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Review
. 2023 Jul 18:11:1193130.
doi: 10.3389/fcell.2023.1193130. eCollection 2023.

Chemogenetic manipulation of astrocyte activity at the synapse- a gateway to manage brain disease

Affiliations
Review

Chemogenetic manipulation of astrocyte activity at the synapse- a gateway to manage brain disease

Maria João Pereira et al. Front Cell Dev Biol. .

Abstract

Astrocytes are the major glial cell type in the central nervous system (CNS). Initially regarded as supportive cells, it is now recognized that this highly heterogeneous cell population is an indispensable modulator of brain development and function. Astrocytes secrete neuroactive molecules that regulate synapse formation and maturation. They also express hundreds of G protein-coupled receptors (GPCRs) that, once activated by neurotransmitters, trigger intracellular signalling pathways that can trigger the release of gliotransmitters which, in turn, modulate synaptic transmission and neuroplasticity. Considering this, it is not surprising that astrocytic dysfunction, leading to synaptic impairment, is consistently described as a factor in brain diseases, whether they emerge early or late in life due to genetic or environmental factors. Here, we provide an overview of the literature showing that activation of genetically engineered GPCRs, known as Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), to specifically modulate astrocyte activity partially mimics endogenous signalling pathways in astrocytes and improves neuronal function and behavior in normal animals and disease models. Therefore, we propose that expressing these genetically engineered GPCRs in astrocytes could be a promising strategy to explore (new) signalling pathways which can be used to manage brain disorders. The precise molecular, functional and behavioral effects of this type of manipulation, however, differ depending on the DREADD receptor used, targeted brain region and timing of the intervention, between healthy and disease conditions. This is likely a reflection of regional and disease/disease progression-associated astrocyte heterogeneity. Therefore, a thorough investigation of the effects of such astrocyte manipulation(s) must be conducted considering the specific cellular and molecular environment characteristic of each disease and disease stage before this has therapeutic applicability.

Keywords: CNS disease; DREADDs; astrocytes; heterogeneity; synaptic plasticity; synaptogenesis.

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Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Figures

FIGURE 1
FIGURE 1
Astrocytes control synapse formation and plasticity (figure represents a general synapse). (A) Synaptogenesis. ① During early development, astrocytes secrete pro-synaptogenic factors thrombospondin 1 and 2 (TSP1 and TSP2) and hevin. TSP1 and TSP2 interact with the neuronal receptor α2δ-1, while hevin bridges neuronal neurexin-1α (NRX1α) and neuroligin-1B (NL1B), inducing structural synapse formation. These factors induce the formation of immature synapses containing synaptic vesicles, post-synaptic density (PSD) and NMDARs, but lacking AMPARs. ② Astrocytes can also secrete a hevin antagonist, SPARC, which inhibits hevin-induced synaptogenesis, controlling the rate of new synapse formation. ③ Astrocyte-secreted molecules, such as glypican 4 and 6 (Gpc4 and Gpc6), contribute to synapse maturation by recruiting AMPARs to the post-synaptic membrane (red dotted arrow). ④ Astrocyte-secreted cholesterol is also crucial during synaptic maturation as it regulates pre-synaptic vesicle exocytosis. ⑤ Astrocyte-neuron cell adhesion molecules (CAM), like protocadherins, provide stability and promote synaptic development via contact-mediated signalling. (B) Synaptic transmission and plasticity. ① Once released, neurotransmitters stimulate mainly ionotropic receptors at the post-synaptic neuron to propagate/suppress synaptic transmission. Following this, specialized transporters, like GLT-1/GLAST, take up excess neurotransmitter, such as glutamate, thus preventing excitotoxicity. ② Neurotransmitters released at the synapse also bind and activate astrocytic metabotropic neurotransmitter receptors, such as mGluR and purinergic P2 receptors, which commonly induces astrocytic Ca2+ levels to rise. ③ Synaptically-evoked Ca2+ increases usually contribute, at least in part, to gliotransmitter release (glutamate, ATP, D-serine, GABA). These gliotransmitters interact with neuronal receptors at the pre- and post-synaptic elements, regulating synaptic activity and affecting neurotransmitter release.
FIGURE 2
FIGURE 2
Targeting astrocyte activity in CNS disease (figure represents a general synapse). ① In several brain diseases, the surface expression of glutamate transporters, such as GLT-1 and GLAST, is significantly decreased. This compromises glutamate uptake from the synaptic cleft, leading to excitotoxicity and neuronal death. ② Most neurodevelopmental diseases show decreased spine density and, in some cases, astrocyte-derived thrombospondin (TSP1) secretion was shown to be decreased. Since astrocyte stimulation via hM4Di has been shown to induce elevations in intracellular Ca2+ as well as TSP1 release in the dorsal striatum, this represents a potential approach to promote structural synapse formation in patients with neurodevelopmental diseases, but beneficial behavioral outcome is still to be established. ③ Neuroinflammation is a common hallmark of neurodegenerative diseases and astrocytes are known to secrete pro-inflammatory molecules which contribute to inflammation propagation. Interestingly, hM4Di-mediated astrocyte activation in the CA1 was reported to suppress inflammation, resulting in an improvement in cognitive function. ④ Selective hM4Di stimulation in striatal astrocytes was also found to phenocopy GPCR activation by increasing Ca2+ signalling, rescuing astrocytic functional impairments and synaptic dysfunction associated with Huntington’s disease. ⑤ Selective activation of hM3Dq was shown to increase Ca2+ levels in astrocytes in the cingulate cortex and hippocampus. Increased Ca2+ in cortical astrocytes rescued neuronal activity and protected against seizures and day/night hyperactivity associated with early Alzheimer’s disease. Additionally, the increased Ca2+ levels driven by hM4Di activation in the hippocampus are thought to lead to D-serine release and improved memory formation. Solid arrows indicate established/tested effects, while dashed lines represent circumstantial/hypothetical links.

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Grants and funding

This work was supported by the KU Leuven Research Council (C14/20/071) and the Research Foundation Flanders Belgium (FWO, G080821N) via research project funding. MGH is currently the ERA Chair (NCBio) at i3S Porto funded by the European Commission (H2020-WIDESPREAD-2018-2020-6; NCBio; 951923).

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