Scientists are discovering that different serotonin-producing neurons matter for health and disease
For decades, researchers often referred to serotonergic neurons as a single, uniform group. These brain cells produce the neurotransmitter serotonin, a molecule that influences mood, appetite, breathing, body temperature and many other physiological functions. Recent work, however, shows that serotonergic neurons are far from identical. Their molecular profiles and developmental origins vary, and those differences appear to be important for specific behaviors and disease processes.
Earlier research from the laboratory of Harvard Medical School genetics professor Susan Dymecki established that a specific subset of serotonergic neurons in mice are uniquely responsible for increasing breathing rate in response to elevated carbon dioxide. Building on that discovery, Dymecki and colleagues have now performed a systematic molecular characterization of the entire serotonergic system in mice, identifying distinct subtypes and linking those subtypes to unique properties and functions.
Published in Neuron, the new study shows that serotonergic neurons segregate into at least six major molecular subtypes, each distinguished by characteristic patterns of hundreds of genes. These subtypes not only differ in gene expression but also vary in developmental lineage, anatomical location within the brainstem, the arrays of receptors displayed on their surfaces and their electrical firing behaviors. In many cases, these subtype distinctions map to different roles in physiology and behavior.
“This work reveals how diverse serotonin neurons are at the molecular level, which may help explain how, collectively, they carry out so many distinct functions,” said Benjamin Okaty, a postdoctoral researcher in the Dymecki lab and co-first author on the paper. Susan Dymecki added that identifying the molecular players that define each subtype provides a practical handle for studying what each cell type does and for selectively manipulating specific subtypes therapeutically.
The study combined several complementary approaches: intersectional genetic fate mapping to trace developmental origins, cell sorting, genome-wide RNA sequencing at both population and single-cell resolution, electrophysiological recordings and targeted interventions in behaving animals. These multi-scale methods allowed the researchers to derive organizing principles for the serotonin system, define subtype-specific gene signatures, predict functional specializations and then test those predictions experimentally.
One important finding is that a serotonergic neuron’s molecular identity and function depend not only on its adult position within the raphe region of the brainstem but also on its progenitor lineage during embryonic development. “We demonstrate that molecular phenotypes of these neurons track closely with their developmental origin, with anatomy making notable additional contributions,” Dymecki said. That linkage between lineage and mature phenotype offers new insight into how molecular diversity arises and why different serotonergic cells play different roles in health and disease.
Although the experiments were carried out in mice, Dymecki and colleagues are optimistic their conclusions will extend to humans because the serotonergic system resides in a highly conserved brain region across vertebrates. If similar molecular subtypes exist in human tissue, researchers could begin to investigate whether particular serotonergic subtypes contribute to conditions such as sudden infant death syndrome (SIDS), sleep-disordered breathing, autism, anxiety or chronic pain.
The practical implications are broad. Knowing subtype-specific markers could improve the interpretation of stem cell differentiation protocols: which serotonergic subtype is produced in vitro, and can protocols be adjusted to generate specific subtypes reliably? The subtype framework could also guide biomarker development and promote more targeted therapies that modulate only the relevant serotonergic population, reducing side effects associated with broad pharmacological manipulation of serotonin signaling.
The authors emphasize that this study is a resource as well as a discovery: it classifies molecular diversity across the serotonergic system, identifies subtype markers and organizing principles, and links molecular profiles to cellular physiology and animal behavior. Using electrophysiology, subtype-specific silencing and conditional gene knockout, the team confirmed that molecularly defined subtypes are functionally distinct.
Funding: This work was supported by grants from the National Institutes of Health (R01 DA034022, P01 HD036379, T32 GM007753, R21 MH083613, R21 DA023643), the American SIDS Institute, a Harvard Stem Cell Institute seed grant, a NARSAD Distinguished Investigator Grant from the Brain & Behavior Foundation, and Harvard’s Blavatnik Biomedical Accelerator, which supports translation of early-stage biomedical technologies toward clinical application. Harvard’s Office of Technology Development has filed a patent application related to aspects of the technology.
Source: David Cameron – Harvard Medical School
Image Credit: Mallory Rice
Original Research: Abstract for “Inappropriate Neural Activity during a Sensitive Period in Embryogenesis Results in Persistent Seizure-like Behavior” by Benjamin W. Okaty, Morgan E. Freret, Benjamin D. Rood, Rachael D. Brust, Morgan L. Hennessy, Danielle deBairos, Jun Chul Kim, Melloni N. Cook, and Susan M. Dymecki in Neuron. Published online November 5, 2015. doi:10.1016/j.neuron.2015.10.007
Abstract
Multi-Scale Molecular Deconstruction of the Serotonin Neuron System
Highlights
• RNA sequencing of 5HT neurons across anatomy and developmental sublineages at population and single-cell scales
• Unbiased analyses reveal 5HT neuron subtypes and organizing principles
• Differential gene expression predicts subtype-specific functions and associations with disease
• In vitro drug responses, sensorimotor gating, and distinct behaviors map to different subtypes
Summary
Serotonergic (5HT) neurons modulate a wide range of behaviors and physiological processes and are implicated in numerous clinical disorders. Molecular diversity among 5HT neurons appears extensive and likely underlies their functional specialization, but a comprehensive, multi-scale molecular characterization linked to cellular phenotypes has been lacking. This study combines intersectional fate mapping, neuron sorting and genome-wide RNA sequencing to deconstruct the mouse 5HT system from anatomical regions to genetic sublineages to individual cells. Unbiased analyses uncover principles of system organization, define 5HT neuron subtypes, identify sets of differentially expressed genes that distinguish subtypes, and generate predictions of subtype-specific roles. Electrophysiology, subtype-specific silencing and conditional gene knockout confirm that molecularly defined 5HT neuron subtypes are functionally distinct. Together, these data classify molecular diversity across the 5HT system and reveal subtype markers and functions with potential disease relevance.
“Inappropriate Neural Activity during a Sensitive Period in Embryogenesis Results in Persistent Seizure-like Behavior” by Benjamin W. Okaty et al., Neuron. Published online November 5, 2015. doi:10.1016/j.neuron.2015.10.007