Lim homeobox genes in the Ctenophore Mnemiopsis leidyi: the evolution of neural cell type specification

Cytoplasmic Polyadenylation Is an Ancestral Hallmark of Early Development in Animals

Differential regulation of gene expression has produced the astonishing diversity of life on Earth. Understanding the origin and evolution of mechanistic innovations for control of gene expression is therefore integral to evolutionary and developmental biology. Cytoplasmic polyadenylation is the biochemical extension of polyadenosine at the 3'-end of cytoplasmic mRNAs. This process regulates the translation of specific maternal transcripts and is mediated by the Cytoplasmic Polyadenylation Element-Binding Protein family (CPEBs). Genes that code for CPEBs are amongst a very few that are present in animals but missing in nonanimal lineages. Whether cytoplasmic polyadenylation is present in non-bilaterian animals (i.e., sponges, ctenophores, placozoans, and cnidarians) remains unknown. We have conducted phylogenetic analyses of CPEBs, and our results show that CPEB1 and CPEB2 subfamilies originated in the animal stem lineage. Our assessment of expression in the sea anemone, Nematostella vectensis (Cnidaria), and the comb jelly, Mnemiopsis leidyi (Ctenophora), demonstrates that maternal expression of CPEB1 and the catalytic subunit of the cytoplasmic polyadenylation machinery (GLD2) is an ancient feature that is conserved across animals. Furthermore, our measurements of poly(A)-tail elongation reveal that key targets of cytoplasmic polyadenylation are shared between vertebrates, cnidarians, and ctenophores, indicating that this mechanism orchestrates a regulatory network that is conserved throughout animal evolution. We postulate that cytoplasmic polyadenylation through CPEBs was a fundamental innovation that contributed to animal evolution from unicellular life.

Nitric oxide signaling in ctenophores

Nitric oxide (NO) is one of the most ancient and versatile signal molecules across all domains of life. NO signaling might also play an essential role in the origin of animal organization. Yet, practically nothing is known about the distribution and functions of NO-dependent signaling pathways in representatives of early branching metazoans such as Ctenophora. Here, we explore the presence and organization of NO signaling components using Mnemiopsis and kin as essential reference species. We show that NO synthase (NOS) is present in at least eight ctenophore species, including Euplokamis and Coeloplana, representing the most basal ctenophore lineages. However, NOS could be secondarily lost in many other ctenophores, including Pleurobrachia and Beroe. In Mnemiopsis leidyi, NOS is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms. Ctenophores also possess soluble guanylyl cyclases as potential NO receptors with weak but differential expression across tissues. Combined, these data indicate that the canonical NO-cGMP signaling pathways existed in the common ancestor of animals and could be involved in the control of morphogenesis, cilia activities, feeding and different behaviors.

Cell polarity signalling at the birth of multicellularity: What can we learn from the first animals

The innovation of multicellularity has driven the unparalleled evolution of animals (Metazoa). But how is a multicellular organism formed and how is its architecture maintained faithfully? The defining properties and rules required for the establishment of the architecture of multicellular organisms include the development of adhesive cell interactions, orientation of division axis, and the ability to reposition daughter cells over long distances. Central to all these properties is the ability to generate asymmetry (polarity), coordinated by a highly conserved set of proteins known as cell polarity regulators. The cell polarity complexes, Scribble, Par and Crumbs, are considered to be a metazoan innovation with apicobasal polarity and adherens junctions both believed to be present in all animals. A better understanding of the fundamental mechanisms regulating cell polarity and tissue architecture should provide key insights into the development and regeneration of all animals including humans. Here we review what is currently known about cell polarity and its control in the most basal metazoans, and how these first examples of multicellular life can inform us about the core mechanisms of tissue organisation and repair, and ultimately diseases of tissue organisation, such as cancer.

Mass spectrometry of short peptides reveals common features of metazoan peptidergic neurons

The evolutionary origins of neurons remain unknown. Although recent genome data of extant early-branching animals have shown that neural genes existed in the common ancestor of animals, the physiological and genetic properties of neurons in the early evolutionary phase are still unclear. Here, we performed a mass spectrometry-based comprehensive survey of short peptides from early-branching lineages Cnidaria, Porifera and Ctenophora. We identified a number of mature ctenophore neuropeptides that are expressed in neurons associated with sensory, muscular and digestive systems. The ctenophore peptides are stored in vesicles in cell bodies and neurites, suggesting volume transmission similar to that of cnidarian and bilaterian peptidergic systems. A comparison of genetic characteristics revealed that the peptide-expressing cells of Cnidaria and Ctenophora express the vast majority of genes that have pivotal roles in maturation, secretion and degradation of neuropeptides in Bilateria. Functional analysis of neuropeptides and prediction of receptors with machine learning demonstrated peptide regulation of a wide range of target effector cells, including cells of muscular systems. The striking parallels between the peptidergic neuronal properties of Cnidaria and Bilateria and those of Ctenophora, the most basal neuron-bearing animals, suggest a common evolutionary origin of metazoan peptidergic nervous systems.

Multigenerational laboratory culture of pelagic ctenophores and CRISPR-Cas9 genome editing in the lobate Mnemiopsis leidyi

Despite long-standing experimental interest in ctenophores due to their unique biology, ecological influence and evolutionary status, previous work has largely been constrained by the periodic seasonal availability of wild-caught animals and difficulty in reliably closing the life cycle. To address this problem, we have developed straightforward protocols that can be easily implemented to establish long-term multigenerational cultures for biological experimentation in the laboratory. In this protocol, we describe the continuous culture of the Atlantic lobate ctenophore Mnemiopsis leidyi. A rapid 3-week egg-to-egg generation time makes Mnemiopsis suitable for a wide range of experimental genetic, cellular, embryological, physiological, developmental, ecological and evolutionary studies. We provide recommendations for general husbandry to close the life cycle of Mnemiopsis in the laboratory, including feeding requirements, light-induced spawning, collection of embryos and rearing of juveniles to adults. These protocols have been successfully applied to maintain long-term multigenerational cultures of several species of pelagic ctenophores, and can be utilized by laboratories lacking easy access to the ocean. We also provide protocols for targeted genome editing via microinjection with CRISPR-Cas9 that can be completed within ~2 weeks, including single-guide RNA synthesis, early embryo microinjection, phenotype assessment and sequence validation of genome edits. These protocols provide a foundation for using Mnemiopsis as a model organism for functional genomic analyses in ctenophores.

Functional differentiation of two lhx8 paralogs and possible regulatory role of lhx8a in Japanese flounder (Paralichthys olivaceus)

Lhx8, belonging to the LIM-Homebox family, is involved in the tooth, nervous system, and primordial follicles development in mammals. However, little is known about the regulatory roles of lhx8 in teleosts. In this study, two lhx8 duplicates were identified in Paralichthys olivaceus, termed Polhx8a and Polhx8b, respectively. Bioinformatic analysis showed that Polhx8a was more likely to be a teleost-specific paralog. According to expression analysis, Polhx8a transcripts were almost exclusively concentrated in the oocytes, while Polhx8b was weakly expressed in the spleen, gill, and some facial organs, indicating sub-functionalization of this gene pair during evolution. Furthermore, Polhx8a mRNA level elevated from perinucleolar oocyte (PNO) stage to vitellogenic oocyte (VO) stage transition and changed after exogenous hormone stimulation, proving that Polhx8a was involved in the oocyte development and could be regulated by sex hormones. Yeast two-hybrid, bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (co-IP) experiments captured the positive protein interactions between PoLhx8a and the other two oocyte-specific transcription factors: PoFigla and PoNobox. After knocking down lhx8a in embryos or adult ovaries in vivo, the expression of oocyte-associated genes was significantly down-regulated (P < 0.05). Our findings suggest the evolution and functional differentiation of lhx8 genes, and shed light on the potential role of lhx8a in protein interactions and gene regulation in teleosts.

Tumor suppressor LHX6 upregulation contributes to the inhibitory effect of miR-346 knockdown on colorectal cancer cell growth

Colorectal cancer (CRC) is one of the prevalent types of human malignancies and ranks as the second leading cause of cancer-associated death worldwide. Dysregulated miRNAs have been promulgated as oncogenes or tumor-suppressive genes participating in the initiation and progression of CRC. A recent study reported that miR-346 was highly expressed in CRC patients. However, the biological role and underlying mechanism of miR-346 in CRC remain elusive. qRT-PCR and western blot assays were employed to detect miR-346 and LIM homeobox domain 6 (LHX6) expression in CRC cells. Cell proliferation was evaluated by CCK-8 and BrdU assays. Apoptosis was evaluated by TUNEL assay. The interaction between miR-346 and LHX6 was assessed by luciferase reporter assay. Results showed that miR-346 expression was increased and LHX6 expression was reduced in CRC cells. miR-346 knockdown and LHX6 overexpression inhibited proliferation and promoted apoptosis of CRC cells. Additionally, we found that miR-346 negatively regulated LHX6 expression in CRC cells by directly targeting LHX6. LHX6 knockdown partially attenuated anti-miR-346-induced proliferation reduction and apoptosis promotion in CRC cells. Furthermore, miR-346 knockdown inhibited the protein kinase B (Akt)/mechanistic target of rapamycin (mTOR) pathway in CRC cells by targeting LHX6. The present study indicated that miR-346 knockdown repressed cell growth in CRC cells by upregulating LHX6, and this was associated with inactivation of the Akt/mTOR pathway.

Evolution of synapses and neurotransmitter systems: The divide-and-conquer model for early neural cell-type evolution

Nervous systems evolved around 560 million years ago to coordinate and empower animal bodies. Ctenophores - one of the earliest-branching lineages - are thought to share a few neuronal genes with bilaterians and may have evolved neurons convergently. Here we review our current understanding of the evolution of neuronal molecules in nonbilaterians. We also reanalyse single-cell sequencing data in light of new cell-cluster identities from a ctenophore and uncover evidence supporting the homology of one ctenophore neuron-type with neurons in Bilateria. The specific coexpression of the presynaptic proteins Unc13 and RIM with voltage-gated channels, neuropeptides and homeobox genes pinpoint a spiking sensory-peptidergic cell in the ctenophore mouth. Similar Unc13-RIM neurons may have been present in the first eumetazoans to rise to dominance only in stem Bilateria. We hypothesise that the Unc13-RIM lineage ancestrally innervated the mouth and conquered other parts of the body with the rise of macrophagy and predation during the Cambrian explosion.

A developmental perspective on the evolution of the nervous system

The evolution of nervous systems in animals has always fascinated biologists, and thus multiple evolutionary scenarios have been proposed to explain the appearance of neurons and complex neuronal centers. However, the absence of a robust phylogenetic framework for animal interrelationships, the lack of a mechanistic understanding of development, and a recapitulative view of animal ontogeny have traditionally limited these scenarios. Only recently, the integration of advanced molecular and morphological studies in a broad range of animals has allowed to trace the evolution of developmental and neuronal characters on a better-resolved animal phylogeny. This has falsified most traditional scenarios for nervous system evolution, paving the way for the emergence of new testable hypotheses. Here we summarize recent progress in studies of nervous system development in major animal lineages and formulate some of the arising questions. In particular, we focus on how lineage analyses of nervous system development and a comparative study of the expression of neural-related genes has influenced our understanding of the evolution of an elaborated central nervous system in Bilateria. We argue that a phylogeny-guided study of neural development combining thorough descriptive and functional analyses is key to establish more robust scenarios for the origin and evolution of animal nervous systems.

Integrating Embryonic Development and Evolutionary History to Characterize Tentacle-Specific Cell Types in a Ctenophore

The origin of novel traits can promote expansion into new niches and drive speciation. Ctenophores (comb jellies) are unified by their possession of a novel cell type: the colloblast, an adhesive cell found only in the tentacles. Although colloblast-laden tentacles are fundamental for prey capture among ctenophores, some species have tentacles lacking colloblasts and others have lost their tentacles completely. We used transcriptomes from 36 ctenophore species to identify gene losses that occurred specifically in lineages lacking colloblasts and tentacles. We cross-referenced these colloblast- and tentacle-specific candidate genes with temporal RNA-Seq during embryogenesis in Mnemiopsis leidyi and found that both sets of candidates are preferentially expressed during tentacle morphogenesis. We also demonstrate significant upregulation of candidates from both data sets in the tentacle bulb of adults. Both sets of candidates were enriched for an N-terminal signal peptide and protein domains associated with secretion; among tentacle candidates we also identified orthologs of cnidarian toxin proteins, presenting tantalizing evidence that ctenophore tentacles may secrete toxins along with their adhesive. Finally, using cell lineage tracing, we demonstrate that colloblasts and neurons share a common progenitor, suggesting the evolution of colloblasts involved co-option of a neurosecretory gene regulatory network. Together these data offer an initial glimpse into the genetic architecture underlying ctenophore cell-type diversity.

Embracing Uncertainty in Reconstructing Early Animal Evolution

The origin of animals, one of the major transitions in evolution, remains mysterious. Many key aspects of animal evolution can be reconstructed by comparing living species within a robust phylogenetic framework. However, uncertainty remains regarding the evolutionary relationships between two ancient animal lineages - sponges and ctenophores - and the remaining animal phyla. Comparative morphology and some phylogenomic analyses support the view that sponges represent the sister lineage to the rest of the animals, while other phylogenomic analyses support ctenophores, a phylum of carnivorous, gelatinous marine organisms, as the sister lineage. Here, we explore why different studies yield different answers and discuss the implications of the two alternative hypotheses for understanding the origin of animals. Reconstruction of ancient evolutionary radiations is devilishly difficult and will likely require broader sampling of sponge and ctenophore genomes, improved analytical strategies and critical analyses of the phylogenetic distribution and molecular mechanisms underlying apparently conserved traits. Rather than staking out positions in favor of the ctenophores-sister or the sponges-sister hypothesis, we submit that research programs aimed at understanding the biology of the first animals should instead embrace the uncertainty surrounding early animal evolution in their experimental designs.

Evolution of Animal Neural Systems

Nervous systems are among the most spectacular products of evolution. Their provenance and evolution have been of interest and often the subjects of intense debate since the late nineteenth century. The genomics era has provided researchers with a new set of tools with which to study the early evolution of neurons, and recent progress on the molecular evolution of the first neurons has been both exciting and frustrating. It has become increasingly obvious that genomic data are often insufficient to reconstruct complex phenotypes in deep evolutionary time because too little is known about how gene function evolves over deep time. Therefore, additional functional data across the animal tree are a prerequisite to a fuller understanding of cell evolution. To this end, we review the functional modules of neurons and the evolution of their molecular components, and we introduce the idea of hierarchical molecular evolution.

Dynamics of genomic innovation in the unicellular ancestry of animals

Which genomic innovations underpinned the origin of multicellular animals is still an open debate. Here, we investigate this question by reconstructing the genome architecture and gene family diversity of ancestral premetazoans, aiming to date the emergence of animal-like traits. Our comparative analysis involves genomes from animals and their closest unicellular relatives (the Holozoa), including four new genomes: three Ichthyosporea and Corallochytrium limacisporum. Here, we show that the earliest animals were shaped by dynamic changes in genome architecture before the emergence of multicellularity: an early burst of gene diversity in the ancestor of Holozoa, enriched in transcription factors and cell adhesion machinery, was followed by multiple and differently-timed episodes of synteny disruption, intron gain and genome expansions. Thus, the foundations of animal genome architecture were laid before the origin of complex multicellularity – highlighting the necessity of a unicellular perspective to understand early animal evolution.

DOI:http://dx.doi.org/10.7554/eLife.26036.001

Hundreds of millions of years ago, some single-celled organisms gained the ability to work together and form multicellular organisms. This transition was a major step in evolution and took place at separate times in several parts of the tree of life, including in animals, plants, fungi and algae.

Animals are some of the most complex organisms on Earth. Their single-celled ancestors were also quite genetically complex themselves and their genomes (the complete set of the organism’s DNA) already contained many genes that now coordinate the activity of the cells in a multicellular organism.

The genome of an animal typically has certain features: it is large, diverse and contains many segments (called introns) that are not genes. By seeing if the single-celled relatives of animals share these traits, it is possible to learn more about when specific genetic features first evolved, and whether they are linked to the origin of animals.

Now, Grau-Bové et al. have studied the genomes of several of the animal kingdom’s closest single-celled relatives using a technique called whole genome sequencing. This revealed that there was a period of rapid genetic change in the single-celled ancestors of animals during which their genes became much more diverse. Another ‘explosion’ of diversity happened after animals had evolved. Furthermore, the overall amount of the genomic content inside cells and the number of introns found in the genome rapidly increased in separate, independent events in both animals and their single-celled ancestors.

Future research is needed to investigate whether other multicellular life forms – such as plants, fungi and algae – originated in the same way as animal life. Understanding how the genetic material of animals evolved also helps us to understand the genetic structures that affect our health. For example, genes that coordinate the behavior of cells (and so are important for multicellular organisms) also play a role in cancer, where cells break free of this regulation to divide uncontrollably.

DOI:http://dx.doi.org/10.7554/eLife.26036.002

Interactions between LHX3- and ISL1-family LIM-homeodomain transcription factors are conserved in Caenorhabditis elegans

LIM-Homeodomain (LIM-HD) transcription factors are highly conserved in animals where they are thought to act in a transcriptional ‘LIM code’ that specifies cell types, particularly in the central nervous system. In chick and mammals the interaction between two LIM-HD proteins, LHX3 and Islet1 (ISL1), is essential for the development of motor neurons. Using yeast two-hybrid analysis we showed that the Caenorhabditis elegans orthologs of LHX3 and ISL1, CEH-14 and LIM-7 can physically interact. Structural characterisation of a complex comprising the LIM domains from CEH-14 and a LIM-interaction domain from LIM-7 showed that these nematode proteins assemble to form a structure that closely resembles that of their vertebrate counterparts. However, mutagenic analysis across the interface indicates some differences in the mechanisms of binding. We also demonstrate, using fluorescent reporter constructs, that the two C. elegans proteins are co-expressed in a small subset of neurons. These data show that the propensity for LHX3 and Islet proteins to interact is conserved from C. elegans to mammals, raising the possibility that orthologous cell specific LIM-HD-containing transcription factor complexes play similar roles in the development of neuronal cells across diverse species.

Think like a sponge: The genetic signal of sensory cells in sponges

A complex genetic repertoire underlies the apparently simple body plan of sponges. Among the genes present in poriferans are those fundamental to the sensory and nervous systems of other animals. Sponges are dynamic and sensitive animals and it is intuitive to link these genes to behaviour. The proposal that ctenophores are the earliest diverging metazoan has led to the question of whether sponges possess a 'pre-nervous' system or have undergone nervous system loss. Both lines of thought generally assume that the last common ancestor of sponges and eumetazoans possessed the genetic modules that underlie sensory abilities. By corollary extant sponges may possess a sensory cell homologous to one present in the last common ancestor, a hypothesis that has been studied by gene expression. We have performed a meta-analysis of all gene expression studies published to date to explore whether gene expression is indicative of a feature's sensory function. In sponges we find that eumetazoan sensory-neural markers are not particularly expressed in structures with known sensory functions. Instead it is common for these genes to be expressed in cells with no known or uncharacterized sensory function. Indeed, many sensory-neural markers so far studied are expressed during development, perhaps because many are transcription factors. This suggests that the genetic signal of a sponge sensory cell is dissimilar enough to be unrecognizable when compared to a bilaterian sensory or neural cell. It is possible that sensory-neural markers have as yet unknown functions in sponge cells, such as assembling an immunological synapse in the larval globular cell. Furthermore, the expression of sensory-neural markers in non-sensory cells, such as adult and larval epithelial cells, suggest that these cells may have uncharacterized sensory functions. While this does not rule out the co-option of ancestral sensory modules in later evolving groups, a distinct genetic foundation may underlie the sponge sensory system.

Ctenophores: an evolutionary-developmental perspective

Ctenophores are non-bilaterian metazoans of uncertain phylogenetic position, some recent studies placing them as sister-group to all other animals whereas others suggest this placement is artefactual and ctenophores are more closely allied with cnidarians and bilaterians, with which they share nerve cells, muscles and gut. Available information about developmental genes and their expression and function in ctenophores is reviewed. These data not only unveil some conserved aspects of molecular developmental mechanisms with other basal metazoan lineages, but also can be expected to enlighten the genomic and molecular bases of the evolution of ctenophore-specific traits, including their unique embryonic development, complex anatomy and high cell type diversity.

From nerve net to nerve ring, nerve cord and brain--evolution of the nervous system

The puzzle of how complex nervous systems emerged remains unsolved. Comparative studies of neurodevelopment in cnidarians and bilaterians suggest that this process began with distinct integration centres that evolved on opposite ends of an initial nerve net. The 'apical nervous system' controlled general body physiology, and the 'blastoporal nervous system' coordinated feeding movements and locomotion. We propose that expansion, integration and fusion of these centres gave rise to the bilaterian nerve cord and brain.

Independent origins of neurons and synapses: insights from ctenophores

There is more than one way to develop neuronal complexity, and animals frequently use different molecular toolkits to achieve similar functional outcomes. Genomics and metabolomics data from basal metazoans suggest that neural signalling evolved independently in ctenophores and cnidarians/bilaterians. This polygenesis hypothesis explains the lack of pan-neuronal and pan-synaptic genes across metazoans, including remarkable examples of lineage-specific evolution of neurogenic and signalling molecules as well as synaptic components. Sponges and placozoans are two lineages without neural and muscular systems. The possibility of secondary loss of neurons and synapses in the Porifera/Placozoa clades is a highly unlikely and less parsimonious scenario. We conclude that acetylcholine, serotonin, histamine, dopamine, octopamine and gamma-aminobutyric acid (GABA) were recruited as transmitters in the neural systems in cnidarian and bilaterian lineages. By contrast, ctenophores independently evolved numerous secretory peptides, indicating extensive adaptations within the clade and suggesting that early neural systems might be peptidergic. Comparative analysis of glutamate signalling also shows numerous lineage-specific innovations, implying the extensive use of this ubiquitous metabolite and intercellular messenger over the course of convergent and parallel evolution of mechanisms of intercellular communication. Therefore: (i) we view a neuron as a functional character but not a genetic character, and (ii) any given neural system cannot be considered as a single character because it is composed of different cell lineages with distinct genealogies, origins and evolutionary histories. Thus, when reconstructing the evolution of nervous systems, we ought to start with the identification of particular cell lineages by establishing distant neural homologies or examples of convergent evolution. In a corollary of the hypothesis of the independent origins of neurons, our analyses suggest that both electrical and chemical synapses evolved more than once.

Where is my mind? How sponges and placozoans may have lost neural cell types

Recent phylogenomic evidence suggests that ctenophores may be the sister group to the rest of animals. This phylogenetic arrangement opens the possibility that sponges and placozoans could have lost neural cell types or that the ctenophore nervous system evolved independently. We critically review evidence to date that has been put forth in support of independent evolution of neural cell types in ctenophores. We observe a reluctance in the literature to consider a lost nervous system in sponges and placozoans and suggest that this may be due to historical bias and the commonly misconstrued concept of animal complexity. In support of the idea of loss (or modification beyond recognition), we provide hypothetical scenarios to show how sponges and placozoans may have benefitted from the loss and/or modification of their neural cell types.

Evo-devo of non-bilaterian animals

The non-bilaterian animals comprise organisms in the phyla Porifera, Cnidaria, Ctenophora and Placozoa. These early-diverging phyla are pivotal to understanding the evolution of bilaterian animals. After the exponential increase in research in evolutionary development (evo-devo) in the last two decades, these organisms are again in the spotlight of evolutionary biology. In this work, I briefly review some aspects of the developmental biology of nonbilaterians that contribute to understanding the evolution of development and of the metazoans. The evolution of the developmental genetic toolkit, embryonic polarization, the origin of gastrulation and mesodermal cells, and the origin of neural cells are discussed. The possibility that germline and stem cell lineages have the same origin is also examined. Although a considerable number of non-bilaterian species are already being investigated, the use of species belonging to different branches of non-bilaterian lineages and functional experimentation with gene manipulation in the majority of the non-bilaterian lineages will be necessary for further progress in this field.

Evolution: ctenophore genomes and the origin of neurons

Recent sequencing of ctenophore genomes opens a new era in the study of this unique and phylogenetically distant group. The presence of neurodevelopmental genes, pre- and postsynaptic modules, and transmitter molecules is consistent with a single origin of neurons.

Retinoic acid signaling and neuronal differentiation

The identification of neurological symptoms caused by vitamin A deficiency pointed to a critical, early developmental role of vitamin A and its metabolite, retinoic acid (RA). The ability of RA to induce post-mitotic, neural phenotypes in various stem cells, in vitro, served as early evidence that RA is involved in the switch between proliferation and differentiation. In vivo studies have expanded this "opposing signal" model, and the number of primary neurons an embryo develops is now known to depend critically on the levels and spatial distribution of RA. The proneural and neurogenic transcription factors that control the exit of neural progenitors from the cell cycle and allow primary neurons to develop are partly elucidated, but the downstream effectors of RA receptor (RAR) signaling (many of which are putative cell cycle regulators) remain largely unidentified. The molecular mechanisms underlying RA-induced primary neurogenesis in anamniote embryos are starting to be revealed; however, these data have been not been extended to amniote embryos. There is growing evidence that bona fide RARs are found in some mollusks and other invertebrates, but little is known about their necessity or functions in neurogenesis. One normal function of RA is to regulate the cell cycle to halt proliferation, and loss of RA signaling is associated with dedifferentiation and the development of cancer. Identifying the genes and pathways that mediate cell cycle exit downstream of RA will be critical for our understanding of how to target tumor differentiation. Overall, elucidating the molecular details of RAR-regulated neurogenesis will be decisive for developing and understanding neural proliferation-differentiation switches throughout development.

Convergent evolution of neural systems in ctenophores

Neurons are defined as polarized secretory cells specializing in directional propagation of electrical signals leading to the release of extracellular messengers - features that enable them to transmit information, primarily chemical in nature, beyond their immediate neighbors without affecting all intervening cells en route. Multiple origins of neurons and synapses from different classes of ancestral secretory cells might have occurred more than once during ~600 million years of animal evolution with independent events of nervous system centralization from a common bilaterian/cnidarian ancestor without the bona fide central nervous system. Ctenophores, or comb jellies, represent an example of extensive parallel evolution in neural systems. First, recent genome analyses place ctenophores as a sister group to other animals. Second, ctenophores have a smaller complement of pan-animal genes controlling canonical neurogenic, synaptic, muscle and immune systems, and developmental pathways than most other metazoans. However, comb jellies are carnivorous marine animals with a complex neuromuscular organization and sophisticated patterns of behavior. To sustain these functions, they have evolved a number of unique molecular innovations supporting the hypothesis of massive homoplasies in the organization of integrative and locomotory systems. Third, many bilaterian/cnidarian neuron-specific genes and 'classical' neurotransmitter pathways are either absent or, if present, not expressed in ctenophore neurons (e.g. the bilaterian/cnidarian neurotransmitter, γ-amino butyric acid or GABA, is localized in muscles and presumed bilaterian neuron-specific RNA-binding protein Elav is found in non-neuronal cells). Finally, metabolomic and pharmacological data failed to detect either the presence or any physiological action of serotonin, dopamine, noradrenaline, adrenaline, octopamine, acetylcholine or histamine - consistent with the hypothesis that ctenophore neural systems evolved independently from those in other animals. Glutamate and a diverse range of secretory peptides are first candidates for ctenophore neurotransmitters. Nevertheless, it is expected that other classes of signal and neurogenic molecules would be discovered in ctenophores as the next step to decipher one of the most distinct types of neural organization in the animal kingdom.

Overexpression of Lhx8 inhibits cell proliferation and induces cell cycle arrest in PC12 cell line

LIM-homeobox genes play a pivotal function in tissue patterning and differentiation, Lhx8 is a member of LIM-homeobox gene family, and it is selectively expressed in embryonic basal forebrain and is a key factor for the determination of cholinergic cells fate. However, besides cholinergic differentiation, little is known about the potential role of Lhx8 in cell biology. In this study, we transfected Lhx8 complementary DNA (cDNA) into PC12 cell line using lentiviral vectors to acquire the cells which stably expressed high level of Lhx8, and we provide the experimental evidence that overexpression of Lhx8 inhibits cell proliferation and induces cell cycle arrest but not apoptosis in vitro. In conclusion, besides cholinergic differentiation, our results suggest that Lhx8 also plays as a suppressor gene of proliferation in cell biology.

Islet is a key determinant of ascidian palp morphogenesis

The anterior-most ectoderm of ascidian larvae contains the adhesive papillae, or palps, which play an important role in triggering the metamorphosis of swimming tadpoles. In Ciona intestinalis, the palps consist of three conical protrusions within a field of thickened epithelium that form late in embryogenesis, as tailbuds mature into larvae. The palp protrusions express the LIM-homeodomain transcription factor Islet. Protrusion occurs through differential cell elongation, probably mediated by Islet, as we find that ectopic expression of Islet is sufficient to promote cell lengthening. FGF signaling is required for both Islet expression and palp morphogenesis. Importantly, we show that Islet expression can rescue the palp-deficient phenotype that results from inhibition of FGF signaling. We conclude that Islet is a key regulatory factor governing morphogenesis of the palps. It is conceivable that Islet is also essential for the cellular morphogenesis of placode-derived sensory neurons in vertebrates.

Did the ctenophore nervous system evolve independently?

Recent evidence supports the placement of ctenophores as the most distant relative to all other animals. This revised animal tree means that either the ancestor of all animals possessed neurons (and that sponges and placozoans apparently lost them) or that ctenophores developed them independently. Differentiating between these possibilities is important not only from a historical perspective, but also for the interpretation of a wide range of neurobiological results. In this short perspective paper, I review the evidence in support of each scenario and show that the relationship between the nervous system of ctenophores and other animals is an unsolved, yet tractable problem.

Evidence for involvement of Wnt signalling in body polarities, cell proliferation, and the neuro-sensory system in an adult ctenophore

Signalling through the Wnt family of secreted proteins originated in a common metazoan ancestor and greatly influenced the evolution of animal body plans. In bilaterians, Wnt signalling plays multiple fundamental roles during embryonic development and in adult tissues, notably in axial patterning, neural development and stem cell regulation. Studies in various cnidarian species have particularly highlighted the evolutionarily conserved role of the Wnt/β-catenin pathway in specification and patterning of the primary embryonic axis. However in another key non-bilaterian phylum, Ctenophora, Wnts are not involved in early establishment of the body axis during embryogenesis. We analysed the expression in the adult of the ctenophore Pleurobrachia pileus of 11 orthologues of Wnt signalling genes including all ctenophore Wnt ligands and Fz receptors and several members of the intracellular β-catenin pathway machinery. All genes are strongly expressed around the mouth margin at the oral pole, evoking the Wnt oral centre of cnidarians. This observation is consistent with primary axis polarisation by the Wnts being a universal metazoan feature, secondarily lost in ctenophores during early development but retained in the adult. In addition, local expression of Wnt signalling genes was seen in various anatomical structures of the body including in the locomotory comb rows, where their complex deployment suggests control by the Wnts of local comb polarity. Other important contexts of Wnt involvement which probably evolved before the ctenophore/cnidarian/bilaterian split include proliferating stem cells and progenitors irrespective of cell types, and developing as well as differentiated neuro-sensory structures.

The genome of the ctenophore Mnemiopsis leidyi and its implications for cell type evolution

An understanding of ctenophore biology is critical for reconstructing events that occurred early in animal evolution. Toward this goal, we have sequenced, assembled, and annotated the genome of the ctenophore Mnemiopsis leidyi. Our phylogenomic analyses of both amino acid positions and gene content suggest that ctenophores rather than sponges are the sister lineage to all other animals. Mnemiopsis lacks many of the genes found in bilaterian mesodermal cell types, suggesting that these cell types evolved independently. The set of neural genes in Mnemiopsis is similar to that of sponges, indicating that sponges may have lost a nervous system. These results present a newly supported view of early animal evolution that accounts for major losses and/or gains of sophisticated cell types, including nerve and muscle cells.

The LIM and POU homeobox genes ttx-3 and unc-86 act as terminal selectors in distinct cholinergic and serotonergic neuron types

Transcription factors that drive neuron type-specific terminal differentiation programs in the developing nervous system are often expressed in several distinct neuronal cell types, but to what extent they have similar or distinct activities in individual neuronal cell types is generally not well explored. We investigate this problem using, as a starting point, the C. elegans LIM homeodomain transcription factor ttx-3, which acts as a terminal selector to drive the terminal differentiation program of the cholinergic AIY interneuron class. Using a panel of different terminal differentiation markers, including neurotransmitter synthesizing enzymes, neurotransmitter receptors and neuropeptides, we show that ttx-3 also controls the terminal differentiation program of two additional, distinct neuron types, namely the cholinergic AIA interneurons and the serotonergic NSM neurons. We show that the type of differentiation program that is controlled by ttx-3 in different neuron types is specified by a distinct set of collaborating transcription factors. One of the collaborating transcription factors is the POU homeobox gene unc-86, which collaborates with ttx-3 to determine the identity of the serotonergic NSM neurons. unc-86 in turn operates independently of ttx-3 in the anterior ganglion where it collaborates with the ARID-type transcription factor cfi-1 to determine the cholinergic identity of the IL2 sensory and URA motor neurons. In conclusion, transcription factors operate as terminal selectors in distinct combinations in different neuron types, defining neuron type-specific identity features.

Cell-specific actions of a human LHX3 gene enhancer during pituitary and spinal cord development

The LIM class of homeodomain protein 3 (LHX3) transcription factor is essential for pituitary gland and nervous system development in mammals. In humans, mutations in the LHX3 gene underlie complex pediatric syndromes featuring deficits in anterior pituitary hormones and defects in the nervous system. The mechanisms that control temporal and spatial expression of the LHX3 gene are poorly understood. The proximal promoters of the human LHX3 gene are insufficient to guide expression in vivo and downstream elements including a conserved enhancer region appear to play a role in tissue-specific expression in the pituitary and nervous system. Here we characterized the activity of this downstream enhancer region in regulating gene expression at the cellular level during development. Human LHX3 enhancer-driven Cre reporter transgenic mice were generated to facilitate studies of enhancer actions. The downstream LHX3 enhancer primarily guides gene transcription in α-glycoprotein subunit -expressing cells secreting the TSHβ, LHβ, or FSHβ hormones and expressing the GATA2 and steroidogenic factor 1 transcription factors. In the developing nervous system, the enhancer serves as a targeting module active in V2a interneurons. These results demonstrate that the downstream LHX3 enhancer is important in specific endocrine and neural cell types but also indicate that additional regulatory elements are likely involved in LHX3 gene expression. Furthermore, these studies revealed significant gonadotrope cell heterogeneity during pituitary development, providing insights into the cellular physiology of this key reproductive regulatory cell. The human LHX3 enhancer-driven Cre reporter transgenic mice also provide a valuable tool for further developmental studies of cell determination and differentiation in the pituitary and nervous system.

Expression of the Lhx genes apterous and lim1 in an errant polychaete: implications for bilaterian appendage evolution, neural development, and muscle diversification

BACKGROUND

Arthropod and vertebrate appendages appear to have evolved via parallel co-option of a plesiomorphic gene regulatory network. Our previous work implies that annelids evolved unrelated appendage-forming mechanisms; we therefore found no support for homology of parapodia and arthropodia at the level of the whole appendage. We expand on that study here by asking whether expression of the LIM homeobox (Lhx) genes apterous and lim1 in the annelid Neanthes arenaceodentata supports homology of the dorsal branches as well as the proximodistal axes of parapodia and arthropodia. In addition, we explore whether the neural expression of apterous and lim1 in Neanthes supports the putative ancestral function of the Lhx gene family in regulating the differentiation and maintenance of neuronal subtypes.

RESULTS

Both genes exhibit continuous expression in specific portions of the developing central nervous system, from hatching to at least the 13-chaetiger stage. For example, nerve cord expression occurs in segmentally iterated patterns consisting of diffuse sets of many lim1-positive cells and comparatively fewer, clustered pairs of apterous-positive cells. Additionally, continuous apterous expression is observed in presumed neurosecretory ganglia of the posterior brain, while lim1 is continuously expressed in stomatogastric ganglia of the anterior brain. apterous is also expressed in the jaw sacs, dorsal parapodial muscles, and a presumed pair of cephalic sensory organs, whereas lim1 is expressed in multiple pharyngeal ganglia, the segmental peripheral nervous system, neuropodial chaetal sac muscles, and parapodial ligules.

CONCLUSIONS

The early and persistent nervous system expression of apterous and lim1 in Neanthes juveniles supports conservation of Lhx function in bilaterian neural differentiation and maintenance. Our results also suggest that diversification of parapodial muscle precursors involves a complementary LIM code similar to those generating distinct neuronal identities in fly and mouse nerve cords. Expression of apterous and lim1 in discrete components of developing parapodia is intriguing but does not map to comparable expression of these genes in developing arthropod appendages. Thus, annelid and arthropod appendage development apparently evolved, in part, via distinct co-option of the neuronal regulatory architecture. These divergent patterns of apterous and lim1 activity seemingly reflect de novo origins of parapodia and arthropodia, although we discuss alternative hypotheses.