Cytoskeleton Dynamics

What is the first step in invadopodia assembly?

Initiation of invadopdia formation is highly complex, being influenced by various signalling cascades and phosphorylation events that occur following detection of a stimulant.

For example, activation of the epidermal growth factor (EGF) receptor by EGF has been shown to trigger and/or enhance invadopodia formation [1] as have platelet-derived growth factor (PDGF) and reactive oxygen species (ROS) (as reviewed in [2]). Following stimulation of the EGF receptor, the tyrosine kinase Src is activated, followed by the Abelson-related nonreceptor tyrosine kinase, Arg [3]. Src has traditionally been known as a vital component in the signaling cascade that governs the initiation of invadopodia formation [4][5][6][7] however more recent studies have also highlighted the importance of other Src family kinases [8] and protein kinase C [9] in this process.

Activation of these kinases are reported to increase the number of free actin barbed ends and lead to the phosphorylation of cortactin [10] and the scaffolding protein Tks5 (tyrosine kinase substrate with 5 SH3 domains) [7]. Together these events promote actin filament polymerization and maturation of the invadopodium [10]. Tks5 is also associated with the production of invadopodia in response to ROS [11].

In addition, the GTPase Cdc42 has been implicated in invadopodia formation via the regulation of neuronal Wiskott-Aldrich Syndrome protein (N-WASP), which acts in concert with the Arp2/3 complex to promote actin polymerization [9]. Similarly, cortactin is able to promote actin polymerization through binding and activating the Arp2/3 complex [12]. Live-cell imaging experiments have shown the accumulation of cortactin in invadopodia precedes matrix metalloproteinases (MMPs) accumulation and matrix degradation [5].

Ultimately the outcome of initiating invadopodia formation has been speculated to result in the production of a podosome-like precursor. This precursor structure comprises a small branched actin network, which is suggested to continue to polymerize and extend deeper into the extracellular matrix (ECM) [13] as MMPs are released from the tip of the elongating structure.

How do invadopodia disassemble?

The final step in function of invadopodia is disassembly which primarily involves dismantling the actin core [14]. Several proteins have been implicated in a cascade leading to this, including paxillin, extracellular signal-regulated kinases (Erk) and calpain [14]. The phosphorylation state of tyrosine residues within paxillin localized to invadopodia controls the rate of disassembly. The mutation of specific tyrosine residues in paxillin render these sites non-phosphorylatable and this results in a significant delay in the disassembly of invadopodia. Furthermore, phosphorylation of paxillin was shown to promote the activation of Erk, which promotes the activation of calpain [14]. Calpains are calcium dependent, non-lysosomal, cysteine proteases. The activation of calpain 2 has been shown to be required for the degradation of cortactin, a component of the actin core and this subsequently promotes invadopodia disassembly [15]. The inhibition of calpain [14] or the expression of calpain-resistant cortactin in cells lacking endogenous cortactin [15], results in a decrease in the rate of invadopodia disassembly. This is akin to the role of calpain in the degradation of talin [16] and focal adhesion kinase [17], which has been shown to promote focal adhesion disassembly.

References

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