Gunnera tinctoria invasions increase, not decrease, earthworm abundance and diversity

Invasive plants often modify soil biotic communities through changes in soil physicochemical characteristics or the amount and/or quality of litter inputs. We assessed the impacts of Gunnera tinctoria invasions on soil and the earthworm community, on Achill Island, Co. Mayo, Ireland. We compared replicated (n = 5) areas invaded by G. tinctoria with uninvaded semi-natural grasslands, as well as with areas subjected to mechanical removal or herbicide treatment. Modifications in physiochemical properties included lower soil temperatures and higher soil pH during the summer in invaded areas, yet little effect on C and N stocks, or soil moisture. Marked differences in litter were observed, however, with invaded areas having c. 20-fold higher (above-ground) litter input than uninvaded ones, as well as lower C:N ratio (17 vs. 29). This was associated with a significantly higher overall abundance and biomass of earthworms in invaded plots (375 individuals m–2, 115 g biomass m–2), compared to the uninvaded control (130 individuals m–2, 45 g biomass m–2), with removal treatments having intermediate values. Earthworm communities comprised 10 species, typical for Irish grasslands, dominated by the common endogeic species Allolobophora chlorotica, Aporrectodea caliginosa and Aporrectodea rosea. Both earthworm species richness and Shannon diversity were significantly higher in invaded areas, but only in spring samples. Based on this new information, plant invaders may increase the abundance and diversity of earthworms, mainly due to much larger litter inputs, increased soil pH and possibly lower soil temperatures in the summer.typical of Irish grasslands

Post-Invasion Recovery of Plant Communities Colonised by Gunnera tinctoria after Mechanical Removal or Herbicide Application and Its Interaction with an Extreme Weather Event

The interventions that are required for both the control and post-invasion restoration of native plant communities depends on several factors, including the efficacy of the measures that are used and how these interact with environmental factors. Here, we report on the results of an experiment on the effects of mechanical removal and herbicide application on the invasive plant Gunnera tinctoria and how an extreme weather event impacted on the invader and on the recovery of native coastal grassland communities. Both removal protocols were largely effective in eradicating mature plants, but the mechanical removal treatment resulted in a major increase in the number of G. tinctoria seedlings, which was exacerbated by the extreme event. Nine months after removal, the number of native species had recovered to c. 80% of that in uninvaded grasslands. In contrast to seedlings, mature plants of G. tinctoria showed a significant reduction in above-ground production after the extreme weather event, although these had largely recovered after six months. Overall, our results indicate that post-control restoration of the plant community may be possible without further significant management interventions. Nevertheless, since some invasive plants survived, further monitoring is required to ensure that recolonisation does not occur.

Alien plant introductions and greenhouse gas emissions: Insights from Gunnera tinctoria invasions

Plant invasions represent a major global change in land/vegetation cover with the potential to significantly modify greenhouse gas (GHG) emissions. To get a better understanding of the impacts of terrestrial invasive plants on soil GHG emissions we report, firstly, on experiments conducted on invasive populations of the N-fixing herbaceous species Gunnera tinctoria in Ireland, and secondly, compare our results with published information based on a systematic review of the literature. For G. tinctoria populations, there was a >50% reduction in soil CO₂ emissions, mainly due to a reduction in autotrophic respiration, but with little impact on annual N₂O or CH₄ budgets. One year after the removal of G. tinctoria, soil GHG emissions returned to values comparable to uninvaded areas and this was associated with the reestablishment of the vegetation and an increased root biomass per unit area. If G. tinctoria covered 10% of abandoned agricultural land in Ireland, this could be associated with a reduction of approximately 8% (or 4.988 Mt CO₂eq y^-1) of the country's national CO₂ emissions. Comparisons of these results with literature values were difficult because of the often low and limited sampling effort of previous investigations, a failure to assess all three major GHGs and because of marked seasonal variations. We found 46 studies that documented results for 16 species. From the studies that measured soil respiration, it was enhanced in only 45% of cases, questioning the assumption that invasive plants always increase soil CO₂ emissions. In 25 cases that analysed methane, CH₄ emissions increased in 76% of them, but all of these were conducted in wetlands. In only two cases were N-fixing species associated with enhanced N₂O emissions. Our results argue for more detailed and comprehensive assessments of the effect of plant invasions on GHG emissions and their global impact.

Hyperbaric oxygen inhibits stimulus-induced proinflammatory cytokine synthesis by human blood-derived monocyte-macrophages

Hyperbaric oxygen (HBO) is 100% oxygen administered at elevated atmospheric pressure to patients with inflammatory diseases. We developed an in vitro model to investigate the effects of HBO on stimulus-induced proinflammatory cytokine transcription and translation. Human blood-derived monocyte-macrophages were stimulated before being transferred to an HBO chamber where they were incubated at 97.9% O2, 2.1% CO2, 2.4 atmospheres absolute, 37 degrees C. Controls were maintained in the same warm room at normoxia at sea level, hyperoxia or increased pressure alone. A 90-min HBO exposure inhibited IL-1beta synthesized in response to lipopolysaccharide by 23%, lipid A by 45%, phytohaemagglutinin A (PHA) by 68%, and tumour necrosis factor (TNF)-alpha by 27%. HBO suppressed lipopolysaccharide-, lipid A- and PHA-induced TNF-alpha by 29%, 31% and 62%, respectively. HBO transiently reduced PHA-induced steady state IL-1beta mRNA levels. Hyperoxia alone and pressure alone did not affect cytokine production. The immunosuppressive effect of HBO was no longer evident in monocyte-macrophages exposed to HBO for more than 3 h. Interestingly, cells exposed to HBO for 12 h synthesized more IL-1beta than cells cultured under control conditions. In summary, HBO exposure transiently suppresses stimulus-induced proinflammatory cytokine production and steady state RNA levels.

Hyperbaric oxygen enhances apoptosis in hematopoietic cells

Hyperbaric oxygen (HBO) is 100% oxygen administered at elevated atmospheric pressure. In this study, we examined the effect of HBO on hematopoietic cell apoptosis. Cells exposed to HBO were incubated in a chamber containing 97.9% O(2) and 2.1% CO(2) at 2.4 atmospheres absolute (ATA). HBO enhanced spontaneous HL-60 cell apoptosis in a time-dependent manner; a 12 h exposure increased apoptosis by 42%. Exposing these cells to hyperoxia at standard atmospheric pressure (95% O(2), 5% CO(2) at 1 ATA) or increased pressure alone (8.75% O(2), 2.1% CO(2) at 2.4 ATA) had minimal effect on apoptosis. HBO also enhanced stimulus-induced apoptosis. HL-60 cells stimulated to die using gamma radiation underwent 33% more apoptosis than cells exposed to radiation alone. HBO enhanced melphalan, camptothecin, and chlorambucil-induced apoptosis by 22%, 13%, and 8%, respectively. Jurkat cells stimulated to die with anti-Fas antibody underwent 44% more apoptosis when exposed to HBO. Spontaneous apoptosis was increased by 15% in HBO-exposed murine thymocytes. HBO's effect on apoptosis did not require new protein synthesis. As expected, HBO exposure increased the intracellular concentration of H(2)O(2). Incubating HL-60 cells in the presence of dehydroascorbic acid partially abrogated HBO-induced increases in intracellular H(2)O(2) and apoptosis. In summary, HBO enhances spontaneous and stimulus-induced apoptosis in hematopoietic cells, at least in part, by enhancing the intracellular accumulation of H(2)O(2).

Early Vlambda diversification in sheep

This study examined a number of tissues during early gestation in foetal sheep to determine the earliest site of Vlambda expression and time of generation of the Vlambda repertoire. Tissues, including spleen, liver, gut, blood and bone marrow, were obtained from 48, 55, 60 and 63 gestational day (g.d.) ovine foetuses and cDNA libraries were prepared from them by reverse transcription-polymerase chain reaction. Clones were randomly selected from cDNA libraries and subjected to sequencing. Analysis of these sequences and comparison with a pool of germline genes led to the following conclusions. The expression of Vlambda occurs earlier in spleen (48 g.d.) than in all of the other tissues examined. Also, diversity is seen earlier and at higher levels in early foetal spleen than in all of the other tissues examined. In this regard, it is notable that splenic Vlambda expression is readily apparent even before such gut-associated lymphoid tissue as the ileal Peyer's patch (IPP) has developed. Two germline Vlambda genes, 5.1 and 5.3 predominate in early immunoglobulin lambda light-chain gene rearrangement. Examination of Jlambda usage revealed the existence of a new Jlambda gene and its utilization during the early phases of the development of the ovine antibody repertoire. This study indicates that sites other than the IPP contribute to the diversification of the Vlambda repertoire in sheep. We suggest that it is likely that foetal spleen may provide a partially diversified B-cell repertoire before the IPP becomes active as a major site for massive clonal expansion and extensive diversification of B cells.

Transcriptional control of T cell development

Transcriptional control of T cell development is a complex and rapidly moving area of investigation. Recent advances reveal critical roles for several transcription factors in T cell commitment, differentiation and selection. In particular, new roles for E proteins as well as members of the Notch signaling pathway have been described. Additionally, a unique function of Ikaros in chromatin remodeling reveals a novel mechanism by which transcriptional control may be exerted.

Apoptosis and the proteasome

The ubiquitin-proteasome pathway is responsible for the regular turnover of a wide variety of proteins and is a critical regulator of many cellular processes. Although this pathway is abundant and ubiquitous, it is also discriminating. This specificity is achieved because there are multiple levels of regulation at work in the pathway. X-ray crystallographic data on the eukaryotic 20S proteasome suggest that substantial rearrangement of the alpha rings, probably mediated by the association of additional regulatory complexes, is required to allow access of substrates into the inner core of the complex. The associated complexes also confer a ubiquitin-dependence on the proteasome, requiring that potential substrates be tagged with chains of ubiquitin proteins. The presence of multiple ubiquitinating enzymes that favor distinct substrates provides a way for a cell to regulate what proteins are to be ubiquitinated. In some cases ubiquitination is not required, but we now know that other modifications, such as phosphorylation and protein-protein interactions, are also important for targeting proteins for degradation. Even with the existence of so many regulatory controls, it is difficult to imagine how one complex can perform so many tasks. As more information is gathered about the proteasome, we begin to understand that all proteasomes are not exactly the same. For example, there is strong evidence that proteasomes involved in antigen presentation differ in both composition and function from proteasomes involved in other processes. The past image of the proteasome as a static structure is being shed, and a new image is emerging that portrays the complex as dynamic and flexible, able to tailor its composition and function to meet a particular need. With this new image of the proteasome in mind, investigators are looking at the potential involvement of the proteasome in cell death. Inhibitor studies have demonstrated a requirement for proteasomes during apoptosis in noncycling and differentiated cells. Similar studies in cycling cells suggest that the proteasome may regulate a cell's decision to proliferate, differentiate, or die. It will be necessary in the future to supplement the peptide and lactacystin studies with work that is not inhibitor-driven since the specificity of an inhibitor for a particular protease is always in question. In addition, a real understanding of how proteasomes may regulate this process awaits the identification of its substrates. With cell death investigators showing increased interest in proteasomes, it may be possible in the next few years to determine the precise role of the proteasome in the pathways that lead to the death of a cell.

Costimulatory signals are required for induction of transcription factor Nur77 during negative selection of CD4(+)CD8(+) thymocytes

A major question in end-stage T cell development is how T cell receptor(TCR) ligation on immature CD4(+)CD8(+) double positive thymocytes is translated into either survival (positive selection) or apoptotic (negative selection) signals. Because different types of antigen-presenting cells (APCs) induce positive or negative selection in the thymus and express different costimulatory molecules, involvement of such costimulatory molecules in determining cell fate of DP thymocytes is considered here. If TCR-generated signals are modulated by APCs, this should be reflected in the activation of distinct biochemical pathways. We here demonstrate that costimulatory signals involved in negative selection also are required for induction of protein expression of Nur77 and its family members. These transcription factors are critically involved in negative but not positive selection. In contrast, the signals that costimulate negative selection are not required for induction of several molecular events associated with positive selection. These include activation of the immediate early gene Egr-1, the mitogen-activated protein kinase ERK2, and surface expression of the CD69 marker. Thus, costimulation for negative selection selectively provides signals for activation of apoptotic mediators. These data provide molecular insights into how TCR-engagement by ligands on different thymic APCs can determine cell fate.

Cutting edge: protective effects of notch-1 on TCR-induced apoptosis

The Notch receptor protein was originally identified in Drosophila and is known to mediate cell to cell communication and influence cell fate decisions. Members of this family have been isolated from invertebrates as well as vertebrates. We isolated mouse Notch-1 in a yeast two-hybrid screen with Nur77, which is a protein that has been shown previously to be required for apoptosis in T cell lines. The data presented below indicate that Notch-1 expression provides significant protection to T cell lines from TCR-mediated apoptosis. These data demonstrate a new antiapoptotic role for Notch-1, providing evidence that, in addition to regulating cell fate decisions, Notch-1 can play a critical role in controlling levels of cell death in T cells.

Multiple sites of V lambda diversification in cattle

Ig repertoire diversification in cattle was studied in the ileal Peyer's patch (IPP) follicles of young calves and in the spleens of late first-trimester bovine fetuses. To investigate follicular diversification, individual IPP follicles were isolated by microdissection; VA diversity was examined by RT-PCR and subsequent cloning and sequencing. When 52 intrafollicular sequences from a 4-wk-old calf were determined and compared, two major groups, one of 23 members and the other of 25, could be delineated. An examination of these groups revealed clear genealogic relationships that implicated in situ diversification of V lambda sequences within the confines of an IPP follicle. V lambda expression was also examined in early (95 and 110 gestational day) fetal bovine spleens. Although earlier studies in cattle and sheep implicated the IPP as a likely site of Ab diversification, a close investigation of V lambda sequences in late first-trimester fetal calves revealed that diversity appears in the early fetal spleen before the establishment of a diverse repertoire in the ileum. When the sequences for the fetal spleen were compared with an existing pool of germline sequences, we found evidence of possible gene conversion events and possible untemplated point mutations occurring in sequences recovered from fetal spleens. We conclude that IPP is not the sole site of VA diversification in cattle. Also, as suggested for rabbits, cattle may use both gene conversion and untemplated somatic point mutation to diversify their primary VA repertoire.

Mouse/human T-cell hybrids rosetting with sheep erythrocytes

Hybrid cells have been recovered from selective culture medium after fusion of concanavalin-A-activated human lymphocytes with an AKR mouse thymoma (BW 5147). After 6 months of culture twenty-seven out of forty-nine clones still contained human chromosomes. Human chromosome 6 was present in 89% of these clones, and human X in 70%. Clones from one hybrid line contained several human chromosomes. In twelve of the clones carrying human chromosomes, the rosetting with sheep erythrocytes (SRBC) was 3 times as high as in the BW 5147 cell line. All these clones carried the human chromosome 6, and eight clones contained the human X chromosome as well. In some of these clones (25%) chromosome 6 was the only human one present. In the two clones in which human chromosome 6 was completely missing, the rosetting with SRBC was at the level of the BW line. We therefore suggest that genes on human chromosome 6 are responsible for rosetting with SRBC.

Gene regulation associated with apoptosis

Apoptosis, one of the best-studied forms of programmed cell death processes, plays an important role during the development and life-cycle of most multicellular organisms. The mechanisms underlying the initiation and manifestation of apoptotic cell death are the focus of the most recent cell death research. Generally, it is believed that cells are eliminated via a highly ordered and controlled program. This program might consist of the successive activation of unique apoptosis-specific genes, which are solely involved in the regulation of the programmed cell death. However, more and more evidence is accumulating that novel genes are not activated or induced during apoptosis, but rather many well-known genes previously described for their roles in processes such as proliferation and differentiation and belonging, for example, to the protein families of immediate-early genes and transcription factors become activated. The death-specific feature is achieved thereby by the extent, combination, and specific timing of gene expression. The involvement of the three different transcription factors glucocorticoid receptor (GR), nur77, and activator protein 1 (AP-1) in such a scenario is the focus of this review.

Immunoglobulin gene diversification in cattle

Research in several species has revealed that different types of mammals have evolved divergent molecular and cellular strategies for generating immunoglobulin diversity. Other chapters in this text have highlighted the specific characteristics unique to chicken, rabbit, mouse, human and sheep B lymphocyte development; namely indicating differences in the mechanisms of diversity and the site of primary B cell development. Studies of the bovine system have indicated that like the sheep system, the ileal Peyer's patch (IPP) is a likely chicken bursal equivalent, and is a site of primary B lymphocyte development. Substantial investigation in sheep has indicated that Ig diversity is created by untemplated somatic mutation and intense selective pressure (Reynaud et al., 1991). The frequency of alteration in the sheep Ig light chain gene locus also is characteristic of the bovine system, however, recent evidence from sequencing of bovine lambda light chain genes indicates that one mechanism that contributes to diversity is gene conversion, utilizing several pseudogenes located in the Ig locus (Parng et al., 1996). The mechanism by which this hyperalteration of Ig genes occurs in both sheep and cattle is poorly understood and is thus the focus of considerable investigation. The study of events in the IPP may also have informative ramifications for secondary diversification of the Ig repertoire by somatic hyperalteration in germinal centers.

Gene conversion contributes to Ig light chain diversity in cattle

In humans and mice, extensive gene rearrangement is the major mechanism of diversification of the primary Ig repertoire. This study shows that cattle depart from this pattern because rearrangement in the light chain locus is sharply limited. Furthermore, in cattle, gene conversion contributes to the diversification of the primary light chain repertoire. Sequencing of germ-line and expressed Vlambda genes revealed three important features. First, the germ line contained a number of Vlambda pseudogenes. In fact, 14 (70%) of the 20 germ-line genes identified and sequenced were pseudogenes, because they had one or more of the following defects: lack of recombination signal sequences at the 3' end, stop codons within the reading frame or truncations, and/or insertions or deletions that resulted in loss of reading frame. Second, Vlambda cDNA from ileal Peyer's patch B cells demonstrated that the light chain repertoire arises from only a small number of V(J) rearrangements. Even though two J genes were identified in the germ line, all of the expressed Vlambda genes examined contained the same J segment, indicating that only a single J gene participates in rearrangement at the lambda locus. Third, a significant number of departures from the germ-line sequences of rearranged Vlambda can be traced to donor sequences of one or more Vlambda pseudogenes. We conclude that a limited number of rearrangements and gene conversion play a role in contributing to the diversification of the primary lambda repertoire. Furthermore, while clear indications of a role for somatic mutation in lambda diversification was seen, V gene rearrangement was not a major factor.

Gene conversion contributes to Ig light chain diversity in cattle

In humans and mice, extensive gene rearrangement is the major mechanism of diversification of the primary Ig repertoire. This study shows that cattle depart from this pattern because rearrangement in the light chain locus is sharply limited. Furthermore, in cattle, gene conversion contributes to the diversification of the primary light chain repertoire. Sequencing of germ-line and expressed Vlambda genes revealed three important features. First, the germ line contained a number of Vlambda pseudogenes. In fact, 14 (70%) of the 20 germ-line genes identified and sequenced were pseudogenes, because they had one or more of the following defects: lack of recombination signal sequences at the 3' end, stop codons within the reading frame or truncations, and/or insertions or deletions that resulted in loss of reading frame. Second, Vlambda cDNA from ileal Peyer's patch B cells demonstrated that the light chain repertoire arises from only a small number of V(J) rearrangements. Even though two J genes were identified in the germ line, all of the expressed Vlambda genes examined contained the same J segment, indicating that only a single J gene participates in rearrangement at the lambda locus. Third, a significant number of departures from the germ-line sequences of rearranged Vlambda can be traced to donor sequences of one or more Vlambda pseudogenes. We conclude that a limited number of rearrangements and gene conversion play a role in contributing to the diversification of the primary lambda repertoire. Furthermore, while clear indications of a role for somatic mutation in lambda diversification was seen, V gene rearrangement was not a major factor.

Proteasomes play an essential role in thymocyte apoptosis

Cell death in many different organisms requires the activation of proteolytic cascades involving cytosolic proteases. Here we describe a novel requirement in thymocyte cell death for the 20S proteasome, a highly conserved multicatalytic protease found in all eukaryotes. Specific inhibitors of proteasome function blocked cell death induced by ionizing radiation, glucocorticoids or phorbol ester. In addition to inhibiting apoptosis, these signals prevented the cleavage of poly(ADP-ribose) polymerase that accompanies many cell deaths. Since overall rates of protein degradation were not altered significantly during cell death in thymocytes, these results suggest that the proteasome may either degrade regulatory protein(s) that normally inhibit the apoptotic pathway or may proteolytically activate protein(s) than promote cell death.

The role of oxygen in thymocyte apoptosis

Signals generated by T cell receptor (TCR) cross-linking (or phorbol 12-myristate-13-acetate + Ca2+ ionophore), glucocorticoids or ionizing radiation all stimulate apoptotic cell death in thymocytes by signals that are initially distinct from each other. However, when these stimuli were administered to thymocyte cultures that were maintained under an atmosphere containing less than 20 ppm oxygen as opposed to one that contained 18.5% molecular oxygen, cell death was inhibited or abrogated, suggesting that the induction of death by all three different stimuli depend on the presence of molecular oxygen. Studies of the effects of the cysteine analog N-acetyl cysteine (NAC) with normal thymocytes demonstrated that this antioxidant inhibited the induction of death by each of the different stimuli in a manner the paralleled anaerobiosis. Furthermore, studies with thymocytes demonstrated that the induction of nur77, a gene shown to be involved in thymocyte apoptosis signaled through the TCR or its surrogates, is not inhibited by NAC or dependent upon molecular oxygen. The possible implications for negative selection of NAC-mediated inhibition of TCR-signaled thymocyte cell death was examined in thymic organ culture. Treatment of these cultures with NAC provided significant protection against staphylococcal enterotoxin B-mediated deletion of V beta 8-expressing thymocytes.

Apoptosis and the maintenance of homoeostasis in the immune system

The regulation of cell proliferation and the selection against autoreactive cells in the lymphoid system both occur through the induction of apoptosis. Many of the signals that induce apoptosis in lymphocytes are now well defined. Interactions between Fas and its ligand have emerged as a major mechanism for the deletion of activated peripheral T cells and autoreactive B cells. Although the signal-transduction pathway leading from engagement of Fas to apoptosis is not entirely clear, significant advances have been made recently. There has also been progress in the elucidation of the mechanisms that regulate apoptosis in the immune system.

Photosynthesis and nutrient-use efficiency of barley in response to low arbuscular mycorrhizal colonization and addition of phosphorus

The effects of arbuscular mycorrhizal infection by Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe on growth and photosynthesis of barley (Hordeum vulgare L. cv. Manitou) were investigated in sand culture at five levels of calcium phosphate (50, 100, 200, 400 and 800 mg P kg(-1) ). Mycorrhizal infection was low and varied with P supply, declining from 3.3% at 50 mg P kg(-1) to 1.5% at the highest P concentration. In general, there were small differences in biomass between mycorrhizal (+AM) and non-mycorrhizal (-AM) barley but a significant reduction in dry mass of senesced leaves occurred in the +AM plants. Leaf P concentrations increased with P application, but did not differ between + AM and -AM plants. Although there were no differences in dry mass between + AM and -AM plants at 50 mg P kg(-1) , it was at this lowest P supply that +AM plants had higher rates of photosynthesis and greater P-use and N-use efficiencies. The mycorrhizal enhancement of maximum photosynthetic rate at the lowest P level was associated with a higher stomatal conductance, but was not related to increased leaf P or to changes in photon yield or the ratio of variable (FV) to maximum (FM) chlorophyll fluorescence.

Genes that regulate apoptosis in the mouse thymus

Elimination of self-reactive T lymphocytes occurs during T-cell development in the thymus by a process known as negative selection. The mechanism that drives negative selection is apoptosis. To identify genes that regulate apoptosis in the mouse thymus, a library of negatively selected T cells was constructed and, by subtractive screening, several differentially regulated genes were isolated. Two transcripts that are repressed during cell death were identified, in addition to two induced transcripts. Further experiments demonstrated that cell death in thymocytes can occur via several induction pathways and each pathway appears to be regulated by a unique cascade of genes.

Cell death suffers a TKO

The cytokine interferon-gamma (IFN-gamma), initiates both cell cycle arrest and cell death in certain cell lines. Through a novel strategy of cell transfection with episomal vectors expressing antisense cDNAs, Deiss et al. have demonstrated that it is possible to isolate genes that are required for the initiation of cell death by the cytokine IFN-gamma. This approach, referred to as TKO, for Technical Knock Out, has identified several genes whose activity appears to be essential for the induction of apoptosis by IFN-gamma in HeLa cells. Interestingly, these genes appear to mediate IFN-gamma-induced apoptosis in HeLa cells, but their inhibition by antisense does not ameliorate the antiproliferative effects of IFN-gamma in these cells. The clever strategy employed by these authors holds promise for others who wish to isolate genes required for other differentiative processes in cultured cell lines.

Induction of genes during apoptosis: examples from the immune system

The immune system provides many examples of cell populations that are susceptible to the induction of apoptosis. Self-reactive immature cells are deleted by triggering apoptosis. Additionally, mature peripheral lymphocytes are induced to undergo apoptosis, particularly when hyperactivated. In the past few years, several genes have been linked to cell death in lymphoid cells. However, only a handful of these genes has been shown to be required for cell death to occur in the immune system. This review focuses on signals known to mediate apoptosis in the immune system and those genes demonstrated to be required for the induction of cell death.

Transient transfection assays to examine the requirement of putative cell death genes

In conclusion, this chapter provides a convenient and efficient method for the detection and analysis of transiently transfected cells. Such strategies allow a fast and simple analysis of the requirement for particular genes that have been identified as being induced during apoptosis. Our experience has been that, when screening for "cell death genes," it is easy to isolate genes induced during apoptosis but far more difficult to determine the requirement for any given gene. These protocols have rendered such determinations much simpler to perform.

Essential genes that regulate apoptosis

The expression of several genes has been associated with the induction of apoptosis in a wide variety of vertebrate and invertebrate organisms. However, relatively few gene products have been demonstrated to be required for cell death. This review highlights genes that are required for apoptosis and proposes mechanisms by which the proteins encoded by these genes might function.

Quantitative analysis of the B cell repertoire by limiting dilution analysis and fluorescent in situ hybridization

We have made a quantitative and concurrent analysis of B cell frequencies and VH gene family expression to study the influence of tissue type and age on the development and establishment of the primary B cell repertoire. Using LPS-mediated limiting dilution analysis and a panel of antigens we show that the newly generated B cell specificities from bone marrow get distributed without a bias to peripheral tissues such as spleen and Peyer's patches throughout the lifetime of the animal. Comparison of the B cell frequencies in animals of four different age groups (2-4 days old, 3, 12, and 18 months old) reveals that while the neonatal repertoire is comparable to that of adults, there was a selective twofold increase in the generation and distribution of B cells reactive with autologous mouse red blood cells in older mice compared to young ones. By means of a novel technique that employs fluorescent in situ hybridization and flow cytometry, we have also compared the VH gene family usage in large numbers of single B cells from these mice. Analysis of the same cell population (surface Ig+, functional B lineage cells) for expression of 7183, J558, and S107 VH families shows a preferential twofold increase in the use of VH 7183 in neonates compared to adults, while all three families show no significant difference in levels of expression during adult life or between primary and secondary lymphoid tissues. Taken together, our data indicate that specific and selective changes occur in both VH gene usage and antibody frequencies during murine ontogeny.

Negative selection of CD4+CD8+ thymocytes by T cell receptor-induced apoptosis requires a costimulatory signal that can be provided by CD28

CD4+CD8+ thymocytes expressing self-reactive T cell antigen receptors (TCR) are deleted in the thymus as a consequence of TCR/self-antigen/major histocompatibility complex interactions. However, the signals that are necessary to initiate clonal deletion have not yet been clarified. Here we demonstrate that TCR engagement does not efficiently induce apoptosis of CD4+CD8+ thymocytes, although it generates signals that increase expression of CD5, a thymocyte differentiation marker. In fact, TCR signals fail to induce thymocyte apoptosis even when augmented by simultaneous engagement with CD4 or lymphocyte function 1-associated molecules. In marked contrast, signals generated by engagement of both TCR and the costimulatory molecule CD28 potently induce apoptosis of CD4+CD8+ thymocytes. Thus, the present results define a requirement for both TCR and costimulatory signals for thymocyte apoptosis and identify CD28 as one molecule that is capable of providing the necessary costimulus. These results provide a molecular basis for differences among cell types in their ability to mediate negative selection of developing thymocytes.

Apoptotic signals delivered through the T-cell receptor of a T-cell hybrid require the immediate-early gene nur77

Engagement of the T-cell antigen receptor (TCR) on immature thymic T cells induces death by apoptosis. Although several lines of evidence indicate that apoptosis requires de novo gene expression, little is known about the molecular pathways that mediate this response. Here we show that nur77 (refs 4-7), a zinc-finger transcription factor, is expressed in response to TCR engagement in immature T cells and T-cell hybrids. Antisense inhibition of nur77 expression prevents apoptosis in TCR-stimulated cells. nur77 is also expressed in response to mitogens, but in this case transcription is regulated by 5' upstream elements that are distinct from those used for induction of apoptosis. In addition, polyadenylation is only observed on nur77 transcripts found in condemned cells. These data support a role for nur77 in cell death that may be distinct from that of activation.

Programmed cell death, apoptosis and killer genes

A cursory examination of the literature reveals that the study of programmed cell death and apoptosis is increasing exponentially. Most contributors to this field have come either from developmental biology or immunology and view programmed cell death from different perspectives, leading both to confusion and an inability to fully appreciate the literature from other disciplines. Here, Lawrence Schwartz and Barbara Osborne define the terms and ideas relevant to the study of cell death in a way that will be accessible to investigators from all fields.

p53 is required for radiation-induced apoptosis in mouse thymocytes

The p53 tumour suppressor gene is the most widely mutated gene in human tumorigenesis. p53 encodes a transcriptional activator whose targets may include genes that regulate genomic stability, the cellular response to DNA damage, and cell-cycle progression. Introduction of wild-type p53 into cell lines that have lost endogenous p53 function can cause growth arrest or induce a process of cell death known as apoptosis. During normal development, self-reactive thymocytes undergo negative selection by apoptosis, which can also be induced in immature thymocytes by other stimuli, including exposure to glucocorticoids and ionizing radiation. Although normal negative selection involves signalling through the T-cell receptor, the induction of apoptosis by other stimuli is poorly understood. We have investigated the requirement for p53 during apoptosis in mouse thymocytes. We report here that immature thymocytes lacking p53 die normally when exposed to compounds that may mimic T-cell receptor engagement and to glucocorticoids but are resistant to the lethal effects of ionizing radiation. These results demonstrate that p53 is required for radiation-induced cell death in the thymus but is not necessary for all forms of apoptosis.

Do all programmed cell deaths occur via apoptosis?

During development, large numbers of cells die by a nonpathological process referred to as programmed cell death. In many tissues, dying cells display similar changes in morphology and chromosomal DNA organization, which has been termed apoptosis. Apoptosis is such a widely documented phenomenon that many authors have assumed all programmed cell deaths occur by this process. Two well-characterized model systems for programmed cell death are (i) the death of T cells during negative selection in the mouse thymus and (ii) the loss of intersegmental muscles of the moth Manduca sexta at the end of metamorphosis. In this report we compare the patterns of cell death displayed by T cells and the intersegmental muscles and find that they differ in terms of cell-surface morphology, nuclear ultrastructure, DNA fragmentation, and polyubiquitin gene expression. Unlike the T cells, which are known to die via apoptosis, we find that the intersegmental muscles display few of the features that characterize apoptosis. These data suggest that more than one cell death mechanism is used during development.

Immunoglobulin VH usage analysis by fluorescent in situ hybridization and flow cytometry

We have devised a flow cytometry-based fluorescent in situ hybridization assay that permits analysis of gene expression in a large number of single cells. In this technique, fixed and permeabilized cells are incubated with biotinylated single-stranded RNA probes and by means of a fluorescently labelled second-step reagent, the cells are analyzed by flow cytometry. This is a rapid and simple method that allows all of the steps in the procedure to be performed on cells in suspension. Using this approach, we demonstrate here that immunoglobulin heavy chain variable region (VH) gene expression can be analyzed among individual cells using particular VH family-specific probes. This technique has a high degree of accuracy (greater than 97%) in detecting the fraction of cells expressing a specific message in a population and is sensitive enough to detect immunoglobulin message in LPS activated B cells. The technique has been applied successfully to monitor gene expression in homogeneous and heterogeneous populations. It also allows concurrent analysis of cell surface proteins and gene expression through two-color flow cytometry. This method of monitoring gene expression in individual cells may have a number of applications in immunology and cell biology.

Analysis of a novel VHS107 haplotype in CLA-2 and WSA mice. Evidence for gene conversion among IgVH genes in outbred populations

Gene conversion has been suggested as the basis for many VH allelic differences, particularly in the murine VHS107 family. Whether conversion among IgVH genes is likely to have occurred in outbred populations has not been directly addressed. The CLA-2/Cn and WSA strains, which were recently and independently derived from a feral population exhibiting low responsiveness to PC, provide the opportunity to approach this question. In previous studies, the heavy chain cDNA sequence of a PC-specific hybridoma derived from CLA-2/Cn suggested gene conversion events within the VHS107 family. Accordingly, we have examined the germline VHS107 genes of CLA-2/Cn and WSA. The results indicate that: (a) The CLA-2 and WSA strains bear an identical but novel VHS107 family haplotype, which lacks a V3 element and contains a V1, a V13, and two V11 genes; (b) low PC responsiveness in these populations is unlikely due to an inability to express the V1 member of the VHS107 gene family; and (c) when compared with the other known VHS107 haplotypes, the proportion of differences consistent with gene conversion greatly exceeds that expected by random base substitution. Thus, gene conversion events appear to have occurred with considerable frequency in the evolution of the murine VHS107 family, especially among the V3, V13, and V11 members.

Interaction and sequence diversity among T15 VH genes in CBA/J mice

Nucleotide sequences of the four genes composing the T15 heavy chain variable region (VH) family of the CBA/J mouse have been determined. Comparison of these sequences with their published BALB/c and C57BL/10 homologues reveals that nucleotide differences found between given alleles of two strains, i.e., CBA/J and BALB/c, are observed in other family members of the same strain. We suggest that these patterns of sequence variation are most readily explained by gene interaction (conversion). Additionally, the sequence of a CBA/J hybridoma, 6G6, proposed to have been generated by gene conversion, is directly encoded by the CBA/J V11 gene indicating that the putative conversion has occurred meiotically in the germline. These results are consistent with the premise that gene correction is occurring frequently among members of this family and that such processes may contribute significantly to the evolution of Ig variable region genes even in the relatively short time frame of inbred strain derivation.

Evolution of the IgA heavy chain gene in the genus Mus

To examine questions of immunoglobulin gene evolution, the IgA alpha heavy chain gene from Mus pahari, an evolutionarily distant relative to Mus musculus domesticus, was cloned and sequenced. The sequence, when compared to the IgA gene of BALB/c or human, demonstrated that the IgA gene is evolving in a mosaic fashion with the hinge region accumulating mutations most rapidly and the third domain at a considerably lower frequency. In spite of this pronounced accumulation of mutations, the hinge region appears to maintain the conformation of a random coil. A marked propensity to accumulate replacement over silent site changes in the coding regions was noted, as was a definite codon bias. The possibility that these two phenomena are interrelated is discussed.

Two-dimensional gel analysis of swine histocompatibility antigens

Miniature swine MHC antigens from three inbred herds were examined by two-dimensional gel electrophoresis. These antigens were found to constitute a series of complex glycoproteins displaying haplotype-specific patterns that allowed the distinction of both class I and class II molecules among the three haplotypes. Selected outbred pig antisera reacted with a subset of class I antigens, suggesting the presence of at least two distinct molecular species among these antigens. Similarly, alloantisera reacting with mouse Ia antigens and a monoclonal anti-human DR were shown to immunoprecipitate a subset of class II molecules. Examination of the cells from two recombinant haplotypes demonstrated that both independent recombinational events took place between the class I and class II genes.

Two-dimensional gel analysis of swine histocompatibility antigens

Miniature swine MHC antigens from three inbred herds were examined by two-dimensional gel electrophoresis. These antigens were found to constitute a series of complex glycoproteins displaying haplotype-specific patterns that allowed the distinction of both class I and class II molecules among the three haplotypes. Selected outbred pig antisera reacted with a subset of class I antigens, suggesting the presence of at least two distinct molecular species among these antigens. Similarly, alloantisera reacting with mouse Ia antigens and a monoclonal anti-human DR were shown to immunoprecipitate a subset of class II molecules. Examination of the cells from two recombinant haplotypes demonstrated that both independent recombinational events took place between the class I and class II genes.

Murine thymocyte and splenocyte Ia antigens are indistinguishable by two-dimensional gel electrophoresis

Ia antigens have been found on the surface of B lymphocytes, macrophages, epidermal Langerhans cells and on certain transformed cells. Ia antigens have also been detected on the surface of thymocytes but the biosynthesis of these antigens by thymocytes has been difficult to demonstrate. We describe the labeling of murine thymocytes with 35S-methionine and the subsequent analysis of Ia antigens by two-dimensional polyacrylamide gel electrophoresis. Cell elimination experiments demonstrated that the Ia antigens detected were not of B cell origin and were synthesized by a Thy-1-positive thymocyte. Ia antigens from thymocytes were found to be indistinguishable from spleen Ia preparations. Since T cell I region determinants have been postulated to be involved in cellular recognition phenomena, models addressing this recognition must allow for the observation that T and B cell I region molecules detected by antisera such as A. TH anti-A. TL are indistinguishable by two-dimensional gel analysis and are thus unlikely to be involved in the generation of specificity in recognition.

Murine thymocyte and splenocyte Ia antigens are indistinguishable by two-dimensional gel electrophoresis

Ia antigens have been found on the surface of B lymphocytes, macrophages, epidermal Langerhans cells and on certain transformed cells. Ia antigens have also been detected on the surface of thymocytes but the biosynthesis of these antigens by thymocytes has been difficult to demonstrate. We describe the labeling of murine thymocytes with 35S-methionine and the subsequent analysis of Ia antigens by two-dimensional polyacrylamide gel electrophoresis. Cell elimination experiments demonstrated that the Ia antigens detected were not of B cell origin and were synthesized by a Thy-1-positive thymocyte. Ia antigens from thymocytes were found to be indistinguishable from spleen Ia preparations. Since T cell I region determinants have been postulated to be involved in cellular recognition phenomena, models addressing this recognition must allow for the observation that T and B cell I region molecules detected by antisera such as A. TH anti-A. TL are indistinguishable by two-dimensional gel analysis and are thus unlikely to be involved in the generation of specificity in recognition.