A new front in cell invasion: The invadopodial membrane

Abstract

Invadopodia are F-actin-rich membrane protrusions that breach basement membrane barriers during cell invasion. Since their discovery more than 30 years ago, invadopodia have been extensively investigated in cancer cells in vitro, where great advances in understanding their composition, formation, cytoskeletal regulation, and control of the matrix metalloproteinase MT1-MMP trafficking have been made. In contrast, few studies examining invadopodia have been conducted in vivo, leaving their physiological regulation unclear. Recent live-cell imaging and gene perturbation studies in C. elegans have revealed that invadopodia are formed with a unique invadopodial membrane, defined by its specialized lipid and associated protein composition, which is rapidly recycled through the endolysosome. Here, we provide evidence that the invadopodial membrane is conserved and discuss its possible functions in traversing basement membrane barriers. Discovery and examination of the invadopodial membrane has important implications in understanding the regulation, assembly, and function of invadopodia in both normal and disease settings.

Keywords: Basement membrane; C. elegans; Cell invasion; Endocytic membrane trafficking; Invadopodia; Invadopodial membrane.

Fig. 1. Models of Cofilin and GDI-1 function in invadopodial membrane trafficking in the C. elegans anchor cell

(A) Schematic images of invadopodial membrane trafficking within the anchor cell during anchor cell invasion into the vulval cells. F-actin and actin regulators (red) are coupled with invadopodial membrane trafficking (green) to form invadopodia at the anchor cell-basement membrane (BM, shown in magenta) interface prior to invasion. Both cofilin (encoded by the C. elegans unc-60a gene) and GDI-1 regulate trafficking of the invadopodial membrane through the endolysosome system. RNAi-mediated knockdown of cofilin results in a static, non-trafficked invadopodial membrane that is localized internally, an accumulation of stagnant F-actin along the invasive cell membrane, and an absence of invadopodia formation. Loss of GDI-1 results in mis-targeting of invadopodial membrane to all cell membranes and a reduction in the number of invadopodia. (B) A grayscale image of the invadopodial membrane observed with mCherry::PLCδ^PH, which binds to the invadopodial membrane component PI(4,5)P₂ (highlighted by white arrows). In wild-type animals, the invadopodial membrane is localized along the invasive cell membrane. Loss of cofilin (through cofilin RNAi) results in a static, non-trafficked internal membrane and reduction of GDI-1 protein (through gdi-1 RNAi) causes the invadopodial membrane to be trafficked to all cell membranes. (C) Left, RNAi-mediated loss of the cofilin or GDI-1 results in invasion defects as seen by an inability of the anchor cell (green—visualized with either the invadopodial membrane probe mCherry::PLCδ^PH or the F-actin probe mCherry::MoeABD) to breach the basement membrane (magenta; visualized with laminin::GFP) overlaid on DIC. Right, a grayscale image of laminin::GFP. Arrow shows position of basement membrane breach in wild-type and a lack of breach after loss of cofilin or GDI-1. Bars, 5 μm.