Digging a little deeper: the stages of invadopodium formation and maturation

Abstract

Invadopodia are actin-rich protrusions that degrade the extracellular matrix and are required for penetration through the basement membrane, stromal invasion and intravasation. Invadopodia are enriched in actin regulators, such as cortactin, cofilin, N-WASp, Arp2/3 and fascin. Much of the work to date has centered around identifying the proteins involved in regulating actin polymerization and matrix degradation. Recently, there have been significant advances in characterization of the very early stages of invadopodium precursor assembly and the role of adhesion proteins, such as β1 integrin, talin, FAK and Hic-5, in promoting invadopodium maturation. This review summarizes these findings in the context of our current model of invadopodial function and highlights some of the important unanswered questions in the field.

Keywords: Arg; Cdc42; Cofilin; Invadopodia; Invasion; Metastasis; Moesin; NHE-1; Talin; β1 integrin.

Figure 1. Stages of invadopodium maturation

Stage 1 (early precursor stage): invadopodia initially form as non-degradative precursors that consist of a core structure containing actin, cortactin, cofilin, N-WASp, Tks5 and other proteins. Stage 2 (late precursor stage): kinases are activated, β1 integrin and talin are recruited and Tks5 anchors the precursor to PI(3,4)P₂. Stages 3-4 (mature invadopodium stage): in stage 3, actin polymerization is activated by stimulation of the NHE-1-cofilin pathway, and continued actin polymerization drives invadopodial elongation and stabilization. In stage 4, microtubule and intermediate filament recruitment facilitates further elongation of the protrusion, and matrix proteases are recruited to degrade the ECM (modified from Artym et al., 2006; Oser et al., 2009; Schoumacher et al., 2010; Sharma et al., 2013).

Figure 2. Integrative signaling diagram of invadopodial assembly and maturation

Invadopodia initially form as precursors in response to EGF or other stimuli (e.g. TGF-β or PDGF). Src is activated either directly by EGFR or by PTP1B (Cortesio et al., 2008). These stimuli induce Cdc42 activation, leading to assembly of the precursor core structure (red text within circle; Razidlo et al., 2014; Yamaguchi et al., 2005). The invadopodium precursor is then anchored by binding to PI(3,4)P₂ and further stabilized by β1 integrin-mediated adhesion to the ECM (Beaty et al., 2013; Sharma et al., 2013). Invadopodium maturation begins as β1 integrin activates Arg, which phosphorylates cortactin on Y421 to recruit Nck1 (Beaty et al., 2013; Oser et al., 2010). Talin localizes to the structure and recruits a complex of moesin and NHE-1 through a direct binding interaction with moesin (Beaty et al., 2014). The intracellular pH increases in response to NHE-1 activity, which disrupts the inhibitory interaction between cortactin and cofilin (Busco et al., 2010; Magalhaes et al., 2011). Cofilin severs F-actin to form barbed ends that are used to elongate filaments, on which Nck1 induces N-WASp-Arp2/3-dependent dendritic nucleation (DesMarais et al., 2004). Actin polymerization is required for MMP-dependent matrix degradation at invadopodia, possibly through MMP recruitment (Oser et al., 2009; Sakurai-Yageta et al., 2008; Yamaguchi et al., 2005). MT1-MMP is delivered by the IQGAP1-WASH-exocyst complex and fuses to the membrane via the v-SNARE VAMP7, resulting in matrix degradation (Monteiro et al., 2013; Sakurai-Yageta et al., 2008; Steffen et al., 2008). Cortactin, cttn; synaptojanin2, synj2.