Astrocytic adenosine receptor A2A and Gs-coupled signaling regulate memory.
Astrocytes express a variety of G protein-coupled receptors and might influence cognitive functions, such as learning and memory.However, the roles of astrocytic Gs-coupled receptors in cognitive function are not known.Together, these findings establish a regulatory role for astrocytic Gs-coupled receptors in memory and suggest that AD-linked increases in astrocytic A2A receptor levels contribute to memory loss.
Affiliation: 1] Gladstone Institute of Neurological Disease, San Francisco, California, USA.  Department of Neurology, University of California, San Francisco, California, USA.
Astrocytes express a variety of G protein-coupled receptors and might influence cognitive functions, such as learning and memory. However, the roles of astrocytic Gs-coupled receptors in cognitive function are not known. We found that humans with Alzheimer's disease (AD) had increased levels of the Gs-coupled adenosine receptor A2A in astrocytes. Conditional genetic removal of these receptors enhanced long-term memory in young and aging mice and increased the levels of Arc (also known as Arg3.1), an immediate-early gene that is required for long-term memory. Chemogenetic activation of astrocytic Gs-coupled signaling reduced long-term memory in mice without affecting learning. Like humans with AD, aging mice expressing human amyloid precursor protein (hAPP) showed increased levels of astrocytic A2A receptors. Conditional genetic removal of these receptors enhanced memory in aging hAPP mice. Together, these findings establish a regulatory role for astrocytic Gs-coupled receptors in memory and suggest that AD-linked increases in astrocytic A2A receptor levels contribute to memory loss.
- Gene Expression Regulation/physiology*
- Memory, Long-Term/physiology*
- Receptor, Adenosine A2A/genetics/metabolism*
- Receptors, Serotonin, 5-HT4/genetics/metabolism*
- Signal Transduction/physiology*
- Alzheimer Disease/pathology
- Animals, Newborn
- Cytoskeletal Proteins/genetics/metabolism
- Exploratory Behavior/drug effects/physiology
- Glial Fibrillary Acidic Protein/genetics/metabolism
- Maze Learning/physiology
- Mice, Inbred C57BL
- Mice, Transgenic
- Nerve Tissue Proteins/genetics/metabolism
- Recognition (Psychology)/drug effects/physiology
- Serotonin Antagonists/pharmacology
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Figure 6: Untreated young and aging GFAP-Rs1 mice have reduced memory(a–e) Untreated GFAP-tTA (Con) and GFAP-Rs1 mice were tested in the Morris water maze at 5–7 months of age. (a) Distance traveled to reach the platform during hidden platform training. Two-way ANOVA: F(4, 120) = 1.00, P = 0.41 for interaction effect. n = 16 mice per genotype. (b–d) Crossings of the target and equivalent non-target (other) locations during probe trials conducted at indicated times after training. Two-way ANOVA: (b) F(1, 60) = 0.37, P = 0.54 for interaction effect; (c) F(1, 59) = 2.48, P = 0.12 for interaction effect; (d) F(1, 57) = 7.92, P = 0.007 for interaction effect. n = 16 mice (b–c) and 15 mice (d) per genotype. Student’s t-test with Welch’s correction (Target vs. Other of matching genotype): (b) P = 0.0028 (Con), P = 0.0055 (GFAP-Rs1); (c) P = 0.0009 (Con), P = 0.076 (GFAP-Rs1); (d) P = 0.018 (Con), P = 0.28 (GFAP-Rs1). (e) Representative swim paths during the probe on day 8. Squares indicate target (red) and non-target (black) locations. (f) Rs1 mRNA levels in the cortex of young (2–4 months) and old (19–22 months) GFAP-Rs1 mice. Actb (β-actin) mRNA served as a loading control. Rs1/Actb ratios were normalized to the average ratio in young mice. Student’s t-test with Welch’s correction (Young vs. Old): P = 0.012. n = 10 young and 9 old GFAP-Rs1 mice. (g) Representative photomicrographs of pERK-immunostained (green) brain sections of young (2–5 months) and old (18– 22 months) GFAP-tTA (Con) and GFAP-Rs1 mice. n = 2 Young Con, 2 Young GFAP-Rs1, 2 Old Con, and 8 old GFAP-Rs1 mice. Scale bar: 100 μm. Inset (i): magnified view of the boxed region coimmunostained for glutamine synthetase (GS, red) and cell nuclei (blue). DGmol: dentate gyrus (DG) molecular layer; DGgl: dentate gyrus granular layer. (h) Levels of phosphorylated ERK in different brain regions of GFAP-tTA (Con) and GFAP-Rs1 mice. Densitometric quantification of pERK/tERK ratios in the DG and CA regions of the hippocampus and in the cortex (CTX) of young (2–5 months) and old (18–22 months) mice. pERK/tERK ratios were normalized to the average ratio within each brain region of Con mice. n = 6 Young/Con/DG, 6 Young/GFAP-Rs1/DG, 6 Young/Con/CA, 6 Young/GFAP-Rs1/CA, 5 Old/Con/DG, 11 Old/GFAP-Rs1/DG, 6 Old/Con/CA, 10 Old/GFAP-Rs1/CA, 6 Old/Con/CTX, and 5 Old/GFAP-Rs1/CTX mice. Student’s t-test with Welch’s and FDR corrections (Con vs. GFAP-Rs1): P = 0.0138 (DG), P = 0.043 (CA), P = 0.018 (CTX). (i–k) Untreated GFAP-tTA (Con) and GFAP-Rs1 mice were tested in the Morris water maze at 14–18 months of age.(i) Distance traveled to reach the platform during hidden platform training. Two-way ANOVA: F(4, 192) = 0.60, P = 0.66 for interaction effect. n = 28 Con and 22 GFAP-Rs1 mice. (j–k) Crossings during probe trials conducted at indicated times after completion of training. Two-way ANOVA: (j) F(1, 46) = 0.25, P = 0.62 for interaction effect. n = 11 Con and 13 GFAP-Rs1 mice. (k) F(1, 90) = 12.27, P = 0.0007 for interaction effect. n = 26 Con and 21 GFAP-Rs1 mice. Student’s t-test with Welch’s correction (Target vs. Other of matching genotype): (j) P < 0.0001 (Con), P = 0.0002 (GFAP-Rs1); (k) P = 0.0002 (Con), P = 0.616 (GFAP-Rs1). *P < 0.05, **P < 0.01, ***P < 0.001 (Student’s t-test with Welch’s correction). #P < 0.05, ##P < 0.01, ###P < 0.001 vs. Con (Bonferroni test). Values are means ± s.e.m.
In addition to its robust activation by synthetic ligands, Rs1 is known to exhibit ligand-independent constitutive Gs-coupled activity.31 Consistent with these reports, Rs1-expressing cultured mouse astrocytes showed increased basal intracellular cAMP and phosphorylated CREB levels, but no increases in basal intracellular calcium levels (Supplementary Fig. 8a–d). To test if this constitutive activity is sufficient to impair memory in GFAP-Rs1 mice, we tested untreated mice in the Morris water maze after increasing the delay between the completion of hidden platform training and the subsequent probe trials. Untreated GFAP-Rs1 mice learned well and showed robust memory in a probe trial conducted one day post-training (Fig. 6a–b). However, GFAP-Rs1 mice lost target preference by 8 days post-training (Fig. 6c–e). In contrast, control mice showed significant target preference on day 8 and lost target preference by day 15 post-training (Fig. 6d–e and Supplementary Fig. 8e). Untreated GFAP-Rs1 mice also showed reduced performance in a novel object-recognition task (Supplementary Fig. 8f–g), which depends on the ability to recall familiar objects, suggesting that the reduced memory in GFAP-Rs1 mice is not restricted to the Morris water maze or spatial memory tasks.