title: Procoagulant surface exposure and apoptosis in rabbit platelets: association with shortened survival and steady-state senescence.
abstract: BACKGROUND
  The signal(s) for removal of senescent platelets from the circulation are not fully understood; phosphatidylserine (PS) expression on platelets and another marker of apoptosis, loss of mitochondrial inner membrane potential (DeltaPsim), have been implicated in platelet clearance.
  
  
  OBJECTIVE
  To investigate whether shortened platelet survival and steady-state platelet senescence are associated with increased surface exposure of PS and DeltaPsim collapse.
  
  
  METHODS
  Survival of in-vitro biotinylated rabbit platelets treated with thrombin or Ca(2+)-ionophore A23187 was tracked by flow cytometry after injection. Steady-state platelet senescence was investigated by infusing biotin to label a platelet cohort. PS expression and DeltaPsim of in-vitro biotinylated platelets and of the aging platelet cohort biotinylated in-vivo were measured by flow cytometry using annexin V-FLUOS and the DeltaPsim-sensitive dye CMXRos, respectively.
  
  
  RESULTS
  Although PS expression, DeltaPsim and survival of thrombin-degranulated platelets were similar to those of control platelets, increasing concentrations of A23187 caused increased surface exposure of PS and progressive shortening of platelet survival; only one-sixth of PS-expressing platelets also exhibited DeltaPsim loss. The cohort of senescent, biotinylated platelets remaining in the circulation at 96 h had increased exposure of PS and collapsed DeltaPsim; of the 17% of PS-expressing platelets, one-third did not exhibit DeltaPsim loss. There was also an increase in platelets with collapsed DeltaPsim but not expressing PS.
  
  
  CONCLUSIONS
  Platelets with shortened survival and senescent platelets have increased surface exposure of PS, that may be involved in their clearance. PS expression can occur independently of DeltaPsim collapse and conversely, in aged platelets, DeltaPsim loss can occur independently of PS expression.
authors.affiliation: Division of Haematology/Oncology and Integrative Biology Program, The Hospital for Sick Children, Toronto, Ontario, Canada. margaret.rand@sickkids.ca
authors.forename: M L
authors.initials: ML
authors.lastname: Rand
authors.forename: H
authors.initials: H
authors.lastname: Wang
authors.forename: K W A
authors.initials: KW
authors.lastname: Bang
authors.forename: K S V
authors.initials: KS
authors.lastname: Poon
authors.forename: M A
authors.initials: MA
authors.lastname: Packham
authors.forename: J
authors.initials: J
authors.lastname: Freedman
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D001775:Blood Circulation; D001792:Blood Platelets / Q000378:metabolism / Q000502:physiology*; D002462:Cell Membrane / Q000502:physiology*; D002470:Cell Survival; D016922:Cellular Senescence; D006487:Hemostasis; D008564:Membrane Potentials / Q000502:physiology; D015335:Molecular Probes; D010718:Phosphatidylserines / Q000378:metabolism / Q000502:physiology; D011817:Rabbits
chemical_list: D015335:Molecular Probes; D010718:Phosphatidylserines