Background and purpose: We previously showed that stimulation of RANKL (receptor activator of nuclear factor-B ligand)/ RANK (receptor for RANKL) signal with novel RANKL-based partial peptide, MHP1, worked as anti-inflammatory in microglia and macrophages without induction of osteoclast activation, but the effects of systemic injection of MHP1 in ischemic brain is still unclear. Here, we examined the effects of systemically injected MHP1 in ischemic brain.
Methods: FITC-conjugated MHP1 was injected via jugular vein 4 hrs after transient middle cerebral artery occlusion (tMCAo) in C57BL6J mice to check whether MHP1 could penetrate into infarct parenchyma. To examine whether MHP1 has therapeutic effects, MHP1 was injected intravenously (IV) followed by successive subcutaneous injection with Alzet pump beginning at 4 or 6 hrs after tMCAo. The effects of MHP1 for osteoclast activation in the radial bone in paralyzed upper arm were examined by TRAP staining.
Results: Immunohistochemistry for FITC showed that MHP1 could successfully penetrated into infarct parenchyma 5 min after IV injection of MHP1, but the penetrated MHP1 was not observed 1 hr after injection. These results indicated that MHP1 should be administered continuously to achieve successive penetration into infarct parenchyma. The mice treated with MHP1 showed less neurological severity score and infarct volume at 48 hrs after tMCAo even when the treatment was started at 6 hrs after MCAo. TRAP staining showed that osteoclast activation in paralyzed radial bone was inhibited in MHP1-treated mice, suggesting that MHP1 could prevent osteoclast activation as well as ischemic injury.
Conclusion: Systemically injected MHP1 could successfully penetrated into infarct parenchyma and prevent exacerbation of paralysis and infarct size after tMCAo. Although further studies for improvement of its stability are needed, MHP1 might be a novel agent to treat ischemic brain.