THE DEVELOPMENT OF THE GLUTAMATERGIC NEURONS IS SHAPED BY IL-1BETA

The developing brain is exquisitely sensitive to immune system activation that, shaping the organism’s response to infections, may impact the development of the nervous system increasing susceptibility to behavioural and neurological diseases later in life. Experimental studies, in particular, link dysregulated production of pro-inflammatory cytokines (i.e. IL6 and IL-1β) to the onset of neurodevelopmental diseases later in life. Neonatal immune activation, through cytokines production, can have direct long-term effects on neuronal function by interfering with neurotransmitter function, altering the expression of synaptic proteins, producing differential neuronal activation. We addressed this specific topic in vitro by means of primary hippocampal neurons shortly exposed to a pulse of IL-1beta (0,05 ng/ml for 30 min) at different developmental stages, 3,7, and 14 days in vitro (DIV). Maturation of the glutamatergic system and any possible interference by IL-1beta has been evaluated in terms of expression of NMDAR subunits (GluN2A and GluN2B) and AMPAR subunits (GluA1 and GluA2), distribution at the post-synaptic site and ability to control intracellular Ca2+ homeostasis at full maturation (DIV 21). The exposure of hippocampal neurons to IL-1β at DIV 7 leads to a reduced GluN2A expression at DIV 21 that result in an increased GluN2B/GluN2A ratio, favouring the GluN2B-containing NMDARs. This effect is coupled to a reduced mRNA GluN2A transcription and is prevented blocking the Src family of tyrosine kinases suggesting the involvement of a GluN2B over-activation induced by IL-1β. These alterations, confirm at the post-synapses, are not evident when neurons are exposed to the cytokine at DIV 3 and 14, suggesting that IL-1β influences neuronal development only in a specific vulnerable period of growth. The effect is specifically operated by IL-1β, since no effects were assessed using another pro-inflammatory cytokine, TNF-α. The different NMDAR composition and distribution observed after IL-1β treatment at DIV 7 is coupled with an altered calcium homeostasis regulation under the control of NMDAR, with a reinforced GluN2B subunit contribution coupled with a weaker GluN2A subunit contribution. Increased expression of GluN2A over a prevalence of NMDAR sharing a GluN2B subunit is believe to be a crucial factor to prompt synaptic spine maturation from stubby to mushrooms spines. Accordingly to the delayed GluN2B/GluN2A shift, IL-1beta treatment at DIV 7 favours an higher amount of stubby and a lower amount of mushroom-shaped spines compared to controls. Our findings suggest the hypothesis that a transient increase of IL-1β during the early postnatal neuronal development leads to “long-term” functional and structural alteration of the glutamatergic system development providing a molecular link between neuroinflammation and “long-term” alteration of neuronal activity that could potentially predispose to neurodevelopmental disorders later in life.