Bone marrow cells produce a novel TSHbeta splice variant that is upregulated in the thyroid following systemic virus infection

Neuroendocrinology of bone

The past decade has witnessed significant advances in our understanding of skeletal homeostasis and the mechanisms that mediate the loss of bone in primary and secondary osteoporosis. Recent breakthroughs have primarily emerged from identifying disease-causing mutations and phenocopying human bone disease in rodents. Notably, using genetically-modified rodent models, disrupting the reciprocal relationship with tropic pituitary hormone and effector hormones, we have learned that pituitary hormones have independent roles in skeletal physiology, beyond their effects exerted through target endocrine glands. The rise of follicle-stimulating hormone (FSH) in the late perimenopause may account, at least in part, for the rapid bone loss when estrogen is normal, while low thyroid-stimulating hormone (TSH) levels may contribute to the bone loss in thyrotoxicosis. Admittedly speculative, suppressed levels of adrenocorticotropic hormone (ACTH) may directly exacerbate bone loss in the setting of glucocorticoid-induced osteoporosis. Furthermore, beyond their established roles in reproduction and lactation, oxytocin and prolactin may affect intergenerational calcium transfer and therefore fetal skeletal mineralization, whereas elevated vasopressin levels in chronic hyponatremic states may increase the risk of bone loss.. Here, we discuss the interaction of each pituitary hormone in relation to its role in bone physiology and pathophysiology.

Pituitary crosstalk with bone, adipose tissue and brain

Traditional textbook physiology has ascribed unitary functions to hormones from the anterior and posterior pituitary gland, mainly in the regulation of effector hormone secretion from endocrine organs. However, the evolutionary biology of pituitary hormones and their receptors provides evidence for a broad range of functions in vertebrate physiology. Over the past decade, we and others have discovered that thyroid-stimulating hormone, follicle-stimulating hormone, adrenocorticotropic hormone, prolactin, oxytocin and arginine vasopressin act directly on somatic organs, including bone, adipose tissue and liver. New evidence also indicates that pituitary hormone receptors are expressed in brain regions, nuclei and subnuclei. These studies have prompted us to attribute the pathophysiology of certain human diseases, including osteoporosis, obesity and neurodegeneration, at least in part, to changes in pituitary hormone levels. This new information has identified actionable therapeutic targets for drug discovery.

The interplay of thyroid hormones and the immune system - where we stand and why we need to know about it

Over the past few years, growing evidence suggests direct crosstalk between thyroid hormones (THs) and the immune system. Components of the immune system were proposed to interfere with the central regulation of systemic TH levels. Conversely, THs regulate innate and adaptive immune responses as immune cells are direct target cells of THs. Accordingly, they express different components of local TH action, such as TH transporters or receptors, but our picture of the interplay between THs and the immune system is still incomplete. This review provides a critical overview of current knowledge regarding the interaction of THs and the immune system with the main focus on local TH action within major innate and adaptive immune cell subsets. Thereby, this review aims to highlight open issues which might help to infer the clinical relevance of THs in host defence in the context of different types of diseases such as infection, ischemic organ injury or cancer.

Thyroid-stimulating hormone decreases the risk of osteoporosis by regulating osteoblast proliferation and differentiation

BACKGROUND

As the incidence of secretory osteoporosis has increased, bone loss, osteoporosis and their relationships with thyroid-stimulating hormone (TSH) have received increased attention. In this study, the role of TSH in bone metabolism and its possible underlying mechanisms were investigated.

METHODS

We analyzed the serum levels of free triiodothyronine (FT3), free thyroxine (FT4), and TSH and the bone mineral density (BMD) levels of 114 men with normal thyroid function. In addition, osteoblasts from rat calvarial samples were treated with different doses of TSH for different lengths of time. The related gene and protein expression levels were investigated.

RESULTS

A comparison of the BMD between the high-level and low-level serum TSH groups showed that the TSH serum concentration was positively correlated with BMD. TSH at concentrations of 10 mU/mL and 100 mU/mL significantly increased the mRNA levels of ALP, COI1 and Runx2 compared with those of the control (P < 0.05, P < 0.01). Bone morphogenetic protein (BMP)2 activity was enhanced with both increased TSH concentration and increased time. The protein levels of Runx2 and osterix were increased in a dose-dependent manner.

CONCLUSIONS

The circulating concentrations of TSH and BMD were positively correlated with normal thyroid function in males. TSH promoted osteoblast proliferation and differentiation in rat primary osteoblasts.

Unconventional Actions of Glycoprotein Hormone Subunits: A Comprehensive Review

The glycoprotein hormones (GPH) are heterodimers composed of a common α subunit and a specific β subunit. They act by activating specific leucine-rich repeat G protein-coupled receptors. However, individual subunits have been shown to elicit responses in cells devoid of the receptor for the dimeric hormones. The α subunit is involved in prolactin production from different tissues. The human chorionic gonadotropin β subunit (βhCG) plays determinant roles in placentation and in cancer development and metastasis. A truncated form of the thyrotropin (TSH) β subunit is also reported to have biological effects. The GPH α- and β subunits are derived from precursor genes (gpa and gpb, respectively), which are expressed in most invertebrate species and are still represented in vertebrates as GPH subunit paralogs (gpa2 and gpb5, respectively). No specific receptor has been found for the vertebrate GPA2 and GPB5 even if their heterodimeric form is able to activate the TSH receptor in mammals. Interestingly, GPA and GPB are phylogenetically and structurally related to cysteine-knot growth factors (CKGF) and particularly to a group of antagonists that act independently on any receptor. This review article summarizes the observed actions of individual GPH subunits and presents the current hypotheses of how these actions might be induced. New approaches are also proposed in light of the evolutionary relatedness with antagonists of the CKGF family of proteins.

Dynamic Interactions Between the Immune System and the Neuroendocrine System in Health and Disease

The immune system and the neuroendocrine system share many common features. Both consist of diverse components consisting of receptors and networks that are widely distributed throughout the body, and both sense and react to external stimuli which, on the one hand control mechanisms of immunity, and on the other hand control and regulate growth, development, and metabolism. It is thus not surprising, therefore, that the immune system and the neuroendocrine system communicate extensively. This article will focus on bi-directional immune-endocrine interactions with particular emphasis on the hormones of the hypothalamus-pituitary-thyroid (HPT) axis. New findings will be discussed demonstrating the direct process through which the immune system-derived thyroid stimulating hormone (TSH) controls thyroid hormone synthesis and bone metamorphosis, particularly in the context of a novel splice variant of TSHβ made by peripheral blood leukocytes (PBL). Also presented are the ways whereby the TSHβ splice variant may be a contributing factor in the development and/or perpetuation of autoimmune thyroid disease (AIT), and how systemic infection may elicit immune-endocrine responses. The relationship between non-HPT hormones, in particular adipose hormones, and immunity is discussed.

The Human TSHβ Subunit Proteins and Their Binding Sites on the TSH Receptor Using Molecular Dynamics Simulation

To gain further insight into the binding of the normal and variant human TSHβ subunits (TSHβ and TSHβv), we modeled the 2 monomeric proteins and studied their interaction with the TSH receptor ectodomain (TSHR-ECD) using molecular dynamics simulation Furthermore, analyzed their bioactivity in vitro using recombinant proteins to confirm that such binding was physiologically relevant. Examining the interaction of TSHβ and TSHβv with the TSHR-ECD model using molecular dynamic simulation revealed strong binding of these proteins to the receptor ECD. The specificity of TSHβ and TSHβv binding to the TSHR-ECD was examined by analyzing the hydrogen-bonding residues of these subunits to the FSH receptor ECD, indicating the inability of these molecules to bind to the FSH receptors. Furthermore, the modelling suggests that TSHβ and TSHβv proteins clasped the concave surface of the leucine rich region of the TSHR ECD in a similar way to the native TSH using dynamic hydrogen bonding. These mutually exclusive stable interactions between the subunits and ECD residues included some high-affinity contact sites corresponding to binding models of native TSH. Furthermore, we cloned TSHβ and TSHβv proteins using the entire coding ORF and purified the flag-tagged proteins. The expressed TSHβ subunit proteins retained bioactivity both in a coculture system as well as with immune-purified proteins. In summary, we showed that such interactions can result in a functional outcome and may exert physiological or pathophysiological effects in immune cells.

Thyroid stimulating hormone β-subunit splice variant is expressed in all fractional subsets of bone marrow hematopoietic cells and peripheral blood leukocytes and is modulated during bacterial infection

Thyroid stimulating hormone (TSH), a hormone produced in the anterior pituitary, is used to regulate thyroid hormone secretion. It has been known for over three decades that TSH is made by the cells of the immune system; however, the functional role of immune system TSH is unclear. We previously demonstrated that an alternatively-spliced isoform of TSHβ, referred to as the TSHβ splice variant (TSHβv), is the primary form of TSHβ made by hematopoietic cells in mice and humans. Most studies have linked TSHβv expression to myeloid cells of the immune system; however, it has recently been demonstrated that plasma cells in patients with Hashimoto's thyroiditis may be a source of immune system TSHβv. Here, we demonstrate that TSHβv is expressed in bone marrow precursors of lymphoid cells, monocytes, and granulocytes, as well as in mesenteric lymph node (MLN) cells. Plasma cells generated by in vitro culture with bacterial lipopolysaccharide (LPS), and MLN cells from mice infected with L. monocytogenes expressed TSHβv. There was an increase in the intensity of intracellular TSHβv expression in MLN cells following exposure to LPS, and in the proportion of TSHβv⁺ CD138⁺ MLN cells following L. monocytogenes infection. The number of TSHβv⁺ cells increased in MLN cells, particularly among CD138⁺ cells, following bacterial infection. This was confirmed by an increase in gene expression of BLIMP-1, the transcription factor for CD138, following infection. Levels of circulating thyroxine dropped significantly in mice 24 hrs post-infection. These findings suggest that immune system TSHβv may contribute to the host immune response during bacterial infection.

HPT axis‑independent TSHβ splice variant regulates the synthesis of thyroid hormone in mice

Thyroid stimulating hormone (TSH) consists of an α‑subunit and a unique β‑subunit. The first in‑frame TSHβ splice variant produced by the cells of immune system was identified in 2009. The TSHβ splice variant and native TSHβ exhibit different expression profiles, and research has been conducted to elucidate the role of the TSHβ splice variant in different diseases. However, understanding of the fundamental physiological characteristics of the TSHβ splice variant is currently limited. To verify whether the TSHβ splice variant has the potential to induce thyroid follicular cells to synthesize thyroid hormone, in vivo and in vitro stimulation experiments were conducted in the present study. A total of 60 C57BL/6 mice were divided into control‑, 5 and 10 µg TSHβ splice variant‑treated groups at random. Mice were sacrificed at 0.5, 1 and 4 h after intraperitoneal injection, and serum levels of tri‑iodothyronine (T3) and thyroxine (T4) were determined using a radioimmunoassay. Thyroid follicular cells were isolated from the thyroids of mice, and stimulated with 2 µg/ml TSHβ splice variant. Supernatants were collected, and the levels of T3 and T4 were detected. The protein expression levels of the sodium‑iodide symporter, thyroperoxidase and thyroglobulin in thyroid follicular cells were quantified using western blot analysis. To verify whether the TSHβ splice variant expression was regulated by the hypothalamus‑pituitary‑thyroid (HPT) axis, similar to native TSHβ, a total of 60 C57BL/6 mice were equally divided into control, 2 mg/kg T3 intraperitoneal injection and 0.05 mg/kg thyroid‑releasing hormone intraperitoneal injection groups at random. Mice were sacrificed at 1 and 4 h after injection. Alterations in the expression of the TSHβ splice variant in the pituitary, thyroid, peripheral blood leukocytes and spleen tissues were detected using western blot analysis. The present study demonstrated that the TSHβ splice variant is not regulated by the HPT axis and may affect thyroid hormone synthesis. Modifications in the expression of the TSHβ splice variant may occur in a uniquely regulated manner to provide peripheral immunological compartments with a source of activated cells, particularly under immune stress.

Novel Splicing of Immune System Thyroid Stimulating Hormone β-Subunit-Genetic Regulation and Biological Importance

Thyroid stimulating hormone (TSH), a glycoprotein hormone produced by the anterior pituitary, controls the production of thyroxine (T₄) and triiodothyronine (T₃) in the thyroid. TSH is also known to be produced by the cells of the immune system; however, the physiological importance of that to the organism is unclear. We identified an alternatively-spliced form of TSHβ that is present in both humans and mice. The TSHβ splice variant (TSHβv), although produced at low levels by the pituitary, is the primary form made by hematopoietic cells in the bone marrow, and by peripheral leukocytes. Recent studies have linked TSHβv functionally to a number of health-related conditions, including enhanced host responses to infection and protection against osteoporosis. However, TSHβv also has been associated with autoimmune thyroiditis in humans. Yet to be identified is the process by which the TSHβv isoform is produced. Here, a set of genetic steps is laid out through which human TSHβv is generated using splicing events that result in a novel transcript in which exon 2 is deleted, exon 3 is retained, and the 3' end of intron 2 codes for a signal peptide of the TSHβv polypeptide.

Pituitary Diseases and Bone

Neuroendocrinology of bone is a new area of research based on the evidence that pituitary hormones may directly modulate bone remodeling and metabolism. Skeletal fragility associated with high risk of fractures is a common complication of several pituitary diseases such as hypopituitarism, Cushing disease, acromegaly, and hyperprolactinemia. As in other forms of secondary osteoporosis, pituitary diseases generally affect bone quality more than bone quantity, and fractures may occur even in the presence of normal or low-normal bone mineral density as measured by dual-energy X-ray absorptiometry, making difficult the prediction of fractures in these clinical settings. Treatment of pituitary hormone excess and deficiency generally improves skeletal health, although some patients remain at high risk of fractures, and treatment with bone-active drugs may become mandatory. The aim of this review is to discuss the physiological, pathophysiological, and clinical insights of bone involvement in pituitary diseases.

Actions of pituitary hormones beyond traditional targets

Studies over the past decade have challenged the long-held belief that pituitary hormones have singular functions in regulating specific target tissues, including master hormone secretion. Our discovery of the action of thyroid-stimulating hormone (TSH) on bone provided the first glimpse into the non-traditional functions of pituitary hormones. Here we discuss evolving experimental and clinical evidence that growth hormone (GH), follicle-stimulating hormone (FSH), adrenocorticotrophic hormone (ACTH), prolactin, oxytocin and arginine vasopressin (AVP) regulate bone and other target tissues, such as fat. Notably, genetic and pharmacologic FSH suppression increases bone mass and reduces body fat, laying the framework for targeting the FSH axis for treating obesity and osteoporosis simultaneously with a single agent. Certain 'pituitary' hormones, such as TSH and oxytocin, are also expressed in bone cells, providing local paracrine and autocrine networks for the regulation of bone mass. Overall, the continuing identification of new roles for pituitary hormones in biology provides an entirely new layer of physiologic circuitry, while unmasking new therapeutic targets.

Thyroid Hormone and Skeletal Development

Thyroid hormone (TH) is essential for skeletal development from the late fetal life to the onset of puberty. During this large window of actions, TH has key roles in endochondral and intramembranous ossifications and in the longitudinal bone growth. There is evidence that TH acts directly in skeletal cells but also indirectly, specially via the growth hormone/insulin-like growth factor-1 axis, to control the linear skeletal growth and maturation. The presence of receptors, plasma membrane transporters, and activating and inactivating enzymes of TH in skeletal cells suggests that direct actions of TH in these cells are crucial for skeletal development, which has been confirmed by several in vitro and in vivo studies, including mouse genetic studies, and clinical studies in patients with resistance to thyroid hormone due to dominant-negative mutations in TH receptors. This review examines progress made on understanding the mechanisms by which TH regulates the skeletal development.

Thyroid hormone metabolism in innate immune cells

Thyroid hormone (TH) metabolism and thyroid status have been linked to various aspects of the immune response. There is extensive literature available on the effects of thyroid hormone on innate immune cells. However, only recently have authors begun to study the mechanisms behind these effects and the role of intracellular TH metabolism in innate immune cell function during inflammation. This review provides an overview of the molecular machinery of intracellular TH metabolism present in neutrophils, macrophages and dendritic cells and the role and effects of intracellular TH metabolism in these cells. Circulating TH levels have a profound effect on neutrophil, macrophage and dendritic cell function. In general, increased TH levels result in an amplification of the pro-inflammatory response of these cells. The mechanisms behind these effects include both genomic and non-genomic effects of TH. Besides a pro-inflammatory effect induced by extracellular TH, the cellular response to pro-inflammatory stimuli appears to be dependent on functional intracellular TH metabolism. This is illustrated by the fact that the deiodinase enzymes and in some cell types also thyroid hormone receptors appear to be crucial for adequate innate immune cell function. This overview of the literature suggests that TH metabolism plays an important role in the host defence against infection through the modulation of innate immune cell function.

Expanding the Role of Thyroid-Stimulating Hormone in Skeletal Physiology

The dogma that thyroid-stimulating hormone (TSH) solely regulates the production of thyroid hormone from the thyroid gland has hampered research on its wider physiological roles. The action of pituitary TSH on the skeleton has now been well described; in particular, its action on osteoblasts and osteoclasts. It has also been recently discovered that the bone marrow microenvironment acts as an endocrine circuit with bone marrow-resident macrophages capable of producing a novel TSH-β subunit variant (TSH-βv), which may modulate skeletal physiology. Interestingly, the production of this TSH-βv is positively regulated by T3 accentuating such modulation in the presence of thyroid overactivity. Furthermore, a number of small molecule ligands acting as TSH agonists, which allosterically modulate the TSH receptor have been identified and may have similar modulatory influences on bone cells suggesting therapeutic potential. This review summarizes our current understanding of the role of TSH, TSH-β, TSH-βv, and small molecule agonists in bone physiology.

Neuroendocrine-immune interaction: Evolutionarily conserved mechanisms that maintain allostasis in an ever-changing environment

It has now become accepted that the immune system and neuroendocrine system form an integrated part of our physiology. Immunological defense mechanisms act in concert with physiological processes like growth and reproduction, energy intake and metabolism, as well as neuronal development. Not only are psychological and environmental stressors communicated to the immune system, but also, vice versa, the immune response and adaptation to a current pathogen challenge are communicated to the entire body, including the brain, to evoke adaptive responses (e.g., fever, sickness behavior) that ensure allocation of energy to fight the pathogen. This phenomenon is evolutionarily conserved. Hence it is both interesting and important to consider the evolutionary history of this bi-directional neuroendocrine-immune communication to reveal phylogenetically ancient or relatively recently acquired mechanisms. Indeed, such considerations have already disclosed an extensive "common vocabulary" of information pathways as well as molecules and their receptors used by both the neuroendocrine and immune systems. This review focuses on the principal mechanisms of bi-directional communication and the evidence for evolutionary conservation of the important physiological pathways involved.

Beyond Reproduction: Pituitary Hormone Actions on Bone

The long-held belief that pituitary hormones act solely on master targets was first questioned when we documented G protein-coupled receptors for thyroid-stimulating hormone, follicle-stimulating hormone, adrenocorticotrophic hormone, oxytocin, and vasopressin on bone cells. These evolutionarily conserved hormones and their receptors are known to have primitive roles, and exist in invertebrate species as far down as coelenterates. It is not surprising therefore that each such hormone has multiple hitherto unrecognized functions in mammalian integrative physiology, and hence, becomes a potential target for therapeutic intervention. Here we discuss the skeletal actions of pituitary hormones.

T3 Regulates a Human Macrophage-Derived TSH-β Splice Variant: Implications for Human Bone Biology

TSH and thyroid hormones (T3 and T4) are intimately involved in bone biology. We have previously reported the presence of a murine TSH-β splice variant (TSH-βv) expressed specifically in bone marrow-derived macrophages and that exerted an osteoprotective effect by inducing osteoblastogenesis. To extend this observation and its relevance to human bone biology, we set out to identify and characterize a TSH-β variant in human macrophages. Real-time PCR analyses using human TSH-β-specific primers identified a 364-bp product in macrophages, bone marrow, and peripheral blood mononuclear cells that was sequence verified and was homologous to a human TSH-βv previously reported. We then examined TSH-βv regulation using the THP-1 human monocyte cell line matured into macrophages. After 4 days, 46.1% of the THP-1 cells expressed the macrophage markers CD-14 and macrophage colony-stimulating factor and exhibited typical morphological characteristics of macrophages. Real-time PCR analyses of these cells treated in a dose-dependent manner with T3 showed a 14-fold induction of human TSH-βv mRNA and variant protein. Furthermore, these human TSH-βv-positive cells, induced by T3 exposure, had categorized into both M1 and M2 macrophage phenotypes as evidenced by the expression of macrophage colony-stimulating factor for M1 and CCL-22 for M2. These data indicate that in hyperthyroidism, bone marrow resident macrophages have the potential to exert enhanced osteoprotective effects by oversecreting human TSH-βv, which may exert its local osteoprotective role via osteoblast and osteoclast TSH receptors.

Role of Thyroid Hormones in Skeletal Development and Bone Maintenance

The skeleton is an exquisitely sensitive and archetypal T3-target tissue that demonstrates the critical role for thyroid hormones during development, linear growth, and adult bone turnover and maintenance. Thyrotoxicosis is an established cause of secondary osteoporosis, and abnormal thyroid hormone signaling has recently been identified as a novel risk factor for osteoarthritis. Skeletal phenotypes in genetically modified mice have faithfully reproduced genetic disorders in humans, revealing the complex physiological relationship between centrally regulated thyroid status and the peripheral actions of thyroid hormones. Studies in mutant mice also established the paradigm that T3 exerts anabolic actions during growth and catabolic effects on adult bone. Thus, the skeleton represents an ideal physiological system in which to characterize thyroid hormone transport, metabolism, and action during development and adulthood and in response to injury. Future analysis of T3 action in individual skeletal cell lineages will provide new insights into cell-specific molecular mechanisms and may ultimately identify novel therapeutic targets for chronic degenerative diseases such as osteoporosis and osteoarthritis. This review provides a comprehensive analysis of the current state of the art.

Splenic Leukocytes Traffic to the Thyroid and Produce a Novel TSHβ Isoform during Acute Listeria monocytogenes Infection in Mice

The thyroid stimulating hormone beta-subunit (TSHβ) with TSHα form a glycoprotein hormone that is produced by the anterior pituitary in the hypothalamus-pituitary-thyroid (HPT) axis. Although TSHβ has been known for many years to be made by cells of the immune system, the role of immune system TSH has remained unclear. Recent studies demonstrated that cells of the immune system produce a novel splice variant isoform of TSHβ (TSHβv), but little if any native TSHβ. Here, we show that within three days of systemic infection of mice with Listeria monocytogenes, splenic leukocytes synthesized elevated levels of TSHβv. This was accompanied by an influx of CD14⁺, Ly6C⁺, Ly6G⁺ cells into the thyroid of infected mice, and increased levels of intrathyroidal TSHβv gene expression. Adoptive transfer of carboxyfluorescein succinimidyl ester (CFSE)-labeled splenic leukocytes from infected mice into non-infected mice migrated into the thyroid as early as forty-eight hours post-cell transfer, whereas CFSE-labeled cells from non-infected mice failed to traffic to the thyroid. These findings demonstrate for the first time that during bacterial infection peripheral leukocytes produce elevated levels of TSHβv, and that spleen cells traffic to the thyroid where they produce TSHβv intrathyroidally.

Functional human TSHβ splice variant produced by plasma cell may be involved in the immunologic injury of thyroid in the patient with Hashimoto's thyroiditis

Hashimoto's Thyroiditis (HT) is the most common cause of hypothyroidism in areas of the world where iodine levels are sufficient. However, the pathogenesis of HT has not been completely elucidated. The first functional human TSHβ splice variant was supposed to be involved in the pathology of Hashimoto's thyroiditis. The question remains as to which kind of intrathyroid cells expresses functional TSHβ splice variant and whether there are expression variations of functional TSHβ splice variant in the injured thyroid of patient with HT. For the answer to this question, immune-injured thyroids were obtained from 30 patients with HT. Localization study of functional TSHβ splice variant in injured thyroid was done by immunofluorescence double staining. Transcription and translation level of functional TSHβ splice variant were detected by using qRT-PCR and semi-quantitative immunohistochemistry method, respectively. The correlation between expression level of functional TSHβ splice variant and degree of thyroid follicles damage was assessed. It was firstly identified that functional TSHβ splice variant was predominately expressed by plasma cells infiltrated around follicles and germinal center in injured thyroid of patient with HT. Of particular interest, the TSHβ splice variant was expressed at significantly higher levels in the thyroid tissues of patients with HT than that in the normal thyroid tissues, furthermore, expression level of TSHβ splice variant was positive related with the degree of follicles damage in thyroid of patient with HT. These findings defined the immune-derived functional TSHβ splice variant that resided in the thyroid of patient with HT, which exerted the unique effects on the pathogenesis of HT, meanwhile, we considered these findings to have significant implications for understanding immune-endocrine interactions in a number of ways.

Splicing Regulation of Pro-Inflammatory Cytokines and Chemokines: At the Interface of the Neuroendocrine and Immune Systems

Alternative splicing plays a key role in posttranscriptional regulation of gene expression, allowing a single gene to encode multiple protein isoforms. As such, alternative splicing amplifies the coding capacity of the genome enormously, generates protein diversity, and alters protein function. More than 90% of human genes undergo alternative splicing, and alternative splicing is especially prevalent in the nervous and immune systems, tissues where cells need to react swiftly and adapt to changes in the environment through carefully regulated mechanisms of cell differentiation, migration, targeting, and activation. Given its prevalence and complexity, this highly regulated mode of gene expression is prone to be affected by disease. In the following review, we look at how alternative splicing of signaling molecules—cytokines and their receptors—changes in different pathological conditions, from chronic inflammation to neurologic disorders, providing means of functional interaction between the immune and neuroendocrine systems. Switches in alternative splicing patterns can be very dynamic and can produce signaling molecules with distinct or antagonistic functions and localization to different subcellular compartments. This newly discovered link expands our understanding of the biology of immune and neuroendocrine cells, and has the potential to open new windows of opportunity for treatment of neurodegenerative disorders.

Thyrostimulin Regulates Osteoblastic Bone Formation During Early Skeletal Development

The ancestral glycoprotein hormone thyrostimulin is a heterodimer of unique glycoprotein hormone subunit alpha (GPA)2 and glycoprotein hormone subunit beta (GPB)5 subunits with high affinity for the TSH receptor. Transgenic overexpression of GPB5 in mice results in cranial abnormalities, but the role of thyrostimulin in bone remains unknown. We hypothesized that thyrostimulin exerts paracrine actions in bone and determined: 1) GPA2 and GPB5 expression in osteoblasts and osteoclasts, 2) the skeletal consequences of thyrostimulin deficiency in GPB5 knockout (KO) mice, and 3) osteoblast and osteoclast responses to thyrostimulin treatment. Gpa2 and Gpb5 expression was identified in the newborn skeleton but declined rapidly thereafter. GPA2 and GPB5 mRNAs were also expressed in primary osteoblasts and osteoclasts at varying concentrations. Juvenile thyrostimulin-deficient mice had increased bone volume and mineralization as a result of increased osteoblastic bone formation. However, thyrostimulin failed to induce a canonical cAMP response or activate the noncanonical Akt, ERK, or mitogen-activated protein kinase (P38) signaling pathways in primary calvarial or bone marrow stromal cell-derived osteoblasts. Furthermore, thyrostimulin did not directly inhibit osteoblast proliferation, differentiation or mineralization in vitro. These studies identify thyrostimulin as a negative but indirect regulator of osteoblastic bone formation during skeletal development.

Oxytocin and bone

One of the most meaningful results recently achieved in bone research has been to reveal that the pituitary hormones have profound effect on bone, so that the pituitary-bone axis has become one of the major topics in skeletal physiology. Here, we discuss the relevant evidence about the posterior pituitary hormone oxytocin (OT), previously thought to exclusively regulate parturition and breastfeeding, which has recently been established to directly regulate bone mass. Both osteoblasts and osteoclasts express OT receptors (OTR), whose stimulation enhances bone mass. Consistent with this, mice deficient in OT or OTR display profoundly impaired bone formation. In contrast, bone resorption remains unaffected in OT deficiency because, even while OT stimulates the genesis of osteoclasts, it inhibits their resorptive function. Furthermore, in addition to its origin from the pituitary, OT is also produced by bone marrow osteoblasts acting as paracrine-autocrine regulator of bone formation modulated by estrogens. In turn, the power of estrogen to increase bone mass is OTR-dependent. Therefore, OTR(-/-) mice injected with 17β-estradiol do not show any effects on bone formation parameters, while the same treatment increases bone mass in wild-type mice. These findings together provide evidence for an anabolic action of OT in regulating bone mass and suggest that bone marrow OT may enhance the bone-forming action of estrogen through an autocrine circuit. This established new physiological role for OT in the maintenance of skeletal integrity further suggests the potential use of this hormone for the treatment of osteoporosis.

Biological Impact of the TSHβ Splice Variant in Health and Disease

Thyroid stimulating hormone (TSH), a glycoprotein hormone composed of α and β chains, is produced by thyrotrope cells of the anterior pituitary. Within the conventional endocrine loop, pituitary-derived TSH binds to receptors in the thyroid, resulting in the release of the thyroid hormones thyroxine (T₄) and triiodothyronine (T₃). T₄ and T₃ in turn regulate nearly every aspect of mammalian physiology, including basal metabolism, growth and development, and mood and cognition. Although TSHβ has been known for years to be produced by cells of the immune system, the significance of that has remained largely unclear. Recently, a splice variant of TSHβ (TSHβv), which consists of a truncated but biologically functional portion of the native form of TSHβ, was shown to be produced by bone marrow cells and peripheral blood leukocytes, particularly cells of the myeloid/monocyte lineage. In contrast, full-length native TSHβ is minimally produced by cells of the immune system. The present article will describe the discovery of the TSHβv and will discuss its potential role in immunity and autoimmunity, inflammation, and bone remodeling.

Thyroid and bone: macrophage-derived TSH-β splice variant increases murine osteoblastogenesis

It is now firmly established that TSH may influence the physiology and patho-physiology of bone by activating osteoblasts and inhibiting osteoclast activity resulting in relative osteoprotection. Whether this influence is directly exerted by pituitary-derived TSH in vivo is less certain, because we have previously reported that the suppression of pituitary TSH does not remove such protection. Here, we have characterized the functional relevance of a novel form of the TSH-β subunit, designated TSH-βv, known to be produced by murine bone marrow cells. We found that fresh bone marrow-derived macrophages (MØs) preferentially produced TSH-βv and, when cocultured with CHO cells engineered to overexpress the full-length TSH receptor, were able to generate the production of intracellular cAMP; a phenomenon not seen in control CHO cells, such results confirmed the bioactivity of the TSH variant. Furthermore, cocultures of MØs and osteoblasts were shown to enhance osteoblastogenesis, and this phenomenon was markedly reduced by antibody to TSH-β, suggesting direct interaction between MØs and osteoblasts as observed under the electron microscope. These data suggest a new paradigm of local modulation of bone biology by a MØ-derived TSH-like molecule and raise the question of the relative contribution of local vs pituitary-derived TSH in osteoprotection.

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 TSHβ. 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.

Hyperthyroid-associated osteoporosis is exacerbated by the loss of TSH signaling

The osteoporosis associated with human hyperthyroidism has traditionally been attributed to elevated thyroid hormone levels. There is evidence, however, that thyroid-stimulating hormone (TSH), which is low in most hyperthyroid states, directly affects the skeleton. Importantly, Tshr-knockout mice are osteopenic. In order to determine whether low TSH levels contribute to bone loss in hyperthyroidism, we compared the skeletal phenotypes of wild-type and Tshr-knockout mice that were rendered hyperthyroid. We found that hyperthyroid mice lacking TSHR had greater bone loss and resorption than hyperthyroid wild-type mice, thereby demonstrating that the absence of TSH signaling contributes to bone loss. Further, we identified a TSH-like factor that may confer osteoprotection. These studies suggest that therapeutic suppression of TSH to very low levels may contribute to bone loss in people.

A newly identified TSHβ splice variant is involved in the pathology of Hashimoto's thyroiditis

Thyrotropin (TSH) is a protein that plays a key role in the control of thyroid function. TSH consists of a common α-subunit and a unique β-subunit; the latter is responsible for hormone specificity. A novel splice variant of human TSHβ was identified in 2009. To date, only the tissue distribution of the human TSHβ splice variant mRNA has been studied. Therefore, we aimed to characterize the protein translated from this splice variant. Salting-out, dialysis and concentration of serum proteins were followed by immunoprecipitation to identify the hTSHβ splice variant in serum. Stable CHO cell lines expressing the hTSHβ splice variant and V5-hTSHα were generated. After co-culture, co-immunoprecipitation was used to determine if the hTSHβ splice variant can dimerise with TSHα. We showed for the first time that the hTSHβ splice variant exists in human serum and dimerises with TSHα. To explore the relationship between the TSHβ splice variant and the pathogenesis of autoimmune thyroiditis, we assessed variations in the mRNA expression of the TSHβ splice variant in the peripheral blood leukocytes (PBLs) of Hashimoto's thyroiditis (HT) patients using quantitative RT-PCR. We found that the mRNA expression levels of the TSHβ splice variant were higher in the PBLs of HT patients who were not undergoing prednisone therapy (n = 10, P < 0.0001) and in the PBLs of HT patients with a longer duration of illness (>18 months) who were undergoing prednisone therapy (n = 5, P = 0.023) than in those of the control group. This pattern was reversed in the PBLs of HT patients with a shorter duration of illness (<9 months) who were undergoing prednisone therapy (n = 8, P < 0.0001). Dexamethasone inhibition of the TSHβ splice variant mRNA expression occurred in a dose- and time-dependent manner. These results demonstrated that the TSHβ splice variant may participate in the pathogenesis of HT.

Bone marrow oxytocin mediates the anabolic action of estrogen on the skeleton

Estrogen uses two mechanisms to exert its effect on the skeleton: it inhibits bone resorption by osteoclasts and, at higher doses, can stimulate bone formation. Although the antiresorptive action of estrogen arises from the inhibition of the MAPK JNK, the mechanism of its effect on the osteoblast remains unclear. Here, we report that the anabolic action of estrogen in mice occurs, at least in part, through oxytocin (OT) produced by osteoblasts in bone marrow. We show that the absence of OT receptors (OTRs) in OTR(-/-) osteoblasts or attenuation of OTR expression in silenced cells inhibits estrogen-induced osteoblast differentiation, transcription factor up-regulation, and/or OT production in vitro. In vivo, OTR(-/-) mice, known to have a bone formation defect, fail to display increases in trabecular bone volume, cortical thickness, and bone formation in response to estrogen. Furthermore, osteoblast-specific Col2.3-Cre(+)/OTR(fl/fl) mice, but not TRAP-Cre(+)/OTR(fl/fl) mice, mimic the OTR(-/-) phenotype and also fail to respond to estrogen. These data attribute the phenotype of OTR deficiency to an osteoblastic rather than an osteoclastic defect. Physiologically, feed-forward OT release in bone marrow by a rising estrogen concentration may facilitate rapid skeletal recovery during the latter phases of lactation.

Skeletal receptors for steroid-family regulating glycoprotein hormones: A multilevel, integrated physiological control system

Pituitary glycoprotein hormone receptors, including ACTH-R, TSH-R, and FSH-R, occur in bone. Their skeletal expression reflects that central endocrine control is evolutionarily recent. ACTH receptors, in osteoblasts or the adrenal cortex, drive VEGF synthesis. VEGF is essential to maintain vasculature. In bone, ACTH suppression by glucocorticoids can cause osteonecrosis. TSH receptors occur on osteoblasts and osteoclasts, in both cases reducing activity. Thus, TSH directly reduces skeletal turnover, consistent with evolutionary adaptation to stress. FSH receptors accelerate bone resorption, whereas estrogen promotes bone formation, the forces usually balancing. With ovarian failure, low estrogen with high FSH causes rapid bone loss. The skeletal FSH effect in the menopause seems paradoxical, but it is a logical adaptation in lactation, where prolonged FSH elevation also occurs. In addition to receptors, there is some synthesis of pituitary glycoproteins at distributed sites; this is not well studied, but it may further modify the paradigm of central endocrine regulation.

Extrapituitary production of anterior pituitary hormones: an overview

Protein hormones from the anterior pituitary gland have well-established endocrine roles in their peripheral target glands. It is, however, now known that these proteins are also produced within many of their target tissues, in which they act as local autocrine or paracrine factors, with physiological and/or pathophysiological significance. This emerging concept is the focus of this brief review.

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.

Regulated production of the pituitary hormone oxytocin from murine and human osteoblasts

Oxytocin (OT) is a primitive neurohypophyseal hormone that plays a primary and indispensible role in mammalian lactation. We have shown recently that OT also regulates bone remodeling, mainly bone formation, with remarkable sensitivity. We now show that OT, apart from its neurohypophyseal origin, is produced in abundance by both human and murine osteoblasts. Production of osteoblast OT is under the control of estrogen, which acts by activating the MAP kinase Erk. This non-genomic mechanism of estrogen action is in stark contrast to its genomic control of OT receptor (OTR) expression. We surmise that there is a local feed-forward loop in bone marrow through which the OT so produced from osteoblasts in response to estrogen acts upon its receptor to exert a potent anabolic action.

Interleukin-19: multiple roles in immune regulation and disease

First reported in 1999, IL-19 remains a mystery in many ways. Despite appearing in many genome scans and candidate gene studies, and having been searched for specifically as part of the IL-10 family, its function is still to be defined. Nonetheless, a pattern of Th2 promotion is coalescing from this nebulous body of work, supported by increasing evidence for a role in asthma. Similarly, a clear but less intuitive role as a subtle immunomodulator is emerging in psoriasis and chronic inflammatory disorders in general. Indeed, several human diseases and their animal models have highlighted a role for IL-19. Key questions remain, relating to the nature of its receptor, its function (if any) on leukocytes and how its effects are distinguished by the cell from those of IL-20 and IL-24. In this review, I shall attempt to bring together a summary of the known work - disparate as it may be - as well as presenting a picture of these two important clinical disorders and the potential involvement of this somewhat enigmatic cytokine.

Immunological regulation of metabolism--a novel quintessential role for the immune system in health and disease

The hypothalamus-pituitary-thyroid (HPT) axis is an integrated hormone network that is essential for maintaining metabolic homeostasis. It has long been known that thyroid stimulating hormone (TSH), a central component of the HPT axis, can be made by cells of the immune system; however, the role of immune system TSH remains enigmatic and most studies have viewed it as a cytokine used to regulate immune function. Recent studies now indicate that immune system-derived TSH, in particular, a splice variant of TSHβ that is preferentially made by cells of the immune system, is produced by a subset of hematopoietic cells that traffic to the thyroid. On the basis of these and other findings, we propose the novel hypothesis that the immune system is an active participant in the regulation of basal metabolism. We further speculate that this process plays a critical role during acute and chronic infections and that it contributes to a wide range of chronic inflammatory conditions with links to thyroid dysregulation. This hypothesis, which is amenable to empirical analysis, defines a previously unknown role for the immune system in health and disease, and it provides a dynamic connection between immune-endocrine interactions at the organismic level.

Splicing variants impact in thyroid normal physiology and pathological conditions

RNA splicing is an essential, precisely regulated process that occurs after gene transcription and before mRNA translation, in which introns may be removed and exons, retained. Variability in splicing patterns is a major source of protein diversity from the genome and function to generate a tremendously diverse proteome from a relatively small number of genes. Changes in splice site choice can determine different effects on the encoded protein. Small changes in peptide sequence can alter ligand binding, enzymatic activity, allosteric regulation, or protein localization. Errors in splicing regulation have been implicated in a number of different disease states. This study reviewed the mechanisms of splicing and their repercussion in endocrinology, emphasizing its importance in some thyroid physiological and pathological conditions.

Increased generation of fibrocytes in thyroid-associated ophthalmopathy

CONTEXT

The pathogenic basis for Graves' disease (GD) continues to elude our understanding. Specifically why activating antibodies are generated against self-antigens remains uncertain as does the identity of the antigen(s) that provokes orbital involvement in GD, a process known as thyroid-associated ophthalmopathy (TAO).

OBJECTIVE

The aim of the study was to determine whether CD34(+) fibrocytes are generated more frequently in GD, whether they infiltrate orbital connective tissues in TAO, and whether they express the thyrotropin receptor (TSHR).

DESIGN/SETTING/PARTICIPANTS

Generation of fibrocytes from peripheral blood mononuclear cells was examined in samples from 70 patients with GD and 25 healthy control subjects. Fibrocytes were characterized by flow cytometry. Orbital tissues and fibroblast culture strains were examined for their presence.

MAIN OUTCOME MEASURES

The frequency of CD34(+) fibrocyte generation from peripheral blood cells, characterization of their phenotype, cytokine production, and their presence in affected orbital tissues were analyzed.

RESULTS

CD34(+)CXCR4(+)Col I(+) fibrocytes expressing IGF-I receptor are far more frequently generated from cultured peripheral blood mononuclear cells of donors with GD compared with healthy subjects. They express TSHR at high levels and TSH induces fibrocytes to produce IL-6 and TNF-alpha. Numerous CD34(+) fibrocytes were detected in orbital tissues in TAO but were absent in healthy orbits. Tissue-infiltrating fibrocytes express TSHR in situ and comprise a subpopulation of TAO-derived orbital fibroblasts.

CONCLUSIONS

Our findings suggest that fibrocytes may participate in the pathogenesis of TAO because they express relevant autoantigens such as IGF-I receptor and functional TSHR and differentially accumulate in orbital tissue in TAO.

A novel thyroid stimulating hormone beta-subunit isoform in human pituitary, peripheral blood leukocytes, and thyroid

Thyroid stimulating hormone (TSH) is produced by the anterior pituitary and is used to regulate thyroid hormone output, which in turn controls metabolic activity. Currently, the pituitary is believed to be the only source of TSH used by the thyroid. Recent studies in mice from our laboratory have identified a TSHbeta isoform that is expressed in the pituitary, in peripheral blood leukocytes (PBL), and in the thyroid. To determine whether a human TSHbeta splice variant exists that is analogous to the mouse TSHbeta splice variant, and whether the pattern of expression of the splice variant is similar to that observed in mice, PCR amplification of RNAs from pituitary, thyroid, PBL, and bone marrow was done by reverse-transcriptase PCR and quantitative realtime PCR. Human pituitary expressed a TSHbeta isoform that is analogous to the mouse TSHbeta splice variant, consisting of a 27 nucleotide portion of intron 2 and all of exon 3, coding for 71.2% of the native human TSHbeta polypeptide. Of particular interest, the TSHbeta splice variant was expressed at significantly higher levels than the native form or TSHbeta in PBL and the thyroid. The TSHalpha gene also was expressed in the pituitary, thyroid, and PBL, but not the BM, suggesting that the TSHbeta polypeptide in the thyroid and PBL may exist as a dimer with TSHalpha. These findings identify an unknown splice variant of human TSHbeta. They also have implications for immune-endocrine interactions in the thyroid and for understanding autoimmune thyroid disease from a new perspective.