The Scf/Kit pathway implements self-organized epithelial patterning

Crb3 is required to organize the apical domain of multiciliated cells

Cell shape changes mainly rely on the remodeling of the actin cytoskeleton. Multiciliated cells (MCCs) of the mucociliary epidermis of Xenopus laevis embryos, as they mature, dramatically reshape their apical domain to grow cilia, in coordination with the underlying actin cytoskeleton. Crumbs (Crb) proteins are multifaceted transmembrane apical polarity proteins known to recruit actin linkers and promote apical membrane growth. Here, we identify the homeolog Crb3.L as an important player for the migration of centrioles or basal bodies (collectively centrioles/BBs) and apical domain morphogenesis in MCCs. Crb3.L is present in cytoplasmic vesicles close to the ascending centrioles/BBs, where it partially colocalizes with Rab11a. Crb3.L morpholino-mediated depletion in MCCs caused abnormal migration of centrioles/BBs, a reduction of their apical surface, disorganization of their apical actin meshwork and defective ciliogenesis. Rab11a morpholino-mediated depletion phenocopied Crb3.L loss-of-function in MCCs. Thus, the control of centrioles/BBs migration by Crb3.L might be mediated by Rab11a-dependent apical trafficking. Furthermore, we show that both phospho-activated ERM (pERM; Ezrin-Radixin-Moesin) and Crb3.L are recruited to the growing apical domain of MCCs, where Crb3.L likely anchors pERM, allowing actin-dependent expansion of the apical membrane.

Inhibition of Soluble Stem Cell Factor Promotes Intestinal Mucosal Repair

BACKGROUND

Incidences of inflammatory bowel disease (IBD), including Crohn's disease and ulcerative colitis, are escalating worldwide and can be considered a global public health problem. Given that the gold standard approach to IBD therapeutics focuses on reducing the severity of symptoms, there is an urgent unmet need to develop alternative therapies that halt not only inflammatory processes but also promote mucosal repair. Previous studies have identified increased stem cell factor (SCF) expression in inflamed intestinal mucosal tissues. However, the role that SCF plays in mediating intestinal inflammation and repair has not been explored.

METHODS

Changes in the expression of SCF were evaluated in the colonic tissue of healthy mice and during dextran sodium sulfate (DSS)-induced colitis. Furthermore, mucosal wound healing and colitis severity were analyzed in mice subjected to either mechanical biopsy or DSS treatment, respectively, following intestinal epithelial cell-specific deletion of SCF or anti-SCF antibody administration.

RESULTS

We report robust expression of SCF by intestinal epithelial cells during intestinal homeostasis with a switch to immune cell-produced SCF during colitis. Data from mice with intestinal epithelial cell-specific deletion of SCF highlight the importance of immune cell-produced SCF in driving the pathogenesis of colitis. Importantly, antibody-mediated neutralization of total SCF or the specific SCF248 isoform decreased immune cell infiltration and enhanced mucosal wound repair following biopsy-induced colonic injury or DSS-induced colitis.

CONCLUSIONS

These data demonstrate that SCF functions as a pro-inflammatory mediator in mucosal tissues and that specific neutralization of SCF248 could be a viable therapeutic option to reduce intestinal inflammation and promote mucosal wound repair in individuals with IBD.

Actomyosin contractility in olfactory placode neurons opens the skin epithelium to form the zebrafish nostril

Despite their barrier function, epithelia can locally lose their integrity to create physiological openings during morphogenesis. The mechanisms driving the formation of these epithelial breaks are only starting to be investigated. Here, we study the formation of the zebrafish nostril (the olfactory orifice), which opens in the skin epithelium to expose the olfactory neurons to external odorant cues. Combining live imaging, drug treatments, laser ablation, and tissue-specific functional perturbations, we characterize a mechanical interplay between olfactory placode neurons and the skin, which plays a crucial role in the formation of the orifice: the neurons pull on the overlying skin cells in an actomyosin-dependent manner which, in combination with a local reorganization of the skin epithelium, triggers the opening of the orifice. This work identifies an original mechanism to break an epithelial sheet, in which an adjacent group of cells mechanically assists the epithelium to induce its local rupture.

Multiciliated cells use filopodia to probe tissue mechanics during epithelial integration in vivo

During embryonic development, regeneration, and homeostasis, cells have to migrate and physically integrate into the target tissues where they ultimately execute their function. While much is known about the biochemical pathways driving cell migration in vivo, we are only beginning to understand the mechanical interplay between migrating cells and their surrounding tissue. Here, we reveal that multiciliated cell precursors in the Xenopus embryo use filopodia to pull at the vertices of the overlying epithelial sheet. This pulling is effectively used to sense vertex stiffness and identify the preferred positions for cell integration into the tissue. Notably, we find that pulling forces equip multiciliated cells with the ability to remodel the epithelial junctions of the neighboring cells, enabling them to generate a permissive environment that facilitates integration. Our findings reveal the intricate physical crosstalk at the cell-tissue interface and uncover previously unknown functions for mechanical forces in orchestrating cell integration.

Lrrcc1 and Ccdc61 are conserved effectors of multiciliated cell function

Ciliated epithelia perform essential functions across animal evolution, ranging from locomotion of marine organisms to mucociliary clearance of airways in mammals. These epithelia are composed of multiciliated cells (MCCs) harbouring myriads of motile cilia, which rest on modified centrioles called basal bodies (BBs), and beat coordinately to generate directed fluid flows. Thus, BB biogenesis and organization is central to MCC function. In basal eukaryotes, the coiled-coil domain proteins Lrrcc1 and Ccdc61 were shown to be required for proper BB construction and function. Here, we used the Xenopus embryonic ciliated epidermis to characterize Lrrcc1 and Ccdc61 in vertebrate MCCs. We found that they both encode BB components, localized proximally at the junction with striated rootlets. Knocking down either gene caused defects in BB docking, spacing, and polarization. Moreover, their depletion impaired the apical cytoskeleton, and altered ciliary beating. Consequently, cilia-powered fluid flow was greatly reduced in morphant tadpoles, which displayed enhanced mortality when exposed to pathogenic bacteria. This work illustrates how integration across organizational scales make elementary BB components essential for the emergence of the physiological function of ciliated epithelia.

A live-imaging protocol to track cell movement in the Xenopus embryo

Tracking individual cell movement during development is challenging, particularly in tissues subjected to major remodeling. Currently, most live imaging techniques in Xenopus are limited to tissue explants and/or to superficial cells. We describe here a protocol to track immature multiciliated cells (MCCs) moving within the inner epidermal layer of a whole embryo. In addition, we present a data processing protocol to uncouple the movements of individual cells from the coplanar drifts of the tissue in which they are embedded. For complete details on the use and execution of this protocol, please refer to Chuyen et al. (2021).

Tubulin acetylation promotes penetrative capacity of cells undergoing radial intercalation

Post-translational modification of tubulin provides differential functions to microtubule networks. Here, we address the role of tubulin acetylation on the penetrative capacity of cells undergoing radial intercalation, which is the process by which cells move apically, insert between outer cells, and join an epithelium. There are opposing forces that regulate intercalation, namely, the restrictive forces of the epithelial barrier versus the penetrative forces of the intercalating cell. Positively and negatively modulating tubulin acetylation in intercalating cells alters the developmental timing such that cells with more acetylation penetrate faster. We find that intercalating cells preferentially penetrate higher-order vertices rather than the more prevalent tricellular vertices. Differential timing in the ability of cells to penetrate different vertices reveals that lower-order vertices represent more restrictive sites of insertion. We shift the accessibility of intercalating cells toward more restrictive junctions by increasing tubulin acetylation, and we provide a geometric-based mathematical model that describes our results.

Emergence of self-organizational patterning at the mesoscopic scale

In this issue of Developmental Cell, Chuyen et al. (2021) suggest that the Scf/Kit pathway controls mutual repulsion of multiciliated cells and their affinity for epidermal cell junctions through soluble and membrane-associated Scf ligands, respectively. Effective self-organizational patterning emerges at the mesoscopic scale as a small set of effective behaviors.