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PubMed1988

The nature of the primary photochemical events in rhodopsin and isorhodopsin.

The nature of the primary photochemical events in rhodopsin and isorhodopsin is studied by using low temperature actinometry, low temperature absorption spectroscopy, and intermediate neglect of differential overlap including partial single and double configuration interaction (INDO-PSDCI) molecular orbital theory. The principal goal is a better understanding of how the protein binding site influences the energetic, photochemical, and spectroscopic properties of the bound chromophore. Absolute quantum yields for the isorhodopsin (I) to bathorhodopsin (B) phototransformation are assigned at 77 K by using the rhodopsin (R) to bathorhodopsin phototransformation as an internal standard (phi R----B = 0.67). In contrast to rhodopsin photochemistry, isorhodopsin displays a wavelength dependent quantum yield for photochemical generation of bathorhodopsin at 77 K. Measurements at seven wavelengths yielded values ranging from a low of 0.089 +/- 0.021 at 565 nm to a high of 0.168 +/- 0.012 at 440 nm. An analysis of these data based on a variety of kinetic models suggests that the I----B phototransformation encounters a small activation barrier (approximately 0.2 kcal mol-1) associated with the 9-cis----9-trans excited-state torsional-potential surface. The 9-cis retinal chromophore in solution (EPA, 77 K) has the smallest oscillator strength relative to the other isomers: 1.17 (all-trans), 0.98 (9-cis), 1.04 (11-cis), and 1.06 (13-cis). The effect of conformation is quite different for the opsin-bound chromophores. The oscillator strength of the lambda max absorption band of I is observed to be anomalously large (1.11) relative to the lambda max absorption bands of R (0.98) and B (1.07). The wavelength-dependent photoisomerization quantum yields and the anomalous oscillator strength associated with isorhodopsin provide important information on the nature of the opsin binding site. Various models of the binding site were tested by using INDO-PSDCI molecular orbital theory to predict the oscillator strengths of R, B, and I and to calculate the barriers and energy storage associated with the photochemistry of R and I for each model. Our experimental and theoretical investigation leads to the following conclusions: (a) The counterion (abbreviated as CTN) is not intimately associated with the imine proton in R, B, or I. The counterion lies underneath the plane of the chromophore in R and I, and the primary chromophore-counterion electrostatic interactions involve C15-CTN and C13-CTN. These interactions are responsible for the anomalous oscillator strength of I relative to R and B. (b) The presence of a small activation barrier (~0.2 kcal mol-1) in the 9-cis - 9-trans excited-state surface is associated with the location of the counterion as well as the intrinsic photophysical properties of the 9-cis chromophore. The principal difference between the 1 1-cis -c 1 -transphoto reaction surface and the 9-cis - 9-trans photoreaction surface is the lack of effective electrostatic stabilization of distorted 9 = 10 conformations due to incomplete charge polarization. (c) Hydrogen bonding to the imine proton, ifpresent, does not involve the counterion. We conclude that water in the active site, or secondary interactions with the protein (not involving the CTN), are responsible. (d) All photochemical transformations involve one-bond photoisomerizations.This prediction is based on the observation of a very small excited state barrier for the I -- B photoreaction and a negative barrier for the R - B phototransformation, coupled with the theoretical prediction that all two-bond photoisomerizations have significant S, barriers while one-bond photoisomerizations have small to negative S, barriers.(e) Rhodopsin is energetically stabilized relative to isorhodopsin due to both electrostatic interactions and conformational distortion, both favoring stabilization of R. The INDO-PSDCI calculations suggest that rhodopsin chromophore-CTN electrostatic interactions provide an enhanced stabilization of -2 kcal mol-1 relative to I. Conformational distortion of the 9-cis chromophore-lysine system accounts for -3 kcal mol-1. (f) Energy storage in bathorhodopsin is-60% conformational distortion and 40% charge separation. Our model predicts that the majority of the chromophore protein conformational distortion energy involves interaction of the C,3(-CH3)=CI4--C,5=N-lysine moiety with nearby (unknown) protein residues. (g) Strong interactions between the counterion and the chromophore in R and I will generate weak, but potentially observable charge-transfer bands in the near infrared. The key predictions are the presence of an observable charge-transfer transition at 859 nm (1 1,640 cm- 1) in I and an analogous, but slightly weaker band at 897 nm (11,150 cm-1) in R. Both transitions involve the transfer of an electron from the counterion into low-lying l theta* molecular orbitals.

Binding Sites↗
PubMed2004

X-linked high myopia associated with cone dysfunction.

OBJECTIVE: Bornholm eye disease (BED) consists of X-linked high myopia, high cylinder, optic nerve hypoplasia, reduced electroretinographic flicker with abnormal photopic responses, and deuteranopia. The disease maps to chromosome Xq28 and is the first designated high-grade myopia locus (MYP1). We studied a second family from Minnesota with a similar X-linked phenotype, also of Danish descent. All affected males had protanopia instead of deuteranopia. METHODS: X chromosome genotyping, fine-point mapping, and haplotype analysis of the DNA from 22 Minnesota family individuals (8 affected males and 5 carrier females) and 6 members of the original family with BED were performed. Haplotype comparisons and mutation screening of the red-green cone pigment gene array were performed on DNA from both kindreds. RESULTS: Significant maximum logarithm of odds scores of 3.38 and 3.11 at theta = 0.0 were obtained with polymorphic microsatellite markers DXS8106 and DXYS154, respectively, in the Minnesota family. Haplotype analysis defined an interval of 34.4 cM at chromosome Xq27.3-Xq28. Affected males had a red-green pigment hybrid gene consistent with protanopia. We genotyped Xq27-28 polymorphic markers of the family with BED, and narrowed the critical interval to 6.8 cM. The haplotypes of the affected individuals were different from those of the Minnesota pedigree. Bornholm eye disease-affected individuals showed the presence of a green-red hybrid gene consistent with deuteranopia. CONCLUSIONS: Because of the close geographic origin of the 2 families, we expected affected individuals to have the same haplotype in the vicinity of the same mutation. Mapping studies, however, suggested independent mutations of the same gene. The red-green and green-red hybrid genes are common X-linked color vision defects, and thus are unrelated to the high myopia and other eye abnormalities in these 2 families. CLINICAL RELEVANCE: X-linked high myopia with possible cone dysfunction has been mapped to chromosome Xq28 with intervals of 34.4 and 6.8 centimorgan for 2 families of Danish origin.

Adolescent↗
PubMed1997

Syk kinase, tyrosine-phosphorylated proteins and actin filaments accumulate at forming phagosomes during Fcgamma receptor-mediated phagocytosis.

Phagocytosis mediated by Fcgamma receptors (FcgammaRs) is thought to be regulated by a cascade of tyrosine phosphorylation events that finally leads to the rearrangement of submembranous actin-based cytoskeleton and internalization of particles. Suggestions concerning the functional relationship between protein tyrosine kinases, their substrates, and actin filament reorganization prompted us to determine cellular distribution of these elements during uptake of IgG-coated particles in murine thio-macrophages. We found that the onset of uptake of the particles was accompanied by tyrosine phosphorylation of several proteins, among which 90, 50, 40, 30, and 25 kDa polypeptides were distinguished. In most of the proteins the tyrosine hyperphosphorylation persisted up to 3 min of the uptake; however, kinetics of the phosphorylation of individual proteins varied. Immunofluorescence data showed that the phosphotyrosine-bearing proteins were localized in regions of the particle uptake, being concentrated at phagocytic cups and nascent phagosomes. The local enrichment in tyrosine phosphorylated proteins was correlated with accumulation of actin filaments at these early stages of phagosome formation. During phagosome maturation, both tyrosine phosphorylated proteins and microfilaments disappeared from the periphagosomal regions. Syk, one of the tyrosine kinases, was translocated to the regions where FcgammaR-mediated phagocytosis had started. On the contrary, no enrichment in phosphatidylinositol 3-kinase was detected in these places.

Actins↗
PubMed1996

Neuronal development in the Drosophila compound eye: photoreceptor cells R1, R6, and R7 fail to differentiate in the retina aberrant in pattern (rap) mutant.

The compound eye of Drosophila is a reiterated pattern of 800 unit eyes known as ommatidia. In each ommatidium there are eight photoreceptor neurons (R1-R8) and an invariant number of accessory cells organized in a precise manner. In the developing eye, specification of cell fates is triggered by sequential inductive events mediated by cell-cell interactions. The R8 photoreceptor neuron is the first cell to differentiate and is thought to play a central role in the recruitment of the remaining photoreceptor cells. Our previous work demonstrated that mutations in the retina aberrant in pattern (rap) locus lead to abnormal pattern formation in the compound eye. Genetic mosaic experiments demonstrated that for normal retinal patterning to occur, rap gene function is required only in the photoreceptor cell R8. In this study we analyzed the R cell composition of developing as well as the adult eyes of rap mutants employing a variety of R cell specific markers. We show that in rap mutants, although some of the R8-specific markers show normal expression patterns, other aspects of the R8 cell differentiation are abnormal. In addition, the cells R1, R6, and R7 fail to differentiate properly in rap mutants. These results suggest that the rap gene encodes an R8-specific function that plays a role in the determination of the photoreceptor cells R1, R6, and R7.

Animals↗
PubMed2003

Origin of the vertebrate visual cycle: genes encoding retinal photoisomerase and two putative visual cycle proteins are expressed in whole brain of a primitive chordate.

The absorption of light by rhodopsin leads to the cis-to-trans isomerization of the chromophore to generate all-trans-retinal. In the visual cycle, the resultant all-trans-retinal is converted back into the 11-cis-retinal. In the mammalian eye, the retinal pigment epithelium (RPE) plays an essential role in the visual cycle. We have identified cDNA clones encoding three putative visual cycle proteins, homologs of mammalian retinal G-protein-coupled receptor (RGR), cellular retinaldehyde-binding protein (CRALBP) and beta-carotene 15,15'-monooxygenase (BCO)/RPE65 in a primitive chordate, ascidian Ciona intestinalis. The mRNAs for these proteins are specifically expressed in the central nervous system during embryonic development. In the larva, the transcripts were widely distributed in the brain vesicle and visceral ganglion. Since visual pigment, Ci-opsin1, is solely expressed in photoreceptor cells, the visual cycle in this primitive chordate may take place in two compartments, which are coupled into a cycle by the direct flow of retinoids though the intercellular matrix. The Ci-opsin3, an ascidian homolog of mammalian RGR, was expressed in HEK 293S cells and purified after binding of retinal. The chromophore of Ci-opsin3 is in an all-trans-retinal and it is isomerized to an 11-cis-form upon absorption of light. Mammalian CRALBP and BCO/RPE65 are believed to play critical roles in the process of reisomerization of all-trans-retinoid to 11-cis-retinoid in RPE. The present data suggest that isomerization of all-trans-retinoid to 11-cis-retinoid occurs in the brain vesicle and visceral ganglion of a primitive chordate.

Amino Acid Sequence↗
PubMed2003

Retinal ganglion cell projections to the hamster suprachiasmatic nucleus, intergeniculate leaflet, and visual midbrain: bifurcation and melanopsin immunoreactivity.

The circadian clock in the suprachiasmatic nucleus (SCN) receives direct retinal input via the retinohypothalamic tract (RHT), and the retinal ganglion cells contributing to this projection may be specialized with respect to direct regulation of the circadian clock. However, some ganglion cells forming the RHT bifurcate, sending axon collaterals to the intergeniculate leaflet (IGL) through which light has secondary access to the circadian clock. The present studies provide a more extensive examination of ganglion cell bifurcation and evaluate whether ganglion cells projecting to several subcortical visual nuclei contain melanopsin, a putative ganglion cell photopigment. The results showed that retinal ganglion cells projecting to the SCN send collaterals to the IGL, olivary pretectal nucleus, and superior colliculus, among other places. Melanopsin-immunoreactive (IR) ganglion cells are present in the hamster retina, and some of these cells project to the SCN, IGL, olivary pretectal nucleus, or superior colliculus. Triple-label analysis showed that melanopsin-IR cells bifurcate and project bilaterally to each SCN, but not to the other visual nuclei evaluated. The melanopsin-IR cells have photoreceptive characteristics optimal for circadian rhythm regulation. However, the presence of moderately widespread bifurcation among ganglion cells projecting to the SCN, and projection by melanopsin-IR cells to locations distinct from the SCN and without known rhythm function, suggest that this ganglion cell type is generalized, rather than specialized, with respect to the conveyance of photic information to the brain.

Animals↗
PubMed2006

Central projections of melanopsin-expressing retinal ganglion cells in the mouse.

A rare type of ganglion cell in mammalian retina is directly photosensitive. These novel retinal photoreceptors express the photopigment melanopsin. They send axons directly to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus (OPN), thereby contributing to photic synchronization of circadian rhythms and the pupillary light reflex. Here, we sought to characterize more fully the projections of these cells to the brain. By targeting tau-lacZ to the melanopsin gene locus in mice, ganglion cells that would normally express melanopsin were induced to express, instead, the marker enzyme beta-galactosidase. Their axons were visualized by X-gal histochemistry or anti-beta-galactosidase immunofluorescence. Established targets were confirmed, including the SCN, IGL, OPN, ventral division of the lateral geniculate nucleus (LGv), and preoptic area, but the overall projections were more widespread than previously recognized. Targets included the lateral nucleus, peri-supraoptic nucleus, and subparaventricular zone of the hypothalamus, medial amygdala, margin of the lateral habenula, posterior limitans nucleus, superior colliculus, and periaqueductal gray. There were also weak projections to the margins of the dorsal lateral geniculate nucleus. Co-staining with the cholera toxin B subunit to label all retinal afferents showed that melanopsin ganglion cells provide most of the retinal input to the SCN, IGL, and lateral habenula and much of that to the OPN, but that other ganglion cells do contribute at least some retinal input to these targets. Staining patterns after monocular enucleation revealed that the projections of these cells are overwhelmingly crossed except for the projection to the SCN, which is bilaterally symmetrical.

Animals↗
PubMed1989

Localization of L-glutamic acid decarboxylase mRNA in monkey and human retina by in situ hybridization.

Immunocytochemical studies with gamma-aminobutyric acid (GABA) antibodies and glutamic acid decarboxylase antibodies have shown that the primate retina contains GABAergic amacrine, interplexiform, and displaced amacrine cells. In addition, subpopulations of photoreceptors and horizontal cells have also been suggested to be GABAergic in this retina. In the present study, we have used in situ hybridization to localize GABAergic neurons in human and monkey retinas. In situ hybridizations were carried out with 35S-labeled DNA and RNA probes derived from human and cat glutamic acid decarboxylase cDNA clones. In the monkey retina, labeled cells were present in the inner nuclear and ganglion cells layers. The outer nuclear layer or the inner segment layer had only background levels of labeling. In the inner nuclear layer, all labeled somata were located in the vitread-half bordering the inner nuclear layer/inner plexiform layer boundary. These cells constituted approximately 83% of all labeled cells. Labeled cells were also seen in the ganglion cell layer. In the human retina, labeled somata were observed only in the inner nuclear and the ganglion cell layers. In the inner nuclear layer, the majority of labeled cells were located close to the inner nuclear layer/inner plexiform layer boundary although a minor population of labeled somata (approximately 20%) were found deeper in the inner nuclear layer. The distribution of glutamic acid decarboxylase mRNA-containing cells we observed is in good agreement with the known location of GABAergic neurons. We, however, did not find glutamic acid decarboxylase mRNA in either horizontal cells or photoreceptors in monkey and human retina.

Animals↗
PubMed1989

Fibroblast growth factor phosphorylation and receptors in rod outer segments.

Acidic and basic fibroblast growth factors (aFGF and bFGF) have been isolated and purified from rod outer segments (ROS). aFGF is tightly bound to ROS membranes and can be specifically released by ATP. We show that this mechanism is dependent on the phosphorylation of aFGF itself. Phorbol 12-myristate 13-acetate (PMA) enhances this phenomenon independently of rhodopsin phosphorylation. This demonstrates that aFGF release from ROS membranes is dependent on its phosphorylation by endogenous kinase C. In addition specific binding sites for exogenous FGFs have been identified on ROS and disc membranes. A single high affinity site with a Kd of 40 pM was present in intact ROS while an additional low affinity site with a Kd of 300-600 pM was present in leaky ROS or in disc membranes. Light or ATP modified neither these Kd nor the apparent number of sites. The presence of specific receptors for FGFs and the kinase C dependent release of endogenous membrane bound aFGF suggest an autocrine mechanism which may be involved in photoreceptor cell biology.

Animals↗
PubMed1992

The polymorphic photopigments of the marmoset: spectral tuning and genetic basis.

The marmoset (Callithrix jacchus jacchus), a South American monkey, is polymorphic for the middle- to long-wave cone photopigments: the three variant pigments have spectral peaks at 543, 556 and 563 nm. Comparisons of the deduced amino acid sequences of these pigments indicate that the variations in spectral sensitivity are associated with the presence or absence of hydroxyl-bearing residues at sites 180 and 285; but, in contrast to the additive hypothesis of Neitz et al. (1991), we propose that adjustments at site 233 may also be required to produce viable long-wave and middle-wave pigments. Within a family group of monkeys, we find that a restriction site polymorphism in the photopigment gene segregates in a way that is consistent with the single X-linked gene hypothesis previously proposed on the basis of the photopigment types present in male and female marmosets.

Amino Acid Sequence↗
PubMed2005

Angiopoietin 1 prevents retinal detachment in an aggressive model of proliferative retinopathy, but has no effect on established neovascularization.

Vascular endothelial growth factor (VEGF) plays a central role in vasoproliferative diseases in the retina, however, other gene products modulate its effects. The angiopoietins are particularly important in this regard. Angiopoietin 2 (Ang2) collaborates with VEGF to stimulate neovascularization (NV) in some situations, but in other situations causes regression of NV. Ang2 also causes a transient increase in vascular density during retinal vascular development. In this study, we sought to determine if Ang1 has similar activities. The effects of Ang1 were tested in double transgenic mice with inducible expression of Ang1. Increased expression of Ang1 in the retina during retinal vascular development did not cause a detectable alteration in vascular density. Also, unlike Ang2, increased expression of Ang1 had no effect on established retinal or choroidal NV. However, when Ang1 expression was initiated simultaneously with that of VEGF, it strongly suppressed VEGF-induced NV and prevented retinal detachment. These data indicate that the timing of Ang1 expression is a critical determinate of its effects on VEGF-induced NV in the retina; it effectively blocks the initiation and progression of NV, but cannot reverse established NV or reduce leakage from NV. These data suggest that increased expression of Ang1 may be a good strategy for prophylaxis of retinal NV, but is unlikely to be effective as monotherapy of established NV.

Angiopoietin-1↗
PubMed2001

Photoreception and circadian clock system of the chicken pineal gland.

Chicken pinealocytes contain three major components of the circadian clock system: 1) a self-sustained oscillator, 2) a photic-input pathway to the oscillator, and 3) an overt output represented by the rhythmic production of melatonin. Even under cultured conditions of isolated pineal gland or dissociated pinealocytes, the input-oscillator-output functions are well maintained. Because of these experimental advantages, chicken pineal gland has been one of the best models for the study of the circadian clock system. Since the finding of a pineal-specific photoreceptive molecule, pinopsin, we have characterized the endogenous phototransduction pathway in the pinealocytes. On the other hand, despite the long history of chick pineal research, the molecular mechanism underlying the pineal clock oscillation has been largely unknown. Our recent characterization of the chick pineal clock genes strongly suggests that they constitute a transcription/translation-based autoregulatory feedback loop, which is very similar to that generating circadian rhythmicity in mammalian SCN.

Animals↗
PubMedDate not supplied

Prenatal human ocular degeneration occurs in Leber's congenital amaurosis (LCA2).

BACKGROUND: Leber's congenital amaurosis (LCA) encompasses the most precocious and severe forms of inherited retinal dystrophy, displaying very significant visual handicap at or soon after birth. Among the currently identified mutations, alterations in the gene coding for retinal pigment epithelium 65-kDa protein (RPE65) lead to LCA2. Existing animal models for LCA2 (RPE65(-/-) null mice and naturally occurring RPE65(-/-) Briard dogs) exhibit near normal retinal histology at birth, although no recordable photofunction can be detected. Structural degeneration in both cases occurs with delayed onset, cone death generally preceding that of rods. METHODS: We obtained retinal tissue from a voluntarily aborted embryo of an LCA2 carrier in order to compare histopathology and immunohistochemistry with age-matched normal foetal retina. RESULTS: Compared to normal retinas, affected retina displayed cell loss and thinning of the outer nuclear (photoreceptor) layer, decreased immunoreactivity for key phototransduction proteins, and aberrant synaptic and inner retinal organisation. The gene mutation abolished detectable expression of RPE65 within the retinal pigment epithelium (RPE) of affected eyes, and ultrastructural examination revealed the presence of lipid and vesicular inclusions not seen in normal RPE. In addition, mutant eyes demonstrated thickening, detachment and collagen fibril disorganisation in the underlying Bruch's membrane, and the choroid was distended and abnormally vascularised, in comparison with controls. CONCLUSIONS: Such data contrast with the late-onset ocular changes observed in animal models, indicating caution should be exercised when inferring human retinal pathophysiology from information based on other species.

Aborted Fetus↗
PubMed2004

Hedgehog and Fgf signaling pathways regulate the development of tphR-expressing serotonergic raphe neurons in zebrafish embryos.

Serotonin (5HT) plays major roles in the physiological regulation of many behavioral processes, including sleep, feeding, and mood, but the genetic mechanisms by which serotonergic neurons arise during development are poorly understood. In the present study, we have investigated the development of serotonergic neurons in the zebrafish. Neurons exhibiting 5HT-immunoreactivity (5HT-IR) are detected from 45 h postfertilization (hpf) in the ventral hindbrain raphe, the hypothalamus, pineal organ, and pretectal area. Tryptophan hydroxylases encode rate-limiting enzymes that function in the synthesis of 5HT. As part of this study, we cloned and analyzed a novel zebrafish tph gene named tphR. Unlike two other zebrafish tph genes (tphD1 and tphD2), tphR is expressed in serotonergic raphe neurons, similar to tph genes in mammalian species. tphR is also expressed in the pineal organ where it is likely to be involved in the pathway leading to synthesis of melatonin. To better understand the signaling pathways involved in the induction of the serotonergic phenotype, we analyzed tphR expression and 5HT-IR in embryos in which either Hh or Fgf signals are abrogated. Hindbrain 5HT neurons are severely reduced in mutants lacking activity of either Ace/Fgf8 or the transcription factor Noi/Pax2.1, which regulates expression of ace/fgf8, and probably other genes encoding signaling proteins. Similarly, serotonergic raphe neurons are absent in embryos lacking Hh activity confirming a conserved role for Hh signals in the induction of these cells. Conversely, over-activation of the Hh pathway increases the number of serotonergic neurons. As in mammals, our results are consistent with the transcription factors Nk2.2 and Gata3 acting downstream of Hh activity in the development of serotonergic raphe neurons. Our results show that the pathways involved in induction of hindbrain serotonergic neurons are likely to be conserved in all vertebrates and help establish the zebrafish as a model system to study this important neuronal class.

Animals↗
PubMed2005

Prediction of interfaces for oligomerizations of G-protein coupled receptors.

Several lines of biochemical and pharmacological evidence have suggested that some G-protein-coupled receptors (GPCRs) form homo oligomers, hetero oligomers or both. The GPCRs oligomerizations are considered to be related to signal transduction and some diseases. Therefore, an accurate prediction of the residues that interact upon oligomerization interface would further our understanding of signal transduction and the diseases in which GPCRs are involved. One of the complications for such a prediction is that the interfaces differ with the subtypes, even within the same GPCR family. Focusing on the distribution of residues conserved on the molecular surface in a particular subtype, we developed a new method to predict the interface for the GPCR oligomers, and applied it to several subtypes of known GPCRs to check the sensitivity. Subsequently, we found that predicted interfaces of rhodopsin, D(2) dopamine receptor and beta(2) adrenergic receptor agreed with the experimentally suggested interfaces, despite difference in the interface region among the three subtypes. Moreover, a highly conserved residue detected from the D(2) dopamine receptor corresponded to a residue involved in a missense change found in the large family of myoclonus dystonia. Our observation suggests the possibility that the disease is caused by the disorder of the oligomerization, although the molecular mechanism of the disease has not been revealed yet. The benefits and the pitfalls of the new method will be discussed, based on the results of the applications.

Algorithms↗
PubMed1999

Activin family members in the developing chick retina: expression patterns, protein distribution, and in vitro effects.

We have investigated whether the activin family of growth factors is involved in the regulation of retinal cell differentiation. Immunocytochemistry and in situ hybridization have shown that activin/inhibin subunits alpha, betaA, and betaB; receptors II and IIB; follistatin; and a follistatin-like gene are expressed in different regions of the chick embryo retina in developmentally regulated patterns. When tested in dissociated retinal cultures, activin did not appear to affect cell survival or proliferation, but it exerted marked inhibitory effects on the differentiation of photoreceptors, while stimulating the differentiation of nonphotoreceptor neurons; both effects were concentration-dependent and follistatin-sensitive. The results are consistent with the possibility that activin family members play significant roles in the regulation of retinal development.

Activin Receptors↗