Summer 2019 – GoMRI Researcher Interview with Dr. Danielle McDonald

A Gulf toadfish. Photo Credit: Dr. Maria C. Cartolano.

A Gulf toadfish. Photo Credit: Dr. Maria C. Cartolano.

(From Summer 2019 Newsletter) Dr. Danielle McDonald from the University of Miami’s Rosenstiel School of Marine and Atmospheric Science (RSMAS) Department of Marine Biology and Ecology answered a few questions about her RFP-VI project, The Impact of Deepwater Horizon Oil Exposure on the Vertebrate Stress Response.

1. Please tell us about your RFP-VI project, “The Impact of Deepwater Horizon Oil Exposure on the Vertebrate Stress Response.” What are the goals of the project?

In 2014, Lori Schwacke and coworkers reported that bottlenose dolphins that were exposed to Deepwater Horizon (DWH) oil were found to have an impaired stress response and didn’t secrete as much cortisol, a stress hormone, as dolphins that were not exposed to oil. This is a bad thing, since the secretion of cortisol helps dolphins to overcome stressors. Since fish have a similar stress axis as marine mammals and have the same stress hormone, cortisol, we followed up their study with some work looking at the stress response of Gulf toadfish (Reddam et al., 2017). In our study, we found that toadfish also have problems with their stress response and the secretion of cortisol. Thus, the goal of our GoMRI work is to further study the Gulf toadfish and determine where along the stress axis the problem is occurring. We also want to know if the impairment is due to a hyperactivation and then subsequent fatigue of the stress axis and/ or if the impact was mediated by the aryl hydrocarbon receptor (AhR). Our findings in toadfish could lend some insight into what is going on in bottlenose dolphins.

2. What are the stress axis and the aryl hydrocarbon receptor, and what role do they play in stress response?

The stress axis, and more specifically the glucocorticoid stress axis, refers to the brain and two endocrine glands working together to respond to an environmental stressor. Essentially, the brain hypothalamus perceives an environmental stressor and in response secretes a hormone called corticotropin-releasing hormone, CRH (also called corticotropin- releasing factor, CRF). CRH then binds to receptors in the pituitary gland which tells the gland to secrete a second hormone, called adrenocorticotropic hormone (ACTH). ACTH then travels in the bloodstream and binds to a receptor that, in fish, is located on the kidney interrenal cells and in mammals, is located on the adrenal gland and tells the interrenal cells (or adrenal gland) to secrete the hormone cortisol. Cortisol then travels through the blood and binds to cortisol receptors in various tissues that will turn genes on or off and result in a response to the environmental stressor.

The aryl hydrocarbon receptor (AhR) responds to PAHs by changing the transcription of different genes including upregulating cytochrome P4501A1 (often referred to as CYP1A). This pathway is essentially an organism’s way of trying to detoxify some types of toxicants. CYP1A can then goes on to affect other processes and so the upregulation of CYP1A is often used as a biomarker of PAH exposure. It is possible that the downregulation of the stress response may start with the activation of the AhR and upregulation of CYP1A and part of our study is determining whether the downregulation of the stress axis is via the AhR.

3. Why is the toadfish specifically a good comparison species for the bottlenose dolphins?

Toadfish, like other fish, have a similar stress axis and the same main stress hormone, cortisol, as mammals. That in of itself is an interesting thing because not all animals use cortisol as their stress hormone. Fish, dolphins, and humans do but rats, for example, use something called corticosterone. Another reason why we thought that toadfish would make a good comparison species for bottlenose dolphins was because they have a very pronounced stress response. Toadfish secrete cortisol at levels that are at least twice as high as other fish and many times higher than dolphins. We believed that this pronounced response would allow for an enhanced sensitivity when looking for disturbances in the function of the stress axis.

4. What is your background, and how did you get involved with this kind of work?

I am a fish physiologist and have been studying stress physiology for a lot of my career. The Reddam et al. (2017) study was part of the Deepwater Horizon Natural Resource Damage Assessment (NRDA) investigation (in collaboration with Dr. Martin Grosell) – that was the first time I linked my research on stress with oil pollution.

5. What are some of the most significant or exciting findings so far in your GoMRI- funded research?

We are still analyzing and interpreting a lot of our results. We have completed both a 7-day and a 28-day DWH oil exposure using a saltwater flow-through exposure system that we have developed. This system maintains steady polycyclic aromatic hydrocarbons (PAH) concentrations in tanks for up to one month. In our 7-day experiment, Gulf toadfish, Opsanus beta, were exposed for 7 days to 4 different total PAH concentrations (actual 0.006, 0.009, 0.06, 2.8 µg/L) and then allowed to recover for 7 days. In our 28-day experiment, toadfish were exposed for 28 days to total PAH concentrations similar to the acute experiment and then allowed to recover for 28 days. In both experiments, there was a second group of toadfish that were subjected to a standardized simulated predation stress before being sacrificed to determine if PAH exposure compromised the ability to mount a stress response.

Our most significant findings are:

(1) Toadfish that are exposed to DWH oil have difficulties in mounting a stress response in response to a simulated predator after 7 days of oil exposure and this effect persists even after 7 days of recovery. So, toadfish do not appear to be able to recover from this impact.

(2) We have evidence that these fish are not perceiving the oil as a stressor, as exposure itself does not result in elevated stress hormone levels. So, impairment due to hyperactivation followed by fatigue has been ruled

(3) There is a significant upregulation in CYP1A mRNA expression (which indicates AhR activation) within the stress axis (the brain hypothalamus, the pituitary gland, and the kidney interrenal tissue) as well as in the liver in response to fairly low PAH concentrations (0.06 µg/L or higher) after 4 hours of exposure that persists even after 7 days of However, CYP1A mRNA expression returns to control levels by 7 days of recovery (even though the impairment in the stress response persists).

(4) We have not found the mechanism of impairment yet. After 7-days exposure, there is no significant change in the mRNA expression of key proteins that are involved in cortisol production (StAR, P450scc, and 11-beta hydroxylase). We are still analyzing samples to determine whether cholesterol levels are an issue (which is the precursor to cortisol) and whether a particular receptor may be We are also still analyzing the data from our 28-day exposure experiment; however, one result that has come to light with respect to the 28-day exposure is that simultaneous exposure to a chronic stressor during DWH oil exposure exacerbates the impact of PAHs on the stress response. We are currently doing further experiments to clarify that result.

6. Please tell us about some of your outreach activities (visit Dr. McDonald’s Toadfish Lab Facebook page!).

Most of our outreach involves talking to children about our research on toadfish, fish stress, using fish as models for mammal health, and pollution in the environment. We do presentations to kids that are passing through the lab (on tours with RSMAS in general), we bring our toadfish on the road to talk to elementary school kids (G.W. Carver Elementary), underprivileged kids (Ocean Kids), or kids at summer camp (Miami Seaquarium, Frost Science Summer Camp). I have also given several lectures to faculty, staff, and students at RSMAS, as well as to the general public (John Pennekamp Delicate Balance of Nature Lecture Series).

7. If funding were not an issue, what would you add to your GoMRI-funded project?

This is a really good question!! We are intrigued by the fact that fish (and bottlenose dolphins) are not appearing to recover once the exposure has ended. We would really love to look at potential epigenetic impacts of PAH exposure on genes related to either cholesterol production or stress hormone synthesis to see if they could explain the lack of recovery.

[Back to the Summer 2019 Newsletter]