Structural and functional properties of rhodopsin from rod outer segment disk and plasma membrane

New views on phototransduction from atomic force microscopy and single molecule force spectroscopy on native rods

By combining atomic force microscopy (AFM) imaging and single-molecule force spectroscopy (SMFS), we analyzed membrane proteins of the rod outer segments (OS). With this combined approach we were able to study the membrane proteins in their natural environment. In the plasma membrane we identified native cyclic nucleotide-gated (CNG) channels which are organized in single file strings. We also identified rhodopsin located both in the discs and in the plasma membrane. SMFS reveals strikingly different mechanical properties of rhodopsin unfolding in the two environments. Molecular dynamic simulations suggest that this difference is likely to be related to the higher hydrophobicity of the plasma membrane, due to the higher cholesterol concentration. This increases rhodopsin mechanical stability lowering the rate of transition towards its active form, hindering, in this manner, phototransduction.

Cholesterol in the rod outer segment: A complex role in a "simple" system

The rod outer segment (ROS) of retinal photoreceptor cells consists of disk membranes surrounded by the plasma membrane. It is a relatively uncomplicated system in which to investigate cholesterol distribution and its functional consequences in biologically relevant membranes. The light sensitive protein, rhodopsin is the major protein in both membranes, but the lipid compositions are significantly different in the disk and plasma membranes. Cholesterol is high in the ROS plasma membrane. Disk membranes are synthesized at the base of the ROS and are also high in cholesterol. However, cholesterol is rapidly depleted as the disks are apically displaced. During this apical displacement the disk phospholipid fatty acyl chains become progressively more unsaturated, which creates an environment unfavorable to cholesterol. Membrane cholesterol has functional consequences. The high cholesterol found in the plasma membrane and in newly synthesized disks inhibits the activation of rhodopsin. As disks are apically displaced and cholesterol is depleted rhodopsin becomes more responsive to light. This effect of cholesterol on rhodopsin activation has been shown in both native and reconstituted membranes. The modulation of activity can be at least partially explained by the effect of cholesterol on bulk lipid properties. Cholesterol decreases the partial free volume of the hydrocarbon region of the bilayer and thereby inhibits rhodopsin conformational changes required for activation. However, cholesterol binds to rhodopsin and may directly affect the protein also. Furthermore, cholesterol stabilizes rhodopsin to thermal denaturation. The membrane must provide an environment that allows rhodopsin conformational changes required for activation while also stabilizing the protein to thermal denaturation. Cholesterol thus plays a complex role in modulating the activity and stability of rhodopsin, which have implications for other G-protein coupled receptors.

An inducible expression system to measure rhodopsin transport in transgenic Xenopus rod outer segments

We developed an inducible transgene expression system in Xenopus rod photoreceptors. Using a transgene containing mCherry fused to the carboxyl terminus of rhodopsin (Rho-mCherry), we characterized the displacement of rhodopsin (Rho) from the base to the tip of rod outer segment (OS) membranes. Quantitative confocal imaging of live rods showed very tight regulation of Rho-mCherry expression, with undetectable expression in the absence of dexamethasone (Dex) and an average of 16.5 µM of Rho-mCherry peak concentration after induction for several days (equivalent to >150-fold increase). Using repetitive inductions, we found the axial rate of disk displacement to be 1.0 µm/day for tadpoles at 20 °C in a 12 h dark /12 h light lighting cycle. The average distance to peak following Dex addition was 3.2 µm, which is equivalent to ~3 days. Rods treated for longer times showed more variable expression patterns, with most showing a reduction in Rho-mCherry concentration after 3 days. Using a simple model, we find that stochastic variation in transgene expression can account for the shape of the induction response.

The role of cholesterol in rod outer segment membranes

The photoreceptor rod outer segment (ROS) provides a unique system in which to investigate the role of cholesterol, an essential membrane constituent of most animal cells. The ROS is responsible for the initial events of vision at low light levels. It consists of a stack of disk membranes surrounded by the plasma membrane. Light capture occurs in the outer segment disk membranes that contain the photopigment, rhodopsin. These membranes originate from evaginations of the plasma membrane at the base of the outer segment. The new disks separate from the plasma membrane and progressively move up the length of the ROS over the course of several days. Thus the role of cholesterol can be evaluated in two distinct membranes. Furthermore, because the disk membranes vary in age it can also be investigated in a membrane as a function of the membrane age. The plasma membrane is enriched in cholesterol and in saturated fatty acids species relative to the disk membrane. The newly formed disk membranes have 6-fold more cholesterol than disks at the apical tip of the ROS. The partitioning of cholesterol out of disk membranes as they age and are apically displaced is consistent with the high PE content of disk membranes relative to the plasma membrane. The cholesterol composition of membranes has profound consequences on the major protein, rhodopsin. Biophysical studies in both model membranes and in native membranes have demonstrated that cholesterol can modulate the activity of rhodopsin by altering the membrane hydrocarbon environment. These studies suggest that mature disk membranes initiate the visual signal cascade more effectively than the newly synthesized, high cholesterol basal disks. Although rhodopsin is also the major protein of the plasma membrane, the high membrane cholesterol content inhibits rhodopsin participation in the visual transduction cascade. In addition to its effect on the hydrocarbon region, cholesterol may interact directly with rhodopsin. While high cholesterol inhibits rhodopsin activation, it also stabilizes the protein to denaturation. Therefore the disk membrane must perform a balancing act providing sufficient cholesterol to confer stability but without making the membrane too restrictive to receptor activation. Within a given disk membrane, it is likely that cholesterol exhibits an asymmetric distribution between the inner and outer bilayer leaflets. Furthermore, there is some evidence of cholesterol microdomains in the disk membranes. The availability of the disk protein, rom-1 may be sensitive to membrane cholesterol. The effects exerted by cholesterol on rhodopsin function have far-reaching implications for the study of G-protein coupled receptors as a whole. These studies show that the function of a membrane receptor can be modulated by modification of the lipid bilayer, particularly cholesterol. This provides a powerful means of fine-tuning the activity of a membrane protein without resorting to turnover of the protein or protein modification.

Photoreceptor renewal: a role for peripherin/rds

Visual transduction begins with the detection of light within the photoreceptor cell layer of the retina. Within this layer, specialized cells, termed rods and cones, contain the proteins responsible for light capture and its transduction to nerve impulses. The phototransductive proteins reside within an outer segment region that is connected to an inner segment by a thin stalk rich in cytoskeletal elements. A unique property of the outer segments is the presence of an elaborate intracellular membrane system that holds the phototransduction proteins and provides the requisite lipid environment. The maintenance of normal physiological function requires that these postmitotic cells retain the unique structure of the outer segment regions--stacks of membrane saccules in the case of rods and a continuous infolding of membrane in the case of cones. Both photoreceptor rod and cone cells achieve this through a series of coordinated steps. As new membranous material is synthesized, transported, and incorporated into newly forming outer segment membranes, a compensatory shedding of older membranous material occurs, thereby maintaining the segment at a constant length. These processes are collectively referred to as ROS (rod outer segment) or COS (cone outer segment) renewal. We review the cellular and molecular events responsible for these renewal processes and present the recent but compelling evidence, drawn from molecular genetic, biochemical, and biophysical approaches, pointing to an essential role for a unique tetraspanning membrane protein, called peripherin/rds, in the processes of disk morphogenesis.

Complex intermolecular interactions maintain a stable linkage between the photoreceptor connecting cilium axoneme and plasma membrane

Microtubule-membrane cross-linkers in motile and nonmotile cilia are supramolecular structures, held together by strong interactions between the constituent molecules. We have characterized these interactions in the photoreceptor connecting cilium, where cross-linkers co-fractionate and maintain their in situ location after Triton X-100 extraction of axonemes. In bovine photoreceptor cells, the transmembrane assemblage that is cross-linked to the connecting cilium axoneme contains three high molecular mass glycoconjugates of 425, 600, and 700 kDa (Horst et al., 1987). The relative amounts of the three glycoconjugates, as judged from band intensity in electrophoretograms, depend strongly on sample treatment prior to electrophoresis. The electrophoretic pattern was reproducible after several weeks of storage of the axoneme fraction in extraction buffer containing 50% sucrose. Removal of sucrose from the buffer by dialysis eliminated the 600 kDa and 700 kDa, and decreased the detected amount of the 425 kDa glycoconjugate. When samples were incubated in Laemmli sample buffer at increasing temperatures (23 degrees, 60 degrees, 95 degrees C), a gradual reduction in the intensity of the three bands was observed. The quantitative reduction of high molecular mass glycoconjugates was accompanied by the appearance of novel protein species of lower molecular mass, as detected by lectin and antibody overlays of axonemal transblots. These results suggest that the previously characterized cross-linker glycoconjugates are complex, SDS-resistant multi-molecular conglomerates. We have further used fluorescent lectins to monitor the presence of glycoconjugates on whole-mounted axonemes, in conditions aimed to selectively solubilize the cross-linkers. The cross-linker complexes could not be dissociated from the axoneme by incubation with buffers containing 1 M of either Na2SO4 or NaI. The results indicate that the connecting cilium-specific cross-linker complexes are bound via high-affinity interactions to both axoneme and overlying plasma membrane.