cytoplasmic (Cyt) domains and becomes a substrate for γ-secretase (GS), which can cleave the cytoplasmic domain to allow it to translocate to the nucleus. Alternatively (left), the metalloprotease (M) domain can be cleaved to produce a non-proteolytic ADAM, which is the substrate of two kinases, GSK3 and Polo like kinase (Plk). The function of that form of ADAM13 is unknown, but the phosphorylation is critical for CNC migration.

Figure 2. ADAM molecular functions in neural crest cells

ADAMs have been shown to regulate the Wnt/β-catenin signaling pathway in two ways. ADAM13 can cleave the Ephrin B ligand, thereby blocking signaling that represses β-catenin. Thus, ADAM13 cleavage of Ephrin B increases β-catenin signaling, resulting in the robust expression of β-catenin target genes such as Snail2, CyclinD1 and myc. Second, by cleaving cadherins that associate with β-catenin at the membrane, ADAMs can initiate a series of proteolytic processing events, leading to the release of the cadherin cytoplasmic domain/C-terminal fragment (CTF2) still associated with β-catenin and thus stimulating this pathway. By cleaving cell adhesion molecules such as cadherins, ADAMs positively control the epithelial-to-mesenchymal transition (EMT) as well as negatively affect contact inhibition of locomotion (CIL). EMT is also stimulated by the activation of β-catenin target genes, which can occur through the formation of a CTF2/β-catenin complex. In the quail trunk neural crest, ADAM10 processes N-cadherin, forming a CTF2 that increases expression of β-catenin and CyclinD1, while chick cranial neural crest cells use ADAM10 and ADAM19 to cleave Cadherin-6B, liberating a CTF2/β-catenin complex that associates with chromatin and activates these and other EMT effector genes, including Snail2. This pro-EMT role for a cadherin cleavage fragment is also observed for the Cadherin-6B NTF, which furthers EMT by enhancing Laminin (LN) degradation, likely through activation of MMPs. ADAM13 cleavage of Cadherin-11 produces an extracellular fragment, EC1-3, that binds to the ErbB2/3 complex and stimulates the phosphorylation of AKT in CNC and eventual CNC migration. EC1-3 increases the persistence of migration while AKT phosphorylation has been shown to increase N-cadherin, a protein critical for CIL and controlling the onset of CNC migration. ADAM13 also controls the nuclear translocation of Arid3a and its ability to induce Tfap2α to define the neural plate border and, subsequently, the neural crest. At least two genes, PCNS and Calpain8a, which lie downstream of ADAM13/Arid3a/Tfap2α, are critical for CNC migration.