Epithelial cell extrusion: Pathways and pathologies

Abstract

To remove dying or unwanted cells from an epithelium while preserving the barrier function of the layer, epithelia use a unique process called cell extrusion. To extrude, the cell fated to die emits the lipid Sphingosine 1 Phosphate (S1P), which binds the G-protein-coupled receptor Sphingosine 1 Phosphate receptor 2 (S1P₂) in the neighboring cells that activates Rho-mediated contraction of an actomyosin ring circumferentially and basally. This contraction acts to squeeze the cell out apically while drawing together neighboring cells and preventing any gaps to the epithelial barrier. Epithelia can extrude out cells targeted to die by apoptotic stimuli to repair the barrier in the face of death or extrude live cells to promote cell death when epithelial cells become too crowded. Indeed, because epithelial cells naturally turn over by cell death and division at some of the highest rates in the body, epithelia depend on crowding-induced live cell extrusion to preserve constant cell numbers. If extrusion is defective, epithelial cells rapidly lose contact inhibition and form masses. Additionally, because epithelia act as the first line of defense in innate immunity, preservation of this barrier is critical for preventing pathogens from invading the body. Given its role in controlling constant cell numbers and maintaining barrier function, a number of different pathologies can result when extrusion is disrupted. Here, we review mechanisms and signaling pathways that control epithelial extrusion and discuss how defects in these mechanisms can lead to multiple diseases. We also discuss tactics pathogens have devised to hijack the extrusion process to infect and colonize epithelia.

Keywords: Epithelial extrusion; Microtubules; Piezo1; Rho; Sphingosine-1-phosphate; Sphingosine-1-phosphate receptor 2.

Figure 1. Extrusion removes cells from the epithelium while maintaining barrier function

A) Apoptotic stimulus activates basolateral actomyosin contraction to squeeze the dying cell apically out of the epithelial layer and preserve the barrier function. B) During normal epithelial turnover, mechanical crowding from cell proliferation activates the stretch-activated channel Piezo1 to trigger some live cells to extrude. Live extruded cells later die by anoikis, or apoptosis due to loss of matrix-dependent survival signaling. The S1P/S1P₂ signaling pathway is critical for both apoptotic and live cell extrusion.

Figure 2. Signaling mechanisms controlling apical and basal extrusion

During apical extrusion, the cell destined to die produces and exports the bioactive lipid Sphingosine-1-phosphate (S1P) which binds to a G-protein coupled receptor, S1P receptor 2 (S1P₂), in the neighboring cells (inset). S1P₂ activates Rho to trigger actin and myosin recruitment and contraction along the basolateral surface of the extruding and neighboring cells to squeeze the extruding cell out apically. During wild-type apical extrusion, both the extruding and the neighboring cells reorient their microtubules basolaterally. Microtubule reorientation in the extruding cell presumably targets S1P to the base of the cell. In the neighboring cells, microtubules target p115 Rho-GEF (see text) to activate actomyosin contraction basolaterally and initiate apical extrusion. Loss of S1P₂, disruption of microtubule dynamics due to mutations in adenomatous polyposis coli (APC) or expression of oncogenic K-Ras (K-Ras^V12) blocks apical extrusion and drives extrusion basally.

Figure 3. Loss of Adenomatous Polyposis Coli (APC) or S1P₂, and oncogenic K-Ras expression causes cells to extrude basally

A) During wild-type extrusion, microtubule reorientation and S1P/S1P₂ signaling promote apical extrusion. B) Mutations in APC disrupt microtubule dynamics and cause the cell to extrude basally in a cell-autonomous manner. C) Expression of oncogenic K-Ras (K-Ras^V12) blocks apical extrusion by degrading S1P through autophagy and downregulating S1P₂, thus driving extrusion basally.

Figure 4. Pathogens hijack the extrusion process to infect and colonize epithelia

A) Listeria monocytogenes (Lm) invade at sites of cell extrusion by attaching to epithelial junctions through the synergistic action of two bacterial surface proteins Internalin A (InlA) and Internalin B (InlB), which bind to host E-cadherin and c-met, respectively (Inset). Endocytosis of junctions is accelerated at sites of extrusion, assisting E-cadherin-bound Lm internalization into the host cell. B) Shigella, Salmonella, and enterohaemorrhagic Escherichia coli (EHEC) inhibit extrusion by delivering the protein OspE (or homologs) into host cells. OspE binds the host Integrin Linked Kinase (ILK), which promotes and stabilizes focal adhesions, increases cell adhesion to the matrix, and inhibits cell extrusion (inset). These pathogens can either activate or block apoptotic pathways during the initial stages of infection to make cells more susceptible to invasion. C) Respiratory Syncytial Virus (RSV) protein NS2 causes cell shedding by suppressing IFN-mediated host antiviral response. The RSV envelope fusion protein (F) (Inset) causes p53-dependent apoptosis in polarized epithelial cell and contributes epithelial shedding, airway obstruction, and inflammation culture leading to accelerated extrusion of apoptotic cells. While epithelial cell shedding clears out RSV-infected cells to reduce viral titers, excessive cell shedding leads to blocking of smaller bronchiolar airways and bronchiolitis.

Figure 5. Mutations or downregulation of cell-cell adhesion molecules may cause defects in cell extrusion

The loss of key adhesion proteins (such as E-cadherin) or those regulating cell-cell adhesions (such as Lkb1) can result in aberrant extrusion, disruption of epithelial integrity, and possibly diseases such as celiac and inflammatory bowel disease (IBD).