Nuclear Proximity of Mtr4 to RNA Exosome Restricts DNA Mutational Asymmetry.

The distribution of sense and antisense strand DNA mutations on transcribed duplex DNA contributes to the development of immune and neural systems along with the progression of cancer. Because developmentally matured B cells undergo biologically programmed strand-specific DNA mutagenesis at focal DNA/RNA hybrid structures, they make a convenient system to investigate strand-specific mutagenesis mechanisms. We demonstrate that the sense and antisense strand DNA mutagenesis at the immunoglobulin heavy chain locus and some other regions of the B cell genome depends upon localized RNA processing protein complex formation in the nucleus. Both the physical proximity and coupled activities of RNA helicase Mtr4 (and senataxin) with the noncoding RNA processing function of RNA exosome determine the strand-specific distribution of DNA mutations. Our study suggests that strand-specific DNA mutagenesis-associated mechanisms will play major roles in other undiscovered aspects of organismic development.

SUPPRESSOR OF MAX2 1-LIKE (SMXL) homologs are MAX2-dependent repressors of Physcomitrium patens growth.

SUPPRESSOR OF MAX2 (SMAX)1-LIKE (SMXL) proteins are a plant-specific clade of type I HSP100/Clp-ATPases. SMXL genes are present in virtually all land plant genomes. However, they have mainly been studied in angiosperms. In Arabidopsis (Arabidopsis thaliana), 3 functional SMXL subclades have been identified: SMAX1/SMXL2, SMXL345, and SMXL678. Of these, 2 subclades ensure endogenous phytohormone signal transduction. SMAX1/SMXL2 proteins are involved in KAI2 ligand (KL) signaling, while SMXL678 proteins are involved in strigolactone (SL) signaling. Many questions remain regarding the mode of action of these proteins, as well as their ancestral roles. We addressed these questions by investigating the functions of the 4 SMXL genes in the moss Physcomitrium patens. We demonstrate that PpSMXL proteins are involved in the conserved ancestral MAX2-dependent KL signaling pathway and negatively regulate growth. However, PpSMXL proteins expressed in Arabidopsis cannot replace SMAX1 or SMXL2 function in KL signaling, whereas they can functionally replace SMXL4 and SMXL5 and restore root growth. Therefore, the molecular functions of SMXL proteins are conserved, but their interaction networks are not. Moreover, the PpSMXLC/D clade positively regulates SL signal transduction in P. patens. Overall, our data reveal that SMXL proteins in moss mediate crosstalk between the SL and KL signaling pathways.

Chemistry of Strigolactones, Key Players in Plant Communication.

Today, the use of artificial pesticides is questionable and the adaptation to global warming is a necessity. The promotion of favorable natural interactions in the rhizosphere offers interesting perspectives for changing the type of agriculture. Strigolactones (SLs), the latest class of phytohormones to be discovered, are also chemical mediators in the rhizosphere. We present in this review the diversity of natural SLs, their analogs, mimics, and probes essential for the biological studies of this class of compounds. Their biosynthesis and access by organic synthesis are highlighted especially concerning noncanonical SLs, the more recently discovered natural SLs. Organic synthesis of analogs, stable isotope-labeled standards, mimics, and probes are also reviewed here. In the last part, the knowledge about the SL perception is described as well as the different inhibitors of SL receptors that have been developed.

The Physcomitrium (Physcomitrella) patens PpKAI2L receptors for strigolactones and related compounds function via MAX2-dependent and -independent pathways.

In angiosperms, the α/ß hydrolase DWARF14 (D14), along with the F-box protein MORE AXILLARY GROWTH2 (MAX2), perceives strigolactones (SL) to regulate developmental processes. The key SL biosynthetic enzyme CAROTENOID CLEAVAGE DIOXYGENASE8 (CCD8) is present in the moss Physcomitrium patens, and PpCCD8-derived compounds regulate moss extension. The PpMAX2 homolog is not involved in the SL response, but 13 PpKAI2LIKE (PpKAI2L) genes homologous to the D14 ancestral paralog KARRIKIN INSENSITIVE2 (KAI2) encode candidate SL receptors. In Arabidopsis thaliana, AtKAI2 perceives karrikins and the elusive endogenous KAI2-Ligand (KL). Here, germination assays of the parasitic plant Phelipanche ramosa suggested that PpCCD8-derived compounds are likely noncanonical SLs. (+)-GR24 SL analog is a good mimic for PpCCD8-derived compounds in P. patens, while the effects of its enantiomer (-)-GR24, a KL mimic in angiosperms, are minimal. Interaction and binding assays of seven PpKAI2L proteins pointed to the stereoselectivity toward (-)-GR24 for a single clade of PpKAI2L (eu-KAI2). Enzyme assays highlighted the peculiar behavior of PpKAI2L-H. Phenotypic characterization of Ppkai2l mutants showed that eu-KAI2 genes are not involved in the perception of PpCCD8-derived compounds but act in a PpMAX2-dependent pathway. In contrast, mutations in PpKAI2L-G, and -J genes abolished the response to the (+)-GR24 enantiomer, suggesting that PpKAI2L-G, and -J proteins are receptors for moss SLs.

IgH 3' regulatory region increases ectopic class switch recombination.

DNA lesions inflicted by activation-induced deaminase (AID) instrumentally initiate the processes reshaping immunoglobulin genes in mature B-cells, from local somatic hypermutation (SHM) to junctions of distant breaks during class switch recombination (CSR). It remains incompletely understood how these divergent outcomes of AID attacks are differentially and temporally focused, with CSR strictly occurring in the Ig heavy chain (IgH) locus while SHM concentrates on rearranged V(D)J regions in the IgH and Ig light chain loci. In the IgH locus, disruption of either the 3'Regulatory Region (3'RR) super-enhancer or of switch (S) regions preceding constant genes, profoundly affects CSR. Reciprocally, we now examined if these elements are sufficient to induce CSR in a synthetic locus based on the Igκ locus backbone. Addition of a surrogate "core 3'RR" (c3'RR) and of a pair of transcribed and spliced Switch regions, together with a reporter system for "κ-CSR" yielded a switchable Igκ locus. While the c3'RR stimulated SHM at S regions, it also lowered the local SHM threshold necessary for switch recombination to occur. The 3'RR thus both helps recruit AID to initiate DNA lesions, but then also promotes their resolution through long-distance synapses and recombination following double-strand breaks.

Three mutations repurpose a plant karrikin receptor to a strigolactone receptor.

Uncovering the basis of small-molecule hormone receptors' evolution is paramount to a complete understanding of how protein structure drives function. In plants, hormone receptors for strigolactones are well suited to evolutionary inquiries because closely related homologs have different ligand preferences. More importantly, because of facile plant transgenic systems, receptors can be swapped and quickly assessed functionally in vivo. Here, we show that only three mutations are required to turn the nonstrigolactone receptor, KAI2, into a receptor that recognizes the plant hormone strigolactone. This modified receptor still retains its native function to perceive KAI2 ligands. Our directed evolution studies indicate that only a few keystone mutations are required to increase receptor promiscuity of KAI2, which may have implications for strigolactone receptor evolution in parasitic plants.

Histone Deacetylases Regulate MORE AXILLARY BRANCHED 2-Dependent Germination of Arabidopsis thaliana.

Under specific conditions, the germination of Arabidopsis thaliana is dependent on the activation of the KARRIKIN INSENSITIVE 2 (KAI2) signaling pathway by the KAI2-dependent perception of karrikin or the artificial strigolactone analogue, rac-GR24. To regulate the induction of germination, the KAI2 signaling pathway relies on MORE AXILLARY BRANCHED 2- (MAX2-)dependent ubiquitination and proteasomal degradation of the repressor protein SUPPRESSOR OF MAX2 1 (SMAX1). It is not yet known how the degradation of SMAX1 proteins eventually results in the regulation of seed germination, but it has been hypothesized that SMAX1-LIKE generally functions as transcriptional repressors through the recruitment of co-repressors TOPLESS (TPL) and TPL-related, which in turn interact with histone deacetylases. In this article, we show the involvement of histone deacetylases HDA6, HDA9, HDA19 and HDT1 in MAX2-dependent germination of Arabidopsis, and more specifically, that HDA6 is required for the induction of DWARF14-LIKE2 expression in response to rac-GR24 treatment.

Unraveling the MAX2 Protein Network in Arabidopsis thaliana: Identification of the Protein Phosphatase PAPP5 as a Novel MAX2 Interactor.

The F-box protein MORE AXILLARY GROWTH 2 (MAX2) is a central component in the signaling cascade of strigolactones (SLs) as well as of the smoke-derived karrikins (KARs) and the so far unknown endogenous KAI2 ligand (KL). The two groups of molecules are involved in overlapping and unique developmental processes, and signal-specific outcomes are attributed to perception by the paralogous α/ß-hydrolases DWARF14 (D14) for SL and KARRIKIN INSENSITIVE 2/HYPOSENSITIVE TO LIGHT (KAI2/HTL) for KAR/KL. In addition, depending on which receptor is activated, specific members of the SUPPRESSOR OF MAX2 1 (SMAX1)-LIKE (SMXL) family control KAR/KL and SL responses. As proteins that function in the same signal transduction pathway often occur in large protein complexes, we aimed at discovering new players of the MAX2, D14, and KAI2 protein network by tandem affinity purification in Arabidopsis cell cultures. When using MAX2 as a bait, various proteins were copurified, among which were general components of the Skp1-Cullin-F-box complex and members of the CONSTITUTIVE PHOTOMORPHOGENIC 9 signalosome. Here, we report the identification of a novel interactor of MAX2, a type 5 serine/threonine protein phosphatase, designated PHYTOCHROME-ASSOCIATED PROTEIN PHOSPHATASE 5 (PAPP5). Quantitative affinity purification pointed at PAPP5 as being more present in KAI2 rather than in D14 protein complexes. In agreement, mutant analysis suggests that PAPP5 modulates KAR/KL-dependent seed germination under suboptimal conditions and seedling development. In addition, a phosphopeptide enrichment experiment revealed that PAPP5 might dephosphorylate MAX2 in vivo independently of the synthetic SL analog, rac-GR24. Together, by analyzing the protein complexes to which MAX2, D14, and KAI2 belong, we revealed a new MAX2 interactor, PAPP5, that might act through dephosphorylation of MAX2 to control mainly KAR/KL-related phenotypes and, hence, provide another link with the light pathway.

Lotus japonicus karrikin receptors display divergent ligand-binding specificities and organ-dependent redundancy.

Karrikins (KARs), smoke-derived butenolides, are perceived by the α/ß-fold hydrolase KARRIKIN INSENSITIVE2 (KAI2) and thought to mimic endogenous, yet elusive plant hormones tentatively called KAI2-ligands (KLs). The sensitivity to different karrikin types as well as the number of KAI2 paralogs varies among plant species, suggesting diversification and co-evolution of ligand-receptor relationships. We found that the genomes of legumes, comprising a number of important crops with protein-rich, nutritious seed, contain two or more KAI2 copies. We uncover sub-functionalization of the two KAI2 versions in the model legume Lotus japonicus and demonstrate differences in their ability to bind the synthetic ligand GR24ent-5DS in vitro and in genetic assays with Lotus japonicus and the heterologous Arabidopsis thaliana background. These differences can be explained by the exchange of a widely conserved phenylalanine in the binding pocket of KAI2a with a tryptophan in KAI2b, which arose independently in KAI2 proteins of several unrelated angiosperms. Furthermore, two polymorphic residues in the binding pocket are conserved across a number of legumes and may contribute to ligand binding preferences. The diversification of KAI2 binding pockets suggests the occurrence of several different KLs acting in non-fire following plants, or an escape from possible antagonistic exogenous molecules. Unexpectedly, L. japonicus responds to diverse synthetic KAI2-ligands in an organ-specific manner. Hypocotyl growth responds to KAR1, KAR2 and rac-GR24, while root system development responds only to KAR1. This differential responsiveness cannot be explained by receptor-ligand preferences alone, because LjKAI2a is sufficient for karrikin responses in the hypocotyl, while LjKAI2a and LjKAI2b operate redundantly in roots. Instead, it likely reflects differences between plant organs in their ability to transport or metabolise the synthetic KLs. Our findings provide new insights into the evolution and diversity of butenolide ligand-receptor relationships, and open novel research avenues into their ecological significance and the mechanisms controlling developmental responses to divergent KLs.

Transcriptional Analysis in the Arabidopsis Roots Reveals New Regulators that Link rac-GR24 Treatment with Changes in Flavonol Accumulation, Root Hair Elongation and Lateral Root Density.

The synthetic strigolactone (SL) analog, rac-GR24, has been instrumental in studying the role of SLs as well as karrikins because it activates the receptors DWARF14 (D14) and KARRIKIN INSENSITIVE 2 (KAI2) of their signaling pathways, respectively. Treatment with rac-GR24 modifies the root architecture at different levels, such as decreasing the lateral root density (LRD), while promoting root hair elongation or flavonol accumulation. Previously, we have shown that the flavonol biosynthesis is transcriptionally activated in the root by rac-GR24 treatment, but, thus far, the molecular players involved in that response have remained unknown. To get an in-depth insight into the changes that occur after the compound is perceived by the roots, we compared the root transcriptomes of the wild type and the more axillary growth2 (max2) mutant, affected in both SL and karrikin signaling pathways, with and without rac-GR24 treatment. Quantitative reverse transcription (qRT)-PCR, reporter line analysis and mutant phenotyping indicated that the flavonol response and the root hair elongation are controlled by the ELONGATED HYPOCOTYL 5 (HY5) and MYB12 transcription factors, but HY5, in contrast to MYB12, affects the LRD as well. Furthermore, we identified the transcription factors TARGET OF MONOPTEROS 5 (TMO5) and TMO5 LIKE1 as negative and the Mediator complex as positive regulators of the rac-GR24 effect on LRD. Altogether, hereby, we get closer toward understanding the molecular mechanisms that underlay the rac-GR24 responses in the root.

A structural homologue of the plant receptor D14 mediates responses to strigolactones in the fungal phytopathogen Cryphonectria parasitica.

Strigolactones (SLs) are plant hormones and important signalling molecules required to promote arbuscular mycorrhizal (AM) symbiosis. While in plants an α/ß-hydrolase, DWARF14 (D14), was shown to act as a receptor that binds and cleaves SLs, the fungal receptor for SLs is unknown. Since AM fungi are currently not genetically tractable, in this study, we used the fungal pathogen Cryphonectria parasitica, for which gene deletion protocols exist, as a model, as we have previously shown that it responds to SLs. By means of computational, biochemical and genetic analyses, we identified a D14 structural homologue, CpD14. Molecular homology modelling and docking support the prediction that CpD14 interacts with and hydrolyses SLs. The recombinant CpD14 protein shows α/ß hydrolytic activity in vitro against the SLs synthetic analogue GR24; its enzymatic activity requires an intact Ser/His/Asp catalytic triad. CpD14 expression in the d14-1 loss-of-function Arabidopsis thaliana line did not rescue the plant mutant phenotype. However, gene inactivation by knockout homologous recombination reduced fungal sensitivity to SLs. These results indicate that CpD14 is involved in SLs responses in C. parasitica and strengthen the role of SLs as multifunctional molecules acting in plant-microbe interactions.

Immunoglobulin light-chain toxicity in a mouse model of monoclonal immunoglobulin light-chain deposition disease.

Light chain (LC) deposition disease (LCDD) is a rare disorder characterized by glomerular and peritubular amorphous deposits of a monoclonal immunoglobulin LC, leading to nodular glomerulosclerosis and nephrotic syndrome. We developed a transgenic model using site-directed insertion of the variable domain of a pathogenic human LC gene into the mouse immunoglobulin κ locus, ensuring its production by all plasma cells (PCs). High free LC levels were achieved after backcrossing with mice presenting increased PC differentiation and no immunoglobulin heavy chain production. Our mouse model recapitulates the characteristic features of LCDD, including progressive glomerulosclerosis, nephrotic-range proteinuria, and finally kidney failure. The variable domain of the LC bears alone the structural properties involved in its pathogenicity. RNA sequencing conducted on PCs demonstrated that LCDD LC induces endoplasmic reticulum stress, likely accounting for the high efficiency of proteasome inhibitor-based therapy. Accordingly, reduction of circulating pathogenic LC was efficiently achieved and not only preserved renal function but also partially reversed kidney lesions. Finally, transcriptome analysis of presclerotic glomeruli revealed that proliferation and extracellular matrix remodeling represented the first steps of glomerulosclerosis, paving the way for future therapeutic strategies in LCDD and other kidney diseases featuring diffuse glomerulosclerosis, particularly diabetic nephropathy.

MAX2-dependent competence for callus formation and shoot regeneration from Arabidopsis thaliana root explants.

Although the division of the pericycle cells initiates both lateral root development and root-derived callus formation, these developmental processes are affected differently in the strigolactone and karrikin/KARRIKIN INSENSITIVE 2 (KAI2) ligand signalling mutant more axillary growth 2 (max2). Whereas max2 produces more lateral roots than the wild type, it is defective in the regeneration of shoots from root explants. We suggest that the decreased shoot regeneration of max2 originates from delayed formation of callus primordium, yielding less callus material to regenerate shoots. Indeed, when incubated on callus-inducing medium, the pericycle cell division was reduced in max2 and the early gene expression varied when compared with the wild type, as determined by a transcriptomics analysis. Furthermore, the expression of the LATERAL ORGAN BOUNDARIES DOMAIN genes and of callus-induction genes was modified in correlation with the max2 phenotype, suggesting a role for MAX2 in the regulation of the interplay between cytokinin, auxin, and light signalling in callus initiation. Additionally, we found that the in vitro shoot regeneration phenotype of max2 might be caused by a defect in KAI2, rather than in DWARF14, signalling. Nevertheless, the shoot regeneration assays revealed that the strigolactone biosynthesis mutants max3 and max4 also play a minor role.

Noncanonical Strigolactone Analogues Highlight Selectivity for Stimulating Germination in Two Phelipanche ramosa Populations.

Strigolactones (SLs) are plant hormones exuded in the rhizosphere with a signaling role for the development of arbuscular mycorrhizal (AM) fungi and as stimulants of seed germination of the parasitic weeds Orobanche, Phelipanche, and Striga, the most threatening weeds of major crops worldwide. Phelipanche ramosa is present mainly on rape, hemp, and tobacco in France. P. ramosa 2a preferentially attacks hemp, while P. ramosa 1 attacks rapeseed. The recently isolated cannalactone (14) from hemp root exudates has been characterized as a noncanonical SL that selectively stimulates the germination of P. ramosa 2a seeds in comparison with P. ramosa 1. In the present work, (-)-solanacol (5), a canonical orobanchol-type SL exuded by tobacco and tomato, was established to possess a remarkable selective germination stimulant activity for P. ramosa 2a seeds. Two cannalactone analogues, named (±)-SdL19 and (±)-SdL118, have been synthesized. They have an unsaturated acyclic carbon chain with a tertiary hydroxy group and a methyl or a cyclopropyl group instead of a cyclohexane A-ring, respectively. (±)-SdL analogues are able to selectively stimulate P. ramosa 2a, revealing that these minimal structural elements are key for this selective bioactivity. In addition, (±)-SdL19 is able to inhibit shoot branching in Pisum sativum and Arabidopsis thaliana and induces hyphal branching in the AM fungus Rhizophagus irregularis, like SLs.

Locus suicide recombination actively occurs on the functionally rearranged IgH allele in B-cells from inflamed human lymphoid tissues.

B-cell activation yields abundant cell death in parallel to clonal amplification and remodeling of immunoglobulin (Ig) genes by activation-induced deaminase (AID). AID promotes affinity maturation of Ig variable regions and class switch recombination (CSR) in mature B lymphocytes. In the IgH locus, these processes are under control of the 3' regulatory region (3'RR) super-enhancer, a region demonstrated in the mouse to be both transcribed and itself targeted by AID-mediated recombination. Alternatively to CSR, IgH deletions joining Sµ to "like-switch" DNA repeats that flank the 3' super-enhancer can thus accomplish so-called "locus suicide recombination" (LSR) in mouse B-cells. Using an optimized LSR-seq high throughput method, we now show that AID-mediated LSR is evolutionarily conserved and also actively occurs in humans, providing an activation-induced cell death pathway in multiple conditions of B-cell activation. LSR either focuses on the functional IgH allele or is bi-allelic, and its signature is mainly detected when LSR is ongoing while it vanishes from fully differentiated plasma cells or from "resting" blood memory B-cells. Highly diversified breakpoints are distributed either within the upstream (3'RR1) or downstream (3'RR2) copies of the IgH 3' super-enhancer and all conditions activating CSR in vitro also seem to trigger LSR although TLR ligation appeared the most efficient. Molecular analysis of breakpoints and junctions confirms that LSR is AID-dependent and reveals junctional sequences somehow similar to CSR junctions but with increased usage of microhomologies.

Nanoscopic interactions of colloidal particles can suppress millimetre drop splashing.

The splashing of liquid drops onto a solid surface is important for a wide range of applications, including combustion and spray coating. As the drop hits the solid surface, the liquid is ejected into a thin horizontal sheet expanding radially over the substrate. Above a critical impact velocity, the liquid sheet is forced to separate from the solid surface by the ambient air, and breaks up into smaller droplets. Despite many applications involving complex fluids, their effects on splashing remain mostly unexplored. Here we show that the splashing of a nanoparticle dispersion can be suppressed at higher impact velocities by the interactions of the nanoparticles with the solid surface. Although the dispersion drop first shows the classical transition from deposition to splashing when increasing the impact velocity, no splashing is observed above a second higher critical impact velocity. This result goes against the commonly accepted understanding of splashing, that a higher impact velocity should lead to even more pronounced splashing. Our findings open new possibilities to deposit large amount of complex liquids at high speeds.

Design and visualization of second-generation cyanoisoindole-based fluorescent strigolactone analogs.

Strigolactones (SLs) are a family of terpenoid allelochemicals that were recognized as plant hormones only a decade ago. They influence a myriad of both above- and below-ground developmental processes, and are an important survival strategy for plants in nutrient-deprived soils. A rapidly emerging approach to gain knowledge on hormone signaling is the use of traceable analogs. A unique class of labeled SL analogs was constructed, in which the original tricyclic lactone moiety of natural SLs is replaced by a fluorescent cyanoisoindole ring system. Biological evaluation as parasitic seed germination stimulant and hypocotyl elongation repressor proved the potency of the cyanoisoindole strigolactone analogs (CISAs) to be comparable to the commonly accepted standard GR24. Additionally, via a SMXL6 protein degradation assay, we provided molecular evidence that the compounds elicit SL-like responses through the natural signaling cascade. All CISAs were shown to exhibit fluorescent properties, and the high quantum yield and Stokes shift of the pyrroloindole derivative CISA-7 also enabled in vivo visualization in plants. In contrast to the previously reported fluorescent analogs, CISA-7 displays a large similarity in shape and structure with natural SLs, which renders the analog a promising tracer to investigate the spatiotemporal distribution of SLs in plants and fungi.

Detecting Rare AID-Induced Mutations in B-Lineage Oncogenes from High-Throughput Sequencing Data Using the Detection of Minor Variants by Error Correction Method.

In B-lineage cells, the cytidine deaminase AID not only generates somatic mutations to variable regions of Ig genes but also inflicts, at a lower frequency, mutations to several non-Ig genes named AID off-targets, which include proto-oncogenes. High-throughput sequencing should be in principle the method of choice to detect and document these rare nucleotide substitutions. So far, high-throughput sequencing-based methods are impaired by a global sequencing error rate that usually covers the real mutation rate of AID off-target genes in activated B cells. We demonstrate the validity of a per-base background subtraction method called detection of minor variants by error correction (DeMinEr), which uses deep sequencing data from mutated and nonmutated samples to correct the substitution frequency at each nucleotide position along the sequenced region. Our DeMinEr method identifies somatic mutations at a frequency down to 0.02% at any nucleotide position within two off-target genes: Cd83 and Bcl6 Biological models and control conditions such as AID- and UNG-deficient mice validate the specificity and the sensitivity of our method. The high resolution and robustness of DeMinEr enable us to document fine effects such as age-dependent accumulation of mutations in these oncogenes in the mouse.

Mesangial Deposition Can Strongly Involve Innate-Like IgA Molecules Lacking Affinity Maturation.

BACKGROUND: IgA nephropathy (IgAN) often follows infections and features IgA mesangial deposition. Polymeric IgA deposits in the mesangium seem to have varied pathogenic potential, but understanding their pathogenicity remains a challenge. Most mesangial IgA1 in human IgAN has a hypogalactosylated hinge region, but it is unclear whether this is required for IgA deposition. Another important question is the role of adaptive IgA responses and high-affinity mature IgA antibodies and whether low-affinity IgA produced by innate-like B cells might also yield mesangial deposits. METHODS: To explore the effects of specific qualitative variations in IgA and whether altered affinity maturation can influence IgA mesangial deposition and activate complement, we used several transgenic human IgA1-producing models with IgA deposition, including one lacking the DNA-editing enzyme activation-induced cytidine deaminase (AID), which is required in affinity maturation. Also, to explore the potential role of the IgA receptor CD89 in glomerular inflammation, we used a model that expresses CD89 in a pattern observed in humans. RESULTS: We found that human IgA induced glomerular damage independent of CD89. When comparing mice able to produce high-affinity IgA antibodies with mice lacking AID-enabled Ig affinity maturation, we found that IgA deposition and complement activation significantly increased and led to IgAN pathogenesis, although without significant proteinuria or hematuria. We also observed that hinge hypoglycosylation was not mandatory for IgA deposition. CONCLUSIONS: In a mouse model of IgAN, compared with high-affinity IgA, low-affinity innate-like IgA, formed in the absence of normal antigen-driven maturation, was more readily involved in IgA glomerular deposition with pathogenic effects.

Initiation of arbuscular mycorrhizal symbiosis involves a novel pathway independent from hyphal branching.

The arbuscular mycorrhizal symbiosis is a very common association between plant roots and soil fungi, which greatly contributes to plant nutrition. Root-exuded compounds known as strigolactones act as symbiotic signals stimulating the fungus prior to root colonization. Strigolactones also play an endogenous role in planta as phytohormones and contribute to the regulation of various developmental traits. Structure-activity relationship studies have revealed both similarities and differences between the structural features required for bioactivity in plants and arbuscular mycorrhizal fungi. In the latter case, bioassays usually measured a stimulation of hyphal branching on isolated fungi of the Gigaspora genus, grown in vitro. Here, we extended these investigations with a bioassay that evaluates the bioactivity of strigolactone analogs in a symbiotic situation and the use of the model mycorrhizal fungus Rhizophagus irregularis. Some general structural requirements for bioactivity reported previously for Gigaspora were confirmed. We also tested additional strigolactone analogs bearing modifications on the conserved methylbutenolide ring, a key element of strigolactone perception by plants. A strigolactone analog with an unmethylated butenolide ring could enhance the ability of R. irregularis to colonize host roots. Surprisingly, when applied to the isolated fungus in vitro, this compound stimulated germ tube elongation but inhibited hyphal branching. Therefore, this compound was able to act on the fungal and/or plant partner to facilitate initiation of the arbuscular mycorrhizal symbiosis, independently from hyphal branching and possibly from the strigolactone pathway.