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Microglial dysfunction is a key pathological feature of Alzheimer's disease (AD), but little is known about proteome-wide changes in microglia during the course of AD and their functional consequences. Here, we performed an in-depth and time-resolved proteomic characterization of microglia in two mouse models of amyloid β (Aβ) pathology, the overexpression APPPS1 and the knock-in APP-NL-G-F (APP-KI) model. We identified a large panel of Microglial Aβ Response Proteins (MARPs) that reflect heterogeneity of microglial alterations during early, middle and advanced stages of Aβ deposition and occur earlier in the APPPS1 mice. Strikingly, the kinetic differences in proteomic profiles correlated with the presence of fibrillar Aβ, rather than dystrophic neurites, suggesting that fibrillar Aβ may trigger the AD-associated microglial phenotype and the observed functional decline. The identified microglial proteomic fingerprints of AD provide a valuable resource for functional studies of novel molecular targets and potential biomarkers for monitoring AD progression or therapeutic efficacy., eLife digest Alzheimer’s disease is a progressive, irreversible brain disorder. Patients with Alzheimer’s have problems with memory and other mental skills, which lead to more severe cognitive decline and, eventually, premature death. This is due to increasing numbers of nerve cells in the brain dying over time. A distinctive feature of Alzheimer’s is the abnormally high accumulation of a protein called amyloid-β, which forms distinctive clumps in the brain termed ‘plaques’. The brain has a type of cells called the microglia that identify infections, toxic material and damaged cells, and prevent these from building up by clearing them away. In Alzheimer’s disease, however, the microglia do not work properly, which is thought to contribute to the accumulation of amyloid-β plaques. This means that people with mutations in the genes important for the microglia activity are also at higher risk of developing the disease. Although problems with the microglia play an important role in Alzheimer’s, researchers still do not fully understand why microglia stop working in the first place. It is also not known exactly when and how the microglia change as Alzheimer’s disease progresses. To unravel this mystery, Sebastian Monasor, Müller et al. carried out a detailed study of the molecular ‘fingerprints’ of microglia at each key stage of Alzheimer’s disease. The experiments used microglia cells from two different strains of genetically altered mice, both of which develop the hallmarks of Alzheimer’s disease, including amyloid-β plaques, at similar rates. Analysis of the proteins in microglia cells from both strains revealed distinctive, large-scale changes corresponding to successive stages of the disease – reflecting the gradual accumulation of plaques. Obvious defects in microglia function also appeared soon after plaques started to build up. Microscopy imaging of the brain tissue showed that although amyloid-β plaques appeared at the same time, they looked different in each mouse strain. In one, plaques were more compact, while in the other, plaques appeared ‘fluffier’, like cotton wool. In mice with more compacted plaques, microglia recognized the plaques earlier and stopped working sooner, suggesting that plaque structure and microglia defects could be linked. These results shed new light on the role of microglia and their changing protein ‘signals’ during the different stages of Alzheimer’s disease. In the future, this information could help identify people at risk for the disease, so that they can be treated as soon as possible, and to design new therapies to make microglia work again.

Background Excessive and hypersynchronous neuronal discharges are key characteristics in the pathophysiology of neurological disorders such as epilepsy. Owing to their ability of regulating neuronal excitability, small conductance calcium-activated potassium (SK) channels have been implicated in several diseases of the brain, and their activation provided neuroprotection in different paradigms of cell death, including oxidative stress and excitotoxicity. Objectives In our study, we investigated the function and regulation of SK channel expression in different models of epilepsy and excessive neuronal firing. Methods As a model for hypersynchronous neuronal firing in vitro, we used primary cortical neurons either challenged with glutamate or deprived of magnesium to increase neuronal firing. In addition, we used perforant pathway stimulation (PPS) to induce hippocampal seizures in vivo. We investigated neuronal firing using multielectrode array recordings, analyze SK channel expression by Western blot, and assess mitochondrial performance by evaluating mitochondrial complex activity. MicroRNA135a-dependent effects on SK3 channels were investigated using a dual-luciferase assay. Results and Conclusions In vitro, analysis of neuronal firing in magnesium deprived primary neuronal cultures revealed that SK channel activation fully blocked the increase in neuronal activity, and restored homeostatic signaling. We found reduced SK3 channel expression following glutamate-induced excitotoxicity in vitro, and following PPS in the rat hippocampus in vivo. Further, PPS in vivo impaired the performance of mitochondrial complex I. Interestingly, we identified miRNA-135a as a key regulator of SK3 channel expression in primary neurons. Thus, we provide strong evidence that SK3 channels are involved in epilepsy(-like) conditions which are characterized by enhanced neuronal firing and an impairment of mitochondrial function, and the miRNA135a-dependent regulation of SK3 channel expression was unraveled as a new regulatory mechanism.