Calcium-sequestering smooth endoplasmic reticulum in retinula cells of the blowfly

Fractional Ca(2+) currents through TRP and TRPL channels in Drosophila photoreceptors

Light responses in Drosophila photoreceptors are mediated by two Ca(2+) permeable cation channels, transient receptor potential (TRP) and TRP-like (TRPL). Although Ca(2+) influx via these channels is critical for amplification, inactivation, and light adaptation, the fractional contribution of Ca(2+) to the currents (Pf) has not been measured. We describe a slow (τ ∼ 350 ms) tail current in voltage-clamped light responses and show that it is mediated by electrogenic Na(+)/Ca(2+) exchange. Assuming a 3Na:1Ca stoichiometry, we derive empirical estimates of Pf by comparing the charge integrals of the exchanger and light-induced currents. For TRPL channels, Pf was ∼17% as predicted by Goldman-Hodgkin-Katz (GHK) theory. Pf for TRP (29%) and wild-type flies (26%) was higher, but lower than the GHK prediction (45% and 42%). As predicted by GHK theory, Pf for both channels increased with extracellular [Ca(2+)], and was largely independent of voltage between -100 and -30 mV. A model incorporating intra- and extracellular geometry, ion permeation, diffusion, extrusion, and buffering suggested that the deviation from GHK predictions was largely accounted for by extracellular ionic depletion during the light-induced currents, and the time course of the Na(+)/Ca(2+) exchange current could be used to obtain estimates of cellular Ca(2+) buffering capacities.

RdgB proteins: Functions in lipid homeostasis and signal transduction

The RdgBs are a group of evolutionarily conserved molecules that contain a phosphatidylinositol transfer protein (PITP) domain. However in contrast to classical PITPs (PITPalpha) with whom they share the conserved PITP domain, these proteins also contain several additional sequence elements whose functional significance remains unknown. The founding member of the family DrdgB alpha (Drosophila rdgB) appears to be essential for sensory transduction and maintenance of ultra structure in photoreceptors (retinal sensory neurons). Although proposed to support the maintenance of phosphatidylinositol 4, 5 bisphosphate [PI (4, 5) P(2)] levels during G-protein coupled phospholipase C activity in these cells, the biochemical mechanism of DrdgB alpha function remains unresolved. More recently, a mammalian RdgB protein has been implicated in the maintenance of diacylglycerol (DAG) levels and secretory function at Golgi membranes. In this review we discuss existing work on the function of RdgB proteins and set out future challenges in understanding this group of lipid transfer proteins.

Cellular localization of calcium, heavy metals, and metallothionein in lobster (Homarus americanus) hepatopancreas

This investigation combines confocal microscopy with the cation-specific fluorescent dyes Fluo-3 and BTC-5N to localize calcium and heavy metals along the length of intact lobster (Homarus americanus) hepatopancreatic tubules and isolated cells. A metallothionein-specific antibody, developed in mollusks with cross-reactivity in crustaceans, showed the tissue-specific occurrence of this metal-binding protein in several organ systems in lobster and in single cell types isolated from lobster hepatopancreas. Individual lobster hepatopancreatic epithelial cell types were separated into pure single cell type suspensions for confocal and antibody experiments. Intact hepatopancreatic tubules showed high concentrations of both calcium and heavy metals at the distal tips of tubules where mitotic stem cells (E-cells) are localized. In addition, a concentrated distribution of calcium signal within isolated single premolt E-cells in solution was disclosed that might suggest an endoplasmic reticulum compartmentation of this cation within these stem cells. Both E- and R-cells showed significantly (P < 0.05) greater intracellular calcium concentrations in premolt than intermolt, suggesting the accumulation of this cation in these cells prior to the molt. Antibody studies with lobster tissues indicated that the hepatopancreas possessed 5-10 times the metallothionein concentration as other lobster organ systems and that isolated E-cells from the hepatopancreas displayed more than twice the binding protein concentrations of other cells of this organ or those of blood cells. These results suggest that crustacean hepatopancreatic stem cells (E-cells) and R-cells play significant roles in calcium and heavy metal homeostasis in this tissue. Interactions between the four hepatopancreatic cell types in this regulatory activity remain to be elucidated.

Calcium homeostasis in fly photoreceptor cells

In fly photoreceptor cells, two processes dominate the Ca2+ homeostasis: light-induced Ca2+ influx through members of the TRP family of ion channels, and Ca2+ extrusion by Na+/Ca2+ exchange. Ca2+ release from intracellular stores is quantitatively insignificant. Both, the light-activated channels and the Ca2+-extruding exchangers are located in or close to the rhabdomeric microvilli, small protrusions of the plasma membrane. The microvilli also contain the molecular machinery necessary for generating quantum bumps, short electrical responses caused by the absorption of a single photon. Due to this anatomical arrangement, the light-induced Ca2+ influx results in two separate Ca2+ signals that have different functions: a global, homogeneous increase of the Ca2+ concentration in the cell body, and rapid but large amplitude Ca2+ transients in the microvilli. The global rise of the Ca2+ concentration mediates light adaptation, via regulatory actions on the phototransduction cascade, the voltage-gated K+ channels and small pigment granules controlling the light intensity. The local Ca2+ transients in the microvilli are responsible for shaping the quantum bumps into fast, all-or-nothing events. They achieve this by facilitating strongly the phototransduction cascade at early stages ofthe light response and subsequently inhibiting it. Many molecular targets of these feedback mechanisms have been identified and characterized due to the availability of numerous Drosophila mutant showing defects in the phototransduction.

Calcium imaging demonstrates colocalization of calcium influx and extrusion in fly photoreceptors

During illumination, Ca(2+) enters fly photoreceptor cells through light-activated channels that are located in the rhabdomere, the compartment specialized for phototransduction. From the rhabdomere, Ca(2+) diffuses into the cell body. We visualize this process by rapidly imaging the fluorescence in a cross section of a photoreceptor cell injected with a fluorescent Ca(2+) indicator in vivo. The free Ca(2+) concentration in the rhabdomere shows a very fast and large transient shortly after light onset. The free Ca(2+) concentration in the cell body rises more slowly and displays a much smaller transient. After approximately 400 ms of light stimulation, the Ca(2+) concentration in both compartments reaches a steady state, indicating that thereafter an amount of Ca(2+), equivalent to the amount of Ca(2+) flowing into the cell, is extruded. Quantitative analysis demonstrates that during the steady state, the free Ca(2+) concentration in the rhabdomere and throughout the cell body is the same. This shows that Ca(2+) extrusion takes place very close to the location of Ca(2+) influx, the rhabdomere, because otherwise gradients in the steady-state distribution of Ca(2+) should be measured. The close colocalization of Ca(2+) influx and Ca(2+) extrusion ensures that, after turning off the light, Ca(2+) removal from the rhabdomere is faster than from the cell body. This is functionally significant because it ensures rapid dark adaptation.

Distribution of ryanodine receptor Ca(2+) channels in insect photoreceptor cells

Physiological studies have provided evidence for the existence of ryanodine receptor (RyR) Ca(2+) channels in compound eyes of insects. The present study identifies and localizes RyR in insect photoreceptors by use of an affinity-purified antibody against lobster muscle RyR. Western blotting and indirect immunofluorescence staining confirm cross-reactivity of the antibody with insect muscle RyR. In both honeybee and fly eyes, the antibody identifies a single protein that comigrates with muscle RyR on sodium dodecylsulfate (SDS) polyacrylamide gels demonstrating that RyR is present in this tissue. By confocal immunofluorescence microscopy on honeybee eyes, RyR is detected within the photoreceptors and shows a nonhomogeneous distribution over the endoplasmic reticulum (ER). Double labeling studies have demonstrated further that RyR is localized at distinct ER elements close to the light-sensitive microvilli and juxtaposed to adherens junctions. RyR has also been observed within the remaining soma of honeybee photoreceptors, being concentrated on ER cisternae close to mitochondria and the nonreceptive plasma membrane. For comparative purposes, the distribution of RyR has also been assayed in compound eyes of flies. In both Calliphora and Drosophila photoreceptors, the anti-RyR antibody provides punctate labeling throughout the cell body. The submicrovillar ER cisternae associated with the base of the microvilli, however, are only lightly labeled for RyR. These results suggest that RyR is involved with Ca(2+) regulation in the nonreceptive cell area of both fly and honeybee photoreceptors, but that it may contribute to Ca(2+) regulation close to the phototransduction compartment only in the latter cell.

Calcium transients in the rhabdomeres of dark- and light-adapted fly photoreceptor cells

The light response of fly photoreceptor cells is modulated by changes in free Ca(2+) concentration. Fly phototransduction and most processes regulating it take place in or very close to the rhabdomere. We therefore measured the kinetics and the absolute values of the free Ca(2+) concentration in the rhabdomere of fly photoreceptor cells in vivo by making use of the natural optics of the fly's eye. We show that Ca(2+) flowing into the rhabdomere after light stimulation of dark-adapted cells causes fast Ca(2+) transients that reach peak values higher than 200 microM in <20 msec. Approximately 500 msec later, the free Ca(2+) concentration has declined again to approximately 20 microM. The duration of the Ca(2+) transients becomes still shorter, and their size reduced, when the photoreceptor cell is light-adapted. This reduction in duration and size of the Ca(2+) transients is graded with the intensity of the adapting light. The kinetics and absolute values of the free calcium concentration found to occur in the rhabdomere are suitable to mediate the fast feedback signals known to act on the fly phototransduction cascade.

Visual transduction in Drosophila

The Drosophila phototransduction cascade has emerged as an attractive paradigm for understanding the molecular mechanisms underlying visual transduction, as well as other G protein-coupled signaling cascades that are activated and terminated with great rapidity. A large collection of mutants affecting the fly visual cascade have been isolated, and the nature and function of many of the affected gene products have been identified. Virtually all of the proteins, including those that were initially classified as novel, are highly related to vertebrate homologs. Recently, it has become apparent that most of the proteins central to Drosophila phototransduction are coupled into a supramolecular signaling complex, signalplex, through association with a PDZ-containing scaffold protein. The characterization of this complex has led to a re-evaluation of the mechanisms underlying the activation and deactivation of the phototransduction cascade.

Does a decrease in subplasmalemmal Ca2+ explain how store-operated Ca2+ channels are opened?

The phenomenon of store-activated Ca2+ inflow (capacitative Ca2+ entry) in which the depletion of Ca2+ in the endoplasmic reticulum (ER) increases the probability of opening of store-operated Ca2+ channels (SOCs) located in the plasma membrane is ubiquitous in 'non-excitable' animal cells and is also found in some 'excitable' cells. At present, neither the structures of SOCs nor the mechanism(s) by which a decrease in Ca2+ in the lumen of the ER activates SOCs are well understood. This paper discusses the hypothesis that a decrease in the concentration of Ca2+ in restricted regions of the subplasmalemmal space (bounded by the plasma membrane and peripheral regions of the ER) is responsible for the activation of SOCs. The hypothesis rests on observations made by others that Ca2+ is a strong feed-back inhibitor of SOCs and of the endoplasmic reticulum (Ca(2+)+Mg2+)-ATPases (SERCAs), and on the concepts (developed previously by others) of a subplasmalemmal space and the directed flow of Ca2+ through SOCs into the lumen of the ER and from there to the deep cytoplasmic space. The way in which the hypothesis might explain the actions of agonists (acting via inositol 1,4,5-trisphosphate) and thapsigargin (an inhibitor of SERCAs) in activating SOCs under physiological conditions is described. The proposed involvement of thapsigargin-insensitive SERCAs, and possible limitations of the hypothesis are discussed.

Distribution of Na+,K(+)-ATPase in photoreceptor cells of insects

Light stimulation of insect photoreceptors causes opening of cation channels and an inward current that is partially carried by Na+ ions. There is also an efflux of K+ ions upon photostimulation. Na+ and K+ gradients across the photoreceptor membrane are reestablished by the activity of the enzyme Na+,K(+)-ATPase. About two-thirds of the total amount of ATP consumed in response to a light stimulus is attributed to the activity of this ion pump, demonstrating the importance of this enzyme for photoreceptor function. Insect photoreceptor cells are polarized epithelial cells; their plasma membrane is organized into two domains having a distinct morphology, molecular composition, and function. The visual pigment rhodopsin and the molecular components of the transduction machinery are localized in the rhabdomere, an array of densely packed microvilli, whereas Na+,K(+)-ATPase resides in the nonrhabdomeric membrane. Comparative immunolocalization studies on compound eyes of diverse insect species have demonstrated subtle variations in the distribution patterns of Na+,K(+)-ATPase. These may be accounted for by differences in the mechanisms responsible for Na+,K(+)-ATPase positioning.

The Limulus ventral photoreceptor: light response and the role of calcium in a classic preparation

The ventral nerve photoreceptor of the horseshoe crab Limulus polyphemus has been used for many years to investigate basic mechanisms of invertebrate phototransduction. The activation of rhodopsin leads in visual cells of invertebrates to an enzyme cascade at the end of which ion channels in the plasma membrane are transiently opened. This allows an influx of cations resulting in a depolarization of the photoreceptor cell. The receptor current of the Limulus ventral photoreceptor consists of three components which differ in several aspects, such as the time course of activation, the time course of recovery from light adaptation, and the reversal potential. Each component is influenced in a different, characteristic way by various pharmacological manipulations. In addition, at least two types of single photon-evoked events (bumps) and three elementary channel conductances are observed in this photoreceptor cell. These findings suggest that the receptor current components are controlled by three different light-activated enzymatic pathways using three different ligands to increase membrane conductance. Probably one of these ligands is cyclic GMP, another one is activated via the IP3-cascade and calcium, the third one might be cyclic AMP. Calcium ions are very important for the excitation and adaptation of visual cells in invertebrates. The extracellular and intracellular calcium concentrations determine the functional state of the visual cell. A rise in the cytosolic calcium concentration appears to be an essential step in the excitatory transduction cascade. Cytosolic calcium is the major intracellular mediator of adaptation. If the cytosolic calcium level exceeds a certain threshold value after exposure to light it causes the desensitization of the visual cell. On the other hand, from a slight rise in cytosolic calcium facilitation results, i.e. increased sensitivity of the photoreceptor.

The phosphatidylinositol transfer protein domain of Drosophila retinal degeneration B protein is essential for photoreceptor cell survival and recovery from light stimulation

The Drosophila retinal degeneration B (rdgB) gene encodes an integral membrane protein involved in phototransduction and prevention of retinal degeneration. RdgB represents a nonclassical phosphatidylinositol transfer protein (PITP) as all other known PITPs are soluble polypeptides. Our data demonstrate roles for RdgB in proper termination of the phototransduction light response and dark recovery of the photoreceptor cells. Expression of RdgB's PITP domain as a soluble protein (RdgB-PITP) in rdgB2 mutant flies is sufficient to completely restore the wild-type electrophysiological light response and prevent the degeneration. However, introduction of the T59E mutation, which does not affect RdgB-PITP's phosphatidylinositol (PI) and phosphatidycholine (PC) transfer in vitro, into the soluble (RdgB-PITP-T59E) or full-length (RdgB-T59E) proteins eliminated rescue of retinal degeneration in rdgB2 flies, while the light response was partially maintained. Substitution of the rat brain PITPalpha, a classical PI transfer protein, for RdgB's PITP domain (PITPalpha or PITPalpha-RdgB chimeric protein) neither restored the light response nor maintained retinal integrity when expressed in rdgB2 flies. Therefore, the complete repertoire of essential RdgB functions resides in RdgB's PITP domain, but other PITPs possessing PI and/or PC transfer activity in vitro cannot supplant RdgB function in vivo. Expression of either RdgB-T59E or PITPalpha-RdgB in rdgB+ flies produced a dominant retinal degeneration phenotype. Whereas RdgB-T59E functioned in a dominant manner to significantly reduce steady-state levels of rhodopsin, PITPalpha-RdgB was defective in the ability to recover from prolonged light stimulation and caused photoreceptor degeneration through an unknown mechanism. This in vivo analysis of PITP function in a metazoan system provides further insights into the links between PITP dysfunction and an inherited disease in a higher eukaryote.

Mammalian homolog of Drosophila retinal degeneration B rescues the mutant fly phenotype

Mutations in the Drosophila rdgB gene, which encodes a transmembrane phosphatidylinositol transfer protein (PITP), cause a light-enhanced retinal degeneration. Cloning of mammalian rdgB orthologs (mrdgB) reveal predicted proteins that are 39% identical to rdgB, with highest homology in the N-terminal PITP domain (62%) and in a region near the C terminus (65%). The human mrdgB gene spans approximately 12 kb and maps to 11q13.1, a locus where several retinal diseases have also been mapped. Murine mrdgB maps to a syntenic region on the proximal region of chromosome 19. MrdgB is specifically expressed in the retina and brain. In the retina, MrdgB protein is localized to photoreceptor inner segments and the outer and inner plexiform layers. Expression of murine mrdgB in mutant flies fully rescues both the rdgB-dependent retinal degeneration and abnormal electroretinogram. These results suggest the existence of similarities between the invertebrate and mammalian retina that were not previously appreciated and also identify mrdgB as a candidate gene for retinal diseases that map to 11q13.1.

Elementary and global aspects of calcium signalling

Images

The roles of trp and calcium in regulating photoreceptor function in Drosophila

Invertebrate photoreceptors use the ubiquitous inositol-lipid signaling pathway for phototransduction. This pathway depends on Ca2+ release from internal stores and on Ca2+ entry via light-activated channels to replenish the loss of Ca2+ in those stores. The Drosophila transient receptor potential (TRP) protein is essential for the high Ca2+ permeability and other biophysical properties of these light-activated channels, which affect both excitation and adaptation in photoreceptor cells. Physiological and heterologous expression studies indicate that TRP is a putative subunit of a surface membrane channel that can be activated by depletion of internal Ca2+ stores. Furthermore, trp is an archetypal member of a multigene family whose products share a structure that is highly conserved throughout evolution, from worms to humans.

A quantitative estimate of the maximum amount of light-induced Ca2+ release in Drosophila photoreceptors

Simultaneous measurements of the light-induced current (LIC) and cytosolic Ca2+ (using INDO-1) were made in Drosophila photoreceptors. In the presence of 1.5 mM Cao2+, the UV light used to measure INDO-1 fluorescence saturated the LIC and induced a large Ca2+ rise. In the absence of extracellular Ca2+ and with Na+ replaced by N-methyl-D-glucamine, the light-induced Ca2+ rise was virtually abolished. A residual rise of about 20 nM is regarded as an upper estimate of Ca2+ released from internal stores. To estimate the Ca2+ flux required to generate such a rise, Ca2+ influx signals in response to weak light steps (500 ms LED stimulus) were measured in the presence of external Ca2+. The relationship between [Ca(in)] and the total charge carried during the LIC had a slope of 2.7 nM pC-1. Assuming that 50% of the LIC is carried by Ca2+ and that the single-channel Ca2+ current carried by the InsP3 receptor is 0.04 pA, it was estimated that about 350 InsP3 receptors should have been sufficient to generate a Ca2+ rise of 20 nM within 500 ms. By contrast, the current activated by the UV measuring light was equivalent to the activation of at least 5000 quantum bumps, making it unlikely that InsP3-induced Ca2+ release could have been the causal event for excitation under these conditions.

INDO-1 measurements of absolute resting and light-induced Ca2+ concentration in Drosophila photoreceptors

Absolute Ca2+ levels in dissociated Drosophila photoreceptors were measured using the ratiometric indicator dye INDO-1 loaded via patch pipettes, which simultaneously recorded whole-cell currents. In wild-type photoreceptors, the ultraviolet (UV) excitation light used to measure fluorescence elicited a massive Ca2+ influx that saturated the dye (>10 microM Ca2+), but lagged the electrical response by 2.8 msec. Resting Ca2+ levels in the dark, measured during the latent period before the response, averaged 160 nM in normal Ringer's (1.5 mM Ca2+). Ca2+ increases in response to weak illumination were estimated (1) by using a weak adapting stimulus before the UV excitation light and measuring Ca2+ during the latent period; and (2) by using ninaE mutants with greatly reduced rhodopsin levels. Ca2+ rose linearly as a function of the time integral of the light-sensitive current with a slope of 2.7 nM/pC. In the transient receptor potential (trp) mutant, which lacks a putative light-sensitive channel subunit, the slope was only 1.1 nM/pC, indicating a 2.5-fold reduction in the fractional Ca2+ current. From these data, it can also be estimated that >99% of the Ca2+ influx is effectively buffered by the cell. In Ca2+-free Ringer's, resting cytosolic Ca2+ was reduced (to 30-70 nM), but contrary to previous reports, significant light-induced increases (approximately 250 nM) could be elicited. This rise was reduced to <20 nM when extracellular Na+ was replaced with N-methyl-D-glucamine, suggesting that it could be attributed to Na+ influx altering the Na/Ca exchanger equilibrium. It is concluded that any light-induced release from internal stores amounts to <20 nM.

Capacitative calcium entry

Images

Phosphoinositide-mediated phototransduction in Drosophila photoreceptors: the role of Ca2+ and trp

Drosphoinate photoreceptors, represent a paradigm for the genetic dissection of phototransduction and, more generally for Ca2+ signaling. As in most invertebrates, phototransduction in Drosophila is mediated by the phosphoinositide (PI) cascade and is completely blocked by null mutations of the norpA gene which encodes a phospholipase C-beta isoform. The light-activated conductance in Drosophila is normally highly permeable to Ca2+, but in null mutants of the trp gene Ca2+ permeability is greatly reduced. Furthermore, the trp gene sequence shows homologies with voltage gated Ca2+ channels, suggesting that trp encodes a light-sensitive channel subunit. Ca2+ influx via these channels is instrumental in light adaptation, and profoundly influences phototransduction via positive and negative feedback at multiple molecular targets including protein kinase C. The mechanism of activation of the light-sensitive channels remains unresolved. A requirement for Ca2+ release from internal stores is suggested by the finding that Drosophila photoreceptors cannot sustain a maintained response under various conditions which might be expected to result in depletion of Ca2+ stores. However, Ca2+ release cannot be detected by Ca2+ indicator dyes and raising Ca2+ by photorelease of caged Ca2+ fails to mimic excitation. Recent studies, both in situ and with heterologously expressed trp protein, suggest that the trp-dependent channels may be activated by a process analogous to 'capacitative Ca2+ entry', a widespread, but poorly understood mode of PI-regulated Ca2+ influx in vertebrate cells.

Structure and cellular physiology of Ca2+ stores in invertebrate photoreceptors

Invertebrate microvillar photoreceptors contain an extensive, morphologically continuous endoplasmic reticulum (ER) that comprises several distinct subregions. Most prominent is the smooth submicrovillar ER, a sponge-like cisternal network underneath the photoreceptive microvillar membrane. The submicrovillar ER spatially separates the microvilli and a narrow space of submicrovillar cytoplasm from the remaining cell body, and, thus, defines a transduction compartment. In bee and locust photoreceptors, the shape and position of these submicrovillar ER cisternae is maintained by interaction with actin filaments. The structural layout of the ER is either rather static, or, in some invertebrate species, the ER undergoes dramatic rearrangements during illumination. The submicrovillar ER has a high Ca content in dark-adapted cells (47.5 mmol/kg dry weight in bee photoreceptors), and acts as a source and sink for Ca2+ mobilized by illumination. About 50% of the Ca content is released by a 3 s, non-saturating light stimulus, and an almost equimolar amount of Mg is taken up to maintain electroneutrality within the ER. Ca2+ release is initiated by Ins(1,4,5)P3. In addition, the submicrovillar ER contains a heparin-insensitive, caffeine- and ryanodine-sensitive Ca2+ release pathway in bee photoreceptors. Both the Ins(1,4,5)P3-dependent and the ryanodine-sensitive Ca2+ release mechanism are modulated by cytosolic Ca2+, but at different Ca2+ concentrations. The presence of two release pathways with different Ca2+ sensitivities may be a prerequisite for highly localized, exceptionally fast and large Ca2+ elevations during the illumination of invertebrate photoreceptors.

The pineal gland of the aging rat: calcium localization and variation in the number of pinealocytes

In the present study, we investigated the population of pinealocytes in the pineal gland of aging rats. Dark and light pinealocytes were analyzed as to their calcium content. Calcium localization was realized in dark and light cells by means of cytochemistry and X-ray microanalysis. Calcium was mainly localized in dark pinealocytes characterized by many ultrastructural signs of degeneration. The number of pinealocytes per square surface of aged rats (28 months) was compared to young ones (3-4 months). While there is a significant increase in the number of dark pinealocytes there is a decrease in the total number of pinealocytes in aged rats. This age-related loss of pinealocytes may explain the age-related functional decline of the pineal gland activity (e.g., the decrease of the nocturnal melatonin production).

Localization of subrhabdomeric haemolymph lacunae in the retina of Drosophila melanogaster and Calliphora erythrocephala

The haemolymph of flies and other insects contains tyrosinase (EC 1.14.18.1), the rate-limiting enzyme in mammalian melanogenesis. After incubation with 5 mM L-DOPA for several hours the endogenous tyrosinase of the haemolymph forms an electron dense reaction product. This method was used to localize spaces in the retina of the wild type and the white (w) mutant of Drosophila melanogaster that are filled with haemolymph. A network of subrhabdomeric haemolymph lacunae was found. Moreover it was found that these haemolymph lacunae also form extensions into the photoreceptor cells and are connected with small haemolymph lacunae that cross the retinal basement membrane. In a second set of experiments L-DOPA was injected into the thoraces of Calliphora erythrocephala. Half-an-hour after injections the flies were killed and the eyes were embedded for electron microscopy. The small molecule of DOPA or its product dopachrome, surprisingly, penetrated the retinal basement membrane and reached the subrhabdomeric haemolymph lacunae and the ommatidial cavity.

Calphotin: a Drosophila photoreceptor cell calcium-binding protein

Monoclonal antibody 23E9 identifies a calcium-binding protein, calphotin, in photoreceptor cells of the Drosophila melanogaster compound eyes and ocelli. The antigen is restricted to a defined cytoplasmic region; it is not present in the rhabdomeres, nuclei, mitochondria, or rough endoplasmic reticulum. A corresponding cDNA recognizes a 3-kb mRNA with retinal specificity similar to the antigen and maps to band 86E/F-87A/B on chromosome 3. An open reading frame of 2595 bp encodes an estimated 85-kDa protein of unusual amino acid composition, with > 50% proline, alanine, and valine and very few basic residues. The C-terminal segment contains a leucine zipper motif uninterrupted by prolines. We found no significant similarities with the GenBank or National Biomedical Resource Foundation data bases. The location of the protein within a distinct cytoplasmic region suggests that it might function as a calcium-sequestering "sponge" to regulate the amount of free cytoplasmic calcium.

Degeneration of photoreceptors in rhodopsin mutants of Drosophila

Five different, well-characterized mutants of the R1-6 rhodopsin gene (ninaE), which corresponds to the rod opsin gene of vertebrates, have been examined morphologically as a function of age (up to 9 weeks) to determine whether or not the photoreceptors degenerate and to assess the pattern of degeneration. Structural deterioration of R1-6 photoreceptors with age has been found in all five mutants. The structural pattern of degeneration is similar in the five mutants, but the time course of degeneration is allele dependent and varies greatly among the five, with the strongest alleles causing the fastest degeneration. The degeneration appears to be independent of either the illumination cycle to which the animals are exposed or the presence of screening pigments in the eye. Although the degeneration first appears in R1-6 photoreceptors, eventually R7/8 photoreceptors, which correspond to cones of vertebrates, are also affected. In many of these mutants, striking proliferations of membrane processes have been observed in the subrhabdomeric region of R1-6 photoreceptors. It is hypothesized that (1) this accumulation of membranes may be caused by the failure of newly synthesized membranes that are inserted into the base of microvilli to be assembled into R1-6 rhabdomeres and (2) this failure may be caused by the extremely low concentration of normal R1-6 rhodopsin in the ninaE mutants.

The trp gene is essential for a light-activated Ca2+ channel in Drosophila photoreceptors

Invertebrate phototransduction is an important model system for studying the ubiquitous inositol-lipid signaling system. In the transient receptor potential (trp) mutant, one of the most intensively studied transduction mutants of Drosophila, the light response quickly declines to baseline during prolonged intense light. Using whole-cell recordings from Drosophila photoreceptors, we show that the wild-type response is mediated by at least two functionally distinct classes of light-sensitive channels and that both the trp mutation and a Ca2+ channel blocker (La3+) selectively abolish one class of channel with high Ca2+ permeability. Evidence is also presented that Ca2+ is necessary for excitation and that Ca2+ depletion mimics the trp phenotype. We conclude that the recently sequenced trp protein represents a class of light-sensitive channel required for inositide-mediated Ca2+ entry and suggest that this process is necessary for maintained excitation during intense illumination in fly photoreceptors.

Intracellular free calcium concentration in the blowfly retina studied by Fura-2

The retinal tissue of blowflies was loaded with the fluorescent Ca2+ indicator Fura-2 by incubating cut heads in saline solutions which contained the membrane permeable acetoxymethylester of Fura-2 (Fura-2/AM). The spectral analysis of the tissue fluorescence showed that Fura-2/AM was intracellularly hydrolysed to Fura-2. In order to monitor the intracellular free Ca2+ concentration ([Ca2+]i) the Fura-2 fluorescence was excited by short light flashes. The fluorescence was calibrated by incubating the tissue in Ca2+ buffers of high buffering capacity and subsequent disruption of the cell membranes by freeze/thawing, which gave a dissociation constant for the Ca(2+)-Fura-2 complex of 100 nM. When the extracellular Ca2+ concentration ([Ca2+]o) was altered [Ca2+]i reversibly changed. The changes were most pronounced when [Ca2+]o was varied in the millimolar range, e.g. [Ca2+]i increased from 0.07 microM at [Ca2+]o = 0.1 mM to 1 microM at [Ca2+]o = 10 mM. When extracellular Na+ was replaced by Li+ or other monovalent ions, [Ca2+]i rapidly increased which supports the view that electrogenic Na+/Ca2+ exchange contributes to the control of [Ca2+]i. However, [Ca2+]i decreased again when the tissue was superfused with Na(+)-free media for longer periods, which points to a Ca(2+)-transporting system different from Na+/Ca2+ exchange. Light adaptation had only a small effect on [Ca2+]i. Even after intense stimulation [Ca2+]i increased by a factor of 1.5 only, which is in line with results obtained in the photoreceptors of Balanus and Apis.

Phototransduction: different mechanisms in vertebrates and invertebrates

The photoreceptor cells of invertebrate animals differ from those of vertebrates in morphology and physiology. Our present knowledge of the different structures and transduction mechanisms of the two animal groups is described. In invertebrates, rhodopsin is converted by light into a meta-rhodopsin which is thermally stable and is usually re-isomerized by light. In contrast, photoisomerization in vertebrates leads to dissociation of the chromophore from opsin, and a metabolic process is necessary to regenerate rhodopsin. The electrical signals of visual excitation have opposite character in vertebrates and invertebrates: the vertebrate photoreceptor cell is hyperpolarized because of a decrease in conductance and invertebrate photoreceptors are depolarized owing to an increase in conductance. Single-photon-evoked excitatory events, which are believed to be a result of concerted action (the opening in invertebrates and the closing in vertebrates) of many light-modulated cation channels, are very different in terms of size and time course of photoreceptors for invertebrates and vertebrates. In invertebrates, the single-photon events (bumps) produced under identical conditions vary greatly in delay (latency), time course and size. The multiphoton response to brighter stimuli is several times as long as a response evoked by a single photon. The single-photon response of vertebrates has a standard size, a standard latency and a standard time course, all three parameters showing relatively small variations. Responses to flashes containing several photons have a shape and time scale that are similar to the single-photon-evoked events, varying only by an amplitude scaling factor, but not in latency and time course. In both vertebrate and invertebrate photoreceptors the single-photon-evoked events become smaller (in size) and faster owing to light adaptation. Calcium is mainly involved in these adaptation phenomena. All light adaptation in vertebrates is primarily, or perhaps exclusively, attributable to calcium feedback. In invertebrates, cyclic AMP (cAMP) is apparently another controller of sensitivity in dark adaptation. The interaction of photoexcited rhodopsin with a G-protein is similar in both vertebrate and invertebrate photoreceptors. However, these G-proteins activate different photoreceptor enzymes (phosphodiesterases): phospholipase C in invertebrates and cGMP phosphodiesterase in vertebrates. In the photoreceptors of vertebrates light leads to a rapid hydrolysis of cGMP which results in closing of cation channels. At present, the identity of the internal terminal messenger in invertebrate photoreceptors is still unsolved.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective illumination of single photoreceptors in the house fly retina: local membrane turnover and uptake of extracellular horseradish peroxidase (HRP) and lucifer yellow

Single photoreceptor cells in the compound eye of the housefly Musca domestica were selectively illuminated and subsequently compared electron-microscopically with the unilluminated photoreceptors in the immediate surroundings. The rhabdomeres of the illuminated cells remain largely unaffected, but the cells show an increase in the number of coated pits, various types of vesicles, and degradative organelles; some of the latter organelles are described for the first time in fly photoreceptors. Coated pits are found not only at the bases of the microvilli, but also in other parts of the plasma membrane. Degradative organelles, endoplasmic reticulum (ER) and mitochondria aggregate in the perinuclear region. The rough ER and smooth ER are more elaborate, the number of Golgi stacks, free ribosomes and polysomes is increased, and the shape and distribution of heterochromatin within the nuclei are altered. Illuminated photoreceptors also interdigitate extensively with their neighbouring secondary pigment cells. These structural changes in illuminated fly photoreceptor cells indicate an increase in membrane turnover and cellular metabolism. When applied to the eye, Lucifer Yellow spreads into the extracellular space and is taken up only by the illuminated photoreceptor cells. These cells show the same structural modifications as above. Horseradish peroxidase applied in the same way is observed in pinocytotic vesicles and degradative organelles of the illuminated cells. Hence, the light-induced uptake of extracellular compounds takes place in vivo at least partially as a result of an increase in pinocytosis.

Retinomotor movements in the frog retinal pigment epithelium: dependence of pigment migration on Na+ and Ca2+

The ionic dependence of the screening-pigment migrations in the frog retinal epithelium (RPE) was quantitatively studied with eyecups incubated in media of different compositions. Typical migrations in response to light and darkness, equivalent to those observed in the intact animal, were fully accomplished and maintained for up to 6 hr by the isolated organ bathed in Ringer solution rich with O2. Pigment migration in either direction was completed under the appropriate illumination conditions at any time during the day, indicating that circadian influences, if present in the intact animal, can be overridden in the isolated organ by light or darkness alone. Pigment aggregation toward the dark-adapted position was inhibited by: (a) low external Ca2+, (b) high external Na+, and (c) drugs expected to increase the cytoplasmic levels of either Na+, or Ca2+, like ouabain, caffeine and the ionophore A23187. However, the inhibition caused by low Ca2+ did not occur if Na+ was also reduced in the incubation medium. On the other hand, an increase in the concentration of external Ca2+ or the addition of Co2+ to the normal Ringer facilitated pigment aggregation in the dark. Pigment dispersion to the light-adapted position was unaffected by any of the above conditions. This is the first report of full and stable pigment responses in the RPE of vertebrate eyes incubated under simple physiological conditions. The results seem to conciliate a discrepancy of previous reports on the Ca2+ dependence of RPE movements, and are compatible with current views on ionic mechanisms in analogous systems of intracellular transport.

Lamellar bodies are markers of cholinergic neurons in ferret nucleus basalis

Lamellar bodies are composed of stacks of closely-packed, ribosome-free cisterns which are in continuity with the rough endoplasmic reticulum. In the ferret nucleus basalis stained for choline acetyltransferase it was shown, by correlating light with electron microscopy, that only the cholinergic cells there possess lamellar bodies. The significance of lamellar bodies in the cholinergic neurons of the nucleus basalis is not known, but these structures may reflect a peculiar aspect of the functioning of the cholinergic cells which will need to be investigated further.

Structure of the subrhabdomeric cisternae in the photoreceptor cells of Drosophila melanogaster

The structure of subrhabdomeric cisternae (SRC) and related structures in the photoreceptor cells (retinular cells) of Drosophila melanogaster in normal flies and visual mutants were compared by electron microscopic observation of semithin sections of osmium-impregnated specimens. The three-dimensional organization of SRC and the other cell organelles was demonstrated by stereoscopy. Both light- and dark-adapted normal retinular cells contained elaborate networks of anastomosing tubules of SRC immediately beneath the rhabdomeres. Tubules connecting the SRC and rough endoplasmic reticulum were frequently seen. The SRC were absent from the retinular cells of rdgAKS60 whose rhabdomeres degenerate gradually after eclosion. Instead, numerous smooth vesicles were observed in the subrhabdomeric regions. In rdgBEE170, in which rhabdomere degeneration is light dependent, the SRC appeared normal in the dark-adapted flies. But their SRC gradually disintegrated after exposure to light. In norpASB37, whose rhabdomeres are small but do not degenerate, SRC appeared normal. These results suggest that the SRC is a significant structure for the maintenance of the structure of photoreceptive membrane in the retinular cells of Drosophila.

Programmed cell death: cytochemical and X-ray microanalytical characterization of calcium compartments in neuromuscular junctions during the normal breakdown of the intersegmental muscles in the giant silkmoth Antheraea polyphemus

Calcium stores were cytochemically demonstrated using a combined oxalate-pyroantimonate method in the neuromuscular junctions of the degenerating intersegmental muscles in the giant silkmoth Antheraea polyphemus. The elemental composition of punctate precipitates of the reaction product was determined by electron probe X-ray microanalysis of unstained thin sections by energy-dispersive spectrometry and wavelength-dispersive spectrometry. The wavelength-dispersive spectra collected over terminal axons demonstrate a significant calcium signal and a trace of antimony. During the rapid lytic phase of spontaneous muscle degeneration, the calcium punctate deposits were detected in presynaptic terminals in the following sites: the synaptic vesicles and the mitochondria. Calcium precipitates were also found in the dense bodies and the mitochondria encountered in the glial convolutions. No calcium deposit was seen in the synaptic clefts and intercellular spaces of the subsynaptic reticulum of type I and type II. A comparison of calcium to antimony ratios between the terminal axons and the sarcoplasmic lysosomes revealed highly significant differences (P less than 0.001). Such a variability of the calcium to antimony ratio may be related to different conditions of precipitation or antimony diffusion in the different cell compartments. It was concluded that such synaptic terminals do not appear damaged in spite of the muscle degeneration and presumably continue to perform vital functions while the muscles are no longer contractile 20 h after adult ecdysis.

Frontiers in electron probe microanalysis: application to cell physiology

The application of electron probe microanalysis techniques, using X-ray and electron energy loss instruments, to problems in cell physiology is reviewed. The details of the special methodological requirements for the analysis of cryosections at high spatial resolution in an analytical electron microscope are discussed together with a comprehensive review of data obtained on major organ systems and cell types.

Distribution and calcium-sequestering ability of smooth endoplasmic reticulum in olfactory axon terminals of frog brain

In the present study, the structural and functional role of smooth endoplasmic reticulum was investigated in bullfrog olfactory axon terminals. Structural evidence obtained from this study indicated that this vesiculotubular organelle becomes a more elaborate network of anastomosing tubules near the nerve terminal, located in the olfactory lobe of frog brain. Further structural evidence suggested that membranes of the smooth endoplasmic reticulum pinch off to give rise to some electron-lucent vesicles of approximately 50 nm diameter (microvesicles). Ultrastructural cytochemistry was employed in the present study to demonstrate that olfactory axon terminal smooth endoplasmic reticulum actively sequesters Ca2+. However, a variable amount of electron-dense product (calcium oxalate) was associated with microvesicles located at a distance from the synapse, in contrast to those clustered near the synapse which usually did not contain this reaction product. Results from Ca2+-Mg2+-adenosine-5'-triphosphatase (ATPase) cytochemistry showed a similar pattern of distribution, with smooth endoplasmic reticulum being densely labeled with ATPase reaction product (lead phosphate), but aggregated microvesicles in the nerve terminal generally lacking this electron-dense product. Therefore, it is concluded that olfactory axonal smooth endoplasmic reticulum plays a role in the regulation of intraneuronal Ca2+ levels, and that the Ca2+-sequestering activity of this membranous organelle is dependent upon enzymatic hydrolysis of ATP. Conversely, the microvesicles, particularly those accumulated near the synapse, lack this Ca2+-pumping capacity. Thus, if some of the microvesicles originate from smooth endoplasmic reticulum membranes which are capable of pumping Ca2+, but these vesicles themselves lack this capacity, one can postulate that the Ca2+ pumps are either removed from the newly formed microvesicle membranes or are somehow incapacitated in situ in the membrane.