Strippers Reveal Their Depressing Secrets: Removing AMPA Receptors
Authors: Graham L. Collingridge, Stephane Peineau
Journal: Neuron (2014)

How do microglia regulate synaptic function? In this issue of Neuron, Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar describe a novel form of long-term depression of AMPA receptor-mediated synaptic transmission in the hippocampus involving the activation of microglia. How do microglia regulate synaptic function? In this issue of Neuron, Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar describe a novel form of long-term depression of AMPA receptor-mediated synaptic transmission in the hippocampus involving the activation of microglia. Microglia, the immunocompetent cells of the CNS, are involved in numerous diseases of the nervous system (Hagberg et al., 2012Hagberg H. Gressens P. Mallard C. Ann. Neurol. 2012; 71: 444-457Crossref PubMed Scopus (394) Google Scholar, Perry and Teeling, 2013Perry V.H. Teeling J. Semin. Immunopathol. 2013; 35: 601-612Crossref PubMed Scopus (384) Google Scholar). In response to insults, they change from a monitoring to an activated state in which a major function is the phagocytosis of damaged tissue. Microglia are in dynamic contact with neurons, where they also serve to either promote or inhibit neuronal survival. One critical function of activated microglia is to remove dysfunctional synapses, via a process termed "synaptic stripping" (Kettenmann et al., 2013Kettenmann H. Kirchhoff F. Verkhratsky A. Neuron. 2013; 77: 10-18Abstract Full Text Full Text PDF PubMed Scopus (802) Google Scholar). It is likely that these mechanisms operate both under physiological conditions, in particular for the pruning of superfluous synapses during development (Paolicelli et al., 2011Paolicelli R.C. Bolasco G. Pagani F. Maggi L. Scianni M. Panzanelli P. Giustetto M. Ferreira T.A. Guiducci E. Dumas L. et al.Science. 2011; 333: 1456-1458Crossref PubMed Scopus (2399) Google Scholar, Schafer et al., 2012Schafer D.P. Lehrman E.K. Kautzman A.G. Koyama R. Mardinly A.R. Yamasaki R. Ransohoff R.M. Greenberg M.E. Barres B.A. Stevens B. Neuron. 2012; 74: 691-705Abstract Full Text Full Text PDF PubMed Scopus (2244) Google Scholar), as well as under pathological conditions to eliminate synapses of damaged neurons. Clearly, a process that functions to eliminate synapses needs to be tightly regulated. Untoward activation of this process would lead to devastating effects for neuronal function. Indeed, there are good reasons to believe that a dysregulation of this process may lead to aberrant synaptic elimination and could contribute to the early stages of both acute and chronic neurodegenerative disorders, such as stroke and Alzheimer's disease (Wake et al., 2013Wake H. Moorhouse A.J. Miyamoto A. Nabekura J. Trends Neurosci. 2013; 36: 209-217Abstract Full Text Full Text PDF PubMed Scopus (325) Google Scholar, Perry and Teeling, 2013Perry V.H. Teeling J. Semin. Immunopathol. 2013; 35: 601-612Crossref PubMed Scopus (384) Google Scholar). Consequently, the determination of how microglia contribute to synaptic elimination is central to the understanding of both important physiological functions and major brain disorders. The process of long-term depression (LTD) results in synaptic weakening and, if not reversed by, for example, long-term potentiation (LTP), can progress to the elimination of synapses. LTD has been extensively studied both regarding its physiological and pathological functions. LTD comes in various forms, the best characterized of which are triggered by the synaptic or pharmacological activation of either N-methyl-D-aspartate (NMDA) or metabotropic glutamate (mGlu) receptors (Collingridge et al., 2010Collingridge G.L. Peineau S. Howland J.G. Wang Y.T. Nat. Rev. Neurosci. 2010; 11: 459-473Crossref PubMed Scopus (669) Google Scholar). In this issue of Neuron, Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar have made a significant advance in the understanding of how microglia are involved in the rapid regulation of synaptic strength in response to neuronal insults. They show that the combination of hypoxia and an inflammatory stimulus (lipopolysaccharide [LPS]) results, within minutes, in LTD in the hippocampus. The induction of this form of pathological LTD (pathLTD) involves the activation of complement receptor 3 (CR3) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. The pathLTD is expressed by the internalization of AMPA receptors, resulting in a weakening of AMPA receptor-mediated synaptic transmission. The authors conclude that this novel form of pathLTD may contribute to memory impairments and synaptic disruption in neuroinflammatory brain disorders. Interestingly, neither hypoxia nor LPS alone was sufficient to induce LTD. Hypoxia (8% O2) caused a transient depression of synaptic transmission, whereas LPS (10–15 μg/ml) had no effect. Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar pointed out that this combination mimics the pathological condition, in which inflammatory neuropathies are often associated with hypoxia due to reductions in cerebral blood flow. They go on to provide an explanation for the synergy. LPS activates microglial CR3 receptors to stimulate NADPH oxidase, which in turn results in superoxide production and release from microglia. Superoxide then stimulates PP2A within neurons to drive AMPA receptor endocytosis and thereby reduce synaptic strength. Hypoxia leads to the formation of lactic acid, which increases the availability of NADPH. This synergy is interesting since it provides a potential explanation of what may happen in the brain during insults and disease, but it should be noted that under different experimental conditions, LPS alone can be sufficient to affect AMPA receptor-mediated synaptic transmission (Pascual et al., 2012Pascual O. Ben Achour S. Rostaing P. Triller A. Bessis A. Proc. Natl. Acad. Sci. USA. 2012; 109: E197-E205Crossref PubMed Scopus (470) Google Scholar). In all likelihood, a myriad of factors may interact under different circumstances to affect AMPA receptors and synaptic function. Cataloging and explaining these will be a major challenge but essential for understanding synaptic function in health and disease. Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar address the mechanism of expression of the LTD by using a well-characterized peptide (Ahmadian et al., 2004Ahmadian G. Ju W. Liu L. Wyszynski M. Lee S.H. Dunah A.W. Taghibiglou C. Wang Y. Lu J. Wong T.P. et al.EMBO J. 2004; 23: 1040-1050Crossref PubMed Scopus (236) Google Scholar) that inhibits AMPA receptor endocytosis (Collingridge et al., 2010Collingridge G.L. Peineau S. Howland J.G. Wang Y.T. Nat. Rev. Neurosci. 2010; 11: 459-473Crossref PubMed Scopus (669) Google Scholar). They show convincingly that pathLTD involves the internalization of AMPA receptors. It is well established that AMPA receptor endocytosis is important for the expression of many, but not all, forms of LTD in the CNS (Collingridge et al., 2010Collingridge G.L. Peineau S. Howland J.G. Wang Y.T. Nat. Rev. Neurosci. 2010; 11: 459-473Crossref PubMed Scopus (669) Google Scholar). Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar therefore test whether their LTD utilizes the same pool of AMPA receptors as those triggered by NMDA receptors and mGlu receptors. They do this by comparing the level of pathLTD induced in naive neurons, with the level of pathLTD induced after the induction of either NMDA receptor-triggered LTD (NMDAR-LTD) or mGlu receptor-triggered LTD (mGluR-LTD). Since the levels of pathLTD were similar in all cases, this suggests that pathLTD involves a different AMPA receptor pool. Although this conclusion should remain tentative until occlusion experiments are performed on saturated levels of LTD, it is pertinent to speculate how different forms of LTD, as defined by their induction triggers, may target different populations of AMPA receptors. A possible explanation is compartmentalized signaling, either involving different synaptic populations or microdomains within individual synapses. A contributory factor could be the role of different phosphatases. It is established that NMDAR-LTD involves the Ser/Thr phosphatases PP2B and PP1, whereas mGluR-LTD involves Tyr phosphatases, such as STEP (Collingridge et al., 2010Collingridge G.L. Peineau S. Howland J.G. Wang Y.T. Nat. Rev. Neurosci. 2010; 11: 459-473Crossref PubMed Scopus (669) Google Scholar). In the present report, Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar present evidence that pathLTD involves the activation of PP2A by superoxide. Highly compartmentalized phosphorylation processes may therefore determine the pools of AMPA receptors that are regulated. Further research aimed at identifying the targets of these phosphatases during the various forms of LTD would help with this issue. Does pathLTD reflect a process that is entirely pathological and for which no equivalent mechanism exists under normal physiological conditions? Or does pathLTD represent the aberrant activation of a normal physiological process. The link between physiological and pathological plasticity has been studied in most detail with respect to NMDAR-LTD. Here there is good evidence that NMDAR-LTD serves a normal physiological role in synaptic weakening and elimination during synaptic development and for information storage in the mature brain (Collingridge et al., 2010Collingridge G.L. Peineau S. Howland J.G. Wang Y.T. Nat. Rev. Neurosci. 2010; 11: 459-473Crossref PubMed Scopus (669) Google Scholar). There is also emerging evidence that dysregulation of NMDAR-LTD, leading to its uncontrolled activation, leads to synaptic injury and may constitute an initial step in neurodegenerative conditions, such as Alzheimer's disease (AD). This idea is supported by the findings that molecules that are strongly implicated in the etiology of AD, such as casapse-3 and GSK-3beta (Collingridge et al., 2010Collingridge G.L. Peineau S. Howland J.G. Wang Y.T. Nat. Rev. Neurosci. 2010; 11: 459-473Crossref PubMed Scopus (669) Google Scholar), and most recently tau (Kimura et al., 2014Kimura T. Whitcomb D.J. Jo J. Regan P. Piers T. Heo S. Brown C. Hashikawa T. Murayama M. Seok H. et al.Philos. Trans. R. Soc. Lond. B Biol. Sci. 2014; 369: 20130144Crossref PubMed Scopus (189) Google Scholar), are components of the NMDAR-LTD cascade. By analogy, it seems likely that pathLTD studied by Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar is engaging a normal physiological process in which microglia are activated for synaptic stripping as part of the processes of developmental and synaptic plasticity and the response to brain pathology. In which case, identifying the regulatory mechanisms that limit synaptic stripping to when it is required and how these become dysregulated in acute and chronic neurodegenerative disorders would seem to be a priority area of future research. So how do activated microglia and neurons communicate? The study by Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar clearly demonstrates the importance of superoxide. In their experiments, superoxide is both necessary and sufficient to induce LTD. However, this molecule is likely to be only one of a variety of different molecules that mediate neuron-microglia crosstalk. Activated microglia release many other substances that potentially can influence synapses, including NO, growth factors, and pro- and anti-inflammatory cytokines (Kettenmann et al., 2013Kettenmann H. Kirchhoff F. Verkhratsky A. Neuron. 2013; 77: 10-18Abstract Full Text Full Text PDF PubMed Scopus (802) Google Scholar). Indeed, a recent study has shown how microglia may sense glutamate release via intrinsic NMDA receptors and release a host of cytokines, including TNFalpha, IL-1beta, and IL-6, as well as NO and superoxide. In this study, neurotoxicity induced by NMDA receptor stimulation was prevented by an inhibitor of TNFalpha. Significantly, conditional knockout of microglial NMDARs offered substantial neuroprotection against ischemic insults, highlighting the significant role played by this cell type in neuronal damage (Kaindl et al., 2012Kaindl A.M. Degos V. Peineau S. Gouadon E. Chhor V. Loron G. Le Charpentier T. Josserand J. Ali C. Vivien D. et al.Ann. Neurol. 2012; 72: 536-549Crossref PubMed Scopus (151) Google Scholar). The complexity of cytokine signaling is highlighted in another recent study that showed that IL-1beta inhibits LTD (Nisticò et al., 2014Nisticò R. Mori F. Feligioni M. Nicoletti F. Centonze D. Philos. Trans. R. Soc. Lond. B Biol. Sci. 2014; 369: 20130162Crossref PubMed Scopus (41) Google Scholar). In this context, it is perhaps relevant that two of the major effector pathways of cytokines, PI3K-Akt (Collingridge et al., 2010Collingridge G.L. Peineau S. Howland J.G. Wang Y.T. Nat. Rev. Neurosci. 2010; 11: 459-473Crossref PubMed Scopus (669) Google Scholar) and JAK/STAT (Nicolas et al., 2012Nicolas C.S. Peineau S. Amici M. Csaba Z. Fafouri A. Javalet C. Collett V.J. Hildebrandt L. Seaton G. Choi S.-L. et al.Neuron. 2012; 73: 374-390Abstract Full Text Full Text PDF PubMed Scopus (157) Google Scholar), are integral components of NMDAR-LTD. Thus, cytokines have the capacity to directly influence LTD via pathways that would be predicted to promote synaptic survival and synaptic weakening, respectively. Establishing the relative roles of cytokines, reactive oxidative species and other factors secreted by microglia in synaptic weakening and stripping remains a challenge for the future. The study of Zhang et al., 2014Zhang J. Malik A. Choi H.B. Ko R.W.Y. Dissing-Olesen L. MacVicar B.A. Neuron. 2014; 82 (this issue): 195-207Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar has substantially increased our understanding of how microglia may participate in synaptic plasticity, in particular by demonstrating the involvement of CR3 in this regulation. There is an interesting parallel with a previous study from the Dublin team of Anwyl and Rowan (Wang et al., 2004Wang Q. Rowan M.J. Anwyl R. J. Neurosci. 2004; 24: 6049-6056Crossref PubMed Scopus (184) Google Scholar) that showed that the activation of microglia, including the formation of superoxide, is required for the acute inhibition of NMDA receptor-mediated LTP by Abeta oligomers. These two studies add further weight to the emerging concept that the signaling mechanisms that are activated by Abeta to inhibit LTP are the same as those that are involved in various forms of LTD. Clearly a greater understanding of the signaling mechanisms involved in synaptic plasticity and how these are modified during various insults is going to impact upon the search for improved therapeutic strategies for AD as well as for other acute and chronic neurodegenerative disorders. In this context, the role of microglia needs to be taken fully into account. Figure 1 presents a scheme that attempts to link some of the pertinent LTD signaling cascades within neurons to the activity of microglia. Clearly, this is just a start of what is likely to be a highly complex picture that will need to take account of both temporal and spatial aspects of the interaction between neurons and microglia. In conclusion, microglia serve essential physiological functions but are dangerous beasts if their control mechanisms go awry. In terms of the pathLTD that has been characterized in the present article, it is possible that the removal of AMPA receptors is part of a neuroprotection mechanism to limit excessive synaptic excitation during the neuronal insult. However, if the synaptic depression is not reversed, then the microglia turn into strippers and get down to their business of eliminating synapses. Strippers serve an essential function, but they must be properly regulated. Microglial CR3 Activation Triggers Long-Term Synaptic Depression in the Hippocampus via NADPH OxidaseZhang et al.NeuronMarch 27, 2014In BriefInflammation of the brain and hypoxia are linked to neurodegeneration and cognitive impairment. Zhang et al. describe their discovery of a pathway by which inflammation and hypoxia act on microglia, the brain's immune cells, to depress synaptic function. Full-Text PDF Open Archive