Effects of irradiance during growth on tolerance of geranium to sub- and supra-optimal boron supply

In our previous study on geranium, we showed that increases in growth irradiance from sub-optimal to near-optimal could delay boron deficiency effects on photosynthesis. In this study, we further investigated the effects of growth irradiance on tolerance to B stress by growing geranium (Pelargonium × hortorum cv. Maverick White) under sub- to supra-optimal B concentrations (4.5, 45, and 450 μM) and under three irradiances of 100, 300, or 500 μmol m^?2 s^?1 PAR for 30 d. In general, at low and medium irradiances, sub- and supra-optimal B availability decreased root and shoot dry masses, but at high irradiance, the B stress was prevented. Net photosynthetic rate decreased by the supra-optimal B concentration at the high irradiance only suggesting B-related photoinhibition. Tissue B content and root specific B uptake only modestly decreased by the low B treatment, but increased greatly by the high B availability, and the higher irradiance decreased the tissue B content and the root B uptake only at the low and medium B supplies. Interestingly, the increases in irradiance decreased the content and uptake of all other nutrients, except Fe uptake. Effects of the B stress on the content of other nutrients were variable, but the B stress often exacerbated decreases in nutrient content with the increasing irradiance which would be especially important under nutrient-limiting conditions. Hence, in this study, the B stress effects on growth were mitigated by the increases in growth irradiance, which offset negative effects on physiology, and the protective effects of irradiance were likely caused by its positive effects on plant carbon/energy status rather than on tissue B content or B uptake.     

Boron deficiency in maize

Boron (B) deficiency depresses wheat, barley and triticale yield through male sterility. On the basis of field responses to B fertilization, maize (Zea mays L.) is affected by B deficiency in five continents. In a series of sand culture trials with maize subject to B0 (nil added B) and B20 (20???M added B) treatments, we described how B deficiency depressed maize grain yield while showing an imperceptible effect on vegetative dry weight. With manual application of pollen to the silk of each plant, B0 plants produced 0.4 grain ear^?1 compared with 410 grains ear^?1 in B20 plants. Symptoms of B deficiency was observed only in B0 plants, which exhibited symptoms of narrow white to transparent lengthwise streaks on leaves, multiple but small and abnormal ears with very short silk, small tassels with some branches emerging dead, and small, shrivelled anthers devoid of pollen. Tassels, silk and pollen of B0 plants contained only 3?C4?mg B kg^?1 DW compared with twice or more B in these reproductive tissues in B20 plants. A cross-fertilization experiment showed that, although the tassels and pollen were more affected, the silk was more sensitive to B deficiency. Pollen from B20 plants applied to B0 silk produced almost no grains, while pollen from B0 on B20 silk increased the number of grains to 37% of the 452 grains plant^?1 produced from B20 pollen on B20 silk. Therefore, the silk of the first ear may be targeted for precise diagnosis of B status at maize reproduction, for timely correction by foliar B application, and even for B-efficient genotype selection.     

Biocontrol of root-knot nematode Meloidogyne incognita by the isolates of Bacillus on tomato

Fifteen isolates of Bacillus, isolated from the root-knot nematode suppressive soils, were used for the biocontrol of Meloidogyne incognita on tomato. Bacillus isolates B1, B4, B5 and B11 caused greater inhibitory effect on hatching of M. incognita than caused by other isolates. In addition, these isolates (B1, B4, B5 and B11) caused greater colonisation of tomato roots and also caused greater increase in the growth of tomato seedling than caused by other isolates. All the isolates of Bacillus were able to increase growth of tomato and caused reduction in galling and nematode multiplication in green house tests. Isolates B1, B4, B5 and B11 caused a greater increase in growth of tomato and higher reduction in galling and nematode multiplication than other isolates in a green house test. These isolates were also tested for hydrogen cyanide (HCN) and indole acetic acid productions. Only one isolate (B13) produced HCN out of 15 tested. On the other hand, isolates B5, B11, B4 and B1 showed greater production of IAA than the other 11 isolates tested. This study suggests that Bacillus isolates B5, B11, B4 and B1 may be used for the biocontrol of M. incognita on tomato.     

Utilization of F1 monosomics for genetic analyses involving awn expression,glume color,seed setting,and seed abortion in crosses of tetraploid and hexaploid wheats

Summary F₁ plants, monosomic for chromosomes 1A to 7B, from crosses of three lines of Triticum durum var. Khapli with the Chinese series were investigated together with their backcrosses to normal Chinese Spring. The three Khapli lines were designated K1-A, K1-B, and K1-D. Five parameters were analyzed: awn development, glume color, degree of selfing, crossing ability, and seed abortion.Morphological examination of F₁ monopentaploid plants revealed that, in the three lines, chromosomes 5A, 1B, 3B, 4B, 5B, and 6B had promotor genes and 2A, 6A, and 2B had inhibitor genes for awn development. Results on glume color suggested the presence of at least three inhibitor genes on 1B, 5B, and 7B, and three promotor genes on 3A, 6A, and 6B chromosomes of Chinese Spring.The first backcross of interspecific hybrids seemed to indicate that some chromosomes (1A, 1B, and 4B) originating from the Khapli lines possessed promotor genes, others (2B and 4A) carried inhibitor genes for seed setting. Similarily, the genetic factor (s) carried by chromosome 5A increased seed abortion whereas those on 2B and 6B reduced it.Self fertility in K1-D hybrids was probably the result of the inhibitor factor (s) on 7A and promotor genes on 3B, 4B, 5B, and 6B chromosomes coming from K1-D.     

Long‐term monitoring of B‐chromosome invasion and neutralization in a population of Prospero autumnale (Asparagaceae)

B chromosomes have been reported in about 15% of eukaryotes, but long‐term dynamics of B chromosomes in a single natural population has rarely been analyzed. Prospero autumnale plants collected in 1981 and 1983 at Cuesta de La Palma population had shown the presence of B chromosomes. We analyze here seven additional samples collected between 1987 and 2015, and show that B frequency increased significantly during the 1980s and showed minor fluctuations between 2005 and 2015. A mother–offspring analysis of B chromosome transmission, at population level, showed significant drive on the male side (k_B = 0.65) and significant drag on the female side (k_B = 0.33), with average B transmission rate being very close to the Mendelian rate (0.5). No significant effects of B chromosomes were observed on a number of vigor and fertility‐related traits. Within a parasite/host framework, these results suggest that B chromosomes’ drive on the male side is the main pathway for B chromosome invasion, whereas B chromosome drag on the female side might be the main manifestation of host genome resistance in this species. Prospero autumnale thus illuminates a novel evolutionary pathway for B chromosome neutralization by means of a decrease in B transmission through the nondriving sex.     

Response of Populus tremula to heterogeneous B distributions in soil

Background

Poplars accumulate inordinate amounts of B in their leaves and are candidate plants for the remediation of B contaminated soil. We aimed to determine the effect of heterogeneous B distribution in soil by comparing the growth and B accumulation of young Populus tremula trees growing in soil with heterogeneous and homogeneous B distributions.

Methods

The first of two experiments focused on the tolerance and B accumulation of P. tremula under heterogeneous soil B distributions, while the second was designed to study fine root growth under such conditions in detail.

Results

Growth and B accumulation of P. tremula were unaffected by the spatial distribution of B. Root and shoot growth were both reduced simultaneously when leaf B concentrations increased above 800 mg kg^?1. In the heterogeneous soil B treatments, root growth was more reduced in spiked soil portions with B concentrations >20 mg kg^?1. Fine root length growth was stronger inhibited by B stress than secondary growth.

Conclusions

The root growth responses of P. tremula to B are primarily a systemic effect induced by shoot B toxicity and local toxicity effects on roots become dominant only at rather high soil B concentrations. Local heterogeneity in soil B should have little influence on the phytoremediation of contaminated sites.     

Systematic revision of the living species of Bullidae (Mollusca: Gastropoda: Cephalaspidea), with a molecular phylogenetic analysis

Bullidae are a worldwide family of marine shelled cephalaspidean gastropods with a mainly tropical distribution, but also with some representatives in temperate waters. The taxonomy of the group has in the past been based only on shell characters, and the few anatomical accounts available have not addressed more than one to three species, so there has been no agreement about the number of valid species. Seventy‐two specific names and 16 varietal names have been proposed worldwide. The systematics of the family Bullidae are revised, based not only on shells but also on anatomy of all extant species and on DNA sequence data. Twelve species are recognized worldwide, including one new species here described, and all are assigned to the genus Bulla. Two species occur in the eastern Atlantic, B. striata and B. mabillei; two in the western Atlantic, B. occidentalis and B. solida; two in the eastern Pacific, B. gouldiana and B. punctulata; and six in the Indo‐West Pacific, B. ampulla, B. arabica sp. nov. , B. orientalis, B. peasiana, B. quoyii and B. vernicosa. Full synonymies and taxonomic histories are provided for each species. In order to promote taxonomic stability, neotypes are designated for B. striata, B. solida, B. nebulosa (valid name B. gouldiana) and B. vernicosa, and lectotypes for B. occidentalis, B. mabillei, B. punctulata, B. ampulla and B. quoyii. The type locality of B. ampulla is restricted to Mauritius. Bullidae show a general morphological stasis, with anatomy being very similar between species. However, there are high levels of intraspecific variability in the shell, radula and male genital system. In some cases species could only be separated based on molecular data . After defining the characters and geographical range of each species it became clear that sympatric species (a maximum of three) show distinctive shells and reproductive structures, which makes identification straightforward. This study employs an integrative approach, combining information on shells, anatomy, DNA and geographical distribution, in order to resolve the systematics of a difficult taxonomic group. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153 , 453–543.     

Effects of boron on pollen viability in wheat

Grain set failure in wheat, caused by boron (B) deficiency, is associated with poorly developed pollen and anthers. This paper presents results of a study of the effect of B on pollen viability when it was supplied "internally" through the roots and externally in an agar medium for in vitro germination.There was no major effect of B supply to wheat plants on the number of pollen anther^-1 or the percentage of pollen with positive reaction to iodine. Pollen germination in the medium was, however, responsive to both internal and external B supply. When B was not added to the medium, germination was poor, regardless of the level of B supplied to the plant, in both a B deficiency sensitive (SW41) and a B deficiency tolerant (Sonora 64) genotypes. The percentage of germinated pollen and length of the pollen tube increased with increasing medium B. With 20–100 mg H₃BO₃ L^-1 in the medium, the percentage of germinated pollen and length of the pollen tube responded positively to increasing B supply to the plant.No difference was found between sensitive and tolerant genotypes in the effect of B on their pollen viability. On the other hand, without added B in the nutrient solution applied to the plant, grain set was depressed in the B deficiency sensitive SW41 and not in the B deficiency tolerant Sonora 64. A difference in B supply to the germinating pollen in the stigma and style is one possible explanation for this variation in the response to B among wheat genotypes.     

Comparison of performance on different host plants between the B biotype and a non-B biotype of Bemisia tabaci from Zhejiang, China

The capacity of the B biotype of the whitefly, Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae), to invade has often been linked to its presumably wider host range than the non‐B indigenous biotypes. However, there are few experimental studies of the relative performance of the B biotype and non‐B biotypes on different host‐plant species. Here, we compared the performance of the B biotype and an indigenous non‐B biotype (China‐ZHJ‐1) of B. tabaci from Zhejiang, China on five commonly cultivated host plants, each from a different family: cotton, tobacco, cabbage, squash, and kidney bean. We also examined the effect of rearing host plants on the performance of the B biotype. Overall, the performance of the B biotype on the five species of plants was much better than that of the indigenous non‐B population. On tobacco, cabbage, and kidney bean, no individuals of ZHJ‐1 completed development to adulthood, whereas the B biotype developed successfully from egg to adult on all three plants. On squash, the B biotype survived better, developed to adulthood earlier and had a higher fecundity than ZHJ‐1. The two biotypes performed more equally on cotton, but even on this plant the B biotype female adults lived nearly twice as long as that of ZHJ‐1 and may have realized a higher life‐time fecundity. The B biotype also showed a substantial capacity to acclimatize to alternative host plants for improved survival and reproduction, on both highly suitable and marginally suitable host plants. We conclude that the host range of the B biotype of B. tabaci may be much wider than those of some indigenous biotypes, and this advantage of the B biotype over the non‐B biotypes may assist in its invasion and displacement of some indigenous biotypes in the field.     

A High-Density Genetic Map Identifies a Novel Major QTL for Boron Efficiency in Oilseed Rape (Brassica napus L.)

Low boron (B) seriously limits the growth of oilseed rape (Brassica napus L.), a high B demand species that is sensitive to low B conditions. Significant genotypic variations in response to B deficiency have been observed among B. napus cultivars. To reveal the genetic basis for B efficiency in B. napus, quantitative trait loci (QTLs) for the plant growth traits, B uptake traits and the B efficiency coefficient (BEC) were analyzed using a doubled haploid (DH) population derived from a cross between a B-efficient parent, Qingyou 10, and a B-inefficient parent, Westar 10. A high-density genetic map was constructed based on single nucleotide polymorphisms (SNPs) assayed using Brassica 60 K Infinium BeadChip Array, simple sequence repeats (SSRs) and amplified fragment length polymorphisms (AFLPs). The linkage map covered a total length of 2139.5 cM, with 19 linkage groups (LGs) and an average distance of 1.6 cM between adjacent markers. Based on hydroponic evaluation of six B efficiency traits measured in three separate repeated trials, a total of 52 QTLs were identified, accounting for 6.14–46.27% of the phenotypic variation. A major QTL for BEC, qBEC-A3a, was co-located on A3 with other QTLs for plant growth and B uptake traits under low B stress. Using a subset of substitution lines, qBEC-A3a was validated and narrowed down to the interval between CNU384 and BnGMS436. The results of this study provide a novel major locus located on A3 for B efficiency in B. napus that will be suitable for fine mapping and marker-assisted selection breeding for B efficiency in B. napus.     

Role and mechanism of homocysteine in affecting hepatic protein-tyrosine phosphatase 1B

BackgroundElevated homocysteine is epidemiologically related to insulin resistance. Protein-tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin signaling. However, the effect of homocysteine on PTP1B remains unclear.MethodsS-homocysteinylated PTP1B was identified by LC-ESI-MS/MS. The ability of thioredoxin system to recover active PTP1B from S-homocysteinylated PTP1B was confirmed by RNA interference. To address the mechanism for homocysteine to affect PTP1B activity, we performed 5-IAF insertion, activity assays, Western blotting, co-immunoprecipitation and glucose uptake experiments.ResultsThe thiol-containing form of homocysteine (HcySH) suppressed phosphorylation of insulin receptor-β subunit, but enhanced PTP1B activity. This phenomenon was partially related to the fact that HcySH promoted PTP1B expression. Although the disulfide-bonded form of homocysteine (HSSH) modified PTP1B to form an inactive S-homocysteinylated PTP1B, HcySH-induced increase in the activities of cellular thioredoxin and thioredoxin reductase, components of thioredoxin system, could recover active PTP1B from S-homocysteinylated PTP1B. Thioredoxin system transferred electrons from NADPH to S-homocysteinylated PTP1B, regenerating active PTP1B in vitro and in hepatocytes. The actions of HcySH were also related with decrease in hepatic glucose uptake.ConclusionsThe effect of HcySH/HSSH on PTP1B activity depends, at least partially, on the ratio of active PTP1B and S-homocysteinylated PTP1B. High HcySH-induced an increase in thioredoxin system activity is beneficial to de-S-homocysteinylation and is good for PTP1B activity.General significanceOur data provide a novel insight into post-translational regulation of PTP1B, and expand the biological functions of thioredoxin system.     

Mapping of QTL for Fusarium head blight resistance and morphological and developmental traits in three backcross populations derived from Triticum dicoccum?×?Triticum durum

Breeding for resistance to Fusarium head blight (FHB) in durum wheat continues to be hindered by the lack of effective resistance sources. Only limited information is available on resistance QTL for FHB in tetraploid wheat. In this study, resistance to FHB of a Triticum dicoccum line in the background of three Austrian T. durum cultivars was genetically characterized. Three populations of BC₁F₄-derived RILs were developed from crosses between the resistant donor line T. dicoccum-161 and the Austrian T. durum recipient varieties DS-131621, Floradur and Helidur. About 130 BC₁F₄-derived lines per population were evaluated for FHB response using artificial spray inoculation in four field experiments during two seasons. Lines were genetically fingerprinted using SSR and AFLP markers. Genomic regions on chromosomes 3B, 4B, 6A, 6B and 7B were significantly associated with FHB severity. FHB resistance QTL on 6B and 7B were identified in two populations and a resistance QTL on 4B appeared in three populations. The alleles that enhanced FHB resistance were derived from the T. dicoccum parent, except for the QTL on chromosome 3B. All QTL except the QTL on 6A mapped to genomic regions where QTL for FHB have previously been reported in hexaploid wheat. QTL on 3B and 6B coincided with Fhb1 and Fhb2, respectively. This implies that tetraploid and hexaploid wheat share common genomic regions associated with FHB resistance. QTL for FHB resistance on 4B co-located with a major QTL for plant height and mapped at the position of the Rht-B1 gene, while QTL on 7B overlapped with QTL for flowering time.     

Assessing the effects of low boron diets on embryonic and fetal development in rodents using in vitro and in vivo model systems

To date, boron (B) essentiality has not been conclusively shown in mammals. This article summarizes the results of a series of in vitro and in vivo experiments designed to investigate the role of B in mammalian reproduction. In the first study, rat dams were fed either a low (0.04 μg B/g) or an adequate (2.00 μg B/g) B diet for 6 wk before breeding and through pregnancy; reproductive outcome was monitored on gestation day 20. Although low dietary B significantly lowered maternal blood, liver, and bone B concentrations, it had no marked effects on fetal growth or development. The goal of the second study was to assess the effects of B on the in vitro development of rat postimplantation embryos. Day 10 embryos collected from dams fed either the low or adequate B diets for at least 12 wk were cultured in serum collected from male rats exposed to one of the two dietary B treatments. Dams fed the low B diet had a significantly reduced number of implantation sites compared to dams fed the B-adequate diet. However, embryonic growth in vitro was not affected by B treatment. The aim of study 3 was to define the limits of boric acid (BA) toxicity on mouse preimplantation development in vitro. Two-cell mouse embryos were cultured in media containing graded levels of BA (from 6 to 10,000 μM). Impaired embryonic differentiation and proliferation were observed only when embryos were exposed to high levels of BA (>2000 μM), reflecting a very low level of toxicity of BA on early mouse embryonic development. Study 4 tested the effects of low (0.04 μg B/g) and adequate (2.00 μg B/g) dietary B on the in vitro development of mouse preimplantation embryos. Two-cell embryos obtained from the dams were cultured in vitro for 72 h. Maternal exposure to the low B diet for 10, 12, and 16 wk was associated with a reduction in blastocyst formation, a reduction in blastocyst cell number, and an increased number of degenerates. Collectively, these studies support the concept that B deficiency impairs early embryonic development in rodents.     

Cryptic association of B7‐2 molecules and its implication for clustering

T‐cell co‐stimulation through CD28/CTLA4:B7‐1/B7‐2 axis is one of the extensively studied pathways that resulted in the discovery of several FDA‐approved drugs for autoimmunity and cancer. However, many aspects of the signaling mechanism remain elusive, including oligomeric association and clustering of B7‐2 on the cell surface. Here, we describe the structure of the IgV domain of B7‐2 and its cryptic association into 1D arrays that appear to represent the pre‐signaling state of B7‐2 on the cell membrane. Super‐resolution microscopy experiments on heterologous cells expressing B7‐2 and B7‐1 suggest, B7‐2 form relatively elongated and larger clusters compared to B7‐1. The sequence and structural comparison of other B7 family members, B7‐1:CTLA4 and B7‐2:CTLA‐4 complex structures, support our view that the observed B7‐2 1D zipper array is physiologically important. This observed 1D zipper‐like array also provides an explanation for its clustering, and upright orientation on the cell surface, and avoidance of spurious signaling.     

Morphology and phylogenetics of two holoparasitic plants, <Emphasis Type="Italic">Balanophora japonica</Emphasis> and <Emphasis Type="Italic">Balanophora yakushimensis</Emphasis> (Balanophoraceae), and their hosts in Taiwan and Japan

Balanophora japonica and B. yakushimensis are two putatively agamospermic taxa previously reported from southern Japan. Their inflorescences superficially represent those of B. laxiflora and B. fungosa. In this study we confirmed their presence in Taiwan by morphological and phylogenetic analysis using nuclear 18S rDNA and nrITS sequences with related taxa. B. japonica, B. yakushimensis, and B. laxiflora formed a well-supported clade that is distinct from other Balanophora. All three taxa also show considerable differences on morphological and nucleotide sequence differences, therefore the name of B. yakushimensis is retained. The results provide new insights on the intrageneric classification of Balanophora and suggest the positioning of female flowers should be down-weighted. We also successfully identify the hosts of B. japonica and B. yakushimensis by amplifying chloroplast matK sequences from the connected root tissues. The results showed that B. japonica parasitizes on Symplocos species, and that B. yakushimensis parasitizes on Distylium racemosum in Japan and Schima superba in Taiwan’s population.     

Human Parvovirus B19 NS1 Protein Aggravates Liver Injury in NZB/W F1 Mice

Human parvovirus B19 (B19) has been associated with a variety of diseases. However, the influence of B19 viral proteins on hepatic injury in SLE is still obscure. To elucidate the effects of B19 viral proteins on livers in SLE, recombinant B19 NS1, VP1u or VP2 proteins were injected subcutaneously into NZB/W F1 mice, respectively. Significant expressions of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) were detected in NZB/W F1 mice receiving B19 NS1 as compared to those mice receiving PBS. Markedly hepatocyte disarray and lymphocyte infiltration were observed in livers from NZB/WF 1 mice receiving B19 NS1 as compared to those mice receiving PBS. Additionally, significant increases of Tumor Necrosis Factor –α (TNF-α), TNF-α receptor, IκB kinase –α (IKK-α), nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor (IκB) and nuclear factor-kappa B (NF-κB) were detected in livers from NZB/W F1 mice receiving B19 NS1 as compared to those mice receiving PBS. Accordingly, significant increases of matrix metalloproteinase-9 (MMP9) and U-plasminogen activator (uPA) were also detected in livers from NZB/W F1 mice receiving B19 NS1 as compared to those mice receiving PBS. Contrarily, no significant variation on livers from NZB/W F1 mice receiving B19 VP1u or VP2 was observed as compared to those mice receiving PBS. These findings firstly demonstrated the aggravated effects of B19 NS1 but not VP1u or VP2 protein on hepatic injury and provide a clue in understanding the role of B19 NS1 on hepatic injury in SLE.     

Dynamics of diamondback moth oviposition in the presence of a highly preferred non-suitable host

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), highly prefers to oviposit on yellow rocket, Barbarea vulgaris (R. Br.) (Cruciferae) var. arcuata, despite larvae not being able to survive on it, suggesting it may have potential as a trap crop. In a no‐choice greenhouse experiment, P. xylostella laid 28% more eggs on B. vulgaris than on cabbage. Within the B. vulgaris plant, P. xylostella laid 3.7 times more eggs on younger than older leaves. Furthermore, we demonstrated that in the presence of B. vulgaris volatiles, P. xylostella laid 23% more eggs on cabbage plants than when B. vulgaris volatiles were absent. Because increased oogenesis in the presence of B. vulgaris could complicate the use of this host as a trap crop for P. xylostella, we wanted to examine levels of oogenesis in varying mixtures of cabbage and B. vulgaris. In outdoor screenhouse experiments, P. xylostella laid a decreasing percentage of eggs on cabbage as the percentage of B. vulgaris increased. However, the total number of eggs laid on cabbage did not differ among treatments, suggesting that the presence of B. vulgaris may have stimulated P. xylostella oviposition. In the field, total oviposition in cabbage plots containing B. vulgaris was 6.3 times higher than in cabbage plots without B. vulgaris. However, in plots with B. vulgaris, P. xylostella laid 99% of the eggs on B. vulgaris and oviposition on cabbage plants was 6.2 times lower than in the plots without B. vulgaris. The results of this study are discussed according to P. xylostella egg‐laying behavior and life history as it relates to its interaction with B. vulgaris.     

Surface antigen changes during B-lymphocyte activation in primates

It is shown that B-cell-specific surface antigens are conserved on lymphocytes from phylogenetically distant primate species. Characterization of the expression of those antigens on the surface of simian B lymphocytes has led to two observations with important implications for human B-cell physiology. First, lectin stimulation in vitro or antigen stimulation in situ in lymph nodes drives a population of human B lymphocytes to express the B2 but not the B1 antigen on its surface. Second, under pathologic circumstances, this activated B cell can be found in the peripheral blood of monkeys. Thus, the “B2 only” cell defines an activated B lymphocyte whose presence may provide useful diagnostic information concerning pathologic processes.     

Kinin-Stimulated B1 Receptor Signaling Depends on Receptor Endocytosis Whereas B2 Receptor Signaling Does Not

Kinins are potent pro-inflammatory peptides that act through two G protein-coupled receptor subtypes, B1 (B1R) and B2 (B2R). Kinin-stimulated B2R signaling is often transient, whereas B1R signaling is sustained. This was confirmed by monitoring agonist-stimulated intracellular Ca²⁺ mobilization in A10 smooth muscle cells expressing human wild-type B2R and B1R. We further studied the role of receptor membrane trafficking in receptor-mediated phosphoinositide (PI) hydrolysis in model HEK293 cell lines stably expressing the receptors. Treatment of cells with brefeldin A, to inhibit maturation of de novo synthesized receptors, or hypertonic sucrose, to inhibit receptor endocytosis, showed that the basal cell surface receptor turnover was considerably faster for B1R than for B2R. Inhibition of endocytosis, which stabilized B1R on the cell surface, inhibited B1R signaling, whereas B2R signaling was not perturbed. Signaling by a B1R construct in which the entire C-terminal domain was deleted remained sensitive to inhibition of receptor endocytosis, whereas signaling by a B1R construct in which this domain was substituted with the corresponding domain in B2R was not sensitive. B2R and B1R co-expression, which also appeared to stabilize B1R on the cell surface, presumably by receptor hetero-dimerization, also inhibited B1R signaling, whereas B2R signaling was slightly enhanced. Furthermore, the B2R-specific agonist bradykinin (BK) directed both receptors through a common endocytic pathway, whereas the B1R-specific agonist Lys-desArg⁹-BK was unable to do so. These results suggest that B1R-mediated PI hydrolysis depends on a step in receptor endocytosis, whereas B2R-mediated PI hydrolysis does not. We propose that B1R uses at least part of the endocytic machinery to sustain agonist-promoted signaling.     

Murine B cell development and antibody responses to model antigens are not impaired in the absence of the TNF receptor GITR

The Glucocorticoid-Induced Tumor necrosis factor Receptor GITR, a member of the tumor necrosis factor receptor superfamily, has been shown to be important in modulating immune responses in the context of T cell immunity. B lymphocytes also express GITR, but a role of GITR in humoral immunity has not been fully explored. To address this question, we performed studies to determine the kinetics of GITR expression on naïve and stimulated B cells and the capacity of B cells to develop and mount antibody responses in GITR^−/− mice. Results of our studies indicate that all mature B cells express GITR on the cell surface, albeit at different levels. Expression of GITR on naïve mature B cells is upregulated by BCR signaling, but is counteracted by helper T cell-related factors and other inflammatory signals in vitro. In line with these findings, expression of GITR on germinal center and memory B cells is lower than that on naïve B cells. However, the expression of GITR is strongly upregulated in plasma cells. Despite these differences in GITR expression, the absence of GITR has no effect on T cell-dependent and T cell-independent antibody responses to model antigens in GITR^−/− mice, or on B cell activation and proliferation in vitro. GITR deficiency manifests only with a slight reduction of mature B cell numbers and increased turnover of naïve B cells, suggesting that GITR slightly contributes to mature B cell homeostasis. Overall, our data indicate that GITR does not play a significant role in B cell development and antibody responses to T-dependent and independent model antigens within the context of a GITR-deficient genetic background.