Gene name - Rab-protein 11
Cytological map position - 93B12--13
Function - signaling
Symbol - Rab11
FlyBase ID: FBgn0015790
Genetic map position - 3R
Classification - GTP-binding protein domain
Cellular location - cytoplasmic
In developing Drosophila photoreceptors, rhodopsin is trafficked to the rhabdomere, a specialized domain within the apical membrane surface. Rab11, a small GTPase implicated in membrane traffic, immunolocalizes to the trans-Golgi network, cytoplasmic vesicles and tubules, and the base of rhabdomeres. One hour after release from the endoplasmic reticulum, rhodopsin colocalizes with Rab11 in vesicles at the base of the rhabdomere. When Rab11 activity is reduced by three different genetic procedures, rhabdomere morphogenesis is inhibited and rhodopsin-bearing vesicles proliferate within the cytosol. Rab11 activity is also essential for development of multivesicular body (MVB) endosomal compartments; this is probably a secondary consequence of impaired rhabdomere development. Furthermore, Rab11 is required for transport of TRP, another rhabdomeric protein, and for development of specialized membrane structures within Garland cells. These results establish a role for Rab11 in the post-Golgi transport of rhodopsin and of other proteins to the rhabdomeric membranes of photoreceptors, and in analogous transport processes in other cells (Satoh, 2005).
Rab11 also functions during oogenesis and during cellularization of Drosophila embryos. The Nuclear fallout and Rab11 function in membrane trafficking and actin remodeling during the initial stages of furrow formation during cellularization. Membrane addition is mediated via endosomal-mediated membrane delivery to the site of furrow formation. Thus Rab11 regulates endosomes as key trafficking intermediates that control vesicle exocytosis and membrane growth during cellularization (Riggs, 2003; Pelissier, 2003). Rab11 is required in endocytic recycling and in the organization of posterior membrane compartments during oogenesis. Rab11 is also required in the organization of microtubule plus ends and osk mRNA localization and translation at the posterior pole (Jankovics, 2001). It is proposed that microtubule plus ends and, possibly, translation factors for osk mRNA are anchored to posterior membrane compartments that are defined by Rab11-mediated trafficking and reinforced by Rab11-Osk interactions (Dollar, 2002).
During photoreceptor terminal differentiation, massive biosynthetic membrane traffic delivers rhodopsin and other phototransduction proteins to an apical plasma membrane subdomain to form photosensory organelles, invertebrate rhabdomeres and vertebrate outer segments. The proper targeting of rhodopsin to this domain is crucial for normal development and, if impaired, leads to retinal degeneration. Rab proteins and their effectors are known to control membrane traffic and maintain distinct organelle identities (Deneka, 2003). Indeed, observations in Xenopus (Deretic, 1995; Moritz, 2001) and Drosophila (Satoh, 1997; Shetty, 1998) support a role for particular Rab proteins in rhodopsin transport to the photosensitive organelles (Satoh, 2005 and references therein).
Surprisingly, these studies on rhodopsin trafficking identified Rab11 as active in exocytosis. Rab11 is thought to regulate endosomal/plasma membrane interactions by controlling membrane traffic through recycling endosomes. These endosomes receive endocytosed plasma membrane and either return it to the cell surface or direct it to degradative pathways. Rab11 localizes to the pericentriolar recycling endosome, the trans-Golgi network (TGN), and post-Golgi vesicles (Chen, 1998; Deretic, 1997; Ullrich, 1996). Dominant-negative Rab11a inhibits apical recycling and basolateral-to-apical transcytosis in polarized MDCK cells (Wang, 2000), blocks stimulus-induced recruitment of endosome-sequestered H+-K+ ATPase-rich membrane to the apical membrane of acid-secreting parietal cells (Duman, 1999), and inhibits exosome release in human leukemic K562 cells (Savina, 2002; Satoh, 2005 and references therein).
In addition to these extensive reports linking Rab11 activity to endocytic pathways, other reports suggest a role for Rab11 in biosynthetic exocytic membrane traffic. In PC12 cells, Rab11 was detected in association with TGN and TGN-derived secretory vesicles (Urbe, 1993). In baby hamster kidney cells, overexpression of dominant-negative Rab11S25N decreased delivery of the basolaterally targeted vesicular stomatitis virus (VSV) G protein to the cell surface (Chen, 1998), and expression of wild-type Rab11a accelerated delivery of new protease activated receptors to kidney epithelial cell surfaces following trypsin exposure (Roosterman, 2003). Of particular interest to this study, the movement of rhodopsin to the apical surface may also be dependent on Rab11. Rhodopsin has been detected in Rab11-positive post-Golgi vesicles of Xenopus retina cell-free extracts (Deretic, 1997). Immature rhodopsin, which is indicative of defective rhodopsin transport, accumulated in Drosophila photoreceptors that expressed dominant-negative Rab11N124I (Satoh, 1998; Satoh, 2005 and references therein).
The movement of rhodopsin and other rhabdomeric membrane proteins has been characterized in the developing Drosophila photoreceptor. This experimental system allows the role of Rab11 in this process to be defined. Vigorous light-dependent endocytosis competes with exocytosis from the outset of rhabdomere morphogenesis. Independent of a requirement in endosomal recycling, Rab11 activity is essential for the initial exocytic rhodopsin delivery to the growing rhabdomere. Loss of Rab11 activity disrupts endocytic pathways, but this is likely to be a secondary consequence of attenuated exocytic delivery. Thus, these results demonstrate Rab11 promotes the trans-Golgi to rhabdomere membrane traffic responsible for elaboration of the sensory membranes of these cells (Satoh, 2005).
Drosophila rhabdomere differentiation begins in mid-pupal life with the establishment of a morphologically and molecularly distinct apical plasma membrane subdomain, which is then amplified and specialized for phototransduction by targeted membrane delivery (Karagiosis, 2004; Longley, 1995). Between ~50% and 60% of pupal development (% pd), nascent rhabdomeres begin to load with TRP, a light-activated Ca2+ channel that serves phototransduction. TRP is first immunodetectable in photoreceptor cytoplasm beginning at about 45% pd and accumulates in large cytoplasmic vesicles by 50% pd. Expression of the major rhodopsin (Rh1), the photosensory protein of photoreceptors R1-6, initiates later, at about 70% pd. Rh1 is first detected as faint diffuse signals and small puncta spread throughout the cytoplasm. In animals raised in standard 12/12 hour light/dark conditions, Rh1 concentrates in large (>200 nm) cytoplasmic vesicles, 'Rh1-containing large vesicles' (RLVs), prior to its appearance in the rhabdomere. During the stage when Rh1 and TRP synthesis overlap, the proteins colocalize in RLVs, suggesting the same vesicle accommodates both rhabdomere proteins. One or more small dots of F-actin decorate each RLV, resembling the actin patches associated with vesicles that mediate transport in yeast and other systems. In both fixed and living preparations, RLVs appear tethered via actin patches to the rhabdomere terminal web (RTW), a specialization of the cortical actin cytoskeleton (Satoh, 2005).
Immunogold electron microscopy using anti-Rh1 identified RLVs as multivesicular bodies (MVBs). The delicate detail of MVBs is poorly preserved by fixation protocols that retain Rh1 antigenicity. However, MVBs are the only organelle observed in the EM with the size, shape and distribution characteristic of RLVs observed in confocal immunofluorescence studies. RLVs label with GFP-tagged Rab7, an endosomal protein that marks the limiting membrane of MVBs in Drosophila. RLVs also immunostain for Hrs, the endosome-associated Hepatocyte growth factor-regulated tyrosine kinase substrate associated with MVBs in Drosophila and other systems. These observations identify RLVs as MVBs (Satoh, 2005).
The early detection of rhodopsin and TRP within MVBs was not anticipated because MVBs are generally considered to be late endosomal compartments, delivering cargo retrieved from the plasma membrane to the lysosomes for degradation. Thus, the presence of Rh1 in an endocytic degradative organelle during the time the cell is increasing its sensory membrane is noteworthy. However, light-dependent endocytosis of Rh1 is well documented in adult photoreceptors, and it is possible that the Rh1 found in RLVs has already been retrieved from the developing rhabdomere (Satoh, 2005).
To investigate this possibility, Rh1 transport was observed in dark-reared flies. Rh1 begins to accumulate in the rhabdomere just after Rh1 expression starts at 70% pd. There are few RLVs in all stages in dark-reared flies. RLVs form within 30 minutes of light exposure, and disappear within 13 hours of return to dark. These observations suggest that in light-reared flies, Rh1 is first transported to the rhabdomere, but light-induced internalization quickly transports Rh1 into RLVs. Thus, even during the developmental period in which the photoreceptor cell is increasing rhabdomeric volume and Rh1 content, vigorous endocytosis can exceed the rate of biosynthetic delivery of Rh1 (Satoh, 2005).
To position Rab11 in the sensory membrane transport pathway, flies expressing the Golgi marker, CFP-galactosyl-transferase and Rab11 were immunolocalized in developing photoreceptors. Rab11 is present on small vesicles (<200 nm in diameter) scattered throughout the cytoplasm. Many of these appear associated with Golgi structures; others are located at the base of developing rhabdomere. Rab11 is not associated with RLVs. To further characterize the Golgi association of Rab11, developing photoreceptors were immunostained for Rab11 and the cis-Golgi marker, Rab1. Rab1 localizes to the convex side of the Golgi, while Rab11 localizes to the opposite, concave, side. Triple staining of CFP-labeled Golgi directly shows Rab1 and Rab11 localize to opposite sides of the Golgi. Therefore, Rab11 must localize to the trans-Golgi surface (Satoh, 2005).
To investigate the possibility that Rab11 is associated with maturing Rh1, blue light was used to trigger synchronized release of Rh1 accumulated in the ER because of lack of the correct chromophore isomer. Rh1 and Rab11 were then immunolocalized in pupal eyes 0, 40, 60, 90 and 180 minutes after blue-light irradiation. Prior to blue light (0 minutes), Rh1 was distributed throughout photoreceptor cytoplasm, colocalizing with an ER marker. At 40 and 60 minutes, most Rh1 colocalized with the Golgi. By 40 minutes, some Rh1 was concentrated in Rab11 immunopositive vesicles, some of these at the rhabdomere base. At both 60 and 90 minutes, Rh1 and Rab11 show extensive colocalization in vesicles at the rhabdomere base. By 90 minutes, there are few Rh1 positive cytoplasmic vesicles, and by 180 minutes most Rh1 is found in the rhabdomeres (Satoh, 2005).
These immunofluorescence results suggest Rh1, upon exit from the ER, associates first with the Golgi, then within Rab11-positive vesicles, before being deposited in the rhabdomere. Thus, the Rab11 localization is consistent with a role in TGN--->rhabdomere transport. Transport visualized in these studies is completed by 180 minutes, in good agreement with previous immunoblot data showing intermediate Rh1 is completely processed into mature 35K Rh1 within 180 minutes (Satoh, 1997). RLVs are not prominent at any time during ER--->rhabdomere transport, consistent with the proposal that RLVs do not participate in biosynthetic traffic (Satoh, 2005).
Thus Rab11 is essential for trafficking two membrane proteins, TRP and Rh1, to the Drosophila photosensory organelle, the rhabdomere. When Rab11 activity is reduced in developing photoreceptors, Golgi-derived TRP- and Rh1-bearing vesicles accumulate in photoreceptor cytoplasm instead of exocytosing to expand the growing rhabdomere. Thus Rab11 activity supports a distinct plasma membrane compartment, the apical plasma membrane subdomain specialized for phototransduction (Satoh, 2005).
Rab11 has been implicated in control of membrane traffic through the pericentriolar recycling endosome. In cultured baby hamster kidney (BHK) cells, return of internalized transferrin receptor to the cell surface is inhibited by dominant-negative Rab11 expression (Ullrich, 1996). During cellularization of Drosophila embryos, apical membrane redeployed to the growing basolateral surface transits a Rab11-dependent recycling endosome (Pelissier, 2003). Rab11 has also been implicated in trans-Golgi to plasma membrane transport. In non-polarized BHK cells in culture, expression of dominant-negative Rab11S25N inhibits transport of a basolateral marker protein marker, vesicular stomatitis virus G protein, but has no impact on delivery of an apical marker protein, influenza hemagglutinin (Chen, 1998). Recent observation that recycling endosomes can serve as an intermediate during transport from the Golgi to MDCK cell plasma membranes raises the possibility that biosynthetic traffic transits a recycling endosome and the site of Rab11 action is at the recycling endosome. However, no pericentriolar endosome has been observed in Drosophila photoreceptors, and Rh1 moves directly from the trans-Golgi to the rhabdomere when released into the biosynthetic pathway by blue light. Thus, there is no evidence for Rh1 moving through an intermediate compartment when en route to the rhabdomere (Satoh, 2005).
Rh1-bearing post-Golgi vesicles and recycling endosome-derived vesicles may both traffick to the cell surface because they share common Rab11 effectors. Rab11 interacts with unconventional class V myosins and expression of dominant-negative Myo-Vb inhibits delivery from early endosomes to the cell surface (Lapierre, 2001). An extensive F-actin terminal web, the RTW, extends from the rhabdomere base into photoreceptor cell cytoplasm (Chang, 2000) and disruption of the photoreceptor actin cytoskeleton inhibits the vesicular traffic that builds crab rhabdomeres. Rab11, together with a Myo-V effector, may promote post-Golgi vesicle motility along the actin RTW to focus delivery to the rhabdomere (Satoh, 2005).
Loss of Rab11 activity also disrupts normal photoreceptor MVB morphology. MVBs are often identified as late endosomal compartments, delivering cargo destined for lysosomal degradation. However, several recent studies show MVBs can be also exocytic carriers, delivering endosomal contents to the cell surface. Examples include the secretory lysosomes of immune system cells, melanosomes of pigment cells, exosomes of maturing red blood cells and secreted vesicles mediating cell-cell signaling. The accumulation of newly synthesized MHCII receptors within MVBs of unstimulated dendritic cells, and the stimulus-induced reorganization of MVBs and appearance of MHCII receptors at the plasma membrane, has led to consideration of MVBs as an exocytic compartment. Autoradiography of crayfish eyes following 3H-leucine injection has shown newly synthesized protein first in the cytoplasm, then in MVBs and then in rhabdomere rhabdomeres, prompting the conjecture that MVBs are a post-Golgi organelle of biosynthetic traffic (Satoh, 2005).
The work reported in this study discounts the possibility that MVBs are exocytic vesicles in Drosophila photoreceptors. It was shown that appearance of Rh1 in the MVBs is dependent on light treatment. Previously, light was shown to trigger endocytosis of rhabdomeric membrane, so this light dependency suggests MVBs originate from an endocytic process. Depletion of Rab5 activity, which is known to regulate the fusion between endocytic vesicles and early endosomes, also eliminates Rh1 and TRP containing MVBs, without affecting Rh1 and TRP transport to the rhabdomere. Thus, all the results support the view that MVBs are endocytic vesicles. The early and rapid appearance of Rh1 and TRP in these vesicles is remarkable, showing that the machinery of light-dependent receptor internalization is fully operational at the outset of morphogenesis. Vigorous light-dependent endocytosis competes with exocytosis from the outset of rhabdomere morphogenesis, internalizing rhodopsin and TRP from the growing sensory membrane even as exocytosis expands it (Satoh, 2005).
Rab5 loss-of-function analysis also supports the view that Rab11 acts before Rab5. In Rab5, Rab11 double mutants, photoreceptors retain the Rab11 phenotype. These results are consistent with a role of Rab11 in the exocytic process, but not with an exclusive role in endocytic recycling. Yet, Rab11 activity is required for accumulation of MVBs. It is proposed that this is an indirect effect of the Rab11 requirement in the exocytic pathway. Rab11 inhibition 'starves' the rhabdomere, the target of Rab11-mediated transport, of required proteins, which in turn slows the rate of endocytosis and eliminates endocytosis-dependent MVBs. In support of this view, Rab11 activity was shown to be required for the presence of labyrinthine channels on Garland cells, membrane specializations that promote vigorous endocytosis. Rab11 loss plausibly depletes membrane components that sustain vigorous endocytosis (Satoh, 2005).
Drosophila and vertebrate photoreceptors share fundamental cellular and molecular mechanisms and Rab family members and their functions are strongly conserved across eukaryotes. Rab11 has been identified in rhodopsin-containing post-Golgi vesicles formed within a vertebrate retina cell-free system (Deretic, 1997), raising the likelihood vertebrate photoreceptors also contain a Rab11-dependent vesicular compartment essential for rhodopsin transport and outer segment development. Failure to traffic Rh1 in Drosophila leads to retinal degeneration, and similar mechanisms are implicated in rhodopsin mutations and other mutations causing the human disease retinitis pigmentosa. The involvement of Rab11 in the post-Golgi processes provides an entry point to discover the cellular components and pathways responsible for elaborating the specialized photosensitive membranes. These events are likely to be key regulators of normal cellular development and the triggering events of retinal disease (Satoh, 2005).
date revised: 15 May 2005
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