Super-Resolution Imaging to Study Co-Localization of Proteins and Synaptic Markers in Primary Neurons

Synapses are the functional elements of neurons and their defects or losses are at the basis of several neurodegenerative and neurological disorders. ...

Istituto di Ricerche Farmacologiche Mario Negri IRCCS

Creutzfeldt-Jakob disease (CJD) is a neurodegenerative disorder caused by prion protein (PrP) misfolding, clinically recognized by cognitive and motor deficits, electroencephalographic abnormalities, and seizures. Its neurophysiological bases are not known. To assess the potential involvement of NMDA receptor (NMDAR) dysfunction, we analyzed NMDA-dependent synaptic plasticity in hippocampal slices from Tg(CJD) mice, which model a genetic form of CJD. Because PrP depletion may result in functional upregulation of NMDARs, we also analyzed PrP knock-out (KO) mice. Long-term potentiation (LTP) at the Schaffer collateral-commissural synapses in the CA1 area of ∼100-d-old Tg(CJD) mice was comparable to that of wild-type (WT) controls, but there was an inversion of metaplasticity, with increased GluN2B phosphorylation, which is indicative of enhanced NMDAR activation. Similar but less marked changes were seen in PrP KO mice. At ∼300 d of age, the magnitude of LTP increased in Tg(CJD) mice but decreased in PrP KO mice, indicating divergent changes in hippocampal synaptic responsiveness. Tg(CJD) but not PrP KO mice were intrinsically more susceptible than WT controls to focal hippocampal seizures induced by kainic acid. IL-1β-positive astrocytes increased in the Tg(CJD) hippocampus, and blocking IL-1 receptor signaling restored normal synaptic responses and reduced seizure susceptibility. These results indicate that alterations in NMDA-dependent glutamatergic transmission in Tg(CJD) mice do not depend solely on PrP functional loss. Moreover, astrocytic IL-1β plays a role in the enhanced synaptic responsiveness and seizure susceptibility, suggesting that targeting IL-1β signaling may offer a novel symptomatic treatment for CJD. Dementia and myoclonic jerks develop in individuals with Creutzfeldt-Jakob disease (CJD), an incurable brain disorder caused by alterations in prion protein structure. These individuals are prone to seizures and have high brain levels of the inflammatory cytokine IL-1β. Here we show that blocking IL-1β receptors with anakinra, the human recombinant form of the endogenous IL-1 receptor antagonist used to treat rheumatoid arthritis, normalizes hippocampal neurotransmission and reduces seizure susceptibility in a CJD mouse model. These results link neuroinflammation to defective neurotransmission and the enhanced susceptibility to seizures in CJD and raise the possibility that targeting IL-1β with clinically available drugs may be beneficial for symptomatic treatment of the disease.

Inhibition of IL-1β Signaling Normalizes NMDA-Dependent Neurotransmission and Reduces Seizure Susceptibility in a Mouse Model of Creutzfeldt-Jakob Disease.

Traumatic brain injury is a risk factor for subsequent neurodegenerative disease, including chronic traumatic encephalopathy, a tauopathy mostly associated with repetitive concussion and blast, but not well recognized as a consequence of severe traumatic brain injury. Here we show that a single severe brain trauma is associated with the emergence of widespread hyperphosphorylated tau pathology in a proportion of humans surviving late after injury. In parallel experimental studies, in a model of severe traumatic brain injury in wild-type mice, we found progressive and widespread tau pathology, replicating the findings in humans. Brain homogenates from these mice, when inoculated into the hippocampus and overlying cerebral cortex of naïve mice, induced widespread tau pathology, synaptic loss, and persistent memory deficits. These data provide evidence that experimental brain trauma induces a self-propagating tau pathology, which can be transmitted between mice, and call for future studies aimed at investigating the potential transmissibility of trauma associated tau pathology in humans.

Induction of a transmissible tau pathology by traumatic brain injury.

α-Synuclein oligomers are crucial players in the pathogenesis of Parkinson's disease. Some mechanisms involved in α-synuclein oligomer detrimental effects include membrane damage, neuroinflammation and protein-protein interactions. Recently, the cellular prion protein (PrPC) emerged as an interactor of α-synuclein oligomers, apparently mediating their detrimental activities. Through direct in vivo and in vitro approaches we herein investigated the existence of a direct cross-talk between α-synuclein oligomers and PrPC. In vitro, we assessed α-synuclein oligomer toxicity by comparing the effect in Prnp+/+ versus PrPC knockout (Prnp0/0) hippocampal neurons. Through an in vivo acute mouse model, where α-synuclein oligomers injected intracerebroventricularly induce memory impairment and neuroinflammation, we verified whether these detrimental effects were preserved in Prnp0/0 mice. In addition, PrPC-α-synuclein oligomer direct binding was investigated through surface plasmon resonance. We found that PrPC was not mandatory to mediate α-synuclein oligomer detrimental effects in vitro or in vivo. Indeed, α-synuclein oligomer toxicity was comparable in Prnp+/+ and Prnp0/0 neurons and both Prnp+/+ and Prnp0/0 mice injected with α-synuclein oligomers displayed memory deficit and hippocampal gliosis. Moreover, surface plasmon resonance analyses ruled out PrPC-α-synuclein oligomer binding. Our findings indicate that PrPC neither binds α-synuclein oligomers nor mediates their detrimental actions. Therefore, it is likely that PrPC-dependent and PrPC-independent pathways co-exist in Parkinson's disease.

Cellular prion protein neither binds to alpha-synuclein oligomers nor mediates their detrimental effects.

Prion diseases are devastating neurodegenerative disorders for which no drugs are available. The successful development of therapeutics depends on drug screening platforms and preclinical models that recapitulate key molecular and pathological features of the disease. Innovative experimental tools have been developed over the last few years that might facilitate drug discovery, including cell-free prion replication assays and prion-infected flies. However, there is still room for improvement. Animal models of genetic prion disease are few, and only partially recapitulate the complexity of the human disorder. Moreover, we still lack a human cell culture model suitable for high-content anti-prion drug screening. This review provides an overview of the models currently used in prion research, and discusses their promise and limitations for drug discovery.

Experimental models to study prion disease pathogenesis and identify potential therapeutic compounds.

To determine whether preventive trials in genetic prion disease could be designed to follow presymptomatic mutation carriers to onset of disease.

Age at onset in genetic prion disease and the design of preventive clinical trials.

Prion diseases typically involve brain deposition of abnormally folded prion protein, which is associated with activated glia and increased cytokine production. Cyclophilin A (CypA) is a ubiquitous protein with peptidyl prolyl cis-trans isomerase activity, which regulates protein folding, and can be secreted by cells in response to inflammatory stimuli. On the basis of in vitro studies, CypA was proposed to mediate glial activation during prion infection. To investigate the role of CypA in vivo, we inoculated CypA, CypA and CypA mice with the RML prion strain, and recorded the time to onset of neurological signs and to terminal disease, and the astrocyte and microglia response at presymptomatic and symptomatic stages. Time to onset of disease and survival were significantly shorter in CypA-deficient mice than CypA-expressing controls. CypA-deficient mice had significantly greater microglial activation in the presymptomatic stage, and analysis of anti- and pro-inflammatory microglial markers indicated a shift towards a pro-inflammatory phenotype. There was no difference in astrocyte activation. This suggests that CypA contributes to dampening the pro-inflammatory microglial response during the early stage of RML-induced prion disease.

Cyclophillin A deficiency accelerates RML-induced prion disease.

Editorial overview: Prion disease: From drug discovery tools to clinical trials.

The ubiquitously expressed SUMO proteins regulate a plethora of cellular pathways and processes. While they have a predominantly nuclear localization, extranuclear roles of SUMO isoforms at the synapse have also been described, making SUMOylation one of the major post-translational regulators of nerve functions. These findings have however recently been challenged, at least for SUMO1, by the analysis of knock-in mice expressing His-HA-SUMO1, where the authors failed to detect the protein at the synapse. In the ongoing dispute, the subcellular distribution in neurons of SUMO2/3 and of the E2 SUMO ligase Ubc9 has not been examined. To investigate whether SUMO proteins do or do not localize at the synapse, we studied their localization in hippocampal primary neurons by super resolution microscopy. We found that SUMO1, SUMO2/3, and Ubc9 are primarily nuclear proteins, which also colocalize partially with pre- and post-synaptic markers such as synaptophysin and PSD95.

Super Resolution Microscopy of SUMO Proteins in Neurons.

Alterations in the expression of glutamate/aspartate transporter (GLAST) have been associated with several neuropathological conditions including Alzheimer's disease and epilepsy. However, the mechanisms by which GLAST expression is altered are poorly understood. Here we used a combination of pharmacological and genetic approaches coupled with quantitative PCR and Western blot to investigate the mechanism of the regulation of GLAST expression by a Ca/calmodulin-activated phosphatase calcineurin (CaN). We show that treatment of cultured hippocampal mouse and fetal human astrocytes with a CaN inhibitor FK506 resulted in a dynamic modulation of GLAST protein expression, being downregulated after 24-48 h, but upregulated after 7 days of continuous FK506 (200 nM) treatment. Protein synthesis, as assessed by puromycin incorporation in neo-synthesized polypeptides, was inhibited already after 1 h of FK506 treatment, while the use of a proteasome inhibitor MG132 (1 μM) shows that GLAST protein degradation was only suppressed after 7 days of FK506 treatment. In astrocytes with constitutive genetic ablation of CaN both protein synthesis and degradation were significantly inhibited. Taken together, our data suggest that, in cultured astrocytes, CaN controls GLAST expression at a posttranscriptional level through regulation of GLAST protein synthesis and degradation.

Calcineurin Controls Expression of EAAT1/GLAST in Mouse and Human Cultured Astrocytes through Dynamic Regulation of Protein Synthesis and Degradation.

Prion protein (PrP) mutations are linked to genetic prion diseases, a class of phenotypically heterogeneous neurodegenerative disorders with invariably fatal outcome. How mutant PrP triggers neurodegeneration is not known. Synaptic dysfunction precedes neuronal loss but it is not clear whether, and through which mechanisms, disruption of synaptic activity ultimately leads to neuronal death. Here we show that mutant PrP impairs the secretory trafficking of AMPA receptors (AMPARs). Specifically, intracellular retention of the GluA2 subunit results in synaptic exposure of GluA2-lacking, calcium-permeable AMPARs, leading to increased calcium permeability and enhanced sensitivity to excitotoxic cell death. Mutant PrPs linked to different genetic prion diseases affect AMPAR trafficking and function in different ways. Our findings identify AMPARs as pathogenic targets in genetic prion diseases, and support the involvement of excitotoxicity in neurodegeneration. They also suggest a mechanistic explanation for how different mutant PrPs may cause distinct disease phenotypes.

Mutant prion proteins increase calcium permeability of AMPA receptors, exacerbating excitotoxicity.

Sleep is severely impaired in patients with Alzheimer's disease. Amyloid-β deposition in the brain of Alzheimer's disease patients is a key event in its pathogenesis and is associated with disrupted sleep, even before the appearance of cognitive decline. Because soluble amyloid-β oligomers are the key mediators of synaptic and cognitive dysfunction in Alzheimer's disease and impair long-term memory in rodents, the first aim of this study was to test the hypothesis that amyloid-β oligomers would directly impair sleep in mice. The cellular prion protein is a cell surface glycoprotein of uncertain function. Because cellular prion protein binds oligomeric amyloid-β with high affinity and mediates some of its neurotoxic effects, the second aim of the study was to test whether amyloid-β oligomer-induced sleep alterations were mediated by cellular prion protein. To address these aims, wild-type and cellular prion protein-deficient mice were given acute intracerebroventricular injections (on different days, at lights on) of vehicle and synthetic amyloid-β oligomers. Compared to vehicle, amyloid-β oligomers significantly reduced the amount of time spent in non-rapid eye movement sleep by wild-type mice during both the light and dark phases of the light-dark cycle. The amount of time spent in rapid eye movement sleep was reduced during the dark phase. Sleep was also fragmented by amyloid-β oligomers, as the number of transitions between states increased in post-injection hours 9-24. No such effects were observed in cellular prion protein-deficient mice. These results show that amyloid-β oligomers do inhibit and fragment sleep, and that these effects are mediated by cellular prion protein.

Roberto Chiesa

Department of Neuroscience

Istituto di Ricerche Farmacologiche Mario Negri IRCCS

Super-Resolution Imaging to Study Co-Localization of Proteins and Synaptic Markers in Primary Neurons

Roberto Chiesa

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Neuroscience

Istituto di Ricerche Farmacologiche Mario Negri IRCCS

Super-Resolution Imaging to Study Co-Localization of Proteins and Synaptic Markers in Primary Neurons

Department of Neuroscience