How Environmental Chemicals can Alter the Endocrine System… and What Zebrafish can do about it!

Use of biomarkers in endocrine-disrupting compounds screening and testing with zebrafish

By Javier Terriente, Co-Founder/CDD at ZeClinics and Co-Founder/CSO at ZeCardio Therapeutics.

Javier is an expert in zebrafish biology and drug discovery. Co-Founder of ZeClinics and ZeCardioTX. 20 year of research experience. 7 years supervising research teams. VP at Asebio.

Harmful effects of chemicals on human health

Human progress is a double-sided sword. On the one hand, it has brought instant communication and antibiotics, among the multiple advances that have facilitated our lives during the last centuries. On the other hand, it has altered the climate and polluted our ecosystems, bringing profound alterations to the fragile equilibrium that allows the life of plants and animals in our planet; and posing a high risk to our own survival. An aspect of this pollution is the fact that thousands of synthetic chemicals, derived from our industrial activity, are disposed of in our environment… every second, everywhere. Some of those chemicals might be harmless for plants and animals. But many others have to be monitored closely because of their potential damage to the ecosystems and animal and human health. Among those, endocrine-disrupting compounds (EDCs) are of particular concern.

The scientific community, together with international health and research organizations (i.e.: WHO, OCDE, EPA, etc.), are making huge efforts to develop and implement analytical tools for evaluating the effects in animals associated with the exposure to these chemicals. This effort is derived from the fact that EDCs have an impact that goes beyond risking individual lives: EDCs interference with sexual development and animal behavior poses a danger for the survival of entire species. In the case of humans, EDCs have been found to alter fertility and metabolism, and to promote carcinogenesis, among some of their nasty repercussions.

The disruption of the endocrine system

Now, let’s understand a bit better what we are discussing here. The endocrine system is the group of organs and tissues that secrete hormones and respond to them. Hormones are biological products that act as chemical messengers inside the body, activating multiple biological processes. They are secreted from hormone releasing organs, called glands (i.e.: suprarenal glands), into the bloodstream. Once in the vasculature, they can reach target organs/tissues to modulate functions that include the regulation of growth rate and cellular metabolism, the development and function of sexual organs and some aspects of animal behavior. The endocrine system is divided in different axes, which group together all the biological processes that occur from the production and maturation of the hormone – at the gland – to the impact that same hormone might have on its target tissue. Among those axes, we found the estrogenic axis – required for the generation and function of female hormones; the androgenic axis – the male counterpart; and the hypothalamus-pituitary-thyroid (HPT) axis – involved in the control of growth, nervous system development and body energy consumption. The first two axes are fundamental for sexual differentiation, development and function. As such, their disruption has an important impact in animal reproductive capacity, impacting aquatic and terrestrial ecosystems, as well as in our reproductive capacity. On the other hand, disrupting the HPT axis might alter embryonic development, if mothers are exposed during pregnancy; or affect growth and metabolism if individuals are exposed after birth.

Figure adapted from www.epa.gov/endocrine-disruption/what-endocrine-system

Can zebrafish help evaluate EDCs?

There is an important pool of knowledge regarding the biological mechanisms involved in the synthesis, maturation and release of hormones from glands, as well as the processes that hormones will trigger when they reach their target tissues. This understanding has allowed the discovery of biomarkers – specific genes and proteins – that allow evaluating the activity of a particular endocrine axis. The analysis of the same biomarkers, under certain chemical conditions, might also reveal the disruption of such axes. Interestingly, the same biological mechanisms and biomarkers are vastly conserved across the animal kingdom. From humans to fishes.

Thanks to this evolutionary conservation, and as explained in previous blog articles, we can exploit the biology of the zebrafish larvae to evaluate the impact of chemicals in other animals, including humans. To allow the evaluation of the EDCs, we have developed an experimental strategy that relies on the quantification of gene expression biomarkers associated with the activity of the three endocrine axes described above. To measure how these biomarkers respond to EDC exposure, we looked at their transcriptional profile, through the use of quantitative PCR. In order to develop this analytical tool, we first evaluated a broader set of potential biomarkers, so we could select those that respond more robustly to known EDCs. This strategy allowed us to select a panel of gene expression biomarkers in order to analyze the estrogen axis, the androgen axis, and HPT axis disruption. Those biomarkers were selected according to their responses after exposure to natural ligands 17-estradiol, testosterone and 3,30,5-triiodo-L-thyronine (T3), respectively. The biomarker panel was subsequently validated using chemical compounds reported as endocrine disruptors in previous studies. Cross-talk effects were also evaluated for all compounds. The complete study has been published (Jarque et al, 2019) and you can check it here.

Figure adapted from Jarque et al, 2019.

The biological translatability and experimental throughput provided by this analytical tool has allowed us to develop a very robust method for evaluating EDCs. If you are interested in evaluating the endocrine disruption potential of your compounds of interest, you might want to look at our endocrine disruption assay.

REFERENCES

[1] Jarque S, Ibarra J, Rubio-Brotons M, García-Fernández J, Terriente J. Multiplex Analysis Platform for Endocrine Disruption Prediction Using Zebrafish. Int J Mol Sci. 2019 Apr 8;20(7):1739. https://www.mdpi.com/1422-0067/20/7/1739

Chemical toxicityendocrine disruptionendocrine systemenvironmental chemicalsZebrafish