TSH Pulses Finely Tune Thyroid Hormone Release and TSH Receptor Transduction

Detection of circulating TSH is a first-line test of thyroid dysfunction, a major health problem (affecting about 5% of the population) that, if untreated, can lead to a significant deterioration of quality of life and adverse effects on multiple organ systems. Human TSH levels display both pulsatile and (nonpulsatile) basal TSH secretion patterns; however, the importance of these in regulating thyroid function and their decoding by the thyroid is unknown. Here, we developed a novel ultra-sensitive ELISA that allows precise detection of TSH secretion patterns with minute resolution in mouse models of health and disease. We characterized the patterns of ultradian TSH pulses in healthy, freely behaving mice over the day-night cycle. Challenge of the thyroid axis with primary hypothyroidism because of iodine deficiency, a major cause of thyroid dysfunction worldwide, results in alterations of TSH pulsatility. Induction in mouse models of sequential TSH pulses that mimic ultradian TSH profiles in periods of minutes were more efficient than sustained rises in basal TSH levels at increasing both thyroid follicle cAMP levels, as monitored with a genetically encoded cAMP sensor, and circulating thyroid hormone. Hence, this mouse TSH assay provides a powerful tool to decipher how ultradian TSH pulses encode thyroid outcomes and to uncover hidden parameters in the TSH-thyroid hormone set-point in health and disease.

Metabolic Stress Impairs Pericyte Response to Optogenetic Stimulation in Pancreatic Islets

Pancreatic islets are highly vascularized micro-organs ensuring whole body glucose homeostasis. Islet vascular cells play an integral part in sustaining adequate insulin release by beta cells. In particular, recent studies have demonstrated that islet pericytes regulate local blood flow velocity and are required for maintenance of beta cell maturity and function. In addition, increased metabolic demand accompanying obesity alters islet pericyte morphology. Here, we sought to explore the effects of metabolic stress on islet pericyte functional response to stimulation in a mouse model of type 2 diabetes, directly in the pancreas in vivo . We found that high fat diet induced islet pericyte hypertrophy without alterations in basal local blood flow. However, optogenetic stimulation of pericyte activity revealed impaired islet vascular responses, despite increased expression of genes encoding proteins directly or indirectly involved in cell contraction. These findings suggest that metabolic stress impinges upon islet pericyte function, which may contribute to beta cell failure during T2D.

Pituitary stem cells produce paracrine WNT signals to control the expansion of their descendant progenitor cells

In response to physiological demand, the pituitary gland generates new hormone-secreting cells from committed progenitor cells throughout life. It remains unclear to what extent pituitary stem cells (PSCs), which uniquely express SOX2, contribute to pituitary growth and renewal. Moreover, neither the signals that drive proliferation nor their sources have been elucidated. We have used genetic approaches in the mouse, showing that the WNT pathway is essential for proliferation of all lineages in the gland. We reveal that SOX2⁺ stem cells are a key source of WNT ligands. By blocking secretion of WNTs from SOX2⁺ PSCs in vivo, we demonstrate that proliferation of neighbouring committed progenitor cells declines, demonstrating that progenitor multiplication depends on the paracrine WNT secretion from SOX2⁺ PSCs. Our results indicate that stem cells can hold additional roles in tissue expansion and homeostasis, acting as paracrine signalling centres to coordinate the proliferation of neighbouring cells.

Maternal hypothyroidism in mice influences glucose metabolism in adult offspring

AIMS/HYPOTHESIS

During pregnancy, maternal metabolic disease and hormonal imbalance may alter fetal beta cell development and/or proliferation, thus leading to an increased risk for developing type 2 diabetes in adulthood. Although thyroid hormones play an important role in fetal endocrine pancreas development, the impact of maternal hypothyroidism on glucose homeostasis in adult offspring remains poorly understood.

METHODS

We investigated this using a mouse model of hypothyroidism, induced by administration of an iodine-deficient diet supplemented with propylthiouracil during gestation.

RESULTS

Here, we show that, when fed normal chow, adult mice born to hypothyroid mothers were more glucose-tolerant due to beta cell hyperproliferation (two- to threefold increase in Ki67-positive beta cells) and increased insulin sensitivity. However, following 8 weeks of high-fat feeding, these offspring gained 20% more body weight, became profoundly hyperinsulinaemic (with a 50% increase in fasting insulin concentration), insulin-resistant and glucose-intolerant compared with controls from euthyroid mothers. Furthermore, altered glucose metabolism was maintained in a second generation of animals.

CONCLUSIONS/INTERPRETATION

Therefore, gestational hypothyroidism induces long-term alterations in endocrine pancreas function, which may have implications for type 2 diabetes prevention in affected individuals.

Imaging and Manipulating Pituitary Function in the Awake Mouse

Extensive efforts have been made to explore how the activities of multiple brain cells combine to alter physiology through imaging and cell-specific manipulation in different animal models. However, the temporal regulation of peripheral organs by the neuroendocrine factors released by the brain is poorly understood. We have established a suite of adaptable methodologies to interrogate in vivo the relationship of hypothalamic regulation with the secretory output of the pituitary gland, which has complex functional networks of multiple cell types intermingled with the vasculature. These allow imaging and optogenetic manipulation of cell activities in the pituitary gland in awake mouse models, in which both neuronal regulatory activity and hormonal output are preserved. These methodologies are now readily applicable for longitudinal studies of short-lived events (e.g., calcium signals controlling hormone exocytosis) and slowly evolving processes such as tissue remodeling in health and disease over a period of days to weeks.

SAT-547 Pituitary Thyrotrophs: A Generator of Repetitive Calcium Waves in Freely-Moving Animal Models

Hypothyroidism refers to the pathological state of thyroid hormone (TH) deficiency. It affects between 0.3% and 3.7% of the general population in the USA and between 0.2% and 5.3% in Europe. Hypothyroid patients suffer from extreme fatigue, altered heart rate, depression, weight gain, and difficulty in conceiving. Also, maternal hypothyroidism, which affects 0.5% of all pregnant women, is the leading preventable cause of mental retardation in offspring. Importantly, diagnosis of hypothyroidism is based on a single thyroid-stimulating-hormone (TSH) measurement. TSH secretion by pituitary thyrotrophs is under the positive control of hypothalamic thyrotropin-releasing-hormone (TRH) and negative feedback exerted by THs. Despite the undeniable implication of thyrotrophs in the hypothalamus-pituitary-thyroid (HPT) axis regulation, little is known about the mechanisms underlying adaptive TSH secretion. Being the smallest endocrine cell population of the pituitary (2-4%), thyrotrophs display high plasticity in cell shape, number and network topology throughout development and in disease. Moreover, enzymatically-dispersed thyrotrophs lose their ability to express and secrete TSH in both basal and TRH-stimulated conditions. Altogether, these findings led us to the hypothesis that thyrotrophs form a very finely regulated and highly plastic functionally-organized cell population capable of adapting to physiological (i.e. pregnancy) and pathological (i.e. hypothyroidism) demand. The aim of this study was to investigate how thyrotrophs function in their native environment. Using gradient-index (GRIN) lenses implanted at the pituitary level and a 2g head-mounted miniscope, multicellular calcium activities of the thyrotroph population were monitored in freely-moving TSHβ-crexR26fl-flGCaMP6f mice. Strikingly, thyrotrophs displayed both short-lived calcium spikes of high amplitude, and slowly-evolving calcium waves propagating among the thyrotroph population (n=3). Such calcium waves recurred every 80min during episodes lasting 3min and were interspaced by short-lived activity/silent periods. As TSH displays a 2.722min half-life (n=6), repetitive bouts of calcium-dependent TSH exocytosis would lead to cumulative increase in circulating TSH levels. Hence, the thyrotroph population functions in vivo as a robust generator of repetitive calcium waves which would orchestrate the generation of ultradian TSH fluctuations. Such signaling events could be monitored during longitudinal studies (weeks to months) in which individual animals could be their own controls.