"Breaching the Basement Membrane: Netrin Receptor Oscillations Help Guide the Way"

Cell invasion through basement membrane, the thin, dense, highly cross-linked extracellular matrix that surrounds most tissues, is critical in many developmental processes and underlies the lethal progression of most cancers. Despite its significance, the mechanisms that control invasive behavior are poorly understood. During C. elegans larval development, the anchor cell, a specialized gonadal cell, breaches the gonadal and ventral epidermal basement membranes to contact the vulval precursor cells and initiate uterine-vulval connection. We have found that the netrin pathway plays a key role in regulating the polarization of the anchor cell towards its basement membrane target. The netrin ligand and its receptor DCC, directs diverse polarizing activities in animals. How DCC polarizes towards netrin, however, is poorly understood. By performing live-cell imaging of the C. elegans DCC ortholog UNC-40 during anchor cell invasion, we have unexpectedly found that UNC-40 clusters, recruits F-actin effectors and generates F-actin in the absence of UNC-6 (netrin). Time-lapse analysis revealed that polarized UNC-40 clusters assemble, disassemble and reform again at periodic intervals in random regions of the cell membrane. This oscillatory behavior indicates that UNC-40 polarizes through a self-organizing mechanism involving interlinked positive (formation) and negative (disassembly) feedback. Though not required for UNC-40 self-organizing behavior, we show that endogenous UNC-6 and ectopically provided UNC-6 orients and stabilizes UNC-40 clustering. Furthermore, the UNC-40 binding protein MADD-2/Mid1 promotes self-organized clustering and is required for UNC-40 polarization towards UNC-6. Together, our data suggests netrin directs polarized responses by stabilizing the localization of randomly directed self-organized DCC clustering to regions of the cell in contact with netrin.