How Do Cortical Excitatory Neurons Terminate Their Migration at the Right Place? Critical Roles of Environmental Elements

Novel insights into the regulation of neuron migration by axon guidance proteins

Neural circuit development requires precisely coordinated guidance of migrating neurons to their targets within the nervous system. A diverse array of molecular cues has been implicated in neuron migration, including signals originally identified for their ability to dictate the trajectories of growing axons, i.e. axon guidance proteins. These proteins are now known to have pleiotropic effects affecting different stages of neuron migration, from promoting cell mobility to acting as stop signals. In this review, we discuss recent advances in our understanding of how canonical axon guidance proteins influence migrating neurons with a particular focus on recent insights into how neuron migration is controlled in the GnRH system and cortex, and the multifunctional role of Netrin-1. At the molecular level, tight control of receptor expression and crosstalk, and interactions with the extracellular matrix have recently been implicated in neuron migration control.

Serotonin signaling at cilia synapses

Serotonin (5-HT) is a key neuromodulator influencing cognition, mood, and sleep, yet the structural and molecular mechanisms of serotonergic signaling remain incompletely understood. Recent findings have identified a novel mode of serotonergic transmission via axo-ciliary synapses, where serotonergic axons directly contact the primary cilia of hippocampal pyramidal neurons. These synapses facilitate localized 5-HT release, activating ciliary 5-HT6R receptors and triggering intracellular signaling cascades distinct from conventional synaptic mechanisms. This pathway leads to chromatin remodeling and transcriptional regulation, providing a direct link between serotonergic signaling and neuronal gene expression. Additional volume electron microscopy studies have revealed the prevalence of axo-ciliary contacts across different brain regions, suggesting a broad role in neuromodulation. Further investigation into axo-ciliary synapses may provide critical insights into serotonergic function and its implications for neuropsychiatric disorders.

Critical Role of the Presynaptic Protein CAST in Maintaining the Photoreceptor Ribbon Synapse Triad

The cytomatrix at the active zone-associated structural protein (CAST) and its homologue, named ELKS, being rich in glutamate (E), leucine (L), lysine (K), and serine (S), belong to a family of proteins that organize presynaptic active zones at nerve terminals. These proteins interact with other active zone proteins, including RIMs, Munc13s, Bassoon, and the β subunit of Ca²⁺ channels, and have various roles in neurotransmitter release. A previous study showed that depletion of CAST/ELKS in the retina causes morphological changes and functional impairment of this structure. In this study, we investigated the roles of CAST and ELKS in ectopic synapse localization. We found that the involvement of these proteins in ribbon synapse distribution is complex. Unexpectedly, CAST and ELKS, in photoreceptors or in horizontal cells, did not play a major role in ribbon synapse ectopic localization. However, depletion of CAST and ELKS in the mature retina resulted in degeneration of the photoreceptors. These findings suggest that CAST and ELKS play critical roles in maintaining neural signal transduction in the retina, but the regulation of photoreceptor triad synapse distribution is not solely dependent on their actions within photoreceptors and horizontal cells.