title: Receptor downregulation and desensitization enhance the information processing ability of signalling receptors.
abstract: BACKGROUND
  In addition to initiating signaling events, the activation of cell surface receptors also triggers regulatory processes that restrict the duration of signaling. Acute attenuation of signaling can be accomplished either via ligand-induced internalization of receptors (endocytic downregulation) or via ligand-induced receptor desensitization. These phenomena have traditionally been viewed in the context of adaptation wherein the receptor system enters a refractory state in the presence of sustained ligand stimuli and thereby prevents the cell from over-responding to the ligand. Here we use the epidermal growth factor receptor (EGFR) and G-protein coupled receptors (GPCR) as model systems to respectively examine the effects of downregulation and desensitization on the ability of signaling receptors to decode time-varying ligand stimuli.
  
  
  RESULTS
  Using a mathematical model, we show that downregulation and desensitization mechanisms can lead to tight and efficient input-output coupling thereby ensuring synchronous processing of ligand inputs. Frequency response analysis indicates that upstream elements of the EGFR and GPCR networks behave like low-pass filters with the system being able to faithfully transduce inputs below a critical frequency. Receptor downregulation and desensitization increase the filter bandwidth thereby enabling the receptor systems to decode inputs in a wider frequency range. Further, system-theoretic analysis reveals that the receptor systems are analogous to classical mechanical over-damped systems. This analogy enables us to metaphorically describe downregulation and desensitization as phenomena that make the systems more resilient in responding to ligand perturbations thereby improving the stability of the system resting state.
  
  
  CONCLUSIONS
  Our findings suggest that in addition to serving as mechanisms for adaptation, receptor downregulation and desensitization can play a critical role in temporal information processing. Furthermore, engineering metaphors such as the ones described here could prove to be invaluable in understanding the design principles of biological systems.
authors.affiliation: Computational Biology and Bioinformatics Group, Pacific Northwest National Laboratory, Richland WA 99352, USA. harish.shankaran@pnl.gov
authors.forename: Harish
authors.initials: H
authors.lastname: Shankaran
authors.forename: H Steven
authors.initials: HS
authors.lastname: Wiley
authors.forename: Haluk
authors.initials: H
authors.lastname: Resat
mesh_terms: D001665:Binding Sites; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D003198:Computer Simulation; D015536:Down-Regulation; D004705:Endocytosis / Q000502:physiology*; D066246:ErbB Receptors / Q000378:metabolism*; D008954:Models, Biological; D008956:Models, Chemical; D011485:Protein Binding; D043562:Receptors, G-Protein-Coupled / Q000737:chemistry* / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*
chemical_list: D043562:Receptors, G-Protein-Coupled; D066246:ErbB Receptors