Immunocytochemical localization of opsin, visual arrestin, myosin III, and calmodulin in Limulus lateral eye retinular cells and ventral photoreceptors

The photoreceptors of the horseshoe crab Limulus polyphemus are classical preparations for studies of the photoresponse and its modulation by circadian clocks. An extensive literature details their physiology and ultrastructure, but relatively little is known about their biochemical organization largely because of a lack of antibodies specific for Limulus photoreceptor proteins. We developed antibodies directed against Limulus opsin, visual arrestin, and myosin III, and we have used them to examine the distributions of these proteins in the Limulus visual system. We also used a commercial antibody to examine the distribution of calmodulin in Limulus photoreceptors. Fixed frozen sections of lateral eye were examined with conventional fluorescence microscopy; ventral photoreceptors were studied with confocal microscopy. Opsin, visual arrestin, myosin III, and calmodulin are all concentrated at the photosensitive rhabdomeral membrane, which is consistent with their participation in the photoresponse. Opsin and visual arrestin, but not myosin III or calmodulin, are also concentrated in extra-rhabdomeral vesicles thought to contain internalized rhabdomeral membrane. In addition, visual arrestin and myosin III were found widely distributed in the cytosol of photoreceptors, suggesting that they have functions in addition to their roles in phototransduction. Our results both clarify and raise new questions about the functions of opsin, visual arrestin, myosin III, and calmodulin in photoreceptors and set the stage for future studies of the impact of light and clock signals on the structure and function of photoreceptors.

Cellular distributions and functions of histamine, octopamine, and serotonin in the peripheral visual system, brain, and circumesophageal ring of the horseshoe crab Limulus polyphemus

The data reviewed here show that histamine, octopamine, and serotonin are abundant in the visual system of the horseshoe crab Limulus polyphemus. Anatomical and biochemical evidence, including new biochemical data presented here, indicates that histamine is a neurotransmitter in primary retinal afferents, and that it may be involved in visual information processing within the lateral eye. The presence of histamine in neurons of the central nervous system outside of the visual centers suggests that this amine also has functions unrelated to vision. However, the physiological actions of histamine in the Limulus nervous system are not yet known. Octopamine is present in and released from the axons of neurons that transmit circadian information from the brain to the eyes, and octopamine mimics the actions of circadian input on many retinal functions. In addition, octopamine probably has major functions in other parts of the nervous system as octopamine immunoreactive processes are widely distributed in the central nervous system and in peripheral motor nerves. Indeed, octopamine modulates functions of the heart and exoskeletal muscles as well as the eyes. A surprising finding is that although octopamine is a circulating neurohormone in Limulus, there is no structural evidence for its release into the hemolymph from central sites. The distribution of serotonin in Limulus brain suggests this amine modulates the central processing of visual information. Serotonin modulates cholinergic synapses in the central nervous system, but nothing further is known about its physiological actions.

A myosin III from Limulus eyes is a clock-regulated phosphoprotein

The lateral eyes of the horseshoe crab Limulus polyphemus undergo dramatic daily changes in structure and function that lead to enhanced retinal sensitivity and responsiveness to light at night. These changes are controlled by a circadian neural input that alters photoreceptor and pigment cell shape, pigment migration, and phototransduction. Clock input to the eyes also regulates photomechanical movements within photoreceptors, including membrane shedding. The biochemical mechanisms underlying these diverse effects of the clock on the retina are unknown, but a major biochemical consequence of activating clock input to the eyes is a rise in the concentration of cAMP in photoreceptors and the phosphorylation of a 122 kDa visual system-specific protein. We have cloned and sequenced cDNA encoding the clock-regulated 122 kDa phosphoprotein and show here that it is a new member of the myosin III family. We report that Limulus myosin III is similar to other unconventional myosins in that it binds to calmodulin in the absence of Ca2+; it is novel in that it is phosphorylated within its myosin globular head, probably by cAMP-dependent protein kinase. The protein is present throughout the photoreceptor, including the region occupied by the photosensitive rhabdom. We propose that the phosphorylation of Limulus myosin III is involved in one or more of the structural and functional changes that occur in Limulus eyes in response to clock input.

Calcium/calmodulin-dependent protein kinase II and arrestin phosphorylation in Limulus eyes

In rhabdomeral photoreceptors, light stimulates the phosphorylation of arrestin, a protein critical for quenching the photoresponse, by activating a calcium/calmodulin-dependent protein kinase (CaM PK). Here we present biochemical evidence that a CaM PK that phosphorylates arrestin in Limulus eyes is structurally similar to mammalian CaM PK II. In addition, cDNAs encoding proteins homologous to mammalian and Drosophila CaM PK II in the catalytic and regulatory domains were cloned and sequenced from a Limulus lateral eye cDNA library. The Limulus sequences are unique, however, in that they lack most of the association domain. The proteins encoded by these sequences may phosphorylate arrestin.

Pattern of vitellogenesis and follicle maturational competence during the ovarian follicular cycle of Fundulus heteroclitus

The patterns of vitellogenesis and follicle maturational competence were examined across the semilunar spawning cycle of Fundulus heteroclitus. Daily egg collection showed spawning cyclicity in six experimental groups, with a mean period between spawnings of 14.9 +/- 0.3 days, indicated by the nonlinear regression sine-curve matching analysis. Each cycle was then dated from Day -7 to Day 7, with Day 0 as the peak egg-collection day. Females from each group were sampled on selected days during two to three consecutive spawning cycles, and these days were each chronologically given a temporal relation to Day 0 to pool the data into a composite. The analysis of the size-frequency distribution of ovarian follicles > or = 0.5-mm diameter across the composite revealed a constant recruitment of small follicles (0.5- to 0.7-mm diameter) into vitellogenesis, which was supported by the continuous presence of vitellogenin (Vtg) I mRNA in the liver of the females. The plasma levels of Vtg were also essentially constant across the cycle, except for a progressive decrease from Day -7 through Day 3 that could be related to a more active Vtg uptake by a dominant population of follicles up to 1.7 mm in diameter. A second and more selective recruitment of full-grown follicles (1.3- to 1.4-mm diameter) toward maturation was noted at Days -5, -4, which appeared associated with high plasma levels of estradiol-17 beta. However, the responsiveness of those follicles undergoing oocyte maturation in vitro after gonadotropin and maturation-inducing steroid (MIS), 17, 20 beta-dihydroxy-4-pregnen-3-one, stimulation dramatically declined from Days -1, 0, 1 to Days 4, 5, concomitantly with an increase of the population of the largest follicles (1.8- to 2.1-mm diameter) in the ovary. These findings extend previous observations on the process of follicular recruitment in F. heteroclitus and indicate that full-grown follicles may be recruited into maturation by a mechanism that modulates the oocyte sensitivity to the MIS.

Isolation and expression of an arrestin cDNA from the horseshoe crab lateral eye

Electrophysiological studies of photoreceptors from the horseshoe crab Limulus polyphemus continue to provide fundamental new knowledge of the photoresponse in invertebrates. Therefore, it is of particular interest to characterize the molecular components of the photoresponse in this system. Here we describe an arrestin cloned from a cDNA library constructed using poly(A)+ RNA isolated from Limulus lateral eyes. The protein, deduced from the arrestin cDNA, is most similar to arrestin from locust antennae (56% identity) and Drosophila phosrestin I (53% identity). Limulus arrestin was expressed in a heterologous system, and its properties were compared with those of a 46-kDa light-regulated phosphoprotein (pp46A) in Limulus photoreceptors described in previous studies from this laboratory. Arrestin and pp46A (a) have the same apparent molecular weight on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, (b) have an isoelectric point in the basic pH range, (c) require calmodulin and elevated Ca2+ levels for phosphorylation, (d) are immunoreactive with monoclonal antibody C10C10 directed against a sequence in bovine arrestin (S-antigen) that is perfectly conserved in the deduced arrestin protein, and (e) are associated with photoreceptors. We conclude that the arrestin described here and pp46A are the same protein. The results of this and previous studies show that in Limulus photoreceptors, light regulates the phosphorylation of arrestin in complex ways.

Localization of actin filaments and microtubules in the cells of the Limulus lateral and ventral eyes

The ommatidia of the lateral eye of the horseshoe crab, Limulus polyphemus, undergo rhythmic changes in structure that are driven by diurnal lighting and efferent neural activity from a circadian clock in the brain. This study uses cytochemical probes to investigate the cytoskeletal elements mediating these responses and to develop models for their control. Antibodies to actin and phallodin, a specific F-actin probe, label the rhabdom of lateral eye ommatidia, the cone cells of the ommatidial aperture, the ommatidial sheath, and the peripheral regions of the photoreceptor (retinular cell) cytoplasm. These probes also label the rhabdomere of ventral photoreceptors. Antibodies to tubulin label the eccentric cell dendrite and soma in each lateral eye ommatidium, the cone cells of the aperture, and the peripheral retinular cell cytoplasm. Models are proposed for the cytoskeletal mechanisms involved in controlling aperture and rhabdom shape, pigment movement, and shedding of rhabdomeral membrane.

Central projections of Limulus photoreceptor cells revealed by a photoreceptor-specific monoclonal antibody

Studies of lateral, median, and ventral eyes of the chelicerate arthropod Limulus polyphemus (the common American horseshoe crab) are providing important basic information about mechanisms for information processing in the peripheral visual system and for the modulation of visual responses by light and circadian rhythms. The processing of visual information in Limulus brain is less well understood in part because the specific central projections of the various classes of visual neurons are not known. This study describes a mouse monoclonal antibody, 3C6A3, which binds to Limulus photoreceptor cell bodies, their axons, and terminals, but not to any other cell type in the central nervous system. This antibody, and intracellular injection of biocytin, are used to demonstrate the central projections of each type of photoreceptor. Our main conclusions are that: 1) the photoreceptors (retinular cells) of the lateral eye project only to the lamina; 2) the photoreceptors of the lateral rudimentary eye project to both the lamina and medulla; 3) the photoreceptors of the median ocellus project only to the ocellar ganglion; and 4) the photoreceptors of the rudimentary median (endoparietal) eye project to the ocellar ganglion and also into the optic tract. These results, along with previous studies, allow us to infer the projections of the secondary cells. The eccentric cells of the lateral eye project to the lamina, medulla, optic tract, and ocellar ganglion. The arhabdomeral cells of the median ocellus project through the ocellar ganglion and to optic tract to the medulla.

Central origin of the efferent neurons projecting to the eyes of Limulus polyphemus

Circadian rhythms affect the anatomy, physiology, and biochemistry of the visual cells in the eyes of the horseshoe crab (Limulus polyphemus). These rhythms are mediated by the activity of efferent neurons that project from the central nervous system to all of the eyes. In this study, the optic nerves of Limulus were backfilled with Neurobiotin revealing the location of efferent cell bodies and their projections through the central nervous system. We propose that this efferent system mediates the circadian changes in visual functions in Limulus. Whether these cells are the circadian pacemaker neurons is unknown. The cell bodies of the efferent neurons are ovoid and have a diameter of 40-80 microns. They lie within the cheliceral ganglion of the tritocerebrum, just posterior to the protocerebrum. This ganglion is on the lateral edge of the circumesophageal ring, near the middle of the dorsal-ventral axis of the ring. Each optic nerve contains axons from both ipsilateral and contralateral efferent cells, and some, possibly all, of them project bilaterally and to more than one type of optic nerve. The efferent axons form a tract that projects anteriorly from the cell bodies to the protocerebrum, and bifurcates just lateral to the protocerebral bridge. One branch crosses the midline and projects anteriorly to the optic tract and medulla on the side contralateral to the cell of origin; the other branch follows a symmetric pathway on the ipsilateral side. Small branches arising from the major efferent axons in the optic tract project through the ocellar ganglia to the median optic nerves. The efferent axons branch again in the medulla, and some of these branches innervate the ventral optic nerves. The major branches of the efferent axons continue through the lamina and enter the lateral optic nerve.

Histamine: a putative afferent neurotransmitter in Limulus eyes

Histamine has been proposed as a photoreceptor neurotransmitter in two major groups of arthropods, the insects and the crustacea. In this study biochemical and immunocytochemical approaches were used to examine the synthesis, endogenous content, and cellular distribution of histamine in the visual system of the horseshoe crab Limulus polyphemus, an ancient chelicerate arthropod. Studies with this animal have been critical to our understanding of the basic processes of vision. High-voltage paper electrophoresis was used to assay for histamine synthesis in Limulus tissues incubated with radiolabeled histidine; histamine synthesis was detected in the lateral, median, and ventral eyes and optic nerves and in the visual centers in the brain. Endogenous histamine, assayed as its orthophthalaldehyde derivative by high-performance liquid chromatography and electrochemical detection, was also detected in these tissues. Immunocytochemical analyses, with an antiserum directed against a protein conjugate of histamine, revealed histamine-like immunoreactivity in the somata of photoreceptors in each of the eyes and in the regions of the brain where the photoreceptors terminate. Histamine-like immunoreactivity was also intense in the cell bodies and axon collaterals of eccentric cells in the lateral eye and in eccentric cell projections in the brain. These results show that histamine is a major biogenic amine in the Limulus visual system, and they suggest that this amine is involved in transmitting visual information from the eyes to the brain and in lateral inhibition, a fundamental mechanism for processing visual information in the lateral eye.

Subcellular localization of neutral red staining in Limulus ventral photoreceptors

Limulus ventral photoreceptors are vitally stained by neutral red. In other systems such staining has been correlated with the presence of monoamines or neuropeptides. The stained cellular components in ventral photoreceptors are clusters of small ovoids which have been identified as residual bodies. These structures are unlikely candidates for monoamine or neuropeptide synthesis or storage sites, but may be part of the cyclic synthesis and degradation of photosensitive membrane. While vital staining with neutral red is a particularly useful method for identifying certain classes of neurons in vivo, in the case of ventral photoreceptors, the association of the vital staining property with the presence of a particular class of neurotransmitter candidates has proven difficult. Neutral red is useful, however, for visualizing the segmentation of ventral photoreceptors in vivo.

Distinct lobes of Limulus ventral photoreceptors. II. Structure and ultrastructure

The structure of Limulus ventral photoreceptors fixed in situ has been investigated using light and electron microscopy and computer-assisted reconstruction and planimetry. Photoreceptors occur singly and in clusters. All photoreceptors have two types of lobes. The rhabdomeral lobe (R lobe) appears to be specialized for light sensitivity, containing the rhabdomere, which completely covers its external surface and forms infoldings into the lobe. The structure of the external rhabdom differs from that within infoldings. The other main structures of the R lobe are the palisades along the rhabdom, multivesicular bodies, lamellar bodies, and mitochondria. The arhabdomeral lobe (A lobe) bears the axon and contains the nucleus, clusters of residual bodies, lamellar arrays of endoplasmic reticulum, masses of glycogen, lipid droplets, and Golgi complexes. The R lobe and A lobe are analogous to the outer and inner segments of vertebrae photoreceptors. In single photoreceptors A and R lobes are separated by an indentation filled with glial processes. Computer reconstructions of cell clusters reveal that each cell has both types of lobes and an axon. Most of the rhabdom is formed from abutting arrays of external rhabdom from the R lobes of different members of the cluster. Efferent fibers containing characteristic angular granules penetrate single cells and clusters in glial invaginations. The main, if not exclusive, target of the efferent fibers is the internal rhabdom.