Rac1 Drives Melanoblast Organization during Mouse Development by Orchestrating Pseudopod- Driven Motility and Cell-Cycle Progression

Abstract

Summary During embryogenesis, melanoblasts proliferate and migrate ventrally through the developing dermis and epidermis as single cells. Targeted deletion of Rac1 in melanoblasts during embryogenesis causes defects in migration, cell cycle progression and cytokinesis. Rac1 null cells migrate markedly less efficiently, but surprisingly, global steering, crossing the dermal/epidermal junction and homing to hair follicles occur normally. Melanoblasts navigate in the epidermis using two classes of protrusion: short stubs and long pseudopods. Short stubs are distinct from blebs and are driven by actin assembly, but are independent of Rac1, Arp2/3 complex, myosin or microtubules. Rac1 positively regulates the frequency of initiation of long pseudopods, which promote migration speed and directional plasticity. Scar/WAVE and Arp2/3 complex drive actin assembly for long pseudopod extension, which also depends on microtubule dynamics. Myosin contractility balances the extension of long pseudopods by effecting retraction and allowing force generation for movement through the complex 3D epidermal environment.

The FERM protein EPB41L5 regulates actomyosin contractility and focal adhesion formation to maintain the kidney filtration barrier

“…5 and Movies S1-S5). Pseudopods represent a specialized cellular protrusion type mainly observed in chemotactic cell types, and Rac1 activation has been shown to be involved in the initiation and propagation of pseudopod formation (43). In light of this, the pseudopod phenotype of EPB41L5 knockout cells might reflect the disturbed balance of GTPase activation and insufficient FA maturation, ultimately culminating in the generation of numerous unstable cellular projections.…”

Microtubules in 3D cell motility

“…Although microtubule disassembly in response to nocodazole causes premature breakdown of the basement membrane during gastrulation (Nakaya et al, 2008), microtubule functions during mesoblast migration in 3D are poorly understood. Studies that used paclitaxel and nocodazole on migrating melanoblasts -cells that originate from neural crest cells -provided an elegant in vivo example of the role microtubules have in developmental cell motility in 3D (Li et al, 2011a;Thomas and Erickson, 2008). Similarly to other mesenchymal cells in 3D cultures, drug-based microtubule network disorganization caused mouse melanoblasts to lose their long pseudopods and stop migrating (Li et al, 2011a).…”

“…Studies that used paclitaxel and nocodazole on migrating melanoblasts -cells that originate from neural crest cells -provided an elegant in vivo example of the role microtubules have in developmental cell motility in 3D (Li et al, 2011a;Thomas and Erickson, 2008). Similarly to other mesenchymal cells in 3D cultures, drug-based microtubule network disorganization caused mouse melanoblasts to lose their long pseudopods and stop migrating (Li et al, 2011a). For neural crest cells, the correlation between pseudopod loss and motility impairment appears to be specific for the 3D situation as nocodazole-treated neural crest cells in 2D cultures still move, although they round up (Moore et al, 2013).…”

Migration des cellules du lignage mélanocytaire

“…Son invalidation spécifique dans les mélanoblastes de souris génère un ventre blanc. L'absence de Rac1 provoque une réduction de la vitesse de migration des mélanoblastes, mais n'abolit pas le passage des mélano-cytes du derme vers l'épiderme, ni la colonisation des follicules pileux [11]. L'importance de Rac1 lors de la migration a été confortée par la démonstration que l'invalidation chez la souris de P-Rex1, un membre de la famille GEF (Rho GTPase guanine nucleotide exchange factors) activant la formation de Rac-GTP, se traduit également par l'apparition d'un ventre blanc [12].…”