Neutralization of Lipocalin-2 Diminishes Stroke-Reperfusion Injury

Abstract

Oxidative stress is a key contributor to the pathogenesis of stroke-reperfusion injury. Neuroinflammatory peptides released after ischemic stroke mediate reperfusion injury. Previous studies, including ours, have shown that lipocalin-2 (LCN2) is secreted in response to cerebral ischemia to promote reperfusion injury. Genetic deletion of LCN2 significantly reduces brain injury after stroke, suggesting that LCN2 is a mediator of reperfusion injury and a potential therapeutic target. Immunotherapy has the potential to harness neuroinflammatory responses and provides neuroprotection against stroke. Here we report that LCN2 was induced on the inner surface of cerebral endothelial cells, neutrophils, and astrocytes that gatekeep the blood-brain barrier (BBB) after stroke. LCN2 monoclonal antibody (mAb) specifically targeted LCN2 in vitro and in vivo, attenuating the induction of LCN2 and pro-inflammatory mediators (iNOS, IL-6, CCL2, and CCL9) after stroke. Administration of LCN2 mAb at 4 h after stroke significantly reduced neurological deficits, cerebral infarction, edema, BBB leakage, and infiltration of neutrophils. The binding epitope of LCN2 mAb was mapped to the β3 and β4 strands, which are responsible for maintaining the integrity of LCN2 cup-shaped structure. These data indicate that LCN2 can be pharmacologically targeted using a specific mAb to reduce reperfusion injury after stroke.

Keywords: Lipocalin-2; immunotherapy; neutrophils; oxidative stress; reperfusion injury; stroke.

Figure 1

Immunolocalization of LCN2 in vascular endothelial cells in the ipsilateral cortex after transient middle cerebral artery occlusion (tMCAo). Brain slices isolated from naive mice (A–C) and at 23 h after tMCAo (D–L) were labeled with LCN2 antibody (green), tomato lectin (red, blood vessel), and GFAP antibody (blue, astrocyte). (D) Neutrophils detected within the blood vessel (arrowheads) and in ischemic brain parenchyma (arrows) labeled with LCN2 antibody (green). (G–L) Merged and amplified images showing the induction of LCN2 (green) on the inner surface of vascular endothelial cells (red) surrounded by astrocytic end-feet (blue). The shaded area in the inset indicates the infarcted region. Scale bars, 50 μm for the main images (A–I), and 10 μm for the amplified images (J–L).

Figure 2

Immunolocalization of LCN2 in astrocytes and neutrophils in the ipsilateral cortex after tMCAo. Mouse brain slices isolated at 23 h after tMCAo were labeled with LCN2 antibody (green, A), Tomato Lectin (red, blood vessel, B), and GFAP antibody (blue, astrocyte, C). (D) Merged image showing the expression of LCN2 in an astrocyte whose end-feet encircle blood vessels (arrowheads). Brain slices isolated at 23 h after tMCAo were stained with antibodies recognizing LCN2 (green, E) and a specific marker for neutrophils (anti-Ly-6B.2 clone 7/4) (red, F). Nuclei were labeled with DAPI (blue, G). (H) Merged image showing the colocalization of LCN2 with 7/4 in yellow. The shaded area in the inset indicates the infarcted region. (I) The percentage of LCN2-positive cell types (n = 5). Scale bars, 10 μm (A–D), 50 μm (E–H).

Figure 3

LCN2 monoclonal antibody (mAb) specifically immunoprecipitated recombinant and endogenous LCN2 proteins. (A) Representative Western blots showing that LCN2 mAb reduced the level of LCN2 protein by immunoprecipitation. Increasing concentrations of LCN2 mAb (0, 0.1, 0.5, and 2.5 μg) bound to the Dynabeads were incubated with a fixed amount of mouse recombinant LCN2 protein (0.1 μg). LCN2 mAb bound to the Dynabeads, immunoprecipitated LCN2 protein, and unbound LCN2 protein in the supernatant after the immunoprecipitation are shown in the top, middle, and bottom panels, respectively; (B,C) LCN2 mAb specifically immunoprecipitated the LCN2 protein that was induced after tMCAo. Ipsilateral hemisphere lysates (B) and blood sera (C) collected from naive LCN2^+/+ and LCN2^−/− mice (+/+ con and −/− con) and at 23 h after tMCAo (+/+ tMCAo and −/− tMCAo) were immunoprecipitated with LCN2 mAb and analyzed by Western blotting using a polyclonal antibody that recognized LCN2 protein; (D) Total RNA isolated from ipsilateral hemispheres of naive LCN2^+/+ and LCN2^−/− mice (+/+ con and −/− con), at 23 h after tMCAo (+/+ tMCAo and −/− tMCAo), and LCN2^+/+ mice treated with LCN2 mAb at 4 h after tMCAo (+/+ tMCAo LCN2 mAb) was analyzed by real-time RT-PCR (n = 6 per group). Relative mRNA expression of LCN2 in the brain homogenates was compared between the mice groups using a one-way ANOVA and Newman–Keuls post hoc tests. LCN2 mRNA levels were significantly induced after tMCAo (*** p < 0.001) as compared with those in naive LCN2^+/+ mice. LCN2 mRNA levels in mice that were treated with LCN2 mAb were significantly reduced (* p < 0.05) as compared those in LCN2^+/+ mice after tMCAo; (E,F) Mice were treated with an isotype control IgG (con) or LCN2 mAb at 4 h after tMCAo. We analyzed the concentration of LCN2 in the ipsilateral hemispheres (n = 5 per group, E) and blood sera (n = 9–10 per group, F) at 23 h after reperfusion using ELISA. The concentration of LCN2 in the brains of mice treated with LCN2 mAb were significantly decreased (* p < 0.05) as compared with that in the brains of mice that received the control IgG (one-tailed, unpaired t test). The serum concentration of LCN2 in mice that received LCN2 mAb were also significantly decreased (** p < 0.01) as compared with that in mice that received the control IgG (two-tailed, unpaired t-test).

Figure 4

LCN2 mAb attenuated neurological deficits and cerebral infarction after tMCAo. Neurological deficit scoring (A) and corner test (B) were performed at 20 h after one hour of tMCAo in mice treated with isotype control IgG (con) and LCN2 mAb (n = 7 per group). (C) Representative images of TTC-stained brain slices from mice treated with control IgG and LCN2 mAb after 23 h of reperfusion. Viable tissue is stained in red color, whereas the infarcted area remains unstained (white). Total infarct volume (D) and brain swelling percentage (E) in mice treated with LCN2 mAb were significantly decreased 23 h after reperfusion as compared with those in mice treated with the control IgG (n = 5 per group). ** p < 0.01, *** p < 0.001 compared with treatments with control IgG (two-tailed, unpaired t-test).

Figure 5

LCN2 mAb limited blood–brain barrier leakage and infiltration of neutrophils after tMCAo. Representative images (A) and quantification (B) of Evans blue extravasation in the ipsilateral hemispheres of mice treated with control IgG (con) and LCN2 mAb (n = 5 per group) after one hour of tMCAo and 23 h after reperfusion. The concentration of Evans blue dye in the ipsilateral hemispheres of mice treated with LCN2 mAb was significantly decreased (** p < 0.01) as compared with that in the ipsilateral hemispheres of mice treated with control IgG (two-tailed, unpaired t test); (C) The expression level of the tight junction protein claudin-5 was analyzed after treatments with control IgG and LCN2 mAb (n = 4 per group). The ipsilateral (i) and contralateral (c) hemispheres isolated at 23 h after tMCAo were analyzed by Western blotting using antibodies against claudin-5. Representative Western blot showing the expression of claudin-5 (~22 kDa) in brain homogenates. β-actin served as a loading control; (D) The level of claudin-5 immunoreactivity normalized to β-actin (claudin-5/actin) in the ipsilateral hemispheres in mice treated with LCN2 mAb was significantly higher than that in the ipsilateral hemispheres of mice that received the control IgG (* p < 0.05, one-tailed, unpaired t test); (E,F) Neutrophil infiltration was analyzed by measuring the levels of MPO in brain homogenates. The ipsilateral (i) and contralateral (c) hemispheres of mice treated with control IgG (con) and LCN2 mAb (n = 4 per group) isolated at 23 h after tMCAo were analyzed by Western blotting using antibodies against MPO; (E) Representative Western blots show the expression of MPO heavy chain (~55 kD) in brain homogenates; (F) The level of MPO immunoreactivity normalized to β-actin (MPO/actin) was significantly reduced in the ipsilateral hemispheres of mice treated with LCN2 mAb (* p < 0.05, one-tailed, unpaired t test) as compared with that in the ipsilateral hemisphere of mice that received control IgG.

Figure 6

LCN2 mAb attenuated the induction of chemokines and cytokines after tMCAo. We analyzed total RNA isolated from ipsilateral hemispheres of naive LCN2^+/+ and LCN2^−/− mice (+/+ con and −/− con), as well as at 23 h after tMCAo (+/+ tMCAo and −/− tMCAo), and LCN2^+/+ mice treated with LCN2 mAb at 4 h after tMCAo (+/+ tMCAo LCN2 mAb) using real-time RT-PCR (n = 6 per group). We compared differences in the relative mRNA expression of iNOS (A), IL-6 (B), CCL2 (C), and CCL9 (D) between the five groups using a one-way ANOVA and Newman–Keuls post hoc tests. The relative brain mRNA levels of iNOS, IL-6, CCL2, and CCL9 after tMCAo were significantly increased as compared with those in naive LCN2^+/+ mice. Additionally, the mRNA levels of iNOS, IL-6, CCL2, and CCL9 in LCN2^−/− mice or mice treated with LCN2 mAb were significantly reduced as compared with those in LCN2^+/+ mice after tMCAo (*** p < 0.001, ** p < 0.01, * p < 0.05).

Figure 7

Epitope mapping of LCN2 mAb. (A) ELISA quantification of relative binding of LCN2 mAb to 56 peptides covering the mouse LCN2 protein (n = 3); (B) The combined sequences of peptides 22–24 are shown in red at amino acids 64–84 of the mouse LCN2 protein. The N-terminal signal peptide that was cleaved prior to the secretion of the protein is underlined and not included in the epitope mapping. Cys-78 and Cys-177 forming an intramolecular disulfide bond (blue); (C) Alignment of the LCN2 mAb epitope sequences from mouse, rat, and human LCN2 homologs. Conserved amino acids are underlined. Amino acids interacting with siderophore are highlighted in pink. Conserved cysteines are highlighted in blue; (D) The crystal structure of human LCN2 protein containing a siderophore (black) and an iron (orange). The epitope of LCN2 mAb, highlighted in red, is located in the β3 and β4 strands. The side chains of amino acids bound with siderophore are colored in pink. An intramolecular disulfide bond formed between Cys-78 in the epitope and Cys-177 near the C-terminal (blue); (E) Representative Western blot showing that increased concentrations of epitope peptides from mouse (YNVTSILVRDQDQGCRYWIRT) and human (YNVTSVLFRKKKCDYWIRT) tissue reduced the level of immunoprecipitated mouse (mLCN2) or human LCN2 protein (hLCN2).