Microglia from LPS-treated
Cx3cr1-/- mice (b) showed
increased activation relative to microglia
from LPS-treated Cx3cr1+/-
mice (a).
Like man's best friend, microglia behave best when on a short leash. Cardona et al. report that the chemokine fractalkine restrains microglial activation and protects neurons in a recent article in Nature Neuroscience.
Fractalkine is a transmembrane glycoprotein constitutively expressed in neurons. The authors localized expression of the fractalkine receptor (Cx3cr1) to microglia. They substituted the fractalkine receptor (Cx3cr1) gene with a green fluorescent protein (GFP) reporter. Both Cx3cr1-/- and Cx3cr1+/- mice expressed GFP at Cx3cr1-expression sites, and Cx3cr1+/- mice maintained functional Cx3cr1. GFP expression in Cx3cr1+/- brains colocalized with the microglial marker calcium binding protein adapter molecule-1, but did not colocalize with the neuronal marker NeuN or the astrocytic marker glial fibrillary acidic protein.
The authors injected the endotoxin lipopolysaccharide (LPS) to induce systemic inflammation. LPS activated microglia to a greater extent in Cx3cr1-/- mice relative to Cx3cr1+/- mice. LPS also induced more neuronal loss in Cx3cr1-/- than in Cx3cr1+/- mice, as shown by annexin V staining for apoptotic cells. These data suggest that the enhanced microglial activation in Cx3cr1-/- mice was neurotoxic.
The authors confirmed the neurotoxicity of the activated microglia in Cx3cr1-/- mice in transplant experiments. They transplanted microglia from LPS-injected Cx3cr1-/- and Cx3cr1+/- mice into the brains of wild-type mice. The Cx3cr1+/- microglia migrated away, but Cx3cr1-/- microglia remained at the injection site. In contrast, microglia from saline-injected Cx3cr1-/- mice migrated away from the injection site, suggesting that fractalkine is important in the migration of microglia activated by inflammation. Apoptotic neurons were found near the injection site in mice injected with activated Cx3cr1-/-, but not activated Cx3cr1+/- microglia, showing that the aggregated microglia from LPS-injected Cx3cr1-/- mice were neurotoxic to the host animal.
Microglial activation exacerbates neurodegenerative disorders. The neurotoxin MPTP kills substantia nigra neurons and is used to model Parkinson disease. Cx3cr1-/- mice lost more neurons than did wild-type or Cx3cr1+/- mice following MPTP injection. GFP immunohistochemistry showed enhanced microglial activation in the substantia nigra of Cx3cr1-/- relative to Cx3cr1+/- mice. Similar to people with amyotropic lateral sclerosis, transgenic mice that express the human superoxide dismutase (SOD1) gene show age-dependent motor neuron loss. The authors found decreased neuron density in spinal cords of Cx3cr1-/- mice expressing SOD1 relative to SOD1-expressing Cx3cr1+/- and wild-type mice. The authors observed a steeper decline in hindlimb strength in Cx3cr1-/- relative to Cx3cr1+/- mice, and SOD1-expressing Cx3cr1-/- mice died at a younger age than Cx3cr1+/- mice.
Together, these data suggest that fractalkine restrains microglial activation and is neuroprotective. A fractalkine agonist might therefore mediate neuroprotection, especially in people with neurodegenerative disorders. However, fractalkine mediates or exacerbates atherosclerosis, suggesting that careful targeting of fractalkine delivery would be necessary for such a drug to be clinically beneficial.