Fluorescence relaxation in 3D from diffraction-limited sources of PAGFP or sinks of EGFP created by multiphoton photoconversion.

The relaxation of fluorescence from diffraction-limited sources of photoactivatable green fluorescent protein (PAGFP) or sinks of photobleached enhanced GFP (EGFP) created by multiphoton photo-conversion was measured in solutions of varied viscosity (eta), and in live, spherical Chinese hamster ovary (CHO) cells. Fluorescence relaxation was monitored with the probing laser fixed, or rapidly scanning along a line bisected by the photoconversion site. Novel solutions to several problems that hamper the study of PAGFP diffusion after multiphoton photoconversion are presented. A theoretical model of 3D diffusion in a sphere from a source in the shape of the measured multiphoton point-spread function was applied to the fluorescence data to estimate the apparent diffusion coefficient, D(ap). The model incorporates two novel features that make it of broad utility. First, the model includes the no-flux boundary condition imposed by cell plasma membranes, allowing assessment of potential impact of this boundary on estimates of D(ap). Second, the model uses an inhomogeneous source term that, for the first time, allows analysis of diffusion from sources produced by multiphoton photoconversion pulses of varying duration. For diffusion in aqueous solution, indistinguishable linear relationships between D(ap) and eta(-1) were obtained for the two proteins: for PAGFP, D(aq)= 89 +/- 2.4 microm2 s(-1) (mean +/- 95% confidence interval), and for EGFP D(aq)= 91 +/- 1.8 microm2 s(-1). In CHO cells, the application of the model yielded D(ap)= 20 +/- 3 microm2 s(-1) (PAGFP) and 19 +/- 2 microm2 s(-1) (EGFP). Furthermore, the model quantitatively predicted the decline in baseline fluorescence that accompanied repeated photobleaching cycles in CHO cells expressing EGFP, supporting the hypothesis of fluorophore depletion as an alternative to the oft invoked 'bound fraction' explanation of the deviation of the terminal fluorescence recovery from its pre-bleach baseline level. Nonetheless for their identical diffusive properties, advantages of PAGFP over EGFP were found, including an intrinsically higher signal/noise ratio with 488-nm excitation, and the requirement for approximately 1/200th the cumulative light energy to produce data of comparable signal/noise.

Remineralization of demineralized albumin-calcium phosphate coatings.

Calcium phosphate and bovine serum albumin were coprecipitated (under physiological conditions of temperature and pH) upon the surfaces of titanium-alloy samples, which thereby became coated with a dense, proteinaceous mineral layer 30-50 microm in thickness. Dissolution of the inorganic phase by treatment with acidic saline yielded a self-supporting protein scaffold, 7-10 microm in thickness. Energy-dispersive X-ray analysis and Fourier-transform infrared spectroscopy confirmed the absence of inorganic components from the demineralized albumin scaffolds. When titanium-alloy samples bearing these demineralized protein scaffolds were immersed in a supersaturated solution of calcium phosphate (again at physiological temperature and pH), they remineralized. These redux albumin-calcium phosphate layers corresponded in thickness to the original coatings. When titanium-alloy discs bearing the demineralized protein scaffolds were implanted ectopically (subcutaneously) in mice, they, too, remineralized. No uniform mineral layer was deposited upon the surfaces of naked titanium-alloy implants. To the best of our knowledge, this is the first demonstration of remineralization within the interstices of a noncollagenous protein scaffold, either in vitro or in vivo.

Constitutive "light" adaptation in rods from G90D rhodopsin: a mechanism for human congenital nightblindness without rod cell loss.

A dominant form of human congenital nightblindness is caused by a gly90-->asp (G90D) mutation in rhodopsin. G90D has been shown to activate the phototransduction cascade in the absence of light in vitro. Such constitutive activity of G90D rhodopsin in vivo would desensitize rod photoreceptors and lead to nightblindness. In contrast, other rhodopsin mutations typically give rise to nightblindness by causing rod cell death. Thus, the proposed desensitization without rod degeneration would be a novel mechanism for this disorder. To explore this possibility, we induced mice to express G90D opsin in their rods and then examined rod function and morphology, after first crossing the transgenic animals with rhodopsin knock-out mice to obtain appropriate levels of opsin expression. The G90D mouse opsin bound the chromophore and formed a bleachable visual pigment with lambda(max) of 492 nm that supported rod photoresponses. (G+/-, R+/-) retinas, heterozygous for both G90D and wild-type (WT) rhodopsin, possessed normal numbers of photoreceptors and had a normal rhodopsin complement but exhibited considerable loss of rod sensitivity as measured electroretinographically. The rod photoresponses were desensitized, and the response time to peak was faster than in (R+/-) animals. An equivalent desensitization resulted by exposing WT retinas to a background light producing 82 photoisomerizations rod(-1) sec(-1), suggesting that G90D rods in darkness act as if they are partially "light-adapted." Adding a second G90D allele gave (G+/+, R+/-) animals that exhibited a further increase of equivalent background light level but had no rod cell loss by 24 weeks of age. (G+/+, R-/-) retinas that express only the mutant rhodopsin develop normal rod outer segments and show minimal rod cell loss even at 1 year of age. We conclude that G90D is constitutively active in mouse rods in vivo but that it does not cause significant rod degeneration. Instead, G90D desensitizes rods by a process equivalent to light adaptation.

Membrane protein diffusion sets the speed of rod phototransduction.

Retinal rods signal the activation of a single receptor molecule by a photon. To ensure efficient photon capture, rods maintain about 109 copies of rhodopsin densely packed into membranous disks. But a high packing density of rhodopsin may impede other steps in phototransduction that take place on the disk membrane, by restricting the lateral movement of, and hence the rate of encounters between, the molecules involved. Although it has been suggested that lateral diffusion of proteins on the membrane sets the rate of onset of the photoresponse, it was later argued that the subsequent processing of the complexes was the main determinant of this rate. The effects of protein density on response shut-off have not been reported. Here we show that a roughly 50% reduction in protein crowding achieved by the hemizygous knockout of rhodopsin in transgenic mice accelerates the rising phases and recoveries of flash responses by about 1.7-fold in vivo. Thus, in rods the rates of both response onset and recovery are set by the diffusional encounter frequency between proteins on the disk membrane.

Phototransduction in transgenic mice after targeted deletion of the rod transducin alpha -subunit.

Retinal photoreceptors use the heterotrimeric G protein transducin to couple rhodopsin to a biochemical cascade that underlies the electrical photoresponse. Several isoforms of each transducin subunit are present in the retina. Although rods and cones seem to contain distinct transducin subunits, it is not known whether phototransduction in a given cell type depends strictly on a single form of each subunit. To approach this question, we have deleted the gene for the rod transducin alpha-subunit in mice. In hemizygous knockout mice, there was a small reduction in retinal transducin alpha-subunit content but retinal morphology and the physiology of single rods were largely normal. In homozygous knockout mice, a mild retinal degeneration occurred with age. Rod-driven components were absent from the electroretinogram, whereas cone-driven components were retained. Every photoreceptor examined by single-cell recording failed to respond to flashes, with one exception. The solitary responsive cell was insensitive, as expected for a cone, but had a rod-like spectral sensitivity and flash response kinetics that were slow, even for rods. These results indicate that most if not all rods use a single transducin type in phototransduction.

Morphological, physiological, and biochemical changes in rhodopsin knockout mice.

Mutations in rod opsin, the visual pigment protein of rod photoreceptors, account for approximately 15% of all inherited human retinal degenerations. However, the physiological and molecular events underlying the disease process are not well understood. One approach to this question has been to study transgenic mice expressing opsin genes containing defined mutations. A caveat of this approach is that even the overexpression of normal opsin leads to photoreceptor cell degeneration. To overcome the problem, we have reduced or eliminated endogenous rod opsin content by targeted gene disruption. Retinas in mice lacking both opsin alleles initially developed normally, except that rod outer segments failed to form. Within months of birth, photoreceptor cells degenerated completely. Retinas from mice with a single copy of the opsin gene developed normally, and rods elaborated outer segments of normal size but with half the normal complement of rhodopsin. Photoreceptor cells in these retinas also degenerated but did so over a much slower time course. Physiological and biochemical experiments showed that rods from mice with a single opsin gene were approximately 50% less sensitive to light, had accelerated flash-response kinetics, and contained approximately 50% more phosducin than wild-type controls.

Role for the target enzyme in deactivation of photoreceptor G protein in vivo.

Heterotrimeric guanosine 5'-triphosphate (GTP)-binding proteins (G proteins) are deactivated by hydrolysis of the GTP that they bind when activated by transmembrane receptors. Transducin, the G protein that relays visual excitation from rhodopsin to the cyclic guanosine 3',5'-monophosphate phosphodiesterase (PDE) in retinal photoreceptors, must be deactivated for the light response to recover. A point mutation in the gamma subunit of PDE impaired transducin-PDE interactions and slowed the recovery rate of the flash response in transgenic mouse rods. These results indicate that the normal deactivation of transducin in vivo requires the G protein to interact with its target enzyme.

Onset of feedback reactions underlying vertebrate rod photoreceptor light adaptation.

Light adaptation in vertebrate photoreceptors is thought to be mediated through a number of biochemical feedback reactions that reduce the sensitivity of the photoreceptor and accelerate the kinetics of the photoresponse. Ca2+ plays a major role in this process by regulating several components of the phototransduction cascade. Guanylate cyclase and rhodopsin kinase are suggested to be the major sites regulated by Ca2+. Recently, it was proposed that cGMP may be another messenger of light adaptation since it is able to regulate the rate of transducin GTPase and thus the lifetime of activated cGMP phosphodiesterase. Here we report measurements of the rates at which the changes in Ca2+ and cGMP are followed by the changes in the rates of corresponding enzymatic reactions in frog rod outer segments. Our data indicate that there is a temporal hierarchy among reactions that underlie light adaptation. Guanylate cyclase activity and rhodopsin phosphorylation respond to changes in Ca2+ very rapidly, on a subsecond time scale. This enables them to accelerate the falling phase of the flash response and to modulate flash sensitivity during continuous illumination. To the contrary, the acceleration of transducin GTPase, even after significant reduction in cGMP, occurs over several tens of seconds. It is substantially delayed by the slow dissociation of cGMP from the noncatalytic sites for cGMP binding located on cGMP phosphodiesterase. Therefore, cGMP-dependent regulation of transducin GTPase is likely to occur only during prolonged bright illumination.

Rhodopsin kinase inhibition by recoverin. Function of recoverin myristoylation.

Recoverin is a Ca(2+)-binding protein that may play a role in vertebrate photoreceptor light adaptation by imparting Ca2+ sensitivity to rhodopsin kinase. It is heterogeneously acylated (mostly myristoylated) at its amino-terminal glycine. Recent studies have shown that recoverin myristolyation is necessary for its Ca(2+)-dependent membrane association and cooperative Ca2+ binding. We have addressed several issues concerning the role of recoverin myristoylation with respect to inhibition of rhodopsin kinase. We find that 1) myristoylation of recoverin is not necessary for inhibition of rhodopsin kinase, 2) myristoylation of recoverin induces a cooperative Ca(2+)-dependence for rhodopsin kinase inhibition, and 3) each Ca(2+)-binding site on the nonmyristoylated recoverin partially inhibits rhodopsin kinase. The available data suggest that the functions of recoverin myristoylation in the living rod are to induce a sharp Ca2+ dependence of rhodopsin kinase inhibition and to bring this dependence into the rod's physiological Ca2+ concentration range.

Inhibition of rhodopsin kinase by recoverin. Further evidence for a negative feedback system in phototransduction.

Recoverin is a 23-kDa Ca(2+)-binding protein found predominantly in vertebrate photoreceptor cells. Recent electrophysiological and biochemical studies suggest that recoverin may regulate the photoresponse by inhibiting rhodopsin phosphorylation. We find in both cell homogenates and reconstituted systems that the inhibition of rhodopsin phosphorylation by recoverin occurs over a significantly higher free Ca2+ range than previously reported. Half-maximal inhibition occurs at 1.5-3 microM free Ca2+ and is cooperative with a Hill coefficient of approximately 2. Measurements of transducin activation demonstrate that this inhibition prolongs the lifetime of catalytically active rhodopsin. Ca(2+)-recoverin directly inhibits rhodopsin kinase activity, and Ca(2+)-dependent binding of recoverin to rod outer segment membranes is not required for its action. Extrapolation of the in vitro data to in vivo conditions based on simple mass action calculations places the Ca(2+)-recoverin regulation within the physiological free Ca2+ range in intact rod outer segment. The data are consistent with a model in which the fall in free Ca2+ that accompanies rod excitation exerts negative feedback by relieving inhibition of rhodopsin phosphorylation.

Rod outer segment structure influences the apparent kinetic parameters of cyclic GMP phosphodiesterase.

Cyclic GMP hydrolysis by the phosphodiesterase (PDE) of retinal rod outer segments (ROS) is a key amplification step in phototransduction. Definitive estimates of the turnover number, kcat, and of the Km are crucial to quantifying the amplification contributed by the PDE. Published estimates for these kinetic parameters vary widely; moreover, light-dependent changes in the Km of PDE have been reported. The experiments and analyses reported here account for most observed variations in apparent Km, and they lead to definitive estimates of the intrinsic kinetic parameters in amphibian rods. We first obtained a new and highly accurate estimate of the ratio of holo-PDE to rhodopsin in the amphibian ROS, 1:270. We then estimated the apparent kinetic parameters of light-activated PDE of suspensions of disrupted frog ROS whose structural integrity was systematically varied. In the most severely disrupted ROS preparation, we found Km = 95 microM and kcat = 4,400 cGMP.s-1. In suspensions of disc-stack fragments of greater integrity, the apparent Km increased to approximately 600 microM, though kcat remained unchanged. In contrast, the Km for cAMP was not shifted in the disc stack preparations. A theoretical analysis shows that the elevated apparent Km of suspensions of disc stacks can be explained as a consequence of diffusion with hydrolysis in the disc stack, which causes active PDEs nearer the center of the stack to be exposed to a lower concentration of cyclic GMP than PDEs at the disc stack rim. The analysis predicts our observation that the apparent Km for cGMP is elevated with no accompanying decrease in kcat. The analysis also predicts the lack of a Km shift for cAMP and the previously reported light dependence of the apparent Km for cGMP. We conclude that the intrinsic kinetic parameters of the PDE do not vary with light or structural integrity, and are those of the most severely disrupted disc stacks.

Innovative materials processing strategies: a biomimetic approach.

Many organisms construct structural ceramic (biomineral) composites from seemingly mundane materials; cell-mediated processes control both the nucleation and growth of mineral and the development of composite microarchitecture. Living systems fabricate biocomposites by: (i) confining biomineralization within specific subunit compartments; (ii) producing a specific mineral with defined crystal size and orientation; and (iii) packaging many incremental units together in a moving front process to form fully densified, macroscopic structures. By adapting biological principles, materials scientists are attempting to produce novel materials. To date, neither the elegance of the biomineral assembly mechanisms nor the intricate composite microarchitectures have been duplicated by nonbiological processing. However, substantial progress has been made in the understanding of how biomineralization occurs, and the first steps are now being taken to exploit the basic principles involved.

A novel high-efficiency crystal/polymer composite material for nonlinear optics.

FOR practical applications in optoelectronic devices, nonlinear optical materials should ideally combine appropriate optical properties (that is, a nonlinear response to an electric field, characterized by second-harmonic generation) with the mechanical properties, such as strength and rigidity, required for ease of processibility. As reported here, we have developed a new class of material that combines these attributes, by growing aligned crystals of an optically nonlinear organic compound in a transparent polymer matrix. The host conveys desirable mechanical characteristics to the otherwise fragile organic crystals. The composites are transparent and non-scattering, with a refractive index that can be varied by modification of the polymer host. Given, in addition, the high chemical stability of these materials, we believe that they will have an important part to play in the development of optoelectronic devices.