Rhabdomeric Structures Research Articles

Ectopic Expression of Mouse Melanopsin in Drosophila Photoreceptors Reveals Fast Response Kinetics and Persistent Dark Excitation

The intrinsically photosensitive M1 retinal ganglion cells (ipRGC) initiate non-image-forming light-dependent activities and express the melanopsin (OPN4) photopigment. Several features of ipRGC photosensitivity are characteristic of fly photoreceptors. However, the light response kinetics of ipRGC is much slower due to unknown reasons. Here we used transgenic Drosophila, in which the mouse OPN4 replaced the native Rh1 photopigment of Drosophila R1-6 photoreceptors, resulting in deformed rhabdomeric structure. Immunocytochemistry revealed OPN4 expression at the base of the rhabdomeres, mainly at the rhabdomeral stalk. Measurements of the early receptor current, a linear manifestation of photopigment activation, indicated large expression of OPN4 in the plasma membrane. Comparing the early receptor current amplitude and action spectra between WT and the Opn4-expressing Drosophila further indicated that large quantities of a blue absorbing photopigment were expressed, having a dark stable blue intermediate state. Strikingly, the light-induced current of the Opn4-expressing fly photoreceptors was ∼40-fold faster than that of ipRGC. Furthermore, an intense white flash induced a small amplitude prolonged dark current composed of discrete unitary currents similar to the Drosophila single photon responses. The induction of prolonged dark currents by intense blue light could be suppressed by a following intense green light, suggesting induction and suppression of prolonged depolarizing afterpotential. This is the first demonstration of heterologous functional expression of mammalian OPN4 in the genetically emendable Drosophila photoreceptors. Moreover, the fast OPN4-activated ionic current of Drosophila photoreceptors relative to that of mouse ipRGC, indicates that the slow light response of ipRGC does not arise from an intrinsic property of melanopsin.

Loss of Tau results in defects in photoreceptor development and progressive neuronal degeneration in Drosophila.

Accumulations of Tau, a microtubule-associated protein (MAP), into neurofibrillary tangles is a hallmark of Alzheimer's disease and other tauopathies. However, the mechanisms leading to this pathology are still unclear: the aggregates themselves could be toxic or the sequestration of Tau into tangles might prevent Tau from fulfilling its normal functions, thereby inducing a loss of function defect. Surprisingly, the consequences of losing normal Tau expression in vivo are still not well understood, in part due to the fact that Tau knockout mice show only subtle phenotypes, presumably due to the fact that mammals express several MAPs with partially overlapping functions. In contrast, flies express fewer MAP, with Tau being the only member of the Tau/MAP2/MAP4 family. Therefore, we used Drosophila to address the physiological consequences caused by the loss of Tau. Reducing the levels of fly Tau (dTau) ubiquitously resulted in developmental lethality, whereas deleting Tau specifically in neurons or the eye caused progressive neurodegeneration. Similarly, chromosomal mutations affecting dTau also caused progressive degeneration in both the eye and brain. Although photoreceptor cells initially developed normally in dTau knockdown animals, they subsequently degenerated during late pupal stages whereas weaker dTau alleles caused an age-dependent defect in rhabdomere structure. Expression of wild type human Tau partially rescued the neurodegenerative phenotype caused by the loss of endogenous dTau, suggesting that the functions of Tau proteins are functionally conserved from flies to humans.

Soft Anatomy of the Early Cambrian ArthropodIsoxys curvirostratusfrom the Chengjiang Biota of South China with a Discussion on the Origination of Great Appendages

An updated reconstruction of the body plan, functional morphology and lifestyle of the arthropod Isoxys curvirostratus is proposed, based on new fossil specimens with preserved soft anatomy found in several localities of the Lower Cambrian Chengjiang Lagerstatte. The animal was 2–4 cm long and mostly encased in a single carapace which is folded dorsally without an articulated hinge. The attachment of the body to the exoskeleton was probably cephalic and apparently lacked any well-developed adductor muscle system. Large stalked eyes with the eye sphere consisting of two layers (as corneal and rhabdomeric structures) protrude beyond the anterior margin of the carapace. This feature, together with a pair of frontal appendages with five podomeres that each bear a stout spiny outgrowth, suggests it was raptorial. The following 14 pairs of limbs are biramous and uniform in shape. The slim endopod is composed of more than 7 podomeres without terminal claw and the paddle shaped exopod is fringed with at least 17 ...

Cofilin/ADF is required for retinal elongation and morphogenesis of the Drosophila rhabdomere

Drosophila photoreceptors undergo marked changes in their morphology during pupal development. These changes include a five-fold elongation of the retinal cell body and the morphogenesis of the rhabdomere, the light sensing structure of the cell. Here we show that twinstar ( tsr), which encodes Drosophila cofilin/ADF (actin-depolymerizing factor), is required for both of these processes. In tsr mutants, the retina is shorter than normal, the result of a lack of retinal elongation. In addition, in a strong tsr mutant, the rhabdomere structure is disorganized and the microvilli are short and occasionally unraveled. In an intermediate tsr mutant, the rhabdomeres are not disorganized but have a wider than normal structure. The adherens junctions connecting photoreceptor cells to each other are also found to be wider than normal. We propose, and provide data supporting, that these wide rhabdomeres and adherens junctions are secondary events caused by the inhibition of retinal elongation. These results provide insight into the functions of the actin cytoskeleton during morphogenesis of the Drosophila eye.

The nauplius eye complex in ‘conchostracans’(Crustacea, Branchiopoda: Laevicaudata, Spinicaudata, Cyclestherida) and its phylogenetic implications

The nauplius eye in Cyclestherida, Laevicaudata and Spinicaudata (previously collectively termed Conchostraca) consists of four cups of inverse sensory cells separated by a pigment layer and a tapetum layer. There are two lateral and two medial cups, a ventral medial cup and a posterior medial cup. The pigment and tapetum layers contain two different kinds of pigment granules, the inner pigment layer relatively large, dark (and electron dense) granules, and the outer tapetum layer light, reflective pigment granules. The presence of four cups and two different kinds of pigment granules are interpreted as autapomorphies of Phyllopoda. The position and shape of the nauplius eye in Spinicaudata is very distinct and herein interpreted as an autapomorphy of this taxon. Additional frontal eyes might be present dorsally or ventrally in varying proximity to the nauplius eye, but they have separate nerves from their sensory cells to the nauplius eye centre in the protocerebrum. Rhabdomeric structures are present in all these frontal eyes, evidencing their light sensitivity. In Lynceus biformis and L. tatei (Laevicaudata), two pairs of frontal eyes were found. In Cyclestheria hislopi (Cyclestherida), an unpaired ventral frontal eye is present. We did not find additional frontal eyes in Limnadopsis parvispinus and Caenestheriella sp. (Spinicaudata).

The Drosophila rhodopsin cytoplasmic tail domain is required for maintenance of rhabdomere structure

The ninaE-encoded Rh1 rhodopsin is the major light-sensitive pigment expressed in Drosophila R1-6 photoreceptor cells. Rh1 rhodopsin localizes to and is essential for the development and maintenance of the rhabdomere, the specialized membrane-rich organelle that serves as the site of phototransduction. We showed previously that the vertebrate bovine rhodopsin (Rho) is expressed and properly localized in Drosophila photoreceptor cells. Drosophila photoreceptors expressing only Rho have normal rhabdomere structure at young ages, but the rhabdomeres are not maintained and show extensive disorganization by 7-10 days of age. A series of Rho-Rh1 opsin chimeric rhodopsins were used to identify Rh1 domains required for maintenance of rhabdomeric structure. The results show that the Rh1 rhodopsin cytoplasmic tail domain, positioned to interact with cytoplasmic structural components, plays a major role in promoting rhabdomeric organization.

Ultrastructure and localization of a visual Gq protein in ied epitoke ocelli of Perinereis brevicirris (Polychaeta, Annelida)

Functional ultrastructural changes in the rhabdomeric photoreceptors of the cerebral ocelli are described for normal and sexually mature (epitoke) Perinereis brevicirris (Polychaeta, Annelida). With sexual maturation, the cerebral ocelli hypertrophied, increasing in volume to 5.5 times that of ocelli in the normal state, and the thickness of the retinal layer increased up to 10 times. Perinereis ocelli have a pigmented retinal layer consisting of at least two cell types: photoreceptor cell (PR) and pigmented supporting cells (PS). In epitoke ocelli, PR bear well-developed rhabdomeric microvilli, multilamellar bodies, and numerous cytoplasmic membranous structures, including vesicles, smooth endoplasmic reticulum, and secondary lysosomes. Localization of a visual Gq protein in the ocelli was studied with anti-GqC antibody. The antibody strongly labeled not only microvilli and multilamellar bodies throughout the retinal layer, but also secondary lysosomes and vesicles in the cytoplasm of the PR in the epitoke ocelli, although labeling was observed only in the microvilli and multilamellar bodies in normal ocelli. Reverse transcription/polymerase chain reaction analysis revealed that the amount of G protein alpha subunit mRNA in the epitoke head increased by roughly twice that of the normal head. Since Gq protein is essential for phototransduction in Perinereis ocelli, these results suggest that the sites are involved in photoreceptive membrane turnover, which occurs much more extensively in epitoke ocelli. Thus, epitoke ocelli may represent a model system for studying rhabdomeric photoreceptive membrane turnover.

A new type of arthropod photoreceptor

A new type of photoreceptor for the phylum Arthropoda, found in the class Collembola (Arthropoda, Hexapoda) is reported. This new light-sensitive structure consists of a pair of interocular vesicles present in the genus Vesicephalus Richards, 1964 and is anatomically related to the cluster of ommatidia. The absence of a lens, the presence of a rabdome in the upper part of the vesicle and the reflection and refraction of light by a hemolymph bubble with incidence to the rhabdomeric structure are the main traits of this new photoreceptor.

Cerebral extraocular photoreceptors in beetles

Cell clusters containing rhabdomeric structures and screening pigments are located in the optic lobes of pupae and adult beetles of Tenebrio molitor and Hylotrupes bajulus. Due to their resemblance with the compound eyes, the cell clusters were called cerebral extraocular photoreceptors (CEPs) and can readily be detected macroscopically as black spots spread between the lamina and medulla. CEPs of T. molitor responded to bright light with a transient depolarisation followed by a sustained plateau. In contrast, CEPs of H. bajulus responded to bright light either with small depolarisations or did not respond to the range of light intensities presented. CEP cells labelled with Lucifer yellow had axons projecting toward the medullar region. The maximum spectral sensitivity of CEPs was broadly tuned to the green region ( λ max at 524 nm). The role of CEPs in insects is still a matter of wide speculation, due to the lack of consistent systematic behavioral studies about insects carrying photoreactive CEPs. Here we suggest that the scattered distribution of CEPs in the optic lobes could allow a temporal pooling of information about the surrounding illumination, thereby entraining some rhythmic photobehavioural responses.

Nonsense suppression of the major rhodopsin gene of Drosophila.

We placed UAA, UAG and UGA nonsense mutations at two leucine codons, Leu205 and Leu309, in Drosophila's major rhodopsin gene, ninaE, by site-directed mutagenesis, and then created the corresponding mutants by P element-mediated transformation of a ninaE deficiency strain. In the absence of a genetic suppressor, flies harboring any of the nonsense mutations at the 309 site, but not the 205 site, show increased rhodopsin activity. Additionally, all flies with nonsense mutations at either site have better rhabdomere structure than does the ninaE deficiency strain. Construction and analysis of a 3'-deletion mutant of ninaE indicates that translational readthrough accounts for the extra photoreceptor activity of the ninaE309 alleles and that truncated opsins are responsible for the improved rhabdomere structure. The presence of leucine-inserting tRNA nonsense suppressors DtLa Su+ and DtLb Su+ in the mutant strains produced a small increase (less than 0.04%) in functional rhodopsin. The opal (UGA) suppressor derived from the DtLa tRNA gene is more efficient than the amber (UAG) or opal suppressor derived from the DtLb gene, and both DtLa and DtLb derived suppressors are more efficient at site 205 than 309.

Developmental and physiological analysis of a conditional mutation affecting photoreceptor and optic lobe development in Drosophila melanogaster.

Developmental studies have shown that elavJ1 mutants are temperature-sensitive for defects in retinula cell and optic lobe structure. Anatomical and physiological examination of mutants, shifted between permissive and restrictive temperatures, shows that the early pupal period is phenocritical for producing defects in retinula cells. Although this period precedes visible development of the rhabdomere, brief pulses to 29 degrees C during this time result in characteristic abnormalities in rhabdomere structure. These studies also show that the temperature-sensitive effects on mutant optic lobe development begin prior to the effects on the retina, suggesting that the elav product participates directly in optic lobe as well as in retinula cell development.

Rhabdome-bearing cells within the corpus cardiacum of Melanogryllus desertus (Orthoptera, Gryllidae)

Cells with rhabdomeric structures characteristic of photoreceptor cells occur within the corpus cardiacum of Melanogryllus desertus. These cells are few in number and are larger than intrinsic secretory cells. Each has numerous rhabdomeric microvilli forming a rhabdome within the cell. The arrangement of large mitochondria under the rhabdomeric microvilli around the dilated endoplasmic reticulum membranes and multivesiculated bodies is distinctive. Rhabdome cells have synaptic contacts with the numerous neurosecretory axons that surround them. Other neurosecretory axons come in close contact with the membranes of microvilli.

The fine structure of the compound eye of the African armyworm moth, Spodoptera exempta Walk. (Lepidoptera, Noctuidae).

The morphology of the compound eye of the noctuid moth Spodoptera exempta was investigated by electron microscopy. This eucone superposition eye is composed of about 8000 ommatidia. Each ommatidium is surrounded by six secondary pigment cells showing pigment movement according to the state of adaptation. It contains four crystalline cone cells forming together a crystalline cone and tract, two primary pigment cells, which encompass the crystalline cone, and usually eight retinula cells. On the basis of their rhabdomeric structure, three types of retinula cells can be distinguished. According to the structure of the rhabdom, two types of ommatidia are found in different regions of the eye. The rhabdom of the lobed type, providing more than 80% of ommatidia, is composed of V-shaped rhabdomeres with fanwise arranged microvilli. The rhabdom of the square type, found in a small area in the dorsal region of the eye, consists of triangular rhabdomeres with parallel microvilli. The functional significance of this difference is discussed.

New aspects of polarized light detection in the bee in view of non-twisting rhabdomeric structures

Polarization orientation in the bee is limited to short wavelengths (Frisch, 1967; Heiversen and Edrich, 1974). Although each ommatidium contains three UV-receptors (two long and a short retinula cell) only the short and proximally located ninth retinula cell has a high sensitivity to the plane of polarized light (Menzel and Snyder, 1974). A map over the dorsal eye of the microvillar orientations of the ninth cells was constructed using special fixation techniques that results in minimum distortion of rhabdomeric structures (Ribi, 1979). In a region between the 5th and the 45th horizontal ommatidial row (counted from the top) three ninth cell types (X, V, Y) according to their microvilli directions, were found. One third of 119 observed ninth cells (type X cells) have their microvilli arranged 52°±7° S.D. to the z-axis. A second third of ninth cells (type V cells) have their microvilli 89°±7° S.D. to the z-axis whereas the remaining third of ninth cells (type Y cells) have their microvilli 127°±7° S.D. to the z-axis. The analysis of polarization information based on simultaneous measurements requires at least three inputs for an unambiguous e-vector detection (Kirschfeld, 1972). The pattern of ommatidia as described here is such that three adjacent ommatidia always have ninth cells with three different microvilli directions. It is suggested that such a triplet of ommatidia represents a unit capable of unambiguous e-vector detection.

Coloured screening pigments cause red eye glow hue in pierid butterflies

The fine structure of the tracheal tapetum lucidum was investigated in six members of the diurnal lepidopteran family Pieridae. These observations were compared with the eye glow hue on intact eyes, the reflection colour of the incident illuminated tapetum and the distribution of the red screening pigments found in some receptor cells. The eye glow hue of dark adapted pierid eyes appears dark red. Whereas in the speciesColias crocea, C. australis andGonepteryx rhamni the red eye glow is observed all over the eye, appear the uppermost 35 horizontal eye rows in the speciesPieris brassicae, P. napi andP. rapae turquoise to yellow-green. The reflection colour of the tracheal tapetum of all examined pierid eyes is, however, uniformly turquoise to yellow-green all over the eye. In addition no regional structural differences in the tracheal tapetum at the proximal end of the rhabdom were observed. One knows, however, that inPieris brassicae, P. napi andP. rapae in the ventral and medial part of the eye, inColias crocea, C. australis andGonepteryx rhamni in all ommatidia the four proximally located receptor cells 5–8 contain red screening pigment which is arranged close to and even within the rhabdomeric structures (Ribi, 1978a). These observations show that the red eye glow observed in various pierid butterflies depends on coloured retinula cell screening pigment and not on the tracheal and rhabdomeric structures. Morphological and functional aspects of the Lepidopteran tapetum lucidum and the role of the coloured retinula cell pigmentation are discussed.

Do the rhabdomeric structures in bees and flies really twist?

Electron microscopic observations following careful tissue treatment show that the microvilli in visual cells of both the bee and the fly do not twist (Figs. 1 and 2). Theories of how bees analyse polarized light are reexamined in the light of this finding.

Ultrastructure and migration of screening pigments in the retina of Pieris rapae L. (Lepidoptera, Pieridae).

The retinal morphology of the butterfly, Pieris rapae L., was investigated using light and electron microscopy with special emphasis on the morphology and distribution of its screening pigments. Pigment migration in pigment- and retinula cells was analysed after light-dark adaptation and after different selective chromatic adaptations. The primary pigment cells with white- to yellow-green pigments symmetrically surround the cone process and the distal half of the crystalline cone, whilst the six secondary pigment cells, around each ommatidium, contain dark brown pigment granules. The nine retinula cells in one ommatidium can be categorised into four types. Receptor cells 1-4, which have microvilli in the distal half of the ommatidium only, contain numerous dark brown pigment granules. On the basis of the pigment content and morphology of their pigment granules, two groups of cells, cells 1, 2 and cells 3, 4 can be distinguished. The four diagonally arranged cells (5-8), with rhabdomeric structures and pigments in the proximal half of the cells, contain small red pigment granules of irregular shape. The ninth cell, which has only a small number of microvilli, lacks pigment. Chromatic adaptation experiments in which the location of retinula cell pigment granules was used as a criterium reveal two UV-receptors (cells 1 and 2), two green receptors (cells 3 and 4) and four cells (5-8) containing the red screening pigment, with a yellow-green sensitivity.

Fine structure of ocelli in sphinx moths

The internal ocelli of sphinx moths have receptor cells with a rhabdomere structure that is unique for insects. The rhabdomere consists of an invagination of a single receptor cell membrane to produce a cavity lined with microvilli and containing small lumen.

Electrophysiological evidence of photoreceptors in the epistellar body of Eledone moschata.

IN 1936 Young1 described a structure in several genera of octopods (cephalopods) appearing on the ventral surface of both stellate ganglia (near the base of the stellar nerve most distal to the mantle connective) which he termed the “epistellar body”. From histological considerations he hypothesized that this body might have a neurosecretory function. Nishioka et al.2 reinvestigated this structure in Octopus in more detail and, by electron microscopy, made the significant observation that the epistellar body contains cells the surfaces of which display microvilli suggesting rhabdomeric structures as seen typically in molluscan and arthropod visual photoreceptors. Later Nishioka et al.3 extended their investigations to include Eledone moschata and presented not only further electron micrograph data on the rhabdomeric ultrastructure but also biochemical data indicating the presence of a rhodopsin photopigment. Thus these investigators argued that the neurone-like cells of the epistellar body could be photo-receptors. We present here electrophysiological evidence clearly indicating the presence of photoreceptor cells in the epistellar body. There is already electrophysiological evidence for extracellular photoreceptors in arthropods4–7, molluscs8,9 and for photoreceptors in the diencephalon of the frog10,11.

The projection of the optical environment on the screen of the rhabdomere in the compound eye of the Musca

The dioptrics of the Musca ommatidium acts as an inverting lens system. The distal endings of the rhabdomeres at the basis of the dioptric apparatus are separated and arranged in a typical asymmetric pattern. The optical axes of the individual rhabdomeres of one ommatidium are the geometric projections of the distal rhabdomere endings into the environment, inverted by 180° by the dioptric apparatus. The divergence angles between the optical axes of the rhabdomeres of one ommatidium correspond to divergence angles between the appropriate set of ommatidia in such a way, that seven rhabdomeres of seven ommatidia are looking at one point in the environment (in the intermediate region between dorsal and ventral part of the eye: eight to nine rhabdomeres of a set of eight to nine ommatidia). These facts were established from sections of living eyes and confirmed by using special optical methods in the intact animal. The pattern of the decussation of individual retinulacell axons between retina and lamina was predicted adopting the hypothesis that the fibres of retinulacells number one to six, whose rhabdomers are looking at one point in the environment, project into a single “cartridge” in the lamina. These predicted connections were confirmed by other investigators. We have therefore a one to one correspondence between a lattice of points in the environment and the lattice of “cartridges” in the lamina. It is shown that the unfused rhabdomeric structure of the Musca ommatidium increases the effective entrance pupil of the eye by a factor of seven (resp. eight to nine in the intermediate region between dorsal and ventral part of the eye) compared to the classical apposition eye. —The Musca compound eye can be regarded as a “neural superposition eye”.