1
|
Żak M, Støle TP, Plagnol V, Daudet N. Regulation of otic neurosensory specification by Notch and Wnt signalling: insights from RNA-seq screenings in the embryonic chicken inner ear. Front Cell Dev Biol 2023; 11:1245330. [PMID: 37900277 PMCID: PMC10600479 DOI: 10.3389/fcell.2023.1245330] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/26/2023] [Accepted: 09/25/2023] [Indexed: 10/31/2023] Open
Abstract
The Notch and Wnt signalling pathways play key roles in the formation of inner ear sensory organs, but little is known about their transcriptional effectors and targets in this context. Here, we perturbed Notch and Wnt activities in the embryonic chicken otic vesicle using pharmacological treatment or in ovo electroporation of plasmid DNA, and used RNA-Seq to analyse the resulting changes in gene expression. Compared to pharmacological treatments, in ovo electroporation changed the expression of fewer genes, a likely consequence of the variability and mosaicism of transfection. The pharmacological inhibition of Notch activity induced a rapid change in the expression of known effectors of this pathway and genes associated with neurogenesis, consistent with a switch towards an otic neurosensory fate. The Wnt datasets contained many genes associated with a neurosensory biological function, confirming the importance of this pathway for neurosensory specification in the otocyst. Finally, the results of a preliminary gain-of-function screening of selected transcription factors and Wnt signalling components suggest that the endogenous programs of otic neurosensory specification are very robust, and in general unaffected by the overexpression of a single factor. Altogether this work provides new insights into the effectors and candidate targets of the Notch and Wnt pathways in the early developing inner ear and could serve as a useful reference for future functional genomics experiments in the embryonic avian inner ear.
Collapse
Affiliation(s)
- Magdalena Żak
- UCL Ear Institute, University College London, London, United Kingdom
| | - Thea P. Støle
- UCL Ear Institute, University College London, London, United Kingdom
| | - Vincent Plagnol
- Genetics Institute, University College London, London, United Kingdom
| | - Nicolas Daudet
- UCL Ear Institute, University College London, London, United Kingdom
| |
Collapse
|
2
|
Zhu XN, Wang YZ, Li C, Wu HY, Zhang R, Hu XX, Zhang R, Hu XX. Chicken chromatin accessibility atlas accelerates epigenetic annotation of birds and gene fine-mapping associated with growth traits. Zool Res 2023; 44:53-62. [PMID: 36317479 PMCID: PMC9841184 DOI: 10.24272/j.issn.2095-8137.2022.228] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/04/2023] Open
Abstract
The development of epigenetic maps, such as the ENCODE project in humans, provides resources for gene regulation studies and a reference for research of disease-related regulatory elements. However, epigenetic information, such as a bird-specific chromatin accessibility atlas, is currently lacking for the thousands of bird species currently described. The major genomic difference between birds and mammals is their shorter introns and intergenic distances, which seriously hinders the use of humans and mice as a reference for studying the function of important regulatory regions in birds. In this study, using chicken as a model bird species, we systematically compiled a chicken chromatin accessibility atlas using 53 Assay of Transposase Accessible Chromatin sequencing (ATAC-seq) samples across 11 tissues. An average of 50 796 open chromatin regions were identified per sample, cumulatively accounting for 20.36% of the chicken genome. Tissue specificity was largely reflected by differences in intergenic and intronic peaks, with specific functional regulation achieved by two mechanisms: recruitment of several sequence-specific transcription factors and direct regulation of adjacent functional genes. By integrating data from genome-wide association studies, our results suggest that chicken body weight is driven by different regulatory variants active in growth-relevant tissues. We propose CAB39L (active in the duodenum), RCBTB1 (muscle and liver), and novel long non-coding RNA ENSGALG00000053256 (bone) as candidate genes regulating chicken body weight. Overall, this study demonstrates the value of epigenetic data in fine-mapping functional variants and provides a compendium of resources for further research on the epigenetics and evolution of birds and mammals.
Collapse
Affiliation(s)
- Xiao-Ning Zhu
- State Key Laboratory of Agrobiotechnology, College of Biological Sciences, China Agricultural University, Beijing 100193, China
| | - Yu-Zhe Wang
- State Key Laboratory of Agrobiotechnology, College of Biological Sciences, China Agricultural University, Beijing 100193, China,National Research Facility for Phenotypic and Genotypic Analysis of Model Animals (Beijing), China Agricultural University, Beijing 100193, China,E-mail:
| | - Chong Li
- State Key Laboratory of Agrobiotechnology, College of Biological Sciences, China Agricultural University, Beijing 100193, China
| | - Han-Yu Wu
- State Key Laboratory of Agrobiotechnology, College of Biological Sciences, China Agricultural University, Beijing 100193, China,National Research Facility for Phenotypic and Genotypic Analysis of Model Animals (Beijing), China Agricultural University, Beijing 100193, China
| | - Ran Zhang
- State Key Laboratory of Agrobiotechnology, College of Biological Sciences, China Agricultural University, Beijing 100193, China
| | - Xiao-Xiang Hu
- State Key Laboratory of Agrobiotechnology, College of Biological Sciences, China Agricultural University, Beijing 100193, China,National Research Facility for Phenotypic and Genotypic Analysis of Model Animals (Beijing), China Agricultural University, Beijing 100193, China,
| | | | | | | | | |
Collapse
|
3
|
Olmos-Carreño CL, Figueres-Oñate M, Scicolone GE, López-Mascaraque L. Cell Fate of Retinal Progenitor Cells: In Ovo UbC-StarTrack Analysis. Int J Mol Sci 2022; 23:ijms232012388. [PMID: 36293245 PMCID: PMC9604099 DOI: 10.3390/ijms232012388] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2022] [Revised: 10/10/2022] [Accepted: 10/12/2022] [Indexed: 11/28/2022] Open
Abstract
Clonal cell analysis outlines the ontogenic potential of single progenitor cells, allowing the elucidation of the neural heterogeneity among different cell types and their lineages. In this work, we analyze the potency of retinal stem/progenitor cells through development using the chick embryo as a model. We implemented in ovo the clonal genetic tracing strategy UbC-StarTrack for tracking retinal cell lineages derived from individual progenitors of the ciliary margin at E3.5 (HH21-22). The clonal assignment of the derived-cell progeny was performed in the neural retina at E11.5-12 (HH38) through the identification of sibling cells as cells expressing the same combination of fluorophores. Moreover, cell types were assessed based on their cellular morphology and laminar location. Ciliary margin derived-cell progenies are organized in columnar associations distributed along the peripheral retina with a limited tangential dispersion. The analysis revealed that, at the early stages of development, this region harbors multipotent and committed progenitor cells.
Collapse
Affiliation(s)
- Cindy L. Olmos-Carreño
- Instituto de Biología Celular y Neurociencias “Prof. E. De Robertis” (IBCN), CONICET and Departamento de Biología Celular, Histología, Embriología y Genética, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires 1121, Argentina
- Instituto Cajal-CSIC, Molecular, Cellular and Developmental Neurobiology Department, 28002 Madrid, Spain
| | - María Figueres-Oñate
- Instituto Cajal-CSIC, Molecular, Cellular and Developmental Neurobiology Department, 28002 Madrid, Spain
- Correspondence: (M.F.-O.); (L.L.-M.)
| | - Gabriel E. Scicolone
- Instituto de Biología Celular y Neurociencias “Prof. E. De Robertis” (IBCN), CONICET and Departamento de Biología Celular, Histología, Embriología y Genética, Facultad de Medicina, Universidad de Buenos Aires, Buenos Aires 1121, Argentina
| | - Laura López-Mascaraque
- Instituto Cajal-CSIC, Molecular, Cellular and Developmental Neurobiology Department, 28002 Madrid, Spain
- Correspondence: (M.F.-O.); (L.L.-M.)
| |
Collapse
|
4
|
Bachu VS, Kandoi S, Park KU, Kaufman ML, Schwanke M, Lamba DA, Brzezinski JA. An enhancer located in a Pde6c intron drives transient expression in the cone photoreceptors of developing mouse and human retinas. Dev Biol 2022; 488:131-150. [PMID: 35644251 PMCID: PMC10676565 DOI: 10.1016/j.ydbio.2022.05.012] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2022] [Revised: 04/29/2022] [Accepted: 05/19/2022] [Indexed: 02/04/2023]
Abstract
How cone photoreceptors are formed during retinal development is only partially known. This is in part because we do not fully understand the gene regulatory network responsible for cone genesis. We reasoned that cis-regulatory elements (enhancers) active in nascent cones would be regulated by the same upstream network that controls cone formation. To dissect this network, we searched for enhancers active in developing cones. By electroporating enhancer-driven fluorescent reporter plasmids, we observed that a sequence within an intron of the cone-specific Pde6c gene acted as an enhancer in developing mouse cones. Similar fluorescent reporter plasmids were used to generate stable transgenic human induced pluripotent stem cells that were then grown into three-dimensional human retinal organoids. These organoids contained fluorescently labeled cones, demonstrating that the Pde6c enhancer was also active in human cones. We observed that enhancer activity was transient and labeled a minor population of developing rod photoreceptors in both mouse and human systems. This cone-enriched pattern argues that the Pde6c enhancer is activated in cells poised between rod and cone fates. Additionally, it suggests that the Pde6c enhancer is activated by the same regulatory network that selects or stabilizes cone fate choice. To further understand this regulatory network, we identified essential enhancer sequence regions through a series of mutagenesis experiments. This suggested that the Pde6c enhancer was regulated by transcription factor binding at five or more locations. Binding site predictions implicated transcription factor families known to control photoreceptor formation and families not previously associated with cone development. These results provide a framework for deciphering the gene regulatory network that controls cone genesis in both human and mouse systems. Our new transgenic human stem cell lines provide a tool for determining which cone developmental mechanisms are shared and distinct between mice and humans.
Collapse
Affiliation(s)
- Vismaya S Bachu
- Department of Ophthalmology, University of Colorado Anschutz Medical Campus, Aurora, CO, USA; Johns Hopkins University School of Medicine, Baltimore, MD, USA
| | - Sangeetha Kandoi
- Department of Ophthalmology, University of California San Francisco, San Francisco, CA, USA; Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California San Francisco, San Francisco, CA, USA
| | - Ko Uoon Park
- Department of Ophthalmology, University of Colorado Anschutz Medical Campus, Aurora, CO, USA
| | - Michael L Kaufman
- Department of Ophthalmology, University of Colorado Anschutz Medical Campus, Aurora, CO, USA
| | - Michael Schwanke
- Department of Ophthalmology, University of Colorado Anschutz Medical Campus, Aurora, CO, USA
| | - Deepak A Lamba
- Department of Ophthalmology, University of California San Francisco, San Francisco, CA, USA; Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, University of California San Francisco, San Francisco, CA, USA
| | - Joseph A Brzezinski
- Department of Ophthalmology, University of Colorado Anschutz Medical Campus, Aurora, CO, USA.
| |
Collapse
|
5
|
MYCN induces cell-specific tumorigenic growth in RB1-proficient human retinal organoid and chicken retina models of retinoblastoma. Oncogenesis 2022; 11:34. [PMID: 35729105 PMCID: PMC9213451 DOI: 10.1038/s41389-022-00409-3] [Citation(s) in RCA: 4] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/10/2022] [Revised: 06/03/2022] [Accepted: 06/07/2022] [Indexed: 11/09/2022] Open
Abstract
Retinoblastoma is a rare, intraocular paediatric cancer that originates in the neural retina and is most frequently caused by bi-allelic loss of RB1 gene function. Other oncogenic mutations, such as amplification and increased expression of the MYCN gene, have been found even with proficient RB1 function. In this study, we investigated whether MYCN over-expression can drive carcinogenesis independently of RB1 loss-of-function mutations. The aim was to elucidate the events that result in carcinogenesis and identify the cancer cell-of-origin. We used the chicken retina, a well-established model for studying retinal neurogenesis, and established human embryonic stem cell-derived retinal organoids as model systems. We over-expressed MYCN by electroporation of piggyBac genome-integrating expression vectors. We found that over-expression of MYCN induced tumorigenic growth with high frequency in RB1-proficient chicken retinas and human organoids. In both systems, the tumorigenic cells expressed markers for undifferentiated cone photoreceptor/horizontal cell progenitors. The over-expression resulted in metastatic retinoblastoma within 7–9 weeks in chicken. Cells expressing MYCN could be grown in vitro and, when orthotopically injected, formed tumours that infiltrated the sclera and optic nerve and expressed markers for cone progenitors. Investigation of the tumour cell phenotype determined that the potential for neoplastic growth was embryonic stage-dependent and featured a cell-specific resistance to apoptosis in the cone/horizontal cell lineage, but not in ganglion or amacrine cells. We conclude that MYCN over-expression is sufficient to drive tumorigenesis and that a cell-specific resistance to apoptosis in the cone/horizontal cell lineage mediates the cancer phenotype. ![]()
Collapse
|
6
|
Daghsni M, Aldiri I. Building a Mammalian Retina: An Eye on Chromatin Structure. Front Genet 2021; 12:775205. [PMID: 34764989 PMCID: PMC8576187 DOI: 10.3389/fgene.2021.775205] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/13/2021] [Accepted: 10/08/2021] [Indexed: 11/13/2022] Open
Abstract
Regulation of gene expression by chromatin structure has been under intensive investigation, establishing nuclear organization and genome architecture as a potent and effective means of regulating developmental processes. The substantial growth in our knowledge of the molecular mechanisms underlying retinogenesis has been powered by several genome-wide based tools that mapped chromatin organization at multiple cellular and biochemical levels. Studies profiling the retinal epigenome and transcriptome have allowed the systematic annotation of putative cis-regulatory elements associated with transcriptional programs that drive retinal neural differentiation, laying the groundwork to understand spatiotemporal retinal gene regulation at a mechanistic level. In this review, we outline recent advances in our understanding of the chromatin architecture in the mammalian retina during development and disease. We focus on the emerging roles of non-coding regulatory elements in controlling retinal cell-type specific transcriptional programs, and discuss potential implications in untangling the etiology of eye-related disorders.
Collapse
Affiliation(s)
- Marwa Daghsni
- Department of Ophthalmology, School of Medicine, University of Pittsburgh, Pittsburgh, PA, United States
| | - Issam Aldiri
- Department of Ophthalmology, School of Medicine, University of Pittsburgh, Pittsburgh, PA, United States.,Department of Developmental Biology, School of Medicine, University of Pittsburgh, Pittsburgh, PA, United States.,Louis J. Fox Center for Vision Restoration, University of Pittsburgh, Pittsburgh, PA, United States
| |
Collapse
|
7
|
Chen X, Emerson MM. Notch signaling represses cone photoreceptor formation through the regulation of retinal progenitor cell states. Sci Rep 2021; 11:14525. [PMID: 34267251 PMCID: PMC8282820 DOI: 10.1038/s41598-021-93692-w] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2021] [Accepted: 06/25/2021] [Indexed: 11/29/2022] Open
Abstract
Notch signaling is required to repress the formation of vertebrate cone photoreceptors and to maintain the proliferative potential of multipotent retinal progenitor cells. However, the mechanism by which Notch signaling controls these processes is unknown. Recently, restricted retinal progenitor cells with limited proliferation capacity and that preferentially generate cone photoreceptors have been identified. Thus, there are several potential steps during cone genesis that Notch signaling could act. Here we use cell type specific cis-regulatory elements to localize the primary role of Notch signaling in cone genesis to the formation of restricted retinal progenitor cells from multipotent retinal progenitor cells. Localized inhibition of Notch signaling in restricted progenitor cells does not alter the number of cones derived from these cells. Cell cycle promotion is not a primary effect of Notch signaling but an indirect effect on progenitor cell state transitions that leads to depletion of the multipotent progenitor cell population. Taken together, this suggests that the role of Notch signaling in cone photoreceptor formation and proliferation are both mediated by a localized function of Notch in multipotent retinal progenitor cells to repress the formation of restricted progenitor cells.
Collapse
Affiliation(s)
- Xueqing Chen
- Biology PhD Program, The Graduate Center, The City University of New York, New York, NY, 10016, USA
- Department of Biology, The City College of New York, The City University of New York, New York, NY, 10031, USA
| | - Mark M Emerson
- Biology PhD Program, The Graduate Center, The City University of New York, New York, NY, 10016, USA.
- Department of Biology, The City College of New York, The City University of New York, New York, NY, 10031, USA.
- Biochemistry PhD Program, The Graduate Center, The City University of New York, New York, NY, 10016, USA.
| |
Collapse
|
8
|
Lonfat N, Wang S, Lee C, Garcia M, Choi J, Park PJ, Cepko C. Cis-regulatory dissection of cone development reveals a broad role for Otx2 and Oc transcription factors. Development 2021; 148:dev198549. [PMID: 33929509 PMCID: PMC8126413 DOI: 10.1242/dev.198549] [Citation(s) in RCA: 7] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/13/2020] [Accepted: 03/31/2021] [Indexed: 11/20/2022]
Abstract
The vertebrate retina is generated by retinal progenitor cells (RPCs), which produce >100 cell types. Although some RPCs produce many cell types, other RPCs produce restricted types of daughter cells, such as a cone photoreceptor and a horizontal cell (HC). We used genome-wide assays of chromatin structure to compare the profiles of a restricted cone/HC RPC and those of other RPCs in chicks. These data nominated regions of regulatory activity, which were tested in tissue, leading to the identification of many cis-regulatory modules (CRMs) active in cone/HC RPCs and developing cones. Two transcription factors, Otx2 and Oc1, were found to bind to many of these CRMs, including those near genes important for cone development and function, and their binding sites were required for activity. We also found that Otx2 has a predicted autoregulatory CRM. These results suggest that Otx2, Oc1 and possibly other Onecut proteins have a broad role in coordinating cone development and function. The many newly discovered CRMs for cones are potentially useful reagents for gene therapy of cone diseases.
Collapse
Affiliation(s)
- Nicolas Lonfat
- Department of Genetics, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
- Department of Ophthalmology, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
| | - Su Wang
- Department of Biomedical Informatics, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
| | - ChangHee Lee
- Department of Genetics, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
| | - Mauricio Garcia
- Department of Genetics, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
- Department of Ophthalmology, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
| | - Jiho Choi
- Department of Genetics, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
- Department of Ophthalmology, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
| | - Peter J. Park
- Department of Biomedical Informatics, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
| | - Connie Cepko
- Department of Genetics, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
- Department of Ophthalmology, Blavatnik Institute; Harvard Medical School, Boston, MA 02115, USA
- Howard Hughes Medical Institute, Harvard Medical School, Boston, MA 02115, USA
| |
Collapse
|
9
|
Schick E, Gonzalez KC, Dutta P, Hossain K, Ghinia Tegla MG, Emerson MM. Early cis-regulatory events in the formation of retinal horizontal cells. Dev Biol 2021; 476:88-100. [PMID: 33774011 DOI: 10.1016/j.ydbio.2021.03.016] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2020] [Revised: 03/17/2021] [Accepted: 03/18/2021] [Indexed: 11/25/2022]
Abstract
During retinal development, multipotent and restricted progenitor cells generate all of the neuronal cells of the retina. Among these are horizontal cells, which are interneurons that modulate the light-induced signal from photoreceptors. This study utilizes the identification of novel cis-regulatory elements as a method to examine the gene regulatory networks that direct the development of horizontal cells. Here we describe a screen for cis-regulatory elements, or enhancers, for the horizontal cell-associated genes PTF1A, ONECUT1 (OC1), TFAP2A (AP2A), and LHX1. The OC1ECR22 and Tfap2aACR5 elements were shown to be potential enhancers for OC1 and TFAP2A, respectively, and to be specifically active in developing horizontal cells. The OC1ECR22 element is activated by PTF1A and RBPJ, which translates to regulation of OC1 expression and suggests that PTF1A is a direct activator of OC1 expression in developing horizontal cells. The region within the Tfap2aACR5 element that is responsible for its activation was determined to be a 100 bp sequence named Motif 4. Both OC1ECR22 and Tfap2aACR5 are negatively regulated by the nuclear receptors THRB and RXRG, as is the expression of OC1 and AP2A, suggesting that nuclear receptors may have a role in the negative regulation of horizontal cell development.
Collapse
Affiliation(s)
- Estie Schick
- Biology PhD Program, Graduate Center, City University of New York, New York, NY, 10016, USA
| | - Kevin C Gonzalez
- Department of Biology, The City College of New York, City University of New York, New York, NY, 10031, USA
| | - Pooja Dutta
- Department of Biology, The City College of New York, City University of New York, New York, NY, 10031, USA
| | - Kazi Hossain
- Department of Biology, The City College of New York, City University of New York, New York, NY, 10031, USA
| | - Miruna G Ghinia Tegla
- Department of Biology, The City College of New York, City University of New York, New York, NY, 10031, USA
| | - Mark M Emerson
- Biology PhD Program, Graduate Center, City University of New York, New York, NY, 10016, USA; Department of Biology, The City College of New York, City University of New York, New York, NY, 10031, USA; Biochemistry PhD Program, Graduate Center, City University of New York, New York, NY, 10016, USA.
| |
Collapse
|
10
|
Transcription Factors of the bHLH Family Delineate Vertebrate Landmarks in the Nervous System of a Simple Chordate. Genes (Basel) 2020; 11:genes11111262. [PMID: 33114624 PMCID: PMC7693978 DOI: 10.3390/genes11111262] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/19/2020] [Accepted: 10/12/2020] [Indexed: 02/07/2023] Open
Abstract
Tunicates are marine invertebrates whose tadpole-like larvae feature a highly simplified version of the chordate body plan. Similar to their distant vertebrate relatives, tunicate larvae develop a regionalized central nervous system and form distinct neural structures, which include a rostral sensory vesicle, a motor ganglion, and a caudal nerve cord. The sensory vesicle contains a photoreceptive complex and a statocyst, and based on the comparable expression patterns of evolutionarily conserved marker genes, it is believed to include proto-hypothalamic and proto-retinal territories. The evolutionarily conserved molecular fingerprints of these landmarks of the vertebrate brain consist of genes encoding for different transcription factors, and of the gene batteries that they control, and include several members of the bHLH family. Here we review the complement of bHLH genes present in the streamlined genome of the tunicate Ciona robusta and their current classification, and summarize recent studies on proneural bHLH transcription factors and their expression territories. We discuss the possible roles of bHLH genes in establishing the molecular compartmentalization of the enticing nervous system of this unassuming chordate.
Collapse
|