Rescue of mutant rhodopsin traffic by metformin-induced AMPK activation accelerates photoreceptor degeneration.

Protein misfolding caused by inherited mutations leads to loss of protein function and potentially toxic 'gain of function', such as the dominant P23H rhodopsin mutation that causes retinitis pigmentosa (RP). Here, we tested whether the AMPK activator metformin could affect the P23H rhodopsin synthesis and folding. In cell models, metformin treatment improved P23H rhodopsin folding and traffic. In animal models of P23H RP, metformin treatment successfully enhanced P23H traffic to the rod outer segment, but this led to reduced photoreceptor function and increased photoreceptor cell death. The metformin-rescued P23H rhodopsin was still intrinsically unstable and led to increased structural instability of the rod outer segments. These data suggest that improving the traffic of misfolding rhodopsin mutants is unlikely to be a practical therapy, because of their intrinsic instability and long half-life in the outer segment, but also highlights the potential of altering translation through AMPK to improve protein function in other protein misfolding diseases.

Retinitis pigmentosa mutants provide insight into the role of the N-terminal cap in rhodopsin folding, structure, and function.

Autosomal dominant retinitis pigmentosa (ADRP) mutants (T4K, N15S, T17M, V20G, P23A/H/L, and Q28H) in the N-terminal cap of rhodopsin misfold when expressed in mammalian cells. To gain insight into the causes of misfolding and to define the contributions of specific residues to receptor stability and function, we evaluated the responses of these mutants to 11-cis-retinal pharmacological chaperone rescue or disulfide bond-mediated repair. Pharmacological rescue restored folding in all mutants, but the purified mutant pigments in all cases were thermo-unstable and exhibited abnormal photobleaching, metarhodopsin II decay, and G protein activation. As a complementary approach, we superimposed this panel of ADRP mutants onto a rhodopsin background containing a juxtaposed cysteine pair (N2C/D282C) that forms a disulfide bond. This approach restored folding in T4K, N15S, V20G, P23A, and Q28H but not T17M, P23H, or P23L. ADRP mutant pigments obtained by disulfide bond repair exhibited enhanced stability, and some also displayed markedly improved photobleaching and signal transduction properties. Our major conclusion is that the N-terminal cap stabilizes opsin during biosynthesis and contributes to the dark-state stability of rhodopsin. Comparison of these two restorative approaches revealed that the correct position of the cap relative to the extracellular loops is also required for optimal photochemistry and efficient G protein activation.

Highly conserved tyrosine stabilizes the active state of rhodopsin.

Light-induced isomerization of the 11-cis-retinal chromophore in the visual pigment rhodopsin triggers displacement of the second extracellular loop (EL2) and motion of transmembrane helices H5, H6, and H7 leading to the active intermediate metarhodopsin II (Meta II). We describe solid-state NMR measurements of rhodopsin and Meta II that target the molecular contacts in the region of the ionic lock involving these three helices. We show that a contact between Arg135(3.50) and Met257(6.40) forms in Meta II, consistent with the outward rotation of H6 and breaking of the dark-state Glu134(3.49)-Arg135(3.50)-Glu247(6.30) ionic lock. We also show that Tyr223(5.58) and Tyr306(7.53) form molecular contacts with Met257(6.40). Together these results reveal that the crystal structure of opsin in the region of the ionic lock reflects the active state of the receptor. We further demonstrate that Tyr223(5.58) and Ala132(3.47) in Meta II stabilize helix H5 in an active orientation. Mutation of Tyr223(5.58) to phenylalanine or mutation of Ala132(3.47) to leucine decreases the lifetime of the Meta II intermediate. Furthermore, the Y223F mutation is coupled to structural changes in EL2. In contrast, mutation of Tyr306(7.53) to phenylalanine shows only a moderate influence on the Meta II lifetime and is not coupled to EL2.

Free backbone carbonyls mediate rhodopsin activation.

Conserved prolines in the transmembrane helices of G-protein-coupled receptors (GPCRs) are often considered to function as hinges that divide the helix into two segments capable of independent motion. Depending on their potential to hydrogen-bond, the free C=O groups associated with these prolines can facilitate conformational flexibility, conformational switching or stabilization of the receptor structure. To address the role of conserved prolines in family A GPCRs through solid-state NMR spectroscopy, we focus on bovine rhodopsin, a GPCR in the visual receptor subfamily. The free backbone C=O groups on helices H5 and H7 stabilize the inactive rhodopsin structure through hydrogen-bonds to residues on adjacent helices. In response to light-induced isomerization of the retinal chromophore, hydrogen-bonding interactions involving these C=O groups are released, thus facilitating repacking of H5 and H7 onto the transmembrane core of the receptor. These results provide insights into the multiple structural and functional roles of prolines in membrane proteins.

Retinal orientation and interactions in rhodopsin reveal a two-stage trigger mechanism for activation.

The 11-cis retinal chromophore is tightly packed within the interior of the visual receptor rhodopsin and isomerizes to the all-trans configuration following absorption of light. The mechanism by which this isomerization event drives the outward rotation of transmembrane helix H6, a hallmark of activated G protein-coupled receptors, is not well established. To address this question, we use solid-state NMR and FTIR spectroscopy to define the orientation and interactions of the retinal chromophore in the active metarhodopsin II intermediate. Here we show that isomerization of the 11-cis retinal chromophore generates strong steric interactions between its ß-ionone ring and transmembrane helices H5 and H6, while deprotonation of its protonated Schiff's base triggers the rearrangement of the hydrogen-bonding network involving residues on H6 and within the second extracellular loop. We integrate these observations with previous structural and functional studies to propose a two-stage mechanism for rhodopsin activation.

Construction of stable mammalian cell lines for inducible expression of G protein-coupled receptors.

The large-scale expression of many membrane proteins, including the members of the G protein-coupled receptor superfamily, in a correctly folded and fully functional form remains a formidable challenge. In this chapter, we focus on the construction of stable mammalian cell lines to overcome this hurdle. First, we will outline the steps for establishing a tightly regulated gene expression system in human HEK293S cells. This system utilizes separate plasmids containing components of well-defined genetic control elements from the Escherichia coli tetracycline operon to control the powerful cytomegalovirus immediate early enhancer/promoter. Next, we describe the assembly of this expression system into HEK293S cells and a derivative cell line devoid of complex N-glycosylation. Finally, we describe methods for the growth of these cells lines in scalable suspension culture for the preparation of milligram amounts of recombinant protein.

Magic angle spinning nuclear magnetic resonance spectroscopy of G protein-coupled receptors.

G protein-coupled receptors (GPCRs) represent the largest family of membrane receptors and mediate a diversity of cellular processes. These receptors have a common seven-transmembrane helix structure, yet have evolved to respond to literally thousands of different ligands. In this chapter, we describe the use of magic angle spinning solid-state NMR spectroscopy for characterizing the structure and dynamics of GPCRs. Solid-state NMR spectroscopy is well suited for structural measurements in both detergent micelles and membrane bilayer environments. We first outline the methods for large-scale production of stable, functional receptors containing (13)C- and (15)N-labeled amino acids. The expression methods make use of eukaryotic HEK293S cell lines that produce correctly folded, fully functional receptors. We subsequently describe the basic methods used for magic angle spinning solid-state NMR measurements of chemical shifts and dipolar couplings, which reveal detailed information on GPCR structure and dynamics.