Directed cell invasion and migration during metastasis

Abstract

Metastasis requires tumor cell dissemination to different organs from the primary tumor. Dissemination is a complex cell motility phenomenon that requires the molecular coordination of the protrusion, chemotaxis, invasion and contractility activities of tumor cells to achieve directed cell migration. Recent studies of the spatial and temporal activities of the small GTPases have begun to elucidate how this coordination is achieved. The direct visualization of the pathways involved in actin polymerization, invasion and directed migration in dissemination competent tumor cells will help identify the molecular basis of dissemination and allow the design and testing of more specific and selective drugs to block metastasis.

Figure 1. Invasion, migration and intravasation in tumors

Tumor cells undergoing EMT form invadopodia and acquire a migratory phenotype. Degradation of basement membranes and extracellular matrix during migration is achieved by invadopodia. Cells migrate on the extracellular matrix (ECM) forming pseudopodia. There is evidence that invadopodia are involved in directional migration and chemotaxis at this step. Once tumor cells reach a blood vessel they are believed to again use invadopodia to degrade the basement membrane and enter the blood stream.

Figure 2. Invadopodia and locomotory protrusions are functionally coupled during both 2 and 3-D migration

In 2-D invadopodia and pseudopodia (lamellipodia) are separated in space. In 3-D both types of protrusions are found together at the cell front. Common signaling and cytoskeletal pathways used by both motility supporting protrusions such as pseudopodia and invasion specific protrusions such as invadopodia indicate a high degree of molecular integration and cross talk between pseudopodia and invadopodia allowing efficient invasion coupled migration in 2 and 3D.

Figure 3. Examples of Rho-family GTPase biosensors based on FRET

In all examples, the guanine nucleotide loading state (GDP versus GTP) confers either a conformational change within the molecule or to bring the binding pairs together to affect FRET between the cyan and yellow fluorescent proteins. A: The fluorescent protein pair for FRET is placed at the terminal ends of the molecule, necessitating an attachment of a CAAX-box at the C-terminus for the plasma membrane insertion [67^•]. This design compromises the GDI-GTPase interaction that requires the C-terminal lipid-moiety in addition to the Switch I/II accessibility by the GDI. B: The Rho GTPase biosensor based on a design that maintains the GDI-GTPase interaction by placing a full-length RhoA at the C-terminus of the molecule [65^•]. The RhoC GTPase biosensor is based on a similar design as B [48] . C: Bimolecular FRET biosensor for Rho GTPases [63]. This approach maximizes the total dynamic range of FRET while making the data processing and interpretation more challenging due to the non-equimolar distribution of the biosensor components in living cells.

Figure 4. A model for the spatial regulation of RhoC activity during invadopodium protrusion

Formation of a focused invadopodium is mediated by the spatiotemporal localization of RhoC activity outside invadopodia. RhoC activity increases around the invadopodial core structure, as shown by this plot of the maximum projection over time of RhoC activity. Pseudo-color shows low RhoC activity levels (blue) to high RhoC activity levels (red) in relation to low (white) and high (brown) cortactin intensity where cortactin marks the central core of the invadopodium. This activation pattern is achieved by activation of p190RhoGEF (green) outside and p190RhoGAP inside (yellow), restricting RhoC activity just outside the invadopodium core. This spatial restriction localizes active cofilin to the core of the structure and focuses actin polymerization so as to achieve optimum protrusion elongation and invasion (bottom part adapted from [68]).