Today's issue of Molecular Cell
has two complementary papers on biological signal processing that look interesting (subscription required):
"The Incoherent Feedforward Loop Can Provide Fold-Change Detection in Gene Regulation"
Many sensory systems (e.g., vision and hearing) show a response that is proportional to the fold-change in the stimulus relative to the background, a feature related to Weber's Law. Recent experiments suggest such a fold-change detection feature in signaling systems in cells: a response that depends on the fold-change in the input signal, and not on its absolute level. It is therefore of interest to find molecular mechanisms of gene regulation that can provide such fold-change detection. Here, we demonstrate theoretically that fold-change detection can be generated by one of the most common network motifs in transcription networks, the incoherent feedforward loop (I1-FFL), in which an activator regulates both a gene and a repressor of the gene. The fold-change detection feature of the I1-FFL applies to the entire shape of the response, including its amplitude and duration, and is valid for a wide range of biochemical parameters.
And "Evidence that Fold-Change, and Not Absolute Level, of β-Catenin Dictates Wnt Signaling"
In response to Wnt stimulation, β-catenin accumulates and activates target genes. Using modeling and experimental analysis, we found that the level of β-catenin is sensitive to perturbations in the pathway, such that cellular variation would be expected to alter the signaling outcome. One unusual parameter was robust: the fold-change in β-catenin level (post-Wnt/pre-Wnt). In Xenopus, dorsal-anterior development and target gene expression are robust to perturbations that alter the final level but leave the fold-change intact. These suggest, first, that despite cellular noise, the cell responds reliably to Wnt stimulation by maintaining a robust fold-change in β-catenin. Second, the transcriptional machinery downstream of the Wnt pathway does not simply read the β-catenin level after Wnt stimulation but computes fold-changes in β-catenin. Analogous to Weber's Law in sensory physiology, some gene transcription networks must respond to fold-changes in signals, rather than absolute levels, which may buffer stochastic, genetic, and environmental variation.
I haven't read them yet, so I can't comment in detail, but what's nice about this approach is that the researchers use a physical model to explain a phenomenon (sensitivity to fold-change in signal, not absolute levels) that can't easily be explained with non-quantitative reasoning. I hope to return to these papers later this week.
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