Altered synaptic connectivity and brain function in mice lacking microglial adapter protein Iba1

Abstract

Growing evidence indicates that microglia impact brain function by regulating synaptic pruning and formation, as well as synaptic transmission and plasticity. Iba1 (Ionized Ca+2-binding adapter protein 1), encoded by the Allograft inflammatory factor 1 (Aif1) gene, is an actin-interacting protein in microglia. Although Iba1 has long been used as a cellular marker for microglia, its functional role remains unknown. Here, we used global Iba1-deficient (Aif1-/-) mice to characterize microglial activity, synaptic function and behavior. Microglial imaging in acute hippocampal slices and fixed tissues from juvenile mice revealed that Aif1-/- microglia display reductions in ATP-induced motility and ramification, respectively. Biochemical assays further demonstrated that Aif1-/- brain tissues exhibit an altered expression of microglial-enriched proteins associated with synaptic pruning. Consistent with these changes, juvenile Aif1-/- mice displayed deficits in excitatory synapse number and synaptic transmission assessed by neuronal labeling and whole-cell patch-clamp recording in acute hippocampal slices. Unexpectedly, microglial synaptic engulfment capacity was diminished in juvenile Aif1-/- mice. During early postnatal development when synapse formation is a predominant event in the hippocampus, excitatory synapse number was still reduced in Aif1-/- mice. Together these findings support an overall role of Iba1 in excitatory synaptic growth in juvenile mice. Lastly, postnatal synaptic deficits persisted in the adulthood and correlated with significant behavioral changes in adult Aif1-/- mice, which exhibited impairments in object recognition memory and social interaction. These results suggest that Iba1 critically contributes to microglial activity underlying essential neuro-glia developmental processes that may deeply influence behavior. SignificanceAbnormal microglia-neuron interaction is increasingly implicated in neurodevelopmental and neuropsychiatric conditions such as autism spectrum disorders and schizophrenia, as well as in neurodegenerative disorders such as Alzheimers disease. This study demonstrates that deletion of the microglia-specific protein Iba1, which has long been utilized as a selective microglial marker but whose role has remained unidentified, results in microglial structural and functional impairments that significantly impact synaptic development and behavior. These findings not only highlight the importance of microglia in brain function but may also suggest that modifying microglial function could provide a therapeutic strategy for treatment of neurodevelopmental, neuropsychiatric and neurodegenerative disorders.