Advancing stroke recovery: unlocking the potential of cellular dynamics in stroke recovery

Abstract

Stroke stands as a predominant cause of mortality and morbidity worldwide, and there is a pressing need for effective therapies to improve outcomes and enhance the quality of life for stroke survivors. In this line, effective efferocytosis, the clearance of apoptotic cells, plays a crucial role in neuroprotection and immunoregulation. This process involves specialized phagocytes known as "professional phagocytes" and consists of four steps: "Find-Me," "Eat-Me," engulfment/digestion, and anti-inflammatory responses. Impaired efferocytosis can lead to secondary necrosis and inflammation, resulting in adverse outcomes following brain pathologies. Enhancing efferocytosis presents a potential avenue for improving post-stroke recovery. Several therapeutic targets have been identified, including osteopontin, cysteinyl leukotriene 2 receptor, the µ opioid receptor antagonist β-funaltrexamine, and PPARγ and RXR agonists. Ferroptosis, defined as iron-dependent cell death, is now emerging as a novel target to attenuate post-stroke tissue damage and neuronal loss. Additionally, several biomarkers, most importantly CD163, may serve as potential biomarkers and therapeutic targets for acute ischemic stroke, aiding in stroke diagnosis and prognosis. Non-pharmacological approaches involve physical rehabilitation, hypoxia, and hypothermia. Mitochondrial dysfunction is now recognized as a major contributor to the poor outcomes of brain stroke, and medications targeting mitochondria may exhibit beneficial effects. These strategies aim to polarize efferocytes toward an anti-inflammatory phenotype, limit the ingestion of distressed but viable neurons, and stimulate efferocytosis in the late phase of stroke to enhance post-stroke recovery. These findings highlight promising directions for future research and development of effective stroke recovery therapies.

Fig. 1. A schematic illustration of post-stroke efferocytosis.

After the occurrence of a brain stroke, local microglia (resident macrophages) and blood-derived monocytes enter the insulted area and undergo various genetic and molecular differentiations, resulting in two main phenotypes of activated macrophages: M1 and M2 (a). The M2 phenotype exerts anti-inflammatory activity (b), while the M1 phenotype is primarily a pro-inflammatory phagocyte (c), initiating extensive inflammation, ROS production, and tissue damage (d). The activated M2 microglia and other activated efferocytes, such as astrocytes, dendritic cells, neutrophils, and oligodendrocytes, irreversibly eliminate distressed neurons to avoid escalation of inflammation and tissue damage (e). Effective efferocytosis protects bystander healthy neurons from being phagocytosed and may even rescue distressed but viable neurons via cytokine production (e.g., IL-10 and -4). * Recent studies have investigated the efficacy of some medications on M2/M1 polarization, such as OPN, glucose-lowering agents (metformin, TZDs), CysLT2R antagonists, NF-κB inhibitors, β-funaltrexamine, miR-98, PPARγ, RXR agonists, CDP-choline, and IL-4. Although their exact mechanism of action is still unclear, the majority of these medications may have some inhibitory effects on the NF-κB signaling pathway. Among non-pharmacological treatments, IHT may increase HIF-1, ERK, and Akt signaling pathways and reduce M1 polarization, ROS production, and tissue damage. Physical rehabilitation, especially if not initiated early after the occurrence of stroke, may have inhibitory effects on inflammation, probably through the PI3K/Akt and FoxO1 signaling pathways. Post-stroke hypothermia has various anti-inflammatory effects; one important target might be Annexin A1 in PMN cells. ROS reactive oxygen species, PMN polymorphonuclear cells, OPN osteopontin, TZDs thiazolidinediones, CysLT2R cysteinyl leukotriene receptor 2, NF-κB nuclear factor kappa-light-chain-enhancer of activated B cells, HIF-1 hypoxia-inducible factor 1, ERK extracellular signal-regulated kinase, Akt protein kinase B, PI3K phosphoinositide 3-kinase, FoxO1 forkhead box protein O1, miR-98 microRNA-98, PPARγ peroxisome proliferator-activated receptor gamma, RXR retinoid X receptor, CDP-choline cytidine-5V-diphosphocholine, IL interleukin.

Fig. 2. A schematic illustration of post-stroke ferroptosis and related therapeutic targets.

After a hemorrhagic stroke, numerous RBCs are released in the affected region. Moreover, BBB integrity is disrupted following an ischemic stroke, leading to extravasation of RBCs (a). These RBCs are either phagocytosed by local immune cells (b) or undergo lysis (f). CD36 and Axl are two important regulators of erythrophagocytosis, upregulated by RXR and PPARγ agonists (b). The phagocytosed RBCs are digested in the phagolysosome, and the resultant heme is exported via HRG-1 (c). Heme is then hydrolyzed into Fe²⁺, CO, and biliverdin by HO-1. Fe²⁺ ions are then incorporated into ferritin or transported into extracellular space through FPN (d). Fe²⁺ is then oxidized into Fe³⁺ and forms Tf-Fe³⁺ complexes, which are uptaken by local neurons (e). Iron chelators reduce iron load (d) and diminish its neurotoxic effects. In addition, lysed RBCs (f) release massive amounts of Heme and Hb, which are uptaken by both efferocytes and neurons via CD91 and CD163, respectively (g). The Fe²⁺ load in the neurons accelerates ROS production and lipid peroxidation through several pathways (h). A protective pathway, cysteine-gluthatione-GPX4, scavenges the ROS and ameliorates lipid peroxidation (i). The path is the target of many medications, enhancing intracellular anti-oxidant activity. Finally, the massive production of ROS and lipid peroxidation leads to cell death and completes the ferroptosis process (j). BBB blood-brain barrier, RBC red blood cell, RXR retinoid X receptor, PPARγ peroxisome proliferator-activated receptor gamma, Hb hemoglobin, HRG-1 heme-responsive gene 1, CO carbon monoxide, FPN ferroportin, HO-1 and -2 heme oxygenase-1 and -2, Tf transferrin, TFR1 transferrin receptor 1, STEAP3, the six-transmembrane epithelial antigen of prostate family member 3, DMT1 divalent metal transporter 1, NAC N-acetylcysteine, IRN isorhynchophylline, Fer-1 ferrostatin-1, CDP-choline cytidine-5V-diphosphocholine, LOXs lipoxygenase, PUFA poly-unsaturated fatty acid, PL phospholipid, Se selenium, CUR curcumin, GPX4 glutathione peroxidase 4, ROS reactive oxygen species, SLC7A11 solute carrier family 7 member 11, SLC3A2 solute carrier family 3 member 2, SLC11A2 solute carrier family 11 member 2.