Cross-talk between GABAergic postsynapse and microglia regulate synapse loss after brain ischemia.

Microglia interact with neurons to facilitate synapse plasticity; however, signal(s) contributing to microglia activation for synapse elimination in pathology are not fully understood. Here, using in vitro organotypic hippocampal slice cultures and transient middle cerebral artery occlusion (MCAO) in genetically engineered mice in vivo, we report that at 24 hours after ischemia, microglia release brain-derived neurotrophic factor (BDNF) to downregulate glutamatergic and GABAergic synapses within the peri-infarct area. Analysis of the cornu ammonis 1 (CA1) in vitro shows that proBDNF and mBDNF downregulate glutamatergic dendritic spines and gephyrin scaffold stability through p75 neurotrophin receptor (p75NTR) and tropomyosin receptor kinase B (TrkB) receptors, respectively. After MCAO, we report that in the peri-infarct area and in the corresponding contralateral hemisphere, similar neuroplasticity occurs through microglia activation and gephyrin phosphorylation at serine-268 and serine-270 in vivo. Targeted deletion of the Bdnf gene in microglia or GphnS268A/S270A (phospho-null) point mutations protects against ischemic brain damage, neuroinflammation, and synapse downregulation after MCAO.

Docosahexaenoic acid (DHA): a modulator of microglia activity and dendritic spine morphology.

BACKGROUND: Recent studies have revealed that excessive activation of microglia and inflammation-mediated neurotoxicity are implicated in the progression of several neurological disorders. In particular, chronic inflammation in vivo and exposure of cultured brain cells to lipopolysaccharide (LPS) in vitro can adversely change microglial morphology and function. This can have both direct and indirect effects on synaptic structures and functions. The integrity of dendritic spines, the postsynaptic component of excitatory synapses, dictates synaptic efficacy. Interestingly, dysgenesis of dendritic spines has been found in many neurological diseases associated with ω-3 polyunsaturated fatty acid (PUFA) deficiency and cognitive decline. In contrast, supplemented ω-3 PUFAs, such as docosahexaenoic acid (DHA), can partly correct spine defects. Hence, we hypothesize that DHA directly affects synaptic integrity and indirectly through neuron-glia interaction. Strong activation of microglia by LPS is accompanied by marked release of nitric oxide and formation of lipid bodies (LBs), both dynamic biomarkers of inflammation. Here we investigated direct effects of DHA on synaptic integrity and its indirect effects via microglia in the hippocampal CA1 region. METHODS: Microglia (N9) and organotypic hippocampal slice cultures were exposed to the proinflammagen LPS (100 ng/ml) for 24 h. Biochemical and morphological markers of inflammation were investigated in microglia and CA1 regions of hippocampal slices. As biomarkers of hyperactive microglia, mitochondrial function, nitric oxide release and LBs (number, size, LB surface-associated proteins) were assessed. Changes in synaptic transmission of CA1 pyramidal cells were determined following LPS and DHA (25-50 µM) treatments by recording spontaneous AMPA-mediated miniature excitatory postsynaptic currents (mEPSCs). RESULTS: Microglia responded to LPS stimulation with a significant decrease of mitochondrial function, increased nitric oxide production and an increase in the formation of large LBs. LPS treatment led to a significant reduction of dendritic spine densities and an increase in the AMPA-mediated mEPSC inter-event interval (IEI). DHA normalized the LPS-induced abnormalities in both neurons and microglia, as revealed by the restoration of synaptic structures and functions in hippocampal CA1 pyramidal neurons. CONCLUSION: Our findings indicate that DHA can prevent LPS-induced abnormalities (neuroinflammation) by reducing inflammatory biomarkers, thereby normalizing microglia activity and their effect on synaptic function.