Secreted protein lipocalin‐2 promotes microglial M1 polarization

Abstract

Activated macrophages are classified into two different forms: classically activated (M1) or alternatively activated (M2) macrophages. The presence of M1/M2 phenotypic polarization has also been suggested for microglia. Here, we report that the secreted protein lipocalin 2 (LCN2) amplifies M1 polarization of activated microglia. LCN2 protein (EC 1 μg/ml), but not glutathione S-transferase used as a control, increased the M1-related gene expression in cultured mouse microglial cells after 8-24 h. LCN2 was secreted from M1-polarized, but not M2-polarized, microglia. LCN2 inhibited phosphorylation of STAT6 in IL-4-stimulated microglia, suggesting LCN2 suppression of the canonical M2 signaling. In the lipopolysaccharide (LPS)-induced mouse neuroinflammation model, the expression of LCN2 was notably increased in microglia. Primary microglial cultures derived from LCN2-deficient mice showed a suppressed M1 response and enhanced M2 response. Mice lacking LCN2 showed a markedly reduced M1-related gene expression in microglia after LPS injection, which was consistent with the results of histological analysis. Neuroinflammation-associated impairment in motor behavior and cognitive function was also attenuated in the LCN2-deficient mice, as determined by the rotarod performance test, fatigue test, open-field test, and object recognition task. These findings suggest that LCN2 is an M1-amplifier in brain microglial cells.

Lipocalin 2 modulates the cellular response to amyloid beta

“…27 Interestingly, also microglial cells were shown to respond to LCN2, which seems to favor their polarization into a M1-activated phenotype. 28 Here, we show that LCN2 is not produced by microglia in response to Ab , both in astrocyte-enriched cultures (o5% microglia) and in microglia-enriched cultures.…”

“…Previous studies have shown that different cell populations from the CNS are able to upregulate the expression of Cxcl10 after treatment with LCN2, which, in turn, promotes the activation and migration of glial cells within the CNS. 16,28 Moreover, Ab led to an overexpression of Hmox1 and Fth in CP epithelial cells and in astrocytes. It is well established that both CP epithelial cells and astrocytes have an important role in brain iron metabolism and changes in the expression of ironrelated genes are often correlated with alterations in the level of intracellular/extracellular iron ratio under stressful conditions.…”

“…However, taking into account recent observations suggesting that LCN2 alone is not able to induce apoptosis in astrocytes, 26 we cannot discard an indirect modulation of astrocytes' survival by molecules secreted by activated microglia (o5% in astrocyte-enriched cultures), because of their ability to respond to LCN2. 28 In fact, others have shown that in response to LPS, microglia are able to influence the survival of brain cells, oligodendrocytes in particular, by secreting proinflammatory molecules like TNF, 40 and that this process is modulated, in vivo, by LCN2. 28 Of notice, neuronal apoptosis in response to Ab 1-42 is independent of their ability to produce LCN2 as the survival of Lcn2( þ / þ ) and Lcn2( À / À ) neurons significantly decreased in response to Ab .…”

Role of Lipocalin-2 in Brain Injury after Intracerebral Hemorrhage

“…Mice lacking LCN2 have a markedly reduced M1-related gene expression in microglia after lipopolysaccharide injection. 26 Expression of LCN2 was sensitive to cytotoxic agents, and inflammatory activation of microglia can lead to LCN2 upregulation. Meanwhile, LCN2 expression in BV-2 microglia induces cell morphologic changes.…”

“…Although LCN2 is considered as being mainly expressed in astrocytes, 12,14,23,24 where its secretion may be neurotoxic, 24 other cell types may express LCN2. Thus, inflammatory cells, such as microglia and neutrophils 15,25,26 can also express LCN2. The effects of LCN2 on microglia are complex.…”