Induction of astrocytic Slc22a3 regulates sensory processing through histone serotonylation

Neuronal activity drives alterations in gene expression within neurons, yet how it directs transcriptional and epigenomic changes in neighboring astrocytes in functioning circuits is unknown. We found that neuronal activity induces widespread transcriptional up-regulation and down-regulation in astrocytes, highlighted by the identification of Slc22a3 as an activity-inducible astrocyte gene that encodes neuromodulator transporter Slc22a3 and regulates sensory processing in the mouse olfactory bulb. Loss of astrocytic Slc22a3 reduced serotonin levels in astrocytes, leading to alterations in histone serotonylation. Inhibition of histone serotonylation in astrocytes reduced the expression of γ-aminobutyric acid (GABA) biosynthetic genes and GABA release, culminating in olfactory deficits. Our study reveals that neuronal activity orchestrates transcriptional and epigenomic responses in astrocytes while illustrating new mechanisms for how astrocytes process neuromodulatory input to gate neurotransmitter release for sensory processing. Description Editor’s summary Astrocytes can respond to input from neuromodulators. However, the roles for neuromodulators in astrocytes in functioning brain circuits are poorly defined. Sardar et al. discovered that loss of the astrocytic neuromodulator transporter Slc22a3 resulted in reduced levels of intracellular serotonin and impaired calcium ion–mediated serotonin responses (see the Perspective by Vasile and Rouach). These deficits in serotonin manifest in astrocytes with a Slc22a3 deletion that have reduced histone serotonylation. The expression of genes for the synthesis of γ-aminobutyric acid (GABA) is regulated by histone serotonylation. These changes in GABA gene expression are accompanied by reduced GABA release from astrocytes that lack Slc22a3. Inhibition of histone serotonylation in olfactory bulb astrocytes leads to reduced GABA release and impaired olfactory sensory processing. —Peter Stern Manipulating neuronal activity influences astrocyte transcriptomes in mice. INTRODUCTION Astrocytes are intimately associated with neurons and participate in a host of essential roles that facilitate synaptic transmission and circuit function. In neurons, heightened activity induces the expression of “immediate early genes,” which are predominately transcription factors that modify gene expression programs and activity-dependent epigenomic states, ultimately regulating circuit activity, plasticity, and associated behavioral outputs. However, whether heightened neuronal activity induces an analogous immediate early gene–like response in mature astrocytes and how this sculpts astrocytic transcriptional and epigenomic responses to regulate circuit function remain unclear. RATIONALE Prior studies have shown that non-neuronal cells undergo transcriptional changes in response to neuronal activity. A series of recent studies demonstrated that astrocytic transcription factors regulate region specific–circuits in the adult brain, indicating that diverse transcriptional states in astrocytes can influence neuronal activity. Therefore, we used astrocyte-specific transcription factors to interrogate drivers of neuronal activity–dependent changes in astrocytes. This revealed astrocyte transcription factor targets that are critical for maintaining astrocyte-neuron communication, and, using a model of olfactory processing in combination with an astrocyte conditional knockout mouse model, we further investigated how an astrocytic activity–dependent target regulates epigenomic states in astrocytes to regulate animal behavior. RESULTS We show that neuronal activity induces widespread transcriptional changes in astrocytes, and, in the olfactory bulb, these transcriptional changes are driven by alterations in DNA binding of the transcription factor Sox9. Using Sox9 as a molecular entry point, we screened through neuronal activity–dependent targets in astrocytes, which led to the identification of Slc22a3 as an activity-inducible astrocyte gene that encodes neuromodulator transporter Slc22a3, which regulates sensory processing in the olfactory bulb. Region-specific and cell type–specific deletion of olfactory bulb astrocytic Slc22a3 led to defects in olfactory bulb circuits and odor processing, implying that the loss of astrocytic Slc22a3 affects astrocyte-neuron communication in the olfactory bulb. Loss of astrocytic Slc22a3 led to reduced astrocyte morphological complexity and diminished calcium activity in the presence of neurotransmitters and neuromodulators. Mechanistically, Slc22a3 regulates the transport of serotonin into olfactory bulb astrocytes and coordinates histone serotonylation (H3-5HT) to regulate the expression of astrocytic γ-aminobutyric acid (GABA)–associated genes and olfactory behaviors. Finally, attenuation of astrocytic H3-5HT in the olfactory bulb was achieved using mutant histone variant 3.3 (H3.3Q5A), which was compared against control (H3.3). This led to reduced expression of astrocytic GABA biosynthetic genes and astrocytic tonic GABA release, culminating in olfactory deficits. CONCLUSION Our study reveals how neuronal activity orchestrates transcriptional responses in astrocytes and identifies the astrocyte transcription factor Sox9 as a mediator of these responses. We identify Slc22a3 as an odor-responsive, astrocyte gene in the olfactory bulb that encodes neuromodulator transporter Slc22a3 and is required to maintain astrocyte-neuron communication and olfactory sensory processing. Mechanistically, Slc22a3 uses epigenomic processes that involve the transport of the monoamine neuromodulator serotonin into olfactory bulb astrocytes, which subsequently regulates gene expression through histone serotonylation. Taken together, these results identify new mechanisms for how astrocytes integrate neuromodulator signaling to gate neurotransmitter release for sensory processing. Neuronal activity–dependent astrocyte transcriptional and epigenomic changes control olfactory processing. Neuronal activity induces widespread transcriptional changes in astrocytes (1), which is exemplified by Sox9 DNA-binding changes after odor-evoked neuronal activation (2). Slc22a3 (3) represents a Sox9-regulated and activity-dependent olfactory bulb target that controls astrocytic serotonin (5HT) levels to coordinate histone serotonylation (4), a previously uncharacterized epigenomic mediator of astrocytic GABA release that ultimately affects olfactory processing (5). GFP, green fluorescent protein; OB, olfactory bulb; ChIP-seq, chromatin immunoprecipitation coupled with next-generation sequencing.