The influence of plasma proteins on the distribution of leucocytes within the brain parenchyma in a murine model of stroke

Downstream Toll-like receptor signaling mediates adaptor-specific cytokine expression following focal cerebral ischemia

Background

Deletion of some Toll-like receptors (TLRs) affords protection against cerebral ischemia, but disruption of their known major downstream adaptors does not. To determine whether compensation in the production of downstream effectors by one pathway when the other is disrupted can explain these findings, we examined cytokine/chemokine expression and inflammatory infiltrates in wild-type (WT), MyD88^−/− and TRIF-mutant mice following permanent middle cerebral artery occlusion (pMCAO).

Methods

Cytokine/chemokine expression was measured with a 25-plex bead array in the serum and brains of all three groups of mice at baseline (no surgery/naïve) and at 3 hours and 24 hours following pMCAO. Brain inflammatory and neutrophil infiltrates were examined 24 hours following pMCAO.

Results

IL-6, keratinocyte chemoattractant (KC), granulocyte colony-stimulating factor (G-CSF) and IL-10 were significantly decreased in MyD88^−/− mice compared to WT mice following pMCAO. Significantly, decreased levels of the neutrophil chemoattractants KC and G-CSF corresponded with a trend toward fewer neutrophils in the brains of MyD88^−/− mice. IP-10 was significantly decreased when either pathway was disrupted. MIP-1α was significantly decreased in TRIF-mutant mice, consistent with TRIF-dependent production. Finally, MyD88^−/− mice showed elevations of a number of Th2 cytokines, such as IL-13, at baseline, which became significantly decreased following pMCAO.

Conclusions

Both MyD88 and TRIF mediate pathway-specific cytokine production following focal cerebral ischemia. Our results also suggest a compensatory Th2-type skew at baseline in MyD88^−/− mice and a paradoxical switch to a Th1 phenotype following focal cerebral ischemia. Finally, the MyD88 pathway directs the expression of neutrophil chemoattractants following cerebral ischemia.

Activated natural killer cells adhere to cultured hippocampal neurons and affect the dendritic morphology

To examine the manner of interactions between immune cells and central nervous system (CNS) neurons, mouse hippocampal neurons were co-cultured with lymphokine (IL-2)-activated killer (LAK) cells. Immunocytochemical and time-lapse observations indicated that LAK cells migrated along neuronal processes and made adhesive contacts with them. In addition to the direct physical effects, LAK cells released glutamate, induced the formation of beads-like structure in the dendrites of about 14% of hippocampal neurons and caused the reduction of dendritic protrusions. These results suggest that infiltrating immune cells can form direct adhesive connections with CNS neurons and affect their dendritic morphology.

Characterization of two novel proteins, NgRH1 and NgRH2, structurally and biochemically homologous to the Nogo-66 receptor

Nogo-66 receptor (NgR) has recently been identified as the neuronal receptor of the myelin-associated proteins Nogo-A, oligodendrocyte protein (OMgp) and myelin-associated glycoprotein (MAG), and mediates inhibition of axonal regeneration both in vitro and in vivo. Through database searches, we have identified two novel proteins (NgRH1 and NgRH2) that turned out to be homologous in their primary structures, biochemical properties and expression patterns to NgR. Like NgR, the homologues contain eight leucine-rich repeats (LRR) flanked by a leucine-rich repeat C-terminus (LRRCT) and a leucine-rich repeat N-terminus (LRRNT), and also have a C-terminal GPI signal sequence. Northern blot analysis showed predominant expression of NgRH1 and NgRH2 mRNA in the brain. In situ hybridization and immunohistochemistry on rat brain slices revealed neuronal expression of the genes. NgRH1 and NgRH2 were detected on the cell surface of recombinant cell lines as N-glycosylated GPI anchored proteins and, consistent with other GPI anchored proteins, were localized within the lipid rafts of cellular membranes. In addition, an N-terminal proteolytic fragment of NgR comprising the majority of the ectodomain was found to be constitutively secreted from cells. Our data indicate that NgR, NgRH1 and NgRH2 constitute a novel receptor protein family, which may play related roles within the CNS.

Monocyte chemoattractant protein-1 deficiency is protective in a murine stroke model

Inflammatory processes have been implicated in the pathogenesis of brain damage after stroke. In rodent stroke models, focal ischemia induces several proinflammatory chemokines, including monocyte chemoattractant protein-1 (MCP-1). The individual contribution to ischemic tissue damage, however, is largely unknown. To address this question, the authors subjected MCP-1-deficient mice (MCP-1-/-) to permanent middle cerebral artery occlusion (MCAO). Measurement of basal blood pressure, cerebral blood flow, and blood volume revealed no differences between wild-type (wt) and MCP-1-/- mice. MCAO led to similar cerebral perfusion deficits in wt and MCP-1-/- mice, excluding differences in the MCA supply territory and collaterals. However, compared with wt mice, the mean infarct volume was 29% smaller in MCP-1-/- mice 24 hours after MCAO (P = 0.022). Immunostaining showed a reduction of phagocytic macrophage accumulation within infarcts and the infarct border in MCP-1-/- mice 2 weeks after MCAO. At the same time point, the authors found an attenuation of astrocytic hypertrophy in the infarct border and thalamus in MCP-1-/- mice. However, these effects on macrophages and astrocytes in MCP-1-/- mice occurred too late to suggest a protective role in acute infarct growth. Of note: at 6 hours after MCAO, MCP-1-/- mice produced significantly less interleukin-1beta in ischemic tissue; this might be related to tissue protection. The results of this study indicate that inhibition of MCP-1 signaling could be a new acute treatment approach to limit infarct size after stroke.

Neuropathologic characterization of a rodent model of closed head injury--addition of clinically relevant secondary insults does not significantly potentiate brain damage

We have characterized the early brain pathology in Sprague-Dawley rats subjected to a modified Richmond impact acceleration model of closed head injury (CHI). This model was modified to produce maximal traumatic brain injury (TBI) in the absence of skull fracture, extracerebral or intracerebral hemorrhage, or brain contusion. We then used this model to assess the neuropathologic effects of superimposed secondary insults, which were designed to reflect a clinically relevant combination of hypotension and pyrexia. Acute neuronal injury, blood-brain barrier (BBB) integrity, axonal injury (AI), and glial activation were studied 4 1/2 hours following either CHI (group A), CHI plus secondary insults (group B), secondary insults alone (group C), or sham control injury (group D). There was evidence of limited AI following CHI in the lower medulla and upper cervical cord region, which was not modified by addition of secondary insult. Loss of dendritic microtubule-associated protein MAP2 immunoreactivity proved a reliable marker of acute neuronal damage, which was confined to subimpact and inferolateral cortical locations following CHI and was widespread after secondary insult. The pattern of plasma protein extravasation paralleled that of acute neuronal injury. We found no evidence of microglial activation, either local or generalized, by 4 1/2 hours. However, by this time CHI and secondary insults had combined to produce evidence of subimpact astrocyte activation, which was not apparent with either insult or injury alone. We conclude that in this modified Richmond model of CHI, when combined with secondary insults, there is no convincing potentiation of brain damage with the minor exception of astrocyte activation.