Clearance of Neutrophils From ICH-Affected Brain by Macrophages Is Beneficial and Is Assisted by Lactoferrin and CD91.

BACKGROUND: Within hours after intracerebral hemorrhage (ICH) onset, masses of polymorphonuclear neutrophils (PMNs) infiltrate the ICH-affected brain. After degranulation involving controlled release of many toxic antimicrobial molecules, the PMNs undergo rapid apoptosis and then are removed by phagocytic microglia/macrophages (MΦ) through a process called efferocytosis. Effective removal of PMNs may limit secondary brain damage and inflammation; however, the molecular mechanisms governing these cleanup activities are not well understood. We propose that scavenger receptor CD91 on myeloid phagocytes especially in presence of CD91 ligand, LTF (lactoferrin, protein abundant in PMNs), plays an important role in clearance of dead apoptotic PMNs (ANs). METHODS: Mice/rats were subjected to an autologous blood injection model of ICH. Primary cultured microglia were used to assess phagocytosis of ANs. Immunohistochemistry was employed to assess CD91 expression and PMN infiltration. CD91 knockout mice selectively in myeloid phagocytes (Mac-CD91-KO) were used to establish the CD91/LTF function in phagocytosis and in reducing ICH-induced injury, as assessed using behavioral tests, hematoma resolution, and oxidative stress. RESULTS: Masses of PMNs are found in ICH-affected brain, and they contain LTF. MΦ at the outer border of hematoma are densely packed, expressing CD91 and phagocytosing ANs. Microglia deficient in CD91 demonstrate defective phagocytosis of ANs, and mice deficient in CD91 (Mac-CD91-KO) subjected to ICH injury have increased neurological dysfunction that is associated with impaired hematoma resolution (hemoglobin and iron clearance) and elevated oxidative stress. LTF that normally ameliorates ICH injury in CD91-proficient control mice shows reduced therapeutic effects in Mac-CD91-KO mice. CONCLUSIONS: Our study suggests that CD91 plays a beneficial role in improving ANs phagocytosis and ultimately post-ICH outcome and that the beneficial effect of LTF in ICH is in part dependent on presence of CD91 on MΦ.

Young Astrocytic Mitochondria Attenuate the Elevated Level of CCL11 in the Aged Mice, Contributing to Cognitive Function Improvement.

Aging drives cognitive decline, and mitochondrial dysfunction is a hallmark of age-induced neurodegeneration. Recently, we demonstrated that astrocytes secrete functional mitochondria (Mt), which help adjacent cells to resist damage and promote repair after neurological injuries. However, the relationship between age-dependent changes in astrocytic Mt function and cognitive decline remains poorly understood. Here, we established that aged astrocytes secret less functional Mt compared to young astrocytes. We found the aging factor C-C motif chemokine 11 (CCL11) is elevated in the hippocampus of aged mice, and that its level is reduced upon systemic administration of young Mt, in vivo. Aged mice receiving young Mt, but not aged Mt improved cognitive function and hippocampal integrity. Using a CCL11-induced aging-like model in vitro, we found that astrocytic Mt protect hippocampal neurons and enhance a regenerative environment through upregulating synaptogenesis-related gene expression and anti-oxidants that were suppressed by CCL11. Moreover, the inhibition of CCL11-specific receptor C-C chemokine receptor 3 (CCR3) boosted the expression of synaptogenesis-related genes in the cultured hippocampal neurons and restored the neurite outgrowth. This study suggests that young astrocytic Mt can preserve cognitive function in the CCL11-mediated aging brain by promoting neuronal survival and neuroplasticity in the hippocampus.

Increased Expression of Interferon-Induced Transmembrane 3 (IFITM3) in Stroke and Other Inflammatory Conditions in the Brain.

Microglia, the resident innate immune cells of the brain, become more highly reactive with aging and diseased conditions. In collaboration with other cell types in brains, microglia can contribute both to worsened outcome following stroke or other neurodegenerative diseases and to the recovery process by changing their phenotype toward reparative microglia. Recently, IFITM3 (a member of the "interferon-inducible transmembrane" family) has been revealed as a molecular mediator between amyloid pathology and neuroinflammation. Expression of IFITM3 in glial cells, especially microglia following stroke, is not well described. Here, we present evidence that ischemic stroke causes an increase in IFITM3 expression along with increased microglial activation marker genes in aged brains. To further validate the induction of IFITM3 in post-stroke brains, primary microglia and microglial-like cells were exposed to a variety of inflammatory conditions, which significantly induced IFITM3 as well as other inflammatory markers. These findings suggest the critical role of IFITM3 in inducing inflammation. Our findings on the expression of IFITM3 in microglia and in aged brains following stroke could establish the basic foundations for the role of IFITM3 in a variety of neurodegenerative diseases, particularly those that are prevalent or enhanced in the aged brain.

Transplantation of astrocytic mitochondria modulates neuronal antioxidant defense and neuroplasticity and promotes functional recovery after intracerebral hemorrhage.

Astrocytes release functional mitochondria (Mt) that play regulatory and pro-survival functions upon entering adjacent cells. We recently demonstrated that these released Mt could enter microglia to promote their reparative/pro-phagocytic phenotype that assists in hematoma cleanup and neurological recovery after intracerebral hemorrhage (ICH). However, a relevance of astrocytic Mt transfer into neurons in protecting brain after ICH is unclear. Here, we found that ICH causes a robust increase in superoxide generation and elevated oxidative damage that coincides with loss of the mitochondrial enzyme manganese superoxide dismutase (Mn-SOD). The damaging effect of ICH was reversed by intravenous transplantation of astrocytic Mt that upon entering the brain (and neurons), restored Mn-SOD levels and reduced neurological deficits in male mice subjected to ICH. Using an in vitro ICH-like injury model in cultured neurons, we established that astrocytic Mt upon entering neurons prevented reactive oxygen species-induced oxidative stress and neuronal death by restoring neuronal Mn-SOD levels, while at the same time promoted neurite extension and upregulation of synaptogenesis-related gene expression. Furthermore, we found that Mt genome-encoded small peptide humanin (HN) that is normally abundant in Mt, could simulate Mt-transfer effect on neuronal Mn-SOD expression, oxidative stress, and neuroplasticity under ICH-like injury. This study demonstrates that adoptive astrocytic Mt transfer enhances neuronal Mn-SOD-mediated anti-oxidative defense and neuroplasticity in the brain, which potentiate functional recovery following ICH.SIGNIFICANCE STATEMENTMitochondrial dysfunction and antioxidant defense play essential role in brain damage after intracerebral hemorrhage (ICH). Astrocytes release functional mitochondria (Mt) that enter adjacent cells to help brain homeostatic function. Here, we show that systemic transplantation of astrocytic Mt restores ICH-impaired neuronal anti-oxidative defense, enhances neurite outgrowth, and improves stroke recovery after ICH. Our study suggests that systemic transplantation of astrocytic Mt could be considered as a novel and potentially promising strategy for ICH treatment.

The Mitochondria-Derived Peptide Humanin Improves Recovery from Intracerebral Hemorrhage: Implication of Mitochondria Transfer and Microglia Phenotype Change.

Astrocytes are an integral component of the neurovascular unit where they act as homeostatic regulators, especially after brain injuries, such as stroke. One process by which astrocytes modulate homeostasis is the release of functional mitochondria (Mt) that are taken up by other cells to improve their function. However, the mechanisms underlying the beneficial effect of Mt transfer are unclear and likely multifactorial. Using a cell culture system, we established that astrocytes release both intact Mt and humanin (HN), a small bioactive peptide normally transcribed from the Mt genome. Further experiments revealed that astrocyte-secreted Mt enter microglia, where they induce HN expression. Similar to the effect of HN alone, incorporation of Mt by microglia (1) upregulated expression of the transcription factor peroxisome proliferator-activated receptor gamma and its target genes (including mitochondrial superoxide dismutase), (2) enhanced phagocytic activity toward red blood cells (an in vitro model of hematoma clearance after intracerebral hemorrhage [ICH]), and (3) reduced proinflammatory responses. ICH induction in male mice caused profound HN loss in the affected hemisphere. Intravenously administered HN penetrated perihematoma brain tissue, reduced neurological deficits, and improved hematoma clearance, a function that normally requires microglia/macrophages. This study suggests that astrocytic Mt-derived HN could act as a beneficial secretory factor, including when transported within Mt to microglia, where it promotes a phagocytic/reparative phenotype. These findings also indicate that restoring HN levels in the injured brain could represent a translational target for ICH. These favorable biological responses to HN warrant studies on HN as therapeutic target for ICH.SIGNIFICANCE STATEMENT Astrocytes are critical for maintaining brain homeostasis. Here, we demonstrate that astrocytes secrete mitochondria (Mt) and the Mt-genome-encoded, small bioactive peptide humanin (HN). Mt incorporate into microglia, and both Mt and HN promote a "reparative" microglia phenotype characterized by enhanced phagocytosis and reduced proinflammatory responses. Treatment with HN improved outcomes in an animal model of intracerebral hemorrhage, suggesting that this process could have biological relevance to stroke pathogenesis.

Beneficial Role of Neutrophils Through Function of Lactoferrin After Intracerebral Hemorrhage.

BACKGROUND AND PURPOSE: Intracerebral hemorrhage (ICH) is a devastating disease with a 30-day mortality of ~50%. There are no effective therapies for ICH. ICH results in brain damage in 2 major ways: through the mechanical forces of extravasated blood and then through toxicity of the intraparenchymal blood components including hemoglobin/iron. LTF (lactoferrin) is an iron-binding protein, uniquely abundant in polymorphonuclear neutrophils (PMNs). After ICH, circulating blood PMNs enter the ICH-afflicted brain where they release LTF. By virtue of sequestrating iron, LTF may contribute to hematoma detoxification. METHODS: ICH in mice was produced using intrastriatal autologous blood injection. PMNs were depleted with intraperitoneal administration of anti-Ly-6G antibody. Treatment of mouse brain cell cultures with lysed RBC or iron was used as in vitro model of ICH. RESULTS: LTF mRNA was undetectable in the mouse brain, even after ICH. Unlike mRNA, LTF protein increased in ICH-affected hemispheres by 6 hours, peaked at 24 to 72 hours, and remained elevated for at least a week after ICH. At the single cell level, LTF was detected in PMNs in the hematoma-affected brain at all time points after ICH. We also found elevated LTF in the plasma after ICH, with a temporal profile similar to LTF changes in the brain. Importantly, mrLTF (recombinant mouse LTF) reduced the cytotoxicity of lysed RBC and FeCl3 to brain cells in culture. Ultimately, in an ICH model, systemic administration of mrLTF (at 3, 24, and 48 hours after ICH) reduced brain edema and ameliorated neurological deficits caused by ICH. mrLTF retained the benefit in reducing behavioral deficit even with 24-hour treatment delay. Interestingly, systemic depletion of PMNs at 24 hours after ICH worsened neurological deficits, suggesting that PMN infiltration into the brain at later stages after ICH could be a beneficial response. CONCLUSIONS: LTF delivered to the ICH-affected brain by infiltrating PMNs may assist in hematoma detoxification and represent a powerful potential target for the treatment of ICH.