Open Forum on Cell Signaling
Highlights from ASCB Symposium II (02 December 2007)
5 December 2007
Nancy R. Gough
At the Sunday morning symposium entitled "Architecture of Signaling Systems", the speakers were Richard Losick (Harvard), Pamela Silver (Harvard), and Tobias Meyer (Stanford).
Dr. Losick led off with a discussion of stochasticity in cell fate determination in which he described four examples from bacteria and one from the fruitfly, Drosophila melanogaster. The Drosophila color perception example and the example that Dr. Losick called "growth versus competence" have been described in detail in the STKE Review by Samoilov et al. The other bacterial examples included "swimming versus chaining", "eating or being eaten" (a story of bacterial cannabilism, a mechanism to regulate sporulation), and "individual versus community" (the formation of biofilms). In each of the bacterial examples, green fluorescent protein (GFP) or its variants was used to report on the expression of a particular gene promoter in individual bacterial cells, which revealed the stochastic differences among individuals in the population. The systems described all involve a transcriptional system with a positive feedback mechanism that allows the system to behave as a bistable switch. Dr. Losick emphasized that stochastic cell fate determination for the bacteria allows the cells to "hedge their bets" so that they are poised to adapt to changing environmental conditions.
Dr. Silver gave a very interesting talk on synthetic biology and posed the question of whether engineering biology can be as exciting as building a robot (see iGEM). After explaining why biological systems are attractive from engineering and design perspectives, she illustrated three applications of synthetic biology. The first she called designing a cellular memory system, in which yeast were created with a synthetic transcription system (a synthetic transcription factor that responded to an external stimulus and reporter gene with an engineered promoter). These engineered yeast activated the reporter gene in response to the cognate stimulus and this response persisted even after the stimulus was removed and after multiple rounds of cell division, thereby providing what Dr. Silver termed a "memory" of the stimulus.
In the second example, she described how the reduction of dimensionality in a biological process can be leveraged to engineer a fusion protein that targeted interferon (IFN) selectively to cancer cells. Biological systems routinely limit the diffusion of molecules to enable specific molecular interactions, for example the interaction of proteins with cellular membranes or residents of cellular membranes (such as receptors) restricts the mobility of the protein. In this example, knowledge of the structure of IFN and its interaction with its receptor allowed point mutations to be introduced that compromised its ability to bind to its receptor and then fusion to ligand specific to certain types of cancer cells (EGF) allowed the interaction of this "crippled" IFN with its receptor to only occur on the cancer cells following recognition of the EGF part of the fusion protein. This example certainly proves that higher affinity isn't always better.
In the final example, a group in her lab has tackled the ambitious goal of solving the energy crisis by engineering a new metabolic pathway in yeast that will allow the production of hydrogen from biomass.
The final speaker, Dr. Meyer, gave a two-part talk. In the beginning, he described his view of the cell signaling landscape in which he identifies ~40 signaling modules, ~3000 signaling components, and 200 unique cell types. He emphasized that fluorescent biosensors have been instrumental in dissecting signaling modules and he described how silencing RNA (siRNA) techniques have provided a genetic toolkit for studying mammalian biology. Using siRNA screens combined with fluorescent biosensors, Dr. Meyer described how a model for a signaling module of store-operated calcium (SOC) entry, which occurs when the calcium in the endoplasmic reticulum (ER) is depleted, was developed. The model consists of 6 core components: (i) calcium, (ii) Orai (the plasma membrane calcium channel activated by store depletion), (iii) PMCA (the plasma membrane calcium pump), (iv) SERCA (the ER calcium pump), (v) STIM proteins (the transmembrane calcium sensors of the ER), and (vi) the IP3 receptor (an endoplasmic reticulum calcium channel activated by inositol trisphosphate).
For the calcium signaling aficionados, STIM1 appears to be the high-affinity calcium sensor that is activated upon ER calcium depletion; whereas STIM2 appears to be a low-affinity calcium sensor that is active under basal conditions. Redistribution of these proteins to puncta of ER and plasma membrane junctions in response to ER calcium depletion requires STIM dimer- or oligomerization. This oligomerization requirement is likely because the polybasic region of STIM proteins that mediates the interaction with the plasma membrane has too few polybasic residues to mediate the interaction of individual STIM proteins.
See the ASCB program for the titles of these presentations and other information about the Saturday and Sunday events.
Science Signaling. ISSN 1937-9145 (online), 1945-0877 (print). Pre-2008: Science's STKE. ISSN 1525-8882