Gregory G. Garwin

Visual Cycle Impairment in Cellular Retinaldehyde Binding Protein (CRALBP) Knockout Mice Results in Delayed Dark Adaptation

Mutations in the human CRALBP gene cause retinal pathology and delayed dark adaptation. Biochemical studies have not identified the primary physiological function of CRALBP. To resolve this, we generated and characterized mice with a non-functional CRALBP gene (Rlbp1(-/-) mice). The photosensitivity of Rlbp1(-/-) mice is normal but rhodopsin regeneration, 11-cis-retinal production, and dark adaptation after illumination are delayed by >10-fold. All-trans-retinyl esters accumulate during the delay indicating that isomerization of all-trans- to 11-cis-retinol is impaired. No evidence of photoreceptor degeneration was observed in animals raised in cyclic light/dark conditions for up to 1 year. Albino Rlbp(-/-) mice are protected from light damage relative to the wild type. These findings support a role for CRALBP as an acceptor of 11-cis-retinol in the isomerization reaction of the visual cycle.

Multiple phosphorylation of rhodopsin and the in vivo chemistry underlying rod photoreceptor dark adaptation

Dark adaptation requires timely deactivation of phototransduction and efficient regeneration of visual pigment. No previous study has directly compared the kinetics of dark adaptation with rates of the various chemical reactions that influence it. To accomplish this, we developed a novel rapid-quench/mass spectrometry-based method to establish the initial kinetics and site specificity of light-stimulated rhodopsin phosphorylation in mouse retinas. We also measured phosphorylation and dephosphorylation, regeneration of rhodopsin, and reduction of all-trans retinal all under identical in vivo conditions. Dark adaptation was monitored by electroretinography. We found that rhodopsin is multiply phosphorylated and then dephosphorylated in an ordered fashion following exposure to light. Initially during dark adaptation, transduction activity wanes as multiple phosphates accumulate. Thereafter, full recovery of photosensitivity coincides with regeneration and dephosphorylation of rhodopsin.

Reduction of all-trans-retinal limits regeneration of visual pigment in mice

Absorption of photons by pigments in photoreceptor cells results in photoisomerization of the chromophore, 11-cis-retinal, to all-trans-retinal and activation of opsin. Photolysed chromophore is converted back to the 11-cis-configuration via several enzymatic steps in photoreceptor and retinal pigment epithelial cells. We investigated the levels of retinoids in mouse retina during constant illumination and regeneration in the dark as a means of obtaining more information about the rate-limiting step of the visual cycle and about cycle intermediates that could be responsible for desensitization of the visual system. All-trans-retinal accumulated in the retinas during constant illumination and following flash illumination. Decay of all-trans-retinal in the dark following constant illumination occurred without substantial accumulation of all-trans-retinal, generated by constant approximately equal to visual pigment regeneration (t1/2 approximately 5 and t1/2 approximately 7 min, respectively). All-trans-retinal, generated by constant illumination, decayed approximately 3 times more rapidly than that generated by a flash and, as shown previously, the rate of rhodopsin regeneration following a flash was approximately 4 times slower than after constant illumination. The retinyl ester pool (> 95% all-trans-retinyl ester) did not show a statistically significant change in size or composition during illumination. In addition, constant illumination increased the amount of photoreceptor membrane-associated arrestin. The results suggest that the rate-limiting step of the visual cycle is the reduction of all-trans-retinal to all-trans-retinol by all-trans-retinol dehydrogenase. The accumulation of all-trans-retinal during illumination may be responsible, in part, for the reduction in sensitivity of the visual system that accompanies photobleaching and may contribute to the development of retinal pathology associated with light damage and aging.

Properties and immunocytochemical localization of three retinoid-binding proteins from bovine retina.

Cellular retinal-binding protein (CRALBP) complexed with 11-cis-retinal has several properties characteristic of a visual pigment. Interaction of the protein and retinoid results in a bathochromic shift in the absorption spectrum of the chromophore from 380 to 425 nm, accompanied by a decrease in the extinction coefficient (25,000-15,000 M-1 cm-1). Illumination of the complex results in the progressive loss of absorbance at 425 nm and an increase at 375 nm, consistent with the production of a geometrical isomer of retinal that lacks affinity for the binding protein. Analysis by HPLC of the retinoids after illumination reveals that the basis of the spectral transition is a photoisomerization of 11-cis-retinal to all-trans-retinal. Only small amounts (less than 10%) of 13-cis-retinal are produced during the photoisomerization, showing the stereospecificity of the process. Although CRALBP has the spectral characteristics of a blue-sensitive visual pigment, there is no evidence that this is related to its function. This protein may serve as a model for the interactions of 11-cis-retinal and protein. Eleven-cis-retinol bound to CRALBP is a better substrate for esterification by microsomes from retinal pigment epithelium (RPE) than all-trans-retinol bound to cellular retinol-binding protein (CRBP). The product of the reaction, retinyl ester, does not remain bound to either binding protein but becomes associated with the microsomal fraction. Esterification is the first described process, occurring in the dark, by which retinoids can be removed from CRBP and CRALBP. Antibodies to bovine CRBP have been produced in rabbits following injection of the performic acid-oxidized protein.(ABSTRACT TRUNCATED AT 250 WORDS)

[19] High-performance liquid chromatography analysis of visual cycle retinoids

Publisher Summary Analysis of visual cycle retinoids under various experimental conditions has provided a wealth of information regarding the fundamentals of the visual process. This chapter describes the advances in column packings and quality and in instrument design has resulted in considerable improvements in separations and sensitivities, and the topic of high-performance liquid chromatography (HPLC) analysis of visual cycle retinoids is dealt with. In addition, several of the common chemical procedures employed during retinoid analysis are described on a micromole scale. All procedures involving retinoids are conducted under dim, red illumination or in aluminum foil-covered vials to minimize photoisomerization and photodecomposition. Reactions are carried out in screwcap vials fitted with Teflon cap liners. In extraction of retinoids from tissues, all procedures are carried out under dim, red light or in aluminum foil-covered, screwcap vials. Tissues are carefully homogenized to avoid oxygenation of the sample. Retinoic acids are poorly extracted from tissues by the preceding procedure because of the limited solubility of the carboxylate anion in hexane. The yield can be improved by acidifying the homogenate with 0.1 vol of 1 M sodium phosphate, pH 5, before the extraction.

Cellular retinaldehyde-binding protein is expressed by oligodendrocytes in optic nerve and brain

Cellular retinaldehyde‐binding protein (CRALBP) is abundant in the retinal pigment epithelium and Müller glial cells of the retina, where it forms complexes with endogenous 11‐cis‐retinoids. We examined the distribution of CRALBP in extra‐retinal tissues using polyclonal antibodies (pAb) and monoclonal antibodies (mAb). A protein was detected by immunoblot analysis in extracts of bovine and rat brain and optic nerve but not in several other tissues. This protein had electrophoretic, chromatographic, and retinoid‐binding properties identical to those of CRALBP from bovine retina. Comparison of the masses of tryptic peptides and of partial amino acid sequences derived from brain and retinal CRALBP indicated that the two proteins are probably identical. Immunoperoxidase cytochemistry and double labeling immunofluorescence revealed CRALBP(+) cells in brain that resembled oligondendrocytes and not astrocytes, microglial cells, or pinealocytes. In 11‐day‐old rat brain, approximately 11% of the CRALBP(+) cells were labeled with the Rip antibody, a marker for oligodendroglia. In developing rat optic nerve, the temporal appearance of CRALBP(+) cells corresponded to that of oligodendrocytes and not that of astrocytes. In adult rat and mouse optic nerves, the CRALBP(+) somata showed the same distribution as oligodendrocytes. No endogenous retinoids were associated with CRALBP isolated from dark‐dissected adult bovine brain. The results suggest that CRALBP has functions in addition to retinoid metabolism and visual pigment regeneration. GLIA 21:259–268, 1997.

Characterization and localization of an aldehyde dehydrogenase to amacrine cells of bovine retina

An enzyme of bovine retina that catalyzes oxidation of retinaldehyde to retinoic acid was purified to homogeneity and a monoclonal antibody (mAb H-4) was generated. MAb H-4 recognized a single component (Mr = 55,000) in extracts of bovine retina and other bovine tissues. The antibody showed no cross-reactivity with extracts of rat, monkey, or human retinas. A 2067 bp cDNA was selected from a retina cDNA expression library using mAb H-4. The cDNA hybridized with a similarly sized, moderately abundant mRNA prepared from bovine retina. Nucleotide sequence analysis indicated that the cDNA contained a single open reading frame encoding 501 amino acids that have 88% sequence identity with the amino-acid sequence of human hepatic Class 1 aldehyde dehydrogenase. Amino-acid sequence analysis of purified enzyme demonstrated that the cDNA encodes the isolated enzyme. MAb H-4 specifically labeled the somata and processes of a subset of amacrine cells in bovine retinal sections. Labeled amacrine somata were located on both sides of the inner plexiform layer, and their processes ramified into two laminae within the inner plexiform layer. The inner radial processes of Müller (glial) cells were weakly reactive with mAb H-4. Weak immunostaining of amacrine cells was found in monkey retina with mAb H-4, but no signal was detected in rat or human retina. The results provide further evidence for metabolism and function of retinoids within cells of the inner retina and define a novel class of retinal amacrine cells.

Relationships among Visual Cycle Retinoids, Rhodopsin Phosphorylation, and Phototransduction in Mouse Eyes during Light and Dark Adaptation

Phosphorylation and regeneration of rhodopsin, the prototypical G-protein-coupled receptor, each can influence light and dark adaptation. To evaluate their relative contributions, we quantified rhodopsin, retinoids, phosphorylation, and photosensitivity in mice during a 90 min illumination followed by dark adaptation. During illumination, all-trans-retinyl esters and, to a lesser extent, all-trans-retinal accumulate and reach the steady state in <1 h. Each major phosphorylation site on rhodopsin reaches a steady state level of phosphorylation at a different time during illumination. The dominant factor that limits dark adaptation is isomerization of retinal. During dark adaptation, dephosphorylation of rhodopsin occurs in two phases. The faster phase corresponds to rapid dephosphorylation of regenerated rhodopsin present at the end of the illumination period. The slower phase corresponds to dephosphorylation of rhodopsin as it forms by regeneration. We conclude that rhodopsin phosphorylation has three physiological functions: it quenches phototransduction, reduces sensitivity during light adaptation, and suppresses bleached rhodopsin activity during dark adaptation.

[38] Analysis of visual cycle in normal and transgenic mice

Publisher Summary This chapter presents biochemical protocols for determining visual cycle function in normal mice and in mice bearing targeted disruptions of specific genes. The levels of rhodopsin and of visual cycle retinoids are measured in dark-adapted animals and in animals recovering from a flash or from constant illumination. The flash subjects the resting visual cycle to a sudden pulse of substrate (all-trans-retinal), whereas constant illumination establishes a steady cycling state from which recovery to the dark-adapted state can be measured. In each protocol, the illumination conditions have been chosen to bleach approximately 40% of the visual pigment, thus avoiding the complete saturation of the cycle with all-trans-retinal, which occurs with prolonged, total bleaching. Considerable variation in the absolute levels of retinoids in mice of different ages and strains was encountered in the study and bleaching regimens introduced further poorly controlled variables. However, it was found that the data were reproducible and consistent when reported as percentages of either total or polar retinoids.

Paclitaxel improves outcome from traumatic brain injury.

Pharmacologic interventions for traumatic brain injury (TBI) hold promise to improve outcome. The purpose of this study was to determine if the microtubule stabilizing therapeutic paclitaxel used for more than 20 years in chemotherapy would improve outcome after TBI. We assessed neurological outcome in mice that received direct application of paclitaxel to brain injury from controlled cortical impact (CCI). Magnetic resonance imaging was used to assess injury-related morphological changes. Catwalk Gait analysis showed significant improvement in the paclitaxel group on a variety of parameters compared to the saline group. MRI analysis revealed that paclitaxel treatment resulted in significantly reduced edema volume at site-of-injury (11.92 ± 3.0 and 8.86 ± 2.2mm(3) for saline vs. paclitaxel respectively, as determined by T2-weighted analysis; p ≤ 0.05), and significantly increased myelin tissue preservation (9.45 ± 0.4 vs. 8.95 ± 0.3, p ≤ 0.05). Our findings indicate that paclitaxel treatment resulted in improvement of neurological outcome and MR imaging biomarkers of injury. These results could have a significant impact on therapeutic developments to treat traumatic brain injury.