Phd in LIFE SCIENCE

Thesis title: Antibiotics Treatment Modulates Microglia–Synapses Interaction

‘Dysbiosis’ of the adult gut microbiota, in response to challenges such as infection, birth delivery, altered diet, stress, and antibiotics (ABX) treatment has been recently linked to pathological alteration of brain function and behaviour. Moreover, gut microbiota composition constantly controls microglia maturation, as revealed by morphological observations and gene expression analysis. However, it is unclear whether microglia functional properties and crosstalk with neurons, known to shape and modulate synaptic development and function, are influenced by the gut microbiota. Here, we investigated how antibiotic-mediated alteration of the gut microbiota influences microglial, astrocytic and neuronal functions in adult mice hippocampus. Hippocampal microglia from adult mice treated with oral antibiotics exhibited increased microglia density, altered basal patrolling activity, and impaired process rearrangement in response to damage. Moreover, astrocytes derived from ABX-treated mice displayed an increased density and C3 reactivity, even if in the absence of neuroinflammatory process. Patch clamp recordings at CA3-CA1 synapses revealed that antibiotics treatment alters neuronal functions, reducing spontaneous postsynaptic glutamatergic currents and decreasing synaptic connectivity, without reducing dendritic spines density. Antibiotics treatment was unable to modulate synaptic function in CX3CR1-deficient mice, pointing to an involvement of microglia–neuron crosstalk through the CX3CL1/CX3CR1 axis in the effect of dysbiosis on neuronal functions. Real time PCR analysis of metabolic genes revealed a significant increase in metabolic-gene related expression in hippocampal samples derived from antibiotic-treated mice compared to control. In addition we found out a reduced expression of lipid droplets (LDs) in both microglia and astrocytes derived from ABX-treated mice. Together, our findings show that antibiotic alteration of gut microbiota impairs microglia functionality, thus altering synaptic efficacy, astrocytes homeostasis and altered lipid metabolism, highlighting that CX3CL1/CX3CR1 signalling supports microglia as a major player in the gut–brain axis, and in particular in the gut microbiota-to-neuron communication pathway.