Lipocalin-2 in Stroke

Wen-Hai Chou, Yi-Chinn Weng, Varun Kumar

DOI: https://doi.org/10.17140/noj-2-109 · Published 2015-04-17 · Licence: https://creativecommons.org/licenses/by/4.0 · © 2015 The Author(s)

Also available as PDF (figures and tables appear in the PDF).

Keywords: Lipocalin-2; NGAL; 24p3; Stroke; Reperfusion injury; Neutrophil; PKC; Phosphorylation; Biomarker; Lipocalin-2; NGAL; 24p3; Stroke; Reperfusion injury; Neutrophil; PKC; Phos- th Published: April 17th, 2015 phorylation; Biomarker

Introduction

Stroke is a sudden loss of neurological function due to ischemia or hemorrhage in

the brain.1 It is the fifth leading cause of death and a major cause of long-term disability in the

United States.2 There are two main types of stroke: ischemic and hemorrhagic strokes. Isch-

emic stroke accounts for approximately 87% of all strokes and results from blockage of blood

flow into the brain by thrombus or embolus. Hemorrhagic stroke, caused by rupture of cerebral

blood vessels, is less common (13%) than ischemic stroke but accounts for 50% of stroke

3-6 death.3-6 Currently, there is no proven medical or surgical treatment for hemorrhagic stroke.

Thrombolysis with fibrinolytic agents such as tissue Plasminogen Activator (tPA) is

the only FDA approved therapy to reverse ischemic stroke.7 However, only 5% of patients

receive the treatment because tPA must be given within 3 to 4.5 hours after the occurrence of

stroke. Delayed treatment may increase the risk of serious side effects such as hemorrhagic

transformation and reperfusion injury.8 Ischemia initiates cerebral infarction during ischemic

stroke, but reperfusion after recanalization may promote secondary injury and worsen neuro-

8,9 logical outcomes.8,9 Reperfusion injury includes a series of inflammatory events with activa-

tion and infiltration of circulating neutrophils, macrophages, and T-cells into infarcted brain

10,11 tissue. Post-stroke inflammation has detrimental effects, but may be needed for repairing

processes.11 In order to reduce stroke-reperfusion injury and develop effective and balanced

therapeutic methods, it is important to identify neurotoxic and neuroprotective molecules of

post-stroke inflammation. In the acute stage of stroke (within 24 hours), infiltrating immune

8,9,11 ruption of blood-brain barrier (BBB). However, some of these molecules have a differ-

ent role in the later stage of stroke (after 24 hours). For example, MMP-9 enhances ischemic

of brain tissues in the later stage of stroke.12 Therefore, detailed

mechanistic studies of post-stroke inflammation are needed.

Lipocalin-2 (Lcn2) In Ischemic Stroke

Lipocalin-2 (LCN2), also known as 24p3 or neutrophil

gelatinase-associated lipocalin (NGAL), is a 25 kDa protein se-

creted from activated neutrophils.13 Using a chemical-genetics

approach, LCN2 was identified as one of PKCδ phosphoryla-

14-16 tion substrates in neutrophils. PKCδ directly phosphorylates

LCN2 at Thr-115, and mediates the secretion of LCN2 from ac-

tivated neutrophils in vitro and after cerebral ischemia in vivo.16

Plasma level of LCN2 is elevated at 1-3 days in pa-

17-19 tients with ischemic stroke. LCN2 is acutely induced after

transient middle cerebral artery occlusion (tMCAO) in rodents

20-23 (a model of ischemic stroke). LCN2 appears in mouse sera as

early as one hour, peaks at 23 hours, and diminishes by 48 to 72

hours after tMCAO.23 Due to the short time window for effective

thrombolytic therapy, it is of great interest to diagnose stroke

early and reduce the risk of cerebral hemorrhage. The early

induction of LCN2 suggests the possibility of using LCN2 as an

early blood biomarker to detect stroke.

In addition to blood plasma, LCN2 is also induced in

the penumbra of ipsilateral hemispheres after tMCAO. The

induction of LCN2 in mouse brain initiates at 6 hours, reaches

a peak at 24 hours, and reduces at 48 hours after reperfusion.

Induced LCN2 protein is identified in a subset of reactivated

astrocytes, cerebral endothelial cells, and infiltrated neutrophils

21,23,26 after tMCAO. Cerebral infarction, neurological deficits, in-

filtration of immune cells, BBB permeability, proinflammatory

cytokines, chemokines, and adhesion molecules are reduced af-

21,23 ter tMCAO in LCN2 null mice. Recombinant LCN2 protein

is able to stimulate neutrophil migration as well as promote cell

death in primary neurons but not in astrocytes, microglia and

21,23,27,28 oligodendrocytes. These results suggest that LCN2 is a

proinflammatory mediator during the acute stage of ischemic

stroke. Therefore, LCN2 inhibitors or anti-LCN2 antibodies

may prove useful to reduce post-stroke inflammation and brain

injury. At later time point (3 days) after ischemic stroke in rats

and humans, LCN2 is expressed in injured neurons and may be

released as a “help me signal” to condition microglia and as-

trocytes for recovery.22 These studies demonstrate the diverse

functions of LCN2 during the acute and later stages of ischemic

stroke.

Lipocalin-2 (Lcn2) In Hemorrhagic Stroke

Hemorrhagic stroke is a devastating form of stroke

3-6 with high mortality.3-6 There are two major types of hemorrhagic

stroke: intracerebral hemorrhage (ICH) and subarachnoid hem-

orrhage (SAH). ICH is associated with bleeding in the brain

4-6 parenchyma.4-6 SAH is often caused by intracranial aneurysm

with blood leakage in subarachnoid space.3 LCN2 is induced

mainly in astrocytes after rodent models of ICH and SAH.

LCN2 induction was detected in the ipsilateral hemispheres at 1,

3, 7 days after ICH in rats and 24 hours after SAH in mice.

Neuro

4-6 brain injury.4-6 LCN2 is capable of transporting irons through

31-33 siderophore. Injection of iron upregulates the expression of

LCN2 in the brain, while systemic treatment of an iron chelator

(deferoxamine) reduces ICH-induced LCN2 upregulation.29 The

results suggest that LCN2 may function as an important regula-

tor of iron homeostasis after ICH. White matter injury and mark-

ers for axonal damage and myelin degradation are increased af-

ter SAH in wild type mice, but scarcely developed in LCN2 null

16 30 mice.30 The result suggests that LCN2 may facilitate the devel-

opment of white matter injury after SAH.

Several studies we summarized in this review suggest

that LCN2 promotes brain injury as a proinflammatory molecule

16,21,23,26,29,30 in the acute stage of stroke. Interestingly, LCN2 may

also support the neurovascular recovery by enhancing angio-

genesis and serving as a “help me signal” in the later stage of

22,34 stroke. Therefore, a comprehensive understanding of time-

24,25 dependent functions of LCN2 is a prerequisite for developing

effective therapeutic interventions for the treatment of ischemic

and hemorrhagic strokes.

Conclusion

21-23,26

LCN2 has been identified as an important mediator of

stroke-reperfusion injury and white matter injury after ischemic

and hemorrhagic strokes. Future studies are needed to reveal the

detailed mechanisms of LCN2-mediated signaling and to devel-

op potential LCN2-based therapy.

Competing Interests

The authors declare that they have no competing interests.

Acknowledgements

This work was supported by AHA 13BGIA13820011 to WC

Chou.

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