Conserved regulation of proximodistal limb axis development by Meis1/Hth

Transcription factor Meis1b regulates craniofacial morphogenesis in zebrafish

BACKGROUND

Meis family of transcription factors operates in Pbx-Meis-Hox regulatory network controlling development of various tissues including eye, limbs, heart, hindbrain or craniofacial skeletal elements originating from the neural crest. Although studies in mouse provide abundant information about Meis factors function in embryogenesis, little is known about their role in zebrafish.

RESULTS

We generated zebrafish lines carrying null mutations in meis1a, meis1b, meis2a, and meis2b genes. Only meis1b mutants are lethal at larval stage around 13 dpf whereas the other mutant lines are viable and fertile. We focused on development of neural crest-derived craniofacial structures such as tendons, cranial nerves, cartilage and accompanying muscles. Meis1b mutants displayed morphogenetic abnormalities in the cartilage originating from the first and second pharyngeal arches. Meckel's cartilage was shorter and wider with fused anterior symphysis and abnormal chondrocyte organization. This resulted in impaired tendons and muscle fiber connections while tenocyte development was not largely affected.

CONCLUSIONS

Loss-of-function mutation in meis1b affects cartilage morphology in the lower jaw that leads to disrupted organization of muscles and tendons.

The methylomic landscape of human articular cartilage development contains epigenetic signatures of osteoarthritis risk

Increasing evidence is emerging to link age-associated complex musculoskeletal diseases, including osteoarthritis (OA), to developmental factors. Multiple studies have shown a functional role for DNA methylation in the genetic mechanisms of OA risk using articular cartilage samples taken from aged individuals, yet knowledge of temporal changes to the methylome during human cartilage development is limited. We quantified DNA methylation at ∼700,000 individual CpGs across the epigenome of developing human chondrocytes in 72 samples ranging from 7 to 21 post-conception weeks. We identified significant changes in 3% of all CpGs and >8,200 developmental differentially methylated regions. We further identified 24 loci at which OA genetic variants colocalize with methylation quantitative trait loci. Through integrating developmental and mature human chondrocyte datasets, we find evidence for functional effects exerted solely in development or throughout the life course. This will have profound impacts on future approaches to translating genetic pathways for therapeutic intervention.

A chromatin code for limb segment identity in axolotl limb regeneration

The salamander limb correctly regenerates missing limb segments because connective tissue cells have segment-specific identities, termed "positional information". How positional information is molecularly encoded at the chromatin level has been unknown. Here, we performed genome-wide chromatin profiling in mature and regenerating axolotl limb connective tissue cells. We find segment-specific levels of histone H3K27me3 as the major positional mark, especially at limb homeoprotein gene loci but not their upstream regulators, constituting an intrinsic segment information code. During regeneration, regeneration-specific regulatory elements became active prior to the re-appearance of developmental regulatory elements. In the hand, the permissive chromatin state of the homeoprotein gene HoxA13 engages with the regeneration program bypassing the upper limb program. Comparison of regeneration regulatory elements with those found in other regenerative animals identified a core shared set of transcription factors, supporting an ancient, conserved regeneration program.

Mechanisms and translational applications of regeneration in limbs: From renewable animals to humans

BACKGROUND

The regenerative capacity of organisms declines throughout evolution, and mammals lack the ability to regenerate limbs after injury. Past approaches to achieving successful restoration through pharmacological intervention, tissue engineering, and cell therapies have faced significant challenges.

OBJECTIVES

This review aims to provide an overview of the current understanding of the mechanisms behind animal limb regeneration and the successful translation of these mechanisms for human tissue regeneration.

RESULTS

Particular attention was paid to the Mexican axolotl (Ambystoma mexicanum), the only adult tetrapod capable of limb regeneration. We will explore fundamental questions surrounding limb regeneration, such as how amputation initiates regeneration, how the limb knows when to stop and which parts to regenerate, and how these findings can apply to mammalian systems.

CONCLUSIONS

Given the urgent need for regenerative therapies to treat conditions like diabetic foot ulcers and trauma survivors, this review provides valuable insights and ideas for researchers, clinicians, and biomedical engineers seeking to facilitate the regeneration process or elicit full regeneration from partial regeneration events.

Disparity of gene expression in coronary artery disease: insights from MEIS1, HIRA, and Myocardin

INTRODUCTION

Coronary artery disease (CAD) in young adults can have devastating consequences. The cardiac developmental gene MEIS1 plays important roles in vascular networks and heart development. This gene effects on the regeneration capacity of the heart. Considering role of MEIS1 in cardiac tissue development and the progression of myocardial infarction this study investigated the expression levels of the MEIS1, HIRA, and Myocardin genes in premature CAD patients compared to healthy subjects and evaluated the relationships between these genes and possible inflammatory factors.

METHODS AND RESULTS

The study conducted a case-control design involving 35 CAD patients and 35 healthy individuals. Peripheral blood mononuclear cells (PBMCs) were collected, and gene expression analysis was performed using real-time PCR. Compared with control group, the number of PBMCs in the CAD group exhibited greater MEIS1 and HIRA gene expression, with fold changes of 2.45 and 3.6. The expression of MEIS1 exhibited a negative correlation with IL-10 (r= -0.312) expression and positive correlation with Interleukin (IL)-6 (r = 0.415) and tumor necrosis factor (TNF)-α (r = 0.534) gene expression. Moreover, there was an inverse correlation between the gene expression of HIRA and that of IL-10 (r= -0.326), and a positive correlation was revealed between the expression of this gene and that of the IL-6 (r = 0.453) and TNF-α (r = 0.572) genes.

CONCLUSION

This research demonstrated a disparity in expression levels of MEIS1, HIRA, and Myocardin, between CAD and healthy subjects. The results showed that, MEIS1 and HIRA play significant roles in regulating the synthesis of proinflammatory cytokines, namely, TNF-α and IL-6.

Epigenetic mechanisms of osteoarthritis risk in human skeletal development

The epigenome, including the methylation of cytosine bases at CG dinucleotides, is intrinsically linked to transcriptional regulation. The tight regulation of gene expression during skeletal development is essential, with ~1/500 individuals born with skeletal abnormalities. Furthermore, increasing evidence is emerging to link age-associated complex genetic musculoskeletal diseases, including osteoarthritis (OA), to developmental factors including joint shape. Multiple studies have shown a functional role for DNA methylation in the genetic mechanisms of OA risk using articular cartilage samples taken from aged patients. Despite this, our knowledge of temporal changes to the methylome during human cartilage development has been limited. We quantified DNA methylation at ~700,000 individual CpGs across the epigenome of developing human articular cartilage in 72 samples ranging from 7-21 post-conception weeks, a time period that includes cavitation of the developing knee joint. We identified significant changes in 8% of all CpGs, and >9400 developmental differentially methylated regions (dDMRs). The largest hypermethylated dDMRs mapped to transcriptional regulators of early skeletal patterning including MEIS1 and IRX1. Conversely, the largest hypomethylated dDMRs mapped to genes encoding extracellular matrix proteins including SPON2 and TNXB and were enriched in chondrocyte enhancers. Significant correlations were identified between the expression of these genes and methylation within the hypomethylated dDMRs. We further identified 811 CpGs at which significant dimorphism was present between the male and female samples, with the majority (68%) being hypermethylated in female samples. Following imputation, we captured the genotype of these samples at >5 million variants and performed epigenome-wide methylation quantitative trait locus (mQTL) analysis. Colocalization analysis identified 26 loci at which genetic variants exhibited shared impacts upon methylation and OA genetic risk. This included loci which have been previously reported to harbour OA-mQTLs (including GDF5 and ALDH1A2), yet the majority (73%) were novel (including those mapping to CHST3, FGF1 and TEAD1). To our knowledge, this is the first extensive study of DNA methylation across human articular cartilage development. We identify considerable methylomic plasticity within the development of knee cartilage and report active epigenomic mediators of OA risk operating in prenatal joint tissues.

Fgf signalling triggers an intrinsic mesodermal timer that determines the duration of limb patterning

Complex signalling between the apical ectodermal ridge (AER - a thickening of the distal epithelium) and the mesoderm controls limb patterning along the proximo-distal axis (humerus to digits). However, the essential in vivo requirement for AER-Fgf signalling makes it difficult to understand the exact roles that it fulfils. To overcome this barrier, we developed an amenable ex vivo chick wing tissue explant system that faithfully replicates in vivo parameters. Using inhibition experiments and RNA-sequencing, we identify a transient role for Fgfs in triggering the distal patterning phase. Fgfs are then dispensable for the maintenance of an intrinsic mesodermal transcriptome, which controls proliferation/differentiation timing and the duration of patterning. We also uncover additional roles for Fgf signalling in maintaining AER-related gene expression and in suppressing myogenesis. We describe a simple logic for limb patterning duration, which is potentially applicable to other systems, including the main body axis.

Molecular mechanism of synovial joint site specification and induction in developing vertebrate limbs

The vertebrate appendage comprises three primary segments, the stylopod, zeugopod and autopod, each separated by joints. The molecular mechanisms governing the specification of joint sites, which define segment lengths and thereby limb architecture, remain largely unknown. Existing literature suggests that reciprocal gradients of retinoic acid (RA) and fibroblast growth factor (FGF) signaling define the expression domains of the putative segment markers Meis1, Hoxa11 and Hoxa13. Barx1 is expressed in the presumptive joint sites. Our data demonstrate that RA-FGF signaling gradients define the expression domain of Barx1 in the first presumptive joint site. When misexpressed, Barx1 induces ectopic interzone-like structures, and its loss of function partially blocks interzone development. Simultaneous perturbations of RA-FGF signaling gradients result in predictable shifts of Barx1 expression domains along the proximo-distal axis and, consequently, in the formation of repositioned joints. Our data suggest that during early limb bud development in chick, Meis1 and Hoxa11 expression domains are overlapping, whereas the Barx1 expression domain resides within the Hoxa11 expression domain. However, once the interzone is formed, the expression domains are refined and the Barx1 expression domain becomes congruent with the border of these two putative segment markers.

A single-cell census of mouse limb development identifies complex spatiotemporal dynamics of skeleton formation

Limb development has long served as a model system for coordinated spatial patterning of progenitor cells. Here, we identify a population of naive limb progenitors and show that they differentiate progressively to form the skeleton in a complex, non-consecutive, three-dimensional pattern. Single-cell RNA sequencing of the developing mouse forelimb identified three progenitor states: naive, proximal, and autopodial, as well as Msx1 as a marker for the naive progenitors. In vivo lineage tracing confirmed this role and localized the naive progenitors to the outer margin of the limb, along the anterior-posterior axis. Sequential pulse-chase experiments showed that the progressive transition of Msx1⁺ naive progenitors into proximal and autopodial progenitors coincides with their differentiation to Sox9⁺ chondroprogenitors, which occurs along all the forming skeletal segments. Indeed, tracking the spatiotemporal sequence of differentiation showed that the skeleton forms progressively in a complex pattern. These findings suggest an alternative model for limb skeleton development.

An example of parenchymal renal sparing in the context of complex malformations due to a novel mutation in the PBX1 gene

INTRODUCTION

PBX1 encodes the pre-B cell leukemia factor 1, a Three Amino acid Loop Extension (TALE) transcription factor crucial to regulate basic developmental processes. PBX1 loss-of-function variants have been initially described in association with renal malformations in both isolated and syndromic forms.

CASE REPORT

Herein, we report a male infant presenting multiple organ malformations (cleidosternal dysostosis, micrognathia, left lung hypoplasia, wide interatrial defect, pulmonary hypertension, total anomalous pulmonary venous return, intestinal malrotation) and carrying the heterozygous de novo c.868C > T (p.Arg290Trp) variant in PBX1. This novel variant affects the highly conserved homeodomain of the protein, leading to a non-conservative substitution and consequently altering its tridimensional structure and DNA-binding capacity.

CONCLUSION

So far, PBX1 has been reported in association with a broad spectrum of renal anomalies. However, given the role of this gene in many different developing processes, whole-exome sequencing can detect mutations in PBX1 even in patients with different phenotypes, not necessarily involving the renal primordium. This report presents a novel PBX1 variant with a predicted strong deleterious effect. The mutation leads to a non-conservative substitution in a very highly conserved domain of the protein, thus altering its tertiary structure and DNA-binding capacity.

When the end modifies its means: the origins of novelty and the evolution of innovation

The origin of novel complex traits constitutes a central yet largely unresolved challenge in evolutionary biology. Intriguingly, many of the most promising breakthroughs in understanding the genesis of evolutionary novelty in recent years have occurred not in evolutionary biology itself, but through the comparative study of development and, more recently, the interface of developmental biology and ecology. Here, I discuss how these insights are changing our understanding of what matters in the origin of novel, complex traits in ontogeny and evolution. Specifically, my essay has two major objectives. First, I discuss how the nature of developmental systems biases the production of phenotypic variation in the face of novel or stressful environments toward functional, integrated and, possibly, adaptive variants. This, in turn, allows the production of novel phenotypes to precede (rather than follow) changes in genotype and allows developmental processes that are the product of past evolution to shape evolutionary change that has yet to occur. Second, I explore how this nature of developmental systems has itself evolved over time, increasing the repertoire of ontogenies to pursue a wider range of objectives across an expanding range of conditions, thereby creating an increasingly extensive affordance landscape in development and developmental evolution. Developmental systems and their evolution can thus be viewed as dynamic processes that modify their own means across ontogeny and phylogeny. The study of these dynamics necessitates more than the strict reductionist approach that currently dominates the fields of developmental and evolutionary developmental biology.

Positional Memory in Vertebrate Regeneration: A Century's Insights from the Salamander Limb

Salamanders, such as axolotls and newts, can regenerate complex tissues including entire limbs. But what mechanisms ensure that an amputated limb regenerates a limb, and not a tail or unpatterned tissue? An important concept in regeneration is positional memory-the notion that adult cells "remember" spatial identities assigned to them during embryogenesis (e.g., "head" or "hand") and use this information to restore the correct body parts after injury. Although positional memory is well documented at a phenomenological level, the underlying cellular and molecular bases are just beginning to be decoded. Herein, we review how major principles in positional memory were established in the salamander limb model, enabling the discovery of positional memory-encoding molecules, and advancing insights into their pattern-forming logic during regeneration. We explore findings in other amphibians, fish, reptiles, and mammals and speculate on conserved aspects of positional memory. We consider the possibility that manipulating positional memory in human cells could represent one route toward improved tissue repair or engineering of patterned tissues for therapeutic purposes.

Tig1 regulates proximo-distal identity during salamander limb regeneration

Salamander limb regeneration is an accurate process which gives rise exclusively to the missing structures, irrespective of the amputation level. This suggests that cells in the stump have an awareness of their spatial location, a property termed positional identity. Little is known about how positional identity is encoded, in salamanders or other biological systems. Through single-cell RNAseq analysis, we identified Tig1/Rarres1 as a potential determinant of proximal identity. Tig1 encodes a conserved cell surface molecule, is regulated by retinoic acid and exhibits a graded expression along the proximo-distal axis of the limb. Its overexpression leads to regeneration defects in the distal elements and elicits proximal displacement of blastema cells, while its neutralisation blocks proximo-distal cell surface interactions. Critically, Tig1 reprogrammes distal cells to a proximal identity, upregulating Prod1 and inhibiting Hoxa13 and distal transcriptional networks. Thus, Tig1 is a central cell surface determinant of proximal identity in the salamander limb.

The mechanisms by which cells determine their position within the 3D space are poorly understood. Research in salamanders offers fresh insights into this question, uncovering Tig1 as a central determinant of proximo-distal identity in regeneration.

De novo PBX1 variant in a patient with glaucoma, kidney anomalies, and developmental delay: An expansion of the CAKUTHED phenotype

An infant was referred for evaluation of congenital glaucoma and corneal clouding. In addition, he had a pelvic kidney, hypotonia, patent ductus arteriosus, abnormal pinnae, and developmental delay. Exome sequencing identified a previously unpublished de novo single nucleotide insertion in PBX1 c.400dupG (NM_002585.3), predicted to cause a frameshift resulting in a truncated protein with loss of function (p.Ala134Glyfs*65). Identification of this loss of function variant supports the diagnosis of congenital anomalies of the kidney and urinary tract syndrome with or without hearing loss, abnormal ears, or developmental delay (CAKUTHED). Here, we propose glaucoma as an extra-renal manifestation associated with PBX1-related disease due to the relationship of PBX1 with MEIS1, MEIS2, and FOXC1 transcription factors associated with eye development.

Co-option of the limb patterning program in cephalopod eye development

Background

Across the Metazoa, similar genetic programs are found in the development of analogous, independently evolved, morphological features. The functional significance of this reuse and the underlying mechanisms of co-option remain unclear. Cephalopods have evolved a highly acute visual system with a cup-shaped retina and a novel refractive lens in the anterior, important for a number of sophisticated behaviors including predation, mating, and camouflage. Almost nothing is known about the molecular-genetics of lens development in the cephalopod.

Results

Here we identify the co-option of the canonical bilaterian limb patterning program during cephalopod lens development, a functionally unrelated structure. We show radial expression of transcription factors SP6-9/sp1, Dlx/dll, Pbx/exd, Meis/hth, and a Prdl homolog in the squid Doryteuthis pealeii, similar to expression required in Drosophila limb development. We assess the role of Wnt signaling in the cephalopod lens, a positive regulator in the developing Drosophila limb, and find the regulatory relationship reversed, with ectopic Wnt signaling leading to lens loss.

Conclusion

This regulatory divergence suggests that duplication of SP6-9 in cephalopods may mediate the co-option of the limb patterning program. Thus, our study suggests that this program could perform a more universal developmental function in radial patterning and highlights how canonical genetic programs are repurposed in novel structures.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01182-2.

Intronic elements associated with insomnia and restless legs syndrome exhibit cell type-specific epigenetic features contributing to MEIS1 regulation

A highly evolutionarily conserved MEIS1 intronic region is strongly associated with restless legs syndrome (RLS) and insomnia. To understand its regulatory function, we dissected the region by analyzing chromatin accessibility, enhancer-promoter contacts, DNA methylation, and eQTLs in different human neural cell types and tissues. We observed specific activity with respect to cell type and developmental maturation, indicating a prominent role for distinct highly conserved intronic elements in forebrain inhibitory neuron differentiation. Two elements were hypomethylated in neural cells with higher MEIS1 expression, suggesting a role of enhancer demethylation in gene regulation. MEIS1 eQTLs showed a striking modular chromosomal distribution, with forebrain eQTLs clustering in intron 8/9. CRISPR interference targeting of individual elements in this region attenuated MEIS1 expression, revealing a complex regulatory interplay of distinct elements. In summary, we found that MEIS1 regulation is organized in a modular pattern. Disease-associated intronic regulatory elements control MEIS1 expression with cell type and maturation stage specificity, particularly in the inhibitory neuron lineage. The precise spatiotemporal activity of these elements likely contributes to the pathogenesis of insomnia and RLS.

HOX Protein Activity Regulation by Cellular Localization

While the functions of HOX genes have been and remain extensively studied in distinct model organisms from flies to mice, the molecular biology of HOX proteins remains poorly documented. In particular, the mechanisms involved in regulating the activity of HOX proteins have been poorly investigated. Nonetheless, based on data available from other well-characterized transcription factors, it can be assumed that HOX protein activity must be finely tuned in a cell-type-specific manner and in response to defined environmental cues. Indeed, records in protein–protein interaction databases or entries in post-translational modification registries clearly support that HOX proteins are the targets of multiple layers of regulation at the protein level. In this context, we review here what has been reported and what can be inferred about how the activities of HOX proteins are regulated by their intracellular distribution.

Placental Expression of the Forelimb Patterning Transcription Factor MEIS2 in Trisomy 15

BackgroundObservations of first trimester human trisomy 15 (T15) embryos have identified meromelic changes in the upper limbs. These changes are similar to those observed in animal studies investigating the effects of overexpression of Meis2, a signaling transcription factor expressed during forelimb development. Although it would be exceedingly difficult to assess MEIS2 expression in the human embryonic arm, MEIS2 has been reported as consistently expressed in first trimester placental villus stroma. Methods: This study addresses whether gene dosage effect might underlie meromelia in T15 by comparing MEIS2 expression in placentas from T15 and euploid spontaneous abortions employing manual and automated assessment of MEIS2 immunohistochemical scoring. Results: Average MEIS2 expression is increased in T15 first trimester placental tissue compared to euploid controls but that the difference is marginal. Manual and automated scoring showed moderately strong correlation. Conclusion: Extrapolation of these results suggests MEIS2 overexpression may not be required for meromelia in T15.

Genome-Wide Association Study of Growth Performance and Immune Response to Newcastle Disease Virus of Indigenous Chicken in Rwanda

A chicken genome has several regions with quantitative trait loci (QTLs). However, replication and confirmation of QTL effects are required particularly in African chicken populations. This study identified single nucleotide polymorphisms (SNPs) and putative genes responsible for body weight (BW) and antibody response (AbR) to Newcastle disease (ND) in Rwanda indigenous chicken (IC) using genome-wide association studies (GWAS). Multiple testing was corrected using chromosomal false detection rates of 5 and 10% for significant and suggestive thresholds, respectively. BioMart data mining and variant effect predictor tools were used to annotate SNPs and candidate genes, respectively. A total of four significant SNPs (rs74098018, rs13792572, rs314702374, and rs14123335) significantly (p ≤ 7.6E-5) associated with BW were identified on chromosomes (CHRs) 8, 11, and 19. In the vicinity of these SNPs, four genes such as pre-B-cell leukaemia homeobox 1 (PBX1), GPATCH1, MPHOSPH6, and MRM1 were identified. Four other significant SNPs (rs314787954, rs13623466, rs13910430, and rs737507850) all located on chromosome 1 were strongly (p ≤ 7.6E-5) associated with chicken antibody response to ND. The closest genes to these four SNPs were cell division cycle 16 (CDC16), zinc finger, BED-type containing 1 (ZBED1), myxovirus (influenza virus) resistance 1 (MX1), and growth factor receptor bound protein 2 (GRB2) related adaptor protein 2 (GRAP2). Besides, other SNPs and genes suggestively (p ≤ 1.5E-5) associated with BW and antibody response to ND were reported. This work offers a useful entry point for the discovery of causative genes accountable for essential QTLs regulating BW and antibody response to ND traits. Results provide auspicious genes and SNP-based markers that can be used in the improvement of growth performance and ND resistance in IC populations based on gene-based and/or marker-assisted breeding selection.

MEIS1 in Hematopoiesis and Cancer. How MEIS1-PBX Interaction Can Be Used in Therapy

Recently MEIS1 emerged as a major determinant of the MLL-r leukemic phenotype. The latest and most efficient drugs effectively decrease the levels of MEIS1 in cancer cells. Together with an overview of the latest drugs developed to target MEIS1 in MLL-r leukemia, we review, in detail, the role of MEIS1 in embryonic and adult hematopoiesis and suggest how a more profound knowledge of MEIS1 biochemistry can be used to design potent and effective drugs against MLL-r leukemia. In addition, we present data showing that the interaction between MEIS1 and PBX1 can be blocked efficiently and might represent a new avenue in anti-MLL-r and anti-leukemic therapy.

Conserved Mechanisms, Novel Anatomies: The Developmental Basis of Fin Evolution and the Origin of Limbs

The transformation of paired fins into tetrapod limbs is one of the most intensively scrutinized events in animal evolution. Early anatomical and embryological datasets identified distinctive morphological regions within the appendage and posed hypotheses about how the loss, gain, and transformation of these regions could explain the observed patterns of both extant and fossil appendage diversity. These hypotheses have been put to the test by our growing understanding of patterning mechanisms that regulate formation of the appendage axes, comparisons of gene expression data from an array of phylogenetically informative taxa, and increasingly sophisticated and elegant experiments leveraging the latest molecular approaches. Together, these data demonstrate the remarkable conservation of developmental mechanisms, even across phylogenetically and morphologically disparate taxa, as well as raising new questions about the way we view homology, evolutionary novelty, and the often non-linear connection between morphology and gene expression. In this review, we present historical hypotheses regarding paired fin evolution and limb origins, summarize key aspects of central appendage patterning mechanisms in model and non-model species, address how modern comparative developmental data interface with our understanding of appendage anatomy, and highlight new approaches that promise to provide new insight into these well-traveled questions.

Control of mouse limb initiation and antero-posterior patterning by Meis transcription factors

Meis1 and Meis2 are homeodomain transcription factors that regulate organogenesis through cooperation with Hox proteins. Elimination of Meis genes after limb induction has shown their role in limb proximo-distal patterning; however, limb development in the complete absence of Meis function has not been studied. Here, we report that Meis1/2 inactivation in the lateral plate mesoderm of mouse embryos leads to limb agenesis. Meis and Tbx factors converge in this function, extensively co-binding with Tbx to genomic sites and co-regulating enhancers of Fgf10, a critical factor in limb initiation. Limbs with three deleted Meis alleles show proximal-specific skeletal hypoplasia and agenesis of posterior skeletal elements. This failure in posterior specification results from an early role of Meis factors in establishing the limb antero-posterior prepattern required for Shh activation. Our results demonstrate roles for Meis transcription factors in early limb development and identify their involvement in previously undescribed interaction networks that regulate organogenesis.

Hoxd13/Bmp2-mediated mechanism involved in zebrafish finfold design

The overexpression of hoxd13a during zebrafish fin development causes distal endochondral expansion and simultaneous reduction of the finfold, mimicking the major events thought to have happened during the fin-to-limb transition in Vertebrates. We investigated the effect of hoxd13a overexpression on putative downstream targets and found it to cause downregulation of proximal fin identity markers (meis1 and emx2) and upregulation of genes involved in skeletogenesis/patterning (fbn1, dacha) and AER/Finfold maintenance (bmps). We then show that bmp2b overexpression leads to finfold reduction, recapitulating the phenotype observed in hoxd13a-overexpressing fins. In addition, we show that during the development of the long finfold in leo^t1/lof^dt1 mutants, hoxd13a and bmp2b are downregulated. Our results suggest that modulation of the transcription factor Hoxd13 during evolution may have been involved in finfold reduction through regulation of the Bmp signalling that then activated apoptotic mechanisms impairing finfold elongation.

A developmental perspective of homology and evolutionary novelty

The development and evolution of multicellular body plans is complex. Many distinct organs and body parts must be reproduced at each generation, and those that are traceable over long time scales are considered homologous. Among the most pressing and least understood phenomena in evolutionary biology is the mode by which new homologs, or "novelties" are introduced to the body plan and whether the developmental changes associated with such evolution deserve special treatment. In this chapter, we address the concepts of homology and evolutionary novelty through the lens of development. We present a series of case studies, within insects and vertebrates, from which we propose a developmental model of multicellular organ identity. With this model in hand, we make predictions regarding the developmental evolution of body plans and highlight the need for more integrative analysis of developing systems.

Reduced RING finger protein 10 expression in macrophages is associated with aging-related inflammation

Age‐associated decline of the immune system is referred to as immunosenescence. The E3 ligase RING finger 10 (RNF10) has long been associated with the innate immune response, although a potential role in immunosenescence has not previously been reported. In the present study, we identified that RNF10 expression is lower in aged mouse macrophages than in young cells. After lipopolysaccharide stimulation, RNF10 expression remained at a basal low level in aged mouse cells, but declined sharply in young mouse cells. Knockdown of RNF10 enhanced both the nuclear factor‐κB and interferon regulatory factor 3 signaling pathways and thus enhanced proinflammatory cytokines and type I interferons in macrophages, promoting clearance of Listeria monocytogenes. These findings indicate that dysregulated expression of RNF10 is associated with age‐associated immune dysfunction, and RNF10 may thus be a potential target for the treatment of age‐related inflammatory diseases.

Role of Retinoic Acid Signaling, FGF Signaling and Meis Genes in Control of Limb Development

The function of retinoic acid (RA) during limb development is still debated, as loss and gain of function studies led to opposite conclusions. With regard to limb initiation, genetic studies demonstrated that activation of FGF10 signaling is required for the emergence of limb buds from the trunk, with Tbx5 and RA signaling acting upstream in the forelimb field, whereas Tbx4 and Pitx1 act upstream in the hindlimb field. Early studies in chick embryos suggested that RA as well as Meis1 and Meis2 (Meis1/2) are required for subsequent proximodistal patterning of both forelimbs and hindlimbs, with RA diffusing from the trunk, functioning to activate Meis1/2 specifically in the proximal limb bud mesoderm. However, genetic loss of RA signaling does not result in loss of limb Meis1/2 expression and limb patterning is normal, although Meis1/2 expression is reduced in trunk somitic mesoderm. More recent studies demonstrated that global genetic loss of Meis1/2 results in a somite defect and failure of limb bud initiation. Other new studies reported that conditional genetic loss of Meis1/2 in the limb results in proximodistal patterning defects, and distal FGF8 signaling represses Meis1/2 to constrain its expression to the proximal limb. In this review, we hypothesize that RA and Meis1/2 both function in the trunk to initiate forelimb bud initiation, but that limb Meis1/2 expression is activated proximally by a factor other than RA and repressed distally by FGF8 to generate proximodistal patterning.

Axial skeleton anterior-posterior patterning is regulated through feedback regulation between Meis transcription factors and retinoic acid

Vertebrate axial skeletal patterning is controlled by co-linear expression of Hox genes and axial level-dependent activity of HOX protein combinations. MEIS transcription factors act as co-factors of HOX proteins and profusely bind to Hox complex DNA; however, their roles in mammalian axial patterning remain unknown. Retinoic acid (RA) is known to regulate axial skeletal element identity through the transcriptional activity of its receptors; however, whether this role is related to MEIS/HOX activity remains unknown. Here, we study the role of Meis in axial skeleton formation and its relationship to the RA pathway in mice. Meis elimination in the paraxial mesoderm produces anterior homeotic transformations and rib mis-patterning associated to alterations of the hypaxial myotome. Although Raldh2 and Meis positively regulate each other, Raldh2 elimination largely recapitulates the defects associated with Meis deficiency, and Meis overexpression rescues the axial skeletal defects in Raldh2 mutants. We propose a Meis-RA-positive feedback loop, the output of which is Meis levels, that is essential to establish anterior-posterior identities and patterning of the vertebrate axial skeleton.

The Geometry of Limb Motor Innervation is Controlled by the Dorsal-Ventral Compartment Boundary in the Chick Limbless Mutant

Precise control of limb muscles, and ultimately of limb movement, requires accurate motor innervation. Motor innervation of the vertebrate limb is established by sequential selection of trajectories at successive decision points. Motor axons of the lateral motor column (LMC) segregate at the base of the limb into two groups that execute a choice between dorsal and ventral tissue: medial LMC axons innervate the ventral limb, whereas lateral LMC axons innervate the dorsal limb. We investigated how LMC axons are targeted to the limb using the chick mutant limbless (ll), which has a dorsal transformation of the ventral limb mesenchyme. In ll the spatial pattern of motor projections to the limb is abnormal while their targeting is normal. While extensive, the dorsal transformation of the ll ventral limb mesenchyme is incomplete whereas the generation, specification and targeting of spinal motor neurons are apparently unaffected. Thus, the dorsal-ventral motor axon segregation is an active choice that is independent of the ratio between dorsal and ventral tissue but dependent on the presence of both tissues. Therefore, the fidelity of the motor projections to the limb depends on the presence of both dorsal and ventral compartments, while the geometry of motor projections is controlled by the position of limb dorsal-ventral compartment boundary.

IRX3/5 regulate mitotic chromatid segregation and limb bud shape

Pattern formation is influenced by transcriptional regulation as well as by morphogenetic mechanisms that shape organ primordia, although factors that link these processes remain under-appreciated. Here we show that, apart from their established transcriptional roles in pattern formation, IRX3/5 help to shape the limb bud primordium by promoting the separation and intercalation of dividing mesodermal cells. Surprisingly, IRX3/5 are required for appropriate cell cycle progression and chromatid segregation during mitosis, possibly in a nontranscriptional manner. IRX3/5 associate with, promote the abundance of, and share overlapping functions with co-regulators of cell division such as the cohesin subunits SMC1, SMC3, NIPBL and CUX1. The findings imply that IRX3/5 coordinate early limb bud morphogenesis with skeletal pattern formation.

In vivo analysis of the evolutionary conserved BTD-box domain of Sp1 and Btd during Drosophila development

The Sp family of transcription factors plays important functions during development and disease. An evolutionary conserved role for some Sp family members is the control of limb development. The family is characterized by the presence of three C2H2-type zinc fingers and an adjacent 10 aa region with an unknown function called the Buttonhead (BTD) box. The presence of this BTD-box in all Sp family members identified from arthropods to vertebrates, suggests that it plays an essential role during development. However, despite its conservation, the in vivo function of the BTD-box has never been studied. In this work, we have generated specific BTD-box deletion alleles for the Drosophila Sp family members Sp1 and buttonhead (btd) using gene editing tools and analyzed its role during development. Unexpectedly, btd and Sp1 mutant alleles that lack the BTD-box are viable and have almost normal appendages. However, in a sensitized background the requirement of this domain to fully regulate some of Sp1 and Btd target genes is revealed. Furthermore, we have also identified a novel Sp1 role promoting leg vs antenna identity through the repression of spineless (ss) expression in the leg, a function that also depends on the Sp1 BTD-box.

Apical ectodermal ridge regulates three principal axes of the developing limb

Understanding limb development not only gives insights into the outgrowth and differentiation of the limb, but also has clinical relevance. Limb development begins with two paired limb buds (forelimb and hindlimb buds), which are initially undifferentiated mesenchymal cells tipped with a thickening of the ectoderm, termed the apical ectodermal ridge (AER). As a transitional embryonic structure, the AER undergoes four stages and contributes to multiple axes of limb development through the coordination of signalling centres, feedback loops, and other cell activities by secretory signalling and the activation of gene expression. Within the scope of proximodistal patterning, it is understood that while fibroblast growth factors (FGFs) function sequentially over time as primary components of the AER signalling process, there is still no consensus on models that would explain proximodistal patterning itself. In anteroposterior patterning, the AER has a dual-direction regulation by which it promotes the sonic hedgehog (Shh) gene expression in the zone of polarizing activity (ZPA) for proliferation, and inhibits Shh expression in the anterior mesenchyme. In dorsoventral patterning, the AER activates Engrailed-1 (En1) expression, and thus represses Wnt family member 7a (Wnt7a) expression in the ventral ectoderm by the expression of Fgfs, Sp6/8, and bone morphogenetic protein (Bmp) genes. The AER also plays a vital role in shaping the individual digits, since levels of Fgf4/8 and Bmps expressed in the AER affect digit patterning by controlling apoptosis. In summary, the knowledge of crosstalk within AER among the three main axes is essential to understand limb growth and pattern formation, as the development of its areas proceeds simultaneously.

Establishing the pattern of the vertebrate limb

The vertebrate limb continues to serve as an influential model of growth, morphogenesis and pattern formation. With this Review, we aim to give an up-to-date picture of how a population of undifferentiated cells develops into the complex pattern of the limb. Focussing largely on mouse and chick studies, we concentrate on the positioning of the limbs, the formation of the limb bud, the establishment of the principal limb axes, the specification of pattern, the integration of pattern formation with growth and the determination of digit number. We also discuss the important, but little understood, topic of how gene expression is interpreted into morphology.

Proximo-distal positional information encoded by an Fgf-regulated gradient of homeodomain transcription factors in the vertebrate limb

The positional information theory proposes that a coordinate system provides information to embryonic cells about their position and orientation along a patterning axis. Cells interpret this information to produce the appropriate pattern. During development, morphogens and interpreter transcription factors provide this information. We report a gradient of Meis homeodomain transcription factors along the mouse limb bud proximo-distal (PD) axis antiparallel to and shaped by the inhibitory action of distal fibroblast growth factor (FGF). Elimination of Meis results in premature limb distalization and HoxA expression, proximalization of PD segmental borders, and phocomelia. Our results show that Meis transcription factors interpret FGF signaling to convey positional information along the limb bud PD axis. These findings establish a new model for the generation of PD identities in the vertebrate limb and provide a molecular basis for the interpretation of FGF signal gradients during axial patterning.

Development of the Proximal-Anterior Skeletal Elements in the Mouse Hindlimb Is Regulated by a Transcriptional and Signaling Network Controlled by Sall4

The vertebrate limb serves as an experimental paradigm to study mechanisms that regulate development of the stereotypical skeletal elements. In this study, we simultaneously inactivated Sall4 using Hoxb6Cre and Plzf in mouse embryos, and found that their combined function regulates development of the proximal-anterior skeletal elements in hindlimbs. The Sall4; Plzf double knockout exhibits severe defects in the femur, tibia, and anterior digits, distinct defects compared to other allelic series of Sall4; Plzf We found that Sall4 regulates Plzf expression prior to hindlimb outgrowth. Further expression analysis indicated that Hox10 genes and GLI3 are severely downregulated in the Sall4; Plzf double knockout hindlimb bud. In contrast, PLZF expression is reduced but detectable in Sall4; Gli3 double knockout limb buds, and SALL4 is expressed in the Plzf; Gli3 double knockout limb buds. These results indicate that Plzf, Gli3, and Hox10 genes downstream of Sall4, regulate femur and tibia development. In the autopod, we show that Sall4 negatively regulates Hedgehog signaling, which allows for development of the most anterior digit. Collectively, our study illustrates genetic systems that regulate development of the proximal-anterior skeletal elements in hindlimbs.

The acute myeloid leukemia variant DNMT3A Arg882His is a DNMT3B-like enzyme

We have previously shown that the highly prevalent acute myeloid leukemia (AML) mutation, Arg882His, in DNMT3A disrupts its cooperative mechanism and leads to reduced enzymatic activity, thus explaining the genomic hypomethylation in AML cells. However, the underlying cause of the oncogenic effect of Arg882His in DNMT3A is not fully understood. Here, we discovered that DNMT3A WT enzyme under conditions that favor non-cooperative kinetic mechanism as well as DNMT3A Arg882His variant acquire CpG flanking sequence preference akin to that of DNMT3B, which is non-cooperative. We tested if DNMT3A Arg882His could preferably methylate DNMT3B-specific target sites in vivo. Rescue experiments in Dnmt3a/3b double knockout mouse embryonic stem cells show that the corresponding Arg878His mutation in mouse DNMT3A severely impairs its ability to methylate major satellite DNA, a DNMT3A-preferred target, but has no overt effect on the ability to methylate minor satellite DNA, a DNMT3B-preferred target. We also observed a previously unappreciated CpG flanking sequence bias in major and minor satellite repeats that is consistent with DNMT3A and DNMT3B specificity suggesting that DNA methylation patterns are guided by the sequence preference of these enzymes. We speculate that aberrant methylation of DNMT3B target sites could contribute to the oncogenic potential of DNMT3A AML variant.

Remembering where we are: Positional information in salamander limb regeneration

Fifty years ago, Lewis Wolpert defined an important question in developmental biology: how are cell fates determined by the positions of cells within a system? He proposed that cells retain positional values as if they lie within a coordinate system and that the interpretation of these values produces patterns in development. He referred to this concept as positional information. Though initially controversial, this concept of positional information has proven to be profoundly influential in developmental biology. One area in which the influence of Wolpert's theoretical work can be clearly demonstrated is the study of limb regeneration in salamanders. Here, we review the work in limb regeneration leading up to Wolpert defining the concept of positional information and how his theory has guided regeneration research over the subsequent 50 years.

Homeobox gene Meis1 modulates cardiovascular regeneration

Regeneration of cardiomyocytes, endothelial cells and vascular smooth muscle cells (three major lineages of cardiac tissues) following myocardial infarction is the critical step to recover the function of the damaged heart. Myeloid ecotropic viral integration site 1 (Meis1) was first discovered in leukemic mice in 1995 and its biological function has been extensively studied in leukemia, hematopoiesis, the embryonic pattering of body axis, eye development and various genetic diseases, such as restless leg syndrome. It was found that Meis1 is highly associated with Hox genes and their cofactors to exert its regulatory effects on multiple intracellular signaling pathways. Recently with the advent of bioinformatics, biochemical methods and advanced genetic engineering tools, new function of Meis1 has been found to be involved in the cell cycle regulation of cardiomyocytes and endothelial cells. For example, inhibition of Meis1 expression increases the proliferative capacity of neonatal mouse cardiomyocytes, whereas overexpression of Meis1 results in the reduction in the length of cardiomyocyte proliferative window. Interestingly, downregulation of one of the circular RNAs, which acts downstream of Meis1 in the cardiomyocytes, promotes angiogenesis and restores the myocardial blood supply, thus reinforcing better regeneration of the damaged heart. It appears that Meis1 may play double roles in modulating proliferation and regeneration of cardiomyocytes and endothelial cells post-myocardial infarction. In this review, we propose to summarize the major findings of Meis1 in modulating fetal development and adult abnormalities, especially focusing on the recent discoveries of Meis1 in controlling the fate of cardiomyocytes and endothelial cells.

MEIS1 and Restless Legs Syndrome: A Comprehensive Review

Restless legs syndrome (RLS) is a common sleep-related disorder for which the underlying biological pathways and genetic determinants are not well understood. The genetic factors so far identified explain less than 10% of the disease heritability. The first successful genome-wide association study (GWAS) of RLS was reported in 2007. This study identified multiple RLS associated risk variants including some within the non-coding regions of MEIS1. The MEIS1 GWAS signals are some of the strongest genetic associations reported for any common disease. MEIS1 belongs to the homeobox containing transcriptional regulatory network (HOX). Work in C. elegans showed a link between the MEIS1 ortholog and iron homeostasis, which is in line with the fact that central nervous system (CNS) iron insufficiency is thought to be a cause of RLS. Zebrafish and mice have been used to study the MEIS1 gene identifying an RLS-associated-SNP dependent enhancer activity from the highly conserved non-coding regions (HCNR) of MEIS1. Furthermore, this gene shows a lower expression of mRNA and protein in blood and thalamus of individuals with the MEIS1 RLS risk haplotype. Simulating this reduced MEIS1 expression in mouse models resulted in circadian hyperactivity, a phenotype compatible with RLS. While MEIS1 shows a strong association with RLS, the protein's function that is directly linked to an RLS biological pathway remains to be discovered. The links to iron and the enhancer activity of the HCNRs of MEIS1 suggest promising links to RLS pathways, however more in-depth studies on this gene's function are required. One important aspect of MEIS1's role in RLS is the fact that it encodes a homeobox containing transcription factor, which is essential during development. Future studies with more focus on the transcriptional regulatory role of MEIS1 may open novel venues for RLS research.

MEIS transcription factors in development and disease

MEIS transcription factors are key regulators of embryonic development and cancer. Research on MEIS genes in the embryo and in stem cell systems has revealed novel and surprising mechanisms by which these proteins control gene expression. This Primer summarizes recent findings about MEIS protein activity and regulation in development, and discusses new insights into the role of MEIS genes in disease, focusing on the pathogenesis of solid cancers.

Evolution of limb development in cephalopod mollusks

Cephalopod mollusks evolved numerous anatomical novelties, including arms and tentacles, but little is known about the developmental mechanisms underlying cephalopod limb evolution. Here we show that all three axes of cuttlefish limbs are patterned by the same signaling networks that act in vertebrates and arthropods, although they evolved limbs independently. In cuttlefish limb buds, Hedgehog is expressed anteriorly. Posterior transplantation of Hedgehog-expressing cells induced mirror-image limb duplications. Bmp and Wnt signals, which establish dorsoventral polarity in vertebrate and arthropod limbs, are similarly polarized in cuttlefish. Inhibition of Bmp2/4 dorsally caused ectopic expression of Notum, which marks the ventral sucker field, and ectopic sucker development. Cuttlefish also show proximodistal regionalization of Hth, Exd, Dll, Dac, Sp8/9, and Wnt expression, which delineates arm and tentacle sucker fields. These results suggest that cephalopod limbs evolved by parallel activation of a genetic program for appendage development that was present in the bilaterian common ancestor.

Role of MEIS1 in restless legs syndrome: From GWAS to functional studies in mice

MEIS1 is a transcription factor playing an important role in the development of several organs, including central and peripheral nervous systems. A genetic locus spanning the MEIS1 coding region has been associated with the risk of RLS in genome-wide association studies, with increasing evidence that MEIS1 is the causal RLS gene. The RLS-linked genetic signal has been mapped to an intronic regulatory element within MEIS1. This element plays a role in the ganglionic eminences of the developing forebrain, with the RLS risk allele related to a reduced activation of the enhancer. This suggests that the ganglionic eminences play an important role in the development of genetic susceptibility to RLS. In addition, rare variants within MEIS1 have been shown to contribute to the disease risk. These variants were identified first in RLS families and later found in further RLS cases by targeted sequencing. Some of these variants alone are sufficient to suppress MEIS1 function in neural development, providing further evidence of the importance of neurodevelopmental processes in the pathological mechanism of MEIS1 in RLS. Heterozygous Meis1 inactivation in mice causes hyperactivity at the onset of the inactive period, consistent with human RLS. In addition, these mice revealed an effect of MEIS1 on the dopaminergic system at both the spinal and supraspinal level. More studies are needed in human genetics to determine the exact role of MEIS1 variants in the risk of RLS, as well as in functional genetics and animal studies to further elucidate the pathological mechanism of MEIS1 in RLS.

Dental Epithelial Stem Cells Express the Developmental Regulator Meis1

MEIS1 is a key developmental regulator of several organs and participates in stem cell maintenance in different niches. However, despite the murine continuously growing incisor being a well described model for the study of adult stem cells, Meis1 has not been investigated in a dental context. Here, we uncover that Meis1 expression in the tooth is confined to the epithelial compartment. Its expression arises during morphogenesis and becomes restricted to the mouse incisor epithelial stem cell niche, the labial cervical loop. Meis1 is specifically expressed by Sox2⁺ stem cells, which give rise to all dental epithelial cell lineages. Also, we have found that Meis1 in the incisor is coexpressed with potential binding partner Pbx1 during both embryonic and adult stages. Interestingly, Meis2 is present in different areas of the forming tooth and it is not expressed by dental epithelial stem cells, suggesting different roles for these two largely homologous genes. Additionally, we have established the expression patterns of Meis1 and Meis2 during tongue, hair, salivary gland and palate formation. Finally, analysis of Meis1-null allele mice indicated that, similarly, to SOX2, MEIS1 is not essential for tooth initiation, but might have a role during adult incisor renewal.

Meis transcription factor maintains the neurogenic ectoderm and regulates the anterior-posterior patterning in embryos of a sea urchin, Hemicentrotus pulcherrimus

Precise body axis formation is an essential step in the development of multicellular organisms, for most of which the molecular gradient and/or specifically biased localization of cell-fate determinants in eggs play important roles. In sea urchins, however, any biased proteins and mRNAs have not yet been identified in the egg except for vegetal cortex molecules, suggesting that sea urchin development is mostly regulated by uniformly distributed maternal molecules with contributions to axis formation that are not well characterized. Here, we describe that the maternal Meis transcription factor regulates anterior-posterior axis formation through maintenance of the most anterior territory in embryos of a sea urchin, Hemicentrotus pulcherrimus. Loss-of-function experiments revealed that Meis is intrinsically required for maintenance of the anterior neuroectoderm specifier foxQ2 after hatching and, consequently, the morphant lost anterior neuroectoderm characteristics. In addition, the expression patterns of univin and VEGF, the lateral ectoderm markers, and the mesenchyme-cell pattern shifted toward the anterior side in Meis morphants more than they did in control embryos, indicating that Meis contributes to the precise anteroposterior patterning by regulating the anterior neuroectodermal fate.

Developmental Bias and Evolution: A Regulatory Network Perspective

Phenotypic variation is generated by the processes of development, with some variants arising more readily than others-a phenomenon known as "developmental bias." Developmental bias and natural selection have often been portrayed as alternative explanations, but this is a false dichotomy: developmental bias can evolve through natural selection, and bias and selection jointly influence phenotypic evolution. Here, we briefly review the evidence for developmental bias and illustrate how it is studied empirically. We describe recent theory on regulatory networks that explains why the influence of genetic and environmental perturbation on phenotypes is typically not uniform, and may even be biased toward adaptive phenotypic variation. We show how bias produced by developmental processes constitutes an evolving property able to impose direction on adaptive evolution and influence patterns of taxonomic and phenotypic diversity. Taking these considerations together, we argue that it is not sufficient to accommodate developmental bias into evolutionary theory merely as a constraint on evolutionary adaptation. The influence of natural selection in shaping developmental bias, and conversely, the influence of developmental bias in shaping subsequent opportunities for adaptation, requires mechanistic models of development to be expanded and incorporated into evolutionary theory. A regulatory network perspective on phenotypic evolution thus helps to integrate the generation of phenotypic variation with natural selection, leaving evolutionary biology better placed to explain how organisms adapt and diversify.

Differential Dopamine D1 and D3 Receptor Modulation and Expression in the Spinal Cord of Two Mouse Models of Restless Legs Syndrome

Restless Legs Syndrome (RLS) is often and successfully treated with dopamine receptor agonists that target the inhibitory D3 receptor subtype, however there is no clinical evidence of a D3 receptor dysfunction in RLS patients. In contrast, genome-wide association studies in RLS patients have established that a mutation of the MEIS1 gene is associated with an increased risk in developing RLS, but the effect of MEIS1 dysfunction on sensorimotor function remain unknown. Mouse models for a dysfunctional D3 receptor (D3KO) and Meis1 (Meis1KO) were developed independently, and each animal expresses some features associated with RLS in the clinic, but they have not been compared in their responsiveness to treatment options used in the clinic. We here confirm that D3KO and Meis1KO animals show increased locomotor activities, but that only D3KO show an increased sensory excitability to thermal stimuli. Next we compared the effects of dopaminergics and opioids in both animal models, and we assessed D1 and D3 dopamine receptor expression in the spinal cord, the gateway for sensorimotor processing. We found that Meis1KO share most of the tested behavioral properties with their wild type (WT) controls, including the modulation of the thermal pain withdrawal reflex by morphine, L-DOPA and D3 receptor (D3R) agonists and antagonists. However, Meis1KO and D3KO were behaviorally more similar to each other than to WT when tested with D1 receptor (D1R) agonists and antagonists. Subsequent Western blot analyses of D1R and D3R protein expression in the spinal cord revealed a significant increase in D1R but not D3R expression in Meis1KO and D3KO over WT controls. As the D3R is mostly present in the dorsal spinal cord where it has been shown to modulate sensory pathways, while activation of the D1Rs can activate motoneurons in the ventral spinal cord, we speculate that D3KO and Meis1KO represent two complementary animal models for RLS, in which the mechanisms of sensory (D3R-mediated) and motor (D1R-mediated) dysfunctions can be differentially explored.

An intrinsic cell cycle timer terminates limb bud outgrowth

The longstanding view of how proliferative outgrowth terminates following the patterning phase of limb development involves the breakdown of reciprocal extrinsic signalling between the distal mesenchyme and the overlying epithelium (e-m signalling). However, by grafting distal mesenchyme cells from late stage chick wing buds to the epithelial environment of younger wing buds, we show that this mechanism is not required. RNA sequencing reveals that distal mesenchyme cells complete proliferative outgrowth by an intrinsic cell cycle timer in the presence of e-m signalling. In this process, e-m signalling is required permissively to allow the intrinsic cell cycle timer to run its course. We provide evidence that a temporal switch from BMP antagonism to BMP signalling controls the intrinsic cell cycle timer during limb outgrowth. Our findings have general implications for other patterning systems in which extrinsic signals and intrinsic timers are integrated.

A direct interaction between two Restless Legs Syndrome predisposing genes: MEIS1 and SKOR1

Restless Legs syndrome (RLS) is a common sleep disorder for which the genetic contribution remains poorly explained. In 2007, the first large scale genome wide association study (GWAS) identified three genomic regions associated with RLS. MEIS1, BTBD9 and MAP2K5/SKOR1 are the only known genes located within these loci and their association with RLS was subsequently confirmed in a number of follow up GWAS. Following this finding, our group reported the MEIS1 risk haplotype to be associated with its decreased expression at the mRNA and protein levels. Here we report the effect of the risk variants of the three other genes strongly associated with RLS. While these variants had no effect on the mRNA levels of the genes harboring them, we find that the homeobox transcription factor MEIS1 positively regulates the expression of the transcription co-repressor SKOR1. This regulation appears mediated through the binding of MEIS1 at two specific sites located in the SKOR1 promoter region and is modified by an RLS associated SNP in the promoter region of the gene. Our findings directly link MEIS1 and SKOR1, two significantly associated genes with RLS and also prioritize SKOR1 over MAP2K5 in the RLS associated intergenic region of MAP2K5/SKOR1 found by GWAS.