To maintain homeostasis, hypothalamic neurons in the arcuate nucleus must dynamically sense and integrate a multitude of peripheral signals. Blood-borne molecules must therefore be able to circumvent the tightly sealed vasculature of the blood–brain barrier to rapidly access their target neurons. However, how information encoded by circulating appetite-modifying hormones is conveyed to central hypothalamic neurons remains largely unexplored. Using in vivo multiphoton microscopy together with fluorescently labeled ligands, we demonstrate that circulating ghrelin, a versatile regulator of energy expenditure and feeding behavior, rapidly binds neurons in the vicinity of fenestrated capillaries, and that the number of labeled cell bodies varies with feeding status. Thus, by virtue of its vascular connections, the hypothalamus is able to directly sense peripheral signals, modifying energy status accordingly.

Amphipols (APols) are short amphipathic polymers that can substitute for detergents to keep integral membrane proteins (MPs) water soluble. In this review, we discuss their structure and solution behavior; the way they associate with MPs; and the structure, dynamics, and solution properties of the resulting complexes. All MPs tested to date form water-soluble complexes with APols, and their biochemical stability is in general greatly improved compared with MPs in detergent solutions. The functionality and ligand-binding properties of APol-trapped MPs are reviewed, and the mechanisms by which APols stabilize MPs are discussed. Applications of APols include MP folding and cell-free synthesis, structural studies by NMR, electron microscopy and X-ray diffraction, APol-mediated immobilization of MPs onto solid supports, proteomics, delivery of MPs to preexisting membranes, and vaccine formulation.

The eight metabotropic glutamate receptors (mGluRs) are key modulators of synaptic transmission and are considered promising targets for the treatment of various brain disorders. Whereas glutamate acts at a large extracellular domain, allosteric modulators have been identified that bind to the seven transmembrane domain (7TM) of these dimeric G-protein-coupled receptors (GPCRs). We show here that the dimeric organization of mGluRs is required for the modulation of active and inactive states of the 7TM by agonists, but is not necessary for G-protein activation. Monomeric mGlu2, either as an isolated 7TM or in full-length, purified and reconstituted into nanodiscs, couples to G proteins upon direct activation by a positive allosteric modulator. However, only a reconstituted full-length dimeric mGlu2 activates G protein upon glutamate binding, suggesting that dimerization is required for glutamate induced activation. These data show that, even for such well characterized GPCR dimers like mGluR2, a single 7TM is sufficient for G-protein coupling. Despite this observation, the necessity of dimeric architecture for signaling induced by the endogenous ligand glutamate confirms that the central core of signaling complex is dimeric.

G protein-coupled receptors (GPCRs) are seven-transmembrane proteins that mediate most cellular responses to hormones and neurotransmitters, representing the largest group of therapeutic targets. Recent studies show that some GPCRs signal through both G protein and arrestin pathways in a ligand-specific manner. Ligands that direct signaling through a specific pathway are known as biased ligands. The arginine-vasopressin type 2 receptor (V2R), a prototypical peptide-activated GPCR, is an ideal model system to investigate the structural basis of biased signaling. Although the native hormone arginine-vasopressin leads to activation of both the stimulatory G protein (Gs) for the adenylyl cyclase and arrestin pathways, synthetic ligands exhibit highly biased signaling through either Gs alone or arrestin alone. We used purified V2R stabilized in neutral amphipols and developed fluorescence-based assays to investigate the structural basis of biased signaling for the V2R. Our studies demonstrate that the Gs-biased agonist stabilizes a conformation that is distinct from that stabilized by the arrestin-biased agonists. This study provides unique insights into the structural mechanisms of GPCR activation by biased ligands that may be relevant to the design of pathway-biased drugs.