Glucocorticoid-mediated activation of DNA degradation in avian lymphocytes

Stress and immunity in poultry: light management and nanotechnology as effective immune enhancers to fight stress

The poultry industry plays a significant role in boosting the economy of several countries, particularly developing countries, and acts as a good, cheap, and affordable source of animal protein. A stress-free environment is the main target in poultry production. There are several stressors, such as cold stress, heat stress, high stocking density, and diseases that can affect birds and cause several deleterious changes. Stress reduces feed intake and growth, as well as impairs immune response and function, resulting in high disease susceptibility. These effects are correlated with higher corticosteroid levels that modulate several immune pathways such as cytokine-cytokine receptor interaction and Toll-like receptor signaling along with induction of excessive production of reactive oxygen species (ROS) and thus oxidative stress. Several approaches have been considered to boost bird immunity to overcome stress-associated effects. Of these, dietary supplementation of certain nutrients and management modifications, such as light management, are commonly considered. Dietary supplementations improve bird immunity by improving the development of lymphoid tissues and triggering beneficial immune modulators and responses. Since nano-minerals have higher bioavailability compared to inorganic or organic forms, they are highly recommended to be included in the bird's diet during stress. Additionally, light management is considered a cheap and safe approach to control stress. Changing light from continuous to intermittent and using monochromatic light instead of the normal light improve bird performance and health. Such changes in light management are associated with a reduction of ROS production and increased antioxidant production. In this review, we discuss the impact of stress on the immune system of birds and the transcriptome of oxidative stress and immune-related genes, in addition, how nano-minerals supplementations and light system modulate or mitigate stress-associated effects.

Local glucocorticoid production in lymphoid organs of mice and birds: Functions in lymphocyte development

Circulating glucocorticoids (GCs) are powerful regulators of immunity. Stress-induced GC secretion by the adrenal glands initially enhances and later suppresses the immune response. GC targets include lymphocytes of the adaptive immune system, which are well known for their sensitivity to GCs. Less appreciated, however, is that GCs are locally produced in lymphoid organs, such as the thymus, where GCs play a critical role in selection of the T cell antigen receptor (TCR) repertoire. Here, we review the roles of systemic and locally-produced GCs in T lymphocyte development, which has been studied primarily in laboratory mice. By antagonizing TCR signaling in developing T cells, thymus-derived GCs promote selection of T cells with stronger TCR signaling. This results in increased T cell-mediated immune responses to a range of antigens. We then compare local and systemic GC patterns in mice to those in several bird species. Taken together, these studies suggest that a combination of adrenal and lymphoid GC production might function to adaptively regulate lymphocyte development and selection, and thus antigen-specific immune reactivity, to optimize survival under different environmental conditions. Future studies should examine how lymphoid GC patterns vary across other vertebrates, how GCs function in B lymphocyte development in the bone marrow, spleen, and the avian bursa of Fabricius, and whether GCs adaptively program immunity in free-living animals.

Locally elevated cortisol in lymphoid organs of the developing zebra finch but not Japanese quail or chicken

Glucocorticoids are important for production of functional lymphocytes and immunity. In altricial neonates, adrenal glands are unresponsive and local glucocorticoid synthesis in lymphoid organs may be necessary to support lymphocyte development. Precocial neonates, in contrast, have fully responsive adrenal glucocorticoid production, and lymphoid glucocorticoid synthesis may not be necessary. Here, we found that in altricial zebra finch hatchlings, lymphoid organs had dramatically elevated endogenous glucocorticoid (and precursor) levels compared to levels in circulating blood. Furthermore, while avian adrenals produce corticosterone, finch lymphoid organs had much higher levels of cortisol, an unexpected glucocorticoid in birds. In contrast, precocial Japanese quail and chicken offspring did not have locally elevated lymphoid glucocorticoid levels, nor did their lymphoid organs contain high proportions of cortisol. These results show that lymphoid glucocorticoids differ in identity, concentration, and possibly source, in hatchlings of three different bird species. Locally-regulated glucocorticoids might have species-specific roles in immune development.

Corticosterone and cortisol binding sites in plasma, immune organs and brain of developing zebra finches: intracellular and membrane-associated receptors

Glucocorticoids (GCs) affect the development of both the immune and nervous systems. To do so, GCs bind to intracellular receptors, mineralocorticoid receptors (MR) and glucocorticoid receptors (GR). In addition, GCs bind to membrane-associated corticosteroid receptors (mCR). Two well-known GCs are corticosterone and cortisol. Whereas corticosterone is the primary GC in zebra finch plasma, cortisol is the primary GC in zebra finch lymphoid organs and is also present in the brain and plasma during development. Here, we characterized binding sites for corticosterone and cortisol in plasma, liver, lymphoid organs, and brain of developing zebra finches. In tissues, we examined both intracellular and membrane-associated binding sites. For intracellular receptors, there were MR-like sites and GR-like sites, which differentially bound corticosterone and cortisol in a tissue-specific manner. For mCR, we found little evidence for membrane-associated receptors in immune organs, but this could be due to the small size of immune organs. Interestingly, cortisol, but not corticosterone, showed a low amount of specific binding to bursa of Fabricius membranes. For neural membranes, corticosterone bound to one site with low affinity but a relatively high B(max), and in contrast, cortisol bound to one site with high affinity but a lower B(max). Our results indicate that intracellular and membrane-associated receptors differentially bind corticosterone and cortisol suggesting that corticosterone and cortisol might have different roles in immune and nervous system development.

Glucocorticoid-induced thymocyte apoptosis in wall lizard Hemidactylus flaviviridis

The aim of the present study is to demonstrate the phenomenon and mechanism of GC-induced cell death of thymocytes in wall lizard Hemidactylus flaviviridis. Lizard thymocytes were treated in vitro with different concentration of corticosterone (CS) for varying durations and the genomic DNA was analyzed both by agarose gel electrophoresis and flow cytometry. Corticosterone induced DNA fragmentation in a dose-dependent manner, as the DNA laddering was more prominent at 10(-14) M than at lower concentrations. However, CS could activate the programmed cell death of thymocytes only when incubated for 48 h and not at 3, 6, 12, or 24 h. Similar results were obtained following the flow cytometric analysis. The highest apoptotic cell death was noted at 10(-14) M concentration. CS-induced programmed cell death was decreased dramatically when the thymocytes were preincubated with RU 486 (10(-9) ng /ml) for 24 h, suggesting the classical receptor-mediated genomic pathway of glucocorticoid action in inducing apoptosis of thymocytes in wall lizard H. flaviviridis.

Porcentagem de parênquima e índice apoptótico da bolsa cloacal em frangos de corte em ambiente de conforto e estresse térmico

Noventa e seis pintos de corte foram mantidos em diferentes temperaturas ambientais (calor, 29ºC; neutro, 24ºC; frio, 15ºC) e oito aves de cada grupo de temperatura (tratamento) foram sacrificadas nos dias 1, 7, 14, 21, 28, 35 e 42, e suas bolsas cloacais (BC) coletadas e processadas segundo técnica de rotina para inclusão em parafina. Cortes de 4m m, corados em HE, foram submetidos à análise histométrica utilizando-se o programa Kontron KS300, V 2.0. De cada corte foram mensurados a porcentagem de parênquima e o índice apoptótico nas BC. As médias da porcentagem de parênquima nas BC das aves com cinco semanas foram significativamente menores quando mantidas em ambiente frio (53,0%), quando comparadas com as do ambiente neutro (78,5%). As médias do índice apoptótico foram significativamente maiores nas BC de aves mantidas no ambiente frio (0,26), em relação às do neutro (0,12). Conclui-se que o estresse térmico induz apoptose de linfócitos e, conseqüentemente, hipotrofia da bolsa cloacal, alterando sua maturação.

Dexamethasone-induced apoptosis in immune cells from peripheral circulation and lymphomyeloid tissues of juvenile clearnose skates, Raja eglanteria

Juvenile clearnose skates (Raja eglanteria) were injected intramuscularly with dexamethasone-21-phosphate at 50, 75, and 100mg/kg body weight. After 24h, skates were sacrificed and lymphomyeloid tissues (thymus, spleen, Leydig organ, and epigonal organ) were removed and whole blood was sampled. Tissues were used fresh for imprints or prepared for histology by solvent fixation or freezing in liquid nitrogen. Apoptosis in fixed tissues was assessed by transmission electron microscopy. Frozen sections and cytospin preparations of peripheral blood leukocytes (PBL) were evaluated by the TUNEL reaction to detect DNA strand breaks. Dexamethasone treatment increased apoptotic activity in all lymphomyeloid tissues as well as in PBL. These studies demonstrate that immune cells of elasmobranchs have the capacity for glucocorticoid-driven apoptosis, and that programmed cell death as a mechanism to regulate immune cell production appears to have been conserved during vertebrate evolution.

Reversible disruption of thymic function by steroid treatment

The effect of steroid treatment on the thymic output of T cells was examined in an avian model. Recent thymic emigrants in chickens transiently express the chicken T cell Ag 1 thymocyte marker, and thymic function can be monitored indirectly by measuring the levels of TCR gene rearrangement excision circles in peripheral T cells. Both parameters were used to show that intensive steroid treatment induces thymic involution and a profound reduction in the supply of naive T cells to the periphery. Conversely, resident T cells in the peripheral lymphocyte pool were relatively spared. Thymopoiesis immediately recovered following cessation of steroid treatment, concurrent with restoration of the thymic output of newly formed T cells. Repopulation of the peripheral T cell pool recapitulated the ontogenetic pattern of gamma delta T cell replenishment before alpha beta T cell reseeding, thereby indicating the complete recovery of thymic function after a course of steroid treatment.

Apoptosis of testicular germ cells induced by exogenous glucocorticoid in rats

The influence of exogenous glucocorticoid, dexamethasone (Dex), on testicular germ cell apoptosis was investigated in rats. The percentages of apoptotic tubules and apoptotic germ cells in the Dex-treated group of rats were about seven-fold and 10-fold higher respectively than in either the control group, or in rats treated with glucocorticoid receptor agonist (GR-A), or in rats treated with both Dex and GR-A. These results suggest that, in rats, apoptosis of testicular germ cells is mediated by glucocorticoid receptors.

Multiparametric assessment of bursal lymphocyte apoptosis

When bursal lymphocytes are placed in cell culture, they undergo an apoptotic form of cell death that can be inhibited by phorbol esters and protein synthesis inhibitors. The goal of the current study was to evaluate the time course of this process and the inhibition of this process using several different assays to detect apoptosis: (1) terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) of lymphocyte DNA strand breaks with dUTP-FITC; (2) propidium iodide (PI) staining of lymphocyte chromatin; (3) chloromethyl-x-rosamine (CMX-Ros) binding to lymphocyte mitochondria; (4) merocyanine-540 (MC-540) binding to the lymphocyte plasma membrane; (5) flow cytometric analysis of light scatter from lymphocytes; (6) analysis of genomic DNA from lymphocytes by agarose gel electrophoresis; and (7) cellular caspase-3 activity of lymphocytes. When bursal lymphocyte apoptosis was analyzed as a function of time, or inhibited by phorbol esters or cycloheximide, all of these assays corroborated the apoptotic process. However, treatment of lymphocytes with a cytotoxic level of the proteinase inhibitor, n-ethylmaleimide (NEM) resulted in a putative, necrotic form of cell death that revealed discrepancies among the various assays in the detection of apoptotic cells. Specifically, the CMX-Ros and MC-540 assays erroneously detected the necrotic cells as being apoptotic cells following NEM treatment. These findings indicate the need for additional assays and appropriate treatment controls to verify the apoptotic process when using the CMX-Ros and MC-540 assays.

Natural and induced apoptosis during lymphocyte development in the axolotl

Lymphocytes apoptosis was characterized in a urodele amphibian, the axolotl, by morphology using electron microscopy and by flow cytometry after propidium iodide staining, as well as by biochemical criteria with the detection of DNA ladders after glucocorticoid treatment. The morphological and biochemical features observed in treated axolotls are in accordance with the criteria of apoptosis found in different models of mammalian lymphocyte programmed cell death. The onset of natural apoptosis was then detected by DNA fragmentation in thymus and in spleen during lymphocyte development and ontogenesis. A typical DNA ladder characteristic of apoptosis is detectable in the thymus as early as 5 months; apoptosis increases and peaks at 8 months, and is no longer detected by 10 months or thereafter. The ability of a superantigen, Staphylococcus aureus enterotoxin B (SEB), to induce T lymphocyte apoptosis in larvae was investigated as well. In vivo exposure of young axolotl larvae to SEB induces, as in mammals, thymocyte apoptosis as indicated by the enhancement of DNA fragmentation. These last results, natural programmed cell death and SEB induced apoptosis during thymic ontogeny, are discussed in correlation with what is known during mammalian thymic selection and apoptosis.

Effects of chicken thymic stromal cells on the growth and differentiation of thymocytes in vitro

We examined contact-mediated effects of chicken thymic stromal cells (TSC) on thymocyte differentiation by co-cultivation of these cell populations. The primary cultures of TSC isolated from thymus mainly have consisted of epithelial cells which were polygonal in shape, possessed long processes and expressed MHC class II antigen. When thymocytes were co-cultured with TSC, 60% to 70% of thymocytes attached to TSC and some of them engulfed underneath TSC. These attached thymocytes were CD4-CD8- and CD4+CD8+ subsets and expressed alpha/beta TCRhigh or gamma/delta TCRlow. Some of the thymocytes attaching to TSC showed an increase of intracellular and nuclear density, fragmentation of cytoplasm and nuclei, and DNA fragmentation. And also, thymocytes attaching to TSC contained a higher percentage of cycling (S and G2 + M phase) cells than nonattaching cells. These results indicate that specific subsets in thymocytes selectively bind to TSC and undergo apoptotic death or proliferation because of interaction with TSC. Chicken TSC may play an important role in thymic differentiation by direct contact within the thymus as in mammals.

Ontogeny of the thymus in a teleost fish, Cyprinus carpio L.: developing thymocytes in the epithelial microenvironment

A monoclonal antibody, WCL9, specific for membrane molecules of a thymocyte subpopulation was used to detect these cells in situ during the ontogeny of thymus. Cryo-sections revealed WCL9+ cells in the rudiment of the thymus (day 4 post fertilization); thereafter, the positive cells were observed exclusively in the cortex from the first appearance of thymic regionalization (week 4 post fertilization) until adult age. Whole-mount immunostaining of the thymus with WCL9 revealed the three-dimensional structure of the cortex by specific staining. The presence and distribution of apoptotic cells during thymus development was studied by in situ end-labelling of fragmented DNA. From week 4 post fertilization onwards, apoptotic cells were more frequently detected in the cortex than medulla, suggesting a continuous selection of thymocytes in the cortex. Ultrastructural studies confirmed the presence of numerous cortical apoptotic cells inside macrophages. Electron microscopy provided evidence for the existence of epithelial heterogeneity in the thymus. During the ontogeny, the differentiation of epithelial cells was followed from the first weeks until the juvenile age. Cell types were classified on the basis of their localization and cytological characteristics as: i) limiting epithelial cells located in subcapsular, perivascular and peritrabecular zones; ii) reticular epithelial cells situated in medullary and cortical zones; iii) nurse-like cells at the border between the cortex and medulla, iiii) Hassall's body-like structures localized in the medulla. This study could suggest the occurrence of a wide range of lympho-epithelial interactions throughout thymocytes differentiation.

Restraint stress-induced thymic involution and cell apoptosis are dependent on endogenous glucocorticoids

The aim of this study was to investigate the specific role of endogenous glucocorticoids (GC) following restraint stress on thymic involution and apoptosis. Restraint stress has been reported to alter physiological and behavioral responses in experimental animals. Exposure of mice to restraint stress led to involution of the thymus, to a decrease of the CD4+ 8+ thymocyte subset, and to fragmentation of thymic DNA. The role of endogenous GC in restraint stress-induced changes in the thymus was studied by three experimental approaches: surgical adrenalectomy, chemical adrenalectomy, and blocking of GC receptors by a specific type II receptor antagonist. In surgically-Adx mice, which lack endogenous GC, the effects of restraint on the thymus were wholly abrogated. Pretreatment of restrained mice with metyrapone (an 11beta hydroxylase inhibitor that specifically inhibits GC biosynthesis) had the same consequence, and blockage of GC receptors with the specific GC type II receptor antagonist RU-38486 attenuated the effects of the stressor. These findings indicate that GC are involved in the restraint-induced effects on the thymus.

Identification of a differential display product associated with apoptosis in chicken thymocytes

To further elucidate the cellular mechanisms that mediate programmed cell death in avian immune cells, differential display analysis was employed to identify differentially expressed genes in chicken thymocytes undergoing apoptosis. Primary cultures of thymocytes were treated with dexamethasone to activate apoptosis and RNA was isolated for differential display analysis. A differential display product designated A1 (479 bp) was identified. This display product was subcloned and induced expression of the genes was confirmed by ribonuclease protection analysis. Nucleotide sequence analysis of A1 revealed a putative 82 amino acid open reading frame that demonstrated limited homology with Bad, an apoptotic regulatory protein. Thus, A1 may represent the avian homolog of Bad.

Physiological levels of testosterone kill salmonid leukocytes in vitro

Adult spring chinook salmon (Oncorhynchus tshawytscha) elaborate high plasma concentrations of testosterone during sexual maturation, and these levels of testosterone have been shown to reduce the salmonid immune response in vitro. Our search for the mechanism of testosterone's immunosuppressive action has led to the characterization of an androgen receptor in salmonid leukocytes. In the present study we examined the specific effects that testosterone had on salmonid leukocytes. Direct counts of viable leukocytes after incubation with and without physiological levels of testosterone demonstrate a significant loss of leukocytes in cultures exposed to testosterone. At least 5 days of contact with testosterone was required to produce significant immunosuppression and addition of a "conditioned media" (supernatant from proliferating lymphocytes not exposed to testosterone) did not reverse the immunosuppressive effects of testosterone. These data lead us to conclude that testosterone may exert its immunosuppressive effects by direct action on salmonid leukocytes, through the androgen receptor described, and that this action leads to the death of a significant number of these leukocytes.

Biochemical analysis of programmed cell death during premeiotic stages of spermatogenesis in vivo and in vitro

Control points of regulator action during spermatogenesis are not completely known. Using the shark testis model, which facilitates analysis of spermatogenesis stage-by-stage in vivo and in vitro, an early biochemical marker of programmed cell death (PCD) was detected. Nucleosomal oligomers were seen in DNA extracts of testis and isolated spermatocysts (clonal germ cell/Sertoli cell units) at premeiotic (PrM), but not meiotic (M) or postmeiotic (PoM), stages. Cell nuclei isolated from M stages of development were susceptible to cleavage by micrococcal nuclease, suggesting that developmental control of factors other than a nuclease-insensitive chromatin structure may account for stage specificity. Cytological features of apoptosis were seen in germ cells, but not Sertoli cells, of a subset of isolated PrM spermatocysts and appeared to be all-or-none in affected clones. In culture, DNA fragmentation occurred on schedule with or without various additives, but the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) decreased accumulation of DNA breakdown products. Identification of the apoptotic form of PCD as a major, variable component of normal spermatogenesis and the use of PrM spermatocysts as an in vitro test system will allow further definition of mechanisms and developmental and physiological controls.

Thymocyte apoptosis induced by elevated endogenous corticosterone levels

A well-known model of apoptosis is induction in thymocytes by injection of pharmacological doses of exogenous steroids. The aim of this study was to investigate whether this process also occurs under physiological conditions, i.e. by stimulation of endogenous glucocorticoid release, using the chicken as an experimental model. Endogenous glucocorticoid levels can be elevated by immunization with exogenous antigens or by injection of conditioned medium, e.g. supernatant of mitogen-stimulated spleen cells. This effect is mediated by so-called glucocorticoid-increasing factors, and is considered to act as an immunoregulatory principle. Thymocyte DNA of so treated birds showed a typical "ladder" pattern after electrophoresis in a 1.8% agarose gel, and degradation could be prevented by RU 38,486. This provides evidence that apoptosis can be induced by elevating endogenous corticosterone levels in vivo. By means of in situ nick translation (ISNT) and simultaneous immunofluorescence tests, it was possible to analyze various thymic subpopulations during apoptosis after treatment with exogenous glucocorticoids. Additionally, using confocal microscopical techniques, apoptosis of the same cells as analyzed by ISNT is shown. The possible role of elevated concentrations of endogenous glucocorticoids in regulating thymocyte cell death and autoimmune diseases is discussed.

Absence of endonuclease activation during acute cell death in renal proximal tubules

The role of endonuclease and poly(ADP-ribose) polymerase activation in various types of cell injuries and death to rabbit renal proximal tubule suspensions was examined. Proximal tubules were exposed to the mitochondrial inhibitor antimycin A (0.1 microM), the protonophore carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP, 1 microM), the calcium ionophore ionomycin (5 microM), or the oxidant t-butyl hydroperoxide (TBHP, 0.5 mM) in the absence or presence of the endonuclease inhibitor aurintricarboxylic acid or the poly(ADP-ribose) polymerase inhibitor 3-aminobenzamide. Lactate dehydrogenase (LDH) release was used as a marker of cell death and analysis of genomic DNA for internucleosomal cleavage was used as a marker of endonuclease activation. Aurintricarboxylic acid and 3-aminobenzamide had no effect on the proximal tubule LDH release produced by 1 h exposure to antimycin A, FCCP, or ionomycin, or 2 h exposure to TBHP. Furthermore, there was no evidence of DNA fragmentation with any compound prior to or after cell death began. As a positive control, proximal tubules exposed to digitonin in the absence of metabolic substrates resulted in the chelator-inhibitable fragmentation of DNA, indicating that the endonuclease is present in proximal tubules. These results show that endonuclease activation did not occur prior to or after cell death began. Furthermore, these results suggest that endonuclease and poly(ADP-ribose) polymerase activation do not play a role in this model of acute renal proximal tubule cell injury and death induced by agents that cause oxidative stress, mitochondrial dysfunction, or increases in cytosolic free calcium.

Programmed cell death in avian thymocytes: role of the apoptotic endonuclease

Apoptosis, or programmed cell death, is a physiological process whereby a target cell dies in response to a specific signal. A prominent model system used to study this process is the glucocorticoid-mediated killing of immature thymocytes. Following glucocorticoid treatment, apoptotic thymocytes undergo a series of distinct morphological alteration including cellular shrinkage, blebbing of the cytoplasmic membrane, and chromatin condensation. The chromatin condensation that occurs during apoptosis is associated with a characteristic endonuclease activity that degrades the genome at internucleosomal sites. To study this characteristic endonuclease activity further, nuclear extracts were prepared from thymocytes of glucocorticoid-treated chicks and nuclease activity present in the protein extracts was analyzed using chicken red blood cell nuclei as a substrate. Using this in vitro assay system, it was demonstrated that the avian endonuclease activity degrades chromatin at internucleosomal sites and can be inhibited by EDTA and zinc ions. Current efforts are focused on purifying the avian apoptotic endonuclease and further characterizing this nuclease activity.

Analysis of thymic lymphocyte apoptosis using in vitro techniques

Thymocyte apoptosis was analyzed in 4-week-old chicks that were treated with dexamethasone. Glucocorticoid treatment of birds resulted in the internucleosomal cleavage of thymocyte DNA as detected by agarose gel electrophoresis and the generation of a population of thymocyte nuclei with a hypodiploid content of cellular DNA when analyzed by flow cytometry. Visualization of apoptotic thymocytes at the ultrastructural level or via confocal microscopy revealed the typical morphological characteristics of cells undergoing apoptosis. Similar techniques were employed to further analyze apoptosis using an in vitro approach where extracts of thymocyte nuclear proteins from glucocorticoid-treated birds were incubated with chicken red blood cell (cRBC) nuclei. The thymocyte nuclear protein extract contained an endonuclease activity that degraded chromatin at internucleosomal sites and generated an amplified population of hypodiploid cRBC nuclei, similar to that detected in vivo in glucocorticoid-treated thymocytes. These data demonstrate a clear correlation between the detection of apoptotic endonuclease activity generated in vivo, where endogenous thymocyte DNA was degraded, with an in vitro assay, where extracted thymocyte nuclear proteins demonstrated a similar type of nuclease activity when cRBC nuclei were employed as an exogenous chromatin substrate.

A biochemical hallmark of apoptosis: internucleosomal degradation of the genome

Apoptosis, or programmed cell death, is an endogenous cellular process whereby an external signal activates a metabolic pathway that results in cell death. This form of cell death appears to be a common feature in many biological processes where cell deletion is a mechanism for altering tissue structure and function. Historically, apoptosis has been studied using histological techniques; however, more recent interest has focused on analyzing this process at the biochemical level. A biochemical hallmark of apoptosis is a characteristic form of DNA degradation in which the genome is cleaved at internucleosomal sites, generating a 'ladder' of DNA fragments when analyzed by agarose gel electrophoresis. A number of assay systems have been developed to study this nuclease activity. For example, nuclease activity has been analyzed by measuring the release of endogenous DNA from apoptotic cells, by flow cytometric analysis of apoptotic cells and by analyzing in situ apoptotic nuclease activity in polyacrylamide gels containing DNA. Use of these assay systems has enabled investigators to study the signal transduction pathways that mediate apoptosis and to characterize the endonuclease itself. Future biochemical studies in this field will focus on isolating the genes and gene products that mediate apoptosis.

Malignant transformation by mutant Rel proteins

A newly described family of transcriptional regulatory proteins, the Rel family, has recently been the subject of much interest. The Rel family includes proteins known to be important in Drosophila development, replication of HIV-1, oncogenesis and general transcriptional control. Nevertheless, there is still much to be learned about their precise mechanism of action, including the process by which the original member of this family, v-Rel, malignantly transforms cells.

Development of an apoptosis endonuclease assay

A biochemical hallmark of cells undergoing programmed cell death, or apopotosis, is the endonucleolytic cleavage of genomic DNA at internucleosomal sites. To study further the nuclease involved in this process, an assay system was developed to measure internucleosomal DNA degradation. Micrococcal nuclease (MNase), a bacterial enzyme that cleaves chromatin at internucleosomal intervals, was used to validate the assay procedure. Thymocyte nuclear proteins obtained from glucocorticoid-treated chickens, a source of internucleosomal DNA-degrading activity, were incubated with chicken red blood cell nuclei, and genomic DNA was subsequently extracted and analyzed by agarose gel electrophoresis. Generation of internucleosomal DNA degradation products by the thymocyte protein extract required ATP and was both time and protein concentration dependent. This nuclease activity could be inhibited by EDTA, EGTA, alkylating agents, or heat denaturation. Addition of purified proteinases, RNases, or other types of nucleases to the assay failed to generate discrete internucleosomal lengths of DNA, thus confirming the nuclease specificity of this assay. On the basis of these data, we believe that this assay system will be instrumental in isolating and characterizing the nuclease(s) associated with apoptosis.

Activation of thymocyte deoxyribonucleic acid degradation by endogenous glucocorticoids

Immature lymphocytes in the thymus gland are killed by treatment with exogenous glucocorticoids. This steroid-mediated lymphocytolysis is preceded by numerous alterations in lymphocyte metabolism, including a DNA-degrading process in which the genome is cleaved at internucleosomal intervals. To date, this process has only been characterized by treating lymphocytes in vitro with glucocorticoids or by exogenous treatment of whole animals with adrenal steroids. To determine whether thymocyte DNA degradation could be activated by endogenous glucocorticoids, 4-wk-old chicks were treated with porcine adrenocorticotropic hormone (ACTH). This procedure elevated serum corticosterone levels approximately 80-fold within 2 h of hormone treatment. Following ACTH administration, thymocyte DNA was isolated and analyzed by agarose gel electrophoresis. The ACTH activated a DNA-degrading process that generated internucleosomal fragments of DNA identical in size to those observed following exogenous treatment with synthetic or naturally occurring glucocorticoids. Furthermore, this response could be inhibited by the glucocorticoid antagonist RU486 (17 beta-hydroxy-11 beta, 4-dimethylaminophenyl-17 alpha-propynl-estra-4,9,diene-3-one), indicating that adrenal steroids activate this process via the glucocorticoid receptor. These results demonstrate that lymphocyte DNA degradation does not result solely from exogenous glucocorticoid treatment; moreover, endogenous glucocorticoids can mediate this process and may thereby play an important role in thymic gland function.

Glucocorticoid activation of deoxyribonucleic acid degradation in bursal lymphocytes

Treatment of animals with exogenous adrenal steroids or elevation of endogenous glucocorticoids results in a profound involution of lymphoid tissue. In rodent species, this lymphoinvolution is accompanied by lymphocyte cell death and extensive degradation of the genome prior to lymphocytolysis. In the present study, this process was investigated in the bursa of Fabricius of domestic fowl. Four-wk-old chicks were treated with a single injection of dexamethasone, and bursal regression and cell viability were monitored over a 72-h period. Following hormone treatment, DNA was extracted from bursal lymphocytes and analyzed by agarose gel electrophoresis. Dexamethasone treatment resulted in a rapid regression of bursal tissue that could be detected as soon as 6 h posttreatment, but lymphocyte viability was not altered until 24 h afterward. The DNA isolated from bursal lymphocytes of glucocorticoid-treated birds appeared to be degraded at internucleosomal sites and generated a "ladder" of discrete DNA fragments when analyzed by agarose gel electrophoresis. This form of hormone-induced cell death, referred to as programmed cell death, may play a key role in glucocorticoid-mediated immunosuppression.