Astrocytes provide structural and functional support for neurons, as well as display neurotoxic or neuroprotective phenotypes depending upon the presence of an immune or inflammatory microenvironment. This study was undertaken to characterize multiple phenotypes of activated astrocytes and to investigate the regulatory mechanisms involved. We report that activated astrocytes in culture exhibit two functional phenotypes with respect to pro- or anti-inflammatory gene expression, glial fibrillary acidic protein expression, and neurotoxic or neuroprotective activities. The two distinct functional phenotypes of astrocytes were also demonstrated in a mouse neuroinflammation model, which showed pro- or anti-inflammatory gene expression in astrocytes following challenge with classical or alternative activation stimuli; similar results were obtained in the absence of microglia. Subsequent studies involving recombinant lipocalin-2 (LCN2) protein treatment or Lcn2-deficient mice indicated that the pro- or anti-inflammatory functionally polarized phenotypes of astrocytes and their intracellular signaling pathway were critically regulated by LCN2 under in vitro and in vivo conditions. Astrocyte-derived LCN2 promoted classical proinflammatory activation of astrocytes but inhibited IL-4–STAT6 signaling, a canonical pathway involved in alternative anti-inflammatory activation. Our results suggest that the secreted protein LCN2 is an autocrine modulator of the functional polarization of astrocytes in the presence of immune or inflammatory stimuli and that LCN2 could be targeted therapeutically to dampen proinflammatory astrocytic activation and related pathologies in the CNS.

Lipocalin-2 (LCN2) is a secreted protein of the lipocalin family, but little is known about the expression or the role of LCN2 in the central nervous system. Here, we investigated the role of LCN2 in ischemic stroke using a rodent model of transient cerebral ischemia. Lipocalin-2 expression was highly induced in the ischemic brain and peaked at 24 hours after reperfusion. After transient middle cerebral artery occlusion, LCN2 was predominantly expressed in astrocytes and endothelial cells, whereas its receptor (24p3R) was mainly detected in neurons, astrocytes, and endothelial cells. Brain infarct volumes, neurologic scores, blood–brain barrier (BBB) permeabilities, glial activation, and inflammatory mediator expression were significantly lower in LCN2-defkient mice than in wild-type animals. Lipocalin-2 deficiency also attenuated glial neurotoxicity in astrocyte/neuron cocultures after oxygen-glucose deprivation. Our results indicate LCN2 has a critical role in brain injury after ischemia/reperfusion, and that LCN2 may contribute to neuronal cell death in the ischemic brain by promoting neurotoxic glial activation, neuroinflammation, and BBB disruption.

Lipocalin-2 (LCN2) is a member of the highly heterogeneous secretory protein family of lipocalins and increases in its levels can contribute to neurodegeneration in the adult brain. However, there are no reports on the role of LCN2 in Parkinson's disease (PD). Here, we report for the first time that LCN2 expression is increased in the substantia nigra (SN) of patients with PD. In mouse brains, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment for a neurotoxin model of PD significantly upregulated LCN2 expression, mainly in reactive astrocytes in both the SN and striatum. The increased LCN2 levels contributed to neurotoxicity and neuroinflammation, resulting in disruption of the nigrostriatal dopaminergic (DA) projection and abnormal locomotor behaviors, which were ameliorated in LCN2-deficient mice. Similar to the effects of MPTP treatment, LCN2-induced neurotoxicity was also observed in the 6-hydroxydopamine (6-OHDA)-treated animal model of PD. Moreover, treatment with the iron donor ferric citrate (FC) and the iron chelator deferoxamine mesylate (DFO) increased and decreased, respectively, the LCN2-induced neurotoxicity in vivo. In addition to the in vivo results, 1-methyl-4-phenylpyridinium (MPP ϩ )-induced neurotoxicity in cocultures of mesencephalic neurons and astrocytes was reduced by LCN2 gene deficiency in the astrocytes and conditioned media derived from MPP ϩ -treated SH-SY5Y neuronal enhanced glial expression of LCN2 in vitro. Therefore, our results demonstrate that astrocytic LCN2 upregulation in the lesioned DA system may play a role as a potential pathogenic factor in PD and suggest that inhibition of LCN2 expression or activity may be useful in protecting the nigrostriatal DA system in the adult brain.

Lipocalin-2 (LCN2) is a key mediator of various cellular processes. Recent studies have indicated that LCN2 also plays an important role in central nervous system (CNS) injuries and neurological diseases, such as spinal cord injury, stroke, experimental autoimmune encephalomyelitis, and neurodegenerative diseases. Here, we investigated the role of LCN2 in a rodent model of lipopolysaccharide (LPS)-induced neuroinflammation. At 24 hours after intraperitoneal injection of LPS, LCN2 expression was strongly induced in the brain; LCN2 was mainly expressed in endothelial cells, astrocytes, and microglia. Next, we used LCN2-deficient mice to further investigate the role of LCN2 in neuroinflammation. LCN2 deficiency attenuated LPS-induced glial activation in the brain. In a mechanistic study employing glia/neuron co-cultures, LCN2 deficiency reduced glial neurotoxicity. Our results indicate that LCN2 plays a central role in the neuroinflammatory responses following LPS administration, and that LCN2 might contribute to the uncontrolled neurotoxic glial activation under excessive and chronic inflammatory conditions.

Background:In the central nervous system, the role of LCN2 as a chemokine inducer has been previously reported. Results: Peripheral nerve injury increased LCN2 expression. Lcn2 deficiency attenuated pain hypersensitivity, microglial activation, and chemokine production. Chemokine expression and pain behavior were induced by LCN2. Conclusion: The LCN2-chemokine axis plays a critical role in the pathogenesis of neuropathic pain. Significance: LCN2 can be targeted for treatment of neuropathic pain.