Atypical E-Cadherin Complex Promotes Melanoblast Protrusivity and Migration through Tightly Adherent Epidermal Junctions

Abstract

During the development of the mammalian epidermis, neural crest derived melanoblasts must adapt how they adhere to their surroundings in order to migrate through the dermis and eventually invade and colonize the tightly adherent epidermis. Previous work suggests that melanoblasts make attachments to the basement membrane to migrate along the epidermal basal layer. However, we observe that melanoblasts must also migrate between tightly adherent basal cells and through the suprabasal layers of the epidermis, held tightly together by desmosomes and E-Cadherin-based adherens junctions (AJs). To further verify that melanoblasts can migrate between keratinocytes layers without appreciable contact with the ECM, we crossed Tyr-Cre;tdT mice to a newly developed basement membrane (BM) reporter mouse line in which endogenous Nidogen 1 is tagged with mNeonGreen (Nid1-mNG). It is unknown how melanoblasts move through tight epithelial spaces without disrupting the skin’s barrier, particularly in the suprabasal layers of the skin. Using embryonic mouse backskin as a model we find that melanoblasts make extensive E-Cadherin based attachments to surrounding keratinocytes. However, unlike keratinocyte-keratinocyte (KC-KC) AJs that are enriched in cytoplasmic catenins, melanoblast AJs largely lack α-, β- and p120-catenin, suggesting that melanoblasts may use E-Cadherin in an atypical way to migrate as individual cells. To test this hypothesis, we use Cre-recombinase mediated knockout to selectively deplete E-Cadherin from melanoblasts (Tyr-Cre) or keratinocytes (K14-Cre). We find that loss of melanoblast E-Cadherin results in decreased melanoblast protrusivity, while increasing cell body circularity. Using live imaging, we note that this morphological change results in decreased melanoblast persistence, displacement and mean straight line speed, ultimately resulting in ventral skin depigmentation. This defect in morphology is recapitulated when wildtype melanoblasts are challenged to migrate between keratinocytes lacking E-Cadherin. Collectively, our data suggest that migratory melanoblasts use E-Cadherin on the surface of keratinocytes to stabilize filopodia and generate traction needed to facilitate directed migration in a tightly adherent epithelial environment. Our findings suggest a mechanism in which keratinocyte E-Cadherin serves as a substrate for melanoblasts to migrate upon without impairing epithelial integrity.