Consequences of developmental and adult neuroendocrine disruption to the hypothalamus and pituitary

Gonzalez, Rachel

Permalink

https://hdl.handle.net/2142/113999 Copy

Description

Title

Consequences of developmental and adult neuroendocrine disruption to the hypothalamus and pituitary

Author(s)

Gonzalez, Rachel

Issue Date

2021-12-03

Director of Research (if dissertation) or Advisor (if thesis)

Raetzman, Lori T

Doctoral Committee Chair(s)

Raetzman, Lori T

Committee Member(s)

• Christian-Hinman, Catherine A
• Flaws, Jodi A
• Mahoney, Megan M

Department of Study

Neuroscience Program

Discipline

Neuroscience

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• neuroscience
• neuroendocrinology
• endocrinology
• hypothalamus
• pituitary
• endocrine disruption
• gestational diabetes

Abstract

The endocrine system is a complex, interconnected web of communication that uses hormones as chemical messengers to allow distinct and geographically distant organs to coordinate physiological homeostasis. The hypothalamus and pituitary are the master regulators of this process. Healthy physiology relies on the hypothalamus and pituitary to respond appropriately to signals communicated by other endocrine organs. When external stimuli influence the development or behavior of neuroendocrine tissue, it can lead to deleterious health consequences. Often, associations between environmental exposures and negative health outcomes are reported in epidemiological data while the question of how they are causally connected remains unclear. This is the case with the two exposures on which this work expands: gestational diabetes mellitus and iodoacetic acid. Using in vivo and in vitro models, we observed the impact these exposures have on the hypothalamus and pituitary to better understand how they lead to endocrine disruption. Gestational diabetes mellitus (GDM), a transient diabetes during pregnancy, has frequently been linked to the development of obesity in offspring, beginning in childhood and continuing into adulthood. The hypothalamus is the master regulator of energy homeostasis and despite the opportunity for an in utero exposure to alter the development of this region, relatively little is known about if or how it does so. The arcuate nucleus (ARC) balances feeding behavior through two competing neuron populations that either promote hunger or satiety. The ARC is bordered by a pool of nutritionally active, stem-like progenitors called tanycytes that can influence the cellular make-up of the ARC as well as mediate the cues it sees from circulation. Additionally, the ARC lies just superior to the median eminence (ME), a region with a leaky blood brain barrier that acts as a conduit for peripheral cues about energy status. We hypothesized that GDM would influence the cellular composition and function of the ARC, ME, and/or tanycytes, potentially driving altered nutritional responsiveness in offspring. Using a genetic model of gestational diabetes, we found a reduction in the number of cells that proliferated during the onset of GDM in the ME of offspring. We also found that post-natal basal insulin signaling is elevated in the β2-tanycytes of offspring. Through a tanycytic neurosphere culture paradigm, we observed that excess insulin, a characteristic of the GDM growth environment, has a trophic effect on cells. We do not see this effect in vivo. This indicates that GDM’s influence is likely multifactorial and ultimately not primarily driven by hyperphysiological insulin levels. Our data together reveal that GDM’s influence on nutritional communication tools, rather than altered stem cell proliferation, is a key driver of hypothalamic consequence. These developmental alterations may underlie high-weight phenotype in offspring. Turning our attention to another timepoint and exposure source, we observe the consequences of adult exposure to the water disinfection byproduct (DBP), iodoacetic acid (IAA). DBPs have been linked to a range of health consequences including increased risk of cancer, birth defects, and reproductive disruption. IAA has high formation potential, created as a byproduct of a reaction between an oxidizing disinfectant used to treat water and iodide present in the water. In previous studies, IAA has been shown to have significant cyto- and genotoxic capabilities, as well as to induce mRNA expression of genes related to cell cycle arrest and promoting apoptosis. IAA also disrupts ovarian follicle development and estradiol synthesis in vitro and estradiol serum levels in vivo. These data suggest IAA acts as toxicant with reproductive-axis disrupting potential. Yet, previously virtually nothing was known about how IAA effects the hypothalamus and pituitary, which guide reproduction through the hypothalamic-pituitary-gonadal axis. We hypothesized that IAA exposure disrupts expression of key neuroendocrine factors and directly induce cell damage in the mouse pituitary. Using an in vivo exposure experiment, we found that IAA significantly elevated mRNA levels of kisspeptin (Kiss1) in the arcuate nucleus of the hypothalamus, while not affecting Kiss1 in the anteroventral periventricular nucleus. It also reduced follicle-stimulating hormone (FSHβ)-positive cell number and Fshb mRNA expression, though not luteinizing hormone (LHβ/Lhb) expression. Evidence for FSHβ/Fshb disruption was substantiated by pituitary explant experiments which revealed the same effect and suggested that IAA acts on the pituitary directly. IAA also introduced toxicity in the pituitary, inducing DNA damage and P21/Cdkn1a expression in vitro and DNA damage and Cdkn1a expression in vivo. These data implicate IAA as a reproductive neuroendocrine disrupter with toxic capabilities in the pituitary and add weight to the call for IAA to be better studied and regulated as a potential public health concern. With this understanding of adult exposure risks, we wanted to determine how a developmental exposure to IAA could affect the pituitary. Prior data indicating cytotoxicity, DNA damage, and cell cycle arrest-related mRNA expression in work from other groups, combined with similar findings from our adult study, suggest IAA may be especially detrimental for developing, proliferative tissue. Prior work from our lab suggests that the pituitary is particularly vulnerable to disruption by endocrine disrupting chemicals during development. If IAA were to induce similar effects, it may permanently alter pituitary regulation of the endocrine system. As such, we wanted to survey the major hormone-building mRNA of the pituitary in this context, as well as observe its toxic effects. Until the current research, no prior data had been collected on early life exposures to IAA in the pituitary, nor, to the best of our knowledge, any tissue. Using an in vivo IAA exposure from conception through weaning, we tested the hypothesis that in the pituitary, developmental exposure to IAA would result in upregulation of the cell-cycle arrest gene Cdkn1a, suppression of the proliferative marker Mki67, and shifts in hormone-building mRNA. Surprisingly, we found no changes to Ckdn1a or to Mki67 suggesting IAA may not be a significant threat to pituitary cellular health developmentally. We also performed qPCR for the major hormone-building mRNAs expressed by the pituitary, finding a significant increase in thyroid stimulating hormone beta subunit (Tshb) and a trend towards increased proopiomelanocortin (Pomc), though no other significant effects. Together with our findings from our adult IAA exposure study, these data suggest that IAA may be a more notable threat to adult pituitaries, though further experiments observing different pituitary functions and whole axes is warranted. The work presented in this document explores the previously under-examined topics of hypothalamic consequences of the gestational diabetes growth environment and neuroendocrine disruption by the water disinfection byproduct iodoacetic acid. Through these studies, we reveal the hypothalamus and pituitary to be vulnerable to environmental exposures in a way that could undermine healthy physiology.

Graduation Semester

2021-12

Type of Resource

Thesis

Permalink

http://hdl.handle.net/2142/113999

Copyright and License Information

Copyright 2021 Rachel Gonzalez

Owning Collections