Abstract
Cell signaling pathways are noisy communication channels, and statistical measures derived from information theory can be used to quantify the information they transfer. Here we use single cell signaling measures to calculate mutual information as a measure of information transfer via gonadotropin-releasing hormone (GnRH) receptors (GnRHR) to extracellular signal-regulated kinase (ERK) or nuclear factor of activated T-cells (NFAT). This revealed mutual information values <1 bit, implying that individual GnRH-responsive cells cannot unambiguously differentiate even two equally probable input concentrations. Addressing possible mechanisms for mitigation of information loss, we focused on the ERK pathway and developed a stochastic activation model incorporating negative feedback and constitutive activity. Model simulations revealed interplay between fast (min) and slow (min-h) negative feedback loops with maximal information transfer at intermediate feedback levels. Consistent with this, experiments revealed that reducing negative feedback (by expressing catalytically inactive ERK2) and increasing negative feedback (by Egr1-driven expression of dual-specificity phosphatase 5 (DUSP5)) both reduced information transfer from GnRHR to ERK. It was also reduced by blocking protein synthesis (to prevent GnRH from increasing DUSP expression) but did not differ for different GnRHRs that do or do not undergo rapid homologous desensitization. Thus, the first statistical measures of information transfer via these receptors reveals that individual cells are unreliable sensors of GnRH concentration and that this reliability is maximal at intermediate levels of ERK-mediated negative feedback but is not influenced by receptor desensitization.
Highlights
Tions despite the fact that this obscures cell-cell variation
We show that gonadotropin-releasing hormone (GnRH) sensing via mGnRHR or hGnRHR is comparable to that via XGnRHR or h.XGnRHR
I(Egr1;stimulus) was greater for PDBu (0.72 Ϯ 0.07) than for GnRH (0.21 Ϯ 0.05). These data revealed that mutual information (MI) can be used to measure information transfer via GnRHR to extracellular signalregulated kinase (ERK) and that for each input, output, and time point considered, the MI values approximately paralleled the dynamic range observed for the population-averaged responses
Summary
Cell Culture and Transfection—HeLa cells (from ECACC) were cultured in DMEM with 10% fetal calf serum (FCS) as described [27, 28]. For most experiments we constructed full concentration response curves (i.e. control and 10Ϫ12–10Ϫ6 M GnRH) at multiple time points (5 min– 6 h) and collected images for 4 –9 fields of view per well. This yielded data for Ͼ10,000 individual cells (for each treatment in each experiment). Stochastic Simulations—We modeled ERK signaling as a dynamical system with two negative feedback loops: a fast one from ppERK to effectors E and E* and a slow one via phosphatase (p-ASE2) expression (Fig. 3, see Table 1 for parameters). The number of signal molecules (S) was randomly drawn from a uniform distribution on the logarithmic scale between 10 and 1000 molecules
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