(237e) Cell-Cell Crosstalk Potentiates Cell Patterning during Development

Completion of several genome projects has produced a catalogue of genes that make up the organisms investigated. Now, the key challenge is to understand how individual gene products work together to produce higher order biological phenomena, such as the patterning of cell fates during development. Correct patterns of cell fates are required for development of functional organs. We have focused on a specific model system, the nematode C. elegans, to understand how a biomolecular network ensures robust fate patterning. An intriguing aspect of this system is that patterning depends not only on a spatial gradient of a soluble factor (a morphogen), but also on direct cell-cell contact. Furthermore, genetic evidence shows that these two fate patterning signals are coupled via an intracellular signaling network. Lateral cell-cell interactions inhibit signaling mediated by the soluble factor; meanwhile, the soluble factor regulates the extent of lateral signaling.

Why are cell-cell interactions needed if a morphogen gradient is already present? How does coupling quantitatively contribute to cell fate patterning? Does an integrated, in silico examination of the intracellular network predict phenotypes observed in experiments? What new phenotypes are possible, and what perturbations render such phenotypes?

To address these and related questions, we developed and analyzed a computational model of the cell population that undergoes patterning in response to the two aforementioned extracellular signals, the soluble factor (LIN-3) and cell-cell interaction mediated by LIN-12. We show that coupling these two extracellular signals amplifies the cellular perception of the LIN-3 gradient and polarizes lateral signaling, both of which enhance fate segregation beyond that achieved by an uncoupled system. Experiments are currently being conducted to test this prediction using a fluorescent reporter responsive to LIN-3 signaling that has been chromosomally integrated in worms. In addition to quantitative predictions regarding biochemical signals, we devised a novel framework for predicting qualitative phenotypic outcomes. Our model accurately predicts both the wild-type phenotype and a remarkably different mutant phenotype [Giurumescu et. al., PNAS (2006)]. We have now extended these results by conducting a broad sweep of parameter values, in effect conducting in silico genetics experiments. Results show that several experimentally-observed mutant phenotypes and the wild-type phenotype are among the most prevalent phenotypes predicted by the model. In fact, the in silico phenotypes require the same mutations as their experimental counterparts. Moreover, our model identifies new ways of producing known experimental mutants and reveals how novel mutant phenotypes may be generated, offering new lead experiments to probe the patterning potential of this organ in C. elegans. Since both LIN-3 (an EGF-like protein) and LIN-12 (a homolog of Notch) are conserved signaling component found in worms, flies and mammals, our study provides new insight into mechanisms guiding development of simple and complex organisms.