title: Intracellular phospho-protein staining techniques for flow cytometry: monitoring single cell signaling events.
abstract: BACKGROUND
  Recent advances in intracellular staining techniques, cytometer technology, fluorescent reagents, and antibody production have expanded the number of intracellular antigens that can be analyzed by flow cytometry. Measurement of protein phosphorylation with phospho-specific antibodies has given insight into kinase signaling cascades. However, available techniques for phospho-epitope staining can differ greatly, making it necessary to understand the differences between the outcomes when such techniques are applied and to develop robust and reproducible methods of application.
  
  
  METHODS
  Ten different cellular fixation and permeabilization techniques were tested for their ability to provide phospho-specific staining. Combinations of formaldehyde, methanol, ethanol, acetone, Triton X-100, and saponin were used as fixation and permeabilization reagents. Phospho-specific antibodies were labeled with Alexa Fluor dyes to provide multicolor analysis of different signaling events simultaneously within individual cells.
  
  
  RESULTS
  Fixing cells with 1.5% formaldehyde followed by permeabilization in methanol gave optimal results for pERK, pp38, pJNK, pStat1, pStat5, and pStat6 staining. Alteration of formaldehyde fixation and methanol permeabilization times affected measurements of phosphorylation induction. Phospho-specific flow cytometric analyses correlated well with Western blotting, providing cross platform validation of the technique.
  
  
  CONCLUSIONS
  Measuring phosphorylation events by flow cytometry provides a rapid and efficient way to measure kinase cascades in individual cells. Stability of phospho-epitopes in methanol allows long-term storage of samples prior to analysis. Multiple signaling cascades can be monitored simultaneously through the use of different fluorophore labels to determine specificity of ligands or inhibitors. Application of optimized techniques to heterogeneous cell types such as peripheral blood or murine splenocytes may allow signaling to be analyzed simultaneously in immune cell subsets.
mesh_terms: D000911:Antibodies, Monoclonal / Q000737:chemistry; D015153:Blotting, Western; D002463:Cell Membrane Permeability; D005434:Flow Cytometry / Q000379:methods* / Q000592:standards; D005456:Fluorescent Dyes / Q000737:chemistry; D006801:Humans; D019169:Jurkat Cells; D000432:Methanol / Q000493:pharmacokinetics; D020928:Mitogen-Activated Protein Kinases / Q000032:analysis / Q000378:metabolism; D010750:Phosphoproteins / Q000737:chemistry* / Q000378:metabolism; D010766:Phosphorylation; D013194:Staining and Labeling / Q000379:methods*; D013696:Temperature; D013997:Time Factors; D020298:U937 Cells
chemical_list: D000911:Antibodies, Monoclonal; D005456:Fluorescent Dyes; D010750:Phosphoproteins; D020928:Mitogen-Activated Protein Kinases; D000432:Methanol