Nuclear receptor corepressor 1 levels differentially impact the intracellular dynamics of mutant thyroid hormone receptors associated with resistance to thyroid hormone syndrome.

Thyroid hormone receptor α1 (TRα1) undergoes nucleocytoplasmic shuttling and mediates gene expression in response to thyroid hormone (T3). In Resistance to Thyroid Hormone Syndrome α (RTHα), certain TRα1 mutants have higher affinity for nuclear corepressor 1 (NCoR1) and may form stable complexes that are not released in the presence of T3. Here, we examined whether NCoR1 modulates intranuclear mobility and nuclear retention of TRα1 or RTHα-associated mutants in transfected human cells, as a way of analyzing critical structural components of TRα1 and to further explore the correlation between mutations in TRα1 and aberrant intracellular trafficking.

Mediator subunit MED1 differentially modulates mutant thyroid hormone receptor intracellular dynamics in Resistance to Thyroid Hormone syndrome.

Thyroid hormone receptor (TR) controls the expression of thyroid hormone (T3)-responsive genes, while undergoing rapid nucleocytoplasmic shuttling. In Resistance to Thyroid Hormone syndrome (RTH), mutant TR fails to activate T3-dependent transcription. Previously, we showed that Mediator subunit 1 (MED1) plays a role in TR nuclear retention.

Getting there: Thyroid hormone receptor intracellular trafficking.

A year ago, when I first contemplated writing this article, my intent was to provide a detailed review of the contributions of the diverse community of talented scientists in my lab to the nuclear receptor research field. In the throes of a deadly pandemic, political turmoil, and Black Lives Matter, however, I found myself compelled to tell a more personal story. While I will still cover milestones in our understanding of the intracellular trafficking of the thyroid hormone receptor, now these will be set against the backdrop of my path as a woman in STEM and on being intentionally inclusive.

Mislocalization of Cancer-associated Thyroid Hormone Receptor Mutants.

The thyroid hormone receptor (TR) is essential for the proper regulation of metabolism and development, as it regulates gene expression in response to thyroid hormone. Nuclear localization signals (NLSs) and nuclear export signals (NESs) allow for TR transport into and out of the nucleus, respectively. Previous research suggests that nuclear import, nuclear retention, and nuclear export of TR are associated with modulation of gene expression, the alteration of which can contribute to various diseases.

Mediator subunit MED1 modulates intranuclear dynamics of the thyroid hormone receptor.

The thyroid hormone receptors (TRs) mediate thyroid hormone (T )-dependent gene expression. The nuclear import and export signals that direct TR shuttling are well characterized, but little is known about factors modulating nuclear retention. We used fluorescence-based nucleocytoplasmic scoring and fluorescence recovery after photobleaching in transfected cells to investigate whether Mediator subunits MED1 and MED13 play a role in nuclear retention of TR.

Acetylation modulates thyroid hormone receptor intracellular localization and intranuclear mobility.

The thyroid hormone receptor (TR) undergoes nucleocytoplasmic shuttling, but is primarily nuclear-localized and mediates expression of genes involved in development and homeostasis. Given the proximity of TR acetylation and sumoylation sites to nuclear localization (NLS) and nuclear export signals, we investigated their role in regulating intracellular localization. The nuclear/cytosolic fluorescence ratio (N/C) of fluorescent protein-tagged acetylation mimic, nonacetylation mimic, and sumoylation-deficient TR was quantified in transfected mammalian cells.

Thyroid hormone receptor localization in target tissues.

The thyroid hormone receptors, TRα1, TRβ1 and other subtypes, are members of the nuclear receptor superfamily that mediate the action of thyroid hormone signaling in numerous tissues to regulate important physiological and developmental processes. Their most well-characterized role is as ligand-dependent transcription factors; TRs bind thyroid hormone response elements in the presence or absence of thyroid hormone to facilitate the expression of target genes. Although primarily residing in the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm.

Nuclear Import and Export of the Thyroid Hormone Receptor.

The thyroid hormone receptors, TRα1 and TRβ1, are members of the nuclear receptor superfamily that forms one of the most abundant classes of transcription factors in multicellular organisms. Although primarily localized to the nucleus, TRα1 and TRβ1 shuttle rapidly between the nucleus and cytoplasm. The fine balance between nuclear import and export of TRs has emerged as a critical control point for modulating thyroid hormone-responsive gene expression.

Nuclear import of the thyroid hormone receptor α1 is mediated by importin 7, importin β1, and adaptor importin α1.

The thyroid hormone receptor α1 (TRα1) is a nuclear receptor for thyroid hormone that shuttles rapidly between the nucleus and cytoplasm. Our prior studies showed that nuclear import of TRα1 is directed by two nuclear localization signals, one in the N-terminal A/B domain and the other in the hinge domain. Here, we showed using in vitro nuclear import assays that TRα1 nuclear localization is temperature and energy-dependent and can be reconstituted by the addition of cytosol.

Multiple exportins influence thyroid hormone receptor localization.

The thyroid hormone receptor (TR) undergoes nucleocytoplasmic shuttling and regulates target genes involved in metabolism and development. Previously, we showed that TR follows a CRM1/calreticulin-mediated nuclear export pathway. However, two lines of evidence suggest TR also follows another pathway: export is only partially blocked by leptomycin B (LMB), a CRM1-specific inhibitor; and we identified nuclear export signals in TR that are LMB-resistant.

Multiple novel signals mediate thyroid hormone receptor nuclear import and export.

Thyroid hormone receptor (TR) is a member of the nuclear receptor superfamily that shuttles between the cytosol and nucleus. The fine balance between nuclear import and export of TR has emerged as a critical control point for modulating thyroid hormone-responsive gene expression; however, sequence motifs of TR that mediate shuttling are not fully defined. Here, we characterized multiple signals that direct TR shuttling.

Using group-specific PCR to detect predation of mayflies (Ephemeroptera) by wolf spiders (Lycosidae) at a mercury-contaminated site.

Bioaccumulation of contaminants can occur across ecosystem boundaries via transport by emergent aquatic insects. In the South River, Virginia, USA, aquatic mercury has contaminated songbirds nesting in adjacent riparian forests. Spiders contribute the majority of mercury to these songbirds' diets.

Recruitment of the oncoprotein v-ErbA to aggresomes.

Aggresome formation, a cellular response to misfolded protein aggregates, is linked to cancer and neurodegenerative disorders. Previously we showed that Gag-v-ErbA (v-ErbA), a retroviral variant of the thyroid hormone receptor (TRα1), accumulates in and sequesters TRα1 into cytoplasmic foci. Here, we show that foci represent v-ErbA targeting to aggresomes.

The pseudophosphatase MK-STYX interacts with G3BP and decreases stress granule formation.

MK-STYX [MAPK (mitogen-activated protein kinase) phospho-serine/threonine/tyrosine-binding protein] is a pseudophosphatase member of the dual-specificity phosphatase subfamily of the PTPs (protein tyrosine phosphatases). MK-STYX is catalytically inactive due to the absence of two amino acids from the signature motif that are essential for phosphatase activity. The nucleophilic cysteine residue and the adjacent histidine residue, which are conserved in all active dual-specificity phosphatases, are replaced by serine and phenylalanine residues respectively in MK-STYX.

Thyroid hormone receptor alpha1 follows a cooperative CRM1/calreticulin-mediated nuclear export pathway.

The thyroid hormone receptor alpha1 (TRalpha) exhibits a dual role as an activator or repressor of its target genes in response to thyroid hormone (T(3)). Previously, we have shown that TRalpha, formerly thought to reside solely in the nucleus bound to DNA, actually shuttles rapidly between the nucleus and cytoplasm. An important aspect of the shuttling activity of TRalpha is its ability to exit the nucleus through the nuclear pore complex.

Activation of the DNA-dependent protein kinase stimulates nuclear export of the androgen receptor in vitro.

The androgen receptor undergoes nuclear import in response to ligand, but the mechanism by which it undergoes nuclear export is poorly understood. We developed a permeabilized cell assay to characterize nuclear export of the androgen receptor in LNCaP prostate cancer cells. We found that nuclear export of endogenous androgen receptor can be stimulated by short double-stranded DNA oligonucleotides.

Oncogenic conversion of the thyroid hormone receptor by altered nuclear transport.

Nuclear receptors (NRs) are transcription factors whose activity is modulated by ligand binding. These receptors are at the core of complex signaling pathways and act as integrators of many cellular signals. In the last decade our understanding of NRs has greatly evolved.

Cancer promoted by the oncoprotein v-ErbA may be due to subcellular mislocalization of nuclear receptors.

The retroviral v-ErbA oncoprotein is a highly mutated variant of the thyroid hormone receptor alpha (TRalpha), which is unable to bind T(3) and interferes with the action of TRalpha in mammalian and avian cancer cells. v-ErbA dominant-negative activity is attributed to competition with TRalpha for T(3)-responsive DNA elements and/or auxiliary factors involved in the transcriptional regulation of T(3)-responsive genes. However, competition models do not address the altered subcellular localization of v-ErbA and its possible implications in oncogenesis.

Nuclear export of the oncoprotein v-ErbA is mediated by acquisition of a viral nuclear export sequence.

v-ErbA, an oncogenic derivative of the thyroid hormone receptor alpha (TRalpha) carried by the avian erythroblastosis virus, contains several alterations including fusion of a portion of avian erythroblastosis virus Gag to its N terminus, N- and C-terminal deletions, and 13 amino acid substitutions. Nuclear export of v-ErbA occurs through a CRM1-mediated pathway. In contrast, nuclear export of TRalpha and another isoform, TRbeta, is CRM1-independent.

Compartment-specific phosphorylation of rat thyroid hormone receptor alpha1 regulates nuclear localization and retention.

The thyroid hormone receptor alpha1 (TRalpha1) is a transcription factor, which can activate or repress gene expression in response to thyroid hormone. In addition, some of its actions, including DNA binding and transcriptional activation, are thought to be regulated by phosphorylation. Results presented here, using Xenopus oocyte microinjection assays, demonstrate that a phosphorylated form of rat TRalpha1 is present in the nucleus, whereas unphosphorylated TRalpha1 remains cytoplasmic.