Neuron-microglia interactions induce aberrant inflammatory mechanisms in schizophrenia

Abstract

Inflammation in the human brain is suggested to contribute to several diseases of the central nervous system. Human microglia, the resident immune cells of the brain, have essential functions for maintenance of the central nervous system, synaptic organization and immune defense. During brain development and until late adolescence, the elimination of weak and inactive synapses is a mandatory process for sculpting mature, neuronal circuits. Excessive synaptic elimination by reactive microglia is suggested to contribute to the pathology of several neurodegenerative, neurodevelopmental and neuropsychiatric disorders, such as autism spectrum disorder or schizophrenia. Schizophrenia is a complex and highly heterogeneous disease with detrimental impairments for affected patients. Aberrant microglial activation, release of pro-inflammatory cytokines and ungoverned phagocytosis of synaptic structures is considered a central cause for the development and progression of schizophrenia. So far, there is no cure for schizophrenia and antipsychotic drug therapy can only reduce symptom severity. Targeting microglia by anti-inflammatory treatment is hypothesized to be highly beneficial for the integrity of neuronal networks in neuropsychiatric diseases.

To better understand how neuroinflammatory processes and excessive synaptic elimination contribute to pathological phenotypes of schizophrenia, somatic fibroblasts from four patients with schizophrenia and three healthy controls were reprogrammed successfully into induced pluripotent stem cells (iPSCs). iPSCs were completely characterized and reproducible protocols for the differentiation into microglia and glutamatergic neurons were established. Both cell types were separately analyzed for mature phenotypes. Neurite outgrowth, intracellular calcium signaling and synaptic density was reduced in schizophrenia patient-derived neurons. Microglia derived from patients with schizophrenia displayed increased expression of microglial activation marker HLA-DR. Finally, the cells generated were introduced in a co-culture system comprising iPSC-derived neurons and microglia to study neuroinflammatory mechanisms in the early development of schizophrenia. Addition of microglia led to reduced synaptic density with microglia from patients with schizophrenia engulfing and eliminating more synapses compared to control microglia. Likewise, neuronal cultures derived from patients with schizophrenia activated microglia in a more pronounced way than healthy control neurons. Pro-inflammatory pre-treatment amplified microglial activation and synaptic pruning by control and patient-derived microglia. Most interestingly, application of the anti-inflammatory antibiotic minocycline could reverse excessive synaptic elimination by microglia derived from patients with schizophrenia.

The established co-culture model of microglia and neurons offers the possibility to study neuroinflammatory processes in the development of schizophrenia and detect pathological mechanisms in patient-derived microglia and neurons.