January has been Thyroid Awareness Month in the US. An estimated 15 million Americans have undiagnosed thyroid problems. We recently spoke with BPS member Grace Brannigan, Rutgers University Camden, Physics Department and Center for Computational and Integrative Biology about her biophysics research related to thyroid function.
What is the connection between your research and thyroid conditions?
In most thyroid conditions, the thyroid produces too much or too little of two thyroid hormones called T3 and T4. An initial blood test of thyroid function will usually measure thyroid-stimulating hormone (TSH), which, if out-of-range, will be followed by a measurement of T3 and T4 in the blood. One of the proteins affected by T3 is a critical inhibitory neurotransmitter receptor in the post-synaptic membrane, the GABA(A) receptor. We wanted to know if T3 could bind directly to the GABA(A) receptor, and if it could, whether the binding mode was similar to that of another class of potent endogenous small molecules called neurosteroids. Some neurosteroids increase GABA(A) function, acting like the body’s own sedatives and anesthetics, while others decrease GABA(A) function (just as T3 does) and can improve memory and cognition.
Why is your research important to those concerned about thyroid conditions?
There’s a well-established connection between pathological thyroid function and disrupted mood, cognition, memory, and sleep. In fact, some studies suggest that supplementing with thyroid hormone improves mental function even in people with normal levels of thyroid hormones, although this is not established. The connection has been attributed largely to a relatively indirect mechanism in which thyroid hormones affect synaptic neurotransmitter concentrations. Our research suggests an alternate, more direct mechanism in which thyroid hormone itself binds directly to the receptor and causes loss of function. Further, we see that when it binds it can displace one of the endogenous sedatives, allopregnanolone, amplifying the effect.
How did you get into this area of research?
I began studying this family of neurotransmitter receptors as a postdoc at the University of Pennsylvania in the lab of Dr. Michael Klein. He had a collaboration with an anesthesiology researcher, Dr. Roderic Eckenhoff, also at Penn, seeking to understand the mechanism of anesthesia. GABA(A) receptors are one of the most widely investigated anesthetic targets and are also modulated by cholesterol-derived neurosteroids. My research on thyroid hormones in particular began when I joined the Physics department faculty at Rutgers Camden, and began collaborating with Dr. Joseph Martin, a PI in the Biology department. Dr. Martin has a long record of studying effects of thyroid hormone on sleep, and he and co-workers had hypothesized in the 1990s that interactions between thyroid hormones and the GABA(A) receptor could explain why too much thyroid hormone can significantly disrupt sleep. Our joint research aims (in part) to test this hypothesis.
How long have you been working on it?
I’ve been working in the general area of ion channel pharmacology since I began my postdoc in 2006, and on mechanisms of thyroid hormones since I became a PI at Rutgers in 2011.
Do you receive public funding for this work? If so, from what agency?
The anesthesiology research that introduced me to this field was (and is) funded by an NIH PO1 grant, led by Dr. Roderic Eckenhoff. Our research on thyroid hormone mechanisms has been funded by a three-year grant from the MCB division of NSF, soon to be up for renewal. The computational resources provided through the NSF XSEDE program have also been critical to this work.
Have you had any surprise findings thus far?
Maybe the most surprising findings came from Molecular Dynamics (MD) Simulations of T3 in the interfacial binding sites indicated by the experiments. These allowed us to visualize bound T3 and estimate relative affinities of pseudosymmetric sites that were indistinguishable in experiments. The GABA(A) receptor is a pentamer; It’s most commonly found as a heteromer with five similar but not identical subunits (and five pseudo-symmetric interfaces).
In the MD simulations, we found that when T3 is bound to the highest affinity interface, it can assume several distinct but equivalently favorable configurations. One of these configurations was favorable primarily because of a hydrogen bond with the protein backbone at a well-conserved helix deformation. These two observations suggested that T3 affinity would be relatively insensitive to most single residue substitutions. This property may extend to other modulators, such as general anesthetics or neurosteroids, for which site-directed mutagenesis studies have yielded numerous ambiguous or inconclusive results. The latter mystery has been primarily technical, but long-standing and a significant barrier to understanding mechanisms of GABA(A) receptor modulation.
What is particularly interesting about the work from the perspective of other researchers?
Site-directed Mutagenesis is one of the most commonly used techniques for determining ligand-binding modes. Yet, as mentioned, it has yielded surprisingly few lasting insights for hydrophobic modulators of GABA(A) receptors. Structural experiments, such as x-ray crystallography and photoaffiinity labeling, are extremely time and/or resource intensive, and in some cases simply not feasible. Our research design circumvented these issues by using an older, inexpensive technique (Schild analysis) to test for competition of thyroid hormone with one of the few hydrophobic ligands with a well-established binding site (ivermectin, determined by crystallography).
What is particularly interesting about the work from the perspective of the public?
Endocrine glands are kind of like the body’s own in-house pharmacy– they produce natural “drugs” (hormones) that can directly affect the same proteins targeted by many pharmaceuticals, even in the brain. And like pharmaceuticals that have side effects, a single hormone can act on many different systems in the body – often, especially in the brain, in ways that are still not well understood. This means that endocrine research is naturally both basic science and health-related: the ideas and approaches developed from studying hormones can be directly applied to understanding how pharmaceuticals cause their effects and side effects – and how they can be improved.