Researchers at Johns Hopkins University School of Medicine have created a single-nucleus RNA-sequencing resource to generate molecular signatures of cell types in five key regions of the human brain that form the reward circuitry.
Until now, single-cell gene expression technologies—a powerful tool to study cell types in the human brain—have mostly focused on cells in the cerebral cortex, the outer layer of the brain responsible for thinking and sensing, or the hippocampus, a C-shaped structure important for memory.
The current study sequences over 70,000 cellular nuclei that constitute junctions in the brain’s reward circuit and play a key role in addiction and neuropsychiatric disorders. These brain regions are the nucleus accumbens, amygdala, subgenual anterior cingulate cortex, hippocampus, and dorsolateral prefrontal cortex.
Based on their molecular classification, the team identifies new sub-populations of interneurons and medium spiny neurons (MSNs) in the nucleus accumbens and characterizes neurons in the amygdala that express the inhibitory neurotransmitter, GABA (gamma-amino butyric acid).
Through joint analyses across 107 types of cells, the authors uncover subcellular patterns of transcriptomic dynamics. For instance, in the medium spiny neurons of the nucleus accumbens, the scientists identify sub-populations that express the dopamine receptor D1 versus D2. The authors also map genetic risk factors associated with neuropsychiatric disorders and addiction to the D1 and D2 medium spiny neurons of the nucleus accumbens.
The study conducted by neuroscientists at the Lieber Institute for Brain Development (LIBD) at the Johns Hopkins Medical Campus in Baltimore, is published this week in an article in the journal Neuron titled, “Single-nucleus transcriptome analysis reveals cell-type-specific molecular signatures across reward circuitry in the human brain.” The researchers say these findings provide a strong foundation for new approaches to neuropsychiatric treatment.
“This study is the first, to our knowledge, to systematically profile and compare across multiple interconnected cortical and subcortical human brain areas, selected for their function and association with risk for neuropsychiatric disorders and addiction. We placed special emphasis on analyses in the nucleus accumbens and the amygdala, given their roles in emotional processing and reward signaling and the lack of any current human single nucleus RNA-seq reference data in these regions,” the authors note.
The team also evaluates cross-species conservation of neurons in the nucleus accumbens and amygdala between humans and rodents, focusing on comparisons of subsets of medium spiny neurons that play key roles in reward processing and addiction.
“Many studies in rodents have suggested that specific cell types are implicated in reward signaling associated with addiction and neuropsychiatric disorders. However, translating these findings into treatments targeted at treating human disorders has been hindered by not knowing the similarities and differences between human and rodent cell types in brain areas associated with reward and addiction,” says Keri Martinowich, PhD, associate professor of psychiatry and neuroscience at the LIBD and co-corresponding author on the paper.
“Our studies provide confidence in prioritizing specific cell types and identifying the molecular composition of these cell types in the human brain. This provides a strong foundation for future investigations targeting these cells. Our resource also provides one of the most complete sets of cell-type-specific gene expression for assessing the cellular specificity of human traits within and across brain regions.”
One of the major impediments in molecular studies on reward and motivation in the context of human addiction is that, since these are conducted on postmortem tissue of study-participants who had taken drugs during their lifetimes, it is not possible to discern whether changes in gene expression are the cause or the effect of addiction.
By integrating genetic studies for substance use and neuropsychiatric disorders, the authors delineate the cell-type association of the changes in gene expression of risk loci-associated genes, with several neuropsychiatric and substance use disorders. This highlights the importance of understanding cell-type- and region-specific gene expression in the human brain for the treatment of addiction and neuropsychiatric disorders since changes in gene expression identified in patient postmortem tissue is difficult to interpret without an in-depth understanding of normal gene expression in the neurons of the regions concerned.
This resource constitutes a major step toward constructing a single-nucleus transcriptomic atlas of the entire human brain and shows how a structural and functional understanding of diverse cell populations is critical in gaining insights into biology and disease.
“While our study identified the molecular composition of many key cell populations, the positioning of these cell types within the spatial topography of these brain regions is not known. Given the close relationship between brain structure and function, precisely assigning gene expression to the spatial coordinates of individual cell populations within the cytoarchitecture would significantly advance our understanding of how dysregulation in these areas contributes to addiction and neuropsychiatric disorders associated with altered reward signaling,” says Martinowich.
“The next steps include expanding the studies to investigate spatial gene expression, which will allow us to map the locations of key cell types to precise locations within reward circuits.”