Abstract
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.