Molecular cell biology
Lab leaders
Professor Paul Fisher
Associate Dean (Research), Faculty of Science, Technology and Engineering
Dr Sarah Annesley
Lecturer, Faculty of Science, Technology and Engineering
Our main research interest is the molecular genetics of signal transduction in the cellular slime mould Dictyostelium discoideum, one of a handful of non-mammalian model organisms recognized by the NIH for their importance in biomedical research. We study signal transduction during phototaxis and thermotaxis in the multicellular slug stage and discovered that these behaviours are highly sensitive to genetic defects affecting the mitochondria.
Dictyostelium discoideum is a cellular slime mould whose natural habitat is soil and leaf litter where it predates bacteria by phagocytosis, grows and divides by mitosis. You can hear more about it in radio interviews broadcast on 29th August, 2009 (ABC Science Show interview by Robyn Williams) and the 25th June 2000 on "Einstein-A-Go-Go", the weekly Sunday science broadcast from the Melbourne community radio station 3RRR.
The Dictyostelium discoideum life cycle begins with differentiation of starving amoebae to a form where they become capable of synthesizing, secreting and being attracted by extracellular cAMP. The resulting aggregation process forms a multicellular migratory organism, the 'slug', which migrates through a cellulose/protein extracellular matrix, the 'slime sheath', that collapses behind to form a trail. Slugs are phototactic, thermotactic and weakly chemotactic. After a variable period of migration the slug stops and forms a fruiting body consisting, to a first approximation, of a droplet of spores supported by a tapered stalk and basal disc.
The behaviour and morphogenetic movements are controlled by the tip via what are believed to be extracellular tip activation and inhibition signals. The tip activation signal is probably carried by three-dimensional scroll waves of cAMP emanating from the tip. The tip inhibition signal has been proposed to be carried by a small non-volatile, diffusible molecule (Slug Turning Factor, STF), and/or ammonia, and/or adenosine. Phototactic and thermotactic behaviour seem to be controlled by modulation of the tip activation/inhibition system. We are investigating the signal transduction pathways in slug behaviour using a combination of pharmacological, genetic, cell physiological and molecular biological approaches. Our recent work has revealed a protein signalling complex for phototaxis in which the participating proteins are assembled on a scaffold provided by the actin-binding protein filamin.
We discovered that signal transduction for phototaxis and thermotaxis in slugs is more sensitive to the presence of mitochondrial defects than other cellular activities such as growth and division. Thus phototaxis and thermotaxis are impaired by mitochondrial mutations created by plasmid insertions in a minority of the mitochondrial genomes in the cell. The same defects are observed when the folding of proteins in the mitochondria is impaired by antisense inhibition of the expression of chaperonin 60. Chaperonin 60 is encoded on a nuclear gene and is required for the proper folding of proteins in the mitochondria. Undersupply of chaperonin 60 therefore causes serious mitochondrial disease. The severity of the undersupply caused by antisense inhibition is determined by the number of copies of the antisense inhibition construct and this is different in every cell line carrying the construct. This allows the generation of genetic dose-response curves [PDF 34.17KB] relating phenotype to the severity of the underlying genetic defect. In addition to the phototaxis and thermotaxis defects, mitochondrial disease in Dictyostelium causes slow growth (without affecting the rate of uptake of nutrients by phagocytosis or pinocytosis), a misdirection of cells into the stalk differentiation (programmed cell death) pathway, and less efficient aggregation. All of the defects are a result of chronic activation of the cellular energy-sensing alarm protein AMPK. The Dictyostelium mitochondrial disease model thus suggests that the complex pathology of human mitochondrial disease might be explained partially by chronic AMPK signalling rather than an energy insufficiency per se. This discovery resulted in my being awarded the Australasian Science Prize for 2007 and provides a completely new understanding of how mitochondrial dysfunction damages cells. You can hear more about this discovery in an interview conducted by Dr Moira Gunn on May 7th, 2007 in a BioTech Nation radio broadcast of September 28th, 2007 and made available on line by ITConversations. A major research interest of the laboratory is therefore to study mitochondrial biogenesis and function and the roles of mitochondria in modulating cellular signal transduction pathways.
The third major project in the laboratory concerns intracellular Ca2+ signals. Using an assay based on expression in Dictyostelium of recombinant aequorin, a Ca2+-sensitive luminescent protein, we are able to measure cytosolic Ca2+ concentrations in a population of cells every 20 msecs, down to concentrations of about 25 nM to within a few nM. Using this assay we are studying signal transduction pathways involving intracellular Ca2+ signals initiated by various extracellular stimuli including the morphogen DIF and the chemoattractants cAMP and folic acid.
You can get more information about Dictyostelium discoideum and the people who work with it from the Dictyostelium World Wide Web Site.
Other sites of interest
- Index of Dictyostelium discoideum entries in SWISS-PROT
- Dictyostelium - model organism in motion
- The Dicty Work Bench
