Rhodopsin is a prototypical G-protein coupled receptor (GPCR) that initiates phototransduction in the retina. The receptor consists of the apoprotein opsin covalently linked to the inverse agonist 11-cis retinal. Rhodopsin and opsin have been shown to form oligomers within the outer segment disc membranes of rod photoreceptor cells. However, the physiological relevance of the observed oligomers has been questioned since observations were made on samples prepared from the retina at low temperatures. To investigate the oligomeric status of opsin in live cells at body temperatures, we utilized a novel approach called FRET spectrometry, which previously has allowed the determination of the stoichiometry and geometry (i.e., quaternary structure) of various GPCRs. In the current study, we have extended the method to additionally determine whether or not a mixture of oligomeric forms of opsin exists and in what proportion. Application of this improved method revealed that opsin expressed in live Chinese hamster ovary (CHO) cells at 37 °C exists as oligomers of various sizes. At lower concentrations, opsin existed in an equilibrium of dimers and tetramers. The tetramers were in the shape of a near-rhombus. At higher concentrations of the receptor, higher order oligomers began to form. Thus, a mixture of different oligomeric forms of opsin is present in the membrane of live CHO cells and oligomerization occurs in a concentration-dependent manner. The general principles underlying the concentration-dependent oligomerization of opsin may be universal and apply to other GPCRs as well.

Mutations in rhodopsin can cause misfolding and aggregation of the receptor, which leads to retinitis pigmentosa, a progressive retinal degenerative disease. The structure adopted by misfolded opsin mutants and the associated cell toxicity is poorly understood. Förster resonance energy transfer (FRET) and Fourier transform infrared (FTIR) microspectroscopy were utilized to probe within cells the structures formed by G188R and P23H opsins, which are misfolding mutants that cause autosomal dominant retinitis pigmentosa. Both mutants formed aggregates in the endoplasmic reticulum and exhibited altered secondary structure with elevated β-sheet and reduced α-helical content. The newly formed β-sheet structure may facilitate the aggregation of misfolded opsin mutants. The effects observed for the mutants were unrelated to retention of opsin molecules in the endoplasmic reticulum itself.

Docosahexaenoic acid (DHA) is enriched in photoreceptor cell membranes. DHA deficiency impairs vision due to photoreceptor cell dysfunction, which is caused, at least in part, by reduced activity of rhodopsin, the light receptor that initiates phototransduction. It is unclear how the depletion of membrane DHA impacts the structural properties of rhodopsin and, in turn, its activity. Atomic force microscopy (AFM) was used to assess the impact of DHA deficiency on membrane structure and rhodopsin organization. AFM revealed that signaling impairment in photoreceptor cells is independent of the oligomeric status of rhodopsin and causes adaptations in photoreceptor cells where the content and density of rhodopsin in the membrane is increased. Functional and structural changes caused by DHA deficiency were reversible.

The largest class of rhodopsin mutations causing autosomal dominant retinitis pigmentosa (adRP) is mutations that lead to misfolding and aggregation of the receptor. The misfolding mutants have been characterized biochemically, and categorized as either partial or complete misfolding mutants. This classification is incomplete and does not provide sufficient information to fully understand the disease pathogenesis and evaluate therapeutic strategies. A Förster resonance energy transfer (FRET) method was utilized to directly assess the aggregation properties of misfolding rhodopsin mutants within the cell. Partial (P23H and P267L) and complete (G188R, H211P, and P267R) misfolding mutants were characterized to reveal variability in aggregation properties. The complete misfolding mutants all behaved similarly, forming aggregates when expressed alone, minimally interacting with the wild-type receptor when coexpressed, and were unresponsive to treatment with the pharmacological chaperone 9-cis retinal. In contrast, variability was observed between the partial misfolding mutants. In the opsin form, the P23H mutant behaved similarly as the complete misfolding mutants. In contrast, the opsin form of the P267L mutant existed as both aggregates and oligomers when expressed alone and formed mostly oligomers with the wild-type receptor when coexpressed. The partial misfolding mutants both reacted similarly to the pharmacological chaperone 9-cis retinal, displaying improved folding and oligomerization when expressed alone but aggregating with wild-type receptor when coexpressed. The observed differences in aggregation properties and effect of 9-cis retinal predict different outcomes in disease pathophysiology and suggest that retinoid-based chaperones will be ineffective or even detrimental.

Rhodopsin is the light receptor in photoreceptor cells that plays a central role in phototransduction and photoreceptor cell health. Mutations in rhodopsin are the leading cause of autosomal dominant retinitis pigmentosa (adRP), a retinal degenerative disease. A majority of mutations in rhodopsin cause misfolding and aggregation of the apoprotein opsin. The pathogenesis of adRP caused by misfolded opsin is unclear. It has been proposed that physical interactions between wild-type opsin and misfolded opsin mutants may underlie the autosomal dominant phenotype. To test whether or not wild-type opsin can form a complex with misfolded opsin mutants, we examined the interactions between wild-type opsin and opsin with a G188R mutation, a clinically identified mutation causing adRP. Förster resonance energy transfer (FRET) was utilized to monitor the interactions between fluorescently tagged opsins expressed in live cells. The FRET assay employed was able to discriminate between properly folded opsin oligomers and misfolded opsin aggregates. Wild-type opsin predominantly formed oligomers and only a minor population formed aggregates. Conversely, the G188R opsin mutant predominantly formed aggregates. When wild-type opsin and G188R opsin were coexpressed in cells, properly folded wild-type opsin did not aggregate with G188R opsin and was trafficked normally to the plasma membrane. Thus, the autosomal dominant phenotype in adRP caused by misfolded opsin mutants is not predicted to arise from physical interactions between wild-type opsin and misfolded opsin mutants.

Phosphorylation of Chk1 by ataxia telangiectasia-mutated and Rad3-related (ATR) is critical for checkpoint activation upon DNA damage. However, how phosphorylation activates Chk1 remains unclear. Many studies suggest a conformational change model of Chk1 activation in which phosphorylation shifts Chk1 from a closed inactive conformation to an open active conformation during the DNA damage response. However, no structural study has been reported to support this Chk1 activation model. Here we used FRET and bimolecular fluorescence complementary techniques to show that Chk1 indeed maintains a closed conformation in the absence of DNA damage through an intramolecular interaction between a region (residues 31-87) at the N-terminal kinase domain and the distal C terminus. A highly conserved Leu-449 at the C terminus is important for this intramolecular interaction. We further showed that abolishing the intramolecular interaction by a Leu-449 to Arg mutation or inducing ATR-dependent Chk1 phosphorylation by DNA damage disrupts the closed conformation, leading to an open and activated conformation of Chk1. These data provide significant insight into the mechanisms of Chk1 activation during the DNA damage response.The genome integrity of eukaryotic cells is threatened by various sources of DNA damage arising in the environment (e.g. UV light) and/or within the cell (e.g. free radical species). Cells have evolved complex networks termed the DNA damage response (DDR) 4 to counter these assaults by inhibiting cell division and repairing damaged DNA (1, 2). Central to the DDR is the Ser/Thr checkpoint kinase Chk1, which plays a key role in responding to a wide range of DNA-damaging agents (3). Activation of Chk1 requires its phosphorylation at two conserved sites, Ser-317 and Ser-345, by the upstream kinase ataxia telangiectasia-mutated and Rad3-related (ATR) (4 -6). Activated Chk1 then phosphorylates a number of downstream targets to control the cell cycle transition and facilitate DNA damage repair (3). However, a key question remains: how does phosphorylation lead to Chk1 activation in cells?A crystal structure of the N-terminal kinase domain of human Chk1 revealed that the catalytic site adopts an active conformation without the need for phosphorylation of a Thr residue at the kinase domain as with cyclin-dependent kinases (7). However, the Chk1 protein displays only a low level of basal activity and does not trigger a checkpoint response under normal growth conditions (5, 6). This suggests that the open conformation of the catalytic site of Chk1 is inhibited in the absence of DNA damage. Studies from several laboratories showed that the C-terminal regulatory domain of Chk1 interacts with its N-terminal kinase domain in vitro or in vivo (8 -13), suggesting that Chk1 may form a "closed" conformation. This intramolecular interaction and the resulting closed conformation make it likely that the C terminus of Chk1 provides a physical hindrance to the open conformation of its catalytic site and restrains its activity under ...

Rhodopsin is the light receptor in rod photoreceptor cells of the retina that plays a central role in phototransduction and rod photoreceptor cell health. Rhodopsin mutations are the leading known cause of autosomal dominant retinitis pigmentosa, a retinal degenerative disease. A majority of rhodopsin mutations cause misfolding and aggregation of the apoprotein opsin. The nature of aggregates formed by misfolded rhodopsin mutants and the associated cell toxicity is poorly understood. Misfolding rhodopsin mutants have been characterized biochemically, and categorized as either partial or complete misfolding mutants. This classification is incomplete and does not provide sufficient information to fully understand rhodopsin aggregation, disease pathogenesis, and evaluate therapeutic strategies. To better understand the aggregation of misfolded rhodopsin mutants, a Förster resonance energy transfer assay has been developed to monitor the aggregation of fluorescently tagged mutant rhodopsins expressed in live cells.

Reveromycin A (RM-A), a small natural product isolated from Streptomyces bacteria, is a potential osteoporosis therapeutic in that it specifically induces apoptosis in osteoclasts but not osteoblasts. The purpose of the study presented here was to further elucidate the intracellular mechanisms of RM-A death effects in mature osteoclasts. A specific clone of RAW264.7 murine macrophages that was previously characterized for its ability to acquire an osteoclast nature on differentiation was differentiated in the presence of receptor activator of nuclear factor kappa B ligand (RANKL). Subsequent staining was performed for tartrate-resistant acid phosphatase to confirm their osteoclast character. These osteoclasts were treated with ten micromolar RM-A for 2, 4, 6, 24, and 48 h at a pH of 5.5. Peak apoptosis induction occurred at 4-6 h as measured by caspase 3 activity. Lactate dehydrogenase release assay revealed no significant RM-A-induced necrosis. Western blot analysis of cytoplasmic extracts demonstrated activation of caspase 9 (2.3-fold at 2 h and 2.6-fold at 4 h, each P < 0.05) and no significant changes in Bcl-XL . In nuclear extracts, NFκB levels significantly increased on differentiation with RANKL but then remained constant through RM-A treatment. Over the extended time course studied, RM-A-induced apoptosis in osteoclasts was not accompanied by necrosis, suggesting that RM-A would likely have limited effects on immediate, neighboring bone cell types. This specific cell death profile is promising for potential clinical investigations of RM-A as a bone antiresorptive.