A serotonergic axon-cilium synapse drives nuclear signaling to maintain chromatin accessibility

Abstract

Chemical synapses between axons and dendrites mediate much of the brain's intercellular communication. Here we describe a new kind of synapse - the axo-ciliary synapse - between axons and primary cilia. By employing enhanced focused ion beam - scanning electron microscopy on samples with optimally preserved ultrastructure, we discovered synapses between the serotonergic axons arising from the brainstem, and the primary cilia of hippocampal CA1 pyramidal neurons. Functionally, these cilia are enriched in a ciliary-restricted serotonin receptor, 5-hydroxytryptamine receptor 6 (HTR6), whose mutation is associated with learning and memory defects. Using a newly developed cilia-targeted serotonin sensor, we show that optogenetic stimulation of serotonergic axons results in serotonin release onto cilia. Ciliary HTR6 stimulation activates a non-canonical GNAQ/11-RhoA pathway. Ablation of this pathway results in nuclear actin and chromatin accessibility changes in CA1 pyramidal neurons. Axo-ciliary synapses serve as a distinct mechanism for neuromodulators to program neuron transcription through privileged access to the nuclear compartment.

Structure, interaction, and nervous connectivity of beta cell primary cilia

“…Finally, the observation of synapses between islet cell cilia and innervating axons points to signaling functions of primary cilia beyond the local islet context. To our knowledge these kinds of connections (serotonergic cilia-axon synapses) have only been described in the brain (45). Thus, our reconstructions are the first to document this phenomenon elsewhere in the body.…”

“…Research has so far focused on the role of islet cell cilia in paracrine signaling between endocrine cells. Recently, however, primary cilia have been shown to form synapses with neurons in brain tissue (45). While screening the aforementioned 8 nm isotropic FIB-SEM dataset of mouse pancreas tissue (44) we found several nerve fibers contacting the islets (Fig.…”

Volume electron microscopy

“…In the mouse brain, ssTEM showed that ultrastructural features of the blood brain barrier, including increased pericyte-brain endothelial cell contact, are altered during aging 233 , and single axons form clustered compound multi-synaptic contacts of distinct morphologies with individual neurons, thus maximizing their influence on the postsynaptic neuron 139 . FIB-SEM has been used to demonstrate that some primary cilia form serotonergic axo-ciliary synaptic contacts in the hippocampus 234 . SBF-SEM was used to reconstruct two individual cells from a large dataset of a leech ganglion, discovering new connections 235 .…”

Techniques for in vivo serotonin detection in the brain: State of the art

“…As such, a variety of designs incorporating distinct scaffolds of GPCR molecules have been used to develop a range of serotonin sensors. For example, a cilia‐targeted serotonin sensor (GRAB‐HTR6‐PM) based on the 5‐HT 6 receptor was developed to observe serotonin release onto cilia by activation of serotonergic axons (Sheu et al, 2021). The sensor was validated in HEK cells, and HaloTag was added to improve the sensor for visualizing cilia (HTR6‐GRAB‐cilia).…”