A Retinoblastoma Orthologue Is Required for the Sensing of a Chalone in Dictyostelium discoideum

Abstract

Retinoblastoma-like proteins regulate cell differentiation and inhibit cell proliferation. The Dictyostelium discoideum retinoblastoma orthologue RblA affects the differentiation of cells during multicellular development, but it is unclear whether RblA has a significant effect on Dictyostelium cell proliferation, which is inhibited by the secreted proteins AprA and CfaD. We found that rblA ؊ cells in shaking culture proliferate to a higher density, die faster after reaching stationary density, and, after starvation, have a lower spore viability than wild-type cells, possibly because in shaking culture, rblA ؊ cells have both increased cytokinesis and lower extracellular accumulation of CfaD. However, rblA ؊ cells have abnormally slow proliferation on bacterial lawns. Recombinant AprA inhibits the proliferation of wild-type cells but not that of rblA ؊ cells, whereas CfaD inhibits the proliferation of both wild-type cells and rblA ؊ cells. Similar to aprA ؊ cells, rblA ؊ cells have a normal mass and protein accumulation rate on a per-nucleus basis, indicating that RblA affects cell proliferation but not cell growth. AprA also functions as a chemorepellent, and RblA is required for proper AprA chemorepellent activity despite the fact that RblA does not affect cell speed. Together, our data indicate that an autocrine proliferation-inhibiting factor acts through RblA to regulate cell density in Dictyostelium, suggesting that such factors may signal through retinoblastoma-like proteins to control the sizes of structures such as developing organs or tumors.

An Autocrine Proliferation Repressor Regulates Dictyostelium discoideum Proliferation and Chemorepulsion Using the G Protein-Coupled Receptor GrlH

“…To identify the AprA receptor, we first determined whether any of an available set of mutants with insertions of a blasticidin resistance cassette in the coding region for a putative G protein-coupled receptor might have phenotypes similar to those of cells lacking AprA. Cells lacking AprA, Gα8, Gβ, or the AprA signal transduction components PakD, RblA, and QkgA exhibit faster proliferation and proliferate to a higher maximal density than wild-type cells ( 5 , 8 , 10 , 11 , 18 ). Examining the proliferation in a shaking culture of cells lacking putative G protein-coupled receptors, we observed that grlH¯ cells showed significantly faster proliferation than wild-type cells, while grlD¯ and fscE¯ cells were slower to proliferate ( Fig.…”

“…Possibly due to a decreased ability to cause cells at the edge of a colony to be repelled from the colony, aprA¯ , pakD¯ , rblA¯ , and qkgA¯ cells form abnormally small colonies starting from single cells on bacterial lawns ( 5 , 10 , 11 , 18 ). The grlB¯ , grlH¯ , fslA¯ , fslB¯ , and fslK ¯ cells also formed abnormally small colonies on bacterial lawns ( Fig.…”

“…3 ). Whereas wild-type cells form colonies in submerged liquid culture with cells dispersed from the colony edges, aprA¯ , rblA¯ , and qkgA¯ cells form colonies with well-defined edges and few dispersed cells ( 5 , 10 , 11 ). We found that grlB¯ , grlH¯ , and fslK¯ cells also formed colonies with well-defined edges ( Fig.…”

“…Both AprA and CfaD are necessary for proliferation inhibition, as recombinant AprA (rAprA) cannot rescue the cfaD¯ phenotype and recombinant CfaD (rCfaD) cannot rescue the aprA¯ phenotype ( 7 , 8 ). Several components of the AprA-induced and/or CfaD-induced proliferation inhibition signaling pathway have been identified, including the ROCO kinase QkgA, the p21-activated kinase (PAK) family member PakD, the PTEN-like phosphatase CnrN, and the tumor suppressor RblA ( 9 , 10 , 11 , 13 , 18 ). Additionally, AprA functions to chemorepel D. discoideum cells, causing cells to move in a biased direction away from a source of AprA ( 12 ).…”

Protease activated-receptor 2 is necessary for neutrophil chemorepulsion induced by trypsin, tryptase, or dipeptidyl peptidase IV

“…Comparatively, much less is known about the mechanisms used by cells to move away from chemorepellents . Based on initial studies on a Dictyostelium chemorepellent, we found that the human extracellular protein dipeptidyl peptidase IV (DPPIV) is a chemorepellent for human and mouse neutrophils . Since chemoattractants activate cell surface receptors, an intriguing possibility is that a neutrophil chemorepellent such as DPPIV may also activate a neutrophil cell‐surface receptor.…”