cGMP Signaling in Brown Adipocytes

Abstract

Obesity is characterized by an imbalance between energy intake and expenditure, resulting in an abnormal accumulation of white adipose tissue (WAT), the primary form of energy storage in humans. Brown adipose tissue (BAT) possesses the unique ability to dissipate energy in the form of heat, making it a promising target for the development of novel obesity treatments. <br /> Earlier studies have shown that the cyclic nucleotide cyclic guanosine monophosphate (cGMP) plays a pivotal role in BAT development and function. While previous research has focused on the role and compartmentalization of cyclic adenosine monophosphate (cAMP), the subcellular architecture of cGMP compartmentalization remains poorly understood and unexplored in BA. Thus, the primary objective of this thesis was to provide a detailed characterization of cGMP subcellular compartmentalization in murine and human BA, as well as their progenitors. To achieve this, a range of live-cell imaging techniques, such as Förster-resonance energy transfer and intensiometric biosensors, were employed and developed. <br /> The experiments conducted in this study reveal that cGMP does not freely diffuse within BA. Instead, cGMP forms distinct subcellular pools that are regulated by different guanylate cyclases (GCs) and phosphodiesterases (PDEs). Moreover, the regulation of these pools by GCs and PDEs changes during the differentiation of BA progenitors into fully mature BA. Additional experiments revealed a pivotal role of PDE1 in specifically regulating natriuretic peptide-mediated cGMP in murine and human BA, while not interacting with nitric oxide induced cGMP. Moreover, PDE1 conveyed a cGMP-pool specific crosstalk between calcium and natriuretic peptide associated cGMP, while showing no interactions with nitric oxide associated cGMP. This observation underscores the potential druggability of individual subcellular cGMP pools by inhibiting specific associated PDEs. The differences between human and murine BA are well documented. However, the subcellular distinctions regarding cGMP compartmentalization remain unclear. The data presented here reveal significant variations in the subcellular architecture of cGMP compartmentalization in human versus murine BA. Nevertheless, cGMP compartmentalization, as well as the specific interaction of calcium with PDE1 and natriuretic peptide-derived cGMP, are consistent in both human and murine BA. <br /> Lastly, the targeted sensors developed during this project revealed another dimension of cGMP compartmentalization in human BA progenitors. The data demonstrate that while nitric oxide does not induce detectable cGMP level increases in the cytosol it instead enhances cGMP levels specifically at the nucleus. Thus, cGMP appears to be compartmentalized into different cytosolic pools but also different organelle-associated pools.