Loss of cholinergic phenotype in septohippocampal projection neurons: Relation to brain versus peripheral IL-2 deficiency

Abstract

In the peripheral immune system, IL-2 is essential for immune homeostasis, normal T regulatory cell function, and self-tolerance. IL-2 knockout (IL-2KO) mice develop spontaneous autoimmunity characterized by increased T cell trafficking to multiple organs. The IL-2 gene is also expressed in the brain, and in vitro studies have shown that IL-2 is a potent modulator of acetylcholine release from septohippocampal neurons and exerts trophic effects on septal neurons in culture. We previously described the apparent loss of cholinergic cell bodies in the medial septum of IL-2KO mice. Here we investigated if loss of brain-derived IL-2, or autoimmunity stemming from loss of peripheral IL-2, is responsible for the alteration in choline acetyltransferase (ChAT) expression in the medial septum of IL-2KO mice. To accomplish this objective, we compared ChAT-positive neurons between wild-type (WT) mice, IL-2KO, and congenic mice with a double gene deletion for the IL-2 and the recombinase activating gene-2 (RAG-2) which are referred to as IL-2KO/RAG-2KO mice (congenic mice which lack mature T and B cells as well as peripheral and brain-derived IL-2). We found that the loss of ChAT staining did not coincide with an overall loss of cells in the medial septum, suggesting that loss of brain IL-2 results in a change in cholinergic phenotype unrelated to cell death. No differences were noted in the endogenous expression of cytokines and chemokines tested in the medial septum. Evaluation of BDNF and NGF levels between WT and IL-2KO mice in medial septal homogenates revealed that IL-2KO mice have markedly higher levels of NGF in the medial septum compared to WT mice. Our findings suggest that brain-derived IL-2 plays an essential role in the maintainance of septohippocampal projection neurons in vivo.

Effects of Brain-Derived IL-2 Deficiency and the Development of Autoimmunity on Spatial Learning and Fear Conditioning

“…Our hypothesis that alterations in learning acquisition in IL-2KO would be associated largely with the loss of brain-derived IL-2 was not confirmed in Experiment 1, as the congenic IL-2KO/SCID and WT/SCID littermates did not differ in Morris water-maze performance. That hypothesis was based on a series of studies from our lab where we found that there is a marked phenotypic loss of the ChAT-positive neurons in the MS/vDB of IL-2KO mice that occurs between weaning and adulthood that results from the absence of brain-derived IL-2 during development [ 16 , 17 , 19 ]. Although the loss of brain-derived IL-2 in IL-2KO/SCID mice was not associated with alterations in spatial learning in the Morris water-maze, the results of Experiment 2 suggest that autoimmunity associated with peripheral IL-2 gene deletion altered learning acquisition in adult IL-2KO mice.…”

“…Our lab has shown previously that IL-2KO mice have altered learning acquisition in the Morris water-maze and related septohippocampal changes including fewer infrapyramidal granule cells, reductions in hippocampal infrapyramidal mossy fiber length, alterations in neurotrophin levels, and marked phenotypic loss of choline acetyltransferase (ChAT)-positive neurons in the medial septum/vertical diagonal band of Broca (MS/vDB) that occurs between weaning and adulthood [ 16 – 19 ]. Moreover, we have found that loss of brain IL-2 gene expression results in dysregulation of the brain’s endogenous neuroimmunological milieu (e.g., increasing chemokines that attract autoreactive T cells into the brain), and may be involved in initiating processes that lead to the progression of CNS autoimmunity [ 6 , 20 – 23 ].…”

“…In the present study, we sought to determine whether cognitive alterations in IL-2KO mice are due to the loss the IL-2 gene in the brain and/or autoimmunity resulting from loss of the gene in the peripheral immune system. Given the aforementioned effects of IL-2 deficiency in the septohippocampal system, and the recent the finding from our lab that loss of brain-derived IL-2 (rather than peripheral of loss of IL-2 and autoimmunity) underlies the phenotypic loss of ChAT-positive cholinergic MS/vDB neurons in mice [ 19 ], we postulated that alterations in learning acquisition in IL-2KO would be associated largely with the loss of brain-derived IL-2. Therefore, Experiment 1 compared the water-maze performance of IL-2 congenic mice on the severe combined immunodeficient (SCID) background that we have bred in our colony, and have used in previous studies to disentangle the effects of the loss of brain-derived IL-2 from peripheral IL-2 on nerve injury and sensorimotor function [ 15 , 23 ].…”

Selective Neuronal and Brain Regional Expession of IL-2 in IL2P 8-GFP Transgenic Mice: Relation to Sensorimotor Gating

“…We also detected modest levels of GFP expression from a subset of cells in the subiculum, the main output structure of the hippocampus, that projects back to the septal nuclei (in addition to other GFP-positive regions including the prefrontal cortex, hypothalamus, mammillary nuclei, entorhinal cortex, and amygdala). Previously, we showed the effects of brain-derived IL-2 deficiency on cholinergic phenotype and neurotrophin expression in the septohippocampal system of IL-2KO mice [ 25 , 26 ]. In addition to the neuropathology in the septum and hippocampus, we have detected changes in several measures of behavior.…”