Project description:The conversion of cell fates is controlled by hierarchical gene regulatory networks (GRNs) that induce remarkable alterations of cellular and transcriptome states. The identification of key regulators within these networks from myriad of candidate genes, however, poses a major research challenge. Here we present Convert-seq, combining single-cell RNA sequencing (scRNA-seq) and pooled (mutiplexed) ectopic gene expression with a new strategy to discriminate sequencing reads derived from exogenous and endogenous transcripts. We demonstrate Convert-seq by associating hundreds of single cells at multiple time-points during direct conversion of human fibroblasts to induced neurons (iN) with exogenous and endogenous transcriptional signatures. Convert-seq simultaneously identified GRNs that modulate the emergence of parallel developmental trajectories during iN conversion and predicted combinatorial interactions of exogenous transcription factors controlling iN subtype specification. Validation of iN subtypes generated by novel combinations of exogenous transcription factors establish Convert-seq as a broadly applicable workflow to rapidly identify key transcription factors and GRNs orchestrating the direct conversion of virtually any cell type.
Project description:The hypothalamus is one of the most complex brain structures whose development involves a plastic process of neuronal fate specification. Progress has been made to decipher the gene regulatory programs that are responsible for hypothalamus development; however, the molecular developmetal trajectory of hyothalamus is largely unknown. To understand how pre- and postmitotic transcriptional programs interact and coordinate to endow neuronal cell subtypes with their characteristic properties during hypothalamic development, we performed single-cell RNA sequencing (scRNA-seq) on single cells derived from Rax+ hypothalamic neuroepithelium at four critical developmental points during hypothalamic development. Our single-cell analysis provides a developmental landscape of mouse hypothalamus. We show that while the fate of radial glial cells (RGCs) is predetermined before differentiation but lack spatial code to distinguish from each other, different clusters of intermediate progenitors (IPCs) emerge to display diversifying fates and subdivide hypothalamic primordium into distinct spatially-restricted progenitor domains. We further characterize the maturation dynamics of hypothalamic neurons and suggest that immature neurons could evolve into multiple peptidergic neuronal subtypes. Finally, we identify sets of transcription factors (TFs) serving as regulons to determine the fate of diverse GABAergic and Glutamatergic neurons in hypothalamus. Together, our study offers a single-cell transcriptional framework for the hypothalamus developmental trajectory and propose a cascade diversifying model to deconstruct the origin of neuronal diversity in hypothalamus.
Project description:Deciphering how neuronal diversity is established and maintained requires a detailed knowledge of neuronal gene expression throughout development. In contrast to mammalian brains, the large neuronal diversity of the Drosophila optic lobes and its connectome are almost completely characterized. However, a molecular characterization of this diversity, particularly during development, has been lacking. We present novel insights into brain development through a nearly exhaustive description of the transcriptomic diversity of the optic lobes. We acquired the transcriptome of 275,000 single-cells at adult and five pupal stages, and developed a machine learning framework to assign them to almost 200 cell-types at all timepoints. We discovered two large neuronal populations that wrap neuropils during development but die just before adulthood, as well as neuronal subtypes that partition dorsal and ventral visual circuits by differential Wnt signaling throughout development. Moreover, we showed that neurons of the same type but produced days apart synchronize their transcriptomes shortly after being produced. We also resolved during synaptogenesis neuronal subtypes that converge to indistinguishable transcriptomic profiles in adults while greatly differing in morphology and connectivity. Our datasets almost completely account for the known neuronal diversity of the optic lobes and serve as a paradigm to understand brain development across species.
Project description:Neuronal diversity is a defining feature of the mammalian brain deemed necessary for realizing the complex function of the nervous system. In order to begin to understand the transcriptional basis of this diversity, we collected more than 170 neuronal and non-neuronal cell type-specific transcriptomes defined operationally by transgenic mouse lines and anatomical regions. The dataset indicates that the genes specifically expressed in neuronal cell types are biased toward long genes. We revealed that these long genes have higher capacities to be differentially expressed between cell types and thus assume an important role in diversification of the neuronal transcriptomes. Since mobile element insertions are the main cause of the gene elongations, we propose that exaptation of the inserted mobile elements significantly contributed to the neuronal diversity.
Project description:Deciphering how neuronal diversity is established and maintained requires a detailed knowledge of neuronal gene expression throughout development. In contrast to mammalian brains, the large neuronal diversity of the Drosophila optic lobes and its connectome are almost completely characterized. However, a molecular characterization of this diversity, particularly during development, has been lacking. We present novel insights into brain development through a nearly exhaustive description of the transcriptomic diversity of the optic lobes. We acquired the transcriptome of 275,000 single-cells at adult and five pupal stages, and developed a machine learning framework to assign them to almost 200 cell-types at all timepoints. We discovered two large neuronal populations that wrap neuropils during development but die just before adulthood, as well as neuronal subtypes that partition dorsal and ventral visual circuits by differential Wnt signaling throughout development. Moreover, we showed that neurons of the same type but produced days apart synchronize their transcriptomes shortly after being produced. We also resolved during synaptogenesis neuronal subtypes that converge to indistinguishable transcriptomic profiles in adults while greatly differing in morphology and connectivity. Our datasets almost completely account for the known neuronal diversity of the optic lobes and serve as a paradigm to understand brain development across species.
Project description:How our brain generates diverse neuron types that assemble into precise neural circuits remains unclear. Using Drosophila lamina neuron types (L1-L5), we show that the primary homeodomain transcription factor (HDTF) Brain-specific homeobox (Bsh) is initiated in progenitors and maintained in L4/L5 neurons to adulthood. Bsh activates secondary HDTFs Ap (L4) and Pdm3 (L5) and specifies L4/L5 neuronal fates while repressing the HDTF Zfh1 to prevent ectopic L1/L3 fates (control: L1-L5; Bsh-knockdown: L1-L3), thereby generating lamina neuronal diversity for normal visual sensitivity. Subsequently, in L4 neurons, Bsh and Ap function in a feed-forward loop to activate the synapse recognition molecule DIP-β, thereby bridging neuronal fate decision to synaptic connectivity. Expression of a Bsh:Dam specifically in L4 reveals Bsh binding to the DIP-β locus and additional candidate L4 functional identity genes. We propose that HDTFs function hierarchically to coordinate neuronal molecular identity, circuit formation, and function. Hierarchical HDTFs may represent a conserved mechanism for linking neuronal diversity to circuit assembly and function.
Project description:Dendritic and axonal morphology reflects the input and output of neurons and is a defining feature of neuronal types, yet our knowledge of the diversity of neuronal morphology, in particular distal axon projection patterns, remains limited. To systematically obtain complete single neuron morphology on a brain-wide scale, we established a platform with five major components: sparse labeling, whole-brain imaging, reconstruction, registration, and classification. We fully reconstructed 1,741 neurons from cortex, claustrum, thalamus, striatum and other brain regions. We identify 11 major projection neuron types with distinct morphological features and corresponding transcriptomic identities. We further reveal extensive morphological diversity within each of these major types, some of which cluster into more refined morphological subtypes. We analyze this diversity at different levels following a set of generalizable organizational rules governing long-range axonal projections, including molecular correspondence, divergent or convergent projection, axon termination pattern, regional specificity, topography, and individual cell variability. We illustrate how these rules manifest in different projection neuron types. Although clear concordance with transcriptomic profiles is evident at major projection type level, fine-grained morphological diversity often does not readily correlate with transcriptomic subtypes derived from unsupervised clustering, highlighting the need for single-cell level cross-modality studies. Overall, our study provides a systematic demonstration of the crucial need for quantitative description of complete single cell anatomy in cell type classification, as the cell type-specific morphological diversity reveals a plethora of ways different cell types and individual neurons may contribute to the function of their respective circuits.
Project description:The mammalian brain is heterogeneous, containing billions of neurons and trillions of synapses forming various neural circuitries, through which sense, movement, thought, and emotion are generated. The cellular heterogeneity of the brain has made it difficult to study the molecular logic of neural circuitry wiring, pruning, activation, and plasticity, until recently, transcriptome analyses with single-cell resolution makes decoding of gene regulatory networks underlying aforementioned circuitry properties possible. Here, we report success in performing both electrophysiological and whole-genome transcriptome analyses on single human neurons in culture. Using Weighted Gene Coexpression Network Analyses (WGCNA), we identified gene clusters highly correlated with neuronal maturation judged by electrophysiological characteristics. A tight link between neuronal maturation and genes involved in ubiquitination and mitochondrial function was revealed. Moreover, we identified a list of candidate genes, which could potentially serve as biomarkers for neuronal maturation. Coupled electrophysiological recording and single-cell transcriptome analysis will serve as powerful tools in the future to unveil molecular logics for neural circuitry functions.