Epithelial polarity and signaling during cytokinesis of animal cells – CYTOSIGN

In metazoans, epithelia are compact tissues that function as mechanical and chemical barriers between the body and the environment. During development and throughout adult life, epithelial tissues keep on growing or regenerate in large part thanks to cell division. Importantly, heterogeneity is a central feature of epithelia: they are composed on one hand of cells that divide symmetrically and contribute to tissue growth, and on the other hand of stem cells or progenitors that can divide asymmetrically to generate daughter cells with distinct identities thanks to cell-cell communication. How epithelia coordinate cell division and signalling while preserving tissue integrity is largely unknown and is at the centre of this proposal.

An emerging concept is that cytokinesis could potentially couple cell division and signalling. Indeed, as they undergo cytokinesis, cells need to remodel neighbouring cellular contacts and remodel their apical basal polarity to form new membrane interfaces with specific biochemical and biophysical properties to enable signalling. Throughout cytokinesis, cells could also transmit diffusible signals trough the intercellular bridge that connects the daughters prior to abscission. In addition, cytokinetic abscission generates a midbody remnant that is released and has been proposed to have the potential to transmit signals to neighbouring or distant cells. However, direct evidence of these novel modes of cell-cell communication and fate acquisition is lacking. In this proposal, we will specifically address these questions using two model systems, the Drosophila pupal notum and mammalian 3D cultures of renal and liver progenitors, as they enable a comparative and comprehensive study of epithelial cell cytokinesis and fate acquisition relying, in particular, on the differential activation of the evolutionarily conserved Notch signalling pathway.

In order to address the fundamental interplay between cytokinesis and signalling in developing epithelia, we will make use of the combined expertise of the two teams including state of the art quantitative and non-invasive live imaging, electron microscopy, advanced biochemical approaches and quantitative proteomics, and genetic dissection of the processes.

Task 1: Investigate how junctions ensuring permeability barrier are remodelled in symmetric and asymmetric cell divisions. We will also address how abscission, the final step of cytokinesis, is regulated in these two types of epithelial cytokinesis and could contribute to signalling in vivo.

Task 2: Explore the functions of the membrane trafficking regulator Rab35 and activators in coupling preservation of tissue integrity and ability to signal. The Rab35 GTPase is located at the cleavage furrow and has been recently and independently identified by the two Partners as a crucial regulator of apical-basal polarity initiation in mammalian cells and of epithelial cell signalling in Drosophila. A key question remains the understanding of how this conserved GTPase is activated and this will be addressed in this task.

Task 3: Explore the function of midbody remnants in providing positional cues and in regulating signalling in epithelial tissues. We will address this question in vivo in epithelial divisions in Drosophila, and directly test experimentally ex vivo the ability of midbodies to influence the Notch-dependent programs of differentiation.

Collectively, the CytoSIGN proposal will examine the molecular mechanisms by which epithelial cytokinesis opens a specific temporal and topological window that enables cell-cell signalling, via the evolutionarily conserved Notch signalling. Based on the synergy of the two teams and experimental models, CytoSIGN is expected to unravel the fundamental molecular mechanisms linking epithelial cytokinesis and initiation of signalling.