Soyon Hong

Complement and microglia mediate early synapse loss in Alzheimer mouse models

Too much cleaning up The complement system and microglia seek out and destroy unwanted cellular debris for the peripheral immune system as well as excess synapses in the developing brain. Hong et al. now show how the system may go haywire in adults early in the progression toward Alzheimers disease (AD). Aberrant synapse loss is an early feature of Alzheimers and correlates with cognitive decline. In mice susceptible to AD, complement was associated with synapses, and microglial function was required for synapse loss. The authors speculate that aberrant activation of this “trash disposal” system underlies AD pathology. Science, this issue p. 712 The immune complement system also attacks brain synapses early in Alzheimer’s disease mouse models. Synapse loss in Alzheimer’s disease (AD) correlates with cognitive decline. Involvement of microglia and complement in AD has been attributed to neuroinflammation, prominent late in disease. Here we show in mouse models that complement and microglia mediate synaptic loss early in AD. C1q, the initiating protein of the classical complement cascade, is increased and associated with synapses before overt plaque deposition. Inhibition of C1q, C3, or the microglial complement receptor CR3 reduces the number of phagocytic microglia, as well as the extent of early synapse loss. C1q is necessary for the toxic effects of soluble β-amyloid (Aβ) oligomers on synapses and hippocampal long-term potentiation. Finally, microglia in adult brains engulf synaptic material in a CR3-dependent process when exposed to soluble Aβ oligomers. Together, these findings suggest that the complement-dependent pathway and microglia that prune excess synapses in development are inappropriately activated and mediate synapse loss in AD.

New insights on the role of microglia in synaptic pruning in health and disease

Recent genome-wide association studies implicate microglia in Alzheimers disease (AD) pathogenesis; however, their biological significance remains poorly understood. Synapse loss is a significant correlate of cognitive decline that serves as a critical hallmark of AD and other neurodegenerative diseases; however, mechanisms underlying synaptic vulnerability remain elusive. Emerging research on microglia function in the healthy brain is providing new insight into fundamental roles of microglia and immune molecules in brain wiring. Among their many roles, microglia prune developing synapses and regulate synaptic plasticity and function. Here, we review and discuss how this emerging work may provide new insight into how disruptions in microglia-synapse interactions could contribute to synapse loss and dysfunction, and consequently cognitive impairment, in AD.

Soluble Aβ oligomers are rapidly sequestered from brain ISF in vivo and bind GM1 ganglioside on cellular membranes

Soluble Aβ oligomers contribute importantly to synaptotoxicity in Alzheimers disease, but their dynamics in vivo remain unclear. Here, we found that soluble Aβ oligomers were sequestered from brain interstitial fluid onto brain membranes much more rapidly than nontoxic monomers and were recovered in part as bound to GM1 ganglioside on membranes. Aβ oligomers bound strongly to GM1 ganglioside, and blocking the sialic acid residue on GM1 decreased oligomer-mediated LTP impairment in mouse hippocampal slices. In a hAPP transgenic mouse model, substantial levels of GM1-bound Aβ₄₂ were recovered from brain membrane fractions. We also detected GM1-bound Aβ in human CSF, and its levels correlated with Aβ₄₂, suggesting its potential as a biomarker of Aβ-related membrane dysfunction. Together, these findings highlight a mechanism whereby hydrophobic Aβ oligomers become sequestered onto GM1 ganglioside and presumably other lipids on neuronal membranes, where they may induce progressive functional and structural changes.

Complement C3-Deficient Mice Fail to Display Age-Related Hippocampal Decline

The complement system is part of the innate immune response responsible for removing pathogens and cellular debris, in addition to helping to refine CNS neuronal connections via microglia-mediated pruning of inappropriate synapses during brain development. However, less is known about the role of complement during normal aging. Here, we studied the role of the central complement component, C3, in synaptic health and aging. We examined behavior as well as electrophysiological, synaptic, and neuronal changes in the brains of C3-deficient male mice (C3 KO) compared with age-, strain-, and gender-matched C57BL/6J (wild-type, WT) control mice at postnatal day 30, 4 months, and 16 months of age. We found the following: (1) region-specific and age-dependent synapse loss in aged WT mice that was not observed in C3 KO mice; (2) age-dependent neuron loss in hippocampal CA3 (but not in CA1) that followed synapse loss in aged WT mice, neither of which were observed in aged C3 KO mice; and (3) significantly enhanced LTP and cognition and less anxiety in aged C3 KO mice compared with aged WT mice. Importantly, CA3 synaptic puncta were similar between WT and C3 KO mice at P30. Together, our results suggest a novel and prominent role for complement protein C3 in mediating aged-related and region-specific changes in synaptic function and plasticity in the aging brain. SIGNIFICANCE STATEMENT The complement cascade, part of the innate immune response to remove pathogens, also plays a role in synaptic refinement during brain development by the removal of weak synapses. We investigated whether complement C3, a central component, affects synapse loss during aging. Wild-type (WT) and C3 knock-out (C3 KO) mice were examined at different ages. The mice were similar at 1 month of age. However, with aging, WT mice lost synapses in specific brain regions, especially in hippocampus, an area important for memory, whereas C3 KO mice were protected. Aged C3 KO mice also performed better on learning and memory tests than aged WT mice. Our results suggest that complement C3, or its downstream signaling, is detrimental to synapses during aging.

Complement C3 deficiency protects against neurodegeneration in aged plaque-rich APP/PS1 mice

C3 deficiency protects against hippocampal neurodegeneration and cognitive decline in aged APP/PS1 mice despite abundant Aβ plaques. Avoiding complements Complement C3 is an immune molecule that protects against pathogens and plays a role in refinement of the developing visual system by removing weak nerve connections (that is, synapses). C3 is up-regulated in Alzheimer’s disease and, therefore, may contribute to the synapse loss that underlies cognitive decline. Shi et al. now report that an aged transgenic mouse model of Alzheimer’s disease that lacks C3 was protected against synapse loss and cognitive decline even in the presence of Aβ plaques, possibly by altering the glial response to Aβ deposition. Thus, modulation of complement signaling may have potential as a new therapeutic strategy for Alzheimer’s disease. The complement cascade not only is an innate immune response that enables removal of pathogens but also plays an important role in microglia-mediated synaptic refinement during brain development. Complement C3 is elevated in Alzheimer’s disease (AD), colocalizing with neuritic plaques, and appears to contribute to clearance of Aβ by microglia in the brain. Previously, we reported that C3-deficient C57BL/6 mice were protected against age-related and region-specific loss of hippocampal synapses and cognitive decline during normal aging. Furthermore, blocking complement and downstream iC3b/CR3 signaling rescued synapses from Aβ-induced loss in young AD mice before amyloid plaques had accumulated. We assessed the effects of C3 deficiency in aged, plaque-rich APPswe/PS1dE9 transgenic mice (APP/PS1;C3 KO). We examined the effects of C3 deficiency on cognition, Aβ plaque deposition, and plaque-related neuropathology at later AD stages in these mice. We found that 16-month-old APP/PS1;C3 KO mice performed better on a learning and memory task than did APP/PS1 mice, despite having more cerebral Aβ plaques. Aged APP/PS1;C3 KO mice also had fewer microglia and astrocytes localized within the center of hippocampal Aβ plaques compared to APP/PS1 mice. Several proinflammatory cytokines in the brain were reduced in APP/PS1;C3 KO mice, consistent with an altered microglial phenotype. C3 deficiency also protected APP/PS1 mice against age-dependent loss of synapses and neurons. Our study suggests that complement C3 or downstream complement activation fragments may play an important role in Aβ plaque pathology, glial responses to plaques, and neuronal dysfunction in the brains of APP/PS1 mice.

Microglia: Phagocytosing to Clear, Sculpt, and Eliminate

Microglia are the primary phagocytes of the central nervous system. They eliminate excess functional connections between neurons to sculpt neuronal circuits during development and throughout adulthood. Understanding how microglia recognize and prune synapses during development is providing insight into synapse loss and dysfunction in disease.

Physical and functional interaction between the α- and γ-secretases: A new model of regulated intramembrane proteolysis

The α-secretase ADAM10 interacts with γ-secretase in a proteolytically functional complex, which may suggest a new mechanism of RIP signaling where the sheddase and intramembrane protease reside together in a complex that can accept and process full-length substrates efficiently.

New Brain Lymphatic Vessels Drain Old Concepts.

The brain has traditionally been considered an immune privileged organ, in part due to the lack of evidence for lymphatic vasculature (Galea et al., 2007). Two centuries ago, the existence of lymphatic vessels on the surface of the human brain was proposed but it has since been widely dismissed (Lukic et al., 2003). Therefore, while T cells leave all other organs via the lymphatic system to reach nearby lymph nodes, the prevailing view has been that infiltrated T cells exit the brain via venous blood circulation, circumventing the lymph nodes (Ransohoff and Engelhardt, 2012). The recent evidence for the existence of a lymphatic vascular system in the brains meninges provided independently by two labs, first by the Kipnis lab (Louveau et al., 2015) and then by the Alitalo lab (Aspelund et al., 2015), challenges this view and raises the intriguing possibility of an alternative gateway for T cells to egress the brain.

Structured Illumination Microscopy for the Investigation of Synaptic Structure and Function

The neuronal synapse is a primary building block of the nervous system to which alterations in structure or function can result in numerous pathologies. Studying its formation and elimination is the key to understanding how brains are wired during development, maintained throughout adulthood plasticity, and disrupted during disease. However, due to its diffraction-limited size, investigations of the synaptic junction at the structural level have primarily relied on labor-intensive electron microscopy or ultra-thin section array tomography. Recent advances in the field of super-resolution light microscopy now allow researchers to image synapses and associated molecules with high-spatial resolution, while taking advantage of the key characteristics of light microscopy, such as easy sample preparation and the ability to detect multiple targets with molecular specificity. One such super-resolution technique, Structured Illumination Microscopy (SIM), has emerged as an attractive method to examine synapse structure and function. SIM requires little change in standard light microscopy sample preparation steps, but results in a twofold improvement in both lateral and axial resolutions compared to widefield microscopy. The following protocol outlines a method for imaging synaptic structures at resolutions capable of resolving the intricacies of these neuronal connections.

COMPLEMENT IN ALZHEIMER'S DISEASE: LESSONS FROM C3-DEFICIENT MICE

Background: Generation of neurotoxic amyloid-ß peptides and their deposition along with neurofibrillary tangle formation represent key pathological hallmarks in Alzheimer’s disease (AD). Recent evidence suggests that inflammation may be a third important component which, once initiated in response to neurodegeneration or dysfunction actively contributes to disease progression and chronicity. Microglia is being activated by binding of aggregated proteins or aberrant nucleic acids to pattern recognition receptors which elicit an innate immune response. The latter is characterized by the release of in€ıflammatory mediators including complement activators and inhibitors, chemokines, cytokines, radical oxygen species and enzyme systems. Exogenous as well as endogenous factors may promote and facilitate neuroinflammation in the AD brain. Thus, degeneration of aminergic brain stem nuclei including the locus ceruleus and the nucleus basalis of Meynert may drive inflammation in their projection areas given the antiinflammatory and neuroprotective action of their key transmitters norepinephrine and acetylcholine. Methods: Analysis of neuroinflammatio and in particular of micro and astroglial cell function in vitro and in vivo. Determination of inflammatory markes from cell culture, murine mouse brain and brain tissue or cerebrospinal fluid of MCI and AD patients. Results: In inflammation may not just occur secondary to degeneration, but actively drive amyloid beta aggregation and APP processing. Inhibition of themicroglia driven innate immune response at key signalling steps may provide protection. Conclusions: Therefore, antiinflammatory treatment strategies should be considered. Data on microglial activation in AD along with suggestions to modify and alter the prointo an antiinflammatory phenotype will be reviewed and discussed.