Neofunctionalization in vertebrates: the example of retinoic acid receptors

C286, an orally available retinoic acid receptor β agonist drug, regulates multiple pathways to achieve spinal cord injury repair

Retinoic acid receptor β2 (RARβ2) is an emerging therapeutic target for spinal cord injuries (SCIs) with a unique multimodal regenerative effect. We have developed a first-in-class RARβ agonist drug, C286, that modulates neuron-glial pathways to induce functional recovery in a rodent model of sensory root avulsion. Here, using genome-wide and pathway enrichment analysis of avulsed rats' spinal cords, we show that C286 also influences the extracellular milieu (ECM). Protein expression studies showed that C286 upregulates tenascin-C, integrin-α9, and osteopontin in the injured cord. Similarly, C286 remodulates these ECM molecules, hampers inflammation and prevents tissue loss in a rodent model of spinal cord contusion C286. We further demonstrate C286's efficacy in human iPSC-derived neurons, with treatment resulting in a significant increase in neurite outgrowth. Additionally, we identify a putative efficacy biomarker, S100B, which plasma levels correlated with axonal regeneration in nerve-injured rats. We also found that other clinically available retinoids, that are not RARβ specific agonists, did not lead to functional recovery in avulsed rats, demonstrating the requirement for RARβ specific pathways in regeneration. In a Phase 1 trial, the single ascending dose (SAD) cohorts showed increases in expression of RARβ2 in white blood cells correlative to increased doses and at the highest dose administered, the pharmacokinetics were similar to the rat proof of concept (POC) studies. Collectively, our data suggests that C286 signalling in neurite/axonal outgrowth is conserved between species and across nerve injuries. This warrants further clinical testing of C286 to ascertain POC in a broad spectrum of neurodegenerative conditions.

Expectations of duplicate gene retention under the gene duplicability hypothesis

BACKGROUND

Gene duplication is an important process in evolution. What causes some genes to be retained after duplication and others to be lost is a process not well understood. The most prevalent theory is the gene duplicability hypothesis, that something about the function and number of interacting partners (number of subunits of protein complex, etc.), determines whether copies have more opportunity to be retained for long evolutionary periods. Some genes are also more susceptible to dosage balance effects following WGD events, making them more likely to be retained for longer periods of time. One would expect these processes that affect the retention of duplicate copies to affect the conditional probability ratio after consecutive whole genome duplication events. The probability that a gene will be retained after a second whole genome duplication event (WGD2), given that it was retained after the first whole genome duplication event (WGD1) versus the probability a gene will be retained after WGD2, given it was lost after WGD1 defines the probability ratio that is calculated.

RESULTS

Since duplicate gene retention is a time heterogeneous process, the time between the events (t1) and the time since the most recent event (t2) are relevant factors in calculating the expectation for observation in any genome. Here, we use a survival analysis framework to predict the probability ratio for genomes with different values of t1 and t2 under the gene duplicability hypothesis, that some genes are more susceptible to selectable functional shifts, some more susceptible to dosage compensation, and others only drifting. We also predict the probability ratio with different values of t1 and t2 under the mutational opportunity hypothesis, that probability of retention for certain genes changes in subsequent events depending upon how they were previously retained. These models are nested such that the mutational opportunity model encompasses the gene duplicability model with shifting duplicability over time. Here we present a formalization of the gene duplicability and mutational opportunity hypotheses to characterize evolutionary dynamics and explanatory power in a recently developed statistical framework.

CONCLUSIONS

This work presents expectations of the gene duplicability and mutational opportunity hypotheses over time under different sets of assumptions. This expectation will enable formal testing of processes leading to duplicate gene retention.

Diversification and Functional Evolution of HOX Proteins

Gene duplication and divergence is a major contributor to the generation of morphological diversity and the emergence of novel features in vertebrates during evolution. The availability of sequenced genomes has facilitated our understanding of the evolution of genes and regulatory elements. However, progress in understanding conservation and divergence in the function of proteins has been slow and mainly assessed by comparing protein sequences in combination with in vitro analyses. These approaches help to classify proteins into different families and sub-families, such as distinct types of transcription factors, but how protein function varies within a gene family is less well understood. Some studies have explored the functional evolution of closely related proteins and important insights have begun to emerge. In this review, we will provide a general overview of gene duplication and functional divergence and then focus on the functional evolution of HOX proteins to illustrate evolutionary changes underlying diversification and their role in animal evolution.

MicroRNAs as Indicators into the Causes and Consequences of Whole-Genome Duplication Events

Whole-genome duplications (WGDs) have long been considered the causal mechanism underlying dramatic increases to morphological complexity due to the neo-functionalization of paralogs generated during these events. Nonetheless, an alternative hypothesis suggests that behind the retention of most paralogs is not neo-functionalization, but instead the degree of the inter-connectivity of the intended gene product, as well as the mode of the WGD itself. Here, we explore both the causes and consequences of WGD by examining the distribution, expression, and molecular evolution of microRNAs (miRNAs) in both gnathostome vertebrates as well as chelicerate arthropods. We find that although the number of miRNA paralogs tracks the number of WGDs experienced within the lineage, few of these paralogs experienced changes to the seed sequence, and thus are functionally equivalent relative to their mRNA targets. Nonetheless, in gnathostomes, although the retention of paralogs following the 1R autotetraploidization event is similar across the two subgenomes, the paralogs generated by the gnathostome 2R allotetraploidization event are retained in higher numbers on one subgenome relative to the second, with the miRNAs found on the preferred subgenome showing both higher expression of mature miRNA transcripts and slower molecular evolution of the precursor miRNA sequences. Importantly, WGDs do not result in the creation of miRNA novelty, nor do WGDs correlate to increases in complexity. Instead, it is the number of miRNA seed sequences in the genome itself that not only better correlate to instances in complexification, but also mechanistically explain why complexity increases when new miRNA families are established.

The impact of whole genome duplications on the human gene regulatory networks

This work studies the effects of the two rounds of Whole Genome Duplication (WGD) at the origin of the vertebrate lineage on the architecture of the human gene regulatory networks. We integrate information on transcriptional regulation, miRNA regulation, and protein-protein interactions to comparatively analyse the role of WGD and Small Scale Duplications (SSD) in the structural properties of the resulting multilayer network. We show that complex network motifs, such as combinations of feed-forward loops and bifan arrays, deriving from WGD events are specifically enriched in the network. Pairs of WGD-derived proteins display a strong tendency to interact both with each other and with common partners and WGD-derived transcription factors play a prominent role in the retention of a strong regulatory redundancy. Combinatorial regulation and synergy between different regulatory layers are in general enhanced by duplication events, but the two types of duplications contribute in different ways. Overall, our findings suggest that the two WGD events played a substantial role in increasing the multi-layer complexity of the vertebrate regulatory network by enhancing its combinatorial organization, with potential consequences on its overall robustness and ability to perform high-level functions like signal integration and noise control. Lastly, we discuss in detail the RAR/RXR pathway as an illustrative example of the evolutionary impact of WGD duplications in human.

Gene duplication is one of the main mechanisms driving genome evolution. The duplication of a genomic segment can be the result of a local event, involving only a small portion of the genome, or of a dramatic duplication of the whole genome, which is however only rarely retained. All vertebrates descend from two rounds of Whole-Genome Duplication (WGD) that occurred approximately 500 Mya. We show that these events influenced in unique ways the evolution of different human gene regulatory networks, with sizeable effects on their current structure. We find that WGDs statistically increased the presence of specific classes of simple genetic circuits, considered to be fundamental building blocks of more sophisticated circuitry and commonly associated to complex functions. Our findings support the hypothesis that these rare, large-scale events have played a substantial role in the emergence of complex traits in vertebrates.

Molecular Characterization of Retinoic Acid Receptor CgRAR in Pacific Oyster (Crassostrea gigas)

Retinoic acid (RA) signaling pathways mediated by RA receptors (RARs) are essential for many physiological processes such as organ development, regeneration, and differentiation in animals. Recent studies reveal that RARs identified in several mollusks, including Pacific oyster Crassostrea gigas, have a different function mechanism compared with that in chordates. In this report, we identified the molecular characteristics of CgRAR to further explore the mechanism of RAR in mollusks. RT-qPCR analysis shows that CgRAR has a higher expression level in the hemocytes and gonads, indicating that CgRAR may play roles in the processes of development and metabolism. The mRNA expression level of both CgRAR and CgRXR was analyzed by RT-qPCR after injection with RA. The elevated expression of CgRAR and CgRXR was detected upon all-trans-RA (ATRA) exposure. Finally, according to the results of Yeast Two-Hybrid assay and co-immunoprecipitation analysis, CgRAR and CgRXR can interact with each other through the C-terminal region. Taken together, our results suggest that CgRAR shows a higher expression level in gonads and hemocytes. ATRA exposure up-regulates the expression of CgRAR and CgRXR. Besides, CgRAR can interact with CgRXR to form a heterodimer complex.

Why PGD2 has different functions from PGE2

Prostaglandin (PG) D₂ and PGE₂ are positional isomers; however, they sometimes exhibit opposite physiological functions, such as in cancer development. Because DP receptors are considered to be a duplicated copy of EP2 receptors, PGD₂ and PGE₂ cross-react with both receptors. These prostanoids may act as biased agonists for each receptor. In reviewing this field, a hypothesis was proposed to explain the opposed effects of these prostanoids from the viewpoints of the evolution of, mutations in, and biased activities of their receptors. Previous findings showing more mutations/variations in DP receptors than EP2 receptors among individuals worldwide indicate that DP receptors are still in a rapid evolutionary stage. The opposing effects of these prostanoids on cancer development may be attributed to the biased activity of PGE₂ for DP receptors, which may incidentally develop during the process of the old ligand, PGE₂ gaining selectivity to newly diverged DP receptors.

An Overview of Duplicated Gene Detection Methods: Why the Duplication Mechanism Has to Be Accounted for in Their Choice

Gene duplication is an important evolutionary mechanism allowing to provide new genetic material and thus opportunities to acquire new gene functions for an organism, with major implications such as speciation events. Various processes are known to allow a gene to be duplicated and different models explain how duplicated genes can be maintained in genomes. Due to their particular importance, the identification of duplicated genes is essential when studying genome evolution but it can still be a challenge due to the various fates duplicated genes can encounter. In this review, we first describe the evolutionary processes allowing the formation of duplicated genes but also describe the various bioinformatic approaches that can be used to identify them in genome sequences. Indeed, these bioinformatic approaches differ according to the underlying duplication mechanism. Hence, understanding the specificity of the duplicated genes of interest is a great asset for tool selection and should be taken into account when exploring a biological question.

New Insights into the Control of Cell Fate Choices and Differentiation by Retinoic Acid in Cranial, Axial and Caudal Structures

Retinoic acid (RA) signaling is an important regulator of chordate development. RA binds to nuclear RA receptors that control the transcriptional activity of target genes. Controlled local degradation of RA by enzymes of the Cyp26a gene family contributes to the establishment of transient RA signaling gradients that control patterning, cell fate decisions and differentiation. Several steps in the lineage leading to the induction and differentiation of neuromesodermal progenitors and bone-producing osteogenic cells are controlled by RA. Changes to RA signaling activity have effects on the formation of the bones of the skull, the vertebrae and the development of teeth and regeneration of fin rays in fish. This review focuses on recent advances in these areas, with predominant emphasis on zebrafish, and highlights previously unknown roles for RA signaling in developmental processes.

An Orthologue of the Retinoic Acid Receptor (RAR) Is Present in the Ecdysozoa Phylum Priapulida

Signalling molecules and their cognate receptors are central components of the Metazoa endocrine system. Defining their presence or absence in extant animal lineages is critical to accurately devise evolutionary patterns, physiological shifts and the impact of endocrine disrupting chemicals. Here, we address the evolution of retinoic acid (RA) signalling in the Priapulida worm, Priapulus caudatus Lamarck, 1816, an Ecdysozoa. RA signalling has been shown to be central to chordate endocrine homeostasis, participating in multiple developmental and physiological processes. Priapulids, with their slow rate of molecular evolution and phylogenetic position, represent a key taxon to investigate the early phases of Ecdysozoa evolution. By exploring a draft genome assembly, we show, by means of phylogenetics and functional assays, that an orthologue of the nuclear receptor retinoic acid receptor (RAR) subfamily, a central mediator of RA signalling, is present in Ecdysozoa, contrary to previous perception. We further demonstrate that the Priapulida RAR displays low-affinity for retinoids (similar to annelids), and is not responsive to common endocrine disruptors acting via RAR. Our findings provide a timeline for RA signalling evolution in the Bilateria and give support to the hypothesis that the increase in RA affinity towards RAR is a late acquisition in the evolution of the Metazoa.

Regulation plays a multifaceted role in the retention of gene duplicates

A gene duplication can lead to all sorts of problems in a cell. However, it can also lead to all sorts of benefits. Beneficial or not, the gene duplicates might be kept in the genome because of several different reasons. For instance, if natural selection works towards optimizing one function of a gene at the expense of another, then gene duplication could resolve this conflict by separating the functions in two genes. Here, we outline evolutionary incentives to keep a duplicated gene in the genome, focusing on divergence in expression and trade-off resolution as featured in a new and exciting paper published in this edition of PLOS Biology.

Genes Encoding Teleost Fish Ligands and Associated Receptors Remained in Duplicate More Frequently than the Rest of the Genome

Signaling through ligand/receptor interactions is a widespread mechanism across all living taxa. During evolution, however, there has been a diversification in multigene families and changes in their interaction patterns. Among the events that led to the creation of new genes is the whole-genome duplication, which made possible some major innovations. Teleost fishes descended from a common ancestor which underwent one such whole-genome duplication.

In our study, we investigated the effect of complete genome duplication on the evolution of ligand–receptor pairs in teleosts. We selected ten teleost species and used bioinformatics programs and phylogenetic tools in order to study the evolution of the human ligands and receptors that have orthologous genes in fishes, as well as the rest of the fish genomes.

We established that since the complete duplication of the fish genomes, the conservation in duplicate copy of ligand and receptor genes is higher than expected. However, the ligand/receptor pair partners did not necessarily evolve in the same way, and a lot of situations occurred in which one of the partners returned in singleton copy when the other one was maintained in duplicate. This suggests that changes in interaction partners may have taken place during the evolution of teleosts. Moreover, the fate of the ligands and receptor coding genes is partly congruent with the phylogeny of teleosts. However, some incongruences can be observed. We suggest that these incongruences are correlated to the environment.

A Behavioral Assay to Study Effects of Retinoid Pharmacology on Nervous System Development in a Marine Annelid

Autonomous animal locomotion, such as swimming, is modulated by neuronal networks acting on cilia or muscles. Understanding how these networks are formed and coordinated is a complex scientific problem, which requires various technical approaches. Among others, behavioral studies of developing animals treated with exogenous substances have proven to be a successful approach for studying the functions of neuronal networks. One such substance crucial for the proper development of the nervous system is the vitamin A-derived morphogen retinoic acid (RA). In the larva of the marine annelid Platynereis dumerilii , for example, RA is involved in the specification and differentiation of individual neurons and responsible for orchestrating the swimming behavior of the developing larva. Here, we report a workflow to analyze the effects of RA on the locomotion of the P. dumerilii larva. We provide a protocol for both the treatment with RA and the recording of larval swimming behavior. Additionally, we present a pipeline for the analysis of the obtained data in terms of swimming speed and movement trajectory. This chapter thus summarizes the methodology for analyzing the effects of a specific drug treatment on larval swimming behavior. We expect this approach to be readily adaptable to a wide variety of pharmacological compounds and aquatic species.

Hindbrain induction and patterning during early vertebrate development

The hindbrain is a key relay hub of the central nervous system (CNS), linking the bilaterally symmetric half-sides of lower and upper CNS centers via an extensive network of neural pathways. Dedicated neural assemblies within the hindbrain control many physiological processes, including respiration, blood pressure, motor coordination and different sensations. During early development, the hindbrain forms metameric segmented units known as rhombomeres along the antero-posterior (AP) axis of the nervous system. These compartmentalized units are highly conserved during vertebrate evolution and act as the template for adult brainstem structure and function. TALE and HOX homeodomain family transcription factors play a key role in the initial induction of the hindbrain and its specification into rhombomeric cell fate identities along the AP axis. Signaling pathways, such as canonical-Wnt, FGF and retinoic acid, play multiple roles to initially induce the hindbrain and regulate Hox gene-family expression to control rhombomeric identity. Additional transcription factors including Krox20, Kreisler and others act both upstream and downstream to Hox genes, modulating their expression and protein activity. In this review, we will examine the earliest embryonic signaling pathways that induce the hindbrain and subsequent rhombomeric segmentation via Hox and other gene expression. We will examine how these signaling pathways and transcription factors interact to activate downstream targets that organize the segmented AP pattern of the embryonic vertebrate hindbrain.

The retinoic acid receptor (RAR) in molluscs: Function, evolution and endocrine disruption insights

Retinoid acid receptor (RAR)-dependent signalling pathways are essential for the regulation and maintenance of essential biological functions and are recognized targets of disruptive anthropogenic compounds. Recent studies put forward the inability of mollusc RARs to bind and respond to the canonical vertebrate ligand, retinoic acid: a feature that seems to have been lost during evolution. Yet, these studies were carried out in a limited number of molluscs. Therefore, using an in vitro transactivation assay, the present work aimed to characterize phylogenetically relevant mollusc RARs, as monomers or as functional units with RXR, not only in the presence of vertebrate bone fine ligands but also known endocrine disruptors, described to modulate retinoid-dependent pathways. In general, none of the tested mollusc RARs were able to activate reporter gene transcription when exposed to retinoic acid isomers, suggesting that the ability to respond to retinoic acid was lost across molluscs. Similarly, the analysed mollusc RAR were unresponsive towards organochloride pesticides. In contrast, transcriptional repressions were observed with the RAR/RXR unit upon exposure to retinoids or RXR-specific ligands. Loss-of-function and gain-of-function mutations further corroborate the obtained results and suggest that the repressive behaviour, observed with mollusc and human RAR/RXR heterodimers, is possibly mediated by ligand biding to RXR.

Gene expression in the social behavior network of the wire-tailed manakin (Pipra filicauda) brain

The vertebrate basal forebrain and midbrain contain a set of interconnected nuclei that control social behavior. Conserved anatomical structures and functions of these nuclei have now been documented among fish, amphibians, reptiles, birds and mammals, and these brain regions have come to be known as the vertebrate social behavior network (SBN). While it is known that nuclei (nodes) of the SBN are rich in steroid and neuropeptide activity linked to behavior, simultaneous variation in the expression of neuroendocrine genes among several SBN nuclei has not yet been described in detail. In this study, we use RNA-seq to profile gene expression across seven brain regions representing five nodes of the vertebrate SBN in a passerine bird, the wire-tailed manakin Pipra filicauda. Using weighted gene co-expression network analysis, we reconstructed sets of coregulated genes, showing striking patterns of variation in neuroendocrine gene expression across the SBN. We describe regional variation in gene networks comprising a broad set of hormone receptors, neuropeptides, steroidogenic enzymes, catecholamines and other neuroendocrine signaling molecules. Our findings show heterogeneous patterns of brain gene expression across nodes of the avian SBN and provide a foundation for future analyses of how the regulation of gene networks may mediate social behavior. These results highlight the importance of region-specific sampling in studies of the mechanisms of behavior.

Adaptive correction of craniofacial defects in pre-metamorphic Xenopus laevis tadpoles involves thyroid hormone-independent tissue remodeling

While it is well-established that some organisms can regenerate lost structures, the ability to remodel existing malformed structures has been less well studied. Thus, in this study we examined the ability of pre-metamorphic Xenopus laevis tadpoles to self-correct malformed craniofacial tissues and found that tadpoles can adaptively improve and normalize abnormal craniofacial morphology caused by numerous developmental perturbations. We then investigated the tissue-level and molecular mechanisms that mediate the self-correction of craniofacial defects in pre-metamorphic X. laevis tadpoles. Our studies revealed that this adaptive response involves morphological changes and the remodeling of cartilage tissue, prior to metamorphosis. RT-qPCR and RNA-Seq analysis of gene expression suggests a thyroid hormone-independent endocrine signaling pathway as the potential mechanism responsible for triggering the adaptive and corrective remodeling response in these larvae that involves mmp1 and mmp13 upregulation. Thus, investigating how malformed craniofacial tissues are naturally corrected in X. laevis tadpoles has led us to valuable insights regarding the maintenance and manipulation of craniofacial morphology in a vertebrate system. These insights may help in the development of novel therapies for developmental craniofacial anomalies in humans.

The ancestral retinoic acid receptor was a low-affinity sensor triggering neuronal differentiation

Retinoic acid (RA) is an important intercellular signaling molecule in vertebrate development, with a well-established role in the regulation of hox genes during hindbrain patterning and in neurogenesis. However, the evolutionary origin of the RA signaling pathway remains elusive. To elucidate the evolution of the RA signaling system, we characterized RA metabolism and signaling in the marine annelid Platynereis dumerilii, a powerful model for evolution, development, and neurobiology. Binding assays and crystal structure analyses show that the annelid retinoic acid receptor (RAR) binds RA and activates transcription just as vertebrate RARs, yet with a different ligand-binding pocket and lower binding affinity, suggesting a permissive rather than instructive role of RA signaling. RAR knockdown and RA treatment of swimming annelid larvae further reveal that the RA signal is locally received in the medial neuroectoderm, where it controls neurogenesis and axon outgrowth, whereas the spatial colinear hox gene expression in the neuroectoderm remains unaffected. These findings suggest that one early role of the new RAR in bilaterian evolution was to control the spatially restricted onset of motor and interneuron differentiation in the developing ventral nerve cord and to indicate that the regulation of hox-controlled anterior-posterior patterning arose only at the base of the chordates, concomitant with a high-affinity RAR needed for the interpretation of a complex RA gradient.

LXRα and LXRβ Nuclear Receptors Evolved in the Common Ancestor of Gnathostomes

Nuclear receptors (NRs) regulate numerous aspects of the endocrine system. They mediate endogenous and exogenous cues, ensuring a homeostatic control of development and metabolism. Gene duplication, loss and mutation have shaped the repertoire and function of NRs in metazoans. Here, we examine the evolution of a pivotal orchestrator of cholesterol metabolism in vertebrates, the liver X receptors (LXRs). Previous studies suggested that LXRα and LXRβ genes emerged in the mammalian ancestor. However, we show through genome analysis and functional assay that bona fide LXRα and LXRβ orthologues are present in reptiles, coelacanth and chondrichthyans but not in cyclostomes. These findings show that LXR duplicated before gnathostome radiation, followed by asymmetric paralogue loss in some lineages. We suggest that a tighter control of cholesterol levels in vertebrates was achieved through the exploitation of a wider range of oxysterols, an ability contingent on ligand-binding pocket remodeling.

Nuclear receptor research in zebrafish

Nuclear receptors (NRs) form a superfamily of transcription factors that can be activated by ligands and are involved in a wide range of physiological processes. NRs are well conserved between vertebrate species. The zebrafish, an increasingly popular animal model system, contains a total of 73 NR genes, and orthologues of almost all human NRs are present. In this review article, an overview is presented of NR research in which the zebrafish has been used as a model. Research is described on the three most studied zebrafish NRs: the estrogen receptors (ERs), retinoic acid receptors (RARs) and peroxisome proliferator-activated receptors (PPARs). The studies on these receptors illustrate the versatility of the zebrafish as a model for ecotoxicological, developmental and biomedical research. Although the use of the zebrafish in NR research is still relatively limited, it is expected that in the next decade the full potential of this animal model will be exploited.

RARβ2 is required for vertebrate somitogenesis

During vertebrate somitogenesis, retinoic acid is known to establish the position of the determination wavefront, controlling where new somites are permitted to form along the anteroposterior body axis. Less is understood about how RAR regulates somite patterning, rostral-caudal boundary setting, specialization of myotome subdivisions or the specific RAR subtype that is required for somite patterning. Characterizing the function of RARβ has been challenging due to the absence of embryonic phenotypes in murine loss-of-function studies. Using the Xenopus system, we show that RARβ2 plays a specific role in somite number and size, restriction of the presomitic mesoderm anterior border, somite chevron morphology and hypaxial myoblast migration. Rarβ2 is the RAR subtype whose expression is most upregulated in response to ligand and its localization in the trunk somites positions it at the right time and place to respond to embryonic retinoid levels during somitogenesis. RARβ2 positively regulates Tbx3 a marker of hypaxial muscle, and negatively regulates Tbx6 via Ripply2 to restrict the anterior boundaries of the presomitic mesoderm and caudal progenitor pool. These results demonstrate for the first time an early and essential role for RARβ2 in vertebrate somitogenesis.

Lineage-specific duplication of amphioxus retinoic acid degrading enzymes (CYP26) resulted in sub-functionalization of patterning and homeostatic roles

BACKGROUND

During embryogenesis, tight regulation of retinoic acid (RA) availability is fundamental for normal development. In parallel to RA synthesis, a negative feedback loop controlled by RA catabolizing enzymes of the cytochrome P450 subfamily 26 (CYP26) is crucial. In vertebrates, the functions of the three CYP26 enzymes (CYP26A1, CYP26B1, and CYP26C1) have been well characterized. By contrast, outside vertebrates, little is known about CYP26 complements and their biological roles. In an effort to characterize the evolutionary diversification of RA catabolism, we studied the CYP26 genes of the cephalochordate amphioxus (Branchiostoma lanceolatum), a basal chordate with a vertebrate-like genome that has not undergone the massive, large-scale duplications of vertebrates.

RESULTS

In the present study, we found that amphioxus also possess three CYP26 genes (CYP26-1, CYP26-2, and CYP26-3) that are clustered in the genome and originated by lineage-specific duplication. The amphioxus CYP26 cluster thus represents a useful model to assess adaptive evolutionary changes of the RA signaling system following gene duplication. The characterization of amphioxus CYP26 expression, function, and regulation by RA signaling demonstrated that, despite the independent origins of CYP26 duplicates in amphioxus and vertebrates, they convergently assume two main roles during development: RA-dependent patterning and protection against fluctuations of RA levels. Our analysis suggested that in amphioxus RA-dependent patterning is sustained by CYP26-2, while RA homeostasis is mediated by CYP26-1 and CYP26-3. Furthermore, comparisons of the regulatory regions of CYP26 genes of different bilaterian animals indicated that a CYP26-driven negative feedback system was present in the last common ancestor of deuterostomes, but not in that of bilaterians.

CONCLUSIONS

Altogether, this work reveals the evolutionary origins of the RA-dependent regulation of CYP26 genes and highlights convergent functions for CYP26 enzymes that originated by independent duplication events, hence establishing a novel selective mechanism for the genomic retention of gene duplicates.

Developmental Mechanism of Limb Field Specification along the Anterior-Posterior Axis during Vertebrate Evolution

In gnathostomes, limb buds arise from the lateral plate mesoderm at discrete positions along the body axis. Specification of these limb-forming fields can be subdivided into several steps. The lateral plate mesoderm is regionalized into the anterior lateral plate mesoderm (ALPM; cardiac mesoderm) and the posterior lateral plate mesoderm (PLPM). Subsequently, Hox genes appear in a nested fashion in the PLPM and provide positional information along the body axis. The lateral plate mesoderm then splits into the somatic and splanchnic layers. In the somatic layer of the PLPM, the expression of limb initiation genes appears in the limb-forming region, leading to limb bud initiation. Furthermore, past and current work in limbless amphioxus and lampreys suggests that evolutionary changes in developmental programs occurred during the acquisition of paired fins during vertebrate evolution. This review presents these recent advances and discusses the mechanisms of limb field specification during development and evolution, with a focus on the role of Hox genes in this process.

Evolutionary diversification of retinoic acid receptor ligand-binding pocket structure by molecular tinkering

Whole genome duplications (WGDs) have been classically associated with the origin of evolutionary novelties and the so-called duplication-degeneration-complementation model describes the possible fates of genes after duplication. However, how sequence divergence effectively allows functional changes between gene duplicates is still unclear. In the vertebrate lineage, two rounds of WGDs took place, giving rise to paralogous gene copies observed for many gene families. For the retinoic acid receptors (RARs), for example, which are members of the nuclear hormone receptor (NR) superfamily, a unique ancestral gene has been duplicated resulting in three vertebrate paralogues: RARα, RARβ and RARγ. It has previously been shown that this single ancestral RAR was neofunctionalized to give rise to a larger substrate specificity range in the RARs of extant jawed vertebrates (also called gnathostomes). To understand RAR diversification, the members of the cyclostomes (lamprey and hagfish), jawless vertebrates representing the extant sister group of gnathostomes, provide an intermediate situation and thus allow the characterization of the evolutionary steps that shaped RAR ligand-binding properties following the WGDs. In this study, we assessed the ligand-binding specificity of cyclostome RARs and found that their ligand-binding pockets resemble those of gnathostome RARα and RARβ. In contrast, none of the cyclostome receptors studied showed any RARγ-like specificity. Together, our results suggest that cyclostome RARs cover only a portion of the specificity repertoire of the ancestral gnathostome RARs and indicate that the establishment of ligand-binding specificity was a stepwise event. This iterative process thus provides a rare example for the diversification of receptor-ligand interactions of NRs following WGDs.

Genomic changes during evolution of animal parasitism in eukaryotes

Understanding how pathogens have evolved to survive in close association with their hosts is an important step in unraveling the biology of host-pathogen interactions. Comparative genomics is a powerful tool to approach this problem as an increasing number of genomes of multiple pathogen species and strains become available. The ever-growing catalog of genome sequences makes comparison of organisms easier, but it also allows us to reconstitute the evolutionary processes occurring at the genomic level that may have led to the acquisition of pathogenic or parasitic mechanisms.

All-Trans Retinoic Acid-Induced Deficiency of the Wnt/β-Catenin Pathway Enhances Hepatic Carcinoma Stem Cell Differentiation

Retinoic acid (RA) is an important biological signal that directly differentiates cells during embryonic development and tumorigenesis. However, the molecular mechanism of RA-mediated differentiation in hepatic cancer stem cells (hCSCs) is not well understood. In this study, we found that mRNA expressions of RA-biosynthesis-related dehydrogenases were highly expressed in hepatocellular carcinoma. All-trans retinoic acid (ATRA) differentiated hCSCs through inhibiting the function of β-catenin in vitro. ATRA also inhibited the function of PI3K-AKT and enhanced GSK-3β-dependent degradation of phosphorylated β-catenin. Furthermore, ATRA and β-catenin silencing both increased hCSC sensitivity to docetaxel treatment. Our results suggest that targeting β-catenin will provide extra benefits for ATRA-mediated treatment of hepatic cancer patients.

Drosophila duplicate genes evolve new functions on the fly

Gene duplication is thought to play a key role in phenotypic innovation. While several processes have been hypothesized to drive the retention and functional evolution of duplicate genes, their genomic contributions have never been determined. We recently developed the first genome-wide method to classify these processes by comparing distances between expression profiles of duplicate genes and their ancestral single-copy orthologs. Application of our approach to spatial gene expression profiles in two Drosophila species revealed that a majority of young duplicate genes possess new functions, and that new functions are acquired rapidly-often within a few million years. Surprisingly, new functions tend to arise in younger copies of duplicate gene pairs. Moreover, we found that young duplicates are often specifically expressed in testes, whereas old duplicates are broadly expressed across several tissues, providing strong support for the hypothetical "out-of-testes" origin of new genes. In this Extra View, I discuss our findings in the context of theoretical predictions about gene duplication, with a particular emphasis on the importance of natural selection in the evolution of novel phenotypes.

Key Role of Amino Acid Repeat Expansions in the Functional Diversification of Duplicated Transcription Factors

The high regulatory complexity of vertebrates has been related to two rounds of whole genome duplication (2R-WGD) that occurred before the divergence of the major vertebrate groups. Following these events, many developmental transcription factors (TFs) were retained in multiple copies and subsequently specialized in diverse functions, whereas others reverted to their singleton state. TFs are known to be generally rich in amino acid repeats or low-complexity regions (LCRs), such as polyalanine or polyglutamine runs, which can evolve rapidly and potentially influence the transcriptional activity of the protein. Here we test the hypothesis that LCRs have played a major role in the diversification of TF gene duplicates. We find that nearly half of the TF gene families originated during the 2R-WGD contains LCRs. The number of gene duplicates with LCRs is 155 out of 550 analyzed (28%), about twice as many as the number of single copy genes with LCRs (15 out of 115, 13%). In addition, duplicated TFs preferentially accumulate certain LCR types, the most prominent of which are alanine repeats. We experimentally test the role of alanine-rich LCRs in two different TF gene families, PHOX2A/PHOX2B and LHX2/LHX9. In both cases, the presence of the alanine-rich LCR in one of the copies (PHOX2B and LHX2) significantly increases the capacity of the TF to activate transcription. Taken together, the results provide strong evidence that LCRs are important driving forces of evolutionary change in duplicated genes.

Evolution of retinoic acid receptors in chordates: insights from three lamprey species, Lampetra fluviatilis, Petromyzon marinus, and Lethenteron japonicum

Background

Retinoic acid (RA) signaling controls many developmental processes in chordates, from early axis specification to late organogenesis. The functions of RA are chiefly mediated by a subfamily of nuclear hormone receptors, the retinoic acid receptors (RARs), that act as ligand-activated transcription factors. While RARs have been extensively studied in jawed vertebrates (that is, gnathostomes) and invertebrate chordates, very little is known about the repertoire and developmental roles of RARs in cyclostomes, which are extant jawless vertebrates. Here, we present the first extensive study of cyclostome RARs focusing on three different lamprey species: the European freshwater lamprey, Lampetra fluviatilis, the sea lamprey, Petromyzon marinus, and the Japanese lamprey, Lethenteron japonicum.

Results

We identified four rar paralogs (rar1, rar2, rar3, and rar4) in each of the three lamprey species, and phylogenetic analyses indicate a complex evolutionary history of lamprey rar genes including the origin of rar1 and rar4 by lineage-specific duplication after the lamprey-hagfish split. We further assessed their expression patterns during embryonic development by in situ hybridization. The results show that lamprey rar genes are generally characterized by dynamic and highly specific expression domains in different embryonic tissues. In particular, lamprey rar genes exhibit combinatorial expression domains in the anterior central nervous system (CNS) and the pharyngeal region.

Conclusions

Our results indicate that the genome of lampreys encodes at least four rar genes and suggest that the lamprey rar complement arose from vertebrate-specific whole genome duplications followed by a lamprey-specific duplication event. Moreover, we describe a combinatorial code of lamprey rar expression in both anterior CNS and pharynx resulting from dynamic and highly specific expression patterns during embryonic development. This ‘RAR code’ might function in regionalization and patterning of these two tissues by differentially modulating the expression of downstream effector genes during development.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0016-4) contains supplementary material, which is available to authorized users.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Evolution of retinoic acid receptors and retinoic acid signaling

Retinoic acid (RA) is a vitamin A-derived morphogen controlling important developmental processes in vertebrates, and more generally in chordates, including axial patterning and tissue formation and differentiation. In the embryo, endogenous RA levels are controlled by RA synthesizing and degrading enzymes and the RA signal is transduced by two retinoid receptors: the retinoic acid receptor (RAR) and the retinoid X receptor (RXR). Both RAR and RXR are members of the nuclear receptor superfamily of ligand-activated transcription factors and mainly act as heterodimers to activate the transcription of target genes in the presence of their ligand, all-trans RA. This signaling pathway was long thought to be a chordate innovation, however, recent findings of gene homologs involved in RA signaling in the genomes of a wide variety of non-chordate animals, including ambulacrarians (sea urchins and acorn worms) and lophotrochozoans (annelids and mollusks), challenged this traditional view and suggested that the RA signaling pathway might have a more ancient evolutionary origin than previously thought. In this chapter, we discuss the evolutionary history of the RA signaling pathway, and more particularly of the RARs, which might have experienced independent gene losses and duplications in different animal lineages. In sum, the available data reveal novel insights into the origin of the RA signaling pathway as well as into the evolutionary history of the RARs.

A mollusk retinoic acid receptor (RAR) ortholog sheds light on the evolution of ligand binding

Nuclear receptors are transcription factors that regulate networks of target genes in response to small molecules. There is a strong bias in our knowledge of these receptors because they were mainly characterized in classical model organisms, mostly vertebrates. Therefore, the evolutionary origins of specific ligand-receptor couples still remain elusive. Here we present the identification and characterization of a retinoic acid receptor (RAR) from the mollusk Nucella lapillus (NlRAR). We show that this receptor specifically binds to DNA response elements organized in direct repeats as a heterodimer with retinoid X receptor. Surprisingly, we also find that NlRAR does not bind all-trans retinoic acid or any other retinoid we tested. Furthermore, NlRAR is unable to activate the transcription of reporter genes in response to stimulation by retinoids and to recruit coactivators in the presence of these compounds. Three-dimensional modeling of the ligand-binding domain of NlRAR reveals an overall structure that is similar to vertebrate RARs. However, in the ligand-binding pocket (LBP) of the mollusk receptor, the alteration of several residues interacting with the ligand has apparently led to an overall decrease in the strength of the interaction with the ligand. Accordingly, mutations of NlRAR at key positions within the LBP generate receptors that are responsive to retinoids. Altogether our data suggest that, in mollusks, RAR has lost its affinity for all-trans retinoic acid, highlighting the evolutionary plasticity of its LBP. When put in an evolutionary context, our results reveal new structural and functional features of nuclear receptors validated by millions of years of evolution that were impossible to reveal in model organisms.

Signaling through retinoic acid receptors in cardiac development: Doing the right things at the right times

Retinoic acid (RA) is a terpenoid that is synthesized from vitamin A/retinol (ROL) and binds to the nuclear receptors retinoic acid receptor (RAR)/retinoid X receptor (RXR) to control multiple developmental processes in vertebrates. The available clinical and experimental data provide uncontested evidence for the pleiotropic roles of RA signaling in development of multiple embryonic structures and organs such eyes, central nervous system, gonads, lungs and heart. The development of any of these above-mentioned embryonic organ systems can be effectively utilized to showcase the many strategies utilized by RA signaling. However, it is very likely that the strategies employed to transfer RA signals during cardiac development comprise the majority of the relevant and sophisticated ways through which retinoid signals can be conveyed in a complex biological system. Here, we provide the reader with arguments indicating that RA signaling is exquisitely regulated according to specific phases of cardiac development and that RA signaling itself is one of the major regulators of the timing of cardiac morphogenesis and differentiation. We will focus on the role of signaling by RA receptors (RARs) in early phases of heart development. This article is part of a Special Issue entitled: Nuclear receptors in animal development.

Restriction and recruitment-gene duplication and the origin and evolution of snake venom toxins

Snake venom has been hypothesized to have originated and diversified through a process that involves duplication of genes encoding body proteins with subsequent recruitment of the copy to the venom gland, where natural selection acts to develop or increase toxicity. However, gene duplication is known to be a rare event in vertebrate genomes, and the recruitment of duplicated genes to a novel expression domain (neofunctionalization) is an even rarer process that requires the evolution of novel combinations of transcription factor binding sites in upstream regulatory regions. Therefore, although this hypothesis concerning the evolution of snake venom is very unlikely and should be regarded with caution, it is nonetheless often assumed to be established fact, hindering research into the true origins of snake venom toxins. To critically evaluate this hypothesis, we have generated transcriptomic data for body tissues and salivary and venom glands from five species of venomous and nonvenomous reptiles. Our comparative transcriptomic analysis of these data reveals that snake venom does not evolve through the hypothesized process of duplication and recruitment of genes encoding body proteins. Indeed, our results show that many proposed venom toxins are in fact expressed in a wide variety of body tissues, including the salivary gland of nonvenomous reptiles and that these genes have therefore been restricted to the venom gland following duplication, not recruited. Thus, snake venom evolves through the duplication and subfunctionalization of genes encoding existing salivary proteins. These results highlight the danger of the elegant and intuitive "just-so story" in evolutionary biology.

Active repression by RARγ signaling is required for vertebrate axial elongation

Retinoic acid receptor gamma 2 (RARγ2) is the major RAR isoform expressed throughout the caudal axial progenitor domain in vertebrates. During a microarray screen to identify RAR targets, we identified a subset of genes that pattern caudal structures or promote axial elongation and are upregulated by increased RAR-mediated repression. Previous studies have suggested that RAR is present in the caudal domain, but is quiescent until its activation in late stage embryos terminates axial elongation. By contrast, we show here that RARγ2 is engaged in all stages of axial elongation, not solely as a terminator of axial growth. In the absence of RA, RARγ2 represses transcriptional activity in vivo and maintains the pool of caudal progenitor cells and presomitic mesoderm. In the presence of RA, RARγ2 serves as an activator, facilitating somite differentiation. Treatment with an RARγ-selective inverse agonist (NRX205099) or overexpression of dominant-negative RARγ increases the expression of posterior Hox genes and that of marker genes for presomitic mesoderm and the chordoneural hinge. Conversely, when RAR-mediated repression is reduced by overexpressing a dominant-negative co-repressor (c-SMRT), a constitutively active RAR (VP16-RARγ2), or by treatment with an RARγ-selective agonist (NRX204647), expression of caudal genes is diminished and extension of the body axis is prematurely terminated. Hence, gene repression mediated by the unliganded RARγ2-co-repressor complex constitutes a novel mechanism to regulate and facilitate the correct expression levels and spatial restriction of key genes that maintain the caudal progenitor pool during axial elongation in Xenopus embryos.

Cloning and characterization of the retinoic acid receptor-like protein in the rock shell, Thais clavigera

The organotin compounds have a high affinity for the retinoid X receptor (RXR), which is a transcriptional factor activated by retinoids that induce imposex in gastropods. However, the molecular mechanisms underlying the regulation of RXR and its related genes in gastropods remain unclear. We isolated a retinoic acid receptor (RAR)-like cDNA (TcRAR) in the rock shell, Thais clavigera, and examined the transcriptional activity of the TcRAR protein by using all-trans retinoic acid (ATRA). However, we did not observe any ligand-dependent transactivation by this protein. We also examined the transcriptional activity of the TcRAR-ligand binding domain fused with the GAL4-DNA binding domain by using retinoic acids, retinol, and organotins and again saw no noteworthy transcriptional induction by these chemicals. Use of a mammalian two-hybrid assay to assess the interaction of the TcRAR protein with the TcRXR isoforms suggested that TcRAR might form a heterodimer with the RXR isoforms. The transcriptional activity of domain-swapped TcRAR chimeric proteins (the A/B domain of TcRAR combined with the D-F domain of human RARα) was also examined and found to be ATRA-dependent. These results suggest that TcRAR is not activated by retinoic acids, but can form a heterodimer with TcRXR isoforms. These data contribute to our understanding of the mechanism by which RXR functions in gastropods.

Retinoid X receptor-mediated transdifferentiation cascade in budding tunicates

In the budding tunicate, Polyandrocarpa misakiensis, retinoic acid (RA) applied to buds promotes transdifferentiation of somatic cells to form the secondary body axis. This study investigated the gene cascade regulating such RA-triggered transdifferentiation in tunicates. Genes encoding retinoic acid receptor (RAR) and retinoid X receptor (RXR) were induced during transdifferentiation, and they responded to all-trans RA or 13-cis RA in vivo, whereas 9-cis RA had the least effects, demonstrating differences in the ligand preference between budding tunicates and vertebrates. In contrast to RAR mRNA, RXR mRNA could induce transdifferentiation-related genes such as RXR itself, ERK, and MYC in an RA-dependent manner and also induced β-catenin (β-CTN) RA-independently when it was introduced in vitro into tunicate cell lines that do not express endogenous RAR or RXR. Small interfering RNA (siRNA) of RXR dramatically attenuated not only RXR but also ERK and β-CTN gene activities. An ERK inhibitor severely blocked wound healing and dedifferentiation. β-CTN siRNA suppressed morphogenesis and redifferentiation, similar to RXR siRNA. These results indicate that in P. misakiensis, the main function of RA is to trigger positive feedback regulation of RXR rather than to activate RAR for unlocking downstream pathways for transdifferentiation. Our results may reflect an ancient mode of RA signaling in chordates.

Neofunctionalization of young duplicate genes in Drosophila

Gene duplication is a key source of genetic innovation that plays a role in the evolution of phenotypic complexity. Although several evolutionary processes can result in the long-term retention of duplicate genes, their relative contributions in nature are unknown. Here we develop a phylogenetic approach for comparing genome-wide expression profiles of closely related species to quantify the roles of conservation, neofunctionalization, subfunctionalization, and specialization in the preservation of duplicate genes. Application of our method to pairs of young duplicates in Drosophila shows that neofunctionalization, the gain of a novel function in one copy, accounts for the retention of almost two-thirds of duplicate genes. Surprisingly, novel functions nearly always originate in younger (child) copies, whereas older (parent) copies possess functions similar to those of ancestral genes. Further examination of such pairs reveals a strong bias toward RNA-mediated duplication events, implicating asymmetric duplication and positive selection in the evolution of new functions. Moreover, we show that young duplicate genes are expressed primarily in testes and that their expression breadth increases over evolutionary time. This finding supports the "out-of-testes" hypothesis, which posits that testes are a catalyst for the emergence of new genes that ultimately evolve functions in other tissues. Thus, our study highlights the importance of neofunctionalization and positive selection in the retention of young duplicates in Drosophila and illustrates how duplicates become incorporated into novel functional networks over evolutionary time.

Elongation factor-1α, a putative single-copy nuclear gene, has divergent sets of paralogs in an arachnid

Identification of paralogy in candidate nuclear loci is an important prerequisite in phylogenetics and statistical phylogeography, but one that is often overlooked. One marker commonly assumed to be a single-copy gene and claimed to harbor great utility for inferring recent divergences is elongation factor-1α (EF-1α). To test this hypothesis, we systematically cloned EF-1α in three disjunct populations of the harvestman Metasiro americanus. Here we show that EF-1α has a large number of paralogs in this species. The paralogs do not evolve in a concerted manner, and the paralogs diverged prior to the population divergence. Moreover, the paralogs of M. americanus are not comparable to the highly divergent EF-1α paralogs found in bees and spiders, which are easily recognized and separated through the use of specific primers. We demonstrate statistically that our detection of paralogs cannot be attributed to amplification error. The presence of EF-1α paralogs in M. americanus prevents its use in statistical phylogeography, and the presence of out-paralogs argues against its use in phylogenetic inference among recently diverged clades. These data contradict the common assumption that EF-1α is for most or all taxa a single-copy gene, or that it has a small number of paralogs that are homogenized through gene conversion, unequal crossing over, or other processes.

Evolutionary, structural and functional interplay of the IκB family members

A primary level of control for nuclear factor kappa B (NF-κB) is effected through its interactions with the inhibitor protein, inhibitor of kappa B (IκB). Several lines of evidence confirm the existence of multiple forms of IκB that appear to regulate NF-κB by distinct mechanisms. Therefore, we performed a comprehensive bioinformatics analysis to understand the evolutionary history and intrinsic functional diversity of IκB family members. Phylogenetic relationships were constructed to trace the evolution of the IκB family genes. Our phylogenetic analysis revealed 10 IκB subfamily members that clustered into 5 major clades. Since the ankyrin (ANK) domain appears to be more ancient than the Rel homology domain (RHD), our phylogenetic analysis suggests that some undefined ancestral set of ANK repeats acquired an RHD before any duplication and was later duplicated and then diverged into the different IκB subfamilies. Functional analysis identified several functionally divergent sites in the ANK repeat domains (ARDs) and revealed that this region has undergone strong purifying selection, suggesting its functional importance in IκB genes. Structural analysis showed that the major variations in the number of ANK repeats and high conformational changes in the finger loop ARD region contribute to the differing binding partner specificities, thereby leading to distinct IκB functions. In summary, our study has provided useful information about the phylogeny and structural and functional divergence of the IκB family. Additionally, we identified a number of amino acid sites that contribute to the predicted functional divergence of these proteins.

Molecular and evolutionary basis of limb field specification and limb initiation

Specification of limb field and initiation of limb development involve multiple steps, each of which is tightly regulated both spatially and temporally. Recent developmental analyses on various vertebrates have provided insights into the molecular mechanisms that specify limb field and have revealed several genetic interactions of signals involved in limb initiation processes. Furthermore, new approaches to the study of the developmental mechanisms of the lateral plate mesoderm of amphioxus and lamprey embryos have given us clues to understand the evolutionary scenarios that led to the acquisition of paired appendages during evolution. This review highlights such recent findings and discusses the mechanisms of limb field specification and limb bud initiation during development and evolution.

Molecular evolution and structural features of IRAK family members

The interleukin-1 receptor-associated kinase (IRAK) family comprises critical signaling mediators of the TLR/IL-1R signaling pathways. IRAKs are Ser/Thr kinases. There are 4 members in the vertebrate genome (IRAK1, IRAK2, IRAKM, and IRAK4) and an IRAK homolog, Pelle, in insects. IRAK family members are highly conserved in vertebrates, but the evolutionary relationship between IRAKs in vertebrates and insects is not clear. To investigate the evolutionary history and functional divergence of IRAK members, we performed extensive bioinformatics analysis. The phylogenetic relationship between IRAK sequences suggests that gene duplication events occurred in the evolutionary lineage, leading to early vertebrates. A comparative phylogenetic analysis with insect homologs of IRAKs suggests that the Tube protein is a homolog of IRAK4, unlike the anticipated protein, Pelle. Furthermore, the analysis supports that an IRAK4-like kinase is an ancestral protein in the metazoan lineage of the IRAK family. Through functional analysis, several potentially diverged sites were identified in the common death domain and kinase domain. These sites have been constrained during evolution by strong purifying selection, suggesting their functional importance within IRAKs. In summary, our study highlighted the molecular evolution of the IRAK family, predicted the amino acids that contributed to functional divergence, and identified structural variations among the IRAK paralogs that may provide a starting point for further experimental investigations.