Organization of actin filaments and immunocolocalization of alpha-actinin in the connecting cilium of rat photoreceptors

NUDC is critical for rod photoreceptor function, maintenance, and survival

NUDC (nuclear distribution protein C) is a mitotic protein involved in nuclear migration and cytokinesis across species. Considered a cytoplasmic dynein (henceforth dynein) cofactor, NUDC was shown to associate with the dynein motor complex during neuronal migration. NUDC is also expressed in postmitotic vertebrate rod photoreceptors where its function is unknown. Here, we examined the role of NUDC in postmitotic rod photoreceptors by studying the consequences of a conditional NUDC knockout in mouse rods (rNudC-/- ). Loss of NUDC in rods led to complete photoreceptor cell death at 6 weeks of age. By 3 weeks of age, rNudC-/- function was diminished, and rhodopsin and mitochondria were mislocalized, consistent with dynein inhibition. Levels of outer segment proteins were reduced, but LIS1 (lissencephaly protein 1), a well-characterized dynein cofactor, was unaffected. Transmission electron microscopy revealed ultrastructural defects within the rods of rNudC-/- by 3 weeks of age. We investigated whether NUDC interacts with the actin modulator cofilin 1 (CFL1) and found that in rods, CFL1 is localized in close proximity to NUDC. In addition to its potential role in dynein trafficking within rods, loss of NUDC also resulted in increased levels of phosphorylated CFL1 (pCFL1), which would purportedly prevent depolymerization of actin. The absence of NUDC also induced an inflammatory response in Müller glia and microglia across the neural retina by 3 weeks of age. Taken together, our data illustrate the critical role of NUDC in actin cytoskeletal maintenance and dynein-mediated protein trafficking in a postmitotic rod photoreceptor.

NUDC is critical for rod photoreceptor function, maintenance, and survival

NUDC ( nu clear d istribution protein C) is a mitotic protein involved in nuclear migration and cytokinesis across species. Considered a cytoplasmic dynein (henceforth dynein) cofactor, NUDC was shown to associate with the dynein motor complex during neuronal migration. NUDC is also expressed in postmitotic vertebrate rod photoreceptors where its function is unknown. Here, we examined the role of NUDC in postmitotic rod photoreceptors by studying the consequences of a conditional NUDC knockout in mouse rods (r NudC -/- ). Loss of NUDC in rods led to complete photoreceptor cell death at six weeks of age. By 3 weeks of age, r NudC -/- function was diminished, and rhodopsin and mitochondria were mislocalized, consistent with dynein inhibition. Levels of outer segment proteins were reduced, but LIS1 (lissencephaly protein 1), a well-characterized dynein cofactor, was unaffected. Transmission electron microscopy revealed ultrastructural defects within the rods of r NudC -/- by 3 weeks of age. We investigated whether NUDC interacts with the actin modulator cofilin 1 (CFL1) and found that in rods, CFL1 is localized in close proximity to NUDC. In addition to its potential role in dynein trafficking within rods, loss of NUDC also resulted in increased levels of phosphorylated CFL1 (pCFL1), which would purportedly prevent depolymerization of actin. Absence of NUDC also induced an inflammatory response in Müller glia and microglia across the neural retina by 3 weeks of age. Taken together, our data illustrate the critical role of NUDC in actin cytoskeletal maintenance and dynein-mediated protein trafficking in a postmitotic rod photoreceptor.

Significance Statement

Nuclear distribution protein C (NUDC) has been studied extensively as an essential protein for mitotic cell division. In this study, we discovered its expression and role in the postmitotic rod photoreceptor cell. In the absence of NUDC in mouse rods, we detected functional loss, protein mislocalization, and rapid retinal degeneration consistent with dynein inactivation. In the early phase of retinal degeneration, we observed ultrastructural defects and an upregulation of inflammatory markers suggesting additional, dynein-independent functions of NUDC.

A Ciliary Branched Actin Network Drives Photoreceptor Disc Morphogenesis

The light-detecting organelle of the photoreceptor cell is a modified primary cilium, called the outer segment. The outer segment houses hundreds of light-sensitive membrane, "discs," that are continuously renewed by the constant formation of new discs at the outer segment base and the phagocytosis of old ones from outer segment tips by the retinal pigment epithelium. In this chapter, we describe how an actin cytoskeleton network, residing precisely at the site of disc formation, provides the driving force that pushes out the ciliary plasma membrane to form each disc evagination that subsequently can mature into a bona fide disc. We highlight the functions of actin-binding proteins, particularly PCARE and Arp2/3, that are known to participate in disc formation. Finally, we describe a working model of disc formation built upon the many studies focusing on the role of actin during disc morphogenesis.

Tmem138 is localized to the connecting cilium essential for rhodopsin localization and outer segment biogenesis

The connecting cilium (CC) of the photoreceptor provides the only route for the trafficking of the outer segment (OS) proteins. Failure of OS protein transport causes degenerative photoreceptor diseases, including retinitis pigmentosa. We demonstrate that Tmem138, a protein linked to ciliopathy, is localized to the photoreceptor CC. Germline deletion of Tmem138 abolished OS morphogenesis, followed by rapid photoreceptor degeneration. Tmem138 interacts with rhodopsin and two additional CC compartment proteins, Ahi1 and Tmem231, likely forming a membrane complex to facilitate trafficking of rhodopsin and other OS-bound proteins across the CC. The study thus implicates a new line of regulation on the delivery of OS proteins through interactions with CC membrane complex(es) and provides insights into photoreceptor ciliopathy diseases.

Photoreceptor connecting cilium (CC) is structurally analogous to the transition zone (TZ) of primary cilia and gates the molecular trafficking between the inner and the outer segment (OS). Retinal dystrophies with underlying CC defects are manifested in a broad array of syndromic conditions known as ciliopathies as well as nonsyndromic retinal degenerations. Despite extensive studies, many questions remain in the mechanism of protein trafficking across the photoreceptor CC. Here, we genetically inactivated mouse Tmem138, a gene encoding a putative transmembrane protein localized to the ciliary TZ and linked to ciliopathies. Germline deletion of Tmem138 abolished OS morphogenesis, followed by rapid photoreceptor degeneration. Tmem138 was found localized to the photoreceptor CC and was required for localization of Ahi1 to the distal subdomain of the CC. Among the examined set of OS proteins, rhodopsin was mislocalized throughout the mutant cell body prior to OS morphogenesis. Ablation of Tmem138 in mature rods recapitulated the molecular changes in the germline mutants, causing failure of disc renewal and disintegration of the OS. Furthermore, Tmem138 interacts reciprocally with rhodopsin and a related protein Tmem231, and the ciliary localization of the latter was also altered in the mutant photoreceptors. Taken together, these results suggest a crucial role of Tmem138 in the functional organization of the CC, which is essential for rhodopsin localization and OS biogenesis.

Effect of lumbopelvic control on landing mechanics and lower extremity muscles' activities in female professional athletes: implications for injury prevention

Background

Lumbopelvic control (LPC) has recently been associated with function, kinesiology, and load distribution on the limb. However, poor LPC has not been studied as a risk factor for lower limb injury in sports requiring frequent jump landings. The present study investigated the effects of LPC on landing mechanics and lower limb muscle activity in professional athletes engaged in sport requiring frequent landing.

Methods

This study was conducted on 34 professional female athletes aged 18.29 ± 3.29 years with the height and body mass of 173.5 ± 7.23 cm and 66.79 ± 13.37 kg, respectively. The landing error scoring system (LESS) and ImageJ software were used to assess landing mechanics. Wireless electromyography was also used to record the activity of the gluteus medius (GMed), rectus femoris, and semitendinosus. Lumbopelvic control was evaluated using the knee lift abdominal test, bent knee fall-out, active straight leg raising, and the PRONE test using a pressure biofeedback unit. Based on the LPC tests results, the participants were divided into two groups of proper LPC (n = 17) and poor LPC (n = 17).

Results

There were significant differences between the groups with proper and poor LPC in terms of the LESS test scores (P = 0.0001), lateral trunk flexion (P = 0.0001), knee abduction (P = 0.0001), knee flexion (P = 0.001), trunk flexion (P = 0.01), and GMed muscle activity (P = 0.03). There were no significant differences in the activity of the rectus femoris and semitendinosus muscles, and ankle dorsiflexion (P > 0.05).

Conclusions

Poor lumbopelvic control affects the kinematics and activity of the lower limb muscles, and may be a risk factor for lower limb injuries, especially of the knee.

Photoreceptor Discs: Built Like Ectosomes

The light-sensitive outer segment organelle of the vertebrate photoreceptor cell is a modified cilium filled with hundreds of flattened 'disc' membranes that provide vast light-absorbing surfaces. The outer segment is constantly renewed with new discs added at its base every day. This continuous process is essential for photoreceptor viability. In this review, we describe recent breakthroughs in the understanding of disc morphogenesis, with a focus on the molecular mechanisms responsible for initiating disc formation from the ciliary membrane. We highlight the discoveries that this mechanism evolved from an innate ciliary process of releasing small extracellular vesicles, or ectosomes, and that both disc formation and ectosome release rely on the actin cytoskeleton.

PCARE and WASF3 regulate ciliary F-actin assembly that is required for the initiation of photoreceptor outer segment disk formation

The outer segments (OS) of rod and cone photoreceptor cells are specialized sensory cilia that contain hundreds of opsin-loaded stacked membrane disks that enable phototransduction. The biogenesis of these disks is initiated at the OS base, but the driving force has been debated. Here, we studied the function of the protein encoded by the photoreceptor-specific gene C2orf71, which is mutated in inherited retinal dystrophy (RP54). We demonstrate that C2orf71/PCARE (photoreceptor cilium actin regulator) can interact with the Arp2/3 complex activator WASF3, and efficiently recruits it to the primary cilium. Ectopic coexpression of PCARE and WASF3 in ciliated cells results in the remarkable expansion of the ciliary tip. This process was disrupted by small interfering RNA (siRNA)-based down-regulation of an actin regulator, by pharmacological inhibition of actin polymerization, and by the expression of PCARE harboring a retinal dystrophy-associated missense mutation. Using human retinal organoids and mouse retina, we observed that a similar actin dynamics-driven process is operational at the base of the photoreceptor OS where the PCARE module and actin colocalize, but which is abrogated in Pcare-/- mice. The observation that several proteins involved in retinal ciliopathies are translocated to these expansions renders it a potential common denominator in the pathomechanisms of these hereditary disorders. Together, our work suggests that PCARE is an actin-associated protein that interacts with WASF3 to regulate the actin-driven expansion of the ciliary membrane at the initiation of new outer segment disk formation.

Photoreceptor disc membranes are formed through an Arp2/3-dependent lamellipodium-like mechanism

The light-sensitive outer segment of the vertebrate photoreceptor is a highly modified primary cilium filled with disc-shaped membranes that provide a vast surface for efficient photon capture. The formation of each disc is initiated by a ciliary membrane evagination driven by an unknown molecular mechanism reportedly requiring actin polymerization. Since a distinct F-actin network resides precisely at the site of disc morphogenesis, we employed a unique proteomic approach to identify components of this network potentially driving disc morphogenesis. The only identified actin nucleator was the Arp2/3 complex, which induces the polymerization of branched actin networks. To investigate the potential involvement of Arp2/3 in the formation of new discs, we generated a conditional knockout mouse lacking its essential ArpC3 subunit in rod photoreceptors. This knockout resulted in the complete loss of the F-actin network specifically at the site of disc morphogenesis, with the time course of ArpC3 depletion correlating with the time course of F-actin loss. Without the actin network at this site, the initiation of new disc formation is completely halted, forcing all newly synthesized membrane material to be delivered to the several nascent discs whose morphogenesis had already been in progress. As a result, these discs undergo uncontrolled expansion instead of normal enclosure, which leads to formation of unusual, large membrane whorls. These data suggest a model of photoreceptor disc morphogenesis in which Arp2/3 initiates disc formation in a "lamellipodium-like" mechanism.

The route of the visual receptor rhodopsin along the cilium

The photoreceptor outer segment is the most elaborate primary cilium, containing large amounts of rhodopsin (RHO) in disk membranes that grow from a connecting cilium. The movement of RHO along the connecting cilium precedes formation of the disk membranes. However, the route that RHO takes has not been clearly determined; some reports suggest that it follows an intracellular, vesicular route along the axoneme, possibly as an adaptation for the high load of delivery or the morphogenesis of the disk endomembranes. We addressed this question by studying RHO in cilia of IMCD3 cells and mouse rod photoreceptors. In IMCD3 cilia, fluorescence recovery after photobleaching (FRAP) experiments with fluorescently tagged RHO supported the idea of RHO motility in the ciliary plasma membrane and was inconsistent with the hypothesis of RHO motility within the lumen of the cilium. In rod photoreceptors, FRAP of RHO-EGFP was altered by externally applied lectin, supporting the idea of plasmalemmal RHO dynamics. Quantitative immunoelectron microscopy corroborated our live-cell conclusions, as RHO was found to be distributed along the plasma membrane of the connecting cilium, with negligible labeling within the axoneme. Taken together, the present findings demonstrate RHO trafficking entirely via the ciliary plasma membrane.This article has an associated First Person interview with the first author of the paper.

Gelsolin dysfunction causes photoreceptor loss in induced pluripotent cell and animal retinitis pigmentosa models

Mutations in the Retinitis Pigmentosa GTPase Regulator (RPGR) cause X-linked RP (XLRP), an untreatable, inherited retinal dystrophy that leads to premature blindness. RPGR localises to the photoreceptor connecting cilium where its function remains unknown. Here we show, using murine and human induced pluripotent stem cell models, that RPGR interacts with and activates the actin-severing protein gelsolin, and that gelsolin regulates actin disassembly in the connecting cilium, thus facilitating rhodopsin transport to photoreceptor outer segments. Disease-causing RPGR mutations perturb this RPGR-gelsolin interaction, compromising gelsolin activation. Both RPGR and Gelsolin knockout mice show abnormalities of actin polymerisation and mislocalisation of rhodopsin in photoreceptors. These findings reveal a clinically-significant role for RPGR in the activation of gelsolin, without which abnormalities in actin polymerisation in the photoreceptor connecting cilia cause rhodopsin mislocalisation and eventual retinal degeneration in XLRP.

Mutations in the Retinitis Pigmentosa GTPase Regulator (RPGR) cause retinal dystrophy, but how this arises at a molecular level is unclear. Here, the authors show in induced pluripotent stem cells and mouse knockouts that RPGR mediates actin dynamics in photoreceptors via the actin-severing protein, gelsolin.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.

Rhodopsin Trafficking and Mistrafficking: Signals, Molecular Components, and Mechanisms

Rhodopsin is a seven-transmembrane G protein-coupled receptor (GPCR) and is the main component of the photoreceptor outer segment (OS), a ciliary compartment essential for vision. Because the OSs are incapable of protein synthesis, rhodopsin must first be synthesized in the inner segments (ISs) and subsequently trafficked across the connecting cilia to the OSs where it participates in the phototransduction cascade. Rapid turnover of the OS necessitates a high rate of synthesis and efficient trafficking of rhodopsin to the cilia. This cilia-targeting mechanism is shared among other ciliary-localized GPCRs. In this review, we will discuss the process of rhodopsin trafficking from the IS to the OS beginning with the trafficking signals present on the protein. Starting from the endoplasmic reticulum and the Golgi apparatus within the IS, we will cover the molecular components assisting the biogenesis and the proper sorting. We will also review the confirmed binding and interacting partners that help target rhodopsin toward the connecting cilium as well as the cilia-localized components which direct proteins into the proper compartments of the OS. While rhodopsin is the most critical and abundant component of the photoreceptor OS, mutations in the rhodopsin gene commonly lead to its mislocalization within the photoreceptors. In addition to covering the trafficking patterns of rhodopsin, we will also review some of the most common rhodopsin mutants which cause mistrafficking and subsequent death of photoreceptors. Toward the goal of understanding the pathogenesis, three major mechanisms of aberrant trafficking as well as putative mechanisms of photoreceptor degeneration will be discussed.

Submembrane assembly and renewal of rod photoreceptor cGMP-gated channel: insight into the actin-dependent process of outer segment morphogenesis

The photoreceptor outer segment (OS) is comprised of two compartments: plasma membrane (PM) and disk membranes. It is unknown how the PM renewal is coordinated with that of the disk membranes. Here we visualized the localization and trafficking process of rod cyclic nucleotide-gated channel α-subunit (CNGA1), a PM component essential for phototransduction. The localization was visualized by fusing CNGA1 to a fluorescent protein Dendra2 and expressing in Xenopus laevis rod photoreceptors. Dendra2 allowed us to label CNGA1 in a spatiotemporal manner and therefore discriminate between old and newly trafficked CNGA1-Dendra2 in the OS PM. Newly synthesized CNGA1 was preferentially trafficked to the basal region of the lateral OS PM where newly formed and matured disks are also added. Unique trafficking pattern and diffusion barrier excluded CNGA1 from the PM domains, which are the proposed site of disk membrane maturation. Such distinct compartmentalization allows the confinement of cyclic nucleotide-gated channel in the PM, while preventing the disk membrane incorporation. Cytochalasin D and latrunculin A treatments, which are known to disrupt F-actin-dependent disk membrane morphogenesis, prevented the entrance of newly synthesized CNGA1 to the OS PM, but did not prevent the entrance of rhodopsin and peripherin/rds to the membrane evaginations believed to be disk membrane precursors. Uptake of rhodopsin and peripherin/rds coincided with the overgrowth of the evaginations at the base of the OS. Thus F-actin is essential for the trafficking of CNGA1 to the ciliary PM, and coordinates the formations of disk membrane rim region and OS PM.

Cone outer segments: a biophysical model of membrane dynamics, shape retention, and lamella formation

An hypothesis is developed to explain how the unique, right circular conical geometry of cone outer segments (COSs) in Xenopus laevis and other lower vertebrates is maintained during the cycle of axial shortening by apical phagocytosis and axial elongation via the addition of new basal lamellae. Extension of a new basal evagination (BE) applies radial (lateral) traction to membrane and cytoplasmic domains, achieving two coupled effects. 1), The bilayer domain is locally stretched/dilated, creating an entropic driving force that draws membrane components into the BE from the COS's distributed bilayer phase, i.e., plasmalemma and older lamellae (membrane recycling). Membrane proteins, e.g., opsins, are carried passively in this advective, bilayer-driven process. 2), With BE stretching, hydrostatic pressure within the BE cytoplasm is reduced slightly with respect to that of the axonemal cytoplasmic reservoir, allowing cytoplasmic flow into the BE. Attendant lowering of the reservoir's hydrostatic pressure facilitates the subsequent transfer of cytoplasm from lamellar domains to the reservoir (cytoplasmic recycling). The geometry of the BE reflects the membrane/cytoplasm ratio needed for its construction, and essentially specifies the ratio of components recycled from older lamellae. Length and taper angle of the COS reflect the ratio of recycled/new components constructing a new BE. The model also integrates the trajectories and dynamics of lamella open margin lattice components. Although not fully evaluated, the initial model has been assessed against the relevant literature, and three experimental predictions are derived.

α-Actinin-2 deficiency results in sarcomeric defects in zebrafish that cannot be rescued by α-actinin-3 revealing functional differences between sarcomeric isoforms

α-Actinins are actin-binding proteins that can be broadly divided into Ca(2+)-sensitive cytoskeletal and Ca(2+)-insensitive sarcomeric isoforms. To date, little is known about functional differences between the isoforms due to their indistinguishable activities in most in vitro assays. To identify functional differences in vivo between sarcomeric isoforms, we employed computational and molecular approaches to characterize the zebrafish (Danio rerio) genome, which contains orthologoues of each human α-actinin gene, including duplicated copies of actn3. Each isoform exhibits a distinct and unique pattern of gene expression as assessed by mRNA in situ hybridization, largely sharing similar expression profiles as seen in humans. The spatial conservation of expression of these genes from lower invertebrates to humans suggests that regulation and subsequent functions of these genes are conserved during evolution. Morpholino-based knockdown of the sarcomeric isoform, actn2, leads to skeletal muscle, cardiac, and ocular defects evident over the first week of development. Remarkably, despite the high degree of sequence conservation between actn2 and actn3, the phenotypes of α-actinin-2 deficient zebrafish can be rescued by overexpression of α-actinin-2 but not by α-actinin-3 mRNAs from zebrafish or human. These data provide functional evidence that the primary sequences of α-actinin-2 and α-actinin-3 evolved differences to optimize their functions.

Heterotrimeric kinesin-II is necessary and sufficient to promote different stepwise assembly of morphologically distinct bipartite cilia in Drosophila antenna

Structurally diverse sensory cilia have evolved from primary cilia, a microtubule-based cellular extension engaged in chemical and mechanical sensing and signal integration. The diversity is often associated with functional specialization. The olfactory receptor neurons in Drosophila, for example, express three distinct bipartite cilia displaying different sets of olfactory receptors on them. Molecular description underlying their assembly and diversification is still incomplete. Here, we show that the branched and the slender olfactory cilia develop in two distinct step-wise patterns through the pupal stages before the expression of olfactory receptor genes in olfactory neurons. The process initiates with a thin procilium growth from the dendrite apex, followed by volume increment in successive stages. Mutations in the kinesin-II subunit genes either eliminate or restrict the cilia growth as well as tubulin entry into the developing cilia. Together with previous results, our results here suggest that heterotrimeric kinesin-II is the primary motor engaged in all type-I sensory cilia assembly in Drosophila and that the cilia structure diversity is achieved through additional transports supported by the motor during development.

The translocation of signaling molecules in dark adapting mammalian rod photoreceptor cells is dependent on the cytoskeleton

In vertebrate rod photoreceptor cells, arrestin and the visual G-protein transducin move between the inner segment and outer segment in response to changes in light. This stimulus dependent translocation of signalling molecules is assumed to participate in long term light adaptation of photoreceptors. So far the cellular basis for the transport mechanisms underlying these intracellular movements remains largely elusive. Here we investigated the dependency of these movements on actin filaments and the microtubule cytoskeleton of photoreceptor cells. Co-cultures of mouse retina and retinal pigment epithelium were incubated with drugs stabilizing and destabilizing the cytoskeleton. The actin and microtubule cytoskeleton and the light dependent distribution of signaling molecules were subsequently analyzed by light and electron microscopy. The application of cytoskeletal drugs differentially affected the cytoskeleton in photoreceptor compartments. During dark adaptation the depolymerization of microtubules as well as actin filaments disrupted the translocation of arrestin and transducin in rod photoreceptor cells. During light adaptation only the delivery of arrestin within the outer segment was impaired after destabilization of microtubules. Movements of transducin and arrestin required intact cytoskeletal elements in dark adapting cells. However, diffusion might be sufficient for the fast molecular movements observed as cells adapt to light. These findings indicate that different molecular translocation mechanisms are responsible for the dark and light associated translocations of arrestin and transducin in rod photoreceptor cells.

A role for cytoskeletal elements in the light-driven translocation of proteins in rod photoreceptors

PURPOSE

Light-driven protein translocation is responsible for the dramatic redistribution of some proteins in vertebrate rod photoreceptors. In this study, the involvement of microtubules and microfilaments in the light-driven translocation of arrestin and transducin was investigated.

METHODS

Pharmacologic reagents were applied to native and transgenic Xenopus tadpoles, to disrupt the microtubules (thiabendazole) and microfilaments (cytochalasin D and latrunculin B) of the rod photoreceptors. Quantitative confocal imaging was used to assess the impact of these treatments on arrestin and transducin translocation. A series of transgenic tadpoles expressing arrestin truncations were also created to identify portions of arrestin that enable arrestin to translocate.

RESULTS

Application of cytochalasin D or latrunculin B to disrupt the microfilament organization selectively slowed only transducin movement from the inner to the outer segments. Perturbation of the microtubule cytoskeleton with thiabendazole slowed the translocation of both arrestin and transducin, but only in moving from the outer to the inner segments. Transgenic Xenopus expressing fusions of green fluorescent protein (GFP) with portions of arrestin implicates the C terminus of arrestin as an important portion of the molecule for promoting translocation. This C-terminal region can be used independently to promote translocation of GFP in response to light.

CONCLUSIONS

The results show that disruption of the cytoskeletal network in rod photoreceptors has specific effects on the translocation of arrestin and transducin. These effects suggest that the light-driven translocation of visual proteins at least partially relies on an active motor-driven mechanism for complete movement of arrestin and transducin.

RP1 is required for the correct stacking of outer segment discs

PURPOSE

Mutations in RP1 are a common cause of dominant retinitis pigmentosa (RP), but the mechanism by which the identified mutations lead to photoreceptor cell death and blindness has not been determined. To investigate the function of the RP1 protein in photoreceptors and gain insight into the mechanism of disease, gene-targeting techniques were used to produce mice with a mutant Rp1 allele that mimics the truncation alleles found to cause disease.

METHODS

RT-PCR was used to amplify illegitimate RP1 transcripts from lymphoblasts. Gene targeting was used to create mice with a mutant Rp1-myc allele. Confocal immunofluorescence microscopy was used to identify the location of the mutant Rp1-myc protein in photoreceptors. The structure of the photoreceptors in the resultant Rp1-myc mice was studied by light and electron microscopy. The retinal function of the mutant mice was investigated using analysis of full-field ERGs.

RESULTS

Wild-type and mutant RP1 mRNA were both detected in lymphoblasts from patients with RP1 disease. Rp1-myc mice produced a truncated version of the Rp1 protein, containing the N-terminal 662 amino acids, which localized correctly to the axoneme of the photoreceptor outer segments. Mice homozygous for the mutant Rp1-myc allele underwent a rapid-onset retinal degeneration characterized by incorrectly oriented outer segment discs that failed to stack properly into outer segments. In contrast, the photoreceptors of heterozygous mice remained relatively healthy.

CONCLUSIONS

The presence of mutant RP1 mRNA in lymphoblasts from patients with RP1 disease implies that the mutant message can escape nonsense-mediated mRNA decay and that a truncated RP1 protein may be produced in the retina. The truncated Rp1-myc protein appears to be nonfunctional, and not to exert a dominant negative effect in the photoreceptors of heterozygous mice. Results from homozygous Rp1-myc mice indicate that RP1 is required for the correct orientation and higher order stacking of outer segment discs.

Interaction of the betaIV-tubulin isotype with actin stress fibers in cultured rat kidney mesangial cells

Microtubules and actin filaments are two of the major components of the cytoskeleton. There is accumulating evidence for interaction between the two networks. Both the alpha- and beta-subunits of tubulin exist as numerous isotypes, some of which have been highly conserved in evolution. In an effort to better understand the functional significance of tubulin isotypes, we used a double immunofluorescence labeling technique to investigate the interactions between the tubulin beta-isotypes and the actin stress fiber network in cultured rat kidney mesangial cells, smooth-muscle-like cells from the renal glomerulus. Removal of the soluble cytoplasmic and nucleoplasmic proteins by detergent extraction caused the microtubule network to disappear while the stress fiber network was still present. In these extracted cells, the betaI- and betaII-tubulin isotypes were no longer present in the cytoplasm while the betaIV-isotype co-localized with actin stress fibers. Co-localization between betaIV-tubulin and actin stress fibers was also observed when the microtubule network was disrupted by the anti-tubulin drug colchicine and also by microinjection of the betaIV-tubulin antibody. Our results suggest that the betaIV isotype of tubulin may be involved in interactions between microtubules and actin.

Myosin VIIa as a common component of cilia and microvilli

The distribution of myosin VIIa, which is defective or absent in Usher syndrome 1B, was studied in a variety of tissues by immunomicroscopy. The primary aim was to determine whether this putative actin-based mechanoenzyme is a common component of cilia. Previously, it has been proposed that defective ciliary function might be the basis of some forms of Usher syndrome. Myosin VIIa was detected in cilia from cochlear hair cells, olfactory neurons, kidney distal tubules, and lung bronchi. It was also found to cofractionate with the axonemal fraction of retinal photoreceptor cells. Immunolabeling appeared most concentrated in the periphery of the transition zone of the cilia. This general presence of a myosin in cilia is surprising, given that cilia are dominated by microtubules, and not actin filaments. In addition to cilia, myosin VIIa was also found in actin-rich microvilli of different types of cell. We conclude that myosin VIIa is a common component of cilia and microvilli.

Rhodopsin transport in the membrane of the connecting cilium of mammalian photoreceptor cells

The transport of the photopigment rhodopsin from the inner segment to the photosensitive outer segment of vertebrate photoreceptor cells has been one of the main remaining mysteries in photoreceptor cell biology. Because of the lack of any direct evidence for the pathway through the photoreceptor cilium, alternative extracellular pathways have been proposed. Our primary aim in the present study was to resolve rhodopsin trafficking from the inner to the outer segment. We demonstrate, predominantly by high-sensitive immunoelectron microscopy, that rhodopsin is also densely packed in the membrane of the photoreceptor connecting cilium. Present prominent labeling of rhodopsin in the ciliary membrane provides the first striking evidence that rhodopsin is translocated from the inner segment to the outer segment of wild type photoreceptors via the ciliary membrane. At the ciliary membrane rhodopsin co-localizes with the unconventional myosin VIIa, the product of human Usher syndrome 1B gene. Furthermore, axonemal actin was identified in the photoreceptor cilium, which is spatially co-localized with myosin VIIa and opsin. This actin cytoskeleton of the cilium may provide the structural bases for myosin VIIa-linked ciliary trafficking of membrane components, including rhodopsin.

Characterization of human retinal fascin gene (FSCN2) at 17q25: close physical linkage of fascin and cytoplasmic actin genes

Retinal fascin is a newly identified photoreceptor-specific paralog of the actin-bundling protein fascin. Fascins crosslink f-actin into highly ordered bundles within dynamic cell extensions such as neuronal growth cone filopodia. We have isolated cDNA and genomic clones of human retinal fascin and characterized the structure of the human retinal fascin gene (FSCN2). The cDNA predicts a protein of 492 amino acids and molecular mass 55,057 that shows 94% identity to bovine retinal fascin and 56% identity to human fascin. Promoter analysis reveals a consensus retinoic acid response element and several potential binding sites for transcription factors Crx and Nrl, which correlates with the retina-specific expression of FSCN2 mRNA. Fluorescence in situ hybridization analysis and genomic clone sequencing indicate that the FSCN2 gene lies within 200 kb of the actin gene ACTG1 at 17q25. Database searches revealed that the human fascin gene FSCN1 and actin gene ACTB at 7p22 also coexist within a 200-kb genomic clone. The close physical linkage of these fascin/actin gene pairs suggests that they derive from a common gene duplication event and allows comparison of fascin and actin phylogenetic analyses. Finally, a possible link to the retinitis pigmentosa 17 allele (RP17) at distal 17q was excluded by demonstration of multiple independent segregation events in two RP17 kindreds. Informative FSCN2 polymorphisms were identified and will serve as useful markers in future linkage studies. The likely function of retinal fascin, in light of known fascin roles in other cell types, is to assemble actin microfilaments in support of photoreceptor disk morphogenesis.

Rhodopsin trafficking and its role in retinal dystrophies

We review the sorting/targeting steps involved in the delivery of rhodopsin to the outer segment compartment of highly polarized photoreceptor cells. The transport of rhodopsin includes (1) the sorting/budding of rhodopsin-containing vesicles at the trans-Golgi network, (2) the directional translocation of rhodopsin-bearing vesicles through the inner segment, and (3) the delivery of rhodopsin across the connecting cilium to the outer segment. Several independent lines of evidence suggest that the carboxyl-terminal, cytoplasmic tail of rhodopsin is involved in the post-Golgi trafficking of rhodopsin. Inappropriate subcellular targeting of naturally occurring rhodopsin mutants in vivo leads to photoreceptor cell death. Thus, the genes encoding mutations in the cellular components involved in photoreceptor protein transport are likely candidate genes for retinal dystrophies.

Myosin VIIa participates in opsin transport through the photoreceptor cilium

Two types of Usher syndrome, a blindness-deafness disorder, result from mutations in the myosin VIIa gene. As for most other unconventional myosins, little is known about the function or functions of myosin VIIa. Here, we studied the photoreceptor cells of mice with mutant myosin VIIa by electron immunomicroscopy and microscopic autoradiography. We found evidence that myosin VIIa functions in the connecting cilium of each photoreceptor cell and participates in the transport of opsin through this structure. These findings provide the first direct evidence that opsin travels along the connecting cilium en route to the outer segment. They demonstrate that a myosin may function in a cilium and suggest that abnormal opsin transport might contribute to blindness in Usher syndrome.

Disk membrane initiation and insertion are not required for axial disk displacement in Xenopus laevis rod outer segments

PURPOSE

Mechanisms that maintain the close coupling between the formation of photoreceptor disk membranes and the displacement of disk membranes toward the pigment epithelium are poorly understood. This study was designed to determine whether the axial displacement of disk membranes requires the assembly and insertion of new disk lamellae.

METHODS

Retinal detachment and treatment with cytochalasin D were employed to interrupt the normal formation of disk membranes in cultured Xenopus laevis retinas. The effect of disrupting disk initiation and assembly upon disk displacement was documented and quantified.

RESULTS

Isolating retinas from the retinal pigment epithelium prevented the normal morphogenesis of disks, but previously formed disks moved distally at a rate that is greater than or equal to the rate in attached retinas or in vivo. Treatment of attached retinas in eyecups with cytochalasin D blocked initiation of new disks and resulted in the formation of ectopic, disk-like membranes, but it did not stop axial displacement of previously formed disks. Rod cells in retinas that were cultured while slightly elevated from the retinal pigment epithelium sometimes formed disks of a smaller diameter than normal, even though the rate of initiation and displacement of disks was the same as in vivo.

CONCLUSIONS

Observations on detached retinas and or retinas treated with cytochalasin D suggest that disk displacement does not depend upon normal disk formation and that the motive mechanism does not involve filamentous actin. The formation of small diameter disks in elevated retinas suggests that disk initiation and displacement is independent of the completion of normal diameter disks.

Isolation of a cDNA encoding a photoreceptor cell-specific actin-bundling protein: retinal fascin

We have isolated a novel retina-specific gene, retinal fascin, encoding a new member of actin-bundling protein gene family, from a bovine retina cDNA library. The cDNA encodes a 492 amino acid protein which shows 36-57% amino acid identity with three vertebrate fascins, echinoid fascin and Drosophila singed gene. Northern blot analysis revealed that retinal fascin mRNA was exclusively expressed in the eye and not seen in other tissues examined. In situ hybridization analysis indicated that retinal fascin mRNA signals were found only in the inner segment of the photoreceptor layer and outer nuclear layer, indicating that retinal fascin was specifically expressed in photoreceptor cells. As fascins are actin-bundling proteins important for constructing several intracellular structures, retinal fascin might play a pivotal role in photoreceptor cell-specific events, such as disk morphogenesis.

Microtubule-microfilament synergy in the cytoskeleton

This review describes examples of structural and functional synergy of the microtubule and actin filament cytoskeleton. An analysis of basal body (centriole)-associated fibrillar networks includes studies of ciliated epithelium, neurosensory epithelium, centrosomes, and ciliated protozoa. Microtubule and actin filament interactions in cell division and development are illustrated by centrosome motility, cleavage furrow positioning, centriole migration, nuclear migration, dynamics in the phragmoplast, growth cone motility, syncytial organization, and ring canals. Model systems currently used for studies on organelle transport are described in relation to mitochondrial transport in axons and vesicular transport in polarized epithelium. Evidence that both anterograde and retrograde motors are associated with one organelle is also discussed. The final section reviews proteins that bind both microtubules and actin filaments and are possible regulators of microtubule-microfilament interactions. Regulatory roles for posttranslational modifications, microtubule and microfilament dynamics, and multisubunit complexes are considered.

The actin network in the ciliary stalk of photoreceptors functions in the generation of new outer segment discs

Cytochalasin D (CD) interferes with the morphogenesis of outer segment disc membrane in photoreceptors. Disruption of either the actin network in the ciliary stalk, where membrane evagination is initiated, or the actin core of the calycal processes, whose position could define the disc perimeter, could be responsible. We have attempted to determine which of these local F-actin populations is involved in membrane morphogenesis and what step in the process is actin-dependent. Biocytin accumulation in nascent discs, detected by fluorescent avidin and laser scanning confocal microscopy (LSCM), provided a means of labeling abnormal discs and a measure of disc membrane addition. F-actin content and distribution were assessed using fluorescent phalloidin and LSCM. First, we examined the effects of a range of CD dosages (0.1, 1.0, or 10.0 microM) on rod photoreceptors in Xenopus laevis eyecup cultures. Ectopic outgrowth of discs, evaluated by LSCM and transmission electron microscopy (TEM), occurred at each concentration. Phalloidin labeling intensified in the ciliary stalk with increasing CD concentration, indicating F-actin aggregation. In contrast, it diminished in the calycal processes, indicating dispersal; TEM showed that calycal process collapse ensued. Disruption was evident at a lower concentration in the ciliary stalk (0.1 microM) than in the calycal processes (1.0 microM). TEM confirmed that the calycal processes remained intact at 0.1 microM. Thus, CD's action on the ciliary stalk network is sufficient to disrupt disc morphogenesis. Second, we examined the effect of CD on temperature-induced acceleration of the rate of disc formation. In the absence of CD, a 10 degrees C temperature shift increased the disc formation rate nearly three-fold. CD (5 microM) caused a 94% inhibition (P < 0.025) of this response; yet, the rate of membrane addition to ectopically growing discs exhibited the expected three-fold increase. Thus, CD's action interferes with the generation of new discs.

Electrodynamics of microtubular motors: the building blocks of a new model

Microtubules are ubiquitous components of the cytoskeleton. They participate in many motility processes ranging from intracellular transport or chromosome movement during mitosis to ciliary and flagellar beating. The biophysical mechanism inherent in the generation and control of movement in all these motility phenomena has not yet been entirely elucidated. The authors propose a new model based on a charge transfer mechanism capable of shedding a new light on the molecular foundations of all motility processes. Electron transfer along the microtubular lattice is responsible for activation and control of all microtubule-associated ATPases (i.e. force generating enzymes). Microtubules are thus shown to be the basic motors of cell dynamics. The model is first applied to intracellular transport and ciliary and flagellar beating. Through two additional examples, the authors show the heuristic capabilities of the suggested hypothesis. The application of charge transfer control to the Protozoan Euglena gracilis leads to a plausible model capable of accounting for its phototactic response mechanism. Furthermore, the model allows a new interpretation of the electrophysiological response in vertebrate photoreceptors.

Centrin in the photoreceptor cells of mammalian retinae

Photoreceptor cells of vertebrate retinae are highly specialized ciliary cells. Their non-motile ciliated structure is restricted to the so-called connecting cilium at the joint between the light sensitive outer segment and the metabolically active inner segment. Extensive bidirectional intracellular transport between both segments is forced to occur through this tight connecting cilium. In the present study it is shown that the CA2+-binding, phospho-protein centrin is present in mammalian retinae. Western blot and immunoprecipitation reveal that anti-centrin antibodies react with purified photoreceptor cell fractions of retinae in bands at a molecular weight of 20 kDa, the molecular weight of centrins found in other cells. Indirect immunofluorescence analysis of cryosections through retinae of different mammalian species show that centrin is present only in centrosomes and basal bodies but also more extensively at the linkage between the inner and the outer segment of the photoreceptor cells. Immunocytological studies on isolated rod cells and immunoelectron microscopy clearly demonstrate a unique presence of centrin in the connecting cilium of photoreceptor cells. High molecular identity between centrins in lower eukaryotes and mammals indicates that centrin may play a role in cellular motility and/or in microtubule severing in the mammalian retina.

Complex intermolecular interactions maintain a stable linkage between the photoreceptor connecting cilium axoneme and plasma membrane

Microtubule-membrane cross-linkers in motile and nonmotile cilia are supramolecular structures, held together by strong interactions between the constituent molecules. We have characterized these interactions in the photoreceptor connecting cilium, where cross-linkers co-fractionate and maintain their in situ location after Triton X-100 extraction of axonemes. In bovine photoreceptor cells, the transmembrane assemblage that is cross-linked to the connecting cilium axoneme contains three high molecular mass glycoconjugates of 425, 600, and 700 kDa (Horst et al., 1987). The relative amounts of the three glycoconjugates, as judged from band intensity in electrophoretograms, depend strongly on sample treatment prior to electrophoresis. The electrophoretic pattern was reproducible after several weeks of storage of the axoneme fraction in extraction buffer containing 50% sucrose. Removal of sucrose from the buffer by dialysis eliminated the 600 kDa and 700 kDa, and decreased the detected amount of the 425 kDa glycoconjugate. When samples were incubated in Laemmli sample buffer at increasing temperatures (23 degrees, 60 degrees, 95 degrees C), a gradual reduction in the intensity of the three bands was observed. The quantitative reduction of high molecular mass glycoconjugates was accompanied by the appearance of novel protein species of lower molecular mass, as detected by lectin and antibody overlays of axonemal transblots. These results suggest that the previously characterized cross-linker glycoconjugates are complex, SDS-resistant multi-molecular conglomerates. We have further used fluorescent lectins to monitor the presence of glycoconjugates on whole-mounted axonemes, in conditions aimed to selectively solubilize the cross-linkers. The cross-linker complexes could not be dissociated from the axoneme by incubation with buffers containing 1 M of either Na2SO4 or NaI. The results indicate that the connecting cilium-specific cross-linker complexes are bound via high-affinity interactions to both axoneme and overlying plasma membrane.

Characterization of calpain II in the retina and photoreceptor outer segments

Calpain II was purified to apparent homogeneity from bovine neural retinas. It was found to be biochemically similar to brain calpain II, purified by the same procedure, with respect to: subunit mobility in SDS-polyacrylamide gel electrophoresis; Ca2+ sensitivity; inhibition by calpeptin and other cysteine protease inhibitors; and optimal pH. Semithin cryosections were immuno-labeled with antibodies specific for the catalytic subunit of calpain II. Calpain II was detected in most layers of the retina, with the most pronounced label present in the plexiform layers (synaptic regions) and the photoreceptor outer segments. In dark-adapted retinas, the label was distributed throughout the outer segments. In light-adapted retinas, outer segment labeling was concentrated in the connecting cilium, and the inner segments were labeled. A partially pure preparation of calpain II from isolated rod outer segments was found to have the same biochemical characteristics as calpain II prepared in the same way from the whole retina. The enzyme was distributed fairly evenly between the cytosolic and cytoskeletal fractions of isolated rod outer segments. Immunoblots of the rod outer segment cytoskeleton were used to determine the susceptibility of known components of the actin-based cytoskeleton to proteolysis by calpain II in vitro. Actin was not proteolyzed at all, alpha-actinin was only slowly degraded, but myosin II heavy chain was rapidly proteolyzed. Actin filaments have been shown previously to be associated with myosin II and alpha-actinin in a small domain within the connecting cilium, where they play an essential role in the morphogenesis of new disk membranes. The localization of calpain II in the connecting cilium after light exposure, combined with the in vitro proteolysis of myosin II, suggests that calpain II could be involved in light-dependent regulation of disk membrane morphogenesis by proteolysis of myosin II.

Morphogenesis of the photoreceptor outer segment during postnatal development in the mouse (BALB/c) retina

Disc formation of rod photoreceptor cells in developing BALB/c mice retinas was studied by rapid freeze, freeze-substitution, freeze-etching, immunocytochemistry, and myosin S-1 decoration methods. Freeze-substituted photoreceptor cells contained variously shaped vesicles in the apical swelling of the connecting cilium or the base of the outer segment during postnatal development. Rapid freezing successfully arrested pinocytosis; the fusion of small vesicles to give large ones, and the compression of certain vesicles (0.3-0.6 micron) appears to lead gradually to the formation of the so-called discs. We therefore propose that membranous discs are formed by the fusion of small pinocytotic vesicles and their subsequent compression. Discs formed in this way were partially stacked, but were ordered at random during the early developmental stages. During development, a partial stack of discs was progressively rearranged to a regular form as seen in mature outer segments. Cytoskeletal actin was expected to be involved in the disc formation; it was demonstrated in the distal axoneme of the connecting cilium during development and showed no change in its distribution. However, the polarity of the actin filaments, as revealed by myosin S-1 decoration in early developmental stages, was much more variable than in the adult. Barbed ends of actin filaments were associated with the plasma membrane or the membrane of vesicles. We also found actin filaments coiled up helically on ciliary microtubules.

Localization of peripherin/rds in the disk membranes of cone and rod photoreceptors: relationship to disk membrane morphogenesis and retinal degeneration

The outer segments of vertebrate rod photoreceptor cells consist of an ordered stack of membrane disks, which, except for a few nascent disks at the base of the outer segment, is surrounded by a separate plasma membrane. Previous studies indicate that the protein, peripherin or peripherin/rds, is localized along the rim of mature disks of rod outer segments. A mutation in the gene for this protein has been reported to be responsible for retinal degeneration in the rds mouse. In the present study, we have shown by immunogold labeling of rat and ground squirrel retinas that peripherin/rds is present in the disk rims of cone outer segments as well as rod outer segments. Additionally, in the basal regions of rod and cone outer segments, where disk morphogenesis occurs, we have found that the distribution of peripherin/rds is restricted to a region that is adjacent to the cilium. Extension of its distribution from the cilium coincides with the formation of the disk rim. These results support the model of disk membrane morphogenesis that predicts rim formation to be a second stage of growth, after the first stage in which the ciliary plasma membrane evaginates to form open nascent disks. The results also indicate how the proteins of the outer segment plasma membrane and the disk membranes are sorted into their separate domains: different sets of proteins may be incorporated into membrane outgrowths during different growth stages of disk morphogenesis. Finally, the presence of peripherin/rds protein in both cone and rod outer segment disks, together with the phenotype of the rds mouse, which is characterized by the failure of both rod and cone outer segment formation, suggest that the same rds gene is expressed in both types of photoreceptor cells.

Double immunogold localization of opsin and actin in the cilium of developing mouse photoreceptors

A 9 + 0 cilium represents the only connection between the light-sensitive rod outer segment (ROS) and the visual cell body. Differentiation of a ROS derives from a remodeling of the plasma membrane at the distal end of the cilium. Prior to this event, an actin-rich domain can be demonstrated within the distal cilium using immunocytochemical techniques. This actin is in the filamentous form and is also observed in mature photoreceptors where it has been implicated in ROS disc morphogenesis. In separate studies, the visual pigment protein, opsin, has also been localized to the distal ciliary membrane before disc synthesis begins. For the current report, we have used double label immunoelectron microscopy to investigate the presence of opsin and actin in the cilia of developing mouse photoreceptors during the period preceding ROS differentiation. Initially, we used post-embedding immunolabeling for the localization of both proteins on ultrathin sections of Lowicryl embedded tissues. However, increased sensitivity for the detection of membrane opsin was obtained when the retinas were immersion labeled prior to resin embedment. Although it remains unclear whether the appearance of ciliary opsin and actin are synchronized, the results of this study confirm that opsin and actin are each sequestered within their respective ciliary domains prior to the differentiation of an ROS.

Actin filaments in the photoreceptor cilium of the rds mutant mouse

An actin filament meshwork was recently demonstrated within the ciliary axoneme at the base of the photoreceptor outer segment (OS) in rat retina. Individual filaments of a uniform polarity extended from the cilium and entered into the bottom of the OS disc stack where they associated with the plasma membrane in the region of new disc assembly. This and other studies have indicated that an actin-mediated mechanism may regulate OS disc morphogenesis. The homozygous rds mouse exhibits an absence of OS formation, although cilia do develop and opsin is contained within the ciliary plasma membrane. The rds abnormality is believed to result from a defect in OS disc assembly. Immunogold labeling has shown that actin is situated within the distal end of rds photoreceptor cilia, as well as in the distal cilium of normal mice prior to the onset of OS differentiation. However, anti-actin antibodies do not distinguish between monomer and filamentous actin. In the current study, neural retinas from rds and control mice were permeabilized with saponin, incubated with myosin subfragment-1 (S-1), and prepared for electron microscopy. Following this treatment, a meshwork of myosin S-1 decorated actin filaments could be observed within the axoneme in the distal end of each rds photoreceptor cilium. As in normal visual cells, actin filaments exited the axoneme by passing between pairs of microtubule doublets. These filaments had the correct polarity, with all arrowheads pointing toward the axoneme, and they associated with the ciliary plasma membrane in the region where OS disc morphogenesis would normally occur. (ABSTRACT TRUNCATED AT 250 WORDS)

Actin filaments and photoreceptor membrane turnover

The shape and turnover of photoreceptor membranes appears to depend on associated actin filaments. In dipterans, the photoreceptor membrane is microvillar. It is turned over by the addition of new membrane at the bases of the microvilli and by subsequent shedding, mostly from the distal ends. Each microvillus contains actin filaments as a component of its cytoskeletal core. Two myosin I-like proteins co-localize with the actin filaments. It is suggested that one of the myosin I-like proteins might be linked to the microvillar membrane. By interacting with the actin filaments, this motor should move the membrane of a microvillus in a distal direction, thus providing a possible mechanism for the turnover of the membrane. A vertebrate photoreceptor cell contains a small cluster of actin filaments in its connecting cilium at the site where new transductive disk membranes are formed. Disruption of the actin filaments perturbs disk morphogenesis. The most likely explanation for this perturbation is that the process of initiating a new disk is inhibited. Conventional myosin (myosin II) is found in the connecting cilium with the same distribution as actin. A simple model is proposed to illustrate how the actin-myosin system of the connecting cilium might function to initiate the morphogenesis of a disk membrane.

Cytoskeletal specializations at the rod photoreceptor distal tip

We have examined microtubules and microtubule-like elements within the toad rod photoreceptor outer segment in order to define regional specializations of the photoreceptor cytoskeleton. "Ciliary" microtubules were localized within the rod outer segment (ROS) by using thin section electron microscopy, immunofluorescence, and rapid-freeze deep-etch microscopy. All three methods showed that ciliary microtubules stop short of the extreme ROS distal tip, although abundant microtubule-like structures distinct from the ciliary microtubules were found within the distal 10-15 microns of the ROS tip. These heretofore undescribed "distal ROS tubules" are clustered at the clefts or incisures of the disk membrane stack and resemble microtubules in overall size and shape, although they are not closely related antigenically to tubulin. The distal ROS tubules are more abundant in green rods than red rods and vary in number during the daily light/dark cycle. Quantitation of these tubules at two time points during the light/dark cycle suggests that there are three- to fourfold more tubules in the ROS tip one hour after light onset than one hour before light onset. Retinas prevented from normal disk membrane shedding by separation of the retina from the adjacent pigment epithelium, failed to develop increased numbers of tubules after light onset. This suggests that the newly described distal ROS tubules may modulate or be modulated by light-induced interactions between the photoreceptors and pigment epithelium, such as those that occur during the disk shedding phase of membrane turnover.

Identification of actin filaments in the rhabdomeral microvilli of Drosophila photoreceptors

The phototransductive microvilli of arthropod photoreceptors each contain an axial cytoskeleton. The present study shows that actin filaments are a component of this cytoskeleton in Drosophila. Firstly, actin was detected in the rhabdomeral microvilli and in the subrhabdomeral cytoplasm by immunogold labeling with antiactin. Secondly, the rhabdomeres were labeled with phalloidin, indicating the presence of filamentous actin. Finally, the actin filaments were decorated with myosin subfragment-1. The characteristic arrowhead complex formed by subfragment-1 decoration points towards the base of the microvilli, so that the fast growing end of each filament is at the distal end of the microvillus, where it is embedded in a detergent-resistant cap. Each microvillus contains more than one actin filament. Decorated filaments extend the entire length of each microvillus and project into the subrhabdomeral cytoplasm. This organization is comparable to that of the actin filaments in intestinal brush border microvilli. Similar observations were made with the photoreceptor microvilli of the crayfish, Procambarus. Our results provide an indication as to how any myosin that is associated with the rhabdomeres might function.