Cordycepin confers long-term neuroprotection via inhibiting neutrophil infiltration and neuroinflammation after traumatic brain injury.BACKGROUND
 The secondary injury caused by traumatic brain injury (TBI), especially white matter injury (WMI), is highly sensitive to neuroinflammation, which further leads to unfavored long-term outcomes.  Although the cross-talk between the three active events, immune cell infiltration, BBB breakdown, and proinflammatory microglial/macrophage polarization, plays a role in the vicious cycle, its mechanisms are not fully understood.  It has been reported that cordycepin, an extract from Cordyceps militaris, can inhibit TBI-induced neuroinflammation although the long-term effects of cordycepin remain unknown.  Here, we report our investigation of cordycepin's long-term neuroprotective function and its underlying immunological mechanism.


 TBI mice model was established with a controlled cortical impact (CCI) method.  Cordycepin was intraperitoneally administered twice daily for a week.  Neurological outcomes were assessed by behavioral tests, including grid walking test, cylinder test, wire hang test, and rotarod test.  Immunofluorescence staining, transmission electron microscopy, and electrophysiology recording were employed to assess histological and functional lesions.  Quantitative-PCR and flow cytometry were used to detect neuroinflammation.  The tracers of Sulfo-NHS-biotin and Evans blue were assessed for the blood-brain barrier (BBB) leakage.  Western blot and gelatin zymography were used to analyze protein activity or expression.  Neutrophil depletion in vivo was performed via using Ly6G antibody intraperitoneal injection.


 Cordycepin administration ameliorated long-term neurological deficits and reduced neuronal tissue loss in TBI mice.  Meanwhile, the long-term integrity of white matter was also preserved, which was revealed in multiple dimensions, such as morphology, histology, ultrastructure, and electrical conductivity.  Cordycepin administration inhibited microglia/macrophage pro-inflammatory polarization and promoted anti-inflammatory polarization after TBI.  BBB breach was attenuated by cordycepin administration at 3 days after TBI.  Cordycepin suppressed the activities of MMP-2 and MMP-9 and the neutrophil infiltration at 3 days after TBI.  Moreover, neutrophil depletion provided a cordycepin-like effect, and cordycepin administration united with neutrophil depletion did not show a benefit of superposition.


 CONCLUSIONS
 The long-term neuroprotective function of cordycepin via suppressing neutrophil infiltration after TBI, thereby preserving BBB integrity and changing microglia/macrophage polarization.  These findings provide significant clinical potentials to improve the quality of life for TBI patients.
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