Social interaction and memory are crucial for healthy relationships and overall well-being. Social memory deficits are hallmarks of numerous neurodevelopmental and neuropsychiatric disorders, including autism and schizophrenia. The hippocampus plays a critical role in social memory, with specific subregions implicated in social recognition memory storage. However, the contributions of glial cells, like astrocytes, and the underlying molecular mechanisms remain largely unknown. Astrocytes are increasingly recognized for their active roles in brain development and function, regulating various synaptic processes, including synaptogenesis, ion homeostasis, synaptic transmission, and plasticity. Perturbations in astrocytic function can impact LTP, a crucial form of synaptic plasticity associated with learning and memory. While astrocytes' roles in spatial and fear memory are established, their involvement in social memory is less understood. Astrocytes exhibit activity-dependent Ca2+ responses, releasing gliotransmitters such as glutamate, ATP/adenosine, and D-serine, which modulate synaptic signaling and plasticity. Neuroligins (NLGs), cell adhesion proteins, are implicated in several neurodevelopmental and neuropsychiatric disorders. While neuronal NLGs' roles in synapse maturation and plasticity are well-studied, the functions of NLGs in astrocytes and their contribution to learning and memory remain largely unexplored. This research focuses on manipulating NLG3 in adult ventral hippocampal astrocytes to determine its role in regulating synaptic plasticity and social memory.

Existing literature establishes the critical role of the hippocampus, particularly specific subregions like dorsal CA2, ventral CA1, and dorsal DG, in social recognition memory. Studies in rodents have highlighted the importance of these areas in social memory storage. However, the roles of other cell types, particularly astrocytes, in these processes remain under-investigated. While the importance of astrocytes in synaptic plasticity and various forms of memory (spatial and fear) is recognized, their contribution to social memory and related disorders needs further exploration. The existing literature on neuroligins primarily focuses on their neuronal roles in synapse development and function, with limited understanding of their astrocytic roles. Studies have shown that NLGs are highly expressed in astroglia and are required for astrocyte morphogenesis and synapse development during development; however, the role in mature synapses and in adult memory processes remains unknown. Therefore, this study aims to bridge this gap in knowledge by focusing on the astrocytic role of NLG3 in adult social memory.

The study used NLG3 floxed mice crossed with hGFAP-CreERT2 mice to generate astrocyte-specific NLG3 knockout (GFAP-NLG3 KO) mice. Tamoxifen was administered after postnatal development to avoid developmental effects. Behavioral tests, including the three-chamber social test, open field test, elevated plus maze test, novel object recognition test, buried food test, and contextual fear memory test were conducted to assess social memory, locomotor activity, anxiety, and olfactory function. To determine the specific hippocampal region involved, AAV viruses expressing GFAP-EGFP-T2A-Cre were injected into either the dorsal or ventral hippocampus of adult NLG3fl/fl mice. Western blotting confirmed NLG3 deletion. To investigate the mechanism, Ca2+ imaging using GCaMP6f in hippocampal slices and in vivo fiber photometry were used to assess astrocytic activity in response to high-frequency stimulation (HFS) and social interaction. Western blotting and immunostaining assessed the expression levels of EAAT2, EAAT1, EAAT3, purinergic receptors (P2X, P2Y1), mGluR5, and Kir4.1. Electrophysiology recordings examined basal synaptic transmission (input/output curves, paired pulse facilitation), mEPSCs, AMPAR/NMDAR EPSC ratios, and LTP. The DREADD system was employed to manipulate astrocytic activity chemogenetically. Microdialysis measured extracellular adenosine levels. Finally, local drug injections (TFB-TBOA, CGS 21680, SCH 58261) were used to manipulate glutamate transporters and adenosine signaling.

Astrocytic deletion of NLG3 specifically impaired social recognition memory in male mice without affecting sociability or other behavioral parameters. This impairment was observed only when NLG3 was deleted in the ventral hippocampus, suggesting region-specific involvement. The deletion of astrocytic NLG3 impaired activity-dependent astrocytic Ca2+ signals in response to both HFS and social interaction, both in vitro and in vivo. Increased EAAT2 expression was found in GFAP-NLG3 KO mice, leading to reduced extracellular glutamate levels. Chemogenetic activation of astrocytes rescued social memory deficits, indicating that impaired astrocytic activity contributes to the observed memory impairment. LTP was abolished in GFAP-NLG3 KO mice, but restored by inhibiting EAAT2. Adenosine, acting through A2a receptors, was crucial for rescuing LTP deficits, suggesting that impaired adenosine signaling contributes to the social memory deficits. Systemic administration of the A2a receptor agonist CGS 21680 rescued the social memory deficits in GFAP-NLG3 KO mice. Deleting neuronal A2a receptors also impaired social memory and LTP, confirming the importance of neuronal A2a receptors in these processes. The study also found that NLG3 regulates EAAT2 protein expression via suppression of a mTOR-dependent translational mechanism.

This study provides compelling evidence that astrocytic NLG3 in the ventral hippocampus is critical for social recognition memory. The findings demonstrate that astrocytic NLG3 regulates activity-dependent astrocyte activation, which is essential for social memory. The impaired astrocytic activation in GFAP-NLG3 KO mice is linked to increased EAAT2 expression and subsequent reduction of extracellular glutamate. The adenosine/A2a signaling pathway plays a central role in mediating the effects of astrocytic NLG3 on LTP and social memory. The impaired glutamate signaling appears to be responsible for impaired astrocyte activation, leading to a lack of adenosine release and reduced A2a receptor activation, which in turn causes memory deficits. These findings highlight the importance of astrocytes in social memory and identify potential therapeutic targets for social deficits in neurodevelopmental and neuropsychiatric disorders.

This study demonstrates a crucial role for astrocytic NLG3 in regulating social memory and synaptic plasticity in male mice. The findings reveal a novel mechanism involving the modulation of EAAT2 expression, glutamate homeostasis, and adenosine/A2a signaling. These results have implications for understanding the pathophysiology of social memory deficits in neurodevelopmental and neuropsychiatric disorders and suggest potential therapeutic avenues by targeting astrocyte activity, glutamate transporters, and adenosine signaling pathways. Future studies could explore the sex-specific effects, the precise mechanisms of NLG3 regulation of EAAT2, and the translation of these findings into potential therapeutic interventions.

The study utilized a GFAP-CreERT2 mouse line, which might not be entirely astrocyte-specific, potentially affecting other cell types. The study focused on adult mice; therefore, the findings may not fully reflect the role of astrocytic NLG3 in developmental processes. The study primarily investigated male mice, and thus the findings may not be generalizable to female mice. The mechanisms of NLG3 regulation of EAAT2 translation needs further investigation.