Extracellular Polyphosphate Accumulation Inhibits Proliferation In Dictyostelium Discoideum

Abstract

Cancer is a collection of diseases that display abnormal cell proliferation with the potential to spread throughout the body. Much remains to be understood about the regulation of cell proliferation. Regulation of proliferation is difficult to study in complex mammalian systems, but can be studied using a simpler system such as the model organism Dictyostelium discoideum. Using Dictyostelium we have discovered that inorganic polyphosphate is acting as a signal to help regulate cell proliferation. This research project focuses on regulation of cell proliferation in Dictyostelium and consists of examining the phenotypes of mutants that we have identified as having abnormal responses to polyphosphate, measuring the extracellular polyphosphate accumulation of existing mutants to see if polyphosphate is being synthesized by these mutants, and assaying identified mutants for binding to polyphosphate using biotinylated polyphosphate. My work will elucidate the mechanism whereby polyphosphate regulates cell proliferation in Dictyostelium. This research will then hopefully shed light on how cell proliferation is regulated in mammalian systems. It was found that tested mutants reacted as either hypersensitive, normal response, or exhibited resistance to polyphosphates inhibitory proliferation signal. Cells lacking PPK1, an enzyme responsible for the production of intracellular polyphosphate, exhibited complete resistance to inhibition suggesting that intracellular polyphosphate is needed for the inhibition of proliferation by extracellular polyphosphate. Binding assays provided evidence that hypersensitive mutants are becoming saturated with polyphosphate at lower concentrations suggesting that these mutants may play a role in receptor regulation or the intracellular regulation mechanism. Extracellular polyphosphate synthesis concentrations were recorded and this data suggested that initial exposure to increasing concentrations of endogenous polyphosphate decreases polyphosphate production from Dictyostelium. Through testing the identification of mutants as well as binding and synthesis of polyphosphate in Dictyostelium, the effects on the regulation of cell proliferation in addition to the mechanism behind this regulation in mammalian systems may be learned and applied to areas such as cancer treatment.