Unraveling the neuro-immunological implications of the Ikaros family in schizophrenia and neuroinflammation

Abstract

[eng] The Ikaros family of transcription factors has been largely described in the context of immune system function and immune system cells development. In the last years, some of its members were found in neural cells, particularly during perinatal life, associated with CNS development and proper function of some brain regions, such as the hippocampus. Dysregulations of the Ikaros family members have been linked to a wide range of immune-related disorders. However, thei mplication of the Ikaros family in brain-related disorders remains a widely underexplored area. Schizophrenia is a psychiatric condition affecting around 21 million people worldwide with an annual associated cost of around 16.500€ per patient in Europe. Schizophrenia symptoms are classified into three broad categories: Positive (e.g. hallucinations, delusions and thought disorder), Negative (e.g. withdrawal and lack of motivation) and Cognitive (e.g. deficiencies in executive functions and working memory). Some of the key-affected brain regions in schizophrenia are the pre-frontal cortex and the hippocampus. Genetic and environmental factors play a role in the risk of developing this condition and, in many cases, these factors act through the immune system. Indeed, one of the most prominent discoveries in schizophrenia research in the last years has been the association of immune dysfunction with its pathogenesis, proposing an aberrant neuro-immune crosstalk in this condition. In that context, two major ways the immune system is proposed to contribute to the appearance of schizophrenia are: neuroinflammation and T cells dysfunction. Microglia cells are one of the main populations orchestrating neuroinflammation processes within the brain. In schizophrenia, microglia density and cell number have been shown to be generally increased while alterations in density and number of other glia cells seem to be minor. Besides, many pro-inflammatory cytokines, such as IL-6 or TNF-α, have been found to be increased in schizophrenia patients’ post-mortem brains. On the other side, T cell mediated immunity dysregulations have also been proposed in schizophrenia, with abnormal invasion of T cells within patients’ brains and possibly alterations of dopamine-T cells crosstalk. Therefore, T cells population could be considered as a candidate mediating the aberrant neuro-immune crosstalk in schizophrenia possibly involving neurotoxic effects and contributing to neuroinflammation. Previous studies have shown that Ikaros and Helios are essential for the proper function of some immune cell types. On the other hand, SPNs of IKAROS gene have been found to be related to the age of onset of schizophrenia and Helios protein deficiency have been shown to alter molecular pathways which are also altered in schizophrenia. Furthermore, preliminary data from our lab indicated that Ikaros and Helios are dysregulated in immune cells of schizophrenia patients. Consequently, we aimed to evaluate the potential involvement of Ikaros and Helios in the pathophysiology of schizophrenia. In the first article, we have found downregulated levels of Ikaros and Helios in PBMCs of schizophrenia patients while their levels were not different from controls in post-mortem brain samples. A doble mutant animal model mimicking the double downregulation of Ikaros and Helios also showed schizophrenia-like behavior in the three categories of symptoms. Using the secretome of the PBMCs from schizophrenia patients, we induced several schizophrenia-like phenotypes in in-vitro and in-vivo translational models. We have characterized the molecular profile of the PBMCs secretome and we identified IL-4 and CXCL10 as possible candidates mediating its effects. In the second article, we have identified Ikaros as a factor involved in microglia homeostasis, particularly in inflammation-associated conditions. Using an Ikaros total Knockout model we identified deficiencies linked to a hippocampal-dependent task. Thereafter, we focused on the hippocampus. The Ikaros KO model presented microglial morphological changes associated to inflammatory conditions such as elevated expression of TNFα and NF-kβ and long-term potentiation defects. Besides, we tested the level of Ikaros in other conditions of neuroinflammation and we found augmented levels of Ikaros in all of them. Correspondingly, microglia phagocytic activity was also disrupted under the modulation of Ikaros in vitro. The elevated levels of some inflammasome members and alterations in the acetylation-methylation ratios found in the KO model give some ideas about the mechanisms by which Ikaros could modulate microglia function. The final model about how the downregulation of Ikaros and Helios in PBMCs of schizophrenia patients could be contributing to the pathogenesis of the condition could be summarized as follows: The combination of the downregulations of Ikaros and Helios in different PBMCs subsets possibly generates elevated levels of IFN-γ and reduced immune suppression capacity of Treg cells. IFN-γ is priming microglia cells that upon second immune stimuli generate oversimplified and neurotoxic inflammatory responses over years deteriorating the nervous tissue until the manifestation of the first psychotic episode. A possibility is that Ikaros dysregulation within microglia is exacerbating this circuit.