Development of a genetically encoded melanocortin sensor for high sensitivity intravital imaging

The central melanocortin system, composed of peptides derived from pro-opiomelanocortin (POMC) such as the melanocyte-stimulating hormones (-, {beta}-, {gamma}-MSH) and melanocortin 4 receptors (MC4R), along with the agouti-related protein (AgRP), plays a pivotal role in controlling energy balance. To elucidate the dynamic role of -MSH release in regulating appetite, specific, sensitive, and spatiotemporally resolved genetic sensors are required. The melanocortin 1 receptor (MC1R) scaffold was leveraged for its robust plasma membrane expression, high affinity for melanocortin peptides and low affinity for AgRP to design a -MSH selective sensor for in vivo use. This was achieved by integrating circularly permuted green fluorescent protein (cpGFP) into the receptor, which we named Fluorescence Amplified Receptor sensor for Melanocortin (FLAREMC). The FLAREMC sensor shows high potency and selectivity in heterologous and homologous expressing cells for -MSH and the synthetic melanocortin agonist MTII but not to the inverse agonist AgRP. The sensor exhibited impaired signaling, with reduced G protein activation, no {beta}-arrestin coupling, and failed to internalize upon agonist stimulation. In vitro, FLAREMC displayed high photostability and reversible photoactivation. These properties suggest that the FLAREMC is suitable for long-term activity recording in the brain without desensitizing or interfering with endogenous melanocortin receptor signaling. When expressed in the paraventricular nucleus (PVN) of the mouse hypothalamus, the primary site of anorexigenic -MSH signaling, FLAREMC demonstrated its effectiveness in detecting changes associated with melanocortin responses in vivo. FLAREMC enables the study of melanocortin system in cultured cells and intravitally. This first of its class highly sensitive melanocortin sensor will serve as a valuable tool to advance our understanding of the complex dynamics governing melanocortin-dependent appetite regulation and related processes in the brain.