A single cell transcriptome atlas of the developing zebrafish hindbrain

Short-range Fgf signalling patterns hindbrain progenitors to induce the neurogenesis-to-oligodendrogenesis switch

In the vertebrate nervous system, neurogenesis generally precedes gliogenesis. The mechanisms driving the switch in cell type production and generation of the correct proportion of cell types remain unclear. Here, we show that Fgf20 signalling patterns progenitors to induce the switch from neurogenesis to oligodendrogenesis in the zebrafish hindbrain. Fgf20 emanating from earlier-born neurons signals at a short range to downregulate proneural gene expression in the segment centre with high spatial precision along both anterior-posterior and dorsal-ventral axes. This signal induces oligodendrocytes in the segment centre by upregulating olig2 and sox10 expression in pre-patterned competent progenitors. We show that the magnitude of proneural gene downregulation and the quantity of oligodendrocyte precursor cells specified is dependent on the extent of Fgf20 signalling. Overexpression of fgf20a induces precocious specification and differentiation of oligodendrocytes among olig2+ progenitors, resulting in an increase in oligodendrocytes at the expense of neurogenesis. Thus, Fgf20 signalling defines the proportion of each cell type produced. Taken together, Fgf20 signalling from earlier-born neurons patterns hindbrain segments spatially and temporally to induce the neurogenesis-to-oligodendrogenesis switch.

Motor neurons are dispensable for the assembly of a sensorimotor circuit for gaze stabilization

Sensorimotor reflex circuits engage distinct neuronal subtypes, defined by precise connectivity, to transform sensation into compensatory behavior. Whether and how motor neuron populations specify the subtype fate and/or sensory connectivity of their pre-motor partners remains controversial. Here, we discovered that motor neurons are dispensable for proper connectivity in the vestibular reflex circuit that stabilizes gaze. We first measured activity following vestibular sensation in pre-motor projection neurons after constitutive loss of their extraocular motor neuron partners. We observed normal responses and topography indicative of unchanged functional connectivity between sensory neurons and projection neurons. Next, we show that projection neurons remain anatomically and molecularly poised to connect appropriately with their downstream partners. Lastly, we show that the transcriptional signatures that typify projection neurons develop independently of motor partners. Our findings comprehensively overturn a long-standing model: that connectivity in the circuit for gaze stabilization is retrogradely determined by motor partner-derived signals. By defining the contribution of motor neurons to specification of an archetypal sensorimotor circuit, our work speaks to comparable processes in the spinal cord and advances our understanding of principles of neural development.

Ancient developmental genes underlie evolutionary novelties in walking fish

A critical question in biology is how new traits evolve, but studying this in wild animals remains challenging. Here, we probe the genetic basis of trait gain in sea robin fish, which have evolved specialized leg-like appendages for locomotion and digging along the ocean floor. We use genome sequencing, transcriptional profiling, and interspecific hybrid analysis to explore the molecular and developmental basis of leg formation. We identified the ancient, conserved transcription factor tbx3a as a major determinant of sensory leg development. Genome editing confirms that tbx3a is required for normal leg formation in sea robins, and for formation of enlarged central nervous system lobes, sensory papillae, and adult digging behavior. Our study establishes sea robins as a model organism for studying the evolution of major trait gain and illustrates how ancient developmental control genes can underlie novel organ formation.

Celsr3 drives development and connectivity of the acoustic startle hindbrain circuit

In the developing brain, groups of neurons organize into functional circuits that direct diverse behaviors. One such behavior is the evolutionarily conserved acoustic startle response, which in zebrafish is mediated by a well-defined hindbrain circuit. While numerous molecular pathways that guide neurons to their synaptic partners have been identified, it is unclear if and to what extent distinct neuron populations in the startle circuit utilize shared molecular pathways to ensure coordinated development. Here, we show that the planar cell polarity (PCP)-associated atypical cadherins Celsr3 and Celsr2, as well as the Celsr binding partner Frizzled 3a/Fzd3a, are critical for axon guidance of two neuron types that form synapses with each other: the command-like neuron Mauthner cells that drive the acoustic startle escape response, and spiral fiber neurons which provide excitatory input to Mauthner cells. We find that Mauthner axon growth towards synaptic targets is vital for Mauthner survival. We also demonstrate that symmetric spiral fiber input to Mauthner cells is critical for escape direction, which is necessary to respond to directional threats. Moreover, we identify distinct roles for Celsr3 and Celsr2, as Celsr3 is required for startle circuit development while Celsr2 is dispensable, though Celsr2 can partially compensate for loss of Celsr3 in Mauthner cells. This contrasts with facial branchiomotor neuron migration in the hindbrain, which requires Celsr2 while we find that Celsr3 is dispensable. Combined, our data uncover critical and distinct roles for individual PCP components during assembly of the acoustic startle hindbrain circuit.

Single-cell analysis of innate spinal cord regeneration identifies intersecting modes of neuronal repair

Adult zebrafish have an innate ability to recover from severe spinal cord injury. Here, we report a comprehensive single nuclear RNA sequencing atlas that spans 6 weeks of regeneration. We identify cooperative roles for adult neurogenesis and neuronal plasticity during spinal cord repair. Neurogenesis of glutamatergic and GABAergic neurons restores the excitatory/inhibitory balance after injury. In addition, a transient population of injury-responsive neurons (iNeurons) show elevated plasticity 1 week post-injury. We found iNeurons are injury-surviving neurons that acquire a neuroblast-like gene expression signature after injury. CRISPR/Cas9 mutagenesis showed iNeurons are required for functional recovery and employ vesicular trafficking as an essential mechanism that underlies neuronal plasticity. This study provides a comprehensive resource of the cells and mechanisms that direct spinal cord regeneration and establishes zebrafish as a model of plasticity-driven neural repair.

Neural progenitor-derived Apelin controls tip cell behavior and vascular patterning

During angiogenesis, vascular tip cells guide nascent vascular sprouts to form a vascular network. Apelin, an agonist of the G protein-coupled receptor Aplnr, is enriched in vascular tip cells, and it is hypothesized that vascular-derived Apelin regulates sprouting angiogenesis. We identify an apelin-expressing neural progenitor cell population in the dorsal neural tube. Vascular tip cells exhibit directed elongation and migration toward and along the apelin-expressing neural progenitor cells. Notably, restoration of neural but not vascular apelin expression in apelin mutants remedies the angiogenic defects of mutants. By functional analyses, we show the requirement of Apelin signaling for tip cell behaviors, like filopodia formation and cell elongation. Through genetic interaction studies and analysis of transgenic activity reporters, we identify Apelin signaling as a modulator of phosphoinositide 3-kinase and extracellular signal-regulated kinase signaling in tip cells in vivo. Our results suggest a previously unidentified neurovascular cross-talk mediated by Apelin signaling that is important for tip cell function during sprouting angiogenesis.

scMultiome analysis identifies a single caudal hindbrain compartment in the developing zebrafish nervous system

BACKGROUND

A key step in nervous system development involves the coordinated control of neural progenitor specification and positioning. A long-standing model for the vertebrate CNS postulates that transient anatomical compartments - known as neuromeres - function to position neural progenitors along the embryonic anteroposterior neuraxis. Such neuromeres are apparent in the embryonic hindbrain - that contains six rhombomeres with morphologically apparent boundaries - but other neuromeres lack clear morphological boundaries and have instead been defined by different criteria, such as differences in gene expression patterns and the outcomes of transplantation experiments. Accordingly, the caudal hindbrain (CHB) posterior to rhombomere (r) 6 has been variably proposed to contain from two to five 'pseudo-rhombomeres', but the lack of comprehensive molecular data has precluded a detailed definition of such structures.

METHODS

We used single-cell Multiome analysis, which allows simultaneous characterization of gene expression and chromatin state of individual cell nuclei, to identify and characterize CHB progenitors in the developing zebrafish CNS.

RESULTS

We identified CHB progenitors as a transcriptionally distinct population, that also possesses a unique profile of accessible transcription factor binding motifs, relative to both r6 and the spinal cord. This CHB population can be subdivided along its dorsoventral axis based on molecular characteristics, but we do not find any molecular evidence that it contains multiple pseudo-rhombomeres. We further observe that the CHB is closely related to r6 at the earliest embryonic stages, but becomes more divergent over time, and that it is defined by a unique gene regulatory network.

CONCLUSIONS

We conclude that the early CHB represents a single neuromere compartment that cannot be molecularly subdivided into pseudo-rhombomeres and that it may share an embryonic origin with r6.

Elevated phagocytic capacity directs innate spinal cord repair

Immune cells elicit a continuum of transcriptional and functional states after spinal cord injury (SCI). In mammals, inefficient debris clearance and chronic inflammation impede recovery and overshadow pro-regenerative immune functions. We found that, unlike mammals, zebrafish SCI elicits transient immune activation and efficient debris clearance, without causing chronic inflammation. Single-cell transcriptomics and inducible genetic ablation showed zebrafish macrophages are highly phagocytic and required for regeneration. Cross-species comparisons between zebrafish and mammalian macrophages identified transcription and immune response regulator ( tcim ) as a macrophage-enriched zebrafish gene. Genetic deletion of zebrafish tcim impairs phagocytosis and regeneration, causes aberrant and chronic immune activation, and can be rescued by transplanting wild-type immune precursors into tcim mutants. Conversely, genetic expression of human TCIM accelerates debris clearance and regeneration by reprogramming myeloid precursors into activated phagocytes. This study establishes a central requirement for elevated phagocytic capacity to achieve innate spinal cord repair.

Aspartame Causes Developmental Defects and Teratogenicity in Zebra Fish Embryo: Role of Impaired SIRT1/FOXO3a Axis in Neuron Cells

Aspartame, a widely used artificial sweetener, is present in many food products and beverages worldwide. It has been linked to potential neurotoxicity and developmental defects. However, its teratogenic effect on embryonic development and the underlying potential mechanisms need to be elucidated. We investigated the concentration- and time-dependent effects of aspartame on zebrafish development and teratogenicity. We focused on the role of sirtuin 1 (SIRT1) and Forkhead-box transcription factor (FOXO), two proteins that play key roles in neurodevelopment. It was found that aspartame exposure reduced the formation of larvae and the development of cartilage in zebrafish. It also delayed post-fertilization development by altering the head length and locomotor behavior of zebrafish. RNA-sequencing-based DEG analysis showed that SIRT1 and FOXO3a are involved in neurodevelopment. In silico and in vitro analyses showed that aspartame could target and reduce the expression of SIRT1 and FOXO3a proteins in neuron cells. Additionally, aspartame triggered the reduction of autophagy flux by inhibiting the nuclear translocation of SIRT1 in neuronal cells. The findings suggest that aspartame can cause developmental defects and teratogenicity in zebrafish embryos and reduce autophagy by impairing the SIRT1/FOXO3a axis in neuron cells.

Spatially resolved single-cell atlas of ascidian endostyle provides insight into the origin of vertebrate pharyngeal organs

The pharyngeal endoderm, an innovation of deuterostome ancestors, contributes to pharyngeal development by influencing the patterning and differentiation of pharyngeal structures in vertebrates; however, the evolutionary origin of the pharyngeal organs in vertebrates is largely unknown. The endostyle, a distinct pharyngeal organ exclusively present in basal chordates, represents a good model for understanding pharyngeal organ origins. Using Stereo-seq and single-cell RNA sequencing, we constructed aspatially resolved single-cell atlas for the endostyle of the ascidian Styela clava. We determined the cell composition of the hemolymphoid region, which illuminates a mixed ancestral structure for the blood and lymphoid system. In addition, we discovered a cluster of hair cell-like cells in zone 3, which has transcriptomic similarity with the hair cells of the vertebrate acoustico-lateralis system. These findings reshape our understanding of the pharynx of the basal chordate and provide insights into the evolutionary origin of multiplexed pharyngeal organs.

Hindbrain boundaries as niches of neural progenitor and stem cells regulated by the extracellular matrix proteoglycan chondroitin sulphate

The interplay between neural progenitors and stem cells (NPSCs), and their extracellular matrix (ECM) is a crucial regulatory mechanism that determines their behavior. Nonetheless, how the ECM dictates the state of NPSCs remains elusive. The hindbrain is valuable to examine this relationship, as cells in the ventricular surface of hindbrain boundaries (HBs), which arise between any two neighboring rhombomeres, express the NPSC marker Sox2, while being surrounded with the membrane-bound ECM molecule chondroitin sulphate proteoglycan (CSPG), in chick and mouse embryos. CSPG expression was used to isolate HB Sox2+ cells for RNA-sequencing, revealing their distinguished molecular properties as typical NPSCs, which express known and newly identified genes relating to stem cells, cancer, the matrisome and cell cycle. In contrast, the CSPG- non-HB cells, displayed clear neural-differentiation transcriptome. To address whether CSPG is significant for hindbrain development, its expression was manipulated in vivo and in vitro. CSPG manipulations shifted the stem versus differentiation state of HB cells, evident by their behavior and altered gene expression. These results provide further understanding of the uniqueness of hindbrain boundaries as repetitive pools of NPSCs in-between the rapidly growing rhombomeres, which rely on their microenvironment to maintain their undifferentiated state during development.

Motor neurons are dispensable for the assembly of a sensorimotor circuit for gaze stabilization

Sensorimotor reflex circuits engage distinct neuronal subtypes, defined by precise connectivity, to transform sensation into compensatory behavior. Whether and how motor neuron populations specify the subtype fate and/or sensory connectivity of their pre-motor partners remains controversial. Here, we discovered that motor neurons are dispensable for proper connectivity in the vestibular reflex circuit that stabilizes gaze. We first measured activity following vestibular sensation in pre-motor projection neurons after constitutive loss of their extraocular motor neuron partners. We observed normal responses and topography indicative of unchanged functional connectivity between sensory neurons and projection neurons. Next, we show that projection neurons remain anatomically and molecularly poised to connect appropriately with their downstream partners. Lastly, we show that the transcriptional signatures that typify projection neurons develop independently of motor partners. Our findings comprehensively overturn a long-standing model: that connectivity in the circuit for gaze stabilization is retrogradely determined by motor partner-derived signals. By defining the contribution of motor neurons to specification of an archetypal sensorimotor circuit, our work speaks to comparable processes in the spinal cord and advances our understanding of general principles of neural development.

scMultiome analysis identifies embryonic hindbrain progenitors with mixed rhombomere identities

Rhombomeres serve to position neural progenitors in the embryonic hindbrain, thereby ensuring appropriate neural circuit formation, but the molecular identities of individual rhombomeres and the mechanism whereby they form has not been fully established. Here, we apply scMultiome analysis in zebrafish to molecularly resolve all rhombomeres for the first time. We find that rhombomeres become molecularly distinct between 10hpf (end of gastrulation) and 13hpf (early segmentation). While the embryonic hindbrain transiently contains alternating odd- versus even-type rhombomeres, our scMultiome analyses do not detect extensive odd versus even molecular characteristics in the early hindbrain. Instead, we find that each rhombomere displays a unique gene expression and chromatin profile. Prior to the appearance of distinct rhombomeres, we detect three hindbrain progenitor clusters (PHPDs) that correlate with the earliest visually observed segments in the hindbrain primordium that represent prospective rhombomere r2/r3 (possibly including r1), r4, and r5/r6, respectively. We further find that the PHPDs form in response to Fgf and RA morphogens and that individual PHPD cells co-express markers of multiple mature rhombomeres. We propose that the PHPDs contain mixed-identity progenitors and that their subdivision into individual rhombomeres requires the resolution of mixed transcription and chromatin states.

Hindbrain rhombomere centers harbor a heterogenous population of dividing progenitors which rely on Notch signaling

Tissue growth and morphogenesis are interrelated processes, whose tight coordination is essential for the production of different cell fates and the timely precise allocation of stem cell capacities. The zebrafish embryonic brainstem, the hindbrain, exemplifies such coupling between spatiotemporal cell diversity acquisition and tissue growth as the neurogenic commitment is differentially distributed over time. Here, we combined cell lineage and in vivo imaging approaches to reveal the emergence of specific cell population properties within the rhombomeres. We studied the molecular identity of hindbrain rhombomere centers and showed that they harbor different progenitor capacities that change over time. By clonal analysis, we revealed that cells within the center of rhombomeres decrease the proliferative capacity to remain mainly in the G1 phase. Proliferating progenitors give rise to neurons by asymmetric and symmetric neurogenic divisions while maintaining the pool of progenitors. The proliferative capacity of these cells differs from their neighbors, and they are delayed in the onset of Notch activity. Through functional studies, we demonstrated that they rely on Notch3 signaling to be maintained as non-committed progenitors. In this study, we show that cells in rhombomere centers, despite the neurogenic asynchrony, might share steps of a similar program with the rhombomere counterparts, to ensure proper tissue growth.

Zbtb16 mediates a switch between Fgf signalling regimes in the developing hindbrain

Developing tissues are sequentially patterned by extracellular signals that are turned on and off at specific times. In the zebrafish hindbrain, fibroblast growth factor (Fgf) signalling has different roles at different developmental stages: in the early hindbrain, transient Fgf3 and Fgf8 signalling from rhombomere 4 is required for correct segmentation, whereas later, neuronal Fgf20 expression confines neurogenesis to specific spatial domains within each rhombomere. How the switch between these two signalling regimes is coordinated is not known. We present evidence that the Zbtb16 transcription factor is required for this transition to happen in an orderly fashion. Zbtb16 expression is high in the early anterior hindbrain, then gradually upregulated posteriorly and confined to neural progenitors. In mutants lacking functional Zbtb16, fgf3 expression fails to be downregulated and persists until a late stage, resulting in excess and more widespread Fgf signalling during neurogenesis. Accordingly, the spatial pattern of neurogenesis is disrupted in Zbtb16 mutants. Our results reveal how the distinct stage-specific roles of Fgf signalling are coordinated in the zebrafish hindbrain.

Etv5a Suppresses Neural Progenitor Cell Proliferation by Inhibiting sox2 Transcription

Neural progenitor cells are self-renewable, proliferative, and multipotent cell populations that generate diverse types of neurons and glia to build the nervous system. Transcription factors play critical roles in regulating various cellular processes; however, the transcription factors that regulate the development of neural progenitors are yet to be identified. In the present study, we demonstrated that zebrafish etv5a is expressed in the neural progenitor cells of the neuroectoderm. Downregulation of endogenous Etv5a function by etv5a morpholino or an etv5a dominant-negative variant increased the proliferation of sox2-positive neural progenitor cells, accompanied by inhibition of neurogenesis and gliogenesis. These phenotypes in Etv5a-depleted embryos could be rescued by a co-injection with etv5a cRNA. Etv5a overexpression reduced sox2 expression. Direct binding of Etv5a to the regulatory elements of sox2 was affirmed by chromatin immunoprecipitation. These data revealed that Etv5a directly suppressed sox2 expression to reduce the proliferation of neural progenitor cells. In addition, the expression of foxm1, a putative target gene of Etv5a and a direct upstream transcription factor of sox2, was upregulated in Etv5a-deficient embryos. Moreover, the suppression of Foxm1 function by the foxm1 dominant-negative construct nullified the phenotype of upregulated sox2 expression caused by Etv5a deficiency. Overall, our results indicated that Etv5a regulates the expression of sox2 via direct binding to the sox2 promoter and indirect regulation by inhibiting foxm1 expression. Hence, we revealed the role of Etv5a in the transcriptional hierarchy that regulates the proliferation of neural progenitor cells.

A molecular atlas of adult C. elegans motor neurons reveals ancient diversity delineated by conserved transcription factor codes

Motor neurons (MNs) constitute an ancient cell type targeted by multiple adult-onset diseases. It is therefore important to define the molecular makeup of adult MNs in animal models and extract organizing principles. Here, we generated a comprehensive molecular atlas of adult Caenorhabditis elegans MNs and a searchable database (http://celegans.spinalcordatlas.org). Single-cell RNA-sequencing of 13,200 cells revealed that ventral nerve cord MNs cluster into 29 molecularly distinct subclasses. All subclasses are delineated by unique expression codes of either neuropeptide or transcription factor gene families. Strikingly, we found that combinatorial codes of homeodomain transcription factor genes define adult MN diversity both in C. elegans and mice. Further, molecularly defined MN subclasses in C. elegans display distinct patterns of connectivity. Hence, our study couples the connectivity map of the C. elegans motor circuit with a molecular atlas of its constituent MNs, and uncovers organizing principles and conserved molecular codes of adult MN diversity.

Spatiotemporal transcriptomic maps of whole mouse embryos at the onset of organogenesis

Spatiotemporal orchestration of gene expression is required for proper embryonic development. The use of single-cell technologies has begun to provide improved resolution of early regulatory dynamics, including detailed molecular definitions of most cell states during mouse embryogenesis. Here we used Slide-seq to build spatial transcriptomic maps of complete embryonic day (E) 8.5 and E9.0, and partial E9.5 embryos. To support their utility, we developed sc3D, a tool for reconstructing and exploring three-dimensional 'virtual embryos', which enables the quantitative investigation of regionalized gene expression patterns. Our measurements along the main embryonic axes of the developing neural tube revealed several previously unannotated genes with distinct spatial patterns. We also characterized the conflicting transcriptional identity of 'ectopic' neural tubes that emerge in Tbx6 mutant embryos. Taken together, we present an experimental and computational framework for the spatiotemporal investigation of whole embryonic structures and mutant phenotypes.

Neuronal birthdate reveals topography in a vestibular brainstem circuit for gaze stabilization

Across the nervous system, neurons with similar attributes are topographically organized. This topography reflects developmental pressures. Oddly, vestibular (balance) nuclei are thought to be disorganized. By measuring activity in birthdated neurons, we revealed a functional map within the central vestibular projection nucleus that stabilizes gaze in the larval zebrafish. We first discovered that both somatic position and stimulus selectivity follow projection neuron birthdate. Next, with electron microscopy and loss-of-function assays, we found that patterns of peripheral innervation to projection neurons were similarly organized by birthdate. Finally, birthdate revealed spatial patterns of axonal arborization and synapse formation to projection neuron outputs. Collectively, we find that development reveals previously hidden organization to the input, processing, and output layers of a highly conserved vertebrate sensorimotor circuit. The spatial and temporal attributes we uncover constrain the developmental mechanisms that may specify the fate, function, and organization of vestibulo-ocular reflex neurons. More broadly, our data suggest that, like invertebrates, temporal mechanisms may assemble vertebrate sensorimotor architecture.

scMultiome analysis identifies embryonic hindbrain progenitors with mixed rhombomere identities

Rhombomeres serve to position neural progenitors in the embryonic hindbrain, thereby ensuring appropriate neural circuit formation, but the molecular identities of individual rhombomeres and the mechanism whereby they form have not been fully established. Here we apply scMultiome analysis in zebrafish to molecularly resolve all rhombomeres for the first time. We find that rhombomeres become molecularly distinct between 10hpf (end of gastrulation) and 13hpf (early segmentation). While the mature hindbrain consists of alternating odd- versus even-type rhombomeres, our scMultiome analyses do not detect extensive odd versus even characteristics in the early hindbrain. Instead, we find that each rhombomere displays a unique gene expression and chromatin profile. Prior to the appearance of distinct rhombomeres, we detect three hindbrain progenitor clusters (PHPDs) that correlate with the earliest visually observed segments in the hindbrain primordium and that represent prospective rhombomere r2/r3 (possibly including r1), r4 and r5/r6, respectively. We further find that the PHPDs form in response to Fgf and RA morphogens and that individual PHPD cells co-express markers of multiple mature rhombomeres. We propose that the PHPDs contain mixed-identity progenitors and that their subdivision into individual mature rhombomeres requires resolution of mixed transcription and chromatin states.

shox2 is required for vestibular statoacoustic neuron development

Homeobox genes act at the top of genetic hierarchies to regulate cell specification and differentiation during embryonic development. We identified the short stature homeobox domain 2 (shox2) transcription factor that is required for vestibular neuron development. shox2 transcripts are initially localized to the otic placode of the developing inner ear where neurosensory progenitors reside. To study shox2 function, we generated CRISPR-mediated mutant shox2 fish. Mutant embryos display behaviors associated with vestibular deficits and showed reduced number of anterior statoacoustic ganglion neurons that innervate the utricle, the vestibular organ in zebrafish. Moreover, a shox2-reporter fish showed labeling of developing statoacoustic ganglion neurons in the anterior macula of the otic vesicle. Single cell RNA-sequencing of cells from the developing otic vesicle of shox2 mutants revealed altered otic progenitor profiles, while single molecule in situ assays showed deregulated levels of transcripts in developing neurons. This study implicates a role for shox2 in development of vestibular but not auditory statoacoustic ganglion neurons.

Single-cell profiles reveal tumor cell heterogeneity and immunosuppressive microenvironment in Waldenström macroglobulinemia

BACKGROUND

Waldenström macroglobulinemia (WM) is a rare and incurable indolent B-cell malignancy. The molecular pathogenesis and the role of immunosuppressive microenvironment in WM development are still incompletely understood.

METHODS

The multicellular ecosystem in bone marrow (BM) of WM were delineated by single-cell RNA-sequencing (scRNA-seq) and investigated the underlying molecular characteristics.

RESULTS

Our data uncovered the heterogeneity of malignant cells in WM, and investigated the kinetic co-evolution of WM and immune cells, which played pivotal roles in disease development and progression. Two novel subpopulations of malignant cells, CD19+CD3+ and CD138+CD3+, co-expressing T-cell marker genes were identified at single-cell resolution. Pseudotime-ordered analysis elucidated that CD19+CD3+ malignant cells presented at an early stage of WM-B cell differentiation. Colony formation assay further identified that CD19+CD3+ malignant cells acted as potential WM precursors. Based on the findings of T cell marker aberrant expressed on WM tumor cells, we speculate the long-time activation of tumor antigen-induced immunosuppressive microenvironment that is involved in the pathogenesis of WM. Therefore, our study further investigated the possible molecular mechanism of immune cell dysfunction. A precursor exhausted CD8-T cells and functional deletion of NK cells were identified in WM, and CD47 would be a potential therapeutic target to reverse the dysfunction of immune cells.

CONCLUSIONS

Our study facilitates further understanding of the biological heterogeneity of tumor cells and immunosuppressive microenvironment in WM. These data may have implications for the development of novel immunotherapies, such as targeting pre-exhausted CD8-T cells in WM.

Sequential and additive expression of miR-9 precursors control timing of neurogenesis

MicroRNAs (miRs) have an important role in tuning dynamic gene expression. However, the mechanism by which they are quantitatively controlled is unknown. We show that the amount of mature miR-9, a key regulator of neuronal development, increases during zebrafish neurogenesis in a sharp stepwise manner. We characterize the spatiotemporal profile of seven distinct microRNA primary transcripts (pri-mir)-9s that produce the same mature miR-9 and show that they are sequentially expressed during hindbrain neurogenesis. Expression of late-onset pri-mir-9-1 is added on to, rather than replacing, the expression of early onset pri-mir-9-4 and -9-5 in single cells. CRISPR/Cas9 mutation of the late-onset pri-mir-9-1 prevents the developmental increase of mature miR-9, reduces late neuronal differentiation and fails to downregulate Her6 at late stages. Mathematical modelling shows that an adaptive network containing Her6 is insensitive to linear increases in miR-9 but responds to stepwise increases of miR-9. We suggest that a sharp stepwise increase of mature miR-9 is created by sequential and additive temporal activation of distinct loci. This may be a strategy to overcome adaptation and facilitate a transition of Her6 to a new dynamic regime or steady state.

Nrf2 dictates the neuronal survival and differentiation of embryonic zebrafish harboring compromised alanyl-tRNA synthetase

tRNA synthetase deficiency leads to unfolded protein responses in neuronal disorders; however, its function in embryonic neurogenesis remains unclear. This study identified an aars1cq71/cq71 mutant zebrafish allele that showed increased neuronal apoptosis and compromised neurogenesis. aars1 transcripts were highly expressed in primary neural progenitor cells, and their aberration resulted in protein overloading and activated Perk. nfe2l2b, a paralog of mammalian Nfe2l2, which encodes Nrf2, is a pivotal executor of Perk signaling that regulates neuronal phenotypes in aars1cq71/cq71 mutants. Interference of nfe2l2b in nfe2l2bΔ1/Δ1 mutants did not affect global larval development. However, aars1cq71/cq71;nfe2l2bΔ1/Δ1 mutant embryos exhibited increased neuronal cell survival and neurogenesis compared with their aars1cq71/cq71 siblings. nfe2l2b was harnessed by Perk at two levels. Its transcript was regulated by Chop, an implementer of Perk. It was also phosphorylated by Perk. Both pathways synergistically assured the nuclear functions of nfe2l2b to control cell survival by targeting p53. Our study extends the understanding of tRNA synthetase in neurogenesis and implies that Nrf2 is a cue to mitigate neurodegenerative pathogenesis.

GATA4/5/6 family transcription factors are conserved determinants of cardiac versus pharyngeal mesoderm fate

GATA4/5/6 transcription factors play essential, conserved roles in heart development. To understand how GATA4/5/6 modulates the mesoderm-to-cardiac fate transition, we labeled, isolated, and performed single-cell gene expression analysis on cells that express gata5 at precardiac time points spanning zebrafish gastrulation to somitogenesis. We found that most mesendoderm-derived lineages had dynamic gata5/6 expression. In the absence of Gata5/6, the population structure of mesendoderm-derived cells was substantially altered. In addition to the expected absence of cardiac mesoderm, we confirmed a concomitant expansion of cranial-pharyngeal mesoderm. Moreover, Gata5/6 loss led to extensive changes in chromatin accessibility near cardiac and pharyngeal genes. Functional analyses in zebrafish and the tunicate Ciona, which has a single GATA4/5/6 homolog, revealed that GATA4/5/6 acts upstream of tbx1 to exert essential and cell-autonomous roles in promoting cardiac and inhibiting pharyngeal mesoderm identity. Overall, cardiac and pharyngeal mesoderm fate choices are achieved through an evolutionarily conserved GATA4/5/6 regulatory network.

scInTime: A Computational Method Leveraging Single-Cell Trajectory and Gene Regulatory Networks to Identify Master Regulators of Cellular Differentiation

Trajectory inference (TI) or pseudotime analysis has dramatically extended the analytical framework of single-cell RNA-seq data, allowing regulatory genes contributing to cell differentiation and those involved in various dynamic cellular processes to be identified. However, most TI analysis procedures deal with individual genes independently while overlooking the regulatory relations between genes. Integrating information from gene regulatory networks (GRNs) at different pseudotime points may lead to more interpretable TI results. To this end, we introduce scInTime-an unsupervised machine learning framework coupling inferred trajectory with single-cell GRNs (scGRNs) to identify master regulatory genes. We validated the performance of our method by analyzing multiple scRNA-seq data sets. In each of the cases, top-ranking genes predicted by scInTime supported their functional relevance with corresponding signaling pathways, in line with the results of available functional studies. Overall results demonstrated that scInTime is a powerful tool to exploit pseudotime-series scGRNs, allowing for a clear interpretation of TI results toward more significant biological insights.

From Cell States to Cell Fates: How Cell Proliferation and Neuronal Differentiation Are Coordinated During Embryonic Development

The central nervous system (CNS) exhibits an extraordinary diversity of neurons, with the right cell types and proportions at the appropriate sites. Thus, to produce brains with specific size and cell composition, the rates of proliferation and differentiation must be tightly coordinated and balanced during development. Early on, proliferation dominates; later on, the growth rate almost ceases as more cells differentiate and exit the cell cycle. Generation of cell diversity and morphogenesis takes place concomitantly. In the vertebrate brain, this results in dramatic changes in the position of progenitor cells and their neuronal derivatives, whereas in the spinal cord morphogenetic changes are not so important because the structure mainly grows by increasing its volume. Morphogenesis is under control of specific genetic programs that coordinately unfold over time; however, little is known about how they operate and impact in the pools of progenitor cells in the CNS. Thus, the spatiotemporal coordination of these processes is fundamental for generating functional neuronal networks. Some key aims in developmental neurobiology are to determine how cell diversity arises from pluripotent progenitor cells, and how the progenitor potential changes upon time. In this review, we will share our view on how the advance of new technologies provides novel data that challenge some of the current hypothesis. We will cover some of the latest studies on cell lineage tracing and clonal analyses addressing the role of distinct progenitor cell division modes in balancing the rate of proliferation and differentiation during brain morphogenesis. We will discuss different hypothesis proposed to explain how progenitor cell diversity is generated and how they challenged prevailing concepts and raised new questions.

Toward a granular molecular-anatomic map of the blood vasculature - single-cell RNA sequencing makes the leap

Single-cell RNA sequencing (scRNAseq) marks the birth of a new era in physiology and medicine. Within foreseeable future, we will know exactly what genes are expressed - and at what levels - in all the different cell types and subtypes that make up our bodies. We will also learn how a particular cell state, whether it occurs during development, tissue repair, or disease, reflects precise changes in gene expression. While profoundly impacting all areas of life science, scRNAseq may lead to a particular leap in vascular biology research. Blood vessels pervade and fulfill essential functions in all organs, but the functions differ. Innumerable organ-specific vascular adaptations and specializations are required. These, in turn, are dictated by differential gene expression by the two principal cellular building blocks of blood vessels: endothelial cells and mural cells. An organotypic vasculature is essential for functions as diverse as thinking, gas exchange, urine excretion, and xenobiotic detoxification in the brain, lung, kidney, and liver, respectively. In addition to the organotypicity, vascular cells also differ along the vascular arterio-venous axis, referred to as zonation, differences that are essential for the regulation of blood pressure and flow. Moreover, gene expression-based molecular changes dictate states of cellular activity, necessary for angiogenesis, vascular permeability, and immune cell trafficking, i.e. functions necessary for development, inflammation, and repair. These different levels of cellular heterogeneity create a nearly infinite phenotypic diversity among vascular cells. In this review, I summarize and exemplify what scRNAseq has brought to the picture in just a few years and point out where it will take us.

Primary culture of adult cortical neurons from reptile Gekko japonicus

Adult neurons of several reptiles still retain the ability of axonal regeneration in contrast to the low intrinsic regenerative capacity of those in the central nervous system (CNS) in mammals. This feature of the reptilian neurons has provided a perfect model for elucidating the regenerative mechanism lost in the mammalian counterparts. However, little information is available on the primary culture method of adult reptilian neurons, which greatly limits their valuable applications. In the present study, we introduced a simple and easy method for the isolation, culture, and identification of neurons from the cerebral cortex using the adult geckos. The cultured cells were further identified by immunofluorescence using antibodies against neuron-specific markers β-Ⅲ-tubulin and NeuN. The cortical neurons from adult gecko displayed spindle-shaped, bipolar, or multipolar morphology with a plump soma. This primary culture method for adult reptilian neurons will be beneficial for comparative studies of neuronal biology in various vertebrates.

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

Conserved genetic signatures parcellate cardinal spinal neuron classes into local and projection subsets

Motor and sensory functions of the spinal cord are mediated by populations of cardinal neurons arising from separate progenitor lineages. However, each cardinal class is composed of multiple neuronal types with distinct molecular, anatomical, and physiological features, and there is not a unifying logic that systematically accounts for this diversity. We reasoned that the expansion of new neuronal types occurred in a stepwise manner analogous to animal speciation, and we explored this by defining transcriptomic relationships using a top-down approach. We uncovered orderly genetic tiers that sequentially divide groups of neurons by their motor-sensory, local-long range, and excitatory-inhibitory features. The genetic signatures defining neuronal projections were tied to neuronal birth date and conserved across cardinal classes. Thus, the intersection of cardinal class with projection markers provides a unifying taxonomic solution for systematically identifying distinct functional subsets.

Zebrafish as a Neuroblastoma Model: Progress Made, Promise for the Future

For nearly a decade, researchers in the field of pediatric oncology have been using zebrafish as a model for understanding the contributions of genetic alternations to the pathogenesis of neuroblastoma (NB), and exploring the molecular and cellular mechanisms that underlie neuroblastoma initiation and metastasis. In this review, we will enumerate and illustrate the key advantages of using the zebrafish model in NB research, which allows researchers to: monitor tumor development in real-time; robustly manipulate gene expression (either transiently or stably); rapidly evaluate the cooperative interactions of multiple genetic alterations to disease pathogenesis; and provide a highly efficient and low-cost methodology to screen for effective pharmaceutical interventions (both alone and in combination with one another). This review will then list some of the common challenges of using the zebrafish model and provide strategies for overcoming these difficulties. We have also included visual diagram and figures to illustrate the workflow of cancer model development in zebrafish and provide a summary comparison of commonly used animal models in cancer research, as well as key findings of cooperative contributions between MYCN and diverse singling pathways in NB pathogenesis.

An atlas of neural crest lineages along the posterior developing zebrafish at single-cell resolution

Neural crest cells (NCCs) are vertebrate stem cells that give rise to various cell types throughout the developing body in early life. Here, we utilized single-cell transcriptomic analyses to delineate NCC-derivatives along the posterior developing vertebrate, zebrafish, during the late embryonic to early larval stage, a period when NCCs are actively differentiating into distinct cellular lineages. We identified several major NCC/NCC-derived cell-types including mesenchyme, neural crest, neural, neuronal, glial, and pigment, from which we resolved over three dozen cellular subtypes. We dissected gene expression signatures of pigment progenitors delineating into chromatophore lineages, mesenchyme cells, and enteric NCCs transforming into enteric neurons. Global analysis of NCC derivatives revealed they were demarcated by combinatorial hox gene codes, with distinct profiles within neuronal cells. From these analyses, we present a comprehensive cell-type atlas that can be utilized as a valuable resource for further mechanistic and evolutionary investigations of NCC differentiation.

Uncovering the genetic blueprint of the C. elegans nervous system

Despite rapid advances in connectome mapping and neuronal genetics, we lack theoretical and computational tools to unveil, in an experimentally testable fashion, the genetic mechanisms that govern neuronal wiring. Here we introduce a computational framework to link the adjacency matrix of a connectome to the expression patterns of its neurons, helping us uncover a set of genetic rules that govern the interactions between neurons in contact. The method incorporates the biological realities of the system, accounting for noise from data collection limitations, as well as spatial restrictions. The resulting methodology allows us to infer a network of 19 innexin interactions that govern the formation of gap junctions in Caenorhabditis elegans, five of which are already supported by experimental data. As advances in single-cell gene expression profiling increase the accuracy and the coverage of the data, the developed framework will allow researchers to systematically infer experimentally testable connection rules, offering mechanistic predictions for synapse and gap junction formation.

Emergence of Neuronal Diversity during Vertebrate Brain Development

Neurogenesis comprises many highly regulated processes including proliferation, differentiation, and maturation. However, the transcriptional landscapes underlying brain development are poorly characterized. We describe a developmental single-cell catalog of ∼220,000 zebrafish brain cells encompassing 12 stages from embryo to larva. We characterize known and novel gene markers for ∼800 clusters and provide an overview of the diversification of neurons and progenitors across these time points. We also introduce an optimized GESTALT lineage recorder that enables higher expression and recovery of Cas9-edited barcodes to query lineage segregation. Cell type characterization indicates that most embryonic neural progenitor states are transitory and transcriptionally distinct from neural progenitors of post-embryonic stages. Reconstruction of cell specification trajectories reveals that late-stage retinal neural progenitors transcriptionally overlap cell states observed in the embryo. The zebrafish brain development atlas provides a resource to define and manipulate specific subsets of neurons and to uncover the molecular mechanisms underlying vertebrate neurogenesis.

Insights Into the Early Gene Regulatory Network Controlling Neural Crest and Placode Fate Choices at the Neural Border

The neural crest (NC) cells and cranial placodes are two ectoderm-derived innovations in vertebrates that led to the acquisition of a complex head structure required for a predatory lifestyle. They both originate from the neural border (NB), a portion of the ectoderm located between the neural plate (NP), and the lateral non-neural ectoderm. The NC gives rise to a vast array of tissues and cell types such as peripheral neurons and glial cells, melanocytes, secretory cells, and cranial skeletal and connective cells. Together with cells derived from the cranial placodes, which contribute to sensory organs in the head, the NC also forms the cranial sensory ganglia. Multiple in vivo studies in different model systems have uncovered the signaling pathways and genetic factors that govern the positioning, development, and differentiation of these tissues. In this literature review, we give an overview of NC and placode development, focusing on the early gene regulatory network that controls the formation of the NB during early embryonic stages, and later dictates the choice between the NC and placode progenitor fates.

Gsx2 is required for specification of neurons in the inferior olivary nuclei from Ptf1a-expressing neural progenitors in zebrafish

Neurons in the inferior olivary nuclei (IO neurons) send climbing fibers to Purkinje cells to elicit functions of the cerebellum. IO neurons and Purkinje cells are derived from neural progenitors expressing the proneural gene ptf1a In this study, we found that the homeobox gene gsx2 was co-expressed with ptf1a in IO progenitors in zebrafish. Both gsx2 and ptf1a zebrafish mutants showed a strong reduction or loss of IO neurons. The expression of ptf1a was not affected in gsx2 mutants, and vice versa. In IO progenitors, the ptf1a mutation increased apoptosis whereas the gsx2 mutation did not, suggesting that ptf1a and gsx2 are regulated independently of each other and have distinct roles. The fibroblast growth factors (Fgf) 3 and 8a, and retinoic acid signals negatively and positively, respectively, regulated gsx2 expression and thereby the development of IO neurons. mafba and Hox genes are at least partly involved in the Fgf- and retinoic acid-dependent regulation of IO neuronal development. Our results indicate that gsx2 mediates the rostro-caudal positional signals to specify the identity of IO neurons from ptf1a-expressing neural progenitors.

Investigation of alpl expression and Tnap-activity in zebrafish implies conserved functions during skeletal and neuronal development

Hypophosphatasia (HPP) is a rare genetic disease with diverse symptoms and a heterogeneous severity of onset with underlying mutations in the ALPL gene encoding the ectoenzyme Tissue-nonspecific alkaline phosphatase (TNAP). Considering the establishment of zebrafish (Danio rerio) as a new model organism for HPP, the aim of the study was the spatial and temporal analysis of alpl expression in embryos and adult brains. Additionally, we determined functional consequences of Tnap inhibition on neural and skeletal development in zebrafish. We show that expression of alpl is present during embryonic stages and in adult neuronal tissues. Analyses of enzyme function reveal zones of pronounced Tnap-activity within the telencephalon and the mesencephalon. Treatment of zebrafish embryos with chemical Tnap inhibitors followed by axonal and cartilage/mineralized tissue staining imply functional consequences of Tnap deficiency on neuronal and skeletal development. Based on the results from neuronal and skeletal tissue analyses, which demonstrate an evolutionary conserved role of this enzyme, we consider zebrafish as a promising species for modeling HPP in order to discover new potential therapy strategies in the long-term.