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Membrane Assembly in Retinal Photoreceptors III 0270.6474/E/0504-1035$02.00/O The Journal of Neuroscience CopyrIght 0 Society for Neuroscience Vol. 5. No. 4, pp. 1035-1048 Printed m U.S.A. April 1985 Membrane Assembly in Retinal Photoreceptors III. Distinct Membrane Domains of the Connecting Cilium of Developing Rods’ JOSEPH C. BESHARSE,2 DONNA M. FORESTNER, AND DENNIS M. DEFOE Department of Anatomy, Emory University School of Medicine, Atlanta, Georgia 30322 Abstract Retinal photoreceptors provide a unique opportunity for the study of neuronal polarity related to membrane turnover and the formation To investigate the putative role of the photoreceptor con- of functional membrane domains. The membranous rod outer seg- necting cilium in the delivery of opsin to forming discs and in ment @OS) consists of photosensitive discs surrounded by a plasma the maintenance of membrane domains (Besharse, J. C., and membrane and is connected to the cell body by a non-motile cilium K. H. Pfenninger (1980) J. Cell Biol. 87: 451-483), we have (Rohlich, 1975). The dominant intrinsic protein of the ROS is opsin, studied developing photoreceptors of neonatal rats during the apoprotein of visual pigment (Hall et al., 1969). Whereas opsin the period of initial disc formation using conventional freeze- is largely restricted to the ROS (Jan and Revel, 1974; Papermaster fracture, immunocytochemistry, and lectin cytochemistry. et al., 1978) an Na+/K+ ATPase responsible for generation of the Specific anti-opsin-binding sites were localized in the distal photoreceptor dark current (Hagins, 1972) is restricted to the inner cilium, the developing outer segment plasma membrane, and segment (Stirling and Lee, 1980). The zone of demarcation separat- at focal sites on the inner segment plasma membrane at all ing these distinctive membrane domains appears to be in the region developmental stages examined, including the period prior of the connecting cilium. to the onset of disc morphogenesis. The proximal ciliary Throughout life the ROS turns over by a process involving assem- shaft generally lacked anti-opsin-binding sites or exhibited bly of new discs from ciliary plasmalemma, their transport distally, them in extremely low density. The distribution of anti-opsin- and their eventual loss from the ROS distal tip (Young and Bok, binding sites corresponded in a general way to the distribu- 1969; Young, 1976). Opsin and other membrane components are tion of large intramembranous particles (IMPS) in freeze- synthesized in the inner segment, transported to the periciliary fracture replicas like those seen in the rod outer segment region, and rapidly consumed in the process of disc assembly (ROS). The proximal zone corresponded in freeze-fracture (Young, 1967; Young and Droz, 1968; Papermaster et al., 1975). images to a zone of consecutive horizontal rows of intramem- Opsin is a transmembrane glycoprotein (Fukuda et al., 1979; Har- brane particles (ciliary necklaces) and axoneme-membrane grave, 1982) that is initially synthesized on rough endoplasmic cross-linkers. Although protoplasmic face leaflet IMPS similar reticulum (RER) (Young and Droz, 1968; Hall et al., 1969). It is co- to those of the distal cilium and outer segment were less translationally inserted into RER membrane (Goldman and Blobel, abundant in the inner segment and proximal cilium than in 1981) and then transferred to the Golgi complex for further process- the distal cilium and ROS, they were detected in these zones ing prior to transport to the cilium (Papermaster et al., 1978). at low frequency. Cytochemistry with concanavalin A and Throughout its sojourn in the cell, opsin appears to be associated wheatgerm agglutinin revealed the presence of a well devel- with membranous organelles. A fundamental question for which only oped glycocalyx in the proximal zone. Although opsin binds both lectins, the results suggest heterogeneity among the provisional data are available relates to the mechanism of delivery and concentration of opsin in the ROS while maintaining the distinc- glycoconjugates of the three membrane domains. Our data define distinct membrane domains of the developing photo- tive properties of inner and outer segment membranes. receptor cilium that have important implications for the mech- Recent morphological, freeze-fracture, immunocytochemical, and anisms for delivering and sequestering opsin in the outer autoradiographic data (Holtzman et al., 1977; Kinney and Fisher, segment. They also establish that the mechanism of opsin 1978; Papermaster et al., 1979, 1985; Besharse and Pfenninger, delivery to the distal zone occurs well in advance of the 1980; Besharse and Forestner, 1981; Defoe and Besharse, 1985) period of disc morphogenesis. have identified a population of smooth membranous vesicles and cisternae in the periciliary region as the principal organelles of opsin transport to the site of disc assembly. The events occurring in the ciliary region that lead to disc morphogenesis are less well defined. Largely on the basis of freeze-fracture analysis we have suggested Received May 25, 1984; Revised October 22, 1984; that opsin-containing vesicles fuse with the inner segment plasma Accepted October 24, 1984 membrane near the cilium and that membrane components destined ’ Our hearty thanks to David Papermaster for a generous supply of for incorporation into discs are transferred to the disc-forming region antibody and to Win Sale for many useful discussions. This work was via the ciliary plasmalemma (Besharse and Pfenninger, 1980). The supported by National institutes of Health Research Grant EY03222, Re- elaborately developed periciliary ridge complex of frog photorecep- search Career Development Award EY00169 to J. C. B, and Postdoctoral tors may play a role in exocytotic fusion and shunting of components Fellowship EY05646 to D. M. D. Preliminary reports of these results have into the cilium (Andrews, 1982; Peters et al., 1983). Such a mecha- appeared in abstract form (Besharse and Forestner, 1983; Forestner and nism implies that the ciliary region is capable of concentrating Besharse, 1983). membrane components distally against a concentration gradient ’ To whom correspondence should be addressed. and restricting the lateral mobility of those components in order to 1035 1036 Besharse et al. Vol. 5, No. 4, Apr. 1985 The Journal of Neuroscience Membrane Domains of Connecting Cilium of Developing Rods 1037 maintain the distinctive characteristics of inner and outer segment used affinity-purified, biotinylated sheep anti-bovine opsin IgG at concentra- membranes. tions of 0.03 to 0.18 mg/ml as the first stage ligand. This antibody has been To analyze further the role of the cilium in these processes, we characterized extensively (Papermaster et al., 1978) and was generously have carried out a morphological analysis of the cilium of neonatal provided by Dr. David S. Papermaster. Purification was carried out with purified bovine opsin coupled to sepharose (Papermaster et al., 1978). It was rats using freeze-fracture, immunocytochemistry with anti-opsin, and biotinylated using N-hydroxysuccinimide ester, according to the procedure lectin cytochemistry. We chose neonatal rats for these studies of Heitzman and Richards (1974) and was detected by avidin-ferritin pre- because the entire process of ROS formation occurs postnatally pared by the method of Heitzman and Richards (1974) or obtained directly (Weidman and Kuwabara, 1969; Galbavy and Olson, 1979) and from Vector Laboratories (Burlingame, CA). because the small size and high density of photoreceptors in their To examine antibody specificity under conditions similar to those of our retinas enabled us to observe large numbers of cilia in sections. Our immunocytochemistry experiments we separated proteins of a crude prep- principal results are that the ciliary plasma membrane consists of aration of rat outer segments prepared as in the preceding paper (Defoe and distinct proximal and distal domains with unique structural properties Besharse, 1985) or whole retina on 8 to 18% polyacrylamide gradient gels and that those domains have different binding affinities for anti-opsin using essentially the method of Laemmli (1970). lmmunoreplicas were pre- and lectins. Even during the period prior to disc formation opsin is pared and stained using the procedures described in the preceding paper (Defoe and Besharse, 1985). The only protein staining specifically in any of concentrated in the distal region from which discs form. The results the immunoreplicas was opsin (37,000 daltons) and its dimer, trimer, and suggest that the unique proximal zone of the cilium plays a role in tetramer. The latter characteristically form in our discontinuous SDS-polyacryl- transport of opsin to the distal cilium, as well as the maintenance of amide system. distinct membrane domains. Controls for specific binding of anti-opsin in the immunocytochemistry experiments were carried out in each separate labeling experiment. These Materials and Methods involved use of biotinylated IgG of the preimmune serum at the same IgG Animals and fixation of retinas. Fisher strain rats were obtained at 3, 5, concentrations as those for specific IgG followed by avidin-ferritin, or treat- 6, 7, 9, 11, and 15 days of postnatal development; three additional experi- ment with avidin-ferritin without prior treatment with antibody. Ferritin grains ments were carried out on 3-day-old Sprague-Dawley rats. It is important to were rare on tissues treated by either procedure. After labeling with anti- point out that we studied photoreceptors
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