Field cyanobacterial blooms producing retinoid compounds cause teratogenicity in zebrafish embryos

Cyanobacteria routinely release potentially harmful bioactive compounds into the aquatic environment. Several recent studies suggested a potential link between the teratogenicity of effects caused by cyanobacteria and production of retinoids. To investigate this relationship, we analysed the teratogenicity of field-collected cyanobacterial bloom samples by means of an in vivo zebrafish embryo test, an in vitro reporter gene bioassay and by the chemical analysis of retinoids. Extracts of biomass from cyanobacterial blooms with the dominance of Microcystis aeruginosa and Aphanizomenon klebahnii were collected from water bodies in the Czech Republic and showed significant retinoid-like activity in vitro, as well as high degrees of teratogenicity in vivo. Chemical analysis was then used to identify a set of retinoids in ng per gram of dry weight concentration range. Subsequent fractionation and bioassay-based characterization identified two fractions with significant in vitro retinoid-like activity. Moreover, in most of the retinoids eluted from these fractions, teratogenicity with malformations typical for retinoid signalling disruption was observed in zebrafish embryos after exposure to the total extracts and these in vitro effective fractions. The zebrafish embryo test proved to be a sensitive toxicity indicator of the biomass extracts, as the teratogenic effects occurred at even lower concentrations than those expected from the activity detected in vitro. In fact, teratogenicity with retinoid-like activity was detected at concentrations that are commonly found in biomasses and even in bulk water surrounding cyanobacterial blooms. Overall, these results provide evidence of a link between retinoid-like activity, teratogenicity and the retinoids produced by cyanobacterial water blooms in the surrounding environment.

Endogenous retinoids in the pathogenesis of alopecia areata

Alopecia areata (AA) is an autoimmune disease that attacks anagen hair follicles. Gene array in graft-induced C3H/HeJ mice revealed that genes involved in retinoic acid (RA) synthesis were increased, whereas RA degradation genes were decreased in AA compared with sham controls. This was confirmed by immunohistochemistry in biopsies from patients with AA and both mouse and rat AA models. RA levels were also increased in C3H/HeJ mice with AA. C3H/HeJ mice were fed a purified diet containing one of the four levels of dietary vitamin A or an unpurified diet 2 weeks before grafting and disease progression followed. High vitamin A accelerated AA, whereas mice that were not fed vitamin A had more severe disease by the end of the study. More hair follicles were in anagen in mice fed high vitamin A. Both the number and localization of granzyme B-positive cells were altered by vitamin A. IFNγ was also the lowest and IL13 highest in mice fed high vitamin A. Other cytokines were reduced and chemokines increased as the disease progressed, but no additional effects of vitamin A were seen. Combined, these results suggest that vitamin A regulates both the hair cycle and immune response to alter the progression of AA.

Immunochemical recognition of A2E, a pigment in the lipofuscin of retinal pigment epithelial cells

The autofluorescent lipofuscin pigment A2E accumulates in retinal pigment epithelial cells with age and is particularly abundant in some retinal disorders. To generate a polyclonal antibody that recognizes this pyridinium bisretinoid molecule, we immunized rabbits with bovine serum albumin (BSA) conjugates in which the protein was linked to the A2E molecule via its pyridinium ethanolamine moiety. Analysis by matrix-assisted laser desorption ionization/time of flight mass spectrometry (MALDI-TOF MS) of the A2E-BSA conjugate indicated the presence of five intact A2E molecules covalently linked to BSA, thus deeming it a suitable antigen for immunization. By immunocytochemical staining, the rabbit polyclonal antibody recognized A2E that had accumulated in cultured cells, whereas dot-blot analysis revealed binding to both A2E and A2E-rabbit serum albumin (A2E-RSA) conjugate but no cross-reactivity with various retinoids. Preimmune serum was nonreactive. In fluorescence spectroscopy studies, antibody-A2E binding was evidenced by a fluorescence increase and by a blue-shift in the emission maximum consistent with a change in A2E milieu upon antibody binding. The changes in fluorescence emission upon antibody binding could reflect several processes including restrictions on trans-cis isomerization and intersystem crossing of photo-excited A2E.

Characterization of dihydro-A2PE: an intermediate in the A2E biosynthetic pathway

Bisretinoid lipofuscin pigments that accumulate in retinal pigment epithelial cells are implicated in the etiology of several forms of macular degeneration, including juvenile onset Stargardt disease, Best vitelliform macular degeneration, and age-related macular degeneration. One of these compounds, A2E, is generated by phosphate hydrolysis of a phosphatidyl-pyridinium bisretinoid (A2PE) that forms within photoreceptor outer segments. Here, we demonstrate that the formation of the aromatic pyridinium ring of A2PE follows from the oxidation of a dihydropyridinium intermediate. Time-dependent density functional theory calculation, based on the structure of dihydro-A2E, produced a simulated UV-visible absorbance spectrum characterized by maxima of 494 and 344 nm. Subsequently, a compound exhibiting similar UV-visible absorbance maxima (lambdamax 490 and 330 nm) was identified in the A2E biomimetic reaction mixture. By liquid chromatography-mass spectrometry (LC-MS) this bischromophore had the expected mass of the dihydro-pyridinium bisretinoid. The compound also exhibited the behavior of a biosynthetic intermediate since it formed in advance of the final product A2E and was consumed as A2E accumulated. Moreover, under deoxygenated conditions, conversion to the aromatic pyridinium bisretinoid was inhibited. Taken together, these findings indicate that A2E biosynthesis involves the oxidation of a dihydropyridinium intermediate dihydro-A2PE. An understanding of the biosynthetic pathways of retinal pigment epithelial lipofuscin pigments is critical to the development of therapies for macular degeneration that are based on limiting the formation of these damaging compounds.

Identification of protein modulation by the synthetic retinoid CD437 in lung carcinoma cells using high throughput immunoblotting

The novel synthetic retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (CD437) induces growth arrest and apoptosis in various tumor cell lines including non-small cell lung cancer (NSCLC) cells. CD437 binds retinoic acid receptor gamma (RARgamma) selectively, and can enhance receptor-dependent transcriptional activation of various genes. However, some of the effects of this retinoid on cell growth inhibition and apoptosis appear to be receptor-independent. To gain a better understanding of the mechanism by which CD437 exerts its effects, we employed a high throughput western blotting method (PowerBlottrade mark) using 760 monoclonal antibodies to compare the levels of their target cellular signaling proteins in untreated and CD437-treated NSCLC H460 cells. CD437 (1 microM, 24 h) increased the levels of 70 proteins and decreased the levels of 28 proteins. These proteins play a role in fundamental cellular processes including: DNA synthesis and repair, transcription and DNA-binding, cell cycle, apoptosis, cytoskeleton assembly, cell adhesion, endocytosis, growth and signal transduction. Some proteins identified by this approach have been implicated previously in the effect of CD437 (e.g., p53, Bax, cyclin B, CDK2). Additionally we identified proteins that are novel candidates for mediating the cellular responses to CD437 (e.g., FAF1, Bid, caspase 8, cdk1, KAP, NDR, RBBP, 53BP2, Grb2, PLCgamma1, p70s6k, PP2Cdelta, PKBalpha/AKT, PDK1, and several DNA repair enzymes).

The homeobox gene Gsh2 is required for retinoid production in the embryonic mouse telencephalon

We have examined the role of the homeobox gene Gsh2 in retinoid production and signaling within the ventral telencephalon of mouse embryos. Gsh2 mutants exhibit altered ventral telencephalic development,including a smaller striatum with fewer DARPP-32 neurons than wild types. We show that the expression of the retinoic acid (RA) synthesis enzyme,retinaldehyde dehydrogenase 3 (Raldh3, also known as Aldh1a3), is reduced in the lateral ganglionic eminence (LGE) of Gsh2 mutants. Moreover,using a retinoid reporter cell assay, we found that retinoid production in the Gsh2 mutants is markedly reduced. The striatal defects in Gsh2 mutants are thought to result from ectopic expression of Pax6 in the LGE. Previously, we had shown that removal of Pax6 from the Gsh2 mutant background improves the molecular identity of the LGE in these double mutants; however, Raldh3 expression is not improved. The Pax6;Gsh2 double mutants possess a larger striatum than the Gsh2 mutants, but the disproportionate reduction in DARPP-32 neurons is not improved. These findings suggest that reduced retinoid production in the Gsh2 mutant contributes to the striatal differentiation defects. As RA promotes the expression of DARPP-32 in differentiating LGE cells in vitro, we examined whether exogenous RA can improve striatal neuron differentiation in the Gsh2 mutants. Indeed,RA supplementation of Gsh2 mutants, during the period of striatal neurogenesis, results in a significant increase in DARPP-32 expression. Thus,in addition to the previously described role for Gsh2 to maintain correct molecular identity in the LGE, our results demonstrate a novel requirement of this gene for retinoid production within the ventral telencephalon.

Retinoids and binding proteins in the cerebellum during lifetime

The vertebrate nervous system is one of the main sites where retinoids and related binding proteins play their critical roles. The cerebellum is subjected to the effects of retinoids over its lifetime. The retinoid binding proteins are differently expressed during life: CRBP I, expressed in the adult cerebellum, is possibly involved in plasticity during memory processes. CRABP I, expressed in the embryonic cerebellum, is involved in the development of the organ and in cellular differentiation. In particular, since the expression of CRABP I coincides with earliest cerebellar differentiation, it is probably involved in this critical step, participating in a complex network of regulatory genes expressed in the mes/methencephalon such as Fgf8, Wnt1, En1/2, Pax2/5, Otx2 and Gbx2. Finally, the way for metabolic synchronization of isthmic cell populations and the putative existence of alternative pathways for CRABP I to regulate RA are discussed.

Alpha-tocopherol and ascorbic acid decrease the production of beta-apo-carotenals and increase the formation of retinoids from beta-carotene in the lung tissues of cigarette smoke-exposed ferrets in vitro

Previously, we found that exposing ferrets to cigarette smoke enhanced oxidative excentric cleavage of beta-carotene. In the present study, we examined whether alpha-tocopherol, ascorbic acid, or the two combined can prevent smoke-altered beta-carotene metabolism. In vitro incubation of beta-carotene (10 micro mol/L) with lung postnuclear fractions from ferrets exposed to cigarette smoke was carried out in the absence or presence of alpha-tocopherol (50 micro mol/L), ascorbic acid (10 or 50 micro mol/L), or both vitamins to evaluate their effects on the production of beta-apo-carotenals and retinoids from beta-carotene. The oxidative cleavage metabolites of beta-carotene, beta-apo-carotenals (beta-apo-14', beta-apo-12', beta-apo-10', and beta-apo-8'), retinoic acid (RA), and retinal were analyzed by HPLC. We found that the smoke-enhanced production of individual beta-apo-carotenals was significantly decreased by 36-77% when alpha-tocopherol (50 micro mol/L) and ascorbic acid (50 micro mol/L) were added together to the incubation mixture. alpha-Tocopherol alone had a modest effect. Ascorbic acid in the presence of alpha-tocopherol inhibited the production of beta-apo-carotenals in a dose-dependent manner, although ascorbic acid alone had no effect. In contrast, the production of RA and retinal among smoke-exposed ferrets was substantially increased ( approximately 3-fold, P < 0.05) when both alpha-tocopherol and ascorbic acid were added to the incubation mixtures. However, when ascorbic acid or alpha-tocopherol alone was added, the production of RA among smoke-exposed ferrets increased only modestly (80%, P < 0.05) and did not differ from the RA levels in control ferrets. In conclusion, these data indicate that alpha-tocopherol and ascorbic acid may act synergistically in preventing the enhanced oxidative excentric cleavage of beta-carotene induced by smoking exposure, thereby facilitating the conversion of beta-carotene into RA and retinal.

A2E: A Component of Ocular Lipofuscin¶

The presence of lipofuscin in postmitotic cells is considered a hallmark of the aging process. In the retinal pigment epithelium (RPE), lipofuscin is found as micrometer-sized spherical particles and characterized by its yellow autofluorescence when exposed to blue light. This exposure to light is also known to produce reactive oxygen intermediates (ROI), but the particular molecular constituent(s) responsible for this phototoxicity have yet to be completely identified. Resulting mostly from the autophagocytosis of intracellular organelles, the composition of lipofuscin is poorly defined but known to contain protein, lipids and several fluorophores. The subsequent identification of one of the fluorophores in lipofuscin, A2E, generated much interest and resulted in a variety of studies to understand its potential role in the phototoxicity of lipofuscin. Several modes of toxicity have been suggested through which A2E can affect the health of RPE cells. These modes include photoinduced production of ROI, which places additional oxidative stress on RPE cells, the disruption of membrane integrity through its natural role as an amphiphilic detergent and inhibition of key cellular functions. This article presents the current understanding of the photochemistry of A2E and its involvement as a phototoxic agent in RPE cells.

Retinoids in myelopoiesis

The vitamin A derivative retinoic acid plays a critical role during the differentiation of myeloid progenitors towards the neutrophil lineage. This role is primarily mediated by binding of retinoic acid to retinoic acid receptor alpha (RARalpha), a nuclear receptor that modulates the expression of multiple downstream targets via retinoic acid response elements. The importance of this signalling pathway in myelopoiesis is evidenced by the recurrent disruption of the RARalpha gene by chromosomal rearrangements in all cases of acute promyelocytic leukemia (APL). Biochemical evidence suggests RARalpha performs two opposing functions, one as a repressor of gene expression in the absence of ligand, the second as a transcriptional activator in the presence of ligand, each controlled by multimeric complexes of transcription corepressors and coactivators, respectively. Here the molecular mechanisms activated by retinoic acid during myelopoiesis in the context of neutrophil development will be reviewed, together with some of the more recently identified targets of the retinoic signalling pathway.

Biosynthetic studies of A2E, a major fluorophore of retinal pigment epithelial lipofuscin

We have examined questions related to the biosynthesis of A2E, a fluorophore that accumulates in retinal pigment epithelial cells with aging and in some retinal disorders. The use of in vitro preparations revealed that detectable levels of A2-PE, the A2E precursor, are formed within photoreceptor outer segments following light-induced release of endogenous all-trans-retinal. Moreover, experiments in vivo demonstrated that the formation of A2-PE in photoreceptor outer segment membrane was augmented by exposing rats to bright light. Whereas the generation of A2E from A2-PE by acid hydrolysis was found to occur very slowly, the detection in outer segments of a phosphodiesterase activity that can convert A2-PE to A2E may indicate that some portion of the A2-PE that forms in the outer segment membrane may undergo hydrolytic cleavage before internalization by the retinal pigment epithelial cell. The identities of additional minor components of retinal pigment epithelium lipofuscin, A2E isomers with cis olefins at positions other than the C13-C14 double bond, are also described.

Effects of osmotic and light stimulation on 3H-taurine efflux from isolated rod outer segments and synthesis of tauret in the frog retina

After injection of 3H-taurine into eyeballs of frogs and maintenance for 3 h in darkness by a gentle shaking, an almost homogenous fraction of rod outer segments (ROS) was prepared. About a 22% decrease in tonicity caused by reducing NaCl in isotonic 225 mOsm normal solution caused a rapid increase in the rate coefficient of efflux of 3H-taurine from the ROS fraction. The peak level of increased efflux rate coefficient was 7-times higher than the basal isotonic level. This indicates that taurine could contribute essentially to the volume regulation, either via selective channels or a carrier transporter-mediated pathways. For further clarifying if taurine fluxes in the ROS are sensitive to the light, other experiments were performed. Neither light stimulation of dark-adapted ROSs fractions or dark stimulation of weakly illuminated ROSs revealed any detectable changes in the efflux rate coefficient of 3H-taurine. These results indicate that light-induced taurine efflux, if present in the ROS, must be small, compared with hypoosmotic induced efflux. Thus the question of light-induced release of taurine from ROS still remains to be clarified. In the second part of this study, using TLC (thin layer chromatography) in combination with 3H-taurine measurements we have tried to clarify whether frogs (Rana ridibunda) eye structures can synthesize tauret (retinylidenetaurine). In isolated retinal preparations almost no any noticeable radioactivity was detected compared with background level. The capability of the eye structures to synthesize tauret from 3H-taurine was revealed in the second whole eye injection experiment. About 0.3% of the total 3H-taurine pool taken up was converted into 3H-tauret in the dark-adapted frog retina. In the retina of frogs adapted to light compared with those which were dark adapted tauret quantities were remarkable lower--on average about half. These results are in agreement with our recent data obtained by HPLC, which indicate tauret levels several times higher in the dark-adapted frog retinae compared with those after long lasting light adaption. Taking into account these results one can conclude that the main structure able to synthesize 3H-tauret is probably pigment epithelium rather than retina.

Retinal is not formed in vitro by enzymatic central cleavage of beta-carotene

Rat intestinal mucosa was prepared and incubated with beta-carotene by the procedure of Goodman and Olson [Goodman, DeW. S., & Olson, J.A. (1969) Methods Enzymol. 15, 462-475] to determine beta-carotene cleavage activity. A new detection system for the reaction products of the described enzyme beta-carotene 15,15'-dioxygenase (EC 1.13.11.21) employs solvent extraction of retinoids and carotenoids followed by high-performance liquid chromatography separation and photometric detection of the pigments. It has not detected any newly formed retinal or other retinoids in the intestinal protein preparations from normal or vitamin A deficient rats. The latter were chosen as a possible source of more active enzyme preparations. With corresponding blank samples subjected to identical conditions of incubation but without added protein, small amounts of beta-apocarotenals could be detected. They were previously reported as cleavage products of beta-carotene [Ganguly, J., & Sastry, P.S. (1985) World Rev. Nutr. Diet. 45, 198-220] but are clearly not formed as a result of an enzymatic reaction. The failure to detect in vitro enzymatic central or random cleavage of the beta-carotene molecule in extracts of rat intestinal mucosa emphasizes the need to reevaluate the existing theory of conversion of beta-carotene into vitamin A.

Biosynthesis of 11-cis-retinoids and retinyl esters by bovine pigment epithelium membranes

Previously, we have shown that retina/pigment epithelium membranes from the amphibian can synthesize 11-cis-retinoids from added all-trans-retinol [Bernstein, P.S., Law, W.C., & Rando, R.R. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1849-1853]. The activity was largely localized to the pigment epithelium. Here it is shown that, in the bovine system, the activity resides exclusively in the membranes of the pigment epithelium. Subcellular fractionation does not reveal a particular organelle where the activity resides. Washed bovine pigment epithelium membranes, which are devoid of retinoid redox activity, convert added all-trans-retinol to a mixture of 11-cis-retinol and its palmitate ester. all-trans-Retinal and all-trans-retinyl palmitate are not converted into 11-cis-retinoids by the membranes. The membranes show substantial ester synthetase activity, producing large amounts of all-trans-retinyl palmitate. Diverse chemical reagents, such as ethanol, hydroxylamine, and p-(hydroxymercuri)benzoate, inhibit both ester synthetase and isomerase activities in a roughly parallel fashion, suggesting a possible functional linkage between the two activities.

Uptake and metabolism of retinol in cultured Sertoli cells: evidence for a kinetic model

When cultured Sertoli cells derived from 20-day-old weanling rats were supplied [3H]retinol bound to serum retinol binding protein-transthyretin complex, [3H]retinol was rapidly incorporated and [3H]retinyl esters were synthesized. Within 28 h after administration, 83% of the labeled retinoids were accounted for as retinyl esters (64% as retinyl palmitate). Sertoli cells derived from vitamin A deficient rats and supplied [3H]retinol in culture under identical conditions likewise incorporated [3H]retinol and synthesized retinyl esters. In contrast to normal Sertoli cells, vitamin A deficient Sertoli cells eventually metabolized virtually all of the cellular [3H]retinol to retinyl esters. The primary metabolic fate of retinol administered to Sertoli cell cultures was the synthesis of retinyl esters under all conditions tested. However, administration of [3H]retinol bound to serum retinol binding protein gave metabolic profiles having a higher proportion of retinyl esters and lower proportions of unresolved polar material than administration of [3H]retinol alone. The kinetics of retinol uptake and intracellular retinyl ester synthesis in cultured Sertoli cells was complex. An initial, rapid phase of [3H]retinol incorporation lasting 30 min was followed by a slower rate of incorporation and a concomitant decrease in the intracellular concentration of [3H]retinol. During the time course the specific activity of [3H]retinyl palmitate eventually exceeded that of intracellular [3H]retinol. These observations suggest that two intracellular pools of retinol may exist in Sertoli cells.

Nonstereospecific biosynthesis of 11-cis-retinal in the eye

[3H]-all-trans-Retinol injected intraocularly into rats is processed to [3H]-11-cis-retinal, the visually active retinoid that binds to opsin. After 18 h, virtually all (93%) of the radioactive retinals recovered were in the form of 11-cis-retinal. At earlier times, however, both all-trans- and 13-cis-retinals, the latter being a nonphysiological isomer, were formed. Both of these isomers disappeared concomitant with the formation of 11-cis-retinal. The rise and fall of 13-cis-retinal suggest that this isomer can be converted into 11-cis-retinal either directly or indirectly in vivo and, hence, that the biosynthesis of the latter is nonstereospecific. This hypothesis was verified by showing that in double-labeling experiments [14C]-13-cis-retinol was converted into 11-cis-retinal nearly as well (approximately 70%) as [3H]-all-trans-retinol. These studies show that the biosynthesis of 11-cis-retinal can be nonstereospecific and, hence, that the process may be chemically rather than enzymatically mediated in vivo. In contrast, double-labeling studies with [14C]-9-cis-retinol and [3H]-all-trans-retinol showed that very little, if any, of the 9-cis isomer was processed to 11-cis-retinal in vivo although it did form isorhodopsin. This is consistent with what is known about the relative chemical stabilities of 9-cis-retinoids from model studies. The isomerization of 9-cis-retinoids is much slower than that of their all-trans, 13-cis, or 11-cis congeners. These results are discussed in terms of a possible mechanism for the biosynthesis of 11-cis-retinal in vivo and suggest that the isomerization event need not necessarily be enzyme mediated.

A photoproduct with 13-cis retinal generated by irradiation with violet light in the octopus retina

The photoconversion between rhodopsin (R) and metarhodopsin (MR) was investigated in the retina of Octopus ocellatus by measurements of the fast photovoltage (FPV) in conjunction with high pressure liquid chromatography (HPLC). Following conversion of most of R to MR by short-term exposure to an intense violet light, a large vitreous negative FPV due to photon absorption by MR was observed with an orange test flash. By continuing the exposure to the same violet light, however, the negative FPV became very small, though long-term irradiation with blue-green light produced a recovery. With the violet light exposure a substance other than R and MR is produced, which hardly contributes to FPV generation. Analysis of retinal isomers with HPLC show that such an exposure produces a substance with 13-cis retinal as its chromophore and that it was significantly reduced after exposure to blue-green light.