Effects of lipocalin-2 on brain endothelial adhesion and permeability

Mechanistic insights into Lipocalin-2 in ischemic stroke and hemorrhagic brain injury: Integrating animal and clinical studies

Brain injuries, including strokes and traumatic brain injuries (TBI), are a major global health concern, contributing significantly to both mortality and long-term disability. Recent research has identified lipocalin-2 (LCN2), a glycoprotein secreted by various brain cells, as a key factor in influencing brain injury outcomes. Evidence from animal and clinical studies firmly establishes the pivotal role of LCN2 in driving the inflammatory responses triggered by damage to brain tissue. Furthermore, increased LCN2 promotes cellular differentiation, blood-brain barrier breakdown, and decreases cell viability. Interventions with LCN2 inhibitors attenuated brain injury through a reduction in the inflammation process and enhanced cellular viability. Potential mechanisms of LCN2 involve several pathways including the Janus kinase-2 (JAK2)-signal transducers and the transcription-3 (STAT3) signaling, hypoxia-inducible factor 1-alpha (HIF-1α)-LCN2-vascular endothelial growth factor alpha (VEGFα), and the PKR-like ER kinase (PERK) pathways. LCN2 itself interacts with diverse inflammatory cytokines in TBI and intracranial hemorrhage (ICH), resulting in disruption of the blood-brain barrier, increased programmed cell death, and an imbalance in iron homeostasis. Clinical studies have also shown that increased LCN2 level can act as a prognostic biomarker of outcomes following brain injuries. Therefore, this review aims to comprehensively evaluate the role and underlying mechanisms of LCN2 in brain injuries, including stroke and TBI, and explore potential therapeutic interventions targeting LCN2 in these conditions.

SLC22A17 as a Cell Death-Linked Regulator of Tight Junctions in Cerebral Ischemia

BACKGROUND

Beyond neuronal injury, cell death pathways may also contribute to vascular injury after stroke. We examined protein networks linked to major cell death pathways and identified SLC22A17 (solute carrier family 22 member 17) as a novel mediator that regulates endothelial tight junctions after ischemia and inflammatory stress.

METHODS

Protein-protein interactions and brain enrichment analyses were performed using STRING, Cytoscape, and a human tissue-specific expression RNA-seq database. In vivo experiments were performed using mouse models of transient focal cerebral ischemia. Human stroke brain tissues were used to detect SLC22A17 by immunostaining. In vitro experiments were performed using human brain endothelial cultures subjected to inflammatory stress. Immunostaining and Western blot were used to assess responses in SLC22A17 and endothelial tight junctional proteins. Water content, dextran permeability, and electrical resistance assays were used to assess edema and blood-brain barrier (BBB) integrity. Gain and loss-of-function studies were performed using lentiviral overexpression of SLC22A17 or short interfering RNA against SLC22A17, respectively.

RESULTS

Protein-protein interaction analysis showed that core proteins from apoptosis, necroptosis, ferroptosis, and autophagy cell death pathways were closely linked. Among the 20 proteins identified in the network, the iron-handling solute carrier SLC22A17 emerged as the mediator enriched in the brain. After cerebral ischemia in vivo, endothelial expression of SLC22A17 increases in both human and mouse brains along with BBB leakage. In human brain endothelial cultures, short interfering RNA against SLC22A17 prevents TNF-α (tumor necrosis factor alpha)-induced ferroptosis and downregulation in tight junction proteins and disruption in transcellular permeability. Notably, SLC22A17 could repress the transcription of tight junctional genes. Finally, short interfering RNA against SLC22A17 ameliorates BBB leakage in a mouse model of focal cerebral ischemia.

CONCLUSIONS

Using a combination of cell culture, human stroke samples, and mouse models, our data suggest that SLC22A17 may play a role in the control of BBB function after cerebral ischemia. These findings may offer a novel mechanism and target for ameliorating BBB injury and edema after stroke.

Antagonism of the brain P2X7 ion channel attenuates repeated social defeat induced microglia reactivity, monocyte recruitment and anxiety-like behavior in male mice

Chronic stress is linked to increased anxiety. Repeated social defeat (RSD) in mice causes anxiety that is dependent on activated neurons, reactive microglia, and accumulation of monocytes in the brain. This response requires interactions between the immune system and central nervous system (CNS). Neuronal activation within threat appraisal regions is a key response to RSD, however, it is unclear how microglia become activated. One potential explanation is that microglia express a purinergic non-selective ligand gated adenosine-triphosphate (ATP) receptor 7 (P2X7). Activation of P2X7 promotes the release of chemokines and cytokines, and recruitment of monocytes to the brain. Thus, the purpose of this study was to determine if a novel P2X7 antagonist blocked neuronal and microglia interactions and the corresponding anxiety following RSD. Male mice were administered (i.p.) a P2X7 antagonist, JNJ-54471300, prior to each cycle of RSD. Fourteen hours after RSD, behavioral deficits including social avoidance and anxiety-like were determined. Moreover, several immune parameters were assessed. RSD caused neuronal activation in stress-responsive regions, monocyte production and release, splenomegaly, and social avoidance. These parameters were unaffected by P2X7 antagonism. RSD-associated proportional area of Iba-1+ microglia, monocyte accumulation in the brain, IL-1β mRNA expression in enriched myeloid cells, plasma IL-6, and anxiety-like behavior were ameliorated by P2X7 antagonism. Gene expression analysis in the hippocampus and amygdala showed regional specific responses to RSD and some were reversed with P2X7 antagonism. Overall, blocking P2X7 activation attenuated RSD-induced microglia reactivity with corresponding reduction in neuroinflammation, monocyte accumulation, and anxiety-like behavior in male mice.

Lipocalin 2 receptors: facts, fictions, and myths

The human 25-kDa Lipocalin 2 (LCN2) was first identified and purified as a protein that in part is associated with gelatinase from neutrophils. This protein shows a high degree of sequence similarity with the deduced sequences of rat α2-microglobulin-related protein and the mouse protein 24p3. Based on its typical lipocalin fold, which consists of an eight-stranded, anti-parallel, symmetrical β-barrel fold structure it was initially thought that LCN2 is a circulating protein functioning as a transporter of small lipophilic molecules. However, studies in Lcn2 null mice have shown that LCN2 has bacteriostatic properties and plays a key role in innate immunity by sequestering bacterial iron siderophores. Numerous reports have further shown that LCN2 is involved in the control of cell differentiation, energy expenditure, cell death, chemotaxis, cell migration, and many other biological processes. In addition, important roles for LCN2 in health and disease have been identified in Lcn2 null mice and multiple molecular pathways required for regulation of Lcn2 expression have been identified. Nevertheless, although six putative receptors for LCN2 have been proposed, there is a fundamental lack in understanding of how these cell-surface receptors transmit and amplify LCN2 to the cell. In the present review we summarize the current knowledge on LCN2 receptors and discuss inconsistencies, misinterpretations and false assumptions in the understanding of these potential LCN2 receptors.

Association between lipocalin-2 and mild cognitive impairment or dementia: A systematic review and meta-analysis of population-based evidence

BACKGROUND

The associations between lipocalin-2 (LCN2) with mild cognitive impairment (MCI) and dementia have gained growing interest. However, population-based studies have yielded inconsistent findings. Therefore, we conducted this essential systematic review and meta-analysis to analyze and summarize the existing population-based evidence.

METHODS

PubMed, EMBASE, and Web of Science were systematically searched until Mar 18, 2022. Meta-analysis was performed to generate the standard mean difference (SMD) of peripheral blood and cerebrospinal fluid (CSF) LCN2. A qualitative review was performed to summarize the evidence from postmortem brain tissue studies.

RESULTS

In peripheral blood, the overall pooled results showed no significant difference in LCN2 across Alzheimer's disease (AD), MCI and control groups. Further subgroup analysis revealed higher serum LCN2 levels in AD compared to controls (SMD =1.28 [0.44;2.13], p = 0.003), while the difference remained insignificant in plasma (SMD =0.04 [-0.82;0.90], p = 0.931). Besides, peripheral blood LCN2 were higher in AD when age difference between AD and controls ≥ 4 years (SMD =1.21 [0.37;2.06], p = 0.005). In CSF, no differences were found in LCN2 across groups of AD, MCI and controls. However, CSF LCN2 was higher in vascular dementia (VaD) compared to controls (SMD =1.02 [0.17;1.87], p = 0.018), as well as compared to AD (SMD =1.19 [0.58;1.80], p < 0.001). Qualitative analysis supported that LCN2 was increased in the brain tissue of AD-related areas, especially in astrocytes and microglia; while LCN2 increased in infarct-related brain areas and over-expressed in astrocytes and macrophages in mixed dementia (MD).

CONCLUSION

The difference in peripheral blood LCN2 between AD and controls may be affected by the type of biofluid and age. No differences were found in CSF LCN2 across AD, MCI and controls groups. In contrast, CSF LCN2 was elevated in VaD patients. Moreover, LCN2 was increased in AD-related brain areas and cells in AD, while in infarcts-related brain areas and cells in MD.

Role of lipocalin 2 in stroke

Stroke is the second leading cause of death worldwide; however, the treatment choices available to neurologists are limited in clinical practice. Lipocalin 2 (LCN2) is a secreted protein, belonging to the lipocalin superfamily, with multiple biological functions in mediating innate immune response, inflammatory response, iron-homeostasis, cell migration and differentiation, energy metabolism, and other processes in the body. LCN2 is expressed at low levels in the brain under normal physiological conditions, but its expression is significantly up-regulated in multiple acute stimulations and chronic pathologies. An up-regulation of LCN2 has been found in the blood/cerebrospinal fluid of patients with ischemic/hemorrhagic stroke, and could serve as a potential biomarker for the prediction of the severity of acute stroke. LCN2 activates reactive astrocytes and microglia, promotes neutrophil infiltration, amplifies post-stroke inflammation, promotes blood-brain barrier disruption, white matter injury, and neuronal death. Moreover, LCN2 is involved in brain injury induced by thrombin and erythrocyte lysates, as well as microvascular thrombosis after hemorrhage. In this paper, we review the role of LCN2 in the pathological processes of ischemic stroke; intracerebral hemorrhage; subarachnoid hemorrhage; and stroke-related brain diseases, such as vascular dementia and post-stroke depression, and their underlying mechanisms. We hope that this review will help elucidate the value of LCN2 as a therapeutic target in stroke.

Vitreous inflammatory and angiogenic factors on patients with proliferative diabetic retinopathy or diabetic macular edema: the role of Lipocalin2

PURPOSE

Quantitative analysis of vitreous inflammatory and angiogenic factors from patients with proliferative diabetic retinopathy (PDR) or diabetic macular edema (DME).

MATERIALS AND METHODS

Collection of undiluted vitreous samples from 20 diabetic patients: 13 with proliferative diabetic retinopathy (PDR) and 7 with diabetic macular edema (DME). DME patients had suboptimal response to anti-VEGF treatment. Samples from 11 control patients, with vitreomacular interface pathology such as idiopathic epiretinal membrane (iERM) (n = 4), vitreomacular traction syndrome (VMT) (n = 3) and full thickness macular hole (FTMH) (n = 3), were also collected. The levels of IL1b, IL6, IL8, IL27, TNFα, ICAM-1, VCAM, MCP-1, VEGFA and LCN2 were measured using cytometry flow analysis. Median values were compared with Mann-Whitney test since the distributions were skewed. Statistical analysis was performed with the Statistical Package for Social Sciences software (IBM Corp. Released 2012. IBM SPSS Statistics for Windows, Version 21.0. Armonk, NY: IBM Corp.).

RESULTS

The median concentration of LCN2, IL6, IL8, IL1b, IL27, ICAM, VCAM-1, MCP-1, TNFa and VEGFA was higher in PDR patients than in controls. Similarly, the median concentration of LCN2, IL6, IL8, IL27, ICAM, VCAM-1, TNFa and VEGFA was higher in DME patients than in controls. In particular, median LCN2 concentration in diabetic patients was 5,711 pg/ml (interquartile range [IR] = 2,534), while in controls was 2,586 pg/ml (IR = 2,345). Moreover, median LCN2 was 6,534 pg/ml in the DME group (IR = 6,850) and 4,785 pg/ml in the PDR group (IR = 2,608), (p = 0.025).

CONCLUSION

Various inflammatory and angiogenic factors are involved in the pathophysiology of PDR and DME. Elevated vitreous levels of LCN2 in PDR and especially in DME patients reveal a potential pathogenic association. More extended studies could verify LCN2 as an alternative therapeutic target.

The role of lipocalin 2 in brain injury and recovery after ischemic and hemorrhagic stroke

Ischemic and hemorrhagic stroke (including intracerebral hemorrhage, intraventricular hemorrhage, and subarachnoid hemorrhage) is the dominating cause of disability and death worldwide. Neuroinflammation, blood–brain barrier (BBB) disruption, neuronal death are the main pathological progress, which eventually causes brain injury. Increasing evidence indicated that lipocalin 2 (LCN2), a 25k-Da acute phase protein from the lipocalin superfamily, significantly increased immediately after the stroke and played a vital role in these events. Meanwhile, there exists a close relationship between LCN2 levels and the worse clinical outcome of patients with stroke. Further research revealed that LCN2 elimination is associated with reduced immune infiltrates, infarct volume, brain edema, BBB leakage, neuronal death, and neurological deficits. However, some studies revealed that LCN2 might also act as a beneficial factor in ischemic stroke. Nevertheless, the specific mechanism of LCN2 and its primary receptors (24p3R and megalin) involving in brain injury remains unclear. Therefore, it is necessary to investigate the mechanism of LCN2 induced brain damage after stroke. This review focuses on the role of LCN2 and its receptors in brain injury and aiming to find out possible therapeutic targets to reduce brain damage following stroke.

The role of neutrophils in the dysfunction of central nervous system barriers

Leukocyte migration into the central nervous system (CNS) represents a central process in the development of neurological diseases with a detrimental inflammatory component. Infiltrating neutrophils have been detected inside the brain of patients with several neuroinflammatory disorders, including stroke, multiple sclerosis and Alzheimer’s disease. During inflammatory responses, these highly reactive innate immune cells can rapidly extravasate and release a plethora of pro-inflammatory and cytotoxic factors, potentially inducing significant collateral tissue damage. Indeed, several studies have shown that neutrophils promote blood-brain barrier damage and increased vascular permeability during neuroinflammatory diseases. Recent studies have shown that neutrophils migrate into the meninges and choroid plexus, suggesting these cells can also damage the blood-cerebrospinal fluid barrier (BCSFB). In this review, we discuss the emerging role of neutrophils in the dysfunction of brain barriers across different neuroinflammatory conditions and describe the molecular basis and cellular interplays involved in neutrophil-mediated injury of the CNS borders.

Lipocalin 2 attenuates oligodendrocyte loss and immune cell infiltration in mouse models for multiple sclerosis

Multiple sclerosis (MS) is a central nervous system disease characterized by both degenerative and inflammatory processes. Various mediators are involved in the interplay of degeneration and innate immunity on one hand and peripheral adaptive immunity on the other hand. The secreted protein lipocalin 2 (LCN2) is an inflammatory modulator in a variety of pathologies. Although elevated intrathecal levels of LCN2 have been reported in MS patients, it's functional role is widely unknown. Here, we identified a subpopulation of astrocytes as a source of LCN2 in MS lesions and respective animal models. We investigated the functional role of LCN2 for both autoimmune and degenerative aspects in three MS mouse models including both wild type (WT) and Lcn2^-/- mouse strains. While the experimental autoimmune encephalomyelitis (EAE) model reflects primary autoimmunity, the cuprizone model reflects selective oligodendrocyte loss and demyelination. In addition, we included a combinatory Cup/EAE model in which primary cytodegeneration is followed by inflammatory lesions within the forebrain. While in the EAE model, the disease outcome was comparable in between the two mouse strains, cuprizone intoxicated Lcn2^-/- animals showed an increased loss of oligodendrocytes. In the Cup/EAE model, Lcn2^-/- animals showed increased inflammation when compared to WT mice. Together, our results highlight LCN2 as a potentially protective molecule in MS lesion formation, which might be able to limit loss of oligodendrocytes immune-cell invasion. Despite these findings, it is not yet clear which glial cell phenotype (and to which extent) contributes to the observed neuroprotective effects, that is, microglia and/or astroglia or even endothelial cells in the brain.

Aerobic Physical Exercise as a Non-medical Intervention for Brain Dysfunction: State of the Art and Beyond

Brain disorders, including stroke, Alzheimer's disease, depression, and chronic pain, are difficult to effectively treat. These major brain disorders have high incidence and mortality rates in the general population, and seriously affect not only the patient's quality of life, but also increases the burden of social medical care. Aerobic physical exercise is considered an effective adjuvant therapy for preventing and treating major brain disorders. Although the underlying regulatory mechanisms are still unknown, systemic processes may be involved. Here, this review aimed to reveal that aerobic physical exercise improved depression and several brain functions, including cognitive functions, and provided chronic pain relief. We concluded that aerobic physical exercise helps to maintain the regulatory mechanisms of brain homeostasis through anti-inflammatory mechanisms and enhanced synaptic plasticity and inhibition of hippocampal atrophy and neuronal apoptosis. In addition, we also discussed the cross-system mechanisms of aerobic exercise in regulating imbalances in brain function, such as the “bone-brain axis.” Furthermore, our findings provide a scientific basis for the clinical application of aerobic physical exercise in the fight against brain disorders.

Lipocalin-2 and Cerebral Stroke

Stroke is a common and devastating disease with an escalating prevalence worldwide. The known secondary injuries after stroke include cell death, neuroinflammation, blood-brain barrier disruption, oxidative stress, iron dysregulation, and neurovascular unit dysfunction. Lipocalin-2 (LCN-2) is a neutrophil gelatinase-associated protein that influences diverse cellular processes during a stroke. The role of LCN-2 has been widely recognized in the peripheral system; however, recent findings have revealed that there are links between LCN-2 and secondary injury and diseases in the central nervous system. Novel roles of LCN-2 in neurons, microglia, astrocytes, and endothelial cells have also been demonstrated. Here, we review the evidence on the regulatory roles of LCN-2 in secondary injuries following a stroke from various perspectives and the pathological mechanisms involved in the modulation of stroke. Overall, our review suggests that LCN-2 is a promising target to promote a better understanding of the neuropathology of stroke.

Surface Plasmon Resonance Identifies High-Affinity Binding of l-DOPA to Siderocalin/Lipocalin-2 through Iron-Siderophore Action: Implications for Parkinson's Disease Treatment

l-3,4-Dihydroxyphenylalanine (l-DOPA), the dopamine precursor, remains the frontline treatment for Parkinson's disease (PD). With the treatment progress, l-DOPA efficacy decreases, necessitating higher and more frequent doses, with higher risks of dyskinesia. l-DOPA chelates iron through its catechol group, forming the l-DOPA:Fe complex; however, the fate of this complex is unknown. Catechol siderophore-like compounds are known to bind siderocalin (Scn)/lipocalin-2 to form stable siderophore:Fe:Scn complexes. Scn is upregulated in PD patients' substantia nigra and may play a role in PD pathophysiology. Therefore, in this study, we used the surface plasmon resonance (SPR) technique to examine the binding properties of l-DOPA to Scn. We found that l-DOPA formed a stable complex with Scn in the presence of Fe³⁺. Our analysis of the binding properties of l-DOPA precursors and metabolites indicates that the catechol group is necessary but not sufficient to form a stable complex with Scn. Finally, the affinity constant (K_d) of DOPA:Fe³⁺ binding with Scn (0.8 μM) was lower than l-DOPA plasma peak concentrations in l-DOPA preparations in the past six decades. Our results speculate a significant role for the l-DOPA-Scn complex in the decreased bioavailability of l-DOPA with the progress of PD.

Role of Lipocalin-2 in Amyloid-Beta Oligomer-Induced Mouse Model of Alzheimer's Disease

Lipocalin-2 (LCN2) is an inflammatory protein with diverse functions in the brain. Although many studies have investigated the mechanism of LCN2 in brain injuries, the effect of LCN2 on amyloid-toxicity-related memory deficits in a mouse model of Alzheimer’s disease (AD) has been less studied. We investigated the role of LCN2 in human AD patients using a mouse model of AD. We created an AD mouse model by injecting amyloid-beta oligomer (AβO) into the hippocampus. In this model, animals exhibited impaired learning and memory. We found LCN2 upregulation in the human brain frontal lobe, as well as a positive correlation between white matter ischemic changes and serum LCN2. We also found increased astrocytic LCN2, microglia activation, iron accumulation, and blood–brain barrier disruption in AβO-treated hippocampi. These findings suggest that LCN2 is involved in a variety of amyloid toxicity mechanisms, especially neuroinflammation and oxidative stress.

Lipocalin 2 as a link between ageing, risk factor conditions and age-related brain diseases

Chronic (neuro)inflammation plays an important role in many age-related central nervous system (CNS) diseases, including Alzheimer's disease, Parkinson's disease and vascular dementia. Inflammation also characterizes many conditions that form a risk factor for these CNS disorders, such as physical inactivity, obesity and cardiovascular disease. Lipocalin 2 (Lcn2) is an inflammatory protein shown to be involved in different age-related CNS diseases, as well as risk factor conditions thereof. Lcn2 expression is increased in the periphery and the brain in different age-related CNS diseases and also their risk factor conditions. Experimental studies indicate that Lcn2 contributes to various neuropathophysiological processes of age-related CNS diseases, including exacerbated neuroinflammation, cell death and iron dysregulation, which may negatively impact cognitive function. We hypothesize that increased Lcn2 levels as a result of age-related risk factor conditions may sensitize the brain and increase the risk to develop age-related CNS diseases. In this review we first provide a comprehensive overview of the known functions of Lcn2, and its effects in the CNS. Subsequently, this review explores Lcn2 as a potential (neuro)inflammatory link between different risk factor conditions and the development of age-related CNS disorders. Altogether, evidence convincingly indicates Lcn2 as a key constituent in ageing and age-related brain diseases.

Neuropathophysiology of coronavirus disease 2019: neuroinflammation and blood brain barrier disruption are critical pathophysiological processes that contribute to the clinical symptoms of SARS-CoV-2 infection

Coronavirus disease 2019 (COVID-19) is caused by the novel SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) first discovered in Wuhan, Hubei province, China in December 2019. SARS-CoV-2 has infected several millions of people, resulting in a huge socioeconomic cost and over 2.5 million deaths worldwide. Though the pathogenesis of COVID-19 is not fully understood, data have consistently shown that SARS-CoV-2 mainly affects the respiratory and gastrointestinal tracts. Nevertheless, accumulating evidence has implicated the central nervous system in the pathogenesis of SARS-CoV-2 infection. Unfortunately, however, the mechanisms of SARS-CoV-2 induced impairment of the central nervous system are not completely known. Here, we review the literature on possible neuropathogenic mechanisms of SARS-CoV-2 induced cerebral damage. The results suggest that downregulation of angiotensin converting enzyme 2 (ACE2) with increased activity of the transmembrane protease serine 2 (TMPRSS2) and cathepsin L in SARS-CoV-2 neuroinvasion may result in upregulation of proinflammatory mediators and reactive species that trigger neuroinflammatory response and blood brain barrier disruption. Furthermore, dysregulation of hormone and neurotransmitter signalling may constitute a fundamental mechanism involved in the neuropathogenic sequelae of SARS-CoV-2 infection. The viral RNA or antigenic peptides also activate or interact with molecular signalling pathways mediated by pattern recognition receptors (e.g., toll-like receptors), nuclear factor kappa B, Janus kinase/signal transducer and activator of transcription, complement cascades, and cell suicide molecules. Potential molecular targets and therapeutics of SARS-CoV-2 induced neurologic damage are also discussed.

Immune Cells in the BBB Disruption After Acute Ischemic Stroke: Targets for Immune Therapy?

Blood-Brain Barrier (BBB) disruption is an important pathophysiological process of acute ischemic stroke (AIS), resulting in devastating malignant brain edema and hemorrhagic transformation. The rapid activation of immune cells plays a critical role in BBB disruption after ischemic stroke. Infiltrating blood-borne immune cells (neutrophils, monocytes, and T lymphocytes) increase BBB permeability, as they cause microvascular disorder and secrete inflammation-associated molecules. In contrast, they promote BBB repair and angiogenesis in the latter phase of ischemic stroke. The profound immunological effects of cerebral immune cells (microglia, astrocytes, and pericytes) on BBB disruption have been underestimated in ischemic stroke. Post-stroke microglia and astrocytes can adopt both an M1/A1 or M2/A2 phenotype, which influence BBB integrity differently. However, whether pericytes acquire microglia phenotype and exert immunological effects on the BBB remains controversial. Thus, better understanding the inflammatory mechanism underlying BBB disruption can lead to the identification of more promising biological targets to develop treatments that minimize the onset of life-threatening complications and to improve existing treatments in patients. However, early attempts to inhibit the infiltration of circulating immune cells into the brain by blocking adhesion molecules, that were successful in experimental stroke failed in clinical trials. Therefore, new immunoregulatory therapeutic strategies for acute ischemic stroke are desperately warranted. Herein, we highlight the role of circulating and cerebral immune cells in BBB disruption and the crosstalk between them following acute ischemic stroke. Using a robust theoretical background, we discuss potential and effective immunotherapeutic targets to regulate BBB permeability after acute ischemic stroke.

Ultra-Early Cerebral Thrombosis Formation After Experimental Subarachnoid Hemorrhage Detected on T2* Magnetic Resonance Imaging

BACKGROUND AND PURPOSE

The mechanisms of brain damage during ultra-early subarachnoid hemorrhage (SAH) have not been well studied. The current study examined the SAH-induced hyperacute brain damage at 4 hours using magnetic resonance imaging and brain histology in a mouse model.

METHODS

SAH was induced by endovascular perforation in adult mice. First, adult male wild-type mice underwent magnetic resonance imaging T2 and T2* 4 hours after an endovascular perforation or a sham operation and were euthanized to assess brain histology. Second, male and female adult lipocalin-2 knockout mice had SAH. All animals underwent magnetic resonance imaging at 4 hours, and the brains were harvested for brain histology.

RESULTS

T2* hypointensity vessels were observed in the brain 4 hours after SAH in male wild-type mice. The numbers of T2*-positive vessels were significantly higher in SAH brains than in sham-operated mice. Brain histology showed thrombosis and erythrocyte plugs in the T2*-positive cerebral vessels which may be venules. The number of T2*-positive vessels correlated with SAH grade and the presence of T2 lesions. Brain thrombosis was also accompanied by albumin leakage and neuronal injury. LCN2 deficient male mice had lower numbers of T2*-positive vessels after SAH compared with wild-type male mice.

CONCLUSIONS

SAH causes ultra-early brain vessel thrombosis that can be detected by T2* gradient-echo sequence at 4 hours after SAH. LCN2 deficiency decreased the number of T2*-positive vessels.

Lipocalin-2 Deficiency Reduces Oxidative Stress and Neuroinflammation and Results in Attenuation of Kainic Acid-Induced Hippocampal Cell Death

The hippocampal cell death that follows kainic acid (KA)-induced seizures is associated with blood–brain barrier (BBB) leakage and oxidative stress. Lipocalin-2 (LCN2) is an iron-trafficking protein which contributes to both oxidative stress and inflammation. However, LCN2′s role in KA-induced hippocampal cell death is not clear. Here, we examine the effect of blocking LCN2 genetically on neuroinflammation and oxidative stress in KA-induced neuronal death. LCN2 deficiency reduced neuronal cell death and BBB leakage in the KA-treated hippocampus. In addition to LCN2 upregulation in the KA-treated hippocampus, circulating LCN2 levels were significantly increased in KA-treated wild-type (WT) mice. In LCN2 knockout mice, we found that the expressions of neutrophil markers myeloperoxidase and neutrophil elastase were decreased compared to their expressions in WT mice following KA treatment. Furthermore, LCN2 deficiency also attenuated KA-induced iron overload and oxidative stress in the hippocampus. These findings indicate that LCN2 may play an important role in iron-related oxidative stress and neuroinflammation in KA-induced hippocampal cell death.

Satellite glia as a critical component of diabetic neuropathy: Role of lipocalin-2 and pyruvate dehydrogenase kinase-2 axis in the dorsal root ganglion

Diabetic peripheral neuropathy (DPN) is a common complication of uncontrolled diabetes. The pathogenesis of DPN is associated with chronic inflammation in dorsal root ganglion (DRG), eventually causing structural and functional changes. Studies on DPN have primarily focused on neuronal component, and there is limited knowledge about the role of satellite glial cells (SGCs), although they completely enclose neuronal soma in DRG. Lipocalin-2 (LCN2) is a pro-inflammatory acute-phase protein found in high levels in diverse neuroinflammatory and metabolic disorders. In diabetic DRG, the expression of LCN2 was increased exclusively in the SGCs. This upregulation of LCN2 in SGCs correlated with increased inflammatory responses in DRG and sciatic nerve. Furthermore, diabetes-induced inflammation and morphological changes in DRG, as well as sciatic nerve, were attenuated in Lcn2 knockout (KO) mice. Lcn2 gene ablation also ameliorated neuropathy phenotype as determined by nerve conduction velocity and intraepidermal nerve fiber density. Mechanistically, studies using specific gene KO mice, adenovirus-mediated gene overexpression strategy, and primary cultures of DRG SGCs and neurons have demonstrated that LCN2 enhances the expression of mitochondrial gate-keeping regulator pyruvate dehydrogenase kinase-2 (PDK2) through PPARβ/δ, thereby inhibiting pyruvate dehydrogenase activity and increasing production of glycolytic end product lactic acid in DRG SGCs and neurons of diabetic mice. Collectively, our findings reveal a crucial role of glial LCN2-PPARβ/δ-PDK2-lactic acid axis in progression of DPN. Our results establish a link between pro-inflammatory LCN2 and glycolytic PDK2 in DRG SGCs and neurons and propose a novel glia-based mechanism and drug target for therapy of DPN. MAIN POINTS: Diabetes upregulates LCN2 in satellite glia, which in turn increases pyruvate dehydrogenase kinase-2 (PDK2) expression and lactic acid production in dorsal root ganglia (DRG). Glial LCN2-PDK2-lactic acid axis in DRG plays a crucial role in the pathogenesis of diabetic neuropathy.

Lipocalin 2 as a potential systemic biomarker for central serous chorioretinopathy

No systemic biomarker of Central Serous Chorioretinopathy (CSCR) has been identified. Lipocalin 2 (LCN2 or NGAL), alone or complexed with MMP-9 (NGAL/MMP-9), is increased in several retinal disorders. Serum levels of LCN2 and NGAL/MMP-9 were measured in CSCR patients (n = 147) with chronic (n = 76) or acute/recurrent disease (n = 71) and in age- and sex-matched healthy controls (n = 130). Samples with CRP > 5 mg/L, creatinine > 100 µmol/L, and/or urea > 7.5 mmol/L were excluded. Serum LCN2 was lower in CSCR patients than controls (81.4 ± 48.7 vs 107.3 ± 44.5 ng/ml, p < 0.0001), and lower in acute/recurrent CSCR than controls (p < 0.001) and chronic CSCR (p = 0.006). Serum NGAL/MMP-9 was lower in CSCR patients than controls (47.2 ± 40.7 vs 74.1 ± 42.6, p < 0.0001), and lower in acute/recurrent CSCR than controls (p < 0.001) and chronic CSCR (p = 0.002). A ROC curve showed that for LCN2 serum levels, the 80-ng/ml cutoff value allows to discriminate acute/recurrent CSCR from controls with 80.3% sensitivity and 75.8% specificity, and for NGAL/MMP-9 serum levels, a 38-ng/ml cutoff value allows to discriminate acute/recurrent CSCR from controls with 69.6% sensitivity and 80.3% specificity. In both acute and chronic CSCR, low serum LCN2 and NGAL/MMP-9, provide a biological link between the two CSCR forms, and potential susceptibility to oxidative stress and innate immune dysregulation in CSCR.

Inflammatory Responses of Astrocytes Are Independent from Lipocalin 2

The central nervous system (CNS) responds to diverse neurologic injuries with a vigorous activation of astrocytes. In addition to their role in the maintenance of CNS homeostasis and neuronal function, astrocytes are thought to participate in the regulation of innate and adaptive immune responses in the CNS. Following antigen recognition, reactive astrocytes may participate in the initiation of innate immune responses, and modulate adaptive immune response leading to the recruitment of peripheral immune cells. Among activation, astrocytes undergo morphological changes and express several molecules, e.g., chemokines. Lipocalin 2 (LCN2) is involved in the control of innate immune responses, regulation of excess iron, and reactive oxygen production. Here, we investigated the influence of LCN2 on basic astrocytic functions linked to inflammatory responses. In vitro studies revealed a similar chemokine expression pattern in wild-type and Lcn2-deficient astrocyte cultures after treatment with lipopolysaccharides (LPS). Increased wound closure and morphological changes upon LPS treatment are independent of Lcn2 expression. We conclude that LCN2 is not necessary for basic astrocytic functions in the context of inflammation. However, CNS-derived LCN2 might have a regulatory effect on other cells, e.g., endothelial cells of the blood-brain barrier.

Lipocalin 2 induces neuroinflammation and blood-brain barrier dysfunction through liver-brain axis in murine model of nonalcoholic steatohepatitis

BACKGROUND

Recent clinical and basic research implicated a strong correlation between NAFLD/NASH phenotypes with ectopic manifestations including neuroinflammation and neurodegeneration, but the mediators and critical pathways involved are not well understood. Lipocalin 2 (Lcn2) is one of the important mediators exclusively produced in the liver and circulation during NASH pathology.

METHODS

Using murine model of NASH, we studied the role of Lcn2 as a potent mediator of neuroinflammation and neurodegeneration in NASH pathology via the liver-brain axis.

RESULTS

Results showed that high circulatory Lcn2 activated 24p3R (Lipocalin2 receptor) in the brain and induced the release of high mobility group box 1 (HMGB1) preferably from brain cells. Released HMGB1 acted as a preferential ligand to toll-like receptor 4 (TLR4) and induced oxidative stress by activation of NOX-2 signaling involving activated p65 protein of the NF-κB complex. Further, the HMGB1-derived downstream signaling cascade activated NLRP3 inflammasome and release of proinflammatory cytokines IL-6 and IL-1β from brain cells. In addition, to advance our present understanding, in vitro studies were performed in primary brain endothelial cells where results showed high circulatory Lcn2 influenced HMGB1 secretion. Mechanistically, we also showed that elevated Lcn2 level in underlying NASH might be a likely cause for induction of blood-brain barrier dysfunction since the adipokine decreased the expression of tight junction protein Claudin 5 and caused subsequent elevation of pro-inflammatory cytokines IL-6 and IL-1β.

CONCLUSION

In conclusion, the NASH-induced brain pathology might be because of increased Lcn2-induced release of HMGB1 and accompanying neuroinflammation.

3nLcn2, a teleost lipocalin 2 that possesses antimicrobial activity and inhibits bacterial infection in triploid crucian carp

Lipocalin 2 (Lcn2) has been identified in mammals, however, the in vivo function of fish Lcn2 is essentially unknown. Triploid crucian carp (3n = 150) of red crucian carp (female, 2n = 100) and allotetraploid (male, 4n = 200) shows better resistance to pathogenic infections. To elucidate the antimicrobial mechanism of triploid crucian carp, we examined the function of a novel Lcn2 from triploid crucian carp (3nLcn2). 3nLcn2 is 183 residues in length and contains a conserved lipocalin domain. Quantitative real time reverse transcription PCR (qRT-PCR) analysis showed that 3nLcn2 expression occurred in multiple tissues and was upregulated by bacterial infection in a time-dependent manner. We found that purified recombinant 3nLcn2 (r3nLcn2) exerted bactericidal activity to Aeromonas hydrophila and Escherichia coli. qRT-PCR detected increased expression of pro-inflammatory cytokines and tight junctions in fish with 3nLcn2 overexpression. Fish administered with 3nLcn2 exhibited enhanced intestinal barrier and resistance against bacterial infection. These results provide the first evidence that 3nLcn2 is a functional lipocalin with antimicrobial activity and plays a positive role in the immune defense during bacterial infection.

White matter T2 hyperintensities and blood-brain barrier disruption in the hyperacute stage of subarachnoid hemorrhage in male mice: The role of lipocalin-2

AIMS

The current study examined whether white matter injury occurs in the hyperacute (4 hours) phase after subarachnoid hemorrhage (SAH) and the potential role of blood-brain barrier (BBB) disruption and an acute phase protein, lipocalin 2 (LCN2), in that injury.

METHODS

Subarachnoid hemorrhage was induced by endovascular perforation in adult mice. First, wild-type (WT) mice underwent MRI 4 hours after SAH to detect white matter T2 hyperintensities. Second, changes in LCN2 expression and BBB disruption associated with the MRI findings were examined. Third, SAH-induced white matter injury at 4 hours was compared in WT and LCN2 knockout (LCN2 KO) mice.

RESULTS

At 4 hours, most animals had uni- or bilateral white matter T2 hyperintensities after SAH in WT mice that were associated with BBB disruption and LCN2 upregulation. However, some disruption and LCN2 upregulation was also found in mice with no T2-hyperintensity lesion. In contrast, there were no white matter T2 hyperintensities in LCN2 KO mice after SAH. LCN2 deficiency also attenuated BBB disruption, myelin damage, and oligodendrocyte loss.

CONCLUSIONS

Subarachnoid hemorrhage causes very early BBB disruption and LCN2 expression in white matter that is associated with and may precede T2 hyperintensities. LCN2 deletion attenuates MRI changes and pathological changes in white matter after SAH.