Lipocalin 2 and lipocalin 2 receptor in neuron-glia interactions following injury

• ¹ National University of Singapore, Department of Pharmacology, Yong Loo Lin School of Medicine, Singapore
• ² National University of Singapore, Neurobiology and Ageing Programme, Life Sciences Institute, Singapore
• ³ National University of Singapore, Department of Anatomy, Yong Loo Lin School of Medicine, Singapore
• ⁴ National University of Singapore, Singapore Institute for Neurotechnology (SINAPSE), Singapore

Bacteria release siderophores to acquire soluble iron essential for their growth. Mammalian lipocalin 2 (LCN2) is expressed as part of an innate immune response to bind bacterial siderophores and deprive bacteria of iron. The putative lipocalin 2 receptor (LCN2R) is the Solute Carrier Family 22, Member 17 (SLC22A17) also known as the organic cation transporter (BOCT), BOCT1, 24p3R or neutrophil gelatinase-associated lipocalin receptor (NGALR). In the absence of bacterial infection, LCN2 may be involved in iron homeostasis by capturing iron bound to mammalian siderophores such as 2,5-dihydroxy benzoic acid. We have reported that intracerebroventicular injection of kainic acid (KA), which induces lesions of the hippocampus, results in increased expression of LCN2 mRNA and protein in the rat hippocampus. Immunofluorescence showed that LCN2 is present in astrocytes in the olfactory bulb, brainstem and cerebellum of the normal brain, and is upregulated in reactive astrocytes in KA-lesioned hippocampus. In contrast, LCN2R was expressed in normal hippocampal neurons. Three days and 2 weeks after KA injections loss of LCN2R was observed due to degeneration of neurons although surviving neurons continued to express LCN2R. Meanwhile, induction of LCN2R was observed in OX-42 positive microglia. Expression of the pro-apoptotic marker, Bim, was increased in both neurons and microglia after KA injection. However, TUNEL staining showed that apoptosis was predominantly limited to Bim-expressing neurons and largely absent in microglia. Holo-LCN2, consisting of LCN2:iron:enterochelin complex, increased Bim mRNA expression and decreased neuronal survival. In contrast, apo-LCN2 (without iron) did not significantly alter neuronal Bim expression or cell survival. Together, our findings suggest that LCN2 and LCN2R may play a role in excitotoxic neuronal injury, with LCN2 being expressed by reactive astrocytes and the receptor LCN2R being expressed by both neurons and microglia but being more strongly associated with Bim-mediated apoptotic cell death in neurons.