Regulation of Contact Inhibition of Locomotion and Collective Cell Migration in tumor cells by a tetrameric JAM - Tetraspanin - αvβ5 integrin complex

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Cell migration requires the polarized formation of membrane protrusions, in particular lamellipodia, at the leading edge of the cell. The formation of lamellipodia is a highly dynamic process and is regulated predominantly by the Arp2/3 complex, which triggers the formation of new actin filaments at the lateral sides of preexisting ones thus generating a branched network of actin filaments. When individually migrating cells develop new cell-cell contacts, for example during development of during wound healing, lamellipodia formation must be halted, a process referred to as contact inhibtion of locomotion (CIL). When cells migrate as clusters, called collective cell migration (CCM), lamellipodia activity is highly polarized being restricted to the leading edges of the cells within the cluster (cryptic lamellipodia) but absent at the lateral edges of the cells. Both processes, CIL and CCM, point to the existence of cell-cell contact-based mechanisms that inhibit of lamellipodia formation at the interface of bounded cells. The focus of our work are Junctional Adhesion Molecules (JAMs). We have identified a tetrameric protein complex, in which JAM-A is linked to αvβ5 integrin through the two tetraspanins CD9 and CD81 (JAM-Tspn-αvβ5 complex). Disrupting the complex, either by simultaneous depletion of CD9 and CD81 or by depletion of JAM-A, increases the activities of c-Src, Erk1/2 and Abi1, which are part of a signalling cascade that regulates the Arp2/3 complex. Our findings therefore strongly suggest that the JAM-Tspn-αvβ5 complex negatively regulates the Arp2/3 complex and lamellipodial activity. In accordance with this, simultaneous depletion of CD9 and CD81 increased cell motility, and depletion of JAM-A or expression of a JAM-A mutant that does not localize at cell junctions resulted in multilayered growth of cells reminiscent of impaired CIL. The aim of this study is to characterize the role of the JAM-Tspn-αvβ5 complex during CIL and CCM in detail. We will biochemically characterize the JAM-Tspn-αvβ5 complex and identify the downstream targets through which the complex regulates the Arp2/3 complex. We will study its role in lamellipodia dynamics of single cells during processes that depend on the Arp2/3 complex, including cell spreading, cell migration in 2D and in 3D, and during mechanical force generation. We will analyze its contribution to the downregulation of lamellipodial activity in response to cell-cell contact formation during CIL. And we will address its putative role in inhibiting lamellipodial activity at the lateral borders of cells migrating in a cluster during CCM. We expect to characterize a molecular mechanism that prevents lamellipodial activity at cell-cell contacts, which has strong implications for processes like embryonic development, wound healing and tumor dissemination.