Alzheimer’s disease (AD) is a progressive neurodegenerative disease in which the formation of extracellular aggregates of amyloid beta (Aβ) peptide, fibrillary tangles of intraneuronal tau and ...microglial activation are major pathological hallmarks. One of the key molecules involved in microglial activation is galectin-3 (gal3), and we demonstrate here for the first time a key role of gal3 in AD pathology. Gal3 was highly upregulated in the brains of AD patients and 5xFAD (familial Alzheimer’s disease) mice and found specifically expressed in microglia associated with Aβ plaques. Single-nucleotide polymorphisms in the
LGALS3
gene, which encodes gal3, were associated with an increased risk of AD. Gal3 deletion in 5xFAD mice attenuated microglia-associated immune responses, particularly those associated with TLR and TREM2/DAP12 signaling. In vitro data revealed that gal3 was required to fully activate microglia in response to fibrillar Aβ. Gal3 deletion decreased the Aβ burden in 5xFAD mice and improved cognitive behavior. Interestingly, a single intrahippocampal injection of gal3 along with Aβ monomers in WT mice was sufficient to induce the formation of long-lasting (2 months) insoluble Aβ aggregates, which were absent when gal3 was lacking. High-resolution microscopy (stochastic optical reconstruction microscopy) demonstrated close colocalization of gal3 and TREM2 in microglial processes, and a direct interaction was shown by a fluorescence anisotropy assay involving the gal3 carbohydrate recognition domain. Furthermore, gal3 was shown to stimulate TREM2–DAP12 signaling in a reporter cell line. Overall, our data support the view that gal3 inhibition may be a potential pharmacological approach to counteract AD.
To investigate the role of astrocytes in regulating synaptic transmission, we generated inducible transgenic mice that express a dominant-negative SNARE domain selectively in astrocytes to block the ...release of transmitters from these glial cells. By releasing adenosine triphosphate, which accumulates as adenosine, astrocytes tonically suppressed synaptic transmission, thereby enhancing the dynamic range for long-term potentiation and mediated activity-dependent, heterosynaptic depression. These results indicate that astrocytes are intricately linked in the regulation of synaptic strength and plasticity and provide a pathway for synaptic cross-talk.
Although metabotropic glutamate receptor 5 (mGluR5) is essential for cocaine self-administration and drug-seeking behavior, there is limited knowledge of the cellular actions of this receptor in the ...nucleus accumbens (NAc). Although mGluR5 has the potential to regulate neurons directly, recent studies have shown the importance of mGluR5 in regulating Ca²⁺ signaling in astrocytes and, as a consequence, the Ca²⁺-dependent release of excitatory transmitters from these glia. In this study, we demonstrate that activation of mGluR5 induces Ca²⁺ oscillations in NAc astrocytes with the correlated appearance of NMDA receptor-dependent slow inward currents detected in medium spiny neurons (MSNs). Photolysis of caged Ca²⁺ loaded specifically into astrocytes evoked slow inward currents demonstrating that Ca²⁺ elevations in astrocytes are responsible for these excitatory events. Pharmacological evaluation of these glial-evoked NMDA currents shows that they are mediated by NR2B-containing NMDA receptors, whereas synaptic NMDA receptors rely on NR2A-containing receptors. Stimulation of glutamatergic afferents activates mGluR5-dependent astrocytic Ca²⁺ oscillations and gliotransmission that is sustained for minutes beyond the initial stimulus. Because gliotransmission is mediated by NMDA receptors, depolarized membrane potentials exhibited during up-states augment excitation provided by gliotransmission, which drives bursts of MSN action potentials. Because the predominant mGluR5-dependent action of glutamatergic afferents is to cause the sustained activation of astrocytes, which in turn excite MSNs through extrasynaptic NMDA receptors, our results raise the potential for gliotransmission being involved in prolonged mGluR5-dependent adaptation in the NAc.
Brain-derived neurotrophic factor (BDNF) is a potent regulator of neuronal activity, neurogenesis, and depressive-like behaviors; however, downstream effectors by which BDNF exerts these varying ...actions remain to be determined. Here we reveal that BDNF induces long-lasting enhancements in the efficacy of synaptic inhibition by stabilizing γ2 subunit-containing GABA
A
receptors (GABA
A
Rs) at the cell surface, leading to persistent reductions in neuronal excitability. This effect is dependent upon enhanced phosphorylation of tyrosines 365 and 367 (Y365/7) in the GABA
A
R γ2 subunit as revealed using mice in which these residues have been mutated to phenyalanines (Y365/7F). Heterozygotes for this mutation exhibit an antidepressant-like phenotype, as shown using behavioral-despair models of depression. In addition, heterozygous Y365/7F mice show increased levels of hippocampal neurogenesis, which has been strongly connected with antidepressant action. Both the antidepressant phenotype and the increased neurogenesis seen in these mice are insensitive to further modulation by BDNF, which produces robust antidepressant-like activity and neurogenesis in wild-type mice. Collectively, our results suggest a critical role for GABA
A
R γ2 subunit Y365/7 phosphorylation and function in regulating the effects of BDNF.
Amyloid-beta (Aβ) deposition is a defining feature of Alzheimer’s disease (AD). The toxicity of Aβ aggregation is thought to contribute to clinical deficits including progressive memory loss and ...cognitive dysfunction. Therefore, Aβ peptide has become the focus of many therapeutic approaches for the treatment of AD due to its central role in the development of neuropathology of AD. In the past decade, taking the advantage of multiphoton microscopy and molecular probes for amyloid peptide labeling, the dynamic progression of Aβ aggregation in amyloid plaques and cerebral amyloid angiopathy has been monitored in real time in transgenic mouse models of AD. Moreover, amyloid plaque-associated alterations in the brain including dendritic and synaptic abnormalities, changes of neuronal and astrocytic calcium homeostasis, microglial activation and recruitment in the plaque location have been extensively studied. These studies provide remarkable insight to understand the pathogenesis and pathogenicity of amyloid plaques in the context of AD. The ability to longitudinally image plaques and related structures facilitates the evaluation of therapeutic approaches targeting toward the clearance of plaques.
GABABreceptors are heterodimeric G protein-coupled receptors composed of R1 and R2 subunits that mediate slow synaptic inhibition in the brain by activating inwardly rectifying K+channels (GIRKs) and ...inhibiting Ca2+channels. We demonstrate here that GABABreceptors are intimately associated with 5'AMP-dependent protein kinase (AMPK). AMPK acts as a metabolic sensor that is potently activated by increases in 5'AMP concentration that are caused by enhanced metabolic activity, anoxia, or ischemia. AMPK binds the R1 subunit and directly phosphorylates S783 in the R2 subunit to enhance GABABreceptor activation of GIRKs. Phosphorylation of S783 is evident in many brain regions, and is increased dramatically after ischemic injury. Finally, we also reveal that S783 plays a critical role in enhancing neuronal survival after ischemia. Together our results provide evidence of a neuroprotective mechanism, which, under conditions of metabolic stress or after ischemia, increases GABABreceptor function to reduce excitotoxicity and thereby promotes neuronal survival.
Intravenous anesthetics exert a component of their actions via potentiating inhibitory neurotransmission mediated by γ-aminobutyric type-A receptors (GABAARs). Phasic and tonic inhibition is mediated ...by distinct populations of GABAARs, with the majority of phasic inhibition by subtypes composed of α1-3βγ2 subunits, whereas tonic inhibition is dependent on subtypes assembled from α4-6βδ subunits. To explore the contribution that these distinct forms of inhibition play in mediating intravenous anesthesia, we have used mice in which tyrosine residues 365/7 within the γ2 subunit are mutated to phenyalanines (Y365/7F). Here we demonstrate that this mutation leads to increased accumulation of the α4 subunit containing GABAARs in the thalamus and dentate gyrus of female Y365/7F but not male Y365/7F mice. Y365/7F mice exhibited a gender-specific enhancement of tonic inhibition in the dentate gyrus that was more sensitive to modulation by the anesthetic etomidate, together with a deficit in long-term potentiation. Consistent with this, female Y365/7F, but not male Y365/7F, mice exhibited a dramatic increase in the duration of etomidate- and propofol-mediated hypnosis. Moreover, the amnestic actions of etomidate were selectively potentiated in female Y365/7F mice. Collectively, these observations suggest that potentiation of tonic inhibition mediated by α4 subunit containing GABAARs contributes to the hypnotic and amnestic actions of the intravenous anesthetics, etomidate and propofol.
Fast synaptic inhibition in the brain is largely mediated by γ-aminobutyric acid receptors (GABAAR). While the pharmacological manipulation of GABAAR function by therapeutic agents, such as ...benzodiazepines can have profound effects on neuronal excitation and behavior, the endogenous mechanisms neurons use to regulate the efficacy of synaptic inhibition and their impact on behavior remains poorly understood. To address this issue, we created a knock-in mouse in which tyrosine phosphorylation of the GABAARs γ2 subunit, a posttranslational modification that is critical for their functional modulation, has been ablated. These animals exhibited enhanced GABAAR accumulation at postsynaptic inhibitory synaptic specializations on pyramidal neurons within the CA3 subdomain of the hippocampus, primarily due to aberrant trafficking within the endocytic pathway. This enhanced inhibition correlated with a specific deficit in spatial object recognition, a behavioral paradigm dependent upon CA3. Thus, phospho-dependent regulation of GABAAR function involving just two tyrosine residues in the γ2 subunit provides an input-specific mechanism that not only regulates the efficacy of synaptic inhibition, but has behavioral consequences.
Estrogens have long been implicated in influencing cognitive processes, yet the molecular mechanisms underlying these effects and the roles of the estrogen receptors alpha (ERalpha) and beta (ERbeta) ...remain unclear. Using pharmacological, biochemical and behavioral techniques, we demonstrate that the effects of estrogen on hippocampal synaptic plasticity and memory are mediated through ERbeta. Selective ERbeta agonists increased key synaptic proteins in vivo, including PSD-95, synaptophysin and the AMPA-receptor subunit GluR1. These effects were absent in ERbeta knockout mice. In hippocampal slices, ERbeta activation enhanced long-term potentiation, an effect that was absent in slices from ERbeta knockout mice. ERbeta activation induced morphological changes in hippocampal neurons in vivo, including increased dendritic branching and increased density of mushroom-type spines. An ERbeta agonist, but not an ERalpha agonist, also improved performance in hippocampus-dependent memory tasks. Our data suggest that activation of ERbeta can regulate hippocampal synaptic plasticity and improve hippocampus-dependent cognition.
Cognitive dysfunction is a common symptom in many neuropsychiatric disorders and directly correlates with poor patient outcomes. The majority of prolonged inhibitory signaling in the brain is ...mediated via GABAB receptors (GABABRs), but the molecular function of these receptors in cognition is ill defined. To explore the significance of GABABRs in neuronal activity and cognition, we created mice with enhanced postsynaptic GABABR signaling by mutating the serine 783 in receptor R2 subunit (S783A), which decreased GABABR degradation. Enhanced GABABR activity reduced the expression of immediate-early gene-encoded protein Arc/Arg3.1, effectors that are critical for long-lasting memory. Intriguingly, S783A mice exhibited increased numbers of excitatory synapses and surface AMPA receptors, effects that are consistent with decreased Arc/Arg3.1 expression. These deficits in Arc/Arg3.1 and neuronal morphology lead to a deficit in spatial memory consolidation. Collectively our results suggest a novel and unappreciated role for GABABR activity in determining excitatory neuronal architecture and spatial memory via their ability to regulate Arc/Arg3.1.
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