Biomedical subjects

B Burnside

Publications and source records attributed to B Burnside.

At least 19 recordsLinked to original sources

PubMed1996

Stimulation of cell elongation in teleost rod photoreceptors by distinct protein kinase inhibitors.

The rod photoreceptors of teleost retinas elongate in the light. To characterize the role of protein kinases in elongation, pharmacological studies were carried out with rod fragments consisting of the motile inner segment and photosensory outer segment (RIS-ROS). Isolated RIS-ROS were cultured in the presence of membrane-permeant inhibitors that exhibit selective activity toward specific serine/threonine protein kinases. We report that three distinct classes of protein kinase inhibitors stimulated elongation in darkness: (1) cyclic-AMP-dependent protein kinase (PKA)-selective inhibitors (H-89 and KT5720), (2) a protein kinase C (PKC)-selective inhibitor (GF 109203X) that affects most PKC isoforms, and (3) a kinase inhibitor (H-85) that does not affect PKC and PKA in vitro. Other kinase inhibitors tested neither stimulated elongation in darkness nor inhibited light-induced elongation; these include the myosin light chain kinase inhibitors ML-7 and ML-9, the calcium-calmodulin kinase II inhibitor KN-62, and inhibitors or activators of diacylglycerol-dependent PKCs (sphingosine, calphostin C, chelerythrine, and phorbol esters). The myosin light chain kinase inhibitors as well as the PKA and PKC inhibitors H-89 and GF 109203X all enhanced light-induced elongation. These observations suggest that light-induced RIS-ROS elongation is inhibited by both PKA and an unidentified kinase or kinases, possibly a diacylglycerol-independent form of PKC.

Animals↗

PubMed1995

Expression of kinesin heavy chain isoforms in retinal pigment epithelial cells.

To examine the possible role of kinesin in pigment granule migration in the retinal pigment epithelium (RPE) of teleosts, we investigated the expression and distribution of kinesin heavy chain (KHC) in RPE. Blots of fish RPE lysates probed with two well-characterized antibodies to KHC (H2 and HD) displayed a prominent band at 120 kD. A third KHC antibody (SUK4) recognized a band at 118 kD. The 118 kD band was also occasionally present in blots probed with H2, suggesting the presence of two KHC isoforms in teleost RPE. Reverse transcriptase-polymerase chain reaction (RT-PCR) of mRNA from RPE using primers homologous to conserved regions of the KHC motor domain resulted in the identification of two putative KHC genes (FKIF1 and FKIF5) based on partial amino acid sequences. Previous studies had demonstrated a requirement for microtubules in pigment granule aggregation in RPE. In addition, the reported microtubule polarity orientation in RPE apical projections is consistent with a role for kinesin in pigment granule aggregation. Immunofluorescent localization of KHC in isolated RPE cells using H2 revealed a mottled distribution over the entire cell body, with no detectable selective association with pigment granules, even in cells fixed while aggregating pigment granules. Microinjected KHC antibodies had no effect on pigment granule aggregation or dispersion, although each of the three antibodies has been shown to block kinesin function in other systems. Thus we found no evidence for KHC function in RPE pigment granule aggregation. However, the two KHC isoforms may participate in other microtubule-dependent processes in RPE.

Amino Acid Sequence↗

PubMed1995

Preparation of teleost rod inner and outer segments.

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Animals↗

PubMed1995

Evidence for D4 receptor regulation of retinomotor movement in isolated teleost cone inner-outer segments.

In the retinas of teleost fish, cone photoreceptors change shape in response to light and circadian signals. They elongate in the dark, contract in the light, and under conditions of constant darkness undergo appropriate movements at expected dusk and dawn. Dopamine induces cones to contract, thus mimicking the effect of light or expected dawn. To identify the receptor subtype responsible for mediating dopamine regulation of cone retinomotor movements, we have carried out pharmacological studies using isolated fragments of teleost cones consisting of cone inner segments-cone outer segments (CIS-COS). Isolated CIS-COS retain the ability to elongate in dark culture and contract when subsequently exposed to light or dopamine. We report that dark-induced elongation of CIS-COS was inhibited by dopamine and its agonists with an effectiveness ranking of dopamine = quinpirole > bromocriptine >>> SKF-38393. After 60 min of elongation in dark culture, CIS-COS myoids contracted when subsequently cultured in the dark with dopamine or quinpirole. Quinpirole-induced inhibition of elongation and quinpirole-induced contraction were completely blocked by clozapine at 1 microM or by sulpiride at 100 microM. These effectiveness profiles for dopamine agonists and antagonists suggest that dopamine regulation of cone retinomotor movement is mediated by a D4-like receptor.

Animals↗

PubMed1995

Phosducin and PP33 are in vivo targets of PKA and type 1 or 2A phosphatases, regulators of cell elongation in teleost rod inner-outer segments.

Teleost rod photoreceptors elongate in the light and shorten in darkness. We are investigating the role of cAMP-dependent protein kinase (PKA), phosphatases and target phosphoproteins in the regulation of photoreceptor cell shape. Preparations of rod fragments, consisting of the motile inner segment with attached photosensory outer segment (RIS-ROS), undergo light-stimulated elongation in culture. The PKA-selective inhibitor, H89, enhanced RIS-ROS elongation in both light and darkness, suggesting that elongation is associated with dephosphorylation of PKA substrates. Okadaic acid and calyculin A, inhibitors of type 1 and 2A phosphatases, blocked light-dependent and light-independent elongation with relative potencies suggesting that elongation requires dephosphorylation by type 1 phosphatase in light and type 2A phosphatase in darkness. To identify targets of PKA and phosphatases, RIS-ROS were isolated from retinas prelabeled with 32P-orthophosphate, and then incubated in the presence of kinase inhibitors or phosphatase inhibitors. Two phosphoproteins, PP33 and PP35, were phosphorylated by PKA and dephosphorylated by type 1 or 2A phosphatases in light- and dark-cultured RIS-ROS. PP35 (but not PP33) was immunoprecipitated by an antibody to phosducin, a PKA-regulated modulator of phototransduction (Lee et al., 1992); PP35 was also phosphorylated in vitro by a Ca2+ calmodulin-activated kinase. PP33 further differed from PP35 in its phosphopeptide maps and phosphorylation by PKC. We conclude that RIS-ROS elongation is correlated with the dephosphorylation of PKA substrates by type 1 or 2A phosphatases. Candidate mediator proteins include PP35, a fish phosducin homolog, and PP33, a newly described photoreceptor phosphoprotein.

Adaptation, Ocular↗

PubMed1994

Myosin I localizes to the midbody region during mammalian cytokinesis.

During cytokinesis, daughter cells are cleaved in two by the constriction of an actin-rich contractile ring which encircles the equator of the dividing cell. Filamentous myosin II is present in the contractile ring and necessary for constriction of the furrow, as shown in several cell types [Satterwhite and Pollard, 1992: Curr. Opin. Cell Biol. 4:43-52]. However, no functional role nor distinctive localization has been previously identified for non-filamentous "unconventional" myosins, such as myosin I, during cytokinesis. Using antibodies to adrenal medullary myosin I, we report that myosin I is localized in 3T3 fibroblasts to the mid-equatorial plane during late-cytokinesis, as well as to the polar edges as previously described in ameboid cells [Fukui et al., 1989: Nature 341:328-331]. Confocal microscopy revealed that myosin I is concentrated at the midbody region in a nearly continuous transverse disk, extending from the cortical region of the furrow through the midbody itself. These findings suggest that, in addition to the accepted role of filamentous myosin II in constriction of the contractile ring, nonfilamentous myosin I might contribute to motile events occurring late in cytokinesis.

3T3 Cells↗

PubMed1994

Myosin-I in retinal pigment epithelial cells.

PURPOSE: Myosin-I is a nonfilamentous motor protein associated with the actin cytoskeleton and cellular membranes in several cell types. The occurrence and subcellular distribution of myosin-I in mammalian and fish RPE were investigated to examine the possible role of myosin-I in retinal pigment epithelium (RPE) motility processes. METHODS: Antibodies directed against myosin-I proteins from bovine adrenal medulla or chicken intestinal brush border were used to examine cultured fetal human RPE cells and freshly isolated bovine or green sunfish RPE by Western immunoblots and immunocytochemistry, using both conventional and confocal fluorescence microscopy. RESULTS: The monoclonal antibody directed against bovine adrenal myosin-I identified a single strong immunoreactive band at 116 kD in Western blots of homogenates of cultured human RPE cells and 114 kD in bovine RPE sheets. An immunoreactive band of similar molecular weight was also observed in bovine and rabbit retina. Cell fractionation studies of bovine RPE cells revealed that myosin-I was present in all fractions that included cell membranes. The polyclonal antibody directed against chicken brush border myosin-I identified doublet immunoreactive bands at 115/110 kD in Western blots of homogenates of fish retina but identified a strong predominant immunoreactive band at 140 kD in fish RPE and brain homogenates; minor bands at 115/110 kD were identified in fish RPE homogenates. Immunocytochemistry of cultured human RPE cells using the bovine adrenal myosin-I antibody revealed a broad distribution of myosin-I that appeared to be most concentrated along the length of the lateral membranes; no colocalization was seen with actin-rich stress fibers. CONCLUSIONS: Proteins immunoreactive with myosin-I antibodies are present in both RPE and retina of mammals and green sunfish. In confluent cultures of human RPE cells, myosin-I is concentrated along the lateral cell membranes of the cuboidal cells.

Animals↗

PubMed1994

Suppression of cAMP-induced pigment granule aggregation in RPE by organic anion transport inhibitors.

PURPOSE: To investigate the mechanism(s) by which intracellular cAMP levels are elevated to induce pigment granule aggregation in teleost retinal pigment epithelium (RPE). METHODS: Pigment granule migration was studied in vitro using RPE sheets isolated from dark-adapted green sunfish, Lepomis cyanellus. After preculture to allow pigment granule dispersion, RPE sheets were incubated with various agents to test their ability to induce pigment granule aggregation. RPE sheets were then fixed, and pigment granule position was assessed microscopically. RESULTS: Pigment granule aggregation was induced by nonderivatized cAMP. At maximally effective concentration (1 mM), cAMP was as effective as its more membrane-permeant analogs dbcAMP and 8-Br-cAMP. Forskolin (1 or 10 microM), a stimulator of adenylyl cyclase, was also effective at inducing pigment aggregation. Two inhibitors of organic anion transport, probenecid and sulfinpyrazone, inhibited cAMP-induced aggregation by approximately 80% but had no effect on forskolin-induced aggregation. Several agents shown to stimulate RPE adenylyl cyclase in other species failed to induce pigment aggregation in isolated RPE sheets. CONCLUSIONS: Our observations strongly suggest that exogenously applied, nonderivatized cAMP can gain access to the cytoplasm of isolated RPE cells via organic anion transporters. Thus, if cAMP were secreted by retinal cells into the subretinal space, it could be taken up by RPE cells and subsequently act as an intracellular messenger to activate dark-adaptive physiological processes such as pigment granule aggregation.

Animals↗

PubMed1993

Light-activation of teleost rod photoreceptor elongation.

Rod photoreceptors in the retinas of teleost fish undergo changes in cell length in response to changing ambient light intensities. In the dark rods shorten and in the light rods elongate. These movements are mediated by actin-dependent processes which occur in the ellipsoid and myoid of the inner segment. As an approach to examining the underlying intracellular signaling pathways that link light absorption to actin-dependent motility in the inner segment, we have investigated the quantitative aspects of the light stimulus required to activate elongation in isolated rod inner/outer segments (RIS-ROS) of the green sunfish (Lepomis cyanellus). The intensity thresholds and strength-duration characteristics of the light stimulus required to activate teleost rod elongation were found to differ from those reported to activate vertebrate rod membrane hyperpolarization. In response to brief pulses of light, RIS-ROS elongated in a graded manner, both as a function of increasing light pulse intensity and light pulse duration. Half maximal activation of light-induced RIS-ROS elongation was produced by a stimulus of roughly 6 x 10(15) photons cm-2, which is calculated to bleach approximately 20% of the photopigment molecules in green sunfish rod outer segments. This degree of photopigment bleach is approximately 6-7 orders of magnitude greater than that required to elicit half maximal changes in membrane potential in other vertebrate rod preparations. Furthermore, the reciprocal relationship between light pulse intensity and duration in eliciting an equal elongation response held for relatively long light pulse durations.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1993

Identification of cyclic nucleotide-regulated phosphoproteins, including phosducin, in motile rod inner-outer segments of teleosts.

In teleost retinas, rods elongate in the light and shorten in the dark. Rod motility is mediated by the actin cytoskeleton of the inner segment and is regulated by cyclic AMP- or cyclic GMP-stimulated phosphorylation of target proteins. In this study, we have identified the target proteins of cyclic nucleotide-dependent kinases in rods, using preparations of isolated, motile rod inner-outer segments (RIS-ROS). Five proteins found in Percoll-purified RIS-ROS were phosphorylated in the presence of cAMP (> 10 nM), cGMP (> or = 10 microM) and exogenous catalytic subunit of cAMP-dependent protein kinase (PKA). The PKA inhibitor, PKI, blocked stimulation of phosphorylation by both cAMP and cGMP. Three cAMP-stimulated phosphoproteins were detected in cytoskeletal fractions of light- and dark-adapted RIS-ROS. One of these, PP33, appears to be a fish homologue of mammalian phosducin, based on immunolabeling by two different antibodies against mammalian phosducin and on electrophoretic characteristics in 2-D gels. Two additional phosducin immunoreactive bands were detected in Western blots. One, at 35 kDa, comigrated with a second cAMP-stimulated RIS-ROS phosphoprotein, PP35, which was also detected in the cytoskeleton. The other, at 37 kDa, was present in whole teleost retinas but not in purified RIS-ROS. Our results suggest that the effects of both cAMP and cGMP on teleost rod motility are mediated through PKA modulation of target phosphoproteins. These phosphoproteins include a cytoskeleton-associated phosducin homologue.

Animals↗

PubMed1993

Retinomotor movements in isolated teleost retinal cone inner-outer segment preparations (CIS-COS): effects of light, dark and dopamine.

Teleost cone inner segments elongate and contract in response to light and circadian signals. Previous studies have shown that teleost cone contraction is triggered by light or dopamine, while cone elongation is triggered by darkness or experimental elevation of cAMP. We have developed procedures for isolating and purifying motile cone fragments consisting of inner and outer segments (CIS-COS) to permit more detailed analysis of light and dopamine regulation of cone retinomotor movements. When retinas are dissected from long-term dark-adapted fish, CIS-COS break off at the base of the ellipsoid and remain attached to the RPE. CIS-COS can be detached from the RPE by brief protease treatment, thereby generating a highly enriched CIS-COS suspension. CIS-COS retain normal morphology and extend new myoids when cultured in darkness or in light plus forskolin, an activator of adenylate cyclase. The microtubule and actin cytoskeletons of the new myoids resemble those of intact cone myoids in vivo. Light inhibits CIS-COS myoid elongation, suggesting that light reception by the outer segment can directly influence cone motility. In dark-cultured CIS-COS, myoid elongation is inhibited half-maximally by nanomolar concentrations of dopamine, suggesting that dopamine effects on motility are mediated by D2-family receptors present on the cone inner and/or outer segment. After dark-induced elongation in culture, CIS-COS myoids can be induced to contract by subsequent culture in the light or with dopamine. Thus isolated cone inner and outer segments possess sufficient cytoskeletal and regulatory machinery to exhibit light- and dopamine-regulation retinomotor movement similar to that observed in intact cones in situ.

Animals↗

PubMed1993

Cyclic nucleotide regulation of teleost rod photoreceptor inner segment length.

Retinal rod photoreceptors of teleost fish elongate in the light and shorten in the dark. Rod cell elongation and shortening are both mediated by actin-dependent mechanisms that occur in the inner segment myoid and ellipsoid. The intracellular signaling pathways by which light and dark regulate the actin cytoskeleton in the inner segment are unknown. To investigate the intracellular signals that regulate teleost rod motility, we have been using mechanically isolated rod inner/outer segments (RIS-ROS) obtained from the retinas of green sunfish, Lepomis cyanellus. In culture, RIS-ROS retain the ability to elongate in response to light; myoids elongate 15-20 microns in length during 45 min of light culture. A pharmacological approach was taken to investigate the role of cyclic nucleotides, cyclic nucleotide-dependent kinases, and protein phosphatases in the regulation of RIS-ROS motility. Millimolar concentrations of cAMP and cGMP analogues were both found to inhibit light-induced myoid elongation and two cyclic nucleotide analogues, SpCAMPS and 8BrcGMP, promoted myoid shortening after RIS-ROS had elongated in response to light. The cyclic nucleotide-dependent kinase inhibitor, H8, mimicked light by promoting myoid elongation in the dark. The effects of H8 were dose dependent, with maximal elongation occurring at concentrations of approximately 100 microM. Similar to the effects of cyclic nucleotide analogues, the phosphatase inhibitor, okadaic acid (0.1-10 microM), inhibited light-induced elongation and promoted shortening. The results presented here suggest that RIS-ROS motility is regulated by protein phosphorylation: phosphorylation in the dark by cyclic nucleotide-dependent protein kinases promotes rod shortening, while dephosphorylation in the light promotes rod elongation.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗

PubMed1993

Light and circadian modulation of teleost retinal tyrosine hydroxylase activity.

PURPOSE: The authors examined the effects of both light and circadian phase on the synthetic activity of the dopamine producing enzyme, tyrosine hydroxylase (TH), in the retina of the Midas cichlid (Cichlasoma citrinellum). METHODS: TH activity was assayed in the retina-retinal pigment epithelium (RPE) complex of fish entrained to cyclic light regimens. The animals were killed throughout cyclic light and continuous dark cycles and after light exposure following short-term dark adaptation to assess the effects of both diurnal and circadian phase and light exposure on TH activity. Variations in retina-RPE TH activity were compared with cone retinomotor position. RESULTS: The authors report that TH activity was influenced both by light and by circadian phase. In both cyclic light and continuous darkness, TH activity fluctuated in cyclic fashion, increasing to peak values at night, declining through dawn, and reaching minimal values at midday. In continuous dark-adapted retinae, the peak activity at night was approximately sixfold greater than the minimum activity during the day. In cyclic light-adapted retinae at midday, the TH activity was 2.4-fold higher than that determined at the same time point in continuous dark-adapted retinae. Light onset enhanced the TH activity in previously dark-adapted animals by 2.2-fold at midday and 1.7-fold at dusk. Thus, retinal TH activity exhibits fluctuations of approximately sixfold in response to circadian signals and is stimulated approximately twofold by light. CONCLUSIONS: These observations indicate that dopamine production in the retina is regulated both by light and by circadian phase and that there are cyclic changes in dopamine production during the 24-hr cycle both in normal light and dark cycles and under conditions of constant darkness.

Animals↗

PubMed1992

Actin-dependent myoid elongation in teleost rod inner/outer segments occurs in the absence of net actin polymerization.

In the retinas of teleost fish, rod photoreceptors elongate in response to light. Light-activated elongation is mediated by the myoid of the rod inner segment and is actin-dependent. Inner segment F-actin filaments form bundles running parallel to the cell's long axis. We examined the mechanism of rod elongation using mechanically-detached rod fragments, consisting of the motile inner segment and sensory outer segment (RIS-ROS). When RIS-ROS are isolated from dark-adapted green sunfish and cultured in the light, they elongate 15 microns at 0.3-0.6 microns/min. Elongation was inhibited 65% by 0.1 microM Cytochalasin D, suggesting a requirement for actin assembly. To determine the extent of assembly during elongation, we used three approaches to measure the F-actin content in RIS-ROS: detection of pelletable actin by SDS-PAGE after detergent-extraction of RIS-ROS; quantification of fluorescein-phalloidin binding by fluorimetry, fluorescence-activated cell sorting and image analysis; estimation of total F-actin filament length by electron microscopy. All three assays indicated that no net assembly of RIS-ROS F-actin accompanied myoid elongation. An increase in F-actin content within the elongated myoid was counterbalanced by a decrease in F-actin content within the 13 microvillus-like calycal processes located at the end of the inner segment opposite to the growing myoid. O'Connor and Burnside (Journal of Cell Biology 89:517-524, 1981) showed that minus-ends of rod F-actin filaments are oriented towards the elongating myoid while plus-ends are oriented towards the shortening calycal processes. Our observations suggest that RIS-ROS elongation entails actin polymerization at the minus-ends of filaments coupled with depolymerization at the filament plus-ends.

Actin Cytoskeleton↗

PubMed1992

A role for endogenous dopamine in circadian regulation of retinal cone movement.

Cone movements in the retina of the Midas cichlid (Cichlasoma citrinellum) take place in response both to light and endogenous circadian signals. In the normal light/dark cycle (LD) cone myoids are long at night (50-55 microns), begin to contract before expected dawn, and with light onset contract to their fully contracted positions (5 microns) which are retained throughout the day. In continuous darkness (DD) cone myoids are fully elongate at night, but undergo pre-dawn contractions to partially contracted positions which they retain throughout expected day (20-25 microns). To investigate the mechanisms by which circadian signals modulate cone myoid movements in teleost retinas, we have tested the effects on circadian cone movements of optic nerve section, intraocular injection of dopamine agonists or antagonists, and intraocular injection of melatonin. We report here that both light-induced and circadian-driven cone myoid movements can occur in the absence of efferent input from higher centres: both are retained with full amplitude after optic nerve section in vivo. Intraocular injection studies suggest that circadian regulation of cone myoid movement is mediated locally within the eye by dopamine acting via a dopaminergic D2-receptor. Cone myoid contraction can be induced at midnight in LD or DD animals by intraocular injection of dopamine or the D2-receptor agonist LY171555. The partially contracted cones of DD animals at expected mid-day can be induced to fully contract by intraocular injection of dopamine or the D2-receptor agonist, or to elongate by intraocular injection of the dopamine D2-antagonist sulpiride. Furthermore, the pre-dawn cone myoid contraction observed in both LD and DD animals in response to circadian signals can be completely blocked in DD animals by intraocular injection of the D2-antagonist sulpiride shortly before the time of expected light onset. In contrast, circadian cone myoid movements were unaffected by intraocular injection of the D1-receptor agonist SCH23390, or the D1-receptor antagonist SKF38393. In addition, we report that intraocularly injected melatonin had no effect on cone position when injected in the light at mid-day, in darkness at midnight or in darkness just before expected light onset at dawn. However, both melatonin and iodomelatonin induced cone myoid contraction (the light-adaptive movement) when injected in darkness at expected mid-day in DD animals. This paradoxical result is not consistent with observations from other species in which melatonin induces dark-adaptive photoreceptor responses.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1992

Shortening of the calycal process actin cytoskeleton is correlated with myoid elongation in teleost rods.

Light activates the elongation of rods within teleost retinas. Rod cell elongation is mediated by actin-dependent length changes of the myoid portion of the inner segment. The actin cytoskeleton of the inner segment consists of filament bundles, which run parallel to the long axis of the rod, from the calycal processes, through the ellipsoid and into the myoid. In isolated rod inner/outer segments (RIS-ROS), myoid elongation was found to occur in the absence of net polymerization of actin into filaments. Outgrowth of actin filaments within the myoid was counterbalanced by a shortening of actin filaments within the calycal processes. In this study, we have further examined light-activated modifications of the rod cytoskeleton using rhodamine-phalloidin to stain actin filaments within retinal cryosections as well as in isolated RIS-ROS. In RIS-ROS isolated from dark-adapted green sunfish, the phalloidin-stained calycal processes appeared as long, brush-like structures, averaging 4.2 microns in length. In light-cultured RIS-ROS populations, the calycal process actin cytoskeleton shortened from 4.2 microns to 1.7 microns. In control, dark-cultured populations, RIS-ROS that did not elongate maintained long calycal process actin cytoskeletons. However, in cases where dark-cultured RIS-ROS did elongate, despite the absence of a light stimulus, myoid elongation was accompanied by a shortening of the calycal process actin cytoskeleton, suggesting that the two events are correlated with one another. In light-adapted green sunfish and in light-cultured retinas from green sunfish and the Midas cichlid, the calycal process cytoskeleton of intact rods shortened by 40-60%. Within the two-tiered retina of green sunfish, shortening of the calycal process cytoskeleton, from 5.1 microns to 2.1-3.1 microns, was only evident in the shorter, inner tier of rods. The calycal process actin cytoskeleton did not appear to shorten within the longer, outer tier of rods; here, stained processes were short, averaging 2.3 microns in length, within dark-adapted retinas. Using scanning and transmission electron microscopy, we present evidence to suggest that the plasmalemmal surface of the calycal processes shortens along with the cytoskeletal actin core. We conclude that calycal processes of teleost rods are dynamic structures which shorten during light-activated myoid elongation.

Animals↗

PubMed1991

Posttranslational modifications of tubulin in teleost photoreceptor cytoskeletons.

1. Posttranslational modifications of tubulin by acetylation and detyrosination have been correlated previously with microtubule stability in numerous cell types. 2. In this study, posttranslational modifications of tubulin and their regional distribution within teleost photoreceptor cones and rods are demonstrated immunohistochemically using antibodies specific for acetylated, detyrosinated, or tyrosinated tubulin. 3. Immunolocalization was carried out on isolated whole cones and mechanically detached rod and cone inner/outer segments. 4. Acetylated tubulin within rods and cones is found only in microtubules of the ciliary axoneme of the outer segment. Detyrosinated tubulin is also enriched in axonemes of both rod and cone outer segments. 5. Distributions of tyrosinated and detyrosinated cytoplasmic microtubules differ within cones and rods. In cones, detyrosinated and tyrosinated tubulins are both abundant throughout the cell body. In rods, the ellipsoid and myoid contain much more tyrosinated tubulin than detyrosinated tubulin. Comparisons between whole cones and cone fragments suggest that detyrosinated microtubules are more stable than tyrosinated microtubules in teleost photoreceptors. 6. Our findings provide further evidence that microtubules of teleost cones differ from rod microtubules in their stabilities and rapidity of turnover within the photoreceptor inner segment.

Acetylation↗

PubMed1991

Effects of circadian phase on cone retinomotor movements in the Midas cichlid.

In retinas of the Midas cichlid (Cichlasoma citrinellum), cone myoid lengths change in response to both light and circadian signals. Cone myoids are short in day (or in the light), and long at night (or in darkness). During the normal light/dark cycle, cone myoids elongate to 58 microns after light offset at dusk, and remain long until just before dawn, at which time they begin to contract before the time of expected light onset and eventually (after light onset) assume daytime lengths of 5 microns. In continuous light, cone myoids remain fully contracted regardless of the time in the cycle. In continuous darkness, cone myoids undergo circadian movements; they elongate fully at night and contract partly during expected day to 23 microns (65% of the contraction seen at dawn in a normal light/dark cycle). To clarify further the regulation of these retinomotor movements, we have investigated the effects of circadian phase on cone movements induced by light or dark onset. Circadian phase had no significant effect on either the initial rate or the final extent of light-induced cone myoid contraction: at mid-dark cones began to contract immediately after light onset and contracted fully at 3.0 +/- 0.5 microns min-1; at mid-night cones contracted immediately and fully at 4.0 +/- 0.2 microns min-1. At all of the relatively high intensities of light tested (30-5000 lx) cones contracted at similar rates and to similar final extents.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗