Fabrication of an affordable and sensitive corticosteroid-binding globulin immunosensor based on electrodeposited gold nanoparticles modified glassy carbon electrode

Herein, we fabricated an ultrasensitive electrochemical immunosensor for the quantitative detection of corticosteroid-binding globulin (CBG). CBG is a protein that regulates glucocorticoid levels and is an important biomarker for inflammation. A decrease in CBG levels is a key biomarker for inflammatory diseases, such as septic shock. To enhance the electrochemical performance and provide a large surface area for anti-CBG immobilization, we functionalized the glassy carbon electrode surface with AuNPs. Electrochemical characterization methods including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to examine the construction of the fabricated immunosensor. The electrochemical signal demonstrated a remarkable sensitivity to the CBG antigen, with a detection range from 0.01 to 100 μg/mL and a limit of detection of 0.012 μg/mL, making it suitable for both clinical and research applications. This label-free immunosensor offers significant advantages, including high sensitivity, low detection limits and excellent selectivity, making it a promising tool for detecting CBG in complex biological samples. Its potential applications include early disease diagnosis, treatment monitoring and studying CBG-related physiological processes.

Effects of estradiol- and ethinylestradiol-based contraceptives on adrenal steroids: A randomized trial

OBJECTIVES

Ethinylestradiol (EE)-based combined oral contraceptives (COC) affect adrenal function by altering steroid and corticosteroid-binding globulin (CBG) synthesis that may contribute to adverse effects related to these drugs. The effects of COCs containing natural estrogens remain unclear. We compared the effects of COCs containing estradiol valerate (EV) and EE on cortisol and other adrenal steroid hormones.

STUDY DESIGN

A spin-off study of a randomized, open-label trial. Fifty-nine healthy women were allocated to groups that engaged in the continuous use of EV+dienogest (DNG), EE+DNG, or DNG only for 9 weeks. We measured changes in adrenal steroids, CBG, and the free cortisol index (FCI).

RESULTS

Treatment with EE+DNG increased total cortisol (mean increment 668 nmol/L, p < 0.001) and cortisone (10 nmol/L, p= 0.001) levels, whereas the change from the baseline was insignificant for the EV+DNG and DNG-only groups. Dehydroepiandrosterone sulfate decreased by 24% in the EE+DNG group but remained unchanged in the EV+DNG and DNG-only groups. Aldosterone and 17-hydroxyprogesterone levels did not differ between the groups. All preparations increased CBG, but the increase in the EE+DNG group (median increment 42 µg/mL, p < 0.001) was 9- and 49-fold higher than that in the EV+DNG and DNG-only groups, respectively. The FCI remained unchanged in all study groups, indicating that cortisol and CBG mainly increased in parallel, although some individuals demonstrated larger alterations in the cortisol-CBG balance.

CONCLUSION

In COCs, EV had a milder effect on circulating CBG and adrenal steroid levels than EE; however, further research is necessary to determine the long-term effects.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02352090 IMPLICATIONS: EV-based COC had reduced effects on circulating CBG and adrenal steroids compared to EE, probably due to a lower hepatic impact. Whether the sensitization of the adrenals to ACTH varies according to COC contents and whether it relates to experienced side effects needs to be investigated. These results encourage further research and development of contraceptives containing natural estrogens.

Assay temperature affects corticosteroid-binding globulin and free corticosterone estimates across species

Glucocorticoid hormones are often measured to assess how organisms physiologically respond to challenges in their environment. In plasma, glucocorticoids circulate in two forms: bound to corticosteroid-binding globulins (CBG) or unbound (free). Measuring CBG allows us to estimate the amount of free glucocorticoids present in a plasma sample. However, free glucocorticoid estimates are affected by the assay temperature used when measuring CBG, with colder temperatures maximizing specific binding but likely underestimating glucocorticoid's affinity for CBG. Here, we test how a biologically relevant incubation temperature (41 °C) changes the disassociation constant (K_d; used to estimate free glucocorticoid levels) when compared to the traditional 4 °C incubation temperature, across four commonly studied avian species. We then apply the new K_d's calculated at 41 °C to existing data sets to examine how the change in K_d affects free corticosterone estimates and data interpretation. K_d's were generally higher (lower affinity for CORT) at warmer incubation temperatures which resulted in higher levels of estimated free CORT in all four species but differed among subspecies. This increase in free CORT levels did not qualitatively change previously reported statistical relationships, but did affect variance and alpha (P) values. We suggest that future assays be run at biologically relevant temperatures for more accurate estimates of free CORT levels in vivo and to increase the chances of detecting biological patterns of free-CORT that may not be revealed with the classic methodology that tends to underestimate free CORT levels.

Avian corticosteroid-binding globulin: biological function and regulatory mechanisms in physiological stress responses

Corticosteroid-binding globulin (CBG) is a high-affinity plasma protein that binds glucocorticoids (GCs) and regulates their biological activities. The structural and functional properties of CBG are crucial to understanding the biological actions of GCs in mediating stress responses and the underlying mechanisms. In response to stress, avian CBGs modulate the free and bound fractions of plasma corticosterone (CORT, the main GC), enabling them to mediate the physiological and behavioral responses that are fundamental for balancing the trade-off of energetic investment in reproduction, immunity, growth, metabolism and survival, including adaptations to extreme high-elevation or high-latitude environments. Unlike other vertebrates, avian CBGs substitute for sex hormone-binding globulin (SHBG) in transporting androgens and regulating their bioavailability, since birds lack an Shbg gene. The three-dimensional structures of avian and mammalian CBGs are highly conserved, but the steroid-binding site topographies and their modes of binding steroids differ. Given that CBG serves as the primary transporter of both GCs and reproductive hormones in birds, we aim to review the biological properties of avian CBGs in the context of steroid hormone transportation, stress responses and adaptation to harsh environments, and to provide insight into evolutionary adaptations in CBG functions occurred to accommodate physiological and endocrine changes in birds compared with mammals.

Seasonal changes in acute stressor-mediated plasma glucocorticoid regulation in New World flying squirrels

Southern flying squirrels have higher circulating cortisol levels than most vertebrates. However, regulation of tissue exposure to cortisol by the hormone's carrier protein, corticosteroid-binding globulin (CBG), appears to be altered due to lower-than-expected CBG expression levels, and a reduced affinity for cortisol. To assess the capacity of flying squirrels to regulate acute stress-mediated cortisol levels, we used the dexamethasone (DEX) suppression test followed by the adrenocorticotropic hormone (ACTH) stimulation test in both the breeding and non-breeding seasons, and quantified resultant changes in plasma cortisol and relative CBG levels. Regulation of cortisol via negative feedback, and the acute stress response appeared to function as they do in other vertebrates during the breeding season, but response to DEX in the non-breeding season showed that the sensitivity of the negative feedback mechanism changed across seasons. The relatively high concentrations of DEX required to induce negative feedback suggests that southern flying squirrels have a reduced sensitivity to cortisol compared with other vertebrates, and that high circulating cortisol levels may be required to compensate for low target tissue responsiveness in this species. Cortisol, but not CBG levels, were higher during the non-breeding than breeding season, and females had higher cortisol and CBG levels than males. Our data suggest that flying squirrel cortisol levels are regulated by negative feedback at a higher set point than in related species. Seasonal changes in cortisol levels, target tissue sensitivity to DEX, and in the capacity to respond to stressors appear to be part of the underlying physiology of southern flying squirrels, and may be required to maximize fitness in the face of tradeoffs between survival and reproduction.

Analysis and validation of a new extended method for estimating plasma free cortisol including neutrophil elastase and competition from other steroids

A non-linear mechanistic model for the distribution of cortisol in plasma on free and bound forms is proposed. The influence of progesterone, testosterone and neutrophil elastase on the cortisol distribution in the blood is investigated. The activity of neutrophil elastase is directly included in the model with the concentration of elastase and the kinetic constants describing the activity of elastase collected in one single input variable. The model is very sensitive towards this input variable and fits data excellently, when it is allowed to be subject specific. The analysis shows that steroids such as testosterone with low affinity for corticosteroid-binding globulin (CBG) do not significantly influence the concentration of free cortisol. Progesterone has a high affinity for CBG, but low plasma concentrations compared to cortisol. Contrary to expectations, progesterone is shown to impact the distribution of cortisol in plasma both under circumstances with high levels as seen in pregnancy and during the normal menstrual cycle of women. Comparing the predictions of our model with predictions made with the equilibrium models by Coolens et al. [1], Dorin et al. [2] and Nguyen et al. [3] shows that the models differ considerably not only in their predictions for free cortisol, but also for cortisol on bound forms; i.e. bound to albumin, intact CBG and elastase-cleaved CBG. Disregarding some of the smallest terms of the model equations a reduced version of the model in form of a fourth order polynomial equation is obtained. The reduced version of the model performs almost identically to the full version and serves as a new formula for calculating the plasma free cortisol concentration.

Glucocorticoids and CBG during pregnancy in mammals: diversity, pattern, and function

Pregnancy is one of the defining characteristics of placental mammals. Key in the growth and development of the fetus during pregnancy are the dynamics of glucocorticoids (GCs) and their binding protein,corticosteroid-binding globulin (CBG), which determines how much of the GCs are free and biologically active. Out of more than 5000 species of placental mammals in 19 different orders, our understanding of the dynamics of maternal GCs and CBG during pregnancy is largely limited to the detailed study of 3 groups - sheep, laboratory rodents, and humans. The assumption is often made that what we see in these few species applies to the rest. To examine this generality, we compared patterns of maternal GCs over pregnancy from all placental mammals where data is available: in the blood of 13 species from 5 different orders and in metabolites in excreta in an additional 20 species from 9 orders. We found that maternal free GCs increase by late pregnancy in most taxa. This increase is achieved by either an increase in total GC secretion or a decrease in CBG. A major exception is found in the even-toed ungulates (sheep, cows, etc.) where maternal GCs and CBG remain stable, but where the fetal adrenals mature in late pregnancy and produce the majority of their own GCs. We conclude that patterns of change in maternal GCs and CBG during pregnancy are species-specific but are alternative means to the same end: increased fetal exposure to GCs in late pregnancy, which is essential for development.

Monoclonal antibodies to the reactive centre loop (RCL) of human corticosteroid-binding globulin (CBG) can protect against proteolytic cleavage

Corticosteroid-binding globulin (CBG) binds most of the cortisol in circulation and is a non-functional member of the family of serine protease inhibitors (serpins) with an exposed elastase sensitive reactive centre loop (RCL). The RCL can be cleaved by human neutrophil elastase, released from activated neutrophils, and can also be cleaved at nearby site(s) by elastase released by Pseudomonas aeruginosa, and at two further sites, also within the RCL, by bovine chymotrypsin. Cleavage of the RCL results in a conformational change accompanied by a marked decrease in affinity for cortisol and hence its release at the site of proteolysis. These cleavages are irreversible and the similar half-lives of cleaved and intact CBG could mean that there may be some advantage in slowing the rate of CBG cleavage in acute inflammation thereby increasing the proportion of intact CBG in circulation. Here we show, for the first time, that pre-incubation of tethered human CBG with two monoclonal antibodies to the RCL of CBG protects against cleavage by all three enzymes. Furthermore, in plasma, pre-incubation with both RCL monoclonal antibodies delays neutrophil elastase cleavage of the RCL and one of these RCL monoclonal antibodies also delays bovine chymotrypsin cleavage of the RCL. These findings may provide a basis and rationale for the concept of the use of RCL antibodies as therapeutic agents to effectively increase the proportion of intact CBG in circulation which may be of benefit in acute inflammation.

Seasonal programming, not competition or testosterone, drives stress-axis changes in a partially-semelparous mammal

Animals must make tradeoffs between reproduction and longevity. This is particularly pronounced in male arctic ground squirrels (Urocitellus parryii), that compete aggressively for territories and mates during a three-week breeding season. Breeding males have high rates of severe wounding, high mortality rates, and high free cortisol levels, along with downstream consequences of chronic stress (weight loss, reduced immune function) that appear to contribute to their early death. The elevated cortisol levels are thought to be a result of the intense intrasexual competition. An alternative hypothesis, however, is that the hormonal change is a seasonal adaptation facilitating the tradeoff of immediate competitive advantage at the expense of long-term survival. We tested a two-part hypothesis: first, that elevated free cortisol during the breeding period is a seasonal change that will still occur in the absence of actual competition, and second, that testosterone maintains this increase. We measured plasma cortisol, corticosteroid-binding globulin, and fecal glucocorticoid metabolites in three groups: wild male ground squirrels, captive males prevented from fighting, and captive castrated males. There were no differences amongst these three groups in free and total plasma cortisol, fecal glucocorticoids, or downstream measures of chronic stress. This suggests that high free cortisol and its effects on breeding males are not a consequence of contest competition during the breeding season, but rather a generalized seasonal change. We found no evidence that testosterone plays a role in maintaining elevated free cortisol in arctic ground squirrel males.

Coping with pregnancy after 9months in the dark: Post-hibernation buffering of high maternal stress in arctic ground squirrels

Pregnancy and lactation are key times in the life of female mammals when energetic resources must be brought to bear to produce and nurture offspring. Changes in glucocorticoid (GC) levels are central to this objective, due to their roles in modulating development and physiology and in mediating energetic tradeoffs. We examined GC changes over reproduction in a species living in a harsh seasonal environment: the arctic ground squirrel (Urocitellus parryii). Females become pregnant immediately after emerging from a ∼9month hibernation, and then must begin this pregnancy during an additional month of freezing temperatures and limited food availability. We measured plasma levels of total cortisol, corticosteroid-binding globulin (CBG), and free cortisol in unstressed females at three stages: not-visibly pregnant, visibly pregnant, and lactating. Total cortisol levels were similar in all stages, but CBG levels increased 4-fold from the not-visibly pregnant stage to visibly pregnant and lactating stage. As a result, the free cortisol fraction declined from 51% of total cortisol when females were not-visibly pregnant to only 5% when they were visibly pregnant (remaining low and stable throughout pregnancy) and 10% when they were lactating. This pattern is markedly different from that seen in other mammals, where 10% or less of GCs are free and these tend to increase during gestation. We postulate that the high free cortisol just prior to visible pregnancy is a seasonal adaptation relating either to the pronounced physiological changes the female must undergo after emerging from hibernation and immediately getting pregnant, or to the mobilization of body reserves for energy to permit pregnancy, or both. Thereafter, high CBG levels may shield the developing offspring from the negative effects of cortisol overexposure.

Corticosteroid-binding globulin (CBG) reactive centre loop antibodies and surface plasmon resonance interrogate the proposed heat dependent "flip-flop" mechanism of human CBG

Corticosteroid-binding globulin (CBG) is the predominant carrier of cortisol in circulation and is a non-inhibitory member of the serpin superfamily of serine protease inhibitors. In the stressed or "S" conformation, CBG possesses an intact exposed reactive centre loop (RCL) that can be irreversibly cleaved by elastase released from activated human neutrophils whereupon it adopts a relaxed or "R" conformation. The latter conformation has decreased affinity for cortisol, allowing the release of the majority of cortisol at sites of inflammation. Recently there has been speculation that mild increments in heat such as found in pyrexia (39-40°C) may also induce a reversible "flip-flop" of the RCL into the body of the protein structure, without cleavage, facilitating a reversible temperature-dependent release of cortisol. Here we raised a new monoclonal antibody to the RCL of human CBG and used this in concert with an existing RCL antibody and show by surface plasma resonance that, at temperatures up to 40°C, the RCL of purified CBG and the RCL of CBG in intact plasma is accessible to these two antibodies. Together, the epitopes of these antibodies span 11 consecutive amino acids (STGVTLNLTSK) of the 18 residues of the RCL. This adequate antibody cover of the RCL sequence leads to the conclusion that the proposed temperature-dependent "flip-flop" of the RCL of CBG is doubtful.

Determinants and significance of corticosterone regulation in the songbird brain

Songbirds exhibit significant adult neuroplasticity that, together with other neural specializations, makes them an important model system for neurobiological studies. A large body of work also points to the songbird brain as a significant target of steroid hormones, including corticosterone (CORT), the primary avian glucocorticoid. Whereas CORT positively signals the brain for many functions, excess CORT may interfere with natural neuroplasticity. Consequently, mechanisms may exist to locally regulate CORT levels in brain to ensure optimal concentrations. However, most studies in songbirds measure plasma CORT as a proxy for levels at target tissues. In this paper, we review literature concerning circulating CORT and its effects on behavior in songbirds, and discuss recent work suggesting that brain CORT levels are regulated independently of changes in adrenal secretion. We review possible mechanisms for CORT regulation in the avian brain, including corticosteroid-binding globulins, p-glycoprotein activity in the blood-brain barrier and CORT metabolism by the 11ß hydroxysteroid dehydrogenases. Data supporting a role for CORT regulation within the songbird brain have only recently begun to emerge, suggesting that this is an avenue for important future research.

Corticosteroid-binding globulin cleavage is paradoxically reduced in alpha-1 antitrypsin deficiency: Implications for cortisol homeostasis

High-affinity corticosteroid-binding globulin (haCBG) is cleaved by neutrophil elastase (NE) resulting in permanent transition to the low cortisol-binding affinity form (laCBG), thereby increasing cortisol availability at inflammatory sites. Alpha-1 antitrypsin (AAT) is the major inhibitor of NE. AAT deficiency (AATD) predisposes patients to early-onset emphysema due to increased proteolytic destruction from the inherent proteinase-antiproteinase imbalance. We hypothesized that AATD may result in increased CBG cleavage in vivo. We collected demographic data and blood samples from 10 patients with AATD and 28 healthy controls measuring total CBG and haCBG levels by parallel in-house ELISAs, as well as AAT, total and free cortisol levels. haCBG was higher (median [range]); 329 [210-551] vs. 250 [175-365] nmol/L; P<0.005, and laCBG lower; 174 [68-229] vs. 220 [119-348] nmol/L; P=0.016 in the AATD group, compared with controls. The ratio of haCBG:total CBG was also higher in AATD; 72 [53-83] vs. 54 [41-72] %; P=0.0001). There was a negative correlation between haCBG:total CBG and AAT levels (P<0.05, R=-0.64). Paradoxically, proteolytic cleavage of CBG was reduced in AATD, despite the recognized increase in NE activity. This implies that NE activity is not the mechanism for systemic CBG cleavage in basal, low inflammatory conditions. Relatively low levels of laCBG may have implications for cortisol action in AATD.

The half-lives of intact and elastase cleaved human corticosteroid-binding globulin (CBG) are identical in the rabbit

Corticosteroid-binding globulin (CBG) is a non-inhibitory member of the serpin superfamily of serine protease inhibitors and carries the majority of cortisol in circulation. It can be cleaved by neutrophil elastase at its exposed reactive centre loop which decreases its affinity for cortisol allowing the release of most of the cortisol at sites of inflammation. Intact and elastase cleaved CBG can be distinguished from each other and can coexist in circulation but with unknown half-lives. Here we treated a portion of purified human CBG with elastase, terminated the digestion and then combined this portion with intact human CBG and measured their respective half-lives in rabbits by ELISA. This investigation shows for the first time that the half-lives of intact and elastase cleaved CBG are identical (∼10h). This is an important finding as it implies that in conditions such as sepsis and septic shock where levels of intact CBG are low and the proportion of cleaved CBG is high that this is likely sustained which may affect the CBG mediated targeted delivery of cortisol to sites of inflammation. Furthermore the residual binding of cortisol to cleaved CBG may alter the overall buffering capacity of CBG for cortisol resetting the baseline concentration of free cortisol.

The reactive centre loop of corticosteroid-binding globulin (CBG) is a protease target for cortisol release

Corticosteroid-binding globulin (CBG) binds more than 90% of circulating cortisol and is a non-inhibitory member of the family of serine protease inhibitors (SERPINS) with an exposed elastase sensitive reactive centre loop (RCL). At sites of inflammation neutrophil activation can release elastase which may cleave the RCL and result in cortisol release from CBG. The RCL sequence also has two theoretical chymotrypsin cleavage sites and we used a monoclonal antibody with specificity for the RCL to investigate chymotrypsin cleavage of CBG. Here we show, for the first time, rapid chymotrypsin cleavage of the RCL of CBG, resulting in undetectable levels of intact CBG, whereas total CBG levels were unchanged. Coincident with both chymotrypsin and elastase cleavage there was an increase in the free cortisol fraction of serum to levels similar to when CBG had been inactivated by heat indicating total cortisol release from CBG. These findings demonstrate a new mechanism for cortisol release from its binding globulin.

The biology of the glucocorticoid receptor: new signaling mechanisms in health and disease

Glucocorticoids are primary stress hormones necessary for life that regulate numerous physiologic processes in an effort to maintain homeostasis. Synthetic derivatives of these hormones have been mainstays in the clinic for treating inflammatory diseases, autoimmune disorders, and hematologic cancers. The physiologic and pharmacologic actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), a member of the nuclear receptor superfamily of ligand-dependent transcription factors. Ligand-occupied GR induces or represses the transcription of thousands of genes through direct binding to DNA response elements, physically associating with other transcription factors, or both. The traditional view that glucocorticoids act through a single GR protein has changed dramatically with the discovery of a large cohort of receptor isoforms with unique expression, gene-regulatory, and functional profiles. These GR subtypes are derived from a single gene by means of alternative splicing and alternative translation initiation mechanisms. Posttranslational modification of these GR isoforms further expands the diversity of glucocorticoid responses. Here we discuss the origin and molecular properties of the GR isoforms and their contribution to the specificity and sensitivity of glucocorticoid signaling in healthy and diseased tissues.