title: Potential for control of signaling pathways via cell size and shape.
abstract: BACKGROUND
  In order for signals generated at the plasma membrane to reach intracellular targets, activated messengers, such as G proteins and phosphoproteins, must diffuse through the cytoplasm. If the deactivators of these messengers, GTPase activating proteins (GAPs) and phosphatases, respectively, are sufficiently active in the cytoplasm, then the signal could in principle decay before reaching the target and a stable spatial gradient in phosphostate would be generated. Recent experiments document the existence of such gradients in living cells and suggest a role for them in mitotic spindle morphogenesis and cell migration. However, how such systems behave theoretically when embedded in a cell of varying size or shape has not been considered.
  
  
  RESULTS
  Here we use a simple mathematical model to explore the theoretical consequences of a plasma membrane bound activator (i.e., guanine nucleotide exchange factor, GEF, or kinase) and a cytoplasmic deactivator (i.e., GAP or phosphatase), and we find that as a model cell grows, the substrate becomes progressively dephosphorylated as a result of decreased proximity to the activator. Conversely, as a cell spreads and flattens, the substrate becomes globally phosphorylated because of increased proximity of the substrate to the activator. Similarly, in the leading edge of polarized cells and in protrusions such as lamellipodia or filopodia, the substrate is highly phosphorylated. As a specific test of the model, we found that the experimentally observed preferential activation of the G protein Cdc42 in the periphery of fibroblasts that was recently reported is consistent with model predictions.
  
  
  CONCLUSIONS
  We conclude that cell-signaling pathways can theoretically be turned on and off, both locally and globally, in response to alterations in cell size and shape.
authors.affiliation: Department of Biomedical Engineering, University of Minnesota, Minneapolis, Minnesota 55455, USA.
authors.forename: Jason
authors.initials: J
authors.lastname: Meyers
authors.forename: Jennifer
authors.initials: J
authors.lastname: Craig
authors.forename: David J
authors.initials: DJ
authors.lastname: Odde
mesh_terms: D048430:Cell Shape / Q000502:physiology*; D048429:Cell Size; D020690:GTPase-Activating Proteins / Q000502:physiology; D020662:Guanine Nucleotide Exchange Factors / Q000502:physiology; D008954:Models, Biological; D010766:Phosphorylation; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D020690:GTPase-Activating Proteins; D020662:Guanine Nucleotide Exchange Factors