Neuronal Production of Lipocalin-2 as a Help-Me Signal for Glial Activation

Abstract

Background and purpose We explored the hypothesis that injured neurons release lipocalin-2 as a help-me signal. Methods In vivo lipocalin-2 responses were assessed in rat focal cerebral ischemia and human stroke brain samples using a combination of ELISA and immunostaining. In vitro, microglia and astrocytes were exposed to lipocalin-2 and various markers and assays of glial activation were quantified. Functional relevance of neuron-to-glia lipocalin-2 signaling was examined by transferring conditioned media from lipocalin-2-activated microglia and astrocytes onto neurons to see whether activated glia could protect neurons against oxygen-glucose deprivation and promote neuroplasticity. Results In human stroke samples and rat cerebral ischemia, neuronal expression of lipocalin-2 was significantly increased. In primary cell cultures, exposing microglia and astrocytes to lipocalin-2 resulted in glial activation. In microglia, lipocalin-2 converted resting ramified shapes into a long-rod morphology with reduced branching, increased interleukin-10 release, and enhanced phagocytosis. In astrocytes, lipocalin-2 upregulated GFAP, BDNF and thrombospondin-1. Conditioned media from lipocalin-2-treated astrocytes upregulated synaptotagmin, and conditioned media from lipocalin-2-treated microglia upregulated synaptophysin and PSD95 and protected neurons against oxygen-glucose deprivation. Conclusions These findings provide proof-of-concept that lipocalin-2 is released by injured neurons as a “help-me” distress signal that activates microglia and astrocytes into potentially pro-recovery phenotypes.

Role of Lipocalin-2 in Thrombin-Induced Brain Injury

“…Most of the LCN2-positive cells were astrocytes. This finding is consistent with previous reports 9, 11, 19, 20 , although neurons, microglia, and endothelial cells can also express LCN2 12, 21 . Our data also found that thrombin results in a significant neutrophil infiltration which may also contribute to increased brain LCN2 levels (LCN2 is also known as neutrophil gelatinase-associated lipocalin).…”

Help-me signaling: Non-cell autonomous mechanisms of neuroprotection and neurorecovery

“…Emerging data now appears to recast some of these extracellular factors as neuronal “help-me” signals (Kyritsis et al 2012; Mizuno et al 2011; Noda et al 2011; Noda et al 2014; Xing et al 2014). After brain injury, neurons can release unique “help-me” signals, including chemokines, cytokines, growth factors etc, which will interact with receptors expressed in microglia to guide microglial activation into a beneficial phenotype of neuroprotection and neurorecovery (Figure 3) .…”

Intercellular cross-talk in intracerebral hemorrhage

“…Accumulating evidence suggests that LCN2 is mainly secreted astrocytes, and it has been implicated as having a role in brain injury after ICH and subarachnoid hemorrhage (Figure 5) (Dong et al, 2013; Egashira et al, 2014). Interestingly, after ischemic stroke, neurons in the penumbra express LCN2, and LCN2 produced by neurons act as a “help me” signal for astrocytes and microglia (Jin et al, 2014; Xing et al, 2014). The role of LCN2 mediated cellular interaction in ICH-induced brain injury merits investigation.…”