The occurrence of actinlike filaments in association with migrating pigment granules in frog retinal pigment epithelium

Actin-dependent motility of melanosomes from fish retinal pigment epithelial (RPE) cells investigated using in vitro motility assays

Melanosomes (pigment granules) within retinal pigment epithelial (RPE) cells of fish and amphibians undergo massive migrations in response to light conditions to control light flux to the retina. Previous research has shown that melanosome motility within apical projections of dissociated fish RPE cells requires an intact actin cytoskeleton, but the mechanisms and motors involved in melanosome transport in RPE have not been identified. Two in vitro motility assays, the Nitella assay and the sliding filament assay, were used to characterize actin-dependent motor activity of RPE melanosomes. Melanosomes applied to dissected filets of the Characean alga, Nitella, moved along actin cables at a mean rate of 2 microm/min, similar to the rate of melanosome motility in dissociated, cultured RPE cells. Path lengths of motile melanosomes ranged from 9 to 37 microm. Melanosome motility in the sliding filament assay was much more variable, ranging from 0.4-33 microm/min; 70% of velocities ranged from 1-15 microm/min. Latex beads coated with skeletal muscle myosin II and added to Nitella filets moved in the same direction as RPE melanosomes, indicating that the motility is barbed-end directed. Immunoblotting using antibodies against myosin VIIa and rab27a revealed that both proteins are enriched on melanosome membranes, suggesting that they could play a role in melanosome transport within apical projections of fish RPE.

A burst of differentiation in the outer posterior retina of the eleven-week human fetus: an ultrastructural study

Many studies on human retinal development have cited the third gestational month as a period when the posterior retina undergoes rapid differentiation and maturation, including a lining up of cone precursors. Ultrastructural data on the posterior retina during the third month are very limited, and totally lacking for the cone monolayer. We have examined two human fetal retinas between ten and 11 gestational weeks. Before the appearance of the cone monolayer, the outer neural retina consists of a homogeneous population of undifferentiated neuroblasts. Mitotic figures are still evident, even posteriorally. There is no outer plexiform layer (OPL). The interface of neural retina to retinal pigment epithelium (RPE) is largely featureless. By 11 weeks, the posterior retina has a thin OPL that separates the many rows of cells in the developing inner nuclear layer from the single tier of macular cone precursors. The RPE monolayer consists of cuboidal cells whose apical surface elaborates ridges of cytoplasm and branched processes that project into the subretinal space. The large, cuboidal cones are linked to each other and Müller cells at the outer limiting membrane. They show definitive signs of the structural polarity typical of vertebrate photoreceptors. Their apical cytoplasm contains many organelles common to the inner segment, while the basal cytoplasm has synaptic ribbons and vesicles, and receives invaginating contacts from processes in the OPL neuropil arising from differentiating second-order neurons. Lateral cone surfaces are mutually underlain by large subsurface cisterns.

Immunogold localization of actin in developing photoreceptor cilia of normal and rds mutant mice

Recent immunocytochemical studies have localized actin to the distal cilium of mature vertebrate photoreceptors. Since this is the site where the ciliary plasma membrane evaginates to form new outer segment discs, the results suggest that an actin-mediated contractile mechanism or cytoskeletal network might regulate some aspect of outer segment disc morphogenesis. In the present study, immunoelectron microscopy was used to localize actin to the developing cilia of normal and rds mutant mouse photoreceptors. In normal mice, actin could not be localized to newly projecting cilia, but an actin-rich domain was demonstrated within the distal, bulbous ending of elongated cilia just prior to outer segment development. These results suggest that actin is not important for ciliary growth, but that it may be necessary for the subsequent differentiation of an outer segment. In the rds mutant mouse, there is an absence of outer segment formation, although cilia appear to develop normally. Rhodamine phalloidin staining of cryostat sections demonstrated a normal F-actin distribution within the rds retina. Utilizing immunogold labeling of developing rds photoreceptors, actin was localized to the distal, bulbous ending of elongated cilia. This result indicates that actin is situated within its normal domain in rds cilia.

Immunochemistry of the outer retina

The immunochemistry of the outer retina is discussed with particular reference to photoreceptor cells, the retinal pigment epithelium and the interphotoreceptor space. The antigens identified and the techniques utilised are summarised.

Membrane turnover in rod photoreceptors: ensheathment and phagocytosis of outer segment distal tips by pseudopodia of the retinal pigment epithelium

We have documented the ultrastructural changes that occur within the photoreceptor outer segment and the retinal pigment epithelium (RPE) during photosensitive membrane turnover. We employed an in vitro eyecup preparation from Xenopus laevis in which a large shedding event was induced by adding the excitatory amino acid L-aspartate (Greenberger & Besharse 1985; J. comp. Neurol. 239, 361-372). We found that during L-aspartate-induced shedding the RPE cells formed, on their apical domains, previously undescribed processes that were directly involved in disc phagocytosis. These processes are structurally similar to processes formed by macrophages during phagocytosis and are accordingly referred to as pseudopodia. Pseudopodia were distinguishable from the apical villous process normally extended from the RPE in that they were closely applied to the surface of the outer segment, had a cytoplasmic matrix of low electron density that was devoid of most cellular organelles and were enriched in thin (7 nm diameter) filaments. Filament size, specific pseudopodial staining with the actin-specific probe rhodamine phalloidin and inhibition of pseudopod formation by cytochalasin D suggested that the thin filaments were composed of actin. Pseudopodial formation also occurs during a normal light-initiated shedding event. However, the low frequency of shedding, the asynchrony of the individual shedding events and the transient appearance of the pseudopodia prevented a full appreciation of their role during normal disc shedding. Associated with massive shedding and pseudopodial formation, there was an increased adherence between retina and RPE. During L-aspartate treatment, the apical portions of the RPE cells partitioned with the distal outer segment during retinal isolation. This effect was directly related to the development of pseudopodia and may reflect alteration of surface features of the rod outer segment (ROS)-RPE interface related to phagocytosis. Our observations show that transiently forming pseudopodia are the organelles of phagocytosis and that they may play a role in disc detachment as well.

The cytoskeleton of chick retinal pigment epithelial cells in situ

Gelatin-coated slides were used to obtain en face preparations of retinal pigment epithelium (RPE) from 6- to 21-day-old chick embryos in order to study the distribution of F-actin in microfilaments (MF) and the MF-associated proteins, myosin, tropomyosin, alpha-actinin and vinculin in situ at different stages of development by fluorescence microscopy. The epithelial sheets were fixed in formaldehyde and then extracted in a solution containing 0.1% Triton X-100. NBD-Phallacidin was used to visualize the F-actin in MF, and antisera against myosin, tropomyosin, alpha-actinin and vinculin were used to determine the distribution of these four MF-associated proteins. F-actin, myosin, tropomyosin, alpha-actinin and vinculin were present in cortical rings around the apical ends of the RPE cells throughout this period of development. Of these proteins, only F-actin was identified in the apical processes of RPE cells. The increase in the amount of F-actin could be followed as the length and the number of apical processes increased with age and maturation of RPE cells. F-actin was first detected in numerous short apical processes on the surface of each RPE cell on day 12. From day 12 to day 17, they were at an intermediate stage of elongation and from day 17 onward all of the RPE cells had long F-actin-containing apical processes. These results indicate that the F-actin-containing MF assemble much later in the apical processes than in the cortical rings.(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in actin contents and elongation of apical projections in retinal pigmented epithelial cells during development of the chicken eye

The structural and biochemical changes of cytoskeletal components of retinal pigmented epithelial cells were studied during the development of chicken eyes. When the cytoskeletal components of the pigmented epithelial cells from various stages of development were examined by SDS PAGE, actin contents in the cells markedly increased between the 15-d-old and hatching stages. Immunofluorescence microscopy showed that chicken pigmented epithelial cells have two types of actin bundles. One is the circumferential bundle associated with the zonula adherens region as previously reported (Owaribe, K., and H. Masuda, 1982, J. Cell Biol., 95:310-315). The other is the paracrystalline bundle forming the core of the apical projections. The increase in actin contents after the 15-d-old stage is accompanied by the formation and elongation of core filaments of apical projections in the cells. During this period the apical projections extend into extracellular space among outer and inner segments of photoreceptor cells. Accompanying this change is an elongation of the paracrystalline bundles of actin filaments in the core of the projection. By electron microscopy, the bundles decorated with muscle heavy meromyosin showed unidirectional polarity, and had transverse striations with approximately 12-nm intervals, as determined by optical diffraction of electron micrographs. Since the shape of these bundles was not altered in the presence or absence of Ca2+, they seemed not to have villin-like proteins. Unlike the circumferential bundles, the paracrystalline bundles did not contract when exposed to Mg-ATP. These observations indicate that the paracrystalline bundles are structurally and functionally different from the circumferential actin bundles.

Actin in the photoreceptor connecting cilium: immunocytochemical localization to the site of outer segment disk formation

Actin has been localized in Rana pipiens retinas that were fixed and embedded in aldehyde cross-linked BSA. Thin sections were reacted sequentially with (a) affinity-purified antiactin antibodies induced in rabbits; (b) biotinyl-sheep anti-rabbit antibodies; and (c) avidin-ferritin conjugates. As expected, antiactin labeling density was high in the apical pigment epithelial cell processes and in the calycal processes of photoreceptors. Actin was also localized in a new site. The connecting cilium that joins the inner and outer segments of both rods and cones was heavily labeled by antiactin at its outer segment (OS), or distal, end. In this region of the cilium, the plasma membrane evaginates to form new OS disks and these basal disks were labeled in some instances. Below the new disks in rods, the cytoplasm of liplike expansions of the distal cilium was also heavily labeled. The plasma membrane and interior of the connecting cilium and the remainder of the OS were unlabeled. These findings suggest that actin may participate in the vectorial transport of opsin and other intrinsic membrane proteins that are incorporated into newly forming OS disks. The results also implicate actin in the membrane expansion involved with OS disk formation.

Cytoplasmic transport in keratocytes: direct visualization of particle translocation along microtubules

We report the first direct demonstration that the cytoplasmic transport of organelles and vesicles (collectively called particles) takes place along microtubules. Living keratocytes from the corneal stroma of the frog, Rana pipiens were observed with Allen video-enhanced contrast, differential interference contrast (AVEC-DIC) microscopy [Allen et al, 1981]. In sufficiently thin regions of these cells a network of linear elements was visible. When particles were observed in motion, they always moved along these linear elements. The linear elements remained intact and in focus on the microscope when lysed in a cell lysis solution that stabilized microtubules. Preparations were then fixed in formaldehyde, washed with phosphate-buffered saline (PBS), incubated with rabbit antitubulin, washed with PBS, stained with rhodamine-conjugated goat antirabbit, and washed with PBS. The extracted cells continued to remain in place and in focus on the microscope throughout these procedures. The same cells were then observed using epifluorescence optics and a silicon-intensified target (SIT) video camera. A network of fluorescent linear elements was seen to correspond in number, form, and position to the linear elements seen in the live AVEC-DIC image. Taken together, the AVEC-DIC and fluorescence microscopy observations prove that the linear elements along which particles move are microtubules (MTLEs). The observed particle speeds, pause times, and distances moved varied widely, even for the same particle on the same microtubule. Particles were also observed to switch from one microtubule to another as they were transported. The polarity of the microtubules did not seem to affect the particle direction, since particles were observed to move in both directions on the same MTLE. When not in motion these particles behaved as if anchored to the microtubules since they showed negligible Brownian motion. Finally, it was observed that an elongate particle could move onto two intersecting linear elements such that it was deformed into an inverted "Y" shape. This indicates that there may be more than a single site of attachment between the force generator and the particle.

Microtubules and 10 nm-filaments in the retinal pigment epithelium during the diurnal light-dark-cycle

Microtubules and 10 nm-filaments appear to be involved in the functions of the retinal pigment epithelium (RPE). The presence of microtubules in the RPE of light-adapted eyes, but not in dark-adapted eyes, suggests that they may be involved primarily in organelle movement. On the other hand, the random and constant presence of 10 nm-filaments within the basal portion of the PE implies a cytoskeletal role for these filaments.

The identification and localization of actin and actin-like filaments in lactating guinea pig mammary gland alveolar cells

Cytochalasin B, a microfilament-altering drug, inhibits lactose synthesis in lactating guinea pig mammary gland [Biochim. Biophys. Acta 392:20, 1975] but not primary by inhibiting glucose transport [Eur. J. Cell Biol. 20:150, 1979]. In order to study the possible role of microfilaments in lactose synthesis and secretion, we isolated both the alveolar (milk-secreting) and myoepithelial (contractile) cells from lactating mammary gland. Light microscopy shows that the alveolar cell fraction (viability approximately 71%) is homogenous and that the cells retain strong polarity of secretory structures in the apical region. Two proteins were extracted from the alveolar cell fraction. One (mol wt 42,000) comigrates with skeletal muscle actin on SDS-PAGE gels. The other, a high-molecular-weight (180,000) protein (HMWP) may be analogous to actin-binding protein or clathrin. An extract from the myoepithelial cell fraction also contains a protein that comigrates with actin but no HMWP. Whole tissue extract contains the 42K protein, and a 185K HMWP. Examination of the alveolar cell extract by electron microscopic (EM) negative staining revealed meshworks of multistranded, interconnecting filaments, with attached globular structures (100-200 A) (possibly the HMWP) and single filaments (40-60 A diameter) branching off. To localize these filamentous structures in situ, whole tissue was glycerinated and incubated with rabbit skeletal muscle heavy meromyosin (HMM). Masses of filaments in myoepithelial cells served as convenient standards for HMM decoration. Decorated filaments have cross-arms or projections, unlike the narrow, smooth filaments of control tissue. Decorated filaments in alveolar cells are located beneath the plasma membrane, in close association with secretory vacuoles, and near the Golgi apparatus; filaments near the latter two are often oriented perpendicular to the plasma membrane. Microvesicles are embedded in meshworks under the plasmalemma and near the Golgi apparatus. Intermediate-sized (85-115 A diameter), non-decorated filaments diverge from the meshworks of decorated filaments. Micro-vesicles are associated with intermediate-sized filaments as well. The association of actin-like filaments with secretory vacuoles and microvesicles and their location in areas of the cell concerned with biosynthetic activities suggest a possible function in the intracellular transport of secretory products.

Actin microfilaments in melanophores of Fundulus heteroclitus. Their possible involvement in melanosome migration

In melanophores of Fundulus heteroclitus, hormone-stimulated melanosome aggregation is accompanied by cytoplasmic flow from the cellular processes to the perikaryon, and reversal of these events takes place upon hormone-induced melanosome dispersion. These cells contain parallel arrays of microtubules, the majority of which are located in the perikaryon and in cortical regions of the processes. Studies with heavy meromyosin binding demonstrated two types of actin filaments: 11 a decorated meshwork of filaments similar to those usually found in close association with plasma membranes, and 2) filaments decorated in a manner similar to that of stress fibers. There is an apparent increase in the amount of filaments during melanosome aggregation. These results are discussed in relation to intracellular movement.

Ultrastructural demonstration of hormone-induced movement of carotenoid droplets and endoplasmic reticulum in xanthophores of the goldfish, Carassius auratus L

The hormone-induced pigment dispersion in primary cultures of xanthophores of goldfish (Carassius auratus L.) has been shown to involve the dispersion of not only carotenoid droplets but also of smooth endoplasmic reticulum. The dispersion of these organelles is inhibited by cytochalasin B and is accompanied by thinning of the cell body, thickening of the processes, and also overall changes in cellular morphology (process extension) under certain conditions. Electron microscopic examination of heavy meromyosin treated glycerinated xanthophores in scales revealed the presence of actin filaments in these cells.

The photoreceptors and pigment epithelim of the adult Xenopus retina: morphology and outer segment renewal

Outer segment renewal and the fine structure of photoreceptors and pigment epithelium (p. e.) were studied in the adult Xenopus retina by light microscopic autoradiography and electron microscopy. Following the injection of [ 3 H]leucine, the pattern of labelling observed in receptor outer segments was typical of that reported in other adult retinae: only diffuse labelling was found in cones, but in rods a discrete band of label accumulated at the base of the outer segment and migrated sclerally with time. The rate of band displacement and thus disk addition in Xenopus rods was 1.86 μm/day (or 78 disks/day), which is more than twice that reported for red rods in Rana under similar experimental conditions, although these species have similar metabolic rates. Average rod outer segment (r. o. s.) length did not change, demonstrating a balance between disk addition and shedding. R. o. s. renewal time was about 24 days, corresponding to the time when labelled phagosomes were first found in the p. e. Ultrastructurally, one kind of (red) rod and one kind of cone were found whose outer segments differed in membrane topology. Although microfilaments were found in the apical processes of the p. e. and its cytoplasm contained both pigment granules and myeloid bodies, pigment granules did not migrate into these processes during light adaptation. In addition to possible morphological evidence for phago­somes of cone origin, both large and small rod phagosomes were observed in the p. e. The latter appear to represent small stacks of partial disks shed from individual r. o. s. scallops.

Actin like filaments in the peritubular cells of human testis chemical extraction and binding with heavy meromyosin

Two types of the cytoplasmic filaments are found in human peritubular cells; thin filaments (50-80 A) and 100 A filaments. The thin filaments are numerous and tightly packed in cytoplasma of peripheral cell processes, and form bundles. These bundles are arranged in parallel with the long axis of the cells. Dense areas are occasionally seen among the filaments, or underneath the cell membrane to which the thin filaments seem to be anchored. The 100 A filaments are generally located around the perinuclear region or in the middle region of cytoplasma. The thin filaments are not altered after incubation with Hasselbach-Schneider's solution. These filaments, however, disappear in cytoplasma when extracted with 0.6 M KI solution. And also, the thin filaments form chracteristic arrowhead complexes, when incubated with HMM, while these complexes are never found, when incubated with HMM plus ATP. The 100 A filaments do not react to HMM. Thus it is suggested that the thin filaments are actin or actin like and that a contractile system exists in human peritubular cells, which may be involved in transport of nonmotile sperm or permeability of the seminiferous walls.

Localization of myosin and actin in ocular nonmuscle cells. Immunofluorescence-microscopic, biochemical, and electron-microscopic studies

Myosin and actin were localized by indirect immunofluorescence microscopy using specific antibodies prepared in rabbits against highly purified gizzard myosin and actin. A strong fluorescence staining with both antibodies was observed in rat corneal epithelial cells, anterior lens epithelial cells, rod inner segments, and in rat and frog pigment epithelial cells. The immunohistochemical localization of myosin in corneal epithelial cells was further supported by the electrophoretic and immunological identification of smooth muscle type myosin heavy chain in pure corneal epithelial abrasions. Electron-microscopic observations revealed a clear correlation between staining with actin antibodies and the presence of numerous thin cytoplasmic filaments (50-80 A in diameter). The functional and biochemical nature of 90-110 A filaments occurring in corneal and lens epithelial cells, as well as the ultrastructural localization of myosin in ocular nonmuscle cells under study remains obscure.

Actin-like filaments in the Sertoli cell junctional specializations in the swine and mouse testis

Microfilaments at the junctional specializations between adjacent Sertoli cells and between the Sertoli cell and the late spermatid of the mouse and swine testes bind HMM and form arrowhead complexes with a periodicity of about 35 nm. The arrowhead formation is inhibited when the tissues are treated with HMM in the presence of ATP. These observations show that the microfilaments are actin-like in nature. The functional significance of these filaments in the Sertoli cell is discussed.

Association of microfilament bundles with lysosomes in polymorphonuclear leukocytes

The juxtaposition of microfilament bundles and lysosomes seen both in thin-sectioned cells in the transmission electron microscope and in cryofractured cells in the scanning electron microscope, and the presence of short filamentous structures between lysosomes and microfilament bundles, suggest that microfilaments may be attached to lysosomal membranes and that these filaments may be involved in lysosomal movements. Further work is in progress to test these hypotheses.

Actin-like filaments amd membrane rearrangement in oxyntic cells

The secretory pole of vertebrate oxyntic cells possesses two distinct membrane systems: the apical plasma membrane which presents numerous infoldings, microvilli and processes, and a complex tubulovesicular system located in close proximity to the plasma membrane. These two membrane systems are generally believed to be interconvertible in relation to the functional state of the cell. To determine the role that filaments may play in the interconversion process, the secretory pole of rat and toad oxyntic cells was examined by electron microscopy under conditions designed to demonstrate filamentous structures, i.e., slight cellular swelling and incubation with heavy meromyosin. Filaments 50-80 A in diameter are present in close association with the plasma membrane to which they are connected by regularly spaced bridges. Heavy meroxyosin-treated material reveals "decorated" filaments in topographically corresponding locations. No filaments are seen in association with membranes of the tubulovesicular system. These findings suggest that association with actin-like filaments is a step in the translocation of membranes from the tubulovesicular system to the plasma membrane.

Involvement of vesicle coat material in casein secretion and surface regeneration

The ultrastructure of the apical zone of lactating rat mammary epithelial cells was studied with emphasis on vesicle coat structures. Typical 40-60 nm ID "coated vesicles" were abundant, frequently associated with the internal filamentous plasma membrane coat or in direct continuity with secretory vesicles (SV) or plasma membrane proper. Bristle coats partially or totally covered membranes of secretory vesicles identified by their casein micelle content. This coat survived SV isolation. Exocytotic fusion of SV membranes and release of the casein micelles was observed. Frequently, regularly arranged bristle coat structures were identified in those regions of the plasma membrane that were involved in exocytotic processes. Both coated and uncoated surfaces of the casein-containing vesicles, as well as typical "coated vesicles", were frequently associated with microtubules and/or microfilaments. We suggest that coat materials of vesicles are related or identical to components of the internal coat of the surface membrane and that new plasma membrane and associated internal coat is produced concomitantly by fusion and integration of bristle coat moieties. Postexocytotic association of secreted casein micelles with the cell surface, mediated by finely filamentous extensions, provided a marker for the integrated vesicle membrane. An arrangement of SV with the inner surface of the plasma membrane is described which is characterized by regularly spaced, heabily stained membrane to membrane cross-bridges (pre-exocytotic attachment plaques). Such membrane-interconnecting elements may represent a form of coat structure important to recognition and interaction of membrane surfaces.

Alpha-actinin: immunofluorescent localization of a muscle structural protein in nonmuscle cells

Antibodies specific for the skeletal muscle structural protein alpha-actinin are used to localize this protein by indirect immunofluorescence in nonmuscle cells. In cultured nonmuscle cells, alpha-actinin is localized along or between actin filament bundles producing an almost regular periodicity. The protein is also detected in the form of fluorescent plaques at some ends of actin filament bundles, as well as in a filamentous form in some overlap areas of cells. In spreading rat embryo cells, alpha-actinin assumes a focal distribution which corresponds to the vertices of a highly regular actin filament network. The results suggest that alpha-actinin may be involved in the organization of actin filament bundles, in the attachment of actin filaments to the plasma membrane, and in the assembly of actin filaments in areas of cell to cell contact.