title: The role of microglia and the TLR4 pathway in neuronal apoptosis and vasospasm after subarachnoid hemorrhage.
abstract: BACKGROUND
  Although microglia and the Toll-like receptor (TLR) pathway have long been thought to play a role in the pathogenesis of aneurysmal subarachnoid hemorrhage (aSAH), thus far only correlations have been made. In this study, we attempted to solidify the relationship between microglia and the TLR pathway using depletion and genetic knockouts, respectively.
  
  
  METHODS
  Subarachnoid hemorrhage was induced in TLR4-/-, TRIF-/-, MyD88-/- and wild type C57BL/6 mice by injecting 60 μl of autologous blood near the mesencephalon; animals were euthanized 1 to 15 days after SAH for immunohistochemical analysis to detect microglia or apoptotic cells. Lastly, microglial depletion was performed by intracerebroventricular injection of clodronate liposomes.
  
  
  RESULTS
  On post operative day (POD) 7 (early phase SAH), neuronal apoptosis was largely TLR4-MyD88-dependent and microglial-dependent. By POD 15 (late phase SAH), neuronal apoptosis was characterized by TLR4- toll receptor associated activator of interferon (TRIF)-dependence and microglial-independence. Similarly, vasospasm was also characterized by an early and late phase with MyD88 and TRIF dependence, respectively. Lastly, microglia seem to be both necessary and sufficient to cause vasospasm in both the early and late phases of SAH in our model.
  
  
  CONCLUSIONS
  Our results suggest that SAH pathology could have different phases. These results could explain why therapies tailored to aSAH patients have failed for the most part. Perhaps a novel strategy utilizing immunotherapies that target Toll like receptor signaling and microglia at different points in the patient's hospital course could improve outcomes.
authors.affiliation: Division of NeuroCritical Care, Department of Neurology, Harvard Medical School, Beth Israel Deaconess Medical Center, The Center for Life Science, 3 Blackfan Circle, Boston, MA 02215, USA. khanafy@bidmc.harvard.edu
authors.forename: Khalid A
authors.initials: KA
authors.lastname: Hanafy
mesh_terms: D033942:Adaptor Proteins, Vesicular Transport / Q000096:biosynthesis / Q000235:genetics; D000818:Animals; D017209:Apoptosis / Q000502:physiology*; D004797:Enzyme-Linked Immunosorbent Assay; D007150:Immunohistochemistry; D020287:In Situ Nick-End Labeling; D007276:Injections, Intraventricular; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D017628:Microglia / Q000502:physiology*; D053594:Myeloid Differentiation Factor 88 / Q000096:biosynthesis / Q000235:genetics; D009474:Neurons / Q000502:physiology*; D061251:Primary Cell Culture; D015398:Signal Transduction / Q000502:physiology; D013345:Subarachnoid Hemorrhage / Q000150:complications / Q000473:pathology*; D051197:Toll-Like Receptor 4 / Q000502:physiology*; D014409:Tumor Necrosis Factor-alpha / Q000378:metabolism; D020301:Vasospasm, Intracranial / Q000209:etiology / Q000473:pathology*
chemical_list: D033942:Adaptor Proteins, Vesicular Transport; D053594:Myeloid Differentiation Factor 88; D051197:Toll-Like Receptor 4; D014409:Tumor Necrosis Factor-alpha; C507780:Myd88 protein, mouse; C476286:Ticam1 protein, mouse