Imaging of corticosteroid receptors in live cells

Adrenal corticosteroids (cortisol in humans or corticosterone in rodents) exert numerous effects on the central nervous system that regulate the stress response, mood, learning, and memory, and various neuroendocrine functions. The actions of corticosterone (CORT) in the brain are mediated via two receptor systems: the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR). It has been shown that the MR and GR are highly co-localized in the hippocampus. These receptors are mainly localized in the cytoplasm, and are translocated into the nucleus, after binding with hormones, to act as transcriptional factors. Thus, the subcellular dynamics of both receptors constitute one of the most important issues to be clarified. Given the differential actions of MR and GR in the central nervous system, it is of great consequence to elucidate how MR and GR are trafficked between the cytoplasm and nucleus and how their interactions are regulated by hormones and/or other molecules to exert their functional activities. In this chapter, we describe our recent studies of corticosteroid receptor dynamics in living cells, focusing on three points: (1) time-lapse imaging of green fluorescent protein (GFP)-labeled corticosteroid receptors; (2) intranuclear dynamics of GFP-labeled corticosteroid receptors using the technique of fluorescence recovery after photobleaching; and (3) the possibility of heterodimer formation using the fluorescence resonance energy transfer technique. We discuss various factors affecting the dynamics of these receptors. Furthermore, we present future directions in the in vivo molecular imaging of corticosteroid receptors at the whole brain level.