Mechanism and Function Study of RpS12 Dependent Cell Competition

Abstract

Cell competition is a phenomenon in which cells compete with each other based on their relative fitness within the same tissue. The ‘less-fit’ cells are by themselves viable but eliminated by other cells with better fitness during cell competition. Cell competition has been observed in Drosophila and mouse. For example, in Drosophila imaginal discs, cells heterozygous for certain ribosomal protein genes (Rp+/–) are by viable, however, they are out-competed when they are surrounded by wild type cells. Since mosaicism could occur spontaneously, cell competition is proposed to function in maintaining the tissue health and serve as a growth control mechanism. Defects in cell competition could cause aging as well as tumorigenesis.

Insights into the molecular mechanism and physiological function of cell competition came from an unbiased genetic screen that identified a ribosomal protein, RpS12, as a key player of cell competition. RpS12 defines the competitiveness of Rp+/– cells during cell competition. In this thesis, we have generated RpS12 specific antibodies and observed that RpS12 is auto-regulated. Both cell competition and auto-regulation of RpS12 are dependent on the Gly97 residue and impaired by the Asp97 mutation, a single amino acid substitution. We also demonstrated that this auto-regulatory machinery is facilitated at the mRNA level, and the first intron is important for RpS12 expression. In addition, snoRNAs encoded in intron regions of rpS12 gene are dispensable for RpS12 expression and function.

In addition, through genetic and developmental studies of RpS12, we have assessed several physiological functions of cell competition. We show that cell competition serves as a surveillance mechanism to select out cells with segmental aneuploidy. A prior hypothesis suggested that cells with segmental aneuploidy are potentially Rp+/–, since ribosomal protein genes are randomly distributed across the genome. Therefore, segmental aneuploid cells could be recognized by cell competition and eliminated by surrounding wild type cells. In order to prove this idea, we employed the Flp-out system to generate cells with large-scale genetic deletions and we found that only cells with a heterozygous deletion containing certain Rp genes were eliminated, whereas cells with a large-scale deletion lacking specific Rp genes survived when they were surrounded by wild type cells. In addition, when rpS12 is mutated to rpS12G97D, by which the cell competition is blocked, both types of cells with large-scale genetic deletions survived. These observations indicate that RpS12 dependent cell competition plays a role in selecting out cells with large-scale genetic changes. Moreover, we have also observed that damaged cells induced by IR irradiation are mostly eliminated by p53-dependent cell death. However, some cells containing large-scale genetic changes are eliminated by p53-independent, but RpS12 dependent competitive cell death. In addition, flies absence of cell competition have a relatively shorter life span compared to wild type flies.

Taken together, the Gly97 residue of RpS12 is critical to both cell competition and auto-regulation function, and RpS12 dependent cell competition functions as a surveillance mechanism by sensing the ribosomal protein imbalance. Cell competition can select out cells with segmental aneuploidy to maintain the tissue health and extends life span of Drosophila.