Rab proteins and post-Golgi trafficking of rhodopsin in photoreceptor cells

Structural basis of ELKS/Rab6B interaction and its role in vesicle capturing enhanced by liquid-liquid phase separation

ELKS proteins play a key role in organizing intracellular vesicle trafficking and targeting in both neurons and non-neuronal cells. While it is known that ELKS interacts with the vesicular traffic regulator, the Rab6 GTPase, the molecular basis governing ELKS-mediated trafficking of Rab6-coated vesicles has remained unclear. In this study, we solved the Rab6B structure in complex with the Rab6-binding domain of ELKS1, revealing that a C-terminal segment of ELKS1 forms a helical hairpin to recognize Rab6B through a unique binding mode. We further showed that liquid-liquid phase separation (LLPS) of ELKS1 allows it to compete with other Rab6 effectors for binding to Rab6B and accumulate Rab6B-coated liposomes to the protein condensate formed by ELKS1. We also found that the ELKS1 condensate recruits Rab6B-coated vesicles to vesicle releasing sites and promotes vesicle exocytosis. Together, our structural, biochemical, and cellular analyses suggest that ELKS1, via the LLPS-enhanced interaction with Rab6, captures Rab6-coated vesicles from the cargo transport machine for efficient vesicle releasing at exocytotic sites. These findings shed new light on the understanding of spatiotemporal regulation of vesicle trafficking through the interplay between membranous structures and membraneless condensates.

Diffuse or hitch a ride: how photoreceptor lipidated proteins get from here to there

Photoreceptors are polarized neurons, with specific subcellular compartmentalization and unique requirements for protein expression and trafficking. Each photoreceptor contains an outer segment (OS) where vision begins, an inner segment (IS) where protein synthesis occurs and a synaptic terminal for signal transmission to second-order neurons. The OS is a large, modified primary cilium attached to the IS by a slender connecting cilium (CC), the equivalent of the transition zone (TZ). Daily renewal of ~10% of the OS requires massive protein biosynthesis in the IS with reliable transport and targeting pathways. Transport of lipidated (‘sticky’) proteins depends on solubilization factors, phosphodiesterase δ (PDEδ) and uncoordinated protein-119 (UNC119), and the cargo dispensation factor (CDF), Arf-like protein 3-guanosine triphosphate (ARL3-GTP). As PDE6 and transducin still reside prominently in the OS of PDEδ and UNC119 germline knockout mice, respectively, we propose the existence of an alternate trafficking pathway, whereby lipidated proteins migrate in rhodopsin-containing vesicles of the secretory pathway.

Rab11, a vesicular trafficking protein, affects endoreplication through Ras-mediated pathway in Drosophila melanogaster

Rab11, a small monomeric GTPase associated with recycling endosomes, is a key molecule in the regulation of vesicular trafficking and is involved in the development and differentiation of many Drosophila tissues through interaction with diverse signaling pathways. In this study, we report for the first time that Rab11 affects endoreplication through a Ras-mediated pathway. Suppression of Rab11 activity in salivary glands, an endoreplicating tissue, leads to reduction in size of salivary glands with cells having a small nucleus. Endoreplication-regulating proteins, CycE, E2f1 and Gem, are also down-regulated in Rab11 knocked-down salivary glands suggesting that Rab11 has a role in the process of endoreplication, possibly indirectly through other pathways that regulate cell cycle progression. Ras signaling plays an important role in cell cycle progression through G/S phase transition. Ectopic expression of activated Ras in salivary glands of Rab11 down-regulated individuals rescues the small-sized glands to intermediate size. Furthermore, we observed altered localization of Ras in Rab11 down-regulated salivary glands. It is likely that the low level of endoreplication in the Rab11 down-regulated condition is Ras-mediated.

Biology and therapy of inherited retinal degenerative disease: insights from mouse models

Retinal neurodegeneration associated with the dysfunction or death of photoreceptors is a major cause of incurable vision loss. Tremendous progress has been made over the last two decades in discovering genes and genetic defects that lead to retinal diseases. The primary focus has now shifted to uncovering disease mechanisms and designing treatment strategies, especially inspired by the successful application of gene therapy in some forms of congenital blindness in humans. Both spontaneous and laboratory-generated mouse mutants have been valuable for providing fundamental insights into normal retinal development and for deciphering disease pathology. Here, we provide a review of mouse models of human retinal degeneration, with a primary focus on diseases affecting photoreceptor function. We also describe models associated with retinal pigment epithelium dysfunction or synaptic abnormalities. Furthermore, we highlight the crucial role of mouse models in elucidating retinal and photoreceptor biology in health and disease, and in the assessment of novel therapeutic modalities, including gene- and stem-cell-based therapies, for retinal degenerative diseases.

Rab11 plays an indispensable role in the differentiation and development of the indirect flight muscles in Drosophila

Rab11, an evolutionary conserved, ubiquitously expressed subfamily of small monomeric GTPase has been known to regulate diverse cellular and developmental events, by regulating the exocytotic and transcytotic events inside the cell. Our studies show that Rab11 regulates Drosophila adult myogenesis by controlling proliferation and differentiation of the Adult muscle precursors (AMPs). Blocking Rab11 in the AMPs, which fuse to form the Indirect Flight Muscles (IFMs) of fly, renders flies completely flightless and non-viable. The indirect flight musculature, comprising of the differentially patterned dorsal longitudinal muscles (DLMs) and dorsal ventral muscles (DVMs), is affected to different extents. Abrogating or knocking down normal Rab11 function results in severely disrupted IFMs. DLMs forming from larval templates are reduced in number along with a significant reduction in their fibre size. The de novo developing DVMs are frequently absent. The DLMs in Rab11 hypomorphs are highly reduced, showing as a small constricted mass in one half of the thorax. Further, Rab11 function is essential for growth of these muscles during later half of adult myogenesis, as down regulation of Rab11 in IFMs results in degenerated muscles and broken fibres. Finally, we show that loss of Rab11 activity in the AMPs result in acquisition of migratory characteristic of myoblast as they show cellular protrusion at their polar ends accompanied with loss of cell-cell contacts. Our data provide the first evidence of a trafficking protein playing an indispensable role in regulating early stages of adult muscle development.

Mutations in RAB28, encoding a farnesylated small GTPase, are associated with autosomal-recessive cone-rod dystrophy

The majority of the genetic causes of autosomal-recessive (ar) cone-rod dystrophy (CRD) are currently unknown. A combined approach of homozygosity mapping and exome sequencing revealed a homozygous nonsense mutation (c.565C>T [p.Glu189*]) in RAB28 in a German family with three siblings with arCRD. Another homozygous nonsense mutation (c.409C>T [p.Arg137*]) was identified in a family of Moroccan Jewish descent with two siblings affected by arCRD. All five affected individuals presented with hyperpigmentation in the macula, progressive loss of the visual acuity, atrophy of the retinal pigment epithelium, and severely reduced cone and rod responses on the electroretinogram. RAB28 encodes a member of the Rab subfamily of the RAS-related small GTPases. Alternative RNA splicing yields three predicted protein isoforms with alternative C-termini, which are all truncated by the nonsense mutations identified in the arCRD families in this report. Opposed to other Rab GTPases that are generally geranylgeranylated, RAB28 is predicted to be farnesylated. Staining of rat retina showed localization of RAB28 to the basal body and the ciliary rootlet of the photoreceptors. Analogous to the function of other RAB family members, RAB28 might be involved in ciliary transport in photoreceptor cells. This study reveals a crucial role for RAB28 in photoreceptor function and suggests that mutations in other Rab proteins may also be associated with retinal dystrophies.

Regulation of G protein-coupled receptor trafficking and signaling by Rab GTPases

Rab GTPases play an essential role in the regulation of intracellular transport including the budding, tethering, and fusion of vesicles as well as organelle motility. The regulation of G protein-coupled receptor (GPCR) trafficking by Rab GTPases has traditionally been regarded as a non-specific process that facilitates the movement of the receptors between intracellular membrane compartments. Thus, alterations in GPCR signal transduction and trafficking following the overexpression of constitutively active and dominant negative Rabs were originally considered to be solely the passive by-product of perturbations in intracellular compartmental dynamics. Recently, an explosion of experimental studies has provided increasingly convincing evidence that receptor trafficking actively affects the signal transduction of cargo proteins and that the signaling of GPCR vesicular cargo can in turn modulate Rab GTPase regulated intracellular transport processes. This research is revealing how different Rabs coordinate with themselves and other regulatory molecules to mediate protein trafficking, as well as uncovers novel functions for traditional Rabs, while illustrating the active role these trafficking molecules play in pathology of disease. Recently published in the Journal of Neuroscience, Esseltine et al., present a novel role for the typified exocytic small G protein Rab8 in the intracellular trafficking and signal transduction of metabotropic glutamate receptor 1.

Rab8 modulates metabotropic glutamate receptor subtype 1 intracellular trafficking and signaling in a protein kinase C-dependent manner

Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors (GPCRs) that are activated by glutamate, the primary excitatory neurotransmitter in the CNS. Alterations in glutamate receptor signaling are implicated in neuropathologies such as Alzheimer's disease, ischemia, and Huntington's disease among others. Group 1 mGluRs (mGluR1 and mGluR5) are primarily coupled to Gα(q/11) leading to the activation of phospholipase C and the formation of diacylglycerol and inositol 1,4,5-trisphosphate, which results in the release of intracellular calcium stores and protein kinase C (PKC) activation. Desensitization, endocytosis, and recycling are major mechanisms of GPCR regulation, and the intracellular trafficking of GPCRs is linked to the Rab family of small G proteins. Rab8 is a small GTPase that is specifically involved in the regulation of secretory/recycling vesicles, modulation of the actin cytoskeleton, and cell polarity. Rab8 has been shown to regulate the synaptic delivery of AMPA receptors during long-term potentiation and during constitutive receptor recycling. We show here that Rab8 interacts with the C-terminal tail of mGluR1a in an agonist-dependent manner and plays a role in regulating of mGluR1a signaling and intracellular trafficking in human embryonic kidney 293 cells. Specifically, Rab8 expression attenuates mGluR1a-mediated inositol phosphate formation and calcium release from mouse neurons in a PKC-dependent manner, while increasing cell surface mGluR1a expression via decreased receptor endocytosis. These experiments provide us with an understanding of the role Rabs play in coordinated regulation of mGluR1a and how this impacts mGluR1a signaling.

A new vesicular scaffolding complex mediates the G-protein-coupled 5-HT1A receptor targeting to neuronal dendrites

Although essential for their neuronal function, the molecular mechanisms underlying the dendritic targeting of serotonin G-protein-coupled receptors are poorly understood. Here, we characterized a Yif1B-dependent vesicular scaffolding complex mediating the intracellular traffic of the rat 5-HT(1A) receptor (5-HT(1A)R) toward dendrites. By combining directed mutagenesis, GST-pull down, and surface plasmon resonance, we identified a tribasic motif in the C-tail of the 5-HT(1A)R on which Yif1B binds directly with high affinity (K(D) ≈ 37 nM). Moreover, we identified Yip1A, Rab6, and Kif5B as new partners of the 5-HT(1A)R/Yif1B complex, and showed that their expression in neurons is also crucial for the dendritic targeting of the 5-HT(1A)R. Live videomicroscopy revealed that 5-HT(1A)R, Yif1B, Yip1A, and Rab6 traffic in vesicles exiting the soma toward the dendritic tree, and also exhibit bidirectional motions, sustaining their role in 5-HT(1A)R dendritic targeting. Hence, we propose a new trafficking pathway model in which Yif1B is the scaffold protein recruiting the 5-HT(1A)R in a complex including Yip1A and Rab6, with Kif5B and dynein as two opposite molecular motors coordinating the traffic of vesicles along dendritic microtubules. This targeting pathway opens new insights for G-protein-coupled receptors trafficking in neurons.

A role for prenylated rab acceptor 1 in vertebrate photoreceptor development

Background

The rd1 mouse retina is a well-studied model of retinal degeneration where rod photoreceptors undergo cell death beginning at postnatal day (P) 10 until P21. This period coincides with photoreceptor terminal differentiation in a normal retina. We have used the rd1 retina as a model to investigate early molecular defects in developing rod photoreceptors prior to the onset of degeneration.

Results

Using a microarray approach, we performed gene profiling comparing rd1 and wild type (wt) retinas at four time points starting at P2, prior to any obvious biochemical or morphological differences, and concluding at P8, prior to the initiation of cell death. Of the 143 identified differentially expressed genes, we focused on Rab acceptor 1 (Rabac1), which codes for the protein Prenylated rab acceptor 1 (PRA1) and plays an important role in vesicular trafficking. Quantitative RT-PCR analysis confirmed reduced expression of PRA1 in rd1 retina at all time points examined. Immunohistochemical observation showed that PRA1-like immunoreactivity (LIR) co-localized with the cis-Golgi marker GM-130 in the photoreceptor as the Golgi translocated from the perikarya to the inner segment during photoreceptor differentiation in wt retinas. Diffuse PRA1-LIR, distinct from the Golgi marker, was seen in the distal inner segment of wt photoreceptors starting at P8. Both plexiform layers contained PRA1 positive punctae independent of GM-130 staining during postnatal development. In the inner retina, PRA1-LIR also colocalized with the Golgi marker in the perinuclear region of most cells. A similar pattern was seen in the rd1 mouse inner retina. However, punctate and significantly reduced PRA1-LIR was present throughout the developing rd1 inner segment, consistent with delayed photoreceptor development and abnormalities in Golgi sorting and vesicular trafficking.

Conclusions

We have identified genes that are differentially regulated in the rd1 retina at early time points, which may give insights into developmental defects that precede photoreceptor cell death. This is the first report of PRA1 expression in the retina. Our data support the hypothesis that PRA1 plays an important role in vesicular trafficking between the Golgi and cilia in differentiating and mature rod photoreceptors.

Ciliary transition zone (TZ) proteins RPGR and CEP290: role in photoreceptor cilia and degenerative diseases

INTRODUCTION

Primary cilia are microtubule-based extensions of the plasma membrane in nearly all cell types. In vertebrate photoreceptors, the sensory cilium develops as outer segment (OS) that contains the photopigment rhodopsin and other proteins necessary for phototransduction. The distinct composition of proteins and lipids in the OS membrane is maintained by the selective barrier located at the border between the basal body and the ciliary compartment, called the transition zone (TZ).

AREAS COVERED

In this review, we will discuss the identification and function of two ciliary TZ proteins, RPGR (retinitis pigmentosa GTPase regulator) and CEP290. Mutations in these proteins account for a majority of retinopathies due to ciliary dysfunction. We will also discuss the potential of such information in designing therapeutic approaches to treat cilia-dependent photoreceptor degenerative diseases.

EXPERT OPINION

RPGR and CEP290 perform overlapping yet distinct functions in regulating trafficking of cargo via the TZ of photoreceptors. While RPGR modulates the trafficking by acting as a GEF for the small GTPase RAB8A, CEP290 may be involved in maintaining the polarized distribution of proteins in the OS by modulating intracellular levels of selected proteins involved in inhibiting OS formation.

Targeting of mouse guanylate cyclase 1 (Gucy2e) to Xenopus laevis rod outer segments

Photoreceptor guanylate cyclase (GC1) is a transmembrane protein and responsible for synthesis of cGMP, the secondary messenger of phototransduction. It consists of an extracellular domain, a single transmembrane domain, and an intracellular domain. It is unknown how GC1 targets to the outer segments where it resides. To identify a putative GC1 targeting signal, we generated a series of peripheral membrane and transmembrane constructs encoding extracellular and intracellular mouse GC1 fragments fused to EGFP. The constructs were expressed in Xenopus laevis rod photoreceptors under the control of the rhodopsin promoter. We examined the localization of GFP-GC1 fusion proteins containing the complete GC1 sequence, or partial GC1 sequences, which were membrane-associated via either the GC1 transmembrane domain or the rhodopsin C-terminal palmitoyl chains. Full-length GFP-GC1 targeted to the rod outer segment disk rims. As a group, fusion proteins containing the entire cytoplasmic domain of GC1 targeted to the OS, whereas other fusion proteins containing portions of the cytoplasmic or the extracellular domains did not. We conclude that GC1 likely has no single linear peptide-based OS targeting signal. Our results suggest targeting is due to either multiple weak signals in the cytoplasmic domain of GC1, or co-transport to the OS with an accessory protein.

Immunocytochemical evidence of Tulp1-dependent outer segment protein transport pathways in photoreceptor cells

Tulp1 is a protein of unknown function exclusive to rod and cone photoreceptor cells. Mutations in the gene cause autosomal recessive retinitis pigmentosa in humans and photoreceptor degeneration in mice. In tulp1-/- mice, rod and cone opsins are mislocalized, and rhodopsin-bearing extracellular vesicles accumulate around the inner segment, indicating that Tulp1 is involved in protein transport from the inner segment to the outer segment. To investigate this further, we sought to define which outer segment transport pathways are Tulp1-dependent. We used immunohistochemistry to examine the localization of outer segment proteins in tulp1-/- photoreceptors, prior to retinal degeneration. We also surveyed the condition of inner segment organelles and rhodopsin transport machinery proteins. Herein, we show that guanylate cyclase 1 and guanylate cyclase activating proteins 1 and 2 are mislocalized in the absence of Tulp1. Furthermore, arrestin does not translocate to the outer segment in response to light stimulation. Additionally, data from the tulp1-/- retina adds to the understanding of peripheral membrane protein transport, indicating that rhodopsin kinase and transducin do not co-transport in rhodopsin carrier vesicles and phosphodiesterase does not co-transport in guanylate cyclase carrier vesicles. These data implicate Tulp1 in the transport of selective integral membrane outer segment proteins and their associated proteins, specifically, the opsin and guanylate cyclase carrier pathways. The exact role of Tulp1 in outer segment protein transport remains elusive. However, without Tulp1, two rhodopsin transport machinery proteins exhibit abnormal distribution, Rab8 and Rab11, suggesting a role for Tulp1 in vesicular docking and fusion at the plasma membrane near the connecting cilium.

Retinal degeneration and failure of photoreceptor outer segment formation in mice with targeted deletion of the Joubert syndrome gene, Ahi1

Vertebrate photoreceptors have a modified cilium composed of a basal body, axoneme and outer segment. The outer segment includes stacked membrane discs, containing opsin and the signal transduction apparatus mediating phototransduction. In photoreceptors, two distinct classes of vesicles are trafficked. Synaptic vesicles are transported down the axon to the synapse, whereas opsin-containing vesicles are transported to the outer segment. The continuous replacement of the outer segments imposes a significant biosynthetic and trafficking burden on the photoreceptors. Here, we show that Ahi1, a gene that when mutated results in the neurodevelopmental disorder, Joubert syndrome (JBTS), is required for photoreceptor sensory cilia formation and the development of photoreceptor outer segments. In mice with a targeted deletion of Ahi1, photoreceptors undergo early degeneration. Whereas synaptic proteins are correctly trafficked, photoreceptor outer segment proteins fail to be transported appropriately or are significantly reduced in their expression levels (i.e., transducin and Rom1) in Ahi1(-/-) mice. We show that vesicular targeting defects in Ahi1(-/-) mice are cilium specific, and our evidence suggests that the defects are caused by a decrease in expression of the small GTPase Rab8a, a protein required for accurate polarized vesicular trafficking. Thus, our results suggest that Ahi1 plays a role in stabilizing the outer segment proteins, transducin and Rom1, and that Ahi1 is an important component of Rab8a-mediated vesicular trafficking in photoreceptors. The retinal degeneration observed in Ahi1(-/-) mice recapitulates aspects of the retinal phenotype observed in patients with JBTS and suggests the importance of Ahi1 in photoreceptor function.

Rab10 associates with primary cilia and the exocyst complex in renal epithelial cells

Rab10, a mammalian homolog of the yeast Sec4p protein, has previously been associated with endocytic recycling and biosynthetic membrane transport in cultured epithelia and with Glut4 translocation in adipocytes. Here, we report that Rab10 associates with primary cilia in renal epithelia in culture and in vivo. In addition, we find that Rab10 also colocalizes with exocyst proteins at the base of nascent cilia, and physically interacts with the exocyst complex, as detected with anti-Sec8 antibodies. These data suggest that membrane transport to the primary cilum may be mediated by interactions between Rab10 and an exocyst complex located at the cilium base.

Differential role of Rab proteins in ciliary trafficking: Rab23 regulates Smoothened levels

The structure and function of the primary cilium as a sensory organelle depends on a motor-protein-powered intraflagellar transport system (IFT); defective IFT results in retinal degeneration and pleiotropic disorders such as the Bardet Biedl syndrome (BBS) and defective hedgehog (HH) signaling. Protein transport to the cilium involves Rab GTPases. Rab8, together with a multi protein complex of BBS proteins, recruits cargo to the basal body for transport to the cilium. Loss of Rab23 in mice recapitulates the HH phenotype but its function in HH signaling is unknown. Here we established a novel protocol, based on fluorescence recovery after photo-bleaching (FRAP), allowing the quantitative analysis of protein transport into the cilium of MDCK cells. We compared the effect of Rab8, Rab5 and Rab23 on the ciliary transport of the HH-associated transmembrane receptor Smoothened, the microtubular tip protein EB1, and the receptor protein Kim1. Ciliary FRAP confirmed the role of Rab8 in protein entry to the cilium. Dominant negative Rab5 had no impact on the ciliary transport of Smoothened or EB1, but slowed the recovery of the apical protein Kim1 in the cilium. Depletion of Rab23 or expression of dominant-negative Rab23 decreased the ciliary steady state specifically of Smoothened but not EB1 or Kim1, suggesting a role of Rab23 in protein turnover in the cilium.

Melatonin reprogrammes proteomic profile in light-exposed retina in vivo

Melatonin, a small organic molecule synthesized by the pineal gland and the retina, has a variety of physiologic functions such as circadian clock pacemaker and antioxidant. Retinal melatonin is down-regulated by light and is barely detectable during the day. The absence of melatonin in the retina during prolonged light exposure may contribute to light-induced retinal degeneration. We sought to investigate the impact of melatonin in the light-exposed retina using proteomic approaches. We exposed mice to either light (250-300lux) for 12h followed by 12h of darkness or the same intensity of continuous light for 7 days. In half of the animals exposed to continuous light, melatonin was injected each night. Proteomic analysis of the retina from these three groups of animals showed that five proteins prominently up-regulated by constant light were down-regulated by melatonin treatment. These five proteins were identified as vimentin, serine/threonine-protein phosphatase 2A, Rab GDP dissociation inhibitor alpha, guanine nucleotide-binding protein G(o) alpha, and retinaldehyde-binding protein. These five proteins are known to be involved in several cellular processes that may contribute to light-induced retinal degeneration. Identification of melatonin target proteins in our study provides a basis for future studies on melatonin's potential in preventing or treating light-induced retinal degeneration.

Cell type-specific and light-dependent expression of Rab1 and Rab6 GTPases in mammalian retinas

The Ras-like Rab1 and Rab6 GTPases modulate protein traffic along the early secretory pathway and are involved in the regulation of maturation of rhodopsin in the outer retina. However, Rab GTPases have not been studied in the inner retinas. Here, we analyzed the anatomatic distribution and expression of Rab1 and Rab6 in the mouse and rat retinas by immunohistochemistry and immunoblotting. We found that Rab1 was specifically expressed in the rod bipolar cells, while Rab6 was expressed in a different cell type(s) from rod bipolar cells in the inner retina. We also demonstrated that expression of Rab1 and Rab6 was increased with light. These data provided the first evidence implicating that Rab1 and Rab6 may be involved in the regulation of the retinal adaptation.

Roles of endosomal trafficking in neurite outgrowth and guidance

Membrane trafficking and cargo delivery are essential for axonal and dendritic growth and guidance. Neurons have numerous diverse post-Golgi vesicles and recent advances have clarified their identity and regulation. Combinatorial approaches using in vivo imaging of 'intracellular cargo address labels' and functional perturbation have provided insight into these processes. In particular, the UNC-51 kinase regulates the trafficking of early endosomes and their axon guidance molecular cargos in several types of neurons in multiple organisms. Vesicular compartments bearing features of recycling endosomes, late endosomes or lysosomes also contribute to membrane addition and protein trafficking during neurite outgrowth and extension. New work shows that ubiquitylation of cargos and Rab effectors further specifies the trafficking routes of post-Golgi vesicles. These findings have begun to provide a more detailed view of the molecular mechanisms involved in neurite outgrowth and guidance. Additionally, high-resolution light microscopy imaging promises greater temporal and spatial understanding of vesicular exchange and maturation in neurons in the near future.

Mutation in Rab11 results in abnormal organization of ommatidial cells and activation of JNK signaling in the Drosophila eye

Rab11(mo), a P insertion line of Rab11 showed degenerated ommatidia and excess cell death in larval/pupal eyes. Here, we demonstrate that Rab11 is essential for normal organization of ommatidial cells and their survival in Drosophila, and a mutation in this gene results in cytoskeleton disruption and activation of JNK signaling in the eye. The spatial organization of various cell types in compound eye, viz., cone, photoreceptor, pigment and bristle cells, were disrupted in Rab11 mutants as revealed by immunostaining of F-actin and adherens and septate junction proteins. Genetic interaction studies indicated that mutation in Rab11 upregulates Drosophila apoptotic genes, rpr, hid and grim. In order to study the pathway that causes excessive cell death in Rab11 mutants, the JNK pathway was chosen and genetic interaction analyses were carried out between Rab11 and candidates of the JNK signaling pathway. A downregulation of JNK signaling rescued the phenotype in Rab11 mutant eyes significantly while overexpression of JNK in the eyes using UAS-eiger, UAS-dtak1 or EP(2)0578, resulted in enhancement of the eye phenotype indicating a link between Rab11 and the JNK signaling pathway.

Cellular trafficking of lipoteichoic acid and Toll-like receptor 2 in relation to signaling: role of CD14 and CD36

Lipoteichoic acid (LTA) is a central inducer of inflammatory responses caused by Gram-positive bacteria, such as Staphylococcus aureus, via activation of TLR2. Localization of TLR2 in relation to its coreceptors may be important for function. This study explores the signaling, uptake, and trafficking pattern of LTA in relation to expression of TLR2 and its coreceptors CD36 and CD14 in human monocytes. We found TLR2 expressed in early endosomes, late endosomes/lysosomes, and in Rab-11-positive compartments but not in the Golgi apparatus or endoplasmic reticulum (ER). Rapid internalization of fluorescently labeled LTA was observed in human monocytes, colocalizing with markers for early and late endosomes, lysosomes, ER, and Golgi network. Blocking CD14 and CD36 with antibodies inhibited LTA binding and LTA-induced TNF release from monocytes, emphasizing an important role for both molecules as coreceptors for TLR2. Importantly, blocking CD36 did not affect TNF release induced by N-palmitoyl-S-[2,3-bis(palmitoyloxy)-(2R,S)-propyl]-(R)-cysteinyl-seryl-(lysyl)3-lysine or LPS. Expression of CD14 markedly enhanced LTA binding to the plasma membrane and also enhanced NF-kappaB activation. LTA internalization, but not NF-kappaB activation, was inhibited in Dynamin-I K44A dominant-negative transfectants, suggesting that LTA is internalized by receptor-mediated endocytosis but that internalization is not required for signaling. In fact, immobilizing LTA and thereby inhibiting internalization resulted in enhanced TNF release from monocytes. Our results suggest that LTA signaling preferentially occurs at the plasma membrane, is independent of internalization, and is facilitated by CD36 and CD14 as coreceptors for TLR2.

Proteomics of photoreceptor outer segments identifies a subset of SNARE and Rab proteins implicated in membrane vesicle trafficking and fusion

The outer segment is a specialized compartment of vertebrate rod and cone photoreceptor cells where phototransduction takes place. In rod cells it consists of an organized stack of disks enclosed by a separate plasma membrane. Although most proteins involved in phototransduction have been identified and characterized, little is known about the proteins that are responsible for outer segment structure and renewal. In this study we used a tandem mass spectrometry-based proteomics approach to identify proteins in rod outer segment preparations as an initial step in defining their roles in photoreceptor structure, function, renewal, and degeneration. Five hundred and sixteen proteins were identified including 41 proteins that function in rod and cone phototransduction and the visual cycle and most proteins previously shown to be involved in outer segment structure and metabolic pathways. In addition, numerous proteins were detected that have not been previously reported to be present in outer segments including a subset of Rab and SNARE proteins implicated in vesicle trafficking and membrane fusion. Western blotting and immunofluorescence microscopy confirmed the presence of Rab 11b, Rab 18, Rab 1b, and Rab GDP dissociation inhibitor in outer segments. The SNARE proteins, VAMP2/3, syntaxin 3, N-ethylmaleimide-sensitive factor, and Munc 18 detected in outer segment preparations by mass spectrometry and Western blotting were also observed in outer segments by immunofluorescence microscopy. Syntaxin 3 and N-ethylmaleimide- sensitive factor had a restricted localization at the base of the outer segments, whereas VAMP2/3 and Munc 18 were distributed throughout the outer segments. These results suggest that Rab and SNARE proteins play a role in vesicle trafficking and membrane fusion as part of the outer segment renewal process. The data set generated in this study is a valuable resource for further analysis of photoreceptor outer segment structure and function.

Sec15 interacts with Rab11 via a novel domain and affects Rab11 localization in vivo

Sec15, a component of the exocyst, recognizes vesicle-associated Rab GTPases, helps target transport vesicles to the budding sites in yeast and is thought to recruit other exocyst proteins. Here we report the characterization of a 35-kDa fragment that comprises most of the C-terminal half of Drosophila melanogaster Sec15. This C-terminal domain was found to bind a subset of Rab GTPases, especially Rab11, in a GTP-dependent manner. We also provide evidence that in fly photoreceptors Sec15 colocalizes with Rab11 and that loss of Sec15 affects rhabdomere morphology. Determination of the 2.5-A crystal structure of the C-terminal domain revealed a novel fold consisting of ten alpha-helices equally distributed between two subdomains (N and C subdomains). We show that the C subdomain, mainly via a single helix, is sufficient for Rab binding.

Rab11 mediates post-Golgi trafficking of rhodopsin to the photosensitive apical membrane of Drosophila photoreceptors

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 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.

Phosphoinositides, ezrin/moesin, and rac1 regulate fusion of rhodopsin transport carriers in retinal photoreceptors

The post-Golgi trafficking of rhodopsin in photoreceptor cells is mediated by rhodopsin-bearing transport carriers (RTCs) and regulated by the small GTPase rab8. In this work, we took a combined pharmacological-proteomic approach to uncover new regulators of RTC trafficking toward the specialized light-sensitive organelle, the rod outer segment (ROS). We perturbed phospholipid synthesis by activating phospholipase D with sphingosine 1-phosphate (S1P) or inhibiting phosphatidic acid phosphohydrolase by propranolol (Ppl). S1P stimulated the overall rate of membrane trafficking toward the ROS. Ppl stimulated budding of RTCs, but blocked membrane delivery to the ROS. Ppl caused accumulation of RTCs in the vicinity of the fusion sites, suggesting a defect in tethering, similar to the previously described phenotype of the rab8T22N mutant. Proteomic analysis of RTCs accumulated upon Ppl treatment showed a significant decrease in phosphatidylinositol-4,5-bisphosphate-binding proteins ezrin and/or moesin. Ppl induced redistribution of moesin, actin and the small GTPase rac1 from RTCs into the cytosol. By confocal microscopy, ezrin/moesin and rac1 colocalized with rab8 on RTCs at the sites of their fusion with the plasma membrane; however, this distribution was lost upon Ppl treatment. Our data suggest that in photoreceptors phosphatidylinositol-4,5-bisphosphate, moesin, actin, and rac1 act in concert with rab8 to regulate tethering and fusion of RTCs. Consequentially, they are necessary for rhodopsin-laden membrane delivery to the ROS, thus controlling the critical steps in the biogenesis of the light-detecting organelle.

Evectins: vesicular proteins that carry a pleckstrin homology domain and localize to post-Golgi membranes

We have identified two vesicular proteins, designated evectin (evt)-1 and -2. These proteins are approximately 25 kDa in molecular mass, lack a cleaved N-terminal signal sequence, and appear to be inserted into membranes through a C-terminal hydrophobic anchor. They also carry a pleckstrin homology domain at their N termini, which potentially couples them to signal transduction pathways that result in the production of lipid second messengers. evt-1 is specific to the nervous system, where it is expressed in photoreceptors and myelinating glia, polarized cell types in which plasma membrane biosynthesis is prodigious and regulated; in contrast, evt-2 is widely expressed in both neural and nonneural tissues. In photoreceptors, evt-1 localizes to rhodopsin-bearing membranes of the post-Golgi, an important transport compartment for which specific molecular markers have heretofore been lacking. The structure and subcellular distribution of evt-1 strongly implicate this protein as a mediator of post-Golgi trafficking in cells that produce large membrane-rich organelles. Its restricted cellular distribution and genetic locus make it a candidate gene for the inherited human retinopathy autosomal dominant familial exudative vitreoretinopathy and suggest that it also may be a susceptibility gene for multiple sclerosis.

Retinal targets for calmodulin include proteins implicated in synaptic transmission

Ca2+ influxes regulate multiple events in photoreceptor cells including phototransduction and synaptic transmission. An important Ca2+ sensor in Drosophila vision appears to be calmodulin since a reduction in levels of retinal calmodulin causes defects in adaptation and termination of the photoresponse. These functions of calmodulin appear to be mediated, at least in part, by four previously identified calmodulin-binding proteins: the TRP and TRPL ion channels, NINAC and INAD. To identify additional calmodulin-binding proteins that may function in phototransduction and/or synaptic transmission, we conducted a screen for retinal calmodulin-binding proteins. We found eight additional calmodulin-binding proteins that were expressed in the Drosophila retina. These included six targets that were related to proteins implicated in synaptic transmission. Among these six were a homolog of the diacylglycerol-binding protein, UNC13, and a protein, CRAG, related to Rab3 GTPase exchange proteins. Two other calmodulin-binding proteins included Pollux, a protein with similarity to a portion of a yeast Rab GTPase activating protein, and Calossin, an enormous protein of unknown function conserved throughout animal phylogeny. Thus, it appears that calmodulin functions as a Ca2+ sensor for a broad diversity of retinal proteins, some of which are implicated in synaptic transmission.

Regulation of sorting and post-Golgi trafficking of rhodopsin by its C-terminal sequence QVS(A)PA

Several mutations that cause severe forms of the human disease autosomal dominant retinitis pigmentosa cluster in the C-terminal region of rhodopsin. Recent studies have implicated the C-terminal domain of rhodopsin in its trafficking on specialized post-Golgi membranes to the rod outer segment of the photoreceptor cell. Here we used synthetic peptides as competitive inhibitors of rhodopsin trafficking in the frog retinal cell-free system to delineate the potential regulatory sequence within the C terminus of rhodopsin and model the effects of severe retinitis pigmentosa alleles on rhodopsin sorting. The rhodopsin C-terminal sequence QVS(A)PA is highly conserved among different species. Peptides that correspond to the C terminus of bovine (amino acids 324-348) and frog (amino acids 330-354) rhodopsin inhibited post-Golgi trafficking by 50% and 60%, respectively, and arrested newly synthesized rhodopsin in the trans-Golgi network. Peptides corresponding to the cytoplasmic loops of rhodopsin and other control peptides had no effect. When three naturally occurring mutations: Q344ter (lacking the last five amino acids QVAPA), V345M, and P347S were introduced into the frog C-terminal peptide, the inhibitory activity of the peptides was no longer detectable. These observations suggest that the amino acids QVS(A)PA comprise a signal that is recognized by specific factors in the trans-Golgi network. A lack of recognition of this sequence, because of mutations in the last five amino acids causing autosomal dominant retinitis pigmentosa, most likely results in abnormal post-Golgi membrane formation and in an aberrant subcellular localization of rhodopsin.