GSK3 and lamellipodin balance lamellipodial protrusions and focal adhesion maturation in mouse neural crest migration

Cell extrusion drives neural crest cell delamination

Neural crest cells (NCC) comprise a heterogeneous population of cells with variable potency that contribute to nearly every tissue and organ throughout the body. Considered unique to vertebrates, NCC are transiently generated within the dorsolateral region of the neural plate or neural tube during neurulation. Their delamination and migration are crucial for embryo development as NCC differentiation is influenced by their final resting locations. Previous work in avian and aquatic species revealed that NCC delaminate via an epithelial-mesenchymal transition (EMT), which transforms these progenitor cells from static polarized epithelial cells into migratory mesenchymal cells with fluid front and back polarity. However, the cellular and molecular mechanisms facilitating NCC delamination in mammals are poorly understood. Through time-lapse imaging of NCC delamination in mouse embryos, we identified a subset of cells that exit the neuroepithelium as isolated round cells, which then halt for a short period prior to acquiring the mesenchymal migratory morphology classically associated with delaminating NCC. High-magnification imaging and protein localization analyses of the cytoskeleton, together with measurements of pressure and tension of delaminating NCC and neighboring neuroepithelial cells, revealed that round NCC are extruded from the neuroepithelium prior to completion of EMT. Furthermore, cranial NCC are extruded through activation of the mechanosensitive ion channel, PIEZO1. Our results support a model in which cell density, pressure, and tension in the neuroepithelium result in activation of the live cell extrusion pathway and delamination of a subpopulation of NCC in parallel with EMT, which has implications for cell delamination in development and disease.

The NHSL1-A complex interacts with the Arp2/3 complex and controls cell migration efficiency and chemotaxis

Cell migration is crucial for development and deregulation causes diseases. The Scar/WAVE complex promotes mesenchymal cell migration through Arp2/3 mediated lamellipodia protrusion. We previously discovered that all isoforms of Nance-Horan Syndrome-like 1 (NHSL1) protein interact directly with the Scar/WAVE complex and the NHSL1-F1 isoform negatively regulates Scar/WAVE-Arp2/3 activity thereby inhibiting 2D random cell migration. Here, we investigate the NHSL1-A1 isoform, which contains a Scar homology domain (SHD). The SHD in Scar/WAVE mediates the formation of the Scar/WAVE complex. We found that the SHD of NHLS1-A is sufficient for the formation of an NHSL1-A complex composed of the same proteins as the Scar/WAVE complex, but NHSL1-A replaces Scar/WAVE. NHSL1-A SHD recruits the NHSL1-A complex to lamellipodia, where also the Scar/WAVE complex resides. Scar/WAVE contains a WCA domain, which is phosphorylated by CK2 and recruits and activates the Arp2/3 complex to nucleate branched actin networks supporting lamellipodial protrusion. We identified a WCA domain in NHSL1 which interacts with the Arp2/3 complex. The NHSL1 WCA domain is phosphorylated by GSK3, and this increases the interaction with the Arp2/3 complex. In contrast to NHSL1-F1, the NHSL1-A complex promotes cell migration speed but not cell persistence via the Scar/WAVE complex and potentially via its WCA domain. In addition, the NHSL1-A complex is required for chemotaxis. Mechanistically, the NHSL1-A complex may increase lamellipodial Arp2/3 activity and lamellipodial speed while reducing lamellipodial persistence. Our findings reveal an additional layer of Arp2/3 complex control essential for mesenchymal cell migration highly relevant for development and disease.

Cell extrusion - a novel mechanism driving neural crest cell delamination

Neural crest cells (NCC) comprise a heterogeneous population of cells with variable potency, that contribute to nearly every tissue and organ system throughout the body. Considered unique to vertebrates, NCC are transiently generated within the dorsolateral region of the neural plate or neural tube, during neurulation. Their delamination and migration are crucial events in embryo development as the differentiation of NCC is heavily influenced by their final resting locations. Previous work in avian and aquatic species has shown that NCC delaminate via an epithelial-mesenchymal transition (EMT), which transforms these stem and progenitor cells from static polarized epithelial cells into migratory mesenchymal cells with fluid front and back polarity. However, the cellular and molecular drivers facilitating NCC delamination in mammals are poorly understood. We performed live timelapse imaging of NCC delamination in mouse embryos and discovered a group of cells that exit the neuroepithelium as isolated round cells, which then halt for a short period prior to acquiring the mesenchymal migratory morphology classically associated with most delaminating NCC. High magnification imaging and protein localization analyses of the cytoskeleton, together with measurements of pressure and tension of delaminating NCC and neighboring neuroepithelial cells, revealed these round NCC are extruded from the neuroepithelium prior to completion of EMT. Furthermore, we demonstrate that cranial NCC are extruded through activation of the mechanosensitive ion channel, PIEZO1, a key regulator of the live cell extrusion pathway, revealing a new role for PIEZO1 in neural crest cell development. Our results elucidating the cellular and molecular dynamics orchestrating NCC delamination support a model in which high pressure and tension in the neuroepithelium results in activation of the live cell extrusion pathway and delamination of a subpopulation of NCC in parallel with EMT. This model has broad implications for our understanding of cell delamination in development and disease.