Basement membrane turnover drivesfilopodial protease-independent invasion.

Basement membranes (BMs) are specialized, nanoporous extracellular matrices (ECMs) mainly composed of collagen IV fibres and laminins. Because collagen IV fibres form covalent cross-links (CCLs), cells cannot freely cross BMs. BM breaching marks the transition from in situ to invasive carcinoma, commonly attributed to protease activity. However, recent data show tumour cells crossing BMs through protease-independent mechanisms. Experimental evidence suggests that filopodia can play an active role in pore enlargement in extracellular matrices by generating plastic mechanical deformation. Moreover, plasticity can be regulated by CCL concentration, which we hypothesize responds to collagen IV turnover, thus generating transient weak spots. To test this, we developed a biophysical, mathematical model describing the interaction between a tumour-cell cluster (TCC) and a BM considering experimentally measured parameters of filopodial activity and collagen IV turnover taken from the literature. The cluster is represented as an evolving energetic surface, while the BM is described as a point set representing active and inactive links. Simulations show that synchronization of collagen IV turnover, coupled with filopodium extension, drives pore enlargement, providing a mechanistic basis for protease-independent invasion. Consistent with experimental observations, simulations identify two complementary filopodium groups: one driving global degradation and another promoting local pore enlargement.