Remote ischemic conditioning attenuates transneuronal degeneration and promotes stroke recovery via CD36-mediated efferocytosis

Abstract

AbstractBACKGROUNDRemote ischemic limb conditioning (RIC) has been implicated in cross-organ protection in cerebrovascular disease, including stroke. However, the lack of a consensus protocol and controversy over the clinical therapeutic outcomes of RIC suggest inadequate mechanistic understanding of RIC. The current study identifies RIC-induced molecular and cellular events in the blood that enhance long-term functional recovery in experimental cerebral ischemiaMETHODSNaive mice or mice subjected to transient ischemic stroke were randomly selected to receive sham conditioning or RIC in the hind limb at 2 h post-stroke. At 3d post-stroke, monocyte composition in the blood was analyzed, and brain tissue was examined for monocyte-derived macrophages (Mφ), levels of efferocytosis, and CD36 expression. Mouse with conditional deletion of CD36 in Mφ (cKOMMφ) was used to establish the role of CD36 in RIC-mediated modulation of efferocytosis, transneuronal degeneration, and recovery following stroke.RESULTSRIC applied 2h after stroke increased entry of monocytes into the injured brain. In the post-ischemic brain, Mφ had increased levels of CD36 expression and efferocytosis. These changes in brain Mφ were derived from RIC-induced changes in circulating monocytes. In the blood, RIC increased CD36 expression in circulating monocytes and shifted monocytes to a proinflammatory LY6CHighstate. Conditional deletion of CD36 in Mφ abrogated the RIC-induced monocyte shift in the blood and efferocytosis in the brain. During the recovery phase of stroke, RIC rescued the loss of the volume and of tyrosine hydroxylase+ neurons in substantia nigra (SN) as well as behavioral deficits in WT mice, but not in cKOMMφmice.CONCLUSIONSRIC induces a shift in monocytes to a proinflammatory state with elevated CD36 levels, and this is associated with CD36-dependent efferocytosis in Mφs that rescues delayed transneuronal degeneration in the post-ischemic brain and promotes stroke recovery. Together, these findings provide novel insight into our mechanistic understanding of how RIC improves in post-stroke recovery.Novelty and SignificanceWhat Is Known?Infiltrated monocyte-derived macrophages (Mφ) into the post-ischemic brain cause neural inflammation, but they also engage in efferocytosis that promotes tissue repair in the injured CNS.Remote ischemic limb conditioning (RIC) changes monocyte composition and enhances functional recovery in experimental brain ischemia.The application of RIC is safe, feasible, and tolerable in stroke patients, but clinical outcomes remain inconsistent.What New Information Does This Article Contribute?We provide experimental evidence that RIC modifies peripheral monocyte composition and molecular expression, and leads to favorable changes in debris clearance, structure integrity, transneuronal degeneration, and behavior following stroke.Protective effects of RIC disappear in the absence of CD36 in Mφ, suggesting an essential mechanistic role for CD36 in RIC-induced endogenous protective outcomes.The current study demonstrates that immune-mediated RIC mechanisms facilitate inflammatory and recovery processes in the injured CNS. Given the challenges in directly manipulating the brain after stroke, the study suggests that RIC is a promising alternative strategy by inducing changes in peripheral monocytes that can influence injury progression and recovery. Moreover, RIC-induced peripheral changes uncovered by this study may serve as biomarkers to establish an optimal RIC protocol.