The endocrine and nervous systems work in a highly integrated fashion to regulate virtually all functions in the human body (e.g. body temperature, myocardial contractility, glucose and lipid metabolism, bone mineralization and reproduction). Pituitary hormones, secreted by the anterior pituitary in response to hypothalamic release factors, provide the link between the central nervous system and the endocrine system. They regulate the function of the thyroid (TSH), gonads (LH and FSH), adrenal (ACTH) and mammary (prolactin) glands as well as growth and metabolism (GH). With a couple of notable exceptions, hypothalamic release factors and pituitary hormones signal through the activation of specific G-protein-coupled receptors (GPCR) located on the surface of target cells. Given their key relevance in maintaining body and cell homeostasis, alterations of these hormones and their receptors are implicated in several human diseases. Moreover, they constitute relevant pharmacological targets.
In our group, we are using a combination of biochemical and advanced optical methods, such as fluorescence resonance energy transfer (FRET) and total internal reflection fluorescence (TIRF) microscopy, to study the spatio-temporal dynamics of GPCR signaling directly in living endocrine cells. By using such a strategy, we are addressing fundamental and still controversial issues regarding GPCR signaling in general and, more specifically, the mechanisms of action of pituitary hormones and their possible pathophysiological implications. These include: the mechanisms of GPCR activation, the extent and relevance of GPCR dimerization, the role of signal compartmentalization and the functional consequences of GPCR internalization. In order to study GPCR signaling under physiologically relevant conditions, we employ primary cells as well as three-dimensional tissue models such as acute pituitary slices or intact thyroid and ovarian follicles.
Utilizing this approach, we were recently able to show that the TSH receptor, and possibly other GPCRs, continues to signal to cAMP after internalization into endosomes, leading to persistent cAMP accumulation and apparently to specific signaling outcomes (Calebiro et al., PLoS Biology 2009). These data suggest that endosomes could constitute specialized intracellular platforms for GPCR signaling, thus implying novel and important functions for receptor internalization. Efforts are currently underway to further elucidate the physiological relevance and possible pathophysiological implications of GPCR signaling in the endosomal compartment.