Epigenetic Regulators Of Development In The Social Amoeba Dictyostellium Discoideum : The Roles Played By Histone Deacetylases And Heat Shock Protein 90

Abstract

The major evolutionary transition from single-celled to multicellular life is believed to have occurred independently of the main metazoan lineages in the cellular slime moulds, of which Dictyostelium discoideum is the best-studied species. Unusually, in this case multicellular development is a facultative trait and part of an asexual life cycle. It is triggered by starvation and involves aggregation of hitherto independent and possibly unrelated free-living cells. The consequences of multicellularity in D.discoideum are strongly influenced by the environment and meaningful external perturbations are easily carried out. This makes the organism ideally suited to a study of epigenetic factors that regulate development. In an attempt to understand how conserved epigenetic pathways are integrated within the developmental framework, two likely players were chosen for investigation - heat shock protein 90 (Hsp90) and histone deacetylases (HDACs). Hsp90 has been implicated in diverse biological processes such as protein folding, cell cycle control, signal transduction, and morphological evolution. The role of Hsp90 in D.discoideum life cycle was studied using a specific inhibitor, geldanamycin. Inhibition of Hsp90 function in D.discoideum caused a delay in aggregation and an arrest of development at the ‘mound’ stage. A reduction in Hsp90activity in starving cells of D.discoideum resulted in the generation of a range of phenotypes. The study suggests that Hsp90 is required for a specific developmental transition of the social amoeba and is important in generating a reliable outcome of the developmental process. Histone acetylation regulates gene expression and leads to the establishment and maintenance of cellular phenotypes during development of plants and animals. To study the roles of HDACs in D.discoideum, biochemical, pharmacological and genetic approaches were employed. The inhibition of HDAC activity by trichostatin A resulted in histone hyperacetylation and a delay in cell aggregation and differentiation. Cyclic AMP oscillations were normal in starved amoebae treated with trichostatin A but the expression of a subset of cAMP-regulated genes was delayed. Bioinformatic analysis indicated that there are four genes encoding putative HDACs in D.discoideum. One of these four genes, hdaB, was found to be dispensable for growth and development under laboratory conditions; but formed spores with lower efficiency than the wild type in chimeras. The work shows that HDAC activity is important for regulating two aspects of multicellular development: (a) heterochrony, namely the relative timing of developmental events, and (b) modulating the behaviour of single cells in a manner that is sensitive to their social environment.