The mineralocorticoid receptor knockout in medaka is further validated by glucocorticoid receptor compensation

Molecular mechanisms of the stress-induced regulation of the inflammatory response in fish

Stressors in the environment of aquatic organisms can profoundly affect their immune system. The stress response in fish involves the activation of the hypothalamus-pituitary-interrenal (HPI) axis, leading to the release of several stress hormones, among them glucocorticoids, such as cortisol, which bind and activate corticosteroid receptors, namely the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR). These receptors are highly expressed on immune cells, thereby allowing stress to have a potent effect that is classically considered to suppress immune function. In this review, we highlight the conserved structure and function of GR and MR among vertebrates and describe their role in modulating inflammation by regulating the expression of pro-inflammatory and anti-inflammatory genes. In particular, the involvement of MR during inflammation is reviewed, which in many studies has been shown to be immune-enhancing. In recent years, the use of zebrafish as a model organism has opened up new possibilities to study the effects of stress on inflammation, making it possible to investigate knockout lines for MR and/or GR, in combination with transgenic models with fluorescently labeled leukocyte subpopulations that enable the visualization and manipulation of these immune cells. The potential roles of other hormones of the HPI axis, such as corticotrophin-releasing hormone (Crh) and adrenocorticotropic hormone (Acth), in immune modulation are also discussed. Overall, this review highlights the need for further research to elucidate the specific roles of GR, MR and other stress hormones in regulating immune function in fish. Understanding these mechanisms will contribute to improving fish health and advancing our knowledge of stress signalling.

Conditional Ablation of Glucocorticoid and Mineralocorticoid Receptors from Cochlear Supporting Cells Reveals Their Differential Roles for Hearing Sensitivity and Dynamics of Recovery from Noise-Induced Hearing Loss

Endogenous glucocorticoids (GC) are known to modulate basic elements of cochlear physiology. These include both noise-induced injury and circadian rhythms. While GC signaling in the cochlea can directly influence auditory transduction via actions on hair cells and spiral ganglion neurons, evidence also indicates that GC signaling exerts effects via tissue homeostatic processes that can include effects on cochlear immunomodulation. GCs act at both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Most cell types in the cochlea express both receptors sensitive to GCs. The GR is associated with acquired sensorineural hearing loss (SNHL) through its effects on both gene expression and immunomodulatory programs. The MR has been associated with age-related hearing loss through dysfunction of ionic homeostatic balance. Cochlear supporting cells maintain local homeostatic requirements, are sensitive to perturbation, and participate in inflammatory signaling. Here, we have used conditional gene manipulation techniques to target Nr3c1 (GR) or Nr3c2 (MR) for tamoxifen-induced gene ablation in Sox9-expressing cochlear supporting cells of adult mice to investigate whether either of the receptors sensitive to GCs plays a role in protecting against (or exacerbating) noise-induced cochlear damage. We have selected mild intensity noise exposure to examine the role of these receptors related to more commonly experienced noise levels. Our results reveal distinct roles of these GC receptors for both basal auditory thresholds prior to noise exposure and during recovery from mild noise exposure. Prior to noise exposure, auditory brainstem responses (ABRs) were measured in mice carrying the floxed allele of interest and the Cre recombinase transgene, but not receiving tamoxifen injections (defined as control (no tamoxifen treatment), versus conditional knockout (cKO) mice, defined as mice having received tamoxifen injections. Results revealed hypersensitive thresholds to mid- to low-frequencies after tamoxifen-induced GR ablation from Sox9-expressing cochlear supporting cells compared to control (no tamoxifen) mice. GR ablation from Sox9-expressing cochlear supporting cells resulted in a permanent threshold shift in mid-basal cochlear frequency regions after mild noise exposure that produced only a temporary threshold shift in both control (no tamoxifen) f/fGR:Sox9iCre+ and heterozygous f/+GR:Sox9iCre+ tamoxifen-treated mice. A similar comparison of basal ABRs measured in control (no tamoxifen) and tamoxifen-treated, floxed MR mice prior to noise exposure indicated no difference in baseline thresholds. After mild noise exposure, MR ablation was initially associated with a complete threshold recovery at 22.6 kHz by 3 days post-noise. Threshold continued to shift to higher sensitivity over time such that by 30 days post-noise exposure the 22.6 kHz ABR threshold was 10 dB more sensitive than baseline. Further, MR ablation produced a temporary reduction in peak 1 neural amplitude one day post-noise. While supporting cell GR ablation trended towards reducing numbers of ribbon synapses, MR ablation reduced ribbon synapse counts but did not exacerbate noise-induced damage including synapse loss at the experimental endpoint. GR ablation from the targeted supporting cells increased the basal resting number of Iba1-positive (innate) immune cells (no noise exposure) and decreased the number of Iba1-positive cells seven days following noise exposure. MR ablation did not alter innate immune cell numbers at seven days post-noise exposure. Taken together, these findings support differential roles of cochlear supporting cell MR and GR expression at basal, resting conditions and especially during recovery from noise exposure.

Analysis of stress responses in Astyanax larvae reveals heterogeneity among different populations

Stress responses are conserved physiological and behavioral outcomes as a result of facing potentially harmful stimuli, yet in pathological states, stress becomes debilitating. Stress responses vary considerably throughout the animal kingdom, but how these responses are shaped evolutionarily is unknown. The Mexican cavefish has emerged as a powerful system for examining genetic principles underlying behavioral evolution. Here, we demonstrate that cave Astyanax have reduced behavioral and physiological measures of stress when examined at larval stages. We also find increased expression of the glucocorticoid receptor, a repressible element of the neuroendocrine stress pathway. Additionally, we examine stress in three different cave populations, and find that some, but not all, show reduced stress measures. Together, these results reveal a mechanistic system by which cave-dwelling fish reduced stress, presumably to compensate for a predator poor environment.

The mineralocorticoid receptor contributes to barrier function of a model fish gill epithelium

Cortisol-induced epithelial tightening of a primary cultured rainbow trout gill epithelium model occurs in association with reduced paracellular permeability and increased abundance of select barrier-forming tight junction (TJ) proteins. Corticosteroid receptor (CR) pharmacological blocker studies have suggested that to produce this tightening effect, cortisol acts on the mineralocorticoid receptor (MR) as well as glucocorticoid receptors (GRs). This study considered how cortisol influences model gill epithelium permeability and TJ properties by transcriptional knockdown of the gene encoding the MR (mr-KD) using double-stranded RNA. Following mr-KD, a significant reduction in MR protein abundance was observed in the epithelium. The mr-KD epithelium demonstrated reduced transepithelial resistance (TER) and an increase in the paracellular flux of [³H]polyethylene glycol (MW 400 kDa, PEG-400). Concurrently, mRNA abundance of gr2 and 11βhsd increased, indicating a possible compensatory response to mr-KD. Transcript abundance of claudin (cldn)-6, -8d, -23a and -28b decreased while that of cldn-20a increased in mr-KD preparations. Cortisol-induced epithelial tightening was enhanced in mr-KD preparations, suggesting that alterations in CRs and TJ composition augmented model epithelium barrier function in response to lowered MR abundance. Cortisol treatment significantly increased the transcript and protein abundance of TJ proteins such as Cldn-8d and -28b. However, in mr-KD preparations, Cldn-28b protein abundance did not significantly alter in response to cortisol treatment, while Cldn-8d abundance was significantly elevated. Data suggest that mr-KD compromises normal barrier function of a primary cultured rainbow trout gill epithelium in both the presence and absence of cortisol and that Cldn-28b protein abundance may be modulated by cortisol via the MR only.

'Central' Actions of Corticosteroid Signaling Suggested by Constitutive Knockout of Corticosteroid Receptors in Small Fish

This review highlights recent studies of the functional implications of corticosteroids in some important behaviors of model fish, which are also relevant to human nutrition homeostasis. The primary actions of corticosteroids are mediated by glucocorticoid receptor (GR) and mineralocorticoid receptor (MR), which are transcription factors. Zebrafish and medaka models of GR- and MR-knockout are the first constitutive corticosteroid receptor-knockout animals that are viable in adulthood. Similar receptor knockouts in mice are lethal. In this review, we describe the physiological and behavioral changes following disruption of the corticosteroid receptors in these models. The GR null model has peripheral changes in nutrition metabolism that do not occur in a mutant harboring a point mutation in the GR DNA-binding domain. This suggests that these are not “intrinsic” activities of GR. On the other hand, we propose that integration of visual responses and brain behavior by corticosteroid receptors is a possible “intrinsic”/principal function potentially conserved in vertebrates.

A possible principal function of corticosteroid signaling that is conserved in vertebrate evolution: Lessons from receptor-knockout small fish

Corticosteroid receptors are critical for homeostasis maintenance, but understanding of the principal roles of the glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) throughout vertebrates is limited. Lines of constitutive GR-knockout zebrafish and MR-knockout medaka have recently been generated as the first adult-viable corticosteroid receptor-knockout animals, in contrast to the lethality of these receptor knockouts in mice. Here, we describe behavioral and physiological modifications following disruption of corticosteroid receptor function in these animal models. We suggest these data point toward a potentially conserved function of corticosteroid receptors in integrating brain-behavior and visual responses in vertebrates. Finally, we discuss how future work in cartilaginous fishes (Chondrichthyes) will further advance understanding of the unity and diversity of corticosteroid receptor function, since distinct orthologs of GR and MR derived from an ancestral corticoid receptor appear in these basal jawed vertebrates.