Biology subjects

Luo, L.

Publications and source records attributed to Luo, L..

At least 181 records · Page 10Linked to original sources

bioRxiv2020

On the shuttling across the blood-brain barrier via tubules formation: mechanism and cargo avidity bias

The blood-brain barrier is made of polarised brain endothelial cells (BECs) phenotypically conditioned by the central nervous system (CNS). Transport across BECs is of paramount importance for nutrient uptake as well as to rid the brain of waste products. Nevertheless, currently we do not understand how large macromolecular cargo shuttles across and how BECs discriminate between the brain-bound and own nutrients. Here, we study the low-density lipoprotein receptor-related protein 1 (LRP1) an essential regulator of BEC transport, and show that it is associated with endocytic effectors, endo-lysosomal compartments as well as syndapin-2, a member of the Bin/Amphiphysin/Rvs (BAR) domain superfamily known to stabilise tubular carriers. We employed synthetic self-assembled vesicles, polymersomes, as a multivalent system with tunable avidity as a tool to investigate the mechanism of transport across BECs. We used a combination of conventional and super-resolution microscopy, both in vivo and in vitro, accompanied with biophysical modelling of transport kinetics and membrane-bound interactions. Our results demonstrate that the avidity of the ligand-receptor interaction (the overall cargo binding energy) determines the mechanism of sorting during the early stages of endocytosis and consequent trafficking. We show that high avidity cargo biases the LRP1 towards internalisation and fast degradation in BECs, while mid avidity augments the formation of syndapin-2 stabilised tubular carriers and promotes fast shuttling across BECs. Thus, we map out a very detailed mechanism where clathrin, actin, syndapin-2, dynamin and SNARE act synergistically to enable fast shuttling across BECs.

neuroscience↗

bioRxiv2020

Revealing the functions of supra-temporal and insular auditory responsive areas in humans

The human auditory sensory area, which includes primary and non-primary auditory cortices, has been considered to locate in the supra-temporal lobe for more than a century. Recently, accumulating evidence shows that the posterior part of insula responses to sounds under non-task states with relevant short latencies. However, whether posterior insula (InsP) contribute to forming auditory sensation remains unclear. Here we addressed this issue by recording and stimulation directly on the supra-temporal and insular areas via intracranial electrodes from 53 epileptic patients. During passive listening to a non-speech sound, the high-{gamma} (60-140 Hz) active rate of InsP (68.8%) was approximate to the non-primary auditory areas (72.4% and 79.0%). Moreover, we could not distinguish InsP from supra-temporal subareas by either activation, latency, temporal pattern or lateral dominance of sound induce high-{gamma}. On the contrary, direct electrical stimulation evoked auditory sensations effectively on supra-temporal subareas (> 65%), while sparsely on InsP (9.49%). The results of cortico-cortical evoked potentials (CCEPs) showed strong bidirectional connectivity within supra-temporal areas, but weak connectivity between supra-temporal areas and InsP. These findings suggest that even the InsP has similar basic auditory response properties to the primary or non-primary cortex, it may not directly participate in the formation of auditory perception.

neuroscience↗

bioRxiv2020

Differential encoding in prefrontal cortex projection neuron classes across cognitive tasks

Single-cell transcriptomics has been widely applied to classify neurons in the mammalian brain, while systems neuroscience has historically analyzed the encoding properties of cortical neurons without considering cell types. Here we examine how specific transcriptomic types of mouse prefrontal cortex (PFC) projection neurons relate to axonal projections and encoding properties across multiple cognitive tasks. We found that most types projected to multiple targets, and most targets received projections from multiple types, except PFC[->]PAG (periaqueductal gray). By comparing Ca2+-activity of the molecularly homogeneous PFC[->]PAG type against two heterogeneous classes in several two-alternative choice tasks in freely-moving mice, we found that all task-related signals assayed were qualitatively present in all examined classes. However, PAG-projecting neurons most potently encoded choice in cued tasks, whereas contralateral PFC-projecting neurons most potently encoded reward context in an uncued task. Thus, task signals are organized redundantly, but with clear quantitative biases across cells of specific molecular-anatomical characteristics.

neuroscience↗

bioRxiv2020

A single cell atlas reveals distinct immune landscapes in transplant and primary tumors that determine response or resistance to immunotherapy

Despite impressive responses in some patients, immunotherapy fails to cure most cancer patients. Preclinical studies indicate that radiotherapy synergizes with immunotherapy, promoting radiation-induced antitumor immunity. Nearly all preclinical immunotherapy studies utilize transplant tumor models, but cure rates of transplant tumor models treated with immunotherapy often overestimate patient responses. Here, we show that transplant tumors are cured by PD-1 blockade and radiotherapy, but identical treatment fails in autochthonous tumors. We generated a single-cell atlas of tumor-infiltrating immune cells from transplant and primary tumors treated with radiation and immunotherapy, which reveals striking differences in their immune landscapes. Although radiotherapy remodels myeloid cell phenotypes in primary and transplant tumors, only transplant tumors are enriched for CD8+ T cells that mediate tumor clearance while mice with primary sarcomas demonstrate tumor-specific tolerance. These results identify distinct microenvironments in tumors that coevolve with the immune system, which promote tolerance that must be overcome for immune-mediated cancer cures.

cancer biology↗

bioRxiv2020

Integrated profiling of single cell epigenomic and transcriptomic landscape of Parkinson's disease mouse brain

Parkinsons disease (PD) is a neurodegenerative disease leading to the impairment of execution of movement. PD pathogenesis has been largely investigated, but either restricted in bulk level or at certain cell types, which failed to capture cellular heterogeneity and intrinsic interplays among distinct cell types. To overcome this, we applied single-nucleus RNA-seq and single cell ATAC-seq on cerebellum, midbrain and striatum of PD mouse and matched control. With 74,493 cells in total, we comprehensively depicted the dysfunctions under PD pathology covering proteostasis, neuroinflammation, calcium homeostasis and extracellular neurotransmitter homeostasis. Besides, by multi-omics approach, we identified putative biomarkers for early stage of PD, based on the relationships between transcriptomic and epigenetic profiles. We located certain cell types that primarily contribute to PD early pathology, narrowing the gap between genotypes and phenotypes. Taken together, our study provides a valuable resource to dissect the molecular mechanism of PD pathogenesis at single cell level, which could facilitate the development of novel methods regarding diagnosis, monitoring and practical therapies against PD at early stage.

cell biology↗

bioRxiv2020

fMRI-guided white matter connectivity in fluid and crystallized cognitive abilities in healthy adults

This study examined within-subject differences among three fluid abilities that decline with age: reasoning, episodic memory and processing speed, compared with vocabulary, a crystallized ability that is maintained with age. The data were obtained from the Reference Ability Neural Network (RANN) study from which 221 participants had complete behavioral data for all 12 cognitive tasks, three per ability, along with fMRI and diffusion weighted imaging data. We used fMRI task activation to guide white matter tractography, and generated mean percent signal change in the regions associated with the processing of each ability along with diffusion tensor imaging measures, fractional anisotropy (FA) and mean diffusivity (MD), for each cognitive ability. Qualitatively brain regions associated with vocabulary were more localized and lateralized to the left hemisphere whereas the fluid abilities were associated with brain activations that were more distributed across the brain and bilaterally situated. Using continuous age, we observed smaller correlations between MD and age for white matter tracts connecting brain regions associated with the vocabulary ability than that for the fluid abilities, suggesting that vocabulary white matter tracts were better maintained with age. Furthermore, after multiple comparisons correction, the mean percent signal change for the episodic memory showed positive associations with behavioral performance, and the associations between MD and percent signal change differed by age such that, when divided into three age groups to further explore this interaction, only the oldest age group show a significant negative correlation between the two brain measures. Overall, the vocabulary ability may be better maintained with age due to the more localized brain regions involved, which places smaller reliance on long distance white matter tracts for signal transduction. These results support the hypothesis that functional activation and white matter structures underlying the vocabulary ability contribute to the abilitys greater resistance against aging.

neuroscience↗

bioRxiv2019

Phagocytic glia are obligatory intermediates in transmission of mutant huntingtin aggregates across neuronal synapses

Emerging evidence supports the hypothesis that pathogenic protein aggregates associated with neurodegenerative diseases spread from cell to cell through the brain in a manner akin to infectious prions. Here, we show that mutant huntingtin (mHtt) aggregates associated with Huntington disease transfer anterogradely from presynaptic to postsynaptic neurons in the adult Drosophila olfactory system. Trans-synaptic transmission of mHtt aggregates is inversely correlated with neuronal activity and blocked by inhibiting caspases in presynaptic neurons, implicating synaptic dysfunction and cell death in aggregate spreading. Remarkably, mHtt aggregate transmission across synapses requires the glial scavenger receptor Draper and involves a transient visit to the glial cytoplasm, indicating that phagocytic glia act as obligatory intermediates in aggregate spreading between synaptically-connected neurons. These findings expand our understanding of phagocytic glia as double-edged players in neurodegeneration--by clearing neurotoxic protein aggregates, but also providing an opportunity for prion-like seeds to evade phagolysosomal degradation and propagate further in the brain.

cell biology↗

bioRxiv2019

Cell-Surface Proteomic Profiling in the Fly Brain Uncovers New Wiring Regulators

Molecular interactions at the cellular interface mediate organized assembly of single cells into tissues, and thus govern the development and physiology of multicellular organisms. Here, we developed a cell-type-specific, spatiotemporally-resolved approach to profile cell-surface proteomes in intact tissues. Quantitative profiling of cell-surface proteomes of Drosophila olfactory projection neurons (PNs) in pupae and adults revealed a global down-regulation of wiring molecules and an up-regulation of synaptic molecules in the transition from developing to mature PNs. A proteome-instructed in vivo screen identified 20 new cell-surface molecules regulating neural circuit assembly, many of which belong to evolutionarily conserved protein families not previously linked to neural development. Genetic analysis further revealed that the lipoprotein receptor LRP1 cell-autonomously controls PN dendrite targeting, contributing to the formation of a precise olfactory map. These findings highlight the power of temporally-resolved in situ cell-surface proteomic profiling in discovering new regulators of brain wiring.

neuroscience↗

bioRxiv2019

Mapping Mesoscale Axonal Projections in the Mouse Brain Using A 3D Convolutional Network

The projection targets of a neuronal population are a key feature of its anatomical characterization. Historically, tissue sectioning, confocal microscopy, and manual scoring of specific regions of interest have been used to generate coarse summaries of mesoscale projectomes. We present here TrailMap, a 3D convolutional network for extracting axonal projections from intact cleared mouse brains imaged by light-sheet microscopy. TrailMap allows region-based quantification of total axon content in large and complex 3D structures after registration to a standard reference atlas. The identification of axonal structures as thin as one voxel benefits from data augmentation but also requires a loss function that tolerates errors in annotation. A network trained with volumes of serotonergic axons in all major brain regions can be generalized to map and quantify axons from thalamocortical, deep cerebellar, and cortical projection neurons, validating transfer learning as a tool to adapt the model to novel categories of axonal morphology. Speed of training, ease of use, and accuracy improve over existing tools without a need for specialized computing hardware. Given the recent emphasis on genetically and functionally defining cell types in neural circuit analysis, TrailMap will facilitate automated extraction and quantification of axons from these specific cell types at the scale of the entire mouse brain, an essential component of deciphering their connectivity.

neuroscience↗

bioRxiv2019

LIS1 determines cleavage plane positioning by regulating actomyosin-mediated cell membrane contractility

Heterozygous loss of human PAFAH1B1 (coding for LIS1) results in the disruption of neurogenesis and neuronal migration via dysregulation of microtubule (MT) stability and dynein motor function/localization that alters mitotic spindle orientation, chromosomal segregation, and nuclear migration. Recently, human induced pluripotent stem cell (iPSC) models revealed an important role for LIS1 in controlling the length of terminal cell divisions of outer radial glial (oRG) progenitors, suggesting cellular functions of LIS1 in regulating neural progenitor cell (NPC) daughter cell separation. Here we examined the late mitotic stages NPCs in vivo and mouse embryonic fibroblasts (MEFs) in vitro from Lis1-deficient mutants. Lis1-deficient neocortical NPCs and MEFs similarly exhibited cleavage plane displacement with mislocalization of furrow-associated markers, associated with actomyosin dysfunction and cell membrane hyper-contractility. Thus, it suggests LIS1 acts as a key molecular link connecting MTs/dynein and actomyosin, ensuring that cell membrane contractility is tightly controlled to execute proper daughter cell separation.

developmental biology↗

bioRxiv2019

Serotonergic plasticity in the dorsal raphe nucleus characterizes susceptibility and resilience to anhedonia

Chronic stress induces anhedonia in susceptible, but not resilient individuals, a phenomenon observed in humans as well as animal models, but the molecular mechanisms underlying susceptibility and resilience are not well understood. We hypothesized that the serotonergic system, which is implicated in stress, reward and antidepressant therapy, may play a role. We found that plasticity of the serotonergic system contributes to the differential vulnerability to stress displayed by susceptible and resilient animals. Stress-induced anhedonia was assessed in adult male rats using social defeat and intracranial self-stimulation (ICSS), while changes in serotonergic phenotype were investigated using immunohistochemistry and in situ hybridization. Susceptible, but not resilient, rats displayed an increased number of neurons expressing the biosynthetic enzyme for serotonin, tryptophan-hydroxylase-2 (TPH2), in the ventral subnucleus of the dorsal raphe nucleus (DRv). Further, a decrease in the number of DRv glutamatergic neurons was observed in all stressed animals. This neurotransmitter plasticity is dependent on DR activity, as was revealed by chemogenetic manipulation of the central amygdala, a stress-sensitive nucleus that forms a major input to the DR. Activation of amygdalar corticotropin releasing hormone (CRH)+ neurons abolished the increase in DRv TPH2+ neurons and ameliorated stress-induced anhedonia in susceptible animals. These findings show that activation of amygdalar projections induces resilience, and suppresses the gain of serotonergic phenotype in the DR that is characteristic of susceptible animals. This molecular signature of vulnerability to stress-induced anhedonia and the active nature of resilience could be a target of new treatments for stress-related disorders like depression.\n\nSIGNIFICANCE STATEMENTDepression and other mental disorders can be induced by chronic or traumatic stressors. However, some individuals are resilient and do not develop depression in response to chronic stress. A complete picture of the molecular differences between susceptible and resilient individuals is necessary to understand how plasticity of limbic circuits is associated with the pathophysiology of stress-related disorders. Using a rodent model, our study identifies a novel molecular marker of susceptibility to stress-induced anhedonia, a core symptom of depression, and a means to modulate it. These findings will guide further investigation into cellular and circuit mechanisms of resilience, and the development of new treatments for depression.

neuroscience↗

bioRxiv2019

Serotonin Neurons in the Dorsal and Medial Raphe Nuclei: from Single-Cell Transcriptomes to Whole-Brain Projections

Serotonin neurons of the dorsal and medial raphe nuclei (DR and MR) collectively innervate the entire forebrain and midbrain, modulating diverse physiology and behavior. To gain a fundamental understanding of their molecular heterogeneity, we used plate-based single-cell RNA-sequencing to generate a comprehensive dataset comprising eleven transcriptomically distinct serotonin neuron clusters. Systematic in situ hybridization mapped specific clusters to the principal DR, caudal DR, or MR. These transcriptomic clusters differentially express a rich repertoire of neuropeptides, receptors, ion channels, and transcription factors. We generated novel intersectional viral-genetic tools to access specific subpopulations. Whole-brain axonal projection mapping revealed that DR serotonin neurons co-expressing vesicular glutamate transporter-3 preferentially innervate the cortex, whereas those co-expressing thyrotropin-releasing hormone innervate subcortical regions in particular the hypothalamus. Reconstruction of 50 individual DR serotonin neurons revealed segregated axonal projection patterns at the single-cell level. Together, these results provide a molecular foundation of the heterogenous serotonin neuronal phenotypes.

neuroscience↗

bioRxiv2019

Essential roles of Hdac1 and 2 in lineage development and genome-wide DNA methylation during mouse preimplantation development

Epigenetic modifications, including DNA methylation and histone modifications, are reprogrammed considerably following fertilization during mammalian early embryonic development. Incomplete epigenetic reprogramming is a major factor leading to poor developmental outcome in embryos generated by assisted reproductive technologies, such as somatic cell nuclear transfer. However, the role of histone modifications in preimplantation development is poorly understood. Here, we show that co-knockdown (cKD) of Hdac1 and 2 (but not individually) resulted in developmental failure during the morula to blastocyst transition. This outcome was also confirmed with the use of small-molecule Hdac1/2-specific inhibitor FK228. We observed reduced cell proliferation and increased incidence of apoptosis in cKD embryos, which were likely caused by increased acetylation of Trp53. Importantly, both RNA-seq and immunostaining analysis revealed a failure of lineage specification to generate trophectoderm and pluripotent cells. Among many gene expression changes, a substantial decrease of Cdx2 may be partly accounted for by the aberrant Hippo pathway occurring in cKD embryos. In addition, we observed an increase in global DNA methylation, consistent with increased DNA methyltransferases and Uhrf1. Interestingly, deficiency of Rbbp4 and 7 (both are core components of several Hdac1/2-containing epigenetic complexes) results in similar phenotypes as those of cKD embryos. Overall, Hdac1 and 2 play redundant functions required for lineage specification, cell viability and accurate global DNA methylation, each contributing to critical developmental programs safeguarding a successful preimplantation development.\n\nSignificanceSubstantial changes to epigenetic modifications occur during preimplantation development and can be detrimental when reprogrammed incompletely. However, little is known about the role of histone modifications in early development. Co-knockdown of Hdac1 and 2, but not individually, resulted in developmental arrest during morula to blastocyst transition, which was accompanied by reduced cell number per embryo and increased incidence of apoptosis. Additionally, we observed a failure of first lineage specification to generate trophectoderm and pluripotent cells, which were associated with reduced expression of key lineage-specific genes and aberrant Hippo pathway. Moreover, an increase in global DNA methylation was found with upregulated Dnmts and Uhrf1. Thus, Hdac1 and 2 play overlapping roles in lineage development, apoptosis, and global methylation during preimplantation development.

developmental biology↗

bioRxiv2019

Functional divergence of Plexin B structural motifs in distinct steps of Drosophila olfactory circuit assembly

Plexins exhibit multitudinous, evolutionarily conserved functions in the development of nervous systems. However, how Plexins employ their diverse structural motifs in vivo to perform distinct roles in the stepwise assembly of neural circuits is unclear. Here, we systematically mutagenized structural motifs of Drosophila Plexin B (PlexB) and examined the function of these variants at multiple PlexB-mediated neurodevelopmental processes in olfactory receptor neurons: axon fasciculation, trajectory choice, and synaptic partner selection. We found that the extracellular Sema domain is essential for all three processes, the catalytic site of the intracellular RapGAP is engaged in none, and the intracellular GTPase-binding motifs are essential for trajectory choice and synaptic partner selection, but are dispensable for fasciculation. Moreover, extracellular PlexB cleavage serves as a regulatory mechanism of PlexB signaling. Thus, PlexB structural motifs have divergent roles in distinct steps of neural development, altogether contributing to the functional versatility of PlexB in neural circuit assembly.

neuroscience↗

bioRxiv2019

APOL1-G0 protects podocytes in a mouse model of HIV-associated nephropathy

BackgroundAfrican polymorphisms in the gene for Apolipoprotein L1 (APOL1) confer a survival advantage against lethal trypanosomiasis but also an increased risk for several chronic kidney diseases (CKD) including HIV-associated nephropathy (HIVAN). APOL1 is expressed in renal cells, however, the pathogenic events that lead to renal cell damage and kidney disease are not fully understood.\n\nMethodsThe podocyte function of APOL1-G0 versus APOL1-G2 in the setting of a known disease stressor was assessed using transgenic mouse models. Survival, renal pathology and function, and podocyte density were assessed in an intercross of a mouse model of HIVAN (Tg26) with two mouse models that express either APOL1-G0 or APOL1-G2 in podocytes.\n\nResultsMice that expressed HIV genes developed heavy proteinuria and glomerulosclerosis, and had significant losses in podocyte numbers and reductions in podocyte densities. Mice that co-expressed APOL1-G0 and HIV had preserved podocyte numbers and densities, with fewer morphologic manifestations typical of HIVAN pathology. Podocyte losses and pathology in mice co-expressing APOL1-G2 and HIV were not significantly different from mice expressing only HIV. Podocyte hypertrophy, a known compensatory event to stress, was increased in the mice co-expressing HIV and APOL1-G0, but absent in the mice co-expressing HIV and APOL1-G2. Mortality and renal function tests were not significantly different between groups.\n\nConclusionsAPOL1-G0 expressed in podocytes may have a protective function against podocyte loss or injury when exposed to an environmental stressor. This function appears to be absent with APOL1-G2 expression, suggesting APOL1-G2 is a loss-of-function variant.

pathology↗

bioRxiv2019

Trans-synaptic Fish-lips Signaling Prevents Misconnections between Non-synaptic Partner Olfactory Neurons

Our understanding of the mechanisms of neural circuit assembly is far from complete. Identification of new wiring molecules with novel mechanisms of action will provide new insights into how complex and heterogeneous neural circuits assemble during development. Here, we performed an RNAi screen for cell-surface molecules and identified the leucine-rich-repeat containing transmembrane protein, Fish-lips (Fili), as a novel wiring molecule in the assembly of the Drosophila olfactory circuit. Fili contributes to the precise targeting of both olfactory receptor neuron (ORN) axons as well as projection neuron (PN) dendrites. Cell-type-specific expression and genetic analyses suggest that Fili sends a trans-synaptic repulsive signal to neurites of non-partner classes that prevent their targeting to inappropriate glomeruli in the antennal lobe.\n\nSignificance StatementIn the fruit fly olfactory system, 50 classes of olfactory receptor neurons (ORNs) make precise synaptic connections with 50 classes of corresponding projection neurons (PNs). Identification of wiring molecules in this circuit can provide insight into understanding neural circuit assembly. This paper reports the role of a transmembrane protein, Fish-lips (Fili), in forming specific connections in this circuit. We found that some ORN axons are repelled by Fili, which is present on dendrites of non-matching PN class, preventing them from targeting inappropriate glomeruli. Similarly, some PN dendrites are repelled by Fili expressed by non-matching ORN class for their correct targeting. Together, these results suggest that Fili mediates repulsion between axons and dendrites of non-synaptic partners to ensure precise wiring patterns.

neuroscience↗

bioRxiv2019

Coordinating Receptor Expression and Wiring Specificity in Olfactory Receptor Neurons

The ultimate function of a neuron is determined by both its physiology and connectivity, but the transcriptional regulatory mechanisms that coordinate these two features are not well understood1-4. The Drosophila Olfactory receptor neurons (ORNs) provide an excellent system to investigate this question. As in mammals5, each Drosophila ORN class is defined by the expression of a single olfactory receptor or a unique combination thereof, which determines their odor responses, and by the single glomerulus to which their axons target, which determines how sensory signals are represented in the brain6-10. In mammals, the coordination of olfactory receptor expression and wiring specificity is accomplished in part by olfactory receptors themselves regulating ORN wiring specificity11-13. However, Drosophila olfactory receptors do not instruct axon targeting6, 14, raising the question as to how receptor expression and wiring specificity are coordinated. Using single-cell RNA-sequencing and genetic analysis, we identified 33 transcriptomic clusters for fly ORNs. We unambiguously mapped 17 to glomerular classes, demonstrating that transcriptomic clusters correspond well with anatomically and physiologically defined ORN classes. We found that each ORN expresses ~150 transcription factors (TFs), and identified a master TF that regulates both olfactory receptor expression and wiring specificity. A second TF plays distinct roles, regulating only receptor expression in one class and only wiring in another. Thus, fly ORNs utilize diverse transcriptional strategies to coordinate physiology and connectivity.

neuroscience↗

bioRxiv2019

The Developmental Transcriptome for Lytechinus variegatus Exhibits Temporally Punctuated Gene Expression Changes

Embryonic development is arguably the most complex process an organism undergoes during its lifetime, and understanding this complexity is best approached with a systems-level perspective. The sea urchin has become a highly valuable model organism for understanding developmental specification, morphogenesis, and evolution. As a non-chordate deuterostome, the sea urchin occupies an important evolutionary niche between protostomes and vertebrates. Lytechinus variegatus (Lv) is an Atlantic species that has been well studied, and which has provided important insights into signal transduction, patterning, and morphogenetic changes during embryonic and larval development. The Pacific species, Strongylocentrotus purpuratus (Sp), is another well-studied sea urchin, particularly for gene regulatory networks (GRNs) and cis-regulatory analyses. A well-annotated genome and transcriptome for Sp are available, but similar resources have not been developed for Lv. Here, we provide an analysis of the Lv transcriptome at 11 timepoints during embryonic and larval development. The data indicate that the gene regulatory networks that underlie specification are well-conserved among sea urchin species. We show that the major transitions in variation of embryonic transcription divide the developmental time series into four distinct, temporally sequential phases. Our work shows that sea urchin development occurs via sequential intervals of relatively stable gene expression states that are punctuated by abrupt transitions.

developmental biology↗