The adaptive aging brain

Epilepsy is one of the most common neurological disorders characterized by recurrent seizures. The mechanism of epilepsy remains unclear and previous studies suggest that N-methyl-D-aspartate receptors (NMDARs) play an important role in abnormal discharges, nerve conduction, neuron injury and inflammation, thereby they may participate in epileptogenesis. NMDARs belong to a family of ionotropic glutamate receptors that play essential roles in excitatory neurotransmission and synaptic plasticity in the mammalian CNS. Despite numerous studies focusing on the role of NMDAR in epilepsy, the relationship appeared to be elusive. In this article, we reviewed the regulation of NMDAR and possible mechanisms of NMDAR in epilepsy and in respect of onset, development, and treatment, trying to provide more evidence for future studies.

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Roles of N-Methyl-D-Aspartate Receptors (NMDARs) in Epilepsy

Furin is an important mammalian proprotein convertase that catalyzes the proteolytic maturation of a variety of prohormones and proproteins in the secretory pathway. In the brain, the substrates of furin include the proproteins of growth factors, receptors and enzymes. Emerging evidence, such as reduced FURIN mRNA expression in the brains of Alzheimer's disease patients or schizophrenia patients, has implicated a crucial role of furin in the pathophysiology of neurodegenerative and neuropsychiatric diseases. Currently, compared to cancer and infectious diseases, the aberrant expression of furin and its pharmaceutical potentials in neurological diseases remain poorly understood. In this article, we provide an overview on the physiological roles of furin and its substrates in the brain, summarize the deregulation of furin expression and its effects in neurodegenerative and neuropsychiatric disorders, and discuss the implications and current approaches that target furin for therapeutic interventions. This review may expedite future studies to clarify the molecular mechanisms of furin deregulation and involvement in the pathogenesis of neurodegenerative and neuropsychiatric diseases, and to develop new diagnosis and treatment strategies for these diseases. 

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The emerging role of furin in neurodegenerative and neuropsychiatric diseases

Alzheimer's disease (AD) is the most common cause of dementia, affecting 35 million people worldwide. One pathological feature of progressing AD is the loss of synapses. This is the strongest correlate of cognitive decline. Astrocytes, as an essential part of the tripartite synapse, play a role in synapse formation, maintenance, and elimination. During AD, astrocytes get a reactive phenotype with an altered gene expression profile and changed function compared to healthy astrocytes. This process likely affects their interaction with synapses. This systematic review aims to provide an overview of the scientific literature including information on how astrocytes affect synapse formation and elimination in the brain of AD patients and in animal models of the disease. We review molecular and cellular changes in AD astrocytes and conclude that these predominantly result in lower synapse numbers, indicative of decreased synapse support or even synaptotoxicity, or increased elimination, resulting in synapse loss, and consequential cognitive decline, as associated with AD. Preventing AD induced changes in astrocytes might therefore be a potential therapeutic target for dementia. Systematic Review Registration: https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=148278, identifier [CRD148278]. Graphical Abstract AD astrocytes decrease the number of synapses via a variety of pathways. We distinguished a direct and indirect effect of astrocytes on synapse number. Direct: phagocytic capacity is impaired, affected by astrocyte expression of MERTK, MEGF10, C3, and ApoE4. Indirect: impaired glutamate transport & signaling, impaired Ca2+ signaling, and decreased TSP1 expression all can lead to a synaptotoxic environment, which indirectly reduces synapse number. Impaired glutamate transport is caused by a decrease in GLT1 and GLAST expression and leads to an increase in glutamate in the synaptic cleft, resulting in impaired NMDAR activity and excitotoxicity. Besides affecting phagocytosis, ApoE4 also increases inflammation, and Tau and Aβ aggregation, thereby further promoting synapse loss. The astrocyte (green) interacts with a synapse on a neurite (blue). The decrease in synapse number is visualized by more transparent synapses.

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The Role of Astrocytes in Synapse Loss in Alzheimer's Disease: A Systematic Review

We aim to investigate the mechanisms underlying the beneficial effects of exercise rehabilitation (ER) and/or astragaloside (AST) in counteracting amyloid-beta (Aβ) pathology. Aβ oligomers were microinjected into the bilateral ventricles to induce Aβ neuropathology in rats. Neurobehavioral functions were evaluated. Cortical and hippocampal expressions of both BDNF/TrkB and cathepsin D were determined by the western blotting method. The rat primary cultured cortical neurons were incubated with BDNF and/or AST and ANA12 followed by exposure to aggregated Aβ for 24 h. In vivo results showed that ER and/or AST reversed neurobehavioral disorders, downregulation of cortical and hippocampal expression of both BDNF/TrkB and cathepsin D, neural pathology, Aβ accumulation, and altered microglial polarization caused by Aβ. In vitro studies also confirmed that topical application of BDNF and/or AST reversed the Aβ-induced cytotoxicity, apoptosis, mitochondrial distress, and synaptotoxicity and decreased expression of p-TrkB, p-Akt, p-GSK3β, and β-catenin in rat cortical neurons. The beneficial effects of combined ER (or BDNF) and AST therapy in vivo and in vitro were superior to ER (or BDNF) or AST alone. Furthermore, we observed that any gains from ER (or BDNF) and/or AST could be significantly eliminated by ANA-12, a potent BDNF/TrkB antagonist. These results indicate that whereas ER (or BDNF) and/or AST attenuate Aβ pathology by reversing BDNF/TrkB signaling deficits and mitochondrial dysfunction, combining these two potentiates each other's therapeutic effects. In particular, AST can be an alternative therapy to replace ER. 

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Exercise Rehabilitation and/or Astragaloside Attenuate Amyloid-beta Pathology by Reversing BDNF/TrkB Signaling Deficits and Mitochondrial Dysfunction

Synapse degeneration correlates strongly with cognitive impairments in Alzheimer's disease (AD) patients. Soluble Amyloid-beta (Aβ) oligomers are thought as the major trigger of synaptic malfunctions. Our earlier studies have demonstrated that Aβ oligomers interfere with synaptic function through N-methyl-D-aspartate receptors (NMDARs). Our recent in vitro study found the neuroprotective role of astrocytic GluN2A in the promotion of synapse survival and identified nerve growth factor (NGF) derived from astrocytes, as a likely mediator of astrocytic GluN2A buffering against Aβ synaptotoxicity. Our present in vivo study focused on exploring the precise mechanism of astrocytic GluN2A influencing Aβ synaptotoxicity through regulating NGF. We generated an adeno-associated virus (AAV) expressing an astrocytic promoter (GfaABC1D) shRNA targeted to Grin2a (the gene encoding GluN2A) to perform astrocyte-specific Grin2a knockdown in the hippocampal dentate gyrus, after 3 weeks of virus vector expression, Aβ were bilaterally injected into the intracerebral ventricle. Our results showed that astrocyte-specific knockdown of Grin2a and Aβ application both significantly impaired spatial memory and cognition, which associated with the reduced synaptic proteins PSD95, synaptophysin and compensatory increased NGF. The reduced astrocytic GluN2A can counteract Aβ-induced compensatory protective increase of NGF through regulating pNF-κB, Furin and VAMP3, which modulating the synthesis, mature and secretion of NGF respectively. Our present data reveal, for the first time, a novel mechanism of astrocytic GluN2A in exerting protective effects on synapses at the early stage of Aβ exposure, which may contribute to establish new targets for AD prevention and early therapy.

Knockdown of astrocytic Grin2a aggravates β-amyloid-induced memory and cognitive deficits through regulating nerve growth factor.

BACKGROUND Soluble oligomeric amyloid-β (Aβ)-induced synaptic dysfunction is an early event in Alzheimer's disease (AD) pathogenesis. Mounting evidence has suggested N-methyl-D-aspartate receptors (NMDARs) play an important role in Aβ-induced synaptotoxicity. Originally NMDARs were believed to be expressed exclusively in neurons; however, recent two decades studies have demonstrated functional NMDARs present on astrocytes. Neuronal NMDARs are modulators of neurodegeneration, while our previous initial study found that astrocytic NMDARs mediated synaptoprotection and identified nerve growth factor (NGF) secreted by astrocytes, as a likely mediator, but how astrocytic NMDARs protect neurons against Aβ-induced synaptotoxicity through regulating NGF remains unclear. OBJECTIVE To achieve further insight into the mechanism of astrocytic NMDARs oppose Aβ-induced synaptotoxicity through regulating NGF. METHODS With the primary hippocampal neuronal and astrocytic co-cultures, astrocytes were pretreated with agonist or antagonist of NMDARs before Aβ142 oligomers application to neuron-astrocyte co-cultures. Western blot, RT-PCR, etc., were used for the related proteins evaluation. RESULTS Activation of astrocytic NMDARs can significantly mitigate Aβ142-induced loss of PSD-95 and synaptophysin through increasing NGF release. Blockade of astrocytic NMDARs inhibited Aβ-induced compensatory protective NGF increase in protein and mRNA levels through modulating NF-κB of astrocytes. Astrocytic NMDARs activation can enhance Aβ-induced Furin increase, and blockade of astrocytic NMDARs inhibited Aβ-induced immunofluorescent intensity elevation of vesicle trafficking protein VAMP3 and NGF double-staining. CONCLUSION Astrocytic NMDARs oppose Aβ-induced synaptotoxicity through modulating the synthesis, maturation, and secretion of NGF in astrocytes. This new information may contribute to the quest for specific targeted strategy of intervention to delay the onset of AD.

Astrocytic N-Methyl-D-Aspartate Receptors Protect the Hippocampal Neurons Against Amyloid-β 142-Induced Synaptotoxicity by Regulating Nerve Growth Factor

Xinyue Chen

Papers

Knockdown of astrocytic Grin2a aggravates β-amyloid-induced memory and cognitive deficits through regulating nerve growth factor.

Astrocytic N-Methyl-D-Aspartate Receptors Protect the Hippocampal Neurons Against Amyloid-β 142-Induced Synaptotoxicity by Regulating Nerve Growth Factor