ROLE OF LIPOCALIN 2 IN KAINATE-INDUCED NEURODEGENERATION

Abstract

Lipocalin 2 (LCN2) is a member of the lipocalin family, which has diverse roles in infections, iron homeostasis, apoptosis, tumourigenesis, inflammation, renal physiology and pathology. Yet, little is known about the functions of LCN2 in the central nervous system (CNS). In view of recent studies that showed an association of LCN2 with apoptosis via LCN2R, the present study investigates the role of LCN2 in kainate (KA)-induced neurodegeneration. Intracerebroventricular injection of KA in rats induces neuronal loss in the hippocampal formation. Therefore, the KA model was used as a neurodegeneration model to elucidate the role of LCN2 in neurodegeneration in the hippocampus. Despite microarray studies reporting substantial upregulation of LCN2 mRNA expression after some forms of brain insults, the expression and localisation of LCN2 had not been characterised physiologically. Thus, LCN2¿s tissue distribution and expression were analysed in the normal rat brain. High LCN2 expression in the olfactory bulb, cerebellum and brainstem suggested that these brain regions may be potential routes of entry for microorganisms since LCN2 is known as a bacteriostatic agent. After KA injury, LCN2 was highly upregulated and expressed in the astrocytes in the lesioned hippocampal region. Since the interaction of LCN2 with its receptor (LCN2R) has been suggested to have an effect on apoptosis via Bim, a pro-apoptotic protein, it was important to examine LCN2R¿s cellular localisation. LCN2R was expressed in the hippocampal pyramidal neurons physiologically. After KA injury, LCN2R neuronal expression was reduced, but upregulated in the activated microglia. Coincidentally, Bim was also upregulated in activated microglia after KA treatment, but these microglia were not apoptotic. Only at 1 day post-KA injection, selective neurons with high Bim expression were apoptotic. However, it is unclear if the upregulation of Bim was an effect of the interaction between LCN2 and LCN2R. The effect of LCN2 on Bim-mediated apoptosis in neurons was further examined when primary hippocampal neurons were treated with recombinant LCN2 (rLCN2, apo-LCN2), and rLCN2:iron:enterochelin (holo-LCN2) to mimic the substantial LCN2 release after KA injury. Both treatments interacted with the LCN2R expressed on the neurons and were internalised into the neuronal cell bodies. Apo-LCN2 had no effect on apoptosis, while upregulation of Bim and decreased cell survival were detected in holo-LCN2-treated neurons, indicating the pro-apoptotic effect of holo-LCN2. It is possible that holo-LCN2 may import iron into the neurons, where it increases intracellular free iron to promote ROS production, thus increasing their vulnerability to oxidative stress and apoptosis. The present study addressed the expression and localisation of LCN2 and its receptor, LCN2R, in the hippocampus after KA injury and their association with Bim-mediated apoptosis. Besides Bim, the apoptotic role of LCN2 was also dependent on its iron status as only iron-loaded LCN2 produced a pro-apoptotic effect. This pro-apoptotic effect of iron-loaded LCN2 may account for a select population of cell demise in KA-induced neurodegeneration. Elucidation of the role of LCN2 in the KA-induced neurodegeneration model could serve as a stepping stone to understand the role of LCN2 in other neurodegenerative diseases as interventions can be targeted at the interaction between LCN2 and LCN2R to retard neurodegeneration in general.