Many hormones repress or enhance protein expression by binding to transcription factors called nuclear receptors. Other hormones regulate processes by binding to cell surface receptors that induce cell signaling cascades.
A. Represent these two different processes for cell-cell communication by creating a flow chart showing the direction of information in a process regulating homeostasis such as the control of blood sugar or water loss. Annotate your diagram to describe the key elements of each step in the particular example that you choose.
B. Justify the claim that a developmental process is more likely to involve nuclear receptors, while regulation of metabolism is more likely to involve cell surface receptors.
The lancelet is a chordate, fish-like filter feeder that buries itself in marine sands. Their ancestors and vertebrates diverged approximately 500 million years ago. Lancelet and human are members of a superphylum of animals with bilateral symmetry, Deuterostomia. In one model of animal evolution, the other superphylum, Protostomia, includes fruit fly and nematode. In an alternative model, insects are included with lancelet and human in a superphylum called the Coelomata, and the nematode is separate. Molecular studies of fruit fly, lancelet, human, and nematode show that homologous genes, when present, are strongly conserved. For example, the genes for the receptors of germ cell nuclear factor (GCNF) and retinoids, hormones that regulate cellular differentiation during development, are present in each species. Homologous genes for nuclear receptor estrogen and thyroid hormone are present in lancelet and human but missing in nematode and fruit fly. Genes for the vitamin D receptor are absent in fruit fly but are present in the other species.
Many neuropeptide hormones found in human, fruit fly and nematode such as GnRH (gonadotropin-releasing hormone) and the gonadotropins luteinizing and follicle stimulating hormones (LH and FSH) have not been found in lancelet. However, genes for the cell surface receptors activated by these hormones are expressed. The same is true for many different hormones and their corresponding receptors. A large number of such “orphan” nuclear receptors have been reported in animals where there is no evidence of the presence of the hormone to which they bind.
C. Analyze these data (a table to organize the data is suggested) for supporting evidence of the alternative superphylum, Coelomata. Consider:
- separate selection of signaling molecules and the proteins that detect them
- constitutive (always transcribed) expression
- the possible effect of gene deletion
D. Evaluate the legitimacy of a claim that a hormone is not present although the receptor for that hormone has been found.
Genes for receptors and the hormones to which they bind are often found on different chromosomes and have co-evolved. Other ligands may have an affinity for orphaned receptors. By understanding the three-dimensional shape of the ligand binding site of a nuclear receptor, new drugs can be developed to match the site and regulate expression when the endocrine system fails. Even greater flexibility in drug design is allowed when small molecules, called positive (PAM) and negative (NAM) allosteric modulators, change the shape of the ligand binding site. The receptors for follicle stimulating hormone (FSH) and luteinizing hormone (LH) were subjects in a drug discovery program using PAMs and NAMs (Nataraja et al., Frontiers in Endocrinology, 6, 2015).
E. Explain how the refinement of nuclear receptor by attachment of a molecule other than the ligand of the receptor can increase the level of expression of genes targeted by the transcription factor.
Gonadotrophin-releasing hormone (GnRH) stimulates the production of follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The concentration of these hormones over time in females regulates the menstrual cycle. GnRH expressing neurons (GEN) are scattered throughout the forebrain and hypothalamus and secrete into ducts leading to the pituitary gland, causing the secretion of FSH and LH into the bloodstream. Ca+2 concentrations oscillate in time, changing the electric membrane potential of the GnRH neurons, GEN. The period of the oscillation is only a few seconds in duration. The mechanism controlling the oscillating membrane potential is unknown, but during maturation the frequency of the oscillations of the Ca+2 concentration in the GEN increases.
A. Explain how the absence of direct contact among the GnRH neurons and the coordinated release of FSH and LH suggest another timing mechanism.
An association of high concentrations of anti-Mullerian hormone (AMH) and elevated LH levels among women with a common reproductive disorder called polycystic ovary syndrome (PCOS) caused investigators to study the effects of AMH on membrane potentials of GnRH expressing neurons. The researchers (Cimino et al., Nature Communications, 7, 2016) found that very small concentrations of AMH initiated oscillations.
B. The graph illustrates the time dependence of LH and estrogen in the blood of a female. A goal of research in human reproductive physiology is to construct a model that accounts for the spike in luteinizing hormone at approximately 14 days. Predict how a positive feedback loop involving the release of a hormone such as AMH could be used to account for this response. Explain why estrogen is probably not a good candidate for use in this model.
FSH and LH inhibit production of GnRH. Exposure of ovarian follicles to high levels of FSH and LH causes the follicle to rupture, releasing an oocyte to the oviduct. Birth control drugs suppress the LH surge by maintaining a constant blood concentration of progesterone (PG). After 40 years of use, the mechanism of suppression is still unknown. However, we do know that during pregnancy progesterone levels remain elevated and that cells in both the pituitary gland and GnRH expressing neurons (GEN) have progesterone nuclear receptors (PGR) that act as transcription factors to either i) inhibit production of GnRH possibly by blocking transcription of GNRH1, the gene coding for GnRH, or ii) inhibit production of LH by blocking transcription in the GnRH neurons of the gene, LH1, coding for LH.
C. Construct two alternative explanations of the role of progesterone as a birth control drug. Your explanation can be in the form of descriptions or in terms of diagrams involving the key molecular components PG, PGR, LH, LH1, GEN, GnRH, and GnRH1.
An endocrine disruptor is a molecule with a structure that mimics a hormone and can interfere with regulation. The chemical bis-phenol A (BPA) has been used for many years in the production of plastics and is now widespread. BPA mimics estrogen. BPA is present in most humans at the concentration of 5 micrograms per kilogram of body weight. Whether or not that concentration is harmful is uncertain.
In a study of Ca+2 oscillations in GnRH expressing neurons, Klenke and co-workers (Endocrinology, 157, 2016) presented evidence that BPA switches off oscillations even at these very low concentrations. This suggests a mechanism for the results from an earlier investigation in which female rats were fed low and high levels of estrogen (EE) and BPA during pregnancy. Measurements were made of the levels of expression of an estrogen nuclear receptor in the hypothalamus of the pups just after birth (Cao et al., Toxicological Sciences, 133, 2013). Their data are shown in the graph below.
Using the data, the graph at the top compares expression in a “do-nothing” control (Naïve) and a control (Vehicle) in which a feeding tube is used to deliver nutrients during pregnancy. The graph at the bottom shows expression levels in pups born to female rats with four different treatments of estrogen (EE) and bis-phenol A (BPA).
D. Compared to the naïve control where homeostasis is not disrupted, qualitatively analyze these data for the disruption of the endocrine system in offspring of a female rat exposed to both physical and chemical stresses.
E. In female sheep, the estrus cycle is coordinated by the length of the day and does not begin until day length shortens. The gestation period of sheep is roughly 150 days.
- Predict the effect on fitness of this control of gene expression by an external stimulus.
- The “ram effect” is a livestock management practice in Australia to induce ovulation by exposing an anestrus (when the estrus cycle is inactive) ewe to a ram. Compare this phenomenon with the results of delivering nutrients and hormones using a feeding tube to construct a possible explanation for the ram effect.