Increased dosage of DYRK1A leads to congenital heart defects in a mouse model of Down syndrome

Down syndrome (DS) is caused by trisomy of human chromosome 21 (Hsa21). DS is a gene dosage disorder that results in multiple phenotypes including congenital heart defects. This clinically important cardiac pathology is the result of a third copy of one or more of the approximately 230 genes on Hsa21, but the identity of the causative dosage-sensitive genes and hence mechanisms underlying this cardiac pathology remain unclear. Here, we show that hearts from human fetuses with DS and embryonic hearts from the Dp1Tyb mouse model of DS show reduced expression of mitochondrial respiration genes and cell proliferation genes. Using systematic genetic mapping, we determined that three copies of the dual-specificity tyrosine phosphorylation-regulated kinase 1A (Dyrk1a) gene, encoding a serine/threonine protein kinase, are associated with congenital heart disease pathology. In embryos from Dp1Tyb mice, reducing Dyrk1a gene copy number from three to two reversed defects in cellular proliferation and mitochondrial respiration in cardiomyocytes and rescued heart septation defects. Increased dosage of DYRK1A protein resulted in impairment of mitochondrial function and congenital heart disease pathology in mice with DS, suggesting that DYRK1A may be a useful therapeutic target for treating this common human condition.

Day-to-day spontaneous social behaviours is quantitatively and qualitatively affected in a 16p11.2 deletion mouse model

Background

Autism spectrum disorders affect more than 1% of the population, impairing social communication and increasing stereotyped behaviours. A micro-deletion of the 16p11.2 BP4-BP5 chromosomic region has been identified in 1% of patients also displaying intellectual disabilities. In mouse models generated to understand the mechanisms of this deletion, learning and memory deficits were pervasive in most genetic backgrounds, while social communication deficits were only detected in some models.

Methods

To complement previous studies, we itemized the social deficits in the mouse model of 16p11.2 deletion on a hybrid C57BL/6N × C3H.Pde6b+ genetic background. We examined whether behavioural deficits were visible over long-term observation periods lasting several days and nights, to parallel everyday-life assessment of patients. We recorded the individual and social behaviours of mice carrying a heterozygous deletion of the homologous 16p11.2 chromosomic region (hereafter Del/+) and their wild-type littermates from both sexes over two or three consecutive nights during social interactions of familiar mixed-genotype quartets of males and of females, and of same-genotype unfamiliar female pairs.

Results

We observed that Del/+ mice of both sexes increased significantly their locomotor activity compared to wild-type littermates. In the social domain, Del/+ mice of both sexes displayed widespread deficits, even more so in males than in females in quartets of familiar individuals. In pairs, significant perturbations of the organisation of the social communication and behaviours appeared in Del/+ females.

Discussion

Altogether, this suggests that, over long recording periods, the phenotype of the 16p11.2 Del/+ mice was differently affected in the locomotor activity and the social domains and between the two sexes. These findings confirm the importance of testing models in long-term conditions to provide a comprehensive view of their phenotype that will refine the study of cellular and molecular mechanisms and complement pre-clinical targeted therapeutic trials.

An Aphid-Transmitted Virus Reduces the Host Plant Response to Its Vector to Promote Its Transmission

The success of virus transmission by vectors relies on intricate trophic interactions between three partners, the host plant, the virus, and the vector. Despite numerous studies that showed the capacity of plant viruses to manipulate their host plant to their benefit, and potentially of their transmission, the molecular mechanisms sustaining this phenomenon has not yet been extensively analyzed at the molecular level. In this study, we focused on the deregulations induced in Arabidopsis thaliana by an aphid vector that were alleviated when the plants were infected with turnip yellows virus (TuYV), a polerovirus strictly transmitted by aphids in a circulative and nonpropagative mode. By setting up an experimental design mimicking the natural conditions of virus transmission, we analyzed the deregulations in plants infected with TuYV and infested with aphids by a dual transcriptomic and metabolomic approach. We observed that the virus infection alleviated most of the gene deregulations induced by the aphids in a noninfected plant at both time points analyzed (6 and 72 h) with a more pronounced effect at the later time point of infestation. The metabolic composition of the infected and infested plants was altered in a way that could be beneficial for the vector and the virus transmission. Importantly, these substantial modifications observed in infected and infested plants correlated with a higher TuYV transmission efficiency. This study revealed the capacity of TuYV to alter the plant nutritive content and the defense reaction against the aphid vector to promote the viral transmission.

Bioluminescence Production by Turnip Yellows Virus Infectious Clones: A New Way to Monitor Plant Virus Infection

We used the NanoLuc luciferase bioluminescent reporter system to detect turnip yellows virus (TuYV) in infected plants. For this, TuYV was genetically tagged by replacing the C-terminal part of the RT protein with full-length NanoLuc (TuYV-NL) or with the N-terminal domain of split NanoLuc (TuYV-N65-NL). Wild-type and recombinant viruses were agro-infiltrated in Nicotiana benthamiana, Montia perfoliata, and Arabidopsis thaliana. ELISA confirmed systemic infection and similar accumulation of the recombinant viruses in N. benthamiana and M. perfoliata but reduced systemic infection and lower accumulation in A. thaliana. RT-PCR analysis indicated that the recombinant sequences were stable in N. benthamiana and M. perfoliata but not in A. thaliana. Bioluminescence imaging detected TuYV-NL in inoculated and systemically infected leaves. For the detection of split NanoLuc, we constructed transgenic N. benthamiana plants expressing the C-terminal domain of split NanoLuc. Bioluminescence imaging of these plants after agro-infiltration with TuYV-N65-NL allowed the detection of the virus in systemically infected leaves. Taken together, our results show that NanoLuc luciferase can be used to monitor infection with TuYV.

Differential gene expression in aphids following virus acquisition from plants or from an artificial medium

BACKGROUND

Poleroviruses, such as turnip yellows virus (TuYV), are plant viruses strictly transmitted by aphids in a persistent and circulative manner. Acquisition of either virus particles or plant material altered by virus infection is expected to induce gene expression deregulation in aphids which may ultimately alter their behavior.

RESULTS

By conducting an RNA-Seq analysis on viruliferous aphids fed either on TuYV-infected plants or on an artificial medium containing purified virus particles, we identified several hundreds of genes deregulated in Myzus persicae, despite non-replication of the virus in the vector. Only a few genes linked to receptor activities and/or vesicular transport were common between the two modes of acquisition with, however, a low level of deregulation. Behavioral studies on aphids after virus acquisition showed that M. persicae locomotion behavior was affected by feeding on TuYV-infected plants, but not by feeding on the artificial medium containing the purified virus particles. Consistent with this, genes potentially involved in aphid behavior were deregulated in aphids fed on infected plants, but not on the artificial medium.

CONCLUSIONS

These data show that TuYV particles acquisition alone is associated with a moderate deregulation of a few genes, while higher gene deregulation is associated with aphid ingestion of phloem from TuYV-infected plants. Our data are also in favor of a major role of infected plant components on aphid behavior.

Dyrk1a from Gene Function in Development and Physiology to Dosage Correction across Life Span in Down Syndrome

Down syndrome is the main cause of intellectual disabilities with a large set of comorbidities from developmental origins but also that appeared across life span. Investigation of the genetic overdosage found in Down syndrome, due to the trisomy of human chromosome 21, has pointed to one main driver gene, the Dual-specificity tyrosine-regulated kinase 1A (Dyrk1a). Dyrk1a is a murine homolog of the drosophila minibrain gene. It has been found to be involved in many biological processes during development and in adulthood. Further analysis showed its haploinsufficiency in mental retardation disease 7 and its involvement in Alzheimer's disease. DYRK1A plays a role in major developmental steps of brain development, controlling the proliferation of neural progenitors, the migration of neurons, their dendritogenesis and the function of the synapse. Several strategies targeting the overdosage of DYRK1A in DS with specific kinase inhibitors have showed promising evidence that DS cognitive conditions can be alleviated. Nevertheless, providing conditions for proper temporal treatment and to tackle the neurodevelopmental and the neurodegenerative aspects of DS across life span is still an open question.

Atypical molecular features of RNA silencing against the phloem-restricted polerovirus TuYV

In plants and some animal lineages, RNA silencing is an efficient and adaptable defense mechanism against viruses. To counter it, viruses encode suppressor proteins that interfere with RNA silencing. Phloem-restricted viruses are spreading at an alarming rate and cause substantial reduction of crop yield, but how they interact with their hosts at the molecular level is still insufficiently understood. Here, we investigate the antiviral response against phloem-restricted turnip yellows virus (TuYV) in the model plant Arabidopsis thaliana. Using a combination of genetics, deep sequencing, and mechanical vasculature enrichment, we show that the main axis of silencing active against TuYV involves 22-nt vsiRNA production by DCL2, and their preferential loading into AGO1. Moreover, we identify vascular secondary siRNA produced from plant transcripts and initiated by DCL2-processed AGO1-loaded vsiRNA. Unexpectedly, and despite the viral encoded VSR P0 previously shown to mediate degradation of AGO proteins, vascular AGO1 undergoes specific post-translational stabilization during TuYV infection. Collectively, our work uncovers the complexity of antiviral RNA silencing against phloem-restricted TuYV and prompts a re-assessment of the role of its suppressor of silencing P0 during genuine infection.

Dyrk1a gene dosage in glutamatergic neurons has key effects in cognitive deficits observed in mouse models of MRD7 and Down syndrome

Perturbation of the excitation/inhibition (E/I) balance leads to neurodevelopmental diseases including to autism spectrum disorders, intellectual disability, and epilepsy. Loss-of-function mutations in the DYRK1A gene, located on human chromosome 21 (Hsa21,) lead to an intellectual disability syndrome associated with microcephaly, epilepsy, and autistic troubles. Overexpression of DYRK1A, on the other hand, has been linked with learning and memory defects observed in people with Down syndrome (DS). Dyrk1a is expressed in both glutamatergic and GABAergic neurons, but its impact on each neuronal population has not yet been elucidated. Here we investigated the impact of Dyrk1a gene copy number variation in glutamatergic neurons using a conditional knockout allele of Dyrk1a crossed with the Tg(Camk2-Cre)4Gsc transgenic mouse. We explored this genetic modification in homozygotes, heterozygotes and combined with the Dp(16Lipi-Zbtb21)1Yey trisomic mouse model to unravel the consequence of Dyrk1a dosage from 0 to 3, to understand its role in normal physiology, and in MRD7 and DS. Overall, Dyrk1a dosage in postnatal glutamatergic neurons did not impact locomotor activity, working memory or epileptic susceptibility, but revealed that Dyrk1a is involved in long-term explicit memory. Molecular analyses pointed at a deregulation of transcriptional activity through immediate early genes and a role of DYRK1A at the glutamatergic post-synapse by deregulating and interacting with key post-synaptic proteins implicated in mechanism leading to long-term enhanced synaptic plasticity. Altogether, our work gives important information to understand the action of DYRK1A inhibitors and have a better therapeutic approach.

The Dual Specificity Tyrosine Phosphorylation Regulated Kinase 1A, DYRK1A, drives cognitive alterations with increased dose in Down syndrome (DS) or with reduced dose in DYRK1A-related intellectual disability syndromes (ORPHA:268261; ORPHA:464311) also known as mental retardation, autosomal dominant disease 7 (MRD7; OMIM #614104). Here we report that specific and complete loss of Dyrk1a in glutamatergic neurons induced a range of specific cognitive phenotypes and alter the expression of genes involved in neurotransmission in the hippocampus. We further explored the consequences of Dyrk1a dosage in glutamatergic neurons on the cognitive phenotypes observed respectively in MRD7 and DS mouse models and we found specific roles in long-term explicit memory with no impact on motor activity, short-term working memory, and susceptibility to epilepsy. Then we demonstrated that DYRK1A is a component of the glutamatergic post-synapse and interacts with several component such as NR2B and PSD95. Altogether our work describes a new role of DYRK1A at the glutamatergic synapse that must be considered to understand the consequence of treatment targeting DYRK1A in disease.

Cauliflower mosaic virus protein P6-TAV plays a major role in alteration of aphid vector feeding behaviour but not performance on infected Arabidopsis

Emerging evidence suggests that viral infection modifies host plant traits that in turn alter behaviour and performance of vectors colonizing the plants in a way conducive for transmission of both nonpersistent and persistent viruses. Similar evidence for semipersistent viruses like cauliflower mosaic virus (CaMV) is scarce. Here we compared the effects of Arabidopsis infection with mild (CM) and severe (JI) CaMV isolates on the feeding behaviour (recorded by the electrical penetration graph technique) and fecundity of the aphid vector Myzus persicae. Compared to mock-inoculated plants, feeding behaviour was altered similarly on CM- and JI-infected plants, but only aphids on JI-infected plants had reduced fecundity. To evaluate the role of the multifunctional CaMV protein P6-TAV, aphid feeding behaviour and fecundity were tested on transgenic Arabidopsis plants expressing wild-type (wt) and mutant versions of P6-TAV. In contrast to viral infection, aphid fecundity was unchanged on all transgenic lines, suggesting that other viral factors compromise fecundity. Aphid feeding behaviour was modified on wt P6-CM-, but not on wt P6-JI-expressing plants. Analysis of plants expressing P6 mutants identified N-terminal P6 domains contributing to modification of feeding behaviour. Taken together, we show that CaMV infection can modify both aphid fecundity and feeding behaviour and that P6 is only involved in the latter.

Modeling Down syndrome in animals from the early stage to the 4.0 models and next

The genotype-phenotype relationship and the physiopathology of Down Syndrome (DS) have been explored in the last 20 years with more and more relevant mouse models. From the early age of transgenesis to the new CRISPR/CAS9-derived chromosomal engineering and the transchromosomic technologies, mouse models have been key to identify homologous genes or entire regions homologous to the human chromosome 21 that are necessary or sufficient to induce DS features, to investigate the complexity of the genetic interactions that are involved in DS and to explore therapeutic strategies. In this review we report the new developments made, how genomic data and new genetic tools have deeply changed our way of making models, extended our panel of animal models, and increased our understanding of the neurobiology of the disease. But even if we have made an incredible progress which promises to make DS a curable condition, we are facing new research challenges to nurture our knowledge of DS pathophysiology as a neurodevelopmental disorder with many comorbidities during ageing.

In Vitro Acquisition of Specific Small Interfering RNAs Inhibits the Expression of Some Target Genes in the Plant Ectoparasite Xiphinema index

Xiphinema index is an important plant parasitic nematode that induces direct damages and specifically transmits the Grapevine fanleaf virus, which is particularly harmful for grapevines. Genomic resources of this nematode species are still limited and no functional gene validation technology is available. RNA interference (RNAi) is a powerful technology to study gene function and here we describe the application of RNAi on several genes in X. index. Soaking the nematodes for 48 h in a suspension containing specific small interfering RNAs resulted in a partial inhibition of the accumulation of some targeted mRNA. However, low reproducible silencing efficiency was observed which could arise from X. index silencing pathway deficiencies. Indeed, essential accustomed proteins for these pathways were not found in the X. index proteome predicted from transcriptomic data. The most reproducible silencing effect was obtained when targeting the piccolo gene potentially involved in endo-exocytosis of synaptic molecules. This represents the first report of gene silencing in a nematode belonging to the Longidoridae family.

Cbs overdosage is necessary and sufficient to induce cognitive phenotypes in mouse models of Down syndrome and interacts genetically with Dyrk1a

Identifying dosage-sensitive genes is a key to understand the mechanisms underlying intellectual disability in Down syndrome (DS). The Dp(17Abcg1-Cbs)1Yah DS mouse model (Dp1Yah) shows cognitive phenotypes that need to be investigated to identify the main genetic driver. Here, we report that three copies of the cystathionine-beta-synthase gene (Cbs) in the Dp1Yah mice are necessary to observe a deficit in the novel object recognition (NOR) paradigm. Moreover, the overexpression of Cbs alone is sufficient to induce deficits in the NOR test. Accordingly, overexpressing human CBS specifically in Camk2a-expressing neurons leads to impaired objects discrimination. Altogether, this shows that Cbs overdosage is involved in DS learning and memory phenotypes. To go further, we identified compounds that interfere with the phenotypical consequence of CBS overdosage in yeast. Pharmacological intervention in Tg(CBS) mice with one selected compound restored memory in the NOR test. In addition, using a genetic approach, we demonstrated an epistatic interaction between Cbs and Dyrk1a, another human chromosome 21-located gene (which encodes the dual-specificity tyrosine phosphorylation-regulated kinase 1a) and an already known target for DS therapeutic intervention. Further analysis using proteomic approaches highlighted several molecular pathways, including synaptic transmission, cell projection morphogenesis and actin cytoskeleton, that are affected by DYRK1A and CBS overexpression. Overall, we demonstrated that CBS overdosage underpins the DS-related recognition memory deficit and that both CBS and DYRK1A interact to control accurate memory processes in DS. In addition, our study establishes CBS as an intervention point for treating intellectual deficiencies linked to DS.

Split green fluorescent protein as a tool to study infection with a plant pathogen, Cauliflower mosaic virus

The split GFP technique is based on the auto-assembly of GFP when two polypeptides–GFP1-10 (residues 1–214; the detector) and GFP11 (residues 215–230; the tag)–both non-fluorescing on their own, associate spontaneously to form a fluorescent molecule. We evaluated this technique for its efficacy in contributing to the characterization of Cauliflower mosaic virus (CaMV) infection. A recombinant CaMV with GFP11 fused to the viral protein P6 (a key player in CaMV infection and major constituent of viral factory inclusions that arise during infection) was constructed and used to inoculate transgenic Arabidopsis thaliana expressing GFP1-10. The mutant virus (CaMV_11P6) was infectious, aphid-transmissible and the insertion was stable over many passages. Symptoms on infected plants were delayed and milder. Viral protein accumulation, especially of recombinant 11P6, was greatly decreased, impeding its detection early in infection. Nonetheless, spread of infection from the inoculated leaf to other leaves was followed by whole plant imaging. Infected cells displayed in real time confocal laser scanning microscopy fluorescence in wild type-looking virus factories. Thus, it allowed for the first time to track a CaMV protein in vivo in the context of an authentic infection. 11P6 was immunoprecipitated with anti-GFP nanobodies, presenting a new application for the split GFP system in protein-protein interaction assays and proteomics. Taken together, split GFP can be an attractive alternative to using the entire GFP for protein tagging.

Plant infection by two different viruses induce contrasting changes of vectors fitness and behavior

Insect-vectored plant viruses can induce changes in plant phenotypes, thus influencing plant-vector interactions in a way that may promote their dispersal according to their mode of transmission (i.e., circulative vs. noncirculative). This indirect vector manipulation requires host-virus-vector coevolution and would thus be effective solely in very specific plant-virus-vector species associations. Some studies suggest this manipulation may depend on multiple factors relative to various intrinsic characteristics of vectors such as transmission efficiency. In anintegrative study, we tested the effects of infection of the Brassicaceae Camelina sativa with the noncirculative Cauliflower mosaic virus (CaMV) or the circulative Turnip yellows virus (TuYV) on the host-plant colonization of two aphid species differing in their virus transmission efficiency: the polyphagous Myzus persicae, efficient vector of both viruses, and the Brassicaceae specialist Brevicoryne brassicae, poor vector of TuYV and efficient vector of CaMV. Results confirmed the important role of virus mode of transmission as plant-mediated effects of CaMV on the two aphid species induced negative alterations of feeding behavior (i.e., decreased phloem sap ingestion) and performance that were both conducive for virus fitness by promoting dispersion after a rapid acquisition. In addition, virus transmission efficiency may also play a role in vector manipulation by viruses as only the responses of the efficient vector to plant-mediated effects of TuYV, that is, enhanced feeding behavior and performances, were favorable to their acquisition and further dispersal. Altogether, this work demonstrated that vector transmission efficiency also has to be considered when studying the mechanisms underlying vector manipulation by viruses. Our results also reinforce the idea that vector manipulation requires coevolution between plant, virus and vector.

Phloem-Triggered Virus-Induced Gene Silencing Using a Recombinant Polerovirus

The phloem-limited poleroviruses infect Arabidopsis thaliana without causing noticeable disease symptoms. In order to facilitate visual infection identification, we developed virus-induced gene silencing (VIGS) vectors derived from Turnip yellows virus (TuYV). Short sequences from the host gene AtCHLI1 required for chlorophyll biosynthesis [42 nucleotides in sense or antisense orientation or as an inverted-repeat (IR), or an 81 nucleotide sense fragment] were inserted into the 3' non-coding region of the TuYV genome to screen for the most efficient and robust silencing vector. All recombinant viruses produced a clear vein chlorosis phenotype on infected Arabidopsis plants due to the expression inhibition of the AtCHLI1 gene. The introduction of a sense-oriented sequence into TuYV genome resulted in a virus exhibiting a more sustainable chlorosis than the virus containing an IR of the same length. This observation was correlated with a higher stability of the sense sequence insertion in the viral genome. In order to evaluate the impact of the TuYV silencing suppressor P0 in the VIGS mechanism a P0 knock-out mutation was introduced into the recombinant TuYV viruses. They induced a similar but milder vein clearing phenotype due to lower viral accumulation. This indicates that P0 does not hinder the performances of the TuYV silencing effect and confirms that in the viral infection context, P0 has no major impact on the production, propagation and action of the short distance silencing signal in phloem cells. Finally, we showed that TuYV can be used to strongly silence the phloem specific AtRTM1 gene. The TuYV-derived VIGS vectors therefore represent powerful tools to easily detect and monitor TuYV in infected plants and conduct functional analysis of phloem-restricted genes. Moreover this example indicates the potential of poleroviruses for use in functional genomic studies of agronomic plants.

Production of a Beet chlorosis virus full-length cDNA clone by means of Gibson assembly and analysis of biological properties

Beet chlorosis virus (genus Polerovirus, family Luteoviridae), which is persistently transmitted by the aphid Myzus persicae, is part of virus yellows in sugar beet and causes interveinal yellowing as well as significant yield loss in Beta vulgaris. To allow reverse genetic studies and replace vector transmission, an infectious cDNA clone under cauliflower mosaic virus 35S control in a binary vector for agrobacterium-mediated infection was constructed using Gibson assembly. Following agroinoculation, the BChV full-length clone was able to induce a systemic infection of the cultivated B. vulgaris. The engineered virus was successfully aphid-transmitted when acquired from infected B. vulgaris and displayed the same host plant spectrum as wild-type virus. This new polerovirus infectious clone is a valuable tool to identify the viral determinants involved in host range and study BChV protein function, and can be used to screen sugar beet for BChV resistance.

The Aphid-Transmitted Turnip yellows virus Differentially Affects Volatiles Emission and Subsequent Vector Behavior in Two Brassicaceae Plants

Aphids are important pests which cause direct damage by feeding or indirect prejudice by transmitting plant viruses. Viruses are known to induce modifications of plant cues in ways that can alter vector behavior and virus transmission. In this work, we addressed whether the modifications induced by the aphid-transmitted Turnip yellows virus (TuYV) in the model plant Arabidopsis thaliana also apply to the cultivated plant Camelina sativa, both belonging to the Brassicaceae family. In most experiments, we observed a significant increase in the relative emission of volatiles from TuYV-infected plants. Moreover, due to plant size, the global amounts of volatiles emitted by C. sativa were higher than those released by A. thaliana. In addition, the volatiles released by TuYV-infected C. sativa attracted the TuYV vector Myzus persicae more efficiently than those emitted by non-infected plants. In contrast, no such preference was observed for A. thaliana. We propose that high amounts of volatiles rather than specific metabolites are responsible for aphid attraction to infected C. sativa. This study points out that the data obtained from the model pathosystem A. thaliana/TuYV cannot be straightforwardly extrapolated to a related plant species infected with the same virus.

Transmission of Turnip yellows virus by Myzus persicae Is Reduced by Feeding Aphids on Double-Stranded RNA Targeting the Ephrin Receptor Protein

Aphid-transmitted plant viruses are a threat for major crops causing massive economic loss worldwide. Members in the Luteoviridae family are transmitted by aphids in a circulative and non-replicative mode. Virions are acquired by aphids when ingesting sap from infected plants and are transported through the gut and the accessory salivary gland (ASG) cells by a transcytosis mechanism relying on virus-specific receptors largely unknown. Once released into the salivary canal, virions are inoculated to plants, together with saliva, during a subsequent feeding. In this paper, we bring in vivo evidence that the membrane-bound Ephrin receptor (Eph) is a novel aphid protein involved in the transmission of the Turnip yellows virus (TuYV, Polerovirus genus, Luteoviridae family) by Myzus persicae. The minor capsid protein of TuYV, essential for aphid transmission, was able to bind the external domain of Eph in yeast. Feeding M. persicae on in planta- or in vitro-synthesized dsRNA targeting Eph-mRNA (dsRNA_Eph) did not affect aphid feeding behavior but reduced accumulation of TuYV genomes in the aphid's body. Consequently, TuYV transmission efficiency by the dsRNA_Eph-treated aphids was reproducibly inhibited and we brought evidence that Eph is likely involved in intestinal uptake of the virion. The inhibition of virus uptake after dsRNA_Eph acquisition was also observed for two other poleroviruses transmitted by M. persicae, suggesting a broader role of Eph in polerovirus transmission. Finally, dsRNA_Eph acquisition by aphids did not affect nymph production. These results pave the way toward an ecologically safe alternative of insecticide treatments that are used to lower aphid populations and reduce polerovirus damages.

Trans-species synthetic gene design allows resistance pyramiding and broad-spectrum engineering of virus resistance in plants

To infect plants, viruses rely heavily on their host's machinery. Plant genetic resistances based on host factor modifications can be found among existing natural variability and are widely used for some but not all crops. While biotechnology can supply for the lack of natural resistance alleles, new strategies need to be developed to increase resistance spectra and durability without impairing plant development. Here, we assess how the targeted allele modification of the Arabidopsis thaliana translation initiation factor eIF4E1 can lead to broad and efficient resistance to the major group of potyviruses. A synthetic Arabidopsis thaliana eIF4E1 allele was designed by introducing multiple amino acid changes associated with resistance to potyvirus in naturally occurring Pisum sativum alleles. This new allele encodes a functional protein while maintaining plant resistance to a potyvirus isolate that usually hijacks eIF4E1. Due to its biological functionality, this synthetic allele allows, at no developmental cost, the pyramiding of resistances to potyviruses that selectively use the two major translation initiation factors, eIF4E1 or its isoform eIFiso4E. Moreover, this combination extends the resistance spectrum to potyvirus isolates for which no efficient resistance has so far been found, including resistance-breaking isolates and an unrelated virus belonging to the Luteoviridae family. This study is a proof-of-concept for the efficiency of gene engineering combined with knowledge of natural variation to generate trans-species virus resistance at no developmental cost to the plant. This has implications for breeding of crops with broad-spectrum and high durability resistance using recent genome editing techniques.

Identification of host factors potentially involved in RTM-mediated resistance during potyvirus long distance movement

The long distance movement of potyviruses is a poorly understood step of the viral cycle. Only factors inhibiting this process, referred to as "Restricted TEV Movement" (RTM), have been identified in Arabidopsis thaliana. On the virus side, the potyvirus coat protein (CP) displays determinants required for long-distance movement and for RTM-based resistance breaking. However, the potyvirus CP was previously shown not to interact with the RTM proteins. We undertook the identification of Arabidopsis factors which directly interact with either the RTM proteins or the CP of lettuce mosaic virus (LMV). An Arabidopsis cDNA library generated from companion cells was screened with LMV CP and RTM proteins using the yeast two-hybrid system. Fourteen interacting proteins were identified. Two of them were shown to interact with CP and the RTM proteins suggesting that a multiprotein complex could be formed between the RTM proteins and virions or viral ribonucleoprotein complexes. Co-localization experiments in Nicotiana benthamiana showed that most of the viral and cellular protein pairs co-localized at the periphery of chloroplasts which suggests a putative role for plastids in this process.

Systemic Propagation of a Fluorescent Infectious Clone of a Polerovirus Following Inoculation by Agrobacteria and Aphids

A fluorescent viral clone of the polerovirus Turnip yellows virus (TuYV) was engineered by introducing the Enhanced Green Fluorescent Protein (EGFP) sequence into the non-structural domain sequence of the readthrough protein, a minor capsid protein. The resulting recombinant virus, referred to as TuYV-RTGFP, was infectious in several plant species when delivered by agroinoculation and invaded efficiently non-inoculated leaves. As expected for poleroviruses, which infect only phloem cells, the fluorescence emitted by TuYV-RTGFP was restricted to the vasculature of infected plants. In addition, TuYV-RTGFP was aphid transmissible and enabled the observation of the initial sites of infection in the phloem after aphid probing in epidermal cells. The aphid-transmitted virus moved efficiently to leaves distant from the inoculation sites and importantly retained the EGFP sequence in the viral genome. This work reports on the first engineered member in the Luteoviridae family that can be visualized by fluorescence emission in systemic leaves of different plant species after agroinoculation or aphid transmission.

Comparative Analysis of RNAi-Based Methods to Down-Regulate Expression of Two Genes Expressed at Different Levels in Myzus persicae

With the increasing availability of aphid genomic data, it is necessary to develop robust functional validation methods to evaluate the role of specific aphid genes. This work represents the first study in which five different techniques, all based on RNA interference and on oral acquisition of double-stranded RNA (dsRNA), were developed to silence two genes, ALY and Eph, potentially involved in polerovirus transmission by aphids. Efficient silencing of only Eph transcripts, which are less abundant than those of ALY, could be achieved by feeding aphids on transgenic Arabidopsis thaliana expressing an RNA hairpin targeting Eph, on Nicotiana benthamiana infected with a Tobacco rattle virus (TRV)-Eph recombinant virus, or on in vitro-synthesized Eph-targeting dsRNA. These experiments showed that the silencing efficiency may differ greatly between genes and that aphid gut cells seem to be preferentially affected by the silencing mechanism after oral acquisition of dsRNA. In addition, the use of plants infected with recombinant TRV proved to be a promising technique to silence aphid genes as it does not require plant transformation. This work highlights the need to pursue development of innovative strategies to reproducibly achieve reduction of expression of aphid genes.

Transcription of densovirus endogenous sequences in the Myzus persicae genome

Integration of non-retroviral sequences in the genome of different organisms has been observed and, in some cases, a relationship of these integrations with immunity has been established. The genome of the green peach aphid, Myzus persicae (clone G006), was screened for densovirus-like sequence (DLS) integrations. A total of 21 DLSs localized on 10 scaffolds were retrieved that mostly shared sequence identity with two aphid-infecting viruses, Myzus persicae densovirus (MpDNV) and Dysaphis plantaginea densovirus (DplDNV). In some cases, uninterrupted potential ORFs corresponding to non-structural viral proteins or capsid proteins were found within DLSs identified in the aphid genome. In particular, one scaffold harboured a complete virus-like genome, while another scaffold contained two virus-like genomes in reverse orientation. Remarkably, transcription of some of these ORFs was observed in M. persicae, suggesting a biological effect of these viral integrations. In contrast to most of the other densoviruses identified so far that induce acute host infection, it has been reported previously that MpDNV has only a minor effect on M. persicae fitness, while DplDNV can even have a beneficial effect on its aphid host. This suggests that DLS integration in the M. persicae genome may be responsible for the latency of MpDNV infection in the aphid host.

A protein kinase binds the C-terminal domain of the readthrough protein of Turnip yellows virus and regulates virus accumulation

Turnip yellows virus (TuYV), a phloem-limited virus, encodes a 74kDa protein known as the readthrough protein (RT) involved in virus movement. We show here that a TuYV mutant deleted of the C-terminal part of the RT protein (TuYV-∆RTCter) was affected in long-distance trafficking in a host-specific manner. By using the C-terminal domain of the RT protein as a bait in a yeast two-hybrid screen of a phloem cDNA library from Arabidopsis thaliana we identified the calcineurin B-like protein-interacting protein kinase-7 (AtCIPK7). Transient expression of a GFP:CIPK7 fusion protein in virus-inoculated Nicotiana benthamiana leaves led to local increase of wild-type TuYV accumulation, but not that of TuYV-∆RTCter. Surprisingly, elevated virus titer in inoculated leaves did not result in higher TuYV accumulation in systemic leaves, which indicates that virus long-distance movement was not affected. Since GFP:CIPK7 was localized in or near plasmodesmata, CIPK7 could negatively regulate TuYV export from infected cells.

Discovery of a Small Non-AUG-Initiated ORF in Poleroviruses and Luteoviruses That Is Required for Long-Distance Movement

Viruses in the family Luteoviridae have positive-sense RNA genomes of around 5.2 to 6.3 kb, and they are limited to the phloem in infected plants. The Luteovirus and Polerovirus genera include all but one virus in the Luteoviridae. They share a common gene block, which encodes the coat protein (ORF3), a movement protein (ORF4), and a carboxy-terminal extension to the coat protein (ORF5). These three proteins all have been reported to participate in the phloem-specific movement of the virus in plants. All three are translated from one subgenomic RNA, sgRNA1. Here, we report the discovery of a novel short ORF, termed ORF3a, encoded near the 5’ end of sgRNA1. Initially, this ORF was predicted by statistical analysis of sequence variation in large sets of aligned viral sequences. ORF3a is positioned upstream of ORF3 and its translation initiates at a non-AUG codon. Functional analysis of the ORF3a protein, P3a, was conducted with Turnip yellows virus (TuYV), a polerovirus, for which translation of ORF3a begins at an ACG codon. ORF3a was translated from a transcript corresponding to sgRNA1 in vitro, and immunodetection assays confirmed expression of P3a in infected protoplasts and in agroinoculated plants. Mutations that prevent expression of P3a, or which overexpress P3a, did not affect TuYV replication in protoplasts or inoculated Arabidopsis thaliana leaves, but prevented virus systemic infection (long-distance movement) in plants. Expression of P3a from a separate viral or plasmid vector complemented movement of a TuYV mutant lacking ORF3a. Subcellular localization studies with fluorescent protein fusions revealed that P3a is targeted to the Golgi apparatus and plasmodesmata, supporting an essential role for P3a in viral movement.

In order to maximize coding capacity, RNA viruses often encode overlapping genes and use unusual translational control mechanisms. Plant viruses express proteins required for movement of the virus through the plant, often from non-canonically translated open reading frames (ORFs). Viruses in the economically important Luteoviridae family are confined to the phloem (vascular) tissue, probably due to their specialized phloem-specific movement proteins. These proteins are translated from one viral mRNA, sgRNA1, via initiation at more than one AUG codon to express overlapping genes, and by ribosomal read-through of a stop codon. Here, we describe yet another gene translated from sgRNA1, ORF3a. Translation of ORF3a initiates at a non-standard (not AUG) start codon. We found that ORF3a is not required for viral genome replication, but is required for long-distance movement of the virus in the plant. The movement function could be restored in trans by providing the ORF3a product, P3a, from another viral or plasmid vector. P3a localizes in the Golgi apparatus and adjacent to the plasmodesmata, supporting a role in intercellular movement. In summary, we used a powerful bioinformatic tool to discover a cryptic gene whose product is required for movement of a phloem-specific plant virus, revealing multiple levels of translational control that regulate expression of four proteins from a single mRNA.

Opposite phenotypes of muscle strength and locomotor function in mouse models of partial trisomy and monosomy 21 for the proximal Hspa13-App region

The trisomy of human chromosome 21 (Hsa21), which causes Down syndrome (DS), is the most common viable human aneuploidy. In contrast to trisomy, the complete monosomy (M21) of Hsa21 is lethal, and only partial monosomy or mosaic monosomy of Hsa21 is seen. Both conditions lead to variable physiological abnormalities with constant intellectual disability, locomotor deficits, and altered muscle tone. To search for dosage-sensitive genes involved in DS and M21 phenotypes, we created two new mouse models: the Ts3Yah carrying a tandem duplication and the Ms3Yah carrying a deletion of the Hspa13-App interval syntenic with 21q11.2-q21.3. Here we report that the trisomy and the monosomy of this region alter locomotion, muscle strength, mass, and energetic balance. The expression profiling of skeletal muscles revealed global changes in the regulation of genes implicated in energetic metabolism, mitochondrial activity, and biogenesis. These genes are downregulated in Ts3Yah mice and upregulated in Ms3Yah mice. The shift in skeletal muscle metabolism correlates with a change in mitochondrial proliferation without an alteration in the respiratory function. However, the reactive oxygen species (ROS) production from mitochondrial complex I decreased in Ms3Yah mice, while the membrane permeability of Ts3Yah mitochondria slightly increased. Thus, we demonstrated how the Hspa13-App interval controls metabolic and mitochondrial phenotypes in muscles certainly as a consequence of change in dose of Gabpa, Nrip1, and Atp5j. Our results indicate that the copy number variation in the Hspa13-App region has a peripheral impact on locomotor activity by altering muscle function.

Both structural and non-structural forms of the readthrough protein of cucurbit aphid-borne yellows virus are essential for efficient systemic infection of plants

Cucurbit aphid-borne yellows virus (CABYV) is a polerovirus (Luteoviridae family) with a capsid composed of the major coat protein and a minor component referred to as the readthrough protein (RT). Two forms of the RT were reported: a full-length protein of 74 kDa detected in infected plants and a truncated form of 55 kDa (RT*) incorporated into virions. Both forms were detected in CABYV-infected plants. To clarify the specific roles of each protein in the viral cycle, we generated by deletion a polerovirus mutant able to synthesize only the RT* which is incorporated into the particle. This mutant was unable to move systemically from inoculated leaves inferring that the C-terminal half of the RT is required for efficient long-distance transport of CABYV. Among a collection of CABYV mutants bearing point mutations in the central domain of the RT, we obtained a mutant impaired in the correct processing of the RT which does not produce the RT*. This mutant accumulated very poorly in upper non-inoculated leaves, suggesting that the RT* has a functional role in long-distance movement of CABYV. Taken together, these results infer that both RT proteins are required for an efficient CABYV movement.

Divergence of host range and biological properties between natural isolate and full-length infectious cDNA clone of the Beet mild yellowing virus 2ITB

Plant infection by poleroviruses is restricted to phloem tissues, preventing any classical leaf rub inoculation with viral RNA or virions. Efficient virus inoculation to plants is achieved by viruliferous aphids that acquire the virus by feeding on infected plants. The use of promoter-driven infectious cDNA is an alternative means to infect plants and allows reverse genetic studies to be performed. Using Beet mild yellowing virus isolate 2ITB (BMYV-2ITB), we produced a full-length infectious cDNA clone of the virus (named BMYV-EK) placed under the control of the T7 RNA polymerase and the Cauliflower mosaic virus 35S promoters. Infectivity of the engineered BMYV-EK virus was assayed in different plant species and compared with that of the original virus. We showed that in vitro- or in planta-derived transcripts were infectious in protoplasts and in whole plants. Importantly, the natural aphid vector Myzus persicae efficiently transmitted the viral progeny produced in infected plants. By comparing agroinoculation and aphid infection in a host range assay, we showed that the engineered BMYV-EK virus displayed a similar host range to BMYV-2ITB, except for Nicotiana benthamiana, which proved to be resistant to systemic infection with BMYV-EK. Finally, both the BMYV-EK P0 and the full-length clone were able to strongly interfere with post-transcriptional gene silencing.

Formation of virions is strictly required for turnip yellows virus long-distance movement in plants

Viral genomic RNA of the Turnip yellows virus (TuYV; genus Polerovirus; family Luteoviridae) is protected in virions formed by the major capsid protein (CP) and the minor component, the readthrough (RT*) protein. Long-distance transport, used commonly by viruses to systemically infect host plants, occurs in phloem sieve elements and two viral forms of transport have been described: virions and ribonucleoprotein (RNP) complexes. With regard to poleroviruses, virions have always been presumed to be the long-distance transport form, but the potential role of RNP complexes has not been investigated. Here, we examined the requirement of virions for polerovirus systemic movement by analysing CP-targeted mutants that were unable to form viral particles. We confirmed that TuYV mutants that cannot encapsidate into virions are not able to reach systemic leaves. To completely discard the possibility that the introduced mutations in CP simply blocked the formation or the movement of RNP complexes, we tested in trans complementation of TuYV CP mutants by providing WT CP expressed in transgenic plants. WT CP was able to facilitate systemic movement of TuYV CP mutants and this observation was always correlated with the formation of virions. This demonstrated clearly that virus particles are essential for polerovirus systemic movement.

Viral and cellular factors involved in Phloem transport of plant viruses

Phloem transport of plant viruses is an essential step in the setting-up of a complete infection of a host plant. After an initial replication step in the first cells, viruses spread from cell-to-cell through mesophyll cells, until they reach the vasculature where they rapidly move to distant sites in order to establish the infection of the whole plant. This last step is referred to as systemic transport, or long-distance movement, and involves virus crossings through several cellular barriers: bundle sheath, vascular parenchyma, and companion cells for virus loading into sieve elements (SE). Viruses are then passively transported within the source-to-sink flow of photoassimilates and are unloaded from SE into sink tissues. However, the molecular mechanisms governing virus long-distance movement are far from being understood. While most viruses seem to move systemically as virus particles, some viruses are transported in SE as viral ribonucleoprotein complexes (RNP). The nature of the cellular and viral factors constituting these RNPs is still poorly known. The topic of this review will mainly focus on the host and viral factors that facilitate or restrict virus long-distance movement.

Closely related poleroviruses depend on distinct translation initiation factors to infect Arabidopsis thaliana

In addition to being essential for translation of eukaryotic mRNA, translation initiation factors are also key components of plant-virus interactions. In order to address the involvement of these factors in the infectious cycle of poleroviruses (aphid-transmitted, phloem-limited viruses), the accumulation of three poleroviruses was followed in Arabidopsis thaliana mutant lines impaired in the synthesis of translation initiation factors in the eIF4E and eIF4G families. We found that efficient accumulation of Turnip yellows virus (TuYV) in A. thaliana relies on the presence of eIF (iso)4G1, whereas Beet mild yellowing virus (BMYV) and Beet western yellows virus-USA (BWYV-USA) rely, instead, on eIF4E1. A role for these factors in the infectious processes of TuYV and BMYV was confirmed by direct interaction in yeast between these specific factors and the 5' viral genome-linked protein of the related virus. Although the underlying molecular mechanism is still unknown, this study reveals a totally unforeseen situation in which closely related viruses belonging to the same genus use different translation initiation factors for efficient infection of A. thaliana.

The in vivo Down syndrome genomic library in mouse

Mouse models are key elements to better understand the genotype-phenotype relationship and the physiopathology of Down syndrome (DS). Even though the mouse will never recapitulate the whole spectrum of intellectual disabilities observed in the DS, mouse models have been developed over the recent decades and have been used extensively to identify homologous genes or entire regions homologous to the human chromosome 21 (Hsa21) that are necessary or sufficient to induce DS cognitive features. In this chapter, we review the principal mouse DS models which have been selected and engineered over the years either for large genomic regions or for a few or a single gene of interest. Their analyses highlight the complexity of the genetic interactions that are involved in DS cognitive phenotypes and also strengthen the hypothesis on the multigenic nature of DS. This review also addresses future research challenges relative to the making of new models and their combination to go further in the characterization of candidates and modifier of the DS features.

The App-Runx1 region is critical for birth defects and electrocardiographic dysfunctions observed in a Down syndrome mouse model

Down syndrome (DS) leads to complex phenotypes and is the main genetic cause of birth defects and heart diseases. The Ts65Dn DS mouse model is trisomic for the distal part of mouse chromosome 16 and displays similar features with post-natal lethality and cardiovascular defects. In order to better understand these defects, we defined electrocardiogram (ECG) with a precordial set-up, and we found conduction defects and modifications in wave shape, amplitudes, and durations in Ts65Dn mice. By using a genetic approach consisting of crossing Ts65Dn mice with Ms5Yah mice monosomic for the App-Runx1 genetic interval, we showed that the Ts65Dn viability and ECG were improved by this reduction of gene copy number. Whole-genome expression studies confirmed gene dosage effect in Ts65Dn, Ms5Yah, and Ts65Dn/Ms5Yah hearts and showed an overall perturbation of pathways connected to post-natal lethality (Coq7, Dyrk1a, F5, Gabpa, Hmgn1, Pde10a, Morc3, Slc5a3, and Vwf) and heart function (Tfb1m, Adam19, Slc8a1/Ncx1, and Rcan1). In addition cardiac connexins (Cx40, Cx43) and sodium channel sub-units (Scn5a, Scn1b, Scn10a) were found down-regulated in Ts65Dn atria with additional down-regulation of Cx40 in Ts65Dn ventricles and were likely contributing to conduction defects. All these data pinpoint new cardiac phenotypes in the Ts65Dn, mimicking aspects of human DS features and pathways altered in the mouse model. In addition they highlight the role of the App-Runx1 interval, including Sod1 and Tiam1, in the induction of post-natal lethality and of the cardiac conduction defects in Ts65Dn. These results might lead to new therapeutic strategies to improve the care of DS people.

Characterization of PTZ-induced seizure susceptibility in a down syndrome mouse model that overexpresses CSTB

Down syndrome (DS) is a complex genetic syndrome characterized by intellectual disability, dysmorphism and variable additional physiological traits. Current research progress has begun to decipher the neural mechanisms underlying cognitive impairment, leading to new therapeutic perspectives. Pentylenetetrazol (PTZ) has recently been found to have positive effects on learning and memory capacities of a DS mouse model and is foreseen to treat DS patients. But PTZ is also known to be a convulsant drug at higher dose and DS persons are more prone to epileptic seizures than the general population. This raises concerns over what long-term effects of treatment might be in the DS population. The cause of increased propensity for epilepsy in the DS population and which Hsa21 gene(s) are implicated remain unknown. Among Hsa21 candidate genes in epilepsy, CSTB, coding for the cystein protease inhibitor cystatin B, is involved in progressive myoclonus epilepsy and ataxia in both mice and human. Thus we aim to evaluate the effect of an increase in Cstb gene dosage on spontaneous epileptic activity and susceptibility to PTZ-induced seizure. To this end we generated a new mouse model trisomic for Cstb by homologous recombination. We verified that increasing copy number of Cstb from Trisomy (Ts) to Tetrasomy (Tt) was driving overexpression of the gene in the brain, we checked transgenic animals for presence of locomotor activity and electroencephalogram (EEG) abnormalities characteristic of myoclonic epilepsy and we tested if those animals were prone to PTZ-induced seizure. Overall, the results of the analysis shows that an increase in Cstb does not induce any spontaneous epileptic activity and neither increase or decrease the propensity of Ts and Tt mice to myoclonic seizures suggesting that Ctsb dosage should not interfere with PTZ-treatment.

Controlled somatic and germline copy number variation in the mouse model

Changes in the number of chromosomes, but also variations in the copy number of chromosomal regions have been described in various pathological conditions, such as cancer and aneuploidy, but also in normal physiological condition. Our classical view of DNA replication and mitotic preservation of the chromosomal integrity is now challenged as new technologies allow us to observe such mosaic somatic changes in copy number affecting regions of chromosomes with various sizes. In order to go further in the understanding of copy number influence in normal condition we could take advantage of the novel strategy called Targeted Asymmetric Sister Chromatin Event of Recombination (TASCER) to induce recombination during the G2 phase so that we can generate deletions and duplications of regions of interest prior to mitosis. Using this approach in the mouse we could address the effects of copy number variation and segmental aneuploidy in daughter cells and allow us to explore somatic mosaics for large region of interest in the mouse.

The telomeric part of the human chromosome 21 from Cstb to Prmt2 is not necessary for the locomotor and short-term memory deficits observed in the Tc1 mouse model of Down syndrome

Trisomy 21 or Down syndrome (DS) is the most common form of human aneuploid disorder. Increase in the copy number of human chromosome 21 genes leads to several alterations including mental retardation, heart and skeletal dysmorphologies with additional physiological defects. To better understand the genotype and phenotype relationships, several mouse models have been developed, including the transchromosomic Tc1 mouse, which carries an almost complete human chromosome 21, that displays several locomotor and cognitive alterations related to DS. In this report we explore the contribution of the genetic dosage of 47 mouse genes located in the most telomeric part of Hsa21, using a novel model, named Ms4Yah, carrying a deletion of the 2.2Mb Ctsb–Prmt2 genetic interval. We combine this deletion with the Tc1 Hsa21 in a rescue experiment. We could recapitulate most of the Tc1 phenotypes but we found no phenotypes induced by the Ms4Yah and no contribution to the Tc1-induced phenotypes even if we described new alteration in social preference but not in olfaction. Thus we conclude that the genes conserved between mouse and human, found in the most telomeric part of Hsa21, and trisomic in Tc1, are not contributing to the major Tc1 phenotypes, suggesting that the Cstb–Prmt2 region is not playing a major role in locomotor and cognitive deficits found in DS.

Phloem protein partners of Cucurbit aphid borne yellows virus: possible involvement of phloem proteins in virus transmission by aphids

Poleroviruses are phytoviruses strictly transmitted by phloem-feeding aphids in a circulative and nonpropagative mode. During ingestion, aphids sample virions in sieve tubes along with sap. Therefore, any sap protein bound to virions will be acquired by the insects and could potentially be involved in the transmission process. By developing in vitro virus-overlay assays on sap proteins collected from cucumber, we observed that approximately 20 proteins were able to bind to purified particles of Cucurbit aphid borne yellows virus (CABYV). Among them, eight proteins were identified by mass spectrometry. The role of two candidates belonging to the PP2-like family (predominant lectins found in cucurbit sap) in aphid transmission was further pursued by using purified orthologous PP2 proteins from Arabidopsis. Addition of these proteins to the virus suspension in the aphid artificial diet greatly increased virus transmission rate. This shift was correlated with an increase in the number of viral genomes in insect cells and with an increase of virion stability in vitro. Surprisingly, increase of the virus transmission rate was also monitored after addition of unrelated proteins in the aphid diet, suggesting that any soluble protein at sufficiently high concentration in the diet and acquired together with virions could stimulate virus transmission.

A genomic analysis of transcytosis in the pea aphid, Acyrthosiphon pisum, a mechanism involved in virus transmission

Aphids are the primary vectors of plant viruses. Transmission can occur via attachment to the cuticle lining of the insect (non-circulative transmission) or after internalization in the insect cells with or without replication (circulative transmission). In this paper, we have focused on the circulative and non-propagative mode during which virions enter the cell following receptor-mediated endocytosis, are transported across the cell in vesicles and released by exocytosis without replicating. The correct uptake, transport and delivery of the vesicles cargo relies on the participation of proteins from different families which have been identified in the Acyrthosiphon pisum genome. Assemblage of this annotated dataset provides a useful basis to improve our understanding of the molecules and mechanisms involved in virus transmission by A. pisum and other aphid species.

A new mouse model for the trisomy of the Abcg1-U2af1 region reveals the complexity of the combinatorial genetic code of down syndrome

Mental retardation in Down syndrome (DS), the most frequent trisomy in humans, varies from moderate to severe. Several studies both in human and based on mouse models identified some regions of human chromosome 21 (Hsa21) as linked to cognitive deficits. However, other intervals such as the telomeric region of Hsa21 may contribute to the DS phenotype but their role has not yet been investigated in detail. Here we show that the trisomy of the 12 genes, found in the 0.59 Mb (Abcg1–U2af1) Hsa21 sub-telomeric region, in mice (Ts1Yah) produced defects in novel object recognition, open-field and Y-maze tests, similar to other DS models, but induces an improvement of the hippocampal-dependent spatial memory in the Morris water maze along with enhanced and longer lasting long-term potentiation in vivo in the hippocampus. Overall, we demonstrate the contribution of the Abcg1–U2af1 genetic region to cognitive defect in working and short-term recognition memory in DS models. Increase in copy number of the Abcg1–U2af1 interval leads to an unexpected gain of cognitive function in spatial learning. Expression analysis pinpoints several genes, such as Ndufv3, Wdr4, Pknox1 and Cbs, as candidates whose overexpression in the hippocampus might facilitate learning and memory in Ts1Yah mice. Our work unravels the complexity of combinatorial genetic code modulating different aspect of mental retardation in DS patients. It establishes definitely the contribution of the Abcg1–U2af1 orthologous region to the DS etiology and suggests new modulatory pathways for learning and memory.

Role of vector-transmission proteins

Most phytoviruses rely on vectors for their spread and survival. Although a great variety of virus vectors have been described, there are relatively few different mechanisms mediating virus transmission by vectors: virions can either be internalized into vector cells where replication may or may not take place or they can simply be adsorbed on the vector's surface or cuticle. Virus transmission by vectors requires tight associations between viral proteins, generally capsid proteins, and vector compounds, usually referred to as receptors. This review will focus on the viral determinants involved in virus transmission. Only the best-known models for which molecular data are available are described.

Modeling the monosomy for the telomeric part of human chromosome 21 reveals haploinsufficient genes modulating the inflammatory and airway responses

Monosomy 21 is a rare human disease due to gene dosage errors disturbing a variety of physiological and morphological systems including brain, skeletal, immune and respiratory functions. Most of the human condition corresponds to partial or mosaic monosomy suggesting that Monosomy 21 may be lethal. In order to search for dosage-sensitive genes involved in the human pathology, we generated by chromosomal engineering a monosomic mouse for the Prmt2-Col6a1 interval corresponding to the most telomeric part of human chromosome 21. Haploinsufficiency of the 13 genes, located in the 0.5 Mb genetic interval and conserved in man and mouse, caused apparently no morphological defect as observed in patients. However, monosomic mice displayed an enhanced inflammatory response after local intranasal lipopolysaccharide administration with enhanced recruitment of neutrophils and secretion of cytokines such as tumor necrosis factor-alpha (TNF-alpha), IL-1beta, IL-12p70 and IFN-gamma in the lung as well increased TNF-alpha production after systemic administration. Further analysis demonstrates that monosomic macrophages were involved and that a few genes, Prmt2, Pcnt2, Mcm3ap and Lss located in the region were candidate for the inflammatory response. Altogether, these results demonstrate the existence of dosage-sensitive genes in the Prmt2-Col6a1 region that control the inflammation and the lung function. Furthermore, they point out that similar partial Monosomies 21 in human might have eluded the diagnosis due to the very specific defects observed in this murine model.

Electron microscopy studies on luteovirid transmission by aphids

Transmission electron microscopy (TEM) observations have been extensively applied to follow the route of luteovirids in their vectors. Luteovirids are icosahedral plant viruses which are phloem-limited and strictly transmitted in a circulative manner by aphids. Virus particles, acquired by aphids while feeding on an infected plant, circulate in the aphid's body without replication and are internalized during this process in two different cell types (intestinal and accessory salivary gland cells). The endocytosis mechanism at the gut level seems to rely on a clathrin-mediated entry process and virions are observed in the aphid's gut cells in various vesicular structures. After exocytosis from intestinal cells, virions are released in the aphid's body cavity where they are thought to bind to symbionin, an endosymbiotic protein. Transcytosis of the accessory salivary gland cells occurs similarly as at the gut level but in the reverse direction. Using engineered virus mutants, viral proteins required for transmission and involved in virus retention in the hemocoel have been identified. Virus mutants poorly or non aphid-transmitted have also been localized in the aphid's body by TEM. These observations reveal the crucial role of the minor capsid protein in gut internalization. While not strictly required, this protein seems to play an important role in the efficiency of this process by interacting with putative virus receptors localized on the gut apical membrane. More recently, some aphid proteins have also been shown to exhibit in vitro virus binding capacity and could potentially be components of the endocytotic apparatus.

Cre/loxP-mediated chromosome engineering of the mouse genome

Together with numerous other genome modifications, chromosome engineering offers a very powerful tool to accelerate the functional analysis of the mammalian genome. The technology, based on the Cre/loxP system, is used more and more in the scientific community in order to generate new chromosomes carrying deletions, duplications, inversions and translocations in targeted regions of interest. In this review, we will present the basic principle of the technique either in vivo or in vitro and we will briefly describe some applications to provide highly valuable genetic tools, to decipher the mammalian genome organisation and to analyze human diseases in the mouse.

Modeling chromosomes in mouse to explore the function of genes, genomic disorders, and chromosomal organization

One of the challenges of genomic research after the completion of the human genome project is to assign a function to all the genes and to understand their interactions and organizations. Among the various techniques, the emergence of chromosome engineering tools with the aim to manipulate large genomic regions in the mouse model offers a powerful way to accelerate the discovery of gene functions and provides more mouse models to study normal and pathological developmental processes associated with aneuploidy. The combination of gene targeting in ES cells, recombinase technology, and other techniques makes it possible to generate new chromosomes carrying specific and defined deletions, duplications, inversions, and translocations that are accelerating functional analysis. This review presents the current status of chromosome engineering techniques and discusses the different applications as well as the implication of these new techniques in future research to better understand the function of chromosomal organization and structures.

F-box-like domain in the polerovirus protein P0 is required for silencing suppressor function

Plants employ small RNA-mediated posttranscriptional gene silencing as a virus defense mechanism. In response, plant viruses encode proteins that can suppress RNA silencing, but the mode of action of most such proteins is poorly understood. Here, we show that the silencing suppressor protein P0 of two Arabidopsis-infecting poleroviruses interacts by means of a conserved minimal F-box motif with Arabidopsis thaliana orthologs of S-phase kinase-related protein 1 (SKP1), a component of the SCF family of ubiquitin E3 ligases. Point mutations in the F-box-like motif abolished the P0-SKP1 ortholog interaction, diminished virus pathogenicity, and inhibited the silencing suppressor activity of P0. Knockdown of expression of a SKP1 ortholog in Nicotiana benthamiana rendered the plants resistant to polerovirus infection. Together, the results support a model in which P0 acts as an F-box protein that targets an essential component of the host posttranscriptional gene silencing machinery.

The polerovirus minor capsid protein determines vector specificity and intestinal tropism in the aphid

Aphid transmission of poleroviruses is highly specific, but the viral determinants governing this specificity are unknown. We used a gene exchange strategy between two poleroviruses with different vectors, Beet western yellows virus (BWYV) and Cucurbit aphid-borne yellows virus (CABYV), to analyze the role of the major and minor capsid proteins in vector specificity. Virus recombinants obtained by exchanging the sequence of the readthrough domain (RTD) between the two viruses replicated in plant protoplasts and in whole plants. The hybrid readthrough protein of chimeric viruses was incorporated into virions. Aphid transmission experiments using infected plants or purified virions revealed that vector specificity is driven by the nature of the RTD. BWYV and CABYV have specific intestinal sites in the vectors for endocytosis: the midgut for BWYV and both midgut and hindgut for CABYV. Localization of hybrid virions in aphids by transmission electron microscopy revealed that gut tropism is also determined by the viral origin of the RTD.

Posterior midgut and hindgut are both sites of acquisition of Cucurbit aphid-borne yellows virus in Myzus persicae and Aphis gossypii

Members of the family Luteoviridae ('luteovirids') rely strictly on aphid vectors for plant-to-plant transmission. This interaction operates according to a persistent and circulative manner, which implies that the virions are being endocytosed and exocytosed across two epithelial barriers (alimentary tract and accessory salivary glands) in the vector's body. In several luteovirid-aphid vector species combinations, the route of virions in the insect has been investigated ultrastructurally by transmission electron microscopy (TEM). Here, we used TEM to follow the route of Cucurbit aphid-borne yellows virus (CABYV; genus Polerovirus) in its two efficient vector species, Myzus persicae and Aphis gossypii. We demonstrated that CABYV particles are acquired from the gut lumen to the haemocoel through two different sites in both aphid species, i.e. the posterior midgut (as for Beet western yellows virus in M. persicae) and the hindgut (as for Barley yellow dwarf virus complex in cereal aphids). This 'dual' tissue specificity of CABYV represents an original situation among viruses in the family Luteoviridae examined so far by TEM. A variety of virion-containing structures (e.g. clathrin-coated and tubular vesicles, endosome-like bodies) are found in intestinal cells of both types in both aphids. Release of virus particles from midgut and hindgut cells into the haemolymph was confirmed by immunotrapping using CABYV-specific antibodies. In accessory salivary glands, transport of CABYV virions across the cells was similar in each aphid species, and occurred by a transcytosis mechanism involving formation of tubular and coated vesicles before release of free virions in the salivary canal.