The Rosenblatt Lab studies both cell death and cell division and the roles that the actin and microtubule cytoskeletons play in both processes. Our lab is investigating if extrusion (a contraction that squeezes dying cells out of tissue) could drive cell death in order to control cell numbers.
The cells comprising the epithelium constantly turn over via cell division and death. For an adult tissue to maintain homeostasis, the numbers of cells that divide must match those that die. If more cells die than divide, barrier function diseases like asthma and colitis could result, whereas if more divide than die, epithelial cancers (or carcinomas) may result. We have found that mechanical tensions control cell numbers: if too many cells accumulate, cell crowding activates a process called extrusion that squeezes out unwanted cells that later die.
Epithelia provide a protective barrier for the organs they encase. Dying cells could potentially disrupt the barrier function of epithelia. We discovered a process termed ‘epithelial extrusion’ that eliminates unwanted cells while preserving a functional barrier. To extrude, a cell destined to die produces and emits a lipid, Sphingosine 1 Phosphate (S1P) that binds the S1P2 receptor in its live neighboring cells to form and contract an actin and myosin ring that squeezes the dying cell out.
To investigate how cells normally die during normal homeostatic turnover, we imaged numerous different epithelial tissues without inducing cell death and found that cells normally first extrude live cell at regions of high cellular crowding. In all epithelia observed, cells divide at defined sites, migrate from these sites, and extrude in zones of convergence where cells are 1.6-1.8X more crowded.
The stretch-activated signal senses crowding of epithelial cells and enables activation of the S1P-S1P2-Rho pathway to initiate crowding. If Piezo is blocked, cells amass at sites where they would have extruded and been lost.
Since extrusion normally promotes cell death during normal epithelial cell turnover, we have found that it is often mis-regulated in cancer. Expressing oncogenic mutations, like APC or K-Ras mutations, blocks extrusion but also allows cells to basally extrude. Basal extrusion could enable transformed cells to invade the underlying epithelium to initiate metastasis to other sites in the body.
We have found that zebrafish epidermis provides an excellent model for the epithelia that coat our organs—but one that can be imaged live and where gene expression can be regulated in specific cells and specific times. George Eisenhoffer, in collaboration with Chi-Bin Chien and Paul Marcos at Gene Tools, has developed a set of tools to express transgenes or knockdown gene function, using photo-activatable morpholinos, in a variety of different cells within the epidermis.
Zebrafish expressing Cldb-GFP in epidermis shows cell movements during development.