Signal responses to neutral TSH receptor antibody - A cycle of damage in the pathophysiology of Graves' disease.

BACKGROUND: Graves' disease is associated with TSH receptor (TSHR) antibodies of variable bioactivity including "neutral" antibodies (N-TSHR-Ab) that bind to the hinge region of the TSHR ectodomain. We have previously found that such antibodies induced thyroid cell apoptosis via excessive mitochondrial and ER stress with elevated reactive oxygen species (ROS). However, the detailed mechanisms by which excess ROS was induced remained unclear. OBJECTIVES: To determine how ROS is induced by N-TSHR-monoclonal antibodies (mAb, MC1) mediated signaling and to measure stress in polyorganelles. METHODS: Total ROS and mitochondrial ROS was measured by fluorometry of live rat thyrocytes. Live-cell imaging of labelled organelles was carried out using red or green fluorescent dyes. Proteins were detected by Li-Cor Western immunoblots and immunocytochemistry. RESULTS: Endocytosis of N-TSHR-mAb induced ROS, disturbed vesicular trafficking, damaged organelles and failed to induce lysosomal degradation and autophagy. We found that the endocytosis triggered signaling cascades involving Gα13 and PKC-δ leading to intrinsic thyroid cell apoptosis. CONCLUSIONS: These studies define the mechanism of ROS induction in thyroid cells following the endocytosis of N-TSHR-Ab/TSHR complexes. We suggest that a viscous cycle of stress initiated by cellular ROS and induced by N-TSHR-mAbs may orchestrate overt intra-thyroidal, retro-orbital, and intra-dermal inflammatory autoimmune reactions in patients with Graves' disease.

Blocking FSH induces thermogenic adipose tissue and reduces body fat.

Menopause is associated with bone loss and enhanced visceral adiposity. A polyclonal antibody that targets the ß-subunit of the pituitary hormone follicle-stimulating hormone (Fsh) increases bone mass in mice. Here, we report that this antibody sharply reduces adipose tissue in wild-type mice, phenocopying genetic haploinsufficiency for the Fsh receptor gene Fshr. The antibody also causes profound beiging, increases cellular mitochondrial density, activates brown adipose tissue and enhances thermogenesis. These actions result from the specific binding of the antibody to the ß-subunit of Fsh to block its action. Our studies uncover opportunities for simultaneously treating obesity and osteoporosis.

Rescue of thyroid cells from antibody induced cell death via induction of autophagy.

BACKGROUND: Graves' disease (GD) is associated with thyroid stimulating hormone (TSH) receptor (TSHR) antibodies of variable bioactivity. We have previously characterized "neutral" TSHR antibodies (N-TSHR-Abs) that bind to the hinge region of the TSHR ectodomain. We showed that an N-TSHR monoclonal antibody (mAb) failed to induce any G proteins to sustain survival signaling and lead to excessive stress and apoptosis. Furthermore, the addition of TSH, or the antioxidant N-acetyl-l-cysteine (NAC), rescued N-TSHR-mAb-induced apoptotic death. However, the detailed mechanisms of this rescue remained unclear. METHODS: Autophagy is activated in response to diverse stress related stimuli so we have, therefore, studied the autophagy response in rat thyroid cells (FRTL-5) during N-TSHR-mAb induced thyrocyte stress and apoptosis using the In Cell Western technique for quantitation along with immunocytochemistry. RESULTS: Under starvation conditions with N-TSHR-mAb the addition of TSH or NAC prevented thyroid cell death by enhancing autophagy. This was evidenced by elevated levels of autophagy related proteins including beclin 1, LC3A, LC3B, ULK1, p62, and also activated pink and perkin mitophagy related proteins. The phenomenon was further confirmed by image analyses using Cyto-ID and Mito-ID autophagy detection systems. We also found that either TSH or NAC enhanced PKA, Akt, mTORC, AMPK, Sirtuins, PGC1α, NRF-2, mitofusin-2, TFAM and catalase in the N-TSHR-mAb stressed cells. Thus TSH or NAC restored cell survival signaling which reduced cell stress and enhanced mitochondrial biogenesis. The N-TSHR-mAb also activated cytochrome-C, Bax, caspase-9, caspase-3A, and had less effect on FADD or caspase-8 indicating activation of the intrinsic pathway for apoptosis. CONCLUSIONS: These findings indicated that TSH or antioxidant can rescue thyroid cells from N-TSHR-mAb induced apoptosis via enhanced autophagy. These observations signify that N-TSHR-mAb in GD under low TSH conditions caused by the hyperthyroidism could be detrimental for thyrocyte survival which would be another factor able to precipitate ongoing autoinflammation.

Autoimmune conditions in the World Trade Center general responder cohort: A nested case-control and standardized incidence ratio analysis.

BACKGROUND: The World Trade Center (WTC) general responder cohort (GRC) was exposed to environmental toxins possibly associated with increased risk of developing autoimmune conditions. OBJECTIVES: Two study designs were used to assess incidence and risks of autoimmune conditions in the GRC. METHODS: Three clinically trained professionals established the status of possible GRC cases of autoimmune disorders adhering to diagnostic criteria, supplemented, as needed, by specialists' review of consenting responders' medical records. Nested case-control analyses using conditional logistic regression estimated the risk associated with high WTC exposure (being in the 9/11/2001 dust cloud or ≥median days' response worked) compared with low WTC exposure (all other GRC members'). Four controls were matched to each case on age at case diagnosis (±2 years), sex, race/ethnicity, and year of program enrollment. Sex-specific and sensitivity analyses were performed. GRC age- and sex-adjusted standardized incidence ratios (SIRs) were compared with the Rochester Epidemiology Project (REP). Complete REP inpatient and outpatient medical records were reviewed by specialists. Conditions meeting standardized criteria on ≥2 visits were classified as REP confirmed cases. RESULTS: Six hundred and twenty-eight responders were diagnosed with autoimmune conditions between 2002 and 2017. In the nested case-control analyses, high WTC exposure was not associated with autoimmune domains and conditions (rheumatologic domain odds ratio [OR] = 1.03, 95% confidence interval [CI] = 0.77, 1.37; rheumatoid arthritis OR = 1.12, 95% CI = 0.70, 1.77). GRC members had lower SIR than REP. Women's risks were generally greater than men's. CONCLUSIONS: The study found no statistically significant increased risk of autoimmune conditions with WTC exposures.

Functions of vasopressin and oxytocin in bone mass regulation.

Prior studies show that oxytocin (Oxt) and vasopressin (Avp) have opposing actions on the skeleton exerted through high-affinity G protein-coupled receptors. We explored whether Avp and Oxtr can share their receptors in the regulation of bone formation by osteoblasts. We show that the Avp receptor 1α (Avpr1α) and the Oxt receptor (Oxtr) have opposing effects on bone mass: Oxtr(-/-) mice have osteopenia, and Avpr1α(-/-) mice display a high bone mass phenotype. More notably, this high bone mass phenotype is reversed by the deletion of Oxtr in Oxtr(-/-):Avpr1α(-/-) double-mutant mice. However, although Oxtr is not indispensable for Avp action in inhibiting osteoblastogenesis and gene expression, Avp-stimulated gene expression is inhibited when the Oxtr is deleted in Avpr1α(-/-) cells. In contrast, Oxt does not interact with Avprs in vivo in a model of lactation-induced bone loss in which Oxt levels are high. Immunofluorescence microscopy of isolated nucleoplasts and Western blotting and MALDI-TOF of nuclear extracts show that Avp triggers Avpr1α localization to the nucleus. Finally, a specific Avpr2 inhibitor, tolvaptan, does not affect bone formation or bone mass, suggesting that Avpr2, which primarily functions in the kidney, does not have a significant role in bone remodeling.

Bisphosphonates inactivate human EGFRs to exert antitumor actions.

Bisphosphonates are the most commonly prescribed medicines for osteoporosis and skeletal metastases. The drugs have also been shown to reduce cancer progression, but only in certain patient subgroups, suggesting that there is a molecular entity that mediates bisphosphonate action on tumor cells. Using connectivity mapping, we identified human epidermal growth factor receptors (human EGFR or HER) as a potential new molecular entity for bisphosphonate action. Protein thermal shift and cell-free kinase assays, together with computational modeling, demonstrated that N-containing bisphosphonates directly bind to the kinase domain of HER1/2 to cause a global reduction in downstream signaling. By doing so, the drugs kill lung, breast, and colon cancer cells that are driven by activating mutations or overexpression of HER1. Knocking down HER isoforms thus abrogates cell killing by bisphosphonates, establishing complete HER dependence and ruling out a significant role for other receptor tyrosine kinases or the enzyme farnesyl pyrophosphate synthase. Consistent with this finding, colon cancer cells expressing low levels of HER do not respond to bisphosphonates. The results suggest that bisphosphonates can potentially be repurposed for the prevention and therapy of HER family-driven cancers.

Genetic confirmation for a central role for TNFα in the direct action of thyroid stimulating hormone on the skeleton.

Clinical data showing correlations between low thyroid-stimulating hormone (TSH) levels and high bone turnover markers, low bone mineral density, and an increased risk of osteoporosis-related fractures are buttressed by mouse genetic and pharmacological studies identifying a direct action of TSH on the skeleton. Here we show that the skeletal actions of TSH deficiency are mediated, in part, through TNFα. Compound mouse mutants generated by genetically deleting the Tnfα gene on a Tshr(-/-) (homozygote) or Tshr(+/-) (heterozygote) background resulted in full rescue of the osteoporosis, low bone formation, and hyperresorption that accompany TSH deficiency. Studies using ex vivo bone marrow cell cultures showed that TSH inhibits and stimulates TNFα production from macrophages and osteoblasts, respectively. TNFα, in turn, stimulates osteoclastogenesis but also enhances the production in bone marrow of a variant TSHß. This locally produced TSH suppresses osteoclast formation in a negative feedback loop. We speculate that TNFα elevations due to low TSH signaling in human hyperthyroidism contribute to the bone loss that has traditionally been attributed solely to high thyroid hormone levels.

Blocking antibody to the ß-subunit of FSH prevents bone loss by inhibiting bone resorption and stimulating bone synthesis.

Low estrogen levels undoubtedly underlie menopausal bone thinning. However, rapid and profuse bone loss begins 3 y before the last menstrual period, when serum estrogen is relatively normal. We have shown that the pituitary hormone FSH, the levels of which are high during late perimenopause, directly stimulates bone resorption by osteoclasts. Here, we generated and characterized a polyclonal antibody to a 13-amino-acid-long peptide sequence within the receptor-binding domain of the FSH ß-subunit. We show that the FSH antibody binds FSH specifically and blocks its action on osteoclast formation in vitro. When injected into ovariectomized mice, the FSH antibody attenuates bone loss significantly not only by inhibiting bone resorption, but also by stimulating bone formation, a yet uncharacterized action of FSH that we report herein. Mesenchymal cells isolated from mice treated with the FSH antibody show greater osteoblast precursor colony counts, similarly to mesenchymal cells isolated from FSH receptor (FSHR)(-/-) mice. This suggests that FSH negatively regulates osteoblast number. We confirm that this action is mediated by signaling-efficient FSHRs present on mesenchymal stem cells. Overall, the data prompt the future development of an FSH-blocking agent as a means of uncoupling bone formation and bone resorption to a therapeutic advantage in humans.

Thyroid-stimulating hormone induces a Wnt-dependent, feed-forward loop for osteoblastogenesis in embryonic stem cell cultures.

We have shown that the anterior pituitary hormone, thyroid-stimulating hormone (TSH), can bypass the thyroid to exert a direct protective effect on the skeleton. Thus, we have suggested that a low TSH level may contribute to the bone loss of hyperthyroidism that has been attributed traditionally to high thyroid hormone levels. Earlier mouse genetic, cell-based, and clinical studies together have established that TSH inhibits osteoclastic bone resorption. However, the direct influence of TSH on the osteoblast has remained unclear. Here, we have used a model system developed from murine ES cells, induced to form mature mineralizing osteoblasts, and show that TSH stimulates osteoblast differentiation primarily through the activation of protein kinase Cδ and the up-regulation of the noncanonical Wnt components frizzled and Wnt5a. We predict that a TSH-induced, fast-forward short loop in bone marrow permits Wnt5a production, which, in addition to enhancing osteoblast differentiation, also stimulates osteoprotegerin secretion to attenuate bone resorption by neighboring osteoclasts. We surmise that this loop should uncouple bone formation from bone resorption with a net increase in bone mass, which is what has been observed upon injecting TSH.

Thyrotrophin receptor signaling dependence of Braf-induced thyroid tumor initiation in mice.

Mutations of BRAF are found in ∼45% of papillary thyroid cancers and are enriched in tumors with more aggressive properties. We developed mice with a thyroid-specific knock-in of oncogenic Braf (LSL-Braf(V600E)/TPO-Cre) to explore the role of endogenous expression of this oncoprotein on tumor initiation and progression. In contrast to other Braf-induced mouse models of tumorigenesis (i.e., melanomas and lung), in which knock-in of Braf(V600E) induces mostly benign lesions, Braf-expressing thyrocytes become transformed and progress to invasive carcinomas with a very short latency, a process that is dampened by treatment with an allosteric MEK inhibitor. These mice also become profoundly hypothyroid due to deregulation of genes involved in thyroid hormone biosynthesis and consequently have high TSH levels. To determine whether TSH signaling cooperates with oncogenic Braf in this process, we first crossed LSL-Braf(V600E)/TPO-Cre with TshR knockout mice. Although oncogenic Braf was appropriately activated in thyroid follicular cells of these mice, they had a lower mitotic index and were not transformed. Thyroid-specific deletion of the Gsα gene in LSL-Braf(V600E)/TPO-Cre/Gnas-E1(fl/fl) mice also resulted in an attenuated cancer phenotype, indicating that the cooperation of TshR with oncogenic Braf is mediated in part by cAMP signaling. Once tumors were established in mice with wild-type TshR, suppression of TSH did not revert the phenotype. These data demonstrate the key role of TSH signaling in Braf-induced papillary thyroid cancer initiation and provide experimental support for recent observations in humans pointing to a strong association between TSH levels and thyroid cancer incidence.

The full-length TSH receptor is stabilized by TSH ligand.

The receptor for thyroid stimulating hormone (TSHR), a GPCR, is the primary antigen in autoimmune hyperthyroidism (Graves' disease) caused by stimulating TSHR antibodies. While we have previously published a full length model of the TSHR, including its leucine rich domain (LRD), linker region (LR) and transmembrane domain (TMD), to date, only a partial LRD (aa 21-261) stabilized with TSHR autoantibodies has been crystallized. Recently, however, cryo-EM structures of the full-length TSHR have been published but they include only an incomplete LR. We have now utilized the cryo-EM models, added disulfide bonds to the LR and performed longer (3000 ns) molecular dynamic (MD) simulations to update our previous model of the entire full-length TSHR, with and without the presence of TSH ligand. As in our earlier work, the new model was embedded in a lipid membrane and was solvated with water and counterions. We found that the 3000 ns Molecular Dynamic simulations showed that the structure of the LRD and TMD were remarkably constant while the LR, known more commonly as the "hinge region", again showed significant flexibility, forming several transient secondary structural elements. Analysis of the new simulations permitted a detailed examination of the effect of TSH binding on the structure of the TSHR. We found a structure-stabilizing effect of TSH, including increased stability of the LR, which was clearly demonstrated by analyzing several intrinsic receptor properties including hydrogen bonding, fluctuation of the LRD orientation, and radius of gyration. In conclusion, we were able to quantify the flexibility of the TSHR and show its increased stability after TSH binding. These data indicated the important role of ligands in directing the signaling structure of a receptor.

Hypothyroidism Induced by a TSH Receptor Peptide-Implications for Thyroid Autoimmunity.

Background: The "neutral" thyrotropin receptor autoantibodies (N-TSHR-Ab) directed at the TSHR ectodomain's hinge region have been shown to induce thyroid cell damage in vitro. During these earlier studies, we developed a mouse monoclonal antibody (MC1) specific for a peptide (amino acid 322-340) in the region (MC1-Mab) which was able to induce thyroid cell stress and apoptosis when administered in vivo. Methods: In order to examine the effect of in vivo generated N-TSHR-Abs, rather than an acutely administered monoclonal antibody, we immunized Balb/c mice with the hinge region peptide over 18 weeks. Serum TSHR antibodies, specific TSHR hinge region antibodies, serum thyroglobulin (TG) and anti-TG as well as thyroxine and thyrotropin (TSH) levels were examined to evaluate the response to the immunization. Histological examination of the thyroid glands and flow cytometry of spleen T cells, B cells and macrophages were also performed to explore the underlying mechanisms. Results: We found that TSHR-peptide immunized mice developed N-TSHR-Abs against the peptide which resulted in thyroid damage shown by thyroid follicular destruction with follicular cell apoptosis, M1 macrophage infiltration, thyroglobulin release, and induction of thyroglobulin antibodies. This resulted in hypothyroidism with increased TSH levels. Conclusion: This study demonstrated that endogenous neutral antibodies to the TSHR could induce thyroid cell damage from apoptosis and M1 macrophage infiltration and resulted in hypothyroidism.

TSH Receptor Oligomers Associated With the TSH Receptor Antibody Reactome.

The TSH receptor (TSHR) and its many forms are the primary antigens of Graves' disease as evidenced by the presence of TSHR antibodies of differing biological activity. The TSH holoreceptor undergoes complex posttranslational changes including cleavage of its ectodomain and oligomer formation. We have previously shown that the TSHR exists in both monomeric and dimeric structures in the thyroid cell membrane and have demonstrated, by modeling, that the transmembrane domains (TMD) can form stable dimeric structures. Based on these earlier simulations of the TSHR-TMD structure and our most recent model of the full-length TSHR, we have now built models of full-length TSHR multimers with and without TSH ligand in addition to multimers of the extracellular leucine-rich domain, the site of TSH and autoantibody binding. Starting from these models we ran molecular dynamics simulations of the receptor oligomers solvated with water and counterions; the full-length oligomers also were embedded in a dipalmitoylphosphatidylcholine bilayer. The full-length TSHR dimer and trimer models stayed in the same relative orientation and distance during 2000 ns (or longer) molecular dynamics simulation in keeping with our earlier report of TMD dimerization. Simulations were also performed to model oligomers of the leucine-rich domain alone; we found a trimeric complex to be even more stable than the dimers. These data provide further evidence that different forms of the TSHR add to the complexity of the immune response to this antigen that, in patients with autoimmune thyroid disease, generate an autoantibody reactome with multiple types of autoantibody to the TSHR.

How one TSH receptor antibody induces thyrocyte proliferation while another induces apoptosis.

Thyroid stimulating hormone (TSH) activates two major G-protein arms, Gsα and Gq leading to initiation of down-stream signaling cascades for survival, proliferation and production of thyroid hormones. Antibodies to the TSH receptor (TSHR-Abs), found in patients with Graves' disease, may have stimulating, blocking, or neutral actions on the thyroid cell. We have shown previously that such TSHR-Abs are distinct signaling imprints after binding to the TSHR and that such events can have variable functional consequences for the cell. In particular, there is a great contrast between stimulating (S) TSHR-Abs, which induce thyroid hormone synthesis and secretion as well as thyroid cell proliferation, compared to so called "neutral" (N) TSHR-Abs which may induce thyroid cell apoptosis via reactive oxygen species (ROS) generation. In the present study, using a rat thyrocyte (FRTL-5) ex vivo model system, our hypothesis was that while N-TSHR-Abs can induce apoptosis via activation of mitochondrial ROS (mROS), the S-TSHR-Abs are able to stimulate cell survival and avoid apoptosis by actively suppressing mROS. Using fluorescent microscopy, fluorometry, live cell imaging, immunohistochemistry and immunoblot assays, we have observed that S-TSHR-Abs do indeed suppress mROS and cellular stress and this suppression is exerted via activation of the PKA/CREB and AKT/mTOR/S6K signaling cascades. Activation of these signaling cascades, with the suppression of mROS, initiated cell proliferation. In sharp contrast, a failure to activate these signaling cascades with increased activation of mROS induced by N-TSHR-Abs resulted in thyroid cell apoptosis. Our current findings indicated that signaling diversity induced by different TSHR-Abs regulated thyroid cell fate. While S-TSHR-Abs may rescue cells from apoptosis and induce thyrocyte proliferation, N-TSHR-Abs aggravate the local inflammatory infiltrate within the thyroid gland, or in the retro-orbit, by inducing cellular apoptosis; a phenomenon known to activate innate and by-stander immune-reactivity via DNA release from the apoptotic cells.

Functional Water Channels Within the TSH Receptor: A New Paradigm for TSH Action With Disease Implications.

The thyroid-stimulating hormone receptor (TSHR) transmembrane domain (TMD) is found in the plasma membrane and consists of lipids and water molecules. To understand the role of TSHR-associated water molecules, we used molecular dynamic simulations of the TMD and identified a network of putative receptor-associated transmembrane water channels. This result was confirmed with extended simulations of the full-length TSHR with and without TSH ligand binding. While the transport time observed in the simulations via the TSHR protein was slower than via the lipid bilayer itself, we found that significantly more water traversed via the TSHR than via the lipid bilayer, which more than doubled with the binding of TSH. Using rat thyroid cells (FRTL-5) and a calcein fluorescence technique, we measured cell volumes after blockade of aquaporins 1 and 4, the major thyroid cell water transporters. TSH showed a dose-dependent ability to influence water transport, and similar effects were observed with stimulating TSHR autoantibodies. Small molecule TSHR agonists, which are allosteric activators of the TMD, also enhanced water transport, illustrating the role of the TMD in this phenomenon. Furthermore, the water channel pathway was also mapped across 2 activation motifs within the TSHR TMD, suggesting how water movement may influence activation of the receptor. In pathophysiological conditions such as hypothyroidism and hyperthyroidism where TSH concentrations are highly variable, this action of TSH may greatly influence water movement in thyroid cells and many other extrathyroidal sites where the TSHR is expressed, thus affecting normal cellular function.

Thyroid Stem Cell Speciation-a Major Role for PKC.

Instructive signals that delineate the formation of thyroid follicles by thyrotropin (TSH) in stem cells are complex. Here, we have examined the role of protein kinase C (PKC) by using a unique Gαq/11 biased small molecule (MSq1) to develop thyroid progenitor cells. Mouse embryonic stem cells (mESCs) were differentiated into anterior endoderm cells and treated with either TSH or MSq1 in the presence or absence of PKC inhibitors. The transcriptional and translational response of key thyroid markers-sodium iodide symporter (NIS), thyroglobulin (TG), and thyrotropin receptor (TSHR) as well as potential signaling molecules-were then analyzed. The data confirmed that MSq1 is a potent Gαq/11 activator with a major increase in Gαq/11 signaling when compared to TSH. MSq1 activation resulted in an increase in thyroid-specific genes, demonstrating that enhanced PKC signaling was able to induce their expression. The specificity of the PKC signals over the protein kinase A (PKA) pathway in regulating thyroid gene expression was shown by using a specific PKC enzyme inhibitor. The data revealed that TG and NIS expression were suppressed in the presence of the PKC inhibition but, in contrast, were not influenced by PKA inhibition. This indicated that PKC activation was the dominant pathway in the inductive process for thyroid hormone production. Furthermore, by examining PKC isoforms we found that PKCξ was the predominant form in the ES cells that mediated the effects. Since PKCξ can lead to activation of transforming growth factor-ß-activated kinase (pTAK1), and its downstream effector nuclear factor κB (NFκB) complex, this demonstrated the involvement of the TAK1/NFκB pathway in thyroid speciation.

Bone circuitry and interorgan skeletal crosstalk.

The past decade has seen significant advances in our understanding of skeletal homeostasis and the mechanisms that mediate the loss of bone integrity in disease. Recent breakthroughs have arisen mainly from identifying disease-causing mutations and modeling human bone disease in rodents, in essence, highlighting the integrative nature of skeletal physiology. It has become increasingly clear that bone cells, osteoblasts, osteoclasts, and osteocytes, communicate and regulate the fate of each other through RANK/RANKL/OPG, liver X receptors (LXRs), EphirinB2-EphB4 signaling, sphingolipids, and other membrane-associated proteins, such as semaphorins. Mounting evidence also showed that critical developmental pathways, namely, bone morphogenetic protein (BMP), NOTCH, and WNT, interact each other and play an important role in postnatal bone remodeling. The skeleton communicates not only with closely situated organs, such as bone marrow, muscle, and fat, but also with remote vital organs, such as the kidney, liver, and brain. The metabolic effect of bone-derived osteocalcin highlights a possible role of skeleton in energy homeostasis. Furthermore, studies using genetically modified rodent models disrupting the reciprocal relationship with tropic pituitary hormone and effector hormone have unraveled an independent role of pituitary hormone in skeletal remodeling beyond the role of regulating target endocrine glands. The cytokine-mediated skeletal actions and the evidence of local production of certain pituitary hormones by bone marrow-derived cells displays a unique endocrine-immune-skeletal connection. Here, we discuss recently elucidated mechanisms controlling the remodeling of bone, communication of bone cells with cells of other lineages, crosstalk between bone and vital organs, as well as opportunities for treating diseases of the skeleton.

Emerging roles of brain tanycytes in regulating blood-hypothalamus barrier plasticity and energy homeostasis.

Seasonal changes in food intake and adiposity in many animal species are triggered by changes in the photoperiod. These latter changes are faithfully transduced into a biochemical signal by melatonin secreted by the pineal gland. Seasonal variations, encoded by melatonin, are integrated by third ventricular tanycytes of the mediobasal hypothalamus through the detection of the thyroid-stimulating hormone (TSH) released from the pars tuberalis. The mediobasal hypothalamus is a critical brain region that maintains energy homeostasis by acting as an interface between the neural networks of the central nervous system and the periphery to control metabolic functions, including ingestive behavior, energy homeostasis, and reproduction. Among the cells involved in the regulation of energy balance and the blood-hypothalamus barrier (BHB) plasticity are tanycytes. Increasing evidence suggests that anterior pituitary hormones, specifically TSH, traditionally considered to have unitary functions in targeting single endocrine sites, display actions on multiple somatic tissues and central neurons. Notably, modulation of tanycytic TSH receptors seems critical for BHB plasticity in relation to energy homeostasis, but this needs to be proven.

Modeling TSH Receptor Dimerization at the Transmembrane Domain.

Biophysical studies have established that the thyrotropin (TSH) receptor (TSHR) undergoes posttranslational modifications including dimerization. Following our earlier simulation of a TSHR-transmembrane domain (TMD) monomer (called TSHR-TMD-TRIO) we have now proceeded with a molecular dynamics simulation (MD) of TSHR-TMD dimerization using this improved membrane-embedded model. The starting structure was the TMD protein with all extracellular and intracellular loops and internal waters, which was placed in the relative orientation of the model originally generated with Brownian dynamics. Furthermore, this model was embedded in a DPPC lipid bilayer further solvated with water and added salt. Data from the MD simulation studies showed that the dimeric subunits stayed in the same relative orientation and distance during the 1000 ns of study. Comparison of representative conformations of the individual monomers when dimerized with the conformations from the monomer simulation showed subtle differences as represented by the backbone root mean square deviations. Differences in the conformations of the ligand-binding sites, suggesting variable affinities for these "hot spots," were also revealed by comparing the docking scores of 46 small-molecule ligands that included known TSHR agonists and antagonists as well as their derivatives. These data add further insight into the tendency of the TSHR-TMD to form dimeric and oligomeric structures and show that the differing conformations influence small-molecule binding sites within the TMD.

Computational model of the full-length TSH receptor.

(GPCR)The receptor for TSH receptor (TSHR), a G protein coupled receptor (GPCR), is of particular interest as the primary antigen in autoimmune hyperthyroidism (Graves' disease) caused by stimulating TSHR antibodies. To date, only one domain of the extracellular region of the TSHR has been crystallized. We have run a 1000 ns molecular dynamic simulation on a model of the entire TSHR generated by merging the extracellular region of the receptor, obtained using artificial intelligence, with our recent homology model of the transmembrane domain, embedded it in a lipid membrane and solvated it with water and counterions. The simulations showed that the structure of the transmembrane and leucine-rich domains were remarkably constant while the linker region (LR), known more commonly as the 'hinge region,' showed significant flexibility, forming several transient secondary structural elements. Furthermore, the relative orientation of the leucine-rich domain with the rest of the receptor was also seen to be variable. These data suggest that this LR is an intrinsically disordered protein. Furthermore, preliminary data simulating the full TSHR model complexed with its ligand (TSH) showed that (a) there is a strong affinity between the LR and TSH ligand and (b) the association of the LR and the TSH ligand reduces the structural fluctuations in the LR. This full-length model illustrates the importance of the LR in responding to ligand binding and lays the foundation for studies of pathologic TSHR autoantibodies complexed with the TSHR to give further insight into their interaction with the flexible LR.