The discovery of TREM2 as a myeloid-specific Alzheimer's disease (AD) risk gene has accelerated research into the role of microglia in AD. While TREM2 mouse models have provided critical insight, the ...normal and disease-associated functions of TREM2 in human microglia remain unclear. To examine this question, we profile microglia differentiated from isogenic, CRISPR-modified TREM2-knockout induced pluripotent stem cell (iPSC) lines. By combining transcriptomic and functional analyses with a chimeric AD mouse model, we find that TREM2 deletion reduces microglial survival, impairs phagocytosis of key substrates including APOE, and inhibits SDF-1α/CXCR4-mediated chemotaxis, culminating in an impaired response to beta-amyloid plaques in vivo. Single-cell sequencing of xenotransplanted human microglia further highlights a loss of disease-associated microglial (DAM) responses in human TREM2 knockout microglia that we validate by flow cytometry and immunohistochemistry. Taken together, these studies reveal both conserved and novel aspects of human TREM2 biology that likely play critical roles in the development and progression of AD.
An increasing number of studies show that an altered epigenetic landscape may cause impairments in regulation of learning and memory-related genes within the aged hippocampus, eventually resulting in ...cognitive deficits in the aged brain. One such epigenetic repressive mark is trimethylation of H3K9 (H3K9me3), which is typically implicated in gene silencing. Here, we identify, for the first time, an essential role for H3K9me3 and its histone methyl transferase (SUV39H1) in mediating hippocampal memory functions. Pharmacological inhibition of SUV39H1 using a novel and selective inhibitor decreased levels of H3K9me3 in the hippocampus of aged mice, and improved performance in the objection location memory and fear conditioning tasks and in a complex spatial environment learning task. The inhibition of SUV39H1 induced an increase in spine density of thin and stubby but not mushroom spines in the hippocampus of aged animals and increased surface GluR1 levels in hippocampal synaptosomes, a key index of spine plasticity. Furthermore, there were changes at BDNF exon I gene promoter, in concert with overall BDNF levels in the hippocampus of drug-treated animals compared with control animals. Together, these data demonstrate that SUV39H1 inhibition and the concomitant H3K9me3 downregulation mediate gene transcription in the hippocampus and reverse age-dependent deficits in hippocampal memory.
Cognitive decline is a debilitating condition associated with not only neurodegenerative diseases but also aging in general. However, effective treatments have been slow to emerge so far. In this study, we demonstrate that epigenetic regulation of key synaptic proteins may be an underlying, yet reversible, cause of this decline. Our findings suggest that histone 3 trimethylation is a probable target for pharmacological intervention that can counteract cognitive decline in the aging brain. Finally, we provide support to the hypothesis that, by manipulating the enzyme that regulates H3K9me3 (using a newly developed specific inhibitor of SUV39H1), it is possible to alter the chromatin state of subjects and restore memory and synaptic function in the aging brain.
Highlights • Cytokines can affect learning and memory by modulating LTP. • Cytokines modulate LTP directly in neurons, and indirectly via cell-cytokine networks. • Direct cytokine’s effect on LTP can ...be tested in cultured neurons and in synaptosomes. • Synaptosomes allow testing LTP modulation by cytokines directly at the synapse.
Evolving evidence suggests that brain inflammation and the buildup of proinflammatory cytokine increases the risk for cognitive decline and cognitive dysfunction. Interleukin-1β (IL-1β), acting via ...poorly understood mechanisms, appears to be a key cytokine in causing these deleterious effects along with a presumably related loss of long-term potentiation (LTP)-type synaptic plasticity. We hypothesized that IL-1β disrupts brain-derived neurotrophic factor (BDNF) signaling cascades and thereby impairs the formation of filamentous actin (F-actin) in dendritic spines, an event that is essential for the stabilization of LTP. Actin polymerization in spines requires phosphorylation of the filament severing protein cofilin and is modulated by expression of the immediate early gene product Arc. Using rat organotypic hippocampal cultures, we found that IL-1β suppressed BDNF-dependent regulation of Arc and phosphorylation of cofilin and cAMP response element-binding protein (CREB), a transcription factor regulating Arc expression. IL-1β appears to act on BDNF signal transduction by impairing the phosphorylation of insulin receptor substrate 1, a protein that couples activation of the BDNF receptor TrkB to downstream signaling pathways regulating CREB, Arc, and cofilin. IL-1β upregulated p38 mitogen-activated protein kinase (MAPK), and inhibiting p38 MAPK prevented IL-1β from disrupting BDNF signaling. IL-1β also prevented the formation of F-actin in spines and impaired the consolidation, but not the induction, of BDNF-dependent LTP in acute hippocampal slices. The suppressive effect of IL-1β on F-actin and LTP was prevented by inhibiting p38 MAPK. These findings define a new mechanism for the action of IL-1β on LTP and point to a potential therapeutic target to restore synaptic plasticity.
CNS inflammatory responses are linked to cognitive impairment in humans. Research in animal models supports this connection by showing that inflammatory cytokines suppress long-term potentiation ...(LTP), the best-known cellular correlate of memory. Cytokine-induced modulation of LTP has been previously studied in vivo or in brain slices, two experimental approaches containing multiple cell populations responsive to cytokines. In their target cells, cytokines commonly increase the expression of multiple cytokines, thus increasing the complexity of brain cytokine networks even after single-cytokine challenges. Whether cytokines suppress LTP by direct effects on neurons or by indirect mechanisms is still an open question. Here, we evaluated the effect of a major set of inflammatory cytokines including tumor necrosis factor-α (TNFα), interleukin-1β (IL-1β) and interleukin-18 (IL-18) on chemically-induced LTP (cLTP) in isolated hippocampal synaptosomes of mice, using fluorescence analysis of single-synapse long-term potentiation (FASS-LTP). We found that TNFα and IL-1β suppress synaptosomal cLTP. In contrast, cLTP was not affected by IL-18, at a concentration previously shown to block LTP in hippocampal slices. We also found that IL-18 does not impair cLTP or brain-derived neurotrophic factor (BDNF) signaling in primary hippocampal neuronal cultures. Thus, using both synaptosomes and neuron cultures, our data suggest that IL-18 impairs LTP by indirect mechanisms, which may depend on non-neuronal cells, such as glia. Notably, our results demonstrate that TNFα and IL-1β directly suppress hippocampal plasticity via neuron-specific mechanisms. A better understanding of the brain’s cytokine networks and their final molecular effectors is crucial to identify specific targets for intervention.
This scientific commentary refers to ‘Striatal synaptic bioenergetic and autophagic decline in premotor experimental parkinsonism’ by Merino-Galán et al. (https://doi.org/10.1093/brain/awac087).
Death-mediating proteases such as caspases and caspase-3 in particular, have been implicated in neurodegenerative processes, aging and Alzheimer＇s disease. However, emerging evidence suggests that in ...addition to their classical role in cell death, caspases play a key role in modulating synaptic function. It is remarkable that active caspases-3, which can trigger widespread damage and degeneration, aggregates in structures as delicate as synapses and persists in neurons without causing acute cell death. Here, we evaluate this dichotomy, and discuss the hypothesis that caspase-3 may be a bifurcation point in cellular signaling, able to orient the neuronal response to stress down either pathological/apoptotic pathways or towards physiological cellular remodeling. We propose that temporal, spatial and other regulators of caspase activity are key determinants of the ultimate effect of caspase-3 activation in neurons. This concept has implications for differential roles of caspase-3 activation across the lifespan. Specifically, we propose that limited caspase-3 activation is critical for synaptic function in the healthy adult brain while chronic activation is involved in degenerative processes in the aging brain.
In the aged brain, synaptic plasticity and memory show increased vulnerability to impairment by the inflammatory cytokine interleukin 1β (IL-1β). In this study, we evaluated the possibility that ...synapses may directly undergo maladaptive changes with age that augment sensitivity to IL-1β impairment. In hippocampal neuronal cultures, IL-1β increased the expression of the IL-1 receptor type 1 and the accessory coreceptor AcP (proinflammatory), but not of the AcPb (prosurvival) subunit, a reconfiguration that potentiates the responsiveness of neurons to IL-1β. To evaluate whether synapses develop a similar heightened sensitivity to IL-1β with age, we used an assay to track long-term potentiation (LTP) in synaptosomes. We found that IL-1β impairs LTP directly at the synapse and that sensitivity to IL-1β is augmented in aged hippocampal synapses. The increased synaptic sensitivity to IL-1β was due to IL-1 receptor subunit reconfiguration, characterized by a shift in the AcP/AcPb ratio, paralleling our culture data. We suggest that the age-related increase in brain IL-1β levels drives a shift in IL-1 receptor configuration, thus heightening the sensitivity to IL-1β. Accordingly, selective blocking of AcP-dependent signaling with Toll–IL-1 receptor domain peptidomimetics prevented IL-1β–mediated LTP suppression and blocked the memory impairment induced in aged mice by peripheral immune challenge (bacterial lipopolysaccharide). Overall, this study demonstrates that increased AcP signaling, specifically at the synapse, underlies the augmented vulnerability to cognitive impairment by IL-1β that occurs with age.
Exercise has emerged as a powerful variable that can improve cognitive function and delay age-associated cognitive decline and Alzheimer's disease (AD); however, the underlying mechanisms are poorly ...understood. To determine if protective mechanisms may occur at the transcriptional level, we used microarrays to investigate the relationship between physical activity levels and gene expression patterns in the cognitively intact aged human hippocampus. In parallel, hippocampal gene expression patterns associated with aging and AD were assessed using publicly available microarray data profiling hippocampus from young (20–59 years), cognitively intact aging (73–95 years) and age-matched AD cases. To identify “anti-aging/AD” transcription patterns associated with physical activity, probesets significantly associated with both physical activity and aging/AD were identified and their directions of expression change in each condition were compared. Remarkably, of the 2210 probesets significant in both data sets, nearly 95% showed opposite transcription patterns with physical activity compared with aging/AD. The majority (>70%) of these anti-aging/AD genes showed increased expression with physical activity and decreased expression in aging/AD. Enrichment analysis of the anti-aging/AD genes showing increased expression in association with physical activity revealed strong overrepresentation of mitochondrial energy production and synaptic function, along with axonal function and myelin integrity. Synaptic genes were notably enriched for synaptic vesicle priming, release and recycling, glutamate and GABA signaling, and spine plasticity. Anti-aging/AD genes showing decreased expression in association with physical activity were enriched for transcription-related function (notably negative regulation of transcription). These data reveal that physical activity is associated with a more youthful profile in the hippocampus across multiple biological processes, providing a potential molecular foundation for how physical activity can delay age- and AD-related decline of hippocampal function.
•Physical activity is extensively associated with gene expression in the aged human hippocampus.•Gene expression patterns associated with physical activity restore a more youthful state in the hippocampus and opposed transcription patterns associated with aging and Alzheimer's disease.•Gene expression is primarily increased, with genes targeting multiple core cellular functions.•Genes showed robust enrichment for mitochondrial energy production, synaptic signaling mechanisms, axon and white matter health, and control of transcription.
Brain inflammation including increases in inflammatory cytokines such as IL-1β is widely believed to contribute to the pathophysiology of Alzheimer's disease. Although IL-1β-induced impairments in ...long-term potentiation (LTP) in acute hippocampal slices and memory functions in vivo have been well documented, the neuron-specific molecular mechanisms of IL-1β-mediated impairments of LTP and memory remain unclear.
This study uses an in vitro approach in primary hippocampal neurons to evaluate the effect of IL-1β on chemical LTP (cLTP)-induced structural plasticity and signaling.
We found that IL-1β reduces both the surface expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA1 and the spine growth following cLTP. These effects of IL-1β were mediated by impairing actin polymerization during cLTP, as IL-1β decreased the cLTP-induced formation of F-actin, and the effect of IL-1β on cLTP-induced surface expression of GluA1 can be mimicked by latrunculin, a toxin that disrupts dynamics of actin filaments, and can be prevented by jasplakinolide, a cell-permeable peptide that stabilizes F-actin. Moreover, live-cell imaging demonstrated that IL-1β decreased the stability of the actin cytoskeleton in spines, which is required for LTP consolidation. We further examined the role of sphingolipid signaling in the IL-1β-mediated impairment of spine plasticity and found that both the neutral sphingomyelinase inhibitor GW4869 and the inhibitor of Src kinase PP2 attenuated the IL-1β-mediated suppression of cLTP-induced surface expression of GluA1 and actin polymerization.
These findings support a mechanism by which IL-1β, via the sphingomyelinase/ceramide/Src pathway, impairs structural spine remodeling essential for LTP consolidation and memory.
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